Tertiary building units: Synthesis, structure, and porosity of a metal-organic dendrimer framework (MODF-1)

Full text of DOI:10.1021/ja011692+

11482
J. Am. Chem. Soc. 2001, 123, 11482-11483
Zn4O(TTA)(DMA)2‚(DMA)3(H2O)21 (DMA ) N,N-dimethyl-
acetamide), which contains zinc ions linking a carboxy-terminated
triarylamine dendrimer 4,4′,4′′-tris(N,N-bis(4-carboxylphenyl)-
amino)triphenylamine (H6TTA).
The carboxy-terminated, first-generation dendrimer with tri-
arylamine branches and a triarylamine core was prepared (eq 1)
using recently developed palladium-catalyzed amination meth-
odology, which provides rapid access to a variety of triarylamine
materials.7 We prepared the H6TTA linker containing six periph-
eral carboxyl groups to coordinate metal ions by coupling of
4,4′,4′′-tribromotriphenylamine with 4,4′-di-tert-butylcarboxy di-
phenylamine and subsequent thermal conversion of the tert-butyl
carboxylates to the free acids.
Tertiary Building Units: Synthesis, Structure, and
Porosity of a Metal-Organic Dendrimer Framework
(MODF-1)
Hee K. Chae,| Mohamed Eddaoudi,† Jaheon Kim,†
Sheila I. Hauck,‡ John F. Hartwig,‡ Michael O’Keeffe,§ and
Omar M. Yaghi*,†
Department of Chemistry, UniVersity of Michigan
930 North UniVersity AVenue
Ann Arbor, Michigan 48109
Department of Chemistry, Yale UniVersity
New HaVen, Connecticut 06520
Department of Chemistry
Arizona State UniVersity, Tempe, Arizona 85287
ReceiVed July 11, 2001
Studies on the assembly of metal-organic frameworks (MOFs)
have uncovered methods to build extended structures from
molecular building blocks.1-3 The synthesis of a new type of
porous material using strategies based on the expansion and
decoration of vertexes in basic nets has demonstrated the wide
scope of this chemistry.4 In particular, crystalline MOFs that
maintain their open structure in the absence of guests can perform
highly selective separations.5
Most MOFs have been constructed from simple unbranched
connectors, such as 1,4-benzenedicarboxylate and 1,3,5-benzene-
tribenzoate.4a In contrast, branched connectors, such as small
dendrimers, have not been used to prepare MOFs because these
connectors are more flexible. This flexibility could impede
crystallization and characterization by single-crystal X-ray dif-
fraction.6 Yet, crystalline metal-organic dendrimer frameworks
(MODFs) generated from branched linkers such as small den-
drimers would (a) provide information on dendrimer structure
and (b) allow construction, from nanometer-sized building blocks,
of well-defined frameworks with chemical and physical properties
derived from those of the organic units. We report the synthesis,
crystal structure, and sorption chemistry of the first MODF:
To prepare MODF-1, H6TTA (0.002 g, 2.0 × 10-6 mol) and
Zn(NO3)2‚6H2O (0.012 g, 4.0 × 10-5 mol) were dissolved in a
mixture of DMA/EtOH/H2O (1.5/0.4/0.1 mL), and the resulting
solution was placed in a quartz tube (10 mm × 8 mm o.d. × i.d.,
140 mm length). The sample was heated at a constant rate of 0.1
°C/min to 75 °C, held at 75 °C for 24 h, cooled at a constant rate
of 0.1 °C/min to 65 °C, held at this temperature for 10 h, and
cooled to room temperature. Light-green crystals of MODF-1 were
formed and isolated by washing with a mixture of DMA and
EtOH (3 × 4 mL) and drying briefly in air (ca. 1 min) (0.002 g,
56%). The infrared spectrum of this compound showed that the
carboxylic acid units had been deprotonated as indicated by
absence of υÃ-Η stretch and a shifted of υCdO stretch from 1690.4
cm-1 in the free carboxylic acid to 1599.0 cm-1 in MODF-1, as
for other Zn-bound carboxylates.8 The structure was deduced from
chemical microanalysis and single-crystal X-ray diffraction
data.9,10
* E-mail: oyaghi@umich.edu.
† University of Michigan.
‡ Yale University.
§ Arizona State University.
| Visiting Professor at University of Michigan from Hankuk University of
Foreign Studies, Korea.
All crystal structures may be viewed and manipulated at http://
www.umich.edu/∼yaghigrp/structures.html.
(1) For recent reviews: (a) Batten, S. R.; Robson, R. Angew. Chem., Int.
Ed. 1998, 37, 1460-1494. (b) Stein, A.; Keller, S. W.; Mallouk, T. E. Science
1993, 259, 1558-1564. (c) Ozin, G. A.; Bowes, C. L. AdV. Mater. 1996, 8,
13-28. (d) Cheetham, A. K.; Fe´rey, G.; Loiseau, T. Angew. Chem., Int. Ed.
1999, 38, 3268-3292. (e) Yaghi, O. M.; Li, H.; Davis, C.; Richardson, D.;
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J.; Ward, M. D. Sci. Am. 1992, 267, 48-53.
(6) To our knowledge no crystalline network materials built from organic
dendrimers have been reported. For structures of discrete organometallic
dendrimers, see: (a) Kriesel, J. W.; Konig, S.; Freitas, M. A.; Marshall, A.
G.; Leary, J. A.; Tilley, T. D. J. Am. Chem. Soc. 1998, 120, 12207-12215.
(b) Lambert, J. B.; Hongwei, W. Organometallics 1998, 17, 4904-4909. (c)
Mekelburger, H.-B.; Rissanen, K.; Vogtle, F. Chem. Ber. 1993, 126, 1161-
1169. For references and reviews on organometallic dendrimers, see: (d)
Knapen, J. W. J.; van der Made, A. W.; de Wilde, J. C.; van Leeuwen, P. W.
N. M.; Wijkens, P.; Grove, D. M.; van Koten, G. Nature 1994, 372, 659. (e)
Onitsuka, K.; Takahashi, S. J. Synth. Org. Chem. Jpn. 2000, 58, 988. (f)
Brunner, H. J. Organomet. Chem. 1995, 500, 39.
(7) (a) Louie, J.; Hartwig, J. F. J. Am. Chem. Soc. 1997, 119, 11695. (b)
Yamamoto T.; Nishiyama, M.; Koie, Y. Tetrahedron Lett. 1998, 39, 2367.
(c) Harris, M. C.; Buchwald, S. L. J. Org. Chem. 2000, 65, 5327.
(8) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Wiley: New
York, 1958.
(9) The preparation is fully reproducible for MODF-1 and for other MODFs
derivatives employing Cu (II), which we are currently studying. Elemental
microanalysis for Zn4O(TTA)(DMA)2‚(DMA)3(H2O)5 (MODF-1): Calcd C,
53.13; H, 5.08; N, 6.97. Found C, 53.53; H, 5.61; N, 6.68. Discrepancy in
formulation of guests in MODF-1 using single-crystal X-ray diffraction and
elemental microanalysis is due to inherent problems associated with their facile
loss at room temperature and their extensive disorder in the solid state.
(10) A green needle crystal of MODF-1 was analyzed: orthorhombic, space
group P212121 with a ) 20.3300(9) Å, b ) 23.1692(11) Å, c ) 32.0081(15)
Å, V ) 15076.8 (12) Å3, Z ) 4, dcalc ) 0.900 g/cm3, and µ(Mo KR) ) 0.685
mm-1. All measurements were made on a SMART CCD area detector with
graphite-monochromated Mo KR radiation. R1 ) 0.0885 and Rw (all data) )
0.2723.
(2) For nitrogen-containing linkers, see, for example: (a) MacGillivray,
L. R.; Subramanian, S.; Zaworotko, M. J. Chem. Commun. 1994, 1325. (b)
Klein, C.; Graf, E.; Hossieni, M. W.; De Cian, A. New J. Chem. 2001, 25,
207-209. (c) Evans, O. R.; Wang, Z.; Xiong, R. G.; Foxman, B. M.; Lin, W.
Inorg. Chem. 1999, 38, 2969-2973. (d) Carlucci, L.; Ciani, G.; Macchi, P.;
Proserpio, D. M.; Rizzato, S. Chem. Eur. J. 1999, 5, 237-243. (e) Kiang,
Y.-H.; Lee, S.; Xu, Z.; Choe, W.; Gardner, G. B. AdV. Mater. 2000, 12, 767-
770. (f) Lu, J.; Harrison, W. T. A.; Jacobson, A. J. Inorg. Chem. 1996, 35,
4271-4273.
(3) For carboxylate linkers, see, for example: (a) Seki, K.; Takamizawa,
Mori, W. Chem. Lett. 2001, 122-123. (b) Serpaggi, F.; Fe´rey, G. J. Mater.
Chem. 1998, 8, 2737-2731. (c) Yaghi, O. M.; Li, H.; Groy, T. L. J. Am.
Chem. Soc. 1996, 118, 9096. (d) Yaghi, O. M.; Li, G.; Li, H. Nature 1995,
378, 703-706.
(4) (a) Eddaoudi, M.; Moler, D. B.; Li, H.; Chen, B.; Reineke, T. M.;
O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, 319-330. (b) O’Keeffe,
M.; Eddaoudi, M.; Li, H.; Reineke, T. M.; Yaghi, O. M. J. Solid State Chem.
2000, 152, 3-20.
(5) (a) Li, H.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. Nature 1999,
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2000, 122, 1391-1397.
10.1021/ja011692+ CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/26/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 46, 2001 11483
Figure 1. Crystal structure of MODF-1 showing the connectivity around (a) the TTA dendrimer and (b) the basic zinc acetate cluster building blocks
(Zn, orange in (a) and magenta polyhedra in (b); C, gray in (a) but shown in space-filling style with six different colors in (b); O, red; N, blue), which
respectively form (c) trigonal prismatic TBUs (blue) and octahedral SBUs (orange) that are reticulated into a NiAs-type structure having (d) a 3-D chiral
channel system. (all hydrogen atoms, guests, and DMA ligands have been omitted for clarity).
The crystal structure of MODF-1 shows a 3-D porous
framework that is created by linking the branched organic unit
with Zn(II) ions at each of the six carboxylate termini (Figure
1). The asymmetric unit is composed of one TTA linker, one
Zn4O cluster, and two DMA ligands. Each carboxylate of TTA
bridges two zinc ions that are each part of a basic zinc acetate-
type cluster5a,11 containing four tetrahedral Zn centers surrounding
a central oxide. Two terminal DMA ligands are bound to one of
the zinc centers in the cluster (Figure 1a). In other words, each
carboxylate lies on the edge of a Zn4O group to produce an
octahedral arrangement of carboxylates (Figure 1b).
Triarylamines crystallize with propeller structures, and this
feature is present in the linkers of MODF-1. Within the TTA
dendrimer, the central N lies in the same plane as the three outer
N atoms. Each aryl ring of the triarylamine unit is canted by 15-
45° to relieve repulsion between ortho-H atoms of the benzene
units and create C-H‚‚‚π interactions (3.827 (8)-3.841 (8) Å).
The N atoms are nearly planar and lie only short distances from
the triangular plane of the three ipso C atoms (0.097 (2)-0.250
(3) Å) bound to the nitrogen.
The dihedral angles between the benzene units create the
linker’s 3-D structure in which the set of carboxylate C atoms
form a slightly distorted, large trigonal prism with edge dimen-
sions ranging from 9.693 to 15.893 Å (Figure 1c). The trigonal
prismatic units of MODF-1 contain nine branching phenylene
units, which can be considered tertiary building units (TBUs).12
The presence of trigonal prismatic and octahedral building units4b
rationalizes the NiAs topology13 found in MODF-1 (Figure 1c).
The resulting framework contains a labyrinth of chiral channels
that are between 8 and 17 Å in diameter and contain at least three
DMA and 21 water molecules per formula unit (Figure 1d).
A division of the crystal volume into void volume (70%),
organic matter (25%), and Zn4O clusters (5%) shows that the bulk
of the material is void volume and organic matter. To evaluate
the stability of MODF-1, we conducted, under inert atmosphere,
thermal gravimetric analysis of crystalline MODF-1 (5.328 mg).
We heated the sample from 20 to 700 °C at 10 °C/min. The
sample lost weight immediately. A sharp decrease in weight was
observed at 50 °C, and this decrease continued slowly up to 300
°C. The total mass loss was 28.3%, which corresponds to
dissociation of two DMA ligands and the three DMA and five
H2O guests in each formula unit (Calcd: 28%). A further decrease
in weight 53.3% occurred between 400 and 600 °C, corresponding
to loss of the TTA unit (55%) and decomposition of the material.
To confirm the stability of this evacuated framework and
evaluate its permanent porosity, we measured its gas sorption
isotherm. MODF-1 (40 mg, ca. 0.01 mm size particles) was
evacuated at 5 × 10-5 Torr and 25 °C for 20 h in a microbalance
(CAHN 1000) using published methods.5 Sorption isotherms for
gases or vapors of N2, Ar, CH2Cl2, CCl4, C6H6, and C6H12. All
isotherm plots of (mg sorbed)/[g of MODF-1] vs P/P0 (P0 )
saturation pressure of sorbate) showed type-I sorption behavior,
indicating that evacuated MODF-1 has permanent porosity.
Similar to those of zeolites and porous MOFs, the isotherms of
MODF-1 are reversible and show no hysteresis upon desorption
of sorbates from the pores. A pore volume of 0.26 cm3/g and an
estimated surface area of 740 m2/g were calculated from the N2
isotherm.
This report demonstrates the potential of using branched organic
building blocks to link inorganic clusters and provide crystalline
frameworks.
Acknowledgment. The Department of Energy and National Science
Foundation support to J.F.H. (DE-FG02-96ER14678), M.O’K (DMR-
9804817) and O.M.Y. (DMR-9980469) is gratefully acknowledged.
H.K.C. is supported by the Korean Science and Engineering Foundation
(KOSEF 2000-1-12200-002-3).
Supporting Information Available: Preparation and characterization
of H6TTA, crystallographic data, bond lengths and bond angles, including
FT-IR data and sorption isotherms for MODF-1 (PDF). An X-ray
crystallographic file (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
(11) Clegg, W.; Harbron, D. R.; Homan, C. D.; Hunt, P. A.; Little, I. R.;
Straughan, B. P. Inorg. Chim. Acta 1991, 186, 51-60.
(12) The distinction between SBUs and TBUs is made here because SBUs
contain bond sequences that converge to form rings, whereas TBUs branch.
(13) O’Keeffe, M.; Hyde, B. G. Crystal Structures. I. Patterns and
Symmetry; Mineralogical Society of America: Washington, DC, 1996.
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