Di- and Tetrametallic Hafnocene Oxamidides Prepared from CO-Induced N2 Bond Cleavage and Thermal Rearrangement to Hafnocene Cyanide Derivatives

Abstract

Carbonylation of the hafnocene dinitrogen complex [(η5-C5H2-1,2,4-Me3)2Hf]2(μ2,η2:η2-N2) with 4 atm of carbon monoxide yielded the tetrametallic hafnocene oxamidide complex [(η5-C5H2-1,2,4-Me3)2Hf(NCO)]4, a new structural motif arising from CO-induced N2 cleavage. The more commonly observed dimeric hafnocene oxamidide [(η5-C5H2-1,2,4-Me3)2Hf]2(N2C2O2) was observed by multinuclear NMR spectroscopy when the carbonylation was performed at lower (∼1 atm) CO pressure. Over the course of 1 h at 23 °C, the dimeric hafnocene oxamidide undergoes dimerization to the tetrametallic compound, establishing its intermediacy for synthesis of the latter. Additional functionalization of the hafnium–nitrogen bonds in the tetrametallic complex was accomplished by cycloaddition of tBuNCO or 1,2-addition of CySiH3. The former example maintains a tetrametallic hafnocene where only two of the four Hf–N bonds have undergone [C═O] cycloaddition of the heterocumulene. In contrast, the primary silane yielded a dimeric hafnocene product where all of the hafnium–nitrogen linkages have undergone 1,2-addition. Thermolysis of [(η5-C5H2-1,2,4-Me3)2Hf(NCO)]4 at 110 °C provided a route to a new μ-oxo hafnocene complex with both terminal isocyanate and cyanide ligands. This process is general among hafnocene oxamidides and provides a route to rare hafnium cyanide complexes that undergo preferential [CN] rather than [NCO] group transfer.