Strong Ligand Stabilization Based on π-Extension in a Series of Ruthenium Terpyridine Water Oxidation Catalysts

The substitution behavior of the monodentate Cl ligand of a series of ruthenium(II) terpyridine complexes (terpyridine (tpy)=2,2′:6′,2′′-terpyridine) has been investigated. ¹H NMR kinetic experiments of the dissociation of the chloro ligand in D₂O for the complexes [Ru(tpy)(bpy)Cl]Cl ( 1 , bpy=2,2’-bipyridine) and [Ru(tpy)(dppz)Cl]Cl ( 2 , dppz=dipyrido[3,2-a:2′,3′-c]phenazine) as well as the binuclear complex [Ru(bpy)₂(tpphz)Ru(tpy)Cl]Cl₃ ( 3 b , tpphz=tetrapyrido[3,2-a:2′,3′-c:3′′,2′′-h:2′′′,3′′′-j]phenazine) were conducted, showing increased stability of the chloride ligand for compounds 2 and 3 due to the extended π-system. Compounds 1 – 5 ( 4 =[Ru(tbbpy)₂(tpphz)Ru(tpy)Cl](PF₆)₃, 5 =[Ru(bpy)₂(tpphz)Ru(tpy)(C₃H₈OS)/(H₂O)](PF₆)₃, tbbpy=4,4′-di-tert-butyl-2,2′-bipyridine) are tested for their ability to run water oxidation catalysis (WOC) using cerium(IV) as sacrificial oxidant. The WOC experiments suggest that the stability of monodentate (chloride) ligand strongly correlates to catalytic performance, which follows the trend 1 > 2 > 5 ≥ 3 > 4 . This is also substantiated by quantum chemical calculations, which indicate a stronger binding for the chloride ligand based on the extended π-systems in compounds 2 and 3 . Additionally, a theoretical model of the mechanism of the oxygen evolution of compounds 1 and 2 is presented; this suggests no differences in the elementary steps of the catalytic cycle within the bpy to the dppz complex, thus suggesting that differences in the catalytic performance are indeed based on ligand stability. Due to the presence of a photosensitizer and a catalytic unit, binuclear complexes 3 and 4 were tested for photocatalytic water oxidation. The bridging ligand architecture, however, inhibits the effective electron-transfer cascade that would allow photocatalysis to run efficiently. The findings of this study can elucidate critical factors in catalyst design.     

Activating a Low Overpotential CO2 Reduction Mechanism by a Strategic Ligand Modification on a Ruthenium Polypyridyl Catalyst

The introduction of a simple methyl substituent on the bipyridine ligand of [Ru(tBu₃tpy)(bpy)(NCCH₃)]²⁺ (tBu₃tpy=4,4′,4′′‐tri‐tert‐butyl‐2,2′:6′,2′′‐terpyridine; bpy=2,2′‐bipyridine) gives rise to a highly active electrocatalyst for the reduction of CO₂ to CO. The methyl group enables CO₂ binding already at the one‐electron reduced state of the complex to enter a previously not accessible catalytic cycle that operates at the potential of the first reduction. The complex turns over with a Faradaic efficiency close to unity and at an overpotential that is amongst the lowest ever reported for homogenous CO₂ reduction catalysts.     

Photophysical Properties of Ruthenium(II) Polypyridyl DNA Intercalators: Effects of the Molecular Surroundings Investigated by Theory

The environmental effects on the structural and photophysical properties of [Ru(L)₂(dppz)]²⁺ complexes (L=bpy=2,2′‐bipyridine, phen=1,10‐phenanthroline, tap=1,4,5,8‐tetraazaphenanthrene; dppz=dipyrido[3,3‐a:2′,3′‐c]phenazine), used as DNA intercalators, have been studied by means of DFT, time‐dependent DFT, and quantum mechanics/molecular mechanics calculations. The electronic characteristics of the low‐lying triplet excited states in water, acetonitrile, and DNA have been investigated to decipher the influence of the environment on the luminescent behavior of this class of molecules. The lowest triplet intra‐ligand (IL) excited state calculated at λ≈800 nm for the three complexes and localized on the dppz ligand is not very sensitive to the environment and is available for electron transfer from a guanine nucleobase. Whereas the lowest triplet metal‐to‐ligand charge‐transfer (³MLCT) states remain localized on the ancillary ligand (tap) in [Ru(tap)₂(dppz)]²⁺, regardless of the environment, their character is drastically modified in the other complexes [Ru(phen)₂(dppz)]²⁺ and [Ru(bpy)₂(dppz)]²⁺ upon going from acetonitrile (MLCT_dppz/phen or MLCT_dppz/bpy) to water (MLCT_dppz) and DNA (MLCT_phen and MLCT_bpy). The change in the character of the low‐lying ³MLCT states accompanying nuclear relaxation in the excited state controls the emissive properties of the complexes in water, acetonitrile, and DNA. The light‐switching effect has been rationalized on the basis of environment‐induced control of the electronic density distributed in the lowest triplet excited states.     

Structure of the Complex of [Ru(tpm)(dppz)py]2+ with a B‐DNA Oligonucleotide—A Single‐Substituent Binding Switch for a Metallo‐Intercalator

We report the synthesis of three new complexes related to the achiral [Ru(tpm)(dppz)py]²⁺ cation (tpm=tripyridazole methane, dppz=dipyrido[3,2‐a:2′,3′‐c]phenazine, py=pyridine) that contain an additional single functional group on the monodentate ancillary pyridyl ligand. Computational calculations indicate that the coordinated pyridyl rings are in a fixed orientation parallel to the dppz axis, and that the electrostatic properties of the complexes are very similar. DNA binding studies on the new complexes reveal that the nature and positioning of the functional group has a profound effect on the binding mode and affinity of these complexes. To explore the molecular and structural basis of these effects, circular dichroism and NMR studies on [Ru(tpm)(dppz)py]Cl₂ with the octanucleotides d(AGAGCTCT)₂ and d(CGAGCTCG)₂, were carried out. These studies demonstrate that the dppz ligand intercalates into the G²–A³ step, with {Ru(tpm)py} in the minor groove. They also reveal that the complex intercalates into the binding site in two possible orientations with the pyridyl ligand of the major conformer making close contact with terminal base pairs. We conclude that substitution at the 2‐ or 3‐position of the pyridine ring has little effect on binding, but that substitution at the 4‐position drastically disrupts intercalative binding, particularly with a 4‐amino substituent, because of steric and electronic interactions with the DNA. These results indicate that complexes derived from these systems have the potential to function as sequence‐specific light‐switch systems.     

Design and spectroscopic study of new ruthenium(II) complexes containing ligands derived from terpyridine and dipyrido[3,2‐a:2′,3′‐c]phenazine: {Ru(4′‐Rph‐tpy) [dppz(COOH)]Cl}PF6 with R = NO2, Br,Cl

A series of polypyridine ruthenium complexes of the general formula {Ru(Rph‐tpy)[dppz(COOH)]Cl} PF₆ with R = Br ( 1 ), Cl ( 2 ), NO₂ ( 3 ) where Rph‐tpy is 4′‐(4‐Rphenyl‐2,2′:6′,2″‐terpyridine and dppz(COOH) is dipyrido[3,2‐a:2′,3′‐c]phenazine‐2‐carboxylic acid were prepared and characterized. These complexes display intense metal‐to‐ligand charge‐transfer (MLCT) bands centered about 500 nm. The effect of pH on the absorption spectra of these complexes consisting of protonatable ligands has been investigated in water solution by spectrophotometric titration. The electrochemistry shows oxidation potentials for the Ru(II)–Ru(III) couple at +0.881 ( 1 ), +0.907 ( 2 ) and +0.447 V ( 3 ), respectively. Copyright © 2006 John Wiley & Sons, Ltd.     

DNA Interactions of the Functionalized (Mixed Polypyridine)ruthenium(II) Complex Bis(2,2′‐bipyridine‐κN1,κN1′)(methyl dipyrido[3,2‐a : 2′,3′‐c]phenazine‐11‐carboxylate‐κN4,κN5)ruthenium(2+) ([Ru(bpy)2(dppz‐11‐CO2Me)]2+)

A novel polypyridine ligand, dipyrido[3,2‐a:2′,3′‐c]phenazine‐11‐carboxylic acid methyl ester (=dppz‐11‐CO₂Me), and its ruthenium(II) complex, [Ru(bpy)₂(dppz‐11‐CO₂Me)]²⁺ ( 1 ), were synthesized and characterized. The binding properties of this complex to calf‐thymus DNA (CT‐DNA) were investigated by different spectrophotometric methods and viscosity measurements. The results suggest that the complex binds to DNA in an intercalative mode and serves as a molecular ‘light switch’ for DNA. When irradiated at 365 nm, the complex 1 promoted the photocleavage of plasmid pBR‐322 DNA.     

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.     

π‐Stacking Modulates the Luminescence of [(dppz)Ni(Mes)Br] (dppz = dipyrido[3,2‐a:2′,3′‐c]phenazine,Mes = 2,4,6‐trimethylphenyl)

The red colour of the novel organonickel complex [(dppz)Ni(Mes)Br] (dppz = dipyrido[3,2‐a:2′,3′‐c]phenazine, Mes = 2,4,6‐trimethylphenyl) originates from long‐wavelength MLCT/L′LCT charge transfer bands. However, luminescence in dilute solution comes presumably from the ³π‐π* (phenazine) excited state. The red‐shifted emission exhibited in concentrated solutions is assigned to dimers. In the solid state emission is quenched. The crystal structure reveals two different types of π‐π stacking along the crystallographic a axis.     

Synthesis,characterization, and evaluation of biological activities of new 4′‐substituted ruthenium (II) terpyridine complexes: Prospective anti‐inflammatory properties

The synthesis and characterization of Ru (II) terpyridine complexes derived from 4′ functionalized 2,2′:6′,2″‐terpyridine (tpy) ligands are reported. The heteroleptic complexes comprise the synthesized ligands 4′‐(2‐thienyl)‐ 2,2′:6′,2″‐terpyridine) or (4′‐(3,4‐dimethoxyphenyl)‐2,2′:6′,2″‐terpyridine and (dimethyl 5‐(pyrimidin‐5‐yl)isophthalate). The new complexes [Ru(4′‐(2‐thienyl)‐2,2′:6′,2″‐terpyridine)(5‐(pyrimidin‐5‐yl)‐isophthalic acid)Cl₂] ( 9 ), [Ru(4′‐(3,4‐dimethoxyphenyl)‐2,2′:6′,2″‐terpyridine)(5‐(pyrimidin‐5‐yl)‐isophthalic acid)Cl₂] ( 10 ), and [Ru(4′‐(2‐thienyl)‐2,2′:6′,2″‐terpyridine)(5‐(pyrimidin‐5‐yl)‐isophthalic acid)(NCS)₂] ( 11 ) were characterized by ¹H‐ and ¹³C‐NMR spectroscopy, C, H, N, and S elemental analysis, UPLC‐ESI‐MS, TGA, FT‐IR, and UV‐Vis spectroscopy. The biological activities of the synthesized ligands and their Ru (II) complexes as anti‐inflammatory, antimicrobial, and anticancer agents were evaluated. Furthermore, the toxicity of the synthesized compounds was studied and compared with the standard drugs, namely, diclofenac potassium and ibuprofen, using hemolysis assay. The results indicated that the ligands and the complex 9 possess superior anti‐inflammatory activities inhibiting albumin denaturation (89.88–100%) compared with the standard drugs (51.5–88.37%) at a concentration of 500 μg g^?1. These activities were related to the presence of the chelating N‐atoms in the ligands and the exchangeable chloro‐ groups in the complex. Moreover, the chloro‐ and thiophene groups in complex 9 produce a higher anticancer activity compared with its isothiocyanate derivative in the complex 11 and the 3,4‐dimethoxyphenyl moiety in complex 10 . Considering the toxicity results, the synthesized ligands are nontoxic or far less toxic compared with the standard drugs and the metal complexes. Therefore, these newly synthesized compounds are promising anti‐inflammatory agents in addition to their moderate unique broad antimicrobial activity.     

Effect of Metal Ions on the Structures of Coordination Polymers Based on Biphenyl‐2,2′,4,4′‐Tetracarboxylate

Two new coordination polymers, {[Cd₂(btc)(2,2′‐bpy)₂] · H₂O}_n ( 1 ) and [Zn₂(btc)(2,2′‐bpy)(H₂O)]_n ( 2 ) (H₄btc = biphenyl‐2,2′,4,4′‐tetracarboxylic acid, 2,2′‐bpy = 2,2′‐bipyridine), were synthesized hydrothermally under similar conditions and characterized by elemental analysis, IR spectra, TGA, and single‐crystal X‐ray diffraction analysis. In complexes 1 and 2 , the (btc)^4– ligand acts as connectors to link metal ions to give a 2D bilayer network of 1 and a 3D metal‐organic framework of 2 , respectively. The differences in the structures are induced by diverging coordination modes of the (btc)^4– ligand, which can be attributed to the difference metal ions in sizes. The results indicate that metal ions have significant effects on the formation and structures of the final complexes. Additionally, the fluorescent properties of the two complexes were also studied in the solid state at room temperature.     

Synthesis and Characterization of Heteroleptic Ru(II) Complexes Based on 4,4′‐Bis((E)‐styryl)‐2,2′‐bipyridine as Ancillary Ligand and Application for Dye‐Sensitized Solar Cells

Heteroleptic Ru(II) complexes were designed based on 4,4′‐bis((E)‐styryl)‐2,2′‐bipyridine (bsbpy) as an ancillary ligand for dye‐sensitized solar cells (DSSCs), and those Ru(II) sensitizers, [Ru(L)(bsbpy)(NCS)₂][TBA] (TBA; tetrabutylammonium), were synthesized according to a typical one‐pot reaction of [RuCl₂(p‐cymene)]₂ with the corresponding anchoring ligands (where L = 4,4′‐dicarboxy‐2,2′‐bipyridine (dcbpy), 4,4′‐bis((E)‐carboxyvinyl)‐2,2′‐bipyridine (dcvbpy), 4,7‐dicarboxy‐1,10‐phenanthroline (dcphen), or 4,7‐bis((E)‐carboxyvinyl)‐1,10‐phenanthroline (dcvphen)). The new Ru(II) dyes, [Ru(L)(bsbpy)(NCS)₂][TBA] that incorporated vinyl spacer(s) into ancillary and/or anchoring ligand displayed red‐shifted bands over the overall UV/VIS region relative to the absorption spectra of N719 . A combination of bsbpy ancillary and dcphen anchoring ligand showed the best result for the overall power conversion efficiency (η); i.e., a DSSC fabricated with [Ru(dcphen)(bsbpy)(NCS)₂][TBA] exhibited a power conversion efficiency (η) of 2.98% (compare to N719 , 4.82%).     

Synthesis and properties of ruthenium(II) complexes of 3,3′-polymethylene-2-(pyrid-2′-yl)benzo[b]-1,10-phenanthrolines

Coordination abilities of unsymmetrical tridentate ligands, 3,3′-polymethylene-2-(pyrid-2′-yl)-benzo[b]-1,10-phenanthrolines (4) were studied. Reactions of the 3,3′-di- and 3,3′-trimethylene-2-(pyrid-2′-yl)benzo[b]-1,10-phenanthrolines (4b and 4c) with RuCl₃ ? 3H₂O afforded [Ru(4b)₂]²⁺ and [Ru(4c)₂]²⁺ in 57% and 78% yield, respectively, while reactions of the parent non-bridged ligand (4a), tetramethylene-bridged ligand (4d), and fully aromatized ligand (4e) afforded a messy mixture. Reactions of 4 with Ru(tpy)Cl₃ (tpy = 2,2′;6′,2″-terpyridine) afforded [Ru(tpy)(4)]²⁺ in 61–72% yields. UV absorption spectra of the ligands showed four ligand-centered (LC) π–π* transitions and their Ru complexes showed four LC π–π* transitions and one Ru(d_π) → ligand(π*) MLCT. The ligands showed three major emission maxima (λ _emission) in the region of 393–418, 416–445, and 437–471 nm in which λ _emission is highly dependent on the length of the methylene bridge connecting C3 of benzo[b]-1,10-phenanthroline and C3 of pyridine. Ru complexes with fully aromatic ligand, [Ru(tpy)(4e)]²⁺, and the most distorted ligand, [Ru(tpy)(4d)]²⁺, showed two emission maxima at 410 and 444–446 nm, while the others showed one emission at 410 nm. Each of the emission maxima is bathochromatically shifted from the complex with the most distorted ligand (4d) to the complex with fully aromatized planar ligand (4e) indicating lower energy emission.     

Photophysical behavior of Ru(II) and Os(II) terpyridyl phenylene vinylene complexes: perturbation of MLCT state by intra-ligand charge-transfer state

The photophysical behavior of novel bimetallic Ru(II) and Os(II) complexes having a bridging ligand consisting of two terpyridyl moieties covalently linked in the 4′ position through a distyrylbenzene bridge (tp vp vpt) is reported. The Ru(II) complex has a unique red emission with an excited state lifetime nearly 2000-times longer than the parent complex, [Ru(mpt)₂](PF6)₂ (mpt=4′-(methylphenyl)-2,2′,6′,2″-terpyridine). Combined spectral data suggest the presence of an emissive intra-ligand charge-transfer (ILCT) state lower in energy than the metal-to-ligand charge transfer (MLCT) state. The Os(II) complex exhibits red emission that is similar to that of the parent complex [Os(mpt)₂](PF₆)₂. However, the excited state absorption spectrum reveals a unique transient absorption in the far red that suggests perturbation of the MLCT state by the ILCT state.     

Honeycomb‐like Ordered Assembly of DNA Induced by Flexible Binuclear Ruthenium(II)–Polypyridyl Complexes

A series of binuclear ruthenium(II)–polypyridyl complexes of the type [Ru₂(N‐N)₄(BPIMBp)]⁴⁺, in which N‐N is 2,2′‐bipyridine (bpy; 1 ), 1,10‐phenanthroline (phen; 2 ), dipyrido[3,2‐d:2′,3‐f] quinoxaline (dpq; 3 ), dipyrido[3,2‐a:2′,3′‐c] phenanzine (dppz; 4 ), and 1,4′‐bis[(2‐pyridin‐2‐yl)‐1H‐imidazol‐1‐yl)methyl]‐1,1′‐biphenyl (BPIMBp) is a bridging ligand, have been synthesized and characterized. These complexes are charged (4+) cations and flexible due to the ?CH₂ group of the bridging ligand and possess terminal ligands with variable intercalative abilities. The interaction of complexes 1 – 4 with calf thymus DNA (CT‐DNA) was explored by using UV/Vis absorption spectroscopy, steady‐state emission, emission quenching with K₄[Fe(CN)₆], ethidium bromide displacement assay, Hoechst displacement assay, and viscosity measurements and revealed a groove‐binding mode for all the complexes through a spacer and an intercalative mode for complexes 3 and 4 . A decrease in the viscosity of DNA revealed bending and coiling of DNA, an initial step toward aggregation. Interestingly, a distinctive honeycomb‐like ordered assembly of the DNA–complex species was visualized by fluorescence microscopy in the solution state. The use of SEM and AFM confirmed the disordered self‐organization of the DNA–complex adduct on evaporation of the solvent. The small orderly nanosized DNA aggregates were confirmed by means of circular dichroism, dynamic light scattering (DLS), and TEM. These complexes are moderately cytotoxic against three different cell lines, namely, MCF‐7, HeLa, and HL‐60.     

Photodriven Multi‐electron Storage in Disubstituted RuII Dppz Analogues

Four derivatives of the laminate acceptor ligand dipyrido‐[3,2‐a:2′,3′‐c]phenazine (dppz) and their corresponding ruthenium complexes, [Ru(phen)₂(dppzX₂)]²⁺, were prepared and characterized by NMR spectroscopy, ESI‐MS, and elemental analysis. The new ligands, generically denoted dppzX₂, were symmetrically disubstituted on the distal benzene ring to give 10,13‐dibromodppz (dppz‐p‐Br), 11,12‐dibromodppz (dppz‐o‐Br), 10,13‐dicyanodppz (dppz‐p‐CN), 11,12‐dicyanodppz (dppz‐o‐CN). Solvated ground state MO calculations of the ruthenium complexes reveal that these electron‐withdrawing substituents not only lower the LUMO of the dppz ligand (dppz(CN)₂<dppzBr₂<dppz), but that the para disubstitution results in a lower LUMO than the ortho disubstitution (dppz‐p‐CN<(dppz‐o‐CN), and dppz‐p‐Br<dppz‐o‐Br). The validity of the calculations was confirmed experimentally using cyclic voltammetry. Of the complexes evaluated in this study, only the dicyanodppz complexes showed multiple dppz‐based reductions prior to reduction of the phen ligands. The capacity to form singly and doubly reduced dppz‐based anions at modest reduction potentials was confirmed using a combination of spectroelectrochemical and chemical titration methods. When subjected to photolysis with visible light in the presence of a sacrificial donor, such as triethylamine, both cyano complexes showed multi‐electron reduction. The other complexes only show a single reduction.     

Shielding of Electron Acceptors Coordinated to Rhenium(I) by Carboxylato Groups. Intraligand and Charge‐Transfer Excited‐State Properties of fac‐(‘Anthraquinone‐2‐carboxylato')(2,2′‐bipyridine)tricarbonylrhenium (fac‐[ReI(aq‐2‐CO2)(2,2′‐bipy)(CO)3])

The photophysical and photochemical properties of (OC‐6‐33)‐(2,2′‐bipyridine‐κN¹,κN¹′)tricarbonyl(9,10‐dihydro‐9,10‐dioxoanthracene‐2‐carboxylato‐κO)rhenium (fac‐[Re^I(aq‐2‐CO₂)(2,2′‐bipy)(CO)₃]) were investigated and compared to those of the free ligand 9,10‐dihydro‐9,10‐dioxoanthracene‐2‐carboxylate (=anthraquinone‐2‐carboxylate) and other carboxylato complexes containing the (2,2′‐bipyridine)tricarbonylrhenium ([Re(2,2′‐bipy)(CO)₃]) moiety. Flash and steady‐state irradiations of the anthraquinone‐derived ligand (λ_exc 337 or 351 nm) and of its complex reveal that the photophysics of the latter is dominated by processes initiated in the Re‐to‐(2,2′‐bipyridine) charge‐transfer excited state and 2,2′‐bipyridine‐ and (anthraquinone‐2‐carboxylato)‐centered intraligand excited states. In the reductive quenching by N,N‐diethylethanamine (TEA) or 2,2′,2″‐nitrilotris[ethanol] TEOA, the reactive states are the 2,2′‐bipyridine‐centered and/or the charge‐transfer excited states. The species with a reduced anthraquinone moiety is formed by the following intramolecular electron transfer, after the redox quenching of the excited state: [Re^I(aq−2−CO₂)(2,2′‐bipy^.)(CO)₃]⁻⇌[Re^I(aq−2−CO₂^.)(2,2′‐bipy)(CO)₃]⁻ The photophysics, particularly the absence of a Re^I‐to‐anthraquinone charge‐transfer excited state photochemistry, is discussed in terms of the electrochemical and photochemical results.     

Efficient DNA Condensation Induced by Ruthenium(II) Complexes of a Bipyridine‐Functionalized Molecular Clip Ligand

Complexes of the type [Ru(bxbg)₂(N‐N)]²⁺, where N‐N denotes 2,2′‐bipyridine (bpy) ( 1 ), 1,10‐phenanthroline (phen) ( 2 ), dipyrido[3,2‐d:2′,3‐f] quinoxaline (dpq) ( 3 ), and dipyrido[3,2‐a:2′,3′‐c]phenazine (dppz) ( 4 ), incorporating bis(o‐xylene)bipyridine‐glycoluril (bxbg) as an ancillary “molecular clip” ligand, have been synthesized and characterized. These ruthenium(II) complexes of bis(o‐xylene)bipyridine‐glycoluril self‐associate in water through specific molecular recognition processes to form polycationic arrays. These arrays containing electrostatic binders as well as intercalator ligands at micromolar doses rapidly condense free DNA into globular nanoparticles of various sizes. The DNA condensation induced by these complexes has been investigated by electrophoretic mobility assay, dynamic light scattering, and transmission electron microscopy. The cellular uptake of complex–DNA condensates and the low cytotoxicity of these complexes satisfy the requirements of a gene vector.     

Judicious Ligand Design in Ruthenium Polypyridyl CO2 Reduction Catalysts to Enhance Reactivity by Steric and Electronic Effects

A series of Ru^II polypyridyl complexes of the structural design [Ru^II(R?tpy)(NN)(CH₃CN)]²⁺ (R?tpy=2,2′:6′,2′′‐terpyridine (R=H) or 4,4′,4′′‐tri‐tert‐butyl‐2,2′:6′,2′′‐terpyridine (R=tBu); NN=2,2′‐bipyridine with methyl substituents in various positions) have been synthesized and analyzed for their ability to function as electrocatalysts for the reduction of CO₂ to CO. Detailed electrochemical analyses establish how substitutions at different ring positions of the bipyridine and terpyridine ligands can have profound electronic and, even more importantly, steric effects that determine the complexes’ reactivities. Whereas electron‐donating groups para to the heteroatoms exhibit the expected electronic effect, with an increase in turnover frequencies at increased overpotential, the introduction of a methyl group at the ortho position of NN imposes drastic steric effects. Two complexes, [Ru^II(tpy)(6‐mbpy)(CH₃CN)]²⁺ (trans‐[ 3 ]²⁺; 6‐mbpy=6‐methyl‐2,2′‐bipyridine) and [Ru^II(tBu?tpy)(6‐mbpy)(CH₃CN)]²⁺ (trans‐[ 4 ]²⁺), in which the methyl group of the 6‐mbpy ligand is trans to the CH₃CN ligand, show electrocatalytic CO₂ reduction at a previously unreactive oxidation state of the complex. This low overpotential pathway follows an ECE mechanism (electron transfer–chemical reaction–electron transfer), and is a direct result of steric interactions that facilitate CH₃CN ligand dissociation, CO₂ coordination, and ultimately catalytic turnover at the first reduction potential of the complexes. All experimental observations are rigorously corroborated by DFT calculations.     

Switching the Mechanism of NADH Photooxidation by Supramolecular Interactions

A series of three Ru(II) polypyridine complexes was investigated for the selective photocatalytic oxidation of NAD(P)H to NAD(P)⁺ in water. A combination of (time-resolved) spectroscopic studies and photocatalysis experiments revealed that ligand design can be used to control the mechanism of the photooxidation: For prototypical Ru(II) complexes a ¹O₂ pathway was found. Rudppz ([(tbbpy)₂Ru(dppz)]Cl₂, tbbpy=4,4'-di-tert-butyl-2,2'-bipyridine, dppz=dipyrido[3,2-a:2′,3′-c]phenazine), instead, initiated the cofactor oxidation by electron transfer from NAD(P)H enabled by supramolecular binding between substrate and catalyst. Expulsion of the photoproduct NAD(P)⁺ from the supramolecular binding site in Rudppz allowed very efficient turnover. Therefore, Rudppz permits repetitive selective assembly and oxidative conversion of reduced naturally occurring nicotinamides by recognizing the redox state of the cofactor under formation of H₂O₂ as additional product. This photocatalytic process can fuel discontinuous photobiocatalysis.     

Characterization of the Oxygen Binding Motif in a Ruthenium Water Oxidation Catalyst by Vibrational Spectroscopy

For homogeneous mononuclear ruthenium water oxidation catalysts, the Ru–O₂ complex plays a crucial role in the rate determining step of the catalytic cycle, but the exact nature of this complex is unclear. Herein, the infrared spectra of the [Ru(tpy)(bpy)(O₂)]²⁺ complex (tpy=2,2′:6′,2′′‐terpyridine; bpy=2,2′‐bipyridine) are presented. The complex [Ru(tpy)(bpy)(O₂)]²⁺, formed by gas‐phase reaction of [Ru(tpy)(bpy)]²⁺ with molecular O₂, was isolated by using mass spectrometry and was directly probed by cryogenic ion IR predissociation spectroscopy. Well‐resolved spectral features enable a clear identification of the O?O stretch using ¹⁸O₂ substitution. The band frequency and intensity indicate that the O₂ moiety binds to the Ru center in a side‐on, bidentate manner. Comparisons with DFT calculations highlight the shortcomings of the B3LYP functional in properly depicting the Ru–O₂ interaction.