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1. Stable abnormal N-heterocyclic carbenes and their applications

   https://doi.org/10.1039/c9cs00866G  

   Sau, Samaresh Chandra; Hota, Pradip Kumar; Mandal, Swadhin K.; Soleilhavoup, Michele; Bertrand, Guy (February 2020, Chemical Society Reviews)

   Although N-heterocyclic carbenes (NHCs) have been known as ligands for organometallic complexes since the 1960s, these carbenes did not attract considerable attention until Arduengo et al. reported the isolation of a metal-free imidazol-2-ylidene in 1991. In 2001 Crabtree et al. reported a few complexes featuring an NHC isomer, namely an imidazol-5-ylidene, also termed abnormal NHC (aNHCs). In 2009, it was shown that providing to protect the C-2 position of an imidazolium salt, the deprotonation occurred at the C-5 position, affording imidazol-5-ylidenes that could be isolated. Over the last ten years, stable aNHCs have been used for designing a range of catalysts employing Pd( ii ), Cu( i ), Ni( ii ), Fe(0), Zn( ii ), Ag( i ), and Au( i / iii ) metal based precursors. These catalysts were utilized for different organic transformations such as the Suzuki–Miyaura cross-coupling reaction, C–H bond activation, dehydrogenative coupling, Huisgen 1,3-dipolar cycloaddition (click reaction), hydroheteroarylation, hydrosilylation reaction and migratory insertion of carbenes. Main-group metal complexes were also synthesized, including K( i ), Al( iii ), Zn( ii ), Sn( ii ), Ge( ii ), and Si( ii / iv ). Among them, K( i ), Al( iii ), and Zn( ii ) complexes were used for the polymerization of caprolactone and rac -lactide at room temperature. In addition, based on the superior nucleophilicity of aNHCs, relative to that of their nNHCs isomers, they were used for small molecules activation, such as carbon dioxide (CO 2 ), nitrous oxide (N 2 O), tetrahydrofuran (THF), tetrahydrothiophene and 9-borabicyclo[3.3.1]nonane (9BBN). aNHCs have also been shown to be efficient metal-free catalysts for ring opening polymerization of different cyclic esters at room temperature; they are among the most active metal-free catalysts for ε-caprolactone polymerization. Recently, aNHCs successfully accomplished the metal-free catalytic formylation of amides using CO 2 and the catalytic reduction of carbon dioxide, including atmospheric CO 2 , into methanol, under ambient conditions. Although other transition metal complexes featuring aNHCs as ligand have been prepared and used in catalysis, this review article summarize the results obtained with the isolated aNHCs. 

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2. Atomically Dispersed Dual‐Metal Site Catalysts for Enhanced CO ₂ Reduction: Mechanistic Insight into Active Site Structures

   https://doi.org/10.1002/anie.202205632  

   Li, Yi; Shan, Weitao; Zachman, Michael_J; Wang, Maoyu; Hwang, Sooyeon; Tabassum, Hassina; Yang, Juan; Yang, Xiaoxuan; Karakalos, Stavros; Feng, Zhenxing; et al (May 2022, Angewandte Chemie International Edition)

   Abstract Carbon‐supported nitrogen‐coordinated single‐metal site catalysts (i.e., M−N−C, M: Fe, Co, or Ni) are active for the electrochemical CO₂reduction reaction (CO₂RR) to CO. Further improving their intrinsic activity and selectivity by tuning their N−M bond structures and coordination is limited. Herein, we expand the coordination environments of M−N−C catalysts by designing dual‐metal active sites. The Ni‐Fe catalyst exhibited the most efficient CO2RR activity and promising stability compared to other combinations. Advanced structural characterization and theoretical prediction suggest that the most active N‐coordinated dual‐metal site configurations are 2N‐bridged (Fe‐Ni)N₆, in which FeN₄and NiN₄moieties are shared with two N atoms. Two metals (i.e., Fe and Ni) in the dual‐metal site likely generate a synergy to enable more optimal *COOH adsorption and *CO desorption than single‐metal sites (FeN₄or NiN₄) with improved intrinsic catalytic activity and selectivity. 

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3. Use of Intramolecular Quinol Redox Couples to Facilitate the Catalytic Transformation of O ₂ and O ₂ -Derived Species

   https://doi.org/10.1021/acs.accounts.4c00645  

   Farnum, Byron H; Goldsmith, Christian R (January 2025, Accounts of Chemical Research)

   The redox reactivity of transition metal centers can be augmented by nearby redox-active inorganic or organic moieties. In some cases, these functional groups can even allow a metal center to participate in reactions that were previously inaccessible to both the metal center and the functional group by themselves. Our research groups have been synthesizing and characterizing coordination complexes with polydentate quinol-containing ligands. Quinol is capable of being reversibly oxidized by either one or two electrons to semiquinone or para-quinone, respectively. Functionally, quinol behaves much differently than phenol, even though the pKa values of the first O−H bonds are nearly identical. The redox activity of the quinol in the polydentate ligand can augment the abilities of bound redox-active metals to catalyze the dismutation of O2−• and H2O2. These complexes can thereby act as high-performing functional mimics of superoxide dismutase (SOD) and catalase (CAT) enzymes, which exclusively use redox-active metals to transfer electrons to and from these reactive oxygen species (ROS). The quinols augment the activity of redox-active metals by stabilizing higher-valent metal species, providing alternative redox partners for the oxidation and reduction of reactive oxygen species, and protecting the catalyst from destructive side reactions. The covalently attached quinols can even enable redox-inactive Zn(II) to catalyze the degradation of ROS. With the Zn(II)-containing SOD and CAT mimics, the organic redox couple entirely substitutes for the inorganic redox couples used by the enzymes. The ligand structure modulates the antioxidant activity, and thus far, we have found that compounds that have poor or negligible SOD activity can nonetheless behave as efficient CAT mimics. Quinol-containing ligands have also been used to prepare electrocatalysts for dioxygen reduction, functionally mimicking the enzyme cytochrome c oxidase. The installation of quinols can boost electrocatalytic activity and even enable otherwise inactive ligand frameworks to support electrocatalysis. The quinols can also shift the product selectivity of O2 reduction from H2O2 to H2O without markedly increasing the effective overpotential. Distinct control of the coordination environment around the metal center allows the most successful of these catalysts to use economic and naturally abundant first-row transition metals such as iron and cobalt to selectively reduce O2 to H2O at low effective overpotentials. With iron, we have found that the electrocatalysts can enter the catalytic cycle as either an Fe(II) or Fe(III) species with no difference in turnover frequency. The entry point to the cycle, however, has a marked impact on the effective overpotential, with the Fe(III) species thus far being more efficient. 

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4. Influence of spin state and electron configuration on the active site and mechanism for catalytic hydrogenation on metal cation catalysts supported on NU-1000: insights from experiments and microkinetic modeling

   https://doi.org/10.1039/d0cy00394h  

   Shabbir, Hafeera; Pellizzeri, Steven; Ferrandon, Magali; Kim, In Soo; Vermeulen, Nicolaas A.; Farha, Omar K.; Delferro, Massimiliano; Martinson, Alex B.; Getman, Rachel B. (June 2020, Catalysis Science & Technology)

   null (Ed.)

   The mechanism of ethene hydrogenation to ethane on six dicationic 3d transition metal catalysts is investigated. Specifically, a combination of density functional theory (DFT), microkinetic modeling, and high throughput reactor experiments is used to interrogate the active sites and mechanisms for Mn@NU-1000, Fe@NU-1000, Co@NU-1000, Ni@NU-1000, Cu@NU-1000, and Zn@NU-1000 catalysts, where NU-1000 is a metal–organic framework (MOF) capable of supporting metal cation catalysts. The combination of experiments and simulations suggests that the reaction mechanism is influenced by the electron configuration and spin state of the metal cations as well as the amount of hydrogen that is adsorbed. Specifically, Ni@NU-1000, Cu@NU-1000, and Zn@NU-1000, which have more electrons in their d shells and operate in lower spin states, utilize a metal hydride active site and follow a mechanism where the metal cation binds with one or more species at all steps, whereas Mn@NU-1000, Fe@NU-1000, and Co@NU-1000, which have fewer electrons in their d shells and operate in higher spin states, utilize a bare metal cation active site and follow a mechanism where the number of species that bind to the metal cation is minimized. Instead of binding with the metal cation, catalytic species bind with oxo ligands from the NU-1000 support, as this enables more facile H 2 adsorption. The results reveal opportunities for tuning activity and selectivity for hydrogenation on metal cation catalysts by tuning the properties that influence hydrogen content and spin, including the metal cations themselves, the ligands, the binding environments and supports, and/or the gas phase partial pressures. 

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5. Time-resolved β-lactam cleavage by L1 metallo-β-lactamase

   https://doi.org/10.1038/s41467-022-35029-3  

   Wilamowski, M.; Sherrell, D. A.; Kim, Y.; Lavens, A.; Henning, R. W.; Lazarski, K.; Shigemoto, A.; Endres, M.; Maltseva, N.; Babnigg, G.; et al (December 2022, Nature Communications)

   Abstract Serial x-ray crystallography can uncover binding events, and subsequent chemical conversions occurring during enzymatic reaction. Here, we reveal the structure, binding and cleavage of moxalactam antibiotic bound to L1 metallo-β-lactamase (MBL) from Stenotrophomonas maltophilia . Using time-resolved serial synchrotron crystallography, we show the time course of β-lactam hydrolysis and determine ten snapshots (20, 40, 60, 80, 100, 150, 300, 500, 2000 and 4000 ms) at 2.20 Å resolution. The reaction is initiated by laser pulse releasing Zn 2+ ions from a UV-labile photocage. Two metal ions bind to the active site, followed by binding of moxalactam and the intact β-lactam ring is observed for 100 ms after photolysis. Cleavage of β-lactam is detected at 150 ms and the ligand is significantly displaced. The reaction product adjusts its conformation reaching steady state at 2000 ms corresponding to the relaxed state of the enzyme. Only small changes are observed in the positions of Zn 2+ ions and the active site residues. Mechanistic details captured here can be generalized to other MBLs. 

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