Synthesis and Crystal Structure of Hexakis(imidazole) nickel (II) O,O′-diphenyldithiophosphate [Ni(Im)6](Ph2O2PS2)2

The crystal and molecular structures of [Ni(Im)6](dtp)2 (Im = imidazole, dtp = O,O′-diphenyldithiophosphate) have been determined by X-ray crystallography. It crystallizes in the triclinic system, space group Pī, with cell parameters a = 9.375 (2), b = 12.324(3), c = 13.285(3) Å, α = 107.86(3), β = 102.28(3), γ = 109.24(3), and Z = 1. The crystal structure of the title compound is built up of discrete monomeric molecules of [Ni(Im)6](dtp)2. The nickel (II) ion is hexacoordinated by six imidazole molecules and the coordination environment of Ni (II) is of octahedral geometry. In the solid state, a network of N-H∙∙∙S intermolecular hydrogen bonds connect the Ni(Im)6 moieties and O,O′-diphenyl- dithiophosphate molecules, forming a three-dimensional structure.


Introduction
Imidazole is of considerable interest as a ligand in that its presence in many biological systems provides a potential binding site for metal ions [1,2]. Imidazole itself is usually a unidentate ligand and forms complexes with metal ions through its tertiary nitrogen atom. Some complexes of imidazole and its derivatives with transition-metal ions have been reported [3][4][5].
Complexes of nickel (II) and imidazole ligand have been studied as models for copper proteins that contain both functionalities in the side chain [6]. Some of these nickel (II) complexes were found to exhibit a variety of pharmacological activity and superoxide dismutase activities [7]. The adducts and their formation reactions have also been found useful in a variety of ways, such as in biological applications and as both ultraviolet absorbers and antioxidants. Recently, the structure of hexakis(imidazole) nickel (II) disalicylate, [Ni(Im) 6 ](sal) 2 , has been reported [8]. In this paper, we describe the synthesis, and crystal structure of the title compound, hexakis(imidazole) nickel (II) O,O′diphenyldithiophosphate, [Ni(Im) 6 ](Ph 2 O 2 PS 2 ) 2 .

X-ray Crystal Structure of the Title Compound
The X-ray structure of the complex [Ni(Im) 6 ](dtp) 2 is built up of discrete monomeric molecules. Figure 1 shows a perspective view of the title compound with atomic numbering scheme and Figure 2 a perspective view of the crystal packing in the unit cell. Table 1 contains atomic positions and equivalent temperature factors for non-hydrogen atoms. Selected bond lengths and angles are presented in Table 2.  (Table 3), the whole complexes forming a 3dimensional network structure.

IR Spectroscopy
The IR spectrum of the complex shows characteristic absorption bands at 3401 and 552 cm -1 , assigned to ν(N-H) and ν(Ni-N) vibrations, respectively. This indicates that the atom coordinated with Ni(II) is the double bond nitrogen (=N) in the imidazole molecule. This result is in agreement with that of X-ray crystal structure determination.   All commercially available chemicals were of analytical reagent grade and used directly without further purification. The C, N and H contents were determined by using an Elementar Vario EL analyzer. Infrared spectra were recorded from KBr pellets on a Midac Prospet IR spectrometer. All operations were carried out under ambient conditions.

Synthesis of the complex [Ni(Im) 6 ](dtp) 2
To a warm solution of excess imidazole (1.0 g, 15 mmol) in EtOH (50 mL) was added with stirring nickel (II) O, O′-diphenyl dithiophosphate, and the mixture was refluxed for 40 min. The blue solution was filtered and the filtrate was left to stand undisturbed. Upon slow evaporation at room temperature, a blue crystalline solid suitable for X-ray structure determination appeared several weeks later and was separated by filtration. Crystallographic Data and Structure Determination [9] The selected crystal of [Ni(Im) 6 ](dtp) 2 was mounted on a Nonius CAD4 diffractometer, reflection data for the unit cell determination were measured at 293 K using MoKα radiation (λ= 0.71073 Å) with a graphite monochromator. The technique used was ω-scan with 0.995<θ<25.04 º for the complex. Absorption correction was made with π-scans. The structure was solved by direct method and refined by full-matrix least-squares method on F 2 obs by using the SHELXTL software package [10]. All non-H atoms were anisotropically refined. The hydrogen atoms were geometrically fixed and allowed to ride on the parent atoms to which they are attached. The final conventional R = 0.0604, wR = 0.1388 for 3843 reflections with I > 2σ(I); w = 1/[σ 2 (F 2 o )+(0.0600P) 2 +1.4652P] where P = (F 2 o +2(F 2 c ))/3. The molecular graphics were created by SHELXTL. Atomic scattering factors and anomalous dispersion correction were taken from International Table for X-Ray Crystallography [11]. A summary of the key crystallographic information is given in Table 4.