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.
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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.
CHEM. COMMUN., 2003, 336–337
337