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The formation of this suprastructure was also confirmed by mass
spectrometry.
11.5.2
[3]Catenanes Based on
Cis
Palladium Complexes
A [3]catenane is a structure composed of three interlocked rings,
where one of them is interwoven by two macrocycles. An efficient and
preparative synthesis of oligocatenanes remains elusive, probably
due to the difficulty of synthesizing large enough macrocycles and
the formation of catenanes of lower and/or higher order.
The [3]catenanes are obtained by metal-directed self-assembly
of three different building blocks: 1,1
′
-methylenebis-4,4
′
-
bipyridinium (
L5
), a dioxoaryl cyclophane (
DB24C8, BPP34C10
II
II
or
DN38C10
), and a square-planar Pd
or Pt
complex in a 2:2:2
(M = Pd, Pt) was chosen as the two triflate
ligands can be easily substituted. On the one hand, metal centers
react with 1,1
ratio [31]. M(en)(OTf)
2
-bipyridinium that can adopt a
L-shape conformation to form a square dinuclear metallocycle
(
′
-methylenebis-4,4
′
), which presents the ideal cavity size for the inclusion of
two molecules of the guest (see Section 11.4.2). On the other hand,
the formation of [3]catenanes with dioxoaryl cyclophanes could
be favored by: (a) π-π interactions between
M5a,b
-
bipyridinium systems and dioxoaryl rings and (b) hydrogen bonds
between the methylene link of the ligand and oxygen atoms from the
cyclophane. These types of interactions have been clearly observed in
the pseudorotaxane structures formed from dioxoaryl cyclophanes
and
N
-monoalkyl-4,4
′
L5
. The interaction between
L5
and dioxoaryl cyclophanes was
examined because the insertion of
into the crown ether might be
a prerequisite for the formation of the [3]catenane.
L5
The equimolar solutions of ligand
L5
·2PF
and cyclic polyethers
6
DB24C8
are red, yellow, and violet
colored,respectively,indicatingthepresenceofabroadintermolecular
charge-transfer band centered on 480, 356, and 535 nm, respectively.
This kind of interaction has usually a very low contribution to the
complex stability, but the charge-transfer bands are very useful for
the determination of the association constant by means of UV-vis
spectroscopy. The UV-vis titration method was used to determine
the association constants in acetonitrile for the pseudorotaxanes of
L5
,
BPP34C10
, and
DN38C10
·2PF
with
BPP34C10
,
DB24C8
, and
DN38C10
(
K
= 2160
±
110
6
a
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