الملخص الإنجليزي
This research endeavour was motivated to a large extent by the paucity of
thiosemicarbazone-based ternary complexes of copper(II) and oxovanadium(IV) with
bidentate co-ligands (N,N-donor) such as 1,10-phenanthroline (phen), 2,2ˊ-bipyridine
(bipy), dipyridoquinoxaline (dpq) and dipyridophenazine (dppz), or their derivatives.
Even rarer are phenolate-possessing thiosemicarbazone complexes with these
aforementioned transition metals. Considering the inherent pharmacological
properties of thiosemicarbazones and the N,N-donor co-ligands as well as the
bioactivities of copper and vanadium, such ternary complexes, [CuII(L)(N,N-donor)]n+
and [VIVO(L)(N,N-donor)]n+, were anticipated to exhibit potentially fascinating
biological properties. Given the electrochemically accessible multiple electrovalencies
of copper and vanadium, redox processes of thiosemicarbazone complexes of these
transition metals were envisaged to play a key role in the generation of reactive oxygen
species (ROS) responsible for apoptotic cytotoxicity. Additionally, the development
of high-valent vanadium coordination chemistry based on thiosemicarbazones was
another important aspect of this work with emphasis on interesting structural,
spectroscopic, redox and antiproliferative properties.
The anticancer agent di(2-pyridyl) ketone 4,4-dimethyl-3-thiosemicarbazone
(HDpk44mT), a series of variously-substituted salicylaldehyde 4,4-dimethyl-3-
thiosemicarbazones [H2(3,5-R
1
,R2
)-sal44mT: R1 = R2 = H; R1 = H, R2 = OMe; R1 = H,
R
2 = NO2; R1 = H, R2 = t-Bu; R1 = t-Bu, R2 = H; R1 = R2 = t-Bu] and 3,5-di-tertbutylsalicyladehyde 4-ethyl-3-thiosemicarbazone [H2(3,5-t-Bu2)-sal4eT] were
produced straightforwardly by the usual one-pot Schiff-base condensation reaction and
isolated in the thio-keto (thione) tautomer as evidenced by vibrational spectroscopy
which gave stretching frequencies in the range 3100–3250 cm–1
attributable to the
hydrazinic thio-amide N–H bonds. 1H-NMR spectroscopy (700 MHz) revealed that
the structures of these ligands remain intact in solution. The single-crystal X-ray
structures of H2(3,5-t-Bu2)-sal44mT and H2(3,5-t-Bu2)-sal4eT not only confirmed the
existence of the thione tautomer (C=S: 1.701 and 1.703 Å, respectively), but also
showed that these molecules crystallise in the E-configuration; however, they adopt
different conformations with different orientations of the thio-amide functionality. In
both structures, the ligands exhibit intramolecular H-bonding between the phenol
proton and the imine nitrogen. Hence, for all the ligands, the chemical shift of the
phenol proton is the most downfield, followed by that of the thio-amide N–H proton.
A clear trend of the electronic effects of the substituent groups (R1
and R2
on the
chemical shifts is established. In the case of the di(2-pyridyl)-based ligand
HDpk44mT, the two pyridyl moieties are nonequivalent: the proton adjacent to the
pyridyl nitrogen which is H-bonded to the thio-amide N–H group is more deshielded
(δ 8.8 vs. 8.6).
Reactions of the phenolic thiosemicarbazones with [VO(acac)2] in the presence of a
bidentate co-ligand in equimolar amounts afforded a series of mononuclear molecular
oxovanadium(IV) complexes of the type [VO{(3,5-R
1
,R2
)-sal44mT}(N,N-donor)] and
[VO{(3,5-R
1
,R2
)-sal4eT}(N,N-donor)] (where N,N-donor = phen, 4,4ˊ-Me2-2,2ˊbipy). Evidently, the thiosemicarbazones underwent tautomeric transformations
concomitantly with deprotonation for charge-neutrality. [VO{(3,5-t-Bu2)-
v
sal44mT}(phen)], [VO{(3-t-Bu2)-sal44mT}(phen)] and [VO{(3,5-t-Bu2)-
sal4eT}(phen)] are the first examples of crystallographically elucidated ternary
complexes of their kind (excluding the only three known for the closely related
thiosemicarbazate class of ligands). The geometry at the metal centre is distorted
octahedral with phen adopting an axial-equatorial coordination mode. The transinfluence of the vanadyl oxygen [d(V=O) ~1.60] causes a conspicuous difference
between the V–Nphen distances [Δd(V–N) ~0.19–0.23 Å]. The crystal lattice of all three
complexes exhibits π-π stacking interactions involving the phen ligand. As expected,
the well-resolved X-band ESR spectra of the representative complexes [VO{(3,5-tBu2)-sal44mT}(phen)] and [VO{(3,5-t-Bu2)-sal4eT}(phen)] are axial with Az>>Ax,y
for the [Ar]3d
1
configuration consistent with a dxy ground state. Treatment of H2(3-tBu)-sal44mT and H2(3,5-t-Bu2)-sal44mT with half a molar equivalent of [VO(acac)2]
in MeCN caused spontaneous deoxygenation of the vanadyl ion to produce the nonoxo vanadium(IV) bis(chelate) binary complexes [V{(3-t-Bu)-sal44mT}2] and
[V{(3,5-t-Bu2)-sal44mT}2], respectively. These have been crystallographically
characterized and are the first examples of thiosemicarbazone complexes of this kind.
Previously, two papers reported similar thiosemicarbazate complexes. The
coordination geometries are intermediate between distorted octahedral and trigonal
prismatic, but the distortion is leaning more in favour of the latter. Indeed, in frozen
DMF solution, X-band ESR spectroscopy detected the trigonal prismatic structure
(Ax,y >> Az: dz2 ground state). Cyclic voltammetry revealed two redox processes
attributable to the anodic VIV/VV
and cathodic VIV/VIII redox couples. Reactions of the
phenolic thiosemicarbazones with equimolar equivalents of [VO(acac)2] in MeOH
afforded the oxovanadium(V) methoxide complexes [VO{(3,5-R
1
,R2
)-
sal44mT}(OMe)], of which [VO{(3-t-Bu)-sal44mT}(OMe)] and [VO{(5-OMe)-
sal44mT}(OMe)] were characterized by X-ray crystallography. When these reactions
were conducted in the presence of Et3N, the ionic complexes of dioxovanadium(V)
[Et3NH][VO2{(3,5-R
1
,R2
)-sal44mT}] were formed. The 3-D structure of the complex
[Et3NH][VO2{(3,5-t-Bu2)-sal44mT}] was determined definitively by X-ray analysis.
The coordinate bond to the oxygen hydrogen-bonded of the triethylammonium counter
cation was considerably weakened. In the case of HDpk44mT, the formation of
[VO2(Dpk44mT)] occurred in the absence of a base. The structure of
[VO2(Dpk44mT)] was also determined by X-ray crystallography. Each of the
abovementioned vanadium(V) complexes are diamagnetic and were characterized by
1H-NMR spectroscopy; additionally, a selected few were characterized by 51V-NMR
spectroscopy.
Each of the ligands H2(3,5-R
1
,R2
)-sal44mT was treated with (CH3COO)2Cu·H2O
followed immediately by the addition of a bidentate co-ligand (N,N-donor = phen, 2,9-
Me2-phen, bipy or 4,4ˊ-2,2´-bipy) in stoichiometric amounts to produce the
mononuclear ternary copper(II) complexes [Cu{(3,5-R
1
,R2
)-sal44mT}(N,N-donor)].
However, in the case of the ligand H2(3,5-t-Bu2)-sal4eT, the reaction afforded the
dinuclear complex, [Cu2{(3,5-
tBu2)-sal4eT}2(phen)]. In these reactions, the acetate ion
served as a base that assisted tautomerisation. As an illustration of this point, the
reaction of H2(3,5-t-Bu2)-sal44mT with Cu(ClO4)2·6H2O in the presence of phen
under the same conditions, gave the ionic complex [Cu{H(3,5-t-Bu2)-
sal44mT}(phen)]ClO4 in which the ligand remained in the thione tautomeric form;
only the phenol moiety was deprotonated. The X-ray structures of the mononuclear
ternary complexes show meridional coordination of the donor atoms of the
thiosemicarbazone ligand and axial-equatorial coordination of the donor atoms of the
vi
co-ligand giving rise to a distorted square pyramidal geometry at the metal centre with
the trigonality index [τ = (β – α)/60º] varying from 0.018 for the unsubstituted complex
to 0.375 for the tert-butyl-substituted complexes. All of these copper(II) complexes
are subject to the Jahn-Teller effect as evidenced by the elongation of the axial Cu–
Nco-ligand causing a difference in the two Cu–Nco-ligand distances ranging from 0.247 to
0.313 Å. X-band ESR spectroscopy has demonstrated that the 3-D structure is
maintained in solution given the axial spectra (gǁ > gꞱ > 2.0; Aǁ > AꞱ) which, together
with the X-ray structures, are indicative of half occupancy of the dx2–y2 orbital in the
ground state. The dinuclear complex [Cu2{(3,5-
tBu2)-sal4eT}2(phen)] exhibits a
distorted square planar coordination sphere at one copper(II) centre and a distorted
square pyramidal geometry (τ = 0.33) at the other copper(II) centre. This is one of only
two known structurally characterised dinuclear copper(II) phenolate-based
thiosemicarbazone complexes of this type. The mononuclear copper(II) complexes
have RT effective magnetic moments in the range 1.66 to 1.89 µB as expected for a d9
system. The value of µeff for the dinuclear complex (2.38 µB) is very close to the spinonly value for two non-interacting copper(II) paramagnetic centres. All the copper(II)
complexes display colours ranging from light yellow to orange-yellow largely on
account LMCT near-visible or visible absorptions.
The cytotoxicity of selected thiosemicarbazone ligands and corresponding complexes
with copper(II) and vanadium(IV/V) was investigated in two cancer cell lines, namely
human cervical carcinoma (HeLa) and human breast adenocarcinoma (MCF-7),
together with the normal (noncancerous) human breast epithelial cell line (MCF-10A)
in some cases, using the MTT cell viability assay. Whereas the di(2-pyridyl)-based
ligand HDpk44mT was highly antiproliferative towards these cancer cells, the
phenolic thiosemicarbazone ligands gave IC50 values greater than 100 µM indicative
of lack of activity. However, the phenolate-possessing thiosemicarbazone complexes
exhibited anticancer activity. Generally, the copper(II) complexes exhibited higher
cytotoxicity than their vanadium counterparts. It is also noteworthy that the ternary
complexes containing bipy or 4,4`-Me2-bipy were less potent than those having phen
or 2,9-Me2-phen. A comparative photocytotoxicity investigation using the HeLa
cancer cell line revealed that [VO{(3,5-
tBu2)-sal44mT}(phen)] exhibits apoptotic
photodynamic behavior whereas [Cu{(3,5-
tBu2)-sal44mT}(phen)] does not. Finally,
most of the complexes showed inhibitory activity against either Escherichia coli or
Staphylococcus aureus or both.
