Commercial 4,4′-bipyridine is a popular scaffold that is primarily employed as a linker in 3D self-assembled architectures such as metallo-organic frameworks or as a connector in 2D networks. The introduction of alkyl substituents on the bipyridine skeleton is instrumental when 4,4′bipyridines are used as linkers to form 2D self-assembled patterns on surfaces. Here, several synthetic strategies to access 4,4′-bipyridines functionalized at various positions are described. These easily scalable reactions have been used to introduce a range of alkyl substituents at positions 2 and 2′ or 3 and 3′ and at positions 2,2′ and 6,6′ in the case of tetra-functionalization. Scanning tunneling microscopy studies of molecular monolayers physisorbed at the graphite−solution interface revealed different supramolecular patterns whose motifs are primarily dictated by the nature and position of the alkyl chains.
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, 3H 4 and 4H 4 . 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 3H 4 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 4H 4 with Ti(OiPr) 4 afforded a tetranuclear complex Ti 4 (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 TiO 6 nodes that is converted to the thermodynamically stable tetranuclear complex with two TiO 6 nodes and two TiO 5 N units with enhanced covalent character.
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