Spectroscopic and Computational Studies of Spin States of Iron(IV) Nitrido and Imido Complexes

High-oxidation-state
metal complexes with multiply bonded ligands are of great interest
for both their reactivity as well as their fundamental bonding properties.
This paper reports a combined spectroscopic and theoretical investigation
into the effect of the apical multiply bonded ligand on the spin-state
preferences of threefold symmetric iron­(IV) complexes with tris­(carbene)
donor ligands. Specifically, singlet (S = 0) nitrido
[{PhB­(ImR)3}­FeN], R = tBu (1), Mes (mesityl, 2) and the related triplet
(S = 1) imido complexes, [{PhB­(ImR)3}­Fe­(NR′)]+, R = Mes, R′ = 1-adamantyl
(3), tBu (4), were investigated
by electronic absorption and Mössbauer effect spectroscopies.
For comparison, two other Fe­(IV) nitrido complexes, [(TIMENAr)­FeN]+ (TIMENAr = tris­[2-(3-aryl-imidazol-2-ylidene)ethyl]­amine;
Ar = Xyl (xylyl), Mes), were investigated by 57Fe Mössbauer
spectroscopy, including applied-field measurements. The paramagnetic
imido complexes 3 and 4 were also studied
by magnetic susceptibility measurements (for 3) and paramagnetic
resonance spectroscopy: high-frequency and -field electron paramagnetic
resonance (for 3 and 4) and frequency-domain
Fourier-transform (FD-FT) terahertz electron paramagnetic resonance
(for 3), which reveal their zero-field splitting parameters.
Experimentally correlated theoretical studies comprising ligand-field
theory and quantum chemical theory, the latter including both density
functional theory and ab initio methods, reveal the key role played
by the Fe 3dz2 (a1) orbital in these systems: the nature of its interaction with the
nitrido or imido ligand dictates the spin-state preference of the
complex. The ability to tune the spin state through the energy and
nature of a single orbital has general relevance
to the factors controlling spin states in complexes with applicability
as single molecule devices.