Research on inorganic activators of dibromo Co-terpyridine complex precatalyst for hydrosilylation.

The search for a stable, inexpensive, and easy-to-handle activator toward the catalyst precursor [Co(tpy)Br2] in the hydrosilylation of olefins with hydrosilane revealed that K2CO3 is an effective activator. This inorganic salt is available on substrates with some functional groups and can be readily removed by simple filtration or centrifugation after the reaction. After examining and comparing the activator abilities of various salts, it was proposed that low MX lattice energy, high X-nucleophilicity, and a strong Si-X bond are necessary for an inorganic salt (MX) to be an excellent activator.

Effect of Micropores of a Porous Coordination Polymer on the Product Selectivity in RuII Complex-catalyzed CO2 Reduction.

To develop an efficient CO2 reduction catalyst, hybridizing a molecular catalyst and a porous coordination polymer (PCP) is a promising strategy because it can combine both advantages of the precise reactivity control of the former and the CO2 adsorption property of the latter. Although several PCP hybrid catalysts have been reported to date, the CO2 sorption behavior and the CO2 reduction reactivity have been investigated separately, and the CO2 enrichment during the catalysis is still unclear. We report CO2 photoreduction under different temperatures and pressures using a PCP-RuII complex hybrid catalyst. The product selectivity (CO or HCOOH) varied depending on the reaction conditions. The altered selectivity could be interpreted in terms of the CO2 capture in the micropores of a PCP.

Base Metal-terpyridine Complex Immobilized on Stationary Phase Aimed as Reusable Hydrosilylation Catalyst.

The catalytic activity of a base metal-terpyridine complex immobilized on silica gel (M(tpy)X2 @SiO2 /H2 O: M=Mn, Fe, Co, Ni, Cu; X=Cl, Br) for hydrosilylation was investigated. Co(tpy)Br2 @SiO2 /H2 O in the presence of NaBHEt3 exhibited the highest catalytic activity for hydrosilylation of 1-octene with diphenylsilane (Ph2 SiH2 ) to form the anti-Markovnikov-type hydrosilylation compound as the main product. The reusability of Co(tpy)Br2 @SiO2 /H2 O activated by NaBHEt3 was examined. It was found that the catalytic activity decreased with repeated use because of the peeling off of the Co complex anchor portion from the silica gel surface upon the attack of NaBHEt3 . The introduction of Co(OAc)2 instead of CoBr2 to silica gel formed Co(tpy)(OAc)2 - and Co(tpy)(OH)2 -immobilized silica gel, which exhibited catalytic activity for the hydrosilylation in the absence of an activator such as NaBHEt3 . The glassware in which Co(tpy)(OH)2 was immobilized on the inner wall was prepared. It was found that the hydrosilylation catalytically occurred on the surface of a pretreated glassware and that the catalytic activity did not decrease even after 10 repeated uses.

Hydrosilylation of Ketones Catalyzed by Iron Iminobipyridine Complexes and Accelerated by Lewis Bases.

Fe-iminobipyridine complexes ((R BPIAr,R' )FeBr2 , R BPIAr,R' =iminobipyridine derivatives) were found to exhibit good catalytic activity for hydrosilylation of ketones. The highest TOF (turnover frequency) was obtained for the hydrosilylation of 2-octanone with phenylsilane (4190 min-1 ). The reactions of various 4-substituted acetophenone derivatives revealed that the introduction of an electron-withdrawing group at the 4-position retarded the reaction. The TOF of the hydrosilylation of 4-chloroacetophenone with diphenylsilane was quite low (30 min-1 ), however the addition of a catalytic amount of Lewis base, especially pyridine, dramatically accelerated this hydrosilylation (980 min-1 ). Comparison of this additive effect for several N-donor ligands revealed that the coordination ability of the N-donor ligand was responsible for the acceleration. The rate determining step in the hydrosilylation of ketones appeared to be the reductive elimination of alkoxy and silyl groups from the iron center, which was facilitated by the coordination of N-donor ligand to the iron. This coordination ability of the N-donor ligand, however, inhibited olefin hydrosilylation. Addition of KOt Bu instead of N-donor also showed the same acceleration and inhibition effects on ketone and olefin hydrosilylations, respectively.

Electrochemical behavior of a Rh(pentamethylcyclopentadienyl) complex bearing an NAD+/NADH-functionalized ligand.

A RhCp* (Cp* = pentamethylcyclopentadienyl) complex bearing an NAD+/NADH-functionalized ligand, [RhCp*(pbn)Cl]Cl ([1]Cl, pbn = (2-(2-pyridyl)benzo[b]-1,5-naphthyridine)), was synthesized. The cyclic voltammogram of [1]Cl in CH3CN shows two reversible redox waves at E1/2 = -0.58 and -1.53 V (vs. the saturated calomel electrode (SCE)), which correspond to the RhIII/RhI and pbn/pbn˙- redox couples, respectively. The addition of acetic acid to the solution afforded the proton-coupled two-electron reduction of [1]Cl at -0.62 V, from which [RhCp*(pbnHH)Cl]+ was selectively generated, probably via a hydride transfer from a RhIII-hydride intermediate to the pbn ligand. Complex [1]Cl is stable under acidic conditions, whereas a methyl proton of the Cp* moiety dissociates under basic conditions. The resulting anionic methylene group attacks the para carbon of the free pyridine of pbn, accompanied by protonation of the nitrogen atom of the ligand. As a result, treatment of [1]Cl with a base produces selectively the cyclic complex [1CH]Cl, which bears a reduced pbn framework (pbnCH). [1CH]Cl forms 1 : 1 adducts with PhCOO-via hydrogen bonding. A similar adduct, formed by a Ru-pbnHH scaffold and RCOO- (R = CH3, C6H5), has been reported to react with CO2 to produce HCOO- under concomitant regeneration of Ru-pbn. The adduct of [1CH]Cl with PhCOO-, however, lacks such hydride-donor ability, due to a steric barrier in the molecular structure of [1CH]Cl, which hampers the hydride transfer.

Catalytic Hydride Transfer to CO2 Using Ru-NAD-Type Complexes under Electrochemical Conditions.

The catalytic performance of Ru-NAD-type complexes [Ru(tpy)(pbn)(CO)]2+ ([1]2+; tpy = 2,2';6',2″-terpyridine; pbn = 2-(pyridin-2-yl)benzo[b][1,5]naphthyridine) and the Ru-CO-bridged metallacycle [2]+ was investigated in the context of the electrochemical reduction of CO2 in H2O/CH3CN at room temperature. A controlled-potential electrolysis of [1]2+ and [2]+ afforded formate (HCOO-) as the main product, under concomitant formation of minor amounts of CO and H2. Metallacycle [2]+ showed a higher selectivity toward the formation of HCOO- than [1]2+ (HCOO-/CO for [1]2+, 2.7; HCOO-/CO for [2]+, 7). The generation of HCOO- via a catalytic hydride transfer from the NADH-type ligands of [1]2+ and [2]+ to CO2 was supported by the experimental results and a comparison with the reduction of CO2 catalyzed by [Ru(tpy)(bpy)(CO)]2+ under similar conditions. A mechanism for the catalytic reduction of CO2 by [1]2+ and [2]+ was proposed based on the experimental evidence. The thus-obtained results may help to expand the field of NADH-assisted reduction reactions.

Base assisted C-C coupling between carbonyl and polypyridyl ligands in a Ru-NADH-type carbonyl complex.

A reaction of a ruthenium(ii) NAD-type complex, [Ru(tpy)(pbn)(Cl)]+ (tpy = 2,2':6',2''-terpyridine; pbn = 2-(pyridin-2-yl)benzo[b][1,5]naphthyridine), with pressurized CO (2 MPa) at 150 °C in H2O selectively produced a two-electron reduced ruthenium(ii)-NADH-type carbonyl complex, [Ru(tpy)(pbnHH)(CO)]2+ (pbnHH = 2-(pyridin-2-yl)-5,10-dihydrobenzo[b][1,5]naphthyridine), rather than the oxidized [Ru(tpy)(pbn)(CO)]2+ complex. Indeed, [Ru(tpy)(pbnHH)(CO)]2+ was quantitatively oxidized to [Ru(tpy)(pbn)(CO)]2+ upon treatment with one equiv. of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ). The reactivity of [Ru(tpy)(pbnHH)(CO)]2+ with various bases was studied herein. Treatment of [Ru(tpy)(pbnHH)(CO)]2+ with a suitable organic base, 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), resulted in the formation of a new five-membered Ru-CO-bridge metallacycle quantitatively in acetonitrile under air at room temperature. A probable mechanism was proposed for this reaction based on UV-vis, NMR, and EPR spectral studies and other experimental data. Furthermore, a reaction of the five membered Ru-CO-bridge metallacycle with NH4PF6 in CH3CN : H2O (1 : 1) under air smoothly produced another new six-membered Ru-OCO-bridge complex. A mechanism for the formation of a Ru-OCO-bridge complex was also proposed here on the basis of H2O18 experiments, DDQ treatment and other experimental data. These newly synthesized complexes appended with NAD-type ligands may have potential use as renewable hydride sources for organic reductions.

Four-Electron Reduction of a New Ruthenium Dicarbonyl Complex Having Two NAD Model Ligands through Decarboxylation in Water.

Three Ru-CO complexes, [Ru(pbn)2(CO)2]2+, [Ru(pbn)2(CO)(COOH)]+, [Ru(pbn)2(CO)(COO)]0 [pbn = 2-(pyridin-2-yl)benzo[b]-1,5-naphthyridine], exist as equilibrium mixtures in aqueous solutions. Thermal decarboxylation of [Ru(pbn)2(CO)(COOH)]+ and/or [Ru(pbn)2(CO)(COO)]0 induces a two-electron reduction of pbn to form [Ru(pbn)(pbnHH)(CO)(OH2)]2+ [pbnHH = 2-(pyridin-2-yl)-5,10-dihydrobenzo[b]-1,5-naphthyridine] in H2O.

Photochemical Properties and Reactivity of a Ru Compound Containing an NAD/NADH-Functionalized 1,10-Phenanthroline Ligand.

An NAD/NADH-functionalized ligand, benzo[b]pyrido[3,2-f][1,7]-phenanthroline (bpp), was newly synthesized. A Ru compound containing the bpp ligand, [Ru(bpp)(bpy)2](2+), underwent 2e(-) and 2H(+) reduction, generating the NADH form of the compound, [Ru(bppHH)(bpy)2](2+), in response to visible light irradiation in CH3CN/TEA/H2O (8/1/1). The UV-vis and fluorescent spectra of both [Ru(bpp)(bpy)2](2+) and [Ru(bppHH)(bpy)2](2+) resembled the spectra of [Ru(bpy)3](2+). Both complexes exhibited strong emission, with quantum yields of 0.086 and 0.031, respectively; values that are much higher than those obtained from the NAD/NADH-functionalized complexes [Ru(pbn)(bpy)2](2+) and [Ru(pbnHH)(bpy)2](2+) (pbn = (2-(2-pyridyl)benzo[b]-1.5-naphthyridine, pbnHH = hydrogenated form of pbn). The reduction potential of the bpp ligand in [Ru(bpp)(bpy)2](2+) (-1.28 V vs SCE) is much more negative than that of the pbn ligand in [Ru(pbn)(bpy)2](2+) (-0.74 V), although the oxidation potentials of bppHH and pbnHH are essentially equal (0.95 V). These results indicate that the electrochemical oxidation of the dihydropyridine moiety in the NADH-type ligand was independent of the π system, including the Ru polypyridyl framework. [Ru(bppHH)(bpy)2](2+) allowed the photoreduction of oxygen, generating H2O2 in 92% yield based on [Ru(bppHH)(bpy)2](2+). H2O2 production took place via singlet oxygen generated by the energy transfer from excited [Ru(bppHH)(bpy)2](2+) to triplet oxygen.

Photochemical Reduction of Low Concentrations of CO2 in a Porous Coordination Polymer with a Ruthenium(II)-CO Complex.

Direct use of low pressures of CO2 as a C1 source without concentration from gas mixtures is of great interest from an energy-saving viewpoint. Porous heterogeneous catalysts containing both adsorption and catalytically active sites are promising candidates for such applications. Here, we report a porous coordination polymer (PCP)-based catalyst, PCP-Ru(II) composite, bearing a Ru(II) -CO complex active for CO2 reduction. The PCP-Ru(II) composite showed improved CO2 adsorption behavior at ambient temperature. In the photochemical reduction of CO2 the PCP-Ru(II) composite produced CO, HCOOH, and H2 . Catalytic activity was comparable with the corresponding homogeneous Ru(II) catalyst and ranks among the highest of known PCP-based catalysts. Furthermore, catalytic activity was maintained even under a 5 % CO2 /Ar gas mixture, revealing a synergistic effect between the adsorption and catalytically active sites within the PCP-Ru(II) composite.

Reactivity of CO2 Activated on Transition Metals and Sulfur Ligands.

Dicationic dicarbonyl [Ru(bpy)2(CO)2](2+) (bpy = 2,2'- bipyridyl) exists as equilibrium mixtures with [Ru(bpy)2(CO)(COOH)](+) and [Ru(bpy)2(CO)(CO2)](0) depending on the pH in H2O. Those three complexes work as the precursors to CO, HCOOH production, and CO2 carrier, respectively, in electro- and photochemical CO2 reduction in aqueous solutions. However, [Ru(bpy)2(CO)2](2+) loses the catalytic activity toward CO2 reduction under aprotic conditions because [Ru(bpy)2(CO)2](2+) is not regenerated from [Ru(bpy)2(CO)(CO2)](0) in the absence of proton sources. Analogous monocarbonylruthenium complexes such as [Ru(tpy)(bpy)(CO)](2+) and [Ru(bpy)2(qu)(CO)](2+) catalyze CO2 reduction in the absence and presence of proton sources. Both complexes are reproduced through oxide transfer from the corresponding Ru-CO2 to CO2 in CO2 reduction and produce the same amount of CO and CO3(2-) in the absence of proton donors. The reduction of CO2 catalyzed by polypyridylrhenium complexes in the presence of proton sources takes place via essentially the similar mechanism as that in the case of ruthenium complexes. On the other hand, CO evolution in CO2 reduction under aprotic conditions is ascribed to the dissociation of CO from a dimeric Re-C(O)OC(O)O-Re scaffold. Visible-light irradiation to a catalytic system composed of [Ru(bpy)2(CO)2](2+)/[Ru(bpy)3](2+)/Me2NH2(+)/Me2NH as the catalyst, photosensitizer, proton donor, and nucleophile in addition to the electron donor, respectively, in CO2-saturated CH3CN selectively produces N,N-dimethylformamide without concomitant CO and HCOOH formation. Structurally robust µ3-S of reduced metal-sulfur clusters provides a suitable site for reductive activation of CO2 with retention of the framework. Indeed, CO2 activated on µ3-S of [Fe6Mo2S8(SEt)3](5-) is fixed at the carbonyl carbon of thioesters trapped on a neighboring iron of the cluster, and α-keto acids are produced catalytically. Furthermore, two-electron reduction of [(CpMen)3M3S3](2+) (n = 1, M = Co; n = 5, M = Rh, Ir) creates the catalytic ability to produce oxalate through the coupling of two CO2 molecules possibly activated on µ3-S and a metal ion.

Proton-induced generation of remote N-heterocyclic carbene-Ru complexes.

The proton-induced Ru-C bond variation, which was previously found to be relevant in the water oxidation, has been investigated by using cyclometalated ruthenium complexes with three phenanthroline (phen) isomers. The designed complexes, [Ru(bpy)2 (1,5-phen)](+) ([2](+)), [Ru(bpy)2 (1,6-phen)](+) ([3](+)), and [Ru(bpy)2 (1,7-phen)](+) ([4](+)) were newly synthesized and their structural and electronic properties were analyzed by various spectroscopy and theoretical protocols. Protonation of [4](+) triggered profound electronic structural change to form remote N-heterocyclic carbene (rNHC), whereas protonation of [2](+) and [3](+) did not affect their structures. It was found that changes in the electronic structure of phen beyond classical resonance forms control the rNHC behavior. The present study provides new insights into the ligand design of related ruthenium catalysts.

Selective generation of formamides through photocatalytic CO2 reduction catalyzed by ruthenium carbonyl compounds.

The selective formation of dialkyl formamides through photochemical CO2 reduction was developed as a means of utilizing CO2 as a C1 building block. Photochemical CO2 reduction catalyzed by a [Ru(bpy)2(CO)2](2+) (bpy: 2,2'-bipyridyl)/[Ru(bpy)3](2+)/Me2NH/Me2NH2(+) system in CH3CN selectively produced dimethylformamide. In this process a ruthenium carbamoyl complex ([Ru(bpy)2(CO)(CONMe2)](+)) formed by the nucleophilic attack of Me2NH on [Ru(bpy)2(CO)2](2+) worked as the precursor to DMF. Thus Me2NH acted as both the sacrificial electron donor and the substrate, while Me2NH2(+) functioned as the proton source. Similar photochemical CO2 reductions using R2NH and R2NH2(+) (R = Et, nPr, or nBu) also afforded the corresponding dialkyl formamides (R2NCHO) together with HCOOH as a by-product. The main product from the CO2 reduction transitioned from R2NCHO to HCOOH with increases in the alkyl chain length of the R2NH. The selectivity between R2NCHO and HCOOH was found to depend on the rate of [Ru(bpy)2(CO)(CONR2)](+) formation.

Approach to multi-electron reduction beyond two-electron reduction of CO2.

Dicationic [Ru(bpy)2(CO)2](2+) (bpy = 2,2'-bipyridyl) exists as an equilibrium mixture of [Ru(bpy)2(CO)(COOH)](+) and [Ru(bpy)2(CO)(CO2)] in H2O. Photo- and electrochemical reduction of [Ru(bpy)2(CO)2](2+) in the presence of proton sources under CO2 produced CO and HCOOH with generation of [Ru(bpy)2(CO)(CO2)], which spontaneously comes to an equilibrium of [Ru(bpy)2(CO)(COOH)](+) and [Ru(bpy)2(CO)2](2+). Two-electron reduction of CO2, giving CO and/or HCOOH depending on the proton concentration, is ascribed to reductive cleavage of M-COOH and M-CO bonds derived from M-CO2 complexes. Catalytic DMF production in the electrochemical reduction of [Ru(bpy)2(CO)2](2+) in the presence of Me2NH under CO2 is also explained by reductive cleavage of the Ru-C(O)NMe2 bond. On the other hand, the treatment of [Ru(bpy)2(CO)2](2+) with BH4(-) produced CH3OH via [Ru(bpy)2(CO)(CHO)](+) and [Ru(bpy)2(CO)(CH2OH)](+), indicating that reduction of M-CO complexes with renewable hydride donors is a feasible pathway to realize catalytic six-electron reduction of CO2 affording CH3OH. Along these lines, [Ru(bpy)2(pbn)](2+) (pbn = 2-(2-pyridyl)benzo[b]-1,5-naphthyridine) was prepared to simulate the biological role of the NAD/NADH redox couple. Irradiation of visible light on[Ru(bpy)2(pbn)](2+) in the presence of a sacrificial electron donor produced smoothly the two-electron reduced form, [Ru(bpy)2(pbnHH)](2+) in a quantum yield of 20%. Furthermore, hydride transfer from [Ru(bpy)2(pbnHH)](2+) to CO2 took place in the presence of base to regenerate [Ru(bpy)2(pbn)](2+) accompanied by HCOO(-) formation.

Fabrication and placement of a ring structure of nanoparticles by a laser-induced micronanobubble on a gold surface.

We have developed a new fabrication method for a ring structure of assembled nanoparticles on a gold surface by the use of continuous Nd:YAG laser light. A micronanobubble on a gold surface, created by laser local heating, acts as a template for the formation of the ring structure. Both Marangoni convection flow and capillary flow around the micronanobubble are responsible for the driving force to assemble nanoparticles such as CdSe Q-dots into the ring structure from the solution. Because a single micronanobubble was generated by the Nd:YAG laser focusing point, the precise positioning of the ring structure was feasible directly on the gold surface, which makes it possible to fabricate various patterns of rings such as arrays and letters and even a double-ring structure without any photomasks or any templates.

Proton-induced tuning of metal-metal communication in rack-type dinuclear Ru complexes containing benzimidazolyl moieties.

Ru complexes bearing a bis-tridentate benzimidazolyl ligand have been synthesized. The dinuclear ones act as a bibasic acid with pK(a1)=4.36 and pK(a2)=5.90. The protonated form of the dinuclear complex exhibited two one-electron oxidations at +0.91 and +1.02 V versus the ferrocenium/ferrocene (Fc/Fc(+)) couple (the potential difference (ΔE)=0.11 V), but the di-deprotonated form showed two waves at +0.50 and +0.58 V versus Fc/Fc(+) (ΔE=0.08 V). Since the potential difference between two waves reflects the strength of the metal-metal interaction, the deprotonation of the benzimidazole moieties in the complexes weakened the Ru-Ru communication. The degree of electronic coupling between two metal centers, estimated from the intervalence charge transfer (IVCT) band, was greater for the protonated form. DFT calculations for the protonated and deprotonated forms of the dinuclear complex suggest that the Ru(II)-L(H(2)) π* interaction plays a key role in the Ru-Ru interaction.

Proton-induced dynamic equilibrium between cyclometalated ruthenium rNHC (remote N-heterocyclic carbene) tautomers with an NAD+/NADH function.

Cyclometalated ruthenium(II) complexes having acridine moieties have been synthesized and characterized by spectroscopic methods. Protonation of the acridine nitrogen of the ruthenium(II) complexes not only causes dynamic equilibrium with remote N-heterocyclic carbene Ru═C complexes but also generates the NAD(+)/NADH redox function driven by a proton-coupled two-electron transfer accompanying a reversible C-H bond formation in the pyridinium ring.

Observation of DNA pinning at laser focal point on Au surface and its application to single DNA nanowire and cross-wire formation.

We report a new technique for fabricating a single DNA nanowire at a desired position in a sequential manner using the micronanobubble generated by laser local heating at the Au/water interface. In our previous report, we found the reversible pull-in/shrinkage of one end immobilized DNA strands near a Nd:YAG laser focal point on an Au surface. In further experiments, the pinning of DNA strands in the stretched state was observed on the Au surface only when the bubble has touched the free end of DNA. This pinning phenomenon was observed even on the alkane thiol modified Au surface as self-assembled monolayers (SAMs) such as hexanethiol, mercaptohexanol, and hexadecanethiol. However, no pinning was observed on the bovine serum albumin (BSA) modified surface. Since optical tweezers can manipulate a DNA modified bead (radius=1.87 µm), the bead was firstly fixed on a solid surface by being compressed with the optical tweezers, and the pulling and pinning of DNA on the bead were achieved. As a consequence, the laser local heating on the Au surface enables us to control the number and position of the one end immobilized DNA strands as DNA nanowires.

Syntheses and photophysical properties of optical-active blue-phosphorescent iridium complexes bearing asymmetric tridentate ligands.

Novel blue-phosphorescent iridium complexes bearing 1-(2-pyridyl)-3-(N-methylimidazol-2-yl)-4,6-difluorobenzene(LH) as an asymmetric tridentate ligand have been optically resolved into a pair of enantiomers that exhibits opposite circularly polarized luminescence (CPL) spectra.