Metal-ligand cooperativity has emerged as a viable strategy for promoting σ-bond activation, with promising applications in catalysis. By directly involving ligand-based electrons in σ-bond cleavage events, potentially challenging metal-based redox changes are obviated. To this end, our group has developed tridentate pincer ligands comprised of a central N-heterocyclic phosphenium (NHP+) moiety and two chelating phosphine sidearms that enforce coordination to metal centers while imparting stability on the resulting transition metal complexes. When coordinated to a metal center, the rigidity of the ligand scaffold allows the central NHP+ of the (PPP) ligand to be reduced to the anionic NHP- phosphido form, which now has a lone pair of electrons poised for participation in cooperative bond activation processes. This presentation will focus on the coordination chemistry and redox profiles of a series of (PPP)Co complexes, along with a discussion of ongoing explorations of their reactivity. For example, (PPP)CoI(PMe3) has been shown to activate a variety of E-H bonds (E = H, O, S, B) across the Co-PNHP bond, include a facile H2 activation process that occurs readily at room temperature to generate (PPHP)Co(H)(PMe3). Although (PPHP)Co(H)(PMe3) is rendered catalytically inactive by the strongly coordinating PMe3 ligand, a catalytically active (PPP)Co complex can be generated in situ without added phosphine using additives such as KHBEt3 and KOtBu. These conditions allow us to achieve remarkably rapid and selective catalytic hydroboration of terminal alkenes at just 1 mol % (PPP)Co catalyst loading at room temperature in less than 60 minutes, with &gt;95% conversion and &gt;95% selectivity for the linear anti-Markovnikov product. The (PPP)Co complex can also catalytically hydrogenate terminal alkenes at 5 mol % catalyst loading with a conversion of 90% in 2 hours at room temperature. Insights into the catalytic mechanism will be presented, with particular attention paid to the role of metal-ligand cooperativity in catalytic turnover.

Pincer-ligated cobalt complexes and their catalytic activity for alkene hydroboration: Case for metal-ligand phosphorus ligand cooperativity?

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.