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 [(η5-C5Me5)Re(NO)(NCCH3)(CO)]+ BF4− and phosphines PR3 (R=C6H5a; 4-C6H4CH3b; 4-C6H4-t-C4H9c; 4-C6H4C6H5d; 4-C6H4OCH3e; c-C6H11f) give the phosphine carbonyl complexes [(η5-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 (η5-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 (ΔE°=0.07, 0.07, 0.01, 0.09, 0.22 V; CH2Cl2). The radical cations 2radical dot+ X− can be generated with Ag+ X− or (η5-C5H5)2Feradical dot+ 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 (η5-C5Me5)Re(NO)(η2-TCNE)(CH3), which is crystallographically characterized and proposed to form by initial electron transfer followed by radical chain substitution.