Synthesis of an unsymmetrical N-functionalized triazacyclononane ligand and its Cu(II) complex

Article abstract of DOI:10.1016/j.ica.2013.12.036

The unsymmetrical 1,4-bis(2-aminophenyl)-7-(pyridin-2-ylmethyl)-1,4,7- triazacyclononane ligand (L3) has been prepared and characterized by NMR spectroscopy. The L3 ligand is based on the triazamacrocycle ring bearing one flexible 2-pyridylmethyl linked to the macrocycle group via the methyl group, and two rigid 2-aminophenyl pendant donor groups linked to the macrocycle via the aromatic carbon atoms. Reaction of this ligand with Cu(ClO₄) ₂·6H₂O afforded the corresponding complex [Cu(L3)](ClO₄)₂·H₂O (4) which was structurally characterized both in solid state and in solution. The crystal structure of 4 consists of a discrete monomeric [Cu(L3)]²⁺ in which the Cu(II) ion is six coordinated with three nitrogen atoms of the macrocycle ring, two of the aminophenyle and one of the pyridine appended functions. The triazacyclonane macrocycle ring is facially coordinated and the N-donor atoms of the three pendant groups (two aniline and one pyridine groups), are disposed in the same side of the basal macrocyclic ring, leading to a distorted and elongated CuN₄N₂ octahedron. UV-Vis spectroscopy of complex 4 in acetonitrile displays a d-d transition band at λ = 673 nm. The voltammetric studies show that the [Cu(L3)]²⁺ cation can be reduced quasi-reversibly and oxidized irreversibly, both process being monoelectronic.

Full text of DOI:10.1016/j.ica.2013.12.036

Inorganica Chimica Acta xxx (2014) xxx–xxx
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis of an unsymmetrical N-functionalized triazacyclononane
ligand and its Cu(II) complex
⇑
Mélissa Roger, Véronique Patinec , Raphaël Tripier, Smail Triki, Nicolas Le Poul, Yves Le Mest
UMR 6521 CNRS – Université de Brest, SFR ScInBioS, UFR des Sciences et Techniques, 6 avenue Victor le Gorgeu, C.S. 93837, 29238 Brest Cedex 3, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online xxxx
Metals In Supramolecular Chemistry Special
Issue
The unsymmetrical 1,4-bis(2-aminophenyl)-7-(pyridin-2-ylmethyl)-1,4,7-triazacyclononane ligand (L3)
has been prepared and characterized by NMR spectroscopy. The L3 ligand is based on the triazamacrocy-
cle ring bearing one flexible 2-pyridylmethyl linked to the macrocycle group via the methyl group, and
two rigid 2-aminophenyl pendant donor groups linked to the macrocycle via the aromatic carbon atoms.
Reaction of this ligand with Cu(ClO4)2ꢀ6H2O afforded the corresponding complex [Cu(L3)](ClO4)2ꢀH2O (4)
which was structurally characterized both in solid state and in solution. The crystal structure of 4 consists
of a discrete monomeric [Cu(L3)]2+ in which the Cu(II) ion is six coordinated with three nitrogen atoms of
the macrocycle ring, two of the aminophenyle and one of the pyridine appended functions. The triazacy-
clonane macrocycle ring is facially coordinated and the N-donor atoms of the three pendant groups (two
aniline and one pyridine groups), are disposed in the same side of the basal macrocyclic ring, leading to a
distorted and elongated CuN4N2 octahedron. UV–Vis spectroscopy of complex 4 in acetonitrile displays a
d–d transition band at k = 673 nm. The voltammetric studies show that the [Cu(L3)]2+ cation can be
reduced quasi-reversibly and oxidized irreversibly, both process being monoelectronic.
Keywords:
Macrocyclic ligand
Copper complex
X-ray crystal structure
Electrochemistry
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
transition metal ion depends strongly on the geometry and on
these pendant arms. Therefore, such N-functionalized groups im-
More and more attentions are being attracted by the research
on copper metal complexes due to their implications in many dif-
ferent domains, such as nuclear medicine for imaging diagnosis
(PET) and radioimmunotherapy (RIT) [1,2], biochemistry for mim-
icking active sites of metalloenzymes [3–8], sensing for cation
detection [9–12]. These various applications usually necessitate
the design and tailoring of ligands which fit at best in the require-
ments. The polyazacycloalkane ligands such as tacn (1,4,7-triaza-
cyclononane) are very much investigated in these different
research fields owing to their ability to form very stable complexes
with transition metals cations [13–15] and their possible easy
transformation by C- or N-functionalization of the macrocyclic
platform [16,17]. The small azamacrocycle tacn is known for coor-
dinating transition metals as copper (II) in a facial manner, with
the cation located above the plane formed by the three nitrogen
atoms of the cycle. Additional pendant arms containing donor
atoms on the triamine macrocycle complete the coordination
sphere of the cation leading to the high stability of the complexes
[13–15]. Configurational environment of the ligand around the
pact on the cation coordination sphere nature and geometry and,
then on the complex properties. Among the already described
[CuN6] tacn complexes, two complexes exhibiting five-membered
chelate rings, caught our attention regarding the flexibility degree
of the triazamacrocyclic coordinating arms. The first one is based
on the macrocycle ligand bearing three flexible 2-pyridylmethyl
which are linked to the N3-macrocycle group via methylene groups
(L1) [18,19]; the second one is based on the macrocycle ligand
involving three rigid 2-aminophenyl pendant donor groups, linked
to the macrocycle group via the aromatic carbon atoms (L2) [20]
(Scheme 1).
Herein, we report the synthesis and characterization of the
intermediary system based on unsymmetrical 1,4,7-triazacyclo-
nonane (L3) and of its Cu(II) complex [Cu(L3)](ClO4)2ꢀH2O (4),
including its spectroscopic and electrochemical properties.
2. Experimental
2.1. Reagents and techniques
⇑
Solvents and reagents were obtained from commercial suppli-
ers, and were used without further purification. Starting tacn was
Corresponding author. Tel.: +33 298017927; fax: +33 298017001.
E-mail address: veronique.patinec@univ-brest.fr (V. Patinec).
http://dx.doi.org/10.1016/j.ica.2013.12.036
0020-1693/Ó 2014 Elsevier B.V. All rights reserved.
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2
M. Roger et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx
N
NH2
NH2
N
N
N
NH2
NH2
N
N
N
N
N
N
N
N
N
NH2
L3
L1
L2
Scheme 1. Ligands discussed in this paper.
purchased from CheMaTech (Dijon, France). Elemental analyses
were performed by the ‘‘Service Central d’Analyses du CNRS’’,
Gif-sur-Yvette, France. Infrared spectra were recorded in the range
4000–200 cmꢁ1 on a FT-IR BRUKER ATR VERTEX70 Spectrometer.
Diffraction analyses were performed using an Oxford Diffraction
Xcalibur -CCD diffractometer. NMR and MALDI mass spectra were
carried out by the ‘‘Services communs’’ of the University of Brest.
MALDI mass spectra were recorded with an Autoflex MALDI TOF
III LRF200 CID spectrometer. NMR spectra were recorded on a
with CHCl3 (2 ꢂ 20 mL) allowed to eliminate traces of organic
impurities. The aqueous solution was made basic (pH >12) with
NaOH pellets and extracted with CHCl3 (3 ꢂ 20 mL), dried over
MgSO4, filtered and the solvant evaporated to yield the compound
(2) as a coloured oil (740 mg, 75%). NMR (CDCl3, 300 MHz) 1H
2.44–2.45 (m, 8H, CH2tacn) 2.56–2.58 (m, 4H, CH2tacn) 3.11 (bs,
2H, NH) 3.65 (s, 2H, CH2pyr) 6.89 (m, 1H, CHpyr) 7.17 (m, 1H, CHpyr
)
7.39 (m, 1H, CHpyr) 8.28 (m, 1H, CHpyr); 13C (CDCl3, 75 MHz) 45.9
46.3 52.4 (CH2tacn) 61.6 (CH2pyr) 121.5 122.5 135.9 148.5 (CHpyr
)
Brucker Avance 400 (400 MHz) or
a
Bruker AMX-3 300
159.3 (Cpyr).
(300 MHz). UV–Vis–NIR spectroscopy was performed with a JASCO
V-670 spectrophotometer with a classical cell holder. The electro-
chemical studies in acetonitrile were performed in a glovebox
(Jacomex) (O2 <1 ppm, H2O <1 ppm) with a home-designed 3-elec-
trodes cell (WE: vitreous carbon electrode, RE: Pt wire in a solution
of CH3CN/NBu4PF6 containing equimolar amounts of ferrocene and
ferrocenium hexafluorophosphate, CE: Pt). Acetonitrile (CH3CN)
(99.9% BDH, VWR) was distilled over CaH2 and stored after
freeze-pumping in the glovebox under argon. NBu4PF6 was synthe-
sized from NBu4OH (Fluka) and HPF6 (Aldrich). It was then purified,
dried under vacuum for 48 h at 100 °C, then kept under N2 in the
glovebox. The potential of the cell was controlled by an AUTOLAB
PGSTAT 302 (Ecochemie) potentiostat monitored by a computer.
Ferrocene (Fc) was added at the end of each experiment to deter-
mine accurate redox potential values.
2.2.1.3. Preparation of 1,4-bis(2-nitrophenyl)-7-(pyridin-2-ylmethyl)-
1,4,7-triazacyclononane (3). 1-fluoro-2-nitrobenzene (370 L,
l
3.5 mmol) was added to 355 mg of 1-(pyridin-2-ylmethyl)-1,4,7-
triazacyclononane (1.6 mmol) and an excess of potassium carbon-
ate (1.10 g, 8.0 mmol (5 eq)) in distilled acetonitrile (20 mL). The
reaction mixture was stirred at reflux under nitrogen atmosphere
during 12 h. The hot solution was filtrated and the filtrate evapo-
rated under reduced pressure. The residue was purified by silica
gel chromatography (Hexane then Hexane/CHCl3:1/1) to yield an
orange oil (690 mg, 93%). NMR (CDCl3, 300 MHz) 1H 2.89 (m, 4H,
CH2tacn) 3.35 (m, 4H, CH2tacn) 3.68 (bs, 4H, CH2tacn) 3.77 (s, 2H,
CH2pyr) 6.81 (t, 2H, CHPhe) 7.00 (d, 2H, CHPhe) 7.08 (t, 1H, CHpyr
7.34 (m, 3H, CHPhe + CHpyr) 7.53 (t, 1H, CHpyr) 7.59 (dd, 2H, CHPhe
8.44 (d, 1H, CHpyr); 13C (CDCl3, 75 MHz) 53.8 54.5 55.1 (CH2tacn
)
)
)
Cautions! Perchlorate salts of metal complexes are potentially
explosive and should be handled with care in small quantities.
64.2 (CH2pyr) 118.6 119.4 (CHPhe) 121.9 123.3 (CHpyr) 126.1 132.8
(CHPhe 136.2 (CHpyr)141.1 143.7(C) 148.8 (CHpyr)159.1(C);
MALDI-TOF: m/z 463.2 [M+1+].
)
2.2. Synthetic procedures
2.2.1.4.
Preparation
of
1,4-bis(2-aminophenyl)-7-(pyridin-2-
2.2.1. Synthesis of 1,4-bis(2-aminophenyl)-7-(pyridin-2-ylmethyl)-
1,4,7-triazacyclononane (L3)
2.2.1.1. Preparation of 10-phenyl-1,4,7-triazabicyclo[5.2.1] decane
(1). 5 mmol of 1,4,7-triazacyclononane (645 mg) and benzalde-
ylmethyl)-1,4,7-triazacyclononane
(L3). 1,4-bis(2-nitrophenyl)-7-
(pyridin-2-ylmethyl)-1,4,7-triazacyclononane (325 mg, 0.80 mmol)
in absolute ethanol (30 mL) was stirred at reflux under nitrogen
atmosphere during 3 days with 10 mL (excess) of hydrazine monohy-
drate and activated carbon. After cooling, the solution was filtrated
and the filtrate was evaporated under reduced pressure. The residue
was dissolved in CHCl3 (20 mL) and MgSO4 was added. After filtra-
tion, elimination of the solvent under reduced pressure yielded the
product as a brown oil (205 mg, 73%). NMR (CDCl3, 400 MHz) 1H
hyde (508 lL, 5 mmol) were stirred at room temperature in dis-
tilled ethanol (80 mL) containing molecular sieve for 4 h. The
solution was filtered and evaporated under reduced pressure to
yield the aminal product as a white solid (980 mg, 4.5 mmol,
90%). NMR (CDCl3) 1H (300 MHz) 2.89–2.93 (m, 4H,CH2tacn) 2.99–
3.03 (m, 2H,CH2tacn) 3.07–3.17 (m, 4H,CH2tacn) 3.32–3.39 (m,
2.99–3.02 (m, CH2tacn, 4H) 3.29–3.31 (m, CH2tacn, 4H) 3.35 (s, CH2tacn
4H) 3.94 (s, CH2pyr, 2H) 6.65–6.72 (m, 4H, CHPhe) 6.89 (t, 2H, CHPhe
,
)
2H,CH2tacn) 5.66 (s, 1H, Haminal) 7.18 (t, 1H, HPhe) 7.29 (t, 2H, HPhe
)
7.50 (2H, d, HPhe); 13C (75 MHz) 49.3 49.6 58.8 (CH2tacn) 88.3
(Caminal) 126.6, 126.7 128.2 (CHPhe) 145.8 (CPhe).
7.07 (d, 2H, CHPhe) 7.18 (t, 1H, CHpyr) 7.46 (d, 1H, CHpyr) 7.67 (t, 1H,
CHpyr) 8.56 (d, 1H, CHpyr); 13C (CDCl3, 100 MHz) 55.8 56.0 56.7
(CH2tacn) 64.9 (CH2pyr) 115.4 118.2 (CHPhe) 122.0 (CHpyr) 123.3 (CHPhe
)
123.4 (CHpyr) 124.4 (CHPhe) 136.3 (CHpyr) 140.9 142.7 (C) 149.1 (CHpyr
)
2.2.1.2. Preparation of 1-(pyridin-2-ylmethyl)-1,4,7-triazacyclo-
nonane (2). 10-phenyl-1,4,7-triazabicyclo[5.2.1] decane (1)
(4.5 mmol, 980 mg) and 575 mg of 2-methylpyridine chloride
(4.5 mmol) were stirred with potassium carbonate (3 g, excess)
in distilled acetonitrile at room temperature for 4 days. Filtration
and solvant elimination gave a brown oily product. After hydrolysis
in HCl 1 M (15 mL) at room temperature for 3 h, extraction at pH1
159.4 (C); MALDI-TOF: m/z 403.2 [M+1+].
2.2.2. Synthesis of [Cu(L3)](ClO4)2ꢀH2O (4) complex
An aqueous solution (5 mL) of L3 (0.05 mmol, 20.13 mg) was
added progressively, under continuous stirring, to an aqueous
solution (10 mL) of Cu(ClO4)2ꢀ6H2O (0.05 mmol, 18.53 mg). Slow
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M. Roger et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx
3
evaporation of the resulting blue solution gave blue crystals of 4.
Yield: 24.2 mg, 71.0%. Anal. Calc. for C24H32Cl2CuN6O9: C, 42.2; H,
4.7; N, 12.3. Found: C, 41.9; H, 4.8; N, 12.5%. IR (cmꢁ1): 3267(br),
3169(br), 1614(m), 1570(m), 1496(m), 1481(m), 1456(m),
1366(w), 1278(w), 1228(w), 1061(s), 1012(w), 924(w), 830(m),
810(m), 766 (w), 741(w), 618(s), 559(w), 532(w), 458(w). Single
crystals of 4 were used for crystallographic study and for the prep-
aration of an acetonitrile solution of 4 to perform UV–Vis spectro-
scopic and electrochemical studies.
tion were done with the CRYSALIS-CCD and CRYSALIS-RED programs on
the full set of data [21,22]. The crystal structures were solved by
direct methods and successive Fourier difference syntheses, and
were refined on F2 by weighted anisotropic full-matrix least-
square methods [23]. All non-hydrogen atoms were refined aniso-
tropically, while the hydrogen atoms were calculated and therefore
included as isotropic fixed contributors to Fc. All other calculations
were performed with standard procedures (WINGX) [24]. Crystal
data, structure refinement and collection parameters are listed in
Table 1.
2.3. X-ray crystallography
2.4. Solution studies
Crystallographic study of 4 was performed at 170 K, using an
Oxford Diffraction Xcalibur
a graphite monochromated Mo K
full sphere data collections were performed using 1.0° -scans with
an exposure time of 60 s per frame. Data collection and data reduc-
j
-CCD diffractometer equipped with
Spectroscopic and electrochemical studies were performed
using acetonitrile solution of single crystals of 4 (0.8 mM) contain-
ing electrolyte salt (NBu4PF6 0.1 M).
a
radiation (k = 0.71073 Å). The
3. Results and discussion
Table 1
Crystal data and structural refinement parameters for [Cu(L3)](ClO4)2ꢀH2O (4).
3.1. Synthesis
Empirical formula
Molecular weight
C24H32Cl2CuN6O9
683.00
The strategy used for L3 synthesis is based on the initial forma-
tion of 1-(pyridine-2-ylmethyl)-1,4,7-triazacyclononane (2) fol-
lowed by the introduction of two reductible 2-nitrophenyl arms
on the remaining secondary amine functions of the triazamacrocy-
cle. 1-(pyridin-2-ylmethyl)-1,4,7-triazacyclononane (2) has been
synthesised by exploiting the aminal protection route [25]: reac-
tion of tacn with benzaldehyde allowed the formation of 10-phe-
nyl-1,4,7-triazabicyclo[5.2.1] decane (1) leaving one secondary
amine function free to react with 2-methylpyridine chloride
according to a nucleophilic substitution. Acidic hydrolysis of the
corresponding product 4-(pyridin-2-ylmethyl)-10-phenyl-1,4,7-
triazabi-cyclo[5.2.1] decane with HCl 1 M allowed to recover the
1-(2-pyridylmethyl)(tacn) (2). The reaction of (2) with two equiv-
alents of 1-fluoro-2-nitrobenzene gave easily the tacn derivative
bearing two 2-nitrophenyl and one 2-pyridylmethyl arms (3) with
a very good yield (93%). Reduction of nitro into amino groups was
carried out by hydrazine hydrate in ethanol giving the ligand L3
with good yield (73%) (Scheme 2). NMR characterization and mass
spectroscopy are in fully accordance with L3 structure.
Space group
a (Å)
b (Å)
P21/n
10.6442(3)
16.4183(5)
16.6388(5)
93.255(3)
2903.1(2)
4
1.563
9.98
1412
23005
5.54–54.00
6318/0.0639
3799
379
0.0471/0.1174
0.976
c (Å)
b (°)
V (Å3)
Z
qcalc (g cmꢁ3
)
l
(cmꢁ1
F(000)
)
Reflections measured
2h range (°)
Reflections unique (Rint
Reflections with I > 2r(I)
)
Nv
aR1/bwR2
cGoodness-of-fit (GOF)
D
qmax,min (e Åꢁ3
)
+0.601, ꢁ0.353
P
a
R1 = |Fo ꢁ Fc|/Fo.
1=2
P
P
wR2 ¼ f ½wðF2o ꢁ Fc2Þ2ꢃ= ½wðF2oÞ2ꢃg
.
b
GOF ¼ f ½wðF2o ꢁ F2Þ2ꢃ=ðNobs ꢁ NvarÞg1=2
.
P
c
c
Cl
H
CHO
H
H
N
N
N
N
1
NH
NH
N
N
NH HN
(ii)
(iii)
(i)
2
N
iv)
NO2
NH2
NO2
N
N
3
F
NH2
NO2
(v)
N
N
N
N
N
N
L3
Scheme 2. Five steps synthesis of ligand L3. (i) Ethanol, molecular sieve, RT, 4 h; (ii) CH3CN, K2CO3, RT, 4 days; (iii) HCl 1 M, RT, 3 h/OHꢁ; (iv) CH3CN, K2CO3,
(v) NH2NH2, H2O/ethanol, activated carbon, , 3 days under N2.
D, 12 h under N2;
D
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4
M. Roger et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx
Table 2
Selected bond distances (Å) and bon angles (°) for complex 4.
Cu–N1
2.287(3)
Cu–N4
2.095(3)
Cu–N2
2.095(3)
Cu–N5
1.998(3)
Cu–N3
2.055(3)
Cu–N6
2.258(3)
N1–Cu–N2
N1–Cu–N3
N1–Cu–N4
N1–Cu–N5
N1–Cu–N6
N2–Cu–N3
N2–Cu–N4
N2–Cu–N5
82.17(10)
81.78(10)
79.33(10)
102.07(11)
160.90(10)
85.00(11)
160.50(11)
84.51(11)
N2–Cu–N6
N3–Cu–N4
N3–Cu–N5
N3–Cu–N6
N4–Cu–N5
N4–Cu–N6
N5–Cu–N6
96.44(11)
98.31(11)
168.21(11)
79.13(11)
93.37(11)
103.06(11)
96.72(12)
3.2. Crystal structure description
Single crystals of 4 were obtained by slow evaporation of an
aqueous solution containing triazamacrocyclic ligand (L3) and
Cu(ClO4)2 in a 1/1 ratio. The [Cu(L3)](ClO4)2ꢀH2O (4) complex crys-
tallizes in the monoclinic P21/n space group. The unit cell parame-
ters, crystal and refinement data, and the pertinent bond distances
and bond angles are summarized in Tables 1 and 2, respectively.
The crystal structure consists of
a discrete monomeric
[Cu(L3)]2+ cation (Fig. 1), two uncoordinated perchlorate anions
and one water molecule. The Cu(II) ion is six coordinated with
three nitrogen atoms of the macrocycle (N1, N2 and N3), two of
the phenylamine rings (N4 and N5) and one of the pyridine ring
(N6). Examination of the six Cu–N bonds allows us to describe
the metal environment as a distorted and elongated CuN4N2 octa-
hedron. The two apical positions (Cu–N1: Cu–N1: 2.287(3) Å; Cu–
N6: 2.258(3) Å) are occupied by one nitrogen atom belonging to
the macrocycle (N1) and one nitrogen atom (N6) of the 2-pyridyl-
methyl pendant arm; while the equatorial plane involves two short
(Cu–N3 and Cu–N5: 2.055(3) and 1.998(3) Å, respectively) and two
equivalent intermediate Cu–N (Cu–N2 and Cu–N4: 2.095(3) Å)
distances.
Fig. 1. Structure of cationic complex of 4 showing the atom labelling scheme and
the coordination environment of the Cu(II) ion (N1 and N6 are the two apical
positions of the elongated CuN4N2 octahedron).
As observed in other similar tacn N-functionalized macrocycle
ligands (L1 et L2) [18–20], the macrocycle is facially coordinated
and the N-donor atoms of the three pendant groups (two aniline
and one pyridine groups), are disposed in the same side of the basal
macrocyclic ring. This fact results from the relatively small size of
the tacn ring forcing the metal ion to remain well above the mean
ring plane. The distance of the Cu(II) metal ion to the perfect plane
generated by the three nitrogen atoms of the macrocycle ring (N1,
N2 and N3) is 1.37 Å.
Reports based on the parent Cu(II) discrete complexes involving
the tacn ring with similar pendant arms, reveal only two examples:
[(CuL1)2](ClO4)4 (5) [18,19] and [CuL2](ClO4)2 (6) [20]; where L1 li-
gand involves three flexible 2-pyridylmethyl pendant arms, while
L2 ligand involves three rigid 2-aminophenyl pendant groups
(see Scheme 1). Both complexes 5 and 6 have been reported as
exhibiting discrete structures [18–20]. However a perusal of the
Table 3
Cu–N distance in Cu(II) tacn N-functionalized macrocycle based complexes.
5
6
4
Fig. 2. UV–Vis spectrum of complex 4 (0.8 mM) in CH3CN/NBu4F6 (0.1 M).
Cu–N(amine)
Cu–N(py)
2.110(8)
2.111(8)
2.192(8)
2.041(8)
2.180(8)
2.239(9)
2.024(9)
2.193(8)
2.218(9)
2.055(3)
2.095(3)
2.287(3)
data for the metal environments in the two complexes (Table 3) re-
veals that they differ strongly by their metal environments: the
asymmetric unit of 5 consists of two different [CuL1]2+ cations;
the first one displays a regular elongated CuN4N2 octahedron, with
two long axial distances (2.192(8) and 2.198(8) Å) and four shorter
equatorial distances in the range 2.080–2.138 Å (see column 1 in
Table 3), nearly similar to that described for complex 4. However,
the second Cu(II) environment of 5 and that of complex 6 can be
viewed as CuN3N3 distorted octahedrons, since their corresponding
2.080(8)
2.138(8)
2.198(8)
1.995(8)
2.115(8)
2.211(9)
–
–
2.258(3)
Cu–N(aniline)
References
2.032(8)
2.081(9)
2.211(9)
1.998(3)
2.095(3)
–
[18,19]
[20]
This work
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M. Roger et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx
5
Table 4
3.3. Solution studies
UV–Vis and electrochemical data for complex 4 and analogous Cu complexes in
CH3CN/NBu4PF6.
The spectroscopic and electrochemical properties of the com-
plex 4 were investigated in acetonitrile containing electrolyte salt
(NBu4PF6 0.1 M) at room temperature. The UV–Vis spectrum of
the complex displays an absorption band at k = 683 nm
Complex
kmax (nm)
emax (Mꢁ1 cmꢁ1))
E0(1)/V
vs. Fc
Refs.
(
4
683 (110)
612 (210)a
693 (c)a
ꢁ0.71
This work
[26]
[18]
[20]
[27]
ꢁ0.81b
(e
= 110 Mꢁ1 cmꢁ1) (Fig. 2). The band at 683 nm can be ascribed
[Cu(dmptacn)] (ClO4)2
5
6
[Cu((BzNH2)3tacn)] (ClO4)2
[Cu(iPr3tacn)(CH3CN)2] (SbF6)2
c
to a d-d transition as classically observed for Cu(II) complexes. This
wavelength value is close to that found for analogous complexes in
the same solvents (Table 4). It confirms the tetragonal character of
the N-based coordination sphere surrounding the cupric ion, in
agreement with the solid state structure.
Voltammetric studies in CH3CN/NBu4PF6 at a vitreous carbon
electrode were performed to rationalize the effect of the L3 ligand
on the coordination of the Cu(II) ion, in comparison to existing
data. Fig. 3A displays the cyclic voltammetric behaviour of the
complex when scanning negatively until ꢁ2.1 V versus Fc (plain
line).
695 (107)a
605 (276)a
669 (170)d
c
c
ꢁ0.04e,f
[28,29]
a
b
c
d
e
f
Measured in CH3CN.
Measured in CH3CN/NBu4ClO4.
Not determined.
In CH2Cl2.
Assuming E0(Fc+/Fc) = +0.40 V vs. SCE.
Data determined from Cu(I) complex.
Two reduction peaks were detected at ca ꢁ0.8 V and ꢁ1.0 V and
crystallographic data reveals, in each case, two axial and one equa-
torial Cu-N distances, much more longer than the three other ones
(see columns 2 and 3 in Table 3), giving rise to a meridional (mer)
configuration.
the backscan of the more negative process indicated a Cu(0) anodic
⁄
redissolution peak (peak , Fig. 3A), as typically observed for Cu(I)
complexes upon reduction. This oxidation peak disappeared when
Fig. 3. Voltammetry at a vitreous carbon electrode in CH3CN/NBu4PF6 0.1 M (E/V vs. Fc): (A) Complex 4 (0.8 mM) (v = 0.1 V sꢁ1) with first vertex potential at ꢁ2.1 V (plain line)
⁄
and ꢁ0.9 V (dotted line); peak corresponds to Cu0 redissolution; inset: Rotating-Disk Electrode Voltammogram (1000 t minꢁ1); (B) Complex 4 (0.8 mM) (plain line) with
negative initial scan (v = 0.1 V sꢁ1) and ligand L3 (0.5 mM) (dotted line) with positive scan; (C) Complex 4 (0.8 mM) for 0.02 V sꢁ1 < v < 0.5 V sꢁ1 with negative initial scan,
inset (v = 5 V sꢁ1); (D) Complex 4 (0.8 mM) (v = 0.1 V sꢁ1) with positive initial scan. Inset: plots of |ipc(1)| and ipa(2) vs. v1/2
.
Please cite this article in press as: M. Roger et al., Inorg. Chim. Acta (2014), http://dx.doi.org/10.1016/j.ica.2013.12.036

6
M. Roger et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx
Epc(1)
NH2
NH2
NH2
N
Cu
N
Cu
N
1 e-
1 e-
+
1 e-
1 e-
+
-
I
CuIII
II
H2N
H2N
H2N
N
N
N
N
N
N
-
Epa(1)
N
Epa(2)
N
N
4
4
4
Scheme 3. Proposed-redox processes for complex 4.
reversing the scan at potential values (E) higher than ꢁ0.9 V
(Fig. 3A, dotted line). For ꢁ1.2 V < E < 1.3 V, rotating-disk electrode
voltammetry (RDEV) displayed two reduction and one oxidation
waves of almost equal current intensities (inset Fig. 3A). By corre-
lation with cyclic voltammetric results, the two reduction waves
are ascribed to the Cu(II)/Cu(I) and Cu(I)/Cu(0) processes (the high-
er slope of the wave for the second process supports the formation
of Cu(0) onto the electrode surface). On this basis, we restricted our
voltammetric studies on complex 4 and ligand L3 to potential val-
ues higher than ꢁ0.9 V. As depicted in Fig. 3B, the complex 4 (plain
Epa(1) and Epa(3) are correlated with the variation of the scan rate.
Such behaviour is indicative of a chemical equilibrium involving
two oxidizable species. This is frequently observed for Cu(II)/
Cu(I) systems as associated with the geometric reorganization be-
tween each redox state (five- or six-coordinate for Cu(II) against
four-coordinate for Cu(I)) [30].
In the present case, conformational changes may be related to
the decoordination of one or two coordinating amino arms by
reduction at the Cu(I) state, as previously observed for analogous
complexes [31–33]. As when
v is increased, the ipa(3)/ipa(1) ratio
0
0
line) shows a quasi-reversible system at E (1) = ꢁ0.71 V versus Fc
is also increased, shown by CV at variable scan rates (Fig. 3C),
the forward chemical reaction (decoordination) which follows
the electrochemical reduction would be faster than the backward
reaction (re-binding of the arm(s)). These processes are under
investigation for a full understanding of the mechanism.
Another interesting information which is given by voltammetry
is the irreversible oxidation process occurring at Epa(2). As already
mentioned the good match between peak heights at various scan
rates for the systems (1) and (2) suggests that the oxidation is
monoelectronic (Scheme 3). In each case, plots of ipa(2) and ipc(1)
versus v1/2 yield a linear slope as expected for a diffusion-limited
process (inset Fig. 3D). The detection of Cu(III) species by electro-
chemistry has been mainly reported for N/O macrocyclic systems
appending strong donating ligands [31–33]. In the present exam-
ple, the oxidation potential is relatively high in comparison to
these systems in analogous conditions. Interestingly, the energy
gap between Epa(2) and Epc(1) (ca 1.8 V) is relatively low compared
(D
Ep(1) = Epa(1) ꢁ Epc(1) = 110 mV), and two irreversible oxidation
peaks on the back scan at Epa(2) = +0.93 V and Epa(3) = +0.43 V
when starting negatively from the zero-current potential (0 V).
When starting positively (Fig. 3D), the oxidation peak at Epa(3) dis-
appeared while the systems (1) and (2) remained present. On the
other hand, the free ligand L3 showed two irreversible oxidation
peaks at 0.01 V and 0.43 V when scanning positively from
ꢁ0.90 V (Fig. 3B, dashed line). At last, the increase of the scan rate
(0.02 V sꢁ1 < v < 5 V sꢁ1) induced the decrease of intensity of the
peak at Epa(1) whereas the irreversible peak at Epa(3) increased
and the peak at Epa(2) remained present (Fig. 3C). At v = 5 V sꢁ1 (in-
set Fig. 3C), total irreversibility was detected for Epc(1). Exhaustive
electrolysis of the solution at E = ꢁ0.9 V on a graphite rod led to the
deposition of Cu on the electrode surface. The resulting RDEV and
0
0
CV showed the presence of the three systems at E (1), Epa(2) and
Epa(3) with predominance of the system at Epa(3). Exhaustive elec-
trolysis of the initial solution at E = 1.0 V lead to disappearance of
all systems.
to existing data (typically
DE > 2 V [34–38]). This effect may result
from the constraint exerted by the short spacer between the ani-
line moiety and the tacn core which destabilizes the Cu(II) redox
state and favours high reduction and low oxidation potentials.
RDEV and CV data show clearly that two redox processes can be
assigned to the complex 4: Cu(II)/Cu(I) for the reduction at Epc(1)
and Cu(III)/Cu(II) for the oxidation at Epa(2). This is inferred for
the latter as the similar peak heights imply that electronic transfer
involves the same number of charge in each process (insets Fig. 3A
and Scheme 3). This proposal is in agreement with previous elec-
trochemical analysis of [Cu(dmptacn)]2+ (two picolyl groups and
three amino from the tacn core) [26]. The CV behaviour of complex
4 indicates that both amino moieties are coordinated to the Cu(II)
ion as no free amine oxidation peaks are observed, (Fig. 3B and 3D).
4. Conclusions
The new functionalized 1,4-bis(2-aminophenyl)-7-(pyridin-2-
ylmethyl)-1,4,7-triazacyclononane ligand (L3) has been synthe-
sized using the aminal route for specifically grafting one 2-pyridyl-
methyl arm and then two aniline groups on the tacn core. Its
corresponding Cu(II) complex, [Cu(L3)](ClO4)2ꢀH2O (4), has been
prepared and structurally characterized. As the parent complexes
previously reported, the [Cu(L3)]2+ complex displays a discrete
monomeric [Cu(L3)]2+ in which the macrocycle ring is facially
coordinated and the N-donor atoms of the three pendant groups
(two aniline and one pyridine groups), are disposed on the same
side of the basal macrocyclic ring. However, complex 4 exhibits a
regular elongated CuN4N2 octahedron different from the two Cu(II)
complexes based on symmetrically functionalized tacn ligand (see
L1 and L2 ligands in Scheme 1) for which the metal environment
displays a CuN3N3 distorted geometry, close to a meridional
(mer) configuration. In addition to the nature of the metal ion, this
0
0
This is also supported by the fact that the E (1) value for 4 is much
lower than that obtained for the [Cu(iPr3tacn)]+ complex and sim-
ilar to that found for the five-coordinated [Cu(dmptacn)]2+ com-
plex (Table 4).
Focusing now on the reduction process occurring at Epc(1), CV
shows that the electrogenerated Cu(I) complex is not stable under
these experimental conditions and evolves rapidly towards a new
species which displays an oxidation potential peak at Epa(3). This
new species only results from the reduction of the initial Cu(II)
complex since the peak at Epa(3) does not appear when scanning
positively (Fig. 3D). Moreover, the relative intensities of peaks at
Please cite this article in press as: M. Roger et al., Inorg. Chim. Acta (2014), http://dx.doi.org/10.1016/j.ica.2013.12.036

M. Roger et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx
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Acknowledgments
The authors acknowledge the CNRS (‘‘Centre National de la
Recherche Scientifique’’), the Brest University, the french ‘‘Ministè-
re de l’Enseignement Supérieur et de la Recherche’’, the ‘‘Ministère
des Affaires Etrangères et Européennes’’ (PHC Maghreb project No.
30255ZJ) and the ‘‘Agence Nationale de la Recherche’’ (ANR project
BISTA-MAT: ANR-12-BS07-0030-01). Authors especially thank the
financial support from the Région Bretagne, France and also the
‘‘Service Commun’’ of NMR facilities of the University of Brest.
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NMR spectra (1H, 13C) of L3 ligand are provided in supplemen-
tary data. Crystallographic data have been deposited in the Cam-
bridge Crystallographic Data Centre under the CCDC number
958291 that contains the supplementary crystallographic data for
compound 4. These data can be obtained free of charge via the
web application at www.ccdc.cam.ac.uk/conts/retrieving.html [or
from the Cambridge Crystallographic Data Center, 12 Union Road,
Cambridge CB2 1EZ, UK: Fax: +44 1223 336 033; E-mail: depos-
it@ccdc.cam.ac.uk]. Supplementary data associated with this arti-
cle can be found, in the online version, at http://dx.doi.org/
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