Tetranuclear Copper(II) Complexes Bridged by α-d-Glucose-1-Phosphate and Incorporation of Sugar Acids through the Cu4 Core Structural Changes

Abstract

Tetranuclear copper(II) complexes containing α-d-glucose-1-phosphate (α-d-Glc-1P), [Cu4(μ-OH){μ-(α-d-Glc-1P)}2(bpy)4(H2O)2]X3 [X = NO3 (1a), Cl (1b), Br (1c)], and [Cu4(μ-OH){μ-(α-d-Glc-1P)}2(phen)4(H2O)2](NO3)3 (2) were prepared by reacting the copper(II) salt with Na2[α-d-Glc-1P] in the presence of diimine ancillary ligands, and the structure of 2 was characterized by X-ray crystallography to comprise four {Cu(phen)}2+ fragments connected by the two sugar phosphate dianions in 1,3-O,O‘ and 1,1-O μ4-bridging fashion as well as a μ-hydroxo anion. The crystal structure of 2 involves two chemically independent complex cations in which the C2 enantiomeric structure for the trapezoidal tetracopper(II) framework is switched according to the orientation of the α-d-glucopyranosyl moieties. Temperature-dependent magnetic susceptibility data of 1a indicated that antiferromagnetic spin coupling is operative between the two metal ions joined by the hydroxo bridge (J = −52 cm-1) while antiferromagnetic interaction through the Cu−O−Cu sugar phosphate bridges is weak (J = −13 cm-1). Complex 1a readily reacted with carboxylic acids to afford the tetranuclear copper(II) complexes, [Cu4{μ-(α-d-Glc-1P)}2(μ-CA)2(bpy)4](NO3)2 [CA = CH3COO (3), o-C6H4(COO)(COOH) (4)]. Reactions with m-phenylenediacetic acid [m-C6H4(CH2COOH)2] also gave the discrete tetracopper(II) cationic complex [Cu4{μ-(α-d-Glc-1P)}2(μ-m-C6H4(CH2COO)(CH2COOH))2(bpy)4](NO3)2 (5a) as well as the cluster polymer formulated as {[Cu4{μ-(α-d-Glc-1P)}2(μ-m-C6H4(CH2COO)2)(bpy)4](NO3)2}n (5b). The tetracopper structure of 1a is converted into a symmetrical rectangular core in complexes 3, 4, and 5b, where the hydroxo bridge is dissociated and, instead, two carboxylate anions bridge another pair of CuII ions in a 1,1-O monodentate fashion. The similar reactions were applied to incorporate sugar acids onto the tetranuclear copper(II) centers. Reactions of 1a with δ-d-gluconolactone, d-glucuronic acid, or d-glucaric acid in dimethylformamide resulted in the formation of discrete tetracopper complexes with sugar acids, [Cu4{μ-(α-d-Glc-1P)}2(μ-SA)2(bpy)4](NO3)2 [SA = d-gluconate (6), d-glucuronate (7), d-glucarateH (8a)]. The structures of 6 and 7 were determined by X-ray crystallography to be almost identical with that of 3 with additional chelating coordination of the C-2 hydroxyl group of d-gluconate moieties (6) or the C-5 cyclic O atom of d-glucuronate units (7). Those with d-glucaric acid and d-lactobionic acid afforded chiral one-dimensional polymers, {[Cu4{μ-(α-d-Glc-1P)}2(μ-d-glucarate)(bpy)4](NO3)2}n (8b) and {[Cu4{μ-(α-d-Glc-1P)}2(μ-d-lactobionate)(bpy)4(H2O)2](NO3)3}n (9), respectively, in which the d-Glc-1P-bridged tetracopper(II) units are connected by sugar acid moieties through the C-1 and C-6 carboxylate O atoms in 8b and the C-1 carboxylate and C-6 alkoxy O atoms of the gluconate chain in 9. When complex 7 containing d-glucuronate moieties was heated in water, the mononuclear copper(II) complex with 2-dihydroxy malonate, [Cu(μ-O2CC(OH)2CO2)(bpy)] (10), and the dicopper(II) complex with oxalate, [Cu2(μ-C2O4)(bpy)2(H2O)2](NO3)2 (11), were obtained as a result of oxidative degradation of the carbohydrates through C−C bond cleavage reactions.