Semiconductive and magnetic one-dimensional coordination polymers of Cu(II) with modified nucleobases

Article abstract of DOI:10.1021/ic401758w

Four new copper(II) coordination complexes, obtained by reaction of CuX₂ (X = acetate or chloride) with thymine-1-acetic acid and uracil-1-propionic acid as ligands, of formulas [Cu(TAcO)₂(H ₂O)₄]·4H₂O (1), [Cu(TAcO) ₂(H₂O)₂]_n (2), [Cu ₃(TAcO)₄(H₂O)₂(OH)₂] _n·4H₂O (3), and [Cu₃(UPrO) ₂Cl₂(OH)₂(H₂O)₂] _n (4) (TAcOH = thymine-1-acetic acid, UPrOH = uracil-1-propionic acid) are described. While 1 is a discrete complex, 2-4 are one-dimensional coordination polymers. Complexes 2-4 present dc conductivity values between 10^-6 and 10^-9 S/cm^-1. The magnetic behavior of complex 2 is typical for almost isolated Cu(II) metal centers. Moderate-weak antiferromagnetic interactions have been found in complex 3, whereas a combination of strong and weak antiferromagnetic interactions have been found in complex 4. Quantum computational calculations have been done to estimate the individual "J" magnetic coupling constant for each superexchange pathway in complexes 3 and 4. Compounds 2-4 are the first known examples of semiconductor and magnetic coordination polymers containing nucleobases.

Full text of DOI:10.1021/ic401758w

Article
pubs.acs.org/IC
Semiconductive and Magnetic One-Dimensional Coordination
Polymers of Cu(II) with Modified Nucleobases
,†
‡
§
†
,∥
́
Pilar Amo-Ochoa,* Oscar Castillo, Carlos J. Gomez-García, Khaled Hassanein, Sandeep Verma,*
∥
,†
Jitendra Kumar, and Fel
†
́
ix Zamora*
Departamento de Química Inorgan
́
́
ica, Universidad Auton
ica, Universidad del País Vasco (UPV/EHU), Apartado 644, E-48080 Bilbao, Spain
Instituto de Ciencia Molecular (ICMol), Parque Científico, Universidad de Valencia, Catedratico Jose Beltran, 2, 46980 Paterna
́
oma de Madrid, 28049 Madrid, Spain
‡
Departamento de Química Inorgan
§
́
́
́
Valencia, Spain
∥
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016 (UP), India
*
S Supporting Information
ABSTRACT: Four new copper(II) coordination complexes, obtained by reaction of
CuX (X = acetate or chloride) with thymine-1-acetic acid and uracil-1-propionic acid as
2
ligands, of formulas [Cu(TAcO) (H O) ]·4H O (1), [Cu(TAcO) (H O) ] (2),
2
2
4
2
2
2
2 n
[
(
Cu (TAcO) (H O) (OH) ] ·4H O (3), and [Cu (UPrO) Cl (OH) (H O) ] (4)
3 4 2 2 2 n 2 3 2 2 2 2 2 n
TAcOH = thymine-1-acetic acid, UPrOH = uracil-1-propionic acid) are described.
While 1 is a discrete complex, 2−4 are one-dimensional coordination polymers.
−6
−9
−1
Complexes 2−4 present dc conductivity values between 10 and 10 S/cm . The
magnetic behavior of complex 2 is typical for almost isolated Cu(II) metal centers.
Moderate−weak antiferromagnetic interactions have been found in complex 3, whereas
a combination of strong and weak antiferromagnetic interactions have been found in
complex 4. Quantum computational calculations have been done to estimate the
individual “J” magnetic coupling constant for each superexchange pathway in complexes
3
and 4. Compounds 2−4 are the first known examples of semiconductor and magnetic
coordination polymers containing nucleobases.
6
INTRODUCTION
viewpoints. In particular most of the studies have been
focused on aspects related to their porosity, large inner surface
■
DNA nanotechnology provides one of the most interesting
approaches to form tailored complex structures with precise
control over molecular features. However, its intrinsic
7
area, tunable pore sizes, and topologies, leading to various
8
9,10
1
architectures and promising applications
such as gas
11
12
13
adsorption and storage, heterogeneous catalysis, nano-
electrical conductivity does not give the basis for electronic
functions. The DNA molecule needs to be functionalized with
electronic materials in order to increase its electrical
conductivity and to assemble functional electronic devices.
The ability of nucleic acid arrays to arrange other molecules
allows potential applications in molecular-scale electronics since
the assembly of a nucleic acid lattice may template the assembly
of molecular electronic elements such as molecular wires. The
nucleic acid nanostructures could provide a method for
nanometer-scale control of the placement and overall
architecture of these components, essentially using nucleic
acid structures as a molecular breadboard. DNA has been
mainly used as a template upon which to organize more highly
14
15
16
17
particles, luminescence, 1 nonlinear optics, magnetism,
8
and electrical conductivity. Recently, a breakthrough pointed
out the potential use of 1D-CPs as molecular nanowires thanks
to the excellent conductivity values reported for some
19−21
nanostructures of these compounds.
Our approach tries to combine the molecular recognition
capability of the DNA with the (multi)functional properties of
certain coordination polymers. We plan to form (multi)-
functional 1D coordination polymers containing nucleobases
and then connect those polymers to selected sequences of
oligonucleotides. The self-assembly between the DNA and the
coordination polymer will produce new nanohybrid materials
with new interesting properties with the feasibility of using well-
known DNA structural capabilities, including origami DNA,
toward complex nanocircuit formation.
2
,3
conductive materials such as (i) metal clusters on DNA, (ii)
metallization (electrostatic binding of metal ions and
4
reduction), (iii) formation of metallic-DNA (insertion of
Despite the importance of these potential applications as
advanced functional materials, the scarce studies with one-
metal ions by coordination between complementary nucleo-
2
bases on DNA), and (iv) decoration of DNA with semi-
5
conducting or surface-functionalized metal nanoparticles.
Coordination polymers (CPs) have attracted remarkable
attention in recent years from structural and properties
Received: July 8, 2013
©
XXXX American Chemical Society
A
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Article
dimensional and high dimensional polymeric nucleobase
structures have primarily focused on biological and biomimetic
aspects or in the construction of new metal organic
Powder X-ray diffraction has been done using a Diffractometer
PANalyticalX’Pert PRO theta/2theta primary monochromator and
detector with fast X’Celerator. The samples have been analyzed with
scanning theta/2theta.
Preliminary direct current (DC) electrical conductivity measure-
ments were performed on two different single crystals of compounds 2
and 3 and in two different pellets for compound 4, with carbon paint
at 300 K and two contacts. The contacts were made with platinum
wires (25 μm diameter). The samples were measured at 300 K
applying an electrical current with voltages form +10 to −10 V. The
measurements were performed in the compounds along the
crystallographic a axis.
2
2−37
frameworks.
Surprisingly, the study of one-dimensional coordination
polymers with nucleobases exhibiting magnetic or conductive
38
properties is also very scarce. With the aim to extend the
potential of these compounds to the fabrication of complex
conductive structures, we have explored the idea to use CPs
3
9,40
containing nucleobases
with basic structural aspects
39
resembling those expected for M-DNA. The results obtained,
although promising, suggest that new systems have to be
prepared to improve their electrical properties.
The DC electrical conductivity measurements were carried out also
with the four contacts method on two single crystals of compound 2
and four of compound 3 in the temperature range 300−400 K since
the resistance at room temperature exceeded the detection limit of our
On the basis of these principles, as a first step we focus on
the design and synthesis of one-dimensional (1D) coordination
polymers with selected transition metals and modified
nucleobases in order to study their electrical and magnetic
properties. We have selected carboxylic functionalized
nucleobases, (i) thymine-1-acetic acid (TAcOH) and (ii)
uracil-1-propionic acid (UPrOH), which are excellent candi-
dates to coordinate to Cu(II) via the carboxylate group (Figure
11
equipment (5 × 10 Ω), precluding its study at low temperatures. The
contacts were made with Pt wires (25 μm diameter) using graphite
paste. The samples were measured in a Quantum Design PPMS-9
equipment connected to an external voltage source (Keithley model
2400 source-meter) and amperometer (Keithley model 6514 electro-
meter). All the conductivity quoted values have been measured in the
voltage range where the crystals are Ohmic conductors. All the
measured crystals showed similar conductivity values and thermal
behaviors. The cooling and warming rates were 0.5 and 1 K/min, and
the results were similar in the cooling and warming scans.
1). This metal ion could provide the material with interesting
Magnetic measurements were performed on polycrystalline samples
of the complexes taken from the same uniform batches used for the
structural determinations with a Quantum Design MPMS-XL5
SQUID susceptometer in the temperature range 5−300 K with an
applied magnetic field of 5000 G. The susceptibility data were
corrected for the sample holder previously measured using the same
conditions and for the diamagnetic contribution of the salt as deduced
4
1
by using Pascal’s constant tables.
The X-ray diffraction data collections and structure determinations
were done at 100(2) K on an Oxford Diffraction Xcalibur
diffractometer. All the structures were solved by direct methods
4
2
using the SIR92 program and refined by full-matrix least-squares on
2
43
F
including all reflections (SHELXL97). All calculations were
44
performed using the WINGX crystallographic software package. All
non-hydrogen atoms were refined anisotropically. The hydrogen
atoms of the 3-(1-uracilyl)propanoate and thyminyl ligands were
either positioned geometrically and allowed to ride on their parent
atoms or located at the Fourier difference map and fixed at that
position (hydroxyl and water O−H). Crystal parameters and details of
the final refinements of compounds 1−4 are summarized in Table S1.
For complex 4, the water and hydroxide hydrogen atoms were
located on a Fourier map; however constraints were applied to fix the
O−H distance and H−O−H angle.
Figure 1. Schematic representation of the thymine-1-acetic acid
TAcOH) (left) and the uracil-1-propionic acid (UPrOH) (right).
(
1
8
magnetic or electrical properties. The direct reactions
between the two building blocks, metal ion and nucleobases,
has allowed us to isolate three unprecedented 1D coordination
polymers of Cu(II)-containing nucleobases. Herein, we present
their structural, magnetic, and electrical studies.
Syntheses. Synthesis of Ethyl-3-(1-uracilyl)propanoate. Uracil
EXPERIMENTAL SECTION
(
10 g, 89.2 mmol) and ethyl acrylate (13.4 mL, 125.8 mmol) were
suspended in 150 mL of dry DMF, and N-methylimidazole (1.0 mL)
was added. The white reaction mixture was stirred under a N
■
Materials and Methods. Copper(II) acetate monohydrate,
copper(II) chloride, thymine-1-acetic acid, uracil, N-methyl imidazole,
and other chemicals were purchased from standard chemical suppliers
and used as received. IR spectra were recorded on a PerkinElmer
spectrum 100 spectrophotometer using a universal ATR sampling
accessory and on a Bruker FT-IR Vector 22 model from 4000 to 400
2
atmosphere for 18 h (color changes from white to colorless). The
completion of the reaction was monitored by TLC analysis. The
solvent was evaporated under reduced pressure, affording an oily
product. To the residue was added 50 mL of diethyl ether and
sonicated for 10 min. The resulting suspension was cooled at 0 °C
overnight to induce precipitation. The precipitate was filtered and
dried to afford the desired product as a white powder (16.0 g, 85%
−
1
cm as KBr pellets. Elemental analyses were carried out by the
Microanalytical Service of the Autonoma University of Madrid and on
a Thermoquest CE instrument CHNS-O elemental analyzer (Model
́
1
13
+
EA/10) at IIT Kanpur. H and C NMR spectra were recorded either
on a JEOL-JNM LAMBDA 400 model operating at 400 and 100 MHz,
respectively, or on JEOL ECX-500 model operating at 500 and 125
MHz, respectively. The chemical shifts were referenced with respect to
tetramethylsilane. The high resolution mass spectra [HRMS] were
recorded in a Q-Tof Premier Micromass HAB 213 mass spectrometer
using a capillary voltage of 2.6−3.2 kV. L-SIMS spectra were obtained
with a Waters/Autospec mass spectrometer, using m-NBA (m-
nitrobenzyl alcohol) as a matrix. Peak identifications were based on
the m/z values and the isotopic distribution patterns.
yield). HRMS (M + Na) calculated: 235.07. Found: 235.07. M.P.
1
114−116 °C. H NMR (400 MHz, CDCl
, 25 °C, TMS): δ (ppm)
3
1.17 (t, 3H, CH ), 2.71 (t, 2H, CH ), 3.94 (t, 2H, CH ), 4.07 (q, 2H,
3
2
2
CH ), 5.61 (d, 1H, C5−H), 7.36 (d, 1H, C6−H), 10.25 (broad, 1H,
2
13
N3−H). C NMR (100 MHz, CDCl , 25 °C, TMS): δ (ppm) 13.93,
3
32.86, 45.07, 60.92, 101.60, 145.67, 150.95, 164.166, 171.21. Anal.
Calcd (found) for C H N O : C, 50.94 (52.36); H, 5.70 (5.59); N,
9
12
2
4
13.20 (13.67).
Synthesis of 3-(1-uracilyl)propanoic Acid (UPrOH). The above
ester (2.0 g, 9.4 mmol) was suspended in 40 mL of 5 M HCl and
B
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Figure 2. (a) ORTEP representation of the asymmetric unit of complex 1 at the 35% probability level. (b) Complex entity found in compound 1
with coordination polyhedron around the Cu center (C, gray; H, light gray; N, blue; O, red; Cu, turquoise).
refluxed for 10 h. The reaction mixture was cooled to room
allowed to crystallize at 25 °C. After several days, blue crystals were
formed, filtered, washed with water, and dried in the air (0.11 g, 34%
yield based on the metal). Anal. (%) Calcd (found) for
C H Cu N O : C, 31.56 (31.18); H, 3.94 (3.92); N, 10.52
(10.82). IR selected data (KBr, cm ): 3166(m), 3040(w), 1696(s),
1562(s), 1432(s), 1402(m), 1231(w), 901(w), 832(w), 763(w). The
analysis of powder X-ray diffraction of the first solid isolated in this
reaction is consistent with a mixture of compounds 1 and 3.
temperature and then neutralized with Na CO up to pH 6. The
2
3
solution was concentrated under reduced pressure to 20 mL, and
DMF (20 mL) was added to precipitate the sodium chloride salt. The
DMF layer was cooled to 0 °C for 1 h and filtered. Then, the filtrate
was evaporated under reduced pressure, and the viscous compound
thus obtained was dispersed with 20 mL of diethyl ether and kept at 0
2
8
42
3
8
24
−1
°C overnight, which afforded the target compound as a crystalline
precipitate. HRMS [M − 1]¯ calculated: 183.04. Found: 183.04. M.P.
Synthesis of [Cu
0.27 mmol.), CuCl ·2H
2
3
(UPrO)
2 2 2 2 2 n
Cl (OH) (H O) ] (4). UPrOH (0.050 g,
1
1
2
1
3
60−162 °C. H NMR (400 MHz, CD OD, 25 °C, TMS): δ (ppm)
2
O (0.070 g, 0.41 mmol), and distilled water (5
3
.50 (t, 2H, CH ), 3.95 (t, 2H, CH ), 5.60 (d, 1H, C5−H,), 7.65 (d,
mL) were placed in a 10 mL Teflon liner stainless steel bomb and
heated at 110 °C for 6 h under autogenous pressure. The reaction
mixture was cooled down to ambient temperature, which afforded a
light blue color clear solution. The solution was filtered and kept for
slow evaporation. Blue crystals of complex 4 were obtained after
2
2
1
3
H, C6−H). C NMR (125 MHz, DMSO-d , 25 °C, TMS): δ (ppm)
6
6.38, 46.45, 100.90, 147.63, 151.56, 165.65, 176.41. Anal. Calcd
(
(
found) for C H N O : C, 45.66 (45.57); H, 4.38 (4.33); N, 15.21
7 8 2 4
15.14).
Synthesis of [Cu(TAcO) (H O) ]·4H O (1). Copper(II) acetate
several days (65 mg, 18% yield, based on HL precursor). HRMS (ES+
2
2
4
2
+
monohydrate (0.100 g, 0.5 mmol) was added to a solution of
thymine-1-acetic acid (0.184 g, 1 mmol) and potassium hydroxide
mode): [L + Cu + H
2
O] = calculated, 263.99; found, 264.03. [2L+Cu
+
+H] = calculated, 430.03; found, 430.02. (ES−) mode: [3L+Cu]¯ =
calculated, 612.06 and 614.05; found, 612.04 and 614.04. Anal. (%)
(
0.058 g, 1 mmol) in 10 mL of distilled water (pH = 5). The resulting
mixture was refluxed and stirred for 3 h. Upon cooling, blue crystals
appeared, which were filtered off; washed with water, ethanol, and
ether; and dried in the air (0.112 g, 50% yield based on the metal). L-
Calcd (found) for C14H20Cl Cu N O12: C, 24.09 (23.76); H, 2.89
2 3 4
−1
(3.10); N, 8.03 (7.83). IR selected data (KBr, cm ): 3412(w),
3034(w), 2832(m) 1682(s), 1436(s), 1361(s), 864(s).
+
+
SIMS (ES mode): [L + Cu] = calculated, 246.65; found, 245.97. [2L
+
+
Cu+H] = calculated, 431.15; found, 430.03. Anal. (%) Calcd
RESULTS AND DISCUSSION
■
(
9
1
found) for C H CuN O : C, 29.29 (30.01); H, 5.23 (5.07); N,
14
30
4
16
−
1
.76 (9.71). IR selected data (KBr, cm ): 3430(m), 1678(s),
The reaction of thymine-1-acetic acid with copper(II) acetate
or copper(II) chloride carried out at 25 °C and at different pH
values led to the formation of one-dimensional coordination
polymers (pH 2 for 2 and pH 5 for 3 and 4). More energetic
conditions, reflux or solvothermal, increase the yield of the
mononuclear complex 1. However, the reaction of uracile-1-
propionic acid with copper(II) chloride under solvothermal
conditions led selectively to compound 4.
1
603(w), 1479(w), 1404(w), 1350(w), 1247(w), 869(w). H NMR
(
3
400 MHz, D O, 25 °C, TMS, pH = 5.5): δ (ppm) 1.96 (d, 3H, CH ),
2
3
.57 (s, 2H, CH ), 7.44 (s, 1H, C6−H). The purity of the crystal
2
sample was checked by powder X-ray diffraction.
Synthesis of [Cu(TAcO) (H O) ] (2). A mixture of CuCl ·2H O
0.100 g, 0.58 mmol), thymine-1-acetic acid (0.216 g, 1.17 mmol), and
2
2
2 n
2
2
(
potassium hydroxide (0.065 g, 1.17 mmol) was dissolved in 20 mL of
distilled water and stirred for 3 h at 25 °C (pH = 2). The blue mixture
obtained was filtered off. The blue solid obtained was washed with
water, methanol, and ether and dried in the air (0.056 g, 21% yield
based on the metal). The purity of the blue solid has been checked
using powder X-ray diffraction. Blue crystals suitable for X-ray
diffraction were obtained from the mother solution upon standing for
The IR spectra of the obtained compounds (1−4) show
significant shifts in the CO and C−O stretching vibrations
relative to those found in the free ligands. Thus, in general
upon copper coordination to the carboxylic groups the C−O
vibrations undergoes a shift to lower frequencies, υ (C−O) of
as
3
weeks at 25 °C. They were filtered off; washed with water, ethanol,
−1
−1
ca. 10−40 cm and υ (C−O) of ca. 40−100 cm . This is in
agreement with the coordination of the carboxylic groups from
the ligands with copper(II) ions.
sim
and ether; and dried in the air (0.042 g, 15.5% yield based on the
metal). Anal. (%) Calcd (found) for C H CuN O : C, 36.1 (36.2);
1
4
18
4
10
−1
H, 3.89 (3.89); N, 12.51 (12.39). IR selected data (KBr, cm ):
Structural Description. The crystal structure of compound
3
1
165(m), 3038(m), 1672(s), 1562(s), 1426(m), 1408(m), 1353(w),
254(w), 898(w).
1
consists of discrete [Cu(TAcO-κO) (H O) ] complexes and
2 2 4
crystallization water molecules (Figure 2). The metal center
(green polyhedra) presents a CuO coordination environment
Synthesis of [Cu (TAcO) (H O)(OH)] ·4H O (3). The reaction was
1
.5
2
2
n
2
carried out as described above for compound 1 but at 25 °C instead.
The blue suspension obtained was filtered off and the blue solution
6
with the usual Jahn−Teller tetragonally elongated octahedral
C
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Figure 3. (a) ORTEP representation of the asymmetric unit of complex 2. (b) Fragment of the polymeric chain of compound 2. (c) T:T base
pairing between adjacent polymeric chains to provide supramolecular sheets.
geometry (Table S2). The basal plane implies two carboxylate
oxygen atoms from two TAcO ligands and two coordinated
water molecules (O1w and O1w ). The two apical positions are
in such a way that it is coordinated through O1 to the
equatorial plane of one copper(II) atom and through O2 to the
apical position of an adjacent metal center (1.945(2) and
2.624(2) Å, respectively). The carboxylate group is almost
coplanar with the Cu1−O1 bond but almost perpendicular to
i
occupied by additional coordination water molecules (O2w and
i
O2w ) with longer coordination bond distances (2.5309(14) vs
ii
1.9413(14) and 1.9595(13) Å). The functionalized thymine
Cu1 −O2. Two trans water molecules complete the coordina-
moiety coordinates only through one of its carboxylic oxygen
atoms (Cu1−O1: 1.9595(13) Å), whereas the other is involved
in an intramolecular hydrogen bonding interaction with the
apical coordination water molecule. The thymine molecule not
involved in metal coordination establishes an intricate network
of hydrogen bonding interactions with the water molecules that
provides cohesion to the 3D crystal building. There is no
evidence of direct hydrogen bonding or aromatic interaction
among the nucleobases.
Although compound 2 crystallizes under similar aqueous
conditions, it presents a double bridged 1D polymeric complex
in which the copper(II)/water ratio is lower, the TAcO ligand
being forced to occupy the empty coordination sites of the
elongated octahedral geometry (Figure 3). The carboxylato
group of the thymine adopts a μ-1κO:2κO′ coordination mode
tion equatorial plane. These coordinated water molecules
further contribute to the cohesion within the chain by means of
intrachain hydrogen bonds that they establish as donors with
the thymine O2′ and the carboxylate O2 oxygen atoms as
acceptors. The copper···copper distance within the chain is
4.5896(3) Å. The thymine bases are projected outward from
the 1D chain, allowing the establishment of complementary
hydrogen bonding interactions (N3−H···O4, d
= 1.99 Å)
H···O
between thymine fragments belonging to adjacent chains. This
interaction provides 2D supramolecular sheets of complex
chains that are held together by means of weak van der Waals
interactions (Figure 3c).
Compound 3 corresponds to a 1D polymeric complex in
which hydroxide anions help the TAcO ligands to connect the
copper(II) metal centers. In fact, two crystallographically
D
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independent copper(II) centers are present in the crystal
structure of this compound. Both are octahedrally coordinated,
but Cu1, sited on a symmetry center, is bonded to two
hydroxide anions and four oxygen atoms belonging to the
carboxylate group of four TAcO ligands, whereas Cu2, located
in a general position, completes its coordination polyhedron
with two hydroxide anions, a water molecule, and three oxygen
carboxylate atoms from three TAcO ligands. Both the
hydroxide and the TAcO ligands act as bridges linking the
metal centers within the chain. The hydroxide anion connects
three metal centers in an almost symmetrical fashion (bond
distances: 1.963−1.966 Å, Table S2). There are two crystallo-
graphically independent TAcO ligands that bridge adjacent
metal centers showing different coordination modes. The first
ligand, TAcO-1, uses its two carboxylate oxygen atoms to
bridge three metal centers (μ -1κO:2κO:3κO′), whereas the
3
second ligand, TAcO-2, uses only one carboxylate oxygen atom
to bridge two metal centers (μ-1κO:2κO) (Figure 4). The
equatorial positions of the Cu coordination spheres are
occupied by the carboxylato oxygen atoms of the TAcO-1
ligand, the hydroxide anion, and the water molecule with short
coordination bond distances (1.93−1.99 Å). The TAcO-2
ligand and one carboxylato oxygen atom from the TAcO-1
ligand occupy the apical positions (2.34−2.68 Å). This
connectivity scheme leads to a linear array of fused metal
triangles that share an edge and the opposite vertex of each
triangle where three different metal···metal distances could be
ii
identified (Cu1···Cu2, 3.003(2); Cu1···Cu2 , 3.302(2); and
ii
Cu2···Cu2 , 2.9734(14) Å). The thymine residue of the TAcO
ligands remains, like in compounds 1 and 2, free to establish
additional supramolecular interactions that ensure the 3D
cohesiveness of the crystal structure.
These supramolecular interactions comprise two different
recognition processes involving thymine residues that hold
chains together: (i) a double hydrogen bond (N23−H···O27,
d
= 2.01 Å) between the TAcO-2 ligands and (ii) two
H···O
hydrogen bonds established between the TAcO-1 ligand and
the coordinated water molecule and a carboxylate oxygen atom
from another TAcO-1 ligand (dH···O = 1.96 and 1.85 Å,
respectively). The crystallization water molecules also reinforce
previously described interactions by means of an extensive
network of hydrogen bonds.
The structure of compound 4 consists of a
[
Cu (UPrO) Cl (OH) (H O) ] one-dimensional polymer
3 2 2 2 2 2 n
running along the a axis. The asymmetric unit of complex 4
is composed of two crystallographically unique copper ions
(Cu1 and Cu2) with Cu2 having half occupancy (Figure 5a).
The nucleobase precursor behaves as a monoanionic ligand
with both carboxylate oxygen atoms coordinated to the copper
ions. The lattice is further neutralized by a μ -bridging hydroxo
3
Figure 4. (a) ORTEP representation of the asymmetric unit of
complex 3. (b) Fragment of the polymeric chain and supramolecular
structure of compound 3. (c) Schematic representation of the chain
topology. Crystallographically unique copper ions are highlighted with
different colors.
anion and a chloride anion (Figure 5a) along with a μ -bridging
2
aqua ligand, also found in the lattice. As can be observed in
Figure 5b, both copper centers show elongated octahedral
geometries (due to the Jahn−Teller effect) with different
coordination spheres. The Cu1 center is coordinated by one
chloride and five oxygen atoms. The equatorial plane is formed
by one oxygen atom from a carboxylate group (O1), two
equivalent hydroxide anions (O4), and the chlorine anion
Jahn−Teller elongated octahedral geometry. The equatorial
plane is formed by two oxygen atoms from two carboxylate
groups (O2 and O2*, with Cu2−O2 = 1.959(3) Å) and two
(
(
Cl1). The axial positions are occupied by a water molecule
O3) and a carboxylate oxygen atom (O2). The Cu−O
equivalent μ -bridging hydroxide anions (O4 and O4*, with
3
equatorial bond lengths (1.951−1.988 Å) are much shorter
than the axial ones (2.457−2.614 Å). The Cu2 ion lies on an
inversion center and is coordinated by six oxygen atoms in a
Cu2−O4 = 1.957(3) Å). The axial positions are occupied by
two water molecules (O3 and O3* with a much longer bond
length of 2.427(4) Å). All the Cu−O and Cu−Cl bond lengths
E
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Inorganic Chemistry
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Figure 5. (a) ORTEP representation of complex 4 at 35% probability level. (b) Part of the lattice of 4 with coordination atmosphere around Cu
centers.
Figure 6. (a) U:U base pairing in the crystal lattice as shown with fragmented bonds (view close to c axis). (b) Involvement of hydroxo and aqua
ligands in the H-bonding. (c) Schematic representation of the chain topology of complex 4.
are in the typical range observed in other Cu(II) octahedral
complexes.
It is interesting to note that both oxygen atoms of the
carboxylate group (O1 and O2) interact with the copper
centers in a different manner. O1 acts as monodentate toward
Cu1, whereas O2 bridges Cu1 and Cu2 centers, giving rise to
aqua ligands are also involved in H-bonding interactions acting
as H-donors for the remaining exocylic oxygen atom (O6′) of
the uracil moiety (Table S3 for H bonding) as shown in Figure
6b. These H-interactions further extend the lattice along the c
axis, increasing the dimensionality of the crystal structure.
Magnetic Properties. Compound 2 presents a magnetic
behavior typical of almost isolated copper(II) metal centers; the
4
5
an overall tridentate (μ -1κO:2κO:3κO′) coordination mode for
3
3
−1
the carboxylate group. All the copper atoms are bridged by μ -
χ T curve remains constant upon cooling (0.425 cm K mol )
m
3
OH, μ -aqua, and μ -carboxylate ligands to generate a ribbon-
like extended Cu−O network where all the interconnected
copper atoms lie on the same plane. As observed in compound
and, only at temperatures below 15 K, decreases slightly (0.410
2
3
3
−1
cm K mol at 5 K; Figure S1). Considering the structural
features, the experimental data were least-square fitted with a
numerical expression proposed for an antiferromagnetic
3, the connectivity array consists of fused metal triangles that
46
share an edge and the opposite vertex of each triangle where
copper(II) uniform chain that leads to J (triplet-singlet gap)
−
1
−6
three different metal···metal distances can be found (Cu1···Cu2
= −0.2 cm , g = 2.14, and R = 1.7 × 10 , R being the
i
iii
2
=
2.962(2) Å, Cu1···Cu2 = 3.220(2) Å, and Cu1···Cu1 =
agreement factor defined as ∑ [(χ T) (i) − (χ T) (i)] /
i
M
obs
M
calc
2
3.099(2) Å, Figure 6c).
Σ [(χ T) (i)] . This almost negligible antiferromagnetic
i
M
obs
Further reinforcement to the lattice comes from the
coupling can be understood in terms of the nature of the
orbital involved in the superexchange interactions, together
with the arrangement of the bridging ligand. The unpaired
electron of the copper(II) ion is essentially described by a
hydrogen bonding interaction. The uracil nucleobases are
positioned on each side of the chain structure and involved in
the well-known U:U base pairing scheme by exploiting N−H···
2
2
O hydrogen bonding (d
= 1.988 Å) with the uracil moiety
magnetic orbital built from the d
metallic orbital and
H···O
x −y
present in the adjacent chain as shown in Figure 6a. This
located mainly in the basal plane. The carboxylato, with a basal
interaction extends the lattice along the b axis. The hydroxo and
(short, 1.945(2) Å)−axial (long, 2.624(2) Å) bridging mode
F
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Inorganic Chemistry
Article
leads to an almost nil overlap between the two metal-centered
magnetic orbitals, resulting in a weak anti- or ferromagnetic
(
when the overlap is zero, accidental orthogonality) cou-
47,48
pling.
Variable temperature susceptibility measurements of com-
pounds 3 and 4 (Figure 7) reveal, despite their structural
Figure 8. Magnetic superexchange pathways in compounds 3 (a) and
4
(b). Thinner bonds represent the elongated coordination bonds due
to the Jahn−Teller effect of the copper(II) metal centers. Only the
carboxylate groups of the thymine (a) and uracile (b) modified ligands
are represented for clarity.
Figure 7. Temperature variable χ T (squares) and χ (circles) curves
per copper(II) atom for compounds 3 (blue) and 4 (red).
m
m
52−57
symmetry (BS) approach.
This approach has proven to be
similarity, a different magnetic behavior. The χ T value per
a powerful tool in the prediction and interpretation of magnetic
properties of a large variety of magnetic systems, as well as for
revealing and tracing magneto-structural correlations. The
m
copper(II) atom for compound 3 at room temperature is 0.430
3
−1
58
cm K mol . This value remains almost constant until 120 K
and shows a sharp decrease upon cooling to reach a value of
broken symmetry treatment results in a solution that is an
3
−1
0
.012 cm K mol at 2 K. The thermal evolution of the
eigenstate of S (with eigenvalue S ) but not of S , which can
z
min
2
magnetic susceptibility shows the presence of a rounded
maximum around 43 K. This behavior is indicative of dominant
moderate antiferromagnetic interactions between the Cu(II)
be written as a weighted average of the energies of the pure spin
multiplets. In order to analyze the results, the approximate
projection method introduced by Yamaguchi and co-workers to
atoms. Compound 4 presents a χ T value at room temperature
account for the spin contamination was employed in the
m
3
−1
53,59,60
of 0.092 cm K mol which is well below the expected value
for a copper(II) metal center. This value slowly decreases upon3
cooling down to 15 K, and then it drops to a value of 0.009 cm
K mol− at 2 K. The magnetic susceptibility curve shows a
approximate spin projected scheme.
The hybrid B3LYP
61
method, as implemented in Gaussian 03, was used in all
calculations. The exact Hartree−Fock-type exchange was
mixed with Becke’s expression for the exchange functional,
62
1
61
63
sharp maximum at 5 K. The low χ T value at room
and the Lang−Yong−Parr correlation functional was used.
The Gaussian implemented 6-31G(d) basis set has been
employed throughout this work. In order to reproduce as close
as possible the chemical surrounding for the computed
superexchange coupling for every adjacent pair of metal
centers, the additional metal centers coordinated to the
bridging ligands were replaced by zinc(II) atoms without any
further change in the coordination bond distances. The
coordination sphere of every metal center is completed by
the original ligand molecules/anions except for uracil-
propionate that was replaced by acetate anions. The results
are summarized in Table 1. Figures of the models and the spin
density distribution for the triplet and singlet states are
provided as Supporting Information (Figure S2).
m
temperature and its slow decrease upon cooling suggests the
presence of very strong antiferromagnetic coupling between
some but not all the copper(II) centers. In fact, from the
maximum in the χ curve at 5 K it can be inferred that the
m
remaining copper(II) atoms present only weak antiferromag-
netic interactions.
Figure 8 depicts the different superexchange coupling
constants taking place in compounds 3 and 4 that should be
taken into account in order to model their magnetic behavior.
As far as we know, no available mathematical expression could
account for the scheme of magnetic interactions taking place
within the polymeric chains of these compounds. Therefore, we
have estimated the individual J magnetic coupling constant for
each magnetic superexchange pathway (J , J , J ) from quantum
The results indicate that the most notorious difference takes
place for the double hydroxide bridge mediated superexchange
interaction (J ) that should be weakly ferromagnetic (J = +28
1
2
3
computational calculations performed over simplified fragments
of the polymeric chains. The magnetic exchange interaction
between the open-shell electrons of a magnetic system within
1
1
−
1
cm ) for compound 3 according to the computational
simulation but strongly antiferromagnetic (J ≈ −300 cm )
for compound 4. The calculated spin-density distribution for
4
9−51
−1
the DFT methods
is based on spin-polarized or
1
unrestricted DFT calculations, in combination with the broken
G
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Inorganic Chemistry
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a
Table 1. Magnetically Relevant Structural Parameters and Computed Magnetic Coupling Constants for Compounds 3 and 4
Compound 3
b
Cu2···Cu2
.97 Å
Cu1···Cu2
.00 Å
Cu1···Cu2
.30 Å
Cu2−O−Cu2
98.3°
Cu1−O−Cu2
99.7°
Cu1−O−Cu2
114.4°
Cu O planarity
H−O···O
53.5°
Cu1−OOH
1.96 Å
Cu2−O
1.97 Å
J1
2
2
−
−
1
1
2
0.0°
Cu1−OOCO
1.99 Å
+28 cm
J2
Cu2−OOCO
1.93 Å
Cu2−OOH
1.97 Å
3
+36 cm
J3
Cu1−OOH
1.96 Å
Cu2−OOH
1.97 Å
−
1
3
−8 cm
Compound 4
Cu1···Cu1
.10 Å
Cu1···Cu2
.96 Å
Cu1···Cu2
.22 Å
Cu2−O−Cu2
103.6°
Cu1−O−Cu2
97.3°
Cu1−O−Cu2
110.7°
Cu O planarity
H−O···O
44.7°
Cu1−OOH
1.99 Å
Cu1−O
J1
2
2
−
1
1
3
0.0°
Cu1−OOCO
1.95 Å
1.96/1.99 Å
Cu2−OOH
1.96 Å
−295 cm
J2
Cu2−OOCO
1.96 Å
−
2
−18 cm
J3
Cu1−OOH
1.96 Å
Cu2−OOH
1.96 Å
−
1
3
−27 cm
a
b
̂
̂
̂
The spin Hamiltonian is defined by H = −J∑S ·S . Cu O planarity defined as the dihedral angle between the two OCuO planes in the Cu O
central core.
i i+1
2
2
2
2
the ground state (S = 0) is shown in Figure 9. These results are
in good agreement with previous experimental results that show
pounds 3 and 4. For compound 3, the most similar
magnetically characterized compound corresponds to a 1D
polymeric compound, [Cu (μ-adipato) (μ -OH) (μ-H O) ]
3
2
3
2
2
4 n
(
Cu−O−Cu, 98.5°; Cu O planarity, 0.0°; H−O···O, 47.3°;
2
2
−1
70
Cu−O, 1.97 Å) for which a J value of −54.5 cm is reported.
The nature of the magnetic interaction in this example is the
opposite of the ferromagnetic behavior estimated from DFT
calculations, but the energy difference lies inside the accuracy
for this kind of calculation. On the other hand, the
ferromagnetic interaction predicted in compound 3 for the
second superexchange pathway (J ) is explained by means of
2
the orbital counter-complementarity between the μ-carbox-
71
ylato-κO:κO′ bridge and the hydroxide bridge with φ > 98°.
For φ angles below this threshold, the μ-hydroxido induced
SOMO energy difference is so small in comparison to that
generated by the carboxylato ligand that the antiferromagnet-
ism dominates this superexchange pathway as computed for
compound 4.
The Cu O central core with closer structural resemblance to
2
2
that of compound 4 corresponds to a dinuclear compound
II
II
[
LCu (μ-OH) Cu L](BF ) [L = tris(3-isopropyl-4,5-
2 4 2
trimethylenepyrazolyl)methane] (Cu−O−Cu, 103.3 and
Figure 9. Calculated spin-density distribution of the ground state (S =
1
04.1°; Cu O planarity, 2.7°; H−O···O, 56.3 and 53.1°;
2
2
0
) of compound 4 with a threshold level of 0.0015. The blue spheres
Cu···Cu, 3.06 Å; Cu−O, 1.93−1.95 Å) where a J value of −633
correspond to Zn(II) atoms.
−1
72
cm has been found. This last example reinforces the
hypothesis of the presence of very strong antiferromagnetic
interactions between the metal centers bridged by the double
the superexchange coupling constants mediated by hydroxide
−
1
64−66
hydroxido bridge in compound 4. Therefore, the χ T value at
ligands range from −700 to +300 cm .
For di-μ-
m
3
00 K would correspond only to the weakly antiferromagneti-
hydroxido-bridged Cu(II) binuclear complexes, the classical
magneto-structural correlation between the Cu−O−Cu bond
angle (φ) and the experimental superexchange constant (J) (J =
74.538φ + 7270 cm ) indicates that those complexes are
antiferromagnetic for φ > 98° but ferromagnetic for smaller
cally coupled copper(II) metal center, which is not coordinated
to this hydroxido double bridge (just one-third of the total
metal centers).
−1
−
Electrical Conductivity Studies. Two probe direct current
dc) electrical conductivity measurements at 300 K were
6
7
(
angles. However, more recently advanced theoretical
calculations using different density functional methods have
demonstrated that other structural parameters such as small
variances in the Cu−O(bridge) distances, the out of plane
displacement of the hydroxo hydrogen atom, the nonplanarity
of the Cu O core, and the distance from copper atom to the
performed in six single crystals of compound 2, four of
compound 3, and in two pressed pellets of compound 4 (due to
the impossibility to obtain suitable crystals for these measure-
ments). The conductivity values at 300 K were obtained
applying voltages from +10 to −10 V. For complexes 2−4, the
2
2
−9
basal plane can play an important role on the fine-tuning of the
room temperature dc conductivity values are 1.0 × 10 , 6.8 ×
6
8,69
−6
−8
−1
superexchange coupling.
10 , and 7.7 × 10 S·cm , respectively, suggesting a
semiconductor behavior in all cases. For compounds 2 and 3,
we have also performed dc electrical measurements with the
four contacts method in the temperature range 300−400 K in
In fact, a perusal of the CSD database allows us to locate the
compounds with structural parameters around the double
hydroxido magnetic superexchange pathway closer to com-
H
dx.doi.org/10.1021/ic401758w | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
order to confirm the semiconducting behavior and determine
the activation energy. These measurements show activation
energies of ca. 1.3 eV for both complexes (Figures S4 and S5).
As expected, at 400 K the conductivity is higher with values of 1
MAT2008-05690/MAT, CTQ-2011-26507, FIS2009-12721,
and ACI2009-0969), Comunidad de Madrid (project S-0505/
MAT/0303), Gobierno Vasco (IT-280-07), Generalitat Va-
lenciana (Projects Prometeo 2009/95 and ISIC), and EU (FP6-
029192). Work in India is supported by DAE-SRC Outstanding
Investigator Award and DST J. C. Bose National Fellowship to
S.V.
−6
−7
−1
×
10 and 5 × 10 S cm in complexes 2 and 3, respectively.
Note that the crystals used for the conductivity measurements
at high temperatures present some microfractures after the
thermal cycles under a vacuum, indicative of a degradation of
the quality of the single crystals that justify the fact that in
compound 3 the conductivity in the cooling scan is lower than
in the heating scan (Figure S4) and also that the conductivity at
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