Metallation of unactivated methyl groups. Platinum(II) derivatives with 6-alkyl-2,2’-bipyridines

The reaction of K2[PtCl4] with three 6-alkyl-2,2-bipyridines
HL, (N2C10H7R; R = CH2Me, HLet; CHMe2, HLip; CH2CMe3,
HLnp) affords five- and six-membered cyclometallated complexes
[Pt(L)Cl] where L is a terdentate N,N,C anionic ligand
which originates from HL through direct activation of a
C(sp3)−H bond. In the case of HLip, metallation generates a
stereogenic carbon atom β to the metal. Complex 5,
[Pt(Lnp)Cl], having an uncommon six-membered ring containing
a Pt−C(sp3) bond, was studied in depth in solution, by
means of 1- and 2- dimensional NMR spectroscopy, and in
the solid state by X-ray diffraction. Differences are observed
in the behaviour of these ligands: thus with HLet an adduct,
[Pt(HLet)Cl2], was also isolated and its structure was determi-ned by X-ray diffraction. In the metallated species, the fourth
ligand, Cl, can easily be substituted by neutral or anionic
ligands (CO, PPh3, CN−). The metallated species react with
potentially bidentate ligands L−L [Ph2PCH2PPh2 (dppm),
Ph2P(CH2)2PPh2 (dppe), Ph2P(CH2)2AsPh2 (dppae)] to give,
with different Pt:L ratios, mononuclear [Pt(L)(L−L)]+ or dinuclear
[(L)Pt(μ−L−L)Pt(L)]++ cationic species. In the second
case an uncommon unsupported L−L bridge is present.
The cyclometallated species are stable in air and moisture,
and the N,N,C system is rather robust