Di- and tetrametallic hafnocene oxamidides prepared from CO-induced N ₂ bond cleavage and thermal rearrangement to hafnocene cyanide derivatives

Carbonylation of the hafnocene dinitrogen complex [(η ⁵- C ₅H ₂-1,2,4-Me ₃) ₂Hf] ₂(μ ₂,η ²:η ²-N ₂) with 4 atm of carbon monoxide yielded the tetrametallic hafnocene oxamidide complex [(η ⁵-C ₅H ₂-1,2,4-Me ₃) ₂Hf(NCO)] ₄, a new structural motif arising from CO-induced N ₂ cleavage. The more commonly observed dimeric hafnocene oxamidide [(η ⁵-C ₅H ₂-1,2,4-Me ₃) ₂Hf] ₂(N ₂C ₂O ₂) 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 CySiH ₃. 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 [(η ⁵-C ₅H ₂-1,2,4-Me ₃) 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.