AmericanChemicalSociety.com

Specific project objectives

Objective I: Chemical and photochemical generation of highly reactive porphyrin- ruthenium(V)-oxo species

Objective II: Comprehensive kinetic studies and mechanistic investigation of high-valent metal-oxo species

Scientific and Technical Description of the Results

Chemical generation of ruthenium(V)-oxo porphyrin species

As proposed, we tried to produce and detect the ruthenium(V)-oxo species by oxidation of the porphyrin ruthenium(III) precursors with two-electron sacrificial oxidants. Thus, we followed the reported procedure to synthesize the Ru^III(TPP)(OEt), confirmed by the known UV-vis absorption and by a typical EPR signal of a ruthenium(III) species. However, treatment of the ruthenium(III) precursor by the sacrificial oxidants, including peroxyacetic acid, m-CPBA, iodosylbenzene and 2,6-dichlorpyridine, does not result in any change of UV-vis absorption, and no detectable intermediate was observed even at low temperature (-40 ^oC). These results suggested that the chemical oxidation of ruthenium(III) is not viable to generate the detectable ruthenium(V)-oxo species presumably due to its high reactivity and low concentration.

Photo-induced ligand cleavage reactions

During the current period, we have studied the photo-induced ligand cleavage reactions to generate the putative ruthenium(V)-oxo species.  Firstly, we tested the photo-labile precursor 2, which was prepared by the reaction of 1 with an excess of pyridine-N-oxide (Scheme 1 and Figure 1A).  Irradiation of 2 with 355 nm laser light in CH₂Cl₂ solution gave a transient that has a Soret band absorbance at ca. 395 nm. When excess cyclohexene was present, the decay of the formed species accelerated linearly with the substrate concentration (Figure 1C). The plot slope gave a secondary rate constant (k₂) of 740 M^-1 s^-1, which is 4-5 orders of magnitude greater than that of reaction of the well characterized trans-dioxoruthenium(VI) complex. The generated transient species was tentatively assigned the ruthenium(V)-oxo structure 3 based on its unique UV-visible spectrum and its very high reactivity. With 2,6-dichloropyridine N-oxide for precursor, we obtained similar result.

Scheme 1: Photo-induced ligand cleavage reactions

Figure 1: (A) UV-visible spectra of 2 (solid lines) and 1 (dotted lines) in CH₂Cl₂. (B) Time-resolved spectrum for 100 ms following 355 nm irradiation of species 2 in CH₂Cl₂ at 25 ^oC. (C) Observed rate constants for reactions with cyclohexene in CH₂Cl₂ at 25 ^oC, monitored at 395 nm.

Secondly, we also investigated the photolysis of porphyrin-ruthenium(IV) chlorate (5) which typically undergoes homolytic cleavage of the O-Cl.  The precursor was prepared from the dichlororuthenium(IV) complex 4 by exchanging of the counterion with Ag(ClO₃) in CH₃CN. However, irradiation of complex 5 with 355 nm laser light gave a relatively stable species with a Soret band of l_max at 418 nm, which was characteristic absorption for the well-known trans-dioxoruthenium(VI) porphyrin complex, i.e.  Ru^VI(O)₂TPP (Scheme 1).

Photo-disproportionation of a diruthenium(IV)-m-oxo bisporphyrin complexes

In literature, photo-disproportionation of the m-oxo iron(III) dimer was well established to form iron(II) and porphyrin-iron(IV)-oxo species concomitantly. We suggest that the photo-disproportionation of diruthenium(IV)- m-oxo bisporphyrins may also produce the terminal ruthenium(V)-oxo intermediate and ruthenium(III) porphyrin (Eq. 1):

   (1)

To test above hypothesis, the diruthenium(IV)- m-oxo bis[5,10,15,20-tetraphenylporphyrin(TPP)] precursor (7) was synthesized. The initial hydroxyl axial ligand could be replaced with chloride to form [Ru^IV(TPP)Cl]₂O (Figure 2A). At ambient temperature, irradiation of the complex [Ru^IV(TPP)OH]₂O with 355 nm laser light in CH₃CN solution produced a shot-lived transient (Figure 2B), very similar to that formed in photo-induced reaction, as determined by UV-vis spectra and kinetic behavior. The species decayed over 100 ms to form a species at 410 nm, known for Ru^III species (Figure 2B). The k₂ of 1800 M^-1 s^-1 with cyclohexene was obtained in CH₃CN (Figure 1C). When the photo-disproportionation reaction was carried out in CH₂Cl₂, a second rate constant (k₂ = 670 M^-1 s^-1) was obtained, confirming that the same oxidizing species was produced from both methods.

Figure 2: (A) UV-visible spectra of [Ru^IV(TPP)OH]₂O (solid lines) and [Ru^IV(TPP)Cl]₂O  (dotted lines) in CH₃CN.  (B) Time-resolved difference spectrum over 100 ms following 355 nm irradiation of [Ru^IV(TPP)OH]₂O in CH₃CN at 25 ^oC. (C) Observed rate constants for reactions with cyclohexene in CH₃CN at 25 ^oC, monitored at 390 nm.

Photo-disproportionation of a diiron(IV)-m-oxo biscorrole complexes

We extended the scope of the photo-disproportionation reactions with the utilization of iron corroles. In our published results, we demonstrated that iron(IV) m-oxo corrole dimers could form a reactive iron(V)-oxo intermediate, and potentially to provide another aerobic catalytic protocol as well. The bis-corrole diiron(IV)-oxo complex 8 was prepared and characterized by UV-visible, ¹H NMR, and ¹⁹F NMR spectra that matched those previously reported in 2001 by Simkhovich et al.   The UV-visible spectra of complex 8 and its corrole-iron(III) precursor are shown in Figure 3A.

Scheme 2: Formation of an iron(V) corrole species by photolysis of either m-oxo iron(IV) biscorrole dimer (8) or a iron(IV) chlorate (10).

Irradiation of complex [Fe^IV(TPFP)]₂O with 355 nm laser light in CH₃CN solution produced a highly reactive transient with a Soret band absorbance with l_max at 415 nm (Figure 3B), the same as that of the species formed by photo-induced ligand cleavage reaction, reported by Newcomb and coworkers in 2005. The observed rate constant for cyclooctene is 5.6 ± 10³ M^-1 s^-1, again, consistent with the previously reported one. Kinetic results with other substrates confirmed that same oxidizing species was produced from both routes.

Figure 3: (A) UV-visible spectra of [Fe^IV(TPFC)]₂O (solid lines) and Fe^III(TPFC)(OEt)₂ (dotted lines) in CH₃CN.  (B) Time-resolved difference spectrum over 10 ms following 355 nm irradiation of [Fe^IV(TPFC)]₂O in CH₃CN at 25 ^oC.

The potential for a “green” catalytic system was evaluated in the aerobic oxidation of cis-cyclooctene (Scheme 3).  The reaction system consisted of 1 mmol of 8 in 5 mL of acetonitrile containing 5 mmol of cis-cyclooctene.  Dry air was bubbled through the solution as it was irradiated with 400-500 nm light.  After 48 hours of photolysis, cis-cyclooctene oxide was obtained as the only identifiable oxidation product (>95% by GC) with ca. 200 turnovers of catalyst.

Scheme 3: Catalytic oxidation by the diiron(IV) m-oxo biscorroles using visible light and O₂