Easy activation of the aryl-sulfur bond by platinum(ii)

Article abstract of DOI:10.1039/c3cc43303j

The reagents 1,2-C₆H₄(CHNR)(SMe), R = CH ₂CH₂NMe₂ or Ph, react with [Pt ₂Me₄(μ-SMe₂)₂] by oxidative addition of the aryl-sulfur bond to give the correspondi

Full text of DOI:10.1039/c3cc43303j

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Easy activation of the aryl–sulfur bond by platinum(II)†
Seowoo Kim, Paul D. Boyle, Matthew S. McCready, Kyle R. Pellarin and
Richard J. Puddephatt*
Cite this: Chem. Commun., 2013,
49, 6421
Received 3rd May 2013,
Accepted 4th June 2013
DOI: 10.1039/c3cc43303j
www.rsc.org/chemcomm
Q
The reagents 1,2-C6H4(CH NR)(SMe), R = CH2CH2NMe2 or Ph, react
with [Pt2Me4(l-SMe2)2] by oxidative addition of the aryl–sulfur bond to
give the corresponding crystalline binuclear platinum(IV) compounds
2
Q
[Pt2Me4(l-SMe)2(j -C,N-C6H4-2-CH NR)2], as the isomers with Ci
(R = CH2CH2NMe2 or Ph) or C1 (R = Ph) symmetry. These first examples
of C–S bond activation at platinum(II) occur easily at room temperature,
and the reactions give complex equilibria of isomeric products, from
which the isolated compounds crystallise.
The activation of carbon–sulfur bonds by transition metal
complexes has been studied in depth, driven primarily by the
role of this reaction in both hydrodesulfurisation (HDS) of
petroleum and in cross-coupling reactions of thioesters.1 The
C–S bond activation at platinum is important, given the use of
platinum catalysts in petroleum reforming and HDS catalysis, but
there are relatively few studies with platinum complexes and none
that involve oxidative addition of the C–S bond to platinum(II)
complexes.1,2 Indeed, there are relatively few platinum(IV) thiolates,3
and thiols are known to reduce several platinum(IV) prodrugs to the
more bioactive platinum(II) complexes.4 Most information on
reactivity and mechanism in C–S bond activation by oxidative
addition has been obtained from studies of thiophene with
rhodium(I), iridium(I) and platinum(0) complexes.1,2,5 Aryl–sulfur
bonds are stronger than alkyl– or vinyl–sulfur bonds and are
typically inert in thioether derivatives and their complexes.6 It is
noteworthy that thiophene derivatives with platinum(II) typically
react by C–H rather than C–S bond activation.2,7 We are therefore
prompted to report the easy and selective cleavage of the aryl–sulfur
bond in aryl(methyl)thioether compounds by platinum(II).
Scheme 1 Proposed reaction products and intermediates in the reaction of
NNS or NS with complex 1. The isomers of 6 and 7, with their symmetry point
groups, are shown below.
The new ligands, abbreviated as NNS and NS, were easily
prepared according to eqn (1),† and their reactions with the
complex [Pt2Me4(m-SMe2)2], 1, are shown in Scheme 1. The
reactions were more complex than predicted.
As monitored by 1H NMR spectroscopy, the reaction of NNS
with complex 1 at ꢀ20 1C led to a mixture of platinum(II)
complexes 2, 3a, 4a and 5, but further reaction occurred
between 10–20 1C to give the binuclear platinum(IV) complex
6, as a mixture of isomers 6a–6d. The reaction of 1 with the
ligand NS occurred in a similar way to give first a mixture of 2,
3b and 4b, and then a mixture of isomers 7a–7d (Scheme 1).
Department of Chemistry, University of Western Ontario, London, Canada N6A 5B7.
E-mail: pudd@uwo.ca
† Electronic supplementary information (ESI) available: Details of the experi-
mental and theoretical procedures and spectroscopic properties. X-ray data are
available in CIF format. CCDC 933211, 933212 and 937141. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c3cc43303j
c
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The imine is the strongest donor in the ligands NNS and NS,
and its coordination in all products is shown by the CHQN
resonance with coupling constant 3J(PtNQCH) in the range
34–46 Hz.7,8 † The new platinum(II) complexes 3–5 cannot be
isolated and were characterized by their 1H NMR spectra. In
particular, complexes 3a and 3b show 3J(PtH) coupling of 24 Hz
to the coordinated SMe protons but not to the NMe2 protons in
3
3a, complex 5 shows J(PtH) coupling of 21 Hz to the coordi-
nated NMe2 protons but not to the SMe protons, and complexes
3
4a and 4b do not show J(PtH) coupling to either the NMe2 or
SMe protons. Each of these platinum(II) complexes gives two
methylplatinum resonances with coupling constants in the
range 84–91 Hz.7,8 Complex 2 is known to be formed by
reaction of 1 with SMe2, which is released by the reaction of 1
with NNS or NS.7,8
Oxidative addition of C–X bonds to the platinum(II) complex 1
has been studied with ligands 2-X-C6H4CHQNR (X = halogen),
which are analogous to NNS or NS, but these give mononuclear
platinum(IV) complexes [PtXMe2(k3-C,N,N0-C6H4-2-CHQNCH2CH2-
NMe2)] or [PtXMe2(k2-C,N-C6H4-2-CHQNPh)(SMe2)] when R =
CH2CH2NMe2 or Ph respectively.7,8 In the present system, coordina-
tion of the ligand NMe2 group or dimethylsulfide is not observed,
but the thiolate-bridged complexes 6 or 7 are observed exclusively.
The complication is that they are observed as an unusual mixture of
isomers. Scheme 1 shows the four possible isomers [{Pt(m-SMe)-
Me2(k2-C,N-C6H4-2-CHQNR)}2] which contain the stable fac-PtC3
coordination and the most symmetrical arrangement of pyramidal
Fig. 1 Views (30% probability ellipsoids) of the structures of (a) complex 6a,
symmetry equivalent: 2 ꢀ x, 1 ꢀ y, 1 ꢀ z; (b) complex 7a, symmetry equivalent:
1 ꢀ x, ꢀy, 1 ꢀ z; (c) complex 7b.
Pt2(m-SMe)2 groups. The complexes all appear to give rapid inversion Equilibration of 6a in solution with other isomers was slow at
at sulfur so that, although there are more isomers that differ from room temperature. Complex 6a gives a single MeS resonance,
those shown by having different arrangements of the SMe groups, and one other isomer also gave only one MeS resonance and so
these are not observed in the NMR spectra.3,9 For a similar reason, this could be assigned as 6d which has C2 symmetry. The third
the isomer 6b or 7b with only C1 symmetry, has effective C2 isomer, which gives two MeS resonances, is therefore 6b or 6c;
symmetry because of the rapid inversion at sulfur. From this structure 6c is tentatively assigned. Complex 7 was formed as a
analysis, the 1H NMR spectrum of each isomer should contain mixture of all four possible isomers (Scheme 1, initial ratio
two methylplatinum resonances and one imine resonance, and the 7a : 7b : 7c : 7d ca. 3 : 2 : 1 : 2). The complex crystallized to give a
isomers with Ci and C2 symmetry should give one methylsulfur mixture of the Ci and C1 isomers, 7a and 7b, whose structures
resonance while those with Cs or C1 symmetry should give two were determined (Fig. 1). Complex 7b has the syn conformation
methylsulfur resonances. A complete solution to the assignment of the Pt2(m-SMe)2 unit, whereas 6a and 7a have the anti
was possible only after structure determinations were obtained conformation. Once the 1H NMR spectra of 7a and 7b were
(Fig. 1).
known, the remaining resonances of the initial product mixture
Complex 6 was formed as a mixture of three isomers [initial could be assigned to 7c and 7d. We note that the MeS groups in
ratio 6a : 6c : 6d ca. 3 : 2 : 3], but crystallisation gave isomer 6a, the Ci and C2 isomers are trans to one methyl and one aryl
which has Ci symmetry.‡ The 1H NMR spectrum of 6a was fully group, whereas those in the Cs and C1 isomers (Scheme 1) are
consistent with the structure 6a, and could be fully assigned. trans to either two methyl groups or two aryl groups, but the
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could be formed by combination of two units of either 8b or 9b,
while complexes 7b and 7c could be formed by combination of
8b with 9b. The complexes all contain chiral centres and 7a or
7d could be formed from two units of 8b or 9b in the AC, CA or
CC, AA enantiomeric forms respectively (C = clockwise, A =
anticlockwise), while 7b or 7c could be formed from 8b and 9b
in the AC, CA or CC, AA enantiomeric forms respectively.
This work shows that easy activation of aryl–sulfur bonds
can occur if the C–S bond is arranged so that directed metalla-
tion is favoured. A theoretical study indicates that the key step
occurs by pivoting of the aryl group in the flexible 6-membered
chelate ring of complex 3b to allow concerted, non-polar
oxidative addition to platinum(II). It is likely that oxidative
addition of aryl–halogen bonds to platinum(II) occurs in a
similar way, though the intermediates analogous to 3b cannot
be detected.7,8 Catalytic cross-coupling using aryl–sulfur bonds
might be possible using similar bond activation strategies.
Notes and references
‡ X-ray data: 6a, C28H48N4Pt2S2, fw 895.00, monoclinic, P21/c, a =
9.525(3), b = 15.830(5), c = 11.853(4), b = 118.167(10), V = 1575.6(9),
Z = 2, T = 110 K, reflns total 5890, R1 (all data) 0.0341, wR2 (all data)
%
0.0399; 7a, C32H38N2Pt2S2, fw 904.94, triclinic, P1, a = 8.461(5), b =
9.198(6), c = 11.156(5), a = 93.302(8), b = 109.27(3), g = 108.338(14), V =
765.2(8), Z = 1, T = 150 K, reflns total 6292, R1 (all data) 0.0367, wR2 (all
data) 0.0507; 7b, C32H38N2Pt2S2, fw 904.94, monoclinic, Cc, a =
20.627(6), b = 9.269(3), c = 16.440(4), b = 111.186(9), V = 2930.7(14),
Z = 4, T = 110 K, reflns total 9219, R1 (all data) 0.0275, wR2 (all data)
0.0464, flack = ꢀ0.004.
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Scheme 2 Proposed mechanism of formation of complexes 7a–7d (above) and
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3
separate couplings were not resolved, with all values of J(PtH)
in the narrow range 19–21 Hz, and the MeS resonances appear
as 1 : 8 : 18 : 8 : 1 multiplets due to coupling to 195Pt.
Although the product mixtures were complex, the isomers
are likely to be formed by a common mechanism as shown in
Scheme 2, illustrated for the case with R = Ph. The 6-membered
chelate ring in 3b is flexible and the aryl group can slide from
sulfur to platinum by concerted oxidative addition to give the
5-coordinate square pyramidal platinum(IV) complex 8b, in
which the thiolate is cis to the aryl group. The activation energy
is calculated by DFT to be 38 kJ molꢀ1, when R = Ph. The sulfur
atom moves only slightly during this step of the reaction. 8 T. Calvet, M. Crespo, M. Font-Bardia, S. Jansat and M. Martinez,
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isomer 9b, in which the thiolate is trans to the aryl group and
9 E. W. Abel, S. K. Bhargava and K. G. Orrell, Prog. Inorg. Chem., 1984,
which is calculated to be more stable. Complexes 7a and 7d
32, 1; K. G. Orrell, Coord. Chem. Rev., 1989, 96, 1.
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