Synthesis and Characterisation of Two New Iron(II) Spin‐Crossover Complexes with N 4 O 2 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 N 2 O 2 2– ‐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 Mössbauer spectroscopy and X‐ray 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, 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)