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

A systematic investigation of the reactions of Cu(ClO4)2 · 6H2O with maleamic acid (H2L) 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 [Cu2(HL)2(bpy)2(H2O)2](ClO4)2 (1), [Cu2(HL)2(bpy)2(H2O)2](ClO4)2 · 2H2O (1 · 2H2O), [Cu(L′′)(bpy)]n · 2nH2O (2 · 2nH2O), [Cu2(L′′)(bpy)2(H2O)2]n(ClO4)2n · 0.5nH2O (3 · 0.5nH2O), [Cu2(L′′)2(bpy)2] · 2MeOH (5 · 2MeOH), [Cu2(L′)2(bpy)2(ClO4)2] (6) and [Cu(ClO4)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 · 2nH2O and 6, respectively, via metal ion-assisted processes involving hydrolysis (2 · 2nH2O) and methanolysis (6) of the primary amide group. The reaction that leads to 6 takes place through the formation of the mononuclear complex [Cu(ClO4)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 · 2H2O consists of two CuII atoms bridged by the carboxylate groups of the two HL− ligands, each exhibiting the less common η2 coordination mode; a chelating bpy molecule and a H2O ligand complete square pyramidal coordination at each metal centre. The structure of the dinuclear repeating unit in the 1D coordination polymer 3 · 0.5nH2O consists of two CuII atoms bridged by two syn,synη1:η1:μ2 carboxylate groups belonging to two L′′2− ions; each ligand bridged two neighboring [CuII,II2] units thus promoting the formation of a helical chain. The structure of the dinuclear molecule of complex 5 · 2MeOH consists of two CuII atoms bridged by two η2 carboxylate groups from two L′′2− ligands; the second carboxylate group of each maleate(−2) ligand is monodentately coordinated to CuII, 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,synη1:η1:μ2 coordination mode; a chelating bpy molecule and a coordinated ClO4− complete five-coordination at each CuII 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.

Graphical abstractThe reactions of Cu(ClO4)2 · 6H2O with the maleamate(−1) ligand, in the presence of 2,2′-bipyridine give dinuclear and polymeric complexes; in some reactions, interesting CuII-assisted/promoted transformations of the maleamate(−1) ion to the maleate(−2) and monomethyl maleate(−1) ligands are observed.Figure optionsDownload full-size imageDownload as PowerPoint slide