The preparation of cis-folded macrocyclic nickel(II) complexes and the kinetics of their base-catalysed conversion to planar species

Abstract

The violet folded cis-macrocyclic complexes [NiL(en)][ClO₄]₂[L = 1,4,8,11-tetra-azacyclotetradecane (L¹), its C-meso-5,12-dimethyl analogue, or 1,4,7,10-tetra-azacyclotridecane] have been prepared. In basic solution these complexes are converted to the yellow planar complexes [NiL]²⁺ which are believed to have the RSSR(SRRS) chiral nitrogen configuration. Kinetic studies establish that rate =k_OH[complex][OH^–]. Values of k_OH have been obtained at various temperatures and activation parameters determined. Interconversion of the folded cis complexes having the RRRR(SSSS) chiral nitrogen configuration requires the inversion of two chiral sec-NH centres to give the RSSR(SRRS) configuration found in the planar species. The isomerisation of cis-[NiL¹(H₂O)₂]²⁺ shows a first-order dependence on the hydroxide ion concentration up to pH 12.6. Possible mechanisms involving intermolecular attack by hydroxide, or intramolecular attack by co-ordinated hydroxide at the sec-NH centres of the complex are considered. The relevance of these studies to the formation of macrocyclic nickel(II) complexes is discussed.