American Chemical Society
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Cyclopalladation of 6-Substituted-2,2‘-bipyridines. Metalation of Unactivated Methyl Groups vs Aromatic C−H Activation

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journal contribution
posted on 2000-09-09, 00:00 authored by Antonio Zucca, Maria Agostina Cinellu, Maria Vittoria Pinna, Sergio Stoccoro, Giovanni Minghetti, Mario Manassero, Mirella Sansoni
6-Alkyl-2,2‘-bipyridines, HL, (N2C10H7R; R = CH2Me, HLet; CHMe2, HLip; CMe3, HLtb, CH2CMe3, 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(CH2OC(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.