carbene ligands - carbenes - rhodium - phophorus
Significance
<P>The preparation of the first stable P-heterocyclic carbenes (PHCs) is reported.
A formal [3+2] cycloaddition of a diphosphaallylic cation, generated from the corresponding
phosphalkane with a dipolarophile leads to the PHCs in moderate yields. Crystallographic
studies reveal that the phosphorus centers deviate only slightly from planarity, suggesting
a possible increase in π-donating capability of the phosphorus atoms, thus increasing
the ligand’s δ-donating ability. Carbonyl stretching frequencies for the
cis-[RhCl(CO)
2PHC] complex indicate that the metal center is a weaker π-donor compared to that in
the analogous NHC complex.</P>
Comment
<P>N-heterocyclic carbenes (NHCs) are now accepted as extremely useful ligands in
the field of transition metal catalysis, especially in the very active methathesis
chemistry (F. K. Zinn, M. S. Viciu, S. P. Nolan
Annu. Rep. Prog. Chem., Sect. B 2004,
100, 231-249). Their ability to strongly coordinate to metal centers increases resistance
of the complex toward air, moisture, and oxidation and avoids the need to use an excess
amount of the ligand. As analogues of NHCs, PHCs may prove similarly useful as ligands
in catalytic systems requiring the need for electron-rich ligands. In view of the
simple synthesis of the reported PHCs, further activity in this area may be anticipated.</P>
