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 υ­(CNR) and υ­(B−H) spectral regions on Tp*Rh­(CNCH2CMe3), and their reaction mechanisms were modeled by density functional theory (DFT) on TpRh­(CNMe). The major intermediate species were: κ3-η1-alkane complex (1); κ2-η2-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 Tp*Rh­(CNR) and Tp*Rh­(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 Cp*Rh­(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 Cp*Rh­(CO) fragments, because the reduced electron density in dechelated κ2-η2-alkane Tp′ complexes stabilizes the d 8 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.