Reactivity of 6-(2-tolyl)- and 6-(2,6-xylyl)-2,2′-bipyridines with palladium(II) derivatives. Selective C(sp3)H vs. C(sp2)H activation

Two new 6-substituted 2,2′-bipyridines, L, 6-(2-tolyl)bipy, L1, and 6-(2,6-xylyl)bipy, L2, have been synthesized. Their reactions with Na2[PdCl4] or {Pd(OAc)2} afford either 1:1 adducts [Pd(L)X2] (X=Cl, OAc) or five-membered cyclometallated derivatives [Pd(L1-H)X] arising from C(sp2)H activation. From the chloro-alkyl intermediates [Pd(L)(Me)Cl], in the presence of Na[BAr′4] (Ar′=3,5-(CF3)2C6H3), cationic species [Pd(L)(Me)(S)]+ (L=L1, L2; S=CH3CN) can be obtained. At variance, in less coordinating solvents, e.g. dichloromethane, unexpected activation of a C(sp3)H bond occurs with loss of methane, to afford 6-membered cyclometallated derivatives. The latter species were isolated as [Pd(L-H)(PPh3)][BAr′4].