No abstract

The electrooxidation of ethanol to acetate is achieved with Rh(I) diolefin amine complexes of the general formula [Rh(Y)(trop 2 NH)(L)] (L ¼ PPh 3 , P(4-n-BuPh) 3 ; Y ¼ triflate, acetate; Bu ¼ butyl) in direct alcohol fuel cells that have the peculiarity of containing a molecular anode electrocatalyst and, hence, are denoted as OrganoMetallic Fuel Cells (OMFCs). Changing the carbon black support from Vulcan XC-72 (Cv) to Ketjenblack EC 600JD (Ck) and/or the axial phosphane to produce non crystalline complexes has been found to remarkably change the electrochemical properties of the organorhodium catalysts, especially in terms of specific activity and durability. An in-depth study has shown that either Ck or P(4-n-butylPh) 3 favour the formation of an amorphous Rh-acetato phase on the electrode, leading to a much more efficient and recyclable catalyst as compared to a crystalline Rhacetate complex which is formed on Cv with PPh 3 as the ligand. The ameliorating effect of the amorphous phase has been ascribed to its higher number of surface complex molecules as compared to the crystalline phase. A specific activity as high as 10 000 A g Rh À1 has been found in a half cell, which is the highest value ever reported for ethanol electrooxidation.

The oxidation of alcohols with N2O as the hydrogen acceptor was achieved with low catalyst loadings of a rhodium complex that features a cooperative bis(olefin)amido ligand under mild conditions. Two different methods enable the formation of either the corresponding carboxylic acid or the ester. N2 and water are the only by-products. Mechanistic studies supported by DFT calculations suggest that the oxygen atom of N2O is transferred to the metal center by insertion into the Rh-H bond of a rhodium amino hydride species, generating a rhodium hydroxy complex as a key intermediate.

The amino diolefin complex [Rh(trop2NH)(TMIY)]+(OTf)− (OTf−=CF3SO3−; trop=5‐H‐dibenzo[a,d]cyclohepten‐5yl), incorporating N‐heterocyclic carbene ligand 1,3,4,5‐tetramethylimidazole‐2‐ylidene (TMIY), has been prepared. The structure is determined by single‐crystal X‐ray diffraction, which shows a butterfly structure with the NH and TMIY groups in the apical and the two olefinic binding sites in the equatorial position of a trigonal bipyramid with one vacant site in the equatorial plane. This complex can be deprotonated to give the amide [Rh(trop2N)(TMIY)], which cleaves H2 heterolytically across the RhN bond to give the amino hydride [RhH(trop2NH)(TMIY)]. In dimethylsulfoxide (DMSO), both the amide and amino hydride complexes are active as catalysts in the dehydrogenative coupling (DHC) of various primary alcohols with water to give the corresponding acids under aerobic conditions. O2 serves as hydrogen acceptor. One O atom is converted into water while the other is transferred to DMSO as oxygen acceptor, to yield Me2SO2. Hence, the net reaction RCH2OH+2 O2+2 DMSO→RCOOH+2 Me2SO2+H2O is catalyzed by the butterfly‐shaped rhodium amide complexes.

Organometallic fuel cells catalyze the selective electrooxidation of renewable diols, simultaneously providing high power densities and chemicals of industrial importance. It is shown that the unique organometallic complex [Rh(OTf)(trop2NH)(PPh3)] employed as molecular active site in an anode of an OMFC selectively oxidizes a number of renewable diols, such as ethylene glycol , 1,2-propanediol (1,2-P), 1,3-propanediol (1,3-P), and 1,4-butanediol (1,4-B) to their corresponding mono-carboxylates. The electrochemical performance of this molecular catalyst is discussed, with the aim to achieve cogeneration of electricity and valuable chemicals in a highly selective electrooxidation from diol precursors.