No abstract

We describe an efficient homogeneous ruthenium catalyst for the dehydrogenation of ammonia borane (AB). This catalyst liberates greater than 2 equivalents of H2 and up to 4.6 system wt% H2 from concentrated AB suspensions under air. Importantly, this catalyst is robust, delivering several cycles of dehydrogenation at high [AB] without loss of catalytic activity, even with exposure to air and water.

A series of three phosphorescent mononuclear (NHC)-Cu(I) complexes were prepared and characterized. Photophysical properties were found to be largely controlled by the NHC ligand chromophore. Variation of the NHC ligand leads to emission colour tuning over 200 nm range from blue to red, and emission efficiencies of 0.16-0.80 in the solid state.

Today and into the foreseeable future, our world will continue to run on the conversion of fossilized hydrocarbons to energy and materials ( Fig. 7.1) Energy production (fuels for propulsion, electrical power generation, heating, etc.) is the major use of hydrocarbon feedstocks but materials (chemical, petrochemical, plastics, rubber industries) are also essential for modern life. Given the potential issues with CO 2 emissions and global warming, there is a need to develop non-carbon based energy sources. However, any alternatives will take many 235 Activation of Small Molecules. Edited by William B. Tolman

A key challenge to developing selective, low-temperature hydrocarbon oxidation catalysts based on the CH activation reaction is integration with a compatible functionalization reaction. 1,2 We recently reported a CH activation reaction (Figure 1) with an alkoxo complex, M-OR, that simultaneously generates a functionalized product, ROH, and a metal alkyl, M-R, where M is Ir(III). 3a,b As shown in Figure 1, a catalytic cycle for the conversion of RH to ROH could be possible by regeneration of M-OR from M-R with O-atom donors, YO. Pt(IV) or Hg(II) alkyls are M-C σ+ polarized and readily undergo reductive functionalization with O-nucleophiles. 1b,4 However, M-Rs of more electropositive metals, such as Ir or Re, in the lower oxidation states useful for C-H activation do not undergo facile reductive functionalizations and are likely M-C σ-polarized. Consequently, functionalization of these M-Rs may be more facile in nonredox, insertion reactions with electrophilic YOs (Figure 1) if free-radical pathways or formal oxidation of the metal centers could be minimized. 5 Conversion of Y to YO with O 2 could complete the overall catalytic cycle for the overall oxidation of RH with O 2 .The conversion of M-R to M-OR is not well-known, and the few reported examples proceed with O 2 by free-radical pathways 6 or by slow redox reactions involving alkyl to metal oxo migration. 7 Consequently, identification of a facile pathway for this reaction, especially with non-peroxo 8 YOs that could potentially be recycled with O 2 , could be useful. We report here combined experimental and theoretical evidence for a facile Re-R to Re-OR bond conversion with non-peroxo YOs that proceeds via a low-energy, Baeyer-Villiger (BV)-type, electrophilic O-atom insertion.BV and alkyl borane oxidation reactions to generate oxyesters and alkoxy boranes, respectively, are well-known organic reactions involving electrophilic O-insertions with YOs. Significantly, both peroxo and non-peroxo YOs can be utilized, and the reactions proceed without free radicals or formal redox changes. 9 Methyltrioxorhenium, MTO, with peroxo YOs is well-known to catalyze olefin epoxidation and other oxidation reactions likely via Re η 2 -peroxo intermediates. 10 A reported observation that attracted our attention was that an undesirable side reaction is the decomposition of a MTO catalyst to methanol at room temperature. 11 We were intrigued because, despite the high Re(VII) oxidation state, unlike Pt(IV) alkyls, 1b,4 treatment of MTO in basic or acidic water does not generate Re(V) and methanol. Consistent with the observations of the initial investigators, 11 we find that the formation of methanol from MTO in water requires added H 2 O 2 as the oxidant. The reaction is facile, selective, quantitative, and significantly proceeds without a change in oxidation state of the Re to generate the ReO 4 -anion.In the initial studies on decomposition of MTO to methanol, only H 2 O 2 was investigated and two non-BV-type mechanisms proposed: reaction via a η 2 -peroxo intermediate or...