Synthesis and oxidation of chiral rhenium phosphine methyl complexes of the formula (η5-C5Me5)Re(NO) (PR3)(CH3):: in search of radical cations with enhanced kinetic stabilities

Reactions of racemic [(eta (5)-C5Me5)Re(NO)(NCCH3)(CO)](+) BF4- and phosphines PR3 (R = C6H5 a; 4-C6H4CH3, b; 4-C6H4-t-C4H9 c; 4-C6H4C6H5 d; 4-C6H4OCH3 e; c-C6H11 f) give the phosphine carbonyl complexes [(eta (5)-C5Me5)Re(NO)(PR3)(CO)](+) BF4- (5a-5f(+) BF4-; 55-95%). These are treated with LiEt3BH and then BH3. THF to give the phosphine methyl complexes (eta (5)-C5Me5)Re(NO)(PR3)(CH3) (2a-2f, 50-86%). Cyclic voltammetry shows that the new compounds 2b-2f undergo chemically reversible one-electron oxidations that are thermodynamically more favorable than that of 2a (DeltaE degrees = 0.07, 0.07, 0.01, 0.09, 0.22 V; CH2Cl2). The radical cations 2(.+) X- can be generated with Ag+ X- or (eta (5)-C5H5)(2)Fe.+ X- (X- = PF6-, SbF6-), as evidenced by:IR and ESR spectra, but are labile and efforts to isolate pure salts fail. Reaction of 2a and TCNE give (eta (5)-C5Me5)Re(NO)(eta (2)-TCNE)(CH3), which is crystallographically characterized and proposed to form by initial electron transfer followed by radical chain substitution. (C) 2000 Elsevier Science B.V. All rights reserved.