N^N^C platinum(II) and palladium(II) cyclometallates of 6,6′-diphenyl-2,2′-bipyridine, L: Crystal and molecular structure of [Pd(L–H)Cl]

Reaction of K2[PtCl4] or Na2[PdCl4] with 6,6′-diphenyl-2,2′-bipyridine, L, gives the cyclometallated species [Pt(L–H)Cl], 1, and [Pd(L–H)Cl], 2, respectively, where L–H is a terdentate N^N^C anionic ligand originated by direct activation of a C(sp2)–H bond. The crystal structure of 2 has been solved by X-ray diffraction and compared to that of the analogous complex [Pd(L′–H)Cl] L′ = 6-phenyl-2,2′-bipyridine. The second phenyl ring in 2 entails a considerable distortion of the coordination around the metal. A similar distortion is also to be expected in the analogous compound 1, due to the almost equal covalent radii of palladium(II) and platinum(II). From the complexes 1 and 2 the chloride can be displaced with AgBF4 and substituted by CO or PPh3 to give the corresponding cationic species. By reaction of 1 with Na[BH4] substitution of H− for Cl− can be achieved: the rare hydrido complex [Pt(L–H)H], stabilized only by nitrogen ligands, was isolated in the solid state and fully characterized in solution. It is noteworthy that in the case of the 6-phenyl-2,2′-bipyridine the analogous terminal hydride [Pd(L′–H)H] is unstable. In platinum chemistry the reaction of 6-substituted 2,2′-bipyridines is known to give either N^N^C or N′^C(3) rollover cyclometallation, depending on the nature of the metal precursor. In the case of 6,6′-Ph2-2,2′-bipy cyclometallation was also shown to undergo multiple C–H activation giving the C^N^C pincer complex [Pt(L-2H)(DMSO)]. The latter species can be related to complex 1: indeed its reaction with HCl produces complex 1 and [Pt(L–H)(DMSO)Cl], a rollover species with a pendant phenyl substituent.