The Effect of Pyridine-2-thiolate Ligands on the Reactivity of Tungsten Complexes toward Oxidation and Acetylene Insertion

Intending to deepen our understanding of tungsten acetylene (C2H2) chemistry, with regard to the tungstoenzyme acetylene hydratase, here we explore the structure and reactivity of a series of tungsten acetylene complexes, stabilized with pyridine-2-thiolate ligands featuring tungsten in both +II and +IV oxidation states. By varying the substitution of the pyridine-2-thiolate moiety with respect to steric and electronic properties, we examined the details and limits of the previously reported intramolecular nucleophilic attack on acetylene followed by the formation of acetylene inserted complexes. Here, we demonstrate that only the combination of high steric demand and electron-withdrawing features prevents acetylene insertion. Nevertheless, although variable synthetic approaches are necessary for their synthesis, tungsten acetylene complexes can be stabilized predictably with a variety of pyridine-2-thiolate ligands.


■ INTRODUCTION
Tungsten is the metal of choice for several challenging enzymatic reactions. 1 Besides being in the active site of the metalloenzymes that catalyze redox reactions, it is also essential for the function of acetylene hydratase (AH). This is a unique example of a tungstoenzyme catalyzing the nonredox hydration of acetylene to acetaldehyde. 1−5 This reaction is the first metabolic step of the mesophilic bacterium Pelobacter acetylenicus, which consumes acetylene as its only carbon and energy source. 2 The only other known enzyme that accepts acetylene as a substrate is nitrogenase, which reduces acetylene to ethylene. 6 The mechanism of the catalysis of AH remains elusive, as there are no reported crystal structures of the enzyme containing substrate or any inhibitor. Since one of the mechanistic ideas of AH suggests coordination of acetylene to the tungsten(IV) center, 7−12 we aim to synthesize and understand tungsten acetylene adducts with ligands similar to those in tungstoenzymes. 13,14 The first tungsten(II) acetylene species bearing dithiocarbamate ligands was reported in 1978; 15 thereafter, only a few other W−C 2 H 2 adducts have been synthesized. 16−26 For example, Templeton et al. reported a tungsten(IV) oxo acetylene complex containing an N-donor boron-based scorpionate ligand. 20 In our group, the use of bioinspired S,N-bidentate ligands, such as SPhoz (2-(4′4′-dimethyloxazoline-2′-yl)thiophenolate), 21 PyS (pyridine-2-thiolate), 24 and 6-MePyS (6-methylpyridine-2-thiolate), 26 allows the preparation of W(II) and W(IV) complexes and ensures a sulfur-rich environment, more closely resembling the one in the native enzyme. Additionally, complexes bearing those S,N-bidentate ligands allowed for important insight into the nature of the W−C 2 H 2 chemistry. For instance, [WO(C 2 H 2 )(SPhoz) 2 ] is capable of reversible binding of C 2 H 2 with the release of acetylene being triggered by irradiation. 21 In addition, coordination and subsequent insertion of a second molecule of C 2 H 2 into the tungsten−nitrogen bond take place in [W(CO)(C 2 H 2 )(PyS) 2 ] showing that the alkyne is activated toward the reaction with nucleophiles. The acetylene insertion was studied using C 2 D 2 and revealed coordination of the second acetylene before insertion. 24 This represents the first example of a nucleophilic attack on a W-coordinated C 2 H 2 . Similar behavior has previously been observed for tungsten complexes but only with substituted alkynes. 27−34 To sterically prevent the insertion from taking place, and to favor the attack from an external nucleophile, we introduced a methyl group in position 6 of the PyS ligand. The resulting complex [W(CO)(C 2 H 2 )(6-MePyS) 2 ] also reacts with the second molecule of acetylene, but insertion occurs only partially. Moreover, nucleophilic attack of PMe 3 on coordinated acetylene was observed for W(II) and W(IV) complexes bearing 6-MePyS ligands leading to carbyne and vinyl complexes, respectively. 26 Herein, we explore the influence of different pyridine-2thiolate ligands, namely, 4-methylpyridine-2-thiolate (4-MePyS), 3-chloropyridine-2-thiolate (3-ClPyS), and 5-nitro-6-methylpyridine-2-thiolate (5-NO 2 -6-MePyS) (Figure 1), on the oxidation and acetylene insertion reactivity.

■ RESULTS AND DISCUSSION
Choice of Ligands. Ligand design is based on PyS and 6-MePyS which previously allowed the preparation of tungsten acetylene complexes of the types [W(CO)(C 2 H 2 )(SN) 2 ] and [WO(C 2 H 2 )(SN) 2 ], (SN= bidentate pyridine-2-thiolate moiety). 24,26 To investigate the insertion reaction, various substituents at the pyridine heterocycle were introduced as displayed in Figure 1. Since the electronic effects of substituents in positions 2, 4, or 6 in pyridine are known to be similar, 35 we chose 4-MePyS for comparison with 6-MePyS to elucidate whether the insertion is prevented for electronic or steric reasons. 36,37 The introduction of a nitro group in position 5 in 6-MePyS increases the electron-withdrawing properties, rendering the nitrogen donor less nucleophilic. A similar approach was used by introducing a chloro substituent at the PyS moiety. With both ligands, a reduced reactivity toward insertion is expected. The initial attempts to introduce the Cl to position 6 of the PyS moiety were not successful, so 3-ClPyS was prepared instead. Moreover, coordination of the known ligands 6-tert-butylpyridine-2-thiolate (6-t BuPyS) 38 and 6-trifluoromethylpyridine-2-thiolate (6-CF 3 PyS) to the tungsten(II) precursor [WBr 2 (CO) 3 (MeCN) 2 ] was attempted, but it turned out to be unsuccessful due to steric hindrance of the large groups in position 6.
Introduction of the Ligand. Following the synthetic procedure for complexes employing the ligands PyS and 6-MePyS, 24   However, on the way to tungsten oxo acetylene complexes, the isolation and purification of tricarbonyl compounds turned out to be an unnecessary step. Indeed, in most of the cases, we chose an in situ approach in which the initial reaction of [WBr 2 (CO) 3 (MeCN) 2 ] with the ligand salt is immediately followed by the treatment with acetylene.
Reaction with Acetylene. The reaction solutions of tricarbonyl complexes were purged with acetylene and subsequently stirred, after which the products were isolated as described in the SI. Exposing a toluene solution of 1a to an acetylene atmosphere (1 atm) leads to the formation of [W(CO)(C 2 H 2 )(4-MePyS)(CHCH-4-MePyS)] (3a) where one molecule of acetylene has inserted into the W−N bond and a second C 2 H 2 is coordinated. This is similar to what has recently been found for [W(CO) 3 (PyS) 2 ], 38 while only partial insertion occurred when position 6 was blocked by a methyl group as with 6-MePyS. 26 The electronic effects of substituents in positions 2, 4, or 6 in pyridine are known to be similar 35 indicating that the methyl group in 6-MePyS prevents the insertion for steric reasons only. Although the pK a values of the pyridine heterocycles indicate that 4-MePy (6.0) with an additional methyl group is a stronger base than the parent Py (5.2), 37 complexes bearing their respective thiolate ligand form show the same reactivity toward the insertion. Accordingly, we expected 1b containing the 3-ClPyS ligand to react to the inserted product [W(CO)(C 2 H 2 )(3-ClPyS)(CHCH-3-ClPyS)] (3b) upon exposure to C 2 H 2 . However, after full consumption of the starting material, we consistently observe a mixture of [W(CO)(C 2 H 2 )(3-ClPyS) 2 ] (2b) and 3b in a 2:3 ratio. Nevertheless, after 24 h of stirring under acetylene atmosphere, 3b could be isolated after purification by filtration through silica gel and recrystallization from dichloromethane/ heptane in 40% yield as a purple solid. Interestingly, we noticed that stirred solutions of 3b in dichloromethane gradually change the color from purple to brown. Spectroscopic characterization of this brown mixture (SI) revealed the partial formation of [W(CO)(C 2 H 2 )(3-ClPyS) 2 ] (2b) due to the release of the inserted acetylene. Full conversion to 2b was possible by stirring a dichloromethane solution of 3b under reflux for 6 h. Thus, the most convenient procedure to prepare 2b was performed by isolating the initial reaction mixture of 2b and 3b, subsequent purification by filtration through silica gel, and stirring under reflux for 6 h, which allowed the isolation of 2b as a brown powder in 60% yield. The release of C 2 H 2 was also observed with [W(CO)( 26 but only partially and accompanied by the formation of polyacetylene. Thus, the use of a sterically demanding, electron-deficient ligand should prevent insertion reactions altogether. Indeed, the reaction of [WBr 2 (CO) 3 (MeCN) 2 ] with 2.1 equiv of K(5-NO 2 -6-MePyS) for 45 min and subsequent flushing with acetylene for another 45 min gave [W(CO)(C 2 H 2 )(5-NO 2 -6-MePyS) 2 ] (2c) after workup as a brown powder in 80% yield. In contrast to all other investigated pyridine-2-thiolate systems, even after stirring a solution of 2c in CH 2 Cl 2 under acetylene atmosphere for 24 h, no inserted complex was detected. We ascribe this to the synergistic effect of the steric demand of the methyl group in the 6-position and the electron-withdrawing properties of the nitro-group. The prevention of insertion is relevant for the oxidation to the biological oxidation state +IV, as all our attempts to oxidize inserted products were futile.
Oxidation to Tungsten(IV) Complexes. Compounds 2a−d can be oxidized to complexes of the type [WO(C 2 H 2 )-(SN) 2 ] (4a−d) by a slight excess of pyridine-N-oxide (PyNO) as an oxygen source in dichloromethane (Scheme 1). The required reaction time for full conversion is highly dependent on the pyridine-2-thiolate ligand and ranges from 10 min in the case of the nitro-substituted 2c to several hours starting from 2a, 2b, and 2d. All oxido acetylene compounds can be isolated in high yields (4a 68%, 4b 87%, 4c 75%, and 4d 91%) as yellow powders. Furthermore, complex 4b can also be obtained directly from the reaction of PyNO with 3b due to the observed reversibility of the insertion in the latter. Compound 2c bearing 5-NO 2 -6-MePyS is, in comparison to the other W(II) systems, oxidized significantly faster. This can be explained by a decrease in π-back-donation of tungsten to CO weakening the tungsten carbonyl bond, thereby facilitating the release of CO and oxidation of the metal center. This is supported by IR and X-ray data upon comparison of complex 2c (ν (CO) 1919 cm −1 , W−CO: 1.969 Å) with its analogue lacking the nitro group [W(CO)(C 2 H 2 )(6-MePyS) 2 ] (ν (CO) 1891 cm −1 , W−CO: 1.958 Å). 26 Spectroscopic Data. Coordination of acetylene was confirmed by 1 H and 13 C NMR spectroscopy. In the case of type 2 complexes, acetylenic protons resonate as two singlets in the region 12−14 ppm due to the asymmetry of coordination (Table 1). Complexes 2a and 2b are found as a mixture of two isomers in solution, while complex 2c appears in isomerically pure form. For type 3 complexes, 1 H NMR spectra show the presence of side-on coordinated (η 2 -C 2 H 2 ) and an inserted acetylene (η 1 -C 2 H 2 ). Side-on coordinated acetylene resonates in the same region as those in type 2, while the η 1 -C 2 H 2 resonates in the form of two doublets in the aromatic region. Both inserted complexes 3a and 3b are isomerically pure. Due to C 2 H 2 release from 3b and its low solubility, it was not possible to record a meaningful 13 C NMR spectrum. In the case of tungsten(IV) oxo species (type 4), acetylenic proton resonances are upfield-shifted compared to their W(II) analogues and flanked by 183 W satellites. For complexes 4a−c, the acetylenic protons appear in the form of two singlets. Differently, complex 4d shows only one singlet for both C 2 H 2 protons, presumably due to the dynamic behavior of coordinated acetylene. All type 4 complexes, except 4c, show an additional set of signals related to the presence of a second isomer in solution. As expected, signals are downfield-shifted when 5-NO 2 -6-MePyS is bound to the tungsten center. Even though most of the complexes exhibit two isomers in solution, only one could be crystallized (vide infra). Upon dissolving single crystals, the same ratio of the two isomers is observed pointing toward an equilibrium in solution.
The reactivity and synthetic approaches for obtaining complexes 2, 3, and 4 vary significantly depending on the ligand, which is however hardly reflected when comparing NMR and IR data of the respective complexes. Thus, the acetylenic proton shifts in the tungsten(II) acetylene complexes 2a−c are very similar. This can possibly be ascribed to the long distance from the ligand substituents to the acetylenic protons. It suggests that the reactivity differences are primarily influenced by steric effects.  Table 2. Full crystallographic details such as structure refinement data as well as    The center of the η 2 CC bond occupies the sixth position, which is always located cis to the carbonyl or oxo groups. A significant loss of triple bond character and linearity is observed in all η 2 -bound acetylene molecules, which shows that the actual bonding situation is between the η 2 -adduct and metallacyclopropene resonance structures. Monoacetylene carbonyl complexes (2a−c) show similar structural properties in terms of bond lengths and angles. Compounds 2a and 2b crystallized in S,S-trans configuration, while 2c crystallized as a S,S-cis isomer, as reported for similar compounds. The carbonyl ligand in 2a−c is trans to the N atoms of the pyridine-2-thiolate ligand. Moreover, the CO shows the common parallel arrangement with coordinated C 2 H 2 . 15 In complex 2a, the carbonyl ligand and the acetylene ligand are disordered over two orientations. The complex lies on a twofold rotation axis parallel to the c-axis. Moreover, in the case of 2c, the η 2 -acetylene ligand is trans to the rather long-distant atom S1 (W1−S1 2.5872(6) Å vs W1−S2 2.4024(6) Å). The planes of the nitro groups enclose angles of 28.1(3)°and 29.2(3)°with the pyridine rings they are bonded to.
The geometry of type 2 complexes is likely influencing the reactivity toward acetylene, especially since the second molecule of C 2 H 2 seems to coordinate to the W(II) center prior to insertion. 24 The latter occurs more easily when the metal center is not shielded by methyl groups in position 6. Moreover, carbonyl acetylene complexes bearing 6-MePyS 26 and 5-NO 2 -6-MePyS (2c) ligands exist as a single isomer in solution (S,S-cis), and this orientation may be too rigid to undergo the coplanar rearrangement necessary for the migratory insertion. In contrast, other pyridine-2-thiolate based complexes with no substituent at position 6 show the presence of two isomers in solution. This suggests that for the insertion, the S,S-trans configuration is required. Flexible isomerization between the two configurations might facilitate the reactivity toward the coordination of the second molecule of acetylene.

■ CONCLUSION
Here, we report the influence of different substitutional patterns on the reactivity of W(II) complexes toward the acetylene insertion and oxidation process. Comparison of the electronically similar 4-MePyS with 6-MePyS systems allows the conclusion that steric factors govern the reactivity toward insertion. The presence of the methyl group in the 6-MePyS complex prevents insertion of acetylene, while with 4-MePyS, the inserted compound [W(CO)(C 2 H 2 )(CHCH-4-MePyS)-(4-MePyS)] was obtained. No insertion is observed by combining the steric effect of the methyl group and the electronic effect of the nitro group in 5-NO 2 -6-MePyS. The predominance of steric effects is also demonstrated by the 3-ClPyS system where the presence of an electron-withdrawing group in position 3 does not prevent insertion. However, the inserted complex is significantly more sensitive toward releasing C 2 H 2 in solution compared to 3a, which exhibits a methyl group in position 4. Moreover, the tendency to release inserted acetylene from type 3 complexes decreases with the C−C bond lengths of the η 1 -C 2 H 2 moiety. Tungsten(IV) oxo acetylene complexes [WO(C 2 H 2 )(SN) 2 ] can be synthesized from W(II) acetylene species with pyridine-N-oxide. When using 5-NO 2 -6-MePyS, π-back-donation from tungsten to the carbonyl in type 2 is reduced, thereby weakening the tungsten carbonyl bond and thus leading to a shorter oxidation reaction time. Complexes with pyridine-2-thiolate ligands substituted in position 6 are prone to adopt the S,S-cis orientation, while the sterically less demanding systems tend to crystallize in the S,Strans configuration. We assume that the flexibility of the ligand coordination in W(II) complexes with 4-MePyS and 3-ClPyS ligands allows further reactivity with a second molecule of acetylene required for insertion. To our surprise, regardless of the substitutional patterns of pyridine-2-thiolate ligands, all the complexes containing η 2 -C 2 H 2 have similar structural properties within their groups.
Full X-ray data, NMR spectra, and experimental procedures (PDF)

■ ACKNOWLEDGMENTS
Financial support by the Austrian Science Fund (FWF, grant number P31583) and NAWI Graz is gratefully acknowledged. S,S-trans S,S-cis S,S-trans S,S-trans S,S-cis