Synthesis and Electronic Ground-State Properties of Pyrrolyl-Based Iron Pincer Complexes: Revisited

The
pyrrolyl-based iron pincer compounds [(tBuPNP)­FeCl] (1), [(tBuPNP)­FeN2] (2), and [(tBuPNP)­Fe­(CO)2] (3) were prepared and structurally characterized.
In addition, their electronic ground states were probed by various
techniques including solid-state magnetic susceptibility and zero-field 57Fe Mössbauer and X-band electron paramagnetic resonance
spectroscopy. While the iron­(II) starting material 1 adopts
an intermediate-spin (S = 1) state, the iron­(I) reduction
products 2 and 3 exhibit a low-spin (S = 1/2) ground state. Consistent
with an intermediate-spin configuration for 1, the zero-field 57Fe Mössbauer spectrum shows a characteristically large
quadrupole splitting (ΔEQ ≈
3.7 mm s–1), and the solid-state magnetic susceptibility
data show pronounced zero-field splitting (|D| ≈
37 cm–1). The effective magnetic moments observed
for the iron­(I) species 2 and 3 are larger
than expected from the spin-only value and indicate an incompletely
quenched orbital angular momentum and the presence of spin–orbit
coupling in the ground state. The experimental findings are complemented
by density functional theory computations, which are in good agreement
with the experimental data. Most notably, these calculations reveal
a low-lying (S = 2) excited state for complex 1; furthermore, the computed Mössbauer parameters for
all complexes studied herein are in excellent agreement with the experimental
findings.