Tetrakis(μ3-2-{[1,1-bis(hydroxymethyl)-2-oxidoethyl]iminomethyl}-6-methoxyphenolato)tetranickel(II) tetrahydrate

The title complex, [Ni4(C12H15NO4)4]·4H2O, has crystallographic fourfold inversion symmetry, with each NiII ion coordinated in a slightly distorted square-pyramidal coordination environment and forming an Ni4O4 cubane-like core. In the crystal structure, intermolecular O—H⋯O hydrogen bonds connect complex and water molecules to form a three-dimensional network. The O atom of one of the unique hydroxymethyl groups is disordered over two sites, with the ratio of occupancies being approximately 0.79:0.21.

The title complex, [Ni 4 (C 12 H 15 NO 4 ) 4 ]Á4H 2 O, has crystallographic fourfold inversion symmetry, with each Ni II ion coordinated in a slightly distorted square-pyramidal coordination environment and forming an Ni 4 O 4 cubane-like core. In the crystal structure, intermolecular O-HÁ Á ÁO hydrogen bonds connect complex and water molecules to form a threedimensional network. The O atom of one of the unique hydroxymethyl groups is disordered over two sites, with the ratio of occupancies being approximately 0.79:0.21.
A few structurally characterized multinuclear complexes containing Schiff base ligands has been reported( e.g. ; Dong, Li, Xu, Cui & Wang (2007); Nihei et al., 2003). Herein, we report the synthesis and crystal structure of a novel tetranickel(II) complex with a tridentate Schiff base ligand derived from the condensation of ovanillin and trihydroxymethylaminomethane.
The title compound contains a tetranuclear cubane core based on an approximately cubic array of alternating nickel and oxygen atoms (Fig.1). Each Ni II ion is in a distorted square-pyramidal coordination environment with one nitrogen and two oxygen atoms from one Schiff base ligand and two oxygen atoms from the symmetry related units of the cubane core. The Ni atom deviates from the basal plane (formed by O1, N1, O3 and O3 i , symmetry code (i) y -7/4, -x + 3/4, -z + 7/4) by 0.1299 (33) Å, with a significantly longer Ni-O apical bond distance (Table 1). In the molecular structure, the Ni-Ni distances (3.472 (4) Å, 3.182 (3) Å) are longer than some reported values (Koikawa et al., 2005). In addition, there are four H 2 O solvent molecules, which are involved in intermolecular O-H···O hydrogen bonds (Fig. 2, Table 2) which stabilize the crystal atructure along with van der Waals forces.

S2. Experimental
Trihydroxymethylaminomethane(1 mmol, 121.14 mg) was dissolved in hot methanol (10 ml) and added successively to a methanol solution(3 ml) of o-vanillin (1 mmol, 152.15 mg). The mixture was then stirred at 323 K for 2 h. Subsequently, an aqueous solution(2 ml) of nickel chlorate hexahydrate(1 mmol, 237.66 mg) was added dropwise and stirred for another 5 h. The solution was held at room temperature for ten days, whereupon green blocky crystals suitable for X-ray diffraction were obtained.

S3. Refinement
Difference Fourier maps revealed that one of the hydroxymethyl group is distorted over two sites. The subsequent refinement of their occupancies gave the value of 0.791 (3) and 0.209 (3), respectively. All the H atoms were placed in geometrically calculated positions (C-H = 0.93 -0.97 Å, O-H = 0.82 Å) and allowed to ride on their respective parent atoms, with U iso (H) = 1.2U eq (C) or 1.5U eq (C methyl ).   Part of the crystal structure with hydrogen bonds shown as dashed lines. The disorder is not shown. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.23 e Å −3 Δρ min = −0.70 e Å −3

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (