KOOPMHH^HOHHAS XHMH3, 2011, moM 37, № 3, c. 169-172
y%K 541.49
MONONUCLEAR ZINC(II) COMPLEX WITH 2,3,5,6-TETRA(2-PYRIDYL)PYRAZINE
© 2011 A. Sh. Saljooghi1, * and S. J. A. Fatemi2
department of Chemistry, Ferdowsi University of Mashhad, Mashhad 91735-654, Iran 2Department of Chemistry, Shahid Bahonar University of Kerman, Kerman 76169, Iran *E-mail: amir.saljooghi@yahoo.com Received June 7, 2010
Complex [Zn(Tppz)Cl2] (I), where Tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine, has been synthesized and characterized by elemental analyses, IR, 1H NMR, cyclic voltammetry, and electronic spectral studies. Solid state structures of the complex have been determined by single-crystal X-ray crystallography. The structural determination shows that the mononuclear complex I is a 1D coordination polymer. Also an ORTEP drawing of complex I shows that the coordination geometry around the Zn(II) center is slightly distorted from regular square-based pyramidal. Crystal data for I: triclinic, spase group P1, a = 10.171(2), b = 10.3550(13), c = 12.239 (2) A, a = 64.839(9)°, P = 85.736(8)°, y = 77.842(10)°, V = 1140.3(4) A3, Z = 2.
INTRODUCTION
Tridentate ligands offer the possibility of controlling the stereochemistry of octahedral metal complexes. When a symmetric tridentate ligand is coordinated to a metal ion, there is only one possible stereoisomer. Ifsym-metric tridentate bridging ligands are used in the construction ofpolymetallic systems, there is only one possible isomer for the whole polymetallic system. Furthermore, the whole complex is a linear rod-like system. It fixes the metal—metal distance, which is essential in understanding of the energy and electron transfer. Triden-tate bridged systems, therefore, offer many advantages over bidentate bridged systems, where the metal—metal distance and photophysical properties can vary depending on the coordination of the ligands. Although triden-tate ligands offer advantages in transition metal complexes, there has been less work done with tridentate ligands than that with bidentate ligands [1]. Many different types of polyazine tridentate bridging ligands have been investigated [2-4].
The ligand 2,3,5,6-tetra-2-pyridyl-pyrazine (Tppz) was first been reported by Goodwin & Lyons [5]. At least eight kinds of coordination modes have been characterized. These compounds involve mono-, di-, and trinu-clear metal complexes in various coordination modes [6-8]. We describe herein the synthesis, structural characterization, cyclic voltammetry and electronic spectral of the five-coordinate mononuclear zinc complex [Zn(Tppz)Cl2] (I) in a glass test tube. Until now, no crystal structures of mono-tridentate Zn(II) complexes of Tppz have been documented. The Tppz bridged bimetallic complex [{ZnCl}2(^-Tppz)] • H2O have also been previously studied [9]. The first Tppz bridged bimetallic complex ^-2,3,5,6-tetra(2-pyridyl)pyrazine-bis[dichlo-
romercury(II)], [Hg2Cl4(C24H16N6)], that has been prepared in a glass test tube have been studied [10].
EXPERIMENTAL
Materials and general methods. All the synthetic manipulations were performed under oxygen-free nitrogen atmosphere. The solvents were dried and distilled before use following standard procedures. Zinc chloride and Tppz were procured from Aldrich and used as received. FT-IR spectrum was recorded as KBr pellets on a Bruker FT-IR spectrometer, and electronic spectra were recorded on a PerkinElmer 550S UV/VIS spectrophotometer. 1H NMR spectra were recorded on a Bruker DRX-500 MHz Avance spectrometer in DMSO-d6. Cyclic voltammograms were recorded by using a SAMA M-500 Research Analyzer. Three electrodes were utilized in this system, a glassy carbon disk working electrode (diameter 5 mm), a platinum wire auxiliary electrode, and an Ag/AgCl reference electrode. The platinum disk working electrode was manually cleaned with 1-^m diamond polish prior to each scan. The supporting electrolyte, tetrabutylammonium hexafluorophosphate (TBAH), was recrystallized twice from ethanol-water (1: 1) and vacuum-dried at 110°C overnight. DMSO was distilled over CaH2 and degassed under vacuum prior to use in cyclic voltammetery. The solutions were deoxygen-ated by bubbling with Ar for 15 min. The procedure performed at room temperature, and Ar atmosphere was maintained over the solution during measurements. Ferrocene (Eo = 0.665 V versus NHE, AEp = 60 mV) was used as an internal reference [11]. The range ofpotentials studied was between + 1 and -2.2 V
Synthesis of complex I. In a test tube, an acetone (4 ml) solution of ZnCl2 (27 mg, 0.2 mmol) was carefully
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SALJOOGHI, FATEMI
Table 1. Crystallographic data and structure refinement summary for [Zn(Tppz)Cl2] (I)
Parameter Value
Formula weight 524.70
Crystal system Triclinic
Space group p1
Unit cell dimensions:
a, A 10.171 (2)
b, A 10.3550 (13)
c, A 12.239 (2)
a, deg 64.839 (9)
ß, deg 85.736 (8)
Y, deg 77.842 (10)
V A3 1140.3 (4)
Z 2
Pcalcd mg cm-3 1.528
Absorption coefficient, mm-1 1.336
Crystal size, nm 0.30 x 0.12 x 0.08
Type of scan Multiscan
Qma® deg 25.03
Index ranges -12 < h < 12, -12 < k < 12, -14 < l < 14
T /T Amin/ Amax 0.6899/0.9064
Measured reflections 28377
Independent reflections (Äint) 3468 (Riat = 0.0388)
Reflections with I > 2o(I) 2752
Refined parameters 298
Refinement on F2 (F2 > 2a(F2)) R1 = 0.0687, wR2 = 0.176
Largest diff. peak and hole, e A-3 0.608, -0.647
layered on the top ofa chloroform (4 ml) solution ofTppz (39 mg, 0.1 mmol) using ethylene glycol (2 ml) as an in-terlayer. After three days at room temperature, colorless block-shaped single crystals suitable for X-ray investigation appeared at the boundary in a yield of 85%.
For C24Hi6N6Cl2Zn
anal. calcd., %: Found, %:
C, 55.11; C, 55.45;
H, 3.06; H, 3.11;
N, 16.06. N, 16.14.
Table 2. Selected bond lengths (Â) and angles (deg) for structure I
Bond d, A
Zn(1)-N(29) 2.144(4)
Zn(1)-N(1) 2.172(4)
Zn(1)-N(28) 2.203(4)
Zn(1)-Cl(11) 2.2455(13)
Zn(1)-Cl(12) 2.2592(15)
N(1)-C(2) 1.338(6)
N(1)-C(6) 1.341(6)
C(2)-C(3) 1.386(7)
C(2)-H(2) 0.9500
Angle ro, deg
N(29)Zn(1)N(1) 73.95(14)
N(29)Zn(1)N(28) 73.11(15)
N(1)Zn(1)N(28) 147.06(16)
N(29)Zn(1)Cl(11) 125.70(10)
N(1)Zn(1)Cl(11) 102.69(10)
N(28)Zn(1)Cl(11) 96.60(10)
N(29)Zn(1)Cl(12) 117.92(10)
N(1)Zn(1)Cl(12) 97.00(11)
N(28)Zn(1)Cl(12) 98.04(11)
Cl(11)Zn(1)Cl(12) 116.29(6)
C(2)N(1)C(6) 118.3(4)
XH NMR (DMSO-d6; 500 MHz; 298 K; 8, ppm): 8.35 (d., 4H), 8.11 (d., 4H), 7.72 (d.d., 4H), 7.34 (d.d., 4H).
X-ray crystal determination. Intensity data were collected on a Bruker AXS diffractometer using graphite-monochromated Mo^a radiation (X = 0.71073 Â) at 150(2) K. The colorless crystals of I were grown at the boundary of a Zn(II) solution in acetone and a Tppz solution in CHCl3 in the presence of ethylene glycol as an interlayer solvent. The structures were solved by direct
and Fourier methods and refined by full-matrix least-squares methods based on F2 using the SHELXS-97 and SHELXL-97 programs [12]. All non-hydrogen atoms were readily located and refined using anisotropic thermal parameters. All hydrogen atom positions were refined in the isotropic approximation in a riding model with the ^(H) parameters equal to 1.5 Ueq(C) for methyl groups and 1.2 Ueq(C) for other carbon atoms, where U(C) are the equivalent thermal parameters of the atoms to which the corresponding H atoms are bonded. The deepest electron density hole is located at 0.90 A from atom Zn. Crystallographic parameters and summary ofda-ta collection and refinement for complexes I and II are given in Table 1. The selected bond lengths and angles of the complex I is listed in Table 2.
Supplementary material has been deposited with the Cambridge Crystallographic Data Centre (no. 757088; deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
RESULTS AND DISCUSSION
There are three general classes of structures that arise from the tridentate-coordination mode of Tppz ligand, including the [M(Tppz)], [M(Tppz)2], and [M2(^-Tppz)] structures. The simplest ofthese are the [M(Tppz)] structures that involve coordination of one mono-tridentate
MONONUCLEAR ZINC(II) COMPLEX
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Fig. 1. Structure of one of the independent complex molecules I.
Tppz to a single metal, leaving the other side free. Complex I is neutral mononuclear (Fig. 1), in which the Tppz functions in a mono-tridentate ligand to link one Zn ion. Each Zn2+ center is five-coordinated by tree N donors of Tppz in one plane and two Cl atoms normal to this plane; the ClZnCl angle is 116.29(6)°. The molecular structures of five-coordinate transition metal complexes show an extensive range from regular trigonal bipyramidal (RTBP) to regular square-based pyramidal (RSBP). The structure index parameter, t = (P — a/60), was evaluated by the two large angles (a < P) in the five-coordinated geometry, where t = 1.0 for a RTBP geometry and t = 0.0 for a RSBP [13]. The coordination geometry around Zn(II) is slightly distorted from regular square-based pyramidal, as ascertained by the observed t value of 0.35. Therefore, the molecular structure of I shows distorted square-based pyramidal geometry around the zinc center and is a mononuclear complex with the ligand coordinated in a tridentate manner. The Zn-N(Pz) (where Pz = = pyrazine) distances Zn(1)-N(29) is 2.144(4) A. The Zn-N(Py) (where Py = pyridine) distances Zn(1)—N(1) and Zn(1)-N(28) are 2.172(4) and 2.203(4) A, respectively. This conformation probably minimizes steric strain while at the same time maximizing the bonding interaction of the Tppz ligand with the zinc ion. The shortness of the Zn-N(Pz) bonds compared with the Zn-N(Py) bonds is suggested to be due to the stronger n-accepting properties of the pyrazine ring. The Zn(II)—Cl distances of2.2455(13) and 2.2592(15) A are well compared with the reported Zn(II)—Cl distances. The unit cell drawing in Fig. 2 shows that complex I interacts with neighboring complexes through the bridging chloride ligand to form a distorted squa
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