“Induced Fit” in Chiral Recognition: Epimerization upon Dimerization in the Hierarchical Self‐Assembly of Helicate‐type Titanium(IV) Complexes

Abstract

Undecided! A titanium(IV) triscatecholate bearing chiral ester groups shows different CD spectra in methanol and DMSO. The observation of different stereoisomers lies in the preferred conformation of the side groups in the monomer and dimer, which leads to different chiral induction in the different species.

Pseudotetrahedral Zn(ii)-(R or S)-dihalogen-salicylaldiminato complexes with Λ- or Δ-chirality induction at-metal

“…36,38 Inversion of metal centered chirality by solvent polarity or pH of the solution was reported in Ni/ Ti/Re-complexes. [39][40][41] Similarly, a redox reaction (electrontransfer) induces chirality inversion at-metal in Co/Cu(II)complexes. [42][43][44] Our recent studies of X-ray structure determinations, 25,[29][30][31][32][33][34][35][36][37][38] the most reliable and quantitative tools to assign the absolute configuration of the metal centre, suggest only the formation of a single diastereomer Λ-M(R-N,O) 2 or Δ-M(S-N,O) 2 in the enantiopure crystals of pseudotetrahedral [M(R or S-N,O) 2 ] (M = Co, Ni, Cu, and Zn; N,O = (Ar)-salicylaldiminato/ naphthaldiminato 45,46 ).…”

Dynamic control of chirality and self-assembly of double-stranded helicates with light

“…Following the seminal work of Lehn and co-workers 30,31 on the spontaneous selfassembly of metal double-stranded helicates 32,33 from oligopyridine ligands, a number of elegant strategies for the control of dynamic helicates using chemical signals (metal binding, redox, pH and solvent) have evolved [34][35][36] . Control of the handedness of the helicates was achieved by introducing chirality into the backbone of the ligands 37,38 or via chiral templates 39 . However, fully reversible handedness switching of double helicates remains highly challenging so far.…”

A Helicate‐Based Three‐State Molecular Switch

“…To obtain a lithium‐dependent molecular switch, two monomeric complex units of the hierarchically assembled helicates are connected by appropriate spacers (Figure b,c). The stereochemistry of the helicate can be well controlled, if the enantiomerically pure ligand 4 ‐H 4 is used and it is possible to address three different states (compressed left‐handed, expanded right‐handed and expanded left‐handed). Although some systems are already described which show inversion of stereochemistry upon an external input, the behavior of the present switch is unprecedented.…”

“…Helicates [(3/4) 3 Ti 2 ] 4À are able to adopt compressed and expanded forms depending on the countercations.B ased on as eries of X-ray structure analyses [11,12] and on the structure of [Li 3 (2) 3 Ti 2 ] À the length in the compressed form is estimated to be around 10 .T he expanded form is approximately 24-25 long based on model considerations.…”