Chiral porous solids based on lamellar lanthanide phosphonates [5]

Full text of DOI:10.1021/ja0163772

J. Am. Chem. Soc. 2001, 123, 10395-10396
10395
Chiral Porous Solids Based on Lamellar Lanthanide
Phosphonates
Owen R. Evans, Helen L. Ngo, and Wenbin Lin*
Department of Chemistry, CB #3290
UniVersity of North Carolina
Chapel Hill, North Carolina 27599
ReceiVed June 8, 2001
There has been tremendous recent interest in the design and
synthesis of functional solids based on metal-organic coordina-
tion networks.1-7 In contrast to inorganic zeolites, metal-organic
frameworks are typically synthesized under mild conditions and
thus allow rational design of novel materials by incorporating
bridging ligands with desired size, shape, chirality, and electronic
properties. Over the past few years, many metal-organic
coordination networks have been shown to exhibit unique
properties including functional group/size selective sorption,
catalysis, gas storage, molecular recognition, and second harmonic
generation.2,6-7 With a few notable exceptions,6 homochiral
metal-organic frameworks have not been explored for applica-
tions in heterogeneous asymmetric catalysis and enantioselective
separation. Motivated by elegant work of Mallouk et al. on
preparative-scale chiral separation of a racemic mixture of
naphthylamines using R-Zr(HPO4)2 intercalated with chiral cy-
clophanes,5 we have recently explored the design and synthesis
of thermally and hydrolytically robust, single-crystalline, chiral
porous metal-organic frameworks based on metal bisphospho-
nates. Herein we wish to report the synthesis, structures, chiral
separation, and catalytic properties of a series of homochiral
porous lamellar lanthanide bisphosphonates.
Figure 1. Coordination environment of 1f. The asymmetric unit
(excluding water guest molecules) is shown with ellipsoids at 30%
probability.
intense and broad O-H stretching vibrations between 3700 and
3200 cm-1. Thermogravimetric analyses show that 1a-g lose
11.7-17.4% of total weight by 90 °C, corresponding to the loss
of 9-14 water molecules per formula unit (expected 11.6-
17.2%).9 These formulations have been supported by microanaly-
sis results (Supporting Information).
A single-crystal X-ray diffraction study performed on [Gd(R-
L-H2)(R-L-H3)(H2O)4]‚12H2O (R-1f) reveals a 2D lamellar struc-
ture consisting of 8-coordinate Gd centers and bridging binaph-
thylbisphosphonate groups. 1f crystallizes in the chiral space group
P212121.10 The asymmetric unit of 1f consists of one Gd center,
two bridging binaphthylbisphosphonate groups, four coordinated
water molecules and 12 water guest molecules (Figure 1). The
Gd center adopts a square anti-prismatic geometry by coordinating
to four water molecules and four phosphonate oxygen atoms of
four different binaphthylbisphosphonate ligands. Three of the four
crystallographically independent phosphonate groups (P1-P3) are
monodeprotonated and coordinate to the Gd center in a mono-
dentate fashion, while the fourth phosphonate group (P4) remains
protonated and also coordinates to the Gd center in a monodentate
fashion.11 If we disregard the coordinated water molecules, the
four phosphonate groups coordinate to the Gd center in a highly
distorted tetrahedral geometry with O-Gd-O angles ranging from
81.4 to 147.4°. The binaphthyl subunits have dihedral angles of
118.2 and 121.0° for L-H2 and L-H3, respectively. The skewed
configuration of the binaphthyl subunits in combination with the
distorted tetrahedral phosphonate coordination allows the forma-
tion of an elongated 2D rhombohedral grid lying in the ac plane
(Figure 2a). The 2D grid has Gd-Gd-Gd angles of 153.6 and
26.7°, and Gd-Gd separations of 16.83 and 16.78 Å. Such 2D
Homochiral lanthanide bisphosphonates with the general
formula of [Ln(L-H2)(L-H3)(H2O)4]‚xH2O (Ln ) La, Ce, Pr, Nd,
Sm, Gd, Tb, x ) 9-14, 1a-g) were synthesized by slow
evaporation of an acidic mixture of nitrate or perchlorate salts of
Ln(III) and 2,2′-diethoxy-1,1′-binaphthalene-6,6′-bisphosphonic
acid(L-H4)8 in methanol at room temperature (eq 1). The IR
(6) 1H NMR spectra of a digested mixture of Na2(EDTA) and 1a in D2O
show the absence of peaks characteristic of methanol, indicating that the
volatile guest species in 1a-g are water molecules.
spectra of 1a-g display strong phosphorus-oxygen stretches at
950-1150 cm-1. In addition, the IR spectra of 1a-g also exhibit
(7) (a) Seo, J. S.; Whang, D.; Lee, H.; Jun, S. I.; Oh, J.; Jeon, Y. J.; Kim,
K. Nature 2000, 404, 982. (b) Kepert, C. J.; Prior, T. J.; Rosseinsky, M. J. J.
Am. Chem. Soc. 2000, 122, 5158.
(1) (a) Desiraju, G. R. Crystal Engineering: The Design of Organic Solids;
Elsevier: New York, 1989. (b) Lehn, J.-M. Supramolecular Chemistry:
Concepts and PerspectiVes; VCH Publishers: New York, 1995. (c) Batten,
S. R.; Robson, R. Angew. Chem., Int. Ed. 1998, 37, 1461. (d) Zawarotko, M.
J. Chem. Soc. ReV. 1994, 283.
(2) (a) Fujita, M.; Kwon, Y. J.; Washizu, S.; Ogura, K. J. Am. Chem. Soc.
1994, 116, 1151. (b) Eddaoudi, H. L.; O’Keeffe, M.; Yaghi, O. M. Nature
1999, 402, 276. (c) Gardner, G. B.; Kiang, Y. H.; Lee, S.; Asgaonkar, A.;
Venkataraman, D. J. Am. Chem. Soc. 1996, 118, 6946. (d) Kiang, Y. H.;
Gardner, G. B.; Lee, S.; Xu, Z. T.; Lobkovsky, E. B. J. Am. Chem. Soc. 1999,
121, 8204.
(3) (a) Clearfield, A. Prog. Inorg. Chem. 1998, 47, 371. (b) Clearfield, A.
Chem. Mater. 1998, 10, 2801. (c) Alberti, G. ComprehensiVe Supramolecular
Chemistry; Pergamon Press: New York, 1996; Vol. 7, pp 151-185. (d) Cao,
G.; Lynch, V. M.; Swinnea, J. S.; Mallouk, T. E. Inorg. Chem. 1990, 29,
2112.
(4) (a) Mallouk, T. E.; Gavin, J. A. Acc. Chem. Res. 1998, 31, 209. (b)
Zhang, Y.; Frink, K. J.; Clearfield, A. Chem. Mater. 1993, 5, 495. (c) Frink,
K. J.; Wang, R.-C.; Colon, J. L.; Clearfield, A. Inorg. Chem. 1991, 30, 1438.
(d) Johnson, J. W.; Jacobsen, A. J.; Butler, W. M.; Rosenthal, S. E.; Brady,
J. F.; Lewandowski, J. T. J. Am. Chem. Soc. 1989, 111, 381.
(5) Garcia, M. E.; Naffin, J. L.; Deng, N.; Mallouk, T. E. Chem. Mater.
1995, 7, 1968.
(8) (a) Lin, W.; Evans, O. R.; Xiong, R.-G.; Wang, Z. J. Am. Chem. Soc.
1998, 120, 13272. (b) Lin, W.; Wang, Z.; Ma, L. J. Am. Chem. Soc. 1999,
121, 11249. (c) Evans, O. R.; Xiong, R.-G.; Wang, Z.; Wong, G. K.; Lin, W.
Angew. Chem., Int. Ed. 1999, 38, 536. (d) Lin, W.; Ma, L.; Evans, O. R.
Chem. Commun. 2000, 2263. (e) Evans, O. R.; Lin, W. Chem. Mater. 2001,
13, 2705.
(9) L-H4 was synthesized via a Pd-catalyzed phosphonation reaction
between 6, 6′-dibromo-2,2′-diethoxy-1,1′-binaphthalene and diethyl phosphite.
See: (a) Jaffre`s, P.-A.; Bar, N.; Villemin, D. J. Chem. Soc., Perkin Trans. 1
1998, 2083. (b) Hirao, T.; Masunaga, T.; Ohshiro, Y.; Agawa, T. Synthesis
1981, 56.
(10) X-ray single-crystal diffraction data for 1f was collected on a Bruker
SMART CCD diffractometer. Crystal data for 1: orthorhombic, space group
P212121, with a ) 7.771(1) Å, b ) 23.900(2) Å, and c ) 32.676(2) Å, V )
6068.8(6) Å3, Z ) 4, Dcalc ) 1.63 g/cm3, T ) 213 K, Mo KR radiation (λ )
0.71073 Å). Least-squares refinement based on 8025 reflections with I > 2σ-
(I) and 708 parameters led to convergence, with a final value of R1 ) 0.097
and wR2 ) 0.188. Flack parameter ) 0.03(2).
(11) The P-O distances about P4 are consistent with this formulation. The
P4-O11 distance is 1.47 Å, whereas the P-OH distances are 1.54 and 1.55
Å, respectively.
10.1021/ja0163772 CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/28/2001

10396 J. Am. Chem. Soc., Vol. 123, No. 42, 2001
Communications to the Editor
Figure 3. XRPD for 1e before removal of water guest molecules (top),
after removal of water guest molecules (middle), and upon subsequent
exposure to water vapor (bottom).
conditions. In all three cases, the products were essentially racemic
(ee < 5%).12 We have also examined the use of 1e in the Lewis-
acid promoted ring opening of meso-anhydrides. Treatment of
meso-2,3-dimethylsuccinic anhydride (1 mmol) with methanol (10
mmol) in a dry THF (5 mL) suspension of powdered and
desolvated 1e (0.1 mmol) afforded the corresponding hemiester
in 81% yield with a disappointingly low ee of <5%.13 Both
cyanosilylation and ring-opening reactions are apparently cata-
lyzed heterogeneously since no reaction took place with the
supernatant obtained from a suspension of 1e.14 Moreover, 31P
NMR spectroscopy shows that these supernatants are free from
ligand contamination. Upon completion of all reactions, the
catalysts could be recovered in quantitative yields (>98%) and
used repeatedly without the loss of catalytic activity.
A preliminary attempt of enantioselective separation of racemic
trans-1,2-diaminocyclohexane with ammonia-treated R-1e at a
substrate/host ratio of 1.4 gives an enantio-enrichment of 13.6%
in S,S-1,2-diaminocyclohexane in the beginning fractions and an
enantio-enrichment of 10.0% in R,R-1,2-diaminocyclohexane in
the ending fractions.15 Despite the limited enantioselectivity, 1a-g
represent a new generation of robust recyclable chiral porous
solids that are capable of chiral separation and heterogeneous
catalysis. With a precise knowledge of their single crystal
structures and facile tunability of their building blocks, the present
research holds great promise in the development of novel chiral
porous materials capable of separation and catalysis with practi-
cally useful enantioselectivity.
Figure 2. (a) A view of 2D framework of 1f down the b axis. Ethoxy
groups have been omitted for clarity. (b) A view of stacking of 2D
framework of 1f along the c axis showing interdigitation of binaphthyl
rings from adjacent layers. The coordination environments of P atoms
and Gd atoms are represented with blue and orange polyhedra, respec-
tively. (c) A space-filling model of 1f viewed down the a axis.
grids stack along the b axis via interdigitation of the binaphthyl
rings from the adjacent layers (Figure 2b). There is still void space
formed between the lamellae after such interdigitation of the
binaphthyl rings, and a space-filling model of 1f viewed along
the a axis clearly indicates the presence of large asymmetric
channels with a largest dimension of ∼12 Å. X-ray powder
diffraction studies indicate that 1a-g are isomorphous, while CD
spectra show that 1e samples made from R- and S-L-H4 are
supramolecular enantiomers (Supporting Information).
We have studied the framework stability of 1a-g using powder
X-ray diffraction techniques. Upon evacuation at room temper-
ature and 10-2 Torr for 24 h, compounds 1a-g experienced
weight losses consistent with the removal of 9-14 water
molecules per formula unit. While the major peaks in the XRPD
patterns of desolvated 1a-g have broadened significantly, XRPD
patterns identical to those of the pristine samples were obtained
upon exposing desolvated 1a-g to water vapor (Figure 3). Such
behavior is consistent with a distortion of the long-range lamellar-
type structure with preservation of local coordination environ-
ments of 1a-g during desolvation.
Good framework stability and reversibility of the dehydration
processes of 1a-g have prompted us to explore their applications
in heterogeneous catalysis and chiral separation. The presence
of both Lewis and Bro¨nsted acid sites in 1a-g has rendered them
capable of catalyzing several organic transformations including
cyanosilylation of aldehydes and ring opening of meso-carboxylic
anhydrides. Treatment of benzaldehyde (1 mmol) and cyanotri-
methylsilane (2 mmol) with a CH2Cl2 (5 mL) suspension of
powdered and desolvated 1e (0.1 mmol) afforded mandelonitrile
in 69% yield after acidic workup. Similar reactions with 1-naph-
thaldehyde and propionaldehyde afforded the corresponding
cyanohydrin products in 55 and 61% yield, respectively. The
indiscriminate catalytic efficiency between aldehydes with varying
sizes suggests that the lamellar lanthanide phosphonates can
readily swell to facilitate substrate transport under reaction
Acknowledgment. We acknowledge NSF (CHE-9875544) for finan-
cial support. We also thank Dr. Richard Staples at Harvard University
for X-ray data collection. W.L. is an Alfred P. Sloan Fellow, an Arnold
and Mabel Beckman Young Investigator, and a Cottrell Scholar of
Research Corp.
Supporting Information Available: Experimental procedures, ana-
lytical data, five figures, and one table (PDF). An X-ray crystallographic
file (CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
JA0163772
(12) The enantiomeric excesses of the products were determined from 1H
NMR analysis on the (+)-R-methoxy-R-trifluoromethylphenylacetic acid [(+)-
MTPA] esters of the cyanohydrins.
(13) The ee was assessed from the 1H NMR analysis of the amide formed
from the reaction of the hemiester with (R)-(+)-1-(1-naphthyl)ethylamine in
the presence of thionyl chloride.
(14) 1e also efficiently catalyzed Diels-Alder reaction between methyl
acrylate and cyclopentadiene.
(15) The ee was determined by chiral HPLC analysis after derivatization
with m-toluoyl chloride.