Interception of Elusive Cationic Hf-Al and Hf-Zn Heterobimetallic Adducts with Mixed Alkyl Bridges Featuring Multiple Agostic Interactions

Heterobimetallic complexes with inequivalent bridging alkyl chains are very often invoked as key intermediates in many catalytic processes, yet their interception and structural characterization are lacking. Such complexes have been prepared from reactions of the cationic cyclometalated hafnocene [CpPrCpCH2CH2CH2 Hf][B(C6F5)4] (1) with main group metal alkyls to afford the corresponding heterobridged cationic products, [CpPrCpCH2CH2ðm-CH2ÞHf(m-R)E(R)n] [B(C6F5)4] (E=Al or Zn; R=Me, Et, or iBu). NMR and DFT studies demonstrate that both bridging alkyls establish agostic interactions with Hf, which are appreciably stronger for ethyl rather than methyl groups. Hf–Al and Hf–Zn distances are surprisingly short and only slightly longer than computed Hf–Al or Hf–Zn single bond lengths (2.80 a). Finally, a reaction of [CpPrCpCH2CH2ðm-CH2ÞHf(m-Me)Zn(Me)][B(C6F5)4] with excess ZnMe2 yields an unprecedented heterotrimetallic species, [(CpPr)2Hf(m-Me)(ZnMe)(m3-CH2)ZnMe][B(C6F5)4], the detailed structure of which is elucidated by a combination of NMR spectroscopic methods and molecular calculations.