Long‐Range Stereocontrol in the Self‐Assembly of Two‐Nanometer‐Dimensioned Triple‐Stranded Dinuclear Helicates

Abstract

A series of bisimine-bridged dicatechol ligands 2-H(4)-5-H(4) were synthesized and were used to prepare triple-stranded dinuclear helicate-type complexes with a length of up to more than 2 nm. X-ray structural analyses of Na(4)[(2)(3)V(2)], Na(4)[(3)(3)Ti(2)], Na(4)[(4)(3)Ti(2)], and Na(4)[(5)(3)Ti(2)], as well as temperature-dependent NMR investigations of Na(4)[(4)(3)Ti(2)] and Na(4)[(5)(3)Ti(2)] show that, in the case of the rigid linear ligands 2 and 3, and of the ligand 5, which possesses C(2h) symmetry in its idealized structure, homochiral helicates are diastereoselectively formed. Ligand 4, on the other hand, with idealized C(2v) symmetry, leads with surprisingly high selectivity to the formation of the heterochiral meso-helicate. This is attributed to the ability of ligand 4 to adopt a less-restricted conformation in the meso compound than in the helical complex. NMR investigations indicate that both complex units of Na(4)[(4)(3)Ti(2)] invert (LambdaDelta-->DeltaLambda) simultaneously, while in the case of Na(4)[(5)(3)Ti(2)] a stepwise racemization proceeds.

Symmetry Driven Self-Assembly of Metallo-Supramolecular Architectures

“…This example shows, that stereo control over long distances in supramolecular chemistry can be achieved by very simple mechanisms. 56,57 The described self-assembly processes of helicates are highly diastereoselective. However, it is important to obtain the helicates not as racemic mixtures but as the enantiomerically pure left-or right-handed helices.…”

Formation of Triple‐Stranded Dinuclear Helicates with Dicatecholimine Ligands: The Influence of Steric Hindrance at the Spacer

“…. [14] These three examples demonstrate the influence of the spacer on the formation of the complexes. Ligand 3a-H 4 , with four methyl groups at the phenyl spacer, exclusively forms triple-stranded dinuclear helicates with lithium and sodium only.…”

“…In the solid state, two sodium cations are bound in the interior of the helicate, each binding to the internal catecholate oxygen atoms and to three DMF molecules. [14] Following this, it is reasonable to suggest that ligand 3b-H 4 with its two additional methyl groups is able to form triple-stranded dinuclear complexes with lithium, sodium or potassium cations. The situation changes somewhat by using ligand 3c-H 4 .…”

“…Ligand 3h-H 4 possesses a biphenyl spacer that is similar to ligand 3c-H 4 but without the methyl substituents. [14] As we have shown, ligand 3h-H 4 leads quantitatively to the formation of a triple-stranded dinuclear helicate M 4 [(3h) 3 Ti 2 ]. The X-ray structure of the complex Na 4 [(3h) 3 Ti 2 ] shows (Figure 2) that the linear rigid ligands form a triple-stranded helicate with two similarconfigured titanium() complex units.…”

“…In this conformation the C=N double bond is directed towards the outside of the cavity of the complex. [14] This conformation is preferred because there are electrostatic attractions between the internal catecholate oxygen atoms and the imine proton. Additional repulsion takes place between the oxygen and the imine-nitrogen lone pairs.…”