Density functional theory study of substituent effects on gas‐phase heterolytic Fe–O and Fe–S bond energies of m‐G‐C6H4OFe(CO)2(η5‐C5H5) and m‐G‐C6H4SFe(CO)2(η5‐C5H5)

Abstract

The knowledge of accurate bond strengths is a fundamental basis for a proper analysis of chemical reaction mechanisms. Quantum chemical calculations at different levels of theory have been used to investigate heterolytic Fe–O and Fe–S bond energies of (meta‐substituted phenoxy)dicarbonyl(η5‐cyclopentadienyl) iron [m‐G‐C6H4OFp (1)] and (meta‐substituted benzenethiolato)dicarbonyl(η5‐cyclopentadienyl) iron [m‐G‐C6H4SFp (2)] complexes. In this study, Fp is (η5‐C5H5)Fe(CO)2, and G is NO2, CN, COMe, CO2Me, CF3, Br, Cl, F, H, Me, MeO, and NMe2. The results show that Tao–Perdew–Staroverov–Scuseria and Becke's power‐series ansatz from 1997 with dispersion corrections functionals can provide the best price/performance ratio and accurate predictions of ΔHhet(Fe–O)'s and ΔHhet(Fe–S)'s. The excellent linear free energy relations [r = 1.00 (g, 1e), 1.00 (g, 2b)] among the ΔΔHhet (Fe–O)'s and δΔG0 of OH bonds of m‐G‐C6H4OH or ΔΔHhet(Fe–S)'s and ΔpKa's of SH bonds of m‐G‐C6H4SH imply that the governing structural factors for these bond scissions are similar. And, the linear correlations [r = −0.97 (g, 1 g), −0.97 (g, 2 h)] among the ΔΔHhet (Fe–O)'s or ΔΔHhet(Fe–S)'s and the substituent σm constants show that these correlations are in accordance with Hammett linear free energy relationships. The inductive effects of these substituents and the basis set effects influence the accuracy of ΔHhet(Fe–O)'s or ΔHhet(Fe–S)'s. The ΔΔHhet(Fe–O)'s(g) (1) and ΔΔHhet(Fe–S)'s(g)(2) follow the capto‐dative Principle. The substituent effects on the Fe–O bonds are much stronger than those on the less polar Fe–S bonds. Insight from this work may help the design of more effective catalytic processes. Copyright © 2016 John Wiley & Sons, Ltd.