Homobinuclear sulfato-bridged and mononuclear nitrato complexes of Cu(II) with thiophen-2-yl-dipicolylamine; Structure and anion-dependent absorption spectra

Article abstract of DOI:10.1016/j.inoche.2011.02.028

A new ligand, bispyridin-2-ylmethyl-thiophen-2-ylmethylamine, 2ThDPA (1), was synthesized and its complexes of Cu(II) were isolated. When Cu(NO ₃)₂ was used as the metal source, a mononuclear complex with the formula [Cu(1)(NO₃)₂], 2, was isolated featuring both nitro groups coordinated bidentate to the Cu. 2 crystallizes in the monoclinic P21/c space group with a = 14.848, b = 8.212, c = 32.752 ?, β = 102.52° and V = 3898.3 ?³. When CuSO₄ was used as the metal source, a homobinuclear sulfato-bridged complex was isolated with the formula [Cu(1)SO₄]₂?CH ₃CN, 3. 3 crystallizes in the monoclinic P21/n space group with a = 10.457, b = 17.035, c = 12.425 ?, β = 108.72° and V = 2096.17 ?³. The structures of 2 and 3 are shown to be stabilized by an extended system of π-π stacking and hydrogen-bonding interactions. Comparison of the electronic absorption spectra of the ligand and complexes shows that complexation is maintained in solution and that the ligand absorption is attenuated in the complexes, depending on the counter-anion utilized.

Full text of DOI:10.1016/j.inoche.2011.02.028

Inorganic Chemistry Communications 14 (2011) 753–758
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Homobinuclear sulfato-bridged and mononuclear nitrato complexes of Cu(II) with
thiophen-2-yl-dipicolylamine; structure and anion-dependent absorption spectra
⁎
Joshua R. Zimmerman, Ana de Bettencourt-Dias
Department of Chemistry, University of Nevada, Reno, NV 89557–0216, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
A new ligand, bispyridin-2-ylmethyl-thiophen-2-ylmethylamine, 2ThDPA (1), was synthesized and its
complexes of Cu(II) were isolated. When Cu(NO3)2 was used as the metal source, a mononuclear complex
with the formula [Cu(1)(NO3)2], 2, was isolated featuring both nitro groups coordinated bidentate to the Cu. 2
crystallizes in the monoclinic P21/c space group with a=14.848, b=8.212, c=32.752 Å, β=102.52° and
V=3898.3 Å3. When CuSO4 was used as the metal source, a homobinuclear sulfato-bridged complex was
Received 23 December 2010
Accepted 26 February 2011
Available online 6 March 2011
Keywords:
Copper(II)
N3S-type ligands
Bridging-bidentate sulfate
·
isolated with the formula [Cu(1)SO4]2 CH3CN, 3. 3 crystallizes in the monoclinic P21/n space group with
a=10.457, b=17.035, c=12.425 Å, β=108.72° and V=2096.17 Å3. The structures of 2 and 3 are shown to
be stabilized by an extended system of π–π stacking and hydrogen-bonding interactions. Comparison of the
electronic absorption spectra of the ligand and complexes shows that complexation is maintained in solution
and that the ligand absorption is attenuated in the complexes, depending on the counter-anion utilized.
© 2011 Elsevier B.V. All rights reserved.
Introduction
the course of our work, we developed a new N3S donor tripodal ligand
with dpa and methylthiophen-3-yl as the third arm attached to the
Copper's physiological presence, along with its interesting magnetic
properties, has kept it as one of the most researched elements [1–5]. Di-
(2-picolyl)amine (dpa) has shown to coordinate to various transition
metals as a tridentate ligand through the amine nitrogen and two
pyridine nitrogen atoms with over 2500 hits in the CCDC [6]. Recently,
the incorporation of Cu(II) into dpa based molecules has been reported
[1–3,7–11]. Zhu and co-workers [1] synthesized a copper-sensitive
fluoroionophore by modifying stilbene with dpa. High sensitivity and
selectivity toward Cu(II) and Cu(I) was observed by significant
fluorescence quenching upon binding with copper ions, even in the
central amine nitrogen [16]. Solid-state structures of Cu(II) complexes
with this tripodal ligand, the effect of the counter-anion, and the
extensive hydrogen-bonding network present within the resulting
molecules were reported. We were interested in how ligand
conformation affects the structure of the compounds. Thus, we
synthesized a similar tripodal ligand with methylthiophen-2-yl as
the third arm. We report here its characterization and the isolation
and structural and photophysical characterization of its Cu(II)
complexes, as well as the effect of the counter-anion on the molecular
structure and on the absorption properties.
presence of metal ions such as Ag+, Al3+, Ba2+, Ca2+, Co2+, Cr3+, Fe2+
,
Fe3+, Hg2+, K+, Mg2+, Mn2+, Na+, Ni2+, Pb2+, Sn2+, and Zn2+. Mikata
et al. [10] studied dpa-based complexes of Cu(II) and investigated the
factors affecting intramolecular ether-oxygen coordination in their solid
state structures. These authors noticed that the anion of the starting
material plays a crucial role in this intramolecular coordination as well
as the overall coordination environment of the metal ion. Contrary to
this, Kumar et al. [11] studied Cu(II) metalorganic frameworks and
found no influence of the counter anion on the primary framework
structure, even though hydrogen-bonding interactions were observed
with the counter anion.
Experimental
All commercially available reagents were of analytical grade and
used as received. Solvents were dried by standard methods [17]. NMR
spectra were recorded on a Varian 400 spectrometer with chemical
shifts (δ, ppm) reported against tetramethylsilane. Absorption spectra
were obtained on a Perkin-Elmer Lambda 35 spectrometer in 1 cm
quartz cells at 25.0 0.1 °C. Methanol solutions of the compounds
were prepared at about 10−4 M.
Our research group focuses on luminescent metal ion complexes
for applications such as light-emitting diodes and sensors [12–15]. In
Synthesis of bispyridin-2-ylmethylthiophen-2-ylmethylamine, 2ThDPA (1)
2-Thiophenemethanol (2.0 g, 18 mmol) in diethyl ether was
added under inert atmosphere to a solution of PBr3 (7.1 g,
26.3 mmol) in diethyl ether. The solution was stirred at room
temperature for 30 min, after which H2O was slowly added to quench
⁎
Corresponding author.
E-mail address: abd@unr.edu (A. de Bettencourt-Dias).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.02.028

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J.R. Zimmerman, A. de Bettencourt-Dias / Inorganic Chemistry Communications 14 (2011) 753–758
Table 2
the reaction. The organic layer was separated and then washed with
brine and dried with MgSO4. The solvent was removed under reduced
pressure to give 2-bromomethylthiophene as an almost colorless oil
in 70% yield (2.16 g). This product in diethyl ether was added to a
solution of N,N-dipicolylamine (2.4 g, 12 mmol) and triethylamine
(4.8 g, 48 mmol) in diethyl ether at 0 °C. The reaction mixture was
stirred at this temperature for 5 min and then at room temperature for
18 h. The resulting red solution was filtered and the solvent was
removed under reduced pressure to yield a red oil. Purification by flash
chromatography using 90:10 THF/CH2Cl2 as the elutent gave 1 as a red
oil in 45% overall yield (2.29 g). 1H NMR (400 MHz, CDCl3, TMS, ppm) δ
8.52 (d, 3J=4.8 Hz, 2H), 7.67 (t, 3J=8.0 Hz, 4H), 7.23 (d, 3J=4.8 Hz,
1H), 7.15 (t, 3J=7.8 Hz, 2H), 6.94 (d, 3J=5.2 Hz, 2H), 3.90 (s, 2H), 3.85
(s, 4H). λmax [nm] (ε [M−1 cm−1]): 233 (13,000), 262 (8560).
Selected bond lengths [Å] for 2.
Cu1–O1
Cu1–O2
Cu1–O4
Cu1–O5
Cu1–N1
Cu1–N2
Cu1–N3
2.202(5)
2.862
Cu2–O7
Cu2–O9
Cu2–O10
Cu2–O11
Cu2–N6
Cu2–N7
Cu2–N8
2.135(4)
2.824
2.142(4)
2.616
1.959(5)
2.075(5)
1.946(5)
2.084(4)
2.653
1.947(5)
2.082(5)
1.948(5)
retrieved using SMART [18] software and refined using SAINTplus [19]
software. Absorption corrections were applied using SADABS [20]. The
structures were solved by direct methods and refined by least-squares
methods on F2 using the SHELXTL [21] program package. All non-
hydrogen atoms were refined anisotropically. The hydrogen atoms were
added geometrically, and their parameters constrained to the parent site.
Synthesis of the metal complexes
[Cu(2ThDPA)](NO3)2 (2). Cu(NO3)2 (0.16 g, 8.5 mmol) in MeOH
was added to a solution of 1 (250 mg, 8.5 mmol) in MeOH. The dark
blue solution was filtered after 30 min and the solvent removed under
reduced pressure to yield a light blue powder (190 mg) in 52% yield.
X-ray quality crystals formed by vapor diffusion of Et2O into a solution
of 2 in MeCN within 1 day. λmax [nm] (ε [M−1 cm−1]): 261 (11,400),
305 (4430), 663 (100).
Results and discussion
The ligand described here, bispyridin-2-ylmethylthiophen-2-
ylmethylamine (2ThDPA), was synthesized as shown in Scheme 1.
Bromination of 2-thiophenemethanol with PBr3 in diethyl ether under
inert atmosphere for 30 min yields 2-bromomethylthiophene. Stirring
this compound with N,N-dipicolylamine in diethyl ether in the
presence of triethylamine as base gives 1 in 45% overall yield.
Two different Cu(II) complexes with 1 were synthesized by
stirring the ligand with the appropriate copper salt, as illustrated in
Scheme 2. Reaction of 1 with the Cu(II) nitrate or sulfate yields the
metal complexes 2 and 3, respectively. X-ray quality crystals of each
were grown by slow vapor diffusion of diethyl ether into a solution of
MeCN containing the metal complex.
·
[Cu(2ThDPA)](SO4)]2 CH3CN (3). CuSO4 (42 mg, 1.7 mmol) in MeOH
was added to a solution of 1 (50 mg, 1.7 mmol) in MeOH. Slow solvent
evaporation of the dark blue solution yielded blue crystals of 3 in 47%
(45 mg) yield. X-ray quality crystals were grown by vapor diffusion of
Et2O into a solution of 3 in MeCN within 1 day. λmax [nm] (ε [M−1 cm−1]):
237 (29,100), 260 (25,000), 283 (3010), 666 (243).
X-ray crystallographic characterization
Complex 2 crystallizes in the monoclinic space group P21/c.
Crystallographic details for 2 are summarized in Table 1. The
asymmetric unit contains two different molecules of 2, which are
rotated 127.1° from one another. Each Cu atom is coordinated by the
three nitrogen atoms of dipicolyamine as well as two nitrate anions, as
shown in Fig. 1. Both thiophene moieties are disordered, occupying
two positions rotated by 180°, as is seen frequently with this ring [12–
15,22,23]. Both nitrate anions display bidentate coordination, result-
ing in a coordination number of seven for the metal atoms. The
coordination geometry around the central metal in both molecules is a
distorted pentagonal bipyramid. For the molecule containing Cu1, the
base of the pentagonal bipyramid is formed by the four oxygen atoms
of the bidentate nitrate anions, O1, O2, O4, and O5, and the central
amine nitrogen of the tripodal ligand, N2. The capping positions are
occupied by the two pyridine nitrogen atoms, N1 and N3.
Crystal data, data collection, and refinement details for compounds 2
and 3 are given in Table 1. Selected bond lengths and angles for these
complexes as well as selected hydrogen bond distances are given in
Tables 2 and3. Suitable crystals were mounted on a glassfiber and placed
in thelow-temperature nitrogen stream. Data were collected on a Bruker
SMART CCD area detector diffractometer equipped with a low-
temperature device, using graphite-monochromated Mo-Kα radiation
(λ=0.71073 Å). Data were measured using a strategy combining ω and
ϕ scans of 0.3° per frame and an acquisition time of 10 or 20 s per frame.
Multiscan absorption corrections were applied. Cell parameters were
Table 1
Details of the X-ray crystallographic characterization of compounds 2–3.
For the molecule containing Cu2 the base of the bipyramid is
spanned by O7, O9, O10 and O11 from the bidentate nitrate anions,
and N7 from the central amine of the tripodal ligand. The capping
positions are occupied by the two pyridine nitrogen atoms, N6 and N8.
The distances between the two Cu(II) and the ligating atoms are
shown in Fig. 2 and summarized in Table 2. The Cu–Npy distances
range from 1.946 to 1.959 Å, while the Cu–Namine distances are
Complex
2
3
CCDC no.
805496
C
805497
Formula
34H34Cu2N10O12S2 C19H20CuN4O4S2
M/g mol−1
965.91
Monoclinic
14.8479(4)
8.2115(2)
32.7523(9)
90
496.05
Crystal system
a/Å
b/Å
c/Å
Monoclinic
10.4569(2)
17.0349(2)
12.4247(2)
90
α/deg
β/deg
102.519(2)
90
3898.33
100(2)
4
108.7190(10)
90
2096.17
100(2)
Table 3
γ/deg
Selected bond lengths (Å) for 3.
V/Å3
CuA–N1A
CuA–N2A
CuA–N3A
CuA–O1A
CuA–O1B
CuA–O2A
C18–H…O2
C18–H…O4
1.987
1.982
2.023
1.953
2.314
2.709
2.257
2.278
T/K
Z
4
Dc/g cm−1
1.646
1.275
1.572
1.275
μ(Mo−Kα)/mm−1
Independent reflections, Rint [Fo ≥4σ(Fo)] 7475, 0.1569
9518, 0.0449
94,146
0.0392, 0.0876
9518/0/272
0.945
Reflections collected
R1, wR2 [IN2σ(I)]
180,834
⁎
⁎
0.0683, 0.1288
7475/20/530
1.023
Data/restraints/parameters
GoF on F2
⁎
Hydrogen bonding distance between acetoni-
trile and sulfato group (Fig. 7).
Largest diff. peak and hole/e Å3
0.808, −0.684
0.822, −0.822

J.R. Zimmerman, A. de Bettencourt-Dias / Inorganic Chemistry Communications 14 (2011) 753–758
755
and 2.538 Å, significantly shorter than what is observed for 2, as the
longer Cu–O distances average 2.739 Å. Another key difference is that
the longer Cu–O bond lengths are not across from one another as they
are in 2. Finally, the Cu–Npy distances are nearly identical to 2 at 1.948
and 1.958 Å, while the Cu–Namine distance is slightly longer (2.180 Å
vs. ~2.08 Å) due to the difference of the third arm attached to the
amine moiety.
An extended network of π–π stacking interactions between
pyridine and thiophene rings supports the packing structure. The
centroid–centroid π–π stacking distance between two pyridine rings
of adjacent molecules is 3.915 Å and the planes spanned by the rings
intersect at 13.69°, as shown in Fig. 3. The π–π stacking distance
between the centroids of the pyridine rings on the opposite side of the
complex with the same ring of a second molecule within the unit cell
is 3.681 Å and the planes of these rings intersect at an angle of 13.65°.
Thiophene rings are involved in π–π stacking with the thiophene of
neighboring molecules at a distance between centroids of 3.946 Å and
with planes spanned by the thiophene rings intersecting at 8.54°. The
previously reported structure with 3ThDPA also displays π–π stacking
between the pyridine rings and thiophene rings. In this structure
there are two different π–π interactions between the pyridine rings,
showing distances of 2.6095 and 3.1613 Å and with the planes
intersecting at 11.097 and 4.016°. The thiophene rings are separated
by 3.5899 Å, and the planes intersect at 3.048°. These π–π stacking
interactions are all at shorter distances and smaller intersecting angles
than in 2, indicating stronger π–π interactions.
Compound 3 crystallizes in the monoclinic space group P21/n.
Details of the crystallographic characterization are summarized in
Table 1. Compound 3 is a homobinuclear complex, in which half of the
molecule is generated by inversion. It displays the two Cu(II) ions
connected by two bridging-bidentate sulfato anions, similar to the
previously reported structure of [3ThDPACu(II)SO4)]2 [16]. A survey
of the CCDC shows that to date only 5 reported examples of sulfate
bridging between two Cu atoms are known [6]. Most other copper
complexes display only monodentate sulfate coordination, making 3
as well as the recently described complex with 3ThDPA unique. In 3
each Cu(II) is additionally bound to a single ligand of 1 by the nitrogen
atoms N1, N2, and N3 of the two pyridine rings and the tripodal
amine. As in 2, the thiophene does not coordinate to the Cu(II) atoms.
This results in a distorted N3O3 octahedral geometry around the Cu
(II), as shown in Fig. 4. The equatorial plane is comprised of the amine
nitrogen atom, N3, and the three oxygen atoms from the two bridging
sulfate anions. The axial positions of the octahedron are occupied by
the pyridine nitrogen atoms N1 and N2.
Scheme 1. Synthesis of 2ThDPA, 1.
slightly longer at 2.075 and 2.082 Å. Each bidentate nitrate has one
Cu–O longer than the other, the shorter distance in the range 2.083–
2.202 Å and the longer in the range 2.616–2.862 Å. These rank among
the longest reported Cu–O distances, however they are still within the
sum of the van der Waals radii of the binding partners (2.92 Å) and
are thus considered bonding interactions [6].
These distances are nearly identical to the previously reported
structure [3ThDPACu(NO3)2], where 3ThDPA is the bispyridin-2-
ylmethylthiophen-3-ylmethylamine ligand [16]. The previously
reported complex displays two independent molecules in the
asymmetric unit, as is the case in 2. However, the nitrate anions
display two different types of coordination. One molecule displays Cu
bound to two bidentate coordinated nitrate anions and in the other
molecule one nitrate is bidentate and the other is monodentate. The
bidentate coordinated nitrate anions have one Cu–O bond longer than
the other, with the shorter distances in the range 2.103 – 2.190 Å and
the longer distances in the range 2.622–2.871 Å. The Cu–Namine
distances for the previously reported compound are 2.082 and 2.084 Å
and the Cu–Npy distances are in the range 1.957 – 1.965 Å, slightly
longer than the distances found for 2.
There are seven other reported structures that feature copper
coordinated to two nitrate anions and a tripodal dpa derivative ligand
with non-coordinating arms on the central amine [7–10,24–26]. Only
one of these features both nitrates coordinated bidentate [8]. This is
however a Cu(III) complex, which also displays longer Cu–O distances
in the range 2.126–2.538 Å. Frey et al [9]. reported a unique Cu(II)
complex, in which three Cu(II) atoms coordinate in individual
tridentate dpa pockets on a single ligand. One of the Cu(II) atoms is
bound to two nitrate anions, one monodentate and one bidentate. The
Cu(II)–O distances of the bidentate nitrate are 2.705 and 2.312 Å and
2.013 Å for the monodentate nitrate. Almesåker et al. [8] isolated a
structure very similar to 2 with the dpa-based ligand N,N-4-tris
(pyridine-2-ylmethyl)aniline (tmpa) and Cu(NO3)3 featuring bis-
nitro-bidentate coordination. This complex has a similar pentagonal
bipyramidal coordination geometry about the Cu center with a NO4
base and capped by the two pyridine nitrogen atoms of dpa in the
axial positions. The Cu–O bonds for these nitrates display short
lengths of 2.126 and 2.356 Å, while the longer Cu–O bonds are 2.487
The Cu–O and Cu–N bond lengths are summarized in Table 3 and
are shown in Fig. 5. The Cu–Npy distances are in the range 1.982–
1.987 Å and are shorter than the Cu–Namine distance of 2.023 Å. The
Cu–Npy distances of 3 are slightly longer than those in 2, where as the
Cu–Namine bond length in 3 is shorter than that of 2. This could be due
to the fact that in 3 one of the sulfate oxygen atoms, more
electronegative than nitrogen, is opposite from the Namine. The Cu–N
Scheme 2. Synthesis of Cu(II) complexes 2 and 3.

756
J.R. Zimmerman, A. de Bettencourt-Dias / Inorganic Chemistry Communications 14 (2011) 753–758
O10
Cu2
O7
N6
O9
N8
O11
N7
N1
O2
N3
N2
Cu1
N2
O1
N1
Cu1
O5
O2
O1
N3
O4
Fig. 1. Thermal ellipsoid plot of 2 with thermal ellipsoids at 50% probability. Hydrogen
atoms are omitted for clarity. Only the thiophene ring with major occupancy in each
ligand is displayed for clarity.
Fig. 4. Thermal ellipsoid plot of 3 with thermal ellipsoids at 50% probability. Hydrogen
atoms and solvent molecule omitted for clarity.
O9
O2
N2
N8
2.862
2.824
O2A
N2A
N7
N1 1.947
1.946
2.082
1.947
O7
2.202Cu1
O1
2.075
2.134
2.142
Cu2
2.709
O1A
1.959
N3
2.083
N6
1.982
2.653
1.953
2.616
Cu1A
O10
O4
O5
2.023
2.314
O1B
O11
1.987
N1A
N3A
Fig. 2. Coordination environment around the two central Cu(II) atoms of 2. Selected
bond lengths are indicated in Å.
distances compare well with the previously reported Cu(II) com-
plexes with 3ThDPA where the Cu–Npy distances are in the range
1.972–1.983 Å and the Cu–Namine distances are slightly shorter in the
range 2.013–2.016 Å.
Fig. 5. Ball and stick diagram of 3 showing the distances (Å) of the coordination sphere
around the Cu(II) center.
Almesåker et al. [8] reported a CuSO4 complex with tpma, [Cu
(tpma)(μ-SO4)], with a very similar coordination environment to 2
Cu2
4.225
3.622
Cu2
Cu1
3.681
3.915
Cu1
3.946
3.946
Cu2
Cu1
3.681
3.915
Cu1
3.622
4.225
Fig. 3. Partial ball-and-stick diagram showing the π–π stacking interactions in 2.
Hydrogen atoms are omitted for clarity. Interplanar π–π distances (centroid–centroid)
are indicated in Å.
Fig. 6. Ball and stick diagram of 3 showing the π–π stacking interactions between the
pyridine rings and thiophene. Centroid–centroid distances are indicated in Å. Hydrogen
atoms have been omitted for clarity.

J.R. Zimmerman, A. de Bettencourt-Dias / Inorganic Chemistry Communications 14 (2011) 753–758
757
Table 4
Electronic absorption spectroscopic data for ThDPA ligands and Cu(II) complexes.
λmax [nm[ (ε [M−1 cm−1])
2.278
2ThDPA, 1
3ThDPA
233 (13,000) 262 (8560)
238 (10,800) 262 (9960)
[Cu(2ThDPA)(NO3)2], 2
[Cu(3ThDPA)(NO3)2]
–
261 (11,400) 305 (4430) 663 (100)
259 (10,100) 292 (2930) 656 (109)
–
[Cu(2ThDPA)(SO4)]2, 3 237 (29,100) 260 (25,000) 283 (6800) 666 (244)
[Cu(3ThDPA)(SO4)]2 238 (12,400) 256 (10,500) 289 (3020) 658 (129)
2.257
2.278
2.257
2.278
centroid distance of 3.6 Å it is considered a weak π-stacking
interaction [27].
2.278
One acetonitrile molecule crystallizes in the asymmetric unit of 3
and through two of its hydrogen atoms it is involved in hydrogen-
bonding with the bridging sulfate anions, as shown in Fig. 7 and
summarized in Table 3. At the same time, the bridging and the non-
coordinated oxygen atoms of the sulfate anion are each interacting
with a different acetonitrile molecule. The hydrogen-bond distance to
the non-bonded oxygen atom is 2.278 Å and the bond distance to the
bridging oxygen atom is slightly shorter at 2.257 Å.
2.278
2.257
2.257
Electronic absorption
Fig. 7. Partial ball-and-stick packing diagram of 3 showing the H-bonding interactions
between the acetonitrile and sulfato molecules. H-bonds shown in dashed lines with
selected distances in Å. All hydrogen atoms not involved in hydrogen-bonding
interactions were omitted for clarity.
UV/visible absorption spectra were obtained for both tripodal
ligands 3ThDPA, which was previously reported [16], and 2ThDPA, as
well as the Cu(II) complexes with nitrate and sulfate counter-anions.
The spectra are shown in Fig. 8 and data are summarized in Table 4.
Electron–donor–π–acceptor conjugated systems generally exhibit
and 3. The complex, however, only displays monodentate sulfato
bridging, resulting thus in 5-coordinate copper atoms. All of the bond
lengths for [Cu(tpma)(μ-SO4)] are longer than those observed for 3 in
the range 1.957–2.888 Å for Cu–O and 1.996–2.029 Å for Cu–N.
3 also displays an extensive network of π–π stacking interactions,
as seen in Fig. 6. The centroid–centroid distance between the pyridine
rings is 4.225 Å with the planes spanned by the rings intersecting at an
angle of 10.8°. This is much longer than similar interactions observed
in the complexes with 3ThDPA, with centroid–centroid distances of
2.8193 and 3.7028 Å and an angle of intersection between the
pyridine planes of 2.320 Å. A weak π–π stacking interaction is also
present between one of the pyridine rings and the thiophene ring. The
centroid–centroid distance is 3.622 Å with an angle of intersection of
26.1°. This angle is large considering π systems, but with a centroid–
⁎
characteristic electronic absorption bands associated with π–π
intraligand charge transfer (ICT). Electronic or structural perturba-
tions on the amine lone pair electrons influence the ICT properties and
thus the electronic spectra, providing a useful signal transduction
method for the design of chemosensors [1,28]. The ligands, 2ThDPA
and 3ThDPA, exhibit absorption peaks at 233 and 262 nm, shown in
the region common for dpa-substituted ligands [7,29,30]. These are
assigned as the ligand–based ICT transitions. Both complexes with
nitrate show almost complete disappearance of these bands, while
they are still visible in the sulfate complexes. The metal complexes
show the appearance of two new peaks, around 300 and 660 nm
(Fig. 8 inset), with the latter assigned to a blue shift of the Cu d–d
transition from λmax =808 nm for the hydrated Cu(II) ion [7,31].
These d–d transitions have very low ε values (100–243 M−1 cm−1
)
compared to the ligand based π–π ICT (8560–29,100 M−1 cm−1). The
d–d transition, however, does prove that complexation is maintained in
solution by the N10-fold increase in ε when compared to [Cu(H2O)6]2+
and the blue shift common for the exchange from O-donors to N-donors
[31]. Kirin et al. [7] reported similar results with aromatic and aliphatic
substituted dpa ligands, showing the d-d bands at~650 nm (ε=100–
130 M−1 cm−1) and the ligand based absorption at 240 nm for the
aliphatic substituent and 252 nm for the aromatic and approximate ε
values of 10,000 and 20,000 M−1 cm−1, respectively.
⁎
Conclusion
A new dpa-based ligand with methylthiophen-2-yl and its
complexes with Cu(II) were synthesized and characterized through
X-ray crystallography and electronic absorption spectroscopy. The Cu
(II) complexes display a similar coordination geometry to the
previously reported Cu(II) complexes of 3ThDPA and structures
which depend on the counter-anion utilized [16]. In the case of
nitrate a mononuclear complex was formed with bidentate nitrate
anions, resulting in a pentagonal bipyramidal coordination sphere
around the copper atoms. In the case of the sulfate counter-anion, a
homobinuclear sulfato bridged complex was formed, in which each
sulfate is coordinated bidentate to one Cu(II) center while bridging to
Fig. 8. Absorption spectra of
1
(2ThDPA), 3ThDPA,
2
[Cu(2ThDPA)(NO3)2] (2-Cu
(NO3)2), [Cu(3ThDPA)(NO3)2] (3-Cu(NO3)2),
3 [Cu(2ThDPA)SO4]2 (2-CuSO4), [Cu
(3ThDPA)(SO4)]2 (3-CuSO4) in methanol at ~[1.0×10−4 M]. Inset shows d–d transition
in the range 550–750 nm.

758
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another Cu(II). The electronic absorption spectra revealed that these
ligands possess intense π–π* transitions in the range 233–260 nm,
which are partially quenched in the complexes. Additionally, the
spectra of the complexes displayed a blue shifted d–d transition peak
for Cu(II) at 660 nm, verifying thus complex formation.
Acknowledgements
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The University of Nevada, Reno, and the National Science
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Appendix A. Supplementary data
Supplementary data to this article can be found online at
doi:10.1016/j.inoche.2011.02.028.
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