A triple helix of double helicates: Three hierarchical levels of self-assembly in a single structure

Article abstract of DOI:10.1039/c2cc30197k

The bis-bidentate bridging ligand L reacts with Ag(i) ions to form a conventional dinuclear [Ag₂L₂]²⁺ double helicate; individual double helicate units assemble via Ag...Ag interactions into infinite chains, three of which

Full text of DOI:10.1039/c2cc30197k

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COMMUNICATION
A triple helix of double helicates: three hierarchical levels of self-assembly
in a single structurew
Andrew Stephenson and Michael D. Ward*
Received 10th January 2012, Accepted 14th February 2012
DOI: 10.1039/c2cc30197k
The bis-bidentate bridging ligand L reacts with Ag(I) ions to form a
conventional dinuclear [Ag2L2]2+ double helicate; individual double
helicate units assemble via AgꢀꢀꢀAg interactions into infinite chains,
three of which wrap around a central spine of anions to give a triple
helical braid, which is therefore an infinite triple helix composed of
molecular double helicate subunits.
by one coordination polymer chain, which can be controlled to
some extent by judicious ligand design, and (ii) weak inter-
actions between two or more chains which result in their
association in the crystals, which is beyond our power to
control reliably.2
As these two manifestations of helicates have such different
origins they are independent of one another. Molecular helicate
complexes generally crystallise in an unremarkable way as
clearly distinct molecules in the crystal, as they do not have the
capacity to form coordination polymers unless there are
specific strong interactions between molecular units such as
Agꢀ ꢀ ꢀAg contacts.5 The separate chains that make up multi-
stranded helical coordination polymers are based on a combi-
nation of metal ion and bridging ligand that is designed to
form a one-dimensional chain, and therefore they bear no
particular chemical relationship to helical complexes which are
discrete molecular species.
Multi-stranded helices as structural motifs in supramolecular
chemistry1,2 retain the fascination that they have had since the
early examples of double3 and triple4 helical coordination
complexes were reported. There are several reasons for this,
including (i) the obvious parallel with the structure of DNA;
(ii) the use of helical complexes as a test-bed for improving our
understanding of self-organisation and molecular recognition in
relatively simple artificial systems; and (iii) the provision of a way
of introducing chirality into molecules and molecular assemblies
for applications ranging from catalysis to opto-electronics.
The helical structural motif appears in synthetic inorganic
systems in two quite different circumstances and on different
length scales. The first type of occurrence is in the well-known
set of molecular complexes which arise from wrapping two or
three flexible, multitopic ligands around a central spine of
metal ions, each of which coordinates to one binding site from
each ligand, as in Lehn’s original examples3 and countless
others.1 The second is less common and occurs in the realm of
crystalline coordination polymers, when two or more infinite
one-dimensional strands each adopt a helical twist and braid
around one another.2 In the first case the main driving force is
provided by the matching of the coordination preferences of
the metal ions which the number and arrangement of binding
sites in the ligand: as these factors are relatively strong,
directional, and susceptible to some synthetic control, design
and synthesis of molecular helicates is now well known.1 The
second manifestation of helicates is rarer and much less
susceptible to synthetic control, relying as it does on two
things: (i) the adoption of a helically twisted conformation
We report here a remarkable example of a crystalline
assembly which combines both forms of helical motif on quite
different length scales: it comprises triple helical infinite chains
in which each of the three infinite strands in the triple helix is
formed from end-to-end association of double helical molecular
complexes joined by Agꢀ ꢀꢀAg contacts. It is, in effect, a triple
helix of double helicates. It is based on the bridging ligand L
in which two bidentate chelating pyrazolyl-pyridine units are
connected to a central benzophenone spacer via flexible
methylene hinges. With two bidentate sites separated by a
flexible spacer this ligand is ideally disposed for formation of
dinuclear double helicates with ions such as Ag(I) that commonly
form four-coordinate complexes, and the ability of this ligand to
form such a helicate will be facilitated by the non-planar diaryl
ketone spacer which is a component of other ligands known to
form helical structures.6
The ligand L was prepared in the usual way7 by reaction of
3-(2-pyridyl)pyrazole with 4,40-bis(bromomethyl)benzo-phenone
in basic conditions, and gave satisfactory analytical data.z
A crystal structure was also obtained and is shown in ESI.wy
It is unremarkable with a transoid arrangement of the two
near-planar rings in each pyrazolyl-pyridine fragment, and an
Department of Chemistry, University of Sheffield, S3 7HF, UK.
E-mail: m.d.ward@shef.ac.uk; Fax: +44 114 2229346;
Tel: +44 114 2229484
w Electronic supplementary information (ESI) available: CIF for the
crystal structures (CCDC numbers 862638–862640) and figures for
the structures of L and [Ag2L2](BF4)2ꢀMeNO2ꢀ(H2O)0.33. See DOI:
10.1039/c2cc30197k
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Fig. 1 ORTEP view of the double helical [Ag2L2]2+ unit of
[Ag2L2](ClO4)2ꢀMeNO2, showing thermal ellipsoids at the 30%
probability level; one ligand is shown with paler colours for clarity.
Fig. 2 Assembly of double helical [Ag2L2]2+ units into infinite one-
dimensional helical chains via Agꢀ ꢀ ꢀAg contacts: (a) a view showing
the interaction between two adjacent helical units; (b) a view showing
six helical units which form one complete turn of the helical chain. In
both diagrams, alternate ligands are coloured separately for clarity.
angle of 55.41 between the two aromatic rings of the benzo-
phenone unit which imparts a twist to the ligand.
Reaction of L with AgClO4 or AgBF4 in a 1 : 1 ratio in
MeOH/CH2Cl2 for 24 h afforded a white precipitate of
[Ag2L2]X2 (X = BF4, ClO4) in each case according to ES mass
spectrometry.z X-ray quality crystals were grown by diffusion
of ethyl acetate vapour into solutions of the complexes in
MeNO2.8 The two crystal structures are essentially identical; the
perchlorate salt [Ag2L2](ClO4)2ꢀMeNO2 has a better refinement
so the discussion is focussed on that.
over 8 sites (site occupancy 50% each); the Clꢀ ꢀ ꢀCl separation
between adjacent perchlorate ions is 6.44 A, which is one
quarter of the unit cell length in that direction. Although the
O atoms are disordered it is clear that they must lie sufficiently
close to inwardly-directed H atoms from the ligands L to
participate in CHꢀ ꢀ ꢀO hydrogen-bonding (non-bonded Cꢀ ꢀ ꢀO
separations E 3.3 A), particularly involving the methylene
groups C(26) and C(36) whose H atoms are inwardly directed.
In addition contacts characteristic of CHꢀ ꢀ ꢀp and p–p inter-
actions between ligand fragments in different strands can be
identified. Interestingly the sense of twist of the supramolecular
triple helicate (each strand proceeds clockwise as it moves into
the page, Fig. 3b) is opposite to the sense of twist in the
molecular helical subunits (Fig. 1 and 2). The chiral space
group means that all triple-helical strands have the same
The core of the structure is a conventional dinuclear double
helicate (Fig. 1) in which two bridging ligands wrap around
two Ag(I) ions, each of which is four coordinate from two
pyrazolyl-pyridine chelating units. Agꢀ ꢀ ꢀN distances lie in the
range 2.23–2.38 A and the angle between the two AgNN
planes is 57.91. The Agꢀ ꢀ ꢀAg separation is 11.48 A, due to
the length of the benzophenone spacer, which is oriented such
that the ketonic O atoms are directed externally. There are
weak inter-ligand interactions such as a CHꢀ ꢀ ꢀp contact
between H(62) of one ligand and the aromatic ring
C(51)–C(56) of the other [separation between H(62) and plane
of aromatic ring towards which it is directed, 3.25 A]. This
double helical unit lies on a twofold axis such that both metal
ions are crystallographically equivalent, as are both ligands.
More interesting than the structure of the double helicate
is the fact that short Agꢀ ꢀ ꢀAg contacts connect these units
end-to-end to form a 1-D chain which is itself helical (Fig. 2).
This intermolecular Agꢀ ꢀ ꢀAg separation is 2.99 A, well within
the range of an attractive argentophilic interaction,8 and in
addition results in aromatic p-stacking between the pyrazolyl-
pyridine groups of each Ag(I) centre (i.e. between adjacent
double helical subunits, Fig. 2a). The chain is oriented along
the c axis and forms a shallow helical spiral with a pitch length
of 77.25 A which corresponds to three unit cell lengths in that
direction; this involves six double helical complex units to
make one complete turn.
Fig. 3 The triple helical array arising from wrapping of three separate
infinite strands of double helical subunits (cf. Fig. 2) around each other,
with each main strand coloured separately for clarity. (a) A space-filling
view emphasising of the threefold helical array; (b) an end-on view of
the same fragment showing the central cavity down the centre of the
helix; (c) a stick diagram from the same perspective as (b), but showing
the anions.
Three of these (homochiral) helical chains are entwined
around each other to complete the supramolecular organi-
sation in the crystal (Fig. 3). This results in a quasi-cylindrical
assembly with a channel down the centre which is occupied
by perchlorate ions. The O atoms of these are disordered
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y Crystal data for L: C31H24N6O, M = 496.56 g molꢁ1, orthorhombic,
space group Pbcn, a = 8.5827(3), b = 13.3565(5), c = 21.0613(8) A,
U = 2414.36(15) A3, Z = 4, T = 100(2)K, l(Mo-Ka) = 0.71073 A,
m = 0.086 mmꢁ1. 19035 reflections were collected (2ymax = 55.21)
which after merging afforded 2792 independent reflections with
Rint = 0.0715. Final R1 [I > 2s(I)] = 0.049; wR2 (all data) = 0.173
(ref. 11).
z Synthesis of complexes. A solution of Ag(ClO4) (0.018 g, 0.079 mmol)
in MeOH (7 cm3) was added to a solution of L (0.040 g, 0.079 mmol) in
CH2Cl2 (7 cm3). The mixture was stirred at room temperature for 24 h,
and the resultant precipitate was filtered off, washed with both MeOH
and CH2Cl2, and dried in vacuo to give {[Ag2L2](ClO4)2}N as a white
powder in 67% yield. ESMS: m/z 1307.9, [Ag2L2](ClO4)+; 604.5,
[Ag2L2]2+. The tetrafluoroborate salt was prepared similarly using
AgBF4. ESMS: m/z 1295.7, [Ag2L2](BF4)+; 604.5, [Ag2L2]2+. X-ray
quality crystals in each case were grown by slow diffusion of ethyl
acetate into a solution of the complex in nitromethane: the crystals were
hygroscopic when removed from the mother liquor and gave variable
elemental analytical data.
chirality in the crystal which has therefore spontaneously
resolved on formation.
The structure of [Ag2L2](BF4)2ꢀMeNO2ꢀ(H2O)0.33 is presented
in ESI:w8 it differs from that of the perchlorate salt only in being
in a lower symmetry space group such that each [Ag2L2]2+ unit
has no internal symmetry with the two Ag(I) ions (and the two
ligands) being crystallographically independent. The AgꢀꢀꢀAg
contacts between [Ag2L2]2+ units, and the organisation of three
of the resulting strands into a triple helical arrangement around a
core of counter-ions, are essentially identical to what is observed
in [Ag2L2](ClO4)2ꢀMeNO2 and again a chiral space group is
adopted.
Individually the three different types of interaction resulting
in formation of this structure are well known in other contexts.
Bridging ligands that wrap around metal ions to assemble into
double helicates are commonplace.1 Argentophilic interactions
that connect Ag(I) complexes into chains are also well known,2b
with significant recent examples being assembly of Ag(I)-containing
double helicates into one dimensional oligomers or polymers
(as here).5 And finally the presence of weak interactions
between coordination polymer chains, leading to formation
of multi-stranded helical assemblies in crystals, is known with
many recent examples.2 What is remarkable about the structures
reported here however is the presence of all three levels of
supramolecular organisation occurring in the same compound
in a clear hierarchical sequence in which there is a nice parallel
with the different levels of organisation in a protein. If the
structure of the ligand L is analogous to the primary covalent
sequence of a protein, then (i) formation of local order (assembly
of the double helicate molecule) is akin to secondary protein
structure; (ii) Agꢀꢀ ꢀAg contacts bringing together the locally-
ordered components into a complete chain corresponds to the
tertiary structure (cf. formation of a complete protein subunit);
and (iii) the wrapping of three such chains around each other
promoted by weak ligand/ligand and ligand/anion interactions,
to give an infinite triple helix of molecular double helicates,
corresponds to the quaternary structure of proteins in which
subunits associate via weak interactions between them. This
combination of different types of self assembly at both the
molecular and crystal growth levels illustrates the power of
self-assembly to achieve order on different scales if only the
‘rules’ behind self-assembly can be fully understood.
8 Crystal data for [Ag2L2](ClO4)2ꢀMeNO2: C63H51Ag2Cl2N13O12
,
M = 1468.8 g molꢁ1, hexagonal, space group P6322, a = b =
21.5080(7), c = 25.7514(13) A, U = 10316.5(7) A3, Z = 6,
T = 100(2)K, l(Mo-Ka) = 0.71073 A, m = 0.714 mmꢁ1. 118981
reflections were collected (2ymax = 46.51) which after merging
afforded 4963 independent reflections with Rint = 0.0786. Final
R1 [I > 2s(I)] = 0.095; wR2 (all data) = 0.260; absolute structure
parameter = 0.07(10) (ref. 11).
Crystal data for [Ag2L2](BF4)2ꢀMeNO2ꢀ(H2O)0.33: C63H51.67Ag2B2F8-
N13O4.33, M = 1449.5 g molꢁ1, hexagonal, space group P63, a = b =
21.2675(6), c = 25.9142(10) A, U = 10150.8(6) A3, Z = 6, T = 100(2)K,
l(Mo-Ka) = 0.71073 A, m = 0.656 mmꢁ1. 154617 reflections were
collected (2ymax = 55.31) which after merging afforded 15618 inde-
pendent reflections with Rint = 0.0629. Final R1 [I > 2s(I)] = 0.120;
wR2 (all data) = 0.367; absolute structure parameter = 0.20(5) (ref. 11).
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Notes and references
z Synthesis of L. A mixture of 4,40-bis(bromomethyl)benzophenone
(ref. 9) (1.00 g, 2.7 mmol) and 3-(2-pyridyl)pyrazole (ref. 10) (0.78 g,
5.4 mmol; 2 equivalents) in THF (60 cm3) containing aqueous NaOH
(2.16 g in 10 cm3 H2O) was heated to reflux for 20 h. After cooling the
solution was filtered, dried with MgSO4 and reduced to dryness to
yield a white powder which was washed with diethyl ether and dried
(0.93 g, 1.9 mmol, 70%). 1H NMR (400 MHz, CDCl3): d8.65 (2H, ddd,
J = 5.2, 1.2, 0.8; pyridyl H6), 7.95 (2H, dt, J = 7.9, 1.0; pyridyl H3), 7.77
(4H, d, J = 8.3; benzophenone H), 7.73 (2H, td, J 7.9, 1.8; pyridyl H4),
7.49 (2H, d, J 2.3; pyrazolyl H5), 7.33 (4H, d, J 8.3; benzophenone H),
7.22 (2H, m; pyridyl H5), 6.96 (2H, d, J 2.3 Hz; pyrazolyl H4), 5.49
(4H, s; CH2). ESMS: m/z 497 (M+H)+. Anal. Calcd for C31H24N6O:
C 75.0; H, 4.9; N, 16.9%. Found: C, 75.1; H, 4.7; N, 16.7%.
(b) P. Pykko, Chem. Rev., 1997, 97, 597.
¨
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This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3605–3607 3607