Synthesis, characterization, and hydrolysis products of (η2-tBu2pz)AlH(μ:η1, η1-tBu2pz)2AlH2 - Structural characterization of a complex containing η1-, η2-, and μ:η1,η1-Pyrazolato ligands and a complex containing a terminal hydroxo ligand

Article abstract of DOI:10.1002/ejic.200400660

Treatment of alane-ethyldimethylamine with 3,5-di-tert-butylpyrazole (tBu₂pzH) in a 2:3 molar ratio afforded [(η²-tBu ₂pz)AlH(μ:η¹,η¹tBu ₂pz)₂AlH₂] in 57% yield. Hydrolysis of [(η²-tBu₂pz)AlH(μ:η¹. η¹-tBu₂pz)₂AlH₂] afforded variable mixtures of Al(tBu₂pz)₃, [(η²- tBu₂pz)AlH(μ:η¹,η¹-tBu ₂pz)₂AlH(OH)], and [(η²-tBu ₂pz)AlH(μ:η¹,η¹-tBu ₂pz)₂AlH(η¹-tBu₂pz)]. The structures of all new complexes were assigned from spectral and analytical data. In addition, the X-ray crystal structures of [(η²-tBu ₂pz)AlH(μ:η¹,η¹-tBu ₂pz)₂AlH(OH)] and [(η²-tBu ₂pz)AlH(μ:η¹,η¹-tBu ₂pz)₂AlH(η¹-tBu₂pz)] were determined. [(η²-tBu₂pz)AlH(μ:η¹, η¹-tBu₂pz)₂AlH(OH)] crystallizes as a dimeric molecule, and contains two bridging pyrazolato ligands, one η²-pyrazolato ligand, as well as terminal hydrido and hydroxo ligands. The hydroxo and η²-pyrazolato ligands possess a syn-relationship within the dimer. The hydroxy group proton does not participate in dihydrogen bonding, and instead appears to be intramolecularly hydrogen-bonded to the π-cloud of the η²-pyrazolato ligand. The overall structure of [(η²-tBu₂pz)AlH(μ: η¹,η¹-tBu₂pz)₂AlH(η ¹-tBu₂pz)] is very similar to that of [(η²- tBu₂pz)AlH(μ:η¹,η¹-tBu ₂pz)₂AlH(OH)], except that the η¹- and η²-pyrazolato ligands have an anti-disposition within the dimer. Molecular orbital calculations were carried out on [(η²-tBu ₂pz)AlH(μ:η¹,η¹-tBu ₂pz)₂AlH(OH)] to understand the hydrogen bonding and the η²-pyrazolato ligand coordination. The calculations predict that there is a 1.4 kcal/mol energy difference between η¹- and η²-pyrazolato ligand coordination, which implies that the observed η²-pyrazolato ligand occurs due to accommodation of the bulky tert-butyl groups. The intramolecular hydrogen bond between the hydroxo ligand proton and the π-cloud of the η²-pyrazolato ligand is estimated to have a bond strength of 3.7 kcal/mol. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.

Full text of DOI:10.1002/ejic.200400660

FULL PAPER
Synthesis, Characterization, and Hydrolysis Products of
(η2-tBu2pz)AlH(µ:η1,η1-tBu2pz)2AlH2 ؊ Structural Characterization of a
Complex Containing η1-, η2-, and µ:η1,η1-Pyrazolato Ligands and a Complex
Containing a Terminal Hydroxo Ligand
Zhengkun Yu,[a][‡] John E. Knox,[a] Andrey V. Korolev,[a][‡‡] Mary Jane Heeg,[a]
H. Bernhard Schlegel,[a] and Charles H. Winter*[a]
Keywords: Hydrogen bond / N ligands / Aluminum / Pyrazolato ligands / Molecular orbital calculations
Treatment of alane−ethyldimethylamine with 3,5-di-tert-bu-
bonded to the π-cloud of the η2-pyrazolato ligand. The over-
tylpyrazole (tBu2pzH) in a 2:3 molar ratio afforded [(η2- all structure of [(η2-tBu2pz)AlH(µ:η1,η1-tBu2pz)2AlH(η1-
tBu2pz)AlH(µ:η1,η1-tBu2pz)2AlH2] in 57% yield. Hydrolysis of
[(η2-tBu2pz)AlH(µ:η1,η1-tBu2pz)2AlH2] afforded variable mix-
tBu2pz)] is very similar to that of [(η2-tBu2pz)AlH(µ:η1,η1-
tBu2pz)2AlH(OH)], except that the η1- and η2-pyrazolato li-
tures of Al(tBu2pz)3, [(η2-tBu2pz)AlH(µ:η1,η1-tBu2pz)2Al- gands have an anti-disposition within the dimer. Molecular
H(OH)],
and
[(η2-tBu2pz)AlH(µ:η1,η1-tBu2pz)2AlH(η1- orbital
calculations
were
carried
out
on
[(η2-
tBu2pz)]. The structures of all new complexes were assigned
from spectral and analytical data. In addition, the X-ray crys-
tal structures of [(η2-tBu2pz)AlH(µ:η1,η1-tBu2pz)2AlH(OH)]
and [(η2-tBu2pz)AlH(µ:η1,η1-tBu2pz)2AlH(η1-tBu2pz)] were
determined. [(η2-tBu2pz)AlH(µ:η1,η1-tBu2pz)2AlH(OH)] crys-
tallizes as a dimeric molecule, and contains two bridging pyr-
azolato ligands, one η2-pyrazolato ligand, as well as terminal
tBu2pz)AlH(µ:η1,η1-tBu2pz)2AlH(OH)] to understand the hy-
drogen bonding and the η2-pyrazolato ligand coordination.
The calculations predict that there is a 1.4 kcal/mol energy
difference between η1- and η2-pyrazolato ligand coordina-
tion, which implies that the observed η2-pyrazolato ligand
occurs due to accommodation of the bulky tert-butyl groups.
The intramolecular hydrogen bond between the hydroxo li-
hydrido and hydroxo ligands. The hydroxo and η2-pyrazolato gand proton and the π-cloud of the η2-pyrazolato ligand is
ligands possess a syn-relationship within the dimer. The hy-
droxy group proton does not participate in dihydrogen bond-
ing, and instead appears to be intramolecularly hydrogen-
estimated to have a bond strength of 3.7 kcal/mol.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
Introduction
tBu2pzH,[3f] and has also described many reactions of this
complex.[3b,3d,3e,3g] As part of our work relating to the
synthesis of aluminum complexes with bridging hydride li-
gands,[4a] we have discovered a family of dimeric pyrazolato
complexes that are derived from the reaction of
alaneϪethyldimethylamine with tBu2pzH in a 2:3 stoichi-
ometry. These complexes are of the general formula [(η2-
tBu2pz)AlH(µ:η1,η1-tBu2pz)2Al(X)H], where X ϭ H, OH,
or η1-tBu2pz. In addition to providing additional rare ex-
amples of structurally characterized group 13 complexes
containing η2-pyrazolato ligands, this work describes the
first crystal structure of a complex containing η1-, η2-, and
µ:η1,η1-pyrazolato ligands as well as a structurally charac-
terized aluminum complex containing a terminal hydroxo
ligand. Aluminum complexes containing terminal hydroxo
ligands are very rare,[5] and have been of significant recent
interest as models for species that might be present in meth-
ylalumoxanes used in olefin polymerization processes.[5,6]
The terminal hydroxoaluminum ligand described herein is
stabilized by formation of an intramolecular hydrogen bond
to the π-cloud of an adjacent pyrazolato ligand, which con-
stitutes a novel hydrogen bonding interaction for a terminal
hydroxo complex.
The chemistry of aluminum complexes containing the
bulky 3,5-di-tert-butylpyrazolato (tBu2pz) ligand has been
an active area of research over the past several years.[1Ϫ4]
Advances that have emerged from this work include the first
examples of η2-pyrazolato ligand coordination to a group
13 element,[2,3c] structural models for ligand species that
occur in alumoxanes,[3b,3g] new coordination modes for
acetylide groups,[3d,3f] as well as stabilization of bridging
alkyl and hydride groups in pyrazolate-bridged dimers.[4]
Roesky has reported the synthesis of the complex
[H2Al(µ:η1,η1-tBu2pz)2AlH2], which was obtained by treat-
ment of equimolar amounts of alaneϪtrimethylamine with
[a]
Department of Chemistry, Wayne State University,
Detroit, Michigan 48202, USA
Fax: (internat.) ϩ 1-313-577-8289
E-mail: chw@chem.wayne.edu
[‡] Current address: Dalian Institute of Chemical Physics, Chinese
Academy of Sciences,
457 Zhongshan Road, Dalian, Liaoning 116023, P. R. China
[‡‡] Current address: SigmaϪAldrich Corporation,
5485 County Rd. V, Sheboygan, Wisconsin 53085, USA
330
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejic.200400660
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Synthesis, Characterization, and Hydrolysis Products of (η2-tBu2pz)AlH(µ:η1,η1-tBu2pz)2AlH2
FULL PAPER
those previously reported by Deacon.[2] The formulations
of 3 and 4 were based upon their spectral and analytical
data, and from their X-ray crystal structures as described
below. The 1H NMR spectrum of 3 in [D8]toluene at ambi-
ent temperature showed resonances for one type of pyrazol-
ato ligand, implying that exchange between the µ:η1,η1- and
the η2-pyrazolato ligands is faster than the NMR timescale.
Upon cooling to Ϫ60 °C, three equal intensity singlets for
the tert-butyl methyl groups are observed at δ ϭ 1.47, 1.43,
and 1.18 ppm and the pyrazolato methine resonances ap-
pear as a broad singlet at δ ϭ 6.22 ppm. In addition, ex-
tremely broad resonances are observed at δ ϭ 5.5 and
4.4 ppm in a 1:2 ratio, which correspond to some combi-
nation of the AlϪH and AlϪOH fragments. The low tem-
Results
Synthetic Chemistry
Scheme 1 outlines the synthetic approaches to the com-
plexes that were prepared in this study. Treatment of
alaneϪethyldimethylamine with tBu2pzH in a 2:3 stoichi-
ometry led to the dimeric pyrazolato complex 1 in 57%
crystallized yield. 1H NMR spectroscopic analysis of the
reaction product demonstrated that 1 was the only product
formed, when extreme care was taken to exclude water from
the reaction. The structure of 1 is proposed based upon
spectral and analytical data and from correlation with the
structurally characterized hydrolysis products 3 and 4 de-
scribed below.
1
perature H NMR spectrum of 3 is very similar to that of
1 and indicates similar structures. The infrared spectrum of
3 shows a strong hydroxy stretch at 3227 cmϪ1 and
aluminumϪhydrogen stretches at 1961, 1908, and 1860
cmϪ1. The low position of the hydroxy stretch is consistent
with hydrogen bonding, yet there is no evidence in the crys-
tal structure for dihydrogen bonding or intermolecular hy-
drogen bonding. Instead, it is likely that the hydroxy hydro-
gen atom is intramolecularly hydrogen-bonded to the π-
cloud of the adjacent η2-pyrazolato ligand. This possibility
is explored below in more detail using molecular orbital
1
calculations. The H NMR spectra of 4 show little differ-
ence at room temperature and Ϫ80 °C, and reveal two equal
intensity pyrazolato ligands. These data are consistent with
slow exchange between the µ:η1,η1-pyrazolato ligands and
the η1- and η2-pyrazolato ligand sites, but with rapid ex-
change between the η1- and η2-pyrazolato ligand sites. The
infrared spectrum of 4 shows an aluminumϪhydrogen
stretch at 1937 cmϪ1
.
The yield of 3 could be increased to 45% in a one-pot
reaction of alaneϪethyldimethylamine with 3,5-di-tert-bu-
tylpyrazole in a 2:3 ratio, followed by addition of water (1
equiv.), stirring for 2 h, workup, and then fractional crystal-
lization from hexane. Attempts to increase the yield of 4
Scheme 1. Preparation and hydrolysis of 1
1
In the H NMR spectrum of 1 in [D8]toluene at Ϫ90 °C, did not meet with much success. Treatment of
the tert-butyl methyl groups appear as three equal intensity alaneϪethyldimethylamine with tBu2pzH in a 2:4 ratio, fol-
singlets at δ ϭ 1.73, 1.43, and 1.22 ppm. The hydride reson- lowed by workup, afforded a crude product consisting of 2
ances are observed as 1:1:1 intensity broad singlets at δ ϭ (68%), 3 (24%), and 4 (8%).
6.1, 5.4, and 3.3 ppm, and the pyrazolato CH groups ap-
pear as singlets at δ ϭ 6.32 and 6.20 ppm in a 1:2 ratio. The
X-ray Crystal Structures of 3 and 4
infrared spectrum shows the AlϪH absorptions at 1962,
Complexes 3 and 4 were characterized by X-ray crystal-
1905, and 1890 cmϪ1. It was discovered that extended lography in order to understand their three dimensional
standing (Ͼ 1 week) of a hexane solution at 23 °C led to structures. The molecular structures of 3 and 4 are shown
the slow decomposition of 1 to tBu2pzH, 2, and new com- in Figures 1 and 2, respectively. The crystal data are given
pounds. As described below, the new compounds were ob- in Table 1.
tained as pure materials and identified to be 3 and 4 from
the hydrolysis of 1.
Complex 3 consists of a dinuclear unit in a boat confor-
mation held together by two µ:η1,η1-tBu2pz ligands. One
Treatment of 1 with water (1 equiv.) at Ϫ78 °C in toluene aluminum center is bonded to η2-tBu2pz and hydride li-
led to immediate gas evolution and the formation of a gands, while the other aluminum atom is bonded to
white, insoluble precipitate. Workup of the reaction mix- hydroxo and hydride ligands. The hydroxo and η2-pyrazol-
ture, followed by fractional crystallization from hexane, af- ato ligands are syn to each other within the dimer. The
forded pure samples of 2 (8%), 3 (15%), and 4 (23%) as aluminumϪnitrogen bond lengths to five-coordinate Al(1)
˚
colorless crystalline solids. The identity of 2 was confirmed range between 1.936Ϫ1.968 A, and are slightly shorter for
1
by comparison of its H and 13C{1H} NMR spectra with four-coordinate Al(2) [1.914(3), 1.920(3) A]. The
˚
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331

Z. Yu, J. E. Knox, A. V. Korolev, M. J. Heeg, H. B. Schlegel, C. H. Winter
FULL PAPER
η2-pyrazolato ligands are anti to each other within the di-
nuclear unit to minimize steric interactions. The non-bridg-
ing pyrazolato ligand planes are nearly perpendicular to
each other due to the steric hindrance of the tert-butyl
groups. The five-coordinate Al(1)ϪN bond lengths range
˚
between 1.892Ϫ2.082 A, and are slightly longer than those
˚
of the four-coordinate Al(2)ϪN bonds (1.871Ϫ1.933 A).
˚
The Al(1)ϪN(7) bond length [1.892(3) A] is much shorter
˚
than the Al(1)ϪN(8) bond length [2.082(4) A], indicating
unsymmetrical coordination of the η2-pyrazolato
ligand.[3c,7] The Al(2)ϪN(6) distance is 2.559(3) A,
˚
which clearly demonstrates η1-coordination. The
N(7)ϪAl(1)ϪN(8) angle is 40.55(13)°, which is slightly
smaller than the value in 3. The Al(1)ϪAl(2) distance is
˚
3.414(2) A, which is slightly longer than in 3, possibly due
to increased steric crowding in 4. Additionally, the intra-
molecular hydrogen bond in 3 may serve to reduce the
aluminumϪaluminum distance.
Figure 1. Perspective view of 3 with thermal ellipsoids at the 50%
probability level
Molecular Orbital Calculations of 3
To gain better insight into the nature of the pyrazolato
ligand bonding and the intramolecular hydrogen bond in 3,
molecular orbital calculations were performed. All calcu-
lations were carried out using the Gaussian suite of pro-
grams.[8] First we studied the model system 5, in which the
tert-butyl groups on the pyrazolato ligands were replaced
by hydrogen atoms. Previous work from our laboratories
indicated that replacement of alkyl groups by hydrogen
atoms in pyrazolato ligands does not significantly affect the
electronic structure of the molecule.[9] Optimization of 5 at
the B3LYP/6-311ϩϩG(d,p) level of theory[10Ϫ13] led to the
transformation of the η2-pz ligand in 5 to an η1-pz ligand.
In addition, the π-cloud of the η1-pyrazolato ligand twisted
away from the hydroxy group hydrogen atom on the adjac-
ent Al atom. Hence, 5 was not an acceptable theoretical
model for 3, which suggested that the bulky tert-butyl
groups on the pyrazolato ligands in 3 may play an import-
ant role in determining the overall structure.
Figure 2. Perspective view of 4 with thermal ellipsoids at the 50%
probability level; the tert-butyl methyl groups have been omitted
for clarity
1
1
˚
aluminumϪoxygen bond length is 1.867(3) A. The µ:η ,η -
pyrazolato ligands place the aluminum centers far enough
˚
apart [Al(1)ϪAl(2) 3.289(2) A], so that hydrogen or oxygen
atoms do not bridge in 3. The N(5)ϪAl(1)ϪN(6) angle is
42.32(11)°, which is typical of η2-pyrazolato ligands bonded
to aluminum.[2,3c] All intermolecular OH···H(hydride) con-
˚
˚
tacts are Ͼ 6 A, and the closest intermolecular OH···O con-
We next sought to carry out molecular orbital calcu-
tact is Ͼ 4.3 A. Thus, intermolecular dihydrogen bonding lations on the full experimental molecule 3. Since 3 has a
or intermolecular hydrogen bonding in 3 can be ruled out. large number of atoms, an ONIOM approach was em-
The low hydroxy stretch noted above in the infrared spec- ployed to reduce the cost of the overall calculation.[14] The
trum of 3, however, may be due to intramolecular hydrogen high level portion of 3 was the same as the model system
bonding between the hydroxy hydrogen atom and the π- and was calculated by B3LYP/6-311ϩϩG(d,p). The tert-bu-
cloud of the η2-pyrazolato ligand (OH···centroid distance ϭ tyl groups on the pyrazolato ligands were included in the
˚
2.574 A).
low-level portion and were calculated by the AM1 semi-
The molecular structure of 4 is similar to that of 3 except empirical method.[15,16] Figure 3 presents the results of this
that it contains an η1-tBu2pz ligand instead of a hydroxo treatment. Optimization of 3 afforded a less dramatic move-
ligand on the four-coordinate aluminum atom. The η1- and ment of the η2-pyrazolato ligand to the η1-bonding mode
332
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2005, 330Ϫ337

Synthesis, Characterization, and Hydrolysis Products of (η2-tBu2pz)AlH(µ:η1,η1-tBu2pz)2AlH2
FULL PAPER
Table 1. Summary of crystallographic data for 3 and 4
3
4
Empirical formula
Formula mass
Temp. [K]
C33H60Al2N6O
610.83
295(2)
C44H78Al2N8
773.10
295(2)
Crystal system
Space group
Triclinic
Triclinic
¯
¯
P1
P1
˚
a [A]
11.7811(11)
13.1426(13)
14.6042(14)
63.993(2)
83.503(3)
68.072(2)
1881.8(3)
2
1.078
0.109
9.9494(14)
11.7026(16)
21.031(3)
93.582(3)
90.742(3)
90.646(4)
2442.5(6)
2
˚
b [A]
˚
c [A]
α [°]
β [°]
γ [°]
Ϫ3
˚
V [A
]
Z
Dcalcd. [g/cm3]
Abs coeff [mmϪ1
F(000)
1.051
0.096
848
]
668
Cryst size [mm]
θ range [°]
0.35 ϫ 0.20 ϫ 0.12
1.83 to 28.32
Ϫ14 Յ h Յ 15
Ϫ15 Յ k Յ 16
0 Յ l Յ19
13657
8616 (Rint ϭ 0.035)
8616/0/406
0.837
0.20 ϫ 0.20 ϫ 0.15
1.94 to 28.30
Ϫ12 Յ h Յ12
Ϫ15 Յ k Յ15
0 Յ l Յ27
17562
11119 (Rint ϭ 0.040)
11119/0/536
0.820
Index ranges
No. of reflections collected
No. of independent reflections
No. of data/restraints/parameters
GOF/F2
R indices [I Ͼ 2σ(I)]
wR2 ϭ 0.1519
R indices (all data)
wR2 ϭ 0.1853
R1 ϭ 0.0621
wR2 ϭ 0.1448
R1 ϭ 0.1822
wR2 ϭ 0.1837
0.264/Ϫ0.622
R1 ϭ 0.0605
R1 ϭ 0.1957
0.574/Ϫ0.252
Ϫ3
˚
Largest diff peak/hole [e·A
]
R(F) ϭ ΣʈFo͉ Ϫ ͉Fcʈ/Σ͉Fo͉; Rw(F) ϭ [Σw(F2o Ϫ F2c)2/Σw(F2o)2]1/2
(Figure 3, a). The computed aluminumϪnitrogen bond ligand to the atoms in the adjacent pyrazoato ligand ranged
˚
˚
˚
˚
lengths were 1.89 A and 2.43 A, compared to 1.96 A deter- from 3.72Ϫ5.11 A. Removing the ability to hydrogen bond
mined experimentally for both bonds. A constrained system from the system led to an ONIOM energy increase of 3.7
was optimized in which the aluminumϪnitrogen bonds kcal/mol. This may be a slight underestimate of the true
were fixed at the experimentally observed distances. This bond strength due to some steric differences in the low-level
treatment led to an energy increase of only 1.4 kcal/mol portion of the ONIOM calculation.
(Figure 3, c), and subsequent unconstrained optimization
of this system afforded the structure shown in Figure 3 (a).
This small energy difference indicates that the stabilization
for the η2-configuration is within the energy of crystal
Discussion
The significance of this work resides principally in the
packing, and suggests that the η2-tBu2pz ligand in 3 orig- unusual structures of 3 and 4. Complex 4 is the first species
inates due to accommodation of the bulky tert-butyl groups. that contains η1-, η2-, and µ:η1,η1-pyrazolato ligands
In both cases it is evident that there is an intramolecular within a single molecule. Many pyrazolato complexes have
hydrogen bond. In the fixed bond structure (Figure 3, c), been reported that contain two of these coordination mo-
the range of distances for the hydroxy hydrogen atom to the des,[1,3b,17] but none until now has contained all three.
atoms in the pyrazolato ring core is 2.68Ϫ3.18 A. In the Aluminum complexes containing η2-pyrazolato ligands re-
˚
case where the aluminumϪnitrogen bond lengths were not main extremely rare,[2,3c] so 3 and 4 are additionally novel
frozen (Figure 3, a), these numbers have a slightly higher from this perspective.
˚
range of 2.61Ϫ3.56 A. To obtain an estimate of the stabiliz-
Complex 3 is a rare example of an aluminum compound
ation energy resulting from the hydrogen bond, the hydro- that contains a terminal hydroxo ligand.[18] The first struc-
gen atom on the hydroxy group was turned away from the turally characterized aluminum complexes containing ter-
pyrazolato ligand so that the hydroxy hydrogen atom was minal hydroxo ligands were reported in 1987.[5d,5e] A ter-
not in the proximity of the π-cloud of the η2-pyrazolato minal hydroxo ligand was structurally documented in
ligand (Figure 3, b). The optimized geometry for this sys- [Al4Si4H8O20·24H2O]4Ϫ, and has a hydroxoaluminumϪ
[5d]
˚
˚
tem had aluminumϪnitrogen bond lengths of 1.88 A and oxygen bond length of 1.783 A.
2.32 A. The distances from the oxygen atom of the hydroxo extensively hydrogen-bonded to interstitial water molecules.
Each hydroxo ligand is
˚
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Z. Yu, J. E. Knox, A. V. Korolev, M. J. Heeg, H. B. Schlegel, C. H. Winter
FULL PAPER
Figure 3. The three configurations of the model system with the DFT contribution as ball-and-stick and the AM1 portion as line drawing;
(a) model system with intramolecular hydrogen bond; (b) model system with hydroxy group reversed so there is no intramolecular
hydrogen bond; (c) model system with the aluminumϪnitrogen bond lengths constrained to experimental values
The other initial report described the structure of quinuclid- constitutes a unique coordination environment for the
inium hydroxo(2,2Ј,2ЈЈ-nitrilotriphenoxy)aluminate, which hydroxo ligand. Apparently, the tBu2pz ligands in 3 are so
contains an anionic aluminum center bonded to a terminal bulky that intermolecular hydrogen or dihydrogen bonding
hydroxo ligand.[5e] The quinuclidinium ion is also associ- is not possible, and the only hydrogen-bond donor available
ated with the oxygen atom through a NϪH···O hydrogen is the π-cloud of the adjacent η2-pyrazolato ligand. To
˚
bond. The aluminumϪoxygen bond length is 1.765(2) A. the best of our knowledge, this is the first example of a
Recently, Roesky and co-workers have reported a series of hydrogen bond to the π-cloud of a pyrazolato ligand.
aluminum complexes containing terminal hydroxo Hydrogen bonding between nitrogenϪhydrogen and
ligands.[5aϪ5c,18] The complexes are of the formula LArAl- oxygenϪhydrogen bonds and the π-clouds of various neu-
(OH)2,[5a] {LArAl(OH)}2(µ-O),[5b] and LArAl(OH)OAl(LAr)- tral aromatic compounds is well precedented.[20] Since the
(OCHϭNtBu),[5c] where LAr
ϭ
HC{(CMe)(2,6- tBu2pz ligand is aromatic and possibly more electron rich
iPr2C6H3N)}2. The aluminumϪoxygen bond lengths in LAr
-
than a neutral aromatic compound due to the formal nega-
[5a]
˚
Al(OH)2 are 1.6947(15) and 1.7107(16) A. In the solid tive charge, its π-system is probably a good hydrogen-bond
state, this molecule forms dimeric units with Al2O4H2 rings acceptor. Molecular orbital calculations suggest that there
that contain intramolecular AlϪOϪH···O(H)(Al) hydrogen is a preference for the formation of the OϪH···pzπ hydrogen
bonds. {LArAl(OH)}2(µ-O) exists as a monomer in the solid bond, and that an estimate of the bond strength for this
state, with aluminumϪoxygen bond lengths of 1.738(2) and interaction is about 3.7 kcal/mol. This bond strength is in
[5b]
˚
1.741(3) A.
Infrared spectroscopy suggests that the the range of a typical weak hydrogen bond, but is enough to
hydroxy groups do not participate in hydrogen bonding. It orient the hydroxo ligand hydrogen atom. Molecular orbital
was proposed that the hydroxo ligand hydrogen atoms re- calculations suggest that there is a very small energy differ-
side within the shielding cone of the LAr aromatic rings, due ence (1.4 kcal/mol) between η1- and η2-pyrazolato ligands
to an unusually upfield resonance for these hydrogen atoms in 3. It is possible that the hydrogen bond in 3 helps to
(δ ϭ Ϫ0.30 ppm). LArAl(OH)OAl(LAr)(OCHϭNtBu) exists stabilize the η2-pyrazolato ligand, since the molecular or-
in the solid state with an eight-membered Al2CHNO3 ring bital calculations suggested that there may be stronger hy-
that includes a strong OϪH···N hydrogen bond.[5c] The drogen bonding to this ligand coordination mode compared
aluminumϪoxygen bond length is 1.727(2) A. Parkin re- to the η1-pyrazolato ligand mode. The hydrogen bond in 3
˚
ported the synthesis of [HB(Me2pz)3]Al(OH)2 in 1990, al- requires that the hydroxo ligand hydrogen atom is pointed
though this complex was not structurally characterized and directly at the shielding cone of the aromatic tBu2pz ligand.
1
it is therefore not clear if it contains terminal hydroxo li- Thus, the H NMR chemical shift of this hydrogen atom
gands.[5f]
should be considerably upfield of where it would appear in
The metrics and stabilization of the terminal hydroxo li- the absence of this interaction. There is some ambiguity
gand in 3 can be compared with those of the previously regarding the assignment of this resonance in 3, since it
reported terminal hydroxo complexes noted above. The appears in the same region as the aluminum-bound hydrido
˚
aluminumϪoxygen bond length in 3 is 1.867(3) A, which is ligands. However, it resonates in the region of δ ϭ
considerably longer than the values in related terminal 4.6Ϫ5.5 ppm. For comparison, the 1H NMR chemical shift
hydroxo complexes. However, the aluminumϪoxygen bond of the hydroxo ligand protons in LArAl(OH)2 was δ
ϭ
length in 3 is similar to values observed in recently reported 12.47 ppm.[5a] In {LArAl(OH)}2(µ-O), where it was pro-
aluminum complexes containing bridging hydroxo ligands posed that the hydroxo ligand protons reside within the
[19]
˚
(1.81Ϫ2.09 A). The hydroxo ligand hydrogen atom in 3 shielding cone of the LAr aromatic rings, the hydroxo pro-
participates in hydrogen bonding to the π-cloud of the ton resonance appeared at δ ϭ Ϫ0.30 ppm.[5c] Roesky has
adjacent η2-pyrazolato ligand within the dimer, and thus argued that terminal AlϪOH protons are strong Brønsted
334
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2005, 330Ϫ337

Synthesis, Characterization, and Hydrolysis Products of (η2-tBu2pz)AlH(µ:η1,η1-tBu2pz)2AlH2
FULL PAPER
1
32.44 [s, C(CH3)3], 103.49 (s, pz CϪH), 164.37 [s, CϪC(CH3)3]
ppm. C33H60Al2N6 (594.84): calcd. C 66.63, H 10.17, N 14.13;
found C 66.84, H 10.10, N 13.95.
acids, and should resonate at low fields in the H NMR
spectra in the absence of other effects.[5a] This speculation
supports the low field position of the hydroxo ligand reson-
ance in LArAl(OH)2. The hydroxo ligand resonance in 3 is
considerably upfield of that in LArAl(OH)2, and therefore
may be shifted due to the interaction with the shielding
cone of the pyrazolato π-cloud.
Hydrolysis of 1 to Afford 2, 3, and 4: A 100-mL Schlenk flask con-
taining a solution of 1 (0.479 g, 0.805 mmol) in toluene (20 mL)
was treated with water (14.5 mg, 0.805 mmol) at Ϫ78 °C. Gas evol-
ution was immediately noted and small amount of insoluble white
material precipitated. The mixture was stirred at Ϫ78 °C for 3 h
and was then warmed to ambient temperature and was further
stirred for 15 h. The volatile components were then removed under
reduced pressure to afford a white solid. Dissolution of this solid
in hexane (20 mL), followed by filtration through a 2-cm pad of
Celite on a coarse glass frit to remove a small amount of insoluble
white solid, afforded a colorless solution. Fractional crystallization
of this solution at Ϫ20 °C afforded colorless crystals of 2 (0.035 g,
7.7%), 3 (0.075 g, 15%), and 4 (0.145 g, 23%). The order of crystal-
lization was 3, 2, and 4. The isolated yields were diminished due to
the care that was required to achieve pure samples. NMR spectro-
scopic analysis of the white solid removed by filtration showed it
to contain tBu2pzH as well as a solid that was insoluble in [D1]chlo-
roform.
Complex 3 is relevant to alumoxane chemistry,[12] since
terminal hydroxo ligands may be present in some alumox-
anes and the acidic hydroxo ligand hydrogen atom may acti-
vate polymerization precatalysts through protonation of a
metalϪalkyl bond. The existence of 3 must be related to
the presence of the extremely bulky tBu2pz ligands, which
block further reactions such as protonation of hydrido or
pyrazolato ligands. The stabilization of terminal hydroxo
ligands in the previously reported aluminum complexes[5] is
generally achieved only in the presence of very bulky ancil-
lary ligands, which can block further reactions. Thus, ter-
minal hydroxo ligands, if they occur in methylalumoxanes,
probably occupy extremely hindered sites. The high
Brønsted acidity of the aluminum-bound hydroxo ligand
proton leads to a strong preference for hydrogen bond for-
mation. As noted above, most of the known terminal
hydroxo complexes of aluminum are stabilized by hydrogen
bonding and have unique hydrogen-bond acceptors. The
only exception is {LArAl(OH)}2(µ-O), for which infrared
spectroscopy suggested a non-hydrogen-bonded hydroxo li-
gand. In 3, we have documented the presence of unique
intramolecular OϪH···pzπ hydrogen bonding.
Complex 2: This compound was identified by comparison of its 1H
and 13C NMR chemical shifts with the reported values.[2]
Data for 3: M.p. 145 °C. IR (Nujol): ν˜ ϭ 3227 (s, νAlOH), 1961 (m,
νAlH), 1908 (m, νAlH), 1860 (m, νAlH) cmϪ1. 1H NMR ([D8]toluene,
23 °C): δ ϭ 1.41 [s, 54 H, C(CH3)3], 5.5, 4.6 (2 br. s, 3 H, 2 AlϪH,
AlϪOH), 6.16 (s, 3 H, pz CϪH) ppm. 13C{1H} NMR ([D8]toluene,
23 °C): δ ϭ 31.14 [s, C(CH3)3], 32.58 [s, C(CH3)3], 103.61 (s, pz
CϪH), 164.55 (s, CϪC(CH3)3] ppm. C33H60Al2N6O (610.83):
calcd. C 64.87, H 9.90, N 13.76; found C 64.59, H 9.76, N 13.94.
Experimental Section
Data for 4: M.p. 135 °C (dec). IR (Nujol): ν˜ ϭ 1937 (w, νAlH) cmϪ1
.
1H NMR ([D8]toluene, 23 °C): δ ϭ 1.12 (s, 36 H, 4 C(CH3)3], 1.41
(s, 36 H, 4 C(CH3)3], 4.9 (br. s, 2 H, AlϪH), 6.02 (s, 2 H, pz CϪH),
6.22 (s, 2 H, pz CϪH) ppm. 13C{1H} NMR ([D8]toluene, 23 °C):
δ ϭ 31.00 [s, C(CH3)3], 31.39 [s, C(CH3)3], 31.94 [s, C(CH3)3], 33.31
[s, C(CH3)3], 101.12 (s, pz CϪH), 106.93 (s, pz CϪH), 162.12 [s,
CϪC(CH3)3], 168.69 [s, CϪC(CH3)3] ppm. C44H78Al2N8 (773.10):
calcd. C 68.36, H 10.17, N 14.49; found C 68.22, H 10.10, N 14.27.
General: All reactions were performed under dry argon using
standard Schlenk and dry-box techniques. Solvents were distilled
from various drying agents under argon prior to use.
AlaneϪethyldimethylamine was purchased from commercial
sources. 3,5-Di-tert-butylpyrazole was prepared according to a lit-
erature procedure.[21] 1H and 13C{1H} NMR spectra were obtained
in dry, degassed [D8]toluene, and were referenced to the residual
proton or carbon atom resonances of this solvent. Infrared spectra
were obtained using Nujol as the medium. Elemental analyses were
performed by Midwest Microlab, Indianapolis, IN. Melting points
were obtained on a Haake-Buchler HBI digital melting point ap-
paratus and are uncorrected.
X-ray Crystallography: Crystals of 3 (M ϭ 610.83) and 4 (M ϭ
773.10) were grown as described above and crystallized in the tri-
¯
clinic crystal system in the space group P1. Relevant details and
data are summarized in Tables 1Ϫ3. The crystals were mounted in
thin glass capillaries under nitrogen. Diffraction data were collected
with a Bruker P4 four-circle diffractometer coupled to a Bruker
Synthesis of 1: A 100-mL Schlenk flask was charged with a 0.5 m
solution of AlH3(NMe2Et) in toluene (6.0 mL, 3.0 mmol) and was
further diluted with toluene (40 mL). tBu2pzH (0.811 g, 4.5 mmol) Mo-Kα radiation (λ ϭ 0.71073 A). The structures were solved from
was added to this solution at 23 °C in small portions over 0.25 h.
The mixture was stirred at 23 °C for 16 h. At this point, the volatile
CCD area detector at 295(2) K, with graphite-monochromated
˚
the Patterson maps using SHELX-97[22] and refined against F2 on
all data by full-matrix least-squares with the same program. All
components were removed under reduced pressure to afford a white non-hydrogen atoms were refined anisotropically. The hydrogen
solid. This solid was dissolved in hexane (30 mL) and the solution
was filtered through a 2-cm pad of Celite on a coarse glass frit.
The filtered solution was placed in a Ϫ20 °C freezer for 24 h, dur-
atoms were included in the model at their geometrically calculated
positions and held riding or observed on a difference fourier and
refined. CCDC-185702 (3) and -185701 (4) contain the supplemen-
ing which time crystallization occurred. Removal of the solvent by tary crystallographic data for this paper. These data can be ob-
cannula, followed by vacuum drying, afforded 1 as colorless crys- tained free of charge on application to CCDC, 12 Union Road,
tals (0.505 g, 57%); m.p. 110 °C (dec). IR (Nujol): ν˜AlH ϭ 1961 (m), Cambridge CB21EZ, UK [Fax: (internat.) ϩ 44-1223-336-033; E-
1
1906 (m), 1885 (m) cmϪ1. H NMR ([D8]toluene, 23 °C): δ ϭ 1.40 mail: deposit@ccdc.cam.ac.uk]. Structure searches were conducted
[s, 54 H, C(CH3)3], 4.5 (br. s, 3 H, AlϪH), 6.16 (s, 3 H, pz CϪH) using the Cambridge Crystallographic Database, Version 5.25, No-
ppm. 13C{1H} NMR ([D8]toluene, 23 °C): δ ϭ 31.00 [s, C(CH3)3],
vember 2003.
Eur. J. Inorg. Chem. 2005, 330Ϫ337
www.eurjic.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
335

Z. Yu, J. E. Knox, A. V. Korolev, M. J. Heeg, H. B. Schlegel, C. H. Winter
FULL PAPER
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Al(1)ϪN(5)
Al(1)ϪN(6)
Al(2)ϪN(2)
Al(2)ϪN(4)
Al(2)ϪO(1)
Al(1)ϪH(1)
1.964(3)
1.936(3)
1.954(3)
1.920(3)
1.914(3)
1.867(3)
1.423(5)
1.534(5)
2.574
3.289(2)
95.11(11)
42.32(11)
102.64(11)
119.45(14)
115.19(15)
152.9
[3e]
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H(O1)···centroid[η2-pz]
Al(1) ··Al(2)
N(1)ϪAl(1)ϪN(3)
N(5)ϪAl(1)ϪN(6)
N(2)ϪAl(2)ϪN(4)
N(2)ϪAl(2)ϪO(1)
N(4)ϪAl(2)ϪO(1)
O(1)ϪH(O1)-centroid(η2-pz)
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Al(1)ϪN(4)
Al(1)ϪN(7)
Al(1)ϪN(8)
Al(2)ϪN(2)
Al(2)ϪN(3)
Al(2)ϪN(5)
Al(2)ϪN(6)
Al(1)ϪH
Al(2)ϪH
Al(1)···Al(2)
N(1)ϪAl(1)ϪN(4)
N(1)ϪAl(1)ϪN(7)
N(1)ϪAl(1)ϪN(8)
N(7)ϪAl(1)ϪN(8)
N(2)ϪAl(2)ϪN(3)
N(2)ϪAl(2)ϪN(5)
N(3)ϪAl(2)ϪN(5)
1.947(3)
1.952(3)
1.892(3)
2.082(4)
1.922(3)
1.933(3)
1.871(3)
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for support of this research (C. H. W.: Grant Nos. CHE-9807269,
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Eur. J. Inorg. Chem. 2005, 330Ϫ337

Synthesis, Characterization, and Hydrolysis Products of (η2-tBu2pz)AlH(µ:η1,η1-tBu2pz)2AlH2
FULL PAPER
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