In aqueous solutions nickel(II) complexes of tetraazamacrocycles have been understood to exist in equilibrium as a mixture of orange square-planar and blue trans-diaqua octahedral species according to the following equation.The evidence for the equilibrium is the observation of lower molar extinction coefficients for the square-planar nickel(II) complexes in coordinating solvents than those in noncoordinating solvents. 1,6 Although the equilibrium is susceptible to temperature, addition of salts, ionic strength, etc., it is difficult to shift the above equilibrium completely to the octahedral side.1,2,7 Therefore, the isolated macrocyclic nickel(II) complexes are usually square-planar, and this is especially true when the perchlorate or hexafluorophosphate salts are added to the aqueous solutions of macrocyclic nickel(II) complexes in equilibrium. Later, the successful isolation of a trans-diaqua octahedral nickel(II) cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane) has been appeared with its solid state X-ray structure as well as single crystal visible spectrum by Mochizuki and Kondo. 4 Similarly, the equilibrium between a square-planar nickel(II) complex and a trans-diacetonitrile octahedral nickel (II) complex has been reported by spectrophotometric titration of the macrocyclic nickel(II) complex with acetonitrile in nitromethane solutions.8 Then, the diacetonitrile coordinated octa- ,14-dimethyl-2,6,13,17-tetraazatricyclo [14,4,0 1.18 ,0 7.12 ]docosane) has been isolated and structurally characterized.5 None the less, the examples of isolated and structurally characterized diaqua or diacetonitrile coordinated octahedral nickel(II) teraazamacrocyclic complexes are extremely rare.In this report we describe the synthesis and structure of a new macrocyclic nickel(II) complex [Ni(L1)(H2O)2][Ni(L1)bis (H2thftc)]·4H2O (1) (thftc = tetrahydrofuran-2r,3t,4t,5c-tetracarboxylate) in which each nickel(II) ion is under an octahedral coordination environment. Thus, the coordination environment of nickel(II) ion in the [Ni(L1)(H2O)2] 2+ dication is composed of a tetraazamacrocycle L1 and axially coordinated aqua ligands, and the nickel(II) ion in the dianion [Ni(L1)bis(H2thftc)] 2-is coordinated by the macrocycle L1 and two monodentate H2thftc ligands axially. The macrocyclic nickel(II) dication is connected to the macrocyclic nickel(II) dianion by the mediation of lattice water molecules through hydrogen bonds.
