Multimetallic Cooperativity in Uranium-Mediated CO₂ Activation

The metal-mediated redox transformation of CO₂ in mild conditions is an area of great current interest. The role of cooperativity between a reduced metal center and a Lewis acid center in small-molecule activation is increasingly recognized, but has not so far been investigated for f-elements. Here we show that the presence of potassium at a U, K site supported by sterically demanding tris­(<i>tert</i>-butoxy)­siloxide ligands induces a large cooperative effect in the reduction of CO₂. Specifically, the ion pair complex [K­(18c6)]­[U­(OSi­(O^tBu)₃)₄], <b>1</b>, promotes the selective reductive disproportionation of CO₂ to yield CO and the mononuclear uranium­(IV) carbonate complex [U­(OSi­(O^tBu)₃)₄(μ-κ²:κ¹-CO₃)­K₂(18c6)], <b>4</b>. In contrast, the heterobimetallic complex [U­(OSi­(O^tBu)₃)₄K], <b>2</b>, promotes the potassium-assisted two-electron reductive cleavage of CO₂, yielding CO and the U­(V) terminal oxo complex [UO­(OSi­(O^tBu)₃)₄K], <b>3</b>, thus providing a remarkable example of two-electron transfer in U­(III) chemistry. DFT studies support the presence of a cooperative effect of the two metal centers in the transformation of CO₂

Tuning of Ferromagnetic Spin Interactions in Polymeric Aromatic Amines via Modification of Their π‑Conjugated System

Polyarylamine containing <i>meta–para–para</i>-aniline units in the main chain and <i>meta–para</i>-aniline units in the pendant chains was synthesized. The polymer can be oxidized to radical cations in chemical or electrochemical ways. The presence of <i>meta</i>-phenylenes in the polymer chemical structure allows for the ferromagnetic coupling of electronic spins, which leads to the formation of high spin states. Detailed pulsed-EPR study indicates that the <i>S</i> = 2 spin state was reached for the best oxidation level. Quantitative magnetization measurements reveal that the doped polymer contains mainly <i>S</i> = 2 spin states and a fraction of <i>S</i> = 3/2 spin states. The efficiency of the oxidation was determined to be 74%. To the best of our knowledge, this polymer is the first example of a linear doped polyarylamine combining such high spin states with high doping efficiency

Tuning of Ferromagnetic Spin Interactions in Polymeric Aromatic Amines via Modification of Their π‑Conjugated System

Polyarylamine containing <i>meta–para–para</i>-aniline units in the main chain and <i>meta–para</i>-aniline units in the pendant chains was synthesized. The polymer can be oxidized to radical cations in chemical or electrochemical ways. The presence of <i>meta</i>-phenylenes in the polymer chemical structure allows for the ferromagnetic coupling of electronic spins, which leads to the formation of high spin states. Detailed pulsed-EPR study indicates that the <i>S</i> = 2 spin state was reached for the best oxidation level. Quantitative magnetization measurements reveal that the doped polymer contains mainly <i>S</i> = 2 spin states and a fraction of <i>S</i> = 3/2 spin states. The efficiency of the oxidation was determined to be 74%. To the best of our knowledge, this polymer is the first example of a linear doped polyarylamine combining such high spin states with high doping efficiency

Tuning of Ferromagnetic Spin Interactions in Polymeric Aromatic Amines via Modification of Their π‑Conjugated System

Polyarylamine containing <i>meta–para–para</i>-aniline units in the main chain and <i>meta–para</i>-aniline units in the pendant chains was synthesized. The polymer can be oxidized to radical cations in chemical or electrochemical ways. The presence of <i>meta</i>-phenylenes in the polymer chemical structure allows for the ferromagnetic coupling of electronic spins, which leads to the formation of high spin states. Detailed pulsed-EPR study indicates that the <i>S</i> = 2 spin state was reached for the best oxidation level. Quantitative magnetization measurements reveal that the doped polymer contains mainly <i>S</i> = 2 spin states and a fraction of <i>S</i> = 3/2 spin states. The efficiency of the oxidation was determined to be 74%. To the best of our knowledge, this polymer is the first example of a linear doped polyarylamine combining such high spin states with high doping efficiency

Tuning of Ferromagnetic Spin Interactions in Polymeric Aromatic Amines via Modification of Their π‑Conjugated System

Polyarylamine containing <i>meta–para–para</i>-aniline units in the main chain and <i>meta–para</i>-aniline units in the pendant chains was synthesized. The polymer can be oxidized to radical cations in chemical or electrochemical ways. The presence of <i>meta</i>-phenylenes in the polymer chemical structure allows for the ferromagnetic coupling of electronic spins, which leads to the formation of high spin states. Detailed pulsed-EPR study indicates that the <i>S</i> = 2 spin state was reached for the best oxidation level. Quantitative magnetization measurements reveal that the doped polymer contains mainly <i>S</i> = 2 spin states and a fraction of <i>S</i> = 3/2 spin states. The efficiency of the oxidation was determined to be 74%. To the best of our knowledge, this polymer is the first example of a linear doped polyarylamine combining such high spin states with high doping efficiency

Tuning of Ferromagnetic Spin Interactions in Polymeric Aromatic Amines via Modification of Their π‑Conjugated System

Polyarylamine containing <i>meta–para–para</i>-aniline units in the main chain and <i>meta–para</i>-aniline units in the pendant chains was synthesized. The polymer can be oxidized to radical cations in chemical or electrochemical ways. The presence of <i>meta</i>-phenylenes in the polymer chemical structure allows for the ferromagnetic coupling of electronic spins, which leads to the formation of high spin states. Detailed pulsed-EPR study indicates that the <i>S</i> = 2 spin state was reached for the best oxidation level. Quantitative magnetization measurements reveal that the doped polymer contains mainly <i>S</i> = 2 spin states and a fraction of <i>S</i> = 3/2 spin states. The efficiency of the oxidation was determined to be 74%. To the best of our knowledge, this polymer is the first example of a linear doped polyarylamine combining such high spin states with high doping efficiency

Mammalian Frataxin Controls Sulfur Production and Iron Entry during de Novo Fe₄S₄ Cluster Assembly

Iron–sulfur (Fe–S) cluster-containing proteins are essential components of cells. In eukaryotes, Fe–S clusters are synthesized by the mitochondrial iron–sulfur cluster (ISC) machinery and the cytosolic iron–sulfur assembly (CIA) system. In the mammalian ISC machinery, preassembly of the Fe–S cluster on the scaffold protein (ISCU) involves a cysteine desulfurase complex (NFS1/ISD11) and frataxin (FXN), the protein deficient in Friedreich’s ataxia. Here, by comparing the biochemical and spectroscopic properties of quaternary (ISCU/NFS1/ISD11/FXN) and ternary (ISCU/NFS1/ISD11) complexes, we show that FXN stabilizes the quaternary complex and controls iron entry to the complex through activation of cysteine desulfurization. Furthermore, we show for the first time that in the presence of iron and l-cysteine, an [Fe₄S₄] cluster is formed within the quaternary complex that can be transferred to mammalian aconitase (mACO2) to generate an active enzyme. In the absence of FXN, although the ternary complex can assemble an Fe–S cluster, the cluster is inefficiently transferred to ACO2. Taken together, these data help to unravel further the Fe–S cluster assembly process and the molecular basis of Friedreich’s ataxia

Radical <i>S</i>‑Adenosyl‑l‑Methionine Enzyme PylB: A C‑Centered Radical to Convert l‑Lysine into (3<i>R</i>)‑3-Methyl‑d‑Ornithine

PylB is a radical S-adenosyl-l-methionine (SAM) enzyme predicted to convert l-lysine into (3R)-3-methyl-d-ornithine, a precursor in the biosynthesis of the 22nd proteogenic amino acid pyrrolysine. This protein highly resembles that of the radical SAM tyrosine and tryptophan lyases, which activate their substrate by abstracting a H atom from the amino-nitrogen position. Here, combining in vitro assays, analytical methods, electron paramagnetic resonance spectroscopy, and theoretical methods, we demonstrated that instead, PylB activates its substrate by abstracting a H atom from the Cγ position of l-lysine to afford the radical-based β-scission. Strikingly, we also showed that PylB catalyzes the reverse reaction, converting (3R)-3-methyl-d-ornithine into l-lysine and using catalytic amounts of the 5′-deoxyadenosyl radical. Finally, we identified significant in vitro production of 5′-thioadenosine, an unexpected shunt product that we propose to result from the quenching of the 5′-deoxyadenosyl radical species by the nearby [Fe4S4] cluster