Synthesis and Characterization of Molybdenum (0) and Tungsten (0

Aug 28, 2014 - Aerosol-assisted chemical vapor deposition of WS 2 from the single source precursor WS(S 2 )(S 2 CNEt 2 ) 2. Nathaniel E. Richey , Chan...
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Synthesis and Characterization of Molybdenum(0) and Tungsten(0) Complexes of Tetramethylthiourea: Single-Source Precursors for MoS2 and WS2 John P. Shupp,† Adam S. Kinne,† Hadi D. Arman, and Zachary J. Tonzetich* Department of Chemistry, University of Texas at San Antonio, San Antonio, Texas 78249, United States S Supporting Information *

ABSTRACT: The molybdenum(0) and tungsten(0) carbonyl complexes, [M(CO)n(TMTU)6−n] (M = Mo and W; TMTU = tetramethylthiourea; n = 4 and 5), have been prepared by addition of TMTU to the appropriate carbonyl precursors. In the solid state, the compounds are monomeric and feature octahedral coordination geometries about the metal. In the case of [M(CO)4(TMTU)2], the TMTU ligands are bound in a cis fashion. Dissolution of cis-[M(CO)4(TMTU)2] produces an equilibrium mixture of cis-[M(CO)4(TMTU)2], free TMTU, and the putative dimeric species, cis,cis-[M2(CO)8(μ-TMTU)2]. Attempts to prepare the tricarbonyl complex, [Mo(CO)3(TMTU)3], by addition of TMTU to fac-[Mo(CO)3(η6-cycloheptatriene)] results in isolation of cis-[Mo(CO)4(TMTU)2], demonstrating that exchange of CO and TMTU ligands is facile in these compounds. Each of the compounds decomposes readily in the solid state above 110 °C, ultimately forming the tetravalent metal disulfide. Thermolysis reactions of [M(CO)n(TMTU)6−n] complexes at 300 °C under an argon atmosphere produces amorphous MS2, establishing the utility of these compounds as precursors to molybdenum and tungsten disulfide materials under relatively mild conditions.

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INTRODUCTION Molybdenum and tungsten disulfide (MoS2 and WS2) are presently of great interest due to their remarkable versatility in a range of applications from hydrogen evolution1−13 and nanodevices14−18 to their use as mechanical lubricants.19 These materials feature pronounced metal−sulfur covalency, leading to a unique layered structure of trigonal-prismatic metal centers with weak interlayer S−S bonding. Much like graphene, the metal disulfides display structure-dependent properties.20 As a result, a large body of work has been directed at controlling their synthesis and deposition, especially as nanoparticles and monolayers.21−27 A variety of procedures have been reported for the preparation of MoS2 and WS2, although surprisingly few have involved small molecule organometallics28 as single-source precursors.29−37 Organometallic compounds offer several advantages over traditional metal precursors such as halides, oxides, and thiometallates, including their high purity, solubility, and potential volatility.38−40 Of the organometallic singlesource precursors examined for group VI disulfides to date, only examples involving molybdenum have been reported. We were therefore motivated to determine if a family of organometallic compounds of both molybdenum and tungsten bearing a common sulfur-based ligand could be prepared and utilized as precursors to group VI disulfides. In order to design an appropriate precursor for Mo and W disulfides, we sought a sulfur-based ligand of low molecular © 2014 American Chemical Society

weight that would decompose to volatile products at moderate to low temperatures while still giving rise to isolable metal complexes. Accordingly, we elected to examine tetramethylthiourea (TMTU). There is precedent for the use of TMTU in the formation of metal sulfides,41,42 as well as in the synthesis of transition-metal complexes in a range of oxidation states.43−51 The versatility of the thiourea ligand versus other neutral sulfur donors (e.g., thioethers) stems in part from its ability to function as a good sigma donor by virtue of the thione/thiolate resonance forms displayed in Scheme 1. Given the documented utility of [M(CO)6] (M = Mo and W) to serve as precursors to MS2 in the presence of sulfur Scheme 1. Thione (I) and Thiolate (II) Resonance Forms of Substituted Thioureas

Received: May 27, 2014 Published: August 28, 2014 5238

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sources such as H2S,13,16 we judged that carbonyl complexes of Mo and W containing TMTU ligands would provide the ideal combination of ease of handling and low thermal stability desired in a single-source precursor. Examples of group VI carbonyl complexes featuring thiourea or imidazolethione ligands have been known for several decades.52−64 The vast majority of such compounds are of the form [M(CO)5L], although Cotton reported the synthesis of the tris-thiourea complex, fac-[Mo(CO)3{SC(NH2)2}3], in 1962.65 Despite this extensive history, no comprehensive single study has examined the chemistry of a family of group VI carbonyl complexes containing a common thiourea ligand. Herein, we describe the synthesis and reactivity of a series of [M(CO)n(TMTU)6−n] (M = Mo, W; n = 3−5) complexes and their decomposition behavior to yield MoS2 and WS2.

n-heptane resulted in substantial decomposition during the course of reflux. Much like [Mo(CO)6], the TMTU complex is soluble in common organic solvents, including alkanes. In contrast to [Mo(CO)6], however, compound 1 does not sublime readily, and all attempts to purify the compound by sublimation under laboratory conditions (∼0.1 mbar, 80−100 °C) were unsuccessful. The crystal structure of compound 1 is depicted in Figure 1. The complex displays octahedral geometry about Mo with TMTU coordinated through sulfur at a distance of 2.5894(6) Å. The Mo(1)−S(1)−C(1) bond angle of 113.11(8)° and S(1)−C(1) bond distance of 1.723(2) Å are consistent with previously reported TMTU complexes and demonstrate that the thiourea ligand is best described by thione resonance form I in Scheme 1.45,47,66 Also notable from Figure 1 is the weaker trans influence of the TMTU ligand in comparison to that of CO (c.f. Mo(1)−C(8) vs Mo(1)−C(6) distances). IR spectra of 1 in both solution and the solid state (film and KBr disc; see the Supporting Information) show three prominent peaks assigned to C−O stretching modes, νCO, as expected for a pentacarbonyl compound with approximate C4v symmetry (2A1 + E normal modes). As a consequence of the lower symmetry of the TMTU ligand, however, a weak peak can also be observed in film spectra at 1983 cm−1, consistent with the formally forbidden νCO mode of B1 symmetry. The tungsten analogue, [W(CO)5(TMTU)] (2), could not be accessed thermally and was instead prepared by photolysis of [W(CO)6] in THF in the presence of TMTU (eq 2). The solid-state structure of 2 is nearly isostructural with that of the Mo congener (Figure 2), and its IR spectroscopic features are likewise similar. We next turned our attention to bis-TMTU complexes, envisioning that such compounds would be ideal precursors to group VI metal disulfides since they possess the correct MS2 stoichiometry. Displacement reactions of TMTU with [M(CO)4(COD)] (M = Mo, W) proceeded very cleanly in diethyl

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RESULTS AND DISCUSSION Synthesis of Mo and W Complexes. Treatment of [Mo(CO)6] with 1 equiv of TMTU in refluxing hexanes afforded the desired mono-TMTU complex, [Mo(CO)5(TMTU)] (1, eq 1), as a yellow crystalline solid. This

compound has been reported previously, although its complete characterization and solid-state structure have never been published.60 Hexanes was found to be the optimal solvent for formation of the compound, as higher boiling solvents such as

Figure 1. Thermal ellipsoid (50%) drawings of the solid-state structures of [Mo(CO)5(TMTU)] (compound 1, left) and [W(CO)5(TMTU)] (compound 2, right). Selected bond distances (Å) and angles (deg) for 1: Mo(1)−S(1) = 2.5894(6); Mo(1)−C(8) = 1.973(3); Mo(1)−C(6) = 2.042(3); S(1)−C(1) = 1.723(2); C(1)−N(1) = 1.345(3); Mo(1)−S(1)−C(1) = 113.11(8). For 2: W(1)−S(1) = 2.582(2); W(1)−C(8) = 1.970(9); W(1)−C(6) = 2.070(10); S(1)−C(1) = 1.726(9); C(1)−N(1) = 1.355(9); W(1)−S(1)−C(1) = 113.0(3). 5239

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rather acute S−M−S angle of ca. 78° similar to that observed for the cationic manganese analogue, cis-[Mn(CO) 4 (TMTU)2](ClO4).67 The solution dynamics of the bis-TMTU complexes were found to be quite different from those of related [M(CO)4L2] compounds (e.g., L = PR3).68 In no instance was any evidence found for isomerization of the cis isomers to the sterically lessencumbered trans isomers. Instead, solution NMR and IR spectra of the cis-[M(CO)4(TMTU)2] complexes demonstrated formation of a mixture of two metal carbonyl species immediately after dissolution (see the Supporting Information, Figures S9 and S10). Similar IR spectra were also obtained when THF solutions of cis-[M(CO)4(TMTU)2] were evaporated to give thin films. The spectroscopic features of the new metal carbonyl species of both molybdenum and tungsten are inconsistent with trans isomers. We therefore propose that compounds 3 and 4 undergo dissociation of TMTU in solution to afford a mixture of cis-[M(CO)4(TMTU)2], free TMTU, and the putative TMTU-bridged dimer, cis,cis-[M2(CO)8(μTMTU)2] (eq 4). We favor the more symmetric structure depicted in eq 4 over one featuring a μ-S,N-bridged TMTU ligand, such as that reported by Alper and Chan for [Fe2(CO)6(μ-TMTU)],69 based on the observation of only two resonances for the CO groups by 13C NMR spectroscopy. Comparable symmetric bridging modes of substituted thiorea ligands have been observed previously in manganese and copper chemistry.70,71 In order to gain more insight into the nature of the solution dynamics of the bis-TMTU complexes, we attempted the synthesis of the bimetallic molybdenum species by addition of 1 equiv of TMTU to cis-[Mo(CO)4(COD)] (5, eq 5). The reaction afforded a dark-brown material that displayed IR and 13 C NMR features consistent with several of those observed for compound 3 when dissolved in solution. Furthermore, an IR titration of TMTU into a benzene-d6 solution of the brown

ether, affording both Mo (3) and W (4) complexes in ca. 70− 80% isolated yields as yellow crystalline solids (eq 3). The bisTMTU complexes were found to be less soluble than the mono-TMTU complexes but still demonstrated sufficient solubility in diethyl ether to facilitate recrystallization.

The solid-state structures of both bis-TMTU complexes are very similar and display a cis disposition of the TMTU ligands about octahedral metal centers (Figure 2). This coordination geometry is corroborated by IR spectra of solid samples (KBr pellet) that display four νCO peaks as expected for approximate C2v symmetry. The metric parameters about the thiourea ligands are in line with those of the mono-TMTU complexes. Interestingly, however, the bis-TMTU complexes feature a

Figure 2. Thermal ellipsoid (50%) drawings of the solid-state structures of cis-[Mo(CO)4(TMTU)2] (compound 3, left) and cis[W(CO)4(TMTU)2] (compound 4, right). Selected bond distances (Å) and angles (deg) for 3: Mo(1)−S(1) = 2.6250(9); Mo(1)−S(2) = 2.5897(8); Mo(1)−C(11) = 1.951(3); Mo(1)−C(12) = 2.029(3); S(1)−C(1) = 1.715(3); S(1)−Mo(1)−S(2) = 78.78(2); Mo(1)−S(1)−C(1) = 114.52(9); Mo(1)−S(2)−C(6) = 117.16(10). For 4: W(1)−S(1) = 2.6093(19); W(1)−S(2) = 2.5683(18); W(1)−C(12) = 1.920(8); W(1)− C(11) = 2.040(8); S(1)−C(1) = 1.717(7); S(1)−W(1)−S(2) = 78.26(6); W(1)−S(1)−C(1) = 114.5(3); W(1)−S(2)−C(6) = 116.9(3). 5240

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parent ion with an m/z ratio consistent with [K(CH3OH){Mo2S2(CO)6}]−, confirming the presence of adventitious sodium in the original samples. Although we have no information concerning the structures of the species formed during ESI, one attractive proposal is the symmetric μ2-S2 species shown in Scheme 2.72−74 Scheme 2. Proposed Structure of Bimetallic Species Identified by ESI-MS

We also attempted the preparation of a molybdenum trisTMTU complex from the precursor fac-[Mo(CO)3(η6-cycloC7H8)]. Reactions of 3 equiv of TMTU with the olefinic precursor in Et2O were unsuccessful, producing a mixture of carbonyl species, as judged by IR spectroscopy. However, treatment of fac-[Mo(CO)3(η6-cyclo-C7H8)] with a substoichiometric amount of TMTU (1.5 equiv) in toluene led to precipitation of a new complex in low yield that we assign as the tricarbonyl species, 6, based on the presence of IR resonances at 1893 and 1768 cm−1 (see Figure S15, Supporting Information). Unfortunately, compound 6 was found to rapidly transform into a mixture of different Mo species upon dissolution and therefore could not be purified (Scheme 3). The majority of the mixture demonstrated IR resonances consistent with 3 and 5. Such a result is not surprising given the facile dissociation of TMTU in solutions of the bis-TMTU complex (vide supra). Because of the difficulty in preparing the molybdenum tris-TMTU complex, the tungsten analogue was not pursued. Decomposition Behavior. We next investigated the ability of the TMTU complexes to serve as precursors to group VI metal disulfides. Compounds 1−4 were all found to demonstrate an onset of decomposition by 120 °C, as judged by melting point determinations. Only compound 1 demonstrated a clean melting point prior to decomposition. TGA curves for the TMTU complexes appear in the Supporting Information (Figures S16−S19) and demonstrate an onset of mass loss for each compound beginning at or below ca. 100 °C. Decomposition is precipitous between 100 and 170 °C, accounting for the majority of mass loss. An additional slower event is observed for each compound above 170 °C corresponding to a further 8−12% loss in mass. The complex nature of the TGA curves suggests that several intermediate species may be involved in the thermal decomposition of the TMTU complexes. However, at this time, we have no information concerning the possible structures for such species. Final mass percentages for each material are in the range expected for formation of MS2. In the case of the mono-TMTU complexes, the metal−sulfur stoichiometry limits a maximal yield of 50% for the corresponding disulfide. We propose that the remainder of the metal is lost by evaporation in the form of [M(CO)6], consistent with reported sublimation data for both molybdenum and tungsten hexacarbonyl.75 Results from the thermogravimetric analyses intimated that the desired metal disulfides could be accessed through straightforward thermolysis reactions of the solid TMTU

material resulted in the disappearance of νCO modes assigned to the dimeric species and enhancement of νCO modes consistent with 3 (see the Supporting Information, Figure S11). Unfortunately, efforts to purify compound 5 by recrystallization were unsuccessful, prohibiting definitive characterization of this species. NMR experiments aimed at determining the equilibrium constant for the reaction in eq 4 (M = Mo) were pursued but proved difficult to interpret due to the coincidence of resonances for both the dimeric species and free TMTU, and the thermal instability of the compound(s) in solution above 40 °C. Nonetheless, variable-temperature 1H NMR spectra in benzene-d6 over a small temperature range (10−40 °C) were used to calculate thermodynamic parameters of ΔH° = −8.7 ± 1.0 kcal/mol and ΔS° = −28 ± 3 e.u. (see the Supporting Information). The negative entropy is somewhat surprising given that the chemical reaction in eq 4 results in increased molecularity. However, formation of the dimeric species may dominate the entropic term, leading to an overall negative ΔS°. Further demonstrating the complex solution-phase behavior of the thiourea complexes are the results of electrospray ionization (ESI) mass spectrometry measurements. Mass spectra obtained from methanol solutions of both [M(CO)5(TMTU)] and cis-[M(CO)4(TMTU)2] were found to be identical for a given metal (see Figures S13 and S14, Supporting Information). The m/z ratios for the parent ions are consistent with anions of the formula [Na{M2S2(CO)6}]−, indicating that, even under the relatively mild conditions of ESI, decomposition of the TMTU complexes is taking place in solution to afford bimetallic species. In the case of Mo, addition of potassium bromide to the methanol solution produced a 5241

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Scheme 3. Attempted Synthesis of fac-[Mo(CO)3(TMTU)3] (6)

Figure 3. Representative SEM images and EDS spectra of MoS2 (left) and WS2 (right) obtained from thermolysis reactions of TMTU complexes. Asterisks denote peaks due to light atoms (C, N, O) resulting from the adhesive used to fix the material to the aluminum stage.

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complexes. Indeed, heating each of the compounds in a tube furnace at 300 °C for 1 h under an argon flow produced iridescent-black flaky solids. IR spectra of the solids displayed no resonances between 500 and 3200 cm−1, indicating complete loss of all functional groups containing light atoms. Energy-dispersive X-ray spectra (EDS) obtained from scanning electron microscopy (SEM) images of the materials confirmed the identity of the solids as the desired metal disulfides (Figure 3). Similar spectra were observed for disulfides formed from both the mono- (1 and 2) and bis-TMTU (3 and 4) precursor complexes. The microstructure of the materials produced in this fashion is amorphous, as judged by the lack of any observable diffraction peaks in the powder XRD spectra out to a 2θ value of 120°.

CONCLUSIONS

In conclusion, we have described the synthesis and characterization of a family of zerovalent Mo and W carbonyl complexes containing the TMTU ligand. Each of the complexes is octahedral in the solid state, featuring κ1-S coordination of the TMTU ligand. The bis-TMTU complexes, cis-[M(CO)4(TMTU)2] (M = Mo and W), undergo rapid dissociation of the thiourea ligand in solution, giving rise to putative dimeric species of the formula [M2(CO)8(μ-TMTU)2]. Consistent with facile loss of TMTU is the observed instability of the trisTMTU complex, fac-[Mo(CO)3(TMTU)3]. Despite the complex solution behavior, each of the TMTU complexes can be isolated in good yield from readily available starting materials and behaves as a convenient single-source precursor for the preparation of MoS2 and WS2. Future studies will more 5242

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ray diffraction were grown by slow cooling of a saturated diethyl ether solution at −30 °C. mp: 104−106 °C. IR (KBr disc, cm−1): 3018 (vw), 2962 (w), 2935 (w), 2862 (vw), 2806 (vw), 2064 (s, νCO), 1915 (vs, νCO), 1871 (vs, νCO), 1543 (m), 1493 (m), 1464 (m), 1435 (m), 1387 (m), 1373 (m), 1267 (w), 1214 (vw), 1153 (m), 1110 (m), 1097 (m), 1057 (w), 883 (w), 664 (vw), 607 (m), 593 (s), 537 (w). 1H NMR: δ 2.37 (s, 24 NMe2). 13C{1H} NMR: δ 214.34, 206.81, 201.41, 43.06. Anal. Calcd for C10H12MoN2O5S: C, 32.62; H, 3.28; N, 7.61. Found: C, 32.68; H, 3.49; N, 7.73. [W(CO)5(TMTU)], 2. A flask was charged with 0.1185 g (0.337 mmol) of [W(CO)6], 0.0450 g (0.340 mmol) of TMTU, and 4 mL of THF. The resulting colorless solution was transferred to a photoreactor and vigorously sparged with nitrogen for 1 min before turning on the lamp. The photolysis was allowed to proceed for 3 h, after which time the resulting yellow solution was transferred to a new flask. All volatiles were removed in vacuo, and the remaining yellow residue was washed with pentane to afford 0.1207 g (78%) of a yellow microcrystalline solid. In addition to the desired TMTU complex, the microcrystalline solid was found to contain small quantities of [W(CO)6] and free TMTU, as judged by IR and NMR spectroscopy. Flash chromatography of the crude product on alumina (100% toluene as eluent) afforded analytically pure material after reprecipitation from diethyl ether. Crystals suitable for X-ray diffraction were grown by slow cooling of a saturated diethyl ether solution at −30 °C. mp: 118−120 °C (dec). IR (KBr disc, cm−1): 3018 (vw), 2963 (w), 2933 (w), 2861 (vw), 2062 (s, νCO), 1982 (m, νCO), 1911 (vs, νCO), 1887 (vs, νCO), 1867 (vs, νCO), 1548 (m), 1495 (m), 1466 (m), 1445 (m), 1413 (vw), 1406 (vw), 1389 (m), 1380 (m), 1267 (w), 1213 (vw), 1156 (m), 1112 (m), 1058 (w), 880 (w), 668 (w), 608 (m), 593 (s), 539 (vw). 1 H NMR: δ 2.29 (s, 24 NMe2). 13C{1H} NMR: δ 201.85, 199.92 (JCW = 128), 191.50 (JCW = 126), 43.05. Anal. Calcd for C10H12N2O5SW· 0.5Et2O: C, 29.22; H, 3.47; N, 5.68. Found: C, 29.61; H, 3.12; N, 5.73. cis-[Mo(CO)4(TMTU)2], 3. A flask was charged with 0.535 g (1.69 mmol) of cis-[Mo(CO)4(COD)], 0.444 g (3.36 mmol) of TMTU, and 20 mL of diethyl ether. The resulting yellow solution was allowed to stir at ambient temperature (25 °C) under nitrogen for 16 h, during which time a yellow precipitate formed. The precipitate was isolated by filtration and washed with pentane to afford 0.659 g (83%) of the desired complex as a yellow microcrystalline solid. Crystals suitable for X-ray diffraction were grown by slow cooling of a saturated diethyl ether solution at −30 °C. mp: 102−104 °C (dec). IR (KBr disc, cm−1): 3003 (vw), 2960 (w), 2928 (w), 2862 (vw), 2800 (vw), 2005 (m, νCO), 1886 (s, νCO), 1867 (s, νCO), 1797 (s, νCO), 1538 (m), 1493 (w), 1467 (w), 1442 (w), 1374 (m), 1264 (w), 1213 (vw), 1152 (w), 1114 (m), 1097 (m), 1059 (w), 882 (w), 668 (vw), 657 (vw), 639 (vw), 599 (vw), 583 (w), 560 (vw). 13C{1H} NMR: δ 222.83 (monomer), 222.37 (dimer), 220.76 (monomer), 210.48 (dimer), 208.89 (dimer), 206.81 (monomer), 195.84 (TMTU), 43.50, 43.43, 43.09. Anal. Calcd for C14H24MoN4O4S2: C, 35.59; H, 5.12; N, 11.86. Found: C, 35.32; H, 4.79; N, 11.78. cis-[W(CO)4(TMTU)2], 4. A flask was charged with 0.300 g (0.743 mmol) of cis-[W(CO)4(COD)], 0.191 g (1.44 mmol) of TMTU, and 20 mL of diethyl ether. The reaction was allowed to stir for 18 h at ambient temperature, during which time it became yellow and a precipitate formed. The precipitate was collected by filtration and washed with pentane to afford 0.273 g (67%) of the desired complex as a yellow microcrystalline solid. Crystals suitable for X-ray diffraction were grown by room-temperature vapor diffusion of pentane into a saturated 2-MeTHF solution of the complex. At concentrations required for 13C NMR spectroscopy, only the dimer and free TMTU were detected in solution. mp: 111−112 °C (dec). IR (KBr disc, cm−1): 3007 (vw), 2963 (w), 2927 (w), 2858 (vw), 2802 (vw), 1999 (m, νCO), 1878 (s, νCO), 1860 (s, νCO), 1790 (s, νCO), 1540 (m), 1493 (w), 1467 (w), 1440 (w), 1375 (m), 1261 (w), 1211 (vw), 1155 (w), 1114 (m), 1100 (m), 1058 (w), 983 (vw), 944 (vw), 881 (w), 656 (vw), 622 (w), 581 (w), 567 (w). 13C{1H} NMR: δ 212.9 (dimer), 207.4 (dimer), 199.9 (dimer), 196.1 (TMTU), 43.4, 43.0. Anal. Calcd for C14H24N4O4S2W: C, 30.01; H, 4.32; N, 10.00. Found: C, 29.98; H, 4.20; N, 10.10.

fully explore the properties of the MoS2 and WS2 produced from these precursors and further examine the utility of these compounds in alternative preparative techniques such as chemical vapor deposition.

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EXPERIMENTAL SECTION

General Comments. Manipulations of air- and moisture-sensitive materials were performed under an atmosphere of purified nitrogen gas using standard Schlenk techniques or in a Vacuum Atmospheres glovebox. Tetrahydrofuran, diethyl ether, pentane, and toluene were purified by sparging with argon and passage through two columns packed with 4 Å molecular sieves. Benzene-d6 was dried over sodium ketyl and vacuum-distilled prior to use. Photolysis experiments were conducted in an Ace Glass quartz micro photochemical reaction assembly using a low-pressure mercury lamp (5.5 W, 254 nm line). Melting points were obtained in glass capillaries sealed with a plug of silicone grease and are reported as uncorrected values. 1H and 13 C{1H} NMR spectra were recorded in benzene-d6 on a Varian spectrometer operating at 500 MHz (1H). Chemical shift values were referenced to the residual 1H (δ 7.16 ppm) or 13C (δ 128.39 ppm) resonance of benzene-d6. Coupling constants (J) are reported in units of Hz. FT-IR spectra were recorded with a ThermoNicolet iS 10 spectrophotometer. Solid samples were pressed into KBr discs or evaporated from THF as thin films on a NaCl plate. Solution spectra were obtained using a Specac Omni cell with KBr windows and 0.1 mm spacers. IR absorption intensities are denoted as very weak (vw), weak (w), medium (m), strong (s), or very strong (vs). Electron microscopy measurements were conducted on a JEOL JSM-6510 scanning electron microscope. Energy-dispersive X-ray spectra were obtained at an accelerating voltage of 15.0 kV. Thermal gravimetric analyses (TGA) were performed in platinum pans under a nitrogen flow (20 mL/min) with a heating rate of 3 °C/min using a Shimadzu TGA-50 thermogravimetric analyzer. Elemental analyses were performed by Atlantic Microlab of Norcross, GA, and ESI-MS was performed at the UTSA mass spectrometry core. Materials. cis-[Mo(CO)4(COD)],76 cis-[W(CO)4(COD)],77 and fac-[Mo(CO)3(η6-cycloheptatrienyl)]78 were prepared according to published procedures or slight modifications thereof. Updated synthetic procedures for cis-[Mo(CO)4(COD)] and cis-[W(CO)4(COD)] appear in the Supporting Information. [Mo(CO)6], [W(CO)6], and 1,1,3,3,-tetramethylthiourea (TMTU) were purchased from commercial suppliers and used as received. X-ray Data Collection and Structure Solution Refinement. Crystals suitable for X-ray diffraction were mounted in Paratone oil onto a glass fiber and frozen under a nitrogen cold stream maintained by an X-Stream low-temperature apparatus. The data were collected at 98(2) K using a Rigaku AFC12/Saturn 724 CCD fitted with Mo Kα radiation (λ = 0.71073 Å). Data collection and unit cell refinement were performed using the Crystal Clear software.79 Data processing and absorption correction, giving minimum and maximum transmission factors, were accomplished with Crystal Clear and ABSCOR, respectively.80 All structures were solved by direct methods and refined on F2 using full-matrix, least-squares techniques with SHELXL-97.81,82 All non-hydrogen atoms were refined with anisotropic displacement parameters. All carbon-bound hydrogen atom positions were determined by geometry and refined by a riding model. Crystallographic data and refinement parameters appear in the Supporting Information. [Mo(CO)5(TMTU)], 1. This complex has been reported previously but never fully characterized.60 Our synthetic procedure is as follows: A flask was charged with 2.00 g (7.58 mmol) of [Mo(CO)6], 1.00 g (7.56 mmol) of TMTU, and 100 mL of deoxygenated hexanes. The mixture was heated to reflux under an atmosphere of nitrogen and allowed to stir for 2 h, during which time the mixture became a homogeneous canary yellow solution. The solution was then allowed to cool to ca. 40 °C, at which point a yellow solid precipitated. The solid was collected by filtration under nitrogen while still warm and washed twice with 30 mL of pentane to afford 1.94 g (70%) of the desired complex as fluffy yellow microcrystals. Crystals suitable for X5243

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cis,cis-[Mo2(CO)8(μ-TMTU)2], 5. A flask was charged with 0.147 g (0.465 mmol) of cis-[Mo(CO)4(COD)], 0.061 g (0.458 mmol) of TMTU, and 10 mL of diethyl ether. The mixture was allowed to stir at ambient temperature for 1 h, during which time a precipitate formed. The mixture was placed in the freezer at −30 °C for 18 h to encourage further precipitation. The yellow supernatant solution was decanted, and the remaining solid was washed with pentane and dried in vacuo to afford 0.043 g (27%) of a red-brown powder. Attempts to crystallize the material were unsuccessful, and therefore, no elemental analysis was obtained. 13C NMR spectroscopy of the material indicated the presence of several species (possibly syn and anti isomers), one of which concurred with the data recorded for cis-[Mo(CO)4(TMTU)2] above. mp: 104 °C (dec). IR (KBr disc, cm−1): 3013 (vw), 2926 (w), 2856 (vw), 2802 (vw), 2064 (w, νCO), 1983 (w, νCO), 1915 (vs, νCO), 1891 (s, νCO), 1872 (s, νCO), 1550 (m), 1500 (w), 1466 (m), 1442 (w), 1376 (m), 1268 (w), 1209 (vw), 1154 (w), 1138 (w), 1112 (m), 1096 (m), 1057 (w), 968 (m), 878 (w), 669 (m), 608 (w), 592 (m), 540 (vw). 13C{1H} NMR: δ 222.3, 210.5, 206.8, 43.3. Thermolysis Reactions. Solid-state thermolysis reactions of the thiourea complexes were conducted in a tube furnace at 300 °C under an argon flow for 1 h. In a typical experiment, 60−80 mg of the complex was loaded into a pyrex cup before being placed in the furnace. Recovery yields for the resulting disulfides were between 80− 90% for the bis-TMTU precursors, and between 30−40% for the mono-TMTU precursors.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental procedures, figures, spectra, tabulated crystallographic data, and crystallographic information (CIF) files. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.J.T.). Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by UTSA and by a grant from the Welch Foundation (AX-1772). The authors acknowledge Dr. David Black for assistance with MS measurements, Mr. James Boyd for assistance with SEM measurements, and Dr. Walter Gray for assistance with powder XRD measurements. Mass spectrometer facilities at UTSA are supported by a grant from the NIMHD (G12MD007591).

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