Alkylaluminum-complexed zirconocene hydrides: Identification of hydride-bridged species by NMR spectroscopy

Article abstract of DOI:10.1021/ja8054723

Reactions of unbridged zirconocene dichlorides, (R_nC ₅H_5-n)₂ZrCl₂ (n=0, 1, or 2), with diisobutylaluminum hydride (HAI'Bu₂) result in the formation of tetranuclear trihydride clusters of the type (R_nC₅H _5-n)₂Zr(μ-H)₃(Al ⁱBu2)3(μ-CI) 2, which contain three [Al ⁱBu₂] units. Ring-bridged ansa-zirconocene dichlorides, Me2E(R_nC₅H _4-n)₂ZrCl₂ with E=C or Si, on the other hand, are found to form binuclear dihydride complexes of the type Me2E(R _nC₅H_4-n)₂Zr(CI)(μ-H) ₂Al ⁱBu₂ with only one [AIBu₂] unit. The dichotomy between unbridged and bridged zirconocene derivatives with regard to tetranuclear versus binuclear product formation is proposed to be connected to different degrees of rotational freedom of their C₅-ring ligands. Alkylaluminum-complexed zirconocene dihydrides, previously observed in zirconocene-based precatalyst systems activated by methylalumoxane (MAO) upon addition of HAI'Bu₂ or Al ⁱBu₃, are proposed to be species of the type Me₂Si(ind)₂Zr(Me)(μ-H) ₂Al ⁱBu₂, stabilized by interaction of their terminal Me group with a Lewis acidic site of MAO.

Full text of DOI:10.1021/ja8054723

Published on Web 11/25/2008
Alkylaluminum-Complexed Zirconocene Hydrides:
Identification of Hydride-Bridged Species by NMR
Spectroscopy
Steven M. Baldwin,† John E. Bercaw,*,† and Hans H. Brintzinger*,‡
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of
Technology, Pasadena, California 91125, and Fachbereich Chemie, UniVersita¨t Konstanz,
D-78457 Konstanz, Germany
Received July 15, 2008; E-mail: Bercaw@caltech.edu; Hans.Brintzinger@uni-konstanz.de
Abstract: Reactions of unbridged zirconocene dichlorides, (RnC5H5-n)2ZrCl2 (n ) 0, 1, or 2), with
diisobutylaluminum hydride (HAliBu2) result in the formation of tetranuclear trihydride clusters of the type
(RnC5H5-n)2Zr(µ-H)3(AliBu2)3(µ-Cl)2, which contain three [AliBu2] units. Ring-bridged ansa-zirconocene
dichlorides, Me2E(RnC5H4-n)2ZrCl2 with E ) C or Si, on the other hand, are found to form binuclear dihydride
complexes of the type Me2E(RnC5H4-n)2Zr(Cl)(µ-H)2AliBu2 with only one [AliBu2] unit. The dichotomy between
unbridged and bridged zirconocene derivatives with regard to tetranuclear versus binuclear product formation
is proposed to be connected to different degrees of rotational freedom of their C5-ring ligands. Alkylaluminum-
complexed zirconocene dihydrides, previously observed in zirconocene-based precatalyst systems activated
by methylalumoxane (MAO) upon addition of HAliBu2 or AliBu3, are proposed to be species of the type
Me2Si(ind)2Zr(Me)(µ-H)2AliBu2, stabilized by interaction of their terminal Me group with a Lewis acidic site
of MAO.
Introduction
In view of their manifold catalytic properties, a great variety
of zirconocene hydride complexes as well as adducts of some
of these entities with organoaluminum or other organometal
hydrides have been isolated and characterized by spectroscopic
and/or crystallographic methods.1,2 From some reaction systems,
mononuclear or binuclear zirconocene hydride cations have been
isolated.3-5Mostabundant,however,appeartobeneutralsbinuclear
or oligonuclearszirconocene hydride complexes, the Zr(IV)
centers of which are incorporated in a [ZrH3] core structure.
[ZrH3] coordination patterns have been crystallographically
characterized, e.g., in many dimeric zirconocene dihydrides,6
in adducts of a zirconocene hydride moiety with another neutral
metal hydride species,7 e.g., with AlH3,8 or in an anionic
[(C5Me5)2ZrH3]- entity in contact with suitable cations such as
Li+ or K+.9 Reaction systems derived from zirconocene dichlo-
ride or dihydride complexes by reaction with organoaluminum
Figure 1. [ZrH3] coordination geometries, proposed for alkylaluminum-
complexed zirconocene hydride complexes in refs 10-15.
hydrides, which catalyze alkene hydrometallations or polymer-
izations, have been reportedsbased mainly on spectroscopic
evidencesto contain oligonuclear species with hydride-bridged
[ZrAl2] or [Zr2Al2] cores, the Zr centers of which adopt, again,
a [ZrH3] coordination geometry (Figure 1, structures A and B,
respectively).10-15
(6) (a) Jones, S. B.; Peterson, J. L. Inorg. Chem. 1981, 20, 2889. (b)
Choukroun, R.; Dahan, F.; Larsonneur, A. M.; Samuel, E.; Peterson,
J.; Meunier, P.; Sornay, C. Organometallics 1991, 10, 37. (c)
Larsonneur, A. M.; Choukroun, R.; Jaud, J. Organometallics 1993,
12, 3216. (d) Lee, H.; Desrosiers, P. J.; Guzei, I.; Rheingold, A. L.;
Parkin, G. J. Am. Chem. Soc. 1998, 120, 3255. (e) Bai, G.; Mu¨ller,
P.; Roesky, H. W.; Uson, I. Organometallics 2000, 19, 4675. (f) Chirik,
P. J.; Henling, L. M.; Bercaw, J. E. Organometallics 2001, 20, 534.
(g) Grossman, R. B.; Doyle, R. A.; Buchwald, S. L. Organometallics
1991, 10, 1501. (h) Arndt, P.; Spannenberg, A.; Baumann, W.;
Burlakov, V. V.; Rosenthal, U.; Becke, S.; Weiss, T. Organometallics
2004, 23, 4792.
† California Institute of Technology.
‡ Universita¨t Konstanz.
(1) Review: Chen, E. Y.-X.; Rodriguez-Delgado, A. In ComprehensiVe
Organometallic Chemistry III; Mingos, D. M. P., Crabtree, R. H.,
Bochmann, M., Eds.; Elsevier: Amsterdam, The Netherlands, 2007;
Vol. 4, p 878.
(2) Review on Zr(III) zirconocene hydride complexes: Lancaster, S. J. In
ComprehensiVe Organometallic Chemistry III; Mingos, D. M. P.,
Crabtree, R. H., Bochmann, M., Eds.; Elsevier: Amsterdam, The
Netherlands, 2007; Vol. 4, p 753.
(3) Yang, X.; Stern, C. L.; Marks, T. Angew. Chem., Int. Ed. Engl. 1992,
31, 1375.
(7) (a) Khan, K.; Raston, C. L.; Grady, J. E.; Skelton, B. W.; White, A. H.
Organometallics 1997, 16, 3252. (b) Wehmschulte, R. J.; Power, P. P.
Polyhedron 1999, 18, 1885.
(4) Jordan, R. F. AdV. Organomet. Chem. 1991, 32, 325.
(5) (a) Carr, A. G.; Dawson, D. M.; Thornton-Pett, M.; Bochmann, M.
Organometallics 1999, 18, 2933. (b) Bryliakov, K. P.; Talsi, E. P.;
Semikolenova, N. V.; Zakharov, V. A.; Brand, J.; Alonso-Moreno,
C.; Bochmann, M. J. Organomet. Chem. 2007, 692, 859.
(8) (a) Etkin, N.; Stephan, D. W. Organometallics 1998, 17, 763. (b) Sizov,
A. I.; Zvukova, T. M.; Belsky, V. K.; Bulychev, B. M. J. Organomet.
Chem. 2001, 619, 36.
9
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2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 17423–17433 17423

A R T I C L E S
Baldwin et al.
Figure 2. 1H NMR spectrum of a 50 mM solution of (C5H5)2ZrCl2 (1) in benzene-d6 at 25 °C after addition of 3 equiv of HAliBu2.
Scheme 1
Recently, however, zirconocene hydride complexes which
deviate from the commonly observed [ZrH3] coordination
pattern have been encountered in methylalumoxane (MAO)-
activated zirconocene precatalyst systems upon addition of
HAliBu2 or AliBu3.16 The 1H NMR spectra of these species show
only a single hydride resonance with an integral corresponding
to two hydride units per Zr center, which indicates the presence
of a [ZrH2] unit with two equivalent H ligands.16b In order to
characterize the species responsible for this NMR signal and
the factors which govern the formation of [ZrH2]-type as
opposed to [ZrH3]-type zirconocene hydride complexes, we have
first re-evaluated some previously described reactions of un-
bridged zirconocene complexes with diisobutylaluminum hy-
dride, HAliBu2,17 and subsequently studied corresponding
reactions of several ansa-zirconocene dichlorides.
an intensity of 1H, and a doublet at -2.06 ppm with an intensity
of 2H per zirconocene unit (Figure 2). This signal pattern has
previously been ascribed to
a
trinuclear cluster,
(C5H5)2ZrH3(AliBu2)2Cl, containing a [(C5H5)2ZrH3] moiety in
contact with a chloride-bridged bis(diisobutylaluminum) unit
(structure A in Figure 1).11 This species has recently been
proposed to be an (inactive) component in zirconocene-based
reaction systems which catalyze the hydroalumination of
R-olefins.12
Results and Discussion
1. Unbridged Zirconocene Complexes. In accord with earlier
reports, we observe that excess HAliBu2 reacts with (C5H5)2ZrCl2
(1) in benzene-d6 at 25 °C to give a species with two
characteristic hydride NMR signals, a triplet at -0.89 ppm with
The assignment of structure A to the product formed from
(C5H5)2ZrCl2 and HAliBu2 does not appear to be correct,18
however. Careful integration of the isobutyl signals versus the
C5H5 signal in reaction systems containing complex 1 and
HAliBu2 consistently yields a [AliBu2]/[Zr] ratio of (3.02 (
0.03):1. Even when 1 is used in excess, only the product with
three [AliBu2] units per zirconocene unit is observed, with an
NMR spectrum identical to that described above, while unre-
acted 1 remains behind as a solid. The reaction of 1 with
HAliBu2 thus appears to lead to the immediate and complete
(9) (a) Etkin, N.; Hoskin, A. J.; Stephan, D. W. J. Am. Chem. Soc. 1997,
119, 11420. (b) Fermin, M. C.; Stephan, D. W. J. Am. Chem. Soc.
1995, 117, 12645.
(10) Wailes, P. C.; Weigold, H.; Bell, A. P. J. Organomet. Chem. 1972,
43, C29.
(11) Shoer, L. I.; Gell, K. I.; Schwartz, J. J. Organomet. Chem. 1977, 136,
C19.
(12) (a) Parfenova, L. V.; Pechatkina, S. V.; Khaliliov, L. M.; Dzhemilev,
U. M. Russ. Chem. Bull. 2005, 54, 316. (b) Parfenova, L. V.;
Vil’danova, R. F.; Pechatkina, S. V.; Khaliliov, L. M.; Dzhemilev,
U. M. J. Organomet. Chem. 2007, 692, 3424.
formation of
a
tetranuclear cluster of composition
(C5H5)2ZrH3(AliBu2)3Cl2 (2, Scheme 1).
(13) Tritto, I.; Zucchi, D.; Destro, M.; Sacchi, M. C.; Dall’Occo, T.;
Galimberti, M. J. Mol. Catal. A: Chem. 2000, 160, 107.
(14) Go¨tz, C.; Rau, A.; Luft, G. J. Mol. Catal. A: Chem. 2002, 184, 95.
(15) Gonzales-Hernandes, R.; Chai, J.; Charles, R.; Perez-Camacho, O.;
Kniajanski, S.; Collins, S. Organometallics 2006, 25, 5366.
(16) (a) Babushkin, D. E.; Brintzinger, H. H. Chem. Eur. J. 2007, 13, 5294.
(b) Babushkin, D. E.; Panchenko, V. N.; Zakharov, V. A.; Brintzinger,
H. H. Macromol. Chem. Phys. 2008, 209, 1210.
(18) An earlier formulation of this complex as a [ZrH3Al2Cl] cluster was
based mainly on cryoscopy data which gave a molar mass close to
that expected for 2 (ref 11). These cryoscopy measurements are likely
to be complicated, however, by the low solubility of the dichloride
starting compound and by the presence of some free (HAliBu2)3 in
equilibrium with it.
(17) For simplicity, diisobutylaluminum hydride is written here and in the
following as a monomer, even though it is undoubtedly preponderantly
present as a trimer in solution under our experimental conditions:(a)
Ziegler, K.; Kroll, W.-R.; Larbig, W.; Steudel, O.-W. Liebigs Ann.
Chem. 1960, 629, 53. (b) Hoffmann, E. G. Liebigs Ann. Chem. 1960,
629, 104. (c) Vestin, R.; Vestin, U.; Kowalewski, J. Acta Chem. Scand.,
Ser. A 1985, 39, 767.
(19) Wailes, P. C.; Weigold, H. J. Organomet. Chem. 1970, 24, 405.
(20) Bickley, D. G.; Hao, N.; Bougeard, P.; Sayer, B. G.; Burns, R. C.;
McGlinchey, M. J. J. Organomet. Chem. 1983, 246, 257.
(21) Attempts to isolate any crystalline products from the reaction systems
described above were unsuccessful so far and are not likely to meet
with success in view of the instability and the multiple degrees of
conformational freedom of the diisobutylaluminum adducts considered.
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Alkylaluminum-Complexed Zirconocene Hydrides
A R T I C L E S
Scheme 2
a
Table 1. 1H NMR Signals of Unbridged Zirconocene Complexes and of Their Reaction Products with HAliBu2
Zr-H
AliBu2
ligand
(C5H5)2ZrCl2 (1)
5.89 (s, 10H)
5.66 (s, 10H)
(C5H5)2Zr(µ-H)3(AliBu2)3(µ-Cl)2 (2)
-0.89 (t, 1H, 7.4 Hz)
-2.03 (d, 2H, 6.8 Hz)
0.45 (d, 12H, 6.9 Hz)
1.15 (d, 36H, 6.3 Hz)
2.10 (m, 6H, 6.6 Hz)
((C5H5)2ZrH)2(µ-H)2 (3)b
-3.45 (t, 2H, 7.3 Hz)
3.85 (t, 2H, 7.3 Hz)
-2.11 (br, 2H)
5.75 (s, 20H)
5.40 (s, 20H)
((C5H5)2ZrH· · ·HAliBu2)2(µ-H)2 (4)c
0.39 (d, 8H, 6.5 Hz)
1.18 (d, 24H, 5.0 Hz)
2.36 (br, 4H)
0.44 (br, 12H)
1.25 (br, 36H)
-3.05 (br, 2H)
(C5H5)2Zr(µ-H)3(AliBu2)3(µ-H)2 (5)c
(nBu-C5H4)2ZrCl2 (6)
-1.46 (t, 1H, 15.5 Hz)
-2.33 (d, 2H, 16.5 Hz)
5.34 (s, 10H)
2.16 (br, 6H)
0.84 (t, 6H, 7.5 Hz)
1.22 (s, 4H, 7.4 Hz)
1.43 (p, 4H, 7.4 Hz)
2.64 (t, 4H, 7.8 Hz)
5.74 (pt, 4H, 2.7 Hz)
5.92 (pt, 4H, 2.6 Hz)
0.86 (t, 6H, 7.5 Hz)
2.24 (t, 4H, 7.1 Hz)d
5.65 (br, 4H)
(nBu-C5H4)2Zr(µ-H)3(AliBu2)3(µ-Cl)2 (7)
-0.58 (t, 1H, 6.9 Hz)
-1.54 (d, 2H, 6.9 Hz)
0.51 (d, 12H, 7.2 Hz)
1.18 (d, 36H, 6.6 Hz)
2.15 (m, 6H, 6.6 Hz)
5.94 (br, 4H)
(1,2-Me2-C5H3)2ZrCl2 (8)
5.47 (t, 2H, 3.0 Hz)
5.62 (d, 4H, 3.0 Hz)
5.58 (t, 2H, 3.0 Hz)
5.65 (d, 4H, 3.0 Hz)
(1,2-Me2-C5H3)2Zr(µ-H)3(AliBu2)3(µ-Cl)2 (9)
-0.20 (t, 1H, 7.2 Hz)
-1.13 (d, 2H, 6.3 Hz)
0.52 (d, 12H, 7.2 Hz)
1.19 (d, 36H, 5.7 Hz)
2.18 (br, 6H)
(Me3Si-C5H4)2ZrCl2 (10)
0.33 (s, 18H)
5.93 (pt, 4H, 2.5 Hz)
6.39 (pt, 4H, 2.5 Hz)
0.00 (s, 18H)
5.96 (br, 4H)
6.11 (br, 4H)
(Me3Si-C5H4)2Zr(µ-H)3(AliBu2)3(µ-Cl)2 (11)c
-1.31 (br, 1H)
-2.09 (br, 2H)
all broad
(C5Me5)2ZrCl2 (12)
(C5Me5)2ZrH2 (13)
(C5Me5)2ZrHiBu (14)
1.85 (s, 30H)
2.02 (s, 30H)
1.91 (s, 30H)
7.50 (s, 2H)
6.13 (s, 1H)
-0.03 (d, 2H, 6.9 Hz)
1.02 (d, 6H, 6.6 Hz)
2.39 (m, 1H, 6.5 Hz)
0.69 (d, 4H, 7.2 Hz)
1.25 (d, 12H, 6.6 Hz)
2.22 (m, 2H, 6.4 Hz)
(C5Me5)2ZriBu2 (15)
1.85 (s, 30H)
d
a In benzene-d6 at 25 °C. b From ref 20. c In toluene-d8 at -75 °C. Two resonances overlap with Bu signals.
i
In order to verify that three [AliBu2] units and two Cl bridges
are indeed present in the reaction product, we have designed
the experiment outlined in Scheme 2: the zirconocene dihydride,
3,19,20 reacts with 1 equiv of HAliBu2 per zirconocene unit to
form species 4, characterized by a 1:1 pair of hydride signals
at -2.12 and -3.06 ppm in toluene-d8 solution at -75 °C (Table
1), which had been seen also for adducts of the dimeric
zirconocene dihydride complex 3 with various aluminum trialkyl
compounds (structure B in Figure 1).10-12,14 Addition of 1 equiv
of ClAliBu2 per zirconocene to such a reaction systemswhich
would be required to form species Asgives instead four broad
signals between -1 and -3 ppm, which indicate the presence
of several mutually interconverting hydride products (Supporting
Information Figure S1). Clean formation of the trihydride species
2 with sharp hydride signals at -0.89 ppm (t, 1H) and -2.06
ppm (d, 2H) is observed, however, when exactly 2 equiv of
ClAliBu2 are added to the reaction system containing 1 equiv
(22) Geometry optimizations, based on Hartree-Fock methods (Spartan
software package, 3-21G basis set), yield for a simplified model for
compound 2, with isobutyl groups replaced by methyl groups, a
geometry close to the C2-symmetric structure shown in Scheme 1 but
with both staggered C5-rings rotated by 18°. Calculations by methods
based on density functional theory (DFT), performed at the University
of Hamburg, yield an energy, lower by ca. 6 kcal/mol than that of the
C2-symmetric geometry shown in Scheme 1, for an unsymmetric
structure with one four-membered Zr(µ-H)2Al ring and a tetracoor-
dinate geometry of the middle Al center (ref 23).
(23) Burger, P. Private communication, 2007.
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A R T I C L E S
Baldwin et al.
Scheme 3
Scheme 4
each of (C5H5)2ZrH2 and of HAliBu2 (Supporting Information
Figure S2). The exclusive and complete formation of species 2
via this alternative route (Scheme 2) corroborates its composition
(C5H5)2ZrH3(AliBu2)3Cl2.
Further support for this assignment comes from the observa-
tion that reaction of the dimeric dihydride 3 with 2 equiv of
ClAliBu2 yields a 2:1 mixture of the hydride cluster 2 and the
dichloride 1, in accord with the stoichiometry expected from
eq 1.
are in accord with the presence of tetranuclear trihydride clusters,
7, 9, and 11 (Table 1, Supporting Information Figures S4-S6),
respectively. In comparison to their unsubstituted analogue 2,
the Zr-H signals of all these complexes appear shifted to lower
fields by ca. 0.5-1 ppm, presumably due to changes in the
electron density of their substituted ring ligands.
1/2((C5H5)2ZrH2)2 +
2ClAliBu2 f 2/3(C5H5)2ZrH3(AliBu2)3Cl2 + 1/3(C5H5)2ZrCl2
(1)
In distinction to the reaction systems discussed so far,
permethylzirconocene dichloride, (C5Me5)2ZrCl2 (12), does not
give a hydride complex when reacted with 2 equiv of HAliBu2
in benzene-d6 solution at 25 °C. Instead, a slight shift of its 1H
NMR signal indicates that its Cl ligands might form an adduct
with HAliBu2. The dihydride complex (C5Me5)2ZrH2 (13), on
the other hand, reacts with 1-2 equiv of AliBu3 to give mixtures
of the monohydride monoisobutyl complex (C5Me5)2ZrHiBu
(14) and the diisobutyl complex (C5Me5)2ZriBu2 (15) (Support-
ing Information Figure S7). A 10-fold excess of either AliBu3
or HAliBu2 in benzene-d6 solution at 25 °C converts the
dihydride 13 practically completely to the diisobutyl complex
(C5Me5)2ZriBu2 (15).24 Remarkably enough, bulky iBu groups,
rather than hydride units, are thus preferentially transferred from
Al to the sterically shielded Zr center of (Me5C5)2ZrCl2.
Apparently, formation of a trihydridic [ZrH3] unit in com-
bination with an array of spatially demanding [AliBu2] units is
disfavored by steric factors in this case. Hydride units present
in these reaction systems thus appear to be more favorably
accommodated in hydride-bridged alkylaluminum clusters,
rather than in a Zr-H moiety, if formation of the otherwise
preferred [Zr-H-Al] bridges is precluded by steric factors.
2. Bridged Zirconocene Complexes. Neutral, alkylaluminum-
complexed zirconocene dihydride complexes have been ob-
served, by a ZrH2 signal at ca. -2.0 ppm, to form in catalyst
systems containing rac-Me2Si(1-indenyl)2ZrCl2, ((SBI)ZrCl2,
16), and MAO upon addition of excess AliBu3 or HAliBu2.16b
In the following, we seek to ascertain structural assignments
for these alkylaluminum-complexed ansa-zirconocene hydrides,
which differ from their unbridged congeners discussed above
by the presence of only twosinstead of threeshydride ligands
at their Zr centers.
Although we were not able to obtain crystals of complex 2
suitable for a diffractometric structure determination,21 we
propose for this complex the structure shown in Schemes 1 and
2, as a representation of itsspresumably time-averagedsC2-
symmetric NMR characteristics. Whether the central Al center
of this complex is indeed five-coordinate, as represented in
Scheme 1, must be left to future studies.22
When the dimeric zirconocene dihydride 3 is reacted, rather
than with ClAliBu2, with 3 equiv of HAliBu2 per zirconocene
unit in toluene-d8, one observes, at -75 °C, two hydride NMR
signals with an intensity ratio of 2:1, a doublet at -2.32 ppm
and a triplet at -1.46 ppm (Supporting Information Figure S3).
These signals closely resemble those assigned to the [ZrH3] core
of species 2 (Table 1). On the basis of this similarity and on
the observation of an additional signal at 3.18 ppm with an
integral of 2H, which we assign to two bridging [Al-H-Al]
units, the product of this reaction is likely to be the cluster 5,
i.e., an analogue to complex 2 with hydride instead of chloride
bridges (Scheme 3).
When the temperature of this reaction system is increased
from -75 to 0 °C, the Zr-H signals of complex 5 at -2.32
and -1.46 ppm coalesce with each other and with the
[Al-H-Al] signal at 3.18 ppm. At 25 °C finally, all of these
signals become so broad as to be hardly detectable. Facile
exchange between lateral and central Zr-H positions and of
both of these with the peripheral [Al-H-Al] units is indicated
by this observation. The entirely H-bridged species 5 thus
appears much more prone to undergo exchange reactions than
its congener 2, which contains two Al-Cl-Al bridges. At
above-ambient temperatures, the appearance of violet colorations
indicates that decomposition occurs in this system.
Similar spectra as for the system 1/HAliBu2 are observed
when unbridged zirconocene dichlorides with substituted C5-
rings are reacted with HAliBu2 in benzene or toluene solution
(Table 1). Reactions of (nBu-C5H4)2ZrCl2 (6), (1,2-Me2-
C5H3)2ZrCl2 (8), or (Me3Si-C5H4)2ZrCl2 (10) with 2 equiv of
HAliBu2 each give a species with a doublet and a triplet Zr-H
signal at ca. -1 ppm and -0.2 ppm, respectively (Scheme 4).
Integrals of these and the respective iBu and ring ligand signals
(SBI)ZrCl2 was found to react with a 2-5-fold excess of
HAliBu2 in benzene or toluene solutions at room temperature
to form a hydride product, which obviously contains a [ZrH2]
group, as indicated by a broad singlet at -1.29 ppm with an
integral corresponding to two protons per zirconocene unit.16b
(24) Reaction of the dihydride complex 13 with excess AlMe3 gives an
unstable intermediate, which decomposes to a yet unidentified product.
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Alkylaluminum-Complexed Zirconocene Hydrides
A R T I C L E S
a
Table 2. 1H NMR Signals of Bridged Zirconocene Complexes and Their Reaction Products with HAliBu2 and/or ClAliBu2
Zr-H
AliBu2b
ligand C5-Hc
rac-Me2Si(indenyl)2ZrCl2 ((SBI)ZrCl2, 16)
rac-Me2Si(indenyl)2ZrCl(µ-H)2AliBu2
5.76 (d, 2H, 3.3 Hz)
6.81 (d, 2H, 3.3 Hz)
5.86 (d, 2H, 3.0 Hz)
6.92 (d, 2H, 2.7 Hz)
-1.29 (br, 2H)
0.41
1.16
2.10
rac-C2H4(indenyl)2ZrCl2 ((EBI)ZrCl2, 17)
rac-C2H4(indenyl)2ZrCl(µ-H)2AliBu2
5.75 (d, 2H, 3.3 Hz)
6.47 (d, 2H, 3.3 Hz)
5.88 (br, 2H)
-0.80 (br, 2H)
-1.36 (br, 2H)
-1.75 (br, 2H)
-0.77 (br, 2H)
-1.72 (s, 2H)
0.42
1.17
2.10
6.32 (br, 2H)
Me2C(C5H4)2ZrCl2 (19)
5.19 (pt, 4H, 3 Hz)
6.42 (pt, 4H, 3 Hz)
5.23 (br, 4H)
Me2C(C5H4)2ZrCl(µ-H)2AliBu2
0.47
1.09
2.04
6.13 (br, 4H)
Me2Si(C5H4)2ZrCl2 (20)
5.52 (pt, 4H, 2.9 Hz)
6.81 (pt, 4H, 2.9 Hz)
5.52 (br, 4H)
Me2Si(C5H4)2ZrCl(µ-H)2AliBu2
0.47
1.13
2.08
6.40 (br, 4H)
Me2Si(2,4-Me2-C5H2)2ZrCl2 (21)
5.05 (d, 2H, 2 Hz)
6.39 (d, 2H, 2 Hz)
5.03 (br, 2H)
Me2Si(2,4-Me2-C5H2)2ZrCl(µ-H)2AliBu2
0.48
1.08
2.02
6.34 (br, 2H)
(Me2Si)2(C5H3)2ZrCl2 (22)d
6.14 (t, 2H, 2.8 Hz)
6.72 (d. 4H, 2.7 Hz)
5.98 (br, 2H)
d, e
(Me2Si)2(C5H3)2ZrCl(µ-H)2AliBu2
0.42
1.12
2.08
6.76 (br, 4H)
(Me2Si)2(2,4-iPr2-C5H)(C5H3)ZrCl2 (23)
6.36 (t, 1H, 2.7 Hz)
6.46 (s, 1H)
6.75 (d, 2H, 2.7 Hz)
6.06 (br, 1H)
d, f
(Me2Si)2(2,4-iPr2-C5H)(C5H3)ZrCl(µ-H)2AliBu2
-1.27 (s, 2H)
0.3-0.5g
1.0-1.3
2.0-2.25
6.42 (br, 1H)
6.71 (br, 2H)
rac-C2H4(4,5,6,7-tetrahydroindenyl)2ZrCl2 ((EBTHI)ZrCl2, 18)
rac-C2H4(4,5,6,7-tetrahydroindenyl)2ZrH)2(µ-H)2 (24)
5.27 (d, 2H, 3 Hz)
6.36 (d, 2H, 3 Hz)
5.07 (br, 2H)
-1.29 (t, 2H, 7.5 Hz)
5.18 (t, 2H, 7.2 Hz)
5.27 (d, 2H, 2.4 Hz)
6.37 (d, 2H, 2.7 Hz)
6.57 (br, 2H)
rac-C2H4(4,5,6,7-tetrahydroindenyl)2ZrH(µ-H)2AliBu2 (25)d, f
rac-C2H4(4,5,6,7-tetrahydroindenyl)2ZrCl(µ-H)2AliBu2 (26)
-1.17 (br, 1H)
-0.53 (br, 1H)
-4.57 (br, 1H)
0.4-0.7g
1.1-1.4
2.0-2.4
4.94 (br, 1H)
5.15 (br, 1H)
5.73 (br, 1H)
6.34 (br, 1H)
-0.09 (br, 2H)
0.53
1.20
2.18
5.49 (br, 2H)
6.16 (d, 2H, 2.7 Hz)
rac-Me2C(indenyl)2ZrCl2 (27)
5.67 (d, 2H, 3.6 Hz)
6.50 (d, 2H, 3.6 Hz)
5.67 (d, 2H, 2.7 Hz)
6.76g
rac-Me2C(indenyl)2ZrCl(µ-H)2AliBu2 (29)
-1.35 (br, 2H)
0.40
1.16
2.08
meso-Me2C(indenyl)2ZrCl2 (28)
5.53 (d, 2H, 3.6 Hz)
6.54 (d, 2H, 3.3 Hz)
6.11 (br, 2H)
meso-Me2C(indenyl)2ZrCl(µ-H)2AliBu2 (30)
-1.89 (br, 1H)
-0.87 (br, 1H)
0.46
1.09
2.01
6.23 (br, 2H)
rac-Me2Si(2-Me3Si-4-Me3C-C5H2)2ZrCl2 (31)
6.17 (d, 2H, 1.8 Hz)
7.23 (d, 2H, 2.1 Hz)
5.75 (d, 1H, 2.1 Hz)
5.93 (d, 2H, 1.5 Hz)
6.36 (d, 1H, 1.8 Hz)
5.56 (t, 2H, 3.0 Hz)
5.89 (t, 2H, 2.3 Hz)
6.86 (dd, 2H, 3.0, 2.1 Hz)
5.12 (br, 2H)
rac-Me2Si(2-Me3Si-4-Me3C-C5H2)2ZrH(µ-H)2AliBu2 (32)
-1.56 (br, 1H)
-0.60 (d, 1H, 8.2 Hz)
2.68 (dd, 1H, 5.5, 9.4 Hz)
0.3-0.5
1.0-1.25
1.9-2.3
meso-Me2Si(3-Me3C-C5H3)2ZrCl2 (33)
meso-Me2Si(3-Me3C-C5H3)2ZrH(µ-H)2AliBu2 (34)
-2.17 (d, 1H, 5 Hz)
-0.21 (d, 1H, 10.2 Hz)
3.31 (dd, 1H, 5.1, 9.9 Hz)
0.54 (d, 4H, 7.2 Hz)
1.21 (m, 12H)
2.19 (m, 2H, 6.8 Hz)
5.79 (br, 2H)
5.94 (br, 2H)
H4C2(C5H4)2ZrCl2 (35)
5.46 (pt, 4H, 2.5 Hz)
6.50 (pt, 4H, 2.5 Hz)
5.72 (br, 4H)
H4C2(C5H4)2Zr(µ-H)3(AliBu2)3(µ-Cl)2 (37)
-1.11 (br, 2H)
-0.32 (br, 1H)
0.47
1.13
2.1
6.07 (br, 4H)
Me4C2(C5H4)2ZrCl2 (36)
5.65 (pt, 4H, 2.7 Hz)
6.51 (pt, 4H, 2.7 Hz)
5.84 (br, 4H)
Me4C2(C5H4)2Zr(µ-H)3(AliBu2)3(µ-Cl)2 (38)
-1.14 (br, 2H)
-0.15 (br, 1H)
0.49
1.17
2.12
6.16 (br, 4H)
b
c
a In benzene-d6 at 25 °C. Complex-bound and free XAliBu2 exchange-averaged. Remaining resonances omitted because of overlap with XAliBu2.
d In toluene-d8. e At -75 °C. f At -50 °C. Complex-bound and free XAliBu2 partly decoalesce.
g
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A R T I C L E S
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Figure 3. 1H NMR spectrum of a 50 mM solution of (SBI)ZrCl2 (16) in benzene-d6 at 25 °C after addition of 3 equiv of HAliBu2.
Scheme 5
The (SBI) ligand resonances of this product are shifted only
slightly from their positions in the dichloride complex 16 (Figure
3, Table 2), thus indicating the presence of a neutral zirconocene
species with an intact (SBI) ligand framework.25 Integration of
the iBu signals indicates that approximately four [AliBu2] units
are present in solution per zirconocene unit. Use of lower ratios
of HAliBu2 over (SBI)ZrCl2 leaves parts of the latter unreacted.
A finite concentration of free HAliBu2 thus has to be present in
these solutions to bring the conversion of the ansa-zirconocene
dichloride into the dihydride complex to completion.
Products with a single high-field signal due to a ZrH2 group
are observed also upon reaction of HAliBu2 with several other
singly or doubly bridged zirconocene dichlorides, shown in
Scheme 5, such as rac-C2H4(indenyl)2ZrCl2 ((EBI)ZrCl2, 17),
rac-C2H4(4,5,6,7-tetrahydroindenyl)2ZrCl2
((EBTHI)ZrCl2,
18),26 Me2C(C5H4)2ZrCl2 (19), Me2Si(C5H4)2ZrCl2 (20), rac-
Me2Si(2,4-Me2-C5H2)2ZrCl2 (21), (Me2Si)2(C5H3)2ZrCl2 (22), or
(Me2Si)2(3,5-iPr2-C5H)(C5H3)ZrCl2 (23).
Formation of a dihydride complex from each of these ansa-
zirconocene dichlorides by reaction with HAliBu2 (Table 2,
Supporting Information Figures S8-S14) requires the concurrent
formation of 2 equiv of the chloroaluminum species ClAliBu2.
This Lewis acidic byproduct is likely to remain in association
with the comparatively Lewis basic zirconocene dihydride. In
order to decide whether one or both of the resulting ClAliBu2
entities are complexed to each of the ansa-zirconocene dihydride
species, we have investigated related, Cl-free reaction systems,
using an ansa-zirconocene dihydride as a starting material.
Particularly useful in this regard proved the easily accessible
and well-characterized dimeric dihydride, ((EBTHI)ZrH)2(µ-
H)2 (24).6g,h The Zr-H signals of this compound, a triplet at
5.17 ppm for its terminal Zr-H groups and another triplet at
-1.29 ppm for its Zr-H-Zr bridges, disappear when 1 equiv
of HAliBu2 is added to a solution of 24. A new species is
formed, which gives rise, in toluene-d8 solution at -75 °C, to
two bridging Zr-H signals, at -1.17 ppm and -0.53 ppm, a
terminal Zr-H signal at 4.57 ppm, and four separate ligand
C5-H signals (Figure 4, Table 2). A gradient correlation
spectroscopy (gCOSY) analysis shows coupling between the
terminal and each of the two bridging [Zr-H] units but not
between the latter two (Supporting Information Figure S15).
On the basis of the similarity of this Zr-H NMR pattern to
that reported by Wehmschulte and Powers for the structurally
t
characterized species (C5H5)2Zr(H)(µ-H)2Al(H)C6H2 Bu3, we
assign the signals shown in Figure 4 to the trihydride (EBT-
HI)ZrH(µ-H)2AliBu2 (25, Scheme 6).
The signals shown in Figure 4 are strongly broadened at
higher temperatures, with the onset of this coalescence being
critically dependent on the [HAliBu2]/[Zr] ratio used. The signals
of (EBTHI)ZrH(µ-H)2AliBu2 are not observable at any temper-
ature in the presence of 4 equiv of HAliBu2, whereas in the
presence of 2 equiv of HAliBu2 the observation of sharp signals
requires cooling to -90 °C. Sharp signals as in Figure 4 can be
observed even at room temperature, however, together with
those of the initial ((EBTHI)ZrH)2(µ-H)2, when only 0.5 equiv
of HAliBu2 is added per zirconocene, so as to minimize the
(25) Ligand and hydride signals shown in Figure 3 were found to remain
essentially unchanged down to -75 °C by variable-temperature 1H
NMR.
(26) This observation contradicts a report in ref 6h, which claims that 18
does not react with HAliBu2.
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17428 J. AM. CHEM. SOC. VOL. 130, NO. 51, 2008

Alkylaluminum-Complexed Zirconocene Hydrides
A R T I C L E S
Scheme 8
Scheme 9
Figure 4. 1H NMR spectrum of a 15 mM solution of ((EBTHI)ZrH)2(µ-
H)2 (24) in toluene-d8 at -50 °C after addition of 1 equiv of HAliBu2 (/,
solvent impurity).
Scheme 6
product, whereas 1 equiv of ClAliBu2 is apparently released into
solution (Scheme 8). Accordingly, addition of a second equiv
of ClAliBu2 to the reaction system made up of the dihydride 24
and 2 equiv of HAliBu2 and 1 equiv of ClAliBu2 per Zr leaves
the positions and integrals of the previous signals unchanged
(Supporting Information Figure S17). It causes, however, a
sharpening of the signals assigned to the dihydride cluster 26,
quite obviously by accelerating an associative side exchange
of this species analogous to that described in Scheme 7.
The view that addition of HAliBu2 to one of the ansa-
zirconocene dichlorides shown in Scheme 5 gives the corre-
sponding zirconocene monochloride dihydride clusters contain-
ing one AliBu2 unit is supported by experiments with a pair of
racemic and meso-configured Me2C(indenyl)2ZrCl2 complexes,
rac-27 and meso-28 (Scheme 9). The racemic dihydride cluster
29 gives, as expected, a single ZrH2 signal at -1.35 ppm
(Supporting Information Figure S18). Its meso counterpart 30,
on the other hand, in which the (µ-H)2AliBu2 moiety is likely
to stay on the open side of the complex, yields two separate
ZrH signals at -1.89 and -0.87 ppm (Supporting Information
Figure S19). The close coincidence of the average of these two
values with the chemical shift of the ZrH2 group in the racemic
isomer supports the notion that the latter has a similarly
unsymmetric structure as the meso isomer and that the appear-
ance of a single ZrH2 resonance in thissand by implication also
in the othersracemic complexes arises from a rapid side
exchange of the type shown in Scheme 9.
Scheme 7
presence of any free HAliBu2 in the reaction system. This
acceleration of exchange processes by excess HAliBu2 indicates
that an associative exchange reaction with free HAliBu2 induces
a dynamic side exchange in complex 25 (cf., Scheme 7).
On the basis of our understanding of this reaction system,
we surmise that the dihydride complex formed upon addition
of HAliBu2 to the dichloride (EBTHI)ZrCl2 (18) is the cluster
26, i.e., an analogue of the trihydride complex 25 in which the
terminal hydride is replaced by a chloride, while both hydride
ligands are bridging to an [AliBu2] unit, as indicated by their
high-field chemical shift. When this view is tested by adding,
instead of HAliBu2, 1 equiv of ClAliBu2 per Zr to a toluene-d8
solution of ((EBTHI)ZrH)2(µ-H)2, we observe, together with the
expected dihydride cluster 26, a mixture of the dichloride 18,
the trihydride cluster 25, and free HAliBu2, i.e., a decay of 26
under release of HAliBu2. When this decay of 26 is suppressed,
however, by addition of 2 equiv of HAliBu2, one observes indeed
exactly the same signals as in the reaction system containing
the dichloride 18 and excess HAliBu2 (Supporting Information
Figure S16). This reaction sequences shows unequivocally that
of the 2 equiv of chloride, originally contained in the zir-
conocene dichloride complex, only one is retained in the reaction
A remarkable mode of reaction with HAliBu2, which differs
from that described above for the bridged dichloride complexes
t
17-24, is observed for the Bu-substituted complexes 31 and
33 (Scheme 10). When complex 31, rac-Me2Si(2-Me3Si-4-
Me3C-C5H2)2ZrCl2, in which both C5-ring ligands are shielded
by a Me3Si and a Me3C group, is reacted with 2 equiv of
HAliBu2 in toluene-d8 at -50 °C, about half of it is converted
to a new species with three Zr-H signals at -1.67, -0.62, and
2.65 ppm. A gCOSY analysis (Supporting Information Figure
S20), which shows coupling of the signal at 2.65 ppm with both
of the high-field Zr-H signals but no coupling between these
9
J. AM. CHEM. SOC. VOL. 130, NO. 51, 2008 17429

A R T I C L E S
Baldwin et al.
Scheme 10
high-field signals, and the observation of duplicate sets of CH,
Me3C, and Me3Si ligand signals are closely analogous to the
NMR pattern described above for the trihydride complex
(EBTHI)ZrH(µ-H)2AliBu2 (25). We can thus conclude that a
trihydride complex carrying one [AliBu2] unit, rac-Me2Si(2-
Me3Si-4-Me3C-C5H2)2ZrH(µ-H)2AliBu2 (32), is formed by reac-
tion of HAliBu2 with the dichloride 31 (Scheme 10). In this
reaction system, 2 equiv of ClAliBu2 are apparently displaced
by HAliBu2 from the Zr center and released into solution. This
Cl/H displacement appears to be an equilibrium reaction, as
shown by an almost complete (ca. 90%) conversion of 31 to
32 in the presence of a 10-fold excess of HAliBu2 (Figure 5).
In distinction to the (EBTHI)Zr trihydride complex 25, the
hydride signals of complex 32 do not coalesce or broaden even
at room temperature in the presence of excess HAliBu2. Side
exchange, while still observable here in polarization transfer
experiments (Supporting Information Figure S21), is undoubt-
edly greatly hindered by the R-positioned Me3Si groups, which
block the approach of HAliBu2 required for the associative
exchange outlined in Scheme 7.
Me2Si(3-Me3C-C5H3)2ZrH(µ-H)2AliBu2 (34) is characterized, in
toluene-d8 at -50 °C, by three sharp Zr-H signals at -2.25,
-0.28, and 3.22 ppm and by a single set of three C5-ring CH
and one tert-butyl signal, in accord with its Cs symmetry
(Supporting Information Figure S22). The NMR spectrum
observed for complex 34 at -50 °C persists essentially
unchanged also at temperatures up to 25 °C. Here, as in the
meso-configured bisindenyl complex 30 discussed above, we
see no signs for any side exchange of the Zr(µ-H)2AliBu2 moiety
(Supporting Information Figure S24). Apparently, the latter is
locked into one side of the complex molecule by the tert-butyl
groups, which make the other side of the complex inaccessible.27
In distinction to all other cases discussed so far, we observe
here even at room temperature separate iBu signals for 2 equiv
of free ClAliBu2 (0.48 (d 8H, 7.0 Hz), 1.05 (d 24 H, 6.7 Hz)
and 2.03 ppm (m, 4H, 6.8 Hz)) and for 1 equiv of a complex-
bound [AliBu2] unit (0.54 (d 4H, 7.2 Hz), 1.21 (m 12H) and
2.19 ppm (m, 2H, 6.8 Hz)). This attests to the efficient shielding
of complex 34 against associative exchange reactions and
confirms the stoichiometry of the reaction shown in Scheme
10.
The observation that bridged zirconocene dichloride com-
plexes with bulky substituents in a ꢀ-position of each C5-ring
react with HAliBu2 to give the trihydride clusters Cpx2ZrH(µ-
H)2AliBu2, instead of the otherwise prevailing chlorodihydride
clusters Cpx2ZrCl(µ-H)2AliBu2, can be explained by the serious
mutual repulsion, which Cl ligands and ꢀ-tBu substituents in
ansa-zirconocenes are known to suffer.28 Much of this repulsion
is likely to be released when a Zr-Cl bond with a length of ca.
245 pm is replaced by a much shorter Zr-H bond (ca. 195
pm), which allows the hydride ligand to escape from repulsive
contacts by staying on the “inside” of the ꢀ-tBu substituent.
This steric shielding appears to favor the formation of the
trihydride cluster over its chlorodihydride congener; at the same
time, it will largely suppress an associative approach of HAliBu2
or ClAliBu2 from the side of the terminal hydride.
A related case concerns a reaction system which contains,
together with 2 equiv of HAliBu2, the dichloride meso-Me2Si(3-
Me3C-C5H3)2ZrCl2 (33). This complex differs from the previous
representative 31 by the absence of the R-positioned Me3Si
groups and by the placement of both tert-butyl substituents on
the same side of the complex. The reaction product meso-
3. Bridged versus Unbridged Zirconocene Complexes. All
of our results presented so far thus indicate a clear dichotomy
between unbridged and bridged zirconocene complexes: all
unbridged representatives discussed in section 1 of this report
(27) A significant NOE, which connects the signals of the tert-butyl and
the terminal Zr-H groups in complex 34, indicates that these groups
are in close proximity.
(28) (a) Wiesenfeldt, H.; Reinmuth, A.; Barsties, E.; Evertz, K.; Brintzinger,
H. H. J. Organomet. Chem. 1989, 369, 359. (b) Brintzinger, H. H.;
Prosenc, M. H.; Schaper, F.; Weeber, A.; Wieser, U. J. Mol. Struct.
1999, 485-486, 409.
Figure 5. 1H NMR spectrum of a 48 mM solution of rac-Me2Si(2-Me3Si-
4-Me3C-C5H2)2ZrCl2 (31) in toluene-d8 at 25 °C after addition of 10 equiv
of HAliBu2.
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Alkylaluminum-Complexed Zirconocene Hydrides
A R T I C L E S
Figure 6. 1H NMR spectrum of a 30 mM solution of Me4C2(C5H4)2ZrCl2 (36) in benzene-d6 at 25 °C after addition of 4 equiv of HAliBu2.
regard to their reactions with HAliBu2 is connected to different
Scheme 11
degrees of rotational freedom of the C5-ring ligands in these
two classes of complexes. In order to test this hypothesis, we
have studied reactions of HAliBu2 with two ethanediyl-bridged
zirconocenes, 35 and 36 (Scheme 11). In distinction to their
congeners with Me2C- and Me2Si-bridges, which enforce a
strictly eclipsed conformation of their C5-ring ligands, ethanediyl-
bridged titanocene and zirconocene complexes are known to
react with HAliBu2 to form clusters containing three [AliBu2]
units and a [ZrH3] coordination core.29 All the bridged
deviate significantly from an eclipsed C5-ring conformation.
Crystal structures reported for complexes 35 and 3630,32 reveal
complexes shown in Scheme 5, on the other hand, react with
HAliBu2 to give an adduct with only one [AliBu2] unit. The
vast majority of these contain a [ZrClH2] coordination core; only
the most heavily congested ansa-zirconocenes with ꢀ-tBu
substituents are found to contain a [ZrH3] core in contact with
a single AliBu2 unit. In the following, we search for possible
causes for this dichotomy between unbridged and bridged
zirconocene complexes.
dihedral bridgehead-centroid-centroid-bridgehead angles of
21° and 15°, respectively. This and the observation of 1H NMR
spectra with time-averaged C2V symmetry in solutions of 35 and
36 indicate an essentially unobstructed fluctuation between right-
and left-handed deviations from C2V symmetry, i.e., a significant
measure of torsional freedom of the C5-rings in these ethanediyl-
bridged complexes.
Reactions of complexes 35 and 36 with 4 equiv of HAliBu2
do indeed give the trialuminum trihydrides H4C2(C5H4)2Zr(µ-
H)3(AliBu2)3(µ-Cl)2 (37) and Me4C2(C5H4)2Zr(µ-H)3(AliBu2)3(µ-
Cl)2 (38), respectively, as judged by their typical NMR doublet
and triplet signals between -0.5 and -2 ppm with integral ratios
of 2:1, which are characteristic for complexes of this type
(Figure 6, Table 2, Supporting Information Figure S25). The
outcome of this test reaction, which sets complexes 35 and 36
clearly apart from their single-atom bridged congeners 19, 20,
and 22, strongly indicates that the accessibility of a staggered
C5-ring conformation is the decisive criterion for the dichotomy
between unbridged zirconocene complexes vis-a`-vis those with
a single-atom bridge with regard to their respective reactions
with HAliBu2.33
Formation of a trialuminum cluster [Zr(µ-H)3(AliBu2)3(µ-Cl)2]
apparently requires that the Al-bound CH2 groups of its [AliBu2]
units fit snugly into spatial niches in the C-H periphery of the
C5-ring ligands. This appears to be achieved more favorably
with staggered than with eclipsed C5-rings. The view that the
Al-bound CH2 groups of these complexes are in close contact
with C5-ring H atoms, is supported by the observation of a
significant nuclear Overhauser effect (NOE) signal for complex
38, which connects these Al-CH2 groups with the ꢀ-H atoms
With regard to steric factors, bridged zirconocenes should
be at least as accessible for contacts with further [AliBu2] units
as their unbridged counterparts. Especially the totally unsub-
stituted ansa-zirconocenes 19 and 20 would appear to be more
open than their unbridged congeners 1, 6, 8, or 10, yet even
they differ from the latter by an apparent incapability to expand
their binuclear [Zr(Cl)(µ-H)2AliBu2] coordination geometry to
the tetranuclear [Zr(µ-H)3(AliBu2)3(µ-Cl)2] alternative. The
preference of unbridged complexes for the latter coordination
mode does not appear to be based on electronic factors either:
Several observables, such as reduction potentials of dichloride
complexes or CO stretching frequencies in the corresponding
Zr(II) dicarbonyl derivatives, as well as single-electron affinities
determined by DFT-based calculations, all point toward a net
electron-withdrawing effect of a Me2C or Me2Si bridge30 and,
hence, toward a greater tendency of bridged than of unbridged
zirconocenes to accept an electron-rich hydride in exchange for
a less donating chloride ligand.31
The only remaining explanation would thus appear to be that
the dichotomy between unbridged and bridged zirconocenes with
(29) An exception to this rule is the complex (C5H5)2Zr(H)(µ-H)2Al(H)(1,3,5-
tBu3C6H3) described in ref 7b, where the exceptional steric demand
of the supermesityl group appears to preclude the approach of further
AlR2X units.
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J. AM. CHEM. SOC. VOL. 130, NO. 51, 2008 17431

A R T I C L E S
Baldwin et al.
of the C5-ring ligands (Supporting Information Figure S26).34
In ethanediyl-bridged complexes with bisindenyl and bis(tet-
rahydroindenyl) ligands, such as 17 and 18, however, the
increased steric bulk of the annulated/substituted ring ligands
appears to be sufficient to restrict reactions with HAliBu2 again
to the formation of [ZrCl(µ-H)2AliBu2] rather than [Zr(µ-
H)3(AliBu2)3(µ-Cl)2] clusters.
Scheme 12
Our results show that either a chloride or a hydride can
occupy the terminal position in hydride-bridged monoaluminum
clusters formed by typical ansa-zirconocenes, depending on the
ligand framework of the zirconocene complex studied. In order
to assess the scope of related hydride-complex formation
reactions, other representatives of the general class of heter-
obinuclear ZrX(µ-H)2AliBu2 clusters were sought for. Reaction
of the fluoride analogue of complex 18, (EBTHI)ZrF2, with 1
or 2 equiv of HAliBu2 is found to yield, in accord with previous
studies,6h the dihydride 24 as the only zirconocene product, with
no indication for an intermediate of the kind (EBTHI)ZrF(µ-
H)2AliBu2 (Supporting Information Figure S27). This might thus
be a case where the coexistence of “hard” fluoride and “soft”
hydride coligands is unfavorable, such that the reaction leads
to the disjoined products, i.e., to ((EBTHI)ZrH2)2 and (FAliBu2)n.
the terminal Cl ligand of (SBI)ZrCl(µ-H)2AliBu2,35 rather than
the sought-for Me-versus-Cl exchange product (Scheme 12).
When the dimethyl complex (SBI)ZrMe2 is reacted with
slightly more than 2 equiv of HAliBu2 in benzene or toluene
solution, at room temperature or at -75 °C, several Zr-H
signals appear between ca. -0.5 and -2 ppm. This observation,
and an intense darkening of the reaction mixture in the course
of 1-2 h,36 indicate that any (SBI)ZrMe(µ-H)2AliBu2 formed
in this reaction would decay to other species, possibly to
complexes which contain their Zr center in a reduced oxidation
state. The sought-for species (SBI)ZrMe(µ-H)2AliBu2 might be
in equilibrium with HAliBu2 and (SBI)Zr(Me)H; the latter is
likely to form, by reductive elimination of methane,37,38 the
Zr(II) species (SBI)Zr. This would then most probably lead to
further decay reactions under the prevailing reaction conditions.
With a view toward “softer” coligands, we have tried to find
evidence for the existence of clusters of the type [ZrMe(µ-
H)2AlR2] containing a methyl group as a terminal ligand. When
the chloride-containing complex (SBI)ZrCl(µ-H)2AliBu2 is
treated with AlMe3, a shift of its Zr-H signal from 1.22 ppm
to a limiting value of 1.65 ppm, reached at high excess of AlMe3
([AlMe3]/[Zr] ) 128), indicates the occurrence of some reaction.
Product formation, as measured by the change in chemical shift,
∆δ(Zr-H), is found to be a function of the absolute concentra-
tion [AlMe3], however, rather than of the ratio [AlMe3]/[Zr]
(Supporting Information Appendix S-1). The product thus
appears to be an adduct, most likely with AlMe3 attached to
Conclusions
Our results show that reactions with HAliBu2 give different
reaction productsstetranuclear trihydrides [Zr(µ-H)3(AliBu2)3(µ-
Cl)2] versus binuclear dihydrides [ZrCl(µ-H)2AliBu2]sfrom
unbridged zirconocenes and from zirconocene dichlorides with
a single-atom bridge, respectively. Binuclear complexes [ZrH(µ-
H)2AliBu2], which contain a Zr-H instead of a Zr-Cl unit, are
formed when HAliBu2 is reacted with a zirconocene dihydride
or with a zirconocene dichloride carrying particularly congested
ring ligands. A terminal Zr-Me group instead of a Zr-Cl or
Zr-H unit, however, appears to render otherwise analogous
binuclear dihydrides prone to decay, possibly due to reductive
CH4 elimination.38
(30) Zachmanoglou, C. E.; Docrat, A.; Bridgewater, B. M.; Parkin, G.;
Brandow, C. G.; Bercaw, J. E.; Jardine, C. N.; Lyall, M.; Green, J. C.;
Keister, J. B. J. Am. Chem. Soc. 2002, 124, 9525.
On the basis of these results, we would propose that the
zirconocene dihydride complexes, which are formed in (SBI)-
(31) Neither do hydride affinities, calculated by DFT-based methods as
enthalpy differences for the hypothetical reaction (C5H5)2ZrH2 + H-
f (C5H5)2ZrH3-, indicate any preferred hydride uptake when the
interanullar wedge angle is increased to values characteristic for ansa-
zirconocenes complexes such as 19 or 20 (ref 23).
i
(35) Either Me or Bu can occupy terminal Zr(µ-H)2Al-alkyl positions in
i
such a complex. A closely related exchange of terminal Bu and Me
groups between the cation (SBI)Zr(µ-Me)2AlMe2-xiBux+ and mixed
alkylaluminum dimers, Al2(µ-Me)2Me4-xiBux, has been shown to occur
to an extent which is close to statistical expectations (ref 16a). This
Me vs iBu exchange was found to leave the chemical shift of the µ-Me
groups unchanged; in the present case, such an exchange is thus
unlikely to cause the changing chemical shifts of the (µ-H)2 signal.
(36) Similar decay reactions appear to proceed upon addition of HAliBu2
to mixtures of (SBI)ZrMe2 and AlMe3 in various ratios. Addition of
HAliBu2 to 1:1 mixtures of (SBI)ZrMe2 and B(C6F5)3, in the absence
as in the presence of excess AlMe3, appears to lead to partial transfer
of C6F5 groups from boron to aluminum, as indicated by signals at-
124.8,-155.2, and-163.7 ppm in the 19F NMR spectra of the reaction
mixtures (cf., Bochmann, M.; Sarsfield, M. J. Organometallics 1998,
17, 5908).
(32) Bu¨hl, M.; Hopp, G.; von Philipsborn, W.; Beck, S.; Prosenc, M.-H.;
Rief, U.; Brintzinger, H. H. Organometallics 1996, 15, 778.
i
(33) Possible steric interactions of the Al-bound Bu groups directly with
the interannular bridging units would be expected to be stronger with
a Me4C2 bridge, due to its greater spatial extension, than with a C2H4
bridgesprobably more comparable to that with a Me2C bridge. The
observation that the reactions of 35 and 36 with HAliBu2 result in
hydride complexes of the same type would thus rule out direct steric
interactions between AliBu2 groups and bridging units as a decisive
criterion for the relative stabilities of [ZrCl(µ-H)2AliBu2] vs [Zr(µ-
H)3(AliBu2)3(µ-Cl)2] species.
(34) A significant NOE signal connects the signal at 0.875 ppm due to the
CH3 groups of the interanullar bridge of complex 38 with the C5-H
signal at 5.839 ppm and identifies the latter as being due to the
R-positioned H atoms and that at 6.161 ppm as the resonance of the
ꢀ-H atoms. This order of the chemical shifts of R- and ꢀ-positioned
C5-H atoms, which we find also for the dimeric dihydride 24 and for
the dihydride cluster 26 (see the Supporting Information), groups these
five-coordinate hydride-bridged zirconocene complexes together with
typical four-coordinate, 16-electron dichloride complexes (cf., Gut-
mann, S.; Burger, P.; Prosenc, M.-H.; Brintzinger, H. H. J. Organomet.
Chem. 1990, 397, 21). This indicates that the Zr centers of these
hydride-bridged complexes do not reach a full 18-electron complement
and might explain why pentacoordination, rather than an even more
electron-deficient tetracoordination, is prevalent in these hydride-
bridged complexes.
(37) ((EBTHI)ZrH2)2 in C6D6 reacts with excess AlMe3sin the absence as
in the presence of MAOsto give mainly the dimethyl complex
(EBTHI)ZrMe2, together with some hydride species with a resonance
at-1.542 ppm; several C5-H signals between 5 and 6 ppm indicate
the formation of additional products. In these cases, evolution of CH4
is evident by the appearance of its characteristic sharp signal at 0.16
ppm, while this spectral region is obscured by the Me2Si resonances
in reactions systems involving (SBI)Zr derivatives.
(38) Alkane elimination from zirconocene alkyl hydrides: (a) Pool, J. A.;
Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2003, 125, 2241. (b)
Chirik, P. J.; Bercaw, J. E. Polym. Prepr. (Am. Chem. Soc., DiV. Polym.
Chem.) 2000, 41, 393. (c) McAllister, D. R.; Erwin, D. K.; Bercaw,
J. E. J. Am. Chem. Soc. 1978, 100, 5966.
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17432 J. AM. CHEM. SOC. VOL. 130, NO. 51, 2008

Alkylaluminum-Complexed Zirconocene Hydrides
A R T I C L E S
22,57 23,58 24,6g,h 31,59 33,28a,60 and 3530,61). Trimethylaluminum,
triisobutylaluminum, diisobutylaluminum hydride, and diisobuty-
laluminum chloride were used as obtained from Aldrich Chemical
Co., Milwaukee. CAUTION: alkylaluminum compounds are py-
rophoric and must be handled with special precautions (see, e.g.,
Shriver, D.F. The Manipulation of Air-sensitiVe Compounds; Robert
E. Krieger Publishing Company; Malabar, Florida, 1982).
Reaction mixtures for NMR measurements were prepared by
dissolving a weighed amount of solid zirconocene starting com-
pound, in an oven-dried J Young NMR tube, in benzene-d6 or
toluene-d8 under inert atmosphere. Aluminum reagents were added
as neat liquids via microliter syringes. NMR spectra were obtained
using Varian Inova 400, 500, or Mercury 300 spectrometers.
Chemical shifts are referenced to residual solvents peaks, 7.160
ppm for benzene and 7.000 ppm for the central aromatic proton
resonance of toluene.
ZrCl2/MAO reaction systems upon addition of HAliBu2,16b are
species of the type (SBI)Zr(Me · · ·MAO)(µ-H)2AlMe2, in which
the terminal Zr-Me group is bound, without being cationized,39
to a Lewis acidic Al site of MAO.40 Interaction of excess
AlMe3salways present in these reaction systemsswith minor
fractions of a MAO-free species (SBI)Zr(Me)(µ-H)2AlMe2, on
the other hand, might make side exchange in this species fast
enough to give the symmetrized ZrH2 signal observed for
zirconocene dihydride complexes in these reaction media.16b
Such a “mild” sequestration of the Zr-Me group would be
analogous to that observed in reaction systems (C5H5)2ZrMe2/
MAO at relatively low [Al]MAO/[Zr] ratios of 20-50,41 and
might be sufficient to suppress an otherwise prevalent methane
elimination, at least to a substantial degree. A residual extent
of CH4 release even from these MAO-stabilized complexes
might explain the ubiquitous formation of methane from
“working” zirconocene-based polymerization catalysts42 and,
likewise, the still enigmatic formation of zirconocene species
with a long-wavelength absorbance, at ca. 600 nm, possibly due
to a zirconocene species containing Zr in a reduced oxidation
state,43 in reaction systems containing (SBI)ZrCl2 and MAO
upon addition of HAliBu2.16b Further fates of such reduced
zirconocene species, i.e., their contribution to irreversible catalyst
deactivation and/or their reconversion to active cationic catalyst
species, remain a topic for future investigations.
Acknowledgment. This research was supported by USDOE
Office of Basic Energy Sciences (Grant No. DE-FG03-85ER13431),
by Fonds der Chemischen Industrie, and by BASELL Polyolefine
GmbH. We are grateful to Professor Peter Burger (University of
Hamburg) for DFT calculations on problems related to the work
reported here, to Dr. Heike Gregorius (BASELL, Frankfurt/Main)
for a gift of a sample of complex 16, and to Dr. Markus Ringwald
(MCAT, Konstanz) for gifts of samples of several zirconocene
complexes.
Supporting Information Available: Additional figures and
appendix outlining the analysis of changes in the chemical shift
of the ZrH2 signal of (SBI)ZrCl(µ-H)2AliBu2 upon addition of
Al2Me6. This material is available free of charge via the Internet
at http://pubs.acs.org.
Experimental Section
All operations were carried out under a protective dinitrogen
atmosphere, either in a glovebox or on a vacuum manifold.
Benzene-d6, toluene-d8, and other solvents used were dried by
vacuum transfer either from sodium benzophenone or from “ti-
tanocene”.44 Zirconocene complexes used as starting materials were
either purchased from Strem Chemicals, Newburyport (1, 6, 12,
and 17), obtained as gifts from Dr. M. Ringwald, MCAT, Konstanz
((EBTHI)ZrF2,45 27, 28,46 and 3647) and from BASELL Polyolefins,
Frankfurt/Main (1648), or prepared in our laboratories according
to published procedures (3,49 8,50 10,51 13,52 18,5319,54 20,55 21,56
JA8054723
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species are close to those of other neutral complexes, such as
(SBI)ZrCl2 and (SBI)ZrMe2, rather than to those of the cationic species
[(SBI)Zr(µ-Me)2AlMe2]+ MeMAO- or [(SBI)ZrMe+ · · ·MeMAO-]
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systems are quite similar to each other; both show broad absorbance
bands around 380 and 340 nm (ref 16b).
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9
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