Electronically Divergent Triscyclometalated Iridium(III) 2-(1-naphthyl)pyridine Complexes and Their Application in Three-Component Methoxytrifluoromethylation of Styrene
Rachael E.N. Njogu
a
Centre for Analysis and Synthesis, Department of Chemistry, P.O. Box 124, SE-22100 Lund University, Sweden Email: ola.wendt@chem.lu.se
d
Department of Chemistry, University of Nairobi, P.O Box 30197, KE-00100 Nairobi, Kenya
b
Department of Chemistry and Molecular Biology, P.O. Box 462, SE-40530 University of Gothenburg, Sweden Email: carl.wallentin@chem.gu.se
› Author AffiliationsWe would like to acknowledge funding from the International Science Program (KEN 01), the Swedish Research Council (2014-04046, 621-2014-3935, 2017-00748), and the Carl Tryggers Foundation (CTS 16: 490) for a fellowship for P.F.
Published as part of the Special Section 10th EuCheMS Organic Division Young Investigator Workshop
Abstract
A systematic study of the photophysical and electrochemical properties of triscyclometalated homoleptic iridium(III) complexes based on 2-(1-naphthyl)pyridine (npy) ligands is presented. A systematic investigation of ligand substitution patterns showed an influence on the lifetime of the excited state, with slight changes in the absorption and emission spectral features. Specifically, the emission lifetime of a complex of an npy ligand substituted with a strongly electron-withdrawing trifluoromethyl group was longer than that of the corresponding complex with the electronically nonperturbed ligand (3.7 μs versus 1.5 μs). Electronically complementary ligands and complexes with orthogonal configurations showed slightly shorter excited state lifetimes compared with unsubstituted npy (1.4–3.0 μs). All complexes displayed reversible or quasireversible redox-couple processes, with the complex of the trifluoromethylated ligand showing the highest ground-state oxidation potential E1/2ox [Ir(III)/Ir(IV) = 0.95 V vs. SCE in CH2Cl2]. This study showed that these complexes can be used as efficient photoredox catalysts, as demonstrated by their application in a regioselective methoxytrifluoromethylation in which the npy complexes showed equal or better performance compared with the archetypical photoredox catalyst tris[2-phenylpyridinato]iridium(III).
1 Current address: Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering and Jiangsu Key Laboratory of Vehicle Emissions Control, Nanjing University, Nanjing, P. R. of China.
2
Baldo MA,
O’Brien DF,
You Y,
Shoustikov A,
Sibley S,
Thompson ME,
Forrest SR.
Nature 1998; 395: 151
10
Flamigni L,
Barbieri A,
Sabatini C,
Ventura B,
Barigelletti F.
In
Photochemistry and Photophysics of Coordination Compounds II
.
Balzani V,
Campagna S.
Springer; Berlin: 2007: 143
232-(1-Naphthyl)pyridine (L1); Typical Procedure
A Straus flask equipped with a stirrer bar was charged with 2-bromopyridine (1.0 equiv.), 1-naphthylboronic acid (1.1 equiv), Pd(PPh3)4 (4 mol%), 2 M aq Na2CO3 (2.5 mL/mmol), toluene (3.8 mL/mmol), and ethanol (1.3 mL/mmol). The mixture was degassed by three freeze–pump–thaw cycles and then refluxed under inert conditions overnight. The mixture was cooled to r.t. and extracted with EtOAc (3 × 20 mL). The organic phases were combined, dried (MgSO4), and concentrated. The pure ligand was obtained by flash column chromatography [silica gel, EtOAc–hexane (1:4)].
Compound was isolated as viscous liquid that solidified upon standing. Yield: 1.6 g (99%).
1H NMR (400 MHz, CDCl3): δ = 8.81 (ddd, J = 4.9, 1.9, 1.0 Hz, 1 H), 8.11 (dd, J = 8.5, 1.3 Hz, 1 H), 7.93 (dt, J = 7.2, 1.3 Hz, 2 H), 7.82 (td, J = 7.7, 1.8 Hz, 1 H), 7.63–7.54 (m, 3 H), 7.54–7.46 (m, 2 H), 7.33 (ddd, J = 7.6, 4.9, 1.2 Hz, 1 H). 13C NMR (101 MHz, CDCl3): δ = 159.27, 149.56, 138.51, 136.44, 133.96, 131.18, 128.93, 128.39, 127.51, 126.52, 125.90, 125.62, 125.33, 125.09, 122.06. HRMS (ESI+): m/z: [M + H]+ calcd for C15H12N: 206.0970; found: 206.095.
24Iridium(III) Complexes C1–6; General Procedure
A Straus flask was charged with the ligand L1–6 (12.0 equiv), Na2CO3 (6.0 equiv), IrCl3 (1.0 equiv), and deionized H2O (60 mL). The mixture was degassed by three freeze–pump–thaw cycles, and the sealed Straus flask was heated at 205 °C for 48 h. The mixture was then cooled and extracted with CH2Cl2 (3 × 20 mL). The extracts were dried (MgSO4), filtered through a pad of Celite, and concentrated. The product was isolated by flash chromatography [silica gel, EtOAc–hexane (1:3) (for ligand), then CH2Cl2 (for complexes)].
25 CCDC 1875304 contains the supplementary crystallographic data for compound C1. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/getstructures.
26
Jiang X,
Peng J,
Wang J,
Guo X,
Zhao D,
Ma Y.
ACS Appl. Mater. Interfaces 2016; 8: 3591
37Oxytrifluoromethylation of Styrene; General Procedure
The oxytrifluoromethylation was performed according to the reported procedure; see ref. 33. An oven-dried microwave vial was cooled to r.t. under high vacuum then charged with 5-(trifluoromethyl)dibenzo[b,d]thiophenium tetrafluoroborate (42.0 mg, 1.1 equiv), the appropriate catalyst (0.5 mol%), and styrene (10.4 mg, 0.1 mmol), which had been filtered through a pad of basic Al2O3 before addition. The microwave vial was sealed and carefully degassed in by three vacuum–nitrogen cycles. A mixture of MeOH (0.2 mL) and CH2Cl2 (1.8 mL), previously degassed by three freeze–pump–thaw cycles, was added. The resulting mixture was irradiated with a blue LED light strip (7 W) at a distance of 2 cm with stirring for 4 h. The vial was then opened, and the internal standard trimethoxybenzene was added. The reaction mixture with the internal standard was briefly stirred and then transferred to an NMR tube equipped with a sealed D2O capillary, which was used for locking and shimming of the spectrometer. The yield was determined by using 128 scans with a relaxation time of 60 s. The reported yield is based on the integration of the most upfield signals (δ = 2.27 and 2.43 ppm) of the product. The experiments were performed twice, and the conversions and yields are reported as average values.
38
Griffin JD,
Zeller MA,
Nicewicz DA.
J. Am. Chem. Soc. 2015; 137: 11340
