Structural and theoretical insights into solvent effects in an iron(iii) SCO complex

Abstract

Alcohol effects in a series of iron(III) spin crossover complexes [Fe(qsal-Cl)₂]NO₃·ROH (R = Me 1, Et 2, 1-Pr 3) are explored. Despite the solvents differing from each other by only one or two CH₂ groups, unique packing motifs are observed for each complex. While 1 crystallizes in triclinic P with two independent iron(III) centres, connecting them in a tight undulating 1D chain with a rectangular cross-section, 2 (monoclinic P2₁/c) and 3 (monoclinic I2/a) crystallize with only one iron center. For 2, a linear 1D chain is observed with a square cross-section, while 3 exhibits a looser 1D chain formed by dimers. The 3D supramolecular networks are distinctive with 1 showing identical parallelogram shaped 2D sheets, whereas 2 and 3 show alternating 2D sheets with differing staggering between the layers (AB). These results are supported by Hirshfeld surface analysis. The magnetic studies show no significant SCO for 1 and 3. However, 2 shows an incomplete gradual spin transition (T_1/2 = 200 K). Moreover, upon partial ethanol desolvation, a 25 K hysteresis near room temperature (T_1/2↓ = 275 K, T_1/2↑ = 300 K) is observed. The packing observed in 2 is present in many other [Fe(qsal-X)₂]⁺ complexes with hysteresis and abrupt SCO. DFT calculations using the r²SCAN functional quantify the differences for 1 and 2 in the low-high spin energy and their cooperative effects. This study illustrates that even small changes in the solvent can dramatically influence the crystal packing and therefore the SCO properties.