The effect of remote substitution on the formation of preferential isomers of cobalt(iii)-tetrazolate complexes by microwave assisted cycloaddition

Abstract

The 1,3-dipolar cycloaddition reaction of cis-[Co(N₃)₂(en)₂]NO₃1 with different organonitriles (NCR) under focussed microwave irradiation produced bis-tetrazolate complexes [Co(N₄CR)₂(en)₂](NO₃). Interestingly, in the case of 3-cyano pyridine the reaction produced both cis- and trans-isomers (cis-2 and trans-2), whereas for 4-cyano pyridine the compound obtained was exclusively cis (cis-3) and for 4-bromobenzonitrile it was only the trans- (trans-4) compound which was isolated. This indicates a probable role of remote substitution of the phenyl ring in dictating the formation of the preferential isomer. When starting from the trans-isomer of the diazido complex (trans-[Co(N₃)₂(en)₂]ClO₄, 1a), upon reacting with different nitriles a mixture of cis- and trans-isomers of [Co(N₄CR)₂(en)₂]ClO₄ was produced in each case, with a greater preference towards cis-geometry [R = 4-NC₅H₄ (cis-5 and trans-5), 4-BrC₆H₄ (cis-6 and trans-6) and C₆H₅ (cis-7 and trans-7)]. The preferential formation of the cis-analogue of compound trans-4 when starting from the trans-precursor was quite curious. A theoretical investigation of compounds trans-4 and cis-6 reveals that the greater stability of the trans-complex 4 may arise from additional van der Waals interactions in the solid state because of the presence of an extra DMF molecule as solvent of crystallization. However, an interacting counter-anion and a probable halogen–halogen interaction may also contribute to the formation of preferential isomers for cycloaddition complexes, even in the solution state.