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Inorganic Chemistry Communications 11 (2008) 572–575

A tricobalt(II) coordination polymer incorporating in situ generated5-methyltetrazolate ligands

Yang Chen a, You Song b, Yong Zhang a, Jian-Ping Lang a,b,*

a School of Chemistry and Chemical Engineering, Suzhou University, Suzhou 215123, Jiangsu, People’s Republic of Chinab State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, Jiangsu, People’s Republic of China

Received 5 February 2008; accepted 16 February 2008Available online 21 February 2008

Abstract

A tricobalt(II) coordination polymer {Na0.5[Co3(l4-Mtta)1.5(l2-OAc)3(l3-OAc)(l3-OH)]}n (1) (Mtta = 5-methyl tetrazolate) was pre-pared from the solvothermal reaction of Co(OAc)2 � 4H2O with NaN3 in MeCN in the presence of water. Complex 1 was characterizedby elemental analysis, IR, and X-ray crystallography. 1 consists of a unprecedented 3D hydrogen-bound supramolecular structure inwhich a 2D layer with unusual (326272) topology holds the adjacent layers together in a ABAB sequence via unusual O–H� � �Na pse-duo–agostic interactions. Complex 1 displayed the characteristics of a weak antiferromagnetic exchange interactions between Co2+ ionsin the system of Co3(Mtta)3 trigonal unit.� 2008 Elsevier B.V. All rights reserved.

Keywords: Cobalt; 5-Methyltetrazolate; Solvothermal synthesis; Structure; Magnetic properties

In the past decade, design and syntheses of new coordi-nation polymers containing paramagnetic metal ions linkedby organic components have received much attentionbecause of their fascinating molecular topologies and out-standing properties for potential applications in magneticmaterials [1,2]. These compounds are mainly preparedfrom routine solution reactions or hydro(solvo)thermalreactions of paramagnetic metal salts with readymadeorganic ligands [1]. However, employment of in situmetal/ligand reactions has been less explored [3]. Amongthe in situ reactions, so-called Demko-Sharpless [2 + 3]cycloaddition reaction is the typical one [4]. For example,some tetrazolate ligands could be prepared in situ through[2 + 3] cycloaddition reactions of organicnitriles with azideanion in the presence of metal ions such as Zn2+, Cd2+,Ag+, and Cu+ [5]. However, other transition metal ionssuch as Co2+ have not been explored yet. Is it possible

1387-7003/$ - see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.inoche.2008.02.015

* Corresponding author. Address: School of Chemistry and ChemicalEngineering, Suzhou University, Suzhou 215123, Jiangsu, People’sRepublic of China. Tel.: +86 512 65882865; fax: +86 512 65880089.

E-mail address: jplang@suda.edu.cn (J.-P. Lang).

for cobalt(II) ions to catalysis such [2 + 3] reactions? If itworks, what about their structures and magnetic proper-ties? With these ideas in mind, we chose Co(OAc)2 � 4H2Oand carried out its solvothermal reactions with NaN3 andMeCN in the presence of water and an interesting cobal-t(II) coordination polymer {Na0.5[Co3(l4-Mtta)1.5(l2-OAc)3(l3-OAc)(l3-OH)]}n (1) (Mtta = 5-methyl tetrazo-late) was isolated therefrom. Herein we report its synthesis,structural characterization and magnetic properties.

The solvothermal reaction of Co(OAc)2 � 4H2O withequimolar NaN3 and excess MeCN in the presence of waterat 150 �C for 70 h followed by cooling to ambient temper-ature afforded purple–red hexagonal plates of 1 in 51%yield [6]. The elemental analysis of 1 is consistent with itschemical formula. In the IR spectrum of 1, the absenceof the C„N stretching vibration at ca. 2200 cm�1 andthe appearance of a new stretching vibration of tetrazolateanion at ca. 1400 cm�1 are line with the in situ formation oftetrazolate ligand generated by a [2 + 3] cycloaddition reac-tion of MeCN and N�3 [5a]. In addition, the strong bands at1568 and 1386 cm�1 for 1 are assigned to the C@O asym-metric and symmetric stretching bands of bridging

Y. Chen et al. / Inorganic Chemistry Communications 11 (2008) 572–575 573

acetates. Compound 1 also shows a broad hydroxylstretching band at 3441 cm�1. It is noted that the reactiontime, temperature and solvent remarkably affected the yieldof 1. Short reaction time (e.g. 1000 min) formed tiny crys-tals of 1 in a low yield (24%) while long reaction time (e.g.7000 min) produced cracked crystals of 1 in 45% yield. Theyield of 1 became as poor as <1% when the temperaturewas either elevated above 180 �C or lowered below120 �C. Without water, the yield of 1 was also low(<1%), which may be due to the low solubility ofCo(OAc)2 � 4H2O and NaN3 in MeCN. Solid 1 is stabletowards air and moisture and virtually insoluble in com-mon organic solvents such as DMF and MeCN. The ther-mogravimetric analysis (TGA) revealed that 1 was stableup to 300 �C. The TGA curve of 1 showed a sharp weightloss of 57.23% in the range of 313–400 �C, which corre-sponds roughly to the loss of the Mtta ligands, acetateand hydroxyl groups (calculated 57.55%). The decomposi-tion residual species was assumed to be a mixture of6CoO + 0.5Na2O (42.77% versus calculated 42.45%)according to X-ray fluorescence analysis (see Supplemen-tary Material).

1 crystallizes in the trigonal space group R-3c and theasymmetric unit contains one third of the anion [Co3-(l4-Mtta)1.5(l-OAc)3(l3-OAc)(l3-OH)]�1/2 and one sixthof Na+ ion [7]. The anion consists of a trigonal [Co3(l3-OH)(l3-OAc)] unit that is bridged by l4-Mtta and l-OAc

Fig. 1. Perspective view of Na0.5[Co3(l4-Mtta)1.5(l3-OH)(l3-OAc)-(l-OAc)3] of 1, where only one set of the disordered l3-OH and l-OAcis shown. All hydrogen atoms along with methyl groups of the l-OAcligands are omitted for clarity. Symmetry transformations used togenerate equivalent atoms: A, 2 � y, 1 + x � y, z; B: 1 � x + y, 2 � x, z;C: 2/3 + x � y, 4/3 � y, 11/6 � z; D: �1/3 + y, 1/3 + x, 11/6 � z. Co, C,N, O and Na atoms are represented by pink, black, blue, red and greenspheres, respectively. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

ligands (Fig. 1). There is a C3 axis running through theO1 and C5 and C6 atoms. The local coordination geometryaround each Co center can be best described as a distortedoctahedron with two N atoms from two l4-Mtta ligandsand four O atoms of one l3-OH, one l3-OAc ion, andtwo l-OAc ions. In each [Co3(l3-OH)(l3-OAc)] unit, oneacetate carrying C5 and C6 atoms is disordered over theC3 axis, which makes it an unusual triply-bridging ligandcapping over the [Co3(l3-OH)] core with the Co–O (l3-OAc) distance of 2.134(8) A. At the center of the [Co3(l3-OH)] core is sitting one disordered OH� group that worksas a triply-bridging ligand to link three Co2+ ions. Such acore was found in [Co3(DBM)3(pz)4(OH)] � 2THF(DBM = dibenzoylmethanate; pz = pyrazole) [8]. Themean Co1–O1(OH) distance (2.113(2) A) is longer thanthat of the corresponding ones in [Co3(DBM)3(p-z)4(OH)] � 2THF (2.033(2) A). Each Co� � �Co contact inthe core of 1 is 3.51 A, which is too long to include anymetal–metal interaction [9]. Each l-OAc is also disorderedover two sites. Pairs of such l-OAc ligands link the Co2+

ions of the neighboring core to form a 2D clover-like net-work extending along the ab plane (Fig. 2a). The averageCo–O bond distance for l-OAc (1.960(7) A) is shorter thanthe literature values (range from 2.10 to 2.25 A) [10].

Each Mtta in 1 works as a l4-g1:g1:g1:g1-bridging ligand[5e]. Around each [Co3(l3-OH)(l3-OAc)] unit are three l4-Mtta ligands, each of which bridges two Co2+ ions with itstwo N atoms, thereby forming a [Co3(l4-Mtta)1.5(l3-OH)(l3-OAc)] fragment. The mean Co–N bond distance(2.108(3) A) is comparable with that observed in{[Co3(IDC)2 (4,40- bipy)(H2O)4] � 2H2O}n (IDC = imidaz-ole 4,5-dicarboxylic acid, Co–NIDC = 2.069(3))[11]. Withanother two N atoms, each Mtta links a pair of Co2+ ionsof the adjacent fragment to form a 2D Mtta–AcO compos-ite layer extending along the ab plane (Fig. 2a). If Co2+ andMtta are treated as a two-connected node and a four-con-nected node, respectively, this 2D net adopts an unprece-dented (326272) Schlafli symbol topology network(Fig. 2b). The distance between two adjacent layers is7.069 A. Such a layer further connects with its neighboringones by the Na� � �H–O pseduo–agostic interactions(Na� � �H 2.069 A, H–O 0.959 A, Na� � �H–O 180�) [12] togive a 3D hydrogen-bound supramolecular structure(Fig. 3).

The magnetic measurements were performed by using aSQUID magnetometer. Magnetic properties of the crystal-line sample1 was measured at an applied field of 2 kOefrom 300 to 1.8 K as shown in Fig. 4 in the forms ofvMT versus T.

At room temperature, vMT is equal to 8.68 emu K mol�1,which is much higher than the spin-only value of5.625 emu K mol�1 based on three Co2+ ion (g = 2 ands = 3/2) due to the prominent orbital contribution. Uponlowering the temperature, vMT continuously decreasesand reaches 0.62 emu K mol�1 at 1.8 K. Above 100 K,the magnetic properties of 1 obey Curie–Weiss law andgives C = 9.57(3) and h = �28.5(6) K. The smaller Weiss

Fig. 2. (a) View of the 2D anionic network of 1 along the ab plane. Hydrogen atoms and one set of disordered OAc and OH groups are omitted for clarity.(b) 2D layered network with a Schlafli symbol (326272). The Mtta ligands are represented by the blue balls. (For interpretation of the references to colour inthis figure legend, the reader is referred to the web version of this article.)

Fig. 3. Packing diagrams of 1 looking along the a axis. H atoms andAcO� groups are omitted for clarity.

Fig. 4. Temperature dependence of magnetic susceptibilities of 1 in theforms of 1/vM and vMT versus T. The solid lines are the fitting results.

574 Y. Chen et al. / Inorganic Chemistry Communications 11 (2008) 572–575

constant than �20 K for non-interacting Co2+ ions [13]indicates the additional antiferromagentic couplingbetween Co2+ ions except the contribution of spin-orbital

coupling in 1. According to the preceding structure descrip-tion of 1, no appropriate model could be used for fitting themagnetic properties of such a system. So, the treatmentmethod reported by Rueff et al. [14] and the simple phe-nomenological Eq. (1) [15] can be used here

vMT ¼ A expð�E1=kT Þ þ B expð�E2=kT Þ ð1Þwhere A + B equals the Curie constant and E1 and E2 rep-resent the ‘‘activation energies” corresponding to the spin-orbit coupling and to the magnetic exchange interaction,respectively. The best fitting results give: �E1/k =�41(1) K and �E2/k = �2.4(1) K with C = 9.55 cm3

K mol�1. Thus, the magnetic coupling constants betweenCo2+ ions are �4.8 K for 1 according to the relationshipof vMT / exp(+J/2kT) [2b,16]. This indicates that theweak antiferromagnetic exchange interaction betweenCo2+ ions with spin-orbital coupling of Co2+ ions domi-nate the magnetic properties in 1.

In summary, the present work demonstrated that a newcobalt(II) coordination polymer 1 was prepared fromsolvothermal reactions of Co(OAc)2 � 4H2O with NaN3 inMeCN in the presence of water. The formation of 1 isinvolved in a [2 + 3] cycloaddition of the azide and aceto-nitrile. Complex 1 displayed an unprecedented 3D hydro-gen-bound structure in which 2D layers with (326272)topology are linked by unusual O–H� � �Na pseduo–agosticinteractions. Investigations of this system are continuing inan effort to examine if other main group metal ions such asLi+, Cs+, Ca2+ could be introduced and, if so, to exploretheir effects on the topological structures and the magneticproperties of the resulting products.

Acknowledgements

This work was supported by the NNSF (No. 20525101),the Specialized Research Fund for the Doctoral Programof Higher Education (No. 20050285004), and the State

Y. Chen et al. / Inorganic Chemistry Communications 11 (2008) 572–575 575

Key Laboratory of Coordination Chemistry of NanjingUniversity, and the Qin-Lan Project of Jiangsu Provincein China. The authors also thank the helpful suggestionsof Prof. B.F. Abrahams in University of Melbourne andProf. G.L. Ma in Suzhou University.

Appendix A. Supplementary material

CCDC 674064 contains the supplementary crystallo-graphic data for 1. These data can be obtained free ofcharge from The Cambridge Crystallographic Data Centrevia www.ccdc.cam.ac.uk/data_request/cif. Supplementarydata associated with this article can be found, in the onlineversion, at doi:10.1016/j.inoche.2008.02.015.

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