Reversible Phase Transitions in a Coordination 1D-Polymer Containing an Unusual Hexatungstate Building Block

Abstract

Two novel compounds, {[Cu(bpy)2]3H2W12O40}·3H2O (1) and {{[Cu(OH2)(bpy)][Cu(bpy)]2W6O21}·4H2O}n (2), were isolated under hydrothermal conditions. Compound 1 is an α-metatungstate Keggin unit decorated by three “{Cu(bpy)2}2+” fragments. Compound 2 is a thermodynamically stable 1D polymer built-up from an unusual hexatungstate moiety decorated by three Cu complexes. Compound 2 can be synthesized either from the starting reagents or via thermal transformation from crystals of 1 (180 °C, 7 d). The hexatungstate in 2 represents the first example containing a W–O bonding pattern with 13 terminal oxygen ligands, four μ2-Obridging, two μ3-Otriple-bridging, and two μ4-Ocorner, symbolizing the fourth W-hexamer geometry characterized so far. Compound 2 provides a rare example that withstands a water desorption–readsorption process (80–130 °C) and a subsequent reversible phase transition (190–230 °C). Both events appear accompanied by a unit-cell contraction, which occurs in the b and c directions involving the short...