CRYSTAL GROWTH & DESIGN
Inorganic-Organic Hybrid Materials with Novel Framework Structures: Synthesis, Structure, and Magnetic Properties of [Ni(py)4]2V10O29 and [Ni2(py)5(H2O)3]V4O12 M. Ishaque Khan,*,† Renata C. Nome,† Sangita Deb,† James H. McNeely,† Brant Cage,† and Robert J. Doedens*,‡
2009 VOL. 9, NO. 6 2848–2852
Department of Biological, Chemical and Physical Sciences, Illinois Institute of Technology, Chicago, Illinois 60616, and Department of Chemistry, UniVersity of California, IrVine, California 92697 ReceiVed February 3, 2009; ReVised Manuscript ReceiVed March 25, 2009
ABSTRACT: Inorganic-organic hybrid materials [Ni(py)4]2V10O29 (1) and [Ni2(py)5(H2O)3]V4O12 (2) have been synthesized and characterized by infrared spectroscopy, thermogravimetry, magnetometry, and complete single crystal structure analysis. The crystal structures of 1 and 2 exhibit novel three-dimensional covalent networks. The framework structure in 1 contains polyoxometallate groupings of stoichiometry {V10O29} which are constructed from two cyclic {V4O12} units bound to centrosymmetric {V2O7} species. V-O-Ni bridges in the a- and b-directions join these units to two crystallographically independent trans-[Ni(py)4O2] octahedra. The nickel- and vanadium-based moieties occupy alternating channels along the c-direction. The structure of 2 consists of octahedral species [Ni(py)3(H2O)O2] and [Ni(py)2(H2O)2O2] which are linked into a three-dimensional covalent network by the sharing of oxygen atom vertices with tetrahedral {VO4} groups. Compounds 1 and 2 are thermally stable up to 200 and 175 °C, respectively. Both compounds show Curie-Weiss type magnetic behavior over the temperature range 1.9-300 K. The effective magnetic moment in both cases is 3.0 µB, revealing the presence of significant orbital contribution. Single ion magnetization as a function of magnetic field showed linear behavior for 1 and 2 over the range of 0-1 T. At a magnetic field of 9 T, 1 approaches saturation at 2 µB per Ni2+ ion, whereas 2 does not approach saturation well. Introduction Inorganic-organic hybrid materials are of current interest.1-9 A variety of these materials have been reported in recent years. Many exhibit interesting host-guest chemistry,10 separation,11-14 catalytic,15-17 and optical properties.18 Some of these systems have been also reported to show promising gas absorption properties for H2, CH4, and CO2.19-24 The properties of the hybrid materials can, in principle, be modified and improved by the amalgamation of the appropriate building blocks with desirable functionalities. However, despite the significant amount of ongoing research efforts in this field which has yielded an impressive array of materials, the rationalized approach for the design and synthesis of hybrid materials with desirable and predictable properties remains underdeveloped. In view of the rich redox chemistry associated with vanadium and the structural variety exhibited by oxovanadates,25,26 we have been interested in developing strategies for the synthesis of vanadium oxide-based hybrid materials. For this purpose, we have employed discrete oxovanadate motifs, vanadium oxide chains, and layered structures in combination with metal-organic complexes of the first row transition metals to prepare hybrid materials composed of mixed metal oxide {V/O/M/O} (M ) Mn, Fe, Co, Ni, Cu, etc.) framework incorporating organic functionalities. In this approach, metal-organic cationic complexes can serve both as cross-linkers for vanadium oxide building units and provide charge balance to generate neutral framework structures. This effort has led to the synthesis and characterization of a series of new materials.27-37 Here we report the synthesis of two new open-framework hybrid materials [Ni(py)4]2V10O29 (1) and [Ni2(py)5(H2O)3]V4O12 (2) (py ) pyridine) with a novel covalent network structure which have * Corresponding author: E-mail:
[email protected] (M.I.K.);
[email protected] (R.J.D.). † Illinois Institute of Technology. ‡ University of California.
been characterized by infrared spectroscopy, thermogravimetry, elemental analysis, magnetometry, and complete single crystal X-ray structure analyses. Experimental Section Materials and Methods. Reagent grade materials from commercial sources were used without further purification in the syntheses of 1-2. The hydrothermal reactions were performed in 23 mL Parr Teflonlined acid digestion bombs. Of the several preparation methods, only those which produced high purity and X-ray quality single crystals in decent yield are reported here. Infrared spectra (KBr pellet: 4000-400 cm-1) were recorded as KBr pellets using a Nexus 470 spectrometer from Thermonicolet and were evaluated by Thermonicolet’s OMNIC software. A Mettler Toledo TGA/SDTA 851E thermogravimetric analyzer (TGA) was used to obtain TGA curves in N2 atmosphere. The TGA results were analyzed by Mettler Toledo STAR 7.01 software.38 Approximately 10 mg samples were placed in a 70 µL alumina pan and heated over the temperature range of 25-1000 °C at a rate of 5 °C per minute in nitrogen atmosphere with a flow rate of 70 mL/min. The residues from TGA studies were examined by IR spectroscopy. The ac and dc magnetic measurements were obtained using a Quantum Design Physical Properties Measurement System (PPMS). Sample sizes were between 20-40 mg measured in a standard gelcap contained within a drinking straw. Samples were cooled to 1.9 K at zero field, a static field of 0.1 T (T) was set, and the temperature was swept from 1.9 to 300 K. No hysteresis in the field-cooled data was observed. The χac magnetization was done at 0.1 T with a 10 Oe alternating field. The estimates given for χD in Table 1 were not corrected for the sample holder assembly. The results of the χdc and χac were in agreement. The parameters and data for temperature dependent studies in Table 1 are from χac due to the superior signalto-noise. Crystal Structure Analyses. Crystallographic data were collected at low temperature on a Bruker SMART-1K CCD diffractometer. Crystals were immersed in hydrocarbon oil and mounted on a thin glass fiber in a cold nitrogen stream. Preliminary unit cell parameters and crystal orientation were determined by standard procedures.39 A full sphere of diffraction data was collected in frames separated by 0.3° increments in ω. The first 50 frames were remeasured at the end of
10.1021/cg9001237 CCC: $40.75 2009 American Chemical Society Published on Web 04/24/2009
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Table 1. Crystal Data and Structure Refinement Summary for 1 and 2
empirical formula fw space group cryst syst a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalcd µ (mm-1) T (K) F(000) GOF R1[I > 2σ(I)] wR2 (F2 all data)
1
2
C40H40N8Ni2O14.50V5 1236.92 P21/n monoclinic 11.306(11) 26.46(3) 16.820(17) 90 107.31(8) 90 4804(14) 4 1.710 1.770 175(2) 2492 1.191 0.0521 0.1373
C25H31N5Ni2O15V4 962.73 Pca2(1) orthorhombic 17.480(4) 11.444(3) 17.450(5) 90 90 90 3490(3) 4 1.832 2.158 169(2) 1936 1.063 0.0183 0.0459
data collection as a check on crystal decay. The data were processed with the program SAINT40 and corrected for absorption (and other effects) with SADABS.41 The structures were solved by direct methods and refined on F2 by full-matrix least-squares techniques. All calculations were performed by the use of the SHELXTL42 package. A summary of the crystal data and details of the intensity data collection and structure refinement are given in Table 1. Full details have been deposited as Suppporting Information. Preparation of [Ni(py)4]2V10O29 (1). After the initial preparation of compound 1 in modest yield,43 the synthetic method was optimized to prepare the compound in high yield monophasic form by the following procedure. A reaction mixture consisting of V2O5 (0.45 mmol), NiCl2 · 6H2O (0.25 mmol), H2O (555.6 mmol), and C5H5N (12.2 mmol) was placed in a 23 mL Teflon-lined Parr autoclave. After the mixture was stirred for 30 s, the reaction vessel was closed and heated in a furnace maintained at 145 °C for 6 days. Afterward, the furnace was turned off and the reaction vessel, left inside the furnace, was cooled slowly to room temperature over a period of 24 h. Dark green needlelike crystals of 1, which deposited at the bottom of the Teflon-bucket, were filtered from the yellow mother liquor. The crystals were dried in air at room temperature to give ∼159 mg of 1 (in 67.4% yield based on vanadium). Anal. calculated for C40H40N8O14.5Ni2V5: C, 38.84; H, 3.26; N, 9.05. Found: C, 38.72; H, 3.15; N, 8.96. Prominent FT-IR bands (KBr pellet 1600 - 400 cm-1): 1600 (vs), 1568 (m), 1481 (s), 1442 (vs), 1211 (vs), 1151 (s), 1073 (vs), 1040 (vs), 1010 (s), 980 (s), 961 (vs), 946 (m), 926 (m), 903 (m), 835 (s), 819 (vs), 759 (m), 705 (vs), 670 (s), 648 (m), 627 (m), 521 (s), 501 (w), 427 (vs). Preparation of [Ni2(py)5(H2O)3]V4O12 (2): Method I. A reaction mixture consisting of NH4VO3 (0.45 mmol), NiCl2.6H2O (0.25 mmol), 1,4-benzenedicarboxylic acid (0.50 mmol), H2O (555.6 mmol), and C5H5N (12.36 mmol) in a molar ratio of 1.8:1:2.32:2222.2:49.46 was placed in a 23 mL Teflon-lined Parr autoclave. The mixture was stirred for 30 s and heated in a Thermoline furnace maintained at 145 °C for 6 days. The reaction vessel was then taken out of the furnace and cooled to room temperature for 1 h. Green rodlike crystals mixed with a very small amount of light blue impurity, which was deposited at the bottom of the reaction vessel, were filtered from the light blue mother liquor. The crystals were washed thoroughly with water and dried in air at room temperature to give 13 mg of 2 (yield ∼ 12% based on vanadium). Anal. Calculated for C25H31N5O15Ni2V4 (962.73 g mol-1): C, 31.18; H, 3.24; N, 7.27. Found: C, 31.21; H, 2.73; N, 7.15. Prominent IR bands (KBr pellet, 4000 - 400 cm-1) 3612 (w), 3298 (b and s), 1603 (m), 1486 (w), 1478 (w), 1454 (vw), 1444 (s), 1418 (vw), 1236 (vw), 1218 (w), 1209 (w), 1156 (vw), 1141 (vw), 1071 (w), 1060 (w), 1043 (m), 1017 (vw), 1005 (vw), 942 (vs), 923 (vs), 896 (m), 882 (m), 840 (vs), 822 (vs), 762 (m), 698 (s), 642 (vs), 445 (vw), 438 (vw), 427 (w). Preparation of [Ni2(py)5(H2O)3]V4O12 (2): Method II. Compound 2 could be prepared in a better yield from a reaction mixture consisting of NH4VO3 (0.50 mmol), NiSO4 · 6H2O (0.25 mmol) or NiCl2 · 6H2O (0.25 mmol), H2O (555.55 mmol), and C5H5N (6.18 mmol) in a molar ratio of 2:1:2222.2:24.72 and following the rest of the procedure as
described in method I. The resulting rodlike green crystals, which were mixed with a small amount of yellow powdery impurity, were filtered from the yellow mother liquor, washed thoroughly with water, and dried in air at room temperature to give 21 mg of 2 (yield ∼ 20% based on vanadium).
Results and Discussion Synthesis and Characterization. Hydrothermal reaction of a mixture of V2O5, NiCl2 · 6H2O, 1,4-C6H4(COOH)2, H2O, and C5H5N at 145 °C for 6 days resulted in the synthesis of the covalent network hybrid material 1. Use of NH4VO3 in place of V2O5 in the reaction mixture led to the preparation of a different species, compound 2. Since 1,4-benzenedicarboxylate is not incorporated in the isolated final products 1 and 2, a series of reactions with varying stoichiometries were carried out in order to rationalize the synthesis and improve the yield and purity of the products. This effort led to the modified synthetic procedures for 1 and 2 described in the Experimental Section. These modified synthetic methods do not require the use of 1,4benzenedicarboxylic acid. And they resulted in a high yield (71.4%) synthesis of pure, crystalline 1 and an improved yield (∼20%) of 2. Compounds 1 and 2 are insoluble in cold water, ethanol, acetonitrile, DMF, DMSO, chloroform, and acetone. 1 is slightly soluble in methanol and hot water and 2 is slightly soluble in hot water. Crystals of 1 lose their shiny faces and change color from dark green to opaque yellow-green within a day upon exposure to air. The infrared spectra of 1 and 2 exhibit four bands between 1600 and 1400 cm-1, which are attributed to ring-stretching vibrations from the pyridine ligand. Additionally, the IR spectra exhibit strong sharp bands at 961 and 923 cm-1, a strong intensity band at 903 and 840 cm-1, and a strong band at 835 and 822 cm-1 for compounds 1 and 2, respectively. The sharp features below 1000 cm-1 are associated with the (VdO) stretching of the terminal vanadium oxide groups. Signatures of asymmetric and symmetric (V-O-V) stretching appear below 800 cm-1. The crystals of 1 and 2 adopt three-dimensional covalent network structures. Crystal data for the two compounds are given in Tables 1. A table of selected bond lengths and bond angles has been deposited as Supporting Information. The structure of 1 may be described as built from trans-[Ni(py)4O2] octahedra and VO4 tetrahedra linked by shared oxygen atom vertices. Both of the nickel-bound oxygen atoms participate in Ni-O-V bridges, and each vanadium atom is linked to one terminal oxygen atom and to three µ2-bridging oxygen atoms. All bond angles and bond lengths are in the expected range. The monoclinic unit cell contains four asymmetric units of formula C40H40N8O14.5Ni2V5. Figure 1 is a view of the asymmetric unit, including atom labels and displacement ellipsoids. The fractional oxygen atom (O10) is located on the inversion center at the cell origin. The unit cell and its contents, projected approximately down the c-axis, are shown in Figure 2. The polyoxometallate groupings, of unusual stoichiometry [V10O29]8-, in the structure of 1 are constructed from two cyclic V4O12 units bound to centrosymmetric V2O7 species, whose linear V-O-V linkage is approximately parallel to a. V-O-Ni bridges in the a- and b-directions join these units to the two crystallographically independent Ni-centered octahedra. The V-O-V bridges between the V2O7 and V4O12 species provide the third dimension of the network. As can be seen in the polyhedral representation of Figure 3, the nickel and vanadiumbased moieties occupy alternating channels along the c-direction.
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Figure 1. The asymmetric unit in the crystal structure of [Ni(py)4]2V10O29 (1) showing atom labels and 50% displacement ellipsoids.
Figure 2. A view of the unit cell contents projected approximately down the c-axis in the crystals of [Ni(py)4]2V10O29 (1) (Carbon atoms are left out for clarity. Atom color codes: yellow, V; green, Ni; blue, N; red, O).
Figure 3. A polyhederal representation of the extended crystal structure of [Ni(py)4]2V10O29 (1) (Carbon atoms are left out for clarity).
Compound 2 crystallizes in the orthorhombic space group Pca21 with four asymmetric units, of stoichiometry C25H31N5O15Ni2V4, per unit cell. Two octahedral Ni2+ species, [Ni(py)3(H2O)O2] and [Ni(py)2(H2O)2O2], are linked into a threedimensional covalent network by the sharing of oxygen atom vertices with tetrahedral VO4 groups. Each Ni atom participates in two Ni-O-V bridges, while the environment of each V atom
Khan et al.
Figure 4. View of the asymmetric unit, including atom labels and displacement ellipsoids in the crystal structure of [Ni2(py)5(H2O)3]V4O12 (2).
Figure 5. A view of the unit cell, projected approximately down the b-axis, in the crystal structure of [Ni2(py)5(H2O)3]V4O12 (2).
includes one terminal O atom, two V-O-V bridges, and one V-O-Ni bridge. Figure 4 is a view of the asymmetric unit, including atom labels and displacement ellipsoids. The unit cell and its contents, projected approximately down the b-axis, are shown in Figure 5. As is evident in this figure, the metal-oxo framework generates large channels along the b-direction, which are occupied by the pyridine and water ligands. Thermogravimetric analysis of 1 revealed its thermal stability up to 200 °C. The thermogram trace showed a 49.5% weight loss in the temperature range from 200 to 396 °C. This approximately corresponds to the removal of all of the pyridine molecules (calculated 51.2%). The TGA trace for 2 showed a two-step weight loss of 53.67%. The major weight loss of 46.67% in the temperature range 175-310 °C corresponds to the release of three water molecules and five molecules of pyridine (calculated 46.69%). The magnetic susceptibility χ (in units of m3/mol) is the sum of the diamagnetic contribution χd and paramagnetic contribution χP such that
χ≡
MZ C ) + χd HZ T-θ
(1)
where MZ is the longitudinal molar magnetization (in units of A · m2/mol), HZ is the applied magnetic field that defines the
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Figure 6. In-phase AC temperature sweep of [Ni(py)4]2V10O29 (1) (blue circles) and [Ni2(py)5(H2O)3]V4O12 (2) (red triangles) correcting for the diamagnetic moment. Table 2. Magnetic Measurement Data for [Ni(py)4]2V10O29 (1) and [Ni2(py)5(H2O)3]V4O12 (2) 1 3
C (m K/mol) C (emu · K/mol) χD (m3/mol) χD (emu/mol) Θ (Κ) g µeff (µB)
2 -5
5.6 × 10 4.5 -1.8 × 10-8 -0.0014 0.8 ( 0.8 2.1 3.0 ( 0.1
2.9 × 10-5 2.3 -2.4 × 10-8 -0.0019 0.1 ( 0.8 2.1 3.0 ( 0.1
z-axis, C is the Curie constant (in units of m3K/mol), T is the bath temperature, and θ is the Curie-Weiss temperature. We use an empirical method outlined previously to obtain the parameters by plotting the inverse total susceptibility plus a constant vs temperature until the best fit is found for a linear dependence on T.44 Figure 6 shows the in-phase component of the ac paramagnetic susceptibility χp over the temperature range of 1.9-300 K for the compounds 1 (blue circles) and 2 (red triangles). We use χac instead of the dc susceptibility due to the superior signal-to-noise ratio. The behavior for both compounds is Curie-Weiss in nature, as evidenced by the inset plot of χp-1 which is linear over the entire range. The inverse slopes provide Curie constants and the x-intercept provides θ; the positive value of θ for both compounds indicates small ferromagnetic interactions. Table 2 gives the parameters for both compounds in SI and CGS units. The Curie constant is related to the Lande g-tensor such that
C ) Ng2µB2J(J + 1)/3k
(2)
where N is the mols of paramagnetic centers, µB is the Bohr magneton, J is the sum of the spin S and orbital L angular momentum, and k is the Boltzmann constant. In the case of Ni2+ centers two unpaired electrons would be present giving S ) 1. Assuming significant quenching of L so that J ) S ) 1 we use g as a fit parameter and obtain g ) 2.1 for the compound 1. The effective magnetic moment is µeff ) 3.0 µB. Since the spin only value for two noninteracting electrons is 1.73 µB, significant orbital contribution is present. These results are in line with standard parameters of octahedral Ni2+ where the µeff is between 2.9 and 3.4 µB with corresponding g-values of about 2.25.45,46 The data for compound 2 are similar and provided in Table 2. In Figure 7 the single ion magnetization as a function of magnetic field is shown. The behavior for both is linear over the range of 0-1 T. At the highest field of 9 T, 1 approaches saturation at 2 µB per Ni ion, consistent with two unpaired
Figure 7. Magnetic saturation of samples [Ni(py)4]2V10O29 (1) and [Ni2(py)5(H2O)3]V4O12 (2) at 1.8 K.
electrons. However, 2 does not approach saturation well. The reason for this is currently unknown. The consistency of the data for Ni2+ centers is consistent with the charge balance requirement of the compounds where vanadium ions are in fully oxidized state. Supporting Information Available: Details of the crystallographic data for 1-2 (CCDC reference number: [Ni(py)4]4V10O29 (1), CCDC 703237; [Ni2(py)5(H2O)3]V4O12 (2) CCDC 703236 with the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge, CB21EZ, UK (
[email protected]); X-ray crystallographic files in CIF format for the structure determinations of [Ni(py)4]4V10O29 (1) and [Ni2(py)5(H2O)3]V4O12 (2); table of selected bond length and bond angles of the compounds. This information is available free of charge via the Internet at http://pubs.acs.org.
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(18) Houbertz, R.; Domann, G.; Cronauer, C.; Schmitt, A.; Martin, H.; Park, J. U.; Frohlich, L.; Buestrich, R.; Popall, M.; Streppel, U.; Dannberg, P.; Wachter, C.; Brauer, A. Thin Solid Films 2003, 442, 194–200. (19) Rowsell, J. L. C.; Spencer, E. C.; Eckert, J.; Howard, J. A. K.; Yaghi, O. M. Science 2005, 309, 1350–1354. (20) Chen, B.; Ockwig, N. W.; Millward, A. R.; Contreras, D. S.; Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 4745–4749. (21) Sudik, A. C.; Millward, A. R.; Ockwig, N. W.; Cote, A. P.; Kim, J.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 7110–7118. (22) Rowsell, J. L. C.; Millward, A. R.; Park, K. S.; Yaghi, O. M. J. Am. Chem. Soc. 2004, 126, 5666–5667. (23) Banerjee, R.; Phan, A.; Wang, B.; Knobler, C.; Furukawa, H.; O’Keeffe, M.; Yaghi, O. M. Science 2008, 319, 939–943. (24) Wong-Foy, A. G.; Matzger, A. J.; Yaghi, O. M. J. Am. Chem. Soc. 2006, 128, 3494–3495. (25) Koene, B. E.; Taylor, N. J.; Nazar, L. F. Angew. Chem., Int. Ed. 1999, 38, 2888–2891, and references therein. (26) Shan, Y. K.; Huang, R. H.; Huang, S. P. D. Angew. Chem., Int. Ed. 1999, 38, 1751–1754. (27) Khan, M. I.; Cevik, S.; Doedens, R. J. In Polyoxometalate Chemistry for Nano-Composite Design; Yamase, T.; Pope, M. T.; Eds.; Kluwer Academic: New York, 2002; pp 27-38. (28) Khan, M. I.; Yohannes, E.; Ayesh, S.; Doedens, R. J. J. Mol. Struct. 2003, 656, 45–53. (29) Khan, M. I.; Giri, S.; Ayesh, S.; Doedens, R. J. Inorg. Chem. Commun. 2004, 7, 721–724. (30) Khan, M. I.; Cevik, S.; Doedens, R. J. Chem. Commun. 2001, 1930, 1931. (31) Khan, M. I.; Tabussum, S.; Doedens, R. J. Chem. Commun. 2003, 532, 533. (32) Khan, M. I.; Tabussum, S.; Doedens, R. J.; Golub, V. O.; O’Connor, C. J. Inorg. Chem. Commun. 2004, 7, 54–57. (33) Khan, M. I.; Yohannes, E.; Doedens, R. J. Angew. Chem., Int. Ed. 1999, 38, 1292–1294. (34) Khan, M. I.; Yohannes, E.; Nome, R. C.; Ayesh, S.; Golub, V. O.; O’Connor, C. J.; Doedens, R. J. Chem. Mater. 2004, 16, 5273–5279.
Khan et al. (35) Khan, M. I.; Yohannes, E.; Golub, V. O.; O’Connor, C. J.; Doedens, R. J. Chem. Mater. 2007, 19, 4890–4895. (36) Khan, M. I.; Tabussum, S.; Doedens, R. J.; Golub, V. O.; O’Connor, C. J. Inorg. Chem. 2004, 43, 5850–5859. (37) Khan, M. I.; Deb, S.; Doedens, R. J. Inorg. Chem. Commun. 2006, 9, 25–28. (38) A Mettler Toledo TGA/SDTA 851E thermogravimetric analyzer (TGA) was used to obtain TGA curves. A 10 mg sample of 1-2 was placed in a 70 mL alumina pan and heated from 25 to 1000 °C at a rate of 5 °C/min in nitrogen atmosphere with a flow rate of 60-90 mL/min. (39) SMART, Version 4.210; Siemens Analytical X-Ray Systems: Madison, WI, 1997. (40) SAINT, Version 6.36A; Bruker AXS: Madison, WI, 2001. (41) Sheldrick, G. M. SADABS, Version 2.10; University of Go¨ttingen: Go¨ttingen, Germany, 2002. (42) Sheldrick, G. M. SHELXTL Version 6.12; Bruker AXS, Madison, WI, 2001. (43) A reaction mixture consisting of V2O5 (0.45 mmol), NiCl2 · 6H2O (0.25 mmol), 1,4-C6H4(COOH)2 (0.55 mmol), H2O (555.6 mmol), and C5H5N (12.2 mmol) was placed in a 23 mL Teflon-lined Parr autoclave. After the mixture was stirred for 30 seconds, the reaction vessel was closed and heated in a furnace maintained at 145 °C for 6 days. The furnace was then turned off, and the reaction vessel, which was left inside the furnace, was cooled slowly to room temperature over a period of 24 h. Dark green needle-like crystals of 1, which were mixed with a brown powdery impurity, were filtered from the yellow mother liquor. The crystals were separated from the powder mechanically and dried in air at room temperature to give ∼50 mg of 1 (yield ∼ 22.5% based on vanadium). (44) Dalal, N. S.; Millar, J. M.; Jagadeesh, M. S.; Seehra, M. S. J. Chem. Phys. 1981, 74, 1916–1923. (45) Cotton, F. A.; Wilkinson G. AdVanced Inorganic Chemistry, 5th ed.; John Wiley & Sons: New York, 1988; pp 743-755. (46) Carlin R. L.; van Duyneveldt A. J. Magnetic Properties of Transition Metal Compounds; Springer-Verlag, NY, 1977; pp 205-207.
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