Metal - Organic frameworks from silicon- and germanium-centered tetrahedral ligands

Article abstract of DOI:10.1021/om701262m

Metal-organic frameworks (MOFs) have been constructed from the reaction of Si(C₆H₄CO₂H)₄ and of Ge(C ₆H₄CO₂H)₄ with Zn(II). The two structures are quite different from each other, and both are different from that of the known MOF of C(C₆H₄CO₂H)₄ with Zn(II). The zinc portion of the carbon system is a dinuclear zinc-oxo cluster with square-planar geometry, that of the silicon system consists of zinc-oxo chains containing two zinc atoms with different geometries, and that of the germanium system consists of linear zinc-oxo clusters containing three zinc atoms with two different geometries. For the silicon system, the secondary building unit around zinc is a distorted tetrahedron, whereas for the germanium system, the secondary building unit around zinc is a bipyramid with eight coordination. The differences in morphology may be due to the more open geometry provided as the M - C bond increases from carbon to germanium. Both MOFs have considerable vacancy in the crystal lattice, calculated to be ca. 42% for the silicon system and ca. 46% for the germanium system. Gas sorption experiments on the germanium system indicated significant uptake of both nitrogen and carbon dioxide.

Full text of DOI:10.1021/om701262m

1464
Organometallics 2008, 27, 1464–1469
Metal-Organic Frameworks from Silicon- and
Germanium-Centered Tetrahedral Ligands
Joseph B. Lambert,* Zhongqiang Liu, and Chunqing Liu
Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208-3113
ReceiVed December 18, 2007
Metal-organic frameworks (MOFs) have been constructed from the reaction of Si(C6H4CO2H)4 and
of Ge(C6H4CO2H)4 with Zn(II). The two structures are quite different from each other, and both are
different from that of the known MOF of C(C6H4CO2H)4 with Zn(II). The zinc portion of the carbon
system is a dinuclear zinc-oxo cluster with square-planar geometry, that of the silicon system consists of
zinc-oxo chains containing two zinc atoms with different geometries, and that of the germanium system
consists of linear zinc-oxo clusters containing three zinc atoms with two different geometries. For the
silicon system, the secondary building unit around zinc is a distorted tetrahedron, whereas for the
germanium system, the secondary building unit around zinc is a bipyramid with eight coordination. The
differences in morphology may be due to the more open geometry provided as the M-C bond increases
from carbon to germanium. Both MOFs have considerable vacancy in the crystal lattice, calculated to be
ca. 42% for the silicon system and ca. 46% for the germanium system. Gas sorption experiments on the
germanium system indicated significant uptake of both nitrogen and carbon dioxide.
Porous materials with large, regular, accessible cages or
Co(II), and Mn(II). The complete rigidity of all components of
the structure permits little flexibility within the crystal. The terms
coordination polymers, organic–inorganic hybrids, organic zeo-
lites, and metal-organic frameworks (MOFs) have been given
to these materials.
The current study has focused on organic components
constructed around a tetrahedral center. With connections
directed tetrahedrally from such a center, complex three-
dimensional networks, or reticulations, are expected. Three such
organic components (1-3) have produced MOFs by reaction
with Zn(II). Despite the similarity of geometry and identity of
channels are increasingly in demand for applications in separa-
tions, catalysis, chemosensing, molecular storage, and nanore-
actors. Pore size, pore shape, and the chemical properties of
the pore lining can determine specificity of admission or
exclusion of materials to the pores and reactivity within the
pores. Traditionally, zeolites and molecular sieves have fulfilled
these roles. Usually found naturally and composed of the most
widely available elements in the earth’s crust, these materials
offer many advantageous properties, including stability, rigidity,
specificity, and low cost. They can be altered only minimally,
however, to optimize their properties. Introduction of organic
components to the surfaces or pore linings offers a semisynthetic
alternative. Far more useful are fully synthetic materials, whose
properties can be tuned by the choice of starting materials.
Such materials have been realized in the form of polymers
made up generally of two multidentate components: metallic
units connected by organic bridges.1–5 Properties could be
altered by selection and variation of both the metal and the
organic bridge. Because each component can bind to more than
one unit of the other, infinite polymers result. If both components
are bidentate, one-dimensional, or linear, polymers result. If at
least one component is tridentate but planar, two-dimensional,
or planar, polymers result. If at least one component is
tetradentate or nonplanar, three-dimensional polymers result.
These polymers often are regular, crystalline, and highly stable
and offer high porosity due to vacancies that result naturally
during their crystallization. The most widespread organic ligands
are polycarboxylic acids and polypyridines.1–5 Numerous transi-
tion metals have been used, such as Cu(II), Zn(II), Cd(II), Ni(II),
* Corresponding author. E-mail: jlambert@northwestern.edu.
(1) Janiak, C. J. Chem. Soc., Dalton Trans. 2003, 2781–2804.
(2) James, S. L. Chem. Soc. ReV. 2003, 32, 276–288.
(3) Kitagawa, S.; Kitaura, R.; Noro, S.-I. Angew. Chem., Int. Ed. 2004,
43, 2334–2375.
the metal ion, the resulting topologies in the crystal proved to
be quite different.6 Differences also are seen when the frame-
works are constructed from the same organic ligands with Cd(II)
(4) Ockwig, N. W.; Delgado-Friedrichs, O.; O’Keefe, M.; Yaghi, O. M.
Acc. Chem. Res. 2005, 38, 176–182.
(5) Hupp, J. T.; Poeppelmeier, K. R. Science 2005, 309, 2040–2042.
10.1021/om701262m CCC: $40.75
2008 American Chemical Society
Publication on Web 03/05/2008

Metal-Organic Frameworks from Si- and Ge-Centered Tetrahedral Ligands
Organometallics, Vol. 27, No. 7, 2008 1465
Table 1. Some Crystallographic Parameters
Table 2. Calculations of Pore Volume
MOF-36
P42/mmc
cluster geometry square-planar
dinuclear zinc-oxo
cluster
C-M-C bond 103.54(6)
angle, deg
Si4A-Zn
Pnna
zinc-oxo chain linear trinuclear
zinc-oxo cluster
Ge4A-Zn
Pnnn
ratio of
pore columns
free
occupied
total
free vol/
space group
vol,a Å3 vol,a Å3 vol,a Å3 total vol, % vol,b
%
5 to 4
Si4A-Zn 1583.04 2280.72 3863.76
Ge4A-Zn 3656.58 4449.49 8106.07
MOF-36 4121.05 2862.78 6983.83
41.0
45.1
59.0
41.8
46.4
61.6
1.02
1.03
1.04
105.09(16)
105.89(11)
a Calculations by Accelrys MS modeling (version 3.2). b Calculations
by Platon for Windows Taskbar (version 1.081).
103.54(79)
112.52(6)
112.52(6)
112.52(6)
105.52(11)
108.79(12)
108.79(12)
114.45(12)
114.45(12)
1.870(3)
108.19(10)
109.23(11)
110.14(10)
111.94(11)
111.24(9)
1.941(2)
112.52(6)
M-C bond
length, Å
1.546(7)
1.546(7)
1.546(7)
1.546(15)
1.870(3)
1.872(4)
1.872(4)
1.944(3)
1.947(2)
1.948(3)
ions.6,7 In the present study we retained the geometry offered
by 3 but changed the central atom M to silicon (4) and
germanium (5). In addition to retaining a constant theme for
the organic geometry, the Si and Ge systems illustrate the ease
with which quaternary centers can be built around these atoms.
The Si system was prepared in an earlier study,7 and the Ge
system is new. The series 3-5 exemplifies the effect of the
single change whereby the M-C bond is lengthened (C to Si
to Ge). These systems were allowed to react with zinc nitrate
to form crystalline MOFs.
Figure 1. Zinc-oxo chain of Si4A-Zn with 50% probability level
thermal ellipsoids.
Results
typical for MOF materials. The BET surface area calculated
from the carbon dioxide isotherms is 284.2 m2 g-1, and the
micropore volume is 0.122 cm3 g-1. See Supporting Information
for sorption data.
Tetrakis(4-carboxyphenyl)silicon (4) was prepared as before.8
Tetrakis(4-carboxyphenyl)germanium (5) was prepared by reac-
tion of 1,4-dibromobenzene with tetrachlorogermanium to
form tetrakis(4-bromophenyl)germanium, which was converted
to the tetralithium derivative for reaction with carbon dioxide.
These tetracarboxylic acids (4 and 5) were allowed to react with
zinc nitrate hexahydrate to produce crystals as described in the
Experimental Section. We designate the MOF formed from 4
and Zn(II) as Si4A-Zn and that from 5 and Zn(II) as Ge4A-Zn
(“4A” denotes the presence of four carboxylic acid groups on
each M). Single crystals were subjected to X-ray diffraction.
Some crystallographic results are presented in Table 1, along
with analogous results for the carbon derivative from the reaction
of 3 with Zn(II) to form MOF-36.6 Details of the crystal
structures are provided in the Supporting Information.
Discussion
The MOF of Si4A-Zn crystallizes in the orthorhombic space
groupPnna.ThesimplestrepeatingstructureisaZn1-O1-Zn2-O1
chain connected to carboxylate groups from the organic
component (Figure 1). One zinc atom (Zn1) has the geometry
of a distorted trigonal bipyramid and is coordinated to a
molecule of water (O5), to a single oxygen atom of one
carboxylate group (O1), and to a single oxygen atom of a second
carboxylate group (O3). Both O1 and O3 are repeated a second
time from an additional carboxylate to bring the coordination
number to five. The second zinc atom (Zn2) is six coordinated.
One carboxyate group (C7) brings coordination from both
oxygen atoms (O1 and O2), and this unit is repeated a second
time. Note that O1 is shared between Zn1 and Zn2 and
constitutes the bridging oxygen for the zinc oxo chain. Ad-
ditionally, Zn2 is connected to a single oxygen atom O4 of a
second carboxylate (C14), and this unit also occurs twice. Thus
Zn2 is coordinated to two O1’s, two O2’s, and two O4’s for
hexacoordination. From the point of view of the carboxylate
groups, one (C14) provides a bridge between the two zinc atoms
(Zn1-O3-C14-O4-Zn2). The other (C7) serves as a bidentate
ligand to Zn2 and a monodentate ligand to Zn1, with the result
that one oxygen (O1) is bridged between the two zinc atoms.
The water molecule (O5) is coordinated solely to Zn1. Any pair
of zinc atoms is coordinated to a total of four carboxylate groups,
so the overall formula of the repeating structure is Zn2(RCO2)4
(H2O). Because each organic molecule possesses four charged
carboxylate groups, the metal/organic ratio is 2:1 to produce
the overall neutral structure. Figure 1 illustrates two units of
the repeating structure. The structure also contains one water
The program Platon9 was used to calculate the solvent accessible
pore volume of the MOFs, after solvent molecules not directly
connected to the metals were removed. Accelrys MS modeling10
was used to calculate the occupied volume and the free volume of
a unit cell. These results are given in Table 2.
Nitrogen and carbon dioxide sorption experiments were
carried out on Ge4A-Zn. Nitrogen sorption indicated type I
behavior, typical for microporous materials, with some hysteresis
between the adsorption and desorption isotherms. The BET
surface area calculated from the nitrogen sorption isotherms is
417.7 m2 g-1, and the micropore volume is 0.277 cm3 g-1
,
(6) Kim, J.; Chen, B.; Reineke, T. M.; Li, H.; Eddaoudi, M.; Moler,
D. B.; O’Keefe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2001, 123, 8239–
8247.
(7) Chun, H.; Kim, D.; Dubtsev, D. N.; Kim, K. Angew Chem., Int. Ed.
2004, 43, 971–974.
(8) Lambert, J. B.; Zhao, Y.; Stern, C. J. Phys. Org. Chem. 1997, 10,
229–232.
(9) Spek, A. L. Platon software 1.081; Utrecht University: The
Netherlands, 2005.
(10) Materials Studio Visualizer by Accelrys, Inc.: Madison, WI, 2005.

1466 Organometallics, Vol. 27, No. 7, 2008
Lambert et al.
these materials were not MOFs. They did not exist as three-
dimensional frameworks because the axial or apical groups (the
C14 carboxyl in Ge4A-Zn) were self-limiting structures like
pyridine without additional ligands to extend the structure. The
middle zinc atom (Zn1) in Ge4A-Zn sits on the crystallographic
inversion point and is attached through oxo bridges to two
identical Zn2 atoms. Zn1 is octahedrally coordinated to two O2
atoms (of the C7 carboxylate group), which bridge to the
respective Zn2 atoms, and two pairs of O5 and O7 atoms from
four carboxylate groups (two pairs each of C21 and C28), which
bridge between the two zinc atoms (Zn-O-C-O-Zn). Zn2
is coordinated to O3 and O4 of the disordered carboxylic acid
(C14). Zn2 also is coordinated to O2 (of carboxylate C7), which
provides the bridge to Zn1. A second carboxylate (C21) and a
third carboxylate (C28) bridge between the two zinc atoms, with
O6 and O8 coordinated to Zn2 and O5 and O7 to Zn1. The
Zn2-O1 (2.670(2) Å) bond may not be sufficiently long to be
counted in the coordination number. The two values of the
disordered Zn2-O3 bond are 1.956(6) and 2.509(7) Å, and of
the disordered Zn2-O4 bond are 1.973(6) and 2.369(5). If the
figures are averaged respectively to 2.233 and 2.171 Å, both
interactions qualify as bonds. Thus the coordination number of
Zn2 may be considered to be four to six, depending on how
one counts the long bonds. The grouping O4-C14-O3 is from
a carboxylic acid (CO2H), whereas the other OCO units are
charged carboxylates (CO2-). The presence of the proton on
O3 or O4 probably contributes to the disorder in the C14 unit.
The trizinc unit is coordinated to a total of six carboxylates
(two each of C7, C21, and C28) and two carboxylic acids, giving
an overall formula of Zn3(RCO2)6(CO2H)2. The structure is
electroneutral, with zinc and carboxylate charges balanced at
six.
Figure 6 shows the reticulated framework of Ge4A-Zn from a
perspective sighting along the b axis of the crystal. The crystal
structure clearly contains two distinct channels. The approximate
diameter of the larger channel is about 9 Å, and that of the smaller
channel about 6 Å. Again it is helpful to consolidate the building
blocks from which the network is constructed. The trizinc grouping
can be represented by a Zn1-centered bipyramid with eight
benzoate groupings irradiating outward (Figure 7, left), six equa-
torially and two apically. Each benzoate terminates at a nearly
regular Ge-centered tetrahedron in green. Figure 7 (right) illustrates
diagrammatically how the yellow bipyramid is surrounded by eight
purple tetrahedra. Because each germanium tetrahedron is attached
to four different Zn-centered bipyramids, the structure propagates
throughout the crystal. Figure 8 shows how each of the eight Ge-
centered tetrahedra that emanate from a single bipyramid bond to
three other bipyramids.
Figure 2. Reticulated framework of Si4A-Zn as viewed along the
b axis, in which zinc is cyan, oxygen is red, carbon is gray, silicon
is green, and nitrogen from dimethylformamide (solvent uncoor-
dinated to the network) is blue.
molecule of solvation and 1.5 DMF molecules of solvation
uncoordinated to the network.
Figure 2 shows the reticulated framework of Si4A-Zn from a
perspective sighting along the b axis of the crystal. To understand
the framework topology, it is helpful to consolidate the building
blocks from which the network is constructed. A pair of zinc atoms
in the zinc oxo chain is bonded to four carboxylate groups and
may be considered roughly to represent a distorted tetrahedron.
As depicted in Figure 3 (left), Zn2 is surrounded by the four
carboxylate groups. The carboxylate groups are connected through
benzene rings to four silicon atoms in green, each of which sits at
the center of the nearly regular tetrahedron of the organic molecule.
Figure 3 at the right shows how a single Zn2-centered distorted
tetrahedron is surrounded by four Si-centered, nearly regular
tetrahedra. The Si-centered tetrahedra in turn are surrounded by
four of the distorted Zn-centered tetrahedra. The reticulated structure
represents the extension of these motifs infinitely into three
dimensions. There are, however, two interconnected such networks,
when both zinc atoms in Figure 1 are taken into consideration.
Zn1 also is surrounded by four carboxylate groups, only two of
which are shown in Figure 3. Figure 4 shows a portion of the two
interconnected networks, one (Zn1-centered) in blue and the other
(Zn2-centered) carried over from Figure 3 (right) in yellow and
green. These two networks are connected through the zinc-oxo
chain and not (directly) via the Si-centered tetrahedral. There is
no interpenetration of independent, identical networks, but rather
a single network comprised of two identical but connected
networks.
The reticulated structures depicted in Figures 2 and 6 have
considerable vacant space. We used two calculational ap-
proaches to assess the degree of vacancy for the series of zinc
MOFs constructed from M(C6H4)CO2H (3-5): MOF-36, Si4A-
Zn, and Ge4A-Zn (Table 2). Platon calculates the volume of
the accessible void with a rolling ball probe (representing a water
molecule, 1.2 Å in diameter). The result is the pore volume
percentages given in the sixth column of Table 2. Accelrys
compares the calculated space occupied by atoms of assigned
van der Waals radii (third column) with the observed total cell
volume (fourth column) to give the free volume (second column)
by subtraction. The percentage of the accessible void then is
calculated from the ratio (fifth column) of the free and total
volumes. The two different methods lead to very similar results
(sixth column). Differences might be expected if some of the
The MOF of Ge4A-Zn crystallizes in the orthorhombic space
group Pnnn. The repeating structure contains a centrosymmetric
linear array of three zinc ions (Figure 5). Although similar trizinc
arrays have been observed previously in crystal structures,11–13
(11) Clec, G. W.; Little, I. R.; Straughan, B. P. J. Chem. Soc., Dalton
Trans. 1986, 1283–1288.
(12) de Hoog, P.; Pacho´n, L. D.; Gamez, P.; Lutz, M.; Spek, A. L.;
Reedijk, J. J. Chem. Soc., Dalton Trans. 2004, 261, 4–2615.
(13) You, Z.-L.; Zhu, H.-L.; Liu, W.-S. Z. Anorg. Allg. Chem. 2004,
630, 1617–1622.

Metal-Organic Frameworks from Si- and Ge-Centered Tetrahedral Ligands
Organometallics, Vol. 27, No. 7, 2008 1467
Figure 3. (left) The distorted tetrahedron with Zn2 at the center, surrounded by four carboxylate ligands, in which zinc is cyan, oxygen is
red, carbon is gray, and silicon is green. The four carboxylate ligands are attached to the nearly regular tetrahedron around silicon. (right)
A single Zn2-centered distorted tetrahedron in yellow surrounded by four Si-centered nearly regular tetrahedra in green.
Figure 4. Network of yellow, Zn2-centered, distorted tetrahedra
and green Si-centered regular tetrahedra, interconnected by a second,
Zn-1-centered network in blue.
Figure 6. Reticulated framework of Ge4A-Zn as viewed along the
b axis, in which zinc is cyan, oxygen is red, carbon is gray, and
germanium is purple.
assumed by the MOF; that is, it is more of a supramolecular
than a molecular property.
The existence of significant voids in the crystals raises the
possibility that they can serve as reservoirs for gaseous species.
Consequently, nitrogen and carbon dioxide adsorption and desorp-
tion experiments were carried out on Ge4A-Zn, the material with
Figure 5. Zinc-oxo chain of Ge4A-Zn with 50% probability level
thermal ellipsoids.
the larger voids (see Supporting Information). The BET surface
area calculated from N2 sorption isotherms was 417.7 m2 g-1, and
void is inaccessible to the rolling ball and hence is ignored by
that calculated from CO2 sorption isotherms was 284.2 m2 g-1
.
Platon. In fact, the results are within 1–3% of each other. The
C-centered MOF offers the largest void percentage and the Si-
centered MOF the least, but all three are in the range 40–60%,
which is large for most crystals.1–4 Since the void order of C >
Ge > Si does not follow vertical placement in the periodic table,
it must be the result primarily of the specific crystal type
The micropore volume calculated from N2 sorption isotherms was
0.277 cm3 g-1 and that calculated from CO2 sorption isoterms was
0.122 cm3 g-1. These results are typical for microporous materials1–5
and demonstrate that Ge4A-Zn is a nonspecific adsorber, since it
receives both N2 and CO2 effectively.

1468 Organometallics, Vol. 27, No. 7, 2008
Lambert et al.
Figure 7. (left) Bipyramid represented by three zinc atoms at the center, surrounded by eight carboxylate ligands, in which Zn is cyan,
oxygen is red, carbon is gray, and germanium is green. The eight carboxylate ligands are attached to the regular tetrahedron around germanium.
(right) A single Zn-centered bipyramid in yellow surrounded by eight Ge-centered regular tetrahedra in purple.
terminal zinc has four to six coordination, depending on how
bonds are counted that are slightly long.
Whereas the secondary building unit for Si4A-Zn has four
coordination around the zinc portion and four coordination
around silicon (Figure 3), the secondary building unit for Ge4A-
Zn has eight coordination around the zinc portion and four
coordination around germanium (Figure 7). Networks with
connectivity larger than six are rare.14 The space group Pnnn
is relatively unusual. There are only two reports of three-
dimensional networks with this space group. These are con-
structed of cyano groups with either silver15 or copper16 ions.
There are no MOFs constructed from carboxylate groups and
metal ions with this space group.
The three MOFs were produced under identical conditions
with a common metal ion and organic ligands that differ only
in the central atom M. The organic ligands have the same
tetrahedral geometry (C-M-C angles in Table 1) and differ
primarily in the M-C bond lengths (Table 1). The identity of
the central atom affects the acidity of the carboxyl groups, but
any role of carboxylic acidity is not obvious. The differences
in morphology may be due to the more open geometry provided
as the M-C bond increases from carbon to germanium. In these
systems the longest bonds provide the highest effective coor-
dination in the secondary building blocks (eight coordination
for germanium). That such small differences in structure and
reactivity can generate such large differences in the crystal
structures illustrates the subtlety of the process of crystal growth.
Figure 8. Connectivity of the yellow Zn-centered bipyramids with
the purple Ge-centered tetrahedra.
Conclusions
Despite the similarity in structure of the organic ligands 3-5
[M(C6H4CO2H)4, M ) C, Si, Ge], their MOFs with Zn(II) have
very different crystal morphologies. The carbon congener, MOF-
36, crystallizes in the tetragonal space group P42/mmc (#131).6
The zinc portion of the structure consists of dinuclear zinc-oxo
clusters in which the geometry around the zinc atoms is square
planar. The silicon congener, Si4A-Zn, crystallizes in the
orthorhombic space group Pnna (#52). The zinc portion consists
of zinc-oxo chains with two different, alternating zinc atoms.
The geometry around one zinc atom is a distorted trigonal
bipyramid (five-coordinate), and that around the other is a
distorted octahedron (six-coordinate). The germanium congener,
Ge4A-Zn, crystallizes in the orthorhombic space group Pnnn
(#48). The zinc portion consists of linear trinuclear zinc-oxo
clusters with an inversion center at the central zinc atom. The
central zinc has octahedral (six-coordinate) geometry, and the
Experimental Section
Elemental analyses were performed by Midwest Microlab, LLC,
Indianapolis, IN.
(14) Halper, S. R.; Cohen, S. M. Inorg. Chem. 2005, 44, 486–488.
(15) Halper, S. R.; Cohen, S. M. Abstracts of Papers, 229th ACS
National Meeting, San Diego, CA, March 13–17, 2005, INOR-334.
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A. S. C. Eur. J. Inorg. Chem. 2004, 3751–3763.

Metal-Organic Frameworks from Si- and Ge-Centered Tetrahedral Ligands
Organometallics, Vol. 27, No. 7, 2008 1469
Tetrakis(4-carboxyphenyl)silicon (4) was prepared according
to the literature.8
were collected and dried under vacuum (10 Torr) at room
temperature for 3 days, with an overall yield of 38%. Anal. Calcd
for (GeC28H17O8)2Zn3(C3H7NO)2(H2O)4.6: C, 48.56; H, 3.76; N,
1.83. Found: C, 48.54; H, 3.67; N, 1.72. Drying procedures for
elemental analysis and X-ray crystallography were different, so that
there are differences in solvation.
Sorption Measurements. The surface areas and micropore
volumes were determined by N2 adsorption at 77 K and by CO2
adsorption at 297 K, respectively, using an Autosorb-1 volumetric
sorption analyzer controlled by Autosorb-1 for Windows 1.19
software (Quantachrome). Prior to the experiments, the samples
were degassed at 80 °C under vacuum for 12 h. Details are given
in the Supporting Information.
Synthesis of Si4A-Zn. To a 20 mL vial were added 33.4 mg
(0.06 mmol) of 4 and 56 mg (0.2 mmol) of zinc nitrate hexahydrate.
DMF (4.5 mL), ethanol (4.5 mL), and water (3.6 mL) were added
to dissolve the organic ligand and the salt. The mixture was shocked
by ultrasonic sound for 10 min. The solution was filtered through
fritted glass (fine) into three 4 mL vials, which were sealed with
Teflon-lined caps. These vials were placed in an oil bath. The bath
was heated slowly (over a day) to 75 °C, held at that temperature
for 3 days, and then cooled to room temperature. Colorless crystals
were collected and dried under vacuum (10 Torr) at room
temperature for three days, with an overall yield of 38%. Anal.
Calcd for (SiC28H16O8)2Zn4(H2O)(C3H7NO)3: C, 51.50; H, 3.66;
N, 2.77. Found: C, 51.58; H, 3.88; N, 2.55. Drying procedures for
elemental analysis and X-ray crystallography were different, so that
there are differences in solvation.
Tetrakis(4-bromophenyl)germanium. To a 250 mL Schlenk
flask equipped with a magnetic stirring bar was added 4,4′-
dibromobenzene (11.8 g, 50 mmol). The flask, sealed with a septum,
was placed under vacuum and filled with dry N2 three times.
Anhydrous ethyl ether (120 mL) was transferred into the flask with
a double-ended needle. The flask then was placed into an ice/acetone
bath and treated dropwise with butyllithium in hexane (20 mL, 50
mmol). The mixture was stirred for 30 min, and GeCl4 (12 mmol
in 20 mL ether) was added dropwise slowly. The ice/acetone bath
was removed, and the mixture was stirred for 8 h at room
temperature. Then 1 N aqueous HCl was added, and the resulting
mixture was extracted with ether. The combined extracts were
washed with water and brine, dried over MgSO4, and filtered. The
solvent was removed by rotary evaporation. Recystallization from
1,2-dichloroethane and benzene gave 5.6 g (67%) of colorless needle
crystals. 1H NMR (CDCl3): δ 7.56 (d, 8 H, 3J ) 7.5 Hz), 7.34 (d,
8 H, 3J ) 7.5 Hz). 13C NMR (CDCl3): δ 136.8, 133.5, 132.0, 125.0.
EI-MS: M+, calcd 695.7, found 695.9. Anal. Calcd: C, 41.38; H
2.31. Found: C, 41.68; H 2.33.
Single Crystal Structure of Si4A-Zn. Crystals were mounted
using oil (Infineum V8512) on a glass fiber. Measurements were
made on a CCD area detector with graphite-monochromated Mo
KR radiation. The structures were solved by direct methods and
expanded using Fourier techniques. The non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were included at calculated
positions but not refined, except as noted in the following. The
reported molecular formula of C32.5H16N1.5O11.5SiZn2 was obtained
as follows. There are two structurally distinct Zn atoms (Figure 1),
to which are attached one molecule of water and four carboxylate
groups of the organic molecule, contributing a formula of
C28H16O9SiZn2. Although the four carboxyl groups are attached to
four Si atoms, those atoms are shared with four other Zn2 centers,
so that only a single Si atom is dedicated on average to each Zn2
center. The hydrogen atoms on the structural water molecule were
not included, as their calculated positions were unknown. In
addition, the crystal structure contains one water of solvation and
1.5 DMF molecules of solvation (one fully occupied, one-half
occupied), contributing O(C3NO)1.5 to the molecular formula, for
the total given above with molecular weight 770 based on two Zn
atoms. Further details are given in the Supporting Information.
Single Crystal Structure of Ge4A-Zn. Data were obtained in
the same fashion as for Si4a-Zn. Hydrogen atoms were included
at calculated positions but not refined, except as noted in the
following. The reported molecular formula of C74H74O22N6Ge2Zn3
was obtained as follows (the actual reported formula was calculated
on the basis of 1.5 Zn atoms, but we have doubled that number in
order to correspond more clearly to the depiction in Figure 5). The
three Zn atoms are attached to eight carboxylate or carboxyl groups,
contributing a formula of C56H32O16Ge2Zn3. The eight carboxylic
groups are attached to eight Ge atoms, each of which is shared
with four other Zn3 centers, so that 8/4 or two Ge atoms are
dedicated to each Zn3 center. The two carboxyl H atoms are not
included in this total, as their calculated positions are unknown.
This formula corresponds to the calculated formula of molecular
weight 1302. Refinements were carried out on this unit by squeezing
out six DMF molecules of solvation. The DMF atoms, (C3H7NO)6,
then were reinstated after refinement to give the reported formula
given above, with the reported molecular weight of 1741 based on
three Zn atoms. Further details are given in the Supporting
Information.
Tetrakis(4-carboxyphenyl)germanium (5). To a 250 mL
Schlenk flask equipped with a magnetic stirring bar was added
tetrakis(4-bromophenyl)germanium (0.70 g, 1 mmol). The flask,
sealed with a septum, was placed under vacuum and filled with
dry N2 three times. Anhydrous THF (100 mL) was transferred into
the flask with a double-ended needle. The flask then was immersed
in a bath of acetone and dry ice. The system was cooled for 10
min, and tert-butyllithium in hexane (5.4 mL, 8.1 mmol) was added
dropwise with a syringe. The mixture was stirred for 30 min, and
then CO2 was bubbled through it. After 30 min, the acetone/dry
ice bath was removed, and the reaction system was warmed to room
temperature. Solvent was removed by rotary evaporation, and water
(50 mL) was added. Dilution with HCl solution (1 N) produced a
white precipitate, which was removed by filtration and dissolved
in a dilute solution of NaOH. The solution was filtered and
reacidified with dilute HCl. The new white solid 5 was collected
1
and dried: 0.32 g (58%). H NMR (acetone-d6): δ 8.136 (d, 8 H,
3
3J ) 8 Hz), 7.728 (d, 8 H, J ) 8 Hz). 13C NMR (acetone-d6): δ
167.0, 140.7, 135.6, 132.2, 129.7. ESI-MS: [M - H]+, calcd, 557.03
(100%), 555.03 (70%), 553.03 (53%); obsd, 557.01 (100%), 555.14
(76%), 553.25 (50%); [2(M - H)]+, calcd, 1112.06; found, 1112.57.
Anal. Calcd: C, 60.37, H 3.62. Found: C, 60.15, H 3.75.
Acknowledgment. This work was supported by the
National Science Foundation (CHE0349412). We thank Prof.
O. M. Yaghi for useful discussions at the inception of this
work and Dr. Charlotte Stern for carrying out the X-ray
experiments.
Synthesis of Ge4A-Zn. To a 20 mL vial were added 33.4 mg
(0.06 mmol) of 5 and 56 mg (0.2 mmol) of zinc nitrate hexahydrate.
DMF (4.5 mL), ethanol (4.5 mL), and water (3.6 mL) were added
to dissolve the organic ligand and the salt. The mixture was shocked
by ultrasonic sound for 10 min. The solution was filtered through
fritted glass (fine) into three 4 mL vials, which were sealed with
Teflon-lined caps. These vials were placed in an oil bath. The bath
was heated slowly (over a day) to 75 °C, held at that temperature
for 3 days, and then cooled to room temperature. Colorless crystals
Supporting Information Available: Sorption plots for Ge4A-
Zn. Crystal data for Si4A-Zn and Ge4A-Zn. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM701262M