The preparation of cis-folded macrocyclic nickel(II) complexes and the kinetics of their base-catalysed conversion to planar species
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 =kOH[complex][OH–]. Values of kOH 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.