Structure and Reactivity of a Bis(μ-acetato-O,O′diiron(II) Complex, [Fe₂(O₂CCH₃)₂(TPA)₂](BPh₄)₂. A Model for the Diferrous Core of Ribonucleotide Reductase

[Fe₂(O₂CCH₃)₂(TPA)₂](BPh₄)₂ (1, TPA = tris(2-pyridylmethyl)amine) serves as a model for the diferrous core of ribonucleotide reductase. 1 crystallizes in the space group P1 (a = 10.923 (8) Å, b = 12.416 (4) Å, c = 13.935 (3) Å, α = 105.03 (3)°, β = 95.38 (3)°, γ = 92.53 (3)°, Z = 1) with an inversion center located in the center of the bis(μ-acetato)diiron(II) core. The acetates coordinate to the two Fe(II) centers in a syn-anti mode, affording an Fe-Fe separation of 4.288 (2) Å, and mediate a weak antiferromagnetic interaction between the metal centers (H = −2JS₁,•S₂, J ~ −1 cm⁻¹). This dimeric structure appears to partially dissociate into monomeric units in solution, as indicated by its NMR and EPR spectra. Exposure of 1 to 02 results in an immediate reaction forming [Fe₂O(O₂CCH₃)₂(TPA)₂](BPh₄)₂ (2), a (μ-oxo)diferric TPA complex with terminal monodentate acetates, which in turn converts readily to the previously characterized [Fe₂O(μ-O₂CCH₃)(TPA)₂]³⁺ complex (9) in the presence of protic solvents. Complexes 1, 2, and 9 represent structural motifs associated with the diiron core of ribonucleotide reductase in its diferrous and diferric oxidation states. The structural changes observed in the autoxidation of 1 mimic putative changes in the diiron site that occur in the O₂-dependent formation of oxidized ribonucleotide reductase from its reduced counterpart.