Isostructural bridging diferrous chalcogenide cores [FeII(μ-E)FeII] (E = O, S, Se, Te) with decreasing antiferromagnetic coupling down the chalcogenide series

Abstract

Iron compounds containing a bridging oxo or sulfido moiety are ubiquitous in biological systems, but substitution with the heavier chalcogenides selenium and tellurium, however, is much rarer, with only a few examples reported to date. Here we show that treatment of the ferrous starting material [(^tBupyrpyrr₂)Fe(OEt₂)] (1-OEt₂) (^tBupyrpyrr₂ = 3,5-^tBu₂-bis(pyrrolyl)pyridine) with phosphine chalcogenide reagents E = PR₃ results in the neutral phosphine chalcogenide adduct series [(^tBupyrpyrr₂)Fe(EPR₃)] (E = O, S, Se; R = Ph; E = Te; R = ^tBu) (1-E) without any electron transfer, whereas treatment of the anionic starting material [K]₂[(^tBupyrpyrr₂)Fe₂(μ-N₂)] (2-N₂) with the appropriate chalcogenide transfer source yields cleanly the isostructural ferrous bridging mono-chalcogenide ate complexes [K]₂[(^tBupyrpyrr₂)Fe₂(μ-E)] (2-E) (E = O, S, Se, and Te) having significant deviation in the Fe-E-Fe bridge from linear in the case of E = O to more acute for the heaviest chalcogenide. All bridging chalcogenide complexes were analyzed using a variety of spectroscopic techniques, including ¹H NMR, UV-Vis electronic absorbtion, and ⁵⁷Fe Mössbauer. The spin-state and degree of communication between the two ferrous ions were probed via SQUID magnetometry, where it was found that all iron centers were high-spin (S = 2) Fe^II, with magnetic exchange coupling between the Fe^II ions. Magnetic studies established that antiferromagnetic coupling between the ferrous ions decreases as the identity of the chalcogen is tuned from O to the heaviest congener Te.

Scheme 1. Reported examples of mono-bridged diiron chalcogenide complexes, DeBeer's bis-chalcogenide bridged diiron system stabilizing intermediate spin states, and this work. Cp^BIG = Cp(p-^tBuPh)₅.

Scheme 2. Synthesis of mononuclear phosphine chalcogenide adducts 1-O^Ph, 1-S^Ph, 1-Se^Ph, and 1-Te^tBu.

Fig. 1. Solid-state molecular structures of 1-O^Ph, 1-S^Ph, 1-Se^Ph, 1-Te^tBu (top) and 2-O, 2-S, 2-Se, and 2-Te (bottom) shown at 50% probability. Hydrogen atoms and solvent molecules have been omitted for clarity.

Scheme 3. Synthesis of dinuclear bridging chalcogenide anions 2-O, 2-S, 2-Se, and 2-Te.

Fig. 2. Zero-field ⁵⁷Fe Mössbauer spectra of compounds 1-E (left) and 2-E (right) recorded at 77 K.

Fig. 3. SQUID magnetization data of representative samples of compounds 1-E (top) and 2-E (bottom). Reproducibility was confirmed by measuring two independently synthesized samples for each compound (see the ESI†). For 1-Te, temperature-independent paramagnetism (TIP = 1.7 × 10⁻³ emu) has been considered and subtracted. Color indicates identity of chalcogen E (O-red, S-orange, Se-green, and Te-brown).

Fig. 4. (Top) Scatter plots of Fe–E bond length (orange circles, Å, left axis), Fe–Fe distance (red circles, Å, left axis), ∠Fe–E–Fe angle (green triangles, degrees, right axis), and ∠N_pyr–Fe–E angle (purple triangles, degrees, right axis) of compounds 2-E as a function of the period of the chalcogenide substituent. The ∠N_pyr–Fe–E angles and Fe–E bond lengths for periods 3–5 (compounds 2-S, 2-Se, and 2-Te) are calculated as the average for the two different iron atoms. (Bottom) Scatter plots of Mössbauer isomer shift (red circles, mm s⁻¹, left axis), Mössbauer Quadrupole Splitting (orange circles, mm s⁻¹, left axis), SQUID μ_eff at 300 K (purple triangles, μ_B, left axis), and SQUID antiferromagnetic coupling J (grey triangles, cm⁻¹, right axis) as a function of the period of the chalcogenide substituent.