Design and synthesis of novel terpyridine-based ligands with one and two terminal aurophilic moieties and their Rh(III) and Ru(II) complexes for the adsorption on metal surfaces

Article abstract of DOI:10.1016/j.poly.2021.115149

Novel terpyridine ligands with one and two terminal aurophilic sulfur containing fragments, (S)-11-(4-([2,2′:6′,2″-terpyridin]-4′-yl)phenoxy)undecyl 5-(1,2-dithiolan-3-yl)pentanoate (7), (S)-((4-([2,2′:6′,2″-terpyridin]-4′-yl)-1,3-phenylene)bis(oxy))bis(u

Full text of DOI:10.1016/j.poly.2021.115149

Polyhedron 200 (2021) 115149
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Design and synthesis of novel terpyridine-based ligands with one and
two terminal aurophilic moieties and their Rh(III) and Ru(II) complexes
for the adsorption on metal surfaces
⇑
Irina O. Salimova, Anna V. Berezina, Ilona A. Shikholina, Nikolai V. Zyk, Elena K. Beloglazkina
M.V. Lomonosov Moscow State University, Chemistry Department, Moscow 119991, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel terpyridine ligands with one and two terminal aurophilic sulfur containing fragments, (S)-11-(4-
([2,20:60,200-terpyridin]-40-yl)phenoxy)undecyl 5-(1,2-dithiolan-3-yl)pentanoate (7), (S)-((4-([2,20:60,200-
terpyridin]-40-yl)-1,3-phenylene)bis(oxy))bis(undecane-11,1-diyl) bis(5-((S)-1,2-dithiolan-3-yl)pentanoate)
(11a), (S)-((4-([2,20:60,200-terpyridin]-40-yl)-1,2-phenylene)bis(oxy))bis(undecane-11,1-diyl) bis(5-((S)-
1,2-dithiolan-3-yl)pentanoate) (11b), (S)-((5-([2,20:60,200-terpyridin]-40-yl)-1,3-phenylene)bis(oxy))bis
(undecane-11,1-diyl) bis(5-((S)-1,2-dithiolan-3-yl)pentanoate) (11c) were synthesized and employed
to synthesis of mononuclear ruthenium(II) and rhodium(III) complexes. Electrochemical studies of the
obtained ligands and coordination compounds and their ability to chemisorb on gold electrodes surfaces
have been carried out. Among the obtained metal complexes, compounds with the minimum time of
chemisorption with the formation of Au-S bonds were revealed, which are complexes of 3,4- and 3,5-sub-
stituted phenylterpyridines.
Received 18 November 2020
Accepted 5 March 2021
Available online 17 March 2021
Keywords:
2.20:60,200-Terpyridine
Aurophilic ligands
Ruthenium
Rhodium
Electrochemistry
Ó 2021 Elsevier Ltd. All rights reserved.
1. Introduction
with three functional groups, while one is able to coordinate a
metal atom, and the other two groups can bind to electrodes.
The interest in the development of metal-containing aurophilic
organic derivatives with one or two terminal sulfur-containing
groups in ligand fragments is associated with the promise of such
molecules for the creation of single-electron single-molecule tran-
sistors or charge-sensitive biosensors [1–7] and luminescent
probes [8,9]. In particular, the polypyridyl complexes of ruthe-
nium(II) and rhodium(III) are used as luminophores in DNA-based
biosensors [10–13].
The presence of two sulfur-containing groups in the organic
ligand, used to obtain the metal complexes, give possibility to such
coordination compounds chemosorb on two closely spaced gold
electrodes to form strong Au-S covalent bonds. One of the promis-
ing using of these compounds may be the single-molecular
sequencing based on charge-sensitive sensors. It is proposed to
use metal complexes fixed between planar nanoelectrodes as a
bridge-molecule in the sensors for detecting the one charge pair
separation as a result of DNA/RNA polymerization [14–16]. The
key to the practical implementation of such sequencing is the
development of reliable methods for producing organic ligands
Coordination compounds of ruthenium with bipyridine ligands
have been offered as sensors for various amino acid fragments
(phenylglycine, valine), as well as phosphate anions [17–19]. There
are works devoted to the creation of single-molecule sensors based
on aurophilic nitrogen-containing coordination compounds 4,40-
bipyridine, N0-bis(6-mercaptohexyl)-(4,40-bipyridine) and N,N0-bis
(6-acetoxythio)-(4,40-bipyridine) [20–22], as well as S-alkyl- and
ruthenium 4-pyridyl substituted terpyridine complexes [23] and
complexes of terpyridines with Zn(II) [24,25].
Coordination compounds of Fe, Co, Cr, Ru with ligands based on
substituted aurophilic terpyridines showed higher conductivity
compared to 4,40-bipyridine and its analogues. These results have
evidenced the terpyridine coordination compounds are highly per-
spective for using in single-molecule devices as an island [26,27].
Earlier, we proposed the method of single-molecule nanotransis-
tors producing based on gold electrodes located at the 4 nm
distance. The gap between electrodes is occupied by the chemi-
cally-formed symmetric aurophilic coordination compound of Rh
(III) with (S)-4-([2,20:60,200-terpyridin]-40-yl)phenyl 5-(1,2-dithi-
olan-3-yl)pentanoate. It is shown that in the obtained molecular
nanotransistors the electrons tunneling was carried out through
the rhodium atom was the only intramolecular charge center [28].
⇑
Corresponding author.
E-mail address: bel@org.chem.msu.ru (E.K. Beloglazkina).
https://doi.org/10.1016/j.poly.2021.115149
0277-5387/Ó 2021 Elsevier Ltd. All rights reserved.

I.O. Salimova, A.V. Berezina, I.A. Shikholina et al.
Polyhedron 200 (2021) 115149
The development of the above-mentioned single-molecule
devices requires the developing a convenient synthetic procedure
for the preparation of suitable organic ligands and their bis-ligand
coordination compounds that can be chemisorbed on two gold
electrodes located at a distance of several nm. It was previously
proved that when such a pair of electrodes is immersed in a solu-
tion of a bis-(S-containing ligand) coordination compound with the
distances between two terminal sulfur-containing groups no less
than the interelectrode gap, some of the molecules of the symmet-
ric aurophilic coordination compound can be adsorbed with Au-S
bonds formation on two different electrodes, connecting them
among themselves [28].
In this article, novel terpyridine ligands with one and two ter-
minal aurophilic fragments was designed to provide the following
criteria: (1) the ligand possibility to bound second-row transition
metal ions, in particular rhodium and ruthenium, forming
mononuclear coordination compounds [29,30]; (2) the presence
of the stable disulfide fragment, which at the same time provides
the possibility of rapid chemosorption on gold electrodes surface
with the formation of Au-S bonds; (3) the distance between the
sulfur-containing fragments is about 4 nm, which corresponds to
the achievable interelectrode gap in the nanoelectrode device
[28]. The target ligands (L) were synthesized and employed to syn-
thesize of ruthenium(II) and rhodium(III) complexes with the Ru(L)
DMSOCl2 or RhM(L)Cl3 composition. The possibility of the resulting
complexes adsorption on the gold electrodes surface was
demonstrated.
20 kV. Samples were applied to a polished steel substrate. Spectra
were recorded in positive ion mode. The resulting spectrum was a
sum of 50 spectra obtained at different points of the sample.
4-([2,20:60,200-Terpyridin]-40-yl)phenol syntheses was described
in [31]; the synthesis of [Rh(DMSO)3]Cl3 and [Ru(DMSO)4]Cl2 were
described in [32–34].
Electrochemical studies were performed at 25 °C using a IPC-
2000 potentiostat with the refinement program complex (devel-
oped in A. N. Frumkin Institute of Physical Chemistry and Electro-
chemistry, RAS; author V. E. Kasatkin, vadim_kasatkin@mail.ru;
see, for example http://www.expo.ras.ru/base/prod_data.asp?
prod_id=4687). Glass carbon and Au disks (both 2 mm in diameter)
polished with Al2O3 (<10 mm) were used as the working elec-
trodes, and a 0.1 m Bu4NClO4 solution in DMSO served as the sup-
porting electrolyte. Ag/AgCl/KCl(sat.) was used as the reference
electrode. The potentials are given with allowance for iR compen-
sation. All measurements were carried out under argon. The sam-
ples were dissolved in pre-deoxygenated solvent. For the ligands
and complexes chemisorption on gold electrode surface electrode
was immersed of in a saturated solution of corresponding disul-
fide-derivatized compound for 1–3 days, then the electrode was
removed, washed 2 times with DMSO and dried in air.
2.2. Synthesis
2.2.1. Synthesis of 4-((11-hydroxyundecyl)oxy)benzaldehyde (2)
To the solution of 4-hydroxybenzaldehyde 1 (0.5 g, 4.1 mmol)
in 50 ml of acetonitrile K2CO3 (1.1 g, 8.2 mmol) were added and
the resulting mixture was heated to 80 °C. After that, 11-bro-
moundecanol (2.2 g, 8.6 mmol) was introduced into the reaction
mixture. The mixture was stirred under heating for 25 h. After
the reaction completed, the solvent was distilled off, the resulting
mixture was suspended in water, extracted with CHCl3, and the
organic fraction was dried over Na2SO4. After the solvent removing
the target substance was recrystallized from a methanol:diethyl
ether mixture (1:1), washed with ethanol and dried in air. Thus,
0.98 g (82%) of compound 2 was obtained as a white powder. M.
p. 68–70 °C.
2. Experimental
2.1. Materials and methods
All starting materials were obtained from commercial sources
and used without additional purification. 1H and 13C NMR spectra
were obtained on a Bruker Avance spectrometer (400 and 100 MHz
respectively) instrument, internal standard was HMDS (d
0.05 ppm).
Preparative chromatographic separation of the reaction mix-
tures was carried out using the INTERCHIM puriFlash 430
chromatograph.
High resolution mass spectra were recorded on the Orbitrap
Elite high resolution mass spectrometer. Solutions of samples in
DMSO with 1% formic acid were introduced into the ionization
source by electrospray. HRMS spectra of rhodium-containing coor-
dination compounds 13 and 14 were recorded with the addition of
1% silver nitrate.
NMR 1H (400 MHz, CDCl3, d, ppm): 9.87 (s. 1H), 7.81–7.83 (m,
2H), 6.99 (d, 2H, J = 8.7 Hz), 4.03 (t, 2H, J = 6.6 Hz), 3.63 (t, 2H,
J = 6.7 Hz), 1.79–1.83 (m, 2H), 1.54–1.58 (m, 2H), 1.29–1.46 (m,
14H).
NMR 13C (100 MHz, CDCl3, d, ppm): 190.62, 164.28, 131.90,
129.88, 114.78, 68.44, 63.01, 33.80, 32.79, 29.50, 29.42, 29.35,
29.26, 29.04, 28.70, 28.13, 25.91, 25.71.
LC-MS: Calculated, m/z: 292.20. C18H28O3. Found: ([M+H]+)
293.24.
Mass spectra with laser ionization (LI MS) were recorded on a
Bruker Autoflex II instrument (FWHM 18000) with the 337 nm
nitrogen laser, a time-of-flight mass analyser operating in reflec-
tron mode, and an accelerating voltage of 20 kV. Samples were
applied to a polished steel substrate. Spectra were recorded in pos-
itive ion mode. The resulting spectrum was a sum of 50 spectra
obtained at different points in the sample.
For HPLC analysis system with Shimadzu Prominence LC-20 col-
umn and a convection fraction collector connected with a single
quadrupole mass spectrometer Shimadzu LCMS-2020 with dual
ionization source DUIS-ESI-APCI were used. The analytical and
preparative column was Phenomenex Luna 3u C18 100A.
Elemental analyses were performed on a Vario MICRO cube
CHNS Elementar.
2.2.2. Synthesis of 11-(4-([2,20:60,200-terpyridin]-40-yl)phenoxy)
undecan-1-ol (3)
To the solution of KOH (0.16 g, 2.06 mmol) in 10 ml of ethanol
4-((11-hydroxyundecyl)oxy)benzaldehyde 2 (0.3 g, 1.03 mmol),
then 2-acetylpyridine (0.235 ml, 2.06 mmol) and after 10 min an
excess (0.4 ml, 10.3 mmol) of 25% aqueous ammonia were added.
The reaction mixture was stirred at 50 °C for two days. After the
reaction completed, the solvent was distilled off, and the residue
was recrystallized from methanol, washed with diethyl ether and
dried in air. Thus, 0.21 g (42%) of the compound 3 was obtained
as a white powder. M.p. 161–163 °C.
NMR 1H (400 MHz, CDCl3, d, ppm): 8.68–8.74 (m, 6H), 7.88–7.90
(m, 4H), 7.37 (d, 2H, J = 3.5 Hz), (dd, 2H, J1 = 8.3 Hz, J2 = 3.5 Hz), 4.04
(d, 2H, J = 4.2 Hz), 3.63–3.67 (m, 2H), 1.74–1.82 (m, 2H), 1.32–1.57
(m, 16H).
Electronic spectra in the UV and visible regions were recorded
on a Hitachi U-2900 instrument. Mass spectra with laser ionization
(LI-MS) recorded on a Bruker Autoflex II instrument (FWHM
18000) with the 337 nm nitrogen laser and a time-of-flight mass
analyzer operating in the reflection mode. Accelerating voltage
NMR 13C (100 MHz, CDCl3, d, ppm): 160.14, 156.45, 155.82,
149.81, 149.07, 136.81, 130.50, 128.47, 123.71, 121.37, 118.25,
114.91, 68.14, 63.00, 32.81, 29.47, 29.26.
2

I.O. Salimova, A.V. Berezina, I.A. Shikholina et al.
Polyhedron 200 (2021) 115149
LC-MS: Calculated, m/z: 495.29. C32H37N3O2. Found: ([M+H]+)
2.2.5. Synthesis of bis((11-hydroxyundecyl)oxy)benzaldehydes
496.30.
(general procedure).
To the solution of dihydroxybenzaldehyde 8a-c in 60 ml of ace-
tonitrile K2CO3 was added and the mixture was heated to 80 °C.
After that, 11-bromoundecanol was introduced into the reaction
mixture. The mixture was stirred under heating for 40 h. After
the reaction complete, the solvent was distilled off, the resulting
mixture was suspended in water and extracted from chloroform.
The combined organic fractions were dried over Na2SO4, the sol-
vent was remover and the residue was recrystallized from a
methanol:diethyl ether mixture (1:1), washed with diethyl ether
and dried in air.
2.2.3. Synthesis of (S)-11-bromoundecyl 5-(1,2-dithiolan-3-yl)
pentanoate (5)
To the solution of lipoic acid (0.206 g; 1 mmol) in 20 ml of
dichloromethane DMAP (0.012 g; 0.1 mmol), EDCꢀHCl (0.210 g;
1.1 mmol) and 11-bromo-undecanol-1 (0.251 ml; 1 mmol) were
added. The mixture was stirred for 24 h in the inert atmosphere.
The solvent was removed under reduced pressure and the residue
was purified by column chromatography (Puriflash 15
l 25 g, elu-
ent: P.E. (100%)/EtOAc (0%) = > P.E. (90%)/EtOAc (10%) for 15 min).
Thus, 0.360 g (82%) of the compound 5 was obtained as yellow oil.
NMR 1H (400 MHz, CDCl3, d, ppm): 4.05 (t, 2H, J = 6.7 Hz), 3.51–
3.62 (m, 1H), 3.40 (t, 2H, J = 6.7 Hz), 3.05–3.23 (m, 2H), 2.41–2.51
(m, 1H), 2.31 (t, 2H, J = 7.4 Hz), 1.80–1.96 (m, 3H), 1.57–1.75 (m,
6H), 1.37–1.54 (m, 4H), 1.23–1.37 (m, 12H).
2.2.5.1. Synthesis of 2,4-bis((11-hydroxyundecyl)oxy)benzaldehyde
(9a). From 1.0 g of 2,4-dihydroxybenzaldehyde 8a (7.25 mmol),1.0
of potassium carbonate (29.0 mmol) and 4.0 g of 11-bromounde-
canol (15.9 mmol),1.32 g (38%) of the compound 9a was obtained
as a light brown powder. M.p. 94–97 °C.
LC-MS: Calculated, m/z: 297.97. C19H35BrO2S2. Found: ([M+H]+)
298.93.
NMR 1H (400 MHz, CDCl3, d, ppm): 9.70 (s, 1H), 7.42 (d, 1H,
J = 8.6 Hz), 6.53 (d, 1H, J = 6.5 Hz), 6.41 (s, 1H), 4.00 (t, 4H,
J = 6.5 Hz), 3.64 (t, 4H, J = 6.6 Hz), 1.30–1.89 (m, 36H).
NMR 13C (100 MHz, CDCl3, d, ppm): 188.59, 165.83, 163.40,
130.21, 118.81, 106.17, 98.90, 68.44, 63.04, 32.78, 29.43, 29.06,
26.02, 25.75.
2.2.4. Synthesis of (S)-11-(4-([2,20:60,200-terpyridin]-40-yl)phenoxy)
undecyl 5-(1,2-dithiolan-3-yl)pentanoate (7)
2.2.4.1. Method I. To the solution of lipoic acid (0.17 g, 0.84 mmol)
in 20 ml of DMF HOBt (0.17 g, 1.26 mmol), HBTU (0.48 g,
1.26 mmol) and DIPEA (0.3 ml, 1.68 mmol) were added and the
mixture was stirred in the inert atmosphere for 40 min. After that
11-(4-([2,20:60,200-terpyridin]-40-yl)phenoxy)undecan-1-ol 3 were
added and the reaction mixture was stirred in the inert atmo-
sphere for 48 h. Then the solvent was removed under reduced
pressure and the residue was purified by column chromatography
LC-MS: Calculated, m/z: 478.37.C29H50O5. Found: ([M+H]+)
479.35.
2.2.5.2. Synthesis of 3,4-bis((11-hydroxyundecyl)oxy)benzaldehyde
(9b). From 1.0 g of 3,4-dihydroxybenzaldehyde 8b (7.25 mmol),
1.00 g of potassium carbonate (29.0 mmol) and 4.0 g of 11-bro-
moundecanol (15.9 mmol) 2.63 g (76%) of the compound 9b was
obtained as a white powder. M.p. 88–90 °C.
(Puriflash 15
l
25 g, eluent: petroleum ether => petroleum ether
NMR 1H (400 MHz, CDCl3, d, ppm): 9.82 (s. 1H), 7.39–7.43 (m,
2H), 6.95 (d, 1H, J = 8.1 Hz), 4.03–4.09 (v, 4H), 3.64 (t, 4H,
J = 6.6 Hz), 2.57 (br.s., 2H), 1.84 (sxt, 4H, J = 7.2 Hz), 1.56 (quin,
4H, J = 6.8 Hz), 1.46–1.49 (m, 4H), 1.25–1.34 (m, 24H).
NMR 13C (100 MHz, CDCl3, d, ppm): 191.14, 154.66, 149.38,
129.80, 126.73, 111.68, 110.81, 69.08, 63.01, 32.78, 29.58, 29.41,
25.98, 25.78.
(50%)/EtOAc + NH3ꢀH2O (50%) =>EtOAc + NH3ꢀH2O (100%) for
26 min. The ratio of EtOAc and NH3ꢀH2O is 1:0.0025. Thus, 0.18 g
(66%) of the compound 7 was obtained as white-green oily crystals.
2.2.4.2. Method II. To the suspension of 4-(2.2:60,200- terpyridin-40-
yl) phenol (0.4 g, 1.23 mmol), and cesium carbonate (4.0 g,
12.3 mmol) in 100 ml of acetonitrile 11-bromoundecyl-5-(1,2-
dithiolan-3-yl)pentanoate 5 (0.81 g, 1.85 mmol) was added. The
resulting mixture was boiled with stirring for 40 h. Then the sol-
vent was distilled off under reduced pressure, the resulting mix-
ture was suspended in water and extracted with chloroform. The
combined organic fractions were dried over Na2SO4 and the sol-
vent was distilled off. The residue was purified by column chro-
LC-MS: Calculated, m/z: 478.37. C29H50O5. Found: ([M+H]+)
479.35.
2.2.5.3. Synthesis of 3,5-bis((11-hydroxyundecyl)oxy)benzaldehyde
(9c). From 1.0 g of 3,5-dihydroxybenzaldehyde 8c (7.25 mmol),
1.00 g of potassium carbonate (29.0 mmol) and 4.0 g of 11-bro-
moundecanol (15.9 mmol) 3.05 g (88%) of the compound 9c was
obtained as a white powder. M.p. 96–99 °C.
matography (Puriflash 15
l
40 g, eluent: P.E. (100%)/
NMR 1H (400 MHz, CDCl3, d, ppm): 9.88 (s. 1H), 6.98 (d, 2H,
J = 2.3 Hz), 6.69 (t, 1H, J = 2.2 Hz), 3.98 (t, 4H, J = 6.5 Hz), 3.64 (t,
4H, J = 6.6 Hz), 2.03 (br.s., 2H), 1.75–1.85 (m, 4H), 1.25–1.58 (m,
32H).
EtOAc + NH3*H2O (0%) => P.E. (50%)/EtOAc + NH3ꢀH2O (50%) => P.
E. (0%)/EtOAc + NH3*H2O (100%) for 20 min. The ratio of EtOAc
and NH3*H2O is 1:0.0025). Thus, 0.46 g (54%) of the compound 7
was obtained as white-green oily crystals.
NMR 13C (100 MHz, CDCl3, d, ppm): 191.95, 160.82, 138.40,
108.17, 107.68, 68.48, 63.01, 32.78, 29.44, 29.26, 29.10, 25.94,
25.71.
NMR 1H (400 MHz, CDCl3, d, ppm): 8.76–8.79 (m, 6H), 7.93–8.00
(m, 4H), 7.43 (dd, 2H, J1 = 6.6 Hz, J2 = 5.3 Hz), 7.01 (d, 2H, J = 8.8 Hz)
4.04 (dt, 4H, J1 = 17 Hz, J2 = 6.6 Hz), 3.53–3.60 (m, 1H), 3.07–3.20
(m, 2H), 2.41–2.49 (m, 2H), 2.31 (t, 2H, J = 7.4 Hz), 1.24–1.94 (m,
24H).
LC-MS: Calculated, m/z: 478.37. C29H50O5. Found: ([M+H]+)
479.35.
NMR 13C (100 MHz, CDCl3, d, ppm): 173.67, 160.11, 156.37,
155.79, 149.81, 149.09, 136.91, 130.43, 128.50, 123.79, 121.40,
118.24, 114.83, 68.11, 64.56, 56.36, 40.23, 38.50, 34.62, 34.14,
29.72, 29.53, 29.28, 28.80, 28.65, 26.07, 25.96, 24.74.
LC-MS: Calculated, m/z: 683.32.C40H39N3O3S2. Found: ([M+H]+)
684.35.
2.2.6. Synthesis of 11,110-(([2,20:60,200-terpyridin]-40-yl)phenylene)bis
(oxy))bis(undecan-1-ol)s (general procedure)
To the solution of KOH in 20 ml of ethanol bis((11-hydroxyun-
decyl)oxy)benzaldehydes 9a-c, then 2-acetylpyridine and after
10 min an excess (0.8 ml, 21.0 mmol) of 25% aqueous ammonia
were added. The reaction mixture was stirred at 50 °C for two days.
After the reaction completed, the solvent was distilled off, the tar-
get substance was recrystallized from methanol, washed with
diethyl ether and dried in air.
HRMS: Calculated ([M+H]+): 684.3294. C40H49N3O3S2. Found:
([M+H]+) 684.3298.
UV–vis: (k, nm, (
e
, lꢀmolꢁ1ꢀcmꢁ1)): 289 (43650).
3

I.O. Salimova, A.V. Berezina, I.A. Shikholina et al.
Polyhedron 200 (2021) 115149
2.2.6.1. Synthesis of 11,110-((4-([2,20:60,200-terpyridin]-40-yl)-1,3-phe-
nylene)bis(oxy))bis(undecan-1-ol) (10a). From 1.0 g of 2,4-bis((11-
hydroxyundecyl)oxy)benzaldehyde 9a (2.1 mmol), 0.23 g of potas-
sium hydroxide (4.2 mmol), 0.47 ml of 2-acetylpyridine (4.2 mmol)
0.66 g (46%) of the compound 10a was obtained as a white powder.
M.p. 170–171 °C.
1,3-phenylene)bis(oxy))bis(undecan-1-ol) 10a (0.44 mmol) 0.27 g
(58%) of the compound 11a was obtained as yellow oil.
NMR 1H (400 MHz, CDCl3, d, ppm): 8.74–8.83 (m, 6H), 8.03 (t,
2H, J = 7.7 Hz), 7.63 (d, 1H, J = 8.4 Hz), 7.45 (t, 2H, J = 5.9 Hz),
6.61 (dd, 1H, J1 = 8.5 Hz, J2 = 2 Hz), 6.55 (d, 1H, J = 2.1 Hz), 4.00–
4.08 (m, 8H), 3.08–3.66 (m, 6H), 2.41–2.49 (m, 2H), 2.31 (t, 4H,
J = 7.4 Hz), 1.10–1.94 (m, 48H).
NMR 1H (400 MHz, CDCl3, d, ppm): 8.56–8.60 (m, 6H), 7.80 (t,
2H, J = 7.7 Hz), 7.41–7.44 (m, 1H), 7.26 (t, 2H, J = 5.6 Hz), 6.50 (d,
2H, J = 8.4 Hz), 3.90 (t, 4H, J = 6.3 Hz), 3.45 (qw, 4H, J = 7.0 Hz),
1.62–1.88 (m, 4H), 1.02–1.46 (m, 32H).
LC-MS: Calculated, m/z: 1057.52. C59H83N3O6S4. Found: ([M
+H]+) 1058.50.
HRMS: Calculated ([M+H]+): 1058.5243, C59H83N3O6S4. Found:
([M+H]+) 1058.5236.
NMR 13C (100 MHz, CDCl3, d, ppm): 160.89, 156.78, 155.01,
149.06, 136.65, 131.21, 123.40, 121.64, 121.21, 105.53, 100.30,
68.56, 68.15, 63.06, 32.81, 29.40, 26.14, 26.00, 25.74.
LC-MS: Calculated, m/z: 681.45. C43H59N3O4. Found: ([M+H]+)
682.45.
UV–vis: (k, nm, (
286 (51400).
e
, lꢀmolꢁ1ꢀcmꢁ1)): 322 (26950), 305 (36650),
2.2.7.2. Synthesis of (S)-((4-([2,20:60,200-terpyridin]-40-yl)-1,2-pheny-
lene)bis(oxy))bis(undecane-11,1-diyl) bis(5-((S)-1,2-dithiolan-3-yl)
pentanoate) (11b). From 0.38 g of lipoic acid (1.8 mmol), DIPEA
(0.6 ml, 3.7 mmol), HOBt (0.37 g, 2.7 mmol), HBTU (1.05 g,
2.7 mmol)and 0.3 g of 11,110-((4-([2,20:60,200-terpyridin]-40-yl)-
1,2-phenylene)bis(oxy))bis(undecan-1-ol) 10b (0.44 mmol) 0.28 g
(61%) of the compound 11b was obtained as light green oil.
NMR 1H (400 MHz, CDCl3, d, ppm): 8.74 (d, 2H, J = 4.7 Hz), 8.66–
8.68 (m, 4H), 7.89 (td, 2H, J1 = 7.7 Hz, J2 = 1,8 Hz), 7.48 (dd, 1H,
J1 = 8.3 Hz, J2 = 2,1 Hz), 7.42 (d, 1H, J = 2.1 Hz), 7.36 (ddd, 2H,
J1 = 7.4 Hz, J2 = 1,8 Hz, J3 = 2.1 Hz), 6.99 (d, 1H, J = 8.3 Hz), 4.04–
4.15 (m, 8H), 3.53–3.60 (m, 2H), 3.08–3.21 (m, 4H), 2.42–2.50
(m, 2H), 2.31 (td, 4H, J1 = 7.4 Hz, J2 = 2.6 Hz), 1.83–1.95 (m, 6H),
1.24–1.74 (m, 44H).
2.2.6.2. Synthesis of 11,110-((4-([2,20:60,200-terpyridin]-40-yl)-1,2-phe-
nylene)bis(oxy))bis(undecan-1-ol) (10b). From 1.0 g of 3,4-bis((11-
hydroxyundecyl)oxy)benzaldehyde 9b (2.1 mmol), 0.23 g of potas-
sium hydroxide (4.2 mmol), 0.47 ml of 2-acetylpyridine (4.2 mmol)
0.65 g (45%) of the compound 9b was obtained as a white powder.
M.p. 182–185 °C.
NMR 1H (400 MHz, CDCl3, d, ppm): 8.56–8.61 (m, 6H), 7.78–7.85
(m, 2H), 7.30–7.36 (m, 4H), 6.84 (d, 1H, J = 8.4 Hz), 3.99 (t, 2H,
J = 6.3 Hz), 3.89 (t, 2H, J = 6.3 Hz), 3.39 (dt, 4H, J1 = 6.5 Hz,
J2 = 3.3 Hz), 3.22 (br.s., 2H), 1.60–1.67 (m, 4H), 1.12–1.35 (m, 32H).
NMR 13C (100 MHz, CDCl3, d, ppm): 156.44, 155.78, 150.36,
149.52, 149.04, 136.90, 131.24, 123.75, 121.48, 120.37, 118.51,
113.87, 113.17, 69.71, 69.25, 62.98, 58.38, 50.73, 32.78, 29.45,
25.99, 25.74, 18.37.
LC-MS: Calculated, m/z: 1057.52. C59H83N3O6S4. Found: ([M
+H]+) 1058.50.
LC-MS: Calculated, m/z: 681.45.C43H59N3O4. Found: ([M+H]+)
682.45.
HRMS: Calculated ([M+H]+): 1058.5243, C59H83N3O6S4. Found
([M+H]+): 1058.5214.
UV–vis: (k, nm, (
306 (41,750), 287 (57,400), 256 (30,300).
e
, lꢀmolꢁ1ꢀcmꢁ1)): 381 (4000), 331 (24,550),
2.2.6.3. Synthesis of 11,110-((5-([2,20:60,200-terpyridin]-40-yl)-1,3-phe-
nylene)bis(oxy))bis(undecan-1-ol) (10c). From 1.0 g of 3,5-bis((11-
hydroxyundecyl)oxy)benzaldehyde 9c (2.1 mmol), 0.23 g of potas-
sium hydroxide (4.2 mmol), 0.47 ml of 2-acetylpyridine (4.2 mmol)
0.65 g (45%) of the compound 10c was obtained as a white powder.
M.p. 187–188 °C.
2.2.8. Synthesis of (S)-((5-([2,20:60,200-terpyridin]-40-yl)-1,3-
phenylene)bis(oxy))bis(undecane-11,1-diyl) bis(5-((S)-1,2-dithiolan-
3-yl)pentanoate) (11c)
From 0.38 g of lipoic acid (1.8 mmol), DIPEA (0.6 ml, 3.7 mmol),
HOBt (0.37 g, 2.7 mmol), HBTU (1.05 g, 2.7 mmol)and 0.3 g of
11,110-((5-([2,20:60,200-terpyridin]-40-yl)-1,3-phenylene)bis(oxy))
bis(undecan-1-ol) 10c (0.44 mmol) 0.29 g (63%) of the compound
11c was obtained as yellow oil.
NMR 1H (400 MHz, CDCl3, d, ppm): 8.71 (d, 2H, J = 4.0 Hz), 8.64–
8.67 (m, 4H), 7.88 (d, 2H, J = 1.5 Hz), 7.36 (d, 2H, J = 1.4 Hz), 7.01 (d,
2H, J = 2.1 Hz), 6.54 (s, 1H),4.03 (t, 4H, J = 6.4 Hz), 3.61(t, 4H,
J = 6.7 Hz), 2.03 (br.s., 2H), 1.29–2.05 (m, 36H).
NMR 13C (100 MHz, CDCl3, d, ppm): 160.78, 156.33, 155.86,
150.59, 149.10, 140.61, 136.84, 123.77, 121.41, 119.10, 106.13,
101.91, 68.28, 63.03, 32.81, 29.50, 29.35, 26.03, 25.72,
LC-MS: Calculated, m/z: 681.45.C43H59N3O4. Found: ([M+H]+)
682.45.
NMR 1H (400 MHz, CDCl3, d, ppm): 8.73 (d, 2H, J = 4.2 Hz), 8.66–
8.70 (m, 4H), 7.88 (td, 2H, J1 = 7.7 Hz, J2 = 1.7 Hz), 7.36 (ddd, 2H,
J1 = 7.4 Hz, J2 = 4.8 Hz, J3 = 1.1 Hz), 7.00 (d, 2H, J = 2.1 Hz), 6.55
(t, 1H, J = 2.1 Hz), 4.02–4.07 (m, 8H), 3.53–3.60 (m, 2H), 3.07–
3.20 (m, 4H), 2.45 (td, 2H, J1 = 12.3 Hz, J2 = 6.5 Hz), 2.31 (t, 4H,
J = 7.4 Hz), 1.86–1.94 (m, 2H), 1.78–1.85 (m, 4H), 1.58–1.74 (m,
14H), 1.24–1.54 (m, 30H).
2.2.7. Synthesis of compounds 11a-c (general procedure)
To the solution of lipoic acid in 20 ml of DMF HOBt, HBTU and
DIPEA were added and the mixture was stirred in the inert atmo-
sphere for 40 min. After that bis(11-hydroxoundecyloxo)-
2.20:60,200-terpyridine 10a-c were added and the reaction mixture
was stirred in the inert atmosphere for 48 h. Then the solvent
was removed under reduced pressure and the residue was purified
NMR 13C (100 MHz, CDCl3, d, ppm): 173.68, 160.73, 156.22,
155.81, 150.57, 149.10, 140.54, 136.97, 123.88, 121.45, 119.10,
106.00, 101.74, 68.24, 64.57, 60.44, 56.36, 40.23, 38.50, 34.62,
34.14, 29.54, 29.44, 29.28, 28.80, 28.64, 26.10, 25.96, 24.73,
21.11, 14.22.
LC-MS: Calculated, m/z: 1057.52. C59H83N3O6S4. Found: ([M
by column chromatography (Puriflash 15
ether => petroleum ether (50%)/EtOAc
=>EtOAc + NH3ꢀH2O (100%) for 26 min. The ratio of EtOAc and NH3-
ꢀH2O is 1:0.0025.
l
25 g, eluent: petroleum
NH3ꢀH2O (50%)
+H]+) 1058.50.
+
HRMS: Calculated ([M+H]+): 1058.5243, C59H83N3O6S4. Found
([M+H]+): 1058.5234.
UV–vis: (k, nm, (
259 (39400).
e
, lꢀmolꢁ1ꢀcmꢁ1)): 317 (19300), 281 (64200),
2.2.7.1. Synthesis of (S)-((4-([2,20:60,200-terpyridin]-40-yl)-1,3-pheny-
lene)bis(oxy))bis(undecane-11,1-diyl) bis(5-((S)-1,2-dithiolan-3-yl)
pentanoate) (11a). From 0.38 g of lipoic acid (1.8 mmol), DIPEA
(0.6 ml, 3.7 mmol), HOBt (0.37 g, 2.7 mmol), HBTU (1.05 g,
2.7 mmol) and 0.3 g of 11,110-((4-([2,20:60,200-terpyridin]-40-yl)-
2.2.9. Synthesis of coordination compounds (general procedure)
The solution of ligands 7, 11a-c in 2 ml of dry EtOH was heated
to 70 °C. After that, the solution of metal salt in 2 ml dry EtOH was
added. The mixture was stirred for 24 h under reflux. The formed
4

I.O. Salimova, A.V. Berezina, I.A. Shikholina et al.
Polyhedron 200 (2021) 115149
precipitate was filtered off, washed with ethanol, chloroform,
diethyl ether and dried in air.
2.2.9.7. Coordination compound 14c. From 0.03 g of ligand 11c
(0.03 mmol), 0.014 g of [Ru(DMSO)4]Cl2(0.03 mmol), 0.016 g
(43%) of the compound 14c was obtained as a dark brown powder.
M.p. > 250 °C.
2.2.9.1. Coordination compound 12a. From 0.03 g of ligand 7
(0.04 mmol), 0.018 g of [Rh(DMSO)3]Cl3(0.04 mmol), 0.02 g (51%)
of the compound 12a was obtained as orange powder. M.
p. > 250 °C.
Elemental analysis, calculated: C, 55.98; H, 3.21; N, 6.85. C61
-
H
89Cl2N3O7RuS5. Found: C, 56.09; H, 4.43; N, 3.32.
LI-MS: Calculated ([M+2H]++CH3CN): 1350,4147 C61H89N3Cl2-
O7RuS5H2. Found ([M+2H]++CH3CN): 1350.4182
Elemental analysis, calculated: C, 55.89; H, 6.60; N, 3.31. C40
-
H49Cl3N3O3RhS2. Found: C, 56.16; H, 6.72; N, 3.62.
HRMS: Calculated ([M+Ag]): 998.0387. C40H49N3Cl3O3AgRhS2.
3. Results and discussion
Found: ([M+Ag]) 998.0372
UV–vis: (k, nm, (
(47500), 259 (31150).
e
, lꢀmolꢁ1ꢀcmꢁ1)): 381 (6250), 313 (26650), 288
3.1. Ligands synthesis
The structure of the organic ligands 7, 11a-c synthesized in this
work is shown in Schemes 1, 2. Compounds 7, 11a-c contain the
terpyridine fragment for metal ion coordination, and two disulfide
groups in the lipoic acid fragment, which allow adsorption on gold
electrodes. The length of linker fragment between the two disulfide
groups was selected in such way as to ensure a distance between
them about 4 nm, corresponding to the achievable inter electrode
gap in the nanoelectrode device [28]. The initial development of
the synthetic scheme was carried out at the example ligand 7 with
one disulfide fragment; this ligand can be used for the synthesis of
mono- and bis-ligand coordination compounds with increased sta-
bility compared to previously described ligands of similar structure
with thioacetate aurophilic moiety [30,31].
The strategy development for obtaining the target ligands using
the example of model ligand 7 was carried out using two synthetic
sequences, designated as Method 1 and Method 2 in Scheme 1.
According to Method 1, the 4-hydroxybenzaldehyde 1 was first
alkylated with 11-bromundecanol in the presence of base
(K2CO3) according to SN2 reactions to give substituted benzalde-
hyde 2, which was further converted to 40-substituted terpyridine
3 [35]; subsequent esterification with lipoic acid at the action of
HOBt/HBTU mixture yielded the target ligand 7. Method 2 con-
sisted in the initial alkylation of lipoic acid with 11-bromunde-
canol to give ester 5, and then the resulting product was reacted
with 4-([2,20:60,200-terpiridin]-40-yl) phenol 6 obtained by the
described procedure [31]. Generally, Method 1, despite a greater
number of synthetic stages, gives a better yield of the target pro-
duct, and is easier than the experimental plan. This method was
further used to obtain ligands with two sulfur-containing
fragments.
The synthetic sequence for the preparation of the target ter-
pyridine ligands 11a-c with two sulfur-containing fragments is
shown in Scheme 2. Considering the data obtained for model
ligand 7, the synthesis of compounds 11a-c was carried out accord-
ing to the initial modification scheme of phenol fragments 2,4-,
3,4- and 3,5-dihydroxybenzaldehyde 8a-c followed by conversion
of the obtained 9a-c esters to the corresponding terpyridines
10a-c in ~45% yields. The ester bond formation with lipoic acid
was carried out using the mixture of HOBt and HBTU in the pres-
ence of the base, similarly to the described methods [36,37].
When optimizing the preparation conditions ligands 11a-c, it
was found in all cases for the formation of bis-functionalization
products of both OH groups of the initial dioles 10a-c, a large
excess of lipoic acid and the corresponding activators have to be
used. Otherwise the mono-esterification productsat one of the
hydroxyl groups of the starting compounds are mainly formed.
Under the optimized conditions, target ligands 11a-c were
obtained in 58–63% yields from diols 10a-c (10%, 21%, 25% from
aldehydes 10a-c). In the synthesis of intermediate compounds
9a-c, the main by-products also were the monoalkylation product
at one of the hydroxyl groups of starting compounds 8; to suppress
this undesirable reaction, a large excess of the starting 11-bro-
2.2.9.2. Coordination compound 12b. From 0.03 g of ligand 7
(0.04 mmol), 0.019 g of [Ru(DMSO)4]Cl2(0.03 mmol), 0.02 g (50%)
of the compound 12b was obtained as dark brown powder. M.
p. > 250 °C.
Elemental analysis, calculated: C, 54.01; H, 5.94; N, 4.50. C42
-
H
55Cl2N3O4RuS3. Found: C, 54.27; H, 5.84; N, 4.38.
LI-MS: Calculated ([M+2H]++CH3CN): 976.2197 C42H55N3Cl2O4-
RuS3H2. Found: ([M+2H]++CH3CN) 976.2195
UV–vis: (k, nm, (
e
, lꢀmolꢁ1ꢀcmꢁ1)): 500 (7400), 332 (33300), 312
(38500), 291 (40800).
2.2.9.3. Coordination compound 13a. Elemental analysis, calculated:
C, 55.89; H, 6.60; N, 3.31. C59H83Cl3N3O6RhS4. Found: C, 56.28; H,
6.48; N, 3.50.
From 0.03 g of ligand 11a (0.03 mmol), 0.013 g of [Rh(DMSO)3]
Cl3(0.03 mmol), 0.015 g (42%) of the compound 13a was obtained
as orange powder. M.p. > 250 °C.
UV–vis: (k, nm, (
(69550), 261 (44000).
e
, lꢀmolꢁ1ꢀcmꢁ1)): 381 (1750), 318 (35100), 285
2.2.9.4. Coordination compound 13b. From 0.03 g of ligand 11b
(0.03 mmol), 0.013 g of [Rh(DMSO)3]Cl3(0.03 mmol), 0.015 g
(42%) of the compound 13b was obtained as orange powder. M.
p. > 250 °C.
Elemental analysis, calculated: C, 55.89; H, 6.60; N, 3.31. C59
-
H83Cl3N3O6RhS4. Found: C, 55.94; H, 6.52; N, 3.09.
HRMS: Calculated ([M+Ag]): Exact Mass: 1372.2336 C59H83N3-
Cl3O6AgRhS4. Found: ([M+Ag]) 1373.2362
UV–vis: (k, nm, (
317 (42950), 284 (79600), 261 (58300).
e
, lꢀmolꢁ1ꢀcmꢁ1)): 383 (11350), 329 (34200),
2.2.9.5. Coordination compound 13c. From 0.03 g of ligand 11c
(0.03 mmol), 0.013 g of [Rh(DMSO)3]Cl3(0.03 mmol), 0.014 g (4%)
of the compound 13c was obtained as orange powder. M.
p. > 250 °C.
Elemental analysis, calculated: C, 55.89; H, 6.60; N, 3.31. C59
-
H83Cl3N3O6RhS4. Found: C, 56.15; H, 6.95; N, 3.35.
HRMS: Calculated ([M+Ag]): Exact Mass: 1372.2336 C59H83N3-
Cl3O6AgRhS4. Found: ([M+Ag]) 1372.2331
UV–vis: (k, nm, (
(15200), 281 (37950), 257 (24450).
e
, lꢀmolꢁ1ꢀcmꢁ1)): 380 (500), 339 (4450), 316
2.2.9.6. Coordination compound 14b. From 0.03 g of ligand 11b
(0.03 mmol), 0.014 g of [Ru(DMSO)4]Cl2(0.03 mmol), 0.015 g
(41%) of the compound 14b was obtained as a dark brown powder.
M.p. > 250 °C.
Elemental analysis, calculated: C, 55.98; H, 3.21; N, 6.85. C61
-
H
89Cl2N3O7RuS5. Found: C, 56.29; H, 3.40; N, 6.58.
LI-MS: Calculated ([M+2H]++CH3CN): 1350,4147 C61H89N3Cl2-
O7RuS5H2. Found: ([M+2H]++CH3CN) 1350.4172
5

I.O. Salimova, A.V. Berezina, I.A. Shikholina et al.
Polyhedron 200 (2021) 115149
Scheme 1. Synthesis of the ligand 7.
Scheme 2. Synthesis of ligands 11a-c.
6

I.O. Salimova, A.V. Berezina, I.A. Shikholina et al.
Polyhedron 200 (2021) 115149
Table 1
mundecanol-1 was used and the reaction time was increased to
40 h.
Red
Ox
Electrochemical reduction (Ep
complexes measured by the CV technique in DMSO on GC and Au electrodes in the
presence of 0.1 M Bu4NClO4. The potential scan rate was 200 mV/s*.
) and oxidation (Ep ) potentials of ligands and
The structures and purity of all obtained organic compounds
were confirmed by 1H and 13C NMR spectra, high performance liq-
uid chromatography and mass-spectrometry. For compounds 2,
9a-c, the characteristic signals in the 1H NMR spectra are the alde-
hyde group signals at ~9.7 ppm and the triplets of CH2O fragments
at ~4.0 ppm with J ~ 6.5 Hz. Compounds 3 and 10a-c are character-
ized by the presence of similar signals at ~4.0 ppm, as well as by
the significant shift proton signals of the aromatic region to the
weakly field region, which confirms the formation of the ter-
pyridine system. The main characteristic features of the 1H NMR
spectra of target ligands 11a-c, as well as model ligand 7, are an
increase in the integrated signal intensity in the region of
~4.0 ppm with the simultaneous change in their multiplicity, as
well as the appearance of lipoic acid characteristic signals: the
CH2COO groups triplet at ~ 2.31 ppm with J = 7.4 Hz, CH-S group
multiplet at ~3.5 ppm and two signals of cyclic CH2 groups at
~3.1 and ~2.5 ppm.
3.2. Synthesis of coordination compounds
Based on the obtained ligand 7 with one aurophilic fragment,
we investigated the possibility to preparate its rhodium-containing
coordination compound using the reaction with rhodium (III) chlo-
ride hydrate. However, the isolation of individual product 12a in
this reaction turned out to be hindered by the extremely low solu-
bility of RhCl3ꢀ3H2O in organic solvents and by the formation of the
ester bond hydrolysis product of ligand 7 when the reaction is car-
ried out in water-containing mixtures. Taking this into account, we
used complexes [Rh(DMSO)3]Cl3 and [Ru(DMSO)4]Cl2, which are
readily soluble in organic solvents, as the sources of Rh(III) and
Ru(II). DMSO-containing complexes were obtained from the corre-
sponding hydrates of metal chlorides using the described proce-
dures [32–34]. Thus, coordination compounds 12a,b were
synthesized according to a procedure similar to that previously
described for the preparation of monoterpyridine rhodium and
ruthenium derivatives [38] by refluxing of the ligand and metal
salt in ethyl alcohol (Scheme 3). To obtain coordination com-
pounds based on ligands with two sulfur-containing fragments
(13a-c, 14b, c; Scheme 4), the same method was used.
The structures of coordination compounds 12 were confirmed
by 1H NMR spectra, where the signals of the ligand terpyridine pro-
tons were shifted to the down field region by approximately
0.8 ppm compared to free ligand. Also, the structures of com-
pounds 12–14 were confirmed by HRMS and UV–vis spectra, and
for the coordination compounds its composition was consistent
with elemental analysis data.
Coordination compounds 12–14 are practically insoluble in
water and in most organic solvents, with the exception of DMSO
and DMF. However, even in these two solvents, the solubility of
the ruthenium complexes does not exceed 10ꢁ5 M. Rhodium-con-
taining complexes are somewhat better soluble and can be dis-
solved in DMSO at a maximum concentration of ~5.10ꢁ4 M.
3.3. Electrochemistry
*
The peaks corresponding to the oxidation and reduction of coordinated metal
The electrochemical behaviour of ligands 7, 11 and complexes
12–14 in DMSO solutions using class carbon (GC) or Au electrodes
was investigated by cyclic voltammetry. The electrochemical data
are summarised in Table 1, typical CV curves are shown in Figs. 1–
3 and in Supplementary Information.
The oxidation and reduction potentials of the studied ligands
and complexes are practically don’t depend on the positions of
the substituents in the benzene ring of the phenylterpyridine
ligand. For all ligands there are two reduction peaks at Epc = ꢁ1.70
ions are shown in blue; peaks of Au-S bond reduction – in red.
to ꢁ2.00 V on CV curves recorded using GC electrodes (Fig. 1a);
these peaks correspond to the reduction of the terpyridine ligands
[38–41].
The same peaks corresponding to the reduction of terpyridine
ligands were also observed on the CV of complexes 12–14. In addi-
7

I.O. Salimova, A.V. Berezina, I.A. Shikholina et al.
Polyhedron 200 (2021) 115149
Scheme 3. Synthesis of coordination compounds 12a, b.
Scheme 4. Synthesis of coordination compounds 13a-c and 14b,c.
Fig. 1. Cyclic voltammograms of ligand 11b (a), its Rh(III) complex 13b (b) and Ru(II) complex 14b (c). GC electrode, 5.10ꢁ4 M, DMSO, Bu4NClO4.
tion to them, on GC electrodes for rhodium (III) complexes 12a and
13 the additional cathodic peaks, corresponding to the RhIII ? RhI
reduction [30,31] appear at CV in the region about ꢁ0.65 to
ꢁ0.96 V (Fig. 1b). For the ruthenium (II) complexes 12b and 14
the additional peaks at ~ 0.73 to 0.96 V were observed in the oxi-
dation region, corresponding to the RuII ? RuIII oxidation [30,31].
No additional peaks of metal reduction were observed in the ano-
dic region for complexes 14, which is consistent with literature
data. At deep reduction of complexes 12–14 (scanning the poten-
tial up to ꢁ2.1 V), a black metal powder deposition was observed
on the electrode surface; thus, the reduced form of the complex
is unable to retain zero-valent rhodium or ruthenium.
formation of self-assembled monolayers (SAM) with the formation
of a stable covalent Au-S bonds [42]. When Au electrodes were
used for CV registration, we observed a changes in the volt-ampere
curves over time. Electrochemical monitoring of the changes when
the gold electrode was kept in a solution of compounds 7, 11 in
DMSO showed that at the first moment after placing the gold elec-
trode in the ligand solution, the CV curves were almost identical
obtained on the GC electrode (Fig. 2, blue and green curves). How-
ever, already after 30 min, a very intense additional peak with a
reduction potential of about ꢁ1.6 V was observed on CV, in the
region less cathodic than the peak of the initial terpyride ligand
(Fig. 2, red curve). Upon further keeping the electrode in the ligand
solution, the intensity of the peak at ~ꢁ1.6 V gradually decreased
and simultaneously a peak at ~ꢁ1.3 V appeared, corresponding
to the reduction of the Au-S bond of the chemisorbed ligand.
Finally, after 1–2 days, the intensity of the peak at ꢁ1.3 V reaches
The use of a gold electrode makes it possible to explore the
adsorption of synthesized compounds on Au surface with the for-
mation of a covalent Au-S bond. It is known that disulfides are cap-
able of spontaneously chemisorbing on the gold surface with the
8

I.O. Salimova, A.V. Berezina, I.A. Shikholina et al.
Polyhedron 200 (2021) 115149
accompanied by a shift in the reduction peaks of the ligand frag-
ment to the region of lower potentials (Scheme 1S). However, then
the reorientation of the ligand relative to the gold surface occurs,
and finally, there is a peak in the reduction of the Au-S bond and
uncoordinated terpyridine on the CV curves.
At the final stage of the electrochemical study, we showed the
stability of the formed SAM of coordination compounds on Au elec-
trodes. For this, the electrodes, kept for a day in the solutions of
complexes 13 and 14, were sequentially washed with a solution
of a supporting electrolyte and then DMSO (3 times) to remove
non-chemisorbed molecules from the surface, dried in air, after
which the electrodes were immersed in a clean solution of a sup-
porting electrolyte and cyclic voltammograms were recorded.
The results obtained showed almost complete identity of the
volt-ampere curves obtained before and after washing and drying
the electrodes (Fig. 3).
Taking into account the better solubility of rhodium complexes
13 as compared to ruthenium complexes 14, in further experi-
ments, the Rh(III) complexes 13a-c were tested. We studied the
time of complete adsorbtion of complexes 13 and their organic
ligands 11 on gold electrodes surfaces in order to identify the opti-
mal complex for potential practical use. To determine the time
required for the complete course of adsorption, Au electrodes were
placed in the solutions of ligands 11 or complexes 13 in DMSO at
the same concentration of 5.10ꢁ4 M for different times (12 h,
24 h, 36 h, etc. up to 3 days), after which the electrodes were
removed from the solution of the testing compound, washed with
DMSO, dried in air, placed in a solution of a supporting electrolyte
in DMSO, and the intensity of the peaks at ~ꢁ1.35 V on the CV, cor-
responding to the reduction of the Au-S, was determined. The
moment of completion of the formation of the adsorption layer
was considered the time after which the intensity of the peaks at
~ꢁ1.35 V ceases to increase with an increase in the electrode keep-
ing time in the solution of the adsorbed compound.
It was found that the complete adsorption on gold for different
isomers of ligands and their complexes occurs at different times
(Table 2). Thus, considering the ligands 11 chemisorption is com-
pleted the fastest for 3,5-disubstituted terpyridine 11c. In the case
of the complexes, compounds 13b and 13c are adsorbed much fas-
ter than the 2,4-disubstituted isomer 13a, for which the peak of
Au-S bond reduction remains extremely low-intensity even after
3 days of keeping the gold electrode in the test compound solution.
Such differences, apparently, can be associated with steric hin-
drances arising during the adsorption of ortho-substituted phenyl-
terpyridines. Thus, taking into account the data obtained, rhodium
complexes of 3,5 and 3,4-disero-substituted terpyridines 13b and
13c are most suitable as a bridge molecule for the sensors.
Fig. 2. Cathode part of cyclic voltammograms of ligand 11c on Au electrode
depending on the time the electrode is kept in solution, Green curve – after 1 min,
red curve – after 30 min, black curve – after 24 h. The blue curve represents the CV on
GC electrode.
a maximum and does not change further, and the peak at ꢁ1.6 V
completely disappeared (Fig. 2, black curve).
To explain these changes, one should take into account the pre-
viously shown ability of terpyridines to adsorb on a gold surface
due to the coordination of nitrogen atoms with Au [43]; this pro-
cess proceeds faster than the breaking of the disulfides SAS bonds
and the formation of the Au-S bonds, but it is reversible. Presum-
ably, when a gold electrode is placed in a solution of terpyridine-
containing ligands 7, 11, the ligand is rapidly adsorbed on gold
with the participation of terpyridine nitrogen atoms, which is
Fig. 3. Cyclic voltammograms of complexes 13c (a) and 14c adsorbed on Au electrode surface. Black curves – before and red curves – after the electrodes washing and drying.
9

I.O. Salimova, A.V. Berezina, I.A. Shikholina et al.
Polyhedron 200 (2021) 115149
Table 2
Time of the ligands 11 and their rhodium complexes 13 adsorption on the surface of gold electrodes with Au-S bond formation*.
Compound
11a
11b
11c
13a
13b
12
13c
Adsorption time, h
>72
48
36
>72
12
*
From a solution in DMSO with a concentration of 5.10ꢁ4 M.
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CRediT authorship contribution statement
Irina O. Salimova: Methodology, Formal analysis, Visualization,
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Elena K. Beloglazkina: Conceptualization, Writing - review & edit-
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Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
We are grateful to the Russian Science Foundation (Project 19-
73-00193) and Russian Foundation of Basic Research (Project 19-
33-90146) for the financial support of this work. This work in part
of NMR study was supported by the M.V. Lomonosov Moscow State
University Program of Development.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.poly.2021.115149.
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