Remote substituent effects on gas-phase homolytic Fe-O and Fe-S bond energies of p-G-C_6H_4OFe(CO)_2(h_5-C_5H_5) and p-G-C_6H_4SFe (CO)_2(h_5-C_5H_5) studied using Hartree-Fock and density functional theory methods

摘要

Metal-ligand bond enthalpy data can afford invaluable insights into important reaction patterns in organometallic chemistry and catalysis. In this paper, the Fe-O and Fe-S homolytic bond dissociation energies [ΔH_(homo)(Fe-O)'s and ΔH_(homo)(Fe-S)'s] of two series of para-substituted phenoxydicarbonyl(h5-cyclopentadienyl) iron [p-G-C6H4OFp (1)] and (para-substituted benzenethiolato)dicarbonyl(h5-cyclopentadienyl) iron [p-G-C_6H_4SFp (2)] were studied using Hartree-Fock and density functional theory (DFT) methods with large basis sets. In this study, Fp is (h_5-C_5H_5)Fe(CO)_2, and G are NO_2, CN, COMe, CO_2Me, CF_3, Br, Cl, F, H, Me, MeO, and NMe_2. The results show that DFT methods can provide the best price/performance ratio and accurate predictions of ΔH_(homo)(Fe-O)'s and ΔH_(homo)(Fe-S)'s. The remote substituent effects on ΔH_(homo)(Fe-O)'s and ΔH_(homo)(Fe-S)'s [ΔΔH_(homo)(Fe-O)'s and ΔΔH_(homo)(Fe-S)'s] can also be satisfactorily predicted. The good correlations [r = 0.98 (g, 1), 0.98 (g, 2)] of ΔΔH_(homo)(Fe-O)'s and ΔΔH_(homo)(Fe-S)'s in series 1 and 2 with the substituent σ_p~+ constants imply that the para-substituent effects on ΔH_(homo)(Fe-O)'s and ΔH_(homo)(Fe-S)'s originate mainly from polar effects, but those on radical stability originate from both spin delocalization and polar effects. ΔΔH_(homo)(Fe-O)'s (1) and ΔΔH_(homo)(Fe-S)'s (2) conform to the captodative principle. Insight from this work may help the design of more effective catalytic processes.