Mechanistic Study of a Photocatalyzed CS Bond Formation Involving Alkyl/Aryl Thiosulfate

This study presents thioether construction involving alkyl/aryl thiosulfates and diazonium salt catalyzed by visible‐light‐excited [Ru(bpy)₃Cl₂] at room temperature in 44–86 % yield. Electron paramagnetic resonance studies found that thiosulfate radical formation was promoted by K₂CO₃. Conversely, radicals generated from BnSH or BnSSBn (Bn=benzyl) were clearly suppressed, demonstrating the special property of thiosulfate in this system. Transient absorption spectra confirmed the electron‐transfer process between [Ru(bpy)₃Cl₂] and 4‐MeO‐phenyl diazonium salt, which occurred with a rate constant of 1.69×10⁹ M ^?1 s^?1. The corresponding radical trapping product was confirmed by X‐ray diffraction. The full reaction mechanism was determined together with emission quenching data. Furthermore, this system efficiently avoided the over‐oxidation of sulfide caused by H₂O in the photoexcited system containing Ru²⁺. Both aryl and heteroaryl diazonium salts with various electronic properties were investigated for synthetic compatibility. Both alkyl‐ and aryl‐substituted thiosulfates could be used as substrates. Notably, pharmaceutical derivatives afforded late‐stage sulfuration smoothly under mild conditions.     

A New Heteroleptic Ruthenium(II) Polypyridyl Complex with Long‐Wavelength Absorption and High Singlet‐Oxygen Quantum Yield

Ruthenium(II) polypyridyl complexes with long‐wavelength absorption and high singlet‐oxygen quantum yield exhibit attractive potential in photodynamic therapy. A new heteroleptic Ru^II polypyridyl complex, [Ru(bpy)(dpb)(dppn)]²⁺ (bpy=2,2′‐bipyridine, dpb=2,3‐bis(2‐pyridyl)benzoquinoxaline, dppn=4,5,9,16‐tetraaza‐dibenzo[a,c]naphthacene), is reported, which exhibits a ¹MLCT (MLCT: metal‐to‐ligand charge transfer) maximum as long as 548 nm and a singlet‐oxygen quantum yield as high as 0.43. Steady/transient absorption/emission spectra indicate that the lowest‐energy MLCT state localizes on the dpb ligand, whereas the high singlet‐oxygen quantum yield results from the relatively long ³MLCT(Ru→dpb) lifetime, which in turn is the result of the equilibrium between nearly isoenergetic excited states of ³MLCT(Ru→dpb) and ³ππ*(dppn). The dppn ligand also ensures a high binding affinity of the complex towards DNA. Thus, the combination of dpb and dppn gives the complex promising photodynamic activity, fully demonstrating the modularity and versatility of heteroleptic Ru^II complexes. In contrast, [Ru(bpy)₂(dpb)]²⁺ shows a long‐wavelength ¹MLCT maximum (551 nm) but a very low singlet‐oxygen quantum yield (0.22), and [Ru(bpy)₂(dppn)]²⁺ shows a high singlet‐oxygen quantum yield (0.79) but a very short wavelength ¹MLCT maximum (442 nm).     

Experimental and Theoretical Studies on DNA‐Binding and Spectral Properties of ‘Light Switch’ Complexes [Ru(L)2(ppn)]2+ (L=2,2′‐Bipyridine and 1,10‐Phenanthroline; ppn=Pteridino[6,7‐f] [1,10]phenanthroline‐11,13‐diamine)

The ligand pteridino[6,7‐f] [1,10]phenanthroline‐11,13‐diamine (ppn) and its Ru^II complexes [Ru(bpy)₂(ppn)]²⁺ ( 1 ; bpy=2,2′‐bipyridine) and [Ru(phen)₂(ppn)]²⁺ ( 2 ; phen=1,10‐phenanthroline) were synthesized and characterized by elemental analysis, electrospray MS, ¹H‐NMR, and cyclic voltammetry. The DNA‐binding behaviors of 1 and 2 were studied by spectroscopic and viscosity measurements. The results indicate that both complexes strongly bind to calf‐thymus DNA in an intercalative mode, with DNA‐binding constants K_b of (1.7±0.4)?10⁶ M ^?1 and (2.6±0.2)?10⁶ M ^?1, respectively. The complexes 1 and 2 exhibit excellent DNA‐‘light switch’ performances, i.e., they do not (or extremely weakly) show luminescence in aqueous solution at room temperature but are strongly luminescent in the presence of DNA. In particular, the experimental results suggest that the ancillary ligands bpy and phen not only have a significant effect on the DNA‐binding affinities of 1 and 2 but also have a certain effect on their spectral properties. [Ru(phen)₂(ppn)]²⁺( 2 ) might be developed into a very prospective DNA‐‘light switch’ complex. To explain the DNA‐binding and spectral properties of 1 and 2 , theoretical calculations were also carried out applying the DFT/TDDFT method.     

Facile CH Bond Formation by Reductive Elimination at a Dinuclear Metal Site

The electronically unsaturated dirhenium complex [Re₂(CO)₈(µ‐AuPPh₃)(µ‐Ph)] ( 1 ) was obtained from the reaction of [Re₂(CO)₈{µ‐η²‐C(H)?C(H)nBu}(µ‐H)] with [Au(PPh₃)Ph]. The bridging {AuPPh₃} group was replaced by a bridging hydrido ligand to yield the unsaturated dirhenium complex [Re₂(CO)₈(µ‐H)(µ‐Ph)] ( 2 ) by reaction of 1 with HSnPh₃. Compound 2 reductively eliminates benzene upon addition of NCMe at 25 °C. The electronic structure of 2 and the mechanism of the reductive elimination of the benzene molecule in its reaction with NCMe were investigated by DFT computational analyses.     

Application of a Structure/Oxidation‐State Correlation to Complexes of Bridging Azo Ligands

Based on data from more than 40 crystal structures of metal complexes with azo‐based bridging ligands (2,2′‐azobispyridine, 2,2′‐azobis(5‐chloropyrimidine), azodicarbonyl derivatives), a correlation between the N? N bond lengths (d_NN) and the oxidation state of the ligand (neutral, neutral/back‐donating, radical‐anionic, dianionic) was derived. This correlation was applied to the analysis of four ruthenium compounds of 2,2′‐azobispyridine (abpy), that is, the new asymmetrical rac‐[(acac)₂Ru1(μ‐abpy)Ru2(bpy)₂](ClO₄)₂ ([ 1 ](ClO₄)₂), [Ru(acac)₂(abpy)] ( 2 ), [Ru(bpy)₂(abpy)](ClO₄)₂ ([ 3 ](ClO₄)₂), and meso‐[(bpy)₂Ru(μ‐abpy)Ru(bpy)₂](ClO₄)₃ ([ 4 ](ClO₄)₃; acac^?=2,4‐pentanedionato, bpy=2,2′‐bipyridine). In agreement with DFT calculations, both mononuclear species 2 and 3 ²⁺ can be described as ruthenium(II) complexes of unreduced abpy⁰, with 1.295(5)<d_NN<1.320(3) Å, thereby exhibiting effects from π back‐donation. However, the abpy ligand in both the asymmetrical diamagnetic compound 1 ²⁺ (d_NN=1.374(6) Å) and the symmetrical compound 4 ³⁺ (d_NN=1.360(7), 1.368(8) Å) must be formulated as abpy^.?. Remarkably, the addition of [Ru^II(bpy)₂]²⁺ to mononuclear [Ru^II(acac)₂(abpy⁰)] induces intracomplex electron‐transfer under participation of the noninnocent abpy bridge to yield rac‐[(acac)₂Ru1^III(μ‐abpy^.?)Ru2^II(bpy)₂]²⁺ ( 1 ²⁺) with strong antiferromagnetic coupling between abpy^.? and Ru^III (DFT (B3LYP/LANL2DZ/6‐31G*)‐calculated triplet–singlet energy separation E_S=1?E_S=0=11739 cm^?1). Stepwise one‐electron transfer was studied for compound 1 ⁿ, n=1?, 0, 1+, 2+, 3+, by UV/Vis/NIR spectroelectrochemistry, EPR spectroscopy, and by DFT calculations. Whereas the first oxidation of compound 1 ²⁺ was found to mainly involve the central ligand to produce an (abpy⁰)‐bridged Class I mixed‐valent Ru1^IIIRu2^II species, the first reduction of compound 1 ²⁺ affected both the bridge and Ru1 atom to form a radical complex ( 1 ⁺), with considerable metal participation in the spin‐distribution. Further reduction moves the spin towards the {Ru2(bpy)₂} entity.     

O2 Activation and Double CH Oxidation by a Mononuclear Manganese(II) Complex

A Mn^II complex, [Mn(dpeo)₂]²⁺ (dpeo=1,2‐di(pyridin‐2‐yl)ethanone oxime), activates O₂, with ensuing stepwise oxidation of the methylene group in the ligands providing an alkoxide and ultimately a ketone group. X‐ray crystal‐structure analysis of an intermediate homoleptic alkoxide Mn^III complex shows tridentate binding of the ligand via the two pyridyl groups and the newly installed alkoxide moiety, with the oxime group no longer coordinated. The structure of a Mn^II complex of the final ketone ligand, cis‐[MnBr₂(hidpe)₂] (hidpe=2‐(hydroxyimino)‐1,2‐di(pyridine‐2‐yl)ethanone) shows that bidentate oxime/pyridine coordination has been resumed. H₂¹⁸O and ¹⁸O₂ labeling experiments suggest that the inserted O atoms originate from two different O₂ molecules. The progress of the oxygenation was monitored through changes in the resonance‐enhanced Raman bands of the oxime unit.     

Tuning Excited State Isomerization Dynamics through Ground State Structural Changes in Analogous Ruthenium and Osmium Sulfoxide Complexes

The complexes [Ru(bpy)₂(pyESO)](PF₆)₂ and [Os(bpy)₂(pyESO)](PF₆)₂, in which bpy is 2,2′‐bipyridine and pyESO is 2‐((isopropylsulfinyl)ethyl)pyridine, were prepared and studied by ¹H NMR, UV–visible and ultrafast transient absorption spectroscopy, as well as by electrochemical methods. Crystals suitable for X‐ray structural analysis were grown for [Ru(bpy)₂(pyESO)](PF₆)₂. Cyclic voltammograms of both complexes provide evidence for S→O and O→S isomerization as these voltammograms are described by an ECEC (electrochemical‐chemical electrochemical‐chemical) mechanism in which isomerization follows Ru²⁺ oxidation and Ru³⁺ reduction. The S‐ and O‐bonded Ru^3+/2+ couples appear at 1.30 and 0.76 V versus Ag/AgCl in propylene carbonate. For [Os(bpy)₂(pyESO)](PF₆)₂, these couples appear at 0.97 and 0.32 V versus Ag/AgCl in acetonitrile, respectively. Charge‐transfer excitation of [Ru(bpy)₂(pyESO)](PF₆)₂ results in a significant change in the absorption spectrum. The S‐bonded isomer of [Ru(bpy)₂(pyESO)]²⁺ features a lowest energy absorption maximum at 390 nm and the O‐bonded isomer absorbs at 480 nm. The quantum yield of isomerization in [Ru(bpy)₂(pyESO)]²⁺ was found to be 0.58 in propylene carbonate and 0.86 in dichloroethane solution. Femtosecond transient absorption spectroscopic measurements were collected for both complexes, revealing time constants of isomerizations of 81 ps (propylene carbonate) and 47 ps (dichloroethane) in [Ru(bpy)₂(pyESO)]²⁺. These data and a model for the isomerizing complex are presented. A striking conclusion from this analysis is that expansion of the chelate ring by a single methylene leads to an increase in the isomerization time constant by nearly two orders of magnitude.     

Experimental and DFT Evidence for the Fractional Non‐Innocence of a β‐Diketonate Ligand

New compounds [Ru(pap)₂(L)](ClO₄), [Ru(pap)(L)₂], and [Ru(acac)₂(L)] (pap=2‐phenylazopyridine, L^?=9‐oxidophenalenone, acac^?=2,4‐pentanedionate) have been prepared and studied regarding their electron‐transfer behavior, both experimentally and by using DFT calculations. [Ru(pap)₂(L)](ClO₄) and [Ru(acac)₂(L)] were characterized by crystal‐structure analysis. Spectroelectrochemistry (EPR, UV/Vis/NIR), in conjunction with cyclic voltammetry, showed a wide range of about 2 V for the potential of the Ru^III/II couple, which was in agreement with the very different characteristics of the strongly π‐accepting pap ligand and the σ‐donating acac^? ligand. At the rather high potential of +1.35 V versus SCE, the oxidation of L^? into L^. could be deduced from the near‐IR absorption of [Ru^III(pap)(L^.)(L^?)]²⁺. Other intense long‐wavelength transitions, including LMCT (L^?→Ru^III) and LL/CT (pap^.?→L^?) processes, were confirmed by TD‐DFT results. DFT calculations and EPR data for the paramagnetic intermediates allowed us to assess the spin densities, which revealed two cases with considerable contributions from L‐radical‐involving forms, that is, [Ru^III(pap⁰)₂(L^?)]²⁺?[Ru^II(pap⁰)₂(L^.)]²⁺ and [Ru^III(pap⁰)(L^?)₂]⁺?[Ru^II(pap⁰)(L^?)(L^?)]⁺. Calculations of electrogenerated complex [Ru^II(pap^.?)(pap⁰)(L^?)] displayed considerable negative spin density (?0.188) at the bridging metal.     

Metal‐catalyzed reversible conversion between chemical and electrical energy designed towards a sustainable society

Proton dissociation of an aqua‐Ru‐quinone complex, [Ru(trpy)(q)(OH₂)]²⁺ (trpy = 2,2′ : 6′,2″‐terpyridine, q = 3,5‐di‐t‐butylquinone) proceeded in two steps (pK_a = 5.5 and ca. 10.5). The first step simply produced [Ru(trpy)(q)(OH)]⁺, while the second one gave an unusual oxyl radical complex, [Ru(trpy)(sq)(O^?.)]⁰ (sq = 3,5‐di‐t‐butylsemiquinone), owing to an intramolecular electron transfer from the resultant O^2? to q. A dinuclear Ru complex bridged by an anthracene framework, [Ru₂(btpyan)(q)₂(OH)₂]²⁺ (btpyan = 1,8‐bis(2,2′‐terpyridyl)anthracene), was prepared to place two Ru(trpy)(q)(OH) groups at a close distance. Deprotonation of the two hydroxy protons of [Ru₂(btpyan)(q)₂(OH)₂]²⁺ generated two oxyl radical Ru‐O^?. groups, which worked as a precursor for O₂ evolution in the oxidation of water. The [Ru₂(btpyan)(q)₂(OH)₂](SbF₆)₂ modified ITO electrode effectively catalyzed four‐electron oxidation of water to evolve O₂ (TON = 33500) under electrolysis at +1.70 V in H₂O (pH 4.0). Various physical measurements and DFT calculations indicated that a radical coupling between two Ru(sq)(O^?.) groups forms a (cat)Ru‐O‐O‐Ru(sq) (cat = 3,5‐di‐t‐butylcathechol) framework with a μ‐superoxo bond. Successive removal of four electrons from the cat, sq, and superoxo groups of [Ru₂(btpyan)(cat)(sq)(μ‐O₂^?)]⁰ assisted with an attack of two water (or OH^?) to Ru centers, which causes smooth O₂ evolution with regeneration of [Ru₂(btpyan)(q)₂(OH)₂]²⁺. Deprotonation of an Ru‐quinone‐ammonia complex also gave the corresponding Ru‐semiquinone‐aminyl radical. The oxidized form of the latter showed a high catalytic activity towards the oxidation of methanol in the presence of base. Three complexes, [Ru(bpy)₂(CO)₂]²⁺, [Ru(bpy)₂(CO)(C(O)OH)]⁺, and [Ru(bpy)₂(CO)(CO₂)]⁰ exist as an equilibrium mixture in water. Treatment of [Ru(bpy)₂(CO)₂]²⁺ with BH₄^? gave [Ru(bpy)₂(CO)(C(O)H)]⁺, [Ru(bpy)₂(CO)(CH₂OH)]⁺, and [Ru(bpy)₂(CO)(OH₂)]²⁺ with generation of CH₃OH in aqueous conditions. Based on these results, a reasonable catalytic pathway from CO₂ to CH₃OH in electro‐ and photochemical CO₂ reduction is proposed. A new pbn (pbn = 2‐pyridylbenzo[b]‐1,5‐naphthyridine) ligand was designed as a renewable hydride donor for the six‐electron reduction of CO₂. A series of [Ru(bpy)_3‐n(pbn)_n]²⁺ (n = 1, 2, 3) complexes undergoes photochemical two‐ (n = 1), four‐ (n = 2), and six‐electron reductions (n = 3) under irradiation of visible light in the presence of N(CH₂CH₂OH)₃. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 169–186; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.200800039     

Three Distinct Redox States of an Oxo‐Bridged Dinuclear Ruthenium Complex

A series of [{(terpy)(bpy)Ru}(μ‐O){Ru(bpy)(terpy)}]ⁿ⁺ ( [RuORu]ⁿ⁺ , terpy=2,2′;6′,2′′‐terpyridine, bpy=2,2′‐bipyridine) was systematically synthesized and characterized in three distinct redox states (n=3, 4, and 5 for Ru^II,III₂ , Ru^III,III₂ , and Ru^III,IV₂ , respectively). The crystal structures of [RuORu]ⁿ⁺ (n=3, 4, 5) in all three redox states were successfully determined. X‐ray crystallography showed that the Ru? O distances and the Ru‐O‐Ru angles are mainly regulated by the oxidation states of the ruthenium centers. X‐ray crystallography and ESR spectra clearly revealed the detailed electronic structures of two mixed‐valence complexes, [Ru^IIIORu^IV]⁵⁺ and [Ru^IIORu^III]³⁺ , in which each unpaired electron is completely delocalized across the oxo‐bridged dinuclear core. These findings allow us to understand the systematic changes in structure and electronic state that accompany the changes in the redox state.     

Controlling Metal–Ligand–Metal Oxidation State Combinations by Ancillary Ligand (L) Variation in the Redox Systems [L2Ru(μ‐boptz)RuL2]n,boptz=3,6‐bis(2‐oxidophenyl)‐1,2,4,5‐tetrazine,and L=acetylacetonate, 2,2′‐bipyridine,or 2‐phenylazopyridine

The new compounds [(acac)₂Ru(μ‐boptz)Ru(acac)₂] ( 1 ), [(bpy)₂Ru(μ‐boptz)Ru(bpy)₂](ClO₄)₂ ( 2 ‐(ClO₄)₂), and [(pap)₂Ru(μ‐boptz)Ru(pap)₂](ClO₄)₂ ( 3 ‐(ClO₄)₂) were obtained from 3,6‐bis(2‐hydroxyphenyl)‐1,2,4,5‐tetrazine (H₂boptz), the crystal structure analysis of which is reported. Compound 1 contains two antiferromagnetically coupled (J=?36.7 cm^?1) Ru^III centers. We have investigated the role of both the donor and acceptor functions containing the boptz^2? bridging ligand in combination with the electronically different ancillary ligands (donating acac^?, moderately π‐accepting bpy, and strongly π‐accepting pap; acac=acetylacetonate, bpy=2,2′‐bipyridine pap=2‐phenylazopyridine) by using cyclic voltammetry, spectroelectrochemistry and electron paramagnetic resonance (EPR) spectroscopy for several in situ accessible redox states. We found that metal–ligand–metal oxidation state combinations remain invariant to ancillary ligand change in some instances; however, three isoelectronic paramagnetic cores Ru(μ‐boptz)Ru showed remarkable differences. The excellent tolerance of the bpy co ‐ ligand for both Ru^III and Ru^II is demonstrated by the adoption of the mixed ‐ valent form in [L₂Ru(μ‐boptz)RuL₂]³⁺, L=bpy, whereas the corresponding system with pap stabilizes the Ru^II states to yield a phenoxyl radical ligand and the compound with L=acac^? contains two Ru^III centers connected by a tetrazine radical‐anion bridge.     

DNA-Targeting RuII-Polypyridyl Complex with a Long-Lived Intraligand Excited State as a Potential Photodynamic Therapy Agent

Subtle ligand modifications on Ru^II-polypyridyl complexes may result in different excited-state characteristics, which provides the opportunity to tune their photo-physicochemical properties and subsequently change their biological functions. Here, a DNA-targeting Ru^II-polypyridyl complex (named Ru1 ) with highly photosensitizing ³IL (intraligand) excited state was designed based on a classical DNA-intercalator [Ru(bpy)₂(dppz)] ⋅ 2 PF₆ by incorporation of the dppz (dipyrido[3,2-a:2′,3′-c]phenazine) ligand tethered with a pyrenyl group, which has four orders of magnitude higher potency than the model complex [Ru(bpy)₂(dppz)] ⋅ 2 PF₆ upon light irradiation. This study provides a facile strategy for the design of organelle-targeting Ru^II-polypyridyl complexes with dramatically improved photobiological activity.     

Photo‐Hydrogen‐Evolving Molecular Catalysts Consisting of Polypyridyl Ruthenium(II) Photosensitizers and Platinum(II) Catalysts: Insights into the Reaction Mechanism

The mechanism of photoinduced hydrogen evolution from water driven by the first photo‐hydrogen‐evolving molecular catalyst ( 1 ), given by a coupling of [Ru(bpy)₂(5‐amino‐phen)]²⁺ and [PtCl₂(4,4′‐dicarboxy‐bpy)] (bpy=2,2′‐bipyridine, phen=1,10‐phenanthroline), was investigated in detail. The H₂ evolution rate was found to obey Michaelis–Menten enzymatic kinetics with regard to the concentration of EDTA (ethylenediamine tetra‐acetic acid disodium salt, sacrificial electron donor), which indicates that an ion‐pair formation between the dicationic 1 and the dianionic form of EDTA (pH 5) is a key step leading to H₂ formation. A 2:1 coupling product of 1 and ethylenediamine (i.e., a {Ru^II₂Pt^II₂} complex 2 ) was found to show significantly higher photo‐hydrogen‐evolving (PHE) activity than 1 , which revealed the validity of the bimolecular activation proposed in our previous study. The PHE activity of 2 was also observed to be linear to the concentration of 2 , which indicates that H₂ formation through the intermolecular path competes with the intramolecular path. Molecular orbital diagrams, conformational features, and Pt???H(water or acetic acid) hydrogen bonds were characterized by DFT calculations.     

Near‐IR Absorbing Ruthenium Complexes of Non‐Innocent 6,12‐Di(pyridin‐2‐yl)indolo[3,2‐b]carbazole: Variation as a Function of Co‐Ligands

This article deals with the hitherto unexplored metal complexes of deprotonated 6,12‐di(pyridin‐2‐yl)‐5,11‐dihydroindolo[3,2‐b]carbazole (H₂L). The synthesis and structural, optical, electrochemical characterization of dimeric [{Ru^III(acac)₂}₂(μ‐L^.?)]ClO₄ ([ 1 ]ClO₄, S=1/2), [{Ru^II(bpy)₂}₂(μ‐L^.?)](ClO₄)₃ ([ 2 ](ClO₄)₃, S=1/2), [{Ru^II(pap)₂}₂(μ‐L^2?)](ClO₄)₂ ([ 4 ](ClO₄)₂, S=0), and monomeric [(bpy)₂Ru^II(HL^?)]ClO₄ ([ 3 ]ClO₄, S=0), [(pap)₂Ru^II(HL^?)]ClO₄ ([ 5 ]ClO₄, S=0) (acac=σ‐donating acetylacetonate, bpy=moderately π‐accepting 2,2’‐bipyridine, pap=strongly π‐accepting 2‐phenylazopyridine) are reported. The radical and dianionic states of deprotonated L in isolated dimeric 1 ⁺/ 2 ³⁺ and 4 ²⁺, respectively, could be attributed to the varying electronic features of the ancillary (acac, bpy, and pap) ligands, as was reflected in their redox potentials. Perturbation of the energy level of the deprotonated L or HL upon coordination with {Ru(acac)₂}, {Ru(bpy)₂}, or {Ru(pap)₂} led to the smaller energy gap in the frontier molecular orbitals (FMO), resulting in bathochromically shifted NIR absorption bands (800–2000 nm) in the accessible redox states of the complexes, which varied to some extent as a function of the ancillary ligands. Spectroelectrochemical (UV/Vis/NIR, EPR) studies along with DFT/TD‐DFT calculations revealed (i) involvement of deprotonated L or HL in the oxidation processes owing to its redox non‐innocent potential and (ii) metal (Ru^III/Ru^II) or bpy/pap dominated reduction processes in 1 ⁺ or 2 ²⁺/ 3 ⁺/ 4 ²⁺/ 5 ⁺, respectively.     

Outer‐Sphere 2 e−/2 H+ Transfer Reactions of Ruthenium(II)‐Amine and Ruthenium(IV)‐Amido Complexes

A diverse set of 2 e⁻/2 H⁺ reactions are described that interconvert [Ru^II(bpy)(en*)₂]²⁺ and [Ru^IV(bpy)(en‐H*)₂]²⁺ (bpy=2,2′‐bipyridine, en*=H₂NCMe₂CMe₂NH₂, en*‐H=H₂NCMe₂CMe₂NH⁻), forming or cleaving different O−H, N−H, S−H, and C−H bonds. The reactions involve quinones, hydrazines, thiols, and 1,3‐cyclohexadiene. These proton‐coupled electron transfer reactions occur without substrate binding to the ruthenium center, but instead with precursor complex formation by hydrogen bonding. The free energies of the reactions vary over more than 90 kcal mol⁻¹, but the rates are more dependent on the type of X−H bond involved than the associated ΔG °. There is a kinetic preference for substrates that have the transferring hydrogen atoms in close proximity, such as ortho ‐tetrachlorobenzoquinone over its para ‐isomer and 1,3‐cyclohexadiene over its 1,4‐isomer, perhaps hinting at the potential for concerted 2 e⁻/2 H⁺ transfers.     

Light‐Harvesting Photocatalysis for Water Oxidation Using Mesoporous Organosilica

An organic‐based photocatalysis system for water oxidation, with visible‐light harvesting antennae, was constructed using periodic mesoporous organosilica (PMO). PMO containing acridone groups in the framework (Acd‐PMO), a visible‐light harvesting antenna, was supported with [Ru^II(bpy)₃²⁺] complex (bpy=2,2′‐bipyridyl) coupled with iridium oxide (IrO_x) particles in the mesochannels as photosensitizer and catalyst, respectively. Acd‐PMO absorbed visible light and funneled the light energy into the Ru complex in the mesochannels through excitation energy transfer. The excited state of Ru complex is oxidatively quenched by a sacrificial oxidant (Na₂S₂O₈) to form Ru³⁺ species. The Ru³⁺ species extracts an electron from IrO_x to oxidize water for oxygen production. The reaction quantum yield was 0.34 %, which was improved to 0.68 or 1.2 % by the modifications of PMO. A unique sequence of reactions mimicking natural photosystem II, 1) light‐harvesting, 2) charge separation, and 3) oxygen generation, were realized for the first time by using the light‐harvesting PMO.     

Synthesis,Characterization, and DNA‐Binding Properties of the Chiral Ruthenium(II) Complexes Δ‐ and Λ‐[Ru(bpy)2(dmppd)]2+ (dmppd = 10,12‐Dimethylpteridino[6,7‐f] [1,10]phenanthroline‐11,13(10H,12H)‐dione; bpy = 2,2′‐Bipyridine)

Two novel chiral ruthenium(II) complexes, Δ‐[Ru(bpy)₂(dmppd)]²⁺ and Λ‐[Ru(bpy)₂(dmppd)]²⁺ (dmppd = 10,12‐dimethylpteridino[6,7‐f] [1,10]phenanthroline‐11,13(10H,12H)‐dione, bpy = 2,2′‐bipyridine), were synthesized and characterized by elemental analysis, ¹H‐NMR and ES‐MS. The DNA‐binding behaviors of both complexes were studied by UV/VIS absorption titration, competitive binding experiments, viscosity measurements, thermal DNA denaturation, and circular‐dichroism spectra. The results indicate that both chiral complexes bind to calf‐thymus DNA in an intercalative mode, and the Δ enantiomer shows larger DNA affinity than the Λ enantiomer does. Theoretical‐calculation studies for the DNA‐binding behaviors of these complexes were carried out by the density‐functional‐theory method. The mechanism involved in the regulating and controlling of the DNA‐binding abilities of the complexes was further explored by the comparative studies of [Ru(bpy)₂(dmppd)]²⁺ and of its parent complex [Ru(bpy)₂(ppd)]²⁺ (ppd = pteridino[6,7‐f] [1,10]phenanthroline‐11,13 (10H,12H)‐dione).     

Ruthenium(II) complexes of 2-phenylimidazo[4,5-f] [1,10]phenanthroline. Synthesis,characterization and third order nonlinear optical properties

Three Ru^II complexes [Ru(bpy)₂(PIP)]²⁺, [Ru(PIP)₂(bpy)]²⁺ and [Ru(PIP)₃]²⁺ (PIP = 2-phenylimidazo[4,5-f][1,10]phenanthroline, bpy = 2,2-bipyridine) were prepared and characterized by electrospray mass spectrometry, ¹H-n.m.r, u.v.–vis. and electrochemistry. The nonlinear optical properties (NLO) of the Ru^II complexes were investigated by Z-scan techniques with 12 ns laser pulses at 540 nm, and all of them exhibit both NLO absorption and self-defocusing effects. The corresponding effective NLO susceptibility |³| of the complexes is in the (4.15 – 4.86) × 10^–12 e.s.u. range.     

Direct evidence for chemically distinct regions within nation films on electrodes

Electrochemical and spectroelectrochemical experiments on the complexes [(bpy)₂(py)Ru^II(OH₂)]²⁺ and [(trpy)(bpy)Ru^II(OH₂)]²⁺ (py is pyridine; bpy is 2,2'-bipyridine; trpy is 2,2',2“-terpyridine) with Nation films coated on electrodes demonstrate that the complexes partition amongst three chemically distinct regions or phases. As Ru(II) the complexes reside both in an electroinactive phase and, based on the pH dependence of the Ru(III)/(II) couples, e.g., [(bpy)₂(py)Ru^III(OH)]²⁺/[(bpy)₂(py)Ru^II(OH₂)]²⁺, in two electroactive phases. Partitioning amongst the three phases depends on the pH of the external solution and on the oxidation state and proton content of the complex. Addition of alcohols releases the complex from the electroinactive phase but at the expense of loss of the bound water molecule and binding to the sulfonate sites by anation and, therefore, to a fourth distinct chemical or physical state in which the complex can exist within Nation films.     

Comparative Study of Ruthenium(II) and Ruthenium(III) Complexes with the Ligand dmbpy (dmbpy = 4, 4′‐Dimethyl‐2, 2′‐bipyridine)

The complex cis‐[Ru^III(dmbpy)₂Cl₂](PF₆) ( 2 ) (dmbpy = 4, 4′‐dimethyl‐2, 2′‐bipyridine) was obtained from the reaction of cis‐[Ru^II(dmbpy)₂Cl₂] ( 1 ) with ammonium cerium(IV) nitrate followed by precipitation with saturated ammonium hexafluoridophosphate. The ¹H NMR spectrum of the Ru^III complex confirms the presence of paramagnetic metal atoms, whereas that of the Ru^II complex displays diamagnetism. The ³¹P NMR spectrum of the Ru^III complex shows one signal for the phosphorus atom of the PF₆^– ion. The perspective view of each [Ru^II/III(dmbpy)₂Cl₂]^0/+ unit manifests that the ruthenium atom is in hexacoordinate arrangement with two dmbpy ligands and two chlorido ligands in cis position. As the oxidation state of the central ruthenium metal atom becomes higher, the average Ru–Cl bond length decreases whereas the Ru–N (dmbpy) bond length increases. The cis‐positioned dichloro angle in Ru^III is 1.3° wider than that in the Ru^II. The dihedral angles between pair of planar six‐membered pyridyl ring in the dmbpy ligand for the Ru^II are 4.7(5)° and 5.7(4)°. The observed inter‐planar angle between two dmbpy ligands in the Ru^II is 89.08(15)°, whereas the value for the Ru^III is 85.46(20)°.