Exploring the Influence of Counterions on a Hysteretic Spin-Transition in Isomorphous Iron(II) Complex Salts

[FeL2]X2·2H2O (L = 2,6-bis{5-methyl-1H-pyrazol-3-yl}pyridine; X‒ = BF4‒ or ClO4‒) are readily dehydrated upon mild heating. Anhydrous [FeL2][BF4]2 exhibits an abrupt spin-transition at T½ = 205 K, with a 65 K thermal hysteresis loop which narrows upon repeated scanning. The isomorphous ClO4‒ salt remains high-spin on cooling, however, which is investigated further in this study. Unlike the iron complex, [ZnL2][ClO4]2·2H2O undergoes single-crystal-to-single-crystal dehydration; the tetragonal anhydrous crystals transform to a new triclinic phase upon cooling. The phase change is apparently sluggish and transition temperatures between 268 K and <200 K were measured by different techniques, implying it depends on the measurement conditions or sample history. Powder diffraction shows the zinc complex is a good model for the structural chemistry of [FeL2][ClO4]2. The spin states of mixed-anion salts of the iron complex [FeL2][BF4]z[ClO4]2‒z (z = 1.5 and 1.0) are also investigated. Their spin-transitions evolve more slowly on repeated scanning, as z decreases, and efficient thermally induced kinetic trapping is observed below 120 K when z = 1.0. Taken together, these data imply structural rearrangements in the anhydrous materials during thermal cycling occur more slowly in the presence of the larger ClO4‒ ion. Hence, rather than reflecting any structural differences with the SCO-active BF4‒ salt, the high-spin nature of [FeL2][ClO4]2 is probably caused by kinetic inhibition of its putative spin-transition.