synthesis and structural characterization of a new 2d coordination polymer [cu(pzta)2]n

Chinese Journal of Chemistry, 2009, 27, 479—482 Full Paper

* E-mail: [email protected]; Tel.: 0086-0991-4332683 Received July 25, 2008; revised October 25, 2008; accepted December 22, 2008. Project supported by the Scientific Research Foundation in Xinjiang Educational Institutions (No. XJEDU2007S24) and the Young Scholar Science

Foundation of Xinjiang Normal University.

© 2009 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Synthesis and Structural Characterization of a New 2D Coordination Polymer [Cu(pzta)2]n

HU, Xin*,a(胡鑫) LIU, Conga(刘丛) WANG, Yongjianga(王永疆) GUO, Jixib(郭继玺)

a College of Life Science & Chemistry, Xinjiang Normal University, Urumqi, Xinjiang 830054, China b Institute of Applied Chemistry, Xinjiang University, Urumqi, Xinjiang 830046, China

A new copper(II) tetrazolate coordination polymer [Cu(pzta)2]n (1) (pzta＝5-pyrazinyltetrazolate) was prepared from the hydrothermal reaction of Cu(OAc)2•H2O with NaN3 and pyrazinecarbonitrile (pzCN) in the presence of ethanol, and characterized by elemental analysis, IR, TGA and X-ray crystallography. The X-ray diffraction analysis of 1 shows that the compound crystallizes in the monoclinic system with space group P2(1)/c and Cu(II) ion center is six-coordinated by four different pzta ligands. The complex 1 features a 2D tetrazole coordination polymer. The thermal analysis of 1 shows that the decomposition of the complex occurs in two regions.

Keywords coordination polymer, crystal structure, pyrazinecarbonitrile, hydrothermal reaction

Introduction

Metal-organic coordination polymers have recently attracted intense attention from chemists, due to their intriguing supramolecular compositions and versatile framework topologies as well as their potential applica-tions as functional materials to molecular magnetism, catalysis, gas sorption, electrical conductivity, optics, etc.1-5 Unlike metal oxides and other traditional inor-ganic materials, metal-organic coordination networks are typically synthesized by combining metal nodes of suitable geometries and desired bridging ligands under very mild conditions.6 A basic strategy to design such metal-organic frameworks is to assemble well-defined and rigid linkers into predetermined motifs by decora-tion and extension of the specific net configuration.7 Due to the diversity of the connecting modes and the high structural stability, the polydentate N donor ligands have been employed extensively as chelating or bridg-ing linkers.8 The tetrazolate-based ligands have been shown to be excellent and versatile building blocks, forming a great variety of supramolecular entities pos-sessing an interesting chemical rearrangement reaction,9 magnetic10 and optical properties.11-13

Since the pioneering work done by Sharpless et al.,14 there have been tremendous research interests focused on the exploration of in situ tetrazole organic ligand synthesis through 2＋3 cycloaddition reactions between organic cyano compounds and inorganic sodium azide (NaN3) in the presence of d10 salts as Lewis acid catalyst in water.15 However, the tetrazolate bridged copper polymers in previous reports were synthesized by the

reaction of Cu(II) salt with azide and nitrile ligands to afford mix-valence copper or Cu(I) tetrazolate com-plexes.16 In this paper, we describe the synthesis, crystal structure and thermal stability of the uniform Cu(II) tetrazolate complex (Scheme 1).

Scheme 1

Experimental

Materials and synthesis

All chemicals used in these syntheses were of re-agent grade and used as purchased without further puri-fication.

The mixture of Cu(OAc)2•H2O (1 mmol), NaN3 (2 mmol) and pyrazinecarbonitrile (pzCN) (2 mmol) in a molar ratio of 1∶2∶2 was dissolved in 8 mL of etha-nol in a Teflon-lined reactor. The reactor was heated in an oven at 120 ℃ for 24 h and then cooled to ambient temperature at a rate of 5 ℃•h－1. Yield: ca. 46% on the basis of Cu. IR (KBr) v: 1427 (s), 1368 (m), 1179 (m), 1161 (m), 1046 (s), 871 (m), 777 (m), 741 (m), 528 (w), 431 (w) cm－1. Anal. calcd for C10H6CuN12: C 33.57, H 1.69, N 33.57; found C 33.52, H 1.61, N 33.62.

Physical measurements

Infrared (IR) spectra were recorded on a Bruker EQUINOX-55 spectrophotometer within 400— 4000

480 Chin. J. Chem., 2009, Vol. 27, No. 3 HU et al.


cm－1 using the samples prepared as pellets with KBr. Elemental analyses were performed on a Perkin-Elmer 2400 elemental analyzer. Thermal analyses were carried out on a Netzsch STA 449C instrument with a heating rate of 20 ℃•min－1 in an atmosphere of flowing air.

X-ray crystallography and structure solution

Suitable single crystal of 1 was mounted on a Rigaku R-axis Spider II diffractometer equipped with graph-ite-monochromated Mo Ka (λ＝0.071073 nm) radiation. Empirical absorption corrections were applied. The unit cell parameters were determined by least squares re-finements of all reflections in the case. The structures were resolved by direct method and refined by full-matrix least-squares on F2. All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were located from the difference map, and then added geo-metrically. All calculations were performed using the SHELXTL-97 program package.17 Crystal data and ex-perimental details for compound 1 are contained in Ta-ble 1. Selected bond distances and angles are listed in Table 2.

Table 1 Crystal data and structure refinement parameters for 1

Empirical formula C10H6CuN12

Formula weight 357.81

Crystal system Monoclinic

Space group P2(1)/c

a/nm 0.97746(5)

b/nm 0.66812(2)

c/nm 0.98524(4)

a/(°) 90.00

β/(°) 92.3110(10)

γ/(°) 90.00

Volume/nm3 0.64290(5)

Crystal size/mm3 0.23×0.14×0.07

Density (calculated)/(g•cm－3 ) 1.848

Z 2

F(000) 358

θ range for data collection/(°) 3.69 to 27.46

Reflections collected 6059

Independent reflections 1474

Goodness-of-fit on F2 1.058

Final R indices [I＞2σ(I)] R1＝0.0210, wR2＝0.0631

R indices (all data) R1＝0.02046, wR2＝0.0595

Results and discussion

Description of the structure of compound 1

The single crystal structure analysis reveals that 1 crystallizes in the monoclinic space group P2(1)/c. The symmetric unit consists of one Cu(II) ion and two pzta ligands. As shown in Figure 1, each Cu ion adopts a

Table 2 Selected bond lengths (nm) and angles (°) for com-pound 1

Bond length

Cu—N(1) 0.19746(12) Cu—N(3B) 0.24777(14)

Cu—N(1A) 0.19746(12) Cu—N(3C) 0.24777(14)

Cu—N(5) 0.20513(13) Cu—N(5A) 0.20513(13)

Angle

N(1)-Cu-N(1A) 180.00(6) N(1)-Cu-N(5A) 99.41(5)

N(1A)-Cu-N(5A) 80.59(5) N(1A)-Cu-N(5) 80.59(5)

N(1A)-Cu-N(5) 99.41(5) N(1)-Cu-N(3B) 85.42(5)

N(5)-Cu-N(5A) 180.00(6) N(5A)-Cu-N(3C) 91.55(5)

N(1)-Cu-N(3C) 94.58(5) N(5)-Cu-N(3C) 88.45(5)

N(1)-Cu-N(3B) 85.42(5) N(5A)-Cu-N(3B) 88.45(5)

N(1A)-Cu-N(3B) 94.58(5) N(5)-Cu-N(3B) 91.55(5)

N(3B)-Cu-N (3C) 180.0

Symmetry codes: A: 1－x, 1－y, 1－z; B: x, 1.5－y, －0.5＋z; C: 1－x, －0.5＋y, 1.5－z.

slightly distorted octahedral geometry and is hexacoor-dinated by six N atoms from four pzta ligands. In the complex, atoms N(1), N(1A), N(5) and N(5A) form the equatorial plane around Cu(II) and the axial positions are occupied by N(3B) and N(3C). The mean plane de-viation formed by the four equatorial atoms is about 0.0000 nm. The copper ion deviation －0.0000 nm from the equatorial plane indicates that it lies in the plane. The bond angle of N(3B)-Cu-N(3C) has an ideal value 180°. The Cu—N bond distances range from 1.9746(12) to 0.24777(14) nm and are comparable to those found in other Cu(II) structures.18 And in com-pound 1, the Cu — N(3B), Cu — N(3C) distances [0.24777(14) nm] are both longer than other Cu—N distances. The differences of bond lengths in the title compound may be attributed to the influence of sur-rounding environments closely dependent on the ligand molecules. Each pzta ligand chelates one Cu(II) center and simultaneously bridges another one, acting as a tri-dentate chelating-bridging ligand. Therefore, the two adjacent Cu(II) centers are linked by one tridentate pzta ligand forming 2D infinite coordination polymer in the ab plane (Figure 2). The copper-copper distances within the chain are all 0.5952 nm. Recently, Li et al.19 re-ported another mononuclear compound of [Cu(pzta)2]• (H2O)3 (1a) obtained by a hydrothermal method using similar chemical reagents. By comparing the composi-tion of 1 with that of 1a, three lattice water molecules were lost in 1. The Cu(II) ion in 1a is four coordinated, while six coordinated in 1. It is noticeable that in com-plex 1a the pzta ligand adopts a bidentate chelating mode through the pyrazinyl N atom and one N atom of tetrazolate, while in complex 1 the pzta ligand adopts a tridentate chelating-bridging coordination mode through the pyrazinyl N atom and two N atoms of tetrazolate, implying that the bridging coordinating mode is inclined to form high dimensional networks. Compound 1 was synthesized with the molar ratio of reagents 1∶2∶2

Coordination polymer Chin. J. Chem., 2009 Vol. 27 No. 3 481


[n(Cu2＋)∶n( 3N－ )∶n(pzCN)], while 1a was synthesized with the molar ratio of reagents 2∶2∶1 [n(Cu2＋)∶n( 3N－ )∶n(pzCN)]. Compound 1 was synthesized in ethanol, while 1a was synthesized in water. Different molar ratio of the starting materials and the solvents maybe influence the hydrolysis reaction of nitrile. As a whole, the flexible pzta ligand was also partially re-sponsible for the formation of 1 and 1a, due to its vari-ous conformations and coordination modes.

Figure 1 A view of the coordination sphere around Cu(II) in 1 with labeling scheme and thermal ellipsoids at the 30% probabil-ity level.

Figure 2 Cu(II) ions are extended by the pzta ligands forming 2D coordination polymers of 1.

Thermal analyses

TG analysis for compound 1 was performed under a flow of air gas showing thermal stability up to about 180 ℃. Degradation of two pyrazine rings maybe takes place in the first stage between 185 and 375 ℃ with a mass loss of 40.72% (Calcd: 43.64%). The maximum rate of mass loss is indicated by the DTG peak at 312 ℃. And the second stage between 375 and 528 ℃ cor-responds to decomposition of the remaining two tetra-zolate-based moieties. The observed mass loss is 38.46%, which is consistent with the theoretical value of 38.59%. The final residue, estimated as copper oxide, has the observed mass 20.37% as against the calculated value of 22.23%.

In summary, a new 2D copper(II) coordination polymer constructed by 5-pyrazinyltetrazolate ligands was successfully synthesized. It is noticeable that the ligand pzta displays a very good flexibility, which demonstrates some different characteristics from the rigid ligand, even the reaction system with a small

change.

Supplementary data

CCDC 694276 contains the supplementary crystal-lographic data for 1. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

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(E0807252 Sun, H.)

synthesis and structural characterization of a new 2d coordination polymer [cu(pzta)2]n

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