One-Dimensional Molybdenum Thiochlorides and Their Use in High

Aug 18, 2014 - Synthesis of Molybdenum Sulfide Chalcogel. To prepare the chalcogel, (NH4)2MoS4 (78 mg, 0.3 mmol) and MoS2Cl3 (53.2 mg,. 0.2 mmol) ...
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One-Dimensional Molybdenum Thiochlorides and Their Use in High Surface Area MoSx Chalcogels Saiful M. Islam, Kota S. Subrahmanyam, Christos D. Malliakas, and Mercouri G. Kanatzidis* Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: New molybdenum thiochlorides, α-MoSCl3 and β-MoSCl3, have been synthesized by chemical vapor transport. A selenium analog, MoSeCl3, could also be prepared in the same way. The crystal structures consist of infinite chains stabilized by weak van der Waals forces. For the α-phase an infinite chain of ∞1[Mo2(S2)Cl4Cl4/2] runs along the crystallographic c axis, while for the β-phase 1 ∞ [Mo2(S2)Cl3Cl6/2] chain passes through the b-axis. MoSeCl3 is isostructural to α-MoSCl3. The formal charge on molybdenum in these thiochlorides have been assigned by X-ray photoelectron spectroscopy (XPS) to be +IV. β-MoSCl3 and already known Mo3S7Cl4 and MoS2Cl3 are weakly paramagnetic. Thermal decomposition of molybdenum thiochlorides leads to MoS2. Red MoS2Cl3 and Mo3S7Cl4 exhibit band gaps of ∼2.07 and ∼2.11 eV, while black α-MoSCl3, β-MoSCl3, and MoSeCl3 exhibit band gaps of ∼1.2, 1.24, and 1.18 eV, respectively. The new thiochlorides can serve as precursors in the synthesis of molybdenum−sulfide gels and aerogels. The MoSx chalcogels have a BET surface area of 353 m2/g and a band gap of 1.5 eV.

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INTRODUCTION Transition-metal thiohalides have been known for more than a century,1 and yet this class of compounds remains an insufficiently characterized group, as evidenced by the long known but inadequately characterized thiohalides, such as MSX3, (M = Nb, Mo, Ta, W; X = Cl, Br),2−6 ReSCl3,3,4 MSCl2 (M = Ti,7 Mn,8 Nb,9 Mo,3,10 W,3 Re11,12), MS2Cl2 (M = Mo,3,13 W,3,13 Re12), and MSCl (M = Ti,14 V,15 Mo16). Atherton et al., in 1979, summarized the synthesis and sparsely known chemistry of transition-metal chalcohalogenides.17 This subject remains a neglected area of research, possibly because of the difficulty in growing suitable crystals for X-ray single-crystal diffraction. In contrast to thiohalides, transition-metal sulfides are well-studied and became technologically interesting with their potential applications, such as hydrodesulfurization catalysis,18 photovoltaics,19 and electronics and optoelectronics.20 Most of these applications derive from the crystal structures, the metal−sulfur interactions, the stability of various oxidation states of transition metals, and the coordination environment of transition metals. The presence of halogens in the structures can be important in tuning and controlling the chemical and physical properties of these materials and it is also the source of chemical functionality by virtue of substitution metathesis chemistry. Therefore, a deeper fundamental understanding of the structure−property relationships in this class of materials is important. In this context, we began a systematic investigation of the system Mo/S/Cl. Among several previously reported molybdenum thiochlorides, only MoS2Cl321 and Mo3S7Cl421 were structurally characterized. Herein we report two new modifications of MoSCl3, which we name as α- and β-phases. In addition, we report a new selenochloride, MoSeCl3, which is isostructural to α-MoSCl3. The crystal structures of α- and β© 2014 American Chemical Society

MoSCl3 consist of one-dimensional chains. We employed X-ray photoelectron spectroscopy (XPS) to resolve the formal charge of molybdenum in various molybdenum thiochlorides. Furthermore, we discuss the magnetic and optical properties of these cluster compounds and show that these compounds are promising starting materials for the rapid solid state synthesis of MoS2. Finally, we show that the (MoxSy)n+ structural unit of the thiochlorides can be used effectively as a building block for the synthesis of novel high surface area porous molybdenum sulfide chalcogels.18b,22

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EXPERIMENTAL SECTION

Synthesis of Molybdenum Thiochlorides. The new molybdenum thiochlorides α- and β-MoSCl3, which are strongly fibrous in nature (Figure 1), were synthesized from a mixture of MoCl3 (Strem Chemicals, 99.5%) and elemental sulfur (5N Plus Inc.) in sealed silica tubes (l ≈ 17 cm, d ≈ 0.9 cm, V ≈ 11 cm3) by the chemical vapor transport (CVT) method.23 The silica tubes were flame-sealed under vacuum (10−4 mbar). α-MoSCl3 was obtained at the sink (lower temperature end) of the tube when a mixture of MoCl3 (202 mg, 1 mmol) and elemental S (32 mg, 1 mmol) was heated for 48 h at 400 → 200 °C. MoSeCl3, which is isotopic to α-MoSCl3, was synthesized in accordance to the experiment performed for α-MoSCl3. In contrast, an analogous experiment at 400 → 140 °C led to the formation of βMoSCl3 in the sink. An increase of sulfur content from 1 to 2.25 mmol against 1 mmol of MoCl3 led to the deposition of MoS2Cl3 in the sink end of the tube together with MoSCl3. However, the reaction of 1 mmol of MoCl3 and 1−1.5 mmol of S led to the synthesis of the new Mo3S4Cl4 in the source at 400 °C, which will be reported elsewhere. Single phases of Mo3S7Cl4 and MoS2Cl3 were obtained by CVT experiments when a mixture of 1 mmol of MoCl3 and 2.5 mmol of S Received: July 17, 2014 Revised: August 7, 2014 Published: August 18, 2014 5151

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Powder X-ray Diffraction. Powder X-ray diffraction data were collected on ground crystalline samples of each product with a flat sample geometry using a silicon-calibrated CPS 120 INEL powder Xray diffractometer (Cu Kα graphite-monochromatized radiation) operating at 40 kV and 20 mA equipped with a position-sensitive detector. Simulated patterns were generated using the CIF of each refined structure and the Visualizer program within FindIt. Scanning Electron Microscopy. Images and semiquantitative energy dispersive X-ray spectroscopy (EDS) analyses were obtained using a Hitachi S-3400 scanning electron microscope equipped with a PGT energy-dispersive X-ray analyzer. Spectra were collected using an accelerating voltage of 10−20 kV and a 90 s accumulation time. Single-Crystal X-ray Crystallography. Data collections were performed on a STOE IPDS II diffractometer using Mo Kα radiation (λ = 0.710 73 Å) operating at 50 kV and 40 mA at 293 K. Integration and numerical absorption corrections were performed on each structure using X-AREA, X-RED, and X-SHAPE. All structures were solved using direct methods and refined by full-matrix least-squares on F2 using the SHELXTL program package.24 A complete list of crystallographic information, data collections, structure refinement, atomic coordinates, and isotropic displacement parameters are given in Tables 1 and 2. In the crystal structures of MoSCl3, the distinction of bridging atoms as chlorine or sulfur becomes challenging by X-ray diffraction, as both the atoms exhibit very similar scatter power. EDS analysis quantitatively indicates the chemical composition as MoSCl3 (Mo = ∼20%, S = ∼20%, and Cl = ∼60%) for both the α- and βphase. EDS analysis of the known Mo3S7Cl4 and MoS2Cl3 corresponds to their correct chemical compositions, and they were used here as calibration standards. In addition to EDS, the Cl and S assignments are consistent with the oxidation state of Mo in the compounds, as determined by X-ray photoelectron spectroscopy (XPS). X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron studies were performed using a Thermo Scientific ESCALAB 250 Xi spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV) and operated at 300 W. Samples were analyzed under vacuum (P < 10−8 mbar), whereas survey scans and high-resolution scans were collected using pass energies of 50 and 25 eV, respectively. Binding energies were referred to the C 1s binding energy at 284.6 eV. A low-energy electron flood gun was employed for charge neutralization. Ion beam etching was performed to clean off some of the surface contamination. Prior to XPS measurements, the crystalline powders were pressed on copper foil, immediately mounted on stubs, and successively put into the entry-load chamber to pump. Prior to the measurement, in situ sputtering was performed to clean the crystallites’ surface. UV−Vis Spectroscopy. Diffuse reflectance spectra were collected in the range of 200−2500 nm using a Shimadzu UV-3101 PC doublebeam, double-monochromator spectrophotometer. The instrument was equipped with an integrating sphere and controlled by a personal computer. BaSO4 was used as a standard and set to 100% reflectance. Samples were prepared by placing ground crystalline products on a bed of BaSO4. Collected reflectance data were converted to absorbance according to the Kubelka−Munk equation: α/S = (1 − R)2 /(2R), where R is the reflectance and α and S are the absorption and scattering coefficients, respectively.25 The band gap was determined as the intersection point between the energy axis and the line extrapolated from the linear portion of the absorption edge in a (α/S)2 vs E plot. Transmission UV−vis spectra were recorded with a PerkinElmer LAMBDA 1050 spectrophotometer. The spectral data collected in the range of 200−800 nm with an average acquisition rate 266 nm/min. Thermogravimetric Analyses (TGA). Thermogravimetric analysis was performed on a Shimadzu TGA-50 thermogravimetric analyzer in aluminum boats under a N2 flow. The samples were heated (600 °C) and cooled at a rate of 10 °C/min. Magnetic Susceptibility. Magnetic measurements were performed from 5 K to room temperature using a Quantum Design Magnetic Properties Measurement System (MPMS) SQUID magnetometer with a magnetic field strength of 1 kOe. Temperaturedependent magnetic susceptibilities were measured in gelatin capsules

Figure 1. SEM images showing needlelike fibrous morphologies of (A) α-MoSCl3, (B) β-MoSCl3, and (C) MoSeCl3. was heated in a temperature gradient at 400 → 200 °C. The source region contains Mo3S7Cl4, whereas MoS2Cl3 forms in the sink. After the experiment, the ampule was removed from the furnace and its high temperature end cooled rapidly with tap water to condense the equilibrium gas phase in the source region. Thus, contamination of the crystals grown in the deposition region (at the lower temperature) was minimized. Attempts to obtain MoSCl3 by conventional isothermal heating of a stoichiometric ratio of MoCl3 and elemental S failed due to the formation of known MoS2Cl3 along with unreacted MoCl3. MoCl3 and S were handled inside a nitrogen-filled glovebox. Synthesis of Molybdenum Sulfide Chalcogel. To prepare the chalcogel, (NH4)2MoS4 (78 mg, 0.3 mmol) and MoS2Cl3 (53.2 mg, 0.2 mmol) were independently dissolved in 1 mL of formamide. The (NH4)2MoS4 solution was added slowly to the solution of MoS2Cl3. The resultant black solution was kept undisturbed for gelation. The solution turned viscous with time and a monolithic black gel formed in 1 week. In order to remove any soluble unreacted precursors and byproducts (NH4Cl), the monolithic gel was soaked in an ethanol and water mixture (1:1 v/v) followed by absolute ethanol for 3 days each. During the soaking period, soaked solvents were exchanged by fresh solvents every 12 h. Aerogels were then prepared by drying the wet gel supercritically using carbon dioxide. For this purpose, a Tousimis Autosamdri-815B Series supercritical fluid dryer was employed. In this process, the wet gel was placed in a custom-built metal basket and successively transferred into the supercritical drying chamber. The remnant alcohol in the gel was exchanged by liquid carbon dioxide (CO2) over a period of 6 h. Fresh CO2 was introduced into the chamber in every 20 min over the soaking period. The supercritical drying of the gel was attained at temperature of 42 °C and a pressure of 1400 psi. 5152

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Table 1. Details Concerning Data Collection and Structure Refinement for α-MoSCl3, MoSeCl3, and β-MoSCl3 parameter crystal system space group a/Å b/Å c/Å β/deg V/Å3 Z μ/mm−1 DX‑ray/g·cm−3 color crystal shape size/mm3 MW/g·mol−1 F(000)

temp/K scan range θ/deg

software measured reflectns independent reflectns parameters, Goof Rint R1a wR2b extinction coefficient weighting scheme Δρmax/Å−3 Δρmin/Å−3 a

α-MoSCl3

β-MoSCl3

MoSeCl3

crystal data monoclinic monoclinic C2/c (15) C2/c (15) 16.235(3) 16.381(3) 6.352(1) 6.540 (1) 12.077(2) 12.107(2) 131.10(3) 130.85 (3) 938.6(3) 981.1 (3) 8 8 4.74 11.53 3.32 3.81 black black needle needle 0.03 × 0.01 × 0.007 0.1 × 0.03 × 0.02 234.35 281.25 872 1016 data collection Mo Kα radiation, λ = 0.710 73 Å, graphite monochromator 293 293 θmax 29.99 θmax 29.99 −22 ≤ h ≤ 22 −21 ≤ h ≤ 15 −8 ≤ k ≤ 8 −8 ≤ k ≤ 0 −16 ≤ l ≤ 16 −15 ≤ l ≤ 15 structure refinement SHELX9724 SHELX9724 8800 2257 1367 1119 47; 1126 with |Fo| > 4σ(Fo), 1.12 46; 980 with Fo > 4σ(Fo), 0.99 residuals 0.080 0.043 0.045 0.016 0.092 0.035 0.000 347 A = 0.0381, B = 3.29 A = 0.0234, B = 0.0 residual electron density 1.03 (close to S1) 0.43 (close to Se1) 0.83 (close to Mo1) −0.66 (close to Mo1)

orthorhombic Pnma (62) 13.956(3) 11.141(2) 13.011(3) 2023.0(7) 16 4.40 3.08 black needle 0.02 × 0.01 × 0.01 234.35 1744.0

293 θmax 29.99 0 ≤ h ≤ 19 −15 ≤ k ≤ 15 0 ≤ l ≤ 18 SHELX9724 9375 3087 101; 2177 with Fo > 4σ(Fo), 0.83 0.029 0.025 0.038 0.001 512 A = 0.0110, B = 0.0 0.94 (close to S1) −0.88 (close to Mo1)

R1 = ∑||Fo| − |Fc||/|Fo|, Fo2 ≥ 2σ(Fo2). bwR2 = 1/[σ2(Fo2) + (AP)2 + BP]; P = (Fo2 + 2Fc2)/3.

containing polycrystalline powders (25−60 mg). No apparent difference between zero-field-cooled and field-cooled temperaturedependent data was observed. Infrared (IR) Spectroscopy. Infrared spectra of molybdenum thiochlorides were acquired using a Thermo Nicolet 6700 FTIR spectrometer. For the analysis the samples were dispersed in KBr pellets and the spectra were recorded at a resolution of 2 cm−1. An attempt to analyze the compounds with Raman failed as the compounds decomposed in a laser beam. Nitrogen Adsorption−Desorption Isotherms. This measurement was carried out to measure the surface area of the chalcogel using a Micromeritics Tristar II system at 77 K. Prior to the analysis, the chalcogel was degassed at 340 K under vacuum for 20 h to remove any adsorbed impurities. Surface area and distribution of pore sizes were estimated from Brunauer−Emmett−Teller (BET) model and Barrett− Joyner−Halenda (BJH) model, respectively.

that these phases can form from a mixture of MoCl3 and elemental S at a molar ratio from 1:1 to 1:1.25. We observed that stoichiometric mixtures of MoCl3 and S deposited crystals of either of the phases in the sink, whereas the source contained a new compound, Mo3S4Cl4. According to X-ray powder diffraction, Mo3S4Cl4 is isotopic to W3S4Cl4.26 Besides MoSCl3, increasing the sulfur content (from 1 mmol upward) against 1 mmol of MoCl3 enhanced the formation MoS2Cl3 in the sink region of the silica tube. The addition of sulfur oxidizes the molybdenum from Mo3+ (MoCl3) → Mo4+ (MoSCl3) → Mo5+ (MoS2Cl3). At MoCl3:S = 1.2.5, we observed that the CVT experiment at 400 → 200 °C allows the synthesis of singlephase products of Mo3S7Cl4 and MoS2Cl3 at the source and the sink, respectively. We caution, however, that while the syntheses of Mo3S7Cl4 and MoS2Cl3 are facile, the syntheses of α- and β-MoSCl3 require strict experimental conditions, as described in the Experimental Section. The α-MoSCl 3 , β-MoSCl 3, MoSeCl 3, Mo 3S 7 Cl4 , and MoS2Cl3 are stable in air and water at least for several days. The compounds α-MoSCl3, β-MoSCl3, and MoS2Cl3 are soluble in polar organic solvents, such as formamide and

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RESULTS AND DISCUSSION Synthesis of Molybdenum Thiochlorides. α- And βMoSCl3 were synthesized in a temperature gradient by the chemical vapor transport method.23 α-MoSCl3 is the hightemperature phase (200 °C) and β-MoSCl3 exhibits at lower temperature (140 °C). Our various CVT experiments indicate 5153

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Table 2. Atomic Coordinates and Isotropic Displacement Parameters for α- MoSCl3, MoSeCl3, and β-MoSCl3a atoms

a

x

Mo1 S1 Cl1 Cl2 Cl3

0.033 87(4) 0.077 84(12) 0.124 99(10) −0.061 92(11) 0.193 95(11)

Mo1 Se1 Cl1 Cl2 Cl3

0.966 68(2) 0.912 83(2) 0.062 84(5) 0.375 77(5) 0.690 53(5)

Mo1 Mo2 S1 S2 Cl1 Cl2 Cl3 Cl4 Cl5 Cl6 Cl7 Cl8 Cl9

0.449 08(2) 0.379 25(2) 0.279 12(5) 0.358 47(6) 0.384 74(8) 0.523 39(5) 0.422 63(9) 0.580 86(8) 0.403 18(9) 0.248 75(7) 0.485 10(7) 0.301 66(6) 0.569 19(6)

y α-MoSCl3 0.397 97(8) 0.124 6(2) 0.596 7(2) 0.283 3(2) 0.274 9(3) MoSeCl3 0.110 79(3) 0.392 49(4) 0.206 43(9) 0.419 81(10) 0.270 51(13) β-MoSCl3 0.097 98(3) −0.099 33(3) 0.046 23(8) −0.052 20(8) 1 /4 0.000 93(8) −1/4 1 /4 1 /4 −1/4 −1/4 −0.010 21(9) 0.009 43(9)

z

δiso

0.387 03(5) 0.292 06(15) 0.325 05(13) 0.472 53(14) 0.615 01(14)

0.007 76(15) 0.011 90(27) 0.010 33(25) 0.010 41(26) 0.014 70(28)

0.112 43(2) 0.203 55(3) 0.025 31(7) 0.174 81(6) 0.117 87(7)

0.016 35(7) 0.025 10(8) 0.022 7(13) 0.021 15(12) 0.033 27(16)

0.721 27(2) 0.812 58(2) 0.727 72(7) 0.631 24(7) 0.845 65(9) 0.865 49(6) 0.947 79(9) 0.744 95(10) 0.588 10(9) 0.787 06(10) 0.725 44(10) 0.955 25(6) 0.617 87(7)

0.023 51(6) 0.022 44(6) 0.030 49(18) 0.029 45(18) 0.029 26(24) 0.028 46(17) 0.030 82(25) 0.033 75(27) 0.031 56(26) 0.030 83(25) 0.027 99(23) 0.035 94(20) 0.035 59(20)

Estimated standard deviations are in parentheses.

Figure 2. Crystal structures of (A) α-MoSCl3 and (B) MoSeCl3.

dimensional polymers composed of discrete Mo2 dimers. Two molybdenum atoms in individual dimetric unit are joined together by a Q22− (Q = S, Se) groups in addition to one chloride ion for β-phase but two chloride ions for α-MoSCl3 and MoSeCl3. In α-MoSCl3, the discrete molybdenum dimer units are connected by two chlorine atoms, whereas three chlorine atoms connect the nearest dimers in the βmodification (Figures 2−4). As a result, the shortest Mo−Mo distance between the nearest neighbor dimers (along the chain) is 3.3346(9) Å for β-MoSCl3, which is significantly shorter than that of α-MoSCl3 at 3.779(1) Å. This can be explained by the fact that three chlorine atoms bridge the discrete Mo2 dimeric units instead of two, as found in the α-phase. The infinite one-dimensional (1D) chains in α-MoSCl3 and MoSeCl3 can be ascribed as ∞1[Mo2(Q2)Cl4Cl4/2] (Q = S, Se) running along the crystallographic the c-axis, whereas for the βphase the 1D chains of ∞1[Mo2(S2)Cl3Cl6/2] run parallel to the

dimethylformamide (DMF). We assume the decomposition products to be [Mo2S4Cl4]2+, [Mo2S2Cl4]2+, and [Mo2S2Cl3]3+, for MoS2Cl3, α-MoSCl3, and β-MoSCl3, respectively, besides the Cl− ions. The evidence for retaining the Mo−Mo cluster unit in the solution was obtained from UV−vis spectra. The solubility of these compounds suggests that solution processing and reaction chemistry will be possible with these compounds as precursors, as we demonstrate below with the synthesis of the novel MoSx chalcogel. In addition, α-MoSCl3, β-MoSCl3, and MoS2Cl3 show moderate solubility in acetone. Mo3S7Cl4 is insoluble in acetone, formamide, and DMF. Crystal Structures. α- And β-MoSCl3 crystallize in the monoclinic and orthorhombic crystal systems, respectively. Xray single crystal structure refinements revealed that both the modifications possess disulfide units (S22−) with S−S distances of ∼2.0 Å. The crystal structures of α-MoSCl3, β-MoSCl3, and MoSeCl3 (Figures 2 and 3) are best described as one5154

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Figure 3. Crystal structure of β-MoSCl3 (A) and the plane at z = 1/4 (B) and 3/4 (C).

b-axis. For the α- and β- phase, the Mo2 dimers are bridged with S22− and chloride ions, whereas dimers in MoS2Cl3 and trimers in Mo3S7Cl4 are connected only with sulfides ions. The crystal structures of these thiochlorides are stabilized by weak van der Waals forces between the 1D chains. The 1D character accounts for the intensely fibrous nature of the crystals, which makes them fragile and difficult to handle. In the α-form the chains stack exactly on the top of each other along the b-axis (Figure 2). In the β-phase, chains are stacked in accordance to an ABAB... pattern along the crystallographic c-axis (Figure 3). The short Mo−Mo bond lengths in the Mo2 cluster are found as d(Mo−Mo) = 2.716(1), 2.6636(7), and 2.7444(8) Å in α-MoIVSCl3, β-MoIVSCl3, and MoIVSeCl3, respectively (Figure 4). These distances are comparable to those in the trinuclear cluster MoIV3S7Cl4, which exhibits Mo−Mo bond distances of ∼2.745(3) Å. By comparison, the Mo2 dimer in MoVS2Cl3 exhibits significantly longer Mo−Mo bond lengths of 2.834(1) Å, consistent with the higher oxidation state of Mo in this compound. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) was performed to study the formal charge of molybdenum in α-MoSCl3, β-MoSCl3, Mo3S7Cl4, and MoS2Cl3. The latter two were used here as standards for comparison. The XPS spectra are shown in Figure 5. Peaks at 231.98 (228.76), 231.90 (228.69), 231.58 (228.39), and 232.38 (228.86) eV originate from Mo 3d3/2 (Mo 3d5/2) electronic states27 for α-MoSCl3, β-MoSCl3, Mo3S7Cl4, and MoS2Cl3, respectively. Mo3S7Cl4 and MoS2Cl3 possess a formal charge of +IV and +V on molybdenum, correspondingly. The binding

energy (BE) for Mo3d3/2 in MoVS2Cl3 is greater than that of MoIV3S7Cl4 by ∼0.8 eV, which is consistent with the higher formal charge on molybdenum in MoVS2Cl3. Conversely, BEs for α-MoSCl3 and β-MoSCl3 are almost identical and lie in between those of MoVS2Cl3 and MoIV3S7Cl4 (shaded region in the spectra). These BEs are consistent with an +IV oxidation states for Mo; however, the Mo atoms being bonded to more Cl atoms than S atoms in α- and β-MoSCl3 compared to those in MoIV3S7Cl4 suggests that the BE should move to slightly higher values. For example, the same shift to higher binding energies is observed for MoCl4 (∼1.5 eV) compared to MoS2.28 The synthesis and structural characterization of analogous MoIVSeCl3 to α-MoIVSCl3 further confirm the formal charge of molybdenum in these phases. Deconvolution of the S 2p peak indicates that it is the result of superposition of different spectra. Doublets in the range from ∼162.2 to 164.5 eV are assigned to S 2p3/2 and 2p1/2 photoelectron peaks for the S22− ion.29 For Mo3S7Cl4, the peak at 161.7 eV arises from S 2p3/2 states in the S2− unit, which is lower than that of α-MoSCl3, β-MoSCl3, and MoVS2Cl3. These results support the absence of monosulfide S2− ions in the structure and provides additional support for +IV oxidation states of molybdenum in α-MoSCl3 and βMoSCl3. The shoulder at ∼226 eV is the signature of S2s photoelectrons. Doublets in the region of 195−200 eV correspond to Cl 2p5/2 and 2p3/2.30 Magnetic Behavior. The magnetic susceptibility as a function of temperature for MoS7Cl4, β-MoSCl3, and MoS2Cl3 shows weak paramagnetic behavior (Figures 6 and S1, 5155

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Figure 5. X-ray photoelectron spectra of molybdenum thiochlorides. Hatched and solid lines indicate experimental and deconvoluted spectra, respectively.

Figure 6. Temperature-dependent magnetic susceptibility data for MoS7Cl4, showing paramagnetic behavior. Red solid lines represent the Curie−Weiss fit. The bump observed at ∼50 K is due to oxygen contamination in the SQUID.

within the Mo2 or Mo3 clusters, as well as strong spin−orbit coupling.31 For α-MoSCl3, the susceptibility is almost temperatureindependent with some anomalies at very low temperature (