The electronic influence of unbridged and ansa-bridged ring substituents on a zirconocene center has been studied by means of IR spectroscopic, electrochemical, and computational methods. With respect to IR spectroscopy, the average of the symmetric and asymmetric stretches (nu(CO(av))) of a large series of dicarbonyl complexes (Cp(R))(2)Zr(CO)(2) has been used as a probe of the electronic influence of a cyclopentadienyl ring substituent. For unbridged substituents (Me, Et, Pr(i), Bu(t), SiMe(3)), nu(CO(av)) on a per substituent basis correlates well with Hammett sigma(meta) parameters, thereby indicating that the influence of these substituents is via a simple inductive effect. In contrast, the reduction potentials (E degrees ) of the corresponding dichloride complexes (Cp(R))(2)ZrCl(2) do not correlate well with Hammett sigma(meta) parameters, thereby suggesting that factors other than the substituent inductive effect also influence E degrees. Ansa bridges with single-atom linkers, for example [Me(2)C] and [Me(2)Si], exert a net electron-withdrawing effect, but the effect is diminished upon increasing the length of the bridge. Indeed, with a linker comprising a three-carbon chain, the [CH(2)CH(2)CH(2)] ansa bridge becomes electron-donating. In contrast to the electron-withdrawing effect observed for a single [Me(2)Si] ansa bridge, a pair of vicinal [Me(2)Si] ansa bridges exerts an electron-donating effect relative to that from the single bridge. DFT calculations demonstrate that the electron-withdrawing effect of the [Me(2)C] and [Me(2)Si] ansa-bridges is due to stabilization of the cyclopentadienyl ligand acceptor orbital, which subsequently enhances back-donation from the metal. The calculations also indicate that the electron-donating effect of two vicinal [Me(2)Si] ansa bridges, relative to that of a single bridge, is a result of it enforcing a ligand conformation that reduces back-donation from the metal.
A series of chiral ansa zirconocene compounds that feature (i) [Me2Si] and [Me4Si2] bridges
and (ii) two tert-butyl substituents on each cyclopentadienyl ring, namely, rac
-[Me2Si(CpBut
2
)2]ZrCl2, rac
-[Me2Si(CpBut
2
)2]Zr(Ph)Cl, rac
-[Me2Si(CpBut
2
)2]ZrMe2, rac
-[Me2Si(CpBut
2
)2]Zr(CO)2, rac
-[Me4Si2(CpBut
2
)2]ZrCl2, and rac
-[Me4Si2(CpBut
2
)2]Zr(CO)2, has been synthesized. In each case,
the complex exists as the racemo isomer due to inter-ring steric interactions between the
But substituents proximal to the ansa bridge that destabilize the meso isomer. rac
-[Me4Si2(CpBut
2
)2]ZrCl2 is the first structurally characterized ansa zirconocene complex with a [Me4Si2] bridge that adopts a racemo geometry. IR spectroscopic studies on the dicarbonyl
complexes (Cp1,3-But
2
)2Zr(CO)2, rac
-[Me2Si(CpBut
2
)2]Zr(CO)2, and rac
-[Me4Si2(CpBut
2
)2]Zr(CO)2
demonstrate that the [Me2Si] and [Me4Si2] ansa bridges have opposite electronic influences
in this system, with the [Me2Si] bridge exerting an electron-withdrawing effect and the [Me4Si2] bridge exerting an electron-donating effect. The [Me2Si] and [Me4Si2] bridges also exert
a different influence with respect to olefin polymerization, with {rac
-[Me2Si(CpBut
2
)2]ZrCl2/MAO} being a more active catalyst system for polymerization of ethylene than is either
{(Cp1,3-But
2
)2ZrCl2/MAO} or {rac
-[Me4Si2(CpBut
2
)2]ZrCl2/MAO}.
A nsa bridges, i.e., substituents that link together two cyclopentadienyl groups, have been shown to exert a profound effect on the reactivity of a metallocene system. For example, we have recently described how a [Me 2 Si] ansa bridge modulates the reactivity of permethylated metallocene complexes (1-10). In each of these examples, the [Me 2 Si] ansa bridge is merely a substituent that modifies the chemistry of the system due to (i) an inductive effect and (ii) a structural effect that displaces the cyclopentadienyl groups from their natural positions. )WH resulting from cleavage and functionalization of the ansa bridge (Scheme 3). The molecular structure of (Cp Me 2 )( 5 , 1 -C 5 H 2 Me 2 SiMe 2 CH 2 )WH has been determined by x-ray diffraction, as illustrated in Fig. 3.A plausible sequence for formation of (Cp Me 2 )( 5 , 1 -C 5 H 2 Me 2 SiMe 2 CH 2 )WH involves initial nucleophilic attack at the silicon atom by MeLi, thereby breaking the Si-cyclopentadienyl bond and forming a trimethylsilylcyclopentadienyl derivative (Scheme 4). Precedent for this transformation is provided by the organolithium induced ring-opening polymerization of the ferrocene derivative [Me 2 Si(Cp) 2 ]Fe (15), although the latter process is Author contributions: C.E.Z. and G.P. designed research; C.E.Z., H.L., S.H.J., and K.P. performed research; C.E.Z., H.L., S.H.J., K.P., and G.P. analyzed data; and C.E.Z., H.L., S.H.J., K.P., and G.P. wrote the paper.
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