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

Abstract

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 χ_MT vs. T curve is associated with a hysteretic crystallographic phase change, occurring around T↓ = 245 K and T↑ = 258 K. The phase change involves a 180° 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(L^SO₂Me)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.