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RSC Advances
Journal Name
DOI: 10.1039/C4RA12783H
Cite this: DOI: 10.1039/c0xx00000x
www.rsc.org/xxxxxx
ARTICLE TYPE
Proline-Functionalized Metal-Organic Frameworks and their Use in
Asymmetric Catalysis: Pitfalls in the MOFs Rush
Jerome Canivet,* and David Farrusseng
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
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DOI: 10.1039/b000000x
Post-functionalisation of Metal-Organic Frameworks is a
very efficient and elegant method for designing tailor-made
chiral solids for selective asymmetric catalysis. However,
erroneous data and misinterpretation can be easily done. We
report some best of practices in amino acid grafting and use.
occur under these conditions. Indeed, we extensively studied the
solidꢀphase peptide coupling applied in MOFs and we published
in 2011 the first report on aminoꢀacid functionalized Metalꢀ
45
Organic
Frameworks
through
covalent
postꢀsynthetic
2
modification. In this study, we demonstrate that, as described
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and comprehensively studied by B. Merrifield on resins,
the
RSC Advances recently published a puzzling research article by
Liu et al. entitled “Catalysis by metal–organic frameworks:
solidꢀphase peptide synthesis in MOFs requires the activation of
the carboxylic acid functions of the amino acid using a soꢀcalled
14ꢀ17
proline and gold functionalized MOFs for the aldol and threeꢀ 50 coupling agent.
This is even more crucial taking into account
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component coupling reactions”. Several key points of this article
caught our attention. The authors claim to have successfully
performed a postꢀsynthetic peptide coupling (Fig. 1) and to have
obtained with their postꢀmodified MOF one of the highest
that the amino groups on the MOF are much less nucleophilic
than their homogenous counterpart due to the coordination of 2ꢀ
aminoterephthalate to metal nodes. This low reactivity was also
highlighted by S. Cohen in his early work on postꢀsynthetic
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activity and selectivity reported for a heterogeneous catalyst in 55 modification (PSM) when he used acyl chloride or acid anhydride
the asymmetric aldol reaction. The reported method for postꢀ
synthetic modification as well as the catalysis data are not
supported by adequate scientific evidences and are in
to react with amino group in aminoꢀMOFs such as IRMOFꢀ3 and
MILꢀ53.
5, 18
Beyond strong doubts on the actual synthesis achievement, key
characterization data are missing while reported data do not
support the achievement of this IRMOFꢀ3ꢀPr(PM). In all
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, 3
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contradiction with those previously by our group
and others.
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First, the synthetic procedure described by Lilli et al. consists in
simply mixing the IRMOFꢀ3 and proline in ethanol overnight,
followed by the evaporation of the solvent to obtained their
IRMOFꢀ3ꢀPr(PM).
reference reports on covalent PSM,
liquid H NMR of the
digested MOF is used in order to assess the functionalization of
the organic ligand and also to determine the ratio of modified
linkers. Moreover N adsorption isotherms (or at least surface
2
area measurements) are always carried out for measuring the
porosity remaining after the PSM process. These two key
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characterizations, H NMR and N adsorption data, are missing in
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the article by Liu et al. As a result, the grafting yield cannot be
determined and thus cannot be used to define the catalyst loading
in the catalytic application described later. Without porous
characterization, we can assume that the proline obstructs the
pore of the IRMOFꢀ3ꢀPr(PM).
Fig. 1. Synthesis of IRMOFꢀ3–Pr(PM). Reprinted with permission from
Other data are also inconsistent about the characterization of
IRMOFꢀ3ꢀPr(PM). The powder Xꢀray diffraction pattern shows a
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0
ref. 1
The authors claimed that this IRMOFꢀ3ꢀPr(PM) is a postꢀ 75 major loss of crystallinity with disappearance and broadening of
modified IRMOFꢀ3 containing prolinamideꢀfunctionalized
terephthalate linkers (Fig. 1). However, one can easily understand
that without purification this material obviously contain
components from the reaction mixture such as, at least, remaining
the main peaks corresponding to the IRMOFꢀ3 structure whereas
the authors only note a slight change.
The authors also present puzzling infrared spectra of these
materials. When they argue for the disappearance of bands at
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ꢀ
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free proline in the pores, which can be responsible for catalytic 80 3473 and 3356 cm attributed to NꢀH stretching band of primary
activity discussed later. It can also possibly contain degradation
products from the MOF in ethanol. Beyond the apparent
simplicity of this coupling methodology, we have to point out
that, to the best of our knowledge, the amide formation between
amine, one can see on the published figure only large broadening
of the signal due to OꢀH stretching band of water with two
ꢀ
1
shoulder remaining at 3473 and 3356 cm (Figure 2). Moreover,
one would expect the appearance of only one band at 3100ꢀ3500
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the amino groups at the MOF walls and an amino acid will never 85 cm corresponding to the (CO)NꢀH stretching band of the
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Canivet, Jerome
