ChineseJournalofCatalysis34(2013)1456–1461
催化学报2013年第34卷第7期|www.chxb.cn a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m j o u r n a l h o m e p a g e : w w w . e l s e v i e r. c o m / l o c a t e / c h n j c Article Synthesis,structuralanalysisandevaluationofthecatalyticactivity
ofanon‐symmetricN‐(salicylidene)diethylenetriaminecomplexof
copper(II)
HassanHOSSEINI‐MONFAREDa,*,SohailaALAVIa,MiloszSICZEKb
DepartmentofChemistry,UniversityofZanjan,45195‐313,Zanjan,IslamicRepublicofIran
FacultyofChemistry,UniversityofWroclaw,Joliot‐Curie14,Wroclaw50‐383,Poland
a
b
A R T I C L E I N F O
A B S T R A C T
Articlehistory:
Received24February2013
Accepted13May2013
Published20July2013
Keywords:
Crystalstructure
Copper(II)complex
Non‐symmetricSchiffbase
Catalyst
Hydrogenperoxide
Anewcopper(II)complexofanon‐symmetricSchiffbase,[CuII(saldien)(H2O)]+(1),hasbeensyn‐
thesized and characterized by elemental analysis and several other spectroscopic methods
(Hsaldien = N‐(salicylidene)diethylenetriamine). The crystal structure of 1 has also been deter‐
minedbyX‐raycrystallography.Thegeometryofthecomplexcationin1wasfoundtobedistorted
squarepyramidalwiththemononegativeSchiffbasecoordinatingtothecopperinatetradentate
modeviatheO,N,N',andN''‐donoratoms.TheremainingcoordinationsitewasoccupiedbytheO
atomofaH2Omoleculeintheaxialposition.Thecatalyticpotentialof1wastestedintheoxidation
reactions of cyclooctene and cyclohexene with aqueous 30% H2O2/NaHCO3 in acetonitrile. These
reactions proceeded smoothly to give the corresponding epoxides with selectivity levels greater
than99%.Thiscatalyticsystemalsoshowedhighlevelsofactivityandselectivitytowardstheoxi‐
dationofcyclohexane(i.e.,cyclohexanol37%andcyclohexanone54%)incomparisonwithmostof
theotherCu‐basedsystemsreportedintheliterature.
©2013,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences.
PublishedbyElsevierB.V.Allrightsreserved.
1. Introduction
The catalytic oxidation of hydrocarbons is of particular in‐
terest to synthetic chemists, and reactions of this type have
been used routinely for some time for the introduction of a
variety of different functionalities, including diols, epoxides,
alcohols,andcarbonyl compounds[1,2].Itis wellknownthat
the derivatives of different transition metals can efficiently
catalyzetheoxygenationofhydrocarbonsusingmolecularox‐
ygenorperoxides,includingtheinexpensiveandenvironmen‐
tally friendly oxidizing agent H2O2 [3–6]. The development of
processes for the selective oxidation of alkanes represents an
important industrial problem because of the inert nature of
hydrocarbons, which makes the activation of their C–H bonds
particularlydifficulttopromote.Reactionsofthistypearegen‐
erally conducted under forcing and aggressive conditions, in‐
cludinghightemperaturesandpressures[7].
Considerableresearcheffortshavebeenfocusedonthede‐
velopment of new catalysts for the oxidization of cyclohexane
under mild conditions [8]. For the commercial oxidation of
cyclohexanebyairat160°CwithaCo(II)catalyst[9],thede‐
sired products (cyclohexanol and cyclohexanone) typically
constitute 85% of the products in the reaction mixture at a
conversionofonly4%.Cyclohexaneisanimportantrawmate‐
rial, and its oxidation products, cyclohexanol and cyclohexa‐
none,areadipicacidprecursors.Largequantitiesofadipicacid
are used in industrial manufacture processes, including the
production of nylon‐6, nylon‐66, urethane foams, lubricant
*Correspondingauthor.Tel:+98‐241‐5152576;Fax:+98‐241‐5283203;E‐mail:[email protected]
DOI:10.1016/S1872‐2067(12)60616‐0|http://www.sciencedirect.com/science/journal/18722067|Chin.J.Catal.,Vol.34,No.7,July2013 HassanHOSSEINI‐MONFAREDetal./ChineseJournalofCatalysis34(2013)1456–1461
additives, pharmaceutical intermediates, insecticides and bac‐
tericides[10,11].Althoughseveraldifferentcyclohexaneoxida‐
tion reactions catalyzed by metallic complexes have been re‐
portedintheliterature[12–17],theproductyieldsareusually
low[17].Inthemajorityofthesecases,thecatalystsused for
theoxidationhavebeen basedonligand systems such aseth‐
ylenediamine[18],bipyridine[19],salen[20],andphthalocya‐
nine [21] ligands. Schiff base metal complexes are one of the
mostimportant stereochemicalmodelsin transitionmetalco‐
ordination chemistry because of their facile preparation and
structural variability [22,23]. The electronic and structural
properties of ligands usually play an important role in deter‐
miningthepropertiesofthecatalyst[24,25].Inthesymmetric
salen Schiff base complexes studied to date, the two identical
salicylaldehyde moieties on both sides of the diamine in the
ligandsmakeequalelectronicandstericcontributions.Incon‐
trast, unsymmetrical combinations allow for the electronic
propertiesononesideofthediamineandthestericeffectson
theothersideofthediaminetobesimultaneouslyandcollec‐
tively tuned to maximize the performance of the
non‐symmetrical Schiff base catalysts. Unsymmetrical Schiff
basescanbindwithmultiplemetalcentersthroughavarietyof
different coordination modes, allowing for the successful syn‐
thesis of homo‐ or heteronuclear metal complexes with inter‐
estingstereochemicalproperties[26–29].
Herein,wereportthesynthesisandstructuralelucidationof
thecopper(II)complexofthenon‐symmetricalSchiffbaselig‐
and N‐(salicylidene)diethylenetriamine (Hsaldien). Further‐
more,thecatalyticactivityofthecomplexwasevaluatedforthe
oxidationofhydrocarbonswithamixtureofH2O2andNaHCO3.
Thereactionconditionswereoptimizedbyvaryingthereaction
temperature,molarratiooftheoxidanttothealkene,andthe
reaction solvent. Following a 5 h reaction period under the
optimized reaction conditions, 91% of the cyclohexane was
convertedtoamixtureofoxidizedproductswithaselectivityof
59% for the cyclohexanone. The ligand used in the current
study has also been used in the synthesis of metal‐organic
complexes [30,31]. The oxidation reactions of phenol and hy‐
droquinone catalyzed by the symmetric copper(II) and oxo‐
vanadium(IV) complexes of N,N′‐bis(salicylidene)diethylene‐
triamine covalently bonded to chloromethylated polystyrene
havebeenreported[32].
2. Experimental Allofthestartingmaterialsusedinthecurrentstudywere
purchased from commercial suppliers and used without fur‐
ther purification. Infrared (IR) spectra were recorded on a
Bruker FT‐IR spectrophotometer as KBr pellets in the
4000–400 cm−1 region. Ultroviolet‐visible (UV‐Vis) solution
spectra were recorded on a Thermo Spectronic Helios Alpha
spectrometer. The products of the oxidation reaction were
analyzed using an HP Agilent 6890 gas chromatograph
equipped with a HP‐5 capillary column (phenyl methyl silox‐
ane,30m×320µm×0.25µm)andaflameionizationdetector.
2.1. SynthesisofN‐(salicylidene)diethylenetriamine(Hsaldien)
1457
A solution of diethylenetriamine (0.103 mg, 0.10 mmol) in
ethanol was addedtoa solutionof salicylaldehyde (0.122 mg,
0.10 mmol) in ethanol, and the resulting yellow solution was
heatedatrefluxfor3h.Thereactionsolventwasthenremoved
undervacuumtogivethedesiredproductasadarkyellowoil
(0.187 g, 91%). IR (KBr, cm–1): 3412 (w, br, O–H), 3062 (w,
NH), 2928 (m), 2842 (m), 1632 (vs, C=N), 1279 (s, Ar–OH),
1214(s).
2.2. Synthesisofthe[CuII(saldien)(H2O)](NO3)0.75(N3)0.25 complex(1)
Schiff base Hsaldien (28.9 mg, 0.14 mmol), copper(II) ni‐
trate hexahydrate (41.4 mg, 0.14 mmol), and sodium azide
(46.8 mg, 0.72 mmol) were placed in the main arm of a
branchedtube(‘branchedtube’method[33]).A1:1(v/v)mix‐
tureofethanolandwaterwasthenaddedtothetubetofillthe
arms. The tube was then sealed and the arm containing the
reagents immersed in an oil bath at 60 °C, whereas the other
armwasheldatambienttemperature.Followinga24hperiod
under these conditions, violet crystals of 1 that were suitable
forX‐rayanalysisweredepositedinthecoolerarminan80%
yield (38.4 mg). IR (KBr, cm–1): 3422 (m, br, O–H), 3312 (m,
N–H),3193(m,N–H),1644(s,C=N),1596(s),1384(vs,NO3–).
UV/Vis(CH3OH,c=1.0×10–4mol/L,λmax(nm)withε(Lmol–1
cm–1)): 228 (25500), 238 (25067), 266 (17640), 363 (5580),
580 (200). Elemental analysis: Calcd. for C11H18CuN4.5O4.25
(344.83);C38.31%,H5.26%,N18.28%,Cu18.43%.FoundC
38.29%,H5.28%,N18.20%,Cu19.00%.
2.3. X‐raycrystallography
X‐ray quality crystals of 1 were obtained from a 1:1 (v/v)
mixtureofethanolandwater.Aviolet‐coloredcrystalof1was
measured at 100(2) Kα on an Oxford XCalibur diffractometer
with monochromated Mo Kα radiation (λ = 0.71073 Å). The
structurewassolvedbydirectmethodswithSHELXS[34]and
refined with full‐matrix least‐squares techniques on F2 with
SHELXL[34].Thecrystaldata andrefinementparametersare
showninTable1.
CCDC 923723 contains the supplementary crystallographic
datafor1.ThesedatacanbeobtainedfreeofchargefromThe
CambridgeCrystallographicDataCentreviawww.ccdc.cam.ac.
uk/data_request/cif.
2.4. Generalprocedurefortheoxidationofhydrocarbons
Theliquidphasecatalyticoxidationswerecarriedoutunder
air (atmospheric pressure) in a 25 ml round bottom flask
equipped with a magnetic stirrer and immersed in a thermo‐
statedoilbath.Inatypicalexperiment,30%H2O2(3.0mmol)
was added to a flask containing the catalyst (0.001 g) and a
representativehydrocarbon(1mmol)inasolvent(3ml)with
n‐octane (0.1g),which wasused as aninternalstandard,and
NaHCO3 (1 mmol), which was used as a co‐catalyst. The pro‐
gressofthereactionwasmonitoredusingagaschromatograph
equippedwithacapillarycolumnandaflameionizationdetec‐
1458
HassanHOSSEINI‐MONFAREDetal./ChineseJournalofCatalysis34(2013)1456–1461
H
N
Table1 Crystaldatafor[CuII(saldien)(H2O)](NO3)0.75(N3)0.25(1).
Netformula Size
Chemicalformulasum
Mr(gmol–1)
Crystalsystem Spacegroup
a(Å)
c(Å) b(Å) β(°) V(Å3)
Z
Tmin,Tmax
Reflectionsmeasured µ(mm–1)
Reflectionsinrefinement Parameters
Restraints
R(Fobs)
Rw(F2)
S
C11H18CuN3O2,0.746(NO3),0.254(N3)
0.4mm×0.25mm×0.25mm
C11H18CuN4.51O4.24
344.74
Monoclinic
P21/c
9.077(3)
14.538(4)
10.768(3)
101.53(3)
1392.3(7)
4
0.723,0.781
7927
1.593
4656
203
0
0.0283
0.0657
1.008
H
N
NH2
CHO
Ethanol
HC
N
NH2
OH
NH2
OH
Hsaldien
Hsaldien + Cu(NO3)2·6H2O + NaN3
Ethanol/H2O
[Cu(saldien)(H2O)](NO3)0.75(N3)0.25
Scheme1.Synthesesofthenon‐symmetricSchiffbase(Hsaldien)and
complex[CuII(saldien)(H2O)](NO3)0.75(N3)0.25.
tor. The reaction products were quantified by gas chromatog‐
raphy and identified through a comparison of their retention
timesandspectraldatawiththoseoftheauthenticsamples.No
degradation of the ligand was observed during the reaction
processandthevioletcolorofthesolutionremainedthesame
followingthe5hreactionperiod.
Control reactions were carried out under the same condi‐
tions in the absence of the catalyst and H2O2. None of the de‐
siredproductswereformedduringthesecontrolreactions.
The complex was found to be stable under the oxidation
conditions over the 5 h reaction period. Although the
blue‐violet color of the catalyst solution became a little faint
following the 5 h reaction period, its UV‐Vis spectrum was
comparablewiththatofthefreshcomplex.
to the ν(O–H) vibrations of the Hsaldien ligand at 3412 cm–1
indicatedthepresenceofanintramolecularhydrogenbonding
interaction (O−H···N) in the ligand [35]. For complex 1, the
absencetheofδ(O–H)(phenolic)band,togetherwithshiftsin
azomethine(C=N,12cm−1)andN–Hbandscomparedwiththe
free Hsaldien ligand, indicated the coordination of (saldien)–
throughthephenolateoxygen,azomethinenitrogen,andamine
oxygenatomsinthecomplex1.TheIRspectrumof1alsocon‐
tainedanintenseabsorptionat1384cm–1,whichwasattribut‐
edtothepresenceofanitratecounterion.Thisresultprovided
furtherconfirmationthat1existedasanioniccomplex.
Followingthecomplexationprocess,theUV‐Visspectrumin
methanolcontainedabsorbancebandsatλmax,(ε,Lmol–1cm–1)
=228(25500),238(25067),266(17640),363(5580),and580
nm(200)(Fig.1).ThecomplexgaveaUV‐Visspectrumsimilar
to that of the ligand. Based on their extinction coefficients,
thesebandswereassignedtointraligandπ→π*(228,238,266
nm)andn→π*(363nm)transitions.Thehigherenergyband
at 363 nm with a high extinction coefficient value was at‐
tributedinparttochargetransfer(LMCT)transitionsbetween
the coordinated phenolate‐O and the Cu(II). Furthermore,
complex 1 exhibited a ligand field d−d transition at 580 nm
(200Lmol–1cm–1)(Fig.1,inset).
3. Resultsanddiscussion
3.2. Crystalstructure
3
2
0.09
Absorbance
The non‐symmetric Schiff base ligand was synthesized in
highyieldandpurityviathemonocondensationofsalicylalde‐
hyde and diethylenetriamine in ethanol. The copper complex
with the Hsaldien ligand was prepared by treating an etha‐
nol/watersolutionoftheligandwithCu(NO3)2·6H2OandNaN3
inamolarratioof1:1:5,respectively(Scheme1).Itwasenvis‐
agedthattheuseofsodiumazidewouldleadtotheformation
of a coordinationpolymer, but this strategy led insteadtothe
formationofamononuclearcopper(II)complex.
The IR spectrum of the free Hsaldien ligand showed
stretching bands at 1279, 1632, and 3062 cm–1, which were
attributedtothephenolicC–OH,C=N,andN–Hbonds,respec‐
tively[35].Thebandat3412cm–1wasassignedtoν(O–H)vi‐
brations associated with intramolecular hydrogen bonding
interactions,whereasthebandat1214cm–1wasattributedto
thephenolicδ(O–H)bond[36].Theweakbandcorresponding
The molecular structure of the catalyst with numbering is
Absorbance
3.1. Synthesisandspectroscopicanalysisof
[CuII(saldien)(H2O)](NO3)0.75(N3)0.25
0.06
0.03
400
1
580
500
600
700
Wavelength (nm)
800
Complex
Ligand
0
200
300
400
500
600
700
800
Wavelength (nm)
Fig. 1. The UV‐Vis spectra of the [CuII(saldien)(H2O)] (NO3)0.75(N3)0.25
complexandtheHsaldienligand(c=0.1mmol/L)recordedinmetha‐
nol.Theinsetshowsthed‐dtransitionintheCu(II).
HassanHOSSEINI‐MONFAREDetal./ChineseJournalofCatalysis34(2013)1456–1461
Fig.2.Thecoordinationenvironmentof1(probability50%).Selected
interatomicbonddistances(Å)andangles(°):Cu–O1,1.925(1);Cu–N1,
1.946(1); Cu–N2, 2.007(1); Cu–N3, 2.014(1); Cu–O2, 2.317(1);
O1–Cu–N1, 93.16(6); O1–Cu–N2, 171.58(5); O1–Cu–N3, 94.82(6);
O1–Cu–O2,95.78(4);N1–Cu–N2,84.75(6);N1–Cu–N3,163.91(5).
depicted in Fig. 2. The compound consisted of a
[CuII(saldien)(H2O)]+cation,aswellasnitrateandazideanions,
which were located in the structural voids of the crystal and
disordered over two positions with occupancies of 0.254N3–
and 0.746NO3–. The geometry of the Cu(II) atom in the cation
wasdistortedsquarepyramidal(τ=0.13accordingtoτ=(dif‐
ferencebetweenthetwolargestangles)/60forfivecoordinat‐
ed metal centers allows for the distinction between trigo‐
nal‐bipyramidal(ideallyτ=1)andsquare‐pyramidal(ideallyτ
=0)[37]),withthecoppercoordinatedtothreenitrogenatoms
andoneoxygenatomfromthenon‐symmetricligandandone
oxygenatomfromacoordinatedwatermolecule.Thefourco‐
ordination atoms from the ligand constituted the basal plane,
whereastheoxygenatomfromtheH2Omoleculeoccupiedthe
axialposition.TheCu(II)atomwaslocated0.186Åabovethe
planeinthedirectionofthewatermolecule.Althoughthebond
lengthsofthethree Cu–N bonds,includingthe Cu(II)contacts
withtheprimaryamine,secondaryamineandimine,weredif‐
ferent from one another, they were in normal bond length
range. The dihedral angle between the equatorial plane (O1,
N1, N2, N3) and the aromatic ring was nearly parallel
(3.08(4)°).Thecrystalstructurewasstabilizedbyintermolec‐
ular hydrogen bonding interactions, which formed a
two‐dimensional network parallel to the (100) plane (Figure
not shown). The complex cations were connected with each
other through the O2–H···O1 and N3–H···O2 interactions to a
formdimer. 3.3. Catalyticactivity
Classical stoichiometric oxidants, such as dichromate and
permanganate, should be replaced with new environmentally
friendlycatalyticprocessesthatusecleanoxidantssuchasmo‐
lecular O2 and H2O2 [5,38]. The catalytic oxidation of cy‐
clooctenewithH2O2wasstudiedinthepresenceof1.Allofthe
reactions were carried out with 1 mmol of cyclooctene in
CH3CN at 80 °C in the presence of catalyst. Cyclooctene oxide
wasformedasthesoleproductofthesereactions.Theresults
ofcontrolexperimentsrevealedthatthe presenceofthecata‐
lyst and the oxidant were essential to the oxidation process.
The oxidation of cyclooctene in the absence of H2O2 did not
occur, whereas in the absence of a catalyst the oxidation only
Cyclooctene conversion (%)
80
1459
Without co-catalyst
With co-catalyst
60
40
20
0
CH3CN
CH3OH
C2H5OH
CH3Cl
Fig. 3. The effect of NaHCO3 as a co‐catalyst towards the oxidation of
cycloocteneby1/H2O2indifferentsolvents.
proceededbyupto5%after24h.Theadditionofaco‐catalyst
(NaHCO3) increased the cyclooctene conversion by 1/H2O2
from 15% to 63% after 5 h (Fig. 3). To further optimize the
processtoachievethemaximumoxidationofcyclooctene,the
effectofdifferentsolventswasstudied(Fig.3).Theorderofthe
catalystactivityintheabsenceorpresenceofNaHCO3wasthe
same. The highest conversion was obtained in acetonitrile
(63%after5h).Theactivityofthecatalystinthedifferentsol‐
ventstestedfollowedtheorderacetonitrile>methanol>eth‐
anol > chloroform. This order reflected the relative dielectric
constants (ε/ε0) of the solvents, which were 37.5, 32.7, 24.3,
and4.9,respectively[39].Thesedatasuggestedthatthehigher
level of conversion observed in acetonitrile was related to its
highdielectricconstant.
To establish the scope for the activity of
[CuII(saldien)(H2O)](NO3)0.75(N3)0.25, the oxidation reactions of
cyclohexene, cyclohexane, and ethylbenzene were also exam‐
inedundertheoptimizedconditions(i.e.,a3:1(mol/mol)mix‐
ture of H2O2 and cyclooctene in acetonitrile at 80 °C with a
co‐catalyst),andtheresultsareshowninTable2.Theoxidation
of cyclohexene by complex 1 was particularly impressive and
showed excellent selectivity towards cyclohexene oxide. Com‐
plex 1 exhibited a high level of selective in comparison with
most of the other systems reported in the literature for the
same transformation [40]. Cyclohexene possesses two allylic
positions,inadditiontodoublebondfunctionalgroup,thatare
activatedtowardsoxidationandthereforeveryeasilyoxidized
[6,41–43].Unfortunately,thecatalystwasfoundtobeinactive
towardstheoxidationofethylbenzene.
The oxidation of cyclohexane under mild conditions is a
topicofconsiderableinterest[44].Usingournewlydeveloped
catalyst 1 in the presence of H2O2 and NaHCO3, cyclohexane
was successfully oxidized to a mixture of cyclohexanol and
cyclohexanone (1.4:1, mol/mol) in a combined yield of 91%
(Table 2, entry 3). This particular result was similar to those
reportedintheliteratureinvolvingtheuseofdinuclearMn(IV)
[45,46].Shul'pinetal.[45,46]reportedtheoxidationofcyclo‐
hexane using the binuclear manganese(IV) complex
([LMnIV(O)3MnIVL]2+, L: 1,4,7‐trimethyl‐1,4,7‐triazacyclonon‐
ane)asacatalyst.Thiscatalystgaveamixtureofcyclohexanol
and cyclohexanone (1.38:1, mol/mol) with a conversion of
1460
HassanHOSSEINI‐MONFAREDetal./ChineseJournalofCatalysis34(2013)1456–1461
Cu2++H2O2→Cu++H++HO2•
Cu++H2O2→Cu2++HO–+HO•
CyH+HO•→Cy•+H2O
Cy•+HO2•→CyOOH
CyOOH→Cy‐one+HO•
Cy•+HO•→Cy‐ol
Table2 Oxidation of different substrates with [CuII(saldien)(H2O)](NO3)0.75‐
(N3)0.25(1)usingH2O2/NaHCO3inacetonitrile.
Entry Substrate
Conversiona Selectivity
(%)
(%)
87
100
Product
1
TONb
300
O
2
63
100
217
91
ketone59
alcohol41
314
5
100
17
O
3
OH
O
4
O
Reaction conditions: Catalyst [CuII(saldien)(H2O)](NO3)0.75(N3)0.25 2.90
mol,substrate1mmol,n‐octane0.1g,acetonitrile3ml,H2O23mmol,
NaHCO31mmol,80°C,5h.
aBasedonsubstrate.
b Turnover number (TON) = mmol of oxidized products per mmol of
metalinthecatalyst.
30%. It is noteworthy that this oxidation reaction was con‐
ductedinthepresenceofaceticacidtopreventthedecomposi‐
tionofH2O2toH2Oandoxygen.
Complex 1 was also used to evaluate the oxidation of
ethylbenzene.Theoxidationproceededselectivelytogiveace‐
tophenone,buttheconversionwaslow(5%).
A variety of different homogeneous and heterogeneous
Cu‐catalysts have been used for the oxidation of cyclohexane
withH2O2andt‐BuOOH.Noneofthesecatalysts,however,have
shownactivity and selectivitylevelsgreaterthanthose exhib‐
ited by [CuII(saldien)(H2O)](NO3)0.75(N3)0.25. Fernandes et al.
[47]reportedtheuseof[Cu(OTf)(L)](OTf),[Cu(L)(H2O)](OTf)2,
[Cu(OTf)(LMe)](OTf), [Cu(Lpy)](OTf)2, [Fe(OTf)2(LMe)], and
[Fe(OTf)(Lpy)](OTf) complexes in the presence of several dif‐
ferent N‐based additives for the oxidation of cyclohexane.
These systems successfully catalyzed the oxidation of cyclo‐
hexanetocyclohexanolandcyclohexanoneinacetonitrilewith
H2O2, with the products being formed in yields of up to 29%
after 6 h (OTf = trifluoromethanesulfonate, L = bis‐ and
tris‐pyridylaminoandiminothioetherligands).Silvaetal.[16]
investigated the catalytic oxidation of cyclohexane using the
Cu(II) complexes [Cu(BMPA)Cl2] and {[Cu(BMPA)Cl2]‐
[Cu(BMPA)(H2O)Cl][Cu(BMPA)Cl][CuCl4]} with H2O2 or
tert‐butyl hydroperoxide in acetonitrile (BMPA =
bis‐(2‐pyridylmethyl)amine)).Thelatterofthesetwocatalysts
gave the best results, providing the products in a combined
yield of 69% after 24 h. Canhota et al. [17] succeeded in pro‐
moting the oxidation of cyclohexane with a series of different
2,2'‐bipyridine Cu(II) complexes. In this particular case, the
[Cu(bipy)3]Cl2, [Cu(bipy)2Cl]Cl, and [Cu(bipy)Cl2] complexes
provided the oxidized products in yields of up to 44% using
H2O2at50°C.
To determine whether the reaction proceeded according
Scheme 2. A possible mechanism of the oxidation of cyclohexane by
1/H2O2/NaHCO3/CH3CN.
to a radical‐based mechanism, an experiment was conducted
involving the addition of the radical inhibitor tert‐butanol to
thereactionmixture.Inthisparticular case,thedesired prod‐
uctwasnotformed.Furthermore,theoxidationofcyclohexane
usingthe1/H2O2/NaHCO3/CH3CNsystemdidnotoccurwhen
the reaction was conducted under a nitrogen atmosphere in‐
stead of an oxygen atmosphere. Taken together, these results
effectively confirmed that free diffusing radicals were present
as intermediates in the reaction mixture. The number of free
diffusingradicalsinamixturecanbeenhancedsignificantlyby
free radical chains in the presence of O2. Similar results have
alsobeenreportedintheliterature[48].Basedontheseresults,
wehaveproposedapossiblemechanismfortheoxidationre‐
action involving the generation of radical species (Scheme 2)
[49]. The increase in the reaction rate observed following the
addition of NaHCO3 has been attributed to an increase in the
equilibriumconcentrationofHOO¯.
4. Conclusions
A new copper(II) complex, [CuII(saldien)(H2O)](NO3)0.75‐
(N3)0.25 (1), of the non‐symmetric Schiff base Hsaldien was
synthesized and characterized by spectroscopic methods. The
structure of the complex was established by X‐ray crystallo‐
graphic analysis. The catalytic activity of 1 was evaluated for
theoxidationofavarietyofdifferenthydrocarbonsusingH2O2
as the terminal oxidant. High levels of selectivity and conver‐
sion were obtained in the oxidation reactions of cycloalkenes
andcyclohexane.
Acknowledgments
The authors would like to thank Professor T. Lis for data
collectionandpreliminarystructuresolving.
WearegratefultotheUniversityofZanjanforprovidingfi‐
nancialsupportforthisstudy.
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1461
GraphicalAbstract
Chin.J.Catal.,2013,34:1456–1461 doi:10.1016/S1872‐2067(12)60616‐0
Synthesis,structuralanalysisandevaluationofthecatalyticactivityofanon‐symmetricN‐(salicylidene)diethylenetriamine
complexofcopper(II)
HassanHOSSEINI‐MONFARED*,SohailaALAVI,MiloszSICZEK
UniversityofZanjan,Iran;UniversityofWroclaw,Poland
N
OH2
NH
O
conversion 87%
O selectivity 100%
OH
H2O2, NaHCO3 , CH3CN
80 °C, 5 h
O
conversion 91%
41%
59% selectivity
Cyclooctene conversion (%)
+
H
N
Cu
80
Without co-catalyst
With co-catalyst
60
40
20
0
CH3CN
CH3OH
H5OH
CH3Cl
Synthesisandcrystalstructuralanalysisofanewcopper(II)complexofanon‐symmetricSchiffbasehavebeenreported.Thecomplex
showedhighlevelsofcatalyticactivityandselectivitytowardstheoxidationofcycloalkenesandcyclohexanebyH2O2andNaHCO3.
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