An important first step in making organic reactions more environmentally benign by design requires processes that are, to a first approximation, simple additions with anything else needed only catalytically. Since so few of the existing reactions are additions, synthesis of complex molecules requires the development of new atom-economic methodology. The prospect for such developments is probed in the context of ruthenium-catalyzed reactions. Using mechanistic reasoning, over 20 new processes of varying complexity have been designed and implemented. While some involved oxidation−reduction processes, most involved C−C bond-forming reactions.

On Inventing Reactions for Atom Economy

Continuous-flow photochemistry is used increasingly by researchers in academia and industry to facilitate photochemical processes and their subsequent scale-up. However, without detailed knowledge concerning the engineering aspects of photochemistry, it can be quite challenging to develop a suitable photochemical microreactor for a given reaction. In this review, we provide an up-to-date overview of both technological and chemical aspects associated with photochemical processes in microreactors. Important design considerations, such as light sources, material selection, and solvent constraints are discussed. In addition, a detailed description of photon and mass-transfer phenomena in microreactors is made and fundamental principles are deduced for making a judicious choice for a suitable photomicroreactor. The advantages of microreactor technology for photochemistry are described for UV and visible-light driven photochemical processes and are compared with their batch counterparts. In addition, different scale-up strategies and limitations of continuous-flow microreactors are discussed.

Photochemical transformations accelerated in continuous-flow reactors : basic concepts and applications

The appearance of ferrocene in the middle of the 20th century has revolutionized organometallic chemistry and is now providing applications in areas as varied and sometimes initially unexpected as optical and redox devices, battery and other materials, sensing, catalysis, including asymmetric and enantioselective catalysis, and medicine. The author presents here a general, although personal, view of ferrocene's chemistry, properties, functions, and applications through a literature survey involving both historical and up-to-date trends and including examples of his group's research in a number of these areas. The review gathers together general features of ferrocene chemistry and representative examples of the salient aspects. Its focus is on ferrocene's basic properties, ferrocene-containing ligands, the ferrocene/ferricinium redox couple, ferrocene mixed-valence and average-valence systems, the ferricinium/ferrocene redox shuttle in catalysis, ligand-exchange reactions, ferrocene-containing polymers, ferrocene-containing structures for cathodic battery and other materials, ferrocenes in supramolecular ensembles, liquid crystals, and nonlinear optical materials, ferrocene-containing stars and their electrostatic effects, ferrocene-containing dendrons, dendrimers, and nanoparticles (NPs) and their application in redox sensing and catalysis, and ferrocenes in nanomedicine.

Why is Ferrocene so Exceptional

Energy conversion cycles are aimed at driving unfavorable, small-molecule activation reactions with a photon harnessed by a transition metal complex. A challenge that has occupied researchers for several decades is to create molecular photocatalysts to promote the production of hydrogen from homogeneous solution. We now report the use of a two-electron mixed-valence dirhodium compound to photocatalyze the reduction of hydrohalic acid to hydrogen. In this cycle, photons break two RhII–X bonds of a LRh0–RhIIX2 core in the presence of a halogen trap to regenerate the active LRh0–Rh0 catalyst, which reacts with hydrohalic acid to produce hydrogen.

https://science.sciencemag.org/content/sci/293/5535/1639.full.pdf

Hydrogen produced from hydrohalic acid solutions by a two-electron mixed-valence photocatalyst.

Publisher Summary This chapter discusses metal–isocyanide chemistry. In the interim, reviews have appeared on specific aspects of isocyanide chemistry. The generally accepted valence bond and molecular orbital (MO) approach to the bonding of metal isocyanides has been well described in Treichel's review, and has been used to rationalize variations in infrared (IR) stretching frequencies between bonded and nonbonded isocyanides and the better π-acceptor qualities of aryl versus alkyl isocyanide groups. A number of new synthetic routes to isocyanide complexes of chromium, molybdenum, and tungsten have been investigated. A number of attempts have been made over the years to develop reproducible synthetic routes to six- and seven-coordinate isocyanide complexes of molybdenum and tungsten. A number of routes have been employed for the synthesis of metal–isocyanide complexes by generating the isocyanide ligand on the metal atom. A number of monomeric complexes have been prepared, which may be considered as substitution products of the [Co(CNR) 5 ] + and [M(CNR) 4 ] + cations. Electrochemical investigations have been reported on a range of homoleptic and mixed carbonyl–isocyanide complexes, in attempts to rationalize substituent effects on the isocyanide with the electronic structure of the metal. Insertion reactions of isonitriles into metal–alkyl or metal–aryl bonds are now well established, occurring with metal–alkyl or –aryl groups from group IVA to IB and, recently, with uranium and thorium carbon bonds.

Metal Isocyanide Complexes

Photochemical properties of the cyclopentadienyl(.eta.6-benzene)ruthenium(II) cation. The synthesis and reactions of a synthetically useful intermediate: the cyclopentadienyltris(acetonitrile)ruthenium(II) cation

Photochemistry of [(.eta.-C5H5)Fe(.eta.-p-xyl)]PF6 in acetonitrile solution. Characterization and reactivity of [(.eta.-C5H5)Fe(MeCN)3]+

Photochemical generation of a reactive transition-metal fragment. Photochemically induced arene replacement reactions of the cyclopentadienyl(p-xylene)iron(II) ion

Synthetic applications of the photolysis of the cyclopentadienyliron-(ρ-xylene) cation

The photoactive species involved in the photochemical hydrogen production in the Rh/sub 2/(bridge)/sub 4//sup 2 +/ (where (bridge) = 1,3-diisocyanopropane) system has not been previously characterized. Now, however, x-ray crystallographic data for the salt obtained on addition of CoCl/sub 2/.6H/sub 2/O to the photoactive solution of Rh/sub 2/(bridge)/sub 4/(BF/sub 4/)/sub 2/ in 12M HC1 may have elucidated the species form. It is believed that the photoactive species differs from the cation ((RH/sub 4/(bridge)/sub 8/Cl)/sup 5 +/) only by the binding of an additional Cl/sup -/ion. The crystal structure and bond distances are discussed in some detail, and it is concluded that the formula for the crystalline salt must be H/sub 3/(Rh/sub 4/(bridge)/sub 8/Cl)(CoCl/sub 4/)/sub 4/.nH/sub 2/O where n is at least 3 and more likely 6. (BLM)

Thomas P. Gill

Papers

Photochemical properties of the cyclopentadienyl(.eta.6-benzene)ruthenium(II) cation. The synthesis and reactions of a synthetically useful intermediate: the cyclopentadienyltris(acetonitrile)ruthenium(II) cation

Photochemistry of [(.eta.-C5H5)Fe(.eta.-p-xyl)]PF6 in acetonitrile solution. Characterization and reactivity of [(.eta.-C5H5)Fe(MeCN)3]+

Photochemical generation of a reactive transition-metal fragment. Photochemically induced arene replacement reactions of the cyclopentadienyl(p-xylene)iron(II) ion

Synthetic applications of the photolysis of the cyclopentadienyliron-(ρ-xylene) cation

X-ray Structural Analysis of H3[Rh4(bridge)8Cl][CoCl4]4•nH2O. The Photoactive Species in the Production of Hydrogen from Aqueous Solutions