One-N, six-membered heterocyclic carbene complexes and the remote heteroatom concept

A whole library of six-membered N-heterocyclic carbene complexes of Ni(II) and Pd(II) were prepared by oxidative substitution. In some of these compounds the heteroatom occurs in a position distant from the carbene donor atom. Combined structural and physical data, especially C-13 NMR results, indicate carbene character in such ligands. DFT quantum mechanical calculation at the RI-BP56/SVP level allowed the extraction of valuable chemical information predicting that rNHC (r = remote) ligands would bond more strongly than their nNHC (n = normal) isomers. This result is also corroborated by calculations on the metal complexes themselves. Orbital overlap (mainly r) follows the order (NHC5)-H-2 < nN(1)HC(6) < rN(1)HC(6) when ligands derived from halo-imidazolium and halo-pyridinium salts are compared. In C-C coupling catalysis using Pd(II) and Ni(II) complexes, the simple one-N, six-membered carbene complexes are superior to simple two-N, five-membered examples but clear differentiation between nNHC and rNHC precatalysts in the former family, is not always possible.