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HO
O
H
2
N
N
O
N
Pt
N
H
2
H
H
N
N
O
N
O
OH
(a)
(b)
Figure 11.5
(a) Structure of
S2b
⊂
M3b·
5PF
showing the [C-H
π]
6
interactions. (b) Crystal structure of an inclusion complex
showing two pairs of bifurcated [C-H
O] hydrogen bonds
p
and four [C-H
] interactions as dashed black lines. Solvent
molecules, counter ions, and hydrogen atoms (not involved
in hydrogen bonding) have been omitted for clarity. Color
code: C, gray; H, light gray; N, blue; O, red; Pt, yellow.
The crystal structure shows the insertion of diol
S2b
into the cavity
of
(Fig. 11.5b). In addition to π-π stacking interactions, the
complex is stabilized by two pairs of bifurcated [C-H
M2a
·6NO
3
O] hydrogen
bonds between two of the β-CH bipyridinium hydrogen atoms and
the free hydroxyl function. There are also four [C-H
π] interactions
between the hydrogens H-4 and H-8 of the guest with the central
aromatic ring of the acridine system ([C-H
π] distances 2.65 and
2.80 Å, angles 152
) and between H-3 and H-7 with one of
the lateral rings of the acridine system ([C-H
°
and 145
°
π] distances 2.65 and
2.68 Å, [C-H
). Inspection of the crystal
structure of the complex revealed a second molecule of
π] angles 156
°
and 153
°
, which
interacts by hydrogen bonding with one of the free hydroxyl groups
of the naphthalene moiety inside the metallocycle.
The association constants measured by UV-vis spectroscopy show
a clear correlation between the stability of the inclusion complex
and the size of the cavity determined by DFT calculations. Thus,
metallocycles
S2b
exhibit the highest binding constants
for their inclusion complexes in both nitromethane and water owing
to their nearly ideal cavity dimensions. On the contrary, no binding
was detected for metallocycles
M2b
and
M3b
. The short distance between
bipyridine subunits in these metallocycles explain these facts. The
equilibrium association constants in water for the formation of the
complexes between
M1a,b
ii
S1b
and the Pd
metallocycles
M2a
and
M3a
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