The Effect of Selenium-Based Ligands on Tungsten Acetylene Complexes

Bioinspired tungsten acetylene complexes containing pyridine-2-selenolato (PySe) or 6-methyl-pyridine-2-selenolato (6-MePySe) ligands were synthesized. ⁷⁷Se NMR spectroscopy allowed for an assessment of the resonance structures in the pyridine-2-selenolato ligands and the rationalization of chemoselectivity observed in regard to 1,2 migratory insertion of HC≡CH. [W(CO)(C₂H₂)(CHCH-PySe)(PySe)] is formed exclusively via insertion of HC≡CH into the W-N bond, while the use of bulkier 6-MePySe allows for the isolation of [W(CO)(C₂H₂)(6-MePySe)₂], which only partially reacts with excess HC≡CH to give [W(CO)(C₂H₂)(CHCH-6-MePySe)(6-MePySe)]. Oxidation of [W(CO)(C₂H₂)(6-MePySe)₂] with pyridine-N-oxide gave the tungsten(IV) complex [WO(C₂H₂)(6-MePySe)₂]. Complexes [W(CO)(C₂H₂)(6-MePySe)₂] and [WO(C₂H₂)(6-MePySe)₂] react with trimethyl phosphine to carbyne complex [W(CO)(CCH₂PMe₃)(PMe₃)₂(6-MePySe)]Cl and alkylidene complex [WO(CHCHPMe₃)(PMe₃)₂(6-MePySe)]Cl, respectively. The addition of substituted alkynes to [W(CO)₃(PySe)₂] via thermal decarbonylation gave complexes [W(CO)(MeC≡CMe)(PySe)₂] and [W(CO)(HC≡Ct-Bu)(PySe)₂], respectively. The here presented complexes are relevant for the modeling of the active site of acetylene hydratase from Pelobacter acetylenicus, in which a tungsten atom is enclosed in a sulfur-rich coordination sphere. A recently published theoretical study concluded that the exchange of sulfur for selenium would increase the activity of the enzyme. Our findings contrast this claim as comparative analysis concludes negligible structural and electronic differences between the selenium-based and previously published sulfur-based complexes.