Soft Scorpionate Hydridotris(2-mercapto-1-methylimidazolyl) borate) Tungsten-Oxido and -Sulfido Complexes as Acetylene Hydratase Models

A series of W^IV alkyne complexes with the sulfur-rich ligand hydridotris(2-mercapto-1-methylimidazolyl) borate) (Tm^Me ) are presented as bio-inspired models to elucidate the mechanism of the tungstoenzyme acetylene hydratase (AH). The mono- and/or bis-alkyne precursors were reacted with NaTm^Me and the resulting complexes [W(CO)(C₂ R₂ )(Tm^Me )Br] (R=H 1, Me 2) oxidized to the target [WE(C₂ R₂ )(Tm^Me )Br] (E=O, R=H 4, Me 5; E=S, R=H 6, Me 7) using pyridine-N-oxide and methylthiirane. Halide abstraction with TlOTf in MeCN gave the cationic complexes [WE(C₂ R₂ )(MeCN)(Tm^Me )](OTf) (E=CO, R=H 10, Me 11; E=O, R=H 12, Me 13; E=S, R=H 14, Me 15). Without MeCN, dinuclear complexes [W₂ O(μ-O)(C₂ Me₂ )₂ (Tm^Me )₂ ](OTf)₂ (8) and [W₂ (μ-S)₂ (C₂ Me₂ )(Tm^Me )₂ ](OTf)₂ (9) could be isolated showing distinct differences between the oxido and sulfido system with the latter exhibiting only one molecule of C₂ Me₂ . This provides evidence that a fine balance of the softness at W is important for acetylene coordination. Upon dissolving complex 8 in acetonitrile complex 13 is reconstituted in contrast to 9. All complexes exhibit the desired stability toward water and the observed effective coordination of the scorpionate ligand avoids decomposition to disulfide, an often-occurring reaction in sulfur ligand chemistry. Hence, the data presented here point toward a mechanism with a direct coordination of acetylene in the active site and provide the basis for further model chemistry for acetylene hydratase.