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A. Paul et al. / Journal of Catalysis 385 (2020) 324–337
2.2.3. Synthesis of [Zn2(1
jN;2
jO-
l-L)2(
j
O4-l4-BTC)]n.3n(DMF).2n
washing it with methanol and drying in air, the new run was initi-
(H2O) (2)
ated by addition of new (see above) amounts of reagents besides
the catalyst. After completion of each run, the products were ana-
lyzed by NMR as described above.
Zn(NO3)2ꢀ6H2O (6.6 mg, 0.022 mmol), HL (5 mg, 0.022 mmol)
and 1,3,5-benzenetricarboxylic acid (H3BTC) (5 mg, 0.022 mmol)
were placed in H2O (1 mL) and DMF (1 mL) in a sealed 8 mL glass
vessel and heated at 75ꢀC for 36 h. The white crystals of 2 which
deposited at the flask were separated by filtration, washed with
deionized water, DMF and then dried in air. 2 is insoluble in any
usual solvents. Yield: 55% (based on Zn). Anal. Calcd for C44H46N7
O17Zn2: C, 49.13; H, 4.31; N, 9.12. Found: C, 49.22; H, 4.40;
N, 9.39. Found: C, 49.22; H, 4.40; N, 9.39. FT-IR (KBr/pellet, cmꢁ1):
3384 (w, br), 1660 (w), 1624 (s), 1596 (m), 1381 (s), 1317 (w),
1244 (w), 1106 (w), 1056 (w), 1015 (m), 927 (m), 847 (m), 761 (s).
2.3.3. Room temperature sonochemical Knoevenagel condensation of
benzaldehyde and malononitrile
To
a capped round bottom Pyrex flask, benzaldehyde
(1.0 mmol), malononitrile (2.0 mmol), 1 or 2 (10.0 mmol, 1 mol%
vs. benzaldehyde) and THF (or MeOH, 2 mL) were added under
ambient conditions. The flask was immersed in the ultrasonic
(US) bath in an open atmosphere for up to 2.5 h. Then, the reaction
mixture was centrifuged and the solids removed by filtration. The
filtrate was evaporated and the residue was dissolved in CDCl3 and
analyzed by 1H NMR. Blank tests were performed in a metal-free
system and no significant amount of products was detected.
For the catalyst recycling experiments, the separated used cat-
alyst was washed with several portions of THF and dried in an oven
at 60 °C. Each cycle was initiated after the preceding one upon
addition of new typical portions of all other reagents. After comple-
tion of each run, the products were separated for analysis (see
above) and the catalyst was recovered (by filtration), washed with
several portions of THF and dried in an oven at 60 °C.
2.3. Procedures for the catalytic applications of MOFs 1 and 2
2.3.1. Solvent-free microwave-assisted oxidation of benzyl alcohol
with t-BuOOH
Benzyl alcohol (2.5 mmol), 1 or 2 (10 mmol, 0.4 mol% vs. sub-
strate) and an aqueous solution of t-BuOOH (5.0 mmol, aq. 70%)
were introduced in a cylindrical Pyrex tube that was subsequently
sealed. The tube was placed in the MW reactor and the system was
stirred and irradiated (5–20 W) at 60 to 100 °C for 0.5–2.0 h. In the
experiments with additives, 2,2,6,6-tetramethyl-piperidinyloxyl
radical (TEMPO, 2.5 mol% vs. substrate) or K2CO3 (2.5 mol% vs. sub-
strate) was added to the reaction mixture. For the assays in the
presence of a radical trap, NHPh2 in a stoichiometric amount rela-
tive to the oxidant was added to the reaction mixture. After cooling
to room temperature, 100 mL of cyclopentanone (as internal stan-
dard) and 2.0 mL of MeCN (to extract the organics from the reac-
tion mixture) were added. This mixture was stirred for ca. 5 min
and a sample (1 mL) was subsequently taken from the organic
phase and analysed by GC. Blank experiments, in the absence of
any catalyst, were performed under the studied reaction conditions
and no appreciable conversion was detected.
Catalyst re-usability in consecutive runs was tested by separat-
ing the used catalyst from the reaction mixture by centrifugation
followed by filtration of the supernatant solution, washing with
methanol and drying in the air; the new run was initiated by addi-
tion of new (see above) amounts of reagents besides the catalyst.
After completion of each run, the products were analyzed by GC
as described above.
3. Crystal structure determination
X-ray quality single crystals of 1 and 2 were immersed in cryo-
oil, mounted in a nylon loop and measured at room temperature.
Intensity data were collected using a Bruker APEX-II PHOTON
100 diffractometer with graphite monochromated Mo-K
a (k
0.71069) radiation. Data were collected using phi and omega scans
of 0.5° per frame and a full sphere of data was obtained. Cell
parameters were retrieved using Bruker SMART [50] software
and refined using Bruker SAINT [50] on all the observed reflections.
Absorption corrections were applied using SADABS [51]. Structures
were solved by direct methods by using the SIR97 program [52]
and refined with SHELX-2014/7 [53] Calculations were performed
using the WinGX System–Version 2014.1 [54]. The hydrogen
atoms attached to carbons were inserted at geometrically calcu-
lated positions and included in the refinement using the riding-
model approximation with Uiso(H) defined as 1.2Ueq of the parent
atoms; Those bound to N were found in the difference Fourier map
and included in the refinement with Uiso(H) = 1.5Ueq(N), eventu-
ally with distances restrains (as in 1). In both compounds, there are
disordered molecules that could not be modelled. PLATON/
SQUEEZE [55] was used to correct the data and potential volumes
of 1037 (1) and 4189 (2) Å3 were found with 189 and 1133 elec-
trons per unit cell worth of scattering. The electron counts may
suggest 4DMF + 3H2O molecules (total of 190 electrons) in the unit
cell of 1, and 3DMF + 2H2O molecules in the asymmetric unit of 2.
In view of the uncertainty of this calculation with DMF easily sub-
stituted by water molecules without any change in the electrons
number, such molecules were not considered in the CIF for the
final refinement. Least square refinements with anisotropic ther-
mal motion parameters for all the non-hydrogen atoms and isotro-
pic ones for the remaining atoms were employed. Crystallographic
data are summarized in Table S1 and selected bond distances and
angles are presented in Table S2 and Table S3 (ESI).
2.3.2. Room temperature nitroaldol (Henry) C-C coupling of
nitroethane and benzaldehyde
To
a capped round bottom Pyrex flask, benzaldehyde
(1.0 mmol), nitroethane (4.0 mmol), 1 or 2 (5.0 mmol, 0.5 mol%
vs. benzaldehyde) and water (or the chosen organic solvent,
2.0 mL) were added under ambient conditions. The reaction mix-
ture was kept under stirring up to 84 h. Then, it was centrifuged
to remove the solids. In the experiments in water, the organic com-
pounds were extracted from the reaction mixture with dichloro-
methane (2.5 mL) and dried over anhydrous sodium sulfate and
the solution was filtered. Subsequent evaporation of the solvent
yielded the crude product. A sample was dissolved in DMSO d6
and analyzed by 1H NMR. The yield in b-nitroethanol (relatively
to benzaldehyde) was established by means of 1H NMR spec-
troscopy [26,27,28,47,48] using 1,2-dimethoxyethane as an inter-
nal standard. For the b-nitroethanol products, the values of the
vicinal coupling constants between the
a-N–C–H and a-O–C–H
protons (J = 7–9 or 3.2–4 Hz, respectively) [49] helped in the iden-
tification of the syn or anti isomers. Blank experiments in the
absence of any metal catalyst were performed and no considerable
(up to 2%) conversion was detected.
Catalyst re-usability in consecutive runs was tested by using the
separated and used catalyst from the reaction mixture. After
4. Computational details
The full geometry optimization of all structures and transition
states was carried out at the DFT level of theory by using the
M06 functional [56] with the help of the Gaussian 09 program
package [57]. No symmetry operations were applied. The