Bioinspired Tungsten Complexes Employing a Thioether Scorpionate

Article pubs.acs.org/IC

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

Bioinspired Tungsten Complexes Employing a Thioether Scorpionate Ligand Madeleine A. Ehweiner, Carina Vidovic,̌ Ferdinand Belaj, and Nadia C. Mösch-Zanetti* Institute of Chemistry, Inorganic Chemistry, University of Graz, Schubertstrasse 1, 8010 Graz, Austria

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S Supporting Information *

ABSTRACT: The synthesis and characterization of a series of novel tungsten complexes employing the bioinspired, sulfur-rich scorpionate ligand [PhTt] (phenyltris((methylthio)methyl)borate) are reported. Starting from the previously published tungsten precursor [WBr2(CO)3(NCMe)2], a salt metathesis reaction with 1 equiv of Cs[PhTt] led to the desired complex [WBr(CO)3(PhTt)] (1), making it the first tungsten complex employing a poly(thioether)borate ligand. Surprisingly, the reaction of [WBr2(CO)3(NCMe)2] with an excess of the ligand gave complex [W(CO)2(η2-CH2SMe)(PhTt)] (2) with a bidentate (methylthio)methanide ligand as the major product. Thereby, phenyldi((methylthio)methyl)borane is formed, which was isolated and characterized by NMR spectroscopy. The bromido ligand in [WBr(CO)3(PhTt)] was further substituted by the S,N-bidentate methimazole in order to make the first coordination sphere more sulfur-rich forming [W(CO)2(mt)(PhTt)] (3). Alkyne tungsten complexes employing the sulfur-rich scorpionate ligand were accessible by reaction of [WBr2(CO)(C2R2)2(NCMe)] (R = Me, Ph) with Cs[PhTt] forming [WBr(CO)(C2R2)2(PhTt-S,S′)] (R = Me 4, Ph 5), with the potentially tridentate ligand coordinated only via two sulfur atoms. In the case of 4, the higher flexibility of the bidentate coordination leads to the formation of two isomers with respect to the sixmembered ring formed by the tungsten center and the two coordinated sulfur atoms of the ligand. All complexes 1−5 were characterized by single-crystal X-ray diffraction analysis.

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INTRODUCTION Generally, the active site of all tungstoenzymes described to date harbors one tungsten atom coordinated by four sulfur atoms deriving from the dithiolene moiety of two molybdopterin cofactors plus oxygen and/or sulfur and/or selenium ligands.1−6 Although their architecture is thus fairly similar, only the nonredox active enzyme acetylene hydratase (AH) of Pelobacter acetylenicus is capable of catalyzing the hydration of acetylene to acetaldehyde.7−11 Apart from nitrogenase, which reduces it to ethylene, acetylene hydratase is the only enzyme known to accept acetylene as a substrate.12,13 In principle, two fundamentally different mechanisms based on theoretical calculations have been proposed so far: a first shell14−16 and a second shell mechanism.17 In the first shell mechanism, acetylene and all reaction intermediates are directly coordinated to the tungsten center, whereas the second shell mechanism includes a tungsten-coordinated, thus electrophilic, water molecule and no tungsten-acetylene adduct. Although the overall reaction of the second shell mechanism was calculated to be exothermic by 21.4 kcal/mol, the first shell mechanism is the only one with realistic energy barriers.14,18,19 Synthetic approaches to elucidate the mechanism of acetylene hydratase and other tungstoenzymes have either been biomimetic or bioinspired: The biomimetic approach implies the similarity of the active site architecture by © XXXX American Chemical Society

employing dithiolene ligands, whereas the bioinspired approach aims to achieve the enzymatic function under ambient conditions by tuning the electronic properties of the tungsten center by utilizing nondithiolene ligands.20,21 So far, various types of dithiolenes have been used in structural modeling chemistry, yet only a few scientists have handled the problem of linking a pterin to the dithiolene moiety to get access to both essential components of molybdopterin.20−24 Although dithiolene ligands in general are supposed to be the closest structural mimics of the cofactor, it has not yet been possible to identify or even isolate a tungsten-dithiolene complex with a coordinated acetylene molecule which would address a first shell mechanism.25,26 Nevertheless, monomeric tungsten(IV) acetylene complexes could be successfully synthesized by utilizing S,S- or S,N-bidentate ligands.27,28 Aside from dithiolene ligands, scorpionate ligands have facilitated the development of a number of important synthetic models for molybdo- and tungstoenzymes.29−37 The advantage of these ligands is not only the fact that coordination to a metal is thermodynamically favored because of an increase of entropy, but complexes are also less prone to hydrolysis. Their synthesis is often much easier, and steric and electronic Received: April 5, 2019

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DOI: 10.1021/acs.inorgchem.9b00973 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry properties can be tuned by minor variations.30 Besides, we consider redox behavior of dithiolene ligands less important for models of AH since the enzymatic reaction represents a nonredox reaction. This renders the class of scorpionate ligands useful for the development of such model complexes. Interestingly, several tungsten(II) acetylene complexes employing the hard scorpionate ligand hydrotris(3,5-dimethyl-1pyrazolyl)borate (Tp′) have been effectively synthesized and investigated.38−40 Some of them could even be oxidized to the corresponding tungsten(IV) complex.41,42 In contrast, only one tungsten(II) alkyne complex employing a sulfur-rich scorpionate ligand is known in the literature, though it has not yet been crystallized.43 Apart from scorpionate ligands, bi- or tridentate thioether ligands are equally able to provide a soft coordination sphere for metal ions, and therefore, they have been utilized in the preparation of tungsten(II) alkyne complexes of the type [WX2(CO)(C2R2)(L-S,S′)] and [WX(CO)(C2R2)(L-S,S′,S″)]X (X = Br, I; R = Me, Ph).44 In order to circumvent the limitation for a S3 donor set and to afford a S3− donor for performing salt metathesis reactions, poly(thioether)borate scorpionate ligands were developed in the 1990s.45,46 The ligand phenyltris((methylthio)methyl)borate, [PhTt], and some modifications of it are now well established in the literature with numerous complexes of Fe(II), Co(II), Ni(II), Cu(I), Zn(II), Mo(0), Mo(II), Pd(II), and Cd(II) but none of tungsten.45−50 Inspired by the diversity of soft scorpionate ligands and their lack in sulfur-rich tungsten alkyne complexes, [PhTt] was chosen to react with three literature-known tungsten(II) precursors. Thus, the versatile tungsten precursor [WBr2(CO)3(NCMe)2] was reacted with [PhTt], and in terms of active site modeling of acetylene hydratase, the reactivity of the triscarbonyl complex [WBr(CO)3(PhTt)] toward the symmetric alkynes acetylene, dimethylacetylene, and diphenylacetylene was intensively investigated. As an alternative synthetic pathway, the previously published tungsten bis-alkyne precursors [WBr2(CO)(C2R2)2(NCMe)] (R = Me, Ph), accessible via reaction of [WBr2(CO)3(NCMe)2] with the respective alkyne, were reacted with the scorpionate ligand [PhTt]. The analogous acetylene (R = H) starting material has not yet been described in the literature. The behavior and benefits of all synthesized compounds were investigated and compared with regard to future trials to mimic the activity of acetylene hydratase or other tungstoenzymes. Within this report, six novel molecular structures of sulfur-rich tungsten complexes are provided. Until now, there are only a few examples of tungsten complexes employing a sulfur-rich scorpionate ligand known in the literature.51,52

acetonitrile, acetone, and DMSO yet insoluble in chlorinated hydrocarbons, which is contrary to the solubility of [Bu4N][PhTt]. Generally, all reactions described below were carried out with both ligand salts, yet only successful reaction conditions are pointed out if not stated otherwise. Complex Synthesis. The versatile tungsten precursor complex [WBr2(CO)3(NCMe)2]54 was synthesized from the dimeric compound [W 2 Br 4 (CO) 7 ] 55 by stirring it in acetonitrile for 1 h. The precursor complexes [WBr2(CO)(C2R2)2(NCMe)] (R = Me, Ph)56,57 were synthesized according to a modified literature procedure. For the synthesis of the sulfur-rich triscarbonyl tungsten complex [WBr(CO) 3 (PhTt)] (1), the reaction of [WBr2(CO)3(NCMe)2] with 1.1 equiv of Cs[PhTt] was performed in acetonitrile according to Scheme 1. Scheme 1. Synthesis of [WBr(CO)3(PhTt)] (1)

After 1 h, the reaction mixture was concentrated to precipitate the product and the supernatant solution was removed. The remaining solid was suspended in CH2Cl2, and after filtration through a pad of Celite to remove insoluble CsBr, 1 was crystallized from acetonitrile in 89% yield. The orange crystals are highly soluble in chlorinated hydrocarbons, THF, and toluene, poorly soluble in acetonitrile and diethyl ether, and largely stable to air and moisture. The corresponding IR spectrum shows three strong carbonyl bands at 2026, 1940, and 1908 cm−1, indicative of rather weak π backbonding from the metal to the carbonyl ligands. 1 H and 13C NMR spectra recorded in various solvents show only one set of signals. Interestingly, only one broad singlet at δ = 223.36 ppm (CD2Cl2) was observed for all three CO ligands in the 13C NMR spectrum revealing rather deshielded, thus electrophilic, carbonyl carbons. The equivalence of the signals is ascribed to the fluxionality of the complex in solution, which is a typical property of seven-coordinate triscarbonyl tungsten(II) complexes.58 Under nitrogen atmosphere, complex 1 is only limitedly stable in solution and starts to decompose after a few hours under formation of an unidentified white precipitate. Furthermore, the stability of 1 toward higher temperatures was investigated by means of in situ IR spectroscopy. Therefore, a concentrated solution of 1 in toluene was continuously heated with an oil bath, monitoring an intensity decrease of the CO absorption bands. The complex solution was held at each temperature for at least 15 min. As shown in Figure S7, full decomposition of complex 1 was observed at 60 °C. Surprisingly, upon using excess of [Bu4N][PhTt] or Cs[PhTt], the reaction with [WBr2(CO)3(NCMe)2] in acetonitrile gave the novel biscarbonyl tungsten complex [WBr(CO)2(η2-CH2SMe)(PhTt)] (2) as the major product according to Scheme 2. Beside the expected coordination of [PhTt], an additional (methylthio)methanide originating from fragmentation of a second borate ligand is coordinated to tungsten, making complex 2 an organometallic compound.

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RESULTS AND DISCUSSION Ligand Synthesis. The ligand salt [Bu4N][PhTt] was prepared according to modified published procedures by reaction of dichlorophenylborane with deprotonated dimethyl sulfide in 44% yield. 1H and 13C NMR spectra are consistent with the literature data.46 Since a salt metathesis reaction of [WBr2(CO)3(NCMe)2] with [Bu4N][PhTt] leads to the formation of [Bu4N]Br, which is highly soluble in most solvents and therefore hard to remove from the reaction mixture, a different cation was introduced. Precipitation with CsCl instead of [Bu4N]Cl in the last step of the workup gave a white powder which was identified as Cs[PhTt] by 1H and 13C NMR spectroscopy.53 The new ligand salt is soluble in B


Article

Inorganic Chemistry Scheme 2. Formation of [WBr(CO)2(η2-CH2SMe)(PhTt)] (2)

accordance with previously published results for nickel complexes.60 Complex 2 is soluble in chlorinated hydrocarbons, toluene, and THF and mostly stable to air in both the solid state and solution. IR spectroscopy revealed two significant CO stretching vibrations at 1925 and 1810 cm−1 characteristic of an electron-rich tungsten center. In the 1H NMR spectrum, the two protons on the carbon atom coordinated to the tungsten center are diastereotopic as evidenced by two symmetric doublets with a roof effect at δ = 3.30 and 3.00 ppm. Furthermore, the proton resonating at δ = 3.30 ppm displays weak coupling with the NMR active tungsten isotope 183W. The two protons of the methylene groups bound to the boron atom are also diastereotopic and display two broad signals at δ = 2.20 and 1.96 ppm integrating for three protons each, whereas the methyl protons of the ligands [PhTt] and (methylthio)methanide have the same chemical shift, respectively. The 13C NMR spectrum shows two sharp singlets with 183W satellites at δ = 224.82 (1JWC = 143.9 Hz) and 217.19 ppm (1JWC = 182.6 Hz) resulting from the nonfluxional carbonyl carbons. The methylene carbon bound to the tungsten center resonates at δ = 36.52 ppm and couples with 183W (1JWC = 19.9 Hz) as well. Complex 1 was investigated regarding the substitution of the remaining bromido ligand and one carbonyl with the S,Nbidentate ligand methimazole (mt) as the utilization of such ligands has previously been proven to be advantageous in terms of acetylene activation.28 In this way, the coordination sphere of the tungsten center would be more sulfur-rich, similar to complex 2. For the synthesis of [W(CO)2(mt)(PhTt)] (3), a solution of Na(mt) was added to a stirred solution of 1 in THF according to Scheme 4. After filtration through Celite, complex 3 could be crystallized from acetonitrile in 51% yield. The dark orange crystals are highly soluble in chlorinated hydrocarbons and THF and stable to air in the solid state. Similar to complex 2, the IR spectrum of 3 shows two strong carbonyl bands at 1922 and 1826 cm−1. Due to the fluxionality of the system, 1H NMR spectra show only one broad singlet for the methylene protons of the scorpionate ligand. For the same reason, carbonyl carbons have never been

This kind of reaction has previously been observed in nickel complexes employing modified [PhTt] ligands.59−61 In this reaction, phenyldi((methylthio)methyl)borane is formed as a byproduct, which was confirmed by 1H and 13C NMR spectroscopy. For comparison with literature data, the pyridine adduct PhB(CH2SMe)2·py was generated in situ by addition of approximately 1 equiv of pyridine to a suspension of PhB(CH2SMe)2 in C6D6 resulting in a colorless solution.62 The synthesis of 2 could be optimized by using exactly 2 equiv of [Bu4N][PhTt] to give the desired product as orange crystals in 38% yield after purification using silica gel. Since NMR spectroscopy of the reaction mixture indicates nearly full conversion of the starting material without any mentionable side product formation, the significant loss of 2 during workup is probably due to interaction with and/or partial decomposition of 2 on the silica gel. However, removal of [Bu4N]Br by precipitation with CH2Cl2/heptane was found to be less efficient than chromatography on silica gel with a yield of less than 30%. Using Cs[PhTt] instead of [Bu4N][PhTt] leads to the same product, however, various unidentified side products are generated during the reaction. Unexpectedly, the direct synthesis of complex 2 by reaction of 1 with LiCH2SMe(TMEDA) in toluene was unsuccessful. A proposed reaction mechanism for the formation of 2 is illustrated in Scheme 3: After formation of complex 1, the second bromido ligand is substituted by a [PhTt] molecule, which is coordinated to tungsten only with one thioether arm. Upon substitution of a carbonyl ligand, a W−C bond is formed while a B−C bond is broken under formation of the neutral borane side product. The proposed mechanism is in

Scheme 3. Proposed Reaction Mechanism for the Formation of [WBr(CO)2(η2-CH2SMe)(PhTt)] (2)

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dichloromethane and generally unstable in solution at room temperature (rt) as it releases its coordinated dimethylacetylenes and decomposes to an unidentified magenta-colored compound after a few hours. However, solutions stored at −35 °C were found to be stable for several days. Single-crystal X-ray diffraction analysis and NMR spectroscopy confirmed the existence of two isomers (4a and 4b) with respect to the sixmembered ring formed by the tungsten center and the two coordinated sulfur atoms of the ligand. 1H NMR spectra recorded in different solvents within 30 min after dissolution of a single crystal of 4a show both isomers in a 3:2 ratio and after a few hours in a 1:1 ratio, which reveals that 4a converts to 4b upon dissolution. In Table 1, IR and 13C NMR data of the carbonyl ligands of complexes 1−5 are summarized for comparison.

Scheme 4. Synthesis of [W(CO)2(mt)(PhTt)] (3) Starting from [WBr(CO)3(PhTt)] (1)

detected by 13C NMR spectroscopy. Both findings are contrary to the chemically similar compound 2, in which the methylene protons as well as the carbonyl carbons exhibit much less dynamic behavior. Eventually, compounds 1−3 were investigated regarding their reactivity toward the symmetric alkynes acetylene, dimethylacetylene, and diphenylacetylene. However, all reactions resulted in unidentified complex mixtures. Neither feasible complexes with one coordinated alkyne nor compounds with two coordinated alkynes could be identified. Consequently, an alternative synthetic pathway was studied. In order to synthesize the sulfur-rich alkyne complexes [WBr(CO)(C2R2)(PhTt)] (R = Me, Ph), a salt metathesis reaction of the previously published bis-alkyne complexes [WBr2(CO)(C2R2)2(NCMe)] (R = Me, Ph) with Cs[PhTt] was carried out. The analogous acetylene starting material (R = H) has not yet been described in the literature. Unexpectedly, the formation of [WBr(CO)(C2R2)2(PhTt-S,S′)] (R = Me 4, Ph 5) with the ligand coordinated only via two sulfur atoms was revealed by NMR spectroscopy and single-crystal X-ray diffraction analysis (Scheme 5). This finding suggests that the scorpionate ligand, while sulfur-rich, is too weakly donating to coordinate in a tridentate fashion and is therefore not able to replace a coordinated alkyne. This is possibly the reason why direct synthesis of alkyne complexes from 1 is not an option as the alkyne might replace the sulfur ligand. For the synthesis of complexes 4 and 5, respectively, Cs[PhTt] was added to a solution of [WBr 2 (CO)(C2R2)2(NCMe)] (R = Me, Ph) in acetonitrile. After stirring for 30 min at −15 °C, the solvent was evaporated in vacuo. Suspending in dichloromethane, filtration through a pad of Celite, and recrystallization from dichloromethane/heptane at −20 °C gave 4 as yellow crystals in 83% yield. In contrast, compound 5 could not be selectively synthesized and purified. Nevertheless, single crystals suitable for X-ray diffraction analysis could be obtained once (vide infra) confirming its principle existence. The isolation of the crystals also allowed the recording of an IR spectrum; however, the compound was found to be too sensitive in solution for NMR spectroscopy. IR spectroscopy of complexes 4 and 5 revealed CO stretching vibrations at 2040 and 2074 cm−1, respectively, which are quite similar to the corresponding starting material (R = Me, 2035 cm−1; R = Ph, 2099 cm−1). Complex 4 is well soluble in

Table 1. Infrared and 13C NMR Data of the Carbonyl Ligands of Complexes 1−5 complex

ν (CO) (cm−1)

δ (CO) (ppm)

1 2 3 4 5

2026, 1940, 1908 1925, 1810 1922, 1826 2040 2074

223.32 224.86, 217.22 208.74

Molecular Structures. Molecular structures of all novel complexes provided in this Article were determined by singlecrystal X-ray diffraction analysis. Molecular views of 1−3 are given in Figure 1 and of complexes 4a, 4b, and 5 in Figure 2. Important bond lengths for complexes 1−3 are given in Table 2 and for complexes 4a, 4b, and 5 in Table 3. Full crystallographic data such as structure refinement and experimental details are provided within the Supporting Information. In complexes 1−3, the tungsten atom is coordinated in a capped octahedral fashion. Complexes 4a, 4b, and 5 contain a formally eight-coordinate tungsten center with a distorted octahedral coordination sphere. Interestingly, the W−S bond lengths of 1−3 vary significantly among themselves (Table 2). This finding can be explained by the large trans influence of a carbonyl ligand which can be observed when a carbonyl carbon and a sulfur enclose an angle of 160−180 °C. Although one sulfur adopts the position trans to a carbonyl carbon (C3− W1−S3 167.00(8)°) in complex 1, the other two sulfur atoms lie opposite the bisector of the angle enclosed by two carbonyl carbons, and it seems that these influences are fairly similar which gives rise to a moderate elongation of all W−S bonds. In complex 2, S2 encloses an angle close to 180° (C2−W1− S2 175.03(4)°) with an opposite carbonyl carbon which stretches the respective W−S bond significantly (W1−S2 2.5804(3) Å). Moreover, the W1−S1 bond in 3 is remarkably

Scheme 5. Synthesis of Complexes [WBr(CO)(C2R2)2(PhTt-S,S′)] (R = Me 4, Ph 5)

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Figure 1. Molecular views (50% probability level) of 1 (top), 2 (bottom left), and 3 (bottom right) showing the atomic numbering scheme. H atoms as well as solvent molecules are omitted for clarity reasons.

triscarbonyl complex [WBr(CO)3(PhTt)] (1). The sulfur-rich organometallic complex [W(CO)2(η2-CH2SMe)(PhTt)] (2) was surprisingly isolated as a major product of the reaction of [WBr2(CO)3(NCMe)2] with excess of [Bu4N][PhTt] or Cs[PhTt]. Thereby, phenyldi((methylthio)methyl)borane is formed as a side product which was isolated and characterized by NMR spectroscopy. In order to make the first coordination sphere even more sulfur-rich, the S,N-bidentate ligand mt was introduced by substitution of one bromido and one carbonyl ligand resulting in the formation of [W(CO)2(mt)(PhTt)] (3). Reaction of all complexes with the symmetric alkynes acetylene, dimethylacetylene, and diphenylacetylene did not lead to the desired alkyne complexes. However, reaction of [WBr2(CO)(C2R2)2(NCMe)] (R = Me, Ph) with Cs[PhTt] gave the alkyne complexes [WBr(CO)(C2R2)2(PhTt-S,S′)] (R = Me 4, Ph 5) with a bidentate [PhTt] ligand. These results demonstrate the flexibility of the ligand which allowed the preparation of alkyne compounds by using a precursor with two already coordinated alkynes, whereas introduction of an alkyne to a tungsten [PhTt] complex by substitution of two carbonyl ligands is hampered. Although the tungsten−sulfur bond is expected to be strong and stable, the here investigated ligand system is apparently too weakly donating which leads to the observed fluxionality of the tungsten [PhTt] system. For the future use of sulfur-rich scorpionate ligands in tungsten alkyne chemistry, the design of a more strongly donating and less flexible ligand should be considered.

stretched due to the strong trans effect of the carbonyl ligand (C1−W1−S1 167.11(7)°). The bond between the tungsten center and the sulfur of the (methylthio)methyl ligand in 2 is the shortest W−S bond of complexes 1−3. In all three complexes 4a, 4b, and 5, the CC bonds of the alkynes are arranged parallel to the CO bond. Due to the strong trans influence of the coordinated carbonyls and alkynes, the W−S bonds opposite these molecules are significantly weakened leading to a difference in bond length of up to 0.2 Å (Table 3). In complexes 4a and 5, the central six-membered ring adopts a chair conformation with the noncoordinating (methylthio)methyl chain almost cis to the carbonyl ligand. In 4b, the central six-membered ring adopts a distorted boat conformation. Due to the significant trans influence of the two coordinated alkynes, the W−S bonds in 4a, 4b, and 5 (2.6473(6)−2.6634(6) Å) are much longer than those found in 1−3 (2.4999(3)−2.5920(5) Å), and therefore, the alkyne complexes are quite prone to decomposition in solution. The W−S bonds in complexes 1−5 are generally rather long compared to W−S bonds in tungsten complexes with coordinated acetylene.28,63 However, the W−S bonds in complexes 1−3 are in the same range as in the chemically similar complex [WI(CO)3(Tm)] employing the sulfur-rich scorpionate ligand hydrotris(methimazolyl)borate (Tm).52

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CONCLUSIONS Herein, the syntheses and molecular structures of four novel sulfur-rich tungsten(II) complexes are reported. The reaction of the versatile tungsten precursor [WBr2(CO)3(NCMe)2] with 1 equiv of the ligand salt Cs[PhTt] led to the desired E


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Inorganic Chemistry

Figure 2. Molecular views (50% probability level) of complexes 4a (top left), 4b (top right), and 5 (bottom) showing the atomic numbering scheme. H atoms as well as solvent molecules are omitted for clarity reasons. acetylene and diphenylacetylene, all were used as received. Acetylene 2.6 was purified by bubbling it through water and concentrated H2SO4 and then dried by passing it through CaCl2 and KOH. Diphenylacetylene was recrystallized from ethanol. Celite was dried at 100 °C prior to use. NMR spectra were recorded on a Bruker Avance III 300 MHz spectrometer at 25 °C. Chemical shifts δ are given in ppm. 1H NMR spectra are referenced to residual protons in the solvent and 13C NMR spectra to the deuterated solvent peak. The multiplicity of peaks is denoted as singlet (s), broad singlet (bs), doublet (d), triplet (t), triplet of triplets (tt), quadruplet (q), or multiplet (m). NMR solvents were stored over molecular sieves. Solid state IR spectra were measured on a Bruker ALPHA ATR-FT-IR spectrometer at a resolution of 2 cm−1. Elemental analyses (C, H, N, S) were performed at the Department of Inorganic Chemistry at the University of Technology in Graz using a Heraeus Vario Elementar automatic analyzer. Values for elemental analyses are given as percentages. X-ray Diffraction Analysis. Single-crystal X-ray diffraction analyses were carried out on a Bruker AXS SMART APEX-II diffractometer equipped with a CCD detector. All measurements were performed using monochromatized Mo-Kα radiation from an Incoatec microfocus sealed tube at 100 K. Absorption corrections were performed semiempirically from equivalents. Molecular structures were solved by direct methods (SHELXS-97)64 and refined by fullmatrix least-squares techniques against F2 (SHELXL-2014/6).65 Ligand Synthesis. [Bu4N][PhTt]46,66 was synthesized in 44% yield according to modified published procedures. The synthesis of Cs[PhTt] was developed by merging two published procedures.46,53 Cs[PhTt]. In an inert 250 mL Schlenk flask, 60 mL of heptane, TMEDA (21 mL, 138 mmol), and (CH3)2S (18 mL, 250 mmol) were combined. Subsequently, n-BuLi (2.5 M in hexane, 42 mL, 105 mmol) was added at −78 °C over 15 min. The resulting solution was allowed to warm to rt and was then stirred for 3 h. Excess (CH3)2S

Table 2. Selected Bond Lengths (Å) for Complexes 1−3 W1−S1 W1−S2 W1−S3 W1−S4 W1−S5 W1−C1 W1−C2 W1−C3 W1−C41

1

2

3

2.5420(6) 2.5654(5) 2.5526(6)

2.5528(3) 2.5804(3) 2.4999(3) 2.4561(4)

2.5920(5) 2.5279(5) 2.5068(5)

1.972(3) 2.001(2) 2.009(3)

1.9824(14) 1.9267(14)

2.6098(6) 1.934(2) 1.967(2)

2.2184(14)

Table 3. Selected Bond Lengths (Å) for Complexes 4 and 5

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W1−S1 W1−S2 W1−C1 W1−C2 W1−C3 W1−C4 W1−C5 C1−C2 C3−C4

4a

4b

5

2.6500(7) 2.6533(7) 2.064(3) 2.110(3) 2.069(3) 2.114(3) 2.019(3) 1.281(4) 1.282(4)

2.6634(6) 2.6473(6) 2.058(3) 2.102(3) 2.080(3) 2.115(2) 2.004(3) 1.292(4) 1.283(4)

2.6532(12) 2.6501(10) 2.077(4) 2.111(4) 2.080(4) 2.091(4) 2.035(5) 1.303(6) 1.313(6)

EXPERIMENTAL SECTION

General. All experiments were performed under N2 atmosphere employing standard Schlenk and glovebox techniques. Solvents were purified via a Pure Solv Solvent Purification System. Chemicals were purchased from commercial sources and, with the exception of F


Article

Inorganic Chemistry

[W(CO)2(mt)(PhTt)] (3). A solution of Na(mt) (242 mg, 1.78 mmol) in 10 mL of THF was added dropwise to a stirred solution of [WBr(CO)3(PhTt)] (1.00 g, 1.62 mmol) in 15 mL of THF. The flask was equipped with a bubbler, and the dark orange reaction mixture was allowed to stir for 1 h, whereupon the solvent was evaporated in vacuo. Subsequently, the solid was suspended in 20 mL of MeCN, and the resulting mixture was filtrated through Celite which was washed with 10 mL of THF afterward. The desired product was recrystallized by slow evaporation and then washed with MeCN (2 × 5 mL) to give [W(CO)2(mt)(PhTt)] (0.52 g, 51%) as orange microcrystalline powder. Single crystals suitable for X-ray diffraction analysis were obtained via recrystallization from MeCN at −35 °C. 1H NMR (CD2Cl2, 300 MHz): δ 7.24 (m, 2H, o-C6H5), 7.19−7.18 (d, 1H, CH), 7.17−7.14 (m, 2H, m-C6H5), 7.04−6.99 (tt, 1H, p-C6H5), 6.60 (d, 1H, CH), 3.34 (s, 3H, NCH3), 2.46 (s, 9H, SCH3), 2.18 (bs, 6H, BCH2) ppm. 13C NMR (CD2Cl2, 75 MHz): δ 161.55−159.47 (q, 1 JBC = 52.3 Hz, BCq), 152.41 (s, CS), 131.35 (s, 2C, o-C6H5), 127.63 (d, 2C, m-C6H5), 126.20 (s, CH), 124.62 (s, p-C6H5), 120.08 (s, CH), 33.28−31.69 (q, 1JBC = 39.9 Hz, 3C, BCH2), 30.86 (s, NCH3), 25.43 (s, 3C, SCH3) ppm. IR (cm−1): 1922 (CO), 1826 (CO). Anal. Calcd for C18H25BN2O2S4W: C, 34.63; N, 4.49; H, 4.04; S, 20.54. Found: C, 34.79; N, 4.58; H, 3.93; S, 20.16. [WBr(CO)(C2Me2)2(PhTt-S,S′)] (4). A solution of Cs[PhTt] (243 mg, 0.60 mmol) in 7 mL of MeCN was added dropwise to a stirred solution of [WBr2(CO)(C2Me2)2(NCMe)] (312 mg, 0.60 mmol) in 7 mL of MeCN. The resulting yellow-green mixture was allowed to stir for 30 min at −15 °C, whereupon the solvent was removed in vacuo. The solid was subsequently suspended in 15 mL of CH2Cl2 at −15 °C, and the resulting mixture was filtrated through Celite. Then, 8 mL of heptane was added to the filtrate. After evaporation to ∼10 mL, the product readily crystallized at −25 °C and was subsequently isolated by filtration and washed with heptane (2 × 2 mL) to give [WBr(CO)(C2Me2)2(PhTt-S,S′)] (332 mg, 82%) as yellow crystals suitable for single-crystal X-ray diffraction analysis. 1H NMR (CD2Cl2, 300 MHz, 4b): δ 7.37−7.35 (m, 2H, o-C6H5), 7.19−7.14 (t, 2H, mC6H5), 7.06−7.01 (tt, 1H, p-C6H5), 3.10 (s, 3H, -CH3), 3.10 (s, 3H, -CH3), 2.89 (s, 3H, -CH3), 2.89 (s, 3H, -CH3), 2.25 (m, 4H, CH2), 1.94 (s, 6H, SCH3), 1.90 (s, 3H, SCH3), 1.59−1.58 (d, 2H, CH2) ppm; 1H NMR (CD2Cl2, 300 MHz, 4a): δ 7.25−7.24 (m, 2H, o-C6H5), 7.11−7.06 (t, 2H, m-C6H5), 6.98−6.93 (tt, 1H, pC6H5), 3.13 (s, 3H, -CH3), 3.13 (s, 3H, -CH3), 3.00 (s, 3H,  -CH3), 3.00 (s, 3H, -CH3), 2.28 (m, 2H, CH2), 2.11 (m, 2H, CH2), 2.06 (s, 3H, SCH3), 1.90 (s, 6H, SCH3), 1.80−1.79 (d, 2H, CH2) ppm. 13C NMR (CD2Cl2, 75 MHz, 4b): δ 208.74 (s, CO), 179.83 (s, CC), 161.81 (s, CC), 132.82 (s, 2C, o-C6H5), 127.02 (s, 2C, mC6H5), 124.29 (s, p-C6H5), 36.77−34.78 (m, 3C, CH2), 26.53 (s, 2C, SCH3), 20.08−2.05 (d, SCH3), 18.92 (s, 6C, -CH3), 18.31 (s, 6C, -CH3) ppm; 13C NMR (CD2Cl2, 75 MHz, 4a): δ 208.74 (s, CO), 179.55 (s, C ≡ C), 161.81 (s, C ≡ C), 131.97 (s, 2C, o-C6H5‑), 127.15 (s, 2C, m-C6H5), 124.23 (s, p-C6H5), 36.77−34.78 (m, 3C, CH2), 26.53 (s, 2C, SCH3), 20.47−2.43 (d, SCH3), 18.98 (s, 6C, -CH3), 18.34 (s, 6C, -CH3) ppm. IR (cm−1): 2040 (CO). Anal. Calcd for C21H32BBrOS3W·0.1 C7H16: C, 38.26; H, 4.97; S, 14.12. Found: C, 38.17; H, 4.90; S, 14.13.

was subsequently removed in vacuo for 15 min. The resulting light yellow, cloudy solution was again cooled to −78 °C, and a solution of PhBCl2 (3.3 mL, 25.0 mmol) in 12 mL of heptane was added dropwise via syringe. The resulting white mixture was allowed to warm to rt and was then stirred for 48 h. The supernatant solution was removed, and the remaining solid was subsequently dried in vacuo. The resulting white residue was treated with 130 mL of H2O and 20 mL of CH2Cl2. The aqueous phase was filtered, and the product was precipitated by adding aqueous CsCl and then leaving the cloudy solution at −10 °C overnight. The precipitate was isolated by filtration, washed with H2O (4 mL) and Et2O (3 × 4 mL), and subsequently dried in vacuo to give Cs[PhTt] (4.30 g, 43%) as white flocculent solid. 1H NMR (CD3CN, 300 MHz): δ 7.40 (m, 2H, oC6H5), 7.07−7.03 (t, 2H, m-C6H5), 6.92−6.87 (tt, 1H, p-C6H5), 1.93 (s, 9H, CH3), 1.79−1.75 (q, 2JBH = 4.1 Hz, 6H, BCH2) ppm. 13C NMR (CD3CN, 75 MHz): δ 165.29−163.29 (q, 1JBC = 50.4 Hz, BCq), 134.06 (d, 3JBC = 1.2 Hz, 2C, o-C6H5), 127.15−127.05 (q, 2JBC = 2.9 Hz, 2C, m-C6H5), 123.70 (s, p-C6H5), 36.94−35.31 (q, 1JBC = 41.1 Hz, 3C, BCH2), 20.35−20.22 (q, 3JBC = 3.4 Hz, 3C, CH3) ppm. Complex Synthesis. In crystalline or microcrystalline forms, all complexes are in principle stable at ambient conditions but ought to be stored in a glovebox for a longer period of time. [WBr(CO)3(PhTt)] (1). A solution of Cs[PhTt] (3.33 g, 8.25 mmol) in 35 mL of MeCN was slowly added to a stirred solution of [WBr2(CO)3(NCMe)2] (3.82 g, 7.50 mmol) in 30 mL of MeCN. After 1 h, the mixture was concentrated to approximately 5 mL. The supernatant solution was removed, and the precipitate was subsequently dried in vacuo. Then, the residue was suspended in 50 mL of CH2Cl2, and the resulting mixture was filtrated through Celite. The desired product was crystallized by addition of 30 mL of MeCN to the filtrate and subsequent slow evaporation. The obtained solid was washed with 10 mL of MeCN and eventually dried in vacuo to give [WBr(CO)3(PhTt)] (3.97 g, 89%) as orange crystals suitable for single-crystal X-ray diffraction analysis. 1H NMR (CD2Cl2, 300 MHz): δ 7.19−7.15 (m, 4H, C6H5), 7.07−7.01 (m, 1H, p-C6H5), 2.79 (s, 9H, CH3), 2.19−2.18 (m, 6H, BCH2) ppm. 13C NMR (CD2Cl2, 75 MHz): δ 223.36 (bs, 3C, CO), 160.24−158.15 (q, 1JBC = 52.8 Hz BCq), 131.18 (s, 2C, o-C6H5), 127.77 (s, 2C, m-C6H5), 124.98 (s, pC6H5), 32.44−30.87 (q, 1JBC = 39.6 Hz, 3C, BCH2), 25.99 (s, 3C, CH3) ppm. IR (cm−1): 2026 (CO), 1940 (CO), 1908 (CO). Anal. Calcd for C15H20BBrO3S3W: C, 29.10; H, 3.26; S, 15.54. Found: C, 29.13; H, 3.15; S, 15.19. [W(CO)2(η2-CH2SMe)(PhTt)] (2). In a 25 mL Schlenk flask, [WBr2(CO)3(NCMe)2] (204 mg, 0.40 mmol) and [Bu4N][PhTt] (411 mg, 0.80 mmol) were dissolved in 12 mL of MeCN. The resulting dark yellow solution was allowed to stir for 24 h, whereupon the reaction was terminated by evaporation of the solvent. The yellow ocher solid was suspended in 6 mL of MeCN, and the resulting mixture was filtrated through Celite to remove insoluble impurities. Then, the solvent was evaporated in vacuo. The dark brown residue was suspended in 3 mL toluene, and the resulting suspension was filtrated through a pad of silica gel. Elution was done with toluene. Then, the solvent was evaporated in vacuo yielding the brownish crude product. Addition of 4 mL of MeCN and subsequent slow evaporation to approximately 1 mL gave [W(CO)2(η2-CH2SMe)(PhTt)] (87 mg, 38%) as orange microcrystalline powder after filtration. Single crystals suitable for X-ray diffraction analysis were obtained via recrystallization from MeCN at rt or −35 °C. 1H NMR (CD2Cl2, 300 MHz): δ 7.20 (m, 2H, o-C6H5), 7.16−7.11 (t, 2H, mC6H5), 7.01−6.97 (tt, 1H, p-C6H5), 3.30 (d, 1H, WCH2), 3.00 (d, 1H, WCH2), 2.67 (bs, 9H, CH3), 2.40 (s, 3H, CH3), 2.20 (m, 3H, BCH2), 1.96 (m, 3H, BCH2) ppm. 13C NMR (CD2Cl2, 75 MHz): 224.82 (s/d, 1JWC = 143.9 Hz, CO), 217.19 (s/d, 1JWC = 182.6 Hz, CO), 161.66−159.60 (q, 1JBC = 51.8 Hz, BCq), 131.46 (s, 2C, oC6H5), 127.54 (d, 2C, m-C6H5), 124.49 (s, p-C6H5), 36.52 (s/d, 1JWC = 19.9 Hz, WCH2), 33.51−31.89 (q, 1JBC = 40.7 Hz, 3C, BCH2), 28.45 (bs, 3C, CH3), 27.29 (s, CH3) ppm. IR (cm−1): 1925 (CO), 1810 (CO). Anal. Calcd for C16H25BO2S4W: C, 33.58; H, 4.40; S, 22.41. Found: C, 33.58; H, 4.27; S, 22.50.

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Nadia C. Mösch-Zanetti: 0000-0002-1349-6725 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by NAWI Graz is gratefully acknowledged. REFERENCES

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Bioinspired Tungsten Complexes Employing a Thioether Scorpionate

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