Synthesis of sterically hindered bis(imidazole) proligands and an exploration of their co-ordination chemistry with copper(II)

Abstract

Five new sterically hindered bis(imidazole) proligands have been synthesised: 1,2-bis(1-ethyl-4-phenylimidazol-2-yl)ethane (L1), bis(1-ethyl-4-phenylimidazol-2-yl)-(L2) and bis(1-methyl-4-phenylimidazol-2-yl)-dimethyl methane (L3), α, α′-bis(4,5′-diphenylimidazol-1-yl)-o-(L4) and m-xylene (L5). They have been characterised analytically and spectroscopically (1H NMR and mass) and L1 and L2 have been crystallised and characterised by X-ray crystallography. Complexation reactions were attempted with Cu(BF4)2·4.5H2O but only L1 formed a simple monomeric complex, [CuL12][BF4]2. The CuII is co-ordinated by four nitrogen atoms, Cu–Nav 2.01 A, in an essentially square-planar environment. The X-band EPR spectrum of solid [CuL12][BF4]2(g⊥≈1.967, g∥≈2.214, A∥≈196 × 10–4 cm–1) is consistent with a tetragonal ligand field but the Q-band spectrum of a frozen solution of the complex in CH2Cl2–toluene (10 : 1) at 77 K indicates a small rhombic distortion (gx≈2.034, gy≈2.052 and gz≈2.229, Az≈198 × 10–4 cm–1). In CH2Cl2 the complex displays a quasi-reversible CuII–CuI couple with an oxidation peak at +0.74 V and a coupled reduction at –0.07 V (scan rate of 0.2 V s–1). The complexes [CuL42][BF4]2·H2O and [CuL52][BF4]2·H2O appear to be polymeric with bridging ligands. Compound L3 reacted with Cu(BF4)2·4.5H2O to form [H2L3][BF4]2 which has been characterised by X-ray crystallography; L2 reacts similarly.