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1. Coordination-Induced Bond Weakening and Electrocatalytic Proton-Coupled Electron Transfer of a Ruthenium Verdazyl Complex

   https://doi.org/10.1021/acs.inorgchem.3c02775  

   Galvin, Conor M. ; Marron, Daniel P. ; Dressel, Julia M. ; Waymouth, Robert M. ( January 2024 , Inorganic Chemistry)

   Coordination of the leucoverdazyl ligand 2,4-diisopropyl-6-(pyridin-2-yl)-1,4-dihydro-1,2,4,5-tetrazin-3(2H)-one VdH to Ru significantly weakens the ligand’s N-H bond. Electrochemical measurements show that the metalated leucoverdazyl Ru(VdH)(acetylacetonate)2 RuVdH has a lower pKa (-5 units), BDFE (-7 kcal/mol), and hydricity (-22 kcal/mol) than the free ligand. DFT calculations suggest that the increased acidity is in part attributable to the stabilization of the conjugate base Vd-. When free, Vd- distorts to avoid an 8πe- antiaromatic state, but it remains planar when bound to Ru. Proton-coupled electron transfer (PCET) behavior is observed for both the free and metalated leucoverdazyls. PCET equilibrium between Vd radical and TEMPOH affords a VdH BDFE that is in good agreement with that obtained from electrochemical methods. RuVd exhibits electrocatalytic PCET donor behavior. Under acidic conditions, it reduces the persistent trityl radical ·CAr3 (Ar = p-tert-butylphenyl) to the corresponding triarylmethane HCAr3 via net 1e-/1H+ transfer from RuVdH. 

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2. Polar molecules catalyze CO insertion into metal-alkyl bonds through the displacement of an agostic C-H bond

   https://doi.org/10.1073/pnas.1816339116  

   Zhou, Tian ; Malakar, Santanu ; Webb, Steven L. ; Krogh-Jespersen, Karsten ; Goldman, Alan S. ( February 2019 , Proceedings of the National Academy of Sciences)

   The insertion of CO into metal-alkyl bonds is the key C-C bond-forming step in many of the most important organic reactions catalyzed by transition metal complexes. Polar organic molecules (e.g., tetrahydrofuran) have long been known to promote CO insertion reactions, but the mechanism of their action has been the subject of unresolved speculation for over five decades. Comprehensive computational studies [density functional theory (DFT)] on the prototypical system Mn(CO)₅(arylmethyl) reveal that the polar molecules do not promote the actual alkyl migration step. Instead, CO insertion (i.e. alkyl migration) occurs rapidly and reversibly to give an acyl complex with a sigma-bound (agostic) C-H bond that is not easily displaced by typical ligands (e.g. phosphines or CO). The agostic C-H bond is displaced much more readily, however, by the polar promoter molecules, even though such species bind only weakly to the metal center and are themselves then easily displaced; the facile kinetics of this process are attributable to a hydrogen bonding-like interaction between the agostic C-H bond and the polar promoter. The role of the promoter is to thereby catalyze isomerization of the agostic product of CO insertion to give an${\eta }^2$-C,O-bound acyl product that is more easily trapped than the agostic species. This ability of such promoters to displace a strongly sigma-bound C-H bond and to subsequently undergo facile displacement themselves is without reported precedent, and could have implications for catalytic reactions beyond carbonylation.

    

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3. C–H bond activation via concerted metalation–deprotonation at a palladium( iii ) center

   https://doi.org/10.1039/D3SC00034F  

   Bouley, Bailey S. ; Tang, Fengzhi ; Bae, Dae Young ; Mirica, Liviu M. ( April 2023 , Chemical Science)

   Herein we report the direct observation of C–H bond activation at an isolated mononuclear Pd( iii ) center. The oxidation of the Pd( ii ) complex ( Me N4)Pd II (neophyl)Cl (neophyl = –CH 2 C(CH 3 ) 2 Ph; Me N4 = N , N ′-dimethyl-2,11-diaza[3.3](2,6)pyridinophane) using the mild oxidant ferrocenium hexafluorophosphate (FcPF 6 ) yields the stable Pd( iii ) complex [( Me N4)Pd III (neophyl)Cl]PF 6 . Upon the addition of an acetate source, [( Me N4)Pd III (neophyl)Cl]PF 6 undergoes Csp 2 –H bond activation to yield the cyclometalated product [( Me N4)Pd III (cycloneophyl)]PF 6 . This metalacycle can be independently prepared, allowing for a complete characterization of both the starting and final Pd( iii ) complexes. The C–H activation step can be monitored directly by EPR and UV-Vis spectroscopies, and kinetic isotope effect (KIE) studies suggest that either a pre-association step such as an agostic interaction may be rate limiting, or that the C–H activation is partially rate-limiting in conjunction with ligand rearrangement. Density functional theory calculations support that the reaction proceeds through a κ 3 ligand coordination and that the flexible ligand structure is important for this transformation. Overall, this study represents the first example of discrete C–H bond activation occurring at a Pd( iii ) center through a concerted metalation–deprotonation mechanism, akin to that observed for Pd( ii ) and Pd( iv ) centers. 

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4. 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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5. 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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