Halogen-bonded tris(2,4-bis(trichloromethyl)-1,3,5-triazapentadienato)-M(iii) [M = Mn, Fe, Co] complexes and their catalytic activity in the peroxidative oxidation of 1-phenylethanol to acetophenone

Article abstract of DOI:10.1039/c4nj00797b

One-pot template condensation of CCl₃CN with ammonia on a metal source [MnCl₂·4H₂O, FeCl₃·6H₂O or Co(CH₃COO)₂·4H₂O] in DMSO led to the formation of tris{2,4-bis(trichloromethyl)-1,3,5-triazapentadienato}-M(iii) complexes, [M{NHC(CCl₃)NC(CCl₃)NH}₃]·n(CH₃)₂SO [M = Mn, n = 1 (1); M = Fe, n = 2 (2); M = Co, n = 2 (3)], which were characterized using elemental analysis, and IR, ESI-MS and single-crystal X-ray analysis. The role of inter- and intramolecular non-covalent halogen and hydrogen bonds in the synthesis of 1-3 is discussed. It is shown that the crystal ionic radii of the metal ions [68.5 (Co) < 69 (Fe) < 72 (Mn), pm] are related to the corresponding Cl?Cl distances [3.178 (3) > 3.155 (2) > 3.133 (1) ?]. Compounds 1-3 and the related di(triazapentadienato)-Cu(ii) complex [Cu{NHC(CCl₃)NC(CCl₃)NH}₂]·2(CH₃)₂SO (4) act as catalyst precursors for the additive-free microwave (MW) assisted homogeneous oxidation of 1-phenylethanol with tert-butylhydroperoxide (TBHP), leading to the formation of acetophenone with yields up to 99% and TONs up to 5.0 × 10³ after 1 h of low power (10 W) MW irradiation. This journal is

Full text of DOI:10.1039/c4nj00797b

NJC
View Article Online
View Journal | View Issue
PAPER
Halogen-bonded tris(2,4-bis(trichloromethyl)-
1,3,5-triazapentadienato)-M(III) [M = Mn, Fe, Co]
Cite this: New J. Chem., 2014,
complexes and their catalytic activity in the
peroxidative oxidation of 1-phenylethanol to
acetophenone†
3
8, 4807
a
a
a
Namiq Q. Shixaliyev, Atash V. Gurbanov, Abel M. Maharramov,
ab
b
bc
b
Kamran T. Mahmudovd, Maximilian N. Kopylovich,* Luısa M. D. R. S. Martins,*
´
d
Vasily M. Muzalevskiy, Valentine G. Nenajdenko and Armando J. L. Pombeiro*
One-pot template condensation of CCl
O or Co(CH COO) O] in DMSO led to the formation of tris{2,4-bis(trichloromethyl)-1,3,5-
ꢀ4H
triazapentadienato}-M(III) complexes, [M{NHQC(CCl )NC(CCl )Q NH} SO [M = Mn, n = 1 (1);
]ꢀn(CH
3
CRN with ammonia on a metal source [MnCl
ꢀ4H
2 2
O, FeCl
3
ꢀ
6
H
2
3
2
2
3
3
3
3 2
)
M = Fe, n = 2 (2); M = Co, n = 2 (3)], which were characterized using elemental analysis, and IR, ESI-MS
and single-crystal X-ray analysis. The role of inter- and intramolecular non-covalent halogen and
hydrogen bonds in the synthesis of 1–3 is discussed. It is shown that the crystal ionic radii of the metal
ions [68.5 (Co) o 69 (Fe) o 72 (Mn), pm] are related to the corresponding Clꢀ ꢀ ꢀCl distances [3.178 (3) 4
3.155 (2) 4 3.133 (1) Å]. Compounds 1–3 and the related di(triazapentadienato)-Cu(II) complex
Received (in Victoria, Australia)
6th May 2014,
1
Accepted 19th July 2014
[
Cu{NHQC(CCl
3
)NC(CCl
3
)Q NH}
2
]ꢀ2(CH
3 2
) SO (4) act as catalyst precursors for the additive-free
microwave (MW) assisted homogeneous oxidation of 1-phenylethanol with tert-butylhydroperoxide
DOI: 10.1039/c4nj00797b
3
(
TBHP), leading to the formation of acetophenone with yields up to 99% and TONs up to 5.0 ꢁ 10 after
www.rsc.org/njc
1 h of low power (10 W) MW irradiation.
NH moiety of the tap ligands and the DMSO molecule is one of the
driving forces of this reaction.
Non-covalent interactions between halogen atoms have also
Introduction
3
Hydrogen bonding assisted synthesis is a rapidly growing area
1
of modern chemistry. Usually, the formation of multiple hydro- attracted much attention recently and have been demonstrated
gen bonds, in particular resonance assisted hydrogen bonding to play a prominent role in supramolecular chemistry, molecular
2
4
(
RAHB), drives the reaction towards stabilized product(s). For recognition, biological systems, functional materials, etc. Halo-
instance, an oxygen atom of a dimethylsulfoxide (DMSO) molecule gen bonding is a rather widespread phenomenon since halogen
can be a bifurcate acceptor centre to create RAHB upon synthesis atoms or ions can form short non-bonded contacts with electron
II
II
II
II
of Cu , Ni , Zn or Pd complexes with 1,3,5-triazapentadiene (tap) acceptors, electron donors or be interconnected due to aniso-
3
ligands. We believe that the formation of H-bonding between the tropic charge distribution in halogen atoms. Hence, an effective
polarization of halogen atoms should lead to stronger halogen
a
Department of Chemistry, Baku State University, Z. Xalilov Str. 23,
Az 1148 Baku, Azerbaijan
bonds. One of the possible ways to reach this goal would be the
introduction of a conjugated system in the nearest proximity to
the halogen atom. Metallacycles provide a simple and conveni-
b
Centro de Qu ´ı mica Estrutural, Instituto Superior T ´e cnico, Universidade de Lisboa,
Av. Rovisco Pais, 1049-001 Lisbon, Portugal. E-mail: kopylovich@yahoo.com,
pombeiro@tecnico.ulisboa.pt
5
ent example of such charged conjugated systems; hence, they
c
can be used to create new halogen bonds and/or to increase their
energy. However, to date, not much attention has been paid to
the halogen bonds in metal complexes and to the analysis of
relations between parameters of such bonds and properties of
Chemical Engineering Department, ISEL, R. Conselheiro Em ´ı dio Navarro,
1
959-007 Lisbon, Portugal. E-mail: lmartins@deq.isel.ipl.pt
d
Department of Chemistry, Moscow State University, Leninskie Gory,
Moscow 119992, Russia
†
Electronic supplementary information (ESI) available: Tables listing bond dis-
3
,4
the central metal ions in coordination compounds.
tances and angles, hydrogen and halogen bond geometry of compounds 1–3. CCDC
89102, 988933 and 986029. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4nj00797b
Another aspect, the study of which is still underdeveloped,
concerns the application of the halogen bonds in the synthesis
9
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem., 2014, 38, 4807--4815 | 4807

View Article Online
Paper
NJC
of new compounds. For instance, if, in the course of a reaction, syntheses; (ii) to analyze some correlations between parameters of
the number and energy of the halogen bonds increase signifi- halogen bonds and properties of the central metal ions; (iii) to
cantly, this should facilitate the formation of the products. study the catalytic activity of the synthesized tap complexes and a
II
6h
In principle, by this way, one can influence the course of a related bi-tap Cu complex in the MW-assisted homogeneous
reaction and its yield. As mentioned above, the introduction of oxidation of 1-phenylethanol to acetophenone.
a metallacycle in the proximity of halogen atoms can increase
the anisotropy of charge distribution, thus extending the
potential of the halogen atoms to form halogen bonds of higher
Results and discussion
energy.
Condensation of trichloroacetonitrile with ammonia on a
In this study we attempt to demonstrate, by a simple but
metal centre
illustrative case, the significance of the halogen bonds in the
synthesis of new coordination compounds. Additionally, some Generally, the synthesis of tap complexes of Co, Fe and Mn
parameters of halogen bonds and their relationship to the proper- metal ions is not a trivial task and is achieved through rather
5
,12
ties of the metal ions will be discussed. As a model reaction, we laborious ligand pre-preparation.
However, it is possible to
choose the synthesis of complexes of 1,3,5-triazapentadienes (tap) prepare tap complexes with Ni and Cu metal ions using a one-
3
b,5
with chlorinated substituents since we have significant experience pot direct synthesis using nitriles as the starting materials.
3,5
with tap complexes and they are easy to deal with from the It was also noted that Ni and Cu tap complexes from halo-
3
b,6h
synthetic and crystallographic points of view. The tap complexes genated nitriles were formed with a high exothermic effect.
with halogenated groups are of particular interest due to the It was supposed that this was related to the activation of the
promotion of their volatility, chemical and thermal stability and cyano carbon atom by the electron-withdrawing halogen-
3
,5,6
solubility in halocarbons;
these properties are important for containing group. However, it seems that the formation of
3a
e.g. catalytic applications. Earlier it was demonstrated that multiple hydrogen and halogen bonding in the products is
chlorinated tap ligands can form complexes with Cu , Co ,
II 3b
II 6h
more significant than the electron-withdrawing ability of a
II 3b,6h
II 6h
II
II 3b
III
III
III
13
Ni ,
complexes have not been synthesized and structurally character- (s
ized. Thus, it is worthwhile to expand the number of this type of that of –CCl
Mn , Zn (ref. 3a) and Pd , but Mn , Fe and Co
substituent. In fact, Hammett’s constant s
= 0.50) and –C (Q O)C (s = 0.43) substituents is similar to
(s = 0.46), but the direct formation of tap
p
of the –C (Q O)CH
3
H
p
6
5
p
3
p
complexes and perform their thorough structural analysis and complexes with those substituents does not proceed and the
application in industrially significant catalytic oxidation reactions known syntheses usually require the preliminary preparation of
7
5
such as the oxidation of alcohols.
The oxidation of alcohols to their corresponding carbonyl driving force of the synthesis.
the tap ligands. Hence, halogen bonding can be the main
compounds constitutes an important reaction in organic synth-
To explore this assumption, we performed a direct synthesis
7
esis. When performed using non-catalytic methods, it generally of new metal-tap complexes using trichloroacetonitrile as the
8
III
III
III
produces hazardous waste. Hence, the development of catalytic starting material and Co , Fe , and Mn as the central metal
oxidation procedures which involve green oxidants, shorter reac- ions. The known examples of tap complexes with these metal
tion times and/or improved selectivity is a matter of current ions are very limited and include only a couple of fully char-
7–9
interest. Recently, some of us have reported that complexes of acterized compounds, all of them being prepared using the pre-
10
12b
arylhydrazones of methylene active compounds
and tap synthesized ligands.
ligands can effectively catalyze the oxidation of 1-phenylethanol complexes of those metal ions are known catalysts for a number
Concerning possible applications, tap
11
5
,11
to acetophenone (Scheme 1), and we believe that the search for of oxidation reactions, in particular, of alcohols.
Hence, a
easy-to-synthesize and stable tap-metal catalysts for that process direct and simple synthesis of such a type of complexes would
should be continued. Moreover, it is known that microwave be of great interest.
irradiation (MW) in many cases provides much more efficient
synthetic methods than conventional heating, and this technique FeCl
has also been applied along the work.
Thus, the reaction of CCl
ꢀ6H O or Co(CH COO)
afforded [M{NHQC(CCl )NC(CCl
3
CN with a metal salt [MnCl
ꢀ4H O] and ammonia in DMSO
)Q NH} SO [M = Mn,
]ꢀn(CH
2
ꢀ4H
2
O,
3
2
3
2
2
3
3
3
3 2
)
Thus, taking in mind the above considerations, in the current
III
work we have pursued the following aims: (i) to prepare new Mn ,
III
III
Fe and Co complexes with chlorinated tap ligands and demon-
strate the significance of non-covalent halogen interactions in their
Scheme 1 MW-assisted oxidation of 1-phenylethanol to acetophenone.
Scheme 2 Synthesis of 1–3.
4808 | New J. Chem., 2014, 38, 4807--4815
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014

View Article Online
NJC
Paper
n = 1 (1); M = Fe, n = 2 (2); M = Co, n = 2 (3)] in 51–73% yield pattern of C–N bond lengths can be seen. For example, for
Scheme 2). The synthesis involves an in situ ligand formation, in complex 1 the N(1)–C(2), C(2)–N(3), N(3)–C(4) and C(4)–N(5)
(
which CCl CN undergoes amination resulting in the formation distances are 1.288(8), 1.366(8), 1.350(8) and 1.257(8) Å, respec-
3
of the tap ligand. The products 1–3 were fully characterized by tively (Table S1, ESI†).
elemental analysis, IR, ESI-MS and single-crystal X-ray analyses.
Due to the stabilization of the formed products 1–3 by
The IR spectra of 1–3 do not exhibit the typical n(CRN) values hydrogen (Table S2, ESI†) and halogen bonds (Table S3, ESI†),
ꢂ1
(
2100–2300 cm range), while new bands due to n(NH) and a highly exothermic effect is observed in the reaction. An
ꢂ1
n(CQN) are observed at 3246–3441 and 1576–1696 cm , respec- extensive interlinkage takes place via intermolecular hydrogen
tively. The ESI-MS spectra confirm the existence of monoproto- bonding between the hydrogen atoms of two imino (CQNH)
nated molecular ion peaks at 970, 971 and 974 for 1, 2 and 3, groups and the oxygen atom of dimethylsulfoxide, giving rise to
respectively. The elemental analyses are consistent with the six-membered H-bond supported cycles (Fig. 1 and Scheme 3).
proposed formulations, which are also supported by X-ray The average CQNHꢀ ꢀ ꢀOQS(CH ) distances of the hydrogen
3
2
crystallography.
bonds in 1–3 fall in the 2.15–2.64 Å range (Table S2, ESI†),
In the molecular structures of 1–3 (Fig. 1), the central metal which is consistent with a strong hydrogen bonding. In all the
atom is located in a markedly distorted octahedral geometry structures, the oxygen atoms of DMSO molecules play an
environment and bonded by six nitrogen atoms from the three acceptor role giving bifurcated or three-centred hydrogen bond
tap ligands. The M–N distances in 1–3 are in the range of 1.892(4)– systems (Fig. 1 and Scheme 3). The NHꢀ ꢀ ꢀO distances become
1.966(5) Å (Table S1 in the ESI†) and increase with the growth of longer in the three-centred hydrogen bonds than in the bifur-
the ionic radius of the metal ion [68.5 (Co) o 69 (Fe) o 72 pm cated ones (Fig. 1).
Mn)]. A similar trend was found for the M–N bond distance in the Extensive halogen bonding between the molecules of com-
(
3b
chlorinated tap ligands. The N–M–N bite angles in the six- plexes is also observed, with –CCl3 groups acting as both
membered planar metallacycles are in the range of 84.7(2)– nucleophiles and electrophiles, which successfully drive the
3
9
8.7(2)1 and are close to those in related compounds. Each formation of a multitude of self-assembled architectures (Fig. 2
bidentate tap ligand forms a six-membered metallacycle MNCNCN and Table S3, ESI†). Thus, the crystal packings of 1–3 are
where, for the NQC–N–CQN moiety, a short–long–long–short similar and reveal infinite strands of the complexes assembled
Fig. 1 Molecular structures of 1–3 (H atoms of DMSO molecules are omitted for clarity). N(1)–C(2) and C(4)–N(5), N(1)–C(2) and C(4)–N(5), N(8)–C(9)
and C(11)–N(12) are 1.288(8) and 1.257(8), 1.295(4) and 1.287(4), 1.268(6) and 1.283(6) Å, respectively, while C(2)–N(3) and N(3)–C(4), C(2)–N(3) and
N(3)–C(4), C(9)–N(10) and N(10)–C(11) are of 1.366(8) and 1.350(8), 1.326(5) and 1.321(4), 1.331(6) and 1.329(6).
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem., 2014, 38, 4807--4815 | 4809

View Article Online
Paper
NJC
Scheme 3 Bifurcated or three-centred hydrogen bond systems in 1–3.
Fig. 2 Clꢀ ꢀ ꢀCl interactions in 1–3; DMSO molecules are omitted for clarity.
by the ClꢀꢀꢀCl intermolecular interactions between chlorine atoms
Microwave-assisted catalytic peroxidative oxidation of
-phenylethanol
distance of 3.221(6)–3.225(6) Å (Table S3, ESI†). Thus, cooperation Complexes 1–3 (Scheme 2) and the known
(
Fig. 2). Additionally, intermolecular NꢀꢀꢀCl interactions are
1
3
observed between the N3 and the Cl atom of –CCl groups at a
1
1
bis(2,4-bis-
of the six-membered hydrogen bonding system and ClꢀꢀꢀCl inter- (trichloromethyl)-1,3,5-triazapentadienato)-Cu(II) (4) were tested
actions (Tables S2 and S3, ESI†) leads to 3D supramolecular as catalyst precursors for the additive-free microwave (MW)
architectures in all structures. The ClꢀꢀꢀCl distance is shorter in assisted homogeneous oxidation of secondary alcohols [1-phenyl-
the case of 1 compared to that in 2 or 3. An increase of the ionic ethanol (chosen as the model substrate), cyclohexanol, 2-hexanol
radius of the metal ion appears to correspond to a decrease in the and 3-hexanol] to the corresponding ketones by aqueous tert-
2
t
ClꢀꢀꢀCl distance (Fig. 3), but no clear linear relationship (r = 0.846) butylhydroperoxide (Bu OOH, TBHP) or hydrogen peroxide under
was found between the mentioned parameters. Although further low power (10 W) irradiation, without an added solvent (Scheme 1).
examples have to be obtained to confirm the trend, these results Selected results are summarized in Table 1.
are indicative that the polarization of the halogen atom depends
on the metal ion involved in the close-by metallacycle.
Under typical additive-free conditions all the compounds
exhibited catalytic activity for the above reactions, leading to
4
810 | New J. Chem., 2014, 38, 4807--4815
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014

View Article Online
NJC
Paper
3
(99%, entry 5, Table 1) and Cu(II) 4 (97%, entry 7, Table 1)
complexes. The activities of the Fe(III) 2 (87%, entry 3, Table 1)
and the Mn(III) 1 (34%, entry 1, Table 1) complexes are lower,
but still significant. Moreover, a high selectivity towards the
formation of acetophenone was observed in these MW-assisted
transformations, since no traces of by-products were detected
by GC-MS analysis of the final reaction mixtures (only the
unreacted alcohol was found, apart from the ketone).
Complexes 1–4 were also tested towards the oxidation of
other secondary alcohols, namely cyclohexanol and the linear
Fig. 3 Plot of the ionic radius of metal ions versus the shortest Clꢀ ꢀ ꢀCl
distances for 1–3.
2- and 3-hexanols. The ketones are the only oxidation products
obtained (Table 1). The aliphatic alcohols are less reactive than
the benzylic derivative, 1-phenylethanol, leading under the
yields (based on the alcohol) of acetophenone up to 99% after same reaction conditions to moderate yields in the 22–68%
7
e,10,11,14,15
1
4
h of irradiation (Table 1 and Fig. 4). The catalyst precursors 2– range (Table 1), as reported in other cases.
Moreover,
3
ꢂ1
were highly active (TOFs up to 5.5 ꢁ 10 h for 3). The best the efficiency of oxidation of those linear aliphatic alcohols is
acetophenone yields were obtained in the presence of the Co(III) not significantly affected by the position (2 or 3) of the OH
a
Table 1 Selected results for the MW-assisted oxidation of 1-phenylethanol using 1–4 as catalyst precursors
Catalyst/substrate
molar ratio (%)
b
c
ꢂ1
Entry
1
Catalyst precursor
Substrate
Reaction time (h)
Yield (%)
TON (TOF (h ))
3
3
1
1
2
2
3
3
4
4
3
3
3
3
3
3
—
3
3
1-Phenylethanol
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.2
0.2
2
0.02
—
0.02
0.02
1
1
1
1
1
1
1
1
0.5
0.25
0.25
0.5
0.25
1
1
1
1
33.5
7.6
1.68 ꢁ 10 (1.68 ꢁ 10 )
d
2
380 (380)
3
3
3
4
86.8
18.9
99.1
24.2
97.4
21.0
54.6
26.1
40.2
56.7
40.7
41.1
7.1
4.32 ꢁ 10 (4.32 ꢁ 10 )
d
945 (945)
3
3
5
6
4.96 ꢁ 10 (4.96 ꢁ 10 )
d
3 3
1.21 ꢁ 10 (1.21 ꢁ 10 )
3 3
7
8
4.87 ꢁ 10 (4.87 ꢁ 10 )
3 3
d
1.05 ꢁ 10 (1.05 ꢁ 10 )
3 3
9
2.73 ꢁ 10 (5.47 ꢁ 10 )
3 3
10
11
12
13
14
15
16
17
1.30 ꢁ 10 (5.21 ꢁ 10 )
2 2
2.01 ꢁ 10 (8.03 ꢁ 10 )
2
2
2.84 ꢁ 10 (5.67 ꢁ 10 )
20 (81)
e
3
3
2.06 ꢁ 10 (2.06 ꢁ 10 )
—
f
4.8
5.3
238 (238)
263 (263)
g
3
3
1
1
2
2
8
9
0
1
1
2
3
4
Cyclohexanol
2-Hexanol
0.02
0.02
0.02
0.02
1
1
1
1
26.5
58.5
67.9
68.4
1.33 ꢁ 10 (1.33 ꢁ 10 )
3 3
2.93 ꢁ 10 (2.93 ꢁ 10 )
3
3
3.40 ꢁ 10 (3.40 ꢁ 10 )
3
3
3.42 ꢁ 10 (3.42 ꢁ 10 )
3
3
2
2
2
2
2
3
4
5
1
2
3
4
0.02
0.02
0.02
0.02
1
1
1
1
22.3
67.0
61.1
35.7
1.12 ꢁ 10 (1.12 ꢁ 10 )
3 3
3.35 ꢁ 10 (3.35 ꢁ 10 )
3
3
3.06 ꢁ 10 (3.06 ꢁ 10 )
3
3
3.40 ꢁ 10 (3.40 ꢁ 10 )
3
3
2
2
2
2
6
7
8
9
1
2
3
4
3-Hexanol
0.02
0.02
0.02
0.02
1
1
1
1
24.7
66.4
59.2
30.5
1.79 ꢁ 10 (1.79 ꢁ 10 )
3 3
3.32 ꢁ 10 (3.32 ꢁ 10 )
3
3
2.96 ꢁ 10 (2.96 ꢁ 10 )
3
3
1.53 ꢁ 10 (1.53 ꢁ 10 )
30
31
32
33
34
35
MnCl
FeCl
ꢀ6H
Co(MeCO
Cu(NO
CoCl
ꢀ6H
CuCl
2
ꢀ4H
2
O
1-Phenylethanol
0.02
0.02
0.02
0.02
0.02
0.02
1
1
1
1
1
1
12.3
45.8
39.3
33.4
27.7
17.2
615 (615) 3
3
3
2
O
2.29 ꢁ 10 (2.29 ꢁ 10 )
3
3
2
)
2
ꢀ4H
2
O
1.97 ꢁ 10 (1.97 ꢁ 10 )
3 3
3
)
2
1.67 ꢁ 10 (1.67 ꢁ 10 )
3 3
3
2
O
1.39 ꢁ 10 (1.39 ꢁ 10 )
2
860 (860)
a
Reaction conditions unless stated otherwise: 5 mmol of substrate, 1–100 mmol (0.02–2 mol% vs. substrate) of 1–4, 10 mmol of TBHP (2 eq., 70% in
b
c
H
2
O), 80 1C, 15–60 min reaction time, MW irradiation (10 W power). Moles of acetophenone per 100 moles of 1-phenylethanol. Turnover
d
2 2
number = number of moles of product per mol of the catalyst precursor; TOF = TON per hour (values in brackets). 10 mmol of H O
e
f
g
2 3
(30% aqueous solution) instead of TBHP. 5 mmol of TBHP. In the presence of Ph NH (10 mmol). In the presence of CBrCl (10 mmol).
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem., 2014, 38, 4807--4815 | 4811

View Article Online
Paper
NJC
irradiation; 0.1 mol% vs. substrate) or a dinuclear m-chlorido-
1
5b
bridged manganese species (acetophenone yield of 5%, TON
of 238, after 3 h of 10 W irradiation; 0.02 mol% vs. substrate).
The relevance of the tap ligands to the catalytic activity of the
obtained compounds is shown by the much lower catalytic
performances of the corresponding metal salts in the oxidation
of the model substrate used (yields in the 12–46% range,
entries 30–35, Table 1) under the same reaction conditions.
The cheaper and environmentally friendly hydrogen per-
oxide (30% aqueous solution) is not as efficient an oxidant as
TBHP, as attested by the marked yield lowering from e.g., 99%
to 24% (3) (entries 5 and 6, respectively, Table 1), in accordance
with the expected decomposition of H O under the used
2
2
2 2
Fig. 4 Effect of the reaction time and oxidant (TBHP or H O ) on the yield
reaction conditions (80 1C). Furthermore, the use of an equi-
molar amount of oxidant to substrate leads to a drastic decrease
of the acetophenone yield (compare entries 5 and 14, Table 1).
Carrying out the reaction in water (up to 2 mL) does not
for the oxidation of 1-phenylethanol to acetophenone catalyzed by 1–4
0
(
means that the oxidant is hydrogen peroxide).
group in the aliphatic chain, as attested by the similar yields of markedly change the yield of acetophenone either using TBHP
- and 3-hexanones (compare e.g., entries 23 and 27, Table 1). or H O under the same conditions. Hence, for example, in the
2
2
2
The MW assisted reaction proceeds rapidly under the low presence of 3 and under the conditions of entry 5 of Table 1,
irradiation power of 10 W used (an increasing of the power up the 99% acetophenone yield achieved without any added
to 25 W did not show a significant yield enhancement) and is solvent decreased to 93% in water. A similar behaviour was
1
4a
highly effective even with very low catalyst amounts. For exam- previously reported
for the Cu(II) complex [Cu(H
2
O)(HL)]ꢀ
ple, in the presence of 1 mmol (0.02 mol% vs. substrate) of 3 a O [H L = (E)-2-(((1-hydroxynaphthalen-2-yl)methylene)amino)-
H
2
2
1
5
fair yield of acetophenone (55%, entry 5, Table 1) was already benzenesulfonic acid]. However, for some Mn systems, the use
achieved in 30 min, whereas an almost quantitative yield was of water as a solvent led to a considerable reduction of the yield
obtained in 1 h (entry 3, Table 1 and Fig. 4) under MW of the oxidized product.
irradiation. An increase in the catalyst amount from 1 mmol
0.02 mol% vs. substrate) to 10 mmol (0.2 mol% vs. substrate) well-known oxygen- or carbon-radical traps, respectively, strongly
results in a yield enhancement for shorter reaction times (from hampers the catalytic activity (entries 16 and 17, Table 1). This
6% to 40%, 15 min, Table 1, entries 10 and 11, respectively) suggests the generation of oxygen and carbon radicals in the
but is not significant after 30 min of irradiation (compare reaction, which are trapped by those radical scavengers. The
The addition to the reaction mixture of Ph NH or CBrCl ,
2 3
16
(
2
t
ꢃ
t
ꢃ
entries 9 and 12, Table 1). Moreover, beyond 10 mmol of the mechanism may involve e.g., BuO and BuOO radicals pro-
1
7
catalyst, the yield remained almost unchanged, even at 15 min duced in the metal (M) promoted decomposition of TBHP
irradiation, leading to the expected TON lowering (entry 13, according to the following equations.
Table 1). Blank tests (in the absence of any catalyst precursor)
n+
t
(nꢂ1)+
t
ꢃ
+
M
+ BuOOH - M
+ BuOO + H
(1)
(2)
(3)
(4)
were performed under common reaction conditions and no
significant conversion was observed (7%, entry 10, Table 1).
Microwave irradiation (10 W) provides a more efficient
synthetic method than conventional heating, thus allowing
the attainment of similar yields in shorter times, as observed
(nꢂ1)+
t
n+
t
ꢃ
M
+ BuOOH - M –OH + BuO
n+
t
n+
t
M –OH + BuOOH - M –OO– Bu + H
2
O
t
ꢃ
0
t
0
ꢃ
BuO + RR CHOH - BuOH + RR C –OH
7
e,11,14,15
in other cases.
For example, in the presence of 3, 99%
t
ꢃ
0
t
0
ꢃ
BuOO + RR CHOH - BuOOH + RR C –OH
(5)
yield of acetophenone was reached after a 1 h reaction under
the MW irradiation, while the reaction under conventional
heating (oil bath) gave only 13% yield. It takes 28 h (instead
of 1 h) to achieve the 99% acetophenone yield using the latter
heating method.
n+
t
0
ꢃ
0
t
(nꢂ1)+
M –OO– Bu + RR C –OH - RR CQO + BuOOH + M
(6)
It may also proceed via the coordination of the alcohol
substrate to an active site of the catalyst, and its deprotonation
to form the alkoxide ligand, followed by a metal-centred
Related Cu(II) compounds with 1,3,5-triazapentadiene ligands
1
1
tested under the same reaction conditions were found to
achieve quantitative yields in half of the reaction time needed
by the present complexes with chlorinated tap ligands, but the
former required a higher amount (0.2 mol% vs. substrate) of the
catalyst. Nevertheless, even the less efficient compound of this
1
7
dehydrogenation.
Conclusions
study, the Mn(III) complex 1, is much more efficient than other In the current study we have prepared a series of structurally
1
5a
known manganese complexes with hydrazone Schiff bases
similar Mn(III), Fe(III) and Co(III) complexes with chlorinated tap
(acetophenone yield of 7%, TON of 27, after 1 h of 10 W ligands, which were characterized using elemental analysis, IR,
4
812 | New J. Chem., 2014, 38, 4807--4815
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014

View Article Online
NJC
Paper
ESI-MS and single-crystal X-ray analyses. The role of hydrogen Syntheses of the complexes
and halogen bonds in the synthesis of these complexes was
To a mixture of 1.6 mL (11 mmol) trichloroacetonitrile and
recognized, while a trend between the ionic radius of the metal
ion and the Clꢀ ꢀ ꢀCl distance (which is related to the strength of
the halogen bonding) was found. The obtained trend can
potentially be used to tune various properties of complexes
with halogenated ligands such as acidity, photoluminescence,
coordination ability, among others, which are known to be
2
1
4
.5 mL (1 M) NH
.1 mmol metal salt [MnCl
O] was added, and the system was stirred for 10 min under
4
OH in 20 mL DMSO, 1 mL water solution of
2
ꢀ4H O, FeCl ꢀ6H O or Co(CH COO)
2
3
2
3
2
ꢀ
H
2
ambient conditions giving a dark red (1), red (2) or light red (3)
precipitate of the respective product which was filtered off and
dried in air. Crystals suitable for X-ray structural analysis were
obtained by recrystallization of 1–3 from DMSO.
4
–6
dependent on halogen bonding.
In this study, we have also successfully explored the catalytic
activity of four metal complexes (1–4) with the chlorinated tap
ligand towards MW-assisted oxidation of secondary alcohols
[
Mn{NHQC(CCl
3
)NC(CCl
3
)Q NH}
3
]ꢀ(CH
3 2
) SO (1): yield 73%; IR
ꢂ1
(KBr, selected bands, cm ): 3375 and 3246 n(N–H), 1696 n(CQN),
+
1617 d(NH). ESI-MS (in methanol): m/z 970 [M-DMSO + H] .
(
1-phenylethanol) with TBHP or H O , in additive-free systems.
2 2
H
9
3
Elemental analysis calculated (%) for C14 12Cl18MnN OS ,
It was demonstrated that the Fe, Co and Cu complexes 2–4 act
as efficient and selective homogeneous catalyst precursors
for the above reaction, which is fast, selective, requires a very
small amount of catalyst precursor and proceeds in the absence
of any additional solvent or additives, which are features of
significance towards the development of a ‘‘green’’ catalytic
process.
M = 1047.46: C 16.05, H 1.15, N 12.03; found: C 15.88, H 1.33,
N 11.79.
[
Fe{NHQC(CCl )NC(CCl )Q NH} ]ꢀ2(CH ) SO (2): yield 65%;
3
3
3
3 2
ꢂ1
IR (KBr, selected bands, cm ): 3441, 3334 and 3255 n(N–H),
576 n(CQN), 1508 d(NH). ESI-MS (in methanol): m/z 971
1
[
C
+
M-2DMSO + H] . Elemental analysis calculated (%) for
18Cl18FeN , M = 1126.50: C 17.06, H 1.61, N 11.19;
found: C 17.02, H 1.91, N 11.03.
16
H
9 2 2
O S
[
Co{NHQC(CCl
3
)NC(CCl
3
)Q NH}
3
]ꢀ2(CH
3 2
) SO (3): yield 51%;
ꢂ1
Experimental
IR (KBr, selected bands, cm ): 3335 n(N–H), 1596 n(CQN),
528 d(NH). ESI-MS (in methanol): m/z 974 [M-2DMSO + H] .
+
1
Materials and methods
Elemental analysis calculated (%) for C H Cl CoN O S ,
M = 1129.59: C 17.01, H 1.61, N 11.16; found: C 16.89, H 1.70,
N 10.95.
1
6
18 18
9 2 2
ꢂ1
Infrared spectra (4000–400 cm ) were recorded on a Vertex 70
(
Bruker) instrument in KBr pellets. Carbon, hydrogen, and
nitrogen elemental analyses were done using a Perkin Elmer
400 CHN Elemental Analyzer. Electrospray mass spectra were
2
X-ray structure determination
run using an ion-trap instrument (Varian 500-MS LC Ion Trap
Mass Spectrometer) equipped with an electrospray (ESI) ion
source. For electrospray ionization, the drying gas and flow rate
were optimized according to the particular sample with 35 p.s.i.
nebulizer pressure. Scanning was performed from m/z 100 to
The diffraction experiments were carried out on a Bruker
SMART APEX-II CCD diffractometer. The program SHELXTL
was used for collecting frames of data, indexing reflections and
1
8a
1
8b
for the determination of the lattice parameters, SAINTP
for
1
8c
integration of the intensity of reflections and scaling, SADABS
for absorption correction, and SHELXTL for the space group
and structure determination, least-square refinements on F
1200 in methanol solution. The compounds were observed in
1
8a
the positive mode (capillary voltage = 80 to 105 V). The catalytic
tests under MW irradiation were performed in a focused
microwave Anton Paar Monowave 300 reactor (10 W), using a
2
.
Crystallographic and selected structural details are listed in
Table 2.
10 mL capacity reaction tube with 13 mm internal diameter,
fitted with a rotational system and an IR temperature detector.
Gas chromatographic (GC) measurements were carried out
using a FISONS Instruments GC 8000 series gas chromatograph
General procedure for the peroxidative oxidation of
1-phenylethanol
with a DB-624 (J&W) capillary column (FID detector) and the In a typical experiment, the alcohol substrate (5.00 mmol),
Jasco-Borwin v.1.50 software. The temperature of injection was TBHP (70% aqueous solution, 10.0 mmol) and the catalyst
2
1
40 1C. The initial temperature was maintained at 120 1C for precursor 1–4 (1–100 mmol, 0.02–2 mol% vs. substrate) were
min, then raised 10 1C min to 200 1C and held at this introduced into a cylindrical Pyrex tube, which was then placed
ꢂ1
temperature for 1 min. Helium was used as the carrier gas. in the focused microwave reactor. The system was stirred and
GC-MS analyses were performed using a Perkin Elmer Clarus irradiated (10 W) for 15 min–1 h at 80 1C. After the reaction, the
600 C instrument (He as the carrier gas). The ionization voltage mixture was allowed to cool down to room temperature. 300 mL
was 70 eV. Gas chromatography was conducted in the temperature- of benzaldehyde (internal standard) and 5 mL of NCMe (to
programming mode, using a SGE BPX5 column (30 m ꢁ 0.25 mm ꢁ extract the substrate and the organic products from the reac-
0.25 mm). Reaction products were identified by comparison of tion mixture) were added. The obtained mixture was stirred
their retention times with known reference compounds, and during 10 min and then a sample (1 mL) was taken from the
by comparing their mass spectra to fragmentation patterns organic phase and analysed by GC (or GC-MS) using the
obtained from the NIST spectral library stored in the computer internal standard method. Blank tests indicate that only 7%
software of the mass spectrometer.
of acetophenone is generated in a metal-free system (Table 1).
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem., 2014, 38, 4807--4815 | 4813

View Article Online
Paper
NJC
Table 2 Crystallographic data and structure refinement details for 1–3
1
2
3
Empirical formula
fw
l (Å)
Temperature (K)
Cryst. syst.
Space group
a (Å)
b (Å)
c (Å)
C
14
H
12Cl18MnN
9
OS
C
16
H
18Cl18FeN
9
O
2
S
2
C
16 9 2 2
H18Cl18CoN O S
1047.43
0.71073
100(2)
Triclinic
P1
14.257(2)
16.101(2)
19.044(3)
104.391(2)
102.766(2)
111.467(2)
3697.2(9)
4
1126.48
0.71073
296(2)
Triclinic
P1%
10.0591(4)
13.5688(5)
16.6349(6)
79.4670(10)
76.0640(10)
75.1320(10)
2112.09(14)
2
1129.54
0.71073
296(2)
Triclinic
P1%
10.171(3)
13.499(4)
16.468(5)
79.309(6)
76.007(6)
74.967(6)
2100.6(10)
2
a (1)
b (1)
g (1)
V (Å )
3
Z
Density
1.882
1.002
0.0722
0.1074
1.771
1.005
0.0749
0.1718
1.786
1.004
0.0770
0.1981
GOOF
a
R
1
(I Z 2s)
(I Z 2s)
b
wR
2
a
b
2
o
2 2
2 2 1/2
R
1
= SJF
o
| ꢂ |F
c
J/S|F
o
|. wR
2
= [S[w(F
ꢂ F
c
) ]/S[w(F
o
) ]]
.
Acknowledgements
3 (a) N. Q. Shixaliyev, A. M. Maharramov, A. V. Gurbanov,
V. G. Nenajdenko, V. M. Muzalevskiy, K. T. Mahmudov and
M. N. Kopylovich, Catal. Today, 2013, 217, 76; (b) N. Q.
Shixaliyev, A. M. Maharramov, A. V. Gurbanov, N. V.
Gurbanova, V. G. Nenajdenko, V. M. Muzalevskiy, K. T.
Mahmudov and M. N. Kopylovich, J. Mol. Struct., 2013,
This work has been partially supported by the Foundation for
Science and Technology (FCT), Portugal [PEst-OE/QUI/UI0100/
2013 and ‘‘Investigador 2013’’ programs], as well as by the Baku
State University, Azerbaijan. K.T.M. and M.N.K. express grati-
tude to FCT for the post-doc fellowship and working contract.
The authors acknowledge the Portuguese NMR Network of
mass-spectrometry (Dr Conceiç ˜a o Oliveira) for the ESI-MS
measurements.
1041, 213.
4
(a) P. Metrangolo, H. Neukirch, T. Pilati and G. Resnati, Acc.
Chem. Res., 2005, 38, 386; (b) P. Metrangolo and G. Resnati,
Halogen Bonding: Fundamentals and Applications, Springer,
Berlin, Germany, 2008; (c) A. Priimagi, G. Cavallo, P. Metrangolo
and G. Resnati, Acc. Chem. Res., 2013, 46, 2686; (d) G. Cavallo,
P. Metrangolo, T. Pilati, G. Resnati, M. Sansotera and G. Terraneo,
Chem. Soc. Rev., 2010, 39, 3772; (e) E. Parisini, P. Metrangolo,
T. Pilati, G. Resnati and G. Terraneo, Chem. Soc. Rev., 2011,
40, 2267.
References
1
Hydrogen bonding in organic synthesis, ed. P. M. Pihko,
Wiley-VCH, Weinheim, 2009.
2
(a) G. Gilli and P. Gilli, The Nature of the Hydrogen Bond:
Outline of a Comprehensive Hydrogen Bond Theory, Oxford
University Press, Oxford, 2009; (b) G. Gilli, F. Bellucci,
V. Ferretti and V. Bertolasi, J. Am. Chem. Soc., 1989,
5 M. N. Kopylovich and A. J. L. Pombeiro, Coord. Chem. Rev.,
2011, 255, 339.
6 (a) J. A. Flores, Y. Kobayashi and H. V. R. Dias, Dalton Trans.,
2011, 40, 10351; (b) H. V. R. Dias, J. A. Flores, M. Pellei,
B. Morresi, G. G. Lobbia, S. Singh, Y. Kobayashi,
M. Yousufuddin and C. Santini, Dalton Trans., 2011,
40, 8569; (c) J. A. Flores and H. V. R. Dias, Inorg. Chem.,
2008, 47, 4448; (d) H. V. R. Dias and S. Singh, Dalton Trans.,
2006, 1995; (e) H. V. R. Dias, S. Singh and T. R. Cundari,
Angew. Chem., Int. Ed., 2005, 44, 4907; ( f ) H. V. R. Dias and
S. Singh, Inorg. Chem., 2004, 43, 7396; (g) H. V. R. Dias and
S. Singh, Inorg. Chem., 2004, 43, 5786; (h) K. Suematsu,
Chem. Pharm. Bull., 1984, 32, 291.
7 (a) G. Tojo and M. Fernandez, Oxidation of Alcohols to
Aldehydes and Ketones, Springer, Berlin, Germany, 2006,
pp. 1–97; (b) M. Hudlicky, Oxidations in Organic Chemistry,
ACS, Washington, DC, 1990, pp. 60–64; (c) J. E. Backwall,
Modern Oxidation Methods, Wiley-VCH, New York, 2004,
pp. 83–118; (d) A. M. Kirillov and G. B. Shul’pin, Coord.
Chem. Rev., 2013, 257, 732; (e) Y. Y. Karabach,
1
11, 1023; (c) M. N. Kopylovich, K. T. Mahmudov, A. Mizar
and A. J. L. Pombeiro, Chem. Commun., 2011, 47, 7248;
d) M. N. Kopylovich, T. C. O. Mac Leod, M. Haukka,
(
G. I. Amanullayeva, K. T. Mahmudov and A. J. L.
Pombeiro, J. Inorg. Biochem., 2012, 115, 72; (e) M. N.
Kopylovich, M. J. Gajewska, K. T. Mahmudov, M. F. C.
Guedes da Silva, M. V. Kirillova, P. J. Figiel, J. Sanchiz and
A. J. L. Pombeiro, New J. Chem., 2012, 36, 1646; ( f ) K. T.
Mahmudov, M. N. Kopylovich and A. J. L. Pombeiro, Coord.
Chem. Rev., 2013, 257, 1244; (g) K. T. Mahmudov,
M. N. Kopylovich, A. M. Maharramov, M. M. Kurbanova,
A. V. Gurbanov and A. J. L. Pombeiro, Coord. Chem. Rev.,
2014, 265, 1; (h) M. N. Kopylovich, K. T. Mahmudov,
M. F. C. Guedes da Silva, A. M. Kirillov and A. J. L.
Pombeiro, Dalton Trans., 2011, 40, 12472; (i) M. N. Kopylovich,
K. T. Mahmudov, M. Haukka, K. V. Luzyanin and
A. J. L. Pombeiro, Inorg. Chim. Acta, 2011, 374, 175.
4
814 | New J. Chem., 2014, 38, 4807--4815
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014

View Article Online
NJC
M. N. Kopylovich, K. T. Mahmudov and A. J. L. Pombeiro,
Paper
A. J. L. Pombeiro, Catal. Commun., 2014, 48, 69; (b) Z. Ma,
L. Wei, E. C. B. A. Alegria, L. M. D. R. S. Martins,
M. F. C. Guedes da Silva and A. J. L. Pombeiro, Dalton
Trans., 2014, 43, 4048.
Microwave-assisted catalytic oxidation of alcohols to carbonyl
compounds, in Advances in Organometallic Chemistry and
Catalysis: The Silver/Gold Jubilee International Conference on
Organometallic Chemistry Celebratory Book, ed. A. J. L. 15 (a) M. Sutradhar, L. M. D. R. S. Martins, M. F. C. Guedes da
Pombeiro, Wiley, 2014, ch. 18, pp. 233–245.
(a) G. W. Parshall and S. D. Ittel, Homogeneous Catalysis,
John Wiley & Sons, New York, 2nd edn, 1992, pp. 255–261;
Silva, E. C. B. A. Alegria, C.-M. Liu and A. J. L. Pombeiro,
Dalton Trans., 2014, 43, 3966; (b) M. Alexandru, M. Cazacu,
A. Arvinte, S. Shova, C. Turta, B. C. Simionescu, A. Dobrov,
E. C. B. A. Alegria, L. M. D. R. S. Martins, A. J. L. Pombeiro
and V. B. Arion, Eur. J. Inorg. Chem., 2014, 120.
8
9
(b) S. Caron, R. W. Dugger, S. G. Ruggeri, J. A. Ragan and
D. H. Ripin, Chem. Rev., 2006, 106, 2943.
S. E. Allen, R. R. Walvoord, R. Padilla-Salinas and M. C. 16 (a) I. N. Moiseeva, A. E. Gekham, V. V. Minin, G. M. Larin,
Kozlowski, Chem. Rev., 2013, 113, 6234.
0 (a) M. N. Kopylovich, K. T. Mahmudov, M. Haukka, P. J.
Figiel, A. Mizar, J. A. L. da Silva and A. J. L. Pombeiro, Eur.
M. E. Bashtanov, A. A. Krasnovskii and I. I. Moiseev, Kinet.
Catal., 2000, 41, 170; (b) J. M. Mattalia, B. Vacher, A. Samat
and M. Chanon, J. Am. Chem. Soc., 1992, 114, 4111.
1
J. Inorg. Chem., 2011, 4175; (b) M. N. Kopylovich, K. T. 17 (a) P. J. Figiel, M. Leskel ¨a and T. Repo, Adv. Synth. Catal.,
Mahmudov, M. F. C. Guedes da Silva, M. L. Kuznetsov,
P. J. Figiel, Y. Y. Karabach, K. V. Luzyanin and A. J. L.
Pombeiro, Inorg. Chem., 2011, 50, 918; (c) M. N. Kopylovich,
Y. Y. Karabach, K. T. Mahmudov, M. Haukka, A. M. Kirillov,
P. J. Figiel and A. J. L. Pombeiro, Cryst. Growth Des., 2011,
2007, 349, 1173; (b) J. U. Ahmad, P. J. Figiel, M. T. R ¨a is ¨a nen,
M. Leskel ¨a and T. Repo, Appl. Catal., A, 2009, 371, 17;
(c) R. A. Sheldon and I. W. C. E. Arends, Adv. Synth. Catal.,
2004, 346, 1051; (d) P. Gamez, I. W. C. E. Arends,
R. A. Sheldon and J. Reedijk, Adv. Synth. Catal., 2004,
346, 805; (e) P. Gamez, I. W. C. E. Arends, J. Reedijk and
R. A. Sheldon, Chem. Commun., 2003, 2414; ( f ) L. Feldberg
and Y. Sasson, Tetrahedron Lett., 1996, 37, 2063; (g) G. Rothenberg,
L. Feldberg, H. Wiener and Y. Sasson, J. Chem. Soc., Perkin Trans. 2,
1998, 2429; (h) V. Mahdavi and M. Mardani, J. Chem. Sci., 2012,
124, 1107.
11, 4247; (d) M. N. Kopylovich, A. Mizar, M. F. C. Guedes da
Silva, T. C. O. Mac Leod, K. T. Mahmudov and A. J. L.
Pombeiro, Chem. – Eur. J., 2013, 19, 588.
1 P. J. Figiel, M. N. Kopylovich, J. Lasri, M. F. C. Guedes da
Silva, J. J. R. Fra u´ sto da Silva and A. J. L. Pombeiro, Chem.
Commun., 2010, 46, 2766.
1
1
2 (a) V. Y. Kukushkin and A. J. L. Pombeiro, Chem. Rev., 2002, 18 (a) SHELXTL Reference Manual, Version 5.1, Bruker AXS,
1
02, 1771; (b) M. Zhou, Y. Song, T. Gong, H. Tong, J. Guo,
Analytic X-ray Systems, Inc., Madison, WI, 1997; (b) SMART
& SAINT Software Reference Manuals, Version 6.22, Bruker AXS
Analytic X-ray Systems, Inc., Madison, WI, 2000; (c) G. M.
Sheldrick, SADABS Software for Empirical Absorption Correc-
tion, University of G ¨o ttingen, Germany, 2000.
L. Weng and D. Liu, Inorg. Chem., 2008, 47, 6692.
1
1
3 C. Hansch, A. Leo and W. R. Taft, Chem. Rev., 1991, 91, 165.
4 (a) A. Sabbatini, L. M. D. R. S. Martins, K. T. Mahmudov,
M. N. Kopylovich, M. G. B. Drew, C. Pettinari and
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem., 2014, 38, 4807--4815 | 4815