COPPER(I) HEMOCYANIN MODELS - VARIABLE COORDINATION-NUMBER AND DISTORTED GEOMETRIES IN BENZIMIDAZOLE CHELATES

Dinuclear copper(I) complexes derived from three benzimidazole-containing binucleating ligands are described within the context of synthetic models for deoxyhemocyanin and carbon monoxyhemocyanin. The neutral, potentially septadentate ligand HL-Et, which contains bis((1-ethyl-2-benzimidazolyl)methyl) moieties at both ends of a 2-hydroxypropylenediamine spacer, forms the CO complex [Cu2(HL-Et)(CO)2][CF3SO3]2 (1) and the acetonitrile complex [Cu2(HL-Et)(MeCN)2][CF3SO3]2 (2). The related tetrakis(1-methylbenzimidazole)-containing ligand HCAB-Me, having a 2,6-disubstituted cresol spacer, forms the acetonitrile complex [Cu2(HCAB-Me)(MeCN)2][CF3SO3]2 (3). The unalkylated benzimidazole analogue of HL-Et, HL-H, forms the acetonitrile complex [Cu2(HL-H)(MeCN)2][ClO4]2•y3Et2O (4) and the CO complex [Cu2(HL-H)(CO)2][BF4]2 (5). Complexes 1–4 have been characterized by X-ray crystallography. All complexes reveal an approximately trigonal array of two benzimidazoles and the added ligand (CO or MeCN) along with a widely varying apical interaction with the tertiary amine N atom. The unusual feature of these nominally four-coordinate trigonal-pyramidal species 1–3 is the large off-normal distortion that is observed in the apical copper–amine bond as it spans the range 2.31–2.73 Å. In complex 4 the off-normal pulling away of the tertiary amine is complete and three-coordination prevails. These complexes further extend the plasticity of coordination number and geometry seen in copper(I) complexes. The superior ligand-constraining ability of proteins suggests that highly irregular coordinate geometries will be the rule rather than the exception in copper proteins. Crystal data: 1, C47H50N10O9Cu2F6S2, triclinic, [formula omitted], a = 16.616 (8) Å, 6 = 1 1.884 (5) Å, c = 15.131 (4) Å, α = 106.69 (3)°, β = 83.25 (3)°, γ = 110.75 (4)°, Z = 2; 2, C49H56N12O7Cu2F6S2, orthorhombic, Pbcn, a = 15.473 (5) Å, b = 16.369 (9) Å, c = 44.178 (17) Å, Z = 8; 3, C51H51O7Cu2F6S2, triclinic, [formula omitted], a = 14.20 (1) Å, b = 13.39 (1) Å,c= 15.31 (1) Å, a = 104.52 (6)°, β = 76.08 (7)°, γ = 90.71 (8)°, Z = 2; 4, C39H40N12O9Cu2Cl2 ⅓C4H10O, monoclinic, C2/c, a = 28.710 (9) Å, b = 7.600 (2) Å, c = 21.857 (9) Å, β = 114.23 (3)°. Benzimidazole Cu-N distances for 1-4 all lie within the range 1.96–2.03 (1) Å. Cu-C distances in 1 are 1.82 (2) and 1.77 (2) Å at Cul and Cu2, respectively. Acetonitrile Cu–N distances lie within the range 1.87–1.96 (1) Å for complexes 2-4. © 1990, American Chemical Society. All rights reserved.