Going Up the Ladder: Stacking Four 4,4'-Bipyridine Moieties within a Ti(IV)-Based Tetranuclear Architecture

Biphenol-based ligands have proven their ability to bind titanium(IV) centers and generate sophisticated self-assembled structures in which auxiliary nitrogen ligands often complete the coordination sphere of the metal and improve stability. Here, a central 4,4'-bipyridine, which acts as both a spacer and a source of monodentate nitrogen to complete the coordination sphere of the Ti(IV) complex, was incorporated within two bis-2,2'-biphenol strands, 3H₄ and 4H₄. Both proligands possess structural features that are well adapted to form self-assembled structures built from titanium-oxygen-nitrogen units; however, their different degrees of torsional freedom strongly influenced the nuclearity of the complexes formed. The presence of a phenyl spacer between the bipyridine and the biphenol moieties of 3H₄ provided enough flexibility for the ligand to wrap around one titanium(IV) center to form a mononuclear complex Ti(3)(DMF)₂ in the presence of dimethylformamide (DMF). Assembly of the more rigid ligand 4H₄ with Ti(OiPr)₄ afforded a tetranuclear complex Ti₄(4)₂(4H)₂(OEt)₂ containing four stacked 4,4'-bipyridine units as shown by the X-ray structure of the complex. Density functional theory studies suggested that the assembly of this tetrametallic complex involves a dimetallic intermediate with TiO₆ nodes that is converted to the thermodynamically stable tetranuclear complex with two TiO₆ nodes and two TiO₅N units with enhanced covalent character.