Synthesis, breathing, and gas sorption study of the first isoreticular mixed-linker phosphonate based metal-organic frameworks

Article abstract of DOI:10.1039/c2cc38092g

The synthesis of the first water stable isoreticular phosphonate based mixed-linker metal-organic frameworks (MOFs) is achieved via the use of the N-donor heterocyclic co-ligands. Furthermore, these isoreticular phosphonate frameworks show selective CO₂/N₂ uptake at low pressures.

Full text of DOI:10.1039/c2cc38092g

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Synthesis, Breathing, and Gas Sorption Study of the First Isoreticular
Mixed-Linker Phosphonate Based Metal-Organic Frameworks
Marco Taddei,a Ferdinando Costantino,*a,b Andrea Ienco,b Angiolina Comottic, Phuong V. Daud and Seth
M. Cohend
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
tetrakis(phosphonomethyl)hexamethylenediamine (hereafter
H8L2) ligands, and Nꢀdonor heterocyclic coꢀligands 4,4’ꢀ
50 bipyridine (bipy) or 1,2ꢀbis(4ꢀpyridyl)ethane (etbipy) (Scheme
1).
The synthesis of the first water stable isoreticular
phosphonate based mixed-linker metal-organic frameworks
(MOFs) is achieved via the use of the N-donor heterocyclic co-
10 ligands. Furthermore, these isoreticular phosphonate
frameworks show selective CO2/N2 uptake at low pressures.
Metalꢀorganic frameworks (MOFs) have been considered a
breakthrough in material sciences due to a wide range of
possible applications.1 MOFs are built by using organic
15 ligands to bridge metal ions or metal clusters to form stable
and porous 3ꢀdimensional frameworks.2 The size and
functionality of the porous channels of MOFs can be easily
tuned by extending ligand length or functionalizing the
ligands, often with retention of the overall topology of the
20 framework. The ability to tune the size and functionality of
MOFs without disturbing the overall structure is termed
‘isoreticular synthesis’.3 However, isoreticular synthesis of
MOFs is largely limited to carboxylate ligands, with fewer
examples with other ligands, particularly phosphonates, due to
25 their high flexibility and coordination variability.4 A recent
review reported the differences between phosphonate and
carboxylate based MOFs, especially with respect to the
influence of connectivity of these ligands on the structure of
the inorganic framework.5 To the best of our knowledge, only
30 two examples of isoreticular and porous phosphonate MOFs
have been reported.6 The functionality and pore size tunability
of phosphonate MOFs remained unexplored, even though they
are very attractive due to their good chemical stability
especially in water, which has been a significant drawback for
35 many carboxylateꢀbased MOFs.
A new family of layered coordination polymers, based on
Cu(II), xylylenediphosphonic ligands, and 4,4’ꢀbipyridine
(bipy) was recently described.7 These compounds have
layered structures because the xylene rings form πꢀπ stacking
40 interactions with the bipy ligands. The similar structures of
these layered coordination polymers suggested the possibility
of using tetraphosphonates with Nꢀdonors heterocyclic coꢀ
ligands to assist the synthesis of isoreticular MOFs. Herein,
we reported the synthesis of the first isoreticular phosphonate
45 based mixedꢀlinkers MOFs. These MOFs are constructed by
Scheme 1. Molecular structure of the ligands used in this study.
These two phosphonate ligands, already reported for the
55 synthesis of layered copper coordination polymers,8 were
combined with copper acetate and one of the two bipyridine
ligands under mild conditions (by simply mixing the reagents
at 80 °C in water, followed by removal of solvent), to produce
three isoreticular MOFs of formula Cu3(H2L1)(bipy)2ꢁ9H2O
60 (1),
Cu3(H2L2)(bipy)2ꢁ11H2O
(2),
and
Cu3(H2L2)(etbipy)2ꢁ24H2O (3). Our strategy uses L1 and L2 as
chelating ligands for the copper ions to restrain one
propagation direction of the organic spacers linked to the
phosphonic moieties. Structural control is ensured by the
65 formation of a trinuclear copper based 1D composite building
units (CBUs) common to the three compounds (Figure 1).
In all three materials, the three copper ions of the CBU are
found in two different coordination environments: the central
atom, Cu1, is located on an inversion centre and it is
70 coordinated by four oxygen atoms, belonging to four different
phosphonic moieties, and by two nitrogen atoms of two
different bipy groups producing
a distorted octahedral
geometry. The external copper atoms, two crystallographically
equivalent Cu2, have square pyramidal coordination, in which
75 three planar vertices are occupied by one nitrogen and two
oxygen atoms belonging to a chelating L1 or L2 moiety, and
the remaining planar coordination site is filled with a nitrogen
atom of the bipy, and the apical vertex is filled by one ꢂ2ꢀ
oxygen atom, shared with Cu1. This CBU is propagated along
80 the aꢀaxis direction via exoꢀbidentate phosphonic groups,
using Cu(II), N,N,N’N’ꢀtetrakis(phosphonomethyl)-
xylylenediamine (hereafter N,N,N’N’ꢀ
α,α’ꢀpꢀ
H8L1),
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creating infinite chains.
These inorganic chains are linked to each other by the organic
moieties, forming rhombic 1D channels of about 22×8 Å,
10 21×12 Å, and 24×12 Å for 1, 2, and 3 respectively (measured
between the atomic centres). The structures of the three
compounds are shown in Figure 2. Their topology resembles
that of other 1DꢀMOFs with infinite parallel channels like
MILꢀ53.9 One difference is that in the title compounds the
15 channel walls are constituted by the combination of the two
different ligands. The 1D channels are polar and hydrophilic
because there are two P=O and two PꢀOH groups for each
cavity that point towards the inner part of the channels and
serve as anchoring point for the water molecules via hydrogen
20 bonds. These channels are filled with water molecules (up to 8
per copper ion).
Figure 1. Polyhedral representation of the copperꢀphosphonate CBU.
The octahedral Cu1 atom is represented in dark blue, the square
pyramidal Cu2 atoms are represented in light blue, and PO3C
tetrahedra are represented in green.
5
Figure 2. Polyhedral representation of the structures of 1, 2, and 3. Octahedral Cu atoms are represented in dark blue, square pyramidal Cu atoms are
represented in light blue, and PO3C tetrahedra are represented in green.
25
Activation of the MOFs by removal of trapped water
molecules affects each MOF differently: 1 undergoes an 55 containing ligand L2 and bipy, has a channel size midway
according to the definition by Kitagawa.13 Compound 2,
irreversible phase change after dehydration (Figure S1). The
irreversible change may be due to the restricted number of
30 degrees of freedom of the xylene groups and the bipy ligands
(Figure S2), which prevents reversible framework ‘breathing’.
between those of 1 and 3 and undergoes a phase transition
similar to the one observed for 3, although the breathing effect
is less pronounced (Figures S4ꢀS5). Due to the quality of the
crystals of 2 after the dehydrating process, we could not
In contrast, 3 shows a strong breathing effect (Figure 3), 60 obtain the dehydrated structure of 2, so we created a structural
which could be due to the higher degree of freedom of the
alkyl chains of L2 and of the ꢀCH2ꢀCH2ꢀ group of the etbipy
35 ligand, compared to 1 (Figure S3). Analogous breathing
effects have been already reported in other pillaredꢀlayered
model on the basis of the 2_lp and treated the diffraction
pattern by means of a Le Bail refinement (Figure S6).14 The
volume difference between 2_lp and 2_np is about 10% of the
volume of the lp phase and the transition of the 2_lp to the
phosphonates containing alkyl chains.10 Two different crystal 65 2_np phase is not reversible, suggesting that again the fewer
structures of 3 in different phases reveal the shrinkage of the
framework after the loss of water molecules (Figure 3a). The
40 phases with fully open and partially collapsed structures are
designated 3_lp and 3_np, respectively (lp = large pores, np =
degrees of freedom with the bipy, in comparison with etbipy,
does not allow the framework to breathe as in 3.
Preliminary gas sorption studies were made on these
materials. While all the frameworks are nonꢀporous toward N2
narrow pores). The volume difference between 3_lp and 3_np 70 at 77K (BET values are 4.6, 16.3, and 8.5 m2 gꢀ1, for 1, 2, and
is remarkably high, being around 26% of the volume of the lp
phase, and the hydration/dehydration dynamic is fully
45 reversible. Other comparable crystalꢀtoꢀcrystal phase
transformations have recently been observed in MOFs with
3, respectively), they do bind CO2 at low pressures and 195K
(Figures S7ꢀS9), with the overall CO2 uptake capacity
increasing as 1 < 3 < 2 (Figure 4). The best performing
compound is 2, which is able to absorb up to 77 cm3 gꢀ1 of
similar 1D cavities.11 Notably, in 3_np the exposed copper 75 CO2. A similar trend in the adsorption capacity of the
ions are able to reversibly bind a water molecule to take on an
octahedral coordination geometry. An analogous reversible
50 water binding has been observed in a coordination polymer of
similar topology.12 This breathing behaviour can be regarded
compounds was observed at room temperature and up to 10
bar (Figure S10). The different behaviour of these MOFs
suggests that the backbone of both the linkers plays a vital
role in their gas sorption properties. The selectivity of these
as midway between the class IV (pillaredꢀlayered with 80 MOFs towards CO2 could be ascribed to two reasons: first,
“accordion effect”) and the class V (shrinking 3D grids)
| Journal Name, [year], [vol], 00–00
they do not retain a permanent porosity to absorb N2 after
2
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activation; and second, the polarity of the PꢀO groups may
promote the affinity to CO2 which has a quadrupole moment.
This work was supported by a grant from Department of
Energy, Office of Basic Energy Sciences, Division of
Materials Sciences and Engineering under Award No. DEꢀ
FG02ꢀ08ER46519 (S.M.C.).
30 Notes and references
a
Dipartimento di Chimica, University of Perugia, Via Elce di Sotto 8,
06123, Perugia, Italy. E-mail: fcost@unipg.it
b CNR – ICCOM, Via Madonna del Piano 10, 50019, Sesto Fiorentino,
Firenze, Italy.
35 c Department of Materials Science, University of Milano Bicocca, Via R.
Cozzi, 53 20125, Milan, Italy
d
Department of Chemistry and Biochemistry, University of California,
San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA.
Electronic Supplementary Information (ESI) available: [Full synthetic
40 procedure; Experimental and instrumental details; Full crystallographic
data; Le Bail plot for 2_np phase; Additional crystal structure figures;
Thermogravimetric curves; Detailed discussion about the phase
transitions; Gas adsorption isotherms]. Crystallographic data (excluding
structure factors) for the compounds reported in this paper have been
45 deposited with the Cambridge Crystallographic Data Centre as
supplementary publication CCDC 909973, to 909976 for 1, 2_lp, 3_lp,
and 3_np, respectively. See DOI: 10.1039/b000000x/
F
igure 3. (a) The 3_lp to 3_np structural transition; (b) the
rearrangement of the layers defined by the inorganic chains and the
bipyridines associated with the transition.
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