Synthesis and characterization of copper(ii) 4′-phenyl-terpyridine compounds and catalytic application for aerobic oxidation of benzylic alcohols

Article abstract of DOI:10.1039/c3dt53054j

The reactions between 4′-phenyl-terpyridine (L) and nitrate, acetate or chloride Cu(ii) salts led to the formation of [Cu(NO₃) ₂L] (1), [Cu(OCOCH₃)₂L]·CH ₂Cl₂ (2·CH₂Cl₂) and [CuCl₂L]·[Cu(Cl)(μ-Cl)L]₂ (3), respectively. Upon dissolving 1 in mixtures of DMSO-MeOH or EtOH-DMF the compounds [Cu(H ₂O){OS(CH₃)₂}L](NO₃)₂ (4) and [Cu(HO)(CH₃CH₂OH)L](NO₃) (5) were obtained, in this order. Reaction of 3 with AgSO₃CF₃ led to [CuCl(OSO₂CF₃)L] (6). The compounds were characterized by ESI-MS, IR, elemental analysis, electrochemical techniques and, for 2-6, also by single crystal X-ray diffraction. They undergo, by cyclic voltammetry, two single-electron irreversible reductions assigned to Cu(ii) → Cu(i) and Cu(i) → Cu(0) and, for those of the same structural type, the reduction potential appears to correlate with the summation of the values of the Lever electrochemical E_L ligand parameter, which is reported for the first time for copper complexes. Complexes 1-6 in combination with TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl radical) can exhibit a high catalytic activity, under mild conditions and in alkaline aqueous solution, for the aerobic oxidation of benzylic alcohols. Molar yields up to 94% (based on the alcohol) with TON values up to 320 were achieved after 22 h.

Full text of DOI:10.1039/c3dt53054j

View Article Online
View Journal
Dalton
Transactions
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: Z. Ma, L. Wei, E.
Alegria, L. Martins, F. Guedes da Silva and A. J. L. Pombeiro, Dalton Trans., 2013, DOI:
10.1039/C3DT53054J.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/dalton

Page 1 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/C3DT53054J
Graphical Abstract
Cu(II)-terpyridine complexes in combination with TEMPO exhibit catalytic activity,
under mild conditions and in alkaline aqueous solution, for the aerobic oxidation of
benzylic alcohols.

Dalton Transactions
Page 2 of 31
View Article Online
DOI: 10.1039/C3DT53054J
Synthesis and Characterization of Copper(II) 4’-phenyl-terpyridine Compounds
and Catalytic Application for Aerobic Oxidation of Benzylic Alcohols
Zhen Ma,a,b Lijuan Wei,a Elisabete C. B. A. Alegria,b,c Luísa M. D. R. S. Martins,b,c M. Fátima
C. Guedes da Silva,b,d Armando J. L. Pombeirob,*
a School of Chemistry and Chemical Engineering, Guangxi University, Guangxi 530004, P. R. China
b Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Universidade de Lisboa,
Av. Rovisco Pais, 1049-001 Lisboa, Portugal. E-mail: pombeiro@tecnico.ulisboa.pt
c Chemical Engeneering Department, ISEL, R. Conselheiro Emídio Navarro, 1959-007 Lisboa,
Portugal.
d Universidade Lusófona de Humanidades e Tecnologias, ULHT Lisbon, Av. do Campo Grande 376,
1749-024, Lisbon, Portugal
Abstract
The reactions between 4’-phenyl-terpyridine (L) and nitrate, acetate or chloride Cu(II) salts led to
the formation of [Cu(NO3)2L] (1), [Cu(OCOCH3)2L]·CH2Cl2 (2·CH2Cl2) and [CuCl2L].[Cu(Cl)(-
Cl)L]2 (3), respectively. Upon dissolving 1 in mixtures of DMSO/MeOH or EtOH/DMF the
compounds [Cu(H2O){OS(CH3)2}L](NO3)2 (4) or [Cu(HO)(CH3CH2OH)L](NO3) (5) were obtained,
in this order. Reaction of 3 with AgSO3CF3 led to [CuCl(OSO2CF3)L] (6). The compounds were
characterized by ESI-MS, IR, elemental analysis, electrochemical techniques and, for 2-6, also by
single crystal X-ray diffraction. They undergo, by cyclic voltammetry, two single-electron
irreversible reductions assigned to Cu(II)  Cu(I) and Cu(I)  Cu(0), and, for those of the same
1

Page 3 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/C3DT53054J
structural type, the reduction potential appears to correlate with the summation of the values of the
Lever electrochemical EL ligand parameter, what is reported for the first time for copper complexes.
Complexes 1-6 in combination with TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl radical) can
exhibit a high catalytic activity, under mild conditions and in alkaline aqueous solution, for the
aerobic oxidation of benzylic alcohols. Molar yields up to 94 % (based on the alcohol) with TON
values up to 320 were achieved after 22 h.
Keywords: copper, 4’-phenyl-terpyridine, catalysis, alcohols oxidation, TEMPO, aqueous medium,
EL parameter, X-ray diffraction
1. Introduction
Multi-pyridyl ligands can coordinate to different metal ions and lead to the formation of a variety of
complexes [1]. An example of such ligands is 4’-phenyl-terpyridine (L) which is able to bind several
transition metals and lead to interesting fluorescence, catalysis or sensorial properties, or even to
quite promising tumor-inhibiting activities [2].
The selective oxidation of alcohols to the corresponding aldehydes and ketones has become an
important research topic [3] due to the wide application of these products in organic synthesis [4].
The application of TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl radical) in combination with
transition metal compounds as catalysts for the aerobic oxidation of benzylic and allylic alcohols
was started by Semmelhack and co-workers, using a CuCl/TEMPO system [5]. Since then, this
stable free radical has been recognized as a mediator or promoter in several organic reactions [6],
and has been widely used in the selective oxidation, with a variety of oxidants, of primary alcohols
to the corresponding aldehydes and ketones. Owing to economic and environmental reasons, a
2

Dalton Transactions
Page 4 of 31
View Article Online
DOI: 10.1039/C3DT53054J
growing attention has been paid to the replacement of the traditionally used, though very active,
oxidants (chromates, hypochlorite and permanganates among others) [7] in the alcohols oxidation by
the environmentally friendly hydrogen peroxide or molecular oxygen (water as by-product).
Another typical source of unwanted waste from these reactions is related with the organic solvents
normally used; ionic and supercritical fluids (SCFs) are thus becoming popular [8] as reaction
media.
Copper(II) complexes with N,N- and N,O-ligands in conjunction with TEMPO were found to be
efficient catalysts or catalyst precursors for the aerobic oxidation of benzyl alcohol in aqueous
media [9]. Other transition metal catalysts, such as Ru [10], Mn/Cu and Mn/Co [11], Cu [12], V/Mo
[13] and Fe [14] were also shown to catalyse the selective oxidation of alcohols to the corresponding
aldehydes in the presence of a stable nitroxyl radical. In particular, systems based on Cu [9,12,15]
metal centres have been successfully explored.
As a continuation of our research line and aiming to achieve novel terpyridine compounds with
promising catalytic or biological properties, we managed to prepare and fully characterize the
copper(II) complexes [Cu(NO ) L] ( , Cu(OCOCH ) L]·CH Cl ( ·CH Cl ), [CuCl L].[Cu(Cl)( -
1) [
2

3 2
3 2
2
2
2
2
2
Cl)L]2 ( ), [Cu(H2O){OS(CH3)2}L](NO3)2 ( ) [Cu(HO)(CH3CH2OH)L](NO3) ( ), and
4 ,
3
5
[CuCl(SO CF )L] ( ). Targeting to contribute towards the development of the application of Cu
6
3
3
compounds as catalysts and/or catalyst precursors for the oxidation of alcohols to the respective
aldehydes, in aqueous medium and with a readily available and environmentally friendly oxidant,
we now report the oxidation, by O (air), of benzylic alcohols catalysed by - /TEMPO systems.
1 6
2
3

Page 5 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/C3DT53054J
2. Results and Discussion
2.1 Synthesis and Characterization
4’-phenyl-terpyridine (L) was synthesized by following a literature procedure [16a], and the
copper(II) complexes [Cu(NO3)2L] (1), [Cu(OCOCH3)2L]·CH2Cl2 (2·CH2Cl2), and
[CuCl2L].[Cu(Cl)(-Cl)L]2 (3), were obtained (Scheme 1) by reacting L (added in a CH2Cl2
solution) with the appropriate Cu(II) salt, i.e. Cu(NO3)2•2.5H2O, Cu(CH3COO)2·H2O or CuCl2,
dissolved in acetonitrile (for 1) or methanol (for
2
and 3). Compounds
[Cu(H2O){OS(CH3)2}L](NO3)2 (4) and [Cu(HO)(CH3CH2OH)L](NO3) (5) were obtained upon
further reaction of 1 with a methanolic solution of DMSO (4) or with 4,4’-bipyridine (4,4’-Bpy) in a
DMF:ethanol mixture (5). In the latter complex, the OH- ligand was generated from adventitious
water. A similar behaviour was previously reported in the reaction of a trinuclear copper compound
with 4,4’-bpy in MeOH, where OH- displaced an acetate ligand [16b]. In the reaction of 3 with
AgSO3CF3 in NCMe, complex [CuCl(SO3CF3)L] (6) was obtained. Complexes 1-6 were achieved
in medium-high yields and characterized by IR, elemental analysis, cyclic voltammetry (CV) and
controlled potential electrolysis (CPE), as well as, for 2 – 6, by single-crystal X-ray diffraction.
Although the IR spectra of the compounds were not very informative, they revealed the bands that
are typical of the 4’-phenyl-terpyridine, in particular the very strong bands in the 1557 – 1417 cm-1
frequency range. The most typical frequency vibrations for nitrate and trifluoromethane sulfonate
anions overlay those of the terpyridine ligand. The UV-vis spectra of the compounds run in DMSO
are identical to those run in water and reveal their stability in those solvents.
2.2. X-ray crystal structures
Crystals suitable for X-ray diffraction analysis were obtained upon slow diffusion of diethyl ether
into a dichloromethane solution (for 2) or a DMSO:methanol mixture (for 4), slow evaporation of a
4

Dalton Transactions
Page 6 of 31
View Article Online
DOI: 10.1039/C3DT53054J
methanol:dichloromethane solution (for 3) or of the reaction mixtures (for 5 and 6) (the schematic
representations of their formulae are shown in Scheme 1). The ellipsoid plots of complexes 2 – 6 are
depicted in Figure 1. Crystallographic data are given in Table 1 and a comparison of selected
bonding parameters in Table 2.
(NO3)2
OH2
N
DMSO
N
Cu
MeOH, 60 ºC
N
S(CH3)2
(NO3)
O
(4)
O NO2
O NO2
N
N
Cu(NO3)2.2.5H2O
MeCN, CH2Cl2
N
Cu
(1)
N
OH
EtOH, DMF
4,4'-Bpy
N
Cu
N
OEt
H
O
(5)
N
OC
CH3
Cu(OCOCH3)2·H2O
MeOH, CH2Cl2
N
N
Cu
O
N
N
N
OC
CH3
(2)
MeOH
CH2Cl2
CuCl2
Cl
N
N
N
N
Cl
AgSO3CF3
Cl
Cl
Cu
N
N
N
Cu
N
Cl
MeOH,
CH2Cl2,DMF
Cu
N
Cu
N
O SO2CF3
N
N
Cl
Cl
(6)
(3)
Scheme 1 - Syntheses of compounds 1-6
The geometries around the copper ions in complexes 4 – 6 can be considered as rectangular
pyramidal, what is supported by the 5 [17] values (Table 2) of 0.07, 0.27 and 0.21, respectively.
5

Page 7 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/C3DT53054J
Although in 2 a value for that parameter is shown (Table 2) the relative orientation of one of the
acetate anions and consequent proximity of the O-atom [Cu2O4 bond distance of 2.902(2) Å] may
lead to a six-coordinate copper complex; in such a situation, the Cu environment will be defined as
N1O3 in the equatorial plan and two N-atoms of the chelating ligand in the axial sites. Such
interaction appears to increase the global electron-donor ability of the ligand (decrease of the EL
parameter; see below, electrochemistry section).
In what concerns 3, it consists of two complexes, a mononuclear and a dinuclear one. While Cu1
stands in a square pyramid environment and belongs to a monomeric complex, the coordination
sphere of the Cu2 atom discloses an octahedron resulting from the long range Cu2Cl4 interaction
[3.1710(8) Å] with a symmetry generated Cu2 moiety, leading to a dimeric complex with an
inversion centre in the middle of a Cu2Cl2 core.
An interesting feature of 2  6 concerns the angle (denoted as angle A) between the terpyridine plane
(defined by the N atoms) and that of the attached phenyl group. In none of the complexes these two
assemblies are co-planar and the twisting of the phenyl rings relative to the terpyridine planes
assume values from 16.82º (in 4) to 31.12º (in the Cu1 molecule of 3).
The disparity of the CuN bond distances, the one involving the central nitrogen atom being shorter
than those concerning the terminal N-atoms (Table 2), is usual and results from chelating ligand
constrains. However, this trend is not followed in complex 6 where the latter is ca. 0.03 Å shorter than
the former; moreover, in this complex the CuN bond length is the shortest one found in the series of
compounds of this work. Also in 6 the CuCl distance is shorter than those in 3.
The interplanar distances of successive terpyridine moieties (shortest values found ranging from
3.5454(9) to 3.843(3) Å, see Table 2) suggested possible π-π interactions in all of the compounds.
Moreover, in 3 and 5 very strong interactions between a metallacycle and a terpyridine ring of a
6

Dalton Transactions
Page 8 of 31
View Article Online
DOI: 10.1039/C3DT53054J
vicinal molecule were detected which, for the former, reached values as short as 2.964 Å. Although
a long range CuCl interaction has been detected for the Cu2-containing molecule in 3 and which
led to dimerization (see above) the Cu∙∙∙Cu separation in this complex [4.0131(6) Å] is shorter than
that found for 6 [3.4031(8) Å]; in the other cases, such contacts exceed 7 Å.
Table 1. Crystal data and structure refinement details for 2 − 6.
2
3
4
5
6
C22H15ClCuF3N3O3 S
557.42
Monoclinic
150(2)
Formula unit
C26H23Cl2CuN3O4 C84H60Cl8Cu4N12
C23H23CuN5O8S C23H22CuN4O5
Formula weight /g mol-1 575.91
1775.20
Triclinic
150(2)
593.06
497.98
Crystal system
T (K)
Triclinic
150(2)
Triclinic
150(2)
Monoclinic
296(2)
Space group
a, Å
b, Å
P-1
P-1
P-1
P 21/n
P 21/c
8.0783(3)
11.2416(3)
14.5084(5)
76.347(2)
89.396(3)
76.328(2)
2
9.7921(8)
13.3622(5)
15.6530(3)
102.603(2)
93.497(3)
104.137(2)
1
9.9081(4)
10.6163(3)
13.7652(5)
101.172(2)
101.232(2)
116.680(3)
2
8.6645(5)
24.9251(2)
9.9251(3)
90.00
103.888(3)
90.00
7.5816(15)
12.192(2)
20.344(4)
90.00
90.60(3)
90.00
c, Å
α, deg
β, deg
γ, deg
Z
4
4
3
1242.61(7)
1924.03(18)
1201.88(9)
2080.80(14)
1880.4(6)
Volume , Å
Dc, g cm-3
1.539
1.133
29287
29287/6545
0.0251
1.532
1.423
27787
7025/5845
0.0350
1.639
1.056
15087
4436/ 4143
0.0282
1.590
1.096
25187
5201/4536
0.0225
1.969
1.481
9285
3299/ 2767
0.0883
μ(Mo Kα), mm-1
Rfls. collected
Rfls. Unique/observed
Rint
Final R1 a, wR2 b (I  2σ)
GOF on F2
0.0514, 0.1371
0.0324, 0.0821
0.0289, 0.0757
0.0466, 0.1399
1.155
0.0414, 0.1110
1.056
1.156
1.059
1.069
2
2 2
a R1 = ||Fo| – |Fc||/|Fo|. b wR2 = [[w(Fo – Fc2)2]/ [w(Fo ) ]]1/2
7

Page 9 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/C3DT53054J
(2)
(4)
(3)
(6)
(5)
8

Dalton Transactions
Page 10 of 31
View Article Online
DOI: 10.1039/C3DT53054J
Figure 1. X-ray molecular structures of complexes 2 – 6 with partial atom numbering schemes.
Ellipsoids are drawn at 50% probability. Dichloromethane of crystallization (in 2) and nitrate
counter ions (in 4 and 5) were omitted for clarity. Symmetry operation to generate equivalent atoms
(in 3): a): 1-x,1-y,2-z.
Table 2 - Selected bond distances (Å) and angles (º) for complexes 2 – 6.
Parameter
2
3
4
5
6
Equatorial environment
Axial atom
N3O
Oacetate
0.02
N3Cl
N3O
Odmso
0.07
N3O
N3Cl
Osulfonate
0.21
Cl
Ohydroxide
0.27
0.00
5
1.958(2) (Cu1)
1.939(2) (Cu2)
2.037(2)
CuNcentral pyridil
1.946(2)
1.9293(16)
1.935(3)
1.947(3)
2.029(2)
-
2.034(16)
-
2.043(3)
-
1.919(2)
CuNterminal pyridils (avg.)
CuClequatorial
2.2469(7) (Cu1)
2.2343(7) (Cu2)
2.4410(7) (Cu1)
2.6570(9) (Cu2)
-
2.2194(12)
-
-
-
-
CuClaxial
1.951(2)
2.114(2)
1.9408(15)
2.2054(14)
1.913(3)
2.149(3)
-
CuOequatorial
CuOaxial
-
2.197(2)
79.62(9)
157.79(9)
78.67(8)
158.40(9)
79.54(7)
159.19(7)
79.32(12)
156.98(13)
79.21(11)
158.35(12)
NCuN (minimum)
NCuN (maximum)
Angle A (i)
28.46
31.12 (Cu1)
19.25 (Cu2)
0.352
16.82
20.75
24.37
Cumean basal plane N3X
(X = Cl or O)
Interplanar distances (ii)
0.312
0.217
0.148
0.123
0.207
3.5816(18)
3.6535(16)
3.5454(9)
3.843(3)
3.7025(18)
(i) Angle between the terpyridine and the attached phenyl planes (see text).
(ii) Involving only rings of the terpyridine ligands.
9

Page 11 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/C3DT53054J
2.3. Electrochemical behaviour
Complexes 1-6 exhibit, by cyclic voltammetry (CV), at a platinum electrode at 25 ºC in a 0.2 M
nBu4NBF4/DMSO solution, two single-electron (as measured by controlled potential electrolysis)
irreversible reduction waves, the former at potential values (Table 3) in the 0.06 to 0.48 V vs.
SCE range, assigned to the Cu(II)  Cu(I) reduction (wave Ired), and the latter in the 1.23 to 1.58
V vs. SCE range assigned to the Cu(I)  Cu(0) process (wave IIred). Upon scan reversal, after the
second cathodic process, an irreversible anodic wave (Iox) at 0.05  0.16 V vs. SCE (Table 3) is
observed, corresponding to the oxidation of cathodically generated (at the reduction level of wave
IIred) Cu(0) species. Expectedly, no anodic waves have been detected for any of the complexes. A
typical cyclic voltammogram is exemplified in Figure 2.
Figure 2 - Cyclic voltammogram of 4 in a 0.2 M nBu4NBF4/DMSO solution, at a Pt disc working
electrode (d = 0.5 mm), run at a scan rate of 200 mVs-1.
The reduction potential of the complexes should reflect the relative electron donor/acceptor abilities
of the ligands [18]. Lever [18d-f] developed an electrochemical parameterization approach,
10

Dalton Transactions
Page 12 of 31
View Article Online
DOI: 10.1039/C3DT53054J
expressed by the equation E = SM  (EL) + IM, in which the redox potential of a complex (E) is
related with electrochemical parameters determined by ligand and metal centre properties; EL is the
sum of the values of the EL ligand parameter for all the ligands (additive effect), SM and IM depend
upon the metal and redox couple, the spin sate and stereochemistry. Such an approach was initially
applied to octahedral-type complexes [18d-f], but subsequently extended to square-planar 16-
electron and square-pyramid 18-electron complexes [19], as well as to half-sandwich type
complexes [18a], but has never been applied to copper(II) centres. Indeed, the SM and IM values for
the Cu(II)/Cu(I) redox couple are not known. Additionally, the EL ligand parameter for the chelating
4,4’-terpyridine ligand is still not available, but since the Cu(II)-L core is common to complexes 1-
6, the variability in the redox potential should be mainly dependent on the co-ligands, i.e., on the
sum of their EL values (EL of the co-ligands). However, only the complexes with the same
stereochemistry are expected to follow the above Lever equation, with constant SM and IM values.
Hence, only complexes 3 – 6 are directly comparable. The dinuclear form in the mixture 3 possibly
dissociates to the mononuclear one in the electrolyte medium, since only single Ired and IIred
reduction waves are observed. Alternatively, both complexes in 3 exhibit the same reduction
potential what does not invalidate the discussion below.
Complexes 1 and 2 are expected to display a different stereochemistry since the acetate and nitrate
ligands can chelate (see the above X-ray diffraction analysis of 2) resulting into a higher Cu
coordination number. In accord, a linear relationship between Ep (Ired) and EL of the co-ligands is
observed for the closely related compounds 3 – 6 (Fig. 3). Nevertheless, one should be cautious
since the reduction potentials are not the thermodynamic ones in view of the irreversible character of
the reduction waves. However, such an approach can be valid for a series of closely related
complexes and has been applied in other cases.[19]
11

Page 13 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/C3DT53054J
Table 3. Cyclic voltammetric dataa for [Cu(NO3)2L] (1), [Cu(OCOCH3)2L]·CH2Cl2 (2·CH2Cl2),
[CuCl2L].[Cu(Cl)(-Cl)L]2 (3), [Cu(H2O){OS(CH3)2}L](NO3)2 (4), [Cu(HO)(CH3CH2OH)L](NO3)
(5) and [CuCl(SO3CF3)L] (6)
EL (co-ligands)c
Complex
Ep (Ired)
Ep (IIred)
Ep (Iox)b
1
-0.29
-1.33
0.11
-0.22
2
3
4
5
6
-0.48
-0.23
-0.06
-0.27
-0.21
-1.43
-1.42
-1.23
-1.58
-1.36
0.13
0.16
0.07
0.05
0.11
-0.48
-0.48
+0.51
ca. -0.55
-0.37
a Potential values in Volt  0.02 vs. SCE, in a 0.2 M Bu4NBF4/DMSO solution, at a Pt disc working electrode
determined by using the Fe(5-C5H5)20/+ redox couple (E1/2ox = 0.44 V vs. SCE [23]) as internal standard at a scan rate
of 200 mVs-1.
b Anodic wave generated upon scan reversal following the second reduction wave.
c
Values in Volt vs. NHE, refer only to the ligands other than 4’-phenyl-terpyridine. EL values for Cl (-0.24 V),
OSO2CF3 (-0.13 V), NO3 (-0.11 V), OH (-0.59 V), water (0.04 V) and DMSO-O (0.47 V) have been indicated in the
original publications [18d-f], and that of CH3COO (-0.24 V) was proposed later [20].
12

Dalton Transactions
Page 14 of 31
View Article Online
DOI: 10.1039/C3DT53054J
Figure 3 – Plot of the redox potential values for the first cathodic wave [Ep (Ired)] vs. the sum of EL
ligand parameters of the co-ligands [EL of the co-ligands] for the 4’-phenyl-terpyridine complexes
[CuCl2L].[Cu(Cl)(-Cl)L]2 (3), [Cu(H2O){OS(CH3)2}L](NO3)2 (4), [Cu(HO)(CH3CH2OH)L](NO3)
(5) and [CuCl(SO3CF3)L] (6).
2.4. Catalytic aerobic oxidation of alcohols
The copper(II) complexes 1-6 catalyse the aerobic oxidation of benzyl alcohol, in alkaline (1 M
K2CO3) aqueous solution and in the presence of the TEMPO (2,2,6,6-tetramethyl-piperidinyloxyl)
radical, to the corresponding aldehyde. The formation of the aldehyde is predominant (yields up to
94 %, moles of product per mole of substrate), the lack of the corresponding carboxylic acid product
conceivably being due to the well-known propensity of TEMPO for scavenging free radicals,
thereby acting as an effective radical trap, avoiding the autoxidation [3e].
13

Page 15 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/C3DT53054J
CHO
CH OH
2
5mol% of TEMPO, 1mol% of [Cu]
Air, H2O, K2CO3, temperature
(X = H, Me, Cl)
X
X
Scheme 2. Aerobic oxidation of benzylic alcohols in water.
The aerobic oxidation of benzyl alcohol (1.5 mmol) in alkaline (1 M K2CO3) aqueous solution, in
the presence of a catalytic amount of the complex 1 (0.015 mmol, 1 mol % based on substrate) and
TEMPO (0.075 mmol, 5 mol % based on substrate) resulted in a yield of 46 % on benzaldehyde
after 6 h of reaction (Table 4, entry 5). Prolongation of the reaction time to 22 h induced a very good
yield on this product (93 %; Table 4, entry 2). Reaction performed under the same conditions but
with copper(II) nitrate resulted in only 2 % of benzaldehyde (Table 4, entry 22). The presence of
TEMPO radical was confirmed to be crucial, since practically no oxidation of benzyl alcohol was
observed in the absence of this radical (2 % yield; Table 4, entry 12).
We have also tested the influence of the amount of catalyst precursor on the oxidation of the alcohol
(Figure 4). In the presence of 1, similar yields of benzaldehyde (93-94 %; Table 4, entries 1-3) were
observed for the substrate/catalyst (S/C) molar ratio from 70/1 to 200/1, but the TON improved from
72 to 186. Subsequent increase of the S/C ratio to 400 (decrease of the catalyst amount) resulted in a
decrease of the yield to 80 % but a TON rising to 320 (Table 4, entry 4), on account of the slowing
down of the reaction upon lowering the catalyst concentration.
14

Dalton Transactions
Page 16 of 31
View Article Online
DOI: 10.1039/C3DT53054J
Attempts to perform the aerobic oxidation of benzyl alcohol at temperatures lower than 70 ºC
resulted in yield reductions (Figure 5, entries 6 and 7). For temperatures higher than that value, the
yield decrease conceivably results from the lower solubility of oxygen in the reaction solution
(Figure 5, entry 8).
The presence of base (K2CO3) was found to be crucial (its absence led to very low yield: 6%, Table
4, entry 9) and its amount was also optimized (Figure 6, entries 10 and 11).
Complex 1 proved to be the most active catalyst under the conditions of this study. Using complexes
2 – 6 as catalyst precursors and under the same conditions resulted also in high selectivities on the
benzaldehyde, but yields in the 57 – 87 % range and TONs not higher than 87 (Table 4, entries 13 –
17).
100
80
60
40
20
0
0.0 0.2 0.4
0.6 0.8 1.0 1.2 1.4 1.6
[catalyst 1] (mol %)
Figure 4 – Effect of catalyst precursor 1 amount (mol % vs. substrate) on benzaldehyde yield in the
aerobic oxidation of benzyl alcohol. Reaction conditions: substrate (1.5 mmol), 0.075 mmol of
TEMPO in 5 mL of 1M K2CO3 aqueous solution, 22 h.
15

Page 17 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/C3DT53054J
100
90
80
70
60
50
40
30
30
40
50
60
70
80
90
Temp (ºC)
Figure 5 – Effect of the temperature on benzaldehyde yield (mol %, vs. substrate) in the aerobic
oxidation of benzyl alcohol, catalyzed by 1. Reaction conditions: substrate (1.5 mmol), 0.015 mmol
of catalyst precursor and 0.075 mmol of TEMPO in 5 mL of 1M K2CO3 aqueous solution, 22 h.
100
80
60
40
20
0
0.0
0.5
1.0
1.5
[K CO ] (M)
2
3
Figure 6 – Yield of benzaldehyde vs. concentration of K2CO3. Reaction conditions: substrate (1.5
mmol), 0.015 mmol of catalyst precursor and 0.075 mmol of TEMPO in different K2CO3 aqueous
solution, 70 ºC, 22 h, 1 atm, air.
16

Dalton Transactions
Page 18 of 31
View Article Online
DOI: 10.1039/C3DT53054J
As it was shown before [9], substituted benzylic alcohols can also be converted to the corresponding
aldehydes with moderate to good yields (Table 4, entries 18 and 19), while the oxidation of an
aliphatic (1-hexanol, entry 20) or secondary benzylic alcohol (1-phenylethanol, entry 21) practically
does not proceed, as also reported for other Cu/TEMPO catalysts [6g,10a,12b]. Our catalytic
systems are expected [3a,12a-12c] to involve the coordination (with deprotonation) of benzyl
alcohol (PhCH2OH) and TEMPO radical to the copper centre, followed by hydrogen abstraction
from the former to the latter with resulting formation of the O-ligated radical PhHC●-O– and
TEMPOH. Intramolecular electron-transfer from ligated PhHC●-O– to CuII leads to the formation of
the aldehyde PhCHO and CuI which is reoxidized to CuII by dioxygen. The TEMPO radical is also
regenerated upon oxidation of TEMPOH.
Table 4. Aerobic oxidation of selected alcohols to the corresponding carbonyl compoundsa
Substrate
Run
Complex
Substrate
Product
Yield b
[%]
TON
Complex
1
2
70/1
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
94
93
93
80
46
38
77
82
6
72
93
186
320
92
38
77
82
6
1
1
1
1
1
1
1
1
1
1
1
100/1
200/1
400/1
200/1
100/1
100/1
100/1
100/1
100/1
100/1
3
4
5c
6d
7e
8f
9g
10h
11i
20
94
20
94
17

Page 19 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/C3DT53054J
12j
13
14
15
16
17
18
19
20
21
22
100/1
100/1
100/1
100/1
100/1
100/1
200/1
200/1
200/1
200/1
200/1
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
4-Me-benzyl alcohol
4-Cl-benzyl alcohol
1-Hexanol
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
4-Me-benzaldehyde
4-Cl-benzaldehyde
Hexanal
2
2
60
75
57
87
67
156
94
0
1
60
75
57
87
67
78
47
0
2
3
4
5
6
1
1
1
1
1-Phenylethanol
Benzyl alcohol
Acetophenone
Benzaldehyde
<1
2
<2
4
Cu(NO3)2
a
Conditions, unless stated otherwise: 1.5 mmol of the substrate, 0.015mmol (1 mol%) of catalyst precursor and
0.075mmol (5 mol%) of TEMPO in 5mL of 1M K2CO3 aqueous solution, 1 atm. air, 22 h. b Yields based on GC
c
d
e
f
g
h
i
analyses, selectivity in all cases >99 %. 6 h. 30ºC. 50ºC. 90ºC. Reaction without K2CO3. 0.5 M K2CO3. 1.5 M
K2CO3. j Reaction without TEMPO.
3. Conclusions
We have synthesized a number of copper(II) complexes with 4’-phenyl-terpyridine ligand and found
that they act as active catalyst precursors for the aerobic oxidation of alcohols, under mild
conditions and in the presence of TEMPO radical. The effects of various parameters have been
investigated and allowed us to achieve yields and TONs of ca. 94% and 320, respectively. The most
active catalyst for the optimized conditions is complex 1 with nitrate ligands.
This study also indicates that the copper(II) complexes may exhibit redox potential-structure
relationships, based on the electrochemical Lever EL ligand parameter, that are similar to those
already established for closed-shell octahedral-type complexes, 16-electron square-planar and 18-
18

Dalton Transactions
Page 20 of 31
View Article Online
DOI: 10.1039/C3DT53054J
electron square-pyramid transition metal d8 complexes, and half-sandwich complexes, what
constitutes a promising and still unexplored subject which deserves further investigation.
4. Experimental
X-ray Diffraction Analysis
The X-ray diffraction data of 2-6 were collected using a Bruker AXS-KAPPA APEX II
diffractometer with graphite monochromated Mo-K 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 [24a] software and refined using Bruker SAINT [24a] on all the observed
reflections. Absorption corrections were applied using SADABS [24a]. Structures were solved by
direct methods using the SHELXS–97 [24b] package and refined with SHELXL-97 [24b]. All
hydrogen atoms were inserted in calculated positions, except those of coordinated water molecule
(H8A and H8B) in 4, and the hydroxide (H1A) and methoxide (H2A) in 5, which were located in
the difference Fourier map and included in the refinement using the riding-model approximation
with coefficients 1.5 times larger than the respective parameters of the parent atoms. There were
disordered molecules present in the structure of complex 3. Since no obvious major site occupations
were found for those molecules, it was not possible to model them. PLATON/SQUEEZE [24d] was
used to correct the data and potential volumes of 152 Å3 were found with 41 electrons per unit cells
worth of scattering. These were removed from the models and not included in the empirical formula.
Least square refinements with anisotropic thermal motion parameters for all the non-hydrogen
atoms and isotropic for most of the remaining atoms were employed.
CCDC 967358 (2) - 967362 (6) contain the supplementary crystallographic data for this paper.
Crystallographic details and selected structural dimensions are listed in Tables 1 and 2 respectively.
19

Page 21 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/C3DT53054J
4.1
Instrumentation and Reagents
All reagents were of analytical grade or purified by standard methods. 4’-phenyl-terpy (L) was
prepared by following a reported procedure [22].
IR spectra were measured with a Perkin–Elmer Spectrum 2000 or a Magna 750 FT-IR
spectrophotometer, in KBr pellets (wavenumbers in cm-1; abbreviations: vs, very strong; s, strong;
m, medium; w, weak; br, broad). The UV-Vis absorption spectra of DMSO and water solutions of 1-
6 in 1.00 cm quartz cells, were recorded at room temperature on a Lambda 35 UV-Vis
spectrophotometer (Perkin-Elmer) by scanning the 200 – 800 nm region at a rate of 240 nm min−1.
Electrospray mass spectra (ESI-MS) were recorded on a Finnigan LCQ Mass Spectrometer using
dimethylformamide-methanol as the mobile phase.
C, H and N elemental analyses were carried out by the Microanalytical Service of the Instituto
Superior Técnico. The electrochemical experiments were performed on an EG&G PAR 273A
potentiostat/galvanostat connected to a personal computer through a GPIB interface. Cyclic
voltammetry (CV) studies were undertaken in 0.2 M nBu4NBF4/DMSO, at a platinum disc
working electrode (d = 0.5 mm) and at room temperature. Controlled-potential electrolyzes (CPE)
were carried out in electrolyte solutions with the above-mentioned composition, in a three-electrode
H-type cell. The compartments were separated by a sintered glass frit and equipped with platinum
gauze working and counter electrodes. For both CV and CPE experiments, a Luggin capillary
connected to a silver wire pseudo-reference electrode was used to control the working electrode
potential. A Pt wire was employed as the counter-electrode for the CV cell. The CPE experiments
were monitored regularly by cyclic voltammetry, thus assuring no significant potential drift occurred
along the electrolysis. The solutions were saturated with N2 by bubbling this gas before each run,
and the redox potentials of the complexes were measured by CV in the presence of ferrocene as the
internal standard, and their values are quoted relative to the SCE by using the Fe(5-C5H5)20/+
20

Dalton Transactions
Page 22 of 31
View Article Online
DOI: 10.1039/C3DT53054J
ox
1/2
redox couple (E = 0.44 V vs. SCE) [23]. Gas chromatographic (GC) measurements were carried
out using a FISONS Instruments GC 8000 series gas chromatograph with a FID detector and a
capillary column (DB-WAX, column length: 30 m; internal diameter: 0.32 mm). The temperature of
injection was 240 ºC. The initial temperature was maintained at 120 ºC for 1 min, then raised 10
ºC/min up to 200 ºC, and held at this temperature for 1 min. Helium was used as the carrier gas.
21

Page 23 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/C3DT53054J
4.2 Syntheses
4.2.1 [Cu(NO3)2L] (1)
Cu(NO3)2•2.5H2O (0.240 g, 1.0 mmol) was dissolved in 20 mL acetonitrile and 20 mL of a
dichloromethane solution of L (0.247g, 0.80 mmol) was added dropwise during half an hour and
stirred for 24 h. The mixture was then filtered to isolate a blue solid which was washed with small
portions of dichloromethane, acetonitrile and finally with methanol (0.347 g, 84 % yield). Anal.
Calcd for C21H15CuN5O6H2O (514.9): calcd. C 49.0, H 3.3, N 13.6; found C 49.1, H 3.2, N 13.7.
MS: [M - 2NO3 - H+]+: (372, 100%), [M - NO3)]+ (434, 35%). IR (KBr disk) ν (cm-1): 1615 (vs,
pyridyl-H), 1557 (s, aryl-H), 1503 (vs, pyridyl-H), 1421 (m, pyridyl-H), 1163 (m), 1098 (m, N-O), 1023 (vs),
974 (m, Ar-H). max (DMSO)/ nm 297 (/ M-1 cm-1 2974), 658 (313); max (H2O)/ nm 294 (/ M-1
cm-1 1374), 655 (384). Blue crystals of 1 were obtained upon diffusion of diethyl ether to a DMF
solution of the complex, but they were not suitable for X-ray analysis.
4.2.2. [Cu(OCOCH3)2L]·CH2Cl2 (2·CH2Cl2)
20 mL of a dichloromethane solution of L (0.155 g, 0.501 mmol) was added dropwise to 20 mL of a
methanol solution of Cu(CH3COO)2·H2O (0.100 g, 0.501 mmol) and the mixture was stirred for 24
h. Diethyl ether was then allowed to diffuse into the solution leading to the formation of green
crystals of 2 (0.216 g, 85% yield), which were suitable for X-ray analysis. Anal. Calcd for
C25H21CuN3O4·CH2Cl2 (575.9): calcd. C 54.2, H 4.0, N 7.3; Found: C 54.4, H 4.2, N 7.0. IR (KBr
disc) (cm-1): 3420 (br, s, νOH), 3059 (w), 2924 (w, νCH3), 1574 (vs, br, C=O ,
pyridyl-H), 1475 (s,
pyridyl-H), 1416 (vs, pyridyl-H), 1333 (m), 1248 (m), 1163 (w), 1021 (m), 898 (w, Ar-H), 794 (m, Ar-H),
768 (s, Ar-H), 730 (m), 684 (s), 657 (m), 628 (w). max (DMSO)/ nm 317 (/ M-1 cm-1 2949), 673
(124); max (H2O)/ nm 315 (/ M-1 cm-1 2937), 670 (101).
22

Dalton Transactions
Page 24 of 31
View Article Online
DOI: 10.1039/C3DT53054J
4.2.3 [CuCl2L].[Cu(Cl)(-Cl)L]2 (3)
CuCl2 (0.312 g, 2.32 mmol) was dissolved in 20 mL methanol and 20 mL of a dichloromethane
solution of L (0.619 g, 2.00 mmol) was added dropwise. The mixture was then stirred for 24 h after
which the green solid was isolated by filtration and washed with methanol (2 × 10 mL), then with
dichloromethane (2 × 10 mL) and dried under vacuum (0.656 g, 57 %). Anal. Calcd for
C21H15Cl2Cu1N30.5H2O0.5CH2Cl2 (495.3): calcd. C 52.1, H 3.5, N 8.5; found C 51.8, H 3.5, N 8.5.
IR (KBr disk) ν (cm-1): 1609 (vs, pyridyl-H), 1558 (s, aryl-H), 1476 (vs, pyridyl-H), 1417 (vs), 1248 (m),
1164 (m), 1102 (w), 1020(vs), 898 (m). max (DMSO)/ nm 292 (/ M-1 cm-1 31683), 736 (290); max
(H2O)/ nm 287 (/ M-1 cm-1 28986), 680 (225). Green crystals of 3 suitable for X-ray analysis were
obtained by dissolving the complex in a 1:1 mixture of methanol and dichloromethane followed by
slow evaporation at room temperature.
4.2.4. [Cu(H2O){OS(CH3)2}L](NO3)2 (4)
Compound 1 (0.10 g, 0.19 mmol) was dissolved in 20 mL of a 1:9 combination of DMSO and
methanol and heated at 60 °C for 2 h. Upon very slow diffusion of diethyl ether to the reaction
mixture, blue crystals were obtained (0.059 g, 51 %), which were suitable for X-ray determination.
Anal. Calcd for C23H23CuN5O8S (593.1): C 46.6, H 3.9, N 11.8; found: C 46.4, H 4.2, N 12.0. IR
(KBr disc) (cm-1): 3404 (br, m, νOH), 3056 (w), 3013 (w), 1763 (m), 1608 (vs, pyridyl-H), 1557 (s),
1476 (s, pyridyl-H), 1418 (vs, br, pyridyl-H), 1305 (w), 1246 (m), 1161 (m), 1105 (w), 1019 (s), 950 (m),
894 (m, Ar-H), 825 (s), 794 (s, Ar-H), 768 (s, Ar-H), 728 (m), 687 (m), 646 (m), 627 (m). max
(DMSO)/ nm 301 (/ M-1 cm-1 3005), 698 (940); max (H2O)/ nm 306 (/ M-1 cm-1 2692), 695 (890).
4.2.5. [Cu(HO)(CH3CH2OH)L](NO3) (5)
Compound 1 (0.18 g, 0.35 mmol) was dissolved in a 20 mL of a 1:1 combination of ethanol and
DMF, and 2 mL of an ethanolic solution of 4,4’-bipyridyl (4,4’-Bpy) (0.0274 g, 0.18 mmol) was
23

Page 25 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/C3DT53054J
added dropwise; this mixture was stirred for 24 h and then refluxed for 3 h, after which it was
filtered and the mother solution allowed to evaporate naturally. Blue crystals (0.078 g, yield 45 %
based on the precursor) were obtained, which were suitable for X-ray determination. Anal. Calcd for
C23H22CuN4O5 (497.9). C: 55.5, H 4. 5, N 11.3.; found: C 55.8, H 4.7, N 11.1. IR (KBr disc) (cm-1):
3403 (br, m, νOH), 3044 (s), 2774 (w), 1763 (m), 1608 (vs, pyridyl-H), 1559 (s), 1475 (s, pyridyl-H),
1385 (s), 1307 (m), 1244 (m), 1156 (m), 1076 (w), 1068 (w), 1044 (w), 1035 (w), 1018 (s), 984 (m),
926 (m), 894 (m, Ar-H), 851 (s), 825 (m), 802 (m), 765 (s, Ar-H), 729 (m), 692 (m), 683 (m), 656 (m),
646 (m), 626 (m), 615 (m). max (DMSO)/ nm 319 (/ M-1 cm-1 2789), 677 (480); max (H2O)/ nm
323 (/ M-1 cm-1 2452), 674 (410).
4.2.6 [CuCl(SO3CF3)L] (6)
0.243 g (0.491 mmol) of compound 3 was dissolved in 75 mL of a 7:7:1 mixture of methanol,
dichloromethane and DMF and 10 mL of an acetonitrile solution of AgSO3CF3 (0.129 g, 0.502
mmol) were added dropwise for a period of 30 minutes. The mixture was then stirred for 24 h, after
which it was filtered and the mother solution allowed evaporating. Blue crystals of 6 (0.173 g, yield
63%) were obtained, which were suitable for X-ray analysis. Anal. Calcd for C22H15ClCuF3N3O3S
(557.4): C 47.4, H 2.7, N 7.5; found C 47.4, H 2.6, N 7.5. IR (KBr disk) ν (cm-1): 1614 (vs, pyridyl-
H), 1564 (s, aryl-H), 1476 (vs, pyridyl-H), 1418 (vs), 1286 (vs), 1150 (m), 1026(vs), 891 (m). max
(DMSO)/ nm 291 (/ M-1 cm-1 30547), 732 (1394); max (H2O)/ nm 287 (/ M-1 cm-1 28986).
4.3. Catalytic Studies
The reactions were carried out according to published procedure [9]. Under typical conditions, they
were performed in flasks fitted with water circulating condensers under atmospheric pressure of air.
1.5 mmol of the substrate, 0.015 mmol (1 mol % based on substrate) of Cu catalyst, 0.075 mmol (5
24

Dalton Transactions
Page 26 of 31
View Article Online
DOI: 10.1039/C3DT53054J
mol%) of TEMPO and 5 mL of 1M K2CO3 aqueous solution were added and the reaction mixture
was stirred for 22 h at 70 ºC after which the addition of 1 M HCl (for neutralization) and 5 mL of
EtOAc (for the extraction of the substrate and of the organic products from the reaction mixture)
were accomplished. Finally, 120 L of cycloheptanone (internal standard) were added. The products
were analysed by GC and quantified by using the internal standard method.
25

Page 27 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/C3DT53054J
Acknowledgements
The authors are grateful for the financial support from the Foundation for Science and Technology
(FCT), Portugal, and its programs (POCI, FEDER funded, Grant No. SFRH/BPD/24691/2005,
Project PTDC/EQU-EQU/122025/2010 and PEst-OE/QUI/UI0100/2013), the Science Foundation
of China (21261002) and the Opening Project of Guangxi Key Laboratory of Petrochemical
Resource Processing and Process Intensification Technology (K008).
References
[1] (a) U. S. Schubert, A. Winter, G. R. Newkome, Terpyridine-based Materials: For catalytic,
Optoelectronic and Life Science Applications, Wiley, 2011; (b) U. S. Schubert, H. Hofmeier,
G. R. Newkome, Modern Terpyridine Chemistry, Wiley, 2006. (c) A. Griffith, T.J. Bandy, M.
Light, E. Stulz, Chem. Commun., 2013, 49, 731-733. (d) J. Du, Z. Huang, X.-Q. Yu, L. Pu,
Chem. Commun., 2013, 49, 5399-5401. (e) F.-X. Zhou, Z. Zheng, H.-P. Zhou, W.-Z. Ke, J.-Q.
Wang, Z.-P. Yu, F. Jin, J.-X. Yang, J.-Y. Wu, Y.-P. Tian, CrystEngComm, 2012, 14, 5613-
5621. (f) Y.-L. Gai, F.-L. Jiang, L. Chen, Y. Bu, M.-Y. Wu, K. Zhou, J. Pan, M.-C. Hong,
Dalton Trans., 2013, 42, 9954-9965. (g) G. Baum, E.C. Constable, D. Fenske, C.E.
Housecroft, T. Kulke, J. Chem. Soc., Chem. Commun. 1998, 2659. (h) E.C. Constable, A.J.
Edwards, G.R. Haire, M.J. Hannon, P.R. Raithby, Polyhedron 1998, 17, 243. (i) L. Hou, D. Li,
Inorg. Chem. Commun. 8, 2005, 128. (j) E.W. Ainscough, A.M. Brodie, S.L. Ingham, J.M.
Waters, J. Chem. Soc., Dalton Trans. 1994, 215. (l) M.J. Hannon, C.L. Painting, E.A.
Plummer, L.J. Childs, N.W. Alcock, Chem. Eur. J. 2002, 8, 2226. (m) E.C. Constable, M.G.B.
Drew, G. Forsyth, M.D. Ward, J. Chem. Soc., Chem. Commun. 1988, 1450. (n) Y. Fu, J. Sun,
Q. Li, Y. Chen, W. Dai, D. Wang, T.C.W. Mak, W. Tang, H. Hu, J. Chem. Soc., Dalton
Trans. 1999, 711. (o) P.K.-K. Ho, S.-M. Peng, K.-Y. Wong, C.-M. Chi, J. Chem. Soc., Dalton
26

Dalton Transactions
Page 28 of 31
View Article Online
DOI: 10.1039/C3DT53054J
Trans. 1996, 1829. (p) J.S. Field, J.-A. Gertenbach, R.J. Haines, L.P. Ledwaba, N.T. Mashapa,
D.R. McMillin, O.Q. Munro, G.C. Summerton, J. Chem. Soc., Dalton Trans. 2003, 1176.
[2] (a) C.-K. Hui, B.W.-K. Chu, N. Zhu, V.W-W. Yam, Inorg. Chem. 2002, 41, 6178. (b) Z. Ma,
Y. Cao, Q. Li, M.F.C. Guedes da Silva, J.J.R. Fraústo da Silva, A.J.L. Pombeiro, J. Inorg.
Biochem. 2010, 104, 704. (c) Z. Ma, Y. Xing, M. Yang, M. Hu, B. Liu, M.F.C. Guedes da
Silva, A.J.L. Pombeiro, Inorg. Chim. Acta, 2009, 362, 2921. (d) Z. Ma, B. Liu, H. Yang, Y.
Xing, M. Hu, J. Sun, J. Coord. Chem., 2009, 62, 3314. (e) V.W.-W. Yam, K.M.-C. Wong, N.-
Y. Zhu, Angew. Chem., Int. Ed. 2003, 42, 1400. (f) A.J. Goshe, I.M. Steele, B. Bosnich, J. Am.
Chem. Soc. 2003, 125, 444. (g) Z. Ma, W. Lu, B. Liang, A. J. L. Pombeiro, New J. Chem.,
2013, 37, 1529.
[3] (a) I.W.C.A. Arends, R.A. Sheldon, Modern Oxidation Methods (Ed.: J.-E. Bäckvall), Wiley,
Weinheim, 2004, p.83. (b) D.H.R. Barton, A.E. Martell, D.T. Sawyer (Eds.), The Activation of
Dioxygen and Homogeneous Catalytic Oxidation, Plenum Press, New York, 1993. (c) H.
Ruijun, L. Ming, W. Hegeng, W. Yanguag, Chin. J. Chem., 2009, 27, 587. (d) P.J. Figiel, A.
Sibaouih, J. U. Ahmad, M. Nieger, M.T. Räisänen, M. Leskelä, T. Repo, Adv. Synth. Catal.
2009, 351, 2625. (e) R.A. Sheldon, I.W.C.E. Arends, Adv. Synth. Catal., 2004, 346, 1051. (f)
R.A. Sheldon, I.W.C.E. Arends, G-Jan T. Brink, A. Dijksman, Acc. Chem. Res. 2002, 35, 774.
[4] (a) G. Tojo, M. Fernandez, Oxidation of Alcohols to Aldehydes and Ketones: A Guide to
Current Common Practice, Springer, New York, 2006; (b) Ullmann’s Encyclopedia of
Industrial Chemistry, 6th edn, Wiley-VCH, Weinheim, 2002. (c) R.A. Sheldon, J.K. Kochi,
Metal-Catalysed Oxidation of Organic Compounds, Academic Press: New York, 1981.
[5] M.F. Semmelhack, C.R. Schmid, D.A. Cortes, C.S. Chou, J. Amer. Chem. Soc., 1984, 106,
3374.
27

Page 29 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/C3DT53054J
[6] (a) T. Vogler, A. Studer, Synthesis-Stuttgart 2008, 1979. (b) J.H. Liu, F. Wang, X.L. Fu, Prog.
Chem. 2007, 19, 1718. (c) M.V.N. De Souza, Mini-Rev. Org. Chem. 2006, 3, 155. (d) R.A.J.
Sheldon, Mol. Catal. A: Chem. 2006, 251, 200. (e) F. Calderon, Synlett. 2006, 657. (f) F.
Minisci, F. Recupero, G.F. Pedulli, M.J. Lucarini, Mol. Catal. A: Chem. 2003, 63, 204. (g) A.
de Mico, R. Margarita, A. Mariani, G. Piancatelli, Tetrahedron Lett., 1996, 37, 1889.
[7] (a) J. March, Advanced Organic Chemistry: Reaction, Mechanisms and Structure, John Wiley
& Sons, New York, 4th ed., 1992. (b) W.J. Mijs, C.R.H.I. de Jonge, Organic Syntheses by
Oxidation with Metal Compounds, Plenum, New York, 1986.
[8] (a) M. Herbert, F. Montilla, A. Galindo, Dalton Trans., 2010, 39, 900. (b) R.A. Sheldon,
Green Chem., 2005, 7, 267. (c) E.J. Beckman, J. Supercrit. Fluids, 2004, 28, 121.
[9] (a) P.J. Figiel, M. Leskelä, T. Repo, Adv. Synth. Cat. 2007, 349, 1173. (b) P.J. Figiel, A.M.
Kirillov, Y.Y. Karabach, M.N. Kopylovich, A.J.L. Pombeiro, J. Mol. Catal. A: Chem. 2009,
305, 178. (c) J.U. Ahmad, P.J. Figiel, M. Räisänen, M. Leskelä, T. Repo, Appl. Catal. A:Gen.
2009, 371, 17. (d) K.T. Mahmudov, M.N. Kopylovich, M.F.C. Guedes da Silva, P.J. Figiel,
Y.Y. Karabach, A.J.L. Pombeiro, J. Mol. Catal. A: Chem. 2010, 318, 44.
[10] (a) A. Dijksman, I.W.C.E. Arends, R.A. Sheldon, Plat. Metals Rev., 2001, 45, 15.
[11] A. Cecchetto, F. Fontana, F. Minisci, F. Recupero, Tetrahedron Lett., 2001, 42, 6651.
[12] (a) P. Gamez, I. W. C. E. Arends, R. A. Sheldon and J. Reedijk, Adv. Synth. Catal., 2004, 346,
805. (b) P. Gamez, I. W. C. E. Arends, J. Reedijk and R. A. Sheldon, Chem. Commun., 2003,
2414. (c) C. Michel, P. Belanzoni, P. Gamez, J. Reedijk, E. J. Baerends, Inorg. Chem., 2009,
48, 11909. d) A. Dijksman, A. Marino-Gonzalez, A. Mairata Paeyeras, I.W.C.E. Arends, R.A.
Sheldon, J. Am. Chem. Soc. 2001, 123, 6826. e) A. Dijksman, I.W.C.W. Arends, R.A.
Sheldon, Chem. Commun., 1999, 1951. (e) W. Brackman, C.J. Gaasbeek, Recl. Trav. Chim.
Pays Bas, 1966, 85, 257.
28