PC Bond Cleavage of (silox)3NbPMe3 (silox = tBu3SiO) under Dihydrogen Leads to (silox)3NbCH2, (silox)3NbPH or (silox)3NbP(H)Nb(silox)3, and CH4

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Abstract
Photolysis of the equilibrium mixture (silox)3NbPMe3 (1) + H2 (1−3 atm) ⇄ (silox)3Nb(Heq)2 (2e, tbp)/(silox)3Nb(Ht)2 (2t, pseudo-Td) + PMe3 causes PC bond cleavage. Depending on conditions, various amounts of (silox)3NbCH2 (3), (silox)3NbPH (5-H), (silox)3NbPMe (5-Me), (silox)3NbP(H)Nb(silox)3 (9, precipitated if N2 is present; X-ray), (silox)3NbH (4, active only through equilibration with 2e,t), and CH4 are produced. Addition of PH3 to 1 provides an independent route to 5-H; its deprotonation gives [(silox)3NbP]Li (6), whose methylation yields 5-Me. Early conversion 3:5-H ratios of ∼3:1 suggest that initial PC bond activation is slow relative to subsequent PC bond cleavages. Addition of HPMe2 and H2PMe to 1 generates (silox)3HNbPMe2 (7) and (silox)3HNbPHMe (8), respectively, and both degrade faster than PMe3. A mechanism based around sequential PC or CH oxidative addition, followed by 1,2-elimination events, is proposed. The limiting step in the decomposition of all PMe3 is a slow hydrogenation of 3 to regenerate 2e,t and produces CH4. Hydrides 2e,t are likely to be the photolytically active species.

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