Structure and phase transitions of azomethine biligand complexes of iron(III) based on 3,4,5-tri(tetradecyloxy)benzoyloxy-4-salicylidene-N′-ethyl-N-ethylenediamine

Article abstract of DOI:10.1134/S0022476616030094

New biligand complexes of iron(III) are synthesized based on 3,4,5-tri(tetradecyloxy)benzoyloxy-4-salicylidene-N′-ethyl-N-ethylenediamine azomethine with the outer sphere NO₃ ^?, PF₆ ^?, Cl^?, BF₄

Full text of DOI:10.1134/S0022476616030094

Journal of Structural Chemistry. Vol. 57, No. 3, pp. 478-490, 2016.
Original Russian Text © 2016 U. V. Chervonova, M. S. Gruzdev, A. M. Kolker, O. B. Akopova.
STRUCTURE AND PHASE TRANSITIONS
OF AZOMETHINE BILIGAND COMPLEXES OF IRON(III)
BASED ON 3,4,5-TRI(TETRADECYLOXY)BENZOYLOXY-
4-SALICYLIDENE-N′-ETHYL-N-ETHYLENEDIAMINE
1 1
U. V. Chervonova , M. S. Gruzdev ,
UDC 54.057:544.17:544.015.4:544.16
1
A. M. Kolker , and O. B. Akopova
2
New biligand complexes of iron(III) are synthesized based on 3,4,5-tri(tetradecyloxy)benzoyloxy-4-
−
−
–
salicylidene-N′-ethyl-N-ethylenediamine azomethine with the outer sphere NO , PF , Cl , BF , ClO ,
−
−
3
6
4
4
–
and CNS anions. All the target compounds are characterized by gel exclusion chromatography, elemental
analysis, and electron, IR, and NMR spectroscopy. The presence of complex-forming ions is confirmed by
FT-IR spectra in the far region. The formation of biligand polychelate complexes with an octahedral
packing of the iron ion is observed. Phase transitions in the resulting coordination compounds are studied
by differential scanning calorimetry and optical polarizing thermomicroscopy. The presence of several
polymorphic crystalline modifications, as well as mesophases, is established. Mesomorphic properties are
found for complexes with chloride and tetrafluoroborate anions.
DOI: 10.1134/S0022476616030094
Keywords: iron(III) complexes, Schiff base, structure, mass spectrometry, mesomorphism.
INTRODUCTION
The development of new types of ligand systems has been a crucial step in the chemistry of metal complexes with
physicochemical properties essential for scientific research and practical applications. The compounds particularly important
as ligands are Schiff bases (azomethines and imines) and their structural analogues, i.e., compounds containing –N=CH
bonds (imino group) [1] because of a wide variability of the resulting ligand structures (depending on the selected aldehyde
and amine), their easy bonding with metals, and their diverse applications [2, 3]. There is a keen interest in studying
dendrimers, i.e., new mesogenic structures with a large internal free volume, which allows one to expand the applications of
liquid crystal materials [4]. In terms of potential applications, the combination of dendrimer properties with the specific
properties of transition metal ions is highly promising. The metal ion in metal dendrimers acts as a structural unit and can be
integrated in different ways: into the dendrimer core [5], as metal complexes that have coordination bonds with the dendrimer
periphery [6], or as periphery units at the branching points of the dendrimer branches [7]. The main areas of research on the
physical properties of metal dendrimers are focused on catalysis [8], light absorption and luminescence properties [9],
electrochemical behavior, sensor, magnetic, and spin dependent properties [10, 11], and nanoobjects in medicine [12]. Here
1
2
Krestov Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, Russia; uch@isc-ras.ru. Ivanovo
State University, Ivanovo, Russia. Translated from Zhurnal Strukturnoi Khimii, Vol. 57, No. 3, pp. 506-519, March-April,
2016. Original article submitted February 12, 2015.
478
0022-4766/16/5703-0478 © 2016 by Pleiades Publishing, Ltd.

we should mention the works where a ferrous iron is coordinated to a nitrogen-containing ligand [13, 14] to obtain liquid
crystal Fe(II) complexes demonstrating spin dependent properties, as a vivid example of polyfunctional materials and
organometallic networks.
The aim of our study was to synthesize a series of metal complexes with a dendrimer periphery, determine their
purity and structure by a number of modern physicochemical analysis techniques, and establish a relatioship between the
structure and the manifested phase transitions and supramolecular structures. Previously, researchers have investigated the
temperature-dependent magnetic properties of iron(III) bis[3,4,5-tri(tetradecyloxy)benzoyloxybenzoyl-4-oxy-salicylidene-N′-
ethyl-N-ethylenediamine]nitrate (1) [15]. They have found that the iron(III) ion in the octahedral coordination node is in
a high-spin state at room temperature. At lower temperatures, the sample becomes magnetically ordered. This situation is
typical of the antiferromagnetic-type ordering in complexes with a dimer supramolecular architecture and the high-spin state
of the central ion [16]. Thus, they showed a solution for the problem of molecular design of coordination compounds in
which liquid crystal phase transitions largely affect the spin-dependent behavior of the central magnetic ion.
EXPERIMENTAL
All the reagents and solvents had the CP grade and were not further purified. The synthesis of 3,4,5-
tri(tetradecyloxy)benzoyloxy-2-hydroxybenzaldehyde was performed according to Scheme 1 by forming an ester bond with
n-hydroxy-salicylic aldehyde, in a similar way as in [17].
Scheme 1. Synthesis of 3,4,5-tri(tetradecyloxy)benzoyloxy-2-hydroxybenzaldehyde.
The target compounds were synthesized according to Scheme 2 in a benzene/ethanol binary solvent [18, 19]; the use
of this solvent enables better dissolution of the initial reagents and a fuller complexation reaction. Azomethine was obtained
Scheme 2. Synthesis of the iron(III) complexes.
479

directly in the solution between 3,4,5-tri(tetradecyloxy)benzoyloxy-2-hydrobenzaldehyde and N′-ethyl-N-ethylenediamine in
the presence of KOH, which accelerates the reaction. The complexation process occurred when the iron(III) salt was added.
The counterion was substituted by adding a double excess of the corresponding potassium or sodium salts.
The synthesized compounds were characterized in terms of purity, individuality, and physicochemical properties by
the methods described below.
The infrared (IR) spectra of the compounds were recorded using a Bruker Vertex 80V device in the range 7500-
–1
–1
1
13
370 cm and 670-190 cm in KBr and CsBr pellets. Н (500.17 MHz) and С (125.76 MHz) NMR spectra were recorded on
a Bruker Avance-500 device, using tetramethylsilane as the internal standard. The elemental analysis of the solid compounds
was performed using a FlashEA 1112 analyzer for C, H, N, and O. Gel exclusion chromatography was performed on
a Shimadzu 10A liquid chromatographer with a two-channel UV detector and a diffractometer, using tetrahydrofuran as an
eluent. The electron absorption spectra (EAS) were recorded on a Cary-100 Varian spectrophotometer in 10-mm thick
cuvettes. The mass spectra were recorded using an Ultraflex III Bruker with a time-of-flight mass analyzer; the spectra were
recorded in a positive-ion mode. The samples were applied onto the target in the following matrices: 2,5-dihydroxy benzoic
acid (DHB) and п-nitroaniline (PNA). The phase transitions in the samples were studied using a Min-8 polarization
microscope equipped with a warm stage and a microphoto attachment. The DSC curves were recorded using a DSC 204 F1
Phoenix differential scanning calorimeter with a μ-sensor (NETZSCH). The scanning rate during heating and cooling was
10 K/min in an argon atmosphere.
Similar synthesis techniques and spectral characteristics of original 3,4,5-tri(tetradecyloxy)benzoyloxy-2-
hydrobenzaldehyde and the complexes are given in Appendix.
APPENDIX
3,4,5-Tris(tetradecyloxy)benzoic acid (a). Weighed portions of tetradecyl bromide (20.22 g) and ethyl ether of
3,4,5-trihydroxybenzoic acid (4.89 g) were dissolved in 200 ml acetone. Then 4% NaOH in С2H5OH was added and boiled
for 8 h. We monitored the reaction using thin layer chromatography. Then the mixture was poured in a 1 M HCl solution, the
solid precipitate was filtered, and the solvent was evaporated. In chromatography Al2O3 was used with diethyl ester as
an eluent. The solvent was distilled off. The yield: 15 g (80%). Found, %: С 76.78, Н 14.52, О 10.63. С49Н90О5.
+
Calculated, %: С 77.52, Н 11.95, О 10.54. MS (MALDI–ToF): m/z: found 781.94; calculated 782.25 [M +Na]. IR spectrum,
–1
ν, cm : 4326, 4250 (m, intermolecular hydrogen bond), 3418 (w, OH vibrations), 3081 (w, aromatic С–H vibrations), 2921-
2850 (s, –CH2 vibrations), 2637-2530 (m, –CH3 vibrations), 1688 (s, С=О vibrations), 1130 (w, –C–O–C–), 989-969 (m,
planar bending C–H vibrations of a 1,3,4,5-substituted aromatic ring), 936 (w, OH vibrations), and 866 (s, nonplanar bending
1
C–H vibrations of a 1,3,4,5-substituted aromatic ring). Н NMR spectrum (СDCl3, TMS) δ, ppm: 0.81 (t, 9Н, CH3–Alk,
J = 7.324 Hz); 1.19 (m, 54Н, –CH2–Alk, J = 44.556 Hz); 1.41 (t, 6Н, Alk–CH2–CH2–O–, J = 6.714 Hz); 1.68 (m, 6Н,
–CH2–Alk, J = 7.324 Hz); 1,75 (m, 6Н, –CH2–Alk, J = 7.935 Hz); 3.95 (m, 6Н, Alk–CH2–O, J = 6.104 Hz); 7.19 (s, 2Н,
13
Ph–H); and 11.98 (s, 1Н, СООН). The С NMR spectrum (СDCl3, TMS) δ, ppm: 14.11 (CH3–), 22.69 (CH3–CH2–), 26.06
(CH3–CH2–CH2–), 29.70 (CH2–Alk), 31.92 (O–CH2–CH2–CH2–), 69.14 (O–CH2–Alk), 73.52 (O–CH2–CH2–), 108.47
(CHarom), 123.58 (Сarom), 143.10 (Сarom), 152.82 (Сarom), and 171.87 (С(О)OH).
3,4,5-Tri(tetradecyloxy)benzoyloxy-2-hydroxybenzaldehyde
(b).
A
weighed
portion
of
3,4,5-
tris(tetradecyloxy)benzoic acid (6 g; 7.9 mmol) was dissolved in chloroform. Then 2,4-dihydrobenzaldehyde (1.09 g;
7.9 mmol) was added and stirred until complete dissolution. Then a weighed portion of dicyclohexylcarbodiimide (DCHCD)
(2.25 g; 10.9 mmol) was added and stirred for 30 min. Then a catalytic amount of dimethylaminopyridine (DMAP) was
added and kept stirring for 12 h. A part of the solvent was distilled off. We used silica gel chromatography with chloroform
as an eluent. The substance was lyophilized from benzene. Yield: 5.6 g (80.6%). Found, %: С 75.93, Н 11.17, О 12.70,
+
С56Н94О7. Calculated, %: С 76.49, Н 10.77, О 12.74. MS (MALDI–ToF): m/z: found 902.33; calculated 902.36 [M +Na]. IR
–1
spectrum, ν, cm : 4329, 4253 (w, intermolecular hydrogen bond), 3433 (w, ОН vibrations), 3091 (w, aromatic CH
vibrations), 2919-2848 (s, –CH2 vibrations), 1727 (s, С=О vibrations), 1617 (s, Ph–OH of salicylic aldehyde), 1386-1274 (s,
Ph–CHO), 1193 (s, Alk–C–O–C(Ph)), 982 (m, planar bending C–H vibrations of a 1,3,4,5-substituted aromatic ring), and 873
480

1
(w, OH vibrations). The Н NMR spectrum (СDCl3, TMS) δ, ppm: 0.81 (t, 9Н, CH3–, J = 6.104 Hz); 1.28-1.19 (m, 54Н,
–CH2–Alk, J = 49.438 Hz); 1.41 (t, 6Н, Alk–CH2–CH2–O–, J = 6.104 Hz); 1.69 (m, 6Н, –CH2–Alk, J = 6.714 Hz); 1.76 (m,
6Н, –CH2–Alk, J = 6.714 Hz); 3.97 (m, 6Н, Alk–CH2–O, J = 6.104 Hz); 6.83-6.79 (d, 2Н, Ph–H, J = 7.935 Hz); 7.19 (s, 1Н,
13
Ph–H); 7.56-7.54 (d, 2Н, Ph–H, J = 8.545 Hz); 9.82 (s, 1Н, ОН–); 11.19 (с, 1Н, СОН). The С NMR spectrum (СDCl3,
TMS) δ, ppm: 14.15 (CH3–), 22.72 (CH3–CH2–), 26.09 (–CH2–), 29.41 (–CH2–), 29.72 (–CH2–), 29.74 (–CH2–), 30.37
(–CH2–), 31.96 (CH3–CH2–CH2–), 69.30 (–O–CH2–Alk), 73.64 (–O–CH2–Alk), 108.64 (CHarom), 110.98 (CHarom), 114.18
(CHarom), 118.69 (Carom), 123.06 (Carom), 134.97 (CHarom), 143.39 (Carom), 153.04 (Carom), 157.82 (Carom), 163.21 (Carom–OH),
164.06 (C(O)O), and 195.52 (CHO).
Bis[3,4,5-tri(tetradecyloxy)benzoyloxybenzoyl-4-oxy-salicylidene-N′-ethyl-N-ethylenediamine]iron(III) nitrate
(1). A weighed portion of 3,4,5-tri(tetradecyloxy)-benzoyloxy-2-hydroxynenzaldehyde (0.9 g) was dissolved in benzene
(6 ml). Then we added N′-ethyl-N-ethylenediamine (0.09 g) dissolved in ethyl alcohol (10 ml), stirred for 5 min, and added
an alcohol solution of KOH (0.113 g, 10 ml). A Fe(NO3)3⋅9H2O solution (0.21 g) in ethanol was added slowly, dropwise, to
the mixture. The resulting mixture was stirred for 2 h, filtered with a fine glass filter, and washed with ethyl alcohol. The
product was reprecipitated from a mixture of dried benzene–ethanol (1/6) solvents with the subsequent lyophilization from
benzene. The product was a fine dark brown powder. Yield: 1.02 g. Found, %: C 70.71, H 11.44, N 3.29, O 10.83.
–1
C120H206N4O12FeNO3. Calculated, %: C 71.53, H 10.30, N 3.47, O 11.91. IR spectrum, ν, cm : 3182.81 (w, aromatic,
−
3
stretching С–H vibrations), 2927.26, 2852.52 (s, –(CH2)n–CH3), 1734.71 (s, С=О), 1635.46 (s, C=N), 1384.23 (s, NO
vibrations), 1193.75, 1120.03 (s, Alk–C–O–C(Ph)), 990.41 (m, NH vibrations), 864.73, 824.91 (s, symmetric vibrations of
the 1,4-disubstituted aromatic ring).
Bis[3,4,5-tri(tetradecyloxy)benzoyloxybenzoyl-4-oxy-salicylidene-N′-ethyl-N-ethylenediamine]iron(III)
hexafluorophosphate (2). The synthesis was similar to that of complex (1). In 15 min after adding an alcohol solution of
Fe(NO3)3⋅9H2O (0.21 g), we added a weighed portion of KPF6 (0.37 g) dissolved in ethanol with several drops of water. The
synthesis continued for 4 h. The precipitate was filtered with a glass filter and reprecipitated from the mixture of dried
benzene–ethanol solvents (1/6) with the subsequent lyophilization from benzene. The product was a fine solid dark brown
powder. Yield: 1.11 g. Found, %: C 69.04, H 9.60, N 3.09, O 9.87. C120H206N4O12FePF6. Calculated, %: C 68.70, H 9.89,
–1
N 2.67, O 9.15. The IR spectrum, ν, cm : 3209.91, 3119.28 (w, aromatic, stretching С–H vibrations), 2916.70, 2849.18 (s,
–(CH2)n–CH3), 1736.78 (s, С=О), 1626.17 (s, C=N), 1192.36, 1121.72 (s, Alk–C–O–C(Ph)), 989.12 (m, NH vibrations), and
−
845.18 (w, PF vibrations).
6
Bis[3,4,5-tri(tetradecyloxy)benzoyloxybenzoyl-4-oxy-salicylidene-N′-ethyl-N-ethylenediamine]iron(III)
chloride (3). The synthesis was similar to that of complex (1). After adding an alcohol solution of KOH (0.113 g, 10 ml), we
added slowly, dropwise, a solution of FeCl3 (0.083 g) in ethanol, stirred for 2 h, filtered with a glass filter, washed with ethyl
alcohol, and reprecipitated from a mixture of dried benzene–ethanol (1/6) solvents with the subsequent lyophilization from
benzene. The product was a fine solid dark brown powder. Yield: 0.89 g. Found, %: C 71.58, H 10.79, N 2.32, O 9.60.
–1
C120H206N4O12FeCl. Calculated, %: C 72.49, H 10.44, N 2.82, O 9.66. IR spectrum, ν, cm : 3078.75 (w, aromatic, stretching
С–H vibrations), 2920.26, 2852.29 (s, –(CH2)n–CH3), 1730.12 (s, С=О), 1630.16 (s, C=N), 1191.69, 1118.42 (s, Alk–C–O–
–
C(Ph)), 988.05 (s, NH-vibrations), and 544.60 (w, Cl vibrations).
Bis[3,4,5-tri(tetradecyloxy)benzoyloxybenzoyl-4-oxy-salicylidene-N′-ethyl-N-ethylenediamine]iron(III)
tetrafluoroborate (4). The synthesis was similar to that of complex (1). In15 min after adding an alcohol solution of
Fe(NO3)3⋅9H2O (0.21 g), we added a weighed portion of NaBF4 (0.22 g) dissolved in several drops of water. The reaction
continued for 6 h, and the reaction mass was kept for 12 h in a refrigerator. The solution was filtered with a glass filter,
washed with ethyl alcohol, and reprecipitated from a mixture of dried benzene–ethanol (1/6) solvents with the subsequent
lyophilization from benzene. The product was a fine solid dark brown powder. Yield:1.03 g. Found, %: C 71.04, H 9.72,
–1
N 3.15, O 11.01. C120H206N4O12FeBF4. Calculated, %: C 70.66, H 10.18, N 2.75, O 9.41. IR spectrum, ν, cm : 3182.81,
3064.19 (w, aromatic, stretching С–H vibrations), 2921.59, 2848.29 (s, –(CH2)n–CH3), 1716.79 (s, С=О), 1624.83 (m, C=N),
−
1213.02, 1190.36, 1111.73 (s, Alk–C–O–C(Ph)), 1033.09 (s, BF vibrations), and 987.78 (m, NH vibrations).
4
481

Bis[3,4,5-tri(tetradecyloxy)benzoyloxybenzoyl-4-oxy-salicylidene-N′-ethyl-N-ethylenediamine]iron(III)
perchlorate (5). The synthesis was similar to that of complex (1). In 15 min after adding an alcohol solution of
Fe(NO3)3⋅9H2O (0.21 g), we added a weighed portion of NaClO4 (0.249 g) dissolved in ethanol. The synthesis continued for
4 h. The precipitate was filtered with a glass filter, washed with ethyl alcohol, reprecipitated from a mixture of dried
benzene–ethanol (1/6) solvents with the subsequent lyophilization from benzene. The product was a fine solid dark brown
powder. Yield: 1.05 g. Found, %: C 69.86, H 9.50, N 2.14, O 13.67. C120H206N4O12FeClO4. Calculated, %: C 70.29, H 10.03,
–1
N 2.73, O 12.48. IR spectrum, ν, cm : 3200.79 (m, aromatic, stretching С–H vibrations), 2917.81, 2852.51 (s, –(CH2)n–
−
4
CH3), 1733.01 (s, С=О), 1628.83 (s, C=N), 1193.47 (s, Alk–C–O–C(Ph)), 1115.73 (s, ClO vibrations), and 989.78 (m, NH
vibrations).
Bis[3,4,5-tri(tetradecyloxy)benzoyloxybenzoyl-4-oxy-salicylidene-N′-ethyl-N-ethylenediamine]iron(III)
thiocyanate (6). The synthesis was similar to that of complex (3). In 10 min after adding an alcohol solution of FeCl3
(0.053 g), we added a weighed portion of KCNS (0.129 g) dissolved in ethanol. The synthesis continued for 4 h. Then we
kept the reaction mass in a refrigerator for one day. The precipitate was filtered with a glass filter, washed with ethyl alcohol,
reprecipitated from a mixture of dried benzene–ethanol (1/6) solvents with the subsequent lyophilization from benzene. The
product was a fine solid dark brown powder. Yield: 0.59 g. Found, %: C 71.43, H 9.78, N 2.99, O 9.01, S 2.03.
–1
120H206N4O12FeCNS. Calculated, %: C 72.27, H 10.32, N 3.48, O 9.55, S 1.59. IR spectrum, ν, cm : 3067.56 (w, aromatic,
C
–
stretching С–H vibrations), 2920.16, 2849.25 (s, –(CH2)n–CH3), 2044.15 (s, CNS vibrations), 1714.61 (s, С=О), 1585.29 (s,
C=N), 1214.03, 1108.35 (s, Alk–C–O–C(Ph)), and 997.11 (m, NH vibrations).
The preparation of single crystals from concentrated solutions of the synthesized compounds did not give the desired
result, which was likely due to the presence of a large number of long alkyl chains.
RESULTS AND DISCUSSION
In order to confirm the structure and purity of the synthesized oligomeric compounds, we conducted
a chromatographic analysis and recorded the electron, IR, NMR, and mass spectra of the samples.
UV spectroscopy. Since the complexation process involves the sedimentation of the desired product in the form of
a fine powder which may be contaminated with the original iron-containing salt, it was necessary to determine the purity of
the chelate compounds. The literature describes applications of electron spectroscopy to identify additions of original
substances in the target compound [20]. The electron absorption spectra were recorded in a dichloromethane solution. As
seen from the electron spectrum of (1) and its comparison with the original salt Fe(NO3)3⋅9H2O, the resulting compound has
no traces of unreacted salts after the sedimentation and additional lyophilization (Fig. 1).
The analysis of the experimental data allows us to conclude that there are no unreacted Fe(NO3)3⋅9H2O and FeCl3
salts. The spectra of all the complexes, except (6), demonstrate four bands. The high-energy band at 227-228 nm is
Fig. 1. UV spectra of complex 1 and Fe(NO3)3⋅9H2O.
482

TABLE 1. Positions of the Maxima of the Electron Absorption Bands and Relative Absorption Coefficients (in parentheses)
at the Band Maxima for Complexes 1-6 and the Original Salts
No.
Compound
No.
Compound
λ1, nm λ2, nm λ3, nm λ4, nm
λ1, nm λ2, nm λ3, nm λ4, nm
1
C120H206N4O12FeNO3
227
(1.19) (0.92) (0.28) (0.17)
227 275 349 393
(1.41) (1.09) (0.29) (0.14)
228 271 350 393
(2.04) (1.83) (0.38) (0.23)
227 270 351 392
(0.83) (0.74) (0.14) (0.08)
273 350 391
5
C120H206N4O12FeClO4
228
274
348
394
(1.62) (1.26) (0.34) (0.18)
2
3
4
C120H206N4O12FePF6
C120H206N4O12FeCl
C120H206N4O12FeBF4
6
C120H206N4O12FeCNS
228
(0.64) (0.74)
240 293
(1.37) (0.81)
249 329
268
–
–
–
–
–
Fe(NO3)3⋅9H2O
FeCl3
361
(2.11) (1.51) (1.75)
responsible for the excitation of π electrons (π–π* transitions) of the aromatic rings. The band at 273-268 nm corresponds to
the intramolecular n–π* transitions of the conjugated aromatic systems. The band at 349-351 nm is associated with the π–π*
transition of the chromorph of the Schiff base (CH=N) [21, 22]. The weak band at 391-394 nm characterizes the
intramolecular energy transfer from р electrons of the donor atomic π orbitals of the ligand to the d metal orbitals [23, 24].
The characteristic bands of the transitions are given in Table 1.
Chromatographic analysis. The chemical purity and homogeneity of metal chelates 1-6 were also analyzed by gel
exclusion chromatography; the samples were eluted with dry tetrahydrofuran. 1,2-dichlorobenzene was used as the standard.
Fig. 2 shows, as an example, the chromatograms of the complexes with tetrafluoroborate 4 and thiocyanate 6 ions. The
eluting time is τ2.
The chromatograms of the coordination compounds have low-intensity peaks (τ1) typical of aggregates with a high
molecular mass [25, 26]. The parameters of the chromatographic distribution of compounds 1-6 are given in Table 2.
A substantial contribution to the polydispersity of the sample comes from the impact of the counterion. The higher
the hydrogen-bonding capacity of the counterion, the higher the polydispersity index of the compound, which may explain
Fig. 2. Chromatograms of complexes 4 and 6.
TABLE 2. Parameters of the Chromatographic Distribution of Compounds 1-6
No.
Compound
М
P
P
z
τ2st
τ2end
i
1
2
3
4
5
6
C120H206N4O12FeNO3
C120H206N4O12FePF6
C120H206N4O12FeCl
C120H206N4O12FeBF4
C120H206N4O12FeClO4
C120H206N4O12FeCNS
2014.93
2097.90
1988.43
2039.75
2050.37
2011.02
8.69
8.71
8.65
8.65
8.74
8.71
9.19
9.40
9.42
9.36
9.42
9.31
1.038
1.040
1.047
1.025
1.038
1.026
1.037
1.035
1.039
1.024
1.033
1.024
P is the polydispersity index; P is the ratio between the retained volume V and the molecular mass M.
i z r
483

Fig. 4. IR spectra of the complexes at 7000-
Fig. 3. Dependence of the output time τ2outp of the
complex on molecular mass (a); dependence of the
–1
4000 cm .
polydispersity index P on the molecular mass of the
i
complex (b).
the nature of the formation of aggregates (τ1) by the complexes. This is confirmed by the mass spectrometry and IR
spectroscopy data. The linear dependence of τ2outp on the molecular weight indicates the identity of the entire homological
series to the one being analyzed (Fig. 3).
1
NMR spectroscopy. The H NMR spectra of the complexes have two series of broadened signals in weak (7-8 pbw)
and strong (0.5-4 pbw) fields, which gives evidence of the paramagnetic nature of the samples, i.e., the effect of the metal ion
on the proton vibrations in the azomethine molecule. Such a dramatic shift, i.e., a broadening in the absence of signal
multiplicity in the NMR spectra of coordination compounds with paramagnetic metal ions, is typical and consistent with the
data in [27].
IR spectroscopy. The IR spectroscopy studies of the samples revealed the presence of a Schiff base and coordinated
–1 –1
Fe(III) ion in the structure of the complexes. The bands at 5850-5839 cm and 5709-5703 cm correspond to the vibrations
of two amines: tertiary and secondary respectively (Fig. 4) [28].
–1 –1
The range 3400-2600 cm is dominated by the 2924-2831 cm bands associated with the stretching vibrations of
–1
the alkyl and methyl fragments. The νH–Ph proton vibrations in the aromatic rings at 3226-3185 cm have a low intensity and
are substantially broadened due to the screening of the alkyl chain vibrations. The formation of the–CH=N– azomethine bond
–1
is evidenced by the vibrational band at ∼1625 cm , which is located near the vibrational band of the carboxyl group C=O
–1
(1736-1732 cm ), Fig. 5 [29, 30].
–1
In the series of complexes 1 to 6, there is a shift and splitting of the C=O group vibrtional band from 1736-1732 cm
–1 –1
to 1716 cm and 1706 cm . These band positions may be attributed to the interaction between the cation part of the complex
–1
and the anion. The vibrational band of the CH=N azomethine bond (1635-1625 cm ) is observed to attenuate. The bands at
–1
1583 cm and 1539 cm demonstrate a symmetric coordination of the Fe ions with two ligand molecules, suggesting that
–1
3+
the metal is coordinated via the nitrogen atoms of the azomethine group [31-33]. The fact that the spectra of the complexes
–1
have no vibrational band associated with the Ph–OH fragment of salicylic aldehyde at 1617 cm indicates that the metal is
–
coordinated via the deprotonated phenol group (Ph–O ) located near the azomethine moiety.
The IR spectra of each complex are observed to have absorption bands typical of the vibration of the counterions
–1
–1
–1
(Fig. 6). These are 825 cm and a broadened signal at ∼1384 cm for the nitrate ion (1), an intense signal at ∼846 cm and
–1
558 cm for the hexafluorophosphate ion (2), ∼544 cm for the chloride ion (3), ∼1033 cm , 539 cm , and 519 cm for the
–1
–1
–1
–1
–1
–1
–1
tetrafluoroborate ion (4), ∼1116 cm and 626 cm for the perchlorate ion (5), and ∼2044 cm for the thiocyanate ion (6)
[34, 35].
484

–1
Fig. 5. IR spectra of the complexes at 1800-1500 cm .
–1
Fig. 6. IR spectra of the complexes at 1500-500 cm .
TABLE 3. Positions of the Maxima of the IR Absorption Bands for Stretching Vibrations of Fe–N and Fe–O Bonds
in Complexes 1-6 in the Far Region
Compound
Fe–N
Fe–O
Compound
Fe–N
Fe–O
C120H206N4O12FeNO3 (1)
C120H206N4O12FePF6 (2)
C120H206N4O12FeCl (3)
541
541
546
419
419
420
C120H206N4O12FeBF4 (4)
C120H206N4O12FeClO4 (5)
C120H206N4O12FeCNS (6)
– *
542
541
419
419
417
* Superimposition of the more intense absorption band of the BF4 counterion.
–1
Vibrations of the bonds of the coordinated Fe(III) ion were recorded in the far region (690-170 cm ) of the IR
spectrum (Table 3) obtained in CsBr pellets [36, 37].
Mass spectrometry. The stability of the complexes and the presence of iron in their structure is confirmed by the
mass spectrometry data [38, 39]. This method is known to be most efficient in determining the masses of heavy ions.
Therefore, it is of interest to apply the method of time-of-flight mass spectrometry in combination with the results of other
physicochemical methods to determine the coordination sphere of the metals. When recorded in the mode of positively
charged particles, the mass spectra of the complexes contain a series of stable ions characterizing the azomethine moiety
+
∼948 a. u. [L] and iron in its nearest environment with the composition “two ligands–iron”; i.e., there is formation of
+
biligand systems [2L·Fe] with a mass of 1949-1952 a. u. The weak interaction of the outer sphere anion with the inner
+
–
coordination sphere is manifested in the absence of stable molecular ions of the composition [2L·Fe] X [40]. For all the
+
compounds, their mass spectra clearly indicate the presence of fragment ions of the composition 972 a.u. [L·Na] and
+
+
1004 a.u. [L·Fe] , which are derived from the decay of the biligand system [2·Fe] . It should be noted that the stability and
+
intensity of the fragment ions relative to the fragment 948 [L ] is rather high: up to 54%, 10%, and 7%, respectively.
Based on these data, we built a structural model of the complexes, which is given, in a general form, in Scheme 3.
–
−
−
–
−
−
–
Scheme 3. Model of the biligand iron(III) complex, X = NO , PF , Cl , BF , ClO , CNS .
3
6
4
4
485

Fig. 7. Fragment of the mass spectrum of complex 5, which suggests the formation of clathrate ions (a);
fragment of the mass spectrum of complex 2, which suggests the formation of a dimer (b).
The mass spectra show the presence of fairly intense clathrate ions with a molecular mass of ∼2130, 2158, and 2364
+
of the composition [2L·Fe] + matrix (sinapinic acid) residue (Fig. 7a) [41] and dimer structures, which is evidenced by the
presence of ions with a molecular mass of ∼4000-4200 in the mass spectrum (Fig. 7b).
The dimerization process is also evidenced by peaks in the gel exclusion chromatograms τ1 (Fig. 2), which show the
output of high-molecular compounds (dimers), and a vibration band associated with strong intermolecular interactions in the
–1
near region of the IR spectra (4350-4250 cm ), which presumably correspond to the interaction of the counterion with the
azomethine proton. Scheme 4 shows a tentative aggregation model for the complex with the tetrafluoroborate anion 4.
Scheme 4. Formation of the dimer of complex 4.
Phase transitions in the complexes. The temperature stability of complexes 1-6 was investigated from the data
obtained by differential scanning calorimetry (DSC) and optical polarization thermomicroscopy. The DSC data on the
temperatures and enthalpies of the phase transitions are summarized in Table 4.
According to the DSC data, complex 1 reveals several reversible solid–solid endothermic phase transitions (Fig. 8).
At negative temperatures, the compound is in the glassy state which changes to the solid-phase state with increasing
temperature (Tmelt1 = 41.14°С) with a subsequent transition at Tmelt2 = 49.52°С. The phase transition at a temperature of
486

TABLE 4. Phase Transition Temperatures and Changes in the Heat Capacity ΔCp and Enthalpy ΔH for Complexes 1-6
in the Heating Cycle
No.
Compound
Tg, °C ΔCp, J/g·K
T
ph1, °C
ΔH, J/g
T
ph2, °C
ΔH, J/g
T
ph3, °C
ΔH, J/g
1
2
3
4
5
C120H206N4O12FeNO3
C120H206N4O12FePF6
C120H206N4O12FeCl
C120H206N4O12FeBF4
C120H206N4O12FeClO4
–53.12
–
0.58
–
41.14
10.53
100.42
48.15
46.02
12.46*
1.80
49.52
40.54
121.34
–
12.46*
22.58
0.89
–
144.23
17.42
139.89
4.42
6.46
–
0.37
–
46.49
76.39
26.20
–
–
–
–
–
–
–54.29
22.36
9.24
0.31
0.17
0.68
123.10
1.20
6
C120H206N4O12FeCNS
41.44
107.09*
46.73
107.09*
–
–
* Total area of the peak under the curve.
melt3 = 144.23°С can be regarded as the transition of the substance into a melt. The thermopolarization microscopy data
T
confirm the conclusions derived from the DSC experiment, namely: at 25°С the substance is in the solid state and at 54°С
there is a transition from one crystal form into another, suggesting the presence of a solid-phase transition.
The thermomicroscopic studies of complex 2 show that at temperatures above 50°С, the sample melts and no
anisotropy is observed. The crystals are partly retained in the isotropic field up to a temperature of 187°С and undergo
visually observable changes at about 140°С.
For complex 3, the thermomicroscopy observations at a temperature of 41°С reveal the formation of a presumably
cubic mesophase (Fig. 9) with a texture and behavior similar to that described in [42].
When heated to T = 45°С, the sample changes to the isotropic state whose transition temperature differs
substantially from the DSC data (Fig. 9a), which presumably suggests the formation of a cubic mesophase, since it usually
has no peaks in the DSC analysis curves [43]. At T = 101.9°С, the sample becomes lighter in color and the liquid becomes
more viscous than that in the isotropic state; the pressing on the sample leads to birefringence. The further heating to
T = 144.0°С leads to increased fluidity, and the sample passes into the isotropic state. The gradual cooling of the sample to
room temperature leads to the formation of finger-type domains in the glassy state (Fig. 10). The presence of these domains is
typical of discotic mesogens with a hexagonal mesophase structure [44]. It is known that liquid crystals (LCs) are
polymorphous. For calamitic LCs, there are examples where the substances are observed to have absolutely different phase
transitions when heated and cooled [45]. There are similar examples for discotic LCs and for the related compounds –
phasmids.
Fig. 8. DSC curves for complex 1.
487

Fig. 9. DSC curves of complex 3 (a); crystals+cubic phase texture, heating cycle, T = 41,5 °C, nicols crossed,
magnification 100 (b).
Fig. 10. Finger-type domains in the texture of
complex 3, cooling cycle, T = 25 °C, nicols
parallel, magnification 200.
In the heating cycle, complex 4 melts at a temperature of 48°С; on cooling, the sample changes from an isotropic
melt to a glassy mesophase (Fig. 11) and then gradually crystallizes.
−
For complex 5 with the ClO counterion there is a distinct solid–solid phase transition at a temperature of 46.02°С
4
(Table 4). The two preceding vitrification stages at Т = –54.29°С and Т = 22.36°С suggest that the compound is in
Fig. 11. DSC curves of complex 4 (a); vitrified mesophase texture, cooling cycle, T = 25 °C, nicols parallel,
magnification ~200 (b).
488

a metastable glassy state, i.e., is amorphous. The melting occurs at a temperature of 123.10°C. In the cooling cycle, the
sample crystallizes at a temperature of –2.57°С. The conclusions based on the DSC data are confirmed by thermopolarization
microscopy. Thus, the substance melts at 120°С and remains, for a long time, in a stable melted state (mesotropic). We were
able to conduct forced crystallization of the sample when it was shifted and the temperature was T = 60°С, which suggests
substantial supercooling of the sample and is evidence of the purity of the obtained complex.
The thermomicroscopic observations of complex 6 show that the substance melts into a brown melt at T = 46°С and
crystallizes into needle-like crystals when cooled.
CONCLUSIONS
We obtained six new biligand complexes of Fe(III) based on 3,4,5-tri(tetradecyloxy)benzoyloxy-4-salicydene-N′-
−
−
–
ethyl-N-ethylenediamine with the NO (1), PF (2), Cl (3), BF (4), ClO (5), and CNS (6) counterions. All the target
−
−
–
3
6
4
4
compounds were characterized by a range of physicochemical methods of analysis. The presence of a complexing ion was
confirmed by FT-IR spectra in the far region. The structure of the compounds was confirmed by mass spectrometric studies
and elemental analysis. The formation of biligand complexes with the octahedral packing of the iron ion was observed. The
study of the phase transformation in the obtained compounds revealed liquid crystal properties for the complexes with
chloride and tetrafluoroborate anions. It is of interest to further examine the effect of the nature of the complexing metal on
the possibility of formation of a mesophase in complexes with the same set (series) of counterions.
The spectral studies and differential scanning calorimetry used the equipment of the Upper Volga Regional Center
for Physicochemical Research (Shared Resource Center).
This work was supported by the President of the Russian Federation (project No. MK-70.2014.3), RFBR (project
No. 14-03-31280_mol_a), and Ministry of Science and Education of the Russian Federation (project No. 4.106.2014K).
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