Hierarchical Assembly of Helicate-Type Dinuclear Titanium(IV) Complexes

The ligands 4−7-H2 were used in coordination studies with titanium(IV) and gallium(III) ions to obtain dimeric complexes Li4[(4−7)6Ti2] and Li6[(4/5a)6Ga2]. The X-ray crystal structures of Li4[(4)6Ti2], Li4[(5b)6Ti2], and Li4[(7a)6Ti2] could be obtained. While these complexes are triply lithium-bridged dimers in the solid state, a monomer/dimer equilibrium is observed in solution by NMR spectroscopy and ESI FT-ICR MS. The stability of the dimer is enhanced by high negative charges (Ti(IV) versus Ga(III)) of the monomers, when the carbonyl units are good donors (aldehydes versus ketones and esters), when the solvent does not efficiently solvate the bridging lithium ions (DMSO versus acetone), and when sterical hindrance is minimized (methyl versus primary and secondary carbon substituents). The dimer is thermodynamically favored by enthalpy as well as entropy. ESI FT-ICR mass spectrometry provides detailed insight into the mechanisms with which monomeric triscatecholate complexes as well as single catechol...