Using p- and d-Block Pairs for the Cooperative Bond Activation of Small Molecules
The valorization of chemical feedstocks into higher-value commodity chemicals through bond activation and subsequent functionalization remains a pillar in the chemical sciences sector. Despite their low abundance, prohibitive cost, and toxicity, precious metals are often utilized in these transformations due to their high catalytic activity. Therefore, sustainable alternatives capable of supplanting them are of academic, environmental, and industrial importance. Redox-noninnocent metal-ligand frameworks comprised of inexpensive and abundant components have shown promise in this regard. Specifically, the cooperative interaction between the redox-active ligand and the Lewis-acidic metal functionality imbues these systems with “nobility,” enabling multi-electron bond activation and functionalization chemistry reminiscent of the 4d/5d metals. Typically, the base metal mediates the relevant bond making/breaking while the appended main-group support facilitates electron transfer during this process. The inherent electronic communication between the two sites is responsible for this reactivity profile. Therefore, we wondered if complementary modes of bond activation and functionalization chemistry are possible by shifting the reactivity locus onto the main-group site while encouraging the appended metal to shuttle the electrons. We are interested in generalizing this phenomenon by probing the electronic structure of tethered p- and d-block combinations. Herein, we present a library of compounds featuring main-group reaction nodes (ex. B, Al) tethered to encapsulated redox-active metals (ex. V, Fe) through linker atoms (ex. N) to investigate their electronic structure and examine their reactivity towards small molecules, which ultimately illuminates the effect of block pairing on the resulting electronic structure. In the first case to be discussed, we detail how tethering a redox-active vanadium imido onto boron results in redox-switchable borane vs. boron radical reactivity. We address the implications of the resulting hybrid electronic structure on the activation of small molecules bearing Group 14–16 element-hydrogen bonds using a suite of spectroscopic, electrochemical, crystallographic, and computational techniques. Building on these results, we adjust the vanadium to main-group stoichiometry, leading to bi- or tri-metallic molecular arrays with expanded electron storage capabilities to target multi-electron transformations of small molecules. Lastly, we explore how tuning the redox-active tether and main-group Lewis acidic or basic partner leads to markedly different bond activation chemistry. These results demonstrate that the prudent choice of main-group/metal pairing leads to interesting electronic structures with applications toward bond activation and functionalization chemistry.