Sulfonated Schiff base copper(ii) complexes as efficient and selective catalysts in alcohol oxidation: syntheses and crystal structures

Article abstract of DOI:10.1039/c5ra19498a

The reaction between 2-aminobenzenesulfonic acid and 2-hydroxy-3-methoxybenzaldehyde produces the acyclic Schiff base 2-[(2-hydroxy-3-methoxyphenyl)methylideneamino]benzenesulfonic acid (H₂L·3H₂O) (1). In situ reactions of this compo

Full text of DOI:10.1039/c5ra19498a

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Sulfonated Schiff Base Copper(II) Complexes as Efficient and Selective
Catalysts in Alcohol Oxidation: Syntheses and Crystal Structures†
Susanta Hazra,*a Luísa M.D.R.S. Martins,*a,b M. Fátima C. Guedes da Silva*a and Armando J.
L. Pombeiroa
a Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049œ001, Lisboa,
Portugal. E-mail: h.susanta@gmail.com and fatima.guedes@tecnico.ulisboa.pt
b
Chemical Engineering Department, Instituto Superior de Engenharia de Lisboa, Instituto Politécnico de Lisboa, R.
Conselheiro Emídio Navarro, 1959-007, Lisboa, Portugal. E-mail: lmartins@deq.isel.ipl.pt
†Electronic Supplementary Information (ESI) available: Figures S1 and S2. Tables S1 and S2. CCDC 1411065 œ 1411069 for 1
œ 5, respectively, contain the supplementary crystallographic data for this paper. 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 or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033 or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this article can be found in the online version.
í

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Abstract
The reaction between 2-aminobenzenesulfonic acid and 2-hydroxy-3-methoxybenzaldehyde
produces the acyclic Schiff base 2-[(2-hydroxy-3-
methoxyphenyl)methylideneamino]benzenesulfonic acid (H2L) (1). In situ reactions of this
compound with Cu(II) salts and, eventually, in the presence of pyridine (py) or 2,2‘-bipyridine
(2,2‘-bipy) lead to the formation of the mononuclear complexes [CuL(H2O)2] (2) and [CuL(2,2‘-
bipy)].DMF.H2O (3) and the diphenoxo-bridged dicopper compounds [CuL(py)]2 (4) and
[CuL(EtOH)]2.2H2O (5). In 2 œ 5 the L2œ ligand acts as a tridentate chelating species by means of
one of the O-sulfonate atoms, the O-phenoxo and the N-atoms. The remaining coordination sites
are then occupied by H2O (in 2), 2,2‘-bipyridine (in 3), pyridine (in 4) or EtOH (in 5). Hydrogen
bond interactions resulted in 466:sv; and in 488:st; graph sets leading to dimeric species (in 2
and 3, respectively), to 1D chain associations (in 2 and 5) or to a 2D network (1). Complexes 2 œ
5 are applied as selective catalysts for the homogeneous peroxidative (with tert-
butylhydroperoxide, TBHP) oxidation of primary and secondary alcohols, under solvent- and
additive-free conditions and under low power microwave (MW) irradiation. A quantitative yield
of acetophenone was obtained by oxidation of 1-phenylethanol with compound 4 [TOFs up to
7.6 þ 103 h-1] after 20 min of MW irradiation, whereas the oxidation of benzyl alcohol to
benzaldehyde is less effective (TOF 992 h-1). The selectivity of 4 to oxidize the alcohol relative
to the ene function is demonstrated when using cinnamyl alcohol as substrate.
Keywords: Sulfonated Schiff base/Mononuclear/Dinuclear/Pyridine/2,2‘-Bipyridine/Alcohol
Oxidation/Microwave.
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Introduction
Over the past few decades there has been a marked development in the coordination
chemistry of Schiff base metal complexes,1 encouraged by the variety of their solid state
structures,2 magnetic,3,4 fluorescence5 and catalytic6 properties. Though a large number of Schiff
bases of various types,1œ7 including those containing the carboxylic acid group,7 have been
reported, sulfonated Schiff bases are rare and only a few metal complexes are known,8 although
covalent or non-covalent (H-bonding) interactions via sulfonate oxygen atoms could generate
interesting multinuclear8aœe or supramolecular structures.8aœc Catalytic properties of sulfonated
Schiff base metal complexes have also been rarely investigated.8a,b,e Thus, the syntheses of a new
sulfonated Schiff base and their copper complexes which could be useful for further catalytic
studies, deserve to be explored and constitute a general aim of the current study.
The oxidation of alcohols to carbonyl compounds is one of the most important reactions
in synthetic organic chemistry as well as in chemical industry.9,10 Moreover, it is of interest for
the development of environmentally benign production processes of new materials and energy
sources.9e,10a
Many catalytic methods have been developed for the oxidation of alcohols.10 A recent
environmentally compatible approach concerns oxidations with copper catalysts and dioxygen,
hydrogen peroxide or tert-butylhydroperoxide (TBHP) as oxidants.10c,10g,11 Hence, inspired by
our previous catalytic applications of a few sulfonated Schiff base copper complexes in the
peroxidative oxidation of cyclohexane8a,e and in the nitro-aldol reaction,8b we have anticipated
that the new copper(II) complexes bearing a sulfonated Schiff base could be active in alcohol
oxidation catalysis.
In
accord,
the
new
sulfonated
Schiff
base
2-[(2-hydroxy-3-
methoxyphenyl)methylideneamino]benzenesulfonic acid H2L (1) (Scheme 1) is prepared from
the condensation reaction of 2-hydroxy-3-methoxybenzaldehyde and 2-aminobenzenesulfonic
acid. Addition of Cu(II) salt(s) to the same reactant mixtures in different solvents (MeOH, EtOH
or DMF) and in the absence or in the presence of an N-heterocycle [pyridine (py) or 2,2‘-
bipyridine (2,2‘-bipy)] produces the mononuclear copper complexes [CuL(H2O)2] (2) and
[CuL(2,2‘-bipy)].DMF.H2O (3) and the diphenoxo-bridged dicopper complexes [CuL(py)]2 (4)
ï

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Results and Discussion
Syntheses and Characterization
The
[1+1]
condensation
of
2-hydroxy-3-methoxybenzaldehyde
and
2-
aminobenzenesulfonic acid in aqueous ethanol leads to the formation of the Schiff base H2L (1)
(Scheme 1), which can be isolated or used in situ for further syntheses of copper(II) complexes
(Scheme 1). Hence, addition of an aqueous methanol (1:1) solution of Cu(OAc)2.H2O and
CuCl2đ2H2O (in 1:1 molar ratio) to that solution of H2L (1) prepared in situ produces the
mononuclear copper(II) complex [CuL(H2O)2] (2). When the reaction is performed in aqueous
ethanol (1:1) and only with Cu(OAc)2.H2O as the copper(II) salt, the diphenoxo bridged
dicopper(II) complex [CuL(EtOH)]2.2H2O (5) is obtained. The mononuclear complex [CuL(2,2•-
bipy)].DMF.H2O (3) is synthesized similarly from the in situ reaction of 1, Cu(OAc)2.H2O and
2,2•-bipyridine (2,2•-bipy) in aqueous DMF (1:1) solution, while the analogous reaction with
pyridine (py) instead of 2,2•-bipy using aqueous methanol solution generates the diphenoxo
bridged dicopper(II) complex [CuL(py)]2 (4). As no product was isolated from the attempted
distinct in situ reactions of 1 with either Cu(OAc)2.H2O or CuCl2đ2H2O in aqueous methanol, we
tried the reaction with both salts together in this solvent and thus compound 2 was isolated.
All the compounds (1 œ 5) were characterized by elemental analysis, IR spectroscopy and
1
single crystal X-ray diffraction study. The Schiff base 1 was also characterized by the H NMR
spectrum. They were isolated in moderate to good yields (69œ82%).
The IR spectrum of the Schiff base H2L (1) exhibits the expected bands at 1629
and 1375 cmœ1 which are indicative of the C=N bond and the sulfonate group, respectively. In the
IR spectra of the metal complexes (2 œ 5), ꢀ(C=N) is observed in the range of 1609œ1618 cmœ1,
whereas the sulfonate groups are evidenced by the medium intense bands at the 1375œ1386 cmœ1
range.8aœd
Description of the Crystal Structures
The crystal structures of the Schiff base and its mono- or dicopper derivatives (1 œ 5) are
shown in Figure 1. Important bond distances and angles are presented in Table 1. Compound 1 is
approximately planar as measured by the angle between the least square planes of the aromatic
ñ

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rings (4.77º, Table 1). Upon O2N chelation to the metal cation by means of the phenolic O-, the
imine N- and a sulfonate O-atoms, the basic form of 1 (L2-) becomes highly distorted as
evidenced by the angle values (30.25œ61.63º, Table 1) of those l.s. planes, following the order 2
< 5 < 4 < 3. Such a distortion is also envisaged through the NœCœCœS and CœNœCœCSO3 torsion
angles which, however, trail in a different way (Table 1; 5 < 4 < 2 < 3 for the former, and 3 < 5 <
4 < 2 for the latter). Despite all these variances, the Nimine-Csp2 bond distances in 1 œ 5 are
similar and vary in the 1.290(4) - 1.315(4) Å range.
To accomplish the pentacoordination sphere of the metal, the copper cations in 2 œ 5 are
then bound to two water molecules (in 2), one 2,2•-bipy chelator (in 3), one py (in 4) and one
ethanol molecule (in 5). Probably as a result of the chelating mode of bipyridine, the L2œ ligand
in 3 occupies both equatorial (O-phenoxo and N-imine atoms) and apical (O-sulfonate) sites,
thus contrasting with the other complexes (2, 4 and 5) in which it occupies three of the equatorial
positions. The 25 descriptor values,12 between 0.08 (in 2) and 0.36 (in 3), may be a result of
steric interferences involving the ligands. Compounds 2 and 3 are mononuclear, while 4 and 5
are dinuclear species with an inversion centre in the middle of the Cu2O2 core. Consequently, the
copper cation in the complexes are involved in 4-membered (in 4 and 5), 5-membered (in 3) and
6-membered (in all cases) metallacycles.
The CuœNL bond distances lie in the 1.956(3) œ 2.012(3) Å range (Table 1) and are
slightly shorter than those involving the other N-ligands [2.019(2) œ 2.075(2) Å]. The equatorial
CuœOphenoxo distances [1.876(2) œ 1.910(2) Å] are shorter than the axial [2.352(2) and 2.5057(15)
Å in 4 and 5, respectively] and the Cu-Osulfonate ones [1.931(2) œ 2.359(2) Å]. Although the
Cu-Owater bond distance of 2.932(4) Å in 2 is the longest found in this work, and slightly longer
than the sum of the Van der Waals radii of copper and oxygen atoms (2.92 Å), all the parameters
are comparable to those found in the literature.8aœd
Several H-bond interactions stabilize structures 1 œ 3 and 5. The molecule of 1 is
involved in two intramolecular H-bond cycles (Table S2) with graph set 555:x; linking the imino
nitrogen atom (as donor) and both the phenolic and sulfonate oxygen atoms (as acceptors).
However, the most interesting feature of the crystal structure of 1 is the extensive H-bond
interactions of the crystallization water molecules leading to the formation of infinite chains
along the crystallographic b axis (Figure 2A). These are further interlinked by H-bonds to the
Schiff base, generating 2D layers (Figure 2B) which are stabilized by ŒđđđđŒ stacking
ò

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interactions. Also resulting from the presence of crystallization water molecules, compounds 2
and 3 form ring graph sets of type 466:sv; and 488:st;, respectively, the former involving one of
the H-atoms of the water molecule (as donor) and an O-methoxide (as acceptor), and the latter
linking both H-atoms of water to two O-sulfonate atoms of the same moiety (Figure S1). The
structure of 2 further generates infinite 1D double chains involving the coordinated water
molecules and vicinal sulfonate O-atoms (Figure 3). The strongest H-interactions in 5 include the
crystallization water molecule, both as donor and as acceptor, and the coordinated ethanol
6: ;
molecule (as donor), giving rise to 46 { graph sets. Overall a 1D chain is formed in the crystal
packing of 5 (Figure S2).
Table 1. Selected bond distances (Å), bond angles and torsion angles (°) for H2L (1) and the
metal complexes 2 - 5.
1
2
3
4
5
Involving the anionic ligand L2œ
NœCsp2
1.290(4)
1.432(4)
-0.8(4)
1.315(4)
1.432(4)
-3.4(5)
1.295(3)
1.430(3)
-11.7(3)
119.9(3)
1.289(4)
1.440(4)
-1.0(4)
1.299(2)
1.432(2)
-0.3(2)
NœCAr
Ø NœCœCœS
Ø CœNœCœCSO3
-172.9(3)
145.9(3)
141.6(3)
137.6(2)
Ø l. s. planes of aromatic
4.77
-
39.25
61.63
53.21
52.48
rings
Involving the metal
Cu coordination sphere
25 descriptor
O4N1
0.08
O2N3
O2N3
0.15
O4N1
0.36
2.359(2)
1.893(2)
-
0.14
MœOSO3
-
-
-
1.931(2)
1.876(2)
-
1.951(2)
1.910(2)
2.352(2)
1.9573(15)
1.9085(14)
2.5057(15)
MœOphenoxo (equatorial)
MœOphenoxo (axial)
1.974(3)
2.932(3)
1.956(3)
MœOother
MœNL
-
-
-
-
-
1.9912(14)
1.9841(16)
-
1.995(2)
2.019(2)
2.075(2)
2.012(3)
2.032(3)
MœNother
-
OSO3œMœOphenoxo
-
-
-
-
-
161.87(12) 116.12(8) 175.55(10)
156.90(12) 175.05(9) 166.69(11)
169.37(7)
177.79(6)
42.9(2)
NLœMœO(N)
-37.3(4)
-
-58.6(3)
-
-39.5(4)
3.2720(11)
6.534(2)
Ø CuœNœCœCSO3
M‡‡‡M (intramolecular)
M‡‡‡M (intermolecular)
3.3606(6)
6.0889(8)
5.188
7.8678(5)
ó

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1
2
3
4
5
Figure 1. Idealized ball and stick presentation of the crystal structure of 1œ5 with atom labeling
schemes. Crystallization solvent molecules in 1, 3 and 5 as well as hydrogen atoms in 4 and 5 are
omitted for clarity. Symmetry codes to generate equivalent atoms: 1œx,1œy,1œz (4) and œx, 2œy,
2œz (5).
ô

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A
B
Figure 2. (A) Representation (along the crystallographic b axis) of an infinite chain of hydrogen-
bonded (dotted lines) water molecules in the structure of 1, and (B) fragment of the 2D
supramolecular packing diagram of 1 constructed by hydrogen bonds between the Schiff base
and the lattice water molecules, also evidencing the ŒđđđŒ stacking between adjacent aromatic
rings.
Figure 3. H-bond supported double chain 1D association in 2.
õ

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It is worthwhile mentioning that sulfonated Schiff bases are of great interest due to their
versatile complex formation abilities; they can form monomers, dimers, tetramers and polymers
(both 1D and 2D).8 In one of our earlier studies we have reported that the Schiff base 2-[(2-
hydroxyphenyl)methylideneamino]benzenesulfonic acid can form solvatomorphs which have
identical basic structure but with different non-coordinated solvents.8d We have also introduced a
bis(ꢁ4-(ae)-cyclohexane-1,4-dicarboxylato-O,O',O'',O'''')-tetracopper8a,e and three pseudohalide
bridged copper complexes8b containing 2-(2-pyridylmethyleneamino)benzenesulfonate. The
tetracopper complex was an efficient catalyst for the cyclohexane oxidation in both conventional
(CH3CN/H2O)8a and non-conventional (ionic liquid) solvents,8e while pseudohalide complexes8b
were found to catalyze the Henry reaction in aqueous medium.
The interesting molecular structures and catalytic properties of such compounds inspired
us (i) to synthesize copper complexes of the new sulfonated Schiff base ligand 2-[(2-hydroxy-3-
methoxyphenyl)methylideneamino]benzenesulfonate (L2œ), as described above, and (ii) to apply
them in alcohol oxidation studies (see below).
Alcohol Oxidation Catalyzed by Copper Complexes 2 œ 5
Complexes 2 œ 5, as well as their salt precursors Cu(OAc)2.H2O and CuCl2.2H2O, were
tested as catalysts for the homogeneous oxidation of 1-phenylethanol and benzyl alcohol (chosen
as models) to acetophenone or benzaldehyde, respectively, using aqueous tert-butyl
hydroperoxide (ButOOH, TBHP) as oxidizing agent, under optimized conditions of 100 ºC, low
power (10 W) microwave irradiation (MW), 20 (or 150 for benzaldehyde) min reaction time and
in a solvent- and additive-free medium (Scheme 2, Table 2). The eventual organocatalytic ability
of the Schiff base H2L (1) was also evaluated (entries 1 and 9, Table 2). Under the assayed
conditions, acetophenone or benzaldehyde for the oxidation of 1-phenylethanol or benzyl
alcohol, respectively, were the only products detected by GCœMS analysis, thus revealing a very
selective oxidation system.
The copper(II) complex catalysts 2 œ 5 exhibit good performance (best TOF value of 7.60
x 103 hœ1 with 4, entry 4 of Table 2) leading to yields (based on the alcohol) of acetophenone or
benzaldehyde in the 52.3 œ 99.6% or 57.6 œ 99.2% ranges, respectively (Table 2, Figure 4).
However, the oxidation of benzyl alcohol was significantly slower as shown by the lower TOF
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compared to the EtOH ligand (in 5), appears to slightly enhance the catalytic activity (compare
entries 4 and 5 or 12 and 13 of Table 2), as observed in other cases.11e,k
The reactions performed under the same conditions but in the presence of the mononuclear
complexes (2 and 3) resulted in lower yields in acetophenone, between 52.3 and 64.0% (entries 2
and 3, Table 2), and also in benzaldehyde, between 57.6 and 78.8% (entries 10 and 11, Table 2).
However, if nuclearities are considered, the mononuclear compounds 2 and 3 become the most
effective ones (higher TOFs per metal atom) under the used conditions.
ñ
ð
ï
î
í
ì
îì
ðì
òì
ôì
íìì
ò]ꢀoꢁ l9
Figure 4. MW-assisted, solvent- and additive-free acetophenone (¢) or benzaldehyde (¢)
production (see Table 2) by oxidation of 1-phenylethanol or benzyl alcohol, respectively,
catalyzed by 1 œ 5.
Blank tests were performed under the same reaction conditions but in the absence of the
copper compounds: no significant conversion of 1-phenylethanol or benzyl alcohol was observed
(entry 8 or 16, Table 2). Moreover, the replacement of the copper(II) complexes (2 œ 5) by their
inorganic salt precursors resulted in a drastic decrease of activity (compare e.g., entries 6 or 7
with entries 5 or 2, respectively, Table 2).
The used low power (10 W) MW irradiation provides a much more efficient synthetic
method than conventional heating (open atmosphere or non-pressurized refluxing, see
Experimental Section below), allowing the attainment of similar or better yields in much shorter
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times (Figure 5, for the oxidation of 1-phenylethanol in the presence of 4). The promoting effect
of MW methods was observed in other cases.11cœg,11k,m,n,13 At 100 ºC, the MW-assisted method
with 4 leads to the almost quantitative oxidation of 1-phenylethanol to acetophenone in only 20
mins, whereas by the conventional method of heating, even at the higher temperature of 150 ºC,
the quantitative conversion was not yet approached after 3.5 h. Attempts to perform the oxidation
at room temperature (without MW or conventional heating) failed, and the minimum required
temperature is 50 ºC. The overall temperature coefficient (in the 50œ100 ºC range) of the MW
oxidations is ca. 1.2.
Figure 5. Influence of the temperature and reaction time on the yield of acetophenone obtained
by MW-assisted (Þ) or conventional (--) oxidation of 1-phenylethanol in the presence of 4.
Our system affords acetophenone upon oxidation of 1-phenylethanol, in the presence of 4
or 5, in comparable yields with those of other successful MW-assisted oxidations of secondary
alcohols involving a Cu(II) complex with a Schiff base and diethanolamine ligands (yields up to
85%, TON = 850),11c alkoxy-1,3,5-triazapentadienate Cu(II) complexes (yields up to 100%,
TON = 500),11f,j but in much higher yields than in the presence of various mononuclear Cu(II)
complexes bearing azathia macrocycles (yields up to 30%, TON = 150)11g,k or aryhydrazones
(yields up to 40%, TON = 98),11m copper-containing metal-organic frameworks (MOFs) based
on 5-(4-pyridyl)tetrazole building blocks (yields up to 79%, TON = 397)11d or coordination
polymers with pyrazolato-based tectons (yields up to 43%, TON = 680).11n Similar yields are
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also observed with bi- or tetranuclear cage-like Cu(II) silsesquioxanes (yields up to 100%, TON
= 475)11i,l with conventional heating.
Moreover, the almost quantitative conversion of benzyl alcohol to benzaldehyde, obtained
with the 4/TBHP/MW system (entry 12, Table 2), is also noticeable, since previous Cu(II)
catalyst/ TBHP/ MW methods led to lower benzaldehyde yields (up to 75%)11n and also needed
higher reaction temperatures (120 ºC11n or 150 ºC11m). Thus, our system could constitute a faster
and eco-friendly alternative to the usually preferred aerobic oxidation in the presence of the
additive nitrosyl TEMPO (2,2,6,6-tetramethyl-piperidinyloxyl) radical and using conventional
heating.10a,11b,13
The selectivity of our best catalytic system 4/TBHP/MW was further investigated by using
cinnamyl alcohol as substrate and water as solvent, under the above optimized conditions (100
ºC, 150 min reaction time, 10 W MW irradiation). Cinnamaldehyde (85% yield, TOF of 851 h-1)
was the unique product detected (Scheme 3), clearly revealing the preference of our oxidation
system for the alcohol function. Moreover, the reported yield in the present work is considerably
higher than the previously obtained (66%)11o by H5IO6, in water, in the presence of Ru(III)
compounds immobilized on silica, at room temperature.
4
OH
O
aq. TBHP, MW
Scheme 3. Homogeneous oxidation of cinnamyl alcohol, by the 4/TBHP/MW system in water.
The alcohol peroxidative oxidation is believed to proceed mainly via a radical mechanism
as proposed for other copper complexes,11 which involves both carbon- and oxygen-centred
radicals.9a,11b,14 In fact, a strong inhibition effect was observed when it is carried out in the
presence of either the carbon-radical trap CBrCl3 or the oxygen-radical trap Ph2NH (e.g.,
t
acetophenone yield decreases to 5.8 and 4.9%, respectively). It may involve e.g., BuO¯ and
tBuOO¯ radicals produced in the Cu promoted decomposition of TBHP11i,15 according to the
following equations (1) œ (6):
íð

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CuII + tBuOOH ç CuI + tBuOO¯ + H+
CuI + tBuOOH ç CuIIœOH + tBuO¯
(1)
(2)
(3)
(4)
(5)
(6)
CuIIœOH + tBuOOH ç CuIIœOOœtBu + H2O
tBuO¯ + R2CHOH ç tBuOH + R2C¯œOH
tBuOO¯ + R2CHOH ç tBuOOH + R2C¯œOH
CuIIœOOœtBu + R2C¯œOH ç R2C =O + tBuOOH + CuI
However, it was not possible to detect any oxygenated intermediate species, conceivably
due to their high reactivity.
It is worth to mention that, to our knowledge, the present 4/TBHP/MW and 5/TBHP/MW
catalytic systems are among the fastest and most effective copper catalyzed MW-assisted,
solvent- and additive-free oxidations of 1-phenylethanol and benzylalcohol and this type of
sulfonated Schiff base copper systems has been applied in alcohol oxidation only for the second
time after
a
mononuclear copper complex containing 2-(((1-hydroxynaphthalen-2-
yl)methylene)amino)benzenesulfonate.11c
In view of the achieved results, the heterogenization of compounds 4 and 5 at different
supports (carbon and zeolitic materials) in order to render them heterogeneous, and therefore
reusable catalysts, is envisaged.
Conclusions
By taking advantage of the coordinating ability of the sulfonate group of the acyclic Schiff
base 2-[(2-hydroxy-3-methoxyphenyl)methylideneamino]benzenesulfonic acid H2L (1), two
mononuclear copper complexes [CuL(H2O)2] (2), [CuL(2,2‘-bipy)].DMF.H2O (3) and two
diphenoxo-bridged dicopper compounds [CuL(py)]2 (4) and [CuL(EtOH)]2.2H2O (5) have been
synthesized. The crystal lattices of all compounds are stabilized by non-covalent H-bond and
ŒđđđŒ stackings interactions that generated 2D polymeric networks (in 1) and a 1D double chain
(in 2).
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These metal complexes (2 œ 5) efficiently catalyze the neat microwave assisted 1-
phenylethanol oxidation reaction to acetophenone, under mild conditions and in the absence of
any additive. Overall 99.6 and 94.3% yields were obtained with the catalysts 4 and 5,
respectively. To our knowledge, the catalytic systems 4/THBP/MW and 5/THBP/MW are among
the fastest and most effective copper catalyzed MW-assisted, solvent- and additive-free
oxidations of secondary alcohols and this type of sulfonated Schiff base systems has been
investigated in alcohol oxidation only for the second time.
The in situ synthetic methodology for the copper(II) complexes 2 œ 5 in different solvents is
quite easy.
In summary, a simple and efficient protocol for the synthesis of four effective and selective
copper(II) catalysts for alcohol oxidation is presented and the extension of this study to the
preparation of further copper complexes with these types of ligands and to their applications in
oxidation catalysis deserves to be explored.
Experimental Section
Materials and Physical Methods
All the reagents and solvents were purchased from commercial sources and used as received.
The water used for all preparations and analyses was double-distilled and de-ionised. Elemental
analyses were performed by the Microanalytical Service of the Instituto Superior Técnico. FT-IR
spectra were recorded in the region 400œ4000 cmœ1 on a Bruker Vertex 70 spectrophotometer
1
with samples as KBr disks. The H NMR spectrum was recorded at ambient temperature on a
Bruker 400 UltraShieldTM spectrometer. The chemical shifts are reported in ppm using
tetramethylsilane as an internal reference.
Syntheses
H2L.3H2O (1). To a hot and stirred water solution (10 mL) of 2-aminobenzenesulfonic acid
(0.692 g, 4.0 mmol) was added dropwise an ethanol solution (20 mL) of 2-hydroxy-3-
methoxybenzaldehyde (0.608 g, 4.0 mmol). The resulted yellow solution was filtered and kept at
room temperature overnight. After 1 day, yellow crystals, suitable for X-ray diffraction analysis,
formed which were collected by filtration and washed with ethanol. Yield 1.185 g (82%). Anal.
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calcd. for C14H19NO8S (361.36): C 46.53, H 5.30, N 3.88%; found: C 46.63, H 5.27, N 3.84%.
FT-IR (cmœ1, KBr): ꢀ(OH), 3449br; ꢀ(NœH), 2923br; ꢀ(C=N), 1629s; ꢀ(CœO), 1228s;
1
ꢀ(sulfonate), 1375s, 1173s. H NMR (DMSO-D6) / (ppm): 10.29 (s, Ar-OH); 9.33 (s, CH=N);
6.92 œ 7.79 (m, 7-Ar-H); 5.90 (br, NœH); 3.85 (s, OœCH3).
[CuL(H2O)2] (2). To a hot and stirred aqueous methanol (1:1) solution (10 mL) of 2-
aminobenzenesulfonic acid (0.173 g, 1.0 mmol) and 2-hydroxy-3-methoxybenzaldehyde (0.152
g, 1.0 mmol), an aqueous methanol (1:1) solution (10 mL) of Cu(OAc)2.H2O (0.100 g, 0.5 mmol)
and CuCl2.2H2O (0.085 g, 0.5 mmol) was added dropwise to obtain a brown solution. The
solution was filtered and kept at room temperature. After 2 days brown crystals, suitable for X-
ray diffraction analysis, formed which were collected by filtration and washed with cold aqueous
methanol solution (1:1). Yield: 0.292 g (72%). Anal. calcd. for C14H15CuNO7S (404.89): C
41.53, H 3.73, N 3.46%; found C 41.63, H 3.76, N 3.42%. FT-IR (cm-1, KBr): ꢀ(OH), 3449br;
ꢀ(C=N), 1618; ꢀ(CœO), 1283s; ꢀ(sulfonate), 1383m, 1174s.
[CuL(2,2•-bipy)].DMF.H2O (3). To a hot and stirred aqueous DMF (1:1) solution (10 mL) of 2-
aminobenzenesulfonic acid (0.173 g, 1.0 mmol) and 2-hydroxy-3-methoxybenzaldehyde (0.152
g, 1.0 mmol), was added dropwise a DMF solution (5 mL) of Cu(OAc)2.H2O (0.199 g, 1.0
mmol). After 30 mins, to the resulted green solution was added dropwise a DMF solution (5 mL)
of 2,2‘-bipyridine (0.234 g, 1.5 mmol) affording a dark green solution. After one hour, the
solution was filtered and kept at room temperature. After a few days dark green crystalline
compounds, suitable for X-ray diffraction analysis, formed which were collected by filtration.
Yield: 0.468 g (76%). Anal. calcd. for C27H28CuN4O7S (616.14): C 52.63, H 4.58, N 9.09%;
found C 52.69, H 4.62, N 9.04%. FT-IR (cmœ1, KBr): ꢀ(OH), 3440br; ꢀ(C=N), 1614; ꢀ(CœO),
1274m; ꢀ(sulfonate), 1386m, 1171s.
[CuL(py)]2 (4). To a hot and stirred aqueous methanol (1:1) solution (15 mL) of 2-
aminobenzenesulfonic acid (0.173 g, 1.0 mmol) and 2-hydroxy-3-methoxybenzaldehyde (0.152
g, 1.0 mmol) was added dropwise a methanol solution (2 mL) of pyridine (0.316 g, 4.0 mmol).
After 2 h, was added dropwise an aqueous methanol (1:1) solution (10 mL) of Cu(OAc)2.H2O
(0.199 g, 1.0 mmol) to obtain a green solution. The solution was filtered and kept at room
temperature. After one day green crystals, suitable for X-ray diffraction analysis, formed which
were collected by filtration and washed with cold methanol. Yield: 0.309 g (69%). Anal. calcd.
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for C38H32Cu2N4O10S2 (895.90): C 50.94, H 3.60, N 6.25%; found C 50.99, H 3.65, N 6.21%.
FT-IR (cm-1, KBr): ꢀ(C=N), 1609; ꢀ(CœO), 1279s; ꢀ(sulfonate), 1381m, 1170s.
[CuL(EtOH)]2.2H2O (5). To a hot and stirred aqueous ethanol (1:1) solution (10 mL) of 2-
aminobenzenesulfonic acid (0.173 g, 1.0 mmol) and 2-hydroxy-3-methoxybenzaldehyde (0.152
g, 1.0 mmol) was added dropwise an aqueous ethanol (1:1) solution (10 mL) of Cu(OAc)2.H2O
(0.199 g, 1.0 mmol) to obtain a green solution. After two hours, the solution was filtered and
kept at room temperature. After 1 day green crystals, suitable for X-ray diffraction analysis,
formed which were collected by filtration and washed with cold ethanol. Yield: 0.311 g (75%).
Anal. calcd. for C32H38Cu2N4O10S2 (829.89): C 46.31, H 4.62, N 6.75%; found C 46.37, H 4.66,
N 6.71%. FT-IR (cmœ1, KBr): ꢀ(OH), 3445br; ꢀ(C=N), 1612s; ꢀ(CœO), 1278m; ꢀ(sulfonate),
1385m, 1176s.
Crystal Structure Determinations
X-ray quality single crystals of the compounds were immersed in cryo-oil, mounted in a
nylon loop and measured at room temperature (1 œ 5). Intensity data were collected using a
Bruker APEX-II PHOTON 100 diffractometer with graphite monochromated Mo-K. (ꢂ
0.71069) radiation. Data were collected using phi and omega scans of 0.5° per frame and a full
sphere of data was obtained. Cell parameters were retrieved using Bruker SMART software and
refined using Bruker SAINT16a on all the observed reflections. Absorption corrections were
applied using SADABS.16a Structures were solved by direct methods by using the SHELXS-97
package16b,c and refined with SHELXL-2014.16b,c Calculations were performed using the WinGX
SystemœVersion 1.80.03.16d The hydrogen atoms attached to carbon atoms and to the nitrogen
atoms were inserted at geometrically calculated positions and included in the refinement using
the riding-model approximation; Uiso(H) were defined as 1.2Ueq of the parent nitrogen atoms or
the carbon atoms for phenyl and methylene residues, and 1.5Ueq of the parent carbon atoms for
the methyl groups. Least square refinements with anisotropic thermal motion parameters for all
the non-hydrogen atoms and isotropic ones for the remaining atoms were employed.
Crystallographic data are summarized in Table S1 (Supplementary Information file) and selected
bond distances and angles are presented in Table 1. CCDC 1411065œ1411069 for 1œ5,
respectively, contain the supplementary crystallographic data for this paper.
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General Procedures for the Oxidation of Alcohols
MW-assisted method: The catalytic tests under microwave irradiation (MW) were
performed in a focused Anton Paar Monowave 300 microwave reactor using a sealed 5 mL
capacity cylindrical Pyrex reaction tube with a 10 mm internal diameter, fitted with a rotational
system and an IR temperature detector. The alcohol (2.5 mmol), TBHP (70 % aqueous solution,
5.0 mmol) and catalyst precursor (1 œ 10 µmol, 0.04œ0.4 mol% vs. substrate) were introduced in
the tube which was then placed in the microwave reactor. In the experiments with radical traps,
CBrCl3 (2.5 mmol) or NHPh2 (2.5 mmol) was added to the reaction mixture. The system was
stirred and irradiated (5 œ 25 W) for 0.15 œ 1.5 h at 50 œ 150 ºC.
Conventional heating method: The catalytic assays were carried out with conventional
heating (oil bath) in air, using 25 mL round-bottomed flaks equipped with a reflux condenser.
The alcohol (2.5 mmol), TBHP (70% aqueous solution, 5.0 mmol) and the catalyst (1 µmol, 0.04
mol% vs. substrate) were introduced in the flask and vigorous stirred during the desired reaction
time.
Products extraction and analysis: After the reaction, the mixture was allowed to cool
down to room temperature. 150 mL of benzaldehyde (internal standard) and 2.5 mL of CH3CN
(to extract the substrate and the organic products from the reaction mixture) were added. The
obtained mixture was stirred for ca. 10 min and then a sample (1 mL) was taken from the organic
phase and analysed by GC or GC-MS. The internal standard method was used to quantify the
organic products.
Chromatographic analyses were undertaken by using a Fisons Instruments GC 8000
series gas chromatograph 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 software. The
temperature of injection was 240 ºC. The initial temperature was maintained at 140 ºC for 1 min,
then raised 10 ºC/min to 220 ºC and held at this temperature for 1 min. Helium was used as the
carrier gas. The products were identified by comparison of their retention times with those of
known reference compounds.
GC-MS analyses were performed using a Perkin Elmer Clarus 600 C instrument (He as
the carrier gas). The ionization voltage was 70 eV. Gas chromatography was conducted in the
temperature-programming mode, using a SGE BPX5 column (30 m þ 0.25 mm þ 0.25 µm).
Reaction products were identified by comparison of their retention times with those of known
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reference compounds, and by comparing their mass spectra to fragmentation patterns obtained
from the NIST spectral library stored in the computer software of the mass spectrometer.
Acknowledgements
Financial supports from the Fundação para a Ciência e a Tecnologia (FCT), Portugal, for the
postdoc grant SFRH/BPD/78264/2011 (S.H.) and for the UID/QUI/00100/2013 project, are
acknowledged. The authors acknowledge the Portuguese NMR Network (IST-UL Centre) for the
access to the NMR facility.
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