Iron(II) complexes of 4-sulfanyl-, 4-sulfinyl- and 4-sulfonyl-2,6-dipyrazolylpyridine ligands. A subtle interplay between spin-crossover and crystallographic phase changes

Oxidation of 4-(methylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine (L-SMe) with hydrogen peroxide or mCPBA yields 4-(methylsulfinyl)-2,6-di(pyrazol-1-yl)pyridine (L-SOMe) and 4-(methylsulfonyl)-2,6-di(pyrazol-1-yl)pyridine (L-SO2Me), respectively. Solid [Fe(L-SMe)(2)][ClO4](2) (1[ClO4](2)) is high-spin at room temperature, and exhibits an abrupt spin-transition at T-1/2 = 256 K. A shoulder on the cooling side of the chi T-M vs. T curve is associated with a hysteretic crystallographic phase change, occurring around T down arrow = 245 K and T up arrow = 258 K. The phase change involves a 180 degrees rotation of around half the methylsulfanyl substituents in the crystal. This contrasts with the previously reported BF4- salt of the same compound, which is isostructural to 1[ClO4](2) at room temperature but transforms to a different crystal phase in its low-spin state. Solid [Fe(L-SOMe)(2)][BF4](2) (2[BF4](2)) and [Fe(LSO2Me)(2)][BF4](2) (3[BF4](2)) both exhibit gradual spin-crossover equilibria centred significantly above room temperature. Solution measurements show that the oxidised sulfur centers in 2[BF4](2) and 3[BF4](2) stabilise the low spin states of those complexes.