Dinitrogen Functionalization: Exploring Dinitrogen Cleavage And Nitrogen-Carbon Bond Forming Reactions With Group 4 Metallocene Compounds

A family of zirconocene and hafnocene compounds bearing side-on bound dinitrogen ligands were prepared and studied for their capability to promote a variety of nitrogen-carbon bond forming reactions. Most notably, these N2 compounds underwent N-N cleavage coupled with C-N and C-C bond formation upon addition of excess carbon monoxide, forming unprecedented oxamidide complexes. For each zirconium example dissociation of N2 and formation of the corresponding dicarbonyl species competes with N-N cleavage. For chiral ansa-metallocene examples, two stereoisomeric oxamidide products were observed in a ratio dependent on the pressure of CO added. In all cases, protonolysis liberated free oxamide, demonstrating synthesis of a useful organic product directly from N2 and CO. For the ansahafnocene, observation and isolation of a bridging isocyanate species at short reaction times prompted a kinetic study of the mechanism for oxamidide formation. Kinetic and isotopic exchange studies are consistent with a mechanism whereby N2 cleavage is fast and precedes C-C bond formation. When stoichiometric CO was added to the hafnocene N2 compounds, new products arising from N-N cleavage coupled with C-H activation of a cyclopentadienyl substituent were observed, suggesting the intermediacy of a [MICRO SIGN]-nitride species. This reactive intermediate was also intercepted with added dihydrogen and terminal acetylenes. The hafnocene oxamidide products were elaborated by the addition of various heterocumulenes, alkyl halides, and silanes resulting in new nitrogen-carbon or nitrogen-silicon bonds. Subsequent protonoylsis of the functionalized hafnocene oxamidato species liberated the corresponding substituted oxamides in high yield. In addition to CO-induced N2 cleavage, other methods of N2 functionalization were also explored. Addition of two equivalents carbon dioxide to ansa-metallocene dinitrogen compounds resulted in selective insertion into each metal-nitrogen bond, forming C2-symmetric N,N'-dicarboxylated hydrazido compounds. These compounds could be elaborated by the addition of trimethyliodosilane or methyl triflate to form additional N-C or N-Si bonds, and protonolysis released the corresponding carboxylated hydrazines. Finally, for one hafnocene dinitrogen compound, methyl triflate addition resulted in formation of a monomeric hafnocene diazenide species arising from direct alkylation of coordinated N2. Further exposure to methyl triflate resulted in additional N-C formation, furnishing a rare triflate hafnocene hydrozanato compound