Elucidating the Structural Chemistry of a Hysteretic Iron(II) Spin‐Crossover Compound From its Copper(II) and Zinc(II) Congeners

Abstract

Annealing [FeL2][BF4]2⋅2 H2O (L=2,6‐bis‐[5‐methyl‐1H‐pyrazol‐3‐yl]pyridine) affords an anhydrous material, which undergoes a spin transition at T1/2=205 K with a 65 K thermal hysteresis loop. This occurs through a sequence of phase changes, which were monitored by powder diffraction in an earlier study. [CuL2][BF4]2⋅2 H2O and [ZnL2][BF4]2⋅2 H2O are not perfectly isostructural but, unlike the iron compound, they undergo single‐crystal‐to‐single‐crystal dehydration upon annealing. All the annealed compounds initially adopt the same tetragonal phase but undergo a phase change near room temperature upon re‐cooling. The low‐temperature phase of [CuL2][BF4]2 involves ordering of its Jahn–Teller distortion, to a monoclinic lattice with three unique cation sites. The zinc compound adopts a different, triclinic low‐temperature phase with significant twisting of its coordination sphere, which unexpectedly becomes more pronounced as the crystal is cooled. Synchrotron powder diffraction data confirm that the structural changes in the anhydrous zinc complex are reproduced in the high‐spin iron compound, before the onset of spin‐crossover. This will contribute to the wide hysteresis in the spin transition of the iron complex. EPR spectra of copper‐doped [Fe0.97Cu0.03L2][BF4]2 imply its low‐spin phase contains two distinct cation environments in a 2:1 ratio.

Iron(II) Complexes of 4-(Alkyldisulfanyl)-2,6-di(pyrazolyl)pyridine Derivatives. Correlation of Spin-Crossover Cooperativity with Molecular Structure Following Single-Crystal-to-Single-Crystal Desolvation

“…The values of ϕ in 1X 2 ·Me 2 CO and for 1X 2 ·sf lie in a range where SCO is possible but is rarely observed in practice . The high-spin molecular geometries of 1X 2 ·sf appear to show a small temperature dependence, as we have observed before in some related compounds. , More detailed investigations would be required to quantify that, however.…”

“…57 The highspin molecular geometries of 1X 2 •sf appear to show a small temperature dependence, as we have observed before in some related compounds. 19,75 More detailed investigations would be required to quantify that, however. The low-spin forms of the compounds have more regular geometries with 174.33(10) ≤ ϕ ≤ 177.61(13)°.…”

Spin-Crossover in a New Iron(II)/Di(pyrazolyl)pyridine Complex with a Terpyridine Embrace Lattice. Thermally Induced Excited Spin State Trapping and Clarification of a Structure–Function Correlation

“…That should reflect the translation of rows of complex molecules along c as the temperature is lowered, so the terpyridine embrace layers become increasingly canted as β gets larger (Figure 2). Such canting of the lattice could be accompanied by distortions of the molecular structure, 52 although that is unclear without full structure refinements. In contrast, β for high-spin 1[ClO 4 ] 2 shows less variation over a wider temperature range, from 95.863(5)°at 250 K to 96.589(2)°at 100 K.…”

Exploring the influence of counterions on a hysteretic spin-transition in isomorphous iron(ii) complex salts

“…However a more recent study of [ZnL 2 ][BF 4 ] 2 ·2H 2 O ( 2a ·2H 2 O), whose crystals survive the dehydration process, shed some light on the question. 48 Dehydrated crystals of 2a (phase B; tetragonal, P 4 2 / n , Z = 2) are isomorphous with anhydrous 1a by powder diffraction. The anhydrous zinc crystal undergoes a phase transition near room temperature to triclinic phase C ( P 1̄, Z = 2).…”

“…The spin-transition in 1a involves a transformation from phase C to a new low-spin phase (phase E), which was not structurally characterised but may involve a tripling of the phase C asymmetric unit. 48…”