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)][BF 4] 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. © 2013 American Chemical Society.