trans‐Configured Ligands Boost Spin Crossover to Room Temperature in Mononuclear Fe(II) Complexes

Seven new mononuclear complexes [Fe(aqin)2(NCS)2] (1), [Fe(aqin)2(NCSe)2] (2), [Fe(aqin)2(NCBH3)2] (3), [Fe(2-pic)2(NCS)2] (4), [Fe(2-pic)2(NCSe)2] (5), [Fe(2-pic)(NCBH3)2] (6), [Fe(tpa)(NCBH3)2] (7), [aqin = 8-aminoquinoline, 2-pic = 2-picolylamine, tpa = tris(2-pyridylmethyl)amine)] are investigated. Structurally, except for 3, which affords a trans-configuration, all the complexes adopt a cis-configuration. Their propensity to crystallize without any solvent molecules facilitates the study of the influence of the "pure" ligand on spin-crossover (SCO) characteristics. Magnetically, 1, 2, and 4 remain in the high-spin state, and 3, 5, 6, and 7 display SCO with transition temperatures T1/2 of approximately 310, 162, 262, and 400 K, respectively. The T1/2 values of all the complexes except 3 follow the sequence of ligand-field strength, whereas the T1/2 value of 3 is increased to room temperature, beyond that of the whole cis-[Fe(2-pic)2(NCE)2] family. Theoretical modeling based on the harmonic approximation via Gaussian and periodic DFT+U+D3 calculations via the QE reveals that the lower distortion effect of the trans-configuration resulted in the LS stabilization increasing T1/2.