Thermal and Light‐Induced Spin Transitions of Fe II Complexes with 4‐ and 5‐(Phenylazo)‐2,2′‐bipyridine Ligands: Intra‐ vs. Intermolecular Effects

Five new spin crossover complexes with 4‐ and 5‐(phenylazo)‐2,2‐bipyridine (4‐/5‐PAbipy) ligands were synthesized and investigated with respect to their spin crossover (SCO) behavior. The results are compared to the thermal and light‐induced spin transition properties of the parent SCO complexes [Fe(bpz) 2 (bipy)] ( 1 ) and [Fe(bipy) 2 (NCS) 2 ] ( 2 ). [Fe(bpz) 2 (4‐PAbipy)] ( 1a ) undergoes a stepwise spin transition whereas [Fe(bpz) 2 (5‐PAbipy)] ( 1b ) exhibits a one‐step transition with a 6 K‐wide hysteresis. For [Fe(bpz) 2 ( t Bu 5‐PAbipy)] ( 1c ) the spin transition to the low‐spin state is incomplete. Qualitatively similar changes of the SCO behavior are observed for the complexes [Fe(4‐PAbipy) 2 (NCS) 2 ] ( 2a ) and [Fe(5‐PAbipy) 2 (NCS) 2 ] ( 2b ). In comparison to the parent system 2 , a strengthening of intermolecular interactions leads to a stabilization of the low‐spin state. Evidence for the LIESST behavior could be obtained for all new compounds by means of magnetic susceptibility measurements as well as UV/Vis and resonance Raman spectroscopy.