Thermodynamic acidity is one of the most widely used quantities for characterizing proton transfer reactions. Measurement of these values for catalytically relevant species can be challenging, often requiring direct observation of equilibria. The C–H bonds of aromatic substrates are proposed to become substantially polarized during electrophilic activation, but quantifying the absolute acidity of the intermediate M­(η2-arene) complexes is highly challenging. Using a system that intercepts nascent protons at electrophilic PdII arene complexes, a combined experimental and computational study has demonstrated these C–H bonds to be far more acidic (pK a CH3CN = 3–6) than many “nonbasic” substrates and additives that are present in electrophilic C–H activation catalysis, and the catalytic roles of these species may need to be reassessed.

Oxidative addition of carbon−halogen bonds at transition metals typically follows either a two-electron pathway (concerted M−R/M−X formation) or a radical chain pathway (stepwise M−R/M−X formation). When the reactive metal species is generated slowly, however, both mechanisms can compete to yield unexpected reactivity paths. The present report highlights the synthesis of rhodium methylidenes from chloroalkanes (e.g., CH 2 Cl 2 and CHCl 3 ) at POP-pincer frameworks (e.g., POP = 4,6-bis(ditert-butylphosphino)dibenzo[b,d]furan) via a cascade of halide abstraction and electron transfer steps. Experimental and computational studies are reported that support the proposed mechanism, including characterization of important reaction intermediates. The overall transformation represents a route toward reactive metal alkylidenes using milder and less-reactive carbenoid precursors than what is presently used.

Pendent nucleophiles are essential partners in the cleavage and formation of bonds with hydrogen (e.g. protonation/deprotonation), but binding of the pendent group to the metal and the potential trapping of complexes in inactive states are a significant problem. The dipyridylmethane-based ligand framework bis(2-pyridyl)-N-pyrrolidinomethane (CPy), bearing a hemilabile pyrrolidine moiety, has been synthesized and complexes of the type [(CPy)M(COD)]X (COD = 1,5-cyclooctadiene) were prepared. The solution-phase ligand dynamics and relative protonation preferences were investigated viaH NMR spectroscopy; although favorable, pendent amine binding does not kinetically inhibit pendent base protonation. Protonation at the metal (with concomitant pyrrolidine binding) has been found to be favorable for Ir, whereas N-protonation is favorable for Rh. DFT calculations predict that the Rh hydrides have much higher relative acidities than their Ir congeners (ΔpK ≃ 7-8 in CHCl), and are also more acidic than the strong acid [H(OEt)][B(CF)].

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<div><div><div><p>Oxidative addition of carbon-halogen bonds at transition metals typically follow either a two-electron pathway (concerted M-R/M-X formation) or a radical chain pathway (stepwise M-R/M-X formation). When the reactive metal species is generated slowly, however, both mechanisms can compete to yield unexpected reactivity paths. The present report highlights the synthesis of rhodium methylidenes from chloroalkanes (e.g. CH2Cl2 and CHCl3) at POP-pincer frameworks (e.g. POP = 4,6-bis(di-tert- butylphosphino)dibenzo[b,d]furan) via a cascade of halide abstraction and electron transfer steps. Experimental and computational studies are reported that support the proposed mechanism, including characterization of important reaction intermediates. The overall transformation represents a route towards reactive metal alkylidenes using milder and less-reactive carbenoid precursors than what is presently used.</p></div></div></div>