Inspired by the proposed inner-sphere mechanism of the tungstoenzyme acetylene hydratase, we have designed tungsten acetylene complexes and investigated their reactivity. Here, we report the first intermolecular nucleophilic attack on a tungsten-bound acetylene (C 2 H 2 ) in bioinspired complexes employing 6-methylpyridine-2-thiolate ligands. By using PMe 3 as a nucleophile, we isolated cationic carbyne and alkenyl complexes.

Upon replacement of molybdenum by tungsten in DMSO reductase isolated from the Rhodobacteraceae family, the derived enzyme catalyzes DMSO reduction faster. To better understand this behavior, we synthesized two tungsten(VI) dioxido complexes [W VI O 2 L 2 ] with pyridine- (PyS) and pyrimidine-2-thiolate (PymS) ligands, isostructural to analogous molybdenum complexes we reported recently. Higher oxygen atom transfer (OAT) catalytic activity was observed with [WO 2 (PyS) 2 ] compared to the Mo species, independent of whether PMe 3 or PPh 3 was used as the oxygen acceptor. [W VI O 2 L 2 ] complexes undergo reduction with an excess of PMe 3 , yielding the tungsten(IV) oxido species [WOL 2 (PMe 3 ) 2 ], while with PPh 3 , no reactions are observed. Although OAT reactions from DMSO to phosphines are known for tungsten complexes, [WOL 2 (PMe 3 ) 2 ] are the first fully characterized phosphine-stabilized intermediates. By following the reaction of these reduced species with excess DMSO via UV–vis spectroscopy, we observed that tungsten compounds directly react to W VI O 2 complexes while the Mo analogues first form μ-oxo Mo(V) dimers [Mo 2 O 3 L 4 ]. Density functional theory calculations confirm that the oxygen atom abstraction from W VI O 2 is an endergonic process contrasting the respective reaction with molybdenum. Here, we suggest that depending on the sacrificial oxygen acceptor, the tungsten complex may participate in catalysis either via a redox reaction or as an electrophile.

Intending to deepen our understanding of tungsten acetylene (C 2 H 2 ) 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.

A series of M(II) and M(IV) (M = Mo, W) alkyne adducts employing two 6-methylpyridine-2-thiolate (6-MePyS) ligands was synthesized and investigated towards the nucleophilic attack of PMe 3 on the coordinated alkynes. For this approach, 2-butyne (C 2 Me 2 ), phenylacetylene (HC 2 Ph), and diphenylacetylene (C 2 Ph 2 ) were used. For the exploration of an intramolecular attack, but-3-yn-1-ol (HCCCH 2 CH 2 OH) was coordinated to the metal centers. A nucleophilic attack of PMe 3 was observed in [W(CO)(HC 2 Ph)(6-MePyS) 2 ] yielding an η 2 -vinyl compound. Reaction of [W(CO)(C 2 Ph 2 )(6-MePyS) 2 ] with excess PMe 3 resulted in the selective coordination of one molecule of PMe 3 concomitant with decoordination of the nitrogen atom of one 6-MePyS ligand. In contrast, the W(IV) complexes did not react with PMe 3 . While no selectivity was observed in the reaction of the Mo(II) compounds with PMe 3 , alkynes in the Mo(IV) compounds were replaced by PMe 3 . Addition of Et 3 N to the but-3-yn-1-ol complexes did not lead to the anticipated formation of 2,3-dihydrofuran.

An increasing number of discovered tungstoenzymes raises interest in the biomimetic chemistry of tungsten complexes in oxidation states +IV, +V, and +VI. Bioinspired (sulfur-rich) tungsten(VI) dioxido complexes are relatively prevalent in literature. Still, their energetically demanding reduction directly correlates with a small number of known tungsten(IV) oxido complexes, whose chemistry is not well explored. In this paper, a reduction of the [WO 2 (6-MePyS) 2 ] (6-MePyS = 6-methylpyridine-2-thiolate) complex with PMe 3 to a phosphine-stabilized tungsten(IV) oxido complex [WO(6-Me-PyS) 2 (PMe 3 ) 2 ] is described. This tungsten(IV) complex partially releases one PMe 3 ligand in solution, creating a vacant coordination site capable of activating dioxygen to form [WO 2 (6-MePyS) 2 ] and OPMe 3 . Therefore, [WO 2 (6-MePyS) 2 ] can be used as a catalyst for the aerobic oxidation of PMe 3 , rendering this complex a rare example of a tungsten system utilizing dioxygen in homogeneous catalysis. Additionally, the investigation of the reactivity of the tungsten(IV) oxido complex with acetylene, substrate of a tungstoenzyme acetylene hydratase (AH), revealed the formation of the tungsten(IV) acetylene adduct. Although this adduct was previously reported as an oxidation product of the tungsten(II) acetylene carbonyl complex, here it is obtained via substitution at the sulfur-rich tungsten(IV) center, mimicking the initial step of the first shell mechanism for AH as suggested by computational studies.