Dithiazolodithiazolyl Radicals: Substituent Effects on Solid State Structures and Properties

A general synthetic route to the pyridine-bridged dithiazolodithiazolyl framework R 2 BPR 1, involving N-alkylation of a 4-substituted 2,6-dichloropyridine, followed by amination and double Herz cyclization with S 2 Cl 2, has been developed. The radicals R 2 BPMe (R 2 = Me, Ph) have been prepared and characterized by EPR spectroscopy and cyclic voltammetry. Their crystal structures have been determined by X-ray crystallography. Both structures consist of undimerized slipped radical π-stacks. Lateral interactions in MeBPMe (space group P 2 1 / c ) generate chainlike arrays with radicals linked by inversion centers; there are no close interchain S−S contacts. By contrast, in PhBPMe (space group P 3 1 21) the radical π-stacks are nested about 3 1 axes, so as to produce an extensive 2-dimensional network of intermolecular S−S interactions. Variable-temperature magnetic susceptibility measurements reveal that MeBPMe is essentially paramagnetic, whereas PhBPMe displays strong antiferromagnetic coupling. A value of J = −149 cm - 1 has been estimated by using a Bonner−Fisher anti-ferromagnetic chain model. Pressed pellet conductivity measurements indicate values of σ RT ≈ 10 - 5 S cm - 1 for both R 2 BPMe compounds (R 2 = Me, Ph), suggesting Mott insulator ground states. The structural results and transport properties are discussed in the light of extended Hückel theory band structure calculations.