  We report the discovery of a correlated insulator with a bulk gap at
two-thirds filling in a geometrically frustrated Hubbard model that describes
the low-energy physics of Mo$_3$S$_7$(dmit)$_3$. This is very different from
the Mott insulator expected at half-filling. We show that the insulating phase,
which persists even for very weak electron-electron interactions ($U$), is
adiabatically connected to the Haldane phase and is consistent with experiments
on Mo$_3$S$_7$(dmit)$_3$
