Hydrogen splitting at a single phosphorus centre and its use for hydrogenation
Recent years have seen a surge in the chemistry of main-group compounds that mimic the behavior of transition metals. For instance, main-group compounds capable of activating chemical bonds through elementary steps such as oxidative addition and forming new bonds through reductive elimination have gained significant interest. This is due to their potential to serve as an alternative or complement transition metals in catalysis.
In the last two decades, a variety of main-group compounds capable of activating dihydrogen have been reported. Some of these compounds activate the H-H bond at a single main-group center, with representative examples being cyclic alkyl amino carbenes (cAACs), borylenes, and their heavier analogs. Other activate H-H bonds between two main-group centers in a process known as frustrated Lewis pair (FLP) chemistry. Importantly, while FLPs have been used as catalysts for hydrogenation reactions, the activation of the H-H bond at a single main-group center and its subsequent use in hydrogenation have never been reported.
The last decade has seen a particular focus on the activation of small molecules by phosphorus-based compounds. Phosphorus's ability to switch between two stable oxidation states (PIII and PV) makes it a prominent candidate for catalysis. Despite this potential, most common neutral PIII compounds cannot activate strong chemical bonds through oxidative addition, limiting their reactivity to very reactive bonds such as dihalogens.
The reactivity of phosphorus centers can be significantly increased by cationization and substitution with strongly electron-withdrawing groups. However, the reactivity of these species is governed by their strong Lewis acidity, while their nucleophilicity is expectedly extremely low.
Another way to enhance the reactivity of the phosphorus center that has become popular recently involves structural deformation of the PIII center from its typical VSEPR geometry by its inclusion into a rigid pincer-type ligand. This structural constraint lowers the HOMO-LUMO gap, rendering the PIII center with both electrophilic and nucleophilic (ambiphilic) reactivity.
Our group developed an approach to further enhance the ambiphilic reactivity of the PIII center by combining both methodologies described above, i.e., structural constraint and cationization of the phosphorus center. In other words, we design new structurally constrained phosphenium cations and study their reactivity with small molecules. In the last few years, we have reported several such species and demonstrated the activation of O-H, N-H, C-N, Si-H, and C-F bonds, while some of these activations could be used in catalytic reactions. However, none of these structurally constrained phosphenium cations were able to activate the H-H bond. In general, the activation of the H-H bond through an oxidative addition to a single phosphorus center has never been shown.
In this work, we decided to use a bis-carborano-pyridine pincer-type ligand to constrain a cationic PIII center. We envisioned that the rigidity of this ligand would efficiently constrain the PIII center, while the electron-withdrawing nature of the ortho-carboranes would further increase the reactivity of this PIII cation. Additionally, the central pyridinyl moiety would stabilize the P-center. After successfully synthesizing this cation, we pressurized it with H2 (~4 atm) and monitored the reaction by NMR over several days. To our great satisfaction, after just a few days, the product of the H-H bond oxidative addition to the P-center was observed. This activation could be sped up by mild heating of the reaction. Remarkably, this represents the first example of the oxidative addition of the H-H bond to a single P-center. Next, we investigated whether our new phosphenium cation could be used as a hydrogenation catalyst, and again we were pleased to find that this new cation can be used for the catalytic hydrogenation of the doubly bonded C=C and fused aromatic species. Although the scope of unsaturated species that could be hydrogenated is quite limited, this reactivity represents the first example of a main-group center that both activates H2 and can be used for catalytic hydrogenation. For more details on the experiments and results, please read our paper: doi.org/10.1038/s41557-024-01569-y
We continue to search for more reactive phosphorus-based and other main-group compounds for the metallomimetic activation of challenging bonds and catalysis.
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