A handful of oxygen-activating enzymes have recently been found to contain Fe/Mn active sites that are closely related to their better characterized diiron cousins, like the Class 1c ribonucleotide reductases. These enzymes are proposed to form high-valent intermediates with Fe–O–Mn cores. Our group has shown that reacting an Fe^IV(O) complex with an appropriate Mn^II complex can form approximately linear Fe^III(μ-O)Mn^III complexes. Titrating these compounds with one equivalent of base leads to the formation of Fe^III(μ-O)(μ-OH)Mn^III complexes. Fe K-edge EXAFS analysis has been used to differentiate the structures of each of compounds. The linear compounds accurately model the Fe - Mn distances found in the Fe/Mn proteins in their resting states, and the doubly bridged diamond core compounds accurately model the distances found in high-valent biological intermediates. Unlike their diiron analogues, the Fe/Mn compounds can be easily analyzed by EPR, revealing S = ½ signals that reflect antiferromagnetic coupling between the high-spin Fe(III) and Mn(III) units of the heterobimetallic centers. These compounds undergo electron-transfer with various ferrocenes, the linear compounds being capable of oxidizing diacetyl ferrocene, a weak reductant, and the diamond core compounds being capable of oxidizing acetyl ferrocene. The diamond core compounds can also perform HAT reactions from substrates with X-H bonds with bond dissociation free energies (BDFEs) up to 70 kcal/mol and are capable of oxidizing hydroquinone to quinone at a rate of 3 M^-1s^-1 at -40 °C, which is comparable to those reported for mononuclear Fe^III-OH and Mn^III-OH complexes. Based on the pK_a and reduction potential of hydroquinone, we propose an electron-transfer proton-transfer mechanism for this oxidation. To our knowledge, this represents the first example of an Fe/Mn compound performing such a reaction.

Patrick M Crossland

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