Spin Crossover around Room Temperature: Effects of Remote Ligand Symmetry Breaking within Jäger‐Type Iron(II) Complexes

In a bottom‐up approach, we studied the feedback of slight molecular modifications on the spin crossover (SCO) properties of otherwise structurally conserved iron(II) complexes. There, the ligand backbone of a C 2v symmetric, planar‐directing N 2 O 2 2‐ Jäger platform was modified through remote substituents (R = CN, F, CF 3 ), reducing symmetry to C s . Both electronic influences, through mesomeric activation/deactivation and inductive electronic effects, were studied. From these ligands, a series of three Jäger‐type iron(II) complexes [Fe 1 ‐ 3 (py) 2 ] has been prepared and characterized by IR spectroscopy, SQUID magnetometry, and 57 Fe Mössbauer spectroscopy. Bulk solid measurements are supplemented by 1 H, 19 F, and 13 C NMR spectroscopy in fluid solution and density functional theory (DFT) modeling of the molecular properties. While full SCO occurs close to room temperature in all three compounds, the SCO profiles lack magnetic bistability previously observed with the symmetric parent complex for R = H. Single crystal XRD analysis of the fluorido substituted complex [Fe 2 (py) 2 ] associates the very gradual SCO profile with the simultaneous effects of syn and anti rotamers of noncoordinate carbonyls and H/F crystallographic site disorder.