Novel Conductive Radical Cation Salts Based on Methylenediselenotetraselenafulvalene (MDSe-TSF): A Sign of Superconductivity in KAPPA-(MDSe-TSF)2Br Below 4 K

Two new families of charge transfer solids based on transition metal bis-maleonitrile dithiolate complexes [M(mnt)2]n- and two tetrathiatulvalene (TTF) derivatives containing external sulfur atoms have been synthesized and characterized as (BMDT-TTF)2[M(mnt)2] and (EDT-TTF) [M(mnt)2], where BMDT-TTF stands for bis(methylenedithio)-tetrathiafulvalene, EDT-TTF for ethylenedithio-tetrathiafulvalene and M=Au, Pt and Ni. The salts of the series (BMDTTTF)2[M(mnt)2] are quasi-isostructural and crystallize forming mixed ADDA stacks along a+b. In the (BMDT-TTF)2[Au(mnt)2] salt, the anion complex is diamagnetic and has a formal charge of -1, whereas in Ni salt the anion has a charge of -2, being also diamagnetic. The magnetic properties of the Au salt follow a 1D antiferromagnetic Heisenberg model modified with a molecular field. The salt with Ni displays very strong antiferromagnetic interactions. The (EDT-TTF)[M(mnt)2] salts with M=Ni and Pt are isostructural and crystallize forming alternated DADA stacks. The anions, with a formal charge of -1, are paramagnetic with a spin ;. Their magnetic susceptibility can be successfully simulated within the antiferromagnetic uniform chain model of Heisenberg. Lastly, (EDT-TTF)[Au(mnt)2] crystallizes in the triclinic space group P(-1) forming also alternated stacks along c in which the EDT-TTF molecules are 50% disordered.