MnII and CuII complexes with arylhydrazones of active methylene compounds as effective heterogeneous catalysts for solvent- and additive-free microwave-assisted peroxidative oxidation of alcohols

Article abstract of DOI:10.1039/c5ra02667a

A one-pot template reaction of sodium 2-(2-(dicyanomethylene)hydrazinyl)benzenesulfonate (NaHL¹) with water and manganese(ii) acetate tetrahydrate led to the mononuclear complex [Mn(H₂O)₆](HL^1a)₂·4H₂O (1), where (HL^1a)^- = 2-(SO₃^-)C₆H₄(NH)NC(CN) (CONH₂) is the carboxamide species derived from nucleophilic attack of water on a cyano group of (HL¹)^-. The copper tetramer [Cu₄(H₂O)₁₀(1κN:κ²O:κO,2κN:κO-L²)₂]·2H₂O (2) was obtained from reaction of Cu(NO₃)₂·2.5H₂O with sodium 5-(2-(4,4-dimethyl-2,6-dioxocyclohexylidene)hydrazinyl)-4-hydroxybenzene-1,3-disulfonate (Na₂H₂L²). Both complexes were characterized by elemental analysis, IR spectroscopy, ESI-MS and single crystal X-ray diffraction. They exhibit a high catalytic activity for the solvent- and additive-free microwave (MW) assisted oxidation of primary and secondary alcohols with tert-butylhydroperoxide, leading to yields of the oxidized products up to 85.5% and TOFs up to 1.90 × 10³ h^-1 after 1 h under low power (5-10 W) MW irradiation. Moreover, the heterogeneous catalysts are easily recovered and reused, at least for three consecutive cycles, maintaining 89% of the initial activity and a high selectivity.

Full text of DOI:10.1039/c5ra02667a

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MnII and CuII complexes with arylhydrazones of
active methylene compounds as effective
heterogeneous catalysts for solvent- and additive-
free microwave-assisted peroxidative oxidation of
alcohols†
Cite this: RSC Adv., 2015, 5, 25979
ab
a
ac
*
*
Kamran T. Mahmudov,
Manas Sutradhar, Luısa M. D. R. S. Martins,
´
M. Fatima C. Guedes da Silva, Alice Ribera,ad Ana V. M. Nunes,e
a
*
´
f
d
a
*
Shahnaz I. Gahramanova, Fabio Marchetti and Armando J. L. Pombeiro
A one-pot template reaction of sodium 2-(2-(dicyanomethylene)hydrazinyl)benzenesulfonate (NaHL1) with
water and manganese(II) acetate tetrahydrate led to the mononuclear complex [Mn(H2O)6](HL1a)2$4H2O
(1), where (HL1a ꢀ
)
¼
2-(SO3ꢀ)C6H4(NH)N]C(C^N) (CONH2) is the carboxamide species derived
from nucleophilic attack of water on a cyano group of (HL1)ꢀ. The copper tetramer [Cu4(H2O)10
-
(1kN:k2O:kO,2kN:kO-L2)2]$2H2O (2) was obtained from reaction of Cu(NO3)2$2.5H2O with sodium 5-(2-
(4,4-dimethyl-2,6-dioxocyclohexylidene)hydrazinyl)-4-hydroxybenzene-1,3-disulfonate (Na2H2L2). Both
complexes were characterized by elemental analysis, IR spectroscopy, ESI-MS and single crystal X-ray
diffraction. They exhibit a high catalytic activity for the solvent- and additive-free microwave (MW)
assisted oxidation of primary and secondary alcohols with tert-butylhydroperoxide, leading to yields of
the oxidized products up to 85.5% and TOFs up to 1.90 ꢁ 103 hꢀ1 after 1 h under low power (5–10 W)
MW irradiation. Moreover, the heterogeneous catalysts are easily recovered and reused, at least for three
consecutive cycles, maintaining 89% of the initial activity and a high selectivity.
Received 11th February 2015
Accepted 4th March 2015
DOI: 10.1039/c5ra02667a
www.rsc.org/advances
drugs, ligands, vitamins, etc., and important intermediates for
many complex syntheses.1–3 A good number of studies has been
1. Introduction
reported on homogeneous or heterogeneous catalytic oxidation
of alcohols with different catalysts and oxidants.4,5 Due to
several disadvantages of heterogeneous systems, namely diffi-
cult catalyst recycling, alcohol oxidation using heterogeneous
catalysts is gathering increasing interest.2,4 Moreover, oxidation
of alcohols with air or peroxides represents an important eld of
contemporary metal complex catalysis.4–6 Galactose oxidase is a
copper-containing enzyme that catalyzes the two-electron
oxidation of primary alcohols to aldehydes using molecular
oxygen as the terminal oxidant.4 Synthetic models of galactose
oxidase, i.e., copper complexes with different types of organic
ligands are also known to be good catalysts in the oxidation of
alcohols by molecular oxygen of air or hydrogen peroxide.4,5
Recently, we have found that water-solubility of arylhy-
drazones of active methylene compounds (AHAMCs) and their
copper(II) complexes can be increased by functionalization of
ligands with hydrophilic polar groups, such as sulfo, carboxy or
nitro, and they can be signicantly active catalysts in the aerobic
and peroxidative oxidation of alcohols or of unsaturated
hydrocarbons to the corresponding organic compounds.7–11 The
modication of AHAMCs ligands with several sulfonic groups
can promote the solubility of the isolated complexes in polar
The development of efficient and selective catalysts for the
oxidation of alcohols into their corresponding carbonyl
compounds is a fundamental issue in organic synthesis, as well
as in the chemical industry.1,2 Carbonyl compounds, such as
aldehydes and ketones, are precursors for the synthesis of many
a
´
´
Centro de Quımica Estrutural, Instituto Superior Tecnico, Universidade de Lisboa, Av.
Rovisco Pais, 1049–001 Lisbon, Portugal. E-mail: kamran_chem@yahoo.com;
kamran_chem@mail.ru; fatima.guedes@tecnico.ulisboa.pt; lmartins@deq.isel.ipl.pt;
pombeiro@tecnico.ulisboa.pt
bBaku State University, Department of Chemistry, Z. Xalilov Str. 23, Az 1148 Baku,
Azerbaijan
c
´
Chemical Engineering Department, ISEL, R. Conselheiro Emıdio Navarro, 1959-007
Lisboa, Portugal
dSchool of Pharmacy, University of Camerino, Via S. Agostino 1, 62032 Camerino, Italy
e
´
ˆ
Requimte/CQFB, Departamento de Quımica, Faculdade de Ciencias e Tecnologia,
Universidade Nova de Lisboa, Campus de Caparica, Caparica 2829-516, Portugal
fInstitute of Catalysis and Inorganic Chemistry named aer acad. M.F. Nagiyev, H.
Cavid 113, Az 1143 Baku, Azerbaijan
† Electronic supplementary information (ESI) available: Analytical data for
compounds NaHL1 and Na2H2L2. Crystallographic and selected structural
details are listed in Tables S1 and S2. CCDC 1023179 and 1023180. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c5ra02667a
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solvents, but hamper their solubility in solvents with low resulting in the formation of the carboxamide (HL1a)ꢀ ¼ 2-
polarity. Hence, these complexes could act as heterogeneous (SO3ꢀ)C6H4(NH)N]C(C^N)(CONH2). A slow evaporation of a
catalysts for the solvent-free and microwave-assisted oxidation mixture of copper(II) nitrate hydrate, Na2H2L2 and HNO3 in
of less polar bulky substituted or long chain alcohols, such as water/methanol mixture furnishes greenish-black crystals of
benzyl alcohol, 1-phenylethanol, hexan-1-ol, heptan-1-ol, etc. to the tetranuclear complex [Cu(H2O)Cu(H2O)4(m-L2)]2$2H2O (2)
the corresponding carbonyl compounds. The application of (Scheme 1).
such an approach to the establishment of an heterogeneous Both complexes were characterized by elemental analysis, IR
catalytic system for alcohol oxidation constitutes the main aim spectroscopy, ESI-MS and X-ray analysis. Elemental analysis and
of the current study.
the ESI-MS peaks at 81.5 [Mn(H2O)6]2+, 163.0 [Mn(H2O)6]+,
Herein, we report the synthesis and characterization of 267.2 (HL1a)ꢀ, and 652.3 [Cu2(H2O)6(L2)+H+] support the
highly water soluble copper(II) and manganese(II) complexes of formulation. In the IR spectrum of 1, n(OH) and n(NH) vibra-
AHAMCs, i.e., 2-(2-(dicyanomethylene)hydrazinyl)benzenesul- tions are observed at 3436 and 3103 cmꢀ1, respectively, while
fonate (HL1)ꢀ and 5-(2-(4,4-dimethyl-2,6-dioxocyclohexylidene) n(C^N) of the unreacted cyano group appears at 2227 cmꢀ1
.
hydrazinyl)-4-hydroxybenzene-1,3 disulfonate (L2)4ꢀ, and their A new peak at 1645 cmꢀ1 indicates that, in the solid state,
catalytic activity (as heterogeneous catalysts) towards solvent- 1 exhibits the carboxamide form which is also supported by
and additive-free peroxidative oxidation of aromatic and of X-ray data (see below). The IR spectrum of 2 displays peaks at
aliphatic cyclic or long chain alcohols under low power micro- 3307, 3217 and 3129 n(OH), 1666 and 1639 n(C]O) and
wave irradiation.
1582 n(C]N) cmꢀ1, which are signicantly shied in relation to
those of the corresponding free ligand at 3287, 3203 and
3105 n(OH), 2965 n(NH), 1648 and 1622 n(C]O) and 1576
n(C]N) cmꢀ1. No band assignable to n(NH) was observed,
indicating that (L2)4– is coordinated in the deprotonated
hydrazone form (see below).
The molecular structures of complexes 1 and 2 have been
established by single-crystal X-ray diffraction (Scheme 1, Fig. 1).
The crystal structure of 1 consists of two anionic organic
moieties, one cationic Mn centre with an almost perfect octa-
2. Results and discussion
2.1. Synthesis and characterization of 1 and 2
Sodium salts of both ligands, NaHL1 and Na2H2L2, were
synthesized by the Japp–Klingemann method7b,12 upon reaction
between the aromatic diazonium salt of a substituted aniline
and malononitrile or 5,5-dimethylcyclohexane-1,3-dione in
water solution containing sodium hydroxide (for analytical data
see ESI†). They were reported earlier by some of us.7b,12
hedral geometry (quadratic elongation of 1.000 and angle vari-
2
ance of 0.52ꢂ
)
13 lled with water ligands, and two crystallization
The reaction of NaHL1 with Mn(CH3COO)2$4H2O in water/
acetone mixture afforded a deep red precipitate which upon
recrystallization from methanol produces deep red crystals of
[Mn(H2O)6](HL1a)2$4H2O (1) in 70% yield (Scheme 1). The
synthesis involves an in situ ligand formation, in which one of
the cyano groups of (HL1)ꢀ undergoes hydrolysis by water
water molecules. The asymmetric unit of 2 contains half of the
tetranuclear cluster molecule, an inversion centre being located
in the middle of the macrocycle. The L2 ligands act as hexa-
dentate 1kN:k2O:kO,2kN:kO chelators to the copper cations that
present distorted N1O4 square pyramidal (Cu1, s5 ¼ 0.14)14 and
Scheme 1 Synthesis and schematic representation of 1 and 2.
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Fig. 1 Crystal structures of 1 and 2. Hydrogen atoms and crystallization water molecule (in 2) were omitted for clarity. Symmetry codes to
generate equivalent atoms: (i) 1 ꢀ x, ꢀy, ꢀz (1); 1 ꢀ x, 1 ꢀ y, 1 ꢀ z (2).
N1O5 octahedral environments (Cu2, quadratic elongation of substrate) and in the solvent- and additive-free medium, the
2
1.058 and angle variance of 591.06ꢂ ).13 Both compounds systems yield 56.4% and 65.5% of cyclohexanone aer 1 h at
present extensive hydrogen bond interactions that lead to 3D 150 ꢂC/10 W MW irradiation (Table 2, entries 1 and 2, for 1 and
supramolecular frameworks.
2, respectively). The linear aliphatic alcohols 2-hexanol and
3-hexanol lead to similar (1) or lower (2) yields (Table 2,
entries 3–6), under similar reaction conditions, as reported in
other cases.5,6h–k,15 However, benzyl alcohol leads to the lowest
product yield in contrast to what is usually observed,16 what can
2.2. Catalytic activity of 1 and 2 in alcohol oxidation
Complexes 1 and 2 were tested as catalysts for the oxidation of
1-phenylethanol (model substrate), using aqueous tert-butylhy-
droperoxide (ButOOH, TBHP, aq. 70%, 2 eq.) as oxidizing agent,
under typical conditions of 150 ꢂC, low power (10 W) microwave
(MW) irradiation, 1 h reaction time and in a solvent- and
additive-free medium (Scheme 2). A good catalytic activity was
observed under the above conditions, leading to yields of ace-
tophenone up to 76.1% (1) or 85.5% (2) (Table 1, entries 12 and
10, respectively), and turnover frequencies (TOF, moles of
product per mol of catalyst per hour) up to 1.83 ꢁ 10ꢀ3 (1) or
1.90 ꢁ 103 hꢀ1 (2, i.e., 4.75 ꢁ 102 hꢀ1 per Cu atom) for a low
catalyst loading (catalyst/substrate molar ratio of 0.04%) aer
1 h under MW irradiation (Table 1, entries 11 and 13,
respectively).
ꢂ
be due to the used temperature of 150 C. In fact, it was previ-
ously found15a that temperatures up to 100 ꢂC are preferable for
the solvent-free peroxidative MW oxidation of this substrate
catalyzed by copper compounds.
While the heterogeneous copper 2/MW/TBHP oxidation
system of the present study yields ketone products in the range
of other previously reported copper homogeneous systems,5,6h–k
the heterogeneous Mn 1/MW/TBHP system is much more effi-
cient than other MW/TBHP homogeneous systems with dinu-
clear Mn(II) complexes with Schiff bases15b in the oxidation of
secondary alcohols, since our system operates effectively under
additive-free conditions and requires much lower loads of
catalyst precursor.
The activity of 1 or 2 in the oxidation of benzyl alcohol is
rather modest since only moderated 19% (1) or 24% (2) yields of
benzaldehyde were obtained under the same reaction condi-
tions (Table 2, entries 7 and 8, for 1 and 2, respectively). The
silica-supported manganese dioxide previously applied16 for the
oxidation of benzyl alcohol under MW solvent-free conditions,
yielded 88% of benzaldehyde aer 20 s irradiation. However,
the MW power and the temperature were not indicated and this
system required a 5 : 1 molar excess of MnO2 relatively to the
substrate.
Moreover, these heterogeneous catalysts could be easily
recovered and reused, at least for three consecutive cycles,
maintaining 89% of the initial activity and a rather high selec-
tivity (see below).
Complexes 1 and 2 were also tested towards the oxidation of
primary (benzyl alcohol) and other secondary alcohols, namely
cyclohexanol and the linear 2- and 3-hexanol. The ketones are
the only oxidation products obtained (Table 2). As expected, the
alicyclic cyclohexanol is less reactive than 1-phenylethanol: in
the presence of a catalytic amount of 1 or 2 (0.4 mol% vs.
The inuences of various reaction parameters, such as the
amounts of catalyst and oxidant, type of oxidant, time,
temperature and presence of additives, were investigated for the
most active substrate (1-phenylethanol), and the results are
summarized in Table 1 and Fig. 2–4.
The MW-assisted alcohol oxidation depends strongly on the
temperature (Fig. 2). The overall temperature coefficient of
the oxidations is ca. 6.8 for 1 or 4.1 for 2 (temperature range
80–150 ꢂC). For example, a high yield of 85.5% of acetophenone
Scheme 2 MW-assisted oxidation of 1-phenylethanol to acetophe-
none catalysed by 1 or 2.
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Table 1 Selected results for the solvent-free MW-assisted oxidation of 1-phenylethanol using 1 or 2 as catalystsa
Catalyst (amount,
mol% vs. substrate)
Reaction time
(h)
Temperature
(ꢂC)
Additive
(mol% vs. substrate)
Yieldb
(%)
TONc
Entry
[TOF (hꢀ1)]
1
2
3
4
5
6
7
8
1 (0.4)
1 (0.4)
1 (0.4)
2 (0.4)
2 (0.4)
2 (0.4)
1 (0.4)
1 (0.4)
2 (0.4)
2 (0.4)
1 (0.04)
1 (0.2)
2 (0.04)
2 (0.2)
1 (0.4)
1 (0.4)
1 (0.4)
1 (0.4)
1 (0.4)
2 (0.4)
2 (0.4)
2 (0.4)
2 (0.4)
2 (0.4)
1 (0.4)
Cu(NO3)2 (0.4)
Mn(CH3COO)2 (0.4)
0.5
1
2
0.5
1
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
80
80
80
80
80
—
—
—
—
—
—
—
—
—
—
—
—
—
—
6.0
11.1
14.7
14.4
21.1
39.2
33.1
75.2
83.2
85.5
73.0
76.1
76.0
78.2
38.1
34.0
56.6
9.7
33.1
19.8
43.7
58.1
5.6
33.7
3.0
18.2
16.9
15 (30)
28 (28)
37 (18)
36 (72)
52 (52)
98 (49)
83 (83)
188 (188)
208 (208)
214 (214)
1.83 ꢁ 103 (1.83 ꢁ 103)
381 (381)
1.90 ꢁ 103 (1.90 ꢁ 103)
391 (391)
95 (95)
85 (85)
142 (142)
24 (24)
83 (83)
50 (50)
80
120
150
120
150
150
150
150
150
150
150
150
150
150
150
150
150
150
150
150
150
150
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25d
26e
27e
K2CO3 (2.5)
HNO3 (2.5)
TEMPO (2.5)
Ph2NH (100)
CBrCl3 (100)
K2CO3 (2.5)
HNO3 (2.5)
TEMPO (2.5)
Ph2NH (100)
CBrCl3 (100)
—
109 (109)
145 (145)
14 (14)
84 (84)
8 (8)
—
—
46 (46)
42 (42)
1
a
Reaction conditions unless stated otherwise: 2.5 mmol of 1-phenylethanol, 1–10 mmol (0.04–0.4 mol% vs. substrate) of 1 or 2, 5 mmol of TBHP
(2 eq., 70% in H2O), 80–150 ꢂC, MW irradiation (5–10 W power). b Molar yield (%) based on substrate, i.e. moles of acetophenone per 100 mol of
1-phenylethanol determined by GC. c Turnover number ¼ number of moles of product per mol of catalyst precursor; TOF ¼ TON per hour (values in
brackets). d 5 mmol of H2O2 (30% aqueous solution) instead of TBHP. e Included for comparative purposes.
is achieved aer 1 h at 150 ꢂC (5 W) for the copper system 2 an amount increase of 1 does not appear to have a signicant
without any additive (Table 1, entry 10), which is much higher effect on the acetophenone yield, the increase of 2 results in a
than the 21.1% yield obtained for the same reaction time but at yield enhancement, e.g. from 76.0% to 85.5% upon changing
80 ꢂC (Table 1, entry 5). Furthermore, at this temperature, only the amount of catalyst from 1 mmol (0.04% vs. substrate) to
39.2% (2) or 14.7% (1) of acetophenone is obtained for the 10 mmol (0.4 mol% vs. substrate). As expected, the increase of
extended reaction time of 2 h (Table 1, entries 6 and 3, the catalysts amounts results in a corresponding TON lowering
respectively).
for both 1 and 2 (Fig. 3).
The inuence of the amount of 1 and 2 on the yield and TON
For 1, experiments with the cheaper and environmentally
(or TOF) is shown in Fig. 3 (entries 8 and 10–14, Table 1). While friendly hydrogen peroxide (30% aqueous solution) as oxidant
Table 2 Selected results for the solvent-free MW-assisted oxidation of selected C6 secondary alcohols and of benzylalcohol using 1 and 2 as
catalystsa
Entry
Catalyst
Substrate
Product
Yieldb (%)
TONc [TOF (hꢀ1)]
1
2
3
4
5
6
7
8
1
2
1
2
1
2
1
2
Cyclohexanol
2-Hexanol
Cyclohexanone
2-Hexanone
3-Hexanone
Benzaldehyde
56.4
65.5
60.3
52.4
55.5
37.1
24.0
19.1
141 (141)
164 (164)
151 (151)
131 (131)
139 (139)
93 (93)
3-Hexanol
Benzylalcohol
60 (60)
48 (48)
a
Reaction conditions: 2.5 mmol of substrate, 5 mmol of TBHP (aq. 70%), 10 mmol (0.4 mol% vs. substrate) of 1 or 2, 150 ꢂC, 1 h of microwave
irradiation (10 W). b Molar yield (%) based on substrate, i.e. moles of product per 100 mol of substrate determined by GC. Turnover number ¼
c
number of moles of product per mol of metal catalyst; TOF ¼ TON per hour (values in brackets).
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Fig. 2 Influence of the temperature on the yield of acetophenone in
the MW-assisted peroxidative oxidation of 1-phenylethanol catalysed
by 1 or 2.
Fig. 4 Influence of different additives (TEMPO, K2CO3, HNO3, radical
traps) on the yield of acetophenone in the MW-assisted peroxidative
oxidation of 1-phenylethanol catalysed by 1 and 2.
are not effective, as attested by the marked yield lowering,
e.g., from 85.5% to 3.0% (entries 10 and 25, Table 1), in accord
with the expected decomposition of H2O2 under the used
TBHP catalysed by bi- or tetra-nuclear cage-like copper(II)
silsesquioxanes.
ꢂ
reaction conditions (150 C).
A very strong inhibition effect is observed (Fig. 4) when the
peroxidative oxidation of 1-phenylethanol is carried out in the
presence of either an oxygen-radical trap such as Ph2NH (Table
1, entries 18 (1) or 23 (2)) or a carbon-radical trap such as CBrCl3
(Table 1, entries 19 (1) or 24 (2)). This suggests that the oxida-
tion reaction proceeds mainly via a radical mechanism
involving both oxygen- and carbon-centred radicals.22 It may
involve, e.g., the tBuOc radical produced in the Mn or Cu
promoted decomposition of TBHP.5 It may proceed via the
coordination of the alcohol substrate to an active site of the
catalyst, and its deprotonation to form the alkoxide ligand,
followed by a metal-centred dehydrogenation.6b,17a,20a,c,d,23
2.2.1. Catalyst recycling. Catalyst recyclability was investi-
gated for up to three consecutive cycles for both catalysts 1 and
2. On completion of each cycle, the product was analyzed as
usually and the solid catalyst was recovered upon ltration of
the reaction mixture, thoroughly washed and dried. The
subsequent cycle was initiated upon addition of new standard
portions of all other reagents. The ltrate was tested in a new
reaction (by addition of fresh reagents), and no oxidation
products were detected. Fig. 5 shows the recyclability of the
systems: in the second cycle, 1 maintains almost (99.5%) the
The inuence of 2,2,6,6-tetramethylpiperidyl-1-oxyl (TEMPO),
a nitroxyl radical can be a promoter in aerobic oxidation catalysis
of alcohols,5,6h,15,17ꢀ19 was also investigated. However, a signicant
yield decrease (Fig. 4) was observed for the 1-phenylethanol
oxidation, from 75.2% or 85.5% in the absence of TEMPO (for 1or
2, respectively, entry 8 or 10, Table 1) to 56.6% (1) or 58.1% (2) in
its presence (entry 17 or 22, Table 1). TEMPO is also known to
inhibit the further oxidation of aldehydes to carboxylic acids
when this reaction occurs via a radical mechanism.19c Moreover,
the previously recognised promoting effect of basic additive-
s5,6a,b,e,17a,20 is also not observed for the present catalytic systems;
in contrast, addition of 1 M aqueous solution of K2CO3 hampers
the catalysis (Table 1, entries 15 and 20; Fig. 4), eventually as a
result of reaction with the aqua-metal centres. The presence of
HNO3 also exhibits an inhibitory effect on the acetophenone
yield (Table 1, entries 16 and 21, Fig. 4), as found, e.g., for the
MW-assisted oxidation of 1-phenylethanol with TBHP catalysed
by Cu(II) complexes bearing Schiff base6h or 1,6-bis(20-pyriyl)-2,5-
dithiahexane ligands.6e,21 In fact, the mechanism of alcohol
oxidation with TBHP does not seem to require the presence of
acid as also veried6f,g in the oxidation of 1-phenylethanol with
Fig. 3 Effect of the amount of catalysts 1 and 2 on the yield (solid lines) and turnover number (TON) (dashed lines) for the oxidation of
1-phenylethanol to acetophenone.
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proton and carbon-13, respectively. The chemical shis are
reported in ppm using tetramethylsilane as an internal refer-
ence. The infrared spectra (4000–400 cmꢀ1) were recorded on a
BIO-RAD FTS 3000MX instrument in KBr pellets. Carbon,
hydrogen and nitrogen elemental and atomic absorption
(Mn and Cu) analyses were carried out by the Microanalytical
´
Service of the Instituto Superior Tecnico. All of the synthetic
work was performed in air and at room temperature. The
catalytic tests under microwave irradiation (MW) were per-
formed in a focused Anton Paar Monowave 300 microwave
reactor using a 10 mL capacity reaction tube with a 10 mm
internal diameter, tted with a rotational system and an IR
temperature detector. Chromatographic analyses were under-
taken by using a Fisons Instruments GC 8000 series gas chro-
matograph with a DB-624 (J&W) capillary column (DB-WAX,
column length: 30 m; internal diameter: 0.32 mm), FID
detector, and the Jasco-Borwin v.1.50 soware. The temperature
of injection was 240 ꢂC. The initial temperature was maintained
at 140 ꢂC for 1 min, then raised 10 ꢂC minꢀ1 to 220 ꢂC and held
at this temperature for 1 min. Helium was used as the carrier
gas. The internal standard method was used to quantify the
organic products.
Fig. 5 Effect of the catalyst recycling on the yield of acetophenone
from MW-assisted peroxidative oxidation of 1-phenylethanol catalysed
by 1 and 2.
original level of activity, and 2 achives 91.8% of its initial one. In
the third consecutive reaction cycle, 89% of the initial activity is
still exhibited by both complexes, with a rather high selectivity
to acetophenone. Atom absorption analysis (7.1% Mn and
19.5% Cu), IR spectroscopy (Fig. S1 and S2, ESI†) and powder
X-ray diffraction (Fig. S3 and S4, ESI†) for the recovered catalysts
1 and 2 showed that the structural integrity remains unaltered
aer the catalytic reaction.
X-ray powder diffraction patterns (XRPD) were recorded in
the reection mode using a D-8 Bruker AXS diffractometer
operating at 40 kV and 40 mA, using CuKa radiation
3. Conclusions
˚
(l ¼ 1.5418 A). The XRD patterns were collected at 2q values
ranging from 3ꢂ to 70ꢂ with step 0.02ꢂ. Electrospray mass
spectra (ESI-MS) were run with an ion-trap instrument (Varian
500-MS LC Ion Trap Mass Spectrometer) equipped with an
electrospray ion source. For electrospray ionization, the drying
gas and ow rate were optimized according to the particular
sample with 35 p.s.i. nebulizer pressure. Scanning was per-
formed from m/z 50 to 1200 in methanol solution. The
compounds were observed in the negative or positive mode
(capillary voltage ¼ 80–105 V).
We have achieved simple and effective syntheses of hydro-
soluble mononuclear manganese(II) and tetranuclear copper(II)
complexes with AHAMCs ligands, which are not soluble in low
polar solvents, thus with a potential for application as hetero-
geneous catalysts in such media. Their structure and nuclearity
are dependent on the active methylene fragment and on
substituents in the aromatic part of the AHAMCs ligands,
besides the metal ions. The obtained complexes act as efficient
and selective heterogeneous catalyst precursors for the mild
MW-assisted oxidation of secondary alcohols in solvent- and
additive-free systems, thus widening the scope of heteroge-
neous catalytic systems suitable for MW assisted oxidative
transformations of alcohols. A comparative study of their cata-
lytic efficiency has been drawn towards different alcohol
substrates.
Moreover, these heterogeneous catalysts are easily recycled
without considerable loss of activity. Hence, they show the same
advantage, in terms of easy separation and recycling, of sup-
ported metal complex catalysts, but without requiring the use of
any solid support. The approach followed in this study deserves
to be further explored and extended to other oxidation catalyses
and to the synthesis and catalytic applications of metal
complexes of low solubility in common organic solvents.
4.2. Synthesis of complexes
4.2.1. Synthesis of 1. Manganese(II) acetate tetrahydrate,
Mn(CH3COO)2$4H2O (24.5 mg, 0.1 mmol), was added to a
solution of NaHL1 (27.2 mg, 0.1 mmol) in acetone–water (3 : 1,
v/v) mixture (20 mL). The reaction mixture was heated under
reux for 3 h. A deep red precipitate was obtained. The reaction
mixture was then ltered and the ltrate was rejected. The
residue obtained was washed several times with acetone. The
crystals of 1 suitable for X-ray structural analysis were obtained
by slow evaporation of a methanol solution of the deep red
solid.
Yield, 27 mg, 70%. Anal. calcd for C18H34MnN8O18S2 (Mr ¼
769.59): C, 28.09; H, 4.45; N, 14.56. Found: C, 28.42; H, 4.61; N,
14.17%. MS (ESI) m/z: 81.5 [Mn(H2O)6]2+, 163.0 [Mn(H2O)6]+ and
267.2 (HL1a)ꢀ. IR, cmꢀ1: 3436 n(OH), 3103 n(NH), 2227 n(C^N),
1645 n(C]O), 1630 d(OH) of H2O, 1513 n(C]N).
4. Experimental
4.1. Materials and methods
All the chemicals were obtained from commercial sources and
4.2.2. Synthesis of 2. Copper(II) nitrate pentahemihydrate,
used as received. The 1H and 13C NMR spectra were recorded at Cu(NO3)2$2.5H2O (23.2 mg, 0.1 mmol), and 0.05 mL of HNO3
room temperature on a Bruker Avance II 300 (UltraShield™ (65%, w/w) were added to a solution of Na2H2L2 (46.4 mg,
Magnet) spectrometer operating at 300.130 and 75.468 MHz for 0.1 mmol) in MeOH/water (4 : 1 v/v) mixture (16 mL). The
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reaction mixture was le to slow evaporation at room temper- A.V.M.N. express gratitude to FCT for the post-doc fellowships.
ature. Aer two days, greenish-black crystals of 2 were obtained. The authors acknowledge the Portuguese NMR Network
Yield, 24 mg, 74%. Anal. Calcd for C28H48Cu4N4O30S4 (IST-UTL Centre) for access to the NMR facility, and the IST
(Mr ¼ 1303.10) C, 25.81; H, 3.71; N, 4.30. Found: C, 25.75; H, Node of the Portuguese Network of mass-spectrometry (Dr
˜
3.63; N, 4.07. MS (ESI): m/z: 652.3 [Cu2(H2O)6(L2)+H+]. IR, cmꢀ1
:
Conceiçao Oliveira) for the ESI-MS measurements.
3307, 3217, 3129 and 2964 n(OH), 1666 n(C]O), 1639 n(C]O),
1628 d(OH) of H2O, 1582 n(C]N).
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Acknowledgements
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