First-row metal catalysts for controlled radical reactivity

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With its goals of preventing pollution and reducing consumption of nonrenewable resources, green chemistry represents an important opportunity for synthetic inorganic chemists. There is a growing emphasis on developing catalysts that use readily-available transition metals, operate under mild conditions, and use reactants obtained from renewable resources. A critical frontier of green chemistry research is to discover earth-abundant first-row transition metal cross-coupling catalysts that break carbon-oxygen or carbon-hydrogen bonds instead of relying on halogenated reactants. This field is generally perceived as difficult due to the potential involvement of radical chemistry. The first-row metal complexes relevant to catalysis often have unpaired d-electrons. The catalysts are prone to single-electron reactions that can generate organic radicals, especially for reactions conducted under oxidative conditions. The organic molecules bound to the metal centers can themselves gain or lose single electrons, forming ligand-based radicals. The ongoing objective of my research program is to design first-row metal catalysts that control radical reactivity. As catalytic substrates shift from traditional petrochemical sources to renewable resources, new methods will need to be developed to selectively cleave carbon-oxygen bonds at lower temperatures without resorting to harsh reagents. Since 2000, my research group has developed well-defined paramagnetic chromium complexes for metal-mediated radical reactivity. In the period covered by this research proposal, this work will be extended to other first-row metal catalysts, particularly nickel and titanium. The involvement of metal-mediated radical reactivity is often responsible for the distinctive and useful reactivity of first-row metal catalysts compared to their heavier second- and third-row transition metal congeners. Despite the growing and wide-spread recognition of this fact, there are still relatively few inorganic research groups that are actively investigating metal-mediated radical reactivity for catalysis for organic synthesis. The recent focus on green chemistry seems likely to fuel the continuing increase in popularity of earth-abundant first-row metal catalysts. Understanding how their propensity for radical reactivity can be controlled and exploited is particularly crucial for tandem concurrent catalytic systems that rely on more than one metal catalyst, or that operate in combination with light-driven photoredox catalysts.