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

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journal contribution
posted on 03.11.2010, 00:00 authored by Donald J. Knobloch, Emil Lobkovsky, Paul J. Chirik
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.