Use of C-13-H-1 NMR coupling constants to assess the binding of imidazole ring ligands to metals

In this report we initiate analysis of imidazole rings bound to metals usin g one-bond H-1-C-13 couplings. Shift trends in the metal binding of N-donor heterocyclic ligands have been difficult to interpret since signals of clo se-in nuclei, particularly the carbon in the CN2 grouping of imidazole ring s, often have erratic or small shift changes. We exploit the well-studied t rans influence of a series of unidentate axial R and X ligands in cobaloxim e complexes (LCo(DH)(2)(R or X)) since shifts of carbons remote from Co hav e interpretable trends that correlate with Co-N bond lengths. We compare in formation obtained from (1)J(CH) values for two series, one with L = a smal l ligand, N-methylimidazole (N-MeImd), and one with L = a large ligand, 1,5 ,6-trimethylbenzimidazole (Me(3)Bzm). For comparison to our spectroscopic m easurements, 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), CH 3 (2), CH2CN (3), and Cl (4)) doubling the number of structures reported fo r cobaloximes containing N-MeImd. The observation of the B-type orientation of the planar N-MeImd axial ligand in structures I and 2 almost certainly results from the small size of N-MeImd. This B-orientation is rare for coba loximes, which normally have an axial ligand in the A-orientation. The t sc ores obtained from the principal component analysis of cobaloxime data [Ran daccio, L.; Geremia, S.; Zangrando, E.; Ebert, C. Inorg. Chern. 1994, 33, 4 641] were applied for the first time in the analysis of intraligand couplin g constants. The use of these parameters allowed us to identify steric effe cts on the C-13 shifts and (1)J(CH) values for the bulky, lopsided Me(3)Bzm axial ligand in the Me(3)BzmCo(DH)(R or X) series; similar steric effects on spectra were not observed for N-MeImdCo(DH)(R or X). The intraligand (1) J(CH) values for the close-in N2CH carbon of Me(3)Bzm and N-MeImd were foun d to reflect the ligand-to-metal binding even better than the more commonly used C-13 shifts Of carbons remote from the metal. Thus, the use of (1)J(C H) values holds considerable promise in metallobiochemistry.