Complexes derived from the copper(II) perchlorate/maleamic acid/2,2′-bipyridine and copper(II) perchlorate/maleic acid/2,2′-bipyridine reaction systems: Synthetic, reactivity, structural and spectroscopic studies

Abstract

A systematic investigation of the reactions of Cu(ClO 4) 2 · 6H 2O with maleamic acid (H 2L) in the presence of 2,2′-bipyridine (bpy) has been carried out. The chemical and structural identity of the products depends on the solvent, the absence or presence of external hydroxides in the reaction mixture and the molar ratio of the reactants. Various reaction schemes have led to the isolation of the complexes [Cu 2(HL) 2(bpy) 2(H 2O) 2](ClO 4) 2 ( 1), [Cu 2(HL) 2(bpy) 2(H 2O) 2](ClO 4) 2 · 2H 2O ( 1 · 2H 2O), [Cu(L′′)(bpy)] n · 2 nH 2O ( 2 · 2 nH 2O), [Cu 2(L′′)(bpy) 2(H 2O) 2] n (ClO 4) 2 n · 0.5 nH 2O ( 3 · 0.5 nH 2O), [Cu 2(L′′) 2(bpy) 2] · 2MeOH ( 5 · 2MeOH), [Cu 2(L′) 2(bpy) 2(ClO 4) 2] ( 6) and [Cu(ClO 4) 2(bpy)(MeCN) 2] ( 7b), where L′′ 2− and L′ − are the maleate(−2) and monomethyl maleate(−1) ligands, respectively. The HL − ion has been transformed to L′′ 2− and L′ − in the known compounds 2 · 2 nH 2O and 6, respectively, via metal ion-assisted processes involving hydrolysis ( 2 · 2 nH 2O) and methanolysis ( 6) of the primary amide group. The reaction that leads to 6 takes place through the formation of the mononuclear complex [Cu(ClO 4) 2(bpy)(MeOH) 2] ( 7a), whose structure was assigned on the basis of its spectral similarity with the structurally characterized complex 7b. The structures of the cations in 1 and 1 · 2H 2O consists of two Cu II atoms bridged by the carboxylate groups of the two HL − ligands, each exhibiting the less common η 2 coordination mode; a chelating bpy molecule and a H 2O ligand complete square pyramidal coordination at each metal centre. The structure of the dinuclear repeating unit in the 1D coordination polymer 3 · 0.5 nH 2O consists of two Cu II atoms bridged by two syn,syn η 1: η 1: μ 2 carboxylate groups belonging to two L′′ 2− ions; each ligand bridged two neighboring [Cu II,II 2] units thus promoting the formation of a helical chain. The structure of the dinuclear molecule of complex 5 · 2MeOH consists of two Cu II atoms bridged by two η 2 carboxylate groups from two L′′ 2− ligands; the second carboxylate group of each maleate(−2) ligand is monodentately coordinated to Cu II, creating a remarkable seven-membered chelating ring. The L′ − ion behaves as a carboxylate-type ligand in 6, with the carboxylate group being in the familiar syn,syn η 1: η 1: μ 2 coordination mode; a chelating bpy molecule and a coordinated ClO 4 − complete five-coordination at each Cu II centre. The crystal structures of the complexes are stabilized by various H-bonding patterns. Characteristic IR bands of the complexes are discussed in terms of the known structures and the coordination modes of the ligands.