The 'Trinity' helix: Synthesis and structural characterisation of a C3-symmetric tris-bidentate ligand and its coordination to Ag(I)

Article abstract of DOI:10.1039/b302652c

The synthesis and structural characterisation of a novel C₃-symmetric tris-bidentate ligand, L, featuring a triphenylamine core appended by pyridylimine coordination sites is reported: ¹H NMR compleximetric titration studies with Ag(I) and ESMS indicate the presence of [Ag₃L₂]³⁺ species in solution, consistent with the formation of a trinuclear double helicate complex: the Trinity helix.

Full text of DOI:10.1039/b302652c

The ‘Trinity’ helix: synthesis and structural characterisation of a
C -symmetric tris-bidentate ligand and its coordination to Ag(
3
I)†
Brian Conerney, Paul Jensen, Paul E. Kruger* and Conchúir MacGloinn
Department of Chemistry, Trinity College, Dublin 2, Ireland. E-mail: paul.kruger@tcd.ie
Received (in Cambridge, UK) 7th March 2003, Accepted 25th March 2003
First published as an Advance Article on the web 23rd April 2003
The synthesis and structural characterisation of a novel C
3
-
2 4
N H –Pd/C mediated reduction of tris-4-nitrotriphenylamine
symmetric tris-bidentate ligand, L, featuring a triphenyla-
followed by condensation of the resulting tri-amine with
pyridine-2-carbaldehyde.‡ Recrystallisation from toluene gave
orange crystals suitable for a single crystal X-ray diffraction
mine core appended by pyridylimine coordination sites is
1
reported: H NMR compleximetric titration studies with
3+
12
Ag(I) and ESMS indicate the presence of [Ag
3
L
2
]
species in
study§ from which its structure was determined (ESI†). The
solution, consistent with the formation of a trinuclear double
helicate complex: the Trinity helix.
ligand structure is as anticipated with three pyridylimine arms
radiating from the amine core. The ligand is chiral due to the
propeller nature of these arms, although the crystal is racemic as
both D and L enantiomers are present. The ligand forms a
number of self-complementary H-bond (C–H…N) interactions,
and C–H…p contacts, which create a 3D network containing
channels running down the crystallographic b-axis which are
occupied by disordered toluene molecules (ESI).
Current interest within supramolecular chemistry involves the
development of strategies to control the outcome of metal-
1
assisted self-assembly processes. In this respect, ligands have
been tailored to recognise intrinsic geometric preferences of
particular metal ions, or to enforce structural constraints upon
coordinated ions. Fine tuning of these ligand systems has
resulted in the isolation of many structural types including
Treatment of a CHCl
AgPF (2 : 3) led to the formation of a deep red solution from
which an orange solid precipitated following the addition of
NH PF .¶ Partial microanalytical data indicate an
[Ag ](PF formulation and ESMS revealed a cluster of
peaks centred around m/z 479.762, with an isotopic distribution
3
solution of L with a MeCN solution of
6
2
3
4
5
6
grids, boxes, cylinders, molecular polyhedra and helicates.
Amongst these, however, the helical topography has attracted
the most interest because of their prevalence and importance in
nature, the ready information they provide about self-assembly
processes, and because of their aesthetic beauty.6c
Ligands employed in the synthesis of metallo-helicates are
2
predominantly based upon C -symmetric polypyridyl or related
4
6
3
L
2
6 3
)
3+
pattern consistent with [Ag
3
L
2
]
ions in solution (ESI).
1
Furthermore, the H NMR spectrum is particularly well
resolved and relatively simple and indicates the presence of a
single species that retains a high degree of symmetry on the
backbones, and consist of bi- or tri-dentate binding sites linked
in series.6 Recently, we have employed more synthetically
facile Schiff’s-base ligand types in the formation of metallo-
NMR timescale, Fig. 1. Coordination of Ag( ) is apparent as all
I
protons shift to some degree. Most noticeable amongst these are
the pyridyl protons, b and c, and the imine proton, e, which shift
downfield by ca. 0.25 ppm. The pyridyl proton, d, shifts by a
similar amount upfield, whilst a shifts marginally downfield by
ca. 0.02 ppm. The protons g and f, of the central phenyl rings,
shift upfield and downfield by ca. 0.06 and 0.10 ppm,
respectively. These observations are again consistent with the
7
helicates and have latterly extended this approach to incorpo-
8
rate anion binding, reporting the first dinuclear anion helicate.
Whilst great advances continue to be made, a possible limitation
to the development of the field is the reliance upon linearly
disposed binding sites. We redress this situation here and
3
describe the synthesis of a C -symmetric ligand, L, consisting
3+
of three pyridylimine binding sites, and report its coordination
presence of [Ag
3
L
2
]
in solution. It should be noted that there
3+
to Ag(
I), to form [Ag
3
L
2
] . We believe this to be the first report
is no indication of any ligand dissociation nor of any fluxional
processes operating under these conditions, which is in direct
dealing specifically with the use of a rigid trigonal ligand in the
formation of helical complexes.
contrast to the behaviour observed for similar systems based
upon linear C
2
symmetric ligands.6
c,9a,13
3 2
To investigate the solution chemistry of [Ag L
]3+ further
and to better understand the processes leading to its formation
we performed a compleximetric titration experiment, tracking
the progress of complex formation through observing shifts
1
2a
25
within the H NMR spectra. To this end, a 1.06 3 10
M
CD CN–CDCl (4 : 1) solution of L was treated with successive
3
3
The tris-bidentate ligand, L, incorporates the binding features
found in the bis-bidentate ligand, L1, which forms dinuclear
double and triple helicates.7 We first developed C
,9
-symmetric
3
ligands based around a tetrahedral central carbon, by using a tri-
anilinomethane scaffold, obtained through the reduction of
parafuschin10 and subsequent condensation with pyridine-
2
-carbaldehyde, L2. Whilst coordination to Ag( ) was success-
I
11
ful, the sensitivity of this scaffold toward oxidation (visibly
noticeable through formation of deep-purple coloured solu-
tions) led us to develop the more robust ligand, L, based around
a triphenylamine core. L was synthesised in two steps via
†
Electronic supplementary information (ESI) available: experimental
Fig. 1 1H NMR spectra of L (bottom) and [Ag
3
assignments (400 MHz, CDCN–CDCl 298 K). * residual CHCl .
3 2 6 3
L ](PF ) (top) with
details. See http://www.rsc.org/suppdata/cc/b3/b302652c/
3
1
274
CHEM. COMMUN., 2003, 1274–1275
This journal is © The Royal Society of Chemistry 2003

additions of a 0.25 M solution of AgPF
regime (equating to sequential ca. 0.12 mol equivalents of Ag(
6
within the same solvent
tion to Ag(
from the traditionally employed C
dominate the field of metallo-helicate chemistry and provides
valuable insight into metal directed self-assembly processes.
We are currently extending this series further.
We thank Enterprise Ireland for financial support, Drs A.C.
Lees and T. Gunnlaugsson for helpful discussion and Drs J.
O’Brien (NMR) and M. Feeney (ESMS) for assistance.
I
). This approach represents a significant move away
I
)
2
-symmetric ligands that
with each addition), Fig. 2. Several points of note can be taken
from this experiment. Firstly, the final spectrum is identical to
that obtained above. Secondly, those protons that undergo the
largest shifts (b, c, d, e) do so before the addition of 1.5
equivalents, whereupon no further appreciable shifting is
observed up to and beyond 2.5 equivalents. Thirdly, a slight
inflection point may be discerned within the plots of those
protons undergoing marginal shifts (a, f, g) at ca. 1 : 1
stoichiometry.
1
4
Notes and references
‡
Full synthetic details will be given elsewhere. Selected data for L: Found
C, 72.22; H, 4.55; N, 16.31; [C36 ]·2.25 H O requires C, 72.28; H,
.30; N, 16.39%. H NMR (400 MHz; 4 : 1 CD CN–CDCl ): d 8.71 (3 H,
d, J = 4.0 Hz, H ), 8.66 (3 H, s, H ), 8.19 (3 H, d, J = 8.0 Hz, H ), 7.89 (3
H, dd, J = 8.0, 6.0 Hz, H ), 7.45 (3 H, dd, J = 6.0, 4.0 Hz, H ), 7.35 (6 H,
d, 7.0 Hz, H ), 7.19 (6 H, d, J = 7.0 Hz, H ). nmax/cm 1625(s), 1581(s),
H N
27 7
2
1
5
3
3
a
e
d
c
b
21
f
g
+
1
§
(
499(s), 1319(s). ESMS m/z 558.24 [L] .
39.5 31 7
Crystal data for L·(toluene)0.5 C H N , M = 603.71, orange prism
¯
0.25 3 0.20 3 0.08), triclinic P1, a = 10.1332(8) Å, b = 11.0554(8) Å,
c = 16.6857(13) Å, a = 70.954(2)°, b = 75.751(2)°, g = 74.992(2)°, V =
3
23
1
0
679.7(2) Å , T = 153(2) K, Z = 2, Dcalc = 1.194 g cm , m(Mo–Ka) =
1
2
.073 mm , Bruker SMART APEX CCD diffractometer, Mo–Ka
radiation (l = 0.71073 Å). 3144 independent reflections, 2049 observed (I
> 2s(I)). R = 0.0987, wR = 0.2723 for observed reflections. CCDC
05745. See http://www.rsc.org/suppdata/cc/b3/b302652c/ for crystallo-
graphic data in .cif or other electronic format.
Selected data for [Ag ][PF : Found: C, 46.09; H, 3.06; N, 10.60.
Ag (C36 ][PF requires C, 45.86; H, 3.53; N, 10.40%. H NMR
(400 MHz; 4 : 1 CD CN–CDCl ): d 8.81 (6 H, s, H ), 8.72 (6 H, d, J = 5.0
Hz, H ), 8.13 (6 H, dd, J = 7.5, 6.0 Hz, H ), 7.96 (6 H, d, J = 7.5 Hz, H ),
.70 (6 H, dd, J = 6.0, 5.0 Hz, H ), 7.32 (12 H, d, J = 8.5 Hz, H ), 7.01 (12
Fig. 2 Compleximetric titration of L (1.06 3 102 M) with AgPF
5
1
2
6
(0.25 M)
2
showing Dd of assigned protons with successive additions {400 MHz,
CDCN–CDCl (4 : 1) 298 K}.
3
¶
[
3
L
2
6 3
]
1
These observations may be rationalised in the following way:
3
H
27
N
)
7 2
6 3
]
2+
3
3
e
2 2
successive additions firstly lead to [Ag L ] , a dinuclear
a
c
d
species at 1 : 1 stoichiometry. The presence of this species may
account for the inflection within the plots for a, f, and g, as these
protons begin to feel through-space effects of the neighbouring
strand. This species, potentially a dinuclear double helicate, has
7
b
f
3+
H, d, J = 8.5 Hz, H
g 3 2
). ESMS, m/z 479.762 [Ag L ] .
1
J.-M. Lehn, Supramolecular Chemistry, Concepts and Perspectives,
VCH, Weinheim, 1995, pp 139–160; R. Robson, in Comprehensive
Supramolecular Chemistry, eds. J. L. Atwood, J. E. D. Davies, D. D.
MacNicol, F. Vögtle and J.-M. Lehn, Pergamon, Oxford, 1997, Vol. 6,
p. 733; G. F. Swiegers and T. J. Malefeste, Chem. Rev., 2000, 100,
3483.
two vacant coordination sites. Further addition of Ag(
I) binds
the two vacant sites together to give the tri-nuclear product:
3
+
[Ag
3
L
2
] , which is robust in the presence of excess Ag( ) and
I
1
non-fluxional as the H NMR spectrum is well resolved.
But what does [Ag
3+
3
L
2
]
look like? Based upon the above
2
3
(a) A. Marquis, J. P. Kintzinger, R. Graff, P. N. W. Baxter and J.-M.
Lehn, Angew. Chem., Int. Ed., 2002, 41, 2760; (b) M. B. Zaman, K.
Udachin, M. Akhtaruzzaman, Y. Yamashita and J. A. Ripmeester,
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Y. Suenaga, T. Kuroda-Sowa, M. Maekawa and M. Munakata, J. Chem.
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Chem. Commun., 1998, 1, 147; M. Albrecht and S. Kotila, Chem.
Commun., 1996, 2309.
evidence we propose, and molecular modelling supports, that
3+
[
Ag
complex is reflected in each arm, as they cross over the Ag–Ag
axis. Helical complexes based upon C -symmetric ligand types
3 2
L ] is a helicate complex, Fig. 3. The helical nature of the
2
may exist in either a helical form (rac-isomer) or as a metallo-
cyclophane (meso-isomer, the ligands do not cross the M–M
axis), and may interconvert between each (as evidenced by their
fluxional behaviour in solution).6c Whilst this is an option here,
4 D. W. Johnson, J. Xu, R. W. Saalfrank and K. N. Raymond, Angew.
Chem., Int. Ed., 1999, 38, 2882.
the three-armed nature of the complex ensures that a helical
species must exist even if the meso-isomer is formed i.e. it is
impossible to coordinate three arms without at least two arms
crossing the Ag–Ag axis (ESI).
5
B. J. Holliday and C. A. Mirkin, Angew. Chem., Int. Ed., 2001, 40, 2022;
M. Fujita, Struct. Bonding, 2000, 96, 177; P. J. Stang and B. Olenyuk,
Acc. Chem. Res., 1997, 30, 502.
6
(a) O. Mamula and A. von Zelewsky, J. Chem. Soc., Dalton Trans.,
In conclusion, we have shown for the first time that a rigid
2
000, 219; (b) C. Piguet, G. Bernardinelli and G. Hopfgartner, Chem.
C
3
-symmetric ligand readily forms helical species on coordina-
Rev., 1997, 97, 2005; (c) M. Albrecht, Chem. Rev., 2001, 101, 3457; (d)
E. C. Constable, Tetrahedron, 1992, 48, 10013.
7
P. E. Kruger, N. Martin and M. Nieuwenhuyzen, J. Chem. Soc., Dalton
Trans., 2001, 1966; J. Keegan, P. E. Kruger, M. Nieuwenhuyzen and N.
Martin, Cryst. Growth Design, 2002, 2, 329.
8
9
J. Keegan, P. E. Kruger, M. Nieuwenhuyzen, J. O’Brien and N. Martin,
Chem. Commun., 2001, 2092.
(a) N. Yoshida, K. Ichikawa and M. Shiro, J. Chem. Soc., Perkin Trans
2
, 2000, 17; (b) M. J. Hannon, C. L. Painting, A. Jackson, J. Hamblin
and W. Errington, Chem. Commun., 1997, 1807.
1
1
1
1
0 D. Hellwinkel and H. Fritsch, Chem. Ber., 1990, 123, 2207; O. Fischer,
A. Fritzen and S. Eilles, J. Prakt. Chem., 1909, 79, 563.
1 B. Conerney, P. Jensen, P. E. Kruger and C. MacGloinn, unpublished
work.
2 SMART and SAINT-NT, Bruker-AXS, Madison, WI, 1998; G. M.
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2
023.
Fig. 3 Proposed structure of [Ag
3
L
2
]3+ showing the helical nature of each
14 N. Fatin-Rouge, S. Blanc, A. Pfeil, A. Rigault, A.-M. Albrecht-Gray and
strand.
J.-M. Lehn, Helv. Chim. Acta, 2001, 84, 1694.
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