Palladium N(CH2CH2PiPR2)2-Dialkylamides: Synthesis, Structural Characterization, and Reactivity

Palladium(II) aminodiphosphine PNP pincer complexes [PdR(PNPH]PF6 (1(R); R = Cl Me, Ph; PNPH = HN((CH2CH2PPr2)-Pr-i)(2)) were prepared. Deprotonation with (KOBu)-Bu-t affords dialkylamides [PdR(PNP)] (2(R); R = Cl Me, Ph; PNP = ((NCH2CH2PPr2)-Pr-i)(2)) in high yield which are stable toward beta-H elimination. While AgPF6 oxidizes the amides, cationic amido complexes [PdL(PNP)]PF6 (3(L); L = (CNBu)-Bu-t, PMe3) were obtained upon chloride abstraction from 1(Cl) with TIPF6. The reaction of amide 2(Cl) with MeOTf results in N-methylation yielding [PdCl(PNPMe)]OTf (5) quantitatively. N-H acidities of the amino complexes 1(Me) (pK(a) = 24.2(1)) and 1(Ph) (pK(a) = 23.2(1)) were determined in dmso. Complexes 1(Cl), 1(Me), 2(Cl) 2(Me), 3(CNf8u), and 5 were structurally characterized by single crystal X-ray diffraction. The amido complexes feature pyramidal nitrogen atoms in the solid state. The molecular structures, high N-basicity, and reactivity of the amido complexes can be explained with Pd-N-amido bonding that is characterized by strong N -> Pd sigma-donation and repulsive d(pi)-p(pi) pi-interactions. This interpretation was confirmed by density functional theory (DFT) calculations of 2(Cl).