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Relating the Radical Stabilization Energy and Steric Bulk of a Hydrocarbyl Group to the Strength of its Bonds with Metals and Hydrogen. A Theoretical Study

2007; American Chemical Society; Volume: 26; Issue: 7 Linguagem: Inglês

10.1021/om0700361

1520-6041

Mariusz P. Mitoraj, Hungjuan Zhu, Artur Michalak, Tom Ziegler,

Radical Photochemical Reactions

We have analyzed the strength of the XR bond for a number of hydrocarbyl groups (R) attached to X = hydrogen, X = (CNCH3)(H)(Tp)Rh [Tp = H-B(pyrazolyl)3], and X = (OSi(CH3)3)(OSi(CH3)3)(NH-Si(CH3)3)Ti with the help of a density functional theory (DFT) based energy decomposition scheme (EDA). The hydrocarbyl groups included had a radical sp3 carbon with unsaturated aromatic or olefinic bonds (R1 = Ph-CH2, mesityl, Me-allyl, allyl) or saturated alkyl substituents (R2 = Me, Et, Pe, i-Pr, t-Bu), a radical carbon as part of a saturated ring (R3 = c-Pr, c-Bu, c-Pe, c-He), or a radical sp2 carbon (R4 = Ph, t-Bu-vinyl, Me-vinyl, vinyl). The EDA scheme was used to rationalize the relative order of the X−Ri bond energies between groups (i = 1,4), within groups (same i), and between metals X = Rh, Ti, and hydrogen (X = H). It was found that the average bond energy within each group increases as R4 > R3 ∼ R2 > R1 for the two metals as well as H. This trend correlates with the radical stabilization energy of Ri that decreases in absolute terms as R4 < R3 ∼ R1 < R2. This trend enables one to make rough correlations between M−C bonds and M−H links on going from one group to another. Within each of the XRi systems where i = 1, 3, 4 there is also a correlation between the trend in the X−Ri bond energies and the Ri distortion energy. However, for the R2 group trends in the X−R2 bond energies are not determined by the radical stabilization but directly (X = H) or indirectly (X = Rh, Ti) by increasing steric bulk on R2. For X = H, steric bulk directly destabilizes the H−R2 bond by increasing the steric interaction between H and R2 without significantly changing the H−C bond distance. For X = Rh, Ti steric bulk indirectly destabilizes the M−R2 bond by increasing the M−C bond distance in order to relieve the steric strain. This leads to a reduction in the M−C bonding overlap and the M−C bond strength. We note finally that the Rh−Ri and Ti−Ri bonds are some 60 kcal/mol weaker than the H−Ri link. The major contributing factor here is the poorer overlap and larger energy gap between the orbitals involved in forming the M−C bond compared to the H−C link. Additional factors are larger steric interactions and the need for some distortion of the Rh and Ti fragments.