Molybdenum(VI) Bis(imido) Complexes: From Frustrated Lewis Pairs to Weakly Coordinating Cations.

Molybdenum(VI) bis(imido) complexes [Mo(NtBu) (L ) ] (R=H 1 a; R=CF 1 b) combined with B(C F ) (1 a/B(C F ) , 1 b/B(C F ) ) exhibit a frustrated Lewis pair (FLP) character that can heterolytically split H-H, Si-H and O-H bonds. Cleavage of H and Et SiH affords ion pairs [Mo(NtBu)(NHtBu)(L ) ][HB(C F ) ] (R=H 2 a; R=CF 2 b) composed of a Mo(VI) amido imido cation and a hydridoborate anion, while reaction with H O leads to [Mo(NtBu)(NHtBu)(L ) ][(HO)B(C F ) ] (R=H 3 a; R=CF 3 b). Ion pairs 2 a and 2 b are catalysts for the hydrosilylation of aldehydes with triethylsilane, with 2 b being more active than 2 a. Mechanistic elucidation revealed insertion of the aldehyde into the B-H bond of [HB(C F ) ] . We were able to isolate and fully characterize, including by single-crystal X-ray diffraction analysis, the inserted products Mo(NtBu)(NHtBu)(L ) ][{PhCH O}B(C F ) ] (R=H 4 a; R=CF 4 b). Catalysis occurs at [HB(C F ) ] while [Mo(NtBu)(NHtBu)(L ) ] (R=H or CF ) act as the cationic counterions. However, the striking difference in reactivity gives ample evidence that molybdenum cations behave as weakly coordinating cations (WCC).