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Fig. 1 Molecular structure of compound 3a showing 50% probability
ellipsoids.
5 G. Dyker, Angew. Chem., Int. Ed., 1999, 38, 1698.
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All compounds were characterised by elemental analyses, mass
spectra, and NMR spectroscopy,13 and compound 3a was also
characterised crystallographically (see Fig. 1).14 In both the H
1
NMR spectra of compounds 3a and 3b, in addition to the main set
of signals, resonances due to minor isomer (abundance ca. 20%
(3a) and 30% (3b)) with the same spectral pattern were observed.
Studies concerning activation of aromatic C–H bonds of toluene4
indicate a selectivity order C–H (meta) . C–H (para) . C–H
(ortho); therefore, the major isomer could be assigned to that with
a meta arrangement between the Pt–C bond and the methyl
substituent (as in the crystal structure of 3a) and the minor isomer
could correspond to the para isomer. A singlet resonance coupled
to platinum at 6.63 (3a) or 6.56 ppm (3b) corresponding to the
major isomer is assigned to the aromatic proton adjacent to the
Pt–C bond.
8 J. Vicente, J. A. Abad, M. J. Lo´pez-Sa´ez and P. G. Jones, Angew.
Chem., Int. Ed., 2005, 44, 6001.
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J. Organomet. Chem., 2002, 650, 157; (b) S. Perez, C. Lopez, A. Caubet,
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Organometallics, 2002, 21, 3305.
In conclusion, the formal insertion of toluene, a typical
‘‘innocent’’ solvent, into platinum–carbon bonds affords seven-
membered metallacycles in the reaction of cis-[PtCl2(dmso)2] with
imines 1a and 1b. This process is an example of intermolecular
C–H activation leading to C–C coupling to produce a metallated
biphenyl. This result can be related to the Shilov mechanism for
platinum(II) mediated C–H bond activation which requires
oxidation of platinum(II) to platinum(IV) and indeed a recent
review2 pointed out that cyclometallated compounds could be
promising candidates for developing C–H activation chemistry of
platinum.
11 M. Crespo, M. Font-Bard´ıa and X. Solans, Organometallics, 2004, 23,
1708.
12 E. Rotondo, A. Giannetto and S. Lanza, J. Organomet. Chem., 1990,
396, 115.
13 Experimental procedures and characterization data for all the
new compounds reported here are given in the Supporting
Information.{ In the 1H NMR spectra of 2a, 2b, 2e, 3a, 3b, 4c and
4d, the coupling to platinum observed for the imine and the methylene
protons as well as for the aromatic hydrogen adjacent to the metallation
site are diagnostic of formation of a bidentate [C,N] chelate system.
The J(H–Pt) values for the imine in the range 112.8–116.8 Hz for 3a, 3b,
4c and 4d or in the range 50.0–52.7 for 2a, 2b and 2e indicate,
respectively, a conformation E or Z around the CLN bond. For 3a
and 3b, the methyl groups of the dimethylsulfoxide ligand and the
methylene protons appear as non-equivalent, consistent with the lack of
planarity of the metallacycle. The presence of the methyl substituent in
the tolyl group is confirmed by the corresponding signal in the 1H NMR
spectra.
Further work aimed at clarifying the toluene insertion described
here, and expanding the process to other aromatic hydrocarbons is
currently in progress.
We thank the Ministerio de Ciencia y Tecnolog´ıa (project BQU
2003-00906) for financial support.
14 CCDC-299686 contain the supplementary crystallographic data for
compound 3a: C23H23Cl2NOPtS, Mw = 627.47, monoclinic, space
˚
˚
˚
group P21/a, a = 10.301 (5) A, b = 22.018(9) A, c = 10.850 (4) A, b =
109.34u, V = 2322.0 (17) A3, Z = 4, m = 6.337 mm21, number of
˚
Notes and references
reflections collected/unique = 18892/5735 [R(int) = 0.0605], final R
indices [I . 2s(I)] R1 = 0.0281, wR2 = 0.0724, R indices (all data) R1 =
0.0410, wR2 = 0.0921, GOF = 1.191. The methyl H atoms at C23 were
generated using the SHELXL HFIX 33 command and their orientation
may not be totally reliable.
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4130 | Chem. Commun., 2006, 4128–4130
This journal is ß The Royal Society of Chemistry 2006