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-4-pyridyl-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 o-phthalate 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 ∼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 changes 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 ∼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-ammine bonding in the loaded phase.