Synthesis, Characterization, and Photophysical Studies of an Iron(III) Catecholate–Nitronylnitroxide Spin-Crossover Complex

Abstract

The synthesis and characterization of an Fe^III catecholate–nitronylnitroxide (CAT-NN) complex (<b>1-NN</b>) that undergoes Fe^III spin-crossover is described. Our aim is to determine whether the intraligand exchange coupling of the semiquinone–nitronylnitroxide Fe^II(SQ-NN) excited state resulting from irradiation of the CAT → Fe^III LMCT band would affect either the intrinsic photophysics or the iron spin-crossover event when compared to the complex lacking the nitronylnitroxide radical (<b>1</b>). X-ray crystallographic analysis provides bond lengths consistent with a ferric catecholate charge distribution. Mössbauer spectroscopy clearly demonstrates Fe^III spin-crossover, hyperfine couplings, and a weak ferromagnetic Fe^III–CAT-NN exchange, and spin-crossover is corroborated by variable-temperature magnetic susceptibility and electronic absorption studies. To explore the effect of the NN radical on photophysical processes, we conducted room-temperature transient absorption experiments. Upon excitation of the ligand-to-metal charge transfer band, an Fe^IISQ state is populated and most likely undergoes fast intersystem crossing to the ligand field manifold, where it rapidly decays into a metastable low-spin Fe^IIICAT state, followed by repopulation of the high-spin Fe^IIICAT ground state. The decay components of <b>1-NN</b> are slightly faster than those obtained for <b>1</b>, perhaps due to the higher number of microstates present within the LMCT and LF manifolds for <b>1-NN</b>. Although the effects of the NN radical are manifest in neither the spin-crossover nor the photophysics, our results lay the groundwork for future studies