Probing the Carbon-Hydrogen Activation of Alkanes Following Photolysis of Tp'Rh(CNR)(carbodiimide): A Computational and Time-Resolved Infrared Spectroscopic Study

摘要

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(CNCH_(2)CMe_(3)), and their reaction mechanisms were modeled by density functional theory (DFT) on TpRh(CNMe). The major intermediate species were: κ~(3)-η~(1)-alkane complex ( 1 ); κ~(2)-η~(2)-alkane complex ( 2 ); and κ~(3)-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 (C_(5)H_(10), C_(6)H_(12), C_(7)H_(14), and C_(8)H_(16)) increase with alkanes size and show a dramatic increase between C_(6)H_(12) and C_(7)H_(14). 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 C_(7)H_(14). 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 κ~(2)-η~(2)-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 κ~(2)-Tp′Rh(CNR)(cycloalkane) species and results in the C–H activation without the assistance of the rechelation.