Understanding the Carbyne Formation from C2H2 Complexes

Nature chooses a high-valent tungsten center at the active site of the enzyme acetylene hydratase to facilitate acetylene hydration to acetaldehyde. However, the reactions of tungsten-coordinated acetylene are still not well understood, which prevents the development of sustainable bioinspired alkyne hydration catalysts. Here we report the reactivity of two bioinspired tungsten complexes with the acetylene ligand acting as a four-: [W­(CO)­(C2H2)­(PymS)2] (1) and a two-electron donor: [WO­(C2H2)­(PymS)2] (3), with PMe3 as a nucleophile to simulate the enzyme’s reactivity (PymS = 4-(trifluoromethyl)-6-methylpyrimidine-2-thiolate). In dichloromethane, compound 1 was found to react to the cationic carbyne [WCCH2PMe3(CO)­(PMe3)2(PymS)]Cl (2-Cl) while 3 reacts to the vinyl compound [WO­(CHCHPMe3)­(PMe3)3(PymS)]Cl (4-Cl). The formation of the latter follows the common rules applied to η2-alkyne complexes, whereas the carbyne formation was not expected due to the challenging 1,2-H shift. To understand these differences in behavior between seemingly similar acetylene complexes, stepwise addition of the nucleophile in various solvents was investigated by synthetic, spectroscopic, and computational approaches. In this manuscript, we describe that only a four-electron donor acetylene complex can react to the carbyne over the η1-vinyl intermediate and that 1,2-H shift can be assisted by an H-transfer reagent (in this case, the decoordinated PymS ligand). Furthermore, to favor the attack of PMe3 at W coordinated acetylene, the metal center needs to be electron-poor and crowded enough to prevent nucleophile coordination. Finally, the intricate role of the anionic PymS ligand in the vicinity of the first coordination sphere models the potential involvement of amino acid residues during acetylene transformations in AH.