Rhodium and iridium complexes of N-heterocyclic carbenes (3a-c and 4a-c) were obtained by transmetalation from the corresponding Ag(I) complexes. The structure of 3b was verified by X-ray diffraction. The compounds display restricted rotation about the metal-carbene bond, the rate of which can be controlled by altering the steric bulk of the auxiliary ligands. Infrared spectroscopy provides an estimate of the electron-donor power of the carbene ligands from ν(CO) of the carbonyl derivatives.

Imidazolium salts are found to bind abnormally via C5 to iridium(I) and iridium(III) to give air-stable monodentate N-heterocyclic carbene complexes. Abnormal ligand binding was verified by X-ray diffraction in both Ir(I) and Ir(III) complexes. In the case of Ir(I), it is necessary to block the C2 and C4 positions to form a stable sterically protected C5-bound complex. Infrared spectroscopy on carbonyl derivatives indicates that abnormally bound N-heterocyclic carbenes are much stronger electron donors than their ubiquitous C2-bound counterparts. The Tolman electronic parameter for 1-isopropyl-2,4-diphenyl-3-methylimidazolin-5-ylidene is 2039 cm-1, compared to ca. 2050 cm-1 for typical NHCs.

The new air-stable and moisture-insensitive Ir catalysts for efficient transfer hydrogenation of ketones contain a chelating bis(N-heterocyclic carbene) ligand. Most other hydrogen transfer catalysts show activity Rh > Ir, but we find Ir > Rh for these cases. Tuning of the ligand wingtip substituents, R, can greatly increase catalyst activity (R = neopentyl) or selectivity (R = isopropyl). Reactivity studies and isotopic labeling are consistent with a monohydride mechanism for the hydrogen transfer.

We show that imidazolium salts do not always give normal or even aromatic carbenes on metalation, and the chemistry of these ligands can be much more complicated than previously thought. N,N'-disubstituted imidazolium salts of type [(2-py)(CH(2))(n)(C(3)H(3)N(2))R]BF(4) react with IrH(5)(PPh(3))(2) to give N,C-chelated products (n = 0, 1; 2-py = 2-pyridyl; C(3)H(3)N(2) = imidazolium; R = mesityl, n-butyl, i-propyl, methyl). Depending on the circumstances, three types of kinetic products can be formed: in one, the imidazole metalation site is the normal C2 as expected; in another, the metalation occurs at the abnormal C4 site; and in the third, C4 metalation is accompanied by hydrogenation of the imidazolium ring. The bonding mode is confirmed by structural studies, and spectroscopic criteria can also distinguish the cases. Initial hydrogen transfer can take place from the metal to the carbene to give the imidazolium ring hydrogenation product, as shown by isotope labeling; this hydrogen transfer proves reversible on reflux when the abnormal aromatic carbene is obtained as final product. Care may therefore be needed in the future in verifying the structure(s) formed in cases where a catalyst is generated in situ from imidazolium salt and metal precursor.

Chelating bis(imidazolium) salts having (CH 2 ) n chains of different lengths (n ) 1-4) linking the diazole rings show very large reactivity differences on metalation with [(cod)RhCl] 2 . Long linkers favor a square-planar Rh(I) product, while short linkers favor octahedral Rh(III). We ascribe the origin of the effect to the restricted rotation of the highly sterically anisotropic diazole rings and the different preferred orientations of these rings as n changes. Defining the x and y axes as the Rh-carbene bond directions, we find that with short linkers the diazole rings tend to be oriented close to the xy plane. This tends to favor Rh(III) because these complexes, [Rh(bis-carbene)I 2 (OAc)], have the lowest steric hindrance in the xy plane. With long linkers, the diazole rings tend to be aligned face to face along the (z axis. This tends to favor Rh(I) because these complexes, [(cod)Rh(bis-carbene)]PF 6 , have the lowest steric hindrance along the (z axis. Crystallographic studies are reported. Electrospray MS data provide evidence for strong metal-carbene binding.