Self-assembly of dinuclear CH2-bridged titanium(IV)/catecholate complexes: Influence of the counterions and of methyl substituents in the ligand

The spontaneous self-assembly of methylene-bridged dinuclear Ti-IV complexes M-4[L3Ti2] (L = 1-3) from CH2-bridged di(catechol) ligands (1-H-4-3-H-4) was found to proceed in the presence of lithium or sodium carbonate acting as a base (M = Li, Na). In contrast, only an unspecific mixture of coordination compounds was obtained in the presence of potassium carbonate. This difference in behavior is due to the ability of Li+ or Na+ to stabilize the triple-stranded dinuclear titanium(IV) species [L3Ti2](4-). In the solid state, Li-4[(1)(3) Ti-2] exhibits a highly symmetric structure with three lithium cations bound to the meso-helicate tetraanion. Li-6 NMR together with H-1 NMR studies at variable temperature revealed that, in solution, an unsymmetrical ''bowl-shaped'' species is formed with only two Li+ bound to the anion. A methyl substituent on the methylene spacer of the di(catechol) ligand (ligand 2) led to supramolecular systems with new stereocenters and thus a higher content of information. However, only one of the four possible diastereoisomeric metal complexes of [(2)(3) Ti-2](4-) was observed. A third type of ligand, with a methyl group attached to one terminus of the linear ligand, was also investigated. The synthesis of such an unsymmetrical ligand, 3-H-4, was readily achieved starting from 2,3-dimethoxybenzyl alcohol (4) and 2,3-dimethoxybenzaldehyde (6). Upon complexation, the ligand 3-H-4, led to a statistical mixture of the two possible isomers of the dinuclear titanium(IV)complex [(3)(3) Ti-2](4-).