Redox reactions between oxomolybdenum(IV) and tetratolylporphinatoiron(III) complexes

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Abstract

The coupling of an MoVIO2→MoIVO oxygen atom transfer reaction to a one-electron reoxidation of the resulting Mo(IV) center by an Fe(III) porphyrin center has been investigated in an attempt to model the first two redox steps in the catalytic cycle of the enzyme sulfite oxidase. Reaction of LMoVIO2Cl (L=hydrotris(3,5-dimethyl-1-pyrazolyl)borate) with excess tri-phenylphosphine in DMF rapidly and cleanly generates LMoIVOCl(DMF). This Mo(IV) species reacts more slowly with FeIIICl(TTP) (TTP=tetratolylporphyrin) to give LMoVOCl2 and FeII(TTP) as the final products. The kinetics of this one-electron redox reaction have been followed by electronic spectroscopy and by 1H NMR. The reaction is zero-order in Fe(III), first-order in Mo(IV), and independent of added chloride ion. The first-order rate constant determined by these methods ranged from 1.7−4.6×10−5 s−1. An inner-sphere (halogen transfer) mechanism is shown to be most consistent with the data.

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