On the synthesis and characterization of Cu[C6H4(NH)2]2 [C6H4(NH)2 = semi-benzoquinonediiminate] as a donor precursor of the semiconducting and ferromagnetic Cu[C6H4(NH)2]2(I3)1.66 charge-transfer complex

The synthesis and characterization of Cu[C6H4(NH)2]2 [C6H4(NH)2 = semi-benzoquinonediiminate, abbreviated s-bqdi] are described and its relevant properties are compared with those of the known d8 and d6 metal bis- or tris-benzoquinonediiminate. The complex is paramagnetic in the solid state with μeff = 1.86 BM. The dominant form of Cu(s-bqdi)2 in aliphatic alcohols like methanol, ethanol and n-butanol or in THF is dimeric or oligomeric, but monomeric in acetonitrile, DMSO and DMF, as proved by UV-vis and ESR spectra. The stability of Cu(s-bqdi)2 to oxygen exposure in different solvents parallels the association behaviour and follows the order: ethanol å DMF å acetonitrile. The electrochemical (voltammetric) response of the [CuN4]z system (z = 0, ± 1, ± 2) in acetonitrile consists of two 0 ⇌ + 1 (E1/2 = − 0.10 V vs SCE) and + 1 ⇌ + 2 (E1/2 = + 0.98 V vs SCE) reversible electron transfers and an irreversible process at Ep,c = − 1.87 V attributed to 0 → − 1 reduction. Iodination of an ethanolic solution of Cu(s-bqdi)2 led to the isolation of polycrystalline Cu(s-bqdi)2I5 which is formulated as Cu(s-bqdi)2(I3)1.66 on the basis of IR, Raman and thermogravimetric analysis. The new material exhibits high, room-temperature conductivity (σ = 6.6 · 10−2 S cm−1) and semiconducting behaviour in the temperature range 70–300 K, with a sharp transition at 215 K. Static magnetic susceptibility measurements provide μeff = 1.23 BM for Cu(s-bqdi)2(I3)1.66 at 295 K [calculated from μeff = 2.83(χT)1/2] which is found to obey the Curie-Weiss law between 70 and 295 K. The electrical and magnetic behaviour of Cu(s-bqdi)2(I3)1.66 is interpreted as being due to strong homomolecular intra- and interstack interactions between donor molecules with radical cationic character.