Synthesis of a base-free hafnium nitride from N₂ cleavage: A versatile platform for dinitrogen functionalization

The synthesis and characterization of a metastable, base-free isocyanato dihafnocene μ-nitrido complex from CO-induced dinitrogen cleavage is described. The open coordination site at hafnium suggested the possibility of functionalization of the nitrogen atom by cycloaddition and insertion chemistry. Addition of the strained, activated alkyne, cyclooctyne, resulted in N-C bond formation by cycloaddition. The alkyne product is kinetically unstable engaging the terminal hafnocene isocyanate and promoting deoxygenation and additional N-C bond formation resulting in a substituted cyanamide ligand. Group transfer between hafnium centers was observed upon treatment with Me₃SiCl resulting in bridging carbodiimidyl ligands. Amidinato-type ligands, [NC(R)N]^3- were prepared by addition of either cyclohexyl or isobutyronitrile to the base free dihafnocene μ-nitrido complex, which also engages in additional N-C bond formation with the terminal isocyanate to form bridging ureate-type ligands. Heterocummulenes also proved reactive as exposure of the nitride complex to CO₂ resulted in deoxygenation and N-C bond formation to form isocyanate ligands. With substituted isocyanates, cycloaddition to the dihafnocene μ-nitrido was observed forming ureate ligands, which upon thermolysis isomerize to bridging carbodiimides. Taken together, these results establish the base free dihafnocene μ-nitrido as a versatile platform to synthesize organic molecules from N₂ and carbon monoxide.