Synthesis and Isolation of Di-<i>n</i>-butylhafnocene and Its Application as a Versatile Starting Material for the Synthesis of New Hafnacycles

The previously described hafnocene alkyne complexes are of limited applicability for the generation of the hafnocene fragment in stoichiometric and catalytic reactions. For this reason di-n-butylhafnocene (1-Hf) was fully characterized and used as a hafnocene source in reactions with alkynes and diynes. With bis(trimethylsilyl)acetylene and pyridine the alkyne complex Cp2Hf(py)(η2-Me3SiC2SiMe3) (2-Hf) was obtained and coordination of di-tert-butylbutadiyne yielded the first structurally characterized hafnacyclocumulene, Cp2Hf(η4-t-BuC4-t-Bu) (4-Hf). By the coupling of 2 equiv of the 1,3-butadiyne RC4R at the hafnocene center, hafnacyclopentadienes were formed regioselectively (R = t-Bu, 5-t-Bu-Hf; R = SiMe3, 5-SiMe3-Hf). In the case of 1,4-diphenylbutadiyne the hafnocene-substituted [4]radialene 6-Hf was formed. Treatment of the latter with acid gave the free [4]radialene 7. In the reaction of 4-Hf with diphenylacetylene, the alkyne inserted into the Hf−Cα bond to give the hafnacyclopentadiene 8.