Metal Salen- and Salphen-Containing Organic Polymers: Synthesis and Applications

Abstract

The properties of organic polymeric materials can be chemically fine-tuned by the implementation of functional groups or units within the backbone. Especially the inclusion of coordinated metal centers offers a nearly infinite toolbox to adjust properties and thus potential applications. In particular, salen and salphen complexes are widely known to be highly efficient homogenous catalysts. They are also used as luminescent materials and devices or as supramolecular building blocks. This review focusses on the class of salen- and salphen-containing organic polymers, from 1D to 3D. Besides the comparison of synthetic polymerization methods, properties and applications are discussed, with an emphasis on porous 2D and 3D polymeric metal salphens and salens for heterogeneous catalysis and gas sorption.

Key words

Porous polymers - heterogeneous catalysis - conductive films - salens - salphens - gas sorption

Biosketches

Sven M. Elbert received his B.Sc. as well as M.Sc. from Ulm University. He conducted his studies for the Master thesis under the supervision of Michael Mastalerz, and got the Barbara-Mez-Starck award for his thesis. He joined the Mastalerz group moving to the Ruprecht-Karls-University of Heidelberg in 2013 to conduct his PhD studies (supported by a scholarship of Studienstiftung des Deutschen Volkes), obtaining the doctoral degree with distinction (summa cum laude) in 2018. Since then, he is the scientific coordinator in the Mastalerz group, focusing his research on porous materials as well as π-extended nonplanar molecules.

Michael Mastalerz studied Chemistry at the Gerhard-Mercator University of Duisburg and after 9 semesters received his diploma (which was awarded by the University) in 2002. He followed Prof. Dr. Gerald Dyker to the Ruhr-University of Bochum to pursue his PhD studies, receiving his doctoral degree in 2005 with distinction (summa cum laude). After a short stay in industry he joined the group of Prof. Dr. Gregory Fu at the Massachusetts Institute of Technology in the United States in 2006 to work on asymmetric cross-coupling reactions. In 2007 he moved back to Germany to Ulm University affiliated to the group of Prof. Dr. Peter Bäuerle to start his independent career working on porous materials and molecules. He received his venia legendi in February 2013 and subsequently (April 2013) got a position at the Ruprecht-Karls-University of Heidelberg as a full professor. Currently, he is an ERC Consolidator Grant holder. His research focusses on organic cage compounds, porous polymeric materials, supramolecular chemistry, nonplanar extended aromatic molecules as well as the development of new synthetic methods.

Introduction

The imine condensation of two equivalents of salicylaldehyde with one equivalent of 1,2-ethylene- or phenylene diamine is the underlying reaction to synthesize the ligand classes called salens and salphens (sometimes also named salophens) ([Figure 1]). Since the first discovery of the structurally related acacen ligand in 1889 by Combes,[1] numerous N₂O₂ ligands[2] such as salans,[3] salalens,[4] salamos,[5] or their thio-derivatives[6] have been developed ([Figure 1]). Among all N₂O₂ ligands, the salens and salphens are probably the most commonly used ones.[7] The most prominent congeners are the Mn(III)-salens developed by Jacobsen (1991) and Katsuki (1994) for the enantioselective epoxidation of unfunctionalized olefins.[8] Inspired by this reaction, metal salen complexes have been successfully used as catalysts for a broad range of asymmetric reactions, such as hydroxylations,[9] cyclopropanations[10] and several other oxidation reactions,[11] aziridinations,[12] epoxide-ring opening reactions,[13] hetero-Diels–Alder reactions,[14] or trimethylsilylcyanations.[7a] [c] [15]

Figure 1 Molecular structures of selected N₂O₂ ligands.[1] [8b]

As mentioned above, all the described ligands provide an N₂O₂ binding site in a square planar geometry, suitable for binding a large variety of metal ions well as ions of main group elements. With a few exceptions, such as the elements of the 7th period or the bigger alkali metals, salphen or salen complexes with nearly all other metals of the periodic system of the elements have been realized ([Figure 2]), such as with Li⁺,[16] Be²⁺,[17] Na⁺,[18] Mg²⁺,[19] Al³⁺,[20] Si⁴⁺,[21] P³⁺,[22] Ca²⁺,[23] Sc³⁺,[24] Ti⁴⁺,[25] V⁴⁺,[26] Cr³⁺,[27] Mn³⁺,[8b] Fe²⁺,[28] Fe³⁺,[29] Co²⁺,[30] Co³⁺,[31] Ni²⁺,[32] Cu²⁺,[33] Zn²⁺,[34] Ga³⁺,[35] Ge^2+/4+,[36] Sr²⁺,[23] Y³⁺,[37] Zr⁴⁺,[38] Nb⁵⁺,[39] Mo⁴⁺,[40] Tc⁵⁺,[41] Ru³⁺,[42] Rh³⁺,[43] Pd²⁺,[44] Ag⁺,[45] Cd²⁺,[46] Sb⁵⁺,[47] Te⁴⁺,[48] La³⁺,[37] Ce⁴⁺,[49] Pr³⁺,[50] Nd³⁺,[51] Sm³⁺,[51] EU³⁺,[52] Gd³⁺,[51] Tb³⁺,[51] Dy³⁺,[51] Ho³⁺,[53] Er³⁺,[51] Tm³⁺,[53] Yb³⁺,[53] Lu³⁺,[53] Hf⁴⁺,[54] Ta⁵⁺,[55] W⁶⁺,[56] Re⁴⁺,[57] Os⁴⁺,[58] Ir³⁺,[59] Pt²⁺,[60] Au³⁺,[61] Hg²⁺,[46] Tl³⁺,[62] Bi³⁺,[63] Th⁴⁺,[64] U⁶⁺,[65] Np⁵⁺,[66] or Pu⁴⁺.[49]

Figure 2 Periodic table of the elements. The elements which ions have been used for salen and salphen complexes are highlighted in green.

In contrast to metal salens, for which comparably simple chiral diamines can be used to introduce chirality, metal salphen complexes lack of sp³-hybridized centers and chirality can be introduced by the use of more sophisticated chiral building blocks such as biaryls,[67] helicenes,[68] and calixarenes,[69] or upon supramolecular chiral induction.[70] Therefore, salphen complexes are rarely used for enantioselective reactions. Despite this disadvantage, metal salphens were able to catalyze polymerizations[71] or fix CO₂ as cyclic carbonates.[72] For the latter, Lewis acidic zinc-salphens have been used, which are able to coordinate reactants to the axial positions of the square-planar coordination sphere and thus activate them for nucleophilic attacks.[73] Besides mononuclear complexes, the formation of salens and salphens was also used to create multinuclear,[74] macrocyclic,[75] or cage-like ligands[76] and their corresponding metal complexes (for selected examples, see [Figure 3]). Such structures have been achieved by dynamic covalent chemistry (DCC).[77]

Figure 3 Top: macrocyclic [3 + 3]- and [6 + 6]-salphen ligands reported by the groups of Nabeshima[75b] and MacLachlan.[75c] Bottom: cage-like [2 + 3]-salphen ligands reported by Nabeshima and coworkers.[76d] [76e]

Another approach to embed salen or salphen moieties into more complex structures is realized by the decoration of molecules with functional groups enabling additional coordinative binding sites and therefore being able to act as building blocks for supramolecular boxes,[78] coordination cages,[79] coordinatively bound macrocycles,[80] or metal–organic frameworks (MOFs).[81] The immobilization of soluble salens or salphens in polymeric materials such as MOFs nicely allows to exploit the good catalytic activity in a heterogeneous approach.[81s] [t] [v] [x] [z] [aa] [ab] [ac] Similar to that, these MOFs were used for gas separation[81u] or storage,[81y] chiral resolution,[81w] and many more. The insoluble salen-containing MOFs can efficiently be recycled after use, which is a common technique when expanding the application of soluble complexes to the heterogeneous phase by immobilization.[82] However, most MOFs including the aforementioned salen MOFs contain metal nodes which suffer from instability against aqueous acidic or basic media. This limits their potential applications and typically stabilization techniques such as post-synthetic coating or guest addition have to be used to circumvent a quick degradation.[83]

This review highlights salen- and salphen-based materials embedded in fully covalent-bonded polymeric backbones. The objective of this review is to summarize and compare the different synthetic strategies, to examine the advantages and disadvantages of the different approaches based on the resulting material properties, and to give an overview of the structures reported to date. Furthermore, the potential applications of these compounds will be highlighted with selected examples. The review does not cover those salphen-containing superstructures based on the formation of coordinative bonds, which have been discussed in other recent excellent reviews.[84]

Structural Dimensionality

We arbitrarily divide salen and salphen polymers into 1D, 2D, or 3D by their idealized topology in terms of the propagation of the monomeric building blocks rather than by the real orientation in space by coiling, folding, steric effects, or contortion. First of all, we give an overview of how the different polymers have been synthesized and then discuss their properties.

1D-Salen and Salphen Polymers

The construction of salen- or salphen-containing polymers can in principal be divided into two main synthetic strategies. One strategy is based on the formation of the salen or salphen pocket during the condensation (polymerization) reaction with or without simultaneous complexation of metal ions. The other strategy is to use the preformed salen or salphen moieties, acting as monomers, in polymerization reactions like electropolymerizations,[85] cross-coupling reactions,[86] or comparable polymerization reactions.[87] Both strategies have been applied in the synthesis of 1D salen and salphen polymers.

The first polysalphens were described in the late 1950s by Marvel and Tarköy, synthesized by the condensation of methylene-[88] and sulfone-bridged[89] bissalicylaldehydes with ortho-phenylene diamine ([Scheme 1]). By treatment of the metal-free polymers with THF-solutions of Zn²⁺, Cu²⁺, Fe²⁺, Ni²⁺, Co²⁺, and Cd²⁺ salts, a large variety of metal-containing salphen polymers were obtained. By the assumption that the polymers are pure, monodisperse, and do not contain solvate or other impurities, the polymers had lengths up to 45 repeating units according to the interpretation of elemental analysis results.[88] The thermal stabilities of the polymers were investigated, showing that the coordinated metal centers had a stabilizing effect. Furthermore, an enhancement in thermal stability was achieved by formal exchange of the methylene bridges by sulfone bridges. This was attributed to the higher acidity of the phenolic oxygen atom due to electron-withdrawing effects of the para-substituted sulfone in comparison to the methylene tethers.[88] [89] It must be noted that at that time the analyses of the thermal stability relied on weight losses exclusively. However, in principle the observations could also be attributed to, for example, the release of bound water molecules.

Scheme 1 Syntheses of the first polysalphens reported by Marvel and Tarköy.[88] [89]

A similar approach based on 2,5-dihydroxyterephthalaldehyde ([Scheme 2]) was reported at the beginning of the 1970s by Manecke and Wille.[90] Here, the structural variety was achieved by using different diamines for the condensation. The conductivity of the fully conjugated 1D-polymers was measured and the authors concluded that the complexation of the metal centers is beneficial for the conductivity by increasing the rigidity of the polymeric backbone. The metalated polymers had up to five times higher conductivities (10⁻¹⁰ S m⁻¹) in comparison to the free ligands, but regarding the nature of the metal used no trend was found.[90b]

Scheme 2 Polysalens and polysalphens based on the condensation of 2,5-dihydroxyterephthalaldehyde with diamines.[90]

Besides the early examples of salphen polymers synthesized by condensation of amines and aldehydes, the second strategy used was the electropolymerization of presynthesized salen and salphen compounds introduced by Goldsby and coworkers in 1988.[91] This so-called potentiostatic method gave a structurally less defined nickel(II) salen-containing film on the surface of a platinum disc electrode ([Scheme 3]).[91] [92] Later, this approach was reinvestigated by others.[93] Within these studies it was revealed that for the aforementioned film “…the electropolymerization of [Ni(salen)] is ultimately a ligand-based process that takes place through a mixture of o- and p-linking of the phenyl rings…” ([Scheme 3]).[93b] Despite these irregular structures, a clear difference of the chemical as well as physical properties of the synthesized polymer films in comparison to aggregated individual Ni-salen complexes was confirmed by IR, UV/vis, and EPR spectroscopy and the redox and charge conduction behaviors were studied.[93b]

Scheme 3 Polysalens and polysalphens synthesized by electropolymerization.[91] [92] [93] [94]

To obtain a 1D polymer chain in a regioselective manner, Reynolds and coworkers used 3,4-diaminothiophene as well as terthiophenes to synthesize Ni-and Cu-salphens, which were electropolymerized to give conjugated polythiophenes with salphens[94a] and salphen/crown-ether moieties[94b] ([Scheme 3]). Swager and coworkers followed a slightly different approach to achieve control over the electrochemical properties of the polymers.[94c] They synthesized a Co-salen moiety with thienyl substituents in the para-position to the phenolic oxygen ([Scheme 3]).[94c] The regioselective electropolymerization on the thiophene's vacant α-position was proven by hindered film growth when this position was blocked by methyl substituents.[94c] This detailed structure–activity relationship was further studied on a series of electropolymerized materials derived from poly(3,4-ethylenedioxythiophene) (PEDOT)-substituted Cu(II), Ni(II), and U(VI)O₂ salens ([Scheme 3]). The interchain spacing was modulated by the use of different sterically bulky ethylene- and phenylene-diamines. It was found that large chain–chain interactions lower the oxidation potentials of the salen units due to destabilizing interactions of the corresponding HOMOs, which lead to the formation of spin-paired π-dimers upon further oxidation. In contrast to that, sterically shielded, noninteracting chains create radical cations and subsequent dications by oxidation.[95] It is worth mentioning that the potentiostatic method has been used exclusively for the synthesis of 1D polymer films so far. Also other nanostructures such as nanowires[96] or nitric oxide sensors are based on this type of polymerization.[97] Additionally, this approach was used to develop composite materials of metal salphen or salen complexes with multiwalled carbon nanotubes (MWCNTs) as capacitive materials[98] and in the synthesis of high-capacity films (see discussion below).[99]

As known from nature, 1D polymers can also create superstructures of high complexity such as the DNA double helices.[100] Already in 1996, Katz and coworkers synthesized fully conjugated 1D helical ladder polymers based on the condensation of a [6]-helicene bissalicylaldehyde with 1,2-phenylene diamine in the presence of nickel(II) acetate ([Scheme 4]).[68a] According to the integration of characteristic signals in ¹H NMR-spectra as well as by gel permeation chromatography (GPC) analyses the average molecular mass of the polymers was estimated to be about 7,000–7,400 Da, which corresponds to approximately eight repeating units. The fully conjugated character of the metal salphen helix was indicated by a bathochromic shift of 114 nm of the metal-to-ligand charge transfer band in comparison to simple nickel(II) salphen complexes in the UV/vis spectra.[68a] At the most redshifted absorption maximum (λ _abs = 595 nm), a large circular dichroism (CD) of Δε = 105 was found. Furthermore, by the variation of the substitution pattern on the enantiopure regioisomeric [6]-helicene bissalicylaldehydes, different winding motifs within the corresponding chiral helicenes were realized as depicted in [Scheme 4].[68b]

Scheme 4 Synthesis of helical polymers from enantiomerically pure bissalicylaldehydes and ortho-phenylene diamine.[68] Sections of the polymeric helices are depicted as MM2-optimized models.

Another example of helical 1D metal salphen polymer was obtained by the condensation of chiral 3,3′-diformylbinaphthol derivatives with various α,ω-diamines and subsequent complexation of Zn²⁺- or Mn²⁺-ions ([Scheme 5]).[101] The chiral helices showed clear differences in CD-spectroscopy by decreased Cotton effects at λ = 230 nm for the helical polymers in comparison to mononuclear model compounds. Furthermore, the aforementioned band at λ = 230 nm, which is assigned to the naphthalene subunits, redshifted approximately about Δλ = 30 nm, which is a hint for a close proximity of adjacent naphthalene units as proposed by geometry optimized models (MMFF94 and MM2) ([Scheme 5]). By GPC analyses, average molecular masses of up to 13,000 Da (approximately 10 repeating units) were estimated.[101b] Interestingly, the helical induction of the binaphthol units is not transferred to polymer chains when copolymerized with biphenyl bissalicylaldehyde in various ratios (from 100:0 to 0:100) in the presence of a soluble phenylenediamine and nickel(II) acetate ([Scheme 5]).[67]

Scheme 5 Synthesis of helical polymers from enantiomerically pure dinaphthyl-bissalicylaldehyde and different α,ω-diamines.[101] Section of the polymeric helix is depicted as a geometry-optimized model (MMFF94, MM2).

Also worth mentioning is a superstructure that was presented by Houjou and coworkers in 2003. Two strands of linear polysalicylimines build a double-helix by the formation of salen-like moieties after complexation of Zn²⁺, Ni²⁺, Co²⁺, and Cu²⁺ ions. The resulting polymers formed microspheres with diameters between 0.5 and 1.4 µm indicating ordered materials on the macroscopic scale ([Scheme 6]).[102]

Scheme 6 Left: synthesis of Houjou's linear polyimines interconnected by salen-like moieties.[102] Right: SEM micrographs of Cu(II) polymers creating microspheres. (Parts of the scheme are adapted with permission from Ref. 102. Copyright © 2003 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim.)

In 2002, Lavastre et al. used palladium-catalyzed Sonogashira–Hagihara cross-coupling reactions the first time to synthesize 1D metal salphen polymers ([Scheme 7]).[103] A library of different linear polysalphens with Ni(II)- as well as Zn(II)-metal centers and various dialkynes as linking units (among several polymers without salphen units) were generated. The corresponding polymers emitted light in solution but were unfortunately no potential candidates for organic light-emitting diodes (OLEDs) due to quenched solid-state emission.[103]

Scheme 7 Synthesis of 1D-polysalphens by palladium-catalyzed Sonogashira–Hagihara cross-coupling.[103] [104]

MacLachlan and coworkers used a similar method to obtain soluble poly(salphen-alkyne)s with Zn²⁺, Ni²⁺ as well as VO²⁺ centers.[104a] GPC analyses revealed high molecular masses of 17,000 Da for the Ni-salphen and up to 84,000 for the VO-salphen polymer ([Scheme 7]). Furthermore, by the variation of the substitution pattern of the halogens at the salphen units, helical or zig-zag-type polysalphen chains with Zn²⁺, Ni²⁺, and Cu²⁺ [104b] as well as with Pt²⁺ centers have been realized.[105] Furthermore, different external substituents like saccharines were used as well.[105] [106]

An interesting example of a 1D salphen polymer with an ordered 3D structure was presented by Liu and coworkers in 2015 by condensing a bissalicylaldehyde and phenazine diamine with zinc acetate.[107] It is suggested that due to the extended aromatic backbone of the precursors, the resulting polymer chains pack in an ordered fashion to form crystalline supramolecular 3D networks. Crystallinity of the material was proven by scanning electron microscopy and powder X-ray diffraction (PXRD; [Scheme 8]). Interestingly, it was reported that these polymers are permanently porous, having a specific surface area of SA_BET = 206 m²/g.[107] The non-local density functional theory (NLDFT) pore size distribution showed a maximum in the microporous regime with d _pore = 1.4 nm and mesoporous proportions with d _pore = 2.5 and 4.4 nm. Unfortunately, no structural model was suggested on how these highly defined pores are generated by the packing of the linear polymer strands in the crystalline state.[107]

Scheme 8 Top: synthesis of Liu's 1D Zn(II) salphen polymer from a naphthalene bissalicylaldehyde and 2,3-diaminophenazine. Bottom: SEM micrographs of the obtained microspheres (a) and rods (b), and PXRD pattern of the corresponding material (c).[107] (Parts of the scheme are adapted with permission from Ref. 107. Copyright © 2015 The Polymer Society of Korea and Springer.)

Other methods such as the epoxy polymerization[108] or Gilch polymerization[109] have been used to synthesize 1D polysalphens. For the polymers derived from these methods as well as for several ionic polymers,[110] no structural suggestions were made.

2D-Salen and Salphen Polymers

By structural modification of the molecular building blocks, 2D salen and salphen polymers can be realized. Although the first 1D polymers were reported in 1954 (see above), half a century had to pass until Gothelf and coworkers presented the first 2D salen polymer in 2005.[111] The condensation of a C ₃-symmetric trissalicylaldehyde and ethylene diamine[111] gave a metal-free network ([Scheme 9]) with at least locally ordered structures as determined by PXRD. Interestingly, when condensed with chiral ethylene diamines, the presence of Mn(II)-acetate was necessary to observe a comparable PXRD signal pattern.[111] Ding, Wang, and coworkers optimized the reaction conditions of the same reaction in 2017 and synthesized a structurally related material with high crystallinity by solvothermal methods (dioxane, ethanol, acetic acid, 120 °C, 3 d).[112] By NLDFT calculations a pore-size distribution with a maximum diameter of 2.5 nm was found, fitting to the expected mesopore size of 2.9 nm according to the proposed eclipsed alignment of the 2D polymer sheets.[112] After subsequent metalation, the M-salen covalent organic frameworks (COFs) with Cu(II)-, Ni(II)-, Zn(II)-, Co(II)-, and Mn(II)-metal centers were obtained. It is worth mentioning that for both the unsubstituted (pH = 1–13) and Co(II)-salen (pH = 2–13) COFs, high chemical stabilities were reported.

Scheme 9 Structural motifs of salen-containing 2D polymers and calculated as well as experimentally measured pore sizes.[111] [112]

Liu and Cui just prior to Yang and coworkers synthesized further salen COFs using salicylaldehydes with 1,3,5-triphenylbenzene as the core structure in condensation reactions ([Scheme 10]).[113] Liu and Cui first synthesized chiral Zn-salen-COFs (CCOF-3) and pointed out that the stability of the COFs differs with changing substituents (H vs. ^tBu) of the salicylaldehyde units with a higher stability of tert-butyl-substituted COFs due to “kinetic blocking.”[113a] After transmetalation,[114] mixed metal networks with Zn(II) and one or two other metal ions were obtained (CCOF-4-M). The exact metal ratios were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES).

Scheme 10 Structural motifs of Liu and Cui's CCOF-4-M series, and Yang's COF-salen-M polymers with triphenylbenzene core structures and calculated as well as experimentally obtained pore sizes.[113]

Yang and coworkers followed the approach of first synthesizing the metal-free COF, which was postmetalated in a second step with Co²⁺, Mn³⁺, Cu²⁺, and Zn²⁺ ions to give the corresponding COF-salen-M. For the COF-salen-M series once more a good agreement of an NLDFT calculated pore-size distribution maximum (1.86 nm) with that from the model (calculated pore diameter = 1.9 nm) was found when a fully eclipsed stacking of the 2D polymer sheets was assumed.[113b] The COF-salen-M series did not decompose in a broad pH range (from 1 M HCl to 1 M NaOH solutions) as proven by PXRD analyses of the immersed COF samples.

As it has been mentioned above, the sheets of the 2D salphen and salen COFs pack in an eclipsed fashion, thus in theory making the metal centers inaccessible for substrates of any kind. To avoid such a packing in combination with an ideal close arrangement of salphen metal centers, we synthesized Ni₃- and Cu₃-MaSOFs ([Scheme 11])[115] based on a triptycene trissalicylaldehyde.[116] Indeed, by single-crystal X-ray diffraction (SCXRD) structure analyses of discrete trinuclear model complexes ([Scheme 11], right), metal-to-metal distances of d _M–M = 6.7 Å were found, which according to Zhang should be ideal to bind CO₂ synergically.[117] Unfortunately, the corresponding polymers Ni₃- and Cu₃-MaSOFs did not give any crystalline material with honeycomb 2D-sheets but were rather amorphous ([Scheme 11]). The absence of highly ordered pores was also confirmed by the QSDFT (quenched solid DFT) pore-size analysis revealing that with 1.1–1.2 nm these are nearly half in size of what would have been expected for the idealized structure. Nevertheless, IR comparison of the MaSOFs with their molecular model compounds suggested the existence of well-defined trinuclear units within the polymers causing a high affinity towards CO₂, which will be discussed below.

Scheme 11 Synthesis and structural representation of the M₃-MaSOF series. Box: single-crystal X-ray structure of a trinuclear nickel model complex with a metal-to-metal distance of 6.74 Å. (Parts of the scheme are adapted with permission from Ref. 115. Copyright © 2019 American Chemical Society.)

Similar to 1D polysalphens, 2D polysalphens have also been synthesized by palladium-catalyzed cross-coupling reactions.[118] Surprisingly, despite the large toolbox of different coupling reactions as well as an unlimited stock of molecular building blocks, to the best of our knowledge, exclusively the Sonogashira–Hagihara cross-coupling of 1,3,5-triethynylbenzene with halogenated salens or salphens has been used for this purpose so far ([Scheme 12]).[118] The polymers were permanently porous and mainly used in heterogeneous catalysis as discussed below.

Scheme 12 Overview of 2D salen- and salphen-containing organic polymers made by Sonogashira–Hagihara cross-coupling reactions. Box: cut-out of the connecting unit.[118]

2D salen and salphen polymers were also obtained by radical polymerizations of vinyl- or allyl-substituted salphens[119] and interfacial polymerization[120] by cross-linking of linearly connected polymer chains. These polymers were of amorphous nature and have been used for the catalytic decomposition of H₂O₂ as in the case of Gupta's N,N-BSPDA-based beads (see discussion below) as well as the activation of molecular oxygen in the aerobic oxidation of cumene.[120]

Scheme 13 Syntheses and structural motifs of Mastalerz's MaSOF-1 and -2, as well as Fang's JUC-508, JUC-509, and the corresponding metalated networks.[123] [124]

3D-Salen and Salphen Polymers

The majority of all COF structures are based on 2D-sheets and a much smaller proportion of COFs is three-dimensional.[121] Therefore, it is not surprising that only a few 3D polysalphens have been reported till date. In 2012 Mastalerz synthesized the first 3D salphen frameworks by the condensation of a tetraphenylmethane tetrakis-salicylaldehyde[122] and 1,2-phenylene diamine in the presence of Ni(II)- or Zn(II)-acetate ([Scheme 13]).[123] The resulting MaSOFs were analyzed by IR as well as ¹³C-CP-MAS spectroscopy, suggesting a high degree of polymerization because nearly no remaining aldehyde moieties were detected. Despite their amorphous nature, both MaSOFs were isostructural in pore size (4.2 Å) and specific surface areas (SA_BET = 630–647 m²/g) (see discussion below).

In 2019, Fang and coworkers reacted halogenated ortho-phenylene diamines with Mastalerz's tetraphenylmethane-based tetrakis-salicylaldehyde under solvothermal conditions and obtained the highly crystalline COFs JUC-508 (Hal = F) and JUC-509 (Hal = Cl) ([Scheme 13]).[124] These networks retained their crystallinity after complexation with Mn²⁺, Cu²⁺, and Eu³⁺ ions. PXRD analyses in combination with modeling revealed for all COFs noninterpenetrated networks of a dia topology.[124] When an extended tetraphenylmethane core with one more phenyl group[122] or a silicon-centered analogue was used, sevenfold interpenetrated salphen COFs resulted, again with the dia topology ([Scheme 14]).[125] In this case, an in-situ metalation upon condensation in addition to the postmetalation of presynthesized COFs gave networks with high degrees of crystallinity.[125]

Scheme 14 Cui's COF-1, COF-2, COF-2-Zn, and COF-2-Zn.[124]

It is worth mentioning that an inverse building block design was reported based on tetraaminotetraphenylmethane with a formyl-substituted Mn-salen ([Scheme 15]).[126] The obtained materials showed gel-like behavior and have been used in the asymmetric kinetic resolution of a racemic mixture of R- and S-1-phenylethanol.

Scheme 15 Synthesis of a chiral Mn-salen imine gel. Inset: photograph of Mn-salen imine gel (0.02 mol/L in DMSO).[126] (Adapted with permission from Ref. 126. Copyright © 2015 The Royal Society of Chemistry.)

Similar to MaSOF-1 and 2 as well as the M₃-MaSOFs, the M-MaSOF_50/100 series was synthesized via the imine condensation approach in the presence of the corresponding metal salts ([Scheme 16]).[127] For this purpose, a triptycene-based hexakis-salicylaldehyde needed to be synthesized by a sequence of a sixfold Suzuki–Miyaura cross-coupling, a sixfold Duff formylation, and finally a sixfold demethylation. This hexakis-salicylaldehyde was used to synthesize a series of nearly isostructural MaSOFs with Zn(II), Ni(II), Cu(II), Pd(II), and Pt(II) metal centers.[127] Independent from the reaction temperature, the MaSOF₅₀ and MaSOF₁₀₀ series showed comparable narrow pore-size distributions with micropores of 0.55–0.59 nm in diameter.

Scheme 16 Syntheses of MaSOF₅₀ and MaSOF₁₀₀ series.[127]

Besides the polymers obtained by imine condensations, the Sonogashira–Hagihara cross-coupling of tetrakis(4-ethynylphenyl)methane with different halogenated salen complexes was also used to synthesize 3D polymers. In 2013, the Son group synthesized the first 3D Cr³⁺- and Al³⁺-salen networks (M-MON series) by cross-coupling methods of tetra(4-ethynylphenyl)methane with trans-cyclohexyldiamine-based diiodo-salens ([Scheme 17]).[128] The insoluble polymers were investigated by ¹³C-MAS-NMR as well as X-ray photoelectron spectroscopy and PXRD, revealing their amorphous nature due to the irreversibility of the cross-coupling conditions. In accordance with their 2D-analogues, the M-MON series possess permanent porosity and were used as heterogeneous catalysts (see discussed below).

Scheme 17 Syntheses of 3D salen- and salphen-containing organic polymers by Sonogashira–Hagihara cross-coupling reactions.[128]

Material Properties and Applications

Depending on the molecular structure, the dimensionality, and the nature of the metal centers of the salen and salphens units, different materials properties result that lead to certain potential applications, which are discussed within the next section.

Conductive Films

Salen or salphen films prepared by electropolymerization commonly find application as conductive materials. One of the first examples was presented by Dahm and Peters in 1994 ([Scheme 3]).[93a] In this study, the catalytic reductions of iodoethane and 2-iodopropane to the corresponding ethyl- and 2-propyl radicals by the polysalphen films were studied and the mixture of subsequent products (different alkanes and the corresponding alkenes) was investigated. A comparison of the product distributions of the nickel(II)-salen coated electrode with those of an uncoated one revealed that the film-coated electrodes exclusively generate products through a radical mechanism while the reaction on the uncoated electrode occurs via a cationic mechanism, demonstrating the significance of the salen polymer film in such transformations.[93a]

In several studies the potential applications of salen or salphen/MWCNT composite materials as supercapacitors have been investigated. A direct relationship of the polysalen/MWCNT-ratio and the capacity of the resulting material was found,[98c] and optimized films possessed between 3.8 and 8 times higher capacities compared to uncoated MWCNTs[98a] [99a] with specific capacitances of up to C = 200 F/g for a poly-Ni(II)salphen coated as well as C = 150 F/g for a poly-Ni(II)salen coated MWCNT electrode.[99b] The latter also gives insight into the modulation of the electronic properties upon incorporation of salen or salphen backbones into conductive polymers. This was nicely exploited, for example, with conductive crown-ether decorated Ni(II)- and Cu(II)-salen films, which have been used for molecular sensing of nanomolar amounts of metal ions (Li⁺, Na⁺, Mg²⁺, Ba²⁺) and other nonionic species.[94b] In another study, nitric oxide (NO_(g)) was detected by a cobalt-salen-based PEDOT-substituted film.[97] For this purpose, the as-mentioned cobalt-salen-based PEDOT polymers were electropolymerized onto an interdigitated microelectrode array (black in [Figure 4], top) and the difference in the resistance (ΔR) between the parent sensor (R ₁) and the sensor under a stream of NO gas (R ₂) was evaluated (black and red curves in [Figure 4], bottom). The comparison to a similar setup with an electropolymerized PEDOT without salen units (blue curve, [Figure 4], bottom) clearly indicated the role of the Co-salen moieties as the recognition sites of the sensor films.[97]

Figure 4 Top: schematic representation of interdigitated microelectrode array with an electropolymerized Co(III)-salen PEDOT copolymer as a sensor layer for NO sensing. Bottom: differential resistance (R ₂/R ₁ , see top figure) against time with various NO_(g) concentrations. Black: unconditioned poly-Co(III)-PEDOT film; red: conditioned (at 0.262 V) poly-Co(III)-PEDOT film: blue: PEDOT film without salen moieties.[97] (Adapted with permission from Ref. 97. Copyright © 2006 American Chemical Society.)

Heterogeneous Catalysts

The use of salens as well as salphens in homogenous catalysis inevitably led to the development of polymeric salens or salphens as heterogeneous catalysts. As early as in 1983, Wöhrle and coworkers demonstrated that a simple Co(II)-salen polymer can catalyze the valence isomerization of quadricyclane to norbornadiene, which has been proved to be superior for example to Co(II)-porphyrin polymers ([Scheme 18]).[129]

Scheme 18 Valence isomerization of norbornadiene to quadricyclane catalyzed by a Co(II) salen polymer.[129]

Gupta et al. constructed N,N-bis(3-allyl salicylidene)o-phenylenediamine (N,N-BSPDA) based polymer beads ([Scheme 19]). It is worth mentioning that the polymer beads were among one of the first polymeric salphen materials with permanent porosity proven by gas sorption ([Table 1]).[119a] Depending on the cross-linker, specific surface areas up 380 m²/g have been obtained. After Co(II) complexation, the beads became catalytically active for the decomposition of H₂O₂.[119a]

Scheme 19 Gupta's N,N-BSPDA-based beads by radical polymerization. Right: SEM micrograph of the corresponding bead after swelling. (Adapted with permission from Ref. 119a. Copyright © 2003 Elsevier Science B. V.)

Table 1 Porous salen- and salphen-containing organic polymers used as heterogeneous catalysts

Name	Metal	SABET [a] [m2/g]	Catalytic reaction	Yield [%] up to[b]	Ref.
JUC-509-Cu	Cu	957	Dismutation of O2 •−	100[c]	[124]
JUC-509-Mn	Mn	1016	Dismutation of O2 •−	∼50[c]	[124]
JUC-509-Eu	Eu	942	Dismutation of O2 •−	∼10[c]	[124]
CCOF-4-Mn	Zn/Mn	614	Asym. epoxidation of alkenes	79[d] (97% ee)[e]	[113a]
CCOF-4-V	Zn/V	547	Asym. cyanation of aldehydes	79[d] (94% ee)[e]	[113a]
CCOF-4-Co	Zn/Co	628	Asym. Diels–Alder reaction	87[d] (96% ee)[e]	[113a]
CCOF-4-Fe	Zn/Fe	566	Asym. epoxidation of alkenes	71[d] (92% ee)[e]	[113a]
CCOF-4-Cr	Zn/Cr	633	Asym. aminolysis of trans-stilbenes	76[d] (86% ee)[e]	[113a]
CCOF-4-Cr-Mn	Zn/Cr/Mn	616	Alkene epoxidation with subsequent epoxide aminolysis	84[d] (91% ee)[e]	[113a]
Co/Salen-COF	Co	854	Henry reaction	92[f]	[112]
COF-salen-Co	Co	1065	CO2 addition to epoxides	91[d]	[113b]
COF-salen-Co(III)	Co(III)	836	Epichlorohydrin hydration	98[d]	[113b]
COF-salen-Mn	Mn	961	Styrene epoxidation	74[d]	[113b]
COF-salen-Cu	Cu	1258	CO2 addition to epoxides	20[d]	[113b]
COF-salen-Zn	Zn	1032	CO2 addition to epoxides	90[d]	[113b]
MsMOP-1	Pd	554	Pd-catalyzed cross-coupling reactions	99[d]	[118a]
Cr-MON	Cr	522	CO2 addition to epoxides	66[g]	[128]
Al-MON	Al	650	CO2 addition to epoxides	71[g]	[128]
Co-MON	Co	580	CO2 addition to epoxides	94[g]	[128]
Co-CMP	Co	965	CO2 addition to epoxides	81[h]	[118b]
Al-CMP	Al	798	CO2 addition to epoxides	91[h]	[118b]
Mn-POP	Mn	836	Alkene epoxidation	>99[d]	[118d]
POP-1-Co	Co	370	CO2 addition to epoxides	97[g]	[118e]
POP-1-Cr	Cr	732	CO2 addition to epoxides	98[g]	[118e]
DVB@ISA	Al	590	CO2 addition to epoxides	99[d]	[110c]
Al-CPOP	Al	136	CO2 addition to epoxides	>99[d]	[132]
Zn@SBMMP	Zn	423	CO2 addition to epoxides	97[f]	[133]
N,N′ -BSPDA Bead Type I	–	56	H2O2 decomposition	–	[119a]
N,N′ -BSPDA Bead Type II	–	72	H2O2 decomposition	–	[119a]
N,N′ -BSPDA Bead Type III	–	94	H2O2 decomposition	–	[119a]
N,N′ -BSPDA Bead Type IV	–	132	H2O2 decomposition	–	[119a]
N,N′ -BSPDA Bead Type V		380	H2O2 decomposition	–	[119a]

^a Derived from N₂ sorption experiments at 77 K.

^b For selected examples.

^c Clearance rate.

^d Determined by GC analyses.

^e Determined by HPLC analyses.

^f Isolated yield based on benzaldehyde.

^g Determined by ¹H-NMR analyses.

^h Isolated yield.

Mn-POP, a 2D salen polymer obtained via the above-described Sonogashira–Hagihara approach (for the structure see discussion above; [Scheme 12]), was also permanently porous (SA_BET = 836 m²/g, [Table 1]) and was capable to catalytically transfer olefins to the corresponding epoxides with conversions of up to 99%, similar to Vancheesan's M(III) polysalphens ([Scheme 20]).[118d] [130]

Scheme 20 Epoxidation of alkenes catalyzed by Mn-salen polymer Mn-POP.[118d] ^aConditions: Mn-POP (0.1 mol%), O₂, isobutyraldehyde (7 eq.), dichloroethane, 25 °C, 5 h. ^bConversion determined by GC.

A large number of metal-salen and -salphen polymers have been used in the synthesis of cyclic carbonates from epoxides upon CO₂ capture, as it is known from the corresponding discrete complexes.[72a] [128] [131] The key advantage to their smaller congeners is their recyclability as a heterogeneous catalyst.[110a] [118e] [119b] [132] For example, Deng et al.'s CMP series (for the structure see discussion above; [Scheme 12]) was able to be used for cyclic carbonate formations in up to 22 cycles even under ambient conditions.[118b]

Polymers such as POP-1-Cr (SA_BET = 732 m²/g),[118e] POP-1-Co (SA_BET = 370 m²/g),[118e] Al-CPOP (SA_BET = 136 m²/g)[132] or Co- (SA_BET = 580 m²/g),[118b] and Al-CMP (SA_BET = 798 m²/g)[118b] possess permanent porosity as indicated by nitrogen sorption (see discussion below; [Table 1]). High turnover frequencies (TOF) of up to 1875 h⁻¹ [113b] and turnover numbers (TONs) of over 200 have been achieved for example in addition reactions of CO₂ to epoxides.[133] The highest TONs (496) were achieved with an ionic ammonium polymer bearing salen Co(III)-salens[110b] or with POP-1-Cr (693).[118e]

One interesting approach to capture CO₂ that is different from the one described above is realized with ionic Zn(II)-salen polymer DVB@ISZ, which converts the CO₂ to synthetically more valuable formamides in high yields of up to 99% from amines using PhSiH₃ as an in-situ reductant ([Scheme 21]).[110c] The high reactivity was attributed to the Lewis acidic nature of the Zn(II) centers in salen and salphen moieties, as has been reported for other Zn(II) salphens too.[72a] [73a] [b] [78a] [134]

Scheme 21 Different methods to capture CO₂.[110c]

It is obvious that asymmetric transformations were realized by using chiral salen polymers. For example, Takata and coworkers reported the asymmetric addition of diethylzinc to aldehydes using a chiral helical Zn(II)-polysalen (structure in [Scheme 5]) in high enantiomeric excess (ee) of 95%, while the use of the related enantiomerically pure mononuclear Zn(II) salphen resulted in a product with an ee of only 5% ([Scheme 22]).[101a] [c]

Scheme 22 Asymmetric addition of Et₂Zn to aldehydes, catalyzed by Takata's poly Zn(II) salen helix.[101a] [101c]

An isostructural chiral helical poly-Mn(III)-salen was used in the asymmetric epoxidation of styrenes, but only delivered a low selectivity of 17% ee,[101b] which is higher than that for the homogenous pendant (1–6% ee), but unfortunately lower than that of other chiral salen complexes (ee up to 98%).[135] This was improved in 2005 by Gothelf and coworkers using also chiral Mn(III) salen polymers (structure in [Scheme 9]; see above), but this time chirality was introduced by enantiomerically pure diamines.[111] The transformation of cis-methyl styrene was performed with a cis/trans-ratio of 12:1 and an ee of 67%. The epoxidation by porous salphen polymers was further improved by Cui and coworkers in 2017.[113a] The porous Mn(III)- (CCOF-4-Mn; SA_BET = 614 m²/g) alongside the Fe(III)-salen polymers (CCOF-4-Fe; SA_BET = 566 m²/g; for structures, see [Scheme 10]) were able to convert 2,2-dimethyl-2H-chromene with iodosylbenzene (or derivatives thereof) as an oxidant to the corresponding epoxides with up to 97% ee ([Scheme 23]).[113a] In this publication, the full potential of embedding different metal ions into the pockets of salens (here CCOF-4-M) for different reactions was demonstrated. For instance, other asymmetric transformations were achieved with high yields and high ee values, such as the cyanosilylation of aldehydes (M = V; up to 94% ee), Diels–Alder reactions of 1-amino-substituted butadienes and α-amino acroleins (M = Co; up to 96% ee), aminolysis of trans-stilbene oxide with anilines (M = Cr; up to 96% ee), or epoxidation of alkenes followed by a subsequent ring opening in the presence of anilines to obtain amino alcohols (M = Cr, Mn; up to 91% ee) ([Scheme 24]).

Scheme 23 Asymmetric epoxidation of styrenes catalyzed by various poly M-salens (for synthesis and structures of the corresponding polymers, see [Scheme 5],[101b] [Scheme 9],[111] and [Scheme 10]).[113a]

All polymers were porous (SA_BET up to 683 m²/g, [Table 1]) and their reusability was exemplarily proven for the cyanation reaction: no leaching of metal ions was detected by ICP-OES and the crystallinity remained even after five runs as investigated by PXRD.[113a]

Other examples are Co(III)-salen polymers catalyzing the Henry reaction of substituted benzaldehydes and nitromethane[112] or the hydration of epichlorohydrin (EPH).[113b]

Scheme 24 Asymmetric transformations catalyzed by different members of Cui's CCOF-4-M series (for synthesis and structures, see [Scheme 10]).[113a]

Interestingly, Pd(II)-salen polymers have been reported to catalyze cross-coupling reactions such as Suzuki–Miyaura reactions of various arylhalides with phenylboronic acid or Heck reactions of arylhalides with styrene or ethyl acrylate in yields up to 99%.[118a] This rare and unusual use of metal salphens in C–C cross-coupling reactions[136] underlines that the broad potential of porous polymetal-salphens still needs to be explored.

Porous Materials for Gas Sorption and Separation

Most of the above-discussed catalytically active metal salen or salphen polymers are permanently porous, with high surface areas, which is favorable to provide accessible catalytic centers. The data of nitrogen sorption experiments of these salen and salphen polymers are summarized in [Table 1]. Among the metal-containing porous salen and salphen polymers, Yang's COF-salen-Cu (see above; [Scheme 10]) that was used for catalytic CO₂ fixation and the corresponding metal-free COF-salen precursor have with SA_BET = 1,258 and 1,646 m²/g, respectively, the highest reported specific surface areas of all here-discussed polymers ([Scheme 24]).[113b] Besides for heterogeneous catalysis, porous salphen and salen polymers were investigated for gas sorption and separation.

As discussed above, Mastalerz et al. reported 3D porous networks based on a tetraphenylmethane unit as the building block in 2012 (structure in [Scheme 13]). The resulting porous salphen polymers were named MaSOF-1 (M = Ni(II), SA_BET = 647 m²/g) and MaSOF-2 (M = Zn(II), SA_BET = 630 m²/g; [Table 2]).[123] Both MaSOFs adsorbed comparable amounts of H₂ at 77 K and 1 bar (4.46 mmol/g, 0.90 wt.% and 4.14 mmol/g, 0.90 wt.%) but the zinc-containing MaSOF-2 showed a slightly higher uptake of CO₂ (2.23 mmol/g, 9.8 wt.%) than the nickel analogue MaSOF-1 (1.83 mmol/g, 8.1 wt.%), which was attributed to the higher Lewis acidity of the incorporated zinc centers ([Table 2]).[123]

Table 2 Comparison of porous salen- and salphen-containing organic polymers used for gas sorption and separation

Name	Metal	SABET [a] [m2/g]	CO2 uptake[b] [mmol/g] (wt.%)	IAST selectivity		Ref.
				CO2/CH4 [c]	CO2/N2 [d]
MaSOF-1	Ni	647	1.83 (8.05)	–[e]	–	[123]
MaSOF-2	Zn	630	2.23 (9.81)	–	–	[123]
Zn-MaSOF100	Zn	638	4.28 (13.6)	10	48	[127]
Zn-MaSOF50	Zn	540	4.02 (12.8)	9.5	45	[127]
Ni-MaSOF100	Ni	816	4.83 (15.6)	9.0	42	[127]
Ni-MaSOF50	Ni	754	4.48 (14.5)	8.8	44	[127]
Cu-MaSOF100	Cu	697	4.66 (14.9)	10.3	52	[127]
Cu-MaSOF50	Cu	725	4.72 (15.1)	9.9	50	[127]
Pd-MaSOF100	Pd	511	4.11 (12.0)	10.5	56	[127]
Pd-MaSOF50	Pd	575	4.24 (12.4)	8.7	43	[127]
Pt-MaSOF100	Pt	572	4.39 (10.9)	9.5	86	[127]
Pt-MaSOF50	Pt	492	4.09 (10.2)	9.5	48	[127]
H2-SOF	–	116	2.12 (7.84)	10.5	32	[127]
Ni3-MaSOFas	Ni	373	2.36 (10.4)	16	89	[115]
Ni3-MaSOFw	Ni	441	2.45 (10.8)	11	64	[115]
Cu3-MaSOFas	Cu	625	3.39 (14.9)	12	69	[115]
Cu3-MaSOFw	Cu	890	5.02 (22.1)	9	40	[115]
MsMOP-Ni	Ni	1087	1.85 (8.15)	–	46[f]	[118c]
MsMOP-Zn	Zn	785	1.17 (5.16)	–	31[f]	[118c]
MsMOP-Pt	Pt	1202	1.67 (7.33)	–	20[f]	[118c]
CMP	–	772	1.61 (7.10)	–	–	[118b]
Al-CMP	Al	798	1.73 (7.65)	–	–	[118b]
Co-CMP	Co	965	1.80 (7.93)	–	–	[118b]
DVB@ISA	Al	590	1.14 (5.04)	–	–	[110c]
Al-CPOP	Al	136	0.54 (2.36)[g]	–	–	[132]
Zn@SBMMP	Zn	423	2.14 (9.40)	–	–	[133]

^a Derived from N₂ sorption experiments at 77 K.

^b At 273 K and 1 bar.

^c For a 50:50 mixture at 273 K and 0.1 bar.

^d For a 20:80 mixture at 273 K and 0.1 bar.

^e Not determined.

^f Ideal selectivity.

^g At 298 K and 1 bar.

For some of the salphens used for CO₂ fixation (see above), the corresponding volumetric uptakes of CO₂ at 1 bar were determined before catalysis testing. Co- as well as Al-CMP for example takes up 1.80 mmol/g (7.93 wt.%; Co-CMP) and 1.73 mmol/g (7.65 wt.%; Al-CMP) at 298 K, respectively ([Table 2]).[118b] Among those salphen polymers used for heterogeneous catalysis for CO₂ addition on epoxides, Zn@SBMMP took up the highest amount of CO₂ (2.14 mmol/g, 9.40 wt.%). This value is only outperformed by salphen networks designed for gas sorption ([Table 2]).[115] [118c] [123] [127] For example, one of the first porous MaSOFs, MaSOF-2, has already been reported to have an uptake of 2.23 mmol/g (9.8 wt.%) of CO₂ at 273 K and 1 bar.[123]

In 2018 the Mastalerz group introduced the next generation of MaSOFs based on a triptycene hexakissalicylaldehyde. In combination with different metal sources, two isostructural series of compounds (M-MaSOF₅₀ and M-MaSOF₁₀₀) with M = Zn(II), Cu(II), Ni(II), Pd(II) as well as Pt(II) were synthesized at 50 and 100 °C (denoted by the suffix in the name). The BET surface areas measured by nitrogen sorption at 77 K ([Figure 5]) of all compounds range between 492 and 816 m²/g. More importantly, all structures have pronounced and defined pores in the microporous regime with a diameter of approximately 5.7 Å ([Figure 5]), thus making these compounds ideal to compare the influence of the metal centers on gas sorption and selectivity. Significantly higher CO₂ uptakes were detected for the M-MaSOF₅₀ and M-MaSOF₁₀₀ series in comparison to the MaSOFs 1 and −2. For instance, Ni-MaSOF₁₀₀ adsorbs the highest amount of CO₂ (4.83 mmol/g, 15.6 wt.%) at 273 K and 1 bar ([Figure 5] and [Table 2]) within this series.[127] For all MaSOFs in these series, high Henry- and ideal adsorbed solution theory (IAST) selectivities of CO₂ against CH₄ or N₂ were determined with Pt-MaSOF₁₀₀ having the highest selectivity for CO₂/N₂ of S _IAST = 86 at 0.1 bar at 273 K.[127] This value is one of the highest reported for all polysalphens, higher than those found in the MsMOP-M series (S _IDEAL up to 46)[118c] and only outperformed by Ni₃-MaSOF_as (S _IAST = 89 (CO₂/N₂) at 0.1 bar; see also discussion below).[115]

Figure 5 Top: nitrogen sorption isotherms (77 K) of MaSOF₅₀ series. Bottom: QSDFT pore-size distributions of the MaSOF₅₀ series.[127] (Adapted with permission from Ref. 127. Copyright © 2018 American Chemical Society.)

The next evolutionary step in the construction of MaSOFs for gas sorption was the development of the M₃-MaSOFs (M = Cu and Ni).[115] Based on previous results in the area of MOFs,[117] [137] the idea was to generate networks that contain trinuclear metal units, where the metals are in close proximity to synergically bind CO₂. Indeed, model complexes of these trinuclear units revealed metal–metal distances (d = 6.65–6.74 Å) that should be ideal for CO₂ sorption (see above; [Scheme 11], [Table 2], and [Figure 6]).[115] For Ni₃-MaSOF_as, an isosteric heat of adsorption of Q _st = 35.6 kJ/mol ([Figure 6] and [Figure 7]) was measured, which is comparable to the values of some of the best-performing MOFs such as Ni-MOF-74 (Q _st = 37–42 kJ/mol),[138] HKUST-1 (Q _st = 15–35 kJ/mol),[139] or MIL-120 (Q _st = 38 kJ/mol).[140] The resulting selectivities are S _IAST = 16 (CO₂/CH₄) and S _IAST = 89 (CO₂/N₂) for Ni₃-MaSOF_as at 0.1 bar and 273 K ([Table 2] and [Figure 7]).[115] To get a deeper insight into the adsorption processes, DFT calculations (def2-SVPD) on the synergic binding of CO₂ were performed ([Figure 7]). A high energy gain for the adsorption of two molecules of CO₂ suggests cooperative effects during the adsorption event. This is experimentally supported by a stepwise adsorption of CO₂ at 195 K ([Figure 7]). Finally, the potential application in gas separation was shown by calculated breakthrough curves which was found to occur after approximately 10 min for an 80:20 mixture of N₂ and CO₂ at 1 bar and 1 L/min flow rate with a hypothetical column of 200 × 30 mm and 60 g of substrate.[115]

Figure 6 Adsorption geometry of one molecule of CO₂ on a [2 + 3]-nickel complex calculated by DFT (def2-SVPD level of theory) as a model complex for the M₃-MaSOF series.[115] (Adapted with permission from Ref. 115. Copyright © 2019 American Chemical Society.)

Figure 7 Top left: IAST selectivities of Ni₃-MaSOF_as (red and green) and Ni₃-MaSOF_w (black and blue) at 273 K. Top right: CO₂ adsorption isotherm of Ni₃-MaSOF_as at 195 K. Bottom: relative adsorption energies calculated by DFT (def2-SVPD level of theory) of three molecules of N₂ and/or CO₂ on a [2 + 3]-nickel model complex. (Adapted with permission from Ref. 115. Copyright © 2019 American Chemical Society.)

Miscellaneous Applications

In addition to the as-discussed applications, salen- and salphen-containing polymers were used for example as light-emitting materials in solution.[103] [104] [105] [106] [141] While some of the polymers were only investigated phenomenologically,[103] MacLachlan's linear 1D Zn(II)-containing poly(salphenyleneethynylenes) showed a fluorescence enhancement upon addition of Lewis bases such as pyridine, proving a potential application as a fluorescent sensor for such.[104b] This was further underlined by a series of poly(Pt-salphen)-(para-phenyleneethynylenes) providing low-energy emission (λ _Em ∼ 580–700 nm) with a selective fluorescence quenching in the presence of Cu²⁺ ions, while other main group and transition metals did not influence the emission properties.[105]

Another example for the use of 1D salen polymers is the metal-free polymer P1, which was used to enantio-discriminate chiral α-hydroxyl carboxylic acids. For instance, for mandelic acid an enantiomeric fluorescence difference ratio (ef)[142] of up to 8.41 in preference to the (L)-enantiomer was found ([Figure 8]).[143] When reduced to the corresponding salan polymer, a “turn-off” of the fluorescence was observed, proving again that minor structural changes in the polymeric backbone can tune the corresponding properties dramatically.

Figure 8 Top: structure of a chiral polymer sensor P1. Bottom: fluorescence enhancement of P1 with (D)- and (L)-mandelic acid. Inset: photographic images of solutions of P1 (left), P1 with (D)-mandelic acid (middle), and P1 with (L)-mandelic acid in toluene/DME.[143] (Adapted with permission from Ref. 143. Copyright © 2012 American Chemical Society.)

Based on defined pores, the COF-1 (SA_BET = 666 m²/g) and COF-1-Zn networks (SA_BET = 460 m²/g; see [Scheme 11]) were used as the stationary phase for HPLC to separate mixtures of ethylbenzene and xylene isomers ([Figure 9]).[125] With metal-free COF-1, baseline separation was observed with separation factors α of up to 2.0, resolution factors R of up to 5.2 as well as high effective theoretical plate numbers of up to 10980. These are in the range or even higher than those of, for example, stationary-phase bases on MOFs.[144]

Figure 9 Representative chromatogram of a column packed with COF-1 ([Scheme 15]) used for the separation of ethylbenzene (EB) and xylene isomers upon different sample loading (depicted on the right).[144] (Adapted with permission from Ref. 144. Copyright © 2019 American Chemical Society.)

Conclusions and Outlook

To summarize, salen and salphen units and their corresponding metal complexes have been introduced into 1D, 2D, and 3D polymers and depending on the dimensionality and properties used for several applications. For instance, most 1D salphen polymers are conjugated compounds and thus it is not surprising that these have been investigated as conductive films, for example, to tune the redox properties of electrodes or the resistivity of multiwalled carbon nanotubes. Furthermore, some of the compounds show fluorescence and in combination with various Lewis acidic metal centers have been used for sensing by fluorescence quenching. If chiral salens instead of salphens are embedded into the polymer backbone, the sensing applications are broadened to chiral discrimination. It is worth mentioning that some of the first chiral helical ladder polymers synthesized by Katz and coworkers are based on the formation of metal salphens. There were three main synthetic approaches towards 1D polysalphens established: electropolymerization, condensation of aldehyde and amine moieties to imines, forming the salphen pockets, or cross-coupling reactions. The two latter have been used to make 2D salphen polymers as well. However, electropolymerization has not been used therefor. The vast majority of 2D as well as 3D salphen polymers are porous and therefore it is not surprising that in combination with the embedded metal centers, these have been used for catalytic applications as well as gas sorption and separation of smaller molecules. In both fields there are some materials that are comparable or even superior to similar compounds. For instance, some of the 2D chiral metal salen COFs are excellent catalysts for a number of asymmetric transformations, giving products in high yields and high ee values, thus perfectly combining heterogeneous reactivity with high surface areas. Due to the fact that almost all of the described 2D-salphene polymers are crystalline and being reported that the sheets are packed in an eclipsed fashion, the vast majority of metal salphen centers are not accessible for catalytic reactions or interactions with gases. Therefore, it is just logical that salphen COFs and polymers will be taken to the third dimension. Till date only a handful of 3D salphen polymers are known, but certainly they will be exploited in the future to enhance catalytic activities and gas sorption properties even more. The large plethora of metals that can be complexed by salen and salphen units as well as organic building blocks to construct polymers allows an infinite access of new materials with exciting properties.

No conflict of interest has been declared by the author(s).

• References

• 1 Combes A. C. R. Acad. Fr. 1889; 108: 1252
• 2a Takeuchi T, Böttcher A, Quezada CM, Meade TJ, Gray HB. Bioorg. Med. Chem. 1999; 7: 815
• 2b Kwiatkowski E, Kwiatkowski M. Inorg. Chim. Acta 1980; 42: 197
• 2c Dedieu A, Rohmer MM, Veillard A. J. Am. Chem. Soc. 1976; 98: 5789
• 2d Böttcher A, Takeuchi T, Hardcastle KI, Meade TJ, Gray HB, Cwikel D, Kapon M, Dori Z. Inorg. Chem. 1997; 36: 2498
• 3a Matsumoto K, Oguma T, Katsuki T. Angew. Chem. Int. Ed. 2009; 48: 7432
• 3b Atwood DA. Coord. Chem. Rev. 1997; 165: 267
• 3c Adão P, Costa Pessoa J, Henriques RT, Kuznetsov ML, Avecilla F, Maurya MR, Kumar U, Correia I. Inorg. Chem. 2009; 48: 3542
• 4a Whitelaw EL, Loraine G, Mahon MF, Jones MD. Dalton Trans. 2011; 40: 11469
• 4b Whitelaw EL, Davidson MG, Jones MD. Chem. Commun. 2011; 47: 10004
• 4c Matsumoto K, Saito B, Katsuki T. Chem. Commun. 2007; 35: 3619
• 4d Whitelaw EL, Jones MD, Mahon MF. Inorg. Chem. 2010; 49: 7176
• 5a Li X-Y, Chen L, Gao L, Zhang Y, Akogun SF, Dong W-K. RSC Adv. 2017; 7: 35905
• 5b Dong Y-J, Dong X-Y, Dong W-K, Zhang Y, Zhang L-S. Polyhedron 2017; 123: 305
• 5c Dong W-K, Li X-L, Wang L, Zhang Y, Ding Y-J. Sens. Actuators, B 2016; 229: 370
• 5d Akine S, Taniguchi T, Nabeshima T. Inorg. Chem. 2004; 43: 6142
• 6a Akine S, Nabeshima T. Inorg. Chem. 2005; 44: 1205
• 6b Yilmaz I, Temel H, Alp H. Polyhedron 2008; 27: 125
• 6c Sowden RJ, Trotter KD, Dunbar L, Craig G, Erdemli O, Spickett CM, Reglinski J. Biometals 2013; 26: 85
• 6d Patchett R, Magpantay I, Saudan L, Schotes C, Mezzetti A, Santoro F. Angew. Chem. Int. Ed. 2013; 52: 10352
• 6e Fernandes RR, Lasri J, Kirillov AM, d. Silva MF. C. G, d. Silva JA. L, d. Silva JJ. R. F, Pombeiro AJ. L. Eur. J. Inorg. Chem. 2011; 2011: 3781
• 6f Cox K, Kariuki BM, Smyth A, Newman PD. Dalton Trans. 2016; 45: 8485
• 7a Katsuki T. Coord. Chem. Rev. 1995; 140: 189
• 7b Whiteoak CJ, Salassa G, Kleij AW. Chem. Soc. Rev. 2012; 41: 622
• 7c Canali L, Sherrington DC. Chem. Soc. Rev. 1999; 28: 85
• 7d McGarrigle EM, Gilheany DG. Chem. Rev. 2005; 105: 1563
• 7e Cozzi PG. Chem. Soc. Rev. 2004; 33: 410
• 8a Sasaki H, Irie R, Hamada T, Suzuki K, Katsuki T. Tetrahedron 1994; 50: 11827
• 8b Jacobsen EN, Zhang W, Muci AR, Ecker JR, Deng L. J. Am. Chem. Soc. 1991; 113: 7063
• 8c Irie R, Noda K, Ito Y, Matsumoto N, Katsuki T. Tetrahedron: Asymmetry 1991; 2: 481
• 8d Irie R, Noda K, Ito Y, Matsumoto N, Katsuki T. Tetrahedron Lett. 1990; 31: 7345
• 9a Hamachi K, Irie R, Katsuki T. Tetrahedron Lett. 1996; 37: 4979
• 9b Nishinaga A, Yamato H, Abe T, Maruyama K, Matsuura T. Tetrahedron Lett. 1988; 29: 6309
• 10 Fukuda T, Katsuki T. Synlett 1995; 1995: 825
• 11a Palucki M, Hanson P, Jacobsen EN. Tetrahedron Lett. 1992; 33: 7111
• 11b Reddy DR, Thornton ER. J. Chem. Soc., Chem. Commun. 1992; 172
• 11c Adam W, Fell RT, Stegmann VR, Saha-Möller CR. J. Am. Chem. Soc. 1998; 120: 708
• 11d Caori S, Kiyohiko N, Masaaki K, Junnosuke F. Bull. Chem. Soc. Jpn. 1991; 64: 1318
• 12 O'Connor KJ, Wey S-J, Burrows CJ. Tetrahedron Lett. 1992; 33: 1001
• 13a Jacobsen EN, Kakiuchi F, Konsler RG, Larrow JF, Tokunaga M. Tetrahedron Lett. 1997; 38: 773
• 13b Wu MH, Jacobsen EN. Tetrahedron Lett. 1997; 38: 1693
• 14 Schaus SE, Brånalt J, Jacobsen EN. J. Org. Chem. 1998; 63: 403
• 15 North M, Orizu C, Tararov VI, Ikonnikov NS, Belokon YN, Hibbs DE, Hursthouse MB, Abdul Malik KM. Chem. Commun. 1998; 387
• 16 Balasanthiran V, Chisholm MH, Durr CB. Dalton Trans. 2015; 44: 8205
• 17 Reglinski J, Kennedy AR, Steel G. New J. Chem. 2015; 39: 2437
• 18 Cui Y, Jiang J, Mao X, Wu J. Inorg. Chem. 2019; 58: 218
• 19 Corazza F, Floriani C, Chiesi-Villa A, Guastini C, Ciurli S. J. Chem. Soc., Dalton Trans. 1988; 2341
• 20 Spassky N, Wisniewski M, Pluta C, Le Borgne A. Macromol. Chem. Phys. 1996; 197: 2627
• 21 Wagler J, Böhme U, Roewer G. Angew. Chem. Int. Ed. 2002; 41: 1732
• 22 Agustin D, Rima G, Gornitzka H, Barrau J. Organometallics 2000; 19: 4276
• 23 Sánchez M, Harvey MJ, Nordstrom F, Parkin S, Atwood DA. Inorg. Chem. 2002; 41: 5397
• 24 Meermann C, Sirsch P, Törnroos KW, Anwander R. Dalton Trans. 2006; 1041
• 25 Floriani C, Solari E, Corazza F, Chiesi-Villa A, Guastini C. Angew. Chem. 1989; 101: 91
• 26 Seangprasertkij R, Riechel TL. Inorg. Chem. 1986; 25: 3121
• 27 Coggon P, McPhail AT, Mabbs FE, Richards A, Thornley AS. J. Chem. Soc. A 1970; 3296
• 28 Wiznycia AV, Desper J, Levy C. J. Chem. Commun. 2005; 4693
• 29 Darensbourg DJ, Ortiz CG, Billodeaux DR. Inorg. Chim. Acta 2004; 357: 2143
• 30 Pizzolato E, Natali M, Posocco B, Montellano Lopez A, Bazzan I, Di Valentin M, Galloni P, Conte V, Bonchio M, Scandola F, Sartorel A. Chem. Commun. 2013; 49: 9941
• 31 Amirnasr M, Schenk KJ, Gorji A, Vafazadeh R. Polyhedron 2001; 20: 695
• 32 Chong JH, Ardakani SJ, Smith KJ, MacLachlan MJ. Chem. Eur. J. 2009; 15: 11824
• 33 Klement R, Stock F, Elias H, Paulus H, Pelikán P, Valko M, Mazúr M. Polyhedron 1999; 18: 3617
• 34 Anselmo D, Salassa G, Escudero-Adán EC, Martin E, Kleij AW. Dalton Trans. 2013; 42: 7962
• 35 Shen Y-Z, Pan Y, Wang L-Y, Dong G, Jin X-P, Huang X-Y, Hu H. J. Organomet. Chem. 1999; 590: 242
• 36 Agustin D, Rima G, Gornitzka H, Barrau J. J. Organomet. Chem. 1999; 592: 1
• 37 Costes J-P, Dupuis A, Laurent J-P. Inorg. Chim. Acta 1998; 268: 125
• 38 Zhu H, Wang M, Ma C, Li B, Chen C, Sun L. J. Organomet. Chem. 2005; 690: 3929
• 39 Floriani C, Solari E, Franceschi F, Scopelliti R, Belanzoni P, Rosi M. Chem. Eur. J. 2001; 7: 3052
• 40 Luts T, Frank R, Suprun W, Fritzsche S, Hey-Hawkins E, Papp H. J. Mol. Catal. A: Chem. 2007; 273: 250
• 41 Jurisson S, Lindoy LF, Dancey KP, McPartlin M, Tasker PA, Uppal DK, Deutsch E. Inorg. Chem. 1984; 23: 227
• 42 Chichak K, Jacquemard U, Branda NR. Eur. J. Inorg. Chem. 2002; 357
• 43 Rogers CA, West BO. J. Organomet. Chem. 1974; 70: 445
• 44 Yoon I, Narita M, Shimizu T, Asakawa M. J. Am. Chem. Soc. 2004; 126: 16740
• 45 Barwiolek M, Sawicka J, Babinska M, Wojtczak A, Surdykowski A, Szczesny R, Szlyk E. Polyhedron 2017; 135: 153
• 46 Tajmir-Riahi HA. Polyhedron 1983; 2: 723
• 47 Sharma P, Rosas N, Ramashankar A. Main Group Met Chem 1995; 18: 621
• 48 Srivastava TN, Chauhan AK. S, Mehrotra GK. Synth. React. Inorg. Met.-Org. Chem. 1982; 12: 705
• 49 Klamm BE, Windorff CJ, Marsh ML, Meeker DS, Albrecht-Schmitt TE. Chem. Commun. 2018; 54: 8634
• 50 Costes J-P, Laussac J-P, Nicodème F. J. Chem. Soc., Dalton Trans. 2002; 2731
• 51 Taha ZA, Ajlouni AM, Al Momani W, Al-Ghzawi AA. Spectrochim. Acta, Part A 2011; 81: 570
• 52 Tsuchimoto M, Ookusa M, Itoh S, Watanabe M, Nakajima K. Polyhedron 2018; 148: 118
• 53 Berardozzi R, Pescitelli G, Di Pietro S, Resta C, Ballistreri FP, Pappalardo A, Tomaselli GA, Di Bari L. Chirality 2015; 27: 857
• 54 Tyagi SB, Singh B, Kapoor RN. Transition Met. Chem. 1987; 12: 405
• 55 Santini-Scampucci C, Wilkinson G. J. Chem. Soc., Dalton Trans. 1976; 807
• 56 Yeung KT, To WP, Sun C, Cheng G, Ma C, Tong GS. M, Yang C, Che CM. Angew. Chem. Int. Ed. 2017; 56: 133
• 57 Ison EA, Cessarich JE, Du G, Fanwick PE, Abu-Omar MM. Inorg. Chem. 2006; 45: 2385
• 58 Che CM, Cheng WK, Mak TC. W. Inorg. Chem. 1986; 25: 703
• 59 Mercati G, Morazzoni F. J. Chem. Soc., Dalton Trans. 1979; 569
• 60 Guo Z, Yiu S-M, Chan MC. W. Chem. Eur. J. 2013; 19: 8937
• 61 Barnholtz SL, Lydon JD, Huang G, Venkatesh M, Barnes CL, Ketring AR, Jurisson SS. Inorg. Chem. 2001; 40: 972
• 62 Pellerito L, Cefalù R, Ruisi G. J. Organomet. Chem. 1972; 44: 243
• 63 Balasanthiran V, Chisholm MH, Durr CB, Gallucci JC. Dalton Trans. 2013; 42: 11234
• 64 Agarwal DD, Srivastava S, Chadha P. Polyhedron 1990; 9: 1401
• 65 Brancatelli G, Pappalardo A, Trusso Sfrazzetto G, Notti A, Geremia S. Inorg. Chim. Acta 2013; 396: 25
• 66 Copping R, Mougel V, Den Auwer C, Berthon C, Moisy P, Mazzanti M. Dalton Trans. 2012; 41: 10900
• 67 Zhang H-C, Huang W-S, Pu L. J. Org. Chem. 2001; 66: 481
• 68a Dai Y, Katz TJ, Nichols DA. Angew. Chem. Int. Ed. 1996; 35: 2109
• 68b Dai Y, Katz TJ. J. Org. Chem. 1997; 62: 1274
• 69 Xu Z-X, Huang Z-T, Chen C-F. Tetrahedron Lett. 2009; 50: 5430
• 70 Escárcega-Bobadilla MV, Salassa G, Martínez Belmonte M, Escudero-Adán EC, Kleij AW. Chem. Eur. J. 2012; 18: 6805
• 71a Zintl M, Molnar F, Urban T, Bernhart V, Preishuber-Pflügl P, Rieger B. Angew. Chem. Int. Ed. 2008; 47: 3458
• 71b Vagin SI, Reichardt R, Klaus S, Rieger B. J. Am. Chem. Soc. 2010; 132: 14367
• 71c Vagin S, Winnacker M, Kronast A, Altenbuchner PT, Deglmann P, Sinkel C, Loos R, Rieger B. ChemCatChem 2015; 7: 3963
• 71d Reichardt R, Vagin S, Reithmeier R, Ott AK, Rieger B. Macromolecules 2010; 43: 9311
• 71e Mandal M, Chakraborty D. J Polym Sci A Polym Chem 2016; 54: 809
• 72a Castro-Gómez F, Salassa G, Kleij AW, Bo C. Chem. Eur. J. 2013; 19: 6289
• 72b Decortes A, Kleij AW. ChemCatChem 2011; 3: 831
• 72c Castro-Osma JA, North M, Wu X. Chem. Eur. J. 2016; 22: 2100
• 72d Martín R, Kleij AW. ChemSusChem 2011; 4: 1259
• 73a Taherimehr M, Decortes A, Al-Amsyar SM, Lueangchaichaweng W, Whiteoak CJ, Escudero-Adan EC, Kleij AW, Pescarmona PP. Catal. Sci. Technol. 2012; 2: 2231
• 73b Escudero-Adán EC, Benet-Buchholz J, Kleij AW. Chem. Eur. J. 2009; 15: 4233
• 73c Kleij AW, Lutz M, Spek AL, van Leeuwen PW. N. M, Reek JN. H. Chem. Commun. 2005; 3661
• 73d Dalla Cort A, Mandolini L, Pasquini C, Rissanen K, Russo L, Schiaffino L. New J. Chem. 2007; 31: 1633
• 74a Theil H, von Richthofen C-GF, Stammler A, Bögge H, Glaser T. Inorg. Chim. Acta 2008; 361: 916
• 74b Sakata Y, Murata C, Akine S. Nat. Commun. 2017; 8: 16005
• 74c Houjou H, Ito M, Araki K. Inorg. Chem. 2009; 48: 10703
• 74d Glaser T, Heidemeier M, Krickemeyer E, Bögge H, Stammler A, Fröhlich R, Bill E, Schnack J. Inorg. Chem. 2009; 48: 607
• 74e Glaser T, Heidemeier M, Fröhlich R, Hildebrandt P, Bothe E, Bill E. Inorg. Chem. 2005; 44: 5467
• 74f Bazarnik M, Bugenhagen B, Elsebach M, Sierda E, Frank A, Prosenc MH, Wiesendanger R. Nano Lett. 2016; 16: 577
• 75a Borisova NE, Reshetova MD, Ustynyuk YA. Chem. Rev. 2007; 107: 46
• 75b Akine S, Taniguchi T, Nabeshima T. Tetrahedron Lett. 2001; 42: 8861
• 75c Hui JK. H, MacLachlan MJ. Chem. Commun. 2006; 2480
• 75d Gallant AJ, Chong JH, MacLachlan MJ. Inorg. Chem. 2006; 45: 5248
• 75e Frischmann PD, Facey GA, Ghi PY, Gallant AJ, Bryce DL, Lelj F, MacLachlan MJ. J. Am. Chem. Soc. 2010; 132: 3893
• 75f Nabeshima T, Miyazaki H, Iwasaki A, Akine S, Saiki T, Ikeda C. Tetrahedron 2007; 63: 3328
• 75g Frischmann PD, Mehr SH. M, Patrick BO, Lelj F, MacLachlan MJ. Inorg. Chem. 2012; 51: 3443
• 76a Sakata Y, Chiba S, Miyashita M, Nabeshima T, Akine S. Chem. Eur. J. 2019; 25: 2962
• 76b Ng CK, Toh RW, Lin TT, Luo H-K, Hor TS. A, Wu J. Chem. Sci. 2019; 10: 1549
• 76c Lai J, Ke X-S, Tang J, Zhang J-L. Chin. Chem. Lett. 2015; 26: 937
• 76d Akine S, Piao S, Miyashita M, Nabeshima T. Tetrahedron Lett. 2013; 54: 6541
• 76e Akine S, Miyashita M, Piao S, Nabeshima T. Inorg. Chem. Front. 2014; 1: 53
• 76f Akine S, Miyashita M, Nabeshima T. Chem. Eur. J. 2019; 25: 1432
• 76g Akine S, Miyashita M, Nabeshima T. J. Am. Chem. Soc. 2017; 139: 4631
• 77a Mastalerz M. Acc. Chem. Res. 2018; 51: 2411
• 77b Zhang G, Mastalerz M. Chem. Soc. Rev. 2014; 43: 1934
• 77c Mastalerz M. Angew. Chem. Int. Ed. 2010; 49: 5042
• 77d Beuerle F, Gole B. Angew. Chem. Int. Ed. 2018; 57: 4850
• 77e Santolini V, Miklitz M, Berardo E, Jelfs KE. Nanoscale 2017; 9: 5280
• 77f Acharyya K, Mukherjee PS. Angew. Chem. Int. Ed. 2019; 58: 8640
• 77g Rue NM, Sun J, Warmuth R. Isr. J. Chem. 2011; 51: 743
• 78a Kuil M, Puijk IM, Kleij AW, Tooke DM, Spek AL, Reek JN. H. Chem. Asian J. 2009; 4: 50
• 78b Kleij AW, Kuil M, Tooke DM, Lutz M, Spek AL, Reek JN. H. Chem. Eur. J. 2005; 11: 4743
• 79a Xuan W, Zhang M, Liu Y, Chen Z, Cui Y. J. Am. Chem. Soc. 2012; 134: 6904
• 79b Tan C, Jiao J, Li Z, Liu Y, Han X, Cui Y. Angew. Chem. Int. Ed. 2018; 57: 2085
• 79c Jiao J, Tan C, Li Z, Liu Y, Han X, Cui Y. J. Am. Chem. Soc. 2018; 140: 2251
• 79d Dong J, Zhou Y, Zhang F, Cui Y. Chem. Eur. J. 2014; 20: 6455
• 80 Dong J, Tan C, Zhang K, Liu Y, Low PJ, Jiang J, Cui Y. J. Am. Chem. Soc. 2017; 139: 1554
• 81a Kitaura R, Onoyama G, Sakamoto H, Matsuda R, Noro S-i, Kitagawa S. Angew. Chem. Int. Ed. 2004; 43: 2684
• 81b Oh M, Mirkin CA. Nature 2005; 438: 651
• 81c Cho S-H, Ma B, Nguyen ST, Hupp JT, Albrecht-Schmitt TE. Chem. Commun. 2006; 24: 2563
• 81d Oh M, Mirkin CA. Angew. Chem. Int. Ed. 2006; 45: 5492
• 81e Heo J, Jeon Y-M, Mirkin CA. J. Am. Chem. Soc. 2007; 129: 7712
• 81f Jeon Y-M, Heo J, Mirkin CA. J. Am. Chem. Soc. 2007; 129: 7480
• 81g Jeon Y-M, Armatas GS, Heo J, Kanatzidis MG, Mirkin CA. Adv. Mater. 2008; 20: 2105
• 81h Jung S, Oh M. Angew. Chem. Int. Ed. 2008; 47: 2049
• 81i Bhunia A, Roesky PW, Lan Y, Kostakis GE, Powell AK. Inorg. Chem. 2009; 48: 10483
• 81j Jung S, Cho W, Lee HJ, Oh M. Angew. Chem. Int. Ed. 2009; 48: 1459
• 81k Li G, Zhu C, Xi X, Cui Y. Chem. Commun. 2009; 2118
• 81l Yuan G, Zhu C, Xuan W, Cui Y. Chem. Eur. J. 2009; 15: 6428
• 81m Song F, Wang C, Falkowski JM, Ma L, Lin W. J. Am. Chem. Soc. 2010; 132: 15390
• 81n Bhunia A, Lan Y, Mereacre V, Gamer MT, Powell AK, Roesky PW. Inorg. Chem. 2011; 50: 12697
• 81o Falkowski JM, Wang C, Liu S, Lin W. Angew. Chem. Int. Ed. 2011; 50: 8674
• 81p Huang Y, Liu T, Lin J, Lü J, Lin Z, Cao R. Inorg. Chem. 2011; 50: 2191
• 81q Roesky PW, Bhunia A, Lan Y, Powell AK, Kureti S. Chem. Commun. 2011; 47: 2035
• 81r Shultz AM, Sarjeant AA, Farha OK, Hupp JT, Nguyen ST. J. Am. Chem. Soc. 2011; 133: 13252
• 81s Song F, Wang C, Lin W. Chem. Commun. 2011; 47: 8256
• 81t Wang C, Zheng M, Lin W. J. Phys. Chem. Lett. 2011; 2: 1701
• 81u Xiang S-C, Zhang Z, Zhao C-G, Hong K, Zhao X, Ding D-R, Xie M-H, Wu C-D, Das MC, Gill R, Thomas KM, Chen B. Nat. Commun. 2011; 2: 204
• 81v Falkowski JM, Liu S, Wang C, Lin W. Chem. Commun. 2012; 48: 6508
• 81w Zhu C, Yuan G, Chen X, Yang Z, Cui Y. J. Am. Chem. Soc. 2012; 134: 8058
• 81x Ren Y, Shi Y, Chen J, Yang S, Qi C, Jiang H. RSC Adv. 2013; 3: 2167
• 81y Li J, Yang J, Liu YY, Ma JF. Chem. Eur. J. 2015; 21: 4413
• 81z Xi W, Liu Y, Xia Q, Li Z, Cui Y. Chem. Eur. J. 2015; 21: 12581
• 81aa Bhunia A, Dey S, Moreno JM, Diaz U, Concepcion P, Van Hecke K, Janiak C, Van Der Voort P. Chem. Commun. 2016; 52: 1401
• 81ab Xia Q, Liu Y, Li Z, Gong W, Cui Y. Chem. Commun. 2016; 52: 13167
• 81ac Zhu C, Xia Q, Chen X, Liu Y, Du X, Cui Y. ACS Catal. 2016; 6: 7590
• 82a McKeown NB. J. Mater. Chem. 2000; 10: 1979
• 82b Manners I. Science 2001; 294: 1664
• 82c Baleizão C, Garcia H. Chem. Rev. 2006; 106: 3987
• 83a Li H, Hill MR, Huang R, Doblin C, Lim S, Hill AJ, Babarao R, Falcaro P. Chem. Commun. 2016; 52: 5973
• 83b Yang S, Peng L, Sun DT, Asgari M, Oveisi E, Trukhina O, Bulut S, Jamali A, Queen WL. Chem. Sci. 2019; 10: 4542
• 84a Wezenberg SJ, Kleij AW. Angew. Chem. Int. Ed. 2008; 47: 2354
• 84b Yuan G, Jiang H, Zhang L, Liu Y, Cui Y. Coord. Chem. Rev. 2019; 378: 483
• 84c Crane AK, MacLachlan MJ. Eur. J. Inorg. Chem. 2012; 2012: 17
• 85 Waltman RJ, Bargon J. Can. J. Chem. 1986; 64: 76
• 86 Boffa LS. Applied Polymer Science: 21st Century. Craver CD, Carraher CE. , eds. Pergamon: Oxford; 2000: 1021-1042
• 87 Odian G. Princiles of Polymerization. 4th edn. John Wiley & Sons, Inc.; Hoboken, NJ: 2004
• 88 Marvel CS, Tarköy N. J. Am. Chem. Soc. 1957; 79: 6000
• 89 Marvel CS, Tarköy N. J. Am. Chem. Soc. 1958; 80: 832
• 90a Manecke VG, Wille WE. Makromol. Chem. 1970; 133: 61
• 90b Manecke G, Wille WE, Kossmehl G. Makromol. Chem. 1972; 160: 111
• 91 Goldsby KA. J. Coord. Chem. 1988; 19: 83
• 92a Hoferkamp LA, Goldsby KA. Chem. Mater. 1989; 1: 348
• 92b Goldsby KA, Blaho JK, Hoferkamp LA. Polyhedron 1989; 8: 113
• 93a Dahm CE, Peters DG. Anal. Chem. 1994; 66: 3117
• 93b Vilas-Boas M, Freire C, de Castro B, Christensen PA, Hillman AR. Inorg. Chem. 1997; 36: 4919
• 94a Reddinger JL, Reynolds JR. Macromolecules 1997; 30: 673
• 94b Reddinger JL, Reynolds JR. Chem. Mater. 1998; 10: 3
• 94c Kingsborough RP, Swager TM. Adv. Mater. 1998; 10: 1100
• 95 Kingsborough RP, Swager TM. J. Am. Chem. Soc. 1999; 121: 8825
• 96 Tchepournaya I, Vasilieva S, Logvinov S, Timonov A, Amadelli R, Bartak D. Langmuir 2003; 19: 9005
• 97 Holliday BJ, Stanford TB, Swager TM. Chem. Mater. 2006; 18: 5649
• 98a Chen C, Zhu Z, Li X, Li J. J. Appl. Polym. Sci. 2017; 134: 44464
• 98b Tedim J, Gonçalves F, Pereira MF. R, Figueiredo JL, Moura C, Freire C, Hillman AR. Electrochim. Acta 2008; 53: 6722
• 98c Gao F, Li J, Kang F, Zhang Y, Wang X, Ye F, Yang J. J. Phys. Chem. C 2011; 115: 11822
• 99a Zhang Y, Li J, Gao F, Kang F, Wang X, Ye F, Yang J. Electrochim. Acta 2012; 76: 1
• 99b Chen C, Li X, Deng F, Li J. RSC Adv. 2016; 6: 79894
• 100a Wittung P, Nielsen PE, Buchardt O, Egholm M, Nordén B. Nature 1994; 368: 561
• 100b Leslie AG. W, Arnott S, Chandrasekaran R, Ratliff RL. J. Mol. Biol. 1980; 143: 49
• 101a Yoshio F, Takeshi M, Takeshi T, Nobuaki M, Toshikazu T. Chem. Lett. 2001; 30: 1020
• 101b Maeda T, Furusho Y, Takata T. Chirality 2002; 14: 587
• 101c Maeda T, Takeuchi T, Furusho Y, Takata T. J. Polym. Sci., Part A: Polym. Chem. 2004; 42: 4693
• 102a Houjou H, Shimizu Y, Koshizaki N, Kanesato M. Adv. Mater. 2003; 15: 1458
• 102b Houjou H, Sasaki T, Shimizu Y, Koshizaki N, Kanesato M. Adv. Mater. 2005; 17: 606
• 103 Lavastre O, Illitchev I, Jegou G, Dixneuf PH. J. Am. Chem. Soc. 2002; 124: 5278
• 104a Leung AC. W, Chong JH, Patrick BO, MacLachlan MJ. Macromolecules 2003; 36: 5051
• 104b Leung AC. W, MacLachlan MJ. J. Mater. Chem. 2007; 17: 1923
• 105 Sun S, Tong W-L, Chan MC. W. Macromol. Rapid Commun. 2010; 31: 1965
• 106 Zhao C, Sun S, Tong W-L, Chan MC. W. Macromolecules 2017
• 107 Wang J, Huang W, Pan L, Wang H, Zhang C, Liu X. Macromol. Res. 2015; 23: 309
• 108 Chantarasiri N, Tuntulani T, Tongraung P, Seangprasertkit-Magee R, Wannarong W. Eur. Polym. J. 2000; 36: 695
• 109 Jiang J, Leung AC. W, MacLachlan MJ. Dalton Trans. 2010; 39: 6503
• 110a Luo R, Zhou X, Chen S, Li Y, Zhou L, Ji H. Green Chem. 2014; 16: 1496
• 110b Leng Y, Lu D, Zhang C, Jiang P, Zhang W, Wang J. Chem. Eur. J. 2016; 22: 8368
• 110c Luo R, Chen Y, He Q, Lin X, Xu Q, He X, Zhang W, Zhou X, Ji H. ChemSusChem 2017; 10: 1526
• 111 Nielsen M, Thomsen AH, Jensen TR, Jakobsen HJ, Skibsted J, Gothelf KV. Eur. J. Org. Chem. 2005; 2005: 342
• 112 Li L-H, Feng X-L, Cui X-H, Ma Y-X, Ding S-Y, Wang W. J. Am. Chem. Soc. 2017; 139: 6042
• 113a Han X, Xia Q, Huang J, Liu Y, Tan C, Cui Y. J. Am. Chem. Soc. 2017; 139: 8693
• 113b Li H, Feng X, Shao PP, Chen J, Li CZ, Jayakumar S, Yang QH. J. Mater. Chem. A 2019; 7: 5482
• 114a Haak RM, Castilla AM, Belmonte MM, Escudero-Adán EC, Benet-Buchholz J, Kleij AW. Dalton Trans. 2011; 40: 3352
• 114b Escudero-Adán EC, Benet-Buchholz J, Kleij AW. Inorg. Chem. 2007; 46: 7265
• 115 Elbert SM, Zhang W-S, Vaynzof Y, Oberhof N, Bernhardt M, Pernpointner M, Rominger F, Schröder RR, Mastalerz M. Chem. Mater. 2019; 31: 6210
• 116 Elbert SM, Rominger F, Mastalerz M. Chem. Eur. J. 2014; 20: 16707
• 117 Johnson JA, Chen S, Reeson TC, Chen Y-S, Zeng XC, Zhang J. Chem. Eur. J. 2014; 20: 7632
• 118a Li H, Xu B, Liu X, , A S, He C, Xia H, Mu Y. J. Mater. Chem. A 2013; 1: 14108
• 118b Xie Y, Wang T-T, Liu X-H, Zou K, Deng W-Q. Nat. Commun. 2013; 4: 1960
• 118c Li H, Li Z, Zhang Y, Luo X, Xia H, Liu X, Mu Y. RSC Adv. 2014; 4: 37767
• 118d Liu TT, Lin ZJ, Shi PC, Ma T, Huang YB, Cao R. ChemCatChem 2015; 7: 2340
• 118e Alkordi MH, Weseliński ŁJ, D'Elia V, Barman S, Cadiau A, Hedhili MN, Cairns AJ, AbdulHalim RG, Basset J-M, Eddaoudi M. J. Mater. Chem. A 2016; 4: 7453
• 119a Gupta KC, Abdulkadir HK, Chand S. J. Mol. Catal. A: Chem. 2003; 202: 253
• 119b Alvaro M, Baleizao C, Carbonell E, El Ghoul M, García H, Gigante B. Tetrahedron 2005; 61: 12131
• 120 Wang R-M, Hao E-X, Shen G-R, He Y-F, Lei Z-Q. J. Appl. Polym. Sci. 2009; 111: 1999
• 121a Côté AP, Benin AI, Ockwig NW, O'Keeffe M, Matzger AJ, Yaghi OM. Science 2005; 310: 1166
• 121b Ding S-Y, Wang W. Chem. Soc. Rev. 2013; 42: 548
• 121c Diercks CS, Yaghi OM. Science 2017 355
• 122 Mastalerz M, Oppel IM. Eur. J. Org. Chem. 2011; 5971
• 123 Mastalerz M, Hauswald H-JS, Stoll R. Chem. Commun. 2012; 48: 130
• 124 Yan S, Guan X, Li H, Li D, Xue M, Yan Y, Valtchev V, Qiu S, Fang Q. J. Am. Chem. Soc. 2019; 141: 2920
• 125 Huang J, Han X, Yang S, Cao Y, Yuan C, Liu Y, Wang J, Cui Y. J. Am. Chem. Soc. 2019; 141: 8996
• 126 Liu H, Feng J, Zhang J, Miller PW, Chen L, Su C-Y. Chem. Sci. 2015; 6: 2292
• 127 Reinhard D, Zhang W-S, Vaynzof Y, Rominger F, Schröder RR, Mastalerz M. Chem. Mater. 2018; 30: 2781
• 128 Chun J, Kang S, Kang N, Lee SM, Kim HJ, Son SU. J. Mater. Chem. A 2013; 1: 5517
• 129 Wöhrle D, Bohlen H, Rothkopf HW. Makromol. Chem. 1983; 184: 763
• 130 Krishnan R, Vancheesan S. J. Mol. Catal. A: Chem. 2000; 157: 15
• 131a Martín C, Fiorani G, Kleij AW. ACS Catal. 2015; 5: 1353
• 131b Decortes A, Castilla AM, Kleij AW. Angew. Chem. Int. Ed. 2010; 49: 9822
• 132 Liu T-T, Liang J, Huang Y-B, Cao R. Chem. Commun. 2016; 52: 13288
• 133 Bhunia S, Molla RA, Kumari V, Islam SM, Bhaumik A. Chem. Commun. 2015; 51: 15732 . It is worth mentioning that for similar transformations much higher TONs of up to 2000 and TOFs of 1000 h⁻¹ were found for MOF-based heterogenous catalysts, as seen in: Zhou, Z.; He, C; Xia, J; Yang, L; Duan, C. J. Am. Chem. Soc. 2015, 137, 15066. For comparable homogenous catalysts, TONs as high as 112000 and TOFs up to 36000 h⁻¹ are reported as seen in: Whiteoak, C.J.; Kleij, A.W. Synlett. 2013, 24, 1748
• 134 Kleij AW. Dalton Trans. 2009; 4635
• 135 Katsuki T. J. Synth. Org. Chem. Jpn. 1995; 53: 940
• 136a Liu P, Feng X-J, He R. Tetrahedron 2010; 66: 631
• 136b Gogoi A, Dewan A, Borah G, Bora U. New J. Chem. 2015; 39: 3341
• 136c Borhade SR, Waghmode SB. Tetrahedron Lett. 2008; 49: 3423
• 137 Li JR, Yu JM, Lu WG, Sun LB, Sculley J, Balbuena PB, Zhou HC. Nat. Commun. 2013; 4: 1538
• 138a Vaidhyanathan R, Iremonger SS, Dawson KW, Shimizu GK. H. Chem. Commun. 2009; 5230
• 138b Dietzel PD. C, Besikiotis V, Blom R. J. Mater. Chem. 2009; 19: 7362
• 138c Dietzel PD. C, Johnsen RE, Fjellvåg H, Bordiga S, Groppo E, Chavan S, Blom R. Chem. Commun. 2008; 5125
• 139a Min Wang Q, Shen D, Bülow M, Ling Lau M, Deng S, Fitch FR, Lemcoff NO, Semanscin J. Microporous Mesoporous Mater. 2002; 55: 217
• 139b Farrusseng D, Daniel C, Gaudillère C, Ravon U, Schuurman Y, Mirodatos C, Dubbeldam D, Frost H, Snurr RQ. Langmuir 2009; 25: 7383
• 140 Volkringer C, Loiseau T, Haouas M, Taulelle F, Popov D, Burghammer M, Riekel C, Zlotea C, Cuevas F, Latroche M, Phanon D, Knöfelv C, Llewellyn PL, Férey G. Chem. Mater. 2009; 21: 5783
• 141 Tong W-L, Lai L-M, Chan MC. W. Dalton Trans. 2008; (11) 1412
• 142 The enantiomeric fluorescence difference ratio (ef) is defined as the ratio in fluorescence intensity of the sensoric polymer at similar concentration of each enantiomer.
• 143 Song F, Wei G, Wang L, Jiao J, Cheng Y, Zhu C. J. Org. Chem. 2012; 77: 4759
• 144a Alaerts L, Maes M, Giebeler L, Jacobs PA, Martens JA, Denayer JF. M, Kirschhock CE. A, De Vos DE. J. Am. Chem. Soc. 2008; 130: 14170
• 144b Finsy V, Verelst H, Alaerts L, De Vos D, Jacobs PA, Baron GV, Denayer JF. M. J. Am. Chem. Soc. 2008; 130: 7110
• 144c Holcroft JM, Hartlieb KJ, Moghadam PZ, Bell JG, Barin G, Ferris DP, Bloch ED, Algaradah MM, Nassar MS, Botros YY, Thomas KM, Long JR, Snurr RQ, Stoddart JF. J. Am. Chem. Soc. 2015; 137: 5706
• 144d Moreira MA, Santos JC, Ferreira AF. P, Loureiro JM, Ragon F, Horcajada P, Shim K-E, Hwang Y-K, Lee UH, Chang J-S, Serre C, Rodrigues AE. Langmuir 2012; 28: 5715
• 144e Moreira MA, Santos JC, Ferreira AF. P, Loureiro JM, Ragon F, Horcajada P, Yot PG, Serre C, Rodrigues AE. Microporous Mesoporous Mater. 2012; 158: 229
• 144f Moreira MA, Santos JC, Ferreira AF. P, Loureiro JM, Ragon F, Horcajada P, Yot PG, Serre C, Rodrigues AE. Langmuir 2012; 28: 3494
• 144g Moreira MA, Santos JC, Ferreira AF. P, Loureiro JM, Rodrigues AE. Ind. Eng. Chem. Res. 2011; 50: 7688
• 144h Moreira MA, Santos JC, Ferreira AF. P, Müller U, Trukhan N, Loureiro JM, Rodrigues AE. Sep. Sci. Technol. 2011; 46: 1995
• 144i Moreira MA, Santos MP. S, Silva CG, Loureiro JM, Chang J-S, Serre C, Ferreira AF. P, Rodrigues AE. Adsorption 2018; 24: 715
• 144j Nicolau MP. M, Bárcia PS, Gallegos JM, Silva JA. C, Rodrigues AE, Chen B. J. Phys. Chem. C 2009; 113: 13173
• 144k Yang C-X, Yan X-P. Anal. Chem. 2011; 83: 7144