Complexes of copper(ii) with 3-(ortho-substituted phenylhydrazo)pentane-2, 4-diones: Syntheses, properties and catalytic activity for cyclohexane oxidation

Article abstract of DOI:10.1039/c0dt01527j

Reactions of copper(ii) with 3-phenylhydrazopentane-2,4-diones X-2-C ₆H₄-NHNC{C(O)CH₃}₂ bearing a substituent in the ortho-position [X = OH (H₂L¹) 1, AsO₃H₂ (H₃

Full text of DOI:10.1039/c0dt01527j

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PAPER
Complexes of copper(II) with 3-(ortho-substituted
phenylhydrazo)pentane-2,4-diones: syntheses, properties and catalytic activity
for cyclohexane oxidation†
Maximilian N. Kopylovich,a Andreia C. C. Nunes,a Kamran T. Mahmudov,a Matti Haukka,b
Tatiana C. O. Mac Leod,a Lu´ısa M. D. R. S. Martins,a,c Maxim L. Kuznetsova and Armando J. L. Pombeiro*a
Received 3rd November 2010, Accepted 21st December 2010
DOI: 10.1039/c0dt01527j
Reactions of copper(II) with 3-phenylhydrazopentane-2,4-diones X-2-C6H4–NHN C{C( O)CH3}2
bearing a substituent in the ortho-position [X = OH (H2L1) 1, AsO3H2 (H3L2) 2, Cl (HL3) 3, SO3H
(H2L4) 4, COOCH3 (HL5) 5, COOH (H2L6) 6, NO2 (HL7) 7 or H (HL8) 8] lead to a variety of complexes
including the monomeric [CuL4(H2O)2]·H2O 10, [CuL4(H2O)2] 11 and [Cu(HL4)2(H2O)4] 12, the dimeric
[Cu2(H2O)2(m-HL2)2] 9 and the polymeric [Cu(m-L6)]n] 13 ones, often bearing two fused six-membered
metallacycles. Complexes 10–12 can interconvert, depending on pH and temperature, whereas the
Cu(II) reactions with 4 in the presence of cyanoguanidine or imidazole (im) afford the monomeric
compound [Cu(H2O)4{NCNC(NH2)2}2](HL4)2·6H2O 14 and the heteroligand polymer [Cu(m-L4)(im)]n
15, respectively. The compounds were characterized by single crystal X-ray diffraction (complexes),
electrochemical and thermogravimetric studies, as well as elemental analysis, IR, 1H and 13C NMR
spectroscopies (diones) and ESI-MS. The effects of the substituents in 1–8 on the HOMO–LUMO gap
and the relative stability of the model compounds [Cu(OH)(L8)(H2O)]·H2O, [Cu(L1)(H2O)2]·H2O and
[Cu(L4)(H2O)2]·H2O are discussed on the basis of DFT calculations that show the stabilization follows
the order: two fused 6-membered > two fused 6-membered/5-membered > one 6-membered
metallacycles. Complexes 9, 10, 12 and 13 act as catalyst precursors for the peroxidative oxidation (with
H2O2) of cyclohexane to cyclohexanol and cyclohexanone, in MeCN/H2O (total yields of ca. 20% with
TONs up to 566), under mild conditions.
spin-coating films3 or be applied for further organic synthesis.4
ortho-Hydroxy substituted ADB were effectively applied for the
Introduction
3-Phenylhydrazopentane-2,4-diones are azoderivatives of b-
diketones (ADB) (Scheme 1)1 potentially with a wide range of
applications,2 e.g. they can be used as optical recording media and
selective determination of copper(II) and iron(III) in various indus-
trial and natural objects.5 It was assumed that these compounds
can exist in several tautomeric forms, and that the tautomeric
equilibria can play an important role for applications e.g. as bistate
molecular switches6 or regulation of the selectivity of analytical
reactions.5b The hydrazo ꢀ enol-azo tautomeric transitions are
connected with a p-electron delocalization within a so-called
“resonance assisted hydrogen bond” (RAHB).6a,7 Usually, in the
solid phase all the structurally studied ADB are stabilized in the
hydrazo form,6,8 while in solution the investigated unsymmetrical
ADB can exist as a mixture of enol-azo and hydrazo tautomers.8e
In spite of the above interest in ADB compounds, their
coordination chemistry still remains underdeveloped, although
some complexes e.g. with copper(II), nickel(II) or sodium(I) have
been reported8b,c and we have synthesized8d,e a few copper(II)
compounds with ortho-hydroxy substituted phenylhydrazo-b-
diketones containing fused six- and five-membered metallacycles
(Scheme 2a). We can anticipate that the five-membered metalla-
cycle is under a higher tension than the six-membered cycle and
that the complexes with a six-membered ring may be more stable.
Scheme 1
aCentro de Qu´ımica Estrutural, Complexo I, Instituto Superior Te´cnico,
TU Lisbon, Av., Rovisco Pais, 1049–001, Lisbon, Portugal. E-mail:
pombeiro@ist.utl.pt
bUniversity of Joensuu, Department of Chemistry, P.O. Box 111, FIN-80101,
Joensuu, Finland
c
´
Area Departamental de Engenharia Qu´ımica, ISEL, R. Conselheiro Em´ıdio
Navarro, 1959-007, Lisboa, Portugal
† Electronic supplementary information (ESI) available. CCDC reference
numbers 798948–798954. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0dt01527j
2822 | Dalton Trans., 2011, 40, 2822–2836
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and studied the related 1 and 6–8, whose preparation and some
properties have been reported earlier.8,12
The ADB compounds can potentially exist in three (hydrazo,
enol-azo and keto-azo) tautomeric forms,5,8 but experimental data
testify that 1–8 are stabilized in the hydrazo form. In fact, their 1H-
NMR spectra in DMSO-d6 solution at room temperature show
only one signal at d 12.4–15.7 which can be assigned to the
proton of the protonated nitrogen atom adjacent to the aryl unit
Scheme 2
(
N–NH– hydrazo form), while the two methyl groups of the
This could be achieved by the introduction of a suitable functional
group, e.g. –AsO3H2, –SO3H, –COOCH3, –COOH or –NO2, into
the ortho-position of the aromatic part of the hydrazo moiety
(Scheme 2b), and such an approach was followed in the present
study.
pentane-2,4-dione moiety yield separate singlets on account of the
formation of an hydrogen bond between one carbonyl group and
the NH of the hydrazone moiety.6,8,12 Weak N–H ◊ ◊ ◊ O bonds are
known to give dN–H values in the 7–9 range, while chemical shifts of
12 ppm and higher are characteristic of a bifurcated (three-centre)
H-bond (Scheme 4).12 Thus, compounds 1–8 bear such a three-
centre H-bond in solution, what is also supported, in the solid
state, by X-ray diffraction analysis of 612a and by their IR data:
n(NH) 3069–3482 cm-1, n(C O) 1678–1638 cm-1, n(C O ◊ ◊ ◊ H)
1646–1601 cm-1 and n(C N) 1601–1575 cm-1.
On the other hand, some copper(II)–ADB complexes have been
successfully applied as catalysts for a few oxidation reactions,
in particular the peroxidative oxidation of cyclohexane.8d,e Func-
tionalization of saturated hydrocarbons under mild conditions
and using environmentally friendly oxidants is currently an
area of great interest.9 Different catalysts have been developed,
and, among them, copper-containing complexes10 are of a high
potential interest, since copper is present in the active sites of
several oxidation enzymes, including the multi-copper particulate
methane monooxidase (PMMO),11 is cheap and widespread in
Nature. Hence, it would be particularly attractive to check the
catalytic activity of the synthesized copper(II) complexes in the
above type of alkane oxidation reaction.
Thus, in this work we focused on the following aims: i) to include
–AsO3H2, –SO3H, –COOCH3, –COOH and –NO2 substituents
into the ortho-position of the aromatic ring of ADB; ii) to study
the influence of these substituents on the tautomeric balance
and coordination ability of the corresponding ligands, namely
towards copper(II), with the formation of two fused six-membered
metallacycles; iii) to study some physicochemical properties of
the ligands and complexes; iv) to check the potential of the
synthesized complexes for further transformations; v) to apply the
complexes as catalyst precursors for the peroxidative oxidation of
cyclohexane.
Scheme 4
By cyclic voltammetry at a Pt disc electrode, in 0.2 M
[nBu4N][BF4]/NCMe, at room temperature, the ADB compounds
1–8 exhibit one irreversible oxidation wave (Iox) and one irre-
versible reduction wave (Ired) (Fig. 1, for 8) at half-peak potential
(Ep/2ox and Ep/2red) values (versus SCE) given in Table 1. In the case
red
of 7, the reduction wave of the nitro group is observed at Ep
-0.92 V. Exhaustive controlled potential electrolysis (CPE) at a
=
potential slightly anodic (or cathodic, for the reduction) to that
Results and discussion
Synthesis and properties of 3-(ortho-substituted
phenylhydrazo)pentane-2,4-diones
The 3-(ortho-substituted phenylhydrazo)pentane-2,4-diones of
this study (Scheme 3) were prepared by a modified (see Experi-
mental) known1 aqueous diazotization of corresponding amines
with subsequent coupling with pentane-2,4-dione. Compounds 2–
5 are novel but, for comparative purposes, we also synthesized
Fig. 1 Cyclic voltammogram (n = 0.2 V s-1) of a 4.1 mM solution of 8,
initiated by the anodic sweep, in 0.2 M [nBu4N][BF4]/NCMe, at a platinum
disc electrode (d = 0.5 mm).
Scheme 3
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Table 1 Cyclic voltammetric data,a calculated vertical ionization poten-
tials and electron affinities for 1–8
oxidation potentials except for 4 which, according to the theory,
should be much harder to oxidize than was observed (Table 1).
There is no satisfactory correlation between the calculated vertical
electron affinities and the experimental reduction potential values,
the irreversible character of the reduction being conceivably the
reason of the discrepancy.
ox
red
Compound
Ep/2
IPb
Ep/2
EAc
1
2
3
4
5
6
7e
8
1.23
—
7.76
8.13
8.09
8.46
7.98
8.09
8.46
8.00
-0.87
0.95
1.14
1.19
1.62
1.14
1.23
1.80
1.05
d
d
—
1.65
1.36
1.59
1.61
1.80
1.49
-1.23
-1.27
-1.25
-1.46
-1.30
-1.35
It is known that azodyes, and in particular ADB, can be used for
optical data storage3 and that the knowledge of thermal properties
is important for such applications. Hence, differential thermal
analyses of 1, 2, 4, 6 and 8 were performed (Fig. S1 and S2,
Table S3†). Compound 1 is thermally stable until 470 K, when an
endothermic decomposition occurs with the maximum mass loss
at 527 K. 2 decomposes in three successive steps: the first one in
the 343–388 K range can be attributed to the loss of the hydrate
water molecules, the second one occurs at 466–504 K and the third
one with maximum mass loss at 575 K. 4 also decomposes in three
steps: dehydration at 356 K and decomposition in the 486–561 and
643–701 K intervals. 6 starts to decompose at 491 K, indicating
the absence of adsorptive water molecules. The calculated Gibbs
free energy of the decomposition of 6 (the calculation details are
given in ESI, Fig. S1 and S2, Table S3†) is mainly contributed to
by the enthalpy factor with an overall process similar to that of
1. The decomposition of 8 occurs at 414–517 K showing a strong
and sharp DTG peak at 499 K with a strong mass loss (82.5%)
and a high activation energy (Ea = 69.2 kJ mol-1). Some of the
ligands are quite thermally stable (1, 6 and 8), and this feature,
the high weight loss rate and the sharp thermal decomposition
threshold make the studied compounds potential candidates for
optical recording materials.3
a Values given in V 0.02 relative to the SCE, scan rate = 0.2 V s-1;
b calculated vertical ionization potentials in eV; c calculated vertical elec-
tron affinities in eV; d no wave has been observed under the experimental
conditions of this study; e an irreversible reduction wave is also observed
red
at Ep = -0.92 V due to the reduction of the nitro group.
of the peak potential indicates the occurrence of a single-electron
oxidation or of a two-electron reduction at the corresponding
waves, during the extended time scale of CPE.
In general, the values of the oxidation potential of 1–8 reflect
the electron-acceptor character of the substituent, the former
tending to increase with the latter, but no clear relationship
with the Hammett’s so or sp or any other related constant was
found, conceivably due to the irreversibility of the redox processes.
This contrasts with the case of the para-substituted ADB for
which we have recently reported13 a correlation between the polar
conjugation sp- substituent constant and the oxidation potential.
Quantum-chemical calculations performed at the B3LYP level
of theory demonstrate that the main contribution to the HOMO
of 1–8 comes from orbitals of the C(3) and N(9) atoms and of the
phenyl group (Fig. 2, Table S2†). The LUMO is centred on the
C(2), C(3), C(4), and N(9) atoms. Additionally, orbitals of N(7)
(1–6 and 8) or C(10) (2, 4–7) atoms and of the NO2 group (7) give
a noticeable contribution to the LUMO. The calculated vertical
ionization potential values roughly correlate with the experimental
Syntheses and characterization of copper(II) complexes
Copper(II) complexes derived from reactions with 2, 4, 6 were
isolated and characterized by elemental analysis, ESI-MS+ and IR
spectroscopy, single crystal X-ray diffraction and thermogravime-
try. A slow evaporation of a mixture of copper(II) nitrate hydrate
Fig. 2 Plots of the HOMO (a) and LUMO (b) of 8 with atom labelling scheme (c).
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and 2 in water furnishes greenish–black crystals of the dimeric
complex [Cu2(m-HL2)2(H2O)2] (9) (Scheme 5). Elemental analysis
and ESI-MS+ peak at 389.8 support the formulation, while IR
reveals n(OH), n(C O) and n(C N) at 3368, 1660 and 1557 cm-1,
respectively.
In the packing structure of 9 (Fig. S4†), the observed mode of
the molecular cross-linking is favoured by an inclination of neigh-
bouring molecules which, however, prevent aromatic interactions.
A different structural motif is found in the Cu(II) complex with
a H2L1 derived ligand,8e which crystallizes from methanol as the
dimer [Cu2(m-L1)2(CH3OH)2] with the methanol molecules located
on the vertex of the quadratic-pyramidal coordination sphere
of the copper(II) ion. In the latter case, the packing structure is
determined by the coordination behaviour of the water molecule,
leading to O–H ◊ ◊ ◊ O bonded molecular zigzag strands which are
further associated by the edge-to-face aromatic interactions.
The thermal behaviour of 9 relates to its structure. Under
heating, it decomposes in four consecutive steps (Fig. S3b,†
Table 3). The first one, in the 454–499 K range, with a strong and
sharp DTG peak at 465 K, most probably concerns the vigorous
elimination of the coordinated water molecules with an activation
energy of 58 kJ mol-1. In the second step (with 33.3% weight
loss), part of the ligand is destroyed with an endothermic peak
at 535 K. The third and fourth steps of the decomposition bear
high positive D‡G values in the 590–622 and 711–716 K ranges,
respectively. All the steps proceed with a negative D‡S suggesting
a low rate of the thermal decomposition.14
A slow evaporation of a solution of copper(II) nitrate hydrate
and 6 in acetone–water (4 : 1, v/v) furnishes greenish–black
crystals of 13 (Scheme 5). Elemental analysis and IR spectrum
support the formulation of [Cu(m-L6)]n, which is also confirmed
by X-ray diffraction analyses. Hence, complex 13 is a 1D polymer
formed by coordination of a carboxyl group to copper(II) (Fig. 3b)
containing a crystallographically imposed centre of inversion.
The coordination sphere of the penta-coordinated copper(II)
is best described as distorted square-pyramidal. The copper
atom is shifted towards the apical position above the basal
O(5)–N(1)–O(7)–O(6i) plane; this apical position is occupied by
a coordinated carboxyl group of an adjacent molecule, thus
one monomer is perpendicular to other. The copper(II) ion
belongs to three different metallacycles: the first C(12)–O(6)–
Cu(1)–O(7) core, which bridges to the neighbour molecule,
and Cu(1)–N(1)–N(2)–C(7)–C(10)–O(5) and Cu(1)–N(1)–C(5)–
C(1)–C(12)–O(7) two six–membered fused metallacycles. The
Scheme 5
The dimer 9 crystallizes in the triclinic space group P–1
comprising of one half of the complex molecule in the asymmetric
entity of the unit cell (Fig. 3a). The coordination environment
Cu(1)O(1)O(1)O(5)N(1) of the copper ion is of a distorted
square planar geometry with the Cu–O(1), Cu–O(5) and Cu–N(1)
˚
distances being 1.9760(19), 1.900(2) and 1.991(2) A, respectively
(Table 2). The pentacoordinated copper(II) atoms belong to
three different metallacycles: one central planar endo Cu2O2
core and two fused six-membered exo rings. In the Cu2O2 core
˚
the Cu1 and Cu1#1 centroid ◊ ◊ ◊ centroid distance is of 3.109 A.
To diminish any intramolecular strain, the aromatic rings are
arranged approximately perpendicular (83.5◦) with respect to
the CuO2N plane. The complex formation causes a decrease in
˚
the C(10)–O(5) bond length (1.255(3) A), whereas the C(10)–
˚
C(7) bond (1.444(4) A) is significantly lengthened, indicating
a change of the p-electron distribution within the pentane-2,4-
˚
dione fragment. C(8)–O(4) (1.225(4) A) is shorter than C(10)–O(5)
˚
(1.255(3) A) due to coordination of O(5) to copper(II). In addition,
the free carbonyl oxygen is associated with the coordinated water
˚
hydrogen H(O6) of an adjacent molecule (H ◊ ◊ ◊ O C, 2.741 A).
Fig. 3 Thermal ellipsoid plots, drawn at the 50% probability level, of the complexes 9 (a) and 13 (b).
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Table 2 Selected structural parameters (distances A, angles ◦) of compounds 9–15
˚
9
10
11
12
Cu(1)–O(5)
Cu(1)–O(1)
Cu(1)–N(1)
Cu(1)–O(6)
Cu(1)–Cu(1)#1
O(5)–C(10)
O(4)–C(8)
O(5)–Cu(1)–O(1)
O(5)–Cu(1)–N(1)
O(1)–Cu(1)–N(1)
O(5)–Cu(1)–O(6)
O(1)–Cu(1)–O(6)
N(1)–Cu(1)–O(6)
O(1)–Cu(1)–O(1)#1
Cu(1)–O(1)–Cu(1)#1
1.900(2)
1.9760(19) Cu(1)–O(1)
Cu(1)–O(5)
1.9001(9)
1.9560(9)
1.9616(10) Cu(1)–O(7)
Cu(1)–O(5)
Cu(1)–N(1)
1.959(4)
1.962(5)
1.987(5)
2.013(5)
2.308(4)
85.36(18)
87.11(18)
172.5(2)
Cu(1)–O(7)
Cu(1)–O(6)
Cu(1)–O(5)
N(1)–N(2)
N(1)–C(6)
N(2)–C(7)
O(5)–C(10)
O(4)–C(8)
1.9434(12)
1.9644(12)
2.4489(11)
1.3077(19)
1.409(2)
1.316(2)
1.2388(19)
1.225(2)
87.61(5)
84.74(4)
95.79(4)
118.97(13)
121.63(14)
1.991(2)
2.257(2)
3.109(2)
1.255(3)
1.225(4)
157.40(9)
90.56(9)
100.10(8)
101.75(8)
97.41(8)
93.73(8)
77.40(8)
102.60(8)
Cu(1)–N(1)
Cu(1)–O(7)
Cu(1)–O(6)
1.9908(9)
2.2462(9)
171.85(4)
89.91(4)
95.96(4)
87.54(4)
85.29(4)
164.37(4)
92.25(4)
93.07(4)
92.93(4)
102.57(4)
Cu(1)–O(6)
Cu(1)–O(1)
O(5)–Cu(1)–O(1)
O(5)–Cu(1)–N(1)
O(1)–Cu(1)–N(1)
O(5)–Cu(1)–O(7)
O(1)–Cu(1)–O(7)
N(1)–Cu(1)–O(7)
O(5)–Cu(1)–O(6)
O(1)–Cu(1)–O(6)
N(1)–Cu(1)–O(6)
O(7)–Cu(1)–O(6)
O(5)–Cu(1)–N(1)
O(5)–Cu(1)–O(7)
N(1)–Cu(1)–O(7)
O(5)–Cu(1)–O(6)
N(1)–Cu(1)–O(6)
O(7)–Cu(1)–O(6)
O(5)–Cu(1)–O(1)
N(1)–Cu(1)–O(1)
O(7)–Cu(1)–O(1)
O(6)–Cu(1)–O(1)
153.18(17) O(7)–Cu(1)–O(6)
93.44(19)
93.3(2)
O(7)–Cu(1)–O(5)
O(6)–Cu(1)–O(5)
110.18(15) N(2)–N(1)–C(6)
90.04(16)
92.48(17)
96.61(16)
N(1)–N(2)–C(7)
N(1)–H(1 N) ◊ ◊ ◊ O(5)
N(1)–H(1 N) ◊ ◊ ◊ O(1)
132.6
122.5
13
14
15
Cu(1)–O(7)
Cu(1)–O(7)#1
Cu(1)–O(5)
Cu(1)–N(1)
Cu(1)–O(6)
O(4)–C(8)
O(5)–C(10)
O(7)–C(12)
N(1)–N(2)
N(1)–C(6)
N(2)–C(7)
O(7)–Cu(1)–O(5)
O(7)–Cu(1)–N(1)
O(5)–Cu(1)–N(1)
O(7)–Cu(1)–O(6)
O(5)–Cu(1)–O(6)
1.875(3)
2.426(3)
1.895(3)
1.951(4)
2.037(3)
1.227(5)
1.248(5)
1.275(5)
1.300(5)
1.438(6)
1.338(6)
Cu(1)–N(3)
Cu(1)–O(6)
Cu(1)–O(8)
O(5)–C(10)
O(4)–C(8)
N(1)–N(2)
N(2)–C(7)
N(3)–C(12)
N(4)–C(12)
N(4)–C(14)
N(1) ◊ ◊ ◊ O(5)
1.953(2)
1.9838(18) Cu(1)–O(1)
2.3956(19) Cu(1)–N(3)
Cu(1)–O(5)
1.9192(18)
1.9754(17)
1.980(2)
1.984(2)
2.2981(18)
1.339(3)
1.220(3)
1.260(3)
91.10(8)
92.85(8)
91.03(8)
92.51(7)
90.48(8)
88.36(8)
175.54(8)
104.55(11)
1.246(3)
1.227(4)
1.312(3)
1.323(4)
1.150(3)
1.306(4)
1.350(3)
2.565(3)
90.96(9)
89.32(8)
88.63(7)
152.6
Cu(1)–N(1)
Cu(1)–O(2)#1
N(2)–C(7)
O(4)–C(8)
O(5)–C(10)
O(1)–Cu(1)–N(1)
N(3)–Cu(1)–O(2)#1
O(1)–Cu(1)–N(3)
O(1)–Cu(1)–O(2)#1
O(5)–Cu(1)–N(1)
O(5)–Cu(1)–N(3)
O(5)–Cu(1)–O(1)
O(1)–S(1)–C(1)
171.76(13) N(3)–Cu(1)–O(6)
94.65(13)
92.57(14)
85.78(12)
86.85(13)
N(3)–Cu(1)–O(8)
O(6)–Cu(1)–O(8)
N(1)–H(1 N) ◊ ◊ ◊ O(5)
N(2)–C(7)–C(10)
124.0(3)
Symmetry transformations used to generate equivalent atoms: #1 x-1,y,z
Table 3 Parameters of thermal decomposition of some copper(II)–ADB complexes
Temperature
DTG peak
Compound
interval (K) Weight loss (%) temperature (K) A (s-1)
Ea (kJ mol-1) D‡H (kJ mol-1) D‡S (J K-1mol-1) D‡G (kJ mol-1)
Cu2(l-L1)2(CH3OH)2 470–531
16.6
33.7
4.1
33.3
4.7
3.4
16.9
37.9
524
601
465
535
599
714
536
580
4.6
38.3
34.0
61.9
53.7
97.4
2.0
58.1
86.9
92.3
-237
-188
-202
-111
-310
-215
-146
-141
158
175
148
157
188
212
165
174
8e
567–608
1.8 ¥ 108 66.9
2.7 ¥ 102 57.6
1.7 ¥ 107 102
8.2 ¥ 10-4 7.0
9
454–499
499–537
590–622
711–716
527–575
574–617
83
64.1
10
13
2.6 ¥ 105 91.3
5.31 ¥ 105 97.1
O(5)–Cu(1)–N(1) and the N(1)–Cu(1)–O(7) angles are of 92.57(14)
and 94.65(13)◦, respectively. Also, the external O(7)–Cu(1)–
O(6) and the O(5)–Cu(1)–O(6) angles are approximately similar
(85.78(12) and 86.85(13)◦, respectively). Hence, a constitutional
modification by an ortho-substituted carboxylic group does
markedly affect the conformational solid-state structure inherent
in the hydrazone framework, and actually helps to stabilize the
1D geometry of the molecule by additional coordination, as well
as its supramolecular lattice structure (Fig. S4e†). In accordance
with the structure, the coordination polymer 13 is thermally more
stable than 9, decomposing in one step at 574–617 K (Fig. S3d,†
Table 3).
The reaction of copper(II) with 4 in water at pH 6 and
393 K allowed the preparation of the mononuclear ADB complex
[CuL4(H2O)2]·H2O (10) (Scheme 6, Route I), which crystallized
directly from the reaction mixture as cubic deep green crystals.
Analytical data support the formulation and X-ray crystallogra-
phy (Fig. 4a) shows a distorted trigonal-bipyramidal coordination,
with two positions being occupied by the O(6) and O(7) oxygen
atoms of water molecules. The presence of the ortho-substituent
at the aromatic ring creates one more six-membered metallacycle
with copper(II). On the other hand, steric effects caused by the
coordinated water molecules lead to anomalous Cu–O distances
˚
˚
of Cu–O(6) (2.2462(9) A) and Cu–O(7) (1.9908(9) A). The
2826 | Dalton Trans., 2011, 40, 2822–2836
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Fig. 4 Thermal ellipsoid plots, drawn at the 50% probability level of the complexes 10 (a), 11 (b), 12 (c), 14 (d) and 15 (e). In 15 the imidazole ring was
disordered over two sites. The disorder was omitted from the drawing.
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Scheme 6
six-membered chelate rings exhibit a slightly distorted half chair
conformation with the copper atom laying outside the plane
defined by the pentane-2,4-dione moiety. The aromatic rings of
the ligands adopt an angle of 58.8◦ with reference to the basal
plane of the coordination polyhedron. The crystal packing of
the complex is characterized by layers of molecules extending in
parallel to the crystallographic a plane. The overall layer structure
(Fig. S4b†) can be regarded as a two dimensional coordination
polymer in which the molecules are nested in such a way that a
close packing results. As all of the strong acceptors are involved
in the complexation, only weak aromatic p-interactions between
copper(II) are occupied by one carbonyl oxygen, one hydrazone
nitrogen, one oxygen of a sulfo group and the oxygens of
two water molecules, and two six-membered metallacycles are
˚
formed. The Cu(1)–O(5) distance (1.959(4) A) is shorter than
˚
the Cu(1)–O(1) one (2.308(4) A), while the C(10) O(5) distance
˚
˚
(1.256(7) A) is longer than C(8) O(4) (1.244(7) A) due to the
coordination of the former moiety to copper. Comparison of
the O(5)–Cu(1)–O(1) angles in 11 and 10 shows a dramatic
increase from 110.18(15)◦ to 171.85(4)◦ due to the interaction
with hydrate water in the latter compound. A regular geometric
configuration was found, with N(1)–Cu(1)–O(6), O(7)–Cu(1)–
O(6), N(1)–Cu(1)–O(1), O(7)–Cu(1)–O(1) and O(6)–Cu(1)–O(1)
angles of 93.44(19), 93.3(2), 90.04(16), 92.48(17) and 96.61(16)◦,
respectively. The crystal structure of 11 is stabilized by strong
hydrogen bond interactions via coordinated water molecules and
the uncoordinated carbonyl group (Fig. S4c†).
˚
the sulfo residues (plane–plane distance of 4.7 A) of consecutive
layers exist. Under heating, complex 10 decomposes in one step,
which occurs in the 527–575 K range (Table 3, Fig. S3c†) with
a strong and sharp DTG peak at 536 K, indicating a rapid and
vigorous decomposition with activation energy of 91.3 kJ mol-1
and a negative D‡S value.
Curiously, if the reaction of copper(II) with 4 is performed
at pH 9 (NH4OH) and at room temperature, the greenish–
black compound [CuL4(H2O)2] 11 (related to 10 but without
crystallization water) is formed (Scheme 6, Route II), while at pH 2
(HCl) the reaction gives another di-ADB monomeric complex,
[Cu(HL4)2(H2O)4] 12 (Scheme 6, Route III). In the structure of
11 (Fig. 4b), similarly to 10, the coordination positions of the
The bulky light greenish–yellow crystals of 12 are of the
2+
monoclinic space group P21/n. The Cu(H2O)4 unit bridges
-
two unideprotonated (SO3 ) ligand molecules (Scheme 6, Fig. 3)
which coordinate via carbonyl groups and do not chelate to
copper(II) since the RAHB H-bond systems N(1)–N(1H)–O(5)
and N(1)–N(1H)–O(1) (Scheme 4) remain intact. The N(1)–
˚
O(5) and N(1)–O(1) distances are 2.6235(18) and 2.9513(18) A,
respectively in two six-membered hydrogen bonded cycles. The
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positive charges of the copper(II) are neutralized by the negative
pentacoordinated copper(II), is formed (Scheme 6, Route IX). The
coordination of one oxygen atom of each sulfo group (Cu(1)–
˚
charges of the sulfo groups. The Cu(1)–O(6) (1.9644(12) A) and
˚
˚
Cu(1)–O(7) (1.9434(12) A) distances are shorter than the Cu(1)–
O(2)#1 distance of 2.2981(18) A) is involved in the polymer for-
˚
O(5) distance (2.4489(11) A), possibly due to steric hindrance of
mation; the O(1)–Cu(1)–O(2)#1 and N(3)–Cu(1)–O(2)#1 angles
are 92.51(7)◦ and 92.85(8)◦, respectively (Fig. 4e). The imidazole
is coordinated perpendicularly to the O(1)–Cu(1)–O(5) plane with
O(1)–Cu(1)–N(3) and N(3)–Cu(1)–O(5) angles of 91.03(8)◦ and
88.36(8)◦, respectively. The metal pertains to two different six-
membered metallacycles, Cu(1)–O(1)–S(1)–C(1)–C(6)–N(1) and
Cu(1)–O(5)–C(10)–C(7)–N(2)–N(1), with O(5)–Cu(1)–N(1) and
O(1)–Cu(1)–N(1) angles of 90.48(8)◦ and 91.10(8)◦, respectively.
The C(10)–O(5) distance is larger than for the uncoordinated
the coordinated water molecules. The overall crystal structure is
stabilized by strong hydrogen bonding interactions between the
coordinated water hydrogens and both the sulfo- and ketone
groups in adjacent units (Fig. S4d†). Curiously, at pH 2, even
at elevated temperatures (393 K), the reaction of copper(II) with
4 gives only 12, indicating that for the destruction of the RAHB
system one has to use an alkali medium.
Apart from the above described reactions, the interconversions
between 12, 11 and 10 (Scheme 6, Routes IV–VII) were also
confirmed (see Experimental). Thus, the conversion of the diligand
monomer 12 to the monoligand monomers 10 and 11 or vice versa
was found to be pH and temperature dependent. These factors
are essential for the destruction of the RAHB system, allowing
the copper(II) ions to enter the ONO chelating pocket of the
ligand. This idea can be extended further by addition of an extra
ligand other than ADB. Thus, when 4 and cyanoguanidine are
added simultaneously to a copper(II) solution at pH 6 and room
temperature, 4 does not coordinate to copper(II) keeping its RAHB
intact and only cyanoguanidine binds to the metal through its
cyano group yielding [Cu(H2O)4{NCNC(NH2)2}2](HL4)2·6H2O
14 (Scheme 6, Route VIII). In 14, the positive charges of copper(II)
are compensated by the negative charges of two sulfonate groups
of the deprotonated 4. The H-bond found in the RAHB N(1)–H(1
˚
carbonyl C(8)–O(4) (1.260(3) and 1.220(3) A, respectively), due
to the coordination of O(5). The packing of 15 is stabilized via
strong intermolecular interactions (Fig. S4g†).
To estimate the relative stability of the various types
of complexes, theoretical calculations of the model
species [Cu(OH)(L8)(H2O)]·H2O, [Cu(L1)(H2O)2]·H2O and
[Cu(L4)(H2O)2]·H2O (Scheme 7) were performed. The
calculations indicate that the negative DGs value of the
formation of [Cu(L4)(H2O)2]·H2O bearing two fused six-
membered metallacycles is the highest (-37.5 kcal mol-1, see
computational details). The formation of [Cu(L1)(H2O)2]·H2O is
less exoergonic (DGs = -24.0 kcal·mol-1) indicating a lower stability
of the five-membered cycle compared to the six-membered one.
[Cu(OH)(L8)(H2O)]·H2O is the least stable complex (the DGs of
formation is -7.7 kcal mol-1) due to the presence of only one
metallacycle in its structure.
˚
N) ◊ ◊ ◊ O(5), with a N ◊ ◊ ◊ O distance of 2.565(3) A, falls within
the N ◊ ◊ ◊ O 2.50–2.62 A distance range observed in previous
The redox properties of [Cu2(m-L1)2(CH3OH)2],8e 9, 10 and
13 have been investigated by cyclic voltammetry, at a Pt disk
electrode, in a 0.2 M [nBu4N][BF4]/NCMe or DMSO solution,
at room temperature. Complexes [Cu2(m-L1)2(CH3OH)2] and 10
exhibit two single-electron (per metal atom) irreversible reduction
waves (Fig. 5), assigned to the CuII → CuI (wave I) and CuI
→ Cu◦ (wave II) reductions, at the reduction peak potential
˚
studies on ADBs.6,8,12,13 Due to the hydrogen bonding involving
O(5), the C(10)–O(5) bond length is longer than that of unco-
˚
˚
ordinated C(8)–O(4) (1.246(3) A and 1.227(4) A, respectively).
The coordination sphere of copper(II) is formed by four coor-
dinated water molecules and two cyanoguanidines. The C(12)–
N(4) distance (1.306(4) A) is shorter than the N(4)–C(14) distance
(1.350(3) A) due to the p-delocalization along N(6)[or N(5)]–
C(14) N(4)–C(12) N(3)→Cu. The angles around copper(II),
N(3)–Cu(1)–O(6), N(3)–Cu(1)–O(8) and O(6)–Cu(1)–O(8), are
90.96(9)◦, 89.32(8)◦ and 88.63(7)◦, respectively.
Another situation is observed when copper(II) reacts with 4 in
the presence of imidazole (im). This facilitates the destruction
of the RAHB, copper(II) enters into the chelating ONO pocket
of 4 and the coordination polymer [Cu(m-L4)(im)]n 15, with
˚
red
values given in Table 4 (I Ep/2 in the range -0.44 to -0.05 V
˚
II
red
vs. SCE, and
E
between -1.08 and -1.48 V vs. SCE). In
p/2
addition, a new irreversible anodic wave (wave a), at ca. -0.15 V
vs. SCE is observed upon scan reversal after the first reduction
wave. It plausibly corresponds to the oxidation of the CuI species
formed at the first reduction process. The sharp shape of the
wave indicates that such a CuI species is adsorbed at the electrode
surface.
Scheme 7
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Table 4 Cyclic voltammetric dataa for some copper(II) complexes with
COOH, respectively, see Table 1) and were not investigated further.
Apparently, there are not any correlations between Hammett’s
and related substituents constant and the oxidation or reduction
potentials. In the case of 9, no wave has been observed under the
experimental conditions of this study.
ADB
red
II
red
ox
oxb
Complexes
X
IEp/2
E
IEp/2
Ep
p/2
[Cu2(m-L1)2(CH3OH)2] OH
-0.44
-0.05
-1.08
-1.48
—
1.16
1.26
1.11
-0.15
-0.17
—
10
13
SO3H
COOH -0.96
Catalytic activity of the copper complexes
a Values given in V 0.02 relative to the SCE, scan rate = 0.2 V s-1. b Anodic
wave (a) generated upon scan reversal following the first reduction wave.
Complexes 9, 10, 12 and 13 act as catalysts or catalyst precursors
for the oxidation of cyclohexane, in acetonitrile, to a cyclohexanol
and cyclohexanone mixture, by aqueous hydrogen peroxide in
acidic medium at room temperature (Scheme 8) with a total yield
of ca. 20% and turnover number (TON) values up to 566 moles of
products per mole of catalyst, for a single batch (Table 5). Control
reactions carried out in the absence of the metal complex catalyst
and with or without the free ligands 2, 4 and 6 (entries 8 and 9),
indicate that no cyclohexane oxidation reaction then occurs, while
the use of Cu(NO3)2 (entry 10) leads to an overall yield of only 5%.
Scheme 8
The higher activity of 12 may be accounted for by the presence
of the four labile water ligands and lower steric hindrance around
the metal centre. The activities of 9 and 13 (dimer and polymer,
respectively) are similar under standard conditions (product yields
ca. 16%, Table 5, entries 1 and 4), slightly higher than that of the
mononuclear complex 10 (entry 2) and lower than that of [Cu2(m-
L1)2(CH3OH)2]8e (entry 7). Dinuclear copper peroxo complexes
can be involved in those types of reactions,15,16,17a and the dimer 9
and polymer 13 bear relatively close copper(II) atoms supported
by ADB bridging ligands, that can assist the formation of such a
type of peroxo species.
Fig. 5 Cyclic voltammogram (n = 0.2 V s-1) of a 1.7 mM solution
of [Cu2(m-L1)2(CH3OH)2], initiated by the cathodic sweep, in 0.2 M
[nBu4N][BF4]/NCMe, at a platinum disk electrode (d = 0.5 mm).
It is known that the peroxidative oxidation of cyclohexane cat-
alyzed by copper(II) complexes usually proceeds more efficiently in
an acidic medium.17 Accordingly, the addition of an acid promoter
The occurrence of a single-electron reduction per CuII (or
CuI) ion has been confirmed by exhaustive controlled potential
electrolysis (CPE) at a potential slightly cathodic to that of the
peak potentials of wave I (or II). CPE at any of the reduction
waves corresponds to a charge consumption of 1 F mole-1 of
complex for 10 or 2 F mole-1 of complex for the dinuclear [Cu2(m-
L1)2(CH3OH)2] compound. The cathodically generated CuI species
appear to be stable in the solvent/electrolyte medium along the
CPE since the corresponding CuI/II oxidation wave is observed at
the end of the electrolysis (Table 4). The CPE performed at the
second reduction wave leads to the deposition of metallic copper.
Moreover, for the dinuclear complex [Cu2(m-L1)2(CH3OH)2] no
metal–metal electronic interaction has been detected, since any of
the cathodic waves involves the reduction of the two metal ions,
without differentiation of distinct waves at different potentials. For
complex 13, only one irreversible reduction wave was observed at
-0.96 V vs. SCE involving 2e per metal ion as indicated by CPE.
The compounds show irreversible or partially reversible
oxidation waves at ca. 1.2 V (Fig. 5 for complex [Cu2(m-
L1)2(CH3OH)2]) which can involve the 3-(ortho-substituted
phenylhydrazo)pentane-2,4-dione ligands (when uncoordinated,
they undergo irreversible oxidations at comparable potentials, e.g.
Table 5 Peroxidative oxidation of cyclohexane to cyclohexanol and
cyclohexanonea
Yieldb of products,%
Entry Catalyst
Alcohol Ketone Totalc
TONd
1
2
3
4
5
6
7
8
9e
6.9
4.3
9.3
7.2
4.4
8.9
7.1
—
9.9
7.1
10.9
9.1
7.0
10.9
16.6
—
16.8
11.4
20.2
16.3
11.4
19.8
23.7
34.2
11.4
35.5
16.7
326
566
23.7
—
10e
12e
13e
10f
12f
[Cu2(m-L1)2(CH3OH)2]
g
—
2,4 or 6
Cu(NO3)2
—
—
g
9
10
—
1.6
—
3.8
—
5.4
5.4
a Selected data; reaction conditions: C6H12 (1 mmol), Cu-catalyst (see
e
footnotes and f), MeCN/H2O 4 mL, n(HNO3)/n(Cat) = 10, H2O2
(10 mmol added as an aqueous 30% solution), 6 h reaction time, 298 K;
b Moles of product/100 moles of C6H12 (Alcohol = cyclohexanol, Ketone =
cyclohexanone); c Cyclohexanol + cyclohexanone; d Overall TON values
(moles of products/mole of catalyst); e 4.9–10.0 mmol; f 0.35 mmol; g Traces,
< 0.3%.
ox
at Ep/2 1.23, 1.36 V and 1.61 vs. SCE for X = OH, SO3H or
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(typically HNO3) leads to higher activities for both 10 and 12,
which achieve the optimum values for the acid-to-catalyst molar
ratio of ca. 10 : 1 (Table S4 and Fig. S5†). A further increase in
the acid additive amount (acid-to-catalyst molar ratio within the
10–60 range) does not result in a higher activity in the case of 10
and even lowers the performance of 12. However, for 9 and 13, the
presence of acid does not appreciably affect the product yields,
which remain practically constant in the n(HNO3)/n(catalyst)
range of 0–60 (Fig. S5†). Although the role of the acid co-catalyst
remains not fully established, it presumably can be associated
with the resulting unsaturation of the metal centre upon ligand
protonation, the enhancement of the oxidative properties of the
metal catalyst, and the hampering of the decomposition of H2O2
to water and oxygen.17
influence their coordination and other properties (e.g. dN–H or
redox potentials) and, depending on X and on the reaction con-
ditions with copper(II), different metal–ADB monomers, dimers
and polymers are formed. The versatility of those compounds as
ligands is well illustrated by the tunable synthesis of the variety of
complexes we have prepared herein with different geometries and
nuclearities. By taking advantage of the coordination ability of the
ortho-substituent of ADB, particually stable complexes with two
fused six-membered metallacycles can be synthesized.
The study also shows that the Cu–ADB complexes act as
efficient and selective catalysts for the peroxidative oxidation of
cyclohexane to the corresponding alcohol and ketone in aqueous
MeCN medium, the ADB thus behaving as quite adequate ligands
for such metal catalyzed reactions. Their use deserves to be further
explored for other types of oxidation catalysis and/or other metals.
Other important factors in the performance of the system
concern the relative amounts of oxidant (hydrogen peroxide) and
catalyst. An increase in the peroxide-to-catalyst molar ratio results
in an yield enhancement (Fig. S6†), e.g. from 7–10 to 12–22% upon
changing that ratio from 500 to 1250 (H2O2 amount increase from
5.0 to 12.5 mmol). A decrease of the catalyst amount below the
typical value of 5–10 mmol results in comparable yields and in
enhancements of the overall TON e.g. from 11 or 36 up to 326 or
566, for complex 10 or 12, respectively (Table 5, entries 2,3,5,6).
The cyclohexane oxidation appears to proceed mainly via
radical mechanisms involving both carbon-centred and oxygen-
centred radicals in view of the pronounced decrease (by ca. 88–
70%) of the catalytic activity when the experiments are performed
in the presence of either a carbon radical trap (e.g., CBrCl3)
or an oxygen radical trap (e.g., Ph2NH).17d Hence, although
the detailed mechanistic pathway is still to be established, it
can possibly proceed, as suggested in other cases,10,17,18 through
H-abstraction from cyclohexane, conceivably by the hydroxyl
radical HO∑ (formed by metal-assisted decomposition of H2O2)
to form the cyclohexyl radical Cy∑. Reaction of Cy∑ with O2
gives the organoperoxyl CyOO∑, or with a metal–hydroperoxo
species yields cyclohexyl hydroperoxide (CyOOH) which can
also be formed upon H-abstraction from H2O2 (or derived
HOO∑) by CyOO∑.10c–e,17a,e–h,18i The organohydroperoxide CyOOH
can undergo metal-assisted decomposition to alkoxyl (CyO∑,
upon O–O bond cleavage) and alkylperoxyl (CyOO∑, upon O–
H bond rupture) radicals from which the final oxygenates can
be formed.10c–e,17a,c,18a,e–h The involvement of CyOOH is recognized
by the increase in the amount of CyOH with the corresponding
decrease of the cycloketone upon treatment of the final reaction
solution with an excess of PPh3 prior to the GC analysis, following
the method reported by Shul’pin.18a,b,f
Experimental
Materials and instrumentation
The 1H and 13C NMR spectra were recorded at room temperature
on a Bruker Avance II + 300 (UltraShieldTM Magnet) spectrometer
operating at 300.130 and 75.468 MHz for proton and carbon-13,
respectively. The chemical shifts are reported in ppm using tetram-
ethylsilane as the internal reference. The infrared spectra (4000–
400 cm-1) was recorded on a BIO-RAD FTS 3000MX instrument
in KBr pellets. Carbon, hydrogen, and nitrogen elemental analyses
were carried out by the Microanalytical Service of the Instituto
Superior Te´cnico. All of the synthetic work was performed in air
and at room temperature. Thermal properties were analyzed with
a Perkin–Elmer Instruction system (STA6000) at a heating rate
of 10 K min-1 under a dinitrogen atmosphere. Electrospray mass
spectra were run with 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 1200 in methanol solution. The compounds were observed in the
positive mode (capillary voltage = 80–105 V). Chromatographic
analyses were undertaken by using a Fisons Instruments GC 8000
series gas chromatograph with a DB-624 (J&W) capillary column
(FID detector) and the Jasco–Borwin v.1.50 software. The internal
standard method was used to quantify the organic products. The
electrochemical experiments were performed on an EG&G PAR
273A potentiostat/galvanostat connected to a personal computer
through a GPIB interface. Cyclic voltammetry (CV) studies were
undertaken at room temperature using a two-compartment three-
electrode cell with a Pt disc working (d = 0.5 mm) and a Pt wire
counter electrode. Controlled-potential electrolyses (CPE) were
carried out in a three-electrode H-type cell. The compartments
were separated by a sintered glass frit and equipped with platinum
gauze working and counter electrodes. For both CV and CPE
experiments, a Luggin capillary connected to a silver wire pseudo-
reference electrode was used to control the working electrode
potential, and a Pt wire was employed as the counter-electrode
for the CV cell. The CPE experiments were monitored regularly
by cyclic voltammetry, thus assuring no significant potential drift
occurred along the electrolyses. The solutions were saturated with
dinitrogen by bubbling through this gas before each run. The
In summary, the overall yields up to ca. 20% in a single
batch achieved in this work are comparable with those ob-
tained by other valuable copper and iron based catalysts18 and
are higher than those achieved with half-sandwich scorpionate
tris(pyrazolyl)methane complexes of vanadium, iron or copper,19
as well as other Cu catalytic systems namely with salen or phthalo-
cyanine ligands,20 although not reaching those reported for a few
remarkably active multicopper triethanolaminate complexes.10a,17b
Conclusions
The functional groups X in 3-(ortho-substituted phenylhy-
drazo)pentane-2,4-diones, X-2-C6H4–NHN C{C( O)CH3}2,
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redox potentials of the compounds were measured in 0.2 M
[nBu4N][BF4]/NCMe or DMSO, using ferrocene as an internal
reference and all reported potentials are quoted relative to the
(Ar–H), 120.47 (Ar–H), 126.09 (Ar–H), 128.84 (Ar–Cl), 129.82
(Ar–H), 134.58 (C N), 137.81 (Ar–NH–N), 196.31 (C O),
197.47 (C O).
5
ox
SCE by using the [Fe(h -C5H5)2]0/+ redox couple (E1/2 = 0.42 or
H2L4 (4). Yield 75% (based on pentane-2,4-dione), yellow
powder soluble in DMSO, water and insoluble in methanol,
ethanol and acetone. Elemental analysis: C11H12N2O5S (M =
284.29); C 45.33 (calc. 46.47); H 4.18 (4.25); N 9.79 (9.85)%.
IR (KBr): 3447 n(NH), 1676 n(C O), 1641 n(C O ◊ ◊ ◊ H), 1578
n(C N) cm-1. 1H-NMR (300.130 MHz) in DMSO, internal TMS,
d (ppm): 1.68 (s, 3H, CH3), 2.44 (s, 3H, CH3), 7.14–7.82 (m, 4H,
0.44 V vs. SCE in NCMe or DMSO, respectively).21
Syntheses of 3-(2-substituted phenylhydrazo)pentane-2,4-diones
(1–8)
The syntheses and some characteristics of 1 and 6–8 were
reported earlier;8e,12 the arylhydrazones 2–5 were synthesized via
a modified13 Japp–Klingemann reaction1–3 between the aromatic
diazonium salt of 2-substituted aniline and pentane-2,4-dione in
water solution containing sodium hydroxide.
Diazotization. 2-Substituted aniline (25 mmol) was dissolved
in 50 mL water, and 0.5 g (12.5 mmol) of NaOH was added. The
solution was cooled in an ice bath to 273 K and 1.725 g (25 mmol)
of NaNO2 were added; 5.00 mL HCl were then added in 0.5 mL
portions for 1 h. The temperature of the mixture should not exceed
278 K.
Azocoupling. NaOH (1.00 g, 25 mmol) was added to a mixture
of 2.55 mL (25 mmol) of pentane-2,4-dione with 50 mL of water.
The solution was cooled in an ice bath to ca. 273 K, and a
suspension of 2-substituted aniline diazonium (see above) was
added in three portions under vigorous stirring for 1 h.
1
Ar–H), 14.71 (s, 1H, N–H). 13C-{ H}-NMR (75.468 MHz) in
DMSO, internal TMS, d (ppm): 26.58 (CH3), 31.07 (CH3), 115.77
(Ar–H), 124.16 (Ar–H), 127.46 (Ar–H), 130.44 (Ar–H), 133.83
(C N), 135.42 (Ar–SO3H), 138.50 (Ar–NH–N), 194.96 (C O),
196.76 (C O).
HL5 (5). Yield 66% (based on pentane-2,4-dione), yellow
powder soluble in DMSO, methanol, ethanol and acetone, and
insoluble in water. Elemental analysis: C13H14N2O4 (M = 262.26);
C 59.42 (calc. 59.54); H 5.29 (5.38); N 10.57 (10.68)%. IR (KBr):
3449 n(NH), 1701 n(C O), 1669 n(C O), 1646 n(C O ◊ ◊ ◊ H),
1601 n(C N) cm-1. 1H-NMR (300.130 MHz) in DMSO, internal
TMS, d (ppm): 2.46 (s, 3H, CH3), 2.50 (s, 3H, CH3), 3.94 (s, 3H,
1
CH3), 7.25–8.04 (m, 4H, Ar–H), 15.08 (s, 1H, N–H). 13C-{ H}-
NMR (75.468 MHz) in DMSO, internal TMS, d (ppm): 26.52
(CH3), 31.16 (CH3), 52.52 (CH3), 115.13 (Ar–CC O), 115.64
(Ar–H), 124.11 (Ar–H), 131.05 (Ar–H), 134.86 (Ar–H), 134.94
(C N), 143.21 (Ar–NH–N), 166.18 (C O), 195.97 (C O),
196.70 (C O).
H2L1 (1). Yield: 72% (based on pentane-2,4-dione), black
powder soluble in methanol, ethanol, acetone and insoluble in
water and chloroform. Anal. Calcd for C11H12N2O3 (M = 220): C,
60.00 (calc. 59.88); H, 5.45 (5.47); N, 12.73 (12.20)%. IR (KBr):
3468 n(OH), 3079 n(NH), 1668 n(C O), 1632 n(C O ◊ ◊ ◊ H),
1599 n(C N) cm-1. 1H-NMR (300.130 MHz) in DMSO, internal
TMS, d (ppm): 2.41 (s, 3H, CH3), 2.48 (s, 3H, CH3), 6.91–7.67
H2L6 (6). Yield 81% (based on pentane-2,4-dione), yellow
powder soluble in DMSO, methanol, ethanol and acetone, and
insoluble in water. IR (KBr): 3482 n(NH), 1677 n(C O), 1638
n(C O), 1601 n(C O ◊ ◊ ◊ H), 1579 n(C N) cm-1. 1H-NMR
(300.130 MHz) in DMSO, internal TMS, d (ppm): 2.45 (s, 3H,
CH3), 2.46 (s, 3H, CH3), 7.17–8.01 (m, 4H, Ar–H), 15.71 (s,
1
(4H, Ar–H), 10.51 (s, 1H, OH), 14.58 (s, 1H, NH). 13C-{ H} NMR
(75.468 MHz) in DMSO, internal TMS, d (ppm): 26.5 (CH3), 31.2
(CH3), 114.9 (Ar–H), 115.8 (Ar–H), 120.2 (Ar–H), 126.2 (Ar–H),
129.3 (Ar–NH–N), 133.2 (C N), 146.3 (Ar–OH), 196.2 (C O),
196.4 (C O).
1
1H, N–H). 13C-{ H}-NMR (75.468 MHz) in DMSO, internal
TMS, d (ppm): 26.47 (CH3), 31.10 (CH3), 114.83 (Ar–CC O),
120.70 (Ar–NH–N), 123.72 (Ar–H), 131.45 (Ar–H), 132.64 (Ar–
H), 134.28 (Ar–H), 143.40 (C N), 168.17 (C O), 194.94 (C O),
196.74 (C O).
H3L2 (2). Yield 67% (based on pentane-2,4-dione), yellow
powder soluble in DMSO, water and insoluble in methanol,
ethanol and acetone. Elemental analysis: C11H13AsN2O5 (M =
328.15); C 40.13 (calc. 40.26); H 4.58 (4.49); N 8.39 (8.54)%.
IR (KBr): 3400 n(OH), 3069 n(NH), 1674 n(C O), 1644
HL7 (7). Yield 79% (based on pentane-2,4-dione), yellow
powder soluble in DMSO, methanol, ethanol and acetone, and
insoluble in water. IR (KBr): 3433 n(NH), 1686 n(C O), 1644
1
n(C O ◊ ◊ ◊ H), 1575 n(C N) cm-1. H-NMR (300.130 MHz) in
DMSO, internal TMS, d (ppm): 2.27 (s, 3H, CH3), 2.72 (s, 3H,
1
n(C O ◊ ◊ ◊ H), 1604 n(C N) cm-1. H-NMR (300.130 MHz) in
1
CH3), 6.75–7.71 (m, 4H, Ar–H), 14.05 (s, 1H, N–H). 13C-{ H}
DMSO, internal TMS, d (ppm): 2.08 (s, 3H, CH3), 2.37 (s, 3H,
NMR (75.468 MHz) in DMSO, internal TMS, d (ppm): 27.64
(CH3), 32.22 (CH3), 118.99 (Ar–H), 123.53 (Ar–H), 127.33 (Ar–
NH–N), 133.13 (Ar–H), 135.63 (Ar–H), 136.24 (C N), 144.94
(Ar–AsO3H2), 199.80 (C O), 202.90 (C O).
1
CH3), 7.15–7.96 (4H, Ar–H), 12.38 (s, 1H, N–H). 13C-{ H}-NMR
(75.468 MHz) in DMSO, internal TMS, d (ppm): 24.02 (CH3),
30.77 (CH3), 117.44 (Ar–H), 117.44 (Ar–H), 122.39 (Ar–NH–N),
125.02 (Ar–H), 125.02 (Ar–H), 134.75 (C N), 138.28 (Ar–NO2),
174.38 (C O), 206.704 (C O).
HL3 (3). Yield 72% (based on pentane-2,4-dione), yellow
powder soluble in DMSO, methanol, ethanol and acetone, and
insoluble in water. Elemental analysis: C11H11Cl1N2O2 (M =
238.67); C 55.29 (calc. 55.36); H 4.65 (4.55); N 11.79 (11.74)%.
IR (KBr): 3436 n(NH), 1669 n(C O), 1636 n(C O ◊ ◊ ◊ H), 1588
n(C N) cm-1. 1H-NMR (300.130 MHz) in DMSO, internal TMS,
d (ppm): 2.07 (s, 3H, CH3), 2.43 (s, 3H, CH3), 7.19–7.85 (m, 4H,
HL8 (8). Yield 86% (based on pentane-2,4-dione), yellow
powder soluble in DMSO, methanol, ethanol and acetone, and
insoluble in water. IR (KBr): 3434 n(NH), 1678 n(C O), 1629
n(C O ◊ ◊ ◊ H), 1603 n(C N) cm-1. 1H-NMR (300.130 MHz)
in DMSO, internal TMS, d (ppm): 2.08 (s, 3H, CH3), 2.43 (s,
1
1
Ar–H), 14.53 (s, 1H, N–H). 13C-{ H}-NMR (75.468 MHz) in
3H, CH3), 7.16–7.56 (5H, Ar–H), 13.93 (s, 1H, N–H). 13C-{ H}-
DMSO, internal TMS, d (ppm): 26.49 (CH3), 31.25 (CH3), 116.21
NMR (75.468 MHz) in DMSO, internal TMS, d (ppm): 28.82
2832 | Dalton Trans., 2011, 40, 2822–2836
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(CH3), 30.71 (CH3), 116.37 (Ar–H), 125.36 (Ar–H), 125.36 (Ar–
H), 129.29 (Ar–H), 129.57 (Ar–H), 133.36 (C N), 142.10 (Ar–
NH–N), 191.06 (C O), 196.36 (C O).
VI): 1 mmol of 10 or 11 was dissolved in water and then 1 mmol
of 4 and 1 M water solution of HCl to reach pH 2 was added. The
system was left at room temperature for slow evaporation; crystals
of 12 suitable for X-rays started to form after ca. 4 d.
Syntheses of copper(II) complexes
[Cu(HL4)2(H2O)4] (12). Yield, 57% (based on Cu). Calcd. for
C11H11CuN2O5S (M = 702.2): C 37.63, H 4.31, N 7.98; found C
37.52, H 4.20, N 7.80. MS (ESI): m/z: 682.7 [M+H]+. IR (KBr):
3437 (s, br) n(OH), 1676 (s) n(C O), 1638 (s) n(C O), 1577 (s)
n(C N) cm-1.
Synthesis of 14 (Scheme 6, Route VIII). 2 mmol of 4 were
dissolved in water, and then 1 mmol of Cu(NO3)2·2.5H2O and
2 mmol of cyanoguanidine were added and stirred under solvent
reflux for 5 min. After ca. 4 d at room temperature, light-green
crystals precipitated which were then filtered off and dried in air.
Syntheses of 9, 13. 1 mmol of 2, 6 was dissolved in water or in an
acetone–water mixture (4 : 1, v/v) in the case of 6, then 1 mmol
of Cu(NO3)2·2.5H2O was added. The mixture was stirred under
solvent reflux for 5 min and left for slow evaporation; the greenish–
black crystals of the product started to form after ca. 4 d at room
temperature; they were then filtered off and dried in air.
[Cu2(H2O)2(l-HL2)2] (9). Yield, 54% (based on Cu). Calcd. for
C11H13AsCuN2O6 (M = 407.7): C 32.41, H 3.21, N 6.87; found C
32.32, H 3.18, N 6.80. MS (ESI): m/z: 389.8 [M+H]+. IR (KBr):
3368 (s, br) n(OH), 1660 (s) n(C O), 1557 (s) n(C N) cm-1.
[Cu(H2O)4{NCNC(NH2)2}2](HL4)2(H2O)6 (14). Yield, 61%
(based on Cu). Calcd. for C26H38CuN12O14S2 (M = 870.3): C 35.88,
H 4.40, N 19.31; found C 36.50, H 4.22, N 19.63. IR (KBr): 3450 (s,
br) n(OH), 2253 (s) n(C N), 2209 (s) n(C N), 1673 (s) n(C O),
1651 (s) n(C O), 1552 (s) n(C N) cm-1.
Synthesis of 15 (Scheme 6, Route VIII or IX). 1 mmol of 4
was dissolved in water, and then 1 mmol of Cu(NO3)2·2.5H2O and
1 mmol of cyanoguanidine or imidazole were added and stirred
under solvent reflux for 5 min. After ca. 4 d the formed grey
crystals were filtered off and dried in air.
[Cu(l-L6)]n (13). Yield, 69% (based on Cu). Calcd. for
C12H10CuN2O4 (M = 309.76): C 46.53, H 3.25, N 9.04; found
C 46.87, H 3.16, N 9.03. MS (ESI): m/z: 309.9 [M+H]+. IR (KBr):
3433 (s, br) n(OH), 1673 (s) n(C O), 1637 (s) n(C O), 1598 (s)
n(C N) cm-1.
Synthesis of 10. Method A (Scheme 6, Route I): 1 mmol of 4
was dissolved in water (pH 6) then 1 mmol of Cu(NO3)2·2.5H2O
was added and the reaction mixture heated to 393 K for 5 min
and then left at room temperature for slow evaporation; crystals
of 10 suitable for X-rays started to form after ca. 4 d. Method B
(Scheme 6, Route VII): 1 mmol of 12 was dissolved in water (pH 6)
and then 1 mmol of Cu(NO3)2·2.5H2O was added. The mixture
was heated to 393 K for 5 min and then left at room temperature
for slow evaporation; deep green crystals of 10 suitable for X-rays
started to form after ca. 5 d.
[Cu(l-L4)(NCHCHNHCH)]n (15). Yield, 53% (based on Cu).
Calcd. for C14H14CuN4O5S (M = 413.9): C 40.63, H 3.41, N 13.54;
found C 40.88, H 3.38, N 13.38. IR (KBr): 3135 (s, br) n(NH),
1668 (s) n(C O), 1653 (s) n(C O), 1588 (s) n(C N) cm-1.
X-ray measurements
[CuL4(H2O)2]H2O (10). Yield, 65% (based on Cu). Calcd. for
C11H16CuN2O8S (M = 399.9): C 33.04, H 4.03, N 7.01; found C
32.62, H 3.90, N 6.93. MS (ESI): m/z: 360.8 [M+H]+. IR (KBr):
3517 (s, br) n(OH), 1644 (s) n(C O), 1586 (s) n(C O), 1547 (s)
n(C N) cm-1.
Synthesis of 11. Method A (Scheme 6, Route II): 1 mmol of 4
was dissolved in water and then 1 mmol of Cu(NO3)2·2.5H2O and
0.1 M water solution of NH4OH to reach pH 9 was added. The
reaction mixture was stirred under solvent reflux for 5 min and left
at room temperature for slow evaporation; crystals of 11 suitable
for X-rays started to form after ca. 4 d. Method B (Scheme 6,
Route II): 1 mmol of 12 was dissolved in water then 1 mmol of
Cu(NO3)2·2.5H2O and 0.1 M water solution of NH4OH to reach
pH 9 were added. The system was then left at room temperature
for slow evaporation; crystals of 12 suitable for X-rays started to
form after ca. 4 d.
The crystals of 9–15 were immersed in cryo-oil, mounted in a
Nylon loop, and measured at a temperature of 100 K. The X-
ray diffraction data were collected on a Bruker Kappa Apex II (9),
Bruker Smart Apex II22 (10, 11, 14), or Bruker Kappa Apex II Duo
(12, 13, 15) CCD diffractometer using Mo–Ka radiation (l = 0.710
˚
73 A). The EvalCCD (9) or SAINT (10–15) programs were used for
cell refinements and data reductions. The structures were solved by
direct methods using the SHELXS-97 (10, 11, 12, 14, 15), SIR97
(9) or SIR2008 (13) programs with the WinGX graphical user
interface.23–25 A semi-empirical absorption correction (SADABS)26
was applied to all data. Structural refinements were carried out
using SHELXL-97.24 in 15 the imidazole ring was disordered over
two sites with occupancies 0.51 and 0.49. The nitrogen atoms
N3 and N3B as well as N4 and N4B were constrained to have
equal coordinates and anisotropic displacement parameters. The
OH, H2O, NH2, and NH (except in 15) hydrogen atoms were
located from the difference Fourier map but constrained to ride
on their parent atom with Uiso = 1.5 Ueq (parent atom). Other
hydrogen atoms were positioned geometrically and constrained
[CuL4(H2O)2] (11). Yield, 60% (based on Cu). Calcd. for
C11H14CuN2O7S (M = 381.8): C 34.60, H 3.70, N 7.34; found
C 34.43, H 4.17, N 7.50. IR (KBr): 3436 (s, br) n(OH), 1677 (s)
n(C O), 1646 (s) n(C O), 1578 (s) n(C N) cm-1.
Synthesis of 12. Method A (Scheme 5, Route III): 2 mmol of 4
were dissolved in water and then 1 mmol of Cu(NO3)2·2.5H2O and
1 M water solution of HCl to reach pH 2 was added. The reaction
mixture was stirred under solvent reflux for 5 min and left at room
temperature for slow evaporation; crystals of 12 suitable for X-rays
started to form after ca. 4 d. Method B (Scheme 6, Routes IV and
˚
to ride on their parent atoms, with C–H = 0.95–0.99 A, N–
˚
H = 0.88 A (the imidazole NH in 15) and Uiso = 1.2–1.5
Ueq (parent atom). The crystallographic details are summarized
in Table 6. CCDC 798948–798954 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.
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Table 6 Crystal data and structure refinement details for compounds 9–15
10 11
9
12
13
14
15
Empirical formula C22H26As2Cu2N4O12 C22H32Cu2N4O16S2 C11H14CuN2O7S C22H30CuN4O14S2 C12H10CuN2O4 C26H50CuN12O20S2 C14H14CuN4O5S
Fw
815.39
100(2)
799.72
100(2)
0.71073
Monoclinic
P21/n
11.3561(3)
9.1757(3)
15.5166(4)
90
110.876(2)
90
1510.69(7)
2
1.758
1.628
23 254
7238
1.024
0.0340
0.0309
0.0765
381.84
100(2)
0.71073
Orthorhombic Monoclinic
Pbca
15.222(7)
7.432(2)
26.225(9)
90
90
90
2966.9(19)
8
1.710
1.649
11 212
2603
1.037
0.0916
0.0535
0.1294
702.16
100(2)
0.71073
309.76
100(2)
0.71073
Monoclinic
P21/c
11.5658(13)
5.2797(6)
18.662(2)
90
90.281(4)
90
1139.6(2)
4
978.44
100(2)
413.89
100(2)
Temp (K)
˚
l(A)
0.71073
Triclinic
0.71073
Triclinic
0.71073
Triclinic
Cryst syst
¯
¯
¯
Space group
P1
P21/n
9.6052(2)
13.5156(2)
11.6369(2)
90
108.9336(8)
90
1428.97(4)
2
1.632
P1
P1
˚
a (A)
7.9753(4)
9.0989(4)
10.3280(6)
112.022(4)
94.776(6)
98.031(4)
680.42(6)
1
6.9100(6)
12.0491(13)
14.337(2)
108.578(7)
100.618(7)
104.892(5)
1046.3(2)
1
4.9947(3)
8.5337(5)
19.0626(10)
87.866(3)
82.974(3)
76.536(3)
784.21(8)
2
˚
b (A)
˚
c (A)
a (deg)
b (deg)
g (deg)
3
˚
V (A )
Z
rcalc (Mg m-3)
1.990
1.805
1.928
6932
2149
1.553
0.713
8178
4674
0.854
0.0473
0.0432
0.0705
1.753
1.561
14341
4783
m(Mo Ka) (mm-1) 4.047
0.987
20 758
3795
No. reflns.
Unique reflns.
GOOF (F2)
Rint
11 269
3962
1.085
0.0340
0.0323
0.0680
1.056
0.970
1.030
0.0304
0.0301
0.0774
0.0624
0.0441
0.1218
0.0397
0.0416
0.0912
R1a (I ≥ 2s)
wR2b (I ≥ 2s)
2
a R1 = RꢀFo| - |Fcꢀ/R|Fo|. b wR2 = [R[w(Fo - Fc2)2]/R[w(Fo2)2]]1/2
.
Computational details
To estimate the relative stabilities of some complexes, the
DGs values of the model reactions of complex formation from
[Cu(OH)2(H2O)2] and the ligand (Scheme 7) have been cal-
culated. The penta-coordinated tetragonal pyramid structures
were selected as the starting geometries of the complexes, in
accordance with the experimental X-ray data.8e In the case of
[Cu(L1)(H2O)2]·H2O and [Cu(L4)(H2O)2]·H2O, the coordination
polyhedron was preserved during the optimization. The optimiza-
tion of [Cu(OH)(L8)(H2O)2] resulted in the extrusion of an axial
water molecule from the inner coordination sphere and formation
of the complex [Cu(OH)(L8)(H2O)]·H2O. Two factors determined
the selection of the model reactions (Scheme 7). First, the charged
species were avoided to minimize computational errors of the
solvent effect. Second, the number of species before and after
reaction was selected to be the same to minimize errors in the
entropy term.
The full geometry optimization of all structures has been carried
out at the DFT/HF hybrid level of theory using the B3LYP
functional27 and 6-31+G(d) basis set with the help of the Gaussian-
9828 program package. Restricted approximations for the struc-
tures with closed electron shells and unrestricted methods for
the structures with open electron shells have been employed. No
symmetry operations have been applied. The Hessian matrix was
calculated analytically for the optimized structures in order to
prove the location of correct minima (no imaginary frequencies),
and to estimate the thermodynamic parameters, the latter being
calculated at 298.15 K. Vertical ionization potentials (IP) and
vertical electron affinities (EA) were calculated using the formulae
(1) and (2):
IP = E(A+) - E(A)
(1)
(2)
EA = E(A) - E(A-)
Oxidation of cyclohexane
where E(A) is the total energy of the neutral structure 1–8, and
E(A+) and E(A-) are the total energies of the oxidized and reduced
species with the geometry corresponding to that of the neutral
structure 1–8.
The reaction mixtures were prepared as follows: to 4.9–10.0 mmol
of the complex 9, 10, 12 or 13 contained in the reaction flask
were added 4 mL of MeCN, 0–0.60 mmol of HNO3, 1.00 mmol
of C6H12 and 5.00–12.5 mmol of H2O2 solution (30% in H2O),
in this order. The reaction mixture was stirred for 6 h at room
temperature (ca. 298 K) and air atmospheric pressure, then 90
mL of cycloheptanone (as internal standard) and 5.0 mL of
diethyl ether (to extract the substrate and the products from
the reaction mixture) were added. The resulting mixture was
stirred for 15 min and then a sample taken from the organic
phase was analyzed by GC. Subsequently, PPh3 was added to
the final organic phase (to reduce the cyclohexyl hydroperoxide, if
formed), the mixture was analyzed again to estimate the amount of
cyclohexyl hydroperoxide, according to Shul’pin’s method.18 Blank
experiments were performed and confirmed that no cyclohexane
Total energies corrected for solvent effects (Es) were estimated at
the single-point calculations on the basis of gas-phase geometries
using the polarizable continuum model29 in the CPCM version30
with water as solvent. The entropic term in solutions (Ss) was
calculated according to the procedure described by Wertz31 and
Cooper and Ziegler32 using equation Ss = Sg + [(-14.3 cal mol-1K-1)
- 0.46(Sg - 14.3 cal mol-1K-1) + 7.98 cal mol-1K-1] where Sg is gas-
phase entropy of solute. The enthalpies and Gibbs free energies in
solution (Hs and Gs) were estimated using the equations Hs = Es +
Hg - Eg and Gs = Hs - TSs where Eg and Hg are gas-phase total
energy and enthalpy.
2834 | Dalton Trans., 2011, 40, 2822–2836
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oxidation products were obtained in the absence of the metal
catalyst.
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Acknowledgements
This work has been partially supported by the Foundation
for Science and Technology (FCT), Portugal, and its PPCDT
(FEDER funded) and “Science 2007” programs. The authors
acknowledge the Portuguese NMR Network for providing access
to the NMR facility.
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