Facilitating multi-electron reactivity at low-coordinate cobalt complexes using redox-active ligands
Smith, Aubrey L.
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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. In a complementary study, use of bidentate or tridentate stabilizing ligands in combination with one redox-active catechol-derived or amidophenol-derived ligand was investigated. With the synthesis of (triphos)CoII(cat) and the one electron oxidized [(triphos)CoII(sq)]+, it is evident that the oxidation occurs at the ligand and not the metal. Reaction of (triphos)CoII(cat) with a Cl+ reagent generated a new material which we tentatively describe as (triphos)CoIII(Cl)(sq). This implies that the two electrons used to create the new cobalt-halide bond are derived from both the ligand and the metal, one from each. We believe the complex is unreactive with organic halides due to the steric bulk surrounding the metal center. Similar cobalt complexes containing tridentate or bidentate phosphine ligands or a tridentate pyrazol ligand in combination with a catechol-derived or amidophenol-derived ligand resulted in unsuccessful synthesis or unstable complexes. Throughout the course of both of these studies, steric crowding at the metal center is a problem disfavoring the facilitated reactivity. We have however shown that the amidophenol ligands have favorable molecular orbital overlap with the cobalt to act as an electron reservoir and facilitate reactivity at the metal center. We have also shown that this combination can create a proclivity to facilitate multi-electron reactions at the metal that is naturally prone to radical reactions.