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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Molecular Routes to Two-Dimensional Metal Dichalcogenides MX2 (M = Mo, W; X = S, Se) Veronika Brune, Corinna Hegemann, and Sanjay Mathur* Institute of Inorganic Chemistry, University of Cologne, Greinstraße 6, D-50939 Cologne, Germany

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S Supporting Information *

ABSTRACT: New synthetic access to two-dimensional transition metal dichalcogenides (TMDCs) is highly desired to exploit their extraordinary semiconducting and optoelectronic properties for practical applications. We introduce here an entirely novel class of molecular precursors, [MIV(XEtN(Me)EtX)2] (MIV = MoIV, WIV, X = S, Se), enabling chemical vapor deposition of TMDC thin films. Molybdenum and tungsten complexes of dianionic tridentate pincer-type ligands (HXEt)2NR (R = methyl, tert-butyl, phenyl) produced air-stable monomeric dichalcogenide complexes, [W(SEtN(Me)EtS)2] and [Mo(SEtN(Me)EtS)2], displaying W and Mo centers in an octahedral environment of 4 S and 2 N donor atoms. Owing to their remarkable volatility and clean thermal decomposition, both Mo and W complexes, when used in the chemical vapor deposition (CVD) process, produced crystalline MoS2 and WS2 thin films. X-ray diffraction analysis and atomic-scale imaging confirmed the phase purity and 2D structural characteristics of MoS2 and WS2 films. The new set of ligands presented in this work open ups convenient access to a scalable and precursor-based synthesis of 2D transition metal dichalcogenides.

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tin19−21 as well as arsenic, antimony, and bismuth.22 This class of ligands offers a remarkable potential of electronic stabilization against the oxidation (MIV → MV, MVI) of molecular complexes through optimal steric shielding and coordinative saturation around the metal center. These intriguing results triggered us to explore further the coordination behavior of one of these ligands with tungsten ([W(SEtN(Me)EtS)2]) and molybdenum ([Mo(SEtN(Me)EtS)2]). In comparison to graphene, transition metal disulfides of general formula MS2 (M = transition metal, e.g., Mo, W) represent stable two-dimensional layered materials with unique physical properties that are interesting for optical,23 optoelectronic,24 and electronic25−27 devices as well as for their technological importance in lubrication,28,29 gas sensing,6 and biological30−32 applications. The direct band gap of these monolayered materials is located in the visible light or nearinfrared range, making them promising in efficient solar energy conversion33,34 and water splitting reactions35−40 and as semiconductor materials41−43 in field-effect transistors.44−47 For the growth of wafer-scale, high-quality TMDC thin films, atomic layer deposition (ALD) and chemical vapor deposition (CVD) are the methods of choice that require volatile and reactive precursors.1,48−53 In this context, chalcogen−metal complexes offer several advantages over traditional metal precursors such as halides, oxides, and thiometallates mainly because of their high purity, solubility, and potential

INTRODUCTION Transition metal−chalcogen complexes containing group VI metal ions are attractive starting materials for two-dimensional transition metal dichalcogenides (TMDCs) that offer great potential for low-power electronics, especially because of their favorable 2D structures and promising electro-optical properties.1−6 TMDCs have been discussed as next-generation electronic and photonic materials because of their direct band gaps, atomically flat surface topography, and lack of dangling bonds as well as structural-network-supported weak van der Waals forces among X−M−X layers (M = Mo, W; X = S, Se) and strong covalent bonds within the layers.7,8 As graphene analogues, TMDCs and their outstanding properties both in bulk and exfoliated forms have been known for several years.9−12 However, the band structure of TMDCs changes dramatically from single layer to multilayer samples, involving a transition from a direct gap in monolayer samples to an indirect gap for multilayer or bulk samples13 calling for new synthesis approaches allowing the controlled fabrication of TMDC materials. In this context, a new class of group VI compounds with preformed metal−chalcogen bonds are attractive precursors for obtaining TMDCs via the lowtemperature decomposition of molecular complexes. However, this approach has not been sufficiently explored because of the lack of suitable precursor chemistry and examples acting as a proof of concept.14,15 To obtain new Mo/W−chalcogen compounds, we turned our attention to the dianionic, tridentate (HXEt)2NR (X = S, Se; R = methyl, tert-butyl, phenyl) ligands that have been used in the coordination chemistry of technetium,16 rhenium,16,17 mercury,18 and © XXXX American Chemical Society

Received: April 13, 2019

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DOI: 10.1021/acs.inorgchem.9b01084 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Scheme 1. Ligand Libary of XNX-Type Ligandsa and Their Reaction to Corresponding Metal Complexes [W(SEtN(Me)EtS)2] 1a and [Mo(SEtN(Me)EtS)2] 2a

a

X = sulfur, selenium, R = -methyl, -tert-butyl, -phenyl.

Scheme 2. General Synthesis Route for (HXEt)2NR (X = S, Se) Ligands

volatility.54 To the best of our knowledge, only a few synthesized single-source precursors (SSPs) for MoS2 and WS2 are known so far.15,55−57 The major advantage of using molecular sources in the CVD and ALD processes is their high purity that prevents TMDC film contamination and the presence of preformed metal−chalcogen bonds in the complex that enables crystallization at lower temperatures.54 In addition, SSPs are often beneficial in preventing the application of hazardous coreactants, such as H2S, and in avoiding premature reactions prior to deposition onto the hot substrates.58 In this article, we report on the optimized synthesis and characterization of the (HXEt)2NR (X = S, Se; R = Me, tBu, Ph)-type ligands as well as their coordination to group IV transition elements Mo and W. The coordination of dianionic ligand 2,2′-(methylazanediyl)bis(ethane-1-thiolate) ({(SEt)2NMe}2−) to metal centers and investigations pertaining to their structural behavior in the solid state and in solution established the chemical identity of [W(SEtN(Me)EtS)2] and [Mo(SEtN(Me)EtS) 2 ]. The results obtained for their thermogravimetric studies and application in CVD experiments confirmed their superior potential as precursors to

desired transition metal disulfide materials such as MoS2 and WS2.

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RESULTS AND DISCUSSION The motivation for this work was to generate single-phase, nanostructured MoS2 and WS2 films via chemical vapor deposition. To prevent the ring closure reactions, well-known among sulfur-containing ligands, new ligand systems were employed to check the polymerization and enable the volatility of metal complexes. The reaction from the corresponding amides of molybdenum and tungsten and the SNS-type ligand delivered stable precursors that were tested in CVD processes to elucidate their application potential. For the synthesis of new Mo and W complexes, a library of the XNX-type ligands (X = chalcogenides S, Se) was developed (Scheme 1) on the basis of the systematic variation of the steric demand of the alkyl rest R and by exchanging the chalcogenides. For the synthesis of the XNX-type ligands, some approaches starting with N-alkyl-diethanthiolamine or Nphenyl-diethanthiolamine have been reported previously59−62 and have been optimized for the synthesis shown in Scheme 2. The class of (HSEt)2NR (R = methyl, tert-butyl, phenyl) ligands (3a−c, 4a) included the replacement of the alcohol B


Article

Inorganic Chemistry

Figure 1. 1H NMR (CDCl3, 300 MHz, rt) spectra of (a) 4a-1 5,12-dimethyl-1,2,8,9-tetraselena-5,12-diaza-cyclotetradecane with molecular structure, (b) 3a-1 5,12-dimethyl-1,2,8,9-tetrathia-5,12-diaza-cyclotetradecane, and (c) 3a N-methyldiethanthiolamine (HSEt)2NMe, chemical shifts of 3a are displayed in red (1H NMR) and in green (13C NMR).

Table 1. Comparison of Selected Cell Parameters of 1a, 2a, and 4a-1 complexes cell parameters

[W(SEtN(Me)EtS)2]

[Mo(SEtN(Me)EtS)2]

((SeEt)2NMe)2

empirical formulas formula weight crystal system space group R(int) R indices (all data) goodness of fit on F2 unit cell dimensions

WS4N2C10H22 482.38 g/mol triclinic P1̅ 0.1026 R1 = 0.0553, wR2 = 0.0777 0.988 a = 7.8311(7) Å b = 9.6503(8) Å c = 11.2664(1) Å α = 82.455(7)° β = 84.509(7)° γ = 66.593(6)° 762.1(2) Å3 2

MoS4N2C10H22 394.47 g/mol triclinic P1̅ 0.1105 R1 = 0.1189, wR2 = 0.1083 0.810 a = 7.8304(8) Å b = 9.615(1) Å c = 11.134(1) Å α = 82.605(8)° β = 84.863(9)° γ = 66.572(8)° 773.7(2) Å3 2

C10H22N2Se4 486.13 g/mol monoclinic P21/n 0.1182 R1 = 0.0350, wR2 = 0.0541 1.026 a = 5.0926(5) Å b = 13.4621(9) Å c = 11.652(1) Å α = 90° β = 93.355(7)° γ = 90° 797.5(1) Å3 2

volume Z

function by a thiol group. For this purpose, the conversion of the diol function to a better leaving group [hydrochloride for 3a, 4a; tosylate for 3b, 3c (These are toxic intermediates! See handling details in the Experimental Section.)] was implemented by adding a chalcogenide source and using alkali treatment afterward, when the colorless ligands were isolated in pure form by distillation (Scheme 2). Selenium-containing ligand

4a-1 was synthesized by a metathesis reaction of the corresponding hydrochloride with in-situ produced NaHSe (Scheme 2). Extraction with organic solvents and drying under reduced pressure produced yellow oil that turned into a crystalline solid upon storage for some days at room temperature. The yellow crystals isolated from the oil were C


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Inorganic Chemistry

Figure 2. (a) Coordination geometry and distances between the methyl groups. Angle of twisted triangular SNS planes. (c) Distances between triangular SNS planes for [W(SEtN(Me)EtS)2] 1a. (d) Endo and exo conformations of both five-membered metallacycles of one chelating ligand in solution and the corresponding protons in endo (highlighted in red) and exo (highlighted in green) positions. (e) Coordination geometry and distances between the methyl groups. (f) Angle of twisted triangular SNS planes. (g) Distances between triangular SNS planes for [Mo(SEtN(Me)EtS)2] 2a. (h) Endo and exo conformations of both five-membered metallacycles of one chelating ligand in solution and the corresponding protons in endo (highlighted in red) and exo (highlighted in green) positions. (The selected bond lengths and angles are listed in the Supporting Information.)

under acidic conditions or hydrazine treatment was reported to afford the corresponding chalcogenols.65,68,69 Three different proton signals were detected in the 1H NMR spectrum of 3a-1 (Figure 1b) in an integrative ratio of 3:4:4. For ligand 3b-1 (SI Figure S1), an integrative proton ratio of 9:4:4 was detected, and ligand 3c-1 (SI Figure S2) showed an integrative proton ratio of 4:4:3:2. The mentioned roof effect by the hindered rotation of the adjacent ethyl protons was detected for all sulfur-containing ligands (3a-1, 3b-1, and 3c1). It has been reported that a color change in dichalcogenidebased compounds from colorless to yellow indicates an oxidation reaction to dimeric or oligomeric structures.67,69 The observation of the sulfur-containing ligands indicated that the sterically demanding substituents influenced the dimerization reactions. This predominantly steric effect is a possible explanation for the isolation of only the open-chain structure with short substituents (R = H, Me, Et, EtS).62 For the phenylcontaining SNS-type ligands (3c), only in-situ reactions are described in the literature because of the high reactivity of the thiol functions in the as-synthesized (HSEt)2NPh compound.60,61 The 1H NMR spectroscopic studies of 3a showed three different proton signals in an integrative ratio of 2:3:8 (Figure 1c), which is attributed to the symmetry of 3a along the methyl function and the nitrogen. The CH2 protons of both ethanthiol chains were observed at 2.24 ppm as a multiplet. The two acidic thiol protons resulted in a broadened signal at 1.69 ppm. The NMR spectroscopic studies of ligand 3a were in agreement with the literature59 and corroborated the proposed structure of the ligand in solution. Ligands 3b and 3c were synthesized under milder conditions than used for ligands 3a and 4a (Scheme 2). The (HXEt)2NR ligands acted as dianionic chelating ligands that can coordinate in a tridentate formation to the metal

characterized by single-crystal X-ray analysis to reveal a dimeric ring structure of ((SeEt)2NMe)2 4a-1 (Figure 1a, Table 1). All synthesized ligands and complexes were analyzed by multinuclear NMR spectroscopy analysis using a combination of 1D and 2D NMR experiments (1H, 13C, 1H,1H COSY, 1 1 H, H NOESY, 1H,13C HSQC, 1H,13C HMBC). Using new ligand systems, we were able to isolate and characterize ring structures ((XEt)2NR)2, X = S, R = Me, tBu, Ph; X = Se, R = Me) of the synthesized disulfide (3a-1, 3b-1, 3c-1) and diselenide (4a-1) XNX-type ligands. The NMR spectra of ligands 3a-1 (Figure 1b), 3b, and 3c (SI Figures 1 and 2) showed comparable resonances (Figure 1a). The NMR analysis confirmed the isolated solid-state structure of ((SeEt)2NMe)2 ligand 4a-1 in solution (Figure 1a). The 1H NMR spectrum of the crystals of 4a-1 displayed a characteristic pattern with three chemically non-equivalent proton signals (Figure 1a), with the singlet at 2.34 ppm corresponding to methyl group at the nitrogen and two triplets at 2.79 and 3.14 ppm demonstrating the adjacent CH2 groups of the ethanthiol chains. The 1H NMR spectrum of the ligand showed a total of 22 protons in the integrative ratio of 3:4:4. The characteristic roof effect of two triplets with same intensity and integrative ratio indicated the hindered rotation of the ethyl chain protons around the CH2CH2 axis. In general, chalcogenides have a tendency to form element− element bonds under polymerization or ring-closure reactions, which can be influenced by different parameters such as UV light, pH values, and substituents. 63−67 In particular, chalcogenols are prone to form dichalcogenide bonds through oxidation reactions that can be reductively cleaved with a reducing agent such as LiAlH4 or Zn/NaOH to form reactive chalcogenide species used for the preparation of the corresponding metal chalcogenolates.66−68 Also, zinc treatment D


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Inorganic Chemistry

Figure 3. Exo- and endo-oriented ring conformations of the coordinated ligand to metal centers (a) tungsten and (b) molybdenum. Endo protons (oriented with respect to the metal center) are highlighted in red; exo protons (oriented away from the metal center) are highlighted in green.

and 2a obtained from a solution of toluene and DME crystallized in the P1̅ triclinic space group with comparable cell parameters (Table 1). Single-crystal X-ray diffraction analysis of isotype complexes [W(SEtN(Me)EtS)2] 1a and [Mo(SEtN(Me)EtS)2] 2a presenting the metal center in a distorted octahedral environment (Figure 2a,e). Two dianionic ligands coordinate the metal center in tridentate fashion, which consequently resulted in an oxidation state of +IV. The coordination sphere around Mo and W is completed by the tridentate coordination of two 2,2′-(methylazanediyl)bis(ethane-1-thiolate) ligands (3a) to form a triangular plane between the two sulfur atoms and the nitrogen atom above and underneath the metal center (plane A, S1−N1−S2; plane B, S3−N2−S4) (Figure 2b,f). Planes A and B were twisted against each other by an angle of ca. 70° (Figure 2b,f) possibly due to the repulsive effect of the methyl groups at each nitrogen atom and the spatial requirements of the lone pairs present on the sulfur atoms in the ligand backbone. In the molecular structure of both complexes (1a and 2a), the sulfur−metal bonds varied over the range of 2.2−2.4 Å and the nitrogen−metal bonds were around 2.3 Å, which are comparable bond length found in other chelating dithiol and nitrogen−dithiol metal complexes (2.3−2.5 Å).73−77 The S−M−S angle in each ligand was around 108° for both transition metal complexes and was in agreement with the reported values (78−80°).73 Each tridentate coordinated ligand formed two five-membered metallacycles with the transition metal center ((1) W1−S1− C1−C2−N1) and Mo1−S1−C1−C2−N1, (2) W1−S2−C3− C4−N1 and Mo1−S2−C3−C4−N1) (Figure 2 d,h). The NMR spectra of the metal complexes in solution showed a total of 22 protons (1a) with 8 chemically non-equivalent signals in the integrative ratio of 4:2:2:2:2:6:2:2, whereas the 1 H NMR spectra for 2a displayed 9 chemically non-equivalent signals in an integrative proton ratio of 2:2:2:2:2:2:6:2:2 (SI Figure S3) apparently due to the exo- and endo-conformation of the five-membered rings. This ligand orientation appears to be a reasonable explanation for the restricted rotation of the hydrogen atoms and the resulting complex 1H NMR spectra of both complexes 1a and 2a in solution.22 The endo protons signals were shifted downfield in the NMR studies, and the exo proton signals were shifted upfield (SI Figure S3).78 This conformation of the complexes in solution was confirmed by 1H,1H NOESY correlation spectral analysis of 1a and 2a. The NMR studies showed the correlation of the protons trough space in which the methyl

center, yielding stable complexes. The reaction of the reported ligand system with the transition metal amides (Scheme 1) led to the formation of a new class of single-source precursors to transition metal dichalcogenide. Precursors hexakis(dimethylamido) tungsten(VI)70 ([W(NMe2)6]) 5 and tetrakis(diethylamido) molybdenum(IV)71 ([Mo(NEt2)4]) 6 were synthesized according to the literature. The red solution of 5 in toluene and the purple solution of 6 in ether instantaneously turned brownish upon addition of the ligand solutions. Bis((2,2′-(methylazanediyl)bis(ethane-1-thiolate)) tungsten(IV) [W(SEtN(Me)EtS)2] 1a and Bis((2,2′(methylazanediyl)bis(ethane-1-thiolate)) molybdenum(IV) [Mo(SEtN(Me)EtS)2] 2a were isolated as black crystals out of the reaction medium and washed several times with nheptane and toluene, followed by drying under reduced pressure. Regardless of the stoichiometric ratios in the reaction solution between 5 or 4 and 3a (1:1, 1:2, or 1:3 ratio [W(NMe2)6] or [Mo(NEt2)4]/(HSEt)2NMe)), transition metal complexes [W(SEtN(Me)EtS)2] and [Mo(SEtN(Me)EtS)2] were preferentially formed and were isolated as crystalline products. This observation of product formation was investigated by determining the cell parameters of the obtained crystalline material. Redox chemistry between transition metals and thiol containing coreactants is reported for applications such as atomic layer deposition; however, the mechanism of the reduction of the tungsten center from +VI (WCl6) to + IV (WS2) or the oxidation of the tungsten center from 0 ([W(CO)6]) to +IV (WS2) in the presence of a sulfur source during the decomposition step is still not completely understood.72 The observed reduction of the tungsten center from +VI [W(NMe2)6] to +IV [W(SEtN(Me)EtS)2] in this study is possibly due to the concomitant oxidation of the thiol function of the ligand from −I to 0 facilitated by the formation of sulfur-containing structures. This was confirmed by mass analysis and IR measurements of the sulfur-rich residue obtained from the mother liquor. The oxidative ring closing or oligomerization reactions of thiol-containing ligands have been reported in the literature.67 The elemental analysis of carbon, hydrogen, sulfur, and nitrogen compared to the calculated weight percent of the dimeric ring structure of ligand 3a-1 also suggested the formation of a sulfur-rich oxidized ligand species. The tungsten and molybdenum disulfide precursors were further analyzed by thermogravimetric measurements, singlecrystal analysis, and NMR spectroscopy. Single crystals of 1a E


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Inorganic Chemistry

Figure 4. (a) TGA−DSC measurement of synthesized single-source precursor [W(SEtN(Me)EtS)2] 1a with the proposed decomposition steps during the TGA−DSC analysis. (b) XRD pattern of the residue after TGA−DSC measurement of 2a with the 2H−WS2 reference (PDF no. 80237).

Figure 5. (a) TGA−DSC measurement of synthesized single-source precursor [Mo(SEtN(Me)EtS)2] 2a with the proposed decomposition steps during TGA−DSC analysis. (b) XRD pattern of the residue of 2a after TGA−DSC measurement with the 2H−MoS2 reference (PDF no. 37-1492) (* = unknown peak).

group of the first ligand interacted with the CH2 protons of the ethyl chains of the second coordinated ligand. Only one ethyl chain (C3/C6 for 1a and C2/C5 for 2a) was not detectable in the 1H,1H NOESY spectra, apparently because it is turned away from the rest of the molecule. The high stability and strong coordination of the ligands in as-synthesized complexes were demonstrated by the NMR studies that ruled out any configurational dynamics in the solution. The decomposition behavior of precursors 1a and 2a was studied by thermogravimetric analysis up to 1100 °C. The TGA curve displayed a multistep decomposition profile for [W(SEtN(Me)EtS)2] 1a (Figure 4a). The complex is thermally stable up to approximately 200 °C, where the decomposition started and was accompanied by an endothermic step resulting in a weight loss of 36% that could be assigned to cleavage of the coordinated ligand, followed by several decomposition steps corresponding to ligand fragmentation and combustion (exothermic events). This was indicated by the change in heat capacity of the DSC curve that finally resulted in the formation of phase-pure tungsten disulfide (2H−WS2). The formation of single-phase WS2 was confirmed by XRD measurements of the solid residue left in the crucible (Figure 4b). Owing to the monocrystallinity of the sample, the X-ray diffraction peaks were slightly broadened. The TGA−DSC analysis of [Mo(SEtN(Me)EtS)2] 2a showed a multistep decomposition, which differed from the pattern of precursor 1a (Figure 5a) as a result of the lower

thermal stability of 2a compared to that of complex 1a. The first decomposition corresponded to a weight loss of 11%, and the second decomposition of the complex was accompanied by an endothermic step. Four more decomposition steps with a cumulative weight loss of 40% were observed at 1100 °C. This product was analyzed by XRD measurements of the residue obtained after TGA−DSC analysis that confirmed the formation of the hexagonal 2H−MoS2 phase (Figure 5b). The broadening of XRD peaks is possibly due to the small crystal size in the isolated product. Despite extensive differential analysis of the XRD data bank, one minor peak in the XRD pattern (around 10°) could not be identified as any known molybdenum-, carbon-, nitrogen-, sulfur-, or oxygen-containing phases and is marked as such. The application of tungsten complex 1a as a volatile singlesource precursor in CVD processes resulted in thin films of crystalline tungsten disulfide. The deposition onto a silicon substrate at a substrate temperature of 600 °C under different deposition times showed homogeneous morphology and a uniform coating of WS2 (Figure 6). Deposition times of 30 min resulted in trigonal platelets (Figure 6a), which are typical of hexagonal TMDCs.74,75,79,80 Longer deposition times (2 to 3 h) produced a flowerlike structure with vertical WS2 triangular sheets (Figure 6b,c) that suggested kinetically limited film growth. Apparently, the exposure of active edge sites to continuous precursor flux together with the high nucleation density due to higher surface coverage resulted in dendritic F


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Inorganic Chemistry

Figure 6. Top-view SEM images of the deposition with [W(SEtN(Me)EtS)2] 1a via CVD on a silicon substrate: (a) deposition time (0.5 h); (b) deposition time (2 h); (c) deposition time (3 h); and (d) XRD pattern of 3 h deposited [W(SEtN(Me)EtS)2] with a reference (PDF no. 8-0237, 2H−WS2).

morphology with vertically oriented triangular WS2 flakes. The X-ray diffraction analysis of thin films deposited on silicon substrates using [W(SEtN(Me)EtS)2] as a precursor showed the formation of the desired hexagonal crystalline tungsten disulfide film (2H−WS2, PDF no. 8-0237, P63/mmc space group) with a preferred oriented growth direction (002) (Figure 6d).81 Furthermore, the SEM images (Figure 6a−c) and AFM measurements (Figures 7a−c) showed that the WS2 sheets are formed by stacking several thinner layers along the same crystal plane of (002). Investigations on surface topography by AFM measurements confirmed the structural features observed in the SEM images (Figure 6b). The topographical features of WS2 films deposited for 2 h on a silicon substrate are shown in Figure 7. The AFM images (Figure 7a) measured in noncontact mode demonstrated an onset of vertical structure formation above a critical height of 45 nm, which confirmed the preferred growth direction of (002) as observed in the XRD pattern (Figure 6d). An exemplary line profile of a triangular WS2 plate revealed a length of around 300 nm and a height of around 40 nm (Figure 7d), corresponding to 57 layers of WS2 calculated on the basis of the dimensions of a WS2 monolayer and considering the van der Waals distance of around 0.7 nm.82 Transmission electron microscopy (TEM) confirmed the formation of crystalline, few-layered WS2 sheets by CVD of

molecular precursors. The high-resolution transmission electron microscopy (HRTEM) image (Figure 8) showed the stacking of WS2 layers with an interplanar spacing of around 0.67 nm between two adjacent lattice planes, which is comparable to the literature known lattice distance.83,84

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CONCLUSIONS This study convincingly demonstrated that ligand system (HXEt)2NR (X = S, Se; R = Me, tBu, Ph) opens up an efficient pathway toward the synthesis of new tungsten and molybdenum dithiolate complexes (1a and 2a). These complexes successfully acted as suitable single-source precursors for the deposition of crystalline and phase-pure WS2 and MoS 2 . On the basis of chelating dithiol ligand (HSEt)2NMe 3a, a general synthesis protocol for the ligand design was developed and a new class of transition metal disulfide complexes ([MIV(SEtN(Me)EtS]; MIV = Mo, W) was established. In addition to their promising physicochemical properties, the tridentate coordination of two dithiol ligands to the transition metal center forced the monomeric structure and prevented the metal center from oxidation and further polymerization reactions that simplified the precursor delivery on the substrate and ensured the stability of the molecules both in solution and in the gas phase. Thermogravimetric measurements indicated their stepwise decomposition and G


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Inorganic Chemistry

Figure 7. AFM measurements of deposited WS2 on a silicon substrate via CVD: (a) substrate overview (10 × 10 μm2); (b) layered triangular WS2 plates (structures of up to 60 nm); (c) WS2 structures above 45 nm (vertical orientation of WS2 plates); and (d) line profile of triangular WS2 plates. and dimethylamine (10% in tetrahydrofuran; 2 mol/L) were purchased from TCI, and N-phenyl-diethanolamine (95%) was purchased from Acros Organics. Thionyl chloride (99.5+%), nbuthyllithium (2.5 M in n-hexane solution), and thiourea (99+%, for analysis) were purchased from Acros Organics. Sodium borohydride was purchased from Fisher Chemical. Selenium powder (99.99%), tungsten hexachloride (99.9%), and molybdenum pentachloride (anhydrous, 99.6%) were purchased from ABCR and were used for the transition metal precursors and the selenium-containing ligand. Toluene, dimethoxyethane (DME), n-heptane, n-pentane, and tetrahydrofuran (THF), which were used for the metal complex syntheses, were dried by standard methods with appropriate desiccating reagents and distilled prior to their use.85 Data collection for single-crystal X-ray structure elucidation was performed on a STOE IPDS II/2T diffractometer using graphite-monochromated Mo Kα radiation (0.71073 Å). The data were corrected for Lorentz and polarization effects. A numerical absorption correction based on crystal-shape optimization was applied for all data. The programs used in this work are STOE’s X-Area, including X-RED86 and X-Shape87 for data reduction and absorption correction, SIR-9288 and SHELXL201489 for structure solution, and SHELXL90,91 for structure refinement. CCDC 1908725−1908726−1910852 contain the supplementary crystallographic data for this article. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. NMR spectra were recorded on a Bruker Avance II 300 spectrometer; chemical shifts are quoted in parts per million relative to external TMS (1H and 13C). Thermal analyses were performed on a TGA/DSC 1 (Mettler-Toledo GmbH, Germany) in a nitrogen atmosphere (25 mL/min) at a rate of 10 °C/min from 30 to 1100 °C. The weight of the sample was in the range of 5−20 mg. Room temperature powder X-ray diffraction (XRD) was obtained on a STOE-STADI MP diffractometer operating in reflection mode using Mo Kα (λ = 0.71073 Å) radiation. Measured peak patterns were compared to reference powder diffraction files (PDF). The size and morphology of deposited films were analyzed using Nova Nano SEM 430, a field-emission scanning electron microscope (SEM), and a JEM-2200FS transmission electron microscope (TEM).

Figure 8. TEM measurements of WS2 flakes. (Inset) Fast Fourier transform (FFT) analysis over a WS2 flake.

verified them as potential precursors for transition metal disulfides (TMDCs), which supported our molecule-based approach to nanostructured materials. By an extension of the ligand library to other chalcogen substituents, diselenol ligands have been synthesized and tested with a view toward new transition metal diselenides, which is currently a work in progress.

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EXPERIMENTAL SECTION

All manipulations of air- and moisture-sensitive materials were carried out under a nitrogen atmosphere using Stock-type all-glass assemblies. All solvents and reactants were used without further purification except in special cases. N-methyl-diethanolamine (98+%) was purchased from Alfa Aesar, N-tert-butyl-diethanolamine (>97.0%) H


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Inorganic Chemistry The syntheses of 1a, 2a, 3, and 4 were carried out under a nitrogen atmosphere at the Stock line, and working under reduced pressure always means 1 × 10−3 mbar. Synthesis of N-Methyl-diethanthiolamine (HSEt)2NMe 3a. (Caution! The reaction of N-methyl-diethanolamine with thionyl chloride generated HCl and SO2 gas, which was bubbled through a wash bottle filled with alkaline solution. It produces a hydrochloride salt of nitrogen mustards that are cytotoxic in nature and require that handling and workup are performed with the necessary precautions and care.) The ligand was synthesized according to optimized reports by HarleyMason et al.,62 Friebe et al.,16 and Sun et al.59 Typically, 19.20 mL (0.17 mol) of N-methyl-diethanolamine in 60 mL of chloroform was stirred in an ice bath while 26.80 mL (0.37 mol) of thionyl chloride in 10 mL of chloroform was slowly added dropwise to the cold solution. Afterward, the reaction mixture was refluxed for 3 h, whereby a clear yellow-orange solution resulted. After removing the solvent under reduced pressure, the colorless crystalline hydrochloride remained. The hydrochloride was solvated in 35 mL of ethanol, and 28.30 g (0.37 mol) of thiourea was added. The mixture was refluxed for 6−8 h until a creamy solid resulted from a clear yellow solution. The solvent was removed under reduced pressure, to the raw product 14.80 g of NaOH and 40 mL of water were added, and the mixture was quickly heated to reflux. During reflux, small portions of water were added to the reaction mixture until a clear solution was formed. The cooled, clear yellow solution was extracted three times with diethyl ether. The collected organic phases were dried over MgSO4, and the solvent was removed under reduced pressure. The remaining yellow oil was distilled under reduced pressure at 70−80 °C oil bath temperature and a head temperature of 45 °C to obtain a colorless oil with a yield of 35% (9.06 g, 0.06 mol). 1 H NMR (CDCl3, 300 MHz, rt, ppm): 2.24 (m, 8H, CH2CH2SH), 1.82 (s, 3H, −NCH3), 1.69 (s, 2H, −SH). 13C NMR (CDCl3, 75 MHz, rt, ppm): 59.9 (HSCH 2 CH 2 −), 41.3 (NCH 3 ), 22.6 (HSCH2CH2−). Synthesis of 5,12-Di-tert-butyl-1,2,8,9-tetrathia-5,12-diazacyclotetradecane ((HSEt)2NtBu)2 3b-1. N-tert-butyl-diethanolamine (10.66 mL, 0.065 mol) in 10 mL of pyridine was stirred under ice bath cooling while 30.00 g (0.16 mol) of paratoluenesulfonyl chloride in pyridine was slowly added dropwise. The reaction mixture was stirred for 3 h at room temperature. Distilled water (100 mL) was added to the formed orange solid and extracted with chloroform (around 100 mL) three times. The collected organic phases were dried with MgSO4, and the solvent was removed under reduced pressure. The residue was refluxed with 9.00 g (0.13 mol) of thiourea and 100 mL of ethanol for 3 h. Sodium hydrogen carbonate (10.92 g, 0.13 mol) and 30 mL of water were added and refluxed for 3 more hours. The watery solution was extracted with chloroform, and the collected organic phases were dried with MgSO4. The solvent was removed under reduced pressure, and the remaining oil was distilled under reduced pressure to obtain a slightly yellow oil in a yield of 15% (1.6 g, 4.2 mmol). 1 H NMR (CDCl3, 300 MHz, rt, ppm): 1.10 (s, 9H, −NtBu), 2.77 (t, 4H, −NCH2−CH2S−), 3.09 (t, 4H, −NCH2−CH2S). 13 C NMR (CDCl3, 75 MHz, 298 K ppm): 27.2 (−CH2−N(tBu)− CH2−), 41.5 (−S−CH2−CH2−NR), 51.4 (−S−CH2−CH2−NR). Synthesis of 5,12-Diphenyl-1,2,8,9-tetrathia-5,12-diaza-cyclotetradecane ((SEt)2NPh)2 3c-1. The ligand was synthesized according to an optimized report by Cheung et al.61 and Lee et al.60 N-phenyl-diethanolamine (18.12 g, 0.10 mol) in 10 mL of pyridine was stirred under ice bath cooling while 40.00 g (0.21 mol) of paratoluenesulfonyl chloride in pyridine was slowly added dropwise. The reaction mixture was stirred for 3 h at room temperature. The clear yellow solution was put in ice-cooled distilled water, whereby a slimy solid appeared. The watery solution was extracted several times with chloroform, and the collected organic phases were dried over MgSO4. The solvent was removed under reduced pressure. The remaining orange oil was refluxed for 2 h with 15.22 g of thiourea and 35 mL of ethanol. Sodium hydrogen carbonate (17.64 g, 0.20 mol) and 60 mL of water were added to the clear orange solution and refluxed again for 3 h. The solution was extracted with chloroform, the collected

organic phases were dried with MgSO4, and the solvent was stripped off under reduced pressure. The remanding yellow oil was distilled under reduced pressure at 110−120 °C oil bath temperature and a head temperature of 100 °C to obtain a slightly yellow oil in a yield of 12% (5.12 g, 1.2 mmol). 1 H NMR (CDCl3, 300 MHz, rt, ppm): 2.59 (t, 4H, −NCH2− CH2S−), 3.39 (t, 4H, −NCH2−CH2S−), 6.76 (m, 3H, −CH2−(Ph)− CH2−). 7.15 (m, 2H, −CH2−(Ph)−CH2−). 13 C NMR (CDCl3, 75 MHz, rt, ppm): 27.2 (−S−CH2−CH2−NR), 52.0 (−S−CH2−CH2−NR), 117.3 (−CH2−N(Ph)−CH2−), 120.1 (−CH2−N(Ph)−CH2−), 129.4 (−CH2−N(Ph)−CH2−), 151.7 (−CH2−N(Ph)−CH2−). Synthesis of 5,12-Dimethyl-1,2,8,9-tetraselena-5,12-diazacyclotetradecane ((SeEt)2NMe)2 4a-1. NaHSe was synthesized according the literature92 by the reaction of 3.03 g (0.08 mol) of NaBH4 and 6.32 g (0.08 mol) of selenium powder in ethanol and flushing with nitrogen. Referring to the work of Levanova et al.,67 4.98 g (0.04 mol) of N-methyl-dichlorodiethylamine hydrochloride in ethanol was added slowly, stirred at room temperature for 3 h, and refluxed for 2 h afterward. The clear yellow solution was separated from the dark residue, and the solvent was removed under reduced pressure to obtain a yellow oil. Yellow crystals could be obtained from this yellow oil. 1 H NMR (CDCl3, 300 MHz, rt, ppm): 2.34 (s, 3H, −NCH3), 2.79 (m, 4H, −NCH2−CH2S−), 3.14 (m, 4H, −NCH2−CH2S−). Synthesis of Hexakis(dimethylamido) tungsten(VI) [W(NMe2)6] 5. Hexakis(dimethylamido) tungsten was synthesized following an optimized description by Chrisholm et al.70 Liquidnitrogen-cooled 2 M dimethylamine in THF (15.00 mL, 30.00 mmol) was stirred with 12.00 mL (30.00 mmol) of 2.5 M n-BuLi solution and allowed to warm to room temperature during another 12 h of stirring. The solvent in the resulting colorless LiNMe2 was removed under reduced pressure, and 1.85 g (5.00 mmol) of tungsten hexachloride was added to the dry LiNMe2. Afterward, 15.00 mL of toluene and 8.00 mL of n-heptane were added, and the reaction mixture was stirred at room temperature for a few minutes. To the suspension, 6.00 mL of dimethoxyethane (DME) was added, whereby the exothermic reaction mixture started and resulted in a brownish solution. After 6 h of stirring at room temperature, the solvent was removed under reduced pressure. Sublimation under reduced pressure (oil bath temperature 100 °C) led to crystalline red [W(NMe2)6]93 in a yield of 22% (0.33 g, 0.70 mmol). Synthesis of Bis(2,2′-(methylazanediyl)bis(ethane-1-thiolate) tungsten(IV) [W(SEtN(Me)EtS)2] 1a. [W(NMe2)6] (0.26 g, 0.60 mmol) was dissolved in 25.00 mL of toluene, and 0.27 g (1.8 mmol) of N-methyldiethanthiolamine was added. The red solution immediately turned brown by adding the ligand. The reaction mixture was stirred for 30 min at room temperature. The clear, dark-brown solution was decanted from a brownish solid. The solution was constrained, and the flask was stored for crystallization. Dark, nearly black crystals were obtained out of the reaction solution and after washing the crystalline product several times with n-heptane in a yield of 77% (0.22 g, 0.46 mmol). Because of the high sensitivity of the product, no further analysis was done. 1 H NMR (CDCl3, 300 MHz, rt, ppm): 3.22−3.30 and 2.82−2.96 (m, 2H and 2H, −S−CH2−CH2−NR2), 3.32 (s, 6H, N−CH3), 2.43− 2.53 (m, 4H, −S−CH2−CH2−NR2), 3.17−3.25 and 3.86−4.00 (m, 2H and 2H, R2N−CH2−CH2−S), 2.59−2.73 and 4.44−4.56 (m, 2H and 2H, R2N−CH2−CH2−S). 13 C NMR (CDCl3, 75 MHz, rt, ppm): 47.9 (−S−CH2−CH2− NR2), 55.3 (R−N(CH3)−R), 65.2 (−S−CH2−CH2−NR2), 70.4 (R2NCH2CH2−S), 97.1 (R2N−CH2−CH2−S). Synthesis of Tetrakis(diethylamido)molybdenum(IV) [Mo(NEt2)4] 6. Tetrakis(diethylamido) molybdenum was synthesized following a modified description by Chrisholm et al.71 n-BuLi solution (8.00 mL, 20.00 mmol, 2.5 M) was combined with 2.20 mL (20.00 mmol) of liquid-nitrogen-cold diethylamine. The solution was allowed to warm to room temperature and stirred overnight. Colorless LiNEt2 was cooled with liquid nitrogen, and 1.10 g (4.00 mmol) of molybdenum pentachloride and 20 mL of dry n-pentane were added I


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Inorganic Chemistry and stirred for 3.5 h at room temperature. Afterward, the solvent was removed under reduced pressure, and the purple oil was obtained by distillation under reduced pressure at 100 °C. Synthesis of Bis(2,2′-(methylazanediyl)bis(ethane-1-thiolate) molybdenum(IV) [Mo(SEtN(Me)EtS)2] 2a. [Mo(SEtN(Me)EtS)2] was synthesized in-situ by the distillation of [Mo(NEt2)4] under reduced pressure into a dry-ice-cooled solution of the (HSEt)2NMe ligand in DME. The reaction mixture immediately turned brown by adding [Mo(NEt2)4] to the ligand solution and was stirred for 30 min at room temperature. The reaction flask was stored for crystallization. Dark-brown crystals could be obtained from the reaction solution. 1 H NMR (CDCl3, 300 MHz, rt, ppm): 4.16 and 2.11 (m, 2H and 2H, −S−CH2−CH2−NR2), 5.00 (s, 6H, N−CH3), 2.45 and 1.31 (m, 2H and 2H, −S−CH2−CH2−NR2), 4.36 and 3.54 (m, 2H and 2H, R2N−CH2−CH2−S), 6.26 and 5.63 (m, 2H and 2H, R2N−CH2− CH2−S). 13 C NMR (CDCl3, 75 MHz, rt, ppm): 77.0 (−S−CH2−CH2− NR2), 66.5 (R−N(CH3)−R), 64.4 (−S−CH2−CH2−NR2), 70.3 (R2NCH2CH2−S), 120.4 (R2N−CH2−CH2−S).

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(2) Groven, B.; Heyne, M.; Nalin Mehta, A.; Bender, H.; Nuytten, T.; Meersschaut, J.; Conard, T.; Verdonck, P.; Van Elshocht, S.; Vandervorst, W.; et al. Plasma-Enhanced Atomic Layer Deposition of Two-Dimensional WS2 from WF6, H2 Plasma, and H2S. Chem. Mater. 2017, 29 (7), 2927−2938. (3) Jurca, T.; Moody, M. J.; Henning, A.; Emery, J. D.; Wang, B.; Tan, J. M.; Lohr, T. L.; Lauhon, L. J.; Marks, T. J. Low-Temperature Atomic Layer Deposition of MoS2 Films. Angew. Chem., Int. Ed. 2017, 56 (18), 4991−4995. (4) Tan, H.; Fan, Y.; Rong, Y.; Porter, B.; Lau, C. S.; Zhou, Y.; He, Z.; Wang, S.; Bhaskaran, H.; Warner, J. H. Doping Graphene Transistors Using Vertical Stacked Monolayer WS2 Heterostructures Grown by Chemical Vapor Deposition. ACS Appl. Mater. Interfaces 2016, 8 (3), 1644−1652. (5) Alharbi, A.; Shahrjerdi, D. Electronic Properties of Monolayer Tungsten Disulfide Grown by Chemical Vapor Deposition. Appl. Phys. Lett. 2016, 109 (19), 193502−193502−193505. (6) Ko, K. Y.; Song, J. G.; Kim, Y.; Choi, T.; Shin, S.; Lee, C. W.; Lee, K.; Koo, J.; Lee, H.; Kim, J.; et al. Improvement of Gas-Sensing Performance of Large-Area Tungsten Disulfide Nanosheets by Surface Functionalization. ACS Nano 2016, 10 (10), 9287−9296. (7) Butler, S. Z.; Hollen, S. M.; Cao, L.; Cui, Y.; Gupta, J. A.; Gutierrez, H. R.; Heinz, T. F.; Hong, S. S.; Huang, J.; Ismach, A. F.; Johnston-Halperin, E.; Kuno, M.; Plashnitsa, V. V.; Robinson, R. D.; Ruoff, R. S.; Salahuddin, S.; Shan, J.; Shi, L.; Spencer, M. G.; Terrones, M.; Windl, W.; Goldberger, J. E.; et al. Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene. ACS Nano 2013, 7 (4), 2898−2926. (8) Xu, M.; Liang, T.; Shi, M.; Chen, H. Graphene-Like TwoDimensional Materials. Chem. Rev. 2013, 113 (5), 3766−3798. (9) Frindt, R. F. Single Crystals of MoS2 Several Molecular Layers Thick. J. Appl. Phys. 1966, 37 (4), 1928−1929. (10) Frindt, R. F.; Yoffee, A. D. Physical Properties of Layer Structures: Optical Properties and Photoconductivity of Thin Crystals of Molybdenum Disulphide. R. Soc. 1963, 273 (1352), 69−83. (11) Frindt, R. F. Optical Absorption of a Few Unit-Cell Layers of MoS2. Phys. Rev. 1965, 140 (2A), A536−A539. (12) Chianelli, R. R.; Ruppert, A. F.; Jose-Yacaman, M.; VazquezZavala, A. HREM Studies of Layered Transition Metal Sulfide Catalytic Materials. Catal. Today 1995, 23, 269−281. (13) Roldán, R.; Silva-Guillén, J. a.; López-Sancho, M. P.; Guinea, F.; Cappelluti, E.; Ordejón, P. Electronic Properties of Single-Layer and Multilayer Transition Metal Dichalcogenides MX2 (M = Mo, W and X = S, Se). Ann. Phys. 2014, 526 (9−10), 347−357. (14) Richey, N. E.; Haines, C.; Tami, J. L.; McElwee-White, L. Aerosol-Assisted Chemical Vapor Deposition of WS2 from the Single Source Precursor WS(S2)(S2CNEt2)2. Chem. Commun. 2017, 53 (55), 7728−7731. (15) Tedstone, A. A.; Lewis, E. A.; Savjani, N.; Zhong, X. L.; Haigh, S. J.; Brien, P. O.; Lewis, D. J. Single-Source Precursor for Tungsten Dichalcogenide Thin Films: Mo1−xWxS2 (0 ≤ x ≤ 1) Alloys by Aerosol-Assisted Chemical Vapor Deposition. Chem. Mater. 2017, 29 (No. 29), 3858−3862. (16) Friebe, M.; Mahmood, A.; Spies, H.; Berger, R.; Johannsen, B.; Mohammed, A.; Eisenhut, M.; Bolzati, C.; Davison, A.; Jones, A. G. 3 + 1” Mixed-Ligand Oxotechnetium(V) Complexes with Affinity for Melanoma: Synthesis and Evaluation in Vitro and in Vivo. J. Med. Chem. 2000, 43 (14), 2745−2752. (17) Baird, I. R.; Mosi, R.; Olsen, M.; Cameron, B. R.; Fricker, S. P.; Skerlj, R. T. 3 + 1” Mixed-Ligand Oxorhenium(V) Complexes and Their Inhibition of the Cysteine Proteases Cathepsin B and Cathepsin K. Inorg. Chim. Acta 2006, 359 (9), 2736−2750. (18) Segal, I.; Zablotskaya, A.; Kniess, T.; Shestakova, I. Synthesis and Cytotoxicity of Pyridine and Quinoline Oxorhenium (V) Complexes With Tridentate (NS2, S3)/ Monodentate (S) Coordination. Chem. Heterocycl. Compd. 2012, 48 (2), 296−300. (19) Boyland, E.; Nery, R. 136. The Reaction between N-Methyldi(2-Chloroethyl)Amine and Thiosulphate. J. Chem. Soc. 1961, 679− 683.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01084. 1 H NMR spectra of 1a, 2a, 3a-1, 3b-1, and 3c-1, detailed assignment of proton and carbon signals of 1a and 2a, and table of selected bond lengths and angles of 1a and 2a (PDF)

Accession Codes

CCDC 1908725−1908726 and 1910852 contain the supplementary crystallographic data for this article. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sanjay Mathur: 0000-0003-2765-2693 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial and infrastructural support provided by the University of Cologne and the excellence cluster “Quantum Matter and Materials” (QM2). Special thanks are due to Dr. Thomas Fischer, Nurgül Tosun, and Dr. Isabel Gessner for TEM and SEM measurements and Silke Kremer for single-crystal analysis. V.B. gratefully acknowledges the help of Markus Schütz for thermogravimetric measurements, Daniel Stadler for AFM measurements, and Michael Haiduk and David Graf for X-ray diffraction analysis of thin films.

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