Syntheses and crystal structures of benzene-sulfonate and -carboxylate copper polymers and their application in the oxidation of cyclohexane in ionic liquid under mild conditions

Article abstract of DOI:10.1039/c6dt02271e

The syntheses, crystal structures and catalytic activities of the polymers derived from 2-(2-pyridylmethyleneamino)benzenesulfonic acid (HL), viz. [CuL(H₂tma)]_n (1) and [{Cu₂L₂(H₂pma)}·(8H₂

Full text of DOI:10.1039/c6dt02271e

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Syntheses and crystal structures of benzeneꢀsulfonate and ꢀcarboxylate
copper polymers and their application to oxidation of cyclohexane in
ionic liquid under mild conditions†
Susanta Hazra,a Ana P. C. Ribeiro,a M. Fátima C. Guedes da Silva,a Carlos A. Nieto de Castrob
and Armando J. L. Pombeiro*a
a Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,
1049ꢀ001 Lisboa, Portugal. Eꢀmail: pombeiro@tecnico.ulisboa.pt
b Centro de Química Estrutural, Faculdade de Ciências, Universidade de Lisboa, 1749ꢀ016 Lisboa, Portugal
†Electronic Supplementary Information (ESI) available: Tables S1–S4. Figs. S1–S8. CCDC 1042662 and 1042663 for 1
and 2, respectively, contain the supplementary crystallographic data for this paper. For crystallographic data in CIF or
other electronic format see the internet at www.rsc.org. These CIF data can also be obtained free of charge from the
Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.
Abstract:
Syntheses, crystal structures and catalytic activities of the polymers derived from 2ꢀ(2ꢀ
pyridylmethyleneamino)benzenesulfonic acid (HL), viz. [CuL(H2tma)]n (1) and [{Cu2L2(H2pma)}·(8H2O)]n (2)
[H3tma
= benzeneꢀ1,3,5ꢀtricarboxylic (trimesic) acid and H4pma = benzeneꢀ1,2,4,5ꢀtetracarboxylic
(pyromellitic) acid], are presented. Despite the comparable combinations and compositions of ligands (sulfonate
and carboxylate) in these two polymers the bridging moiety in 1 is sulfonate while in 2 is carboxylate.
Complexes 1 and 2 act as catalysts in the peroxidative oxidation of cyclohexane under mild conditions using
either the ionic liquid 1ꢀbutylꢀ3ꢀmethylimidazolium hexafluorophosphate [bmim][PF6] or acetonitrile as
solvent. The ionic liquid medium leads to increases in the yields and in the turnover numbers, achieved in
shorter reaction times, in comparison to those in the conventional acetonitrile solvent. Simple recycling of the
catalysts in the ionic liquid medium is achieved without loss of activity and selectivity.
Introduction
Over the past few years there has been an increasing interest in the coordination chemistry of
inorganic polymeric compounds containing organic carboxylates1 and phosphonates.2 Organosulfonic
acids3 that are known to form various types of staked or pillared layered compounds with metal ions,
such as silver(I),3,4 alkali and alkaline earth metals,3,5 transitional metals and lanthanide(III) ions,3,6,7
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have also been the object of a growing attention. Moreover, various functional groups, such as amine,
hydroxyl and carboxylate, have been attached to organosulfonic acids to build new inorganicꢀorganic
hybrid materials.3–10 The combination of sulfonated ligands with organic carboxylates as auxiliary
ones may bring flexibility to the systems and produce interesting coordination polymers with eventual
applications in green chemistry, namely in catalysis.
The use of ionic liquids (ILs) as a green alternative to a volatile organic solvent in several
processes, has received great attention mainly due to their nonꢀmeasurable vapour pressure under
ambient conditions and since they can dissolve transitional metal complexes for catalytic purposes.11–
19
Since ILs are made of ions, they have a high polarity and can act as a favourable medium for
stabilizing free radicals produced, e.g. in an oxidation process.16b,19a Significant improvements on the
catalytic performances, in terms of yield and selectivity, have been observed when using an IL
medium.18b–d,19 In addition to the role of solvent, an IL can perform multiple functions in catalytic
processes, acting as a catalyst, a coꢀcatalyst, a support or a ligand,18 therefore influencing markedly the
catalytic reaction pathway.11a In view of the particular nature of an IL, its use can lead to a biphasic
system and to the immobilization of the catalyst which can facilitate separations. Their stability to
water, namely of the methylimidazolium family [Cnmim][PF6] (with n = 4, 6 and 8), has been
reported.11b,c Nevertheless, the application of ILs to the demanding alkane oxidation, using mild
conditions and a green oxidant (such as H2O2), is still very scarce.16a,b,19
Cyclohexane oxidation to cyclohexanol and cyclohexanone is an important industrial reaction,
since such products are intermediates, e.g., in the manufacture of Nylon polymers.20 Currently, the
industrial chemical process, which uses O2 as oxidant and a Co catalyst, leads to a very low product
yield in order to achieve a good selectivity and requires a considerably high temperature.20 Therefore,
it is of great practical interest to obtain catalytic systems more efficient, operating under milder and
environmentꢀfriendly conditions for this oxidation process.
Based on the above mentioned points, the aim of this work is to obtain metal complex catalysts
for cycloalkane oxidation in the still underexplored IL medium, namely in 1ꢀbutylꢀ3ꢀ
methylimidazolium hexafluorophosphate ([bmim][PF6], Scheme 1) as a "green" solvent and using
hydrogen peroxide as a "green" oxidant which has well known19–24,25a applications in alkane oxidation
catalysis.
Scheme 1. 1ꢀbutylꢀ3ꢀmethylimidazolium hexafluorophosphate ([bmim][PF6]).
2

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Hence, inspired by the successful application of the bis(ꢁ4ꢀ(ae)ꢀcyclohexaneꢀ1,4ꢀ
dicarboxylatoꢀO,O',O'',O'''')ꢀtetracopper(II) as a catalyst for the peroxidative oxidation of cyclohexane
to cyclohexanol and cyclohexanone,16b,25a we now report the syntheses, structures and catalytic
activities of the ꢁ2ꢀ(2ꢀ(2ꢀpyridylmethyleneamino)benzene sulfonatoꢀO,O')ꢀcopper(II) polymer
[CuL(H2tma)]n (1) and the bisꢀꢁ4ꢀ(benzeneꢀ1,2,4,5ꢀtetracarboxylatoꢀO,O,O'',O'')ꢀdicopper(II) polymer
[{Cu2L2(H2pma)}·(8H2O)]n (2) [HL = 2ꢀ(2ꢀpyridylmethyleneamino)ꢀbenzenesulfonic acid, H3tma =
benzeneꢀ1,3,5ꢀtricarboxylic (trimesic) acid and H4pma
=
benzeneꢀ1,2,4,5ꢀtetracarboxylic
(pyromellitic) acid] (Scheme 2). Comparative catalytic studies were carried out in the unconventional
IL [bmim][PF6] medium (in which the complexes are soluble) and in the conventional acetonitrile
solvent.
Scheme 2. Synthesis of [CuL(H2tma)]n (1) and [{L2Cu2(H2pma)}·(8H2O)]n (2) from the parent
dichlorobridged dicopper(II) complex.
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Results and discussion
Syntheses and characterization
The compounds [CuL(H2tma)]n (1) and [{Cu2L2(H2pma)}·(8H2O)]n (2) can be readily
obtained in high yields from the reactions of the dichlorobridged dicopper complex25a,b [CuLCl]2 with
benzeneꢀ1,3,5ꢀtricarboxylic acid (H3tma) and benzeneꢀ1,2,4,5ꢀtetracarboxylic acid (H4pma),
respectively, in aqueous DMF solution. The syntheses proceed via displacements of chloride ligands
in [CuLCl]2 by benzeneꢀ1,3,5ꢀtricarboxylate (H2tma–) and benzeneꢀ1,2,4,5ꢀtetracarboxylate (H2pma2–)
to produce the ꢁ2ꢀsulfonatoꢀO,O'ꢀbridged copper(II) polymer (1) and the ꢁ4ꢀbenzeneꢀ1,2,4,5ꢀ
tetracarboxylatoꢀO,O,O'',O''ꢀbridged dicopper(II) polymer (2), respectively, which were characterized
by elemental microanalysis, IR spectroscopy and single crystal Xꢀray diffraction study.
The IR spectra of the two polymers (1 and 2) exhibit the expected bands of the organic Schiff
base along with those of the H3tma (in 1) and H4pma (in 2) derived ligands, ν(C=N) are observed at
1618 and 1620 cm–1, respectively whereas the sulfonate groups are evidenced by the medium signals
at 1383 and 1380 cm–1, respectively for 1 and 2. The stretching vibrations for C=O (1724 cm–1 for 1
and 1720 cm–1 for 2) and C–COOH (1157 cm–1 for 1 and 1167 cm–1 for 2) are seen in the
corresponding IR spectra. The water stretching occurs as a broad band at 3465 cm–1 for the compound
2.
Description of crystal structures
[CuL(H2tma)]n (1)
Single crystal Xꢀray analysis reveals that compound
1
is
a
ꢁ2ꢀ(2,2ꢀ
((pyridinylmethylene)aminoꢀκN2)benzenesulfonatoꢀO,O')ꢀcopper(II) polymer with monodentate
trimesate groups. Its crystal structure is depicted in Fig. 1 and selected bond distances and angles in
Table S1 (see Electronic Supplementary Information, ESI).
The asymmetric unit of 1 contains a copper(II) cation coordinated by the L– Schiff base and by
the monoanionic H2tma– trimesate moiety. As presented in Fig. 1, the metal centre adopts a distorted
square pyramidal coordination environment (τ
5 = 0.16),26 the basal plane assuming cisoid angle values
in the range 81.90(7) – 95.20(7)º and transoid angles of 167.31(7) and 177.08(7)º. The Npyridyl, Nimine
and Osulfonate atoms of the ligand together with an Ocarboxylate (O21) of H2tma– afford the basal plane for
the copper(II) ion, whereas a symmetry generated Osulfonate (O13i) atom occupies the apical site. The
Cu–Npyridyl bond distance is slightly shorter than the Cu–Nimine [1.9928(18) and 2.0230(18) Å,
respectively] but the Cu–Osulfonate and Cu–Ocarboxylate lengths are almost identical within experimental
error [1.9571(15) and 1.9560(14) Å, in this order]; the apical Cu–Osulfonate bond is significantly longer
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[Cu1–O13i = 2.3432(16) Å]. The sulfonate bridging groups (Fig. 2, pink coloured polyhedra) give rise
to the formation of a 1D chain that runs along the crystallographic b axis, the monodentate H2tma–
ligands being alternatively positioned along this chain.
The carboxylic acid groups in 1 are involved in nonꢀcovalent Hꢀbonding interactions (Figs. 1
and S1) giving rise to a 3D framework: the O23 atoms act as strong Hꢀdonors to symmetry generated
O26 carboxylic atoms, while O22 atoms act as acceptors from O25.
Fig. 1 Ball and stick representation of 1 with atom labelling scheme. Hydrogen bond interactions in
dashed light blue colour. Symmetry code to generate equivalent atom: i) 1–x, –1/2+y, 1/2–z; ii) 2ꢀx,ꢀ
1/2+y,1/2–z; iii) x,1/2–y, –1/2+z; iv) 2–x,1/2+y,1/2–z; v) x,1/2–y,1/2+z.
Fig. 2 Perspective view of the 1D chain in 1. Sulfonate bridging groups are shown as pink coloured
closedꢀface polyhedra. All Hꢀatoms except those of carboxylic acid groups are omitted for clarity.
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[{Cu2L2(H2pma)}·(8H2O)]n (2)
Compound 2 is a H2pmaꢀdicopper(II) polymer with two of the carboxylate groups binding the
metals in a monodentate bridging mode, therefore generating Cu2O2 metallacycles, and with a NNO
chelating L– Schiff base per metal cation. The structure also contains water molecules (four, in the
asymmetric unit). A dinuclear cluster and a fragment of the 1D chain of 2, running along the
crystallographic a axis, are illustrated in Figs. 3 and 4, respectively, whereas selected bond length and
angles are depicted in Table S1, ESI. Due to the presence of µ,η1ꢀcarboxylates of the H2pma2– ligands,
the 1D chains of 2 display the L– Schiff base alternatively positioned along the chain as highlighted
(Fig. 4) by the sulfonate groups polyhedra.
Fig. 3 Ball and stick representation of a fragment of polymer 2, with atom labelling scheme. Water
molecules and all Hꢀatoms, except those of carboxylic groups, are omitted for clarity. Symmetry code
to generate equivalent atom: i) 3–x, 2–y, 1–z; ii) 2–x, 2–y, 1–z.
The metal centres in 2 adopt distorted square pyramidal coordination environments (τ5
=
0.18),26 the basal plane being formed by the Npyridyl, Nimine and Osulfonate atoms of the Schiff base ligand
(L–) together with the Ocarboxylate of the H2pma2– ligand; the apical position is occupied by an Oꢀatom
from a symmetry generates carboxylate moiety. The Cu–Npyridyl and Cu–Nimine bond distances
[1.9743(16) and 2.0126(15) Å, respectively] as well as the Cu–Osulfonate and Cu–Ocarboxylate length
[1.9518(14) and 1.9603(12) Å, in this order] are comparable to those observed for compound 1 and,
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similarly, the apical Cu–Ocarboxylate length [2.2480(12) Å] is longer. The basal cisoid [82.18(8)
97.25(6) ] and transoid angles [166.53(7) and 177.18(6) ] are also identical to those found in 1.
°
°
The crystallization water molecules in the structure of 2 interact among themselves and with
the metalꢀorganic 1D chains through various intermolecular hydrogen bonds, involving also symmetry
generated H2O molecules. Such extensive interactions play a key role in engaging the host metalꢀ
organic 1D chains into a 3D supramolecular framework (Figs. S2 and S3; Table S2) by means of
multiple Hꢀbonds.
Fig. 4 Illustration of the carboxylate bridged one dimensional chain in 2. Sulfonate group are shown as
closedꢀface pink coloured polyhedra. All Hꢀatoms except those of carboxylic acid groups are omitted
for clarity.
According to the CSD database search on Version 5.35, there are a number of copper(II)
complexes containing carboxylate and/or sulfonate bridges, the majority of them being polymeric.7–10
Copper(II) complexes derived from ligands simultaneously containing sulfonate and carboxylate
groups are described with both moieties7 and only carboxylate8 in bridging modes. The concurrent
presence of sulfonate and carboxylate ligands9,10,25a resulted in similar situations. Thus, to our
knowledge, compound 1 is the first example of a sulfonate bridge copper polymeric compound despite
the availability of both carboxylate and sulfonate ligands.
Mild peroxidative oxidation of cyclohexane in a room temperature ionic liquid
The ionic liquid (IL) [bmim][PF6] (Scheme 1) was chosen as medium for the peroxidative
oxidation of cyclohexane (CyH) by H2O2. This IL is air, temperature and moisture stable15,16 and does
16b–d
not compete with CyH neither is oxidised by H2O2
proving to be a suitable medium for alkane
oxidation.16 They can be used for biphasic catalytic procedures by tuning the IL to be immiscible in
water and alkanes.15,16 Moreover, the nonꢀcoordination ability of the IL allows the catalyst active site
to be more accessible to the reagents.16b, 16h
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OOH
OH
O
aq. H2O2, Cu-Catalyst
IL or CH3CN
+
Scheme 3. Peroxidative oxidation of cyclohexane catalyzed by 1 and 2.
In a previously reported tetracopper catalyst16b,25a with comparable benzeneꢀsulfonate and ꢀ
carboxylate ligands, the presence of a basic additive, pyridine, promoted the yield. Therefore we
carried out tests with the same basic additive in our IL, with 1 and 2, but no yield promoting effect
was found. Thus, the reactions were typically performed without pyridine.
At the end of each run, a biphasic mixture was observed (Fig. 5) to which we added diethyl
ether to extract the products. We also observed that the catalyst remained in the IL phase, thus
affording a simple and efficient method for extracting16h, 16i the products and reusing the catalyst. The
water formed as the only byꢀproduct of the decomposition of hydrogen peroxide was removed by
vacuum from the separated IL phase with catalyst for recycling (see below). Selected results are
shown in Table 1.
Fig. 5 Biphasic oxidation of cyclohexane to cyclohexanol and cyclohexanone catalyzed by 1 (a and b)
and 2 (c and d) in the ionic liquid [bmim][PF6]. a and c refer to the highest concentrations of catalysts,
b and d to the lowest ones.
The effect of the catalyst loading on the yield and turnover number (TON) was investigated,
and the highest overall yield (36%, TON = 83) was obtained at the highest used concentration of
catalyst 1 (2x10–3 M, Table 1, entry 2), after 2 h reaction time at 40 ºC. This constitutes a significant
improvement in the IL medium relatively to the optimized system in acetonitrile (see below, Table 2,
entry 17) which, to obtain 31% of total products yield, requires the double reaction time (4 h). With
complex 2 the same behaviour is observed. The maximum yield of 29% (Table 1, entry 5) in the IL,
after 2h, is also higher than that of 22% (Table 2, entry 36) obtained in CH3CN, after 4 h. A
comparison of the obtained turnover numbers (TONs) for the IL and the tested organic solvent, under
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identical experimental conditions, also shows an improvement in the former medium, reaching a
maximum TON of 736 (Table 1, entry 1) or 575 (Table 1, entry 4 for catalyst 1 or 2, respectively.
Although further studies should be undertaken to confirm such a tendency and to understand the
promoting effect of the IL, this can be due do with its nonꢀcoordinating ability (in contrast to
acetonitrile), thus making the catalyst active site more accessible to the substrate.16b
The better yields and TONs, shorter reaction time and the easy biphasic separation are evident
advantages of the use of the IL over that of CH3CN.
The molar ratio between cyclohexanol and cyclohexanone (CyOH/CyO) depends on the used
medium. For IL as solvent, that ratio is higher than for acetonitrile. For the higher catalyst loading of
2, the ratio increases from 1.9 in acetonitrile (Table 2, entry 30) to 8.7 in the IL (Table 1, entry 5). In
the case of the catalyst 1, for the lower loading, that ratio changes from 1.8 in acetonitrile (Table 2,
entry 3) to 7.0 in the IL (Table 1, entry 1). The CyOH/CyO molar ratio decreases with the increase of
loading of catalyst 1, whereas for catalyst 2 the trend is opposite. The increment in that ratio can be an
advantage in the use of ILs, when compared with acetonitrile, of potential commercial significance.17a
Table 1. Peroxidative oxidation of cyclohexane (CyH) to cyclohexanol (CyOH) and cyclohexanone
(CyO) catalyzed by 1 and 2 in [bmim][PF6].a
Yield % (after
PPh3)b
Catalyst
Total [CyOH] Conversion
Entry amount,
d
TONc / [CyO]
%
mol L−1 CyO CyOH Total
Catalyst 1
1
2
2 × 10−4
2 × 10−3
4
7
13
28
29
23
32
36
36
736
83
83
7.0
4.4
1.8
32.3
36.2
36.2
3e
Catalyst 2
4
5
2 × 10−4
2 × 10−3
ꢀ
4
3
13
21
26
16
0.1
2.4
25
29
29
0.8
3.5
575
67
67
ꢀ
5.3
8.7
1.2
0.1
2.2
25.3
29.1
29.1
0.8
6e
7
0.7
8f 2 × 10−3 1.1
8
3.5
a
Reaction conditions, unless stated otherwise: [cyclohexane]0 = 0.46 mol L–1, [total H2O2]0 = 2.2 mol L–1 (50% aqueous),
[bmim][PF6] up to 5 mL total volume, 40 ºC; reaction time: 120 min. b Based on GC analysis, after treatment with PPh3. c
Total (cyclohexanol + cyclohexanone) turnover number (moles of product per mol of catalyst).d Conversion (%) = [1ꢀ
(unreacted moles of reagent after reaction/ initial moles of reagent)]x100. e Yields measured prior to the addition of PPh3. f
Using CuCl2 as catalyst (for comparative purposes).
Under our experimental conditions (Table 1), the oxidation of cyclohexane (CyH) to
cyclohexanol (CyOH) and cyclohexanone (CyO) is via cyclohexyl hydroperoxide (CyOOH, primary
product, Scheme 3) as proved by comparing the alcohol and ketone yields with and without the
addition of PPh3 to the samples before GC according to a method developed by Shul'pin23 and as
observed21−25 in other systems. In fact, prior to the addition of PPh3 (entries 3 and 6, Table 1), the
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alcohol and ketone amounts are lower and higher, respectively, than upon addition of PPh3 (see entries
3 vs. 2 and 6 vs. 5) where the selectivity towards the alcohol increases, indicating that cyclohexyl
hydroperoxide (CyOOH) is present at the end of the reaction (Scheme 3). Reduction of CyOOH by
PPh3 gives CyOH, whereas in the absence of PPh3, CyOOH decomposes in the gas chromatograph
into CyOH + CyO. The estimated amount of CyOOH at the end of reaction is ca. 33% of the total of
all products, for catalyst 1 (Table 1, entries 3 and 2). For catalyst 2, CyOOH was detected in a higher
amount (69%, Table 1, entries 6 and 5).
21–24
The formation of CyOOH (conceivably via reaction of Cy• with O2 to give CyOO•)
suggests a main radical pathway for these catalytic reactions. For unveiling more information, a
carbon radical trap (CBrCl3) and an oxygen radical trap (Ph2NH) were used, resulting in ca. 83−79%
decrease in the product yields of cyclohexane oxidation. This suggests that the reaction proceeds
mainly via radical mechanisms involving both carbonꢀcentred and oxygenꢀcentred radicals promoted
by the generation of hydroxyl radicals acting as active oxidizing species (Scheme 4).20–24
In accord with the mechanisms proposed for other copper catalysts with related ligands in
acetonitrile,19,25a different copper(II) catalysts19,21a–c,25a,27 and other Mn+1/n (e.g., V, Re, Fe or Au)
systems,21d,24b,27 the CyH oxidation can proceed according to Scheme 4. The metalꢀcatalysed
decomposition of hydrogen peroxide leads to the oxygenꢀcentred radicals HOO• and HO• upon
oxidation by Cu(II) or reduction by Cu(I), respectively (reactions 1 and 2). Water is believed to
catalyse the H+ꢀshift steps towards the formation of HO•.21b,c,27h The cyclohexyl radical Cy• is then
formed upon Hꢀabstraction from CyH by HO• (reaction 3). Reaction of Cy• with dioxygen leads to
CyOO• (reaction 4), and CyOOH can then be formed upon Hꢀabstraction from H2O2 by CyOO•
(reaction 5) or upon reduction of the latter to CyOO− by Cu(I) followed by protonation. Metalꢀassisted
decomposition of CyOOH to CyO• and CyOO• (reactions 6 and 7) would then lead ultimately to the
cyclohexanol (CyOH) and cyclohexanone (CyO) products (reactions 8 and 9).21a,e,25a,27e,g
Cu(II) + H2O2 → HOO• + H+ + Cu(I)
Cu(I) + H2O2 → HO• + Cu(II) + HO−
HO• + CyH → H2O + Cy•
(1)
(2)
(3)
(4)
(5)
(6)
(7)
Cy•+ O2 → CyOO•
CyOO• + H2O2 → CyOOH + HOO•
CyOOH + Cu(I) → CyO• + Cu(II) + HO−
CyOOH + Cu(II) → CyOO• + H+ + Cu(I)
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CyO• + CyH → CyOH + Cy•
2CyOO• → CyOH + CyO + O2
(8)
(9)
Scheme 4. Possible pathways for the catalytic oxidation of cyclohexane (CyH) to cyclohexanol
(CyOH) and cyclohexanone (CyO).
This type of mechanism is not hampered by the size of the copper cage in the complexes since in
this free radical mechanism the cyclohexane substrate does not need to interact directly with the metal
centre for the reaction to occur.16b
The higher activity of the complex 1 in comparison with 2 conceivably is accounted for by the
overall stronger electron donation of the set of anionic ligands (one sulfonate + one carboxylate + one
bridging sulfonate in the former versus one sulfonate + two bridging carboxylates in the later), which
promotes the reducing power of the reduced Cu(I) centre towards the reduction (i) of H2O2 to form the
key hydroxyl radical (reaction 2) and (ii) of CyOOH to yield CyO• (reaction 6).
We have also performed a control experiment with the IL, cyclohexane and H2O2, but without
any metal catalyst and no significant amount of product was formed (Table 1, entry 7). Thus, the
investigated IL appears to act as solvent only. We also used CuCl2 as catalyst, obtaining a very low
yield (3.5%, Table 1, entry 8)
In order to check the lifetime and stability of 1 and 2 in the IL, they were recycled in three
consecutive experiments. Each new cycle was initiated upon addition of new portions of all reagents.
After completion of each run, the products were analysed as previously and the IL with the catalyst
was recovered by drying in vacuum overnight at 70 ºC. The activity and selectivity of each catalyst
was maintained (Fig. 6, Table S3), which are significant improvements relative to the homogeneous
process in acetonitrile, where the separation and recyclability of the catalysts are not possible (see
below).
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Fig. 6 Effect of the catalyst recycling on product yield, in the peroxidative oxidation of cyclohexane.
Reaction conditions: [cyclohexane]0 = 0.46 mol L–1, [total H2O2]0 = 2.2 mol L–1 (50% aqueous),
[catalyst] = 2x10–3 mol L–1. [bmim][PF6] up to 5 mL total volume, 40 ºC. Reaction time: 120 min.
Each bar represents a run, and the respective yield value is given in supplementary information.
Studies have been reported17c for cyclohexane oxidation in the mixture [bmim][PF6]/CH2Cl2, at
room temperature, with a manganese(III) porphyrin catalyst and iodobenzene diacetate as oxidant.
Higher yields than ours were obtained, but our system operates with an environmentꢀfriendly oxidant
(H2O2).
We have recently reported the catalytic cyclohexane oxidation with a multinuclear copper
catalyst/ H2O2/ [bmim][PF6] system.16b In comparison with the current work that we now report, that
system, for similar yields, required an amount of catalyst of an order of magnitude higher. In another
study,19 the peroxidative oxidation of cyclohexane in [bmim][BF4] using Cu complexes with AHBD
(arylhydrazones of βꢀdiketones) ligands, in acid free conditions, a maximum product yield of 29.5%
was obtained. Using 1ꢀethylꢀ3ꢀmethylimidazolium tetrafluoroborate ([emim][BF4]) as IL, the
following catalyst/oxidant systems (with respective maximum yields) have been described: titanium
silicalite/tertꢀbutylꢀhydroperoxide (TBHP) (13% yield, after 18 h at 90 ºC);18b ZSMꢀ5 zeolite catalysts
(with different Si/Al ratios) / TBHP (15% yield, after 12 h at 90 ºC);18c FeZSMꢀ5 zeolite / TBHP (20%
yield, after 12 h at 90 ºC).18d In comparison with such reported works, our protocol is advantageous
since the reactions are performed at lower temperatures and require lower reaction time.
Mild peroxidative oxidation of cyclohexane in the conventional acetonitrile medium
We have also tested complexes 1 and 2 as catalysts in the peroxidative oxidation of
cyclohexane, with or without pyridine (py) as additive, in acetonitrile as the typical organic solvent,
for the reactions performed at 40 ºC (Table 2 and Figs. S4–S7).
The yields increased with the concentration of the catalysts and with time, except for the
complex 1 at the highest concentration, without pyridine and for the longest reaction time (4 h) (Table
2, entry 12), conceivably due to overꢀoxidation. For the experiments under typical conditions, the
mass balances indicate that only ca. 1ꢀ3% of the total cyclohexane conversion was not accounted for
by cyclohexanol and cyclohexanone.
Table 2. Oxidation of cyclohexane (CyH) to cyclohexanol (CyOH) and cyclohexanone (CyO)
catalyzed by 1 and 2 in acetonitrile.a
Catalyst Additive
amount (pyridine)
Yield % (after PPh3)b
CyO CyOH Total
Total
TOF
(h–1)
Time
(min)
Total
[CyOH]
/ [CyO]
Entry
TONc
(mol (mol L−1)
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L−1)
Catalyst 1
1
2
3
4
5
6
7
8
9d
10
11
12
13
14
15
16
17
15
60
120
240
15
60
120
240
1.1
1.4
4.5
9.8
0.8
1.2
2.4
3.9
1.8
2.2
8.1
2.9
3.6
12.6 290
67
83
267
83
1.6
1.6
1.8
1.4
3
4.8
5.4
6.4
2 × 10−4
—
145
135
294
159
176
167
13.6 23.4 538
2.4
5.7
3.2
6.9
74
159
2 × 10−4 0.005
12.9 15.3 352
25.1
21
29
29
667
667
11
240
15
60
120
240
15
60
120
240
8
167
45
20
20
10
88
28
20
18
2.6
5.1
1.8
1.5
2.4
2.8
3.3
2.7
2.9
0.8
4.1
5.63 8.71
10.46 17.31 40
11.61 16.46 38
4.9
3.08
6.85
4.85
2.5
2.78
4.64
8
20
2 × 10−3
—
7.1
9.6
22
9.19 11.97 28
12.44 17.08 39
23
18
2 × 10−3 0.005
31
30
71
69
18 d
240
12
17
1.5
Catalyst 2
19
20
21
22
23
24
25
26
15
60
120
240
15
60
120
240
1.3
0.8
2.1
2.5
0.1
1.7
6.7
7.1
2.6
4.5
7.4
1.5
2.5
8.8
9.6
3
33
58
202
220
69
133
58
101
55
0.1
2.1
3.2
2.8
6.5
5
2 × 10−4
—
0.4
0.9
5.8
8.3
276
124
152
106
5.4
13.2 304
124
2 × 10−4 0.005
1.3
1.2
10.2 18.5 426
27 d
240
11.3
7
18.3 421
105
0.6
28
29
30
31
32
33
34
35
36 d
15
60
120
240
15
2.1
2.3
5.2
6.7
5.7
4.8
6.4
7.8
12.3
9.4
9.7
10.1 15.3
11.4 18.1
11.5
12
26
28
35
42
28
33
42
52
51
106
28
18
4.5
4.2
1.9
1.7
1.2
2
1.8
1.9
0.8
2 × 10−3
—
10
6.6
9.4
11.9 18.3
14.6 22.4
12.3
14.1
113
33
21
60
2 × 10−3 0.005
120
240
240
13
10
3.2
ꢀ
22.3
5.4
ꢀ
13
37e 2 × 10−3 0.005
120
120
2.2
ꢀ
13
ꢀ
7
ꢀ
1.5
ꢀ
38
ꢀ
ꢀ
a
Reaction conditions, unless stated otherwise: [cyclohexane]0 = 0.46 mol L–1, [total H2O2]0 = 2.2 mol L–1 (50% aqueous),
[total H2O]0 = 6.4 mol L–1, CH3CN up to 5 mL total volume, 40 ºC. Based on GC analysis, after treatment with PPh3.
b
c
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d
Total (cyclohexanol + cyclohexanone) turnover number (moles of product per mol of catalyst). Yields measured prior to
the addition of PPh3. e Using CuCl2 as catalyst (for comparative purposes).
This behaviour has been reported for other catalytic systems containing copper
complexes.21a,b,25a A maximum yield of 31% was achieved with 1 (Table 1, entry 17) and over 22%
with 2 (Table 2, entry 35), at the highest concentration of the catalyst (2x10–3M) and in the presence of
pyridine. Without this additive, a maximum yield of 23% was reached for the lowest concentration of
catalyst 1 (Table 2, entry 4), while a maximum of more than 18% was obtained with 2 at its highest
concentration (Table 2, entry 26). The improvement with the use of pyridine can be explained by its
ability to promote the protonꢀtransfer steps involved in the formation of the hydroxyl radical from
H2O2.21d
Although the highest TON value was achieved with catalyst 1 (667, Table 2, entry 8), the
major increase in TON and yield, upon addition of pyridine, was attained for 2 with a 92%
improvement (Table 2, entries 22 and 26) for the lowest catalyst concentration, and 81% for the
highest one. Catalyst 2 is thus more susceptible to the presence of pyridine.
Both complexes 1 and 2 show a slight selectivity towards the alcohol, the highest [CyOH] /
[CyO] molar ratios (ca. 6.5) being attained with the lowest concentrations of catalysts and in the
presence of pyridine after 240 min (for 1, Table 2, entry 8) or 15 min (for 2, Table 2, entry 23).
However, the effect of the concentration of catalyst, of the additive and of the reaction time on the
molar ratio of alcohol and ketone is unsystematic, although the highest amount of alcohol being
obtained with the highest concentration of the catalysts and the presence of the additive (Figs. S6 and
S7, ESI).
The formation of CyOOH has been confirmed by Shul’pin’s method23 as in the case of using
the IL (see above). Reduction of CyOOH by PPh3 promotes a higher alcohol/ketone ratio. The
estimated amount of CyOOH at the end of reaction (entries with footnote d in Table 2) is ca. 24% for
the lowest concentration and ca. 27 % of the total of all products, for the highest concentration, for
catalyst 1 (entries 9 and 18, Table 2). For catalyst 2, CyOOH was detected in higher amounts, reaching
40% of the total of all product, for the highest concentration of catalyst (Table 2, entry 36) and 35%
for the lowest concentration of 2 (Table 2, entry 27).
As in the case of using the IL, experiments in the presence of Cꢀ and Oꢀradical traps (CBrCl3
and Ph2NH, respectively) revealed a marked decrease (ca. 70%–85%) in the yield, thus also
suggesting19,25a,27–29 a radical mechanism.
Using CuCl2 as catalyst in CH3CN, a low yield (5.4%, Table 2, entry 37) was obtained, while
without any metal catalyst no product was detected after 120 min (Table 2, entry 38).
A comparison of the product yields from cyclohexane oxidation, using [bmim][PF6] and some
organic solvents, under identical experimental conditions, was undertaken by some of us.16b The
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following order, with their increasing polarity, was observed: acetone < methanol < acetonitrile <
[bmim][PF6]. The promoting effect of this IL could be further related with its nonꢀcoordinating ability
making the catalyst active site more accessible to the substrate.16f,g,21b The charge and the steric
bulkiness of this IL, which can be described as a supramolecular polymeric species with weak
interactions,21b can possibly create an electrostatic and steric colloid type stabilization of the copper(II)
catalysts.
Chen et al. 16g reported the effect of the 1ꢀoctylꢀ3ꢀmethylimidazolium chloride, as well as other
ILs and molecular solvents, in the oxidation of cyclohexanol to cyclohexanone using COSMOꢀRS
theory as support. They concluded that the polarity of solvents influences the catalytic reaction to
some extent and that hydrophobic ILs tend to be more suitable for the reaction (oxidation of
cyclohexanol) with WO3 as catalyst.
Conclusions
Successful replacements of the chloride ligands of the known dichlorobridged dicopper(II)
compound [CuLCl]2 by benzeneꢀ1,3,5ꢀtricarboxylate (H2tma–) and benzeneꢀ1,2,4,5ꢀtetracarboxylate
anions (H2pma2–) produced the new ꢁ2ꢀ(2ꢀ(2ꢀpyridylmethyleneamino)benzene sulfonatoꢀO,O')ꢀ
copper(II) polymer [CuL(H2tma)]n (1) and the bisꢀꢁ4ꢀ(benzeneꢀ1,2,4,5ꢀtetracarboxylatoꢀO,O,O'',O'')ꢀ
dicopper(II) polymer [{Cu2L2(H2pma)}·(8H2O)]n (2), respectively. The crystal structures of these two
polymers revealed sulfonate and carboxylate as the bridging moieties (in 1 and in 2, respectively) even
with comparable combination of ligands. Compound 1 is the first example of a copper(II) polymer
with sulfonate bridge despite the availability of carboxylate as another potentially bridging group.
Compounds 1 and 2 were applied as efficient catalysts for the oxidation of cyclohexane with
aqueous H2O2 under mild conditions. The catalytic study in an ionic liquid (1ꢀbutylꢀ3ꢀ
methylimidazolium hexafluorophosphate, [bmim][PF6]) medium shows an increase in the yields and
in the turnover numbers, with shorter reaction times, in comparison to those when using the
conventional acetonitrile solvent. Simple recycling of these catalysts in the ionic liquid medium was
achieved without loss of activity and selectivity, which constitutes a further advantage.
Hence, a simple and efficient protocol is presented for such a reaction in an ionic liquid
medium, under mild and green conditions, and with relevant advantages relatively to the conventional
medium systems.
Experimental section
Materials and physical methods
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All the reagents and solvents were purchased from commercial sources and used as received.
The water used for all preparations and analyses was doubleꢀdistilled and deꢀionised. The
dichlorobridged dicopper(II) compound [CuLCl]2 was synthesized following the reported
procedure.25a,b 1ꢀButylꢀ3ꢀmethylimidazolium hexafluorophosphate, [bmim][PF6], was prepared from
the 1ꢀbutylꢀ3ꢀmethylimidazolium bromide by anion exchange with Na[PF6], using standard literature
methods18e and used after drying for 24 h at 60 oC under a high vacuum whilst stirring. The 1H and 13C
NMR spectra were recorded at ambient temperature on a Bruker Avance II + 300 (UltraShieldTM
Magnet) spectrometer operating at 300.130 and 75.468 MHz for proton and carbonꢀ13, respectively.
The chemical shifts are reported in ppm using tetramethylsilane as an internal reference. Infrared
spectra (4000–400 cm−1) were recorded on a Vertex 70 (Bruker) instrument in KBr pellets. Elemental
analyses were carried out by the Microanalytical Service of the Instituto Superior Técnico.
Electrospray mass spectra (ESIꢀMS) were run with an ionꢀtrap instrument (Varian 500ꢀMS LC Ion
Trap Mass Spectrometer) equipped with an electrospray ion source. For electrospray ionization, the
drying gas and flow rate were optimized according to the particular sample with 35 p.s.i. nebulizer
pressure. Scanning was performed from m/z 100 to 1200 in methanol solution. The compounds were
observed in the positive mode (capillary voltage = 80–105 V). All the synthetic work was performed
in air and at room temperature. Chromatographic analyses were undertaken by using a Fisons
Instruments GC 8000 series gas chromatograph with a DBꢀ624 (J&W) capillary column (flame
ionization detector) and the JascoꢀBorwin v.1.50 software. The internal standard method was used to
quantify the organic products. UVꢀVis spectra were monitored on a Perkin Elmer Lambdaꢀ35 UV/VIS
spectrometer.
Syntheses
[CuL(H2tma)]n (1). To an aqueous solution (35 mL) of [CuLCl]2 (0.360 g, 0.5 mmol) was added a
DMF solution (5 mL) of benzeneꢀ1,3,5ꢀtricarboxylic (trimesic) acid (H3tma) (0.210 g, 1.0 mmol) to
get a dark green solution. After a day, a dark green crystalline solid containing single crystals suitable
for Xꢀray diffraction precipitated and was collected by filtration. Yield: 0.384 g (72%). Anal. Calcd
for C21H14CuN2O9S (534): C, 47.24; H, 2.64; N, 5.25. Found: C, 47.32; H, 2.60; N, 5.22. FTꢀIR (KBr,
cm–1) data: ν(C=O) 1724s; ν(C=N), 1618m; ν(OCO), 1552m, 1482m; ν(C–O), 1261s; ν(C–COOH),
1157s; ν(sulfonate), 1383m, 775m.
[{Cu2L2(H2pma)}·(8H2O)]n (2). To an aqueous solution (20 mL) of [CuLCl]2 (0.360 g, 0.5 mmol)
was added an DMF solution (5 mL) of benzeneꢀ1,2,4,5ꢀtetracarboxylic (pyromellitic) acid (H4pma)
(0.127 g, 0.5 mmol) to get a green solution. After 12 h, a green crystalline solid containing single
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crystals suitable for Xꢀray diffraction precipitated and was collected by filtration. Yield: 0.392 g
(75%). Anal. Calcd for C34H38Cu2N4O22S2 (1046): C, 39.04; H, 3.66; N, 5.36. Found: C, 39.13; H,
3.62; N, 5.31. FTꢀIR (KBr, cm–1) data: ν(water), 3465br, ν(C=O), 1720s; ν(C=N), 1620m; ν(OCO),
1560s, 1488m; ν(C–O), 1256s; ν(C–COOH), 1167s; ν(sulfonate), 1380m, 776m.
Crystal structure determination
The Xꢀray diffraction data (Table S4, ESI) were collected using a BrukerAPEXꢀII PHOTON
100 with graphite monochromated MoꢀKα radiation. Data were collected using omega scans of 0.5º
per frame, and a full sphere of data was obtained. Cell parameters were retrieved using Bruker
SMART software and refined using Bruker SAINT30a on all the observed reflections. Absorption
corrections were applied using SADABS.30a Structures were solved by direct methods by using the
SHELXSꢀ2014 package30b,c and refined with SHELXLꢀ2014.30b,c Calculations were performed using
the WinGX SystemꢀVersion 2014.1.30b,c Coordinates of hydrogen atoms bonded to carbon atoms were
calculated following the stereochemical rules with C–H distances of 0.93 A for phenyl. The hydrogen
atoms were included in the refinement using the ridingꢀmodel approximation. Uiso(H) were defined as
1.2Ueq of the parent carbon atoms. Hydrogen atoms attached to the water molecules were located in
the difference Fourier synthesis and included in the final refinement using the riding model, with
Uiso(H) defined as 1.5Ueq of the parent oxygen atom. Least square refinements with anisotropic
thermal motion parameters for all the nonꢀhydrogen atoms and isotropic for most of the remaining
atoms were employed. The final refinement converged to R1 (I > 2σ(I)) values 0.0339 and 0.0361 for 1
and 2, respectively.
Peroxidative oxidation of cyclohexane
The alkane oxidations were carried out in air in round bottom flasks with vigorous stirring and
using [bmim][PF6] or CH3CN as solvent (up to 5.0 mL total volume). Typically, the catalyst 1 or 2
was added to the solvent as a solid or in the form of a stock solution in aqueous (10% vol) acetonitrile.
Pyridine (optional) was added in the form of a stock solution in CH3CN (0.44 mol·L–1). After the
addition of cyclohexane (2.3 mmol) the reaction was started when hydrogen peroxide (50% in H2O,
0.68 mL, 11 mmol) was added in one portion. The final concentrations of the reactants in the reaction
mixture were as follows: catalyst precursor 1 or 2 (2x10–4 and 2x10–3 mol L–1), pyridine (0.005 mol L–
1), substrate (0.46 mol L–1) and H2O2 (2.2 mol L–1). The reactions were stopped by cooling. In the case
of the ionic liquid, 1 mL of diethyl ether was then added to the biphasic reaction mixture for extraction
of the products.
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Catalyst recyclability was investigated using the ionic liquid medium, for up to three
consecutive cycles. Each cycle was initiated after the preceding one upon addition of new typical
portions of all other reagents. After completion of each run, the products were analysed as previously
and the ionic liquid with the catalyst was recovered by drying in vacuum overnight at 70 ºC.
All products formed were identified by GC and retention times compared with those of
commercially available products. Nitromethane (0.05 mL) was used as GC internal standard. The
reaction mixtures were analysed twice by GC: with and without adding an excess of solid
triphenylphosphine. The addition of this phosphine to the final organic phase reduces cyclohexyl
hydroperoxide, if present, to the corresponding alcohol, and hydrogen peroxide to water. Comparison
of the results of both analyses allows estimating the amount of cyclohexyl hydroperoxide, following a
method developed by Shul'pin.23 Blank experiments (CH3CN and ionic liquid) were performed and
confirmed that no alkane oxidation products (or traces, below 1%) were obtained in the absence of the
metal catalyst. MS analysis of the catalytic reaction mixture did not detect any byproduct. Moreover,
the conversion of cyclohexane was calculated and only a very small amount (<5%) of byproducts is
formed.
Acknowledgements
Financial supports from the Fundação para a Ciência e a Tecnologia (FCT), Portugal, for fellowships
SFRH/BPD/78264/2011 to S.H. and SFRH/BPD/90883/2012 to A.P.C.R., and also for the
PTDC/QEQꢀQIN/3967/2014 and UID/QUI/00100/2013 projects are gratefully acknowledged. The
authors also acknowledge the Portuguese NMR Network (ISTꢀUL Centre) for access to the NMR
facility, and the IST Node of the Portuguese Network of massꢀspectrometry (Prof. Conceição Oliveira)
for the ESIꢀMS measurements.
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Syntheses and crystal structures of benzene-sulfonate and -carboxylate
copper polymers and their application to oxidation of cyclohexane in ionic
liquid under mild conditions
Susanta Hazra, Ana P. C. Ribeiro, M. Fátima C. Guedes da Silva, Carlos A. Nieto de Castro and Armando
J. L. Pombeiro*
The copper polymers [CuL(H2tma)]n (1) and [{Cu2L2(H2pma)}·(8H2O)]n (2) were synthesized and
applied as efficient catalysts for the oxidation of cyclohexane with H2O2 in an ionic liquid medium under
mild conditions. The use of [bmim][PF6] shows an increase in the yields and turnover numbers, with
shorter reaction times, in comparison with acetonitrile. Catalysts recycling in [bmim][PF6] was achieved
without loss of activity and selectivity.
1
Sulfonate bridge
1 or 2
+
2
Carboxylate bridge