Coordination Polymers of CuII with the Ligand Pyrazine-2,3,5,6-tetracarboxylic Acid

Abstract

The ligand pyrazine-2,3,5,6-tetracarboxylic acid in the presence of CuCl₂ and the buffers CH₃CO₂X/CH₃CO₂H, X = K⁺, Rb⁺, Cs⁺ and (CH₃CO₂)₂Mg/CH₃CO₂H, forms two quite different types of coordination polymers. With the monovalent K⁺, Rb⁺, or Cs⁺ buffer an almost right-angled dimeric unit is formed which polymerizes to form a zig-zag polymer {Cs₄[Cu₂{pztc)₂2H₂O]⋅9H₂O}_∞ (1). This 'dimerizes' about a center of symmetry to form a two-dimensional polymer sheet. With the divalent Mg²⁺ buffer a mononuclear unit polymerizes to form a quasi-linear polymer {Mg(H₂O)₆[Cu(pztc)2H₂O]⋅2H₂O}_∞ (2). The X-ray crystal structures of 1 and 2 indicate that the Cu atom exists in two quite different coordination environments (NO₃, square pyramidal for 1 and N₂O₂, square planar for 2) and that the Cu-N(pyrazine) bond distances are much longer than normal.