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

The pyrrolyl-based iron pincer compounds [((PNP)-P-tBu)FeCl] (1), [((PNP)-P-tBu)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 Fe-57 Mossbauer 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 Mossbauer spectrum shows a characteristically large quadrupole splitting (iE(Q) approximate to 3.7 mm s(-1)), and the solid-state magnetic susceptibility data show pronounced zero-field splitting (|D| approximate to 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 spinorbit 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 Mossbauer parameters for all complexes studied herein are in excellent agreement with the experimental findings.