Guest encapsulations in non-porous crystals of fully fluorinated dinuclear metal complexes with the M2O2 core (M = Fe3+, Co2+, Ni2+)

Article abstract of DOI:10.1039/c9dt01380f

Two different coordination types of fully fluorinated dinuclear metal complexes, [Fe2L4(OMe)2] and [M2L4(OH2)2] (M = Co2+, Ni2+ and HL = bis(pentafluorobenzoyl)methane), were obtained. All of the complexes form non-porous crystals, which act as hosts for the adsorption of various benzene derivatives, e.g., benzene, toluene, p-xylene, anisole, and small gas molecules, e.g., CO2, O2, and NO. The complex of iron selectively adsorbs NO in high amounts and the complex of cobalt is found to store adsorbed O2.

Full text of DOI:10.1039/c9dt01380f

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Guest encapsulations in non-porous crystals of
fully fluorinated dinuclear metal complexes with
the M2O2 core (M = Fe3+, Co2+, Ni2+)†
Cite this: DOI: 10.1039/c9dt01380f
Received 1st April 2019,
Accepted 25th May 2019
Ryu Gonda,a Izabela I. Rzeznicka, a Yuki Kinoshita,b Sayaka Uchida b and
a
DOI: 10.1039/c9dt01380f
Akiko Hori
*
rsc.li/dalton
Two different coordination types of fully fluorinated dinuclear molecules; (2) the large positive quadrupole moment of fluori-
metal complexes, [Fe2L4(OMe)2] and [M2L4(OH2)2] (M = Co2+, Ni2+ nated aromatic ligands generates a polar surface, which pro-
and HL = bis(pentafluorobenzoyl)methane), were obtained. All of vides an opportunity to selectively adsorb molecules with
the complexes form non-porous crystals, which act as hosts for the quadrupole moments of an opposite sign.16 In this respect, we
adsorption of various benzene derivatives, e.g., benzene, toluene, previously reported that the perfluorinated copper complex,
p-xylene, anisole, and small gas molecules, e.g., CO2, O2, and NO. [CuL2] 1, shows reversible encapsulations of vapor of benzene
The complex of iron selectively adsorbs NO in high amounts and the derivatives17 and small gas molecules,18 while the corres-
complex of cobalt is found to store adsorbed O2.
ponding non-fluorinated complex, [Cu(dbm)2] (Hdbm =
dibenzoylmethane), never shows such guest recognition
events. In the study of fluorinated complexes, we also
found that the fully fluorinated metal complex induces two
unique intermolecular arene-perfluoroarene15,19 and metal–π
interactions.20,21 This time, the aim of our work was to obtain
unique dinuclear metal complexes that form discrete crystals
driven by arene–perfluoroarene interactions. The dinuclear
metal complexes have a common fully fluorinated ligand HL
(Scheme 1). They form however two coordination mode crys-
tals. In the di(alkoxo)-bridged [Fe2L4(OMe)2] (2) complex, bis
(pentafluorobenzoyl)-methanido− (L) acts as an ancillary
ligand and in [Co2L4(OH2)2] (3) and [Ni2L4(OH2)2] (4), µ-ligand-
bridged complexes, two water molecules are present at each
metal coordination site.22 All complexes are six-coordinated
but only Co and Ni complexes show unique coordination
modes with two water molecules. These complexes form non-
The structural design of molecular frameworks exhibiting
guest-adaptable cavities is of great importance in the field of
molecular separation, gas storage and catalysis.1–4 In addition
to flexible cavities, such frameworks should possess molecular
recognition properties utilizing various host–guest chem-
istries. Various weak interactions are being exploited in crystal
engineering to create host crystals with selective molecular
recognition properties.5–9 Among molecular frameworks, dis-
crete compounds can provide flexible and reusable cavities for
small molecules.10 However, the presence of such cavities does
not ensure the molecular recognition of guest molecules
because of cavity size mismatches, the formation of interpene-
trating structures, and unexpected conversions.11,12
We have recently found that coordination complexes com-
posed of fully fluorinated aromatic ligands form discrete mole-
cular crystals with adaptable cavities that can reversibly encap-
sulate/adsorb various molecules.13–15 Perfluorination has two
important effects on crystal engineering and crystal properties:
(1) the repulsion between neighboring fluorine atoms makes
the crystal structure more flexible and accessible to solvent
aGraduate School of Engineering and Science, Shibaura Institute of Technology,
Fukasaku 307, Minuma-ku, Saitama 337-8570, Japan.
E-mail: ahori@shibaura-it.ac.jp; Fax: +81-48-687-5013; Tel: +81-48-720-6350
bDepartment of Basic Sciences, School of Arts and Sciences, The University of Tokyo,
3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan
†Electronic supplementary information (ESI) available: Crystallographic data
and thermogravimetric results of iron, cobalt, and nickel complexes. CCDC
1906902–1906909. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c9dt01380f
Scheme 1
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porous discrete crystals with the M2O2 core (M = Fe3+, Co2+, ordinated six membered rings are 33.74, 52.49, 41.11, and
Ni2+) that can selectively recognize several guest molecules.
32.04°, respectively. The remarkable twisted orientation
The target iron complex 2 was synthesized and fully charac- between Ring-2 and the coordination plane is stabilized in the
terized by X-ray crystallographic studies. Typically, the fluori- crystals because of the intramolecular C–F⋯π interactions21
nated ligand HL23 and FeCl2·4H2O were combined in CH3OH between ring-2 and ring-3i (i: −x, −y + 2, −z + 1): the distance
with NaOCH3 to give a bluish purple solution. Subsequent between the centroid of Ring-2 and F11i is 3.179(3) Å. The
extraction using the water/CHCl3 mixture yielded dinuclear packing structure of 2 is shown in Fig. 2a (see the ESI†). No
complex 2. The complex was crystallized from CH3OH by slow remarkable intermolecular interactions were observed between
evaporation to give red block crystals of 2 (isolated yield of molecules, and the unit cell contains no residual solvent acces-
54%). The results of elemental analysis corresponded to the sible voids (Fig. 2a).
formula of 2; calcd for C62H10F40Fe2O10 (%): C, 41.69; H, 0.56:
For all complexes 2–4, several types of aromatic molecules
found: C, 41.60; H, 0.36. Similarly, M(OAc)2·4H2O (M = Co and (guest, G) were found to be reversibly encapsulated in the crys-
Ni) and HL were combined under the same preparation con- tals. In the case of complex 2, cocrystals 2·nG (n = 1–5), con-
ditions to give dinuclear metal complexes 3 and 4, respect- taining up to five guest molecules, were obtained when 2 was
ively.22 These complexes were crystallized in a solution of combined with guest molecules in CH2Cl2, under slow solvent
benzene/CH2Cl2 to give red block crystals of 3·2C6H6 and evaporation conditions at room temperature. In detail, one
green block crystals of 4·2C6H6.22a The same products, 3 and and a half benzene molecules (a), three toluenes (b), one
4, were obtained when 10% water was added to the reaction p-xylene (c), and five anisoles (d) were found for each unit of 2
mixture, irrespective of the polarity of the solvent being used. in the corresponding crystals, yielding four types of single crys-
This fact is in contrast with the general tendency that only tals: 2·1.5a, 2·3b, 2·c, and 2·5d, respectively (Table 1). Crystal 3
mononuclear complexes can be obtained when using bulky also encapsulates guest molecules 3·nG (n = 2 or 3) forming
diketonate ligands and polar media.24 For example, when three new crystals, 3·2b, 3·3c, and 3·3d. The guest encapsulated
M(OAc)2·4H2O (M = Co, Ni) and the non-fluorinated ligand crystals 4·nG were, in most cases, the isomorphs of 3·nG, and
Hdbm were combined following the same synthesis procedure, their structures are summarized in the ESI.† The number of
only the corresponding mononuclear complex was obtained. guest molecules was fully characterized by X-ray crystallo-
In this view, the structures of fluorinated complexes 2–4 are graphic studies and thermogravimetric analysis (TGA). In crys-
unique, and their formation is likely due to fluorine- tallographic studies, no molecular disorders were observed for
substitution.
most of the crystals indicating that G was mainly stabilized
The crystal structure of 2 is shown in Fig. 1. The asymmetric
unit of 2 contains one half of the complex, which is centro-
symmetric, and comprises two Fe3+ ions, four ligands (L), and
two methoxy groups.‡ The same coordination mode is found
in diketonate ligands.25,26 The geometries around the metal
centre are pseudo-octahedral. The metal⋯metal separation is
3.0840(7) Å. The pentafluorophenyl groups are highly twisted
with respect to the coordination planes; the dihedral angles of
Ring-1, Ring-2, Ring-3, and Ring-4 from the corresponding co-
Fig. 2 Packing structures of 2 and 3 with benzene derivatives a–d: (a)
2, (b) 2·1.5a, (c) 2·c, (d) 2·5d, (e) 3·3c and (f) 3·3d. Schematic colours for
the complex: green, C; red, O; light blue, F; and orange, metal and for
the guest molecules: pink, benzene; yellow, p-xylene; and blue, anisole.
The detailed crystal information of eight crystals is shown in the ESI.†
Fig. 1 Crystal structure of
Displacement ellipsoids are drawn at the 50% probability level: sym-
metric operation i = −x, −y + 2, −z + 1.
2 with the atom-numbering scheme.
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Table 1 The number of guest molecules in the crystal of the fluori-
nated complexes 1–4, using X-ray crystallographic and TG studies
Compound
1 (Cu2+ 17
)
2 (Fe3+
)
3 (Co2+
)
4 (Ni2+
)
Benzene (a)
Toluene (b)
p-Xylene (c)
Anisole (d)
3
1.5
3
1
222a
2a
3
2 or 422b
4a
3a
3
1, 2, or 4
1a or 2
5
3
3
a Characterization was performed by TGA.
and fixed in an orientation suitable for the electrostatic arene–
perfluoroarene interactions15,19 of fluorinated 2–4.27 The
selected crystal packing structures of 2·nG and 3·nG are shown
in Fig. 2.
In the case of 2, the amount of encapsulated guest mole-
cules varies and the inclusion site flexibly changes; however
the inclusion amount and the guest position are almost the
same in 3 and also 4. In crystal 2·1.5a, one benzene molecule a
interacts with the pentafluorophenyl group of 2, with the dis-
tance of two centroids between 2 and a measuring 3.610(4) Å
and the corresponding slippage measuring 1.286 Å.27 Such
Fig. 3 (a) N2, (b) CO2, (c) O2, and (d) NO adsorption isotherms at 77,
195, 90, and 121 K, respectively, of the complexes: 2, red squares; 3,
black circles; 4, blue lozenge; filled, adsorption; and open, desorption.
values are found in crystals with the arene–perfluoroarene induced crystalline phase change, which is the characteristic
interaction15 suggesting that these interactions are the driving of discrete molecular-assembled crystals.
forces leading to the encapsulation of aromatic molecules.
CO2 adsorption isotherms in the solids are clearly different
Likewise, the arene–perfluoroarene interactions are observed from those obtained for N2. Fig. 3b shows the adsorption/de-
between guests, c and d, in the host complex 2, but their posi- sorption isotherms of CO2 at 195 K. Some amount of CO2
tions are clearly different because of the flexible orientation of adsorbed in solids of complexes 2–4 is observed, indicating
the pentafluorophenyl groups.27 On the other hand, very that a selective molecular recognition between each complex
similar packing and the corresponding arene–perfluoroarene and CO2 is achieved. The amounts of adsorbed CO2 for 2–4
interactions were observed between guests, c and d, and the are 6.2, 9.1, and 13.1 cm3 g−1, respectively, at 0.90 P/P0. Large
host complex 3, which has two positions for guest insertion: adsorption–desorption hysteresis is observed. The favorable
one above the coordination site of M2O2 and the other cavities insertion of CO2 in fluorinated complexes can be explained by
formed by ligands on the two sides of the complex molecule.
the quadrupole interaction between CO2,29–32 having a nega-
The observed guest encapsulation patterns and crystal tive quadrupole moment (−1.4 × 10−39 C m2), and pentafluoro-
system changes are not limited to single crystals. The revers- phenyl groups of the ligand which have a positive quadrupole
ible guest insertions are also confirmed in the corresponding moment.
powders by TGA and powder XRD.† These results show that
The adsorption behavior of O2 and NO is very different for
both the crystals composed of fluorine-substituted metal com- the coordination mode of solids 2–4, as shown in Fig. 3c and
plexes are not inherently stable, and the structural change can d, respectively. Complex 2 does not exhibit oxygen adsorption,
easily occur due to the uptake of guest molecules.
and on the other hand complex 3 shows a sudden increase in
The adsorption of small gas molecules into the crystals of O2 uptake at the values corresponding to 0.70 P/P0. The
dinuclear metal complexes 2–4 was also investigated. The maximum amount corresponds to 2.7 adsorbed O2 molecules
adsorption/desorption behavior of the complexes was exam- (36.4 cm3 g−1 at 0.90 P/P0) for one molecule of 3. We consider
ined on microcrystalline powder samples, which were activated that this remarkable change of the adsorption is based on the
by heating at 60 °C for 3 h under vacuum. N2 adsorption iso- crystal phase transition due to an increase in the relative
therms are found as type-III for solids 2–4, as shown in Fig. 3a. pressure of oxygen. A similar adsorption behavior is observed
The uptake of N2 for solids 2–4 is very small but steeply for the corresponding nickel complex 4, having the same
increases at 0.90P/P0, reaching 1.0, 1.7, and 2.3 cm3 g−1 for coordination mode. The O2 uptake amounts to 25.9 cm3 g−1 at
solids 2, 3, and 4, respectively. The shape of the adsorption 0.90 P/P0. Large hysteresis and the fact that oxygen remains in
isotherm corresponds to crystals with no voids. The sorption the crystal after the release process suggest that an exchange
curves are analyzed using Brunauer–Emmett–Teller (BET) reaction occurs between the coordinated water and oxygen
theory and the surface areas of solids 2–4 are estimated to be molecules because of the labile nature of Co2+ and Ni2+ ions in
around 0.4–1.0 m2 g−1, respectively.28 A small hysteresis complexes 3 and 4, respectively. However, complex 2 is bridged
observed for both solids may be attributed to the guest- by less bulky methoxy anions and does not have labile ligands.
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The difference in O2 adsorption behavior with the Fe complex
indicates that the peculiar phenomenon is caused by the
difference in the coordination mode of the M2O2 core
complex.
independent reflns = 4670, Rint = 0.0853, GOF = 1.147, R((I) > 2σ(I)) = 0.0456,
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Fig. 3d shows NO adsorption isotherms for 2–4. The
amount of adsorbed NO, a polar molecule,33 is the highest
among all tested solids. For complex 2, the NO adsorption
amount does not change up to 0.40 P/P0; however, it rapidly
increases in two steps reaching about 200 cm3 g−1 at 0.90 P/P0.
The amount of adsorbed NO corresponds to sixteen adsorbed
molecules. In the case of 3 and 4, NO uptake starts immedi-
ately at around 0.10 at 0.40 P/P0, respectively, and then reaches
to 100 and 150 cm3 g−1, respectively, at 0.90 P/P0 which are
equivalent to eight and twelve adsorbed NO molecules.
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Conclusions
We investigated the packing structures and adsorption pro-
perties of dinuclear complexes 2–4 upon crystallization with
appropriate solvents in comparison to compound 1. Eight
crystal structures have been identified for complexes 2–4. It
has been found that all complexes incorporate various aro-
matic and small gas molecules. The molecular recognition of
CO2 is due to the presence of the fluorinated ligand which pos-
sesses negative quadrupole moment. We succeeded to induce
electrostatic interactions peculiar to fluorine substitution even
with six-coordinated metal complexes, such as Fe3+, Co2+, and
Ni2+ ions. Furthermore, we found that the amount of adsorp-
tion of different gasses varies depending on the structure of
the coordination of the M2O2 core of the complexes. In particu-
lar, the Co (3) and Ni (4) complexes show non-reversible O2
uptake by exchange with coordination water and the Fe
complex (2) tends to adsorb NO efficiently. These results
demonstrate that non-porous crystals without defined cavities
can achieve molecular recognition functions when using a
fluorinated ligand. The origin of large hysteresis and the differ-
ences in the amount of adsorbed guest molecules for crystals
with different metal ions are under investigation.
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Conflicts of interest
There are no conflicts to declare.
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This work was supported by Grant-in-Aid for Scientific
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Notes and references
‡Single crystal 2 was obtained from slow evaporation of a CH3OH solution. The
crystal data of 2 (C62H10F40Fe2O10: Mw = 1786.40): monoclinic, P21/n, T = 103 K,
a = 14.3286(12) Å, b = 10.8784(10) Å, c = 19.0990(14) Å, β = 98.836(3)°, V =
2941.7(4) Å3, Z = 2, Dc = 2.017 g cm−3, no. of reflns measured = 33 679, no. of
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