Switchable Heteroleptic Mononuclear Iron(II) Complexes as Versatile Molecular Building Block

A series of photoswitchable precursors based on [Fe(NCX)2L(L′)2], where X = S or Se, L is a bidentate ligand (phenanthroline or bipyrimidine), and L′ a monodentate ligand such as a pyridine or picoline derivative, were synthesized and their spin crossover (SCO) properties investigated in detail. The thermal and light‐induced magnetic properties of a family of [Fe(NCS)2L(py)2] [py = pyridine and L = bipyrimidine (1) or phenanthroline (phen) (2)] is reported. The [Fe(NCX)2(phen)(L′)2] compounds were further explored by changing the counteranion from NCS– to NCSe– (3) or by replacing pyridine with 4‐picoline (4) or 4‐ethylpyridine (5). Complexes 1–4 exhibit thermal and light‐induced SCO, while 5 remains in the high‐spin state. The kinetics governing the photoinduced back‐conversion was investigated for compounds 2 and 4. Complex 2 exhibits LIESST relaxation kinetics involving strongly cooperative (sigmoidal) behavior, whereas complex 4 displays a stretched exponential shape, typical for broad distributions of relaxation times. These results are discussed on the basis of the steric hindrance due to bulkiness and the electronic influence of the substituent effects on the magnetic and photomagnetic properties of mononuclear iron(II) complexes. Ligand design was used to tune the thermal and light‐induced magnetic properties of a family of [Fe(NCX)2(L′)2L] complexes: ([Fe(NCS)2(py)2(bipyrimidine)] (py = pyridine), [Fe(NCS)2(py)2(phen)] (phen = phenanthroline), [Fe(NCSe)2(py)2(phen)], [Fe(NCS)2(4‐picoline)2(phen)], and [Fe(NCS)2(4‐ethylpyridine)2(phen)].