Carbon-sulfur bond reductive coupling from a platinum(II) thiolate complex

Article abstract of DOI:10.1039/c6ra21756g

The room temperature addition of electrophilic alkyl halide reagents (RX = MeI, EtI and PhCH₂Br) to complex [Pt(ppy)(η¹-S-Spy)(PPh₃)], 1a, in which ppyH = 2-phenylpyridine and pySH = pyridine-2-thiol, resulted in a rapid carbon-sulfur (C-S) bond reductive coupling to produce alkyl sulfides and corresponding halide complexes [Pt(ppy)(PPh₃)X], X = I (2a) and Br (2b). A mechanism for this C-S bond formation reaction was suggested based on ¹H and ³¹P {¹H} NMR spectroscopic analyses. In the suggested mechanism, the reaction proceeded through a binuclear intermediate complex [{Pt(ppy)(PPh₃)}₂(μ₂-Spy)]I, 3-I, which was separately synthesized by another counter anion (PF₆) and it was fully characterized by multinuclear NMR spectroscopy and single X-ray crystallography. Also, density functional theory (DFT) calculations were used to theoretically assess the structures of intermediates and transition states in this bond formation reaction.

Full text of DOI:10.1039/c6ra21756g

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DOI: 10.1039/C6RA21756G
CarbonSulfur Bond Reductive Coupling from a Platinum(II) Thiolate
Complex
Maryam Niazi,a Hamid R. Shahsavari,a* Mohsen Golbon Haghighi,b Mohammad Reza
Halvagar,c Samaneh Hatamia and Behrouz Notashb
aDepartment of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Yousef
Sobouti Blvd., Zanjan 45137-6731, Iran.
bDepartment of Chemistry, Shahid Beheshti University, Evin, Tehran 19839-69411, Iran
cChemistry & Chemical Engineering Research Center of Iran, Tehran,14968-13151, Iran
Email: shahsavari@iasbs.ac.ir
Abstract
The room temperature addition of electrophilic alkyl halide reagents (RX = MeI, EtI and
PhCH2Br) to complex [Pt(ppy)(η1-S-Spy)(PPh3)], 1a, in which ppyH = 2-phenylpyridine and
pySH = pyridine-2-thiol, resulted in a rapid CarbonSulfur (CS) bond reductive coupling to
produce alkyl sulfides and corresponding halide complexes [Pt(ppy)(PPh3)X], X = I (2a) and Br
1
31
(2b). A mechanism for this CS bond formation reaction was suggested based on H and P
{1H} NMR spectroscopic analyses. In the suggested mechanism, the reaction proceeded through
a binuclear intermediate complex [{Pt(ppy)(PPh3)}2(µ2-Spy)]I, 3-I, which was separately
synthesized by another counter anion (PF6) and it was fully characterized by multinuclear NMR
spectroscopy and single X-ray crystallography. Also, density functional theory (DFT)
calculations were used to theoretically assess the structures of intermediates and transition states
in this bond formation reaction.
Introduction
Several prominent catalytic or stoichiometric systems were developed for C–H,1, 2 C–
C2-8 and C–X8-17 (X = different heteroatoms) bond formation reactions. Despite these extensive
research efforts, C–S bond formation still has significant limitations identical to poisoning
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and/or deactivation effect of sulfur on reactive species or harsh reaction conditions.14-16 Such
difficulties were addressed in some cases and numerous outstanding systems were established
for C–S bond coupling16 using transition metals such as Ni,18 Cu,17 Pd8, 16 and Pt.19 In addition,
the growing interest in C–S bond construction chemistry is mostly due to its potential
applications in organic synthesis,20 materials science,21 agricultural,15 and pharmaceutical
industry.22
There are only a few reports on the formation of C–S bond using platinum transition
metal complexes.19 This fact has led us to apply complex [Pt(ppy)(η1-S-Spy)(PPh3)], 1a, ppyH
= 2-phenylpyridine and pySH = pyridine-2-thiol, in C–S bond reductive coupling with
electrophilic reagents such as alkyl halides at room temperature. In addition, mechanistic aspect
of this bond formation reaction was studied with multinuclear NMR spectroscopy by observing
an interesting binuclear complex. Density functional theory (DFT) was employed to optimize
structures of the related complexes, which were observed experimentally or proposed
theoretically.
Result and Discussion
Complex [Pt(ppy)(η1-S-Spy)(PPh3)], 1a, was prepared according to our recently
published procedure (Scheme 1 (path I)), 23 involving treatment of complex [Pt(ppy)(PPh3)Cl],
A,24 with an ethanolic solution of sodium pyridine-2-thiolate (NaC5H4NS) under Ar atmosphere.
The reaction of complex 1a with stoichiometric ratio or an excess of MeI at room
temperature was investigated. This reaction was extremely rapid and yielded unexpected
products. As represented in Scheme 1 (path II), the reaction underwent fast CS bond reductive
coupling to cleanly produce an organic compound 2-(methylthio)pyridine (pySMe), O1,25 and
complex [Pt(ppy)(PPh3)I], 2a.
2

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L
L
(III)
Pt
(I)
+
KX
Pt
acetone/CH2Cl2
SH
X
N
Cl
N
E
X = Br; I
E = N; CH Na/EtOH
2a: X = I; L = PPh3
2b: X =Br; L = PPh3
2c: X = I; L = PPhMe2
A: L = PPh3
B: L = PPhMe2
L
L
(II)
acetone
E
RX or X2
S
Pt
+
+
Pt
E
R
S
X
N
N
RX = MeI; EtI; PhCH2Br
X2 = Br2; I2
O1: R = Me;
O2: R = Et;
E = N
E = N
O3: R = PhCH2; E = N
1a: E = N; L = PPh3
1b: E = CH; L = PPh3
1c: E = N; L = PPhMe2
2a: X = I; L = PPh3
2b: X =Br; L = PPh3
2c: X = I; L = PPhMe2
O4: R = Me;
O5: R = Br;
O6: R = I;
E = CH
E = N
E = N
Scheme 1. (I) the synthetic route for the formation of complexes 1; (II) identified products in
the reaction of different alkyl halides with complexes 1; (III) direct route for the formation of
complexes 2 by halide substitution. All reactions were carried out under inert atmosphere at
room temperature.
Complex 2a was identified from the analysis of NMR spectroscopy data and X-ray
31
single crystal diffraction data (see structural determination). As expected, the P {1H} NMR
spectrum of complex 2a (in CDCl3) displayed only a singlet along with platinum satellites at δ
= 22.7 with a significant value of 1JPtP = 4290 Hz. This large coupling constant confirmed that
the phosphine ligand is positioned trans to nitrogen atom, same as complex A.24 In the 1H NMR
spectrum of complex 2a, the existence of a doublet of doublet of doublets for H2 of ppy ligand
at δ = 10.55 ppm with platinum satellites (3JPtH = 30.9 Hz) and also another doublet of doublet
of doublets at δ = 6.67 ppm accompanied by platinum satellites (3JPtH = 52.1 Hz) for H9 of ppy
ligand, confirmed that cyclometalating ligand was chelated in this complex (refer to the
Experimental Section for signal assignments). The later resonance exhibited a CH···π
interaction, as detected in the single X-ray crystal structure of complex 2a. In addition, complex
2a could be achieved directly by a nucleophilic substitution reaction of chloride ligand in
complex A by iodide (I-) ion (Scheme 1 (path III)).
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Other electrophilic substrates have been used to investigate the effect of these kind of
reagents on the above reaction (Scheme 1 (path II)). The obtained products were similar to the
resultant products from methyl iodide species. For example, addition of ethyl iodide (EtI) or
benzyl bromide (PhCH2Br) to complex 1a resulted in products 2-(ethylthio)pyridine (pySEt),
O2,25 and complex 2a or 2-(benzylthio)pyridine O3,25 and complex [Pt(ppy)(PPh3)Br], 2b,
respectively (Scheme 1 (path II)). Complex 2b was fully characterized by NMR spectroscopy
and single X-ray crystallography studies (see structural determination, Experimental Section).
Furthermore, complex 2b was obtained through salt metathesis reaction of complex A with KBr
(Scheme 1 (path III)).
Mechanism of CS Coupling from Complexes 1
In an effort to achieve more insight into the mechanism of reaction (carbonsulfur bond
formation) that was described in Scheme 1 (path II), it was monitored at different temperatures
with NMR spectroscopy. The reactants were mixed and firstly NMR spectra were recorded at
25 °C. In 31P {1H} NMR spectra (Figure 1), immediately after addition of MeI, the signals of
the starting complex 1a (δ = 22.7 ppm) quickly disappeared (Figure 1c) and a signal
corresponding to complex 2a (δ = 22.4 ppm) grew rapidly as time elapsed. Apart from
resonances of complex 1a (precursor) and complex 2a (product), an interesting Pt(II)-Pt(II)
dinuclear intermediate species was detected, suggesting the presence of two different
phosphorus ligands with high platinum coupling constants (Figure 1). Its structure was
determined as a binuclear complex [{Pt(ppy)(PPh3)}2(µ2-Spy)]I, 3-I. Therefore, two singlet
signals, one at δ = 21.5 ppm (1JPtP = 4407 Hz, Pa) and another one at δ = 21.0 ppm (1JPtP = 4245
Hz, Pb) were assigned to complex 3-I and phosphine ligands in this complex perhaps were
located in cis position to the nitrogen and sulfur atoms of bridging thiolate ligand, respectively.
This assignment is related to high cis influence of S atom over N atom.26, 27 As time passed, the
signal intensities of complex 3-I decreased while the signal intensities of complex 2a increased
until full conversion (ca. 6 h) was obtained as compared to the pure form of complex 2a.
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Figure 1. Reaction of complex 1a with 3eq of MeI as monitored by 31P {1H} NMR spectroscopy in CD2Cl2 at room temperature. (a)
Pure complex 1a, (b) immediately after addition of MeI, (c) 5 min (d) 15 min, (e) 30 min, (f) 60 min, (g) 120 min, (h) 180 min, (i) 240
min and (j) 480 min. The signal assignments are shown.
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1
The related H NMR spectra (Figure S1) for this reaction were also obtained
simultaneously at room temperature. This observation confirmed the formation of complex 2a,
and especially compound O1, as products and complex 3-I as an intermediate by observing their
expected signals. As shown in Figure S1, new resonances with increased signals at δ = 2.58 ppm
and 8.46 ppm were assigned to methyl and H6' (CH group adjacent to nitrogen atom) of
compound O1,25 respectively. When complex 1 was treated with CD3I under the same
conditions (CD2Cl2, room temperature), the methyl signal disappeared in the 1H NMR spectrum
(Figure S2).
To detect other plausible intermediates, the reaction was monitored at low temperature
(223 K). In this temperature the rate of reaction became very slow and an excess of MeI was
used. However, no further intermediates were observed (Figure S3) and similar room
temperature NMR spectra were obtained. This spectroscopic observation suggested that other
possible intermediates were very short lived and they were quickly transformed to the products.
Attempts to prepare resemble of complex 3-I were successful. The synthetic strategy
involved treatment of complex A with AgPF6 to abstract the chloride (Cl-) ligand from the
platinum center as AgCl (separated from solution by filtration). Subsequently, complex 1a was
added to the resulting solution and the binuclear complex [{Pt(ppy)(PPh3)}2(µ2-Spy)]PF6, 3-PF6,
was formed as an orange solid. The multinuclear NMR spectroscopy and crystal structure
determination confirmed the formation of this complex. The 31P {1H} NMR spectrum (Figure
2) of complex 3-PF6 exhibited two singlet resonance flanked by platinum satellites at similar
1
chemical shift (with equal coupling JPtP constant) of complex 3-I. It is notable that the
-
counteranion (I- or PF6 ) did not have impact effect on the cationic part of complex 3 in solution
-
and only PF6 facilitated crystallization process. As expected, the 195Pt {1H} spectrum (Figure
2) of complex displayed two doublets at -4081 (Pta) and -4296 (Ptb) ppm with 1JPtP values of
4413 and 4256 Hz, respectively, close to the values obtained from the 31P {1H} NMR spectrum.
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Figure 2. (a) 31P and (b) 31Pt NMR spectra of complex 3-PF6 in CD2Cl2 at room temperature.
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The synthesized complex 3-I was stable in solution for several days and it did not display
any decomposition products or rearrangements to complexes 1a and 2a. Also, complex 3-I was
treated with MeI and it easily was reacted and lead to complex 2a and compound O1. When
complex 3-PF6 was treated with MeI it did not show any reaction after six hours. According to
this data, the binuclear complex 3, perhaps, after the reaction with MeI and subsequent CS
bond coupling needs two coordinating anion like complex 3-I (one from counterion and another
-
from MeI). But complex 3-PF6 has a non-coordination counterion (PF6 ), probably, this point
stops desire bond formation.
Initial coordination site of alkyl halide species in complex 1a is important, because
complex 1a contains three potential sites (nitrogen, sulfur of Spy and platinum center) for initial
coordination of alkyl halide reagents. Therefore, some tests were conducted to clarify the
nucleophilicity of each coordination site in complex 1a against these substrates (see below). The
results from these experiments were helpful in proposing a mechanistic pathway for the
observed CS bond formation reaction in this study.
The free nitrogen of thiolate ligand in complex 1a with alkyl halide reagents could
potentially undergo Menschutkin reaction28 to produce N-alkylated complexes (Scheme S1,
Nitrogen path), although those were not observed in NMR spectroscopy studies (Figure S1).
The possible N-methylated complex 1-N from the reaction of complex 1a with MeI could
display a low field signal owing to the coordinated methyl on nitrogen.26 Supposedly, cationic
complex 1-N was not stable and could not be detected with spectroscopic methods and it was
1
converted to complex 2a and compound O1-t (N-methyl-2-thiopyridone).29 But, H NMR
spectra29 did not show any signal for the formation of compound O1-t and only compound O1
was observed. It is notable that the conversion of compound O1-t to compound O1 (referenced
to the thione-thiol tautomer)29, 30 needs harsher reaction conditions.29 To solve this challenge,
complex [Pt(ppy)(SPh)(PPh3)], 1b,23 in which PhSH = thiophenol, with no pendant nitrogen
atom was selected for reaction with MeI.31 This reaction produced complex 2 and its
corresponding CS coupling product i.e. thioanisole O432 (Scheme 1 (path II)). Therefore, this
test suggested that the uncoordinated nitrogen atom does not have an important role in the
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formation of yielded compounds and perhaps it has some effects on the formation of complex
3-I from complex 1a. Consequently, when the test reaction was followed with NMR
spectroscopy (Figure S4 and Figure S5) it merely displayed transformation of complex 1b to
complex 2a without any detectable intermediates in comparison with complex 1a (Figure 1).
All above observations suggested that the nitrogen site has weak nucleophilicity.31 Comparable
type of systems, each having at least one free nitrogen atom on the coordinated ligand to
platinum center remain intact in reaction with alkyl halide because other sites ofthese complexes
are stronger nucleophiles than N atom.31, 33-35 Consequently, a different site in complexes 1a and
1b is involved in the initial coordination of alkyl halide reagents.
An alternative possibility for preliminary coordination of alkyl halides to complex 1a is
sulfur atom which is known as S-alkylation.36-40 It is related to the metal complexes containing
thiolate ligands especially dithiolate group,38-41 while these are treated with alkyl halides species
(in thermal condition for most cases)38, 39, 42 S-alkylation is a plausible route.36, 37, 43 As shown
in Scheme S1 (Sulfur path), in the case of MeI, the presumed S-methylated complex 1-S ought
to exhibit a signal with platinum satellite in relation to coordinated methyl to sulfur atom but
this was not observed (Figure S1). Thus, another experiment was designed for unravelling this
inconsistency. While complex 1a reacted with iodine or bromine reagents, it produced
complexes 2a or 2b and its corresponding SI or SBr coupling products44 (Scheme 1 (path II)).
This reaction was followed by NMR spectroscopy and an intermediate (complex 3-I) similar to
the reaction of methyl iodide was detected (Figure 1). Therefore, sulfur atom is not incipient
coordination site and S-alkylation can be ruled out.43 Also, there are numerous reports on the
oxidation of metal centers by alkyl substrates45-48 than sulfur oxidation (thiolate alkylation). For
example, Sicilia research group reported that the reaction of half-lantern compound (containing
thiolate ligand) with MeI leaded to the oxidized complex and S-alkylation did not occur.48
Therefore, the proposed routes in Scheme S1 are not appropriate and platinum center is initial
coordination site for alkyl halide substrates and oxidative addition path is adequate for suggested
CS coupling mechanism (Scheme 2).
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We also investigated the effect of phosphine ligand on the CS bond formation by
changing PPh3 with PPhMe2, and complex [Pt(ppy)(η1-S-Spy)(PPhMe2)], 1c, was prepared
(Scheme 1 (path I)). When alkyl halide regent (MeI) was added to complex 1c in acetone-d6 at
room temperature and the reaction was followed by NMR spectroscopy, rapid and complete
conversion to complex [Pt(ppy)(PPhMe2)I], 2c, and O1 was observed (Scheme 1 (path II)). This
reaction pathway is analogous to the reaction with complex 1a which contains PPh3, but CS
bond reductive elimination occurs at a faster rate. The increased rate of reaction is consistent
with the greater nucleophilicity of complex 1c than complex 1a.33, 49
Structural Determination
Single-crystal X-ray diffraction investigation was carried out on complexes A, 2a, 2b
and 3-PF6 to approve their molecular structures. Crystallographic data are collected in Table
S1, and selected bond distances and angles are quoted in Table S2. The perspective drawing of
these complexes are illustrated in Figure 3 and Figure S6.
All complexes A, 2a, 2b and 3-PF6 reveal a distorted square planar coordination
environment around Pt(II) center. This distortion is due to small bite angle [81.0(5)78.7(13)o]
of the ppy cyclometalated ligand. This narrow bite angle with bond distances for cyclometalated
chelate (i.e. Pt-Cppy and Pt-Nppy) are comparable to those observed in other five membered rings
of 2-phenylpyridinate platinum(II) complexes.50-53 In each structure, the metalated carbon atom
of the ppy ligand accommodates the coordinated phosphine ligand in cis arrangement of PtCppy
bond, as a result of high trans influence of carbon atom.54 Therefore, PPh3 ligands are located
trans to nitrogen atom of ppy ligand and bond distances of the PtP are notably shorter than
platinumphosphorous bond lengths, positioned trans to carbon atom of cyclometalated
ligand.50 Furthermore, halogen ligand completes the coordination sphere of platinum atom in
complexes A, 2a and 2b or nitrogen and sulfur atoms of bridging Spy ligand in complex 3-PF6.
Also, in binuclear complex 3-PF6 the pyridine-2-thiolate group acts as a bridging ligand and
binds two Pt centers through S^N coordination. The nitrogen and sulfur atoms of Spy ligand in
this complex occupies a position trans to the PtCσ bond of cyclometalated ligands.
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Additional examination on the molecular packing of the crystal structures displayed the
presence of inter- and intramolecular interactions (see below) such as CH···π interaction, π···π
stacking, without any notable Pt···Pt contacts. All molecular structures reveal a moderate short
CH···π intramolecular interaction (Figure S7) between hydrogen atom adjacent to coordinated
carbon atom of cyclometalated ligand and the phenyl ring of the triphenylphosphine ligand
(CH···π; C2···Cph (PPh3) = 3.320-3.416 Å (A), 3.237-3.401 Å (2a), 3.291-3.365 Å (2b), for
3-PF6, CH···π; C11 or C45···Cph (PPh3) = 3.244-3.493 Å, CH···π; C1···CSpy = 3.462 Å).
The crystal structure of complexes A, 2a and 2b shows that the halogen ligand is
involved in a substantial intramolecular interaction with CH group adjacent to coordinated
nitrogen atom of ppy ligand (Figure S8). This CH···X interaction causes deshielding of the
proton involved in hydrogen bonding55, 56 (~ 1.3 to 1.9 ppm) in 1H NMR (Figure S9) relative to
the hydrogen in free ppy ligand.57 The shift values has also increased according to atomic radii
of halogen ligands in the order of Cl < Br < I.58
In complex 3-PF6 the ring of the pyridine-2-thiolate ligand exhibits weak intramolecular
π···π interaction with one of the phenyl groups of the PPh3 ligand bound to the Pt1 with distance
between the centroids of 3.888 Å (Figure S10). Complexes A and 2b are stacked through short
intermolecular π⋯π interactions with an inter-planar distance of 3.335 Å and 3.329 Å,
respectively. These interactions lead to formation of a dimer in a head to tail style, which are
analogous to other platinum complexes having ppy fragments51 (Figure S11). Furthermore, in
crystal network of complex 2b, each Br atom is bound to one hydrogen atom of the phosphine
ligand from neighboring unit. In this molecular structure, each Pt(ppy) moiety is arranged
through π···π interactions with another Pt(ppy) fragment in nearby chain and thus giving rise to
a 2D network (Figure S12). Therefore, this complex indicates that the CH groups (see above
discussion) are involved in the formation of intra- and intermolecular hydrogen bonds59 through
the coordinated Br ligand with bond distance of 2.583 Å and 3.045 Å, respectively (Figure S8c
and Figure S12). Complex 2a displays the comparatively long interlayer distances between
nearby ppy ligands that are longer than 3.8 Å,60 demonstrating the lack of π···π interactions in
this complex.
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DFT Investigation
In order to verify this approach (according to the experimental data), we calculated the
energy and reaction profiles for the oxidative addition of MeI to complex 1a and carbonsulfur
bond formation from the suggested intermediate structures (Scheme 2 and Figure 4). It is also
worth mentioning that the pyridyl group and its peripheral nitrogen donor atom could be
orientated in four different fashions (Figure S13). In these modes, the py ring and N atom are
located at a plane above or below of the molecule and near or far from the platinum center,
respectively. However, theoretical calculations suggest all possible orientations have small
variation in energy (~ 2 KJ mol-1), perhaps due to free rotation around S−Cpy bond.61 But, the
form a (Figure S13) is preferred over other structures since it is in good agreement with further
theoretical calculations (see below).
The DFT calculations suggest that the electron rich platinum center of complex 1a (as a
nucleophile) attacks on carbon atom of MeI (as an electrophile) to form transition state TS1 and
energy barrier for this state is +52.9 KJ mol-1. This transition state illustrates a linear
arrangement for I···CMeI···Pt with the bond angles close to 90˚ and 180˚ between H–CMeI···Pt
(90-93˚) and I···CMeI···Pt (178.5˚), respectively. The most important modifications in bond
distances are detected for I···CMeI and Pt···CMeI in transition state TS1. These calculations
suggested that the bond distance increases for I···CMeI (2.73 Å) in this transition state as
compared to MeI (2.20 Å), while Pt···CMeI (2.49 Å) bond length is reduced.62, 63 This data
supports the hypothesis that the formation of transition state TS1 is followed by concurrent bond
breaking of methyliodide and a new bond formation between platinum and methyl, then the
cationic 5-coordinate intermediate IM1 is obtained. This intermediate is equilibrated with the
comparable intermediate IM2 with phosphine ligand and Me group being in trans disposition
at platinum center (Figure 4). Both intermediates IM1 and IM2 could potentially convert to the
6-coordinate compounds P1 and P2, respectively (Scheme 2). The calculation indicates that the
energy barriers for these compounds are high (+42.9 and +50.3 KJ mol-1 for compounds P1 and
P2, respectively) and subsequent CS bond coupling is not favorable. It is recommended that
this concerted elimination from the five-coordinate d6 metal complexes is more straightforward
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than direct elimination from the saturated six-coordinate metal complexes.11, 64, 65 Therefore, the
carbonsulfur bond formation from intermediate IM1 (selected based on its good agreement
with theoretical calculations) proceeds via transition state TS2 as a concerted reductive
elimination66 (Scheme 2). In TS2, the bond distances between platinum with methyl (2.42 Å)
or thiolate (2.47 Å) groups are lengthened and the bond angle of CMePtSSpy (61.3˚) is
decreased and it suggested that this elimination is appropriate. Another possibility for the
reductive elimination of CS bond, probably, is the microscopic reverse of the popular SN2
mechanism.66, 67 In this mechanism, pervious mechanistic evidences provided, the nucleophilic
groups like NR-, OR-, I- dissociated from central metal, and then perform SN2 attack upon an
alkyl ligand (Me group) of the cationic intermediates.12, 65, 68-70 Therefore, by addition of excess
of nucleophilic groups to the reaction mixture the rate of reaction should be increased. By this
strategy, when Spy- was added to the reaction mixture, the desire CS bond coupling reaction
was stopped. Due to Spy- was reacted with MeI and compound O1 and KI salt were formed
without accelerating rate of reductive elimination of CS from intermediate IM1. Consequently,
the possibility of SN2 mechanism is not proper for this CS bond formation. As compound O1
dissociates from this transition state (TS2), the new cationic T-shaped intermediate IM311, 65, 69,
71 is produced (Scheme 2). Intermediate IM3 can then react by two routes: (i) migration of iodide
to the platinum center and formation of complex 2a as a very stable complex (-71.3 KJ), or (ii)
reaction with complex 1a to yield a cationic complex 3-I. The latter pathway is slower (deduced
from NMR, Figure 1 and Figure S1) and more favorable (-51.8 KJ, Figure 4). Subsequently,
complex 3-I in the presence of MeI acts as a nucleophile (perhaps through a classical SN2
mechanism)48 and leads to the formation of an intermediate IM448 (Pt(II)-Pt(IV)Me). Finally,
intermediate IM4 through a reductive elimination process (MeSpy, compound O1) generates
the stable final products (Figure 4 and Scheme 2).
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Figure 3. Representations of the X-ray crystal structure of complex [{Pt(ppy)(PPh3)}2(µ2-
Spy)]PF6, 3-PF6, showing all non-hydrogen atoms as 40% thermal ellipsoids. Hydrogen atoms,
PF6 and a CH2Cl2 solvent molecule have been omitted for clarity. Pt1C11 1.982(14), Pt2C45
2.000(17), Pt1N1 2.070(12), Pt2N2 2.066(12), Pt1N3 2.117(11), Pt1P1 2.235(3), Pt2P2
2.226(3), Pt2S1 2.418(4), C11Pt1N1 81.0(5), C45Pt2N2 80.4(6), C11Pt1P1 95.0(4),
C45Pt2P2 97.9(5), C11Pt1N3 172.2(4), C45Pt2S1 160.4(5), N1Pt1P1 173.5(3),
N2Pt2P2 165.7(4), N1Pt1N3 91.2(4), N2Pt2S1 94.6(4), N3Pt1P1 92.8(3),
P2Pt2S1 91.41(13).
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Me
Pt
I
Me
Pt
PPh3
S
I
S
N
N
PPh3
N
N
P1 (Kinetic Product)
P2 (Thermodynamic Product)
I
Me
Pt
N
Pt
N
Me
Pt
PPh3
S
PPh3
S
PPh3
S
trans-cis
PPh3
N
N
I
I
Pt
+ MeI
isomerziation
Me
S
N
N
N
N
1a
TS1
IM1
IM2
Me
PPh3
S
Pt
I
N
N
TS2
PPh3
PPh3
I
N
S
+
Pt
I
Pt
Me
N
N
O1
2a
IM3
O1
+ Complex 1a
Me
Pt
PPh3
S
Ph3P
N
PPh3
S
Pt
N
N
Pt
I2
I
+ MeI
N
N
N
Pt
PPh3
IM4
3-I
Scheme 2. Proposed mechanism for the reaction of complex 1a with methyl iodide.
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Figure 4. Calculated structures and relative energies for probable intermediates and transition states arising from the reaction of complex
1 and MeI in acetone solution.
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Conclusion
The platinum(II) complex 1a reacted readily with electrophilic alkyl sources at ambient
temperature and through carbonsulfur reductive coupling reaction produced corresponding
halide complexes 2a or 2b and alkyl sulfides. This reaction was monitored by NMR
spectroscopy at different temperatures and the results revealed (Scheme 2) an interesting
binuclear complex 3-I was formed during this process. Also, we proposed that the platinum
center is a stronger nucleophile than peripheral nitrogen or sulfur atom. This claim was approved
by selection of complex 1c, which lacks a free nitrogen atom, and it underwent a similar CS
bond formation reaction. Moreover, in this experiment complex 1a did not display any Pt(IV)
compound(s) and perhaps such intermediate(s) is(are) energetically unfavorable as supported
by DFT calculations. Therefore, platinum-thiolate complexes of these nature served as an
appropriate platform for oxidative addition reaction that impressively enhanced the formation
of alkyl sulfides.
Experimental Section
General procedures and materials
All NMR spectra (1H, 31P {1H} and 195P {1H}) were recorded on a Brucker Avance DPX 400
MHz instrument. References were TMS or the residual peak of the solvent, i.e. CD2Cl2, CDCl3
and acetone-d6 (1H), 85% H3PO4 (31P), and aqueous Na2PtCl6 (195Pt). The chemical shifts (δ)
being reported as ppm and coupling constants (J) expressed in Hz. The microanalyses were
performed using a vario EL CHNS elemental analyzer. Electrospray ion mass spectrum (ESI-
MS) was recorded by a HP-5989B spectrometer using methanol–water as the mobile phase. All
solvents were purified and dried according to standard procedures.72 2-phenylpyridine (ppyH),
pyridine-2-thiol (pySH), thiophenol (PhSH), silver hexafluorophosphate (AgPF6),
triphenylphosphine (PPh3) and dimethylphenylphosphine (PPhMe2) were purchased from
Aldrich or Acros. Complexes [Pt(ppy)(DMSO)(Cl)],53 [Pt(ppy)(PPh3)Cl], A,24 [Pt(ppy)(η1-S-
Spy)(PPh3)], 1a,23 [Pt(ppy)(SPh)(PPh3)], 1b,23 were prepared as reported in literature. The NMR
labeling for all ligands are shown in Scheme 3 for clarifying the chemical shift assignments.
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H3
H4
H2
H4'
H5'
H3'
N
H5
H6
Pt
H6'
N
S
Pt
Spy
H7
H9
H8
ppy
Scheme 3. Representative ligands with position labeling.
[Pt(ppy)(PPhMe2)Cl], B.
To a solution of [Pt(ppy)(DMSO)Cl] (100 mg, 0.22 mmol) in acetone (10 mL) was added
PPhMe2 (30.8 μL, 0.22 mmol). The mixture was stirred at room temperature for 1 h. Then, the
solvent was removed under reduced pressure and the residue was triturated with n-hexane (3 ×
2 mL) and the resultant yellow solid was dried under vacuum. Yield: 94 mg, 83%. Elem. Anal.
Calcd for C19H19ClNPPt (522.87): C, 43.64; H, 3.66; N, 2.68; Found: C, 43.91; H, 3.74; N, 2.57.
1H NMR (400 MHz, acetone-d6, 20 °C, δ): 9.80 (d, 3JPtH = 24.1, 3JHH = 5.8, 1H, H2), 8.11-8.04
(m, 4H, H4, H5 and Ho of PPhMe2), 7.69 (dd, 3JHH = 7.8, 4JHH = 1.3, 1H, H6), 7.53-7.48 (m, 4H,
H3, Hp and Hm of PPhMe2), 7.00 (td, 3JHH = 7.8, 4JHH = 1.2, 1H, H7), 6.87 (d, 3JPtH = 56.1, 3JHH
3
4
3
= 7.8, 1H, H9), 6.75 (td, JHH = 7.8, JHH = 1.2, 1H, H8), 2.00 (d, 2JPH = 10.4, JPtH = 39.7 Hz,
31
1
6H, Me group of PPhMe2). P {1H} NMR (162 MHz, acetone-d6, 20 °C, δ): -8.1 (s, JPtP
=
4131, 1P).
[Pt(ppy)(η1-S-Spy)(PPhMe2)], 1c.
Complex B (100 mg, 0.19 mmol) was added to an ethanolic solution of sodium pyridine-2-
thiolate ligand (NaC5H4NS) under inert atmospheric condition [Note: NaC5H4NS was prepared
by dissolving sodium (5.8 mg, 0.25 mmol) in 10 mL of absolute ethanol and subsequent
treatment with pyridine-2-thiol (21.3 mg, 0.19 mmol)]. The resulting orange colored mixture
was allowed to react while stirring for 12 h. Then, solvent was removed under reduced pressure
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the residue was extracted with CH2Cl2 (20 mL). The obtained orange solution was filtered
through Celite and the filtrate was concentrated to a small volume (1 mL). Finally, n-hexane (5
mL) was added to precipitate complex 1c as an orange solid. Yield: 71 mg, 62%. Elem. Anal.
Calcd for C24H23N2PPtS (597.57): C, 48.24; H, 3.88; N, 4.69; Found: C, 47.96; H, 3.74; N, 4.76.
3
1H NMR (400 MHz, acetone-d6, 20 °C, δ): 10.05 (m, JPtH = 26.2, 1H, H2), 8.15-8.03 (m, 5H,
3
4
3
H4, H5, H6' and Ho of PPhMe2), 7.90 (dd, JHH = 7.8, JHH = 1.0, 1H, H6), 7.57 (d, JHH = 8.0,
1H, H3'), 7.48-7.36 (m, 4H, H3, Hp and Hm of PPhMe2), 7.19 (td, 3JHH = 7.8, 4JHH = 1.9, 1H, H7),
7.02 (t, 3JHH = 7.1, 1H, H4'), 6.98 (d, 3JPtH = 46.1, 3JHH = 7.8, 1H, H9), 6.80 (td, 3JHH = 7.8, 4JHH
= 1.7, 1H, H8), 6.71 (dd, 3JHH = 5.1, 4JHH = 0.9, 1H, H5'), 2.01 (d, 2JPH = 10.9, 3JPtH = 40.1 Hz,
31
1
6H, Me group of PPhMe2). P {1H} NMR (162 MHz, acetone-d6, 20 °C, δ): -8.5 (s, JPtP
=
4178, 1P).
[Pt(ppy)(PPh3)I], 2a.
Method A: To a yellow suspension of complex 1a (100 mg, 0.14 mmol) in acetone (15 mL)
was added an excess of MeI (174.3 µL, 2.8 mmol, 20 equiv.) at room temperature. The color of
the mixture turned red instantly and then the mixture was stirred for 2 h. Then, the solvent was
removed under reduced pressure and the residue was triturated with n-hexane (3 × 2 mL). The
resulting green solid was dried under vacuum. Yield: 82 mg, 78%; m.p. 225 ˚C. Elem. Anal.
Calcd for C29H23INPPt (738.03): C, 47.15; H, 3.14; N, 1.90; Found: C, 47.81; H, 3.37; N, 1.97.
3
1H NMR (400 MHz, CDCl3, 20 °C, δ): 10.55 (ddd, JPtH = 30.9, 3JHH = 5.8, 4JHH = 0.9, 4JPH
=
4.3, 1H, H2), 7.87-7.77 (m, 8H, H4, H5, Ho of PPh3), 7.50 (dd, 3JHH = 7.9, 4JHH = 1.1, 1H, H6),
4
4
7.43-7.34 (m, 9H, Hp and Hm of PPh3), 7.20 (tdd, 3JHH = 7.3, JHH = 1.3, JPH = 1.4, 1H, H3),
3
4
3
6.99 (td, JHH = 7.9, JHH = 0.9, 1H, H7), 6.67 (ddd, JPtH = 52.1,3JHH = 7.3, 4JHH = 0.8, 4JPH
=
4.0, 1H, H9), 6.52 (td, 3JHH = 8.0, 4JHH = 1.3, 1H, H8). 31P {1H} NMR (162 MHz, CDCl3, 20 °C,
δ): 22.7 (s, 1JPtP = 4290, 1P). 1H NMR (400 MHz, acetone-d6, 20 °C, δ): 10.54 (ddd, 3JPtH = 31.1,
3JHH = 5.7, 4JHH = 1.0, 4JPH = 4.3, 1H, H2), 8.13-8.11 (m, 2H, H4, H5), 7.86-7.81 (m, 6H, Ho of
3
4
PPh3), 7.70 (dd, JHH = 7.9, JHH = 1.4, 1H, H6), 7.50-7.40 (m, 10H, H3, Hp and Hm of PPh3),
3
4
3
6.99 (td, JHH = 7.9, JHH = 1.1, 1H, H7), 6.69 (ddd, JPtH = 52.7,3JHH = 7.4, 4JHH = 0.8, 4JPH
=
4.0, 1H, H9), 6.49 (td, 3JHH = 79, 4JHH = 1.5, 1H, H8). 31P {1H} NMR (162 MHz, acetone-d6, 20
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3
°C, δ): 22.8 (s, 1JPtP = 4296, 1P). 1H NMR (400 MHz, CD2Cl2, 20 °C, δ): 10.49 (m, JPtH = not
resolved, 1H, H2), 7.92-7.78 (m, 8H, H4, H5, Ho of PPh3), 7.53 (dd, 3JHH = 7.8, 4JHH = 1.1, 1H,
H6), 7.44-7.38 (m, 9H, Hp and Hm of PPh3), 7.25 (t, 3JHH = 7.4, 1H, H3), 7.00 (t, 3JHH = 7.9, 1H,
H7), 6.65 (dd, 3JPtH = 52.4,3JHH = 7.5, 4JPH = 4.1, 1H, H9), 6.52 (t, 3JHH = 7.9, 1H, H8). 31P {1H}
NMR (162 MHz, CD2Cl2, 20 °C, δ): 22.4 (s, 1JPtP = 4288, 1P).
Method B: To a yellow suspension of complex 1a (100 mg, 0.14 mmol) in acetone (15 mL)
was added a solution of I2 (175.8 g, 0.70 mmol, 5 equiv.) in acetone (5 mL) at room temperature.
A fast color change to red was observed. The resulting solution was stirred for 2 h. Then, the
solvent was reduced under vacuum to a small volume (1 mL) and n-hexane (5 mL) was added
to precipitate complex 2a as a green powder (73 mg, 71%).
Method C: To a green suspension of complex A (100 mg, 0.15 mmol) in acetone/CH2Cl2 (10:5
mL) was added an excess of KI (74.7 g, 0.45 mmol, 3 equiv.) in acetone (2 mL) at room
temperature. The resulting mixture was allowed to react while stirring for 24 h. Then, the solvent
was removed under reduced pressure and the residue was extracted with CH2Cl2 (20 mL). The
obtained green solution was filtered through Celite and the filtrate was concentrated to a small
volume (1 mL). Finally, n-hexane (5 mL) was added to yield complex 2a as a green solid which
was dried under vacuum (48 mg, 43%).
[Pt(ppy)(PPh3)Br], 2b.
Method A: To a yellow suspension of complex 1a (100 mg, 0.14 mmol) in acetone (15 mL)
was added benzyl bromide (166.3 µL, 1.4 mmol, 10 equiv.) at room temperature and an orange
solution was obtained quickly. The reaction mixture was stirred for 2 h. Then, the solvent was
evaporated under vacuum and the residue was washed with n-hexane (3 × 2 mL), resulting in a
green solid which was dried under vacuum. Yield: 86 mg, 89%; m.p. 221 ˚C. Elem. Anal. Calcd
1
for C29H23BrNPPt (690.04): C, 50.43; H, 3.36; N, 2.03; Found: C, 51.01; H, 3.59; N, 2.26. H
NMR (400 MHz, CDCl3, 20 °C, δ): 10.18 (ddd, 3JPtH = 30.8, 3JHH = 5.7, 4JHH = 1.0, 4JPH = 4.1,
1H, H2), 7.89-7.77 (m, 8H, H4, H5, Ho of PPh3), 7.51 (dd, 3JHH = 7.8, 4JHH = 1.2, 1H, H6), 7.44-
7.35 (m, 9H, Hp and Hm of PPh3), 7.27 (m, 1H, H3, this signal has overlapping with CHCl3 NMR
solvent), 6.97 (td, 3JHH = 8.0, 4JHH = 0.9, 1H, H7), 6.67 (ddd, 3JPtH = 52.4,3JHH = 7.4, 4JHH = 0.8,
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4JPH = 3.9, 1H, H9), 6.52 (td, 3JHH = 8.0, 4JHH = 1.4, 1H, H8). 31P {1H} NMR (162 MHz, CDCl3,
20 °C, δ): 22.7 (s, 1JPtP = 4328, 1P).
Method B: To a yellow suspension of complex 1a (100 mg, 0.14 mmol) in acetone (15 mL)
was added a solution of Br2 (36 µlit, 0.70 mmol, 5 equiv.) in acetone (5 mL) at room
temperature. A fast color change to red was observed. The resulting solution was stirred for 2 h.
Then, the solvent was reduced under vacuum to a small volume (1 mL) and n-hexane (5 mL)
was added to precipitate complex 2b as a green powder (61 mg, 64%).
Method C: To a green suspension of complex A (100 mg, 0.15 mmol) in acetone/CH2Cl2 (10:5
mL) was added an excess of KBr (53.6 g, 0.45 mmol, 3 equiv.) in acetone (2 mL) at room
temperature. The resulting mixture was allowed to react while stirring for 30 h. Then, the solvent
was removed under reduced pressure and the solid residue was extracted with CH2Cl2 (20 mL).
The obtained green solution was filtered through Celite and the filtrate was concentrated to a
small volume (1 mL). Finally, n-hexane (5 mL) was added to yield complex 2b as a green solid
(39 mg, 38%).
[Pt(ppy)(PPhMe2)I], 2c.
To an orange solution of complex 1c (100 mg, 0.17 mmol) in acetone (15 mL) was added an
excess of MeI (208.3 µL, 3.4 mmol, 20 equiv.) at room temperature. The solution color turned
red instantly and the mixture was stirred for 1 h. Then, the solvent was removed under reduced
pressure and the resulting residue was triturated with n-hexane (3 × 2 mL). Finally, the resulting
green solid was dried under vacuum. Yield: 64 mg, 62%. Elem. Anal. Calcd for C19H19INPPt
1
(614.32): C, 37.15; H, 3.12; N, 2.28; Found: C, 37.31; H, 3.24; N, 2.21. H NMR (400 MHz,
3
acetone-d6, 20 °C, δ): 10.41 (m, JPtH = 32.3, 1H, H2), 8.10-8.05 (m, 4H, H4, H5 and Ho of
PPhMe2), 7.70 (dd, 3JHH = 7.7, 4JHH = 1.4, 1H, H6), 7.51-7.43 (m, 4H, H3, Hp and Hm of PPhMe2),
3
4
3
7.05 (td, JHH = 7.7, JHH = 1.1, 1H, H7), 6.84 (ddd, JPtH = 55.3,3JHH = 7.6, 4JHH = 1.0, 4JPH
=
3.8, 1H, H9), 6.77 (td, 3JHH = 7.7, 4JHH = 1.3, 1H, H8), 2.21 (d, 2JPH = 10.8, 3JPtH = 39.9 Hz, 6H,
31
1
Me group of PPhMe2). P {1H} NMR (162 MHz, acetone-d6, 20 °C, δ): -9.0 (s, JPtP = 4108,
1P).
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[{Pt(ppy)(PPh3)}2(µ2-Spy)]PF6, 3-PF6.
To a solution of complex A (100 mg, 0.15 mmol) in CH2Cl2 (15 mL) was added AgPF6 (51 mg,
0.20 mmol). The reaction mixture was stirred for 5 h in dark at room temperature, and then
filtered through Celite to remove AgCl. The green filtrate was treated with complex 1a (108.3
mg, 0.15 mmol) in CH2Cl2 (5 mL). The resulting orange solution was stirred for 2 h and then
the solvent was removed under reduced pressure and concentrated to a small volume (~ 1 mL)
and n-hexane (3 mL) was added to obtain complex 3-PF6. Yield: 157 mg, 71%; m.p. 217 ˚C.
MS ESI(+): m/z 1332.27 [M-PF6]+. Elem. Anal. Calcd for C63H50F6N3P3Pt2S (1477.22): C,
51.18; H, 3.41; N, 2.84; Found: C, 51.47; H, 3.52; N, 2.91. 1H NMR (400 MHz, CDCl3, 20 °C,
δ): 8.97 (m, 3JPtH = 32.7, 2H, H2), 7.98-6.31 (overlapping multiplets, 48 H). 31P {1H} NMR (162
MHz, CD2Cl2, 20 °C, δ): 21.5 (s, 1JPtP = 4407, 1P, Pa cis with nitrogen atom of Spy ligand), 21.0
(s, 1JPtP = 4245, 1P, Pb cis with sulfur atom of Spy ligand), -144.4 (septet, 1JPF = 712, 1P, P of
PF6). 195Pt {1H} NMR (85.7 MHz, CD2Cl2, 20 °C, δ): -4081 (d, 1JPtP = 4413, 1Pt, Pta bound to
1
the nitrogen atom of Spy ligand), -4296 (d, JPtP = 4256, 1Pt, Ptb bound to the sulfur atom of
Spy ligand).
Monitoring the Reaction of complexes 1a-c with electrophilic reagents (MeI, CD3I, Br2 and
I2) by 1H and 31P {1H} NMR Spectroscopy.
To a solution of complexes 1a-c (0.014 mmol) in CD2Cl2 (0.75 mL) in an NMR tube was added
the appropriate electrophilic reagent at 298 K (0.020 mmol) or at 223 K (0.42 mmol). The tube
was then placed in the probe of the NMR spectrometer, and NMR spectra were obtained at
appropriate time intervals.
Crystal Structure Determination and Refinement. The X-ray diffraction measurements were
carried out on STOE IPDS-2/2T diffractometer with graphite-monochromated Mo Kα radiation.
All single crystals were mounted on a glass fiber and used for data collection. Cell constants
and an orientation matrix for data collection were obtained by least-square refinement of the
diffraction data from 4977, 4763, 4350 and 10828 for A, 2a, 2b and 3-PF6, respectively.
Diffraction data were collected in a series of ω scans in 1° oscillations and integrated using the
Stoe X-AREA73 software package. A numerical absorption correction was applied using X-
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RED74 and X-SHAAPE75 software. The data were corrected for Lorentz and polarizing effects.
The structures were solved by direct methods76 and subsequent difference Fourier maps and
then refined on F2 by a full-matrix least-squares procedure using anisotropic displacement
parameters.77 Atomic factors are from the International Tables for X-ray Crystallography.78 All
non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms
were placed in ideal positions and refined as riding atoms with relative isotropic displacement
parameters. A view of the structure is depicted in Figure 3 (top) and Figure S6. All refinements
were performed using the X-STEP32, SHELXL-2014 and WinGX-2013.3 programs.79-86 Also,
the suitable crystals were obtained from CHCl3/n-hexane solution (complexes A, 2a and 2b) or
CH2Cl2/n-hexane solution (complex 3-PF6) at room temperature.
Theoretical Methods. Gaussian 09 was used87 to fully optimize all the structures at the
DFT/B3LYP level of density functional theory. The solvation energies were calculated by
CPCM model in acetone. The effective core potential of Hay and Wadt with a double-x valence
basis set (LANL2DZ) was chosen to describe Pt and I.88 The 6-31G(d) basis set was used for
other atoms.
Electronic supplementary information (ESI) available: NMR spectra, crystallographic and
computational details (PDF). Crystallographic data (CIF).
Acknowledgments
This work was supported by the Institute for Advanced Studies in Basic Sciences (IASBS)
Research Council. Technical support of the Chemistry Computational Center at Shahid Beheshti
University is gratefully acknowledged. Thanks are also due to Dr. F. Niroomand Hosseini,
Islamic Azad University (Shiraz Branch), for helpful assistance and discussions on DFT
calculations, Dr. A. Neshat, IASBS, for valuable suggestions, Mr. A. Biglari, the operator of
Bruker NMR instrument at IASBS, for recording the NMR spectra and Prof. Elena Lalinde,
Universidad de La Rioja, for ESI-MS measurements.
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Table of Contents Entry
Carbon−Sulfur Bond Reductive Coupling from a Platinum(II) Thiolate
Complex
Complex [Pt(ppy)(η1-S-Spy)(PPh3)], 1, was reacted with different electrophilic alkyl halide
reagents at room temperature. This reaction proceeded via formation of binuclear intermediate
complex [{Pt(ppy)(PPh3)}2(µ2-Spy)]+, 3, and through C−S bond reductive coupling to produce
alkyl sulfides and corresponding halide complexes [Pt(ppy)(PPh3)X], 2, X = I, Br.