Synthesis of bis(indenyl)zirconium dihydrides and subsequent rearrangement to η5,η3-4,5-dihydroindenediyl ligands: Evidence for intermediates during the hydrogenation to tetrahydroindenyl derivatives

Article abstract of DOI:10.1021/ja060472z

Exposure of η⁹,η⁵-bis(indenyl)zirconium sandwich complexes to 4 atm of H₂ resulted in facile oxidative addition to furnish the corresponding zirconocene dihydrides, (η⁵-C₉H₅-1,3-R₂) ₂ZrH₂ (R = SiMe₃, SiMe₂Ph, CHMe ₂). Continued hydrogenation completed conversion to the tetrahydroindenyl derivatives, (η⁵-C₉H ₉-1,3-R₂)₂ZrH₂. Deuterium labeling studies established that dihydrogen (dideuterium) addition to the benzo rings is intramolecular and stereospecific, occurring solely from the endo face of the ligand, proximal to the zirconium. In the absence of dihydrogen, the bis(indenyl)zirconium dihydrides rearranged to new zirconium monohydride complexes containing an unusual η⁵,η³-4,5- dihydroindenediyl ligand, arising from metal-to-benzo ring hydrogen transfer. Mechanistic studies, including a normal, primary kinetic isotope effect measured at 23 °C, are consistent with a pathway involving regio- and stereoselective insertion of a benzo C=C bond into a zirconium hydride. The stereochemistry of the insertion reaction, and hence the η⁵, η³-4,5-dihydroindenediyl product, is influenced by the presence of donor ligands and controlled by the preferred conformation of the indenyl rings. Exposure of the zirconium hydrides containing the η⁵, η³-4,5-dihydroindenediyl rings to 1 atm of dihydrogen afforded the tetrahydroindenyl zirconium dihydride complexes, establishing the intermediacy of this unusual coordination environment during benzo ring hydrogenation.

Full text of DOI:10.1021/ja060472z

Published on Web 04/12/2006
Synthesis of Bis(indenyl)zirconium Dihydrides and
Subsequent Rearrangement to η5,η3-4,5-Dihydroindenediyl
Ligands: Evidence for Intermediates during the
Hydrogenation to Tetrahydroindenyl Derivatives
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 January 27, 2006; E-mail: pc92@cornell.edu
Abstract: Exposure of η9,η5-bis(indenyl)zirconium sandwich complexes to 4 atm of H2 resulted in facile
oxidative addition to furnish the corresponding zirconocene dihydrides, (η5-C9H5-1,3-R2)2ZrH2 (R ) SiMe3,
SiMe2Ph, CHMe2). Continued hydrogenation completed conversion to the tetrahydroindenyl derivatives,
(η5-C9H9-1,3-R2)2ZrH2. Deuterium labeling studies established that dihydrogen (dideuterium) addition to
the benzo rings is intramolecular and stereospecific, occurring solely from the endo face of the ligand,
proximal to the zirconium. In the absence of dihydrogen, the bis(indenyl)zirconium dihydrides rearranged
to new zirconium monohydride complexes containing an unusual η5,η3-4,5-dihydroindenediyl ligand, arising
from metal-to-benzo ring hydrogen transfer. Mechanistic studies, including a normal, primary kinetic isotope
effect measured at 23 °C, are consistent with a pathway involving regio- and stereoselective insertion of
a benzo CdC bond into a zirconium hydride. The stereochemistry of the insertion reaction, and hence the
η5,η3-4,5-dihydroindenediyl product, is influenced by the presence of donor ligands and controlled by the
preferred conformation of the indenyl rings. Exposure of the zirconium hydrides containing the η5,η3-4,5-
dihydroindenediyl rings to 1 atm of dihydrogen afforded the tetrahydroindenyl zirconium dihydride complexes,
establishing the intermediacy of this unusual coordination environment during benzo ring hydrogenation.
halides to (η5-C5H5)2ZrL2 (L ) PPh2Me, PMe2Ph) species.4
Introduction
Detailed mechanistic and spectroscopic studies5 are consistent
Oxidative addition, whereby a transition metal increases in
formal oxidation state by two units, is a fundamental transfor-
mation in organometallic chemistry and constitutes a key step
in numerous catalytic cycles.1 While oxidative addition has been
well studied and amply precedented in late transition metal
chemistry,2 observation of oxidative addition processes with
early transition metal complexes remains rare. This discrepancy
is due, in part, to the paucity of isolable early metal complexes
with low oxidation states.3 Despite the limited number of
examples, oxidative addition reactions with reduced early metal
compounds could serve as a powerful synthetic tool to access
unique structures that are unavailable from alternative prepara-
tive routes.
with a series of one-electron steps involving both metal-centered
and organic radicals. This behavior contrasts the two-electron
processes typically encountered with late metal complexes.2
Oxidative addition of alkenyl halides to Negishi’s reagent, (η5-
n
C5H5)2ZrCl2 treated with 2 equiv of BuLi, has proven useful
in numerous coupling and transmetalation reactions,6 and
isolation and structural characterization of an alkenylzirconocene
chloride product have been achieved.7 In some cases, oxidative
addition is also believed to play a key role in the activation of
fluorocarbons by reduced zirconium complexes.8
Well-defined addition of nonpolar reagents such as silanes
and dihydrogen to reduced group 4 metallocene complexes is
also rare. Exposure of the end-on bound dinitrogen complex,
[(η5-C5Me5)2Zr(η1-N2)]2(µ2,η1,η1-N2), to 1 atm of dihydrogen
results in rapid oxidative addition of H2 to yield the monomeric
dihydride complex, (η5-C5Me5)2ZrH2.9 Similar observations have
In organozirconium chemistry, seminal studies from Schwartz
and co-workers described the formal oxidative addition of alkyl
(1) Collman, J.; Hegedus, L.; Norton, J. R.; Finke, R. G. Principles and
Applications of Organotransition Metal Chemistry; University Science
Books: Mill Valley, CA, 1987; Chapter 5.
(2) For representative examples and reviews, see: (a) Cleary, B. P.; Eisenberg,
R. J. Am. Chem. Soc. 1995, 117, 3510. (b) Sargent, A. L.; Hall, M. B.
Inorg. Chem. 1992, 31, 317. (c) Stang, P. J.; Kowalski, M. H.; Schiavelli,
M. D.; Longford, D. J. Am. Chem. Soc. 1989, 111, 3347. (d) Yamashita,
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1988, 110, 4417. (e) Hill, R. H.; Puddephatt, R. J. J. Am. Chem. Soc. 1985,
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Synthesis of Bis(indenyl)zirconium Dihydrides
A R T I C L E S
been reported with the related “mixed ring” compound, [(η5-
C5Me5)(η5-C5Me4H)Zr(η1-N2)]2(µ2,η1,η1-N2), to furnish (η5-C5-
Me5)(η5-C5Me4H)ZrH2.10 Zirconocene and hafnocene silyl
hydride complexes, (η5-C5H5)2M(SiPh3)(H), have also been
prepared by apparent oxidative addition to phosphine-stabilized
Negishi-type precursors. However, mechanistic studies are
consistent with an addition-elimination pathway rather than
oxidative addition.11 In contrast, oxidative addition of secondary
silanes has been implicated in the catalytic hydrosilation of
olefins promoted by (η5-C5H5)2Zr(η2-CH2dCHEt).12
The interaction of dihydrogen with bis(indenyl)zirconium
compounds is of both fundamental and practical importance,
given the utility of these compounds in olefin polymerization13
and other catalytic bond-forming reactions.14-16 The high-
pressure, PtO2-catalyzed hydrogenation of ansa-bis(indenyl)-
zirconocene dichloride complexes to the corresponding tetrahy-
droindenyl derivatives serves as an important synthetic method
for the separation of racemo and meso isomers (eq 1).17,18
Hydrogenation of propagating zirconocene alkyl species in
R-olefin polymerization has also been identified as an important
and sometimes deleterious chain-transfer pathway, reducing
polymer molecular weights and limiting the industrial utility
of many catalysts.19
been unsuccessful, as H2 addition to the requisite zirconium alkyl
precursor is often accompanied by benzo ring hydrogenation.
For example, hydrogenation of bis(indenyl)zirconium dimethyl
requires high temperature and pressure (80 atm, 140 °C) and
furnishes the bis(tetrahydroindenyl)zirconium dihydride dimer.22
Similar reactivity has been observed in mixed indenyl cyclo-
pentadienyl chemistry, as (η5-C5Me5)(η5-C9H7)ZrMe2 undergoes
hydrogenation at ambient temperature and pressure to yield [(η5-
C5Me5)(η5-C9H11)ZrH2]2.23
Recently, our laboratory has reported the synthesis24 and
characterization25 of the first examples of η9,η5-bis(indenyl)-
zirconium sandwich complexes. In the ground state, η9 coor-
dination of one of the indenyl rings is observed, where all nine
carbons of the carbocycle are bonded to the zirconium. Both
solution spectroscopic25 and computational studies26 have
demonstrated that dissociation of the coordinated benzo ring is
facile in solution and the η5,η5-bis(indenyl)zirconocene is
accessible at ambient temperature (eq 2). While detailed kinetic
and mechanistic studies have yet to be performed, preliminary
reactivity studies support this view, as facile coordination of
CO,24 coupling of olefins and alkynes,25 and C-H activation
of N,N-dimethylaminopyridine (DMAP)24 have been observed
at ambient temperature. In each example, the familiar η5,η5-
hapticity of the indenyl rings has been restored.27
Perhaps more striking is the reactivity of the η9,η5-bis-
(indenyl)zirconium sandwich complexes with principally σ-do-
nating ligands. Addition of cyclic or chelating ethers such as
tetrahydrofuran (THF) and 1,2-dimethoxyethane (DME) pro-
duced an unexpected haptotropic rearrangement, whereby the
zirconium migrates to the benzo ring of one indenyl ligand (eq
3).28 Kinetic studies established a first-order dependence on the
incoming ligand, suggesting a mechanism involving direct attack
of the incoming nucleophile on the complex bearing the strained
η9-indenyl ring. Taken together, the structure and resulting
reactivity of η9,η5-bis(indenyl)zirconium sandwich compounds
Bis(indenyl)zirconium dihydride complexes are attractive
targets, given their potential in both stoichiometric and catalytic
transformations. In addition, complexes of this type could serve
as important mechanistic models for the initiating and propagat-
ing species in olefin polymerization,20 providing insight into
key fundamental steps related to chain propagation, termination,
and stereoselectivity.21 Attempts to prepare such species have
(17) (a) Wild, F. R. W. P.; Zsolnai, G.; Huttner, G.; Brintzinger, H. H. J.
Organomet. Chem. 1985, 288, 63. (b) Wild, F. R. W. P.; Wasiucionek,
M.; Huttner, G.; Brintzinger, H. H. J. Organomet. Chem. 1989, 369, 359.
(18) Elegant synthetic protocols for the stereoselective synthesis of ansa-
zirconocenes that do not require benzo ring hydrogenation have been
developed: (a) LoCoco, M. D.; Zhang, X. W.; Jordan, R. F. J. Am. Chem.
Soc. 2004, 126, 15231. (b) LoCoco, M. D.; Jordan, R. F. J. Am. Chem.
Soc. 2004, 126, 13918. (c) Diamond, G. M.; Rodewald, S.; Jordan, R. F.
Organometallics 1995, 14, 5. (d) Yang, Q.; Jensen, M. D. Synlett 1996,
147.
(19) Resconi, L.; Camurati, L.; Sudmeijer, O. Top. Catal. 1999, 7, 145.
(20) Chirik, P. J.; Bercaw, J. E. Organometallics 2005, 24, 5407.
(21) Gilchrist, J. H.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 12021.
(22) Weigold, H.; Bell, A. P.; Willing, R. I. J. Organomet. Chem. 1974, 73,
C23.
(23) Chirik, P. J.; Day, M. W.; Bercaw, J. E. Organometallics 1999, 18, 1873.
(24) Bradley, C. A.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2003, 125,
8110.
(25) Bradley, C. A.; Keresztes, I.; Lobkovsky, E.; Young, V. G.; Chirik, P. J.
J. Am. Chem. Soc. 2004, 126, 16937.
(26) Veiros, L. F. Chem. Eur. J. 2005, 11, 2505.
(9) Manriquez, J. M.; McAlister, D. R.; Sanner, R. D.; Bercaw, J. E. J. Am.
Chem. Soc. 1978, 100, 2716.
(10) Pool, J. A.; Bernskoetter, W. H.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 14326.
(11) Kreutzer, K. A.; Fisher, R. A.; Davis, W. M.; Spaltenstein, E.; Buchwald,
S. L. Organometallics 1991, 10, 4031.
(12) Takahashi, T.; Hasegawa, M.; Suzuki, N.; Saburi, M.; Rousset, C. J.;
Fanwick, P. E.; Negishi, E. J. Am. Chem. Soc. 1991, 113, 8564.
(13) (a) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. ReV. 2000,
100, 1253. (b) Coates, G. W. Chem. ReV. 2000, 100, 1223. (c) Brintzinger,
H. H.; Fisher, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth, R. M. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1143.
(14) Hoveyda, A. H.; Morken, J. P. In The Metallocenes: Synthesis, ReactiVity
and Applications; Togni, A., Halterman, R. L., Eds.; Wiley-VCH: Wein-
heim, 1998; Chapter 10.
(15) (a) Willoughby, C. A.; Buchwald, S. L. J. Am. Chem. Soc. 1992, 114, 7562.
(b) Willoughby, C. A.; Buchwald, S. L. J. Am. Chem. Soc. 1994, 116,
8952. (c) Willoughby, C. A.; Buchwald, S. L. J. Am. Chem. Soc. 1994,
116, 11703.
(16) Troutman, M. V.; Apella, D. H.; Buchwald, S. L. J. Am. Chem. Soc. 1999,
121, 4916.
(27) For discussions of transition metal-indenyl coordination modes, see: (a)
Calhorda, M. J.; Roma˜o, C. C.; Veiros, L. F. Eur. J. Inorg. Chem. 2002,
8, 868. (b) Trnka, T. M.; Bonanno, J. B.; Bridgewater, B. M.; Parkin, G.
Organometallics 2001, 20, 3255. (c) Stradiotto, M.; McGlinchey, M. J.
Coord. Chem. ReV. 2001, 219, 311.
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A R T I C L E S
Bradley et al.
Table 1. 1H NMR Chemical Shifts of Bis(indenyl)- and
Bis(tetrahydroindenyl)zirconium Dihydrides
highlight the ability of the 10-π-electron indenyl ligand to
smoothly adjust hapticity to meet the electronic requirements
of the metal.
compound
δ
Zr−
H (ppm)a
compound
δ Zr−
H (ppm)a
1-H2
2-H2
5-H2
6.49
6.95
6.86
1-THI-H2
2-THI-H2
5-THI-H2
6.61
6.78
7.07
a Spectra recorded in benzene-d6 at 23 °C.
Oxidative addition of dihydrogen to the η9,η5-bis(indenyl)-
zirconium sandwich complexes seemed a viable synthetic
method for the preparation of elusive bis(indenyl)zirconocene
dihydrides. This reaction class was also of interest to determine
the hapticity of the indenyl rings in the final product as well as
the preferred mechanistic pathway to reach this configuration.
Here we describe the synthesis and crystallographic character-
ization of the first examples of η5,η5-bis(indenyl)zirconium
dihydride complexes prepared by formal oxidative addition of
H2 to the corresponding η9,η5-sandwich compounds. In the
absence of H2, the resulting dihydrides undergo facile regio-
and stereoselective hydride-to-benzo ring migration to furnish
unusual η5,η3-4,5-dihydroindenediyl ligands. A combination of
structural, spectroscopic, isotopic labeling, and kinetic studies
have been used to elucidate the mechanism of the rearrangement
and identify these complexes as intermediates in the hydrogena-
tion to the corresponding tetrahydroindenyl derivatives.
Results and Discussion
Synthesis of η5,η5-Bis(indenyl)- and η5,η5-Bis(tetrahy-
droindenyl)zirconium Dihydrides. Our investigations into
oxidative addition reactions with η9,η5-bis(indenyl)zirconium
sandwich compounds commenced with the addition of 4 atm
of H2 to (η9-C9H5-1,3-R2)(η5-C9H5-1,3-R2)Zr (R ) SiMe3, 1;
SiMe2Ph, 2; CHMe2, 5).29 In each case, a color change from
burgundy to bright yellow was observed immediately upon
exposure of the compound to dihydrogen at 23 °C. Analysis of
the resulting benzene-d6 solutions by 1H and 13C NMR
spectroscopy indicated complete and quantitative conversion to
new C2V-symmetric products, assigned as the η5,η5-bis(indenyl)-
zirconium dihydrides (η5-C9H5-1,3-R2)2ZrH2 (R ) SiMe3, 1-H2;
SiMe2Ph, 2-H2; CHMe2, 5-H2) (eq 4).
Figure 1. (a) Molecular structure of 5-H2 at 30% probability ellipsoids.
Hydrogen atoms, except for the zirconium hydrides, are omitted for clarity.
(b) Top view of the molecule at 30% probability ellipsoids. Isopropyl methyl
substituents and hydrogen atoms, except for the zirconium hydrides, are
omitted for clarity.
monomers in solution.23 Confirmation of these peak assignments
has been accomplished by deuterium labeling and by exposure
of the dihydride complexes to additional H2. The presence of
even trace amounts of H2 resulted in broadening and ultimately
disappearance of the metal hydride resonance, establishing
exchange between the Zr-H bonds and the free gas on the NMR
time scale. This behavior is typical of monomeric zirconocene
dihydrides.23
The solid-state structure of 5-H2 was determined by X-ray
diffraction, a representation of which is shown in Figure 1.
Selected metrical parameters are reported in Table 2. The data
were of sufficient quality that all of the hydrogen atoms,
including the zirconium hydrides, were located and freely
refined. In the solid state, the indenyl ligands adopt an essentially
1
Notably, H and 13C NMR spectra recorded in benzene-d6
exhibit the appropriate number of aromatic resonances, dem-
onstrating that the benzo rings have remained intact and did
not undergo hydrogenation. At typical NMR concentrations of
0.02-0.04 M, downfield-shifted Zr-H resonances were ob-
served in the 1H NMR spectra (Table 1), consistent with
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Synthesis of Bis(indenyl)zirconium Dihydrides
A R T I C L E S
Table 2. Selected Bond Distances and Angles for 5-H2 and
5-H2(PMe3)
no evidence for the bis(indenyl)zirconocene dihydride, 1-H2,
suggesting that initial hydrogenolysis of the zirconium alkyl
proceeds with a higher kinetic barrier than subsequent ring
hydrogenation. Performing a similar procedure with 5-Me2
produced no reaction, even after heating to 85 °C for 2 days.
parameter
5-H2
5-H2(PMe3)
Zr(1)-H(1M)
1.74(2) Å
1.362(2) Å
1.371(2) Å
95.6(2)°
33.2(3)°
146.8(3)°
144.8(3)°
-1
1.741(19) Å; 1.785(19) Å
C(6)-C(7)
C(8)-C(9)
H(1M)-Zr(1)-H(1M′)
129.0(9)°
47.6(2)°
132.4(2)°
136.3(2)°
2
R
The bis(tetrahydroindenyl)zirconium dihydrides, 1-THI-H2
and 5-THI-H2, were readily prepared by continued hydrogena-
tion of the corresponding bis(indenyl)zirconium dihydrides at
23 °C (eq 6). Preparative scale reactions of 5-THI-H2 were
typically carried out at 65 °C for convenience. Special care must
be taken during the preparation of 1-H2 and 2-H2, as exposure
of the η9,η5-bis(indenyl)zirconium sandwich to dihydrogen for
more than a few minutes leads to significant benzo ring
hydrogenation. In contrast, the hydrogenation of 5-H2 to 5-THI-
H2 was much slower, requiring days to reach completion at 23
°C and 1 atm of H2, making isolation of the bis(indenyl)-
zirconium dihydride relatively straightforward. The observation
of ring hydrogenation from 5-H2 but not 5-Me2 further
underscores the higher barrier associated with Zr-Me hydro-
genolysis as compared to ring reduction.
â
γ
τ
rotational anglea
88.7(3)°
163.2(3)°
a Dihedral angle formed between the planes defined by the metal and
C(2) indenyl carbon and the midpoint of the bond in the ring fusion.
gauche conformation, with a rotational angle of 88.7(3)° and
an idealized C2 axis bisecting the two zirconium hydride
positions. The isopropyl substituents are oriented with the
methine hydrogens directed toward the zirconium, minimizing
transannular steric interactions. One isopropyl group on each
ring lies over the zirconocene wedge and most likely serves to
prevent formation of hydride bridges and subsequent dimeriza-
tion. The carbon-carbon bond distances in the planar six-
membered rings range between 1.362(2) and 1.418(2) Å,
confirming their identity as sp2-hybridized carbons with intact
benzo rings.
Formal oxidative addition of H2 to the η9,η5-bis(indenyl)-
zirconium sandwich compounds provides the first access to
zirconium dihydride complexes where the indenyl rings remain
intact.30 To our knowledge, the only other examples of early
metal hydride complexes with indenyl ligands are a family of
ansa-lanthanidocene hydrides, prepared by treatment of the
monoamide complexes with diisobutylaluminum hydride.31
However, isomerization of the ansa ligand to a “flyover”
structure, whereby the two [SiMe2]-linked indenyl rings bridge
two metal centers, accompanies preparation of the metal
hydrides.
To determine if an impurity in 1-H2 was catalyzing benzo
ring hydrogenation and producing artificially faster rates, the
hydrogenation of a nearly equimolar mixture of 1-H2 and 5-H2
1
was monitored by H NMR spectroscopy. Gratifyingly, each
To illustrate the importance of the oxidative addition route
to bis(indenyl)zirconocene dihydrides, the traditional method
for the synthesis of zirconocene hydrides, namely hydrogenation
of a dialkyl precursor, was also explored. Exposure of the bis-
(indenyl)zirconocene dimethyl complex, (η5-C9H5-1,3-(SiMe3)2)2-
ZrMe2 (1-Me2), to 4 atm of H2 resulted in hydrogenation to the
bis(tetrahydroindenyl) derivative, 1-THI-H2 (eq 5). Monitoring
the hydrogenation reaction by 1H NMR spectroscopy provided
compound in the mixture hydrogenated to the tetrahydroindenyl
derivative at a rate similar to that of the independent trials,
suggesting that the observed differences in hydrogenation rates
are intrinsic to the compounds rather than a result of impurities.
Regardless of the synthetic method used, the bis(tetrahy-
droindenyl)zirconium dihydrides were isolated in high yield as
off-white powders. Hydrogenation of the benzo rings was readily
identified by 1H NMR spectroscopy. Resonances in the vicinity
of 1.7-2.8 ppm were observed for the saturated six-membered
rings. Downfield-shifted Zr-H peaks (Table 1) were noted in
each case, indicative of monomers in benzene-d6 solution. As
with the analogous bis(indenyl) complexes, exchange with free
H2 was observed on the NMR time scale.
(28) Bradley, C. A.; Lobkovsky, E.; Keresztes, I.; Chirik, P. J. J. Am. Chem.
Soc. 2005, 127, 10291.
(29) The numbering scheme used throughout this paper is adopted from the
following: Bradley, C. A.; Flores-Torres, S.; Lobkovsky, E.; Abrun˜a, H.
D.; Chirik, P. J. Organometallics 2004, 23, 5332. The nomenclature for
8-H2 and subsequent rearranged products is taken from ref 25.
(30) Examples of metal hydrides with indenyl ligands: Fe: (a) Ahmed, H.;
Brown, D. A.; Fitzpatrick, N. J.; Glass, W. K. Inorg. Chim. Acta 1989,
164, 5. (b) Ahmed, H.; Brown, D. A.; Fitzpatrick, N. J.; Glass, W. K. J.
Organomet. Chem. 1991, 418, C14. Tl: (c) Kumar, N.; Sharma, R. K. J.
Inorg. Nucl. Chem. 1974, 36, 2625. Ce (d) Kapur, S.; Kalsotra, B. L.;
Multani, R. K. J. Inorg. Nucl. Chem. 1974, 36, 932.
The silyl-substituted bis(tetrahydroindenyl)zirconium dihy-
dride, 1-THI-H2, was also characterized by X-ray diffraction.
A representation of the molecular structure is shown in Figure
2, and selected metrical parameters are reported in Table 3. In
the solid state, nearly ideal C2 symmetry is observed, with the
(31) Klimpel, M. G.; Sirsch, P.; Scherer, W.; Anwander, R. Angew. Chem., Int.
Ed. 2003, 42, 574.
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A R T I C L E S
Bradley et al.
hydrogenation at 23 °C was observed by two different synthetic
routes, the stereochemistry of hydrogen addition to the benzo
group was explored. Additional motivation for this study was
provided by the observation of faster rates of benzo ring
hydrogenation with the more sterically hindered zirconocene
dihydride, 1-H2. Few studies aimed at understanding the direct
interaction of H2 and bis(indenyl)zirconium compounds have
appeared. Pino and co-workers32 have reported the stereochem-
istry of PtO2-catalyzed deuterium addition to rac-ethylenebis-
(indenyl)zirconium dichloride and found that benzo ring hy-
drogenation (deuteration) occurred exclusively from the exo face
of the ligand array.
To explore the possibility of intramolecular ring hydrogena-
tion, 1-D2 was prepared and exposed to additional deuterium
gas to achieve complete conversion to 1-THI-D2 (eq 7). As
1
illustrated in the H NMR spectrum of 1-THI-D2 presented in
Figure 3, the tetrahydroindenyl resonances centered at 2.09 and
2.68 ppm were absent in the isotopically labeled compound.
These resonances were directly observed by 2H NMR spectros-
copy. On the basis of NOESY NMR experiments, which were
used to completely assign the 1H NMR spectrum of 1-THI-H2
(see Supporting Information), deuterium was selectively and
exclusively incorporated in the endo positions33 of the six-
membered ring. In contrast to PtO2-catalyzed benzo ring
hydrogenations, where H2 addition occurs solely from the exo
face of the ligand,32 incorporation of deuterium solely in endo
positions is consistent with intramolecular ring hydrogenation
promoted by 1-H2(D2). Endo selectivity was also observed in
the deuteration of 5-D2 to 5-THI-D2 (eq 7).
Figure 2. (a) Molecular structure of 1-THI-H2 at 30% probability
ellipsoids. (b) Top view of the molecule at 30% probability ellipsoids.
Trimethylsilyl substituents and hydrogen atoms, except for the zirconium
hydrides, are omitted for clarity.
Table 3. Selected Bond Distances and Angles for 1-THI-H2
Zr(1)-H(1M)
Zr(1)-H(2M)
C(6)-C(7)
1.85(3) Å
1.73(3) Å
1.362(2) Å
1.371(2) Å
101(1)°
A similar deuterium labeling experiment was conducted with
1-Me2. Exposure of the dimethyl complex to 1 atm of D2 gas
at 65 °C for 3 days furnished 1-THI-D2. Analysis of the
isotopically labeled zirconocene dihydride by a combination of
C(8)-C(9)
H(1M)-Zr(1)-H(1M′)
rotational angle
79.4(7)°
2
1H and H NMR spectroscopy established D2 addition princi-
principal axis bisecting the zirconium-hydride bonds. As a
consequence, the indenyl ligands are in a gauche conformation,
with a rotational angle of 79.4(7)°. In contrast to 5-H2, the
hydrogenated benzo rings are oriented over the metallocene
wedge and serve to block dimerization. The zirconium-hydride
bonds were located and refined and have distances of 1.73(3)
and 1.85(3) Å. The C-C bond distances in the six-membered
rings range between 1.4984(38) and 1.5303(33) Å, consistent
with saturated, sp3-hybridized carbons. By way of comparison,
these distances range between 1.362(2) and 1.418(2) Å in 5-H2.
In 1-THI-H2, deviations between 13.9(1) and 18.3(3)° are
observed for the distal tetrahydroindenyl carbons from the
idealized plane of the five-membered ring, indicative of a
chairlike conformation of the six-membered ring.
pally from the endo face of the ligand, the same stereochemistry
observed from deuteration of the 1-D2. In this case, substantially
more contamination (∼25%) from hydrogen was observed in
the endo positions of the deuterated ring. Control experiments
involving intermediates in the hydrogenation reaction (vide
infra) indicate that cyclometalation of an [SiMe3] group is
competitive with ring reduction and serves as a source of
hydrogen, rapidly exchanging with the Zr-D positions at
65 °C, ultimately placing H- rather than D-atoms in the
tetrahydroindenyl ligand.
Rearrangement of Bis(indenyl)zirconium Dihydrides: Iso-
lation of Complexes Bearing η5,η3-4,5-Dihydroindenediyl
(32) Waymouth, R. M.; Bangerter, F.; Pino, P. Inorg. Chem. 1988, 27, 758.
(33) The terms exo and endo refer to the orientation of the hydrogen atoms
with respect to the zirconium center. Those directed toward the metal are
designated endo, while those pointing away are exo.
Stereochemistry and Mechanism of H2 Addition and
Observation of Hydrogenation Intermediates. Because ring
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Synthesis of Bis(indenyl)zirconium Dihydrides
A R T I C L E S
Figure 3. 1H NMR spectra of (a) 1-THI-H2 and (b) 1-THI-D2.
(DHID) Ligands. Once examples of monomeric bis(indenyl)-
zirconocene dihydrides were isolated, the solution stability of
this new class of compounds was evaluated. Monitoring a
benzene-d6 solution of 1-H2 at 23 °C by 1H NMR spectroscopy
revealed complete consumption of the zirconocene dihydride
over the course of 1 h with concomitant growth of two new
products (eq 8). If the reaction was performed under vacuum,
variable amounts (10-20%) of the η9,η5-bis(indenyl)zirconium
sandwich compound, 1, were observed, arising from reductive
elimination of dihydrogen from 1-H2. Observation of this side
reaction is in agreement with previous observations of rapid
alkane reductive elimination from the corresponding bis-
(indenyl)zirconium alkyl hydrides.24
The major species, 1-syn-DHID-H, accounts for approxi-
mately 90% of the product mixture and has been identified as
a coordinatively saturated zirconium hydride complex containing
one traditional η5-indenyl ring and an unusual η5,η3-4,5-
dihydroindenediyl (DHID) ligand, arising from metal-to-benzo
ring hydrogen transfer. The minor product, formed in 10% yield,
has been assigned as its stereoisomer, 1-anti-DHID-H, differing
only in the disposition of the zirconium hydride and the newly
formed methylene carbon in the six-membered ring of the DHID
ligand.
Identification of 1-syn-DHID-H was accomplished by a
combination of multinuclear and two-dimensional NMR experi-
ments and isotopic labeling studies. In benzene-d6, the 1H NMR
spectrum (Figure 4) of 1-syn-DHID-H exhibits the number of
resonances for a C1-symmetric compound with two different
rings. Selected peak assignments are reported in Table 4. In
addition to four inequivalent [SiMe3] singlets and typical benzo
multiplets in the aromatic region, five multiplets are shifted
upfield and have been identified as protons on the six-membered
ring of the η5,η3-4,5-dihydroindenediyl ligand (Figure 4). The
sp3-hybridized methylene carbon displays exo and endo hydro-
gens centered at 1.93 and 2.47 ppm, respectively. Furthermore,
9
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A R T I C L E S
Bradley et al.
Figure 4. 1H NMR spectra of (a) 1-syn-DHID-H and (b) 1-anti-DHID-H at 23 °C in benzene-d6. D designates the 4,5-dihydroindenediyl ligand, while B
indicates benzo resonances from the indenyl ligand. One DHID resonance and the Zr-H resonances are not observed due to peak overlap in 1-anti-DHID-
H.
a zirconium-hydride resonance was identified at 1.25 ppm,
shifted substantially upfield from the value found in 1-H2,
indicative of a coordinately saturated metal center.34 NOESY
cross-peaks between the zirconium hydride and the endo
methylene hydrogen in the six-membered ring establish the
stereochemistry of the major isomer as syn, wherein the sp3-
hybridized carbon and the Zr-H are adjacent in the molecule.
Deuterium labeling studies were used to confirm the assign-
ment of the zirconium hydride and the endo methylene
hydrogen. Allowing a benzene-d6 solution of 1-D2 to stand at
23 °C resulted in facile conversion to 1-syn-DHID-D. Notably,
the resonances at 1.25 (Zr-H) and 2.47 ppm (endo-CH2) were
1
absent in the H NMR spectrum. These peaks were observed
directly by 2H NMR spectroscopy. No evidence for incorpora-
tion of deuterium into any other part of the ligands was observed.
The minor isomer, 1-anti-DHID-H, exhibited similar NMR
spectral features in benzene-d6 (Table 4). NOESY spectra and
crystallographic studies on a related compound (vide infra)
established the anti disposition of the ring methylene hydrogens
and the zirconium hydride. For this isomer, the Zr-H resonance,
identified by deuterium labeling and two-dimensional NMR
experiments, appears at 0.33 ppm, significantly upfield-shifted
from the syn isomer. The exo and endo methylene hydrogens
on the DHID ligand were observed at 2.55 and 3.04 ppm. It is
(34) Hillhouse, G. L.; Bulls, A. R.; Santarsiero, B. D.; Bercaw, J. E. Organo-
metallics 1988, 7, 1309.
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Synthesis of Bis(indenyl)zirconium Dihydrides
A R T I C L E S
Table 4. Selected 1H NMR Chemical Shifts (ppm) of Zirconocene Hydride Complexes Containing η5,η3-4,5-Dihydroindenediyl Ligands
resonance
1-syn-DHID-H
2-syn-DHID-H
5-syn-DHID-H
1-anti-DHID-H
2-anti-DHID-H
5-anti-DHID-H
Zr-H
1.25
1.93
2.47
5.21
4.77
3.20
1.53
1.78
2.46
5.01
4.65
3.22
1.81
2.05
2.64
4.34
4.34
3.29
0.33
2.55
3.04
6.44
4.78
1.49
0.70
2.35
2.95
6.22
4.54
NLb
0.92
2.05
2.60
6.86
4.56
1.35
a
exo-CH2
a
endo-CH2
H1
H2
H3
a Refers to the sp3-hybridized carbon on the η5,η3-4,5-dihydroindenediyl ligand. b NL ) not located.
curious that the relative stereochemistry of the sp3-hybridized
carbon and the zirconium hydride dramatically shifts the
resonances for the η5,η3-bound ring and the zirconium hydride.
Similar rearrangement chemistry has been observed with the
other bis(indenyl)zirconium dihydrides, 2-H2 and 5-H2. Pertinent
1H NMR spectroscopic features are reported in Table 4. The
silyl-substituted complex, 2-H2, converted at a rate similar to
1-H2 and also yielded a similar product mixture. For the alkyl-
substituted dihydride, 5-H2, slow conversion to 5-syn-DHID-H
and 5-anti-DHID-H occurred at 23 °C. Complete reaction was
observed by warming the solution to 45 °C for 2 days.
Significantly, the rearrangement of 5-H2 to 5-syn-DHID-H was
slightly more selective, with 5-anti-DHID-H accounting for less
than 5% of the product mixture (eq 9). As with the silylated
metallocene, small amounts of 5 were observed upon heating.
The benzene-d6 solution NMR spectral features of 5-syn-
DHID-H are similar to those of 1-syn-DHID-H (Table 4). The
zirconium hydride was observed at 1.81 ppm, while the exo
and endo protons on the DHID ligand appeared as multiplets
centered at 2.05 and 2.64 ppm, respectively. As with 1-syn-
DHID-H, the NMR assignments are based on a combination
of isotopic labeling studies and two-dimensional experiments.
The solid-state structure of 5-syn-DHID-H was determined
by X-ray diffraction and is presented in Figure 5. Selected
metrical parameters are reported in Table 5. The data were of
sufficient quality that all of the hydrogen atoms, including the
zirconium hydride, were located and freely refined. In the solid
state, the indenyl rings adopt a nearly gauche conformation with
a rotational angle of 82.5(5)°, minimizing transannular steric
interactions of the isopropyl substituents. The X-ray data also
support the assignment of the syn isomer whereby the zirconium
hydride and sp3-hybridized carbon are proximal.
Figure 5. (a) Molecular structure of 5-syn-DHID-H at 30% probability
ellipsoids. (b) Top view of the molecule. Isopropyl methyl groups and
hydrogen atoms, except for the zirconium hydride and methylene hydrogens,
are omitted for clarity.
Hydrogen transfer to the C(9) carbon of the benzo ring is
also supported by the metrical data. Both hydrogens were
located, and the bond angles are consistent with sp3 hybridiza-
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A R T I C L E S
Bradley et al.
Table 5. Selected Bond Distances (Å) for 5-syn-DHID-H and
5-anti-DHID-H
hydrogenation and subsequent rearrangement to 1-anti-DHID-
H. A 1:2 mixture of syn and anti products was observed when
no additional THF was present. Attempts to detect the dihydride
complex, 1-H2, or the corresponding THF adduct, 1-H2(THF),
have been unsuccessful, as isomerization is more rapid than in
5-syn-DHID-H
5-anti-DHID-H
Zr(1)-H(1M)
Zr(1)-C(6)
Zr(1)-C(7)
Zr(1)-C(8)
Zr(1)-C(9)
C(6)-C(7)
C(7)-C(8)
C(8)-C(9)
1.796(17)
2.4310(13)
2.4947(12)
2.5529(13)
3.054(1)
1.4179(18)
1.3966(19)
1.5142(17)
1.79(2)
2.895(4)
2.482(3)
2.518(3)
2.625(4)
1.480(5)
1.398(5)
1.380(5)
1
the absence of THF. The H and 13C NMR spectroscopic data
indicated exclusive formation of 1-anti-DHID-H (eq 10).
Similar observations have been made for the rearrangements
of both 2-H2 and 5-H2. Although 5 does not coordinate THF,
addition of approximately 50 equiv of the cyclic ether to 5-H2
1
provided evidence for 5-H2(THF) by H NMR spectroscopy
prior to formation of 5-anti-DHID-H. Notably, the zirconium-
hydride resonance shifts upfield to 6.38 ppm in THF-d8,
consistent with an appreciable concentration of a coordinatively
saturated zirconocene dihydride. As with 1, THF catalyzed the
selective rearrangement to 5-anti-DHID-H over the course of
2 h at 23 °C (eq 10).
tion. The C(4)-C(9) and C(8)-C(9) distances of 1.5185(17)
and 1.5142(17) Å are typical for carbon-carbon single bonds
and are elongated from distances of 1.36-1.40 Å found in the
traditional η5-indenyl rings in 5-H2. The Zr(1)-C(8) bond
distance of 2.5529(13) Å is slightly elongated compared to the
values of 2.4947(12) and 2.4310(13) Å observed for Zr(1)-
C(7) and Zr(1)-C(6), respectively, and is within the range found
in the analogous benzo carbons of η9-indenyl ligands.25 The
Zr(1)-C(9) distance of 3.054(1) Å is well outside the sum of
the covalent radii and the bond lengths typically found between
zirconium and polycyclic aromatic ligands. In addition, the C(9)
carbon dips 42.7(1)° below the idealized plane of the other five
carbons in the six-membered ring, also consistent with hydrogen
transfer.
As was observed in the solid-state structure of η9-coordinated
indenyl ligands,25 the η5,η3-dihydroindenediyl fragment is
significantly buckled as a result of coordination to the zirconium
center. The “hinge angle” is defined as the angle formed between
the planes defined by the five-membered ring and the four sp2
carbons contained in the six-membered ring. The sp3 carbon
was omitted because of the substantial deviation from the
idealized plane. For 5-syn-DHID-H, this value is 44.7(1)°, larger
than the buckling found in the corresponding sandwich com-
pound containing an η9-indenyl ligand,25 which may be a
consequence of the anionic character of the former. To our
knowledge, the compounds presented in this work are the first
examples of η5,η3-4,5-dihydroindenediyl ligands coordinated to
a transition metal. Examples of dihydroindene ligands, C9H9,
have been observed in iron35 and ruthenium36 chemistry and
are typically prepared from synthetic modification of the
corresponding bis(cyclopentadienyl) complex rather than partial
reduction of an indenyl ligand.
Because the anti isomers were observed only in low yield
from the rearrangement of the bis(indenyl)zirconium dihydrides
in benzene-d6, an alternative synthetic route to the compounds
was desired. Our laboratory has previously reported an unusual
THF-induced haptotropic rearrangement of one of the indenyl
ligands in silyl-substituted η9,η5-bis(indenyl)zirconium sand-
wiches to the corresponding η6,η5-bis(indenyl)zirconium THF
compounds.28 As a result, the influence of THF on the
stereochemistry of bis(indenyl)zirconium dihydride complexes
was investigated. This synthetic protocol was also of interest
as the THF complexes, 1-THF and 2-THF, are more crystalline
and easier to isolate in pure form than more lipophilic 1 and 2.
Addition of 1 atm of H2 to a benzene-d6 solution of 1-THF
with an excess (>50 equiv) of THF present resulted in rapid
In addition to diagnostic NMR spectral characteristics (Table
4), confirmation of the stereochemistry of 5-anti-DHID-H was
accomplished by X-ray diffraction. The solid-state structure is
presented in Figure 6, and selected metrical data are reported
in Table 5. As anticipated from the solution data, the zirconium
hydride is directed away from the sp3-hybridized carbon, C(6),
opposite of the configuration observed in the solid-state structure
of 5-syn-DHID-H. The zirconium-carbon distances in the η3
portion of the benzo ring are slightly elongated from the syn
isomer, ranging between 2.482(3) and 2.625(4) Å. Accordingly,
the nonbonded carbon in the benzo ring is slightly closer to the
metal at 2.895(4) Å, as compared to the distance of 3.054(1) Å
for the syn. The hinge angle, as defined for the syn isomer, is
44.5(1)°, indicative of a substantial distortion from planarity.
Likewise, the methylene carbon is distorted 39.6(2)° from the
idealized plane defined by the remaining carbons in the six-
membered ring of 5-anti-DHID-H.
Mechanism of Dihydride Rearrangement. The observation
of the rearrangement of bis(indenyl)zirconocene dihydrides to
complexes containing η5,η3-4,5-dihydroindenediyl ligands and
the unusual influence of THF on the rate and stereochemical
outcome of the reaction prompted investigation into the mech-
anism of isomerization. Two possible mechanisms are proposed
in Figure 7. For the following discussion, 1-H2 will be used as
a representative example of a bis(indenyl)zirconium dihydride.
The first pathway involves insertion of the CdC bond of the
benzo ring into the zirconium-hydride in analogy to well-
established hydrozirconation reactions.37 An alternative mech-
anism involves reversible reductive elimination of H2 from 1-H2,
regenerating 1. The liberated H2 then adds across the zirconium-
(35) (a) Izumi, T.; Tamura, F.; Sasaki, K. Bull. Chem. Soc. Jpn. 1992, 65, 2784.
(b) Trainor, G. L.; Breslow, R. J. Am. Chem. Soc. 1981, 103, 154.
(36) Hofer, O.; Schlo¨gl, K. Tetrahedron Lett. 1967, 36, 3485.
(37) Labinger, J. A. In ComprehensiVe Organic Synthesis; Trost, B. M., Fleming,
I., Eds.; Pergamon Press: Oxford, 1991; Chapter 3.9.
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Synthesis of Bis(indenyl)zirconium Dihydrides
A R T I C L E S
above, rearrangement of bis(indenyl)zirconocene dideuterides,
1-D2 and 5-D2, selectively places the deuterium in the endo
position of the methylene carbon (eq 11). Unfortunately,
stereoselective deuteration of this position is consistent with both
proposed mechanisms, as both the hydrogenolysis and the
insertion pathways are expected to yield the observed products.
The relative rates of isomerization are also not useful in
distinguishing between the two mechanistic possibilities. In the
absence of THF, 1-H2 rearranges faster than 5-H2. If insertion
were operative, this result would be expected, as the rate of
cyclohexene insertion with 1-H2 is faster than that for 5-H2, a
consequence of the increased electrophilicity of the silyl-
substituted zirconocene.38 A similar trend is expected for the
reductive elimination pathway, as previous studies have estab-
lished that silylated zirconocenes undergo reductive elimination
faster than their alkylated counterparts.39
A normal, primary kinetic isotope effect, kH/kD ) 1.7(3), has
been measured at 23 °C for the isomerization of 1-H2(D2) to
1-syn-DHID-H(D). The determination was made by monitoring
the conversion of zirconocene dihydride to the product in parallel
NMR tubes. An effect of this direction and magnitude is similar
to the values reported for olefin insertion with zirconocene
dihydrides (dideuterides)20 and is consistent with an insertion
pathway. However, it should be noted that a similar value could
be expected for the mechanism presented in Path B: the overall
transformation is a composite of reductive coupling, H2 loss,
and hydrogenolysis of metal-carbon bonds. Because the rate-
determining step is unknown, the observed kinetic isotope,
potentially being a composite of these individual transforma-
tions, does not alone distinguish this mechanism from Path A.
In principle, performing the isomerization reaction under
conditions where dihydrogen could be removed as it is formed
could possibly distinguish the different mechanistic pathways.
If reductive elimination were operative, performing the isomer-
ization reaction under a vacuum should ideally lead to exclusive
formation of the η9,η5-bis(indenyl)zirconium sandwich rather
than the rearranged product. Allowing 1-H2 to stand in benzene-
d6 at 23 °C under vacuum furnished an equilibrium mixture of
1-syn-DHID-H and 1-anti-DHID-H, essentially in the same
ratio as in the presence of N2. The same ratio was also obtained
regardless of whether the reaction was carried out with ap-
proximately 1 mL of headspace in a sealed NMR tube or with
300 mL in a glass vessel. Performing the same reaction in a
large volume of toluene under constant dynamic vacuum also
yielded the rearranged products. In this experiment, small
(∼20%) quantities of 1 were observed, most likely arising from
competitive (vide infra) reductive elimination of H2.
Figure 6. (a) Molecular structure of 5-anti-DHID-H at 30% probability
ellipsoids. (b) Top view of the molecule. Isopropyl methyl groups and
hydrogen atoms, except for the zirconium hydride and methylene hydrogens,
are omitted for clarity.
Figure 7. Possible mechanisms for the formation of η5,η3-4,5-dihydroin-
denediyl ligands from the corresponding dihydrides.
carbon bond of the η9 ring. If operative, this mechanism suggests
that the bis(indenyl)zirconocene dihydrides are merely byprod-
ucts and not intermediates to the rearranged complexes. This
pathway would also account for the observation of trace amounts
of sandwich compounds observed during rearrangement.
A series of experiments were conducted in an attempt to
distinguish between the two proposed pathways. As presented
(38) Bradley, C. A.; Keresztes, I.; Lobkovsky, E.; Chirik, P. J. Organometallics
2006, 25, 2080.
(39) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2003, 125,
2241.
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A R T I C L E S
Bradley et al.
Figure 8. Crossover experiments conducted with bis(indenyl)zirconocene dihydrides.
The experiments conducted under vacuum, while not con-
Although the insertion mechanism seems plausible and is in
agreement with the experimental data, questions remain about
the influence of THF on both the stereochemistry and the rate
of rearrangement. Specifically, why does the insertion yield
predominantly syn products in non-coordinating solvents such
as pentane and benzene yet produce the anti isomer in the
presence of THF? Perhaps more difficult to reconcile is the rate
enhancement observed upon addition of a coordinating Lewis
base.
clusively excluding the reductive elimination pathway, support
insertion. To gain additional experimental evidence for this
possibility, a series of crossover experiments were conducted.
Mixing equimolar quantities of 1-H2 and 2 resulted in nearly
quantitative formation of the rearranged products derived from
1 (Figure 8). Small amounts (∼20%) of 2-syn-DHID-H were
detected and most likely arise from competitive reductive
elimination of H2 to form 2-H2, which undergoes subsequent
rearrangement. If reductive elimination were the sole pathway
operative, a statistical mixture of the two different rearranged
products would be expected. In a related experiment, 5-H2 was
mixed with 2, and 5-syn-DHID-H was formed exclusively. In
this case, it is important to note that 2 reacts with H2 much
faster than 5, and if reductive elimination to form free
dihydrogen were operative, significant quantities of 2-H2 (or
its rearranged products) would be expected. Control experiments,
whereby mixtures of bis(indenyl)zirconocene dihydrides and
deuterides of 1, 2, and 5 were prepared and produced little
isotopic exchange, ruled out dimerization through formation of
hydride bridges.
On the basis of the experimental data, the hydride rearrange-
ments most likely proceed via an intramolecular olefin insertion
process. The absence of significant crossover during the
rearrangement of 1-H2 and 5-H2 offers the strongest evidence
in support this assertion. The observation of trace crossover with
1-H2 and not 5-H2 is a result of the relative rates of reductive
elimination in the case where the more electrophilic silyl-
substituted zirconocenes undergo more facile reductive elimina-
tion than their alkylated counterparts. In addition, the observation
of two stereoisomers from rearrangement also argues against a
reductive elimination-hydrogenolysis mechanism, particularly
since formation of 1-anti-DHID-H requires a trans addition of
H2 across the zirconium-carbon bond. While exclusive forma-
tion of the syn isomer followed by isomerization could possibly
account for the second stereoisomer, such a process would
require a different mechanism for isomerization and, as will be
presented below, contrasts the thermodynamic preference for
the syn isomer. The preferred insertion mechanism entails pre-
coordination of a benzo CdC bond to the zirconium, requiring
a distortion from planarity. While it may seem unlikely that a
conjugated carbocycle like indenyl would bend, crystallographic
characterization of η9-hapticity provides precedent for the
geometric flexibility of this ligand class.25
Several experiments were conducted in an attempt to address
these issues. One possibility was that the observed differences
were a result of solvent polarity rather than coordination effects.
To probe this possibility, the rearrangement of 1-H2 was
independently carried out in the presence of a large excess (∼50
equiv) of THF and 2,5-dimethyl-THF. Bergman has previously
established that the two solvents have similar polarities but
vastly different coordination abilities.40 Rearrangement in the
presence of 2,5-dimethyl-THF proceeded with the same selec-
tivity as in benzene-d6, demonstrating that the observed rate
and stereochemical differences are a result of THF coordination
rather than a change in polarity of the medium. Similarly,
performing the rearrangement of 1-H2 in the presence of a large
excess of diethyl ether also produced the same outcome as the
reaction in neat benzene-d6.
Because THF coordination appears to be the origin of the
differences observed in the isomerization reactions, ligand
adducts of the bis(indenyl)zirconocene dihydride complexes
were studied in more detail. For 1-H2, a discrete THF adduct
of the dihydride compound has not been observed due to the
rapid rate of conversion to 1-anti-DHID-H. In the case of 5-H2,
where the rate of rearrangement is slower, addition of a large
excess of THF produced an upfield shifting of the hydride
resonance, consistent with formation of 5-H2(THF). Because
the rearrangement of 5-H2/5-H2(THF) was relatively slow at
23 °C, the rate of isomerization as a function of THF
concentration was determined. Performing the conversion of
5-H2 to 5-anti-DHID-H in benzene-d6 in the presence of 50
equiv of THF produced an observed first-order rate constant of
1.7(1) × 10-4 s-1. This value increases to 2.5(1) × 10-4 s-1
upon increasing the THF concentration to 75 equiv. At 100
equiv, the observed rate constant is 2.7(1) × 10-4 s-1
,
(40) Wax, M. J.; Bergman, R. G. J. Am. Chem. Soc. 1981, 103, 7028.
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Synthesis of Bis(indenyl)zirconium Dihydrides
A R T I C L E S
to be more facile.41,42 This latter pathway may also account for
the observed rate enhancement and suggests the importance of
a pre-equilibrium involving the zirconocene dihydride and the
corresponding THF adduct. THF coordination would place the
indenyl rings in a preferred arrangement for an interchange
associative attack on the zirconium center. However, based on
the experimental data available, this conclusion is tenuous and
should be treated with caution, as slippage to η3 intermediates
along the reaction coordinate and other possibilities have not
been excluded.
When both stereoisomers of the rearranged products were
isolated, the relative thermodynamic stabilities of the two
isomers were determined. Removing the THF from benzene-d6
solutions of the pure anti isomers resulted in conversion to the
corresponding syn compound over the course of hours at 23
°C (eq 12). Likewise, addition of an excess of THF to a 9:1
mixture of 1-syn-DHID-H to 1-anti-DHID-H produced no
change. According to the principle of microscopic reversibility,
the mechanism for conversion of the anti to the syn isomer
proceeds by â-hydrogen elimination43 to generate the zir-
conocene dihydride complex, followed by reinsertion. The
observed isomer preference in the absence of THF supports the
conformational argument, where gauche rotamers lead to syn
rearranged products arising from lateral insertion.
Figure 9. Origin of syn versus anti isomers in the rearrangement of bis-
(indenyl)zirconium dihydride complexes. Indenyl substituents are omitted
for clarity.
statistically invariant from the value at 75 equiv, suggesting
saturation has been achieved.
Concurrent with the studies reported here, our laboratory has
also been exploring the chemistry of other ligand adducts of
bis(indenyl)zirconocene dihydrides. Addition of PMe3 to both
1-H2 and 5-H2 yielded isolable phosphine hydride complexes,
(η5-C9H5-1,3-R2)2ZrH2(PMe3) (R ) SiMe3, 1-H2(PMe3); CHMe2,
5-H2(PMe3)).38 NMR spectroscopic characterization of both
complexes in solution, in combination with the solid-state
structure of 5-H2(PMe3), established that coordination of the
phosphine ligand induced a change in indenyl ligand conforma-
tion. In contrast to the solid-state structure of 5-H2, where
essentially gauche rings are observed, the phosphine dihydride
complexes contain nearly anti indenyl rings with one benzo
substituent oriented directly over the metallocene wedge (Table
2). Another difference between the two structures is the “tilt”
of the indenyl ligands away from the zirconocene hydride
positions. The values for both the interplanar ring angle (R)
and the â parameter (Table 2) demonstrate a significant bending
of the ligands “back” upon coordination of PMe3. Despite these
structural differences, only a slight deviation from η5-hapticity
is observed in the metrical parameters, far from the accepted
values typically ascribed to η3-coordination.
These conformational changes are likely the origin of the
divergent stereochemistries observed in the presence versus in
the absence of THF. As presented in Figure 9, syn isomers are
obtained from coordination of the benzo CdC bond to the lateral
position of the metallocene wedge. Preference for the gauche
isomer in the absence of THF places the incipient CdC bond
directly over the lateral coordination site, in proximity to the
1a1 orbital of the zirconocene.41 In contrast, anti products can
result from two different trajectories derived from the anti
rotamer. Approach of the CdC bond from the benzo ring over
the metallocene wedge leads to coordination between the two
zirconium hydrides, maintaining mirror plane symmetry. An
alternative pathway involves backside attack of the CdC bond
of the opposite benzo ring on the zirconium center, resulting in
olefin coordination in the lateral position of the metallocene
wedge. This trajectory may also be favored by the increased
bending of the indenyl ligands away from the metallocene wedge
observed upon ligand complexation (Table 2). While ligand
coordination in the central position is thermodynamically
preferred, insertion reactions from the lateral sites are believed
To gain further insight into the kinetic and mechanistic
preferences for hydrogen transfer to the benzo ring of the indenyl
ligand, a mixed alkyl-silyl bis(indenyl)zirconium dihydride was
synthesized. Addition of 1 atm of H2 to (η9-C9H5-1,3-(CHMe2)2)-
(η5-C9H5-1,3-(SiMe3)2)Zr (8)25 furnished the corresponding
dihydride complex, 8-H2. At 23 °C, hydride rearrangement
afforded a 5:1 mixture of two isomers of 8-syn-DHID-H (eq
13). Allowing the solution to stand at ambient temperature for
24 h produced no change, suggesting that the mixture was at
equilibrium. A combination of two-dimensional NMR experi-
ments allowed assignment of the major isomer as the one arising
from hydride transfer to the alkylated indenyl ligand while the
minor is the opposite case, where the sp3-hybridized carbon is
on the silylated indenyl ring.
The stereochemistry of the isomerization of 8-H2 was also
studied in the presence of a large excess (∼50 equiv) of THF.
(41) Lauher, J. W.; Hoffmann, R. J. Am. Chem. Soc. 1976, 98, 1729.
(42) Erker, G.; Rosenfeldt, F. Angew. Chem., Int. Ed. Engl. 1978, 17, 608.
(43) Because â-hydrogen elimination occurs from formally coordinatively
saturated compounds, opening of a coordination site either through ring
slippage or olefin dissociation is likely.
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A R T I C L E S
Bradley et al.
At 23 °C, a 5:1 mixture of two isomers of 8-anti-DHID-H were
observed after 3 min. The ratio of products remains unchanged
upon the solution standing at ambient temperature for 3 h. As
with the syn isomer, two-dimensional NMR experiments
established a preference for hydrogen transfer to the alkylated
rather than silylated indenyl ring (eq 14). This preference is
opposite to that observed for ligand-induced haptotropic rear-
rangement, where silylated indenyls are more likely to adopt
η6-hapticity.28
rotamers, which ultimately leads to syn-rearranged products.
The presence of σ-donating ligand such as THF favors formation
of the anti rotamer, leading to the anti isomer. Isomerization of
the anti to the syn compound occurs through â-hydrogen
elimination and establishes the thermodynamic preference for
the syn isomer at ambient temperature. Both stereoisomers of
the η5,η3-4,5-dihydroindenediyl products undergo further hy-
drogenation to the corresponding tetrahydroindenyl derivatives,
consistent with intramolecular insertion events being responsible
for benzo ring hydrogenation.
Experimental Section44
Preparation of (η5-C9H5-1,3-(CHMe2)2)2ZrH2 (5-H2). A thick-
walled glass vessel was charged with 0.145 g (0.300 mmol) of 5, and
approximately 5 mL of pentane was added. The vessel was submerged
in liquid nitrogen, the contents were evacuated, and 1 atm of dihydrogen
was admitted. The vessel was then quickly warmed to ambient
temperature and stirred until a color change to yellow occurred. The
vessel was again submerged in liquid nitrogen, and the volatiles were
removed in vacuo. Solvent removal, followed by recrystallization from
pentane at -35 °C, afforded 0.091 g (63%) of a yellow solid identified
as 5-H2. Anal. Calcd for C30H40Zr: C, 73.26; H, 8.20. Found: C, 73.17;
H, 7.93. 1H NMR (benzene-d6): δ ) 1.12 (d, J ) 7 Hz, 12H, CHMe2),
1.17 (d, J ) 7 Hz, 12H, CHMe2), 2.77 (m, 4H, CHMe2), 6.79 (m, 4H,
Benzo), 6.86 (s, 2H, Zr-H), 6.97 (m, 4H, Benzo), 7.68 (s, 2H, Cp).
13C NMR (benzene-d6): δ ) 22.11, 26.43, 26.51 (CHMe2), 109.98
(Cp), 121.95, 122.11, 122.55, 125.55 (Cp/Benzo). IR (pentane): νZr-H
The observed preference for hydrogen transfer to the alkylated
rather than the silyl ring in the rearrangement to both syn and
anti products is most likely a result of the more electron rich
and hence nucleophilic olefin arising from isopropyl substitution.
The different steric environments of the two ligands also likely
play a role. While 1-H2 undergoes rearrangement faster than
5-H2, the insertion of the alkylated ring at ambient temperature
suggests that the silylated indenyl ring imparts a sufficiently
electrophilic zirconium center to favor rearrangement at 23 °C.
Thus, the two rings work in concert to provide the observed
selectivity.
The Intermediacy of η5,η3-4,5-Dihydroindenediyl Ligands
in Hydrogenation to Tetrahydroindenyl Derivatives. The
possible intermediacy of the η5,η3-4,5-dihydroindenediyl ligands
in the hydrogenation of indenyl rings to the corresponding
tetrahydroindenyl derivatives was explored. Treatment of either
isomer of 1-DHID-H or 5-DHID-H with 1 atm of H2 resulted
in hydrogenation of both rings to furnish 1-THI-H2 and 5-THI-
H2, respectively (eq 15). These results account for the observa-
tion of endo deuterium incorporation in the preparation of
tetrahydroindenyl complexes and implicate η5,η3-4,5-dihydroin-
denediyl ligands and subsequent hydrogenated products as key
intermediates in the addition of H2 to indenyl ligands.
) 1465 cm-1; νZr-D ) 1068 cm-1
.
Preparation of (η5-C9H9-1,3-(SiMe3)2)2ZrH2 (1-THI-H2). A thick-
walled glass vessel was charged with 0.340 g (0.560 mmol) of 1, and
5 mL of toluene was added. The vessel was sealed, removed from the
drybox, frozen in liquid nitrogen, and degassed on a vacuum line. One
atmosphere of hydrogen was then added to the reaction at liquid
nitrogen temperature. The reaction mixture was warmed to room
temperature and stirred for 12 h at 60 °C. After this time, the volatiles
were removed on a vacuum line, and subsequent recrystallization from
pentane at -35 °C yielded 0.166 g (48%) of an off-white solid identified
as 1-THI-H2. Anal. Calcd for C30H56Si4Zr: C, 58.08; H, 9.10. Found:
1
C, 58.45; H, 8.85. H NMR (benzene-d6): δ ) 0.27 (s, 36H, SiMe3),
1.75 (br, 4H, THI), 2.09 (br, 4H, THI), 2.68 (br, 4H, THI), 2.79 (br,
4H, THI), 5.88 (s, 2H, Cp), 6.61 (s, 2H, Zr-H). 13C NMR (benzene-
d6): δ ) 1.73 (SiMe3), 24.47, 28.42 (THI), 113.40, 118.42, 138.57
(Cp).
Preparation of (η5-C9H5-1,3-(CHMe2)2)(η5,η3 -C9H6-1,3-(CHMe2)2)-
ZrH (5-syn-DHID-H). A thick-walled glass vessel was charged with
0.150 g (0.310 mmol) of 5 and approximately 10 mL of toluene. The
vessel was removed from the drybox, frozen in liquid nitrogen, and
evacuated on a high-vacuum line. Dihydrogen was added to the vessel,
and the reaction mixture was subsequently warmed to ambient
temperature and stirred for 15 min. After this time, the reaction mixture
was again frozen in liquid nitrogen and the vessel evacuated. Upon
thawing, the vessel was heated in a 45 °C oil bath for 3 days. Solvent
removal in vacuo, followed by recrystallization from pentane, yields
0.048 g (32%) of 5-syn-DHID-H as a yellow solid. Anal. Calcd for
C30H50Zr: C, 73.26; H, 8.20. Found: C, 73.25; H, 8.19. 1H NMR
(benzene-d6): δ ) 1.00 (br, 6H, 2CHMe2), 1.05 (d, J ) 8 Hz, 3H,
CHMe2), 1.12 (d, J ) 8 Hz, 3H, CHMe2), 1.25 (d, J ) 8 Hz, 3H,
CHMe2), 1.37 (d, J ) 8 Hz, 3H, CHMe2), 1.48 (d, J ) 8 Hz, 3H,
CHMe2), 1.54 (d, J ) 8 Hz, 3H, CHMe2), 1.81 (s, 1H, Zr-H), 2.05 (d,
J ) 10 Hz, 1H, CH2), 2.19 (m, 1H, CHMe2), 2.52 (m, 1H, CHMe2),
2.64 (m, 1H, allyl CH), 3.01 (m, 1H, CHMe2), 3.18 (m, 1H, CHMe2),
3.29 (br, 1H, allyl CH), 4.34 (br, 1H, allyl CH), 4.35 (br, 1H, allyl
Concluding Remarks
Oxidative addition of dihydrogen to η9,η5-bis(indenyl)-
zirconium sandwich compounds has provided a convenient
synthetic route to bis(indenyl)zirconocene dihydrides, where the
unsaturation of the benzo substituents remains intact. In the
absence of H2, the CdC bond of the benzo substituents
undergoes regio- and essentially stereospecific insertion into one
of the zirconium-hydride bonds, providing the first examples
of η5,η3-4,5-dihydroindenediyl ligands. The stereochemistry of
the insertion reaction and hence the resulting zirconocene
monohydride product is dictated by the preferred conformation
of the indenyl rings. In the absence of donor ligands, crystal-
lographic studies have established a preference for gauche
(44) General considerations and additional experimental procedures are contained
in the Supporting Information.
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6466 J. AM. CHEM. SOC. VOL. 128, NO. 19, 2006

Synthesis of Bis(indenyl)zirconium Dihydrides
A R T I C L E S
CH), 5.42 (s, 1H, Cp), 6.76 (m, 1H, Benzo), 6.86 (s, 1H, Cp), 6.93 (m,
1H, Benzo), 6.99 (d, J ) 8 Hz, 1H, Benzo), 7.24 (d, J ) 8 Hz, 1H,
Benzo). 13C NMR (benzene-d6): δ ) 20.85, 21.69, 21.82, 22.82, 23.92,
24.46, 26.20, 26.43, 27.40, 27.87, 27.90, 28.33, 28.99 (CHMe2/CH2),
46.07, 89.30, 100.92, 102.34, 106.10, 112.15, 115.03, 116.09, 117.63,
118.14, 120.83, 120.93, 121.11, 121.63, 122.92, 122.98, 125.29 (Cp/
Benzo).
Research Corporation for a Cottrell Scholarship and the Packard
Foundation for a fellowship in Science and Engineering.
Supporting Information Available: Additional experimental
procedures, detailed NMR spectral characterization, and selected
kinetic data (PDF); crystallographic data for 5-H2, 1-THI-H2,
5-syn-DHID-H, and 5-anti-DHID-H (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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
JA060472Z
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J. AM. CHEM. SOC. VOL. 128, NO. 19, 2006 6467