A reversible CuII/CuI electron transfer centre based on a flexible di-imidazole ligand

Abstract The synthesis and the structure of the new ligand bis(1-methyl-4,5-diphenylimidaz-2-oyl)carbinol (bimOH) is reported; bimOH, C 33 H 28 N 4 O, 496.61, triclinic P 1¯, a = 10.020(9), b = 14.848(8), c = 9.57(1) A, α = 104.98(5), β = 96.86(8), γ = 76.95(6)°, V = 1337(2) A 3 , Z = 2, R = 0.067, R w = 0.075, bimOH reacts with Cu II forming Cu(bimOH) 2 2+ and salts have been synthesised with the counter-anions CuCl 4 2 , CuBr 4 2 , BF 4 and PF 6 . The structures of Cu(bimOH) 2 CuCl 4 · H 2 O and Cu(bimOH) 2 PF 6 2 · 2EtOH have been determined. Cu(bimOH) 2 CuCl 4 · H 2 O. C 66 H 56 N 8 Cl 4 Cu 2 O 2.5 , 1270.12, monoclinic C 2/ c , a = 27.584(9), b = 18.50(2), c = 12.677(5) A, β = 91.71(3)°, V = 6467(9) A, Z = 4, R = 0.069, R w = 0.082: Cu(bimOH) 2 PF 6 2 · 2EtOH, C 70 H 68 N 8 CuF 12 O 4 P 2 , 1436.2, orthorhombic Pbca, a = 23.928(5), b = 23.570(6), c = 11.587(9) A, V = 6535(8) A 3 , Z = 4, R = 0.084, R w = 0.093. In both of these compounds, the Cu II N 4 centres possess an almost square-planar geometry. The powder EPR spectra of these compounds are characteristic of magnetically dilute Cu II N 4 square-planar centres: g = ∼2.21, g v, = ∼2.05, A = ∼200 × 10 −4 cm 1 . The cyclic voltammograms of these Cu II compounds display a one-electron reversible process attributed to the Cu(bimOH) 2 2 1/4 couple at a potential of ca. +0.36 V in CH 2 Cl 2 versus SCE. Reaction of bimOH with Zn(BF 4 ) 2 · x H 2 O yields Zn(bimOH) 2 BF 4 2 : C 66 H 56 N 8 B 2 F 8 O 2 Zn, 1176.18, triclinic P 1¯, a = 14.861(4), b = 16.408(4), c = 14.336(2) A, α = 112.85(2), β = 98.72(2), γ = 105.75(2)°, V = 2967(1) A 3 , Z = 2, R = 0.106, R w = 0.117; and the Zn II N 4 centre possesses a geometry reasonably close to tetrahedral and this is related to the geometry proposed for the Cu I N 4 centre with the bimOH ligand. The implications of these results for the Cu centres in proteins are discussed.