Spectroscopic (UV/VIS, resonance Raman) and spectroelectrochemical study of platinum(II) complexes with 2,2′-bipyridine and aromatic thiolate ligands

Article abstract of DOI:10.1039/a803225d

A series of complexes [Pt(bpy)(4-XC₆H₄S)₂] (bpy = 2,2′-bipyridine; X = NO₂, H, MeO or Me₂N) have been synthesized and characterised spectroscopically. All absorb moderately in the visible region; the molar absorption coefficients of the solvatochromic absorption band lie below the values for the corresponding dithiolate complexes, indicating a slightly distorted planar geometry and a smaller HOMO - LUMO overlap. Resonance Raman, cyclic voltammetric and UV/VIS spectroelectrochemical experiments were carried out to confirm the charge transfer-to-diimine character of the visible electronic transition(s) directed from a p(S)-π(Ph)/d(Pt) delocalised orbital manifold to the π₁* LUMO of the 2,2′-bipyridine ligand. Consistent with this bonding situation: (i) the reduction of [Pt(bpy)(4-XC₆H₄S)₂] is initially localised on the bpy ligand; for X = NO₂ the second and third added electrons enter the vacant π* orbitals on the 4-NO₂ substituents; (ii) the resonance Raman spectra of [Pt(bpy)(4-MeOC₆H₄S)₂] show, besides the internal bpy modes, only a weak effect for intrathiolate (Ph-S) and v(Pt-S) vibrations; and (iii) the oxidation is initially a one-electron process due to electronic interaction of the thiolate ligands through the platinum centre. The strongly thiolate-dependent oxidation potentials indicate a prevailing thiolate character of the HOMO.

Full text of DOI:10.1039/a803225d

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Spectroscopic (UV/VIS, resonance Raman) and spectroelectrochemical
study of platinum(II) complexes with 2,2Ј-bipyridine and aromatic
thiolate ligands†
Julia A. Weinstein,*,a Natalia N. Zheligovskaya,a Michael Ya. Mel’nikova and Frantis˘ek Hartl*,b
a Department of Chemistry, Moscow State University, Moscow 199899, Russia
b Anorganisch Chemisch Laboratorium, Institute of Molecular Chemistry,
Universiteit van Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands
A series of complexes [Pt(bpy)(4-XC6H4S)2] (bpy = 2,2Ј-bipyridine; X = NO2, H, MeO or Me2N) have been
synthesized and characterised spectroscopically. All absorb moderately in the visible region; the molar absorption
coefficients of the solvatochromic absorption band lie below the values for the corresponding dithiolate
complexes, indicating a slightly distorted planar geometry and a smaller HOMO Ϫ LUMO overlap. Resonance
Raman, cyclic voltammetric and UV/VIS spectroelectrochemical experiments were carried out to confirm the
charge transfer-to-diimine character of the visible electronic transition(s) directed from a p(S)–π(Ph)/d(Pt)
delocalised orbital manifold to the π1* LUMO of the 2,2Ј-bipyridine ligand. Consistent with this bonding
situation: (i) the reduction of [Pt(bpy)(4-XC6H4S)2] is initially localised on the bpy ligand; for X = NO2 the second
and third added electrons enter the vacant π* orbitals on the 4-NO2 substituents; (ii) the resonance Raman spectra
of [Pt(bpy)(4-MeOC6H4S)2] show, besides the internal bpy modes, only a weak effect for intrathiolate (Ph᎐S) and
ν(Pt᎐S) vibrations; and (iii) the oxidation is initially a one-electron process due to electronic interaction of the
thiolate ligands through the platinum centre. The strongly thiolate-dependent oxidation potentials indicate a
prevailing thiolate character of the HOMO.
Transition-metal diimine complexes have been intensively
investigated over the past two decades as prospective objects for
vectorial energy/electron transfer,1 which may be incorporated
as building blocks in molecular wires and switches. Among
others, square-planar platinum() diimine complexes with a var-
iety of anionic, in particular organic dithiolate, donor ligands
have attracted considerable attention in this respect.2,3 A gen-
eral requirement for this kind of application is the existence
of a long-lived charge-separated excited state of the metallo-
organic chromophore; its stabilisation is, therefore, a problem
of great interest. One of the possible solutions is to prevent
rapid back electron transfer by introducing a large energy bar-
rier which results from facile structural reorganisation follow-
ing the primary interligand photoelectron transfer in mixed-
ligand complexes. Platinum() diimine bis(thiolate) derivatives
seem to be promising objects for this strategy as their charge-
transfer excited states are stable towards dissociation‡,3h and
structural reorganisation probably occurs in the excited state at
the thiolate site of the complex, involving formation of a three-
assigned3e as charge transfer to diimine by nature, with the
HOMO of mixed S(p)–Pt(d) and the LUMO of diimine(π*)
character. The question has arisen whether the same bonding
situation, in particular with regard to the HOMO parentage,
applies to their bis(thiolate) derivatives. It should be noted
that for [Pt(dpphen)(4-XC6H4S)2] (dpphen = 4,7-diphenyl-1,10-
phenanthroline; X = MeO or NH2) the parentages of the
frontier orbitals have been suggested3g to correspond to those
of platinum() diimine complexes containing dithiolate ligands.
The nature of the frontier orbitals involved in the photo-
electron transfer process can often be elucidated by application
of electrochemical and spectroelectrochemical methods.6,7 To
our knowledge no detailed (spectro)electrochemical investi-
gation of platinum() diimine complexes with aromatic thiolate
ligands has been carried out. In this paper we present the results
of a (spectro)electrochemical study of a series of complexes
[Pt(bpy)(4-XC6H4S)] (bpy = 2,2Ј-bipyridine; X = NO2, H, MeO
or Me2N). Systematic variation of the electron-withdrawing/
releasing properties of the thiolate substituent X enabled us to
obtain strong evidence for a dominant contribution of the
thiolate ligands to the HOMO of the complexes.
electron S᎐Pt᎐S bond.4
The description of the low-energy excited states of this class
of complexes requires first the assignment of their frontier
orbitals, in particular the HOMO and LUMO. In general, sev-
eral close-lying excited states of different nature can be
expected within the excited-state manifolds of transition-metal
complexes, in particular those complexes with mixed donor and
acceptor ligands.1a,5 For platinum() diimine complexes with
bidentate dithiolate ligands the lowest excited state has been
Experimental
Materials
The solvents tetrahydrofuran (thf; Acros Chimica) and dimeth-
ylformamide (dmf; Acros Chimica) were freshly distilled under
a dry nitrogen atmosphere from a sodium–benzophenone
mixture and calcium hydride, respectively. 4-Nitrobenzenethiol
(Aldrich) was recrystallised from absolute ethanol, benz-
enethiol (Aldrich) and 4-methoxybenzenethiol were used as
received, and 4-dimethylaminobenzenethiol was synthesized
according to a literature method.8 Hexamethylenetetraammine
(urotropine) (hmt; KazanЈ Chemical Pharmaceutical Factory)
was used as received. A modified literature procedure9 was
employed to synthesize [Pt(bpy)Cl2] from K2[PtCl4] and 2,2Ј-
† Non-SI unit employed: eV ≈ 1.60 × 10Ϫ19 J.
‡ No thiolate-radical formation was registered under 308 and 532 nm
laser excitation of deaerated dmf solutions of [Pt(bpy)(4-XC6H4S)2]
(X = MeO or Me2N), in contrast to the 365 nm, LLCT excitation of the
corresponding complexes [Zn(phen)(4-XC6H4S)2] producing the 4-
XC6H4S radicals, detected with time-resolved UV/VIS absorption
spectroscopy and as thiophenoxyl adducts with the 2-methyl-2-nitroso-
propane (mnp) spin trap [see ref. 3(i)].
J. Chem. Soc., Dalton Trans., 1998, Pages 2459–2466
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bipyridine (Aldrich). For the (spectro)electrochemical experi-
ments PhSH, 4-O2NC6H4SH (both Fluka), 4-MeOC6H4SH
(Aldrich), ferrocene (BDH) and cobaltocenium hexafluoro-
phosphate (Aldrich) were used as received. The supporting
electrolyte NBu4PF6 (Aldrich) was recrystallised twice from
absolute ethanol and dried in vacuo at 80 ЊC for 10 h.
vacuum. The product, obtained in 85% yield, was used without
further purification. H NMR [(CD3)3SO]: δ 9.64 (d, J 5.60, 2
H), 8.66 (d, J 8.23, 2 H), 8.37 (t, J 8.00, 2 H), 7.79 (t, J 6.45,
2 H), 7.48 (d, J 7.88, 4 H), 6.91 (t, J 7.45, 4 H) and 6.80 (t,
J 7.22 Hz, 2 H) (Found: C, 46.58; H, 3.19; N, 4.60. Calc.
for C22H18N2PtS2: C, 46.41; H, 3.16; N, 4.92%).
1
[Pt(bpy)(4-MeOC6H4S)2]. This was prepared using the same
procedure as for [Pt(bpy)(PhS)2], except that any heating had
to be avoided to prevent the secondary elimination of 2,2Ј-
bipyridine. The reaction time required to complete the co-
ordination of [4-MeOC6H4S]Ϫ was 30 min at room temperature.
The resulting dark red [Pt(bpy)(4-MeOC6H4S)2] precipitate was
recrystallised from acetone to remove the insoluble [Pt(4-
MeOC6H4S)2] side product. Yield 85%. 1H NMR [(CD3)2SO]: δ
9.64 (d, J 5.70, 2 H), 8.62 (d, J 7.90, 2 H), 8.35 (t, J 7.88, 2 H),
7.77 (t, J 6.45, 2 H), 7.35 (d, J 8.88, 4 H), 6.54 (d, J 8.88 Hz,
4 H) and 3.60 (s, 6 H) (Found: C, 45.74; H, 3.44; N, 4.50. Calc.
for C24H22N2O2PtS2: C, 45.78; H, 3.50; N, 4.45%).
Methods and instrumentation
Electronic absorption spectra were recorded on Shimadzu UV-
2401 or software-updated Perkin-Elmer Lambda 5 spectro-
1
photometers, H NMR spectra on a Varian VXR-400 spec-
trometer. Resonance Raman measurements were performed
using a Dilor Modular XY spectrometer which employs a back-
scattering geometry and a CCD detection system. Spinning
KNO3 pellets (214 mg KNO3 ϩ 18.5 mg platinum complex) at
room temperature were excited at 457.9, 488.0 and 514.5 nm
using lines obtained from an SP model 2016 Arϩ laser. The
incident laser power was varied between 20 and 30 mW.
The spectroelectrochemical samples were carefully prepared
under an atmosphere of dry nitrogen or argon, using Schlenk
techniques. The UV/VIS spectroelectrochemical experiments
were carried out with an optically transparent thin-layer elec-
trochemical (OTTLE) cell10 (0.2 mm optical pathlength)
equipped with a platinum minigrid electrode and quartz win-
dows. The solutions were typically 3 × 10Ϫ1  in supporting
electrolyte and 2 × 10Ϫ3  in [Pt(bpy)(4-XC6H4S)2] or 4-
O2NC6H4SH. The potential control at the working electrode
was achieved by a PA4 (EKOM) potentiostat. A thin-layer
cyclic voltammogram was recorded during each potential scan
at ν = 2 mV sϪ1. Bulk electrolyses of 4-XC6H4SH (X = H, NO2
or MeO) to produce the corresponding thiolate anions were
carried out in a gas-tight cell with a platinum flag (120 mm2)
working electrode in the middle compartment separated at the
bottom by S4 frits from two lateral compartments containing
platinum-gauze auxiliary and SCE reference electrodes, respec-
tively. Conventional cyclic voltammetry was performed with an
EG&G PAR model 283 potentiostat, using a gas-tight single-
compartment cell equipped with platinum-disc (0.49 mm2)
working, platinum-gauze auxiliary and silver-wire pseudo-
reference electrodes. The disc was repeatedly polished with a
0.25 µm grain diamond paste (PRAMET, Czech Republic). The
cell contained thf solutions of 3 × 10Ϫ1  NBu4PF6 and 10Ϫ3 
[Pt(bpy)(4-XC6H4S)2] or reduced [4-XC6H4S]Ϫ. The redox
potentials are reported against the standard11 ferrocene–
[Pt(bpy)(4-Me2NC6H4S)2]. The dark red complex was pre-
pared using the same procedure as for [Pt(bpy)(4-MeO-
1
C6H4S)2]. Yield 80%. H NMR [(CD3)2SO]: δ 9.67 (d, J 5.60,
2 H), 8.60 (d, J 8.00, 2 H), 8.32 (t, J 8.00, 2 H), 7.74 (t, J 6.36,
2 H), 7.29 (d, J 8.80, 4 H), 6.40 (d, J 8.80 Hz, 4 H) and
2.73 (s, 12 H) (Found: C, 47.62; H, 4.34; N, 8.56. Calc. for
C26H28N4PtS2: C, 47.66; H, 4.27; N, 8.54%).
[Pt(bpy)(4-O2NC6H4S)2]. The bright orange complex was
prepared using the same procedure as for [Pt(bpy)(PhS)2],
except that heating was not necessary. The substitution of the
chloride ligands in [Pt(bpy)Cl2] was completed during 30 min at
room temperature. Yield 85%. 1H NMR [(CD3)2SO]: δ 9.47 (d,
J 5.90, 2 H), 8.73 (d, J 7.70, 2 H), 8.44 (t, J 7.70, 2 H), 7.86 (t,
J 6.80, 2 H), 7.78 (d, J 9.35, 4 H) and 7.70 (d, J 8.50 Hz, 4 H)
(Found: C, 40.10; H, 2.45; N, 8.33. Calc. for C22H16N2O4PtS2:
C, 40.08; H, 2.43; N, 8.49%).
All the complexes [Pt(bpy)(4-XC6H4S)2] are non-electrolytes,
as was demonstrated by the measured molar conductivities
of 6.0–8.0 S cm2 molϪ1. They are stable in the solid state and
photostable in deaerated dmf solutions on irradiation at wave-
lengths longer than 365 nm. In air-saturated acetonitrile, dmf
or dmso solutions the complexes thermally decompose within
several hours at room temperature into yellow [Pt(solv)2(4-
XC6H4S)2] detectable by HPLC. The formation of unco-
ordinated 2,2Ј-bipyridine in (CD3)2SO was followed by 1H
NMR spectroscopy: δ 8.68 (d, H1, 2 H), 8.38 (d, H4, 2 H), 7.94
(t, H3, 2 H) and 7.45 (t, H2, 2 H). The secondary reaction (1)
₁
ferrocenium couple used as an internal reference: E = 0.575 V
vs. SCE in thf. For [Pt(bpy)(4-MeOC6H4S)2] the cobaltocene–
₂
cobaltocenium redox couple served as the internal reference
standard:12 E = Ϫ1.34 V vs. ferrocene–ferrocenium.
₁
₂
[Pt(bpy)(4-XC6H4S)2] → [Pt(solv)2(4-XC6H4S)2] ϩ bpy (1)
Syntheses of [Pt(bpy)(4-XC6H4S)2] (X ؍ H, NO2, MeO or
Me2N)
becomes facilitated with increasing σ-donor ability of the
thiolate ligands in the order X = H < MeO < Me2N, in accord
with the stronger thiolate trans effect which labilises the Pt᎐N
bonds and results in the complete elimination of the 2,2Ј-
bipyridine ligand. Argon-saturated solutions of the complexes
remain stable at room temperature for several hours. The elim-
ination of 2,2Ј-bipyridine becomes, however, strongly acceler-
ated on elevating the temperature. Warming a solution of
[Pt(bpy)(4-MeOC6H4S)2] in ethanol or dmf under argon results
in rapid precipitation of the insoluble yellow complex [Pt-
(4-MeOC6H4S)2] according to equation (2) as was shown by
All these complexes were prepared from [Pt(bpy)Cl2]. Hexa-
methylenetetraammine was employed as a specific ‘catalyst’
for the inner-sphere substitution of the chloride ligands13 by
[4-XC6H4S]Ϫ. The products were characterised by 1H NMR
and UV/VIS spectroscopy, elemental analysis and molar
conductivity.
[Pt(bpy)(PhS)2]. A solution of hmt (280 mg, 2 mmol) in
EtOH (15 cm3) was added to a suspension of [Pt(bpy)Cl2] (422
mg, 1 mmol) in EtOH (100 cm3). In a separate flask PhSH (264
mg, 2.4 mmol) was added to an ethanolic solution of KBut (2.4
mmol in 20 cm3) under an argon atmosphere. The resulting
solution of [PhS]Ϫ was then slowly cannulated into the suspen-
sion of [Pt(bpy)Cl2]. The reaction mixture was heated for 2 h on
a water-bath with continuous stirring and bubbling with argon.
The addition of the thiol solution resulted in a deep red precipi-
tate of [Pt(bpy)(PhS)2] which was filtered off, washed three
times with hot water, twice with ethanol and dried under
[Pt(solv)2(4-XC6H4S)2] → [Pt(4-XC6H4S)2] (insoluble) (2)
elemental analysis of the precipitate. No decomposition was
observed on the timescale of hours in thf. This solvent, there-
fore, was chosen for the (spectro)electrochemical experiments.
Complexes in chlorinated solvents exposed to daylight can
decompose, probably by a slow photochemical oxidation as
reported3b,f,14 for related platinum() dithiolate complexes.
2460
J. Chem. Soc., Dalton Trans., 1998, Pages 2459–2466

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Table 1 Electronic absorption spectra of the complexes [Pt(bpy)-
(4-XC6H4S)2] (X = NO2, H, MeO or Me2N), unco-ordinated 4-
O2NC6H4SH, and their reduction products; all in thfa at 293 K, unless
stated otherwise
Compound
λmax/nm (ε/103 Ϫ1 cmϪ1
)
[Pt(bpy)(4-O2NC6H4S)2]
243 (35), 306 (18), 314 (sh), 426 (36),b
ca. 510 (sh)c
[Pt(bpy)(PhS)2]
[Pt(bpy)(4-MeOC6H4S)2]
530 (2.14),d 337
543 (2.20),e 341
[Pt(bpy)(4-Me2NC6H4S)2]
557 (2.30),f 370 (sh)
Ϫ
[Pt(bpy )(4-O2NC6H4S)2]Ϫ
259 (29.5), 342 (11.5), 457 (42.9), 725
(sh), 798 (2.2)
272 (27.5), 382 (52.0), 464 (11.1), 501
(11.7), 560 (sh), 725 (sh), 798 (2.2),
>850
271, 364, 454, 484, 706, 787
347 (16.6), 468 (4.8), 502 (6.4), 737
(sh), 816 (1.7), >900
᝽
Ϫ
[Pt(bpy )(4-O2NC6H4S)2]3Ϫ
᝽
Ϫ
[Pt(bpy )(PhS)2]Ϫ
᝽
Ϫ
[Pt(bpy )(4-Me2NC6H4S)2]Ϫ
᝽
4-O2NC6H4SH
225 (sh), 318 (12.7)
250 (7.9), 272 (7.3), 325 (sh), 495
(24.4)
[4-O2NC6H4S]Ϫ
[4-Ϫ O2NC6H4S]2Ϫ
298 (8.9), 384 (13.2), 498 (1.1)
᝽
a The absorption coefficients of [Pt(bpy)(4-XC6H4S)2] correspond to
5 × 10Ϫ5  solutions (see Results). b At 424 nm in CH2Cl2, 432 nm in
dmf. c Obscured by the intense π → π* (NO2) band. d At 512 nm
in CH2Cl2, 498 nm in dmf. e At 567 nm in o-xylene, 521 nm in CH2Cl2,
509 nm in dmf. f At 546 nm in CH2Cl2, 542 nm in dmf.
[Pt(bpy)(4-XC6H4S)2] therein. A more profound asymmetry of
the visible absorption band and a small rigidochromic effect
were observed for [Pt(bpy)(4-XC6H4S)2] on lowering the temper-
ature to 210 K and at 77 K in a 2-methyltetrahydrofuran glass,
respectively. In thf, λmax and the half-width of the band are
independent of the complex concentration varied in the range
cPt = 10Ϫ5–5 × 10Ϫ3 ; however, the corresponding molar
absorption coefficients decrease from the values reported in
Table 1 for 5 × 10Ϫ5  solutions by ca. 10–30% on increasing
the concentration to 3 × 10Ϫ3 . The latter feature may point to
the occurrence of aggregation of [Pt(bpy)(4-XC6H4S)2] in the
ground state at higher concentrations, e.g. via Pt ؒ ؒ ؒ Pt contacts.
No such concentration effect was observed in the presence of
the supporting electrolyte NBu4PF6.
Fig. 1 (a) Electronic absorption spectra of 5 × 10Ϫ5  [Pt(bpy)(4-
XC6H4S)2] in thf at 293 K. X = NO2 (1, –ؒ–ؒ), H (2, ——), MeO (3, -----)
or Me2N (4, –––). (b) Correlation between the charge-transfer (CT)
absorption energy (Eabs, in cmϪ1) of [Pt(bpy)(4-MeOC6H4S)2] in the
visible region and the unitless empirical solvent parameters Pt(SS)-
(NN)3e (open symbols) and E*MLCT
(solid symbols). Solvents
15
used [Eabs, Pt(SS)(NN), E*MLCT]: ᭿, CCl4 (17 857, 0, 0.12); ᭹, thf
(18 868, 0.494, 0.59); ᭡, CHCl3 (19 084, 0.610, 0.42); ᭢, CH2Cl2
(19 157, 0.765, 0.67); ᭜, dmf (19 608, 0.901, 0.95); ଙ, dmso (19 763,
0.973, 1.00); linear approximation (least-squares analysis) fitted to
Pt(SS)(NN), dashed line (slope δ = 1898 cmϪ1 = 0.23 eV; r2 = 99.2%),
and E*MLCT, solid line (slope δ = 1931 cmϪ1 = 0.24 eV; r2 = 94.4%)
The shorter-wavelength absorption band of [Pt(bpy)(4-
XC6H4S)2] is centred at 320–380 nm [see Fig. 1(a)] and its pos-
ition also depends on the donor ability of the thiolate ligands;
switching from X = H to Me2N induces a red (hypsochromic)
shift of 1900 cmϪ1
.
Results
Electronic absorption spectra
Resonance Raman spectra
The resonance Raman spectra of [Pt(bpy)(4-MeOC6H4S)2] dis-
persed in KNO3 were obtained with 457.9, 488 and 514.5 nm
excitation into the visible absorption band of the complex. The
spectra exhibit peaks at 1603, 1560, 1491, 1321, 1277, 1177,
1031 and 663 cmϪ1 which are assigned straightforwardly to the
internal modes of the co-ordinated 2,2Ј-bipyridine.16 The peak
at 1491 cmϪ1 possesses a higher intensity relative to those at
1603 and 1560 cmϪ1. Such an intensity pattern is characteristic
of charge-transfer electronic transitions directed to the lowest
empty orbital of the 2,2Ј-bipyridine ligand, i.e. π1*(bpy).16 In
addition, very weak Raman bands were observed at 1092, 733,
642, 452, 428, 414, 395, 374 and 280 cmϪ1 which may be
assigned to internal thiolate, mainly phenyl-ring17 vibrations.
The band at 374 cmϪ1 may also belong to the ν(Pt᎐S) or
ν(Pt᎐N) modes expected17,18 in this case between 320 and 380
The electronic absorption spectra of the [Pt(bpy)(4-XC6H4S)2]
complexes exhibit in the visible region an asymmetric absorp-
tion band of medium intensity, centred at 500–540 nm in dmf
and at 520–570 nm in thf [see Table 1 and Fig. 1(a)]. For
X = NO2 this band is obscured by the intense π → π*(NO2)
absorption. Increasing the σ,π-donor capacity of the thiolate
ligands by use of more electron-releasing substituents X shifts
the visible band to a lower energy and enhances the shoulder on
the low-energy side. Obviously, at least two electronic transi-
tions are hidden under the band envelope. It is noteworthy that
a similar solvent-dependent asymmetric band shape was
observed for [Pt(dmbpy)(PhS)2] (dmbpy = 4,4Ј-dimethyl-2,2Ј-
bipyridine).3f All [Pt(bpy)(4-XC6H4S)2] complexes show a
negative solvatochromism of their absorption maxima, which
correlates well with Manuta and Lees’ E*MLCT {based on
[W(CO)4(bpy)]} and Cummings and Eisenberg’s solvent scales
{based on [Pt(dbbpy)(tdt)] (dbbpy = 4,4Ј-di-tert-butyl-2,2Ј-
bipyridine, tdt = toluene-3,4-dithiolate)}3e [see Fig. 1(b)].
Non-polar solvents could not be used for the solvato-
chromic measurements due to the negligible solubilities of
cmϪ1
.
(Spectro)electrochemistry
The redox behaviour of the complexes [Pt(bpy)(4-XC6H4S)2]
and of selected free thiolates were studied in thf by cyclic
J. Chem. Soc., Dalton Trans., 1998, Pages 2459–2466
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Fig. 4 Cyclic voltammogram of [Pt(bpy)(4-O2NC6H4S)2]. Conditions:
see Fig. 2
Fig. 2 Cyclic voltammogram of 10Ϫ3  [Pt(bpy)(4-Me2NC6H4S)2].
Conditions: 10Ϫ3  solution in thf–3 × 10Ϫ1  NBu4PF6, 293 K, plat-
The subsequent one-electron reduction of the radical anions
producing [Pt(bpy)(4-XC6H4S)2]2Ϫ (X = H, MeO or Me2N) is
chemically reversible on the voltammetric timescale defined by
ν > 100 mV sϪ1; it is however electrochemically quasi-reversible,
as indicated by the slightly larger and scan-rate dependent ∆Ep
values relative to ∆Ep(ferrocene–ferrocenium) (see Table 2).
During the UV/VIS spectroelectrochemical experiments at
293 K, [Pt(bpy)(4-Me2NC6H4S)]2Ϫ was only detected as a
short-lived transient absorbing in thf at ca. 600 nm. Its decom-
position was not studied in detail. In view of the nearly invari-
able reduction potentials of the radical anions (see Table 2) we
propose that also the second cathodic step is initially localised
on the 2,2Ј-bipyridine ligand.
inum-disc electrode (0.49 mm2 apparent surface area), ν = 100 mV sϪ1
.
The dotted curve corresponds to the anodic scan at 210 K
The reduction path of [Pt(bpy)(4-O2NC6H4S)2] is partly dif-
ferent due to the presence of the reducible conjugated p-NO2
substituents on the thiolate ligands. The cyclic voltammogram
of the latter complex exhibits a reversible one-electron cathodic
process which is followed by two overlapping one-electron
waves (see Table 2 and Fig. 4). The diffusion coefficient of the
complex, DPt = 3.0 × 10Ϫ6 cm2 sϪ1, was determined relative to
that of the internal ferrocene standard (DFc = 8.0 × 10Ϫ6 cm2 sϪ1
in thf 22), and from direct application of equation (3) where Ip,c
2
–
5
¹ ¹
3
²
²
Ip,c = 2.69 × 10 n AD ν C
(3)
is the peak current of the first cathodic wave, A the real surface
area of the cathode and C the bulk concentration of the
complex.23
Fig. 3 The UV/VIS spectral changes accompanying the 1e reduction
of [Pt(bpy)(4-Me2NC6H4S)2]. Conditions: 2 × 10Ϫ3  solution in thf–
3 × 10Ϫ1  NBu4PF6, 293 K; electrolysis within an OTTLE cell10
From the voltammetric response it was not clear whether
the initial one-electron step involves reduction of the 2,2Ј-
bipyridine ligand followed by parallel one-electron reduction of
both p-NO2 groups, or if one of the [4-O2NC6H4S]Ϫ ligands
is reduced first. The latter possibility may get support from
the more positive reduction potential of unco-ordinated [4-
O2NC6H4S]Ϫ relative to that of 2,2Ј-bipyridine (see Table 2). An
unambiguous assignment of the electron-transfer sequence
proved to be possible on the basis of the corresponding UV/VIS
spectroelectrochemical information. The one-electron reduc-
tion of [Pt(bpy)(4-O2NC6H4S)2] led to the appearance of weak
voltammetry and UV/VIS spectroelectrochemistry. The redox
potentials of the compounds are listed in Table 2.
Reduction. A typical cyclic voltammogram of [Pt(bpy)(4-
XC6H4S)2] (X = H, MeO or Me2N) is shown for X = Me2N in
Fig. 2. All three complexes undergo chemically and electro-
chemically reversible one-electron reduction the half-wave
potential of which slightly decreases with increasing donor
capacity of the thiolate ligands. The corresponding UV/VIS
spectroelectrochemical experiment established that the electron
transfer from the platinum cathode is directed to the π1*(bpy)
LUMO, as judged from the appearance of the characteristic
bands at 725 (sh), 798 and 850 nm attributable to the IL transi-
Ϫ
tions of the co-ordinated [bpy] radical anion. The other IL
᝽
bands between 400 and 500 nm are obscured by the intense
π → π* (NO2) band of [Pt(bpy Ϫ)(4-O2NC6H4S)2]Ϫ at 457
᝽
bands of [Pt(bpy Ϫ)(4-Me2NC6H4S)2]Ϫ at λmax = 468, 502, 737
nm, which had shifted to lower energy compared to that of the
neutral parent complex (see Fig. 5 and Table 1). The unresolved
second and third one-electron reduction steps produced the tri-
anion [Pt(bpy)(4-O2NC6H4S)2]3Ϫ the UV/VIS spectrum (see
Fig. 6) of which exhibits a characteristic sharp IL band of the
᝽
(sh) and 816 nm, together with an additional band below 900
nm (see Fig. 3), belonging to the characteristic21 π* → π*
Ϫ
intraligand (IL) electronic transitions of the [bpy] radical
᝽
anion. For [Pt(bpy Ϫ)(PhS)2]Ϫ these IL transitions shift to
᝽
higher energies, viz. to λmax = 454, 484, 706 and 787 nm.
singly reduced dianionic ligand [4-Ϫ O2NC6H4S]2Ϫ at 382 nm.
᝽
2462
J. Chem. Soc., Dalton Trans., 1998, Pages 2459–2466

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Fig. 5 The UV/VIS spectral changes accompanying the 1e reduction
of [Pt(bpy)(4-O2NC6H4S)2]. Conditions: see Fig. 3
Fig. 7 The UV/VIS spectra of unco-ordinated 4-O2NC6H4SH (dotted
line) and its consecutive reduction products, the anion [4-O2NC6H4S]Ϫ
(dashed line) and the radical dianion [4-Ϫ O2NC6H4S]2Ϫ (solid line). A
᝽
spectroelectrochemical experiment under conditions given in Fig. 3
(4-XC6H4S)2], respectively, points to the one-electron character
of the oxidation step on the timescale defined by ν у 100 mV
s
Ϫ1. Importantly, the one-electron oxidation of [Pt(bpy)-
(4-Me2NC6H4S)2] in thf becomes partly chemically reversible
on cooling to 210 K (see Fig. 2). This result sharply contrasts
with the oxidation of [Pt(bpy)(4-O2NC6H4S)2] which remains
totally chemically irreversible at this temperature.
Discussion
The one-electron reduction of all the complexes [Pt(bpy)(4-
XC6H4S)2] under study is localised on the LUMO which pos-
sesses, as could be anticipated,3,7 an almost exclusive π1*(bpy)
0/
character. The gradual positive shift of E (bpy Ϫ) by 180 mV
₁
₂
᝽
on proceeding from the strongly donating thiolate ligand with
X = Me2N to that with the electron-withdrawing NO2 substitu-
ent can reasonably be ascribed to the increased bpy→Pt σ
donation in the latter case. The same conclusions can be drawn
for the second one-electron reduction step, with the exception
Ϫ)(4-O2NC6H4S)2]Ϫ which reduces at the thiolate
Ϫ
of [Pt(bpy
Fig. 6 The UV/VIS spectra of [Pt(bpy )(4-O2NC6H4S)2]Ϫ (dotted
᝽
᝽
Ϫ
line) and [Pt(bpy )(4-O2NC6H4S)2]3Ϫ (solid line). The trianion was
ligands.
᝽
The comparatively large variation of the oxidation potentials
of [Pt(bpy)(4-XC6H4S)2] and those of the corresponding free
anions [4-XC6H4S]Ϫ as reference compounds points to a dom-
inant thiolate character of the HOMO of the complexes. The
same conclusion was made, e.g. for [PtII(bpy)(µ-S2MVIS2)]
(M = Mo or W)2e and for [Pt(bpy)L] [L = tdt or catecholate
(cat)],24 in the latter case being substantiated by a large hypso-
chromic shift (2000 cmϪ1) of the lowest-energy charge-transfer
band on replacing the catecholate by the more easily oxidisable
tdt ligand.25 In contrast to this assignment, Eisenberg and co-
workers3 have ascribed the HOMO of a large number of Pt(α-
diimine)(dithiolate) complexes to a strongly mixed Pt(d)–S(p)
orbital, and classified the HOMO → LUMO electronic tran-
sition as a ‘charge transfer to diimine’ process. This general
term has been adopted also for platinum() diimine complexes
with aromatic thiolate ligands, e.g. for [Pt(dpphen)(4-XC6H4S)2]
(X = MeO or Me2N).3g
generated under conditions given in Fig. 3
This band is found at 384 nm in the UV/VIS spectrum of the
unco-ordinated dianion generated on stepwise reduction of the
precursor 4-O2NC6H4SH and [4-O2NC6H4S]Ϫ (see Fig. 7 and
Table 1). The UV/VIS spectrum of [Pt(bpy)(4-O2NC6H4S)2]3Ϫ
also clearly shows the characteristic π* → π* IL bands of the
Ϫ
[bpy] ligand at λmax = 464 and 501 nm; the lower-lying IL
᝽
Ϫ
bands of [bpy] do not change their position on the two-
᝽
electron reduction of [Pt(bpy Ϫ)(4-O2NC6H4S)2]Ϫ (see Fig. 6).
᝽
Oxidation. Cyclic voltammetry reveals that the [Pt(bpy)(4-
XC6H4S)2] complexes rapidly decompose during oxidation at
room temperature (see Figs. 2 and 4). The oxidation potential
Ep,a shifts considerably negatively with increasing donor cap-
acity of the thiolate ligands. Notably, the difference between the
Ep,a values for [Pt(bpy)(4-XC6H4S)2] is equal to or even larger
than that observed in the case of the chemically irreversible
oxidation of the unco-ordinated anions [4-XC6H4S]Ϫ (gener-
ated by preparative electrolysis of the corresponding thio-
phenols; see Table 2). A comparison of the cathodic and anodic
peak currents due to the reduction and oxidation of [Pt(bpy)-
In view of the strongly thiolate-dependent oxidation poten-
tials of [Pt(bpy)(4-XC6H4S)2], the HOMO of the complexes
possesses a large contribution from the p(S) and π(C6H4X)
orbitals, which can be even larger than predicted by extended
Hückel (EH) calculations on the model complex with X = H,
J. Chem. Soc., Dalton Trans., 1998, Pages 2459–2466
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Table 2 Cyclic voltammetric data of the investigated complexes [Pt(bpy)(4-XC6H4S)2] (X = NO2, H, MeO or Me2N), unco-ordinated 2,2Ј-
bipyridine, corresponding thiolate anions and thiophenols, and related [Pt(bpy)L2] complexes; all in thf at 293 K, unless stated otherwisea
ox
red I
red II
₁
₁
Ep,a
E
E
∆Ep (ferrocene–
₂
₂
Compound
(∆Ep)
(∆Ep)
(∆Ep)
ferrocenium)
[Pt(bpy)(4-O2NC6H4S)2]
ϩ0.58
Ϫ1.60 (110)
Ϫ1.59c (100)
Ϫ1.72 (110)
Ϫ1.67d (100)
Ϫ1.76 (100)
Ϫ1.78 (120)
Ϫ1.67c (100)
Ϫ2.72 (120)
Ϫ2.08i
Ϫ1.87b (140)
Ϫ1.83b,c (150)
Ϫ2.37 (130)
Ϫ2.33d (150)
Ϫ2.39 (120)
Ϫ2.41 (140)
Ϫ2.30c (140)
Ϫ3.29g,h
110
100
110
100
100
120
100
120
ϩ0.66c
ϩ0.22
[Pt(bpy)(PhS)2]
ϩ0.31d
Ϫ0.04
[Pt(bpy)(4-MeOC6H4S)2]
[Pt(bpy)(4-Me2NC6H4S)2]
Ϫ0.23
Ϫ0.21c,e (125)
bpyf
[4-O2NC6H4S]Ϫ
[PhS]Ϫ
Ϫ0.18
Ϫ0.54
Ϫ0.65
[4-MeOC6H4S]Ϫ
4-O2NC6H4SH
PhSH
Ϫ1.46g–i
Ϫ1.70g,h
Ϫ1.80g,h
Ϫ1.64 (83)
Ϫ2.05
4-MeOC6H4SH
[Pt(bpy)Cl2]
[Pt(bpy)(mes)2]j
[Pt(bpy)(tdt)]l
ϩ0.67
Ϫ2.30g
Ϫ2.73
ϩ0.45k (60)
Ϫ0.024e
Ϫ1.739
ox
a Definitions: Ep,a = anodic peak potential of chemically irreversible oxidation, E red = half-wave potential of chemically reversible reduction in V;
₁
₂
∆Ep = cathodic-to-anodic peak separation in mV. b Overall transfer of 2e; two overlapping one-electron reductions of the 4-NO2 groups of the
thiolate ligands. c At 210 K. d In dmf at 293 K. e Chemically partly reversible. f From ref. 19. g Chemically irreversible. h Cathodic peak potential.
i Data in acetonitrile reported in ref. 20. j From ref. 7. k Chemically reversible. l From ref. 3(e).
showing strongly mixed Ph(π)–S(p)–Pt(d) characters for the
closely spaced HOMO, HOMO-1 (∆E = 0.06 eV) and HOMO-
2 (∆E = 0.22 eV).3f In this respect it is important to note that
the EH calculations on [Pt(bpy)(PhS)2] were based only on
X-ray data on dithiolate complexes, e.g. almost planar [Pt-
(bpym)(mnt)] (bpym = 2,2Ј-bipyrimidine, mnt = maleonitrile-
dithiolate),26 and similar compounds, but not on any Pt(α-
diimine)(thiolate)2 structure which may deviate from planarity
to some extent, as discussed below. Consequently, the true par-
entages of the occupied frontier orbitals still remain a matter of
discussion, although the mixed character of the HOMO with
prevailing contribution from the thiolate ligands is indisputable.
We assume that the thiolate contribution to the HOMO is not
constant and may vary with the donor strength of X. It is
probable that in such a case a profound dependence of the
oxidation potentials of [Pt(bpy)(4-XC6H4S)2] on the donor prop-
erties of the thiolate ligands will still be observed independent
of the variable HOMO parentage. For example, the HOMO of
related [Pt(bpy)(edt)] (edt = ethane-1,2-dithiolate) possesses a
41% Pt and 56% edt character, whereas with the significantly
more electron-withdrawing maleonitriledithiolate ligand the
HOMO becomes apparently more dithiolate-localised (27% Pt/
72% mnt).3f The But derivative [Pt(dbbpy)(edt)] then oxidises
internal electron acceptor benzyl viologen (1,1Ј-dibenzyl-4,4Ј-
bipyridinium) to occur.27a The one-electron electrochemical
oxidation of [Pt(bpy)(4-XC6H4S)2] is probably also followed
by the formation of the aforementioned S᎐Pt᎐S bond. The
energy barrier for the latter process is then probably relatively
high in the presence of the donor substituent X = 4-Me2N,
which may explain the partial chemical reversibility of the
one-electron oxidation of [Pt(bpy)(4-Me2NC6H4S)2] at low
temperatures. The subsequent electron-transfer step from the
S᎐Pt᎐S-bound transient to the anode ultimately produces free
disulfide, as was also observed during the photooxidation
reactions.§
The solvent-dependent asymmetric visible absorption band
of [Pt(bpy)(4-XC6H4S)2] obviously belongs to charge-transfer
electronic transitions directed to the empty π1*(bpy) orbital, as
was also confirmed by resonance Raman spectroscopy in the
case of [Pt(bpy)(4-MeOC6H4S)2]. The very weak resonance
Raman effect of the internal thiolate (Ph ring) and possibly also
ν(Pt᎐S) modes implies that the bonds within the platinum thi-
olate moiety are hardly affected during the electronic transition.
This result again supports the rather delocalised Ph(π)–S(p)–
Pt(d) bonding situation in this complex (see above). The alter-
native assignment of the visible absorption band to a largely
Pt(d)→bpy(π1*) charge-transfer transition can be excluded in
view of the absence of a corresponding low-energy absorption
band in the UV/VIS spectra of yellow [Pt(bpy)Cl2]. The EH
calculations performed3f on [Pt(bpy)(PhS)2] indeed revealed
several occupied orbitals delocalised over the Pt᎐SPh moiety,
which lie in close proximity to each other (see above). In this
respect we cannot exclude the possibility that the resonance
significantly more negatively than [Pt(dbbpy)(mnt)]: ∆Ep,a
=
Ϫ0.513 V.3e In our case the fairly strong Pt(d)–S(p)–Ph(π)
mixing in the HOMO of [Pt(bpy)(4-XC6H4S)2] is reflected in the
one-electron character of the primary oxidation step, as was
unambiguously confirmed for X = Me2N at low temperatures.
We can reasonably expect that the absence of electronic
communication between the two thiolate ligands through the
platinum would result in their nearly parallel one-electron
oxidation, i.e. in the transfer of two electrons. Thus, the re-
duction of the non-interacting 4-NO2 substituents in [Pt(bpy)-
(4-O2NC6H4S)2] is indeed an unresolved two-electron step.
The higher stability of the one-electron oxidised complex
[Pt(bpy)(4-Me2NC6H4S)2]ϩ at low temperatures in comparison
with the unstable cation [Pt(bpy)(4-O2NC6H4S)2]ϩ corresponds
with the more efficient photooxidation of the parent complex
[Pt(bpy)(4-O2NC6H4S)2].27a In the charge-separated excited
Raman experiments with [Pt(bpy)(4-MeOC6H4S)2] (λexc
=
457.8, 488 and 514.5 nm) were performed on excitation into a
visible charge transfer-to-diimine transition which originates
from a strongly platinum thiolate delocalised donor orbital
underlying the HOMO, e.g. the HOMO-2 as described for
[Pt(bpy)(PhS)2].3f Thus, matrix analysis28 applied to a series of
UV/VIS spectra of 10Ϫ3–10Ϫ5  [Pt(bpy)(4-MeOC6H4S)2] in thf
revealed two absorption bands centred at 530 and 630 nm,
independent of concentration, which exhibit equal solvato-
state of the latter complex probably a three-electron S᎐Pt᎐S
bond5 is formed which is stabilised by the electron-withdrawing
4-NO2 substituent. Its presence increases sufficiently the
excited-state lifetime. This allows electron transfer to the
§ Photooxidation of [Pt(phen)(PhS)2] in air-saturated butyronitrile
resulted in the appearance of the characteristic27b intense absorption
band of unco-ordinated PhSSPh at 242 nm (ε = 1.3 × 104  cmϪ1).
2464
J. Chem. Soc., Dalton Trans., 1998, Pages 2459–2466

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chromism. As already noted, UV/VIS spectra of [Pt(dmbpy)-
(PhS)2] also show a broad visible absorption band with a
shoulder on its long-wavelength side which becomes particu-
larly apparent in apolar solvents3f and may belong to the
HOMO → LUMO transition. This shoulder of the main vis-
ible band may alternatively be assigned to a spin-forbidden
singlet → triplet charge transfer-to-diimine transition which
has sufficient intensity due to the presence of the heavy 5d8
metal centre. We prefer the latter assignment which is also
consistent with the charge-transfer absorption characteristics
of some Pt(diimine)(dithiolate) complexes studied by
Schanze and co-workers,27 and Pt(diimine)(mes)2 (mes = 2,4,6-
trimethylphenyl).7
Acknowledgements
This work was supported by the Russian Fund of Basic
Research (grant no. 96-03-32530), the Netherlands Foundation
of Chemical Research (SON) and the Netherlands Organiz-
ation for the Advancement of Pure Science (NWO). J. A. W.
is grateful for financial support within the framework of the
Exchange Agreement between the University of Amsterdam
and the Moscow State University to support her stay in
Amsterdam. We owe thanks to Theo L. Snoeck (University of
Amsterdam) for recording the resonance Raman spectra, and
to Professor Derk J. Stufkens (University of Amsterdam) and
Dr. Alex Galin (Moscow State University) for valuable discus-
sions and interest in this work.
The relatively small molar absorption coefficients for the
main visible absorption band of [Pt(bpy)(4-XC6H4S)2] (X = H,
MeO or Me2N) (εmax ≈ 2000 Ϫ1 cmϪ1) compared to those of
the corresponding dithiolate complexes (εmax = 5000–15 000 Ϫ1
References
3
cmϪ1
)
are noteworthy. For [Pt(dpphen)(4-MeOC6H4S)2] with
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the more rigid dpphen ligand a value εmax = 3000 Ϫ1 cmϪ1 was
reported.3g In view of our results, the characteristic value
εmax у 10 000 Ϫ1 cmϪ1 measured by Eisenberg and co-
workers3f for a series of Pt(diimine)(dithiolate) complexes is
unlikely to apply to [Pt(dmbpy)(PhS)2]. We may assume that the
torsion angle between the 2,2Ј-bipyridine and thiolate ligands is
slightly larger than in the case of the more rigid dithiolate lig-
ands, for example due to the thermal excitation of torsional
vibrations. In addition, rotation of the Ph rings of the thiolate
ligands out of the Pt(bpy) plane might cause a poorer orbital
overlap between the frontier orbitals involved in the optical
electron transfer and, consequently, the lower intensity of the
visible charge-transfer absorption band of [Pt(bpy)(4-
XC6H4S)2]. These arguments are indeed supported by the
recently reported first crystal structure of platinum() diimine
bis(monothiolate) complex, where diimine = dpphen and
thiolate = 1,2-dicarba-closo-dodecaborane(12)-1-thiolate.30
The higher-lying absorption band of [Pt(bpy)(4-XC6H4S)2]
centred in dmf at 330–370 nm also belongs to an interligand
charge-transfer electronic transition, as documented by its red
shift on increasing the donor capacity of the thiolate ligands.
The separation between the vacant π1* and π2* orbitals of co-
ordinated 2,2Ј-bipyridine is known to vary between 7000 and
9000 cm .
Ϫ1 16,31 This value corresponds well to the energetic dif-
ference between the main visible band and the higher-energy
one, which amounts to 9670 and 9200 cmϪ1 for [Pt(bpy)(4-
XC6H4S)2], X = H and Me2N, respectively. The absorption band
at 330–370 nm is, therefore, attributed to an electronic transi-
tion from the mixed π(Ph)–p(S)/d(Pt) orbital manifold to the
π2*(bpy) orbital.
6 (a) M. P. Aarnts, M. P. Wilms, K. Peelen, J. Fraanje, K. Goubitz,
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Conclusion
The resonance Raman and (spectro)electrochemical data pre-
sented in this study suggest significant p(S)–π(Ph) and d(Pt)
orbital contributions to the HOMO of [Pt(bpy)(4-XC6H4S)2],
and a dominant π1*(bpy) contribution to the LUMO. In this
respect these complexes resemble their dithiolate derivatives.
Further research, in particular single-crystal X-ray studies,30 is
needed to gain evidence for the anticipated position of the thio-
late Ph rings out of the Pt(bpy) plane and for a larger torsion
angle between the 2,2Ј-bipyridine and the thiolate ligands,
compared to the corresponding dithiolate complexes. These
structural differences are thought to be responsible for the
less intense visible charge transfer-to-diimine absorption of
[Pt(bpy)(4-XC6H4S)2] relative to the dithiolate derivatives. The
X-ray data would also allow the nature of the frontier orbitals
of the bis(thiolate) complexes to be calculated with higher pre-
cision. For example, density functional calculations,6a suitable
for an unambiguous interpretation of electrochemical and UV/
VIS absorption data, could be undertaken.
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Received 29th April 1998; Paper 8/03225D
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