Chemistry of the copper(I)–water bond. Some new observations

Abstract

One-dimensional polymeric copper(I) complexes of the type {[CuL(H₂O)]BF₄·H₂O}_n, where L = 2,3-diphenylquinoxaline, and {[CuL′(H₂O)]X}_n, where L′ = 2,3-dimethylquinoxaline and X⁻ = ClO₄⁻ or BF₄⁻, containing a rare copper(I)–water bond were synthesised. From the X-ray crystal structures of two of them, the copper(I) centres in these complexes are found to have a planar T-shaped N₂O coordination sphere. It is concluded from the observed Cu(I)–O(water) bond lengths [2.167(7)–2.307(14) Å] that the copper(I)–water bonds in these complexes are rather weak. With L, a monomeric complex of the type CuL₂ClO₄ has also been synthesised. But it has not been possible to obtain such a monomeric copper(I) complex with the BF₄⁻ anion or the ligand L′. In CuL₂ClO₄ the metal is also found to have, from the X-ray crystal structure, a planar T-shaped N₂O coordination sphere with the perchlorate anion very weakly bound to the metal through an oxygen atom [Cu(I)–O(perchlorate) = 2.442(8) Å]. While in the solid state electronic spectra, CuL₂ClO₄ displays a band at 346 nm, the aqua complexes show additional band(s) in the 400–480 nm range. CuL₂ClO₄ reacts with water in dichloromethane to yield an aqua copper(I) complex: CuL₂ClO₄ + H₂O  → [CuL₂(H₂O)]ClO₄ → 1/n{[CuL(H₂O)]ClO₄}_n + L. In cyclic voltammetry at a glassy carbon electrode in anhydrous dichloromethane under N₂ atmosphere, CuL₂ClO₄ shows a quasi-reversible Cu^II/I couple with a very high redox potential of 0.91 V vs. SCE, which is lowered to 0.79 V vs. SCE upon addition of water. This indicates that binding of water destabilises copper(I), a result expected on the basis of Pearson's HSAB Principle.