N-Heterocyclic Carbene−Transition Metal Complexes: Spectroscopic and Crystallographic Analyses of π-Back-bonding Interactions

Abstract

The ability of N-heterocyclic carbenes (NHCs) to participate in π-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 1H NMR and IR spectroscopies. In the former series, the 1H 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 π-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 π-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)2RhCl, a more π-acidic analogue. X-ray analysis of the aforementioned series of (NHC)(cod)RhCl complexes indicated changes in N−Ccarbene bond lengths that were consistent with greater π-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 π-back-bonding but that this interaction may be tuned by changing the π-acidity of the substituents on the imidazole ring.