Mössbauer Spectroscopy of the Spin Coupled Fe2+−{FeNO}7 Centers of Nitrosyl Derivatives of Deoxy Hemerythrin and Density Functional Theory of the {FeNO}7(S = 3/2) Motif

Abstract

The mixed-valence diiron centers of nitrosyl derivatives of deoxy hemerythrin have been studied by variable-temperature variable-field Mössbauer spectroscopy. The adduct formed by the reaction of deoxy hemerythrin (deoxyHr) with nitric oxide (deoxyHrNO) produced spectra consistent with a binuclear center of the form Fe2+(S1 = 2)−{FeNO}7(S2 = 3/2) where the two iron species are antiferromagnetically coupled. We have analyzed spectra within the framework of the spin Hamiltonian ℋ = JS1•S2 + = {Si•D̃i•Si + βSi•g̃i•H + Si•ãi•Ii + Ii•P̃i•Ii − βngnH•Ii} that includes exchange, zero-field splitting, electronic Zeeman, magnetic hyperfine, electric quadrupole, and nuclear Zeeman interactions, respectively. We have obtained the following parameters that reproduce simultaneously Mössbauer spectra and EPR g values: J = 27.8 cm-1, D1 = +5.96 cm-1, E1 = +0.18 cm-1, D2 = +18.93 cm-1, E2 = +2.65 cm-1, ã1/gnβn = −(18.5,10.4,13.8) T, ã2/gnβn = −(29.7,25.0,∼22) T, ℛ(ã1 → P̃1) = (18,12,67)°, ℛ(ã2 → P̃2) = (55,53,29)° and ℛ(D̃1 → D̃2) = (0,90,28)°. In addition, at 100 K, we determined the isomer shifts (δFe) 1.21 and 0.68 mm/s and quadrupole splittings (ΔEQ) +2.66 and +0.61 mm/s for the Fe2+ and {FeNO}7 sites, respectively. We have also analyzed spectra with the Seff = 1/2 Hamiltonian for the ground state ℋeff = βSeff•g̃eff•H + {Seff• •Ii + Ii•P̃i•Ii − βngnH•Ii} and have obtained parameters in the effective spin representation. In addition, we discuss the spectra of a second adduct of deoxyHr obtained by its reaction with NO in the presence of fluoride (deoxyHrFNO) which exhibits similar, but not equal, parameters to those of deoxyHrNO. Contrary to the parent form (deoxyHr) where the two ferrous ions are Mössbauer equivalent, the nitrosyl adducts have two iron sites which produce distinctly different spectra and have quite different magnetic and electronic properties. The isomer shifts of the {FeNO}7 sites are significantly reduced with respect to the high-spin ferrous configuration and enhanced with respect to the high-spin ferric configuration. To elucidate the physical origin of Mössbauer parameters from {FeNO}7(S = 3/2) sites we have applied Kohn−Sham density functional theory to the representative complex Fe(C9H21N3)(NO)(N3)2 [Pohl and Wieghardt, J. Chem. Soc., Dalton Trans. 1987, 187]. The unusual isomer shifts have been traced to strong valence electron delocalization within the {FeNO}7 unit, whereby some electrons are almost equally shared by metal (i.e., dxz, and dyz) and NO π* orbitals.