23804
J. Phys. Chem. B 2005, 109, 23804-23807
V4S9Br4: A Novel High-Spin Vanadium Cluster Thiobromide with Square-Planar Metal Core Yuri V. Mironov,† Spartak S. Yarovoi,† Dmitry Y. Naumov,† Svetlana G. Kozlova,†,‡ Vladimir N. Ikorsky,† Reinhard K. Kremer,§ Arndt Simon,§ and Vladimir E. Fedorov*,† NikolaeV Institute of Inorganic Chemistry and BoreskoV Institute of Catalysis, Russian Academy of Sciences, 3, Academician LaVrentieV AVenue, 630090 NoVosibirsk, Russia, and Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany ReceiVed: June 30, 2005; In Final Form: October 6, 2005
The novel vanadium thiobromide, V4S9Br4, with a square-planar metal cluster core was synthesized and characterized by single-crystal X-ray diffraction, measurements of magnetic properties and the heat capacity, and DFT calculations of the electronic structure. At the room temperature, the compound displays paramagnetic properties with an independent spin on each V atom and with a weak exchange constant (J ≈ 10 cm-1). The paramagnetic state is transformed into a low-spin state (AF-type ordering) at low temperatures. This change is accompanied by a heat-capacity anomaly. The observed magnetic and heat-capacity anomalies can be explained by the thermal excitation of electrons on the closely spaced molecular energy levels in the presence of the Jahn-Teller effect.
Introduction Among the group V transition metals, the chemistry of niobium and tantalum chalcogenide cluster compounds is well investigated, but research on vanadium cluster complexes is quite scarce.1 Although some vanadium chalcogenide clusters with varying nuclearity (from binuclear to hexanuclear) are known, the tetranuclear vanadium clusters with square-planar metal cores have not been synthesized until now. Here, we describe the unexpected magnetic and thermal properties of the square-planar tetranuclear vanadium cluster thiobromide V4S9Br4 with polymeric structure [V4(µ4-S)(µ2-S2)4Br8/2]∞. In the literature, several four-nucleus compounds with crystal structures containing square-planar metal clusters that are rather close to V4S9Br4 have been published. The molecular complex [Ta4(µ4-Se)(µ-Se2)4I8]2 contains a cluster core, {Ta4(µ4-Se)(µ2Se2)4}, that is similar to {V4(µ4-S)(µ2-S2)4} in vanadium thiobromide V4S9Br4. The compound (NnBu4)2[V4O(edt)2Cl8] containing the square-planar oxide bridge cluster complex [V4(µ4-O)(µ2-edt)2(µ2-Cl)4Cl4]2- has been synthesized.3 Strong structural similarity was found in the Ln(III) compounds [Ln4(µ4-Se)(µ2-Se2)4(THF)6I2] (Ln ) Tm, Ho, Er, Yb);4 however, these compounds do not form metal-metal bonds. The related square-planar clusters, though coordinated by two µ4-S ligands (a tetragonal bipyramid with sulfur atoms at the apexes), are found in the Nb(III) complexes [Nb4(µ4-S)2(µSPh)8(SPh)4]4- and [Nb4(µ4-S)2(µ-SPh)8(PMe2R)4]; in both compounds, four single Nb-Nb bonds in the clusters are realized.5 The compound K4[Ti4(µ4-O)(µ2-I)8I4] is characterized by a related structure and contains an oxygen-centered Ti square * Corresponding author. Professor Vladimir Fedorov, Nikolaev Institute of Inorganic Chemistry, Russian Academy of Sciences, 3, Acad. Lavrentiev Av., 630090 Novosibirsk, Russia. Fax: +7(383)3309489. Tel.: +7(383)3309253. E-mail:
[email protected]. † Nikolaev Institute of Inorganic Chemistry, Russian Academy of Sciences. ‡ Boreskov Institute of Catalysis, Russian Academy of Sciences. § Max-Planck-Institut fu ¨ r Festko¨rperforschung.
embedded in a cuboctahedron of iodine; there are short Ti-Ti bonds in a square-planar metal cluster.6 Experimental and Computational Details Synthesis. V4S9Br4 was synthesized by the reaction of the elements (the ratio of V/S/Br ) 4:9:4) in an evacuated and sealed silica tube. The tube was heated at 10 °C/h to 450 °C, kept at this temperature for 3 days, and then cooled at 20 °C/h to room temperature. The reaction product was washed with acetone and dried. Single crystals were selected from the reaction mixture. The compound is insoluble in water and common organic solvents. In inert atmosphere, V4S9Br4 decomposes at temperatures above 400 °C with the formation of V3S5. The X-ray powder diffraction pattern of the bulk sample is in excellent agreement with the pattern calculated on the basis of the single-crystal structure. The Raman spectrum of the compound is characterized by several bands (135, 145, 166, 218, 251, 272, 373, 568 cm-1), with the pronounced band at 568 cm-1 being due to the ν(S2) valence mode. Crystallography. Single-crystal X-ray diffraction data were collected using graphite-monochromatized Mo KR radiation (λ ) 0.71073 Å) at 173 K on a Bruker CCD diffractometer with the program SMART. Cell refinement and data reduction were carried out with the program SAINT, and face-indexed absorption corrections were performed numerically with the use of XPREP. Then, the program SADABS was employed to make incident beam and decay corrections. The SHELX-97 program set7 for structure solution (direct methods) and refinement (fullmatrix least-squares on F2) was used for structural analysis. Br4S9V4 (M ) 811.94), 0.1 × 0.08 × 0.06 mm, tetragonal, space group P4/nmm, a ) 10.8639(7) Å, c ) 6.9728(8) Å, V ) 822.96(12) Å3, Z ) 2, Fcalc ) 3.277 g cm-1, µ ) 13.023 mm-1, 2.65° < θ < 28.27°, T ) 173(2) K, face-indexed absorption correction (transmission coefficients 0.3559, 0.5088). Reflections: 4955 collected, 599 unique (Rint ) 0.0231), 552 observed [I > 2σ(I)]; 29 parameters refined with R) 0.0139 [I > 2σ(I)],
10.1021/jp053572k CCC: $30.25 © 2005 American Chemical Society Published on Web 11/30/2005
V4S9Br4: A Novel High-Spin Vanadium Cluster Thiobromide
J. Phys. Chem. B, Vol. 109, No. 50, 2005 23805
Figure 2. Heat capacity of V4S9Br4 at indicated fields together with the estimated lattice contribution (solid line).
Figure 3. fc and zfc measured molar susceptibilities of V4S9Br4 for various magnetic fields. The insert shows 1/χmol vs T. The Curie-Weiss law (solid lines) is fitted to the high-temperature data. Figure 1. (a) Fragment [V4(µ4-S)(µ2-S2)4Br8/2] of the structure of V4S9Br4. Displacement ellipsoids are drawn at the 75% probability level. (b) Packing of the layers in structure in V4S9Br4. (c) Layers of bonded cluster fragments in structure in V4S9Br4.
wR2 ) 0.0289 (all data), GOF ) 1.080, residual electron density ) +0.357, -0.390 e Å-3 Further details of the crystal structure investigations can be obtained from the Fachinformationzentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany, on quoting the depository number CSD-415351. Magnetic Measurements. The magnetic susceptibility was measured in the temperature range from 3 to 300 K in an applied magnetic field of 100 G using a commercial MPMS magnetometer (Quantum Design, 6325 Lusk Boulevard, San Diego, CA). EPR spectra showing a bulk signal were recorded on a Bruker spectrometer in X (3 cm) diapason at 77 and 300 K. Calorimetric Measurements. The heat capacity was measured using a commercial PPMS calorimeter (Quantum Design, 6325 Lusk Boulevard, San Diego, CA) employing the relaxation method. To thermally anchor the crystals to the sample holder platform, a minute amount of Apiezon grease was used. The heat capacity of the platform and the grease was determined in a separate run and subtracted from the measured total heat capacity.
DFT Calculations. Spin-restricted density functional theory (DFT) calculations were carried out using the ADF2003 code.8 The local-exchange VWN correlation potential9 was used for the local density approximation (LDA), Becke’s nonlocal corrections10 was applied to the exchange energy, and Perdew’s nonlocal corrections11 were added to the correlation energy. The ZORA (zeroth-order relativistic approximation) method was used to account for scalar relativistic effects.12 The STO basic set without core potentials was used for all atoms (ADF2003/ ZORA/TZP). The electronic structure calculations and full optimizations were carried out for models [V4S9]4+ of C4V symmetry with full spin S ) 0. Results Structure. The crystal structure of V4S9Br4 is shown in Figure 1a-c. The compound can be described by the crystallo-chemical formula [V4(µ4-S)(µ2-S2)4Br8/2]∞ with a formal oxidation state of vanadium of +3.5. The vanadium atoms form a square with V-V distances of 3.0053(6) Å. All four V atoms are coordinated by µ4-S with distances of 2.3729(6) Å. Four disulfide µ2-S2 ligands are asymmetrically coordinated to the V-V edges, and four sulfur atoms of the S2 ligands lie almost in the V4 plane with V-S distances of 2.3917(5) Å. Another four S atoms are
23806 J. Phys. Chem. B, Vol. 109, No. 50, 2005
Mironov et al.
Figure 4. Calculated (ground state, 1) and proposed (excited state, 2) electronic level schemes for the [V4S9]4+ model and schematic figures of HOMO, HOMO1 and LUMO, LUMO1. Indicated are the numbers and symmetry types of the calculated MOs. Numbers in brackets indicate the percentage of the contributing valence orbitals: V 3d; S 3p, respectively.
below the V4 plane, and the µ4-S atom lies above the plane (V-S ) 2.382 Å). Each V atom has two bridged bromineatom-bonded cluster fragments in a layered polymeric structure (Figure 1b,c). Heat Capacity. Measurements of the low-temperature heat capacity of V4S9Br4 showed that the phase transition occurs at 15 K. The phase transition is complicated by a thermal anomaly. This anomaly is completed at 50 K (Figure 2). Integration of Cp/T indicates a total entropy change on the order of 23 ( 2 J mol-1 K-1. Magnetic Properties. Figure 3 shows the temperature and field dependence of the measured molar susceptibility, χmol. Above approximately 100 K, χmol follows Curie-Weiss behavior with an effective magnetic moment of µeff ) 1.77(3) per V atom (see inset) and a paramagnetic Curie temperature of 10(2) K. Below 100 K, χmol deviates markedly from the CurieWeiss law fitted to the high-temperature data, and the measurement reveals an AF-type ordering at 15 K that shifts to lower temperature with increasing field. Measurements of the zerofield-cooled (zfc) sample with increasing temperature exhibit a significantly different behavior, as the 15 K peak is nearly suppressed and followed by a field-independent transition at approximately 50 K, to become identical with the fc measurement above 70 K. This picture is corroborated by the appearance of substantial excess magnetic heat capacity above the anomaly around 15 K. The surprising difference between the fc and zfc susceptibilities needs further clarification. The EPR spectrum confirms the presence of an independent spin on each V atom: an almost symmetrical line with g0 ) 1.97 and ∆H)270 Oe and without any hyperfine structure is found; the exchange interaction is weak (J ≈ 10 cm-1). Computation. To study the electronic structure of V4S9Br4, the cluster fragment {V4S9}4+ was separated from the polymeric
TABLE 1: Optimized Distances (Å) of the Bond Lengths in the Cluster Core [V4(µ4-S)(µ2-S2)4]4+ bond V-V
V-(µ2-S)
V-(µ4-S)
S-S
calculation
2.882
2.328
2.075
experiment
3.005
2.359 2.401 2.382 2.392
2.373
2.023
network. The ground-state electronic structure (AF-type ordering, S ) 0) was calculated for this fragment. The obtained optimized interatomic distances in the cluster core {V4S9}4+ are in a good agreement with the experimental crystal structure data (Table 1), providing evidence of the validity of the fragmentation. The HOMO with 29e1 symmetry should be completely filled (Figure 4). The electronic structure of the {V4S9}4+ fragment is close to that of the diamagnetic molecular compound Ta4Se9I8.2 However, there is a difference in the filling of states due to the different oxidation states of the metal atoms. In V4S9Br4, the vanadium atoms have a formal mixed-valence state V3.5+ according to 6 d-electrons per cluster V4. Discussion The solution obtained from the calculations does not predict the paramagnetic properties of V4S9Br4. However, the experimental studies discover a presence of unpaired electrons in this compound. Thus, the paramagnetism of V4S9Br4 at higher temperatures should be explained by a different filling of the MOs than the calculated picture. It is reasonable to suggest that the electronic structure of V4S9Br4 is characterized by a highspin state, that is, the electronic system is in the excited state as is shown on the right of Figure 4. The reasons for the
V4S9Br4: A Novel High-Spin Vanadium Cluster Thiobromide
J. Phys. Chem. B, Vol. 109, No. 50, 2005 23807
transition of the given system in this state probably can be explained as following: Four near MOs (two 29e1, 26a1, and 12b1) have similar energies, and electrons prefer to occupy separate orbitals instead of couple on the 29e1 level at high temperatures. It should be noted that all four of these orbitals are formed predominantly by 3d states of vanadium atoms (∼78-98%). Therefore, the occupation of these orbitals by electrons assumes their strong localization on vanadium atoms; this is confirmed by magnetic and EPR measurements. The supposed electronic structure correlates with both the magnetic and thermal properties of V4S9Br4. The heat-capacity anomaly (from 4 to 50 K) might be connected to heat exiting of the electrons (Schottky anomaly) from the 29e1 level to the 26a1 and 12b1 levels. This electron exiting is finished at 50 K and is accompanied by the appearance of a magnetic moment in this system (Figures 2 and 3). Note that such a excited state with the partially filled doubly degenerate level 29e1 is unstable and predicts the Jahn-Teller effect. Probably, the phase transition at 15 K observed in V4S9Br4 can be explained by the Jahn-Teller effect. However, DFT calculations do not explain the experimental data completely. The 12b1-29e1 gap appears to be rather large to allow for a high-spin situation (∼0.3 eV). The reason for the similar discrepancy can be explained by the given quantumchemical cluster approach: An isolated cluster cannot describe the real polymeric solid V4S9Br4 adequately. It is known that MOs draw together in crystals and discrete levels (in the molecular cluster) transfer into bands (in the solid). We hope that, in the near future, the progress of quantum-chemical calculations will allow the suggested model to be defined more exactly. Nevertheless, this model provides qualitative explanations for all observed properties of the title compound. Concerning the properties of metal-metal bonding in V4S9Br4, we can note that the observed V-V distances (3.005 Å) are in a good agreement with the conception of electron-deficient metal-metal bonds. In this cluster, two-electron-four-center bonds (2e-4c) are realized: the 16b2 bonding MO contains d states of all four V atoms, which explains why all V-V bonds in the square cluster are equal. Very often, the V-V distances
in related chalcogenide cluster compounds with electron deficiency in the metal clusters are found to be rather longer than single two-electron V-V bonds. For example, in binuclear clusters {V2(µ2-S2)2}4+, the V-V single bond lengths are 2.838-2.850 Å;13 in the triangular cluster complex {V3(µ3-S)(µ2-S)3}+, where all metal atoms have a d2 electron configuration (3V3+), the availability of six electrons per V3 cluster gives three single V-V bonds with the distances 2.750-2.770 Å.14 In the triangular cluster {V3S4}3+ with mixed-valence vanadium atoms (V3+/2V4+), five electrons per V3 cluster result in longer distances of V-V bonds that vary in the range of 2.881-2.915 Å;15 and in the cuboidal cluster {V4(µ3-S)4}5+, with seven electrons per tetrahedral V4 cluster (3V3+/V4+), the V-V distances are 2.771-2.999 Å.16 References and Notes (1) Sokolov, M. N.; Fedin, V. P. Coord. Chem. ReV. 2004, 248, 925. (2) Sokolov, M. N.; Gushchin, A. L.; Virovets, A. V.; Peresypkina, E. V.; Kozlova, S. G.; Fedin, V. P. Inorg. Chem. 2004, 43, 7966. (3) Rambo, J. R.; Huffman, J. C.; Eisenstein, O.; Christou, G. J. Am. Chem. Soc. 1989, 111, 8027. (4) Kornienko, A.; Melman, J. H.; Hall, G.; Emge, T. J.; Brennan, J. G. Inorg. Chem. 2002, 41, 121. Kornienko, A. Y.; Emge, T. J.; Brennan, J. G. J. Am. Chem. Soc. 2001, 123, 11933. (5) Seela, J. L.; Huffmann, J. C.; Christou, G. J. Chem. Soc., Chem. Commun. 1987, 1258. Babaian-Kibala, E.; Cotton, F. A.; Kibala, P. A. Polyhedron 1990, 9, 1689. (6) Jongen, L.; Mudring, A.-V.; Mo¨ller, A.; Meyer, G. Angew. Chem., Int. Ed. 2004, 43, 3183. (7) Sheldrick, G. M. SHELXS97 and SHELXL97. Programs for the Refinement of Crystal Structures; University of Go¨ttingen: Germany, 1997. (8) Amsterdam Density Functional (ADF) Program, release 2002.02; Vrije Universteit: Amsterdam, The Netherlands, 2003. (9) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (10) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (11) Perdew, J. P. Phys. ReV. B 1986, 33, 8822. (12) Van Lenthe, E.; Ehlers, A. E.; Baerends, E. J. J. Chem. Phys. 1999, 110, 8943. (13) Sokolov, M.; Virovets, A.; Oeckler, O.; Simon, A.; Fedorov, V. Inorg. Chim. Acta 2002, 331, 25. (14) Dean, N. S.; Folting, K.; Lobkovsky, E.; Christou, G. Angew. Chem., Int. Ed. Engl. 1993, 32, 594. (15) Money, J. K.; Huffman, J. C.; Christou, G. IInorg. Chem. 1988, 27, 507. (16) Zhu, H.; Liu, Q.; Chen, C.; Wu, D. Inorg. Chim. Acta 2000, 306, 131.
