4,5-Dicyano-3,6-diethylbenzo-1,2-diselenete, a Highly Stable 1,2-Diselenete: Its Preparation, Structural Characterization, Calculated Molecular Orbitals, and Complexation with Tetrakis(triphenylphosphine)palladium

The
first isolable benzo-1,2-diselenete, 4,5-dicyano-3,6-diethylbenzo-1,2-diselenete
(4), was prepared by the reaction of 4,5-(o-xylylenediseleno)-3,6-diethylphthalonitrile (3) with
aluminum chloride in toluene. X-ray crystallographic analysis demonstrated
that 4 contains a trapezoidal diselenide ring rather
than a benzo-1,2-diselenone structure. In crystal form, 4 undergoes self-assembly and generates structures based on layered
molecular sheets since the unit cell contains only one molecule. While
the cyclic voltammogram of 4 exhibited only one irreversible
peak (Ep = 1.59 V) during oxidation and
two quasireversible couples during reduction, three peaks were observed
in the differential pulse voltammogram of the reduction couples (E1/2 = −1.19, −0.75, and −0.69
V). Although a THF solution of 4 in the presence of sodium
metal was EPR silent, various signals were readily observed in its 1H, 13C, and 77Se NMR spectra. Molecular
orbital calculations for 4 demonstrated that the HOMO
orbital is primarily localized at the two selenium atoms and four
of the benzene carbon atoms while the LUMO orbital is situated solely
on the diselenete ring. It appears that the HOMO and LUMO orbitals
of 4 receive significant stabilization from the nitrile
groups compared to the level of stabilization in the unsubstituted
benzo-1,2-diselenete (BDS). The reaction of 4 with tetrakis­(triphenylphosphine)­palladium in benzene was
found to produce a dinuclear palladium complex (8), and
the structure of this complex was determined by X-ray crystallographic
analysis. The central four membered ring of 8 consists
of the Pd1, Se2, Pd2, and Se3 atoms and is not planar but rather adopts
a folded arrangement.