Synthesis, redox properties, and reactivity of a novel family of electron-rich vinylidene and vinylphosphonium complexes of rhodium(I)

Abstract

The trigonal-bipyramidal Rh(I) vinylidene complexes ((NP{sub 3})Rh=C=C(H)(R)){sup +} have been prepared by treatment of the neutral {sigma}-acetylides ((NP{sub 3})Rh(C{triple bond}CR)) with strong protic acids and isolated as BPh{sub 4}{sup {minus}} or PF{sub 6}{sup {minus}} salts (NP{sub 3} = N(CH{sub 2}CH{sub 2}PPh{sub 2}){sub 3}; R = Ph, CO{sub 2}Et). Reaction of the vinylidene complexes with nucleophiles such as NEt{sub 3}, MeLi, and PhLi leads to deprotonation to restore the parent {sigma}-acetylide compounds. When the vinylidene complexes are treated with NaBH{sub 4} or LiHBEt{sub 3}, the acid-base reaction competes with the nucleophilic addition of H{sup {minus}} to the vinylidene C{sub {alpha}} atom to form the alkenyl derivatives ((E)-(NP{sub 3})Rh(CH=C(H)R)). Two-electron oxidation of the vinylidene complexes by controlled-potential electrolysis results in the deprotonation of the vinylidene ligand. When O{sub 2} is bubbled into CH{sub 2}Cl{sub 2} solutions of the vinylidene complexes, the Rh(III) hydroxide acetylide compounds ((NP{sub 3})Rh(OH)(C{triple bond}CR))BPh{sub 4} are obtained in ca. 60% yield because of extensive decomposition of the starting compounds. For R = Ph, an appreciable amount of phenol is found. Thermal decomposition of the vinylidene complexes in refluxing THF produces H{sub 2} and the paramagnetic Rh(II) derivatives ((NP{sub 3})Rh(C{triple bond}CR)){sup +}. Unlike the NP{sub 3} {sigma}-acetylides, the PP{submore » 3} analogues (PP{sub 3} = P(CH{sub 2}CH{sub 2}PPh{sub 2}){sub 3}) react with strong acids in THF or CH{sub 2}CL{sub 2} to form unusual examples of vinylphosphonium compounds of formula (((Ph{sub 2}PCH{sub 2}CH{sub 2}){sub 2}P(CH{sub 2}CH{sub 2}PPh{sub 2}))Rh(C=C(H)C(O)R))BPh{sub 4}.« less