Benz(2-heteroaryl)cyanoximes and their Tl(i) complexes: New room temperature blue emitters

Article abstract of DOI:10.1039/b803846e

A series of five 2-heteroarylcyanoximes such as: α-oximino-(2- benzimidazolyl)acetonitrile (HBIHCO), α-oximino-(N-methy-l-2- benzimidazolyl)acetonitrile (HBIMCO), α-oximino-(2-benzoxazolyl) acetonitrile (HBOCO), α-oximino-(2-benzothiazolyl)acetonitrile (HBTCO) and α-oximino-(2-quinolyl)acetonitrile (HQCO) and their monovalent thallium(i) complexes were synthesized and characterized using spectroscopic methods (¹H, ¹³C NMR, IR, UV-visible, mass-spectrometry) and X-ray analysis. The HBIMCO (as monohydrate) adopts planar trans-anti configuration in the solid state. The crystal structure of "HBOCO" revealed the presence of nitroso anion a, BOCO^-, and protonated oxime cation b, H₂BOCO⁺, that form a H-bonded dimer in the unit cell. Both molecules adopt planar structures, but different configurations: cis-anti in the molecule a, and trans-anti for b. This is the first reported case of a zwitterionic pair in oximes and the coexistence of the two geometrical cis/trans isomers in the same crystal. All 2-heteroarylcyanoximes form yellow anions upon deprotonation, which exhibit significant negative solvatochromism in solution. Heterogeneous reactions between hot aqueous solutions of Tl ₂CO₃ and solid protonated 2-heteroarylcyanoximes HL afford yellow TlL. The crystal structure of Tl(BTCO) shows the formation of centrosymmetrical dimers, which connect with each other to form a double-stranded one-dimensional coordination polymer. The oxygen atom of the oxime group acts as a bridge between the three different Tl(i) centers. The anion is non-planar and adopts a trans-anti configuration in the complex. The polymeric motif in the complex represents a ladder-type structure. Staggered π-π interactions between benzothiazolyl groups provide additional stabilization of the structure. Both organic ligands and their Tl(i) complexes exhibit strong room temperature blue emission in the solid state.

Full text of DOI:10.1039/b803846e

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www.rsc.org/dalton | Dalton Transactions
Benz(2-heteroaryl)cyanoximes and their Tl(I) complexes:
new room temperature blue emitters†‡
Olesya T. Ilkun,a Stephen J. Archibald,b Charles L. Barnes,c Nikolay Gerasimchuk,*a Richard Biagioni,a
Svitlana Silchenko,d Olga A. Gerasimchuke and Victor N. Nemykin f
Received 5th March 2008, Accepted 3rd July 2008
First published as an Advance Article on the web 8th September 2008
DOI: 10.1039/b803846e
A series of five 2-heteroarylcyanoximes such as: a-oximino-(2-benzimidazolyl)acetonitrile
(HBIHCO), a-oximino-(N-methy-l-2-benzimidazolyl)acetonitrile (HBIMCO), a-oximino-
(2-benzoxazolyl)acetonitrile (HBOCO), a-oximino-(2-benzothiazolyl)acetonitrile (HBTCO) and a-
oximino-(2-quinolyl)acetonitrile (HQCO) and their monovalent thallium(I) complexes were synthesized
and characterized using spectroscopic methods (1H, 13C NMR, IR, UV-visible, mass-spectrometry) and
X-ray analysis. The HBIMCO (as monohydrate) adopts planar trans-anti configuration in the solid state.
The crystal structure of “HBOCO” revealed the presence of nitroso anion a, BOCO-, and protonated
oxime cation b, H2BOCO+, that form a H-bonded dimer in the unit cell. Both molecules adopt planar
structures, but different configurations: cis-anti in the molecule a, and trans-anti for b. This is the first
reported case of a zwitterionic pair in oximes and the coexistence of the two geometrical cis/trans isomers
in the same crystal. All 2-heteroarylcyanoximes form yellow anions upon deprotonation, which exhibit
significant negative solvatochromism in solution. Heterogeneous reactions between hot aqueous solu-
tions of Tl2CO3 and solid protonated 2-heteroarylcyanoximes HL afford yellow TlL. The crystal structure
of Tl(BTCO) shows the formation of centrosymmetrical dimers, which connect with each other to form
a double-stranded one-dimensional coordination polymer. The oxygen atom of the oxime group acts as a
bridge between the three different Tl(I) centers. The anion is non-planar and adopts a trans-anti configu-
ration in the complex. The polymeric motif in the complex represents a ladder-type structure. Staggered
p–p interactions between benzothiazolyl groups provide additional stabilization of the structure. Both
organic ligands and their Tl(I) complexes exhibit strong room temperature blue emission in the solid state.
Introduction
aDepartment of Chemistry, Missouri State University, Temple Hall 456,
Springfield, MO 65897, USA. E-mail: NNGerasimchuk@missouristate.edu;
The continuous interest in one-dimensional coordination poly-
Fax: +1 (417) 836–5165; Tel: +1 (417) 836–5165
mers is primarily focused on molecular electronic devices1 and the
bDepartment of Chemistry, University of Hull, Cottingham Road, Hull, UK
development of new luminescent materials.2 Dimeric, trimeric,
HU6 7RX
cDepartment of Chemistry, University of Missouri-Columbia, Chemistry
3D-polymeric metal compounds and clusters with metallophilic
interactions were found to exhibit photophysical properties use-
ful for optoelectronics, such as metal-based room temperature
luminescence,3 phosphorescence and vaporochromism.4 The latter
Building 125, Columbia, MO 65211, USA
dAbsorbtion Systems Inc., 440 Creamy Way, S.300, Exton, PA 19341, USA
eClariant LSM Inc., W. Bennett St., Springfield, MO, USA
f
Department of Chemistry, University of Minnesota Duluth, Duluth, Min-
property attracts attention due to the possibility of making
new types of sensors for a variety of vapors of organic sub-
stances including industrial solvents5 and organophosphorous
compounds.6 Light-emitting properties of heavy metal complexes
and phosphorescence specifically, has been intensively studied in
recent years due to their applications in high performance organic
light emitting diodes (OLED).7 In fact, phosphorescence can lead
to almost 100% internal quantum efficiency providing high exter-
nal light output of the actual device. This approach shifted focus
to the investigation of heavy metal complexes as phosphorescent
materials since strong spin–orbit coupling induced by the heavy
metal facilitates Sn→Tn intersystem crossing and allows strong
phosphorescence at room temperature.8 The emission color of the
system can be tuned by the introduction of various substituents
on the ligand binding site,9 or by different auxiliary ligands.10 It
should be noted especially that pure blue emitting materials are
in great demand for application in full-colored LCD displays11
nesota 55812-2496, USA
† Electronic supplementary information (ESI) available: Preparation of
HBOCO and HQCO ligands, S1; tabulated room temperature 13C NMR
spectra of all five synthesized benz-(2-heteroaryl)cyanoximes in DMSO-d6,
S2–S4; solid state IR spectroscopic data for HL and TlL (S5, S6); pho-
tographs of crystals of the cyanoxime ligands (S7) and their Tl-complexes
(S20); details of crystal packing in the structure of HBOCO, S8–S10; linear
correlations between UV-visible spectra, pKa of the ligands and Hammett
constants for the benz-(2-heteroaryl)cyanoximes, S11, S12; actual NMR
spectra of HBTCO, HBIMCO and HBIHCO ligands, S13, S14; data of
solution electrical conductivity studies, S15; UV-visible spectra of some
of the protonated and deprotonated cyanoxime ligands, S16; correlations
for solvatochromic series of the BOCO- anion, S17; typical fluorescence
spectra of HL and NaL and signals deconvolution, S18; fluorescence of
HBIHCO in EtOH solution, S19; solid state UV-visible spectra for TlL,
S21; experimental setup for solid state fluorescence measurements, S22;
emission spectra of TlL at different excitations, S23–S23. CCDC reference
numbers 680405–680407. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b803846e
‡ Dedicated to Prof. Vladimir Amirkhanov (Kiev State University,
Ukraine) on the occasion of his 50th birthday.
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since this color is much more difficult to generate than the other
two (red and green) principal colors. Coordination polymers offer
significant advantages over conventional molecular compounds
due to very low solubility in conventional organic solvents and
water, and much higher thermal stability.
There are two important factors that guide the search for one-
dimensional coordination polymers that may exhibit interesting
photophysical properties. These are: 1) design and exploration
of new bridging ligand systems that may provide the formation
of well-organized one-dimensional polymeric motifs such as
columns, chains and double strands (ladders) connected by non-
bonding interactions; 2) the search for close spatial location
of identical (in homometallic complexes) or different metals (in
heterometallic complexes) in the crystal lattice due to direct
metal–metal interactions, or appropriate bridging atoms/groups.
Typically, coinage metals such as Cu,12 Ag,13 Au14 and Pt15 in
homometallic and mixed-metal16 compounds are employed for
building coordination polymers. Thallium has attracted special
interest in recent years due to its pronounced ability to cause
metallophilic interactions17,18 and the formation of metal–metal
bonds with the above metal ions.4a,5a,19
Since ligands were found to play a major role in the formation
of coordination polymers with close metal–metal separations, our
attention turned to cyanoximes – a new versatile group of stable
organic molecules20 with the general formula NC–C(=NOH)–R,
where R represents a variety of electron-withdrawing groups.
Recently, we reported the synthesis, spectroscopic properties
and structures of a large group of Tl(I) complexes based
on monosubstituted arylcyanoximes.21 These complexes have
shown interesting double-stranded, ladder-type one-dimensional
polymeric structures with short Tl–Tl distances comparable to that
Scheme 1
(Spectrum Chemicals, J. T. Baker) were of HPLC grade and used
without additional distillation. Melting points or decomposition
temperatures for synthesized protonated cyanoximes and their
Tl(I) salts were determined using Mel-Temp apparatus (Thomas
Hoover). TLC was carried out on silica coated glass plates (Merck)
containing a fluorescent indicator. Elemental analyses on C,H,N
content were performed at the Atlantic Microlab (Norcross, GA).
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 tetrabutylammonium salts
or Cs+ 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 (Starna, Inc.). Room
temperature (294 K) solid state absorbance spectra of the Tl-
cyanoximates were recorded at the same wavelength intervals
from fine suspensions in silicon oil squeezed between two 10 ¥
40 ¥ 1 mm quartz plates. Solution fluorescence spectra of HL,
deprotonated anionic cyanoximes (as NaL), and TlL (L =
studied cyanoxime ligands) were obtained at 293 K using a
Shimadzu RF-5301 PC spectrofluorometer in the range 200–
900 nm. Fluorescence lifetime measurements at 296 K were carried
out on a Jobin Ivon Fluorolog system using LEDs of variable
wavelengths as excitation sources. Photoluminescence from solid
samples was measured at 294 K using pressed fine powders of
studied cyanoximates (page S22, ESI†). Infrared spectra were
recorded in the range 500–4000 cm-1 using the FT IR Nicolet
Magna 550 spectrophotometer using a KBr matrix for protonated
cyanoximes HL and their cesium salts CsL. However, the IR
spectra of TlL were obtained from fine suspensions in Nujol
due to the solid state exchange reactions between KBr and TlL
during pellet preparation. A mass-spectrometric characterization
(positive FAB) of the protonated 2-heteroarylcyanoximes was
carried out using Autospec Q and ZAB Finigan spectrometers.
meta-Nitrobenzylic alcohol, NBA, was used as a matrix in all
experiments.
˚
in metallic thallium (3.42 A). Amongst all cyanoximes, heteroaryl-
containing ligands attract special attention due to the presence of
donor atoms other than nitrogen such as O,22 S23 and Se,24 and the
ability to provide extra-stabilization of formed lattices because
of the p–p stacking interactions. In general, 2-benz(heteroaryl)
groups25 are increasingly frequently incorporated into a variety
of ligands that form interesting metal complexes. However,
complexes of monovalent thallium with oximes and heterocyclic
cyanoxime ligands in particular are insufficiently studied.
We report in our present paper the synthesis, spectroscopic
and structural characterization of a series of benz(2-heteroaryl)
cyanoximes as a new ligand system, and their thallium(I) com-
plexes.
Experimental
Synthesized benz(2-heteroaryl)cyanoximes with their commonly
used abbreviations26 are shown in Scheme 1.
Materials and methods
Starting materials, thallium(I) carbonate, glacial acetic acid,
sodium nitrite and substituted heteroaryl-actonitriles R–CH2–CN
(R = 2-benzothiazolyl, 2-benzimidazolyl, N-methyl-2-benzimi-
dazolyl groups) were obtained from Aldrich and were used without
additional purification. Precursors for HBOCO and HQCO
ligands were obtained using a three-step procedure26,27 depicted in
the electronic supplementary information (ESI).† Organic solvents
Solution studies
Electrical conductivity of 1 mM solutions of HL (in anhydrous
acetone, DMSO and DMF), CsL and TlL (in DMSO and
water; L = studied cyanoxime anions) was measured at 295 K
using the YSI Conductance-Resistance meter Model 34. Solutions
5716 | Dalton Trans., 2008, 5715–5729
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of tetrabutylammonium bromide and tetraphenylphosphonium
bromide (as 1 : 1 electrolytes) were used for the electrode
calibration. Measurements of pKa values for synthesized benz(2-
heteroaryl)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 solubilizing co-solvent.
AtenololTM and LidocaineTM (from Aldrich) were used for cali-
bration of the instrument. Measurements consisted of three-step
multi-stage titrations in water–DMSO mixtures from 20 wt% to
30 wt% with 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.
IPDS-II imaging plate diffractometer. The X-ray source was a fine
˚
focused sealed Mo tube (Ka radiation; l = 0.71070 A; highly
oriented graphite monochromator). The data were corrected for
decay and absorption using either X-RED or the SADABS 30
program.28 Direct methods structure solution, difference Fourier
calculations, and full-matrix least squares refinements against
F2 were performed with a SHELXL-97 program package. All
non-hydrogen atoms were refined with anisotropic displacement
parameters, while hydrogen atoms were placed in geometrically
idealized positions and refined using a riding model. The crystal
and refinement data for the studied compounds are listed in
Table 1, while selected bond lengths and valence angles in the
structures are presented in Table 2. An ORTEP program package29
was used to draw figures of crystal structures. The respective cif
files are available in the ESI.†
Synthesis of benz-(2-heteroaryl)cyanoximes
X-Ray crystallography
The preparation of these compounds was accomplished using
the modified Meyer reaction30 shown in Scheme 2. This route
represents nitrosation reaction in glacial acetic acid26,31 and leads
to the desired 2-heteroarylcyanoximes with 80–95% yields. Since
all preparations of the protonated cyanoximes HL described here
are similar, synthesis of only one ligand – H(BIHCO) – will be
presented in detail.
Crystal structures were determined for two protonated 2-
heteroarylcyanoximes (HBIMCO, HBOCO) and one monovalent
thallium complex Tl(BTCO). Suitable quality single crystals of
organic ligands were obtained upon slow evaporation of their
water–ethanol or water–acetone solutions at room temperature.
X-Ray quality single crystals of the thallium(I) complex were
obtained upon the slow cooling (~28 h) of a hot aqueous solution
of Tl(BTCO) in a thermostat. Data collection for the studied
crystals was carried out at low temperature using either a Bruker
SMART CCD diffractometer operating in w-scans mode or a Sto¨e
a-Oximino-(2-benzimidazolyl)acetonitrile, HBIHCO. Benzi-
midazolyl-2-acetonitrile, C7H4N2–CH2—CN (1 g, 6.4 mM) was
placed in a heavy walled 50 mL round bottom flask with a
Table 1 Crystal and refinement data for synthesized 2-heteroarylcyanoximates
Parameter
HBIMCO·H2O
H2BOCO+BOCO-
Tl(BTCO)
Empirical formula
Formula weight
Temperature/K
C10H11N4O2
219.23
150 (2)
C9H5N3O2
374.32
173 (3)
C9H4N3OSTl
406.58
150 (2)
Crystal dimensions/mm
Crystal system
Space group
0.15 ¥ 0.12 ¥ 0.1
Triclinic
P-1
0.50 ¥ 0.30 ¥ 0.05
Monoclinic
P21/c
0.15 ¥ 0.07 ¥ 0.06
Monoclinic
P21/c
Unit cell dimensions:
˚
a/A
7.1446(18)
8.587(2)
8.988(2)
96.67(2)
109.370(18)
99.79(2)
503.7(2)
2
1.445
0.105
228
3.10 to 34.75
14.1723(19)
7.1591(10)
17.461(2)
90.00
101.404(2)
90.00
1736.7(4)
4
1.432
14.8621(17)
4.4277(3)
15.0453(15)
90.00
90.15(9)
90.00
990.05(16)
2
2.728
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
Volume/A
Z
Density (calc.)/Mg m-3
Absorption coeff./mm-1
F(000)
0.106
768
1.47 to 27.13
16.496
736
2.71 to 34.78
Theta range for data/◦
Index ranges:
-11 < h < 11
-13 < k < 13
-14 < l < 14
13006
-18 < h < 14
-9 < k < 9
-23 < h < 23
-5 < k < 7
-22 < l < 22
-24 < l < 24
Reflections collected
Independent reflections
Rint
Completeness to theta (%)
Goodness of fit
11651
3815
19093
4236
4301
0.0692
0.0846
0.0950
98.5 (at 34.75◦)
1.012
99.3 (at 27.13◦)
1.027
99.1 (at 34.78◦)
1.042
Final R indices [I > 2s(I)]
R indices (all data)
Extinction coefficient
Highest peak/hole/A-3
R1 = 0.0562, wR2 = 0.1389
R1 = 0.1169 wR2 = 0.1687
0.084(17)
R1 = 0.0708 wR2 = 0.1706
R1 = 0.2128 wR2 = 0.2406
0.0018(13)
R1 = 0.0637 wR2 = 0.1698
R1 = 0.0843 wR2 = 0.1888
0.0144(12)
0.367/-0.364
0.798/-0.543
3.949/-2.528
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◦
a
˚
Table 2 Selected bond lengths (A) and valence angles ( ) in the structures of synthesized 2-heteroarylcyanoximes
Compound
Bonds
Angles
HBIMCO H2O
C(1)–N(1)
C(1)–N(2)
C(1)–C(2)
C(2)–N(3)
C(2)–C(3)
C(3)–N(4)
C(4)–N(1)
C(5)–N(2)
N(3)–O(1)
1.322(2)
1.366(2)
1.460(2)
1.286(2)
1.453(2)
1.142(2)
1.383(2)
1.384(2)
1.3629(19)
N(1)–C(1)–N(2)
N(1)–C(1)–C(2)
N(2)–C(1)–C(2)
N(3)–C(2)–C(3)
N(3)–C(2)–C(1)
C(3)–C(2)–C(1)
N(4)–C(3)–C(2)
C(1)–N(1)–C(4)
113.23(13)
121.25(13)
125.52(13)
120.93(14)
122.55(14)
116.48(13)
177.96(17)
105.17(12)
106.36(12)
111.61(13)
103.4(3)
104.5(4)
115.9(4)
127.7(4)
116.3(4)
103.4(5)
120.8(4)
127.5(4)
111.5(4)
179.0(5)
109.5
104.2(3)
105.3(4)
114.8(4)
127.4(4)
117.7(4)
112.2(4)
121.5(4)
118.6(4)
119.8(4)
177.2(5)
109.5
C(1)–N(2)–C(5)
C(2)–N(3)–O(1)
BOCO-molecule a, nitroso-anion
H2BOCO+molecule b, oxime, cation
Tl(BTCO)c
O(1a)–C(1a)
O(1a)–C(2a)
N(1a)–C(1a)
N(1a)–C(7a)
C(1a)–C(8a)
O(2a)–N(2a)
C(2a)–C(7a)
N(2a)–C(8a)
N(3a)–C(9a)
C(8a)–C(9a)
1.356(5)
1.399(5)
1.296(6)
1.404(6)
1.428(6)
1.259(6)
1.377(6)
1.444(7)
1.149(6)
1.421(7)
C(1a)–O(1a)–C(2a)
C(1a)–N(1a)–C(7a)
N(1a)–C(1a)–O(1a)
N(1a)–C(1a)–C(8a)
O(1a)–C(1a)–C(8a)
O(2a)–N(2a)–C(8a)
C(9a)–C(8a)–C(1a)
C(9a)–C(8a)–N(2a)
C(1a)–C(8a)–N(2a)
N(3a)–C(9a)–C(8a)
N(2a)–O(2a)–H(2a)
C(1b)–O(1b)–C(2b)
C(1b)–N(1b)–C(7b)
N(1b)–C(1b)–O(1b)
N(1b)–C(1b)–C(8b)
O(1b)–C(1b)–C(8b)
C(8b)–N(2b)–O(2b)
N(2b)–C(8b)–C(9b)
N(2b)–C(8b)–C(1b)
C(9b)–C(8b)–C(1b)
N(3b)–C(9b)–C(8b)
N(2b)–O(2b)–H(2b)
N(1)–C(1)–C(2)
O(1b)–C(1b)
O(1b)–C(2b)
N(1b)–C(1b)
N(1b)–C(7b)
C(1b)–C(8b)
O(2b)–N(2b)
C(2b)–C(7b)
N(2b)–C(8b)
N(3b)–C(9b)
C(8b)–C(9b)
1.363(5)
1.397(5)
1.288(5)
1.402(5)
1.439(6)
1.352(4)
1.377(6)
1.294(6)
1.137(6)
1.430(6)
C(1)–N(1)
C(1)–C(2)
C(1)–S(1)
C(2)–N(2)
C(2)–C(3)
C(4)–C(5)
C(3)–N(3)
C(5)–S1
N(2)–O(1)
Tl(1)o–O(1)c1
Tl(1)–O(1)
Tl(1)–O(1)c2
1.297(11)
1.441(14)
1.763(10)
1.319(10)
1.429(12)
1.398(13)
1.149(12)
1.745(9)
1.317(11)
2.859(7)b
2.635(7)b
2.864(8)b
125.4(9)
116.3(8)
118.2(6)
120.8(8)
118.3(8)
120.7(7)
175.0(9)
110.3(8)
116.6(8)
88.3(4)
N(1)–C(1)–S(1)
C(2)–C(1)–S(1)
N(2)–C(2)–C(3)
N(2)–C(2)–C(1)
C(3)–C(2)–C(1)
N(3)–C(3)–C(2)
C(1)–N(1)–C(4)
C(2)–N(2)–O(1)
C(5)–S(1)–C(1)
O(1)–Tl(1)–O(1)c1
O(1)–Tl(1)–O(1)c2
107.3(2)
76.2(2)
a Geometry of the phenyl group adjacent to the heteraryl fragment is normal and not shown. b Details of geometry at the Tl(1) center are presented in
Fig. S10 ESI. c Symmetry operations are for #1: x, y + 1, z; while for #2: -z - 1, -y - 1, -z + 1.
Vacuum/inert gas flushing was applied prior to opening the flask
in order to release any pressure buildup. 35 mL of water were
poured into the reaction mixture, leading to the rapid precipitation
of a pale yellow solid that was filtered, washed with 100 mL of
water (3 portions), and dried in a vacuum dessicator charged with
NaOH pellets. Recrystallization with charcoal afforded a white
solid; yield: 1.164 g (98%). Rf = 0.78 (pure EtOAc) and 0.53
Scheme 2
in EtOAc–hexane = 4 : 1. Upon heating, the color changed to
yellow at 230 ◦C foll◦owed by rapid decomposition to a brown,
tarry product at 280 C. IR spectrum (cm-1): 3345 n(NH); 3284
#19/38 taper joint, and dissolved in 5 mL of glacial acetic acid
at 25 ◦C. Argon was slowly bubbled through the solution for
20 min to replace air. The purged solution was frozen using
an ice bath. Solid NaNO2 (0.8789 g; 12.7 mM, in excess) was
added at once on top of the frozen solution and the flask was
closed with a heavy-duty Teflon valve with a #19/38 taper joint.
The reaction mixture was allowed to thaw slowly for 12 hours.
∫
=
n(OH); 3061 n(C–H) arom; 2239 n(C N); 1614 n(C N) ring; 1533
=
n(C N) oxime; 1065 n(N–O). HBIHCO in DMSO-d6 solution at
298 K represents a mixture of syn- (36%) and anti- (64%) isomers.
The latter isomer also exhibits a dynamic prototropic tautomeric
equilibrium, which is discussed later in this paper. 1H NMR (ppm)
data for syn-isomer: 14.35 (1H, broadened OH), 13.14 (1H, broad
5718 | Dalton Trans., 2008, 5715–5729
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NH), 7.69 (2H, multiplet), 7.31 (2H, multiplet). High-resolution
mass-spectrometry: calc. for C9H6N4O, 186.0542; found, 187.0617
(M + 1).
soluble in light alcohols ROH, acetone, CH3CN, ether, ethyl
acetate and pyridine (with the formation of ionic salts), but were
sparingly soluble in water and CCl4, and insoluble in hexane and
aromatic hydrocarbons.
Other benz-(2-heteroaryl)cyanoximes follow:
a-Oximino-(N-methyl-2-benzimidazolyl)acetonitrile, HBIMCO.
White solid, 95% yield; color change upon heating begins at 195 ◦C
with rapid decomposition to a brown, tarry product at 235 ◦C;
Rf = 0.72 (EtOAc–hexane = 4 : 1). IR spectrum (cm-1): 3570
Synthesis of thallium(I) complexes
There are three practical routes for the synthesis of Tl(I) derivatives
of weak organic acids (Scheme 3). Route 1 represents a clean
heterogeneous reaction that results in the evolution of CO2, which
drives the reaction to completion. Crystals suitable for X-ray
analysis can be obtained upon slow cooling of a hot filtered
reaction mixture in a thermostat. We successfully used all three
routes during years of investigation of Tl(I) cyanoximates in the
past. However, this method 1 was employed in this report, since it
combined reasonable speed of preparation with a good chance of
getting X-ray quality crystals. The typical synthesis of one complex
is described below.
as
s
n(OH); 3023 n(C–H) arom; 2940 n (C–H), 2790 n (C–H) methyl;
∫
=
=
2236 n(C N); 1610 n(C N) ring; 1596 n(C N) oxime; 1090 n(N–
O). Mixture of two isomers (25% syn- and 75% anti-) in DMSO-d6
solution at 297 K. 1H NMR (ppm) for dominant anti-isomer: 14.45
(OH oxime, 1H broadened singlet), 7.73 (2H, doublet), 7.63 (2H,
doublet), 7.36 (2H, triplet), 7.27 (2H, triplet), 3.94 (3H, methyl
group). High-resolution mass-spectrometry: calc for C10H8N4O,
200.0698; found, 201.0763 (M + 1). The compound crystallizes
as monohydrate, and clear HBIMCO ¥ H2O needles suitable for
X-ray analysis were obtained after the slow evaporation of a water–
ethanol solution at 4◦C (page S7, ESI†).
a-Oximino-(2-benzoxazolyl)acetonitrile,
HBOCO. Brown
solid, yield 86%; melting with decomposition at 195 ◦C; Rf =
0.53 (EtOAc–hexane= 1 : 1). IR spectrum (cm-1): 3433 n(OH);
∫
=
3100 n(C–H) arom; 2243 n(C N); 1612 n(C N) ring; 1585
=
n(C N) oxime; 1045 n(N–O). Compound represents a mixture
of syn- (10%) and anti- (90%) isomers in DMSO-d6 solution.
1H NMR (ppm) for anti-isomer: 14.05 (1H, broad oxime OH
singlet), 7.85 (1H, doublet), 7.79 (1H, doublet), 7.50 (1H, triplet),
7.46 (1H, triplet). High-resolution mass-spectrometry: calc for
C9H5N3O2, 186.0542; found, 187.0628 (M + 1). Brown clear
blocks of HBOCO were grown during the slow evaporation of its
acetone–water solution at 4 ◦C.
Scheme 3
Thallium(I) a-oximino-(2-benzoxazolyl)acetonitrile, Tl(BOCO).
Thallium(I) carbonate, Tl2CO3 (250.5 mg; 0.53 mM) was dissolved
under stirring in 10 mL of water at 96 ◦C. Small portions (200.0 mg;
1.06 mM) of solid HBOCO ligand were added within 2 minutes
to a boiling aqueous solution of thallium(I) carbonate. When all
CO2 was evolved, a slightly cloudy solution was hot filtered using
a jacketed, steam heated filter funnel into a large pyrex test tube
that was immersed and fixed inside a Dewar flask containing five
liters of water preheated to 95 ◦C. Slow cooling of the mother
liquor over several days afforded long fibrous golden crystals of
the Tl(BOCO) complex. The compound was filtered and dried in a
dessicator under P4O10. Yield: 214.2 mg (50%); mp ~ 180 ◦C (dec.).
Anal. for C9H4N3O2Tl calculated (found), %: C 27.68 (27.22), H
1.03 (1.20), N 10.76 (10.56).
a-Oximino-(2-benzothiazolyl)acetonitrile,
HBTCO. Tan–
yellow solid, yield 90%; color change upon heating begins at
205 ◦◦C with rapid decomposition to brown, tarry product at
221 C; Rf = 0.30 (EtOAc–hexane= 1 : 1). IR spectrum (cm-1):
∫
3240, 3159 n(OH); 3075 n(C–H) arom; 2263, 2235 n(C N);
=
=
1590, 1556 n(C N) ring, 1575 n(C N) oxime; 1063 n(N–O).
The compound represents a mixture of syn- (10%) and anti-
(90%) isomers in DMSO-d6 solution. H NMR (ppm) for major
1
isomer: 14.82 (1H, broad oxime singlet), 8.11 (2H, multiplet:
two overlapped doublets), 7.54 (2H, multiplet: two overlapped
triplets). High-resolution mass-spectrometry: calc for C9H5N3OS,
203.0153; found, 204.0211 (M + 1).
Other thallium(I) cyanoximates follow.
a-Oximino-(2-quinolyl)acetonitrile, HQCO. White solid, yield
◦
87%; melting with decomposition at 184 C; Rf = 0.38 (EtOAc–
Thallium(I) a-oximino-(2-benzothiazolyl)acetonitrile, Tl(BT-
CO). Yield 72%; at ~190 ◦C complex decomposes. Anal. for
C9H4N3OSTl calculated (found), %: C 26.59 (26.27), H 0.99 (1.11),
N 10.33 (10.38).
hexanes= 1 : 2). IR spectrum (cm-1): 3550 n(OH); 3030 n(C–H)
∫
=
=
arom; 2230 n(C N); 1596 n(C N) ring, 1615 n(C N) oxime; 1030
n(N–O). Compound represents a mixture of syn- (13%) and anti-
(87%) isomers in acetone-d6 solutions at 297 K. 1H NMR (ppm)
data for dominant isomer: 13.15 (1H, broadened oxime singlet),
8.43 (1H, doublet), 8.14 (1H, doublet), 8.05 (1H, doublet), 8.03
(1H, doublet), 7.86 (1H, triplet), 7.70 (1H, triplet). High-resolution
mass-spectrometry: calc for C11H7N3O 197.0589; found, 198.0694
(M + 1).
Thallium(I) a-oximino-(2-benzimidazolyl)acetonitrile, Tl(BI-
◦
HCO). Yield 40%; at ~140 C complex decomposes. Anal. for
C9H5N4OTl calculated (found), %: C 27.75 (27.75), H 1.29 (1.25),
N 14.38 (14.12).
Thallium(I) a-oximino-[(N-methyl)-2-benzimidazolyl]acetoni-
trile, Tl(BIMCO). Yield 42%; at ~160 ◦C complex decomposes.
Anal. for C10H7N4OTl calculated (found),%: C 29.76 (29.95), H
1.75 (1.73), N 13.88 (13.86).
13C NMR spectra for all protonated cyanoximes HL reported
in this paper are tabulated in pages S2–S4 of the ESI.† All
synthesized organic compounds are microcrystalline powders,
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Thallium(I) a-oximino-(2-quinolyl)-acetonitrile, Tl(QCO).
Yield 75%; at ~140 ◦C complex decomposes. Anal. for
C11H6N3OTl calculated (found), %: C 32.98 (32.56), H 1.51 (1.48),
N 10.49 (10.19).
All five obtained monovalent Tl cyanoximates represent yellow
crystalline compounds that are not soluble in ether, ethanol,
propanol, chlorohydrocarbons or aromatic hydrocarbons, but
dissolve in CH3CN, DMF, DMSO, pyridine and hot water.
The IR spectra of synthesized Tl(I) complexes are tabulated
in pages S5 and S6 in the ESI.† All synthesized Tl(I) benz-
(2-heteroaryl)cyanoximates form fibrous or needle-type crystals
(p. S20 ESI†).
SAFETY NOTE! Although we have not encountered any problems
during many years of laboratory work and handling, special care
should be taken during procedures using thallium compounds
because of their toxicity.
Results and discussion
Characterization of the ligands
Solid state structure of HBIMCO·H2O. The molecular struc-
ture of this 2-heteroarylcyanoxime is displayed in Fig. 1, while
values for bonds and valence angles are presented in Table 2.
HBIMCO crystallizes as a monohydrate similarly to the 2-
quinolylcyanoxime, HQCO ligand.31 Contrary to the latter,
HBIMCO adopts a planar trans-anti configuration. Crystalliza-
tion water molecules play an essential role in packing of the
cyanoxime into the crystal. A system of H-bonds is formed
between the nitrogen atom of the imidazolyl group and the oxygen
atom of the oxime group (Fig. 1) with parameters: N1–H1w–Ow =
Fig. 1 Fragment of crystal packing and numbering scheme in the
structure of HBIMCOxH2O showing one layer of molecules in the unit
cell. A – side view that displays the H-bonded chain of the heterocyclic
cyanoxime; B – orthogonal view shows the ligand’s planarity.
normal for this class of compounds.21,31,41 Bond lengths and valence
angles in the heterocyclic fragment of the molecule are normal for
the aromatic compounds as well.
◦
˚
˚
1.853 A, ∠N1–H1w–Ow = 173.3 , Ow–H1(oxime) = 1.617 A
with ∠Ow–H1(oxime)–O1 = 161.2◦. Therefore, water molecules
connect to translation-related HBIMCO molecules along the y
axis into a planar sheet. Formed sheets are held together by “head-
Crystal structure of HBOCO. 2-Benzoxazolylcyanoxime,
HBOCO, is a colored substance contrary to other analogous
cyanoxime molecules described in this paper (S7 ESI†) or sub-
stituted arylcyanoximes.21 This is highly unusual since the vast
majority of oximes (and cyanoximes in particular) are colorless.
The solid-state structure of HBOCO is unique among all known
cyanoximes.31 The molecular structure of this compound is shown
in Fig. 2, while bond lengths and valence angles are presented in
Table 2. The unit cell contains two crystallographically different
HBOCO molecules, a and b, that form H-bonded dimeric units
(Fig. 2). Both molecules a and b are practically planar, but
˚
to-tail” p–p stacking interactions at 3.35 A along the x direction
(Fig. 1). A series of distances between two neighboring molecules
forming p–p stacks in the structure of HBIMCO are the following:
˚
centroids of the phenyl rings are separated by 7.043 A, while
˚
centroids of the imidazole rings are separated by 4.147 A. The
distance between centroids of the phenyl group and imidazole ring
˚
is 5.375 A. Geometrical parameters of the cyanoxime fragment are
Fig. 2 Atomic numbering scheme in the structure of the HBOCO dimer: cationic molecule a is in cis-anti configuration, while anionic molecule b adopts
trans-anti configuration. A – top view, B – side view (~90◦ rotation) of the dimer showing dihedral angle between two planar cyanoxime molecules.
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Table 3 Values of pKa for several cyanoximes and related compounds
have different geometrical configurations: cis-anti and trans-anti
respectively. The dihedral angle between planes of a and b
molecules in the dimer is 45.85◦. There is a significant difference in
the geometry of the oxime fragment in both molecules (Table 2).
Thus, molecule a has typical geometry for the nitroso-anions32
showing a much shorter N2a–O2a than C8a–N2a distance in the
group (Fig. 2). However, molecule b is an oxime (Table 2). It is
commendable that there are unusually large thermal ellipsoids
for N2a and O2a atoms in the molecule a (Fig. 2). This is
evidence of their significant thermal motion as anionic nitroso-
group which is weakly H-bound to the typical protonated cationic
oxime molecule b. These geometrical features, combined with
the intense brown color for the compound, confirm formation
of the cation–anion pair H2BOCO+ ◊ ◊ ◊ BOCO- (Scheme 4). This
autoionization is also reflected in the n→p* transition at ~390 nm –
which is typical for anionic cyanoximates – in the solid state
spectrum of HBOCO. This is an interesting example of an
intermolecular proton transfer leading to autoionization, which
is a fast process,33 while the X-ray structure reflects the averaged
equilibrated position of the involved atoms. Crystal packing of
HBOCO is also quite unusual among all the cyanoximes studied
to date.21,31 Extensive intermolecular H-bonding and short p–p
Compound
pKa1, oxime
pKa2, other sites
Reference
This work
a
4.66
—
a
8.77
5.05
—
55
a
—
This work
a
5.66
—
This work
˚
stacking “head-to-tail” interactions at 3.36 A are responsible for
6.44
5.84
8.65-azomethyne
group, C N
55
the formation of interesting lattice architecture (S8–S10 ESI†).
The series of distances between the two p–p stacked molecules a
and b in the structure of HBOCO is the following: centroids of
=
˚
the phenyl ring a and phenyl ring b are separated by 3.581 A,
3.38-acidic NH
group
This work
centroids of the oxazole ring a and oxazole ring b are separated
˚
by 4.775 A, while centroids of the oxazole ring a and phenyl ring
˚
b are separated by 3.588 A with centroids of the phenyl ring a and
˚
oxazole ring b being 3.868 A apart.
6.18
10.29
This work
a No protonation of the heterocyclic nitrogen atom was observed in the
studied pH range in DMSO–H2O systems.
Scheme 4
of the quinoline group in HQCO can be protonated: its pKa
was found to be 10.3 0.01 (Table 3). The NH-proton in the
HBIHCO is unexpectedly acidic with a measured pKa= 3.38
0.02 for that group. A linear correlation between lmax of the
n→p* transition in the nitroso-chromophore of anionic benz-
(2-heteroarylcyanoximes) L- in ethanol and pKa values of the
oxime group in HL was observed (S11 ESI†). Similarly, linear
relationships between Hammett sp and sm constants36 for the
benz-(2-heteroaryl) fragments and pKa values of the cyanoximes
were found as well (S12 ESI†).
Acid–base properties of benz-(2-heteroaryl)cyanoximes in solu-
tion. Synthesized 2-heteroarylcyanoximes represent monoprotic
acids that are significantly more acidic than aliphatic monoximes
and dioximes.34 The values of measured pKa for HQCO, HBOCO,
HBTCO, HBIHCO and HBIMCO together with literature
data for related compounds are presented in Table 3. Thus,
HBOCO is ~12 900 times more acidic than its analogous benz-(2-
oxazolyl)aldoxime (Table 3). The value of pKa for the oxime proton
in HBOCO is 4.66 0.01 and represents the highest acidity of
the oxime group among the studied 2-heteroarylcyanoximes. The
heterocylic nitrogen atoms in HBOCO, HBTCO, HBIMCO and
HBIHCO could not be protonated under the studied conditions
in mixed aqueous–DMSO (21–35 wt%) solutions with pH ranging
from 3 to 11. This finding is a surprise in the light of the existence of
transition metal complexes with 2-heteroarylcyanoximates where
the latter act as chelating ligands,35 offering the heterocyclic
nitrogen atom for coordination. Nevertheless, the nitrogen atom
Solution structure: NMR spectra. Benz-(2-heteroaryl)cyano-
ximes exhibit rich stereochemistry and exist as several geometrical
isomers, while their nitroso-counterparts exhibit free rotation
around the C–N bond (Scheme 5) reducing the number of possible
species insolution. 13C NMR spectra ofall synthesized compounds
in DMSO-d6 and acetone-d6 contain a double set of signals (S13,
S14 ESI†). This is due to the presence of the two syn- and
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Scheme 5
Scheme 6
anti-geometrical oxime isomers in solution for HBIMCO,
HBOCO, HBTCO, and prototropic tautomerism combined
with the presence of the two isomers for solutions of HBIHCO
(Scheme 6). Partial autoionization with the formation of
zwitterionic species takes place in solutions of HQCO (S15 ESI†).
HBOCO forms a yellow solution in DMSO-d6 and exists as a
mixture of the trans-anti (dominant, 90%) and trans-syn isomers at
room temperature according to 1H NMR spectroscopy data. Benz-
(2-oxazolyl)cyanoxime is only slightly ionized in solution, contrary
to the prediction one could make from the X-ray structure, which
confirmed the autionization of HBOCO in the solid state. 1H
NMR spectra of HBIMCO and HBTCO in DMSO-d6 indicated
the presence of 27% syn-/73% anti- and 12% syn-/88% anti-
isomers in the mixture respectively (S13, S14 ESI†). Nevertheless,
as discussed earlier, HBIMCO and HBTCO crystallize exclusively
as trans-anti isomers in a solid state. Both 1H and 13C NMR
spectra of HBIHCO in DMSO-d6 at 298 K demonstrate the
presence of the two geometrical isomers (S14 ESI†), where the
anti-isomer is involved in a prototropic tautomeric equilibrium as
depicted in Scheme 6. H-bonding is essential for “locking” the
syn-isomer from conversion into an anti-isomer and significantly
slows tautomerism, leading to six narrow 13C signals in the spectra
that evidenced the equivalence of three carbon atoms in the phenyl
ring (S14 ESI†).
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Table 4 Results of UV-visible spectroscopic studies of synthesized 2-
We found that the HBIHCO tautomeric equilibrium is sensitive
to temperature, but is independent of concentration, suggesting
an intramolecular process. A similar case of tautomerism for
the benz-(2-imidazolyl)aminomethyl ligand has been recently
reported in its tris-Co(III) complex.37 The HQCO forms bright-
yellow solutions in acetone-d6 and DMSO-d6 and exists as a mix-
ture of trans-anti (dominant, 88% in the former solvent) and cis-syn
isomers at room temperature according to 1H NMR spectroscopy
data. The latter isomer is stabilized due to the formation of an
intramolecular H-bond and prone to autoionization because of
the favorable geometry in this pseudocyclic structure:
Indeed, both UV-visible spectra and the solution electrical
conductivity studies (see below and S15 ESI†) have confirmed
the formation of ionic species. The cyanoxime anions exhibit the
characteristic n→p* transition in the >CNO chromophore38 at
~380–450 nm, while protonated HL compounds are colorless.
The 13C NMR spectra of HQCO in acetone-d6 and DMSO-
d6 show only a double set of signals that correspond to only
two isomers in solution. Proton migration is a fast process and
can’t be investigated by NMR spectroscopy: there was only
one significantly broadened symmetrical signal of the oxime
hydrogen observed at ~14 ppm. Interestingly, HQCO monohydrate
crystallizes as a single trans-anti isomer.31
heteroarylcyanoximates in EtOH solutions at 296 K
l
max/nm (e, M-1 cm-1)
Cyanoxime anion
UV regiona
Visible regionb
206 (45480); 269 (7400);
328 (25900)
~430 shoulder
204 (48100); 235 (10000);
321 (21700)
396 (139)
202 (41800); 230 (11150);
326 (26300)
432 (109)
219 (20600); 249 (10000);
342 (18100)
423 (128)
Solution conductivity studies. Electrical conductivity of 1mM
solutions of all studied benz-(2-heteroaryl)cyanoximes in acetone,
DMF and DMSO was measured at 298 K and the results tabulated
in S15 ESI.† Thus, HBIHCO, HBTCO and HBIMCO do not
form conductive and colored solutions in the studied polar aprotic
solvents. Solutions of HBOCO show a small degree of conductivity
in these solvents (S15 ESI†). Solutions of HQCO are significantly
more conductive, which implies a larger degree of autoionization
and confirm the formation of the yellow colored anionic species.
Thus, approximately 5–20% of the ionic species formed in acetone,
DMF and DMSO solutions (S15 ESI†).
207 (14300); 235 (11200);
248 (12100) 306 (8740)
~410 shoulder
a 1 mm quartz cuvette, 5 ¥ 10-4 M concentration. b 1 cm quartz cuvette, 5 ¥
10-3 M concentration.
UV-visible spectra. With the exception of the pale-yellow
HBTCO and the brown HBOCO, other obtained benz-(2-
heteroaryl)cyanoximes HBIMCO, HBIHCO, HQCO are colorless
in the solid state. As mentioned before, the deprotonation of
cyanoximes with bases in aqueous solutions or organic solvents
leads to the formation of yellow/orange colored planar conjugated
anions with a substantial charge delocalization.39 The latter is
reflected in the bathochromic shift of bands in the UV-visible
spectra21 of anions L- as compared to protonated HL species (S16
ESI†). Color in the anionic species originates due to the n→p*
transition in the NO-chromophore,38,40 where the values of the
extinction coefficient range between 10 to 200 M-1 cm-1.21,41 UV-
visible spectra of the studied cyanoxime anions are summarized in
Table 4. As part of the characterization of the new ligand systems,
we investigated both protonated forms HL and anionic species L-
in solution using UV-visible spectroscopy. Positions of the bands in
spectra of all benz-(2-heteroaryl)cyanoxime anions showed solvent
dependence consistent with a negative solvatochromic effect.42 The
solvatochromic series for solutions of the BTCO- anion (as tetra-
butylammonium salt) are displayed in Fig. 3. Two groups of sol-
vents are immediately seen in this plot: polar protic solvents ROH
form one group of curves, while polar aprotic solvents strongly
separated form the latter (Fig. 3). The energy difference between
the two groups is ~9 kcal Mol-1 and consistent with the formation
of hydrogen bonds of medium strength. For all studied benz-(2-
heteroaryl)cyanoxime anions, solvent dependence of the n→p*
band position in visible spectra from pKa of the solvent ROH, or
its specific solvation Z parameter34,43 has linear character (S11, S12
+
ESI†). The typical correlation plots for NBu4 BOCO- in alcohols
are presented in S17 ESI.† Previously we have reported
a significant solvent dependence for other studied amide-41
and aryl-21 cyanoxime ligands. Detailed investigation and
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and Cu+ complexes were found to be useful for analyti-
cal applications.44 All ligands presented in this paper readily
form alkali metal (Na, K, Cs) salts, tetraalkylammonium and
tetraphenylphosphonim salts, Ag(I) and Tl(I) complexes.
Characterization of thallium(I) complexes
Only two stoichiometric thallium(I) complexes containing benz-
(2-heteroaryl)cyanoximes have been structurally characterized to
date: Tl(TLCO)45 and Tl(2PCO),46 in which planar nitroso anions
demonstrate both chelate and bridging binding of metal ions
(Scheme 7, binding mode 1). The crystal structure of Tl(I) benz-
(2-thiazolyl)cyanoxime complex is completely different from the
two compounds published earlier45,46 (Scheme 7; binding mode 2).
Fig. 3 Solvatochromic series for isomolar (5 mM) solutions of BTCO-
anion in different solvents at 296 K (1 cm cuvette).
interpretation of this interesting phenomenon for cyanoxime
anions will be presented in a separate publication.
Fluorescence spectra. Three out of five studied ligands exhibit
strong blue fluorescence in solution (Fig. 4) with BIHCO being the
strongest emitter (S19 ESI†). Signals in the overall spectroscopic
envelope are centered at ~490 nm and possess a fine structure,
which can be fitted to minimum three Gaussian lines of p*→p
transitions (S18 ESI†). The 2-quinolylcyanoxime HQCO exhibits
~5 times weaker fluorescence than HBIHCO, HBOCO and
HBIMCO, while HBTCO does not appreciably fluoresce under
identical conditions. Values of measured emission lifetimes of
HBIHCO, HBOCO and HBIMCO in alcohol solutions were in the
range 3.4–9.2 ns, which is consistent with the typical fluorescence
of conjugated cyclic organic compounds.34,56 Deprotonation of
the benz-(2-heteroaryl)cyanoximes and formation of the nitroso-
anions surprisingly decreases the fluorescence intensity by 2 to
6 times.
Scheme 7
Structure of Tl(BTCO). The molecular structure of this com-
plex represents a centrosymmetric dimer shown in Fig. 5, while
bond lengths and valence angles are displayed in Table 2. The
BTCO- anion is not planar and has two planes in the molecule:
cyanoxime fragment N3–C3–C2–N2–O1 and benzothiazolyl frag-
ment with the value of the dihedral angle between them ~13◦. The
[Tl(BTCO)]2 dimeric units connected into a double-stranded one-
dimensional coordination polymer using the oxime oxygen atoms
as the bridge (Fig. 6; Scheme 7, binding mode 2). Individual
planar Tl2O2 rhombs form a corrugated sheet, two orthogonal
projections of which are shown in Fig. 7. It should be noted that
there are two slightly different rhombs in the structure: #1 belongs
to the dimeric unit [Tl(BTCO)]2, while #2 represents the junction
of dimers into a ladder-type polymer (Fig. 7a,b). The value of the
dihedral angle between these rhombs is 107.36◦; other geometrical
parameters for them are presented in Fig. 7c. Intradimeric Tl–
˚
Tl separation in the structure is the shortest: 4.330 A, while the
˚
Tl–Tl distance inside the one zigzag strand is 4.428 A (Fig. 7c).
These distances are considerably larger than Tl–Tl bonds. All
three Tl–O distances in the structure are shorter than the sum
˚
of ionic radii for these elements (ca. 2.94 A from ref. 42, and
˚
2.88 A from ref. 47,48). The observed double-stranded structural
motif of one-dimensional coordination was not expected for
Fig. 4 Fluorescence spectra of 2.4 mM solutions of 2-(benzheteroaryl)-
cyanoximes in ethanol at 296 K recorded in a 1 cm cuvette using
400–425 nm excitation.
Complexation reactions. Several other known benz-(2-
heteroaryl)cyanoximes form stable coordination compounds with
transition metals in solution, and their intensely colored Fe2+
Fig. 5 Molecular structure and numbering scheme in the [Tl(BTCO)2]
centrosymmetric dimer. H-atoms are omitted for clarity. An ORTEP
drawing at 50% thermal ellipsoids probability level.
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cyanoximes established by X-ray analysis in Tl(I) complexes
are summarized in Table 5 and were correlated with their IR
spectra. Binding modes of benz-(2-heteroaryl)cyanoximes in the
reported Tl(I) complexes were deduced using: a) vibrational
spectra of labeled (50% 15N, oxime group) compounds, and b) pre-
viously established IR-spectroscopic criteria of coordination of the
50
=
>C–N O fragment in metal complexes. Data suggests binding
of cyanoxime ligands to the metal center via the oxygen atom
of the oxime-group in Tl(BOCO), Tl(BTCO) and Tl(QCO),
leaving nitrogen atoms of the heteroaryl groups not involved in
=
coordination. There are two distinctive bands of n(C N) and
n(N–O) vibrations at ca ~1410 and ~1150 cm-1 respectively (S5,
S6 ESI†). This conclusion was proved in the crystal structure
of Tl(BTCO) discussed earlier. The bridging binding mode 2
(Scheme 7) is similar to that reported earlier in structures
of thallium(I) arylcyanoximates,21 which contain n(C N) and
=
n(N–O) bands in their IR spectra in the same region. In
Tl(BIMCO) and Tl(BIHCO), nitrogen atoms of the heterocyclic
fragment are involved in chelation of the metal ion similar to that
found in Tl(TLCO)45 and Tl(2PCO)46 (Table 5 and binding mode
Fig. 6 One-dimensional polymeric motif in the structure of Tl(BTCO).
Hydrogen atoms are omitted for clarity.
=
1, Scheme 7). There are multiple bands of the C N and N–O
vibrations in the range of ~1500–1540 cm-1 and ~1240–1185 cm-1
that are characteristic of both chelate and bridging coordination
of the anions to the metal center. We suggest, therefore, that
the structures of the former contain five-membered metallocycles,
leaving the correctness of such a conclusion to be seen when direct
structural information becomes available.
Results of the UV-visible spectroscopic studies of thallium(I)
complexes in H2O and DMSO solutions indicate the presence of
anionic ligands and the loss of solid state structure. Thus, the UV-
visible and photoluminescence spectra of the TlL in these solvents
are identical to those for CsL (L = BTCO-, QCO-). Values of
electrical conductivity of 1 mM solutions of TlL in DMSO were
in the range 20–30 mX-1 M-1 cm-1 and evidenced in the formation
of 1 : 1 electrolytes.51 The solid state UV-visible absorption spectra
of all TlL indicate the presence of an intense broad band of n→p*
transition similar to that in solution spectra of NBu4 L- and CsL
+
centered at ~ 430 nm (S21 ESI†).
The photoluminescence spectra of solid TlL (as finely ground
powders) were recorded at room temperature in the range 300–
900 nm using 300, 350, 400 and 450 nm excitation wavelengths
(S22 ESI†). Regardless of the excitation wavelength, thallium(I)
benz-(2-heteroaryl) cyanoximates are blue emitters and show
structured bands comprised of multiple, ligand-based transitions
centered at ~470 nm (Fig. 8; S23–S25 ESI†). Interestingly,
solid TlL complexes are luminescent at slightly higher energy
(hypsochromic shift ~20 nm) than HL and NaL in solution
(S18 ESI†). The Tl(BIHCO) is by far the strongest emitter,
which is consistent with the brightest photoluminescence of the
uncomplexed cyanoxime HBIHCO observed in solution. This
superior photoluminescence of 2-benz(imidazolyl) cyanoxime
compared to other studied ligands is due to the significant
contribution of the resonance form of HBIHCO involving fast
prototropic tautomerization. Further investigations of photolu-
minescence (including low temperature measurements) of this
interesting ligand and its metal complexes are warranted for better
understanding of properties of compounds and their possible
practical applications. Blue emitters are in great demand for
new OLED technology (for flat screen display manufacturing)
Fig. 7 Two orthogonal views of the double stranded polymeric sheet
comprised of two different Tl2O2 rhombs in the structure of Tl(BTCO).
A – top view; B – side view; C – details of geometry of the two adjacent
rhombs in the ladder-type structure of Tl(BTCO).
benz-(2-heteroaryl)cyanoximes that are capable of chelate binding
of metal ions.35,37,49
Spectroscopic properties of Tl(I) cyanoximates. Tabulated val-
ues of the frequencies in the IR spectra of synthesized complexes
can be found in S5, S6 ESI†. Binding modes of different
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Table 5 Binding modes of the cyanoxime anions in several Tl(I) com-
plexes established by X-ray analysis
Table 5 (Contd.)
Compound
Tl(TDCO)
Coordination of anion to metal
Ref.
52
Compound
Coordination of anion to metal
Ref.
53
Bridging only
Tl(phen)(ACO)
Tl(2PCO)
Tl(PiCO)
46
Tl(phen)(TDCO)
54
50c
Tl(PiCO)
Tl(ACO)
50c
Tl(TLCO)
Tl(TLCO)
45
45
52
Tl(PiCO)
50c
Commonly used abbreviations for cyanoxime anions (e.g. ACO, PiCO,
TDCO, etc.) correspond to given structural formulas; phen = 1,10-
phenanthroline; 18-crown-6 = crown ether; arylcyanoximes = monohalo-
gen substituted a-oximinoacetonitriles.
TlL (L = BOCO,
This work
BTCO, QCO)
colors to generate a white luminescent background in these
devices.
Concluding remarks
Chelate or chelate and bridging
Tl(18-crown-6)(TDCO)
We report a high-yield heterogeneous synthesis of benz-(2-
heteroaryl)cyanoximes in frozen glacial acetic acid using substi-
tuted acetonitriles and solid NaNO2 as a nitrous acid source. The
five cyanoxime ligands obtained were characterized by means of
IR, 1H, 13C NMR, UV-visible and fluorescence spectroscopy and
X-ray crystallography. Studied benz-(2-heteroaryl) cyanoximes are
planar compounds and adopt trans-anti (HBIMCO·H2O) and
both trans-anti and cis-anti (HBOCO) configurations in the solid
state. The presence of color and geometrical features of the CNO
fragment in the latter compound evidenced its autoionization in
the crystal. Three out of five protonated compounds, namely HBI-
HCO, HBOCO and HBIMCO, exhibit strong room temperature
blue emission centered at ~490 nm with the first compound being
the best emitter. All synthesized benz-(2-heteroaryl)cyanoximes
easily deprotonate in solution and their anions showed
22c
22c
Tl(18-crown-6)(TLCO)
since this color is not easily generated in traditionally used
materials, but is necessary for blending with the other two principal
5726 | Dalton Trans., 2008, 5715–5729
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pronounced negative solvatochromism of the n→p* transitions
indicating a substantial degree of charge transfer and its delocal-
ization throughout the anions. The TlL complexes contain bridged
and bridged/chelate coordinated cyanoxime anions according to
IR spectroscopy and X-ray analysis data. The bridging function
of the anion resulted in an unusual (for these ligands) double-
stranded ladder type structure of one-dimensional coordination
polymer. Tl(I) benz-(2-heteroaryl)cyanoximates demonstrate blue
emission in the solid state at room temperature. High intensity
fluorescence of HBIHCO and Tl(BIHCO) have warranted their
further investigation that will include variable temperature mea-
surements and studies of the efficiency of these systems in OLEDs.
Acknowledgements
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