Copper(ii) and iron(iii) complexes with arylhydrazone of ethyl 2-cyanoacetate or formazan ligands as catalysts for oxidation of alcohols

Article abstract of DOI:10.1039/c6nj02161a

The aquasoluble [Cu(1κN,O²:2κO-HL¹)(S)]₂ [S = CH₃OH (1), (CH₃)₂NCHO (2)] and [Cu(κN-HL¹)(en)₂]·CH₃OH·H₂O (3) Cu^II complexes were prepared by the reaction of Cu^II nitrate hydrate with the new ligand (E/Z)-4-(2-(1-cyano-2-ethoxy-2-oxoethylidene)hydrazinyl)-3-hydroxybenzoic acid (H₃L¹), in the presence (for 3) or absence (for 1 and 2) of ethylenediamine (en), while the Fe^III complex [Fe(κN³-HL²)₂] (4) was isolated by treatment of iron(III) chloride hexahydrate with the new ligand (1E,1E)-N′,2-di(1H-1,2,4-triazol-3-yl)diazenecarbohydrazonoyl cyanide (H₃L²), and characterized by elemental analysis, IR spectroscopy and single crystal X-ray diffraction. Cooperative E,Z → E isomerization of H₃L¹, induced by coordination and ionic interactions, occurs upon interaction with Cu^II in the presence of en. Complexes 1-4 act as catalyst precursors for the solvent-free microwave (MW) assisted selective oxidation of primary or secondary alcohols and diols to the corresponding aldehydes, ketones and diketones, respectively, with yields in the 5-99% range (TONs up to 4.96 × 10²) after 60 min of MW irradiation at 120 °C. The influence of temperature, time and organic radicals was studied and also the regioselective oxidation of the catalytic systems involving the primary and secondary alcohols.

Full text of DOI:10.1039/c6nj02161a

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Copper(II) and iron(III) complexes with arylhydrazone of ethyl
2
ꢀcyanoacetate or formazan ligands: E/Z isomerization assisted by
cooperative coordination and ionic interaction, catalysts for oxidation
of alcohols
a
*,a,b
*,a
Nuno M. R. Martins, Kamran T. Mahmudov, M. Fátima C. Guedes da Silva,
*,a,c
*,a
Luísa M. D. R. S. Martins,
Armando J. L. Pombeiro
a
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco
Pais, 1049–001 Lisbon, Portugal
b
Department of Chemistry, Baku State University, Z. Xalilov Str. 23, Az 1148 Baku, Azerbaijan
Chemical Engineering Department, Instituto Superior de Engenharia de Lisboa, Instituto
c
Politécnico de Lisboa, R. Conselheiro Emídio Navarro, 1959ꢀ007 Lisboa, Portugal
*
Corresponding authors. Phone: +351 218419237.
Eꢀmail addresses: kamran_chem@yahoo.com, kamran_chem@mail.ru (Kamran T. Mahmudov),
fatima.guedes@tecnico.ulisboa.pt (M. Fátima C. Guedes da Silva),
lmartins@deq.isel.ipl.pt (Luísa M. D. R. S. Martins),
pombeiro@tecnico.ulisboa.pt (A.J.L. Pombeiro).
1

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Abstract
2
1
The aquasoluble [Cu(1κN,O :2κOꢀHL )(S)]
2
[S = CH
3
OH (1), (CH
3
)
2
NCHO (2)] and [Cu(κNꢀ
1
II
II
HL )(en) ]ꢁCH OHꢁH O (3) Cu complexes were prepared by reaction of Cu nitrate hydrate with
2
3
2
1
3
the new (E/Z)ꢀ4ꢀ(2ꢀ(1ꢀcyanoꢀ2ꢀethoxyꢀ2ꢀoxoethylidene)hydrazinyl)ꢀ3ꢀhydroxybenzoic acid (H L ),
III
in the presence (for 3) or absence (for 1 and 2) of ethylenediamine (en), while the Fe complex
3
2
[
(
Fe(κN ꢀHL )
2
] (4) was isolated by treatment of iron(III) chloride hexahydrate with the new
2
1E,1E)ꢀN',2ꢀdi(1Hꢀ1,2,4ꢀtriazolꢀ3ꢀyl)diazenecarbohydrazonoyl cyanide (H
3
L ), and characterized
by elemental analysis, IR spectroscopy and single crystal Xꢀray diffraction. Cooperative E,Z → E
1
isomerization of H
3
L , induced by coordination and ionic interactions, occurs upon interaction with
II
Cu in the presence of en. Complexes 1−4 are catalyst precursors for the solventꢀfree microwave
(MW) assisted selective oxidation of primary or secondary alcohols and diols to the corresponding
aldehydes, ketones and diketones, respectively, with yields in the 5−99 % range (TONs up to
2
4
.96×10 ) after 60 min of MW irradiation at 120 °C. Temperature, time and organic radical
influence were studied and also the regioselectivity oxidation of the catalytic systems involving the
primary and secondary alcohols.
2+
3+
Keywords: Arylhydrazone of ethyl 2ꢀcyanoacetate; Formazan; Cu or Fe complexes; E/Z
isomerization; Oxidation of alcohols.
Introduction
Arylhydrazones of active methylene compounds (AHAMCs, Scheme 1) are versatile ligands
1
in the synthesis and design of coordination compounds. The preparative procedures for these
complexes are usually rather straightforward giving high yields of the final products. AHAMC
2
2,3
complexes have been applied as i) catalysts for the oxidation of alkanes and alcohols and C−C
4
5a
bond formations (e.g., Henry reaction), ii) as biological active compounds in vitro antifungal and
5b
6a
6b
antiproliferative tests, iii) materials for optical recording media and spinꢀcoating films, and iv)
7
3
molecular switches in response to changes in pH and auxiliary ligands.
A particular property of AHAMCs is their ability to undergo reversible E/Z isomerism
(Scheme 1) with the isomers often being isolable species with good solidꢀstate stability, what makes
them good candidates for the construction of molecular electronics, switches, rotors and similar
8
nanomachines. Thus, E/Z isomerization of AHAMCs can be important for their physical and
pharmacological properties, as well as in their reactions and catalysis. The reported E/Z
7
isomerizations in hydrazones are regulated by pH, resonanceꢀassisted hydrogen bonding [XꢁꢁꢁH−Y
4
9a
9b,c
↔
YꢁꢁꢁH−X (X, Y = N, O, S, etc.), RAHB], coordinationꢀcoupled proton transfer, UV light,
2

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4c
3
2
cooperative RAHB and ionic interaction, auxiliary ligands or solvation, but no cooperative
coordination and ionic interaction assisted control of this process has been reported, and this
possibility is explored in this work.
Scheme 1. E/Z isomerization of AHAMCs.
Formazans, which contain the azoꢀhydrazone bond system (Scheme 2), are colored
compounds due to their E/Z isomerization (Scheme 2) as well as
π–
π* electronic transitions and
1
0
form a distinct class of organic dyes with certain properties. They exhibit a number of biological
11a,b
11c,d
11e
11f
properties, such as antiviral,
antimicrobial,
antiꢀinflammatory, analgesic, antifungal,
1
1g
anticancer and antiꢀHIV ones. The wide variability of substituents in formazan molecules and
relative ease of their synthesis make it possible to study the influence of the ligand fine structure on
the design of metal complexes. In contrast to the wellꢀstudied coordination chemistry of close
analogues of βꢀdiketiminates, the number of publications devoted to structurally characterized
10,12
formazan complexes is very limited.
Thus, in order to promote the versatility of coordination
chemistry of formazan ligands, we decided to focus on a functionalized form, in particular its 1,2,4ꢀ
triazole (heterocycle) substituted derivative (Scheme 2), in view of the promising coordination
feature of this compound towards iron(III).
Scheme 2. A new heterocyclic formazan and its E/Z isomerization.
On the other hand, transition metal complexes catalyzed oxidation of alcohols play an
important role in catalysis and since carbonyl compounds are widely used in organic synthesis, the
development of new oxidative protocols continues to receive attention in spite of the availability of
1
3
several methods to achieve such objectives. Recently, several copper(II) complexes of AHAMCs
2,3
are successfully applied in the oxidation of alcohols to corresponding carbonyl compounds.
However, complexes of formazan ligands have not yet been reported for such a purpose. Hence, one
relevant objective of the current work was the design of a formazan compound that could act as a
3

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multidentate Nꢀdonor ligand for a metal center that would behave as a catalyst for the oxidation of
structurally different alcohols.
II
Taking the above considerations, herein we report: i) the syntheses of new Cu complexes
1
from (E/Z)ꢀ4ꢀ(2ꢀ(1ꢀcyanoꢀ2ꢀethoxyꢀ2ꢀoxoethylidene)hydrazinyl)ꢀ3ꢀhydroxybenzoic acid (H L ); ii)
3
III
the synthesis of
a
new Fe
complex from (1E,1Z)ꢀN',2ꢀdi(1Hꢀ1,2,4ꢀtriazolꢀ3ꢀ
2
yl)diazenecarbohydrazonoyl cyanide (H
3
L ); iii) the cooperation of ionic interactions and
II
coordination for regulating the of E/Z isomerization in AHAMC ligand upon its treatment with Cu ;
II
III
iv) the evaluation of the catalytic activity of the prepared new Cu and Fe complexes in the model
MWꢀassisted and TEMPOꢀmediated solventꢀfree oxidation by peroxide of structurally different
alcohols to the corresponding carbonyl compounds.
Results and Discussion
1
,2
II
III
Synthesis and characterization of H L , Cu and Fe complexes
3
1
(
E/Z)ꢀ4ꢀ(2ꢀ(1ꢀcyanoꢀ2ꢀethoxyꢀ2ꢀoxoethylidene)hydrazinyl)ꢀ3ꢀhydroxybenzoic acid (H L )
3
14
was synthesized via the JappꢀKlingemann reaction
between the 4ꢀcarboxyꢀ2ꢀ
hydroxybenzenediazonium chloride and ethyl 2ꢀcyanoacetate in waterꢀethanol (5/25, v/v)
containing sodium hydroxide and sodium acetate, and characterized by ESIꢀMS, IR and NMR
1
1
3 6
spectroscopies and elemental analysis. The H NMR spectrum of H L in DMSOꢀd at room
temperature consists of two sets of signals at 12.79 and 13.16 ppm indicating the existence of a
mixture of the two isomeric Eꢀ and Zꢀhydrazone forms (Scheme 3), respectively. The mole ratio of
the isomeric forms has been determined by the relative integration of the NNH signal of each
isomer (Figure S1 in Supplementary Information), showing 65 % for the Zꢀhydrazone and 35 % for
the Eꢀisomer. Possibly due to RAHB between carbonyl group and hydrogen of hydrazone moiety
compared to the cyano group (Scheme 3), the Zꢀhydrazone form dominates. These two isomeric
1
3
1
forms were also confirmed by C NMR spectra of H L in DMSOꢀd , which also consists of two
3
6
1
sets of signals, (Figure S2 in Supplementary Information). The IR spectrum of H
3
L reveals ν(OH)
–1
–1
–1
at 3409 and 3088 cm , ν(NH) at 2780 and 2462 cm , ν(C=O) at 1693 and 1610 cm , and ν(C=N)
–1
+
at 1512 cm . Elemental analysis and ESIꢀMS in methanol (peak at m/z 278.2 [Mr+H] ) support the
proposed formulation.
1
The reaction of the E/Zꢀisomeric mixture of H
3
L with copper(II) nitrate hydrate in methanol
and methanolꢀdimethylformamide mixture (20/1, v/v) lead to the dinuclear Zꢀhydrazone complexes
2
1
2
1
[
Cu(1κN,O :2κOꢀHL )(CH
3
OH)]
2
(1)
and
[Cu(1κN,O :2κOꢀHL ){(CH
3
)
2
NCHO}]
2
(2),
1
respectively. The mononuclear complex [Cu(κNꢀHL )(en)
2
]ꢁCH
3
2
OHꢁH O (3) was obtained upon
1
3
E/Z→E conversion by reaction of the E/Zꢀisomeric mixture of H L with the copper salt, in
methanol, in the presence of ethylenediamine (en) (Scheme 3, Figure 1). Compounds 1−3 have been
isolated as airꢀstable crystalline solids and were characterized by IR spectroscopy, ESIꢀMS and
4

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elemental analysis. In the IR spectra of 1, 2 and 3, the ν(C=O) and ν(C=N) signals appear at 1671
–1
and 1577, 1643 and 1567, 1652 and 1584 cm , respectively, values that are significantly shifted
1
relative to the corresponding signals of H
3
L (see above). The ESI mass spectra of 1, 2 and 3
+
dissolved in CH OH, show parent peaks at m/z = 678.52 [Mr−2CH OH+H] , 678.62
3
3
+
+
[
3 2 3 2
Mr−2(CH ) NCHO+H] and 460.02 [Mr−CH OH−H O+H] (Figures S3ꢀS5 in Supplementary
Information). Elemental analyses are consistent with the proposed formulations, which are also
supported by single crystal Xꢀray diffraction (see below).
The cooperation of coordination and ionic interactions provides an unprecedented strategy
1
towards an easy [(E,Z) → E or Z] isomeric resolution of H L to give the copper(II) complexes
3
2
1
2
1
[
Cu(1κN,O :2κOꢀHL )(CH OH)] (1) and [Cu(1κN,O :2κOꢀHL ){(CH ) NCHO}] (2) (Z form)
3
2
3 2
2
1
and [Cu(κNꢀHL )(en)
2
]ꢁCH
3
OHꢁH
2
O (3) (E form). Usually, copper(II) can directly coordinate the
1
AHAMC ligands, destroying the RAHB system and entering the chelating pocket, as obtained in
the cases of 1 and 2. But, to achieve the coordination of those ligands to other transition metal ions,
deprotonating agents have to be used for the weakening/destroying RAHB system, in most of the
1
cases.
1
1,2
Ethylenediamine (en) has been used as auxiliary ligand, deprotonating agent,
2
,4b
nucleophile, etc., in the synthesis and design of coordination compounds of AHMAC ligands. It
II
1
was expected that the reaction of Cu with H
3
L , in the presence of en, would give a copper(II)
complex with the hydrazone ligand in the Zꢀform, as this is isolated in basic medium in many
1
a
cases. However, a different result was obtained (Scheme 3). The isolation of the Eꢀform in
complex 3 in basic medium can be explained on the basis of the higher coordination ability of en to
2
+
2
copper leading to the initial formation of [Cu(en) ] , preventing the expected N,N,Oꢀchelation of
1
H
3
L which thus coordinates in the monodentate fashion via the cyano group. The coordination of
2+
this group promotes deprotonation of the ligand, the positive charges of Cu ion in 3 being
neutralized by the negative charges of the deprotonated nitrogen and oxygen atoms of the
2
+
hydrazone and carboxylate groups (Scheme 3). The intramolecular distance in 3 between Cu and
−
2+
−
N is 5.120 Å (Scheme 3), whereas the intermolecular distance between Cu and O of the
carboxylate group of a neighbouring molecule is 4.170 Å (see below for Xꢀray analysis). Both
15
distances fall in the recognized range of ionic interaction distances. Moreover, deprotonation of
hydrazone moiety gives an intramolecular hydrogen bonding system, which may additionally
support the Eꢀform (Scheme 3). Thus, cooperation of coordination and ionic interaction in the
II
1
reaction of Cu with H L in the presence of en leads to (E,Z) → E isomeric resolution. Isolated
3
7
a
7b
isomers is of particular significance for material and synthetic chemistries. Thus, we found that
cooperative coordination and ionic interaction assisted E/Zꢀisomerization and can be used to control
physical or chemical properties of AHAMC ligands and their complexes.
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1
Scheme 3. E/Z isomerization of H
3
L in the synthesis of 1−3.
It is known that active methylene compounds can be deprotonated and allowed to react with
12a
two equivalents of diazonium salts in aqueous solution to give formazan ligands. Using this
strategy, we also prepared the new formazan (1Z,1E)ꢀN',2ꢀdi(1Hꢀ1,2,4ꢀtriazolꢀ3ꢀyl)diazeneꢀ
2
carbohydrazonoyl cyanide (H
3
L ), from reaction of 1Hꢀ1,2,4ꢀtriazoleꢀ3ꢀdiazonium chloride with
methyl 2ꢀcyanoacetate in basic medium (Scheme 4). The molar ratios of formazan isomers,
tautomers and conformers are dependent on the character and position of the substituents and on the
12
aggregate state. The inclusion of a triazole moiety can provide additional isomers and tautomeric
forms due to rotation around the C=N and N=N double bonds or proton migration to the heteroatom
1
2
of this substituent, respectively. In fact, according to the H NMR spectrum of H L , it has a linear
3
EEꢀconfiguration (Scheme 2) at pH ∼ 7.5, with the NH group proton not involved in hydrogen
13
bonding appearing at 8.15 ppm (Figure S6 in Supplementary Information). In agreement, the Cꢀ
2
NMR spectrum of H
3
L shows only one signal that supports the linear EEꢀconfiguration (Figure S7
2
–1
in Supplementary Information). The IR spectrum of H L shows the ν(C≡N) vibration at 2215 cm ,
3
while ν(NH) and ν(C=N) are observed at 3125 and 2907, and 1695, 1611 and 1539,
correspondingly.
2
III
Slow evaporation of a methanolꢀwater (4/1, v/v) solution at pH 2 (HCl) of H
3
L and Fe
3
2
chloride hydrate furnishes dark red crystals of the monomeric complex [Fe(κN ꢀHL )
2
] (4) (Scheme
+
4
). Elemental analyses and ESIꢀMS (peak at 518.2 [Mr+H] ) support the formulation (Figure S8 in
Supplementary Information), while IR spectrum reveals absorption bands at 3360 and 2923
1
13
ν(N−H), 2223 ν(C≡N), 1647, 1636 and 1545 ν(C=N). No signal was observed in the H or
C
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3
+
NMR spectra of 4, according to the paramagnetic character of the trivalent metal ion (Fe ).
Magnetic susceptibility measurements of 4 via the Evans method at pD of 4 (see Supplementary
1
6
β
Information, Table S1) revealed a susceptibility, ꢂeff (ꢂ ), of 5.1 corresponding to a spin of 5/2.
Moreover, 4 was characterized by cyclic voltammetry at a platinum electrode at 25 °C in a 0.2 M
n
[
4 4
Bu N][BF ]/NCMe solution. It exhibits a singleꢀelectron (as measured by controlled potential
III
II
electrolysis) irreversible reduction wave at 0.03 V vs. SCE (Figure S9) assigned to the Fe → Fe
III
transition. This potential value is very similar to those reported for mononuclear Fe complexes
17
with ligands derived from 3ꢀaminoꢀ2ꢀpyrazinecarboxylic acid.
HN
N
N
N
N
N
H
C
CN
HN
N
N
N
N
N
N
N
N
CH2 1) NaOH/H2O/C2H5OH
N
1) CH3OH, pH 2
2) FeCl3 6H2O
N
N
NH
Fe
N
N
N
O
C
N
N
N
HN
H3L2
N
O
2)
2
N
N
N
Cl
NH
H2C
N
N
N
NC
CH3
N
N
4
NH
2
Scheme 4. Synthesis of H L and 4.
3
1
,2
To determine the dissociation constants of H3L and the stability constants of their
complexes (1−4), pHꢀmetric titrations in aqueous media were performed. The obtained values were
1
pK
1
= 4.30 ± 0.04, pK
2
= 6.77 ± 0.01, pK
3
= 10.51 ± 0.03 (for H
3
L ), pK
1
= 7.75 ± 0.02, pK
2
= 9.70 ±
2
0
1
.05, pK = 11.13 ± 0.03 (for H L ), lgβ = 13.73 ± 0.04 (for 1), lgβ = 13.82 ± 0.06 (for 2), lgβ =
3
3
3.15 ± 0.09 (for 3) and lgβ = 17.85 ± 0.05 (for 4). The details of the performed calculations can be
followed in the Electronic Supplementary Information (Tables S2−S7). According to previous
1c,14a
performed quantumꢀchemical calculations in the Hückel approximation,
we conclude that pK1,
1
pK
2
and pK
3
concern the deprotonation of –COOH, −OH and =N–NH– groups of H
3
L , respectively
2
(
Scheme 3). In the case of H
3
L , pK
1
concerns the proton abstraction from the hydrazone group
(
=N–NH–), while pK and pK refer to the stepwise deprotonation of the –NH– moiety in the triazol
2
3
ring of the molecule (Scheme 4). The results of pHꢀmetric titration show that the complexes are
very stable and their stability follows the order: 4 > 2 > 1 > 3. On account of the stronger
coordination ability of dimethylformamide in comparison to methanol, complex 2 is more stable
than 1. The comparison of stability constants of the complexes with their pHꢀmetric titration curves
+
(
Figure S10) reveals that the more H ions are released during complex formation, the more stable
is the obtained complex.
Xꢀray diffraction analysis
Crystals of 1−4 suitable for Xꢀray diffraction analysis were obtained upon crystallization from
methanol (for 1 and 3), methanolꢀdimethylformamide (20/1, v/v) (for 2) or methanolꢀwater mixtures
7

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(4:1, v/v) (for 4) (Schemes 3 and 4). Representative plots are depicted in Figure 1. Crystallographic
data and refinement parameters are given in Table S8 (Supplementary Information), selected bond
distances and angles in Table S9.
In 1 and 2 (Figure 1) the copper atoms are bridged by the phenolate oxygen atoms of the
1
2−
2 2
chelating (HL ) ligands thus giving rise to Cu O cores with CuꢁꢁꢁCu separations of 2.9830(3) (1)
and 2.96669(8) (2) Å; inversion centres lie in the middle of these cores. The geometry around the
copper atoms are that of basal edgeꢀsharing distorted square pyramids (τ = 0.32 and 0.28, for 1 and
5
18a
2
, respectively), the copper atoms being shifted 0.184 (1) and 0.055 (2) Å above the leastꢀsquare
basal planes towards the apical position occupied by the coordinated methanol or
dimethylformamide molecules. Compound 2 can also be considered as a 1D polymeric species by
way of longer range interactions between each metal cation and the Oꢀcarbonyl atoms of vicinal
1 5
molecules [Cu···O, 2.871(4) Å], the copper cations thus presenting octahedral N O coordination
environments (Figure S11). Such a polymeric chain is stabilized by Hꢀbond contact involving the
hydroxyl moiety of the carboxylic groups and the oxygen atoms of the coordinated
dimethylformamides [O4−H4O−O6: dD···A = 2.668(4) Å, ∠D−H···A = 161(6)º].
2
2−
Contrasting with the aforementioned cases, in 3 (Figure 1) the hydrazone ligand (HL )
2
+
binds the {Cu(en) } complex cation through the cyanide Nꢀatom, the metal thus presenting an allꢀ
2
18a
5
N squareꢀpyramidal (τ = 0.10) coordination geometry. Despite the different coordination modes,
the hydrazone ligands in 1−3 are nearly planar but with the leastꢀsquare plane of the carboxylic (or
carboxylate) groups twisted, by 3.50, 20.31 or 10.77º, in this order, relatively to the plane defined by
the OhydroxylNOketone atoms (the chelating atoms in 1 and 2). The CuNhydrazone bond distances assume
values of 1.920(2) (1), 1.908(3) (2) and 2.329(2) Å (3), the latter clearly evidencing its axial position
in the coordination sphere of the metal. The equatorial CuNen lengths in 3 ranges from 2.005(2) to
2.015(2) Å.
1
2
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3
4
Figure 1. Xꢀray molecular structures of complexes 1−4 with partial atom numbering schemes.
Ellipsoids are drawn at 30% probability level. Solvent molecules in 3 are omitted for clarity.
Symmetry operation to generate equivalent atoms: i) 2ꢀx,ꢀy,1ꢀz (1), ꢀx,ꢀy,1ꢀz (2).
The molecules of 1 are involved in hydrogen bond interactions, the most relevant ones
concerning the mutual interaction of carboxylic acid groups of vicinal molecules giving rise to 8ꢀ
2
2
membered R
rings, and the contacts between the hydroxyl group of coordinated methanol which act
2
2
as donor to the cyanide Nꢀatom of a neighbouring molecule by means of 16ꢀmembered R rings.
These two types of interactions extend the structure of 1 to the second dimension (Figure S11). Also
4
4
the molecules of 2 are involved in Hꢀbond interactions, with relevance for the 12ꢀmembered R
ring
connecting the copper and the crystallization water molecules. The methanol molecules, in turn, act as
acceptors from amine groups and as donors to carboxylate moieties.
Each deprotonated formazan ligand in 4 chelates the iron cation through two Nꢀhydrazone
10d
and one Nꢀtriazole atoms, as it is usual in iron complexes with these species, affording a distorted
6
ꢀcoordinated environment around the metal (octahedral quadratic elongation, OQE, of 1.027;
18b
3
octahedral angle elongation, OAE, of 93.84), and giving rise to four fused CN Fe fiveꢀmembered
metallacycles. The crystal structure of 4 is stabilized by hydrogen bonding interactions between the
triazole hydrogens and the nitrogens of formazan and other triazole moieties in adjacent units
(Figure S11, Supplementary Information).
Catalytic Activity of 1−4 in Microwaveꢀassisted Oxidation of Alcohols to Carbonyl
Compounds
Complexes 1−4 were tested as catalyst precursors for the solventꢀfree microwave (MW) assisted
oxidation of primary or secondary benzyl alcohols (benzyl alcohol, cinnamyl alcohol, 1ꢀ
phenylethanol or benzhydrol), secondary aliphatic alcohols (cyclooctanol, cyclohexanol, isoborneol
or fenchyl alcohol) and secondary aliphatic dialcohols (1,2ꢀcyclohexanediol or 1,4ꢀ
cyclohexanediol) to the corresponding aldehydes, ketones or diketones using aqueous tertꢀbutyl
t
hydroperoxide (Bu OOH) as oxidizing agent, under low power (20 W) MW irradiation (Scheme 5).
Typical homogenous reaction conditions were performed (unless stated otherwise) using 2.5 mmol
9

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t
of alcohol substrate, 5 ꢂmol (0.2 mol % vs. substrate) of 1−4, 5 mmol of Bu OOH (2 equiv, 70 % in
O), 0.5−2 h reaction time in a solventꢀfree medium.
H
2
Scheme 5. MWꢀassisted oxidation of alcohol substrates to the corresponding aldehydes or ketones.
3,19
Influence of two important reaction parameters such as the time and temperature were
investigated using 1ꢀphenylethanol as model substrate (Tables S10 and S11).
Figure 2. Dependence on the temperature for MWꢀassisted solventꢀfree oxidation of 1ꢀ
phenylethanol using 1−4 as catalyst precursors. Reaction conditions: 2.5 mmol of 1ꢀphenylethanol,
1
0

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t
5
ꢂmol (0.2 mol % vs. substrate) of 1−4, 5 mmol of Bu OOH (2 equiv., 70% in H
2
O), 0.5 h reaction
a
time, 80, 100 and 120 °C reaction temperature, MW irradiation (up to 20 W power). Moles of
acetophenone per 100 mol of alcohol substrate (GC yield), >99% selectivity. Turnover number =
b
moles of product/mol of catalyst precursor.
The catalytic performance of 1−4 to produce acetophenone is enhanced increasing
temperature. 1 and 3 exhibited the highest activities at 120 ºC, reaching 94.0 and 95.0% yield,
respectively, with corresponding TON (moles of product per mol of catalyst precursor) values of
2
2
4
.71×10 and 4.66×10 , respectively, after 0.5 h of MW irradiation. High selectivity towards the
formation of acetophenone was found (> 99%), and no traces of byꢀproducts were detected by GC
analyses of the final reaction mixtures (only unreacted alcohol, apart from the ketone product).
Figure 3. Time reaction studies in MWꢀassisted solventꢀfree oxidation of 1ꢀphenylethanol using
1
−4 as catalyst precursors. Reaction conditions: 2.5 mmol of 1ꢀphenylethanol, 5 ꢂmol (0.2 mol %
t
vs. substrate) of 1−4, 5 mmol of Bu OOH (2 equiv., 70% in H O), 0.25, 0.5, 1.0, 1.5 and 2.0 h
reaction time, 120 °C reaction temperature, MW irradiation (up to 20 W power). Moles of
acetophenone per 100 mol of alcohol substrate (GC yield), >99% selectivity. Turnover number =
2
a
b
moles of product/mol of catalyst precursor.
It was found that 1h of MW irradiation is the best compromise since yields reach a
maximum and then slightly decrease conceivably due to overoxidation (Figure 3). 1 and 3 exhibit
2
2
the maximum yields (99.9 %) and highest TON values (4.96×10 and 4.85×10 , respectively).
The molecular structures of 1 and 2 are very similar, only differing in one site of each Cu
coordination sphere, where it can be found the solvent species, methanol or dimethylformamide
(DMF) in 1 and 2, respectively (see Scheme 3). The higher labiality of methanol relative to DMF,
can afford more availability for the metal centre and thus account for the higher acetophenone yield
obtained in the presence of 1.
1
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3f,g
1
Iron(III) in some reports is known to be better than Cu(II) towards alcohol oxidation,
but
in our work 4 exhibited the lowest catalytic performance. Coordination sphere of 4 is saturated by N
donor atoms of the formazan ligands (see Scheme 4), constituting a more stable structure and then
increasing the inertness of this preꢀcatalyst.
The Cu(II) complex bearing one arylhydrazone of ethyl 2ꢀcyanoacetate ligand, coordinated
via metalꢀcyano group (structure 3, Scheme 3), evidenced the higher catalytic activity. This fact is
based on the ability of the coordinated species to leave the metallic centre and dispose free
coordination sites, turning it in a more convenient compound to react with the alcohol substrates
and oxidant. It should be noted that besides 1 and 3 reached similar yield values for production of
acetophenone, structure 1 bears two metal species and 3 just one. Then, if we take into account the
activity per metal centre 3 is the most effective preꢀcatalyst towards oxidation of 1ꢀphenylethanol.
t
The catalytic performance of 1−4 in the homogeneous oxidation using Bu OOH of 1ꢀ
phenylethanol was studied in the presence of the 2,2,6,6ꢀtetramethylpiperidylꢀ1ꢀoxyl (TEMPO)
radical. The results are presented in Figure 4. TEMPO additive provided a slightly decrease in TON
values for all catalytic systems under similar reaction conditions.
Figure 4. MWꢀassisted solventꢀfree oxidation of 1ꢀphenylethanol using 1−4 as catalyst precursors
in absence and presence of TEMPO (2.5 mol % vs. substrate). Reaction conditions: 2.5 mmol of 1ꢀ
t
phenylethanol, 5 ꢂmol (0.2 mol % vs. substrate) of 1−4, 5 mmol of Bu OOH (2 equiv., 70% in
2
H O), 62.5 ꢂmol of TEMPO (2.5 mol % vs. substrate), 1.0 h reaction time, 120 °C reaction
a
temperature, MW irradiation (up to 20 W power). Turnover number = moles of product/mol of
catalyst precursor.
1
,2
1ꢀPhenylethanol blank tests (in the absence of a preꢀcatalyst) and trials using H
3
L
(with no
coordinated metal) were performed under the same optimized reaction conditions and no significant
acetophenone production was observed (Table S10, entries 31ꢀ33).
In order to get insights regarding the mechanism of alcohol oxidation with TBHP, known C
20
and Oꢀradical traps (CBrCl
3 2
and Ph NH) were introduced to the catalytic system. This led to a
significant decrease of the catalytic activity in the presence of 1−4 (Table S10, entries 34ꢀ41). This
1
2

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fact suggests that C· and O· radicals (Hꢀatom abstractor of the alcohol function group) where
t
trapped by the introduced radical scavengers, interrupting the probably metalꢀmediated Bu OO· and
t
21
Bu O· formation. The proposed mechanism is resumed by equations (1ꢀ6) where M is the catalyst
22
metal centre. It should be noted that for the Fe(III) catalyst equations (1) and (2) are in reverse
order. This mechanism was proposed for different transition metals (e.g., M = V, Mn, Fe, Co or
1
9
n+
(n+1)+
Cu) where the ability to undergo a unit change of the metal oxidation state (M /M
) appears
to be particularly favorable. Such a type of radical mechanism has also been reported for copperꢀ
2
2e
22f
catalyzed peroxidative oxidation of other substrates, such as aromatic aldehydes and amines.
n+
t
(n+1)+
n+
t
M + BuOOH → M
–OH + BuO˙
(1)
(2)
(3)
(4)
(5)
(6)
(
n+1)+
n+1)+
t
t
+
M
+ BuOOH → M + BuOO˙ + H
(
t
(n+1)+
t
M
–OH + BuOOH → M
–OO– Bu + H
2
C˙–OH
2
O
t
t
BuO˙ + R
2
CHOH → BuOH + R
t
t
BuOO˙ + R
2
CHOH → BuOOH + R
2
C˙–OH
(
n+1)+
t
t
n+
M
–OO– Bu + R
2
C˙–OH → R
2
C=O + BuOOH + M
For comparison of MW irradiation vs. conventional heating mode (silicone bath), same
optimized reaction parameters were applied on 1 and 3 catalytic systems for the oxidation our
selected alcohol substrate model, 1ꢀphenylethanol (Figure 5). MW irradiation affords an enhanced
acetophenone yield with both precatalysts, since 94.0 % (1) and 95.0 % (3) yield is reach just after
3
0 min and full conversion within 1 h of reaction in contrast with conventional heat at 30 min (52.5
and 53.3 % for 1 and 3, respectively) and 1 h (97.6 and 94.4 % for 1 and 3, respectively). It is
necessary to carry out the reaction to 3 h to obtain full conversion with the conventional heat mode.
It is worthwhile to notice the low MW irradiation power (less than 20 W) also turn this synthetic
protocol more desirable than the conventional heat one, which requires about 1500ꢀ2000 W to
achieve a 120 °C reaction temperature.
Figure 5. Solventꢀfree oxidation of 1ꢀphenylethanol using 1 or 3 as catalyst precursors using
conventional heat or MW irradiation. Reaction conditions: 2.5 mmol of 1ꢀphenylethanol, 5 ꢂmol
1
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t
(
0.2 mol % vs. substrate) of 1 or 3, 5 mmol of Bu OOH (2 equiv., 70% in H
2
O), 0.25, 0.5, 1.0 and
3
.0 h reaction time, 120 °C reaction temperature, conventional heat (silicone bath) or MW
a
irradiation (up to 20 W power). Moles of aldehyde or ketone per 100 mol of alcohol substrate (GC
yield), >99% selectivity.
Since the best catalytic performances were obtained in presence of 1 or 3 at 120 ºC and 1 h
MW irradiation in absence of TEMPO, the oxidation of other alcohol substrates was tested under
the same optimized reaction parameters (Figure 6, Table S11).
Figure 6. MWꢀassisted solventꢀfree oxidation of 1ꢀphenylethanol, benzyl alcohol, cinnamyl
alcohol, bezhydrol, cyclooctanol, cyclohexanol, isoborneol and fenchyl alcohol using 1 or 3 as
catalyst precursors. Reaction conditions: 2.5 mmol of alcohol substrate, 5 ꢂmol (0.2 mol % vs.
t
substrate) of 1 or 3, 5 mmol of Bu OOH (2 equiv., 70% in H O), 1.0 h reaction time, 120 °C
2
a
reaction temperature, MW irradiation (up to 20 W power). Moles of aldehyde or ketone per 100
mol of alcohol substrate (GC yield), >99% selectivity.
Oxidation, with peroxide, of primary aromatic alcohols (benzyl alcohol and cinnamyl
alcohol) end up to be much less profitable than secondary ones (1ꢀphenylethanol and benzhydrol).
Despite the less steric hindrance of primary alcohols that should turn the substrate more suitable to
react, lower conversions were obtained, maybe due the higher stability of the radical intermediate
formed from secondary alcohols (R
equations 4 and 5.
2
C˙–OH> RHC˙–OH) upon the H abstraction illustrated in
For the tested ring membered aliphatic secondary alcohols (Table S11), cyclooctanone was
obtained in higher yields than cyclohexanone, in accord with the greater instability/reactivity of the
8
ꢀcarbon member ring in comparison with the 6 one. Moreover, camphor is produced in higher
yield from isoborneol than fenchaldehyde from fenchyl alcohol. A possible justification relies on
the difference of the steric hindrance of ꢀOH group in the alcohol substrate.
1
4

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It was also studied the regioselective oxidation of 1,2ꢀ and 1,4ꢀcyclohexanediol by 1 and 3
under the optimized reaction conditions (Figure 7). The corresponding diketones were obtained, and
again similar activities were observed for both catalyst precursors. Vicinal diketones (51.5 and
5
6.0% for 1 and 3, respectively) were formed in higher yields than 1,4ꢀdiketones (18.1 and 23.4%
for 1 and 3, respectively). This tendency was expected in vicꢀdiols since after one H· abstraction,
the remaining H of the other –OH functional group has increased acidity (higher pK ) and then
became more easy to be captured by radicals from the peroxide.
a
High selectivity for diketones were achieved. This suggests the presence of only traces of by
products, such as hydroxyl ketones, or even hydroxyoxepanones (BaeyerꢀVilliger mechanism type)
2
3
has reported in other works.
Figure 7. MWꢀassisted solventꢀfree oxidation of 1,2ꢀcyclohexanediol and 1,4ꢀcyclohexanediol
catalysed by 1 or 3. Reaction conditions: 2.5 mmol of alcohol substrate, 5 ꢂmol (0.2 mol % vs.
t
substrate) of 1 or 3, 5 mmol of Bu OOH (2 equiv., 70% in H
2
O), 1.0 h reaction time, 120 °C
a
reaction temperature, MW irradiation (up to 20 W power). Moles of aldehyde or ketone per 100
mol of alcohol substrate (GC yield), ca 94 % selectivity.
Conclusions
Three copper(II) complexes bearing arylhydrazone ligands and an iron(III) compound with
formazan groups were synthesized and fully characterized. A new type of E/Z → E isomerization
assisted by unprecedented cooperation of coordination and ionic interaction was found in the
formation of a copper(II) complex in the presence of ethylenediamine. Although the free
heterocyclic formazan compound occurred in the EEꢀconfiguration, its iron(III) complex showed
that moiety with the EZꢀconfiguration.
The obtained complexes act as effective catalyst precursors, highlighting binuclear Cuꢀ
complexes, for the mild and selective homogeneous oxidation, by peroxide, of benzyl and aliphatic
primary and secondary alcohols and cyclic 1,2ꢀ and 1,4ꢀdiols to corresponding aldehydes, ketones
and diketones in a solventꢀfree MWꢀassisted process. Efforts to increase the catalytic activities
using TEMPO were unsuccessful, which end up to reveal an opposite desired effect. Curiously the
oxidation products from more substituted alcohols (secondary) achieved higher yields and TONs in
comparison with the less substituted ones (primary). The regioselective applicability, minute scale
1
5

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reaction, use of low power MW irradiation and solventꢀfree system are significant factors
converging towards green standards and energy saving catalytic systems aiming more sustainable
scalable chemical processes.
Experimental Section
Materials and instrumentation
All the synthetic work was performed in air and at room temperature. All the chemicals were
obtained from commercial sources (Aldrich) and used as received. Infrared spectra (4000–400
−1
1
13
1
cm ) were recorded on a Vertex 70 (Bruker) instrument in KBr pellets. 1D ( H, C{ H}) NMR
spectra were recorded on Bruker Advance II 300.13 (75.468 carbonꢀ13) and 400.13 (100.61 carbonꢀ
TM
1
3) MHz (UltraShield Magnet) spectrometers at ambient temperature. Magnetic measurements of
1
complex 4 in solution was performed at room temperature (r.t.) by H NMR using the Evans’
1
8
method on a Bruker Avance 300 spectrometer operating at 300.13 MHz at a constant temperature
of 298.15 K. The measurement for 4 was performed in standard 5 mm NMR tube containing the
paramagnetic sample dissolved in D
2
O with an inert reference of tertꢀbutyl alcohol, against a
O) and their shift
reference insert tube filled with the same solvent (tertꢀbutyl alcohol in D
2
measured in Hz. C, H and N elemental analyses were carried out by the Microanalytical Service of
the Instituto Superior Técnico. Electrospray mass spectra (ESIꢀMS) were run with an ionꢀtrap
instrument (Varian 500ꢀMS LC Ion Trap Mass Spectrometer) equipped with an electrospray 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 1200
in methanol solution. The compounds were observed in the positive mode (capillary voltage = 80–
1
05 V). The acidity of the solutions was measured using a CG825 pHꢀmeter with an ESLꢀ43ꢀ07
glass electrode adjusted by standard buffer solutions and an EVLꢀ1M3.1 silverꢀsilver chloride
reference electrode. The ionic strength was maintained constant (0.1 M) by adding a calculated
amount of KCl. The absorbance of the solutions was measured on a Lambda 40 spectrophotometer
(Perkin Elmer) in 1 cm quartz cells. The electrochemical experiments were performed on an EG&G
PAR 273A potentiostat/galvanostat connected to a personal computer through a GPIB interface.
n
Cyclic voltammograms (CV) were obtained in a 0.2 M [ Bu
4
4
N][BF ]/NCMe solution, at a platinum
disc working electrode (d = 0.5 mm). Controlledꢀpotential electrolyses (CPE) were carried out in
electrolyte solutions with the above mentioned composition, in a threeꢀelectrode Hꢀtype 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 pseudoꢀreference electrode was used to control the working electrode potential. The CPE
experiments were monitored regularly by cyclic voltammetry, thus assuring that no significant
potential drift occurred during the electrolyses. Ferrocene was used as an internal standard for the
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6

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measurement of the oxidation potentials of the complexes; the redox potential values are quoted
5
relative to the SCE by using as internal reference the ferrocene/ferricinium couple ([Fe(η ꢀ
0/+
24
5 5 2
C H ) ] ; E = 0.42 V vs. SCE in NCMe).
Synthesis of (E/Z)ꢀ4ꢀ(2ꢀ(1ꢀcyanoꢀ2ꢀethoxyꢀ2ꢀoxoethylidene)hydrazinyl)ꢀ3ꢀhydroxybenzoic acid
1
(H
3
L )
1
14
3
The arylhydrazone H L was synthesized via the JappꢀKlingemann reaction between the
aromatic diazonium salt of 4ꢀaminoꢀ3ꢀhydroxybenzoic acid and ethyl 2ꢀcyanoacetate in water
solution containing sodium acetate.
1
For the synthesis of H
3
L , 4ꢀaminoꢀ3ꢀhydroxybenzoic acid (1.53 g, 10.00 mmol) was
dissolved in 50 mL water, and 0.40 g (10.00 mmol) of NaOH was added. The solution was cooled
in an ice bath to 273 K and 0.69 g (10.00 mmol) of NaNO was added; 2.00 mL HCl was then
2
added in 0.5 mL portions for 1 h. The temperature of the mixture should not exceed 278 K. Ethyl 2ꢀ
cyanoacetate (1.13 mL, 10.00 mmol) was added to 30 mL waterꢀethanol (5/25, v/v) solution of
sodium hydroxide (0.2 g, 10.0 mmol) and sodium acetate (0.82 g, 10.00 mmol). The resulting
solution was stirred and cooled to ca. 273 K, and a suspension of diazonium salt (see above) was
added in three portions under vigorous stirring for 1 h. A yellow precipitate of the title compound
was formed in ca. 1 h, filtered off, dried in air and recrystallized from methanol.
1
H
3
L . Yield: 870 mg (based on ethyl 2ꢀcyanoacetate), yellow powder soluble in water, methanol,
ethanol, acetone and insoluble in chloroform. Anal. Calcd for C H N O (Mr = 277.07): C, 51.99;
1
2
11
3
5
+
H, 4.00; N, 15.16; found: C, 51.81; H, 3.94; N, 15.22 %. ESIꢀMS: m/z: 278.2 [Mr+H] . IR (KBr):
3
1
409 and 3088 ν(OH), 2780 and 2462 ν(NH), 2180 ν(C≡N), 1693 ν(COOH), 1610 ν(C=O) and
−1 1
512 ν(C=N) cm . H NMR of a mixture of isomeric Eꢀ and Zꢀhydrazone forms (300.130 MHz) in
DMSOꢀd , internal TMS, δ (ppm): EꢀHydrazone, 1.27−1.33 (s, 3H, CH ), 3.83 (s, 1H, COOH),
6
3
4
.29−4.35 (2H, CH
2
), 7.48−7.54 (3H, Ar−H), 11.02 (s, 1H, O−H) 12.79 (s, 1H, N−H). Zꢀ
Hydrazone, 1.27−1.33 (s, 3H, CH
Ar−H), 11.03 (s, 1H, O−H) 13.16 (s, 1H, N−H). C{ H} NMR (75.468 MHz, DMSOꢀd
Hydrazone, δ: 13.87 (CH ), 52.93 (CH ), 106.27 (Ar−H), 113.90 (C=N), 115.60 (C≡N), 127.66
), 3.86 (s, 1H, COOH), 4.29−4.35 (2H, CH
), 7.48−7.54 (3H,
3
2
13
1
6
). Eꢀ
3
2
(
2Ar−H), 132.30 (Ar−COOH), 132.33 (Ar−NHN=), 145.31 (Ar−OH), 161.69 (C=O), 166.73
COOH). ZꢀHydrazone, δ: 13.87 (CH ), 62.25 (CH ), 106.45 (Ar−H), 121.79 (C=N), 116.35 (C≡N),
27.70 (2Ar−H), 132.30 (Ar−COOH), 132.33 (Ar−NHN=), 145.33 (Ar−OH), 161.26 (C=O),
66.73 (COOH).
(
3
2
1
1
2
3
Synthesis of (1E,1Z)ꢀN',2ꢀdi(1Hꢀ1,2,4ꢀtriazolꢀ3ꢀyl)diazenecarbohydrazonoyl cyanide (H L )
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2
12a
Synthesis of H
3
L was performed according to a known procedure;
reaction of methyl 2ꢀ
cyanoacetate with two equivalent of 1Hꢀ1,2,4ꢀtriazoleꢀ3ꢀdiazonium chloride lead to formazan
ligand.
Two equivalent of 1Hꢀ1,2,4ꢀtriazolꢀ3ꢀamine (1.68 g, 20.00 mmol) were dissolved in 70 mL
water. The solution was cooled in an ice bath to 273 K and 4.00 mL of HCl was added; 1.38 g
2
(20.00 mmol) of NaNO in 5 mL water was then added in 0.5 mL portions for 1 h. 10 mL water
solution of NaOH (1.20 g, 30.00 mmol) were added to a mixture of 10.00 mmol (0.99 mL) of
methyl 2ꢀcyanoacetate with 20 mL of ethanol. The solution was cooled in an ice bath to ca. 273 K,
and 1Hꢀ1,2,4ꢀtriazoleꢀ3ꢀdiazonium chloride (see above) was added in three portions under vigorous
stirring for 1 h. The reaction mixture was then transferred to a beaker and acidified by HCl (33 %
2
w/w) to pH ∼7.5. The orange product H
3
L precipitated, whereupon it was filtered off, dried in air
and recrystallized from methanol.
2
H
3
L . Yield: 680 mg (based on methyl 2ꢀcyanoacetic acid), yellow powder soluble in water,
methanol, ethanol and insoluble in chloroform. Anal. Calcd for C H N (Mr = 231.18): C, 31.17;
6
5
11
+
H, 2.18; N, 66.65; found: C, 31.04; H, 2.09; N, 66.34 %. ESIꢀMS: m/z: 232.2 [Mr+H] . IR (KBr):
−1 1
3
125 and 2907 ν(NH), 2215 ν(C≡N), 1695, 1611 and 1539 ν(C=N) cm . H NMR (300.130 MHz)
in DMSOꢀd , internal TMS, δ (ppm): EE linear isomer, 8.15 (s, 1H, NH hydrazone), 8.34 and 8.52
6
1
3
1
(
2H, 2CH of triazole moieties), 8.82 and 8.86 (2H, 2NH of triazole moieties). C{ H} NMR
(
75.468 MHz, DMSOꢀd ). EE linear isomer, δ: 110.09 (C≡N), 143.79 (C−NH of triazole moiety),
6
1
45.36 (C−NH of triazole moiety), 145.84 (C−N=N), 157.06 (C−NH), 163.54 (N=C−C≡N).
Syntheses of complexes
1
Syntheses of 1 and 2. 27.7 mg (0.1 mmol) of H L was dissolved in 5 mL methanol (or methanolꢀ
3
dimethylformamide mixture (20/1, v/v) in the case of 2), then 23.3 mg (0.1 mmol) of
3 2 2
Cu(NO ) ꢁ2.5H O were added. The mixture was stirred and heated to 80 °C for 5 min, then left for
slow evaporation at room temperature. Dark blue crystals of the product started to form after ca. 3 d
at room temperature; they were then filtered off and dried in air.
1
: Yield, 4.0 mg, 62 % (based on Cu). Dark blue crystalline compound soluble in ethanol,
methanol, acetonitrile, DMSO and water. Anal. Calcd for C26 12 (Mr = 741.61): C,
2.11; H, 3.53; N, 11.33. Found: C, 42.14; H, 3.39; N, 11.27. ESIꢀMS: m/z: 678.52
2 6
H26Cu N O
4
+
−1
[
Mr−2CH
ν(C=O) and 1577 ν(C=N).
: Yield, 3.3 mg, 51 % (based on Cu). Dark brown crystalline compound soluble in ethanol,
methanol, acetonitrile, DMSO and water. Anal. Calcd for C H Cu N O (Mr = 823.71): C,
3
OH+H] . IR (KBr, selected bands, cm ): 3400 and 2982 ν(O−H), 2224 ν(C≡N), 1671
2
3
0
32
2
8
12
4
3.74; H, 3.92; N, 13.60. Found: C, 43.24; H, 3.61; N, 13.40. ESIꢀMS: m/z: 678.62
1
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+
−1
[
Mr−2(CH
643 ν(C=O) and 1567 ν(C=N).
Synthesis of 3. 27.7 mg (1.0 mmol) of H
mmol) of Cu(NO ꢁ2.5H O and 12 ꢀL (0.2 mmol) ethylenediamine were added. The mixture was
3 2
)
NCHO+H] . IR (KBr, selected bands, cm ): 3497 ν(O−H), 2218 ν(C≡N), 1697 and
1
1
3
L were dissolved in 5 mL methanol, then 23.3 mg (0.1
3
)
2
2
stirred for 30 min at 80 °C and left for slow evaporation; the dark brown crystals of the product
started to form after ca. 3 d at room temperature; they were then filtered off and dried in air.
3
: Yield, 3.0 mg, 47 % (based on Cu). Dark brown crystalline compound soluble in ethanol,
methanol, acetonitrile, DMSO and water. Anal. Calcd for C H CuN O (Mr = 509.02): C, 40.11;
1
7
31
7
7
H, 6.14; N, 19.26. Found: C, 39.94; H, 5.97; N, 19.45. ESIꢀMS: m/z: 460.02
+
−1
[
Mr−CH
3 2
OH−H
O+H] . IR (KBr, selected bands, cm ): 3611, 3298, 3223 and 3144 ν(O−H), 2969,
2
934, 2908 and 2884 ν(N−H), 2363 ν(C≡N), 1652 and 1624 ν(C=O) and 1584 ν(C=N).
2
Synthesis of 4. 23.1 mg (0.1 mmol) of H
3
L were dissolved in 5 mL methanolꢀwater mixture (4/1,
ꢁ6H O were added. The
v/v), then 1 mL 0.01 M HCl (pH 2) and 23.2 mg (0.1 mmol) of FeCl
3
2
mixture was stirred for 5 min, then left for slow evaporation at room temperature, dark red crystals
of 4 suitable for Xꢀrays started to form after ca. 3 d.
4
: Yield, 2.4 mg, 43 % (based on Fe). Dark red crystals soluble in ethanol, acetonitrile, DMSO,
DMF and water. Anal. Calcd for C H FeN (Mr = 517.19): C, 27.87; H, 1.75; N, 59.58. Found: C,
1
2
9
22
+
−1
2
7.71 H, <2.00; N, 59.09. ESIꢀMS: m/z: 518.2 [Mr+H] . IR (KBr, selected bands, cm ): 3360 and
923 ν(N−H), 2223 ν(C≡N), 1647, 1636 and 1545 ν(C=N).
2
Xꢀray structure determinations
Xꢀray quality crystals of 1 ꢀ 4 were immersed in cryoꢀoil, mounted in a Nylon loop and measured at
2
96 K. Intensity data were collected using a Bruker AXSꢀKAPPA APEX II diffractometer with
graphite monochromated MoꢀKα (λ 0.71073Å) radiation. Data were collected using omega scans of
.5º per frame and full sphere of data were obtained. Cell parameters were retrieved using Bruker
0
2
5a
SMART software and refined using Bruker SAINT on all the observed reflections. Data were
25a
corrected for absorption effects using the multiꢀscan method (SADABS). Structures were solved
25b
25c
by direct methods by using SIRꢀ97 package and refined with SHELXLꢀ2014. Calculations
2
5d
were performed using the WinGX Systemꢀv2014.1. The hydrogen atoms attached to carbon
atoms were inserted at geometrically calculated positions and included in the refinement using the
ridingꢀmodel approximation, while those attached to nitrogen atoms were located in the difference
Fourier synthesis but were included in the final refinement at positions calculated from the
geometry of the molecules using the riding model, with isotropic vibration parameters. Uiso(H)
were defined as 1.5Ueq of the parent nitrogen atoms and of the parent carbon atoms for the methyl
groups, or 1.2Ueq of the carbon atoms for phenyl and methylene residues. The hydrogen atoms of
the water and methanol molecules (in 3) have been located from the final Fourier difference map,
1
9

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and the isotropic thermal parameter were set at 1.5 times the average thermal parameter of the
belonging oxygen atom. Some DFIX and DANG restrains were applied for the NH groups and/or
2
5e
the water molecule. PLATON/SQUEEZE
was used to treat disordered molecules in the
asymmetric unit of 3. These were removed from the model and not included in the empirical
formula.
General procedure for the oxidation of alcohols with TBHP
In a typical experiment, the alcohol substrate, 1ꢀphenylethanol, benzylic alcohol, cinnamyl alcohol,
benzhydrol, cyclooctanol, cyclohexanol, isoborneol, fenchyl alcohol, 1,2ꢀcyclohexanediol or 1,4ꢀ
t
cyclohexanediol (2.5 mmol), Bu OOH (70 % aqueous solution, 5.0 mmol) and catalyst precursor
1
−4 (10 µmol, 0.2 mol% vs. substrate) were introduced to a cylindrical Pyrex tube, which was then
placed in the focused microwave reactor. In the simultaneous oxidation of benzylic alcohol and 1ꢀ
phenylethanol experiment an equimolar amount was used (1.25+1.25 mmol). In the TEMPOꢀ
mediated experiments, TEMPO (62.5 µmol, 2.5 mol% vs. substrate) was added to the reaction
mixture. The system was stirred and irradiated (up to 20 W) for 15, 30, 60, 90 or 120 min at 80, 100
or 120 ºC. After the reaction, the mixture was allowed to cool down to room temperature. 150 ꢀL of
benzaldehyde (internal standard) and 2.5 mL of MeCN (to extract the substrate and the organic
products from the reaction mixture) were added. The obtained mixture was stirred during 10 min
and then a sample (0.5 ꢀL) was taken from the organic phase and analysed by GC using the internal
standard method. Blank tests indicate no traces of acetophenone are generated in a preꢀcatalystꢀfree
system. For the conventional heating method, experiments were carried out in 5 mL roundꢀ
bottomed flasks eqquiped with a reflux condenser under silicone heating bath. Reaction parameters
were applied as mentioned above.
Acknowledgements
This work has been partially supported by the Foundation for Science and Technology (FCT),
Portugal [UID/QUI/00100/2013]. M.F.C.G.S. and A.J.L.P. acknowledge the Russian Science
Foundation for support and help in organizing the International Laboratory (grant 14ꢀ43ꢀ00017).
N.M.R.M is thankful to FCT for his CATSUS PhD fellowship (SFRH/BD/52371/2013). The
authors acknowledge the Portuguese NMR Network (ISTꢀUTL Centre) for access to the NMR
facility, and the IST Node of the Portuguese Network of massꢀspectrometry (Dr. Conceição
Oliveira) for the ESIꢀMS measurements.
1
13
1,2
Supporting Information Available: H/ C NMR spectrum of H L . Crystal data, experimental
2
parameters and selected details of the refinement calculations, tables listing bond distances and
angles and hydrogen bond geometry. This material is available free of charge via the Internet or
from the authors. CCDC 1412834−1412837 contain the supplementary crystallographic data for
complexes 1−4, respectively. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223ꢀ336ꢀ033; or eꢀmail:
deposit@ccdc.cam.ac.uk.
2
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TOC graphic and synopsis
Cooperative coordination and ionic interactions assisted E/Z isomerization in arylhydrazone ligand
II
lead to a variety of Cu complexes, which effectively catalyse the homogeneous oxidation of
III
alcohols to carbonyl compounds (see scheme). A new Fe complex with heterocyclic formazan
ligand is also catalytic active in alcohols oxidation.
1