Unusual NMR spectral properties of the 1,2-dicyanoethyl alkyl ligand in organocobaloximes. Structure of the organocobalt complex trans-bis(dimethylglyoximato)-(1,2-dicyanoethyl)(1,5,6-trimethylbenzimidazole) cobalt(III)

Article abstract of DOI:10.1021/om00119a002

The 1,2-dicyanoethyl ligand has been reported to exhibit unusual dynamic processes in certain organocobaloxime complexes. The crystal and molecular structures of the complex trans-bis(dimethylglyoximato)(1,2-dicyanoethyl)(1,5,6-trimethylbenzimidazole) cobalt(III)-0.5-ethanol, Me₃BzmCo-(DH)₂CH(CN)CH₂CN, are reported. The compound crystallizes in space group P1, triclinic, with a = 7.721 (1) ?, b = 11.845 (2) ?, c = 15.078 (3) ?, α = 79.09 (2)°, β = 80.65 (2)°, γ = 75.65 (2)°, V = 1320.0 ?³, Z = 2, D_measd = 1.38 g cm^-3, D_calcd = 1.39 g cm^-3, and R = 0.051 for 5261 independent reflections. The Co-C bond distance of 2.061 (3) ? is slightly shorter than that of 2.076 (2) ? found in the structure of the isopropyl analogue, whereas the Co-N(Me₃Bzm) bond length (2.031 (3) ?) is shorter than that in the isopropyl analogue (2.097 (2) ?). Hence, the 1,2-dicyanoethyl and isopropyl groups have similar bulk properties but different trans influences. The rate constants, k₁, were determined for dissociation of Me₃Bzm from six Me₃BzmCo(DH)₂R complexes where R ranges from weak donors such as CH₂CF₃ to strong donors and CH(CH₃)₂. High-field ¹H NMR spectroscopy showed that the CH(CN)CH₂CN proton pattern, which is distorted by second-order effects, is greatly affected by temperature and the trans influence of L. A deuteration study in pyridine-d₅/D₂O revealed no exchange of the -CH(CN)CH₂CN after 2 months at ambient temperature. These results militate against any unusually rapid dynamic processes involving the CH(CN)CH₂CN group. Rather, previous NMR spectral observations at low field could be understood as arising from the peculiar ¹H NMR spectral characteristics of these compounds.