Reactions of Five-Membered Zirconacyclocumulenes with Diisobutylaluminum Hydride

Different products of the reaction of the five-membered zirconacyclocumulenes (zirconacyclopenta-2,3,4-trienes) Cp*2Zr(η4-1,2,3,4-RC4R) (Cp* = η5-pentamethylcyclopentadienyl,
R = Me (1a), Me3Si (1b), Ph (1c)) with i-Bu2AlH are described which can serve as models
for the elementary reaction steps of the zirconocene-catalyzed hydroalumination of disubstituted buta-1,3-diynes RC⋮CC⋮CR and also for the activation of zirconocene complexes
in the catalytic polymerization of ethylene. In all investigated cases of reactions of complexes
1 with i-Bu2AlH via a cis-hydroalumination of the central double bond, the formation of
zirconacyclopentadienes with i-Bu2Al substituents in the 3-position as intermediates must
be assumed. These complexes were not isolated but subsequently stabilized in a different
manner, depending on the substituents R used. For R = Me, the intermediate reacts with
a second molecule of i-Bu2AlH to give the complex of the substituted zirconacyclopentadiene
with i-Bu2AlH (2). This complex reacts with CO2 to produce the dinuclear complex 3, whose
structure was confirmed by X-ray analysis. For R = Me3Si, the C−C bond in the intermediate
is cleaved (“hydroaluminolysis”) with formation of the alkyne i-Bu2AlC⋮CSiMe3 (detected
by NMR) and the alkyne complex [Cp*2Zr(η2-Me3SiCCH)]. The latter is stabilized by reaction
with i-Bu2AlH to give [Cp*2Zr][(μ-η1:η2-Me3SiCCH)(μ-H)(i-Bu2Al)] (4), which was characterized by X-ray crystallography. For R = Ph, two complexes were isolated after a stepwise
reaction of the unisolated hydroalumination product. The heterobimetallic complex [Cp*2Zr][(μ-η1:η2-PhCC−CHCPh)(μ-H)(i-BuAl)] (5) was formed by interaction with i-Bu2AlH with
elimination of i-Bu3Al. This complex can add a further 1 equiv of i-Bu2AlH to form the
trinuclear bimetallic complex [Cp*2Zr][(μ-η1:η2-PhCC−CHC(i-Bu2Al)(Ph)](μ-H)(i-BuAlH)]
(6). The structures of complexes 5 and 6 were determined by X-ray diffraction analysis.