Antiaromatic Bis(1,2,3-dithiazoles) with Zwitterionic Ground States

Abstract

Interest in the use of 1,3,2-and 1,2,3-dithiazolyl radicals in the design of conductive 1 and magnetic 2 materials is rapidly growing. 3 For bifunctional variants, different spin and orbital arrangements are possible. 4 Thus, while the benzo-bis(1,3,2dithiazole) 1 is biradical, 5,6 the isomeric bis-1,2,3-dithiazole 2 exhibits a quinoidal 7 rather than biradical ground state. In light of these results and the recent finding that hexa-azaanthracenes are zwitterionic, 8 we have pursued the isolation of a zwitterionic 16π-electron bis-1,2,3-dithiazole. Theoretical calculations (B3LYP/ 6-31G**) 9 on potential targets indicated that the triplet state of the benzene bridged system 3 lies 5.1 kcal/mol below the zwitterionic singlet. However, the order is reversed (by 2.1 kcal/ mol) for the pyridine-bridged compound 4.Our synthetic sequence to a derivative of 4 is outlined in Scheme 1. Oxidative thiocyanation 10 of 2,6-pyridinediamine

Prototypal Dithiazolodithiazolyl Radicals: Synthesis, Structures, and Transport Properties

“…The first examples of the R 2 BPR 1 framework were prepared by means of a double Herz condensation of 2,6-diaminopyridine with sulfur monochloride S 2 Cl 2 , which afforded the chloride salt of [ClBPH] + cation, i.e., [ 1 ][Cl] (R 2 = Cl, R 1 = H). Subsequent metathesis to a soluble hexafluorantimonate salt, and treatment of the latter with Proton Sponge liberated a zwitterionic base which, upon alkylation with alkyl triflates R 1 OTf (R 1 = Me, Et, Pr) and reduction of the so-formed triflate salts with decamethylferrocene, yielded ClBPR 1 radicals 1 (R 1 = Me, Et, Pr) . Though successful, this method was lengthy, especially the steps involving anion metathesis and salt purification.…”

What's new in stable radical chemistry?

“…There is a variety of putative bis(1,2,3-dithiazolyl) "diradical" structures which are actually closed shell compounds. A triplet ground state is predicted for the (yet to be synthesized) diradical 113, but the simple replacement of a central carbon atom by nitrogen produces 114 which has been shown (experimentally and computationally) to possess a zwitterionic ground state 173 analogous to "diradicals" 47 and 48. Alternative "diradicals" created either by placing the nitrogens at the para positions of a central aromatic, 174 e.g., 115, or by connecting two C 2 NS 2 rings by exocyclic double bonds, 175 e.g., 116-instead produce closed shell ground state quinoidal species.…”

The Power of Packing: Metallization of an Organic Semiconductor

“…A variety of strategies to reduce the HOMO–LUMO separation and improve charge transport have been considered . Notable targets have included formally antiaromatic (4nπ) systems such as quinonemonoimines 1 , hexaazaanthracenes 2 and bisdithiazoles 3 (Chart ) in which a charge-separated zwitterionic singlet state competes with a triplet or singlet biradical. − These and related materials have been explored extensively, as the HOMO–LUMO gap, and consequent singlet/triplet splitting Δ E ST , can be fine-tuned by variations in substituents and heteroatoms. However, reduction of the band gap to the point where charge carriers can be easily generated by thermal excitation has remained a challenge …”