Theoretical studies of the complex [(BPMEN)Fe(II)(NCCH3)2]2+, precursor of non-heme iron catalysts for olefin epoxidation and cis-dihydroxylation

The density functional theory (DFT) was applied to elucidate the electronic structure of the BPMEN [N,N'-bis(6R-2-pyridyl methyl)-1,2-diaminoethane] iron complex, [(BPMEN)Fe(NCCH3 )(2)](2+), a precursor of catalysts that catalyze the stereospecific olefin oxidation. The low-lying high and low spin states of complex 1 (R = H) are nearly degenerate with a slight preference for the high spin state. For complexes with substituents (R) at the 6-positions of two pyridine rings of BPMEN, the ground state is the high spin (HS) state, with the low spin (LS) state higher in energy by 9.5, 5.3, 8.5, 6.3, and 5.1 kcal/mol for complexes 2 (R = CH3), 3 (R = SiH3), 4 (R = OH), 5 (R = F), and 6 (R = CN), respectively, with the B3LYP method. Our findings for complexes 1 and 2 are in good agreement with the experimental observations. The calculated LS-HS difference of 17.9 kcal/mol for complex 7 (R = t-Bu) clearly demonstrates that the steric effect causes the Fe-N bonds to stretch and favors the high spin state. Comparison of the B3LYP and B3PVV91 results with experiments and ab initio MP2 and CCSD(T) as well as pure DFT results shows that hybrid DFT methods provide a qualitatively correct description of the relative energies of low-lying electronic states of a model of the [(BPMEN)Fe(NCCH3)(2)](2+) complex, while pure DFT methods underestimate the stability of the HS state.