Transition metal complexes with thiosemicarbazide-based ligands. Part 63. Syntheses, structures and physicochemical characterization of the first chromium(III) complexes with pyridoxal semi- and thiosemicarbazones

Article abstract of DOI:10.1016/j.poly.2015.09.017

With pyridoxal semi-/thiosemicarbazones (PLSC/PLTSC) ligands for the first time chromium complexes were obtained. In the reaction of ethanolic solution of Cr(NO₃)₃ and K₃[Cr(NCS)₆] and the ligands in mole ratio 1:1 or 1:2, the following complexes were formed: [Cr(PLSC)(PLSC-H)](NO₃)₂·H₂O (1), K[Cr(PLSC-H)(NCS)₃]·EtOH (2), [Cr(PLTSC)(PLTSC-H)](NO₃)₂·2H₂O (3), [Cr(PLTSC)₂](NO₃)₃ (4) and [Cr(PLTSC)(NCS)₃]·2H₂O (5). All the complexes have mer-octahedral structure which in the cases of the complexes 2, 4 and 5 was proved by single-crystal X-ray diffraction analysis. The Schiff bases coordinate in the usual tridentate ONX manner (X = O/S; PLSC/PLTSC). The pyridoxalic fragment is zwitter ion regardless of the form of the coordinated ligands: neutral (keto/thion) and monoanionic (enolic/thiolic). In addition to the above complexes, X-ray crystallography was used to characterize neutral and protonated forms of PLSC, i.e., PLSC·2H₂O and PLSC·HNCS, the latter one being obtained as a by-product of the reaction of formation of the complex 2 using K₃[Cr(NCS)₆] and PLSC in the mole ratio 1:2. The X-ray analyses of these ligand forms have shown that in the case of PLSC·HNCS the ligators O_phenolic, N_azomethine, O_keto due to strong hydrogen O2-HN3 bond are placed in cis-position to each other (pro-binding conformation) which is not the case with PLSC·2H₂O. The compounds are characterized thoroughly by also IR/UV-Vis spectral analyses, electrochemical and thermal methods.

Full text of DOI:10.1016/j.poly.2015.09.017

Accepted Manuscript
Transition metal complexes with thiosemicarbazide-based ligands. Part 63.
Syntheses, structures and physicochemical characterization of the first chromi-
um(III) complexes with pyridoxal semi- and thiosemicarbazones
Ljiljana S. Vojinović-Ješić, Ljiljana S. Jovanović, Vukadin M. Leovac, Mirjana
M. Radanović, Marko V. Rodić, Berta Barta Holló, Katalin Mészásaros
Szécsényi, Sonja A. Ivković
PII:
S0277-5387(15)00519-7
DOI:
http://dx.doi.org/10.1016/j.poly.2015.09.017
POLY 11531
Reference:
Polyhedron
To appear in:
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Accepted Date:
1 July 2015
5 September 2015
Please cite this article as: L.S. Vojinović-Ješić, L.S. Jovanović, V.M. Leovac, M.M. Radanović, M.V. Rodić, B.B.
Holló, K.M. Szécsényi, S.A. Ivković, Transition metal complexes with thiosemicarbazide-based ligands. Part 63.
Syntheses, structures and physicochemical characterization of the first chromium(III) complexes with pyridoxal
semi- and thiosemicarbazones, Polyhedron (2015), doi: http://dx.doi.org/10.1016/j.poly.2015.09.017
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Transition metal complexes with thiosemicarbazide-based ligands. Part 63.
Syntheses, structures and physicochemical characterization of the first
chromium(III) complexes with pyridoxal semi- and thiosemicarbazones
Ljiljana S. Vojinović-Ješića, Ljiljana S. Jovanovića, Vukadin M. Leovaca*, Mirjana M.
Radanovića, Marko V. Rodića, Berta Barta Hollóa, Katalin Mészásaros Szécsényia, Sonja A.
Ivkovićb
a
Faculty of Sciences, University of Novi Sad, Trg Dositeja Obradovića 3, 21000 Novi Sad,
Serbia
b
Faculty of Environmental Protection, University EDUCONS, V. Putnika bb, 21208 Sremska
Kamenica, Serbia
Abstract
With pyridoxal semi-/thiosemicarbazones (PLSC/PLTSC) ligands for the first time chromium
complexes were obtained. In the reaction of ethanolic solution of Cr(NO3)3 and K3[Cr(NCS)6]
and the ligands in mole ratio 1:1 or 1:2, the following complexes were formed:
[Cr(PLSC)(PLSC–H)](NO3)2·H2O (1), K[Cr(PLSC–H)(NCS)3]·EtOH (2), [Cr(PLTSC)(PLTSC–
H)](NO3)2·2H2O (3), [Cr(PLTSC)2](NO3)3 (4) and [Cr(PLTSC)(NCS)3]·2H2O (5). All the
complexes have mer-octahedral structure which in the cases of the complexes 2, 4 and 5 was
proved by single-crystal X-ray diffraction analysis. The Schiff bases coordinate in the usual
tridentate ONX manner (X=O/S; PLSC/PLTSC). The pyridoxalic fragment is zwitter ion
regardless of the form of the coordinated ligands: neutral (keto/thion) and monoanionic
(enolic/thiolic). In addition to the above complexes, X-ray crystallography was used to
characterize neutral and protonated forms of PLSC, i.e. PLSC·2H2O and PLSC·HNCS, the latter
one being obtained as a by-product of the reaction of formation of the complex 2 using
K3[Cr(NCS)6] and PLSC in the mole ratio 1:2. The X-ray analyses of these ligand forms have
shown that in the case of PLSC·HNCS the ligators Ophenolic, Nazomethine, Oketo due to strong
hydrogen O2-H···N3 bond are placed in cis-position to each other (pro-binding conformation)
which is not the case with PLSC·2H2O. The compounds are characterized thoroughly by also
IR/UV–Vis spectral analyses, electrochemical and thermal methods.
Keywords
pyridoxal semi-/thiosemicarbazone, chromium(III) complexes, crystal structure, spectra, thermal
decomposition, electrochemistry
* Corresponding author. E-mail address: vukadin.leovac@dh.uns.ac.rs (V.M. Leovac)
1

1. Introduction
Chromium has an important role in metabolism of two primary macronutrients—
carbohydrates and lipides [1], but is also one of the most controversal transition metals in the
view of its biological activity [2]. The latter is due to the toxicity of Cr(VI), which exists in the
form of oxyanion, and as such can easily enter the cells and lead to formation of reactive oxygen
species [3]. In contrast, Cr(III) appeared to be more stable and less cytotoxic to cultured human
cells [4], while the insufficiency of Cr(III) in organism causes diabetes and some cardiovascular
diseases [1].
On the other hand, semi- and thiosemicarbazones and their metal complexes are
interesting compounds not only in theoretical view [5–8], but due to the possibility of their
practical application as biologically active molecules [9, 10], analytical reagents [11, 12], etc.
Among them, semi- and thiosemicarbazones of pyridoxal (PLSC and PLTSC, respectively),
which is one of the forms of vitamin B6, are particularly interesting. These ligands and their
complexes stand out in many ways: biological activity [13–15], and interesting and diverse
physicochemical and structural characteristics [13, 16–19]. Since there are no complexes of
Cr(III) with these Schiff bases and considering the biological importance of both the Cr(III) and
ligands, it is of considerable interest to synthesize and characterize their complexes.
2. Experimental
2.1. Materials and physical measurements
All commercially obtained reagent-grade chemicals were used without further
purification. The ligand PLTSC was prepared according to the previously described procedure
[19], while the single crystals of the ligand PLSC·2H2O were obtained in a slightly modified
procedure compared with the one described in [20], since the previously known one have not
yielded in formation of single crystals.
Elemental analyses (C, H, N and S) of air-dried compounds were carried out by standard
micro-methods in the Center for Instrumental Analyses, ICTN in Belgrade. Molar conductivity
measurements of freshly prepared 1 mM solutions were performed on a Jenway 4010
conductivity meter. IR spectra were recorded on a Nicolet Nexus 670 FTIR (Thermo Scientific)
spectrophotometer, in the range of 400–4000 cm−1 by KBr pellet technique. Electronic spectra of
DMF solutions of the ligand and complexes were recorded on a T80+ UV/Vis Spectrometer (PG
Instruments, Ltd.), in the spectral range of 270–1000 nm. Cyclic voltammetric experiments were
carried out on a VOLTALAB PST050 with a GC-disk (diameter 3 mm) working electrode, Pt
wire counter electrode and a reference saturated calomel electrode (SCE). All potentials are
reported against this electrode. The measurements were carried out in analytical grade DMF
solvent which was double distilled after prior drying on molecular sieves. Supporting electrolytes
were 0.1 M tetrabutylammonium perchlorate (TBAP) or LiClO4. Experiments were carried out in
an inert atmosphere provided by purging nitrogen. Thermal data were recorded in the
2

temperature range to 600 °C using TA Instruments’ Q600 SDT Thermal Analyser coupled online
with Hiden HAL RC 301 quadrupole mass spectrometer. Selected ions between m/z = 1–100
were monitored in multiple ion detection mode (MID). Measuring parameters: sample masses ~
5 mg; heating rate 10 °C min–1; argon atmosphere (flow rate 50 cm3 min–1); alumina sample and
empty alumina reference pan. Simultaneous thermogravimetric (TG) and differential scanning
calorimetric (DSC) measurements were carried out under similar conditions but at a heating rate
of 20 °C min–1 in flowing nitrogen. Selected samples were tested in air atmosphere, too.
2.2. Syntheses
2.2.1. PLSC·2H2O
Semicarbazide hydrogenchloride (1.07 g, 10 mmol) was dissolved in warm H2O (10 mL),
to which a warm solution of pyridoxal hydrogenchloride (2.00 g, 10 mmol) in H2O (10 mL) was
added. To this solution Na2CO3·10H2O (3.00 g, 15 mmol) dissolved in H2O (10 mL) was added
in portions, and the mixture was mildly heated for a couple of minutes. The obtained solution
was left at room temperature for about 20 h, after which the single crystals were filtered and
washed with H2O. Yield: 2.30 g (88%). Anal. Calc. for C9H16N4O5: C, 41.54; H, 6.20; N, 21.53.
−1
ꢀ
Found: C, 41.43; H, 6.16; N, 21.50%. Selected IR bands [ /cm ]: 3466, 3381, ν(OH), ν(NH );
ν
2
3202, ν(NH); 2850, ν(NH+); 1697, 1678, ν(C=O); 1583, ν(CN); 1280, ν(COphenolic). UV–Vis
(DMF) [λmax/nm (log ε/M−1 cm−1)]: 289 (4.12), 298sh (4.08), 327bp (3.75). (sh-shoulder, bp-
broad peak)
2.2.2.[Cr(PLSC)(PLSC–H)](NO3)2·H2O (1)
Cr(NO3)3·9H2O (100 mg, 0.25 mmol) was poured with hot EtOH (10 mL) and the warm
suspension of PLSC·2H2O (130 mg, 0.5 mmol) in EtOH (10 mL) was added. This mixture was
slightly heated until the ligand was completely dissolved. After 24 hours red-brown micro-
crystals were filtered and washed with EtOH and dried in vacuo. Yield: 60 mg (38%). Anal.
Calc. for C18H25CrN10O13: C, 33.71; H, 3.93; N, 21.84. Found: C, 33.29; H, 9.95; N, 21.57%.
2
−1
−1
Conductivity [ΛM/Ω−1 cm mol ]: 130 (in DMF). Selected IR bands [ /cm ]: 3436, 3302,
ꢀ
ν
ν(OH), ν(NH2); 3215, ν(NH); 2850, ν(NH+); 1669, ν(C=O); 1637, ν(CN); 1384, ν(NO3); 1300,
ν(COphenolic); 1250, ν(COenolic). UV–Vis (DMF) [λmax/nm (log ε/M−1 cm−1)]: 307 (4.23), 324
(4.20), 359 (4.17), 428 (4.24), 475sh (3.89), 629 (2.20), 699 (2.0), 774 (1.74).
2.2.3. K[Cr(PLSC–H)(NCS)3]·EtOH (2); PLSC·HNCS
The mixture of K3[Cr(NCS)6]·4H2O (250 mg, 0.5 mmol) and PLSC·2H2O (130 mg, 0.5
mmol) was poured with EtOH (10 mL) and refluxed for 5 h on 90 °C. The obtained dark-red
solution was left at the room temperature and after 5 days dark-red laminar single crystals of the
complex were precipitated. Yield: 160 mg (59%). Under the same reaction conditions but at the
molar ratio 1:2, as an admixture yellow monocrystals of the ligand PLSC·HNCS were formed.
Anal. Calc. for KC14H17CrN7O4S3: C, 31.45; H, 3.21; N, 18.34; S, 17.99. Found: C, 31.25; H,
3.19; N, 18.35; S, 18.03%. For chemical check of potassium see SI. Conductivity [ΛM/Ω−1 cm2
3

−1
−1
ꢀ
mol ]: 17 (in DMF); 23 (in MeOH). Selected IR bands [ /cm ]: 3462, 3333, ν(OH), ν(NH );
ν
2
2924, ν(NH+); 2072, ν(NCS); 1666, ν(CN); 1331, ν(COphenolic); 1246, ν(COenolic). UV–Vis
(DMF) [λmax/nm (log ε/M−1 cm−1)]: 308 (4.33), 366sh (3.96), 421bp (3.80), 445 (3.78), 483
(3.32), 556 (2.30), 708 (1.49), 784(1.11).
2.2.4. [Cr(PLTSC)(PLTSC–H)](NO3)2·2H2O (3)
The mixture of Cr(NO3)3·9H2O (100 mg, 0.25 mmol) and PLTSC (120 mg, 0.5 mmol)
was heated in EtOH (20 mL) under reflux for 4h on 80 °C.After 24h staying at the room
temperature from red-brown solution brown micro-crystals were filtered and washed with EtOH.
Yield: 130 mg (37%). Anal. Calc. for C18H27CrN10O12S2: C, 31.26; H, 3.93; N, 20.25; S, 9.27.
Found: C, 31.39; H, 3.52; N, 20.65; S, 9.12%. Conductivity [ΛM/Ω−1 cm2 mol−1]: 110 (in DMF).
−1
+
ꢀ
Selected IR bands [ /cm ]: 3318,3273, ν(OH), ν(NH ); 3164,ν(NH); 2950-2800,ν(NH );
ν
2
1607,ν(CN); 1384,ν(NO3); 841, ν(CS). UV–Vis (DMF) [λmax/nm (log ε/M−1 cm−1)]: 316 (4.27),
360sh (4.05), 465 (3.81), 507 (3.30), 713sh (1.49), 792sh (1.20)
2.2.5.[Cr(PLTSC)2](NO3)3 (4)
A few single crystals of this complex were obtained from the mother liquor of the
previously described complex (3).
2.2.6.[Cr(PLTSC)(NCS)3]·2H2O (5)
The mixture of 250 mg (0.5 mmol) K3[Cr(NCS)6]·4H2O and PLTSC (120 mg, 0.5 mmol)
was poured with EtOH (10 mL) and heated under reflux for 4h at 85 °C.After 7 days dark-red
rod-like single crystals were filtered and washed with EtOH. Yield: 110 mg (42%). Anal. Calc.
for C12H16CrN7O4S4: C, 28.68; H, 3.21; N, 19.51; S, 25.52. Found: C, 28.56; H, 3.20; N, 19.43;
−1
2
−1
−1
ꢀ
S, 25.42%. Conductivity [Λ /Ω cm mol ]: 26 (in DMF). Selected IR bands [ /cm ]: 3479,
ν
M
3430, 3248, ν(OH), ν(NH2); 3162, ν(NH); 2916, 2849, ν(NH+); 2110, 2096, ν(NCS); 1615,
ν(CN); 1307, ν(CO); 828, ν(CS). UV–Vis [λmax/nm (log ε/M−1 cm−1)]: 316 (4.27), 360sh (4.05),
465 (3.81), 507 (3.30), 713sh (1.49), 792sh (1.20)
2.3. X-ray crystal structure determination
Diffraction measurements at room temperature were performed on an Oxford Diffraction
Gemini S diffractometer equipped with Sapphire3 CCD detector. Diffraction data were collected
using graphite monochromatized Cu Kα radiation (λ = 1.5418 Å) for all structures, with the
exception of complex 5, for which Mo Kα radiation (λ = 0.71073 Å) was used. Data collection,
integration, scaling and absorption corrections were processed using the CRYSALISPRO [21].
Structures were solved using SHELXT [22]. The full-matrix least-squares refinement based on F2
was performed with SHELXL-2014 [23] integrated in the SHELXLE graphical user interface
[24]. All non-H atoms were refined anisotropically. C-bonded H-atoms are introduced in
idealized positions and refined as riding on parent atoms, while O and N-bonded H-atoms were
taken from difference Fourier maps and refined with distance restraints. Uiso of all H-atoms were
4

evaluated from Ueq of their parent atoms. In the structure of PLSC·HNCS hydroxyl group was
modeled as disordered over two sites. In the structure of 2, ethanol molecule was modeled as
disordered over two sites with hydrogen atoms generated in idealized positions. Distance and
ADP similarity restraints were applied to maintain chemically reasonable geometry. Structures
were validated using PLATON [25], and Cambridge Structural Database (CSD) (v. 5.36, updates
Nov. 2014) [26] using MERCURY CSD 3.5 [27]. Crystal data and refinement parameters of
ligands and the complexes are given in Table 1.
Crystallographic data for complexes 2, 4, 5, and ligands PLSC·2H2O, PLSC·HNCS, have
been deposited with the Cambridge Crystallographic Data Centre as Supplementary publication
Nos. CCDC 1408327–1408331, respectively. A copy of these data can be obtained, free of
charge, via https://summary.ccdc.cam.ac.uk/structure-summary-form, or by emailing
data_request@ccdc.cam.ac.uk.
3. Results and discussion
3.1. Syntheses and characterization
Scheme 1 shows the synthetic routes to the Cr(III) complexes. All complexes are
obtained in the reactions of warm EtOH solutions of Cr(NO3)3·9H2O or K3[Cr(NCS)6]·4H2O
with PLSC·2H2O and PLTSC, respectively. In case of reactions of Cr(NO3)3·9H2O with ligands
the molar ratio was 1:2, and cationic bis(ligand) complexes 1, 3 and 4 with neutral and
monoanionic form of the ligands are obtained. The reactions of K3[Cr(NCS)6]·4H2O with the
ligands regardless of the stoichiometry (1:1 or 1:2) yielded in only mono(ligand) complexes 2
and 5, whilst in the reaction of PLSC with K3[Cr(NCS)6] in molar ratio 2:1, in addition to the
mono(ligand) complex 2, single crystals of PLSC·HNCS are isolated.
The complexes and ligands are stable in air. All complexes are well soluble in DMF and
MeOH and poorly soluble in EtOH. Unlike the electrolyte-type complexes (1, 2, 3 and 4) which
are soluble in H2O, solubility of 5 is poor.
Molar conductivities of complexes 1 and 3 in DMF are in accordance with their proposed
coordination formulas (i.e. 2:1 type of electrolyte), while conductivity of 2 is lower and that of 5
somewhat higher than expected [28].
3.2. IR and electronic spectra
In these complexes, the ligands are coordinated in a tridentate ONO/ONS mode, via
oxygen atom of phenolic OH-group, nitrogen atoms of azomethine and oxygen/sulphur atom of
amide/thioamide group –C(–NH2)=X (X = O, S). In the case of complexes 2, 4 and 5 this was
confirmed by the single-crystal X-ray analysis (vide infra).
IR spectra of the complexes are in accordance with the afore mentioned ligand
coordination modes. Thus, in the spectra of all complexes, due to the phenolic oxygen
coordination, ν(COphen) band which in the spectra of the ligands is at ca 1285 cm−1, is positively
shifted for 20–40 cm−1 [19, 29–31]. Upon coordination, the band originating from ν(CN)
5

vibrations of azomethine group, which is in the spectra of the ligands present at 1583 (PLSC)
and 1540 cm−1 (PLTSC), is shifted toward high energy region (1618–1666 cm−1) [19, 32, 33].
The carbonyl oxygen coordination can be confirmed by the band of high intensity
ascribed to ν(C=O) vibrations that is in the spectrum of complex 1 moved to lower energies
(1669 cm−1) compared to the spectrum of PLSC (1697 and 1676 cm−1). The absence of this band
in mono-PLSC complex (2), as well as the absence of hydrazinic ν(NH) band in the range 2200–
2120 cm−1 confirm the monoanionic (enolic) form of the ligand. In the spectra of this, as well of
1, which contain one molecule of PLSC in monoanionic (enolic) form, the band ν(COenolic) is
located at 1242 and 1250 cm−1, respectively [34–36]. In the spectra of complexes with PLTSC,
the ν(CS) band, found in lower energy region (ca 835 cm−1) compared with the spectrum of the
ligand (923 cm−1) alone, suggests the coordination via the sulphur atom [19, 32].
Beside the above mentioned bands, in spectra of complexes some additional bands can be
assigned. Thus, in the spectra of all complexes, as well as of both ligands, one or more bands of
different intensities in the range between 2950 and 2800 cm−1 correspond to ν(NH+) vibrations of
the protonated pyridine nitrogen of PL moiety [32, 37]. Finally, in the spectra of the complexes 1
and 3 a very strong band attributed to ν(NO3) vibrations is present at 1384 cm−1, while the
spectra of complex 2 contain the band at 2072 cm−1, which can be assigned to ν(CN) vibrations
of isothiocyanato ligand [38]. This band in the spectrum of complex 5 is present as doublet (2096
and 2110 cm−1).
When the electronic spectra are concerned, although the ligands absorb in one (PLTSC)
or two (PLSC) mainly well defined absorption bands [39], their Cr(III) complexes show more
complicated spectral picture. In DMF all the complexes have spectra with unresolved
absorptions in high-energy region, where the ligand π→π* absorption bands are red-shifted upon
complexation. In addition, several charge-transfer bands appear down to 540 nm with high
intensity. Finally, d–d transitions are better resolved at PLSC complexes than at PLTSC ones.
Their number and intensities witness about distorted geometries of the octahedral Cr(III)
complexes.
3.3. Crystal structures of PLSC·2H2O and PLSC·HNCS
Table 2 summarizes pertinent structural data for PLSC·2H2O and PLSC·HNCS, whereas
molecular structures are shown in Figure 1. The main difference in the structures of the ligands is
that in case of PLSC·HNCS the conformation suitable for coordination is achieved, so that both
ligating O2 and O1 atoms are cis with respect to hydrazinic ligating atom N3, whereas the
neutral ligand PLSC·2H2O has conformations in which both O2 and O1 atoms are trans with
respect to hydrazinic N3. It appears that the nature of PL moiety strongly affects the
conformation of the molecules in a similar way as it was found in thio-analogues of PLSC
(PLTSC) [31]. Namely, due to the protonation of phenolic OH-group and the existence of a
strong intramolecular O2–H2A···N3 bond (Table S1, Supplementary material), the ligand
PLSC·HNCS is stabilized in pro-binding conformation. In contrast to this, no such hydrogen
6

bond is possible in the neutral ligand, since H-atom from phenolic group has migrated to
nitrogen atom of PL fragment forming the zwitter-ion.
The different nature of the PL moiety in two molecules implies some differences in the
bond lengths, such as the elongation (≈ 0.04 Å) of C4–O2 in the cation compared with the
molecule of the neutral ligand. In both structures C1–O1 bond has a value close to the one
characteristic for double C=O bond, while C1–N1 and C1–N2 bonds have partial double bond
character. The value of C5–N4–C7 angle is ca 124° which is in line with the protonation of
nitrogen atom of PL moiety [37].
3.4.Crystal structures of complexes 2, 4, and 5
Selected structural data for complexes 2, 4, and 5 are sumarized in Table 2, whereas the
molecular structures are shown in Figure 2. All structuraly characterized complexes have mer-
octahedral structure, which is in case of the complexes 2 and 5 realized by coordination of an
ONX tridentate ligand (X = O in 2 and X = S in 5), and three NCS– ions, while in 4 the
coordination sphere is comprized of two ONS ligand molecules. It should be noted that
complexes where octahedral coordination of Cr(III) is achieved by three NCS− ions and one
tridentate ligand as found in 2 and 5 are very rare. The search of CSD (v. 5.36 update Nov. 2014)
shows that there are only two such complexes, namely (HL1)[Cr(L1–H)(NCS)3]·MeOH and
[Cr(L2)(NCS)3]·DMF, where L1 = 1-(2-pyridyldiazenyl)-2-naphtholate and L2 = 4′-(2-pyridyl)-
2,2′:6′,2″-terpyridine [40, 41]. In complex 2 the potassium ion is situated in higly deformed
pentagonal bipyramidal environment made by three oxygen atoms at distance 2.840(5)–
2.878(4) Å (two oxygens of ethanol molecules, one oxygen of hidroxymethyl gorup), and three
sulfur atoms belonging to coordinated thiocyanate ions at distances 3.2900(15) to 3.4606(17) Å.
The search of the CSD reveals that hitherto there are no examples where ethanol and thiocyanate
ion are simultaneously bonded to potassium ions, however there are several compounds in which
either ethanol or thiocyanate are coordinated (see Supplementary information for list of
corresponding CSD Refcodes). The lengths of K–S and K–O bonds in 2 are in expected range if
compared to the distances found in the mentioned examples.
In all the complexes the ligands PLSC and PLTSC are coordinated in the usual tridentate
manner, i.e. through oxygen atom of phenolic group, nitrogen of azomethine and carbonyl
oxygen (PLSC), i.e. sulphur (PLTSC) by forming six-membered pyridoxylidene and five-
membered semi/thiosemicarbazide metalocycles. The pyridoxalic fragment in all three
complexes is in the form of dipolar zwitter-ion, which is not surprising because these ligands in
their free (non-coordinated) forms are zwitter-ions. In line with zwitter-ionic structure is the
value of the angle between the pyridine nitrogen atom and the neighbouring carbon atoms in
these complexes (ca 124°) [31, 37]. In complex 2 PLSC is coordinated as monoanion, in which
the deprotonation of hydrazinic nitrogen (N2) takes place.
Of two mutual metal–chelate ligand bonds (Cr1–O2/N3) significantly shorter one is Cr1–
O2 (av. 1.92 Å) compared to Cr1–N3 (av. 2.03 Å). This is in accordance with the literature data
regarding not only these but also the complexes of other metals with these ligands [13]. Very
7

similar bond length to Cr1–N3 have also virtually identical Cr1–NCS bonds (av. 2.00 Å) which
is in accordance with those found in [40, 41]. When Cr1–O1 bond (1.9802(19) Å) is concerned,
it is very close to that of the octahedral Cr(III) complex with semicarbazones of salicyl/pyridine-
2-carbaldehyde, i.e. 2.011(3) Å [42] and 1.997(3) Å [43], respectively. The lengths of Cr1–S1
bonds in both PLTSC complexes have the values similar to those found in also octahedral
complexes of Cr(III) with tridentate thiosemicarbazone ligands [2, 44].
A comparative analysis of the lengths of intraligand bonds in free neutral and coordinated
ligands in addition of great likeness shows in the case of PLSC the differences regarding
primarily the ureido fragment –N2H–C1(=O1)N1H2. Namely, due to deprotonation of this
fragment, i.e. the hydrazinic N2 atom, the C1–N2 bond significantly shortens (1.329(4) Å) in
comparison to the same in the free ligand (1.371(4) Å). Consequently, the C1–O1 bond which in
the free ligand has a character of a double bond (1.247(4) Å), in the complex is elongated
(1.301(4) Å) becoming similar to C–O enolato bonds in other complexes [43, 45–48].
The lengths of C4–O2 phenolato bonds in free and coordinated ligands are almost
identical: (1.29 Å (free ligands) and 1.30 Å (coordinated ligands)). In the case of PLSC, the
azomethine C2–N3 bond after coordination prolongs by 0.02 Å unlike PLTSC where this bond
remains virtually unchanged. The latter regards also the C=S bonds [2].
Even though the ligands alone are almost planar [32], due to the formation of the
metallocycles, they deviate significantly from planarity in complexes 4 and 5, whereas in 2 the
degree of planarity is on the contrary very high. This structural feature of 4 and 5 can be decribed
by the values of appropriate dihedral/torsion angles (Table 2). The mean plane of pyridine ring is
inclined to the mean plane through donor atoms of the chelate ligand and Cr1 which can be
judged from the values of the apropriate dihedral angle δ (Table 2). In both complexes all chelate
rings are puckered. Six-membered metallocycles have conformation close to twist- or screw-boat
with the largest torsion angle τ(C3–C4–O2–Cr1), while the five-membered metallocycles are
twisted on Cr1–N3 or have a conformation of an envelope on Cr1, with the largest torsion angle
τ(C1–N2–N3–Cr1) (puckering parameters are given in Table S2, Supplementary material). The
observed deviation from planarity upon coordination has much resemblance with those found in
transition metal complexes with similar pyridoxal semi- and non-substituted and substituted
thiosemicarbazones (see for example [49, 50]), noting that deviation in complex 4 tends to
extreme values.
3.5. Electrochemistry
Ligands
Both ligands reduce in one 1–e− process at the potentials −1.51 V (PLSC) and −1.37 V
(PLTSC). The processes are quasi-reversible and diffusion-controlled. After reduction, new pairs
of 1–e− peaks arise at the more negative potentials: −1.88 V (PLSC) and −1.97 V (PLTSC)
which are due to slow chemical reactions. On the basis of our previous results for a similar
ligand (pyridoxal aminoguanidine), it can be supposed that the first process is located at the
pyridoxal moiety [51], while the second one corresponds with the first reduction of the
8

semicarbazone and thiosemicarbazone fragments, respectively [52–54]. The first reduction peaks
have Ip/cv1/2 ~ const and ∆Ep/∆ log v ~ −40 mV decade−1 which is consistent with the presence of
a following chemical reaction. Oxidation occurs in a multielectron process at the potentials more
positive than +1.0 V followed by irreversible chemical reactions of ligand decomposition.
Addition of five-fold excess of lithium acetate to the ligands’ solution to form deprotonated
ligand did not affect much their reduction processes, while a dramatic change appeared at the
positive potentials: new complex oxidation peaks at potentials between +0.15 V and +0.50 V
appeared in a similar way with those recorded for some other semicarbazone-based ligands [52].
The obtained species have strong adsorption affinities.
Complexes
The two types of complexes, mono- and bis-ligand, differ in their voltammetric behavior,
both in appearance and electrochemical characteristics.
The voltammograms of the mono(ligand) thiocyanate complex 2 are shown in Figure 3a.
As can be seen, generally two reduction (I and II) and one oxidation process (Ox) can be
observed. In the widest potential amplitude the processes characteristic for the coordinated
ligand oxidation or reduction are also recorded with parameters similar to those of the free
ligand.
The two reduction processes have characteristics of consecutive 1–e− transfers followed
by chemical reactions. A deeper insight into the process at the peak I reveals that it is composed
of two close peaks which are results of electronation of the species involved in the equilibrium
with NCS−. A more detailed picture can be obtained with variation of the potential amplitude and
sweep rate: if the potential is reversed after the peak I, at higher scan rates the presence of
instable species can be recorded both around the peak I’ and at more positive potentials (Figure
3b). Of the two oxidation peaks in the region of the peak I’ (v = 2 V s−1), the more negative one
is the counterpart of the reduced starting complex, i.e. of [Cr(PLSC−H)(NCS)3]2− that is prone to
relax an excess of negative charge by releasing 1–2 NCS−. In this way the complex with a
possible composition [Cr(PLSC−H)(NCS)n(DMF)3−n]
1−n is formed which oxidizes at somewhat
more positive potentials. With lowering the scan rate below 0.5 V s−1 this species dominates and
under suitable conditions of amplitude and sweep rate a new slightly more positive reduction
process (for about 50 mV) can be observed in the second sweep (Figure 3). In parallel with this,
the free NCS– reacting in the peak Ox, according to the current parameters which are dependent
both on the sweep rate and the potential amplitude, seems to be involved in two opposed
chemical reactions. First, fast release after reduction of CrIII complex leading to an increase of
the peak Ox. Then, after reoxidation of CrII to CrIII species in peak I’, a much slower reaction of
its recoordination to form the starting compound is occurring which results in decreasing the
peak Ox.
The peak
I
current function Ip/cv1/2
~
const; peak potential variation
∆Ep/∆ log v ~ −30 mV decade−1 (measured as first sweep). Current functions of the peaks II and
I, i.e. Ip(II)/Ip(I) range from ~ 1 at low sweep rates to ~ 0.8 at v > 0.5 V s−1. The peak Ox has
9

complex behavior: Ip(Ox)/Ip(I) decreases both at low and at high scan rates, reaching its maximal
value of 1.5 at v = 0.20 V s−1 for the smaller amplitude (down to −1.0 V). In the wider amplitude
(to −1.6 V) this parameter increases to a value of 3 which points to a more pronounced NCS−
releasing effect at negative potentials.
In order to establish the effect of the presence of NCS− on the composition and behavior
of the reacting species, we added the stoichiometric amount of Ag+ to precipitate the anion. As
the result, a new pair of quasi-reversible peaks at the potential E0’= −0.55 V (at 100 mV s−1
∆Epa/c = 80 mV) was formed with
a
possible composition of the reactant:
[CrIII(PLSC−H)(DMF)3]2+. In parallel with this, the peak II remained practically unchanged,
while the peak Ox disappeared. On the basis of this, we suppose that in the peak II reacts mainly
the NCS−-free complex.
Generally similar, but less stable complex behavior was observed for mono(ligand)
PLTSC complex. A comparison of the voltammograms for the two NCS− complexes is shown in
Figure 4. The main difference is recorded in the potential range of the peak II where up to three
small peaks arise depending on the sweep rate. A peculiarity of this complex in DMF is that free
NCS− is present in the solution regardless the reduction process. Its current in the amplitude
0.0 V to +0.9 V is practically the same as after reduction at the peak I. This might be the result of
the present NCS− acting to prevent its further dissociation from the reduced complex. When the
peak II process is concerned, it is less defined, since in this particular potential range even three
small peaks appear. They may be ascribed to the reactions of both the NCS−-containing and
NCS−-free CrII species, while the most negative one might be due to the free ligand (see above).
The bis(ligand) complexes show ill-defined voltammograms with several small peaks, at
the potentials ranging from −0.5 V till −1.5 V (Figure 5). On the basis of a similarity with the
NCS-complexes, we suppose that the first two peaks (around −0.6 V) correspond to reactions of
the mono(ligand) species, while the other two to the bis(ligand) ones. To check this assumption,
we added the equivalent amount of K3[Cr(NCS)6] to form the mono(ligand) complex. After this
addition to the PLSC complex solution, the resulted voltammogram had all previously described
peaks for the mono(ligand) NCS− complex at the characteristic potentials. However, a deeper
insight into the nature of these processes for the bis(ligand) complexes is not easy to acquire and,
because of their instabilities in DMF, probably not of scientific interest.
3.6. Thermal decomposition
As thermoanalytical methods belong to standard methods for characterization of
potentially bioactive compounds giving information on their thermal stability, the type and
amount of the solvent, the possible polymorphic transformations [55] or purity [56–58], the
compounds where characterized by coupled TG/MS measurements, too.
According to the elemental analysis data all compounds contain crystal water except of
complex 2 which crystallizes with ethanol and 4 that crystallizes without any solvent. The
solvent evaporation starts practically at room temperature in all the compounds. Only
PLSC·2H2O has a relatively high water evaporation temperature (114 °C onset).
10

The solvent content of PLSC·2H2O (exp. 14.1 %; calcd. 13.84 %) and its complexes 1
(exp. 2.7 %; calcd. 2.81 %), 2 (exp. 8.4 % to the minimum in DTG curve; calcd. 8.62 %) agrees
o
well with the calculated value. The anhydrous ligand and complex 1 are stable over a 70 C
temperature range (to 208 °C and 233 °C onsets, respectively). Compound 2 starts to decompose
before the evaporation of EtOH is completed. The decomposition of the desolvated compounds
is continuous in the whole temperature range taking place practically in one step for PLSC
ligand and 1 while 2 decomposes in a stepwise manner.
PLTSC obtained here does not contain crystal water, however, it is hygroscopic binding
about 2 % water (exp. 1.9 %). The water content of its complexes 3 and 5 is less than the
stoichiometric one (exp. 2.5 % – calcd. 5.21 %; exp 5.8 % – calcd. 7.17 %, respectively) which is
not surprising according to the relatively low onset temperature of dehydration. The stability of
o
the dehydrated PLTSC and 3 is also relatively high (onsets: 218 C, 260oC, respectively) while
the anhydrouos 5 is stable to 191 °C. The following decomposition proceeds with better
separated but still overlapping steps compared to the corresponding PLSC compounds. The
comparison of the decomposition of the compounds is illustrated by the corresponding DTG
curves presented in Figure 6a and b.
TG/MS data refer to the complete water evaporation up to ~160 °C. The peak for nitrate
ion appears at m/z = 30 amu (NO+) in nitrate compounds (1 and 3) while in thiocyanates peak for
HNCS+ at m/z = 59 amu (2 and 4) has been found, in accordance with the proposed/determined
molecular structure. From the ethanolic solvate, 2, peaks for EtOH (CH3O+, C2H5O+, CH2O+ and
C2H5OH with decreasing intensity, in the accordance with literature data [59]) refer to its
complete evaporation above 250 °C only, where the fragmentation of the organic ligand is
already in progress (see Figure 7).
As the ligand molecules are rich in oxygen atoms, peaks for water appear not only at
lower temperatures in hydrates (< 180 °C) but also above 200 °C because oxygen atoms in the
molecules oxidize the fragments at elevated temperatures to H2O and CO2 (m/z = 44). In
addition, in PLTSC compounds there are peaks at m/z = 48 (SO+) and 64 amu (SO2)
characteristic for the ligand decomposition.
As hydrates with PLTSC lose the crystal water at room temperature, they should be dried
at ~120 °C to constant mass prior to elemental analysis in order to check the
stoichiometry/homogeneity of the batches. Compound 2 is not stable; therefore its potential
practical applicability is probably seriously restricted. DSC curves do not refer to any structural
changes or melting of the compounds. However, melting accompanied with the decomposition
was observed in all compounds.
Conclusions
With chromium(III) and tridentate Schiff bases pyridoxal semi/thiosemicarbazones five
complexes were obtained as the first complexes of this metal with the cited ligands. Using
Cr(NO3)3 solely the cationic bis(ligand) complexes containing both neutral and monoanionic
ligand form were isolated, while by means of K3[Cr(NCS)6] the ligand mixed (mono Schiff base-
11

ONX)(trithiocyanato–N)chromium(III) complexes were produced. All the complexes have mer-
octahedral structures which were in the case of three complexes proved by X-ray analyses. By
solving the crystal structures of two different forms of PLSC ligand, neutral PLSC·2H2O and
protonated PLSC·HNCS, it was found that PLSC·HNCS has the same (cis) position of donor
atoms as in the complexes, which is not the case with its neutral form.
Acknowledgments
The research was supported by the Ministry of Education and Science, of the Republic of
Serbia (Grant no. 172014) and the Provincial Secretariat for Science and Technological
Development of Vojvodina.
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14

Table 1. Crystallographic data for PLSC·2H2O, PLSC·HNCS and complexes 2, 4, 5
PLSC·2H2O
PLSC·HNCS
2
4
5
CCDC No.
Molecular formula
M (g mol–1)
C9H16N4O5
260.26
C10H13N5O3S
283.31
C14H17N7O4S3KCr
534.63
C18H24N11O13S2Cr
718.6
C12H16N7O4S4Cr
502.56
Crystal system
Monoclinic
Triclinic
Monoclinic
Triclinic
Orthorhombic
Pna21
Space group
P21/c
P21/n
P 1
P 1
a (Å)
b (Å)
c (Å)
11.6462(9)
7.1123(5)
14.5095(10)
90
4.9927(4)
8.5183(6)
15.7383(11)
74.639(6)
89.759(6)
83.499(8)
641.05(8)
2
8.5575(2)
15.8793(4)
16.0153(4)
90
7.9946(3)
11.1599(5)
17.2273(7)
71.899(4)
89.255(3)
73.877(4)
1398.94(11)
2
17.2793(4)
14.2138(4)
8.5423(2)
90
α (°)
β (°)
101.217(7)
90
96.520(2)
90
90
90
γ (°)
V (Å3)
1178.88(15)
4
1.466
1.029 (Cu Kα)
552
2162.20(9)
4
1.624
9.12 (Cu Kα)
1068
2098.02(9)
4
1.591
8.52 (Cu Kα)
1028
Z
Dcalc (g cm−3)
1.468
2.39 (Cu Kα)
296
1.706
0.64 (Mo Kα)
738
µ (mm−1)
F(000)
Crystal size (mm)
Color / Shape
Temperature (K)
Radiation type; λ(Å)
θ range (°)
0.69 × 0.09 × 0.09
Yellow / Prism
298(2)
Cu Kα; 1.5418
3.9–67.0
4025; 2100
0.024
0.35 × 0.15 × 0.08
Yellow / Irregular
294(2)
Cu Kα;1.5418
2.9–67.0
3677; 2271
0.021
0.41 × 0.13 × 0.07
Dark red / Prism
298(2)
Cu Kα;1.5418
3.9–72.5
8101; 4173
0.022
0.4 × 0.24 × 0.03
Dark red / Plate
294(2)
Mo Kα;0.71073
3.0–30.7
25865; 7824
0.026
0.54 × 0.19 × 0.03
Dark red / Plate
298(2)
Cu Kα; 1.5418
4.0–72.4
13744; 2986
0.030
No. reflections; unique
Rint
No. observed reflections
Restraints; parameters
Goodness-of-fit on F2
R; wR [Fo> 4σ(Fo)]
R; wR [all data]
∆ρmax and ∆ρmin(e Å−3)
1654
11; 192
1.05
0.041 /0.10
0.054 / 0.11
0.23 and −0.19
1960
24; 204
1.05
0.041 / 0.109
0.047 / 0.11
0.26 and –0.24
3651
41; 294
1.08
0.044 / 0.113
0.051 / 0.118
0.62 and –0.62
6456
0; 447
1.126
0.038 /0.11
0.053 / 0.13
0.68 and −0.57
2831
12; 282
1.05
0.028 / 0.075
0.031 / 0.077
0.31 and –0.25
15

Table 2. Selected structural data (Å, °) for PLSC·2H2O, PLSC·HNCS, and complexes 2, 4, and 5
Bonds
PLSC·2H2O
PLSC·HNCS
2
4
5
Cr1–O2
Cr1–O2A
Cr1–N3
Cr1–N3A
Cr1–X1*
Cr1–S1A
Cr1–N5
Cr1–N6
Cr1–N7
C1–N1
C1A–N1A
C1–X1*
C1A–S1A
C1–N2
C1A–N2A
N2–N3
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1.913(2)
–
1.992(2)
–
1.9800(19)
–
1.9131(15)
1.9356(14)
2.0322(16)
2.0345(15)
2.3905(6)
2.3905(6)
–
1.933(2)
–
2.041(3)
–
2.3821(11)
–
2.007(2)
2.009(3)
2.019(3)
1.338(4)
–
1.300(4)
–
1.330(4)
–
1.385(4)
–
1.289(4)
–
1.302(4)
–
1.986(3)
2.000(3)
2.000(3)
1.316(5)
–
1.700(4)
–
1.339(5)
–
1.370(4)
–
1.291(4)
–
1.301(4)
–
–
–
–
–
1.328(5)
–
1.247(4)
–
1.371(5)
–
1.362(4)
–
1.266(5)
–
1.295(4)
–
1.334(9)
–
1.217(8)
–
1.390(8)
–
1.340(7)
–
1.277(8)
–
1.337(7)
–
1.310(3)
1.312(3)
1.711(2)
1.708(2)
1.346(3)
1.346(3)
1.380(2)
1.371(2)
1.296(3)
1.288(2)
1.304(2)
1.300(2)
N2A–N3A
C2–N3
C2A–N3A
C4–O2
C4A–O2A
O2–Cr1–X1*
O2A–Cr1–S1A
N3–Cr1–N3A
N6–Cr1–N7
N3–Cr1–N5
C5–N4–C7
–
–
–
–
–
–
–
–
169.88(8)
–
–
179.28(11)
172.64(10)
123.8(3)
170.70(4)
169.91(5)
175.69(7)
–
172.53(8)
–
–
177.08(14)
173.33(13)
125.3(3)
–
–
–
–
124.2(3)
–
124.3(6)
–
124.42(19)
124.66(18)
C5A–N4A–C7A
τ(C3–C4–O2–Cr1)
τ(C1–N2–N3–Cr1)
δ**
–
–
–
–
–
–
6.5(4)
−3.0(3)
8.7(2)
−32.3(3)
−39.2(3)
26.3(2)
23.3(2)
31.62(6)
24.23(4)
18.1(5)
−11.3(4)
14.32(13)
* X = O in PLSC·2H2O, PLSC·HNCS, and 2; X = S in 4 and 5.
**dihedral angle between mean plane of pyridine ring and mean plane through donor atoms of chelate
ligand and Cr1
16

Figures
Scheme 1. Preparation routes to the Cr(III) complexes
Figure 1. Molecular structures of PLSC·2H2O (a), and PLSC·HNCS (b)
17

Figure 2. Structures of: (a) anionic part of 2 [Cr(PLSC–H)(NCS)3]−; (b) cationic part of 4
[Cr(PLTSC)2]3+; (c) [Cr(PLTSC)(NCS)3]·2H2O (5)
18

a)
Ox
40
20
0
I'
II'
I
-20
II
-1.5
-1.0
-0.5
0.0
0.5
1.0
E(V) vs. SCE
b)
40
20
0
Ox
I'
-20
-40
I
-1.0
-0.5
0.0
0.5
1.0
E(V) vs. SCE
Figure 3. a) Cyclic voltammograms for K[Cr(PLSC−H)(NCS)3]·EtOH; DMF+0.1 M TBAP, GC,
0.10 V s−1. c = 1.12 mM. b) Cyclic voltammograms for K[Cr(PLSC−H)(NCS)3]·EtOH;
DMF+0.1 M TBAP, GC, 0.50 V s−1 (dashed line) and 2 V s−1 (solid line) c = 0.58 mM
19

40
20
0
Ox
I
-20
II
-1,5
-1,0
-0,5
0,0
0,5
1,0
E(V) vs. SCE
Figure 4. Cyclic voltammograms for K[Cr(PLSC−H)(NCS)3]·EtOH (solid line, c = 1.12 mM)
and for [Cr(PLTSC)(NCS)3]·2H2O (dashed line, c = 0.86 mM); DMF+0.1 M TBAP, GC, 0.10 V
s−1.
10
0
-10
-20
-30
-1,5
-1,0
-0,5
0,0
E(V) vs. SCE
Figure 5. Cyclic voltammograms for [Cr(PLTSC)(PLTSC−H)](NO3)2]·2H2O (c = 1.49 mM);
DMF+0.1M TBAP, GC, 0.10 V s−1.
20

Figure 6. a) DTG curves of PLSC, 1 and 2. b) DTG curves of PLTSC, 3 and 5.
21

Figure 7. DTG/MS data for EtOH evaporation (m/z = 31) and selected fragments for the
decomposition of 2 in flowing argon.Graphical abstract
22

Five Cr(III) complexes with tridentate ONX (X = O, S) ligands, i.e. pyridoxal semi-
/thiosemicarbazones, being the first complexes with these ligands, were synthesized and
characterized. All the complexes have mer-octahedral structures, which in the case of 2, 4, 5,
were proved by single crystal X-ray analysis. Detailed electrochemical and thermal analyses of
the studied compounds were performed.
Synopsis
23