Density functional theory study of substituent effects on gas-phase heterolytic Fe-N bond energies of m-G-C6H4NHFe(CO)2(η5-C5H5) and m-G-C6H4N(COMe)Fe(CO)2(η5-C5H5)

Abstract

The nature and strength of metal–ligand bonds in organotransition-metal complexes are crucial to the understanding of organometallic reactions and catalysis. Quantum chemical calculations at different levels of theory have been used to investigate heterolytic Fe-N bond energies of meta-substituted anilinyldicarbonyl(η5-cyclopentadienyl)iron [m-G-C6H4NH(η5-C5H5)Fe(CO)2, abbreviated as m-G-C6H4NHFp (1), where G=NO2, CN, COMe, CO2Me, CF3, Br, Cl, F, H, Me, MeO and NMe2] and meta-substituted α-acetylanilinyldicarbonyl(η5-cyclopentadienyl)iron [m-G-C6H4N(COMe)(η5-C5H5)Fe(CO)2, abbreviated as m-G-C6H4N(COMe)Fp (2)] complexes. The results show that BP86 and TPSSTPSS can provide the best price/performance ratio and more accurate predictions in the study of ΔHhet(Fe-N)′s. ΔΔHhet(Fe-N)′s (1 and 2) conform to the captodative principle. There are excellent linear free energy relations among ΔΔHhet(Fe-N)′s and the experimental and computational substituent effects on acidities of m-G-C6H4NH2, the differences of acidic dissociation constants (ΔpKas) of NH bonds for m-G-C6H4NH2 for series 1 or the substituent σm constants. The former correlations imply that the govering factors for these bond scissions are similar; the latter ones suggest that polar effects of meta-substituents show the dominant role to the magnitudes of ΔHhet(Fe-N)′s. And these correlations are in accordance with Hammett linear free energy relationships. The Fe-N bonds in series 1 are stronger than those in series 2. Therefore m-G-C6H4N(COMe)- are more stable than m-G-C6H4NH-. The absolute magnitudes of AEs, AEα-COMes and AEα-COMe,para-Gs are far larger than MEs, MEα-COMes and MEα-COMe,para-Gs. The influences of MEs, MEα-COMes and MEα-COMe,para-Gs are subtle. A better understanding of organometallic bond energies can suggest whether proposed new catalytic systems may work well.