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

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.