Facile reversible metalation in an agostic complex and hydrogenolysis of a metal aryl complex via a dihydrogen complex

Article abstract of DOI:10.1021/om00037a045

2,6-Diarylpyridines, 2,6-(p-C₆H₄R′)₂C₅H ₃N (diarpyH₂; R′ = H, diphpyH₂; R′ = CH₃, ditolpyH₂) react with [(COD)Ir(PR₃)₂]X (1; PR₃ = PPh₃, P(p-C₆H₄CH₃)₃, P(p-C₆H₄F)₃, PPh₂Me, P-n-Bu₃; X = SbF₆^-, BF₄^-) under H₂ to give complexes with the stoichiometry [Ir(diarpyH)H(PR₃)₂]X (2a-f). The X-ray crystal structure for [Ir(diphpyH)H{P(p-C₆H₄CH₃)₃} ₂]SbF₆ (2b) shows that one phenyl group of the diphpyH is orthometalated and the other is bound to the Ir via an agostic C-H bond. The Ir?C distance of the Ir?H-C group is 2.69 ?, indicating a moderately strong agostic interaction. 2b crystallizes in the triclinic system, space group P1, with a = 11.072 (2) ?, b = 14.450 (2) ?, c = 17.227 (2) ?, α = 92.76 (1)°, β = 105.04 (1)°, γ = 95.00 (1)°, and Z = 2. The complexes 2a-f show dynamic behavior in solution, consistent with rapid oxidative addition of the agostic C-H bond together with reductive elimination of the aryl hydride. Ligands like CO or CH₃CN displace the agostic C-H in 2a to give [Ir(diphpyH)HL(PPh₃)₂]SbF₆ (L = CO, 3; CH₃CN, 4). H₂ reacts with 2d (PR₃ = PPh₂Me) and 2e (PR₃ = P-n-Bu₃) in the same way to give the dihydrogen complexes [Ir(diphpyH)H(η²-H₂)(PR₃) ₂]SbF₆ (5d,e). Reversible Ir-Ar hydrogenolysis occurs with H₂ for 2a-c and 2f; an equilibrium between the dihydrogen complex 5 and the hydrogenolysis product [Ir-(diphpyH₂)(H)₂(PR₃)₂]SbF ₆ (6) is observed at low temperatures. For a given diarpyH ligand, less basic phosphines favor the hydrogenolysis product. Longer treatment of 2a with H₂ leads to complete displacement of the diarpyH₂ ligand from the metal.