Thiolate-bridged diiron(III) complex with spin-crossover behaviour

The reaction of the ligand HL .(HCl)(2) (HL = 2,6-di(aminomethyl)-4-tert-butyl-thiophenol) with MCl2 in methanol in the presence of sodium methanolate and air affords the dinuclear complexes [L3M2][ClO4](3) (M = Fe: 2, Co: 3) in 85% and 68% yield, respectively. Both complexes were characterized by infrared spectroscopy, magnetic susceptibility measurements, and H-1 NMR spectroscopy. By temperature-variable Fe-57 Mossbauer spectroscopy diamagnetic 2 is shown to exhibit a gradual spin transition between the species [L-3(low-spin-Fe)(2)](3+) ((ls,ls)-2) and [L-3(high-spin-Fe)(2)](3+) ((hs,hs)-2). At room temperature the relative concentrations of both species are nearly equal. Well resolved quadrupole doublets at all temperatures for both (ls,ls)-2 (Gamma in the range 0.22(1)-0.28(1) mm s(-1)) and (hs,hs)-2 (Gamma in the range 0.48(1)-0.53(2) mm s(-1)) are indicative of a spin conversion time longer than the half-life of the I=3/2 State of Fe-57. Cyclic voltammetry and square wave voltammetry of 2 in CH3CN solution reveal four quasi-reversible one-electron transfer processes. The first two processes were assigned to metal-centered reductions of (hs,hs)-2 and (ls,ls)-2, respectively, to yield the mixed-valent species [(L3FeFeIII)-Fe-II](3+).