Density functional theory of spin-coupled models for diiron-oxo proteins: Effects of oxo and hydroxo bridging on geometry, electronic structure, and magnetism

Abstract

We have performed a comprehensive study of the electronic structure and magnetic properties of structurally characterized models for diiron-oxo proteins. Results from Kohn–Sham density functional theory show that two complexes, with formula Fe2(μ-O)(μ-O2CCH3)2(HBpz3)2 and [Fe2(μ-OH)(μ-O2CCH3)2(HBpz3)2]+, are strongly and weakly antiferromagnetically coupled, respectively, in agreement with experiment. The physical origin of the stronger and weaker exchange typically measured for oxo- and hydroxo-bridged diiron complexes, respectively, has been elucidated. The main superexchange pathways giving rise to molecular antiferromagnetism in both complexes have been identified. The dominant pathway in the oxo-bridged complex, Fe1(dxz):μ-O(px):Fe2(dxz), was formed by π interactions whereas that of the hydroxo-bridged, Fe1(dz2):μ-OH(p∥):Fe2(dz2), was formed by σ interactions. We also found a pathway mediated by the bridging acetates, Fe1(dx2−y2):bis(μ-acetato):Fe2(dx2−y2), which induces weak antiferromagnetism in the oxo-bridged complex but is significantly more important in the hydroxo-bridged complex. The antiferromagnetic exchange constants that parameterize the Heisenberg Hamiltonian H=JS1⋅S2 have been predicted for both, strongly and weakly, coupled complexes. Overall, the signs, trends, and magnitudes of the theoretical values (Jμ-Ocalc=+152.7 cm−1, Jμ-OHcalc=+23.3 cm−1) were in excellent agreement with experiment. The geometries of the complete molecular structures have been optimized in C2v symmetry and used to calculate molecular properties such as atomic charges and spin densities. The electronic configurations (Fe:4s0.293d5.93,μ-O:2s1.922p4.99;Fe:4s0.303d5.82,μ-OH:2s1.822p5.25,H:1s0.51) of the respective binuclear cores revealed relatively high occupancies for the nominally ferric ions, thus reflecting a donating character of their immediate N3O3 coordination. In addition, the diiron-oxo protein hemerythrin has been discussed. Theoretical and structural considerations indicated that the oxo-bridged diferric complex considered herein models extremely well the antiferromagnetic behavior of azidomet- and azidometmyo-hemerythrin. Finally, the magnetic behavior of closely related oxo-bridged diferric and hydroxo-bridged diferrous complexes containing Me3TACN capping ligands has been explained in light of the results presented in this work.