Self-Assembly and Gelation Behaviour of Pyridyl Urea Complexes

Abstract

Bis(3-pyridyl)ureas with ethylene, propylene, butylene, phenylene and napthy-
lene spacer groups were synthesised by the reaction of 3-isocyanato-pyridine, pre-
pared in situ from nicotinoyl azide, with the appropriate diamine. Each ligand was
crystallised and their structures were solved using X-ray crystallography. The urea
groups of the napthylene-spaced ligand adopt a bifurcated hydrogen-bonding mode,
thereby forming an α-tape whereas in the the propylene-, butylene- and phenylene-
spaced ligands, every other bifurcated interaction is replaced with a NH···Ourea and a
NH···Npyridyl , caused by the presence of a single competing intramolecular CH···Ourea
set-up by the electron-withdrawing pyridyl nitrogen. With the ethylene-spaced urea,
the bifurcated hydrogen-bonding motif is absent. These differences in the intermolec-
ular bonding may be only be due to the packing requirements of the spacer group:
as these interactions become stronger, either by greater van der Waals attraction in
a longer oliomethylene chain or the greater π-π stacking interactions of the napthy-
lene ring, adjacent urea groups become closer and bind preferentially to form the
bifurcated interaction.
The bis(3-pyridyl)ureas were subsequently tested for their ability to form gels,
and in some cases, crystallised in the presence of selected silver(I) salts. Compound
2.1 formed five pseudo-polymorphs of a Borromean-type structure upon slow crys-
tallisation in various solvent mixtures with silver(I) nitrate. Within each struc-
ture, there are layers made up of paired Borromean weaves and the mechanism by
which these layers stack, though unclear, must ultimately be driven by the inter-
actions of the solvent molecules incorporated into the structures. When the coun-
teranion in the crystallisation was replaced by either acetate, tetrafluoroborate or
hexafluorophosphate, several coordination polymers were synthesised but the Bor-
romean structure was not reproduced. A similar dependency on counteranion was
observed with N,N''-pentylene-1,5-diylbis(N'-pyridin-3-ylurea) which was similarly
crystallised with silver(I) nitrate, acetate and tetrafluoroborate. In the presence of
the latter, a quintuple helix formed.
Several bis(3-pyridylureas) with ethylene, pyridine, methylenebisphenylene and
cyclohexylene spacer groups were shown to form gels and crystals in the presence of
silver(I) nitrate. Two macrocyclic structures were grown from gel states which may
be further indication as to how the same molecules aggregate in the gel fibres with
silver(I) nitrate promoting aggregation of the molecules along one direction.