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

Abstract

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 C_Q(⁵⁹Co). Cobalt-59 NMR spectroscopy was used to characterize the cobalt magnetic shielding tensor, to verify the magnitudes of C_Q(⁵⁹Co) and to establish the value of η_Q, 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 C_Q 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 C_Q is negative and that the largest component of the EFG is along the Co–methyl-carbon bond.