Insight into the g ≈ 16 EPR Signals of Reduced Diiron-Oxo Proteins. Structure and Properties of [Fe^II₂BPMP{O₂P(OC₆H₅)₂}₂]Cl

The diiron complexes [Fe^II₂BPMP{O₂P(OC₆H₅)₂}₂]X (1 (X = Cl), 2 (X = BF.,), 3 (X = BPh₄)) and [Fe^II₂BPCP-(O₂CC₂H₅)₂]BPh₄ (4), where BPMP is the anion of 2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol and BPCP is the anion of 2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-chlorophenol, have been synthesized to provide insight into the integer-spin EPR signals found in the diferrous forms of diiron-oxo proteins. Complex 1 crystallizes in the triclinic space group P1̄ with cell constants a = 10.464(8) Å, b = 15.226(7) Å, c = 20.050(10) Å, α = 85.60(4)°, β = 88.38(5)°, γ = 74.98(5)°, V = 3076 ų and Z = 2, with R = 0.049 and R_w = 0.069. It has a (μ-phenoxo)bis(μ-phosphato)diiron core, which affords an Fe-μ-O-Fe angle of 122.7(2)° and an Fe-Fe distance of 3.683(4) Å, values that are significantly larger than those for the corresponding propionate-bridged complex. Complex 4, like [Fe^II₂BPMP(O₂CC₂H₅)₂]BPh₄ (5), exhibits a low field EPR signal near g = 17, similar to that found for deoxyhemerythrin azide. This resonance originates from a ground electronic state with integer spin, indicating that the metal centers are ferromagnetically coupled. Complexes 1–3 differ in two respects. They show EPR signals at g = 15, a resonance position that is incompatible with both strong and weak coupling models earlier proposed to explain the corresponding signals in 5. However, the higher field position can be simulated by a rotation between the easy axes of magnetization of the iron sites in the weak coupling scheme. Secondly, the EPR signals of 1–3 arise from an excited state; thus the coupling interaction between the iron centers is found to be antiferromagnetic. The temperature dependence of the EPR signal indicates that the excited state is 12 cm⁻¹ above the EPR silent ground state. These observations are corroborated by magnetization data for polycrystalline 2. J is found to be 2.5–3.0 cm⁻¹ (ℌ = JS₁·S₂) from fits of the multifield saturation magnetization data and the EPR temperature dependence with the constraint that the zero field splitting tensors of the individual ferrous ions are rotated 60° relative to each other along the easy axis of magnetization. The switch in sign of the iron-iron coupling interaction on going from the propionate-bridged complexes to the phosphate-bridged complexes undoubtedly results from the larger Fe-μ-O-Fe angle found in the latter complexes. The EPR properties observed for these complexes serve to validate the theoretical framework proposed by Hendrich et al. (J. Am. Chem. Soc. 1991, 113, 3039-3044) to rationalize the integer-spin EPR signals observed for the diferrous forms of diiron-oxo proteins and provide a foundation upon which to interpret the g = 15 signal recently observed for the diferrous R2 protein of ribonucleotide reductase.