Stereochemistry in the Bis(1,4,7-triazacyclodecane)cobalt(III) system and force field calculations

Abstract

An analysis of the stereochemical possibilities for the [Co(tacd)₂]³⁺ complex is presented (tacd = 1,4,7-triazacyclodecane). Stereochemical variables are cis (chiral) and trans relative dispositions of the two six-membered rings, combinations of alternative conformations for the six chelate rings in the complex, and alternative configurations R or S about one secondary amine nitrogen in each coordinated ligand. Energy minimization calculations have been carried out on a number of the possible isomers, from which it is deduced that the most stable conformational arrangements in the trans geometry are {λδ chair,A 6 chair} and {I. 6 chair,6 6 chair), and those in the cis geometry are { λδ chair δδ chair}, { λδ chair δδ chair } and { λδ chair δδ chair skew). The calculations give the trans-cis energy difference as 9.3 kJ mol^-1, to be compared with the experimental free energy difference of 8 ± 1 kJ mol^-1.