Synthesis and oxidation of chiral rhenium phosphine methyl complexes of the formula (η 5-C 5Me 5)Re(NO)(PR 3)(CH 3): in search of radical cations with enhanced kinetic stabilities

Abstract

Reactions of racemic [(η 5-C 5Me 5)Re(NO)(NCCH 3)(CO)] + BF 4 − and phosphines PR 3 (R=C 6H 5 a; 4-C 6H 4CH 3 b; 4-C 6H 4- t-C 4H 9 c; 4-C 6H 4C 6H 5 d; 4-C 6H 4OCH 3 e; c-C 6H 11 f) give the phosphine carbonyl complexes [(η 5-C 5Me 5)Re(NO)(PR 3)(CO)] + BF 4 − ( 5a– 5f + BF 4 −; 55–95%). These are treated with LiEt 3BH and then BH 3·THF to give the phosphine methyl complexes (η 5-C 5Me 5)Re(NO)(PR 3)(CH 3) ( 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 (Δ E°=0.07, 0.07, 0.01, 0.09, 0.22 V; CH 2Cl 2). The radical cations 2 + X − can be generated with Ag + X − or (η 5-C 5H 5) 2Fe + X − (X −=PF 6 −, SbF 6 −), as evidenced by IR and ESR spectra, but are labile and efforts to isolate pure salts fail. Reaction of 2a and TCNE give (η 5-C 5Me 5)Re(NO)(η 2-TCNE)(CH 3), which is crystallographically characterized and proposed to form by initial electron transfer followed by radical chain substitution.