Synthesis and Oxidation of Dirhenium C₄, C₆, and C₈Complexes of the Formula (η⁵-C₅Me₅)Re(NO)(PR₃)(C⋮C)<i>_n</i>(R₃P)(ON)Re(η⁵-C₅Me₅) (R = 4-C₆H₄R‘, c-C₆H₁₁): In Search of Dications and Radical Cations with Enhanced Stabilities

Abstract

The racemic methyl complexes (η5-C5Me5)Re(NO)(PR3)(CH3) (R: b, 4-C6H4CH3; c, 4-C6H4-t-C4H9; d, 4-C6H4C6H5; e, c-C6H11), which feature phosphines that are more electron rich and/or bulkier than P(C6H5)3, are elaborated by reactions with HBF4·OEt2/chlorobenzene, HC⋮CH or HC⋮CC⋮CSiMe3, and t-BuOK to give the alkynyl complexes (η5-C5Me5)Re(NO)(PR3)(C⋮CH) (3b,c) and (η5-C5Me5)Re(NO)(PR3)(C⋮CC⋮CSiMe3) (7b−e). The latter are converted (K2CO3/MeOH or wet n-Bu4N+F-) to butadiynyl complexes (8b−e). Homo- or cross-couplings (3b, 8b−e; Cu(OAc)2/pyridine) lead to the μ-butadiynediyl complex (η5-C5Me5)Re(NO)(PR3)(C⋮C)n(R3P)(ON)Re(η5-C5Me5) (4b; n = 2) and analogous μ-hexatriynediyl (10b; n = 3) and μ-octatetraynediyl (9b−e; n = 4) species. Oxidation of 4b by AgSbF6 (ca. 1:1 or 1:2) gives the radical cation 4b•+SbF6- or the dication 4b2+(SbF6-)2. These isolable compounds exhibit delocalized mixed-valence and cumulenic electronic ground states, respectively. The cyclic voltammograms of 9b−e and 10b show oxidations to analogous species, with much better chemical reversibilities than for the P(C6H5)3 analogues. However, these longer chain systems decompose rapidly at −78 °C and could not be isolated or spectroscopically characterized.