New strategy for the simultaneous tuning of catalytic activity and water solubility of complex catalysts is described on the basis of an acid−base equilibrium between pyridinol and pyridinolate as the catalyst ligands. Herein, half-sandwich complexes with 4,4‘-dihydroxy-2,2‘-bipyridine (DHBP) or 4,7-dihydroxy-1,10-phenanthroline (DHPT) served as highly efficient and recyclable catalysts for the hydrogenation of bicarbonate in water. The oxyanion generated from the phenolic hydroxy group shows strong electronic donation and polarity, which play significant roles in the catalytic activity and water solubility, respectively. As a result, turnover frequencies (TOF) up to 42 000 h-1 and turnover numbers (TON) up to 222 000 have been obtained by using iridium catalysts under 6 MPa at 120 °C. Furthermore, an iridium DHPT catalyst was spontaneously precipitated at the end of the reaction. Iridium leaching was found to be 0.11 ppm (1.2% of the loaded catalyst), and the added base was completely consumed. The recovered catalyst could be recycled for four cycles with high catalytic activity. Consequently, the catalyst was homogeneous and highly activated at the beginning of the reaction, whereas it was heterogeneous and deactivated at the end. The catalytic system offers an environmentally benign process with high efficiency, easy separation, catalyst recycling, waste-free process, and aqueous catalysis.

Half-sandwich Ru(II), Ir(III), and Rh(III) complexes with 4,7-dihydroxy-1,10-phenanthroline are highly efficient catalysts for hydrogenation of bicarbonate in alkaline aqueous solution without an amine additive. The generation of an oxyanion by deprotonation of the two hydroxy substituents on the catalyst ligand caused a dramatic enhancement of catalytic activity, due to its strong electron-donating ability, as well as imparting water solubility.The conversion of CO 2 to useful organic products in place of toxic CO still remains an intriguing and challenging subject. 1 From this point of view, the homogeneous catalytic hydrogenation of CO 2 into formic acid catalyzed by transition-metal complexes, usually using phosphine ligand, has been extensively investigated. 2 For example, since 1994, Noyori and co-workers have reported a highly efficient hydrogenation of CO 2 catalyzed by Ru(II) phosphine complexes in supercritical CO 2 . 3 On the other hand, some efforts have been devoted to the investigation of CO 2 hydrogenation in aqueous media. In the most effective aqueous systems using the rhodium complexes with sulfonated phosphine ligand at 4 MPa of pressure (CO 2 :H 2 ) 1:1), a higher temperature (81°C) resulted in the highest initial turnover frequency (TOF) of 7260 h -1 , while a lower temperature (23°C) resulted in the highest turnover number (TON) of 3439. 4 In both the liquid and supercritical reactions, an amine additive is required for high yields. However, the use of such organic additives should be avoided, because they would eventually prevent separation of product and catalyst for recycling. Recently, Joo and coworkers investigated the hydrogenation of CO 2 and bicarbonate catalyzed by [Ru(PTA) 4 Cl 2 ] (PTA ) 1,3,5-triaza-7-phosphaadamantane) in the absence of amines or other organic additives, 5 but this system exhibited a unsatisfactory catalytic efficiency (up to an initial TOF of 807 h -1 ) compared to systems with an amine additive.We have studied the highly water-soluble, halfsandwich bipyridine (bpy) complexes [Cp*M(bpy)Cl]Cl (M ) Rh, Ir; Cp* ) η 5 -pentamethylcyclopentadienyl) and [(C 6 Me 6 )Ru(bpy)Cl]Cl for catalytic transfer hydrogenation, using formic acid as a hydrogen donor in water. 6 In a preliminary experiment, we found that the decomposition of formic acid to CO 2 and H 2 catalyzed by [Cp*Rh(bpy)Cl]Cl proceeded smoothly with a TOF of 238 h -1 at 40°C. The high catalytic activities of the half-sandwich complexes prompted us to investigate the reverse reaction: namely, the hydrogenation of CO 2 or bicarbonate. Herein we report that half-sandwich Ru(II), Ir(III), and Rh(III) complexes with 4,7-dihydroxy-1,10-phenanthroline are highly efficient catalysts for hydrogenation of bicarbonate in alkaline aqueous solution in the absence of an amine additive.We first examined reactions catalyzed by [Cp*M-(phen)Cl]Cl (phen ) 1,10-phenanthroline) and [Cp*M-(bpy)Cl]Cl in aqueous KOH, which was saturated with CO 2 before the reaction was started, 7,8 under 4 MPa of Onozawa-Komatsuzaki, N.; Sugihara...

Transfer hydrogenation catalyzed by an iridium catalyst with 4,4'-dihydroxy-2,2'-bipyridine (DHBP) in an aqueous formate solution exhibits highly pH-dependent catalytic activity and chemoselectivity. The substantial change in the activity is due to the electronic effect based on the acid-base equilibrium of the phenolic hydroxyl group of DHBP. Under basic conditions, high turnover frequency values of the DHBP complex, which can be more than 1000 times the value of the unsubstituted analogue, are obtained (up to 81 000 h(-1) at 80 degrees C). In addition, the DHBP catalyst exhibits pH-dependent chemoselectivity for alpha,beta-unsaturated carbonyl compounds. Selective reduction of the C=C bond of enone with high activity are observed under basic conditions. The ketone moieties can be reduced with satisfactory activity under acidic conditions. In particular, pH-selective chemoselectivity of the C=O versus C=C bond reduction was observed in the transfer hydrogenation of cinnamaldehyde.

Dye-sensitized solar cells (DSSCs) based on nanocrystalline TiO 2 films have attracted considerable attention because of their great potential in terms of low fabrication costs and high solar-light-to-electricity conversion efficiency. [1] Extensive efforts have been focused on the development of new, highly efficient sensitizers, as they play a critical role in cell performance. Sensitizers exhibiting absorption over a wide range of the solar spectrum and a high molecular extinction coefficient have been investigated for improving the conversion efficiency of DSSCs. Among the most successful of the various sensitizers are complex N3, [Ru(dcbpy) 2 (NCS) 2 ] (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine), [2] and complexes of the type [Ru(dcbpy)(L1)(NCS) 2 ], where L1 is a 2,2'-bipyridine with a highly conjugated ancillary group. [3] However, these sensitizers show insufficient light-harvesting efficiencies in the near-IR region. As the solar spectrum has a large photon flux in the near-IR region above 800 nm, the synthesis of efficient near-IR sensitizers is currently one of the most important issues in the development of solar cells.The absorption properties of Ru II complexes can be tuned by careful consideration of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. [4] The absorption band can be extended into a longer wavelength region by either destabilizing the metal t 2g orbital using a strong s-donating ligand or by introducing a ligand with a low-lying p*-level molecular orbital. Complex N749, (TBA) 3 [Ru(tctpy)(NCS) 3 ] (tctpy = 4,4',4''-tricarboxy-2,2':6',2''-terpyridine; TBA = tetra-n-butylammonium), [5] and complexes of type PRT-11-14, (TBA)[Ru(L2)(NCS) 3 ], where L2 is a 4,4'-dicarboxy-2,2':6',2''-terpyridine derivative with a highly conjugated ancillary group, [6] have been reported to exhibit panchromatic sensitization up to 900 nm. Although the introduction of the tctpy ligand improves near-IR sensitization, the main drawbacks of N749 are the inferior incident-photon-to-current conversion efficiency (IPCE) in the shorter wavelength region, and the presence of three NCS ligands. The former problem arises predominantly from the lack of an effective chromophore, whereas the latter is caused by two factors: 1) The linkage isomers of the NCS ligand cause a decrease in the synthetic yield. [5,7] 2) The stability of the complex decreases owing to dye decomposition by weak Ru-NCS bonding. Although NCS-free Ru II complexes with a conversion efficiency of up to 10 % have been reported, [8] these dyes also show relatively low light-harvesting properties over 800 nm.We [9] and others [10] have examined terpyridyl Ru II complexes of the type [Ru(tctpy)(L3)(NCS)] z , where L3 is a bidentate ligand and z = 0 or + 1, in an attempt to optimize near-IR sensitizers. The role of the NCS ligand is to regenerate the sensitizers from the iodine redox. [10a, 11] Among these complexes, cyclometalated Ru II complexes show superior light-harvesting properties in the...

The catalyst recycling in the conversion of CO2 into formate using the iridium complex with 4,7-dihydroxy-1,10-phenanthroline as a catalyst precursor is described. The catalyst precursor was dissolved in an aqueous KOH solution under CO2 pressure prior to the reaction, but was precipitated spontaneously at the end of the reaction. The acidification by the generation of formate caused the transformation from the water-soluble deprotonated form into the water-insoluble protonated form. When the reaction was carried out at 60 degrees C for 20 h using 0.1 M KOH solution under 6 MPa of H2:CO2 (1:1), the catalyst precursor was precipitated spontaneously and the added KOH was consumed completely. The catalyst was recovered by filtration, and the product was obtained by the evaporation of the filtrate. Iridium leaching into the filtrate was found to be 0.11 ppm (<2% of the loaded Ir). The recovered catalyst retained high catalytic activity for four cycles. Consequently, the CO2 conversion using the complex is an environmentally benign process, whose significant features are as follows: (i) catalyst recycling by self-precipitation/filtration, (ii) waste-free process, (iii) the easy isolation of the product, (iv) high efficiency under relatively mild conditions, and (v) aqueous catalysis without the use of organic materials. Furthermore, we have demonstrated the significant roles of the oxyanion generated from the acidic phenolic hydroxyl on the catalyst ligand, which are the catalyst recovery by acid-base equilibrium, as well as the water-solubility by its polarity and the catalyst activation by its electron-donating ability.

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