Formation of a [1 + 1] metallomacrocycle from a heterotritopic ligand containing two terpy and one bipy metal-binding domains

Article abstract of DOI:10.1039/b205233d

Reaction of the heterotritopic ligand L, which contains one bipy and two terpy metal-binding domains linked by a flexible spacer with iron(II) chloride results in the formation of a [1 + 1] metallomacrocycle.

Full text of DOI:10.1039/b205233d

Formation of a [1 + 1] metallomacrocycle from a heterotritopic ligand
containing two terpy and one bipy metal-binding domains
Christopher B. Smith,* Edwin C. Constable,* Catherine E. Housecroft and Benson M. Kariuki
School of Chemical Sciences, The University of Birmingham, Edgbaston, Birmingham, UK B15 2TT.
E-mail: e.c.constable@bham.ac.uk
Received (in Cambridge, UK) 30th May 2002, Accepted 2nd August 2002
First published as an Advance Article on the web 15th August 2002
Reaction of the heterotritopic ligand L, which contains one
bipy and two terpy metal-binding domains linked by a
flexible spacer with iron(II) chloride results in the formation
of a [1 + 1] metallomacrocycle.
Conformation of the product stoichiometry was obtained
through X-ray structural analysis.‡ The complex is a mono-
nuclear [1 + 1] metallomacrocycle (Fig. 2), in which a ring is
formed through the coordination of both terpy domains in a
1
single L ligand to an iron(II) centre. As predicted from the H
Ligands containing multiple bipy or terpy metal-binding
domains have been extensively used in the metal-directed
assembly of novel supramolecules such as helices, grids,
catenanes and knots.1 Recent studies by several groups2,3 have
shown that ligands containing two tpy domains linked through
short, or rigid spacer regions can self-assemble, in the presence
of six-coordinate metal ions to form metallomacrocycles.
However, the investigation of ligands containing longer, more
flexible spacers in metal-directed assembly processes is less
common,4 due to the tendency for formation of polymeric or
polydispersed oligomeric species. In this report, we describe the
synthesis of such a ligand and the outcome of a metal-directed
assembly process.
NMR data, the bipy domain is non-coordinated, with nitrogen
atoms N7 and N8 adopting a transoid arrangement in the solid
The heterotritopic ligand L containing two terpy domains and
one bipy domain linked through polyether spacers, L† was
conveniently prepared in two steps from the 5,5A-disubstituted
2,2A-bipyridine 15 in an overall yield of 56% (Scheme 1).
Reaction of 1 with 2-[2-(2-chloroethoxy)ethoxy] ethanol in
DMF in the presence of potassium carbonate gave diol 2† as a
waxy yellow solid in 70% yield. This compound was efficiently
converted to L in 80% yield by reaction with 4A-chloro-
2,2A+6A,2B-terpyridine6 in DMSO in the presence of potassium
hydroxide.7
The interaction of L with iron(II) chloride was investigated.
Addition of one molar equivalent of aqueous iron(II) chloride to
a solution of L in acetonitrile immediately produced a purple
solution (lmax = 558 nm), typical of a {Fe(terpy)2} chromo-
phore. Analysis of the solution composition by TLC (silica gel,
MeCN-saturated aqueous KNO3–H2O 10+0.5+1.5) after 5 min
showed the formation of several purple products, indicating the
formation of a library of complexes of different stoichiometries.
However, over time several of these components disappeared,
and after 24 h TLC analysis showed the presence of a single
purple species. Addition of NH4PF6 precipitated a purple solid
of composition {FeL(PF6)2}.
The 1H NMR spectrum of a CD3CN solution of the isolated
purple complex indicated high symmetry with only single terpy
and bipy domains being observed. The aromatic region of the
500 MHz 1H NMR spectrum is shown in Fig. 1 together with the
assignments from a COSY experiment and comparison with
model compounds. The doublet at d 8.68 is due to H6A adjacent
to the bipy nitrogen atoms and its resonance to low field is
characteristic of a non-coordinated bipy domain.8 The high RF
of the complex (RF 0.6) also indicates that the product is not a
high nuclearity polymer.
The ESMS of the complex in CH3CN exhibited peaks at m/z
1451 {[FeL][PF6]2 + K}+, 1435 {[FeL][PF6]2 + Na}+, 1141
{[FeL][F]+, and a doubly charged ion at 561 [FeL]2+. While
these are indicative of a 1+1 ligand+metal stoichiometry, the
detection of a low intensity peak at m/z 2680 corresponding to
{Fe2L2(PF6)3}+ raised an ambiguity whether this was due to a
genuine [2 + 2] species of a cluster ion {(FeL)2(PF6)3}+.
Scheme 1 Reagents: (a) 2-[2-(2-chloroethoxy)ethoxy] ethanol, K2CO3,
DMF; (b) 4A-chloro-2,2A:6A,2B-terpyridine, KOH, DMSO.
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state. The metal centre is coordinated to both terpy motifs in the
expected octahedral environment (selected Fe–N bond dis-
tances are shown in the Fig. 2 caption). Additionally, one half of
the macrocyclic cavity is occupied by an acetone molecule,
whose position is stabilised through weak electrostatic inter-
actions between the hydrogen atoms of C65 and C67 and the
oxygen atoms of the spacer linkage (all distances between C65
and C67 and oxygen atoms O1–O4 in the range 3.45–3.75
Å).
Formation of this metallomacrocycle in high yield is very
unexpected, as polymeric structures could be formed through
either coordination to the bipy motif, or through coordination of
each metal centre to binding domains of different ligand
molecules. The formation of a single thermodynamic product is
in this case remarkable due to the far greater length and
flexibility of the spacers in L compared to previously published
examples of multitopic ligands containing two terpy do-
mains.2,3
In conclusion, we have shown that the tritopic ligand L,
which contains mixed metal-binding domains connected
through large flexible spacers, self-assembles to form a
thermodynamically stable [1 + 1] metallomacrocycle in the
presence of iron(II). Current investigations involve varying the
length and flexibility of the spacer regions with a view to
regulating the assembly process.
We would like to thank the University of Birmingham for
support.
Notes and references
†
Selected data for compound 2: ESMS: m/z 627 ([M + Na]+). 1H NMR
(CDCl3, 300 MHz) d 8.92 (d, 2H, H6A), 8.49 (d, 2H, H3A), 8.02 (dd, 2H,
H4A), 7.41 (t, 2H, H5B), 7.25 (m, 2H, H6B), 7.23 (m, 2H, H2B), 6.97 (dd, 2H,
H
4B), 4.23 (t, 4H), 3.91 (t, 4H), 3.75 (m, 12H), 3.63 (t, 4H).
L. ESMS: m/z 1089 ([M + Na]+). 1H NMR (CDCl3, 500 MHz) d 8.88 (d,
2H, H6A), 8.66 (m, 4H, H6D), 8.60 (t, 4H, H3D), 8.45 (d, 2H, H3A), 8.04 (s,
4H, H3C), 7.97 (dd, 2H, H4A), 7.82 (m, 4H, H4D), 7.36 (t, 2H, H5B), 7.31 (m,
4H, H5D), 7.20 (m, 4H, H3B/H6B), 6.95 (dd, 2H, H4B), 4.40 (t, 4H), 4.21 (t,
4H), 3.93 (m, 8H), 3.80 (m, 8H).
‡ X-Ray quality crystals were obtained through diffusion of diethyl ether
into an acetone solution of the complex. Crystal data: C64H58F12Fe-
N8O8P2·C3H6O (purple prisms), M = 1471.05, monoclinic, space group
P21/n, a = 9.4845(5), b = 20.2873(12), c = 33.9574(18), b = 93.440(3),
U = 6522.1(6) Å3, T = 223(2), Z = 4, m(Cu-Ka) = 3.184 mm21. 12903
reflections measured, 7257 unique, 2216 observed with I > 2sI, R(obs.) =
0.1027. CCDC reference number 187005. See http://www.rsc.org/suppdata/
cc/b2/b205233d/ for crystallographic data in CIF or other electronic
format.
Fig. 1 The aromatic region of the 500 MHz 1H NMR spectrum of a CD3CN
solution of [FeL][PF6]2 showing the assignments.
1 Comprehensive Coordination Chemistry, ed. J.-M. Lehn, volume eds. J.-
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4 For self-assembly of a trinuclear iron(II) complex from a ligand
containing three tpy binding domains connected through long flexible
spacers, see: E. C. Constable and D. Phillips, Chem. Commun., 1997,
827.
5 The synthesis of this compound will be presented in detail elsewhere. It
involves Suzuki coupling of 3-methoxyphenylboronic acid and 5,5A-
dibromo-2,2A-bipyridine. For the preparation of 5,5A-dibromo-2,2A-
bipyridine, see: F. M. Romero and R. Ziessel, Tetrahedron Lett., 1995,
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6 E. C. Constable and M. D. Ward, J. Chem. Soc., Dalton Trans., 1990,
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7 For examples of this methodology, see: E. C. Constable, C. E.
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Fig. 2 X-Ray structure of the [1 + 1] metallomacrocycle showing the
encapsulated acetone molecule. Hydrogen atoms and counter ions have
been omitted. Ellipsoids are drawn at the 50% probability level. Selected
Fe–N bond distances (Å): Fe1–N1, 2.014; Fe1–N2, 1.853; Fe1–N3, 1.998;
Fe1–N4, 1.998; Fe1–N5, 1.936; Fe1–N6, 2.012.
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