Reversible breaking and forming of Metal-Ligand coordination bonds: Temperature-Triggered Single-Crystal to Single-Crystal transformation in a Metal-Organic framework

Article abstract of DOI:10.1002/chem.200802385

[Yb(C4H4O4)15] undergoes a temperature-triggered single-crystal to single-crystal transformation. Thermal X-ray single-crystal studies showed a reversibly orchestrated rearrangement of the atoms generated by the breaking/formation of coordination bonds, in which the stoichiometry of the compound remains unchanged. The transformation occurs on heating the crystal at 130 °C. This uncommon behavior was also studied by thermal methods, FTIR spectroscopy, and thermodiffrac-tometry. Both polymorphs, a (room-temperature form) and b (high-temperature form), are proven to be active heterogeneous catalysts; the higher catalytic activity of b is owed to a decrease in the Yb coordination number. A mechanism based on spectroscopic evidence and involving formation of the active species Yb-O-OH is proposed for the sulfide oxidation.

Full text of DOI:10.1002/chem.200802385

DOI: 10.1002/chem.200802385
Reversible Breaking and Forming of Metal–Ligand Coordination Bonds:
Temperature-Triggered Single-Crystal to Single-Crystal Transformation in a
Metal–Organic Framework
Marꢀa C. Bernini,[b] Felipe Gꢁndara,[a] Marta Iglesias,[a] Natalia Snejko,[a]
Enrique Gutiꢂrrez-Puebla,[a] Elena V. Brusau,[b] Griselda E. Narda,[b] and
M. ꢃngeles Monge*[a]
Abstract: [Yb
(C4H4O4)1.5] undergoes a
at ꢀ1308C. This uncommon behavior
was also studied by thermal methods,
FTIR spectroscopy, and thermodiffrac-
tometry. Both polymorphs, a (room-
temperature form) and b (high-temper-
ature form), are proven to be active
heterogeneous catalysts; the higher cat-
alytic activity of b is owed to a de-
crease in the Yb coordination number.
A mechanism based on spectroscopic
evidence and involving formation of
temperature-triggered single-crystal to
single-crystal transformation. Thermal
X-ray single-crystal studies showed a
reversibly orchestrated rearrangement
of the atoms generated by the break-
ing/formation of coordination bonds, in
which the stoichiometry of the com-
pound remains unchanged. The trans-
formation occurs on heating the crystal
Keywords: heterogeneous catalysis ·
metal–organic frameworks · poly-
morphism · structure elucidation ·
ytterbium
ꢁ ꢁ
the active species Yb O OH is pro-
posed for the sulfide oxidation.
Introduction
catalysis, adsorption processes (gas storage), luminescence,
and magnetism.[1–6] A key step for the construction of such
materials is to select not only the experimental synthesis
conditions, but also the appropriate multidentate bridging li-
gands. There has been much interest in aliphatic dicarboxy-
lates because of their varied coordination modes and flexi-
bility, which generally favor the development of three-di-
mensional networks. Rare-earth succinates have been stud-
ied widely and several inorganic–organic frameworks have
been obtained and characterized.[3,7–14] Although the proper-
ties of the rare-earth element and the flexible succinate
anion are combined in these frameworks, the properties of
the compounds such as luminescence[8,14,15] or catalysis[3]
have been evaluated in only a few studies.
However, the transformations of one hybrid polymeric
structure to another are rare in solid-state supramolecular
reactions as the breaking and formation of bonds should
occur in more than one direction simultaneously. Most of
the reversible topotactic solid-state reactions involve the re-
moval of guest solvent molecules from cavities.[16] Although
single-crystal to single-crystal transformations have received
considerable attention in crystal engineering, only limited
examples are known, since it is difficult for crystals to retain
single crystallinity after the solid-state rearrangement of
atoms.[17] The driving forces of such processes are varied—
Metal–organic frameworks (MOFs) continue to attract
much attention because of their interesting structural
chemistry and their potential applications, among which are
[a] F. Gꢀndara, Dr. M. Iglesias, Dr. N. Snejko, Prof. E. Gutiꢁrrez-Puebla,
Prof. M. ꢂ. Monge
Department of Structure and Synthesis of Oxides
Instituto de Ciencia de Materiales de Madrid–CSIC
C/Sor Juana Inꢁs de la Cruz 3, 28049 Madrid (Spain)
Fax : (+34)913-720-623
E-mail: amonge@icmm.csic.es
[b] M. C. Bernini, Dr. E. V. Brusau, Dr. G. E. Narda
Area de Quꢃmica General e Inorgꢀnica ”Dr. G. F. Puelles“
Facultad de Quꢃmica, Bioquꢃmica y Farmacia
Chacabuco y Pedernera
Universidad Nacional de San Luis
5700 San Luis (Argentina)
Supporting Information (complete assignment of the infrared spectra
of the [YbACHTUNGTRENNUNG(C4H4O4)1.5] polymorphs for this article is available on the
WWW under http://dx.doi.org/10.1002/chem.200802385. CCDC-
706459 (a polymorph at 296 K), -706460 (a polymorph at 374 K),
-706461 (b polymorph at 408 K), -706462 (b polymorph at 429 K),
and -706463 (a polymorph at 293 K after heating) contain the supple-
mentary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
4896
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FULL PAPER
for example: a) sorption/desorption of guest molecules in-
duced by vacuum or temperature changes;[18] b) light;[19]
c) rearrangement of solvent molecules;[20] d) a decrease in
temperature;[21,22] e) an increase in temperature.[18b,23] The
single-crystal to single-crystal transformation described in
this report is unusual since it differs from the systems men-
tioned above in two important aspects: that it consists of a
reversibly orchestrated rearrangement of the atoms generat-
ed by the breaking/formation of coordination bonds, this
case being one of the few in which the stoichiometry of the
compound is preserved; and that the single-crystal to single-
crystal phase transition occurs when the crystal is heated at
ꢀ1308C, whereas the only reversible process yet reported in
MOFs takes place on cooling.[22]
Here, we present the synthesis and a complete single-crys-
tal structural study of the thermal hysteresis cycle at 23, 100,
135, 156, and 208C of a novel ytterbium succinate. The
study includes variable-temperature IR spectroscopy, and
variable-temperature powder XRD of the ytterbium succi-
nate in its two polymorphic forms: the a polymorph, at
room temperature; and the b polymorph, at high tempera-
ture. The catalytic activity of the a polymorph has been
evaluated in reactions such as oxidation of methylsulfanyl-
benzene, acetalization of benzaldehyde, or hydrodesulfuriza-
tion of thiophene. This last reaction was selected to corre-
late the structural differences of the two polymorphs with
their catalytic activities.
Results and Discussion
Description and characterization of the two polymorphic
structures: A new crystalline compound is formed during
the hydrothermal reaction of ytterbium nitrate with succinic
acid. From its crystal structure, we found the formula to be
Figure 1. Coordination sphere of YbIII ion in a) the a polymorph; b) the
b polymorph.
[YbACHTUNGTRENNUNG(C4H4O4)1.5]. As explained below, this new compound
may exist in two polymorphic phases, with reversible transi-
tions from one to the other through temperature changes.
The room-temperature phase crystallizes in the triclinic
¯
space group P1. In this structure each metal is surrounded
by eight oxygen atoms, all from carboxylate groups (see Fig-
ure 1a). The Yb coordination sphere in the a polymorph
ꢁ
forms a YbO8 triangulated dodecahedron. Yb O bond
lengths are in the range 2.215(6)–2.530(6) ꢅ, which is consis-
tent with that of previously reported carboxylate-containing
ytterbium complexes.[24]
YbO8 polyhedra are linked together by sharing edges,
giving rise to chains that run in a zigzag along the a direc-
tion (Figures 2a and 3a). These chains are connected by two
crystallographically nonequivalent succinate anions along
the b and c directions, giving a 3D hybrid framework of the
I1O2 type, according to the classification proposed by Rao
et al.[6] The chains of polyhedra are connected along the c
direction by succinate anions in a perfect trans conformation
(torsional angle=180.08; distance between the a- and w-
carbon atoms=3.875(12) ꢅ). Both carboxylate groups of
the trans-succinate anion are linked in a h2m3-h2m3 chelate-
Figure 2. Projection on the ac plane of a) the a polymorph; b) the b poly-
morph.
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M. A. Monge et al.
Figure 3. Perspective view along the (1,0,0) direction of a) the a poly-
morph; b) the b polymorph.
Figure 4. Topological depictions of: a) The a polymorph in its maximum
simplification as an uninodal net, with the dia topology; b) the b poly-
morph as a binodal net, with one of the succinate ligands as a three-con-
nected node and the dimeric ytterbium units as the eight-connected
center, resulting in a tfz-d topology.
bridge mode. In this way, the O2 atom connects two Yb ions
at 2.260(6) and 2.425(6) ꢅ. In the b direction (see Figure 3a)
the chains are linked by a h2m2 bidentate-bridge carboxylate
group from a succinate anion in a gauche conformation (tor-
sional angle=ꢁ75.58; distance between the a- and w-carbon
atoms=3.208(12) ꢅ). The gauche isomer, acting as a h2m2-
h2m3 linker, has the O3 oxygen atom bonded to two Yb
atoms at distances of 2.287(6) and 2.530(6) ꢅ. Thus, the car-
boxylate groups in both types of chelate bridge in the differ-
ent conformers are involved in the development of the poly-
hedron chains. This explains why the distances between ad-
jacent metal centers are not all equivalent, and it is possible
to define two planes to identify the different distances. In
ꢁ
the Yb O2–Yb plane the distance between the metal cen-
ꢁ
ters is 3.877(1) ꢅ and in the Yb O3–Yb plane Yb···Yb=
4.013(1) ꢅ. The two planes form an angle of approximately
97.428 in such a way that the Yb polyhedra are nearly per-
pendicular to each other.
The network can be described topologically as a three-
nodal net. The Yb atoms are six-connected nodes, and the
two different succinate anions are two different four-con-
nected nodes. This network has the point (Schlꢆfli) symbol
Figure 5. TGA/DTA curves of the new material. Inset: the DSC curves.
(42.84)(44.62) (49.66)2. However, the structure can be de-
ACHTUNGTRENNUNG 2ACHTUNGTRENNUNG
scribed more simply by considering the succinate anions just
as linkers connecting the nodes located at the Yb atom posi-
tions. In this simpler representation, the zigzag chains of Yb
polyhedra running along the a axis (as described above) are
the secondary building units (SBUs). The chains are con-
nected by the ligands along the c and b axes. From this de-
piction a uninodal net is defined in which each node is con-
nected directly to two others in the same chain and to two
other nodes in two different chains through the linkers, re-
sulting in a four-connected network of the dia type (Fig-
ure 4a).
A weak endothermic signal not associated with weight
change is observed in the differential thermal analysis
(DTA) curve at 1308C (Figure 5). The organic part decom-
poses at 4108C with an experimental weight loss of 43.2%
(expected value 44.4%); a sharp and intense exothermic
peak in the DTA trace accounts for this process. The first
DTA signal was investigated by differential scanning calo-
rimetry (DSC) measurements in the ranges 25–1908C and
190–258C (Figure 5, inset). The existence of measurable
latent heat or a discontinuous volume change or hysteresis
in the DSC curves may be taken as characteristic of a first-
order transition,[25] in which the high-temperature phases
with high internal energy will also have higher entropy.
Exploration of the possible structural changes associated
with this transition by X-ray powder thermodiffractometry
(XRTD; Figure 6) revealed a reversible phase transition oc-
curring when the sample was heated above 1308C, the initial
structure being recovered when the sample returned to
room temperature. The cell parameters of the high-tempera-
ture phase could be determined by indexing the X-ray
powder pattern, employing the program DICVOL04,[26] in-
cluded in the Fullprof Suite software package.[27] A solution
was obtained in the triclinic system with cell parameters a=
6.45 ꢅ, b=8.18 ꢅ, c=8.62 ꢅ, a=92.848, b=110.218, g=
109.128, and cell volume=396.9 ꢅ3 (figure of merit M(20)=
24.1). As the phase transition occurs without loss of crystal-
linity in the sample, a variable-temperature single-crystal X-
ray diffraction study was carried out. A single crystal of the
a polymorph was heated at 1568C. The cell parameters co-
incided with those obtained from the indexation of the
powder pattern. Furthermore, the quality of the single crys-
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Polymorphic Transformation in a MOF
FULL PAPER
Table 2. Bond lengths of coordination spheres of polymorphs a and b at
different temperatures.
Phase
a at
a at 1008C b at 1358C b at 1558C a at RT after
RT
heating
Bond
Length [ꢅ]
ꢁ
Yb O1
2.433(7) 2.444(7)
2.260(6) 2.266(7)
2.425(6) 2.444(6)
2.287(6) 2.287(7)
2.530(6) 2.545(7)
2.366(6) 2.346(7)
2.215(6) 2.201(7)
2.206(6) 2.201(7)
2.370(6)
2.287(7)
2.446(6)
2.163(7)
3.490[c]
2.226(7)
2.210(7)
2.173(8)
2.380(7)
2.280(6)
2.454(6)
2.162(7)
3.512[c]
2.236(7)
2.212(7)
2.180(7)
2.439(7)
2.259(6)
2.440(6)
2.282(6)
2.537(6)
2.347(6)
2.229(6)
2.208(6)
ꢁ
Yb O2
[a]
[b]
ꢁ
Yb O2
ꢁ
Yb O3
ꢁ
Yb O3
ꢁ
Yb O4
ꢁ
Yb O5
ꢁ
Yb O6
[a] Symmetry transformations used to generate equivalent atoms:
ꢁx,ꢁy+1,ꢁz+1; [b] ꢁx+1,ꢁy+1,ꢁz+1. [c] Not a bond.
seven oxygen atoms (see Figure 1b). The coordination
ꢁ
sphere of b forms a YbO7 pentagonal bipyramid with Yb O
bond lengths in the range 2.162(7)–2.454(6) ꢅ. The YbO7
polyhedra are joined, forming edge-sharing dimeric units
(Figure 2b). These dimers are linked by two crystallographi-
cally nonequivalent succinate anions in the three spatial di-
rections, leading to a type I0O3 coordination polymer, ac-
cording to the classification proposed by Rao et al.[6] The di-
meric units in b are connected along the c direction by the
h2m3-h2m3 succinate anions in a perfect trans conformation, in
exactly the same way as in polymorph a. However, along
the a and b directions (see Figure 3b) the dimers in b are
joined by the h2m2-h2m2 bidentate bridge carboxylate groups
from the succinate anions in a gauche conformation (tor-
sional angle: ꢁ79.48; distance between the a- and w-carbon
atoms=3.255(13) ꢅ). This change in the gauche ligand coor-
dination mode from h2m3-h2m2 in the a polymorph to h2m2-
h2m2 in the b polymorph, together with the subsequent re-
duction in the Yb coordination number, creates the differ-
ence between the two polymorphs.
Figure 6. XRD powder patterns: a) 2q=88–608; b) zoomed in to 2q=
108–408. The patterns at 258C correspond to the a polymorph and the
patterns at 2008C correspond to the b polymorph.
tal remained unaltered, allowing complete data to be col-
lected for solution and refinement of the high-temperature
structure. Subsequently, a complete structural study of the
phase transition was achieved by examining the crystal at
23, 100, 135, and 1558C and again at 208C. The main crystal-
lographic and refinement data are given in Table 1, and
bond lengths in the coordination spheres for both poly-
morphs at each temperature in Table 2.
The cell volume of the b polymorph is about 10% greater
than that of the a form, mainly because the a parameter is
The b polymorph also crystallizes in the triclinic space
¯
ꢁ
group P1. In this structure each metal is surrounded by
higher. This is explained by the breaking of the Yb O3’
bond, and thus the rupture of
the chains along the a direction
Table 1. Main crystallographic data for polymorphs a and b of [YbACHTNUTGRNEUNG
(C4H4O4)1.5] at different temperatures.[a]
to give the dimeric units (see
Figures 1 and 2b). At higher
temperatures, the molecular vi-
bration acquires more energy
and the gauche succinate con-
former becomes more unstable
because of the steric impedi-
Polymorph
a
a
b
b
a
temperature [K]
crystal system/space
group/Z
296
374
408
429
293[b]
¯
triclinic / P1/ 2
a [ꢅ]
b [ꢅ]
c [ꢅ]
a [8]
5.9289(13)
8.0629(17)
8.6516(19)
72.994(3)
72.959(3)
69.464(3)
361.89(14)
1668/1425
5.9775(12)
8.1062(16)
8.6773(17)
73.13(3)
73.05(3)
69.19(3)
6.4334(13)
8.1428(16)
8.5979(17)
92.87(3)
110.20(3)
108.98(3)
392.97(14)
1838/1499
6.4529(13)
8.1694(16)
8.6160(17)
92.79(3)
110.30(3)
108.92(3)
396.27(14)
1854/1514
5.9619(12)
8.0893(16)
8.6744(17)
73.099(3)
73.144(3)
69.326(3)
366.21(13)
1700/1455
ꢁ
ment. When the Yb O3’ bond
is broken the gauche succinate
can relax, changing the torsion-
al angle from ꢁ75.58 to ꢁ79.48.
The O3 atom in the b poly-
morph is no longer shared by
neighboring coordination poly-
hedra. The resulting SBUs after
the phase transition are dimeric
b [8]
g [8]
V [ꢅ3]
367.69(13)
1714/1463
reflections collected/
unique with I>2s(I)
goodness-on-fit on F2
R indices [I>2s(I)]
0.989
R1 =0.0397,
wR2 =0.0756
1.037
R1 =0.0428,
wR2 =0.1011
0.985
R1 =0.0422,
wR2 =0.0952
0.995
R1 =0.0432,
wR2 =0.0994
1.002
R1 =0.0298,
wR2 =0.0666
[a] Molecular weight=347.15 gmolꢁ1 [b] After heating.
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M. A. Monge et al.
units, with Yb···Yb=3.931(1) ꢅ, separated from one another
by 4.90 ꢅ.
The expected increment in the thermal factors with in-
creasing temperature is verified from the structural analysis
at the five selected temperatures. The mean square atomic
displacements of the O3 atom (see Table 3) provide strong
Table 3. Principal mean square atomic displacements, U2, for the O3
atom at different temperatures.
2
2
2
2
2
2
Temperature [8C] Ux
Uy
Uz
DG
D
N
DACHTUNGTRENNUNG
23
0.0284 0.0192 0.0114
0.0417 0.0249 0.0142
0.0615 0.0375 0.0244
0.0673 0.0436 0.0252
0.0354 0.0255 0.0120
0.0133
0.0258
0.0058
0.0057
0.0126
0.0061
0.0028
0.0102
0.0008
100
135
155
ꢁ0.0319 ꢁ0.0211 ꢁ0.0132
20[a]
[a] After heating.
evidence for this. It is striking that the major increment
occurs not with the greatest jump in temperature, but be-
tween 100 and 1358C (see DU2 in Table 3). Furthermore,
i
the major increase is associated with the x component of the
thermal ellipsoid. This surely explains the breakage of the
ꢁ
Yb O3’ bond at ꢀ1308C and consequently the observed
phase transition with the elongation of the a parameter as
the main structural cell change. It is noteworthy that upon
Figure 7. Variable-temperature IR spectra, showing the transition from
the a polymorph (a) to the b polymorph (b).
ꢁ
return to RT, when the Yb O3 bond is formed again, the
2
thermal parameter Ux in the a direction nearly recovers its
original value (Table 3).
The new network cannot be depicted topologically on the
basis of a rod-shaped SBU, as in the room-temperature
polymorph. In this new net, a node is the dimeric unit of the
Yb polyhedra. The trans-succinate is a linker between two
of these nodes, and now the gauche succinate should be con-
sidered as a three-connected center. As a result, the b poly-
morph exhibits a binodal network, eight- and three-connect-
ed, of the tfz-d type (see Figure 4b), with total point (Schlꢆ-
fli) symbol (43)2(46.618.84).
Variable-temperature IR spectroscopy was used to corre-
late the structural changes and thermal vibrations with the
IR spectra. The IR spectra were interpreted by considering
the internal vibrations of carboxylate and methylene groups
and comparing them with those observed in succinic acid,
similar compounds, and related data in the literature.[9,28,29]
The complete assignment of the spectra of both polymorphs
is tabulated in the Supporting Information. The IR spectra
to an exhaustive IR study performed on a holmium succi-
nate framework.[28] The d
ACTHNUTRGENUN(G CH2) mode appears in the spectra
in the 1460–1425 cmꢁ1 range as a strong band split into three
components, of which the one at the highest frequency
could be assigned to the bending mode of the trans-succi-
nate anion whereas the two at lower frequency are tenta-
tively assigned to the dACHTNUGTRENUNG(CH2) mode of CH2 groups in the
gauche succinate anion of each polymorph. The assignment
of this mode took into account steric considerations ex-
plained in the vibrational study cited above. It is remarkable
that certain bands are affected, as shown in Figure 7, by the
structural changes associated with the phase transition, sug-
gesting that the vibrational modes of CH2 and OCO groups
in the gauche succinate anion could be related to those
bands. This relationship is in complete agreement with the
variation in the torsional angle and mode of coordination
observed for this anion. Furthermore, the changes in the
ꢁ
of both polymorphs display the antisymmetric n˜ =
N
stretching n˜ACHTGUNTERNNU(G Yb O) region allow us to assign the band at
stretching mode in the 1610–1550 cmꢁ1 range as a broad
band that includes two components (see Figure 7). This
splitting into two bands can be explained in terms of the dif-
ferent coordination modes (chelate bridge and bidentate
bridge) of the carboxylate groups in the polymorphs. The
550 cmꢁ1 to the Yb O3 stretching mode since it disappears
progressively with increasing temperature and it is absent in
ꢁ
the spectrum of the b polymorph. On the other hand, the
ꢁ1
ꢁ
band at 460 cm , which is assigned to the Yb O stretching
mode, undergoes a slight shift to higher frequencies consis-
ꢁ
band corresponding to the symmetric n˜sym
(OCO) stretching
tent with the reinforcement that some Yb O bonds experi-
mode was assigned in both spectra at ꢀ1400 cmꢁ1 according
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Polymorphic Transformation in a MOF
FULL PAPER
ence (see Table 2). Asterisks in Figure 7 mark the principal
differences between the spectra of the two polymorphs.
Catalytic activity: To test the catalytic activity of the new
Yb succinate polymeric frameworks, they have been em-
ployed as heterogeneous catalysts in different types of reac-
tions. The a polymorph has been tested as a Lewis acid cata-
lyst in the acetalization of aldehydes, and as a redox catalyst
in the oxidation of sulfides (see Scheme 1). These reactions
~
Figure 8. Kinetic curves for acetalization of benzaldehyde ( ) and for ox-
&
idation of methylsulfanylbenzene ( ), with the a polymorph as catalyst.
Lewis acids decompose or become deactivated in the pres-
ence of water, the new Lewis heterogeneous catalyst is
stable both in water and in organic solvents. The TOF (turn-
over frequency; 2700 minꢁ1), indicating how often the sub-
strate interacts with the catalyst, was calculated from the ki-
netic curves and is in agreement with the final conversion
reached.
Scheme 1. a) Oxidation of methylsulfanylbenzene; b) acetalization of al-
dehydes; c) hydrodesulfurization of thiophene.
Oxidation of sulfide: The selective oxidation of sulfides to
sulfoxides is an attractive and important method in organic
chemistry since sulfoxides are useful building blocks, espe-
cially as chiral auxiliaries in organic synthesis.[37] They also
play key roles in the activation of enzymes.[30] They have
been synthesized from the corresponding sulfides by a wide
range of oxidizing systems. Aqueous hydrogen peroxide is a
particularly attractive oxidant since it is cheap, environmen-
tally friendly, and easy to handle, and it produces water as
the only by-product, thus reducing purification require-
ments.[38] The a polymorph was tested as a redox catalyst in
the oxidation of methylsulfanylbenzene with H2O2 as the
oxygen source, which was always in excess (10%) relative to
the amount of substrate. The products of reaction at 608C
were identified by GC. The results are summarized in
Figure 8. The employment of 1.5 equiv of H2O2 and its con-
trolled addition allowed a significant improvement of the
chemoselectivity of the process, and sulfoxide was obtained
as the only product (selectivity=92%). The new material
was efficient and catalyzed selective sulfoxide formation
under mild conditions, with a good yield (50%, TOF=
457 minꢁ1) after 6 h of reaction.
not only constitute attractive and important methods in or-
ganic chemistry, but also participate in relevant green
chemistry processes or play key roles in the activation of en-
zymes.[30] Although lanthanides are receiving increasing at-
tention in the literature, they have scarcely been explored.
Recently we reported catalytic activity for a series of rare
earth MOFs in the methylsulfanylbenzene oxidation.[31] The
Yb succinate a polymorph behaves as heterogeneous cata-
lyst, but the low temperatures at which these two reactions
are carried out (see Experimental Section) make it impossi-
ble to evaluate the catalytic activity of the b polymorph,
which exists only above 1308C. Since the active metallic
centers in the two polymorphs have different coordination
numbers, a difference in the corresponding catalytic activity
is expected. The hydrodesulfurization of thiophene was
chosen to evaluate this difference, since it can occur at tem-
peratures at which the two polymorphs could exist and act
as heterogeneous catalysts.
Acetalization of aldehydes: The formation of acetals is one
of the more effective and frequently used methods of pro-
tecting carbonyl groups.[32] Numerous studies exist of synthe-
sis, protection, and masking of carbonyl groups, because ace-
tals have great importance due to their roles both as inter-
mediates of reaction and as final products.[33–36] The reaction
of benzaldehyde with trimethyl orthoformate (TMOF) in
the presence of the a polymorph produces the correspond-
ing dimethyl acetal in very good yield (90%), in a short re-
action time (5 h), and under mild conditions (608C), indicat-
ing that benzaldehyde was protected successfully employing
the new compound as catalyst (see Figure 8). Whereas most
Recycling experiment: In spite of the activation of the cata-
lyst, an induction period of 1 h was observed in the oxida-
tion reaction. We therefore performed a recycling experi-
ment to investigate the behavior in later reaction cycles.
Furthermore, this test gives us information about the stabili-
ty of the catalyst. A marked increase in the activity is ob-
served from the first to the second cycle of reaction, and a
new minor increase takes place from the second toward the
third cycle (Figure 9). Induction periods were not observed
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4901

M. A. Monge et al.
Figure 9. Kinetic curves for three consecutive cycles of oxidation of meth-
~
:
*
:
ylsulfanylbenzene with the
a
polymorph as catalyst:
cycle 1;
&
cycle 2; : cycle 3.
in the second and third reaction cycles. After this procedure,
the catalyst was recovered by filtration, and then character-
ized by X-ray powder diffraction (XRPD) and IR spectros-
copy (Figure 10). The corresponding powder pattern con-
firms that the compound acts as a heterogeneous catalyst
since the structure is not affected by the reaction cycles.
However, several bands appear in the IR spectrum (marked
with asterisks, Figure 10, right). The band at 1090 cmꢁ1 had
been found previously in the oxidation of methylsulfanyl-
benzene using other Ln-based MOFs that we reported re-
cently.[31] In the present case, three additional new bands
appear at 785, 530, and 504 cmꢁ1. Notably, the last two
bands are found in the Ln–O stretching zone.[39,40] This
Figure 10. Top: X-ray powder diffraction patterns of the a polymorph
a) before and b) after three cycles of oxidation of methylsulfanylbenzene;
c) simulated XRD powder pattern from the single-crystal XRD data.
Bottom: IR spectra of the a polymorph a) before and b) after three
cycles of oxidation of methylsulfanylbenzene. The asterisks indicate the
new bands in the spectra after the catalytic cycles.
ꢁ ꢁ
result supports the hypothesis that a Ln O OH species is
formed during the first reaction cycle and behaves as the
real active intermediate in the catalytic reaction. This would
explain the increase in activity observed after the first reac-
tion cycle. A possible reaction pathway is depicted in
Scheme 2.
center;[41] they were demonstrated to be able to reach very
good yields of conversion, working at lower temperatures
than the traditional catalysts. In the current study, the new
Yb succinate has been employed as catalyst in the HDS of
thiophene at different temperatures, to evaluate the influ-
ence of the structure, and therefore of the coordination en-
vironment of the metal, on the catalytic activity. Thus, from
the percentage of thiophene decomposition after 4 h of
heating under an H2 atmosphere (Figure 11), it is seen how
the activity increases linearly with the temperature as ex-
pected. However, a change in the linear trend is observed
between 120 and 1308C, returning to a good linear fit of the
subsequent points. This change is associated with the phase
transition suffered by the catalyst, and the greater slope of
Hydrodesulfurization of thiophene: Elimination of organic
sulfur compounds is an important process in green chemis-
try, and of special interest for the petrochemical industry. In
the hydrodesulfurization (HDS), the thiophene decomposes
to hydrogen sulfide and 1,3-butadiene, under a hydrogen at-
mosphere (Scheme 1c). Traditionally, Co–Mo-based cata-
lysts are the most often employed in this reaction. As an al-
ternative, we have recently evaluated the activity of hybrid
organic–inorganic materials with ytterbium as the active
Scheme 2. Proposed mechanism for the catalytic oxidation of methylsulfanylbenzene.
4902
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Chem. Eur. J. 2009, 15, 4896 – 4905

Polymorphic Transformation in a MOF
FULL PAPER
Experimental Section
Synthesis: All reagents were purchased at high purity (AR grade) from
Merck (succinic acid) and Strem (Yb
out further purification.
ACHTUGNTRENUN(GN NO3)3·5H2O) and were used with-
[Yb
ACHTUNGTRENNUNG
thermal reaction of YbAHCTUNTGRENNUNG
succinic acid; 1.5 mmol) in water (5 mL) and adjusting the pH to 3.5 with
triethylamine. The mixture of reagents was put in a Teflon-lined digestion
bomb (internal volume 43 mL), heated at 1808C for 90 h, and then
cooled to RT. Prismatic colorless single crystals were collected after
washing with distilled water and acetone.
Crystal structure determination: The cell parameters and the main data
collection and refinement data for [YbACHTNUGTRNEU(GN C4H4O4)1.5] at the selected tem-
peratures are summarized in Table 1. A suitable single crystal was mount-
ed on a Bruker–Siemens Smart CCD diffractometer equipped with a
normal focus, a 2.4 kW sealed-tube X-ray source (MoKa radiation, l=
0.71073 ꢅ) operating at 40 kV and 30 mA. Data were collected over a
hemisphere of reciprocal space by a combination of three sets of expo-
sures. Each set had a different q angle for the crystal and each exposure
of 20 s covered 0.38 in w. The crystal–detector distance was 5.5 cm. Cov-
erage of the unique set was over 99% complete to at least 238 in q. Unit
cell dimensions were determined by a least-squares fit of 30 reflections
and 20 s, with I>20s(I). The first 100 frames of data were collected
again at the end of the data collection process to monitor crystal decay.
The intensities were corrected for Lorentz and polarization effects. Scat-
tering factors for neutral atoms and anomalous dispersion corrections for
Yb, O, and C were taken from the International Tables for Crystallogra-
phy.[42] The structure was solved by direct methods and refined in the tri-
Figure 11. Differences in the linear trends observed in the hydrodesulfuri-
zation of thiophene.
the fitted line beyond this change indicates a higher activity
of the b polymorph. This is due not only to the lower Yb co-
ordination number in b, but also to the unhindered accessi-
bility of the active sites to the reactants. Structurally speak-
ꢁ
ing, the breaking of one Yb O bond per Yb atom which ac-
companies the structural transformation is opening the gate
to the reactants. Finally at 1508C a good conversion of 68%
is achieved after 18 h of heating.
¯
clinic space group P1. Full matrix least-squares refinement with aniso-
tropic thermal parameters for all non-hydrogen atoms was carried out by
2
minimizing wCAHTUNGTRENNNUG
factors (Rw), and all goodness-of-fits S are based on F2, while conven-
tional R factors are based on F. R factors based on F2 are statistically
about twice those based on F, and R factors based on all the data would
be even larger. The conditions for data collection are summarized in
Table 1. The hydrogen atoms of the ligand were fixed at calculated posi-
tions using distance and angle constraints. All calculations were per-
formed using SMART software for data collection, SAINT[43] for data re-
duction, SHELXTL[44] to resolve and refine the structure and to prepare
material for publication, and ATOMS[45] for molecular graphics.
Conclusion
We have successfully prepared a novel Yb MOF, which ex-
hibits a reversible temperature-induced room temperature–
high temperature phase transition without losing or gaining
any solvent or guest molecules, so the two phases are poly-
morphs. Variable-temperature single-crystal X-ray studies
showed a reversibly orchestrated rearrangement of the
Temperature-dependent single-crystal X-ray diffraction measurements:
Single-crystal X-ray diffraction intensities were collected at 101, 135, 156,
and 208C on an Oxford Diffraction Gemini S diffractometer, employing
graphite monochromated MoKa radiation (l=0.71073 ꢅ). All measure-
ments were performed on the same single crystal and the stabilization
time for each one was set at 1 h once the desired temperature was
reached. The structures were solved and refined using the methods and
procedures detailed above.
ꢁ
atoms generated by the breaking/formation of Yb O bonds,
in which the stoichiometry remains unchanged. The transfor-
mation from the a to the b polymorph, which happens at
1308C, involves the change of coordination number from
eight to seven, [YbO8]n rod-shaped SBUs to [YbO7]2 dimeric
SBUs, h2m3-h2m2 to h2m2-h2m2 coordination in the gauche con-
former of succinate, and a dia-type four-connected to a tfa-
d-type binodal network, eight- and three-connected. The a
polymorph has proven to be an active acid and redox heter-
ogeneous catalyst. The influence of the structure on the cat-
alytic activity was tested in HDS of thiophene at different
temperatures, which showed a higher activity for the b poly-
morph. Based on the spectroscopic evidence, a mechanism
Temperature-dependent X-ray powder diffraction: This was carried out
with a Bruker D8 Advance powder diffractometer using CuKa radiation
(l=1.5418 ꢅ, U=40 kV, I=30 mA) equipped with an MRI wide-temper-
ature-range oven-camera (ꢁ195 to 4508C) mounted on a theta–theta go-
niometer. Data were scanned over the range 2q=8.5–608 with 2q step
size=0.058 and counting time=1 s/step. The heating and cooling process
was controlled by Eurotherm 2404. The sample heating rate was
0.28Cminꢁ1 from RT (208C) to 2008C.
ꢁ ꢁ
involving formation of the Ln O OH active species is pro-
Vibrational characterization: The IR spectra were recorded over 4000–
225 cmꢁ1 by the KBr pellet technique with a Nicolet Protꢁgꢁ 460 spec-
trometer, employing a variable-temperature cell with 64 scans; spectral
posed for the sulfide oxidation. This is, to our knowledge,
the first reversible crystal-to-crystal phase transition in an
MOF, in which the hysteresis cycle from one polymorph to
the other goes from room temperature to high temperature:
RT!HT. Additional studies on these materials are ongoing
to evaluate their optical and inclusion properties derived
resolution was 2 cmꢁ1
.
Thermal analysis: Thermogravimetric and differential thermal analyses
(TGA/DTA) were performed using a Seiko TG/DTA 320 apparatus in
the range 25–10008C in an air flow at 20 mLminꢁ1 at a heating rate of
108Cminꢁ1 DSC at 25–1908C (at 108Cminꢁ1 with about 25 mg of
.
ꢁ
from the click of the Yb O bond (opening/closing the gate).
sample) was performed in a Mettler TA3000 system equipped with a
DSC30 unit.
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M. A. Monge et al.
Catalytic reactions
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General procedure for acetalization of aldehydes with TMOF: Catalyst
(0.025 mmol) was added to
a solution of the carbonyl compound
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(2.5 mmol) and TMOF (2.5 mmol) in tetrachloromethane (5 mL) in a
25 mL flask equipped with a magnetic stirrer at 608C. Samples were
taken at intervals and the reaction products were analyzed by gas chro-
matography (GC) on a Hewlett-Packard 5890II chromatograph with a b-
DEX325 capillary column, equipped with an MS detector.
General procedure for oxidation of sulfides with hydrogen peroxide: The
catalyst (0.025 mmol) was activated by stirring it with hydrogen peroxide
(1.5 mmol, 25%) for 1 h at 608C in a 25 mL flask equipped with a mag-
netic stirrer. The reactor with the activated catalyst was then charged
with acetonitrile (5 mL) and the corresponding thioether (methylsulfanyl-
benzene or 2-ethylbutyl phenyl sulfide, 2.52 mmol) and H2O2 (1.5 mmol,
25%) was added dropwise, while the whole suspension was heated at
608C. Samples were taken hourly and analyzed after filtration. Chemical
yields of sulfoxides and sulfones were measured by GC as above.
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thiophene decomposition, substrate (10 mL, 125 mmol) and catalyst
(0.043 g, 0.125 mmol) were mixed and heated at various temperatures,
with stirring. After the selected time intervals, the reactor was cooled,
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Recycling experiments: Polymorph a was recycled by using it in several
methylsulfanylbenzene oxidation reaction cycles. Before reuse, the solid
was filtered from the reaction medium and washed with diethyl ether;
then fresh substrate and solvent were added without further catalyst, for
three consecutive experiments. To verify the heterogeneous nature of the
catalytic reaction we investigated the residual activity of the supernatant
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F.G. acknowledges an FPI fellowship from the Ministerio de Educaciꢇn y
Ciencia (MEC) and the Fondo Social Europeo from the EU. This work
has been supported by the Spanish MCYT Project Mat 2007-60822, CTQ
2007–28909-E/BQU, and Consolider-Ingenio CSD2006–2001. The authors
thank Laura Torre from the X-Ray Group in the Universidad de Oviedo,
for the variable-temperature single-crystal data collections. Dr. M. J. Mar-
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FULL PAPER
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Received: November 17, 2008
Published online: March 25, 2009
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