Title : Cooperative side-on N₂ binding and multielectron reduction of diazenes by a heterobimetallic Zr/Co cluster
Abstract:
Current industrial catalytic processes predominantly rely on second- and third-row transition metals (e.g., palladium), because of their potential to perform two-electron processes. While these metals are highly efficient, often enabling requisite two- electron transformations with high turnover numbers, their scarcity and elevated cost present a significant sustainability challenge for large-scale synthetic applications. First row transition metals are sustainable because of their high natural abundance. However, tuning the electronic structure and reactivity of 1st row transition metals is a challenging task, which can be overcome by metal-metal cooperativity. A bimetallic core with a Lewis acidic, early transition metal and a Lewis basic, late transition metal can help access properties that are inaccessible with their monometallic analogues. The activation and reduction of dinitrogen is an area where the unique reactivity of heterobimetallic complexes can be leveraged. A key similarity between the Haber- Bosch process (Industrial dinitrogen fixation) and nitrogenase enzymes (biological dinitrogen fixation) is the presence of multiple metal centers in the active sites for dinitrogen reduction. Inspired by the biological processes, this study explores the catalytic binding and activation of N2 using a bis(phosphinoamide)-ligated ZrIV/CoI heterobimetallic complex. We have successfully isolated a tetrametallic dinitrogen-bound Zr/Co complex displaying an unprecedented binding mode of N2 with a novel four metal centers cluster, where N2 is bound side-on to the two Co centers and end-on to Zr in a μ3-η1:η2:η2 binding mode defying conventional binding patterns. The N-N bond distance was found to be significantly elongated to 1.380 Å compared to free N2 (1.097Å), which suggests a highly activated dinitrogen ligand. This heterometallic strategy significantly enhances the catalytic reductive silylation of activated nitrogen, producing a turnover number of 548 with respect to N(TMS)3 in 18 h, that are substantially higher than those achieved with previously reported cobalt-only catalysts. This demonstrates that combining different metals can unlock new, highly effective pathways for N2 activation. Further mechanistic insights were gained by exploring N=N bond activation by this Zr/Co complex by using azobenzene, benzo [c] cinnoline and hyrdrazobenzene as substrates to mimic the intermediates observed in conversion of N2 to NH3. These results will be presented in detail in this oral presentation.
