C-H reductive elimination during the reaction of cycloplatinated(ii) complexes with pyridine-2-thione: Kinetic follow up

Article abstract of DOI:10.1039/c5ra01341k

Substitution reactions of the labile SMe₂ ligand in the cycloplatinated(ii) complexes [PtR(ppy)(SMe₂)], 1, in which ppy = 2-phenylpyridinate and R = Me, 1a, or p-MeC₆H₄, 1b, by pyridine-2-thione, C₅H₅SN, were studied. When each of the complexes 1 was treated with 1 equiv. C₅H₅SN, existing as a mixture of thiol (NSH) and thione (HNS) tautomers, a mixture containing the S-bound thiol complex [PtR(ppy)(η¹-S-NH)] (R = Me, 2a, or R = p-MeC₆H₄, 2b) and the dimeric complex [Pt(ppy)(NS)]₂, 3 (having two bridging deprotonated pyridine-2-thione (NS) ligands), was observed along with free R-H. This mixture finally led to pure complex 3 after 3 days. Pure samples of the complexes 2a and 3 were obtained from the abovementioned 2a+3 mixture by using flash chromatography on silica gel. Kinetics of the reactions were investigated by UV-vis spectroscopy (complexes 1 have a MLCT band in the visible region which was used to easily follow the reactions) and ¹H NMR spectroscopy. On the basis of the results, a mechanism was proposed for the related reactions.

Full text of DOI:10.1039/c5ra01341k

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Graphical and textual abstract for the contents pages
C-H reductive elimination during the reaction of cycloplatinated(II)
complexes with pyridine-2-thione: kinetic follow up
Reactions of the cycloplatinated(II) complexes [PtR(ppy)(SMe2)], 1, where ppy = deprotonated
2-phenylpyridine and R = Me or p-MeC6H4, with pyridine-2-thione, C5H5SN, were studied were
1
studied by UV-vis and H NMR spectroscopies and on the basis of the data a mechanism is
proposed.
S
N
R
N
N
1 equiv C5H5SN
- SMe2
Pt
Pt
RH
+
Pt
N
N
SMe2
S
R = Me or p-MeC6H4
C5H5SN = pyridine-2-thione =
N
H
S
N
SH

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C-H reductive elimination during the reaction of
cycloplatinated(II) complexes with pyridine-2-thione:
kinetic follow up
Marzieh Dadkhah Aseman,a S. Masoud Nabavizadeh,*a Hamid R. Shahsavarib and Mehdi
Rashidi*a
aDepartment of Chemistry, College of Sciences, Shiraz University, Shiraz 71467-13565
Iran, bDepartment of Chemistry, Institute for Advanced Studies in Basic Sciences, 45195-
1159, Gava Zang, Zanjan, Iran
Corresponding Author: S. Masoud Nabavizadeh (nabavi@chem.susc.ac.ir); Mehdi
Rashidi (rashidi@chem.susc.ac.ir); Tel: +98 713 613 7110; Fax: +98 713 646 0788.
1

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Abstract
Substitution reactions of the labile SMe2 ligand in the cycloplatinated(II) complexes
[PtR(ppy)(SMe2)], 1, in which ppy = 2-phenylpyridinate and R = Me, 1a, or p-MeC6H4,
1b, by pyridine-2-thione, C5H5SN, were studied. When each of the complexes 1 was
treated with 1 equiv C5H5SN, existing as a mixture of tautomers thiol (N^SH) and thione
(HN^S), a mixture containing the S-bound thiol complex [PtR(ppy)(η1-S-S^NH)] (R =
Me, 2a, or R = p-MeC6H4, 2b) and the dimeric complex [Pt(ppy)(N^S)]2, 3 (having two
bridging deprotonated pyridine-2-thione (N^S) ligands), was observed along with free R-
H. This mixture was finally led to pure complex 3 after 3 days. Pure samples of the
complexes 2a and 3 were obtained from the abovementioned 2a+3 mixture by using flash
chromatography on silica gel. Kinetics of the reactions were investigated by UV-vis
spectroscopy (complexes 1 have a MLCT band in the visible region which was used to
1
easily follow the reactions) and H NMR spectroscopy. On the basis of the results, a
mechanism was proposed for the related reactions.
1. Introduction
The chemistry of cyclometalated complexes has long been the subject of intense study.1-4
Among the cyclometalated complexes, cycloplatinated complexes have been attracted a
great deal of attention as a result of their highly versatile obvious applications in many
fields. 5-8 Monoalkyl cycloplatinated complexes with a solvent or labile ligand have been
reported only rarely.9-16 These complexes reportedly exhibit various reactivities toward
fundamental reactions such as oxidative addition and substitution reactions.17-22
The rich reactive features of complexes with heterodonor ligands, such as 2-
2

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diphenylphosphinopyridine (pyPPh2)21, 23-26 or pyridine-2-thione, C5H5SN,27-38 have been
widely studied for application in different aspects. The latter is an unsymmetrical
bidentate ligand with a nitrogen donor atom and a sulfur donor atom. Its pKa values30
suggest that the parent molecule exists in solution as a mixture containing the thione-thiol
tautomers (Ia and Ib in Figure 1, respectively). This ligand can be deprotonated27, 31
easily to obtain the thiolate form N^S (C5H4SN), indicated as Ic in Figure 1.37 Although
the thione–thiol tautomeric equilibrium has been shown generally to favor the thione
form, the electronic differentiation associated with the hard nitrogen and soft sulfur
donors in this reagent directs its reactivity and coordination behavior when it binds to
39
metal (Figure 2).37,
The reported monomeric platinum(II) complexes containing
pyridine-2-thione have been shown to be S-bound with pendant pyridyl group,40 such as
trans-[Pt(η1-S-C5H4NS)2(PPh3)2] [Pt(η2-N,S-
C5H4NS)(PPh3)2]PF6, 41 with no related example of the N-bound thiol complex.
or
N^S
chelating,
such
as
N
H
S
N
SH
N
S
Thione =
HN^S
N^SH
Thiol =
Thiolate =
N^S
Ic
Ia
Ib
Thione-Thiol Tautomers
Figure 1
3

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N
H
S
N
S
N
S
N
S
M
M
M
M
M
(d)
(a)
(c)
(b)
Figure 2. Coordination modes for C5H5SN and C5H4SN
In the present study, we describe reaction of the cycloplatinated(II) complexes
[PtR(ppy)(SMe2)], 1, (R = Me; 1a, R=p–MeC6H4, 1b, and ppy = deprotonated 2–
phenylpyridine) with pyridine-2-thione (C5H5SN) showing that the monodentate S-bound
complexes [PtR(ppy)(η1-S-S^NH)], 2, with NH pendant group, are formed along with a
dimmeric complex [Pt(ppy)(N^S)]2, 3, resulting from C-H bond reductive elimination.
We have also followed up the kinetics of these reactions by using UV-vis spectroscopy
1
and H NMR spectroscopy in different solvents and suggested a mechanism for the
related reactions.
2. Experimental Section
1
The H NMR spectra of the complexes were recorded on a Bruker Avance DPX 250
MHz or 400 MHz spectrometer, and TMS (0.00) was used as an external reference. All
the chemical shifts and coupling constants are given in units of ppm and Hz, respectively.
Kinetic studies were carried out by using a Perkin-Elmer Lambda 25 spectrophotometer
with temperature control using an EYELA NCB-3100 constant-temperature bath.
Pyridine-2-thione (C5H5SN) was purchased from commercial sources and the starting
complexes, [PtMe(ppy)(SMe2)], 1a,10 [Pt(p-MeC6H4)(ppy)(SMe2)], 1b,10 and
4

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[Pt(ppy)Cl(DMSO)],42 in which DMSO is dimethylsulphoxide, were prepared as
reported. The NMR labeling for ppy and C5H5SN ligands are shown in Scheme 1.43, 44
4'
3'
5'
6'
R
Pt
3
4
N
S
6
3
NH
6
5
4
5
Scheme 1. The NMR labeling
2.1 Reaction of [PtMe(ppy)(SMe2)], 1a, with C5H5SN
To a solution of [PtMe(ppy)(SMe2)], 1a, (100 mg, 0.23 mmol) in dichloromethane (or
benzene) was added 1 equiv pyridine-2-thione (26.1 mg, 0.23 mmol) at room
temperature. The mixture was stirred for 2h (or 24 h in benzene) and the solvent was then
removed under reduced pressure. The residue was triturated with n-hexane to give a pale
red solid, which was separated and dried under vacuum. The product was identified as a
mixture of [PtMe(ppy)(η1-S-S^NH)], 2a, and [Pt(ppy)(N^S)]2, 3, with the ratio 2a/3 being
1/2. Total yield: 76 mg, 69%. Mixture of the compounds 2a and 3 were separated by flash
chromatography with good yields on silica gel eluted with 100:2 CH2Cl2/MeOH, to
afford pure [PtMe(ppy)(η1-S-S^NH)], 2a, and [Pt(ppy)(N^S)]2, 3, identified as follows:
Identification of the complex 2a
M.P. 254 °C (decomp.), Anal. Calcd for C17H16N2SPt, C, 41.45; H, 3.38; N, 5.89. Found:
1
2
C, 41.04; H, 3.06; N, 5.27; H NMR in CDCl3; δ 1.86 (s, JPtH = 69.9 Hz, Me-Pt, 3H),
5

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3
3´
3
3´ 4´
H
aromatic protons: ppy ligand, 6.80 (m, JPtH = 24.5 Hz, JH
= 7.0 Hz, CH group
3
6
3
6 5
adjacent to coordinated C atom, 1H3´ppy), 7.82 (m, JPtH = 15.8 Hz, JH H = 5.7 Hz, CH
3
6 5
group adjacent to coordinated N atom, 1H6ppy), 7.99 (d, JH H = 5.8 Hz, CH group
adjacent to coordinated N atom, H6spy), 8.72 (broad singlet, H atom directly bond to N
1
atom), 6.90-7.81 (other aromatic protons of ppy and S^N groups). H NMR data for the
complex 2a in C6D6, to be used when monitoring the related reaction in benzene (vide
2
infra): δ 2.36 (s, JPtH = 69.9 Hz, Me-Pt, 3H), aromatic protons: ppy ligand, 7.79 (dd,
3JPtH3´ = 24.5 Hz, 3JH3´ 4´ = 7.4 Hz, 4JH3´ 5´ = 1.1 Hz, CH group adjacent to coordinated C
H
H
3
6
3
6
5
4
6 4
atom, 1H3´ppy), 8.76 (dd, JPtH = 16.2 Hz, JH H = 5.7 Hz, JH H = 1.8 Hz, CH group
adjacent to coordinated N atom, 1H6 ppy).
Identification of the complex 3
1
This was identified by comparison of its H NMR spectrum with that of the complex 3
prepared by direct method as follows:
The complex 3 has been reported to be synthesized by the reaction of (Bu4N)[PtCl2(ppy)]
with C5H5SN and was only characterized in solid state by single crystal X-ray
crystallography 45. In the present work, [Pt(ppy)Cl(DMSO)] (100 mg, 0.22 mmol) was
added to an ethanolic solution of sodium pyridine-2-thiolate, NaC5H4NS (see below),
3
6
under inert atmosphere condition. 1H NMR data in CDCl3, δ 8.09 (d, JPtH = 19.1 Hz,
6
3JH H5 = 5.6 Hz, CH group adjacent to coordinated N atom, 2H6ppy), 7.47 (d, 3JPtH3' = 32.8
3
3' 4'
Hz, JH H = 5.7 Hz, CH group adjacent to coordinated C atom, 2H3'ppy), 7.53 (t, 2H),
3
7.49 (d, 2H), 7.86 (d, 3JPtH6 = 16.2 Hz, 3JH H4 = 7.5 Hz, 2H6N^S), 7.13 (t, 4H), 6.91 (t, 4H),
6.74 (t, 2H), 6.71 (t, 2H), 6.43 (t, 2H). Other notes related to complex 3:
1- Single crystal of dimer 3 was grown in a concentrated benzene solution of the 2a+3
6

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mixture product obtained from mixing of complex 1a and C5H5SN by slow diffusion of
diethylether and its structure was confirmed by single crystal X-ray analysis (see Figure
3).
2- NaC5H4NS (as used in direct synthesis of complex 3, see above) was prepared by
dissolving of sodium (6 mg 0.25 mmol) in 10 mL of absolute ethanol following by
treatment with pyridine-2-thione (25 mg, 0.22 mmol) to immediately give a red solution
that after stirring for 6 h at room temperature a red solid was precipitated which was
1
separated and dried under vacuum. Yield: 73 mg, 74%. This compound gave H NMR
45
data similar to the reported data in DMSO-d6
.
3- The complex 3 is not stable in CDCl3 (or CH2Cl2) and slowly forms
45
1
3
6
[Pt(ppy)(N^S)(Cl)]2 complex , giving H NMR data in CDCl3: δ 9.54 (d, JPtH = 20.8
6
3'
Hz, 3JH H5 = 5.6 Hz, CH group adjacent to coordinated N atom, 2H6ppy), 8.15 (d, 3JPtH
=
3
3' 4'
28.2 Hz, JH H = 6.2 Hz, CH group adjacent to coordinated C atom, 2H3'ppy), 7.56 (t,
2H), 7.37 (t, 2H), 7.29 (d, , 2H), 7.17 (t, 4H), 6.97 (t, 4H), 6.90 (t, 2H), 6.71 (t, 2H), 6.77
(t, 2H). These 1H NMR data are similar to those reported for the complex
45
[Pt(ppy)(N^S)(Cl)]2 in DMSO-d6
.
4- When the reaction of [PtMe(ppy)(SMe2)], 1a, with pyridine-2-thione in benzene was
stirred for 3 days, only dimer 3 as a pure product was formed.
1
3
3´ 4´
H
5- H NMR data for the complex 3 in C6D6: ppy ligand, δ 7.49 (m, JH
= 5.3 Hz,
3´ 5´
4JH
H
3
=1.0 Hz, JPtH3´ = 34.2 Hz, CH group adjacent to coordinated C atom, 1H3´ppy),
3
6
5
4
6
4
3
6
8.09 (m, JH H = 6.0 Hz, JH H = 1.0 Hz, JPtH = 20.4 Hz, CH group adjacent to
5
6
coordinated N atom, 1H6ppy); 7.84 (d, 3JH H = 7.8 Hz, br Pt sat, CH group adjacent to
coordinated N atom, 1H6N^S).
7

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Monitoring reaction of 1a with C5H5SN by 1H NMR spectroscopy
To a small sample (10 mg, 0.023 mmol) of 1a dissolved in C6D6 (0.75 mL) in a sealed
NMR tube, 1 equiv pyridine-2-thione (2.6 mg, 0.023 mmol) was added. NMR spectra of
the solution at 27 °C were recorded several times showing gradual conversion to a
mixture containing the complexes 2a and 3 and free CH4.
2.2 Reaction of [Pt(p-MeC6H4)(ppy)(SMe2)], 1b, with C5H5SN
To a solution of [Pt(p-MeC6H4)(ppy)(SMe2)], 1b, (100 mg, 0.19 mmol) in
dichloromethane (15 mL) was added 1 equiv pyridine-2-thione (22.1 mg, 0.19 mmol) at
room temperature. The mixture was stirred for 5h, the solvent was then removed under
reduced pressure and the residue was triturated with n-hexane to give a pale red solid,
which was separated and dried under vacuum. The product was identified as a mixture of
[Pt(p-MeC6H4)(ppy)(η1-S-S^NH)], 2b, and [Pt(ppy)(N^S)]2, 3, with the ratio 2b/3 being
1/1. Total yield: 68 mg, 65%. Due to low solubility, the compounds formed in this
reaction could not be separated and therefore elemental analysis was not useful. 1H NMR
data in CDCl3, 2b: δ 2.29 (s, Me of p-tolyl, 3H), aromatic protons: p-tolyl ligand, 6.88 (d,
m
H
o
3
o
m
3
3JH
= 7.6 Hz, 2Hm), 7.17 (d, JH H = 7.6 Hz, 2Ho); ppy ligand, 6.41 (d, JH3´ 4´ = 7.5
H
3
´
6 5
Hz, JPtH3´ = 24.6 Hz, CH group adjacent to coordinated C atom, 1H3ppy ), 8.60 (d, 3JH H
3
6
= 6.8 Hz, JPtH = 16.1, CH group adjacent to coordinated N atom, 1H6ppy); η1-S-S^NH
3
3
4
3
6
3
6 5
ligand, 9.07 (d, JH H = 5.6 Hz, 1H3). 3: 8.07 (d, JPtH = 19.2 Hz, JH H = 5.6 Hz, CH
3'
group adjacent to coordinated N atom, 1H6ppy), 7.49 (d, 3JPtH3' = 32.0 Hz, 3JH H4' = 5.8 Hz,
3
CH group adjacent to coordinated C atom 1H3'ppy), 7.89 (d, 3JPtH6 = 16.0 Hz, 3JH H4 = 7.5
Hz, H6N^S), 6.50-7.60 (other aromatic protons of ppy and S^N groups).
2.3. Kinetic Study
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In a typical experiment, a solution of complex 1a in dichloromethane (3 mL, 3 ×10-4 M)
in a cuvette was thermostated at 25 °C and a 0.01 molar solution (90 ꢀL) of pyridine-2-
thione was added using a microsyringe. After rapid stirring, the absorbance at λ= 500 nm
was monitored.
2.4. Theoretical Methods
Geometry optimizations were performed with the program suite Gaussian03 at the
DFT/B3LYP level 46. The effective core potential of Hay and Wadt with a double−ξ
valence basis set (LANL2DZ)47, 48 was chosen to describe Pt and the 6-31G* basis set
was used for other atoms. To evaluate and ensure the optimized structures of the
molecules, frequency calculations were carried out using analytical second derivatives.
3. Results and discussion
3.1. Synthesis and characterization of the complexes
As shown in Scheme 2, reaction of the Pt(II) starting complexes
[PtR(ppy)(SMe2)], 1, in which R = Me (1a) or p-MeC6H4 (1b) and ppy = deprotonated 2-
phenylpyridine, with pyridine-2-thione (C5H5SN) in a 1:1 molar ratio, followed by the
replacement of the labile ligand SMe2 to give a mixture containing the S-bound thione
Pt(II) complex [PtR(ppy)(η1-S-S^NH)], 2, in which R = Me (2a) or p-MeC6H4 (2b), the
dimeric complex [Pt(ppy)(N^S)]2, 3, with two bridging deprotonated pyridine-2-thione
(N^S) ligands, and R-H, i.e. CH4 (in case where R = Me) or MeC6H5 (in case where R =
p-MeC6H4). When reaction of 1 with C5H5SN is allowed to stir for 3 days in benzene,
only 3 along with free R-H are formed.
9

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R
S
N
N
N
2h in CH2Cl2
or 24 h in benzne
Pt
+
+
Pt
RH
Pt
S
N
N
S
NH
2a; R = Me
2b; R = p-MeC6H4
R
1 equiv C5H5SN
- SMe2
3
Pt
N
SMe2
1a; R = Me
1b; R = p-MeC6H4
S
N
N
N
3 days in benzene
+
Pt
RH
Pt
N
S
C5H5SN = pyridine-2-thione =
N
H
S
N
SH
3
Scheme 2. Reactions studied in this work.
The products were characterized by 1H NMR and X-ray crystallography (for the complex
1
3) and full data are collected in the Experimental Section. In the H NMR spectrum of
complex [PtMe(ppy)(η1-S-S^NH)], 2a, in C6D6 at room temperature, the methylplatinum
2
resonance occurred at δ = 2.36 as a singlet which coupled to 195Pt with JPtH = 69.9 Hz
(the value in the range expected for a methylplatinum(II) complex with methyl being
trans to nitrogen 49-52). The CH group adjacent to ligating C atom of the ppy ligand, i.e.
H3´, appeared as a doublet of doublets at δ = 7.79 with 3JH3´ 4´ = 7.4 Hz and 4JH3´ 5´ = 1.1
H
H
3´
Hz, and with 3JPtH = 24.5 Hz, while the hydrogen related to the CH group adjacent to
ligating N atom of the ppy ligand, i.e. H6, as expected was appeared further down field as
6
6
6
a doublet of doublets at δ = 8.76 with 3JH H5 = 5.7 Hz and 4JH H4 = 1.8 Hz, and with 3JPtH
= 16.2 Hz. The H3 proton of the η1-S-S^NH ligand appeared as a doublet at δ = 7.92 with
3
4
3JH H = 5.5 Hz, with no observable coupling to Pt center probably due to the related
pyridine plane being perpendicular to square geometry of the complex 18. Notice also
that formation of CH4 is confirmed by the observation of a singlet at 0.16. The complex
10

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[Pt(ppy)(N^S)]2, 3, is also characterized with signals due to H3´ppy, H6N^S and H6ppy at 7.49
(3JPtH3´ = 34.2 Hz), 7.84 (br Pt sat) and 8.09 (3JPtH6 = 20.4 Hz) ppm, respectively.
1
In the H NMR spectrum of complex [Pt(p-MeC6H4)(ppy)(η1-S-S^NH)], 2b, in CDCl3 at
room temperature, a singlet signal was observed at δ = 2.29 for the methyl group on the
para-tolyl ligand. The meta and ortho protons of the para-tolyl ligand appeared as two
m
H
doublets at δ = 6.88 and 7.17, respectively, each with 3JH o = 7.6 Hz. A doublet signal at
δ = 6.41 with 3JH3´ 4´ = 7.5 Hz, accompanied by platinum satellites (3JPtH3´ = 24.6 Hz), is
H
attributed to hydrogen atom of C-H group adjacent to the coordinated C atom of ppy
ligand, i.e. H3´. A doublet signal at δ = 8.60 (with 3JH H5 = 6.8 Hz and 3JPtH6 = 16.1 Hz) is
6
assigned to CH group adjacent to coordinated N atom of ppy ligand, i.e. H6. The H3
3
3 4
proton of the η1-S-S^NH group appeared as a doublet at δ = 9.07 with JH H = 5.6 Hz.
1
The dimer complex, i.e. [Pt(ppy)(N^S)]2, 3, with similar H NMR data as mentioned
above (see also Experimental section), was also detected. Besides, formation of R-H (i.e.
MeC6H5) is confirmed by the observation of a singlet at 2.31.
The dimeric complex [Pt(ppy)(N^S)]2, 3, was also prepared by direct route by reaction of
42
the complex [Pt(ppy)Cl(DMSO)]
with an ethanolic solution of sodium pyridine-2-
thiolate (NaC5H4NS) under inert atmosphere condition and a purified sample of it (as
checked by microanalysis) was characterized by 1H NMR spectroscopy. In the 1H NMR
spectrum of complex 3 in CDCl3, the CH group adjacent to coordinated N atom of ppy
6
5
3
6
appeared at δ = 8.09 as a doublet (3JH H = 5.6 Hz) which coupled to 195Pt with JPtH
=
19.1 Hz. The resonance of CH group adjacent to coordinated C atom of ppy is occurred at
3'
3
3' 4'
δ = 7.47 as a doublet with Pt satellite (3JPtH = 32.8 Hz, JH H = 5.7 Hz) and the CH
group adjacent to coordinated N atom of N^S chelate was observed at δ = 7.86 as a
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3
4
3
6
doublet signal (3JH H = 7.5 Hz, JPtH = 16.2 Hz), which confirmed the coordination of
N^S as chelate. The complex 3 was also characterized by single-crystal X-ray diffraction
analysis as illustrated in Figure 3. 45
Figure 3. Crystal structure of the complex [Pt(ppy)(N^S)]2, 3, from crystallization of the
product obtained from mixing of complex 1a and C5H5SN. Selected bond distances (Å)
and angles (°): Pt2-C22 1.977(11), Pt2-N2 2.064(9), Pt2-N3 2.144(10), Pt2-S2 2.286(3),
Pt2-Pt1 2.8513(11), Pt1-C11 1.982(14), Pt1-N1 2.006(11), Pt1-N4 2.170(9), Pt1-S1
2.287(4), S2-C27 1.758(12), S1-C32 1.715(12), C22-Pt2-N2 80.7(4), C22-Pt2-N3
174.2(4), N2-Pt2-N3 93.4(4), C22-Pt2-S2 95.3(3), N2-Pt2-S2 172.8(3), N3-Pt2-S2
90.5(2), C22-Pt2-Pt1 95.6(3), N2-Pt2-Pt1 99.7(3), N3-Pt2-Pt1 85.4(3), S2-Pt2-Pt1
86.57(9), C11-Pt1-N1 81.1(6), C11-Pt1-N4 173.9(5), N1-Pt1-N4 93.6(5), C11-Pt1-S1
95.4(5), N1-Pt1-S1 172.7(4), N4-Pt1-S1 89.6(3), C11-Pt1-Pt2 98.9(4), N1-Pt1-Pt2
100.9(3), N4-Pt1-Pt2 84.9(3), S1-Pt1-Pt2 85.89(9).
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3.2. Kinetics and mechanism of the reactions
1
On the basis of the data obtained from UV-vis and H NMR spectroscopic studies (see
below), a mechanism (being depicted in Scheme 3) for reaction of the complex
[PtR(ppy)(SMe2)], 1, with 1 equiv pyridine-2-thione (C5H5SN) is proposed. The reaction
is suggested to involve initial displacement of the SMe2 ligand by C5H5SN, during which
S donor site of the thione tautomer (HN^S) attacks Pt center of the complex 1 to displace
the SMe2 ligand, giving either the product 2a (or 2b) or its isomeric form, 2a' (or 2b'), in
which the py ring has twisted by 90º as compared to that in 2a (or 2b); in the latter case,
N-H group has the opportunity to interact with the Pt center to form Pt···H-N hydrogen
1
bonding as confirmed by H NMR studies (vide infra). The complex 2a' (or 2b') then
performs an intramolecular N-H oxidative addition to form the intermediate I (or I')
following by CH3-H (or MeC6H5) reductive elimination to give the complex 3.
R
S
N
N
Pt
Pt
Pt
S
N
N
N
S
NH
3
2a; R = Me
2b; R = p-MeC6H4
R
1 equiv C5H5SN
- SMe2
Pt
R-H Red. Elm.
N
SMe2
H
1a; R = Me
1b; R = p-MeC6H4
Intramolecular
N-H Ox. Add.
R
R
Pt
Pt
N
S
S
N
N
H
N
C5H5SN =
N
H
S
N
SH
Intermediate I; R = Me
Intermediate I' ;R = p-MeC6H4
2a'; R = Me
2b'; R = p-MeC6H4
Scheme 3. Suggested mechanism for reaction of the complex [PtR(ppy)(SMe2)], 1, with
pyridine-2-thione.
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3.2.1 Kinetic studies by UV-vis spectroscopy
Reaction kinetics, using UV-vis spectroscopy, of equimolar amounts of the complex
[Pt(p-MeC6H4)(ppy)(SMe2)], 1b, and pyridine-2-thione in CH2Cl2 solution to form a
mixture containing the complexes [Pt(p-MeC6H4)(ppy)(η1-S-S^NH)], 2b, and
[Pt(ppy)(N^S)]2, 3, along with the free MeC6H5, is typically presented here. A series of
spectra recorded during the reaction is shown in Figure 4. On mixing, the characteristic
peaks of complex 1b (MLCT band at 365 nm) and pyridine-2-thione (n-π* at 375 nm) 53,
54
disappeared and a new broad absorption band due the intermediate 2b' appeared (see
Scheme 3). The decay of 2b' to finally form 3 could be studied by monitoring formation
of absorption band of the final product 3 at 500 nm. Typical plots of absorbance at λ =
500 nm versus time are shown in Figure 5.
1.5
1
0.5
0
(a)
(c)
(b)
350
400
450
500
550
600
λ (nm)
Figure 4. The changes in the UV-vis spectrum during the reaction of [Pt(p-
MeC6H4)(ppy)(SMe2)], 1b, with pyridine-2-thione (each 3 × 10-4 M) in CH2Cl2 at 25 °C:
(a) pure 1b; (b) pure pyridine-2-thione; (c) spectrum at t = 0; successive spectra recorded
at intervals of 1 min.
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The Abs-time data at λ = 500 (i.e. formation of the product 3) is fitted to the equation At =
A + (A0 - A) exp (-kt), and the calculated rate constants at different temperatures are given
in Table 1. The activation parameters were also obtained from measurement at different
temperatures using Eyring equation (Figure 6). Rates of the reactions are sensitive to the
nature of R ligand and it is lower when the tolyl complex 1b is involved as compared to
the case where the methyl complex 1a is used. The intermediate 2b' then performs an
intramolecular N-H oxidative addition reaction to form intermediate I' (see Scheme 3)
following by C-H reductive elimination to form toluene and the dimer complex 3.
0.2
0.15
0.1
0.05
0
0
50
100
150
200
250
time / min
Figure 5. Absorbance (at 500 nm)-time curves for the reaction of [Pt(p-
MeC6H4)(ppy)(SMe2)], 1b, with pyridine-2-thione, using 1:1 stoichiometry, in CH2Cl2 at
temperatures of 15, 20, 25, 30 and 35 °C (temperature increases reading upward).
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-9
-10
-11
-12
-13
-14
-15
-16
(a)
(b)
0.0033
0.00341 0.00352 0.00363
T-1 / K-1
Figure 6. Eyring plots for reductive elimination of R-H and formation of dimer 3 from
(a) complex 2a' (R=Me) and (b) 2b' (R= p-MeC6H4) in CH2Cl2.
Table 1. Rate constantsa and activation parameters for R-H reductive elimination from
the complexes 2a' or 2b' to give dimer 3 in CH2Cl2 solution.
102k/s-1 at different temperatures
ꢁH‡ /
ꢁS‡ /
R
kJ mol-1 JK-1mol-1
5 °C 10 °C 15 °C 20 °C 25 °C 30 °C 35 °C
0.16 0.28 0.53 1.16
1.05b 1.14
Me
53.4 3.9 -105 13
0.012
p-MeC6H4
0.019 0.027 0.039 0.052 50.8 1.9 -142
6
a Estimated errors in k values are 5%. b The value in benzene 102k = 0.32 s-1.
As is shown in Table 1, ꢁH‡ value for methane reductive elimination from the
intermediate I is 53.4 kJ mol-1 being considerably lower than the values of 107.4 and 70
kJ mol-1 reported for CH4 reductive elimination from Pt(IV) complexes [PtMe3(H)(dppe)]
(dppe= 1,2-bis(diphenylphosphino)ethane) 55 and [PtMe(H)(Cl)2(PMe3)2] 56, respectively.
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This lower energy barrier found for [PtMe(H)(ppy)(S^N)] may be related to higher ring
strain, presented in four-membered S^N ring in Pt(IV) intermediate I.
3.2.2. Monitoring the reactions by 1H NMR spectroscopy
As can be seen in Figure 7, immediately after addition of pyridine-2-thione to the
complex [PtMe(ppy)(SMe2)], 1a, at 27 ºC in C6D6, the complexes [PtMe(ppy)(η1-S-
S^NH)], 2a and 2a', were started to appear. The rate of disappearance of complex 1a
1
using the H NMR signal at δ = 1.55 (due to Me group connected directly to Pt in
complex 1a) was found to be 1.90 (0.10)×103 s-1. According to Figure 7, for the complex
2a', a singlet signals for Pt-Me protons was observed at δ 2.02 (with 2JPtH = 68.2 Hz) with
the liberated SMe2 appearing at δ 1.72, in comparison with the coordinated SMe2 ligand
of 1a (at δ 1.85 with 3JPtH = 25.1 Hz). Besides, the presence of a doublet signal, appearing
at δ = 10.52 with accompanying 195Pt satellite signals of 3JPtH = 23.3 Hz (assigned for the
H6 of the pyridine-2-thione ring ) confirms that the pyridine-2-thione ligand is connected
to the Pt center via S atom (which is normally preferred to N atom 41). The observation of
this broad signal at a significantly lower field, in comparison with that for corresponding
hydrogen of the free ligand (in thione form, Ia in Figure 1, appearing at δ = 7.64 39), and
3
with a significant JPtH value of 23.3 Hz complies with the formation of Pt···H-N
hydrogen bonding; this coupling value is smaller than those usually observed in similar
hydrogen bondings which could be attributed to the η1-S-S^NH ligand being rather rigid
and thus there is some force on the corresponding hydrogen to locate itself above the
platinum coordination plane in order to interact with its 5dz2 orbital.57
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The ratio of 2a' to 1a, immediately after addition of pyridine-2-thione to 1a, is close to
1:2. As the time was passing on, the signals due to the starting material 1a was
disappearing while those due to the complex 2a' were growing and later started to fade
away and meanwhile comparatively weak signals due to the complexes 2a and 3 (being
out of the range of Figure 7) were also observed in the first stages. As the reaction was
progressed, the complex 2a' was disappearing while the final complexes 2a and 3 were
forming. As mentioned before, after 3 days the dimer 3 was solely obtained in pure form.
Appearance of the corresponding signal for 2a at δ = 2.36 was used to measure rate of
formation of the complex 2a, which was found to be 1.51(0.05) × 10-4 s-1. The rate of
formation of CH4 (at δ = 0.16) which is equal to disappearance of the intermediate I and
formation of the dimer 3 is measured to be 3.0 × 10-3 s-1, very close to value of 3.2 × 10-3
s-1 obtained from UV-vis study (vide supra).
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1
Figure 7. H NMR spectra (Me region) of reaction of the complex 1a with pyridine-2-
thione at 27 °C in C6D6; (a) pure 1a, (b) immediately after addition of pyridine-2-thione
to 1a, (c) 5 min after addition, (d) 15 min after addition, (e) 1h after addition, (f) 3h after
addition, (g) 5h after addition. Signals with satellites are assigned to the complexes 1a,
2a' and 2a. The peak labeled * is due to water of C6D6 solvent.
We suggest that the formation of the complex 3 proceeds through an intramolecular N-H
oxidative addition, involving the η1-S-S^NH ligand in 2a', to the platinum center 58, 59 to
form the platinum(IV) hydride intermediate [PtMe(H)(ppy)(S^N)], I, followed by
reductive elimination of methane to form the complex 3. Our attempts to observe the
intermediate I (by following the reaction at low temperatures) were not successful.
However, evidence in favor of formation of this hypothesized Pt(IV) complex is the
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methane production, appearing as a singlet signal at δ = 0.16 60-62. Formation of the
dimeric complex [Pt(ppy)(N^S)]2, 3, was confirmed by the observation of a doublet
3
6 5
signal at δ = 7.84 with JH H = 7.6 Hz, due to H6 of the N^S bridge, that experiences
coupling with Pt with the satellites being broad and so the related coupling value was not
measurable. Formation of the dimer 3 was also confirmed by X-ray crystal determination
of suitable crystal of 3 obtained from crystallization of the mixture. Formation of the
product complex [PtMe(ppy)(η1-S-S^NH)], 2a, in which pyridine-2-thione adopts the
thione form 63-66, suggests a pathway including direct substitution of labile SMe2 by
sulfur of thiolate ligand. Same experiment for the reaction of 1a with pyridine-2-thione
was also carried out in CD2Cl2 as NMR solvent and found that the reaction in
dichloromethane is faster than in benzene (in agreement with data obtained from the UV-
vis spectroscopy, see Table 1).
3.3 Geometry optimizations
To get some insights in structures of the synthesized complexes 2 and 3, calculations in
the DFT method were carried out and their geometries were optimized using the B3LYP
function. Optimized geometric parameters of the complexes 2 and 3 are given in Table 2
and the DFT-optimized structures for these complexes are shown in Figure 8. The
computed structural details are in good agreement with corresponding experimental
parameters obtained for the related complexes. Mean error for the bond lengths is 0.026
Å for the complex 3. One reason for the small bond length discrepancy may be due to
differences in the geometrical parameters calculated in the gas phase with those obtained
experimentally in the solid state.
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Figure 8. DFT optimized structures of complexes 2 and 3. The H atoms are omitted for
clarity.
The complex 2a contains a square planar platinum(II) center being connected to the Me
group, ortho C and N atoms of the ppy ligand and S atom of the S^NH. The two carbon
ligating atoms are mutually cis in the square-planar structure, as expected, and largest
deviation from ideal geometry is the angle N(ppy)-Pt-C(ppy) = 79.7, which is associated
with the Pt(ppy) chelate ring. N atom of the S^NH ligand is not coordinated and is
positioned opposite to the platinum center. Similar to 2a, the complex 2b has also a
square-planar stereochemistry at platinum and the p-tolyl ligand lies roughly orthogonal
to the square plane and to the ppy ligand. The Pt-S (S^NH) distances are 2.514 and 2.521
Å in 2a and 2b, respectively. The dimeric complex 3 has a square planar stereochemistry
at each Pt center and has a head-to-tail configuration with PtSNPt coordination, in which
S atom of one of the SN ligands is in a cis arrangement with N atom of the other SN
ligand (Figure 8); the coordination geometry around each platinum center is completed
by nitrogen and ortho C atoms of the ppy ligand. This complex with a head-to-tail
arrangement of the two SN ligands has been formulated as containing a Pt–Pt distance of
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3.009 Å which is close to the experimental value of 2.8513(11) Å. This separation is
amongst the shortest Pt-Pt interactions observed in binuclear platinum complexes bridged
with two ligands.45 The calculated bond distance Pt–N(N^S) = 2.233 Å is slightly longer
than Pt–N(ppy) = 2.093 Å due to a higher trans influence of the coordinating C atom of
ppy as compared to coordinating S donor atom of N^S. These values are comparable with
experimental values of 2.144(10) and 2.064(9) Å for Pt2−N3(N^S) and Pt2-N2(ppy),
respectively. The two nitrogen atoms are cis disposed to one another (N(ppy)PtN(N^S) =
94.9°), indicating that angles around the Pt center are rather close to the ideal angle of
90°. The experimental values of this bond angle is equal to 93.4(4) °.
Table 2. Selected calculated bond distances (Å) and angles (°) for complexes 2 and 3.
3 (exp)
2a
2b
3
2.514
2.064
2.193
2.012
91.4
2.521
2.024
2.190
2.165
173.4
94.9
2.233
2.370
2.009
2.093
3.009
95.0
2.144(10)
2.286(3)
1.977(11)
2.064(9)
2.8513(11)
95.3(3)
Pt−S(S^NH)
Pt−S(S^NH)
Pt−C(Ar)
Pt2−N3(N^S)
Pt2-S2(N^S)
Pt−C(Me)
Pt2-C22(ppy)
Pt2-N2(ppy)
Pt−N(ppy)
Pt−N(ppy)
Pt−C(ppy)
Pt−C(ppy)
C(Me)−Pt−S(S^NH)
S(S^NH)−Pt−N(ppy)
N(ppy)−Pt−C(ppy)
C(ppy)−Pt−C(Me)
C(Ar)−Pt−S(S^NH)
S(S^NH)−Pt−N(ppy)
N(ppy)−Pt−C(ppy)
C(ppy)−Pt−C(Ar)
Pt1−Pt2
95.0
C22(ppy)−Pt2−S2(N^S)
S2(N^S)−Pt2−N3(N^S)
N2(ppy)−Pt2−N3(N^S)
79.7
79.6
89.1
90.5(2)
93.9
95.4
94.9
93.4(4)
3.4. Energy profile for the product formation
To gain more insights into the species involved in the reaction of complexes 1 with
C5H5SN (see Scheme 3), DFT calculations were carried out to show structures and
energies for the related complexes in CH2Cl2 solution (Figures 9 and 10).
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As shown in Figure 9, replacement of labile ligand SMe2 in the cycloplatinated(II)
complex 1a by pyridine-2-thione (C5H5SN) first gives the complex [PtMe(ppy)(η1-S-
S^NH)], 2a, having a NH pendant group, with 33.1 kJmol–1 lower in energy with respect
to that for 1a. This step is facilitated by strong trans–influence of the metalated C atom of
ppy ligand which helps in weakening the Pt–SMe2 bond. The complex 2a is obtained in
equilibrium with its isomeric form, 2a', in which the pyridine ring has twisted by 90º as
1
compared to that in 2a to form Pt···H-N hydrogen bonding (as confirmed by H NMR
studies). The complex 2a' is more stable than 2a by 14.8 kJmol–1. In the next step, the
complex 2a' attends in an intramolecular N–H oxidative addition reaction to give the
organoplatinum(IV) hydride complex [PtHMe(ppy)(N^S)], I, followed by reductive
elimination of methane to give the dimer 3. The final product 3 is 139.0 kJmol–1 more
stable than 1a complying with the experimental finding.
Figure 9. Calculated structures and relative energies of species involved in reaction of
the complex 1a with pyridine-2-thione.
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In the related reaction profile (see Figure 10), the complex 1b reacts with pyridine-2-
thione to form the S-bonded complexes 2b and 2b', which are more stable than 1b by
29.9 and 43.3 kJmol–1, respectively. Coordination of pyridine-2-thione entering ligand
through the N atom is not proposed, because the corresponding isomer [Pt(p-
MeC6H4)(ppy)(η1-N-N^SH)] was calculated to be 42.4 kJmol–1 less stable than 2b'.
Oxidative addition of N–H bond to Pt center of the complex [Pt(p-MeC6H4)(ppy)(η1-S-
S^NH)], 2b′, gives the Pt(IV) intermediate complex [PtH(p-MeC6H4)(ppy)(N^S)], I′,
with 12.9 kJmol–1 higher in energy than 1b. At the final step, reductive elimination of
toluene gives dimer 3 which is more stable than 1b by 126.2 kJmol–1.
Figure 10. Calculated structures and relative energies of species involved in reaction of
the complex 1b with pyridine-2-thione.
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Conclusion
Reaction of the cycloplatinated(II) complexes [PtR(ppy)(SMe2)], 1, with pyridine-2-
thione is suggested to proceed via displacement of the labile SMe2 ligand. In this reaction
pyridine-2-thione, which exists as a mixture of tautomers thiol (N^SH) and thione
(HN^S), attacks Pt center of the complex 1 from S side of the thione form to give the four
coordinated square-planar complex 2a (or 2b) and its isomer 2a' (or 2b'), with a general
formula [PtR(ppy)(η1-S-S^NH)]. DFT calculations show that this behavior is much
preferred to the case of attacking from N side of the thiol form. The following points are
considered for the reaction:
1- Suggestion in the first step is consistent with the “soft” S atom (from the thione
tautomer, HN^S) being preferred to be connected to “soft” Pt(II) center as compared to
that for the “hard” nature of N atom (from the thiol tautomer, N^SH).
2- Formation of intramolecular Pt···H-N hydrogen bonding in one of the product isomers
of the first step, i.e. the complex [PtR(ppy)(η1-S-S^NH)], 2a', (as detected by monitoring
the reaction using 1H NMR spectroscopy) is suggested to provide an extra stabilization as
compared with the product isomer 2a.
3- As was determined by UV-vis spectroscopy (see Table 1), R-H reductive elimination
from the complex 2a' or 2b' to form the dimer complex 3 is significantly faster when R =
Me (with k = 1.05×10-2 s-1 at 25°C, in CH2Cl2) as compared to that when R = p-MeC6H4
(with k = 0.027×10-2 s-1 at 25°C, in CH2Cl2). We attributed this to the easier reductive
elimination of CH4 as compare with toluene.
4- The rate of reaction of [PtMe(ppy)(SMe2)], 1a, with pyridine-2-thione is significantly
faster in CH2Cl2 (k = 1.05×10-2 s-1 at 25°C) than in benzene (k = 0.32×10-2 s-1 at 25°C).
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We attribute this to the possibility that CH2Cl2 solvent molecules make the species more
stable, by solvation, than when solvent molecules are the less polar benzene.
We finally observed the formation of two complexes, the S bounded complex
[PtR(ppy)(η1-S-S^NH)], 2, and the dimeric complex [Pt(ppy)(N^S)]2, 3, along with the
evolution of free R-H. These observations led us to believe that an intramolecular N-H
oxidative addition reaction must have been taken place in the complex [PtR(ppy)(η1-S-
S^NH)], 2a' (or 2b'), to give the Pt(IV) intermediate complex [PtHR(ppy)(N^S)], I (or
I'), in which the reductive elimination of R-H bond is needed to produce complex 3.
Acknowledgements
We thank the Iran National Science Foundation (Grant No. 92028194), the Shiraz
University Research Council and Institute for Advanced Studies in Basic Sciences for
financial support.
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