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1. Isolation and characterization of a covalent CeIV-Aryl complex with an anomalous 13C chemical shift

   https://doi.org/10.1038/s41467-021-21766-4  

   Panetti, Grace B.; Sergentu, Dumitru-Claudiu; Gau, Michael R.; Carroll, Patrick J.; Autschbach, Jochen; Walsh, Patrick J.; Schelter, Eric J. (December 2021, Nature Communications)

   null (Ed.)

   Abstract The synthesis of bona fide organometallic Ce IV complexes is a formidable challenge given the typically oxidizing properties of the Ce IV cation and reducing tendencies of carbanions. Herein, we report a pair of compounds comprising a Ce IV  − C aryl bond [Li(THF) 4 ][Ce IV (κ 2 - ortho -oxa)(MBP) 2 ] ( 3-THF ) and [Li(DME) 3 ][Ce IV (κ 2 - ortho -oxa)(MBP) 2 ] ( 3-DME ), ortho -oxa = dihydro-dimethyl-2-[4-(trifluoromethyl)phenyl]-oxazolide, MBP 2–  = 2,2′-methylenebis(6- tert -butyl-4-methylphenolate), which exhibit Ce IV  − C aryl bond lengths of 2.571(7) – 2.5806(19) Å and strongly-deshielded, Ce IV  − C ipso 13 C{ 1 H} NMR resonances at 255.6 ppm. Computational analyses reveal the Ce contribution to the Ce IV  − C aryl bond of 3-THF is ~12%, indicating appreciable metal-ligand covalency. Computations also reproduce the characteristic 13 C{ 1 H} resonance, and show a strong influence from spin-orbit coupling (SOC) effects on the chemical shift. The results demonstrate that SOC-driven deshielding is present for Ce IV  − C ipso 13 C{ 1 H} resonances and not just for diamagnetic actinide compounds. 

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2. Facile proton-coupled electron transfer enabled by coordination-induced E–H bond weakening to boron

   https://doi.org/10.1039/D1CC02832D  

   Wong, Anthony; Chakraborty, Arunavo; Bawari, Deependra; Wu, Guang; Dobrovetsky, Roman; Ménard, Gabriel (January 2021, Chemical Communications)

   null (Ed.)

   We report the facile activation of aryl E–H (ArEH; E = N, O, S; Ar = Ph or C 6 F 5 ) or ammonia N–H bonds via coordination-induced bond weakening to a redox-active boron center in the complex, (1 − ). Substantial decreases in E–H bond dissociation free energies (BDFEs) are observed upon substrate coordination, enabling subsequent facile proton-coupled electron transfer (PCET). A drop of >50 kcal mol −1 in H 2 N–H BDFE upon coordination was experimentally determined. 

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3. Large transition state stabilization from a weak hydrogen bond

   https://doi.org/10.1039/D0SC02806A  

   Vik, Erik C.; Li, Ping; Maier, Josef M.; Madukwe, Daniel O.; Rassolov, Vitaly A.; Pellechia, Perry J.; Masson, Eric; Shimizu, Ken D. (July 2020, Chemical Science)

   A series of molecular rotors was designed to study and measure the rate accelerating effects of an intramolecular hydrogen bond. The rotors form a weak neutral O–H⋯OC hydrogen bond in the planar transition state (TS) of the bond rotation process. The rotational barrier of the hydrogen bonding rotors was dramatically lower (9.9 kcal mol −1 ) than control rotors which could not form hydrogen bonds. The magnitude of the stabilization was significantly larger than predicted based on the independently measured strength of a similar O–H⋯OC hydrogen bond (1.5 kcal mol −1 ). The origins of the large transition state stabilization were studied via experimental substituent effect and computational perturbation analyses. Energy decomposition analysis of the hydrogen bonding interaction revealed a significant reduction in the repulsive component of the hydrogen bonding interaction. The rigid framework of the molecular rotors positions and preorganizes the interacting groups in the transition state. This study demonstrates that with proper design a single hydrogen bond can lead to a TS stabilization that is greater than the intrinsic interaction energy, which has applications in catalyst design and in the study of enzyme mechanisms. 

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4. Large transition state stabilization from a weak hydrogen bond

   Vik, E. C.; Li, P.; Maier, J. M.; Madukwe, D.O.; Rassolov, V. A.; Pellechia, P. J.; Masson, E.; Shimizu, K. D. (July 2020, Chemical science)

   A series of molecular rotors was designed to study and measure the rate accelerating effects of an intramolecular hydrogen bond. The rotors form a weak neutral O–H/O]C hydrogen bond in the planar transition state (TS) of the bond rotation process. The rotational barrier of the hydrogen bonding rotors was dramatically lower (9.9 kcal mol
   1) than control rotors which could not form hydrogen bonds. The magnitude of the stabilization was significantly larger than predicted based on the independently measured strength of a similar OH/OC hydrogen bond (1.5 kcal mol-1). The origins of the large transition state stabilization were studied via experimental substituent effect and computational perturbation analyses. Energy decomposition analysis of the hydrogen bonding interaction revealed a significant reduction in the repulsive component of the hydrogen bonding interaction. The rigid framework of the molecular rotors positions and preorganizes the interacting groups in the transition state. This study demonstrates that with proper design a single hydrogen bond can lead to a TS stabilization that is greater than the intrinsic interaction energy, which has applications in catalyst design and in the study of enzyme mechanisms. 

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5. Reaction Mechanism Underlying Pd(II)-Catalyzed Oxidative Coupling of Ethylene and Benzene to Form Styrene: Identification of a Cyclic Mono-Pd ^II Bis-Cu ^II Complex as the Active Catalyst

   https://doi.org/10.1021/acs.organomet.2c00183  

   Musgrave, Charles B.; Bennett, Marc T.; Ellena, Jeffrey F.; Dickie, Diane A.; Gunnoe, T. Brent; Goddard, William A. (July 2022, Organometallics)

   A recent advance in the synthesis of alkenylated arenes was the demonstration that the Pd(OAc)2 catalyst precursor gives >95% selectivity toward styrene from ethylene and benzene under optimized conditions using excess Cu(II) carboxylate as the in situ oxidant [ Organometallics 2019, 38(19), 3532−3541]. To understand the mechanism underlying this catalysis, we applied density functional theory (DFT) calculations in combination with experimental studies. From DFT calculations, we determined the lowest-energy multimetallic Pd and Pd–Cu mixed metal species as possible catalyst precursors. From the various structures, we determined the cyclic heterotrinuclear complex PdCu2(μ-OAc)6 to be the global minimum in Gibbs free energy under conditions of excess Cu(II). For cyclic PdCu2(μ-OAc)6 and the parent [Pd(μ-OAc)2]3, we evaluated the barriers for benzene C–H activation through concerted metalation deprotonation (CMD). The PdCu2(μ-OAc)6 cyclic trimer leads to a CMD barrier of 33.5 kcal/mol, while the [Pd(μ-OAc)2]3 species leads to a larger CMD barrier at >35 kcal/mol. This decrease in the CMD barrier arises from the insertion of Cu(II) into the trimetallic species. Because cyclic PdCu2(μ-OAc)6 is likely the predominant species under experimental conditions (the Cu to Pd ratio is 480:1 at the start of catalysis) with a predicted CMD barrier within the range of the experimentally determined activation barrier, we propose that cyclic PdCu2(μ-OAc)6 is the Pd species responsible for catalysis and report a full reaction mechanism based on DFT calculations. For catalytic conversion of benzene and ethylene to styrene at 120 °C using Pd(OAc)2 as the catalyst precursor and Cu(OPiv)2 (OPiv = pivalate) as the oxidant, an induction period of ∼1 h was observed, followed by catalysis with a turnover frequency of ∼2.3 × 10–3 s–1. In situ1H NMR spectroscopy experiments indicate that during the induction period, Pd(OAc)2 is likely converted to cyclic PdCu2(η2-C2H4)3(μ-OPiv)6, which is consistent with the calculations and consistent with the proposal that the active catalyst is the ethylene-coordinated heterotrinuclear complex cyclic PdCu2(η2-C2H4)3(μ-OPiv)6. 

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