Crystal and molecular structure and unusual hydrocarbon solubility properties of (C₅H₅)₂ZrCl(AlCl₄): relevance to Ziegler-Natta polymerization chemistry

(C₅H₅)₂ZrCl₂ and AlCl₃ react in benzene or toluene to form an adduct that under certain conditions readily dissolves in benzene or toluene to form a binary liquidliquid phase system ("liquid clathrate"), the denser of which contains the Cp₂Zr moiety and possesses stoichiometry 1 6.3 and 1 5.4 for benzene and toluene, respectively. Cooling of these liquids, or the corresponding dilute hydrocarbon solutions, results in isolation of the 1 1 neutral adduct (C₅H₅)₂ZrCl(AlCl₄) (4) which is a model for precursors to cationic Ziegler-Natta polymerization catalysts. 4 crystallizes in the monoclinic space group C2 c with 25.568(9), b 12.710(3), c 9.845(2) Å, β 101.47(2)° and Z = 8. An R value of 0.036 was obtained for 1346 observed reflections. The salient feature of the structure is the ZrClAl bridge. The ZrCl bonds are nonequivalent (2.406(3) å terminal vs. 2.605(2) Å bridging) and the ZrClAl angle which, at 126.06(7)°, is quite obtuse. The implication of the structure of 4 is therefore that the AlCl₄^- moiety is somewhat loosely held by the Zr atom, thereby facilitating bond rupture to yield cationic Zr^IV complexes related to those recently shown to be catalytically active for Ziegler-Natta type polymerizations.