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

Abstract

The reaction of the cycloheptatrienylzirconium half-sandwich complex [(η(7)-C(7)H(7))ZrCl(tmeda)] (1) (tmeda = N,N,N',N'-tetramethylethylenediamine) with Li(Im(Dipp)N), generated from bis(2,6-diisopropylphenyl)imidazolin-2-imine (Im(Dipp)NH) with methyllithium, yields the imidazolin-2-iminato complex [(η(7)-C(7)H(7))Zr(Im(Dipp)N)(tmeda)] (2). The corresponding tmeda-free complex [(η(7)-C(7)H(7))Zr(Im(Dipp)N)] (5) can be synthesized via the 1,3-bis(trimethylsilyl)allyl complex [(η(7)-C(7)H(7))Zr{η(3)-C(3)H(3)(TMS)(2)}(THF)] (3; TMS = SiMe(3)), which undergoes an acid-base reaction with Im(Dipp)NH 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 [(η(7)-C(7)H(7))Zr(Im(Dipp)N)(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 [(η(7)-C(7)H(7))Zr{Im(Dipp)N(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.

16-Electron cycloheptatrienyl zirconium allyl complexes

“…The strongly exothermic formation of 1 nicely explains why coordination of THF occurs during the reaction of [(h 7 -C 7 H 7 )ZrCl(tmeda)] and K[C 3 H 3 (TMS) 2 ] in THF (Fig. 6) [11]. Although our experimental results have shown that PMe 3 cleanly replaces THF affording 2 (vide Scheme 3.…”

“…Thus, weak binding can be concluded for the former, whereas comparatively strong bonds are generated for the latter. This is not only exemplified experimentally by the higher energy of activation for phosphine exchange (vide supra), but also by the observation that [(h 7 -C 7 H 7 )Zr(C 5 H 4 TMS)(thf)] readily loses THF under vacuum, although it was crystallized at low temperature [7a], while 1 is sufficiently stable [11].…”

“…Elemental analyses were performed by combustion and gas chromatographic analysis with an Elementar varioMICRO instrument. [(h 7 -C 7 H 7 )Zr{C 3 H 3 (TMS) 2 }(thf)] (1) [11], IMe [16] and H 2 CeIMe [17] were prepared according to the literature. All other reagents were obtained commercially and used as received.…”

“…Furthermore, more sophisticated monoanionic 6p-electron ligands such as pentadienyl [8], phospholyl [9] or boratabenzene [10] were introduced into the cycloheptatrienyl zirconium coordination sphere by taking advantage of the half-sandwich precursor. With the synthesis and characterization of the 16-electron complex [(h 7 -C 7 H 7 )Zr {h 3 -C 3 H 3 (TMS) 2 }(thf)] (1, TMS ¼ SiMe 3 ), we have very recently also shown that it is possible to attach allyl ligands to the (C 7 H 7 )Zr fragment (Scheme 1); however, the lower (by two) number of electrons in the allyl ligand requires the coordination of an additional THF ligand [11]. Here, the use of the extremely bulky 1,3-bis(trimethylsilyl)allyl ligand was a prerequisite for the stability of 1, as was also found for a number of other unusual molecules [12], because the corresponding complex with the unsubstituted C 3 H 5 ligand could not be isolated.…”

“…Here, the use of the extremely bulky 1,3-bis(trimethylsilyl)allyl ligand was a prerequisite for the stability of 1, as was also found for a number of other unusual molecules [12], because the corresponding complex with the unsubstituted C 3 H 5 ligand could not be isolated. Remarkably, the reaction of 1 with 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-imine (Im Dipp NH) resulted in the replacement of THF and subsequent formation of an exceptional pogo stick molecule, [(h 7 -C 7 H 7 )Zr(Im Dipp N)], by an intramolecular deprotonation accompanied by the loss of C 3 H 4 (TMS) 2 (Scheme 1) [11].…”