Nucleophilic Reactivity and Oxo/Sulfido Substitution Reactions of MVIO3Groups (M = Mo, W)

Abstract

The nucleophilic reactivity of oxo ligands in the groups M^VIO₃ in the trigonal complexes [(Me₃tacn)MO₃] (M = Mo (1), W (10)) and [(Bu^t₃tach)MO₃] (M = Mo (5), W (14)) has been investigated. Complexes 1/10 can be alkylated with MeOTf to give [(Me₃tacn)MO₂(OMe)]¹⁺ (2/11), silylated with Prⁱ₃SiOTf to form [(Me₃tacn)MO₂(OSiPrⁱ₃)]⁺ (3/12), and protonated with HOTf to yield [(Me₃tacn)MoO₂(OH)]⁺ (4). Similarly, complexes 5/14 can be silylated to [(Bu^t₃tach)MO₂(OSiPrⁱ₃)]⁺ (6/15) and protonated to [(Bu^t₃tach)MO₂(OH)]⁺ (7/16). Products were isolated as triflate salts in yields exceeding 70%. When excess acid was used, the dinuclear μ-oxo species [(Bu^t₃tach)₂M₂O₅]²⁺ (8/17) were obtained. X-ray structures are reported for 2−4, 6−8, 12, and 15−17. All mononuclear complexes have dominant trigonal symmetry with a rhombic distortion owing to a MOR bond (R = Me, SiPrⁱ₃, H), which is longer than MO oxo interactions; the latter exert a substantial trans influence on MN bond lengths. Oxo ligands in 5/14 undergo replacement with sulfide. Lawesson's reagent effects formation of [(Bu^t₃tach)MS₃] (9/18), 14 with excess B₂S₃ yields incompletely substituted [(Bu^t₃tach)WOS₂] (20), and 5 with excess B₂S₃ yields [(Bu^t₃tach)Mo^IVO(S₄)] (19). The structures of 9, 19, and 20 are reported. Precedents for M^VIS₃ groups in five- and six-coordinate molecules are limited. This investigation is the first detailed study of the behavior of M^VIO₃ groups in nucleophilic and oxo/sulfido substitution reactions and should be useful in synthetic approaches to the active sites of the xanthine oxidase enzyme family and of certain tungstoenzymes. (Bu^t₃tach = 1,3,5-tri-tert-butyl-1,3,5-triazacyclohexane, Me₃tacn = 1,4,7-trimethyl-1,4,7-triazacyclonane; OTf = triflate).