Photocatalytic Aerobic Oxidation by a Bis-porphyrin−Ruthenium(IV) μ-Oxo Dimer: Observation of a Putative Porphyrin−Ruthenium(V)−Oxo Intermediate

Abstract

The title complexes catalyze the aerobic oxidations of hydrocarbons using visible light and atmospheric oxygen as oxygen source in sequences employing photo-disproportionation reactions. The putative oxidants, ruthenium(V)-oxo porphyrin species, can be detected and studied in real time via laser flash photolysis methods.Selective oxidation is a key technology for the synthesis of high value chemicals in the pharmaceutical and petrochemical industries, but oxidations are among the most problematic processes to control. 1 Many stoichiometric oxidants with heavy metals are expensive and/or toxic, and thus impractical. The ideal green catalytic oxidation process would use molecular oxygen or hydrogen peroxide as the primary oxygen source, recyclable catalysts in nontoxic solvents, and an inexpensive energy source. 2 In this context, we have an interest in photochemical generation of highly reactive metal-oxo intermediates which, upon oxidization of substrates, give low-valent metal complexes that can be recycled for catalytic oxidations. One example of a catalytic aerobic oxidation driven by a photo-disproportionation reaction employed a diiron(III)-μ-oxo bisporphyrin complex, 3 and photocatalytic oxidations of hydrocarbons using molecular oxygen as the oxygen source with no added reducing agent for the oxygen have been developed. 4 The low reactivity of the formed iron(IV)-oxo transients and poor quantum efficiency are serious obstacles limiting the use of iron porphyrins as practical photocatalysts, but the catalytic efficiency of diiron(III)-μ-oxo bisporphyrin systems was improved by Nocera and coworkers who employed "Pacman" ligand designs with organic spacer-hinges serving to preorganize two iron centers in a co-facial arrangement. Among the high-valent metal-oxo species, metal(V)-oxo complexes deserve special attention because they are highly reactive although rare and elusive. The known porphyrin manganese (V)-oxo complexes showed higher reactivity than well-studied iron(IV)-oxo porphyrin radical cation analogs. 9, 10 A recent paper by Collins et al. reported spectroscopic evidence for an oxoiron(V) complex supported by a tetraanionic ligand that showed unprecedented reactivity. 11 Putative porphyrin-and corrole-iron(V)-oxo transients produced by laser flash photolysis methods displayed the appropriate high levels of reactivity expected for iron(V)-oxo species, 12, 13 and we recently reported a photodisproportionation of a bis-corrole-iron(IV) μ-oxo dimer that apparently gave the same type of corrole-iron(V)-oxo transient in a system that has potential for light-driven oxidation catalysis. 14 Porphyrin-ruthenium(V)-oxo transients 15,16 are attractive candidates for oxidations, and these species are proposed intermediates in very efficient catalytic processes, 17 although not yet observed directly; computational studies suggest that they are stable with respect to ruthenium (IV)-oxo porphyrin radical cations. 18 In this study, we explore the applicability of ruthenium porphyrins, which are known to dis...

Selective Aerobic Oxidation of 5-Hydroxymethylfurfural in Water Over Solid Ruthenium Hydroxide Catalysts with Magnesium-Based Supports

“…Ruthenium-based catalysts are generally known for their aptitude in aerobic oxidation reactions [15][16][17] including applications for oxidation of alcohols to produce aldehydes or ketones. Hence, homogeneous Ru-complex catalysts have been found to generate aldehydes or ketones in almost quantitative yields when employed in organic solvents [18,19] or ionic liquids [20].…”

Synthesis of LaVO4/TiO2 heterojunction nanotubes by sol–gel coupled with hydrothermal method for photocatalytic air purification

“…Therefore, removal of toluene in the contaminated atmosphere has become an increasingly concerned issue in the field of environment protection [1][2][3][4]. In the past decade, semiconductor photocatalytic oxidation technology, as an advanced oxidation process, has been demonstrated to be potentially advantageous for environmental contaminations degradation because it can completely decompose VOCs into CO 2 and H 2 O under ambient conditions [5][6][7][8][9][10][11].…”

Photochemical generation and kinetic studies of a putative porphyrin-ruthenium(v)-oxo species

“…40 Recently, we have communicated a putative porphyrin-ruthenium(V)-oxo species via a photo-disproportionation reaction that has shown great potential for aerobic photocatalytic oxidation. 41 In the present study, we describe our full findings on the photochemical formation of a highly reactive porphyrin-ruthenium-oxo species ( 1 ) using LFP methods and direct measurements of the rate constants for oxidation reactions. As discussed below, the target oxidant 1 in this work can be formed either by photo-disproportionation of a bis -porphyrin-ruthenium(IV) μ-oxo dimer ( 2 ), or by photolysis of the porphyrin-ruthenium(III) N -oxide adduct ( 3 ) (Figure 1).…”

“…It is very important to point out that its high reactivity generated from two methods in comparison to other metal-oxo complexes supports the original assignment of the structure of the oxidant 1 as a ruthenium(V)-oxo species. 41 …”