Synthesis and Characterisation of Two New Iron(II) Spin-Crossover Complexes with N4O2 Coordination Spheres - Optimizing Preconditions for Cooperative Interactions

Two ne v spin-crossover complexes, [FeL1(phpy)(2)] (1) and [FeL2(phpy)(2)](phpy) (2), with L1 and L2 being tetradentate N2O2--Coordinating Schiff-base-like ligands {L1 = (E,E)-diethyl 2,2'-[1,2-phenylenebis(iminomethylidyne)bis(3-oxobutanoato)(2-)-N,N',O-3,O-3'], L2 = [(3,3')-[1,2-phenylenebis(iminomethylidyne)]bis(2,4-pentanedionato)(2-)-N,N',O-2,O-2'] and phpy = 4-phenylpyridine} have been investigated using temperature-dependent susceptibility and photomagnetic measurements, as well as Mossbauer spectroscopy and Xray structure analysis. Compound 1 shows a cooperative spin transition with a thermal hysteresis loop approximately 4 K wide (T-1/2, = 232 K and T-1/2 = 236 K). The spin transition of 2 is gradual with T-1/2 = 290 K. Results from X-ray structure analysis indicate that the cooperative interactions are due to elastic interactions with several intense C-C contacts in the case of 1. The change of the spin state at the iron centre is accompanied by a change of the O-Fe-O angle, the so-called bite of the equatorial ligand, front 107 degrees in the high-spin state to 89 degrees in the low-spin state. Reflectivity measurements of 1 indicate that the Light-Induced Excited Spin State Trapping (LIESST) effect occurs at the sample surface at low temperatures, In the bulk solid, photomagnetic properties with a photoexcitation level of 30% and it T(LIESST) value of 41 K are observed using a SQUID magnetometer, (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)