Abstract
Rates of substitution of THF or P(OMe)3 by tetracyanoethylene (TCNE) in organometallic complexes (THF)W(CO)5, (THF)Cr(CO)5, [P(OMe)3]Cr(CO)2(C6Me6), (THF)Mn(CO)2(C5Me5) and (THF)Mn(CO)2(C5H4Me) in THF soln., have been detd. As indicated by the calcd. second-order rate consts. k, the manganese complexes (k\textgreater1900 M-1 s-1) react by a factor of at least 104 more rapidly than the W(CO)5 complex (k = 0.043 M-1 s-1). Measurements of recombination kinetics of photodissociated (acpy)Mn(CO)2(C5Me5) (acpy = 4-acetylpyridine) in THF show that the substitution by TCNE of THF in the corresponding solvent complex proceeds faster by a factor of 2$\times$104. Of the two chromium complexes, the pentacarbonyl/THF system has a value of k of 0.39 M-1 s-1 whereas the phosphite ligand in [P(OMe)3]Cr(CO)2(C6Me6) is substituted by TCNE with k = 1.59 M-1 s-1. The results and their correlation with electrochem. data support a self-induced homogeneous electron transfer mechanism: it is proposed that electron transfer between the reaction partners TCNE and the organometallic precursor leads to substitutionally-labile 17 valence electron complexes as essential intermediates in the catalytic chain. Efficient oxidn. of the precursors by the TCNE-substituted 17 VE species is possible because of intramol. metal-to-TCNE electron transfer, esp. in the s-coordinated products with mixed carbonyl/carbocycle ligands. [on SciFinder(R)]
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