Organic–inorganic charge-transfer salts based on the β-[Mo8O26]4– isopolyanion: synthesis, properties and X-ray structure

The complex reaction between the organic electron donor N,N,N′,N′-tetramethyl-p-phenylenediamine (tmpd) and the inorganic acceptor [Mo6O19]2– in air has been investigated. Conversion of the inorganic polyanion to β-[Mo8O26]4–, as well as protonation and/or oxidation of the organic base are possible. Different charge-transfer salts of the type β-[Htmpd]x[tmpd]4–x[Mo8O26] and β-[Htmpd]x[tmpd]3–x[HMo8O26] have been isolated and characterized. They contain positively charged tetramethyl-p-phenylenediamine molecules which may be present, simultaneously or not, as monoprotonated units (Htmpd+) or radical cations (tmpd+). According to optical and magnetic properties, isolated radicals or tightly bound dimers have been found. The X-ray crystal and molecular structure of β-[Htmpd]2[tmpd]2[Mo8O26] has been solved by the heavy-atom method and refined to a final R= 0.057 for 3129 reflections with I 3σ(I); monoclinic, space group P21/c with a= 11.607(5), b= 24.830(9), c= 11.047(4)A, β= 106.82(3)° and Z= 2. The β-octamolybdate polyanion has the well known Lindqvist structure. The radical cations from eclipsed dimeric units with an intradimeric distance 3.35 A and do not interact with the polyanion. The protonated molecules are isolated from one another, but linked to the polyanion through bifurcated hydrogen bonds of 2.87 and 3.11 A.