Copper(I) Schiff-base complexes: reversible carbon monoxide binding, reactions with dioxygen, and the structure of a dinuclear complex containing co-ordinated and unco-ordinated alkene groups

Abstract

Copper(I) complexes of a series of acyclic Schiff-base ligands (L¹–L⁵) derived from the condensation of 2,6-diacetylpyridine with 2 mol of 4-aminobut-1-ene (L¹), allylamine (L²), 1-aminobutane (L³), 1 -aminopropane (L⁴), and 2-(ethylthio)ethylamine (L⁵), have been prepared. Crystals of the dinuclear complex[Cu₂L¹₂][BPh₄]₂ are orthorhombic with a= 17.74(1), b= 17.78(1), c= 21.90(1)Å, Z= 4, and space group Pb2₁a(no. 29). In the dimeric cation each Cu^I atom is bonded to the pyridine nitrogen and one imino nitrogen of one ligand molecule and to one imino nitrogen and one alkene group of the second ligand molecule in a distorted tetrahedral geometry. The second alkene group of each ligand remains unco-ordinated. In order to accommodate to this unusual co-ordination mode one imino group is severely twisted (by 83, 86° in the two ligand molecules) out of the plane of the pyridine ring, leading to two distinct ν(CN) vibrations in the i.r. spectrum. On the basis of i.r. and other properties, it is proposed that the complexes of L²–L⁵, as well as [Cu₂L¹₂][BPh₄]₂ in MeCN solution, have mononuclear structures in which the trimethine pyridyldi-imine group retains its customary planarity and acts as a tridentate ligand. For [CuL³][ClO₄] and [CuL⁴][ClO₄] having no side-arm donor groups, and for [CuL²][ClO₄]·H₂O having propene side chains, a T-shaped three-co-ordinated structure is suggested. For [CuL⁵][ClO₄], as well as for [Cu₂L¹₂][BPh₄]₂ and [CuL²][ClO₄]·H₂O in solution, spectroscopic evidence indicates at least partial side-arm donor group co-ordination leading to four-co-ordinate species. All the complexes bind CO reversibly in MeCN solution to an extent dependent on the competition exercised by the pendant side-arm donor groups, where present. The complexes also bind O₂ in solution in a 0.25:1 O₂: Cu stoicheiometry at rates which parallel the CO affinities. The O₂ binding is partially reversible in some cases as evidenced by electronic spectral and O₂-uptake measurements on successive oxygenation/deoxygenation treatments. Among the ultimate products of the accompanying irreversible oxidation are the bis(ligand) Cu^II complexes [CuL₂]²⁺.