Facile and selective 4e/4H electrochemical reduction of O to HO in aqueous medium has been a sought-after goal for several decades. Elegant but synthetically demanding cytochrome c oxidase mimics have demonstrated selective 4e/4H electrochemical O reduction to HO is possible with rate constants as fast as 10 M s under heterogeneous conditions in aqueous media. Over the past few years, in situ mechanistic investigations on iron porphyrin complexes adsorbed on electrodes have revealed that the rate and selectivity of this multielectron and multiproton process is governed by the reactivity of a ferric hydroperoxide intermediate. The barrier of O-O bond cleavage determines the overall rate of O reduction and the site of protonation determines the selectivity. In this report, a series of mononuclear iron porphyrin complexes are rationally designed to achieve efficient O-O bond activation and site-selective proton transfer to effect facile and selective electrochemical reduction of O to water. Indeed, these crystallographically characterized complexes accomplish facile and selective reduction of O with rate constants >10 M s while retaining >95% selectivity when adsorbed on electrode surfaces (EPG) in water. These oxygen reduction reaction rate constants are 2 orders of magnitude faster than all known heme/Cu complexes and these complexes retain >90% selectivity even under rate determining electron transfer conditions that generally can only be achieved by installing additional redox active groups in the catalyst.

This Article describes the synthesis and characterization of cyclometalated aminoquinoline NiII σ-aryl and σ-alkyl complexes that have been proposed as key intermediates in Ni-catalyzed C–H functionalization reactions. These NiII complexes serve as competent catalysts for the C–H functionalization of aminoquinoline derivatives with I2. They also react stoichiometrically with I2 to form either aryl iodides or β-lactams within minutes at room temperature. Furthermore, they react with AgI salts at −30 °C to afford isolable five-coordinate NiIII species. The NiIII σ-aryl complexes proved inert toward C­(sp2)–I bond-forming reductive elimination under all conditions examined (up to 140 °C in DMF). In contrast, a NiIII σ-alkyl analogue underwent C­(sp3)–N bond-forming reductive elimination at 140 °C in DMF to afford a β-lactam product. However, despite the ability of this latter NiIII species to participate in stoichiometric product formation, the complex was not a competent catalyst for β-lactam formation. Overall, these results suggest against the intermediacy of NiIII species in these C–H functionalization reactions.

This communication describes the synthesis of a series of diphosphine Ni II (Ph)(CF 3 ) complexes and studies of their reactivity toward oxidatively induced Ph−CF 3 bond-forming reductive elimination. Treatment of these complexes with the one-electron outer-sphere oxidant ferrocenium hexafluorophosphate (FcPF 6 ) affords benzotrifluoride, but the yield varies dramatically as a function of diphosphine ligand. Diphosphines with bite angles of less than 92°afforded <10% yield of PhCF 3 . In contrast, those with bite angles between 95 and 102°formed PhCF 3 in yields ranging from 62 to 77%.

No abstract

Metrics & MoreArticle Recommendations * sı Supporting Information F urther analysis of our data revealed that the thermolysis of Ni(III) complex 4b does not lead to C(sp 3 )−N bondforming reductive elimination to form β-lactam 3b-H. As such, our claim that this is "the first directly observable example of C(sp 3 )−N coupling from an isolated Ni III center" is incorrect. Equation 6 should be modified as follows: