Probing the carbon-hydrogen activation of alkanes following photolysis of Tp′Rh(CNR)(carbodiimide): A computational and time-resolved infrared spectroscopic study

Carbon-hydrogen bond activation of alkanes by Tp′Rh(CNR) (Tp′ = Tp = trispyrazolylborate or Tp∗ = tris(3,5-dimethylpyrazolyl)borate) were followed by time-resolved infrared spectroscopy (TRIR) in the v(CNR) and v (B-H) spectral regions on TpRh(CNCH₂CMe₃), and their reaction mechanisms were modeled by density functional theory (DFT) on TpRh(CNMe). The major intermediate species were: κ³-ν¹-alkane complex (1); κ²-ν²-alkane complex (2); and κ³-alkyl hydride (3). Calculations predict that the barrier between 1 and 2 arises from a triplet-singlet crossing and intermediate 2 proceeds over the rate-determining C-H activation barrier to give the final product 3. The activation lifetimes measured for the TpRh(CNR) and TpRh(CO) fragments with n-heptane and four cycloalkanes (C₅H₁₀, C₆H₁₂, C₇H₁₄, and C₈H₁₆) increase with alkanes size and show a dramatic increase between C₆H₁₂ and C₇H₁₄. A similar step-like behavior was observed previously with CpRh(CO) and CpRh(CO) fragments and is attributed to the wider difference in C-H bonds that appear at C₇H₁₄. However, Tp′Rh(CNR) and Tp′Rh(CO) fragments have much longer absolute lifetimes compared to those of CpRh(CO) and CpRh(CO) fragments, because the reduced electron density in dechelated κ²-ν²-alkane Tp′ complexes stabilizes the d⁸ Rh(I) in a square-planar geometry and weakens the metal′s ability for oxidative addition of the C-H bond. Further, the Tp′Rh(CNR) fragment has significantly slower rates of C-H activation in comparison to the Tp′Rh(CO) fragment for the larger cycloalkanes, because the steric bulk of the neopentyl isocyanide ligand hinders the rechelation in κ²-Tp′Rh(CNR)(cycloalkane) species and results in the C-H activation without the assistance of the rechelation.