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 (τ_c) 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.