Iron Complexes Derived from {nacnac-(CH2py)2}− and {nacnac-(CH2py)(CHpy)}n Ligands: Stabilization of Iron(II) via Redox Noninnocence

Nacnac-based tetradentate chelates, {nacnac-(CH2py)2}− ({nn­(PM)2}−) and {nacnac-(CH2py)­(CHpy)}n ({nn­(PM)­(PI)}n) have been investigated in iron complexes. Treatment of Fe­{N­(TMS)2}2(THF) with {nn­(PM)2}H afforded {nn­(PM)2}­FeN­(TMS)2 [1-N­(TMS)2], which led to {nn­(PM)2}­FeCl (1-Cl) from HCl and to {nn­(PM)2}­FeN3 (1-N3) upon salt metathesis. Dehydroamination of 1-N­(TMS)2 was induced by L (L = PMe3, CO) to afford {nn­(PM)­(PI)}­Fe­(PMe3)2 [2-(PMe3)2] and {nn­(PM)­(PI)}­FeCO (3-CO). Substitution of 2-(PMe3)2 led to {nn­(PM)­(PI)}­Fe­(PMe3)­CO [2-(PMe3)­CO], and exposure to a vacuum provided {nn­(PM)­(PI)}­Fe­(PMe3) (3-PMe3). Metathesis routes to {nn­(PM)­(PI)}­FeL2 (2-L2; L = PMe3, PMe2Ph) and {nn­(PM)­(PI)}­FeL (3-L; L = PMePh2, PPh3) from [{nn­(PM)­(PI)}2–]­Li2 and FeBr2(THF)2 in the presence of L proved feasible, and 1e– and 2e– oxidation of 2-(PMe3)2 afforded 2+-(PMe3)2 and 22+-(PMe3)2 salts. Mössbauer spectroscopy, structural studies, and calculational assessments revealed the dominance of iron­(II) in both high-spin (1-X) and low-spin (2-L2 and 3-L) environments, and the redox noninnocence (RNI) of {nn­(PM)­(PI)}n [2-L2, 3-L, n = 2–; 2+-(PMe3)2, n = 1–; 22+-(PMe3)2, n = 0]. A discussion regarding the utility of RNI in chemical reactivity is proffered.