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, HL^et; CHMe₂, HL^ip; CMe₃, HL^tb, CH₂CMe₃, HL^np, CMe₂Ph, HL^dm) react with Na₂[PdCl₄] to give either 1:1 adducts [Pd(HL)Cl₂] (HL^et, HL^ip, HL^np) or cyclometalated complexes [Pd(L)Cl] (HL^tb, HL^dm). 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(L^tb)Cl] and [Pd(L^np)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 HL^ip, three different cyclometalated species have been isolated, [Pd(L)Cl], [Pd(L)Cl]₂, and [Pd{N₂C₁₀H₇[CH(CH₂OC(O)CH₃)(CH₂)]}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.