Discrete and Polymeric Cu(II) Coordination Complexes with a Flexible bis-(pyridylpyrazole) Ligand: Structural Diversity and Unexpected Solvothermal Reactivity

Abstract

Reported here is the synthesis and structural characterisation of five copper complexes derived from the bis-bidentate ligand 4,4′-methylenebis(1-(2-pyridyl)-3,5-dimethylpyrazole), L. Complex 1, [Cu2L(CH3COO)4(OH2)2]·6H2O, is a single stranded unsaturated helical species that forms a highly connected three-dimensional hydrogen-bonding network, whereas [Cu2L(NO3)4], 2, is a coordination polymer derived from [Cu2L] fragments linked together via bridging nitrate anions to yield undulating two-dimensional sheets with (6,3)-topology. Complexes 3, 4, and 5 co-crystallise within a single batch when L is reacted under solvothermal conditions with Cu(NO3)2·2.5H2O in acetonitrile, and each contains a co-ligand formed by either decomposition of the solvent or ligand. Complex 3, [Cu4(NO3)4(µ-CH3COO)2(µ-OH)2L2], forms an unusual discrete cyclic tetrameric species containing acetate co-ligands derived through acetonitrile hydrolysis; whereas complex 4, [CuL(C2O4)(NO3)], forms a one-dimensional coordination polymer containing bridging oxalate co-ligands, formed through hydrolysis and oxidation of acetonitrile. Complex 5, [Cu2L(µ-CN)2], is a two-dimensional coordination polymer with (6,3) topology where bridging between Cu(i) centres is furnished by cyanide co-ligands, suggesting a ligand decomposition pathway for its origin, and produced with concomitant reduction of the Cu(ii) starting reagent. Having initially obtained 3, 4, and 5 serendipitously each were then prepared as pure phases by careful adjustment and control of the reaction conditions (reactant stoichiometry, concentrations, and solvothermal temperature), details of which are discussed.