Remote substituent effects on gas‐phase homolytic Fe–O and Fe–S bond energies of p‐G‐C₆H₄OFe(CO)₂(η⁵‐C₅H₅) and p‐G‐C₆H₄SFe(CO)₂(η⁵‐C₅H₅) studied using Hartree–Fock and density functional theory methods

Abstract

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(h 5 -cyclopentadienyl) iron [p-G-C 6 H 4 OFp (1)] and (para-substituted benzenethiolato)dicarbonyl(h 5 -cyclopentadienyl) iron [p-G-C 6 H 4 SFp (2)] were studied using Hartree-Fock and density functional theory (DFT) methods with large basis sets. In this study, Fp is (h 5 -C 5 H 5 )Fe(CO) 2 , and G are NO 2 , CN, COMe, CO 2 Me, 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 s 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.

Hartree-Fock and density functional theory study of remote substituent effects on gas-phase heterolytic Fe-O and Fe-S bond energies of p -G-C6 H4 OFe(CO)2 (η 5 -C5 H5 ) and p -G-C6 H4 SFe(CO)2 (η 5 -C5 H5 )

“…Although there are scarce reports on the Fe–O and Fe–S bond energies in Fp compounds, there were some indirect data to support the theoretical results. For example, Δ H homo (Fe–O)'s are larger than those in FePorphyrin(O 2 ) and FePorphyrin(imidazole)(O 2 ) whose Δ H homo (Fe–O)'s are 9 and 15 kcal/mol calculated by DFT methods . Δ H homo (Fe–S)'s have the comparable magnitudes with those in S 2 Fe 2 (CO) 6 , S 2 Fe 3 (CO) 9 , (SCH 3 ) 2 Fe 2 (CO) 6 , and (SC 2 H 5 ) 2 Fe 2 (CO) 6 whose Δ H homo (Fe–S)'s are 41.83, 41.11, 46.61, and 46.37 kcal/mol calculated by drop‐microcalorimetric methods .…”

“…For example, Δ H homo (Fe–O)'s are larger than those in FePorphyrin(O 2 ) and FePorphyrin(imidazole)(O 2 ) whose Δ H homo (Fe–O)'s are 9 and 15 kcal/mol calculated by DFT methods . Δ H homo (Fe–S)'s have the comparable magnitudes with those in S 2 Fe 2 (CO) 6 , S 2 Fe 3 (CO) 9 , (SCH 3 ) 2 Fe 2 (CO) 6 , and (SC 2 H 5 ) 2 Fe 2 (CO) 6 whose Δ H homo (Fe–S)'s are 41.83, 41.11, 46.61, and 46.37 kcal/mol calculated by drop‐microcalorimetric methods . Because the superiority of using nonhybrid functionals (e.g., BP86 and TPSSTPSS) in handling Co‐C BDEs is confirmed, we conclude that BP86 and TPSSTPSS can provide the best price/performance ratio and more accurate predictions in the study of Δ H het (Fe–O)'s and Δ H het (Fe–S)'s …”

“…A part of the calculated Δ H het (Fe–O)'s and Δ H het (Fe–S)'s [Eqns and ] are presented in Table . It is clear that the heterolytic Fe–O and Fe–S BDEs of the two types of Fp compounds are all greater than the corresponding homolytic BDEs, which indicate that the homolyses of the Fp–O and Fp–S bonds to generate Fp · and oxygen or sulfur radicals are energetically much more favorable than the heterolyses of the corresponding Fp–O and Fp–S bonds to generate two pairs of ions. In addition, it can be found that Δ H het (Fe–O)'s in column IV are smaller than the corresponding Δ H het (Fe–S)'s in column IV.…”

“…In the B3LYP method, 3‐21G(d), 6‐31G(d) basis sets were used to optimize the geometries of series 1 and 2 . HF, B3LYP, BP86, and TPSSTPSS methods, 3‐21G(d), 6‐31G(d), 6‐31 + G(d,p) and 6‐311 + G(d,p) basis sets were used to calculate the Δ H het (Fe–O)'s and Δ H het (Fe–S)'s.…”

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)

“…Because the Cartesian coordinates of Fp + were derived from the fractional ones of Fp moiety in FpP(CF 3 ) 2 crystallographic structure, its structure could not be modified manually. These little transition states could be regarded as energy minima . And, zero‐point vibrational energies (scaled by the recommended factor of 0.9806) and thermal corrections were obtained.…”