Paramagnetic Salts of Ca, Ba, and Pb with Difurazanopyrazine and Difurazanopyrazine‑N‑oxide Radical Anions

Oxidation of 4H,8H-bis­(1,2,5-oxadiazolo)­[3,4-b:3′,4′-e]­pyrazine H2L in the presence of Ba or Ca oxide leads to the formation of paramagnetic Ba­(II) and Ca­(II) salts, respectively. It was shown that the basicity of the reaction mixture plays an important role in the oxidation of H2L. Molecular and crystal structures of [Ba­(L)2(H2O)5]·2H2O, [Ba2(L)­(HL)2(H2O)6]­(L)·4H2O, [Ba­(HL)­(H2O)6]­(HL)·H2O, and [Ca­(L)­(H2O)6]2Br2 were determined. It was found that paramagnetic [Pb­(L)2(H2O)5]·2H2O can be obtained by H2L oxidation with PbO2 in the absence of alkaline earth metal oxides. It was shown that using two oxidizing agents, PbO2 and Br2, in the reaction of BaO with H2L allows us to obtain paramagnetic salts with a new anion radical bis­(1,2,5-oxadiazolo)­[3,4-b:3′,4′-e]­pyrazine-4-oxyl (LO) and “singlet diradical” LO1. It was found that, depending on the synthetic conditions, [Ba­(LO)2(H2O)6]·2H2O crystallizes in the form of several polymorphic modifications differing in the angles between the planes of LO radicals bound to the Ba ion. Quantum chemical analysis revealed that the presence of the N-oxide fragment in LO leads to a noticeable redistribution of the spin density in comparison with L. The calculated map of the distribution of exchange integrals at different relative displacements in pairs of LO allows us to give a qualitative insight into the magnetic behavior of anion radical stacks.