Synthesis, Characterization, and studies of coordination-polymeres with isomeric pyridylcyanoximes: Route to metal ribbons with very short Tl???Tl separations

Article abstract of DOI:10.1021/cg3000763

Two isomeric pyridylcyanoximes, H(3PCO) and H(4PCO), containing nitrogen atoms of the heterocycle at 3- and 4-positions, respectively, were synthesized and thoroughly characterized using IR (including ¹⁵N labeling), UV-visible, ¹H,¹³C one- and two-dimensional NMR spectroscopy, and X-ray analysis. The H(3PCO) exists as a mixture of syn- and anti-isomers in solutions, contrary to H(4PCO) which is present only as an anti-isomer. Both compounds have planar geometry in the solid state and form elegant zigzag chains via H-bonding between the oxime group and the N-atom of the heterocycle. The H(3PCO) and H(4PCO) upon addition of a base form colored conjugated and solvatochromic anions in solutions. The reaction of the hot aqueous solution of Tl₂CO₃ with solid isomeric pyridylcyanoximes leads to TlL, which were characterized by elemental analyses, IR spectroscopy, and X-ray analysis. Both complexes represent three-dimensional coordination polymers of different complexities where cyanoxime anions act as bridging ligands. Thallium(I) atoms in Tl(3PCO) and Tl(4PCO) form infinite, planar "metal ribbons" with unusually short intermetallic distances: 3.705 A in Tl(3PCO) and 3.635 A in Tl(4PCO) respectively, which are close to that in metallic thallium (3.465 A). The latter compound has the second shortest Tl???Tl thallophillic interaction reported in the literature thus far. Both yellow Tl(3PCO) and orange Tl(4PCO) are insulators at ambient conditions: R ～ 1 × 10¹¹ Ohm.

Full text of DOI:10.1021/cg3000763

Article
pubs.acs.org/crystal
Synthesis, Characterization, and Studies of Coordination-Polymeres
With Isomeric Pyridylcyanoximes: Route to Metal Ribbons With Very
Short Tl···Tl Separations
Daniela Marcano,† Nikolay Gerasimchuk,*,† Victor Nemykin,‡ and Svitlana Silchenko§
†Department of Chemistry, Temple Hall 431, Missouri State University, Springfield, Missouri 65897, United States
‡Department of Chemistry and Biochemistry, University of Minnesota − Duluth, Duluth, Minnesota 55812-2496, United States
§Absorbtion Systems, Inc., 440 Creamy Way, S. 300, Exton, Pennsylvania 19341, United States
S
* Supporting Information
ABSTRACT: Two isomeric pyridylcyanoximes, H(3PCO) and H(4PCO), containing
nitrogen atoms of the heterocycle at 3- and 4-positions, respectively, were synthesized and
1
thoroughly characterized using IR (including 15N labeling), UV−visible, H,13C one- and
two-dimensional NMR spectroscopy, and X-ray analysis. The H(3PCO) exists as a mixture
of syn- and anti-isomers in solutions, contrary to H(4PCO) which is present only as an anti-
isomer. Both compounds have planar geometry in the solid state and form elegant zigzag
chains via H-bonding between the oxime group and the N-atom of the heterocycle. The
H(3PCO) and H(4PCO) upon addition of a base form colored conjugated and
solvatochromic anions in solutions. The reaction of the hot aqueous solution of Tl2CO3
with solid isomeric pyridylcyanoximes leads to TlL, which were characterized by elemental
analyses, IR spectroscopy, and X-ray analysis. Both complexes represent three-dimensional
coordination polymers of different complexities where cyanoxime anions act as bridging
ligands. Thallium(I) atoms in Tl(3PCO) and Tl(4PCO) form infinite, planar “metal
ribbons” with unusually short intermetallic distances: 3.705 Å in Tl(3PCO) and 3.635 Å in Tl(4PCO) respectively, which are
close to that in metallic thallium (3.465 Å). The latter compound has the second shortest Tl···Tl thallophillic interaction reported
in the literature thus far. Both yellow Tl(3PCO) and orange Tl(4PCO) are insulators at ambient conditions: R ∼ 1 × 1011 Ohm.
mechanical properties and solubility in common organic
solvents, the loss of metallic conductivity at low temperatures
due to the Pierls distortions, and insufficient electronic
tunability were main factors that precluded their further
development as low density, nonmetal, and nonalloy 1D
materials for specific molecular electronics applications.
Among main group metals there are several  Sn, Pb, and Bi
 that also exhibit two stable oxidation states that differ by two
electrons, but only thallium is the one that readily forms
coordination polymers. This metal is especially attractive
because its Tl1+ ↔ Tl3+ transition15 involves the top 6p-6s
shells, with those two electrons thought to be coherent and able
to form a Cooper pair.14 In order to provide conditions for the
electron “hopping” between metal centers, certain conditions
should be met, with one of the most important one being a
close mutual spatial location of metal ions in the coordination
polymer formed. Metallophilic interactions play an important
role in bringing metal centers together.16
INTRODUCTION
■
Coordination polymers with unusual structures and properties
have attracted considerable attention due to their interesting
topologies and fascinating potential applications in (a)
molecular electronics, and (b) as photo- and magnetic
materials.1−4 The main focus in development and studies of
coordination polymers was on transition metals, while less
attention was given to main group metals, despite their well-
established importance in electro- and conventional lumines-
cence, fluorescent sensors, and photovoltaic applications.3,5−7
Thus, in the area of molecular electronics search for one-
dimensional (1D) conductive solids, attention was mainly on
mixed valence transition metals homometallic complexes and
especially platinum.8 Two major types of electric conductors
originated from work with the latter metal: MX,9 KCP, and
partially oxidized cyano-platinates (POCP)10 types of 1D
solids. The latter compounds contained a certain mix of Pt(II)
and Pt(IV) states with direct metal−metal contacts in an
identical coordination environment. Further structural deriva-
tives of POCP include dioximates such as BQD,11 PtDMG,12
and others,13 in which there are close --Pt---Pt-- distances in
regular columns of 1D solids. It is well established that the two
electrons “hopping” between those two oxidation states are
responsible for the metallic type room temperature electrical
conductivity in both MX and KCP/POCP solids. Poor
On the other hand, heterometallic Tl···M (M = Au, Pt, Ag)
coordination polymers with short intermetallic separations and
metallophilic interactions demonstrated remarkable photo-
Received: January 16, 2012
Revised: April 24, 2012
Published: April 26, 2012
© 2012 American Chemical Society
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luminescence.17−19 The formation of polymeric homometallic
and heterometallic complexes that form infinite “metallic chains”
based on metallophilic interactions was recently reviewed.20
Thallium is a soft, oxophyllic Lewis acid21 and in general
possesses pronounced halcophylicity readily forming Tl−E
bonds (E = S, Se, Te).22 Most of studied Tl complexes contain
only oxygen donor atoms around a metal center with only a few
examples of other donor atoms.23 During the last several years,
we discovered an interesting group of the oxime-based
thallium(I) 1D and two-dimensional (2D) coordination
polymers.24 The striking feature of some of those complexes
is a formation of a double-stranded zigzag chain with close
intermetallic separations which can be utilized in the formation
of mixed valence Tl1+/Tl3+ species and possibly facilitate 1D
electrical conductivity in these otherwise dielectric solids. A
successful realization of this project may add thermally stable
conducting coordination polymers to the arsenal of tools for
microelectronics and molecular electronics. An apparent
toxicity of Tl-compounds should not preclude their practicality
since similarly toxic arsenides (GaAs, and related), selenides
(ZnSe, CdSe), and tellurides (PbTe, CdTe) are widely used
today in closed electronic devices with limited access to their
components.
Chemicals, J.T. Baker) were of HPLC grade and used without
additional distillation. Isotopomeric sodium nitrite Na15NO2 (∼95%
enrichment) was purchased from Cambridge Isotope Laboratories.
Melting points or decomposition temperatures for synthesized
protonated cyanoximes and their Tl(I) salts were determined using
the Mel-Temp apparatus (Thomas-Hoover). Thin layer chromatog-
raphy (TLC) for ligands was carried out on silica-coated glass plates
(Merk) with an indicator. An elemental analysis on C, H, N content
was performed at the Atlantic Microlab (Norcross, GA).
Solutions Studies. Electrical conductivity of 1 mM solutions of TlL
in water (L = studied isomeric pyridyl-cyanoxime anions) was
measured at 295 K using the YSI Conductance-Resistance meter
model 34. The 1 mM solutions of tetramethylammonium bromide,
potassium sulfate, and thallium carbonate (as 1:1 and 1:2 electrolytes,
respectively) in DI water were used for the electrode calibration.
Measurements of pKa values for synthesized benz(2-hetroaryl)-
cyanoximes were carried out using a Sirius Analytical Instruments
automated titration station (Sussex, UK) equipped with a temperature-
controlled bath. Since protonated cyanoximes HL are poorly soluble in
water, all measurements were conducted in mixed solvent systems
using DMSO as a solubilizing cosolvent. Atenolol and Lidocaine (from
Aldrich) were used for calibration of the instrument. Measurements
consisted of the three-step multistage titration in water/DMSO
mixtures from 20 wt % to 30 wt % with the ionic strength adjusted to
0.15 with KCl. The values were extrapolated to “zero” DMSO content
to obtain an aqueous pKa value using the Yasuda-Shedlovsky
procedure. The pH in titration experiments ranged from 3 to 11.
Spectroscopy. All synthesized organic ligands were characterized at
296 K using 1H, 13C NMR, COSY, and HMQC spectroscopy
(solutions in DMSO-d6 and acetone-d6; TMS was an internal standard;
Varian INova 400 MHz spectrometer). The UV−visible spectra of
protonated ligands HL and their Na+ salts were recorded at room
temperature (293 K) on an HP 8354 spectrophotometer in the range
200−1100 nm, using 1 mm and 10 mm quartz cuvettes. Infrared
spectra were recorded in the range 500−4000 cm−1 using the FT IR
Nicolet Magna 550 spectrophotometer, using KBr matrix for the
protonated cyanoximes HL. The IR spectra of TlL, however, were
recorded from thick fine pastes in Nujol due to the solid state
exchange reactions between KBr and TlL during the pellet
preparation. Also, we used starting cyanoximes labeled 15N (50%
diluted for better visibility of splitted IR bands) and their respective
Tl(I) complexes for the exact assignment of vibrations with
participation of the >C−N−O fragment.
X-ray Crystallography. Yellow thin needles of Tl(3PCO) and
orange needles of Tl(4PCO) were obtained after a slow cooling of the
filtered reaction mixture during the synthesis (Supporting Information,
S1). Suitable single crystals of all obtained compounds  H(3PCO),
H(4PCO), Tl(3PCO), and Tl(4PCO)  were mounted either on
thin glass fibers, or on plastic loops of the Cryollop/MitiGen devices,
and then placed on the goniometer heads. Compounds were studied
using a Bruker APEX2 diffractometer equipped with a SMART CCD
area detector. The intensity data were collected in ω-scan mode using
the Mo tube (Kα radiation; λ = 0.71073 Å) with a highly oriented
graphite monochromator. Intensities were integrated from four series
of 364 exposures, each covering 0.5° in ω within 60 s of acquisition
time, and the total data set being a sphere.29 The space group
determination was done with the aid of XPREP software.30 Numerical
absorption corrections were applied based on crystal face indexing
obtained using actual images recorded by the video-microscope
camera for H(4PCO) and two Tl-cyanoximates (S2). The following
data processing was performed using the SADABS program that was
included in the Bruker AXS software package.31 The structures were
solved by direct methods and refined by least-squares on weighted F2
values for all reflections using the SHELXTL program.30 All atoms
received assigned anisotropic displacement parameters and were
refined without positional constraints. Hydrogen atoms of C−H origin
in structures of organic molecules H(3PCO) and H(4PCO) were
placed in their idealized geometrical positions and refined isotropically
in a riding scheme, while oxime hydrogens O−H were identified on a
difference map. Crystal data for all four studied compounds are
We continued investigations in this field, changing the
ligand’s design in order to keep the favorable double-stranded
polymeric motif in place and at the same time bring the metal
centers as close to each other as possible. In this work we
present a detailed study of two isomeric pyridylcyanoximes
shown in Scheme 1 and their two monovalent Tl complexes
Scheme 1
with unusually short intermetallic separations. Despite an
earlier report on the preparation of these ligands,25 their
characterization, crystal structures, and properties were not
studied in detail at all. Isomeric H(3PCO) and H(4PCO)
pyridylcyanoximes are not able to form chelate complexes
contrary to their H(2PCO) analogue26,27 but may act as
bridging ligands. Moreover, presented here pyridyl cyanoximes
provide an important advantage in comparison with other
typical building block-ligands for the formation of coordination
polymers: these cyanoximes are ionizable. Therefore, they
alleviate the necessity for the counter-anions in the structures
which usually significantly affect the lattice architecture.
In present work we were focused on a search of oximes-based
coordination polymers with short intermetallic thallophilic
separations. Other types of intermolecular interactions in Tl(I)
complexes, such as C···Tl and H···Tl electrostatic contacts,
were out of the scope of our investigations but recently were
summarized by Morsali.23,28 Both reported here Tl(I)
pyridylcyanoximes complexes here can be used as building
blocks or templates for the synthesis of either homometallic
Tl1+/Tl3+ mixed valence polymers or for the preparation of
luminescent heterometallic coordination polymers.
EXPERIMENTAL SECTION
■
Materials and Methods. Starting thallium(I) carbonate, glacial
acetic acid, sodium nitrite, and substituted acetonitriles R-CH2-CN (R
= 3-pyridyl, 4-pyridyl groups) were obtained from Aldrich and were
used without additional purification. Organic solvents (Spectrum
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Table 1. Crystal Data for Synthesized Isomeric Pyridylcyanoximes and Their Tl(I) Complexes
parameter
H(3PCO)
C7H5N3O
H(4PCO)
C7H5N3O
Tl(3PCO)
C7H4N3OTl
Tl(4PCO)
C7H4N3OTl
empirical formula
formula weight
wavelength, Å
147.14
147.14
350.50
350.50
0.71069
0.71069
0.71073
0.71073
diffractometer
Rigaku AFC-7R
296(2)
Bruker APEX2
120(2)
Bruker APEX2
120(2)
Bruker APEX2
173(2)
temperature, K
crystal system
monoclinic
P2(1)/c, #14
a = 3.7810(7)
b = 17.0260(15)
c = 10.941(4)
α = 90
monoclinic
P2(1)/c, #14
a = 3.728(3)
b = 13.938(9)
c = 13.008(9)
α = 90
triclinic
monoclinic
P2(1)/n, #14
a = 3.8990(4)
b = 12.3800(13)
c = 16.7563(17)
α = 90
space group
P1, #2
̅
unit cell dimensions, Å, °
a = 3.7364(8)
b = 9.365(2)
c = 12.156(3)
α = 68.140(2)
β = 86.965(3)
γ = 79.763(3)
388.43(14)
2
β = 97.510(3)
γ = 90
β = 90.474(9)
γ = 90
β = 93.536(2)
γ = 90
volume, Å3
698.3(3)
675.9(8)
807.28(14)
4
Z
4
4
density (calc), Mg/m3
absorption coeff, mm−1
F(000)
1.400
1.446
2.997
2.884
0.100
0.104
20.737
19.956
304
304
312
624
crystal size, mm
θ range for data, °
index ranges
0.80 × 0.10 × 0.04
3.04−27.50
−4 ≤ h ≤ 4
0 ≤ k ≤ 22
−14 ≤ l ≤ 14
3129
0.4 × 0.21 × 0.12
2.14−29.57
−4 ≤ h ≤ 4
−18 ≤ k ≤ 0
0 ≤ l ≤ 16
13268
0.16 × 0.10 × 0.05
1.81−26.73
−4 ≤ h ≤ 4
−11 ≤ k ≤ 11
−15 ≤ l ≤ 15
4453
0.50 × 0.20 × 0.15
2.05−27.10
−4 ≤ h ≤ 4
−15 ≤ k ≤ 14
−21 ≤ l ≤ 21
5537
reflections collected
independent reflections
completeness to θ,%
absorption correction
1598 [R(int) = 0.2682]
99.8
1880 [R(int) = 0.1292]
99.5
1650 [R(int) = 0.0242]
99.6
1755 [R(int) = 0.0597]
99.7
empirical
semiempirical
semiempirical
semiempirical
0.150 and 0.040
Tmax. and Tmin
.
0.996 and 0.9241
0.7461 and 0.5084
0.4488 and 0.1382
refinement method
----------------full-matrix LS on F2-------------------
data/restraints/parameters
goodness-of-fit on F2
1598/0/120
0.888
1880/0/101
1650/0/125
1.092
1755/0/109
1.059
1.038
final R indices [I > 2σ(I)]
R1 = 0.0809
wR2 = 0.1359
R1 = 0.2902
wR2 = 0.2045
0.234 and −0.221
R1 = 0.0792
wR2 = 0.1757
R1 = 0.1373
wR2 = 0.2067
0.243 and −0.340
R1 = 0.0262
wR2 = 0.0600
R1 = 0.0293
wR2 = 0.0614
2.657 /−1.425
R1 = 0.0596
wR2 = 0.1427
R1 = 0.0828
wR2 = 0.1555
3.493/−3.648
R indices (all data)
larg diff. peak/hole, e·Å−3
presented in Table 1, while bond lengths and valence angles are
summarized in Table 2. Figures for the crystal structures of these
complexes were drawn using Mercury 4.1.2 and ORTEP 32 software32
at a 50% thermal ellipsoids probability level. The PLATON checks of
crystallographic data and actual CIF files for reported structures can be
found in the Supporting Information.
Synthesis of Ligands and Tl(I) Complexes and Crystal
Growth. The 4-pyridylcyanoxime, H(4PCO), was obtained according
to the published procedure from commercially available from Aldrich
4-pyridylacetonitrile-hydrochloride, NC-CH2-C5H4N·HCl, and HNO2
(Supporting Information, S6). However, its isomeric 3-pyridylcyanox-
ime, H(3PCO), was obtained at basic conditions using gaseous
methylnitrite according to our recently patented33 method (Support-
ing Information, S7). Thus, a sodium isopropoxide solution was
prepared using 0.20 g of thinly sliced pieces of metallic sodium and 70
mL of 2-propanol under flow of nitrogen. A solution of 0.5 mL (5
mmol) of 3-pyridyl acetonitrile (NC-CH2-C5H4N) in 5 mL of
isopropanol was added to sodium propoxide solution above was stirred
under nitrogen for 20 min. A flow of gaseous methyl nitrite,
CH3ONO, was from a cold (∼0 °C) mixture of 30% H2SO4 and a
solution of NaNO2 (20 g) dissolved in 70 mL of a 1:1 mixture of
CH3OH/H2O. The color of the reaction mixture immediately changed
to yellow, and after 5 min of methylnitrite bubbling through the
reaction mixture it was kept at +4 °C overnight with the following
solvent removal using the rotary evaporator. A thick bright-yellow
yellow solid residue of Na(3PCO) was dissolved in 30 mL of DI water
and acidified with 1 M HCl to pH = 5 and then saturated with solid
NaCl. An off-white flaky solid H(3PCO) was filtered off, washed with
water, and dried in vacuum desiccator over H2SO4 (conc). Yield 80%;
compound decomposes at 235−237 °C. The H(3PCO) is soluble in
propanol, DMSO, and acetone, slightly soluble in toluene, THF,
acetonitrile, and chloroform, and practically insoluble in water,
Solid State Electrical Conductivity Measurements. We attempted
to measure conductivity of single crystals of Tl(3PCO) and Tl(4PCO)
(Supporting Information, S1) at room temperature using the
conventional 2-points method. Thus, thin copper wires were attached
to a selected single crystalline specimen positioned on glass slides of
25 × 25 mm dimensions using Ag-based glue and then dried for 24 h.
Two samples of different size crystals of Tl(3PCO) and Tl(4PCO)
were used for experiments (Supporting Information, S3). All
operations were carried out under the microscope (Meiji) and using
a set of microtools (Hampton Research). After the glue was dried
during 24 h, the copper wires and contact spots, but not the crystals,
were carefully covered with polyurethane to prevent their degradation
upon exposure to gaseous Cl2, or Br2, I2 vapors. Samples were dried
again for 24 h, and then exposed to halogens in a specially designed
glass chamber (Supporting Information, S5). After subjection to
halogen during variable times, samples’ electrical conductivity was
measured at 293 K again, using the Keithley 220 Programmed Current
Source and the Keithley 182 Sensitive Digital voltmeter (Supporting
Information, S4 and S5).
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benzene, carbon tetrachloride, and toluene. Data of the IR spectra
(KBr) for isomeric pyridylcyanoximes are summarized in Table 4 and
Table 2. Selected Geometrical Data for Studied Ligands and
Their Tl(I) Compounds
1
Supporting Information, S8 and S9. The H and 13C NMR spectra of
bond length, Å
valence angle, deg
the H(3PCO) indicate a mixture of syn (32%) and anti (68%)
geometrical isomers in acetone-d6 solution appearing as a double set of
signals.
Synthesis of Thallium(I) Isomeric Pyridylcyanoximates. A large
Dewar flask was filled with 4.5 L of hot water (95°) and kept at this
temperature using an immersed resistance heater connected to a
thermoregulator. A long, side arm Pyrex filter tube, equipped with a
stopper and medium porosity glass filter, was held inside the Dewar
flask with hot water. The 0.100 g (0.68 mmol) of solid H(3PCO) or
H(4PCO) were added at once into a boiling solution of 0.16 g (0.34
H(3PCO)
C(1)−N(1) = 1.302(6)
C(1)−C(2) = 1.433(8)
C(1)−C(3) = 1.457(7)
C(2)−N(2) = 1.152(6)
C(3)−C(4) = 1.362(7)
C(4)−N(3) = 1.359(6)
C(5)−N(3) = 1.333(7)
N(1)−O(1) = 1.382(5)
O(1)-H(1O) = 1.17(7)
N(1)−C(1)−C(2) = 121.0(5)
N(1)−C(1)−C(3) = 119.5(5)
C(2)−C(1)−C(3) = 119.5(5)
N(2)−C(2)−C(1) = 176.6(6)
N(3)−C(4)−C(3) = 123.9(6)
N(3)−C(5)−C(6) = 122.7(6)
C(1)−N(1)−O(1) = 113.5(4)
N(1)−O(1)−H(1O) = 99(3)
H(4PCO)
C(1)−N(1) = 1.287(4)
C(1)−C(2) = 1.430(5)
C(1)−C(3) = 1.455(5)
C(2)−N(2) = 1.133(4)
C(3)−C(4) = 1.385(5)
C(5)−N(3) = 1.335(4)
C(6)−N(3) = 1.324(5)
N(1)−O(1) = 1.362(3)
O(1)−H(1O) = 1.00(4)
N(1)−C(1)−C(2) = 121.0(3)
N(1)−C(1)−C(3) = 118.6(3)
C(2)−C(1)−C(3) = 120.3(3)
N(2)−C(2)−C(1) = 178.6(5)
N(3)−C(5)−C(4) = 122.8(4)
N(3)−C(6)−C(7) = 123.8(4)
C(1)−N(1)−O(1) = 112.0(3)
N(1)−O(1)−H(1O) = 100(2)
mmol) of Tl2CO3 (eq 1). The color of the reaction mixture changed
immediately from colorless to bright yellow and immediately after
carbon dioxide ceased this slightly turbid solution was hot filtered into
the long side arm tube submerged in the Dewar containing 4.5 L of
boiling water. The tube was closed with a rubber septum and left
inside the Dewar for slow crystallization for several days.34 Fine
needles of Tl(3PCO) and Tl(4PCO) suitable for X-ray analysis
appeared in the tube (Supporting Information, S1). The Tl(3PCO), as
yellow crystalline material, was obtained in combined 95% yield.
Compound decomposes above 228 °C. For Tl(3PCO), C7H4N3OTl,
calculated (found): C − 23.99 (23.89), H − 1.15 (1.06), N − 11.99
(11.90). The Tl(4PCO), as large orange crystals, isolated at ∼90%
combined yield. Complex decomposes at 258 °C. For Tl(4PCO),
C7H4N3OTl, calculated (found): C − 23.99 (23.94), H − 1.15 (1.04),
N − 11.99 (11.94). The IR spectra of both TlL are summarized in
Table 3.
Both TlL are sparingly soluble in water, insoluble in alcohols,
acetone, but very soluble in DMF, DMSO, and pyridine with apparent
loss of solid state structures.
Safety note: Although we have not encountered any problems during
many years of laboratory work and handling, special care should be taken
during work with thallium compounds because of their high toxicity.35−37
Both Tl(I) carbonate and cyanoximates are water-soluble compounds and
that emphasizes the absolute necessity for wearing protective gloves at all
times when working with these compounds. However, no respiratory tract
covers are required since Tl(I) compounds are ionic and not volatile.
a
Tl(3PCO)
Anion:
Anion:
C(1)−N(1) = 1.320(9)
C(1)−C(2) = 1.440(10)
C(1)−C(3) = 1.468(10)
C(2)−N(2) = 1.143(10)
C(3)−C(4) = 1.380(10)
C(6)−N(3) = 1.332(10)
C(7)−N(3) = 1.324(10)
N(1)−O(1) = 1.316(8)
Metal center:
N(1)−C(1)−C(2) = 121.0(6)
N(1)−C(1)−C(3) = 120.1(6)
C(2)−C(1)−C(3) = 118.9(6)
N(2)−C(2)−C(1) = 179.8(9)
N(3)−C(6)−C(5) = 124.2(7)
N(3)−C(7)−C(3) = 123.2(7)
O(1)−N(1)−C(1) = 114.6(7)
C(7)−N(3)−C(6) = 117.6(6)
Metal center:
O(1)−Tl(1)#1 = 2.580(6)
O(1)−Tl(1) = 2.742(5)
Tl(1)−O(1)#1 = 2.580(6)
Tl(1)−Tl(1)#2 = 3.7050(8)
Tl(1)−Tl(1)#3 = 3.7364(8)
Tl(1)−Tl(1)#4 = 3.7364(8)
N(1)−O(1)−Tl(1)#1 = 118.5(4)
N(1)−O(1)−Tl(1) = 122.6(4)
Tl(1)#1−O(1)−Tl(1) = 118.4(2)
O(1)#1−Tl(1)−O(1) = 61.6(2)
Tl(1)#2−Tl(1)−Tl(1)#3 = 75.801(9)
Tl(1)#2−Tl(1)−Tl(1)#4 = 104.199(9)
Tl(1)#3−Tl(1)−Tl(1)#4 = 180.00(3)
b
Tl(4PCO)
RESULTS AND DISCUSSION
■
Anion:
Anion:
Organic Ligands. NMR Spectra. According to the 13C{1H}
spectrum, H(3PCO) exists in acetone-d6 as a mixture of two
isomers (syn- and anti-) in solution, since two sets of seven
C(1)−N(1) = 1.291(18)
C(1)−C(2) = 1.46(2)
C(1)−C(3) = 1.45(2)
C(2)−N(2) = 1.12(2)
C(3)−C(4) = 1.400(19)
C(5)−N(3) = 1.37(2)
C(6)−N(3) = 1.43(2)
N(1)−O(1) = 1.322(17)
Metal center:
N(1)−C(1)−C(2) = 118.3(13)
N(1)−C(1)−C(3) = 123.2(14)
C(2)−C(1)−C(3) = 118.5(12)
N(2)−C(2)−C(1) = 178(2)
N(3)−C(6)−C(7) = 124.0(14)
N(3)−C(5)−C(4) = 121.4(14)
O(1)−N(1)−C(1) = 118.6(14)
C(5)−N(3)−C(6) = 117.6(13)
Metal center:
1
signals were observed (Supporting Information, S10). The H
NMR spectrum also revealed two sets of four aromatic proton
signals, but only one signal that corresponds to the proton of
the oxime group, which is highly deshielded and located at
∼13.4 ppm (Supporting Information, S10). COSY and HMQC
(Supporting Information, S11) experiments were recorded in
order to provide unambiguous assignments of signals of
isomers. The ratio between the two geometrical isomers
according to 1H NMR integration was 32:68 = syn/anti. On the
basis of published data for other cyanoximes,38 it was assumed
that the most abundant isomer is anti both in solid state and
solutions.
O(1)−Tl(1)#1 = 2.552(12)
Tl(1)−O(1)#2 = 2.552(13)
Tl(1)−Tl(1)#3 = 3.6346(11)
N(1)−O(1)−Tl(1)#1 = 113.7(9)
O(1)#2−Tl(1)−N(3) = 83.4(4)
O(1)#2−Tl(1)−Tl(1)#3 = 77.6(3)
N(3)−Tl(1)−Tl(1)#3 = 161.0(3)
a
Symmetry operations to generate equivalent atoms: #1 −x + 1, −y, −
b
z + 1; #2 −x, −y, −z + 1. #3 x −1, y, z; #4 x + 1, y, z. Symmetry
transformations used to generate equivalent atoms: #1 x − 1/2, −y +
1/2, z + 1/2; #2 −x, −y, −z + 2; #3 x + 1/2, −y + 1/2, −z − 1/2.
According to the 13C{1H} NMR spectra, the H(4PCO)
compound in acetone-d6 has only one set of five signals
indicating that the only one geometrical isomer is present in
solution. Since the crystal structure of this compound shows its
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Table 3. Tl(I) Coordination Polymers with Closest Intermetallic Separations
for H(3PCO) are 3.25
0.01 (amine) and 6.72
0.01
Table 4. Data of the IR Spectroscopic Studies and X-ray
Analysis for Isomeric Pyridylcyanoximes and Their Tl(I)
Complexes
(oxime), while for H(4PCO) these are 4.00 0.03 and 6.31
0.02, respectively (Supporting Information, S13). The presence
of the CN group in H(3PCO) and H(4PCO) causes a rather
dramatic increase from 4 to 6 orders of magnitude in acidity of
studied ligands, as opposed to traditional monoximes and
dioximes. Moreover, an electron-withdrawing effect of the
cyanoxime fragment in general significantly lowers the basicity
of the pyridine group and brings it almost to the level of chloro-
pyridines (Supporting Information, S13), which are known to
be poor ligands. Nevertheless, H(3PCO) and H(4PCO) are
much better ligands than cyano-pyridines and are able to form a
variety of transition metal complexes.39
UV−Visible Spectra of Anionic Cyanoximes. Pure
organic isomeric pyridylcyanoximes are colorless substances,
as the vast majority of other cyanoximes are, including
substituted arylcyanoximes. However, the deprotonation of
H(3PCO) and H(4PCO) with a base leads to the formation of
colored anions (Figure 1, and Supporting Information, S14).
The π → π* transition in the anion underwent a bathochromic
shift and a significant increase in intensity due to the charge
delocalization compared to the protonated oxime (Figure 1A;
Supporting Information, S14, S16), while the n → π* transition
in the anion exhibited a pronounced solvatochromic effect
(Figure 1B). The wavelength difference between visible spectra
of yellow anions in alcohols and pink-red anions in aprotic
DMF and DMSO is ∼70−80 nm, which corresponds to ∼9 kJ/
mol energy associated with the formation of H-bonds of
medium strength in H-bonded solvates in EtOH or PrOH
(Supporting Information, S15). Spectra of 3PCO− and 4PCO−
anions in aprotic solvents consist of two lines, as opposed to a
vibrational and geometrical parameters of the C−N−O group
bands with participation of the
C−N−O fragment (15N label),
cm−1
C−N−
O
ν(N−
δ(C−
angle
compound ν (CN)
O)
N−O) bond length (Å)
(°)
H(3PCO) 1512
1081
(1071)
764
(755)
C−N = 1.302
113.5
114.6
112.0
(1505)
N−O = 1.382
C−N = 1.320
Tl(3PCO) 1376
1291
(1285)
850
(844)
(1365)
N−O = 1.316
C−N = 1.287
H(4PCO) 1503
1086
(1080)
769
(758)
(1497)
1025
N−O = 1.362
C−N = 1.291
N−O = 1.322
(1017)
Tl(4PCO) 1405
1301
(1296)
832
(828)
118.6
(1399)
anti configuration, the same was assumed for its structure in
1
solution. The H NMR spectra also show one set of three
multiplets; two of those corresponding to two aromatic proton
peaks and the third one corresponding to the proton of an even
more deshielded oxime group at 14.4 ppm (Supporting
Information, S12).
Acidity Studies. Molecules of isomeric pyridyl-cyanoximes
possess both basic and acidic sites (Scheme 1). Values of pKa
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determined despite these difficulties and high R(int) values in
diffraction experiments. Crystal and refinement data for both
compounds are presented in Table 1, while selected bond
distances and angles for both structures are shown in Table 2.
3-Pyridylcyanoxime, H(3PCO). This compound in the solid
state adopts a practically planar trans-anti configuration (Figure
2A). We were unable to find crystals of respective syn-isomer
among specimen selected for crystallographic studies. Perhaps
due to its different solubility than the trans-anti isomes it was
not present in the mixture at the time of crystals selection.
Bond lengths and angles in the heterocyclic 3-pyridyl ring are
normal. The term trans- indicates that the oxime group is on
the opposite side of the N3 atom via the C3−C1 bond. The
cyano group is linear ∠N2−C2−C1 = 176.67°, but is slightly
off the plane of the molecule with a torsion angle of ∠N1−
O1−C1−C3 = 3.99°. Bond lengths in the cyano group is N2−
C2 = 1.153 Å, and in the oxime group are N1−O1 = 1.382 Å
and C1−N1 = 1.302 Å (Table 2). The packing diagram
(Supporting Information, S19A) shows the formation of the
planar chains formed due to the intermolecular hydrogen bonds
between the H1O and N3 atom of the aromatic pyridine ring,
where N3 is the donor atom, with angles of ∠N3−H1O−O1 =
169.99° and distances of N3−H1O = 1.497 Å and H1O−O1 =
1.166 Å.
4-Pyridylcyanoxime, H(4PCO). The cyanoxime also adopts
planar anti configuration (Figure 2B) evidenced by torsion
angles in the molecule less than 1°. Bond lengths and angles in
the heterocyclic 4-pyridyl ring are normal. Bond distances for
the cyano group and the oxime group are N2−C2 = 1.133 Å,
O1−N1 = 1.362 Å, C1−N1 = 1.287 Å, respectively. Values of
the angles O1−N1−C1 and N2−C2−C1 are 111.95° and
178.60° (Table 2), being normal for cyanoximes.42 The packing
diagram for H(4PCO) indicates the formation of a layered
structure with slipped π−π interactions with a separation of 4-
pyridyl π systems distances close to 3.4 Å (Supporting
Information, S19B). Also, there are hydrogen bonds between
the oxime group and the nitrogen atoms of the heterocyclic
ring which connect individual molecules into chains running
parallel to the “c” direction. H-bond parameters: ∠N3−H1O−
O1 = 173.99° and distances of N3(donor)-H1O = 1.600 Å and
H1O−O1 = 1.004 Å.
Figure 1. Electronic spectra of protonated 3-pyridylcyanoxime
H(3PCO) and its anion as Na-salt in n-propanol at 10−5
M
concentrations in 1 mm cell (A), and solvatochromism of 3PCO−
anion (as NBu4+ salt) in several solvents at 5 mM concentrations in 1
cm cuvette (B).
single line in H-bonded solvents (Supporting Information, S17
and S18), which is similar to a negative solvatochromism of
NC-C(NO)-C(O)NH2 anion40 and substituted heteroaryl-
−
and aryl-cyanoximes.41
Crystal Structures of Isomeric Pyridylcyanoxime Ligands.
Crystals of both compounds turned out to be weak diffractors
and exhibited a tremendous propensity for forming a
multidomain specimen upon even a very slow crystallization.
Eventually, after many trials suitable, thin single crystals were
found and structures of both compounds were reliably
Metal Complexes. Properties in Solution. Solutions
conductivity of synthesized isomeric Tl-cyanoximates was
Figure 2. Molecular structures and numbering schemes for H(3PCO) (A) and H(4PCO) (B) showing top and side views. An ORTEP drawing at
50% thermal ellipsoids probability level.
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measured against some common soluble K+ salts due to
similarities of some of chemical properties of monovalent
thallium and potassium. Values of solutions’ conductivity
indicate the formation of 1:1 electrolytes upon compounds’
dissolution. The UV−visible spectra of Tl(3PCO) and
Tl(4PCO) in solutions are identical to those of alkali metal
salts and, therefore, indicate the loss of solid state polymeric
structures.
Crystal Structures of Isomeric Tl(I)-pyridylcyanoximates.
Prior to description of crystal structures of the obtained TlL, it
is important to mention some reference distances43 between
the metal ion and N, O donor atoms of the ligands: the sum of
ionic radii for Tl−O = 2.85 Å and Tl−N = 2.96 Å. Also, Tl(I)
has 6s2 inert electron pair that despite its supposedly spherical
nature shows pronounced spatial directionality.44 Thus, it can
be often visualized in crystal structures in an open cleft nearby
Tl(I) centers.21 Therefore, in numerous monovalent thallium
complexes 6s2 electron pair acts as a lone pair and as such it is
frequently called a stereochemically active pair.17a,22 An addi-
tional explanation for that phenomenon was recently also
summarized by Morsali45 as well.
In our previous work with Tl(I) coordination polymers we
found the formation of a double-stranded, “ladder-type”
coordination-polymers as a dominant motif in crystal structures
with cyanoximes. It was first observed in 1993,24c reported at
several occasions,46 and then recently summarized.21 Also, a
similar polymeric staircase structure consisting of Tl2O2 skewed
rhombs in thallium(I) orthonitrophenolate was later reported
by Harrowfield.47 Other main group heavy metal cations, such
as Ge(II), Pb(II), and Bi(III), also were found to form M2O2
rhombs in a variety of complexes48 extending this peculiar
structural motif well beyond Tl(I) compounds. In all cases the
metal ion has a stereoactive lone pair.
Figure 3. Numbering scheme for 3PCO− anion and its closest cationic
surrounding (A), and one Tl(I) cation and its closest cyanoximes
environment with respected distances (B) in the crystal structure of
Tl(3PCO). The shortest intermetallic separation here is 3.705 Å.
Symmetry operations for #1: x, y, z; for #2: −x, −y, −z. H-atoms in B
are omitted for clarity.
Tl(3PCO). The molecular structure and numbering scheme
for Tl(3PCO) is shown in Figure 3. Selected bond lengths and
valence angles are presented in Table 2. The 3PCO− anion in
this complex acts as a complex unsymmetric tetradentate bridge
between four thallium atoms: three are connected to one
oxygen atom of the CNO group, and one Tl is bound to the N3
atom of the heterocycle (Figure 3A). The unit cell content in
the structure of Tl(3PCO) is shown in Supporting Information,
S20. Organization of Tl(3PCO) crystal structure can be best
described as 1D columns connected with each other by means
of weak extra coordination to the Tl centers N-atoms of the
pyridine group and van der Waals forces as well (Figure 4). In
turn, these columns are formed by centrosymmetric [Tl-
(3PCO)]2 dimers stacked into double-stranded, ladder-type
Tl2O2 sheets (Figure 5A). Two adjacent and slightly different
[Tl2O2] rhombs (labeled as 1 and 2) are bent with an 87.0°
angle between them and represent a building block of such an
interesting ladder-type 1D polymeric motif, and rhombs
geometry is shown in Figure 5B. There are three intermetallic
distances in that building block: Tl1···Tl1_2 (in 2) = 4.571 Å,
Tl1···Tl1 = 3.736, Tl1···Tl1_2 (in 1) = 3.705 Å with the latter
two being very short. Thus, a planar “metal ribbon” is formed as
seen in the space filling representation showing partial overlap
between Tl atoms (Supporting Information, S21A). There are
also three short covalent bonds (less than the sum of van der
Waals and even ionic radii) between the oxygen atom of the
ligand Tl1−O1_2= 2.580 Å, and the oxygen atom of the oxime
group of the neighboring pyridylcyanoxime group Tl1−O1 =
2.742 Å, and finally another, longer contact Tl1_2−O1 = 2.839
Å. (Figures 3 and 5). The nitrogen atom of the neighboring
Figure 4. Organization of crystal structure of Tl(3PCO): perspective
view. Shown formation of columns connected with each other by
means of van der Waals forces. H-atoms are not shown for clarity.
molecule at a distance Tl1−N3 = 2.916 Å provides an
additional, weaker contact that stabilizes the lattice and is
responsible for joining individual 1D columns (Figure 4) into a
3D polymer. Thus, the coordination number of Tl(I) in this
complex is four and can be best described as distorted trigonal
pyramid (Figure 6). The lone 6s2 pair of Tl(I) is stereochemi-
cally active and occupies the space in an open cleft between N3
and O1_2 atoms (Figure 6). The shortest Tl···H distance is
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Figure 5. Three orthogonal projections of the double-stranded
(ladder) polymeric motif in 1D columns on the structure of
Tl(3PCO) (A), and its “building block” − two adjacent different
Tl2O2 rhombs (B).
Figure 7. Numbering scheme for 4PCO− anion and its closest cationic
surrounding (A), and one Tl(I) cation and its closest cyanoximes
environment (B) in the crystal structure of Tl(4PCO). The shortest
intermetallic separation is 3.635 Å. Symmetry operations #1: x, y, z;
#2: −x + 1/2, y + 1/2, −z + 1/2; #4: x + 1/2, −y + 1/2, z + 1/2. H-
atoms in B are omitted for clarity.
from the shorter C1−N1 distance than the N1−O1 bond
(Table 2). Similar to the previous structure, there are three
short covalent (less than the sum of ionic radii) distances
between the oxygen atom of the parent anion Tl1_4−O1 =
2.552 Å, the nitrogen atom of the neighboring pyridyl group
Tl1_1−N3 = 2.813 Å, and the oxygen atom of the oxime group
of neighboring molecule Tl1_4−O1 = 2.839 Å (Figure 7). Also,
there are two more and slightly longer contacts between the
N−O group and Tl center, positioned in a “side-on” fashion
with a formation of a three-membered chelate ring: Tl1_2−N1
and Tl_2−O1 distances are 2.899 Å and 2.898 Å, respectively,
and <N1−Tl1_2−O1 = 26.4° (Figures 7 and 10). Thus, the
coordination number of Tl(I) is five, and the coordination
polyhedron represents a distorted trigonal pyramid capped with
a “side-on” coordinated oxime group from a neighboring
molecule. A sterochemically active lone pair occupies space in
an open cleft opposite O1_2−N1_2−O1_4−N3 plane (Figure
10). The shortest Tl···H distance similar to Tl(3PCO) complex
was found to be also 3.493 Å and is between the 3H-atom of
the ligand and the neighboring polymeric column.
Figure 6. Coordination polyhedron of metal ion in the structure of
Tl(3PCO): two orthogonal views. The 6s2 lone pair of Tl(I) occupies
an open cleft behind the O1−O1_2−N3 plane seen on the right
picture.
3.493 Å between the metal and 4H-hydrogen atom of the
neighboring column of the polymeric complex.
Tl(4PCO). Molecular structure and the numbering scheme
for Tl(4PCO) are shown in Figure 7, while the unit cell content
is presented in Supporting Information, S22. The 4PCO− anion
in the complex also acts as an unsymmetric tetradentate bridge
between four thallium atoms: two metal ions bound only to the
oxygen atom of the CNO-fragment, one metal ion has “side-
on” coordination to the N−O group in the CNO-fragment, and
one Tl ion is bound to the N3 atom of the heterocycle (Figure
7A). The organization of the crystal structure is shown in
Figure 8. The structure of Tl(4PCO) represents a 3D
coordination polymer that is best described as pleated sheets
which are formed by 1D columns connected into the 3D
network by the nitrogen atom of the 4-pyridyl group (Figures 8
and 9). The heterocyclic groups stabilize the lattice due to
slipped π−π stacking, with the shortest distance being ca. 3.61
Å. The cyanoxime ligand is clearly in the oxime form as judged
It should be noted especially that there are two very short
Tl···Tl distances in the structure (Supporting Information,
S21B). The closest Tl···Tl interlayer contact inside the unit’s
1D column is 3.635 Å, while the intralayer separation is 3.899 Å
(Figure 7A). Indeed, they are very close to that in the metallic
thallium, hexagonal α-phase35 with Tl−Tl = 3.465 Å. In fact,
this is the second shortest distance reported in the literature for
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Figure 8. Organization of crystal structure of Tl(4PCO): perspective view of the unit cell content along the a-axis.
intermetallic separations: the first one is 3.594 Å in the case of
heterometallic Tl···Au pentafluorophenole based complex.49 In
that the polymeric compound thallium(I) also forms a
nonplanar symmetric Tl2O2 rhomb as a building block. Results
of the CCDC database search for several complexes with the
shortest intermetallic distances in Tl(I) polymers are
summarized in Table 3. Observed structural arrangement of
Tl-atoms in both structures represent a planar “metal ribbon”
displayed in S21B. These ribbons provide a good incentive for
the future attempt of preparation of homometallic mixed valence
Tl(I)/Tl(III) complexes for studies of their electrical
conductivity, or for the preparation of heterometallic Tl···M
(M = Au, Pt, Ag) compounds that may exhibit photo-
luminescence or vaporochromism. Found short intermetallic
separations in Tl(I)-cyanoximates are due to the “metallophilic”
interactions that lately were increasingly often found in coinage
metal and platinum complexes.17 The latter are known for their
1D electrical conductivity.8−13,55
IR Spectra of 15N Labeled Ligands and Their Tl(I)
Complexes. Since crystal structures of thallium complexes
were determined, it was interesting to find how binding modes
of cyanoximes in these coordination polymeric complexes are
reflected in their IR spectra. For that purpose we synthesized
both ligands and their Tl(I) cyanoximates labeled with ∼50%
15N on the oxime group. This degree of enrichment allows
observation of the split bands with participation of the C−N−
O fragment in the IR spectra of complexes as opposed to the
Figure 9. Two orthogonal views of a double-stranded motif in the
column of Tl(4PCO) with more distant 4-pyridine groups (2.813 Å)
shown bonded to the metal center as dashed lines (A), and the
“building block” − two adjacent Tl2O2 rhombs (B).
Figure 10. Coordination polyhedron of metal ion in the structure of Tl(4PCO): two orthogonal views. The stereochemically active 6s2 lone pair of
Tl(I) occupies an open cleft behind the N3−O1_2−N1_2 plane seen on the left picture.
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Scheme 2
shift only that is often difficult to measure reliably. This
approach proved to be fruitful in our earlier studies of Ag(I)50
and other metallocyanoximates.41 Vibrational bands associated
with the C−N−O fragment, together with its geometrical
features, are summarized in Table 4, while actual spectra are
shown in Supporting Information, S8 and S9. Uncomplexed
starting cyanoximes H(3PCO) and H(4PCO) demonstrate a
low intensity ν(CN) band at ∼1500 cm−1 and an intense
band of ν(N−O) at ∼1090 cm−1, sometimes split into two and
complexes after Tl(I) centers could be partially oxidized to
Tl(III). We attempted a heterogeneous oxidation of our
samples with gaseous of halogens. These are the most common
oxidizers used for such reactions, for example, in well studied
partial oxidation of kappa-phases of tetrathio- and tetraseleno-
fulvalenes. Thus, we investigated the effect of Cl2, and I2 and
Br2 vapors on electrical conductivity of monocrystalline samples
of isomeric Tl(3PCO) and Tl(4PCO) complexes (Supporting
Information, S3−S5). After treatment with Br2 and I2 vapors at
saturating conditions at room temperature within many hours a
small decrease in Tl(4PCO) samples’ resistivity to 1.6 0.2 ×
51
s
assigned as asν(N−O) and ν(N−O). The high intensity of
the latter is due to the considerable dipole moment change dμ/
dx during vibrations in this polar group. The deprotonation of
cyanoximes leads to the significant redistribution of the electron
density in the C−N−O fragment, leading to its “leveling”
(Scheme 2). This is also reflected in changes in bond lengths in
the group from HL to L− where the C−N bond lengthened
with a concomitant decrease of the N−O bond.
1010 and 2.2
0.3 × 1010 Ohm, respectively, was detected.
Also, these samples clearly demonstrated an ohmic behavior as
evidenced from expected voltage changes upon variations in
×10 or ×100 times in the current load. The above values
appeared to be low, but it should be kept in mind that the
sample sizes of single crystals of the studied complexes were
very small: only ∼0.2−0.4 mm across. A small increase in the
conductivity of Tl(4PCO) we attribute to possible reaction
between bromine or iodine and the cyanoxime anions. This
may lead to the formation of either halogenated compounds, or
intercalated with halogens systems with perhaps a narrower
band gap, where some charge motion across π−π stacked
halogenated pyridyl-groups could be probable. It is known that
bromine and iodine atoms can form very close interhalogen
contacts in some organic crystals,52 and even in Tl-complex-
es24a,53 or form polyatomic anions which might be the cause of
their better electrical conductivity. Studied samples did become
slightly darker after prolonged exposure to Cl2 (S5), but not
sufficient for consideration of the formation of mixed valence
Tl1+/3+ complexes that should exhibit low energy CT-band and
gain an intense color.54 Thus, it appears that the effect of dry
halogens on the conductivity of Tl-cyanoximates seems to be
unrelated to metal centers. Since the size of crystalline samples
used for electrical conductivity studies was very small (in the
range 2−4 mg; Supporting Information, S3 and S4) it proved
to be rather difficult to detach them after exposure to halogens
from the glass slide and electrodes. Therefore, no attempts to
investigate possible halogenation of the cyanoxime anion in
Tl(3PCO) and Tl(4PCO) were made at this point.
Rather reasonable correlations between geometrical features
of HL, TlL, and IR spectra were established (Supporting
Information, S23). It is notable that coordination of both
anions to Tl(I) centers significantly increased the rigidity of the
C−N−O fragment that is reflected in the high energy shift of
its δ(C−N−O) bending vibration (Table 4). Thus, the angle at
the nitrogen atom in TlL is approaching 120° suggested for its
sp2 hybridization state in anionic cyanoximes and direct
involvement in the aforementioned redistribution of electron
density in conjugated planar anions (Table 4; Supporting
Information, S23). It is important to note that observed
frequencies in the IR spectra of TlL are different from those in
spectra of silver(I) cyanoximates,50,51 which is in a good
agreement with (a) different binding modes of the ligands in
both coordination-polymeric complexes, (b) different elec-
tronic structure of anions in TlL (in the oxime form) compared
to AgL (in the nitroso form).
Preliminary Data for Solid State Single Crystal
Electrical Conductivity Studies. Solid crystalline samples
of yellow Tl(3PCO) and orange Tl(4PCO) complexes are
insulators at room temperature, and their resistivity is in the
range of (1−2) × 1011 Ohm. We were interested to know
whether very close intermetallic separations in “metallic
ribbons” in structures of Tl(3PCO) and Tl(4PCO) could be
an important factor in the electrical conductivity of these
Our first experiments with dry halogens showed their
inability to oxidize Tl(I) at the studied ambient experimental
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conditions perhaps due to (a) kinetic reasons such as a limited
access of oxidizer to Tl(I) centers because of the tight binding
of protective organic ligands in formed coordination polymers
(Supporting Information, S24, S25); (b) absence of traces of
moisture that frequently provides conditions for transport of
electrons. Although heterogeneous reaction of solid samples of
our Tl-cyanoximates with dry halogens were unsuccessful, we
plan different routes to originate mixed valence species: (1) use
the electrochemical oxidation of thallium(I) in solutions, and
(2) prepare these complexes from the mixture of respective
monovalent TlL and trivalent TlL3 compounds.
pyridyl-cyanoxime anions (S16); full line shape analysis for
both anions (S17 and 18); crystal packing diagrams for both
ligands (S19); unit cell content in the structure of Tl(3PCO)
(S20); “metal ribbons” in Tl(3PCO) and Tl(4PCO)
coordination polymers shown in space filling representation
(S21); unit cell content in the structure of Tl(4PCO) (S22);
correlations between bond length and valence angles in studied
compounds and their IR spectra (S23); space-filling views of
lattices of Tl(3PCO) (S24); and Tl(4PCO) showing shielding
of metal centers by cyanoxime ligands (S25). This material is
available free of charge via the Internet at http://pubs.acs.org.
CONCLUSIONS
AUTHOR INFORMATION
Corresponding Author
*Phone: (417) 836-5165; e-mail: NNGerasimchuk@
MissouriState.edu.
■
■
Two isomeric pyridylcyanoxime ligands with heterocyclic
nitrogen atoms at 3- and 4- positons were synthesized using
developed high yield procedures, and then characterized using
1
UV−visible and H, 13C NMR spectroscopic methods and X-
Notes
ray analysis.
The authors declare no competing financial interest.
Crystal structures of both isometic pyridylcyanoximes
showed their planar geometry and anti-configuration of the
oxime group contrary to some syn-arylcyanoximes studied in
the past.
ACKNOWLEDGMENTS
■
Help from Dr. Kartik Ghosh and Dr. Ram Gupta (Department
of Physics, Materials Science and Astronomy of MSU) with
electrophysical measurements is greatly appreciated. N.G. is
very grateful to Ms. Alex Corbett for technical help during the
manuscript preparation, and to Dr. Charles L. Barnes for the
initial structure determination of Tl(4PCO).
Monovalent thallium complexes Tl(3PCO) and Tl(4PCO)
were obtained in high yield and characterized by solutions
electrical conductivity, UV−visible and IR-spectroscopy, and X-
ray analysis. These compounds are coordination polymers in
which cyanoxime anions act as bridging ligands using the
oxygen atom of the C-NO group and nitrogen atom of the
heterocycle. Both complexes form a double-stranded, “ladder-
type” motif. We found very short intermetallic distances in
these complexes Tl···Tl = 3.705 Å and 3.635 Å in Tl(3PCO)
and Tl(4PCO), respectively, which is close to 3.465 Å in
metallic thallium. The intermetallic separation in the latter is
the second shortest found in the literature. Thus, metal cations
form planar metal ribbons which can be considered as suitable
precursors for mixed metal (Tl···Au, Tl···Pt, or Tl···Ag) or
mixed valence Tl1+/Tl3+ complexes.
We plan in the nearest future: (a) to generate of mixed
valence polymeric compounds in polar aprotic solvents using
electrochemical methods; (b) to prepare separately TlL and
TlL3 complexes with isomeric pyridylcyanoximes and then
cocrystallize them. Solid state electrical conductivity studies of
such systems will follow. Also, we plan to synthesize
heterometallic coordination polymers using TlL and [Au-
(CN)3]−, PtR2(CN)2 which may exhibit metallophilic inter-
actions and possibly show photoluminescence.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Actual photographs of crystal samples of Tl-cyanoximates (S1);
face indexing of single crystals of synthesized compounds (S2);
hardware setup for solid state electrical conductivity measure-
ments crystal samples and hardware for conductivity measure-
ments (S3−S5); synthesis of H(4PCO) (S6); experimental
setup for the nitrosation reaction using gaseous methylnitrite
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(S8); FTIR spectra of labeled and nonlabeled with 15N
H(4PCO) and Tl(4PCO) (S9); 1D and 2D NMR spectra of
H(3PCO) and H(4PCO) (S10−S12); pK values for studied
ligands and related compounds (S13); UV−visible spectra of
anionic 4PCO− in different solvents (S14); tabulated electronic
spectra of isomeric anionic cyanoximes in different solvents
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