Unusual Bond Activations

The unusual reactivity of hafnocene complexes in comparison to their titanium and zirconium analogues was dramatically demonstrated in the recent functionalization of coordinated N2 and the coupling of CO2 and N2 to a substituted hydrazine product. The differences in their structure and bonding were also noticed; for example, the s and p ratio of the diene bonding in (s-cis-h-diene) metallocene complexes is shifted to larger s character for Hf compared to Zr, thus giving shorter Hf C than Zr C bonds. Herein, we report the unusual tandem Si C and C H bond activation of decamethylhafnocene and bis(trimethylsilyl)acetylene, which could not be carried out using titanium and zirconium congeners. The complexation of bis(trimethylsilyl)acetylene by decamethylhafnocene leads to the three-membered hafnacyclopropene [Cp*2Hf(h -Me3SiC2SiMe3)] (1; Cp*= h -C5Me5). [4a] Surprisingly, until very recently only Ti and Zr analogues were known, and these show a very multifaceted chemistry. Our first reduction experiments with [Cp*2HfCl2] and Me3SiC CSiMe3 with magnesium in THF showed that the method used for the successful preparation of titanium and zirconium alkyne complexes failed for hafnocene alkyne complexes. The preferred ring opening of THF was found to be the reason for this failure. However, the reaction of [Cp*2HfCl2] with Me3SiC CSiMe3 and lithium in toluene resulted in the formation of the desired complex 1, which can be isolated in an optimized yield of 45% (Scheme 1). Apparently, the competition between THF and alkyne coordination to the metal causes the attempted synthesis of the hafnium complexes using magnesium in THF to fail. In an attempt to gain further insight into the hafnocene chemistry, the reaction of [Cp*2HfCl2] with Li and Me3SiC CSiMe3 in toluene was studied. By shortening the reaction time, we were able to isolate (in addition to the alkyne complex 1) a light green crystalline solid consisting of complex 2, in which one of the C H bonds of a methyl group of a Cp* ligand has reacted with the metal to form a Hf C bond (Scheme 1). The vinyl ligand is obtained from Me3SiC CSiMe3 by the formal migration of a Me3Si group from one carbon atom to the other (perhaps via vinylidene complex B, Scheme 2). This is a tandem activation of a C H bond of one Cp* methyl group and a Si C bond of the alkyne by the Hf center.