Extended Hypervalent 5c−6e Interactions: Linear Alignment of Five C−Se---O---Se−C Atoms in Anthraquinone and 9-Methoxyanthracene Bearing Arylselanyl Groups at the 1,8-Positions

The structures of 1,8-bis(phenylselanyl)anthraquinone (1a), 1,8-bis(phenylselanyl)-9-methoxyanthracene (2a), and 1,8-bis(phenylselanyl)anthracene (3a) are determined by X-ray crystallographic
analysis, together with the derivatives. The Se−Ci (Ph) bonds in 1a are placed on the anthraquinone
plane (both type B) and the phenyl planes are perpendicular to the anthraquinone plane. The
structure around the Se atoms in 2a is very close to that of 1a:  the conformations of the PhSe
groups are both type B. Consequently, the five Ci−Se- - -O- - -Se−Ci atoms in 1a and 2a align
linearly. The nonbonded Se- - -O distances in 1a and 2a are 2.673−2.688 and 2.731−2.744 Å,
respectively, which are about 0.7 Å shorter than the sum of van der Waals radii of the atoms. The
extended hypervalent σ*(Ci−Se)- - -np(O)- - -σ*(Se−Ci) 5c−6e interactions are strongly suggested
for the origin of the linear alignment of the five atoms in 1a and 2a. The 5c−6e must be constructed
by the connection of the two hypervalent np(O)- - -σ*(Se−Ci) 3c−4e interactions through the central
np(O). The five Ci−Se- - -H- - -Se−Ci atoms never align linearly in 3a. To reveal the nature of 5c−6e in 1a and 2a, QC calculations are performed on HaHbASe- - -O(CH2)- - -BSeHa‘Hb‘ (model a)
and HaHbASe- - -OH2- - -BSeHa‘Hb‘ (model b) with the B3LYP/6-311++G(3df,2pd) method, where
the nonbonded Se- - -O distances are fixed at 2.658 Å. Four conformers, a (AA-cis), a (AA-trans),
a (AB), and a (BB), are optimized to be stable for model a, where a (AA) shows both type A for the
ASe−Hb and BSe−Hb‘ bonds in model a. Three conformers, b (AA-cis), b (AB), and b (BB), are
stable for model b. The bonding models in AA, AB, and BB correspond to 3c−6e, 4c−6e, and 5c−6e, respectively. The models become more stable by 42 ± 5 kJ mol-1, if the type A conformation of
each Se−H bond changes to type B. No noticeable saturation is observed in the stabilization for
each change. QC calculations are also performed on 1a−3a at the B3LYP level. Three conformers
are evaluated to be stable for 1a and 2a. The relative energies of 1a (AA-trans), 1a (AB), and 1a
(BB) are 0.0, −31.5, and −60.6 kJ mol-1, respectively, and those of 2a (AA-cis), 2a (AB), and 2a
(BB) are 0.0, −24.4, and −36.5 kJ mol-1, respectively. These results demonstrate that the origin
of the linear alignment of the five C−Se- - -O- - -Se−C atoms in 1a and 2a is the energy lowering
effect by the extended hypervalent 5c−6e interactions of the σ*(C−Se)←np(O)→σ*(Se−C) type. The
π-conjugation between π(CO) and npz(Se) through the π-framework of anthraquinone must also
contribute to stabilize the BB structure of 1a, where z is the direction perpendicular to the
anthraquinone plane.