The hangman motif provides mechanistic insights into the role of pendant proton relays in governing proton-coupled electron transfer (PCET) involved in the hydrogen evolution reaction (HER). We now show improved HER activity of Ni compared with Co hangman porphyrins. Cyclic voltammogram data and simulations, together with computational studies using density functional theory, implicate a shift in electrokinetic zone between Co and Ni hangman porphyrins due to a change in the PCET mechanism. Unlike the Co hangman porphyrin, the Ni hangman porphyrin does not require reduction to the formally metal(0) species before protonation by weak acids in acetonitrile. We conclude that protonation likely occurs at the Ni(I) state followed by reduction, in a stepwise proton transfer-electron transfer pathway. Spectroelectrochemical and computational studies reveal that upon reduction of the Ni(II) compound, the first electron is transferred to a metal-based orbital, whereas the second electron is transferred to a molecular orbital on the porphyrin ring.renewable | solar fuels | electrocatalysis S olar-to-fuels conversions provide a path to harnessing the ubiquitous albeit intermittent renewable energy resource offered by the sun (1-6). Efficient catalysis of transformations of energy consequence (7-13) mandates the coupling of electron transfer (ET) to proton transfer (PT) in proton-coupled electron transfer (PCET) reactions (14)(15)(16)(17)(18)(19)(20). In the absence of PCET, intermediates possessing equilibrium potentials that are prohibitively large depreciate the storage capacity offered by the solarto-fuels conversion process. The coupling of protons to changes in electron equivalency offers the possibility of restricting the equilibrium potentials of the redox steps to a more narrow potential range, thereby minimizing the overpotential required to sustain catalysis at a desired turnover rate. Thus, the exploitation of PCET pathways to permit potential-leveling effects is a crucial prerequisite for the efficient catalytic conversion reactions of energy relevant molecules.PCET reactions may be classified into stepwise and concerted pathways (14,16,20,21). Stepwise PCET may involve ET first followed by PT (ETPT), or PT followed by ET (PTET). In concerted proton-electron transfers (CPET), the proton and electron traverse a common transition state. Whereas concerted pathways avoid the formation of thermodynamically costly intermediates, CPET reactions may incur kinetic penalties associated with the requirements for proton tunneling (19,20,22). The competition between these dynamics during catalysis determines the most efficient route of reaction. Studies that explore the interplay between these factors are crucial to designing catalytic reactions of high efficiency. Along these lines, the incorporation of proton relays in the second coordination sphere of molecular catalysts has emerged as a useful tool in optimizing PCET transformations (23-29). We have focused on the synthesis and mechanistic investigation of a class of me...

Cobalt hangman porphyrins catalyze the hydrogen evolution reaction (HER). The hangman group is observed to facilitate HER by mediating a proton-coupled electron transfer (PCET) reaction. The details of the PCET pathway have been determined by comparing rate constants associated with the ET and PT processes of the hangman system to those of the corresponding values measured for porphyrins that lack an internal proton relay. A rapid intramolecular proton transfer from the carboxylic acid hanging group to the reduced cobalt centre of 8.5 Â 10 6 s À1 provides a facile pathway for the formation of Co(II)H, which leads directly to H 2 generation.

Open-cage fullerene derivatives have excited organic chemists' creativity over the past decade. These adducts, generated via consecutive cleavage of sigma- and pi-carbon-carbon bonds on the fullerene cage, allow small atoms or molecules to pass through their opening and be placed inside the cavity. Restoration of the ruptured fullerene back to the pristine fullerene cage affords the corresponding endohedral complexes. This "molecular surgery" approach has been proposed as an alternative to the synthesis of endohedral fullerenes via the conventional physical methods of production, which restrict the availability of endohedral fullerenes to milligram quantities after laborious isolation procedures. In this critical review, we survey all published techniques for the creation of an orifice, as well as for the expansion of an existing one, on the fullerene framework. Successful encapsulation experiments employing cage-opened fullerene derivatives are also comprehensively discussed (160 references).

Six aza[60]fullerene monoadducts were synthesized by the thermal reaction between the azafullerene radical C(59)N* and 9-alkyl-substituted fluorenes, 9,10-dihydroanthracene, or xanthene. Unlike fluorenes, dihydroanthracene, and xanthene, the structurally related substituted diphenylmethanes, ethylbenzene, cumene, 1,2-diphenylethane, 5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene, 10,11-dihydro-5H-dibenzo[a,d]cycloheptene, 9-methylanthracene, and 9-benzylanthracene do not lead to the isolation of azafullerene monoadducts. Moreover, 1,2-dichlorobenzene, the most commonly utilized solvent for azafullerene reactions, reacts slowly with the azafullerenyl radical C(59)N* affording the corresponding aza[60]fullerene monoadduct.

[reaction: see text] The vinylcyclopropyl moiety was used as an efficient probe to test mechanistic possibilities of the triazolinedione-alkene ene reaction. In non-hydroxylic solvents, this reaction afforded only the ene adducts via a closed three-membered aziridinium imide (AI) intermediate, whereas in hydroxylic solvents a dipolar intermediate is favored and trapped by the cyclopropyl moiety to form the corresponding cyclopropyl-rearranged solvent-trapped adducts.

The photocycloaddition of vinylcyclopropanes to C60 yields stereospecifically a five-membered [60]fullerene adduct. These results suggest a biradical intermediate of the [2 + 2] photocycloaddition between dienes or arylalkenes and C60. An electron transfer between the triplet excited state of C60 and the unsaturated substrates precedes the formation of the intermediate.

The influence of the solvent on the triazolinedione-alkene ene reaction mechanism has been investigated. Both inter- and intramolecular kinetic isotope effects with tetramethylethylenes and 2,2,2-(trideuterio)methyl-7-methyl-2,6-octadiene-[D3]-1,1,1 provide, for the first time, strong evidence for changes in the mechanism of the reaction on going from non-protic to polar protic solvents. In non-protic polar or apolar solvents, an aziridinium imide that equilibrates to an insignificant extent with an open intermediate (a dipolar or a polarized biradical) is formed irreversibly in the first, rate-determining step of the reaction, which is followed by fast hydrogen abstraction. On the contrary, in polar protic solvents, hydrogen abstraction is rate limiting, allowing the main dipolar intermediate to equilibrate with its open intermediate(s) as well as with the starting reagents.

The synthesis of 10 new open-cage fullerene derivatives with a 16-membered-ring orifice is being reported. These compounds, derived from the regioselective addition reaction of an aromatic hydrazine or hydrazone to isomeric diketone derivatives of C(60), were isolated in moderate to excellent yields.