MECHANISM OF CYCLOMETALLATION OF TRIS(TRIMETHYLPHOSPHINE)NEOPENTYLIRIDIUM(I)

The mechanism of the known cyclometalation of the R group of L3IrR (L = PMe3; R = CH2CMe3 (1), CH2SiMe3 (2)) to form fac-L3Ir(H)(η2-CH2EMe2CH2) (E = C (3), Si (4)) has been investigated. The reactions of both 1 and 2 are first order and show no inhibition by high concentrations of added L. Activation parameters are found to be as follows: formation of 3 at 0 °C ΔH* = 17.8 (0.8) kcal/mol and ΔS* = -10.0 (2.9) eu; formation of 4 at 45 °C, ΔH* = 21.8 (0.2) kcal/mol and ΔS* = -9.8 (0.6) eu. Reaction of L3Ir[CH2Si(CD3)3] (2-d9) reveals an isotope effect of 3.2 on the observed rate. Reactions of L3Ir[CH2E-(CH3)(CD3)2] (l-d6, 2-d9) give nonrandom distributions of deuterium in cyclometalated products 3-d9 and 4-d9 that do not change with time at temperatures of up to 80 °C. At 95 °C the deuterium becomes randomly distributed between the CH2 and CH3 groups of the ligand. When heated at 95 °C with a large excess of P(CD3)3 (L’), 3 and 4 incorporate L’ without any scrambling of deuterium to or from the solvent or into the alkyl or hydride ligands. L’ is incorporated three at a time to go directly to (L’)3Ir(H)(η2-CH2EMe2CH2) (3-d27, 4-d27) only; no d9 or d18 species are detected by 31P NMR spectroscopy. From the rates of 95 °C and from the activation parameters given above, values of Δrecan for the cyclometalations of 1 and 2 are calculated to be -7.7 (2.0) and -4.6 (0.3) kcal/mol, respectively, at 95 °C. These data are most consistent with a mechanism for the reversible cyclometalation that involves direct, concerted oxidative addition and reductive elimination of the C-H bond interconverting square-planar Ir1 and octahedral IrIII centers without any L dissociation. © 1990, American Chemical Society. All rights reserved.