Carbon-Hydrogen Bond Activation by Titanium Imido Complexes. Computational Evidence for the Role of Alkane Adducts in Selective C-H Activation

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This article reports calculations that probe the role of R (hydrocarbon) and R' (ligand substituent) effects on the reaction coordinate for C-H activation.

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7 p.

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Cundari, Thomas R., 1964-; Klinckman, Thomas R. & Wolczanski, Peter T. January 19, 2002.

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This article reports calculations that probe the role of R (hydrocarbon) and R' (ligand substituent) effects on the reaction coordinate for C-H activation.

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7 p.

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Reprinted with permission from the Journal of the American Chemical Society. Copyright 2002 American Chemical Society.

Abstract: This paper reports calculations that probe the role of R (hydrocarbon) and R' (ligand substituent) effects on the reaction coordinate for C-H activation: Ti(OR')₂(=NR') + RH → adduct → transition state → (OR')₂Ti(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' = SiᵗBu₃) 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 activation. Substantial steric modification of the alkane complex is expected from R-R' interactions, given the magnitude of ∆Gadd and the conformational flexibility of the adduct. Molecular mechanics simulations of Ti(OSiᵗBu₃)₂(=NSiᵗBu₃)•••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.

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  • Journal of the American Chemical Society, 2002, Washington DC: American Chemical Society, pp. 1481-1487

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  • Publication Title: Journal of the American Chemical Society
  • Volume: 124
  • Issue: 7
  • Page Start: 1481
  • Page End: 1487
  • Peer Reviewed: Yes

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  • January 19, 2002

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  • Oct. 9, 2012, 10:02 a.m.

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  • July 10, 2013, 2:29 p.m.

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Cundari, Thomas R., 1964-; Klinckman, Thomas R. & Wolczanski, Peter T. Carbon-Hydrogen Bond Activation by Titanium Imido Complexes. Computational Evidence for the Role of Alkane Adducts in Selective C-H Activation, article, January 19, 2002; [Washington, DC]. (digital.library.unt.edu/ark:/67531/metadc107781/: accessed June 27, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.