Reversible and Vapochromic Chemisorption of Ammonia by a Copper(II) Coordination Polymer

The single crystal X-ray structure determination of {[Cu(tpt)(o-phthalate)]·31/3(C2H2Cl4)}n (tpt = 2,4,6-tri-4pyridyl-1,3,5-triazine, C2H2Cl4 = 1,1,2,2-tetrachloroethane = TCE) shows a 3D network in which Cu(II) centres are linked by 3-connecting tpt ligands with the topology of a 12,3 net. Cu(II) centres are further linked by ophthalate dianions.

The copper coordination geometry is square pyramidal, with o-phthalate oxygen donors trans to each other in the basal plane and the remaining positions taken by the pyridines of three linking tpt units. The solvent accessible void space is approximately 65 %. The pale blue-green crystalline desolvate, obtained by heating to 200 °C or washing the TCE solvate with acetone is formulated as [Cu(tpt)(o-phthalate)]n.

Powder X-ray diffraction and electron paramagnetic resonance spectroscopy show that the crystal structure and the Cu(II) geometry change upon desolvation. The crystalline desolvated phase sorbs two equivalents of ammonia per copper ion. The adduct, mauve [Cu(tpt)(o-phthalate)(NH3)2]n, shows reasonable crystallinity and is stable up to ca. 150 °C under ambient conditions before the reversible desorption (minimum 10 cycles) of the guest ammonia.

The colour change and high desorption temperature, along with changes in g-values, is suggestive of chemisorption in two steps with Cu-amine bonding in the loaded phase.