J. Chen, et al.
InorganicChemistryCommunications107(2019)107448
around 78.3% per unit cell (16,767.0 Å3 in total) [18].
and their activity in asymmetric catalysis has been reported by only a
few groups [9–15]. One possible approach to obtain chiral proline-
ordinated H2O molecule or formate with pyridine-containing proline or
proline derivatives [9,10] at metal centers in MOF. An alternative ap-
polycarboxylate skeleton via amide bond before the formation of fra-
mework. For example, Telfer et al. [12] reported a proline functiona-
carboxylate, deprotection of the proline can be triggered by heating to
generate IRMOF-Pro. However, racemization may occur when depro-
tection of the thermolabile Boc-group at a high temperature over
100 °C. Kaskel et al. [13] reported a highly porous chiral MOF (DUT-32-
then catalytic activity of deprotected DUT-32-NHPro was examined in
asymmetric aldol addition between 4-nitrobenzaldehyde and cyclo-
hexanone. Experiments were carried out to identify the temperature
range in this article, where racemization may occur when deprotection
of the thermolabile Boc-group. The results showed only low variations
in the ee-value at 100 °C and 120 °C after defined treatment times, while
the obviously decrease of the ee-value after 1 day or longer at tem-
peratures of 140 °C. Furthermore, the same group [14] also reported
proline functionalized Zr-MOFs (UiO-67 and UiO-68) by pre-synthetic
modification and achieved reversed diastereoselectivity in aldol addi-
tion reactions. Astonishingly, situ composition of the thermal labile Boc
group occurred during the synthesis of Zr-MOFs, attributing to the hard
Lewis-acidity of Zr(IV).
Solvent molecules are extremely disordered, processed by the
SQUEEZE method [19]. Unfortunately, due to the high symmetry and
disorder in the pores of the MOF, the L-proline groups could not be
given the fixed position from the diffraction data. 1H NMR studies of the
PCN-261-NHPro after digestion in DCl and DMSO‑d6 indicated the ex-
istence of the proline groups at the linker portion. Furthermore, IR
analysis was further carried out to prove the existence of proline units
in the framework of PCN-261-NHPro (Figs. S5 and S6). Both observa-
tions confirmed that the chiral groups were successfully incorporated in
the PCN-261-NHPro. Reasonably, the Proline group was appended to
the each H3tcpb-NHPro in the MOF by using Materials Studio 5.0 [20].
Further the geometry optimization was performed in the same program
using a universal force field. Only the rhombohedron shape pores with
the dimension of 13 × 8 Å appended the L-proline, which was based on
the signal of proline electron cloud density (Fig. 1C).
NHPro indicated the phrase purity of its bulky sample, where the dif-
fraction pattern matched the pattern of predicted from the single-
crystal structure. PCN-261-NHPro was exchanged with di-
chloromethane (DCM), followed by drying under vacuum at room
temperature to get a solvent free sample. The results of the powder X-
ray diffraction pattern confirmed that solvent-free PCN-261-NHPro re-
tained the original skeleton with slightly collapsing (Fig. S7). Thermo
gravimetric analysis (TGA) of PCN-261-NHPro showed 45% significant
weight loss by heating from room temperature to 120 °C in the N2 flow,
which was attributed to the loss of solvent molecules. Plateau emerged
when a sample was in the region 120–465 °C, weight loss was not ob-
served. Framework decomposition occurred above 465 °C (Fig. S8).
The nitrogen sorption isotherm for activated PCN-261-NHPro at
77 K shows a behavior of H4-type hysteresis [21] (Fig. S9). The Bru-
nauer–Emmett–Teller surface area of solvent-free PCN-261-NHPro es-
timated by nitrogen sorption isotherms is 48.6 m2 g−1. The low ni-
trogen uptake may attribute both to partly collapsed framework and to
the high concentration of proline groups in the pores.
Ligand H3tcpb-NHPro was synthesized from 2,4,6-tribromoaniline
and 4-methoxybenzeneboronic acid (Scheme S1–S3). Fe2CoO
(CH3COO)6 was synthesized according to the literature methods [16].
H3tcpb-NHPro (12.3 mg, 0.02 mmol) and Fe2CoO(CH3COO)6 (7.5 mg,
0.014 mmol) were added with DMF (1 mL) and 0.1 mL acetic acid as
additive into a Teflon-lined stainless steel autoclave. The reaction
mixture was kept under ultrasonication until it completely dissolved.
The glass bottle was tightened and kept 90 °C for 12 h. The brown
crystals of PCN-261-NHPro were harvested after cooled to room tem-
perature.
Due to the deprotection of the proline group during the chiral linker
synthesis, the PCN-261-NHPro was directly applied to catalyze the
asymmetric aldol addition between 4-nitrobenzaldehyde and cyclo-
hexane. As expected, four different aldol products including two syn-
and two anti-adducts were obtained after the reaction, which can be
identified by 1H NMR analysis and HPLC measurement. Catalytic re-
action was also employed by using homogeneous catalysts of proline
and H3tcpb-NHPro as comparison. As indicated in Table 1, enantios-
catalytic reaction using PCN-261-NHPro. The products catalyzed by
proline or H3tcpb-NHPro were mainly anti-adduct, which was in accord
with the reported results [22]. However, syn-adduct was dominated
catalytic results reported by Kaskel et al. [14]. Maybe the phenomenon
was primarily caused by the high concentration of proline groups in the
pores with relatively small pores, which could be explained by the
participation of several prolines in the transition state [23]. Fe3+ and
Co2+, acted as Lewis acids in the structure, also influenced the ste-
reoselectivity on aldol addition catalysis [15]. As far as we know, this
catalytic peculiarity of PCN-261-NHPro is the third complex except the
two L-proline functionalized complexes of UiO-68-NHPro and UiO-67-
NHPro reported by Kaskel et al.
The chiral proline-functionalized MOF crystals were synthesized
only by using the linkers with Boc-protected proline according to the
previous reports. However, pre-synthetic thermal deprotection of Boc-
proline in the MOF leads to racemization of the chiral center. In our
work, chiral linker of H3tcpb-NHPro (1-L-pyrrolidine-2-carboxamide-
2,4,6-tris(4-carboxyphenyl)benzene) was gained by deprotecting the
thermolabile Boc-group on H3tcpb-NHPro-Boc at room temperature
using trifluoroacetic acid in CH2Cl2 to avoid the undesirable racemi-
zation. The chiral MOF, PCN-261-NHPro, was synthesized via sol-
vothermal reaction of H3tcpb-NHPro with Fe2CoO(CH3COO)6 in DMF,
produced well-faced, brown, cubic crystals of PCN-261-NHPro. The
activated MOF was used as heterogeneous chiral catalyst in asymmetric
aldol reaction.
The structure of the PCN-261-NHPro was confirmed by the analysis
of synchrotron single-crystal XRD. For the whole framework structure,
PCN-261-NHPro is built on a μ3-oxo-Fe2Co basic carboxylate SBU
[Fe2CoO(COO)6] (Fig. 1A) and the tcpb ligand (Fig. 1B), which is iso-
recticular to PCN-261 [17]. In this SBU, two Fe3+ ions, one Co2+ and a
μ3-O atom are on the same plane, forming a six connected node. Each
pair of metal ions are bridged by two carboxylate groups from two
different tcpb ligands, and Fe3+ and Co2+ is coordinated to four car-
boxylate oxygen atoms from different tcpb3−, one O atom on the H2O
molecule, and one O2– ion to form an octahedron pyramid. Each Fe2Co
SBU connects to six tcpb ligands and each tcpb ligand binds three SBUs
to construct the three dimensional network. As shown in Fig. 1D, the
highly porous MOF synthesized in this connection type exhibits three
different pores with the dimensions of approximately 21 × 20, 14 × 11
and 13 × 8 Å, respectively. PLATON calculations indicate that the pore
volume of PCN-261-NHPro is 13,131.4 Å3, corresponding to a void of
In summary, a proline-incorporated tricarboxylic acid was prepared
and utilized to construct a chiral metal-organic framework, PCN-261-
NHPro, which was used to catalyze the asymmetric aldol addition be-
tween 4-nitrobenzaldehyde and cyclohexane. Compared with homo-
catalytic reactions performed by L-proline and H3tcpb-NHPro, PCN-261-
NHPro, a heterogeneous catalyst, showed reversed diastereoselectivity.
2
Chen, Jianxiang
