Design, synthesis and characterization of a Pt-Gd metal-organic framework containing potentially catalytically active sites

Article abstract of DOI:10.1039/b719766g

The heterobimetallic metal-organic framework {[(BPDC)PtCl₂] ₃(Gd(H₂O)₃)₂}?5H₂O (BPDC = 2,2′-bipyridine-5,5′-dicarboxylate) has been designed and synthesized by hydrothermal methods. The new coordination polymer contains subunits of (BPDC)PtCl₂ (1) where both N atoms of the BPDC ligand are attached to a square-planar Pt(ii) center. The two remaining cis coordination sites at Pt(ii) are occupied by chloride ions. The final structure (2) of the polymeric network is obtained when Gd(iii) ions link together the (BPDC)PtCl₂ units, which are organized in sheets, into larger blocks. These blocks are stacked along the crystallographic [010] direction and are held together by a hydrogen bonding scheme that involves carboxylate oxygen atoms and water molecules in the coordination sphere of Gd. The coordination polymer 2 can be obtained in a single-step reaction or in a two-step synthesis where the corresponding Pt complex (1) was first synthesized followed by reacting 1 with Gd(NO₃)₃?6H₂O. In situ high temperature powder X-ray diffraction shows that the crystalline coordination polymer transforms into an anhydrous modification at 100 °C. This modification is stable to 350 °C, at which temperature the structure starts to decompose. The coordination sphere around platinum in the polymer closely resembles organometallic Pt complexes that have been previously found to catalytically or stoichiometrically activate and functionalize hydrocarbon C-H bonds in homogeneous systems. The Royal Society of Chemistry.

Full text of DOI:10.1039/b719766g

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PAPER
www.rsc.org/dalton | Dalton Transactions
Design, synthesis and characterization of a Pt–Gd metal–organic framework
containing potentially catalytically active sites†
Kai C. Szeto,a Kjell Ove Kongshaug,‡a Søren Jakobsen,a Mats Tilset*b and Karl Petter Lilleruda
Received 21st December 2007, Accepted 23rd January 2008
First published as an Advance Article on the web 25th February 2008
DOI: 10.1039/b719766g
The heterobimetallic metal–organic framework {[(BPDC)PtCl2]3(Gd(H2O)3)2}·5H2O (BPDC =
2,2ꢀ-bipyridine-5,5ꢀ-dicarboxylate) has been designed and synthesized by hydrothermal methods. The
new coordination polymer contains subunits of (BPDC)PtCl2 (1) where both N atoms of the BPDC
ligand are attached to a square-planar Pt(II) center. The two remaining cis coordination sites at Pt(II)
are occupied by chloride ions. The final structure (2) of the polymeric network is obtained when Gd(III)
ions link together the (BPDC)PtCl2 units, which are organized in sheets, into larger blocks. These blocks
are stacked along the crystallographic [010] direction and are held together by a hydrogen bonding
scheme that involves carboxylate oxygen atoms and water molecules in the coordination sphere of Gd.
The coordination polymer 2 can be obtained in a single-step reaction or in a two-step synthesis where
the corresponding Pt complex (1) was first synthesized followed by reacting 1 with Gd(NO3)3·6H2O.
In situ high temperature powder X-ray diffraction shows that the crystalline coordination polymer
transforms into an anhydrous modification at 100 ◦C. This modification is stable to 350 ◦C, at which
temperature the structure starts to decompose. The coordination sphere around platinum in the
polymer closely resembles organometallic Pt complexes that have been previously found to catalytically
or stoichiometrically activate and functionalize hydrocarbon C–H bonds in homogeneous systems.
Although the Shilov system serves as a landmark demon-
Introduction
stration that it is indeed possible to achieve mild and catalytic
The direct, selective, catalytic conversion of alkanes, especially
functionalization of methane, this catalytic system suffers from
methane, into value-added products under mild conditions has
limited catalyst lifetime, low reaction rates, and low turnover
become an intensely pursued field of research.1–3 Methane ac-
tivation in homogeneous systems was demonstrated as early as
1969 by Shilov and co-workers,4 who discovered that alkane C–H
numbers. More recently, several research groups have shown that
Pt(II) complexes with cis-coordinated bidentate N-donor organic
ligands can activate and functionalize alkane and arene C–H
bonds, as has beenrecently reviewed.7 Methaneactivation has been
demonstrated by Bercaw and co-workers at a c◦ationic (tmen)Pt(II)
complex in pentafluoropyridine solvent at 85 C,8, 9 by Tilset and
co-workers at a cationic (diimine)Pt(II) complex in trifluoroethanol
at 45 ◦C,10, 11 and by Vedernikov at a Pt(II) complex bearing a sul-
fonated bi(pyridyl)methane ligand in dichloromethane at ambient
temperature.12 Furthermore, catalytic conversion of methane to
methyl bisulfate (which can easily furnish methanol) has been
reported using a Pt(II) bipyrimidine complex in warm, fuming
H2SO4.13 The mechanism of C–H activation at the organometallic
complexes has been investigated both with experimental and
theoretical methods,7 and closely resembles the behaviour of the
classical Shilov system.
bonds can undergo H/D exchange in deuterated acidic aqueous
solution in the presence of K2PtCl4 as a catalyst. Moreover,
catalytic conversion of methane to a mixture of methanol and
chloromethane was achieved in the presence of stoichiometric
quantities of K2PtCl6 as an oxidizing agent by the same group.5
The mechanism of the widely acclaimed Shilov system is believed
to comprise the three steps (a) hydrocarbon C–H activation, by
oxidative addition or electrophilic activation, accompanied by
deprotonation; (b) oxidation of the resulting Pt(II) alkyl to Pt(IV)
alkyl; and (c) nucleophilic attack at the Pt(IV) alkyl to provide the
functionalized hydrocarbon and concomitant regeneration of the
Pt(II) catalyst.6
To find a system that selectively reacts with the normally unre-
active methane C–H bonds in the presence of large quantities of a
solvent without detrimental reactions of this solvent and without
decomposition of the complex (for example by self-degradation
through inter- or intramolecular reactions with its own ligands) is
a great challenge. Other problems that are of particular relevance
for large-scale chemical processes are the difficulties associated
with separating the catalyst from the products, and of products
from the solvent (when a solvent is used). These fundamental
challenges that are more or less inherent to homogeneous catalysis
have motivated us to develop a heterogeneous counterpart to the
aCentre for Materials Science and Nanotechnology, University of Oslo,
P.O. Box 1126, Blindern, N-0318, Oslo, Norway
bCentre for Theoretical and Computational Chemistry, Department of
Chemistry, University of Oslo, P.O. Box 1033, Blindern, N-0315, Oslo,
Norway. E-mail: mats.tilset@kjemi.uio.no
† CCDC reference number 672214. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b719766g
Electronic supplementary information (ESI) available: Digital and SEM
images of the isolated product, powder X-ray pattern for 2, TGA patterns
and TG-MS plot. See DOI: 10.1039/b719766g
‡ Current address: Norwegian Defence Research Establishment, P.O. Box
25, N-2027 Kjeller, Norway.
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homogeneous Pt systems discussed above. The ultimate goal is to
link separate, isolated organometallic Pt(II) complexes with a well-
defined coordination sphere together into a rigid network. The
resulting network structure will constitute an example of the class
of materials that is commonly referred to as coordination polymers
or metal–organic frameworks (MOFs) in the literature.14–17
Coordination polymers have the advantage that it is possible
to control the topology and geometry of the infinite framework.
Hence, materials with different properties can be constructed.14
Coordination polymers with catalytic,18–23 magnetic,24–26 gas
and small molecule adsorption and separation,27–34 small
molecule sensing,35, 36 luminescing,36–38 proton conducting,39 and
semiconducting40, 41 properties, to mention just a few, have been
reported.
Herein, we report on the synthesis and characterization of a
novel coordination polymer containing Pt(II) metal sites where the
coordination sphere around Pt closely resembles the organometal-
lic complexes that have been demonstrated to activate hydrocar-
bon C–H bonds in homogeneous systems. Heterobimetallic Pt-
containing coordination polymers are relatively unexplored42–52
(furthermore, we have recently reported additional aspects of the
chemistry and physical properties of the coordination polymer
reported herein and of a related material53, 54). In the coordi-
nation polymer reported herein, the potentially active sites are
created by cis coordination of the N-atoms in 2,2ꢀ-bipyridine-
5,5ꢀ-dicarboxylate (BPDC) to Pt(II). The remaining coordination
sites around Pt(II) are occupied by chloride ions, resulting in a
(BPDC)PtCl2 building block (1). These units are linked together
through the carboxylate groups by binding at Gd(III) ions to
form an extended framework (2). Incorporating and locking the
active sites into a solid framework hopefully will help prevent the
chemical and practical problems that are associated with catalyst
degradation and product and catalyst separation.
were performed by Ilse Beetz Mikroanalytisches Laboratorium
(Kronach, Germany).
Preparation of 2,2ꢀ-bipyridine-5,5ꢀ-dicarboxylic acid
The commercially available (Aldrich) but fairly expensive com-
pound can be quickly prepared on a large scale using the method
given by Oki and Morgan55 for the preparation of 2,2ꢀ-bipyridine-
4,4ꢀ-dicarboxylic acid, with some modifications: To a stirred
solution of 5,5ꢀ-dimethyl-2,2ꢀ-bipyridine (5.076 g, 27.6 mmol) in
125 mL 95% H2SO4, K2Cr2O7 (24.32 g, 82.7 mmol) was added
in small portions during 20 min. Cooling was applied if the
temperature rose above 80 ◦C. The stirring was continued until the
temperature was 35 ◦C and the solution was poured over 800 mL
of ice water with additional external cooling. The greenish white
solid was separated from the solution either by centrifugation
(preferred) or by filtration through a paper filter (time consuming),
washed with large quantities of water until the water was no
longer discolored, an◦d then once with acetone. The solid was dried
under vacuum at 60 C (yield 6.45 g, 96%). 1H NMR (200 MHz,
D2O/NaOD) d 7.89 (d, J = 8.2 Hz, 2 H), 8.12 (dd, J = 8.2,
1
1.8 Hz, 2 H), 8.79 (d, J = 1.8 Hz, 2 H). 13C{ H} NMR (50 MHz,
D2O/NaOD) d 122.8, 133.3, 139.6, 150.7, 156.7, 173.5. MS m/z
+
−1
=
=
244 (M ). IR (KBr) 1686 (C O), 1593 (arom. C C) cm . Anal.
Calcd for C12H8N2O4: C, 59.02; H, 3.30; N, 11.47. Found: C, 58.40;
H, 3.11; N, 10.92%.
Preparation of Pt(BPDC)Cl2 (1)
To a stirred solution of KPtCl3(C2H4)·H2O (100 mg, 0.27 mmol)
in methanol (3.5 mL) was added 2,2ꢀ-bipyridine-5,5ꢀ-dicarboxylic
acid (66 mg, 0.27 mmol) and the mixture was stirred at ambient
temperature for 5 min before heating at reflux for 15 min. The
mixture was cooled to room temperature. The now orange solid
was separated by filtration, washed with 0.15 M NaOH (7 mL)
which turned the solid red, and then dried in vacuum (yield:
123 mg, 89%), mp decompose at 350 ◦C (Fig. S1, ESI†). 1H NMR
(500 MHz, D2O/KOH) d 8.27 (d, J = 8.2 Hz, 2 H), 8.58 (dd,
Experimental section
General procedures
K2PtCl4, KPtCl3(C2H4)·H2O, Gd(NO3)3·6H2O, and 5,5ꢀ-dimethyl-
2,2ꢀ-bipyridine were obtained commercially (Aldrich). All chemi-
cals were used as received.
J = 8.2, 1.6 Hz, 2 H), 9.38 (d, J = 1.6 Hz, 2 H). 13C{ H} NMR
1
(126 MHz, D2O/KOH) d 126.4, 138.3, 142.2, 150.8, 160.5, 172.8.
195Pt NMR (108 MHz, D2O/KOH) d −1831. IR (KBr) 3553, 1618,
1591, 1557, 1409, 1384 cm−1. Anal. Calcd for C12H8Cl2N2O4Pt: C,
28.25; H, 1.58; Cl, 13.90; N, 5.49; O, 12.54. Found: C, 24.30; H,
1.90; N, 4.69; O, 11.20%. It was not possible to obtain consistent
results from elemental analyses. However, the decomposition in
TGA (Fig. S1, ESI†) is consistent with the given formula. Note
that the BPDC ligand can be dissolved in aqueous NaOH, whereas
the Pt complex cannot.
Thermogravimetric analysis was performed with a Rheometric
Scientific STA 1500 instrument in an N2 atmosphere. The rate
◦
of heating was 2 C min−1. High-temperature powder X-ray
diffraction experiments under N2 were carried out in a Bu¨hler
furnace on a Siemens D500 diffractometer equipped with a Cu
X-ray tube and b-filter. The sample was deposited on a Pt filament
and heated consecutively from 25 ◦C to 900 ◦C. Qualitative
element analysis of the heavier elements and microscopy were
performed with a Philips XL 30 Scanning Electron Microscope.
The sample was mounted on carbon tape and covered with a
thin layer of gold. NMR data were collected on Bruker DPX
200 and 500 MHz instruments at room temperature with 3-
trimethylsilylpropionic acid sodium salt-d4 as internal standard
(1H and 13C NMR) or K2PtCl4 + 2 equiv. NaBr in D2O in a
Preparation of the coordination polymer (2)
Method 1. K2PtCl4 (0.12 g, 0.29 mmol), 2,2ꢀ-bipyridine-5,5ꢀ-
dicarboxylic acid (0.07 g, 0.29 mmol) and Gd(NO3)3·6H2O (0.08 g,
0.18 mmol) were added to a Teflon-lined vessel filled with distilled
water (9.75 g, 0.54 mol). The solution was pre-treated by stirring
for 10 min at ambient temperature in air. Hereafter, the vessel was
closed in a steel autoclave, placed in a pre-heated oven at 100 ◦C
for 16 h, and then cooled in water. The product was collected
by filtration (polycarbonate 5.0 lm pore size membrane filter),
2−
capillary tube, with PtClBr3 set at d −2380 (195Pt NMR). Mass
spectrometry measurements were done on a Waters Micromass
QTOF II spectrometer. IR spectra were recorded on a Nicolet
Magna-IR 550 FT-IR spectrophotometer. Elemental analyses
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washed with water, and dried in air at ambient temperature (yield:
18% based on Pt).
small displacement of weakly connected blocks in the structure
and of non-coordinated water molecules in the voids. Possible
presence of twinning was checked with the program Cell_Now.59
The program can assign 90% of the reflections (based on 999
spots, covering every orientation of the crystal) to a single crystal
and this strongly suggests the absence of twinning. Compound 2
crystallized only as small crystals, thus adding an extra challenge
to the data collection. Despite the long exposure time that was
applied during the acquisition, the quality of the frames was still
poor (weak diffraction and absence of spots at higher angles). This
means that the major structural features of 2 should be correct,
while some of the quantitative crystallographic details may be less
reliable. Due to the poor crystal quality, only the heavier atoms
were refined anisotropically. Hydrogen atoms connected to the
organic ligand were refined as a rigid model in their theoretical
positions while scattering from hydrogen atoms in water was
ignored. The crystallographic data are summarized in Table 1,
and selected bond distances and angles are given in Table 2.
Method 2. K2PtCl4 (63.9 mg, 0.154 mmol), 2,2ꢀ-bipyridine-
5,5ꢀ-dicarboxylic acid (36.5 mg, 0.150 mmol), Gd(NO3)3·6H2O
(291.1 mg, 0.644 mmol) and distilled water (9.64 g, 0.535 mol) were
added to a Teflon-lined vessel. The vessel was closed in a PEEK
(polyether-ether-ketone) autoclave, stirred for 10 min at ambient
temperatureandthenheatedrapidly(heatingrate:100◦C min−1) to
100 ◦C, at which temperature it was held for 10 h using microwaves
while stirring continuously. The product (red solid) was isolated
as described for method 1 (yield: 94.6 mg, 90% based on Pt).
Anal. Calcd for C36H40Gd2Cl6N6O23Pt3: C, 21.23; H, 1.98; N, 4.13.
Found: C, 21.78; H, 2.09; N, 4.32%.
Method 3. Pt complex 1 (0.06 g, 0.12 mmol), Gd(NO3)3·6H2O
(0.05 g, 0.11 mmol), and distilled water (6.90 g, 0.38 mol) were
mixed in a Teflon-lined vessel. The solution was pre-treated by
stirring for 10 min at ambient temperature in air. The vessel
was closed in an autoclave and heated rapidly (heating rate:
◦
80 C min−1) to 100 ◦C, at which temperature it was held for
Results and discussion
10 h. The product was isolated as described for method 1 (yield:
0.05 g, 62% based on Pt).
Structure design
Before starting the synthetic work, certain conditions that the final
structure has to satisfy were identified. One important criterium
is that the compound must have the catalytically active Pt sites
incorporated in the final structure. A Pt(II) source and a cis-
chelating nitrogen-based organic ligand are therefore needed. Rea-
sonably labile ligands coordinated to the two remaining sites of the
square planar Pt(II) center are also required to provide accessible
coordination sites for hydrocarbon binding and activation accord-
ing to the quite well understood7 mechanisms of homogeneous
activation. Due to the limited coordination number and square-
planar geometric preference of Pt(II), a structure containing only
Pt(II) and one type of ligand will probably crystallize as a 2-
dimensional layered structure. Such structures will have difficulties
Structure determination of 2
The structure of 2 was determined by single crystal X-ray
diffraction using a Siemens Bruker Smart CCD diffractometer.
A total of 915 frames were collected (Dx = 0.6◦, 45 s per frame),
covering one hemisphere of the reciprocal space. Data reduction
and empirical absorption correction were carried out using the
programs SAINT+ 56 and SADABS,57 respectively. The structure
was solved by direct methods and refined using the SHELXTL58
program package. We noticed that, despite several data collections
on different crystals performed at low temperature, the refinements
yielded relatively high reliability factors. This is attributed to
relatively poor crystal quality and possible distortions caused by
Table 1 Crystal data and structure refinement for 2
Empirical formula
Formula weight
Temperature/K
C36H40Cl6Gd2N6O23Pt3
2037.21
105
0.71073
˚
Wavelength/A
Crystal system
Space group
Orthorhombic
Pbca (No. 61)
18.021(4)
19.884(4)
28.125(6)
10078(4)
˚
a/A
˚
b/A
˚
c/A
3
˚
Volume/A
Z
8
Calculated density/g cm−3
Absorption coefficient/mm−1
F(000)
2.685
11.300
7568
Crystal size/mm
0.16 × 0.05 × 0.05
Theta range for data collection/◦
Limiting indices
1.68 to 18.67
−16 ≤ h ≤ 16, −17 ≤ k ≤ 17, −25 ≤ l ≤ 25
37175/3833 [R(int) = 0.1994]
3833/204/475
99.5%
1.021
R1 = 0.0488, wR2 = 0.0941
R1 = 0.0933, wR2 = 0.1097
1.438 and −0.864
Reflections collected/unique
Data/restraints/parameters
Completeness to theta = 18.67
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
−3
˚
Largest diff. peak and hole/e A
2056 | Dalton Trans., 2008, 2054–2060
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in accommodating the increase in coordination number caused by
the oxidation of Pt(II) to octahedral 6-coordinate Pt(IV) during
C–H activation by an oxidative addition mechanism without
collapse of the framework. The introduction of an additional metal
center to link the Pt(II) units together may facilitate the desired
stabilization of the desired 3-D network structure. To fulfill this
requirement, BPDC was chosen as the ligand at Pt(II). This ligand
offers two N atoms for binding at the soft Pt center and two
carboxylate groups for binding at a harder (more oxophilic) linker
metal.
The utilization of K2PtCl4 or KPtCl3(C2H4)·H2O (Zeise’s salt)
which are commercially available for the preparation of neutral
Pt(II) chelate complexes has been demonstrated.60,61 Gadolinium
is selected to link the units of 1 into an extended framework.
Gd is strongly oxophilic, has a stable +3 oxidation state, and the
potential for high coordination numbers. The stable oxidation state
is important to preclude oxidation or reduction of the Pt(II) sites
that are needed for catalysis, and the high coordination number
is desirable because it increases the chance that the final structure
will be covalently bonded into a 3-D network that is rigid and
thermally stable.
Fig. 1 Asymmetric unit of 2. Hydrogen atoms and uncoordinated water
molecules are excluded for clarity.
˚
positions. The average bond distances of d(Pt–N) = 2.00(5) A
˚
and d(Pt–Cl) = 2.294(16) A are well in accordance with those
observed for related 5,5ꢀ-disubstituted 2,2ꢀ-bipyridine platinum
complexes.64–66 The contraction of the N–Pt–N angle from the
ideal 90◦ to an average value of 80.7◦ is typical for cis-chelating
bipyridines as is the expansion of the Cl–Pt–N angles to an average
of 95.7◦ from the ideal 90◦.
Syntheses
It is well known that square planar Pt(II) centres have a tendency
to aggregate in the solid state to form complexes that possess short
The BPDC ligand and its PtCl2 complex 1 are important com-
pounds in this system, hence fast and reliable ways of making these
compounds in quite large quantities are needed. Our reported
syntheses are faster and more efficient methods to prepare these
compounds than the previously reported methods.62, 63
˚
Pt · · · Pt contacts (<3.5 A). Most often this results in structures
composed of infinite Pt · · · Pt · · · Pt chains,65–67 but structures
containing discrete dimeric units that are dominated by short
Pt · · · Pt contacts are also well known in the literature.68,69 In 2 there
are discrete trimeric units with short Pt · · · Pt contacts (d(Pt(1)–
The heterobimetallic material 2 can be synthesized either by a
single-step synthesis from K2PtCl4, Gd(NO3)3·6H2O, and BPDC,
described in the Experimental part as method 1 and method
2, or by a reaction between pre-formed 1 and Gd(NO3)3·6H2O
(method 3). The main solid material obtained by using method
1 was the unreacted BPDC ligand, in addition to small amounts
of 2 (Fig. S2, ESI†), including single crystals (Fig. S3, ESI†).
Performing the synthesis in the microwave oven with rapid heating
gave a crystalline powder (Fig. S4, ESI†) of pure 2. Applying
fast heating to this system therefore resulted in improvements in
yield (90 vs. 18%) as well as purity. Qualitative elemental analysis
by SEM confirmed that the sample contained Pt, Gd, and Cl.
The lighter elements (C, N, O, H) could not be identified with
our instrument. Method 3 also gave pure 2, but the yield was
somewhat reduced compared to method 2 (62 vs. 90%). Moreover,
the typical particle size of the method 3 product is believed to be
smaller than that for the product from method 2 since the peaks of
the powder X-ray diffraction pattern are broader (Fig. S5, ESI†)
and the filtration was considerably more time consuming.
˚
Pt(2) = 3.3692(16) and d(Pt(2)–Pt(3)) = 3.2727(16) A), as shown
in Fig. 2. The distance between the trimers is considerably longer
˚
(d(Pt(1)–Pt(3)) = 4.8917(16) A). Crystal structures containing
such trimers are rare, and we could find only three examples
containing square planar Pt(II) units described in the literature.70–72
In the trimeric stack, the BPDC molecule coordinated to the
middle Pt atom is rotated by approximately 133◦ with respect
◦
˚
Table 2 Selected bond distances (A) and angles ( ) for 2
Pt(1)–N(2)
Pt(1)–N(1)
Pt(1)–Cl(2)
Pt(1)–Cl(1)
Pt(2)–N(3)
Pt(2)–N(4)
Pt(2)–Cl(3)
Pt(2)–Cl(4)
Pt(3)–N(6)
Pt(3)–N(5)
Pt(3)–Cl(6)
Pt(3)–Cl(5)
Gd(1)–O(5)
Gd(1)–O(3)
Pt(1)–Pt(2)
N(1)–Pt(1)–N(2)
N(1)–Pt(1)–Cl(2)
N(2)–Pt(1)–Cl(2)
N(1)–Pt(1)–Cl(1)
N(2)–Pt(1)–Cl(1)
Cl(2)–Pt(1)–Cl(1)
N(3)–Pt(2)–N(4)
N(3)–Pt(2)–Cl(3)
N(4)–Pt(2)–Cl(3)
2.010(19)
1.99(2)
2.282(7)
2.297(6)
1.998(19)
1.99(2)
2.293(6)
2.301(6)
1.989(17)
2.016(18)
2.294(7)
2.295(7)
2.232(18)
2.313(16)
3.3692(16)
81.4(8)
Gd(1)–O(11)
Gd(1)–OW5
Gd(1)–OW6
Gd(1)–O(1)
Gd(1)–OW4
Gd(1)–O(2)
Gd(2)–O(6)
Gd(2)–O(9)
Gd(2)–O(4)
Gd(2)–OW1
Gd(2)–OW3
Gd(2)–OW2
Gd(2)–O(8)
Gd(2)–O(7)
Pt(2)–Pt(3)
N(3)–Pt(2)–Cl(4)
N(4)–Pt(2)–Cl(4)
Cl(3)–Pt(2)–Cl(4)
N(6)–Pt(3)–N(5)
N(6)–Pt(3)–Cl(6)
N(5)–Pt(3)–Cl(6)
N(6)–Pt(3)–Cl(5)
N(5)–Pt(3)–Cl(5)
Cl(6)–Pt(3)–Cl(5)
2.335(16)
2.357(18)
2.462(15)
2.479(16)
2.484(16)
2.488(16)
2.307(17)
2.315(15)
2.330(17)
2.326(15)
2.370(15)
2.465(15)
2.485(16)
2.499(15)
3.2727(16)
Crystal structure of the coordination polymer 2
The coordination environments around the Pt and Gd metal
centers are illustrated in Fig. 1. Bond distances and angles are
listed in Table 2. As anticipated, the crystal structure of 2 contains
Pt centers with essentially the same coordination environment
as in the designed molecular precursor 1. Each of the three
crystallographically non-equivalent Pt atoms have a square planar
coordination mode, being coordinated to the two N-atoms of
a BPDC molecule and two chloride ions in the remaining cis
176.2(6)
176.4(6)
95.1(6)
94.9(6)
175.4(6)
88.6(2)
80.0(8)
96.2(6)
87.7(2)
80.6(8)
176.6(5)
96.3(5)
95.4(5)
176.0(5)
87.7(2)
96.1(6)
175.9(6)
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involving carboxylate oxygens and water molecules coordinated
to Gd, resulting in a 2-D layered structure.
Stability of the coordination polymer 2
The isolated coordination polymer 2 shows an exceptionally
good thermal stability under atmospheric, inert, and oxidizing
conditions (Fig. S6, ESI†). This compound can be stored open to
the atmosphere at room temperature for years without any loss of
crystallinity (Fig. S7, ESI†).
The thermal stability of 2 in an N2 atmosphere is depicted in
Fig. 4. The TGA pattern of 2 exposed to O2 (air) is virtually the
◦
same. The first weight loss around 100 C is confirmed (Fig. S8,
ESI†) to be water (observed: 9.11%, calculated: 9.73%). The
TGA curve indicates that the water molecules leave the structure
in two steps, which suggests that there are two different types
of water molecule in the structure. This is in good agreement
with the X-ray diffraction data. The first loss can be associated
with the uncoordinated water molecules while the second loss
corresponds to the water molecules that are coordinated to
Fig. 2 The crystal structure of the trimeric Pt subunit of 2.
to two BPDC molecules above and below. The three Pt atoms do
not form a linear axis in the trimer, but are somewhat displaced
resulting in a Pt(1)–Pt(2)–Pt(3) angle of 165◦.
There are two non-equivalent Gd atoms in the asymmetric unit,
both coordinated to eight oxygen atoms. In both cases, five of
these are carboxylate oxygen atoms, while the remaining three
◦
gadolinium. Around 350 C, the compound begins to transform
to metallic platinum (confirmed with powder X-ray diffraction)
while BPDC and chloride are decomposing (observed: 43.44%,
calculated: 43.75%) to gaseous species. In situ high-temperature
powder X-ray diffraction analyses are consistent with the TGA
data. The patterns are reported in Fig. 5. The structure transforms
to a dehydrated phase at 100 ◦C (red patterns) with small changes
in lattice parameters. The dehydrated phase is stable and remains
crystalline until 350 ◦C, after which the decomposition takes place
(green pattern). Pt(II) is reduced to metallic platinum immediately,
while crystalline Gd2O3 was first observed at 800 ◦C (cyan
patterns).
˚
are terminal water molecules (d(Gd–O): 2.232(18)–2.499(15) A,
˚
d(Gd–OW): 2.326(15)–2.484(16) A). Such a relatively broad dis-
tribution of Gd–O bond lengths is known from the literature.73, 74
In the crystal structure of 2 the Gd atoms form dimers with
˚
d(Gd(1)–Gd(2)) = 5.498(2) A. The metal centers in the dimers
are connected by two carboxylate bridges, both adopting a
syn–syn conformation. In addition to this bridging bidentate
coordination mode, other carboxylate groups are involved in two
other coordination modes to Gd: monodentate and chelating
bidentate.
The asymmetric unit also contains 5 fully occupied water
˚
molecule sites. The Gd dimers are separated by about 13 A in the
[100] direction, creating cages in which these free water molecules
reside. They are involved in an extensive hydrogen-bonding scheme
that also involves the carboxylate oxygen atoms and the water
molecules coordinated to Gd.
The crystal structure of 2 is composed of sheets of 1 stacked
along [010] (Fig. 3). The distance between the sheets is about
˚
3.3 A. In the sheets, the units of 1 are packed along [100] with every
second unit being rotated by 47◦. The Gd dimers serve to link three
successive sheets of 1 into larger blocks. These blocks are stacked
along [010], and are held together by a hydrogen-bonding scheme
Fig. 4 TGA curve for 2 under an N2 atmosphere.
Conclusion
It is an ultimate goal for us to make a heterogeneous ver-
sion of catalytically active organometallic complexes. This work
represents an important step towards this end. In a rationally
designed synthesis, a Pt center that is structurally closely related
to species that are capable of activating hydrocarbon C–H bonds
in homogeneous solutions has been incorporated into the network
Fig. 3 The crystal structure of 2 seen along [100].
2058 | Dalton Trans., 2008, 2054–2060
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Fig. 5 In situ powder X-ray diffraction patterns of 2 at different temperatures under an N2 atmosphere.
structure of a coordination polymer that is thermally robust under
atmospheric, oxidizing, and reducing conditions.
20 C.-D. Wu, A. Hu, L. Zhang and W. Lin, J. Am. Chem. Soc., 2005, 127,
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We acknowledge generous financial support from the Norwegian
Research Council (NFR). We thank Dr Pascal Dietzel for kind
assistance with problems concerning twinning, and Prof. Carl
Henrik Gørbitz for valuable crystallographic discussions.
27 L. Pan, K. M. Adams, H. E. Hernandez, X. Wang, C. Zheng, Y. Hattori
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