Catalysis research of relevance to carbon management: progress, challenges, and opportunities

Approaches to alkane functionalization with Tp′Pt and (nacnac)Pt reagents

In this perspective article we give an overview of our studies of C–H activation with platinum over the past decade. The scorpionate ligand Tp′ (Tp′ = hydrotris(3,5-dimethylpyrazolyl)borate) has exhibited the ability to stabilize both Pt(IV) and Pt(II) complexes due to flexibility between κ3 and κ2 binding modes. Control of the Tp′ coordination mode and Pt oxidation state is tightly coupled with acid/base chemistry. This ability allowed us to model a spectrum of intermediates in C–H activation reactions, including Pt(IV) alkyl hydrides, five-coordinate Pt(IV) silyl hydrides, and Pt(II) benzene complexes. We have been able to observe sturdy Pt(II) benzene complexes and probe their equilibria with the respective C–H activated Pt(IV) species in the presence of an added ligand. We have also taken steps toward functionalizing hydrocarbons with Tp′Pt by dehydrogenation of alkanes and by arylation of olefins. We have recently turned to bidentate nacnac (nacnac = N-phenyl-β-enamineimine) ligands in the hopes of catalytically functionalizing hydrocarbons.Key words: C–H activation, reductive elimination, Tp′Pt, five-coordinate, β-diiminate.

Sustainability in catalytic oxidation: an alternative approach or a structural evolution?

This Review documents some examples of recent innovations in the field of catalytic selective oxidation from an industrial point of view. The use of alkanes as building blocks for the synthesis of bulk chemicals and intermediates is discussed, along with the main properties that catalysts should possess in order to efficiently catalyse the selective oxidation of these hydrocarbons. The currently developed processes for propene oxide and new processes under investigation for the synthesis of adipic acid are also described, highlighting innovative aspects for a better sustainability of the chemical industry.

Roaming is the dominant mechanism for molecular products in acetaldehyde photodissociation

Reaction pathways that bypass the conventional saddle-point transition state (TS) are of considerable interest and importance. An example of such a pathway, termed "roaming," has been described in the photodissociation of H(2)CO. In a combined experimental and theoretical study, we show that roaming pathways are important in the 308-nm photodissociation of CH(3)CHO to CH(4) + CO. The CH(4) product is found to have extreme vibrational excitation, with the vibrational distribution peaked at approximately 95% of the total available energy. Quasiclassical trajectory calculations on full-dimensional potential energy surfaces reproduce these results and are used to infer that the major route to CH(4) + CO products is via a roaming pathway where a CH(3) fragment abstracts an H from HCO. The conventional saddle-point TS pathway to CH(4) + CO formation plays only a minor role. H-atom roaming is also observed, but this is also a minor pathway. The dominance of the CH(3) roaming mechanism is attributed to the fact that the CH(3) + HCO radical asymptote and the TS saddle-point barrier to CH(4) + CO are nearly isoenergetic. Roaming dynamics are therefore not restricted to small molecules such as H(2)CO, nor are they limited to H atoms being the roaming fragment. The observed dominance of the roaming mechanism over the conventional TS mechanism presents a significant challenge to current reaction rate theory.

Reduction of CO2 to CO using low-coordinate iron: formation of a four-coordinate iron dicarbonyl complex and a bridging carbonate complex

A reduced diiron(I) complex reacts with CO(2) to give two iron-containing products. One product has a carbonate bridge, which isomerizes rapidly at -70 degrees C and may be derived from an oxodiiron intermediate. The formation of this product releases free CO, which leads to a four-coordinate iron dicarbonyl complex. This product is the first crystallographically characterized example of a four-coordinate iron dicarbonyl species, a moiety that may be present in the active site of Hmd ("iron-sulfur cluster free") hydrogenase.