Reversible Long-Range Electron Transfer in Ruthenium-Modified Sperm Whale Myoglobin

CpMo(C0),SiMe2CMe2CMe2H + CpMo(CO),' (5) the product ratio for this mechanism is given by [3a]/[2a] = k,[la]/k,, where 3a is the thexylsilyl product in analogy to the group 14 derivative^.^^ In other words, formation of the thexyl product should be favored by high concentrations of la. As no substantial quantity of any other product was observed at hydride concentrations up to ca. 0.9 M in benzene, a finely ground sample of la was treated with neat HMS. Analysis of this reaction mixture by 'H NMR showed the formation of 2a and ca. 13% of 3a, the thexyldimethylsilyl molybdenum complex,26 as predicted by the proposed radical chain mechanism. The trace amounts of molybdenum dimers [CpMo(CO),(L)], formed in eq 1 are consistent with chain termination during the reaction. A ca. IO-fold increase in the initial rate is observed upon addition of a few mole percent of triphenylmethyl radical (as the dimer).27 In a separate experiment, stoichiometric reaction of trityl dimer with la in the absence of HMS produced triphenylmethane and [CPMO(CO)~]~, presumably from the coupling of CpMo(CO), radicals. Attempts to establish the intermolecularity of (1) by means of a cross-over experiment using CpMo(CO),D and (MeCp)Mo(CO),H were thwarted by the extremely rapid isotopic scrambling observed between these complexes in the absence of HMS. Comparison of initial rates measured by using a single batch of la shows no effect on eq I by carbon monoxide (1.5 atm) or tetramethylethylene (0.31 7 M).19 These facts, in conjunction with a negative activation entropy (AS* = -25 f 3 eu),28 preclude reversible dissociation of these species in the rate-controlling step. Changing solvent from benzene to THF also leaves the rate unchanged. In summary, silylene transfer from HMS to molybdenum hydrides is quantitative, and appears to proceed by radical chain ring opening and olefin elimination to yield a transient silyl radical. The apparent stabilization of the silyl radical by metal substitution suggests that radical paths may be more common with metal complexes of silicon ligands than is generally observed in organosilicon chemistry.29 Acknowledgment. We thank the Research Corporation for an Exxon Education Foundation Grant, the University of Pennsylvania Research Fund, and the National Science Foundation (MRL Program, under Grant DMR-8519059) for support of this research.