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 353

A very fruitful self-assembly strategy has proven to be the use
of Pd

II II

complexes in combination with nitrogenated ligands.
The use of these metal centers with 4,4

and Pt

-bipyridine and related
ligands has been especially profitable in the synthesis of 2D and 3D
molecular structures [13-15]. The success of this approach is based
on the structural diversity of pyridyl ligands, the geometry of the
metal centers, and the modulation of the lability of the coordination
bond allowing thermodynamic controlled conditions. However, the
poor π-acceptor ability of 4,4

′

′

-bipyridine limits its participation in
π-acceptor/π-donor interactions for the formation of host-guest
complexes with aromatic substrates (Fig. 11.1).

Figure 11.1 Self-assembly strategies based on 4,4

′

bipyridine derivatives.

In contrast, cyclophanes with a N,N ′ -dialkyl-4,4

′

-bipyridinium or

N,N ′ -dialkyl-2,7-diazapyrenium structure show a strong π-deficient

character, which has been successfully used by Stoddart and co-
workers for the preparation of a myriad of catenanes and rotaxanes
[16,17]. The key step in these processes relies on the electronic
complementarity between dialkyl-4,4 -bipyridinium and dioxoaryl

components. However, the lack of metal-binding units (nitrogen
donor atoms) in dialkylated 4,4

′

-bipyridine derivatives precludes
their use as ligands for the preparation of metallocycles. In these
cases, the formation of the supramolecular structures requires the
N

′

-alkylation of a pyridine ring (under kinetic control conditions),
leading to the desired entity with moderate yields.
An intermediate approach in the middle ground between
those above described, which we have successfully explored in our
research group for the synthesis of self-assembled metallocycles
and their inclusion complexes and catenanes, involves the use of
angular L-shaped ligands based on

′

-bipyridinium
salts (Figs. 11.1 and 11.2). These bidentate ligands display at least
one alkylated nitrogen to ensure the π-deficient character of the

N -monoalkyl-4,4