Cationic, Neutral, and Anionic PNP PdII and PtII Complexes: Dearomatization by Deprotonation and Double-Deprotonation of Pincer Systems

A series of cationic, neutral, and anionic Pd-II and Pt-II PNP (PNP = 2,6-bis-(di-tert-butylphosphinomethyl)pyridine) complexes were synthesized. The neutral, dearomatized complexes [(PNP*)MX] (PNP* = deprotonated PNP; M = Pd, Pt; X = Cl, Me) were prepared by deprotonation of the PNP methylene group of the corresponding cationic complexes [(PNP)MX][Cl] with 1 equiv of base (KN(SiMe3)(2) or (BuOK)-Bu-t), while the anionic complexes [(PNP**)MX]Y--(+) (PNP** = double-deprotonated PNP; Y = Li, K) were prepared by deprotonation of the two methylene groups of the corresponding cationic complexes with either 2 equiv of KN(SiMe3)(2) or an excess of MeLi. While the reaction of [(PNP)PtCl][Cl] with an excess of MeLi led only to the anionic complex without chloride substitution, reaction of [(PNP)PdCl][Cl] with an excess of MeLi led to the methylated anionic complex [(PNP**)PdMe]Li--(+). NMR studies, X-ray structures, and density functional theory (DFT) calculations reveal that the neutral complexes have a broken aromatic system with alternating single and double bonds, and the deprotonated arm is bound to the ring by an exocyclic C=C double bond. The anionic complexes are best described as a pi system comprising the ring carbons conjugated with the exocyclic double bonds of the deprotonated arms. The neutral complexes are reversibly protonated to their cationic analogues by water or methanol. The thermodynamic parameters Delta H, Delta S, and Delta G for the reversible protonation of the neutral complexes by methanol were obtained.