Carbon Monoxide-Induced Dinitrogen Cleavage with Group 4 Metallocenes: Reaction Scope and Coupling to N−H Bond Formation and CO Deoxygenation

The scope of CO-induced N2 cleavage in a series of zirconocene and hafnocene complexes containing activated, side-on bound dinitrogen ligands has been studied. In each case, bridging oxamidide ligands, [N2C2O2]4−, were formed from N−N bond cleavage coupled to N−C and C−C bond assembly. For the zirconium examples, [(η5-C5Me4H)2Zr]2(μ2,η2,η2-N2) and [Me2Si(η5-C5Me4)(η5-C5H3-3-tBu)Zr]2(μ2,η2,η2-N2), dinitrogen loss became competitive with N2 carbonylation, and significant quantities of the zirconocene dicarbonyl accompanied oxamidide formation. In contrast, the hafnocene complex [(η5-C5Me4H)2Hf]2(μ2,η2,η2-N2) underwent clean carbonylative dinitrogen cleavage with no evidence of N2 loss. CO-induced N2 cleavage was also coupled to N−H bond formation by hydrogenation and C−H bond activation, as carbonylation of the zirconocene and hafnocene dinitrogen complexes in the presence of H2 or phenylacetylene furnished isocyanato metallocene complexes with bridging imido (μ-NH) ligands. In the case of the ansa-hafnocene dinitrogen complex, replacing the dihydrogen atmosphere with various primary silanes yielded an isocyanato hafnocene μ-oxo hydride resulting from cleavage of N2 and CO, the diatomics with the two strongest bonds in chemistry.