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

6-Alkyl-2,2¢-bipyridines, HL, (N2C10H7R; R ) CH2Me, HLet; CHMe2, HLip; CMe3, HLtb, CH2-
CMe3, HLnp, CMe2Ph, HLdm) react with Na2[PdCl4] to give either 1:1 adducts [Pd(HL)Cl2]
(HLet, HLip, HLnp) or cyclometalated complexes [Pd(L)Cl] (HLtb, HLdm). Reaction of palladium-
(II) acetate, followed by exchange 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
direct activation of a C(sp3)-H or a C(sp2)-H bond. The structures of [Pd(Ltb)Cl] and [Pd-
(Lnp)Cl], which contain a [5,5] or a [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)(CH2)]}Cl], the latter one arising from activation of both methyl groups of the
substituent. The isolation of two [Pd(Ldm)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.