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
ligands exchange in the dimers. Tandem mass spectrometric experiments in the gas phase show the dimers
to decompose either in a symmetric (Ti) or in an unsymmetric (Ga) fashion when collisionally activated.
The differences between the Ti and Ga complexes can be attributed to different electronic properties and
a charge-controlled reactivity of the ions in the gas phase. The complexes represent an excellent example
for hierarchical self-assembly, in which two different noncovalent interactions of well balanced strengths
bring together eleven individual components into one well-defined aggregate.