Formation of triple-stranded dinuclear helicates with dicatecholimine ligands: The influence of steric hindrance at the spacer

A series of new imine-bridged dicatechol ligands 3a-f-H-4 with sterically demanding groups at the spacers are used for the formation of titanium(IV) complexes M-4[(3)(3)Ti-2]. All three ligands 3a-c-H-4 form triple-stranded dinuclear helicates. When the bulky ligands 3a-H-4 or 3c-H-4 are used with potassium as the countercation, oligomeric or polymeric side products are also observed. The imine-bridged ligand 3e-H-4 quantitatively forms helicates M-4[(3e)(3)Ti-2] and not a M4L6 tetrahedron as observed with Raymond's analogous amide-bridged dicatechol ligand 3i-H-4. NMR spectroscopic investigations at variable temperature show that ligand 3f-H-4, which possesses a spiro fluorenyl group at the central unit of the spacer, forms the meso-helicate M-4[(3f)(3)Ti-2]. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).