Self‐Assembly of Dinuclear CH 2 ‐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 [L 3 Ti 2 ] (L = 1–3 ) from CH 2 ‐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 [L 3 Ti 2 ] 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. 6 Li NMR together with 1 H 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‐dimethoxyben‐zyl alcohol ( 4 ) and 2,3‐dimethoxybenz‐aldehyde ( 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‐ .