A self-assembled macrocyclic tetranuclear molecular square [Co(HL)]44+ [H2L = tetra(2-pyridyl)thiocarbazone]

Article abstract of DOI:10.1039/a607689k

A novel conformationally rigid, cobalt-based cationic molecular square achieved via self-assembly from tetra(2-pyridyl)thiocarbazone is synthesized and structurally characterized.

Full text of DOI:10.1039/a607689k

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4+
A self-assembled macrocyclic tetranuclear molecular square [Co(HL)]4
[H2L = tetra(2-pyridyl)thiocarbazone]
Chun-ying Duan,*a Ze-hua Liu,a Xiao-zeng You,a Feng Xueb and Thomas C. W. Mak*b
a Coordination Chemistry Institute and The State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing
210093, P.R. China
b Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong
A novel conformationally rigid, cobalt-based cationic mole-
cular square achieved via self-assembly from tetra(2-pyr-
idyl)thiocarbazone is synthesized and structurally charac-
terized.
ligand.10 The C–N and N–N bond distances in HL2 are
intermediate between formal single and double bonds (see
Fig. 1), pointing to extensive electron delocalization over the
entire molecular skeleton. The imino H atom of the HL2
monocation could not be located from the difference Fourier
map, and is most likely disordered over both N(1) and N(5)
sites. Except for the uncordinated pyridine rings and the
disordered sulfur atom, the atoms in the monoanion are nearly
coplanar, the mean deviation from the best plane being 0.080 Å.
The dihedral angle between two HL2 ligands coordinated to the
same CoII atom is exactly 90° due to the symmetry of the I41/a
space group, the closest intermolecular distance between two
HL2 planes coordinated to different CoII centres is 4.43 Å,
indicating weak p–p interaction. The crystal structure of 1
Recently, conformationally rigid, tetranuclear macrocyclic ring
systems with approximately 90° bond angles about each corner
atom have been prepared in high yields via self-assembly from
rigidly available precursors, thereby adding a new dimension to
supramolecular as well as potentially biomimetic chemistry.1–7
Hitherto only two related types of supramolecules have been
reported. One is a square molecular box bridged by bifunctional
nitrogen donors such as 4,4A-bipyridine,1 2,6-diazanthracene,2
2,6-diazanthracene-9,10-dione,3 cis- or trans-meso-dipyridyl
porphyrins4 and the uracil monoanion,5 where polyvalent iodine
or heavy transition metals (Pd and Pt) occupy the corners. A
second system is the n 3 n inorganic grid bridged by a linear
polytopic ligand containing bidentate chelating sites for which
the square-matrix arryas are occupied by tetrahedrally coor-
dinating metal centres.6,7
4+
consists of a packing of tetranuclear [Co(HL)]4 cations
N
S
NH2NHCNHNH2
In the present work, we report the synthesis and structural
characterization of the first example of a cobalt-based cationic
molecular square achieved via self-asembly with a novel rigid
pentadentte ligand containing sulfur and nitrogen donor sites.
When di(pyridyl)ketone was treated with thiocarbazide in
methanol at reflux, a yellow solid was readily formed.
Elemental analysis supports the formation of the new penta-
dentate N4S ligand (C5H4N)2CNNN(H)C(S)N(H)NNC-
(C5H4N)2, tetra(2-pyridyl)thiocarbazone, H2L. Interaction of
CoCl2·6H2O with H2L and NaO2CMe in methanol at boiling
temperature gave the tetranuclear complex [Co(HL)]4[O2C-
Me]4·H2O 1 via self-assembly, as shown in Scheme 1.†
Macrocyclic complex 1 is a remarkably stable dark brown
crystalline solid and is soluble in most organic solvents. An
X-ray crystallographic study‡ has unequivocally confirmed the
existence of a tetranuclear cationic molecular square in 1.
In compound 1, the sulfur atom of the HL2 monocation
exhibits twofold positional disorder, and its scattering power is
represented by two ‘half-atoms’ S(1) and S(1A) separated by
0.93 Å. For the sake of clarity, only S(1) is shown in Fig. 1 and
referred to in subsequent discussion. The tetranuclear
+
O
N
N
N
H
H
N
N
N
C
S
N
N
N
H2L
CoCl2 + 2NaO2CMe
N
N
N
N
N
S
N
N
N
Co
Co
N
N
N
N
–
4+
[Co(HL)]4 cation is located at the 4 site with four cobalt(ii)
atoms at the corners of a square of edge length Co···Co of 4.43
Å, each metal centre being octahedrally coordinated by atoms
S(1), N(2) and N(3) of one HL2 ligand and S(1a), N(6a) and
N(7a) from a second ligand (Fig. 1). These two HL2 ligands
bind to cobalt(ii) in the mer configuration (with pairs of S atoms
and pyridyl N atoms each bearing a cis relationship, whereas the
thiocarbazone N atoms are trans to each other) as found in
related octahedral cobalt thiocarbazone complexes.8 The sulfur
atoms lie alternately above and below the mid-points of the
edges of the square, each being connected to two cobalt atoms
with a bond angle Co–S–Co of 151°. The measured C–S bond
distance of 1.814(4) Å is within the normal range of a C–S
single bond,9 indicating that the thiocarbazone moiety HL2
adopts the thiol tautomeric form in acting as a mononegative
S
S
N
N
N
N
Co
N
Co
N
N
S
N
N
N
N
N
[Co(HL)44+ macrocyclic square in 1
Scheme 1
Chem. Commun., 1997
381

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imbedded in a matrix of highly disordered acetate ions and
water molecules.
binding to a first-row transition metal, thereby offering the
potential of broadening the scope of further work in this area.
This work is supported by the National Nature Science
Foundation of China and Hong Kong Research Grants Council
Earmerked Grant CUHK 311/94p.
The UV–VIS spectra of 1 in methanol shows a strong broad
absorption band at ca. 264 nm (log e = 5.3) which is assigned
to intraligand transitions and a quite broad band at ca. 490 nm
(log e = 4.2) which might be assigned to an MLCT transition.
This result suggests that the square molecular box may be
retained in solution. Solid-state spectra in the range 550–850 nm
exhibit only a strong bond at ca. 600 nm, which can be assigned
to d–d transitions of the central metal.
Footnotes
† Complex 1 was synthesized by combining methanol solutions of H2L1
(0.44 g, 1 mmol), CoCl2·6H2O (0.24 g, 1 mmol) and NaO2CMe (0.16 g, 2
mmol). The dark brown crystalline solid which formed after refluxing for 2
h was isolated and dried under vacuum to give 1 in 62% yield. Crystals
suitable for an X-ray structure determination were obtained from slow
evaporation of a DMF solution of 1 in air. Anal. Found: C, 54.1; H, 3.9; N,
19.5. Calc. for C100H80Co4N32O8S4·H2O: C, 53.6; H, 3.7; N, 20.0%.
‡ Crystal data for 2: tetragonal, space group I41/a (no. 88); a = 14.467(1),
The components used in the construction of the cationic
4+
molecular square [Co(HL)]4 by self-assembly are novel in
several respects. Previous studies generally made use of
bidentate nitrogen donor ligands as the edges and iodonium or
heavy transition-metal ions as the corners of the square unit. In
our synthesis of this type of tetranuclear macrocycle we have
introduced a newly designed, rigid pentadentate N4S ligand for
c = 47.411(1) Å, U = 9923(5) Å3, Z = 4, F(000) = 4560, m = 0.82 mm21
;
5240 independent reflections measured, 2662 observed [F > 6.0s(F)],
number of parameters = 408; R = 0.057, Rw = 0.079. The intensities were
collected at 294 K on a Rigaku Raxis IIC imaging-plate diffractometer using
Mo-Ka radiation (l
= 0.710 73 Å) from a rotating-anode generator
operating at 50 kV and 90 mA (2qmax = 55.2°); 60 oscillation frames in the
range 0–180°, exp osure time 8 min per frame. The crystal structure was
solved by direct methods with distance restraints of C–C = 1.38 ± 0.01 Å
and C–N = 1.34 ± 0.01 Å for uncoordinated pyridyl groups. The S atom
[site occupancy factor (sof) for S(1) and S(1A) fixed at 0.5] and acetate ions
[sof = 0.5 for O(1), O(2), C(24) and C(25) of one acetate group, sof = 1/4
for O(3), O(4) C(26), C(27) and O(5), O(6), C(28) and C(29) of the two
other acetate groups with distance restraints of C–C = 1.520 ± 0.005 Å,
C–O = 1.254 ± 0.005 Å, O···O = 2.16 ± 0.01 Å], as well as the water
molecule (sof = 0.25) were found disordered. All non-hydrogen atoms
were refined anisotropically by full-matrix least-squares. Hydrogen atoms
were placed in their calculated positions with C–H = 0.96 Å, assigned fixed
isotropic thermal parameters and allowed to ride on their respective parent
atoms. The contributions of these hydrogen atoms were included in the
structure-factor calculation. All computations were carried out on a PC-486
usig the SHELXTL-PLUS program package. Atomic coordinates, bond
lengths and angles, and thermal parameters have been deposited at the
Cambridge Crystallographic Data Centre (CCDC). See Information for
Authors, Issue No. 1. Any request to the CCDC for this material should
quote the full literature citation and the reference number 182/356.
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Fig. 1 Perspective view of the molecular skeleton of the square macrocycle
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b 1.25 2 y, 0.25 + x, 0.25 2 z.
Received, 12th November 1996; Com. 6/07689K
382
Chem. Commun., 1997