Graeme Allinson, Richard J. Bushby, Malini V. Jesudason, Jean-Louis Paillaud and Norman Taylor
It is known that a two-electron reduction of tetrabutylammonium
3,4-dioxo-4H,8H-dibenzo[cd,mn]pyren-
12-olate gives a trioxy (tri-O-) derivative of the
non-Kekulé polynuclear aromatic compound
dibenzo[cd,mn]pyrene (triangulene). This derivative is
stable in solution and, like triangulene itself, has a triplet ground
state. In exploring the generality of this strategy for the synthesis of
high-spin derivatives of non-Kekulé polynuclear aromatic compounds
we have investigated two electron reductions of
4,8-dioxo-4H,8H-dibenzo[cd,mn]pyrene
(to give a dioxy derivative of triangulene),
7,8-dioxo-7H,8H-dibenzo[de,hi
]naphthacene (to give a dioxy derivative of
dibenzo[de,hi]naphthacene) and
7,9-dioxo-7H,9H-dibenzo[de,jk
]pentacene (to give a dioxy derivative of
dibenzo[de,
jk]pentacene).
Dibenzo[cd,mn]pyrene (triangulene),
dibenzo[de,hi]naphthacene and
dibenzo[de,
jk]pentacene should all have triplet
ground states, but the presence of two O-
substituents on these aromatic nuclei will (
just) lift the
degeneracy of the putative singly occupied molecular orbitals. We have
shown that the splitting this produces is sufficient to ensure that all of
these dioxy derivatives have singlet ground states. Hence the strategy
employed for making and stabilising triplet triangulene as its trioxy
derivative does not provide a paradigm for other high-spin
non-Kekulé polynuclear aromatics. The reduction reactions were
studied by cyclic voltammetry, by UV–VIS spectroscopy, and by EPR
spectroscopy. Improved synthetic routes are described for
7,8-dioxo-7H,8H-dibenzo[de,hi
]naphthacene and for
7,9-dioxo-7H,9H-dibenzo[de,jk
]pentacene. Violent explosions were encountered in attempts to repeat
the literature procedure for the synthesis of
4,6-dichlorobenzene-1,3-dicarboxylic acid.