Preparation and structure of complex [Zn(tren)(Him)](ClO₄)₂
Ren Yanwei, Li Jun, Du Guixiang Zhang Yongjin, Zhang Fengxing
(Shaanxi Key Laboratory of Physico-Inorganic Chemistry, Department of Chemistry, Northwest University, Xi'an 710069, China)

The coordination chemistry of zinc is receiving considerable attention in recent years, for zinc, which is, following iron, the second-most abundant transition metal in biology, and which is represented in metalloenzymes of all six classes of enzymes^[1]. In order to mimic the zinc coordination structure as well as the function of the zinc ion in biological system, many new interesting zinc complexes have been designed and structurally characterized to elucidate the detailed reaction mechanism in these zinc enzymes^[2-8]. In this paper, the synthesis of a new complex Zn(tren)(Him)](ClO₄)₂ and the characterization of its crystal structure were described.

1. EXPERIMENTAL
1.1 Materials and physical measurements
Zinc(II) perchlorate was prepared by ourselves^[9]. The other reagents were of analytical grade from commercial sources and were used without any further purification. The C, H and N elemental analyses were performed using a Vario EL-III instrument. Zinc content was determined by EDTA titrations using xylenol orange as the indicator. Infrared spectrum of the title complex was recorded with Equinox55 spectrophotometer in the 4000--400 cm^-1 region using a powdered sample on a KBr plate. The diffraction data were collected on a Brucker SMART APEX CCD area detector using graphite monochromated MoKa radiation (l=0.071073 nm).
1.2 Preparation of Zn(tren)(Him)](ClO₄)₂
To a ethanol solution (5mL) of Zn(ClO₄)₂·6H₂O (0.82g,2.2mmol) was added tren (0.34g, 2.2mmol). Shortly, much white precipitation formed in the mixed solution, which would be solved by adding 5mL of water, and 0.15g imidazole (2.2mmol) was added. The mixture was stirred for 1 h at room temperature, the solution was filtered and stored in refrigerator(5ºC) until the colorless crystals formed. The product was filtered off, washed with diethyl ether, and dried in air. Yield: 0.695g (66 %). Anal calcd (found) for C₉H₂₂Cl₂N₆O₈Zn: C, 22.76 (22.59); H, 4.56 (4.63); N, 17.44 (17.56), Zn, 13.65 (13.66). IR (KBr, cm^-1): 3433[n (NH₂)], 3134, 1498, 780[n (imidazole)].
1.3 X-ray crystallography
Diffraction data were collected by w-2q scan technique. The total reflections of 8540 were collected, of which independent reflections of 1679 could be observed and used in the structure analysis. The structure was solved by direct methods. The positions of all remaining non-H atoms were obtained from successive Fourier syntheses. The positions of all non-H atoms were refined anisotropically with full-matrix least-squares on F². In the final difference map, the residuals are 871 and -588 e/nm³, respectively.

Table 1 Crystal data and structure analysis parameters

2. RESULTS AND DISCUSSION
Figure 1 shows the ORTEP projection of the complex [Zn(tren)(Him)](ClO₄)₂. The crystallographic data and analysis parameters are showed in Table 1. Selected bond lengths and bond angles are listed in Table 2.

Table 2 Selected bond lengths (nm) and bond angles (^o)

Fig.1 ORTEP projection of [Zn(tren)(Him)](ClO₄)₂

Fig.2 Structure of [Zn(tren)(Him)](ClO₄)₂, showing the arrangement in the crystal lattice

    The structure of [Zn(tren)(Him)]²⁺ has an approximate C₃ symmetry. The zinc ion is five-coordinated with four nitrogen atoms from ligand tren and one nitrogen atom from imidazole, forming a slightly distorted trigonal-bipyramidal configuration. The tertiary amine nitrogen N(1) and the second amine nitrogen N(5) from imidazole are located at the apices, and the three primary amine nitrogen atoms (N(2), N(3), N(4)) lie in the equatorial positions. The very weak axial Zn-N(1) interaction with bond distance of 0.2248(7) nm is significantly longer than the equatorial Zn-N distances with an average of 0.2074 nm. Analogous values have been observed in the previous complexes [Zn(tren)(H₂O)]²⁺ and [Zn(tren)(NCS)]⁺ with the mean equatorial Zn-N distances of 0.2051(5) and 0.2058(5) nm, as well as 0.2193(4) and 0.2292(4) nm for the apical Zn-N distances, respectively^[8,10]. The Zn(1)-N(5) distance is 0.2058(6) nm. In the equatorial plane, the average N-Zn-N angle is 117.3(5)º, close to the ideal value of 120º.The angles N(1)-Zn-N(2), N(1)-Zn-N(3) and N(1)-Zn-N(4) are 83.2(6)º, 81.6(4)º and 77.0(6)º, respectively. The angle N(1)-Zn-N(5) of 175.7(5)º, indicating the distorted trigonal-bipyramidal configuration, is within the values previously observed for other polyamine zinc complexes^[11]. Figure 2 shows the cell packing of the complex Zn(tren)(Him)](ClO₄)₂. One of the oxygen atoms of perchlorate forms hydrogen bond with the primary amine of the tren [N(4)-H…O(1); 0.2960 nm, 119.64º; symmetry code: x, y-1, z].