Spin-Crossover-Triggered Linkage Isomerization by the Pedal-like Motion of the Azobenzene Ligand in a Neutral Heteroleptic Iron(III) Complex.

The temperature dependence of magnetic susceptibility of [FeIII(azp)(qsal-Me)]·0.5CH3OH [Hqsal-Me = 5-methyl-N-(8-quinoyl)salicylaldimine, H2azp = 2,2'-azobisphenol] demonstrated that the spin-crossover (SCO) transition behavior changed from an abrupt transition to consecutive gradual conversions, and moreover, the initial abrupt transition was recovered, keeping the complex at room temperature. The variable-temperature crystal structures revealed that an SCO-triggered linkage isomerization of the azobenzene ligand from one orientation to two disordered orientations and the relaxation from the disordered orientations to the original orientation occurred. The high-spin to low-spin relaxation kinetics and theoretical calculation indicate that the pedal-like motion of the azobenzene ligand can be on in the high-spin state whereas off in the low-spin state.