Electrochemical properties of 2,2′:6′,2″-terpyridine complexes of platinum(II) with arenethiolate ligands. Alkylation of reduced forms of the complexes

Article abstract of DOI:10.1023/A:1024909325619

Three mixed-ligand terpyridine-thiolate Pt^II complexes containing 2,2′:6′,2″-terpyridine (tpy) and para-substituted thiophenolate ions, [(tpy)Pt(SC₆H₄-4-X)]BF₄ (X = NMe₂, H, NO₂) were synth

Full text of DOI:10.1023/A:1024909325619

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Russian Chemical Bulletin, International Edition, Vol. 52, No. 5, pp. 1150—1156, May, 2003
Electrochemical properties of
2,2´:6´,2″ꢀterpyridine complexes of platinum(II)
with arenethiolate ligands. Alkylation of reduced forms of the complexes
R. D. Rakhimov,ꢀ Yu. A. Weinstein, E. V. Lileeva, N. N. Zheligovskaya,
M. Ya. Mel´nikov, and K. P. Butin
Department of Chemistry, M. V. Lomonosov Moscow State University,
Leninskie Gory, 119899 Moscow, Russian Federation.
Fax: +7 (095) 939 5546. Eꢀmail: butin@org.chem.msu.su
Three mixedꢀligand terpyridineꢀthiolate PtII complexes containing 2,2´:6´,2″ꢀterpyridine
(tpy) and paraꢀsubstituted thiophenolate ions, [(tpy)Pt(SC6H4ꢀ4ꢀX)]BF4 (X = NMe2, H,
NO2) were synthesized. The complexes were isolated as chlorides or tetrafluoroborates and
1
characterized by electrochemistry, visible electronic spectroscopy and H NMR spectroscopy,
and elemental analysis. Remarkable influence of the electronꢀdonating/withdrawing properꢀ
ties of the substituent in the thiolate ligand on the physicochemical properties of the comꢀ
plexes, in particular, on the electrochemical reduction potentials, was found. The reduced
forms of the complexes undergo electrochemically initiated alkylation with alkyl halides,
resulting in the formation of Pt alkyl compounds.
Key words: platinum(II), 2,2´:6´,2″ꢀterpyridine, arenethiolates, terpyridine thiolate comꢀ
plexes, electrochemistry, electrochemical alkylation.
In the last two decades, transition metal complexers
tor (X = CN, SO2Me) or πꢀdonor (X = SMe, NMe2)
groups, the substituent was found to exert a strong influꢀ
ence on the firstꢀwave reduction potential of the complex
(in the region of 700 mV),12 which indicates that the
primary electron changes are mainly localized on the tpy
ligand. Spectroelectrochemical measurements13 showed
that both the [(tpy)PtCl]+ complex and its reduced forms
exist as equilibria between the monomeric and dimeric
species in solutions (DMF, 0.1 M Bu4NPF6) and the
oxidized and reduced forms of the molecule interact with
each other to the greatest extent. Analysis of the temperaꢀ
ture dependence of the ESR spectra showed that the first
step of electrochemical reduction involves the acceptor
orbital of the ligand with some contribution of PtII 5dy,z
and 6pz orbitals (4—6 and 3—4%, respectively). The secꢀ
with polypyridines, in particular, platinum(II) complexes
with bipyridine (bpy) and 2,2´:6´,2″ꢀterpyridine (tpy),
have attracted considerable attention of researchers,
mainly for two reasons. First, they can react with bioꢀ
molecules.1—6 Indeed, the [(tpy)PtCl]+ cation is known
as an intercalator for DNA molecules,1—4 which peneꢀ
trates through a double helix and reacts with complemenꢀ
tary bases as with ligands without substantial change in
the helix geometry. Second, such complexes have adjustꢀ
able photophysical properties and are capable of luminesꢀ
cence; therefore, they can be used in various molecular
devices.7 The aboveꢀmentioned chloro platinum comꢀ
plex is oligomerized in aqueous solutions1,8 and in the
solid state;1,8—11 evidence for the existence of both
metal—metal and π—π ligand—ligand interactions beꢀ
tween the monomers in the oligomer was obtained.11
Stacking of [(tpy)PtCl]+ molecules (having a square plaꢀ
nar configuration of the Pt center) in the solid state gives
rise to a lowꢀenergy absorption band in the electronic
absorption spectrum, which is due to charge transfer from
the metal—metal fragment to the tpy ligand (MMLCT
band).11
ond step involves, apparently, the acceptor 5dx ,y2 orbitals
2
of the metal. On the basis of these results, in the subseꢀ
quent discussion of the behavior of our complexes, we
will assume that the first electron is accepted by the ligand
and the second, by the metal.
The purpose of this work is to study the electrochemiꢀ
cal properties of a number of PtII terpyridine complexes
containing arenethiolate ligands from which charge transꢀ
fer to the tpy ligand can take place through the Pt atom.
Particular attention is devoted to the possibility of redox
alkylation (oxidative addition) on treatment of the reꢀ
duced forms of the initial complexes with alkylating reꢀ
Electrochemical study of terpyridine systems was
carried out using Pt,12,13 Zn,12,13 Fe,14 Ru,13 Os,15,16
and Eu17 complexes. For platinum complexes
[(4´ꢀXꢀtpy)PtCl]+ substituted in position 4´ by πꢀaccepꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1089—1095, May, 2003.
1066ꢀ5285/03/5205ꢀ1150 $25.00 © 2003 Plenum Publishing Corporation

Redox properties of platinum complexes
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 5, May, 2003
1151
filtrate. The resulting mixture was cooled to +4 °C over a period
of 1 h. The precipitate was filtered off, washed with small porꢀ
tions of an EtOH—Et2O mixture (1 : 1, v/v), and dried in a
vacuum dessicator over P2O5 at ∼20 °C.
agents. Such electrochemically induced reactions have
not yet been studied for Pt terpyridine complexes.
N,N´,N″ꢀ(2,2´:6´,2″ꢀterpyridinо)(benzenethiolato)plaꢀ
tinum(II) tetrafluoroborate, [(tpy)Pt(SPh)]BF4 (2). The
[(tpy)Pt(SPh)]Cl complex was dissolved in 96% EtOH and a
dilute aqueous solution of HBF4 was added until the medium
was weakly acidic against the universal indicator. The resulting
lilacꢀcolored precipitate was filtered off, washed several times
with distilled water and EtOH, and dried in a vacuum desꢀ
sicator over P2O5. Found (%): C, 40.41; H, 2.56; N, 6.59.
C
21H16BF4N3PtS. Calculated (%): C, 40.38; H, 2.58; N, 6.73.
1H NMR, δ: 9.08 (d, 2 H, J = 5.6 Hz); 8.70 (m, 5 H); 8.47 (td,
2 H, J = 8.0 Hz, J = 1.2 Hz); 7.87 (t, 2 H, J = 6.0 Hz); 7.65 (dd,
2 H, J = 8.4 Hz, J = 1.2 Hz); 7.06 (t, 2 H, J = 7.2 Hz); 6.96 (t,
1 H, J = 7.2 Hz). UVꢀVis (DMF), λmax/nm (log ε): 549 (1500).
N,N´,N″ꢀ(2,2´:6´,2″ꢀTerpyridinо)(4ꢀdimethylaminobenꢀ
zenethiolato)platinum(II) tetrafluoroborate, [(tpy)Pt(SC6H4ꢀ4ꢀ
NMe2)]BF4 (3). The crimsonꢀcolored crystals were prepared
from [(tpy)Pt(SC6H4ꢀ4ꢀNMe2)]Cl by a procedure similar to the
synthesis of complex 2. Found (%): C, 40.28; H, 3.04; N, 8.47.
C23H21BF4N4PtS. Calculated (%): C, 41.39; H, 3.18; N, 8.40.
1H NMR, δ: 8.90 (dd, 2 H, J = 5.6 Hz, J = 1.2 Hz); 8.60 (t, 2 H,
J = 8.4 Hz); 8.43 (td, 2 H, J = 5.6 Hz, J = 1.2 Hz); 8.63 (m,
5 H); 7.40 (d, 2 H, J = 8.4 Hz); 6.51 (d, 2 H, J = 7.2 Hz); 2.78 (s,
6 H). UVꢀVis (DMF), λmax/nm (log ε): 603 (1500).
N,N´,N″ꢀ(2,2´:6´,2″ꢀTerpyridino)(4ꢀnitrobenzenethioꢀ
lato)platinum(II), [(tpy)Pt(SC6H4ꢀ4ꢀNO2)]BF4 (4). Redꢀorange
crystals were prepared from [(tpy)Pt(C6H4ꢀ4ꢀNO2)]Cl by a proꢀ
cedure similar to that described previously for complex 2.
Found (%): C, 37.52; H, 2.09; N, 8.28. C21H15BF4N4O2PtS.
Calculated (%): C, 37.70; H, 2.24; N, 8.37. 1H NMR, δ: 9.03 (d,
2 H, J = 5.2 Hz); 8.70 (m, 5 H); 8.50 (td, 2 H, J = 7.9 Hz; J =
1.41 Hz); 7.88 (m, 6 H). UVꢀVis (DMF), λmax/nm (log ε):
503 (1900).
X = H (2), NMe2 (3), NO2 (4)
Experimental
Dimethylformamide (reagent grade) was stirred with anhyꢀ
drous K2CO3 (20 g L–1) for 4 days at ∼20 °C, decanted from the
solid phase, and then purified by successive refluxing and vacuum
distillation (b.p. 42 °C, 10 Torr) over CaH2 (10 g L–1) and
anhydrous CuSO4 (10 g L–1). The purified solvent was stored
over 4 Å molecular sieves.
2,2´:6´,2″ꢀТеrpyridine (Aldrich), potassium tertꢀbutoxide
(Aldrich), benzenethiol, and 4ꢀnitrobenzenethiol (Aldrich) were
used as received. The [(tpy)PtCl]Cl•2H2O complex was preꢀ
pared by refluxing K2PtCl4 and 2,2´:6´,2″ꢀterpyridine in disꢀ
tilled water for 100 h.18
Electrochemical measurements were carried out using a
PIꢀ50ꢀ1.1 potentiostat. Glassy carbon (d = 1.8 mm) or platinum
(d = 2.8 mm) discs pressed in Teflon served as working elecꢀ
trodes; a 0.05 M solution of Bu4NBF4 served as the supporting
electrolyte, and an Ag/AgCl/KCl(sat.) electrode was the referꢀ
ence electrode. All the measurements were carried out under
argon.
Results and Discussion
We studied platinum(II) complexes with terpyridine
1—4 by cyclic voltammetry (CV) on a glassy carbon elecꢀ
trode in DMF (the number of transferred electrons was
determined using a rotating disc electrode, by comparing
the height of the corresponding wave with the height of
the wave for the singleꢀelectron oxidation of ferrocene of
the same concentration). The results are summarized in
Table 1.
Compounds 1 and 2 exhibit one oxidation peak each,
while 4 has two and 3 has four oxidation peaks. All the
peaks are irreversible. In line with the results obtained
earlier,12 we believe that oxidation processes do not affect
the (tpy)Pt core, being localized on either the outerꢀsphere
chloride ion (in the case of complex 1) or the arenethiolate
ligand (in the case of complexes 2—4).
1H NMR spectra were recorded on a Varian VXRꢀ400 inꢀ
strument in DMSOꢀd6 and UVꢀVis spectra were run on a
Shimadzu UVꢀ2401 PC spectrometer.
N,N´,N″ꢀ(2,2´:6´,2″ꢀTerpyridinо)(arenethiolato)platinum(II)
chlorides, [(tpy)Pt(SC6H4ꢀ4ꢀX)]Cl (general procedure). Terꢀ
pyridine thiolate complexes of PtII were prepared from the chloꢀ
ride complex according to the procedure described previously.19
A solution of paraꢀsubstituted benzenethiol (10% excess) in
EtOH containing an equivalent amount of ButOK was added
dropwise with vigorous stirring under argon over a period of 1 h
to a suspension of [(tpy)PtCl]Cl•2H2O (265 mg, 0.495 mmol)
in 96% EtOH. The addition of the thiolate was accompanied by
dissolution of the initial compound and the change in the soluꢀ
tion color from orange to redꢀvinous (X = NO2), lilac (X = H),
or crimson (X = NMe2). After the addition of the whole solution
of the ligand, stirring was continued for 5—6 h until the startꢀ
ing chloride complex was entirely consumed. Completion of
the reaction required slight heating of the reaction mixture
(X = NO2, H). In the case of X = NMe2, heating resulted in
decomposition. On completion of stirring, the solution was filꢀ
tered, and a double volume of Et2O was slowly added to the
[(tpy)Pt—SAr]+ – e
(tpy)Pt2+ + 0.5 ArSSAr
Compound 3 contains the paraꢀdimethylaminothiolate
ligand rich in electrons (lone electron pairs of the S and

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Russ.Chem.Bull., Int.Ed., Vol. 52, No. 5, May, 2003
Rakhimov et al.
Table 1. Oxidation (EpOx) and reduction (EpRed) potentials of complexes 1—4 in DMF (0.1 М solution of
Bu4NBF4, Pt electrode, Ag/AgCl/KCl (sat.), 20 °C)
Comꢀ
plex
Oxidationa
EpOx/V
Reduction
Red,1
1 b
1 c
Red,2
2
2 b
Red,3 d
–Ep
/V
Ep,a1 – Ep,c
ip,a1/ip,c
–Ep
/V
Ep,a – Ep,c
–Ep
/mV
/mV
/V
1 e
2
3
1.14
1.17
0.87, 1.23,
1.41, 1.68,
1.46, 1.55,
0.60
0.76
0.75
100
90
90
0.60
1.00
0.81
1.14
1.31
1.26
90
70
60
2.04 a
2.04 a
2.26 a
4
0.73
120
1.00
1.22
110
1.39
a All peaks are irreversible.
b The difference between the potentials of the reverse (Ep,a) and forward (Ep,c) peaks.
с Ratio of the heights of reverse and forward peaks.
d On a glassy carbon electrode.
e On a Pt electrode. The following reduction potentials of the [(tpy)PtCl]PF6 complex were determined:12
EpRed,1 = –0.74, EpRed,2 = –1.30 and EpRed,3 = –2.2 V. The former two steps are quasiꢀreversible (Ep,a – Ep,c
80—90 mV); the third step becomes chemically reversible only at temperatures below –20 °C.
=
N atoms) and, therefore, its oxidation gives rise to four
peaks, three of which can be attributed to the hypothetiꢀ
cal reactions shown in Scheme 1.
tive 4 is oxidized at a 290 mV higher potential (i.e., more
difficultly) than unsubstituted compound 2. These results
show that electrooxidation involves particularly the
arenethiolate ligand.
As has been noted previously,12 the effect of substituꢀ
ents in position 4´ of the terpyridine ligand in chloride
complex 1 was studied, and these substituents were shown
to influence only slightly the oxidation potential. Thus,
the tpy ligand remains almost intact during oxidation.
However, the reduction potentials of 4´ꢀsubstituted chloꢀ
ride complexes depend appreciably on the nature of the
substituent,12 whereas the reduction potentials of the comꢀ
plexes we studied are slightly sensitive to the nature of the
paraꢀsubstituents in the arenethiolate ligand (see Table 1).
These data suggest that the first step of reduction involves
the tpy ligand.
Scheme 1
[(tpy)Pt—SC6H4NMe2]+ – e
(tpy)Pt2+ + 0.5 Me2NC6H4SSC6H4NMe2
0.5 Me2NC6H4SSC6H4NMe2 – e
All of these compounds exhibit three reduction peaks in
the cathodic region of potentials, the first two peaks being
reversible or quasiꢀreversible (Ep,a – Ep,c = 60—120 mV).
The third peak, which markedly exceeds the former two
peaks in height, is irreversible at 20 °C. The potential of
the first peak for compounds 2—4 depends only slightly
on the substituent in the arenethiolate ligand. This indiꢀ
cates that the primary electron changes occurring upon
the elctroreduction are not related to this ligand.
The potentials for the first step of reduction of comꢀ
plexes 1—4 are much less negative than the reduction
potentials of free tpy. This effect of coordination to the
metal indicates that electroreduction actually involves
mainly the vacant orbitals of the tpy ligand; this is in line
with the conclusion drawn previously.12 The second step
of reduction corresponds, apparently, to the transfer of
the second electron to give a negatively charged species,
which retains the structure of the initially positively
charged complex. This conclusion is based on the fact
– e
Products
This mechanism of electrooxidation is based on the
following experimental findings. First, the free tpy ligand
is not oxidized in DMF over the accessible range of poꢀ
tentials; second, dimethylaniline is oxidized at a potential
of ∼0.72 V;20 third, bis(N,N´ꢀdimethylaminophenyl) diꢀ
sulfide which we synthesized by the reaction of N,Nꢀdiꢀ
methylaniline with S2Cl2 is oxidized at Ep = 1.22 V, which
is very close to the secondꢀpeak potential (1.23 V) present
in the voltammograms for oxidation of complex 3 (see
Table 1). This electrooxidation mechanism is, apparently,
applicable in general terms to compound 4.
The electrooxidation of compounds 2—4 is subject to
an appreciable influence of substituents in the paraꢀposiꢀ
tion relative to the S atom on the first oxidation potential.
paraꢀDimethylamino derivative 3 is oxidized at a 300 mV
lower potential (i.e., more easily) and paraꢀnitro derivaꢀ

Redox properties of platinum complexes
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 5, May, 2003
1153
that the polarization curves recorded during the reverse
scan always exhibit an anodic peak. However, the ratio of
the heights of the reverse and forward peaks for the secꢀ
ond step is equal to 0.66—0.45 (see Table 1), i.e., partial
destruction still does occur.
Compound 4, which contains two potential reduction
centers, namely, the tpy ligand and the NO2 group, deꢀ
serves particular attention. The first step of reduction may
involve either the ligand or the NO2 group (Scheme 2).
The third irreversible multielectron wave for comꢀ
pounds 1—3 seems to correspond to further reduction of
"(tpy)Pt0" (the other possible electronic structure is
(tpy•–)PtI) to give the negatively charged [(tpy)Pt]– comꢀ
plex with extrusion of platinum metal and further reducꢀ
tion of the terpyridine radical anion (see Scheme 2).
In this connection, note that evidence for the existence
of paramagnetic 19ꢀelectron (bpy)Ni– and (phen)Ni–
complexes (phen is 1,10ꢀphenanthroline) rather stable on
the CV time scale have been obtained recently.21 These
complexes are electrochemically produced from (bpy)Ni0
and (phen)Ni0 at high cathodic potentials (–2.46 and
–2.30 V for the bpy and phen complexes, respectively;
free ligands are reduced at even higher potentials, –2.68
and –2.60 V), and they are efficient homogeneous reducꢀ
ing agents. Owing to the clearly pronounced πꢀacceptor
properties of polypyridine ligands, the electron density in
these complexes should be largely shifted to the ligand;
hence, the electronic structure can be represented more
reasonably as (L•–)Ni0, which is confirmed by ESR
spectra.
Scheme 2
The reduced forms of transition metal complexes conꢀ
taining, for example, Ni0, Pd0, or CoI, are simultaneously
potent nucleophiles and potent reducing agents. These
species, which are usually rather unstable during storage,
are readily generated in situ by electroreduction of approꢀ
priate metal complexes in stable valence states. When
acting as a nucleophile, the reduced complex easily reꢀ
places the nucleofugal groups in organic substrates giving
rise to compounds with a carbon—metal bond. When actꢀ
ing as a reducing agent, this complex converts organic
substrates into reactive radical anions and/or radicals.
Scheme 3 shows two possible reaction routes for the iniꢀ
tial fourꢀcoordinate complex formed by a divalent late
transition metal with the neutral tridentate tpy ligand and
After transfer of the second electron, the same prodꢀ
uct is formed in both cases. This product is shown in
Scheme 2 as a PtII complex with the (tpy)•– radical anion
and with the paraꢀnitrobenzenethiolate radical anion, alꢀ
though the real electronic structure of this species may be
different, for example, (tpy•–)PtI—SC6H4NO2ꢀ4.
Scheme 3
M is metal, L is ligand

1154
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 5, May, 2003
Rakhimov et al.
the anionic monodentate L– ligand. Pathway A correꢀ
sponds to the nucleophilic properties and pathway B repꢀ
resents the reducing properties.
The electrochemical activation of metal complex cataꢀ
lysts of diverse organic reactions has been vigorously studꢀ
ied in the last decade.22,23
Pathway A is most important for organic and organoꢀ
metallic syntheses, because the carbon—metal bond in
the resulting organometallic complex is relatively weak
and the organic group can be transferred (as a cation,
anion, or radical) to other organic or inorganic molecules
or ions.
In this work, the possibility of alkylation of the elecꢀ
trochemically generated lowꢀvalence platinum complexes
was studied using BunI, BunBr, and (dmgH)2Co—Me
(methylcobaloxime) as alkylating agents. There exists a
diagnostic electrochemical test, which allows one to find
out whether the reaction between the reduced forms of
complexes and the alkylating agents follows pathway A or
pathway B (see Scheme 3). If the process follows pathꢀ
way B, the difference between the morphologies of the
CV curves recorded in the absence and in the presence of
an alkylating agent would be an increase in the reduction
current of the starting complex (soꢀcalled catalytic curꢀ
rent) and a decrease or total disappearance of the signs of
reversibility of the reduction of the starting complex
(a decrease or disappearance of the anodic peak when
scanning the potential in the opposite direction). If the
reaction follows pathway A, a new peak corresponding to
reduction of the alkylation product is expected to appear.
This new peak usually appears at more cathodic potentials
relative to the potentials of generation of lowꢀvalence
complexes reactive toward the alkylation. Some excepꢀ
tions are known.24 If reactions along pathways A and B
are slow on the time scale of a given electrochemical
method, no changes will occur in recording the first poꢀ
larization pattern on a clean electrode surface in the presꢀ
ence of an alkylating agent; these changes would be visꢀ
ible only after relatively longꢀterm electrolysis at poꢀ
tentials where species susceptible to alkylation are genꢀ
erated.
When recording CV curves for compounds 1—4 in the
presence of BunBr or BunI, an additional peak appears
already during the first scan for all compounds (Fig. 1, a, b,
peak C), the corresponding potential being –1.31 (comꢀ
plex 1), –1.46 (complex 2), –1.49 (complex 3), and
–1.45 V (complex 4). Simultaneously, the reversibility of
the second reduction peak of complexes 1—4 almost disꢀ
appears, and a peak (or two peaks) for the oxidation of the
corresponding halide ion appears during the reverse anꢀ
odic scan in the anodic region (0.96 V for the use of
BunBr and 0.30 and 0.70 V for the use of BunI). The
reversibility of the first reduction peak decreases insignifiꢀ
cantly in all cases.
In the range of BunI concentrations from 5•10–3 to
1•10–1 mol L–1, only one additional reduction peak for
the product is observed in the polarization patterns. This
means that under these conditions, alkylation starts only
together with the formation of the formally zerovalent
platinum complex (tpy)Pt0 (see Scheme 3). The alkylaꢀ
tion of the singly reduced species, [(tpy)PtISAr], proꢀ
ceeds slowly, if at all.
C
C
a
b
B
B
5 µA
2
2
5 µA
A
A
B
B
A
A
1
1
–0.4
0
0.4 0.8
1.2 1.6 –E/V
0
0.4
0.8
1.2
1.6 –E/V
Fig. 1. Cyclic voltammograms of 1•10–3 М solutions of [(tpy)Pt+Cl]Cl– (1) (a) and [(tpy)Pt+SC6H4ꢀ4ꢀNMe2]BF4– (3) (b) in DMF
(0.05 mol L–1 of Bu4NBF4) in the absence of BunI (1) and in the presence of BunI (0.01 mol L–1) (2).

Redox properties of platinum complexes
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 5, May, 2003
1155
The polarization patterns shown in Fig. 1, a, b can be
interpreted in the following way. The addition of an alkyꢀ
lating agent entails a fast reaction with the doubly reꢀ
duced form of the complex, which can formally be
described as (tpy)Pt0, resulting in the formation of
[(tpy)PtIIAlk]+X–. During further potential scan to the
cathodic region, this product is reduced to give the alkyl
anion Alk– and (tpy)Pt0, the latter being rapidly alkyꢀ
lated. In the presence of an alkylating agent, the concenꢀ
tration of (tpy)Pt0 in the nearꢀelectrode area diminishes;
therefore, during the reverse potential scan to the anodic
region, the peak of (tpy)Pt0 oxidation substantially deꢀ
creases. In the case of rather high concentration of the
alkylating agent, this peak totally disappears. Further scan
to the anodic region implies transition to a potential reꢀ
gion where (tpy)Pt0 no longer exists, but the singly reꢀ
duced (tpy)PtISAr still exists. The latter species is slowly
alkylated; therefore, the anodic peak corresponding to its
oxidation is changed only slightly.
electron reduction of complex 1 takes place (see Scheme 3,
pathway A, reaction (2)).
The twoꢀelectron reduction of complexes 2—4 may
give "(tpy)Pt0" and ArS– in the nearꢀelectrode layer. In
principle, both of these species can be alkylated. In our
opinion, the substantial changes in the CV curves (see
Fig. 1, a, b) are related to alkylation of the doubly reꢀ
duced complex because similar changes are observed not
only for compounds 2—4 but also for compound 1, which
does not contain a thiolate ligand. The alkylation of the
doubly reduced complex involves the metal atom. In the
case of ligand alkylation, the alkylated product would,
probably, have the structure shown in Scheme 4.
Scheme 4
However, when the concentration of the alkylating
agent is >1•10–1 mol L–1, the alkylation of the singly
reduced form becomes sufficiently fast to show itself in
the voltammograms. Thus, a second extra peak with a
potential of –0.95 (compound 1), –1.12 (compound 2),
–1.13 (compound 3), or –1.18 V (compound 4), appears
just after the first reduction peak of the initial complex.
Thus, at high concentrations, one can observe the alkylaꢀ
tion of formally monovalent platinum giving rise to an
alkyl derivative of the formally trivalent platinum (see
Scheme 3, reaction (1)).
In this case, during the reverse potential scan,
the CV curves would exhibit the reversible
(tpy)Pt+—SAr/(tpy)Pt—SAr pair (see Fig. 1, a, b,
peaks A).
Thus, the electrochemical data show that the elecꢀ
tronic structure of the studied complexes is determined by
the charge transfer between the donor thiolate ligands and
the acceptor tpy ligand with participation of the Pt atom.
The electrochemical reduction does not deteriorate subꢀ
stantially the molecular structures of complexes but may
activate them towards alkylation at the metal atom.
A convenient alkylating agent is the dimethylꢀ
glyoximate complex of methylcobalt, which can act as a
carrier for methyl cations (in the case of formation of
(dmgH)2Co–), methyl radicals (in the case of formation
of (dmgH)2CoII), or methyl carbanions (in the case of
formation of (dmgH)2CoIII). The voltammogram of an
equimolar mixture (1•10–3 mol L–1) of (tpy)PtCl2 and
(dmgH)2Co—Me recorded from 0 to –1.4 V does not
show any changes with respect to the voltammograms of
neat specimens; however, switching to anodic polarizaꢀ
tion after Ep = –1.4 V (i.e., after the second reduction
peak of complex 1) brings about two new peaks: an anodic
This work was supported by the Russian Foundation
for Basic Research (Project No. 00ꢀ03ꢀ32888).
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Received May 24, 2002;
in revised form December 24, 2002