Functionalization of the nitrogen atoms in the hafnocene oxamidide complexes [Me2Si(η5-C5Me 4)(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(η 5-C5Me4)(η5-C5H 3-3-tBu)Hf]2(N2C2O 2), 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-C5H 3-3-tBu)Hf(I)(NCO), demonstrating that C-C bond formation is reversible. Alkylation of the oxamidide ligand in [(η5-C 5Me4H)2Hf]2(N2C 2O2) 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-C 5Me4H)2Hf]2(N2C 2O2) was also accomplished with silanes and terminal alkynes, resulting in additional N-Si and N-H bond formation, respectively.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry