From a Cycloheptatrienylzirconium Allyl Complex to a Cycloheptatrienylzirconium Imidazolin-2-iminato “Pogo Stick” Complex with Imido-Type Reactivity

The reaction of the cycloheptatrienylzirconium half-sandwich
complex
[(η7-C7H7)­ZrCl­(tmeda)] (1) (tmeda = N,N,N′,N′-tetramethylethylenediamine)
with Li­(ImDippN), generated from bis­(2,6-diisopropylphenyl)­imidazolin-2-imine
(ImDippNH) with methyllithium, yields the imidazolin-2-iminato
complex [(η7-C7H7)­Zr­(ImDippN)­(tmeda)] (2). The corresponding tmeda-free
complex [(η7-C7H7)­Zr­(ImDippN)] (5) can be synthesized via the 1,3-bis­(trimethylsilyl)­allyl
complex [(η7-C7H7)­Zr­{η3-C3H3(TMS)2}­(THF)] (3; TMS = SiMe3), which undergoes an acid–base
reaction with ImDippNH to form 5 and 1,3-bis­(trimethylsilyl)­propene. 5 exhibits an unusual one-legged piano stool (“pogo
stick”) geometry with a particularly short Zr–N bond
of 1.997(2) Å. Addition of 2,6-dimethylphenyl or tert-butyl isocyanide affords the complexes [(η7-C7H7)­Zr­(ImDippN)­(CNR)] (R = o-Xy, 6; R = t-Bu, 7),
while the reaction with 2,6-dimethylphenyl isocyanate results in a
[2 + 2] cycloaddition to form the ureato(1−) complex [(η7-C7H7)­Zr­{ImDippN­(CO)­N-o-Xy}] (8). 5 can also act as
an initiator for the ring-opening polymerization of ε-caprolactone.
These reactivity patterns together with density functional theory
calculations reveal a marked similarity of the bonding in imidazolin-2-iminato
and conventional imido transition-metal complexes.