Spectroscopic and Electrochemical Properties of (μ-Oxo)diiron(III) Complexes Related to Diiron-Oxo Proteins. Structure of [Fe2O(TPA)2(MoO)4)](ClO4)2
A series of (μ-oxo)diiron(III) complexes of tris(2-pyridylmethy1)amine (TPA), [Fe2O(TPA)2(L)] (C1O4)2, were synthesized and characterized where L represents the bridging tetraoxo anion ligands sulfate, phosphate, arsenate, vanadate, and molybdate. These tetraoxo anion complexes are the first (μ-oxo)diiron(III) complexes that reproduce the protein-tetraoxo anion stoichiometry found in purple acid phosphatases (PAPs). [Fe2O(TPA)2( MoO4)] (C104)2- CH3N (9) crystallizes in the monoclinic space group P21/n (a = 12.74(1) Å, b = 24.69(2) Å, c = 13.733(8) Å, β = 103.41 (7)°, and Z = 4) and consists of two distinct six-coordinate Fe(II1) centers bridged by oxo and molybdate. 9 represents the first (μ-oxo)diiron(III) complex with a single bridging molybdate to be structurally characterized. These new complexes together with previously reported (μ-oxo)diiron(III) TPA complexes constitute a series with a wide range of Fe-μ-0-Fe angles and bridging anion basicities which affect their electronic absorption, resonance Raman, and electrochemical properties. A linear correlation between the Raman vs(FeO-Fe) mode and the energy of the long-wavelength visible absorption band provides a method in which UV-vis spectroscopy can be used to estimate the Fe-O-Fe angle. The electrochemical properties of these complexes show the expected dependence on charge and basicity and thus serve as a basis on which to interpret the redox properties of diiron-xo proteins, particularly uteroferrin, the purple acid phosphatase from porcine uterus. Comparison of the electrochemical properties of the phosphate, arsenate ,and molybdate complexes in the (μ-oxo)diiron(III) TPA series with those of corresponding complexes of uteroferrin suggests that both phosphate and arsenate bridge the diiron core in uteroferrin, while molybdate must bind only to the redox inactive Fe(II1) center. The mixed-valent forms of several of these complexes exhibit EPR signals with gaav < 2, like those observed for the mixed-valent diiron enzymes but with smaller g anisotropies.