Synthesis, structures, surface photovoltage and luminescence properties of two new nickel(II) carboxyphosphonates with a 3D framework structure

By introduction of 4,4'-bipyridine (4,4'-bipy) as a second organic ligand, two new nickel(II) carboxyphosphonates with a 3D framework structure, namely, [Ni-3(H2L)(2)(4,4'-bipy)(H2O)(4)] (1) and [Ni-3(H2L')(2)(4,4'-bipy)(2)(H2O)(2)]center dot 4H(2)O (2) (H5L = 4-{[bis(phosphonomethyl)amino]methyl}benzoic acid; H5L' = N,N-bis(phosphonomethyl)aminoacetic acid) have been synthesized under hydrothermal conditions. Compounds 1 and 2 both feature three-dimensional (3D) framework structures with twodimensional (2D) layers pillared by 4,4'-bipy. For compound 1, {Ni(1)O4N2}, {Ni(2)O-6} and {CPO3} polyhedra form a 2D inorganic layer in the bc-plane via corner-sharing. Then adjacent layers are further pillared by 4,4'-bipy ligands into a 3D pillar-layered structure. In compound 2, {Ni(1)O4N2}, {Ni(2)O4N2} and {CPO3} polyhedra are interconnected by carboxyphosphonate ligands to a 2D layer in the ab-plane. Neighboring layers are further cross-linked via 4,4'-bipy ligands, generating a 3D framework structure. Whereas the carboxylate oxygen atoms of carboxyphosphonate ligands are coordinated to Ni(II) atoms in compound 2 but not in compound 1. The surface photovoltage and luminescence properties of compounds 1 and 2 have been investigated at the same time.