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Abstract
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 1H 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.
Highlights
In solids, weak interactions between molecules are most important for the properties of the respective bulk materials
The focus will be on alkyl, alkenyl- and alkynyl- as well as benzyl-substituted catechol esters as ligands (Figure 4)
EDispB includes the ring fragments of the aromatic catecholates in addition to the hydrocarbon substituents included in the former calculation
Summary
Since 2005, we have been studying the chemistry of hierarchically formed helicates,[14] which are essentially dimeric triple-lithium bridged bis(titanium(IV) triscatecholates).[15] In solution, a monomer–dimer equilibrium can be observed and its energetics can be accurately determined by NMR spectroscopy (Scheme 1).[16]. The stability of the main core of the hierarchical helicates depends more on the binding strength of lithium in the interior of the central complex moiety than compared to the solvation of lithium cation In this case, some main factors play an important role: 1) The carbonyl moiety attached to the catecholate is highly influential. In the present study the focus will be on the influence of primary hydrocarbyl substituents at ester catecholate ligands on the dimerization in DMSO solution as described under points 5–7 and the influence of the “external” cation will be briefly discussed as well. The focus will be on alkyl-, alkenyl- and alkynyl- as well as benzyl-substituted catechol esters as ligands (Figure 4)
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