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 Cu^II is co-ordinated by four nitrogen atoms, Cu–N_av 2.01 Å, 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 (g_x≈2.034, g_y≈2.052 and g_z≈2.229, A_z≈198 × 10^–4 cm^–1). In CH₂Cl₂ the complex displays a quasi-reversible Cu^II–Cu^I 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.