Download or read book Facilitating Multi-electron Reactivity at Low-coordinate Cobalt Complexes Using Redox-active Ligands written by Aubrey L. Smith and published by . This book was released on 2011 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: In this study, we describe a detailed investigation of cobalt complexes containing redox-active ligands. We have prepared an electronic series of the complex in three oxidation states: [CoIII(ap)2]-, CoIII(isq)(ap), and [CoIII(CH3CN)(isq)2]+. Characterization shows that the metal center remains cobalt(III) through the redox changes and indicates that the oxidation state changes occur with gain or loss of electrons from the ligand set. While CoIII(isq)(ap) reacts with halide radicals to form a new cobalt-halide bond in a single electron reaction, [CoIII(ap)2]- appears to be prone to multi-electron reactivity in reactions with sources of "Cl+". Both reactions occur with electrons derived from the ligand set. Mechanistic studies suggest a single, two electron step is responsible for the bond-formation. Similarly, [CoIII(ap)2]- reacts with alkyl halides to pseudo-oxidatively add the alkyl at the cobalt center. The product of the reaction can be isolated and fully characterized and was found to be best assigned as CoIII(alkyl)(isq)2. This assignment indicates that the reaction occurs, again, with the new bond formed with two electrons formally derived from the ligand set and with no change in oxidation state at the metal center. Mechanistic investigations of the pseudo-oxidative addition suggest the reaction is SN2-like. The reaction occurs with a wide scope of alkyl halides, including those containing beta-hydrogens. : The cross-coupling reaction of CoIII(alkyl)(isq)2 with RZnX forms a new carbon-carbon bond. Similarly, the two electron oxidized complex [CoIII(CH3CN)(isq)2]+ reacts with organozinc reagents to couple two carbon nucleophiles and form a new carbon-carbon bond. Both reactions are successful with both sp2 and sp3 carbons. When followed substoichiometrically, the homocoupling reaction can be observed to form CoIII(alkyl)(isq)2. This indicates that the homocoupling and cross-coupling reactions proceed by the same mechanism. However, both reactions have low yields. The yield of the reactions are decreased by steric bulk of the alkyl or aryl fragments or around the metal center created by substituents on the ligand. Also, while the steric congestion disfavors the addition of the first alkyl fragment, the addition of the second alkyl fragment and subsequent rapid elimination of the coupling product is almost completely inhibited. This result also implies that the coupling of the two alkyl fragments is entirely inner-sphere requiring installation of both for coupling.