Solid-state 13C and 59Co NMR spectroscopy of 13C-methylcobalt(iii) complexes with amine ligands

Five octahedral Co(III) cations, [trans-Co(en)2(X)(13CH3)]n+ where en = ethylenediamine, X = CN−, N3−, NH3, NO2− or H2O and n = 1 or 2, as well as [Co(NH3)513CH3]2+, have been investigated by solid-state 13C and 59Co NMR spectroscopy. We show that the determination of the 59Co nuclear quadrupolar parameters both directly via59Co NMR and indirectly via13C NMR provide complementary information that is unavailable if one investigates only one nucleus. Specifically, 1J(59Co,13C) and the orientation of the largest component of the EFG were determined via13C NMR spectroscopy, which also established the negative sign of CQ(59Co). Cobalt-59 NMR spectroscopy was used to characterize the cobalt magnetic shielding tensor, to verify the magnitudes of CQ(59Co) and to establish the value of ηQ, which is difficult to determine indirectly. The measurements show that the EFG tensors are either axially symmetric or close to being so, but there is a wide range of CQ values, from −40 MHz for the complex with X = H2O to −105 MHz with X = CN−. The Co chemical shift tensors are approximately axially symmetric with the spans, δ11−δ33, ranging from 3700 to 5600 ppm for X = H2O and CN−, respectively. The latter measurements also established the relative orientations of the Co EFG and chemical shift tensors. Density functional theory calculations of the 59Co EFG and magnetic shielding tensors as well as of 1J(59Co,13C) for the NO2− and N3− complexes were undertaken. These calculations confirm the experimental observation that the sign of CQ is negative and that the largest component of the EFG is along the Co–methyl-carbon bond.