NMR parameters of cobalt and of the directly bonded organic carbons obtained from solution 59Co and 13C NMR spectra of cobaloximes

Abstract

The one bond cobalt–carbon scalar coupling constants of the cobaloximes [RCo(DH)2PPh3] ((DH)2=bisdimethylglyoximate) (R=Me, n-Pr, CH2Cl) and [MeCo(DH)2L] (L=D2O, N-MeIm, py, P(OMe)3, P(OEt)3) were obtained by the joint use of 59Co and natural abundance 13C NMR spectra in solution. The 1J(Co, C) values of the methyl derivatives show a clear dependence on the axial ligand L. They increase in the order P(OMe)3<P(OEt)3⩽PPh3<py⩽N-MeIm<D2O and reflect changes in the cobalt–carbon binding. Since the reduced constants 1K(Co, C) are about one-half of the 1K(Rh, C) and the directly bonded carbon and phosphorus atoms are notably less shielded than in the corresponding rhodium compounds, important relativistic effects can be hypothesised in the latter. The correlation times for molecular reorientation (τc) of [MeCo(DH)2PPh3] and of [MeCo(DH)2py] were determined at various temperatures from the line-width at half-height of the 59Co NMR signal as its relaxation is inherently quadrupolar. The rotational diffusion exhibits a notable dependence on the solvent, revealing specific interactions with it.