Remote substituent effects on homolytic Fe-C bond energies of p-G-C6H4CH2Fe(CO)2 (η5-C5H5) and p-G-C6H4(H)(CN)CFe(CO)2 (η5-C5H5) studied by Hartree-Fock and density functional theory methods

In general, both stoichiometric and catalytic reactions of organometallic complexes involve breaking and forming metal-ligand bonds. Therefore, an evaluation of the thermodynamics of such reactions requires the knowledge of metal-ligand bond energies (BDEs). The homolytic Fe-C bond dissociation energies [i.e., Delta H-homo(Fe-C)s] of 12 para-substituted benzyldicarbonyl(eta(5)-cyclopentadienyl)iron, p-G-C(6)H(4)CH(2)Fp [1, G = NO2, CN, COMe, CO2Me, CF3, Br, Cl, F, H, Me, MeO, NMe2; Fp = (eta(5)-C5H5)(CO)(2)Fe] and 12 para-substituted alpha-cyanobenzyldicarbonyl (eta(5)-cyclopentadienyl)iron, p-G-PANFp [2, PAN = C6H4CH(CN)] were studied using Hartree-Fock (HF) and density functional theory (DFT) methods with large basis sets. The results show that BP86 and TPSSTPSS can provide the best price/performance ratio and more accurate predictions in the study of Delta H-homo(Fe-C)s. The B3LYP method satisfactorily predicts the alpha and remote substituent effects on Delta H-homo(Fe-C)s [Delta Delta H-homo(Fe-C)s]. The fair correlations [r = 0.97 (g, 1), 0.99(g, 2)] of Delta Delta H-homo(Fe-C)s of series 1 and 2 with the substituent sigma(+)(p) constants imply that the para substituent effects on Delta H-homo(Fe-C)s originate mainly from polar effects, but those on radical stability originate from both spin delocalization and polar effects. The molecule stabilization effects (MEs) causes that not only the magnitude of Delta Delta H-homo(Fe-C)s(1) varies significantly but also the direction changes from S-pattern to O-pattern. Delta Delta H-homo(Fe-C)s(2) were found to conform to the Capto-dative Principle. The detailed knowledge of the factors that determine the Fp-C bond strengths would greatly aid in understanding reactivity patterns in many processes. Copyright (C) 2011 John Wiley & Sons, Ltd.