Role of Water and Solvent in the Formation of Three Mononuclear Copper(II) Crystals: A New Type of Hydrate Isomerism in Coordination Chemistry

Refluxing of the red compound [(CuL1)-L-II subset of(H2O)] (1) in acetone forms two new species: green [(CuL1)-L-II(H2O)] (2) and brown [(CuL1)-L-II] (3) [H2L1 = N,N'-ethylenebis(3-ethoxysalicylaidimine)]. Whereas both 1 and 2 are converted into 3 upon heating, 3 is converted back into 1 when exposed to air. The structures of all three compounds were determined crystallographically, from which the interesting hydrate isomerism of 1 and 2 are identified; in 1 a water molecule is encapsulated into the O-4 section of the ligand through bifurcated hydrogen bonding, whereas in 2 a water molecule coordinates directly to the metal centre. Despite the metal ion binding to the ligand in the same manner in all three cases, the formation of different mononuclear complexes resulting from the presence or absence of water and its coordination or encapsulation is an interesting observation. Thermogravimetric analyses of compounds 1 and 2 showed that the encapsulated water in the former compound is removed at slightly lower temperature than that of the coordinated water molecule, as expected. Although three crystals 1, 2 and 3 were isolated, the composition of species in solution is probably the same as evidenced by H-1 NMR spectroscopy. Therefore, the formation of three crystals and the hydrate isomerism described in this investigation are solid-state phenomena. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)