Framework functionalisation triggers metal complex binding

Article abstract of DOI:10.1039/b718367d

Post-synthetic derivatisation of a porous material produces a functionalized material that binds the metal complex V(O)acac₂, in contrast to the unfunctionalized precursor, which is inactive for complex binding. The Royal Society of Chemistry.

Full text of DOI:10.1039/b718367d

View Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Framework functionalisation triggers metal complex bindingw
Michael J. Ingleson, Jorge Perez Barrio, Jean-Baptiste Guilbaud,
Yaroslav Z. Khimyak and Matthew J. Rosseinsky*
Received (in Cambridge, UK) 28th November 2007, Accepted 14th April 2008
First published as an Advance Article on the web 9th May 2008
DOI: 10.1039/b718367d
useful as a bidentate ligand. Salicylidene formation activates
Post-synthetic derivatisation of a porous material produces a
functionalized material that binds the metal complex V(O)acac2,
in contrast to the unfunctionalized precursor, which is inactive
for complex binding.
the framework towards metal sequestering, demonstrated by
heterogenizing a vanadyl complex; in contrast, the unfunctio-
nalised parent material, possessing only NH2 groups, displays
no propensity for metal binding.
IRMOF3 was prepared as previously reported,13 but to
alleviate the inherent moisture sensitivity post activation by
chloroform,14–16 a further guest exchange for toluene was per-
formed. This generates a toluene solvated material, IRMOF3-
tolx (x E 5 by micro and thermogravimetric (TG) analysis).w
This toluene impregnation enables manipulation of activated
material in ambient conditions (air) for short periods (o30
minutes) with no degradation observed, judged by phase purity
(by X-ray powder diffraction (XRPD)) and high
porosity (consistently 480 weight% CO2 uptake at 195 K and
1 bar CO2 pressure). In addition, samples stored in ‘wet’
(standard analytical reagent grade with no drying) toluene for
months show no decomposition (by XRPD and porosity mea-
surements). An identical procedure can be applied to significantly
increase the moisture resistance of the closely related material
MOF-5, [(Zn4O)(O2C–C6H4–CO2)3]. No analogue of the water-
derived material, MOF69c,17 is observed during similar handling
procedures. The enhanced framework stability afforded by to-
luene guest molecules can be attributed to the combination of
enhanced framework–guest interactions and toluene’s lower
volatility (relative to CHCl3), both of which have the effect of
reducing the effective vapour pressure of the guest molecules. A
highly pertinent calculation revealed that benzene adsorbed in
MOF-5 resides in pockets surrounding the moisture sensitive
Zn4O cluster.18 This is consistent with the observed enhanced
stability of toluene activated IRMOF materials as arene encap-
sulation of the Zn4O cluster would form a hydrophobic barrier,
preventing H2O attack. Derivatisation of IRMOF3-tolx is
achieved by the addition of two equivalents (per NH2 group)
of salicylaldehyde to a toluene suspension of IRMOF3-tolx and
allowed to stand for 7 days, followed by toluene washing.
Functionalisation produced a partially converted MOF,
IRMOF3-sal0.4 (Fig. 1), with empirical formula (Zn4O)-
(O2C–C6H3(NH2)–CO2)2.6 (O2C–C6H3–(NQC(H)C6H4OH)–
CO2)0.4, isolated as the toluene solvate.w The degree of amine
conversion is determined from a desolvated (80 1C, 18 h,
1 Â 10À3 torr) elemental analysis.w Successful imine formation
was initially indicated by micro-crystallite colour change from
cream to yellow, indicative of imine formation, an observation
confirmed by the solid state UV/Vis spectra, with an
IRMOF3-sal0.4 adsorption centred at 450 nm characteristic
of the salicylidene moiety.w Crystallinity is maintained
throughout the conversion (by XRPD); furthermore the
Metal organic frameworks (MOFs) are a relatively new family
of porous materials constructed from multi-dentate organic
building blocks linking metal centres. A significant advantage
in the synthesis of MOF materials over the ubiquitous zeolites
is the large variation possible in the chemical composition of
the linker group.1 This is crucial as the chemical nature of the
chosen linker controls the surface pore environment; thus
specific properties can be engendered simply by linker mod-
ification.2 Of particular interest are frameworks generated
from multi-functional linkers, where one moiety is employed
in material construction (binding to the metal nodes) whilst
the second remains accessible (metal-free) for further use (e.g.,
catalysis, sensing, separation).3–7 The direct, one step, synth-
esis of MOFs with desirable secondary functional groups,
barring a number of notable exceptions,1,8 has proven proble-
matic, principally due to their chemical/thermal sensitivity
combined with an inherent tendency to undergo irreversible
metal complexation.9,10 An alternative methodology for gen-
erating accessible pore wall functionalities is a two step
approach, involving initial framework synthesis followed by
functional group transformation.11,12 This involves pore sur-
face groups that are not integral to the framework structure
being covalently modified using established solution phase
chemistry. The gross extended framework structure will
remain unchanged, but surface functionalisation will generate
a distinct pore topology combined with altered chemical
properties. [(Zn4O)(O2C–C6H3(NH2)–CO2)3]n, IRMOF3
(iso-reticular MOF3), is a porous MOF decorated with pen-
dant amine (–NH2) groups previously developed by Yaghi
et al.1 The amine moieties are not intimately involved in the
framework backbone, and therefore are available to undergo
chemical transformations; indeed during this work the con-
version of the amine group quantitatively to an amide func-
tionality has been reported.11 In this paper we report a more
desirable amine transformation via a simple condensation
reaction producing a salicylidene (R–NQC–C6H4OH) moiety,
Department of Chemistry, University of Liverpool, Liverpool, UK L69
7ZD. E-mail: m.j.rosseinsky@liv.ac.uk; Fax: (+44) 151 794 3598;
Tel: (+44) 151 794 3504
w Electronic supplementary information (ESI) available: Experimental
procedures and full characterization of all materials. See DOI:
10.1039/b718367d
ꢀc
This journal is The Royal Society of Chemistry 2008
2680 | Chem. Commun., 2008, 2680–2682

View Online
solids.19 IR spectroscopy is less revealing with the framework
CO2 and NH2 stretching bands dominating, thus obscuring any
imine (CQN) and O–H stretches. The functionalized frame-
work IRMOF3-sal0.4 remains highly porous, but does adsorb 2
mass% less CO2 (at 195 K and 1 bar CO2) than the starting
material IRMOF3,w confirming the expected decrease in void
space post functionalisation, further corroborated by a reduc-
tion in the degree of solvation of IRMOF3-sal0.4 (by TGA). A
control reaction with amine-free material, MOF-5 (2 eq. of
salicylaldehyde per aryl group, 7 days), resulted in no salicyl-
aldehyde incorporation,w fully consistent with covalent tether-
ing being a prerequisite for salicylaldehyde incorporation and
not simply physisorption by strong host–guest interactions.
The observed sub-stoichiometric conversion (ca. 13% of
amine groups react) corresponds to only 0.4 salicylidene
groups per cavity (there is one cavity per empirical formula
unit). This value is significantly below that expected for void
space saturation, and less than that found for amine to amide
transformation (quantitative)11 and organometallic com-
pound impregnation.20 Significantly longer reaction times
(1 month with no apparent framework decomposition by
pXRD and microanalysis) did not increase the conversion
beyond a maximum of ca. 13%. Numerous attempts to
increase the degree of amine functionalisation whilst main-
taining framework integrity failed (different batches of IR-
MOF3, larger excess of salicylaldehyde, Dean–Stark reactions,
heating to 60 1C and reflux). It is feasible that the framework
decomposition observed under the forcing (reflux or
Dean–Stark) reaction conditions occurs due to the formation
of a significant H2O concentration proximate to the Zn4O
cluster. The molecular species, 2-amino-terephthalic acid, on
condensation with salicylaldehyde is completely converted to
the respective salicylidene under forcing conditions.w Attempts
to synthesize an IRMOF3-sal3 framework utilising this pre-
formed linker with Zn(NO3)2 in diethyl formamide under a
range of conditions were repeatedly unsuccessful. In our
hands, IRMOF3-salx frameworks have to be made in a
stepwise fashion, the incomplete conversion possibly attribu-
table to an equilibrium position favouring amine and sali-
cylaldehyde rather than salicylidene and H2O.
Fig. 1 Left, schematic for the functionalisation of IRMOF3 with
salicylaldehyde and subsequent binding of a vanadyl complex (acac =
acetylacetonate). Right, XRPD (5 to 301 2y) for each material.
similar peak positions and relative intensities observed for
IRMOF3 and IRMOF3-sal0.4 allow us to conclude that there
are minimal changes to the extended lattice. There is no
observable evidence (by XRPD) for any decomposition in-
itiated by the stoichiometric formation of the H2O by-product.
The salicylidene functionalisation can be performed on single
crystals with no reduction in crystallinity, but subsequent
single crystal X-ray diffraction on IRMOF3-sal0.4 proved
uninformative. The inherent positional disorder of the aryl–
NH2 group in IRMOF3 combined with a lack of long range
order that arises due to the partial conversion to imine
frustrated attempts to locate the salicylidene.11
Unambiguous confirmation of imine formation was forth-
coming from the solid-state 1H–13C CP/MAS NMR spectra
(Fig. 2) on evacuated materials. The appearance in the IR-
MOF3-sal0.4 spectrum of two resonances (162 and 158 ppm)
assignable to CQN and aryl C–OH; in addition to the more
À
À
complex CO2 and aryl C–H regions in IRMOF3-sal0.4 are
À
À
consistent with additional overlapped resonances due to ran-
domly arranged functionalized (B13%) and unfunctionalized
(B87%) amino-terephthalates in IRMOF3-sal0.4. Importantly
no aldehyde (CQO) resonance is observed at ca. 195 ppm,
precluding impregnation by unreacted salicylaldehyde. The H
À
1
fast MAS NMR spectrum post-functionalisation is also infor-
mative revealing a resonance at 13.4 ppm attributable to a
hydrogen bonding OH moiety (salicylidene OH hydrogen
bonded to imine N), the chemical shift is fully consistent with
other reported strong hydrogen bonded resonances in organic
To probe IRMOF3-sal0.4 metal binding ability, V(O)acac2Á
H2O (acac = acetylacetonate) solutions (in dry CH2Cl2) were
imbibed into IRMOF3-sal0.4. V(O)acac2 has the appropriate
Fig. 2 Left, solid state 1H MAS NMR spectra (30 kHz) and right, solid state 1H–13C CP/MAS NMR spectra (20 kHz) for IRMOF3 (top) and
IRMOF3-sal0.4 (bottom, materials were desolvated in vacuo at 1 Â 10À3 torr, 80 1C, 18 h). The peaks at 162.3 and 158.4 ppm are in characteristic regions,
assignable to the imine CQN and Caryl–OH, respectively, * = impurity centred at 0.5 ppm we assign to residual toluene solvate and/or silicone grease.
À
À
ꢀc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 2680–2682 | 2681

View Online
CD2Cl2 over the same time period evolved no salicylaldehyde.w
This experiment in combination with the analytical data demon-
strates that only the salicylidene moiety can bind vanadium.
A preliminary catalytic investigation revealed that IRMOF3-
t
Vsal0.4 is active for the oxidation of cyclohexene with BuOOH
(40% conversion of cyclohexene in THF at 60 1C for 72 h).
Catalysis was shown to be unambiguously heterogeneous (filtered
supernatant is inactive) but the poor turnover frequencies com-
bined with loss of framework integrity (judged by XRPD) during
the catalysis limits its applicability in this particular reaction.
Nevertheless, IRMOF3-Vsal0.4 is a rare example of a MOF with
unsaturated metal centres immobilized on the pore surface. The
observed catalysis does provide proof of principle that, via
sequential functionalization, a porous MOF can be chemically
transformed to introduce useful ligating groups (the salicylidene),
activating the material towards metal complex binding and
producing a catalytically active MOF. We emphasize that post-
synthetic chemical transformation is integral to achieving catalyst
heterogenization, with unfunctionalised starting material display-
ing no capacity for metal binding. Solid state NMR spectroscopy
was shown to be a vital diagnostic technique for monitoring
MOF functionalisation. In addition, the facile methodology
introduced to improve the moisture tolerance of the IRMOF
series is highly significant given the widespread use of these
materials and the documented property repeatability issues aris-
ing from H2O sensitivity.16
Fig. 3 1H NMR spectra of IRMOF3-sal0.4 suspended in a CD2Cl2
solution of V(O)acac2ÁH2O. Spectra A: time zero; B: t = 2 days; C:
t = 10 days. * = V(O)acac2ÁH2O resonances, J = resonances for the
two tautomers of acetylacetone, & = salicylaldehyde resonances.
dimensions to freely diffuse through the pore windows present
in IRMOF320 (fixed pore window diameter of B8 A, approx-
imate dimensions of V(O)acac2 = 6.2 Â 8.3 Â 2.5 A).21
Furthermore control reactions established that IRMOF3 does
not adsorb any detectable amounts of V(O)acac2. V(O)acac2
was imbibed into IRMOF3-sal0.4 over 7 days (at 298 K)
followed by repeated washing and subsequent evacuation,
resulting in the formation of a material with empirical formula
(Zn4O)(O2C–C6H3(NH2)–CO2)2.6(O2C–C6H3(NC(H)-
The EPSRC is thanked for funding MJI and JPB
(EP/C511794 and GR/R 59045).
Notes and references
1 M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M. O’Keeffe
and O. M. Yaghi, Science, 2002, 295, 469.
2 S. Kitagawa, S. Noro and T. Nakamura, Chem. Commun., 2006, 701.
3 S. Kitagawa, R. Kitaura and S. Noro, Angew. Chem., Int. Ed.,
2004, 43, 2334.
4 B. Kesanli and W. Lin, Coord. Chem. Rev., 2003, 246, 305.
5 S. L. James, Chem. Soc. Rev., 2003, 32, 276.
6 G. Ferey, C. Mellot-Draznieks, C. Serre and F. Millange, Acc.
Chem. Res., 2005, 38, 217.
C6H4O–[V(O)acac])-CO2)0.4
, IRMOF3-Vsal0.4. The exact
composition of IRMOF3-Vsal0.4 is determined by combined
elemental, TG and EDX analysis, whilst comparable XRPD
patterns for IRMOF3-Vsal0.4 and IRMOF3 confirm the per-
sistence of the extended open framework structure.w Pore void
space decreases after vanadyl binding with less toluene guest
solvent adsorbed further corroborating metal complexation.w
The consistent 10 : 1 Zn : V molar ratio observed by EDX
analysis for all crystallites precludes salicylidene formation as
a minor amorphous impurity. IRMOF3-Vsal0.4 is formed by
the chemisorption of one V(O)acac2 per framework salicyli-
dene functionality. The failure to load V(O)acac2 into IR-
MOF3 precludes complexation via the NH2 functionality.
Solution-state 1H NMR spectroscopy is highly informative for
the analysis of soluble by-products from the loading procedure
(Fig. 3). Initially the only observed resonance is the paramagne-
tically broadened CH3 group of V(O)acac2 and H2O (Fig. 3A).
On standing for several days new resonances appear and grow in
intensity; these correspond to the two tautomers of acetylacetone
(Fig. 3B). The detection of acetylacetone exo-framework con-
firms that the salicylidene moiety has protonated acac, displacing
it from the coordination sphere of vanadium. Longer loading
times increase the intensity of the acetylacetone resonances, but
prolonged standing (47 days) resulted in a slow growth of peaks
corresponding to salicylaldehyde (Fig. 3C). This is attributed to
additional H2O (from V(O)acac2ÁH2O) shifting the equilibrium
position, hydrolysing the imine. IRMOF3-sal0.4 suspended in
7 M. Higuchi, S. Horike and S. Kitagawa, Supramol. Chem., 2007,
19, 75.
8 C.-D. Wu, A. Hu and W. Lin, J. Am. Chem. Soc., 2005, 127, 8940.
9 R. Kitaura, G. Onoyama, H. Sakamoto, R. Matsuda, S.-I. Noro
and S. Kitagawa, Angew. Chem., Int. Ed., 2004, 43, 2684.
10 J. Heo, Y.-M. Jeon and C. A. Mirkin, J. Am. Chem. Soc., 2007,
129, 7712.
11 Z. Wang and S. M. Cohen, J. Am. Chem. Soc., 2007, 129, 12368.
12 J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon and K.
Kim, Nature, 2000, 404, 982.
13 J. L. C. Roswell and O. M. Yaghi, J. Am. Chem. Soc., 2006, 128, 1304.
14 M. Sabo, A. Henschel, H. Frode, E. Klemm and S. Kaskel, J.
Mater. Chem., 2007, 17, 3827.
15 S. Hausdorf, F. Baitalow, J. Seidel and F. O. R. L. Mertens, J.
Phys. Chem. A, 2007, 111, 4259.
16 S. S. Kaye, A. Dailly, O. M. Yaghi and J. R. Long, J. Am. Chem.
Soc., 2007, 129, 14176.
17 N. L. Rosi, J. Kim, M. Eddaoudi, B. Chen, M. O’Keeffe and O. M.
Yaghi, J. Am. Chem. Soc., 2005, 127, 1504.
18 S. Amirjalayer, M. Tafipolsky and R. Schmid, Angew. Chem., Int.
Ed., 2007, 46, 463.
19 G. R. Goward, I. Schnell, S. P. Brown, H. W. Spiess, H.-D. Kim
and H. Ishida, Magn. Reson. Chem., 2001, 39, S5.
20 S. Hermes, F. Schroder, S. Amirjalayer, R. Schmid and R. A.
Fischer, J. Mater. Chem., 2006, 16, 2464.
21 E. Shuter, S. J. Rettig and C. Orvig, Acta Crystallogr., Sect. C:
Cryst. Struct. Commun., 1995, 51, 12.
ꢀc
This journal is The Royal Society of Chemistry 2008
2682 | Chem. Commun., 2008, 2680–2682