Stabilizing Zr and Ti cations by interaction with a ferrocenyl fragment

Article abstract of DOI:10.1021/ja9077446

(Figure Presented) Reaction of the cation Cp₂ZrMe(μ- MeB(C₆F₅)₃) or Cp(tBu₃PN)TiMe(μ- MeB(C₆F₅)₃) with (RC₅H ₄)₂Fe (R = H, Me) yields

Full text of DOI:10.1021/ja9077446

Published on Web 10/13/2009
Stabilizing Zr and Ti Cations by Interaction With a Ferrocenyl Fragment
Alberto Ramos, Edwin Otten, and Douglas W. Stephan*
Department of Chemistry, UniVersity of Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S 3H6
Received September 11, 2009; E-mail: dstephan@chem.utoronto.ca
The discovery of Sinn and Kaminsky1 that active olefin polymer-
ization catalysts are generated by the treatment of zirconocenes with
methylalumoxane initiated exploration of the chemistry of electrophilic
group 4 metal cations. Subsequent work by Jordan et al. resulted in
the structural characterization of the THF stabilized zirconocene cation
[Cp2ZrMe(THF)][BPh4],2 which in the absence of excess THF or donor
solvent is an active olefin polymerization catalyst. Attempts by Turner’s
research group to isolate ‘base-free’ cationic compounds afforded the
spectrum was not accessible. Nonetheless, these data imply a dynamic
exchange process involving an agostic interaction of a Cp C-H bond
with the cationic Zr center. To probe this, the 1,1′-dimethylferrocenyl
analog [Cp2Zr(µ-C5H3Me)Fe(C5H4Me)] [MeB(C6F5)3] 2b was pre-
pared. The 1H NMR spectrum of 2b at 25 °C shows seven inequivalent
CH resonances due to the 1,1′-dimethylferrocenyl fragment. Two of
these are observed at relatively high field (-1.29 and 0.85 ppm). Both
signals broaden into the baseline upon cooling to -80 °C, inferring
that the observed shifts are the average of agostic and nonagostic
contributions arising from exchange between two conformers with an
eclipsed 1,1′-dimethylferrocenyl geometry (Figure S1). X-ray crystal-
lographic studies confirmed the nature of these compounds 2 (Figure
1).12 The anions of 2 were unexceptional. The cations of 2a and 2b
are comprised of a Cp2Zr fragment which is σ-bound to the C5H4 or
C5H3Me unit of a ferrocenyl fragment, respectively. In 2a, the Zr-
and Fe-Cp(centroid) bonds are typical in length averaging 2.207 and
1.660 Å, respectively. The Zr-C σ-bond distance to the C5H4 fragment
is 2.2894(15) Å. The electron deficient nature of the Zr cation results
in a close approach of the Fe center to Zr at a distance of 2.8910(3)
Å, shorter than that in the related neutral [1]-ferrocenophane
(tBuC5H4)2Zr(C5H4)2Fe (2.9621(5) Å).13 This results in a slight
deformation of the ferrocenyl fragment, with a Cp(centroid)-Fe-
Cp(centroid) angle of 172.01(4)° and interplanar angle between the
two C5 rings of 4.72(9)°. In agreement with the NMR data, the structure
shows an agostic interaction, which is characterized by short Zr-C
(2.6151(15) Å) and Zr-H (2.38(2) Å) distances, and the H atom
(which was located in the difference Fourier map) is out of the C5
plane by 16.7°. The metrical parameters for 2b are similar, but the
slightly more electron-rich 1,1′-dimethylferrocenyl group results in a
marginally shorter Zr-Fe distance of 2.8825(7) Å. The Fe-Zr
distances in 2 are significantly longer than the Fe-Ti distance
(2.4907(18) Å) reported by Arnold and co-workers for [LTi(µ-Cl)]22+
(L ) 1,1′-diamidoferrocene).11a It is also noteworthy that aryl-
zirconocene cations are known to be stabilized by an agostic interaction
with an aryl ꢀ-CH bond.14 The cationic zirconocene ferrocenyl
complexes 2 presented herein combine both a Zr-Fe interaction and
species [Cp*2Zr(C6H3R)B(C6H4R)3] (R
) H, Me, Et) and
[Cp′2ZrMe(C2B9H12)] (Cp′ ) Cp*, C5Me4Et).3 Horton and Orpen
described closely related ‘base-free’ zirconocene cations derived from
the insertion of alkyne into the Zr-C bond of [Cp′2ZrMe(NMe2Ph)n]-
[B(4-C6H4F)4)].4 Marks and co-workers5 pioneered and continue to
exploit the use of electrophilic boranes as activators, describing the
single component catalysts [Cp2ZrMe(µ-MeB(C6F5)3)] and [Cp2ZrH(µ-
HB(C6F5)3)] in which the anion is bound to the metal center. A variety
of classical Lewis bases have been employed to stabilize group 4 metal
cations,2,6 but catalytic activity for the resulting species is greatly
diminished. More recently the research groups of Casey, Jordan, and
others have prepared zirconocene cations with a weakly basic alkene
or alkyne group.7 Although these serve as models for polymerization
catalysts in action, the weak donor ability of alkene/alkyne and the
low insertion barrier requires special precautions for their successful
synthesis. Recently, compounds exhibiting unusual dative interactions
of electron-rich metal fragments with Lewis acidic centers have
attracted attention.8 Compounds containing group 13 Lewis acids (e.g.,
BR3) have received the most attention.9 A few examples are known
in which ‘metalloligands’ with M1fM2 dative interactions modulate
the redox chemistry or reactivity of the metal complex.10 Surprisingly,
the ability of electron-rich metal centers to act as dative donors to
electrophilic early transition metals has drawn only limited attention.11
In this communication, we describe the reaction of early metal cations
with ferrocene. The facile CH-activation of ferrocene results in
remarkably stable yet active polymerization catalysts, in which cationic
metal centers are stabilized by interaction with the ferrocenyl moiety.
Treatment of [Cp2ZrMe(µ-MeB(C6F5)3)] 15c with ferrocene in
bromobenzene solution resulted in the clean formation of a new cationic
zirconocene species 2a, which was isolated as an orange solid in 93%
yield upon precipitation with pentane. Loss of CH4 was evident by
1
1H NMR spectroscopy. H and 13C NMR spectral data for 2a show
resonances attributable to the Cp2Zr moiety at 5.92 and 111.2 ppm,
respectively; 1H NMR signals at 4.28, 4.20, and 2.44 ppm in a 2:2:5
ratio indicated C-H-activation of one of the ferrocene Cp-rings. The
most downfield resonance of the corresponding 13C NMR signals for
the metalated ring (171.0, 85.5, and 74.7 ppm) is consistent with a
Zr-C(sp2) bond. These data support the formulation of 2a as [Cp2Zr(µ-
C5H4)FeCp][MeB(C6F5)3]. The 1H NMR resonance of the CpFe
fragment (2.44 ppm) is significantly upfield of typical ferrocene signals
1
(3.5-4.5 ppm). The variable temperature H NMR spectra of 2a in
CD2Cl2 show considerable broadening of this resonance at temperatures
below -25 °C, while other resonances are not affected. The signal
broadens into the baseline upon cooling to -90 °C, but a limiting
Figure 1. Synthesis and POV-ray depiction of cations of 2a and 4.
9
15610 J. AM. CHEM. SOC. 2009, 131, 15610–15611
10.1021/ja9077446 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
agostic CH bond, imparting remarkable stability to these compounds.
For example, samples 2a and 2b remained unchanged on storage for
a week in C6D5Br solution at room temperature.
displaced for subsequent derivatization and reactivity and, thus,
provide a strategy to ‘tame’ highly reactive early transition metal
cations. The cooperation of such Lewis acidic and Lewis basic metal
centers in subsequent chemistry is being explored.
To probe the generality of this approach, the metallocene-cation
analogue [CpTi(NPtBu3)Me(µ-MeB(C6F5)3)]15 5 was prepared
and treated with ferrocene to give [CpTi(NPtBu3)(C5H4)FeCp)]
[MeB(C6F5)3] 6. NMR and crystallographic data were consistent
with a Ti-ferrocenyl interaction similar to that described for the
Zr compounds 2 (Figure 2). In 6, the Ti-Fe distance is 2.7112(4)
Å, while the agostic interaction with the Cp CH fragment gives
rise to Ti-C and Ti-H distances of 2.337(2) and 2.051 Å,
respectively.
Acknowledgment. D.W.S. gratefully acknowledges the financial
support of NSERC of Canada. E.O. is grateful for the support of a
Rubicon postdoctoral fellowship from The Netherlands Organization
for Scientific Research (NWO).
Supporting Information Available: Experimental procedures and
X-ray crystallographic details of 2a, 2b, 4, and 6. This material is
available free of charge via the Internet at http://pubs.acs.org.
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Figure 2. Synthesis and POV-ray depiction of cation of 6.
Despite the stability of these compounds, they remain highly
reactive. For example, 2a acts as a single component catalyst for
the polymerization of ethylene at 25 °C and 1 atm, with an activity
similar to that observed for 1 (2a: 3000, 1; 2800 g/mmol/atm/h).
As this reactivity implies, the dative Zr-ferrocenyl interaction is
readily displaced. Treatment of a C6H5Br solution of 2a with a
stoichiometric amount of a Lewis base such as THF or PMe3 results
in formation of deep blue or purple solutions of the adducts
[Cp2Zr(L)(µ-C5H4)FeCp][MeB(C6F5)3] (L ) THF 3; PMe3 4) in
90 and 91% yield, respectively. NMR spectroscopy is consistent
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in pure form were unsuccessful. The PMe3 adduct 4, obtained as
1
purple crystals, showed H NMR spectroscopic data similar to 3
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bound PMe3 fragment. A crystal structure determination of 4 (Figure
1) confirmed displacement of the Zr-Fe interaction by PMe3,
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3.5146(4) and 3.5981(3) Å.12
DFT optimization (B3LYP/6-31G(d)) of the structure of the cation
2a gave 2acalc, which exhibits metric parameters similar to those
obtained experimentally, with a Zr-Fe contact of 2.955 Å and a
Zr-CH agostic interaction with one of the C5H4 CH bonds (Zr-C:
2.795 Å, Zr-H: 2.510 Å). These latter bond lengths are slightly longer
than the crystallographic values; however, the B3LYP functional is
known to underestimate noncovalent interactions.16 Examination of
the bonding orbitals involving an interaction between Zr and Fe
revealed that primarily the HOMO-3, HOMO-10, and HOMO-11
orbitals are derived from a combination of Zr and Fe d-orbitals,
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enyl-based orbitals (Figure S2). The calculations infer a Wiberg bond
order between Zr and Fe of 0.2540. The decreased positive NBO
charge on the Zr center in 2acalc (+1.10) compared to Cp2ZrMe+
(+1.56) or Cp2ZrPh+ (+1.54) establishes that there is net CpFefZr
electron transfer in the cations 2, albeit small.
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In summary, the present compounds demonstrate the ability of
electron-rich metallocenes to stabilize electrophilic Zr cations by a
combination of dative FefZr interactions and agostic CH bonds.
Despite this stabilization, these donor interactions are readily
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