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

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 Fe 2+ ( S 1 = 2)−{FeNO} 7 ( S 2 = 3 / 2 ) where the two iron species are antiferromagnetically coupled. We have analyzed spectra within the framework of the spin Hamiltonian ℋ = J S 1 • S 2 + = { S i •D̃ i • S i + β S i • g̃ i • H + S i • ã i • I i + I i • P̃ i • I i − β n g n H • I i } 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, D 1 = +5.96 cm - 1, E 1 = +0.18 cm - 1, D 2 = +18.93 cm - 1, E 2 = +2.65 cm - 1, ã 1 / g n β n = −(18.5,10.4,13.8) T, ã 2 / g n β n = −(29.7,25.0,∼22) T, ℛ( ã 1 → P̃ 1 ) = (18,12,67)°, ℛ( ã 2 → P̃ 2 ) = (55,53,29)° and ℛ( D̃ 1 → D̃ 2 ) = (0,90,28)°. In addition, at 100 K, we determined the isomer shifts (δ Fe ) 1.21 and 0.68 mm/s and quadrupole splittings (Δ E Q ) +2.66 and +0.61 mm/s for the Fe 2+ and {FeNO} 7 sites, respectively. We have also analyzed spectra with the S eff = 1 / 2 Hamiltonian for the ground state ℋ eff = β S eff • g̃ eff • H + { S eff • • I i + I i • P̃ i • I i − β n g n H • I i } 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(C 9 H 2 1 N 3 )(NO)(N 3 ) 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., d xz, and d yz ) and NO π* orbitals.