Chemistry Reference
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devices [5,6]. Numerous examples of supramolecular assemblies based on selective metal
ion:ligand interactions have been engineered with the intent of forming symmetrical and
esthetically pleasing structures. Elegant strategies have been elaborated in order to build
these assemblies that include examples of discrete and infinite helical frameworks [7],
hydrogen-bonded [8] or p-p interacting networks [9], three-dimensional supramolecular
complexes displaying well defined channels [10], cucurbituril assemblies [11], highly
symmetric coordination clusters [12], and organometallic polymers [13].
Many of the new inorganic supermolecules generated over the last few years rely on the
coordination of transition metal cations to polypyridine fragments incorporated into
oligomeric ribbons or macroscopic loops [14-16]. This work has combined ingenious
synthetic strategies with state of the art in characterization, in some cases relying on novel
analytical methodologies [17]. By virtue of forming relatively stable and well defined
complexes with metal cations, polypyridine ligands have facilitated assembly of molecu-
lar double- or triple-stranded helicates [18-21], catenates [22], ladders, and related exotic
frameworks [23,24]. Such elaborate molecular architectures are made possible by incor-
porating several polypyridine units into open-chain or macrocyclic multitopic ligands.
Subsequent coordination with cationic metal centers provides the impetus for self-organi-
zation into ordered structures; some of these exhibit useful catalytic [25], mesomorphic
[26], or electronic [27] properties. Some attempts have been made to identify the driving
forces for formation of one structure versus another by systematic variation of the metal
and ligand component, but very little is known about the mechanism of formation of most
of these metal ion-based self-organized structures [28-33]. Quantitative measures of the
thermodynamic stability of such organized scaffoldings are rarely available but are no
doubt fundamental to development of the understanding of possible applications of the
scaffolds in mesophases active in photoconduction, in catalysis, as chemosensors, or as
molecule-based logic gates.
Many long-familiar systems require time-consuming, multistep syntheses from
expensive starting materials and tedious chromatographic separations which afford
the target ligands in low yields. We recently adopted a different strategy based on
addition of coordination sites (imino functions) around a single and central bipyri-
dine coordination shell. The ready availability of these Schiff-base ligands, the sim-
plicity of the purification procedures, the mildness of the reaction conditions, and the
high yields allow the preparation of multigram-scale quantities. The addition of par-
ticular functional groups to the ligand is a very attractive feature since the presence
of two additional donor atoms adjacent to the central bipyridine unit should result in
passage from the simple chelating coordination mode (type I) of the bipyridine to a
non-conventional mode of coordination (type II) where the ligand bridges two metal
centers (Figure 7.1) [34]. An identical situation should be encountered with terpyri-
dine, where again the usual coordination mode (type V) may become non-conven-
tional (type VI). Even where standard coordination modes are adopted by imino
group-functionalized polypyridines, they can still be of interest, since ligands of this
type were amongst the first to be used in metallomesogen synthesis.
Thus, Schiff-base metal complexes feature among the earliest and most widely
studied class of metallomesogens [35]. The advantages of incorporating an imine
functionality stems from: (i) as noted above, the ease of preparation and derivatiza-
tion of the ligands compared to the classical one of type I, (ii) the versatility and
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