SYNTHESIS, REDOX PROPERTIES, AND REACTIVITY OF A NOVEL FAMILY OF ELECTRON-RICH VINYLIDENE AND VINYLPHOSPHONIUM COMPLEXES OF RHODIUM(I)

The trigonal-bipyramidal Rh(I) vinylidene complexes [(NP3)Rh═C═C(H)(R)]+ have been prepared by treatment of the neutral σ-acetylides [(NP3)Rh(C≡CR)] with strong protic acids and isolated as BPh4- or PF6- salts (NP3 = N(CH2CH2PPh2)3; R = Ph, CO2Et). Reaction of the vinylidene complexes with nucleophiles such as NEt3, MeLi, and PhLi leads to deprotonation to restore the parent σ-acetylide compounds. When the vinylidene complexes are treated with NaBH4 or LiHBEt3, the acid-base reaction competes with the nucleophilic addition of H- to the vinylidene Cα atom to form the alkenyl derivatives [(E)-(NP3)Rh(CH═C(H)R)]. Two-electron oxidation of the vinylidene complexes by controlled-potential electrolysis results in the deprotonation of the vinylidene ligand. In this way, the Rh(III) σ-acetylides [(NP3)Rh(C≡CR)]2+ are obtained and isolated in the solid state as the ethanol adducts [(NP3)Rh(C≡CR)(C2H5OH)](BPh4)2. In contrast, one-electron reduction gives the neutral σ-acetylides and H2. The vinylidene compounds can also be oxidized by peroxo acids such as m-chloroperbenzoic acid. As a result, the deprotonation of the vinylidene ligand occurs and the Rh(III) complexes [(NP3)Rh(C≡CR)(O2CC6H4Cl)]BPh4 are formed. When O2 is bubbled into CH2Cl2 solutions of the vinylidene complexes, the Rh(III) hydroxide acetylide compounds [(NP3)Rh(OH)(C≡CR)]BPh4 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 H2 and the paramagnetic Rh(II) derivatives [(NP3)Rh(C≡CR)]+. Unlike the NP3 σ-acetylides, the PP3 analogues (PP3 = P(CH2CH2PPh2)3) react with strong acids in THF or CH2C12 to form unusual examples of vinylphosphonium compounds of formula [((Ph2PCH2CH2)=P(CH2CH2PPh2))Rh(C═C(H)C(O)R)]BPh4. When the protonation reaction is carried out in benzene, the alternative oxidation of the Rh(I) σ-acetylide to the paramagnetic Rh(II) derivatives [(PP3)Rh(C≡CR)]+ occurs. All of the compounds have been characterized by chemical-physical measurements, including IR, ESR, and NMR techniques, and their redox behavior has been studied by electrochemical methods. © 1990, American Chemical Society. All rights reserved.