Synthesis and X-ray Structure of the Novel Aluminum Complex [{η3-HB(3-Phpz)2(5-Phpz)]2Al[AlCl 4]. Catalysis of CO2/Propylene Oxide to Propylene Carbonate by the AlCl4- Anion

Full text of DOI:10.1021/ic951018r

2682
Inorg. Chem. 1996, 35, 2682-2684
Table 1. Crystallographic Data for 1
Synthesis and X-ray Structure of the Novel
Aluminum Complex
formula
fw
C62H62Al2B2Cl4N12O2 density (calcd), g/cm3 0.278
1224.65
abs coeff, mm-1
0.278
0.710 73
163
7.28
17.18
1.016
[{η3-HB(3-Phpz)2(5-Phpz)}2Al][AlCl4]. Catalysis
of CO2/Propylene Oxide to Propylene Carbonate
space group orthorhombic P212121 λ, Å
a, Å
b, Å
c, Å
V, Å3
Z
16.924(2)
17.483(6)
20.572(3)
6087(2)
4
T, K
-
R(F),a %
wR(F2),b %
GOF
by the AlCl4 Anion
Donald J. Darensbourg,* Ernest L. Maynard,
Matthew W. Holtcamp, Kevin K. Klausmeyer, and
Joseph H. Reibenspies
a R(F) ) ∑||Fo| - |Fc||/∑Fo. b wR(F2) ) {[∑w(Fo - Fc2)2]/
2
2
[∑w(Fo )2]}1/2
.
Department of Chemistry, Texas A&M University,
College Station, Texas 77843-3255
The reactivity of 1 with epoxide and carbon dioxide is also
briefly explored.
Experimental Section
ReceiVed August 4, 1995
All manipulations were performed on a double-manifold Schlenk
line under a nitrogen atmosphere or in an argon-filled glovebox.
Solvents were dried and deoxygenated by distillation from the ap-
propriate reagent under a nitrogen atmosphere before use. Potassium
hydrotris(3-phenylpyrazol-1-yl)borate was prepared according to the
published procedure.7 1H and 27Al NMR spectra were recorded on a
Varian XL-200E and XL-200 equipped with a broadband probe,
respectively. Aluminum(III)chloride was purchased from Baker Chemi-
cals and used as received.
The coupling reaction of carbon dioxide and epoxides in the
presence of various catalysts leads to either cyclic carbonates
and/or polycarbonates (eq 1).1 While zinc dicarboxylates have
been shown to be the most active catalysts for polycarbonate
production,2 a metalloporhyrinate system containing aluminum,
although not very active, has proven to be quite interesting.3
This latter catalyst is a living system and hence effective in
block polyether-polycarbonate synthesis. Furthermore, the
addition of 1-methylimidazole rendered the catalyst active for
cyclic carbonate production under mild reaction conditions.
Kisch and co-workers have also reported AlCl3/PPh3 to be an
active catalyst system for cyclic carbonate synthesis at ambient
temperature and atmospheric pressure.4 Hence, these reports
suggest that Al(III) complexes may have a rich chemistry with
epoxides and carbon dioxides.
Synthesis of [{η3-HB(3-Phpz)2(5-Phpz)}2Al][AlCl4]. To 0.56 g
of AlCl3 in 30 mL of THF was cannulated an equimolar quantity of
KHB(3-Phpz)3 in 20 mL of THF. The reaction mixture was stirred
overnight at ambient temperature during which time KCl precipitated
from solution, and was recovered by filtration. The solvent was
removed by vacuum from the filtrate to provide a white solid product
in slightly less than 50% yield, which analyzed for [C54H44B2N12Al]-
[AlCl4]‚2THF/AlCl3‚THF. Anal. Calcd for C66H68B2N12Al3Cl7O3: C,
1
55.51; H, 4.80. Found: C, 56.03; H, 5.05. The H and 13C NMR
spectra of 1 exhibited an array of peaks in the region 6-8 ppm and
120-130 ppm, respectively. Although these are consistent with other
related derivatives, in this instance we were unable to unequivocally
assign the resonances. The complex was more definitely identified by
27Al NMR spectroscopy in CDCl3. The AlCl4- species showed up as
a sharp singlet at δ ) 100 ppm with the cation, [η3-HB(3-Phpz)2(5-
Phpz)]2Al+, displaying a broad resonance centered at 4 ppm. Crystals
of the title complex were grown from a concentrated CH2Cl2/THF
solution of the complex by a slow diffusion of diethylether. The
complex cocrystallized with one molecule of THF and one molecule
of diethyl ether.
Recently, the coordination chemistries of aluminum, gallium,
and indium have been explored with the tridentate derivatives
of pyrazolyl hydroborates.5 A variety of interesting complexes
have been characterized, and the chemistry associated with these
derivatives is still in its infancy. As an ongoing part of our
efforts in carbon dioxide activation we have initiated studies
concerning the reactivity of groups 12 and 13 complexes with
epoxides and carbon dioxide.6 Of particular interest is a
comparison of the reactivities of a range of Al(III) complexes
with various structural motifs. In this regard an investigation
of the reaction of AlCl3 with one equivalent of potassium tris-
(3-phenylpyrazolyl)hydroborate has been carried out, and herein,
we report the X-ray characterization of [{η3-HB(3-Phpz)2(5-
Phpz)}2Al][AlCl4] (1), an interesting and unanticipated product.
Synthesis of [PPN][AlCl4]. A 70 mL of aliquot of tetrahydrofuran
was added to a Schlenk flask containing 0.50g of bis(triphenylphos-
phoranylidene)ammonium chloride ([PPN][Cl]) and an equimolar
quantity of aluminum trichloride, and the reaction mixture was stirred
at ambient temperature for 4 h. The solvent was removed in vacuo
leaving behind a white, solid product which was isolated in near
quantitative yield. Anal. Calcd for C36H30P2NAlCl4: C, 61.12; H, 4.27.
Found: C, 60.87; H, 4.39. An 27Al NMR spectrum of the product
-
exhibited one sharp resonance for the AlCl4 anion at δ ) 100 ppm.
Production of Propylene Carbonate from Propylene Oxide and
CO2. In a typical run, 0.10 g (0.099 mmol) of [{η3-HB(3-Phpz)2(5-
Phpz)}2Al][AlCl4] was placed in a Parr stainless steel reactor along
with approximately 15 mL (0.209 mol) of propylene oxide. The reactor
was charged with 750-1200 psi of carbon dioxide and heated at 60
°C with stirring for 24 h. 1H NMR spectroscopy was used to determine
the conversion of propylene oxide to propylene carbonate. The percent
conversion using complex 1 as catalyst was 38% or 802 turnovers for
a 24-h time period. On the other hand, [PPN][AlCl4] employed as
catalyst resulted in 56.8% conversion of PO/CO2 to propylene carbonate
during the same time period.
(1) (a) Kuran, W.; Listos, T. Macromol. Chem. Phys. 1994, 195, 977. (b)
Darensbourg, D. J.; Holtcamp, M. W. Coord. Chem. ReV., in press.
(2) (a) Soga, K.; Imai, E.; Hattori, I. Polym. J. 1981, 13, 407. (b) U.S.
Patent 4,943,677, 1990, to A. Rokicki, Air Products and Chemicals,
Inc., and Arco Chemical Co. (c) U.S. Patent 5,026,676, 1991, to S.
Motika, Air Products and Chemicals, Inc., Arco Chemical Co., and
Mitsui Petrochemical Industries Ltd.
(3) (a) Takeda, N.; Inoue, S. Makromol. Chem. 1978, 179, 1377. (b) Aida,
T.; Inoue, S. Macromolecules 1982, 15, 682. (c) Aida, T.; Inoue, S.
J. Am. Chem. Soc. 1983, 105, 1304.
(4) Ratzenhofer, M.; Kisch, H. Angew. Chem. 1980, 92, 303.
(5) (a) Frazer, A.; Piggott, B.; Harman, M.; Mazid, M.; Hursthouse, M.
B. Polyhedron 1992, 11, 3013. (b) Frazer, A.; Piggott, B.; Hursthouse,
M. B.; Mazid, M. J. Am. Chem. Soc. 1994, 116, 4127. (c) Dias, H.
V. R.; Huai, L.; Jin, W.; Bott, S. G. Inorg. Chem. 1995, 34, 1975.
(6) (a) Darensbourg, D. J.; Holtcamp, M. W.; Khandelwal, B.; Klausmeyer,
K. K.; Reibenspies, J. H. J. Am. Chem. Soc. 1995, 117, 538. (b)
Darensbourg, D. J.; Holtcamp, M. W. Macromolecules 1995, 28, 7577.
X-ray Crystallography of Complex 1. Crystal data and details of
data collection are given in Table 1. A colorless block for 1 was
mounted on a glass fiber with epoxy cement, at room temperature and
(7) Trofimenko, S.; Calabrese, J. C.; Thompson, J. S. Inorg. Chem. 1987,
26, 1507.
0020-1669/96/1335-2682$12.00/0 © 1996 American Chemical Society

Notes
Inorganic Chemistry, Vol. 35, No. 9, 1996 2683
Scheme 1
cooled to 163 K in a N2 cold stream. Preliminary examination and
data collection were performed on a Rigaku AFC5 diffractometer (Mo
KR λ ) 0.710 73 Å radiation). Cell parameters were calculated from
the least-squares fitting of the setting angles for 25 reflections. The ω
scans for several intense reflections indicated acceptable crystal quality.
Data were collected for 4.0° e 2θ e 50.0°. Three control reflections,
collected every 150 reflections, showed no significant trends. Back-
ground measurement by stationary crystal and stationary counter
technique at the beginning and end of each scan for 0.50 of the total
scan time. Lorentz and polarization corrections were applied to 5923
reflections for 1. An empirical absorption correction was applied (Tmax
) 0.9999, Tmin ) 0.8458). A total of 5923 unique reflections for 1,
with |I| g 2.0σI, were used in further calculations. The structures were
solved by direct methods [SHELXS, SHELXTL-PLUS program
package, Sheldrick (1988)]. Full-matrix least-squares anisotropic
refinement on F2, for all non-hydrogen atoms yielded R(F) ) 0.0728,
wR(F2) ) 0.1718, and S ) 1.016 at convergence for 1 [Shelxl-93,
Shedrick (1993)]. Hydrogen atoms were placed in idealized positions
with isotropic thermal parameters fixed at 0.08. Neutral atom scattering
factors and anomalous scattering correction terms were taken from ref
11.
Figure 1. Thermal ellipsoid drawing of the cation of complex 1 in
50% probability with the atomic numbering scheme.
Table 2. Selected Bond Lengths (Å)a and Angles (deg)a for the
Cation in 1
Cl(1)-Al(2)
Cl(3)-Al(2)
Al(1)-N(7)
Al(1)-N(11)
Al(1)-N(3)
2.128(6)
2.118(6)
1.949(8)
2.037(9)
2.045(9)
Cl(2)-Al(2)
Cl(4)-Al(2)
Al(1)-N(1)
Al(1)-N(5)
Al(1)-N(9)
2.115(6)
2.112(6)
1.951(9)
2.039(8)
2.066(8)
Results and Discussion
In an attempt to synthesize [η3-HB(3-Phpz)3]AlCl2 from AlCl3
and 1 equiv of K[HB(3-Phpz)3] in THF, we have prepared the
product which results from a redistribution process. This was
immediately evident from an 27Al NMR spectrum of the product,
where the strong, sharp signal for AlCl4- was observed at 100
ppm. The analogous reaction carried out using the sterically
less-demanding K[HB(3,5-Me2pz)] reagent had previously been
demonstrated by Cowley and coworkers to afford the salt
resulting from a redistribution reaction (eq 2).8 Nevertheless,
N(7)-Al(1)-N(1)
N(1)-Al(1)-N(11)
N(1)-Al(1)-N(5)
N(7)-Al(1)-N(3)
N(11)-Al(1)-N(3)
N(7)-Al(1)-N(9)
N(11)-Al(1)-N(9)
N(3)-Al(1)-N(9)
Cl(4)-Al(2)-Cl(3)
Cl(4)-Al(2)-Cl(1)
Cl(3)-Al(2)-Cl(1)
N(2)-N(1)-Al(1)
C(12)-N(3)-Al(1)
C(21)-N(5)-Al(1)
N(8)-N(7)-Al(1)
N(10)-N(9)-Al(1)
179.3(4) N(7)-Al(1)-N(11)
89.4(3) N(7)-Al(1)-N(5)
90.9(3) N(11)-Al(1)-N(5)
89.6(3) N(1)-Al(1)-N(3)
95.7(3) N(5)-Al(1)-N(3)
91.7(3) N(1)-Al(1)-N(9)
85.0(3) N(5)-Al(1)-N(9)
178.5(4) Cl(4)-Al(2)-Cl(2)
112.0(3) Cl(2)-Al(2)-Cl(3)
109.3(3) Cl(2)-Al(2)-Cl(1)
107.4(2) C(1)-N(1)-Al(1)
123.3(6) N(4)-N(3)-Al(1)
40.5(7) N(6)-N(5)-Al(1)
139.0(7) C(28)-N(7)-Al(1)
121.4(6) C(39)-N(9)-Al(1)
90.3(3)
89.3(3)
179.4(4)
91.0(3)
84.8(3)
87.7(3)
94.5(3)
108.8(3)
110.2(2)
109.2(3)
129.9(7)
113.9(6)
115.2(6)
132.5(6)
138.8(7)
-
it was anticipated that the bulky nature of the HB(3-Phpz)3
anion would, combined with the small ionic radius of the
aluminum cation, prevent such a reaction from occurring.
Indeed, an X-ray crystallographic study of the reaction product
revealed one phenyl pyrazolyl group belonging to each of the
HB(3-Phpz)3 ligands had isomerized to the 5-position in order
to relieve steric strain (Vide supra).
114.1(6) N(12)-N(11)-Al(1) 116.6(6)
C(48)-N(11)-Al(1) 138.5(7)
a Estimated standard deviations are given in parentheses.
K[HB(3,5-Me2pz)3] + AlCl3 f
Table 2. The geometry about the aluminum center in the cation
is that of a slightly distorted octahedron; i.e., it exhibits a
tetragonal distortion where the AlN6 octahedron is compressed
along the N(1)-Al(1)-N(7) axis relative to the other two. The
Al(1)-N(1) and Al(1)-N(7) bond distances are 1.949(8) and
1.951(9) Å respectively, whereas the other four Al(1)-N bond
distances have an average value of 2.047[9] Å. This most
probably results from an electronic effect, as a consequence of
the electron-withdrawing phenyl substituents migrating to the
5-position. The average deviation from planarity of the three
[{η3-HB(3,5-Me2pz)3}2Al][AlCl4] (2)
X-ray quality single crystals of [{η3-HB(3-Phpz)2(5-Phpz)}2-
Al][AlCl4], (1), were grown from a concentrated dichlo-
romethane/tetrahydrofuran solution of the complex by slow
diffusion of diethylether over a period of several hours. The
structure consists of discrete [{η3-HB(3-Phpz)2(5-Phpz)}2Al]+
and [AlCl4]- ions with no short interionic contacts. One
molecule each of THF and diethylether were also found in the
crystal lattice. Figure 1 illustrates a drawing of the cation, and
selected bond lengths and angles for the cation are listed in
-
AlN4 planes is 0.008Å. The AlCl4 unit is tetrahedral as
expected with an average Al-Cl bond distance of 2.118[6]Å.
Rearrangement of the 3-phenyl substituent is schematically
represented in Scheme 1. A similar isomerization has previously
been observed by Looney and Parkin, where a unimolecular
(8) Cowley, A. H.; Jones, R. A.; Geerts, R. L.; Nunn, C. M. Angew. Chem.,
Int. Ed. Engl. 1988, 27, 277.

2684 Inorganic Chemistry, Vol. 35, No. 9, 1996
Notes
rearrangement of the bis(3-tert-butylpyrazoyl)hydroborate ligand
in (η2-H2B(3-t-butylpz)2)Al(CH3)2 to provide the more stable
isomer {η2-H2B(3-t-butylpz)(5-t-butylpz)}Al(CH3)2 was noted.9
copolymerization of propylene oxide and CO2 to afford poly-
carbonates was observed.10 However, a control experiment
-
employing AlCl4 with the innocent PPN+ counterion as a
1
On the basis of kinetic measurements obtained Via H NMR
catalyst for the PO/CO2 cycloaddition revealed it to be even
more effective, providing a turnover rate of 1200 mol of PC/
mol of [PPN][AlCl4]/24 h. Hence, the presence of the cation
in complex 1 does not significantly affect the catalytic process.
spectroscopy, a free energy of activation of 32.7 kcal/mol was
determined, which is consistent with the proposal that the
reaction occurs by a 1,2-shift of the tert-butylpyrazole group.
In the instance reported upon herein the barrier for rearrange-
ment is somewhat lower, since the process occurs readily at
ambient temperature.
Acknowledgment. The financial support of this research by
the National Science Foundation (Grant CHE91-19737) and the
Robert A. Welch Foundation is greatly appreciated. The authors
wish to thank Professor A. Clearfield for use of the Rigaku
AFC5-R X-ray diffractometer.
Complex 1 was found to be an effective catalyst or catalyst
precursor for the production of propylene carbonate (PC) from
CO2 and propylene oxide (PO) under rather mild reaction
conditions. That is, at 60 °C under CO2 pressures less than
1200 psi, complex 1 catalyzed the conversion of propylene oxide
and carbon dioxide to the corresponding cyclic carbonate with
a turnover rate of 800 mol of PC/mol of 1/24 h. No
Supporting Information Available: Tables of atomic coordinates
and isotropic displacement parameters, anisotropic thermal parameters
and complete bond lengths and angles for complex 1 and ball-and-
stick drawings of complex 1 including solvent molecules and complete
atomic numbering scheme (13 pages). This material is contained in
many libraries on microfiche, immediately follows this article in the
microfilm version of the journal, and can be ordered from the ACS;
see any current masthead page for ordering information.
(9) Looney, A.; Parkin, G. Polyhedron 1990, 9, 265.
(10) Preliminary results from our laboratory indicates the neutral aluminum-
(III) complex, {η3-HB(3,5-Me2pz)3}AlMe2,9 affords, in addition to
cyclic carbonates, some polycarbonates.
(11) International Tables for X-ray Crystallography; Kynoch Press: Bir-
mingham, England, 1974.
IC951018R