N-heterocyclic carbene-transition metal complexes:: Spectroscopic and crystallographic analyses of π-back-bonding interactions

The ability of N-heterocyclic carbenes (NHCs) to participate in pi-back-bonding interactions was evaluated in a range of transition metal complexes. Rh chloride complexes containing a systematic series of various 1,3-dimethyl-4,5-disubstituted-imidazol-2-ylidenes and either 1,5-cyclooctadiene (cod) or two carbon monoxide ligands were synthesized (i.e., (NHC)RhCl(cod) and (NHC)RhCl(CO)(2), respectively) and studied using H-1 NMR and IR spectroscopies. In the former series, the H-1 NMR chemical shifts of the signals attributable to the olefin trans to the NHC ligand were found to shift downfield by up to 0.17 ppm as the pi-acidity of the substituents on the 4,5-positions increased (i.e., H -> Cl -> CN). Similarly, in the latter series, the IR stretching frequencies of the carbonyl groups trans to the NHC ligands were found to increase by 11 +/- 0.5 cm(-1) as it-acidity increased over the same series. Using the nitrile group as a diagnostic handle, the CN stretching frequency of (1,3-dimethyl-4,5-dicyanoimidazol-2-ylidene)(cod)RhCl was found to be 4 +/- 0.5 cm(-1) higher than 1,3-dimethyl-4,5-dicyanoimidazol-2-ylidene)(CO)(2)RhCl, a more pi-acidic analogue. X-ray analysis of the aforementioned series of (NHC)(cod)RhCl complexes indicated changes in N-C-carbene bond lengths that were consistent with greater pi-donation from complexes containing 4,5-dihydroimidazol-2-ylidene relative to the their 4,5-dicyano analogues. Collectively, these results suggest not only that imidazol-2-ylidenes are capable of pi-back-bonding but that this interaction may be tuned by changing the pi-acidity of the substituents on the imidazole ring.