Stereochemical dominance in hierarchically formed helicates

The competition of different chiral ligands in the control of stereochemistry of hierarchically formed helical coordination compounds is investigated. It is found that sterically demanding chiral units can dominate the chiral induction of the helix even if they are present as a minor species. Hereby the relative strength of stereoinduction of different chiral units can be evaluated.

Conversion of a double-tetranuclear cluster silver helicate into a dihelicate via a rare desulfurization process

We show the first example of a bisthiosemicarbazone silver double-tetranuclear cluster helicate [Ag4L2]2 obtained by electrochemical synthesis which undergoes a rare desulfurization process giving rise to a cationic silver dihelicate [Ag2(H2L)2]SO4.

Hierarchically assembled helicates as reaction platform - from stoichiometric Diels-Alder reactions to enamine catalysis

The stereoselectivity of a Diels-Alder reaction within the periphery of hierarchically assembled titanium(IV) helicates formed from mixtures of achiral, reactive and chiral, unreactive ligands was investigated in detail. Following the pathway of the chiral induction, the chiral ligands, solvents as well as substituents at the dienophile were carefully varied. Based on the results of the stoichiometric reaction, a secondary amine-catalyzed nitro-Michael reaction is performed as well which afforded reasonable diastereoselectivities.

Shedding Light on the Interactions of Hydrocarbon Ester Substituents upon Formation of Dimeric Titanium(IV) Triscatecholates in DMSO Solution

The dissociation of hierarchically formed dimeric triple lithium bridged triscatecholate titanium(IV) helicates with hydrocarbyl esters as side groups is systematically investigated in DMSO. Primary alkyl, alkenyl, alkynyl as well as benzyl esters are studied in order to minimize steric effects close to the helicate core. The 1 H NMR dimerization constants for the monomer-dimer equilibrium show some solvent dependent influence of the side chains on the dimer stability. In the dimer, the ability of the hydrocarbyl ester groups to aggregate minimizes their contacts with the solvent molecules. Due to this, most solvophobic alkyl groups show the highest dimerization tendency followed by alkenyls, alkynyls and finally benzyls. Furthermore, trends within the different groups of compounds can be observed. For example, the dimer is destabilized by internal double or triple bonds due to π-π repulsion. A strong indication for solvent supported London dispersion interaction between the ester side groups is found by observation of an even/odd alternation of dimerization constants within the series of n-alkyls, n-Ω-alkenyls or n-Ω-alkynyls. This corresponds to the interaction of the parent hydrocarbons, as documented by an even/odd melting point alternation.

From Hierarchical Helicates to Functional Supramolecular Devices

Catechol ligands with aldehyde, ketone or ester groups attached in 3-position form, in the presence of titanium(IV) triscatecholate, titanium(IV) complexes. If lithium cations are the counterions, they can bind in a successive step to the salicylate units of the complex and form a dimeric triple-lithium-bridged dinuclear helicate. In solution, the dimer is in equilibrium with the monomer and the thermodynamics of the dimerization can be easily evaluated. Thus, the hierarchically assembled titanium(IV) helicates represent a lithium-dependent molecular switch. The investigation of different derivatives of the complex allows for an estimation of the influence of side chain functionalities on the energetics of the dimerization. Thus, the hierarchically assembled helicates can be used as a kind of molecular balance to determine weak interaction energies (solvophobic effects and even dispersive effects). In addition, tethering of two ligands leads to "classical" helicates. Removal or addition of lithium cations allows for a reversible switching between a compressed and expanded state, which in the case of chiral ligands can be even performed stereospecifically.

Kinetic investigation of the dissociation of dinuclear hierarchically assembled titanium(iv) helicates

Hierarchically assembled helicates consisting of lithium-bridged triscatecholate titanium(iv) complexes represent a powerful self-assembled supramolecular system with applications as e.g. molecular balances for the evaluation of weak interactions, stereoselectivity switches in asymmetric synthesis or molecular switches.

Chasing Weak Forces: Hierarchically Assembled Helicates as a Probe for the Evaluation of the Energetics of Weak Interactions

London dispersion forces are the weakest interactions between molecules. Because of this, their influence on chemical processes is often low, but can definitely not be ignored, and even becomes important in cases of molecules with large contact surfaces. Hierarchically assembled dinuclear titanium(IV) helicates represent a rare example in which the direct observation of London dispersion forces is possible in solution even in the presence of strong cohesive solvent effects. Hereby, the dispersion forces do not unlimitedly support the formation of the dimeric complexes. Although they have some favorable enthalpic contribution to the dimerization of the monomeric complex units, large flexible substituents become conformationally restricted by the interactions leading to an entropic disadvantage. The dimeric helicates are entropically destabilized.