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)

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(η 5 -cyclopentadienyl)iron [ m -G-C 6 H 4 NH(η 5 -C 5 H 5 )Fe(CO) 2 , abbreviated as m -G-C 6 H 4 NHFp ( 1 ), where G=NO 2 , CN, COMe, CO 2 Me, CF 3 , Br, Cl, F, H, Me, MeO and NMe 2 ] and meta -substituted α-acetylanilinyldicarbonyl(η 5 -cyclopentadienyl)iron [ m -G-C 6 H 4 N(COMe)(η 5 -C 5 H 5 )Fe(CO) 2 , abbreviated as m -G-C 6 H 4 N(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 Δ H het (Fe-N)′s. ΔΔ H het (Fe-N)′s ( 1 and 2 ) conform to the captodative principle. There are excellent linear free energy relations among ΔΔ H het (Fe-N)′s and the experimental and computational substituent effects on acidities of m -G-C 6 H 4 NH 2 , the differences of acidic dissociation constants (Δp K a s) of N H bonds for m -G-C 6 H 4 NH 2 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 Δ H 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-C 6 H 4 N(COMe) - are more stable than m -G-C 6 H 4 NH - . The absolute magnitudes of AEs, AE α-COMe s and AE α-COMe, para -G s are far larger than MEs, ME α-COMe s and ME α-COMe, para -G s. The influences of MEs, ME α-COMe s 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.

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