Rollover Cyclometalation with 2‑(2′-Pyridyl)quinoline

Rollover cyclometalation of 2-(2′-pyridyl)­quinoline, L, allowed the synthesis of the family of complexes [Pt­(L-H)­(X)­(L′)] and [Pt­(L*)­(X)­(L′)]­[BF4] (X = Me, Cl; L′ = neutral ligand), the former being the first examples of Pt­(II) rollover complexes derived from the ligand L. The ligand L* is a C,N cyclometalated, N-protonated isomer of L, and can also be described as an abnormal-remote pyridylene. The corresponding [Pt­(L-H)­(Me)­(L′)]/[Pt­(L*)­(Me)­(L′)]+ complexes constitute an uncommon Brønsted–Lowry acid–base conjugated couple. The species obtained were investigated in depth through NMR and UV–vis spectroscopy, cyclic voltammetry, and density functional theory (DFT) methods to correlate different chemico-physical properties with the nature of the cyclometalated ligand (e.g., L vs bipy or L* vs L) and of the neutral ligand (DMSO, CO, PPh3). The crystal structures of [Pt­(L-H)­(Me)­(PPh3)], [Pt­(L-H)­(Me)­(CO)] and [Pt­(L*)­(Me)­(CO)]­[BF4] were determined by X-ray powder diffraction methods, the latter being the first structure of a Pt­(II)-based, protonated, rollover complex to be unraveled. The isomerization of [Pt­(L*)­(Me)­(PPh3)]+ in solution proceeds through a retro-rollover process to give the corresponding adduct [Pt­(L)­(Me)­(PPh3)]+, where L acts as a classical N,N chelating ligand. Notably, the retro-rollover reaction is the first process, among the plethora of Pt–C bond protonolysis reactions reported in the literature, where a Pt–C­(heteroaryl) bond is cleaved rather than a Pt–C­(alkyl) one.