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 (ΔE_Q ≈ 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.