Use of 13C−1H NMR Coupling Constants To Assess the Binding of Imidazole Ring Ligands to Metals

Abstract

In this report we initiate analysis of imidazole rings bound to metals using one−bond 1H-13C couplings. Shift trends in the metal binding of N-donor heterocyclic ligands have been difficult to interpret since signals of close-in nuclei, particularly the carbon in the CN2 grouping of imidazole rings, often have erratic or small shift changes. We exploit the well-studied trans influence of a series of unidentate axial R and X ligands in cobaloxime complexes (LCo(DH)2(R or X)) since shifts of carbons remote from Co have interpretable trends that correlate with Co−N bond lengths. We compare information obtained from 1JCH values for two series, one with L = a small ligand, N-methylimidazole (N-MeImd), and one with L = a large ligand, 1,5,6-trimethylbenzimidazole (Me3Bzm). For comparison to our spectroscopic measurements, we have determined X-ray crystal structures for N-MeImdCo(DH)2(R or X) (where DH = monoanion of dimethylglyoxime, R or X = CH2CH3 (1), CH3 (2), CH2CN (3), and Cl (4)) doubling the number of structures reported for cobaloximes containing N-MeImd. The observation of the B-type orientation of the planar N-MeImd axial ligand in structures 1 and 2 almost certainly results from the small size of N-MeImd. This B-orientation is rare for cobaloximes, which normally have an axial ligand in the A-orientation. The t scores obtained from the principal component analysis of cobaloxime data [Randaccio, L.; Geremia, S.; Zangrando, E.; Ebert, C. Inorg. Chem. 1994, 33, 4641] were applied for the first time in the analysis of intraligand coupling constants. The use of these parameters allowed us to identify steric effects on the 13C shifts and 1JCH values for the bulky, lopsided Me3Bzm axial ligand in the Me3BzmCo(DH)(R or X) series; similar steric effects on spectra were not observed for N-MeImdCo(DH)(R or X). The intraligand 1JCH values for the close-in N2CH carbon of Me3Bzm and N-MeImd were found to reflect the ligand-to-metal binding even better than the more commonly used 13C shifts of carbons remote from the metal. Thus, the use of 1JCH values holds considerable promise in metallobiochemistry.