Synthesis and Characterization of a Dinuclear Iron(II) Spin Crossover Complex with Wide Hysteresis

The magnetic properties and results from X-ray structure analysis for a new pair of iron(II) spin-crossover complexes [FeL1(meim)(2)](meim) (1(meim)) and [Fe(2)L2(meim)(4)](meim)(4) (2(meim)(4)), with L1 being a tetradentate N2O22- coordinating Schiff-base-like ligand [([3,3']-[1,2-phenylenebis(iminomethylidyne)]bis(2,4-pentane-dionato)(2-)-N,N' ,O-2,O-2'), L2 being an octadentate, dinucleating N2O22- coordinating Schiff-base-like ligand [3,3',3,3']-[1,2,4,5-phenylenetetra(iminomethylidyne)]tetra(2,4-pentanedionato)(2-)N,N',N,N',O-2,O-2',O-2,O-2'], and meim being N-methylimidazole, are discussed in this work. Crystalline samples of both complexes show a cooperative spin transition with an approximately 2-K-wide thermal hysteresis loop in the case of 1(meim) (T-1/21= 179 K and T-1/21 = 177 K) and an approximately 21-K-wide thermal hysteresis loop in the case of dinuclear complex 2(meim)(4) (T-1/21 = 199 K and T-1/21 = 178 K). For a separately prepared powder sample of 2, a gradual spin transition with T-1/2 = 229 K is observed that was additionally followed by Mossbauer spectroscopy. The results from X-ray structure analysis give a deeper insight into the molecule packing in the crystal and, by this, help to explain the increase of cooperative interactions during the spin transition when going from the mononuclear to the dinuclear complex. Both compounds crystallize in the triclinic space group P (1) over bar, and the X-ray structure was analyzed before and after the spin transition. The change of the spin state at the iron center is accompanied by a change of the O-Fe-O angle, the so-called bite of the equatorial ligand, from about 109 degrees in the high-spin state to 89 degrees in the low-spin state. The cooperative interactions responsible for the thermal hysteresis loop are due to elastic interactions between the complex molecules in both cases. However, due to the higher symmetry of the dinucleating ligand in 2(meim)(4), a 3D network of short contacts is formed, while for mononuclear complex 1 (meim), a 2D layer of linked molecules is observed. The spin transition was additionally followed in solution using H-1 NMR spectroscopy for both complexes. In both cases, a gradual spin transition is observed, and the increase of cooperative interactions when going from the mononuclear to the dinuclear system is solely attributed to the extended network of intermolecular contacts.