The synthesis of singlet ground state derivatives of non-Kekulé polynuclear aromatics

Abstract

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.