Chemistry Reference
In-Depth Information
tic of
bonded CT complexes [26-28] are indicative of strong anion/TCNE
interactions (Table 3).
π
-
π
3.4
Donor/Acceptor (Structural) Effects on π-Halogen Interactions
3.4.1
Intermolecular Separations Relevant to Halogen Bonding
Halogen-carbon separations in
-complexes are significantly less than the
sum of their van der Waals radii. It is notable, however, that the contrac-
tions of
π
0.4
A
relative to equilibrium van der Waals separations, which
are observed in these Br
2
complexes, are somewhat less than those meas-
ured earlier with various n-type donors. For example, the X
∼
Br distance
contraction (relative to the corresponding equilibrium van der Waals sepa-
rations) is 0.55A in the acetone/Br
2
complex (O
···
Br 2.82A), 0.56A in the
···
Br 2.84
A
), 0.57
A
in the [Te
2
Cl
10
]
2-
/Br
2
com-
acetonitrile/Br
2
complex (N
···
Br 3.03
A
), and 0.60
A
in the [Se
2
Br
10
]
2-
/Br
2
complex (Br
plex (Cl
Br
3.10
A
) [41]. A similar tendency is observed in tetrabromomethane com-
plexes with
···
···
-type (aromatic) vs. n-type donors, with the former showing
contractions of up to
π
0.3A relative to the sum of the van der Waals radii,
while the contraction in the oxygen, nitrogen, or halide complexes reach
as much as 0.5-0.8
A
. Most importantly, the shortening of the interatomic
distances within various halogen complexes apparently correlates with the
donor/acceptor strength of the components. Thus, the average C
∼
Br sepa-
ration of 3.156A in the toluene/Br
2
complex is somewhat shorter than that
in the benzene complex (3.18A), as expected from the better donor strength
of toluene [63, 64]. In the carbon tetrabromide complexes with
···
-donors, the
separation between the bromine atom and the benzene plane is decreased
from 3.34
A
in the tetrabromomethane complex with the weak
p
-xylene
donor to 3.21A in the associate with the stronger durene donor, and further
to 3.14A in the complex with OMTP (oxidation potentials for these donors
are 2.01, 1.84 and 1.75, respectively [53]. Such data provide clear indications
of the increase in the halogen-bonding strength (structurally represented as
the interatomic distance) with increasing donor/acceptor strengths that are
similar to those observed in halogen bonding with n-type donors [53].
π
3.4.2
Molecular Geometries of Donor/Acceptor Moieties
···
The C(arene)
Br bonding does not markedly perturbed the geometry of the
dibromine, which is rather sensitive to coordination/ polarization effects and
the bond readily elongates from 2.284
A
in the non-coordinated molecule to
2.53
A
in the [Br
3
]
-
anion [41]. Indeed, the Br - Br bond lengths of 2.301(2)
A
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