Structure and Polymorphism in M (ethylenediamine) 3MoS4 (M= Mn

Nov 24, 2009 - Crystal Growth & Design .... The new structures M(ethylenediamine)3MoS4 (M = Mn, Co) have been determined and are isostructural to the ...
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DOI: 10.1021/cg901087x

Structure and Polymorphism in M(ethylenediamine)3MoS4 (M=Mn, Co, Ni) Hengfeng Tian, Hadley A. Iliff, Lee J. Moore, and Catherine M. Oertel*

2010, Vol. 10 669–675

Department of Chemistry and Biochemistry, Oberlin College, 119 Woodland Street, Oberlin, Ohio 44074 Received September 7, 2009; Revised Manuscript Received November 5, 2009

ABSTRACT: Polymorphism in hybrid inorganic-organic materials has not been explored as extensively as that in organic compounds, yet differences in solid-state structure can significantly affect the physical properties central to application of these materials. A new polymorph of Ni(en)3MoS4 (en = ethylenediamine), a hydrodesulfurization catalyst precursor, has been synthesized solvothermally and structurally characterized by single-crystal X-ray diffraction. The new structure (2) assumes the orthorhombic Pcab space group with a=14.020(5) A˚, b=14.821(7) A˚, and c=16.230(6) A˚. The structure of a polymorph that had been found previously (1) was redetermined at 100 K, confirming the orthorhombic Pna21 structure with a=15.916(13) A˚, b=7.610(3) A˚, and c=14.093(6) A˚. Solvothermal reaction conditions including temperature, solvent water content, and nickel source were important in controlling polymorph formation. Differential scanning calorimetry and solvent-mediated conversion studies were used to compare the stabilities of the two nickel-containing polymorphs. The system was characterized as enantiotropic, with 2 favored at ambient temperature and 1 favored at 120 °C. However, kinetic factors are influential in the intermediate temperature range, and conversion is kinetically hindered under certain conditions. The new structures Co(en)3MoS4 (3) and Mn(en)3MoS4 (4) were determined through single-crystal methods to be isostructural to 2. Compounds 1-4 were also characterized by elemental analysis, infrared spectroscopy, variable-temperature magnetic susceptibility measurements, and thermogravimetric analysis.

Introduction Crystal polymorphism in organic compounds continues to be the subject of a great deal of attention, motivated largely by the significant changes in physical properties that can arise from differences in conformation and packing in the solid state.1-3 Variation in properties including solubility, color, stability, and bioavailability can have significant consequences for compounds with applications as pharmaceutical agents and pigments.4,5 Studies of polymorphism in mixed inorganic-organic systems are considerably less common, even as the number of such hybrid compounds grows and they are developed for commercial applications. Some recent studies of both classical Werner complexes and hybrid inorganic-organic network compounds have shown the effects of phase on characteristics such as porosity, thermal stability, mechanical properties, and electronic structure.6-10 Because these properties are often central to the applications of hybrid materials, more extensive study of polymorphism in these compounds is needed. The tetrathiomolybdate anion, MoS42-, is a versatile building block that can act as a ligand or counteranion in mixed inorganic-organic structures. Because of the decomposition of MoS42- to MoS2 at elevated temperatures, (NH4)2MoS4 and other compounds containing the anion are frequently used as precursors to hydrodesulfurization (HDS) catalysts.11-17 A number of salts consisting of discrete tetrathiomolybdate and organic ammonium ions, associated with one another through NH 3 3 3 S hydrogen bonding, have been prepared.18-27 The ability of MoS42- to coordinate transition metal centers, particularly Cuþ, has also led to its use as a component in clusters and networks.28-33 Polymorphism has so far been observed in a very limited number of tetrathiomolybdate-based

compounds. Two tetragonal forms have been reported for a tetra-n-butylammonium salt of the MoS4(CuCl)42- cluster.34 For the related WS42- anion, polymorphs have been reported for the inorganic compounds KCuWS4 and Cu2WS4.35,36 A material with the stoichiometry Ni(en)3MoS4 (en = ethylenediamine) (1) has been characterized spectroscopically, magnetically, and as an HDS precursor.37-39 A solidstate structure for this compound was determined relatively recently using a single crystal prepared solvothermally in neat en.40 We report here on the synthesis and characterization of a new polymorph, β-Ni(en)3MoS4 (2), as well as two new isostructural compounds, Co(en)3MoS4 (3) and Mn(en)3MoS4 (4). The structure of R-Ni(en)3MoS4 (1) has been redetermined at low temperature to allow rigorous comparison with the new structures. For the nickel system, solvothermal reaction conditions leading to formation of each polymorph have been determined, and the stabilities of the compounds have been compared using thermal analysis and solvent-mediated conversion studies. Experimental Section

*Corresponding author. E-mail: [email protected]. Phone: (440) 775-8989. Fax: (440) 775-6682.

General. Manganese(II) bromide (98%), ammonium tetrathiomolybdate (99.97% metals basis), cobalt(II) bromide hydrate, nickel(II) bromide hydrate, and nickel(II) nitrate hexahydrate were purchased from Sigma Aldrich. Ethylenediamine (Sigma Aldrich), dimethylformamide (Sigma Aldrich), 200 proof ethanol (Aaper), diethyl ether (Fisher), and deionized water were used without further purification. Powder X-ray diffraction (PXRD) patterns were collected on a Philips 3040 MPD diffractometer in BraggBrentano geometry with Cu KR radiation. Thermogravimetric analysis (TGA) was conducted using a TA Instruments SDT 2960 simultaneous DSC-TGA. A ramp rate of 10 °C/min was used, and all experiments were performed under N2(g) at a flow rate of 10 mL/ min. Differential scanning calorimetry (DSC) was carried out in sealed pans using a TA Instruments 2010 DSC with a ramp rate of 10 °C/min. Infrared spectroscopy was performed on KBr pellets using a Thermo Matson Satellite FT-IR. Spectra were collected over a range of 1400-400 cm-1 with a resolution of 2 cm-1. Elemental

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Table 1. Crystallographic Data for Compounds 1-4 data

1

C6H24MoN6NiS4 orthorhombic Pna21 15.916(13) 7.610(3) 14.093(6) 1707.0(17) 1.802 2.320 2.56-28.28 99.1% 3224/33/172 1.048 R1 = 0.0344, wR2 = 0.0613 R1 = 0.0463, R indices (all data)a wR2 = 0.0662 1.114 and -0.595 largest diff. peak and hole (e 3 A˚-3) P P P P a R1 = ||Fo| - |Fc||/ |Fo|; wR2 = [ w(Fo2 - Fc2)2/ w(Fo2)2]1/2. empirical formula crystal system space group a (A˚) b (A˚) c (A˚) V (A˚3) F (g 3 cm-3) μ (mm-1) θ range (deg) for data collection completeness to 28.28° data/restraints/parameters GOF on F2 R indices [I > 2σ(I)]a

analysis was conducted by Robertson Microlit Laboratories, Madison, NJ. Syntheses. All reactions were carried out in 23 mL Teflon cups sealed in stainless steel autoclaves (Parr). In general, autoclaves were placed in at-temperature ovens. To optimize single-crystal growth, a programmed cooling rate was employed. Reaction autoclaves used for routine polycrystalline sample preparation were removed from hot ovens and allowed to cool naturally on the benchtop. After heating, reaction mixtures were filtered with fritted funnels, and isolated solids were washed with water, ethanol, and diethyl ether. All manipulations were performed in air. Synthesis of r-Ni(en)3MoS4 (1). Compound 1 was synthesized using a modification of the procedure published previously.40 Ni(NO3)2 3 6H2O (0.198 g, 0.681 mmol) and (NH4)2MoS4 (0.152 g, 0.584 mmol) were dissolved in 6 mL of ethylenediamine. The mixture was heated to 120 °C for 23 h, yielding 0.241 g (89.1%) of red, phase-pure material containing diffraction-quality single crystals. Elemental analysis found (calculated) for C6H24N6NiMoS4: C 15.72% (15.56%), H 5.32% (5.22%), N 18.14% (18.14%). IR (KBr, cm-1): 3284 (NH2 stretch, m), 3234 (NH2 stretch, m), 2935 (CH2 stretch, w), 2878 (CH2 stretch, w), 1591 (NH2 scissor, sh), 1561 (NH2 scissor, m), 1025 (C-N and C-C stretch, s), 657 (NH2 rock, m), 471 (Mo-S stretch, s). Synthesis of β-Ni(en)3MoS4 (2). Ni(NO3)2 3 6H2O (0.187 g, 0.643 mmol) and (NH4)2MoS4 (0.147 g, 0.565 mmol) were dissolved in a solution of 5 mL of ethylenediamine and 1 mL of H2O. The mixture was heated to 50 °C for 24 h, yielding 0.156 g (59.6%) of red, phasepure polycrystalline powder. Elemental analysis found (calculated) for C6H24N6NiMoS4: C 15.66% (15.56%), H 5.41% (5.22%), N 18.08% (18.14%). IR (KBr, cm-1): 3287 (NH2 stretch, m), 3236 (NH2 stretch, m), 2927 (CH2 stretch, w), 2875 (CH2 stretch, w), 1578 (NH2 scissor, sh), 1562 (NH2 scissor, m), 1027 (C-N and C-C stretch, s), 652 (NH2 rock, m), 472 (Mo-S stretch, s). Diffractionquality single crystals were grown using the same reactant quantities dissolved in a solution of 4.5 mL of ethylenediamine and 1 mL of H2O. This mixture was heated to 60 °C for 24 h and cooled to room temperature at 0.02 °C/min. Synthesis of Co(en)3MoS4 (3). CoBr2 3 H2O (0.097 g, 0.444 mmol) and (NH4)2MoS4 (0.120 g, 0.461 mmol) were dissolved in a solution of 5.5 mL of ethylenediamine and 0.5 mL of H2O. The reaction mixture was heated at 80 °C for 24 h, yielding 0.088 g (84.2%) of red, phasepure polycrystalline powder. Elemental analysis found (calculated) for C6H24N6CoMoS4: C 15.84% (15.55%), H 4.99% (5.23%), N 18.33% (18.13%). IR (KBr, cm-1): 3285 (NH2 stretch, m), 3233 (NH2 stretch, m), 2928 (CH2 stretch, w), 2874 (CH2 stretch, w), 1584 (NH2 scissor, sh), 1560 (NH2 scissor, m), 1014 (C-N and C-C stretch, s), 636 (NH2 rock, m), 468 (Mo-S stretch, s). Diffraction-quality single crystals were grown by dissolving CoBr2 3 H2O (0.148 g, 0.677 mmol) and (NH4)2MoS4 (0.116 g, 0.446 mmol) in 6 mL of ethylenediamine. The reaction mixture was heated at 110 °C for 24 h and cooled to room temperature at 0.1 °C/min.

2

3

4

C6H24MoN6NiS4 orthorhombic Pcab 14.020(5) 14.821(7) 16.230(6) 3373(2) 1.825 2.348 2.36-28.28 100.0% 4189/0/163 1.032 R1 = 0.0411, wR2 = 0.0918 R1 = 0.0718, wR2 = 0.1076 1.888 and -1.124

C6H24MoN6CoS4 orthorhombic Pcab 14.054(4) 14.856(4) 16.218(4) 3385.9(16) 1.818 2.207 2.36-28.28 100.0% 4207/0/163 1.018 R1 = 0.0297, wR2 = 0.0729 R1 = 0.0406, wR2 = 0.0799 1.677 and -1.037

C6H24MoN6MnS4 orthorhombic Pcab 14.305(8) 14.786(8) 16.348(6) 3458(3) 1.765 1.932 2.34-28.28 99.1% 4242/0/163 1.066 R1 = 0.0308, wR2 = 0.0794 R1 = 0.0402, wR2 = 0.0861 1.252 and -0.814

Synthesis of Mn(en)3MoS4 (4). MnCl2 3 4H2O (0.130 g, 0.657 mmol) and (NH4)2MoS4 (0.114 g, 0.437 mmol) were dissolved in 6 mL of ethylenediamine, and ∼0.2 mL of H2O was added. The mixture was heated to 100 °C for 24 h, yielding 0.185 g (81.0%) of red, phase-pure polycrystalline powder. Elemental analysis found (calculated) for C6H24N6MnMoS4: C 15.68% (15.68%), H 5.15% (5.23%), N 18.22% (18.30%). IR (KBr, cm-1): 3289 (NH2 stretch, m), 3233 (NH2 stretch, m), 2927 (CH2 stretch, w), 2874 (CH2 stretch, w), 1583 (NH2 scissor, w), 1557 (NH2 scissor, w), 1003 (C-N and C-C stretch, s), 616 (NH2 rock, m), 475 (Mo-S stretch, s). Diffraction-quality single crystals were grown using the same reagent quantities in a solution of 6 mL of ethylenediamine and ∼0.2 mL of DMF. The mixture was heated to 110 °C for 24 h and cooled to room temperature at 0.1 °C/min. Crystal Structure Determination. Single-crystal X-ray diffraction was performed on a Bruker AXS SMART APEX CCD system using Mo KR radiation at 100(2) K. Data were collected using SMART,41 and data integration and unit cell determination were performed using SAINTþ.42 A SADABS multiscan absorption correction was applied to each data set. The structures were solved using direct methods and refined by full-matrix least-squares on Fo2 using the SHELX-97 suite with WinGX.43,44 For the noncentrosymmetric structure of 1, the absolute configuration was determined using the Flack parameter value of 0.016(19). As in the structure previously published by Bensch and co-workers,40 there was disorder in one of the en ligands on the nickel complex. This was successfully modeled using split positions for C1 and C2. SIMU and DELU commands were applied to the disordered atoms. The relative occupancies of the split positions refined to 0.88:0.12, as compared with 0.70:0.30 for the room-temperature structure.40 The difference in occupancy could result from the lower temperature of the diffraction experiment but may also be sample-dependent. The minor component was refined isotropically, while anisotropic displacement parameters were assigned to all other non-hydrogen atoms. Hydrogen atoms were added using a riding model. For structures 2-4, anisotropic displacement parameters were assigned to all non-hydrogen atoms, and hydrogen atoms were added using a riding model. Crystal structure and refinement data for 1-4 are summarized in Table 1. CCDC entries 746406-746409 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.ac.uk/data_request/cif. Magnetic Characterization. Variable-temperature magnetic susceptibility data were collected using a Quantum Design MPMS 5XL SQUID magnetometer. The temperature range was 2-300 K, and the magnetic field was 1000 Oe. As expected based on the large separations between magnetic ions in the structures, compounds 1-4 behaved as paramagnets across the entire temperature range. Effective magnetic moments (μeff) showed reasonable agreement with predicted spin-only magnetic moments (μs) for all four compounds: μeff=2.91μB (μs=2.83μB) for 1, μeff=3.07μB (μs=2.83μB)

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for 2, μeff=4.08μB (μs=3.87μB) for 3, and μeff=5.84μB (μs=5.92μB) for 4. The compounds obey the Curie-Weiss law, with Weiss constants (θ) of 2.98 K for 1, -4.10 K for 2, 20.9 K for 3, and 2.68 K for 4.

Results and Discussion Syntheses. The synthesis of 1 was carried out using a modification of the method used previously.40 The reaction time was shortened to 24 h from 7 days, producing a phasepure product in a comparable yield. The published procedure included elemental sulfur in the reaction mixture along with the nickel and tetrathiomolybdate salts, and this was found to be unnecessary. The conditions for syntheses of 1 and 2 differed most significantly in reaction temperature, with 2 resulting from lower-temperature reactions. Heating reaction mixtures containing nickel salts and (NH4)2MoS4 to above 140 °C led to decomposition to a black, amorphous solid that could not be redissolved in any solvent tried. A similar decomposition occurred above 120 °C for the cobalt system and above 110 °C for manganese. We believe that these amorphous products result from direct interaction between the first-row transition metal center and a sulfur atom donated by the MoS42- anion. Such products also arise from reaction of first-row transition metal salts and tetrathiomolybdate salts in aqueous solution.45 Though their bulk stoichiometry is “MMoS4” (M = first-row transition metal), previous characterization has shown that they are actually a mixture of the binary sulfides MoS3 and MS. In neat en, the large excess of a chelating ligand prevents this direct metal-sulfur interaction. The difference in the ceiling temperature above which decomposition occurs for Ni, Co, and Mn likely results from differing lability of the coordination of these metals by en. For compounds 2-4, the optimized reaction conditions for preparing phase-pure bulk samples were slightly different than those used to grow diffraction-quality crystals. For 3 and 4, the elevated reaction temperatures needed to bring about single crystal growth led to slight decomposition of the product so that the single crystals were mixed with a small amount of the black material described above. Using lower temperatures prevented this decomposition and led to phasepure polycrystalline products. For each compound, a PXRD trace for the bulk sample was matched with the pattern calculated from the single crystal, demonstrating that the change in reaction conditions does not lead to a change in the product structure. Reaction conditions for the synthesis of the hypothetical R-polymorphs of 3 and 4 could not be identified. Based on observations of the nickel system, such structures would be expected to form at elevated reaction temperatures, and it may be that the lower decomposition temperatures of the Co and Mn reaction mixtures make inaccessible this higher temperature range. In the future, seeding experiments with crystals of 1 may be a means of obtaining cobalt- and manganese-based isostructures. Structures. Each structure consists of discrete M(en)32þ (M=Mn, Co, Ni) and MoS42- units stabilized by hydrogen bonding between amine donors and sulfide acceptors. The structure of 1 redetermined at 100(2) K was compared with the structure previously determined at ambient temperature. The space group and unit cell packing were unchanged, and the low-temperature structure showed a decrease in unit cell parameters on the order of 1%. As expected, most bond lengths were shorter in the structure determined at 100 K (Table 2).

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Table 2. Selected Bond Lengths for 1 Determined at 100 K and Previous Structure Determined at 293 K 100 K

293 Ka

N(1)-Ni(1) N(2)-Ni(1) N(3)-Ni(1) N(4)-Ni(1) N(5)-Ni(1) N(6)-Ni(1)

2.111(4) 2.090(4) 2.124(4) 2.126(4) 2.120(4) 2.116(4)

2.122(3) 2.099(3) 2.133(3) 2.135(2) 2.123(3) 2.133(3)

Mo(1)-S(1) Mo(1)-S(2) Mo(1)-S(3) Mo(1)-S(4)

2.1719(18) 2.1810(15) 2.1841(17) 2.1854(16)

2.1766(9) 2.1791(10) 2.1849(9) 2.1865(11)

a

Reference 40.

Figure 1. Comparison of ligand backbone configurations in Ni(en)32þ units in (a) compound 1 and (b) compound 2.

In both 1 and 2, Ni2þ is in an octahedral coordination environment that is distorted to accommodate formation of a five-membered chelate ring by each of the en ligands. The three N-Ni-N bond angles within the chelate rings are smaller than the ideal 90° (average 81.96° in 1 and 82.01° in 2) and angles between rings are larger than 90° (average 92.86° in both 1 and 2). The Ni-N bond lengths are shorter in 1 than in 2 and vary slightly within each polymorph: from 2.090(4) to 2.126(4) A˚ (average 2.115 A˚) in 1 and from 2.118(4) to 2.148(3) A˚ (average 2.133 A˚) in 2. In both structures, both Δ and Γ enantiomers of the chiral nickel complex are present. However, greater differences are observed when the backbone conformations of the en ligands are considered. While 1 includes Δδλλ and Λλδδ isomers, 2 is made up of Δδδδ and Λλλλ isomers. This difference may be seen by viewing the complexes along a pseudo-3-fold axis of the nickel octahedron. In this view (Figure 1), the carbon atoms on each ligand are nearly eclipsed in 2. In 1, C3 and C4, as well as C5 and C6, are nearly eclipsed, while the bond between C1 and C2 is perpendicular to the viewing direction. The minor component of 1 contributed by the disordered C1 and C2 atoms represents the Δλλλ and Λδδδ isomers. The structures of 1 and 2 may be further differentiated by considering the arrangement of the two optical isomers of Ni(en)32þ within each unit cell (Figure 2). Both cells contain equal numbers of the Δ and Γ enantiomers and are achiral. However, in the noncentrosymmetric, Pna21 structure of 1, rows of like enantiomers are observed along the b direction. In the centrosymmetric, Pcab structure of 2, rows of Ni(en)32þ units extend in a zigzag pattern in the a direction, but neighboring complexes are of opposite handedness. The structures of 1 and 2 are stabilized by NH 3 3 3 S hydrogen bonds. S 3 3 3 H contacts ranging from 2.60 to 2.97 A˚ were identified in 1, and distances ranging from 2.48 to 2.96 A˚ were identified in 2. An upper limit of 3.0 A˚, the sum of the sulfur and hydrogen van der Waals radii, was used in each analysis.

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Tian et al. Table 3. Comparison of Selected Bond Lengths for Isostructural Compounds 2-4 2 (M = Ni)

3 (M = Co)

4 (M = Mn)

M(1)-N(1) M(1)-N(2) M(1)-N(3) M(1)-N(4) M(1)-N(5) M(1)-N(6)

2.129(3) 2.144(4) 2.148(3) 2.118(4) 2.133(3) 2.126(3)

2.170(2) 2.188(2) 2.194(2) 2.162(2) 2.174(2) 2.168(2)

2.276(2) 2.283(2) 2.292(2) 2.261(2) 2.286(2) 2.283(2)

Mo(1)-S(1) Mo(1)-S(2) Mo(1)-S(3) Mo(1)-S(4)

2.1881(14) 2.1644(13) 2.1921(12) 2.1841(13)

2.1874(9) 2.1639(9) 2.1890(9) 2.1794(9)

2.1902(12) 2.1760(10) 2.1942(9) 2.1685(12)

Figure 3. TGA traces for compounds 1-4 showing one mass-loss step due to loss of en.

Figure 2. Comparison of unit cell packing in 1 (top) and 2 (bottom).

N-H 3 3 3 S angles range from 111.6° to 168.3° in 1 and from 133.5° to 170.8° in 2. The significant reduction of these angles from the idealized linear geometry is indicative of generally weak hydrogen bonding in these compounds. Though the minimum contact distance is somewhat lower in 2 than in 1, infrared N-H stretching frequencies for the two polymorphs differ by only 2-3 cm-1, indicating similar strengths of hydrogen-bonding interaction. Both 1 and 2 exhibit structures that are distinct from those of others comprising Ni(en)32þ units and tetrahedral anions. Ni(en)3SO4 and Ni(en)3MoO4 crystallize in the trigonal P3c1 space group and Ni(en)3CdI4 in the trigonal P3c1 space group.46-48 In each of these trigonal structures, none of which are isostructural to one another, there are stronger interactions among the ions than those found in 1 and 2. The NH 3 3 3 O hydrogen bonding present in the sulfate and molybdate structures is stronger than the NH 3 3 3 S bonding in 1 and 2, leading to closer contact between anions and cations. In the tetraiodocadmate structure, noncovalent Cd-I interactions link adjacent tetrahedra. The lack of such

strong interactions between Ni(en)32þ and MoS42- units may be one reason for the lack of a shared structure type with any of the trigonal structures. Compounds 3 and 4 are isostructural to 2. As would be expected based on the radii of the transition metals, the average metal-nitrogen bond length in the cobalt isomorph is longer than that in 1 and 2 at 2.176 A˚ (Table 3). The average metal-nitrogen bond length in 4 is 2.280 A˚, the longest among the three isostructures. Thermal Decomposition. Thermogravimetric analysis of all four compounds showed a single major mass loss step between 220 and 260 °C (Figure 3). The mass loss associated with this step was 42.3% for 1, 39.3% for 2, 41.0% for 3, and 39.2% for 4, in reasonable agreement with the percentage mass decreases expected for loss of all three en ligands (38.9% for 1-3 and 39.2% for 4). The onset temperatures for mass loss as well as the peak temperatures for the mass loss step followed the predicted stability of the compounds based on the Irving-Williams series and on the crystallographically determined metalnitrogen bond lengths. Within the nickel system, 1 decomposed at a slightly higher temperature than 2. A black solid isolated after heating each compound to 280 °C was amorphous to X-rays. The product remaining after ligand loss would be expected to have the bulk stoichiometry “MMoS4 ” (M=Mn, Co, Ni) and is likely a mixture of poorly crystalline MoS3 and MS. MoS3 has been reported to decompose to MoS2 and S at 310 °C,49 and the boiling point of elemental sulfur is 444 °C.50 The gradual mass loss for the compounds beyond the main ligand loss step is likely due to loss of sulfur. Solids isolated after heating 1 and 2 to between

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Figure 4. PXRD traces showing the role of reaction temperature in polymorph formation: (a) pattern calculated for 1 from singlecrystal structure, (b) product of reaction at 120 °C, (c) product of reaction at 100 °C, (d) product of reaction at 80 °C, (e) product of reaction at 60 °C, (f) product of reaction at ambient temperature, (g) pattern calculated for 2 from single-crystal structure. All reactions carried out in pure en.

500 and 600 °C could be identified by PXRD as poorly crystalline MoS2 and NiS. Differential scanning calorimetry was used to further characterize the decomposition of 1 and 2 and to look for conditions leading to interconversion between the polymorphs. Each DSC trace was dominated by a single endothermic feature corresponding to the loss of en ligands. The temperature of this feature was offset to higher temperature as compared with the ligand loss temperature observed by TGA. This observation is explained by the fact that the DSC experiments were carried out in hermetically sealed pans in which the ligand vapor pressure remained high, while the TGA experiments were carried out under flowing nitrogen. Smaller exothermic and endothermic features were observed at higher temperatures and are likely associated with decomposition of MoS3 and loss of S. Notably, no features were observed for either polymorph below the ligand loss temperature, indicating the absence of congruent melting or formation of a new, stable solid phase prior to deamination. The decomposition of 1 and 2 was also observed visually in a melting point apparatus. Darkening of the samples began at approximately 220 °C, and the absence of melting prior to decomposition was confirmed. Formation and Conversion of Polymorphs of Ni(en)3MoS4. As discussed above, reaction temperature was the most important parameter in controlling the selective synthesis of 1 and 2. In addition to the reactions at 60 and 120 °C used to prepare pure samples, reactions were carried out at temperatures ranging from ambient to 120 °C to establish the role of reaction temperature in identity of the prepared phase. In 24-h solvothermal reactions in pure en, temperatures up to 80 °C favored production of 2, while higher temperatures led to production of 1 (Figure 4). Addition of water to the reaction mixture also influenced polymorph formation (Figure 5). The polycrystalline product of a reaction in pure en at 140 °C was 1, whereas addition of 15% v/v H2O led to formation of 2. The choice of nickel source was also important in obtaining the pure polymorphic forms (Figure 6). Use of nickel nitrate allowed

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Figure 5. PXRD traces showing the role of solvent water content in polymorph formation: (a) pattern calculated for 1 from singlecrystal structure, (b) product of reaction at 140 °C in pure en, (c) product of reaction at 140 °C in en with 15% v/v H2O added, (d) pattern calculated for 2 from single-crystal structure.

Figure 6. PXRD traces showing the role of nickel counteranion in polymorph formation: (a) pattern calculated for 1 from single-crystal structure, (b) product of reaction at 60 °C using NiBr2 3 xH2O as nickel source, (c) product of reaction at 60 °C using Ni(NO3)2 3 6H2O as nickel source, (d) pattern calculated for 2 from single-crystal structure.

isolation of pure 2 in a reaction at 60 °C, whereas a reaction using nickel bromide as a starting material resulted in a mixture of the two polymorphs. As discussed above, evidence of solid-solid transformations between the polymorphs was not observed through DSC. It might be expected that such a transformation would be kinetically difficult because translation of the ions as well as changes in ligand conformation are necessary to go from one polymorph structure to the other. It is not uncommon for solid-solid phase conversion kinetics to be slow, and such changes are observed relatively rarely in inorganicorganic coordination polymers.51,52 In hybrid systems, organic ligand loss or decomposition usually occurs upon heating, and below the decomposition temperature, sufficient energy for breaking of ionic or coordination bonds may not be available. Both 1 and 2 decompose through loss of ligands without melting, precluding melting and recrystallization studies for characterization of this polymorphic

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Figure 7. Powder X-ray diffraction traces showing interconversion between 1 and 2: (a) mixed material obtained after 5 min of reaction time at ambient temperature; peaks belonging to 1 are denoted by (, and peaks belonging to 2 by /; (b) material obtained after 35 min of reaction time at ambient temperature shows partial conversion to 2; (c) material obtained after 24 h of reaction time at ambient temperature shows nearly complete conversion to 2; (d) mixed sample added to fresh en and heated at 120 °C for 24 h shows conversion to 1.

system. Solvent-mediated conversion was used to establish the relative stabilities of the polymorphs at ambient temperature and at elevated temperatures. Ethylenediamine was selected for these studies because it is the only solvent among those tried that provided sparing solubility without leading to formation of amorphous side products. Because of the limited solubility of Ni(en)3MoS4 in en, a precipitate forms immediately when nickel nitrate and ammonium tetrathiomolybdate are added to the solvent. A sample isolated by filtration after 5 min at ambient temperature was characterized by PXRD (Figure 7) and elemental analysis as a mixture of 1 and 2. Low peak intensity and broad peaks indicated small crystallite size. In the following discussion, this material is referred to as the “as-precipitated mixture”. Samples isolated after 35 min and 24 h of reaction between nickel nitrate and ammonium tetrathiomolybdate at ambient temperature showed increasing conversion to 2 accompanied by an increase in peak sharpness and intensity (Figure 7). The formation of 2 at the expense of 1 shows that 2 is the thermodynamically preferred form at ambient temperature, with 1 as a metastable form. A sample of the asprecipitated mixture was placed in fresh en in a sealed autoclave and heated to 120 °C for 24 h. This treatment led to nearly complete conversion to 1, indicating that this is the thermodynamically preferred form at this higher temperature. The as-precipitated mixture was also equilibrated in fresh en at 60, 80, and 100 °C in an attempt to identify the temperature at which the free energies of the two polymorphs cross one another. After 24 h at each of these temperatures, PXRD showed that the extent of crystallinity had increased, but a mixture of the polymorphs remained. One interpretation of this result could be that longer reaction times are needed to bring about full conversion at intermediate temperatures. However, there were no clear trends in the direction of polymorph conversion at these temperatures that would have suggested that more time was simply needed. It is likely that in the temperature region surrounding the transition

Tian et al.

temperature, crystallization from solution is affected by solubility and kinetic factors, leading to concomitant polymorphs.3,53 In other words, reaction mixtures in this regime become supersaturated with respect to both polymorphs, and both forms nucleate and grow. One is thermodynamically favored and the other metastable, but conversion to the preferred form can be slow or virtually nonexistent, leading to the persistence of two phases. The results of the solventmediated studies show that the system is enantiotropic, with 2 favored at room temperature and 1 at 120 °C, but that kinetic effects are influential in the intermediate temperature regime. While solvent-mediated conversion between the polymorphs was observed starting with the as-precipitated mixture, it did not occur when other samples were used as starting points. A pure sample of 2 added to fresh en and heated to 120 °C did not convert to 1, and a sample of 1 equilibrated in en at ambient temperature did not convert to 2. At least a small amount of the stable polymorph may need to be present in order to allow nucleation and conversion at the expense of the metastable polymorph. Surprisingly, conversion was also not observed starting from mixtures of 1 and 2 that were prepared by combining equal masses of the two pure solids. Qualitatively, there was no change in the ratio of the polymorphs when such a mixture was heated in en for 24 h at a temperature between 60 and 100 °C. This observation leads to the question of how this mixture and the as-precipitated mixture differ from one another. Peaks in the PXRD pattern of the as-precipitated mixture can be assigned to either 1 or 2, and elemental analysis of this material yielded C 15.91%, H 5.10%, N 18.42%, within 0.4% of the calculated value for each element. These results show that the mixture does not contain significant impurities or additional, as yet unidentified polymorphs. The primary difference appears to be the extent of crystallinity of the samples, as indicated by PXRD peak intensities. The relatively poor crystallinity and small particle size in the as-precipitated mixture could lead to an enhancement of the rate of conversion that is not afforded by purer, more crystalline materials. Conclusions Synthesis and structural characterization of a new polymorph of Ni(en)3MoS4 have been carried out, providing one of the first examples of tetrathiomolybdate-containing polymorphic compounds. The two polymorphs differ in the packing of the Ni(en)32þ and MoS42- units within the unit cells and, in a more detailed way, in the backbone conformations of the en ligands. New Co(en)3MoS4 and Mn(en)3MoS4 structures were also determined, showing that these compounds are isostructural to the new nickel-based polymorph. The ligand loss temperatures and metal-nitrogen bond lengths in the three isostructures follow the expected trend based on the metal radii and the Irving-Williams series. This work demonstrates the suitability of solvothermal synthesis for tetrathiomolybdate-containing materials and shows that the variety of conditions accessible through this method is effective in controlling polymorph formation. Thermal analysis and solvent-mediated conversion studies showed that solid-solid phase tranformations do not occur but that interconversion is possible in an ethylenediamine slurry under certain conditions. The system was characterized as enantiotropic, with β-Ni(en)3MoS4 favored at room temperature and R-Ni(en)3MoS4 favored at 120 °C. Kinetic

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effects are significant in the region surrounding the transition point and prevented determination of this temperature. Thermal characterization of both Ni(en)3MoS4 polymorphs showed that the two have very similar temperatures for decomposition through ligand loss and form the same binary sulfides upon further heating. The distinction between the polymorphs therefore likely does not affect application of this material as an HDS catalyst precursor. Acknowledgment. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research. The authors also thank Oberlin College for additional funding. We acknowledge Dr. Matthias Zeller of the STaRBURSTT CyberInstrumentation Consortium at Youngstown State University for collection of single-crystal diffraction data and for useful discussions. The diffractometer was funded by NSF Grant DUE-0087210, by Ohio Board of Regents Grant CAP-491, and by YSU. Magnetic characterization was carried out with the help of Professor Ram Seshadri at the University of California, Santa Barbara, with the support of a supplement to NSF CAREER grant DMR-0449354. This work made use of MRSEC facilities supported by NSF grant DMR-0520415. Supporting Information Available: Crystallographic information files (CIF) for compounds 1-4, ellipsoid plots for compounds 1-4, comparisons of calculated and experimental powder patterns for compounds 1-4, a normalized Curie-Weiss plot for compounds 1-4, and differential scanning calorimetry (DSC) traces for 1 and 2. This material is available free of charge via the Internet at http:// pubs.acs.org.

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