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.