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 ΔH_het(Fe-N)′s. ΔΔH_het(Fe-N)′s (1 and 2) conform to the captodative principle. There are excellent linear free energy relations among ΔΔH_het(Fe-N)′s and the experimental and computational substituent effects on acidities of m-G-C₆H₄NH₂, the differences of acidic dissociation constants (ΔpK_as) of NH bonds for m-G-C₆H₄NH₂ for series 1 or the substituent σ_m 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 ΔH_het(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, AE_α-COMes and AE_{α-COMe, para-G}s are far larger than MEs, ME_α-COMes and ME_{α-COMe, para-G}s. The influences of MEs, ME_α-COMes and ME_{α-COMe, para-G}s are subtle. A better understanding of organometallic bond energies can suggest whether proposed new catalytic systems may work well.