Supramolecular assembly in a Janus-type urea system

Article abstract of DOI:10.1039/c3cc48603f

A pyrazolyl urea ligand adopts two possible conformations with the urea NH groups directed either outward or inward. Metal coordination enforces the outward conformation resulting in either anion complexation or self-association and hence extended supramolecular assemblies including a hexameric barrel that persists in solution. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc48603f

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Supramolecular assembly in a Janus-type urea
system†
Cite this:DOI: 10.1039/c3cc48603f
Gareth O. Lloyd*a and Jonathan W. Steed*b
Received 11th November 2013,
Accepted 7th December 2013
DOI: 10.1039/c3cc48603f
www.rsc.org/chemcomm
A pyrazolyl urea ligand adopts two possible conformations with the The compound is readily prepared as a mixture of the 1- and
urea NH groups directed either outward or inward. Metal coordination 2-substituted isomers by reaction of 3-amino-5-methylpyrazole with
enforces the outward conformation resulting in either anion complexa- p-tolyl isocyanate. The isomerically pure compound is obtained by
tion or self-association and hence extended supramolecular assemblies recrystallization from hot chloroform. The single crystal X-ray struc-
including a hexameric barrel that persists in solution.
ture of free ligand 1 crystallized from hot chloroform solution (form I)
is shown in Fig. 1a. The disubstituted urea functionality adopts an
Ligands bearing hydrogen bonding functionality represent a powerful anti conformation resulting from an intramolecular hydrogen bond
tool in the design of self-assembling systems.1–6 Both coordination from N(4)–H to the Lewis basic pyrazolyl nitrogen atom.18 The
bonds and hydrogen bonds are strong, directional and synthetically carboxamide N(5)–H group also forms an intra-molecular hydrogen
versatile non-covalent interactions, and in concert can give rise to bond to the same pyrazolyl nitrogen atom. Such intramolecular
stable, self-assembled aggregates with a high degree of complexity. hydrogen bonding is expected based on Etter’s rules which state
Because ligand donor atoms that commonly bind to metal ions are that intra-rather than intermolecular hydrogen bonding should
also generally hydrogen bond acceptors, metal ion coordination can dominate.19,20 It is in contrast to the structure of many disubstituted
be used to mask a potential hydrogen bonding site and hence favour ureas, however, such as N,N0-diphenylurea which adopt a syn
an alternative hydrogen bonding pattern, giving rise to metal– conformation forming the characteristic urea a-tape hydrogen
ion-switchable supramolecular assembly.7 The proximity of a metal bonding motif in which the two NH groups interact with the carbonyl
cation may also enhance hydrogen bond acidity as a result of oxygen atom of an adjacent urea to give a hydrogen bonded
inductive effects.8 This approach has been used to good effect in a
range of creative systems in which metal coordination compounds act
as hosts for anion guests.9,10 Applications include, for example, anion
sensing and medical diagnostic devices.11,12 We and others have
produced a range of pyridyl ligands bearing urea functional groups
exhibiting interesting self-assembly, anion binding and materials
properties, particularly gelation behaviour.13–16 The pyridyl ligand is
relatively bulky, however, and hence we have turned our attention
to urea derivatives bearing the smaller, strongly basic pyrazole17
functionality which contains a 5-membered heterocyclic ring.
Ligand 1 possesses a total of three hydrogen bond donor groups
on the urea functionalities as well as a basic pyrazole nitrogen atom.
a Institute of Chemical Sciences, School of Engineering and Physical Sciences,
William Perkin Building, Heriot-Watt University, Edinburgh, EH14 4AS, Scotland,
UK. E-mail: g.o.lloyd@hw.ac.uk; Fax: +44 (0)131 451 3180; Tel: +44 (0)131 451 8020
b Department of Chemistry, Durham University, South Road, Durham DH1 3LE, UK.
E-mail: jon.steed@durham.ac.uk
† Electronic supplementary information (ESI) available: Experimental details for
the preparation of ligand 1, crystallography and experimental data. CCDC 971034
and 971041–971043. For ESI and crystallographic data in CIF or other electronic
Fig. 1 Crystal packing of 1 in polymorphic forms (a) I and (b) II.
format see DOI: 10.1039/c3cc48603f
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polymer.21 In this case compound 1 adopts an eight-membered
hydrogen bonded ring motif (R22(8) in graph set nomenclature18) There
are only weak C–HꢀꢀꢀO interactions with the carbonyl of the carbox-
amide group leading to a 1D chain of these V-shaped molecules. This
structure is interesting in that it adopts an anti conformation of the urea
group with intramolecular hydrogen bonding, even though the pyr-
azolyl nitrogen atom is acting as a bifurcated acceptor. This means that
the intramolecular interaction to the urea might be expected to be
sterically hindered and hence the acceptor ability of the pyrazolyl
nitrogen atom reduced by taking part in two different hydrogen bonds.
Interestingly, when 1 is crystallised from methanol (or other polar
solvents such as ethanol and acetonitrile), a conformational poly-
morph,22,23 form II results, Fig. 1b. Form II retains the intra-
molecular hydrogen bonding of the carboxamide to the pyrazolyl
nitrogen atom but the urea group adopts a syn conformation
resulting in the formation of a conventional urea a-tape hydrogen
bonding motif. Ligand 1 is thus ‘Janus-like’,24,25 apparently finely
poised between adopting an intra- or intermolecular hydrogen
bonding behaviour and presents a more hydrophilic face in metha-
nol and a more hydrophobic one in chloroform. Calculations using
the UNI force field26 implemented in Mercury27 give a packing
energy for form I of ꢁ165.6 kJ molꢁ1 compared to a much more
substantial ꢁ243.7 kJ molꢁ1 for form II dominated by the urea
a-tape motif which allows an additional intermolecular hydrogen
bonding interaction. This marked difference strongly suggests that
form I is metastable and arises from the conformation adopted by
the molecule in the non-polar crystallization medium.
The conformational behaviour of ligand 1 should be markedly
influenced by metal ion coordination to the hydrogen bond acceptor
pyrazolyl nitrogen atom, with metal cations both polarising the urea
NH groups, enhancing hydrogen bond strength, as well as favouring
the more polar conformer found in the structure of form II by
blocking the hydrogen bond acceptor pyrazolyl nitrogen atom. We
Fig. 2 (a) Asymmetric unit in [{Cu(m-k-O,O,N,N-1–H)(MeOH)}6](MeCO2)6ꢀ
6MeOH showing hydrogen bonding of the acetate anion to the urea group,
and (b) hexameric barrel-shaped assembly based on Cu–O bridges and in
C–Hꢀ ꢀ ꢀp interactions between tolyl groups. Each barrel is linked to its
neighbours by hydrogen bonding from the coordinated methanol to
opted to try metal acetates because the acetate anion is likely to acetate anions, and hence the urea groups of the adjacent assembly.
deprotonate the ligand and should hydrogen bond strongly to the
urea groups. Reaction of 1 with copper(II) acetate in methanol resulted involving the tolyl groups and linking to adjacent assemblies via
in the isolation of a remarkable hexameric, hexacationic assembly of hydrogen bonding to the coordinated methanol. The hexamers stack
formula [{Cu(m-k-O,O,N,N-1–H)(MeOH)}6](MeCO2)6ꢀ6MeOH (Fig. 2). one on top of each other resulting in columns that are packed into the
The structure involves the deprotonation of 1 by the basic acetate trigonal lattice. The urea group is in a syn conformation and directed
counter anion. Deprotonated 1 acts as a tetradentate chelate and away from the metal centre as in form II of the free ligand structure
bridging ligand to the distorted trigonal bipyramidal copper(II) centre. and hence is available to hydrogen bond to the acetate anion, forming
There are two chelate rings present from the coordination of the the well-known R22(8) hydrogen bonding motif.18,30 This anion binding
deprotonated 1; a five-membered ring involving the deprotonated mode resembles analogous exogenous complexation of anions by
carboxamide group and pyrazolyl nitrogen atom and a six-membered ruthenium(II) bound thiourea ligands.31 The strength of the coordina-
ring involving the carbonyl oxygen atom of the urea group and tion interactions holding the assembly together, as well as its close-
pyrazolyl nitrogen atom. The tridentate coordination by the deproto- packed nature, suggest that it should be stable in solution as well as in
nated 1 is similar to some Schiff base ligands and similar ligands the solid state. While the paramagnetism of copper(II) does not permit
used in the supramolecular assembly of multi-metal centred grids.28 detailed study of the assembly by NMR spectroscopy, by ESI-MS of the
The pentacoordination of the metal is completed by interactions to a crystals in MeOH solution showed clear evidence for the persistence
molecule of coordinated methanol. Each metal centre bridges to of the hexamer. The mass spectrum exhibited a peak at 1574 m/z
an adjacent one via the carboxamide carbonyl oxygen atom. The with half-integer isotopic progression consistent with the assembly
Cu–O bond length in this bridging interaction is relatively short29 at [{Cu(1–H)(MeOH)}6](MeCO2)6 in conjunction with two Na+ cations.
1.937(6) Å, reflecting the delocalization of the amidate negative charge Further peaks assigned to fragments of the hexameric assembly were
onto the oxygen atom. These bridging interactions result in a also observed (see ESI†).
remarkable hexameric, barrel-shaped assembly exhibiting crystallo-
Formation of this 1 : 1 complex between deprotonated ligand 1 and
%
3
graphic
symmetry, supported by edge-to-face p-interactions a divalent metal ion naturally leads to anion binding by the syn urea
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polymorphism is easy to rationalise as a response to the crystal-
lization medium in conjunction with the finely balanced nature of
the intramolecular and intermolecular hydrogen bonding interac-
tions. The ligand allows the formation of metallosupramolecular
assemblies held together by a synergic combination of coordination,
hydrogen bonding and aromatic interactions which in the case of the
copper(II) hexameric cluster, forms a robust anion-binding complex
in solution as well as the solid state.
We thank the Association of Commonwealth Universities
and Heriot-Watt University for funding.
Notes and references
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