Stabilization of a helical water chain in a metal-organic host of a trinuclear Schiff base complex

Three heterometallic trinuclear Schiff base complexes, [{CuL1(H2O)}2Ni(CN)4].4H2O (1), [{CuL2(H2O)}2Ni(CN)4] (2), and [{CuL3(H2O)}2Ni(CN)4] (3) (HL1=7-amino-4-methyl-5-azahept-3-en-2-one, HL2=7-methylamino-4-methyl-5-azahept-3-en-2-one, and HL3=7-dimethylamino-4-methyl-5-azahept-3-en-2-one), were synthesized. All three complexes were characterized by elemental analysis, IR and UV spectroscopies, and thermal analysis. Two of them (1 and 3) were also characterized by single crystal X-ray crystallography. Complex 1 forms a hydrogen-bonded one-dimensional metal-organic framework that stabilizes a helical water chain into its cavity, but when any of the amine hydrogen atoms of the Schiff base are replaced by methyl groups, as in L2 and L3, the water chain vanishes, showing explicitly the importance of the host-guest H-bonding interactions for the stabilization of a water cluster.