Synthesis, structure and properties of nickel and copper complexes containing N,O-hydrazone Schiff base ligand

Article abstract of DOI:10.1016/j.ica.2017.04.064

The compound 2-(1-(2-methyl-2-phenylhydrazono)ethyl)phenol, HL^NNMePh, was synthesized by solvent-free condensation of o-hydroxyacetophenone and N,N-methylphenylhydrazine and was analytically and spectroscopically characterized. The reaction of HL^NNMePh with nickel acetate and copper acetate in the presence of NEt₃ resulted the corresponding [M(L^NNMePh)₂] complexes (M = Ni, 1; Cu, 2), which were structurally characterized by X-ray diffraction methods. A four-coordinate square planar environment around the metal centre was found for both compounds, in which the two L^NNMePh ligands act as bidentate N,O-chelators and exhibit a pseudo-trans conformation. These compounds are the first structurally characterized transition metal complexes containing the κ²-N,O-2-(1-(2-methyl-2-phenylhydrazono)ethyl)phenolate ligand, L^NNMePh. Copper complex [Cu(L^NNMePh)₂], 2, was tested as catalyst, in combination with TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl radical), for the aerobic oxidation of benzyl alcohol, as model reaction. Low yields of the corresponding aldehyde were observed by using air at atmospheric pressure as the oxidant in toluene as solvent.

Full text of DOI:10.1016/j.ica.2017.04.064

Accepted Manuscript
Research paper
Synthesis, structure and properties of nickel and copper complexes containing
N,O-hydrazone Schiff base ligand
María del Mar Conejo, Jorge Cantero, Antonio Pastor, Eleuterio Álvarez,
Agustín Galindo
PII:
S0020-1693(17)30189-5
http://dx.doi.org/10.1016/j.ica.2017.04.064
ICA 17575
DOI:
Reference:
Inorganica Chimica Acta
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Accepted Date:
9 February 2017
4 April 2017
10 April 2017
Please cite this article as: M. del Mar Conejo, J. Cantero, A. Pastor, E. Álvarez, A. Galindo, Synthesis, structure
and properties of nickel and copper complexes containing N,O-hydrazone Schiff base ligand, Inorganica Chimica
Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.04.064
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REVISED
Synthesis, structure and properties of nickel and copper complexes
containing N,O-hydrazone Schiff base ligand
a
a
a
Eleuterio Álvarez,b and Agustín
*
*
María del Mar Conejo,
Jorge Cantero, Antonio Pastor,
Galindoa
a
Departamento de Química Inorgánica, Facultad de Química, Universidad de Sevilla, Aptdo
1203, 41071 Sevilla, Spain.
b
Instituto de Investigaciones Químicas, CSIC-Universidad de Sevilla, Avda. Américo Vespucio
49, 41092 Sevilla, Spain
Keywords: copper; nickel; oxidation; catalysis; Schiff base; DFT
Dedicated to Prof. Carlo Mealli, in recognition of his contribution to the understanding of
Chemistry.
* Corresponding authors.
E-mail addresses: mconejo@us.es, anpastor@us.es
1

ABSTRACT
The compound 2-(1-(2-methyl-2-phenylhydrazono)ethyl)phenol, HLNNMePh, was synthesized by
solvent-free condensation of o-hydroxyacetophenone and N,N-methylphenylhydrazine and was
analytically and spectroscopically characterized. The reaction of HLNNMePh with nickel acetate
and copper acetate in the presence of NEt3 resulted the corresponding [M(LNNMePh)2] complexes
(M = Ni, 1; Cu, 2), which were structurally characterized by X-ray diffraction methods. A
four-coordinate square planar environment around the metal centre was found for both
compounds, in which the two LNNMePh ligands act as bidentate N,O-chelators and exhibit a
pseudo-trans conformation. These compounds are the first structurally characterized transition
metal complexes containing the 2-N,O-2-(1-(2-methyl-2-phenylhydrazono)ethyl)phenolate
ligand, LNNMePh. Copper complex [Cu(LNNMePh)2], 2, was tested as catalyst, in combination with
TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl radical), for the aerobic oxidation of benzyl
alcohol, as model reaction. Low yields of the corresponding aldehyde were observed by using air
at atmospheric pressure as the oxidant in toluene as solvent.
1. Introduction
Oxidation catalysis is a topic of permanent interest and therefore the development of new
catalytic systems, based on transition metal compounds and using green oxidants, is an active
research area [1]. In particular, alcohol oxidation is an essential transformation for which
sustainable processes are continually sought [2]. Oxidation by air or molecular oxygen represents
the cleanest, most sustainable option and the catalyst activity of several transition metals in this
transformation is well established [3]. In particular, since the Semmelhack report [4], the
efficiency of copper compounds, with the TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl radical)
co-catalyst, in the selective aerobic alcohol oxidations is unquestionably. Many highly efficient
copper catalysts have been developed [5,6,7,8,9], with new ligands [10,11,12,13], new TEMPO
variants [14], or new reaction conditions [15,16,17,18,19].
2

In continuation of our interest in exploring Cu/TEMPO-catalyzed aerobic oxidation of
alcohols [20], we looked for novel bidentate N,O-ligands for the preparation of new copper
catalyst precursors. Herein, we describe the synthesis, structure and DFT calculations of
compounds [M(LNNMePh)2] (M = Ni, 1; Cu, 2) containing the 2-N,O-2-(1-(2-methyl-2-
phenylhydrazono)ethyl)phenolate ligand, LNNMePh. These compounds are the first structurally
characterized transition metal complexes containing this ligand, according to a CSD search [21].
Copper complex [Cu(LNNMePh)2], 2, was tested as catalyst, in combination with TEMPO, for the
aerobic oxidation of benzyl alcohol, as model reaction. In contrast with other related systems
previously reported by us [20], only low yields of the corresponding aldehyde were observed by
using air at atmospheric pressure as the oxidant.
2. Experimental section
2.1. General
All preparations and other operations were carried out under aerobic conditions. Solvents
were purified and dried appropriately prior to use, using standard procedures. Chemicals were
obtained from commercial sources and used as supplied. Infrared spectra were recorded on a
Perkin-Elmer Spectrum Two spectrophotometer (pressed KBr pellets). NMR spectra were
recorded at the Centro de Investigaciones, Tecnología e Innovación (CITIUS) of the University
1
of Sevilla by using Bruker models Avance III 300 MHz spectrometer. 13C{1H} and H shifts
referenced to the residual signals of deuterated solvents. All data are reported in ppm downfield
from Si(CH3)4. Magnetic moment was measured in the solid state, at room temperature, with a
Sherwood Scientific (Cambridge Research Laboratory) magnetic balance. The gas
chromatograms (GC) were obtained using a Varian Chromatogram CP-3800 with nitrogen as the
carrier gas. The chromatogram used a Varian automatic injector, model CP-8410, flame
ionisation detector (FID), and a Varian column, model CP-8741. Elemental microanalyses (C, H,
N) were carried out on an Elemental LECO CHNS 93 analyzer by the Microanalytical Service of
the University of Sevilla (CITIUS).
3

2.2. Synthesis of 2-(1-(2-methyl-2-phenylhydrazono)ethyl)phenol, HLNNMePh
1-Methyl-1-phenylhydrazine (0.6 ml, 5 mmol) and a catalytic amount of p-toluenesulfonic
acid (5 mg, 0.03 mmol) were added to o-hydroxyacetophenone (0.62 mL, 5 mmol). The mixture
was left to react for 24 h at room temperature and then was purified by chromatography (SiO2,
toluene as eluent). The extract was dried with Na2SO4. After removal of the solvent under low
pressure, the hydrazone HLNNMePh was obtained as a yellow crystalline solid (340 mg, 1.5 mmol,
30 % yield). IR (KBr, , cm-1): 3418, 3059, 2877, 2807, 1944, 1640, 1599, 1492, 1448, 1364,
1303, 1251, 1221, 1158, 1097, 1029, 996, 961, 837, 751, 692, 648, 620, 561, 520, 437. 1H NMR
(CD2Cl2, 300 MHz): δ 2.45 (s, 3H, NC-CH3), 3.21 (s, 3H, NN-CH3), 6.89 (d, 1H, H-5), 6.95 (m,
1H, Hpara), 6.98 (m, 2H, Hortho), 7.03 (dd, J = 1.5, 7.6 Hz, 1H, H-4), 7.26 (m, 2H, Hmeta), 7.37 (dd,
13
J = 1.5, 7.2 Hz, 1H, H-3), 7.56 (dd, J = 1.5, 7.9 Hz, 1H, H-6), 13.5 (br s, OH). C{1H} NMR
(CDCl3, 75 MHz): δ 15.37 (NC-CH3), 43.64 (NN-CH3), 115.7 (Cpara), 118.0 (C-4), 118.9 (C-5),
119,0 (C-1), 121.1 (Cortho), 128.8 (C-6), 129.1 (Cmeta), 132 (C-3), 150.5 (Cipso), 160.5 (C-2), 171.9
1
(C=N). For the notation of H and 13C signals, based on heteronuclear and long range
heteronuclear correlations, see the scheme:
2.3. Synthesis of [M(LNNMePh)2] (M = Ni, 1; Cu, 2) complexes
To a solution of HLNNMePh (0.24 g, 1 mmol) in MeOH (25 ml) was added Et3N (0.14 mL,
1 mmol) and Ni(AcO)2·4H2O (0.13 g, 0.5 mmol). The resulting solution was stirred at room
temperature until the compound [Ni(LNNMePh)2] 1 precipitated. The brown solid was filtered,
4

washed with diethyl ether and dried (0.19 g, 72 % yield). X-ray quality single crystals of 1 were
obtained by crystallization from CH2Cl2. IR (KBr, , cm-1): 3417, 3061, 3029, 2925, 2813, 1923,
1637, 1597, 1560, 1499, 1438, 1349, 1328, 1257, 1247, 1134, 1109, 1033, 999, 942, 875, 846,
756, 743, 691, 624, 587, 533, 452. 1H NMR (CD2Cl2, 300 MHz): δ 2.27 (s, 3H, NC-CH3), 3.57 (s,
3H, NN-CH3), 5.75 (t, J = 7.2 Hz, 1H, H-6), 6.43 (t, J = 7.5 Hz, 1H, H-4), 6.87 (m, 2H, Hpara + H-
13
5), 7.12 (d, J = 7.8 Hz, 1H, Hortho5), 7.27 (m, 2H, H-3 + Hortho6), 7.38 (m, 2H, Hmeta). C{1H}
NMR (CDCl3, 75MHz): δ 18.8 (NC-CH3), 42.2 (NN-CH3), 111.9 (Cortho5), 112.2 (Cortho6), 115.1
(C-4), 117.9 (Cpara), 118.2 (C-1), 120.9 (C-6), 129.6 (Cmeta), 130.2 (C-3), 133.0 (C-5), 149.8
(Cipso), 164.9 (C-2), 173.9 (C=N). Elemental Anal. Calc. for C30H30NiN4O2: C, 67.06; H, 5.63; N
10.92. Found: C, 66.19; H, 5.57; N, 10.28 %.
Following a similar synthetic procedure as described for 1 but using Cu(AcO)2·H2O (0.10
g, 0.5 mmol) as reagent, the compound [Cu(LNNMePh)2] 2 was obtained as red crystals (0.22 g,
81% yield). X-ray quality single crystals of 2 were obtained by crystallization from CH2Cl2. IR
-1
(KBr, , cm ): 3417, 3062, 3017, 2888, 1833, 1637, 1599, 1560, 1522, 1498, 1440, 1350, 1237,
1129, 1110, 932, 868, 754, 693, 573, 525, 442. μeff = 1.61 μB at 298 K. Elemental Anal. Calc. for
C30H30CuN4O2: C, 66.46; H, 5.58; N 10.33. Found: C, 65.36; H, 5.55; N, 10.22 %.
2.4. General procedure for aerobic catalytic oxidation of benzyl alcohol
Reactions performed at atmospheric pressure were conducted in a 50 ml round bottom
flask or in vials of 20 ml capacity, at the desired temperature. The catalyst (0.025 mmol) and
TEMPO (0.05 mmol) were first charged to the flask and the toluene (2 mL) and the benzyl
alcohol (1.0 mmol) were then added and the mixture was stirred for the duration of the reaction.
When the reaction had finished, the reaction mixture was filtered, added an internal standard (50
l of dodecane) and diethyl ether up to 10 mL. The homogenized mixture was analyzed by gas
chromatography. At least, two determinations were carried out for each catalytic essay.
2.5. X-ray structural studies
A summary of the crystallographic data and structure refinement results for [M(LNNMePh)2]
(M = Ni, 1; Cu, 2) compounds is given in Table S1 (Supplementary material). Crystals of these
5

new compounds coated with dry perfluoropolyether were mounted on a glass fiber and fixed
under a cold nitrogen stream. The intensity data were collected on a Bruker-Nonius X8ApexII
CCD area detector diffractometer using Mo-K radiation source ( 0.71073 Å) fitted with a
graphite monochromator. The data collection strategy used was  and  rotations with narrow
frames (width of 0.50 degree). Instrument and crystal stability were evaluated from the
measurement of equivalent reflections at different measuring times and no decay was observed.
The data were reduced using SAINT [22] and corrected for Lorentz and polarization effects, and
a semiempirical absorption correction was applied (SADABS) [23]. The structures were solved
by direct methods using SIR-2002 [24a] and refined against all F2 data by full-matrix least-
squares techniques using SHELXL-2016/6 [24b] minimizing w[Fo2-Fc2]2. All the non-hydrogen
atoms were refined with anisotropic displacement parameters. The hydrogen atoms of
compounds 1 and 2 were included from calculated positions and allowed to ride on the attached
atoms with isotropic temperature factors (Uiso values) fixed at 1.2 times (1.5 times for methyl
groups) those Ueq values of the corresponding attached atoms.
2.6. Computational details
The electronic structure and geometries of the [M(LNNMePh)2] (M = Ni, Cu) compounds
were computed using density functional theory at the B3LYP level [25] and the 6-31G(d,p) basis
set was used for all the atoms. The optimized geometries of all the compounds were characterized
as energy minima by the nonexistence of imaginary frequencies (NImag = 0) in the
diagonalisation of the analytically computed Hessian (vibrational frequencies calculations). The
DFT calculations were performed using the Gaussian 09 suite of programs [26]. Cartesian
coordinates and energies of all optimized compounds are collected in Tables S3 and S4
(Supplementary Material).
3. Results and discussion
3.1. Synthesis and molecular structure of complexes [M(LNNMePh)2] (M = Ni, 1; Cu, 2)
6

The precursor ligand HLNNMePh was prepared by condensation of o-hydroxyacetophenone
and N,N-methylphenylhydrazine in the absence of solvent (Scheme 1) and it was isolated as a
yellow solid in medium yields. A brief mention to the synthesis of this compound has previously
appeared [27]. Analytical and spectroscopic data were in agreement with the proposed
formulation. The IR spectrum showed an intense absorption centered at around 1640 cm-1 that
1
was attributed to the C=N stretching vibration. The H NMR spectrum showed the methyl
resonances of the ketimine, N=C-Me, and the hydrazine, N-N-Me, as a singlets at 2.45 and 3.21
ppm, respectively, while the O-H proton appeared close to 14 ppm. This high downfield signal
shift suggested the presence of an intramolecular hydrogen bond with the nitrogen atom, as
described in other related systems [20a,b,28]. The imino carbon atom C=N resonates at around
172 ppm in the 13C{1H} NMR spectrum, while the aromatic C-H bonds showed the typical
pattern in both 1H and 13C{1H} NMR spectra.
Scheme 1
The reaction of the ligand precursor HLNNMePh with nickel acetate and copper acetate in
the presence of NEt3 afforded the corresponding complexes [M(LNNMePh)2] (M = Ni, 1; Cu, 2) in
good yields (Scheme 2). They were obtained as brown (M = Ni) and red (M = Cu) crystalline
solids. Evidence of coordination of the LNNMePh ligand came from the IR spectra in which the
C=N stretching vibration is slightly shifted to lower wavenumber than that found in the HLNNMePh
compound (1637 cm-1 for 1 and 2, respectively, with respect to 1640 cm-1 found in HLNNMePh).
1
13
[Ni(LNNMePh)2] was diamagnetic and a fully assignation of H and C resonances was achieved
with the help of 2D NMR experiments (HSQC and HMBC). Assignations are reported in the
7

experimental section. The most noteworthy NMR feature is the observation of two different
signals in the 1H NMR spectrum, at 7.12 and 7.27 ppm, for the ortho C-H hydrogen atoms of the
phenyl group joined to nitrogen, indicating that the rotation of the phenyl group around the C-N
bond is blocked. The corresponding signals of the ortho carbons appear at 111.9 and 112.2 ppm
in the 13C{1H} NMR spectrum (see Fig. S1 at the supplementary material). Compound
[Cu(LNNMePh)2] was paramagnetic with an effective magnetic moment of 1.61 μB at 298 K, which
was in agreement with the square-planar geometry of this complex (see below).
Scheme 2
Complexes [M(LNNMePh)2] (M = Ni, 1; Cu, 2) were structurally characterized by X-ray
crystallography. Their molecular structures are shown in Fig. 1, while selected bond distances
and angles are given in Tables 1 and 2 for 1 and 2, respectively. Both compounds show a four-
coordinate square planar environment around the metal centre, which is distorted for the copper
complex. In fact, the τ4 parameters [29] for 1 and 2 are 0 and 0.33º, respectively, while the related
τ4’ parameters [30] are 0 and 0.31º. The two LNNMePh ligands act as bidentate N,O-chelators and
lie in a pseudo-trans conformation. Each LNNMePh ligand creates with the metal a six-membered
metallacycle, which is close to the planarity for 2, but slightly folded for 1. These conformations
have been identified in related complexes as planar and step configurations, respectively (see Fig.
S2, Supplementary material) [31]. The dihedral angle between the two coordination planes
O-Ni-N is 0º for complex 1, in agreement with analogous structurally related Ni-derivatives [32].
By contrast, the same angle for 2 is 32.4º, in harmony with other structurally related
Cu-complexes [33]. As expected, the Ni-O1 and Ni-N1 bond lengths of 1.823(2) and 1.926(2) Å,
respectively, are slightly shorter than Cu-O1 and Cu-N1 ones (1.8681(11) and 2.0017(14) Å,
8

respectively). The relative orientation of the phenyl groups in both complexes is dissimilar,
showing complex 1 an up-down conformation, while for 2 is up-up (Fig. S2).
Fig. 1 Molecular structures of complexes [M(LNNMePh)2] (M = Ni, 1, left; Cu, 2, right).
3.2 DFT analysis of [M(LNNMePh)2] complexes (M = Ni, Cu)
DFT calculations were carried out at the B3LYP level of theory for [M(LNNMePh)2]
complexes (M = Ni, Cu) and Fig. 2 and S3 shows their optimized structures. The combination of
the selected method and basis sets provided a good structural description of both complexes;
according to the comparison of the structural parameters of the optimized geometries with those
of the crystal structures (see Table S2, Supplementary Material). In order to gain additional
information concerning the dissimilar phenyl conformations observed in the experimental
molecular structures, the conformers the up-up for nickel and up-down for copper were also
computed. The optimized resulting structures are shown in Fig. S4 (Supplementary Material).
The calculated energy difference between the two conformations is less than 1 kcal∙mol-1 for both
nickel and copper complexes (see Table S4) and consequently the adoption of one conformer
over the other is simply due by crystal-packing forces.
9

Table 1
Selected bond lengths (Å) and angles (°) for [Ni(LNNMePh)2] complex, 1.
______________________________________________________________________________
Ni(1)-O(1)
Ni(1)-N(1)
O(1)-C(1)
1.823(2)
1.926(2)
1.307(4)
1.423(3)
1.313(4)
N(1)#1-Ni(1)-N(1) 180.0
C(7)-N(1)-N(2)
C(7)-N(1)-Ni(1)
N(2)-N(1)-Ni(1)
C(10)-N(2)-N(1)
C(10)-N(2)-C(9)
N(1)-N(2)-C(9)
113.7(2)
127.3(2)
118.95(18)
116.8(2)
121.1(3)
113.9(2)
N(1)-N(2)
N(1)-C(7)
O(1)-Ni(1)-N(1)
91.29(10)
O(1)-Ni(1)-O(1)#1 180.0
O(1)-Ni(1)-N(1)#1 88.71(10)
______________________________________________________________________________
Symmetry transformations used to generate equivalent atoms: #1 -x+1,-y+2,-z.
Table 2
Selected bond lengths (Å) and angles (°) for [Cu(LNNMePh)2] complex, 2.
____________________________________________________________________________
Cu(1)-O(1)
1.8681(11)
1.8660(12)
2.0017(14)
2.0013(15)
1.305(2)
O(1)-Cu(1)-N(1)
N(3)-Cu(1)-N(1)
C(7)-N(1)-N(2)
C(7)-N(1)-Cu(1)
N(2)-N(1)-Cu(1)
C(10)-N(2)-N(1)
C(10)-N(2)-C(9)
N(1)-N(2)-C(9)
C(22)-N(3)-N(4)
C(22)-N(3)-Cu(1)
N(4)-N(3)-Cu(1)
C(25)-N(4)-N(3)
C(25)-N(4)-C(24)
N(3)-N(4)-C(24)
91.02(5)
Cu(1)-O(2)
158.85(6)
Cu(1)-N(1)
114.66(14)
128.33(12)
116.99(10)
114.09(13)
120.73(14)
113.05(14)
114.70(14)
128.50(12)
116.80(10)
114.09(13)
119.62(14)
112.92(13)
Cu(1)-N(3)
O(1)-C(1)
O(2)-C(16)
1.305(2)
N(1)-N(2)
1.4355(19)
1.305(2)
N(1)-C(7)
N(3)-N(4)
1.433(2)
N(3)-C(22)
1.304(2)
O(2)-Cu(1)-O(1)
O(2)-Cu(1)-N(3)
O(1)-Cu(1)-N(3)
O(2)-Cu(1)-N(1)
155.29(6)
91.17(6)
93.59(5)
93.22(6)
______________________________________________________________________________
11

Fig. 2 Optimized structures of [M(LNNMePh)2] complexes (M = Ni, left; Cu, right).
3.3 Benzyl alcohol oxidation catalyzed by complex [Cu(LNNMePh)2] 2
We recently described the good catalytic activity of compound [Cu(Lallyl)2] (Lallyl
=
N-allyl-o-hydroxyacetophenoniminato) in the TEMPO co-catalyzed aerobic oxidation of alcohols
with air at atmospheric pressure and room temperature [20a]. The similarity of [Cu(LNNMePh)2] 2
with the latter complex prompted to us to investigate the activity of 2 in the same oxidation
reaction. The benzyl alcohol and toluene were selected as the appropriate alcohol substrate and
solvent, respectively, for preliminary assays under the same reaction conditions previously
optimized for [Cu(Lallyl)2] (Scheme 3). Reactions were carried out on the 1 mmol scale in the
presence of a catalytic amount of the copper complex and TEMPO in 2 mL of solvent. The
optimum ratio of [Cu]:TEMPO was demonstrated to be 1:2 in previous related systems [20];
therefore, the ratio 1:0.05:0.025 for alcohol:TEMPO:catalyst was selected as optimal in order to
perform the catalytic essays.
Scheme 3
12

Preliminary results are shown in Table 3 with those known for complex [Cu(Lallyl)2] for an
appropriate comparison. Firstly, control experiments without complex 2 and without TEMPO
produced practically no detectable conversion (entries 1 and 2, respectively), establishing that the
presence of both the copper catalyst and TEMPO as co-catalyst was essential for oxidation to
take place. After 18 h at room temperature the conversion and the yield are low (both in
acetonitrile, entry 3, as in toluene, entry 4) and this situation contrast with that previously
observed for [Cu(Lallyl)2] (entry 5). Better results were also obtained previously by us for the
Cu/TEMPO-catalyzed oxidation carried out in scCO2 or in toluene with other related systems
[20b,c]. By increasing the temperature to 60 ºC the results do not improve (entry 6) being worse
than those reported for [Cu(Lallyl)2] (entry 7).
Table 3 Oxidation of the benzyl alcohol with air catalyzed by copper complexes in the presence
of TEMPO.a
Entry
Catalyst
-
Time (h)
Temperature (0C)
Yield(%)
1
2
3
4
5
6
7
18
18
18
18
18
6
25
25
25
25
25
60
60
0
2
[Cu(LNNMePh)2] 2 b
[Cu(LNNMePh)2] 2 c
[Cu(LNNMePh)2] 2
[Cu(Lallyl)2] d
[Cu(LNNMePh)2] 2
[Cu(Lallyl)2] d
8
10
84 d
14
84 d
6
a Reaction conditions: 0.025 mmol of copper complex, 0.05 mmol of TEMPO, 1 mmol of benzyl alcohol, 2 mL of
b
c
toluene. Air at atmospheric pressure. After work-up, yields were determined by GC. Without TEMPO. With
acetonitrile as solvent. d See reference 20a.
4. Conclusions
The precursor ligand HLNNMePh and the corresponding complexes [M(LNNMePh)2] (M = Ni,
1; Cu, 2) have been synthesized and spectroscopically characterized. [M(LNNMePh)2] complexes
have also been structurally characterized and they exhibit a four-coordinate environment around
the metal centre, where the two LNNMePh ligands act as bidentate N,O-chelators and exhibit a
pseudo-trans conformation. Copper complex [Cu(LNNMePh)2], 2, was tested as catalyst, in
13

combination with TEMPO, for the aerobic oxidation of benzyl alcohol, as model reaction.
Surprisingly, using air at atmospheric pressure as the oxidant only low yields of the
corresponding aldehyde were observed in toluene as solvent. These results are in contradiction
with those observed for other related systems previously described by us [20].
Acknowledgements
Financial support from the Junta de Andalucía (Proyecto de Excelencia FQM-7079) is
gratefully acknowledged. We thank to the Centro de Servicios de Informática y Redes de
Comunicaciones (CSIRC), Universidad de Granada, for providing the computing time.
Appendix A. Supplementary material
CCDC 1531575 (1) and 1531576 (2) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Crystallographic data for [M(LNNMePh)2] (M =
Ni, 1; Cu, 2) complexes, comparison of computed and experimental selected structural
parameters of compounds 1 and 2, and structures, coordinates and energies of the optimized
compounds are available. Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/...
14

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19

SYNOPSIS
Synthesis, structure and properties of nickel and copper complexes
containing N,O-hydrazone Schiff base ligand
a
a
a
María del Mar Conejo, Jorge Cantero, Antonio Pastor, Eleuterio Álvarez,b and Agustín
*
*
Galindoa
Complexes [M(LNNMePh)2] (M = Ni, 1; Cu, 2) were synthesized and characterized. These
compounds are the first structurally characterized transition metal complexes containing the
2-N,O-2-(1-(2-methyl-2-phenylhydrazono)ethyl)phenolate ligand, LNNMePh
.

HIGHLIGHTS
Synthesis, structure and properties of nickel and copper complexes
containing N,O-hydrazone Schiff base ligand
a
a
a
María del Mar Conejo, Jorge Cantero, Antonio Pastor, Eleuterio Álvarez,b and Agustín
*
*
Galindoa
 The compound HLNNMePh are prepared by a solvent-free synthetic procedure.
 Complexes [M(LNNMePh)2] (M = Ni, 1; Cu, 2) are synthesized and characterized.
 The first structurally characterized complexes with the LNNMePh ligand are reported.