Peculiar properties of homoleptic Cu complexes with dipyrromethene derivatives

Article abstract of DOI:10.1039/c3dt51541a

In view of preparing Cu polynuclear complexes with dipyrromethene ligands, the mononuclear complexes [Cu(ii)(dipy)₂] (dipyH = 5-phenyldipyrromethene) and [Cu(ii)(dpdipy)₂] (dpdipyH = 1,5,9-triphenyldipyrromethene) have been prepared and characterized by X-ray crystallography, mass spectrometry and EPR spectroscopy. Their peculiar redox and spectroscopic (absorption/emission) behaviours are discussed. In contrast to Cu^II complexes of 1,1′-bidypyrrin, the reduction electrolysis of [Cu(ii)(dpdipy)₂] leads to decomposition products on a time scale of a few hours. Moreover in relation to this observation, [Cu(i)(dpdipy) ₂]^- could not be synthesized in spite of the Cu ^I core protection by the phenyl substituents in ortho position of the nitrogen atoms. Theoretical calculations provide some explanations for this instability. Interestingly [Cu(ii)(dipy)₂] and [Cu(ii)(dpdipy) ₂] display weak luminescence at room temperature, attributed to a ligand centered emission.

Full text of DOI:10.1039/c3dt51541a

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Peculiar properties of homoleptic Cu complexes with
dipyrromethene derivatives†
Cite this: Dalton Trans., 2013, 42, 14188
Kathleen Servaty,a Emilie Cauët,b Fabrice Thomas,c Javiera Lambermont,a
Pascal Gerbaux,d Julien De Winter,d Margriet Ovaere,e Leen Volker,f
Nathalie Vaeck,b Luc Van Meervelt,e Wim Dehaen,f Cécile Moucheron*a and
Andrée Kirsch-De Mesmaeker*a
In view of preparing Cu polynuclear complexes with dipyrromethene ligands, the mononuclear com-
plexes [Cu(II)(dipy)2] (dipyH = 5-phenyldipyrromethene) and [Cu(II)(dpdipy)2] (dpdipyH = 1,5,9-triphenyldi-
pyrromethene) have been prepared and characterized by X-ray crystallography, mass spectrometry and
EPR spectroscopy. Their peculiar redox and spectroscopic (absorption/emission) behaviours are discussed.
In contrast to CuII complexes of 1,1’-bidypyrrin, the reduction electrolysis of [Cu(II)(dpdipy)2] leads to
decomposition products on a time scale of a few hours. Moreover in relation to this observation, [Cu(I)-
(dpdipy)2]− could not be synthesized in spite of the CuI core protection by the phenyl substituents in
ortho position of the nitrogen atoms. Theoretical calculations provide some explanations for this instabil-
ity. Interestingly [Cu(II)(dipy)2] and [Cu(II)(dpdipy)2] display weak luminescence at room temperature,
attributed to a ligand centered emission.
Received 10th June 2013,
Accepted 29th July 2013
DOI: 10.1039/c3dt51541a
www.rsc.org/dalton
stable in solution due to their oxidation of CuI into CuII,
except if bulky substituents such as phenyl groups are present
Introduction
Cu complexes with polyazaaromatic ligands have attracted in ortho position of the nitrogen atoms of the phen ligand that
several research teams mainly because these compounds protect the CuI centre. Such complexes are usually distorted as
exhibit a peculiar property, which consists of changing the compared to an ideal tetrahedral geometry,5 in which the two
structure or coordination number from CuI to CuII and vice chelating ligands are perpendicular to each other. Paramagnetic
versa. Thus the properties of these complexes depend very CuII complexes in contrast can adopt different geometries6–10
much not only on the ligands, but also on the oxidation state such as distorted octahedra, square pyramids or trigonal bipyra-
of the metal ion. Indeed it was demonstrated several years ago mids. The changes of geometries according to the oxidation
that CuI complexes can be prepared as tetrahedral diamagnetic state adopted by the Cu ion led to interesting and very nice appli-
compounds with two bidentate ligands such as 1,10-phenan- cations highlighted by J. P. Sauvage’s team. Thus Cu complexes
throline (phen) for example.1–4 However they are not very were used as templates in the synthesis of catenanes and
rotaxanes11–13 and their change of structure by electrochemical
oxidation/reduction was exploited for triggering movements.14–18
aOrganic Chemistry & Photochemistry, Université libre de Bruxelles, ULB, CP 160/08,
Such modifications of configuration by a change of the oxi-
50 Avenue F.D. Roosevelt, 1050 Brussels, Belgium. E-mail: akirsch@ulb.ac.be,
dation state of the Cu ion can be induced not only electro-
cmouche@ulb.ac.be; Fax: +32(0)2 650 3018; Tel: +32(0)2 650 3017
chemically but also by light absorption.19 Indeed, this type of CuI
bService de chimie quantique et photophysique, Université libre de Bruxelles, ULB,
complex has a first absorption band in the visible region of the
spectrum corresponding to an MLCT (Metal to Ligand Charge
Transfer) transition. Therefore excitation in this band pro-
motes an electron from CuI to one of the ligands, generating
in this way a CuII transient excited state.20,21 The resulting
movement initiated by this sudden change of oxidation state
and occurring with nonprotecting phen ligands makes the Cu
core accessible to a fifth monodentate nucleophilic ligand such
as Cl− or a water molecule, responsible for the short excited
state lifetimes of these nonprotected CuI complexes.22,23 This
type of excited state quenching is prevented when the phen
50 Avenue F.D. Roosevelt, 1050 Brussels, Belgium
cDépartement de Chimie Moléculaire, Equipe Chimie Inorganique Redox, UMR 5250,
Université Joseph Fourier, BP 53, 38041, Grenoble, France
dGroupe de recherche en spectrométrie de masse, Centre Interdisciplinaire de
Spectrométrie de masse (CISMA), Université de Mons, UMons, 20 Place du Parc,
B-7000 Mons, Belgium
eMolecular and Structural Biology, KU Leuven, Celestijnenlaan 200F, 3001 Heverlee,
Belgium
fMolecular Design & Synthesis, Katholiek Universiteit Leuven, Celestijnenlaan 200F,
3001 Heverlee, Belgium
†Electronic supplementary information (ESI) available. CCDC 909364–909366.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt51541a
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dpdipyH (1,5,9-triphenyldipyrromethene) ligands was the
same as the one described in the literature for similar
Cu complexes based on substituted dipyrromethene
ligands.12,34,38,40,41 Two equivalents of dipyH or dpdipyH were
thus reacted with one equivalent of Cu(OAc)2·H2O in the pres-
ence of a base. While the complexation of two “non-sterically
hindered” dipyrromethene ligands (dipyH) could be per-
formed at room temperature for obtaining [Cu(II)(dipy)2], the
formation of [Cu(II)(dpdipy)2] needed higher temperatures due
to the steric hindrance caused by the phenyl groups of the
dpdipyH ligands. Both complexes were easily purified by silica
column chromatography.
Fig. 1 Structure of the free ligands dipyH (5-phenyldipyrromethene) and
dpdipyH (1,5,9-triphenyldipyrromethene).
For the CuI complexes, as mentioned in the introduction,
the ligand dipyH which has no substituent in ortho position of
the chelating nitrogen atoms cannot sufficiently protect the
metallic CuI core versus oxidation. We anticipated that phenyl
protecting groups were required for the preparation of CuI
complexes. Thus the dpdipyH ligand has been used in order to
obtain the anion [Cu(I)(dpdipy)2]−. The preparation has been
performed at a vacuum line and under Ar according to pro-
cedures described for many other CuI complexes of the litera-
ture and our previous CuI compounds27 based on phen type
ligands.1–4,42 Thus Cu(I)(MeCN)4BF4 was first prepared from
Cu(II)(BF4)2 reduction by Cu(0) in anhydrous MeCN. The so-
obtained Cu(I) intermediate was treated with two equivalents
of dpdipyH ligands always at the vacuum line and under Ar,
under anhydrous conditions and in the presence of triethyl-
amine. After evaporation of the solvent under an Ar atmos-
phere, the crude solid was analyzed by ESI mass spectrometry.
In other trials, the complex was recrystallized in hexane under
Ar at the vacuum line.‡ NMR spectroscopy brought no valuable
structural information on the prepared CuI complex (see
further).
ligands are substituted in ortho position of the chelated nitro-
gen atoms by phenyl groups. Therefore these CuI complexes
are luminescent and have long excited state lifetimes.24–26
In the past in our lab, we have examined the properties of
some CuI complexes27 and their behaviour as SnO2 photo-
sensitizers.28 In the present work, our ultimate goal was to
construct large polynuclear species with CuI/CuII ions and
bridging organic ligands for testing their possible novel pro-
perties. Ligands that would be symmetric and very easily deri-
vatized had thus to be used. In this frame, mononuclear CuII
complexes have already been obtained with dipyrrin ligands
also called dipyrromethene,29–33 as well as polynuclear CuII
complexes with meso-pyridyl dipyrrins.34 Moreover the dipyrro-
+
methene chelates of BF2 , thus the well known BODIPY deriva-
tives, have been shown to exhibit quite remarkable tunable
optical properties by varying the substituents on the
ligands.35–37 For those different reasons, we decided to con-
sider dipyrromethene derivatives for our future polynuclear
CuII and CuI complexes. However, before undertaking such a
study, the knowledge of the properties of the mononuclear Cu
compounds was a prerequisite. Although several dipyrro-
methene CuII complexes have been prepared and examined in
the literature,32,38 the specific properties of the herein targeted
dipyH and dpdipyH ligands (Fig. 1) necessary before starting
the study of polynuclear species have not been described.
Moreover, the literature on the Cu-dipyrromethene complexes
neither discusses the electrochemical and the emission
characteristics of the CuII complexes nor reports the synthesis
of the corresponding CuI complexes. To the best of our knowl-
edge, only tridentate CuI complexes with one tetraphenyl-
azadipyrromethene and one triphenylphosphine have been
synthesized in the literature.39 Therefore, in this work we
report the electrochemical behaviour and electronic absorp-
tion as well as emission spectroscopy of the mononuclear
homoleptic Cu complexes comprising two dipyrromethene
ligands (Fig. 1). Attempts to explain by theoretical consider-
ations the origin of some intriguing results are also presented.
Mass spectroscopy
The first characterizations were performed by mass spectro-
metry. The main mass spectrometry results are shown in
Table 1. Cu has two major isotopes, namely 63Cu (69%) and
65Cu (31%). For the discussion, we will use only 63Cu as the
reference atom for the mass-to-charge ratio description, and
this as well for the CuI as for the CuII complexes.
Upon ESI analysis in the positive ion mode, i.e. ESI(+) of
[Cu(II)(dipy)2] (501 Da) in acetonitrile (entry 1), only decompo-
sition products are detected at m/z 282 and correspond to
[Cu(II)(dipy)]+ ions. When analyzed by MALDI(+), the presence
of the expected [Cu(II)(dipy)2] complex (entry 2) is confirmed
since a unique signal at m/z 501, which corresponds to [Cu(II)-
(dipy)2]+ ions, is observed (Fig. S1†). It is important to empha-
size that, at variance with ESI where protonation is likely to be
the dominant ionization process, the ions detected upon
MALDI correspond to one electron removal from the neutral
complex; this could explain why ions corresponding to the
Results
Syntheses
‡Different parameters have been changed for this synthesis such as (i) time of
The procedure adopted for the syntheses of the CuII complexes
reaction, (ii) absence of base, (iii) order of addition of the reagents and (iv) the
with the dipyH (5-phenyldipyrromethene) and the new use of a commercial anhydrous solvent.
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Table 1 Mass spectrometry results. The complexes are dissolved in acetonitrile. Among the two major isotopes, namely 63Cu (69%) and 65Cu (31%), only 63Cu was
used as the reference atom for the mass-to-charge ratio description
Entry
Targeted complexes
Ionization methods
Detected ions (m/z)
Ion identification
1
2
3
4
5
6
7
8
[Cu(II)(dipy)2]
[Cu(II)(dipy)2]
[Cu(II)(dpdipy)2]
[Cu(II)(dpdipy)2]
ESI(+)
282
501 (S1)
805
806 (S2b)
805 (S2a)
805 (S2c)
806 (S2d)
807 (most intense, Fig. 2a)
[Cu(II)(dipy)]+ decomposition
[Cu(II)(dipy)2]+
MALDI(+)
MALDI(+)
ESI(+)
[Cu(II)(dpdipy)2]+
[Cu(II)(dpdipy)(dpdipyH)]+
[Cu(II)(dpdipy)2]−
[Cu(II)(dpdipy)2]
ESI(−)
[Cu(I)(dpdipy)2]− crude
[Cu(I)(dpdipy)2]− crude
[Cu(I)(dpdipy)2]− small crystals
ESI(−)
[Cu(I)(dpdipy)2]−
ESI(+)
Cf ESI(+) of Cu(II)(dpdipy)2]
ESI(+)
[Cu(I)(dpdipyH)2]+
[Cu(II)(dipy)2] are observed in MALDI and not in ESI. For [Cu(II)-
(dpdipy)2] (entry 3), a unique signal at m/z 805, corresponding
also to an electron removal, is observed in MALDI(+).
The ESI(+) mass spectrum of [Cu(II)(dpdipy)2] in contrast to
that of [Cu(II)(dipy)2] allows the confirmation of the presence
of the targeted complex. Indeed, by ESI(+) the dominant peak
is observed at m/z 806 (entry 4) (Fig. S2b†) which corresponds
to protonation of neutral [Cu(dpdipy)2], affording [Cu(II)-
(dpdipy)(dpdipyH)]+. In this case, no decomplexation process
is observed in contrast to [Cu(II)(dipy)2] (entry 1). This obser-
vation could point to the higher stability of [Cu(II)(dpdipy)2] in
comparison with [Cu(II)(dipy)2] arising from the presence of
the bulky ligands dpdipy. We also examined the behaviour of
the CuII complex upon ESI(−). As demonstrated by the com-
parison of the ESI(+) and ESI(−) data (Fig. S2a and S2b†), the
most dominant peak in ESI(−) corresponds to m/z 805
(entry 5), thus to one electron reduction of [Cu(II)(dpdipy)2]
into [Cu(II)(dpdipy)2]−.
Fig. 2 Attempts to synthesize the [Cu(I)(dpDipy)2]− complex. Mass spec-
trometry analyses of the crystalline products obtained from a toluene solution of
the crude reaction product upon slow addition of heptane: ESI(+) mass spectra
for (a) the thin crystals (Table 1, entry 8) and (b) the large crystals, see text for
more details.
be attributed to the [Cu(I)(dpDipyH)2]+ cations (thus to the
diprotonation of [Cu(I)(dpdipy)2]−).
In conclusion, the mass spectrometry analyses confirm that
the expected CuII complexes have been isolated. As far as the
CuI complex is concerned, no direct evidence of its preparation
has been obtained from mass spectrometry. Only, its presence
could be proposed in the smaller crystals prepared upon
crystallization of the [Cu(I)(dpdipy)2]− complex.
Based on the anionic nature of the prepared CuI complex,
i.e. [Cu(I)(dpdipy)2]− (805 Da), an ESI(−) analysis seems a priori
suitable to probe the presence of the CuI complex. The ESI(−)
mass spectrum (Fig. S2c†) of an acetonitrile solution of the
crude [Cu(I)(dpdipy)2]− (entry 6) reveals the presence of a
signal at m/z 805 which could correspond to the expected
complex [Cu(I)(dpdipy)2]−. Nevertheless, as already revealed
here above, when subjected to ESI(−), the CuII complex (entry
5) also leads to the production of m/z 805 anions (Fig. S2a†). In
other words, the observation of the m/z 805 ions in the ESI(−)
spectrum of the CuI complex does not safely demonstrate the
presence of the expected CuI complex. Finally, an ESI(+) mass
spectrum of the prepared [Cu(I)(dpdipy)2]− (entry 7) was also
recorded. In that case, two protons are required to obtain cat-
ionic species yielding m/z 807 ions. However, when analyzing
the pattern of signals in the ESI(+) of the CuI complex (entry 7)
(Fig. S2d†) the most intense peak is observed at m/z 806 and
not at m/z 807 as expected. Crystallized [Cu(I)(dpdipy)2]− was
also analyzed by mass spectrometry. Two distinct populations
of crystals were obtained and could be described as small and
large crystals. The ESI(+) mass spectrum of the larger crystals
does not show any difference to the ESI(+) mass spectrum of
the CuII complex (Fig. 2b). However, in the ESI(+) mass spec-
trum obtained from the smaller crystals (entry 8) (Fig. 2a), the
pattern of the signals is significantly different from the pre-
vious cases and is dominated by the m/z 807 signal that could
X-ray crystallography
The structures of [Cu(II)(dipy)2] and [Cu(II)(dpdipy)2] were
determined by X-ray diffraction (Fig. 3). The crystals were
obtained by slow evaporation of the complex solution in
MeOH–CHCl3 for [Cu(II)(dipy)2] and in CHCl3–heptane for
[Cu(II)(dpdipy)2]. Like other CuII complexes based on dipyrrin
ligands,38,43,44 [Cu(II)(dipy)2] displays a four-coordinate dis-
torted square-plane geometry (Fig. 3a and Tables 2 and 3). The
asymmetric unit comprises two distinct molecules of [Cu(II)-
(dipy)2]: one exhibits a dihedral angle between the two oppo-
site N–Cu–N planes of 45.8° (Fig. 3a, bottom) and the other
46.2° (Fig. 3a, top). The large deviation from 0 arises from the
steric hindrance of the protons in alpha position of the nitro-
gen atoms.§ For [Cu(II)(dpdipy)2] (Fig. 3b, Tables 2 and 3) the
presence of the two additional phenyl groups leads to an even
more important angle (63.0°) as obtained by the present X-ray
§This angle is comparable to that estimated from spectroscopic data for a
similar complex with a p-nitrophenyl group in the meso position instead of a
phenyl substituent.38 Based on an experimental relation29 between the absorp-
tion coefficient of the most bathochromic band of the complex and this dihedral
angle, Dolphin et al.38 obtained an angle of 48°.
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Table 2 XR data
[Cu(II)(dpDipy)2]−
(elusive)
Cu(II)(Dipy)2
30H22CuN4
Formula
C
C54H38CuN4
806.42
M (g mol−1
)
502.06
Crystal dimensions (mm3)
0.5 × 0.2 × 0.1
Monoclinic
P21/c
17.0388(7)
15.6229(7)
19.7025(7)
90.00
0.4 × 0.3 × 0.2
Monoclinic
P21/n
13.7700(4)
13.5622(4)
21.6416(6)
90.00
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
115.496(2)
90.00
4734.0(3)
4
1.409
71.30
94.943(2)
90.00
4026.6(2)
4
1.330
71.48
γ (°)
V (Å3)
Z
Dcalc (g cm3)
θmax (°)
λ (Å)
F(000)
1.54178 (CuKα)
2072
1.54178 (CuKα)
1676
T (K)
100(2)
47 610
9195
6864
100(2)
39 793
7712
6382
Measured reflections
Unique reflections
Observed reflections
(Io > 2σ(Io))
Parameters refined
R1
632
532
0.0468
0.1094a
0.0686
0.1229
1.068
0.0412
0.0988b
0.0524
0.1059
1.058
a,b
ωR2
R1 (all data)
ωR2 (all data)
GOOF
μ (mm−1
)
1.500
1.097
a Weighting scheme as defined for Cu(II)(Dipy)2: ω = 1/[σ2(Fo2) + (0.0608
P)2 + 1.6118 P], P = (Fo + 2Fc2)/3. b Weighting scheme as defined
2
for Cu(II)(dpDipy)2: ω = 1/[σ2(Fo2) + (0.0530 P)2 + 1.8201 P], P =
(Fo2 + 2Fc2)/3.
Fig. 3 (a) View of crystal structure of [Cu(II)(Dipy)2] (two molecules in the asym-
metric unit shown). (b) View of crystal structure of [Cu(II)(dpDipy)2].
data.¶ For both [Cu(II)(dipy)2] and [Cu(II)(dpdipy)2], the two
pyrrole rings are not coplanar: the angle extends from 1.1 to
8.8° in [Cu(II)(dipy)2] and from 14.8 to 36.1° in [Cu(II)(dpdipy)2]
(Tables S1–4†). Moreover, the phenyl groups in meso position
are not in the same plane as that of the dipyrrin moieties, due
to steric hindrances.
Table 3 Selected bond lengths [Å] and angles [°] for [Cu(II)(Dipy)2] and [Cu(II)-
(dpDipy)2]
[Cu(II)(Dipy)2]
[Cu(II)(dpDipy)2]
Cu1–N2
Cu1–N12
Cu1–N19
Cu1–N29
N2–Cu1–N12
N12–Cu1–N29
N29–Cu1–N19
N19–Cu1–N2
Cu36–N37
Cu36–N47
Cu36–N54
1.950(5)
1.919(6)
1.973(5)
1.920(5)
92.0(2)
96.8(2)
92.0(2)
Cu1–N2
Cu1–N12
Cu1–N31
Cu1–N41
N2–Cu1–N12
N12–Cu1–N41
N41–Cu1–N31
N31–Cu1–N2
1.9582(16)
1.9690(16)
1.9524(16)
1.9711(16)
95.38(7)
105.81(7)
94.76(7)
In the case of [Cu(I)(dpdipy)2]− as mentioned above, the
crude solid obtained after evaporation of the solvent under Ar
was crystallized from a toluene solution by slowly adding
heptane under Ar. Under these conditions, two types of crys-
tals have been obtained and separated. First, thin, small crys-
tals were collected and filtered off under an argon atmosphere.
Afterwards, larger crystals started to be observed from the fil-
trate, which was kept under argon. Only the larger crystals
could be analyzed by X-ray diffraction and they correspond
(Fig. S3†) without ambiguity to the CuII complex [Cu(II)-
97.2(2)
98.16(7)
1.964(6)
1.926(6)
1.979(5)
1.912(6)
91.85(16)
96.5(2)
Cu36–N64
N37–Cu36–N47
N47–Cu36–N64
N64–Cu36–N54
N54–Cu36–N37
92.35(16)
97.6(2)
Dihedral angle between N2Cu1N12 and N19Cu1N29
,
[Cu(II)(Dipy)2]
¶Such an effect of phenyl substituents in those positions of dipyrromethene had
also been evidenced, but in that case without phenyl in meso-position and from
the same experimental relation,29 the authors38 obtained a dihedral angle of
73°.
45.791° (Fig. 3a, bottom). Dihedral angle between N37Cu36N47 and
N54Cu36N64 [Cu(II)(Dipy)2] 46.167° (Fig. 3a, top). Dihedral angle
,
between lines N2Cu1N12 and N31Cu1N41, [Cu(II)(dpDipy)2] 63.043°
(Fig. 3b).
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(dpdipy)2]; they differ only at the phenyl substituents in meso
position. The conclusions from the X-ray analysis are thus in
agreement with those from the mass spectrometry.
EPR spectroscopy
The structures of the dipy and dpdipy CuII complexes have
also been examined in organic solution by EPR spectroscopy.
The X-band EPR spectra of the CuII complexes at 100 K
exhibit an (S = 1/2) signal with axial symmetry (Fig. 4, Table 4).
The observed four-line pattern arises from the hyperfine
interaction between the electronic spin and the copper
nucleus (ICu = 3/2). Because of the broadening of the spectrum
of [Cu(II)(dipy)2] in benzonitrile (likely due to intermolecular
spin exchange), we additionally measured it in a benzonitrile–
methanol (1 : 3) mixture, which allows for a better spin iso-
lation. From simulation of the experimental spectra, the
sets of spin Hamiltonian parameters g┴ = 2.049, g = 2.214
k
with A┴ = 2.0 mT, A = 17.0 mT and g┴ = 2.083, g = 2.269 with
k
k
A┴ = 1.5 mT, A = 11.20 mT could be obtained for [Cu(II)(dipy)2]
k
and [Cu(II)(dpdipy)2] respectively. Interestingly, a superhyper-
fine structure could be observed in the spectrum of [Cu(II)-
(dipy)2], which corresponds to the interaction of the unpaired
electron with the nuclear spins of four equivalent 14N nuclei
(A┴ = 1.28 mT; A = 1.21 mT).
k
The g value is known to be indicative of the extent of tetra-
k
hedral distortion in tetracoordinated copper(II) ions.47,48 For
instance, CuII porphyrins that could be considered as simple
bis(dipyrromethenate) CuII complexes involving
a square
planar metal ion typically exhibit g values within the range, or
k
2−
lower than 2.20. In contrast, the g value reported for CuCl4
k
that could be considered as tetrahedral is close to 2.40.47,48
This behaviour originates from a mixing of the dz2 orbital into
the dx2−y2 orbital that hosts the unpaired electron when tetra-
hedral distortions are present. A comparison between [Cu(II)-
Fig. 4 (a) X-band EPR spectrum of the [Cu(II)(dipy)2] complex at 100 K in
MeOH–benzonitrile (3 : 1). Solid line: experimental spectrum; dotted line: simu-
lation using the parameters given in the text. Microwave Freq.: 9.34 GHz, power:
5 mW, Mod. Amp.: 0.4 mT, Freq.: 100 kHz. (b) X-band EPR spectrum of the [Cu(II)-
(dpdipy)2] complex at 100 K in benzonitrile. Solid line: experimental spectrum;
dotted line: simulation using the parameters given in the text. Microwave Freq.
9.33 GHz, power = 20 mW, Mod. Amp. 0.2 mT, Mod. Freq. 100 kHz.
(dipy)2] and [Cu(II)(dpdipy)2] reveals a larger g value for the
latter complex (2.269 vs. 2.214). This is consistent with an
increased tetrahedral distortion in [Cu(II)(dpdipy)2]. A qualitat-
k
ive relationship between the g value and the dihedral angle
k
between the two N–Cu–N chelate rings of complexes involving
a N4 coordination sphere has been established by Addison
The solutions prepared with the thin, small crystals of the
et al.48 By considering the experimental g values of 2.214 and putative Cu(I) complexes are paramagnetic, indicating that the
k
2.269, the angles of ∼40° and ∼60° are predicted for [Cu(II)- copper is mainly at the (+II) and not (+I) oxidation state.
(dipy)2] and [Cu(II)(dpdipy)2], respectively. These angles are in Double integration of the spectrum and comparison with that
fairly good agreement with the ones calculated on the basis of of genuine Cu(II) samples indicate (Fig. S4a,b†) the presence at
X-ray diffraction data, which indicates that the structures and 95 5% of [Cu(II)(dpdipy)2]. Thus even with the small crystals,
especially the dihedral angles are preserved in solution.
the CuI complex cannot be detected.
Table 4 g values, hyperfine and superhyperfine constants determined for [Cu(II)(dipy)2], [Cu(II)(dpdipy)2] and for reference compounds
g
k
g┴
Ak (Cu) (mT)
A┴(Cu) (mT)
g /Ak (Cu) (cm−1
)
A
k (N) (mT)
A┴(N) (mT)
k
[Cu(II)(Dipy)2]
2.214
2.269
2.189
2.4
2.049
2.083
2.035
17.0
11.20
21.0
2
1.5
3.3
139
191
107
1.21
1.28
[Cu(II)(dpDipy)2]
[Cu(II)(porphyrin)]45
[Cu(II)Cl4]2− 46 a
a Different values of g are observed along the x and y axes and no A value is mentioned in the literature for this complex.
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CuII complexes mentioned above as a reference for a compari-
son, these reduction waves could correspond to the addition of
one electron on one of the two ligands. This seems reasonable
since the reduction wave of [Cu(II)(dpdipy)2] is less negative (by
240 mV) than that of [Cu(II)(dipy)2]. Such a shift can be attribu-
ted to some conjugation induced by the presence of the
additional phenyl groups. Although this assignment seems
reasonable, we performed exhaustive reduction electrolysis
of the dpdipy complex (a priori the most stable in reduction
in comparison to the dipy complex) in order to confirm un-
ambiguously this attribution. The electrolysis of [CuII(dpdipy)2],
1 mM in benzonitrile, in a glove box at a potential corres-
ponding to the first reduction wave has thus been carried out.
Electrolysis was monitored by chrono-amperometry and Coulo-
metry. It was stopped when the residual current was 5% of the
initial value (after 3 h 30 min at room temperature). Coulo-
metry revealed that 0.95 electron was exchanged, consistent
with a monoelectronic redox process. The CV curve of the electro-
chemically generated mono-anion (Fig. 5c) exhibits a reversible
one-electron oxidation wave at −0.59 V/SCE, i.e. a potential
value that matches the Ered value of [CuII(dpdipy)2]. However,
its intensity is about half that of the starting [CuII(dpdipy)2].
In addition, two other reduction waves can be evidenced
(Fig. S7†), one of them being close to that measured for the
free ligand (compare Fig. 5b with 5c). The change in the shape
of the CV curves before and after electrolysis (Fig. 5, S7†)
suggests that during the reduction process, the complex has
evolved, likely by release of one ligand molecule. The solution
after electrolysis also exhibits a change in the absorption spec-
trum (Fig. S6b†) but no absorption band above 600 nm (that
may be indicative of the presence of a ligand radical anion)
can be observed as for the reduction of the CuII complexes
with 1,1′-bidipyrrins.49 The electrolytic solution containing the
reduced species was also analyzed by EPR spectroscopy at
100 K (Fig. S4c†). A quenching of 95% of the EPR signal was
observed after reduction. The amount of quenching correlates
perfectly with the faradic yield of electrolysis, showing that the
reduced species is EPR-silent, which suggests that it corres-
ponds to a CuI complex. Exposure of the EPR tube to air for a
couple of hours restored a CuII EPR spectrum with a yield of
ca. 80%, indicating that the reduced species is not stable
under air.
The redox potentials of [Cu(II)(dipy)2] and [Cu(II)(dpdipy)2]
determined by cyclic voltammetry (CV) in dry deoxygenated
benzonitrile with 0.1 M Bu4NPF6 are shown in Table 5 (Fig. S5
and S6†). When scanning towards the anodic region of poten-
tials the two complexes display a different behaviour. While
[Cu(II)(dipy)2] shows irreversible waves whose intensities
decrease with the number of scans, [Cu(II)(dpdipy)2] exhibits
two reversible monoelectronic oxidation waves. We attribute
them to the sequential oxidation of the two dpdipy ligands, as
for the CuII complexes based on derivatives of the 1,1′-bidipyrrin
ligand (composed of two dipyrrin skeletons linked in their 1
and 1′ positions).49 In the case of [Cu(II)(dipy)2] it might be
possible that a polymerization reaction between the oxidized
dipyrrin skeletons occurs during the successive cycles, as
reported in the literature for free pyrrol groups.50,51 In con-
trast, this polymerization does not occur with [Cu(II)(dpdipy)2],
probably due to the phenyl substituents in alpha positions of
the chelated nitrogens that protect the produced radical
cations from polymerization.
In reduction, [Cu(II)(dipy)2] (Fig. S6†) and [Cu(II)(dpdipy)2]
(Fig. 5a) exhibit a first reversible monoelectronic wave at −0.83
and −0.59 V vs. SCE respectively. If we take the 1,1′-bidipyrrin
Table 5 Electrochemical data determined with
a platinum electrode in a
benzonitrile solution with 0.1 M Bu4NPF6 for [Cu(II)(dipy)2], [Cu(II)(dpdipy)2] and
reference compounds
Complexes
Oxidation, V/SCE
Reduction, V/SCE
[Cu(II)(dipy)2]
[Cu(II)(dpdipy)2]
[Cu(II)(1,1′-
+1.02a +1.15a +1.50a −0.83
+0.80
+0.23
+1.08
+0.70
−0.59
−1.05
bidipyrrin)]49
[Cu(II)(dmp)2]2+ 52 b
—
—
—
+0.58 −1.60 −1.75
a A decrease of these waves is observed during the different scans.
b dmp
=
2,9-dimethyl-1,10-phenanthroline (counter-ions: NO3
;
−
solvent: DMF).
In conclusion, exhaustive reductive electrolysis of [Cu(II)-
(dpdipy)2] shows that the corresponding monoanion is not
stable chemically. This instability is not due to oxidation by
adventitious dioxygen since: (i) slow decomposition (during
the 3 h 30 min electrolysis) is observed under an Ar atmos-
phere in the glove box with probable formation of the
free ligand and (ii) we did not observe the appearance of an
(S = 1/2) copper(II) signal during decomposition. Moreover it
can be concluded that the reduction electrolysis of [Cu(II)-
(dpdipy)2] does not behave like the one of the 1,1′-bidipyrrin
CuII complex described in the literature.49 Indeed in that case,
the authors could demonstrate unambiguously that the
addition of the first electron takes place on the ligand without
decomposition during a reduction electrolysis (occurrence of a
Fig. 5 CV curve of (a) [CuII(dpdipy)2] before reduction electrolysis, (b) the free
ligand Hdpdipy (c) [CuII(dpdipy)2] after reduction electrolysis, with concen-
trations of 1 mM in a benzonitrile solution (+0.1 M TBAPF6) at a platinum disc,
recorded under argon. Scan rate = 0.1 V s−1. T = 298 K. The potential values are
converted against the SCE electrode.
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+
Table 6 Absorption data in CHCl3 at 298
K
of dipyH, dipyH2
,
dpdipyH,
Table 7 Emission maxima of [Cu(II)(dipy)2] and [Cu(II)(dpdipy)2] in different sol-
dpdipyH2+, [Cu(II)(dipy)2], [Cu(II)(dpdipy)2] and reference compounds
vents of increasing polarity
Absorbance, λmax/nm
Emission λmax (nm)
(ε × 103 M−1 cm−1
)
Toluene
Butyronitrile
Chloroform
Ethanol
UV
vis
[Cu(II)(dipy)2]
[Cu(II)(dpdipy)2]
500
593, 628
503
590, 630
504
593, 645
501
a
dipyH
dipyH2
312
368
302
403
436 (31,5)
470 (49,0)
518 (40,6)
576 (121,7)
—
+
a The complex is not soluble in EtOH.
dpdipyH
dpdipyH2
+
Dipyrromethane38
[Cu(II)(dipy)2]
266
323, 370
464 (42,3), 501sh
[Cu(II)(dpdipy)2]
[Cu(II)(NO2dipy)2]38 a
[Cu(II)(tetraphenyl-
azadipyrrine)2]53 b
293, 340sh, 395sh 480, 553 (83,0)
274, 314, 368
300
474 (52,4), 504sh
567, 640 (65,7)
a NO2Dipy
=
5-(4-nitrophenyl)-dipyrromethene. b Tetraphenylazadi-
pyrrine = 1,3,7,9-tetraphenyl-5-aza-dipyromethene.
bathochromic electronic absorption). In contrast, for [Cu(II)-
(dpdipy)2] the reduction electrolysis is accompanied by a slow
release of the ligand and the results suggest that a reduction
on the Cu(II) is more likely than on the ligand.
Concerning the small crystals of targeted [Cu(I)(dpdipy)2]−,
the CV curves of the corresponding solution in benzonitrile
have been recorded on different samples. The CV curves are
similar to those of [Cu(II)(dpdipy)2] but with a somewhat lower
current intensity for
a concentration of elusive [Cu(I)-
(dpdipy)2]− similar to that of [Cu(II)(dpdipy)2], suggesting that
the sample of the elusive compound contains CuII.
Absorption and emission of the CuII complexes
The absorption data are shown in Table 6 (Fig. S8 and 9†)
together with the data for some reference complexes and the
free ligands dipyH and dpdipyH (nonprotonated and proto-
nated). The visible absorption spectra of the complexes are
bathochromically shifted in comparison with those of the non-
protonated free ligands, as also observed in the literature for
similar complexes.34,43,53 The electronic transitions in the
visible region of the spectrum for [Cu(II)(dipy)2] and [Cu(II)-
(dpdipy)2] could correspond to ligand centered (LC) transitions
or LC mixed with Ligand to Metal Charge Transfer (LMCT)
transitions, on the basis of the electrochemical data.
Fig. 6 (a) Comparison between the normalized absorption spectrum (at
298 K) in CHCl3 and emission spectrum (at 77 K) of [Cu(II)(dipy)2] in the mixture
EtOH–MeOH (4/1). (b) Comparison between the normalized absorption spec-
trum (at 298 K) in CHCl3 and emission spectrum (at 110 K) of [Cu(II)(dpdipy)2] in
butyronitrile.
The literature on CuII complexes with dipyH ligand deriva-
tives does not contain any information on the emission
characteristics.43,44 However, the two CuII complexes examined
in this work exhibit weak luminescence at room temperature.k
Table 7 gives the emission λmax values in different solvents and
Fig. 6a and b exhibit the normalized absorption and lumine-
scence spectra (at 77 K and 110 K). It has been verified by
recording the excitation spectrum (Fig. S14†) that the emission
cannot originate from the presence of an impurity such as the
free ligand (in addition to the fact that the emission λmax of
the complex does not correspond to that of the free ligand)
(Fig. S10 and 11). Several arguments indicate that the complex
luminescence is controlled by the dipyrromethene ligands,
and thus by singlet LC states as emitting excited species.
Indeed (i) there is almost no shift of emission λmax with the
polarity of the solvent or from a solution at RT to a glass at
77 K (Fig. S12 and 13†); (ii) the emission and absorption
spectra are quasi symmetrical to each other; (iii) in agreement
kThe quantum yield of emission for [Cu(II)(dpdipy)2] in ButCN at room temp-
erature is estimated to be 1 × 10−3, using the complex [Ru(bpy)3]2+ as the
reference.
1
with a LC fluorescence, the excited state lifetimes are typical
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Table 9 Calculation of charge density for each N atom of [Cu(I)(dpdipy)2]−,
[Cu(II)(dpdipy)2] and the deprotonated free ligand “dpdipy−”, and for [Cu(I)-
(dmp)2]+, [Cu(II)(dmp)2]2+ and the free ligand dmp (dmp = 2,9-dimethyl-1,10-
phenanthroline). The values represent an average of the values obtained for the
N of each compound from the analysis method of Mulliken’s populations
of singlet excited states: at room temperature they are shorter
than or on the order of 1 ns and, at low temperature, shorter
or on the order of 10 ns (Table S5†); (iv) LMCT excited states
usually do not emit and the ligands of the CuII complexes of
this work are not particularly rigid, which might favor lumine-
scence from LMCT states.
[Cu(I)-
[Cu(II)-
(dpdipy)2]
(dpdipy)2]−
“dpdipy−”
−0.557
Computational results
Charge density on each −0.603
N atom (average)
−0.643
Geometry optimization at the DFT/B3LYP level predicts for
[Cu(II)(dpdipy)2] a geometry with a dihedral angle of 68°
between the two dpdipy planes (experimental angle of 63°)
and Cu–N bond distances and angles that are consistent with
the X-ray crystallography results (Tables S6 and S7†). In
addition, the first lowest-lying excited states of [Cu(II)(dpdipy)2]
have been evaluated within the vertical TD-DFT approximation.
The HOMO corresponds to a ligand centered molecular
orbital, thus in agreement with the electrochemical data. In
line with the attribution of the first absorption band and emis-
sion of [Cu(II)(dpdipy)2], the lowest unoccupied molecular orbi-
tals are centered on the ligand, with the exception however of
the LUMO centered on the Cu, which has an extremely small
oscillator strength and the LUMO + 4 which exhibits a very
slight contribution of the Cu (Fig. S15†).
[Cu(I)(dmp)2]+ [Cu(II)(dmp)2]2+ dmp
−0.616 −0.685 −0.528
minus one proton) are given in Table 9. They indicate that the
density of negative charge on the nitrogen atoms is more
important for the CuII complex (−0.643) than for the CuI
complex (−0.603) and for dpdipy− (−0.557). The density of
negative charge on the ligand is thus not correlated with the
fact that the whole molecule is negatively charged or neutral.
As this density is higher for the two complexes than for
dpdipy−, it should originate from the metal ion. Therefore, a
higher charge density might be attributed to better back-
bonding from the metal in the CuII than in the CuI complex.
In order to check this hypothesis of increased stability by back
bonding, we also calculated the negative charge density for
[Cu(I)(dmp)2]+, [Cu(II)(dmp)2]2+, and dmp (dmp = 2,9-dimethyl-
phenanthroline, Table 9). In these cases too, a higher density
of charge on the nitrogen is obtained for the CuII (−0.685)
than for the CuI (−0.616) complex. Moreover, a comparison
between the values for the dpdipy complexes and dmp com-
plexes indicates that the [Cu(I)(dpdipy)2]− has the lowest nega-
tive charge density on its nitrogen atoms; this complex would
thus exhibit the weakest back-bonding. This might be one of
the reasons why it was not possible to isolate this complex. In
conclusion, even if the anion [Cu(I)(dpdipy)2]−, in its equili-
brium geometry (thus according to the total electronic energy),
is more stable than the neutral species, the Cu–N bond break-
ing (thus related to the energy of dissociation) is much easier
in the anion than in the neutral species.
As outlined above, despite our efforts to prepare the CuI
complex [Cu(I)(dpdipy)2]−, we mainly isolated the CuII com-
pound. Therefore we were wondering whether a CuI complex
with two dpdipy ligands would be too unstable for simply
existing from a theoretical point of view. We tested this hypo-
thesis of instability by calculating the total electronic energy of
[Cu(I)(dpdipy)2]− as compared to that of [Cu(II)(dpdipy)2], after
geometry optimization. The calculations predict in gas-phase
(Table 8) a global stabilization of the [Cu(I)(dpdipy)2]− complex
compared to the corresponding CuII complex (relative energy
estimated in vacuo at −2.07 eV). When included, the solvent
effects (dichloromethane and acetonitrile, as in the experi-
ments) favor even more the CuI complex relative to that of CuII.
These results would thus indicate that the CuI complex should
exist at least in the absence of reactive species.
It was thought that a comparison of the atomic charge
density on the ligands in the CuI and CuII complexes could
possibly bring some explanation. The results for the nitrogen
atoms for the two complexes and dpdipy− (the free dpdipyH
Discussion and conclusions
We did not expect problems with the study of Cu-dipyrro-
methene homoleptic complexes since some of them have been
prepared in the literature.29–33 However, it has to be reminded
that in spite of these publications, to the best of our knowl-
edge, no corresponding CuI complexes have been synthesized
and no data on the electrochemical behaviour and lumines-
Table 8 Total electronic energy of [Cu(I)(dpDipy)2]− and [Cu(II)(dpDipy)2] in gas
phase, in CH2Cl2 and in CH3CN, calculated at the DFT/B3LYP level with the basis
6-1G* (for the atoms C, N and H) and the Stuttgart pseudo potential (for the Cu
atom) in DFT. The energies with the solvents have been evaluated with the
model of the continuous solvent COSMO (Conductor-like Screening Model). a.u. cence were published (except for derivatives of 1,1′-bidipyrrin
ligands, which look like porphyrins).49
= atomic units; eV = electron-volts; 1 a.u. = 27.2 eV
A first interesting finding with the ligands dipyH and
[Cu(I)-
[Cu(II)-
(dpdipy)2]/
a.u.
Energy
difference/
a.u.
Energy
difference/
eV
(dpdipy)2]−/
a.u.
dpdipyH is the luminescence of the corresponding homoleptic
CuII complexes, although rather weak. This emission could be
attributed to LC excited states and seems typical of homoleptic
CuII complexes with dipyrromethene ligands since usually the
CuII complexes do not emit at all.
Gas phase
CH2Cl2
CH3CN
−2497.1296
−2497.2112
−2497.2204
−2497.0535
−2497.0812
−2497.0843
−0.0761
−0.1300
−0.1361
−2.07
−3.54
−3.71
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Concerning the synthesis of [Cu(I)(dpdipy)2]−, as indicated from water traces in PhCN and responsible for dechelation of
by the ESI(+) mass analysis (Table 1, entry 8) of the small crys- the ligand as detected in the CV (compare Fig. 5b with 5c). The
tals in MeCN, some CuI complexes could maybe be present. speculated reaction scheme (1)–(4) would thus be compatible
Indeed in the spectrum, the observed ions would correspond with the electrochemical results and with the fact that we
to the diprotonated species [Cu(I)(dpdipyH)2]+ in which each could not isolate [Cu(I)(dpdipy)2]−. The electrochemical results
ligand would form only one Cu–nitrogen bond. The rest of the would also indicate that the LUMO involved in the reduction
analyzed sample contains the homoleptic CuII complex, as process would be different from that controlling the absorp-
shown also by X-ray crystallography of the big crystals originat- tion and emission.**
ing from the elusive [Cu(I)(dpdipy)2]−. If during the synthesis,
In conclusion, we have shown in this work that mono-
[Cu(I)(dpdipy)2]− is formed in situ from the precursor [Cu(I)- nuclear homoleptic Cu complexes comprising two dipyrro-
(MeCN)4]+, it is probably transformed into the CuII corres- methene ligands exhibit unexpected behaviours such as
ponding compound during the isolation procedure, by contact luminescence and properties different from those that could
with air. This is strange since owing to the phenyl substituents be anticipated from bis-phenanthroline or 1,1′-bidipyrrin
in the supposedly tetrahedral geometry of [Cu(I)(dpdipy)2]−, copper complexes.
the CuI core should be protected from oxygen. However as
indicated by the conclusions from the theoretical results, the
back bonding in the CuI complex should be rather weak as
Experimental section
compared to that in the corresponding CuII complex or CuI
complexes with phen ligands. This would yield a very loose
Cu–nitrogen bond, so that it could be speculated that the CuI
species in solution would be better described by an equili-
brium between the two species [Cu(I)(dpdipy)2]− and [Cu(I)-
(dpdipy)(dpDipy′)]− in which dpdipy′ would correspond to a
ligand with only one Cu–nitrogen bond (reaction 1). Under
such conditions, the CuI centre would not be well protected
by the phenyl substituted ligands versus oxidation by oxygen,
so that during the first contact with air during the isolation
procedure, mainly [Cu(II)(dpdipy)2] would be obtained
(reaction 2).
The reagents (Cu(OAc)2·H2O, NH4OH) and solvents (MeOH,
CHCl3, PhCN) were obtained from Sigma-Aldrich and used
without further purification. Water was purified with a Milli-Q
system.
Instrumentation
Mass spectrometry analyses. (i) MALDI mass spectra were
recorded using a Waters QToF Premier mass spectrometer
equipped with a nitrogen laser, operating at 337 nm with a
maximum output of 500 J m−2 delivered to the sample in 4 ns
pulses at 20 Hz repeating rate. Time-of-flight mass analyses
ð1Þ were performed in the reflectron mode at a resolution of about
10 000 at m/z 500. The matrix, trans-2-[3-(4-tert-butyl-phenyl)-2-
½CuðiÞðdpdipyÞ2ꢀꢁ⇄ ½CuðiÞðdpdipyÞðdpdipy′Þꢀꢁ
½CuðiÞðdpdipyÞðdpdipy′Þꢀꢁ þ air ! ½CuðiiÞðdpdipyÞ2ꢀ
ð2Þ
methyl-2-propenylidene] malononitrile (DCTB), was prepared
in chloroform (20 mg ml−1). The matrix solution (1 µL) was
applied to a stainless steel target and air dried. Samples were
dissolved in chloroform (1 mg ml−1). 1 µL aliquots of these
solutions were applied to the target area already bearing the
matrix crystals, and then air dried. For the recording of the
single-stage MALDI-MS spectra, the quadrupole (rf-only mode)
was set to pass ions from m/z 50 to 1000, and all ions were
transmitted into the pusher region of the time-of-flight ana-
lyzer where they were mass analyzed with 1 s integration time.
Data were acquired in continuum mode until acceptable aver-
aged data were obtained. (ii) The ESI analyses were performed
on a Waters QToF2 mass spectrometer. The sample solutions
(1 mg ml−1 in acetonitrile) were delivered to the ESI source by
a Harvard Apparatus syringe pump at a flow rate of 5 µL
min−1. Typical ESI conditions were: capillary voltage, 3.1 kV
(positive or negative ion mode); cone voltage, 30 V; source
This hypothesis could be conciliated with the electrochemi-
cal behaviour of [Cu(II)(dpdipy)2] in reduction. Indeed addition
of the first electron to [Cu(II)(dpdipy)2] leads to a first reversible
wave, meaning that in the CV time scale in PhCN under argon
the reduced complex does not decompose. However, decompo-
sition takes place on a longer time scale since the CV curve
after electrolysis reveals the presence of free ligand. Moreover,
there is no indication of the presence of complexed ligand
radical anion after electrolysis. Based on those data, it could
be speculated that the reduction of [Cu(II)(dpdipy)2] would give
rise to a species like [Cu(I)(dpdipy)(dpdipy′)]− (see reaction 3),
thus similar to the species formed during the synthesis with
a CuI core (reaction 1). Reaction 3 would be reversible at
the electrode and would correspond to the first redox wave
(−0.59 V/SCE) in the negative applied potentials range (Fig. 5).
½CuðiiÞðdpDipyÞ2ꢀ þ eꢁ⇄ ½CuðiÞðdpDipyÞðdpDipy′Þꢀꢁ
ð3Þ
temperature, 80 °C; desolvation temperature, 120 °C. Dry
½CuðiÞðdpDipyÞðdpDipy′Þꢀꢁ þ H2O
! decomposition products
**In contrast, it could also be argued that the first electron (reaction 3) would be
ð4Þ
added to the dpdipy ligand, in agreement with the assignment of the lowest
absorption band and emission to LC transitions. During the electrolysis, the
added electron would finally be localized on the Cu core. However, in such a
case, the observation of the same redox wave after and before electrolysis cannot
In order to explain the weaker intensity of this first revers-
ible wave after the reduction electrolysis, decomposition
should occur (reaction 4) due, for example, to proton attacks be explained.
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nitrogen was used as the ESI gas. The quadrupole was set to Syntheses
pass ions from m/z 50 to 1000 and all ions were transmitted
5-Phenyldipyrromethene was prepared according to literature
procedures.12 1,5,9-Triphenyldipyrromethene was prepared
through an acid catalyzed condensation of benzaldehyde and
2-phenylpyrrole, followed by oxidation with DDQ. To a solution
of 2-phenylpyrrole (2.86 g, 10 mmol) in dichloromethane
(50 ml) under a nitrogen atmosphere was added benzaldehyde
(1.06 g, 10 mmol). A catalytic amount of trifluoroacetic acid
(1 drop) was added to the mixture and the reaction turned
dark immediately. After stirring at room temperature for
30 minutes, DDQ (2.27 g, 10 mmol) was added and stirring
was continued for 2 h at room temperature. The red solution
was evaporated to dryness, and the crude product was purified
chromatographically (Silica, DCM) to yield dark red shiny crys-
tals (1.17 g, 63% yield). 1H-NMR (Fig. S16†): 13.98 (s, br, H,
NH), 7.94 (m, 4H, J = Hz), 7.58–7.37 (m, 11H), 6.85 (d, 2H, J =
3.66 Hz), 6.71 (d, 1H, J = 3.66 Hz) ppm; 13C-NMR: 169.8, 154.4,
142.3, 141.1, 140.0, 137.5, 133.5, 131.2, 130.7, 130.3, 129.4,
128.1, 126.6, 126.5, 126.1, 116.0 ppm; LRMS(EI) 372 (M+), 295
(M+ − Ph); HRMS (EI): Calculated for C27H20N2 372.1626,
found 372.1621; UV-VIS: 517(s), 290(s).
into the pusher region of the time-of-flight analyser for mass-
analysis with 1 s integration time. Data were acquired in
continuum mode until acceptable average data were obtained
(typically 20 scans).
Structure determination. The structures of [Cu(II)(dipy)2]
and [Cu(II)(dpdipy)2] were determined by X-ray diffraction
(Fig. 3). The crystals of [Cu(II)(dipy)2] were obtained by
slow evaporation of the complex solution in MeOH–CHCl3. For
[Cu(II)(dpdipy)2] two types of crystals were obtained from a
CHCl3–heptane solution. X-ray diffraction data were collected
at a temperature of 100 K using CuKα radiation and phi and
omega scans on a SMART 6000 diffractometer equipped with a
CCD detector and crossed Göbel mirrors. Cell refinement and
data reduction were carried out using the program SAINT.54
For the absorption correction the program SADABS was
used.55 The structures were solved by direct methods and
refined by full-matrix least squares on |F2| using the SHELXTL
program package.56 The crystal of Cu(II)(dipy)2 was found to be
twinned (twin law -100 0-10 101). Non-hydrogen atoms were
anisotropically refined and the hydrogen atoms were placed in
calculated positions with temperature factors fixed at 1.2 times
Ueq of the parent atoms. The crystallographic data are given in
Tables 2 and 3.
[Cu(II)(dipy)2] The synthesis of [Cu(II)(dipy)2] was based
on the procedure described in the literature for the [Cu(II)-
(NO2Dipy)2] complex. The reaction product was vacuum-dried
at room temperature and purified by column chromatography
(CHCl3–MeOH, 99/1). Yield: 92.6 mg (88%). Crystals were
grown by slow evaporation of the solvents MeOH–CHCl3
(50/50). MALDI-ToF (CH2Cl2): m/z 501.1.
Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC-909364–909366.
[Cu(II)(dpdipy)2] 100 mg (0.27 mmol) of 1,5,9-triphenyldi-
pyrromethene were dissolved in 80 ml of CH3CN with 33 µL of
NH4OH (8.2 M). 26.5 mg (0.13 mmol) of Cu(OAc)2·H2O dis-
solved in 10 ml of methanol were added to the solution. The
reaction was heated at 85 °C followed by thin layer chromato-
graphy (cyclohexane–ethyl acetate, 70/30) and UV-vis absorp-
tion spectroscopy. After one hour, the reaction product was
vacuum-dried at room temperature and the excess of free
ligand was removed by column chromatography (cyclohexane–
ethyl acetate, 70/30). Yield: 75.3 mg (72%). Crystals were grown
by slow evaporation of the solvents CHCl3–heptane (50/50).
MALDI-ToF (CH2Cl2): m/z 805.2. ESMS (CH2Cl2): m/z 805.2 in
negative mode and m/z 806.0 in positive mode.
The EPR spectra were recorded using an X-Band BRUKER
EMX plus spectrometer equipped with a nitrogen flow cryostat
and a BRUKER Teslameter. The spectra have been recorded at
100 K on 400 µL samples. The spectra were simulated using
the BRUKER Simfonia software.
Cyclic voltammetry was carried out with a platinum disk
working electrode in dried benzonitrile with tetrabutyl-
ammonium hexafluorophosphate (0.1 mol L−1) as a supporting
electrolyte. The potential of the working electrode was con-
trolled by an Autolab PGSTAT 100 potentiostat interfaced with
a PC, with a scan rate of 100 mV s−1 between −2 and +2 V
versus SCE. The counter electrode was a platinum foil and the
reference electrode a Saturated Calomel Electrode (SCE). All
the measurements were performed in a single compartment
Theoretical calculations
cell. The concentration of the complexes was 1 × 10−3 mol L−1
The UV-vis absorption spectra (accuracy
.
2 nm) were For the comparison of the total electronic energy of the dpdipy
recorded using a Perkin-Elmer Lambda 35 UV-vis spectro- complexes with CuI and CuII, the geometries of the two
photometer. The determination of the molar absorption complexes were first optimized in vacuo at the B3LYP level
coefficients was performed by weight and absorption of theory57 with the NWChem computational chemistry
measurements.
package.58 The 6-31G* basis set59 was used for the C, N and H
The emission spectra were recorded using a Shimadzu atoms while the “Stuttgart RSC 1997 ECP”60 relativistic
RF-5001 spectrofluorimeter and the emission lifetimes either effective core potential and associated basis set were employed
using an Edinburgh instruments FL-900 CDT, or an excitation to describe the Cu atom. The bulk solvent effects (dichloro-
source composed of a frequency-tripled (355 nm) Nd:YAG methane and acetonitrile, as in the experiments) have also
Q-switched laser (Continuum Inc.) coupled with an optical been included through the Conductor-like Screening Model
parametric oscillator (Continuum Inc.) and equipped with a (COSMO).61 These calculations were performed at the B3LYP/
CCD camera (Princeton Instruments).
6-31G*/Stuttgart level of theory using the in vacuo optimized
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Dalton Trans., 2013, 42, 14188–14199 | 14197

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Dalton Transactions
geometries of the complexes. The atomic charge density on 12 T. Rohand, E. Dolusic, T. H. Ngo, W. Maes and W. Dehaen,
the ligands in CuI and CuII complexes was calculated by the
ARKIVOC, 2007, x, 307–324.
method of analysis of Mulliken’s populations,62 at the B3LYP/ 13 J. C. Chambron, Rotaxanes: From Random to Transition
6-31G*/MDF10 level of theory using the Gaussian03 program
package.63 In order to determine the assignment of lowest
electronic transitions in absorption, the first lowest-lying
Metal-Templated Threading of Rings at the Molecular
Level, in Perspectives in Supramolecular Chemistry, John
Wiley & Sons, Ltd., 2007, pp. 225–284.
excited states of [Cu(II)(dpdipy)2] have been evaluated within 14 N. Armaroli, V. Balzani, J. P. Collin, P. Gaviña, J. P. Sauvage
the vertical Time-Dependent Density Functional Theory and B. Ventura, J. Am. Chem. Soc., 1999, 121, 4397–4408.
(TD-DFT) approximation using the 6-31G*/MDF10 level of 15 M. C. Jimenez-Molero, C. Dietrich-Buchecker and
theory. These calculations have been performed with the J. P. Sauvage, Chem.–Eur. J., 2002, 8, 1456–1466.
B3LYP functional that is known to be effective for a variety of 16 J. P. Sauvage, Chem. Commun., 2005, 1507–1510.
properties of metal complexes, including absorption spectra.
17 J. P. Sauvage, J. P. Collin, S. Durot, J. Frey, V. Heitz, A. Sour
and C. Tock, C. R. Chim., 2010, 13, 315–328.
18 L. Raehm and J. P. Sauvage, Molecular Machines and
Motors Based on Transition Metal-containing Catenanes
and Rotaxanes, in Structure and Bonding, Springer, 2001,
pp. 55–77.
19 D. R. McMillin, M. T. Buckner and B. T. Ahn, Inorg. Chem.,
1977, 16, 943–945.
20 G. B. Shaw, C. D. Grant, H. Shirota, E. W. Castner,
G. J. Meyer and L. X. Chen, J. Am. Chem. Soc., 2007, 129,
2147–2160.
Acknowledgements
The authors thank the FNRS (Fonds National pour la
Recherche Scientifique) and the IAP (Interuniversities Attrac-
tion Pool) in Belgium for supporting this research. K.S. thanks
the FNRS for a PhD fellowship. The Mons laboratory is grateful
to the FNRS for the financial support for the acquisition of the
Waters QToF Premier mass spectrometer and continuing
support. PG is a Senior Research Associate.
21 M. Iwamura, S. Takeuchi and T. Tahara, J. Am. Chem. Soc.,
2007, 129, 5248–5256.
22 R. M. Everly and D. R. McMillin, Photochem. Photobiol.,
1989, 50, 711–716.
23 L. X. Chen, G. Jennings, T. Liu, D. J. Gosztola, J. P. Hessler,
D. V. Scaltrito and G. J. Meyer, J. Am. Chem. Soc., 2002, 124,
10861–10867.
Notes and references
1 N. Armaroli, G. Accorsi, F. Cardinali and A. Listorti, Photo-
chemistry and Photophysics of Coordination Compounds:
Copper, in Photochemistry and Photophysics of Coordination 24 C. O. Dietrich-Buchecker, P. A. Marnot, J. P. Sauvage,
Compounds I, ed. V. Balzani, S. Campagna, Springer, Berlin,
Heidelberg, 2007, vol. 280, pp. 69–115.
J. R. Kirchhoff and D. R. McMillin, J. Chem. Soc., Chem.
Commun., 1983, 513–515.
2 J. P. Sauvage, Acc. Chem. Res., 1990, 23, 319–327.
3 F. Arnaud-Neu, E. Marques, M. J. Schwing-Weill,
25 D. Felder, J. F. Nierengarten, F. Barigelletti, B. Ventura and
N. Armaroli, J. Am. Chem. Soc., 2001, 123, 6291–6299.
C. Dietrich-Buchecker, J. P. Sauvage and J. Weiss, New 26 A. Lavie-Cambot, M. Cantuel, Y. Leydet, G. Jonusauskas,
J. Chem., 1988, 12, 15–20.
4 C. Moucheron, C. O. Dietrich-Buchecker, J. P. Sauvage and
D. M. Bassani and N. D. McClenaghan, Coord. Chem. Rev.,
2008, 252, 2572–2584.
A. van Dorsselaer, J. Chem. Soc., Dalton Trans., 1994, 885– 27 K. Karlsson, C. Moucheron and A. Kirsch-De Mesmaeker,
893. New J. Chem., 1994, 18, 721–729.
5 M. T. Miller, P. K. Gantzel and T. B. Karpishin, Inorg. 28 A. Kirsch-De Mesmaeker, L. Wilputte-Steinert and
Chem., 1998, 37, 2285–2290. J. Nasielski, Inorg. Chim. Acta, 1980, 45, L37–L39.
6 A. Pasquarello, I. Petri, P. S. Salmon, O. Parisel, R. Car, 29 R. J. Motekaitis and A. E. Martell, Inorg. Chem., 1970, 9,
É. Tóth, D. H. Powell, H. E. Fischer, L. Helm and
A. E. Merbach, Science, 2001, 291, 856–859.
7 R. Hernandez-Molina, I. Kalinina, P. Esparza, M. Sokolov,
1832–1839.
30 Y. Murakami, Y. Matsuda and K. Iiyama, Chem. Lett., 1972,
1069–1072.
J. Gonzalez-Platas, A. Estevez-Braun and E. Perez-Sacau, 31 F. C. March, D. A. Couch, K. Emerson, J. E. Fergusson and
Polyhedron, 2007, 26, 4860–4864. W. T. Robinson, J. Chem. Soc. A, 1971, 440–448.
8 T. Chandra, R. A. Allred, B. J. Kraft, L. M. Berreau and 32 I. V. Sazanovich, C. Kirmaier, E. Hindin, L. Yu,
J. M. Zaleski, Inorg. Chem., 2004, 43, 411–420.
9 G. Murphy, P. Nagle, B. Murphy and B. Hathaway, J. Chem.
Soc., Dalton Trans., 1997, 0, 2645–2652.
10 G. Murphy, C. O’Sullivan, B. Murphy and B. Hathaway,
Inorg. Chem., 1998, 37, 240–248.
D. F. Bocian, J. S. Lindsey and D. Holten, J. Am. Chem. Soc.,
2004, 126, 2664–2665.
33 V. S. Thoi, J. R. Stork, D. Magde and S. M. Cohen, Inorg.
Chem., 2006, 45, 10688–10697.
34 S. R. Halper, M. R. Malachowski, H. M. Delaney and
S. M. Cohen, Inorg. Chem., 2004, 43, 1242–1249.
11 A. Livoreil, J. P. Sauvage, N. Armaroli, V. Balzani,
L. Flamigni and B. Ventura, J. Am. Chem. Soc., 1997, 119, 35 J. Karolin, L. B. A. Johansson, L. Strandberg and T. Ny,
12114–12124.
J. Am. Chem. Soc., 1994, 116, 7801–7806.
14198 | Dalton Trans., 2013, 42, 14188–14199
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
36 K. Tan, L. Jaquinod, R. Paolesse, S. Nardis, C. D. Natale, 54 SAINT, version 5/6.0, Bruker AXS, Madison, USA, 1997.
A. D. Carlo, L. Prodi, M. Montalti, N. Zaccheroni and 55 SADABS, version 5.0, An empirical absorption correction
K. M. Smith, Tetrahedron, 2004, 60, 1099–1106.
37 R. Y. Lai and A. J. Bard, J. Phys. Chem. B, 2003, 107, 5036–
5042.
38 C. Bruckner, V. Karunaratne, S. J. Rettig and D. Dolphin,
Can. J. Chem., 1996, 74, 2182–2193.
program from SAINTPlus NT, Bruker AXS, Madison, USA,
1998.
56 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystal-
logr., 2008, 64, 112–122.
57 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens.
Matter, 1988, 37, 785–789.
39 T. S. Teets, D. V. Partyka, A. J. Esswein, J. B. Updegraff,
M. Zeller, A. D. Hunter and T. G. Gray, Inorg. Chem., 2007, 58 M. Valiev, E. J. Bylaska, N. Govind, K. Kowalski,
46, 6218–6220.
T. P. Straatsma, H. J. J. V. Dam, D. Wang, J. Nieplocha,
E. Apra, T. L. Windus and W. A. de Jong, Comput. Phys.
Commun., 2010, 181, 1477–1489.
40 M. W. Renner, K. M. Barkigia and J. Fajer, Inorg. Chim.
Acta, 1997, 263, 181–187.
41 A. Jaumá, J. A. Farrera and J. M. Ribó, Monatsh. Chem., 59 R. Ditchfield, W. J. Hehre and J. A. Pople, J. Chem. Phys.,
1996, 127, 927–933. 1971, 54, 724–728.
42 C. O. Dietrich-Buchecker, J. P. Sauvage and J. P. Kintzinger, 60 D. Andrae, U. Haeussermann, M. Dolg, H. Stoll and
Tetrahedron Lett., 1983, 24, 5095–5098. H. Preuss, Theor. Chim. Acta, 1990, 77, 123–141.
43 S. R. Halper and S. M. Cohen, Chem.–Eur. J., 2003, 9, 4661– 61 A. Klamt and G. Schuurmann, J. Chem. Soc., Perkin Trans.
4669. 2, 1993, 799–805.
44 S. A. Baudron, D. Salazar-Mendoza and M. W. Hosseini, 62 R. S. Mulliken, J. Chem. Phys., 1962, 36, 3428–3439.
CrystEngComm, 2009, 11, 1245–1254.
63 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr.,
T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam,
S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji,
M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai,
M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross,
V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma,
G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski,
S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas,
D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford,
J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith,
M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,
M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN 03
(Revision E.01), Gaussian, Inc., Wallingford CT, 2004.
45 K. L. Cunningham, K. M. McNett, R. A. Pierce, K. A. Davis,
H. H. Harris, D. M. Falck and D. R. McMillin, Inorg. Chem.,
1997, 36, 608–613.
46 M. Sharnoff, J. Chem. Phys., 1965, 42, 3383–3395.
47 C. M. Wansapura, C. Juyoung, J. L. Simpson, D. Szymanski,
G. R. Eaton, S. S. Eaton and S. Fox, J. Coord. Chem., 2003,
56, 975–993.
48 H. Yokoi and A. W. Addison, Inorg. Chem., 1977, 16, 1341–
1349.
49 M. Bröring, C. D. Brandt, J. Bley-Escrich and
J. P. Gisselbrecht, Eur. J. Inorg. Chem., 2002, 2002, 910–917.
50 M. Zhou and J. Heinze, J. Phys. Chem. B, 1999, 103, 8451–
8457.
51 S. Sadki, P. Schottland, N. Brodie and G. Sabouraud, Chem.
Soc. Rev., 2000, 29, 283–293.
52 G. Sanna, M. I. Pilo, M. A. Zoroddu, R. Seeber and
S. Mosca, Inorg. Chim. Acta, 1993, 208, 153–158.
53 A. Palma, J. F. Gallagher, H. Muller-Bunz, J. Wolowska,
E. J. L. McInnes and D. F. O’Shea, Dalton Trans., 2009, 273–
279.
This journal is © The Royal Society of Chemistry 2013
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