Sulfur nitrogen heterocycles as charge transfer materials

Abstract

A variety of compounds based on either 1,5,2,4,6,8-dithiatetrazocine or 1,2,3-dithiazolyl have been prepared, isolated, and characterized by single crystal X-ray crystallography. Where appropriate, the redox properties of these materials have been studied by cyclic voltammetry and the conductivities and magnetic susceptibilities have been measured. Those compounds with an open shell ground state have been further characterized by electron paramagnetic resonance. Theoretical investigations of several species have been carried out using density functional theory (B3LYP) computational methods on a 6-31G** basis set. The results have been interpreted in the context of designing novel charge transfer acceptors, charge transfer donors, and neutral radical conductors.

Although hydroquinone-like 3,7-diaryl-1,5,2,4,6,8-dithiatetrazocine derivatives can be prepared and isolated, chemical oxidation does not generate a species in whihc there is a quinone-like bond length alternation. For example, the reaction of 3,7-bis(2-bromothiophene)-1,5,2,4,6,8-dithiatetrazocine with tetracyanoethylene oxide oxides only one of the two thienyl groups. The crystal structure of the product 3-(2-bromothiophene)-7-(2,3-bis(dicyanomethylene)thiophene)-1,5,2,4,6,8-dithiatetrazocine has been determined. The experimental results militate against the use of 1,5,2,4,6,8-dithiatetrazocine derivatives as charge transfer acceptors.

When two 1,2,3-dithiazoles are ring-fused to a central phenyl ring such that the nitrogens of the two heterocycles are para- to one another, a stable radical cation oxidation state can be isolated. Charge transfer salts with 1:1, 2:1, and 3:2 cation-to-anion stoichiometric ratios have been characterized by X-ray crystallography. While the 1:1 and 2:1 salts show relatively low conductivities ( CJ :::: 10·5 S cm·1), the conductivities measured for the 3 :2 salts are significantly higher (a:::: 10·2-10 S cm·1).

The ring-fused benzo-bis (1,2,3-dithiazole) in which the nitrogens of the two heterocycles are meta- to one another also has an accessible, stable radical cation oxidation state. The pyridine analog of this species, however, has a remarkably high proton affinity, complicating the isolation of the free base and its oxidized radial cation. When protonated, the closed shell cation salt can be easily isolated. The crystal structure of 4-chloro-1-imidopyridine[2,3-d:6,5-d']-bis(1,2,3-dithiazolylium) hexafluoroantimonate has been determined. Cyclic voltammetry studies show that the stable neutral radical oxidation state of this species is accessible by electroreduction. Upon further electroreduction to the radical anion, proton shifting occurs. The narrow electrochemical window between the anion/neutral redox pair and the neutral/cation redox pair suggests that this compound may have properties favourable for the design of a neutral radical conductor.

Bis(1,2,3-dithiazole)s need not be ring-fused to a central aryl ring. Several compounds in which the two heterocycles are bridged via the carbon C5 have been characterized. These include three closed shell species and a species with a triplet ground state. Of the three closed shell compounds, two have stable radical cation oxidation states. Charge transfer salts of these have been characterized by X-ray diffraction. The diradical species has been isolated and identified by electron paramagnetic resonance. In addition, the analogous 4-chloro-5-(2,3,4,5,6-pentafluorophenyl)-1,2,3-dithiazolyl has been characterized by X-ray crystallography. This is the first simple 1,2,3-dithiazolyl isolated and characterized in the solid state.