Activation of C-H bond by cis-[Pt(NO3)2(PPh 3)2], cyclometallation of tetramethylthiourea

Article abstract of DOI:10.1016/j.inoche.2003.09.024

Reaction of cis-[Pt(NO₃)₂(PPh₃) ₂] with tetramethylthiourea (tmtu) affords [Pt(PPh₃) ₂(tmtu*)] NO₃, in which tmtu* is tetramethylthiourea deprotonated at one methyl group. An X-r

Full text of DOI:10.1016/j.inoche.2003.09.024

Inorganic Chemistry Communications 7 (2004) 97–100
www.elsevier.com/locate/inoche
Activation of C–H bond by cis-[Pt(NO3)2(PPh3)2],
cyclometallation of tetramethylthiourea
a
b,
a,
*
Serena Fantasia , Mario Manassero *, Alessandro Pasini
a
Dipartimento di Chimica Inorganica Metallorganica e Analitica, via G. Venezian 21, 20133 Milano, Italy
b
Dipartimento di Chimica Strutturale e Stereochimica Inorganica, via Venezian 21, 20133 Milano, Italy
Received 16 July 2003; accepted 24 September 2003
Published online: 14 November 2003
Abstract
Reaction of cis-[Pt(NO3)2(PPh3)2] with tetramethylthiourea (tmtu) affords [Pt(PPh3)2(tmtu*)]NO3, in which tmtu* is tetrame-
thylthiourea deprotonated at one methyl group. An X-ray investigation has shown that tmtu* is cyclometallated with Pt–S and
Pt–CH2 bonds. The likely role of dissociation of the labile nitrato ligands, to give cationic intermediates, is outlined.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Cyclometallated tetramethylthiourea; Platinum-nitrato-phosphine complex; X-ray structure
1. Introduction
ment. d values in ppm are relative to Me4Si, H3PO4 and
[PtCl6]2ꢀ. cis-[Pt(NO3)2(PPh3)2] was prepared according
to [9].
Inter- and intramolecular activation of C–H bonds
on platinum(II) is a topic of great interest and contin-
uing investigations [1–4]. Here we present a case of such
an activation of a methyl group of tetramethylthiourea
(tmtu) on the cis-Pt(PPh3)2 species which gives rise to a
Pt(II) complex with cyclometallated tmtu* [tmtu*,
(CH3)2NC(S)N(CH3)CHꢀ2 ]. Although a crystallograph-
ically characterised Pt(IV) derivative of tmtu* has been
reported [5], to our knowledge this is the first example of
a Pt(II) complex with cyclometallated tmtu*.
2.2. Synthesis of bis-triphenilphosphinetetramethylthio-
ureato(C,S)platinum(II) nitrate
Equimolar amounts of tmtu (0.059 g, 0.44 mmol) and
cis-[Pt(NO3)2(PPh3)2] were dissolved in 30 ml of
CH2Cl2. The yellow solution was refluxed for 5 h until
the colour faded. Concentration under reduced pressure
gave 0.305 g (76%) of the product. Elemental analysis:
found: C, 54.0; H, 4.2; N, 4.3. C41H41N3O3P2PtS re-
quires: C, 53.9; H, 4.5; N, 4.6.
2. Experimental
2.3. X-ray crystallography
2.1. General
Crystal data: C43H43Cl6N3O3P2PtS, M ¼ 1151:66,
triclinic, space group P1 (no. 1), a ¼ 9:644ð1Þ, b ¼
All manipulations were carried out in the air. Sol-
vents and reagents were used as received. NMR spectra
were recorded on a Bruker Avance DRX 300 instru-
ꢀ
10:902ð1Þ, c ¼ 12:077ð1Þ A, a ¼ 74:99ð1Þ, b ¼ 85:46ð1Þ,
3
ꢀ
c ¼ 87:42ð1Þ°, U ¼ 1222:2ð2Þ A , z ¼ 1. Bruker SMART
Area Detector, x scan, T ¼ 296 K, full-matrix least
squares refinement on all reflections (9258, Friedel pairs
not merged), R2 ¼ 0:125 and R2w ¼ 0:139 on F 2, con-
ventional R1 ¼ 0:062 on 7200 reflections (I > 2rðIÞ). R2
of the enantiomorph ¼ 0.164.
* Corresponding authors. Tel.: +39-02-26680676; fax: +39-02-
2362748.
E-mail addresses: alessandro.pasini@unimi.it (S. Fantasia),
mario.manassero@unimi.it (M. Manassero).
1387-7003/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2003.09.024

98
S. Fantasia et al. / Inorganic Chemistry Communications 7 (2004) 97–100
The atomic co-ordinates of the structure model have
been deposited with the Cambridge Crystallographic
Data Centre.
An ORTEP view of the cation is shown in Fig. 1.
Selected interatomic distances and angles are reported in
the figure caption. The metal atom is four coordinate
with two cis phosphine groups, one sulfur atom and one
carbon atom of cyclometallate tmtu*. The coordination
geometry is square planar with a slight tetrahedral dis-
tortion, maximum displacements from the best plane
3. Results
ꢀ
3.1. General
being +0.106(12) and )0.103(3) A for C(2) and S, re-
spectively. The penta-atomic metallacycle is essentially
During our work on platinum(II) complexes with
sulfur donor ligands [6–9], we had the occasion of al-
lowing tmtu to react with cis-[Pt(NO3)2(PPh3)2] (1) in
CH2Cl2 or CHCl3. Whichever the tmtu/Pt molar ratio
ratio used (from 0.8 to 2.5), we obtained the same
compound 2, whose analysis was consistent with the
stoichiometry Pt/(PPh3)2/tmtu/NO3, with the nitrato
group anionic [m(NO3) 1380 cmꢀ1]. These results sug-
gested ionisation of tmtu, consequently we grew crystals
suitable for X-ray diffraction studies by diffusion of di-
isopropyl ether into a CHCl3 solution of 2. The crystals
were of poor quality, but we attempted a diffraction
study in order to elucidate the nature of the complex.
planar, with maximum distances from the best plane of
ꢀ
+0.051(12) and )0.068(10) A for C(2) and N(1), re-
spectively. The two Pt–P bond lengths are in the range
found for PPh3 trans to alkyl carbon [10] and to S-
coordinated thiourea [11–13]. Bond parameters involv-
ing the tmtuꢁ ligand can be compared with those found
in the octahedral Pt(IV) complex [Pt(tmtu*)2Br2], 3 [5].
ꢀ
Thepresent Pt–S bond length, 2.298(3) A is very similar
ꢀ
to those found in 3 (2.313(2) A for both tmtu*). On the
ꢀ
contrary, the Pt–C distance found here, 2.15(1) A, is
significantly longer than those found in 3, 2.031(7) and
ꢀ
2.045(9) A, probably because of the stronger trans-in-
fluence of phosphorus with respect to that of bromine
and/or the different oxidation state of Pt. The bites of
the chelating tmtu* ligands are 83.3(4)° here and 83.6(2)
and 86.2(2)° in compound 3.
3.2. Description of the structure of 2.2CHCl3
The structure consists of the packing of [Pt(PPh3)2
(tmtuꢁ)]þ cations, NO3ꢀ anions and CHCl3 molecules in
the molar ratio 1:1:2.
3.3. NMR spectral characterisation
The 31P NMR spectrum (CDCl3 solution) shows two
doublets with JP–P 22.9 Hz, centred at 17.14 ppm (JPt–P
,
2022 Hz) and 22.22 ppm (JPt–P 3462 Hz). The latter
doublet is assigned to P trans to S on the basis of liter-
ature data [11], the other is assigned to P trans to CH2.
Pt–P coupling constants were confirmed in the 195Pt
NMR spectrum (d – 4796.8 ppm, doublet of doublet).
There are three resonances attributable to the tmtu*
1
moiety in the H NMR spectrum (CDCl3 solution): at
3.02 ppm (s, 6H); 3.08 (s, 3H) and a complex signal
centred at 3,50 ppm (2H), which shows 195Pt satellites
(JPt–H 60.0 Hz). Upon irradiation of the 31P resonance at
17.1 ppm (P trans to CH2), such signal becomes a
doublet with a 4.98 Hz separation. A doublet, with 3.73
Hz separation, is also obtained by irradiation of the
resonance of the phosphorus trans to S (i.e. cis to CH2).
Coupling of the CH2 protons with both P atoms was
confirmed by an heteronuclear multiple quantum cor-
relation (HMQC) experiment which showed cross peaks
1
with both P resonances. Both 31P and H spectra are
unchanged in the temperature range 250–320 K.
The 13C NMR spectrum is interesting since all C at-
oms of tmtu* display coupling with the phosphorus at-
oms. Chemical shifts of tmtu* are: 43.5 ppm
(uncoordinated N(CH3)2, JP–C, 1.3 Hz); 46.0 (NCH3
adjacent to CH2,JP–C, 3.9 Hz; JPt–C, 52.0 Hz); 56.2 (CH2,
Fig. 1. ORTEP view of the [Pt(PPh3)2(tmtuꢁ)]þ cation. Ellipsoids are
drawn at the 30% probability level. Principal bond parameters are: Pt–
S 2.298(3), Pt–P(1) 2.293(3), Pt–P(2) 2.326(4), Pt–C(2) 2.15(1), S–C(1)
ꢀ
1.72(1), C(1)–N(1) 1.28(2), N(1)–C(2) 1.46(2) A, S–Pt–P(1) 171.5(1), S–
Pt–P(2) 89.5(1), S–Pt–C(2) 83.3(4), P(1)–Pt–P(2) 97.6(1), P(1)–Pt–C(2)
89.9(4), P(2)–Pt–C(2) 171.1(4), Pt–S–C(1) 100.3(4), S–C(1)–N(1)
120.6(9), C(1)–N(1)–C(2) 122(1)° Pt–C(2)–N(1) 112.3(9)°.
JP–C, 88.9 Hz; JPt–C, 540.9 Hz); 183.0 (C@S, JP–C, 18.2
Hz). Note that the CH2 group is coupled only to one P

S. Fantasia et al. / Inorganic Chemistry Communications 7 (2004) 97–100
99
atom, in contrast to the coupling with both P atoms
observed for the protons of such group. Note also the
small but clearly discernible J of the N(CH3)2 carbon
atoms, which also show barely detectable Pt satellites as
shoulders. No Pt–C coupling was observed for C ¼ S,
the signal is rather weak, however, and Pt satellites may
be buried in the background.
upon dissociation, give rise to cationic complexes, ca-
pable of C–H activation [15,16]. A likely reaction path is
depicted in Scheme 1. Substitution of one nitrato group
by tmtu-S gives intermediate 4, detected by NMR
spectroscopy (see above). Dissociation of the second
NOꢀ3 ligand can give rise to the (undetected) species 5
with an agostic C–H interaction. The high positive
charge of 5 will favour deprotonation of an NCH3
group to give 2 and HNO3.
4
3.4. Formation studies
Two main mechanisms have been proposed for the
activation of C–H bonds on Pt(II) [1–3,17]: oxidative
addition of C–H to give a transient Pt(IV)–H species,
which will eventually give the Pt(II) product by reduc-
tive elimination, and electrophilic activation of a g2-
C–H labile intermediate which could be formed by
dissociation of one nitrato group. Although our results
seem in accordance with the latter mechanism, the for-
mer cannot be ruled out, since the transient Pt(IV)–H
intermediate could be too short living to be detected.
Clearly more detailed studies are needed to wholly un-
derstand the mechanism of this reaction, as well as its
scope.
Formation of compound 2 was followed by 31P NMR
spectroscopy, (at 300 k). Immediately after the addition
of one equivalent of tmtu to a CHCl3 solution of the
nitrato complex 1, (d, 3.62 ppm, JPt–P, 4010 Hz [9]), a
new species, 4, which shows two doublets (JP–P, 19.5
Hz), at 1.70 ppm (JPt–P, 3830 Hz) and 18.95 ppm, (JPt–P
,
3225 Hz) starts to grow. We assign the two doublets to
cis-[Pt(NO3)(PPh3)2(tmtu-S)]þ, (with S-coordinated
tmtu) with the 1.70 signal due to P trans to ONO2, on
account of its chemical shift and Pt–P coupling constant
similar to that of the nitrato complex [9], while the 18.95
ppm doublet is assigned to P trans to S, in accordance
with literature data [11]. Coordination of tmtu is also
confirmed by the shift of the resonance of the CH3
1
protons, in the H NMR spectrum, from 3.09 ppm (in
Supplementary material
free tmtu) to 3.17 ppm (singlet). Both spectra show also
faint traces of the resonances of the product, which
becomes clearly visible after 20 min. No other signal,
including resonances attributable to hydrido species,
was detected.
All crystallographic data have been deposited with
the Crystallographic Data Centre, CCDC No. 222321.
If the reaction mixture is treated with water, its layer
is acidic, suggesting the formation of nitric acid whose
presence in the CH2Cl2 phase was confirmed by mass
spectrometry: injection of the CH2Cl2 solution into a
helium gas flow quadrupole mass spectrometer [14] gave
a signal at m/z 63.
Acknowledgements
We thank Dr Vladimiro Dal Santo, ISTM CNR,
Milano, for the mass spectra. This work was financed by
the Ministero dellÕIstruzione e dellÕUniversita.
ꢁ
4. Discussion
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