Stereocontrol in Dinuclear Triple Lithium-Bridged Titanium(IV) Complexes: Solving Some Stereochemical Mysteries

• Published in: Chemistry : a European journal Vol. 20; no. 22; pp. 6650 - 6658
• Main Authors: Albrecht, Markus, Isaak, Elisabeth, Moha, Verena, Raabe, Gerhard, Fröhlich, Roland
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 26.05.2014; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: Chemistry; circular dichroism; coordination chemistry; Dichroism; Dimers; Esters; Helicity; Methyl alcohol; Monomers; self-assembly; Solvents; Stereochemistry; titanium; stereochemistry; self-assembly; circular dichroism; coordination chemistry; titanium
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201402370

Compounds 1 a–f‐H2 form “monomeric” triscatecholate titanium(IV) complexes [Ti(1 a–f)3]2−, which in the presence of Li cations are in equilibrium with the triple lithium‐bridged “dimers” [Li3(Ti(1 a–f)3)2]−. The equilibrium strongly depends on the donor ability of the solvent. Usually, in solvents with high donor ability, the stereochemically labile monomer is preferred, whereas in nondonor solvents, the dimer is the major species. In the latter, the stereochemistry at the complex units is “locked”. The configuration at the titanium(IV) triscatecholates is influenced by addition of chiral ammonium countercations. In this case, the induced stereochemical information at the monomer is transferred to the dimer. Alternatively, the configuration at the metal complexes can be controlled by enantiomerically pure ester side chains. Due to the different orientation of the ester groups in the monomer or dimer, opposite configurations of the triscatecholates were observed by circular dichroism (CD) spectroscopy for [Ti(1 c–e)3]2− or [Li3(Ti(1 c–e)3)2]−. A surprising exception was found for the dimer [Li3(Ti(1 f)3)2]−. Herein, the dimer is the dominating species in weak donor (methanol), as well as strong donor (DMSO), solvents. This is due to the bulkiness of the ester substituent destabilizing the monomer. Due to the size of the substituent in [Li3(Ti(1 f)3)2]− the esters have to adopt an unusual conformation in the dimer resulting in a stereocontrol of the small methyl group. Following this, opposite stereocontrol mechanisms were observed for the central metal‐complex units of [Li3(Ti(1 c–e)3)2]− or [Li3(Ti(1 f)3)2]−. Supramolecular stereochemistry: Unpredictable seems to be the stereochemical induction of helicity at hierarchically assembled bis(triscatecholate) titanium(IV) helicates. However, it is based on a subtle interplay of preferred conformations and sterical constraints (see figure).