A Novel Coordination Polymer, [Ag4^1 （bpdc）（H2bpdc）（Hbpdc）2]n （bpdc = 2,2＇-bipyridyl-3,3＇-dicarboxylate）： Coordination Models of bpdc Ligands

A novel coordination polymer, [Ag4(bpdc)(H2bpdc)(Hbpdc)2]n (bpdc=2,2'-bipyridyl-3,3'-dicarboxylate), was hydrothermally synthesized at 403 K and structurally characterized by single crystal X-ray diffraction analysis. The compound crystalizes in the monoclinic space group C2/c with α= 1.9516(4) nm, b= 1.9503(4) nm, c= 1.2566(3)nm, and β= 112.48(3)°. In the two-dimensional crystal structure, Ag^I center is coordinated, in a scarce coordination environment, double-capped tetrahedron, by one bpdc ligand to form N-Ag-N chelate bond via two pyridyl N atoms, and other two bpdc ligands to form two O-Ag-O chelate bonds, respectively, via two carboxyl O atoms. The bpdc ligands are present in one non-protonated form, bpdc, and two protonated forms, Hbpdc and H2bpdc, which all act as μ3-ligand in a hexadentate fashion (N, N'; O, O'; O, O') to coordinate with three Ag centers, respectively, through the three chelate bonds. This coordinated fashion of bpdc ligand is first found in the title compound. UV-Vis-NIR reflectance spectroscopy study revealed insulator nature for the crystal with an optical energy gap of 3.1 eV.