Cyclopalladation of 6-substituted-2,2 '-bipyridines. Metalation of unactivated methyl groups vs aromatic C-H activation

6-Alkyl-2,2'-bipyridines, HL, (N2C10H7R; R = CH2Me, HLet; CHMe2, HLip; CMe3 , HLtb, CH2CMe3, HLnp, CMe2Ph, HLdm) react with Na-2[PdCl4] to give either 1:1 adducts [Pd(HL)Cl-2] (HLet, HLip, HLnp) or cyclometalated complexes [Pd (L)Cl] (HLtb, HLdm). Reaction of palladium(II) acetate, followed by exchang e with LiCl, affords a series of cyclopalladated species [Pd(L)Cl] where L is a terdentate anionic N-N-C ligand which originates from HL through direc t activation of a C(sp(3))-H or a C(sp(2))-H bond. The structures of [Pd(L- tb)Cl] and [Pd(L-np)Cl], which contain a [5,5] ora [5,6] fused ring system, respectively, have been determined by X-ray diffraction and are compared. In the case of the ligand HLip, three different cyclometalated species have been isolated, [Pd(L)Cl], [Pd(L)Cl](2), and [Pd{N2C10H7[CH(CH2-OC(O)CH3)(C H2)}Cl] the latter one arising from activation of both methyl groups of the substituent. The isolation of two [Pd(L-dm)Cl] species (compounds 8 and 9) , having an aromatic or an aliphatic carbon-metal bond, respectively, is an example of isomerism, still rare in organometallic chemistry.