Functionalization of Hafnium Oxamidide Complexes Prepared from CO-Induced N₂ Cleavage

Functionalization of the nitrogen atoms in the hafnocene oxamidide complexes [Me₂Si(η⁵-C₅Me₄)(η⁵-C₅H₃-3-^tBu)Hf]₂(N₂C₂O₂) and [(η⁵-C₅Me₄H)₂Hf]₂(N₂C₂O₂), prepared from CO-induced N₂ bond cleavage, was explored by cycloaddition and by formal 1,2-addition chemistry. The ansa-hafnocene variant, [Me₂Si(η⁵-C₅Me₄)(η⁵-C₅H₃-3-^tBu)Hf]₂(N₂C₂O₂), undergoes facile cycloaddition with heterocumulenes such as ^tBuNCO and CO₂ to form new N−C and Hf−O bonds. Both products were crystallographically characterized, and the latter reaction demonstrates that an organic ligand can be synthesized from three abundant and often inert small molecules: N₂, CO, and CO₂. Treatment of [Me₂Si(η⁵-C₅Me₄)(η⁵-C₅H₃-3-^tBu)Hf]₂(N₂C₂O₂) with I₂ yielded the monomeric iodohafnocene isocyanate, Me₂Si(η⁵-C₅Me₄)(η⁵-C₅H₃-3-^tBu)Hf(I)(NCO), demonstrating that C−C bond formation is reversible. Alkylation of the oxamidide ligand in [(η⁵-C₅Me₄H)₂Hf]₂(N₂C₂O₂) was explored due to the high symmetry of the complex. A host of sequential 1,2-addition reactions with various alkyl halides was discovered and both N- and N,N′-alkylated products were obtained. Treatment with Brønsted acids such as HCl or ethanol liberates the free oxamides, H(R¹)NC(O)C(O)N(R²)H, which are useful precursors for N,N′-diamines, N-heterocyclic carbenes, and other heterocycles. Oxamidide functionalization in [(η⁵-C₅Me₄H)₂Hf]₂(N₂C₂O₂) was also accomplished with silanes and terminal alkynes, resulting in additional N−Si and N−H bond formation, respectively.