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 T_1/2 = 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.