STRUCTURE AND REACTIVITY OF A BIS(MU-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 P1BAR (a = 10.923 (8) angstrom, b = 12.416 (4) angstrom, c = 13.935 (3) angstrom, alpha = 105.03 (3)-degrees, beta = 95.38 (3)-degrees, gamma = 92.53 (3)-degrees, Z = 1) with an inversion center located in the center of the bis(mu-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) angstrom, and mediate a weak antiferromagnetic interaction between the metal centers (H = -2JS1.S2, J approximately -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 O2 results in an immediate reaction forming [Fe2O(O2CCH3)2(TPA)2](BPh4)2 (2), a (mu-oxo)diferric TPA complex with terminal monodentate acetates, which in turn converts readily to the previously characterized [Fe2O(mu-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.