Four-Coordinate, 14-Electron Ru^II Complexes: Unusual Trigonal Pyramidal Geometry Enforced by Bis(phosphino)silyl Ligation

Unprecedented diamagnetic, four-coordinate, formally 14-electron (Cy-PSiP)RuX (Cy-PSiP = [κ3-(2-R2PC6H4)2SiMe]−; X = amido, alkoxo) complexes that do not require agostic stabilization and that adopt a highly unusual trigonal pyramidal coordination geometry are reported. The tertiary silane [(2-Cy2PC6H4)2SiMe]H ((Cy-PSiP)H) reacted with 0.5 [(p-cymene)RuCl2]2 in the presence of Et3N and PCy3 to afford [(Cy-PSiP)RuCl]2 (1) in 74% yield. Treatment of 1 with KOtBu led to the formation of (Cy-PSiP)RuOtBu (2, 97% yield), which was crystallographically characterized and shown to adopt a trigonal pyramidal coordination geometry in the solid state. Treatment of 1 with NaN(SiMe3)2 led to the formation of (Cy-PSiP)RuN(SiMe3)2 (3, 70% yield), which was also found to adopt a trigonal pyramidal coordination geometry in the solid state. The related anilido complexes (Cy-PSiP)RuNH(2,6-R2C6H3) (4, R = H; 5, R = Me) were also prepared in >90% yields by treating 1 with LiNH(2,6-R2C6H3) (R = H, Me) reagents. The solid state structure of 5 indicates a monomeric trigonal pyramidal complex that features a C–H agostic interaction. Complexes 2 and 3 were found to react readily with 1 equiv of H2O to form the dimeric hydroxo-bridged complex [(Cy-PSiP)RuOH]2 (6, 94% yield), which was crystallographically characterized. Complexes 2 and 3 also reacted with 1 equiv of PhOH to form the new 18-electron η5-oxocyclohexadienyl complex (Cy-PSiP)Ru(η5-C6H5O) (7, 84% yield). Both amido and alkoxo (Cy-PSiP)RuX complexes reacted with H3B·NHRR′ reagents to form bis(σ-B–H) complexes of the type (Cy-PSiP)RuH(η2:η2-H2BNRR′) (8, R = R′ = H; 9, R = R′ = Me; 10, R = H, R′ = tBu), which illustrates that such four-coordinate (Cy-PSiP)RuX (X = amido, alkoxo) complexes are able to undergo multiple E–H (E = main group element) bond activation steps. Computational methods were used to investigate structurally related PCP, PPP, PNP, and PSiP four-coordinate Ru complexes and confirmed the key role of the strongly σ-donating silyl group of the PSiP ligand set in enforcing the unusual trigonal pyramidal coordination geometry featured in complexes 2–5, thus substantiating a new strategy for the synthesis of low-coordinate Ru species. The mechanism of the activation of ammonia-borane by such low-coordinate (R-PSiP)RuX (X = amido, alkoxo) species was also studied computationally and was determined to proceed most likely in a stepwise fashion via intramolecular deprotonation of ammonia and subsequent borane B–H bond oxidative addition steps.