Pathways to Metal–Ligand Cooperation in Quinoline-Based Titanium(IV) Pincers: Nonelectrophilic N‑methylation, Deprotonation, and Dihydropyridine Formation

A series of titanium­(IV) complexes stabilized by quinoline-based pincer ligands have been synthesized and characterized. The reaction of [TiCp*Me3] with 8-hydroxy-2-quinolinecarboxaldehyde results in [TiCp*Me­{κ3-N,O,O-(OCH)­(8-O-N-Me-quin)}] (1), which shows an uncommon N-methylation/dearomatization of the pyridinic ring; in contrast, when 8-hydroxy-2-quinolinemethanol reacts with the same Ti­(IV) trimethyl derivative, the expected monoalkyl complex [TiCp*Me­{κ3-N,O,O-(OCH2)­(8-O-quin)}] (2) is formed. The pincer ligand in 2 can be dearomatized by deprotonation of the methylene fragment, yielding [TiCp*Me­{κ3-N,O,O-(OCH)­(8-O-quin)}]­[Li­(Et2O)] (3·Et2O) and [TiCp*Me­{k3-N,O,O-(OCH)­(8-O-quin)}]­[Li­(Py)2] (3·2Py), or by incorporation of a hydride group into the para position of the pyridinic ring, giving [TiCp*Me­{κ3-N,O,O-(OCH2)­(4-H-8-O-quin)}]­[Li·THF] (5). Compounds 1, [TiCp*­{κ3-N,O,O-(OCH2)­(4-H-8-O-quin)}]2 (6), and [TiCp*­{κ3-N,O,O-(OCH2)­(4-H-8-O-quin)}]2[Li·THF]2(μ-O)] (7) have been studied by X-ray diffraction. Additionally, DFT quantum mechanical calculations were performed on complexes 1 and 2.