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

Molybdenum(VI) bis(imido) complexes [Mo(NtBu) 2 (L R ) 2 ] (R=H 1 a; R=CF 3 1 b) combined with B(C 6 F 5 ) 3 (1 a/B(C 6 F 5 ) 3 , 1 b/B(C 6 F 5 ) 3 ) exhibit a frustrated Lewis pair (FLP) character that can heterolytically split H-H, Si-H and O-H bonds. Cleavage of H 2 and Et 3 SiH affords ion pairs [Mo(NtBu)(NHtBu)(L R ) 2 ][HB(C 6 F 5 ) 3 ] (R=H 2 a; R=CF 3 2 b) composed of a Mo(VI) amido imido cation and a hydridoborate anion, while reaction with H 2 O leads to [Mo(NtBu)(NHtBu)(L R ) 2 ][(HO)B(C 6 F 5 ) 3 ] (R=H 3 a; R=CF 3 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 6 F 5 ) 3 ] - . We were able to isolate and fully characterize, including by single-crystal X-ray diffraction analysis, the inserted products Mo(NtBu)(NHtBu)(L R ) 2 ][{PhCH 2 O}B(C 6 F 5 ) 3 ] (R=H 4 a; R=CF 3 4 b). Catalysis occurs at [HB(C 6 F 5 ) 3 ] - while [Mo(NtBu)(NHtBu)(L R ) 2 ] + (R=H or CF 3 ) act as the cationic counterions. However, the striking difference in reactivity gives ample evidence that molybdenum cations behave as weakly coordinating cations (WCC).