Novel coordination polymers with (Pyrazolato)-based tectons: Catalytic activity in the peroxidative oxidation of alcohols and cyclohexane

Article abstract of DOI:10.1021/acs.cgd.5b00083

Coupling five rigid or flexible bis(pyrazolato)-based tectons with late transition metal ions allowed us to isolate 18 coordination polymers (CPs). As assessed by thermal analysis, all of them possess a remarkable thermal stability, their decomposition temperatures lying in the range of 340-500 °C. As demonstrated by N₂ adsorption measurements at 77 K, their Langmuir specific surface areas span the rather vast range of 135-1758 m²/g, in agreement with the porous or dense polymeric architectures retrieved by powder X-ray diffraction structure solution methods. Two representative families of CPs, built up with either rigid or flexible spacers, were tested as catalysts in (i) the microwave-assisted solvent-free peroxidative oxidation of alcohols by t-BuOOH, and (ii) the peroxidative oxidation of cyclohexane to cyclohexanol and cyclohexanone by H₂O₂ in acetonitrile. Those CPs bearing the rigid spacer, concurrently possessing higher specific surface areas, are more active than the corresponding ones with the flexible spacer. Moreover, the two copper(I)-containing CPs investigated exhibit the highest efficiency in both reactions, leading selectively to a maximum product yield of 92% (and TON up to 1.5 × 10³) in the oxidation of 1-phenylethanol and of 11% in the oxidation of cyclohexane, the latter value being higher than that granted by the current industrial process.

Full text of DOI:10.1021/acs.cgd.5b00083

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Article
Novel Coordination Polymers with (Pyrazolato)-based Tectons: Catalytic
Activity in the Peroxidative Oxidation of Alcohols and Cyclohexane
Ivan Timokhin, Claudio Pettinari, Fabio Marchetti, Riccardo Pettinari, Francesca Condello,
Simona Galli, Elisabete C. B. A. Alegria, Luísa M.D.R.S. Martins, and Armando J. L. Pombeiro
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b00083 • Publication Date (Web): 23 Mar 2015
Downloaded from http://pubs.acs.org on March 31, 2015
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Novel Coordination Polymers
with (Pyrazolato)ꢀbased Tectons:
Catalytic Activity in the Peroxidative Oxidation of
Alcohols and Cyclohexane
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a
*,a
b
a
Ivan Timokhin, Claudio Pettinari, Fabio Marchetti, Riccardo Pettinari,
a
*,c
d,e
Francesca Condello, Simona Galli, Elisabete C.B.A. Alegria,
d,e
*,e
Luísa M.D.R.S. Martins, Armando J.L. Pombeiro
a
School of Pharmacy, Chemistry Section, University of Camerino,
Via S. Agostino 1, 62032 Camerino, Italy.
b
School of Science and Technology, Chemistry Section, University of Camerino,
Via S. Agostino 1, 62032 Camerino, Italy.
c
Dipartimento di Scienza e Alta Tecnologia, Università dell’Insubria,
Via Valleggio 11, 22100 Como, Italy.
d
Chemical Engineering Department, ISEL,
R. Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal.
e
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa,
Av. Rovisco Pais, 1049 001 Lisboa, Portugal.
‐
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if
required according to the journal that you are submitting your paper to).
*Corresponding Authors
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ABSTRACT
Coupling five, rigid or flexible, bis(pyrazolato)ꢀbased tectons with late transition metal ions allowed
us the isolation of eighteen coordination polymers (CPs). As assessed by thermal analysis, all of
them possess a remarkable thermal stability, their decomposition temperatures lying in the range
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40ꢀ500 °C. As demonstrated by N
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adsorption measurements at 77 K, their Langmuir specific
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surface areas span the rather vast range of 135ꢀ1758 m /g, in agreement with the porous or dense
polymeric architectures retrieved by PXRD structure solution methods. Two representative families
of CPs, built up with either rigid or flexible spacers, were tested as catalysts in i) the microwaveꢀ
assisted solventꢀfree peroxidative oxidation of alcohols by tꢀBuOOH, and ii) the peroxidative
2 2
oxidation of cyclohexane to cyclohexanol and cyclohexanone by H O in acetonitrile. Those CPs
bearing the rigid spacer, concurrently possessing higher specific surface areas, are more active than
the corresponding ones with the flexible spacer. Moreover, the two copper(I)ꢀcontaining CPs
investigated exhibit the highest efficiency in both reactions, leading selectively to a maximum
3
product yield of 92% (and TON up to 1.5×10 ) in the oxidation of 1ꢀphenylethanol and of 11% in
the oxidation of cyclohexane, the latter value being higher than that granted by the current industrial
process.
KEYWORDS
Coordination polymers, pyrazoleꢀbased ligands, thermal stability, Xꢀray powder diffraction,
catalytic peroxidative oxidation, alcohols, cyclohexane.
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INTRODUCTION
Approximately 90% of the chemical processes implemented at the industrial level require the
use of catalysts: as a matter of fact, about 60% of all consumer and industrial products are obtained
by means of catalytic reactions. As such, catalysis generates annual global sales higher than 1.5
trillion dollars. Most catalytic processes are heterogeneous and rely on the use of transition metal
particles dispersed on inert supports with high specific surface area.
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In this regard, (porous) coordination polymers (CPs) may be a versatile alternative to allꢀmetal
catalysts for a variety of organic reactions. A very attractive feature of porous CPs is the evidence
that they can be used as nanoꢀreactors in a number of catalytic processes in which sizeꢀ and shapeꢀ
1
selectivity is granted by the homogeneous dimension of the pores and the decoration of their walls.
Another aspect of porous CPs possibly concurring to favor catalytic activity is the presence of
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exposed metal sites.
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The past two decades have known an incessantly growing interest for the selfꢀassembly of CPs
from (transition) metal ions and organic spacers, because of the outstanding functional properties
that CPs might possess ꢀ e.g. in the fields of nonlinear optics, photosensitivity, magnetism,
molecular recognition, gas adsorption ꢀ which are intimately related to their crystal and molecular
structures. Several advances have been made in the design and rational prediction of CPs crystal
structure. However, this predictive ability is primarily limited to CPs fabricated with rigid ligands.
4
By contrast, flexible spacers may adopt a number of different conformations, also according to the
restrictions imposed by the coordination geometry of the metal ion, thus complicating the
estimation of the final structural architecture. For example, Sun and coꢀworkers have recently
designed and synthesized a family of CPs based on a series of flexible, imidazoleꢀcontaining
5
ligands, the different conformations of which resulted in different topologies.
A key characteristic that CPs should possess for their practical application, irrespective of their
functionality, is a remarkable chemical and thermal inertness. We have a longꢀstanding experience
6
in the synthesis of rigid or flexible poly(pyrazole)ꢀbased tectons which, coupled to transition metal
ions, have typically lead to the formation of materials in which key structural topologies are coupled
7
to high thermal stability and, in specific occasions, chemical resistance to harsh acidic and basic
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conditions or boiling water.
As their poly(carboxylato)ꢀcontaining counterparts, also poly(azolato)ꢀbased CPs have been
tested in catalytic reactions. Examples of CPs containing nitrogenꢀbased ligands which are suitable
to catalyze oxidation reactions are related to the conversion of benzyl alcohol into benzaldehyde by
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means of molecular oxygen, to the synthesis of hydroxy ketones starting from silyl enolates, to
1
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the oxidation of ketones by means of molecular oxygen, and to the oxidation, performed by our
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group and by others, of cycloalkanes (cyclohexane and cyclopentane) to the corresponding
ketones and alcohols by means of H
2
2
O .
In this context, following exclusively solvothermal routes, we have coupled five, rigid or
flexible bis(pyrazole)ꢀbased spacers (Scheme 1) to late transition metal ions, isolating eighteen
novel CPs (Table 1). In the following, for all of them, details about the syntheses, the thermal
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behavior and the adsorption performances toward N at 77 K are provided. The crystal and
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molecular structures of those CPs isolated as polycrystalline powders, retrieved by powder Xꢀray
diffraction methods, are described. Finally, two representative families of CPs, one with a flexible
spacer, the other with a rigid one, have been tested as catalysts in the microwaveꢀassisted solventꢀ
free oxidation of a number of linear and cyclic alcohols with tertꢀbutyl hydroperoxide (tꢀBuOOH),
2 2
and of cyclohexane with hydrogen peroxide (H O ). The efficiency of these reactions has been
investigated. The influence of various parameters, such as reaction time, solvents, type and amount
of catalyst, temperature and presence of additives, has been evaluated. The results of this extensive
study are reported and discussed.
EXPERIMENTAL SECTION
Materials and Methods
All the chemicals and reagents employed were purchased from Sigma Aldrich Co. and used as
received without further purification. All the solvents were distilled prior to use. Melting points
were recorded with an SMP3 Stuart instrument equipped with a capillary apparatus. Elemental
1
analyses (C, H, N) were performed with a Fisons Instruments 1108 CHNSꢀO elemental analyzer. H
NMR spectra were recorded by using a Varian Mercury 400 Plus instrument operating at room
1
temperature and 400 MHz; H chemical shifts are reported in the following in parts per million
ꢀ1
based on SiMe
4
as standard. IR spectra were recorded from 4000 to 650 cm with a PerkinꢀElmer
Spectrum 100 instrument by total reflectance on a CdSe crystal. Thermogravimetric analyses
TGA) were carried out in a N stream with a PerkinꢀElmer STA 6000 simultaneous thermal
(
2
analyzer with a heating rate of 8 °C/min. The catalytic tests under microwave (MW) irradiation
were performed in a focused Anton Paar Monowave 300 microwave reactor fitted with a rotational
system and an IR temperature detector, using a 10 mL capacity reaction tube with a 13 mm internal
diameter. Gas chromatographic (GC) measurements were carried out using a FISONS Instruments
GC 8000 series gas chromatograph equipped with a FID detector and a capillary column (DBꢀ
WAX, column length: 30 m; column internal diameter: 0.32 mm) and run by the software Jascoꢀ
Borwin v.1.50. The temperature of injection was 240 °C. After the injection, the reaction
temperature was maintained at either 100 °C (oxidation of cyclohexane) or 140 °C (oxidation of
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alcohols) for 1 min, then raised, by 10 °C/min, either to 180 °C (oxidation of cyclohexane) or 220
°C (oxidation of alcohols) and held at this temperature for 1 min. Helium was used as the carrier
gas. GCꢀMS analyses were performed using a Perkin Elmer Clarus 600 C instrument equipped with
a 30 m × 0.22 mm × 25 ꢁm BPX5 (SGE) capillary column, using helium as the carrier gas. The
inductively coupled plasma (ICP) analyses were carried out by the Analytical Services of the
Instituto Superior Técnico, Lisboa, Portugal.
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Synthesis of the Ligands. Bis(3,5ꢀdimethylꢀ4ꢀpyrazolyl)methane (H
2
L1), bis(3ꢀmethylꢀ4ꢀ
pyrazolyl)methane (H L2) and bis(4ꢀpyrazolyl)methane (H L3) (Scheme 1) were synthesized
2
2
1
4
according to the method originally reported by Trofimenko. The white solids thus obtained were
1
further recrystallized from boiling water and characterized by elemental analysis, IR, H NMR, and
2
measurement of the melting point. 1,4ꢀBisꢀ4’ꢀ(3’,5’ꢀdimethyl)ꢀpyrazolylbenzene (H L4) and 1,4ꢀ
bisꢀ4’ꢀ(3’,5’ꢀdimethylꢀpyrazolyl)biphenyl (H L5) (Scheme 1) were synthesized according to a
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general procedure already reported in the literature, i.e. by condensation of the proper bis(diketone)
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with hydrazine hydrate in water. Bis(diketones) were prepared following the procedure reported
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by Ramirez in 1967, involving the use of freshly synthesized oxyphosphoranes. The reader is
addressed to the Supplementary Information for all the details upon the syntheses of each ligand.
M M
Synthesis of the CPs. CPs 1 ꢀ5 (Table 1) could be successfully synthesized by adopting the
two different procedures reported in the following. Regardless the synthetic path followed, the
batches obtained for a given material presented analytical data and specific surface areas in very
close agreement.
Low temperature synthetic path. A solution of the metal salt (nitrate or acetate) (0.25 mmol) in
methanol (5 mL) was added to a solution of the bis(pyrazole)ꢀbased ligand (0.25 mmol) in
dimethylformamide (DMF) (5 mL). The resulting clear solution was poured into a highꢀpressure
tube equipped with a magnetic stirring bar. The tube was heated at 140 °C for 2 days under stirring.
Then, it was cooled down to room temperature at a cooling rate of 4 °C/h. The resulting precipitate
was filtered off, washed with DMF and CH Cl and heated to reflux in benzene for 6 h. The excess
2
2
of benzene was removed by decantation. In order to completely remove it, the wet CPs where
frozen, lyophilized in vacuum and outgassed in high vacuum (0.005 mbar) at 60 °C overnight. In
order to obtain single crystals suitable for the determination of the crystal structures, several
syntheses were performed without stirring and heating from room temperature to 140 °C at a rate of
4
°C/h; in spite of this, no crystals of adequate quality were ever obtained.
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High temperature synthetic path. A solution of the metal salt (nitrate or acetate) (0.25 mmol)
in methanol (2 mL) was added to a solution of the bis(pyrazole)ꢀbased ligand (0.25 mmol) in DMF
(
1 mL). The resulting clear solution was poured into a Teflonꢀlined steel autoclave. The reaction
mixture was heated at 200 °C for 2 days and cooled down to room temperature at a cooling rate of 4
C/h. The precipitated CPs were filtered off, washed with methanol and dried in vacuum (ca. 8
mbar), in a vacuum drying oven, at 100 °C.
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Analytical Data for CPs 1 .
[Co(L1)], 1Co. Yield: 65% of blue solid. 1Co is insoluble in all common solvents and in water.
Elem. Anal. Calcd. for C H CoN (FW = 261.18 g/mol): C, 50.58; H, 5.40; N, 21.45%. Found: C,
1
1
14
4
ꢀ1
50.33; H, 5.34; N, 21.35%. IR (cm ): 1571, 1503, 1420, 1375, 1320, 1208, 1134, 987, 813, 765,
2
2
6
60. Langmuir specific surface area: 1021 m /g. BET specific surface area: 730 m /g. Main
crystallographic details: C H CoN , tetragonal, P4 22, Z = 4, a = 9.2523(6) Å, c = 13.449(1) Å, V
11
14
4
1
3
3
ꢀ1
= 1151.3(2) Å ; D
c
= 1.51 g/cm ,
ꢀ
p
(CuKα) = 115.2 cm , F(000) = 540; R = 0.015, Rwp = 0.021 and
R
Bragg = 0.82 for 4851 data and 29 independent parameters.
2
[Cu (L1)], 1Cu. Yield: 43% of white solid. 1Cu is insoluble in all common solvents and in water.
Elem. Anal. Calcd. for C H Cu N (FW = 329.35 g/mol): C, 40.11; H, 4.28; N, 17.01%. Found:
11
14
2
4
ꢀ1
C, 39.96; H, 4.22; N, 16.93%. IR (cm ): 1505, 1426, 1372, 1342, 1216, 1152, 994, 776. Langmuir
2
2
specific surface area: 957 m /g. BET specific surface area: 531 m /g. Main crystallographic details:
11 2 4
C H14Cu N , orthorhombic, Pbca, Z = 8, a = 17.9734(4) Å, b = 17.2179(4) Å, c = 7.9530(4) Å, V =
3
3
ꢀ1
2
461.2(1) Å ; D = 1.78 g/cm ;
ꢀ
(CuKα) = 41.1 cm , F(000) = 1328; R = 0.024, R = 0.040 and
c
p
wp
R
Bragg = 3.30 for 4851 data and 46 independent parameters.
[Zn(L1)], 1Zn. Yield: 70% of white solid. 1Zn is insoluble in all common solvents and in water.
Elem. Anal. Calcd. for C11
H
14
N
4
Zn (FW = 267.65 g/mol): C, 49.36; H, 5.27; N, 20.93%. Found: C,
ꢀ1
4
9.42; H, 5.21; N, 21.02%. IR (cm ): 1571, 1506, 1424, 1377, 1330, 1211, 1150, 991, 765, 662.
2
2
Langmuir specific surface area: 543 m /g. BET specific surface area: 409 m /g.
[
Cd(L1)], 1Cd. Yield: 68% of white solid. 1Cd is insoluble in all common solvents and in water.
Elem. Anal. Calcd. for C11 (FW = 314.67 g/mol): C, 41.99; H, 4.48; N, 17.81%. Found: C,
42.04; H, 4.40; N, 17.68%. IR (cm ): 1497, 1418, 1310, 1206, 1127, 1022, 983, 801, 758, 751.
H14CdN
4
ꢀ1
2
2
Langmuir specific surface area: 135 m /g. BET specific surface area: 21 m /g. Main
crystallographic details: C11
H
4
14CdN , orthorhombic, Pcca, Z = 4, a = 16.9833(5) Å, b = 8.4671(2)
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3
ꢀ1
Å, c = 7.9321(2) Å, V = 1140.63(6) Å ; D
c
= 1.83 g/cm ;
ꢀ
(CuKα) = 151.9 cm , F(000) = 624; R
p
=
0.072, Rwp = 0.098 and RBragg = 5.62 for 4776 data and 35 independent parameters.
Analytical Data for CPs 2
[Co(L2)], 2 . Yield: 75% of blue solid. 2Co is insoluble in all common solvents and in water.
M
.
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Co
Elem. Anal. Calcd. for C
9
H
10CoN
4
(FW = 233.14 g/mol): C, 46.37; H, 4.32; N, 24.03%. Found: C,
ꢀ1
4
6.21; H, 4.25; N, 23.92%. IR (cm ): 1502, 1421, 1376, 1313, 1206, 1132, 983, 897, 765, 755.
2
2
Langmuir specific surface area: 1213 m /g. BET specific surface area: 553 m /g.
[
Cu
2
(L2)], 2Cu. Yield: 43% of white solid. 2Cu is insoluble in all common solvents and in water.
(FW = 301.29 g/mol): C, 35.88; H, 3.35; N, 18.60%. Found: C,
Elem. Anal. Calcd. for C
9 2 4
H10Cu N
ꢀ1
3
5.61; H, 3.21; N, 18.43%. IR (cm ): 1586, 1509, 1436, 1413, 1379, 1299, 1206, 1139, 1050, 1000,
2
2
8
34, 737, 669. Langmuir specific surface area: 678 m /g. BET specific surface area: 462 m /g.
[Cd(L2)], 2Cd. Yield: 80% of white solid. 2Cd is insoluble in all common solvents and in water.
Elem. Anal. Calcd. for C H CdN (FW = 286.61 g/mol): C, 37.72; H, 3.52; N, 19.55%. Found: C,
9
10
4
ꢀ1
37.56; H, 3.40; N, 19.37%. IR (cm ): 1496, 1417, 1367, 1314, 1203, 1127, 1021, 980, 801, 751,
2
2
6
69. Langmuir specific surface area: 426 m /g. BET specific surface area: 167 m /g.
Analytical Data for CPs 3M.
[Co(L3)], 3 . Yield: 78% of blue solid. 3 is insoluble in all common solvents and in water.
Co
Co
Elem. Anal. Calcd. for C
7
H
6
CoN
4
(FW = 205.09 g/mol): C, 41.00; H, 2.95; N, 27.32%. Found: C,
ꢀ1
4
0.81; H, 2.84; N, 27.43%. IR (cm ): 1558, 1441, 1397, 1361, 1282, 1151, 1059, 1009, 848, 832,
2
2
822, 747, 720. Langmuir specific surface area: 849 m /g. BET specific surface area: 576 m /g. Main
crystallographic details: C CoN , monoclinic, C2/c, Z = 4, a = 9.5731(7) Å, b = 14.383(1) Å, c =
= 117.792(4)°, V = 864.2(1) Å ; D
7
H
6
4
3
3
ꢀ1
7
.0945(5) Å,
β
c
= 1.58 g/cm ,
ꢀ
(CuKα) = 151.8 cm , F(000) =
p
412; R = 0.016, Rwp = 0.022 and RBragg = 1.16 for 4726 data and 32 independent parameters.
[
Cu(L3)], 3Cu. Yield: 64% of pale green solid. 3Cu is insoluble in all common solvents and in
water. Elem. Anal. Calcd. for C CuN (FW = 209.07 g/mol): C, 40.09; H, 2.88; N, 26.72%.
Found: C, 40.25; H, 2.79; N, 26.37%. IR (cm ): 1438, 1397, 1371, 1285, 1165, 1077, 1058, 1015,
7
H
6
4
ꢀ1
2
2
8
23, 750, 720, 668. Langmuir specific surface area: 351 m /g. BET specific surface area: 142 m /g.
Main crystallographic details: C CuN , orthorhombic, Cmcm, Z = 4, a = 8.8943(8) Å, b =
2.185(2) Å, c = 7.2837(9) Å, V = 789.4(2) Å ; D
7
H
6
4
3
3
ꢀ1
1
4
c
= 1.76 g/cm ,
ꢀ
(CuKα) = 34.5 cm , F(000) =
20; R = 0.016, R = 0.021 and RBragg = 1.33 for 4726 data and 27 independent parameters.
p
wp
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[Zn(L3)], 3Zn. Yield: 78% of white solid. 3Zn is insoluble in all common solvents and in water.
7 6 4
Elem. Anal. Calcd. for C H N Zn (FW = 211.54 g/mol): C, 39.74; H, 2.86; N, 26.49%. Found: C,
ꢀ1
3
9.85; H, 2.80; N, 26.31%. IR (cm ): 1561, 1398, 1371, 1285, 1158, 1072, 1012, 852, 835, 750,
2
2
718. Langmuir specific surface area: 347 m /g. BET specific surface area: 203 m /g. Main
crystallographic details: C Zn, monoclinic, C2/c, Z = 4, a = 9.5706(7) Å, b = 14.558(1) Å, c =
.1107(6) Å, = 118.259(4)°, V = 872.6(1) Å ; D
7
6 4
H N
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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9
0
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0
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9
0
1
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5
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7
8
9
0
3
3
ꢀ1
7
β
c
= 1.61 g/cm ,
ꢀ
(CuKα) = 35.0 cm , F(000) =
424; R = 0.058, R = 0.075 and RBragg = 1.90 for 4751 data and 36 independent parameters.
p
wp
[
Cd(L3)], 3Cd. Yield: 80% of white solid. 3Cd is insoluble in all common solvents and in water.
Elem. Anal. Calcd. for C CdN (FW = 258.56 g/mol): C, 32.52; H, 2.34; N, 21.67%. Found: C,
32.37; H, 2.41; N, 21.54%. IR (cm ): 1650, 1390, 1359, 1278, 1157, 1061, 1016, 992, 850, 831,
7
H
6
4
ꢀ1
2
2
7
47. Langmuir specific surface area: 1150 m /g. BET specific surface area: 911 m /g. Main
crystallographic details: C14
c = 9.3099(7) Å,
H
12Cd
2
N
8
1
, monoclinic, P2 /a, Z = 4, a = 14.775(1) Å, b = 15.246(1) Å,
3
3
ꢀ1
β
= 79.504(6)°, V = 2062.0(3) Å ; D
c
= 1.67 g/cm ,
ꢀ
(CuKα) = 166.7 cm , F(000)
=
992; R = 0.051, R = 0.067 and RBragg = 3.68 for 4851 data and 58 independent parameters.
p wp
M
Analytical Data for CPs 4 .
[Co(L4)], 4Co. Yield: 80% of blue solid. 4Co is insoluble in all common solvents and in water.
Elem. Anal. Calcd. for C16 (FW = 323.26 g/mol): C, 59.45; H, 4.99; N, 17.33%. Found: C,
9.28; H, 5.08; N, 17.27%. IR (cm ): 1573, 1491, 1423, 1377, 1317, 1283, 1122, 1046, 1012, 846,
H16CoN
4
ꢀ1
5
2
2
788. Langmuir specific surface area: 1573 m /g. BET specific surface area: 791 m /g. Main
crystallographic details: C16
H
4
16CoN , tetragonal, P4
2
/nnm, Z = 4, a = 18.6019(6) Å, c = 7.2745(5)
3
3
ꢀ1
Å, V = 2517.2(2) Å ; D
c
= 0.85 g/cm ,
ꢀ
(CuKα) = 54.0 cm , F(000) = 668; R
p
= 0.036, Rwp = 0.059
and RBragg = 3.97 for 4951 data and 44 independent parameters.
[
Cu
Elem. Anal. Calcd. for C16
C, 49.01; H, 4.15; N, 14.24%. IR (cm ): 1559, 1497, 1425, 1372, 1344, 1291, 1053, 1014, 845,
2
(L4)], 4Cu. Yield: 50% of white solid. 4Cu is insoluble in all common solvents and in water.
H
2 4
16Cu N
(FW = 391.42 g/mol): C, 49.10; H, 4.12; N, 14.31%. Found:
ꢀ1
2
2
6
67. Langmuir specific surface area: 1758 m /g. BET specific surface area: 939 m /g.
[Zn(L4)], 4Zn. Yield: 64% of white solid. 4Zn is insoluble in all common solvents and in water.
Elem. Anal. Calcd. for C16 Zn (FW = 329.72 g/mol): C, 58.28; H, 4.89; N, 16.99%. Found: C,
16 4
H N
ꢀ1
5
8.14; H, 4.74; N, 16.85%. IR (cm ): 1568, 1495, 1430, 1379, 1341, 1285, 1167, 1113, 1048, 1013,
2
2
8
46, 815, 665. Langmuir specific surface area: 1028 m /g. BET specific surface area: 647 m /g.
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[Cd(L4)], 4Cd. Yield: 75% of white solid. 4Cd is insoluble in all common solvents and in water.
Elem. Anal. Calcd. for C16
H16CdN
4
(FW = 376.74 g/mol): C, 51.01; H, 4.28; N, 14.87%. Found: C,
ꢀ1
5
0.88; H, 4.31; N, 14.76%. IR (cm ): 1556, 1492, 1417, 1316, 1282, 1112, 1044, 1012, 860, 843,
2
2
782. Langmuir specific surface area: 1549 m /g. BET specific surface area: 508 m /g.
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Analytical Data for CPs 5
M
.
[
Co(L5)], 5 . Yield: 80% of blue solid. 5 is insoluble in all common solvents and in water.
Co Co
Elem. Anal. Calcd. for C22
H20CoN
4
(FW = 399.35 g/mol): C, 66.17; H, 5.05; N, 14.03%. Found: C,
ꢀ1
6
6.02; H, 4.94; N, 13.97%. IR (cm ): 1570, 1492, 1423, 1377, 1316, 1284, 1260, 1045, 1012, 845,
2
2
8
07, 678. Langmuir specific surface area: 449 m /g. BET specific surface area: 376 m /g.
[
Zn(L5)], 5Zn. Yield: 72% of white solid. 5Zn is insoluble in all common solvents and in water.
Elem. Anal. Calcd. for C22 Zn (FW = 405.81 g/mol): C, 65.11; H, 4.97; N, 13.81%. Found: C,
5.04; H, 4.83; N, 13.76%. IR (cm ): 1551, 1490, 1426, 1324, 1284, 1048, 1011, 824, 756, 672.
20 4
H N
ꢀ1
6
2
2
Langmuir specific surface area: 329 m /g. BET specific surface area: 133 m /g.
[Cd(L5)], 5Cd. Yield: 70% of white solid. 5Cd is insoluble in all common solvents and in water.
Elem. Anal. Calcd. for C H CdN (FW = 452.84 g/mol): C, 58.35; H, 4.45; N, 12.37%. Found: C,
2
2
20
4
ꢀ
1
5
8.32; H, 4.51; N, 12.24%. IR (cm ): 1593, 1553, 1420, 1377, 1315, 1282, 1118, 1047, 1010, 823,
2
2
7
89, 670. Langmuir specific surface area: 510 m /g. BET specific surface area: 427 m /g. Main
1
8
4
crystallographic details: C22H22CdN O, orthorhombic, Pcc2, Z = 2, a = 15.2159(4) Å, b =
3
3
ꢀ1
9.0611(3) Å, c = 7.7533(2) Å, V = 1068.98(6) Å ; D
c
= 1.41 g/cm ,
ꢀ
(CuKα) = 84.1 cm , F(000) =
3
p
86; R = 0.064, Rwp = 0.085 and RBragg = 3.77 for 5001 data and 27 independent parameters.
Powder Xꢀray Diffraction Crystal Structures Determination. Polycrystalline samples of
1
9
M
compounds 1Co, 1Cu, 1Cd, 3 , 4Co and 5Cd were gently ground in an agate mortar. Then, they were
deposited in the hollow of an aluminum sample holder equipped with a zeroꢀbackground plate.
Diffraction data were collected at room temperature by means of overnight scans in the 2θ range 5ꢀ
105°, with steps of 0.02°, on a Bruker AXS D8 Advance diffractometer, equipped with Niꢀfiltered
CuꢀKα radiation (λ = 1.5418 Å), a Lynxeye linear positionꢀsensitive detector, and the following
optics: primary beam Soller slits (2.3°), fixed divergence slit (0.5°), receiving slit (8 mm). The
generator was set at 40 kV and 40 mA. A standard peak search, followed by indexing through the
2
0
21
Singular Value Decomposition approach implemented in TOPASꢀR, allowed to retrieve the
approximate unit cell parameters of the nine species. The space groups were assigned on the basis
of systematic absences. Structure solutions were performed by the simulated annealing technique,
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implemented in TOPASꢀR, employing a rigid, idealized model for the crystallographic independent
2
2
portion of the ligands. When viable, the torsion angles around the exocyclic CꢀC bond were let
refine. The final refinements were carried out by the Rietveld method, maintaining the idealized
models introduced at the solution stage. The peak shapes were described with the fundamental
2
3
parameters approach. When necessary, peak shape anisotropy was modeled with the aid of
spherical harmonics. The background was modeled by a polynomial function. One, refined,
0
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M
isotropic thermal parameter (Biso) was assigned to the metal atoms (B ), lighter atoms being given a
2
Biso = B
M
+ 2.0 Å value. The presence of preferred orientation was taken into account by adopting
2
4
the MarchꢀDollase model for 1Cd (along [010]). The final Rietveld refinement plots are
collectively supplied in Figures S1ꢀS3 of the Supporting Information. Fractional atomic coordinates
are provided in the Supporting Information as CIF files. Xꢀray crystallographic data in CIF format
have been deposited at the Cambridge Crystallographic Data Centre as supplementary publications
no.s 1033388ꢀ1033396. Copies of the data can be obtained free of charge on application to the
Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: +44ꢀ1223ꢀ335033; eꢀmail:
deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
2
Activation of CPs by Supercritical Drying. Supercritical CO drying was performed using a
Polaron Critical Point Dryer. In a typical experiment, the solvent molecules clathrated in a 50ꢀ100
mg sample of as synthesized CP were exchanged with anhydrous ethanol for a period of 1 to 3 days,
refreshing the solution every day. The solventꢀexchanged sample was separated from the excess
ethanol on the glass frit (but not allowed to dry on the filter) and transferred to the Critical Point
Dryer. The latter was flushed with CO
chamber was cooled down to 7ꢀ9 °C in order to make CO
chamber to approximately 60%, the inlet valve was closed and the sample was left to soak into
liquid CO for 10 h, purging liquid CO and refilling the chamber every 2 h. Finally, the drying
2
for 1 min, then the venting valve was closed and the
2
condensing inside. After filling the
2
2
chamber was heated to 45 °C for 2 h and the supercritical fluid was slowly released. After the
supercritical drying, the sample was outgassed at 50 °C in vacuum (0.005 mbar) for 10 h.
Gas Adsorption Measurements. Specific surface areas of the eighteen CPs were determined
2
by N adsorption at 77 K. Adsorption isotherms were acquired by the volumetric method in liquid
nitrogen baths using UHPꢀgrade nitrogen (ALPHAGAZ™ 1 NITROGEN, 99.999% purity), with a
Beckman Coulter SA3100 instrument. Ultraꢀhigh purity He (ALPHAGAZ™ 1 HELIUM, 99.999%
purity) was used for freeꢀspace measurements. After supercritical drying, ca. 50 mg of CP were
introduced into preꢀweighed analysis tubes and outgassed overnight at 50 °C under high vacuum
ꢀ3
(
10 mbar) before carrying out the adsorption experiments. BrunauerꢀEmmettꢀTeller (BET) specific
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surface areas were determined by fitting the lowꢀpressure zone of the N adsorption isotherms by
2
25
2
the BET equation and considering the molecular area of N at 77 K to be 0.162 nm . Langmuir
specific surface areas were determined by the Langmuir equation and considering the molecular
2
area of N
2
at 77 K to be 0.162 nm .
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Catalytic Tests.
Microwave-assisted solvent-free peroxidative oxidation of alcohols. Oxidation reactions of
alcohols were carried out in sealed cylindrical Pyrex tubes under focused microwave irradiation
(
see Materials and Methods Section), by adopting the following procedure: the alcohol (2.5 mmol),
the catalyst (1ꢀ15 ꢀmol) and a 70% aqueous solution of tꢀBuOOH (5 mmol) were introduced into
the tube. This was then placed into the microwave reactor and the system was left under stirring and
irradiation (5 or 20 W), at 80 ºC or 120 ºC, respectively, for 0.08ꢀ15 h. After cooling down to room
temperature, 150 ꢀL of benzaldehyde, adopted as internal standard, were added together with 2.5
mL of acetonitrile, to extract the substrate and the organic products from the reaction mixture. The
obtained mixture was stirred for 10 min; then, an aliquot (1 ꢀL) was taken from the organic phase
and analyzed by GC using the internal standard method. Blank experiments, in the absence of any
catalyst, were performed under the studied reaction conditions for different alcohols: no significant
conversion was observed.
Peroxidative oxidation of cyclohexane with aqueous H
oxidation of cyclohexane was carried out by adopting the following procedure: the catalyst (0.2ꢀ10
mol) was dissolved in MeCN (3.00 mL) under vigorous stirring; thereafter, cyclohexane (0.54 mL,
.00 mmol) and 30% H (1.02 mL, 10.00 mmol) were added. The resulting solution was stirred
for 24 h at room temperature and 1 atm. In the experiments carried out in the presence of radical
traps, either CBrCl (5.00 mmol) or NHPh (5.00 mmol) were added to the reaction mixture. The
2 2
O in acetonitrile. The peroxidative
ꢀ
5
2 2
O
3
2
products analysis was performed as follows: 90 ꢀL of cycloheptanone were added as internal
standard together with 10.00 mL of diethyl ether, to extract the substrate and the organic products
from the reaction mixture, together with an excess of triphenylphosphine, in order to reduce the
formed cyclohexyl hydroperoxide to the corresponding alcohol, and hydrogen peroxide to water,
2
6
following a method developed by Shul’pin. The obtained mixture was stirred for 10 min; then an
aliquot (1 ꢀL) was taken from the organic phase and analyzed by GC using the internal standard
2 2
method. Blank experiments were performed with different amounts of H O and other reagents and
confirmed that no product of cyclohexane oxidation was obtained unless the CPs were present.
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RESULTS AND DISCUSSION
ꢀ5
Synthesis of Species 1
nitrates or acetates with the bis(pyrazole)ꢀbased ligands H
formation of eighteen CPs of formula [M (L)] (x = 2 for 1Cu, 2Cu, and 4Cu; x = 1 in all the other
cases – Table 1). Remarkably, the reactions were successful not only when carried out at 200 °C
high temperature synthetic protocol – see the Experimental Section), but also in the milder
M
M
. The solvothermal reactions of Co(II), Cu(II), Zn(II), and Cd(II)
2
L (Scheme 1) brought about the
x
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
conditions of 140 °C (low temperature synthetic protocol – see the Experimental Section):
regardless the synthetic path adopted, the different batches obtained for a given CP presented
analytical data and specific surface areas in very close agreement.
All the CPs were isolated in reasonable to high yields in the form of airꢀstable powders,
insoluble in the most common solvents and in water, this evidence suggesting their polymeric
nature. As a matter of fact, the absence of N−H stretching bands in their IR spectra indicates the
formation of metalꢀbridging dianions.
For some of the copperꢀcontaining derivatives (1Cu, 2Cu, and 4Cu), the highꢀtemperature
reaction between copper(II) acetate and H L brought about the reduction of Cu(II) to Cu(I). Indeed,
2
the Cu(II)ꢀcontaining green suspension appeared at the beginning of the reaction gradually
transformed into a Cu(I)ꢀbased white precipitate. Reduction of Cu(II)ꢀbased pyrazoleꢀcontaining
1
i
27
complexes promoted by heating has been already observed by us and other authors and attributed
2
8
to the partial oxidation of pyrazole. In the present case, the isolation of 1Cu, 2Cu and 4Cu might
have been accompanied by the oxidation of either the bis(pyrazolato)ꢀbased ligand or the methanol
2
9
employed as solvent.
PXRD Structural Analysis. For all polycrystalline CPs for which a structure determination
19
was feasible, a brief description of the crystal structure is reported in the following.
Crystal and molecular structure of 1Co. Compound 1Co crystallizes in the tetragonal space
2
ꢀ
1
group P4 22. The asymmetric unit is composed by one Co(II) ion and one L1 ligand, both lying on
crystallographic twofold axes. Within a tetrahedral coordination sphere, the Co(II) ions are
coordinated by four nitrogen atoms (Figure 1a) and are maintained about 3.5 Å apart, by bridging
1
pyrazolato moieties, along 1ꢀD helical chains (Figure 1b) winding up along the 4 screw axes
parallel to the crystallographic axis c. By means of the spacers, each chain is connected, along the
crystallographic axes a and b, to four nearby ones. This results into the formation of a bimodal
(
4,4)ꢀconnected 3ꢀD network (Figure 1c) in which the metal centers can be assimilated to
tetrahedral tetraꢀconnected nodes, while the ligands can be visualized as square planar tetraꢀ
connected nodes. The network possesses a negligible percentage of empty volume (2.5% of the unit
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cell volume, corresponding to a pore volume of only 0.02 cm /g). This feature is in contrast with
2
the rather high Langmuir specific surface area estimated for this species from the N adsorption
2
isotherm (1021 m /g, see below); we suggest that either framework flexibility, allowed by the
flexible spacer, is at work during the adsorption process, or a transition phase takes place when the
3
1
material is brought at low temperature.
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Crystal and molecular structure of 1 . Compound 1 crystallizes in the orthorhombic space
Cu
Cu
2
ꢀ
group Pbca. The asymmetric unit is composed by two metal ions and one L1 spacer, all lying on
general positions. Both Cu(I) centers are biꢀcoordinated and show a slightly bent stereochemistry
(
Figure 2a). Bridged by pyrazolato moieties, they alternate, at a distance of about 3.1 Å, along 1ꢀD
chains (Figure 2b) winding up parallel to the crystallographic axis c. Each chain is bridged to two
nearby ones by the ligands, which results into the formation of 2ꢀD corrugated slabs parallel to the
ac plane and stacking along b (Figure 2c). Overall, by assimilating the ligands to square planar
tetraꢀconnected nodes and disregarding the metal ions, which are biꢀconnected, 1Cu can be classified
as a uninodal 4ꢀconnected 2ꢀD layer with sql topology. The methyl groups protrude outside the
slabs occupying the interꢀslab volume, so that the crystal structure features no empty volume.
Again, to explain the observed adsorption performances (see below), either framework flexibility or
31
a low temperature phase transition could be invoked.
Crystal and molecular structure of 1Cd. Compound 1Cd crystallizes in the orthorhombic space
2
ꢀ
group Pcca. The asymmetric unit is composed by one metal ion and one L1 ligand, both lying on a
crystallographic twofold axis. Tetrahedral CdN nodes (Figure 3a) are kept about 3.9 Å apart (c/2)
4
by the spacers along 1ꢀD linear chains running parallel to the crystallographic axis c (Figure 3b).
Each chain is bridged, along the crystallographic axis a, to two neighboring ones ca. 8.5 Å distant
(
a/2). As a result, the overall structural motif consists of 2ꢀD corrugated slabs parallel to the ac
plane and stacking, eclipsed, along b (Figure 3c). As the spacers are not planar, 1ꢀD rhombic
channels parallel to c and hosting the methyl groups are present within the slabs. Overall, the crystal
structure does not feature void volume, in agreement with the negligible specific surface area found.
As it will be evident below, CPs 3Co, 3Cu and 3Zn possess an analogous structural motif: the “bent”
ligand thus acts as a structure directing agent. Overall, by assimilating the ligands to square planar
tetraꢀconnected nodes and the metal ions to tetrahedral tetraꢀconnected nodes, 1Cu can be classified
as a binodal (4,4)ꢀconnected 2ꢀD layer with sql topology.
Crystal and molecular structures of 3Co, 3Cu and 3Zn. In spite of the fact that they crystallize in
different space groups, the Co(II), Cu(II) and Zn(II) derivatives 3 , 3Cu and 3Zn possess the same
Co
2
ꢀD structural motif. Hence, in the following, their crystal and molecular structures will be
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5
6
described jointly. Compound 3Cu crystallizes in the orthorhombic space group Cmcm. Its
asymmetric unit is composed by one metal ion, lying on a crystallographic inversion centre, and one
2
ꢀ
L3 ligand, lying about a crystallographic mirror plane. On the other hand, the isomorphous and
isostructural species 3Co and 3Zn crystallize in the monoclinic space group C2/c (a proper subgroup
2
ꢀ
of Cmcm). Their asymmetric unit contains one metal ion and one L3 ligand, both lying about a
0
1
2
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crystallographic twofold axis. In all of the three species, the metal ions are tetraꢀcoordinated by four
nitrogen atoms of four different pyrazolato rings (Figures S4a and 4a, for and 3Co and 3Cu
,
respectively). Obviously, due to their specific stereochemical requirements, the Co(II) and Zn(II)
ions show a tetrahedral geometry, while the Cu(II) ones possess a square planar coordination
sphere. With one of its pyrazolato moieties, each ligand bridges metal ions about 3.6 Å (c/2) apart.
This leads to the formation of 1ꢀD zigꢀzag (3Co and 3Zn) or linear (3Cu) chains running along the
crystallographic axis c (Figures S4b and 4b). Each ligand employs the second pyrazolato ring to
bridge, along the crystallographic axis a, one nearby chain 8.5ꢀ8.9 Å (a/2) distant. Overall, each
chain is bridged to two neighboring ones, this bringing about the formation of 2ꢀD corrugated slabs
parallel to the ac plane (Figures S4c and 4c). The slabs pile, staggered, along b (Figure S4c and 4c),
with no evidence of interꢀslab non bonding interactions. Again, due to the fact that the spacers are
bent, 1ꢀD rhombic channels running along c are present within the slabs (Figures S4c and 4c). On
the whole, by considering both the ligands and the metal centers as tetraꢀconnected nodes, 3Co, 3Cu
and 3Zn are binodal (4,4)ꢀconnected 2ꢀD layers with sql topology. Differently from 1Cu, in which
methyl groups protrude within the channels, an empty volume of 6.2, 10.0 and 5.9% of the unit cell
30
volume can be estimated in 3Co, 3Cu and 3Zn, respectively, corresponding to a pore volume of
3
0.04, 0.06 and 0.04 cm /g.
Crystal and molecular structure of 3Cd. Compound 3Cd does not share the same structural
motif as 3Co and 3Zn, assembled with the same ligand and with metal centers having the same
stereochemical preferences, nor as 1Cd, in which the two pyrazolato rings of the spacer are bisꢀ
methylated. 3Cd crystallizes in the monoclinic space group P2 /a. Its asymmetric unit is composed
1
2
ꢀ
by two metal ions and two L3 ligands, all lying on general positions. The Cd(II) ions are tetraꢀ
coordinated by four nitrogen atoms of four different pyrazolato moieties and show, as expected, a
tetrahedral coordination sphere (Figures 5a). With one of its pyrazolato rings, each ligand bridges
metal centers about 3.7ꢀ3.9 Å apart. This leads to the formation of 1ꢀD zigꢀzag chains running along
the crystallographic axis a (Figures 5b). Each ligand employs the second pyrazolato ring to bridge
one nearby chain 8.5ꢀ9.5 Å distant. At variance with the other 3 species, in the present case each
M
chain is bridged to four neighboring ones, this implying the formation of a 3ꢀD porous network
30
(Figure 5c) possessing 25.1% of empty volume per unit cell, corresponding to a pore volume of
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3
0.30 cm /g. Both independent metal centers can be visualized as tetrahedral tetraꢀconnected nodes,
while both independent ligands can be visualized as square planar tetraꢀconnected nodes. Hence,
3
Cd can be classified as a twoꢀfold interpenetrated (4,4)ꢀconnected 3ꢀD network with pts topology.
Crystal and molecular structure of 4Co. A DMF solvate of 4Co, labeled MFUꢀ2, was recently
10
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32
published by Tonigold and colleagues. Apart from the symmetry decrease imposed by the
presence of solvent molecules within the channels of MFUꢀ2, the latter and 4Co share the same
structural motif which, for the sake of completeness, will be described hereafter. Compound 4Co
2
crystallizes in the tetragonal space group P4 /nnm. The asymmetric unit contains one Co(II) ion and
2
ꢀ
one L4 spacer, both lying on special positions. Tetrahedral CoN nodes (Figure 6a) are kept 3.6 Å
4
(c/2) apart by the pyrazolato moieties along linear chains running parallel to c (Figure 6b). The
chains are connected along [110] to four neighboring ones within a 3ꢀD porous framework (Figure
6
c). 1ꢀD square channels, hosting cylinders 10 Å wide, run along c and account for a void volume
30
3
as high as 56.6% of the unit cell volume, resulting in a pore dimension of 0.66 cm /g. Apart from
the symmetry modifications imparted by either the functional groups on the spacer or the solvent
molecules trapped in the channels, 4Co shares the same structural motif as a number of porous CPs
3
3
obtained, by us and others, with spacers having the same skeleton ([Co(BDP)] and [Zn(BDP_X)],
3
4
35
H
2
BDP = 1,4ꢀbis(1Hꢀpyrazolꢀ4ꢀyl)benzene, X = H, NH
2
, NO
2
, OH ), or with shorter linkers
7
c
7d
(
[Zn(BPZ)], H
2
BPZ = 4,4′ꢀbipyrazole, and [Zn(Me
4
BPZ)],
H
2
Me BPZ = 3,3′,5,5′ꢀtetramethylꢀ
4
4,4′ꢀbipyrazole). As such, in all these species the metal centers can be visualized as tetrahedral,
tetraꢀconnected nodes, while the ligands can be assimilated to square planar tetraꢀconnected nodes.
This results into the formation of a (4,4)ꢀconnected 3ꢀD network with pts topology.
Crystal and molecular structure of 5Cd. Compound 5Cd crystallizes in the orthorhombic space
2ꢀ
group Pcc2. The asymmetric unit is composed by one metal ion and one L5 ligand, both lying on a
4
twofold axis. The crystal structure features tetrahedral nodes of the kind CdN (Figure 7a) bridged,
2
ꢀ
by the L5 spacers, along [001] and [110]. As a result, 1ꢀD chains of collinear metal ions run along
the crystallographic axis c (Figure 7b). Each chain is connected by the linkers to four nearby ones,
with the consequent formation of a 3ꢀD network featuring ellipsoidal and slitꢀlike channels running
18
parallel to c and hosting the methyl groups and clathrated water molecules. In spite of the steric
30
hindrance of the protruding methyl groups, an empty volume of 11.9% of the unit cell volume,
3
corresponding to a pore dimension of 0.08 cm /g, can be estimated. Finally, assimilating the metal
centers to tetrahedral tetraꢀconnected nodes, and the ligands to square planar tetraꢀconnected nodes,
5Cd can be classified as a (4,4)ꢀconnected 3ꢀD network with pts topology.
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Thermal Behavior. Thermogravimetric analyses (TGA) were performed on CPs 1
M
−5
M
in
order to investigate their thermal behavior. The TG traces acquired on species 1
M
and 4
M
are
displayed as examples in Figure 8a and 8b, respectively. The other TG traces are reported in Figures
S5ꢀS7 of the Supplementary Information.
Overall, the investigated CPs possess a remarkable thermal robustness: the decomposition
process of the less stable species begins at 340 °C, while the highest stability materials survive up to
500 °C. More in detail, species 1Co and 1Zn (Figure 8a) are stable up to about 400 °C, temperature at
0
1
2
3
4
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4
5
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7
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9
0
1
2
3
4
5
6
7
8
9
0
3
6
which a progressive decomposition begins. Compounds 1Cu and 1Cd (Figure 8a) demonstrate an
even higher thermal stability, their degradation beginning only after 450 °C. Also species 2M
(Figure S5), 3Co, 3Zn, 3Cd (Figure S6), 4
M
(Figure 8b) and 5
M
M
(Figure S7) display a high thermal
, 3 and 5 starts only at about 400
robustness: with the exception of 3Cu, the decomposition of 2
M
M
°
C. Even more remarkable, compounds 4 begin to decompose at approximately 500 °C. Finally,
M
compound 3Cu is the less stable material: its decomposition begins at 340 °C.
M M 2
Gas Adsorption Properties. The textural properties of compounds 1 −5 were probed by N
adsorption measurements at 77 K. N adsorption isotherms of CPs 1 , 1 , 1 (Figure 9a), 2M
2
Co
Cu
Zn
3
7
(Figure S8), 3
M
(Figure S9a), 4
M
(Figure 9b) and 5Cd (Figure S9b) can be classified as type I: all
∼ 0.01), corresponding to the
of these isotherms show a sharp knee at low relative pressures (p/p
0
filling of micropore monolayers, followed by a plateau, suggesting that the permanent porosity of
these compounds is mainly due to micropores of quite uniform size. The presence, in all of these
isotherms, of a hysteresis loop might be indicative of textural mesoporosity arising from
interparticle mesoporous voids. On the other hand, the isotherms of species 5Co and 5Zn can be
3
7
classified as type II (Figure S9b), typical of the adsorption on non porous materials or on materials
3
7
with pores larger than micropores. Both isotherms exhibit a type 2 hysteresis loop, connected with
the presence of a wide distribution of pore size radii. Finally, the isotherm of 1Cd can be classified
3
7
as type III (Figure 9a), likely associated to the nonꢀporous crystal structure of the material.
Specific surface areas, estimated using Langmuir and BET methods, are collected, together
with the amount of N
2
adsorbed at STP conditions, in Table 2. As the latter highlights, specific
2
surface areas span a rather vast range, namely 21ꢀ939 m /g (as retrieved with the BET method) or
2
1
35ꢀ1758 m /g (as retrieved with the Langmuir method). Worthy of note, CPs 1Co, 2Co, 3Cd and 4
M
2
possess Langmuir specific surface areas higher than 1000 m /g.
Catalytic Studies. Various (linear and cyclic) alcohols, mainly 1ꢀphenylethanol, and
cyclohexane were used as model substrates to investigate the catalytic properties of CPs 1 and 4
representative of species with flexible and rigid ligands, respectively. All the catalytic tests
M
M
,
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consisted in peroxidative oxidations carried out under mild conditions, using hydrogen peroxide
(H
2
O
2
) or tertꢀbutyl hydroperoxide (tꢀBuOOH) as oxidants.
Microwave-assisted solvent-free peroxidative oxidation of alcohols. The investigation of the
38
M M
catalytic properties of CPs 1 and 4 was undertaken following a procedure developed by us for
10
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59
60
the oxidation of 1ꢀphenylethanol (Scheme 2) as a model substrate, using tꢀBuOOH (aq. 70%, 2 eq)
as the oxidizing agent; the typical conditions adopted consisted of 3 h reaction time in the absence
of any added solvent, at 80 ºC and under lowꢀpower (5 W) microwave irradiation.
The main results of these tests, in terms of yield and of turnover number (TON, moles of
product/mol of catalyst), are summarized in Tables 3 and 4. Whatever CP is adopted as catalyst,
acetophenone is the only product obtained in the assayed conditions. The high selectivity observed
(typically > 98%) was confirmed by mass balance. The best results were obtained with the
copper(I)ꢀbased MOFs 1Cu and 4Cu (Table 3, entries 2 and 6, respectively). Under the above
described conditions, 1Cu and 4Cu heterogeneously catalyze the peroxidative oxidation of 1ꢀ
phenylethanol leading to 29 and 43% of acetophenone and TONs of 465 and 680, respectively,
already with a low catalyst loading (catalyst/substrate molar ratio of 0.06%) and in the absence of
any additive. The reactions performed under the same conditions but in the presence of the other
CPs resulted in lower yields in acetophenone, between 4 and 19% (Table 3). The CPs were easily
separated from the final reaction mixture. ICP determination of the Cu content in the filtered
solution (0.0023% and 0.0034% for 1Cu and 4Cu, respectively) discarded the possibility of
significant lixiviation. However, the Cuꢀcontaining solids recovered at the end of the catalytic cycle
did not show catalytic activity in a subsequent run, suggesting the formation of new, inactive
species. Blank tests were performed under the same reaction conditions but in the absence of CPs:
no significant conversion of 1ꢀphenylethanol was observed. Moreover, replacement of the copper(I)
CP by an inorganic salt, namely CuI, resulted in a drastic decrease of activity (compare entries 12 or
22 with entries 23 or 24, Table 4).
Worthy of note is the fact that, among all the tested CPs, those with the linear and rigid ligand
2ꢀ
L4 and possessing higher specific surface areas (Table 2) are more active than the corresponding
2
ꢀ
ones with the bent and flexible spacer L1 . Moreover, 4Cu, possessing a Langmuir specific surface
2
area as high as 1760 m /g, is, together with 1Cu, the best performing CP in both the peroxidative
oxidation reactions explored (see Tables 3ꢀ6). Nonetheless, by comparing specific surface areas
(Table 2) and yields in the peroxidative oxidation of 1ꢀphenylethanol (Table 3), it clearly emerges
that the two parameters do not possess the same trend, so that other properties of the title CPs,
besides their specific surface areas, must influence their catalytic activity. Given the outstanding
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5
5
6
performances of the copper(I) derivatives, the metal ion, with its specific stereochemical
requirements, should possess a prominent role.
Based on the above results, we chose the most active CPs, 1Cu and 4Cu, for further studies. The
main results, in terms of yield and TON, are gathered in Tables 4 and 5. First of all, the influence of
the amount of 4Cu on yield and TON was investigated (entries 12 and 17ꢀ19 of Table 4; Figure 10).
The increase in the amount of 4Cu from 1 ꢀmol (0.04 mol% vs. substrate) to 15 ꢀmol (0.6 mol% vs.
substrate) resulted in an enhancement of the yield from 18% to 75%. As expected, the increase of
0
1
2
3
4
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8
9
0
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2
3
4
5
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8
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0
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0
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2
3
4
5
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8
9
0
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2
3
4
5
6
7
8
9
0
3
catalyst amount implied a TON lowering from 1.5×10 to 248 (Table 4, entries 17 and 19,
respectively).
The favorable effect of MW irradiation was observed even when applying the low power of 5
38,39
W, as already reported for other systems.
For example, only 15% of product was obtained under
the same conditions (Table 4, entry 12) but using conventional heating (an oilꢀbath). As a further
confirmation of this observation, after 15 h reaction, yields of 89 and 58% were obtained for MW
and conventional heating, respectively, in the presence of 4Cu
.
The use of hydrogen peroxide (aq. 30%) instead of tꢀBuOOH resulted in a marked lowering of
the yield: for example, in the presence of 1 , under the same reaction conditions, a reduction from
Cu
4
2 2
5% to only 8% was observed when passing form tꢀBuOOH to H O (Table 4, entries 1 and 2,
respectively), in agreement with the expected decomposition of the latter.
Performing the reaction with 1Cu in the presence of acetonitrile (other conditions being
preserved) results in a yield lowering (from 45% under solventꢀfree conditions, entry 1 of Table 4,
to 37%, entry 3 of Table 4), while the use of water does not change significantly the yield (44% in
water, entry 4 of Table 4; Figure 11). By contrast, the use of a basic aqueous solution of K
2
CO
3
1 M
(entry 5 of Table 4; Figure 11) results in a significant increase of the yield with respect to the use of
38d,40,41
water tout court (from 44% in water to 72% in basic solution). As it was shown before,
basic
additives facilitate the deprotonation of the alcohol.
We also investigated the influence of 2,2,6,6ꢀtetramethylpiperidylꢀ1ꢀoxyl (TEMPO), a nitroxyl
38e,42,43
radical that is a renown
promoter in the catalytic oxidation of alcohols. Indeed, a significant
yield increase was observed from 45% and 56% in the absence of TEMPO (for 1Cu and 4Cu
,
respectively, entries 1 and 12 of Table 4) to 78% in its presence (for both 1Cu and 4Cu, entries 6 and
2
0 of Table 4).
The MWꢀassisted oxidation of the tested alcohols strongly depends on the reaction temperature.
In the presence of 4Cu and in the absence of additives, the high yield of 83% was achieved in
acetophenone after 30 min at 120 ºC and 20 W (entry 21 of Table 4), which is definitely higher than
the yield of 16% obtained for the same reaction time and under MW irradiation but at 80 ºC (entry
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11 of Table 4). When the reaction was carried out in the presence of 4Cu (0.2 mol%) and of TEMPO
(
2.5 mol%), after 30 min at 120 ºC a yield of 92% in acetophenone was obtained (entry 22, Table
). With our system, the obtained yields of ketones in the presence of 4Cu are comparable to those
of other successful MWꢀassisted oxidations of secondary alcohols involving a Cu(II) complex with
4
3
8e
a Schiff base and diethanolamine ligands, 2,4ꢀalkoxyꢀ1,3,5ꢀ triazapentadienate Cu(II)ꢀcontaining
10
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16
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18
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22
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24
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29
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54
55
56
57
58
59
60
3
9b
39c
complexes, or bisꢀ and trisꢀpyridyl amino and imino thioether copperꢀbased complexes.
In the conditions we adopted, the peroxidative oxidation of 1ꢀphenylethanol is believed to
proceed mainly via a radical mechanism which involves either oxygenꢀ or carbonꢀradicals. As a
matter of fact, a strong inhibiting effect on the catalytic activity was observed upon addition, to the
4
4
reaction mixture, of Ph
2
NH or CBrCl
3
, well known oxygenꢀ or carbonꢀradical traps, respectively
(
entries 7 and 8 of Table 4; Figure 11). Hence, the oxidation mechanism may involve the metalꢀ
●
45
assisted generation of tꢀBuO radicals (upon reduction of tꢀBuOOH by Cu(I) centers) which
45a,cꢀe
behave as hydrogen atom abstractors from the alcohol.
Conceivably, the mechanism is similar
38e,39b,39c
to those proposed in other cases, which are also of a radical type.
The Cu(I) CPs were also tested toward the oxidation of other alcohols, namely cyclohexanol
and benzyl alcohol, modeling a secondary and a primary alcohol, respectively. In the presence of a
catalytic amount of 1Cu or 4Cu (0.2 mol% vs. substrate) and in the absence of any additive, the two
systems yielded 58 and 78% of cyclohexanone after 30 min of reaction at 120 ºC and 20 W (Table
5, entries 2 and 3, for 1Cu and 4Cu, respectively), and 68 and 75% of benzaldehyde under the same
reaction conditions (Table 5, entries 9 and 10, for 1Cu and 4Cu, respectively). Under similar reaction
conditions, linear aliphatic alcohols, namely 2ꢀhexanol and 3ꢀhexanol, lead to lower yields, as
38a,d,45c
already reported in other cases:
namely, the oxidation of 2ꢀhexanol and 3ꢀhexanol yielded 45
or 59% of 2ꢀhexanone and 31 or 48% of 3ꢀhexanone, for 1Cu or 4Cu, respectively, after 30 min of
reaction at 120 ºC (Table 5, entries 4ꢀ7).
Peroxidative oxidation of cyclohexane. The copper(I)ꢀbased CPs 1Cu and 4Cu (mainly the
latter) are also active in the peroxidative oxidation of cyclohexane (CyH) carried out at room
temperature with aqueous hydrogen peroxide. The primary product of this oxidation is cyclohexyl
hydroperoxide (CyOOH), which further evolves to a mixture of cyclohexanol and cyclohexanone
(
final products, Scheme 3). In contrast, under the same reaction conditions, the corresponding
cobalt(II)ꢀbased CPs 1Co and 4Co fail to catalyze the reaction. As a matter of fact, after 24 h stirring
the reaction mixture (CyH, H 30% aq. and 4Cu in acetonitrile) at room temperature, an 11%
2
O
2
overall yield (relative to the alkane) of cyclohexanol and cyclohexanone (0.98 alcohol/ketone ratio)
was obtained with an overall TON of 102 moles of products per mole of 4Cu, i.e. 51 moles of
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1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
product per Cu(I) centre (entry 5 of Table 6). Although the reactions were typically run for 24 h, the
much lower reaction time of 6 h was sufficient to attain 9% of product yield (Figure 12).
The formation of CyOOH (under the conditions of Table 6) was proved by using the method
26
46
proposed by Shul’pin. As observed in other catalytic systems, also in the present case the
addition of PPh prior to GC analysis of the products resulted in a marked increase of the amount of
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
cyclohexanol (due to the reduction of CyOOH by PPh , with the formation of phosphane oxide) and
3
in a corresponding decrease of cyclohexanone.
The obtained yields are considerably lower than those reported for some dinuclear Cu(II)
4
4a
complexes (22ꢀ29%). Nonetheless, in the case of 4 , in spite of the mild reaction conditions used
Cu
in our case, they are higher than those characterizing the industrial process when a good selectivity
13e,47
is aimed to.
Moreover, a high selectivity towards the formation of cyclohexanol and
cyclohexanone is exhibited by both 1Cu and 4 , since no traces of byꢀproducts were detected by
Cu
GCꢀMS analysis of the final reaction mixtures. Another noteworthy feature of the tests performed
on cyclohexane is the fact that even low CP loadings (typically 0.1 mol% vs. substrate) granted high
yields of oxygenated products (Table 6, entry 5).
26,44d,48
The previously recognized promoting effect of an acidic medium
on the peroxidative
oxidation of alkanes is not observed for the present systems (Table 6). Indeed, the presence of
pyrazine carboxylic acid has a strong inhibiting effect on the catalytic activity of 1Cu and 4Cu (Table
6, compare entries 1 and 2 or 5 and 6). A similar behavior was found for Cu(II) complexes bearing
4
9
50
azathia macrocyles and also for Cꢀscorpionate Au(III) complexes.
The effect of the peroxideꢀtoꢀcatalyst molar ratio was also investigated and is depicted in
3
Figure 13. The increase of the peroxide amount up to a n(H O )/n(catalyst) molar ratio of 2×10 led
2
2
to the highest product yield and TON (Table 6, entry 5). Further increase of the oxidant amount
resulted in a drastic yield drop eventually due to overoxidation reactions. Indeed, overoxidation
products, such as 1,4ꢀcyclohexanedione or 1,2ꢀepoxycyclohexane, were detected by GCꢀMS.
Finally, as already observed for Cu(II)ꢀbased and other metalꢀcontaining catalytic
46,49,50
systems,
also in the present case addition of a radical trap (either CBrCl or Ph NH) to the
3 2
reaction mixture results in a considerable suppression of the catalytic activity. This behavior, along
with the formation of cyclohexyl hydroperoxide (typical intermediate product in radicalꢀtype
49,51
reactions) supports the hypothesis that a freeꢀradical mechanism of the type already reported
at work for the cyclohexane oxidations carried out in this study.
is
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3
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5
6
7
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9
CONCLUSIONS
Solvothermal reactions of five, flexible or rigid bis(pyrazolato)ꢀbased tectons with Co(II),
Cu(II), Zn(II) and Cd(II) salts led to the isolation of eighteen coordination polymers. Worthy of
note, all of them possess a remarkable thermal stability, as their decomposition temperatures lie in
the range 340ꢀ500 °C. Their Langmuir specific surface areas span the rather vast range of 135ꢀ1758
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2
m /g. Those CPs for which structure determination was feasible, invariably show (porous or non
porous) polymeric architectures.
Two out of the five families of CPs were employed as catalysts for the peroxidative oxidation
of acyclic and cyclic alcohols (with tꢀBuOOH) and of cyclohexane (with H O ). Among all the CPs
2
2
2
ꢀ
screened, those with the linear and rigid ligand L4 , showing higher specific surface areas, are more
2
ꢀ
active than the corresponding ones with the bent and flexible spacer L1 . Moreover, the copper(I)ꢀ
based systems 1Cu and 4Cu are the most active ones for both oxidation reactions. Noteworthy, using
3
1ꢀphenylethanol as substrate, a maximum yield of 92% and a TON up to 1.5×10 are achieved. This
oxidation occurs rapidly under microwave irradiation: just after 30 min, in the absence of any
solvent or additive, ca. 70% of all the substrate is converted to acetophenone. Notably, the use of
lowꢀpower MW irradiation in a solventꢀfree system for the oxidation of alcohols is of great
significance towards the development of a green catalytic system. Finally, using cyclohexane, a
much more inert substrate, 1Cu and 4Cu allowed us to achieve, at room temperature, without MW
irradiation and with a high selectivity, yields up to 11%, which are higher than those granted by the
present industrial process in order to operate with a good selectivity.
ASSOCIATED CONTENT
Supporting Information. Details about the syntheses of the ligands. Final Rietveld refinement
plots for 1Co, 1Cu, 1Cd, 2Cd, 3
M
, 4Co and 5Cd. TGA traces for 2
crystal and molecular structure for 3Co. Nitrogen adsorption isotherms for 2
Crystallographic data for 1 , 1 , 1 , 2 , 3 , 4 and 5 . in CIF Format. This material is
M
, 3
M
and 5
M
. Representation of the
M
, 3 and 5 .
M
M
Co
Cu
Cd
Cd
M
Co
Cd
available free of charge via the Internet at http://pubs.acs.org.
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1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
AUTHOR INFORMATION
Corresponding authors
*
CP: Tel: +39ꢀ0737402234. Eꢀmail: claudio.pettinari@unicam.it
AJLP: +351ꢀ218419237. Eꢀmail: pombeiro@tecnico.ulisboa.pt
*
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
*SG: Tel. +39ꢀ031ꢀ2386627. Eꢀmail: simona.galli@uninsubria.it
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The Universities of Camerino and Insubria are acknowledged for partial funding. This work has
been partially supported by the Fundação para a Ciência e a Tecnologia (FCT), Portugal, and its
project UID/QUI/00100/2015. Norberto Masciocchi is acknowledged for helpful discussions. SG
acknowledges the precious help of Elisa Lavigna.
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2
3
4
5
6
7
8
9
TABLES
Table 1. Formulas and labels used throughout the manuscript for the title CPs.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CP
Label
1Co
CP
Label
CP
Label
[
Co(L1)]
[Co(L2)]
2Co [Co(L3)] 3Co
[
Cu (L1)] 1Cu [Cu (L2)] 2Cu [Cu(L3)] 3Cu
2
2
[
Zn(L1)]
1Zn
1Cd
4Co
[Cd(L2)]
2Cd [Zn(L3)]
3Zn
[Cd(L1)]
[Cd(L3)] 3Cd
[
Co(L4)]
[Co(L5)]
[Zn(L5)]
[Cd(L5)]
5Co
5Zn
5Cd
[Cu (L4)] 4Cu
2
[
Zn(L4)]
4Zn
4Cd
[Cd(L4)]
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1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 2. Key parameters retrieved from the N
2
adsorption isotherms of 1
M
−5
M
.
CP BET SSA Langmuir SSA Adsorbed N2
2
2
3
(
m /g)
(m /g)
(cm /g STP)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
730
531
409
21
1021
957
235
220
125
31
1
1
1
1
2
2
Co
Cu
Zn
Cd
Co
Cu
543
135
553
462
167
576
142
203
911
791
939
647
508
376
133
427
1213
678
279
156
98
2Cd
426
3
3
Co
Cu
849
195
81
351
3Zn
347
80
3
4
Cd
Co
1150
1573
1758
1028
1549
449
264
362
404
236
356
103
76
4Cu
4
4
Zn
Cd
5Co
5
5
Zn
Cd
329
510
117
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9
Table 3. MWꢀassisted solventꢀfree oxidation of 1ꢀphenylethanol to acetophenone in the
[a]
M M
presence of CPs 1 and 4 .
[c]
Entry CP Yield TON
[
b]
(
%)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
2
3
4
5
6
7
8
1Co
1Cu
1Zn
1Cd
4Co
4Cu
4Zn
4Cd
13
29
4
168
465
71
5
109
204
680
385
470
15
43
15
19
[
a] Reaction conditions: 2.5 mmol of substrate, (0.06% mol CP vs. substrate), 5 mmol of tꢀBuOOH (aq. 70%), 80
ºC, 3 h, MW irradiation (5 W). [b] Molar yield (%) based on the substrate, i.e. moles of product per 100 moles of
substrate, as determined by gas chromatography. [c] Turnover number, i.e. number of moles of product per mol of
catalyst (based on the monomeric unit, i.e. dinuclear in the case of copper or mononulear for the other metals).
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2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
[
a]
Table 4. Oxidation of 1ꢀphenylethanol in the presence of 1Cu or 4Cu
.
Entry
CP
Time
Additive
Yield
TON
[
b]
ꢀ1 [c]
[mol% vs. substr] (h)
[mol% vs. substr] (%)
[TOF(h )]
1
1Cu [0.2%]
1
ꢀ
ꢀ
ꢀ
ꢀ
45
8
112[112]
21[21]
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
[d]
2
1
1
1
1
1
1
1
Cu [0.2%]
Cu [0.2%]
Cu [0.2%]
Cu [0.2%]
Cu [0.2%]
Cu [0.2%]
Cu [0.2%]
1
[
e]
f]
3
4
5
6
7
8
9
1
1
1
37
44
72
78
4
93[92]
[
1
126[126]
380[380]
459[459]
7[7]
1
K
2
CO
TEMPO [2.5]
Ph NH [100]
CBrCl [100]
3
[2.5]
1
1
2
1
3
2
5[5]
[g]
1Cu [0.2%]
0.5
0.08
0.5
1
ꢀ
68
11
16
56
67
82
85
89
331[662]
59[738]
158[316]
281[281]
367[122]
512[85]
586[59]
0
1
4
4
Cu [0.2%]
Cu [0.2%]
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
12
4Cu [0.2%]
1
1
3
4
4
4
Cu [0.2%]
Cu [0.2%]
3
6
15
4Cu [0.2%]
Cu [0.2%]
Cu [0.04%]
4Cu [0.4%]
10
15
1
1
1
6
7
4
644[43]
3
3
4
18 1.5×10 [1.5×10 ]
18
1
68
75
78
83
92
18
28
374[374]
248[248]
525[525]
1
2
9
0
4
4
Cu [0.6%]
Cu [0.2%]
1
1
TEMPO [2.5]
[
g]
g]
3
21
4Cu [0.2%]
Cu [0.2%]
0.5
0.5
1
ꢀ
525[1.1×10 ]
[
3
2
2
2
3
4
TEMPO [2.5]
ꢀ
653[1.3×10 ]
[h]
g,h]
CuI [0.4%]
CuI [0.4%]
94[94]
[
24
0.5
TEMPO [2.5]
138[276]
[
a] Reaction conditions (unless stated otherwise): 2.5 mmol of substrate, CPs 1Cu or 4Cu (1ꢀ15 ꢀmol, 0.04ꢀ0.6
mol% vs. substrate), 5 mmol of tꢀBuOOH (aq. 70%), 80 ºC, MW irradiation (5 W). [b] Molar yield (%) based on
substrate, i.e. moles of product per 100 mol of substrate, as determined by gas chromatography. [c] Turnover number,
i.e. number of moles of product per mol of dinuclear CP; TOF = TON per hour (values in brackets). [d] H
2 2
O aq. 30%
instead of tꢀBuOOH aq. 70%. [e] In acetonitrile. [f] In H O. [g] At 120 ºC and 20 W. [h] For comparative purposes.
2
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Table 5. MWꢀassisted solventꢀfree oxidation of selected secondary alcohols and of benzyl
[a]
alcohol in the presence of 1Cu or 4Cu
.
Entry
CP
Substrate
Time
Yield
TON
[
b]
ꢀ1 [c]
(h)
(%)
[TOF(h )]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
[d]
1
1Cu
1Cu
4Cu
1Cu
4Cu
1Cu
4Cu
1Cu
1Cu
4Cu
1
38
58
78
45
59
31
48
34
68
75
157 [157]
201 [402]
333 [666]
99 [198]
155 [310]
88 [176]
101 [202]
98 [98]
2
3
4
5
6
7
Cyclohexanol
0.5
0.5
0.5
0.5
0.5
0.5
1
2
3
ꢀHexanol
ꢀHexanol
[
d]
8
9
1
Benzylalcohol
0.5
0.5
287 [574]
401 [802]
0
[
a] Reaction conditions (unless stated otherwise): 2.5 mmol of substrate, 5 ꢀmol (0.2 mol% vs. substrate) of 1Cu or
4
Cu, 5 mmol of tꢀBuOOH (aq. 70%), 120 ºC, MW irradiation (20 W). [b] Molar yield (%) based on substrate, i.e. moles
of product per 100 mol of substrate, as determined by gas chromatography. [c] Turnover number, i.e. number of moles
of product per mol of dinuclear catalyst; TOF = TON per hour (values in brackets). [d] Reaction performed at 80 ºC
and 5 W.
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1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
[
a]
Table 6. Peroxidative oxidation of cyclohexane with H
2 2
O catalyzed by 1Cu or 4Cu (selected
data).
[
b]
[c]
Entry
CP
n(Cat)/n(CyH)
n(H
2
O
2
)/n(Cat.)
Yield (%)
TON
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
−3
[d]
×
10
×10
OL
ONE
0.7
0.1
0.3
3.7
5.3
0.1
0.8
0.1
0.2
ꢀ
Tot
1
2
3
4
5
6
7
8
9
1
1Cu
1Cu
1Cu
4Cu
4Cu
4Cu
4Cu
4Cu
4Cu
4Cu
1
2
2
1.3
0.1
0.1
2.8
5.2
0.1
1.9
0.2
0.5
0.1
2.0
0.2
0.4
6.5
10.5
0.2
2.7
0.3
0.7
0.1
20
2
[
e]
1
0.1
2
20
1
35
33
1
2
102
2.2
54
[
[
e]
1
2
0.5
0.1
1
4
20
2
30
f]
[
3.4
1.8
g]
0
1
2
[
a] Reaction conditions (unless stated otherwise): acetonitrile (3 mL), cyclohexane (5.0 mmol), 0.5ꢀ10 ꢀmol of 1Cu
or 4Cu, H (10.0 mmol), 24 h, r.t.; yield and TON determined by gas chromatography upon treatment with PPh (see
text). [b] Molar yield (%) based on substrate, i.e. moles of products [cyclohexanol (OL) and cyclohexanone (ONE)] per
00 mol of cyclohexane. [c] Turnover number, i.e. moles of product per mol of dinuclear catalyst. [d] Moles of
cyclohexanol + cyclohexanone per 100 moles of cyclohexane. [e] Reaction in the presence of pyrazine carboxylic acid
Hpca) [n(Hpca)/n(catalyst) = 25]. [f] Reaction in the presence of CBrCl (5.0 mmol). [g] Reaction in the presence of
2
O
2
3
1
(
3
2
Ph NH (5.0 mmol).
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3
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5
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7
8
9
SCHEMES AND FIGURES
R3
R3
CH3
H C
3
CH2
R5 R5
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
N
N
N
N
N
H
N
N
H
N
H
CH3
H C
3
H L1: R = R = CH
3
2
3
5
H L4
2
H L2: R = CH R = H
2
3
3,
5
H L3: R = R = H
2
3
5
CH3
H C
3
H
N
N
N
N
H
CH3
H C
3
H L5
2
Scheme 1: The bis(pyrazole)ꢀbased ligands adopted in this study.
OH
CH3
O
CP catalyst
CH3
t-BuOOH, MW
Scheme 2. MWꢀassisted, solventꢀfree oxidation of 1ꢀphenylethanol to acetophenone.
OOH
OH
O
1Cu or 4Cu
H O2(aq), MeCN
2
Scheme 3. Peroxidative oxidation of cyclohexane catalyzed by 1Cu or 4Cu
.
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