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

The synthesis and the structure of the new ligand bis(1-methyl-4,5-diphenylimidaz-2-oyl)carbinol (bimOH) is reported; bimOH, C33H28N4O, 496.61, triclinic P (1) over bar, a=10.020(9), b=14.848(8), c=9.57(1) Angstrom, alpha=104.98(5), beta=96.86(8), gamma=76.95(6)degrees, V=1337(2) Angstrom(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 [CuCl4](2-), [CuBr4](2-), [BF4](-) and [PF6](-). The structures of [Cu(bimOH)(2)][CuCl4]. H2O and [Cu(bimOH)(2)][PF6](2) . 2EtOH have been determined. [Cu(bimOH)(2)][CuCl4]. H2O, C66H56N8Cl4Cu2O2.5, 1270.12, monoclinic C2/c, a=27.584(9), b=18.50(2), c=12.677(5) Angstrom, beta=91.71(3)degrees, V=6467(9) Angstrom(3), Z=4, R=0.069, R-w=0.082, [Cu(bimOH)(2)][PF6](2) . 2EtOH, C70H68N8CuF12O4P2, 1436.2, orthorhombic Pbca, a=23.928(5), b=23.570(6), c=11.587(9) Angstrom, V=6535(8) Angstrom(3), Z=4, R=0.084, R-w=0.093. In both of these compounds, the (CuN4)-N-II centres possess an almost square-planar geometry. The powder EPR spectra of these compounds are characteristic of magnetically dilute (CuN4)-N-II square-planar centres: g(z)=similar to 2.21, g(xy)=similar to 2.05, A(z)=similar to 200 X 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+/+) couple at a potential of ca. +0.36 V in CH2Cl2 versus SCE. Reaction of bimOH with Zn(BF4)(2) . xH(2)O yields [Zn(bimOH)(2)][BF4](2):C66H56N8B2F8O2Zn, 1176.18, triclinic P (1) over bar, a=14.861(4), b=16.408(4), c=14.336(2) Angstrom, alpha=112.85(2), beta=98.72(2), gamma=105.75(2)degrees, V=2967(1) Angstrom(3), Z=2, R=0.106, R-w=0.117; and the (ZnN4)-N-II centre possesses a geometry reasonably close to tetrahedral and this is related to the geometry proposed for the (CuN4)-N-I centre with the bimOH ligand. The implications of these results for the Cu centres in proteins are discussed. (C) 1998 Elsevier Science S.A. All rights reserved.