Developing A New Chemical Tool For N_₂ And C-H Activation

Nitrogen (N_₂) is the most abundant gas in Earth’s atmosphere and, as such, the most important feedstock for the production of nitrogen-containing chemicals, such as fertilizers. However, due to its strong triple bond, N_₂ is extremely stable, making its transformation into value-added products a major scientific challenge. Typically, this process requires metal-based catalysts capable of activating N_₂ — that is, making it chemically accessible for further reactions. To develop more efficient catalytic systems, researchers around the world have investigated how N_₂ interacts with metal centers in order to identify the key factors that enable its activation. Despite the rich chemistry of transition metal complexes in the activation of N_₂, catalyst systems based on manganese (Mn) have remained surprisingly absent from this field. A major reason for this is manganese’s strong tendency to form highly stable Mn–Mn bonds upon reduction, which blocks its reactivity towards N_₂. In a recent collaborative study, scientists from Prof. Limberg’s UniSysCat group (Humboldt-Universität zu Berlin) and Prof. Cameron Jones’ group at Monash University, a strategic partner of UniSysCat, have addressed this challenge: They report the synthesis of a new Mn-complex that is able to bind N_₂ — a promising step towards expanding the toolbox of N_₂-activating materials. This achievement stems from the research stay of doctoral student Siad Wolff in the group of Prof. Jones — a great example of international scientific collaboration in action!

The research team used bulky diamide ligands that spatially separate the two Mn(I) centers in such a way that the formation of Mn-Mn bonds is effectively prevented. This strategy yields Mn-complexes that remain fairly reactive and are able to coordinate N_₂. As a result, the team successfully isolated the first anionic manganese diazenide complex.

The experimental approach was inspired by a comparison between low-valent magnesium (Mg) and manganese chemistry — two elements that share more than just similar names: Magnesium, much like manganese, tends to form strong metal–metal bonds (Mg–Mg) upon reduction, which to some extent can limit its utility as a reducing agent. However, in a recent study, the Jones group had demonstrated the first example of N_₂ activation mediated by a low-valent Mg-complex, made possible by a same "bulky ligand framework". Building on this concept, the Limberg group contributed their expertise in transition metal chemistry, adapting the Jones group’s innovative approach to develop the present Mn-based system. The outcome not only advances our understanding of fundamental principles in N_₂ activation, but also offers a fascinating side-by-side comparison of the reactivity of Mn(I) and Mg(I) species.

Interestingly, both the Mn- and Mg-based complexes can activate N_₂ in a similar way, but they exhibit strikingly different reactivity towards benzene, another small molecule they can be activated by both: As a main group metal Mg is limited to "Birch"-type reduction of the aromatic system, while the accessibility of filled d-orbitals in the Mn system allows for an oxidative addition of benzene to cleave a C-H bond. This finding highlights the unique role of d-electrons in transition metal chemistry.

Such a comparison of potential metal catalysts is crucial, as different chemical tools are needed to activate various small molecules depending on the specific context and application. The current work contributes to our understanding of how ligand environment and metal identity jointly determine the activation of small molecules — a key goal for sustainable catalysis.