Synthesis, Structural Characterization, and CO2 Reactivity of a Constitutionally Analogous Series of Tricopper Mono-, Di-, and Trihydrides

The formation of hydrides at heterogeneous copper surfacesresultsin dramatic structural and reactivity changes, yet the morphologiesof these materials and their respective roles in catalysis are notwell understood. Of particular interest is the reactivity of heterogeneouscopper hydrides with carbon dioxide (CO2), an early mechanisticbranching point in the CO2 reduction reaction. Herein,we report the synthesis, characterization, and reactivity of tricoppercompounds supported by a facially biased, chelating tris-(carbene)ligand scaffold. This sterically bulky environment affords accessto an isolable series of tricopper hydrides: [LCu3H](2+) (4), [LCu3H2](+) (3), and LCu3H3 (6). Single-crystal X-ray diffraction and solution NMR spectroscopystudies reveal both geometric flexibility within the Cu-3 core and fluxionality of hydride ligands across the Cu-3 cluster, providing both atomically precise experimental analoguesof static surface species and emulating dynamic ligand behavior proposedfor surfaces. Electronic structure calculations serve as a predictorof hydricity, which was likewise benchmarked experimentally via bothprotonolysis and hydride abstraction reactions. Increased hydridenumber (and commensurately lower cluster charge) results in more hydridiccomplexes, with a thermodynamic hydricity range spanning >30 kcal/mol.These thermochemical studies serve as an accurate predictor of CO2 reactivity. Together, this Cu3H x series exhibits the structure/reactivity relationships proposedfor catalytically active copper surfaces, validating the applicationof carefully designed molecular clusters toward elucidating mechanismsin surface science.

Automated summary

In this study, the synthesis, characterization, and reactivity of tricopper compounds supported by a chelating tris-(carbene) ligand scaffold were reported. The geometric flexibility within the tricopper core and the dynamic behavior of hydride ligands across the cluster were revealed through structural analysis and NMR spectroscopy. The quantity of copper hydrides was found to be correlated with reactivity, and the results serve as an accurate predictor of CO2 reactivity. This study validates the application of carefully designed molecular clusters for elucidating mechanisms in surface science.