Bipyrimidine-bridged dinuclear iron(II) spin crossover compounds

This review reports on the study of the interplay between. magnetic coupling and spin transition in 2,2'-bipyrimidine (bpym)-bridged iron(II) dinuclear compounds. The coexistence of both phenomena has been observed in I[Fe(bpym)(NCS)212(bpym)l, {[Fe(bpym)(NCSe)(2)](2)(bpym)} and {[Fe(bt)(NCS)(2)](2)(bpym)l (bpym=2,2'-bipyrimidine, bt = 2,2'-bithiazoline) by the action of external physical perturbations such as heat, pressure or electromagnetic radiation. The competition between magnetic exchange and spin crossover has been studied in {[Fe(bpym)(NCS)(2)](2)(bpym)} at 0.63 GPa. LIESST experiments carried out on {[Fe(bpym) (NCSe)(2)](2)(bpym)} and {[Fe(bt)(NCS)(2)](2)(bpym)}at 4.2 K have shown that it is possible to generate dinuclear molecules with different spin states in this class of compounds. A special feature of the spin crossover process in the dinuclear compounds studied so far is the plateau in the spin transition curve. Up to now, it has not been possible to explore with a microscopic physical method the nature of the species which constitute such a plateau, due to the relatively high temperatures at which the transition takes place. A two-step spin. transition has been observed for {[Fe(phdia)(2)(NCS)(2)](2)(phdia)] (phdia: 4,7-phenanthroline-5,6-diamine) with T-1/2(1) and T-1/2(2) located at 108 and 80 K, respectively. Due to this low temperature transition we were able to thermally trap, at liquid helium temperatures, the species present in the plateau of the spin transition curve. The results have revealed that the plateau consists mainly of [HS-LS] pairs, and they confirmed the hypothesis formulated earlier that the spin conversion in dinuclear entities proceeds via [HS-HS] <----> [HS-LS] <----> [LS-LS] pairs.