Click synthesis of symmetric bis-triazol ligands and full characterisation of their copper(ii)-complexes

Article abstract of DOI:10.1039/c3nj00570d

Eight novel ligands were prepared from a known symmetric diaza-18-crown-6 (cyclic ligand) and two commercial N,N′-dimethyl-alkyl diamines (acyclic ligands) via the Cu(i)-catalyzed Huisgen dipolar cycloaddition. All C ₂-symmetric isolated ligands readily formed stable crystalline 1:1-copper(ii) complexes with cupric perchlorate. Their structural, electrochemical and physico-chemical properties were fully investigated with the help of X-ray diffraction, cyclic voltammetry, FT-IR, UV-visible, and electron paramagnetic resonance (EPR) spectroscopies. Planar-or nearly planar-arrangement of the two N3-triazole nitrogens and the two tertiary amine pivot nitrogens was found in one single four-coordinated species, in four five-coordinated species, and three six-coordinated species, with one or two solvent molecule(s), or two oxygen atoms of the crown ether, occupying the axial position(s) in the solid sate. The electron-donating or electron-withdrawing effect of the N1-substituent on the triazol was found to influence the Cu(ii)/Cu(i) redox potential of all studied complexes in DMF. The EPR-spectrum of cyclic complexes in frozen DMF at 100 K exhibited two mononuclear species, one of them likely promoting the formation of dinuclear species as a minor component, whereas most acyclic complex spectra were quite similar. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014.

Full text of DOI:10.1039/c3nj00570d

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Click synthesis of symmetric bis-triazol ligands
and full characterisation of their copper(II)-
complexes†
Cite this: New J. Chem., 2014,
38, 419
Zakia Benkhellat,a Mustapha Allali,a Marc Beley,a Emmanuel Wenger,b
Maxime Bernard,c Nathalie Parizel,c Katalin Selmeczia and Jean-Pierre Joly*a
Eight novel ligands were prepared from a known symmetric diaza-18-crown-6 (cyclic ligand) and two
commercial N,N0-dimethyl-alkyl diamines (acyclic ligands) via the Cu(I)-catalyzed Huisgen dipolar cyclo-
addition. All C2-symmetric isolated ligands readily formed stable crystalline 1 : 1-copper(II) complexes
with cupric perchlorate. Their structural, electrochemical and physico-chemical properties were fully
investigated with the help of X-ray diffraction, cyclic voltammetry, FT-IR, UV-visible, and electron para-
magnetic resonance (EPR) spectroscopies. Planar – or nearly planar – arrangement of the two N3-triazole
nitrogens and the two tertiary amine pivot nitrogens was found in one single four-coordinated species, in
four five-coordinated species, and three six-coordinated species, with one or two solvent molecule(s), or
two oxygen atoms of the crown ether, occupying the axial position(s) in the solid sate. The electron-
donating or electron-withdrawing effect of the N1-substituent on the triazol was found to influence the
Cu(II)/Cu(I) redox potential of all studied complexes in DMF. The EPR-spectrum of cyclic complexes in
frozen DMF at 100 K exhibited two mononuclear species, one of them likely promoting the formation of
dinuclear species as a minor component, whereas most acyclic complex spectra were quite similar.
Received (in Montpellier, France)
28th May 2013,
Accepted 31st October 2013
DOI: 10.1039/c3nj00570d
www.rsc.org/njc
growing interest from the chemist community as models of
natural enzymes10,11 (such as superoxide dismutase mimetics12),
Introduction
Copper is second to iron in its prevalence in redox-active but also as new HIV-antiviral agents,13 anti-cancer agents,14,15
metalloproteins,1 mainly as a cofactor of numerous enzymes receptor-mediated tumour imaging agents,16,17 and catalysts of
(e.g. cytochrome-c oxidase,2 copper-thionein,3 ceruloplasmin,4 organic reactions.18 Functionalized 1,4-disubstituted 1,2,3-triazole
dopamine-b-mono-oxygenase,5 galactose oxidase,6 and many units are versatile ligands offering several donor sites or act as
others). Copper is believed to play a significant role in the bridging ligands for copper and some other rare transition
CNS-development of mammalians, too.7 Although this metal metal coordination too.19–27 A large array of mono, bi, tri-, and poly-
is of critical importance in numerous biochemical processes, dentate ligands incorporating 1,4-disubstituted-1,2,3-triazoles has
free copper (i.e. uncomplexed Cu+ or Cu2+ cations) is actually been synthesized since 2007.28–31 Moreover, the Cu(I)-catalyzed
cytotoxic.8,9 Therefore, versatile ligands of copper ions receive version of the [3+2] Huisgen32–35 azide–alkyne cycloaddition
(i.e. the CuAAC ‘click reaction’)36–38 was used to synthesize
ligands endowed with good complexing properties towards
a
´
´
SRSMC UMR 7565 CNRS – Universite de Lorraine, Faculte des Sciences,
zinc, lead, nickel, and ammonium too.39–43 In the meantime,
this powerful synthetic approach was extended to the linkage
of cyclodextrines to various organic azides.44 We reported the
synthesis of a new C2-symmetric macrocyclic ligand (hereafter
L1, a pendant armed 18-crown-2-diaza-4-ether) endowed with
good complexing properties towards copper,45 nickel and zinc
`
BP 70239, F-54506 Vandœuvre-les-Nancy cedex, France.
E-mail: jean-pierre.joly@sucres.uhp-nancy.fr
b
c
´
´
CRM2, UMR 7036 CNRS – Universite de Lorraine, Faculte des Sciences, BP 70239,
`
F-54506 Vandœuvre-les-Nancy cedex, France
´
TGE Reseau National de RPE interdisciplinaire, FR-3443, Institut de Chimie,
´
UMR 7177 CNRS – Universite de Strasbourg, 4 rue Blaise Pascal, CS 90032,
67081 Strasbourg cedex, France
† Electronic supplementary information (ESI) available: Synthesis and full char- cations, too.
acterisation of L2–9 ligands including ES-HRMS+ spectra of all reported com-
Solid-state structures revealed two different coordination
plexes; EPR spectra (Q-band at 100 K) of [CuL9(ClO4)]ClO4 (S1) and [CuL4](ClO4)2
(S2) complexes in PDF format; low resolution crystallographic data & structure
refinement parameters of the [CuL9(ClO4)](ClO4) complex and T1 in CIF format.
CCDC 853231, 851236–851242, 854682. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c3nj00570d
modes, one symmetrical for Cu(II), quite isomorphous with
the Ni(II)-analogue, versus one asymmetric for Zn(II), which were
further investigated via molecular modelling. These results
will be discussed elsewhere.46 During the course of this work,
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Scheme 1 Synthesis of cyclic ligands L2–4 from diaza-crown ether A.
Acyclic ligands L5–9 were isolated in fair to good yields using
a slight modification of the one-pot method reported by Yan
et al.51 from commercial secondary bis-N-methyl amines B and
C (i.e. general procedure II and ESI†).
Fig. 1 Two-step synthesis of the [CuL1](ClO4)2 centro-symmetric
complex in 31% overall yield (symmetry code: (i) 1 ꢁ x, 1 ꢁ y, 1 ꢁ z).45
The tedious isolation of N,N0-dipropargyl bis-tertiary amine
intermediates (after step i) did not improve overall yields
(Scheme 2). Some ligands (e.g. L7 & L8) have to be chromato-
graphed twice over silica and/or neutral alumina to ensure
sufficient purity and unambiguous characterization. All ligand
tetradentate ligands from a N,N0-dimethyl-1,2-diaminoethane
synthesized via click chemistry were reported and their com-
plexes with Mn2+, Ni2+, Zn2+, and Fe2+ generated. Among them,
their Mn2+-complexes displayed interesting catalytic activities
for the epoxidation of various terminal olefins with peracetic
acid.18 We describe herein the ‘click’ syntheses of two series of
new macrocyclic and acyclic ligands via the Cu(I)-catalyzed
version of the Huisgen reaction between a terminal acetylene
and an organic azide. The Cu(II)-complexes of these ligands
were isolated (as their perchlorate salts) and characterised via
different techniques: elemental analysis, FT-IR, UV-visible and
EPR spectroscopies, HRMS+, high resolution X-ray diffraction
(for seven over eight complexes), and cyclic voltammetry.
1
structures were unambiguously ascertained by H & 13C-NMR,
FT-IR, and ES+-HRMS. 1H-NMR spectra of all ligands exhibit the
expected time-averaged 2-fold symmetry and the characteristic
aromatic singlet between 7.5 and 8.0 ppm for the two isochro-
nous triazol protons at the NMR time scale (CDCl3, 298 K).
Results and discussion
Synthesis and characterisation of ligands
Cyclic ligands L2–4 were click-generated in good to fair yields
from known N,N0-dipropargyl-diaza crown A47 and readily avail-
Scheme 2 One-pot synthesis of ligands L5–9 via the Cu(I)-catalyzed
able aryl-azides (i.e. Scheme 1, general procedure I and ESI†).48–50 Huisgen reaction.
Table 1 Crystal data & structure refinement parameters of cyclic Cu(II)-complexes from ligands L2–4 with respect to L145
Complexes
[CuL1](ClO4)2
[CuL2](ClO4)2
[CuL3](ClO4)2
[CuL4](ClO4)2, DMF
Empirical formula
CuC32H44Cl2
N8O12
CuC30H40Cl2
N8O12
CuC32H44Cl2
N8O14
CuC33H45Cl4
N9O13
Formula weight
Colour
Cryst. size [mm]
Crystal system
a [Å]
b [Å]
c [Å]
867.19
839.15
899.20
981.13
Light blue
0.32 ꢀ 0.18 ꢀ 0.14
Triclinic
9.1849(15)
9.2931(8)
11.8913(12)
98.876(4)
108.784(5)
105.480(6)
893.40(19)
%
P1
1
Blue-green
0.12 ꢀ 0.10 ꢀ 0.08
Triclinic
Dark green
0.31 ꢀ 0.19 ꢀ 0.11
Triclinic
Malachite-green
0.21 ꢀ 0.14 ꢀ 0.10
Monoclinic
26.3848(4)
18.7279(3)
27.6457(6)
90.00
143.2590(10)
90.00
8171.8(3)
P21/c
9.390(5)
9.383(5)
10.293(5)
10.426(5)
101.979(5)
108.296(5)
106.798(5)
865.2(7)
%
P1
1
1.618
10.299(8)
11.300(9)
70.19(4)
66.08(3)
73.30(5)
924.7(1)
%
P1
a [deg]
b [deg]
g [deg]
V [Å3]
Space group
Z
1
8
rcalcd [g cmꢁ3
Temp. [K]
R/wR
]
1.612
1.615
1.595
100(2)
0.0400/0.0947
1.025
110(2)
0.0286/0.0696
1.041
110(2)
0.0249/0.0661
1.047
100(2)
0.0344/0.0806
1.062
GOF
CCDC
854682
853231
851236
851237
420 | New J. Chem., 2014, 38, 419--429
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Further information about the isolation and full characterisa-
tion of ligands L2–9 is provided in the ESI.†
Crystallographic characterization
Suitable X-ray diffraction monocrystals of 1 : 1 Cu(II)-complexes
were isolated from ligands L2–8 and Cu(ClO4)2 hexahydrate in
the presence of acetonitrile and dimethylformamide (traces) in
fair to excellent yield within a week (see general procedure III
and ESI†). Their crystallographic data are summarised in
Tables 1 and 3.
Cyclic ligands. Selected bond distances and angles of cyclic
Cu(II)-complexes of ligands L2–4 are summarised in Table 2
with respect to L1.45
Fig. 3 Molecular structure of [CuL3](ClO4)2 showing the hetero-atom num-
bering and the thermal ellipsoids at 50% probability (H-atoms and non-
coordinating anions omitted for clarity; symmetry code: (i) ꢁx, ꢁy, 1 ꢁ z).
The three first cyclic Cu(II)-complexes, i.e. [CuL1–3](ClO4)2,
%
crystallised in the triclinic space group P1 as their perchlorate
salts, with the central Cu-atom lying on the inversion centre.
However, the new [CuL2](ClO4)2 centro-symmetric complex
(see Fig. 2) is quite different from [CuL1](ClO4)2,45 but very
similar to [CuL3](ClO4)2 (see Fig. 3) with close bond distances
and bond angles. It is noteworthy that the N1–Cu1–O1 angle
was significantly smaller in [CuL1](ClO4)2 (i.e. 84.71) than in
[CuL2](ClO4)2 and [CuL3](ClO4)2 (B87.31).
Here again, the hexacoordinated Cu(II)-cation lies on the
inversion centre in the basal plane formed by the four N-atoms
and O1 occupies the axial position in the centro-symmetric
complex (see Fig. 3). Strained 5-membered cycles (i.e. N1–Cu1–N2
and O1–Cu1–N2) induce bond angles diminution and a slightly
distorted six-coordinated octahedral geometry. The Cu1–O1 dis-
tances fall in the range of 2.536 ꢂ 0.011 Å in accordance with the
Jahn–Teller effect.52 Replacement of the EDG-methoxy group by
Fig. 4 Molecular structure of one of the two independent [CuL4](ClO4)2
molecules present in the asymmetric unit showing the hetero-atom
numbering and the thermal ellipsoids at 50% probability (H-atoms and
non-coordinating anions omitted for clarity).
the EWG-chlorine atom in L4 (all else being equal) led to a
completely different solid-state structure.
The tetracoordinated Cu(II) cation lies above (B0.1 Å) the
basal plane formed by N1–N2–N3–N4 in a quite distorted
geometry (N1–Cu1–N2 = 80.81, N3–Cu1–N4 = 83.11); lone pairs
of O1 and O2 oxygen atoms point towards the copper cation
(i.e. red arrows: Cu1–O1 B2.71 Å, Cu1–O2 B2.79 Å), O3 and O4
atoms being too far to interfere with the metal ion (see Fig. 4).
Acyclic ligand Cu(II)-complexes. Their crystallographic data
are summarised in Table 3.
Selected bond distances and angles of acyclic Cu(II)-complexes
from ligands L5–9 are summarised in Table 4.
Unfortunately, structure of the [CuL9(ClO4)](ClO4) complex
could not be fully resolved in spite of numerous attempts under
different conditions. However, its low resolution structure is
given in Fig. 5 for comparison purposes in the acyclic series.
The above 1 : 1-complex is ‘roughly’ symmetric, with the
Fig. 2 Molecular structure of [CuL2](ClO4)2 showing the hetero-atom
numbering and the thermal ellipsoids at 50% probability (H-atoms and
non-coordinating anions omitted for clarity; symmetry code: (i) ꢁx, ꢁy, ꢁz). pentacoordinated Cu(II)-ion lying approximately 0.1 Å over the
Table 2 Selected bond distances (Å) and bond angles (1) in solid state structures of cyclic Cu(II)-complexes from L2–4 with respect to L1
Bond distances (Å)
Bond angles (1)
Cyclic complexes
Cu1–N1
Cu1–N2
Cu1–N3
Cu1–N4
N1–Cu1–N2
N2–Cu1–N3
N3–Cu1–N4
N1–Cu1–N4
[CuL1]2+
[CuL2]2+
[CuL3]2+
[CuL4]2+ b
1.988
2.015
2.011
1.950
2.114
2.096
2.107
2.178
—
—
—
1.941
—
—
—
2.166
80.7
81.1
81.3
80.4
99.3a
98.9a
98.7a
101.3
—
—
—
82.9
—
—
—
95.2
a
b
i.e. N2–Cu–N1i bond angle, N1i and N3 being equivalent via the inversion centre cf. Fig. 1–3. Mean values of two independent molecules.
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Table 3 Crystal data & structure refinement parameters of acyclic Cu(II)-complexes from ligands L5–8
Complexes
[CuL5(MeCN)](ClO4)2
[CuL6(ClO4)](ClO4)
[CuL7(MeCN)](ClO4)2
[CuL8(DMF)2](ClO4)2
Empirical formula
CuC26H33Cl2
N9O8
CuC25H32Cl2
N8O8
CuC25H31Cl2
N9O8
CuC31H46Cl2
N10O12
Formula weight
Colour
Cryst. size [mm]
Crystal system
a [Å]
b [Å]
c [Å]
734.06
707.04
Purple
720.03
885.22
Blue
Deep blue
0.30 ꢀ 0.20 ꢀ 0.12
Triclinic
10.573(4)
10.596(4)
16.788(5)
73.602(4)
79.087(5)
60.693(3)
1570.4(11)
%
P1
2
1.552
100(2)
0.0347/0.0795
1.052
Deep purple
0.20 ꢀ 0.15 ꢀ 0.10
Triclinic
13.8278(5)
14.1824(5)
16.0830(7)
89.792(4)
78.263(4)
87.476(2)
3085.1(2)
%
P1
0.20 ꢀ 0.15 ꢀ 0.13
Orthorhombic
14.8356(4)
25.4907(6)
8.1556(2)
90.00
0.31 ꢀ 0.24 ꢀ 0.11
Triclinic
10.806(3)
12.898(3)
14.296(3)
80.45(1)
85.53(1)
73.19(1)
1880.0(8)
%
P1
a [deg]
b [deg]
90.00
90.00
g [deg]
V [Å3]
3084.2(13)
a
Space group
Z
Cmc21
4
4
2
rcalcd [g cmꢁ3
Temp. [K]
R/wR
]
1.523
1.550
1.564
100(2)
0.0465/0.1239
1.192
100(2)
0.0848/0.18344
1.199
100(2)
0.0474/0.1067
1.063
GOF
CCDC
851241
851242
851238
851239
a
Flack value = 0.4745(8).
Table 4 Selected bond distances (Å) and bond angles (1) in solid state structures of acyclic Cu(II)-complexes from ligands L5–9
Bond distances (Å)
Bond angles (1)
Acyclic complexes
Cu–N1
Cu–N2
Cu–N3
Cu–N4
N1–Cu–N2
N2–Cu–N3
N3–Cu–N4
N1–Cu–N4
[CuL5(MeCN)]2+
[CuL6(ClO4)]+
1.984
1.961
1.985
2.002
1.99–
2.036
2.046
2.055
2.066
2.07–
2.036
—
2.058
2.052
2.08–
1.962
—
1.975
1.986
1.97–
81.5
82.5
81.7
82.5
82.–
86.8
96.7a
96.6
95.9
96.–
82.4
—
82.0
82.1
83.–
103.3
—
98.6
99.5
99.–
[CuL7(MeCN)]2+ b
[CuL8(DMF)2]2+
[CuL9(ClO4)]+ c
a
b
c
i.e. N2–Cu–N2iii bond angle, N2iii and N3 being equivalent by mirror symmetry cf. Fig. 7. Mean values of two independent molecules. See ESI
for the low resolution CIF file and selected crystal data (T1).
Fig. 5 Low resolution molecular structure of [CuL9(ClO4)](ClO4) showing
Fig. 6 Molecular structure of [CuL5(MeCN)](ClO4)2 showing the hetero-
atom numbering and the thermal ellipsoids at 50% probability (H-atoms
and non-coordinating anions omitted for clarity).
the hetero-atom numbering and the thermal ellipsoids at 50% probability
(H-atoms and non-coordinating anions omitted for clarity).
basal plane formed by N1–N2–N3–N4 in a square-pyramidal N1–Cu1–N5 = 97.71) as the 5th ligand in a distorted square pyr-
geometry; one oxygen atom of the perchlorate anion occupies amidal geometry. Strained 5-membered cycles (i.e. N1–Cu1–N2,
the axial position (Cu1–O1 B2.37 Å, N1–Cu1–O1 B94.31) as the N2–Cu1–N3, N3–Cu1–N4) induce bond angles diminution in com-
5th ligand. Thanks to the 6-membered cycle (N2–Cu1–N3) the parison to [CuL9(ClO4)]+ (see Fig. 5) and a significant repulsion of the
resulting structure is more relaxed than that of [CuL5(MeCN)]2+ pentacoordinated Cu-atom above the basal plane (B0.34 Å). An extra
as illustrated in Fig. 6.
This mono-solvated complex is not endowed with true symmetry; pletely different structure for the resulting Cu(II)-complex (see Fig. 7).
N1–N2–N3–N4 forms the basal plane and N5 of one acetonitrile
Thanks to the 6-membered cycle (i.e. –N2–Cu1–N2iii–(CH2)3–)
molecule occupies the axial position (Cu1–N5 = 2.246 Å and the mirror-symmetric [CuL6(ClO4)]+ structure is more relaxed
carbon atom between N2 and N3 atoms in ligand L6 led to a com-
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Fig. 7 Molecular structure of [CuL6(ClO4)]ClO4 showing the hetero-atom
numbering and the thermal ellipsoids at 50% probability (H-atoms and
non-coordinating anions omitted for clarity; symmetry code: (iii) ꢁx, y, z).
Fig. 9 Molecular structure of [CuL8(DMF)2](ClO4)2 showing the hetero-
atom numbering and the thermal ellipsoids at 50% probability (H-atoms
and non-coordinating anions omitted for clarity).
than those of [CuL5(MeCN)]2+. The pentacoordinated Cu(II)-ion
lies approximately 0.13 Å over the basal plane formed by N1–N2–
N2iii–N1iii in a square-pyramidal geometry; O1A of one perchlo-
rate anion occupies the axial position (Cu1–O1A B2.42 Å) as the
5th ligand. Interestingly, direct attachment of the phenyl group
to the triazole moieties (all else being equal) induced a different
coordination of Cu(II) by ligand L7.
were observed in the solid state with respect to the nature of the
ligand (cyclic vs. acyclic) and the nature of the substituent on
the triazoles. One or two solvent molecule(s), or one perchlorate
anion, act(s) eventually as axial ligand(s) in the more open
acyclic series to complete the coordination sphere.
The resulting 1 : 1-complex is here only approximately sym-
metric, the pentacoordinated Cu(II) cation at 0.07 Å centered
over the basal plane formed by N1–N2–N3–N4 in a square-
pyramidal geometry (see Fig. 8); N5 of one acetonitrile molecule
occupies the axial position (N1–Cu1–N5 = 92.11) as the 5th
ligand. Lastly, the bis-solvated 1 : 1-complex formed from the
para-methoxy-phenyl-substituted L8 ligand and Cu(II) in the
presence of DMF is quite different and unique in the acyclic
series (see Fig. 9).
The hexacoordinated Cu(II) cation lies in the basal plane
formed by N1–N2–N3–N4 in a distorted octahedral geometry;
O1A and O1B of two DMF molecules occupy the axial positions
respectively 2.367 Å and 2.548 Å apart from the copper cation,
which fall in the range of reported values.53,54 This structure
can be compared to that of the [CuL3](ClO4)2 complex (Fig. 3) in
the cyclic series. In summary, ligands L2–9 readily form stable 1 :
1-Cu(II) complexes in the presence of Cu(ClO4)2 in solution. Three
different geometries (i.e. tetra-, penta-, and hexa-coordination)
Electrochemical studies
It is noteworthy that the formation of Cuo-species occurs at
about ꢁ1.5 V as an irreversible process and that the formation
of a Cu(III)-complex was not detected before the solvent oxida-
tion which started at +1.5 V under our experimental conditions.
Therefore, we studied the electrochemical behavior starting
potential at +0.2 V towards negative values and kept the
potential range between ꢁ0.4 and +0.8 V to avoid the irrever-
sible formation of Cuo and the oxidation of the solvent through-
out the present study. Table 5 summarizes the electrochemical
results corresponding to the first cathodic peak (1st scan
towards negative potential at three increasing scan rates) which
was assigned to the reduction of the initial and stable geo-
metric form, i.e. Cu(II)st to Cu(I). A reductive-controlled
potential coulometry realized below the 1st cathodic peak at
ꢁ0.4 V showed that the total quantity of electricity exchanged
corresponds to one electron per mole of the copper complex.
Table 5 Electrochemical data for the 1st cathodic peak assigned to the
reduction of Cu(II)st to Cu(I): Epc (mV vs. SCE) and Ipc (mA) in Cu(II)-
complexes from ligands L1–9 at three different scan rates
Cyclic ligands
Acyclic ligands
L4 L5 L6 L7
L1 L2 L3
L8 L9
Epc 25 mV sꢁ1 ꢁ10 +30 +30 +60 ꢁ10 ꢁ25 ꢁ50 ꢁ21 +35
Ipc 25 mV sꢁ1
ꢁ11 ꢁ12 ꢁ11 ꢁ11 ꢁ12 ꢁ10 ꢁ11 ꢁ11 ꢁ11
Epc 100 mV sꢁ1 ꢁ20 +10 +20 +45 ꢁ30 ꢁ45 ꢁ65 ꢁ50 ꢁ10
Ipc 100 mV sꢁ1 ꢁ22 ꢁ24 ꢁ22 ꢁ22 ꢁ24 ꢁ20 ꢁ21 ꢁ22 ꢁ22
Fig. 8 Molecular structure of one of the two independent [CuL7(MeCN)]-
(ClO4)2 molecules present in the asymmetric unit showing the hetero-
atom numbering and the thermal ellipsoids at 50% probability (H-atoms
and non-coordinating anions omitted for clarity).
Epc 500 mV sꢁ1 ꢁ40
0
+5
+40 ꢁ50 ꢁ100 ꢁ120 ꢁ90 ꢁ15
Ipc 500 mV sꢁ1 ꢁ49 ꢁ52 ꢁ50 ꢁ50 ꢁ54 ꢁ45 ꢁ49 ꢁ50 ꢁ50
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First cathodic peak reduction potential (i.e. Epc) analyses
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revealed the ligand-based electrochemical behavior of all pre-
sent complexes. On the one hand, the donor or withdrawing
effect of the para-substituent on the phenyl ring was found to
influence more or less the reduction potential of the complexes.
For instance, [CuL4]2+ and [CuL9(ClO4)]+ display a Cu(II)st -
Cu(I) redox potential at more positive values than the corres-
ponding unsubstituted complexes in cyclic and acyclic series,
i.e. [CuL2]2+ and [CuL7(MeCN)]2+, respectively. Thus, the elec-
tron withdrawing effect of the chlorine substituent decreases
the electronic density around the cupric ion and induces a
positive shift in the reduction potential from Cu(II) to Cu(I). In
the cyclic series (e.g. X = H, [CuL2]2+), the N-triazole doublets are
likely more delocalized than in [CuL1]2+ (no possible p-conjugation).
Accordingly, the donor effect of the triazole moiety significantly
increases the electron density around the metal in [CuL1]2+
with a negative shift (ca. 35 mV) of the Epc value in comparison
Fig. 10 CVs of [CuL4]2+at 3 different scan rates: 25, 100, and 500 mV sꢁ1
in Ar-purged DMF with 0.1 M Bu4NPF6 as a supporting electrolyte at rt.
to [CuL2]2+. In the acyclic series, the reduction potential of The second scan was selected as a typical CV. The electrochemical
[CuL7(MeCN)]2+ (m = 1, n = 0, X = H), including one acetonitrile behavior of [CuL4](ClO4)2 is presented below (Fig. 10) as a relevant
molecule as the axial ligand, is more negative than those of example in the cyclic series.
[CuL8(DMF)2]2+ (m = 1, n = 0, X = OMe) and [CuL6(ClO4)]+
The positive part of CVs obviously displays two oxidation
(m = 1, n = 1, X = H), the donor effect of the acetonitrile ligand waves: a 1st wave, i.e. Epa1, which appears at about +90 mV and
being obviously stronger than the remote methoxy effect in a second one, i.e. Epa2, at about +300 mV under our experi-
[CuL8(DMF)2]2+. The mild donor effect of the methoxy group on mental conditions. After the 1st electrochemical step between
the phenyl also induces the almost same reduction potentials +0.2 and ꢁ0.4 V (Ered), especially when the scan rate was slow,
(Epc B 30 mV) in [CuL2]2+ (X = H) as in [CuL3]2+ (X = OMe). On the chemical step, i.e. Cu(I)met - Cu(I)st, became more com-
the other hand, the reduction of Cu(II)st to Cu(I) is easier in plete and the Cu(I)st/Cu(I)met ratio was much higher. For that
[CuL5(MeCN)]2+ than in [CuL6(ClO4)]+. We attributed the reason, the 1st wave was assigned to the re-oxidation of Cu(I)st
higher measured Epc to the relative destabilization of Cu(II) to Cu(II)met1 and the 2nd one to the formation of Cu(II)met2
in the more distorted geometry of [CuL5(MeCN)](ClO4)2 (m = 0) from Cu(I)met. Thus, we postulated that the re-oxidation of
vs. [CuL6(ClO4)]ClO4 (m = 1).56 It is well-known that there are Cu(I)-complexes, i.e. Cu(I)met and Cu(I)st, produces metastable
important differences in geometrical requirements for coordi- Cu(II)-species via an electron transfer followed by a chemical
nation of Cu(I) in comparison to Cu(II) that produce interesting step that is referred to as the so-called EoxCox mechanism.
dynamic processes in solution. Divalent copper preferentially Indeed, after changing the scan direction at 800 mV, and since
forms stable hexacoordinated, or square pyramidal, complexes 1st reduction potential, i.e. Epc1, remains the same as in the
whereas the monovalent state induces tetrahedral coordina- first scan (i.e. Epc), we assumed that Cu(II)met1 and/or Cu(II)met2
tion. The occurrence of structural rearrangements in the first relax to the initial and more stable form Cu(II)st. As shown in the
coordination shell of Cu(II) to Cu(I) in the redox mechanism has CVs, the chemical rearrangement Cox is complete at 25 mV sꢁ1. At
been largely investigated because many processes in bio- faster scan rates, this chemical rearrangement is not complete
inorganic chemistry, catalysis, and supramolecular chemistry and another reduction wave, i.e. Epc2, appears at about 0.2 V
involve electron-transfer steps in copper complexes.1 As illu- more anodic potential vs. Epc1. As described earlier, we postulate
strated in Table 5, the 1st reduction peak potential of each that the metastable forms of Cu(I) and Cu(II)-complexes should be
complex, i.e. Epc, depends on the scan rate since its value coordinated by solvent molecule(s).54 Fig. 11 displays the CVs
increases as the scan rate decreases. This behaviour suggests obtained with [CuL8(DMF)2](ClO4)2 as a relevant complex in the
that the reduction of the initial and stable geometric form acyclic series. For each scan rate, the CVs in the positive part
Cu(II)st gives intermediate metastable species, i.e. Cu(I)met, display three re-oxidation waves. The 1st wave, i.e. Epa1 at a
which can rearrange into a more stable form, i.e. Cu(I)st, potential of about 110 mV vs. SCE, was attributed to the
according to a two-step EredCred reaction, that is:
re-oxidation of Cu(I)st to Cu(II)met1. The two other waves,
corresponding to Epa2 and Epa3, were ascribed to the re-oxidation
of metastable Cu(I)met1 and Cu(I)met2 species formed by the
reduction of Cu(II)st during the 1st scan.
However, reversing the scan direction at 800 mV induces
only one reduction peak which was clearly observed at 25
and 100 mV sꢁ1 (i.e. Epc1). Moreover, another reduction step
Ered step: Cu(II)st + e # Cu(I)met
%
followed by a
Cred step: Cu(I)met - Cu(I)st
In an attempt to characterize the re-oxidation steps of occurred at about +0.25 V, i.e. Epc2, at 500 mV sꢁ1 with a very
Cu(I)mt and Cu(I)st, CV between ꢁ0.4 and +0.8 V was performed. low current intensity. For each scan rate, the potential values of
424 | New J. Chem., 2014, 38, 419--429
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Fig. 12 EPR spectrum recorded at the X-band (10ꢁ3 M in DMF at 100 K) of
[CuL7(MeCN)]2+ (left) and [CuL8(DMF)2]2+ (right); experimental (upper
traces), simulation (lower traces).
Table 6 Anisotropic EPR-spectral data of Cu(II)-complexes from acyclic
ligands L5–9 (10ꢁ3 M in DMF at 100 K)
Fig. 11 CVs of [CuL8(DMF)2]2+ at 3 different scan rates: 25, 100, and
500 mV sꢁ1 in Ar-purged DMF with Bu4NPF6 0.1 M as a supporting
electrolyte at rt.
g
Tensor Hyperfine coupling tensor to 63Cu
A(Cu)> 10ꢁ4 A(Cu)J 10ꢁ4 gJ/AJ
Complex
g>
gJ
cmꢁ1
cmꢁ1
cm
Epc1 were close to those summarised in Table 5. It was inferred
that the chemical rearrangements after the electronic transfer
may be easier and the chemical reaction constants associated
with the chemical steps Cox are faster in the more flexible
acyclic series than in the cyclic series. However, one should
notice that these systems can be almost totally reversed at scan
rates below 100 mV sꢁ1 and 25 mV sꢁ1 in the acyclic series and
cyclic series, respectively. Comparison of these results (for the
acyclic and cyclic series) with those obtained by others with
analogous mononuclear Cu(II) complexes coordinated by nitro-
gen or sulfur atoms in the ligands show that the Cu(II)/Cu(I)
couples occur within about the same potential domain,
between ca. ꢁ0.4 and ꢁ1.0 V vs. SCE.54 However, chemical
reorganisation steps inducing the formation of metastable
intermediates, i.e. Cu(II)met and Cu(I)met, seem to be easier
with diaza-crown ethers due to the weaker energy of the Cu–O
bond than those of Cu–N or Cu–S bonds.55 More than two
conformational metastable intermediates are generated for
both oxidized and reduced species, preceding or succeeding the
electron transfer steps in diaza-crown ether copper(II) complexes.
Finally, the global mechanism is likely more complicated than the
classical square-scheme mechanism generally proposed for elec-
tron transfer in Cu(II)/Cu(I) systems. This may be attributed mainly
to the weakness of the Cu–O bond strength by comparison to the
Cu–N and Cu–S bond strengths. Altogether, the present electro-
chemical study shows that the electron-transfer process is accom-
panied by bond ruptures (or formations) and changes in
reversible ligand conformation particularly in the acyclic series.
[CuL5(MeCN)]2+ 2.056 2.289 (B60%) 49
2.220 (B40%)
160
163
183
186
185
192
143
136
122
120
121
116
[CuL6(ClO4)]+ 2.054 2.233
[CuL7(MeCN)]2+ 2.058 2.234
[CuL8(DMF)2]2+ 2.058 2.236
[CuL9(ClO4)]+ 2.054 2.237
14
12
13
9
of a Cu(II)-ion coordinated with four N-nuclei within a planar
(or near planar) configuration.57
Since EPR parameters may also reflect the degree of distor-
tion of copper(II) complexes, the gJ/AJ ratio is given as a rough
estimate of coordination geometry. Values of 110–120 are
reported as typical of ‘planar’ complexes while the range
between 130–150 is characteristic of slight to moderate distortion
and 180–250 indicates strong distortion.58 Higher frequency,
e.g. the Q-band EPR spectroscopy, is well known to facilitate the
interpretation of intricate spectra where several interactions are
intermingled.
Although the Q-band was not required to solve the geometry
of [CuL6(ClO4)]+ (i.e. Fig. 13, right), it will serve as a reference
for comparison with more complicated systems. It appears that
the X-band and Q-band parameters for simulations are quite
consistent with the geometry of the complex in the solid state.
These parameters are also very similar to those of L7 and L8
EPR measurements
Acyclic complexes (i.e. from ligands L5–9) will be discussed first
as they displayed much more simple EPR-spectra than cyclic
ones. The X-band EPR spectra of [CuL7(MeCN)]2+ and
[CuL8(DMF)2]2+ complexes recorded at 100 K in frozen DMF
are presented in Fig. 12 together with simulations.
Fig. 13 EPR spectrum recorded at the Q-band and 100 K (10ꢁ3 M in DMF
at 100 K) of [CuL5(MeCN)]2+ (left) and [CuL6(ClO4)]+ (right); experimental
(upper traces), simulation (lower traces).
Fit parameters of acyclic complexes (see Table 6) are typical
of a square-based coordination polyhedra56 and characteristic
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Cu(II)-complexes. The parallel components of the EPR spectra
of L6, L7, L8, and L9 Cu(II)-complexes (see Table 6) confirm the
rather undistorted ‘planar’ geometry in frozen solution consis-
tent with their X-ray crystal structures. [CuL6(ClO4)]+ and
[CuL7(MeCN)]2+ do not show any significant difference in their
EPR parameters despite the supplementary bond between the
triazole and the phenyl ring in L6 and their different coordina-
tion mode in the solid state. Yet, the Q-band EPR spectrum of
[CuL5(MeCN)]2+ in frozen solution (see Fig. 13, left) requires two
components for a satisfactory fit with two slightly different values
of gJ. The major contribution (ca. 60%) points to an increasing Fig. 15 EPR spectrum of [CuL2]2+ recorded at the Q-band (10ꢁ3 M in
DMF at 100 K) experimental (upper trace), simulation (lower trace).
axial character with an increase in gJ and a decrease in AJ as
compared to [CuL7(MeCN)]2+, [CuL8(DMF)2]2+, and [CuL9(ClO4)]+
complexes (see S1 in ESI†). Concerning [CuL5(MeCN)]2+, the
Table 8 Anisotropic EPR-spectral data of [CuL2]2+ (10ꢁ3 M in DMF at 100 K)
higher value of gJ/AJ is indicative of a moderate distortion in
agreement with its crystalline structure which might correspond
S = 1/2 (mononuclear complex, weight: 37%)
to the more stable conformer in DMF at 100 K.59 In the cyclic
series, the Q-band spectra of [CuL3]2+ and [CuL4]2+ are similar in
frozen solution (see Fig. 14, left part for experimental and
simulation spectra of [CuL3]2+ and ESI† S2 for [CuL4]2+). Such
signals are characteristic of a mixture of binuclear species, that
means species in which two close copper nuclei interact via an
intermolecular way, and mononuclear copper species.60–63 Fig. 14’s
right part illustrates the component positions of tensor g (gx, gy
gJ
g>
AJ (10ꢁ4 cmꢁ1
)
gJ/AJ
2.258
2.045
177
127
S = 1/2 (mononuclear complex, weight: 63%)
gJ
g>
AJ (10ꢁ4 cmꢁ1
)
gJ/AJ
2.182
2.061
177
123
The EPR data of mononuclear and dinuclear complexes of
and gz) of binuclear species with the zero-field splitting D and [CuL3]2+ in Table 7 were obtained by simulation of the entire
the hyperfine contribution Az for two equivalent copper nuclei. spectrum.
The index J or > is related to the mononuclear complex. Please,
The gJ/AJ ratio is high compared to the acyclic complexes as
note that AJ is roughly twice as high as Az in agreement with the expected for a more distorted environment of the metal ion.59
delocalization of the electronic cloud over two copper nuclei. The Q-band spectrum of [CuL2]2+ (vide infra Fig. 15) results
The value of 2D was assessed from the splitting of copper from the superimposition of two mononuclear species with very
parallel lines (180 G). Within the point-dipole approximation, it low dipolar interaction between them as revealed by a weak
corresponds to a rCu–Cu spin–spin distance of ca. 5.9 Å.
peak at 1163 mT (-). Such a faint contribution is not visible on
the parallel part of the spectrum. Indeed, all peaks in the low
field zone are equidistant.
EPR data for the [CuL2]2+ complex are summarized in
Table 8.
A tentative explanation is these solutions contain at 100 K a
mixture of two more or less distorted structural species (i.e.
conformers) one of them promoting the formation of dimeric
species. The number and kind of bonds, and the so-called
‘crown effect’, obviously influence the flexibility and the stability
of such complexes in DMF, especially at low temperatures.59,62
Fig. 14 EPR spectrum of [CuL3]2+ recorded at the Q-band (10ꢁ3 M in
DMF at 100 K); left: experimental (upper trace), simulation (lower trace);
right: visualization of the EPR features associated with the di-nuclear
complex (S = 1) labelled x, y, and z and with the mono-nuclear complex
Conclusion
(S = 1/2) labelled and
.
>
J
Novel ligands bearing two 1,4-substituted-triazole moieties as
coordinating side-arms (i.e. L2–9) were easily prepared in one
step from two commercial N,N0-dimethyl-alkyl diamines or
from a known C2-symmetric N,N0-dipropargyl-diaza-18-crown-6
via the Cu(I)-catalyzed Huisgen dipolar cycloaddition. Changing
the alkyl-substituent of the azide (i.e. benzyl, phenyl, p-methoxy-
or p-chloro-phenyl) gave access to stable mononuclear Cu(II)-
complexes with different geometries and coordinations in the
solid state. Their electrochemical studies proved that the elec-
tron transfer induces intricate but reversible reorganisation of
Table 7 Anisotropic EPR-spectral data of [CuL3]2+ (10ꢁ3 M in DMF at 100 K)
S = 1 (dinuclear species, weight: 17%)
gz
gy
gx
Az (10ꢁ4 cmꢁ1
)
D (10ꢁ4 cmꢁ1
)
2.250
2.115
2.003
65
84
S = 1/2 (mononuclear complex, weight: 83%)
gJ
g>
AJ (10ꢁ4 cmꢁ1
107
)
gJ/AJ
2.214
2.061
2.039
207
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the cation coordination shell with a first cathodic peak varying measurement. All potentials were quoted versus SCE. Under
within a maximum 0.16 V range. The EPR-spectrum of cyclic these conditions the redox potential of the couple Fc+/Fc was
[CuL2]2+, [CuL3]2+ & [CuL4]2+ – and to a smaller extent that of found to be 0.47 V. EPR spectra were recorded using a
[CuL5(MeCN)]2+ – exhibited two mononuclear species (or con- continuous-wave EMXplus spectrometer (Bruker Biospin GmbH,
formers) signals in frozen DMF at 100 K, one of them promoting Germany) equipped with a high sensitivity resonator (4119HS-
the formation of dinuclear species as a minor component. W1, Bruker). The spectrometer was tuned such that settings
Altogether these behaviours suggest that such complexes could (modulation coils, incident microwave power) were not distort-
be proposed as catalysts (e.g. for the oxidation of alcohols in ing the EPR signal (X-band Larmor frequency B9.3 GHz and
carbonyl compounds or as artificial enzymes).64 Lastly, the cyto- Q-band Larmor frequency B34 GHz). Measurements were carried
toxicity of some of these complexes towards uterine human out in frozen 10ꢁ3 M DMF (ACS reagent Z 99.8% GC) solutions of
sarcoma cells was investigated and showed promising activities.65 copper(II) complexes held at 100 K with liquid nitrogen. Simula-
tions were generated with the EasySpin free software.67
General procedure I
Experimental section
Synthesis of ligands L2–4. To a stirred suspension of N,N0-bis-
Chemicals and solvents (reagent grade or better) were pur- propargyl-diaza-18-6 crown ether A49 (500 mg, 1.47 mmol) in a
chased from Sigma-Aldrich except spectrophotometric-grade mixture of water–acetonitrile (1 : 1, 25 mL) at rt were added the
DMF which was purchased from Alfa-Aesar [ref. 13808]. Anhy- aryl-azide (2.1 eq.), Na-ascorbate (117 mg, 0.4 eq.), and CuSO4ꢃ
drous solvents were dried by usual procedures and stored over 4 5H2O (74 mg, 0.2 eq.) under Ar. The resulting brownish
Å molecular sieves under Ar before use. Chromatography was solution was stirred overnight at rt under Ar. The reaction
carried out using Grace silicagel (Davisils LC60A 70–200 mm) mixture was filtered, extracted with CH2Cl2 (4 ꢀ 90 mL), and
or basic Al2O3 when stated (Sigma-Aldrich ref. 199443). 1H & the organic phases were combined, washed with 5% aq.
13C-NMR spectra were recorded on a Bruker AC250 spectro- NH4OH (2 ꢀ 20 mL), brine (20 mL), water (3 ꢀ 20 mL), dried
1
meter at 250 MHz for H and 62.9 MHz for 13C in CDCl3 if no over MgSO4, and finally evaporated under reduced pressure.
other conditions are stated. Chemical shifts d were measured The crude material was purified by liquid chromatography on
with respect to the CH3CN signal (2.10 ppm for 1H and 1.89 ppm silica (and/or alumina) with mixtures of CH2Cl2–MeOH as
for 13C-NMR). CHN elemental analyses were performed using eluents.
a Thermo Finnigan EA 1112 Series Flash Elemental Analyzer.
ES+-HRMS spectra were obtained using a Bruker micrOTOFq
General procedure II
mass spectrometer with capillary tensions between ꢁ1200 and Synthesis of ligands L5–9. A solution of symmetric sec.
ꢁ4500 V from diluted MeCN solutions. FT-IR spectra were N,N0-dimethylamine (B or C, 5.0 mmol), triethylamine (1.5 mL,
obtained from KBr-pellets (if not other stated) on a Perkin Elmer 2.15 eq.), and propargyl bromide (80 wt% in toluene, 1.2 mL,
Spectrum 1000 spectrophotometer. UV-visible spectra were 2.15 eq.) in a mixture of acetonitrile–water (1 : 1, 90 mL) was
recorded on a Perkin-Elmer Lambda1050 UV-Vis-NIR spectro- stirred for 90 min at rt under Ar. The organic azide (2.15 eq.),
photometer using a 1 cm-optical-pathlength cell at T = 298 K. Na-ascorbate (0.4 g, 0.4 eq.), and CuSO4ꢃ5H2O (0.25 g, 0.2 eq.)
X-ray structures were determined using a Bruker-Nonius Kappa were then added and the resulting brownish solution was stirred
APEXII or a Agilent SuperNova diffractometer equipped with a overnight at rt under Ar. The reaction mixture was filtered on a
low temperature device using liquid N2 to perform XRD intensity sintered glass, extracted with CH2Cl2 (4 ꢀ 90 mL), and the
measurements near 100 K. X-ray source was a sealed tube giving organic phases were combined, washed with 5% aq. NH4OH
Mo Ka radiation (l = 0.71073 Å). Diffraction pattern frames were (2 ꢀ 20 mL), brine (20 mL), water (3 ꢀ 20 mL), dried over MgSO4,
processed with Apex or CrysAlis software. The structures were and finally evaporated under reduced pressure. The crude result-
solved by direct methods using the SIR software package66a and ing gum was purified by liquid chromatography (LC) on silica or
refined by full matrix least-squares on F squared using SHELXL alumina with mixtures of CH2Cl2–MeOH as eluents.
software.66b In both crystal structure models, non-hydrogen
General procedure III
atoms were located in the difference Fourier syntheses and were
refined with anisotropic thermal parameters. Observed hydrogen Synthesis of Cu-complexes. Warning: perchlorates are poten-
atoms were placed at calculated positions using a riding model. tially explosive compounds. While we have experienced no
Cyclic voltammetry (CV) was performed using a Radiometer problems during our syntheses, extreme care must be taken
PST006 potentiostat using a conventional three-electrode cell when working with perchlorate complexes or salts and only
at rt. The KCl calomel electrode (SCE) was separated from the small quantities should be handled! Cu(ClO4)2ꢃ6H2O (1.2 eq.)
test compartment using a bridge tube. The test solution was was dissolved in a solution of the ligand (50.0 mg) in a mixture
dimethylformamide containing 10ꢁ1 M tetrabutylammonium of acetonitrile–water–EtOH (3 mL, 1 : 1 : 1) at rt. The resulting
hexafluorophosphate as the supporting electrolyte. The working suspension was stirred for 2 h at 70 1C, allowed to cool to rt,
electrode was a 10 mm Pt wire and the counter-electrode a 1 cm2 filtered through a small cotton pad, concentrated to half,
vitreous carbon disc. 5.0 ꢀ 10ꢁ4 M fresh solution of each studied diluted with two drops of DMF (ca. 30 mg), and finally stored
compound was used and purged for 5 min with Ar before each for several days in the dark at 4 1C. Crystallographic-suitable
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18 E. Hao, Z. Wang, L. Jiao and S. Wang, Dalton Trans., 2010,
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rinsed with dist. H2O, CH2Cl2, and finally dried at rt in the dark.
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21 T. L. Mindt, C. Schweinsberg, L. Brans, A. Hagenbach,
Acknowledgements
M. A. gratefully acknowledges financial support from the French
National Research Agency via a post-doctoral grant (ANR-09-BLAN-
0180-01). The authors warmly thank the ‘Institut Jean Barriol’ for
X-ray diffraction measurement facilities and Dr Slimane Dahaoui
(radio-crystallographic measurements and structure determina-
´
U. Abram, D. Tourwe, E. Garcia-Garayoa and R. Schibli,
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