Metal-directed self-assembly of bimetallic dithiocarbamate transition metal cryptands and their binding capabilities

Article abstract of DOI:10.1039/b308629a

A novel family of redox-active dinuclear transition metal based cryptands self-assembled from dithiocarbamate ligands has been synthesised; depending upon the nature of the spacer groups these new cryptand systems have been shown to electrochemically reco

Full text of DOI:10.1039/b308629a

Metal-directed self-assembly of bimetallic dithiocarbamate transition
metal cryptands and their binding capabilities†
Paul D. Beer,* Neil G. Berry, Andrew R. Cowley, Elizabeth J. Hayes, Edward C. Oates and Wallace
W. H. Wong
Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, Oxford, UK OX1 3QR.
E-mail: paul.beer@chem.ox.ac.uk
Received (in Cambridge, UK) 23rd July 2003, Accepted 22nd August 2003
First published as an Advance Article on the web 5th September 2003
A novel family of redox-active dinuclear transition metal
based cryptands self-assembled from dithiocarbamate li-
gands has been synthesised; depending upon the nature of
the spacer groups these new cryptand systems have been
shown to electrochemically recognise the binding of cations
or anions.
obtained were recrystallised to afford the analytically pure
products 1–6 in yields of 13–76 %.
The use of metal-directed self-assembly techniques provides a
facile route to novel host macrocycles capable of binding
charged and neutral guest substrates. Through careful con-
sideration of ligand and preferred metal coordination geometry,
among other factors, complicated and unusual molecular
architectures are attainable.1
Recently the dithiocarbamate (dtc) moiety has proven to be a
useful structural motif, lending itself to the metal-directed
assembly of a range of structures including nano-sized resorcar-
ene-based assemblies, catenanes, assorted macrocycles and
trinuclear cages.2 The incorporation of redox-active metal–
ligand units into the framework of a host introduces the
possibility of using the host as an electrochemical sensor for
guest substrates.3
Herein, we describe the application of dithiocarbamate
ligands for the first time in the construction of metal-directed
self-assembled bimetallic cryptands. The cryptands can be
assembled through careful choice of a suitable octahedral
stereochemical directing metal centre such as Fe(III) and Co(III),
as shown in Scheme 1. A range of redox-active metallo-
cryptands with differing ligand spacer groups are presented,
along with their preliminary electrochemical sensing results.
The starting secondary diamines containing polyether and
alkyl–aryl amide linkages were prepared by straightforward
synthetic methods.† Dithiocarbamate complexes were prepared
in a one-pot synthesis from the parent secondary amines by
reaction with carbon disulfide, base and an appropriate
octahedral transition metal halide salt. The crude powders thus
The new bimetallic cryptands 1–3 containing polyether
cavities of various sizes were designed to complex and sense
Group I metal cations. Initial evidence of metal cation binding
was provided by electrospray mass spectrometry (ESMS) which
revealed the formation of various alkali metal cation–cryptand
complex adducts in the ESMS spectra. Proton NMR spectra of
the diamagnetic cobalt(III) cryptands were too complicated to
enable quantitative titration experiments to be carried out.
Electrochemical recognition studies were undertaken using
cyclic and square wave voltammetry. The electrochemical
properties of 1–3 differ to those of simple acyclic M(Et2CNS2)3
complexes in exhibiting broad overlapping redox waves.
Table 1 Electrochemical Group I metal cation recognition dataa
DEp/mVb
Li+
Na+
K+
Cs+
1a
2a
3a
20
0
0
25
25
20
15
35
30
5
10
45
a Square wave voltammograms recorded in 1 : 1 CH2Cl2–MeCN solutions
containing 0.1 mol dm23 NBu4BF4 as supporting electrolyte. b Anodic shift
of the Co(IV)/Co(III) oxidation potential produced by presence of alkali
metal cations (up to 5 equiv.) added as their perchlorate, hexafluor-
ophosphate and triiodide salts.
Scheme 1 General synthesis of dtc cryptands.
† Electronic supplementary information (ESI) available: selected experi-
mental procedures. See http://www.rsc.org/suppdata/cc/b3/b308629a/
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This journal is © The Royal Society of Chemistry 2003

Table 2 Electrochemical anion recognition dataa
DEp/mVb
whereas with the smaller cryptands 2a and 1a, K+ and Na+
respectively induce the largest anodic shifts. Electrochemical
recognition experiments with Co(Et2CNS2)3 revealed no evi-
dence of interactions with Group I metal cations.4
With a view to constructing cryptands for anion complexa-
tion the new receptors 4–6 containing amide hydrogen bond
donor groups were prepared using the one-pot synthetic
protocol. Proton NMR anion titration experiments in polar
organic solvents with the cobalt(III) cryptands proved problem-
atic because of broadness of spectra. However, electrochemical
2
2
Cl2
OBz2
HSO4
H2PO4
5a
6a
15
60
25
50
25
50
125
90
a Square wave voltammograms recorded in CH2Cl2 solutions containing 0.1
mol dm23 NBu4BF4 as supporting electrolyte. b Cathodic shift of the
Co(IV)/Co(III) oxidation potential produced by presence of anions (up to 10
equiv.) added as their TBA salts.
anion binding studies revealed the respective Co(IV)/Co(III
)
redox couple to undergo significant cathodic perturbations of up
2
to DE = 125 mV with H2PO4 in electrochemical dichloro-
methane solutions (Table 2). No anion induced shifts were seen
with Co(Et2CNS2)3.
An alternative method for bimetallic cryptand synthesis
involving chemical oxidation of a preformed bimetallic nick-
el(II) dtc macrocycle was also investigated. Simple acyclic
square planar nickel(II) (dtc)2 complexes can undergo a two
electron oxidation to form an octahedral nickel(IV) (dtc)3
positively charged species.5
The new bimetallic nickel(II) macrocycle 7 was initially
synthesised in 76% yield (Scheme 2). Crystals of 7 were grown
from a chloroform–acetonitrile solution, and the structure is
shown in Fig. 1. As expected the nickel(II) dithiocarbamate
group forms an almost square planar linkage6 with the nickel
atom lying only 3.2° out of the plane. The macrocycle 7 was
oxidised using two equivalents of N-bromosuccinimide (NBS)
in CHCl3 to produce the novel bimetallic Ni(IV) cryptand 8
which was characterised by UV/visible spectroscopy, electro-
chemistry, infrared, electrospray mass spectrometry (ESMS)
and elemental analysis (Scheme 2). Preliminary anion binding
studies in CHCl3–MeCN (4 : 1) revealed the addition of
chloride and nitrate anions to 8 produced cathodic shifts of the
Ni(IV)/Ni(III) wave of 70 mV and 15 mV respectively.
In conclusion, the dithiocarbamate ligand system has proven
its versatility in providing a route to the metal-directed self-
assembly of a range of novel bimetallic cryptands containing
octahedral metal ions. Initial binding studies show these
systems are useful in the electrochemical sensing of cations and
anions.
We gratefully acknowledge the EPSRC for a postdoctoral
fellowship (EJH) and studentships (NGB and WWHW).
Scheme 2 Synthesis of nickel(IV) cryptand 8.
Notes and references
‡ Crystal data for C50H71N9Ni2O4S8, M = 1236.08, monoclinic, a =
34.0525(6), b = 19.3859(4), c = 9.4251(2) Å, b = 102.058(3)°, V =
6084.6 Å3, T = 150 K, space group C 2/c, Z = 4, 45757 reflections
measured, 7118 unique (Rint = 0.057) were used in all calculations. The
final R1 (all data) was 0.0457 and wR2 (all data) was 0.0524. CCDC
216372. See http://www.rsc.org/suppdata/cc/b3/b308629a/ for crystallo-
graphic data in .cif or other electronic format.
1 R. W. Saalfrank and I. Bernt, Curr. Opin. Solid State Mater. Sci., 1998,
3, 407; M. Fujita, Chem. Soc. Rev., 1998, 27, 417; S. Leininger, B.
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Soc. Rev., 1998, 27, 289.
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2000, 39, 136; M. E. Padilla-Tosta, O. D. Fox, M. G. B. Drew and P. D.
Beer, Angew. Chem., Int. Ed. Engl., 2001, 40, 4235; P. D. Beer, N. Berry,
M. G. B. Drew, O. D. Fox, M. E. Padilla-Tosta and S. Patell, Chem.
Commun., 2001, 199; P. D. Beer, A. G. Cheetham, M. G. B. Drew, O. D.
Fox, E. J. Hayes and T. D. Rolls, Dalton Trans., 2003, 4, 603.
3 P. D. Beer, P. A. Gale and G. Z. Chen, Coord. Chem. Rev., 1999, 185,
3.
4 Preliminary electrochemical Group I metal cation binding studies with
the Fe(III) polyether cryptands revealed insignificant perturbations of the
Fe(IV)/(III) redox couples.
5 J. P. Fackler Jr., A. Avdeef and R. G. Fischer Jr., J. Am. Chem. Soc., 1973,
95, 774.
Fig. 1 Crystal structure of 7.‡
Focusing on the irreversible Co(IV)/Co(III) oxidation redox
couple of 1a, 2a and 3a, the addition of alkali metal cations led
in most cases to significant anodic shifts where the cryptand-
complexed metal cation destabilizes the cobalt(IV) oxidation
state. It is noteworthy that Table 1 shows there is a correlation
between magnitude of anodic shift and complementary metal
cation : cryptand cavity size, and not as commonly observed
with metal cation polarising character. For example with the
large cryptand 3a, Cs+ causes the greatest perturbation of 45 mV
6 M. Bonamico, G. Dessy, C. Mariani, A. Vaciago and L. Zambonelli, Acta
Crystallogr., 1965, 19, 619; M. N. I. Khan, J. P. Fackler Jr., H. H. Murray,
D. D. Heinrich and C. Campana, Acta Crystallogr., Sect. C: Cryst. Struct.
Commun., 1987, 43, 1917.
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