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 [(eta(5)-C3H2-1,2,4-Me-3)(2)Hf](2)(mu(2),eta(2):mu(2)-N-2) with 4 atm of carbon monoxide yielded the tetrametallic hafnocene oxamidide complex [(eta(5)-C5H2-1,2,4-Me-3)(2)Hf(NCO)](4), a new structural motif arising from CO-induced N-2 cleavage. The more commonly observed dimeric hafnocene oxamidide [(eta(5)-C5H2-1,2,4-Me-3)(2)Hf](2)(N2C2O2) was observed by multinuclear NMR spectroscopy when the carbonylation was performed at lower (similar to 1 atm) CO pressure. Over the course of 1 h at 23 degrees 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 (BuNCO)-Bu-t 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 [(eta(5)-C5H2-1,2,4-Me-3)(2)Hf(NCO)](4) at 110 degrees C provided a route to a new mu-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.