Regresar

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

Abstract:

Two novel compounds, {[Cu(bpy) 2 ] 3 H 2 W 12 O 40 }·3H 2 O (1) and {{[Cu(OH 2 )(bpy)][Cu(bpy)] 2 W 6 O 21 }·4H 2 O} 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 μ 2 -O bridging , two μ 3 -O triple-bridging , and two μ 4 -O corner , 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 shortening of the separation of 1D polymers. EPR studies support the shortening in the Cu(II)···Cu(II) distances.