Chalcogenoether complexes of Nb(V) thio- and seleno-halides as single source precursors for low pressure chemical vapour deposition of NbS₂ and NbSe₂ thin films

Abstract

NbSCl₃ was obtained via reaction of NbCl₅ with S(SiMe₃)₂ in anhydrous CH₂Cl₂, whilst in MeCN solution the same reaction gives [NbSCl₃(MeCN)₂]. [NbSeCl₃(MeCN)₂] was obtained similarly from NbCl₅ with Se(SiMe₃)₂. The chalcogenoether complexes, [NbSCl₃(ER₂)] (E = S: R = Me, ⁿBu; E = Se: R = ⁿBu), were obtained from reaction of NbCl₅, ER₂ and S(SiMe₃)₂ in CH₂Cl₂. The structure of the [Nb₂S₂Cl₆(SMe₂)₂] reveals a Cl-bridged dimer with the SMe₂ ligands disposed syn. The Cl bridges are highly asymmetric, with the long Nb–Cl bond trans Nb=S. The complexes, [NbSCl₃(L–L)] (L–L = MeSCH₂CH₂SMe, MeS(CH₂)₃SMe, ⁱPrSCH₂CH₂SⁱPr, MeSe(CH₂)₃SeMe and ⁿBuS(CH₂)₃SⁿBu), were obtained from reaction of L–L with preformed [NbSCl₃(MeCN)₂]. The structures of the Me-substituted complexes reveal distorted octahedral monomers with the neutral ligands trans to S/Cl. Solution ¹H and ⁷⁷Se{¹H} NMR data showed that the neutral ligands are partially dissociated and undergoing fast exchange at ambient temperatures in CH₂Cl₂ solution, consistent with weak Lewis acidity for NbSCl₃. The complexes containing ⁿBu-substituted ligands have been used as single source precursors for low pressure chemical vapour deposition (CVD) of 3R-NbS₂ thin films. 2H-NbSe₂ thin films were also obtained via low pressure CVD using [NbSe₂Cl₃(SeⁿBu₂)]. The thin films were characterised by grazing incidence and in-plane XRD, pole figure analysis, scanning electron microscopy and energy dispersive X-ray analysis.

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Introduction

Layered early transition metal dichalcogenides ME₂ (M = Nb, Ta, V, W etc., E = S, Se or Te), inorganic analogues of graphite or graphene, are highly stable, and their properties can be tuned by varying the metal and the chalcogen.¹ Applications of these materials in areas such as optoelectronics, spintronics, sensors, electrocatalysis and magnetic materials are being actively developed.^1–3 Production of the materials as thin films maximises the anisotropy of their magnetic or electronic properties and thus methods to deposit 2D layers are of great current interest. Chemical vapour deposition (CVD) is a low cost versatile method to deposit such films using either dual or single source precursors.^4–6 Single source precursors potentially offer better control of deposit stoichiometry, cost-effective use of reagents, and in some cases the ability to selectively deposit the ME₂ onto nano-patterned substrates. Reagents for CVD of NbS₂ or NbSe₂ are rare, but include niobium thiolate complexes for the former⁷ and a dual source approach using NbCl₅-Se^tBu₂ for the latter.⁶ We recently reported the deposition of thin films of NbS₂ and NbSe₂ as the 3R-polytype (R3mh) using [NbCl₅(SⁿBu₂)] and [NbCl₅(SeⁿBu₂)], respectively, as single source low pressure chemical vapour deposition (LPCVD) precursors.⁸ The properties of the potential precursors are key to successful applications in this area, requiring appropriate volatility and clean deposition at accessible temperatures (a β-hydride decomposition route for the neutral ligand is often helpful). Our previous studies showed that Nb(III) dimers such as [Nb₂Cl₄(EⁿBu₂)₂(μ-Cl)₂(μ-EⁿBu₂)] did not vaporise under LPCVD conditions, probably due to their high molecular weights,⁹ whilst the Nb(IV) complexes such [NbCl₄(SeⁿBu₂)₂], [NbCl₄{ⁿBuSe(CH₂)₃SeⁿBu}] and [NbCl₄{ⁱPrSCH₂CH₂SⁱPr}], lost the chalogenoether on heating in vacuo, leaving a residue of the (strongly polymerised) NbCl₄.¹⁰

Here we report on the synthesis and properties of chalcogenoether complexes of niobium chalcogenide trichlorides NbECl₃ (E = S or Se) and attempts to use them as LPCVD reagents. Chalcogenohalides are known for many of the transition elements,¹¹ although their coordination chemistry has been much less explored than those of the corresponding halides or oxide halides.¹² Complexes of NbSX₃ with Lewis bases include [NbSCl₄]⁻,¹³ [NbSCl₃(Ph₃PS)],¹⁴ and [NbSX₃(Ph₃PO)₂] (X = Cl, Br),¹⁵ [NbSCl₃(SEt₂)],¹⁵ and [NbSX₃(MeCN)₂],^15,16 However, in other systems rearrangements occur to form compounds with dichalcogenide bridges, including the structurally characterised [Nb₂Cl₄(Se₂)₂(L)₄] (L = SMe₂ or tetrahydrothiophene (tht)) and [Nb₂S₃X₄(tht)₄].^17,18

Results and discussion

Complex synthesis and properties

We obtained crude NbSCl₃ as a dark green solid by reaction of NbCl₅ with S(SiMe₃)₂ in CH₂Cl₂ in an ice-bath, and this solid was used to test the parent compound as an LPCVD reagent (see below). As pointed out by others,¹⁹ the colour of NbSCl₃ is variously described as golden-yellow to green or black in the literature, and it was suggested that the darker coloured samples contain Nb₂S₃Cl₄ impurities. The ν(Nb=S) vibration at 550 cm⁻¹ agrees with the literature value.²⁰

The thio- and seleno-ether complexes of NbSCl₃ were obtained in good yield either by reaction of NbCl₅ with the ligand and S(SiMe₃)₂ in CH₂Cl₂ in an ice-bath, or by reaction of [NbSCl₃(MeCN)₂] with the ligand in CH₂Cl₂ at ambient temperatures. [NbSCl₃(MeCN)₂] was made by the literature method, involving reaction of NbCl₅ with S(SiMe₃)₂ in MeCN solution,¹⁶ or by dissolving the crude NbSCl₃ in MeCN, filtering to remove any insoluble material, and evaporating the filtrate to dryness. The products of the two methods are spectroscopically identical, and the data agree well with the literature.^15,16

[NbSeCl₃(MeCN)₂] was obtained from NbCl₅ and Se(SiMe₃)₂ in MeCN as a brown solid. The IR spectroscopic properties, ν(MeCN) = 2310, 2281, ν(Nb=Se) = 397, ν(Nb–Cl) = 377, 344 cm⁻¹, are as expected, although our assignment of Nb=Se and Nb–Cl vibrations is tentative as they occur at similar frequencies. The ⁹³Nb NMR chemical shift, δ = 923, is at significantly higher frequency than in the thiochloride analogue, reflecting the effect of the selenium coordination.

The reaction of NbCl₅ with SMe₂ and S(SiMe₃)₂ in CH₂Cl₂ cooled to ∼0 °C produced a yellow–green solid of stoichiometry [NbSCl₃(SMe₂)], whilst use of SⁿBu₂ gave a dark oil [NbSCl₃(SⁿBu₂)]. The IR spectrum of [NbSCl₃(SMe₂)] shows a strong vibration at 530 cm⁻¹, assigned as a terminal ν(Nb=S), with ν(Nb–Cl) vibrations at 369, 356, 322 cm⁻¹. In chlorocarbon solution it shows a single δ(Me) resonance in the ¹H NMR spectrum, shifted to high frequency from SMe₂, and, more interestingly, a ⁹³Nb NMR resonance²¹ at δ = 651. The resonances of niobium thiochloride complexes are found to high frequency of those of NbCl₅.^22,23 (Note that the chemical shifts in ref. 22 are reported using the old high frequency negative convention.) The spectroscopic data on [NbECl₃(SⁿBu₂)] were similar, suggesting an analogous structure. The X-ray structure of [NbSCl₃(SMe₂)] showed it to be a dimer, [Nb₂S₂Cl₄(μ-Cl)₂(SMe₂)₂] (Fig. 1). The bond lengths are unexceptional, with the exception that those associated with the bridging Cl ligands lying trans to Nb–S are highly asymmetric. Notably the two SMe₂ ligands are syn-disposed, rather than the much more common anti arrangement found in many centrosymmetric d-block dimers.

Fig. 1 The structure of [Nb₂S₂Cl₆(SMe₂)₂] showing the atom numbering scheme and with ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å): Nb1–S1 = 2.127(2), Nb1–S2 = 2.660(2), Nb1–Cl1 = 2.420(2), Nb1–Cl2 = 2.324(2), Nb1–Cl3 = 2.305(2), Nb1–Cl4 = 2.921(2), Nb2–S3 = 2.123(2), Nb2–S4 = 2.655(2), Nb2–Cl1 = 2.939(2), Nb2–Cl4 = 2.405(2), Nb2–Cl5 = 2.326(2), Nb2–Cl6 = 2.323(2).

The MeCN ligands in [NbSCl₃(MeCN)₂] were cleanly substituted on reaction with an excess (three equivalents) of MeSCH₂CH₂SMe, MeS(CH₂)₃SMe, ⁱPrSCH₂CH₂SⁱPr or MeSe(CH₂)₃SeMe in CH₂Cl₂ solution at ambient temperatures, to give green [NbSCl₃(L–L)] (L–L = MeSCH₂CH₂SMe, MeS(CH₂)₃SMe, ⁱPrSCH₂CH₂SⁱPr) or brown [NbSCl₃{MeSe(CH₂)₃SeMe}] powdered solids. Using ⁿBuS(CH₂)₃SⁿBu produced [NbSCl₃{ⁿBuS(CH₂)₃SⁿBu}] as a green oil. The X-ray crystal structure of [NbSCl₃{MeSe(CH₂)₃SeMe}] showed a distorted octahedral geometry, with the in plane terminal S/Cl disordered. The disorder was successfully modelled using split sites for these two atoms and refined at 50 : 50 occupancies; only one form is shown in Fig. 2. The key bond lengths are Nb1–Cl1 = 2.308(7), Nb1–S1 = 2.197(8) Å, whilst the axial Nb1–Cl2 = 2.3686(6) Å, i.e. significantly longer.

Fig. 2 The structure of [NbSCl₃{MeSe(CH₂)₃SeMe}] showing the atom numbering scheme and with ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Note that S1 and Cl1 are disordered, and only one molecule is shown here. Selected bond lengths (Å) and angles (°): Nb1–Cl1 = 2.308(7), Nb1–Cl2 = 2.3686(6), Nb1–S1 = 2.197(8), Nb1–Se1 = 2.8799(4); Cl1–Nb1–Cl2 = 96.67(15), Cl1–Nb1–Cl2ⁱ = 95.98(15), Cl1–Nb1–S1 = 105.73(5), Cl1–Nb1–Se1ⁱ = 85.29(6), S1–Nb1–Se¹ = 89.90(8), S1–Nb1–Cl2 = 100.1(2), S1–Nb1–Cl2ⁱ = 94.0(2), Cl2–Nb1–Se1 = 79.07(2), Cl2–Nb1–Se1ⁱ = 83.86(2), Se1–Nb1–Se1ⁱ = 79.274(13). Symmetry operation: i = −x, −y + 3/2, z.

The structures of two thioether complexes, [NbSCl₃(MeSCH₂CH₂SMe)] and [NbSCl₃(ⁱPrSCH₂CH₂SⁱPr)], were also determined and are shown in Fig. 3 and 4. The X-ray diffraction data were good quality and careful examination did not provide clear evidence for S/Cl disorder in these cases and the d(Nb=S) = 2.10 Å and d(Nb–Cl) = 2.24 Å are nearly identical in the two structures. Notably, the d(Nb–S) distances trans to the Nb=S and trans to Nb–Cl are different, consistent with a trans influence of Nb=S > Nb–Cl. However, as pointed out by others,^14,17,18 it is not possible to rule out some disorder in the thiochloride systems due to the very similar scattering power of S and Cl atoms. We note in passing that all three structures contain the dichalcogenoether in the DL form.

Fig. 3 The structure of [NbSCl₃(MeSCH₂CH₂SMe)] showing the atom numbering scheme and with ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Nb1–Cl1 = 2.354(1), Nb1–Cl2 = 2.242(1), Nb1–Cl3 = 2.362(1), Nb1–S1 = 2.739(1), Nb1–S2 = 2.717(1), Nb1–S3 = 2.210(1); Cl1–Nb1–Cl2 = 97.52(4), Cl1–Nb1–S1 = 77.05(4), Cl1–Nb1–S2 = 83.36(3), Cl1–Nb1–S3 = 97.13(4), Cl2–Nb1–S1 = 90.37(4), Cl2–Nb1–S3 = 106.56(4), Cl3–Nb1–S2 = 76.78(3), Cl3–Nb1–S3 = 97.00(4), S1–Nb1–S2 = 78.18(3), S2–Nb1–S3 = 85.33(4).

Fig. 4 The structure of [NbSCl₃(ⁱPrSCH₂CH₂SⁱPr)] showing the atom numbering scheme and with ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Nb1–Cl1 = 2.2475(7), Nb1–Cl2 = 2.3588(6), Nb1–Cl3 = 2.3623(6), Nb1–S1 = 2.7813(6), Nb1–S2 = 2.7464(6), Nb1–S3 = 2.2105(7); Cl1–Nb1–Cl2 = 97.78(2), Cl1–Nb1–Cl3 = 97.49(3), Cl1–Nb1–S1 = 86.78(2), Cl1–Nb1–S3 = 103.84(3), Cl2–Nb1–S1 = 84.58(2), Cl2–Nb1–S2 = 76.69(2), Cl2–Nb1–S3 = 98.20(2), Cl3–Nb1–S1 = 76.73(2), Cl3–Nb1–S2 = 83.48(2), Cl3–Nb1–S3 = 97.13(3), S1–Nb1–S2 = 78.56(2), S2–Nb1–S3 = 91.14(2).

The IR spectra of the five complexes exhibit ν(Nb=S) ∼ 525 cm⁻¹ and two ν(Nb–Cl) stretches in the range 350–320 cm⁻¹, sometimes with a shoulder on the higher energy vibration, indicating the same molecular geometry. As expected, they are diamagnetic, confirming the presence of d⁰ Nb(V). The ⁹³Nb NMR spectra each exhibit a broad resonance in the region δ = +500 to +550, significantly to high frequency of NbOCl₃ or NbCl₅–thioether complexes.^10,23 The ¹H NMR spectrum of [NbSCl₃(MeSCH₂CH₂SMe)] in CD₂Cl₂ at 295 K shows two very broad resonances assignable to δ(MeS) and two δ(CH₂) resonances, which upon cooling the solution resolve into resonances due to free dithioether²⁴ and coordinated dithioether, and at 183 K some further splitting is evident, probably due to slowing of pyramidal inversion at the sulfur centres. The data are consistent with some dissociation of the dithioether in solution, and fast exchange between coordinated and ‘free’ dithioether at ambient temperature. This is further evidence for the weaker Lewis acidity of NbSCl₃ compared to NbOCl₃ or NbCl₅. The ¹H NMR spectra of the other four complexes also have broad resonances at ambient temperatures, and show similar changes on cooling the samples. The complex [NbSCl₃(MeSeCH₂CH₂CH₂SeMe)] does not exhibit a ⁷⁷Se{¹H} NMR resonance at 295 K, but on cooling to 223 K, resonances due to the ‘free’ diselenoether (δ = 68),²⁵ and coordinated diselenoether (δ = 163, 70) appear. The singlet at 70 ppm seems anomalous, but is reproducible and may reflect the ‘free’ Se donor of a κ¹-coordinated diselenoether, although it is possible this arises due to the Se donor atom trans to the terminal S.

Attempts to prepare dithio- or diseleno-ether complexes from [NbSeCl₃(MeCN)₂], even using excess chalcogenoethers or long reaction times, were unsuccessful. However, the reaction of NbCl₅ with SeⁿBu₂ in CH₂Cl₂, followed by treatment with (Me₃Si)₂Se, gave a black solid which, on the basis of microanalysis and IR and ¹H NMR spectroscopic analysis, was tentatively identified as [NbSe₂Cl₃(SeⁿBu₂)]. The structure of this material is unknown, but it may be dimeric like [Nb₂Cl₄(Se₂)₂(SMe₂)₂] and contain [Se–Se]²⁻ groups.¹⁷

Application for LPCVD of NbE₂ thin films

Although the gas phase structure of [NbSCl₃] has been determined,¹⁹ indicating some volatility, attempts to use this as a precursor for deposition of NbS₂ in the temperature range 400–750 °C were unsuccessful. However, the complexes, [NbSCl₃(SⁿBu₂)], [NbSCl₃{ⁿBuS(CH₂)₃SⁿBu}] and [NbSe₂Cl₃(SeⁿBu₂)], were likely candidates for use as single source CVD precursors for the low pressure CVD of NbS₂ and NbSe₂ thin films, owing to their volatility and the presence of readily eliminated ⁿBu groups. Thermogravimetric analyses were undertaken for these complexes (ESI Fig. S4–S6†), and while the mass loss observed does not correspond with the residue being simply NbS₂ or NbSe₂, this is unsurprising given the different experimental conditions used for TGA (progressive heating under a flowing argon atmosphere) compared to the vapour transport process involved in the low pressure CVD experiments. However, the TGA experiments do indicate the temperature at which mass loss begins, providing an indication of the low temperature threshold. Low pressure CVD experiments were therefore performed at a pressure of ca. 0.05 mmHg using a range of temperatures around 600–750 °C. No significant deposition was observed at lower temperatures.

Black reflective films were obtained from [NbSCl₃(SⁿBu₂)] at 700 °C, and grazing incidence XRD showed patterns consistent with NbS₂ in space group R3mh (3R-type NbS₂) (Fig. 5). These films appear to be air and moisture stable, and lattice parameters determined by Rietveld refinement of the grazing incidence XRD pattern are: a = 3.317(6) and c = 17.79(4) Å (R_wp = 6.61%, R_p = 4.46%). These are close to literature values for bulk NbS₂ (a = 3.3303(3), c = 17.918(2) Å).²⁶ The grazing incident and in-plane XRD patterns shows considerable variations in the intensities of reflections relative to the bulk pattern. The 003 reflection is the strongest in the grazing incidence XRD pattern, and 101, 012 and 110 reflections are highest in the in-plane XRD pattern, suggesting <00l> preferred orientation.

Fig. 5 In plane XRD (blue), grazing incidence (incidence angle = 1°) XRD (black) from the NbS₂ thin film deposited by LPCVD using [NbSCl₃(SⁿBu₂)] at 700 °C; simulated XRD pattern from bulk NbS₂ (red).²⁶ The broad feature at 2θ ∼22° is from the SiO₂ substrate.

Pole figure measurements were undertaken on a NbS₂ film obtained from the [NbSCl₃(SⁿBu₂)] precursor to establish the film texture. Using 2θ = 14.97°, corresponding to the 0 0 3 reflection, a single, sharp peak (FWHM ∼ 5°) was observed at the centre of the figure with α = 90° (Fig. 6a). The figure taken with 2θ = 31.47°, corresponding to the 1 0 1, exhibits a ring with α = 9° (Fig. 6b). These results confirmed the <00l> preferred orientation in which the ab planes of the crystallites are parallel with the substrate.

Fig. 6 Pole figures with cut line integral graphs for the 0 0 3 (2θ = 14.97°) (a) and 1 0 1 (2θ = 31.47°) (b) reflections of a film of NbS₂ deposited on a SiO₂ substrate.

Scanning electron microscopy (SEM) images reveal that the NbS₂ films have a uniform morphology formed of microcrystalline platelets mainly lying flat on the substrate (Fig. 7), consistent with the orientation inferred from the XRD data. EDX data measured at an accelerating voltage of 15 keV show significant Si and O peaks in addition to peaks due to Nb and S, indicating that the films are thin. EDX data also shows there is no evidence for any residual Cl in the films (Cl K_α = 2.621 keV). Accurate quantification of the Nb : S ratio by EDX is difficult due to the Nb L_α and S K_α peaks overlapping.

Fig. 7 SEM image of a polycrystalline NbS₂ film deposited by LPCVD from [NbSCl₃(SⁿBu₂)] at 700 °C.

Films obtained from low pressure CVD using [NbSCl₃{ⁿBuS(CH₂)₃SⁿBu}] at 700 °C also present diffraction patterns consistent with R3mh (3R-type NbS₂) (Fig. S1†), with lattice parameters a = 3.29(2) and c = 17.8(2) Å (R_wp = 5.54%, R_p = 3.47%). SEM images of these films are presented in the ESI (Fig. S2†).

Low pressure CVD experiments were also performed using [NbSe₂Cl₃(SeⁿBu₂)] at temperatures between 600 and 700 °C (0.05 mmHg). Films obtained at 650 °C present diffraction patterns consistent with P6₃/mmc (2H-type NbSe₂) (Fig. 8). Previously we obtained 3R-NbSe₂ from [NbCl₅(SeⁿBu₂)].⁸ but Parkin and co-workers have reported 2H-NbSe₂ by dual source APCVD from NbCl₅ and Se^tBu₂.⁶ The 002 101 and 110 reflections are the three most intense in both grazing incidence and in-plane XRD patterns, indicating that preferred orientation is not significant in this case. However, using a reduced amount of precursor produces thinner films with evidence of preferred orientation (Fig. S3†). Lattice parameters determined by Rietveld refinement of the grazing incidence XRD pattern are: a = 3.434(7) and c = 12.53(3) Å (R_wp = 2.48%, R_p = 1.87%), comparing to the literature values for bulk NbSe₂ of a = 3.4446(2), c = 12.5444(7) Å.²⁷ The NbSe₂ films appear to be air and moisture stable. The precursor was also tested in LPCVD at 600 °C however, there is no deposition observed. Scanning electron microscopy (SEM) of the same NbSe₂ film shows a polycrystalline film formed of hexagonal platelets mostly oriented with the c axis parallel to the substrate (Fig. 9), although the absence of significant preferred orientation from the XRD data suggests it is likely that there are different crystal orientations within the film. Energy dispersive X-ray spectroscopy (EDX) gives the ratio of Nb : Se in the films of 35.8% : 64.2%, with no detectable Cl contamination.

Fig. 8 In plane XRD (blue), grazing incidence (incidence angle = 1°) XRD (black) from the NbSe₂ thin film deposited by LPCVD using [NbSe₂Cl₃(SeⁿBu₂)] at 650 °C; simulated XRD pattern from bulk NbSe₂ (red).²⁷ The broad feature at 2θ ∼22° is from the SiO₂ substrate.

Fig. 9 SEM images of the NbSe₂ film deposited from [NbSe₂Cl₃(SeⁿBu₂)] at 650 °C.

Conclusions

A series of thioether and selenoether complexes derived from NbSCl₃ is reported. NbSCl₃ has been shown to be a weaker Lewis acid than NbCl₅. Selected complexes containing ⁿBu substituents have been used as single source precursors to produce 3R-NbS₂ thin films via low pressure CVD, with preferred <00l> orientation. In contrast, NbSeCl₃ does not form similar complexes, however treatment of [NbCl₅(SeⁿBu₂)] with Se(SiMe₃)₂ forms the complex [NbSe₂Cl₃(SeⁿBu₂)] which has been used as a single source precursor for low pressure CVD of 2H-NbSe₂ thin films.

Experimental

Syntheses were performed by using standard Schlenk and glove-box techniques under a dry N₂ atmosphere. NbCl₅ and S(SiMe₃)₂ were obtained from Sigma-Aldrich and Se(SiMe₃)₂ from Fluorochem and stored in the glovebox in ampoules under N₂. Solvents were dried by distillation from CaH₂ (CH₂Cl₂, MeCN) or Na/benzophenone ketyl (n-hexane). Dichalcogenoethers, RE(CH₂)_nER (E = S, Se; n = 2, 3; R = Me, ⁱPr, ⁿBu), were prepared via literature methods.^24,25

Physical measurements

Infrared spectra were recorded on a PerkinElmer Spectrum 100 spectrometer in the range 4000–200 cm⁻¹, and samples prepared as Nujol mulls between two CsI plates. ¹H NMR spectra were recorded using a Bruker AV II 400 spectrometer and referenced to the residual protio-resonance of the solvent. Multinuclear (⁷⁷Se, ⁹³Nb) NMR spectra were recorded from CD₂Cl₂, CDCl₃ or CD₃CN solutions using a Bruker AV II 400 spectrometer and referenced to neat external SeMe₂ and [Et₄N][NbCl₆] in MeCN (δ = 0), respectively. Microanalyses on new compounds were undertaken by London Metropolitan University. Thermogravimetric analysis (TGA) data for [NbSCl₃(SⁿBu₂)] (ESI Fig. S4†), [NbSCl₅(ⁿBuSCH₂CH₂CH₂SⁿBu)] (ESI Fig. S5†) and [NbSe₂Cl₃(SeⁿBu₂)] (ESI Fig. 6†) were collected via a Netzsch TG209 F1 Libra analyser under a flow of argon at 65 mL min⁻¹, contained within a dry, nitrogen purged glovebox. The temperature was increased at a rate of 10 °C min⁻¹. Samples were loaded in Al sample pans.

X-ray experimental

Data collections used a Rigaku AFC12 goniometer equipped with an enhanced sensitivity (HG) Saturn724+ detector mounted at the window of an FR-E+ SuperBright molybdenum (λ = 0.71073) rotating anode generator with VHF Varimax optics (70 micron focus) with the crystal held at 100 K (N₂ cryostream). Crystallographic parameters are in the ESI (Table S1†). Structure solution and refinement were performed using SHELX(S/L)97, SHELX-2014/7 and were mostly straightforward, except for the Cl and terminal S atoms of the [NbSCl₃{MeSe(CH₂)₃SeMe}] which were disordered and therefore refined with split occupancies giving a 50 : 50 ratio.^28,29 H atoms were added and refined with a riding model.

Complex/precursor preparations. NbSCl ₃ : NbCl₅ (405 mg, 1.5 mmol) was suspended in CH₂Cl₂ (20 mL) in an ice bath (0 °C). A solution of (Me₃Si)₂S (260 mg, 1.5 mmol) in CH₂Cl₂ (6 mL) was added with stirring. The solution changed from orange to black immediately and was stirred for 30 min at room temperature. The solvent was removed in vacuo, leaving a dark green powder. Yield: 295 mg, 83%. IR (Nujol, cm⁻¹): 550s (Nb=S).

[NbSCl ₃ (MeCN) ₂ ]: Method 1: The complex was prepared by a modified literature method.¹⁶ NbCl₅ (405 mg, 1.5 mmol) was dissolved in MeCN (30 mL) and cooled in an ice bath. A solution of (Me₃Si)₂S (260 mg, 1.5 mmol) in MeCN (20 mL) was added. The reaction mixture was then removed from the ice bath and the colour changed from yellow to green over a few minutes. The solution was stirred for 1 h giving a green solution. The solvent was then removed in vacuo leaving a yellow–green solid. Yield: 337 mg, 72%. Required for C₄H₆N₂Cl₃NbS (313.33): C, 15.33; H, 1.93; N, 8.94. Found: C, 15.26; H, 1.96; N, 8.82%. IR (Nujol, cm⁻¹): 2316, 2287s (MeCN), 530s (Nb=S), 355sh, 343s, 319s (Nb–Cl). ¹H NMR (CD₂Cl₂, 295 K): δ = 1.97 (s, MeCN). ⁹³Nb NMR (CD₃CN, 295 K): δ = 414.

Method 2: NbSCl₃ (83 mg, 0.35 mmol) was dissolved in MeCN (20 mL) with stirred for 30 min and giving a dark green solution. The solution was filtered and the filtrate taken to dryness in vacuo. Green powder. Yield: 100 mg, 90%. The product was spectroscopically identical to that of [NbSCl₃(MeCN)₂] produced by Method 1.

[NbSCl ₃ (SMe ₂ )]: NbCl₅ (135 mg, 0.5 mmol) was suspended in CH₂Cl₂ (10 mL). SMe₂ (1 mL) was added with stirring for 30 min, giving a dark brown solution. The solution was then cooled in an ice bath (0 °C) and a solution of S(SiMe₃)₂ (90 mg, 0.5 mmol) in CH₂Cl₂ (ca. 1 mL) was added slowly with stirring for 30 min. The solution was removed from ice bath and stirred for another 5 min and then the volatiles were removed in vacuo, leaving a light yellow green solid. Yield: 55 mg, 38%. Required for C₂H₆Cl₃NbS₂ (293.46): C, 9.19; H, 2.06. Found: C, 9.29; H, 2.37%. IR (Nujol, cm⁻¹): 530 (Nb=S), 369, 356, 322 (Nb–Cl). ¹H NMR (CDCl₃, 295 K): δ = 2.32 (s, SMe₂). ⁹³Nb NMR (CD₂Cl₂, 295 K): δ = 651. Yellow–green crystals grew by allowing CH₂Cl₂ solution to evaporate slowly under a nitrogen atmosphere.

[NbSCl ₃ (S ⁿ Bu ₂ )]: NbCl₅ (270 mg, 1.0 mmol) was suspended in CH₂Cl₂ (10 mL) and a solution of SⁿBu₂ (147 mg, 1.0 mmol) in CH₂Cl₂ (3 mL) was added with stirring. After 15 min an orange solution formed. A solution of S(SiMe₃)₂ (179 mg, 1.0 mmol) and CH₂Cl₂ (3 mL) was added with stirring. The colour changed to dark green after stirring for 30 min. The solution was taken to dryness in vacuo, leaving a sticky oil which was washed with n-hexane (5 mL), before drying in vacuo. Thick dark oil. Yield: 331 mg, 88%. Required for C₈H₁₈Cl₃NbS₂ (377.62): C, 25.44; H, 4.80. Found: C, 25.58; H, 4.88%. IR (Nujol, cm⁻¹): 554 (Nb=S), 387, 374, 359sh (Nb–Cl). ¹H NMR (CDCl₃, 295 K): δ = 0.97 (t, [6H], Me), 1.48 (m, [4H], CH₂Me), 1.74 (m, [4H], SCH₂CH₂CH₂), 2.94 (t, [4H], SCH₂). ⁹³Nb NMR (CD₂Cl₂, 295 K): δ = 654.

[NbSCl ₃ (Se ⁿ Bu ₂ )]: NbCl₅ (270 mg, 1.0 mmol) was suspended in CH₂Cl₂ (10 mL) and a solution of SeⁿBu₂ (193 mg, 1.0 mmol) in CH₂Cl₂ (3 mL) was added with stirring, giving a red solution, which was stirred for 1 h. S(SiMe₃)₂ (0.21 mL, 1.0 mmol) was then added. The solution turned dark green, and after stirring for 30 min the solvent was removed in vacuo to leave a sticky oil. After washing with hexane (5 mL), the oil was dried in vacuo. Sticky black oil. Yield: 200 mg, 47%. Required for C₈H₁₈Cl₃NbSSe (424.52): C, 22.63; H, 4.27. Found: C, 22.52; H, 4.29%. IR (Nujol, cm⁻¹): 530 (Nb=S), 380, 366, 355sh (Nb–Cl). ¹H NMR (CDCl₃, 295 K): δ = 0.93 (t, [6H], Me), 1.42 (m, [4H], CH₂Me), 1.66 (m, [4H], SeCH₂CH₂), 2.61 (br, [4H], SeCH₂). ⁷⁷Se (CD₂Cl₂, 295 K): no resonance. ⁹³Nb NMR (CD₂Cl₂, 295 K): δ = 694.

[NbSCl ₃ (MeSCH ₂ CH ₂ SMe)]: [NbSCl₃(MeCN)₂] (94 mg, 0.3 mmol) was dissolved in CH₂Cl₂ (10 mL) at room temperature. A solution of MeSCH₂CH₂SMe (110 mg, 0.9 mmol) and CH₂Cl₂ (1 mL) was added and the solution stirred for 30 min. Some green-yellow powder formed in the solution. After filtering, the green solution was evaporated to dryness in vacuo. n-Hexane (10 mL) was added to wash the solid which was separated and dried in vacuo. Yield: 67 mg, 63%. Required for C₄H₁₀Cl₃NbS₃ (353.58): C, 13.59; H, 2.85. Found: C, 13.64; H, 2.93%. IR (Nujol, cm⁻¹): 526 (Nb=S), 361sh, 349, 319 (Nb–Cl). ¹H NMR (CD₂Cl₂, 295 K): δ = 2.22 (br, [3H], SMe_{trans Cl}), 2.78 (br, [3H], SMe_{trans S}) 3.04(s), 3.26(s) ([4H], CH₂); (183 K): 2.06 (s, [6H], Me), 2.63 (s, [4H], CH₂) (both ‘free’ dithoether), 2.12 (s, [3H]), 2.18 (s, [3H]), 2.76, 2.94 (CH₂) (coord. dithioether) (see text). ⁹³Nb NMR (CD₂Cl₂, 295 K): δ = 507. Green crystals grew by allowing a CH₂Cl₂ solution to evaporate under a dinitrogen atmosphere.

[NbSCl ₃ ( ⁱ PrSCH ₂ CH ₂ S ⁱ Pr)]: Was made similarly and isolated as a green powder. Yield: 76%. Required for C₈H₁₈Cl₃NbS₃ (409.69): C, 23.45; H, 4.43. Found: C, 23.27; H, 4.41%. IR (Nujol, cm⁻¹): 527 (Nb=S), 365 (sh), 348, 318 (Nb–Cl). ¹H NMR (CD₂Cl₂, 295 K): δ = 1.32 (br s, [6H], SMe), 1.59 (br s, [6H], SMe), 3.01–3.28 (br, [4H], SCH₂), 3.46 (br, [2H], SCH). ⁹³Nb NMR (CD₂Cl₂, 295 K): δ = 523. Green crystals grew by allowing a CH₂Cl₂ solution to evaporate slowly under a dinitrogen atmosphere.

[NbSCl ₃ {MeS(CH ₂ ) ₃ SMe}]: Was made similarly and isolated as a green solid. Yield: 68% Required for C₅H₁₂Cl₃NbS₃ (367.61): C, 16.34; H, 3.29. Found: C, 16.48; H, 3.21%. IR (Nujol, cm⁻¹): 524 (Nb=S), 369 sh, 345, 323 (Nb–Cl). ¹H NMR (CDCl₃, 295 K): δ = 2.14 (br, [2H], SCH₂CH₂), 2.45 (vbr, [6H], SMe), 2.96 (vbr, [4H], SCH₂). ⁹³Nb NMR (CD₂Cl₂, 295 K): δ = 530.

[NbSCl ₃ { ⁿ BuS(CH ₂ ) ₃ S ⁿ Bu}]: Was made similarly and isolated as a dark green oil after washing with n-hexane, decanting off the washings and drying in vacuo. Yield: 76%. Required for C₁₁H₂₄Cl₃NbS₃ (451.77): C, 29.24; H, 5.35. Found: C, 29.37; H, 5.45%. IR (Nujol, cm⁻¹): 529 (Nb=S), 349, 322 (Nb–Cl). ⁹³Nb NMR (CD₂Cl₂, 295 K): δ = 534.

[NbSCl ₃ {MeSe(CH ₂ ) ₃ SeMe}]: Was made similarly and isolated as a yellow brown powder. Yield: 58%. Required for C₅H₁₂Cl₃NbSSe₂ (461.40): C, 13.02; H, 2.62. Found: C, 13.17; H, 2.74%. IR (Nujol, cm⁻¹): 521 (Nb=S), 342, 320 (Nb–Cl). ¹H NMR (CD₂Cl₂, 295 K): δ = 2.25 (br, [8H], SeMe and CH₂CH₂Se), 2.92 (br, [4H], CH₂Se). ⁷⁷Se{¹H} NMR (CD₂Cl₂, 295 K): no resonance; (223 K): δ = 163, 70, 68. ⁹³Nb NMR (CD₂Cl₂, 295 K): δ = 547. Yellow crystals grew by allowing a CH₂Cl₂ solution to evaporate under a dinitrogen atmosphere.

[NbSeCl ₃ (MeCN) ₂ ]: NbCl₅ (135 mg, 0.5 mmol) was dissolved in MeCN (10 mL). A solution of Se(SiMe₃)₂ (113 mg, 0.5 mmol) in MeCN (5 mL) was then added and the reaction mixture was stirred for 30 min. The colour changed to dark brown. The solvent was removed in vacuo leaving a brown solid. Yield: 150 mg, 83%. Required for C₄H₆Cl₃N₂NbSe (360.33): C, 13.33; H, 1.68; N, 7.77. Found: C, 13.25; H, 1.65; N, 7.57%. IR (Nujol, cm⁻¹): 2310, 2281 (MeCN), 397 (Nb=Se?), 377, 344 (Nb–Cl). ⁹³Nb NMR (CD₂Cl₂, 295 K): δ = 923.

[NbSe ₂ Cl ₃ (Se ⁿ Bu ₂ )]: NbCl₅ (235 mg, 0.88 mmol) was suspended in CH₂Cl₂ (20 mL). A solution of ⁿBu₂Se (177 mg, 0.88 mmol) and CH₂Cl₂ (ca. 1.5 mL) was added with stirring for 1 hour. The reaction mixture changed colour to dark red and all the NbCl₅ dissolved. A solution of Se(SiMe₃)₂ (0.22 mL, 0.88 mmol) and CH₂Cl₂ (ca. 1.7 mL) was added, causing a colour change from dark red to black. The solution was stirred for 15 min before it was taken to dryness in vacuo. The resulting black solid was washed with n-hexane (15 mL) and then dried in vacuo. Yield: 298 mg, 61%. Required for C₈H₁₈Cl₃NbSe₃ (550.37): C, 17.46; H, 3.3. Found: C, 17.59; H, 3.38. IR (Nujol, cm⁻¹): 344, 319, 272 (Nb–Cl). ¹H NMR (CDCl₃, 295 K): δ 0.94 (t, [6H], Me), 1.44 (m, [4H], CH₂Me), 1.70 (m, [4H], SeCH₂CH₂CH₂), 2.71 (t, [4H], SeCH₂).

LPCVD general procedure. The precursor (20–100 mg) was loaded into the end of a silica tube in an N₂ purged glove box. Then the silica substrates (∼1 × 8 × 20 mm³) were loaded in the tube and placed end-to-end. The tube was set in a furnace so that the substrates were in the heated zone and the precursor was ca. 2 cm away from the start of the heated zone. The tube was evacuated to 0.5 mmHg, and the furnace was heated to the requisite temperature between 600 and 800 °C. The tube was then moved into the furnace. The position of the sample was maintained until the all the precursor had evaporated. The tube was then cooled to room temperature and the tiles were unloaded at ambient temperature and stored in air.

LPCVD from [NbSCl₃(SⁿBu₂)], [NbSCl₃{ⁿBuS(CH₂)₃SⁿBu}] and [NbSe₂Cl₃(SeⁿBu₂)] resulted in deposition of reflective black films on tiles positioned in the hotter region of the furnace (i.e. NbS₂ actual deposition temperature ∼670 °C; NbSe₂ actual deposition temperature ∼625 °C – measured by profiling the furnace).

Film characterisation. X-Ray diffraction (XRD) patterns were collected in grazing incidence mode (θ₁ = 1°) or in-plane mode (θ₁ = 0.5°, 2θ_χ scan with the detector scanning in the film plane) using a Rigaku SmartLab diffractometer (Cu-K_α, λ = 1.5418 Å) with parallel X-ray beam and a DTex Ultra 250 1D detector. Phase matching and lattice parameter refinement used the PDXL2 software package³⁰ and diffraction patterns from ICSD.³¹

Scanning electron microscopy (SEM) was performed on samples of NbSe₂ using a Philips XL30 ESEM and with an acceleration voltage of 10 kV or 15 kV, whilst SEM measurements on NbS₂ films used a JEOL JSM6500 and an accelerating voltage of 10 kV.

Energy dispersive X-ray (EDX) data on NbSe₂ samples were obtained at accelerating voltage of 10 and 15 kV with a Thermofisher Ultradry NSS 3 (XL30) detector or at an accelerating voltage of 15 kV with an Oxford INCA x-act X-ray detector (JSM6500) for NbS₂ samples.

Acknowledgements

We thank the EPSRC for funding the SmartLab diffractometer (via EP/K00509X/1 and EP/K009877/1) and the University of Southampton for a VC Scholarship (to Y.-P. C).

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Footnote

† Electronic supplementary information (ESI) available: Crystallographic parameters (Table S1), XRD data (Fig. S1) and SEM images (Fig. S2) for the NbS₂ film obtained from [NbSCl₃{ⁿBuS(CH₂)₃SⁿBu}] and XRD data from a NbSe₂ film obtained by using less [NbSe₂Cl₃(SeⁿBu₂)] precursor (∼50 mg). Cif files for the four crystallographically characterised precursor complexes. CCDC 1542831–1542834. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt01911d

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