Soft synthesis of isocyanate-functionalised metal-organic frameworks

Article abstract of DOI:10.1039/c2dt31977b

We have developed an original synthetic pathway for the conversion of a MIL-68(In)-NH₂ metal-organic framework into its corresponding isocyanate (-NCO) derivative. This two-step soft post-modification technique leads to highly porous isostructural materials.

Full text of DOI:10.1039/c2dt31977b

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Soft synthesis of isocyanate-functionalised Metal Organic Frameworks
Jenny G. Vitillo,*a Tristan Lescouet,b Marie Savonnet,b,c David Farrusseng,b Silvia Bordigaa
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
photoactivation.24ꢀ26 Nitrenes in the triplet spin state are very
5
We have developed an original synthetic pathway for the
50 reactive species that can easily react with gas molecules, solvents
and organic compounds.27, 28 In fact, in the presence of CO and
upon UV irradiation, the transformation of MOFꢀN3 to MOFꢀ
NCO has been reported,24 in analogy to what was reported
previously for matrixꢀisolated aryl azides in cryogenic
55 conditions.29ꢀ31 The possibility of converting azide into isocyanate
has also been reported for the reaction of these species with metal
carbonyls32ꢀ37 or through the Curtius rearrangement of acyl azide
groups. Unfortunately, the azide to isocyanate conversion was
performed over a very long reaction time with a requirement for
60 continuous cooling with liquid nitrogen in order to transform the
inert singletꢀstate azide group into the reactive triplet species.
Moreover, a very low amount of isocyanate was obtained after 12
h.24
conversion of MIL-68(In)-NH2 metal-organic framework into
its corresponding isocyanate (-NCO) derivate. This two-step
soft post-modification technique leads to highly porous
isostructural materials.
10
MetalꢀOrganic Frameworks (MOFs) are the latest generation
of porous crystalline materials.1ꢀ3 Their organicꢀinorganic hybrid
nature allows their cavities to be modified by postꢀsynthetic
methods4ꢀ7 in order to tune their hydrophilicity/hydrophobicity8
or acidity/basicity.9 Even if this characteristic is not a prerogative
15 of MOFs, in this case the versatility and scope of the reactions are
decidedly wider than for zeolites and mesoporous silicates. The
functionalisation of MOFs by postꢀsynthetic methods (PSM) is an
emerging, efficient approach to tuning the properties of porous
coordination polymers at the molecular scale.9ꢀ13 Many postꢀ
20 functionalisation approaches have been developed for MOFs;
they range from simple condensation reactions to more complex
methods involving protection/deprotection steps.14ꢀ18
MOFs constructed from 2ꢀaminoterephthalate are excellent
starting precursors for postꢀsynthetic functionalisation: this is due
25 to the affordability of this linker and because one can employ the
same synthesis conditions
as for their unfunctionalised
counterparts.19ꢀ21 Unfortunately, the weak nucleophilicity of
aromatic amines limits their application. On the contrary, strongly
electrophilic groups such as isocyanates have been shown to be
30 highly versatile in this respect, allowing the introduction of
various carbamate and urea functional groups.22 The direct
synthesis of MOFs possessing such functional groups is difficult,
however, so in general these groups are introduced by postꢀ
synthetic means. Interestingly, it is possible to transform amino
35 into isocyanate groups in MOFs, but the present method requires
the use of diphosgene (route 1 in Scheme 1). In fact, the treatment
of MILꢀ53(Al)ꢀNH2 (MIL: Materials from Institut Lavoisier,
Al(OH)(2ꢀaminoꢀ1,4ꢀbenzenedicarboxylate)) leads to the
corresponding MILꢀ53(Al)ꢀNCO.22, 23This method cannot,
40 however, be applied to most MOFs, because the diphosgene
treatment involves HCl formation, which in turn leads to the
solubilisation of the solid.
65
Scheme 1. Possible synthetic routes to MOF with pendent isocyanate
groups
The present study concerns an original procedure for
70 synthesising MOFꢀNCO from a MOF based on 2ꢀaminoꢀ1,4ꢀBDC
(BDC: 1,4ꢀbenzenedicarboxylate) by a twoꢀstep soft postꢀ
modification technique (route 2 in Scheme 1). This new method
combines two reactions already reported in the literature that here
are juxtaposed for the first time. This combination makes it
75 possible to exploit the advantages of both the amino and
isocyanate precursors in order to derive new MOFs. As a proof of
concept, this study deals with the postꢀmodification of In(OH)(2ꢀ
39
aminoꢀ1,4ꢀBDC), hereafter denoted MILꢀ68(In)ꢀNH2.38,
This
The protectionꢀdeprotection approach is a general concept that
allows this limitation to be circumvented. Very recently,
45 Kitagawa24 and Cohen25 reported the first lightꢀdriven
deprotection of MOFs in order to reveal "dormant" reactive
functionalities.18 In particular, Sato et al.24 have shown that azide
groups in MOFs can be transformed into nitrene species upon
material has been chosen because, despite its good stability in air,
80 it easily hydrolyses in water, making the diphosgene treatment
unsuitable for MILꢀ68. This method can be applied to other MOF
structures with varying degrees of functionalisation: these results
and the properties of the MOFs that can be obtained by exploiting
the rich reactivity of isocyanate groups will be described
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elsewhere.40 The first step involves the conversion of MILꢀ
68(In)ꢀNH2 into the corresponding azide compound MILꢀ68(In)ꢀ
framework modes. The negligible intensity of signals at 3506 and
3393 cmꢀ1 assigned to NꢀH stretching was an indication of the
N3 by following a postꢀsynthetic method reported previously.38, 41 50 total aminoꢀtoꢀazide transformation that was also confirmed by
In the second step, MILꢀ68(In)ꢀN3 is thermally activated in the
presence of CO to form MILꢀ68(In)ꢀNCO. Due to the high
reactivity of the aryl nitrenes, it is important that these groups are
sufficiently widely spaced within the material for reactions
1HꢀNMR (see below). After the reaction with CO, the only
significant change in the IR spectrum (red curve) was the
consumption of ꢀN3 and the growth of a strong band at 2285 cmꢀ1
characteristic of the –NCO group.24 The transformation of –N3
5
between them to be prevented.42 For this reason, an –NH2 diluted 55 species into –NCO is confirmed by the sequence of spectra in
structure was considered as a starting material. This material was
10 synthesised using an 80:20 ratio of BDC and BDCꢀNH2
(MIXMOFꢀMILꢀ68(In)ꢀ20%NH2). The preservation of the ligand
Figure S6, in which an isosbestic point is visible at about 2230
cmꢀ1. An estimation of the degree of conversion was difficult to
be achieved by FTIR because of the high extinction coefficient of
both azide and isocyanate groups.
ratio in the synthesised material and its homogeneity were
44
demonstrated previously.43,
The integration of the aromatic 60 The full conversion of azide into isocyanate groups was
1
proton signals of BDCꢀNH2 (7, 7.35 and 7.75 ppm) in HꢀNMR
15 (Figure 2a) accordingly indicated that the 18.22% of the total
linkers of this material are BDCꢀNH2.
confirmed by 1HꢀNMR.
MILꢀ68(In) materials are MOFs constituted of wires of
InO4(OH)2 octahedra linked by BDCꢀbased linkers, giving rise to
a Kagomé structure characterised by two families of 1D pores of
20 hexagonal (17.8 Å, nucleusꢀnucleus distances) and triangular
shape (7.8 Å). MIXMOFꢀMILꢀ68(In)ꢀ20%NH2 was (i) treated
with tBuONO and TMSN3 in THF overnight at room temperature
to produce the corresponding azide intermediates; (ii) dried in air
and activated under ultraꢀhigh vacuum at 90°C overnight in order
25 to remove all the solvent and water molecules; and (iii) reacted
three times with an excess of CO (300 mbar) at 120°C for 48 h
(total reaction time).
The crystallinity and surface area of each sample were verified
by Xꢀray powder diffraction (XRPD) and nitrogen adsorption and
30 compared with theoretical values (Table S1 and Figure S2). The
transformation of –N3 into –NCO was followed by Fourier
transform infrared (FTIR) and 1HꢀNMR spectroscopies.
Fig. 2. 1HꢀNMR spectra of MIXMOFꢀMILꢀ68(In)ꢀNH2 (a), MIXMOFꢀ
MILꢀ68(In)ꢀN3 (b) and MIXMOFꢀMILꢀ68(In)ꢀNCO (c). The spectrum of
65 MIXMOFꢀMILꢀ53(Al)ꢀNCO obtained via diphosgene treatment is also
reported for comparison (d). Before measurements all samples were
digested in HF/DMSOꢀd6.22 Filled circles refer to amino signals, empty
circles to azide species and filled squares to isocyanate groups.
70 1HꢀNMR was used to follow the modification of the ligand
substitution due to the strong shift of its aromatic protons and to
quantify the degree of conversion of azide into isocyanate groups.
The spectra are reported in Figure 2.
A singlet at 8.02 ppm corresponds to the aromatic protons of
75 pure terephthalic acid. The spectrum of MIXMOFꢀMILꢀ68(In)ꢀ
NH2 (green curve) exhibits two doublets (7 ppm, Hb, and 7.75
ppm, Hc) and a singlet (7.35 ppm, Ha, filled circles) that are not
present after the transformation of amino into azide groups (black
curve).The MIXMOFꢀMILꢀ68(In)ꢀN3 spectrum is characterised
80 by a broad signal around 7.75 ppm (empty circle). The spectrum
of MIXMOFꢀMILꢀ68(In)ꢀNCO (red curve) shows two doublets at
7.6 (Hb) and 8 ppm (Hc) and a singlet at 7.6 ppm (Ha) (squares).
35 Fig. 1. FTIR spectra of MIXMOFꢀMILꢀ68(In)ꢀN3 degassed at 90°C
overnight (black curve) and after reaction with 300 mbar of CO at 120°C
for 48 hours (red curve). In the pictorial scheme of the MOF walls, the
atoms are represented as spheres with the colour code: red (oxygen), grey
(carbon), white (hydrogen), blue (nitrogen) and green (indium).
40
The IR spectra of MIXMOFꢀMILꢀ68(In)ꢀN3 samples before and
after reaction with CO are reported in Figure 1a (black and red
curve, respectively). The spectrum of activated MIXMOFꢀMILꢀ
68(In)ꢀN3 was characterised by a sharp IR absorption at 3662 cmꢀ
These signals can be associated with the formation
of
isocyanates as seen for the spectrum obtained for MIXMOFꢀ
85 MILꢀ53(Al)ꢀNCO (80:20 ratio between BDC and BDCꢀNH2,
orange curve) following the diphosgene route.22 In the
isocyanateꢀmodified materials (red and orange curves) the signals
related to amino groups (filled circles) are observed due to side
1
45 associated with the structural isolated hydroxyls, an intense
band at 2118 cmꢀ1 due to the stretching of azide groups and a
complex set of signals below 1790 cmꢀ1 mainly associated with
2
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reactions. Their weak intensity confirms the very low
concentration of these species, in agreement with FTIR results.
XRPD measurements indicated that the reaction with CO did not
affect the MIXMOFꢀMILꢀ68(In) pattern (Figure S2). These
findings were further confirmed by nitrogen adsorption
measurements that demonstrate that the MOF pore structure is
not modified after the reaction with CO (Figure S1 and Table S1).
In conclusion, this new twoꢀstep synthesis extends the possibility
of postꢀfunctionalisation with isocyanate groups to a higher
10 number of MOFs than was achievable with the diphosgene route,
with these MOFs representing intermediates for the design of
diverse sophisticated and functional porous MOFs. Moreover, the
thermal activation of –N3 groups in MOF materials, by avoiding
the use of solvents, offers the possibility of further extending the
15 nitrene chemistry.29ꢀ31
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Graphical abstract
Green route to isocyanate MOFs: a diphosgene-free method is here described to convert an
amino-MOF into an isocyanate one.