The role played by water in spin state variations among nickel(II) halide complexes containing (7R,14S)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane.

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A complex dihydrate formed with nickel(II) bromide and the title macrocyclic ligand (memo-Me6[14]aneN4) crystallizes in triclinic space group P\bar1 and in orthorhombic space group Pcab. The crystal and molecular structures of the two crystal modifications have been determined by single crystal X-ray diffraction. In each compound, the nickel(II) is surrounded by a square-planar array of nitrogen atoms, giving low-spin complexes. In both compounds, water of crystallization and bromide ions are located above the hydrogen atoms of N–H groups and form hydrogen-bonded chelate rings of the type Ni–N–H···Br−···H–O···H–N. The N–H groups adjacent to the single methyl carbon and the geminal dimethyl carbon interact with bromide ion and water, respectively, in the P\bar1 form, whereas vice versa in the Pcab form. Water molecules in the present crystals and those in the previously reported crystal structure of [Ni(meso-Me6[14]aneN4)]Cl2·2H2O are involved in the same type of specific hydrogen-bonds. Because of the specific hydrogen-bonding effects, the presence of water prevents coordination of not only water itself but also halide ions, yielding thereby low-spin four coordinate complexes.
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