Metal oxo cluster-based coordination polymers with rigid 1,4-naphthalenedicarboxylate and semirigid 1,3-di(4-pyridyl)propane ligands: syntheses, structural topologies, and luminescent properties

Hydrothermal reactions of Zn(NO3)2 or Pb(NO3)2 with 1,4-naphthalenedicarboxylic acid (1,4-H2ndc) and 1,3-di(4-pyridyl)propane (dpp) lead to two novel metal oxo cluster-based coordination polymers, namely, [Zn5(μ3-OH)2(1,4-ndc)4(dpp)2]n (1) and [Pb5(μ4-O)(1,4-ndc)4(dpp)]n (2). The structures exhibit different cluster motifs depending on the nature of the metal ions. Compound 1 features a 3D structure based on the pentanuclear [Zn5(μ3-OH)2] secondary building unit (SBU) where the five zinc atoms are coplanar; its topological structure is an uninodal 10-connected network of (36.434.53.62) topology. The zinc ions in 1 have a coordination number varying from 4 to 6 and are accompanied by a variety of coordination geometries such as tetrahedral, trigonal bipyramidal, and octahedral. Compound 2 has a 3D architecture containing an unusual hexanuclear [Pb6(μ4-O)2] cluster unit composed of two edge-sharing oxygen-centred [Pb4(μ4-O)] tetrahedrons. One explanation for the presence of the centred bridging oxide ligands in 2 is that owing to steric constraints sufficient carboxylate ligands cannot coordinate to the Pb(II) centers to satisfy the positive charges of the Pb(II) cations. The Pb(II) ions in 2 exhibit diverse hemidirected coordination geometries with coordination numbers of 6 and 7. In the solid state, compounds 1 and 2 exhibit photoluminescence with emission maximum at 495 and 420 nm, respectively.