Carbon-hydrogen bond activation by titanium imido complexes. Computational evidence for the role of alkane adducts in selective C-H activation

This paper reports calculations that probe the role of R (hydrocarbon) and W(ligand substituent) effects on the reaction coordinate for C-H activation: Ti(OR')(2)(=NR') + RH --> adduct --> transition state --> (OR')(2)Ti(N(H)R')(R). Compounds with R = H, Me, Et, Vy, cPr, Ph, Cy, Bz, and cubyl are studied using quantum (R'= H, SiH3, SiMe3) and classical (R'= (SiBu3)-Bu-t) techniques. Calculated geometries are in excellent agreement with data for experimental models. There is little variability in the calculated molecular structure of the reactants, products, and most interestingly, transition states as R and R' are changed. Structural flexibility is greatest in the adducts Ti(OR')(2)( NR')...HR. Despite the small structural changes observed for Ti(OR')2(=NR') with different R', significant changes are manifested in calculated electronic properties (the Mulliken charge on Ti becomes more positive and the Ti=N bond order decreases with! larger R'), changes that should facilitate C-H lactivation. Substantial steric modification of the alkane complex is expected from R-R' interactions, given the magnitude of DeltaG(add) and the conformational flexibility of the adduct. Molecular mechanics simulations of Ti((OSiBu3)-Bu-t)(2)(=(NSiBu3)-Bu-t)...isopentane adducts yield an energy ordering as a function of the rank of the C-H bond coordinated to Ti that is consistent with experimental selectivity patterns. Calculated elimination barriers compare very favorably with experiment; larger SiH3 and TMS ligand substituents generally yield better agreement with experiment, evidence that the modeling of the major contributions to the elimination barrier (N-H and C-H bond making) is ostensibly Correct. Calculations indicate that weakening the C-H bond of the hydrocarbon yields a more strongly bound adduct. Combining the different conclusions, the present computational research points to the adduct, specifically the structure and energetics of the substrate/Ti-imido interaction, as the main factor in determining the selectivity of hydrocarbon (R) C-H activation.