Kinetics and mechanism of inversion at asymmetric nitrogen centres in the blue copper(II) complex of C-rac-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetra-azacyclotetradecane

Abstract

In order to investigate the effect of a co-ordinated ligand in the configurational conversion of a tetra-amine macrocyclic ligand complex of copper(II), the kinetics of the blue-to-red interconversion of the copper(II) complex of C-rac-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetra-azacyclotetradecane (tet b) in the presence of monodentate ligands has been examined at 25 °C by using spectrophotometric techniques. The main pathway of this reaction is via a base which is co-ordinated to the metal ion. The co-ordinated base which contains another lone pair in the vicinity of the amine hydrogen catalyzes this reaction by partially removing the proton from nitrogen in the transition state. The reactivity order for the blue-to-red interconversion of blue[Cu(tet b)L]^(2–n)+ is blue[Cu(tet b)(OH)]⁺ > blue[Cu(tet b)(NH₂NH₂)]²⁺ > blue[Cu(tet b)(SH)]⁺ > blue[Cu(tet b)(NH₂OH)]²⁺ > blue[Cu(tet b)(O₂CMe)]⁺ > blue[Cu(tet b)(NO₂)]⁺ > blue[Cu(tet b)(NH₂CSNH₂)]²⁺ > blue[Cu(tet b)(SO₃)] > blue[Cu(tet b)(HPO₄)] > blue[Cu(tet b)(S₂O₃)] blue[Cu(tet b)(CN)]⁺, blue[Cu(tet b)(NCS)]⁺, blue[Cu(tet b)(N₃)]⁺, blue[Cu(tet b)(NH₃)]²⁺, blue[Cu(tet b)(C₆H₅NH₂)]²⁺, blue[Cu(tet b)(C₅H₅N)]²⁺, blue[Cu(tet b)Cl]⁺, blue[Cu(tet b)Br]⁺, blue[Cu(tet b)I]⁺. The bases that fail to catalyze this reaction are of three types: (1) very weak bases such as Cl^–, Br^–, and I^–; (2) those containing only one lone pair such as NH₃, C₅H₅N, and C₆H₅NH₂; and (3) those containing two or more lone pairs which are far away from each other such as N₃^–, NCS^–, and CN^–. For the bases containing at least two lone pairs which are close to each other, the rate of this reaction increases as the basicity of the base increases, and as the negative charge of the base decreases.