Cationic, neutral, and anionic platinum(II) complexes based on an electron-rich PNN ligand. New modes of reactivity based on pincer hemilability and dearomatization

The synthesis and reactivity of new Pt(II) complexes, including anionic d⁸ complexes, based on the electron-rich, hemilabile PNN-type pincer ligand C₅H₃N-2-(CH₂P^tBu ₂)(CH₂NEt₂) are described. Formation of these complexes involves dearomatization/aromatization processes of the ligand. The chloride complex [(PNN)PtCl]⁺Cl^- (1) was prepared and reacted with the base ^tBuOK to give the deprotonated, neutral chloride complex (PNN*)PtCl (2) (PNN* = C₅H ₃N-2-(CHP^tBu₂)(CH₂NEt₂)). Reaction of 2 with ⁿBuLi gave the corresponding neutral hydride complex (PNN*)PtH (3), which was readily protonated by triflic acid to give the cationic hydride complex [(PNN)PtH]⁺OTf^- (4). Unexpectedly, reaction of complex 2 with 1 equiv of RLi resulted in opening of the chelate ring, to give the corresponding anionic, dearomatized complexes Li⁺[(PNN*)Pt(Cl)(R)]^- (R = methyl, 5; phenyl, 6). Notably, these complexes are relatively stable although they bear no stabilizing π acceptors that can lower the electron density at the metal center. Complexes 5 and 6 readily undergo protonation by HCl to form the corresponding neutral, aromatic complexes (PNN)Pt(Cl)(R) (R = methyl, 7; phenyl, 8), in which the hemilabile amine "arm" remains decoordinated and does not undergo protonation. Minor amounts of the dearomatized chloride complex 2 are also formed as a result of elimination of RH. Reaction of complexes 5 and 6 with water results in selective protonation-aromatization to give the corresponding complexes 7 and 8.