Synthesis, crystal structure and charge transfer spectra of dinuclear copper(I) complexes bearing 1,2-bis(arylimino)acenaphthene acceptor ligands

Article abstract of DOI:10.1016/j.ica.2011.02.042

A series of dinuclear halide bridged copper(I) complexes with a central [Cu₂(μ-X)₂] diamond core (X = Cl, Br and I) and substituted bis(imino)acenaphthene (BIAN) ligands acting as strong π-acceptor subunits was prepared and character

Full text of DOI:10.1016/j.ica.2011.02.042

Inorganica Chimica Acta 374 (2011) 632–636
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Inorganica Chimica Acta
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Note
Synthesis, crystal structure and charge transfer spectra of dinuclear
copper(I) complexes bearing 1,2-bis(arylimino)acenaphthene acceptor ligands
Thomas Kern a, Uwe Monkowius a, , Manfred Zabel b, Günther Knör a,
⇑
⇑
a Institut für Anorganische Chemie, Johannes Kepler Universität Linz, Altenbergerstr. 69, A-4040 Linz, Austria
b Zentrale Analytik der Universität Regensburg, Röntgenstrukturanalyse, Universitätsstr. 31, 93053 Regensburg, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 26 February 2011
A series of dinuclear halide bridged copper(I) complexes with a central [Cu2(
Br and I) and substituted bis(imino)acenaphthene (BIAN) ligands acting as strong
l
-X)2] diamond core (X = Cl,
p
-acceptor subunits
was prepared and characterized by elemental analysis, X-ray crystallography and various spectroscopic
techniques. In the visible spectral range, the electronic spectra of these deeply colored air-stable com-
pounds are dominated by low-lying charge transfer (CT) transitions. The corresponding broad absorption
bands display a bathochromic shift and a decreasing amount of negative solvatochromism in the series
X = Cl, Br and I. At the same time, the distance between the two metals becomes shorter and reaches
the range of cuprophilic interactions. These systematic trends are interpreted as a decreasing amount
of charge transfer character and a more pronounced delocalization between the two copper centers
within in the same series.
Dedicated to Professor Wolfgang Kaim on
the occasion of his 60th birthday.
Keywords:
BIAN ligands
Copper
Electronic spectra
Solvatochromism
Charge transfer
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
state properties, which makes them promising candidates for sev-
eral applications including multielectron transfer photocatalysis
Copper is a versatile redox-active bioelement, which can adopt
a variety of different functions and coordination environments in
metalloproteins [1,2]. The development of synthetic model sys-
tems for substrate transformations catalyzed by copper enzymes
is therefore a very active research field in bioinorganic and biomi-
metic chemistry [3–5]. Among several other possibilities, the for-
mation of dinuclear copper complexes coupled by bridging
ligands represents a prominent structural motif in the active sites
of many native metalloenzymes. The bioinorganic chemistry of
such dicopper centers includes the activation and transport of
dioxygen, the controlled hydroxylation of aromatic substrates,
and the acceleration of typical mono-oxygenase reactions such as
the selective oxidation of methane to methanol [2,6,7].
Since the corresponding redox processes are also very attractive
in the context of green chemistry and technology, there is an
increasing interest in the development of novel synthetic catalyst
systems based on cheap, abundant and environmentally benign
metal ions such as copper [8,9]. We have started to explore this
possibility and selected the combination of copper(I) sites with
1,2-bis(arylimino)acenaphthene (Ar–BIAN) derivatives as redox-
active N-donor chelates [10–14]. The corresponding mononuclear
systems are characterized by low-lying metal-to-ligand charge
transfer (MLCT) transitions and display readily tuneable excited
and dye-sensitized solar cells [10]. Here, we wish to report the syn-
thesis, structural characterization and spectroscopic properties of a
series of novel air-stable dinuclear copper(I) complexes of the type
[(Ar–BIAN)CuX]2 (X = Cl, Br and I) with Ar = o,o0-bis(isopropyl)
phenyl.
2. Results and discussion
2.1. Syntheses, characterization and crystal structures
Reaction of the 1,2-bis(o,o0-bis(isopropyl)phenylimino)-
acenaphthene with CuX leads to almost black, in diluted solution
violet colored compounds, which are obtained in high yields as
crystalline materials upon precipitation with n-pentane (Scheme
1). All metal complexes were stable in the solid state with respect
to air and moisture. They were characterized by elemental analysis
and 1H NMR, FTIR and ESI-MS spectroscopic methods. The 1H NMR
spectra of 1 and 2 are very similar and feature broad signals for the
alkyl and aromatic protons reflecting a fluxional behavior in solu-
tion. In contrast, the proton spectrum of 3 is well resolved (see Sec-
tion 3). Upon coordination, the IR-band assigned to stretching
vibrations of the imine C@N bonds is shifted to lower wavenum-
bers from 1671, 1652 and 1642 cmꢀ1 [15] for the free ligand to
ꢁ1645 cmꢀ1 for the copper complexes.
⇑
Corresponding authors.
E-mail addresses: uwe.monkowius@jku.at (U. Monkowius), guenther.knoer@j-
Single crystals suitable for X-ray diffraction of compounds 1–3
obtained from dcm/n-pentane are isomorphous and crystallize in
ku.at (G. Knör).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.02.042

T. Kern et al. / Inorganica Chimica Acta 374 (2011) 632–636
633
Ar
N
Ar
Ar
N
N
X
dcm
r.t.
Cu
Cu
2
+ 2 CuX
X
N
N
N
Ar
Ar
Ar
Ar =
X = Cl (1), Br (2), I (3)
Scheme 1.
Table 1
the monoclinic space group P21/n (Z = 2). The asymmetric unit con-
sists of the moiety [(BIAN)CuX]. Two Cu(I) atoms are bridged by
Bond lengths and angles of 1–3.
two halide ions forming the planar rhombic Cu2(l-X)2 core. Addi-
1
2
3
tionally coordinated by the imine nitrogen atoms of the BIAN
ligand, each Cu atom is in a distorted tetrahedral geometry. With
increasing size of the halide anion the X–Cu–X angle is significantly
increasing from 105.74(3)° to 119.61(2)° whereas the Cu–X–Cu an-
gle decreases from 74.26(3)° to 60.39(2)°. As a result, the copper
atoms are coming closer together. The CuꢂꢂꢂCu distances of the
chloride [2.772(1) Å] and bromide [2.753(1) Å] compounds are
only slightly shorter than the sum of the van der Waals radii of
two copper atoms (2.80 Å [16]). For the iodide compound the
CuꢂꢂꢂCu separation of 2.588(1) Å lies within the range of ‘‘cupro-
philic’’ interactions and is comparable to similar iodide complexes
(2.530–2.612 Å [17–24]). The Cu–N distances are almost unaf-
fected by changing the halide ligands and the bite angles of the
BIAN ligand in compounds 1–3 are nearly identical (N–Cu–N
ꢁ80°) and similar to other Cu–BIAN complexes [10–14]. The crystal
structure of the chloro bridged complex 1 is shown in Fig. 1. Char-
acteristic bond lengths and angles for all three compounds are
summarized in Table 1.
Bond length (Å)
Cu1–Cu1
Cu1–N1
Cu1–N2
Cu1–X1
Cu1–X1
N1–C1
2.772(1)
2.753(1)
2.103(3)
2.117(3)
2.391(1)
2.428(1)
1.288(4)
1.276(4)
2.588(1)
2.102(1)
2.126(1)
2.274(1)
2.319(1)
1.290(3)
1.280(3)
2.114(3)
2.110(3)
2.553(1)
2.593(1)
1.280(4)
1.287(4)
N2–C2
Bond angles (°)
N1–Cu1–N2
X1–Cu1–X1i
Cu1–X1–Cu1i
N1–Cu1–X1
N2–Cu1–X1
80.7(1)
105.74(3)
74.26(3)
119.2(1)
119.1(1)
80.0(1)
80.3(1)
110.31(2)
69.69(2)
117.8(1)
117.0(1)
119.61(2)
60.39(2)
113.2(1)
116.3(1)
Emission
I
Absorption
2.2. Electronic spectra
The absorption spectra of all copper(I) Ar–BIAN complexes
investigated in this study display a series of electronic transitions
below 370 nm and additional broad chromophoric features in the
visible spectral region (Fig. 2, Table 2). The structured bands at
shorter-wavelengths are assigned to intraligand (IL) transitions
Br
involving the conjugated
p
-electron system of the 1,2-diimine
IL transitions
Cl
MLCTs
300
400
500
600
Wavelength [nm]
Fig. 2. Electronic absorption and emission spectra of the [(Ar–BIAN)CuX]2 com-
pounds 1–3 in DCM solution (X = Cl, Br and I; kexc = 350 nm).
fragments, which are only slightly shifted with respect to the cor-
responding UV-absorptions of the free ligand [10].
Upon excitation of the complexes 1–3 in the UV spectral region,
a weak luminescence with a maximum at around 510 nm could be
observed (Fig. 2). This emission shows in the case of 1 and 3 a well-
Fig. 1. Molecular structure of
probability level, H atoms are omitted for clarity).
1
(ORTEP, displacement ellipsoids at the 50%

634
T. Kern et al. / Inorganica Chimica Acta 374 (2011) 632–636
Table 2
26000
MeOH
Cl
UV–Vis spectroscopic data of the complexes 1–3 in DCM solution at 298 K.
EtOH
acetone
kmax (nm) ( (L/(mol cm))
e
1
2
3
408 (13 600), 341 sh (23 900), 326 (27 800), 311 sh (24 600), 270 (51 800),
259 (69 000)
424 (6800), 340 sh (12 400), 326 (15 000), 307 sh (13 000), 271 (26 700),
257 (38 100)
434 (12 600), 341 sh (22 200), 323 (28 500), 318 sh (28 500), 274 (38 800),
259 (59 400)
25000
24000
23000
22000
Br
dmf
CH2Cl2
CHCl3
I
MeCN
toluene
0.3
resolved vibronic pattern attributed to skeleton vibrations of the
aromatic ligand system. Similar luminescence properties have
been reported for aromatic molecules such as decacyclene [25] also
benzene
0.4
0.5
0.6
0.7
0.8
0.9
1.0
containing the 14p-electron chromophore substructure of the Ar–
solvent parameter EM* LCT
BIAN system investigated in our work. Therefore we ascribe this
feature of compounds 1–3 to an intraligand (IL) fluorescence. This
tentative assignment is also supported by the facts that the free
base Ar–BIAN ligand also luminesces under identical conditions
(Fig. S1, Supplementary material), and that a similar emission
behavior has been reported recently for zinc complexes carrying
a related ligand system [26]. Further, much more detailed investi-
gations, however, will be necessary to confirm this tentative
assignment of the observed emission properties as an IL-type
luminescence.
Fig. 3. Correlation between theꢃcharge transfer transition energies of compounds 1
(j), 2 (d) and 3 (N) with the EMLCT values [27] of different solvents.
acceptor ligand has to be taken into consideration. Moreover, a var-
iation of the bridging halide ligands X has a significant effect on the
CuꢂꢂꢂCu distances and the Cu–X–Cu angles of the central Cu2(
l-X)2
core of the three complexes (Table 1). These parameters are well
known to strongly influence the exchange interactions in dinuclear
ligand bridged copper systems [28]. Thus they could also modulate
the properties of the charge transfer bands observed in our com-
pounds by a significant mixing of the transitions with copper(I)-
to-diimine (MLCT) and halide-to-diimine (LLCT) character accom-
panied by an increasing electronic delocalization in the series
X = Cl, Br and I. In order to support such a hypothesis, we also stud-
ied the typical negative solvatochromism of the complexes 1–3,
shifting the visible absorption maxima of all three compounds to
higher energy with increasing solvent polarity. The degree of this
characteristic effect can be quantified by applying different solvent
polarity scales such as the empirical EꢃMLCT-parameters [29] shown
in Fig. 3.
A reasonably linear correlation is observed for the solvatochro-
mic behavior of all compounds investigated.1 The slopes of the
regression line are 6.0 ꢄ 103, 3.4 ꢄ 103 and 1.4 ꢄ 103 cmꢀ1 for X =
Cl, Br and I, respectively. These data reflect a rather strong effect of
the different halide bridges on the dipole moment changes induced
upon excitation in the charge transfer region, which strongly indi-
cates that the dominant MLCT character in the chloride bridged sys-
The additional bands of the copper complexes 1–3 in the visible
spectral region are caused by the presence of charge transfer tran-
sitions. Mononuclear copper(I) systems carrying an individual
Ar–BIAN
p
-acceptor subunit with Ar = o,o0-bis(isopropyl)phenyl
display a broad band maximum at 405 nm with an extinction coef-
ficient of 7500 L/mol cm in DCM, which has been assigned in pre-
vious work as a metal-to-ligand charge transfer (MLCT) transition
[10]. In analogy to these results, the broad chromophoric visible
bands of the chloride-bridged complex 1 (Fig. 2) are ascribed to
charge transfer transitions from predominately copper(I) localized
d-electrons to the lowest unoccupied p⁄-orbitals of the Ar–BIAN li-
gands. The observed band maximum and intensity of these MLCT
transitions in 1 (Table 2) closely matches the features that could
be expected for the presence of two independent mononuclear
copper(I)–BIAN chromophores within the same molecule. It is
therefore plausible to assume that the ground state electronic cou-
pling and delocalization between the two individual halide bridged
copper(I) subunits is not very pronounced in the case of the chlo-
ride complex 1. Since the
p-acceptor properties of 1,2-diimine li-
gands of the Ar–BIAN type are readily fine-tuned by variations of
their aryl-substituents, it is possible to control the excitation
energy of such MLCT states over a wide spectral range. For exam-
ple, a significant red-shift of approximately 5400 cmꢀ1 can be
predicted for the corresponding Ar–BIAN systems carrying
Ar = methoxyphenyl instead of the o,o0-bis(isopropyl)phenyl sub-
stituents selected for the present investigation [10,27].
tem
1 changes into an almost solvent independent transition
between delocalized frontier orbitals in the case of the iodide com-
plex 3. The bromide derivative 2 represents an intermediate situa-
tion, which conserves
a significant degree of charge transfer
character in the lowest-lying excited states. Therefore the effects
of different bridging atoms X together with the possibility of tuning
Compared to the chloride bridged derivative 1, the maxima of
the charge transfer transitions in the bromide and iodide bridged
complexes 2 and 3 systematically shift to longer wavelengths
(Fig. 2), which is in agreement with the regular variations in
electronegativity.
The simplified picture of a predominant MLCT nature of the
charge transfer bands in the dinuclear [(Ar–BIAN)CuX]2 com-
pounds, however, has to be gradually modified in the series
X = Cl, Br and I. In the bromide compound 2 and especially in the
case of the iodide complex 3, the orbital parentage of the lowest-
lying excited states is no longer unambiguously settled. Due to
the more reducing character of the bridging ligands X in 2 and 3,
the occurrence of ligand-to-ligand charge transfer (LLCT) transi-
tions from the halide lone-pairs to the p⁄-orbitals of the diimine
the p-acceptor properties of the Ar–BIAN ligands by variations of the
aryl substituents offers a versatile tool to chemically modify and
control the absorption characteristics and excited state reactivity
patterns of dinuclear copper(I) BIAN complexes and related systems.
This should be a very useful property in terms of many potential
applications such as the development of bio-inspired homogeneous
photocatalysts and novel types of metal complex sensitizers for solar
cell devices based on abundant and environmentally benign
materials.
1
The deviation from the linearity for some solvents might stem from a coordina-
tion of solvent molecules or from some dissociation in different solvent.

T. Kern et al. / Inorganica Chimica Acta 374 (2011) 632–636
635
Table 3
Crystal data, data collection and structure refinement for compounds 1–3.
1
2
3
Formula
C
72H80Cl2Cu2N4
C72H80Br2Cu2N4
1288.30
0.29 ꢄ 0.28 ꢄ 0.22
monoclinic
P21/c
16.493(2)
12.415(2)
16.528(2)
90.00
C72H80I2Cu2N4
1382.30
0.34 ꢄ 0.18 ꢄ 0.14
monoclinic
P21/c
15.1455(2)
13.44971(17)
16.2721(2)
90.00
MW (g/mol)
Crystal size (mm)
Crystal system
Space group
a (Å)
1199.40
0.34 ꢄ 0.22 ꢄ 0.20
monoclinic
P21/c
16.4186(12)
12.4409(10)
16.4453(11)
90.00
b (Å)
c (Å)
a
(°)
b (°)
93.388(8)
90.00
3353.3(4)
1.112
91.616(15)
90.00
3382.9(8)
1.257
91.3972(12)
90.00
3313.68(7)
1.316
c
(°)
V (Å3)
qber (mg cmꢀ3
)
Z
l
2
2
2
(mmꢀ1
)
1.188
1.265
1.385
T (K)
range (°)
k (Å)
296(1)
2.05–25.84
297(1)
2.05–25.88
123
4.04–66.81
H
Reflections collected
Unique reflections
Observed reflections [I > 2r(I)]
24 175
6416 [Rint = 0.0563]
3081
30 789
6219 [Rint = 0.0861]
2910
13 559
5672 [Rint = 0.0331]
4582
Data/restraints/parameters
Absorption correction
Tmin, Tmax
6416/0/361
analytical
0.9171, 0.7729
0.289/ꢀ0.209
0.0355
6219/0/369
analytical
0.8049, 0.5689
0.670/ꢀ0.402
0.0355
5672/0/361
semi-empirical
1.00000, 0.46513
0.795/ꢀ0.619
0.0336
rfin (maximum/minimum) (e Åꢀ3
)
R1 [I P 2r(I)]
wR2
0.0773
0.0937
0.0865
CCDC
810735
810736
810737
ppm. MS (ESI): m/z 1198.00 [M]+, 1161.07 [MꢀCl]+. Anal. Calc. for
3. Experimental section
C
72H80Cl2Cu2N4 (1199.40): C, 72.10; H, 6.72; N, 4.67. Found: C,
71.88; H, 6.39; N, 4.56%. UV–Vis (CH2Cl2): k (log
(69 000), 270 sh (51 800), 311 sh (24 600), 326 (27 800), 341 sh
(27 800), 408 (13 600) nm. IR (KBr) = 2958 (m, –C–H), 1643 (s,
C@N–), 1281 (s, C–N) cmꢀ1
e) = 259 sh
3.1. General methods
m
All chemicals were purchased in reagent grade quality and di-
rectly used as received. Unless otherwise stated, commercially
available organic solvents of standard quality were purified and
dried according to the accepted general procedures. Elemental
analyses were performed at the Centre for Chemical Analysis of
the Faculty of Natural Sciences of the University of Regensburg.
Electronic absorption spectra were recorded with a Cary 300 Bio
UV–Vis spectrophotometer using 1 cm quartz cells. NMR spectra
were recorded with a Bruker Digital Avance NMR-spectrometer
DPX200 (1H: 200.1 MHz; 13C: 50.3 MHz; T = 303 K). The chemical
shifts are reported in ppm relative to external standards (solvent
residual peak), and coupling constants are given in Hertz. The spec-
tra were analyzed as being first order. Error of reported values:
0.01 ppm for 1H NMR and 0.1 Hz for coupling constants. The sol-
vent used is reported for each spectrum. Mass spectra were re-
corded with a LCQ DECA XP Plus (ESI). Infrared spectra were
recorded with a FT-IR-Spectrometer Paragon PC.
.
3.4. [(Pr2–Ph–BIAN)CuBr]2 (2)
Reaction of CuBr (50 mg, 0.35 mmol) and Pr2–Ph–BIAN (174 mg,
0.35 mmol) yielded 162 mg (72%) of a violet powder. 1H NMR
(200 MHz, CDCl3) d = 1.25–1.54 (br), 8.24 (br) ppm. MS (ESI): m/z
1163.20 [MꢀBr]+. Anal. Calc. for C72H80N4Cu2Br2 (1288.30): C,
67.12; H, 6.26; N, 4.35. Found: C, 67.14; H, 6.00; N, 4.31%. UV–
Vis (CH2Cl2): k (log
(13 000), 326 (15 000), 340 sh (12 400), 424 (6800) nm. IR (KBr)
= 2958 (m, –C–H), 1645 (s, C@N–), 1281 (s, C–N) cmꢀ1
e) = 257 sh (38 100), 271 (26 700), 307 sh
m
.
3.5. [(Pr2–Ph–BIAN)CuI]2 (3)
Reaction of CuI (50 mg , 0.26 mmol) and Pr2–Ph–BIAN (131 mg,
0.26 mmol) yielded 130 mg (72%) of a violet powder. 1H NMR
3
(300 MHz, CDCl3) d = 8.04 (d, JHH = 8.45 Hz, 4H), 7.24–7.48 (m,
3.2. Synthesis
Ar), 6.79 (m, 4H), 3.08 (m, Me2C–H, 4H), 0.99–1.54 (m, Me, 48H)
ppm. 13C NMR (50 MHz, CDCl3): d = 22.59 (+, 2C), 22.96 (s, 2C),
28.10 (s, 2C), 122.93 (s), 127.38 (s, 2C), 128.38 (s), 128.99 (s, 2C),
130.31 (s), 134.92 (s, 2C), 147.01 (s, 2C), 160.47 (s), 166.13 (s,
2C) ppm. MS (ESI): m/z 1255.00 [MꢀI]+. Anal. Calc. for
The preparation of all compounds described in the present work
was carried out according to the following general procedure: CuX
(X = Cl, Br and I) and the Pr2–Ph–BIAN ligand were stirred in 20 ml
of dichloromethane at room temperature for 12 h. Approximately
10 ml pentane were added to fully precipitate the complex. The
product was washed with pentane and re-crystallized from DCM/
pentane.
C
72H80N4Cu2I2 (1382.30): C, 62.56; H, 5.83; N, 4.05. Found: C,
62.73; H, 5.70; N, 3.98%. UV–Vis (CH2Cl2): k (log
e) = 259 sh
(59 400), 274 (38 800), 318 sh (28 500), 323 (28 500), 341 sh
(22 200), 434 (12 600) nm. IR (KBr)
m = 2958 (m, –C–H), 1645 (s,
C@N–), 1282 (s, C–N) cmꢀ1
.
3.3. [(Pr2–Ph–BIAN)CuCl]2 (1)
Reaction of CuCl (50 mg, 0.50 mmol) and Pr2–Ph–BIAN (250 mg,
0.50 mmol) yielded 237 mg (79%) of a violet powder. 1H NMR
(200 MHz, CDCl3) d = 8.06 (d, JHH = 10.50 Hz, 4H), 7.27–7.51 (m,
3.6. Crystal structures
3
Diffraction data for crystals of the compounds 1–3 were col-
Ar), 6.80 (m, 4H), 3.08 (m, Isopropyl, 4H), 1.56–1.01 (m, Me, 48H)
lected with a STOE-IPDS diffractometer [30] with graphite-mono-

636
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Acknowledgments
Partial support of this work by the Austrian Science Fund (FWF
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tizers’’) and the European Commission (ERA-Chemistry Project
I316: ‘‘Selective Photocatalytic Hydroxylation of Inert Hydrocar-
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