8
A. J. METHERELL AND M. D. WARD
which could therefore stabilise/template the S4-symmetric
host. We suggest therefore that the initially formed S6
symmetric cage slowly rearranges to the S4-symmetric
version on crystallisation to accommodate and comple-
ment the {Na(BF4)4}3– anion. We note that the variation of
individual metal centres between fac and mer tris-chelate
coordination geometry in cage superstructures has been
highlighted by others (27), and provides additional (and
often unexpected) structural complexity. The general area
of supramolecular transformations within self-assembled
metal/ligand architectures is of considerable topical inter-
est and has recently been reviewed (28).
Unfortunately we were unable to obtain a 1H NMR
spectrum of redissolved crystals of [Co8(Lan)12Na][BF4]17
as these crystals only formed in very small amounts (even
in the presence of excess NaBF4) and had poor solubility.
It is likely however that polar solvents would break the
hydrogen-bonding interactions around the guest complex
{Na(BF4)4}3–, resulting in solvation of the Na+ cation and
fluoroborate anions.
purification procedures. However, caution should be exer-
cised because perchlorate salts of metal complexes with
organic ligands are potentially explosive.
Syntheses
Compound 4
A mixture of 3 (1.88 g, 5.0 mmol), 3-(2-pyridyl)pyrazole
(1.54 g, 10.6 mmol), THF (50 cm3) and aqueous NaOH
(17.5 M, 6 cm3) was stirred at 75 °C for 3 days. The organic
layer was separated, dried over MgSO4 and concentrated
before purification by silica column. Elution with EtOAc/
CH2Cl2 (4:1) → 100% EtOAc yielded two fractions. The first
fraction collected yielded 4 as an off-white solid (Yield:
1.78 g, 3.5 mmol, 70%), and the second fraction yielded
the Boc-deprotected Lan as an off-white solid (yield: 0.41 g,
1.0 mmol, 20%; total yield 90%). 1H-NMR (400 MHz, CDCl3):
δ 8.57 (2H, ddd; pyridyl H6), 7.87 (2H, dt; pyridyl H3), 7.63
(2H, td; pyridyl H4), 7.37 (1H, s; ArH), 7.35 (2H, d; pyrazolyl
H5), 7.20 (2H, s; ArH), 7.14 (2H, ddd; pyridyl H5), 6.84 (2H, d;
pyrazolyl H4), 6.71 (1H, bs; NH), 5.22 (4H, s; CH2), 1.41 (9H,
t
s; Bu). ESMS: m/z 530 [M + Na]+, 508 [M + H]+, 452 [M –
Conclusion
tBu+2H]+, 255 [M + 2H]2+, 227 [M – tBu+3H]2+.
We prepared the new ligand Lan as an amine-functional-
ised analogue of Lm-Ph which might allow access to exter-
nally-functionalised cages; that possibility remains. Initial
studies of its coordination behaviour show some inter-
esting behaviour. Firstly, the amine group can alter the
coordination behaviour of the ligand, which is pentaden-
tate instead of tetradentate in an unusual double-helical
trinuclear complex with Ag(I). Secondly, and in contrast
to this, the amine groups on Lan do not interfere with
formation of a cage structure in the form of the approxi-
mately cubic assembly [Co8(Lan)12][BF4]16 (based on mass
spectrometric and NMR evidence) but become involved
in hydrogen-bonding interactions with the complex anion
guest {Na(BF4)4}3–. Thus the exterior amine groups form
NH•••F interactions with the internal guest, through the
portals in the cage surface; the cage structure undergoes
significant rearrangement from S6 to S4 symmetry to allow
binding of the fourfold symmetric guest.
Lan
To a solution of 4 (1.78 g, 3.5 mmol) in CH2Cl2 (10 cm3) was
added TFA (10 cm3) and the resultant clear yellow solution
was stirred at 25 °C for 14 h. The solvent was removed in
vacuo and the clear brown oil was repeatedly washed with
CH2Cl2/MeOH (1:1) and evaporated to dryness in order to
remove all traces of TFA. The brown solid was washed
with aqueous K2CO3 and the organic layer extracted with
CH2Cl2, dried over MgSO4 and evaporated to dryness, yield-
ing Lan as a white solid (Yield: 1.13 g, 79%). X-ray quality
crystals were grown by slow evaporation of a solution of
L
an in ethyl acetate and hexane. 1H-NMR (400 MHz, CDCl3):
δ 8.62 (2H, ddd; pyridyl H6), 7.92 (2H, dt; pyridyl H3), 7.68
(2H, td; pyridyl H4), 7.39 (2H, d; pyrazolyl H5), 7.18 (2H, ddd;
pyridyl H5), 6.89 (2H, d; pyrazolyl H4), 6.51 (1H, s; ArH), 6.39
(2H, s; ArH), 5.23 (4H, s; CH2), 3.64 (2H, bs; NH2). ESMS: m/z
430 [M + Na]+, 408 [M + H]+, 205 [M + 2H]2+. Found: C, 64.40;
•
H, 5.53; N, 21.92%. Required for C24H21N7 2.25H2O: C, 64.34;
H, 5.74; N, 21.89%.
Experimental section
General details
[Co8(Lan
)12](BF4)16
3-(2-Pyridyl)pyrazole was prepared as reported previ-
ously (29). Instrumentation used for routine NMR and
mass spectrometric analysis has been reported in recent
publications (23–25). Compounds 1 (15), 2 (16) and 3 (16)
were prepared according to the literature methods. The
perchlorate complex included in this work was prepared
in small amounts and was stable for routine synthesis and
A Teflon lined autoclave was charged with Lan (0.050 g,
0.12 mmol), Co(BF4)2•6H2O (0.036 g, 0.10 mmol) and meth-
anol (5 cm3). Heating to 100 °C for 12 h followed by slow
cooling to room temperature yielded a crop of small pink
crystals, which were washed with methanol and dried to
give [Co8(Lan)12](BF4)16 as a pink solid in 76% yield. ESMS: