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Scientific and Technical Description of the Results

Photochemical generation of trans-dioxoruthenium(IV) porphyrin complexes

In literature, trans-dioxoruthenium(VI) porphyrin complexes have been received much attention, and developed as the well-characterized model system for heme-containing enzymes. Typically, trans-dioxoruthenium(VI) porphyrin complexes were prepared  by oxidation of the corresponding ruthenium(II) carbonyl precursors with sacrificial oxidants such as  m-CPBA or PhIO.  Recently, we have successfully developed a new photochemical method that led to generation of the dioxoruthenium(IV) complexes bearing sterically hindered, unhindered and chiral porphyrin ligands (Scheme 1).

Scheme 1. Photochemical synthesis of trans-dioxoruthenium(VI) porphyrins

As shown in Scheme, the dichlororuthenium(IV) complexes (1) were first prepared.  Exchange of the counterions in 1 with Ag(ClO₃) gave the corresponding dichlorate salts 2. Irradiation of dichlorate complexes 2 in anaerobic CH₃CN with visible light resulted in changes in absorption spectra with isosbestic points (Figure 1A).  In Figure 1, 2a was decayed, and a new species 3a was produced, displaying a stronger Soret band at 420 nm and weaker Q band at 518 nm that is characteristic for Ru^VI(TMP)O₂. The spectra signature of Ru^VI(TMP)O₂ was further confirmed by ¹H NMR and IR. Thus, the photolysis reactions of the dichlorate complexes 2 undergo the homolytic cleavage of the O-Cl bond in the two chlorate counterions simultaneously to produce neutral dioxoruthenium(VI) species 3 via two one-electron photooxidation pathways.

 Figure 1 (A) Time–resolved UV-visible spectra of 2a (8 × 10^-6 M) upon irradiation with visible light in anaerobic CH₃CN solution at 22 ^oC over 50 min. (B) UV-visible spectral change of 2b (8.0 × 10^-6 M) upon irradiation over 45 min. (C) Time-resolved spectrum following irradiation of 2c (1.0 × 10^-5 M) over 60 min.

In a similar fashion, the sterically unhindered Ru^VI(TPP)O₂ (3b) and chiral Ru^VI(D₄-Por*)O₂ (3c) were also generated (See Figure 1B  and 1C). The use of other solvents such as CH₂Cl₂ gave the same results. The product degradation was observed when higher-energy UV light (l_max = 350 nm) was used instead of the visible light.

Photocatalytic Aerobic Oxidation      

      As proposed, 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. Previously, we found that the photodisproportionation of a bis-porphyrin-ruthenium(IV) m-oxo dimer apparently gave a putative porphyrin-ruthenium(V)-oxo transient (5) that can be detected by laser flash photolysis methods. Herein we report that ruthenium(IV) m-oxo bisporphyrin complexes catalyze the aerobic oxidation of hydrocarbons using visible light and atmospheric oxygen as oxygen source as shown in Scheme 2.

Scheme 2: Photocatalytic Aerobic Oxidation by a bis-Porphyrin-Ruthenium(IV) µ-Oxo Dimer

A series of ruthenium(IV)-m-oxo bisporphyrins was evaluated in the aerobic oxidation of cis-cyclooctene (Table 1).  After 24 hours of photolysis with visible light (l_max = 420 nm), cis-cyclooctene oxide was obtained as the only identifiable oxidation product (> 95% by GC) with ca. 220 turnovers of catalyst 4a (entry 1). The use of other solvents instead of CH₃CN resulted in reduced TONs (entries 2-4). Catalyst degradation was a problem with higher-energy light, but the use of UV irradiation increased catalytic activity (entry 5). The catalytic activity was enhanced by adding small amounts of anthracene (entries 6 and 9).  Quite surprisingly, the axial ligand on the metal had a significant effect, and the [Ru^IV(TPP)Cl]₂O (entry 7) gave reduced activity compared to [Ru^IV(TPP)OH]₂O. The substituent in the porphyrin ligand gave a noticeable effect on the catalytic activity (entries 1, 8 and 10), with the most electron demanding system, being the most efficient catalyst.

Table 1. Aerobic Photocatalytic Oxidation of cis-Cyclooctene with Diruthenium(IV) m-Oxo Porphyrins ^a

^aThe reaction was carried out with 0.5 µmol of catalyst in 5 mL of solvent containing 4 mmol of cis-cyclooctene. Oxygen-saturated solutions were irradiated with visible light (l_max= 420 nm) or otherwise noted. ^b TON represents the total number of moles of product produced per mole of catalyst. All reactions were run three times, and the data reported are the averages. ^c The major product was cis-cyclooctene oxide, detected in > 95% yield.  ^d UV-visible light (l_max = 350 nm). ^e 5 mg of anthracene was added.

 The photocatalytic oxidations of a variety of organic substrates were examined in a similar way.  Table 2 lists the oxidized products and corresponding TONs using 4b as the photocatalyst. Activated hydrocarbons including triphenylmethane, diphenylmethane, ethylbenzene and xanthenes were oxidized to the corresponding alcohols and/or ketones from over-oxidation with total TONs ranging from 560 to 2900 (entries 3-6). Noticeably, the oxidation of secondary benzylic alcohols gave the highest catalytic activities (entries 7-8). Competitive catalytic oxidation of ethylbenzene and ethlybenzene-d₁₀ revealed a kinetic isotope effect (KIE) of k_H/k_D = 4.8 ± 0.2 at 298 K.

Table 2. Turnover Numbers for Alkenes and Benzylic C-H Oxidations Using 4b as the photocatalyst ^a

^a Typically with 0.25 µmol of 4b in CH₃CN containing 4 mmol of substrate and 5 mg anthracene. ^b Determined for a 24 h photolysis (l_max = 420 nm). ^c > 90% exo isomer. ^d One minor product was detected by GC but not identified.

Conclusions

In summary, we report a new preparation of trans-dioxoruthenium(VI) porphyrin complexes by an extremely easy photochemical approach that bypassed the limitation observed in chemical methods. We have also demonstrated that the ruthenium(IV)-m-oxo bisporphyrins catalyzed efficient aerobic oxidation of alkenes and activated hydrocarbons using visible light and atmospheric oxygen. The observed photocatalytic oxidation is ascribed to a photo-disproportionation mechanism to afford a putative porphyrin-ruthenium(V)-oxo species as the active oxidant.