Synthesis and characterisation of two new iron(II) spin-crossover complexes with N4O2 coordination spheres - optimizing preconditions for cooperative interactions

Two new spin-crossover complexes, [FeL1(phpy)2] (1) and [FeL2(phpy)2](phpy) (2), with L1 and L2 being tetradentate N2O22--coordinating Schiff-base-like ligands {L1 = (E,E)-[diethyl 2,2-[1,2-phenylenebis(iminomethylidyne)]bis(3-oxobutanoato)(2-)-N,N,O3,O3], L2 = [(3,3)-[1,2-phenylenebis(iminomethylidyne)]bis(2,4-pentanedionato)(2-)-N,N,O2,O2]and phpy = 4-phenylpyridine} have been investigated using temperature-dependent susceptibility and photomagnetic measurements, as well as Mössbauer spectroscopy and X-ray structure analysis. Compound 1 shows a cooperative spin transition with a thermal hysteresis loop approximately 4 K wide (T1/2 = 232 K and T1/2 = 236 K). The spin transition of 2 is gradual with T1/2 290 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, from 107° in the high-spin state to 89° 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 a T(LIESST) value of 41 K are observed using a SQUID magnetometer