A rigid cavity containing tetra-cobalt(III) [2 × 2] grid complex

Article abstract of DOI:10.1039/b207039a

The synthesis and structure of a rigid, cavity containing tetra-cobalt(III) [2 × 2] grid complex using an unusual bis(bipyridine)dimethoxynaphthyridine ligand is described.

Full text of DOI:10.1039/b207039a

A rigid cavity containing tetra-cobalt(III) [2 3 2] grid complex
Jeffrey P. Plante,a Paul D. Jones,a Douglas R. Powellb and Timothy E. Glass*a
a Department of Chemistry, Pennsylvania State University, University Park, PA 16802, USA.
E-mail: teg@chem.psu.edu; Fax: 814-863-8403; Tel: 814-865-1898
b Crystallography Laboratory, Department of Chemistry, University of Kansas, Lawrence, KS 66045, USA
Received (in Columbia, MO, USA) 17th July 2002, Accepted 3rd December 2002
First published as an Advance Article on the web 3rd January 2003
The synthesis and structure of a rigid, cavity containing
tetra-cobalt(III) [2 3 2] grid complex using an unusual
bis(bipyridine)dimethoxynaphthyridine ligand is described.
complex confirmed the expected structure of the tetra-Co(III)
grid (Fig. 1).‡ The complex crystallized with six counterions
indicating that two of the ligands had been protonated.11 The
mass spectrum supports this assignment. The protons are most
likely shared between two adjacent pyridone oxygen atoms,
however, their positions in the X-ray structure are not clear. The
four Co atoms line in a plane forming a nearly perfect square
with angles of 89.2 and 90.8° and Co–Co distances of 8.48 and
Modern methods in self assembly have been very useful in the
construction of supramolecular structures based on hydrogen
bonding, hydrophobic interactions and metal ion coordination.1
These structures have potential utility in various aspects of
nanoscience. We, in particular, have been interested in
designing receptors for biologically relevant compounds. In this
regard, metal coordination based self-assembly of receptors is
very attractive as hydrogen bonding is disfavored under
biological (i.e. aqueous) conditions and coordination complexes
are often water soluble. Indeed, self-assembled cavity and cleft
containing compounds have found utility as receptors.2 As part
of a program geared toward the preparation of molecular
capsules, we have been interested in preparing tube shaped
cavities using a novel class of expanded calixarenes based on a
naphthalene core.3 Such molecular tubes with sufficiently large
dimensions may provide pores for shape selective ion and
molecule recognition or transport. As a complement to the
calixarene approach, an appropriately sized [2 3 2] grid
complex appeared to be attractive for construction of such
structures. The formation of several recent coordination based
receptors are templated by the guest and indeed often collapse
around the guest distorting the metal coordination geometry.4
For many receptor applications, a stable, rigid complex is often
deemed desirable. Lehn’s bis-terpyridine [2 3 2] grid com-
plexes are both rigid and substitutionally inert, though lacking a
cavity of useful size.4c,5 Thus, we have explored the use of an
expanded bis-terpyridine ligand which might form a grid
complex with an appropriate cavity size.
We chose to use a 2,7-naphthyridine unit for the expanded
ligand as it is easily prepared6 and has the appropriate
regiochemistry for a terpyridine ligand. Thus, ligand 5 was
prepared as shown in Scheme 1. Knovenagel condensation of 1
with malononitrile followed by acid promoted condensation and
chlorination gave the tetrachloronaphthyridine 2. Treatment of
2 with methoxide resulted in substitution of only the 1 and 8
chlorides giving the dichloride 3.6a Coupling of the remaining
chlorides to bipyridyl stannane 47 via palladium catalysis
produced the bistridentate metal ligand 5. Reaction of 5 with
Co(OAc)28 in methanol/chloroform produced a Co4L4 complex
as indicated by mass spectrometry. Interestingly, the observed
mass suggested that each ligand had been demethylated to give
a pyridone coordinated cobalt. Apparently, cobalt coordination
activated the methoxypyridine units to dealkylation9 by either
acetate or a metal bound solvent.10 The dealkylation was not
complete, as ions for the Co4L4 complex with 1, 2, 3, and 4
remaining methyl groups were also observed in the mass
spectrum. In order to achieve a uniform product, the mixture
was heated with HBr or excess NaBr in acetonitrile to fully
demethylate the complex with concomitant oxidation to Co(III).
Anion metathesis gave the complex as the hexafluorophosphate
salt.†
The NMR spectra of 6 were broad and undefined. X-Ray
quality crystals of 6 were grown by slow diffusion of benzene
into an acetone solution. Single crystal X-ray analysis of the
Scheme 1 Synthesis of complex 6.
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CHEM. COMMUN., 2003, 336–337
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8.62 Å. Opposing naphthyridine rings are canted at a 30–31°
angle due to the geometric constraints of the octahedral Co
coordination. This ‘convergence’ angle has been described in
detail for related rack complexes.12 The convergence angle in 6
produces a cavity which is 11.5 Å deep and rectangular on each
end. The atom-to-atom distance at each end of the cavity is 7.1
Å (C4a–C5aA) by 9.3 Å (O11a–O12aA). This rectangular cavity
is well suited to an aromatic guest and indeed, two benzene
molecules crystallized inside the complex, one at each end of
the cavity. Thus, the contention that complexes such as 6 would
serve as rigid receptors, at least in the solid state, is verified.
Thus, a square [2 3 2] grid has been produced from a
dimethoxynaphthyridine based metal ligand. The complex has a
rectangular internal cavity to the convergence angle of the
ligands, yet traps guest in the solid state. The convergence angle
can be controlled by varying the substitution pattern on the
ligand.12 Thus, it should be possible to substitute the tetra-
chloronaphthyridine 2 with alkoxides which are less prone to
dealkylation in order to manipulate the cavity size and shape.
These alkoxy substituents may also serve as a convenient
handle for the placement of additional functional groups for
specific molecular recognition. The recognition properties of
this type of complex are currently being explored.
This work was supported by the National Institutes of Health.
Richard Koerner is gratefully acknowledged for helpful dis-
cussions.
Notes and references
† Compound 5 (44.5 mg, 0.089 mmol) and Co(OAc)2·4 H2O (22.1 mg,
0.088 mmol) were stirred at a reflux in 1+1 methanol+chloroform (16 ml)
for 24 h. The solvent was removed under vacuum, the residue suspended in
water, and an excess of NH4PF6 was added. The product was isolated as a
yellow powder (64.6 mg 25%). The powder was dissolved in acetonitrile
and HBr (1 ml, 47%) was added and the solution stirred at 60 °C for 2 hours.
This mixture was diluted in H2O (50 ml) and neutralized with aqueous KOH
(pH = 8 ). NH4PF6 (excess) was added and the solution was cooled to 5°
C. The solid product was isolated and washed with H2O to yield a yellow
powder (35.4 mg, 60%). lmax (CH3CN)/nm (e) 402 (22 900), 418 (21 100),
473 (6 770); MS (ESI): 897 (Co4L4 + 2 H+ + 4 PF6, z = 3), 673 (Co4L4 +
2 H+ + 4 PF6, z = 4), 637 (Co4L4 + 2 H+ + 3 PF6, z = 4), 600 (Co4L4 + 2
H+ + 2 PF6, z = 4); Elemental analysis: found C 46.9, H 2.9, N 11.0; cald.
for 6·benzene·acetone C 46.6, H 2.5, N 10.8%.
‡ Crystal Data: for 6·7.12 (C3H6O)·4.88 (C6H6): (C112H64Co-
4N24O8P6F36)·7.12 (C3H6O)·4.88 (C6H6), M = 3772.76, Monoclinic, space
group C2/c, a = 39.027(4), b = 13.9229(13), c = 32.327(3) Å, a = 90°,
b = 113.855(2)°, g = 90°, V = 16065(3) Å3, T = 100 K, Z,ZA = 4, 0.5,
m(Mo Ka, l = 0.71073 Å) = 0.579 mm21, 44663 data measured, 14083
unique, Rint = 0.0974, all data used in refinement against F2 values to give
wR = 0.3067, R = 0.0955 {for 6667 data with F > 4s(F)}. One solvent site
(per unit cell) was found to be disordered as a mixture of acetone and
benzene with occupancies of 0.556(9) for acetone and 0.444(9) for benzene.
As a result, the terminal pyridine of one of the ligands was disordered and
was refined in two orientations, only one of which is depicted in Fig. 1.
CCDC 190379. See http://www.rsc.org/suppdata/cc/b2/b207039a/ for crys-
tallographic files in CIF or other electronic format.
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Fig. 1 ORTEP (50% probability) and CPK representations of complex 6.
The asymmetric unit is comprised of half of the complex. The symmetry
transformations used to generate equivalent atoms (A): 2x + 1, y, 2z +
1/2.
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