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Diboraanthracene-Based Platforms for the Redox Activation of Small Molecules

Creative Commons 'BY-NC-ND' version 4.0 license

The efficient multielectron reduction of small molecules (e.g. CO2, N2) is a key step in the renewable synthesis of both fuels and fertilizers. Due to their intrinsic redox activity and reactivity, transition metals have been targetted as catalysts for these processes. Recently, p-block systems have been developed that affect the activation of small molecules however the limited redox activity of these systems limits their use as catalysts. To address this challenge, we have targeted the 9,10-dihydro-9,10-diboraanthracene (DBA) framework, due to its synthetic modularity and reversible two-electron redox activity. Conventional derivatives of the DBA scaffold require very negative potentials to access their two-electron reduced states (ca. −2.4 V vs Fc/Fc+), limiting their prospects as efficient electrocatalysts. We have targeted two approaches to modulate the redox potentials and facilitate small molecule activation chemistry at DBA derivatives: 1) N-heterocyclic carbene (NHC) stabilization; 2) transition metal coordination via tethered phosphine ligands.

The use of NHCs enables the reduction of DBA at potentials over a volt positive of DBA derivatives with aryl substitution (–1.07 ¬ and –1.40 V vs Fc/Fc+). The reduced species is a rare example of a 1,4-diboron analogue of a parent acene and is capable of binding CO2, ethylene, and O2 via apparent [4+2] cycloaddition reactions across the two boron atoms. This platform captures key features of transition metal complexes despite being comprised exclusively of light elements. Complimentary to the NHC system, we developed a novel diphosphine tethered diboraanthracene ligand (B2P2) and explored the redox chemistry of its transition metal complexes. In the course of these studies, we synthesized the first molecular complex of anionic gold (auride). The auride-B2P2 complex was accessed at modest potentials for a DBA-containing molecule (–2.05 V vs. Fc/Fc+) and was found capable of activating a range of small molecules including CO2, H2, H2O, formaldehyde, benzaldehyde, and acetone. Furthermore, the copper-, silver-, iron-, cobalt- and nickel-B2P2 complexes were synthesized, revealing multiple coordination modes and varying electron-accepting properties of the B2P2 ligand. Collectively, these studies establish the feasibility of redox small molecule activation at conjugated boranes.

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