Photochemical Oxidation of Pt(IV)Me3(1,2-diimine) Thiolates to Luminescent Pt(IV) Sulfinates

Article abstract of DOI:10.1021/acs.inorgchem.0c03553

We report the formation of dinuclear complexes from, and photochemical oxidation of, (CH3)3-Pt(IV)(N^N) (N^N = 1,2-diimine derivatives) complexes of thiophenolate ligands to the analogous sulfinates (CH3)3Pt(N^N)(SO2Ph) and structural, spectroscopic, and

Full text of DOI:10.1021/acs.inorgchem.0c03553

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Photochemical Oxidation of Pt(IV)Me3(1,2-diimine) Thiolates to
Luminescent Pt(IV) Sulfinates
Barbora Mala,∥ Laura E. Murtagh,∥ Charlotte M. A. Farrow, Geoffrey R. Akien, Nathan R. Halcovich,
Sarah L. Allinson, James A. Platts, and Michael P. Coogan*
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ABSTRACT: We report the formation of dinuclear complexes from, and
photochemical oxidation of, (CH3)3-Pt(IV)(N^N) (N^N = 1,2-diimine
derivatives) complexes of thiophenolate ligands to the analogous sulfinates
(CH3)3Pt(N^N)(SO2Ph) and structural, spectroscopic, and theoretical studies of
the latter revealing tunable photophysics depending upon the 1,2-diimine ligands.
Electron-rich thiolate and conjugated 1,2-diimines encourage formation of
thiolate-bridged dinuclear complexes; smaller 1,2-diimines or electron-poor
thiolates favor mononuclear complexes. Photooxidation of the thiolate ligand yields hitherto unreported Pt(IV)-SO2R complexes,
promoted by electron-deficient thiolates such as 4-nitrothiophenol, which exclusively forms the sulfinate complex. Such complexes
exhibit expected absorptions due to π-π* ligand transitions of the 1,2-diimines mixed with spin-allowed singlet MLCT (d-π*) at
relatively high energy (270−290 nm), as well as unexpected broad, lower energy absorptions between 360 and 490 nm. DFT data
indicate that these low energy absorption bands result from excitation of Pt−S and Pt−C σ-bonding electrons to π* orbitals on
sulfinate and 1,2-diimine, the latter of which gives rise to emission in the visible range.
been widely applied in sensing and imaging. The photo-
chemistry and photophysics of the Pt(IV)(N^N) core have
been widely studied,12 and there are several reports of thiolato
derivatives of this unit.13
INTRODUCTION
■
There are many applications of photophysically active
transition-metal complexes in, among other areas, photo-
catalysis,1 LEDs and related devices,2 energy-related research
and photovoltaics,3 and luminescent cell imaging.4 There are
many well-known luminescent platinum-based systems, most
involving d8 Pt(II) complexes, in particular tridentate cyclo-
metallates derived from aromatic heterocycles with high
quantum yields5 and impressive 2-photon cross sections.6
Certain platinum(IV) complexes also have useful photo-
physical properties including photoluminescence, with some
showing quantum yields up to 80%.7 Luminescent bis-
cyclometalated Pt(IV) complexes, of the general formula
[Pt(C^N)2(R)Cl], were first reported in 1984 by Chassot et
al.,8 and since then more brightly luminescent platinum(IV)
complexes with high quantum yields and long luminescence
lifetimes have been reported.7 Cyclometalated Pt(IV) com-
plexes are of particular interest as their properties, such as
solubility and lipophilicity, are easily tunable by only slight
alterations of the ligands.9 Furthermore, some luminescent tris-
cyclometalated Pt(IV) complexes have displayed promising
intense emission which is also highly sensitive to quenching by
oxygen, making these potentially useful for oxygen sensing
applications.10 Pt(IV)Me3I, the first transition-metal σ-alkyl
complex ever reported,11 is commercially available and exists as
an iodide-bridged tetramer and readily reacts with chelating
N^N ligands to give mononuclear complexes of the general
formula PtMe3(N^N)X, analogous to the well-known
luminophores Re(CO)3(N^N)(X) (X = Cl/Br) which have
There is one report exploring the synthesis, photophysics,
and application in fluorescent cell imaging, of platinum(IV)
1,2-diimine trimethyl thiolates.14 Time-dependent density
functional theory calculations revealed that the excited states
of platinum(IV) trimethyl 1,2-diimine iodide complexes arise
from a mixed n and σ-π* Inter-Ligand Charge Transfer
(ILCT) from the lone pair of sulfur and the S−Pt σ-bond, to
the 1,2-diimine π*.14 The small range of 1,2-diimines reported
indicated that absorption and emission were tunable by
variations in ligand substitution, with emission extending into
the NIR in one case. Therefore, in an effort to develop
analogues with a variety of useful photophysical properties, an
investigation of complexes based on this general structure with
a range of 1,2-diimine and thiolate ligands was undertaken.
EXPERIMENTAL SECTION
■
General Experimental. The starting materials, reagents, and
solvents used are all commercially available. H NMR and 13C NMR
1
Received: December 4, 2020
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spectra were recorded at 400 and 100 MHz, respectively, on a Bruker
Avance III 400 and referenced to residual solvent peaks. Chemical
shifts are reported in ppm, and coupling constants in Hz. All
photophysical measurements were recorded in acetonitrile at 20 °C.
UV−vis spectra were recorded on an Agilent Technologies Cary 60.
IR spectra were recorded on an Agilent Technologies Cary 630 FTIR
Spectrometer as solids and are reported in wavenumbers (cm−1).
Mass spectra were recorded on a Shimadzu LCMS-IT-TOF in ESI+
mode.
(4H, m, H5/5′ and H3/3′), 6.92 (2H, d, J = 8.9 Hz H7/7′), 6.16
(2H, d, H6/6′), 1.33 (6H, s, eq-Me; 195Pt satellite J = 70.9 Hz), 0.40
(3H, s, ax-Me; 195Pt satellite J = 66.4 Hz); δ 13C NMR (400 MHz,
CDCl3) 157.4, 146.5 (195Pt satellite J = 15.2 Hz), 145.5, 142.8, 137.2,
133.7 (195Pt satellite J = 5.2 Hz), 130.8 (195Pt satellite J = 6.4 Hz),
127.4, 125.3 (195Pt satellite J = 14.0 Hz), 120.8, −0.0, −5.7 (195Pt
satellite J = 681.8 Hz); IR υmax (cm−1) 2886, 2808, 1567, 1483, 1425,
1219; Crystal data for 27 C21H21N3O4PtS (M = 606.56 g/mol):
monoclinic, space group P21/n (no. 14), a = 6.87831(12) Å, b =
13.65275(18) Å, c = 21.0623(3) Å, β = 97.2931(15)°, V =
1961.91(5) Å3, Z = 4, T = 99.99(10) K, μ(Cu Kα) = 14.681
mm−1, Dcalc = 2.054 g/cm3, 19 671 reflections measured (7.736° ≤ 2Θ
≤ 152.948°), 4087 unique (Rint = 0.0423, Rsigma = 0.0255) which were
used in all calculations. The final R1 was 0.0365 (I > 2σ(I)) and wR2
was 0.0967 (all data). CCDC 2013233.
Steady state emission and excitation spectra were recorded on an
Agilent Technologies Cary Eclipse. Luminescence lifetimes were
recorded on a PicoQuant FluoTime 300 exciting with an LDH-P-C-
375 or LDH-P-C405 ps pulsed laser (PicoQuant) operating at 50 and
80 MHz, respectively. The emission signals were digitized using a
high-resolution TCSPC module (PicoHarp 300, PicoQuant) with 25
ps time width per channel. The time-resolved decay curves were
analyzed using FLUOFIT software (PicoQuant) using a one-
exponential model. Quantum yields were estimated relative to
[Ru(bpy)3]·2PF6 in aerated acetonitrile with titration to equi-
absorbing solutions at the excitation wavelengths at around A =
0.05. Degassed measurements were recorded in a quartz cuvette fused
to a T-piece connecting a round-bottom flask and a J. Young tap
connecting a ground glass joint, following three cycles of freeze-pump-
thaw degassing. Iodotrimethyl(N^N)Pt(IV) complexes 1−5 (N^N =
2,2′-bipyridine (bpy) 1, 4,4′-dimethoxy-2,2′-bipyridine (OMebpy) 2,
1,10-phenanthroline (phen) 3, 4,7-diphenyl-1,10-phenanthroline
(Bphen) 4, dipyrido[3,2-a:2′,3′-c]phenazine (DPPZ) 5) were
prepared by literature procedures.12,14 Complexes 6−25 derived
from the reactions of 1−5 with thiophenol, and derivatives, were
metastable, and for details relating to their detection, partial
characterization by crystallography and in some cases NMR and for
details of the mechanistic studies and the preparation of an authentic
sample of 4-mercaptomethylbenzoate disulfide,15 see the Supporting
Information (SI).
[PtMe3(4,7′-diphenyl-1,10-phenanthroline)SO2PhNO2] 28.
To a flask containing [PtMe3I(4,7′-diphenyl-1,10-phenanthroline)]
4 (25.7 mg, 0.04 mmol), 4-nitrothiophenol (5.9 mg, 0.04 mmol), and
NaOtBu (3.9 mg, 0.04 mmol) were added acetonitrile (4 mL) and
water (1 mL), and the mixture was heated at 60 °C for 30 min, then
cooled to room temperature, and refrigerated overnight, giving a
solution containing orange crystals and a white powder. The mixture
was filtered and washed with water to obtain orange crystals 28 (10.3
mg, 38%).
1
δ H NMR (400 MHz, CDCl3): 9.18 (2H, d, J = 5.2 Hz, H2/2′;
195Pt satellite J = 19.3), 7.91 (2H, s, H4/4′), 7.85 (2H, d, 5.2 Hz, H3/
3′), 7.67−7.51 (10H, m, Ph), 7.11 (2H, 9.0 Hz, d, H9/9′), 6.44 (2H,
6.4 Hz, d, H8/8′), 1.45 (6H, s, eq-Me; 195Pt satellite J = 70.0 Hz),
0.53 (3H, s, ax-Me; 195Pt satellite J = 66.6 Hz); δ 13C NMR (400
MHz, CDCl3) 156.9, 150.5, 146.0, 145.8 (195Pt satellite J = 14.7 Hz),
143.0, 135.6, 134.0, 129.8, 129.5, 129.2, 129.1, 125.6, 125.4, 120.8,
0.1, −5.4 (195Pt satellite J = 678.0 Hz); IR υmax (cm−1) 2890, 2812,
1560, 1489, 1217; Crystal data for 28 C33H29N3O4PtS (M = 758.74
g/mol): monoclinic, space group C2/c (no. 15), a = 21.3317(3) Å, b
= 12.75780(10) Å, c = 24.8460(3) Å, β = 110.718(2)°, V =
6324.47(15) Å3, Z = 8, T = 100.0(3) K, μ(CuKα) = 9.245 mm−1, Dcalc
= 1.594 g/cm3, 48 486 reflections measured (7.608° ≤ 2Θ ≤
152.926°), 6617 unique (Rint = 0.0305, Rsigma = 0.0135) which were
used in all calculations. The final R1 was 0.0437 (I > 2σ(I)) and wR2
was 0.0987 (all data). CCDC 2013234.
[PtMe3(bpy)SO2PhNO2] 26. To a flask containing [PtMe3I(2,2′-
bipyridine)] 1 (101.3 mg, 0.19 mmol), 4-nitrothiophenol (30.0 mg,
0.19 mmol), and NaOtBu (20.6 mg, 0.21 mmol) were added
acetonitrile (4 mL) and water (1 mL), and the mixture was heated at
60 °C for 30 min, then cooled to room temperature, and refrigerated
overnight, giving a solution containing orange crystals and a white
powder. The mixture was filtered and washed with water to obtain
orange crystals 26 (63.1 mg, 57%).
[PtMe3(dipyrido[3,2-a:2′,3′-c]phenazine)SO2PhNO2] 29. To
a flask containing [PtMe3I(dipyrido[3,2-a:2′,3′-c]phenazine)] 5 (25.5
mg, 0.04 mmol), 4-nitrothiophenol (6.2 mg, 0.04 mmol), and
NaOtBu (4.5 mg, 0.04 mmol) were added acetonitrile (4 mL) and
water (1 mL), and the mixture was heated at 60 °C for 30 min, then
cooled to room temperature, and refrigerated overnight, giving a
solution containing orange crystals and a white powder. The mixture
was filtered and washed with water to obtain orange crystals 29 (20.1
mg, 76%).
1
δ H NMR (400 MHz, CD3CN): 8.79 (2H, d, J = 6.5 Hz, H6/6′;
195Pt satellite J = 19.5 Hz), 8.19 (2H, d, J = 8.2 Hz, H3/3′), 8.07 (2H,
t, J = 7.9 Hz, H4/4′), 7.67 (2H, t, J = 6.6, H5/5′), 7.34 (2H, d, J = 8.9
Hz, H8/8′), 6.58 (2H, d, J = 9.1 Hz, H7/7′), 1.17 (6H, s, eq-Me;
195Pt satellite J = 70.2 Hz), 0.32 (3H, s, ax-Me; 195Pt satellite J = 66.2
Hz); 13C NMR 158.8, 154.1, 146.6 (195Pt satellite J = 14.3 Hz) 139.0,
134.2 (195Pt satellite J = 6.3 Hz), 127.1 (195Pt satellite J = 14.4 Hz),
123.9 (195Pt satellite J = 8.5 Hz), 121.5, −0.8 (195Pt satellites obscured
by solvent signal), −6.4 (195Pt satellite J = 677.1 Hz); IR υmax (cm−1)
2892, 2812, 1559, 1491, 1418, 1219; Crystal data for 26
1
δ H NMR (400 MHz, CDCl3): 9.80 (2H, d, J = 8.2 Hz, H4/4′),
9.18 (2H, d, J = 5.1 Hz, H2/2′; 195Pt satellite J = 18.7), 8.50−8.44
(2H, m, H5/5′), 8.10−8.05 (4H, m, H6/6′ and H3/3′), 7.02 (2H, d,
J = 8.9, H8/8′), 6.48 (2H, d, J = 8.9, H7/7′), 1.44 (6H, s, eq-Me;
195Pt satellite J = 70.5 Hz), 0.53 (3H, s, ax-Me; 195Pt satellite J = 64.6
Hz); 29 was insufficiently soluble to collect 13C data. IR υmax (cm−1)
2892, 2812, 1560, 1491, 1418, 1221; Crystal data for 29
C27H23N5O4PtS (M = 708.65 g/mol): monoclinic, space group
P21/m (no. 11), a = 7.98930(10) Å, b = 13.0769(2) Å, c =
12.1911(2) Å, β = 97.1620(10)°, V = 1263.73(3) Å3, Z = 2, T =
215.00(10) K, μ(Cu Kα) = 11.536 mm−1, Dcalc = 1.862 g/cm3, 13 560
̅
C19H21N3O4PtS (M = 582.54 g/mol): triclinic, space group P1 (no.
2), a = 6.96733(19) Å, b = 8.8748(2) Å, c = 15.4326(4) Å, α =
93.374(2)°, β = 92.851(2)°, γ = 92.314(2)°, V = 950.52(4) Å3, Z = 2,
T = 100.00(10) K, μ(Mo Kα) = 7.523 mm−1, Dcalc = 2.035 g/cm3,
21 289 reflections measured (6.802° ≤ 2Θ ≤ 59.066°), 4782 unique
(Rint = 0.0372, Rsigma = 0.0267) which were used in all calculations.
The final R1 was 0.0181 (I > 2σ(I)) and wR2 was 0.0410 (all data).
CCDC 2013232.
[PtMe3(1,10-phenanthroline)SO2PhNO2] 27. To a flask con-
taining [PtMe3I(1,10-phenanthroline)] 3 (24.9 mg, 0.05 mmol), 4-
nitrothiophenol (7.40 mg, 0.05 mmol), and NaOtBu (5.2 mg, 0.05
mmol) were added acetonitrile (4 mL) and water (1 mL), and the
mixture was heated at 60 °C for 30 min, then cooled to room
temperature, and refrigerated overnight, giving a solution containing
orange crystals and a white powder. The mixture was filtered and
washed with water to obtain orange crystals 27 (19.6 mg, 71%).
reflections measured (7.308° ≤ 2Θ ≤ 153.144°), 2755 unique (Rint =
0.0501, Rsigma = 0.0320) which were used in all calculations. The final
R1 was 0.0496 (I > 2σ(I)) and wR2 was 0.1303 (all data). CCDC
2013235.
Computational Procedures. DFT calculations used Gaus-
sian09:16 geometry optimization of 26 was performed using M06-
2X/6-31+G(d,p) with SDD ECP/basis on Pt,17 which retains the
stacking interaction between bipy and Ar-NO2. Predicted absorption
spectra and orbital plots were obtained from CAM-B3LYP18 with the
same basis set as optimization, in PCM simulation of CH3CN.19
1
δ H NMR (400 MHz, CDCl3): 9.14 (2H, d, J = 5.0 Hz, H2/2′;
195Pt satellite J = 19.1), 8.59 (2H, d, J = 8.3 Hz, H4/4′), 8.03−7.98
B
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Crystal structures were solved and refined using OLEX20 running
sulfinate complex in which the coordinated sulfur had acquired
two oxygen atoms. Examples of these are given in Scheme 4
and Figure 2 (thiolate 11) and Scheme 5 and Figure 3
(sulfinate 12), and the rest of the observed species are
summarized in Table 1. Further details are available in the SI.
Again, attempts to purify and characterize these species, with
the exception of those derived from 4-nitrothiophenol, led to
decomposition to complex product mixtures.
Regardless of the unstable nature of the products, crystallo-
graphic characterization of these species was interesting as an
addition to the previously known S-bridged multinuclear
Pt(IV)Me3 species,23 and revealing the first examples of
crystallographically characterized Pt(IV) sulfinate complexes.
Although many of these complexes are metastable and not
fully characterized, the observations of cases in which dinuclear
complexes (7, 9, 14, 16) were apparent in the NMR spectra
suggest that electron-rich thiolate and conjugated 1,2-diimines
encourage formation of thiolate-bridged dinuclear complexes,
whereas the cases in which mononuclear species dominate (11,
13, 15, 21, 30) suggest that smaller 1,2-diimines or electron-
poor thiolates favor mononuclear complexes.
the SHELX21 suite of programs and images generated in ORTEP.22
RESULTS AND DISCUSSION
■
Reactions of Iodotrimethyl Platinum(IV) 1,2-Diimines
with Thiophenolates. Following the literature prece-
dent,12,14 iodotrimethylplatinum(IV) 1,2-diimines were pre-
pared from reaction of [PtMe3I]4 with the relevant 1,2-diimine
ligand (i.e., N^N = 2,2′-bipyridine (bpy) 1, 4,4′-dimethoxy-
2,2′-bipyridine (OMebpy) 2, 1,10-phenanthroline (phen) 3,
4,7-diphenyl-1,10-phenanthroline (Bphen) 4, dipyrido[3,2-
a:2′,3′-c]phenazine (DPPZ) 5) in toluene at reflux in good
to excellent yields (Scheme 1). The DPPZ derivative 5 had low
Scheme 1. Synthesis of Iodotrimethylplatinum(IV)
a
Complexes 1−5
Structural Studies of Thiolate and Sulfinate Com-
plexes of the PtMe3(N^N) Core. X-ray crystallography
showed that all products form distorted octahedra whether in
mononuclear or dinuclear form. Table 2 summarizes the bond
lengths of most interest. The Pt−S bonds are all of similar
lengths; however, the Pt−S bonds in sulfinate complexes (12,
17, 26−29) are shorter that those in the other complexes. A
search of the Cambridge Crystallographic Data Centre for
structures containing the Pt-SO2R unit found only 23 reports
of this substructure, of these 20 were cyclic24−27 (i.e.,
contained a chelating O2S^D unit, D = donor atom), meaning
that only three unsupported Pt-SO2 bonds have previously
been characterized, and there were no Pt(IV) sulfinates
reported.
a
1 = Bpy, 2 = di-OMebpy, 3 = phen, 4 = Bphen, 5 = DPPZ.
solubility in all solvents; however, despite the small size of the
crystals obtained, X-ray data of sufficient quality to establish
connectivity were obtained, confirming the identity of the
product.
Attempts to prepare the thiolate complexes derived from
iodoplatinum species 1−5 following literature precedent
(Scheme 2),14 however, led not to the expected products but
Scheme 2. General Reaction of Iodotrimethylplatinum(IV)
1,2-Diimines with Thiophenols
a
Even though the number of crystal structures containing Pt-
SO2R bonds is limited, a few of the reports highlight Pt−S
bond shortening upon oxidation of sulfur.25,26 This is
unexpected as one would expect this bond to get longer as
the sulfur becomes a weaker sigma donor. However, there are
reported to be three factors that determine the M−S bond
length, from analogy with Ni-S complexes. In these too,
shortening of the Ni−S bond was also observed with increasing
sulfur oxidation.28 Although the sulfur becomes a poorer sigma
donor as it is oxidized, the sulfur atom also contracts in size
upon oxidation, and there is less electronic repulsion between
the metal d-orbitals and sulfur due to loss of its lone pairs.
Overall, the latter two effects dominate over the reduced sigma
donation, thus resulting in a shorter Pt−S bond length.
The dinuclear complexes 7, 9, and 10 have slightly longer
Pt−S bonds, as expected from the bridging nature of the
formally monoanionic thiolate. Interestingly, the two Pt−S
bonds in 7 and 9 are of different lengths, as are their respective
Pt−Cax bonds with, in each case, the segment with the shorter
Pt−S bond showing a longer Pt−Cax and vice versa, which is
assigned to the trans influence. Complex 17 (PtMe3BPhen-
SC6H4CO2Me) displays the longest Pt−S bond along with the
shortest Pt−Cax bond, resulting from the electron withdrawing
−CO2Me group reducing both the donor strength and trans
influence of the thiolate.
a
R, X = H, CO2Me, OMe, NO2.
to a series of metastable complexes which were not fully
characterized, but the identity of some were established by
NMR and X-ray crystallography. NMR (see SI) indicated that
an equilibrium existed between mononuclear and dinuclear
products in many cases, e.g., 6 and 7 from the reaction of 5
with 4-mercaptomethylbenzoate (Scheme 3, Figure 1), and
examples of the dinuclear structures were also confirmed by X-
ray crystallography.
In other cases, NMR analysis indicated the presence of only
mononuclear species, and X-ray crystallography showed either
the expected mononuclear thiophenolate complex or a
Mechanistic Studies of Dinuclear Complex Formation
and Oxidation. In addition to the above observations of the
formation of dinuclear complexes and sulfinates, further
C
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Scheme 3. 2 PtMe3dppzSC6H4-4-CO2Me 6 ⇋ [(PtMe3dppz)2SC6H4-4-CO2Me]+ 7
Figure 1. Thermal ellipsoid plots of Me3dppzPtSC6H4-4-CO2Me 6 (LHS) and the cationic fragment of [(Me3dppzPt)2SC6H4-4-CO2Me]PF6 7
(RHS) (hydrogen atoms omitted for clarity).
Scheme 4. Synthesis of Me3bpyPtSPh 11
Figure 3. Thermal ellipsoid plot of Me3PhenPtSO2-4-C6H4-CO2Me
12 (hydrogen atoms omitted for clarity).
information regarding the degradation of these complexes
came from the study of the degradation of PtMe3BpySC6H4-
CO2Me, 22. 22 had previously been reported to be stable;14
however, a solution of 22 in CDCl3 which had been aged for 6
weeks yielded two different types of crystals which X-ray
Figure 2. Thermal ellipsoid plot of Me3bpyPtSPh 11 (hydrogen
atoms omitted for clarity).
diffraction revealed to be chlorotrimethyl-(2,2′-bipyridine)-
platinum(IV), previously uncharacterized by crystallography,
but closely related to the iodo- and bromo-analogues which
Scheme 5. Synthesis of Me3PhenPtSO2-4-C6H4-CO2Me 12
have been,29 and 4-mercaptomethylbenzoate disulfide.15 The
abstraction of chloride from chloroform and displacement of
the thiolate hinted at a more complex chemistry than simple
aerobic oxidation of coordinated thiols to sulfinates and an
equilibrium of mononuclear and dinuclear species which had
been observed above.
In order to further investigate the degradation of 22, it was
studied in CD3CN in order to avoid halide exchange
phenomena, deliberately exposing the sample to air over 72
1
h and following the changes by H NMR. NMR analysis
revealed that at least three products were present which DOSY
D
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a
Table 1. Indications of Product Nature from Crystallographic and NMR Data
N^N
4-R-Ar-SH, R =
product type
number
crystal structure (counterion; origin)
mononuclear
dinuclear (PF6; added)
NMR data
DPPZ
DPPZ
Bpy
Bpy
Bpy
Phen
Phen
Phen
Phen
BPhen
BPhen
BPhen
OMeBpy
OMeBpy
OMeBpy
Bpy
CO2Me
CO2Me
OMe
OMe
OMe
H
CO2Me
H
OMe
H
OMe
CO2Me
H
OMe
CO2Me
NO2
NO2
NO2
NO2
NO2
mono⇋dinuclear
dinuclear
mononuclear
dinuclear
dinuclear
mononuclear
sulfinate
mono⇋dinuclear
mono⇋dinuclear
mononuclear
mono⇋dinuclear
mono⇋dinuclear
mono⇋dinuclear
mono⇋dinuclear
mononuclear
sulfinate
6
7
8
9
equilibrium 6−7
equilibrium 8−9
dinuclear complex (S2O7; from MgSO4)
dinuclear complex (4-MeO-Ar-SO3−; oxidation)
10
11
12
13
14
15
16
17/18
19
20
21
26
27
28
29
30
mononuclear
mononuclear
mono⇋dinuclear
mono⇋dinuclear
mononuclear
mono⇋dinuclear
mono⇋dinuclear
mono⇋dinuclear
mono⇋dinuclear
mononuclear
sulfinate
thiolate
sulfinate
thiolate
thiolate
sulfinate
sulfinate
sulfinate
sulfinate
mononuclear
Phen
sulfinate
sulfinate
sulfinate
mononuclear
sulfinate
sulfinate
sulfinate
mononuclear
BPhen
DPPZ
OMeBpy
a
See SI Scheme S2 for chemical structures of all substances.
Table 2. Pt−S and Pt−Caxial Bond Lengths Obtained from X-ray Crystallography
complex
N^N ligand
S-ligand
Pt−S (Å)
Pt−Cax(Å)
6
7i
7ii
9i
DPPZ
DPPZ
(dinuclear, PF6)
bpy
(dinuclear, S2O4
bpy
(dinuclear, MeO-C6H4-SO3
bpy
phen
bathophen
bathophen
4,4′-diOMe-bpy
Bpy
S-C6H4-CO2Me
S-C6H4-CO2Me
2.479
2.567
2.504
2.482
2.505
2.493
2.483
2.460
2.391
2.392
2.552
2.480
2.404
2.434
2.411
2.456
2.502
2.073
2.043
2.050
2.071
2.062
2.064
2.069
2.079
2.074
2.092
2.042
2.064
2.086
2.083
2.064
2.112
2.082
S-C6H4-OMe
S-C6H4-OMe
2−
9ii
10
10
11
12
15
17
21
26
27
28
29
30
)
2−
)
S-Ph
SO2-C6H4-CO2Me
SO2-Ph
S-C6H4-CO2Me
S-C6H4-CO2Me
SO2-C6H4-NO2
SO2-C6H4-NO2
SO2-C6H4-NO2
SO2-C6H4-NO2
S-C6H4-NO2
Phen
BPhen
DPPZ
4,4′-diOMe-bpy
suggested had significantly different masses. Although molec-
ular mass calculations from diffusion DOSY data are inaccurate
for systems containing atoms heavier than sulfur, and the
calculation has not been optimized for this solvent, the order of
masses was clear from the diffusion coefficients, and
comparison with the mononuclear complex 12 (previously
characterized), and iodide 1 (previously characterized, no
thiolate peaks), allowed the dinuclear complex (new species
showing both bpy and thiolate peaks in a 2:1 ratio) and other
heavier species to be identified in the mixture (Figure 4,
Scheme 6).
exact chemical shift from, but of similar mass to, the
mononuclear complexes, assigned as the sulfinates. Under
anaerobic conditions, the samples were stable even under
irradiation (300 lm white LED light source; see SI for output
profile), and, in the absence of light, aerobic samples were also
stable. Following the photo-degradation of 22 and 24 and by
comparing signals to known materials, and DOSY estimations
of mass, the identity of the species (thiolate, dinuclear
complex, disulfide, sulfonic acid, and sulfinate complex) and
the sequence of generation were determined (see SI for details
of the reasoning behind identification of peaks).
A series of NMR experiments following the degradation of
22 (Scheme 6) and in situ prepared 24 (Scheme 7, Figure 5)
under aerobic and anaerobic atmospheres and in amber and
clear tubes demonstrated that the conversion of mononuclear
complexes to dinuclear complexes and/or sulfinates required
both light and air and revealed a solution species differing in
Although, in some cases (e.g., 21), appreciable levels of
dinuclear complex were only observed under photooxidation
conditions, it should be noted that all these complexes appear
to exist in equilibrium with the dinuclear complexes, with the
equilibrium position dependent upon substituents. In addition
to the observation above of dinuclear complexes in the DPPZ
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sulfonic acids which dissociate. Meanwhile, the platinum
complex reforms the starting iodide or an aqua complex.
Summary of Steps. On the basis of these results, and the
previous demonstration that the photo-excitation of these
complexes involves charge transfer from the coordinated sulfur
to the N^N ligand, a series of likely steps can be proposed
(Scheme 8):
N^N‐PtMe3‐SR F [(N^N‐PtMe3)2‐SR]+[RS]−
(1)
Mononuclear and dinuclear complex in equilibrium.
N^N‐PtMe3‐SR + hv + O2 → (via N^N•−‐PtMe3‐S•+R)
−
→ [N^N‐PtMe3‐SR]+ + O2
(2)
Photoexcitation to the excited state, followed by SET to O2 (as
both hν and O2 are required) with concomitant reduction of
dioxygen to superoxide (other ROS cannot be excluded).
[(N^N‐PtMe3)2‐SR]+[RS]− + [N^N‐PtMe3‐SR]+
→ [(N^N‐PtMe3)2‐SR]+ + N^N‐PtMe3‐SR]
1
2
+
RSSR
(3)
Figure 4. DOSY NMR showing the diffusion of dinuclear complex
(red, DAV = 1.40 × 10−9 m2/s, mass = 959), mononuclear complex
(green, DAV = 1.66 × 10−9 m2/s, mass = 563), and iodide 1 (yellow,
DAV = 1.86 × 10−9 m2/s, mass = 523).
Oxidation of thiolate anion to disulfide by the radical cation
derived from the photoexcited state of the mononuclear
complex.
[(N^N‐PtMe3)2‐SR]+ + O2 (via ROS)
→ N^N‐PtMe3‐SO2R + [N^N‐PtMe3]+
(4)
complex, 6, NMR experiments in degassed solutions in amber
tubes using Bphen/4-MeOC6H4SH revealed that two major
products, mononuclear and dinuclear, were present in a 3:1
ratio, whereas the Bpy/PhSH pair gives only the mononuclear
complex even under aerobic conditions. Taken together, these
findings suggest that the equilibrium position is ligand-
dependent with more electron-rich thiols encouraging the
formation of dinuclear complexes.
Oxidation of the bridging thiolate (stepwise or concerted) to
the sulfinate complex with loss of the uncordinated Pt core. We
have no data concerning intermediates, but this step is only
observed from the dinuclear complex.
N^N‐PtMe3‐SO2R + ROS → [N^N‐PtMe3]+ + [RSO3]−
(5)
In summary, these platinum 1,2-diimine thiolate complexes
appear to exist in equilibrium with thiol-bridged dinuclear
complexes, with the equilibrium position being substituent-
dependent; under irradiation and in the presence of air, the
equilibrium shifts to favor dinuclear complexes as free thiol is
converted to disulfides. The dinuclear complexes are then
further S-oxidized to mononuclear sulfinate complexes, and
upon prolonged irradiation, the sulfinates are oxidized to the
Oxidation of the sulfinate to the sulfonic acid.
The combination of steps 2 and 3 indicates that the complex
photocatalyzes the oxidation of thiols to disulfides, which was
supported by irradiating an NMR tube containg reduced
glutathione and complex 22 in D2O/d3-MeCN which showed
an irradiation-dependent oxidation to the disulfide form.
4-Nitrophenyl Sulfinate Complexes 26−29. The 4-
nitrosulfinate complexes isolated from the reaction of 4-
Scheme 6. Decomposition of 22
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Scheme 7. Preparation and Photolytic Degradation of Me3PhenPtS-4-C6H4-CO2Me 24
Figure 5. NMRs of 24 (CD3CN) over a period of exposure to air and light: (i) initial product; (ii) 30 min after exposure to air; (iii) irradiation for
1 h; (iv) irradiation for 3 h; (v) irradiation for 4 h; (vi) irradiation for 6.5 h; (vii) irradiation for 8.5 h; (viii) irradiation for 9.5 h; (ix) irradiation for
13.5 h in total.
Scheme 8. Suggested Pathway for the Decomposition of [(N^N)PtMe3SR]
nitrothiophenol with complexes 1−4 were more stable than
complexes previously discussed in this report, allowing partial
characterization. Although these complexes are intrinsically
unstable and degrade even in the solid state over several days,
they were at least long-lived enough to allow NMR and
photophysical characterization on clean materials. NMR
analysis confirmed the mononuclear nature of the complexes,
and IR spectra showed stretches tentatively assigned to SO
at higher wavenumber (average ∼ 1220 cm−1) than the Ni(II)
precedents (average ∼ 1190 cm−1), in line with the higher
oxidation state in the Pt(IV) complexes reducing the back-
bonding and strengthening the SO bonds. The reason for
the higher stability of sulfinate complexes derived from 4-
nitrothiophenol compared to other thiols is not entirely clear,
but may be related to Pt-SO back-bonding. However,
complex 30, in which back-bonding to SO should be
maximized by the electron-donating 4,4′-dimethoxy-2,2′-
bipyridine unit, was itself unstable and not able to be
characterized beyond a crystal structure. The Pt−S bond
lengths in the 4-nitrothiophenolate series (2.40−2.45 Å) are
within the range of the two examples of sulfinate complexes
previously discussed (2.39, 2.55 Å) which is compatible with,
although not empirical evidence for, a back-bonding
explanation. However, it should be noted that stability is a
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Figure 6. Absorption spectra of complexes 26−29 in MeCN.
Figure 7. Expansion of absorption spectra of 26−29 between 325 and 530 nm.
Table 3. Excitation and Emission Data of Complexes 26−29 in MeCN
a
complex
1,2-diimine
2,2′-bipyridine
λmax(abs) (nm); ε (mol−1 dm3 cm−1
)
λmax(ex) (nm)
λmax(em) (nm)
ΘF
τ (ns)
b
26
272(107094)
374(8612)
274(71323)
380(17046)
291(57822)
366(8763)
291(57822)
330
415
8.9 × 10−4
2.3 × 10−4
9.7 × 10−4
4.9 × 10−4
1.24
c
27
28
29
1,10-phenanthroline
336
342
335
407
456
425
N/A
d
4,7-diphenyl-1,10-phenanthroline
dipyrido[3,2-a:2′,3′-c]phenazine
10.3
c
N/A
363(16499)
a
b
c
d
Estimated relative to standard;30 see SI for details. Excited at 405 nm, χ2 = 1.024. No excitation wavelength gave adequate counts. Excited at
375 nm, χ2 = 1.104.
kinetic concept and bond length thermodynamic, and without
much greater insight into the mechanism of loss of the sulfinate
ligand, speculation about the bearing of back-bonding on the
activation energy would be misguided.
Photophysical Characteristics of 4-Nitrophenyl Sulfi-
nate Complexes 26−29. With a series of at least reasonably
stable platinum(IV) trimethyl 1,2-diimine thiophenolate
complexes finally available in the form of the oxidized sulfinate
series 26−29, their absorption and emission properties were
studied in solution. Complexes 26−29 all display similar
absorption spectra across a range of concentrations: they have
a sharp, high energy peak between 273 and 286 nm (Figure 6)
assigned to spin-allowed π-π* ligand transitions of the 1,2-
diimines mixed with spin-allowed singlet MLCT (d-π*) as
previously reported for platinum trimethyl 1,2-diimines.13,14 In
addition, there was a broad, lower energy peak of much lower
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Figure 8. Excitation and emission spectra of complexes 26−29 in MeCN.
intensity between 364 and 485 nm (Figure 7) observed in all
the complexes and which is the absorption band related to the
characteristic colors of the complexes. The nature of this
absorption band is discussed below.
system. The lifetimes of 1−10 ns are short in comparison to
triplet emitting transition-metal complexes, and as sensitivity to
3O2 is a common probe for triplet states, lifetime measure-
ments were recorded on degassed samples. There was very
little change to the average lifetimes recorded (<1 ns),
In addition to the electronic absorption spectra, excitation
and emission spectra were recorded of complexes 26−29 and
revealed broad emission bands with maxima between 410 and
500 nm and broad excitation bands with maxima between 280
and 340 nm (see Table 3, Figure 8). They also show very weak
broad shoulders tailing to low energy (>550 nm) which are
only significant in weakly emitting, over-concentrated samples
where they can appear stronger. This distortion is due to inner
filter and absorption effects whereby, in the case of these
complexes, the higher energy ends of the emission bands have
significant overlap with the lower energy (non-emissive)
absorption bands, biasing detection to the lower energy
shoulders in concentrated samples. The luminescence
quantum yields were determined to be between 0.02 and
0.1%, comparable to the previously reported values for
PtMe3(N^N)thiophenolates of 0.04%.14 TCSPC gave data
which fitted well to single exponential decays solving to give
luminescence lifetimes in the 1−10 ns region. The low energy
shoulders are apparently of the same electronic origin as the
main bands as no second lifetime component could be
detected shifting the detection wavelength (although these
data were poor due to weak counts). These excitation and
emission characteristics are closely related to other reports of
emission from platinum(IV) trimethyl 1,2-diimine com-
plexes13,14 but are of interest as they appear to show that the
low energy absorption detected in the UV−vis absorption
spectrum leads to a non-emissive state, and that only excitation
at higher energy populated an emissive state. The excitation
spectra do not match the absorption spectra, with excitation
maxima matching the low energy shoulder of the strongest
absorption peaks centered around 300 nm, and with the low
energy absorption bands in the visible region not reflected at
all in the excitation spectra. It is expected that the lower energy
shoulder of the strongest absorption bands reflects transitions
in the more conjugated N^N ligands with higher energy
contributions from the less conjugated aryl sulfinate unit,
implying that the emissive state is localized on the conjugated
3
indicating that quenching by O2 is not responsible for the
fast decay of the excited states. As a comparison, the lifetime of
Pt(Me3)(bpy)-4-methoxycarbonylthiophenolate 22 which has
previously been reported as a triplet emitter14 was recorded
and found to be 5.6 ns. Finally, Pt(Me3)(bpy)I, again a known
triplet emitter,12 was found to have a lifetime of 2.8 ns,
indicating that these short lifetimes are typical of this core. In
the light of these comparisons, the magnitude of the quantum
yields, and the large Stokes shifts, it seems most likely that
these complexes emit from triplet states but are quenched by
structural features (e.g., the nine C−H bonds close to the
3
location of the excited state) rather than O2.
The orbital origin of the low energy absorbances was
unclear, as the low energy absorbance bands responsible for
visible light excitation of platinum(IV) trimethyl 1,2-diimine
thiolates have been assigned as originating from excitation of
electrons in a mixed Pt−S σ bonding and sulfur lone pair
nonbonding electrons.14 As the sulfur in the sulfinate
complexes has no nonbonding electrons, it seemed unlikely
that the Pt-S σ orbitals could be the sole donor and further
clarification was sought through theoretical work.
Theoretical Studies of Sulfinate Complexes. DFT
calculations using the CAM-B3LYP level,31 with the 6-
31+G(d,p) basis set on light atoms and SDD on Pt32 in
simulated CH3CN33 indicated the HOMO of 26 to be a
mixture of Pt−S and Pt−C(Me‑trans to S) σ-bonding orbitals and
the LUMO of π* character located on the 4-nitrothiophenolate
ligand rather than the bipyridine (Figure 9). The LUMO+1 of
26 is also of π* character but based on the bipyridine ligand,
and approximately 0.47 eV higher in energy than the LUMO.
HOMO-LUMO absorption is predicted to lie at 319 nm (f =
0.058), while HOMO-LUMO+1 is predicted to absorb at 292
nm (f = 0.021). The HOMO-1 orbital is of π character and
based on the bipyridine ligand, giving a HOMO-1 to LUMO
+1 (i.e., π to π*) transition with strong absorption (f = 0.33) at
271 nm.
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dipole-dipole mechanism to allow relaxation of one state to
populate the excited state of the other, and in the absence of
this, it is common to see phenomena such as dual emission. In
the case of complex 26, the TD-DFT transition dipoles
corresponding to absorptions at 319 and 292 nm are almost
perpendicular (θ = 106.6°), possibly indicating an inefficient
dipole-dipole coupling and therefore inefficient relaxation.
While the explanation for the lack of relaxation through the
non-emissive LUMO (sulfinate-based) excited state is
speculative, a novel excitation mechanism involving a σ (Pt-
S/Pt-C) → π* (Ar-NO2) transition has been revealed by DFT
methods. In addition to this unusual excited state, the emissive
1,2-diimine-based state has also been shown to be of an
unusual origin, again involving a HOMO containing Pt−C σ
bond character. To the best of our knowledge, transitions of
these natures have not been previously reported.
In order to verify that the new transition discovered for
complex 26 was a feature of the nitro-substituted thiophenol
unit, analogous calculations were performed for the (un-
known) nitro-free sulfinate complex PtMe3(bpy)SO2C6H5, 30.
The HOMO was again a mixture of Pt−S and Pt−C(Me‑trans to S)
σ-bonding orbitals, but the LUMO in this case was the
bipyridine π* orbital, giving a HOMO→LUMO allowed
singlet transition of similar character and energy to the
HOMO→LUMO+1 transition of 29 (Figure 10). The
HOMO-1 of 30 is of π* character located on both the
bipyridine and the thiolate, giving a HOMO-1→LUMO
transition of π → π* nature. Thus, while the low energy
transition to the sulfinate π* is unique to the nitro substituted
case, it appears that the HOMOs of the sulfinate complexes are
generally of Pt−S and Pt−C(Me‑trans to S) σ-bonding character.
The existence of this relatively high energy HOMO in all
oxidized complexes is significant in light of the observation that
even oxidized complexes continue to show photooxidation
ability; i.e., the loss of the thiolate lone pair does not preclude
photocatalytic ability.
Figure 9. Molecular orbitals of complex 26.
In the light of these findings, we suggest that the lowest
energy absorption observed in the spectra of 26−29 are
assigned to a spin-allowed singlet HOMO→LUMO σ (Pt-S/
Pt-C) → π* (Ar-NO2) transition, leading to a non-emissive
excited state, while the higher energy excitations combine a
HOMO→LUMO+1 σ (Pt-S/Pt-C) → π* (1,2-diimine)
transition, and a much stronger HOMO-1→LUMO+1 π to
π* transition, which leads to the emissive state, analogous to
those observed in related complexes.12,14 It is unusual for a
higher state not to relax into a lower one, but, given that
relaxation into the Ar-NO2-based state must be to the same
spin multiplicity as the N^N state from which it originates, and
given that the excitation to the Ar-NO2-based singlet state is
itself significantly lower energy than that to the N^N state,
then assuming an Ar-NO2-based triplet of even lower energy,
intersystem crossing may be inefficient due to poor vibrational
overlap of the states, in line with energy-gap law. For efficient
relaxation between different sections of a luminophore without
significant vibrational overlap, there is a requirement for a
CONCLUSIONS
■
Although this study initially set out to explore the luminescent
properties of platinum(IV) trimethyl bipyridyl thiolate
complexes and their potential for luminescence applications,
it is now apparent that there is a rich and complicated
chemistry associated with these systems. These complexes
appear to exist in dynamic equilibrium with dinuclear Pt(IV)
S-bridged complexes, the position of the equilibrium being a
Figure 10. Molecular orbitals of 30.
J
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function of the nature of both the 1,2-diimine and thiolate
ligands. The complexes show photochemical reactivity with
visible light excitation with initial oxidation of the thiolate
counterion trapping them in the dinuclear form, then
photooxidation to the sulfinate complexes. There is precedent
for dinuclear complex formation in Pt(Me3)-1,2-diimine-based
systems,23 but there are no previous reports of Pt(IV)
sulfinates. In a recent report, the closely related Re(CO)3-
1,2-diimine thiophenolates are described as undergoing
degradation in solution to materials which have not been
characterized but in which the NMR signals are similar to
those observed in our photochemical oxidations.34 Although a
number of dinuclear and sulfinate complexes have been
characterized by X-ray crystallography, and an outline
mechanism for the transformation suggested by NMR studies,
the sulfinate complexes are generally unstable and the sulfinate
is displaced by solvent, water, or halides. However, sulfinates
derived from 4-nitrothiophenol give complexes which are
sufficiently stable to allow some degree of characterization to
be undertaken. This stability can be rationalized in terms of
back-bonding into the sulfinate π* by the d6 Pt(IV) core. The
sulfinate complexes show unusual photophysical behavior with
excitations from a HOMO which is based in a mixed σ (Pt-S/
Pt-C) bonding orbital to LUMOs of π* symmetry based on
either the N^N ligand, or, in the case of nitro-substituted
species, the sulfinate aromatic ring. The transition to the
sulfinate ring leads to a non-emissive state, while excitation to
the N^N derived π* orbitals gives emission with large Stokes
shifts, and these states do not interconvert.
Charlotte M. A. Farrow − Department of Chemistry,
University of Lancaster, Lancaster LA1 4YB, United
Kingdom
Geoffrey R. Akien − Department of Chemistry, University of
Lancaster, Lancaster LA1 4YB, United Kingdom
Nathan R. Halcovich − Department of Chemistry, University
of Lancaster, Lancaster LA1 4YB, United Kingdom;
orcid.org/0000-0001-6831-9681
Sarah L. Allinson − Department of Biomedical and Life
Sciences, University of Lancaster, Lancaster LA1 4YG, United
Kingdom
James A. Platts − School of Chemistry, Cardiff University,
Cardiff CF10 3AT, United Kingdom; orcid.org/0000-
0002-1008-6595
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03553
Author Contributions
∥These authors contributed equally. The manuscript was
written through contributions of all authors. All authors have
given approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. David Rochester, Department of Chemistry,
Lancaster University, for mass spectrometry and the Royal
Society of Chemistry for a Royal Society of Chemistry
Undergraduate Research Bursary (to C.M.A.F.).
ASSOCIATED CONTENT
■
sı
* Supporting Information
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■
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AUTHOR INFORMATION
■
Corresponding Author
Michael P. Coogan − Department of Chemistry, University of
Lancaster, Lancaster LA1 4YB, United Kingdom;
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lancaster.ac.uk
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Authors
Barbora Mala − Department of Chemistry, University of
Lancaster, Lancaster LA1 4YB, United Kingdom
Laura E. Murtagh − Department of Chemistry, University of
Lancaster, Lancaster LA1 4YB, United Kingdom
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