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1. 10 ⁶ -fold faster C–H bond hydroxylation by a Co ^III,IV ₂ (µ-O) ₂ complex [via a Co ^III ₂ (µ-O)(µ-OH) intermediate] versus its Fe ^III Fe ^IV analog

   https://doi.org/10.1073/pnas.2307950120  

   Li, Yan; Abelson, Chase; Que, Lawrence; Wang, Dong (December 2023, Proceedings of the National Academy of Sciences)

   The hydroxylation of C–H bonds can be carried out by the high-valent Co^III,IV₂(µ-O)₂complex2asupported by the tetradentate tris(2-pyridylmethyl)amine ligand via a Co^III₂(µ-O)(µ-OH) intermediate (3a). Complex3acan be independently generated either by H-atom transfer (HAT) in the reaction of2awith phenols as the H-atom donor or protonation of its conjugate base, the Co^III₂(µ-O)₂complex1a. Resonance Raman spectra of these three complexes reveal oxygen-isotope-sensitive vibrations at 560 to 590 cm⁻¹associated with the symmetric Co–O–Co stretching mode of the Co₂O₂diamond core. Together with a Co•••Co distance of 2.78(2) Å previously identified for1aand2aby Extended X-ray Absorption Fine Structure (EXAFS) analysis, these results provide solid evidence for their “diamond core” structural assignments. The independent generation of3aallows us to investigate HAT reactions of2awith phenols in detail, measure the redox potential and pK_aof the system, and calculate the O–H bond strength (D_O–H) of3ato shed light on the C–H bond activation reactivity of2a. Complex3ais found to be able to transfer its hydroxyl ligand onto the trityl radical to form the hydroxylated product, representing a direct experimental observation of such a reaction by a dinuclear cobalt complex. Surprisingly, reactivity comparisons reveal2ato be 10⁶-fold more reactive in oxidizing hydrocarbon C–H bonds than corresponding Fe^III,IV₂(µ-O)₂and Mn^III,IV₂(µ-O)₂analogs, an unexpected outcome that raises the prospects for using Co^III,IV₂(µ-O)₂species to oxidize alkane C–H bonds. 

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2. Homo-oligomerization of the human adenosine A2A receptor is driven by the intrinsically disordered C-terminus

   https://doi.org/10.7554/eLife.66662  

   Nguyen, Khanh Dinh; Vigers, Michael; Sefah, Eric; Seppälä, Susanna; Hoover, Jennifer Paige; Schonenbach, Nicole Star; Mertz, Blake; O'Malley, Michelle Ann; Han, Songi (July 2021, eLife)

   null (Ed.)

   G protein-coupled receptors (GPCRs) have long been shown to exist as oligomers with functional properties distinct from those of the monomeric counterparts, but the driving factors of oligomerization remain relatively unexplored. Herein, we focus on the human adenosine A 2A receptor (A 2A R), a model GPCR that forms oligomers both in vitro and in vivo. Combining experimental and computational approaches, we discover that the intrinsically disordered C-terminus of A 2A R drives receptor homo-oligomerization. The formation of A 2A R oligomers declines progressively with the shortening of the C-terminus. Multiple interaction types are responsible for A 2A R oligomerization, including disulfide linkages, hydrogen bonds, electrostatic interactions, and hydrophobic interactions. These interactions are enhanced by depletion interactions, giving rise to a tunable network of bonds that allow A 2A R oligomers to adopt multiple interfaces. This study uncovers the disordered C-terminus as a prominent driving factor for the oligomerization of a GPCR, offering important insight into the effect of C-terminus modification on receptor oligomerization of A 2A R and other GPCRs reconstituted in vitro for biophysical studies. 

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3. Sub-micrometer particle size effects on metastable phases for a photoswitchable Co–Fe Prussian blue analog

   https://doi.org/10.1063/5.0074165  

   Itoi, Miho; Maurin, Isabelle; Boukheddaden, Kamel; Andrus, Matthew J.; Talham, Daniel R.; Elkaim, Erik; Uwatoko, Yoshiya (February 2022, Journal of Applied Physics)

   Metastable phases of the photoswitchable molecular magnet K_0.3Co[Fe(CN)₆]_0.77 ⋅  nH₂O in sub-micrometer particles have been structurally investigated by synchrotron powder x-ray diffraction (PXRD) measurements. The K_0.3Co[Fe(CN)₆]_0.77 ⋅  nH₂O bulk compound (studied here with a sample having average particle size of 500 nm) undergoes a charge transfer coupled spin transition (CTCST), where spin configurations change between a paramagnetic Co^II( S = 3/2) –Fe^III( S = 1/2) high-temperature (HT) state and a diamagnetic Co^III( S = 0) –Fe^II( S = 0) low-temperature (LT) state. The bulk compound exhibits a unique intermediate (IM) phase, which corresponds to a mixture of HT and LT spin states that depend on the cooling rate. Several hidden metastable HT states emerge as a function of thermal and photo stimuli, namely: (1) a quench (Q) state generated from the HT state by flash cooling, (2) a LTPX state obtained by photoexcitation from the LT state derived by thermal relaxation from the Q state, and (3) an IMPX state accessed by photo-irradiation from the IM state. A sample with a smaller particle size, 135 nm, is investigated for which the particles are on the scale of the coherent LT domains in the IM phase within the larger 500 nm sample. PXRD studies under controlled thermal and/or optical excitations have clarified that the reduction of the particle size profoundly affects the structural changes associated with the CTCST. The unusual IM state is also observed as segregated domains in the 135 nm particle, but the collective structural transformations are more hindered in small particles. The volume change decreases to 2%–3%, almost half the value found for 500 nm particles (5%–8%), even though the linear thermal expansion coefficients are larger for the smaller particles. Furthermore, photoexcitation from the IM and LT states does not turn into single phases in the smaller particles, presumably because of the multiple interfaces and/or internal stress generated by the coexistence of small Co^II–Fe^IIIand Co^III–Fe^IIdomains in the lattice. Since the reduced particle size limits cooperativity and domain growth in the lattice, CTCST in the small particle sample becomes less sensitive to external stimuli. 

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4. Palladium(0) complexes of diferrocenylmercury diphosphines: synthesis, X-ray structure analyses, catalytic isomerization, and C–Cl bond activation

   https://doi.org/10.1039/d1dt00641j  

   Tagne Kuate, Alain C.; Lalancette, Roger. A.; Bockfeld, Dirk; Tamm, Matthias; Jäkle, Frieder (April 2021, Dalton Transactions)

   Palladium(0) phosphine complexes are of great importance as catalysts in numerous bond formation reactions that involve oxidative addition of substrates. Highly active catalysts with labile ligands are of particular interest but can be challenging to isolate and structurally characterize. We investigate here the synthesis and chemical reactivity of Pd 0 complexes that contain geometrically adaptable diferrocenylmercury-bridged diphosphine chelate ligands (L) in combination with a labile dibenzylideneacetone (dba) ligand. The diastereomeric diphosphines 1a (p S p R , meso -isomer) and 1b (p S p S -isomer) differ in the orientation of the ferrocene moieties relative to the central Ph 2 PC 5 H 3 –Hg–C 5 H 3 PPh 2 bridging entity. The structurally distinct trigonal LPd 0 (dba) complexes 2a ( meso ) and 2b (p S p S ) are obtained upon treatment with Pd(dba) 2 . A competition reaction reveals that 1b reacts faster than 1a with Pd(dba) 2 . Unexpectedly, catalytic interconversion of 1a ( meso ) into 1b ( rac ) is observed at room temperature in the presence of only catalytic amounts of Pd(dba) 2 . Both Pd 0 complexes, 2a and 2b , readily undergo oxidative addition into the C–Cl bond of CH 2 Cl 2 at moderate temperatures with formation of the square-planar trans -chelate complexes LPd II Cl(CH 2 Cl) ( 3a , 3b ). Kinetic studies reveal a significantly higher reaction rate for the meso -isomer 2a in comparison to (p S p S )- 2b . 

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5. Ligand and Linkage Isomers of Bis(ethylthiocarbamato) Copper Complexes with Cyclic C ₆ H ₈ Backbone Substituents: Synthesis, Characterization, and Antiproliferation Activity

   https://doi.org/10.1002/ejic.202300447  

   Bajaj, Kritika; Andres, Sarah_A; Hofsommer, Dillon_T; Chekwube_Michael, Okolocha; Mashuta, Mark_S; Bates, Paula_J; Buchanan, Robert_M; Grapperhaus, Craig_A (September 2023, European Journal of Inorganic Chemistry)

   Abstract A series of isomeric bis(alkylthiocarbamate) copper complexes have been synthesized, characterized, and evaluated for antiproliferation activity. The complexes were derived from ligand isomers with 3‐methylpentyl (H₂L²) and cyclohexyl (H₂L³) backbone substituents, which each yield a pair of linkage isomers. The thermodynamic products CuL^2a/3ahave two imino N and two S donors resulting in three five‐member chelate rings (555 isomers). The kinetic isomers CuL^2b/3bhave one imino and one hydrazino N donor and two S donors resulting in four‐, six‐, and five‐member rings (465 isomers). The 555 isomers have more accessible Cu^II/Ipotentials (E_1/2=−811/−768 mV vs. ferrocenium/ferrocene) and lower energy charge transfer bands than their 465 counterparts (E_1/2=−923/‐854 mV). Antiproliferation activities were evaluated against the lung adenocarcinoma cell line (A549) and nonmalignant lung fibroblast cell line (IMR‐90) using the MTT assay. CuL^2awas potent (^A549EC₅₀=0.080 μM) and selective (^IMR‐90EC₅₀/^A549EC₅₀=25) for A549. Its linkage isomer CuL^2bhad equivalent A549 activity, but lower selectivity (^IMR‐90EC₅₀/^A549EC₅₀=12.5). The isomers CuL^3aand CuL^3bwere less potent with^A549EC₅₀ values of 1.9 and 0.19 M and less selective with^IMR‐90EC₅₀/^A549EC₅₀ratios of 2.3 and 2.65, respectively. There was no correlation between reduction potential and A549 antiproliferation activity/selectivity. 

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