Hisao Hori

Efficient photocatalytic CO2 reduction using [Re(bpy) (CO)3{P(OEt)3}]+

Abstract [Re(bpy) (CO)3{P(OEt)3}]+ (1+) (bpy, 2,2′-bipyridine) is the most efficient homogeneous photocatalyst for the selective reduction of CO2 to CO reported to date. Both the quantum yield and turnover number of the photocatalytic reaction are strongly dependent on the irradiation light intensity and wavelength because of the unusual stability of the one-electron-reduced species [Re(bpy.−) (CO)3{P(OEt)3}] (1).

Preparation and characterization of [Re(bpy)(CO) 3L][SbF6] (L = phosphine, phosphite)

A series of rhenium complexes [fac-Re(bpy)(CO)3L][SbF6] (bpy = 2,2′-bipyridine, L = P(nBu)3, PEt3, PPh3, P(OMe)Ph2, P(OiPr)3, P(OEt)3, P(OMe)3, P(OPh)3) has been prepared and characterized by the IR, UV-vis, 1H NMR, 31P NMR, X-ray photoelectron spectroscopy and electrochemical techniques. Variations in the electronic properties, i.e. CO stretching, metal-to-ligand charge transfer transition, and 31P NMR chemical shifts were interpreted on the basis of the electron-acceptor strength of L. However, the redox potential corresponding to [Re(bpy)(CO)3L]+/[Re(bpy−)(CO)3L]showed ‘V-character type’ changes after the increase in the electron-acceptor strength of L. Variation of the P(2p) binding energy of the phosphorus atom indicated that the electronic structure of the coordinated phosphorus atom was strongly influenced by the electronic properties of the directly attached substituents.

Photochemical decomposition of pentafluoropropionic acid to fluoride ions with a water-soluble heteropolyacid photocatalyst

Abstract Pentafluoropropionic acid (CF 3 CF 2 COOH; PFPA) was decomposed to F − and CO 2 with a water-soluble homogeneous photocatalyst, H 3 PW 12 O 40 . This is the first example of a photocatalytic system for CF bond cleavage in a perfluorinated acid compound having both CF 3  and CF 2  units. The catalytic reaction proceeds in water at room temperature under irradiation with UV-Vis light in the presence of oxygen. Gas chromatography–mass spectrometry (GC–MS) showed no trace of environmentally undesirable species such as CF 4 , which is often observed in the decomposition of perfluorinated compounds by extremely high energy techniques such as electron beam irradiation. In addition to F − ions, small amounts of trifluoroacetic acid (TFA) were detected in the reaction solution. The proposed reaction mechanism involves redox reactions of PFPA and TFA with the catalyst.

Efficient rhenium-catalyzed photochemical carbon dioxide reduction under high pressure

Abstract The photocatalytic reduction of CO2-to-CO was carried out under high pressure with [fac-Re(bpy)(CO)3Cl] (bpy=2,2′-bipyridine) as the catalyst. The use of high-pressure CO2 gas was much more effective than the addition of excess Cl− ions for increasing the stability of the catalyst and achieving efficient CO formation. The turnover number for CO formation at 2.45 MPa of CO2 reached a maximum of 41.8, which was 5.1 times the turnover number for the conventional normal-pressure system.

Oxygen-induced efficient mineralization of perfluoroalkylether sulfonates in subcritical water.

The decomposition of perfluoroalkylether sulfonates C(2)F(5)OC(2)F(4)SO(3)(-) and C(3)F(7)OC(2)F(4)SO(3)(-) in subcritical water was investigated, and the results were compared with those for perfluorobutanesulfonate (C(4)F(9)SO(3)(-)), which has no ether linkage. This is the first report on the use of subcritical water to decompose perfluoroalkylether sulfonates, which are being developed as alternative surfactants to environmentally persistent and bioaccumulative perfluoroalkylsulfonates. Whereas C(4)F(9)SO(3)(-) showed little reactivity in subcritical water, C(2)F(5)OC(2)F(4)SO(3)(-) decomposed efficiently in subcritical water ( approximately 350 degrees C) in the presence of oxygen gas to form F(-) and SO(4)(2-) in the aqueous phase and CO(2) in the gas phase as major products. Trifluoroacetic acid (CF(3)COOH, TFA) and trifluoromethane (CF(3)H) were also detected as minor products in the aqueous and gas phases, respectively. Similar decomposition behavior was observed for C(3)F(7)OC(2)F(4)SO(3)(-), which decomposed at a rate that was somewhat higher than that of C(2)F(5)OC(2)F(4)SO(3)(-). When argon was used in place of oxygen gas, the time profile of the decrease in the amount of C(2)F(5)OC(2)F(4)SO(3)(-) remained almost unchanged, but the product distribution changed markedly: the amounts of F(-), SO(4)(2-), and CO(2) dramatically decreased, the amounts of TFA and CF(3)H increased, and a new product, HCF(2)SO(3)(-), was detected. These results clearly indicate that treatment with subcritical water in the presence of oxygen gas effectively mineralized perfluoroalkylether sulfonates to F(-), SO(4)(2-), and CO(2). On the basis of a product analysis, we propose a decomposition mechanism.

Photochemistry of[Re(bipy)(CO)3(PPh3)]+(bipy = 2,2′-bipyridine) in thepresence of triethanolamine associated with photoreductive fixation ofcarbon dioxide: participation of a chain reaction mechanism

The complex fac-[Re(bipy)(CO) 3 (PPh 3 )] + 1 + (bipy = 2,2′-bipyridine) was converted into a formate complex fac-[Re(bipy)(CO) 3 {OC(O)H}] 2 in a 52.2% yield by irradiation in the presence of triethanolamine (teoa) and CO 2 . This photochemical fixation of CO 2 proceeds via a unique reaction mechanism: (i) irradiation of 1+ in teoa–dimethylformamide (dmf) resulted in the efficient formation of fac-[Re(bipy)(CO) 3 (teoa)] + 3 + and fac-[Re(bipy)(CO) 3 (dmf)] + 4+ in a quantum yield of 16.9; (ii) the ligand substitution was explained in terms of a chain mechanism involving an initial electron transfer from teoa to excited 1+, followed by substitution of the PPh 3 ligand of 1 with teoa and dmf to give 3 and 4; subsequent electron exchange of 3 and 4 with 1+ leads to the formation of 3+ and 4+ accompanied by the regeneration of 1; (iii) the formate complex 2 was formed in a quantum yield of 1.1 × 10 -3 upon excitation of 3+ and 4+ in the presence of CO 2 ; (iv) further irradiation after the formation of 2 resulted in the photocatalytic reduction of CO 2 to CO in a quantum yield of 0.05.

Efficient carbon dioxide photoreduction by novel metal complexes and its reaction mechanisms

Photoreduction of CO2 to CO using the new catalysts, [Re(bpy)(CO)3{P(OEt)3}]+ (1+) and [Re(bpy)(CO)3(PPh3)]+ (2+) is described. The quantum yields of CO formation are 0.23 and 0.05 for 1+ and 2+, respectively. The value for 1+ is one of the highest reported values for homogeneous CO2 reduction photocatalysts. In-situ UV/vis spectra indicate that one-electron reduced species is an important intermediate.

Photocatalytic mineralization of hydroperfluorocarboxylic acids with heteropolyacid H4SiW12O40 in water

The decomposition of hydroperfluorocarboxylic acids [H-PFCAs; HC(n)F(2)(n)COOH (n=4 and 6)] induced by heteropolyacid photocatalyst H(4)SiW(12)O(40) in water was investigated, and the results are compared with the results for the corresponding perfluorocarboxylic acids (PFCAs; C(n)F(2)(n)(+1)COOH). This is the first report on the photochemical decomposition of H-PFCAs, which are being developed as alternative surfactants to environmentally persistent and bioaccumulative PFCAs. H-PFCAs were not decomposed by irradiation with UV-Visible light (>290 nm) in the absence of H(4)SiW(12)O(40). In contrast, UV-Visible light irradiation of H-PFCAs in the presence of H(4)SiW(12)O(40) efficiently decomposed H-PFCAs to F(-) and CO(2). The decomposition reactions showed pseudo-first-order kinetics, and the decomposition rate constants were 1.8-2.5 times higher than those for the corresponding PFCAs. The reaction mechanism can be explained by elimination of H(+) from the ω-H atom of the H-PFCAs by the excited catalyst, followed by formation of perfluorodicarboxylic acids.

Spectroscopic characterization of iron complexes of methyl pheophorbide with pyridine and its derivatives

Abstract A series of (methyl pheophorbide a)iron(II) and -iron(III) complexes have been prepared and characterized by Mossbauer, 1H NMR and electronic spectra. The central iron(III) ion in (methyl pheophorbide a)iron(III) chloride [Fe(mepheo-a)] is in the high-spin state. Autoreduction of the central iron(III) ion in Fe(mepheo-a)Cl is induced by pyridine (py) and its derivatives (4-Xpy). A connection between isomer shifts and quadrupole splittings of Fe(mepheo-a)(4-Xpy)2 indicates the σ-donation from the axial ligands to the central iron(II) ion is predominant in the axial bonding. The linear dependence of the isomer shifts on Hammetts constants of the pyridine substituents (X) and on the pKa values of the free ligands reveals that the, π-backdonation to the axial ligands enhances synergetically the σ-donation to the central iron(II) ion. The linear correlation between the isomer shift and the electronic absorption wavelength demonstrates that the difference in the π-backdonation to the axial ligands affects the π-π* excitations on the chlorin ring via the central iron(II) ion.

Metal-induced decomposition of perchlorate in pressurized hot water.

Decomposition of perchlorate (ClO(4)(-)) in pressurized hot water (PHW) was investigated. Although ClO(4)(-) demonstrated little reactivity in pure PHW up to 300°C, addition of zerovalent metals to the reaction system enhanced the decomposition of ClO(4)(-) to Cl(-) with an increasing order of activity of (no metal)≈Al < Cu < Zn < Ni << Fe: the addition of iron powder led to the most efficient decomposition of ClO(4)(-). When the iron powder was added to an aqueous ClO(4)(-) solution (104 μM) and the mixture was heated at 150°C, ClO(4)(-) concentration fell below 0.58 μM (58 μg L(-1), detection limit of ion chromatography) in 1 h, and Cl(-) was formed with the yield of 85% after 6 h. The decomposition was accompanied by transformation of the zerovalent iron to Fe(3)O(4). This method was successfully used in the decomposition of ClO(4)(-) in a water sample contaminated with this compound, following fireworks display at Albany, New York, USA.