Structural, EPR, and Mössbauer characterization of (μ-alkoxo)(μ-carboxylato)diiron(II,III) model complexes for the active sites of mixed-valent diiron enzymes

Abstract

To obtain structural and spectroscopic models for the diiron(II,III) centers in the active sites of diiron enzymes, the (μ-alkoxo)(μ-carboxylato)diiron(II,III) complexes [Fe(II)Fe(III)(N-Et-HPTB)(O(2)CPh)(NCCH(3))(2)](ClO(4))(3) (1) and [Fe(II)Fe(III)(N-Et-HPTB)(O(2)CPh)(Cl)(HOCH(3))](ClO(4))(2) (2) (N-Et-HPTB = N,N,N',N'-tetrakis(2-(1-ethyl-benzimidazolylmethyl))-2-hydroxy-1,3-diaminopropane) have been prepared and characterized by X-ray crystallography, UV-visible absorption, EPR, and Mössbauer spectroscopies. Fe1-Fe2 separations are 3.60 and 3.63 Å, and Fe1-O1-Fe2 bond angles are 128.0° and 129.4° for 1 and 2, respectively. Mössbauer and EPR studies of 1 show that the Fe(III) (S(A) = 5/2) and Fe(II) (S(B) = 2) sites are antiferromagnetically coupled to yield a ground state with S = 1/2 (g= 1.75, 1.88, 1.96); Mössbauer analysis of solid 1 yields J = 22.5 ± 2 cm(-1) for the exchange coupling constant (H = JS(A)·S(B) convention). In addition to the S = 1/2 ground-state spectrum of 1, the EPR signal for the S = 3/2 excited state of the spin ladder can also be observed, the first time such a signal has been detected for an antiferromagnetically coupled diiron(II,III) complex. The anisotropy of the (57)Fe magnetic hyperfine interactions at the Fe(III) site is larger than normally observed in mononuclear complexes and arises from admixing S > 1/2 excited states into the S = 1/2 ground state by zero-field splittings at the two Fe sites. Analysis of the "D/J" mixing has allowed us to extract the zero-field splitting parameters, local g values, and magnetic hyperfine structural parameters for the individual Fe sites. The methodology developed and followed in this analysis is presented in detail. The spin Hamiltonian parameters of 1 are related to the molecular structure with the help of DFT calculations. Contrary to what was assumed in previous studies, our analysis demonstrates that the deviations of the g values from the free electron value (g = 2) for the antiferromagnetically coupled diiron(II,III) core in complex 1 are predominantly determined by the anisotropy of the effective g values of the ferrous ion and only to a lesser extent by the admixture of excited states into ground-state ZFS terms (D/J mixing). The results for 1 are discussed in the context of the data available for diiron(II,III) clusters in proteins and synthetic diiron(II,III) complexes.

Figure 1

The substrate-bound active site of mouse MIOX (2HUO).

Figure 2

Thermal ellipsoid plots of A) [Fe^IIFe^III(N-Et-HPTB)(O₂CPh)(NCCH₃)₂]³⁺ (1) and B) [Fe^IIFe^III(N-Et-HPTB)(O₂CPh)(Cl)(HOCH₃)]²⁺ (2) showing 50% probability thermal ellipsoids for all non-hydrogen atoms. H-atoms omitted for clarity. In both complex 1 and 2, Fe1 is Fe^III and Fe2 is Fe^II.

Figure 3

Electronic spectrum of 1 (black dotted line) and 2 (red solid line) in CH₃CN at 298 K.

Figure 4

EPR spectrum of 1 in PrCN recorded at T = 8 K. The theoretical curve (red) was generated for g_x = 1.75, g_y = 1.88 and g_z = 1.96 using the strain parameters σ_x = 0.025, σ_y = 0.015 and σ_z = 0.012. Below, we relate x, y, and z to the molecular frame. Conditions: 9.62 GHz; 20 μW microwave power; 1 mT modulation.

Figure 5

EPR spectra of 1 in PrCN in the low field region recorded at T = 8 K (green), 21 K (blue) and 34 K (red). The blue line in the lower trace is a simulation of the 34 K spectrum with eq 1, using the parameters listed in Table 4. To simulate the approximate line shape, E_B/D_B was assumed to have a Gaussian distribution with σ_E/D = 0.04. The feature at g = 4.3 is due to a minor S = 5/2 contaminant. Conditions: 9.62 GHz, 0.2 mW microwave power, 1 mT modulation; 1 hour accumulation time for the 34 K spectrum.

Figure 6

Mössbauer spectrum of solid 1 recorded at 120 K in zero applied field. The solid line through the data is a simulation involving doublets for ferric site A (43% of Fe, inner bracket), ferrous site B (43%, outer bracket). The solid lines shown above the data indicate the spectra of two diferric contaminants (ΔE_Q = 1.40 mm/s, δ = 0.43 mm/s, 10%; ΔE_Q = 0.66 mm/s, δ = 0.46 mm/s, 4%). Parameters for sites A and B are quoted in Tables 3 and 4.

Figure 7

Mössbauer spectra of solid 1 recorded at 1.8 K (A) and 120 K (B) in a parallel field of 7.63 T. The solid lines through the data are spectral simulations based on eq 1, using the parameters listed in Table 4. The solid line above (A) shows the contribution of the Fe^III site, Fe_A. The solid line above (B) indicates the contribution of the ferrous site, Fe_B. The spectrum in (C) is the same as that shown in (B); the theoretical curve, however, was generated for J = 15 cm⁻¹ rather than J = 22.5 cm⁻¹.

Figure 8

Lowest 3d orbital (doubly occupied) of the Fe^II site of 1 obtained by DFT. The orbital has t_2g parentage and has xy character when the z axis is chosen along the Fe-Fe direction and the y axis is along Fe-N_MeCN. For clarity, the ligands of the FeIII site and some additional atoms have been removed.

Figure 9

Illustration of D/J mixing on the effective g-values of an Fe^IIIFe^II system. For clarity we have made the following simplifying assumptions. ${\mathcal{D}}_{\text{xx}}^{\text{e}}={\mathcal{D}}_{\text{yy}}^{\text{e}}=-{\mathcal{D}}_{\text{zz}}^{\text{e}}/2;g_{\text{x}}^{\text{B}}=g_{\text{y}}^{\text{B}}=2.15,g_{\text{z}}^{\text{B}}=2.0,g^{\text{A}}=2.00$. The solid lines in black and red were obtained by diagonalization of eq 1. The dashed line for g_x = g_y = g_⊥ was generated by using the perturbation expression of eq 8. The difference between the blue horizontal line at $g_{\perp }=-4g_{\text{x},\text{y}}^{\text{B}}/3=1.80$ and the solid line for g_⊥ is due to D/J mixing.