A combination of phase-transfer and traditional alkylation strategies has been employed to synthesise sterically encumbered 1,3-di(cyclohexyl) and 1,3-di(tert-butyl) substituted indenes in multi-gram quantities. These indenyl ligands and sterically demanding alkyl cyclopentadienyl ligands have been used to prepare a series of [(η(7)-C(7)H(7))Zr(η(5)-L)] (L = Cp and Ind) complexes by straightforward salt metathesis between [(η(7)-C(7)H(7))ZrCl(tmeda)] and the corresponding sodium indenide or cyclopentadienide. All of these Zr complexes have been characterized by elemental analysis, NMR spectroscopy and single crystal X-ray diffraction. The structural information derived from these studies was employed to evaluate the steric demand of these ligands in a realistic manner.

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.

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This tutorial review summarizes the organometallic chemistry derived from the half-sandwich complex [(η(7)-C(7)H(7))ZrCl(tmeda)], which was used as an efficient and versatile starting material for the incorporation of monoanionic ligands into the cycloheptatrienyl zirconium coordination sphere by conventional salt metathesis reactions. A broad variety of ligands was employed, affording novel and previously inaccessible cycloheptatrienyl (sandwich) complexes of the type [(η(7)-C(7)H(7))Zr(Y)]; Y comprises pentadienyl, cyclopentadienyl, allyl, phospholyl, boratabenzene, imidazolin-2-iminato and amido systems. The cycloheptatrienyl ring in these systems usually acts as an "innocent spectator ligand", but reactivity can arise from the second ligand Y or the Lewis acidity of the, formally, Zr(+iv) center, which was probed in selected examples and put in perspective to related studies. The corresponding results emphasize why the use of [(η(7)-C(7)H(7))ZrCl(tmeda)] is clearly an advancement in the chemistry of the still fairly unexplored area of cycloheptatrienyl transition metal complexes.

The reactions of Zr(C 7 H 7 )(Cl)(tmeda) (tmeda = tetramethylethylenediamine) with pentadienyl anions lead to formally tetravalent Zr(C 7 H 7 )(Pdl) complexes, for Pdl = C 5 H 7 , 2,4-C 7 H 11 , 6,6-dmch, and c-C 7 H 9 (C 7 H 11 = dimethylpentadienyl, dmch = dimethylcyclohexadienyl, c-C 7 H 9 = cycloheptadienyl). Structural characterizations of the first three have been carried out, revealing much shorter Zr-C distances for the C 7 H 7 ligand and a pattern of Zr-C bond distances for the pentadienyl ligands that is consistent with a formally high (þ4) metal oxidation state, which is also supported by DFT calculations. As had been found for the analogous Cp complexes, these 16-electron species are susceptible to Lewis base coordination, and the 2,6-xylyl isocyanide adducts of the 2,4-C 7 H 11 and 6,6-dmch complexes have been isolated and characterized by IR spectroscopy and single-crystal X-ray diffraction studies. The IR spectroscopic studies indicate that the pentadienyl ligands are serving as better net electron donors than Cp ligands, opposite what is typically found for related but lower valent species. At high temperatures the 16-electron Zr(C 7 H 7 )(C 5 H 7 ) complex undergoes slow conversion to the corresponding Cp complex.