Cooperative iron(II) spin crossover complexes with N₄O ₂ coordination sphere

Two new spin crossover complexes [FeL(py)₂] (1) and [FeL(DMAP)₂] (2) with L being a tetradentate N₂O ₂ ^2- coordinating Schiff-base-like ligand [([3,3′]-[1,2-phenylenebis(iminomethylidyne)]bis(2,4-pentanedionato)(2-) -N,N′,O²,O²′], py = pyridine and DMAP = p-dimethylaminopyridine have been investigated using temperature-dependent susceptibility and thermogravimetric and photomagnetic measurements as well as Mössbauer spectroscopy and X-ray structure analysis. Both complexes show a cooperative spin transition with an approximately 9 K wide thermal hysteresis loop in the case of 2 (T_1/2↑ = 183 K and T_1/2↓ = 174 K) and an approximately 2 K wide thermal hysteresis loop in the case of the pyridine diadduct 1 (T_1/2↑ = 191 K and T_1/2↓ = 189 K). The spin transition was additionally followed by different temperature-scanning calorimetry and Mössbauer spectroscopy for 2, and a good agreement for the transition temperatures obtained with the different methods was found. Results from X-ray structure analysis indicate that the cooperative interactions are due to elastic interactions in both compounds. They are more pronounced in the case of 2 with very short intermolecular iron-iron distances of 7.2 Å and several intense C-C contacts. The change of the spin state at the iron center is accompanied by a change of the O-Fe-O angle, the so-called bit of the equatorial ligand, from 108° in the high-spin state to 90° in the low-spin state. The reflectivity measurements of both compounds give at low temperature indication that at the sample surface the light-induced excited spin state trapping (LIESST) effect occurs. In bulk condition using a SQUID magnetometer the complex 2 displays some photomagnetic properties with an photoexcitation level of 60% and a T(LIESST) value of 53 K.