Chasing the Evasive FeO Stretch and the Spin State of the Iron(IV)–Oxo Complexes by Photodissociation Spectroscopy

We demonstrate the
application of infrared photodissocation spectroscopy
for determination of the FeO stretching frequencies of high-valent
iron­(IV)–oxo complexes [(L)­Fe­(O)­(X)]2+/+ (L = TMC,
N4Py, PyTACN, and X = CH3CN, CF3SO3, ClO4, CF3COO, NO3, N3). We show that the values determined by resonance Raman spectroscopy
in acetonitrile solutions are on average 9 cm–1 red-shifted
with respect to unbiased gas-phase values. Furthermore, we show the
assignment of the spin state of the complexes based on the vibrational
modes of a coordinated anion and compare reactivities of various iron­(IV)–oxo
complexes generated as dications or monocations (bearing an anionic
ligand). The coordinated anions can drastically affect the reactivity
of the complex and should be taken into account when comparing reactivities
of complexes bearing different ligands. Comparison of reactivities
of [(PyTACN)­Fe­(O)­(X)]+ generated in different spin states
and bearing different anionic ligands X revealed that the nature of
anion influences the reactivity more than the spin state. The triflate
and perchlorate ligands tend to stabilize the quintet state of [(PyTACN)­Fe­(O)­(X)]+, whereas trifluoroacetate and nitrate stabilize the triplet
state of the complex.