Ligand-induced haptotropic rearrangements in bis(indenyl)zirconium sandwich complexes

Article abstract of DOI:10.1021/ja052033q

Addition of principally σ-donating ligands such as THF, chelating diethers, or 1,2-bis(dimethyl)-phosphinoethane to η⁹, η⁵-bis(indenyl)zirconium sandwich complexes, (η⁹- C₉H₅-1,3-R₂)(η⁵-C ₉H₅-1,3-R₂)Zr (R = alkyl or silyl), induces haptotropic rearrangement to afford (η⁶-C₉H ₅-1,3-R₂)(η⁵-C₉H ₅-1,3-R₂)ZrL adducts. Examples where L = THF and DME have been characterized by X-ray diffraction and revealed significant buckling of the η⁶ benzo ring, consistent with reduction of the arene, and highlight the importance of the zirconium(IV) canonical form. For the THF-induced haptotropic rearrangements, the thermodynamic driving force for ring migration has been measured as a function of indenyl substituent and demonstrates silylated sandwiches favor THF coordination and the η⁶,η⁵ bonding motif over their alkylated counterparts. In the case of chelating diethers, measurement of the corresponding equilibrium constants establish more stable η⁶, η⁵ adducts with five- over four-membered chelates and with smaller oxygen and carbon backbone substituents. Kinetic studies on both THF and DME addition to (η⁹-C₉H₅-1,3-(SiMe ₃)₂)(η⁵-C₉H₅-1,3- (SiMe₃)₂)-Zr established a first-order dependence on the incoming ligand, consistent with a mechanism involving direct attack of the incoming nucleophile on the η⁹,η⁵ sandwich. These results further highlight the ability of the indenyl ligand to smoothly adjust hapticity to meet the electronic requirements of the metal center.

Full text of DOI:10.1021/ja052033q

Published on Web 07/01/2005
Ligand-Induced Haptotropic Rearrangements in
Bis(indenyl)zirconium Sandwich Complexes
Christopher A. Bradley, Emil Lobkovsky, Ivan Keresztes, and Paul J. Chirik*
Contribution from the Department of Chemistry and Chemical Biology, Baker Laboratory,
Cornell UniVersity, Ithaca, New York 14853
Received March 30, 2005; E-mail: pc92@cornell.edu
Abstract: Addition of principally σ-donating ligands such as THF, chelating diethers, or 1,2-bis(dimethyl)-
phosphinoethane to η9,η5-bis(indenyl)zirconium sandwich complexes, (η9-C9H5-1,3-R2)(η5-C9H5-1,3-R2)Zr
(R ) alkyl or silyl), induces haptotropic rearrangement to afford (η6-C9H5-1,3-R2)(η5-C9H5-1,3-R2)ZrL adducts.
Examples where L ) THF and DME have been characterized by X-ray diffraction and revealed significant
buckling of the η6 benzo ring, consistent with reduction of the arene, and highlight the importance of the
zirconium(IV) canonical form. For the THF-induced haptotropic rearrangements, the thermodynamic driving
force for ring migration has been measured as a function of indenyl substituent and demonstrates silylated
sandwiches favor THF coordination and the η6,η5 bonding motif over their alkylated counterparts. In the
case of chelating diethers, measurement of the corresponding equilibrium constants establish more stable
η6,η5 adducts with five- over four-membered chelates and with smaller oxygen and carbon backbone
substituents. Kinetic studies on both THF and DME addition to (η9-C9H5-1,3-(SiMe3)2)(η5-C9H5-1,3-(SiMe3)2)-
Zr established a first-order dependence on the incoming ligand, consistent with a mechanism involving
direct attack of the incoming nucleophile on the η9,η5 sandwich. These results further highlight the ability
of the indenyl ligand to smoothly adjust hapticity to meet the electronic requirements of the metal center.
Introduction
The indenyl anion, [C9H7]-, and its substituted variants are
versatile ligands in organometallic chemistry, supporting a wide
range of transition metal, lanthanide, actinide, and main group
complexes.1 Pauson and Wilkinson’s synthesis of the first bis-
(indenyl) sandwich compounds, (C9H7)2M (M ) Fe, Co),2
established the similarity between these carbocycles and well-
known cyclopentadienyl ligands. Considering indenyl as merely
benzannulated cyclopentadienyl is often an oversimplification;
metal complexes containing the former exhibit unique chemical
and physical properties as compared to those with the latter
ligand. The origin of this difference is the “haptotropic flex-
ibility” of the indenyl ring where different numbers of metal-
carbon bonds are formed depending on the electronic require-
ments of the metal center.
Commonly encountered indenyl hapticities are presented in
Figure 1. In analogy to metal-cyclopentadienyl complexes,
indenyl ligands are often bound through the five-membered ring
and are classified as having η5 hapticity.3 More careful inspec-
tion of the metrical parameters for many of these compounds
reveals distinct η3,η2 bonding in which two of the five metal-
carbon bonds are significantly longer than the other three.4 While
η3 coordination remains relatively rare in ground-state struc-
Figure 1. Common hapticities for indenyl ligands.
tures,5 ring “slippage” from η5 to η3 hapticity during the course
of a chemical reaction, a phenomenon known as the “indenyl
effect”, is responsible for the rate enhancements observed in
associative substitution reactions.6,7 Complexes with η1 coor-
dinated indenyl ligands, predominantly with late transition
metals, have also been reported (Figure 1).8
The presence of the additional π-system of the indenyl ligand
also offers the possibility for hapticities greater than five,
although complexes with these coordination modes are encoun-
tered less frequently (Figure 2). Deprotonation of the neutral
indene complex, (η6-C9H8)(η5-C9H7)Re, furnished the thermally
stable rhenium anion, [(η6-C9H7)(η5-C9H7)Re]BF4, which showed
no evidence for isomerization to the η5 haptomer.9 In contrast,
(4) (a) Calhorda, M. J.; Roma˜o, C. C.; Veiros, L. F. Eur. J. Inorg. Chem. 2002,
8, 868. (b) Zargarian, D. Coord. Chem. ReV. 2002, 233-234, 157. (c) Trnka,
T. M.; Bonanno, J. B.; Bridgewater, B. M.; Parkin, G. Organometallics
2001, 20, 3255.
(5) (a) Gamelas, C. A.; Herdtweck, E.; Lopes, J. P.; Romao, C. C. Organo-
metallics 1999, 18, 506. (b) Merola, J. S.; Kacmarcik, R. T.; Van Engen,
D. J. Am. Chem. Soc. 1986, 108, 329.
(6) (a) Rerek, M. E.; Basolo, F. J. Am. Chem. Soc. 1984, 106, 5908. (b) Kakkar,
A. K.; Taylor, N. J.; Marder, T. J.; Shen, N.; Hallinan, N.; Basolo, F. Inorg.
Chim. Acta 1992, 198-200, 219. (c) Wescott, S. A.; Kakkar, A. K.; Stringer,
G.; Taylor, N. J.; Marder, T. B. J. Organomet. Chem. 1990, 394, 777.
(7) Calhorda, M. J.; Veiros, L. F. Coord. Chem. ReV. 1999, 185-186, 37.
(8) Stradiotto, M.; McGlinchey, M. J. Coord. Chem. ReV. 2001, 219, 311.
(1) For leading reviews, see: (a) The Metallocenes: Synthesis, ReactiVity and
Applications; Togni, A., Halterman, R. L., Eds.; Wiley-VCH: Weinheim,
1998. (b) Cotton, F. A. Acc. Chem. Res. 1968, 1, 257.
(2) Pauson, P. L.; Wilkinson, G. J. Am. Chem. Soc. 1954, 76, 2024.
(3) (a) Lobanova, I. A.; Zdanovich, V. I. Russ. Chem. ReV. 1988, 57, 967. (b)
Birmingham, J. M. AdV. Organomet. Chem. 1964, 2, 365. (c) Cotton, F.
A. Chem. ReV. 1955, 55, 551.
9
10.1021/ja052033q CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 10291-10304
10291

A R T I C L E S
Bradley et al.
extreme, the zirconium can be considered divalent with a neutral
benzo ligand, while in the other the benzo ring is reduced by
two electrons and serves as an LX2 ligand to a Zr(IV) center.21
The η9 interaction is related to the η4,η3 bonding motif observed
in dihydroindenyl titanium complexes (Figure 2), where the sp3
hybridization at the ring fusion facilitates interaction of the metal
with the diene fragment in the six-membered ring.22
The η9,η5-bis(indenyl)zirconium sandwich complexes are
dynamic in solution, undergoing rapid exchange of the η9 and
η5 indenyl rings. The barriers for ring interconversion range
between 13.8(3) and 19.9(5) kcal/mol at 296 K, depending on
the indenyl substituents, and have entropies of activation
consistent with a unimolecular dissociative process.19 These
results support the intermediacy of a symmetric η5,η5-bis-
(indenyl)zirconium sandwich, arising from dissociation of the
benzo portion of the carbocycle (eq 1). Computational studies
on the unsubstituted model complex, (η9-C9H7)(η5-C9H7)Zr,
support such a pathway with a singlet η5,η5-bis(indenyl)-
zirconium sandwich intermediate. The corresponding S ) 1
sandwich was calculated to be slightly more stable, but the
minimum energy crossing point of the two surfaces was
sufficiently high in energy such that ring interconversion takes
place solely on the singlet reaction coordinate.20
Figure 2. Participation of the benzo group in indenyl coordination.
η6 indenyl complexes of chromium can only be observed at
low temperature, undergoing facile migration to the η5 isomer
upon warming.10 Similar η6 to η5 haptotropic rearrangements
have been observed in flourenyl compounds of iron,11 chro-
mium,12 and manganese.13 Bonding of all nine carbons of the
indenyl ligand to transition metal centers also has precedent in
bimetallic complexes. Reduction of bis(indenyl)vanadium, (η5-
C9H7)2V, affords a dinuclear complex where the indenyl ligands
bridge the two metals and coordinate through the five membered
ring in one case and through the benzo portion of the carbocycle
in the other.14 A similar bonding motif has also been observed
in mixed valent titanium compounds.15 Bimetallic complexes
in which two metals occupy antifacial sites on a bridging indenyl
ligand have also been described.16,17
Recent investigations into the chemistry of low-valent bis-
(indenyl)zirconium complexes have established an unprec-
edented coordination mode for the indenyl ligand. Bis(indenyl)
zirconium sandwich complexes, prepared from either alkane
reductive elimination18 or alkali metal reduction reactions,19
contain η9 coordinated indenyl ligands in which a single metal
center is engaged in bonding to all nine carbon atoms (Figure
2). Independent computational studies concurrent with our
experimental work predicted the stability of the η9 interaction
in the low-valent zirconium sandwich.20 More detailed inves-
tigations into the electronic structure of these compounds from
Veiros20 and our laboratory19 are consistent with a bonding
description intermediate of two limiting canonical forms. In one
Addition of suitable π-basic ligands such as carbon monoxide,
olefins, or alkynes to the η9,η5-bis(indenyl)zirconium sandwich
complexes furnished familiar bent zirconocene derivatives where
the η5,η5 hapticity of the indenyl rings has been restored.18,19
Moreover, the sandwich compounds serve as convenient precur-
sors for classic Negishi-type coupling reactions of olefins and
alkynes.19 In contrast, addition of THF to (η9-C9H5-1,3-
(SiMe3)2)(η5-C9H5-1,3-(SiMe3)2)Zr resulted in an unusual mi-
gration of the metal to the benzo portion of the carbocycle,
forming the η6,η5-bis(indenyl)zirconium THF adduct, (η6-C9H5-
1,3-(SiMe3)2)(η5-C9H5-1,3-(SiMe3)2)Zr(THF) (eq 2).
(9) (a) Green, M. L. H.; Lowe, N. D.; O’Hare, D. J. Organomet. Chem. 1988,
355, 315. (b) For a related Ru example: Wheeler, D. E.; Bitterwolf, T. E.
Inorg. Chim. Acta 1993, 205, 123.
(10) Ceccon, A.; Gambaro, A.; Gottardi, F.; Santi, S.; Venzo, A. J. Organomet.
Chem. 1991, 412, 85.
(11) Johnson, J. W.; Treichel, P. M. J. Am. Chem. Soc. 1977, 99, 1427.
(12) Ustynyuk, N. A.; Oprunenko, Y. F.; Malyugina, S. G.; Trifonova, O. I.;
Ustynyuk, Y. A. J. Organomet. Chem. 1984, 270, 185.
(13) (a) Treichel, P. M.; Fivizzani, K. P.; Haller, K. J. Organometallics 1982,
1, 931. (b) Biagioni, R. N.; Luna, A. D.; Murphy, J. L. J. Organomet.
Chem. 1994, 476, 183.
In this Article, we describe a more detailed investigation into
the scope and mechanism of ligand-induced haptotropic rear-
rangement processes in zirconium sandwich compounds. Chelat-
ing diethers and a bis(phosphine) also induced ring migration,
demonstrating that haptotropic rearrangement is generally
(14) Jonas, K.; Ru¨sseler, W.; Kru¨ger, C.; Raabe, E. Angew. Chem., Int. Ed.
Engl. 1986, 25, 928.
(15) (a) Gauvin, F.; Britten, J.; Samuel, E.; Harrod, J. F. J. Am. Chem. Soc.
1992, 114, 1489. (b) Gyepes, R.; Cisarova, I.; Horacek, M.; Cejka, J.;
Petrusova, L.; Mach, K. Collect. Czech. Chem. Commun. 2000, 65, 1248.
(16) Green, M. L. H.; Lowe, N. D.; O’Hare, D. Chem. Commun. 1986, 1547.
(17) (a) Ceccon, A.; Gambaro, A.; Santi, S.; Valle, G.; Venzo, A. Chem.
Commun. 1989, 51. (b) Kakkar, A. K.; Jones, S. F.; Taylor, N. J.; Collins,
S.; Marder, T. B. Chem. Commun. 1989, 1454.
(21) For a description of the “L” and “X” formalism, see: Crabtree, R. H. The
Organometallic Chemistry of the Transition Metals, 2nd ed.; John Wiley:
New York, 1994; pp 24-38.
(22) (a) Tillack, A.; Baumann, W.; Ohff, A.; Lefeber, C.; Spannenberg, A.;
Kempe, R.; Rosenthal, U. J. Organomet. Chem. 1996, 520, 187. (b) Thomas,
D.; Peulecke, N.; Burlakov, V. V.; Heller, B.; Baumann, W.; Spannenberg,
A.; Kempe, R.; Rosenthal, U. Z. Anorg. Chem. 1998, 624, 919.
(18) Bradley, C. A.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2003, 125,
8110.
(19) Bradley, C. A.; Keresztes, I.; Lobkovsky, E.; Young, V. G.; Chirik, P. J.
J. Am. Chem. Soc. 2004, 126, 16937.
(20) Veiros, L. F. Chem.-Eur. J. 2005, 11, 2505.
9
10292 J. AM. CHEM. SOC. VOL. 127, NO. 29, 2005

Ligand-Induced Haptotropic Rearrangements
A R T I C L E S
no single valence bond depiction will accurately represent the
actual structure of the molecule. Because both structural data
and computational studies support an L2X2 benzo fragment
coordinated to the zirconium, we have opted for the valence
bond representations presented in Figure 3. While this formalism
requires charge separation and suggests a zwitterionic structure,
these representations are not meant to imply charge localization.
In fact, computational studies (vide infra) indicate equal charge
distribution over both the five- and the six-membered rings,
again highlighting the delocalized π-system in these ligands.
In general, the η6,η5 THF adducts were prepared by addition
of approximately 5 equiv of THF to the corresponding sandwich
complex and isolated as red-burgundy solids (eq 3). Monitoring
1
the reactions in situ by H NMR spectroscopy demonstrated
quantitative conversion to the desired products, although isolated
yields are lower because of the lipophilicity of the products.
Figure 3. Bis(indenyl)zirconium THF adducts, their shorthand designations,
and isolated yields.
observed with σ-type ligands. Kinetic studies were performed
to probe the nature of the transition structure for the ring
migration, while equilibration of the η9,η5-sandwiches with the
corresponding η6,η5 adducts provided a measure of the driving
force for these reactions.
Several of the haptotropic rearrangement reactions are worthy
of comment. As described previously,19 sandwich complex, 4,
has been prepared as an equimolar mixture of rac and meso
diastereomers. Accordingly, addition of THF furnished a near
equimolar mixture of the rac and meso isomers of the corre-
sponding adducts, rac and meso-4-THF (Figure 3). Attempts
to separate this mixture on a preparative scale by fractional
recrystallization have been unsuccessful. Examples of “mixed
ring” η6,η5-bis(indenyl) zirconium THF adducts have also been
prepared by addition of THF to 6 and 7. In both cases, a 2:1
mixture of the two possible haptomers was observed. Unfor-
tunately, assignment of the major and minor products by two-
dimensional NMR spectroscopy has not been possible due to
poor resolution of the indenyl resonances.
Results
Synthesis and Characterization of η6,η5-Bis(indenyl)-
zirconium THF Complexes. As was previously communicat-
ed,18 addition of a slight excess (∼3 equiv) of THF to the bis(in-
denyl)zirconium sandwich complex, (η9-C9H5-1,3-(SiMe3)2)(η5-
C9H5-1,3-(SiMe3)2)Zr (1),19 furnished the η6,η5 THF-adduct, (η6-
C9H5-1,3-(SiMe3)2)(η5-C9H5-1,3-(SiMe3)2)Zr(THF) (1-THF), as
a red, crystalline solid. Both solution and solid-state character-
ization established that one of the indenyl ligands had migrated
to the benzo portion of the carbocycle, representing a formal
ligand-induced haptotropic rearrangement.23 Since our initial
report, we have found that 1-THF can be directly synthesized
by reduction of (η5-C9H5-1,3-(SiMe3)2)2ZrCl2 (1-Cl2) with an
excess of 0.5% Na(Hg) in the presence of THF. Likewise,
reductive elimination of isobutane from (η5-C9H5-1,3-(SiMe3)2)2-
Zr(CH2CHMe2)H (1-(CH2CHMe2)H)18 in the presence of THF
also affords 1-THF as the sole organometallic product.
The generality of the THF-induced haptotropic rearrangement
process has subsequently been explored with a family of η9,η5
bis(indenyl)zirconium complexes containing both silyl and alkyl
substituents.19,24 All of the η6,η5-bis(indenyl)zirconium THF
adducts along with their isolated yields and shorthand designa-
tions are presented in Figure 3. At this point, it is worthwhile
to briefly comment on the valence bond depictions of the η6,η5
ligand-adducts used throughout the paper. A more detailed
description of the bonding in these compounds is deferred to
the Discussion section. It is important to keep in mind that the
10 π electron system of the indenyl ligand is delocalized and
THF addition to η9,η5-bis(indenyl)zirconium sandwich com-
plexes with dialkyl-substituted indenyl rings did not yield
detectable quantities of the corresponding η6,η5 adduct. For
example, dissolution of 5 in THF-d8 at ambient temperature
provided no evidence for haptotropic rearrangement by 1H NMR
spectroscopy. Additionally, cooling toluene-d8 solutions contain-
ing a large excess (>25 equiv) of THF to -78 °C also produced
no change. Attempts to access 5-THF by alkali metal reduction
of 5-Cl2 in THF furnished the sandwich complex, 5. Similar
observations were made with 8 and 9 (Figure 3). Reduction in
the presence of THF or addition of large quantities of THF to
the η9,η5-bis(indenyl)zirconium sandwiches produced no ob-
1
servable η6,η5 adducts by H NMR spectroscopy.
Solution and Solid-State Characterization of η6,η5-Bis-
(indenyl)zirconium THF Adducts. Characterization of the
η6,η5-bis(indenyl)zirconium THF adducts was accomplished by
a combination of multinuclear NMR spectroscopy, elemental
analysis, and, in two cases, X-ray diffraction. For 1-THF,
1
2-THF, and 3-THF, the H and 13C NMR spectra recorded in
benzene-d6 at 22 °C exhibit the number of indenyl resonances
anticipated for a Cs symmetric molecule with inequivalent
indenyl rings, consistent with η6,η5 hapticity. As a representative
(23) Albright, T. A.; Hofmann, P.; Hoffmann, R.; Lillya, C. P.; Dobosh, P. A.
J. Am. Chem. Soc. 1983, 105, 3396.
(24) Bradley, C. A.; Flores-Torres, S.; Lobkovsky, E.; Abrun˜a, H. D.; Chirik,
P. J. Organometallics 2004, 23, 5332.
1
example, the H and 13C NMR spectra of 1-THF at 22 °C are
9
J. AM. CHEM. SOC. VOL. 127, NO. 29, 2005 10293

A R T I C L E S
Bradley et al.
Figure 4. 13C (top) and 1H (bottom) NMR spectra of 1-THF in benzene-
d6 at 22 °C. Resonances labeled in blue are associated with the η6 ring,
while those in green are for the η5 ligand. Peaks labeled “B” designate
benzo resonances.
Figure 5. (a) Partially labeled view of the molecular structure of 2-THF
(left) and top view (right). (b) Partially labeled view of the molecular
structure of rac-4-THF (left) and top view (right). Top views of the
molecules with hydrogen atoms and silyl substituents (2-THF) omitted for
clarity. In both cases, the structures are represented with 30% probability
ellipsoids.
presented in Figure 4 and will be described in detail. Peak
assignments are based on the results of g-COSY, g-HSQC,
g-HMBC, and NOESY NMR experiments.
1
Diagnostic, anomalous shifts in both H and 13C spectra are
d6 or toluene-d8. For 1-THF, the carbon proximal to the ring
fusion in the η6 benzo ring is shifted upfield to 85.63 ppm,
consistent with significant reduction by the zirconium center
and development of metal alkyl character.26 The allylic carbon
on the five-membered ring on the η6 bound indenyl is shifted
downfield, resonating at 140.75 ppm as compared to 120.39
ppm for the η5 ring.
The hapticity of the indenyl ligands in 2-THF and rac-4-
THF has also been confirmed by single-crystal X-ray diffraction.
The solid-state structures are presented in Figure 5, while
selected metrical parameters, including those from 1-THF,18
are reported in Table 1. A labeled view of 1-THF has been
included as Figure S7 in the Supporting Information for
reference. For rac-4-THF, only one diastereomer was observed
in the crystal lattice and no disorder was detected in either the
silyl or the tert-butyl substituents, eliminating the possibility
of contamination from the meso isomer. The 1H NMR spectrum
of the bulk crystalline sample in toluene-d8 exhibited peaks for
both diastereomers, suggesting that crystallographic character-
ization of the pure rac isomer is most likely a result of fortuitous
crystal selection, although epimerization to a mixture of isomers
in solution cannot be ruled out.27
useful in identifying the η6,η5 bonding motif. A table compiling
these shifts is presented in the Supporting Information (Table
S1). In the case of 1-THF, two upfield shifted indenyl protons
are observed by 1H NMR spectroscopy. The singlet centered at
3.38 ppm has been assigned as the cyclopentadienyl hydrogen
on the η5 bound indenyl ring, whereas the second upfield
resonance, centered at 3.57 ppm, has been assigned as the benzo
hydrogen distal to the five-membered ring on the η6 indenyl.
Although the latter resonance is obscured by excess THF in
one-dimensional spectra, this peak is observed and readily
assigned in two-dimensional experiments. Significantly, the
anomalous upfield chemical shifts observed for these hydrogens
may not be a direct consequence of the indenyl ring hapticity.
Rather, they arise from transannular ring currents that are a result
of interactions with the π-system of the adjacent indenyl ring,
similar to previous observations made in indenyl iridium
compounds.25 In the current cases, the anti geometry18 of the
sandwich complexes orients the hydrogens ideally for these
interactions. Alternatively, Merola et al. have shown that large
1
changes in H NMR chemical shifts accompany η5 to η3 ring
slippage in iridium indenyl compounds, although the origin of
this effect could also be a result of replacing olefin ligands with
phosphine donors.5b
Diagnostic 13C shifts are also observed for 1-THF and related
η6,η5 adducts. Typically, η5 indenyl complexes of zirconium
exhibit 13C NMR shifts between 95 and 130 ppm in benzene-
As was observed in the solid-state structure of 1-THF,18 each
zirconium complex contains a molecule of THF coordinated in
the metallocene “wedge” essentially coplanar with the indenyl
(26) Wexler, P. A.; Wigley, D. E.; Koerner, J. B.; Albright, T. A. Organome-
tallics 1991, 10, 2319.
(27) Yoder, J. C.; Day, M. W.; Bercaw, J. E. Organometallics 1998, 17, 4946.
(25) Faller, J. W.; Crabtree, R. H.; Habib, A. Organometallics 1985, 4, 929.
9
10294 J. AM. CHEM. SOC. VOL. 127, NO. 29, 2005

Ligand-Induced Haptotropic Rearrangements
A R T I C L E S
Figure 6. Experimentally determined equilibrium constants for THF-induced haptotropic rearrangement as a function of indenyl substituent.
state structures of the η9,η5-bis(indenyl)sandwiches, 3 and 5.
Table 1. Selected Bond Distances (Å) and Angles (deg) for
1-THF, 2-THF, and rac-4-THF
Likewise, the C(2)-C(3) distances contract to ∼1.39 Å, while
compound
1-THF
2-THF
rac-4-THF
the C(1)-C(6) bond lengths expand to 1.44-1.48 Å. The
distortions in the η6 benzo ring are consistent with reduction of
the arene by the formally low-valent zirconium center.26
Zr(1)-O(1)
2.254(2)
2.2304(12)
2.259(2)
Zr(1)-C(1)
Zr(1)-C(2)
Zr(1)-C(3)
Zr(1)-C(4)
Zr(1)-C(5)
Zr(1)-C(6)
2.340(2)
2.472(3)
2.493(2)
2.355(3)
2.468(3)
2.487(3)
2.3540(17)
2.4796(17)
2.4476(17)
2.3573(17)
2.4914(17)
2.4882(17)
2.328(7)
2.474(8)
2.472(8)
2.358(8)
2.496(7)
2.421(7)
The displacement of the zirconium-oxygen bond vector from
the midpoint of the metallocene wedge (Table 1) provides a
measure of the relative steric influence of the indenyl substit-
uents. For Cs symmetric metallocenes such as 1-THF and
2-THF, the Zr-O deviations from the midpoint of the wedge
are close to 0°, a consequence of the identical steric environ-
ments on each side of the metallocene. In the case of rac-4-
THF, the THF ligand is displaced away from the [CMe3] groups
by 12.6(2)°, a result of the shorter C-C bond distance that
places the bulk of the substituent closer to the metal center.
Zr(1)-C(10)
Zr(1)-C(11)
Zr(1)-C(12)
Zr(1)-C(13)
Zr(1)-C(14)
2.623(3)
2.501(3)
2.454(3)
2.494(2)
2.608(3)
2.4913(17)
2.4396(17)
2.4906(17)
2.6237(17)
2.6227(17)
2.498(7)
2.466(7)
2.523(7)
2.591(7)
2.610(7)
C(1)-C(2)
C(1)-C(6)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
1.425(4)
1.458(4)
1.394(4)
1.414(4)
1.466(3)
1.444(3)
1.425(2)
1.452(2)
1.394(3)
1.426(3)
1.450(2)
1.448(2)
1.399(4)
1.479(10)
1.392(11)
1.395(11)
1.462(10)
1.441(10)
Kinetics and Thermodynamics of THF Coordination. The
observation of ring migration in the η9,η5-bis(indenyl)zirconium
sandwich complexes upon addition of THF prompted further
investigation into both the kinetics and the thermodynamics of
the haptotropic rearrangement process. Previous studies19 have
shown that ring exchange in the η9,η5-bis(indenyl)zirconium
sandwich occurs with activation barriers ranging between
13.8(3) and 19.9(5) kcal/mol at 23 °C, depending on the indenyl
substituents. To determine if a similar process was operative in
the η6,η5-bis(indenyl)zirconium THF adducts, a qualitative
EXSY NMR experiment was carried out on the equilibrium
mixture formed by addition of a stoichiometric quantity of THF
to 1 in benzene-d6 at 23 °C. The downfield portion of the EXSY
spectrum (200 ms mixing time) is presented in the Supporting
Information (Figure S1). Cross-peaks are observed between the
benzo, cyclopentadienyl, and trimethylsilyl resonances for the
η9 and η5 rings in 1, consistent with the previously reported
ring interchange process.19 Exchange is also observed between
the free and coordinated THF, suggesting a rapid equilibrium
involving dissociation and recomplexation on the NMR time
scale. Importantly, no correlations are observed between the
distinct ligand resonances in 1-THF, establishing that once THF
is coordinated to the zirconium and haptotropic rearrangement
has occurred, the rings are static on the NMR time scale.
dihedral anglea
rotational angleb
Zr-O deviationc
19.0(1)
177.8(1)
2.5(2)
17.8(1)
178.4(1)
2.0(1)
18.0(2)
176.3(2)
12.6(2)
a Dihedral angle for the planes formed by C(1)-C(2)-C(3)-C(4) and
C(1)-C(6)-C(5)-C(4). b Angle formed between the planes defined by the
midpoint of C(5)-C(6), C(8), and Zr(1) and the C(13)-C(14) midpoint,
C(11), and Zr(1). c Angle formed between the plane defined by Zr(1), C(9),
and C(11) and the line defined by Zr(1) and O(1).
rings. This orientation, while minimizing the potential for
π-bonding, is sterically preferred, allowing the THF ring carbons
to avoid steric interactions with the indenyl substituents. The
zirconium-oxygen bond distances vary between 2.2304(12) and
2.259(2) Å and are well within the range typically observed
for both neutral and cationic zirconocene-THF complexes.28
Unlike the dichloride precursors where primarily gauche indenyl
conformations are observed,24 the rotational angles in 2-THF
and rac-4-THF approach 180° (Table 1) and are consistent with
an anti ligand arrangement that serves to minimize transannular
steric interactions between the bulky indenyl substituents.
In each structure, the η6 bound indenyl ligand is puckered
with dihedral angles ranging from 17.8° to 18.0° for the angle
formed between the planes defined by C(1)-C(2)-C(3)-C(4)
and C(1)-C(6)-C(5)-C(4). This ring buckle is accompanied
by localization in the six-membered ring with Zr(1)-C(1) and
Zr(1)-C(4) distances ranging between 2.32 and 2.36 Å,
contracted from the values of 2.48-2.62 Å observed in the
corresponding five-membered ring. The ring buckle observed
in the η6,η5 THF adducts is more significant than the corre-
sponding distortions of 4.9(2)° and 6.3(2)° observed in the solid-
Because THF coordination is rapid and reversible in benzene-
d6 at 23 °C, the observed distribution of sandwich complex and
THF adduct is at equilibrium and allows measurement of the
thermodynamic driving force for THF-induced haptotropic
rearrangement. Equilibrium constants were measured as a
function of indenyl substituent at 10 °C in toluene-d8 by
integration of the resonances for the sandwich complex, the THF
adduct, and free THF. The results of this study are presented in
Figure 6.
(28) (a) Jordan, R. F.; Bajgur, C. S.; Willett, R.; Scott, B. J. Am. Chem. Soc.
1986, 108, 7410. (b) Crowther, D. J.; Jordan, R. F.; Baenziger, N. C.;
Verma, A. Organometallics 1990, 9, 2574. (c) Jordan, R. F.; LaPointe, R.
E.; Bradley, P. K.; Baenziger, N. C. Organometallics 1989, 8, 2892.
The temperature dependence on the equilibrium constants for
THF-induced haptotropic rearrangement was examined for both
9
J. AM. CHEM. SOC. VOL. 127, NO. 29, 2005 10295

A R T I C L E S
Bradley et al.
1 and 7. Construction of van’t Hoff plots (Figure S2) provided
a ∆H° of -11.3(2) kcal/mol and a ∆S° of -32(4) eu (7 points,
30° range, R2 ) 0.991) for 1 and a ∆H° of -14.2(5) kcal/mol
and a ∆S° of -43(8) eu (8 points, 40° range, R2 ) 0.987) for
7. The large, negative values for ∆S° are consistent with an
addition reaction.
concentration of 1-THF and the experimentally determined
reaction rates. Using this approach, values of 31(3) and 32(3)
s-1 were determined for k-1 in the 0.5 and 1.0 equiv of THF
reactions, respectively. Having computed the value of k-1 and
independently measured the equilibrium constant at 23 °C, the
values for the forward rate constants, k1 and hence kobs where
kobs ) k1[THF], can be extracted. For the 0.5 equiv reaction, a
value of kobs of 12(2) s-1 was computed, while for the 1.0 equiv
With the thermodynamic parameters in hand, the kinetics of
THF addition were investigated. The THF dependence on the
rate of the reaction was desired to establish the rate law for the
haptotropic rearrangement process. Experimental evaluation of
the order in THF is challenging, as the equilibrium between
the sandwich and the THF adduct is established rapidly upon
mixing at -78 °C. As a result, the solution behavior of the
equilibrium mixture of 1 and 1-THF was evaluated using
dynamic NMR techniques over a range of THF concentrations.
Selective, one-dimensional EXSY NMR experiments29 were
used to quantitate the rate of THF addition to the η9,η5-bis-
(indenyl)zirconium sandwich, 1. It was necessary to conduct
these experiments in a narrow range of low THF concentrations
to allow simultaneous observation of both the sandwich
complex, 1, and the adduct, 1-THF. The dynamic NMR
experiments were conducted on equilibrium mixtures of 1 and
1-THF at 23 °C with 0.5 and 1.0 equiv of THF (eq 4). In each
experiment, the concentration of 1-THF was fixed at 0.018 M.
Because THF exchange is competitive (vide infra), only
resonances associated with the indenyl ligands in both zirconium
compounds were used in this analysis. The rate of the reverse
reaction, dissociation of THF from 1-THF, was determined by
selective excitation of the cyclopentadienyl resonances in 1-THF
and plotting the normalized cross-peak areas as a function of
mixing time (Figure S3). The slope of the linear portion of the
curve, prior to attenuation due to relaxation, provided the rate
of the reaction. As expected for a first-order process with equal
1-THF concentrations, statistically invariant rates of 0.46(2) and
0.48(2) M-1 s-1 were measured for the 0.5 and 1.0 equiv
reactions, respectively.
process, kobs was determined as 22(2) s-1
.
Because data are limited due to the experimental difficulties
in measuring reaction rates at variable THF concentrations, an
independent determination of kobs was sought. The forward rates
of the reaction were measured directly to extract the desired
rate constants. Determination of the equilibrium concentration
of 1 in combination with the experimentally determined rates
of reaction furnished second-order rate constants (k1) of 681-
(20) and 697(20) M-1 (s-1)2 for the 0.5 and 1.0 equiv reactions,
respectively. From these values, kobs was found to be 13(2) and
23(2) s-1, respectively, in agreement with the values obtained
from the method described in the previous section.
The kinetics of THF exchange in 1-THF were also exam-
ined. Magnetization transfer NMR experiments30 have proven
particularly useful in probing the mechanism of ligand ex-
change in organometallic chemistry.31 To determine the rates
of THF exchange, separate experiments were conducted in
which the resonances for either free or complexed THF were
selectively inverted using an IBURP232 shaped pulse and the
intensity of the complementary peak was monitored by 1H NMR
spectroscopy as a function of time. The rate of exchange be-
tween the two THF molecules was then modeled with the com-
puter program CIFIT31b and the rate constants extracted from
the fits. This analysis also provided calculated T1 values that
were in agreement with independently measured relaxation
times.
Experiments were conducted at 0.035 M zirconium at 23 °C
with variable amounts of THF ranging between 1 and 25 equiv.
A plot of the observed rate constants versus [THF] concentration
is presented in Figure 7 and is linear with a positive, nonzero
intercept. When greater than 5 equiv of THF is present, only
1
1-THF and free THF are observed by H NMR spectroscopy
with no detectable contribution from 1. EXSY NMR experi-
ments, in addition to magnetization transfer studies, indicate
that exchange between free and coordinated THF remains rapid
on the NMR time scale. The linear relationship between kobs
and the concentration of THF is consistent with an associative
process for THF exchange, proceeding through a coordinatively
saturated bis(THF) intermediate or transition structure, (η6-C9H5-
1,3-(SiMe3)2)(η5-C9H5-1,3-(SiMe3)2)Zr(THF)2 (eq 5). Attempts
to detect such an intermediate at high THF concentrations (>100
equiv) have proven unsuccessful.
The rates of the forward reactions with 0.5 and 1.0 equiv of
THF were measured using a similar procedure. In these cases,
selective irradiation of the cyclopentadienyl peaks in 1 followed
by plotting the normalized cross-peak area as a function of
mixing time provided the desired rates (Figure S3). From this
procedure, rates of 0.45(2) and 0.46(2) M-1 s-1 were measured
for the 0.5 and 1.0 equiv reactions, respectively, and are in
excellent agreement with the independent measurement of the
reverse rates as expected for a reaction at chemical equilibrium,
where rateforward ) ratereverse
.
These independent measures of reaction rates allow calcula-
tion of the respective rate constants for each reaction and hence
the order in THF. Assuming a first-order disappearance of
1-THF, the rate constants for the reverse reaction (k-1) could
be computed in a straightforward manner given the equilibrium
Ligand-Induced Haptotropic Rearrangement with Di-
ethers and 1,2-Bis(dimethyl)phosphinoethane. The observa-
(29) Stott, K.; Keeler, J.; Van, Q. N.; Shaka, A. J. J. Magn. Reson. 1997, 125,
302.
(30) Morris, G. A.; Freeman, R. J. Magn. Reson. 1978, 29, 433.
9
10296 J. AM. CHEM. SOC. VOL. 127, NO. 29, 2005

Ligand-Induced Haptotropic Rearrangements
A R T I C L E S
Figure 8. Molecular structure of 1-DME at 30% probability ellipsoids.
Hydrogen atoms are omitted for clarity (left). Top view of the molecule
with hydrogen atoms and silyl substituents omitted for clarity (right).
Figure 7. Plot of kobs versus [THF] for THF exchange with 1-THF.
tion of THF-induced haptotropic rearrangement prompted
exploration of the reactivity of η9,η5-bis(indenyl)zirconium
sandwich complexes with other σ-donors. The kinetic identifica-
tion of a formally 18 electron, coordinatively saturated η6,η5-
bis(indenyl)zirconium bis(THF) intermediate (or transition
structure) suggested that addition of potentially chelating diethers
may also induce haptotropic rearrangement and furnish isolable
η6,η5 adducts. Addition of 1 equiv of 1,2-dimethoxyethane
(DME) to either 1 or 2 in pentane furnished the dark green η6,η5-
bis(indenyl)zirconium DME adducts, (η6-C9H5-1,3-(SiMe2R)2)-
(η5-C9H5-1,3-(SiMe2R)2)Zr(MeOCH2CH2OMe) (R ) Me,
1-DME; R ) Ph, 2-DME) (eq 6). In contrast to the THF
complexes,addition of a stoichiometric quantity of DME resulted
in complete conversion to the η6,η5-adduct.
resulting from hydrogens that are syn and anti with respect to
the η6 (or η5) indenyl ligand. This phenomenon is not observed
for the methylene hydrogens in the η6,η5 THF adducts, because
of the rapid dissociation and recoordination of the ligand on
the NMR time scale, resulting in broadened signals for the
backbone protons.
EXSY NMR experiments on mixtures of 1-DME in the
presence of excess diether produce no cross-peaks at 23 °C,
establishing the lack of ligand exchange on the NMR time scale
at this temperature. However, chemical exchange is observed
over the course of minutes at ambient temperature upon addition
of 1,2-dimethoxyethane-d10 to a benzene-d6 solution of 1-DME
(eq 7).
The solid-state structure of 1-DME was confirmed by single
crystal X-ray diffraction, and one representation of the molecule
is presented in Figure 8, while the others are contained in the
Supporting Information. Selected metrical parameters are re-
ported in Table 2. Three independent molecules were found in
the asymmetric unit, one of which is positionally disordered
with respect to the orientation of the carbon backbone of the
DME ligand. The orientation of the indenyl ligands in the solid-
state structure of 1-DME is similar to that observed in 1-THF,
with a rotational angle of 176.9(3)°, indicative of an anti
arrangement. Comparison of the metrical parameters for 1-DME
to those for 1-THF reveals the ability of the “haptoflexible”
indenyl ligand to adjust its coordination to meet the electronic
requirements of the metal center. For 1-DME, the zirconium-
carbon bonds to the η6-benzo ring are in general slightly longer
than those in 1-THF and reflect saturation at the metal center
imparted by coordination of a second oxygen atom. Elongated
Zr(1)-C(5) and Zr(1)-C(6) bond distances of 2.556(3) and
2.584(3) Å are observed, respectively. For comparison, the
corresponding distances are 2.468(3) and 2.487(3) Å in 1-THF.
The benzo ring in 1-DME is puckered by 20.4(4)°, indicating
significant reduction by the zirconium center. The Zr-O
1
The H and 13C NMR spectra of both 1-DME and 2-DME
exhibit the diagnostic features associated with η6,η5 coordination
of the indenyl rings. Because the spectra for both compounds
are similar, only the data for 1-DME will be described in detail.
The spectroscopic data for 2-DME is reported in the Supporting
Information. Green benzene-d6 solutions of 1-DME exhibit the
number of resonances expected for a Cs symmetric molecule
with inequivalent indenyl ligands and two bound oxygens.
Upfield shifted cyclopentadienyl and benzo resonances are
observed at 4.11 and 3.85 ppm, respectively, consistent with
transannular ring currents shielding the protons25 or hapticity
change.5b As with the THF adducts, diagnostic 13C resonances
are also observed, indicative of an η6 indenyl ligand. The benzo
carbon proximal to the ring fusion appears upfield at 82.66 ppm,
consistent with reduction of the six-membered ring,26 while a
downfield resonance centered at 140.4 ppm is observed for the
allylic cyclopentadienyl carbon. Distinct methylene protons are
1
also observed at 1.46 and 1.51 ppm in the H NMR spectrum
(31) (a) Sanford, M. J.; Love, J. A.; Grubbs, R. H. J. Am. Chem. Soc. 2001,
123, 6543. (b) Bain, A. D.; Kramer, J. A. J. Magn. Reson. 1996, 118A, 21.
(32) Geen, H.; Freeman, R. J. Magn. Reson. 1991, 93, 93.
9
J. AM. CHEM. SOC. VOL. 127, NO. 29, 2005 10297

A R T I C L E S
Bradley et al.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
1-DME
Zr(1)-O(1)
Zr(1)-O(2)
2.326(3)
2.305(2)
Zr(1)-C(1)
Zr(1)-C(2)
Zr(1)-C(3)
Zr(1)-C(4)
Zr(1)-C(5)
Zr(1)-C(6)
2.368(4)
2.469(4)
2.448(4)
2.340(3)
2.556(3)
2.584(3)
The scope of ligand-induced haptotropic rearrangement was
also studied with other diethers. The silylated bis(indenyl)-
zirconium sandwich, 1, was chosen for the majority of subse-
quent studies because of its relative ease of preparation, high
symmetry, and proclivity to participate in haptotropic rearrange-
ment processes. Addition of 1,2-diethoxyethane (DEE) to a
pentane solution of 1 furnished a green solid identified as
(η6-C9H5-1,3-(SiMe3)2)(η5-C9H5-1,3-(SiMe3)2)Zr(EtOCH2CH2-
OEt) (1-DEE) in 58% yield (eq 9). As with the THF adducts,
dissolution of pure 1-DEE in benzene-d6 at 23 °C afforded an
equilibrium mixture of sandwich 1, free DEE, and the η6,η5
adduct, 1-DEE. Integration of the resonances for each species
in solution yielded an equilibrium constant of 11(2) M-1 at 23
°C, significantly smaller than that observed for 1-DME.
Zr(1)-C(10)
Zr(1)-C(11)
Zr(1)-C(12)
Zr(1)-C(13)
Zr(1)-C(14)
2.508(3)
2.433(4)
2.516(3)
2.649(3)
2.643(3)
C(1)-C(2)
C(1)-C(6)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
1.412(5)
1.457(5)
1.394(5)
1.412(5)
1.455(5)
1.432(5)
dihedral anglea
rotational angleb
20.5(3)
176.9(3)
a Dihedral angle for the planes formed by C(1)-C(2)-C(3)-C(4) and
C(1)-C(6)-C(5)-C(4). b Angle formed between the planes defined by the
midpoint of C(5)-C(6), C(8), and Zr(1) and the C(13)-C(14) midpoint,
C(11), and Zr(1).
Table 3. Half-Lives for the Addition of DME to 1 at -65 °C in
Toluene-d8
[DME] (M)
equiv
half life (s)a
0.10
0.33
1.25
1.2
4.0
15
4445
1050
263
a All reactions were carried out with a 0.083 M solution of 1.
Similar experiments were conducted with 2,3-dimethoxybu-
tane (DMB). Addition of an excess of a near equimolar mixture
of the rac and meso diasteromers of the diether furnished a green
solid identified as (η6-C9H5-1,3-(SiMe3)2)(η5-C9H5-1,3-(SiMe3)2)-
Zr(rac-MeOCH(Me)CH(Me)OMe ) (1-rac-DMB), arising from
preferential coordination of the rac-isomer (Figure 9). Perform-
ing a similar experiment with the pure meso diastereomer also
induced haptotropic rearrangement to yield (η6-C9H5-1,3-(Si-
Me3)2)(η5-C9H5-1,3-(SiMe3)2)Zr(meso-MeOCH(Me)CH(Me)-
OMe) (1-meso-DMB). Exposure of the compound to vacuum
resulted in loss of the diether and regeneration of the η9,η5-
bis(indenyl)zirconium sandwich (Figure 9). Identification of
selective rac coordination was readily accomplished by 1H and
13C NMR spectroscopy. In benzene-d6, 1-rac-DMB exhibits the
number of resonances anticipated for a C1 symmetric compound.
For 1-meso-DMB, two isomeric Cs symmetric compounds are
observed in a 2:1 ratio. Unfortunately, the small chemical shift
dispersion of the resonances for each isomer prohibited iden-
tification of the major and minor products by two-dimensional
NMR spectroscopy.
Methylene bridged diethers were also effective for inducing
haptotropic rearrangement. Addition of a slight excess of
dimethoxymethane (DMM) to either 1 or 2 allowed observation
of the η6,η5-bis(indenyl)zirconium diether adducts (eq 10).
Characterization of the (η6-C9H5-1,3-(SiMe2R)2)(η5-C9H5-1,3-
(SiMe2R)2)Zr(DMM) (R ) Me, 1-DMM; Ph, 2-DMM) adducts
was accomplished by multinuclear NMR spectroscopy, and these
compounds exhibit overall Cs symmetry and diagnostic upfield
shifted indenyl hydrogens (Table S1). Unlike the corresponding
DME adducts, 1-DMM and 2-DMM are unstable to vacuum,
reverting to the respective sandwich complex upon removal of
distances of 2.326(3) and 2.305(2) Å in 1-DME are slightly
longer than the corresponding value of 2.254(2) Å found in
1-THF.
1
Monitoring the addition of DME to 1 by H NMR spectro-
scopy in toluene-d8 at -65 °C revealed a much slower approach
to equilibrium than the THF-induced haptotropic rearrangement.
Even with 1 equiv of THF, the equilibrium between 1 and
1-THF is established rapidly upon mixing at -78 °C. In
contrast, the addition of a stoichiometric quantity of DME to 1
required over 2 h to reach complete conversion at -65 °C.
Measuring the half-lives for the conversion of 1 to 1-DME at
various concentrations of DME revealed an approximate first-
order dependence on the rate of the reaction on the incoming
diether (Table 3). More definitive measures of observed rate
constants could not be obtained due to overlapping resonances
1
in the low temperature H NMR spectrum and the rapid rates
of the reactions at high concentrations of DME. Despite these
experimental complications, there is clearly a DME-dependence
on the rate of the reaction and these haptotropic rearrangements
are much slower than those with THF.
The increased stability of 1-DME relative to 1-THF sug-
gested that a chelating diether may be sufficiently potent to
induce haptotropic rearrangement in an η9,η5 bis(indenyl)-
zirconium sandwich containing a dialkylated indenyl ligand.
Treatment of 5 with a large excess (>15 equiv) of DME allowed
observation of (η6-C9H5-1,3-(CHMe2)2)(η5-C9H5-1,3-(CHMe2)2)-
1
Zr(DME) (5-DME) by H and 13C NMR spectroscopy (eq 8).
Removal of the excess DME resulted in regeneration of the
sandwich 5, complicating isolation of 5-DME in the solid state.
9
10298 J. AM. CHEM. SOC. VOL. 127, NO. 29, 2005

Ligand-Induced Haptotropic Rearrangements
A R T I C L E S
Figure 9. Selective complexation of rac-2,3-dimethoxybutane to 1 (top) and reversible coordination of the meso diastereomer (bottom).
the solvent. In the presence of a large excess of diethoxymethane
(DEM), only trace quantities of (η6-C9H5-1,3-(SiMe3)2)(η5-C9H5-
1,3-(SiMe3)2)Zr(DEM) (1-DEM) are detected (eq 10).
solution of 1 furnished a red solid identified as (η6-C9H5-1,3-
(SiMe3)2)(η5-C9H5-1,3-(SiMe3)2)Zr(DMPE) (1-DMPE) in 71%
isolated yield (eq 12). As with other η6,η5 ligand adducts, the
1H and 13C NMR spectra of 1-DMPE in benzene-d6 exhibit
the number of peaks consistent with a Cs symmetric molecule
with upfield resonances observed at 4.52 and 4.30 ppm.
Additionally, a singlet centered at -1.78 ppm is observed by
31P NMR spectroscopy for the equivalent phosphorus atoms.
The equilibrium constant for DMPE-induced haptotropic rear-
rangement is large, as dissolution of isolated crystals of
1-DMPE in benzene-d6 does not result in loss of phosphine
and regeneration of the sandwich, 1.
Having observed ligand-induced haptotropic rearrangement
with THF and chelating diethers, ring migration with simple
alkyl ethers was also explored. These molecules are expected
to be weaker ligands and hence less likely to furnish isolable
η6,η5-bis(indenyl)zirconium ether adducts because of the steric
inaccessibility of the oxygen. Attempts to observe similar
adducts with common dialkyl ethers such as diethyl ether,
methyl n-butyl ether, or methyl tert-butyl ether resulted in
carbon-oxygen bond scission without observation of a discrete
η6,η5 adduct.33 Addition of a large excess (∼50 equiv) of
dimethyl ether to a benzene-d6 solution of 1 at 23 °C allowed
observation of the η6,η5 adduct, (η6-C9H5-1,3-(SiMe3)2)(η5-
C9H5-1,3-(SiMe3)2)Zr(OMe2) (1-OMe2), at relatively short (∼10
min) reaction times (eq 11). Over time, 1-OMe2 undergoes
reversible C-H activation of the ether to afford (η5-C9H5-1,3-
(SiMe3)2)2Zr(CH2OCH3)H.33
Performing an EXSY NMR experiment on a mixture of 1-
DMPE in the presence of excess DMPE did not produce cross-
peaks between free and coordinated phosphine at 23 °C, dem-
onstrating no exchange between the two species on the NMR
time scale at this temperature. Interestingly, cross-peaks are
observed between the inequivalent phosphorus methyl groups,
consistent with a syn:anti exchange with respect to the η6 (or
η5) indenyl ring. A similar dynamic process is operative for
the hydrogens on the carbon backbone, as cross-peaks are ob-
served between the methylene resonances centered at 0.15 and
0.30 ppm, respectively. Because substitution of the coordinated
DMPE by free phosphine is not competitive on the NMR time
scale, dissociation of one arm of the chelate followed by fast
C-C bond rotation and recoordination is responsible for the
site exchange (eq 13). Another possibility is fast Zr-P bond
rotation, which must be followed by inversion at zirconium and
readdition of the phosphine. A similar methyl exchange process
The possibility of ligand-induced haptotropic rearrangement
was also explored with other σ-donating ligands. Addition of 1
equiv of 1,2-bis(dimethyl)phosphinoethane (DMPE) to a pentane
(33) The results of these studies are the subject of a forthcoming publication.
Bradley, C. A.; Chirik, P. J., unpublished results.
9
J. AM. CHEM. SOC. VOL. 127, NO. 29, 2005 10299

A R T I C L E S
Bradley et al.
has also been observed with [(η5-C5Me5)ZrMe2(THF)(DMPE)]-
Table 4. Equilibrium Constants for the Coordination of Chelating
Diethers, THF, and DMPE to 1 at 23 °C in Benzene-d6
BF4.28b
1
a
abbreviation
Keq (M-
)
EtOCH2OEt
MeOCH2OMe
DEM
DMM
0.3
5
EtOCH2CH2OEt
DEE
11
meso-MeOCH(Me)CH(Me)OMe
THF
rac-MeOCH(Me)CH(Me)OMe
MeOCH2CH2OMe
Me2PCH2CH2PMe2
meso-DMB
THF
rac-DMB
DME
13
22
200
>200
>200
These findings prompted further analysis of the solution
behavior of 1-DME. At 23 °C, the benzene-d6 EXSY NMR
spectrum of the molecule in the presence of excess DME
provided no evidence for intermolecular exchange or dissocation
of one arm of the chelate. In agreement with these observations
is the appearance of the hydrogens on the carbon backbones in
each adduct. For 1-DME, sharp, well-defined multiplets are
observed, consistent with a non-first-order splitting pattern
arising from coupling to adjacent and neighboring hydrogens.
The corresponding resonances in 1-DMPE are broadened at 23
°C, consistent with rapid exchange on the time scale of the NMR
experiment.
Because both DMPE and DME bind tightly, 1H NMR spec-
troscopy is not a useful analytical tool for determining the rel-
ative propensity of each ligand to induce haptotropic rearrange-
ment. To address this issue, competition experiments were con-
ducted. The equilibrium presented in eq 14 was approached from
both sides, where free DMPE was added to 1-DME and DME
added to 1-DMPE. From these experiments, an equilibrium
constant of 1.4(1) was measured at 23 °C, indicating a slight
preference for DMPE-induced haptotropic rearrangement.34 The
experimentally determined equilibrium constants for ligand-
induced haptotropic rearrangement measured for addition of the
diethers, THF, and DMPE to 1 are compiled in Table 4.
DMPE
a Determined at 23 °C, errors are (5%.
limiting resonance structures for the formally 16 electron, η6,η5
adducts. Both A and B are formally zwitterionic structures with
a positive charge on the metal and a negative charge on the
allylic portion of the η6 ring. Canonical forms C and D contain
a neutral five-membered ring and zirconium center. In A, the
benzo ring is formulated as an L2X2-type arene ligand, a
structural type that has been observed in low-valent group 4
and 5 transition metal chemistry.35 Representation B is the
formally divalent version of A, where the η6 benzo ring is
considered as a neutral fragment. The zirconium in resonance
structure C is also formally divalent with an L2X benzo ligand
for the η6 indenyl, similar to transition metal pentadienyl
complexes.36 Application of the Dewar-Chatt-Duncanson
model37 leads to structure C, where the benzo ring of the η6
ligand is formally an LX3 interaction resulting in a tetravalent
zirconium center. Other positional variants of these resonance
structures are important contributors to the hybrid but are not
shown in Figure 10 for simplicity. Shorthand designators for
the two classes of resonance structures are shown below the
individual contributors.
The structural data on the crystallographically characterized
compounds, 1-THF, 2-THF, rac-4-THF, and 1-DME, offer
experimental support for an L2X2 benzo ligand as described by
structure A. While this model has its shortcomings, its resem-
blance to the experimental data was the reason it was chosen
as the preferred form in representing the molecules throughout
the paper. In the solid state, the carbon atoms adjacent to the
ring fusion, C(1) and C(4), have the shortest bonds to zirconium,
on the order of 2.3 Å, contracted approximately 0.2-0.3 Å from
the other four Zr-C bonds to the ring. Concomitant with this
shortening is a 17.8-20.5° buckling of the benzo ligand, which
is accompanied by contraction of C(2)-C(3), consistent with
some degree of localization arising from ring reduction. Full
molecule DFT calculations on 1-THF are generally in agreement
with this bonding picture, and a partial molecule orbital diagram
Discussion
Having isolated and characterized a family of η6,η5-bis-
(indenyl)zirconium ligand adducts with a bis(phosphine) and a
series of oxygen donors, it is worthwhile to begin with a brief
discussion of the bonding and appropriate valence bond
designations for these molecules. Figure 10 presents a series of
(35) (a) Ozerov, O. V.; Ladipo, F. T.; Patrick, B. O. J. Am. Chem. Soc. 2000,
122, 6423. (b) Yoon, M.; Lin, J.; Young, V. G.; Miller, G. J. J. Organomet.
Chem. 1996, 507, 31. (c) Wexler, P. A.; Wigley, D. E.; Koerner, J. B.;
Albright, T. A. Organometallics 1991, 10, 2319.
(36) Ernst, R. D. Comments Inorg. Chem. 1999, 21, 285.
(37) (a) Dewar, M. J. S. Bull. Soc. Chim. Fr. 1951, 18, C71. (b) Chatt, J.;
Duncanson, L. A. J. Chem. Soc. 1953, 2939.
(34) The free energy of formation of free DME and DMPE was not considered
in this analysis, and the observed equilibrium constant could be driven by
formation of the free diether or bis(phosphine).
9
10300 J. AM. CHEM. SOC. VOL. 127, NO. 29, 2005

Ligand-Induced Haptotropic Rearrangements
A R T I C L E S
Figure 10. Examples of different valence bond representations of η6,η5-bis(indenyl)zirconium ligand adducts.
Table 5. Experimentally Determined Equilibrium Constants for
THF Coordination in Toluene-d8
1
a
sandwich
Keq (M-
)
ν
(CO)b
E°
(mV)c
1
2
3
4
5
7
8d
>100e
30
0.6
<0.1f
50
1926
1928
1925
1920
1906
1925
-118
-20
-122
-153
-250
ND
a Determined at 10 °C, errors in the values are (5%. b Average IR
stretching frequencies of (IndRR2)2Zr(CO)2 complexes determined in pentane.
Data taken from ref 24. c Oxidation potential of the corresponding diben-
zoferrocene. Data taken from ref 24. d Value determined from extrapolation
of a van’t Hoff plot. e Complete conversion to 2-THF observed. f No THF
coordination detected.
complex, (η6-C9H7)(η5-C9H7)Zr(THF). The diamagnetic η6,η5
adduct was computed to be a kinetic product that is trapped in
a potential energy well from converting to the thermodynami-
cally preferred (η5-C9H7)2Zr(THF) complex with a triplet (S )
1) ground state.20 The difference in the relative stability of these
two isomers was rationalized on the basis of stronger zirconium
indenyl bonding in the case of η5 hapticity. Thus far, attempts
to experimentally verify this computational prediction have
proven unsuccessful. In all cases examined, the η6,η5 adducts
have been isolated from a variety of different synthetic
pathways: direct THF addition to the η9,η5 sandwich, alkane
reductive elimination, or alkali metal reduction, both performed
in the presence of THF. The discrepancy between the experi-
mental and theoretical results may be a result of a change in
the energy profile by the addition of sterically demanding
substituents versus the model complex or a high kinetic barrier
that has yet to be surmounted.
Figure 11. DFT computed highest occupied molecular orbital for 1-THF.
is presented in the Supporting Information (Figure S8) for
reference. The computed HOMO (Figure 11) suggests an L2X2
2
benzo ligand arising from interaction of a principally dz
zirconium orbital with the appropriate linear combination of
benzo p orbitals. Significantly, the major lobe of the zirconium
orbital interacts with the in-phase combination of p orbitals on
C(1) and C(4), while the toroid combines with the p orbitals
on C(2)-C(3) and C(5)-C(6) that are bonding with respect to
one another but opposite phase of those on C(1) and C(4).
While both structural data and the HOMO of 1-THF support
the importance of structure A (Figure 10), these results should
be treated with caution. A Natural Population Analysis (NPA)38
reported by Veiros20,39 on the model compound, (η6-C9H7)(η5-
C9H7)Zr(THF), reveals equal charge distribution among the
carbons on both the η6 and η5 ligands, once again highlighting
the delocalized nature of the indenyl π-system. Therefore, the
other resonance forms presented in Figure 10 are important
contributors to the overall hybrid. These studies also highlight
the inadequacy of a single valence bond representation to
accurately portray the bonding picture in compounds containing
ligands with highly delocalized π-systems.
Experimental determination of the equilibrium constants for
THF addition to the corresponding η9,η5-bis(indenyl)zirconium
sandwich complexes provided a measure of the thermodynamic
driving force for haptotropic rearrangement. It is important to
realize that these values also contain the zirconium carbon
strengths for the three carbons in the cyclopentadienyl ring as
well as the bond energy for the newly formed zirconium-
oxygen bond. Table 5 presents the experimentally measured
equilibrium constants at 10 °C as a function of indenyl
substituent along with the infrared stretching frequencies of the
CO bands for the corresponding η5,η5 zirconocene dicarbonyl
complexes and the oxidation potentials of the related dibenzo-
Another interesting result from the computational studies
performed by Veiros is the relative stability of the model
(38) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1985, 83, 1736.
(39) Veiros, L. F., private communication.
9
J. AM. CHEM. SOC. VOL. 127, NO. 29, 2005 10301

A R T I C L E S
Bradley et al.
ferrocenes.24 These data have proven to be a useful measure of
the electronic properties about the metal center.24,40
There is a general correlation between the electronic proper-
ties of the zirconocene and the preference for THF coordination
and η6,η5 indenyl hapticity. Relatively electron poor sandwich
complexes containing silyl substituents, such as 1 and 2, undergo
THF-induced ring migration preferentially over alkylated,
electron rich sandwiches. Within the series of silylated sand-
wiches 1-3, the compound containing the most electron
withdrawing substituents affords the most stable η6,η5 THF
adduct. Crystallographic characterization of complexes contain-
t
ing [SiMe2 Bu] or [SiMe2Ph] substituents reveals that the largest
Figure 12. Possible mechanisms for formation of η6,η5-bis(indenyl)-
alkyl group is oriented above and below the ligand planes,
directing the remaining two methyl substituents toward the
interior of the molecule.19,24 As a result, the steric environments
about the zirconium in 2 and 3 are quite similar. A significantly
larger equilibrium constant is observed for 2, demonstrating the
importance of electronic effects on THF-induced haptotropic
rearrangement. Comparison of the equilibrium constants for 1
and 3 indicates that in similar electronic environments, the
increased steric bulk of the indenyl substituent slightly disfavors
haptotropic rearrangement.
The alkylated η9,η5-bis(indenyl)sandwich complexes exhibit
a reduced thermodynamic driving force for THF-induced ring
migration. Sandwiches containing dialkylated indenyls such as
5, 8, and 9 do not furnish detectable quantities of the η6,η5
adduct, even when dissolved in THF-d8. In the case of 5, only
addition of a large excess of a chelating diether, DME, produces
detectable haptotropic rearrangement. Comparing the equilib-
rium constants for THF-induced haptotropic rearrangement for
1 and 7 reveals a slight reduction in the stability of the adduct
as compared to the sandwich, while introduction of a second
[CMe3] group into the ligand scaffold has a slightly more
pronounced effect.
zirconium THF complexes.
Typically the opposite is true; 16 electron complexes with a
vacant molecular orbital coordinate an additional neutral ligand
to attain a closed shell, 18 electron configuration. Computa-
tionally, Veiros has calculated that addition of THF to the
unsubstituted model complex, (η9-C9H7)(η5-C9H7)Zr, to afford
(η6-C9H7)(η5-C9H7)Zr(THF) is indeed thermodynamically fa-
vored by 7.1 kcal/mol.20 The driving force for this reaction was
attributed to the release of strain associated with the bent indenyl
involved in the η9 interaction. The experimental data are in
reasonable agreement with these findings; however, as was
described above, substituent effects are important in attenuating
the values of the observed equilibrium constants.
The addition of a series of chelating diethers to 1 has allowed
determination of the thermodynamic preference for substrate
coordination and ring migration as a function of incoming ligand
(Table 4). In general, ethylene-bridged chelates form more stable
η6,η5 adducts with 1 than the corresponding methylene bridged
diethers. The origin of this difference is most likely the reduced
strain and increased flexibility associated with the formation
of a five- rather than four-membered ring. Increased steric bulk
on either the oxygen atom or the carbon backbone decreases
the equilibrium constant for coordination and ring migration.
The simple change of a methyl group to an ethyl substituent
reduces the stability of the product by a factor of 20. A similar
effect is observed upon addition of methyl groups to the carbon
backbone, as evidenced by the decreased stability of the DMB
complexes relative to 1-DME. A C2 symmetric disposition of
the methyl groups in the diether resulted in an approximately
20-fold increase in the stability of the η6,η5 adduct as compared
to the Cs symmetric meso isomer.
Two limiting mechanistic pathways have been considered for
ligand-induced ring migration (Figure 12). The first, path A,
involves dissociation of the allylic portion of the η9 indenyl
ligand to form a formally 14 electron, η6,η5-bis(indenyl)-
zirconium sandwich that is subsequently trapped by free ligand.
In the second pathway, B, the incoming ligand directly attacks
the η9,η5-bis(indenyl)zirconium sandwich, displacing the allylic
portion of the indenyl ring to furnish the observed products.
Although data collection has been limited due to experimental
complications, the rates of addition of both THF and DME to
1 appear to be dependent on the concentration of the incoming
ether. From these results, a pathway involving rate-determining
dissociation of the allylic portion of the η9 indenyl to form an
η6,η5 sandwich can be excluded. The experimental data are
unable to distinguish between pathways involving a rapid
preequilibrium between an η9,η5 and a η6,η5 sandwich followed
The relative thermodynamic instability of dialkylated indenyls
for THF coordination and η6,η5 bonding is most likely a
combination of several factors. Previous work on the corre-
sponding η9,η5-bis(indenyl)sandwich complexes demonstrated
a preference for these types of ligands to adopt η9 coordination.19
A higher kinetic barrier for ring interchange in 5 coupled with
the observation of essentially one haptomer in sandwiches such
as 8 and 9 established ground state stabilization of complexes
bearing the 1,3-diisopropyl indenyl relative to its silylated
counterparts. In addition to containing the smallest substituents
in the series, the relatively electron rich nature of this indenyl
ligand suggested the importance of π-donation from the formal
diolefin adduct to the electron deficient zirconium center. This
stabilization reduces the proclivity for haptotropic rearrangement
and lessens the likelihood of observing the η6,η5 adduct. The
reduced electrophilicity of the zirconium center may also
discourage coordination of THF and other oxygen donors,
resulting in larger observed equilibrium constants for relatively
electron-deficient silyl-substituted zirconium sandwiches.
At first glance, the conversion of a formally 18 electron,
coordinatively saturated η9,η5-bis(indenyl)sandwich complex
into a 16 electron, unsaturated η6,η5-bis(indenyl) THF adduct
may not seem like a thermodynamically favored process.
(40) Zachmanoglou, C. E.; Docrat, A.; Bridgewater, B. M.; Parkin, G. E.;
Brandow, C. G.; Bercaw, J. E.; Jardine, C. N.; Lyall, M.; Green, J. C.;
Kiester, J. B. J. Am. Chem. Soc. 2002, 124, 9525.
9
10302 J. AM. CHEM. SOC. VOL. 127, NO. 29, 2005

Ligand-Induced Haptotropic Rearrangements
A R T I C L E S
significant cloverleaf d-orbital character and a LUMO+1 with
2
dz parentage. Either of the orbitals may accommodate the
formation of an η6,η5 bis(THF) intermediate or transition
structure. Subsequent isolation of analogous species has been
achieved with a range of chelating diethers.
A natural consequence of an associative mechanism for
ligand-induced haptotropic rearrangement is the conversion of
the η9 coordinated indenyl complex in the starting sandwich
into the η6 carbocycle in the corresponding adduct. The
observation of a 2:1 mixture of the two possible η6,η5 haptomers
of 7-THF prepared from essentially one isomer of 7 may seem
contradictory. However, THF coordination is rapid and revers-
ible in solution at 23 °C, and therefore the observed distribution
of products is under thermodynamic rather than kinetic control.
Thus, the η6 indenyl may indeed derive directly from the η9
ring, but reversible THF coordination results in a thermodynamic
mixture of the two haptomers. Interestingly, the thermodynamic
preference for alkylated indenyls to adopt η9 coordination is
not translated onto η6 hapticity. One explanation for this
observation is that the substituents are farther removed from
the metal-indenyl interaction and may exert less of an influence
on the hapticity preference.
Figure 13. Relative rates of DME complexation with 1.
by rate-determining ligand coordination or direct attack of the
incoming nucleophile on the η9,η5 sandwich. Computationally,
the latter pathway is favored, as conversion to the observed
products is achieved by a single step process where approach
of the THF molecule induces dissociation of the allylic carbons
in the η9 ring through an early, reactant-like transition structure.20
Importantly, formation of a discrete η6,η5-bis(indenyl)zirconium
sandwich was found to be unlikely, requiring an activation
barrier of approximately 14 kcal/mol for the unsubstituted model
complex. In contrast, direct THF addition to (η9-C9H7)(η5-C9H7)-
Zr proceeds with a computed activation energy of 6.5 kcal/mol,
in good agreement with the facile rates observed at both low
and ambient temperatures.
While an associative substitution reaction on a formally 18
electron compound may at first seem unlikely,41 the structures
of the η9,η5-bis(indenyl)zirconium sandwiches offer support for
this possibility. In 3 and 5, the zirconium-carbon distances to
the 2 position of the η9 indenyl ring are 2.752(3) and 2.784(2)
Å, respectively, substantially elongated from the values of 2.445-
(3) and 2.517(2) Å in the corresponding η5 ring. Similar
elongations in the range of 0.1-0.2 Å are observed in the 1
and 3 positions. This bond lengthening may produce a suf-
ficiently electrophilic metal center to allow direct attack by the
incoming ligand. Further support for this hypothesis derives from
the frontier molecular orbitals of the sandwich. Full molecule
The observation of haptotropic rearrangment promoted by
oxygen and phosphine donors raises the question of why do
oxygen and phosphorus donors furnish η6,η5 adducts instead
of more traditional η5,η5 bis(indenyl)zirconium derivatives?
While computational studies have predicted the thermodynamic
preference of the η5,η5-bis(indenyl)zirconium-THF complex for
the unsubstituted model complex, experimentally the η6,η5
haptomer persists with substituted indenyl ligands. It should also
be noted that previous work from our laboratory has established
rapid η9,η5 indenyl exchange in solution, suggesting that an
intermediate η5,η5 bis(indenyl)zirconium sandwich (S ) 0) is
kinetically accessible.
DFT calculations on 519 reveal significant dz character in both
2
the LUMO+1 and the LUMO+2 orbitals, which may accom-
modate the incoming oxygen or phosphorus nucleophile.
The faster rates of THF versus DME addition are also
consistent with a direct attack mechanism. The cyclic ether is
expected to be more nucleophilic than the more hindered dial-
kylated ether and thus provide more facile rates of ligand-in-
duced haptotropic rearrangement. The observation of a larger
equilibrium constant for DME than THF coordination requires
a larger second-order rate constant for the forward reaction in
the DME case. While at first glance this may appear inconsistent
with experimental data of a faster approach to equilibrium by
THF, the forward rate constant for the DME induced haptotropic
rearrangement is actually a composite of two rate constants.
The first and presumably slower reaction affords a putative η6,
η5-bis(indenyl)zirconium κ1 DME intermediate, (η6-C9H5-1,3-
(SiMe3)2)(η5-C9H5-1,3-(SiMe3)2)Zr(κ1-MeOCH2CH2OMe), which
then undergoes rapid coordination of the second oxygen to
afford the observed product, 1-DME (Figure 13). Thus, the
overall second-order rate constant and hence the observed rate
of the reaction is slower than the THF-induced haptotropic
rearrangement.
For 1, the rate of the unimolecular ring interchange process
is slower than bimolecular THF addition, even at low concentra-
tions of incoming ligand. These data support a kinetically distinct
and favored pathway for η6,η5 adduct formation. The origin of
this preference may stem from the approach of the incoming
THF ligand to the η9,η5-bis(indenyl)zirconium sandwich. Partial
dissociation of the allylic fragment, coupled with rapid ring
rotation to allow access to the preferred anti ring rotamer for
the adducts, may facilitate nucleophilic attack. The kinetic, and
perhaps thermodynamic, persistence of the η6,η5 adducts may
also result from a more electron rich zirconium center that arises
from complexation of σ-donating ligands such as a phosphine
or ether. These observations contrast the formation of η5,η5 bis-
(indenyl)zirconium ligand adducts upon addition of π-acids such
as carbon monoxide, olefin, and alkynes. In the latter cases,
the electron withdrawing nature of the ligand may insufficiently
stabilize the η6,η5 bonding motif and hence may cause irrevers-
ible conversion to the observed η5,η5 products. However, it
should be noted that the relative rates of benzo dissociation and
π-acid addition have not been measured and will be the subject
of future investigations in our laboratory.
The kinetic data also establish a second THF-dependent rate
of exchange in the η6,η5 THF adducts. Analysis of the molecular
orbitals of 1-THF by DFT calculations reveals a LUMO with
Concluding Remarks
(41) For examples, see: (a) Basolo, F. Inorg. Chim. Acta 1985, 100, 33. (b)
Basolo, F. Polyhedron 1990, 9, 1503. (c) Poe, A. J. Pure Appl. Chem.
1988, 60, 1209.
This Article describes a combined synthetic, thermodynamic,
and kinetic study of unusual ligand-induced haptotropic rear-
9
J. AM. CHEM. SOC. VOL. 127, NO. 29, 2005 10303

A R T I C L E S
Bradley et al.
1
rangements in η9,η5-bis(indenyl)zirconium sandwich complexes.
Addition of principally σ-donating ether ligands afforded
spectroscopically observable and, in many cases, isolable η6,η5-
bis(indenyl)zirconium complexes, where one of the indenyl
ligands has migrated to the benzo portion of the ring. Crystal-
lographic characterization of both THF and DME adducts
revealed significant distortion in the planarity of the coordinated
benzo ring, indicative of reduction by the zirconium, and
suggests that the Zr(IV) canonical form with an L2X2 benzo
ligand is an important contributor to the resonance hybrid.
The equilibrium constants for THF-induced haptotropic
rearrangement were measured as a function of indenyl substitu-
ent in toluene-d8 at 10 °C. In agreement with previous
spectroscopic, electrochemical, and synthetic studies, silylated
indenyls participated in ring migration over their alkylated
counterparts, demonstrating the thermodynamic preference for
the latter class of ligand to adopt η9 coordination. This
preference does not translate onto η6 coordination, as experi-
ments with mixed indenyl complexes establish a perturbation
in the distribution of haptomers upon treatment with THF.
Kinetic studies have established two THF-dependent pathways
for the solution dynamics of the η6,η5-bis(indenyl)zirconium
THF adducts. Haptotropic rearrangement from the sandwich
complex occurs through an associative-type pathway due to
partial allylic dissociation in the ground state, while at higher
THF concentrations exchange of the THF ligands occurs through
a transient η6,η5-bis(THF) intermediate or transition structure.
Treatment of the sandwich complexes with a variety of diethers
also afforded isolable, coordinatively saturated η6,η5 diether
adducts. Haptotropic rearrangement is thermodynamically fa-
vored with five- over four-membered chelates and with smaller
oxygen substituents, the latter observation arising from reduced
steric interactions with the indenyl ligands. An η6,η5-bis-
(indenyl)zirconium bis(phosphine) adduct was prepared by
addition of 1,2-bis(dimethyl)phosphinoethane and demonstrates
that ligand-induced haptotropic rearrangement processes are not
limited solely to oxygen donors. Importantly, these studies
reinforce the “haptotropic flexibility” of indenyl ligands, where
the presence of a 10 π electron system allows the carbocycle
to smoothly adjust coordination modes to meet the electronic
requirements of the metal center.
for C54H62Si4OZr: C, 69.69; H, 6.72. Found: C, 69.51; H, 6.40. H
NMR (toluene-d8): δ ) 0.20 (s, 6H, SiMe2Ph), 0.46 (s, 12H, SiMe2-
Ph), 0.58 (s, 6H, SiMe2Ph), 1.43 (br s, 4H, OCH2CH2), 2.96 (t, 2H,
Benzo), 3.31 (s, 1H, Cp), 3.56 (br s, 4H, OCH2CH2), 6.27 (s, 1H, Cp),
6.71 (m, 4H, Benzo), 7.11 (br s, 8H, Ph), 7.19 (br s, 8H, Ph), 7.42 (br
s, 2H, Ph), 7.61 (br s, 2H, Ph), 7.42 (m, 2H, Benzo). 13C NMR (toluene-
d8): δ ) -0.15, -0.43, 1.04, 1.56 (SiMe2Ph), 48.87 (OCH2CH2), 73.76
(OCH2CH2), 89.08, 121.93, 123.65 (Cp), 86.83, 104.39, 116.26, 123.28
(Benzo), 124.27, 126.75, 128.85, 129.69, 134.56, 134.86, 136.48,
139.70, 142.40, 143.40, 147.81 (Cp/Benzo/Ph).
Preparation of (η6-C9H5-1,3-(SiMe3)2)(η5-C9H5-1,3-(SiMe3)2)Zr-
(MeOCH2CH2OMe) (1-DME). A 25 mL round-bottomed flask was
charged with 0.264 g (0.43 mmol) of 1 and approximately 5 mL of
pentane. The burgundy solution was degassed on the high vacuum line,
and 72 Torr (0.47 mmol) of 1,2-dimethoxyethane was added with a
100.1 mL calibrated gas bulb at -78 °C. The reaction mixture was
warmed to room temperature and stirred for 30 min. The solvent was
removed in vacuo, the reaction assembly transferred into the drybox,
and the resulting green solid washed with pentane affording 0.240 g
(80%) of 1-DME. Anal. Calcd for C34H56Si4O2Zr: C, 58.30; H, 8.06.
1
Found: C, 57.99; H, 7.69. H NMR (benzene-d6): δ ) 0.19 (s, 18H,
SiMe3), 0.42 (s, 18H, SiMe3), 1.46 (m, 2H, OCH2), 1.51 (m, 2H, OCH2),
3.31 (s, 6H, OCH3), 3.85 (m, 2H, Benzo), 4.11 (s, 1H, Cp), 6.47 (m,
2H, Benzo), 6.84 (s, 1H, Cp), 6.85 (m, 2H, Benzo), 7.61 (m, 2H,
Benzo). 13C NMR (benzene-d6): δ ) 1.49, 2.56 (SiMe3), 67.57 (OCH3),
68.68 (OCH2), 117.49, 140.40 (Cp), 82.66, 104.75, 123.40, 125.54
(Benzo), 85.16, 109.88, 122.01, 136.67 (Cp/Benzo).
Preparation of (η6-C9H5-1,3-(SiMe3)2)(η5-C9H5-1,3-(SiMe3)2)Zr-
(Me2PCH2CH2PMe2) (1-DMPE). A 20 mL scintillation vial was
charged with 0.103 g (0.17 mmol) of 1 and dissolved in approximately
5 mL of pentane. To the burgundy solution, 25 µL (0.15 mmol) of
1,2-bis(dimethyl)phosphinoethane was added, and the resulting reaction
mixture was stirred for 15 min. The solvent was removed in vacuo,
and the resulting blood red solid was washed with pentane to afford
0.080 g (62%) of 1-DMPE. Anal. Calcd for C36H62Si4P2Zr: C, 56.86;
1
H, 8.22. Found: C, 56.63; H, 8.10. H NMR (benzene-d6): δ ) 0.18
(br, 2H, PCH2CH2P), 0.28 (br, 2H, PCH2CH2P), 0.28 (s, 18H, SiMe3),
0.30 (s, 6H, PMe2), 0.50 (s, 18H, SiMe3), 1.46 (s, 6H, PMe2), 4.30 (m,
2H, Benzo), 4.52 (s, 1H, Cp), 6.68 (m, 2H, Benzo), 6.89 (m, 2H,
Benzo), 7.14 (s, 1H, Cp), 7.24 (m, 2H, Benzo). 13C{31P} NMR
(benzene-d6): δ ) 1.69, 2.89 (SiMe3), 17.24, 18.18 (PMe2), 24.99
(PCH2CH2P), 116.35, 144.04 (Cp), 81.05, 101.70, 123.11, 126.37
(Benzo), 82.95, 110.10, 123.38, 133.69 (Cp/Benzo). 31P{1H} NMR
(benzene-d6): δ ) -1.76.
Acknowledgment. We thank the National Science Foundation
(CAREER Award to P.J.C. and pre-doctoral fellowship to
C.A.B.) for financial support. P.J.C. also acknowledges the
Research Corporation for a Cottrell Scholarship and the Packard
Foundation for a fellowship in science and engineering. We also
thank Professor Luis Veiros for sharing computational results
prior to publication.
Experimental Section42
Preparation of (η6-C9H5-1,3-(SiMe2Ph)2)(η5-C9H5-1,3-(SiMe2Ph)2)-
Zr(THF) (2-THF). A 25 mL flame-dried round-bottomed flask was
charged with 0.160 g (0.19 mmol) of 2 and approximately 5 mL of
pentane. A 180° needle valve was attached to the flask, and ap-
proximately 1 mL of THF was added by vacuum transfer. Dissolution
of the burgundy solid produced an immediate color change to deep
red. The resulting solution was stirred for 1 h, and the volatiles were
removed in vacuo. Recrystallization from pentane at -35 °C afforded
0.054 g (31%) of a deep red solid identified as 2-THF. Anal. Calcd
Supporting Information Available: Additional experimental
procedures and spectral characterization. Crystallographic data
for 2-THF, rac-4-THF, and 1-DME including bond distances
and angles (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
(42) General considerations and additional experimental procedures are reported
in the Supporting Information.
JA052033Q
9
10304 J. AM. CHEM. SOC. VOL. 127, NO. 29, 2005