A New Tetranuclear Cobalt Cluster with (1H-Benzimidazol-2-yl)-methanol Synthesis and Crystal Structure①

HUANG Qiu-Ping LI Gui ZHANG Shu-Hua ZHANG Hai-Yang

(C ollege of Chemistry and Bioengineering, Guilin University of Technology, Guilin 541004, China)

1 INTRODUCTION

The rational design and synthesis of polynuclear complexes, aiming at understanding the structural and chemical factors that govern the exchange coupling between paramagnetic centers, are of continuing interest in biology, chemistry and physics[1-3]. Particular interest has focused on the development of single molecule magnets (SMM)[4-16]. An effective and facile approach for the synthesis of such complexes is still the appropriate choice of well-designed organic ligands as bridges or terminal groups with metal ions or metal clusters as nodes.Hbm possessing one oxygen atom of the methanol group and two nitrogen atoms of the benzimidazol group may provide the potential to construct unpredictable and interesting polymetallic clusters. Previous investigations implied that [Co7(bm)12]·(ClO4)2·-13H2O and [Co4(bm)4Cl4(C3H7OH)4] favor the ferromagnetic coupling through μ3-O-bridges[17]. In the reference, we reported anion induced diversification from heptanuclear to tetranuclear clusters. Herein,Hbm reacted with cobalt salts in the presence of different anions (ClO4–, Cl–, HCOO–) in propan-1-ol solvent[17].

In order to further examine the effect of different solvents on the structural assembly, direct reactions of (1H-benzimidazol-2-yl)-methanol with CoCl2·4H2O in methanol solvent under the same hydrothermal conditions were carried out. The complex [Co4-(bm)6Cl2]·(H2O)2·(CH3OH) (1) can be obtained. It is different from the previously reported tetranuclear or heptanuclear clusters, which indicates that the solvents have an important influence on the structures of the bm-bridged cobalt(II) assemblies.

2 EXPERIMENTAL

2. 1 Materials and instruments

All solvents and chemicals were commercial reagents and used without further purification. The elemental analysis was performed on a PE 1700 CHN auto elemental analyzer. The crystal structure was determined by Bruker CCD area-detector and SHELXL crystallographic software of molecular structure. The FT-IR spectra were recorded from KBr pellets in the range of 4000～400 cm–1on a Bio-Rad FTS-7 spectrometer.

2. 2 Syntheses of [Co4(bm)6Cl2]·(H2O)2·(CH3OH) (1)

A mixture of CoCl2·4H2O (0.238 g, 1 mmol),Hbm (0.148 g, 1 mmol) and methanol (10 mL) with a pH adjusted to 7 by the addition of triethylamine was poured into a Teflon-lined autoclave (15 mL)and then heated at 120 ℃ for 4 days. Red crystals of 1 were collected by fi ltration, washed with methanol and dried in air. Phase pure crystals of 1 were obtained by manual separation (yield: 144.6 mg, ca.69% based on Hbm ligans). Anal. Calcd. for 1:C49H50N12Cl2Co4O9(Mr= 1257.63). calcd.: C, 46.79;H, 4.02; N, 13.37%. Found: C, 46.78; H, 4.09; N,13.43%. IR data for 1 (KBr, cm–1, Fig. S1): 3425 s,1626 m, 1458 s, 1274 m, 1069 m, 738 m, 607 m,520 w.

2. 3 Crystal structure determination

A red crystal of complex 1 having approximate dimensions of 0.38mm × 0.32mm × 0.25mm was selected and mounted on a glass fibre. All measurements were made on a Bruker SMART APEX CCD diffractometer with a graphite-monochromatized Cu Ka radiation (λ = 1.54178 ?) by using the ω-θ scan mode in the range of 3.92≤θ≤63.26° at 298(2) K. Raw frame data were integrated with the SAINT program[18]. The structure was solved by direct methods with SHELXS-97 and refined by full-matrix least-squares on F2using SHELXL-97[18].The empirical absorption correction was applied with the program SADABS[18]. All non-hydrogen atoms were refined anisotropically, while all hydrogen atoms were set in the calculated positions and refined by a riding model. The methanol molecule is disordered over two sets of sites in a 0.608(1):0.392(1) ratio. The selected bond distances and bond angles are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°) for 1

Symmetry codes: (i) -x+2, -y, -z+1; (ii) -x+1, -y+2, -z+1

3 RESULTS AND DISCUSSION

3. 1 Crystal structure description

Single-crystal X-ray crystallographic analysis reveals that complex 1 crystallizes in a monoclinic space group P21/n. Complex 1 belongs to monoclinic space group P21/n with a = 21.1713(5), b =12.7948(3), c = 24.0195(9) ?, β = 95.309(3)°, V =6478.6(3) ?3, Z = 4, F(000) = 2568, Dc= 1.289 g·cm-3, Mr= 1257.63, μ = 9.096 mm–1, S = 1.000, the final R = 0.0861 and wR = 0.2552 for 4956 observed reflections with I ＞ 2σ(I). This structure contains two identical but independent molecular units. As shown in Fig. 1, complex 1 is a cen- trosymmetric tetranuclear cobalt cluster. The mole- cule consists of four Co(II), two Cl–anions, six bm ligands, one methanol molecule and two water molecules. The framework of tetranuclear Co4O6cluster, which is bridged by μ2-O and μ3-O alkoxide groups, exhibits two connected face-sharing cubes, each with one vertex missing. All four Co(II) ions are in the +2 oxidation state, as evidenced by the bond valence summation calculations, charge balance considerations and the presence of typical bond lengths for Co(II) ions[19-20]. It is different from the coordination environment between Co(1) and Co(2)and the coordination polyhedron for CoIIion shown as Fig. 2. Co(1) is five-coordinated by two μ2-alkoxide oxygen atoms, one μ3-alkoxide oxygen atom, one benzimidazol N atom and one Cl–anion to form a distorted trigonal bipyramidal geometry with O(3) and Cl(1) located at the apical positions and the O–Co(1)–X (X = O, N) angles of equator position falling in the range of 108.5(4)～116.4(3)° and the O(3)–Co(1)–Cl(1) angle of the apical position to be 171.7(4)°. The Co(2) ion is six-coordinated by two N and two O atoms (O(1), O(2), N(1), N(3)) from two bm ligands of μ3:η1:η2coordination mode(Scheme 1a) and two μ3-O atoms (O(3), O(3A),symmetry code: (i) -x+2, -y, -z+1) from two bm ligands of μ4:η1:η3coor-dination mode (Scheme 1b),forming a distorted octahedral geometry with cis-angles X–Co(2)–X (X = O, N) ranging from 76.0(2) to 100.4(3)° and Co(2)–X (X = O, N) bonds of 1.970(8)～2.072 (8) ?. In intraclusters, the Co–O–Co angles fall in the 101.8(3)～111.4(3)°range, and the adjacent Co··Co distances locate in a narrow range of 3.177～3.252 ?. The two coordination modes of the bm ligand in 1 were observed in the heptanuclear cluster [Co7(bm)12]·(ClO4)2·13H2O[17].

Scheme 1. Coordination modes of the bm ligand

Fig. 3 shows the packing diagram of 1, where the clusters are well isolated by water molecules, Clions and bm ligands. Complex 1 forms a 2-D network through N–H··O hydrogen bonds (Table 2).The apparent holes can be observed in the b direction. As calculated with the PLATON program[21],the total potential solvent area volume is 1406.7 ?3and the value of porosity is 21.7% (All H2O and methanol molecules have been omitted). The hole diameters are 14.39 and 9.56 ? (two groups Cl··Cl distance on the corresponding face to face positions, Fig. 3), respectively.

Fig. 1. Structure of 1 with all water and methanol molecules and all H atoms omitted

Fig. 2. Coordination polyhedron for CoII

Fig. 3. Packing drawing of 1. The water and methanol molecules are omitted

Table 2. Hydrogen Bond Parameters of Complex 1

3. 2 IR spectrum properties

IR spectral data of the Hbm ligand and complex 1 are shown in Fig. S1. Comparison of the IR spectra of complex 1 with those of the free ligand reveals that the medium strong band at 3258 cm–1can be assigned to the N–H of the ligand[22], and that at 3425 cm–1to the uncoordinated water molecules associated with complex 1[23]. The C=N of the free ligand at 1620 cm–1was shifted to higher frequencies (1626 cm–1) in complex 1, suggesting its participation in chelation[24]. The strong bands at 1440 and 1458 cm–1are assigned to C=C benzene ring in the free ligand and complex 1, respectively[25].The C–O of free ligand at 1270, 1208 and 1049 cm–1were shifted to higher frequencies (1274, 1215, 1069 cm–1) in complex 1, suggesting its participation in chelation[26-27]. The bands at 744 and 738 cm–1of the free ligand and complex 1 can be owned to the aromatic C–H out-of-plane bending vibration[25].

4 CONCLUSION

A new tetranuclear cobalt cluster based on the(1H-nenzimidazol-2-yl)-methanol has been successfully synthesized from a conventional solvothermal synthesis method. The framework of tetranuclear{Co4O6} cluster exhibits two connected face-sharing cubes, each with one vertex missing. The apparent holes can be observed along the b direction. The elemental analysis and IR data are consistent with the X-ray analysis results.

(1) Miller, J. S.; Drillon, M. Magnetism: Molecules to Materials II: Molecule-Based Magnets; Wiley-VCH: Weinheim 2001.

(2) Yang, L.; Huang, Q. P.; Zhang, C. L.; Zhao, R. X.; Zhang, S. H. Two disc-like heptanuclear clusters based on Schiff base: syntheses, structure and magnetic properties. Supramol. Chem. 2014, 26, 81–87.

(3) Zhang, S. H.; Li, N.; Ge, C. M.; Feng, C.; Ma, L. F. Structures and magnetism of {Ni2Na2}, {Ni4} and {Ni6IINiIII} 2-hydroxy-3-alkoxy-benzaldehyde clusters. Dalton Trans. 2011, 40, 3000–3007.

(4) Fatemah, H.; Gabriel, B.; Veacheslav, V.; Ilia, K.; Liviu, F. C.; Murugesu, M. Significant enhancement of energy barriers in dinuclear dysprosium single-molecule magnets through electron-withdrawing effects. J. Am. Chem. Soc. 2013, 135, 13242–13245.

(5) Gatteschi, D.; Sessoli, R. Quantum tunneling of magnetization and related phenomena in molecular materials.Angew. Chem., Int. Ed. 2003, 42, 268–297.

(6) Zhang, S. H.; Zhang, Y. D.; Zou, H. H.; Guo, J. J.; Li, H. P.; Song, Y.; Liang, H. A family of cubane cobalt and nickel clusters: syntheses, structures and magnetic properties. Inorg. Chim. Acta 2013, 396, 119–125.

(7) Sessoli, R.; Gattesch, D.; Caneschi, A.; Novak, M. A. Magnetic bistability in a metal-ion cluster. Nature 1993, 365, 141–143.

(8) Beedle, C. C.; Henderson. J. J.; Ho, P. C.; Sayles, T.; Nakano, M.; O’Brien, J. R.; Heroux, K. J.; Barco, E. D.; Maple, M. B.; Hendrickson, D. N.Ferromagnetic ordering and simultaneous fast magnetization tunneling in a Ni4single-molecule magnet. Inorg. Chem. 2010, 49, 5780–5782.

(9) Hirotsu, M.; Shimizu, Y.; Kuwamura, N.; Tanaka, R.; Kinoshita, I.; Takada, R.; Teki, Y.; Hashimoto, H. Anion-controlled assembly of four manganese ions: structural, magnetic, and electrochemical properties of tetramanganese complexes stabilized by xanthene-bridged Schiff base ligands. Inorg. Chem. 2012, 51, 766–768.

(10) Yang, E. C.; Hendrickson, D. N.; Wernsdorfer, W.; Nakano, M.; Zakharov, L. N.; Sommer, R. D.; Rheingold, A. L.; Ledezma-Gairaud, M.; Christou,G. A cobalt single-molecule magnet. J. Appl. Phys. 2002, 91, 7382–7384.

(11) Murrie, M.; Teat, S. J.; Stoeckli-Evans, H.; Güdel, H. U. Synthesis and characterization of a cobalt(II) single-molecule magnet.Angew. Chem., Int. Ed. 2003, 42, 4653–4656.

(12) Cadiou, C.; Murrie, M.; Paulsen, C.; Villar, V.; Wernsdorfer, W.; Winpenny, R. E. P. The first nickel-based single molecule magnet: resonant quantum tunnelling in an S = 12 molecule. Chem. Commun. 2001, 2666–2667.

(13) Ochsenbein, S. T.; Murrie, M.; Rusanov, E.; Stoeckli-Evans, H.; Sekine C.; Güdel, H. U. Synthesis, structure, and magnetic properties of the single-molecule magnet [Ni21(cit)12(OH)10(H2O)10]16-. Inorg. Chem. 2002, 41, 5133–5140.

(14) Guo, Y. N.; Xu, G. F.; Gamez, P.; Zhao, L.; Lin, S. Y.; Deng, R. P.; Tang, J. K.; Zhang, H. J. Two-step relaxation in a linear tetranuclear dysprosium(III) aggregate showing single-molecule magnet behavior. J. Am. Chem. Soc. 2010, 132, 8538–8539.

(15) Rinehart, J. D.; Meihaus, K. R.; Long, J. R.; Observation of a secondary slow relaxation process for the single-molecule magnet U(H2BPz2)3.J. Am. Chem. Soc. 2010, 132, 7572–7573.

(16) Wang, W.; Hai, H.; Zhang, S. H.; Yang, L.; Zhang, C. L.; Qin, X. Y. Microwave-assisted synthesis, crystal structure and magnetic behavior of a Schiff base heptanuclear. Cobalt cluster. J. Clust. Sci. 2013, DOI:1007/s10876-013–0614-z.

(17) Zhang, S. H.; Ma, L. F.; Zhou, H. H.; Wang, Y. G.; Liang, H.; Zeng, M. H. Anion induced diversification from heptanuclear to tetranuclear clusters:syntheses, structures and magnetic properties. Dalton Trans. 2010, 40, 11402–11409.

(18) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr. 2008, A 64, 112–122.

(19) Wang, X. T.; Wang, B. W.; Wang, Z. M.; Zhang, W.; Gao, S. Azide and oxo bridged ferromagnetic clusters: three face-shared tetracubane Ni(II)/Co(II) hexamers and a wheel-shaped SMM-like Co(II) heptamer. Inorg. Chim. Acta 2008, 361, 3895–3902.

(20) Li, Y. G.; Wu, Q.; Lecren, L.; Clerac, R. Synthesis, structure and magnetism of new polynuclear transition metal aggregates assembled with Schiff-base ligand and anionic N-donor ligands. J. Mol. Struct. 2008, 890, 339–345.

(21) Spek, A. L. Structure validation in chemical crystallography. Acta Cryst. 2009, D65, 148–155.

(22) Tyagi, M.; Chandra, S.; Tyagi, P. Mn(II) and Cu(II) complexes of a bidentate Schiff’s base ligand: spectral, thermal, molecular modelling and mycological studies. Spectrochim. Acta Part A 2014, 117, 1–8.

(23) Shebl, M. Synthesis, spectroscopic characterization and antimicrobial activity of binuclear metal complexes of a new asymmetrical Schiff base ligand:DNA binding affinity of copper(II) complexes. Spectrochim. Acta. Part A 2014, 117, 127–137.

(24) Kianfar, A. H.; Paliz, M.; Roushani, M.; Shamsipur, M. Synthesis, spectroscopy, electrochemistry and thermal study of vanadyl tridentate Schiff base complexes. Spectrochim. Acta Part A 2011, 82, 44–48.

(25) Selvakumar, E.; Anandha, Babu, G.; Ramasamy, P.; Rajnikant, Murugesan, V.; Chandramohan, A. Synthesis, growth and spectroscopic investigation of an organic molecular charge transfer crystal: 8-hydroxy quinolinium 4-nitrobenzoate 4-nitrobenzoic acid.Spectrochim. Acta Part A 2014, 117, 259–263.

(26) Naeimi, H.; Moradian, M. Synthesis and characterization of nitro-Schiff bases derived from 5-nitro-salicylaldehyde and various diamines and their complexes of Co(II). J. Coord. Chem. 2010, 63, 156–162.

(27) Shebl, M. Synthesis, spectral and magnetic studies of mono- and bi-nuclear metal complexes of a new bis(tridentate NO2) Schiff base ligand derived from 4,6-diacetylresorcinol and ethanolamine. Spectrochim. Acta Part A 2009, 73, 313–323.