Reaction of cyclopentadienyl zirconium derivatives with sterically encumbered arylaluminum hydrides: X-ray crystal structure of (η5-C5H5)2(H)Zr(μ 2-H)2Al(H)C6H2-2,4,6-Bu t3

Article abstract of DOI:10.1016/S0277-5387(99)00011-X

The novel reactions between Cp₂ZrMe₂ (Cp=η⁵-C₅H₅) and (Mes*AlH)₂ (Mes=C₆H₂-2,4,6-Bu^t₃) to afford Cp₂(H)Zr(μ²-H)₂Al(Me)Mes* (1) and between Cp₂Zr(Cl)H and [Mes*AlH₃Li(THF)₂]₂ to give Cp₂(H)Zr(μ₂-H)₂Al(H)Mes* (2) are described. The X-ray crystal structure and NMR spectroscopy of 1 indicate that, in the crystalline product, a methyl group has been transferred from zirconium to aluminum, and the metals have been connected by two hydrogen bridges. The compounds Cp₂ZrH₂ and Mes*AlMe₂ were also obtained as reaction products in the synthesis of 1. The aluminum center is four-coordinate in contrast to the five- or six-coordination observed in previously published complexes. Compound 2, which is believed to have a very similar structure to that of 1, was obtained in moderate yield as a colorless crystalline material.

Full text of DOI:10.1016/S0277-5387(99)00011-X

Polyhedron 18 (1999) 1885–1888
Reaction of cyclopentadienyl zirconium derivatives with sterically
encumbered arylaluminum hydrides: X-ray crystal structure of
(h5-C5H5)2(H)Zr(m2-H)2Al(H)C6H2-2,4,6-But3
*
Rudolf J. Wehmschulte, Philip P. Power
Department of Chemistry, University of California, Davis, CA 95616, USA
Received 17 August 1998; accepted 12 December 1998
Abstract
The novel reactions between Cp2ZrMe2 (Cp5h5-C5H5) and (Mes*AlH)2 (Mes*5C6H2-2,4,6-But3) to afford Cp2(H)Zr(m2-H)2Al-
(Me)Mes* (1) and between Cp2Zr(Cl)H and [Mes*AlH3Li(THF)2]2 to give Cp2(H)Zr(m2-H)2Al(H)Mes* (2) are described. The X-ray
crystal structure and NMR spectroscopy of 1 indicate that, in the crystalline product, a methyl group has been transferred from zirconium
to aluminum, and the metals have been connected by two hydrogen bridges. The compounds Cp2ZrH2 and Mes*AlMe2 were also
obtained as reaction products in the synthesis of 1. The aluminum center is four-coordinate in contrast to the five- or six-coordination
observed in previously published complexes. Compound 2, which is believed to have a very similar structure to that of 1, was obtained in
moderate yield as a colorless crystalline material.
1999 Published by Elsevier Science Ltd.
Keywords: Arylaluminum hydrides; Cyclopentadienyl zirconium derivatives; Bulky aryl ligands
1. Introduction
lations show that the energy of association between
Cp2ZrH2 and H3AlNH3 is 228.9 kcal mol21 [8]. In
parallel work using the more crowding zirconium pre-
cursor Cp*2ZrH2 (Cp*5h5-C5Me5) [9], Etkin and
Stephan have reported the synthesis of the unique dimer
[Cp*2Zr(H)(m2-H)2Al(H)(m2-H)]2 (4) via the reaction of
Cp*2ZrCl2 with LiAlH4. More recently, the synthesis of
The reaction of the sterically encumbered primary alane
(Mes*AlH2)2 [1] (Mes*5C6H2-2,4,6-But3) with a variety
of molecules has permitted the isolation of several new
types of aluminum compounds [2–5]. Examples include
the quasi-aromatic ring compounds (Mes*AlEPh)3 [2]
(E5P or As), the planar aluminoxane (Mes*AlO)4 [3], and
the sulfides (Mes*AlS)2 [4] and Al4S6(NMe3)4 [5]. The
behavior of (Mes*AlH2)2 and related species toward
transition metal substrates is essentially unexplored, how-
ever. Since the interaction of various aluminum reagents
with group 4 transition metal complexes is extremely
important in a number of catalytic processes [6,7], it is
surprising that studies of group 4-aluminohydride com-
pounds have only lately yielded well-characterized prod-
ucts. Raston et al. have shown that the reaction of
Cp2ZrCl2 (Cp5h5-C5H5) with LiAlH4 and H3GaL, or of
Cp2ZrH2 with H3AlL (L5quinuclidine or NMe3) leads to
a new type of heterobimetallic Lewis-base complex
2
2
9
9
the compounds hCp2Zr(H)(m -H)2j3Al and hCp2ZrH(m -
H)2j2AlH (Cp95h5-C5H4SiMe3) have been described
[10]. Here, we report that the reaction between Cp2ZrMe2
and half an equivalent of (Mes*AlH2)2 leads to the new
complex Cp2(H)Zr(m2-H)2Al(Me)Mes*, 1, and that the
reaction of Cp2Zr(Cl)H with Li(AlH3Mes*) leads to the
closely related novel species containing four-coordinate
aluminum Cp2(H)Zr(m2-H)2Al(H)Mes*, 2.
2. Experimental
2.1. General procedures
Cp2Zr(H)(m2-H)2AlH2(L) (3) with
aluminum atom [8]. Furthermore, density functional calcu-
a five-coordinate
All reactions were performed under N2 by using either
modified Schlenk techniques or a Vacuum Atmospheres
HE-43 drybox. Solvents were freshly distilled from so-
dium–potassium alloy and degassed twice prior to use.
*
Corresponding author. Tel.: 11-530-752-6913; fax: 11-530-752-
8895.
0277-5387/99/$ – see front matter
PII: S0277-5387(99)00011-X
1999 Published by Elsevier Science Ltd.

1886
R.J. Wehmschulte, P.P. Power / Polyhedron 18 (1999) 1885–1888
(Mes*AlH2)2 [1], [Mes*AlH3Li(THF)2]2 [11], Cp2ZrMe2
[12], and Cp2ZrCl(H) [13,14] were prepared according to
literature procedures. The compound Cp2ZrCl2 was pur-
chased commercially and used as received. Infrared spectra
were recorded in the range 4000–200 cm21 as a Nujol
mull between CsI plates, using a Perkin-Elmer PE-1430
spectrometer, and NMR spectra were recorded on a
General Electric GE-300 spectrometer.
liquor to ca. 30 ml, and crystallization at 2208C for 1
week afforded another 0.24 g. Yield, 50%. M.p. turns red
at 135–1408C, darkens to almost black at 1608, and
decomposes with gas evolution at 2438C. Anal. Calcd. for
C28H43AlZr; C, 67.55, H, 8.71. Found: C, 67.91, H, 8.65.
1H-NMR (C6D6): 7.47 (s, m-H, 1H), 7.49 (s, m-H, 1H),
5.50 (s, Cp-H, 5H), 5.07 (s, Cp-H, 5H), 2.57 (dd, Zr-H,
2
1H), JHH56.6 Hz, 1.67 (s, o-CH3, 9H), 1.62 (s, o-CH3,
9H), 1.32 (s, p-CH3, 9H), 21.99 (s, broad, m-H, 1H),
22.81 (s, broad, m-H, 1H). 13Ch1Hj-NMR (C6D6): 159.0,
157.8, (o-C), 150.1 ( p-C), 121.0, 120.9 (m-C), 39.0, 38.8
(o-C(CH3)3), 34.7 ( p-C(CH3)3), 33.6, 33.1 (o-CH3), 31.4
( p-CH3). 27Al–NMR (C6D6, n0578.340477 MHz): 210
ppm (s, broad) w1 / 2¯10,300 Hz. IR: 1837 (st, nAl–H), 1550
(m, nZr–H), 1330 (st, broad, nm–H).
2.2. Preparation of Cp2(H)Zr(m2-H)2 Al(Me)Mes*?0.5
hexane, 1?0.5 hexane
A solution of Cp2ZrMe2 (0.25 g, 1.0 mmol) in hexane
(10 ml) was added to a solution of (Mes*AlH2)2 (0.27 g,
0.5 mmol) in n-hexane (30 ml). The solution immediately
became yellow and then a lime-green color, within ca. 10
min. After ca. 1 h, it had assumed pale orange coloration.
The slightly cloudy solution was then stirred for a further
18 h and the fine solid was separated by decantation and
filtration. The filtrate was concentrated to ca. 15 ml and
cooled in a 2208C freezer overnight to afford 0.07 g of
pale orange plates. Concentration of the mother liquor, and
subsequent cooling in a 2208C freezer for a further week,
gave another 0.04 g of 1?0.5 hexane. Yield, 20% (based on
Al). M.p. turns opaque at 908C (desolvation), darkens to
almost black at 1548C, and decomposes with gas evolution
at 155–1588C. Drying of 1?0.5 hexane under reduced
pressure (ca. 0.01 mm Hg) for 3 h afforded analytically
pure 1 in quantitative yield Anal. Calcd. for C29H45AlZr;
C, 68.00, H, 8.86. Found: C, 68.38, H, 9.01.
2.4. X-ray data collection
X-ray quality crystals of 1?0.5 hexane were removed
from the Schlenk tube and immediately covered with a
layer of hydrocarbon oil. A suitable crystal was selected,
attached to a glass fiber and placed in the cold temperature
N2-stream as previously described [15]. The data were
˚
collected at 130 K with Mo Ka (l50.71073 A) radiation
on a Syntex R3 diffractometer. Crystal data are as follows:
C32H52AlZr, M5554.94, colorless blocks, 0.4430.243
˚
˚
˚
0.11 mm, a59.675(2) A, b512.692(3) A, c514.275(3) A,
a576.81(3)8, b570.89(3)8, g571.25(3)8, Z52, triclinic
3
¯
˚
˚
space group, P1, V51553.5(5) A , Mo Ka (l50.71073 A)
radiation, Syntex R3m/mV diffractometer, absorption
coefficient50.399 mm21, v scans, 2Q range 3–548 with v
scans, R150.0495 for 4675 (I.2s(I)) data; wR250.121
for all 6784 data. Calculations were carried out with the
SHELXTL-PLUS V. 5 set of programs. Scattering factors,
and the correction for anomalous scattering were taken
from common sources [16]. The structure was solved by
direct methods and refined by full-matrix least squares
refinement. Hydrides were located on difference Fourier
maps. Positions of the bridging hydrides were refined, and
the terminal hydrides (disordered) were refined with
1H-NMR (C6D6): 7.48, 7.45 (AB5system, m-H, 2H),
4JHH51.5 Hz, 5.47 (s, Cp, 5H), 5.04 (s, Cp, 5H), 2.52 (dd,
2
Zr-Ht, 1H), JHH59.0 Hz, 5.7 Hz, 1.62 (s, o-CH3, 9H),
1.59 (s, o-CH3, 9H), 1.32 (s, p-CH3, 9H), 1.22 (m,
CH2(hexane), 2H), 0.88 (t, CH3(hexane), 3H), J56.9 Hz,
0.02 (s, Al–Me, 3H), 21.83 (d, broad, m-H, 1H), 22.63 (s,
broad, m-H, 1H). 13Ch1Hj-NMR (C6D6): 159.1, 157.8,
(o-C), 149.7 ( p-C), 139.5 (broad, i-C), 121.2, 121.0 (m-
C), 100.8, 99.8 (Cp), 38.8 (o-C(CH3)3), 34.7 ( p-
C(CH3)3), 33.8, 33.4 (o-CH3), 31.9 (g-CH2(hexane)), 31.4
( p-CH3), 23.0 (b-CH2(hexane)), 14.3 (-CH3(hexane)),
23.0 (Al–Me). 27Al-NMR (C6D6, n0578.340477 MHz):
195 ppm (broad) w1 / 2¯7250 Hz. IR: nZr–H51545 cm21
(m), nAl–H–Zr51350 (broad, st).
˚
restrained, equal Zr–H distances of 1.80(2) A. Their
thermal parameters were riding on the bonded zirconium
atom. An absorption correction was applied using the
program XABS2 [17]. The structure is illustrated in Fig. 1
and selected bond lengths and angles are provided in the
figure caption.
2.3. Preparation of Cp2(H)Zr(m2-H)2 Al(H)Mes*, 2
A solution of [Mes*AlH3Li(THF)2]2 (0.85 g, 1.0 mmol)
in toluene (20 ml) was added to a slurry of Cp2ZrCl(H)
(0.51 g, 2.0 mmol) in toluene (20 ml) at 08C. The reaction
mixture was warmed to room temperature and stirred
overnight. The solvent was then removed under reduced
pressure and the remaining colorless solid was extracted
with n-pentane (100 ml). The pentane solution was cooled
in a 2208C freezer overnight to give 0.25 g of a mi-
crocrystalline colorless solid. Concentration of the mother
3. Results and discussion
Upon mixing (Mes*AlH2)2 and Cp2ZrMe2 in either
n-hexane or benzene solution, an immediate reaction takes
place which is manifested by a color change, first to bright
yellow, then to lime-green and finally to pale orange.
Workup of the reaction mixture gave the new compound

R.J. Wehmschulte, P.P. Power / Polyhedron 18 (1999) 1885–1888
1887
distances display significant variation: Zr–Hbr51.84–2.11
˚
˚
˚
A, Zr–C52.453–2.579 A, and Zr–centroid52.186–2.288
A. These distances are similar to those in recently reported
complexes [8–10]. The Al–H and Al–C distances are in
the normal range for sterically encumbered alanes [1,11],
˚
˚
˚
with Al(1)–H(1)51.70(3) A, Al(1)–H(2)51.73(3) A,
˚
Al(1)–C(1)51.992(3) A, and Al(1)–C(19)51.958(3) A.
The H(1)–Al(1)–H(2) angle is 78.9(15)8, the H–Al–C
angles are in the range 108.6(11)–110.1(11)8, and the
C(1)–Al(1)–C(19) angle is widest, with a value of
129.62(14)8. In addition, there is an angle of 25.68 between
the Al(1)–C(1) and the C(1)–C(4) vector. Such distortion
is a common feature of many compounds having the Mes*
substituent throughout the periodic table [1,11,20–22].
The compound 1?0.5 hexane is the only zirconium
aluminohydride that features four-coordinate aluminum in
the solid state since, in the compounds 3 [8] and 4 [9], the
aluminums are five-coordinate either as a result of Lewis
base complexation or self association. The Al–Zr sepa-
˚
rations in 1 are 2.970(3) (Zr(1)) and 2.977(3) (Zr(1a)) A.
Fig. 1. Computer generated thermal ellipsoid (30%) drawing of 1.
Hydrogen atoms bonded to carbon are omitted for clarity. Hydrogen
atoms bound to aluminum or zirconium are illustrated by circles of
The structure of 1 is maintained intact in solution at
room temperature as shown by 1H and 13C-NMR spec-
troscopy. There are two sets of signals for the inequivalent
o- and m-carbons, hydrogens, and t-butyl substituents of
the Mes* group, as well as for the Cp rings. In addition,
there are three separate signals for the bridging and
terminal hydrides which is also consistent with the pre-
servation of the structure illustrated in Fig. 1. The terminal
hydride at zirconium displays resolved scalar coupling to
the bridging hydrogens. Heating to 858C causes the Cp and
hydride signals to broaden, indicating an exchange process,
probably involving the opening and closing of the hydro-
gen bridges.
˚
arbitrary diameter. Selected distances (A) and angles (8) are as follows:
Zr(1)–H(1) 1.84(3), Zr(1)–H(2) 2.11(3), Zr(1)–H(3) 1.82(2), Zr–C(Cp)
2.432(9)–2.527(13), Al(1)–H(1) 1.70(3), Al(1)–H(2) 1.73(3), Al(1)–
C(1) 1.992(3), Al(1)–C(19) 1.958(3), H(1)–Zr(1)–H(2) 66.4(13), H(1)–
Zr–H(3) 67(2), H(2)–Zr(1)–H(3) 134(2), H(1)–Al(1)–H(2) 578.9(15),
H(1)–Al(1)–C(19) 110.1(11), H(2)–Al(1)–C(19) 109.4(11), C(1)–
Al(1)–C(19) 129.62(14), C(2)–C(1)–C(6) 116.4(2).
Cp2(H)Zr(m2-H)2Al(Me)Mes*, 1, as the only isolable
crystalline species in ca. 20% yield based on aluminum. 1H
and 13C NMR spectroscopic investigations of the glass
obtained after removal of the solvents from the mother
liquor of 1 showed broad signals in the Cp2Zr region and
sharp peaks that are consistent with the presence of
Mes*AlMe2 [18]. Apparently, instead of formation of a
In spite of the fact that samples of freshly isolated 1
were analytically pure, dissolving crystals of 1 in toluene
at room temperature affords a solution which displays an
EPR signal in the form of a complex 13-line multiplet.
This signal is attributable to the presence of a Zr(III)
species of formula Cp2Zr(m2-H)2Al(Me)Mes*. Simulation
of the spectrum of this compound for the coupling of
various nuclei to the unpaired electron affords constants of
3.9 G for the two equivalent bridging hydrogens and 8.7 G
for the aluminum (I55/2). The coupling to zirconium
(91Zr, I55/2, 11%) was not well-resolved, owing to peak
overlap. These values may be compared with those found
[23] for the Zr(III) species Na1[(h5-C5H4CMe3)2ZrH2]2
in THF with a (1H) 58.3 G and a (23Na) 510.1 G. The
generation of Zr(III) from Zr(IV) precursors via reaction
with reducing agents or under reducing conditions has
significant literature precedent. Recent examples include
the above-mentioned [hh5-C5H4C(CH3)3j2ZrH2]2 radical
anion [23], the production of ‘‘Cp2ZrH’’ and
Cp2ZrCH2PPh2 by heating of [Cp2ZrH(CH2PPh)]n
[24,25], and the production of Zr(III) allyl species by the
reaction of Cp2ZrCl2 with LiBu [26].
cation–anion
pair
such
as
[Cp2ZrMe]1
and
[H2(Me)AlMes*]2 a ligand exchange reaction is preferred,
which ultimately leads to the formation of Mes*AlMe2 and
Cp2ZrH2 [19], the latter being an almost insoluble poly-
mer. The compound 1, which may be described as an
adduct of Mes*Al(Me)H and Cp2ZrH2 appears to be an
intermediate in this ligand exchange process.
The colorless blocks of 1?0.5 hexane crystallize in the
¯
triclinic space group P1 with no crystallographically
imposed symmetry. The molecular structure may be de-
scribed as a Mes*AlMe moiety connected via two m2-
hydrogen bridges to a Cp2ZrH unit, as shown in Fig. 1.
The C(1)–Al(1)–C(19) plane is almost perpendicular to
the Al(1)–H(1)–H(2)–Zr(1) (or Zr(1a)) plane. The
Cp2ZrH group displays 50:50 disorder with respect to an
approximate mirror plane through C(1), Al(1), and C(19)
to accommodate both enantiomers. The bridging hydro-
gens, H(1) and H(2) are common to both sets of dis-
ordered atoms. Owing to this disorder, the Zr–H and Zr–C
The compound Cp2(H)Zr(m2-H)Al(H)Mes*, 2, was

1888
R.J. Wehmschulte, P.P. Power / Polyhedron 18 (1999) 1885–1888
conveniently
prepared
by
the
reaction
of
References
[Mes*AlH3Li(THF)2]2 [11] with Cp2ZrHCl. It is obtained
as a colorless microscrystalline solid from pentane solution
in moderate yield. The H-NMR spectrum is very similar
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1
to that of 1, however, the Al–Hterm resonance is not
observed at room temperature. Nonetheless, the IR spec-
trum clearly shows that it is present by displaying a strong
peak at 1837 cm21. As previously mentioned in the
introduction, the reaction of aluminum reagents (including
hydrides such as DIBALH, Bui2AlH) and organic sub-
strates can be catalyzed by transition metal complexes
[27]. Thus, it was thought that 2 might have some
relevance as a model for catalytically active species.
Accordingly, the reaction in toluene solution of 2 with one
equivalent of PhCCPh, or of (Mes*AlH2)2 with one
equivalent of PhCCPh in the presence of 0.1 equivalent of
2, were investigated. No reaction was observed after 3
days at room temperature in either case. Heating to 70–
808C for ca. 20 h led to a deep red solution in the case of
the stoichiometric reaction with 2. For the catalyzed
reaction of (Mes*AlH2)2 with 2, only a pale yellow
solution was obtained under the same conditions. A control
reaction between (Mes*AlH2)2 and PhCCPh showed no
reaction under the same conditions. It was therefore
concluded that 2 and PhCCPh react very slowly. However,
repeated attempts to obtain crystalline products from this
reaction were unsuccessful.
¨
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Supplementary data
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Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic Data
Centre CCDC: [111695 for compound 1. Copies of this
information may be obtained free of charge from the
Director, CCDC, 12 Union Road, Cambridge CB2 1EZ
[Fax: 144 (1223) 336-033].
Acknowledgements
We are grateful to the donors of the Petroleum Research
Fund administered by the American Chemical Society for
financial support.