Reactivity of a Fe(III)-Bound Methoxide Supported with a Tris(thiolato)phosphine Ligand: Activation of C–Cl Bond in CH₂Cl₂ by Nucleophilic Attack of a Fe(III)–OCH₃ Moiety

Abstract

Two mononuclear nonheme Fe(III) complexes, [PPh4][Fe(III)(PS3″)(OCH3)] (1) and [PPh4][Fe(III)(PS3″)(Cl)] (2), supported by a tris(benzenethiolato)phosphine derivative PS3″ (PS3″ = P(C6H3-3-Me3Si-2-S)3(3-)) have been synthesized and characterized. The structures resolved from X-ray crystallography show that Fe(III) centers in both complexes adopt distorted trigonal-bipyramidal geometry with a methoxide or a chloride binding in the axial position. The magnetic data for both are consistent with intermediate-spin Fe(III) centers with a C3 symmetry (S = 3/2 ground state). The bound methoxide in 1 is labile and can be replaced by a CH3CN molecule. The forming Fe(III)-CH3CN species can be further reduced by cobaltcene quantitatively to a stable Fe(II)-CH3CN complex, [Fe(PS3″)(CH3CN)](-). One-electron oxidation of 2 by ferrocenium gave a Fe(IV) analogue, [Fe(IV)(PS3″)(Cl)]. Importantly, the Fe(III)-OCH3 moiety in complex 1 acts as a strong nucleophile that activates the C-Cl bond in CH2Cl2, leading to the formation of complex 2 quantitatively. Complex 1 also reacts with other electrophiles, benzyl chloride and benzyl bromide, to generate Fe(III)-X species (X = Cl or Br). The reactions were investigated and monitored by UV-vis-NIR, NMR, and ESI-MS spectroscopies.

Structural and Spectroscopic Evidence for a Side-on Fe(III)–Superoxo Complex Featuring Discrete O–O Bond Distances

“…The geometry of all iron centers can be viewed as a pseudotrigonal-bipyramid formed by a PS3″ ligand and an η 2 -O 2 moiety (Figure A,B and Figure S2). The average Fe–S distances (2.31–2.32 Å) in all structures are similar and are close to those reported for related trigonal-bipyramidal complexes, [Fe III (PS3″)­L] − (L = Cl – and OCH 3 – ), containing intermediate-spin Fe­(III) centers . However, the two independent molecules in a crystallographic asymmetric unit of the PPh 4 + salt exhibit two distinguishable O–O distances irrespective of the temperature (tables in Figure A).…”

“…The average Fe–S distances (2.31–2.32 Å) in all structures are similar and are close to those reported for related trigonal-bipyramidal complexes, [Fe III (PS3″)L] − (L = Cl – and OCH 3 – ), containing intermediate-spin Fe(III) centers. 61 However, the two independent molecules in a crystallographic asymmetric unit of the PPh 4 + salt exhibit two distinguishable O–O distances irrespective of the temperature (tables in Figure 2 A). Specifically, one with nearly identical Fe–O1 and Fe–O2 bond lengths displays an O1–O2 distance of 1.33–1.35 Å, and the other featuring a slightly asymmetrically coordinated O 2 motif possesses an O3–O4 length of 1.21–1.23 Å.…”

“…Therefore, Mössbauer data clearly indicate that 1 likely contains an intermediate-spin ferric center, as found for [Fe III (PS3″)Cl] − and [Fe III (PS3″)(OCH 3 )] − . 61 The different quadrupole splitting (Δ E Q ) measured for them can be attributed to their distinct coordination geometries.…”

A Mononuclear Non-heme Manganese(III)–Aqua Complex as a New Active Oxidant in Hydrogen Atom Transfer Reactions

“…Lipoxygenases, which are widely distributed in plants, animals, and fungi, employ iron­(III)–hydroxo or manganese­(III)–hydroxo complexes as reactive intermediates to activate allylic C–H bonds via hydrogen atom transfer (HAT), which is believed to proceed via proton-coupled electron transfer (PCET). − These enzymes catalyze the regio- and stereo­specific dioxygenation of poly­unsaturated fatty acids by abstracting a H-atom with the Fe III –OH or Mn III –OH intermediates and generating hydro­peroxides for further reactions. − For example, the use of Fe III –OH as a HAT agent to cleave a substrate C–H bond produces [Fe II –OH 2 R • ] by concerted PCET, in which proton is transferred to the OH ligand to form a H 2 O ligand along with the electron transfer (ET) to the Fe III metal center to produce Fe II –OH 2 . − This strategy can avoid the formation of high-valent metal–oxo intermediates in activating substrate C–H bonds. There have been a number of studies on the HAT reactions using synthetic Fe III –OH and Mn III –OH complexes for HAT as well as PCET. − Synthetic Fe III –OCH 3 , Mn III –OCH 3 , and Mn IV –OH complexes have also been used as lipoxygenase models in activating substrate C–H bonds. − …”

Recent Advances and Promises in Nitrile Hydratase: From Mechanism to Industrial Applications

“…The Oxidation states of the metal ions and cysteine residues at the NHase active site now represent intriguing research hotspots. Studies show that the oxidation states of the metal ions and cysteine residues at the NHase active site could kinetically influence the activity of NHase (Crisóstomo et al, 2010;Swartz et al, 2011;Li et al, 2012;Chang et al, 2015;Kumar et al, 2015;Yano et al, 2015).…”