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1. 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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2. Computational exploration of the copper(I)‐catalyzed conversion of hydrazones to dihalogenated vinyldiazene derivatives

   https://doi.org/10.1002/jcc.27433  

   Askerova, Ulviyya ; Abdullayev, Yusif ; Shikhaliyev, Namiq ; Maharramov, Abel ; Nenajdenko, Valentine G ; Autschbach, Jochen ( May 2024 , Journal of Computational Chemistry)

   This computational study explores the copper (I) chloride catalyzed synthesis of (E)‐1‐(2,2‐dichloro‐1‐phenylvinyl)‐2‐phenyldiazene (2Cl‐VD) from readily available hydrazone derivative and carbon tetrachloride (CCl₄).2Cl‐VDhas been extensively utilized to synthesize variety of heterocyclic organic compounds in mild conditions. The present computational investigations primarily focus on understanding the role of copper (I) andN¹,N¹,N²,N²‐tetramethylethane‐1,2‐diamine (TMEDA) in this reaction, TMEDA often being considered a proton scavenger by experimentalists. Considering TMEDA as a ligand significantly alters the energy barrier. In fact, it is only 8.3 kcal/mol higher compared to the ligand‐free (LF) route for the removal of a chlorine atom to form the radical·CCl₃but the following steps are almost barrierless. This intermediate then participates in attacking the electrophilic carbon in the hydrazone. Crucially, the study reveals that the overall potential energy surface is thermodynamically favorable, and the theoretical turnover frequency (TOF) value is higher in the case of Cu(I)‐TMEDA complex catalyzed pathway.

    

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3. Rapid reversible borane to boryl hydride exchange by metal shuttling on the carborane cluster surface

   https://doi.org/10.1039/C7SC01846K  

   Eleazer, Bennett J. ; Smith, Mark D. ; Popov, Alexey A. ; Peryshkov, Dmitry V. ( January 2017 , Chemical Science)

   In this work, we introduce a novel concept of a borane group vicinal to a metal boryl bond acting as a supporting hemilabile ligand in exohedrally metalated three-dimensional carborane clusters. The (POBOP)Ru(Cl)(PPh 3 ) pincer complex (POBOP = 1,7-OP( i -Pr) 2 - m -2-carboranyl) features extreme distortion of the two-center-two-electron Ru–B bond due to the presence of a strong three-center-two-electron B–H⋯Ru vicinal interaction. Replacement of the chloride ligand with a hydride afforded the (POBOP)Ru(H)(PPh 3 ) pincer complex, which possesses B–Ru, B–H⋯Ru, and Ru–H bonds. This complex was found to exhibit a rapid exchange between hydrogen atoms of the borane and the terminal hydride through metal center shuttling between two boron atoms of the carborane cage. This exchange process, which involves sequential cleavage and formation of strong covalent metal–boron and metal–hydrogen bonds, is unexpectedly facile at temperatures above −50 °C corresponding to an activation barrier of 12.2 kcal mol −1 . Theoretical calculations suggested two equally probable pathways for the exchange process through formally Ru(0) or Ru( iv ) intermediates, respectively. The presence of this hemilabile vicinal B–H⋯Ru interaction in (POBOP)Ru(H)(PPh 3 ) was found to stabilize a latent coordination site at the metal center promoting efficient catalytic transfer dehydrogenation of cyclooctane under nitrogen and air at 170 °C. 

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4. A hemilabile manganese( i )–phenol complex and its coordination induced O–H bond weakening

   https://doi.org/10.1039/d0dt00973c  

   Kadassery, Karthika J. ; Crawley, Matthew R. ; MacMillan, Samantha N. ; Lacy, David C. ( January 2020 , Dalton Transactions)

   The known compound K[( PO ) 2 Mn(CO) 2 ] ( PO = 2-((diphenylphosphino)methyl)-4,6-dimethylphenolate) (K[ 1 ]) was protonated to form the new Mn( i ) complex ( HPO )( PO )Mn(CO) 2 ( H 1 ) and was determined to have a p K a approximately equal to tetramethylguanidine (TMG). The reduction potential of K[ 1 ] was determined to be −0.58 V vs. Fc/Fc + in MeCN and allowed for an estimation of an experimental O–H bond dissociation free energy (BDFE O–H ) of 73 kcal mol −1 according to the Bordwell equation. This value is in good agreement with a corrected DFT computed BDFE O–H of 68.0 kcal mol −1 (70.3 kcal mol −1 for intramolecular H-bonded isomer). The coordination of the protonated O-atom in the solid-state H 1 was confirmed using FTIR spectroscopy and X-ray crystallography. The phenol moiety is hemilabile as evident from computation and experimental results. For instance, dissociation of the protonated O-atom in H 1 is endergonic by only a few kcal mol −1 (DFT). Furthermore, [ 1 ] − and other Mn( i ) compounds coordinated to PO and/or HPO do not react with MeCN, but H 1 reacts with MeCN to form H 1 + MeCN . Experimental evidence for the solution-bound O-atoms of H 1 was obtained from 1 H NMR and UV-vis spectroscopy and by comparing the electronic spectra of bona fide 16-e − Mn( i ) complexes such as [{ PNP }Mn(CO) 2 ] ( PNP = − N{CH 2 CH 2 (P i Pr 2 )} 2 ) and [( Me3SiOP )( PO )Mn(CO) 2 ] ( Me3Si 1 ). Compound H 1 is only meta-stable ( t 1/2 0.5–1 day) and decomposes into products consistent with homolytic O–H bond cleavage. For instance, treatment of H 1 with TEMPO resulted in formation of TEMPOH, free ligand, and [Mn II {( PO ) 2 Mn(CO) 2 } 2 ]. Together with the experimental and calculated weakened BDFE O–H , these data provide strong evidence for the coordination and hemilability of the protonated O-atom in H 1 and represents the first example of the phenolic Mn( i )–O linkage and a rare example of a “soft-homolysis” intermediate in the bond-weakening catalysis paradigm. 

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5. Anomalous chemically induced electron spin polarization in proton-coupled electron transfer reactions: insight into radical pair dynamics

   https://doi.org/10.1039/d0sc02691c  

   Brugh, Alexander M. ; Forbes, Malcolm D. ( June 2020 , Chemical Science)

   Time-resolved electron paramagnetic resonance (TREPR) spectroscopy has been used to study the proton coupled electron transfer (PCET) reaction between a ruthenium complex (Ru(bpz)(bpy) 2 ) and several substituted hydroquinones (HQ). After excitation at 355 nm, the HQ moiety forms a strong hydrogen bond to the exposed N atoms in the bpz heterocycle. At some point afterwards, a PCET reaction takes place in which an electron from the O atom of the hydrogen bond transfers to the metal center, and the proton forming the hydrogen bond remains on the bpz ligand N atom. The result is a semiquinone radical (HQ˙), whose TREPR spectrum is strongly polarized by the triplet mechanism (TM) of chemically induced dynamic electron spin polarization (CIDEP). Closer examination of the CIDEP pattern reveals, in some cases, a small amount of radical pair mechanism (RPM) polarization. We hypothesize that when the HQ moiety has electron donating groups (EDGs) substituted on the ring, S–T − RPM polarization is observed in HQ˙. These anomalous intensities are accounted for by spectral simulation using polarization from S–T − mixing. The generation of S–T − RPM is attributed to slow radical separation after PCET due to stabilization of the positive charge on the ring by EDGs. Results from a temperature dependence support the hypothesis. 

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