Encapsulation controlled single molecule magnetism in tetrathiafulvalene-capped cyanide-bridged cubes

Abstract

New TTF-based (TTF = tetrathiafulvalene) ligands, L1 and L2 (L1 = α-(4′-methyl-4,5-ethylenedithiotetrathiafulvalene-5′-thio)-α′-[tris-2,2,2-(1-pyrazolyl)ethoxy]-p-xylene and L2 = α-(4′-methyl-4,5-dimethylthiotetrathiafulvalene-5′-thio)-α′-[tris-2,2,2-(1-pyrazolyl)ethoxy]-p-xylene), possessing tris-pyrazolyl coordination sites, were synthesized. The reactions of Ni(BF₄)₂·6H₂O with the TTF-ligands (L1 and L2), n-Bu₄N[Fe(CN)₃(tp or pztp)] (tp = hydrotris(pyrazol-1-yl)borate and pztp = tetrakis(pyrazol-1-yl)borate) in the presence of additional counter ions afforded two cyanide-bridged octanuclear complexes: [Fe^III₄Ni^II₄(CN)₁₂(pztp)₄(L1)₄](BF₄)₄ (1) and [Fe^III₄Ni^II₄(CN)₁₂(pztp)₄(L2)₄](PF₆)₄ (2). Using a similar procedure to that employed in the synthesis of complex 2, with the addition of sodium tetraphenylborate, yielded a two-electron-reduced compound, Na[Fe^III₂Fe^II₂Ni^II₄(CN)₁₂(tp)₄(L2)₄](BF₄)₃ (3), in which a sodium ion was encapsulated by the cube. The host–guest complex 3 showed enhanced redox behaviour and while magnetic susceptibility measurements revealed ferromagnetic interactions to be operative in all three complexes, the cation encapsulation behaviour of 3 led it to exhibit single molecule magnet-type properties.