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

Abstract

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 HÀ H, SiÀ H and OÀ 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 BÀ H bond of [HB(C 6 F 5 ) 3 ] À . 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 ] À 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).

Nucleophiles Target the Tungsten Center Over Acetylene in Biomimetic Models

“…1 H NMR spectroscopy shows a downfield shift of acetylenic protons from 11.23 and 10.99 ppm (complex 2 ) to 12.21 and 12.01 ppm due to a more electron-deficient tungsten center. The coordination of the Lewis acid to the oxido group is further confirmed via 19 F NMR spectroscopy as indicated by the shifting of the resonances from uncoordinated borane (−128.14 ppm, −143.81 ppm, −160.95 ppm) to coordinated borane (−133.01 ppm, −159.59 ppm, −165.18 ppm). The W–O–B adduct is highly sensitive toward water, forming known hydroxy borate anion [HO-B­(C 6 F 5 ) 3 ] − and a decomposed complex species.…”