Syntheses, structural analyses and redox kinetics of four-coordinate [CuL2]2+ and five-coordinate [CuL2(solvent)]2+ complexes (L = 6,6′-dimethyl-2,2′-bipyridine or 2,9-dimethyl-1,10-phenanthroline): completely gated reduction reaction of [Cu(dmp)2]2+ in nitromethane

Abstract

[Cu(2,9-dimethyl-1,10-phenanthroline)₂]²⁺ and [Cu(6,6′-dimethyl-2,2′-bipyridine)₂]^2+/+ complexes with no coordinated solvent molecule were synthesized and the crystal structures were analyzed: the coordination geometry around the Cu(I) center was in the D_2d symmetry while a D₂ structure was observed for the four-coordinate Cu(II) complexes. Coordination of a water or an acetonitrile molecule was found in the trigonal plane of the five-coordinate Cu(II) complex in the Tbp (trigonal bipyramidal) structure. Spectrophotometric analyses revealed that the D₂ structure of the Cu(II) complex was retained in nitromethane, although a five-coordinate Tbp species (green in color), was readily formed upon dissolution of the solid (reddish brown) in acetonitrile. The electron self-exchange reaction between D_2d-Cu(I) and D₂-Cu(II), observed by the NMR method, was very rapid with k_ex = (1.1 ± 0.2) × 10⁵ kg mol⁻¹ s⁻¹ at 25 °C (ΔH* = 15.6 ± 1.3 kJ mol⁻¹ and ΔS* = −96 ± 4 J mol⁻¹ K⁻¹), which was more than 10 times larger than that reported for the self-exchange reaction between D_2d-Cu(I) and Tbp-Cu(II) in acetonitrile. The cross reduction reactions of D₂-Cu(II) by ferrocene and decamethylferrocene in nitromethane exhibited a completely gated behavior, while the oxidation reaction of D_2d-Cu(I) by [Ni(1,4,7-triazacyclononane)₂]³⁺ in nitromethane estimated an identically large self-exchange rate constant to that directly obtained by the NMR method. The electron self-exchange rate constant estimated from the oxidation cross reaction in 50% v/v acetonitrile–nitromethane mixture was 10 times smaller than that observed in pure nitromethane. On the basis of the Principle of the Least Motion (PLM) and the Symmetry Rules, it was concluded that gated behaviors observed for the reduction reactions of the five-coordinate Cu(II)–polypyridine complexes are related to the high-energy C_2v → D_2d conformational change around Cu(II), and that the electron self-exchange reactions of the Cu(II)/(I) couples are always adiabatic through the C_2v structures for both Cu(II) and Cu(I) since the conformational changes between D_2d, D₂ and C_2v structures for Cu(I) as well as the conformational change between Tbp and C_2v structures for Cu(II) are symmetry-allowed. The completely gated behavior observed for the reduction reactions of D₂-Cu(II) species in nitromethane was attributed to the very slow conformational change from the ground-state D₂ to the entatic D_2d structure that is symmetry-forbidden for d⁹ metal complexes: the very slow back reaction, the forbidden conformational change from entatic D_2d to the ground-state D₂ structure, ensures that the rate of the reduction reaction is independent of the concentration of the reducing reagent.