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=CF3 1 b) combined with B(C6F5)(3) (1 a/B(C6F5)(3), 1 b/B(C6F5)(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 Et3SiH affords ion pairs [Mo(NtBu)(NHtBu)(L (R))(2)][HB(C6F5)(3)] (R=H 2 a; R=CF3 2 b) composed of a Mo(VI) amido imido cation and a hydridoborate anion, while reaction with H2O leads to [Mo(NtBu)(NHtBu)(L (R))(2)][(HO)B(C6F5)(3)] (R=H 3 a; R=CF3 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(C6F5)(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)][{PhCH2O}B(C6F5)(3)] (R=H 4 a; R=CF3 4 b). Catalysis occurs at [HB(C6F5)(3)](-) while [Mo(NtBu)(NHtBu)(L (R))(2)](+) (R=H or CF3) act as the cationic counterions. However, the striking difference in reactivity gives ample evidence that molybdenum cations behave as weakly coordinating cations (WCC).

Automated summary

Molybdenum(VI) bis(imido) complexes with frustrated Lewis pair characteristics can heterolytically split H-H, Si-H, and O-H bonds. These complexes can be used as catalysts for the hydrosilylation reaction. Mechanistic elucidation revealed the insertion of aldehydes into the B-H bond of the molybdenum complexes, highlighting the weakly coordinating cation behavior of molybdenum.