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 1 H NMR and IR spectroscopies. In the former series, the 1 H 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) 2 RhCl, a more π-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 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.