Syntheses of metal-organic frameworks with protected phosphonate ligands

Article abstract of DOI:10.1039/c2ce25358e

Protected phosphonate groups were embedded onto isophthalic acid and reacted with zinc ion to synthesize metal-organic frameworks (MOFs). Single-crystal X-ray diffraction measurement revealed the crystal structures of obtained MOFs under various temperatures and solvent conditions. The isopropyl groups were left protected when the complexation reaction was executed at 60 °C, while they were partially deprotected in the sample prepared at 120 °C. When the reaction was carried out at 180 °C, the isopropyl groups were totally deprotected. This result provides us with information on the deprotection reaction in a post-synthetic method for constructing MOFs.

Full text of DOI:10.1039/c2ce25358e

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Post-synthetic modification of
coordination networks
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Discovery, development, and functionalization of Zr(IV)-based metal–organic
frameworks
Min Kim and Seth M. Cohen
CrystEngComm, 2012, DOI: 10.1039/C2CE06491J
Controlled modification of the inorganic and organic bricks in an Al-based MOF by
direct and post-synthetic synthesis routes
Tim Ahnfeldt, Daniel Gunzelmann, Julia Wack, Jürgen Senker and Norbert Stock
CrystEngComm, 2012, DOI: 10.1039/C2CE06620C
Conversion of primary amines into secondary amines on a metal–organic framework
using a tandem post-synthetic modification
Andrew D. Burrows and Luke L. Keenan
CrystEngComm, 2012, DOI: 10.1039/C2CE25131K
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PAPER
Syntheses of metal–organic frameworks with protected phosphonate ligands{
Teppei Yamada*a and Hiroshi Kitagawa*ab
Received 13th March 2012, Accepted 3rd May 2012
DOI: 10.1039/c2ce25358e
Protected phosphonate groups were embedded onto isophthalic acid and reacted with zinc ion to
synthesize metal–organic frameworks (MOFs). Single-crystal X-ray diffraction measurement revealed
the crystal structures of obtained MOFs under various temperatures and solvent conditions. The
isopropyl groups were left protected when the complexation reaction was executed at 60 uC, while
they were partially deprotected in the sample prepared at 120 uC. When the reaction was carried out
at 180 uC, the isopropyl groups were totally deprotected. This result provides us with information on
the deprotection reaction in a post-synthetic method for constructing MOFs.
The protection–complexation–deprotection (PCD) method is
Introduction
a sophisticated process for constructing MOFs containing
reactive functional groups.24–26 This method is analogous to
Metal–organic frameworks (MOFs) have achieved impressive
progress recently, and show specific gas adsorption behaviour,
foundry work, as shown schematically in Scheme 1.
showing promise for smart gas separation for many different
Conventional foundry works consists of mould, invest and
gases.1–10 MOFs can contain a wide variety of metal ions and
burn-out processes. The desired cavity shape is designed by
bridging ligands, and their structures can be designed to specific
figuring a mould and an object was constructed by investing
pore sizes, dimensions, and configurations inside their frame-
before eliminating the mould to construct the desired shape. The
works. In the design of the inner environment of these
PCD method also includes three reaction steps. The first step of
frameworks, various functional groups have been modified,
the PCD method is a protection reaction where a reactive group
such as hydrophobic alkyl groups, fluorinated groups,11 acidic
in the ligand is modified by an unreactive organic group to
groups and uncoordinated metal centres (UMCs). An analogy
prevent any unwanted reactions disrupting the pore structure in
with heterogeneous catalysts provided us with the idea that
a framework, as preparing the mould in foundry works. A
uncoordinated metal centres12,13 or acidic groups would
complexation reaction is then executed afterwards, and an MOF
potentially act as active sites for catalytic reactions. However,
is constructed with the designed framework arrangement, and
UMCs or acidic groups have barriers towards their introduction
the prepared mould is inserted into the MOF matrix. Finally, the
into MOF frameworks, and one of these is that MOFs contain
protecting groups are removed chemically, being comparable to
coordination bonds between the metal ions and bridging ligands,
a burn-out process in the foundry process, and the reactive
but UMCs have the ability to coordinate to ligands.
groups reappear. Guest molecules can be inserted into the pores
Uncoordinated acidic sites also tend to coordinate to a metal
that have formed to interact with the functional groups. In this
site, for example, 2,5-dihydroxyterephthalic acid coordinates to
method, highly reactive groups do not have the chance to attack
the metal site in a bidentate fashion to construct the CPO-27
the metal site, and arbitrary functional groups can be introduced
family,12,14 which has a different framework to an isoreticular
into the framework. Several types of group can be introduced,
MOF series.
and thus, isostructural MOFs can be designed easily.
A post-synthetic modification technique is a rational strategy
for modifying acidic functional groups in a framework.10
groups of 2-phosphoterephthalic acid with zinc ions. The
Here, we describe the reaction of protected phosphonate
Aldehyde or amino groups can be introduced into a framework
before reacting with amino, acid anhydride,15–18 isocyanate,19,20
or carbonyl groups.21,22 Sada and co-workers have executed a
click reaction on a framework.23
aDivision of Chemistry, Graduate School of Science, Kyoto University,
Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto, 606-8502, Japan.
E-mail: teppei343@gmail.com; Fax: +81-75-753-4036;
Tel: +81-75-753-4036
bCREST, JST, Sanbancho 5, Chiyoda-ku, Tokyo, 102-0075, Japan.
E-mail: kitagawa@kuchem.kyoto-u.ac.jp
{ CCDC reference numbers 869666–869669. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2ce25358e
Scheme 1 A schematic representation of the PCD method.
This journal is ß The Royal Society of Chemistry 2012
4148 | CrystEngComm, 2012, 14, 4148–4152

protected ligand was synthesized using a two-step complexation
reaction. Single crystal X-ray diffractometry on the MOFs
obtained revealed that the zinc ions assisted in the deprotection
of the phosphonate groups, and novel MOFs were synthesized.
This result provides us with a novel strategy for constructing
secondary building units using known subunits.
and phosphonate groups, and constructs a one-dimensional
framework. One carboxylate group in each Hppip ligand does
not take part in the coordination, but as it is protonated, forms a
hydrogen bond with the crystalline water present.
The crystal structure of 5 is shown in Fig. 3. It is widely known
that N,N-dimethylformamide undergoes hydrolysis to form a
dimethylammonium ion, and the MOF obtained incorporates
this as a counter ion. One isopropyl group is deprotected on each
phosphonate group and the resulting phosphonate groups bridge
two zinc atoms. Two carboxylate groups coordinate to two zinc
atoms monodentately, and thus, each ligand bridges four zinc
atoms. The zinc ions are coordinated by two oxygen atoms from
two carboxylate groups, and the other two oxygen atoms from
Results and discussion
Syntheses and crystal structures
To investigate the reactivity of the protected ligand, we planned
to synthesize diisopropyl 3,5-dicarboxylphenylphosphonate
(H2dppip, 2) as described below. However, there was the
suspicion that esterification of the phosphonate group of 3,5-
dicarboxyphenylphospnonic acid (H4pip) would result in the
esterification of all the carboxyl groups as well as the phosphoxyl
groups. Therefore, we applied another route, as shown in
Scheme 2 for synthesizing H2dppip.
the phosphonate groups, giving
a tetrahedral geometry.
Therefore, a three-dimensional anionic framework is constructed
by the stoichiometric divalent zinc cation and the trivalent anion
ligand. The dimethylammonium cation is introduced inside the
framework for charge compensation, and forms a hydrogen
bond with the oxygen atoms of the carboxylate groups.
The stability of the protecting group was evaluated using NMR
measurements. H2dppip was dissolved in deuterated water and/or
deuterated DMF, and was kept at 60 uC to evaluate the deprotection
ratio using 1H NMR spectroscopy. No peaks emerged up to a
period of 11 days, and the ligand was found to be highly stable.
Synthesis of the MOFs was carried out using H2dppip,
4,49-bipyridine (bpy), and zinc ions with various solvents at
several reaction temperatures. The crystal structure of 3 is shown
in Fig. 1. Compound 3 consists of protected ligands (dppip), bpy,
and zinc ions. The two carboxylate groups of dppip coordinate
monodentately to the zinc ions, and the phosphonate group
remains protected. Each zinc ion is coordinated by two coordina-
tion water molecules, two oxygen atoms from the two carboxylate
groups, and one nitrogen atom of the bpy ligand, and has a
trigonal bipyramidal structure. As a result, a one-dimensional
framework is constructed along the a-axis. These chains make
hydrogen bonds between each other constructing a void between
them, and water and DMF are aligned in interchain as well as
disordered isopropyl groups of the dppip ligand.
When the reaction was carried out in water at 180 uC, the
phosphonate group was fully deprotected, as shown in Fig. 4.
The three oxygen atoms of each phosphonate group of 6 bridge
three zinc ions to form a one-dimensional ladder composed of
zinc ions and phosphonate groups. These ladders are linked with
the bpy ligands to construct a two-dimensional layer, shown by
the colours in Fig. 4(c). The two carboxylate groups remain
uncoordinated and protonated, and hydrogen bonds are formed
between adjacent carboxylic acid sites and crystalline water.
Reaction mechanism
The pH of the reaction mixture of 3 changed from 3.6 to 3.1.
This is rational because complexation or ester hydrolysis
reactions release protons and the solution became more acidic.
On the other hand, the other solutions became basic as a result of
the reactions. This is probably due to the buffering effect of the
ammonium released by the hydrolysis of DMF.
From the results of the NMR study, the protected ligand in 2 is
itself stable under the conditions using a mixture of water and
DMF at high temperature. The MOF obtained from the reaction
carried out at 60 uC resulted in the complexation of the
carboxylate groups and the zinc ions, while the phosphonate
groups remained protected. Therefore, the complexation reaction
at 60 uC was simple, as the carboxylate groups of the ligand and
the bpy groups coordinated to the zinc ions to construct the MOF.
The complexation reaction at 120 uC was different. The
complexation reaction occurred via the partial elimination of the
isopropyl ligands, because these ligands become deprotected in a
simple solution because of the existence of zinc ions. Metal ions
can act as a Lewis acid, and thus, the deprotection occurred from
the interaction of the oxygen atoms on the phosphoxyl groups
with the zinc ions. The reaction at 180 uC resulted in the total
deprotection of the ligand due to the high reaction temperature.
The crystal structure of 4 is shown in Fig. 2. From a reaction
carried out in a mixture of water and ethanol at 120 uC, one of the
isopropyl groups of each ligand becomes deprotected and the
phosphonate groups coordinate to the zinc ions. Each zinc atom is
coordinated by the nitrogen atom of a bpy ligand and three oxygen
atoms from a phosphonate group, a carboxylate group, and a
coordinating water molecule. The isopropyl 3,5-dicarboxylphenyl-
phosphonate (Hppip) ligand bridges the zinc atoms via carboxylate
Conclusions
Diisopropylphosphoxyisophthalic acid was reacted with zinc ions,
and crystal structures of the compounds obtained were determined.
The isopropyl groups were protected when the complexation
reaction was carried out at 60 uC, while they were partially
Scheme 2 Synthesis scheme for 2.
This journal is ß The Royal Society of Chemistry 2012
CrystEngComm, 2012, 14, 4148–4152 | 4149

Fig. 1 (a) The chemical structure of H2dppip and (b–d) the crystal structure of [Zn2(dppip)2(bpy)(H2O)4]?2H2O (3). The grey, pink, blue, yellow, and
purple spheres denote carbon, oxygen, nitrogen, phosphorus, and zinc atoms, respectively. Crystalline water and one of the disordered isopropyl
branches are omitted for clarity. Fig. 1d represents the configuration of disordered oxygen and isopropyl groups.
Fig. 2 (a) The chemical structure of H3ppip and (b,c) the crystal structure of [Zn2(Hppip)2(bpy)(H2O)]?2H2O (4). The white, grey, pink, blue, yellow,
and purple spheres denote hydrogen, carbon, oxygen, nitrogen, phosphorus, and zinc atoms, respectively. Crystalline water and one of the disordered
isopropyl branches are omitted for clarity.
deprotected in the sample prepared at 120 uC. When the reaction was
carried out at 180 uC, the isopropyl groups were fully deprotected.
From these results, four types of MOF were synthesized from the
same starting materials under reactions carried out at different
temperatures and in different solvents. This result provides us with
information on deprotection reactions with metal ions.
Under argon atmosphere, triisopropyl phosphite (3.61 g,
17.3 mmol) was added dropwise to a suspension of 5-bromo-
m-xylene (2.64 g, 14.3 mmol) and nickel bromide (0.61 g,
2.8 m mol) at 150 uC. After a period of 3 h, the reactant was
extracted with ethyl acetate and washed with water. The extract
was dried using anhydrous magnesium sulfate and evaporated at
1 mmHg. Compound 1 was obtained quantitatively and used in
further reactions without further purification.
Experimental
Synthesis of H2dppip (2). A mixture of 1 (1.93 g, 7.1 mmol),
potassium permanganate (8.00 g, 50.6 mmol), potassium
carbonate (2.22 g, 16.1 mmol) and Aliquat 336 (0.12 g) in water
(70 mL) was refluxed for 3 h, and the resulting suspension
Synthetic procedure
Synthesis of diisopropyl-3,5-dimethylphosphonate (1). The
reaction was executed according to a literature procedure.27
4150 | CrystEngComm, 2012, 14, 4148–4152
This journal is ß The Royal Society of Chemistry 2012

Fig. 3 (a) The chemical structure of H3ppip and (b,c) the crystal structure of (dma)[Zn(ppip)] (5). The white, grey, pink, blue, yellow, and purple
spheres denote hydrogen, carbon, oxygen, nitrogen, phosphorus, and zinc atoms, respectively. The dma molecules and one of the disordered isopropyl
branches are omitted for clarity.
Fig. 4 (a) The chemical structure of H4pip and (b,c) the crystal structure of [Zn2(H2pip)2(bpy)]?2H2O (6). The white, grey, pink, blue, yellow, and
purple spheres denote hydrogen, carbon, oxygen, nitrogen, phosphorus, and zinc atoms, respectively. The crystalline water are omitted for clarity.
filtered. The solution was neutralized with conc. hydrochloric
acid and a white precipitate readily formed. The suspension was
cooled, filtered, and dried, and a white powder is obtained.
Yield = 1.55 g (66.1%). Elemental analysis calc. (found)% for
autoclave and allowed to react at 120 uC for 48 h. After cooling,
colourless block crystals were obtained. The pH of the solution
changed from 3.8 to 5.5 by the reaction. Yield = 74 mg (37.3%).
Elemental analysis calc. (found) for ZnC13H18O7NP: C = 39.36
(39.09); H = 4.57 (4.53); N = 3.53 (3.53).
i
C6H3(COOH)2(PO3 Pr2): C = 48.78 (48.88); H = 6.02 (5.87).
Synthesis of [Zn2(dppip)2(bpy)(H2O)4]?2H2O (3). Zinc nitrate
hexahydrate (149 mg, 0.50 mmol), 2 (165 mg, 0.50 mmol),
4,49-bipyridine (78 mg, 0.50 mmol), 4 mL of DMF, and 4 mL of
water were placed in a screw vial and allowed to react at 60 uC
for a period of one week. After cooling, colourless block crystals
were obtained. The pH of the solution changed from 3.8 to 4.1
by the reaction. Yield = 31 mg (9.7%).
Synthesis of [Zn2(H2pip)2(bpy)]?2H2O (6). 6 was synthesized in
almost the same procedure for 3 under elevated temperature.
Zinc nitrate hexahydrate (149 mg, 0.50 mmol), 2 (165 mg,
0.50 mmol), 4,49-bipyridine (78 mg, 0.50 mmol), and 10 mL of
water were placed in a Teflon-lined stainless autoclave and
allowed to react at 180 uC for 72 h. After cooling, colourless
block crystals were obtained. The pH of the solution changed
from 3.8 to 6.1 by the reaction. Yield = 149 mg (36.7%).
Elemental analysis calc. (found) for ZnC13H11O8NP: C = 38.50
(38.21); H = 2.73 (2.79); N = 3.45 (3.40).
Synthesis of [Zn2(Hppip)2(bpy)(H2O)2]?2H2O (4).
4 was
synthesized in almost the same procedure for 3 instead replacing
DMF by ethanol under elevated temperature.
Zinc nitrate hexahydrate (149 mg, 0.50 mmol), 2 (165 mg,
0.50 mmol), 4,49-bipyridine (78 mg, 0.50 mmol), 5 mL of ethanol,
and 5 mL of water were placed in a Teflon-lined stainless
autoclave and allowed to react at 120 uC for 48 h. After cooling,
colourless block crystals were obtained. The pH of the solution
changed from 3.6 to 3.1 by the reaction. Yield = 12 mg (5.0%).
Single-crystal X-ray diffraction measurements
The crystals under analysis were mounted on a glass fibre using
silicone grease, and placed in flowing cold nitrogen gas. The
diffraction data were collected using a SMART APEXII CCD
area detector (Bruker) employing monochromated Mo-Ka
˚
radiation (0.71073 A) from a rotating anode source with a
mirror focusing apparatus. The data were corrected for Lorentz
and polarization effects and absorption using the SADABS
software package. The structure was solved using direct methods
and refined using the full-matrix least-squares method on F2
using the Yadokari-2009 software package28,29 implemented in
Synthesis of (dma)[Zn(ppip)] (5). 5 was synthesized in almost
the same procedure for 3 under elevated temperature.
Zinc nitrate hexahydrate (149 mg, 0.50 mmol), 2 (165 mg,
0.50 mmol), 4,49-bipyridine (78 mg, 0.50 mmol), 5 mL of DMF,
and 5 mL of water were placed in a Teflon-lined stainless
This journal is ß The Royal Society of Chemistry 2012
CrystEngComm, 2012, 14, 4148–4152 | 4151

Table 1 Crystallographic parameters of the compounds
Compound
3
4
5
6
Formula
Formula weight
T/K
Crystal system
Space group
C
41H60N3O22P2Zn2
ZnC16H21PNO10
483.72
100(2)
Triclinic
¯
P1 (#2)
8.463(2)
11.263(3)
11.774(3)
110.960(2)
96.523(3)
108.941(2)
957.2(4)
2
C13H18NO7PZn
396.62
100(2)
Monoclinic
P21/n (#14)
9.7414(11)
16.0486(18)
11.0596(12)
Zn2C26H22P2O16N2
811.14
100(2)
Triclinic
¯
P1 (#2)
5.1360(14)
12.264(3)
12.574(3)
69.555(3)
79.414(3)
82.245(3)
727.3(3)
1
1139.60
100(2)
Triclinic
¯
P1 (#2)
˚
a/A
9.5545(12)
10.3457(13)
13.7876(17)
81.6150(10)
82.8130(10)
70.7230(10)
1268.4(3)
1
˚
b/A
˚
c/A
a (u)
b (u)
c (u)
95.4460(10)
3
˚
V/A
1721.2(3)
4
140 6 80 6 60
3054
233
Z
Crystal size, mm3
Unique reflections
Refined parameters
Dc/g cm23
300 6 200 6 200
5347
419
300 6 300 6 100
3348
346
100 6 50 6 20
3259
215
1.492
1.678
1.531
1.852
F(000)
m(Mo-Ka)/cm21
R1 [|I| . 2s]
593
1.090
0.0788
0.1709
1.086
0.000
1.708
498
1.423
744
1.552
396
1.847
0.0262
0.0759
1.067
0.001
0.554
0.0379
0.0968
1.068
0.001
1.113
0.0384
0.0976
1.036
0.000
0.617
wR2 [all reflections]
Goodness of fit
Max shift/Error
23
˚
Max peak/e A
23
˚
Min peak/e A
CCDC
21.200
869666
20.594
869667
20.655
869668
20.553
869669
the SHELXTL software package (Table 1).30 Most atoms were
refined anisotropically without any restraints; however disor-
dered groups in compounds 3 (isopropyl groups) and 6 (pyridyl
groups) were refined isotropically. Crystallographic data for the
structural analyses have been deposited with the Cambridge
Crystallographic Data Centre, CCDC Nos. 869666–869669 for
3–6, respectively.{
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