Phenalenyls, Cyclopentadienyls, and Other Carbon-Centered Radicals - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 102

moments are canceled by the symmetry of the dimers in the crystal. The finding of the columnar motif for
TBDAP derived by the heteroatomic modification is of great interest in crystal engineering and organic
material sciences.

3.7.3 Hexaazaphenalenyl derivatives
1,3,4,6,7,9-Hexaazaphenalenyl ( HAP ) is a highly symmetrical molecule with full nitrogen substitution in
all sites of the phenalenyl. The lone pair electrons on the six nitrogen atoms are directed radially and

can form coordination and hydrogen bonds to the outside (Figure 3.23a). Quantum chemical calculations
suggest that the HOMO energy level of the anion state decreases substantially in this molecular system
in comparison with the pristine PLY anion (Figure 3.23b). Such a marked decrease in the HOMO energy
is not seen in other azaphenalenyl anions, indicating the exceptionally higher stability of the HAP anion
among the phenalenyl anions. In 1964, Tomlin first described a preparation of the HAP system in his
Ph.D. thesis; however, the structural and physical properties were not examined. 31 In 2005, 1,3,4,6,7,9-
hexaazaphenalene, one of the HAP systems, was prepared according to Tomlin's method with some
modifications (Scheme 3.6), and the potassium salt of the anion is isolated as stable crystals in air at
room temperature. 32 As mentioned above, the stabilities of PLY and TBPLY anions in air are very low in
contrast to the corresponding cation species. Therefore, the HAP anion is the first phenalenyl one isolated
as air-stable crystals. Cyclic voltammetry of the potassium salt gives an irreversible oxidation wave at
around

α

1.2 V (vs Fc/Fc + ), which is assigned to the oxidation of the anion to the neutral radical. This
potential is much higher than that of the pristine PLY (

+

0.9 V vs SCE 17 ), consistent with the decrease
of the HOMO energy of the HAP anion suggested by quantum chemical calculation. 32

−

In a copper (II)

(a) (b)

Figure3.23 (a)Chemicalstructureandligationmodeof HAP systemand(b)MOdiagramsandHOMOpictures
of PLY and HAP anions.CalculationscarriedoutattheB3LYP/6-31 + G*//B3LYP/6-31 + G*leveloftheory.

NH 2

N H

CN

NH 2 CHO

N

N N

N NH 2

N N

Scheme 3.6 One-pot synthetic method for 1,3,4,6,7,9-hexaazaphenalene