Deoxygenations of (silox)(3)WNO (12) and R(3)PO (R = Me, Ph, (t)Bu) by M(silox)(3) (1-M; M = V, NbL (L = PMe(3), 4-picoline), Ta; silox = (t)Bu(3)SiO) reflect the consequences of electronic effects enforced by a limiting steric environment. 1-Ta rapidly deoxygenated R(3)PO (23 degrees C; R = Me (DeltaG degrees (rxn)(calcd) = -47 kcal/mol), Ph) but not (t)Bu(3)PO (85 degrees, >2 days), and cyclometalation competed with deoxygenation of 12 to (silox)(3)WN (11) and (silox)(3)TaO (3-Ta; DeltaG degrees (rxn)(calcd) = -100 kcal/mol). 1-V deoxygenated 12 slowly and formed stable adducts (silox)(3)V-OPR(3) (3-OPR(3)) with OPR(3). 1-Nb(4-picoline) (S = 0) and 1-NbPMe(3) (S = 1) deoxygenated R(3)PO (23 degrees C; R = Me (DeltaG degrees (rxn)(calcd from 1-Nb) = -47 kcal/mol), Ph) rapidly and 12 slowly (DeltaG degrees (rxn)(calcd) = -100 kcal/mol), and failed to deoxygenate (t)Bu(3)PO. Access to a triplet state is critical for substrate (EO) binding, and the S --> T barrier of approximately 17 kcal/mol (calcd) hinders deoxygenations by 1-Ta, while 1-V (S = 1) and 1-Nb (S --> T barrier approximately 2 kcal/mol) are competent. Once binding occurs, significant mixing with an (1)A(1) excited state derived from population of a sigma-orbital is needed to ensure a low-energy intersystem crossing of the (3)A(2) (reactant) and (1)A(1) (product) states. Correlation of a reactant sigma-orbital with a product sigma-orbital is required, and the greater the degree of bending in the (silox)(3)M-O-E angle, the more mixing energetically lowers the intersystem crossing point. The inability of substrates EO = 12 and (t)Bu(3)PO to attain a bent 90 degree angle M-O-E due to sterics explains their slow or negligible deoxygenations. Syntheses of relevant compounds and ramifications of the results are discussed. X-ray structural details are provided for 3-OPMe(3) (90 degree angle V-O-P = 157.61(9) degrees), 3-OP(t)Bu(3) ( 90 degree angle V-O-P = 180 degrees ), 1-NbPMe(3), and (silox)(3)ClWO (9).

The reactivity of highly Lewis acidic perfluorinated borafluorenes C 12 F 8 BR (R ) C 6 F 5 , 1a; CH 3 , 1b) and the nonfluorinated 9-phenyl-9-borafluorene (2) toward [Cp*Al] 4 was investigated. The reaction of 1 with [Cp*Al] 4 leads to the formation of thermally robust η 1 Lewis acid-base adducts 3a,b as the thermodynamically favored products. Use of the less Lewis acidic 2 does not alter the mode of reactivity, with the η 1 Lewis acid-base 4 formed preferentially. Reduction of 2 to the 9-boratafluorene 2‚Li 2 (THF) n is readily accomplished in THF solution. However, reaction of 2‚Li 2 (THF) n with [Cp*AlCl 2 ] 2 or Cp*AlCl 2 (THF), 5, affords aluminum metal, 2‚THF, and Cp*H as the main identifiable products. Compounds 3a, 3b, 4, and 5 were fully characterized including their X-ray structures. A DFT computational study was conducted to probe the reason for the strong preference for η 1 bonding, which essentially stems from the localization of aromaticity in the flanking phenyl rings in the 9-borafluorene ring system.