Introducing Selenium in Single-Component Molecular Conductors Based on Nickel Bis(dithiolene) Complexes

Two
selenated analogues of the all-sulfur single-component molecular
conductor [Ni­(Et-thiazdt)2] (Et-thiazdt = N-ethylthiazoline-2-thione-4,5-dithiolate) have been prepared from
their precursor radical-anion complexes. Replacement of the thione
by a selenone moiety gives the neutral [Ni­(Et-thiazSedt)2] complex. It adopts an unprecedented solid-state
organization (for neutral nickel complexes), with the formation of
perfectly eclipsed dimers and very short intermolecular Se···Se
contacts (81% of the van der Waals contact distance). Limited interactions
between dimers leads to a large semiconducting gap and low conductivity
(σRT = 1.7 × 10–5 S cm–1). On the other hand, going from the neutral [Ni­(Et-thiazdt)2] dithiolene complex to the corresponding [Ni­(Et-thiazds)2] diselenolene complex gives rise to a more
conventional layered structure built out of uniform stacks of the
diselenolene complexes, different, however, from the all-sulfur analogue
[Ni­(Et-thiazdt)2]. Band structure calculations show an
essentially 1D electronic structure with large band dispersion and
a small HOMO–LUMO gap. Under high pressures (up to 19 GPa),
the conductivity increases by 4 orders of magnitude and the activation
energy is decreased from 120 meV to only 13 meV, with an abrupt change
observed around 10 GPa, suggesting a structural phase transition under
pressure.