Inorganic Chemistry, Vol.54, No.9, 4466-4474, 2015
Synthesis, Characterization, and Photophysical Studies of an Iron(III) Catecholate-Nitronylnitroxide Spin-Crossover Complex
The synthesis and characterization of an Fe-III catecholate nitronylnitroxide (CAT-NN) complex (1-NN) that undergoes Fein spin-crossover is described. Our aim is to determine whether the intraligand exchange coupling of the semiquinone nitronylnitroxide Fell(SQ:NN) excited state resulting from irradiation of the CAT -> Fein LMCT band would affect either the intrinsic photophysics or the iron spin-crossover event when compared to the complex lacking the nitronylnitroxide radical (1). X-ray crystallographic analysis provides bond lengths consistent with a ferric catecholate charge distribution. Mossbauer spectroscopy clearly demonstrates Fein spin-crossover, hyperfine couplings, and a weak ferromagnetic Felli 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 FelISQstate is populated and most likely undergoes fast intersystem crossing to the ligand field manifold, where it rapidly decays into a metastable low-spin Fell1CAT state, followed by repopulation of the high-spin FellICAT ground state. The decay components of 1-NN are slightly faster than those obtained for 1, perhaps due to the higher number of microstates present within the LMCT and LF manifolds for 1-NN. 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.