Studies into the mechanism of CO-induced N ₂ cleavage promoted by an Ansa-hafnocene complex and C-C bond formation from an observed intermediate

Carbonylation of the hafnocene dinitrogen complex, [Me ₂Si(η ⁵-C ₅Me ₄)(η ⁵-C ₅H ₃- ^tBu)Hf] ₂(μ ₂, η ², η ²-N ₂), yields the corresponding hafnocene oxamidide compound, arising from N ₂ cleavage with concomitant C-C and C-N bond formation. Monitoring the addition of 4 atm of CO by NMR spectroscopy allowed observation of an intermediate hafnocene complex with terminal and bridging isocyanates and a terminal carbonyl. ¹³C labeling studies revealed that the carbonyl is the most substitutionally labile ligand in the intermediate and that N-C bond formation in the bridging isocyanate is reversible. No exchange was observed with the terminal isocyanate. Kinetic data established that the conversion of the intermediate to the hafnocene oxamidide was not appreciably inhibited by carbon monoxide and support a pathway involving rate-determining C-C coupling of the isocyanate ligands. Addition of methyl iodide to the intermediate hafnocene resulted in additional carbon-carbon bond formation arising from CO homologation following nitrogen methylation. Similar reactivity with ^tBuNCO was observed where C-C coupling occurred upon cycloaddition of the heterocumulene. By contrast, treatment of the intermediate hafnocene with CO ₂ resulted in formation of a μ-oxo hafnocene with two terminal isocyanate ligands.