A Lamellar Coordination Polymer with Remarkable Catalytic Activity

Article abstract of DOI:10.1002/chem.201602157

A positively charged lamellar coordination polymer based on a flexible triphosphonic acid linker is reported. [Gd(H₄nmp)(H₂O)₂]Cl?2 H₂O (1) [H₆nmp=nitrilotris(methylenephosphonic acid)] was obtained by a one-pot approach by using water as a green solvent and by forcing the inclusion of additional acid sites by employing HCl in the synthesis. Compound 1 acts as a versatile heterogeneous acid catalyst with outstanding activity in organic reactions such as alcoholysis of styrene oxide, acetalization of benzaldehyde and cyclohexanaldehyde and ketalization of cyclohexanone. For all reaction systems, very high conversions were reached (92–97 %) in only 15–30 min under mild conditions (35 °C, atmospheric pressure). The coordination polymer exhibits a protonic conductivity of 1.23×10^?5S cm^?1at 98 % relative humidity and 40 °C.

Full text of DOI:10.1002/chem.201602157

DOI: 10.1002/chem.201602157
Full Paper
&
Coordination Polymers
A Lamellar Coordination Polymer with Remarkable Catalytic
Activity
Ricardo F. Mendes,[a] Margarida M. Antunes,[a] Patrꢀcia Silva,[a] Paula Barbosa,[b]
Filipe Figueiredo,[b] Anthony Linden,[c] Jo¼o Rocha,[a] Anabela A. Valente,*[a] and
Filipe A. Almeida Paz*[a]
Abstract: A positively charged lamellar coordination poly-
mer based on a flexible triphosphonic acid linker is reported.
[Gd(H4nmp)(H2O)2]Cl·2H2O (1) [H6nmp=nitrilotris(methylene-
phosphonic acid)] was obtained by a one-pot approach by
using water as a green solvent and by forcing the inclusion
of additional acid sites by employing HCl in the synthesis.
Compound 1 acts as a versatile heterogeneous acid catalyst
with outstanding activity in organic reactions such as alco-
holysis of styrene oxide, acetalization of benzaldehyde and
cyclohexanaldehyde and ketalization of cyclohexanone. For
all reaction systems, very high conversions were reached
(92–97%) in only 15–30 min under mild conditions (358C, at-
mospheric pressure). The coordination polymer exhibits
a protonic conductivity of 1.23ꢁ10ꢀ5 Scmꢀ1 at 98% relative
humidity and 408C.
Introduction
of phosphonic chelating ligands, has been found to induce the
formation of highly robust dense networks,[10] some of which
exhibit true multifunctionality (e.g., photoluminescence com-
bined with catalytic activity).
Research on metal–organic frameworks (MOFs) and coordina-
tion polymers (CPs) is currently driven by the need to employ
such materials in important technological areas for society by
taking advantage of their high versatility.[1] The ability to tailor
specific chemical and physical functionalities into MOFs has
led to a boost of industrial interest in these materials,[2] mostly
driven by the wide diversity of applications and properties:
fabrication of membranes or thin films,[3] application in bio-
medicine,[4] imaging contrast agents,[5] magnetic studies,[6] cat-
alysis,[7] ion exchange[8] and proton conduction.[9] Over the last
few years we have been focusing on the design of networks
based on polyphosphonic acid ligands and rare earth metal
cations. This combination of building units, particularly the use
A common feature of this type of crystalline hybrid materi-
als, particularly the materials designed and developed by us, is
the high concentration of acidic protons and solvent (typically
water) molecules. We are currently looking to take advantage
of this particular structural feature to design better-performing
MOFs that take advantage of proton mobility and/or exchange.
Herein we describe the design and preparation of the crystal-
line layered MOF [Gd(H4nmp)(H2O)2]Cl·2H2O (1, H6nmp=nitri-
lotris(methylenephosphonic acid); see Scheme S1 in the Sup-
porting Information) which proved, on the one hand, to be
a much better and more efficient heterogeneous catalyst in
a wide range of reactions and, on the other, a promising
proton conductor. In the case of H6nmp as organic linker and
Gd3+ as metal center, deliberate inclusion of HCl in the reac-
tion system forced the anchoring of additional catalytic sites in
the resulting MOF, with concomitant inclusion of charge-bal-
ancing chloride anions occupying the interlayer space. Com-
pound 1 was prepared by a one-pot approach (see Experimen-
tal Section for details), which is a more environmentally friend-
ly alternative to the traditional hydrothermal and microwave-
based methods routinely used by us. This approach allows the
preparation of a large quantity of the desired material while
significantly reducing the reaction time and energy consump-
tion compared to hydrothermal methods. Water, the greenest
solvent used in industry, was employed for the preparation of
1, with addition of a small quantity of HCl. As mentioned
above, the use of HCl led to the introduction of acidic centers
into the material with the final aim of improving the catalytic
performance. Besides influencing the chemical properties of
[a] R. F. Mendes, Dr. M. M. Antunes, Dr. P. Silva, Prof. Dr. J. Rocha,
Dr. A. A. Valente, Dr. F. A. Almeida Paz
Department of Chemistry, CICECO - Aveiro Institute of Materials
University of Aveiro, 3810-193 Aveiro (Portugal)
E-mail: atav@ua.pt
filipe.paz@ua.pt
[b] Dr. P. Barbosa, Dr. F. Figueiredo
Department of Materials and Ceramic Engineering
CICECO - Aveiro Institute of Materials, University of Aveiro
3810-193 Aveiro (Portugal)
[c] Prof. Dr. A. Linden
Department of Chemistry, University of Zꢀrich, 8057 Zꢀrich (Switzerland)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201602157. It contains structural characteri-
zation details for compound 1, including single-crystal X-ray diffraction
studies, crystal structure data in CIF format, crystal structure validation,
electron microscopy (EDS and SEM), additional proton conductivity data
and additional data for the heterogeneous catalytic studies and tabulated
data for related MOFs tested under the same reactions as reported in the
manuscript.
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the MOF, the use of HCl also increased the crystallite size by
slowing down the deprotonation of the organic linker and
thus favoring crystal growth over nucleation.
Results and Discussion
Structural description
Compound 1, formulated as [Gd(H4nmp)(H2O)2]Cl·2H2O, crystal-
lizes in the noncentrosymmetric monoclinic space group Ia
(Table 1) and is composed of infinite 2D positively charged
layers, counterbalanced by chloride ions, which are located in
the interlayer spaces. The asymmetric unit consists of one
eight-coordinate Gd3+ center, one bridging and chelating
H4nmp2ꢀ anionic organic linker, two coordinated and two un-
coordinated water molecules and a charge-balancing uncoordi-
nated chloride anion (Supporting Information, Figure S1). The
crystallographic unique Gd3+ center is coordinated to a total
of six phosphonate groups arising from four symmetry-related
H4nmp2ꢀ anionic organic linkers and two coordinated water
molecules, and the {GdO8} coordination polyhedron resembles
a distorted dodecahedron (Figure 1). The GdꢀO bond lengths
are in the range 2.326(3)–2.564(3) ꢃ and are comparable to
those reported for other Gd3+-based phosphonate com-
pounds.
Figure 1. Polyhedral representation of the distorted {GdO8} dodecahedral co-
ordination environment present in 1. Symmetry transformations used to
generate equivalent atoms: i) x+1,y,z; ii) x+1,ꢀy+1/2,z+1/2; iii) x,ꢀy+1/
2,zꢀ1/2.
Table 2. Selected bond lengths [ꢃ] and angles [8] for the Gd3+ coordina-
tion environment in 1.[a]
Gd1ꢀO1
2.371(3)
2.354(3)
2.396(3)
2.382(3)
Gd1ꢀO7ii
2.338(3)
2.326(3)
2.564(3)
2.434(3)
Gd1ꢀO2iii
Gd1ꢀO8i
Gd1ꢀO5iii
Gd1ꢀO1W
Gd1ꢀO6
Gd1ꢀO2W
O1-Gd1-O5iii
O1-Gd1-O6
O1-Gd1-O1W
O1-Gd1-O2W
O2iii-Gd1-O1
O2iii-Gd1-O5iii
O2iii-Gd1-O6
O2iii-Gd1-O1W
O2iii-Gd1-O2W
O5iii-Gd1-O1W
O5iii-Gd1-O2W
O6-Gd1-O5iii
O6-Gd1-O1W
O6-Gd1-O2W
124.50(11)
75.43(11)
126.67(11)
68.54(12)
79.51(11)
73.69(11)
73.45(11)
144.01(11)
104.74(13)
70.86(11)
72.68(12)
136.67(11)
131.97(11)
143.49(12)
O7ii-Gd1-O1
O7ii-Gd1-O2iii
O7ii-Gd1-O5iii
O7ii-Gd1-O6
O7ii-Gd1-O1W
O7ii-Gd1-O2W
O8i-Gd1-O1
O8i-Gd1-O2iii
O8i-Gd1-O5iii
O8i-Gd1-O6
O8i-Gd1-O7ii
O8i-Gd1-O1W
O8i-Gd1-O2W
O2W-Gd1-O1W
79.23(12)
150.53(11)
135.64(12)
81.61(11)
65.37(11)
86.06(13)
149.58(12)
94.94(12)
81.20(11)
74.32(12)
93.16(12)
73.81(12)
140.89(12)
70.41(13)
The internal O-Gd-O polyhedral angles are in the range
65.37(11)–150.53(11)8 (Table 2). H4nmp2ꢀ acts as a hexadentate
organic linker connecting four symmetry-related metal centers.
Table 1. Crystal data collection and structure refinement details for 1.
formula
formula weight
T [K]
C3H18ClGdNO13P3
561.79
160(2)
crystal system
space group
a [ꢃ]
b [ꢃ]
c [ꢃ]
monoclinic
Ia
8.53794(13)
17.4871(3)
10.76816(18)
110.6216(18)
1504.71(5)
4
[a] Symmetry transformations used to generate equivalent atoms: i) x+
1,y,z; ii) x+1,ꢀy+1/2,z+1/2; iii) x,ꢀy+1/2,zꢀ1/2; iv) x,ꢀy+1/2,z+1/2.
b [8]
V [ꢃ3]
Z
All phosphonate groups act as m2-O,O’ bridging moieties, with
two of them being connected to the same pair of metal cen-
ters. As reported for the related [Pr(H3nmp)]·1.5H2O,[11] this
high connectivity of the H4nmp2ꢀ organic ligand is responsible
for the trapping of Gd3+ centers inside a phosphonic-type in-
organic matrix.
1calcd [gcmꢀ3
]
2.480
4.970
m(MoKa) [mmꢀ1
]
crystal type
crystal size [mm]
q range [8]
colourless plate
0.05ꢁ0.25ꢁ0.27
3.519–29.130
ꢀ11 ꢁhꢁ11
ꢀ23ꢁkꢁ23
ꢀ14ꢁlꢁ14
10433
index ranges
The layer arrangement is composed of one-dimensional in-
organic zigzag chains running parallel to the c axis of the unit
cell, formed by Gd3+ polyhedra interconnected by three phos-
phonate groups from two different H4nmp2ꢀ ligand residues
(Gd···Gd 5.6081(3) ꢃ). The organic part of the ligand bridges
these inorganic chains to form a layer parallel to the ac plane
with an additional intermetallic Gd···Gd distance of 8.4834(3) ꢃ
(Figure 2). The organic linker in 1 is a zwitterionic species in
which the central nitrogen atom is protonated and the periph-
eral phosphonate groups each bear a single negative charge
(PO3Hꢀ). Such zwitterionic behavior has already been observed
collected reflections
independent reflections
completeness to q=27.488
final R indices [I>2s(I)][a–c]
4003 (Rint =0.0265)
99.8%
R1=0.0185
wR2=0.0387
R1=0.0202
wR2=0.0396
ꢀ0.012(5)
Final R indices (all data)[a–c]
absolute structure parameter
Largest diff. peak and hole [eꢃꢀ3
]
0.666 and ꢀ0.869
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Â
Ãꢀ
P
2
2
[a] R1=ꢀkFo jꢀjFc k/ꢀjFo j.
[b] wR2 ¼
wðFo2 ꢀ Fc2Þ wðFo2Þ .
2
[c] w ¼ 1=½s2ðF02Þ þ ðmPÞ þ nPꢂ, where p ¼ ðFo2 þ 2Fc2Þ=3.
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Figure 2. Schematic representation of the crystal packing of 1 viewed along the [100] direction of the unit cell (top) and of the 2[Gd(H4nmp)(H2O)2]+ cationic
1
layer (bottom). The four-connected uninodal topology of the cationic layers is shown in both representations.
for other compounds based on aminophosphonate resi-
dues.[11,12] The protonated POH groups and coordinated water
network to central nodes and connecting bridges), the struc-
tures are very similar: on the one hand, the three layered net-
works are assembled by a single Ln3+ center; on the other
hand, the asymmetric unit also contains one H6ꢀxnmpxꢀ (x=3
or 4) residue. Structural differences lie in the quantities of
water molecules and the noncoordinating chloride ions for 1.
Thus, each organic residue is connected to four crystallograph-
ically independent Ln3+ centers and, from a topological view-
point, all compounds are uninodal networks with four-con-
nected nodes and a Schlꢄfli point symbol of {44.62} (Figure 2
and Figure S3 in the Supporting Information).
molecules of the 2[Gd(H4nmp)(H2O)2]+ cationic layer donate
1
their hydrogen atoms to neighboring water molecules of crys-
tallization and to the chloride anion (Supporting Information,
Table S1 and Figure S2).
To the best of our knowledge 1 is the first example of
a phosphonate-based MOF having positively charged 2D
layers, balanced in the interlamellar space by uncoordinated
chloride ions (Figure 2). This structural feature closely resem-
bles those of the naturally occurring layered double hydroxides
(LDHs), which are anionic clay materials with demonstrated
usefulness in a wide range of applications, such as heterogene-
ous catalysts of a variety of organic reactions.[13] In LDHs a varie-
ty of charge-balancing ions can be intercalated to ultimately
improve the catalytic activity. The presence of intercalated
chloride anions in 1 has the same effect: two neutral layered
materials based on the same organic linker and previously re-
ported by us, namely, [La(H3nmp)][14] (obtained by microwave-
assisted synthesis) and [Pr(H3nmp)]·1.5H2O (prepared by hydro-
thermal synthesis),[11] performed poorly as heterogeneous acid
catalysts compared to 1. All compounds have a 2D layered
structure with the difference that 1 has positively charged
sheets involved in strong hydrogen-bonding interactions. Nev-
ertheless, from a topological perspective (i.e., reducing each
Structural differences between
1 and [La(H3nmp)] and
[Pr(H3nmp)]·1.5H2O arise mainly from the supramolecular inter-
actions between the layers. In 1, coordinated water molecules
and protonated POH groups interact with charge-balancing
chloride anions and water molecules of crystallization, and this
ultimately leads to stronger close packing of cationic layers. In
[Pr(H3nmp)]·1.5H2O, which is also a hydrated material, the
supramolecular interactions are limited to hydrogen bonds be-
tween POH groups and water molecules of crystallization.
These interactions are thus weaker than those in 1, so that the
thermal stability of the latter is higher, while in
[Pr(H3nmp)]·1.5H2O the water molecules of crystallization are
completely removed at about 758C immediately accompanied
by a structural modification, whereas 1 is stable up to about
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1508C. For [La(H3nmp)] the interaction between layers is nota-
bly different: without solvent molecules the interaction be-
tween layers is achieved mainly by hydrogen-bonding interac-
tions associated with the protonated POH groups. Although
the connectivity is the same for all three compounds, in the
case of [La(H3nmp)] four metal centers are connected by
a single phosphonate group to form a far more compact layer.
divide the thermogram into three main regions, each corre-
sponding to a combined weight loss. The first region, located
between ambient temperature and about 1958C, is attributed
to the release of four water molecules (two of crystallization
and two of coordination), corresponding to a total weight loss
of 12.6% (calcd 12.8%). When the temperature is increased
the material undergoes a crystalline phase transition, as was
clearly observed by performing variable-temperature powder
X-ray diffraction studies. There is some peak broadening, sug-
gesting loss of crystallinity and reflection dislocations, which
are an indication of variation of the unit-cell dimensions. These
structural modifications may suggest closer proximity between
the gadolinium oxide layers in the temperature-modified struc-
ture, as was observed for [La(H3nmp)].[14]
Thermal analysis and vibrational spectroscopy
The degree of hydration in the crystal structure was investigat-
ed by thermogravimetric analysis between ambient tempera-
ture and about 8008C (Figure 3). Bulk 1 shows consecutive
weight losses with no visible plateaus. Nevertheless, one can
Figure 3. Themogram (top) and thermodiffractometry (bottom) studies of 1 between ambient temperature and about 8008C. EDS analysis showed the ab-
sence of chlorine atoms in the sample calcined at 3508C.
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The next weight loss, in the range of about 195–3558C
(4.4%), can be attributed to the partial release of a hydrochloric
acid residue (calculated 6.5%). The difference in calculated and
observed weight losses may be due to the presence of ag-
glomerates in the sample with entrapped hydrochloric acid,
which is ultimately released at a higher temperature. To cor-
roborate this assumption we performed energy-dispersive X-
ray spectroscopy (EDS) analysis of the material calcined at dif-
ferent temperatures, which clearly showed that up to about
3508C all the chloride anions are released from the material
(Figure 3, inset; Figures S5 and S6 in the Supporting Informa-
tion). It is worth noting that the release of HCl is accompanied
by an overall loss of crystallinity.
cosmetics, in the food and beverage industries,[18–19] pharma-
ceuticals,[18a,b,20] plasticizers,[21] solvents and other intermediate-
s.[18a,c,22] Homogeneous mineral acids (HCl, H3PO4 and H2SO4)
and metal triflates[23] are generally used in these reactions.[24]
Nevertheless, these chemical reactions can be more environ-
mentally friendly and economically feasible if one uses more
efficient heterogeneous catalysts. Compound 1 has remarkable
catalytic activity for these various chemical reactions, which
were studied under atmospheric conditions while using rela-
tively cheap alcohols (methanol and ethanol) as solvents. See
Tables S2, S3 and S5 in the Supporting Information for a com-
parative analysis of the catalytic performance of 1 and various
MOFs reported in the literature. The various parameters that
influence the catalytic reactions are generally different be-
tween studies, so it is difficult to make rigorous, fair compari-
sons. Whenever possible, the catalytic results for 1 in each re-
action system are compared to literature data in the following
discussion.
Above about 3508C additional weight losses of 6.1 and
5.6% in the ranges of about 350–5508C and 550–8008C, re-
spectively, are attributed to complete decomposition of the or-
ganic component. The residue at about 8008C agrees well
with stoichiometric formation of the inorganic residue GdO9P3
(PDF4+ database, release 2012, reference code 04-015-0835).
The vibrational spectrum (Figure S7 in Supporting Informa-
tion) contains a series of bands centered at about 3400 cmꢀ1
attributed to the n(OꢀH) stretching vibrational modes of both
coordinated and solvation water molecules. The typical sym-
metric and asymmetric n(CꢀH) and n(NꢀH) stretching vibra-
tional modes appear in the 3100–2750 cmꢀ1 region of the
spectrum. The latter mode is found at about 3057 cmꢀ1, while
the signals peaking at about 3027, 2982, 2954 and 2778 cmꢀ1
are attributed to the nsym+asym(CꢀH) vibrational modes. In the
Alcoholysis of styrene oxide
Alcoholysis of styrene oxide was performed with methanol and
ethanol, and always gave the corresponding b-alkoxy alcohol
product with 100% selectivity in excellent yields under approx-
imately atmospheric conditions (358C, atmospheric air). Me-
thoxy-2-phenylethanol was obtained in 97% yield after only
a 30 min reaction of styrene oxide with methanol, and 2-
ethoxy-2-phenylethanol was obtained in 100% yield after 4h
of reaction when ethanol was used instead (Figure 4).
region of the spectrum, between 1500 and 1300 cmꢀ1
,
a number of very weak bands can be attributed to n(CꢀH)
modes characteristic of PCH2 groups. The typical POH stretch-
ing modes were observed between 2200 and 2400 cmꢀ1 as
faint and broad bands. In the range of 1500–600 cmꢀ1 the typi-
cal n(CꢀN) stretching vibrational modes of tertiary amines
occur as intense bands peaking at about 1133, 1096 and
1075 cmꢀ1. The n(PꢀC) stretching vibrational modes are also
observed, in particular between about 790 and 690 cmꢀ1. Also
in this region, the n(P=O) stretching vibrational modes are
clearly seen between about 1340 and 1145 cmꢀ1, as well as
To gain insights into the type of acidity, the catalytic per-
formance of 1 was compared to that of the ligand (H6nmp)
and the lanthanide (Gd2O3) precursors. H6nmp produced 2-me-
thoxy-2-phenylethanol as the sole product in 97% yield after
30 min. The lanthanide precursor led, on the other hand, to
negligible styrene oxide conversion after 4 h (Figure S8 in the
Supporting Information). The catalytic activity of 1 resembles
more closely that of the ligand precursor than that of the lan-
thanide precursor, which suggests that the high catalytic activi-
ty of 1 is due to Brønsted acid sites, which may be associated
with its organic component. The Brønsted acidity associated
with the POH groups in 1 can favor protonation of the oxygen
atom of the epoxide group. This leads to favorable nucleophil-
ic attack of the alcohol reagent at the more substituted carbon
atom of the epoxide group, which leads to the corresponding
b-alkoxy alcohol product. A possible reaction mechanism is hy-
pothesized in the Supporting Information. Even though H6nmp
is an effective organocatalyst, the catalytic process is homoge-
neous in nature, which implies demanding catalyst separation/
regeneration.
those of n(PꢀO) between about 1040 and 860 cmꢀ1
.
Heterogeneous catalysis
Compound 1 was tested as a heterogeneous catalyst for vari-
ous acid-catalyzed reactions that are important in both aca-
demia and the chemical industry: epoxide alcoholysis, acetali-
zation of aromatic and aliphatic aldehydes and ketalization of
ketones. Epoxide ring-opening reactions can afford b-alkoxy al-
cohols, which are versatile functional intermediates used in
(in)organic syntheses.[15] Acetalization/ketalization reactions are
important transformations in organic chemistry enabling the
protection of carbonyl groups in the presence of other func-
tional/reactive groups.[16]Acetals and ketals can have advanta-
geously high stability under basic conditions and are stable to
Grignard reagents, metal hydrides, oxidants, halogenating
agents and esterification reagents.[17] Ketals and acetals are
used in the production of perfumes, fragrances and flavors,[18]
Compound 1 was reused in three consecutive reactions with
no considerable change in the conversion rate and retention
of its structural integrity (Figure 5 and Figure S10 in the Sup-
porting Information). The heterogeneous nature of catalyst
1 was assessed by a leaching test at 358C. After separating the
solid catalyst from the reaction mixture after 15 min, the con-
version of styrene oxide ceased to increase and remained at
about 15%), which is an indication of the absence of soluble
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Figure 4. Conversion of a) styrene oxide, b) benzaldehyde, c) cyclohexanone and d) cyclohexaldehyde with methanol or ethanol in the presence of 1 (catalyst
load: 20 gLꢀ1). Dashed lines connecting experimental points are only for illustrative purposes. R=CH3 or CH2CH3.
active species (Figure S9 in the Supporting Information). For
comparison, the reaction of styrene oxide was carried out with
HCl as a homogeneous catalyst (in an amount equivalent to
the Cl added with the MOF in a normal catalytic test), and
gave the b-alkoxy alcohol product in 97% yield after 15 min.
Hence, if the solution from the leaching test for 1 contained
any HCl, conversion should have increased, which was not the
case.
The catalytic activity of 1 is comparable or superior to that
of the 1D MOFs [La2(H3nmp)2(H2O)4]·4.5H2O[25] and
[La(H4bmt)(H5bmt)(H2O)2]·3H2O[26] previously reported by us
(100% conversion after 3 and 6 h reaction, respectively;
Table S2 in the Supporting Information). Although 1 is based
on the same ligand as [La2(H3nmp)2(H2O)4]·4.5H2O,[25] it instead
has a 2D layered structure with electron-withdrawing chloride
ions located in the interlamellar space. Vermoortele et al.[27] re-
ported that electron-withdrawing groups, such as nitro groups
in UIO-66Zr-NO2, may influence the acid properties and induce
(de)stabilizing effects on transition states, which enhance the
catalytic activity. The positive effect of NO2 groups on catalytic
activity when compared to the original MOF was found for var-
ious reactions, namely, the cyclization of (+)-citronellal, conver-
sion of geraniol[27] and reduction of 4-tert-butylcyclohexa-
none.[28] Thus, we envisage that the electron-withdrawing chlo-
ride anions between the layers of 1 account for the enhanced
catalytic activity. The activity of 1 is slightly surpassed by that
of MOF-supported heteropolyacid MIL-101(HPW) (99% conver-
Figure 5. Conversion of styrene oxide in methanol in the presence of 1 at
358C (catalyst load: 20 gLꢀ1) in three consecutive batch runs of the metha-
nolysis of styrene oxide.
sion and 99% selectivity after 0.33 h at 408C).[29] The synthetic
procedure of the latter MOF is, however, much more complex,
requiring higher temperatures, the need to synthesize the het-
eropolyacid and activation under vacuum at 1408C. In addition
it suffered partial loss of catalytic activity over several cycles.
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Acetalization of benzaldehyde
H3PW12O40)[35c] (see Table S5 in the Supporting Information for
further details). Sulfonated metal oxides led to 100% conver-
sion of cyclohexanone after 30 min under similar reaction con-
ditions (e.g., SO42ꢀ/ZrO2, SO42ꢀ/TiO2, SO4ꢀ2/SnO2[37]), although
tetramethylorthoformate was used as a ketalization agent and
tetrachloromethane as the solvent, whereas in the present
work a relatively inexpensive alcohol with the dual function of
reagent and solvent was used.
In the acetalization of benzaldehyde, 100% selectivity towards
the benzaldehyde dialkyl acetal product with excellent yields
was observed: 92% yield of benzaldehyde dimethyl acetal
after 1 h and 56% yield of benzaldehyde diethyl acetal after
6 h for the reactions with methanol and ethanol, respectively
(Figure 4). A possible mechanism of the Brønsted acid-cata-
lyzed alcoholysis of benzaldehyde is depicted in Scheme S3 in
the Supporting Information.[30] A Brønsted acid site protonates
the oxygen atom of the carbonyl group with intermediate for-
mation of a hemiacetal, which, after elimination of water, gives
a carbocation. The latter undergoes nucleophilic attack by an-
other alcohol molecule to give the acetal product. The hemia-
cetal intermediate was not detected, possibly due to its high
reactivity under the catalytic conditions used.
Acetalization of cyclohexanaldehyde
Cyclohexanaldehyde was treated with methanol or ethanol at
35 or 558C. With methanol, 1 led to (dimethoxymethyl)cyclo-
hexane in 88% yield after 93% conversion after 1 h at 358C
(Figure 4). Identified byproducts included cyclohexanecarboxyl-
ic acid and methyl cyclohexanecarboxylate (Table S6 in the
Supporting Information). The reaction with ethanol led to 86%
yield of (diethoxymethyl)cyclohexane at 90% conversion after
1 h at 358C, with byproducts including ethylcyclohexanecar-
boxylate (Table S6 in the Supporting Information). The use of
methanol and an increase of the reaction temperature to 558C
under autogeneous pressure led to methyl cyclohexanecarbox-
ylate as the sole observed product in 76% yield after 1 h. The
mechanism for this reaction is also similar to that for the ace-
talization of benzaldehyde (Scheme S5 in the Supporting Infor-
mation). The hemiacetal was identified by GC-MS for the reac-
tion with ethanol. To the best of our knowledge, catalyst 1 is
the first example of a MOF being investigated for the acetaliza-
tion of cyclohexanaldehyde in acidic medium. Clericiet al.[38] re-
ported the acetalization of cyclohexanaldehyde with MeOH in
the presence of TiCl4 in basic medium, which led to 94–98%
acetal yield after 30 min at 08C. Although this is a rather inter-
esting result, compared to 1, that reaction system was signifi-
cantly more complex and involved the use of relatively toxic
bases such as NH3 and trimethylamine and the need for a re-
frigeration system to maintain low temperatures.
The catalytic results for 1 are far superior to those previously
reported for the 1D MOF [La2(H3nmp)2(H2O)4]·4.5H2O under
identical reaction conditions (94% yield of benzaldehyde di-
methyl acetal after 20 h with a catalyst loading of 20 gLꢀ1
;
Table S3 in the Supporting Information),[25] which may be
partly related to the electron-withdrawing effects reported by
Vermoortele et al. and discussed above.[27] The catalytic activity
of 1 is, in this context, one of the best reported to date, sur-
passing those of [Al2(BDC)3], [Fe(BTC)], [Cu3(BTC)2], MIL-100(Fe)
and UIO-66 (Table S3 in the Supporting Information).[31] Com-
pound 1 performed similarly to MIL-101(PTA),[32] MIL-101-
NO2d[30] and UIO-66-NO2,[33] which, however, required thermal
activation prior to use, whereas no pretreatment was applied
to the material reported herein.
Ketalization of cyclohexanone
Ketalization of cyclohexanone was performed with methanol
or ethanol at 35 and 558C. With methanol, 93% cyclohexanone
conversion was reached after 15 min at 358C (Figure 4) or
5 min at 558C, with the main reaction product being cyclohex-
anone dimethyl ketal (ca. 92–94% selectivity at ca. 93% con-
version). Methoxycyclohexene was formed as a byproduct (ca.
6–8% selectivity, Table S4 in the Supporting Information). With
ethanol, the corresponding ketal was formed with 94% selec-
tivity at 81% conversion after 15 min (358C). Ethoxycyclohex-
ene was also observed as a byproduct (ca. 3% selectivity,
Table S4 in the Supporting Information). The mechanism of the
Brønsted acid-catalyzed ketalization reaction in alcohol media
may be similar to that of the acetalization of benzaldehyde
and is presented in Scheme S4 in Supporting Information.
Only two other MOFs were previously investigated in the ke-
talization of cyclohexanone, namely, [Cu3(BTC)2][31a] and MIL-
101-Cr-NO2d.[30] [Cu3(BTC)2] was less active than 1 (80% conver-
sion after 24 h), and the catalytic results for MIL-101-Cr-NO2d
are roughly comparable to those for 1 (90% conversion after
1.5 h for slightly smaller amounts of catalyst and substrate at
258C). The catalytic performance of 1 compares favorably to
those of other solid acids, such as zeolites (H-Y,[34] H-ZSM5,[35]
H-Mordenite,[35d] H-beta,[35c,d] mesoporous aluminosilicate Al-
General considerations
The conversion versus times curves for the acetalization and
ketalization reactions reached a plateau after about 2 h of reac-
tion due to thermodynamic limitations caused by the presence
of water, formed as a co-product of the catalytic reaction. Simi-
lar kinetic effects were reported by Arrozi et al. for UiO-67 in
benzaldehyde acetalization.[31c] Considerable differences in re-
action rates were observed for 1 when ethanol of PA quality
was used instead of anhydrous ethanol: with anhydrous etha-
nol, the yield of benzaldehyde diethylacetal after 6 h was 56%,
whereas with ethanol PA, the yield was 23%. Ren et al.[39] also
reported that the reaction of benzaldehyde with methanol in
the presence of the 3D MOF [Tb2(dpa)3] (H2dpa=1,4-phenyle-
nediacetic acid) was considerably affected by water. When stoi-
chiometric amounts of water were initially added to the reac-
tor, conversion after 10 h at ambient temperature was 22%, as
opposed to 78% conversion without the addition of water. On
the other hand, Garcia et al.[40] reported for 2D [(K-18-crown-
II
MCM-41[35c,36]
)
and heteropolyacid-based catalysts (e.g.,
6)3][M3 (H2O)4(Ru(ox)3)3] (M=Cu, Co, Fe, Mn and Zr; ox=
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C2O42ꢀ) that the yield of benzaldehyde dimethyl acetal could
be improved slightly (from 50–83% to 56–98%, after 24 h at
708C) by thermal/vacuum pretreatment of the MOF.
and is smaller for 1p. The materials maintain their structure
and crystallinity after these measurements.
The conductivity of 1p and 1p_dry at 408C, plotted as
a function of RH in Figure 6, increases by approximately five
orders of magnitude with increasing RH from 20 to 98%,
which indeed suggests proton transport. The conductivity of
1p is higher than that of 1p_dry, but the differences tend to
decrease with increasing RH. This may indicate an increasing
contribution of proton transport through water molecules ad-
sorbed on the surface of the MOF particles with increasing RH,
whereas bulk transport should predominate for low RH. The
maximum measured conductivities (at 408C and 98% RH) are
1.23ꢁ10ꢀ5 and 3.49ꢁ10ꢀ6 Scmꢀ1 for 1p and 1p_dry, respec-
tively. These values are comparable to those of various other
non-porous phosphonate MOFs and CPs with similar water
content.[42]
The reaction tended to be faster with methanol than with
ethanol. These results agree with literature data for other
MOFs tested as catalysts in the same reaction,[10b,25,41] and may
partly be due to steric hindrance effects. In all tested catalytic
reactions a sixfold decrease in the amount of 1 from 20 to
3.3 glꢀ1 (and methanol as solvent) led to similar outstanding
catalytic results. The catalyst could easily be recovered by
washing/drying, and maintained its structural integrity after
three runs in the alcoholysis of styrene oxide (Figure 5 and Fig-
ure S10 in the Supporting Information), the acetalization of
benzaldehyde, the ketalization of cyclohexanone and the ace-
talization of cyclohexanaldehyde (Figure S11 in the Supporting
Information).
Proton conduction
The mechanism of ion conduction in a solid material is mainly
influenced by its structure, temperature and the amount and
mobility of charge carriers. The presence of phosphonate-con-
taining ligands with hydrogen-bonded chains makes
[Gd(H4nmp)(H2O)2]Cl·2H2O (1) a suitable host structure to
enable ionic transport along interlayer channels, whereby the
adsorption of water molecules facilitated by the phosphonic
acid groups provides paths for the structural diffusion of pro-
tons. Compound 1 was processed into pellets for electrical
measurements. These could easily be obtained free from major
defects from the starting material while preserving crystallinity.
For this study, two pellets were prepared: one was used imme-
diately after processing (1p), and the other was left at 1208C,
overnight (1p_dry).
Figure 6. Protonic conductivity at 408C as a function of RH for 1p and 1p_
dry.
Figure S12 (Supporting Information) shows impedance spec-
tra obtained for these two samples at 408C, which highlight
the effect of relative humidity (RH) on their shape and on the
relaxation frequency of the relevant phenomena contributing
to the impedance. The plots for RH lower than 80% show
a single, slightly depressed semicircle with amplitude corre-
sponding to the bulk resistance of the sample, which was used
to obtain the conductivity. The amplitude of the semicircle de-
creases substantially with increasing humidity, and the shape
changes substantially for 98% RH. Under these conditions of
nearly saturated humidity, the spectrum of 1p_dry shows
a well-defined semicircle at high frequency and a depressed
semicircle at intermediate frequencies, which dominates the
total impedance and strongly overlaps with a capacitive tail at
low frequency. The two contributions at high frequency are ill-
defined in the spectrum of 1p, and appear as a single, highly
depressed semicircle in series with a clearly separated low-fre-
quency tail. The latter may be ascribed to the electrode impe-
dance, whereas the high- and intermediate-frequency contri-
butions are due to the impedance of the pellets. The compari-
son of the distribution of relaxation frequencies for 1p and
1p_dry further suggests that the intermediate-frequency semi-
circle is strongly dependent on the hydration of the sample,
Conclusions
We have reported the sustainable preparation of a cationic la-
mellar coordination polymer by the self-assembly of Gd3+ and
a polyphosphonic acid linker. Though its structure has similar
features to to those of previously reported layered materials
obtained by us, the structural design of positively charged
layers was achieved by using HCl in the synthesis. This led to
the isolation of a new heterogeneous and versatile catalyst ex-
hibiting remarkable activity in four different reactions: alcohol-
ysis of styrene oxide, acetalization of benzaldehyde, ketaliza-
tion of cyclohexanone and acetalization of cyclohexanalde-
hyde. For all reactions, the current material surpassed or at
least equaled the catalytic results reported for other related
materials studied in the literature, which comprise a wide
range of compounds ranging from MOFs to zeo-type materi-
als.
Besides Brønsted acid properties, this new material has inter-
esting proton-conduction properties. At nearly ambient tem-
perature and high RH, the measured conductivity of the as-
prepared material reaches 1.23ꢁ10ꢀ5 Scmꢀ1, which compares
favorably with those of other MOFs reported in the literature.
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We are currently investigating how to process these remark-
able hybrid catalytic materials into functional devices (e.g., thin
films or membranes) while at the same time improving other
functionalities such as proton conduction and photolumines-
cence.
Preparation of [Gd(H4nmp)(H2O)2]Cl·2H2O (1)
A mixture of H6nmp (0.1425 g, 0.477 mmol) and Gd2O3 (0.1587 g,
0.438 mmol) in distilled water (ca. 10.0 mL) and 6m HCl (10.0 mL)
was stirred thoroughly in a round-bottom flask. The resulting solu-
tion was heated at 1208C for 18 h in an oil bath, after which the
vessel was allowed to cool slowly to ambient temperature. Crystals
of [Gd(H4nmp)(H2O)2]Cl·2H2O (1) were obtained after about 15 min,
and a few days later individual crystals could be harvested manual-
ly from the vessels and their structure investigated by single-crystal
Experimental Section
X-ray
diffraction.
Elemental
analysis
(%)
calcd
for
General Instrumentation
[Gd(H4nmp)(H2O)2]Cl·2H2O (M=561.80 gmolꢀ1): C 6.41, H 3.23, N
2.49; found: C 6.40, H 3.22, N 2.34; FTIR: n˜(H2O)coord =3529w;
n˜(H2O)cryst =3447w and 3376w; n˜(NꢀH) and n˜sym+asym(CꢀH)=3052–
2981w; n˜(POH)=2359–2341w; n˜[d(H2O)]=1615w; n˜[d(PꢀCH2)]=
1446–1401 m; n˜(P=O)=1345–1163m–vs; n˜[(CH2)3N]=1130–1000vs;
SEM images were acquired with a Hitachi S4100 field emission gun
tungsten filament instrument working at 25 kV or a high-resolution
Hitachi SU-70 working at 4 kV. Samples were prepared by deposi-
tion on aluminium sample holders followed by carbon coating by
using an Emitech K950X carbon evaporator. EDS and SEM mapping
images were recorded with the latter microscope working at 15 kV
while employing a Bruker Quantax 400 or a Sprit 1.9 EDS micro-
analysis system.
n˜(PꢀC)=767m and 710m cmꢀ1
.
TGA (weight losses) and derivative thermogravimetric peaks (DTG,
in parentheses): 22–1408C, ꢀ9.2% (818C); 140–1958C, ꢀ3.1%
(1648C); 195–3558C, ꢀ4.7% (2458C); 355–5108C, ꢀ6.2% (4188C);
510–7008C, ꢀ4.2% (5858C); 700–8008C, ꢀ1.5%.
TGA was carried out with a Shimadzu TGA 50 instrument from am-
bient temperature to about 8008C at a heating rate of 58Cminꢀ1
under a continuous stream of air at a flow rate of 20 mLminꢀ1
.
FTIR spectra in the range 4000–350 cmꢀ1 were recorded as KBr pel-
lets (2 mg samples were mixed with 200 mg of KBr in a mortar)
with a Bruker Tensor 27 spectrometer by averaging 256 scans at
Catalysis
A 5 mL borosilicate batch reactor, equipped with a magnetic stirrer
(800 rpm) and a valve for sampling, was charged with 1.5 mL of
methanol or ethanol, 0.4m of substrate and the solid catalyst. Sub-
strates studied were styrene oxide, benzaldehyde, cyclohexanone
and cyclohexanaldehyde. The catalytic performance of 1 (3.3–
20 glꢀ1) was compared to that of the ligand and gadolinium pre-
cursors H6nmp and Gd2O3, respectively (5.9 mm, which corresponds
to an equivalent molar amount of ligand or Gd3+ to that present
in a MOF load of 3.3 glꢀ1). Reactions were carried out under at-
mospheric air with the batch reactors immersed in an external
thermostatically controlled oil bath (35 or 558C). Prior to reuse, the
solid catalyst was separated from the reaction mixture by centrifu-
gation (3500 rpm), washed with methanol or ethanol and dried
under atmospheric conditions.
a maximum resolution of 2 cmꢀ1
.
Elemental analyses for C, N and H were performed with a Truspec
Micro CHNS 630-200-200 elemental analyzer at the Department of
Chemistry, University of Aveiro. Analysis parameters: sample
amount between 1 and 2 mg; combustion furnace temperature:
10758C; afterburner temperature: 8508C; detection method: C,H,N
IR absorption; analysis time: 4 min; gases used: carrier: helium,
combustion: oxygen, pneumatic: compressed air.
Routine powder X-ray diffraction (PXRD) data for all prepared ma-
terials were collected at ambient temperature on a Empyrean PAN-
alytical diffractometer (CuKa1,2 radiation, l1 =1.540598 ꢃ, l2 =
1.544426 ꢃ), equipped with an PIXcel 1D detector and a flat-plate
sample holder in a Bragg–Brentano para-focusing optics configura-
tion (45 kV, 40 mA). Intensity data were collected by the step-
counting method (step: 0.018) in continuous mode in the approxi-
mate range 3.5ꢁ2qꢁ508.
Variable-temperature PXRD data were collected with a PANalyticalX’
Pert Powder diffractometer equipped with CuKa1,2 radiation (l1 =
1.540598 ꢃ, l2 =1.544426 ꢃ), a PIXcel 1D detector, a flat-plate
sample holder in a Bragg–Brentano para-focusing optics configura-
tion (40 kV, 50 mA) and a high-temperature Anton Paar HKL 16
chamber controlled by an Anton Paar 100 TCU unit. Intensity data
were collected in the continuous mode (ca. 100 s data acquisition)
in the angular range of about 3.5ꢁ2qꢁ508 (step: 0.018).
A leaching test was carried out for 1 by heating a stirred suspen-
sion of the MOF (3.3 glꢀ1) in methanol and styrene oxide (0.4m)
for 15 min at 358C, and subsequently separating the solid by cen-
trifugation and passing the solution through a 0.20 mm PVDF w/
GMF Whatman membrane. The obtained filtrate was transferred to
a preheated (358C) batch reactor and left to react at the same tem-
perature with stirring.
The progress of the catalytic reactions of styrene oxide and benzal-
dehyde was monitored by using a Varian 3800 GC equipped with
a capillary column (Chrompack, CP-SIL 5 CB, 50 mꢁ0.32 mmꢁ
0.5 mm) and a flame-ionization detector. H2 was used as the carrier
gas. The progress of the catalytic reactions of cyclohexanaldehyde
and cyclohexanone was monitored by GC-MS [Trace GC 2000
Series (Thermo Quest CE Instruments), DSQ II (Thermo Scientific)]
with He gas as the carrier gas. The same instrument was used for
identification of the reaction products for all substrates by using
the commercial databases Wiley 6 and NIST Mainlib and Replib.
The MS data for cyclohexanaldehyde and cyclohexanone and the
corresponding acetal and ketal, respectively, are as follows:
Reagents
Chemicals were obtained from commercial sources and were used
as received without further purification: gadolinium(III) oxide (at
least 99.99%, Jinan Henghua Sci. & Tec. Co. Ltd); nitrilotris(methyle-
nephosphonic acid) (H6nmp, N(CH2PO3H2)3, 97%, Fluka); hydrochlo-
ric acid (37%, analytical-reagent grade, Fisher Chemical); absolute
ethanol (Scharlau ACS, >99.9%, analytical grade); anhydrous abso-
lute ethanol (Carlo Erba, ꢃ99.9%); cyclohexanone (Aldrich, 99.8%);
cyclohexanaldehyde (Aldrich, 98%); styrene oxide (Fluka, purum,
ꢃ97%); methanol (Sigma-Aldrich, chromasolv for HPLC, ꢃ99.9%);
benzaldehyde (Sigma-Aldrich, 99%).
Cyclohexanaldehyde: m/z (relative intensity): 112 [M]+ (18), 97 (7),
94 (57), 84 (9), 83 (95), 81 (14), 79 (30), 77 (5), 71 (8), 70 (30), 69 (6),
68 (46), 67 (12), 66 (5), 57 (9), 56 (10), 55 (100), 53 (7), 41 (22), 39
(9).
Dimethoxymethylcyclohexane: m/z(relative intensity): 127 (30), 95
(40), 76 (7), 75 (100), 67 (6), 47 (8), 45 (6).
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Diethoxymethylcyclohexane: m/z (relative intensity): 141 (30), 104
(6), 103 (100), 95 (29), 75 (26), 55 (5), 47 (15).
associated with the water molecules, both of crystallization and co-
ordinated to Gd3+, were directly located from difference Fourier
maps and included in the structural model with the OꢀH and H···H
distances restrained to 0.95(1) and 1.55(1) ꢃ, respectively. This pro-
cedure ensures that these moieties are refined in chemically rea-
sonable environments. These hydrogen atoms were further includ-
ed in the final structural model with the Uiso values fixed at 1.5Ueq
of the parent oxygen atoms.
Cyclohexanone: m/z (relative intensity): 99 [M]+ (6), 98 (100), 83
(18), 80 (9), 70 (32), 69 (52), 56 (10), 55 (95), 43 (6), 42 (33), 41 (17),
39 (11).
Cyclohexanone dimethyl ketal:m/z (relative intensity): 144 [M]+
(11), 113 (62), 111 (6), 102 (7), 101 (100), 97 (5), 88, (7), 81 (28), 79
(6), 55 (8).
All structural refinements were performed with the graphical inter-
face ShelXle.[45] Structural drawings were created with the software
package Crystal Impact Diamond.[46] Information concerning crys-
tallographic data collection and structure refinement details is
summarized in Table 1. Table 2 and Table S1 of the Supporting In-
formation list the most significant geometrical parameters of the
Gd3+ coordination sphere.
Cyclohexanone diethyl ketal: (m/z): 172 [M]+ (17), 130 (6), 129 (90),
127 (100), 101 (32), 99 (46), 98 (11), 97 (9), 83 (7), 81 (38), 73 (29),
70 (8), 69 (5), 55 (11).
Proton conduction
Proton conductivity s of pelletized samples was studied by electro-
chemical impedance spectroscopy. Disc-shaped samples were ob-
tained after pressing the powders in a uniaxial press at 6.25 MPa
and then isostatically at 200 MPa. The apparent density of these
pellets was obtained from measurements of their weight and the
geometric dimensions. Silver electrodes were applied on both
sides of the pellets by painting with a commercial paste (Agar Sci-
entific). Samples were placed in ceramic tubular sample holders
inside a climatic chamber (ACS DY110) to carry out measurements
at variable temperature (40 and 608C) and RH (20–98%). Before
the measurements, one of the pellets was pretreated overnight at
1208C. Impedance spectra were collected between 20 Hz and
2 MHz with a test signal amplitude of 100 mV by using an Agilent
E4980A LCR meter. Spectra were analyzed with ZView (Version
2.6b, 1990–2002, Scribner Associates) to assess the ohmic resist-
ance R, which was normalized to the geometry of the samples to
calculate the conductivity by using the formula s=L(RA)ꢀ1, where
L is the thickness of the pellets and A is the surface area of the
electrodes.
CCDC 1448427 (1) contains the supplementary crystallographic
data for this paper. These data are provided free of charge by The
Cambridge Crystallographic Data Centre.
Acknowledgements
Funding agencies and projects: We wish to thank Fundażo
para a CiÞncia e a Tecnologia (FCT, Portugal), the European
Union, QREN, FEDER through Programa Operacional Factores
de Competitividade (COMPETE) and CICECO-Aveiro Institute of
Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM/
50011/2013), financed by national funds through the FCT/MEC
and when appropriate co-financed by FEDER under the PT2020
Partnership Agreement. We also thank FCT for funding the
R&D project FCOMP-01-0124-FEDER-041282 (Ref. FCT EXPL/
CTM-NAN/0013/2013). Individual grants and scholarships: FCT
is also gratefully acknowledged for the Ph.D. grants Nos. SFRH/
BD/84231/2012 and SFRH/BD/46601/2008 (to R.F.M. and P.S.,
respectively), the post-doctoral grants Nos. SFRH/BPD/89068/
2012 and SFRH/BPD/96665/2013 (to M.M.A. and P.B., respec-
tively), and the Development grant No. IF/01174/2013 (to F.F.).
Structure determination by single-crystal X-ray diffraction
One-pot synthesis allowed the preparation of several plate-like
crystals, which were inspected and isolated under a Stemi 2000
stereomicroscope equipped with Carl Zeiss lenses. This allowed the
identification of a large colourless plate (dimensions of about
0.05ꢁ0.25ꢁ0.27 mm), which was investigated by single-crystal X-
ray diffraction. This crystal was selected manually and harvested
from the batch powder and mounted on a glass fiber. Preliminary
X-ray diffraction data were collected at 160(2) K with an Oxford Dif-
fraction SuperNova, dual radiation diffractometer equipped with
an MoKa microsource tube (l=0.71073 ꢃ), an Atlas CCDdetector
and an Oxford Instruments CryoJet low temperature device.
Keywords: conducting materials · heterogeneous catalysis ·
layered materials · lanthanides · metal–organic frameworks
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Hydrogen atoms bound to carbon, nitrogen and to the POH
groups were placed at their idealized positions by using appropri-
ate AFIX instructions in SHELXL-2014: 147 for the hydroxy groups
(identified from a combination of the chemical environment and
the PꢀO distances), 23 for the CH2 moieties and 13 for the central
protonated NH group. All hydrogen atoms were included in subse-
quent refinement cycles with isotropic displacement parameters
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Received: May 6, 2016
Published online on && &&, 0000
Chem. Eur. J. 2016, 22, 1 – 12
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FULL PAPER
Layered catalyst: A lamellar Gd coordi-
nation polymer based on a flexible tri-
phosphonic acid linker was obtained by
a one-pot approach by using water as
a green solvent and forcing the inclu-
sion of additional acid sites by employ-
ing HCl in the synthesis. It is a versatile
heterogeneous acid catalyst with out-
standing activity in organic reactions
such as alcoholysis of styrene oxide,
acetalization of benzaldehyde and cy-
clohexanaldehyde and ketalization of
cyclohexanone (see figure).
&
Coordination Polymers
R. F. Mendes, M. M. Antunes, P. Silva,
P. Barbosa, F. Figueiredo, A. Linden,
J. Rocha, A. A. Valente,*
F. A. Almeida Paz*
&& – &&
A Lamellar Coordination Polymer with
Remarkable Catalytic Activity
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Chem. Eur. J. 2016, 22, 1 – 12
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12
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!