PdII and PtII Metal-Directed Self-Assembly of Supramolecular Structures Based on N-Monoalkyl-4,4¢-Bipyridinium Derivatives - Molecular Self-Assembly: Advances and Applications - page 372

(CAT ) into the metallocycle, while the other two are located outside

in

(CAT ). However, the π - π stacking system does not comprise the

out

… CAT

CAT (interplanar distance 3.73 Å) interaction, as in other

catenanes of the series, because these units are displaced 2.86 Å
each other in the orthogonal direction to the mean plane defined
by the four corners of the metallocycle. Owing to this displacement,
the π

in in

π stacking interactions with bipyridinium units are weak.
In addition to the habitual [C-H … O] bonds between the

-

α

-CH

[C-H … O]
hydrogen bonds between methylene protons and the oxygen
atoms of the polyether chain are also detected. Surprisingly, two
external molecules of

bipyridine hydrogens and the oxygen atoms of DB24C8 ,

DB24C8 with a C-shaped conformation are

positioned over the Pd(en) corners in a pseudorotaxane motif

(green macrocycles in Fig. 11.11), in which the metallocycle acts as a
stopper. The

macrocylic ring is held in position by [N-H … O]
hydrogen bonds emanating from the ethylendiamine subunits, each
N-H is involved in a bifurcated hydrogen bonding with two oxygen
atoms of the polyether macrocyclic ring. The π

DB24C8

- π stacking system is

extended by means of π - π interactions between the catechol rings of

each of these external DB24C8 units and the bipyridium units.

11.5.2.1 The Catenation Process Can Be Switched Off and On

It is known that forms a stable complex with potassium ion

so we decided to explore the possibility of reverting the formation
of (

DB24C8

1

DB24C8 )

⊂ M5a

and monitoring the process by H NMR. Thus,

2

when KPF is added to a yellow-colored solution of the catenane, the

color vanished as the catenane is dissociated into metallocycle

6

M5a

1

and the complex between potassium and the crown ether. The H

NMR spectrum shows clearly the disappearance of the catenane and
the appearance of metallocycle

. This process can be reversed
by titration with 18-crown-6 that forms a stronger complex
with potassium than

M5a

rings, which then
reassemble into the catenane (Scheme 11.3). In particular, worthy of
note is the fact that the singlet for amine protons is shifted downfield
by 0.10 ppm, suggesting that

DB24C8 freeing DB24C8

is positioned over the metal
corners also in the solution. A similar shift (

DB24C8

∆ d

= 0.30 ppm) was

observed when two equivalents of DB24C8 were added to a solution

II

of the Pt catenane ( DB24C8 )

⊂ M5b

in CD CN.

2 3