Synthesis and Photophysical Properties of Silver(I) Coordination Polymers Bridged by Dimethylpyrazine: Comparison of Emissive Excited States between Silver(I) and Copper(I) Congeners.

Highly luminescent silver(I) coordination polymers [Ag2X2(PPh3)2(Me2pyz)]n (X = I, Br, Cl; Me2pyz: 2,5-dimethylpyrazine) were prepared together with copper congeners [Cu2X2(PPh3)2(Me2pyz)]n (X = I, Br). All the complexes showed thermally activated delayed fluorescence from the charge-transfer states in the visible region, from blue to red. The isomorphous relationship among the complexes allowed a detailed discussion of the effect of halogenido ligands and crystal packing on their luminescence energy. The relaxation in the emissive excited states (ESs) was determined to be more remarkable in silver complexes than in copper complexes despite their isomorphous structures, and the electronic effect of halogenido ligands was comparable to the effect of relaxation in emissive ESs.

An NAD+-type earth-abundant metal complex enabling photo-driven alcohol oxidation.

For the first time, an NAD+-type earth-abundant metal complex [Zn(pbn)2(H2O)](ClO4)2 (1) was found to exhibit photo-induced oxidizing ability to convert various primary and secondary alcohols to the corresponding aldehyde and ketone compounds. In addition, a two-electron-reduced Zn(II) complex [Zn(pbnH-pbnH)(ClO4)2] (1red) comprising the novel C-C coupling ligand, obtained by the photo-induced oxidation of alcohols by 1, was successfully isolated and completely characterized. We clarified that the photochemical oxidation of alcohols by 1 to produce 1red proceeds via an electron transfer followed by proton transfer mechanism as elucidated by kinetic analysis on the basis of absorption spectroscopic measurements.

Synthesis and Photophysical Properties of Emissive Silver(I) Halogenido Coordination Polymers Composed of {Ag2X2} Units Bridged by Pyrazine, Methylpyrazine, and Aminopyrazine.

Luminescent silver(I) coordination polymers having a {Ag2(µ-X)2} rhombic core (X = I, Br) were prepared using pyrazine (pyz), methylpyrazine (Mepyz), and aminopyrazine (ampyz) as bridging ligands. Photophysical measurements show that the complexes were strongly luminescent in the solid state at room temperature; further, the emissive excited state of the pyz and Mepyz complexes was a triplet charge-transfer (3CT) excited state, similar to that of their copper(I) congeners, whereas that of the ampyz complex was a intraligand (3IL) excited state. The energy of the 3CT excited state of a silver halogenido complex was revealed to be ca. 5000 cm-1 higher than that of the corresponding copper complex.

A Novel Photo-Driven Hydrogenation Reaction of an NAD+-Type Complex Toward Artificial Photosynthesis.

The photocatalytic reduction of carbon dioxide (CO2) to value-added chemicals is an attractive strategy to utilize CO2 as a feedstock for storing renewable energy, such as solar energy, in chemical bonds. Inspired by the biological function of the nicotinamide adenine dinucleotide redox couple (NAD+/NADH), we have been developing transition-metal complexes containing NAD+/NADH-functionalized ligands to create electro- and/or photochemically renewable hydride donors for the conversion of CO2 into value-added chemicals. Our previous findings have provided insights for the development of photocatalytic organic hydride reduction reactions for CO2, however, further examples, as well as investigation, of these photo-driven NAD+/NADH-type hydrogenation and organic hydride transfer reactions are required not only to explore the mechanism in detail but also to develop a highly efficient catalyst for artificial photosynthesis. In this paper, we report the synthesis, characterization, and photo-induced NAD+/NADH conversion properties of a new ruthenium(II) complex, [Ru(bpy)2(Me-pn)](PF6)2 (1), which contains a new NAD+-type ligand, Me-pn (2-methyl-6-(pyridin-2-yl)-1,5-naphthyridine). In addition, we have succeeded in the isolation of the corresponding two-electron reduced ruthenium(II) complex containing the NADH-type ligand Me-pnHH (2-methyl-6-(pyridin-2-yl)-1,4-dihydro-1,5-naphthyridine), i.e., [Ru(bpy)2(Me-pnHH)](PF6)2 (1HH), by the photo-induced hydrogenation reaction of 1. Thus, in this study, a new photo-driven NAD+/NADH-type hydrogenation reaction for possible CO2 reduction using the NAD+/NADH redox function has been constructed.

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.

Novel synthesis of a four-electron-reduced ruthenium(ii) NADH-type complex under water-gas-shift reaction conditions.

A four-electron-reduced ruthenium(ii) NADH-type complex, [Ru(bbnpH4)(CO)2Cl](PF6) (bbnpH4 = 2,2'-(4-(tert-butyl)pyridine-2,6-diyl)bis(5,10-dihydrobenzo[b][1,5]naphthyridine)), has been successfully synthesized by mixing an NAD+-type ligand, bbnp (bbnp = 2,2'-(4-(tert-butyl)pyridine-2,6-diyl)bis(benzo[b][1,5]naphthyridine)), and [Ru(CO)2Cl2] under moderate water-gas-shift reaction conditions, which has been fully characterized by single-crystal X-ray structure analysis, ESI-TOF mass spectrometry, and NMR spectroscopy.

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.

Synthesis, structures and stability of amido gold(iii) complexes with 2,2':6',2''-terpyridine.

Three gold(iii) complexes with terminal amido ligands were prepared by the reaction of [Au(III)(trpy)(OH)](ClO4)2 and primary amines having electron-withdrawing groups such as 2-amino-4-chloropyrimidine, 2-amino-5-chloro-pyridine, and 2-aminopyrimidine. Conversion of the amido ligands into the imido or amine ligands resulted in the decomposition of the complexes by intramolecular redox reaction or the release of amine ligands, respectively.

An organic hydride transfer reaction of a ruthenium NAD model complex leading to carbon dioxide reduction.

Ruthenium will fix it: CO(2) undergoes reduction to HCO(2)(-) when placed over a solution of a ruthenium complex bearing an NADH model ligand 1 (black in right structural formula). The organic hydride transfer is triggered by the addition of benzoate anion, which rapidly forms a complex with 1, a complex that is a stronger reductant than 1. A photocatalytic variant of the reaction using triethanolamine as a sacrificial reagent has also been developed.

Catalytic four-electron oxidation of water by intramolecular coupling of the oxo ligands of a bis(ruthenium-bipyridine) complex.

A bis(ruthenium-bipyridine) complex bridged by 1,8-bis(2,2':6',2''-terpyrid-4'-yl)anthracene (btpyan), [Ru(2)(µ-Cl)(bpy)(2)(btpyan)](BF(4))(3) ([1](BF(4))(3); bpy = 2,2'-bipyridine), was prepared. The cyclic voltammogram of [1](BF(4))(3) in water at pH 1.0 displayed two reversible [Ru(II),Ru(II)](3+)/[Ru(II),Ru(III)](4+) and [Ru(II),Ru(III)](4+)/[Ru(III),Ru(III)](5+) redox couples at E(1/2)(1) = +0.61 and E(1/2)(2) = +0.80 V (vs. Ag/AgCl), respectively, and an irreversible anodic peak at around E = +1.2 V followed by a strong anodic currents as a result of the oxidation of water. The controlled potential electrolysis of [1](3+) ions at E = +1.60 V in water at pH 2.6 (buffered with H(3)PO(4)/NaH(2)PO(4)) catalytically evolved dioxygen. Immediately after the electrolysis of the [1](3+) ion in H(2)(16)O at E = +1.40 V, the resultant solution displayed two resonance Raman bands at nu = 442 and 824 cm(-1). These bands shifted to nu = 426 and 780 cm(-1), respectively, when the same electrolysis was conducted in H(2)(18)O. The chemical oxidation of the [1](3+) ion by using a Ce(IV) species in H(2)(16)O and H(2)(18)O also exhibited the same resonance Raman spectra. The observed isotope frequency shifts (Δnu = 16 and 44 cm(-1)) fully fit the calculated ones based on the Ru-O and O-O stretching modes, respectively. The first successful identification of the metal-O-O-metal stretching band in the oxidation of water indicates that the oxygen-oxygen bond at the stage prior to the evolution of O(2) is formed through the intramolecular coupling of two Ru-oxo groups derived from the [1](3+) ion.

Drastic difference in the photo-driven hydrogenation reactions of ruthenium complexes containing NAD model ligands.

Successful control of photo-driven NAD(+)/NADH type hydrogenation reactions in ruthenium complexes has been accomplished by using a new NAD(+) model ligand with modulated distortion of the ligand taking advantage of the substituent effect.

Photoinduced four- and six-electron reduction of mononuclear ruthenium complexes having NAD+ analogous ligands.

The ruthenium complexes [Ru(bpy)(pbn)(2)](PF(6))(2) ([2](2+); bpy = 2,2'-bipyridine, pbn = 2-(2-pyridyl)benzo[b]-1,5-naphthyridine) and [Ru(pbn)(3)](PF(6))(2) ([3](2+)) were synthesized. Photoirradiation (λ > 420 nm) of [2](2+) and [3](2+) in CH(3)CN/triethanolamine (TEOA) brought about proton coupled four- and six-electron reduction of the complexes to produce [Ru(bpy)(pbnH(2))(2)](PF(6))(2) ([2·H(4)](2+); pbnH(2) = 5,10-dihydro-2-(2-pyridyl)benzo[b]-1,5-naphthyridine) and [Ru(pbnH(2))(3)](PF(6))(2) ([3·H(6)](2+)), respectively. The photoexcited [Ru(III)(bpy)(pbn˙(-))(pbnH(2))](2+) intermediate is quenched by intermolecular electron transfer from TEOA to Ru(III), while intramolecular transfer from pbnH(2) to Ru(III) is negligible. As a result, novel photochemical four- and six-electron reduction of [2](2+) and [3](2+) is achieved through repetition of the two-electron reduction of the Ru-pbn group. The high efficiency photochemical two-, four- and six-electron reductions of [Ru(bpy)(2)(pbn)](2+) ([1](2+)), [2](2+) and [3](2+), respectively, by taking advantage of proton coupled two electron reduction of NAD(+) analogous type ligands such as pbn opens a general pathway for multi-electron reduction of metal complexes via illumination with visible light.

Novel emission properties of melem caused by the heavy metal effect of lanthanides(III) in a LB film.

A novel emissive molecular system is constructed by the intercalation of the fluorophore melem (triamino-tri-s-triazine) within a Langmuir-Blodgett (LB) film of stearic acid with the periodic arrangement of lanthanides (Ln(III)), mainly Pr(III) with supporting of Eu(III). From emission spectra, decay curves, quantum yields and XPS measurements, it is clarified that the external heavy metal effect of Pr(III) on melem is much stronger in the film than in the bulk solid state, resulting in producing an unusual triplet state of melem. The triplet state of melem in the LB film donates the excitation energy to Pr(III) in the LB film, which is completely different from the energy transfer pathway of Pr-melem complex in the solid state through the singlet state of melem.

Molecular distortion effect on ff-emission in a Pr(III) complex with 4,7-diphenyl-1,10-phenanthroline.

The electronic and structural behaviour of a Pr(III) complex with 4,7-diphenyl-1,10-phenanthroline, [Pr(bathophen)(2)(NO(3))(3)], is investigated with respect to the effect of configuration changes on the Pr(III) centre. [Pr(bathophen)(2)(NO(3))(3)] luminesces from the excited states of the ligand and the metal ion. The fluorescence, ff-emission ((1)D(2)-->(3)H(4)), and phosphorescence bands appear at 394, 608.2 and 482 nm, respectively, in the solid state. In acetonitrile, the complex also shows multiple emissions. From the time-resolved emission and the lifetime measurements, the excitation energy-transfer in [Pr(bathophen)(2)(NO(3))(3)] is clarified, that is, the upper excited triplet level of the ligand acts as an energy donor, while the (1)D(2) levels of Pr(III) is the acceptor. Additionally, the emission phenomena of the complex can be modified by molecular distortion, particularly by rotation of the phenyl groups in the ligand.

Sequential reaction intermediates in aliphatic C-H bond functionalization initiated by a bis(mu-oxo)dinickel(III) complex.

The reaction of [Ni2(OH)2(Me2-tpa)2]2+ (1) (Me2-tpa = bis(6-methyl-2-pyridylmethyl)(2-pyridylmethyl)amine) with H2O2 causes oxidation of a methylene group on the Me2-tpa ligand to give an N-dealkylated ligand and oxidation of a methyl group to afford a ligand-based carboxylate and an alkoxide as the final oxidation products. A series of sequential reaction intermediates produced in the oxidation pathways, a bis(mu-oxo)dinickel(III) ([Ni2(O)2(Me2-tpa)2]2+ (2)), a bis(mu-superoxo)dinickel(II) ([Ni2(O2)2(Me2-tpa)2]2+ (3)), a (mu-hydroxo)(mu-alkylperoxo)dinickel(II) ([Ni2(OH)(Me2-tpa)(Me-tpa-CH2OO)]2+ (4)), and a bis(mu-alkylperoxo)dinickel(II) ([Ni2(Me-tpa-CH2OO)2]2+ (5)), was isolated and characterized by various physicochemical measurements including X-ray crystallography, and their oxidation pathways were investigated. Reaction of 1 with H2O2 in methanol at -40 degrees C generates 2, which is extremely reactive with H2O2, producing 3. Complex 2 was isolated only from disproportionation of the superoxo ligands in 3 in the absence of H2O2 at -40 degrees C. Thermal decomposition of 2 under N2 generated an N-dealkylated ligand Me-dpa ((6-methyl-2-pyridylmethyl)(2-pyridylmethyl)amine) and a ligand-coupling dimer (Me-tpa-CH2)2. The formation of (Me-tpa-CH2)2 suggests that a ligand-based radical Me-tpa-CH2* is generated as a reaction intermediate, probably produced by H-atom abstraction by the oxo group. An isotope-labeling experiment revealed that intramolecular coupling occurs for the formation of the coupling dimer. The results indicate that the rebound of oxygen to Me-tpa-CH2* is slower than that observed for various high-valence bis(mu-oxo)dimetal complexes. In contrast, the decomposition of 2 and 3 in the presence of O2 gave carboxylate and alkoxide ligands, respectively (Me-tpa-COO- and Me-tpa-CH2O-), instead of (Me-tpa-CH2)2, indicating that the reaction of Me-tpa-CH2* with O2 is faster than the coupling of Me-tpa-CH2* to generate ligand-based peroxyl radical Me-tpa-CH2OO*. Although there is a possibility that the Me-tpa-CH2OO* species could undergo various reactions, one of the possible reactive intermediates, 4, was isolated from the decomposition of 3 under O2 at -20 degrees C. The alkylperoxo ligands in 4 and 5 can be converted to a ligand-based aldehyde by either homolysis or heterolysis of the O-O bond, and disproportionation of the aldehyde gives a carboxylate and an alkoxide via the Cannizzaro reaction.

Electronic structural changes between nickel(II)-semiquinonato and nickel(III)-catecholato states driven by chemical and physical perturbation.

The selective synthesis of tetracoordinate square-planar low-spin nickel(II)-semiquinonato (Ni(II)-SQ) and nickel(III)-catecholato (Ni(III)-Cat) complexes, 1 and 2, respectively, was achieved by using bidentate ligands with modulated nitrogen-donor ability to the nickel ion. The electronic structures of 1 and 2 were revealed by XPS and EPR measurements. The absorption spectra of 1 and 2 in a noncoordinating solvent, dichloromethane (CH2Cl2), are completely different from those in tetrahydrofuran (THF), being a coordinating solvent. As expected from this result, the gradual addition of N,N-dimethylformamide (DMF), which is also a coordinating solvent like THF, into a solution of 1 or 2 in CH2Cl2 leads to color changes from blue (for 1) and brown (for 2) to light green, which is the same color observed for solutions of 1 or 2 in THF. Furthermore, the same color changes are induced by varying the temperature. Such spectral changes are attributable to the transformation from square-planar low-spin Ni(II)-SQ and Ni(III)-Cat complexes to octahedral high-spin Ni(II)-SQ ones, caused by the coordination of two solvent molecules to the nickel ion.

Equilibrium of low- and high-spin states of Ni(II) complexes controlled by the donor ability of the bidentate ligands.

Low-spin nickel(II) complexes containing bidentate ligands with modulated nitrogen donor ability, Py(Bz)2 or MePy(Bz)2 (Py(Bz)2 = N,N-bis(benzyl)-N-[(2-pyridyl)methyl]amine, MePy(Bz)2 = N,N-bis(benzyl)-N-[(6-methyl-2-pyridyl)methyl]amine), and a beta-diketonate derivative, tBuacacH (tBuacacH = 2,2,6,6-tetramethyl-3,5-heptanedione), represented as [Ni(Py(Bz)2)(tBuacac)](PF6) (1) and [Ni(MePy(Bz)2)(tBuacac)](PF6) (2) have been synthesized. In addition, the corresponding high-spin nickel(II) complexes having a nitrate ion, [Ni(Py(Bz)2)(tBuacac)(NO3)] (3) and [Ni(MePy(Bz)2)(tBuacac)(NO3)] (4), have also been synthesized for comparison. Complexes 1 and 2 have tetracoordinate low-spin square-planar structures, whereas the coordination environment of the nickel ion in 4 is a hexacoordinate high-spin octahedral geometry. The absorption spectra of low-spin complexes 1 and 2 in a noncoordinating solvent, dichloromethane (CH2Cl2), display the characteristic absorption bands at 500 and 540 nm, respectively. On the other hand, the spectra of a CH2Cl2 solution of high-spin complexes 3 and 4 exhibit the absorption bands centered at 610 and 620 nm, respectively. The absorption spectra of 1 and 2 in N,N-dimethylformamide (DMF), being a coordinating solvent, are quite different from those in CH2Cl2, which are nearly the same as those of 3 and 4 in CH2Cl2. This result indicates that the structures of 1 and 2 are converted from a low-spin square-planar to a high-spin octahedral configuration by the coordination of two DMF molecules to the nickel ion. Moreover, complex 1 shows thermochromic behavior resulting from the equilibrium between low-spin square-planar and high-spin octahedral structures in acetone, while complex 2 exists only as a high-spin octahedral configuration in acetone at any temperature. Such drastic differences in the binding constants and thermochromic properties can be ascribed to the enhancement of the acidity of the nickel ion of 2 by the steric effect of the o-methyl group in the MePy(Bz)2 ligand in 2, which weakens the Ni-N(pyridine) bond length compared with that of the nonsubstituted Py(Bz)2 ligand in 1.

Characterization of a stable ruthenium complex with an oxyl radical.

The ruthenium oxyl radical complex, [Ru(II)(trpy)(Bu(2)SQ)O(.-)] (trpy = 2,2':6',2"-terpyridine, Bu(2)SQ = 3,5-di-tert-butyl-1,2-benzosemiquinone) was prepared for the first time by the double deprotonation of the aqua ligand of [Ru(III)(trpy)(Bu(2)SQ)(OH(2))](ClO(4))(2). [Ru(III)(trpy)(Bu(2)SQ)(OH(2))](ClO(4))(2) is reversibly converted to [Ru(III)(trpy)(Bu(2)SQ)(OH-)](+) upon dissociation of the aqua proton (pK(a) 5.5). Deprotonation of the hydroxo proton gave rise to intramolecular electron transfer from the resultant O(2-) to Ru-dioxolene. The resultant [Ru(II)(trpy)(Bu(2)SQ)O(.-)] showed antiferromagnetic behavior with a Ru(II)-semiquinone moiety and oxyl radical, the latter of which was characterized by a spin trapping technique. The most characteristic structural feature of [Ru(II)(trpy)(Bu(2)SQ)O(.-)] is a long Ru-O bond length (2.042(6) A) as the first terminal metal-O bond with a single bond length. To elucidate the substituent effect of a quinone ligand, [Ru(III)(trpy)(4ClSQ)(OH(2))](ClO(4))(2) (4ClSQ = 4-chloro-1,2-benzosemiquinone) was prepared and we compared the deprotonation behavior of the aqua ligand with that of [Ru(III)(trpy)(Bu(2)SQ)(OH(2))](ClO(4))(2). Deprotonation of the aqua ligand of [Ru(III)(trpy)(4ClSQ)(OH(2))](ClO(4))(2) induced intramolecular electron transfer from OH- to the [Ru(III)(4ClSQ)] moiety affording [Ru(II)(trpy)(4ClSQ)(OH.)]+, which then probably changed to [Ru(II)(trpy)(4ClSQ)O(.-)]. The antiferromagnetic interactions (J values) between Ru(II)-semiquinone and the oxyl radical for [Ru(II)(trpy)(Bu(2)SQ)O(.-)] and for [Ru(II)(trpy)(4ClSQ)O(.-)] were 2J = -0.67 cm(-1) and -1.97 cm(-1), respectively.