Novel dirhodium coordination polymers: The impact of side chains on cyclopropanation

Article abstract of DOI:10.1039/c8cy01493k

Seven novel dirhodium coordination polymers (Rh₂-Ln) (n = 1-7) are prepared by employing bitopic ligands to connect dirhodium nodes. The formation of the framework is confirmed by attenuated total reflectance Fourier transform infrared (ATR-FTIR) and ¹H → ¹³C cross polarization magic angle spinning nuclear magnetic resonance (CP MAS NMR) spectroscopy. Defect sites resulting from incomplete ligand substitution are revealed by ¹⁹F MAS NMR. The random stacking behavior of the frameworks in the obtained solid is analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The Rh₂/O interaction in neighboring layers is investigated by diffuse reflectance ultra-violet visible light (DR-UV-vis) spectroscopy and X-ray photoelectron spectroscopy (XPS). This interaction is relevant to understand the catalytic behavior of various Rh₂-Ln catalysts in the cyclopropanation of styrene with ethyl diazoacetate (EDA). In this context, the structure-reactivity relationship is discussed by taking into consideration both interlayer Rh₂/O interactions and steric effects of side chains.

Full text of DOI:10.1039/c8cy01493k

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DOI: 10.1039/C8CY01493K
Novel Dirhodium Coordination Polymers: The Impact of Side Chains on
Cyclopropanation
Jiquan Liu,[a,b] Yeping Xu,[b] Pedro B. Groszewicz,[b] Martin Brodrecht,[b] Claudia Fasel,[c]
Kathrin Hofmann,[b] Xijuan Tan,[d] Torsten Gutmann[b]* and Gerd Buntkowsky[b]*
a Key Laboratory of Synthetic and Natural Functional Molecule Chemistry, College of
Chemistry and Materials Science, Northwest University, 710127 Xi’an, P. R. China
b Eduard-Zintl-Institute for Inorganic Chemistry and Physical Chemistry, Technical
University Darmstadt, Alarich-Weiss-Straße 8, 64287 Darmstadt, Germany
c FB Material- und Geowissenschaften, Technical University Darmstadt, Alarich-Weiss-
Straße 2, 64287 Darmstadt, Germany
d Laboratory of Mineralization and Dynamics, College of Earth Sciences and Land
Resources, Chang’an University, 710054 Xi’an, P. R. China
E-mail: gutmann@chemie.tu-darmstadt.de
Gerd.Buntkowsky@chemie.tu-darmstadt.de
Abstract
Seven novel dirhodium coordination polymers (Rh2-Ln) (n=1-7) are prepared by employing
bitopic ligands to connect dirhodium nodes. The formation of the framework is confirmed by
1
attenuated total reflectance Fourier transform infrared (ATR-FTIR) and H13C cross
polarization magic angle spinning nuclear magnetic resonance (CP MAS NMR) spectroscopy.
Defect sites resulting from incomplete ligand substitution are revealed by 19F MAS NMR. The
random stacking behavior of the frameworks in the obtained solid is analyzed by scanning
electron microscopy (SEM) and X-ray diffraction (XRD). The Rh2/O interaction in neighboring
layers is investigated by diffuse reflectance ultra-violet visible light (DR-UV-vis) spectroscopy
and X-ray photoelectron spectroscopy (XPS). This interaction is relevant to understand the
catalytic behavior of various Rh2-Ln catalysts in the cyclopropanation of styrene with ethyl
diazoacetate (EDA). In this context, the structure-reactivity relationship is discussed by taking
into consideration both interlayer Rh2/O interactions and steric effects of side chains.
Introduction
Homogeneous transition metal catalysts have strong advantages in the industrial scale
production of fine chemicals owing to their high selectivity and activity, but their applications
are prohibited owing to the often challenging recovery and recycling of the catalyst.[1]
Particularly in the synthesis of pharmaceutical products, even smallest metal contaminations
have to be avoided both for medical and legal reasons. In certain cases, isolation of products
such as pharmaceuticals,[2] or of soluble polymer catalysts can be achieved via precipitation
from reaction media.[1a] Furthermore, heterogenization of organometallic catalysts is of
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DOI: 10.1039/C8CY01493K
innovative interest,[1b, 1c] where catalysts can be readily separated from reaction media via
filtration. Finally, fixed-bed reactors have been employed successfully in the production of
pharmaceuticals to overcome separation issues.[3]
Dirhodium (II) complexes are important examples of organometallic catalysts. They have
drawn much attention as efficient catalysts for various carbene transfer reactions including
cyclopropanation, C-H insertion, Si-H insertion.[4] Over the last decades, major efforts have
been devoted to develop recyclable heterogenized dirhodium catalyst by forming covalent
bonds via equatorial[5] or axial binding.[6] Mainly functional silica materials and polymers were
employed as carriers.[4g, 5-6] Recently, some of us achieved two types of supported dirhodium
catalysts using amine-carboxyl bi-functional SBA-15[7] or cellulose nanocrystals[8] as carriers,
where the dirhodium unit was chemically bound via amine and/or carboxyl groups. While these
immobilized catalysts preserve the selectivity of the homogeneous ones, their drawbacks cannot
be ignored, namely the lower catalytic activities deriving from insufficient loading, mass
transfer resistance and blocking of active sites by ligand coordination at axial position.[9]
An alternative approach to the conventional immobilization of homogeneous catalysts, is the
synthesis of self-supported catalysts, who emerged along with the development of coordination
polymers or metal organic frameworks (MOFs).[10] In the heterogenization process typically
bitopic or polytopic ligand systems form a framework with metallic complexes. Such type of
catalysts exhibits unique features, such as high density of catalytic sites, uniform distribution,
high loadings and easy separability.[11] Important examples, such as [Cu(II)3(BTC)2],[12]
[Cu(II)Er(III)2(pdc)2(Hpdc)(H2O)4]·2H2O,[13] [Cu(I)2(4,4’-bpy)2SO4]·6(H2O),[14] and various
polytopic bis(oxazoline)-based Cu(I) (or Cu(II)) coordination polymers[15] were applied in
cyclopropanation reactions. Moreover, Au(III) Schiff base[12] and Ru(III)-salen[16] were
introduced into MOFs and employed as cyclopropanation catalysts.
Dirhodium based coordination polymer catalysts were obtained via solvothermal synthesis or
ligand exchange and applied in hydrogenation,[17] H-D exchange reaction,[17b] cyclopropanation
[20]
reaction[18] and photochemical reduction of water.[19] Su et.al
prepared dirhodium metal-
organic gels and applied them in coupling CO2 with epoxides and intramolecular C-H amination
of vinyl azides.
While most of these studies mainly focused on the synthesis of novel coordination polymers as
catalysts, systematic investigations of the relationship between structure/chemical environment
of the organometallic framework on the catalytic performance are rare. However, as two recent
examples on Pd clusters and PdAu nanoparticles in MOFs showed, they are highly important
for rational control of catalytic sites.[21]
The present study illustrates the role of side chains in the layered dirhodium coordination
polymers and related structure-reactivity relationship. These novel dirhodium coordination
polymers Rh2-Ln (n = 1-7) are prepared by ligand exchange employing ditopic ligands Ln (n =
1
1-7, Scheme 1) that are substituted with different side chains. ATR-FTIR, H13C CP MAS
NMR and 19F MAS NMR are used to probe the local structure of these catalysts. DR-UV-vis
and XPS are employed to investigate the interlayer Rh2/O interaction in the dirhodium
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coordination polymers. Finally, the obtained coordination polymers are applied in thDeOI:m10o.1d0e3l9/C8CY01493K
cyclopropanation between styrene and ethyl diazoacetate (EDA), and the impact of side chains
on the catalytic performance is discussed.
Scheme 1. Ditopic ligand systems employed for the synthesis of novel dirhodium coordination
polymers: H2Ln (n = 1-5) represents modified terephthalic acids with ethers as side chains,
H2Ln (n = 6-7) represents modified terephthalic acids with ester side chains.
Experiment part
Synthesis of Coordination Polymers
General
Rhodium trifluoroacetate dimer (Rh2(TFA)4, Acros), 2,5-dihydroxyterephthalic acid (Sigma-
Aldrich), diethyl 2,5-dihydroxyterephthalate (Sigma-Aldrich), iodomethane (Sigma-Aldrich),
bromoethane (Sigma-Aldrich), 1-bromobutane (Alfa Aesar), 2-bromoethyl methyl ether
(Merck), benzyl bromide (Merck), acetic anhydride (Sigma-Aldrich), butyric anhydride
(Sigma-Aldrich), ethyl diazoacetate (EDA, Sigma-Aldrich) and styrene (Sigma-Aldrich) were
purchased and utilized without further purification.
Linker synthesis
The linkers bearing ether side chains (H2Ln, n = 1-5) were synthesized according to the work
of the Fischer group,[22] and the linkers bearing ester chains (H2Ln, n = 6, 7) were prepared
based on Yamada and Kitagawa’s work (Scheme 2).[23] ATR-FTIR spectra are displayed in ESI
1
Figure S1. H and 13C solution-state NMR spectra are displayed in ESI Figures S2 and S3,
respectively.
H2L1 (2,5-dimethoxyterephthalic acid)
For the preparation of H2L1, 0.25 g diethyl 2,5-dihydroxyterephthalate (0.98 mmol), 0.40 g
K2CO3 (3 eq.), and excess iodomethane (1 mL) were dissolved/suspended in acetonitrile (50
o
mL) in a 100 mL flask. The reaction was performed at 50 C for 24 hours. After evaporating
acetonitrile, 20 mL of water were added to the flask and the pH-value of the solution was
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adjusted to 1 by adding concentrated HCl (37%). The crude product diethyl-2,5-dimDOeIt:h10o.x10y3-9/C8CY01493K
1,4-benzenedicarboxylate was obtained by extracting with ethyl acetate three times. Colorless
powder was obtained after removing the ethyl acetate. Then, 50 g of a mixture of H2O and
EtOH (1:1, w:w) and 0.11 g (3 eq.) NaOH were introduced. The hydrolysis occurred at 50 °C
for one day. After adjusting the pH to 1, the product (H2L1) was obtained by extraction using
ethyl acetate. The product was purified by dissolving it in a mixture of acetone and water. After
evaporation of acetone at room temperature, the purified linker was filtrated and dried at 100 °C
overnight.
1H NMR (300 MHz, D2O-DMF) δ 7.78 (2H, Ar-H), 4.26 (6H, OCH3) ppm; 13C NMR (75.5
MHz, D2O-DMF) δ 167.51, 152.16, 125.37, 115.16, 56.80 ppm. Elemental analysis:
C% = 51.32, H% = 4.368
H2L2 (2,5-diethoxyterephthalic acid)
H2L2 was synthesized similar to H2L1. Bromoethane (0.32 g, 3 eq.) was employed instead of
iodomethane and the etherification reaction was carried out at reflux for 24 h.
1H NMR (300 MHz, DMSO) δ 6.91 (2H, Ar-H), 3.95 (4H, OCH2), 1.18 (6H, OCH3) ppm; 13C
NMR (75.5 MHz, D2O) δ 175.57, 151.34, 130.57,124.36, 72.87, 70.96, 57.99 ppm. Elemental
analysis: C% = 54.70, H% = 5.262
H2L3 (2,5-dibutoxyterephthalic acid)
The preparation of H2L3 was performed similar to H2L2. 1-bromobutane (0.40 g, 3 eq.) was
employed instead of bromoethane.
1H NMR (300 MHz, DMSO) δ 7.27 (s, 2H, Ar-H), 4.33-4.29 (m, 4H, OCH2), 1.96-1.86 (m, 4H,
CH2), 1.59-1.46 (m, 4H, CH2), 1.04-0.99 (m, 6H, CH3) ppm; 13C NMR (75.5 MHz, CDCl3) δ
164.06, 151.73, 122.67, 117.46, 71.01, 30.82, 19.05, 13.65 ppm. Elemental analysis: C% =
61.64, H% = 7.173
H2L4 (2,5-bis(2-methoxyethoxy)terephthalic acid)
The preparation of H2L4 was performed similar to H2L2. 2-bromoethyl methyl ether (0.41 g, 3
eq.) was employed instead of bromoethane.
1H NMR (300 MHz, DMSO) δ 7.36 (s, 2H, Ar-H), 4.20-4.16 (m, 4H, OCH2), 3.70-3.68 (m, 4H,
OCH2), 3.36 (s, 6H, OCH3) ppm; 13C NMR (75.5 MHz, DMSO) δ 166.59, 150.52, 125.70,
116.10, 70.31, 69.18, 58.26 ppm. Elemental analysis: C% = 53.26, H% = 5.649
H2L5 (2,5-bis(benzyloxy)terephthalic acid)
The preparation of H2L5 was performed similar to H2L2. Benzyl bromide (0.50 g, 3 eq.) was
employed instead of bromoethane.
1H NMR (300 MHz, DMSO) δ 7.49-7.28 (m, 12H, Ar-H), 5.16 (m, 4H, OCH2); 13C NMR (75.5
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MHz, DMSO) δ 165.76, 151.73, 136.94, 128.30, 127.66, 127.44, 125.60, 116.01, 70D.4O1I: 1p0p.1m03.9/C8CY01493K
Elemental analysis: C% = 60.32, H% = 4.445
H2L6 (2,5-diacetoxyterephthalic acid)
For preparation of H2L6, 0.25 g 2,5-dihydroxyterephthalic acid (1.26 mmol) was dispersed in
5 mL acetic anhydride in a 10 mL glass bottle. Then, three drops of H2SO4 (98%wt) were added
as catalyst. The reaction was stirred at room temperature overnight. The product (H2L6) was
obtained by filtering the reaction solution. To purify the product, a mixture of acetone and water
was employed to dissolve the obtained raw product as described for the purification of H2L1.
1H NMR (300 MHz, DMSO) δ 7.69 (s, 2H, Ar-H), 2.26 (s, 6H, -CH3) ppm; 13C NMR (75.5
MHz, DMSO) δ 169.13, 164.20, 146.94, 128.58, 126.38, 20.67 ppm. Elemental analysis: C%
= 51.14, H% = 3.572
H2L7 (2,5-bis(butyryloxy)terephthalic acid)
The preparation of H2L7 was performed similar to H2L6. Butyric anhydride (5 mL) was
employed instead of acetic anhydride.
1H NMR (300 MHz, DMSO) δ 7.67 (s, 2H, Ar-H), 2.58-2.50 (m, 4H, CH2), 1.73-1.60 (m, 4H,
13
CH2), 1.00-0.95 (s, 6H, CH3) ppm; C NMR (75.5 MHz, DMSO) δ 171.50, 164.26, 146.84,
128.68, 126.32, 35.17, 17.48, 13.38 ppm. Elemental analysis: C% = 59.19, H% = 5.903
Synthesis of dirhodium coordination polymers
The heterogeneous dirhodium coordination polymers were prepared by ligand exchange
according to refs.[18, 24] The C, H contents were determined by elemental analysis. The Rh
content was determined by thermogravimetry (TG, see ESI Figure S4-S10) according to the
method of Kaskel and co-workers.[17d] The fluorine content was determined by quantitative
analysis of the 19F MAS NMR spectra according to ref.[8]
Rh2-L1
For preparation of Rh2-L1, Rh2(TFA)4 (0.10 g, 0.1520 mmol), H2L1 (0.10 g, 3 eq.) and 75 mL
ethyl acetate was charged into a 100 mL flask. In a Soxhlet extractor, a mixture of K2CO3 and
4Å molecular sieves (SiO2/Al2O3 ≈ 2, Alfa Aesar) was loaded in a cellulose filter tube. The
exchange reaction was conducted for 2 days. The precipitate was then filtered and washed in a
Soxhlet extractor with ethyl acetate for another 2 days. Then, the solid was dried at 80°C under
vacuum overnight to remove residual ethyl acetate.
Composition: C% = 32.93, H%= 3.225, Rh%(TGA) = 25.2 (nominal Rh% = 31.5), F% = 4.1
(2.14 mmol∙g-1)
Rh2-L2
Rh2-L2 was synthesized similar to Rh2-L1 employing H2L2 (0.11 g, 3 eq.) as ligand.
Composition: C% = 37.29, H%= 3.747, Rh%(TGA) = 28.4 (nominal Rh% = 29.0), F% = 9.2
(4.85 mmol∙g-1)
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Rh2-L3
Rh2-L3 was synthesized similar to Rh2-L1 employing H2L3 (0.14 g, 3 eq.) as ligand.
Composition: C% = 45.74, H%= 5.115, Rh%(TGA) = 23.7 (nominal Rh% = 24.9), F% = 0.5
(0.22 mmol∙g-1)
Rh2-L4
Rh2-L4 was synthesized similar to Rh2-L1 employing H2L4 (0.14 g, 3 eq.) as ligand.
Composition: C% = 38.92, H%= 4.180, Rh%(TGA) = 24.2 (nominal Rh% = 24.8), F% = 1.0
(0.52 mmol∙g-1)
Rh2-L5
Rh2-L5 was synthesized similar to Rh2-L1 employing H2L5 (0.17 g, 3 eq.) as ligand.
Composition: C% = 53.86, H%= 3.732, Rh%(TGA) = 20.8 (nominal Rh% = 22.7), F% = 1.2
(0.61 mmol∙g-1)
Rh2-L6
Rh2-L6 was synthesized similar to Rh2-L1 employing H2L6 (0.13 g, 3 eq.) as ligand.
Composition: C% = 32.17, H%= 2.976, Rh%(TGA) = 31.0 (nominal Rh% = 26.9), F% = 0.8
(0.40 mmol∙g-1)
Rh2-L7
Rh2-L7 was synthesized similar to Rh2-L1 employing H2L7 (0.15 g, 3 eq.) as ligand.
Composition: C% = 39.54, H%= 3.836, Rh%(TGA) = 25.2 (nominal Rh% = 23.5), F% = 0.2
(0.12 mmol∙g-1)
Catalytic Cyclopropanation
The cyclopropanation between styrene and ethyl diazoacetate (EDA) was applied to evaluate
the performance of the heterogenized dirhodium catalysts. Generally, an appropriate amount of
Rh2-Ln (i.e., 6.1 mg of Rh2-L1, 6.7 mg of Rh2-L2, 7.7 mg of Rh2-L3, 7.8 mg of Rh2-L4, 9.0 mg
of Rh2-L5, 7.2 mg of Rh2-L6, or 8.2 mg of Rh2-L7), which corresponds in each case to ca. 9.3
mol Rh2 assuming an ideal composition (no defect sites and complete ligand exchange) of the
coordination polymer, was loaded into a 25 mL glass bottle sealed with a septum. Then, 0.52 g
(5 mmol) styrene in 13.4 g dichloromethane (CH2Cl2) were added. This mixture was dispersed
by ultrasonication for 15 min. Then 0.057 g (0.5 mmol) EDA in 4.0 g CH2Cl2 were introduced
via a syringe. Samples were taken after 1 min, 2.5 min, 5 min, 7.5 min, 10 min, 15 min, 20 min,
25 min, 30 min, 60 min and 120 min of reaction. The clear filtered solutions were analyzed by
GC (Agilent Technologies 7820A), which was equipped with a HP-5 (30m × 0.32mm × 0.25μm)
column and FID detector.
Characterization techniques
The C and H contents were determined on an Elemental Analyzer Vario EL III working in CHN
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mode.
DOI: 10.1039/C8CY01493K
Thermogravimetry analyses (TGA) were carried out under oxygen atmosphere (75 ml·min-1)
using the simultaneous thermal analyzer Netzsch STA 429. The temperature was ramped up
with a rate of 10°C /min in the range between 20-800°C. The Rh contents were calculated from
the TGA according to the procedure described by Kaskel and co-workers.[17d] The theoretical
Rh contents were calculated from the fraction of Rh in [Rh2(Ln)2]x (n = 1-7) where each TFA
ligand of the parent Rh2(TFA)4 is replaced by ditopic ligands Ln.
Solution NMR spectra of the synthesized ligands were recorded on a Bruker Avance II
spectrometer at a frequency of 300 MHz for 1H and 75.5 MHz for 13C. H2L1 was dissolved in
DMF with 1-2 drops of D2O, while H2Ln (n=2-7) were dissolved in DMSO-d6.
The attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were recorded
on a Perkin Elmer Spectrum spotlight 200 FT-IR spectrometer with an UATR unit and 4 cm-1
resolution in the range between 4000-650 cm-1.
X-ray Powder diffraction (XRD) patterns were recorded on a X-ray powder diffractometer
(STADIP, Stoe & Cie GmbH, Darmstadt) in transmission geometry employing Cu Kα1 radiation
(Ge[111]-monochromator, λ=1.54060 Å). The sample was placed on a flat sample holder and
measured in the 2θ range of 4 - 60°.
Scanning electron microscopy (SEM) was performed on a Philips XL30 S-FEG microscope to
probe the microstructure of the coordination polymers with 200 K magnification under electron
beam of 25 kV.
1H13C CP MAS NMR spectra were measured at room temperature on a 7 Tesla Bruker
Avance III HD spectrometer equipped with a 4 mm broadband double-resonance probe at a
frequency of 75.47 MHz for 13C and 300.13 MHz for 1H. Cross polarization (CP) experiments
were performed using a 1H excitation pulse of 3.5s and a contact time of 3 ms for polarization
transfer at a spinning rate of 6 kHz. Spectra were recorded with a recycle delay of 1.5 s and
6000-48000 scans. Two-pulse phase modulation decoupling employing 15° phase jumps
(tppm15)[25] was employed during data aquistion. Spectra were referenced to tetramethylsilane
(TMS, δ = 0 ppm) employing adamantane (δCH2 = 38.5 ppm) as external standard.
19F MAS NMR spectra were recorded at 14 Tesla with a Bruker Avance III spectrometer at a
frequency of 564.2 MHz employing a 1.3 mm MAS probe. Spectra were measured at a spinning
frequency of 40 kHz and 256 transients with a recycle delay of 5 s. The 19F chemical shift scale
was referenced to CFCl3 (δ = 0 ppm), and trifluoroacetic acid (δ = -76.5 ppm) was employed as
external standard. Quantitative analysis of 19F MAS NMR was carried out according to ref.[8]
The diffusion reflection ultraviolet-visible spectrum (DR-UV-vis) was recorded on a Shimadzu
UV 2450 spectrophotometer, and BaSO4 was employed as reference.
The X-ray photoelectron spectroscopy (XPS) was measured on a ULVAC PHI PHI5000 Versa
Probe using the Al Kα radiation source. The binding energy of the C1s peak at 284.8 eV was
employed to reference the spectra.
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Result and discussion
Preparation and basic characterization
Seven novel dirhodium coordination polymers were prepared by ligand substitution between
24]
Rh2(TFA)4 and H2Ln (n = 1-7) (Scheme 2).[18,
In a basic characterization step their
compositions were investigated. The differences between experimental rhodium contents and
theoretical ones are less than 20 %, depending on the ligand system employed in the synthesis.
This suggests the presence of guest molecules and/or incomplete ligand exchange.[17c, 17d, 18] The
occurrence of fluorine containing species in the obtained dirhodium coordination polymers,
such as remaining trifluoroacetate coordinating the dirhodium units, is confirmed by the 19F
MAS spectra (Figure 3) of H2Ln (n = 1-7), which allow a quantitative analysis of the fluorine
content. Additionally, partial hydrolysis of the ester groups of the ditopic ligands is assumed
for Rh2-L6 and Rh2-L7. Such a hydrolysis of ester groups was also observed by Yamada et al.
during the preparation of hydroxyl functional MOFs.[23]
Scheme 2. Preparation of modified terephthalic acids with ether (or ester) side chains and
synthesis of dirhodium coordination polymers.
The morphology of the Rh2-Ln frameworks was investigated by scanning electron microscopy
(SEM) and X-ray powder diffraction (XRD). All catalysts exhibit a layered structure as shown
in the SEM images (ESI Figure S11). The XRD patterns (ESI Figure S12) do not show sharp
Bragg reflections, indicating a lack of long range order in the material, caused by a random
stacking of the dirhodium-ligand framework layers. This observation is similar to our previous
one for the Rh2-bdc (bdc = benzene-1,4-dicarboxylate) coordination polymer, where also a
disordered stacking of the layers was obtained.[18]
ATR-FTIR, 1H13C CP MAS NMR and 19F MAS NMR
The ATR-FTIR spectra of Rh2-Ln (n = 1-7) are displayed in Figure 1. The adsorption bands at
3064 (vs=CH), at 1635 and 1499 (skeletal vibrations of phenyl ring vC=C), as well as at 850 cm-1
(out-of plane C-H bending δ=CH) confirm the presence of the phenyl ring in the terephthalate
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backbone. The bands at 1582 (vs) and 1405 (vas) refer to symmetric and asymmetric vDibOrI:a1t0i.o10n3s9/C8CY01493K
of COO-.[18, 22a, 26] Bands for COOH in the parent acid (ESI Figure S1) have disappeared after
ligand substitution, such as the broad band of O-H stretching (3200-2700 cm-1), the overtone or
combination band of C-O (2700-2300 cm-1) and the C=O stretching band of COOH (1677 cm-
1). These observations clearly indicate the success of ligand substitution between Rh2(TFA)4
precursor and the bitopic linkers.
The presence of side chains is revealed by the following adsorption bands. For Rh2-Ln (n = 1-
7), the bands between 2938 and 2834 cm-1 refer to the stretching vibrations (vasCH and vsCH) of -
CH3 and -CH2, while their scissoring vibrations (δCH3 and δCH2) appear at around 1450 and 1354
cm-1. For Rh2-Ln (n = 1-5) containing ether side chains, the peaks at ca.1197 and 1127 cm-1
assign the asymmetric and symmetric vibrations of C-O-C in the ether side chains. In the case
of Rh2-Ln (n =6, 7), the bands at 1754 and 1744 cm-1 refer to C=O vibrations in the ester side
chains. Finally, the adsorption band between 1200 and 1160 cm-1 are attributed to the
asymmetric vibration of the C-C(=O)-O groups.
Figure 1. ATR-FTIR spectra of dirhodium coordination polymers, Rh2-L1 (a), Rh2-L2 (b), Rh2-
L3 (c), Rh2-L4 (d), Rh2-L5 (e), Rh2-L6 (f) and Rh2-L7 (g).
Note: No adsorption bands were observed between 2600 and 1800 cm-1.
The 1H13C CP MAS NMR spectra of the various Rh2-Ln (n = 1-7) coordination polymers are
shown in Figure 2. The chemical shift at 185 ppm, which is visible in all spectra, is assigned to
COO- groups, coordinated to the dirhodium unit.[18] The signals at 116 and 127 ppm are
characteristic for the phenyl ring of the terephthalate backbone. These results corroborate the
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ATR-FTIR data and verify the presence of the terephthalate backbone, which coordiDnOaIt:e1s0.1t0h3e9/C8CY01493K
dirhodium units via the carboxylate groups in all obtained dirhodium coordination polymers.
Additionally, a signal at ca. 172 ppm is visible in the spectra of Rh2-Ln (n = 1=4, Figure 2a-d),
which most probably stems from the carboxylate group of the trifluoroacetate.[7b, 18] This is a
further indication of incomplete ligand substitution during the synthesis.
Information on the ether side chains of Rh2-Ln (n = 1-5) are obtained in the 1H13C CP MAS
spectra. In detail, the signals at ca. 151 ppm (Figure 2a-e) refer to the phenolic carbon involved
in the ether group (CPh-O-). The chemical shift at 55 ppm (Rh2-L1, Figure 2a) is attributed to -
O-CH3. Signals at 65 ppm (Rh2-L2, Figure 2b), 70 ppm (Rh2-L3, Figure 2c), 70 and 58 ppm
(Rh2-L4, Figure 2d), and 70 ppm (Rh2-L5, Figure 2e) refer to -O-CH2- groups. Since in Rh2-
L4, the ether side chain (-O-(CH2)2-O-CH3) contains two ether groups, two different resonance
peaks (70 and 58 ppm) appear in the spectrum. The resonance at 14 ppm in the spectrum of
Rh2-L2 (Figure 2b) is clearly assigned to methyl carbons in the -OCH2CH3 side chain. In the
aliphatic region of Rh2-L3 (Figure 2c), the signals at 31, 19 and 13 ppm are attributed to β-
methylene, γ-methylene and methyl carbons in the side chain -O-CH2(α)-CH2(β)-CH2(γ)-CH3. For
Rh2-L5 (Figure 2e), the signals for the benzyl group in the side chain appear at 136 and 127
ppm with corresponding spinning side bands. The latter one overlaps with the signal of the
phenyl carbon in the terephthalate backbone but can be clearly identified via the spinning side
bands marked with +.
1
Figure 2. H13C CP MAS NMR spectra of dirhodium coordination polymers, Rh2-L1 (a),
Rh2-L2 (b), Rh2-L3 (c), Rh2-L4 (d), Rh2-L5 (e), Rh2-L6 (f) and Rh2-L7 (g).
Note: The peaks marked with # and + are spinning side bands of the phenyl group (136 and 127
ppm) in the ester side chain of Rh2-L5. The spectra do not indicate signals from ethyl acetate.
Spectra are not normalized on the number of scans to enable the visibility of signals also in
cases of lower S/N ratio.
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DOI: 10.1039/C8CY01493K
In the case of dirhodium coordination polymers containing ester side chains (Figure 2f-g), Rh2-
L6 and Rh2-L7 exhibit the typical -(C=O)-O- signal at ca. 174 ppm. The phenolic carbon signals
in the ester group (CPh-O-) emerge at 146 ppm. In addition, two small peaks at ca. 140 and 156
ppm are visible in these spectra, also referring to phenolic carbon. This underlines our
assumption that the ester bond was partly hydrolyzed, which changes the symmetry of the
phenyl ring of the terephthalate backbone. Such a symmetry break of the terephthalate is also
reflected by the broad signal in the region of 116-127 ppm. In the case of Rh2-L6, an additional
signal appears at 193 ppm, which indicates the presence of adsorbed acetic acid or coordinated
acetate.[7a, 18] This further strengthens the hypothesis of the partial decomposition of the ester
side chains during the catalyst synthesis.
19F MAS NMR (Figure 3(I)) provides information on fluorine containing defect sites occurring
in the dirhodium coordination polymers. For Rh2-L1 and Rh2-L2, a 19F signal centered at ca. -
78 ppm is obtained, which contains a weak shoulder peak at -75.5 ppm. The former signal most
probably refers to a TFA group located at a position trans- to a ditopic linker molecule (Figure
3(IIa)), while the latter signal refers to a TFA group located at trans-position to another TFA
group (Figure 3(IIb)).[18]
19
There are some small differences in the chemical shift values of the F MAS NMR signals.
For Rh2-Ln (n = 3-7) coordination polymers, only a single, symmetric 19F signal centered
between -76 and -78 ppm is obtained. For the Rh2-Ln (n = 1, 2) coordination polymers a
superposition of two lines is observed. A weak signal centered at 76 ppm and a strong signal
centered at 77.5 ppm. The most probably reason for this difference are heavy atom effects,
similar to the -gauche effect,[27] which cause a stronger shielding of the fluorine nuclei in the
coordination polymers with the more bulky side chains Rh2-Ln (n = 3-7).
A quantitative analysis of the 19F MAS spectra was carried out to determine the amount of TFA
defect sites in the coordination polymers. In comparison, the content of fluorine in the
coordination polymers Rh2Ln (n = 3-7) (in the range between 0.1-0.7 mmol/g) is much lower
than that in Rh2-Ln (n = 1, 2) (2.1-4.9 mmol/g). This let us assume that TFA groups are more
easily replaced by terephthalate derivatives containing bulky side chains. To verify this
hypothesis, advanced experimental and theoretical efforts are required to understand the
kinetics of the ligand exchange as function of side chain, which are beyond the scope of the
present work.
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DOI: 10.1039/C8CY01493K
Figure 3. (I) 19F MAS NMR spectra of dirhodium coordination polymers, Rh2-L1 (a), Rh2-L2
(b), Rh2-L3 (c), Rh2-L4 (d), Rh2-L5 (e), Rh2-L6 (f) and Rh2-L7 (g). Spectra were referenced to
CFCl3 employing trifluoroacetic acid (δ = -76.5 ppm) as external standard. (II) Illustration of
defect sites: (a) TFA group located at a position trans- to a ditopic linker and (b) TFA group
located at trans-position to another TFA group.
DR-UV-vis and XPS
To investigate the chemical environment of the dirhodium units in the obtained coordination
polymers, diffuse reflectance ultraviolet-visible (DR-UV-vis) spectra for Rh2-Ln (n=1-7) were
recorded (Figure 4). All spectra show two bands which are centered at 605 or 620 nm (band I)
and ca. 450 nm (band II), respectively. While band I represents Rh-Rh π*→Rh-Rh σ*
transitions, band II refers to Rh-Rh π*→Rh-O σ* transitions, according to ref.[26c] These results
confirm that the dirhodium unit is maintained during the ligand exchange process. More
interestingly, the position of band I depends on the ligand system employed in the synthesis. In
detail, band I in Rh2-L1, Rh2-L4, Rh2-L6 and Rh2-L7 is centered at about 605 nm, while those
in Rh2-L2, Rh2-L3 and Rh2-L5 appear at ca. 620 nm.
To understand this observation, simple model systems were investigated (see ESI Figure S13-
16). The solution-state UV-vis spectrum of parent Rh2(TFA)4 was recorded in dichloromethane,
a weak coordinating solvent. Various amounts of ethyl ether (Et2O) were introduced yielding
different molar ratios of Rh2(TFA)4/Et2O (see ESI Figure S13). For this model, a 15 nm blue
shift of band I is observed along adding of Et2O. Similar observations are found for the oxygen-
adducts of Rh2(TFA)4 with ethanol (29 nm blue shift, see ESI Figure S14), acetone (25 nm blue
[18]
shift, see ESI Figure S15), ethyl acetate (15 nm blue shift, see ESI Figure S16 and ref.
)
These observations in solution lead to the assumption that the interaction between Rh2 unit and
oxygen atom of the ligand system influences the position of band I in the spectra of the
dirhodium coordination polymers, and vice versa the position of band I represents the
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interaction between the Rh2 unit and oxygen atom of the ligand system.
DOI: 10.1039/C8CY01493K
According to the different positions of band I, the obtained dirhodium coordination polymers
are classified into two groups, namely Group 1 (Rh2-L1, Rh2-L4, Rh2-L6 and Rh2-L7) and
Group 2 (Rh2-L2, Rh2-L3 and Rh2-L5). When increasing the length of the ether side chain from
methoxy (Rh2-L1, Group 1) to ethoxy (Rh2-L2, Group 2) and then to n-butyloxy (Rh2-L3,
Group 2), their corresponding band I shifts from 605 nm (Figure 4a) to 620 nm (Figure 4b,c).
On the contrary, band I shifts back to 605 nm by replacing the n-butyloxy group (Rh2-L3, Group
2) with the 1-methoxy-2-ethoxy (CH3-O-(CH2)2-O-) group (Rh2-L4, Group 1). This implies that
the wave length of band I is related to the position of the O atom in the ether side chain. Terminal
methoxy groups (in Rh2-L1 and Rh2-L4, Group 1) cause a blue shift of band I. In contrast,
terminal aliphatic (ethyl, n-butyl) or aromatic (phenyl) groups (Rh2-L2, Rh2-L3 and Rh2-L5)
cause a red shift of band I, which represents reduced Rh2/O interlayer interactions in these
catalyst systems.
For Rh2-L6 and Rh2-L7 (Group 1), the partial hydrolysis of the ester groups leads to phenolic
hydroxyl groups that interact with the dirhodium moieties in the neighboring layers, which is
underlined by the blue shift of band I for these catalysts.
Figure 4. DR-UV-vis spectra of dirhodium coordination polymers, Rh2-L1 (a), Rh2-L2 (b), Rh2-
L3 (c), Rh2-L4 (d), Rh2-L5 (e), Rh2-L6 (f) and Rh2-L7 (g).
As a second approach to investigate the chemical environment of the dirhodium units in the
obtained dirhodium coordination polymers, high-resolution XPS spectra in the region of 305-
320 eV were recorded (Figure 5). In all samples, the electron binding energies for Rh3d5/2 are
in the range between 308.9 - 309.2 eV and those for Rh3d3/2 are in the range of 313.8 - 314.0
eV. These values clearly indicate that the oxidation state of the dirhodium unit is +2.[18, 26c]
Together with the above discussed DR-UV-vis data, these results reveal that the dirhodium
moieties stayed intact during the ligand replacement process. For comparison, the high-
resolution XPS spectrum of the parent Rh2(TFA)4 was measured (see ESI Figure S18).[18] The
obtained binding energies for Rh3d5/2 and Rh3d3/2 are 310.2 eV and 314.9 eV, respectively.
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These values are lower by 0.9-1.3 eV compared to those of the dirhodium cooDrdOiIn: 1a0t.1io03n9/C8CY01493K
polymers. This is a further indication for the ligand exchange that occurred during the synthesis
of the coordination polymers.
The electron binding energy is sensitive to the chemical environment of the dirhodium units as
shown for the different dirhodium coordination polymers in Figure 5. In detail, the Rh3d5/2
binding energy of Rh2-L1, Rh2-L4, Rh2-L6 and Rh2-L7 (Group 1) is 308.9 eV, which is 0.3 eV
lower than that of Rh2-L2, Rh2-L3 and Rh2-L5 (Group 2). The Rh3d3/2 binding energy of the
former ones is ca. 313.8 eV, which is 0.2 eV lower than that of the latter ones. These
observations underline that the chemical environment of the dirhodium unit is influenced by
the side chain of the bitopic ligand in the dirhodium coordination polymers. Interestingly, the
obtained tendency is in excellent agreement with the DR-UV-vis data. The Rh3d5/2 and Rh3d3/2
binding energies can be used to study the Rh2/O interaction as well. The electron density of the
rhodium atom increases upon interaction with an O atom of the ether groups that act as electron
donor which is the case for the Rh2-Ln (n = 1, 4, 6, 7, Group 1, black line in Figure 5)
coordination polymers. On the other hand, the side chains of Rh2-L2, Rh2-L3 and Rh2-L5
(Group 2, red line in Figure 5) shield the O atom from the dirhodium unit in the neighboring
layers and thus lower the electron density. This yields a higher binding energy of the dirhodium
unit.
Figure 5. XPS of dirhodium coordination polymers in high-resolution mode, Rh2-L1 (a), Rh2-
L2 (b), Rh2-L3 (c), Rh2-L4 (d), Rh2-L5 (e), Rh2-L6 (f) and Rh2-L7 (g).
Catalytic Tests
The cyclopropanation between styrene and EDA (Figure 6 (I)) was employed to evaluate the
catalytic performances of the seven novel dirhodium coordination polymers. Employing an
excess of the substrate styrene, as major products cis- and trans-1-ethoxycarbonyl-2-
phenylcyclopropane are formed and the amount of side products of this reaction such as
fumarates is less than 7%. The diastereoselectivities (cis/(cis+trans)) are listed in Table 1. For
all catalysts insignificant diastereoselectivity is obtained, which is not very surprising since the
homogeneous Rh2(TFA)4 catalyst shows also no diastereoselective control.[7-8, 18] Nevertheless,
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small deviations of the diastereoselectivities were observed for dirhodium cooDrdOiIn: 1a0t.1io03n9/C8CY01493K
polymers bearing various side chains. As the electrophilicity of the dirhodium unit affects the
diastereoselectivity,[4a] the impact of electron properties of side chains on the dirhodium unit
may account for these tiny variations of diastereoselectivity for the different dirhodium
coordination polymers.
The catalytic performance of dirhodium coordination polymers are compared in Figure 6 (II),
and product yields at a reaction time of 120 min are summarized in Table 1. The corresponding
turnover frequencies (TOFs), calculated from the TGA determined Rh2-amounts (Rh% TGA),
are collected in the ESI (Table S1). The catalytic activity shows the following trend: Rh2-L2 >
Rh2-L3 > Rh2-L6 > Rh2-L5 >> Rh2-L1 ≈ Rh2-L4 ≈ Rh2-L7. For dirhodium coordination
polymers containing ether side chains, this trend illustrates that the catalysts in Group 2 (Rh2-
Ln; n = 2, 3, 5) exhibit higher activity than the catalysts in Group 1 (Rh2-L1 and Rh2-L4). In
detail, by replacing the methoxy side chains (in Rh2-L1, Group 1) with the n-butyloxy groups
(in Rh2-L3, Group 2), the yield of products at 120 min increases from 11% to 75%. However,
the yield drops down to 10% when substituting n-butyloxy groups (in Rh2-L3, Group 2) with
1-methoxy-2-ethoxy (CH3-O-(CH2)2-O-) groups (in Rh2-L4, Group 1), although in both
catalysts the length of side chains is comparable. This shows that the terminal groups in the side
chains drastically influence the catalytic performance of the dirhodium coordination polymers.
These obtained trends of catalytic performances are in excellent agreement with the interlayer
Rh2/O interactions in these catalysts discussed by DR-UV-vis and XPS. Strong interlayer Rh2/O
interactions result in low catalytic efficiency. Related catalysts models are illustrated in Figure 7.
According to the catalytic mechanism proposed in ref. [4a], an intermediate rhodium carbenoid
has to be formed after axial coordination of the substrate to the dirhodium unit. Thus, it is
feasible that the hindrance of forming this rhodium carbenoid by strong interlayer Rh/O
interactions reduces the catalytic performance of the dirhodium coordination polymer. For
dirhodium coordination polymers with ethers as side chains, Rh2-L1 and Rh2-L4 (Group 1)
possessing strong interlayer Rh2/O interactions (Figure 7, Model 1) exhibit the lowest catalytic
performances. In contrast, Rh2-L2, Rh2-L3 and Rh2-L5 (Group 2) show high catalytic efficiency,
most probably due to their weak interlayer Rh2/O interactions (Figure 7, Model 2 and 3).
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Figure 6. (a) Schematic drawing of the model cyclopropanation of styrene with EDA. (II) Time
dependent total yield of cis- and trans-isomer obtained with the Rh2-Ln (n = 1-7) coordination
polymers. Rh2-L1 (a), Rh2-L2 (b), Rh2-L3 (c), Rh2-L4 (d), Rh2-L5 (e), Rh2-L6 (f) and Rh2-L7
(g).
Among the catalysts in Group 2 (Rh2-L2, Rh2-L3 and Rh2-L5), Rh2-L2 exhibits the highest
efficiency while Rh2-L5 shows lowest performance. This may be explained by the additional
steric effect of the side chain (Figure 7, Model 3). The bulky side chains (n-butyloxy in Rh2-L3
and benzyloxy in Rh2-L5) render the frameworks of the catalysts more crowded than that
containing ethoxy side chains (Rh2-L2). This hinders the substrate to reach catalytic sites and
lowers the catalytic performance.
Diastereoselectivity[a]
DR-UV-vis
(band I)
(nm)
XPS
(Rh3d5/2)
(eV)
Yield[a]
(%)
Structure
Model
Catalysts
(cis/(cis+trans))
(%)
54
48
49
54
56
49
54
Rh2-L1
Rh2-L2
Rh2-L3
Rh2-L4
Rh2-L5
Rh2-L6
Rh2-L7
11
91
75
10
27
35
8
605[b]
620[c]
620[c]
605[b]
620[c]
605[b]
605[b]
308.9[b]
309.2[c]
309.2[c]
308.9[b]
309.2[c]
308.9[b]
308.9[b]
Model 1
Model 2
Model 3
Model 1
Model 3
Model 1
Model 1
[a] Yield and diastereoselectivity obtained at a reaction time of 120 min for freshly prepared
catalysts; [b] catalysts refer to Group 1; [c] catalysts refer to Group 2.
Table 1. Comparison of the results obtained from UV-VIS and XPS with the catalytic
performances of the coordination polymers.
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The discussed models are in principle also suitable for the explanation of the reactivitDyOoI:f10R.1h03-9/C8CY01493K
2
L6 and Rh2-L7. In comparison with Rh2-L2, Rh2-L6 exhibits lower catalytic efficiency due to
the strong interaction between the dirhodium node and phenolic oxygen atom which is a result
of partial decomposition of the ester side chain during the catalyst preparation. The higher
activity of Rh2-L6 compared to Rh2-L7 most probably refers to the steric of the ligand system,
which is higher for Rh-L7 compared to Rh-L6.
A closer inspection of the catalytic yield of the three efficient catalysts (Rh2-L2, Fig.6b; Rh2-
L3, Fig.6c and Rh2-L6, Fig.6f) shows that they have practically the same initial conversion rate
up to ca 10 minutes (slope of the curves). After this time the conversion rates of Rh2-L3 and in
particular Rh2-L6 go down. There are two possible explanations of these observation, which
are not mutually exclusive. On the one hand this effect can be caused by different degradation
rates of the catalyst and on the other hand it can be caused by (partial) blocking of active sites
or the access to the active sites by product molecules.
Finally, the heterogeneous nature of the coordination polymer catalysts was studied exemplarily
for Rh2-L2. ESI Figure S19(I) shows that after removal of the catalyst from the reaction mixture
no conversion is obtained in the filtrated solution. Moreover, the rhodium content in this
solution was found to be less than 0.2 ppm according to ICP-MS measurement (see ESI
Table S1). Both outcomes clearly underline that the catalytic active rhodium species are bound
in the coordination polymer and are not dissolved in the reaction medium.
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Figure 7. Schematic models to illustrate possible interlayer interactions in Rh2-Ln (n = 1-7) catalysts
induced by side chains: Model 1 is representative for Rh2-Ln (n = 1, 4, 6, 7). Model 2 is representative
for Rh2-Ln (n = 2). Model 3 is representative for Rh2-Ln (n = 3, 5). Note that a parallel layer stacking
is used to simplify the representation of the interactions. For the real coordination polymer, a
random orientation of the planes in the stack is deduced from the XRD data.
Conclusion
A series of modified terephthalic acid derivatives containing either ether side chains or ester
side chains, respectively, were obtained and employed to synthesize seven novel dirhodium
coordination polymers (Rh2-Ln). ATR-FTIR and 1H13C CP MAS NMR results confirmed the
success of ligand exchange and coordination between the dirhodium unit and carboxylate group.
1
19
Incomplete ligand substitution was verified via H13C CP MAS NMR, F MAS NMR and
XPS. The random stacking of the Rh2-Ln frameworks was revealed by SEM and XRD. The
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relationship between catalytic activity and Rh2/O interactions as well as steric effecDtOsI:o1f0.1t0h3e9/C8CY01493K
ligand systems were analyzed for the cyclopropanation of styrene with EDA. Reduced Rh2/O
interactions and steric effects were found to improve the catalytic performance of dirhodium
coordination polymers. These results give essential insights on the influence of the ligand
system on the catalytic performance of coordination polymers, which will help to design
coordination polymers with efficient catalytic performance in future.
Acknowledgement
This work has been supported by the DFG under contract Bu 911/27-1. The authors further
thank the project iNAPO by the Hessen State Ministry of Higher Education, Research and the
Arts for financial support.
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