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

The
synthesis and characterization of an FeIII catecholate–nitronylnitroxide
(CAT-NN) complex (1-NN) that undergoes FeIII spin-crossover is described. Our aim is to determine whether the
intraligand exchange coupling of the semiquinone–nitronylnitroxide
FeII(SQ-NN) excited state resulting from irradiation of
the CAT → FeIII 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. Mössbauer spectroscopy
clearly demonstrates FeIII spin-crossover, hyperfine couplings,
and a weak ferromagnetic FeIII–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 FeIISQ state is populated and
most likely undergoes fast intersystem crossing to the ligand field
manifold, where it rapidly decays into a metastable low-spin FeIIICAT state, followed by repopulation of the high-spin FeIIICAT 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.