Ligand versus metal protonation of an iron hydrogenase active site mimic

The protonation behavior of the iron hydrogenase active-site mimic [Fe2(mu-adt)(CO)4(PMe3)2] (1; adt=N-benzyl-azadithiolate) has been investigated by spectroscopic, electrochemical, and computational methods. The combination of an adt bridge and electron-donating phosphine ligands allows protonation of either the adt nitrogen to give [Fe2(mu-Hadt)(CO)4(PMe3)2]+ ([1 H]+), the Fe-Fe bond to give [Fe2(mu-adt)(mu-H)(CO)4(PMe3)2]+ ([1 Hy]+), or both sites simultaneously to give [Fe2(mu-Hadt)(mu-H)(CO)4(PMe3)2]2+ ([1 HHy]2 +). Complex 1 and its protonation products have been characterized in acetonitrile solution by IR, (1)H, and (31)P NMR spectroscopy. The solution structures of all protonation states feature a basal/basal orientation of the phosphine ligands, which contrasts with the basal/apical structure of 1 in the solid state. Density functional calculations have been performed on all protonation states and a comparison between calculated and experimental spectra confirms the structural assignments. The ligand protonated complex [1 H]+ (pKa=12) is the initial, metastable protonation product while the hydride [1 Hy]+ (pKa=15) is the thermodynamically stable singly protonated form. Tautomerization of cation [1 H]+ to [1 Hy]+ does not occur spontaneously. However, it can be catalyzed by HCl (k=2.2 m(-1) s(-1)), which results in the selective formation of cation [1 Hy]+. The protonations of the two basic sites have strong mutual effects on their basicities such that the hydride (pK(a)=8) and the ammonium proton (pK(a)=5) of the doubly protonated cationic complex [1 HHy]2+ are considerably more acidic than in the singly protonated analogues. The formation of dication [1 HHy]2+ from cation [1 H]+ is exceptionally slow with perchloric or trifluoromethanesulfonic acid (k=0.15 m(-1) s(-1)), while the dication is formed substantially faster (k>10(2) m(-1) s(-1)) with hydrobromic acid. Electrochemically, 1 undergoes irreversible reduction at -2.2 V versus ferrocene, and this potential shifts to -1.6, -1.1, and -1.0 V for the cationic complexes [1 H]+, [1 Hy]+, and [1 HHy]2+, respectively, upon protonation. The doubly protonated form [1 HHy]2+ is reduced at less negative potential than all previously reported hydrogenase models, although catalytic proton reduction at this potential is characterized by slow turnover.     

Synthesis, electrochemical, and molecular inclusion properties of 'canopied' trinuclear ruthenium complexes with six anchoring groups on an ITO electrode

New trinuclear Ru complexes bearing both 1,3,5-tris(2,2':6',2'-terpyridyl)benzene and bis(benzimidazol-2-yl)pyridine with six phosphonate anchors were synthesised and immobilised on an ITO electrode. The 'canopied' structure was proved by AFM measurements. The immobilised Ru trinuclear complex revealed a one step three-electron oxidation process for the Ru(II/III) couple at around +0.9 V vs. Ag/AgCl, indicating that the Ru-Ru interaction is small. For the Ru immobilised ITO surface, the electron transfer blocking and the electron mediation of [Fe(CN)(6)](4-) oxidation through the Ru(iii) <--> Ru(ii) catalytic cycle are strongly dependent on the alkyl chain length. The spectroelectrochemistry of the Ru trinuclear complex monolayer on the ITO electrode showed a stable electrochromic response under the potential pulse. The present 'canopied' Ru trinuclear complexes hold a small cavity that encapsulates tetrathiafulvalene molecules, which can be proved by a cyclic voltammogram.     

Synthesis, structure, and characterisation of a new phenolato-bridged manganese complex [Mn2(mL)2]2+: chemical and electrochemical access to a new mono-mu-oxo dimanganese core unit

The dinuclear phenolato-bridged complex [(mL)Mn(II)Mn(II)(mL)](ClO(4))(2) (1(ClO(4))(2)) has been obtained with the new [N(4)O] pentadentate ligand mL(-) (mLH=N,N'-bis-(2-pyridylmethyl)-N-(2-hydroxybenzyl)-N'-methyl-ethane-1,2-diamine) and has been characterised by X-ray crystallography. X- and Q-band EPR spectra were recorded and their variation with temperature was examined. All spectra exhibit features extending over 0-800 mT at the X band and over 100-1450 mT at the Q band, features that are usually observed for dinuclear Mn(II) complexes. Cyclic voltammetry of 1 exhibits two irreversible oxidation waves at E(1)(p)=0.89 V and E(2)(p)=1.02 V, accompanied on the reverse scan by an ill-defined cathodic wave at E(1')(p)=0.56 V (all measured versus the saturated calomel electrode (SCE)). Upon chemical oxidation with tBuOOH (10 equiv) at 20 degrees C, 1 is transformed into the mono-mu-oxo species [(mL)Mn(III)-(mu-O)-Mn(III)(mL)](2+) (2), which eventually partially evolves into the di-mu-oxo species [(mL)Mn(III)-(mu-O)(2)-Mn(IV)(mL)](n+) (3) in which one of the aromatic rings of the ligand is decoordinated. The UV/Vis spectrum of 2 displays a large absorption band at 507 nm, which is attributed to a phenolate-->Mn(III) charge-transfer transition. The cyclovoltammogram of 2 exhibits two reversible oxidation waves, at 0.65 and 1.16 V versus the SCE, corresponding to the Mn(III)Mn(III)/Mn(III)Mn(IV) and Mn(III)Mn(IV)/Mn(IV)Mn(IV) oxidation processes, respectively. The one-electron electrochemical oxidation of 2 leads to the mono-mu-oxo mixed-valent species [(mL)Mn(III)-(mu-O)-Mn(IV)(mL)](3+) (2 ox). The UV/Vis spectrum of 2 ox exhibits one large band at 643 nm, which is attributed to the phenolate-->Mn(IV) charge-transfer transition. 2 ox can also be obtained by the direct electrochemical oxidation of 1 in the presence of an external base. The 2 ox and 3 species exhibit a 16-line EPR signal with first peak to last trough widths of 125 and 111 mT, respectively. Both spectra have been simulated by using colinear rhombic Mn-hyperfine tensors. Mechanisms for the chemical formation of 2 and the electrochemical oxidation of 1 into 2 ox are proposed.     

Eilatin as a bridging ligand in ruthenium(II) complexes: synthesis,crystal structures,absorption spectra,and electrochemical properties

The potential of the heptacyclic aromatic alkaloid eilatin (1), that features two nonequivalent binding sites, to serve as a bridging ligand is reported. The nonequivalency of the binding sites allowed the selective synthesis of both mono- and dinuclear complexes. The mononuclear Ru(II) complexes [Ru(dmbpy)(2)(eilatin)](2+) (2) and [Ru(tmbpy)(2)(eilatin)](2+) (3) in which eilatin selectively binds "head-on" were synthesized and employed as building blocks in the synthesis of the dinuclear complexes [[Ru(dmbpy)(2)](2)(mu-eilatin)](4+) (4) and [[Ru(tmbpy)(2)](2)(mu-eilatin)](4+) (5). Complete structure elucidation of the complexes in solution was accomplished by 1D and 2D NMR techniques. The X-ray structures of the mononuclear complex 3 and of the two dinuclear complexes 4 and 5 were solved, and absorption spectra and electrochemical properties of the complexes were explored. Both dinuclear complexes formed as racemic mixtures in a 3:1 diastereoisomeric ratio, the major isomer being the heterochiral one (Delta Lambda/Lambda Delta) as revealed by crystallography. The mononuclear complexes feature an exceptionally low energy MLCT band around 600 nm that shifted to over 700 nm upon the binding of the second Ru(II) center. The mononuclear complexes show one reversible oxidation and several reversible reduction waves, the first two reductions being substantially anodically shifted in comparison with [Ru(bpy)(3)](2+), attributed to the reduction of eilatin, and consistent with its low lying pi* orbital. The dinuclear complexes follow the same reduction trend, exhibiting several reversible reduction waves, and two reversible well-resolved metal centered oxidations due to the nonequivalent binding sites and to a significant metal-metal interaction mediated by the bridging eilatin.     

Effect of pH on the photophysical and redox properties of a ruthenium(II) mixed chelate derived from imidazole-4,5-dicarboxylic acid and 2,2'-bipyridine: an experimental and theoretical investigation

Combined experimental and DFT-TD-DFT computational studies were utilized to investigate the structural and electronic properties of mixed-ligand monometallic ruthenium(II) complexes of compositions [(bpy)(2)Ru(H(2)Imdc)](+) (1(+)), its N-H deprotonated form [(bpy)(2)Ru(HImdc)] (1), and COOH deprotonated form [(bpy)(2)Ru(Imdc)](-) (1(-)), where H(3)Imdc = imidazole-4,5-dicarboxylic acid and bpy = 2,2'-bipyridine. The optimized geometrical parameters for the complexes computed both in the gas phase and in solution are reported and compared with the previously reported X-ray data. The influence of pH on the absorption, emission, and redox properties of [(bpy)(2)Ru(H(2)Imdc)](+) (1(+)) has been thoroughly investigated. The absorption titration data were used to determine the ground state pK values, whereas the luminescence data were utilized for the determination of excited state acid dissociation constants. The proton-coupled redox activity of 1(+) has been studied over the pH range 2-12 in acetonitrile-water (3:2). From the E(1/2) versus pH profile, the equilibrium constants of the variously deprotonated complex species in Ru(II) and Ru(III) oxidation states have been determined. As compared to the protonated complex (1(+)), which undergoes reversible oxidation at 0.96 V (vs Ag/AgCl) in acetonitrile, the redox potential of the fully deprotonated complex (1(-)) is shifted to a much lower value, viz., 0.52 V. Density functional theory (DFT) and time-dependent DFT (TD-DFT) study provides insight into the nature of the ground and excited states with resulting detailed assignments of the orbitals involved in absorption and emission transitions. In particular, the red-shifts of the absorption and emission bands and the cathodic shift in the oxidation potential of 1(+) compared to 1 and 1(-) are also reproduced by our calculations.     

Synthesis and characterization of nitrosyl diruthenium complexes. Interaction between NO and CO across the metal-metal bond

Two neutral diruthenium complexes and one anionic diruthenium complex, Ru2(dpf)4(NO), Ru2(dpf)4(NO)2, and [Ru2(dpf)4(NO)]-, where dpf is diphenylformamidinate anion, were synthesized and characterized as to their electrochemical and spectroscopic properties. Two of the compounds, Ru2(dpf)4(NO) and Ru2(dpf)4(NO)2, were also structurally characterized. Ru2(dpf)4(NO) undergoes reversible one-electron reductions under N2 at E1/2=0.06 and -1.24 V in CH2Cl2, 0.1 M TBABr. These processes are shifted to E1/2=0.18 and -0.78 V under CO due to the trans-coordination of a CO molecule which stabilizes the singly and doubly reduced forms of the metal-metal bonded complexes, thus leading to easier reductions. CO does not coordinate to Ru2(dpf)4(NO), but it does bind to the singly reduced species to generate [Ru2(dpf)4(NO)(CO)]- under a CO atmosphere in solution; characteristic NO and CO bands are seen for this compound at nuNO=1674 cm(-1) and nuCO=1954 cm(-1). Ru2(dpf)4(NO)2 displays a reversible one-electron reduction at E1/2=-1.24 V versus SCE and an irreversible reduction at Epc=-1.96 V in CH2Cl2, 0.1 M TBAP under N2. There are also two reversible one-electron oxidations at E1/2=0.24 and 1.15 V. Spectroelectrochemical monitoring of the Ru2(dpf)4(NO)2 oxidation processes in a thin-layer cell shows only a single NO vibration for each electrogenerated product and nuNO is located at 1726 (neutral), 1788 (singly oxidized), or 1834 (doubly oxidized) cm(-1). Finally, a labile CO complex, [Ru2(dpf)4(NO)(CO)]-, could be generated by passing CO into a solution of [Ru2(dpf)4(NO)]-. Formation of the mixed CO/NO adduct was confirmed by electrochemistry and infrared spectroscopy. Analysis of the NO and CO stretching vibration frequencies for [Ru2(dpf)4(NO)(CO)]- by in-situ FTIR spectroelectrochemistry and comparisons with data for Ru2(dpf)4(NO) and Ru2(dpf)4(CO) reveal the presence of a strong interaction between NO and CO across the Ru-Ru bond.     

Controlling the Direction of the Molecular Axis of Rod‐Shaped Binuclear Ruthenium Complexes on Single‐Walled Carbon Nanotubes

We report the synthesis of a mixed‐valence ruthenium complex, bearing pyrene moieties on one side of the ligands as anchor groups. Composites consisting of mixed‐valence ruthenium complexes and SWNTs were prepared by noncovalent π–π interactions between the SWNT surface and the pyrene anchors of the Ru complex. In these composites, the long axis of the Ru complexes was aligned in parallel to the principal direction of the SWNT. The optimized conformation of these complexes on the SWNT surface was calculated by molecular mechanics. The composites were examined by UV/Vis absorption and FT‐IR spectroscopy, XPS, and SEM analysis. Furthermore, their electrochemical properties were evaluated. Cyclic voltammograms of the composites showed reversible oxidation waves at peak oxidation potentials (Ep_ox) = 0.86 and 1.08 V versus Fc⁺/Fc, which were assigned to the Ru^II‐Ru^II/Ru^II‐Ru^III and the Ru^II‐Ru^III/Ru^III‐Ru^III oxidation events of the dinuclear ruthenium complex, respectively. Based on these observations, we concluded that the electrochemical properties and mixed‐valence state of the dinuclear ruthenium complexes were preserved upon attachment to the SWNT surface.     

Mononuclear and dinuclear complexes of isoeilatin

This work describes the synthesis and characterization of mononuclear and dinuclear Ru(II) and Os(II) complexes based on the symmetrical bridging ligand isoeilatin (1). The crystal structure of 1.[HCl]2 consists of layers of tightly pi-stacked molecules of the biprotonated isoeilatin. The mononuclear complexes [Ru(bpy)2(ieil)]2+ (2(2+)) and [Os(bpy)2(ieil)]2+ (3(2+)) form discrete dimers in solution held together by face-selective pi-stacking interactions via the isoeilatin ligand. Coordination of a second metal fragment does not hinder the pi-stacking completely, as demonstrated by the concentration dependence of the 1H NMR spectra of the dinuclear complexes [{Ru(bpy)2}2{mu-ieil}]4+ (4(4+)), [{Os(bpy)2}2{mu-ieil}]4+ (5(4+)), and [{Ru(bpy)2}{mu-ieil}{Os(bpy)2}]4+ (6(4+)) and supported by the solid-state structure of meso-4.[Cl]4. The bridging isoeilatin ligand conserves its planarity even upon coordination of a second metal fragment, as demonstrated in the solid-state structures of meso-4.[Cl]4, meso-4.[PF6]4, and meso-5.[PF6]4. All of the dinuclear complexes exhibit a preference (3/2-3/1) for the formation of the heterochiral as opposed to the homochiral diastereoisomer. Absorption spectra of the mononuclear complexes feature a low-lying dpi(M) --> pi*iel MLCT band around 600 nm that shifts to beyond 700 nm upon coordination of a second metal fragment. Cyclic and square-wave voltammetry measurements of the complexes exhibit two isoeilatin-based reduction waves that are substantially anodically shifted compared to [M(bpy)3]2+ (M = Ru, Os). Luminescence spectra, quantum yields, and lifetime measurements at room temperature and at 77 K demonstrate that the complexes exhibit 3MLCT emission that occurs in the IR region between 950 and 1300 nm. Both the electrochemical and photophysical data are consistent with the low-lying pi orbital of the isoeilatin ligand. The dinuclear complexes exhibit two reversible, well-resolved, metal-centered oxidation waves, despite the chemical equivalence of the two metal centers, indicating a significant metal-metal interaction mediated by the bridging isoeilatin ligand.     

Synthesis and characterization of novel ferrocene-containing pyridylamine ligands and their ruthenium(II) complexes: electronic communication through hydrogen-bonded amide linkage

Tris(2-pyridylmethyl)amine (TPA) derivatives with one or two ferrocenoylamide moieties at the 6-position of one or two pyridine rings of TPA were synthesized. The compounds, N-(6-ferrocenoylamide-2-pyridylmethyl)-N,N-bis(2-pyridylmethyl)amine (Fc-TPA; L1) and N,N-bis(6-ferrocenoylamide-2-pyridylmethyl)-N-(2-pyridylmethyl)amine (Fc2-TPA; L2), were characterized by spectroscopic methods, cyclic voltammetry, and X-ray crystallography. Their Ru(II) complexes were also prepared and characterized by spectroscopic methods, cyclic voltammetry, and X-ray crystallography. [RuCl(L1)(DMSO)]PF(6) (1) that contains S-bound dimethyl sulfoxide (DMSO) as a ligand and an uncoordinated ferrocenoylamide moiety exhibited two redox waves at 0.23 and 0.77 V (vs ferrocene/ferrocenium ion as 0 V), due to Fc/Fc(+) and Ru(II)/Ru(III) redox couples, respectively. [RuCl(L2)]PF(6) (2) that contains both coordinated and uncoordinated amide moieties showed two redox waves that were observed at 0.27 V (two electrons) and 0.46 V (one electron), assignable to Ru(II)/Ru(III) redox couples overlapped with the uncoordinated Fc/Fc(+) redox couple and the coordinated Fc/Fc(+), respectively. In contrast to 2, an acetonitrile complex, [Ru(L2)(CH(3)CN)](PF(6))(2) (3), exhibited three redox couples at 0.26 and 0.37 V for two kinds of Fc/Fc(+) couples, and 0.83 V for the Ru(II)/Ru(III) couple (vs ferrocene/ferrocenium ion as 0 V). In this complex, the redox potentials of the coordinated and the uncoordinated Fc-amide moieties were discriminated in the range of 0.11 V. Chemical two-electron oxidation of 1 gave [RuIIICl(L1+)(DMSO)](3+) to generate a ferromagnetically coupled triplet state (S = 1) with J = 13.7 cm-1 (H = -JS(1)S(2)) which was estimated by its variable-temperature electron spin resonance (ESR) spectra in CH(3)CN. The electron spins at the Ru(III) center and the Fe(III) center are ferromagnetically coupled via an amide linkage. In the case of 2, its two-electron oxidation gave the same ESR spectrum, which indicates formation of a similar triplet state. Such electronic communication may occur via the amide linkage forming the intramolecular hydrogen bonding.     

Fluorescent and electroactive cyclic assemblies from perylene tetracarboxylic acid bisimide ligands and metal phosphane triflates

Tetraaryloxy-substituted perylene tetracarboxylic acid bisimides with one or two 4-pyridyl receptor substituents at the imide functionality were synthesized and employed in transition metal directed self-assembly with Pd(II) and Pt(II) phosphane triflates. Upon mixing of the components, quantitative formation of functional molecular square-type complexes containing four dye molecules and model complexes of a 2:1 (perylene bisimide ligand:transition metal ion) stoichiometry was observed. The isolated metallosupramolecular squares were characterized by 1H and 31P [1H] NMR spectroscopy as well as conventional electrospray ionization (ESI) and ESI-FTICR mass spectrometry, which gave evidence for the structure and the high stability of these giant cyclic dye assemblies (molecular weight (3a) 8172, Pt-Pt corner diagonal ca. 3.4 nm). Studies of the optical absorption and fluorescence properties and the electrochemistry and spectroelectrochemistry of both the perylene bisimide ligands and the perylene bisimide metal complexes show that Pt(II) coordination does not interfere with the optical and electrochemical properties of the perylene bisimide ligands; this gives squares with high fluorescence quantum yields (phiF (3a)=0.88) and three fully reversible redox couples. The latter could be unambiguously related to quantitative formation of perylene bisimide radical cations (E1/2 = +0.93 V vs. Fc/Fc+), radical anions (E1/2= - 1.01 V vs. Fc/Fc+), and dianions (E1/2 = -1.14 V vs. Fc/Fc+); these redox reactions change the charge state of the cyclic assembly from +12 to zero. In contrast, Pd(II) coordination influenced the electrochemical properties of the assembly because of an irreversible palladium reduction at E1/2= -1.15 V versus Fc/Fc+. Finally, dynamic ligand exchange processes between different metallosupramolecular assemblies were investigated by multinuclear NMR and electrospray mass spectrometry. These studies confirmed the reversible nature of the pyridine-Pt(II)/Pd(II) coordination process.     

Effects of axial coordination on the Ru-Ru single bond in diruthenium paddlewheel complexes

The 1,8-naphthyridine-based (NP-based) ligands with furyl, thiazolyl, pyridyl, and pyrrolyl attachments at the 2-position have been synthesized. Reactions of 3-MeNP (3-methyl-1,8-naphthyridine), fuNP (2-(2-furyl)-1,8-naphthyridine), tzNP (2-(2-thiazolyl)-1,8-naphthyridine), pyNP (2-(2-pyridyl)-1,8-naphthyridine), and prNP(-1) (2-(2-pyrrolyl)-1,8-naphthyridine) with [Ru2(CO)4(CH3CN)6]2+ lead to [Ru2(3-MeNP)2(CO)4(OTf)2] (1), [Ru2(fuNP)2(CO)4]2[BF4]2 (2), [Ru2(tzNP)2(CO)4][ClO4]2 (3), [Ru2(pyNP)2(CO)4][OTf]2 (4), and [Ru2(prNP)2(CO)4] (5). The molecular structures of complexes 1-5 have been established by X-ray crystallographic studies. The modulation of the Ru-Ru single-bond distances with axial donors triflates, furyls, thiazolyls, pyridyls, and pyrrolyls has been examined. A small and gradual increase in the Ru-Ru distance is measured with various donors of increasing strengths. The shortest Ru-Ru distance of 2.6071(9) angstroms is observed for the axially coordinated triflates in complex 1, and the longest Ru-Ru distance of 2.6969(10) angstroms is measured for axial pyrrolyls in complex 5. The Ru-Ru distances in complexes 3 (2.6734(7) angstroms) and 4 (2.6792(9) angstroms), having thiazolyls and pyridyls at axial sites respectively, are similar. The Ru-Ru distance for axial furyls in complex 2 (2.6261(9) angstroms) is significantly shorter than the corresponding distances in 3, 4, and 5. DFT calculations provide insight into the interaction of the Ru-Ru sigma orbital with axial donors. The Ru-Ru sigma orbital is elevated to a higher energy because of the interaction with axial lone pairs. The degree of destabilization depends on the nature of axial ligands: the stronger the ligand, higher the elevation of Ru-Ru orbital. The lengthening of Ru-Ru distances with respect to the axial donors in compounds 1-5 follows along the direction pyrrolyl > pyridyl approximately thiazolyl > furyl > triflate, and the trend correlates well with the computed destabilization of the Ru-Ru sigma orbitals.     

Novel coordinatively unsaturated bimetallic complexes, [(eta5-C5Me5)Ru(mu2-iPrNC(Me)=N(i)Pr)Ru(eta5-C5Me5)]+: a bridging amidinate ligand perpendicular to the metal-metal axis effectively stabilizes the highly reactive cationic diruthenium species.

Coordinatively unsaturated diruthenium complexes, [(eta5-C5Me5)Ru(mu2-iPrNC(Me)=NiPr)Ru(eta5-C5Me5)]+, of which crystallography revealed structures bearing a bridging amidinate ligand perpendicular to the Ru-Ru axis, were synthesized by anion exchange of [(eta5-C3Me5(Ru(mu2-iPrNC(Me)=NiPr)Ru(eta5-C5Me5)]+ Br- by weakly coordinating anions. Variable-temperature NMR showed rapid motion of the bridging amidinate ligand. The coordinatively unsaturated nature of the cationic complexes provides their high reactivity toward a series of two electron donor ligands. Oxidative addition of molecular hydrogen occurred to give [(eta5-C5Me5)Ru(mu2-iPrNC(Me)=NiPr)(mu-H)Ru(eta5-C5Me5)(H)]+, which was isolated and characterized.     

Influence of pH on the photochemical and electrochemical reduction of the dinuclear ruthenium complex, [(phen)2Ru(tatpp)Ru(phen)2)Cl4, in water: proton-coupled sequential and concerted multi-electron reduction

The dinuclear ruthenium complex [(phen)2Ru(tatpp)Ru(phen)2]4+ (P; in which phen is 1,10-phenanthroline and tatpp is 9,11,20,22-tetraaza tetrapyrido[3,2-a:2'3'-c:3',2'-l:2',3']-pentacene) undergoes a photodriven two-electron reduction in aqueous solution, thus storing light energy as chemical potential within its structure. The mechanism of this reduction is strongly influenced by the pH, in that basic conditions favor a sequential process involving two one-electron reductions and neutral or slightly acidic conditions favor a proton-coupled, bielectronic process. In this complex, the central tatpp ligand is the site of electron storage and protonation of the central aza nitrogen atoms in the reduced products is observed as a function of the solution pH. The reduction mechanism and characterization of the rich array of products were determined by using a combination of cyclic and AC voltammetry along with UV-visible reflectance spectroelectrochemistry experiments. Both the reduction and protonation state of P could be followed as a function of pH and potential. From these data, estimates of the various reduced species' pKa values were obtained and the mechanism to form the doubly reduced, doubly protonated complex, [(phen)2Ru(H2tatpp)Ru(phen)2]4+ (H2P) at low pH (< or =7) could be shown to be a two-proton, two-electron process. Importantly, H2P is also formed in the photochemical reaction with sacrificial reducing agents, albeit at reduced yields relative to those at higher pH.     

Mono-, di-, and tetranuclear ruthenium(II) complexes containing 3-(pyridin-2-yl)-as-triazino[5,6-f]1,10-phenanthroline: synthesis, characterization, and electrochemical and photophysical properties

Mono-, di-, and tetranuclear Ru(II) polypyridine complexes based on the bridging ligand pdtp, where pdtp is 3-(pyridin-2-yl)-as-triazino[5,6-f]1,10-phenanthroline, have been synthesized and characterizated. This asymmetric bridging ligand is composed of two nonequivalent coordinating sites: one involves the phenanthroline moiety, and the other one involves the pyridyltriazine moiety. Electrochemical data show that the first redox process in these complexes is pdtp based and the metal-metal interaction in di- and tetranuclear complexes is very weak. The two oxidations (+1.41 and +1.56 V vs SCE) observed in dinuclear complex 2 are mainly ascribed to the different coordination environments of two metal centers. Absorption spectra are essentially the sum of the spectra of the component monometallic species. The emission spectra are measured both at room temperature and at 80 K in a 4:1 (v/v) EtOH/MeOH matrix. The complexes all display luminescence properties which are close to that featured by the parent [Ru(phen)(3)](2+) species. It is also noted that center-to-periphery energy transfer occurs in the dendritic tetranuclear complex 3.     

Syntheses of homochiral multinuclear ru complexes based on oligomeric bibenzimidazoles

The homochiral multinuclear Ru complexes of the oligomeric bibenzimidazoles were synthesized using Lambda-[Ru(bpy)2(py)2][(-)-O,O'-dibenzoyl-l-tartrate].12H2O as an enantiomerically pure building block. The complexations proceed with the retention of configuration to provide well-defined mononuclear, dinuclear, tetranuclear, and octanuclear Ru complexes successfully. The optical purity and the absolute configurations of the complexes were determined by NMR and circular dichroism spectrometry. The rare X-ray structure of a tetranuclear complex Lambda4-[(Ru(bpy)2)4(bis(BiBzIm))](PF6)4 was resolved. The crystallographic analysis reveals that all the four Ru centers have Lambda octahedral configurations, with a Ru-Ru separation of 5.509 A across the bridging bibenzimidazole ligand, which maintains near coplanarity. The UV-vis spectroscopic and electrochemical properties of the homochiral multinuclear assemblies were studied, indicating weak electronic communications between the metal centers.     

Edge-bridging and face-capping coordination of alkenyl ligands in triruthenium carbonyl cluster complexes derived from hydrazines: synthetic, structural, theoretical, and kinetic studies

The reactions of the triruthenium cluster complex [Ru3(mu-H)(mu3-eta2-HNNMe2)(CO)9] (1; H2NNMe2=1,1-dimethylhydrazine) with alkynes (PhC triple bond CPh, HC triple bond CH, MeO2CC triple bond CCO2Me, PhC triple bond CH, MeO2CC triple bond CH, HOMe2CC triple bond CH, 2-pyC triple bond CH) give trinuclear complexes containing edge-bridging and/or face-capping alkenyl ligands. Whereas the edge-bridged products are closed triangular species (three Ru-Ru bonds), the face-capped products are open derivatives (two Ru-Ru bonds). For terminal alkynes, products containing gem (RCCH2) and/or trans (RHCCH) alkenyl ligands have been identified in both edge-bridging and face-capping positions, except for the complex [Ru3(mu3-eta2-HNNMe2)(mu3-eta3-HCCH-2-py)(mu-CO)(CO)7], which has the two alkenyl H atoms in a cis arrangement. Under comparable reaction conditions (1:1 molar ratio, THF at reflux, time required for the consumption of complex 1), some reactions give a single product, but most give mixtures of isomers (not all the possible ones), which were separated. To determine the effect of the hydrazido ligand, the reactions of [Ru3(mu-H)(mu3-eta2-MeNNHMe)(CO)9] (2; HMeNNHMe=1,2-dimethylhydrazine) with PhC triple bond CPh, PhC triple bond CH, and HC triple bond CH were also studied. For edge-bridged alkenyl complexes, the Ru--Ru edge that is spanned by the alkenyl ligand depends on the position of the methyl groups on the hydrazido ligand. For face-capped alkenyl complexes, the relative orientation of the hydrazido and alkenyl ligands also depends on the position of the methyl groups on the hydrazido ligand. A kinetic analysis of the reaction of 1 with PhC[triple chemical bond]CPh revealed that the reaction follows an associative mechanism, which implies that incorporation of the alkyne in the cluster is rate-limiting and precedes the release of a CO ligand. X-ray diffraction, IR and NMR spectroscopy, and calculations of minimum-energy structures by DFT methods were used to characterize the products. A comparison of the absolute energies of isomeric compounds (obtained by DFT calculations) helped rationalize the experimental results.     

Transition metal complexes coordinated by an NAD(P)H model compound and their enhanced hydride-donating abilities in the presence of a base

The ruthenium(II) and rhenium(I) complexes containing an NAD(P)H model compound, 1-benzyl-1,4-dihydronicotinamide (BNAH), as ligand, [Ru(tpy)(bpy)(BNAH)]2+ (1 a) and [Re(bpy)(CO)3(BNAH)]+ (1 b), were quantitatively produced by the reaction of the corresponding metal hydrido complexes with BNA(+) (1-benzylnicotinamidium cation). In the presence of base with pK(a) = 8.9, 1 a and 1 b have much greater reducing power than "free" BNAH. The oxidation potentials of 1 a in the absence and the presence of triethylamine were 0.55 V and -0.04 V, respectively, versus Ag/AgNO(3), whereas that of "free" BNAH was 0.30 V. Spectroscopic results clearly showed that the base extracts a proton from the carbamoyl group on 1 a and 1 b to give the deprotonated BNAH coordinating to the transition-metal complexes [Ru(tpy)(bpy)(BNAH-H+)]+ (3 a) and [Re(bpy)(CO)3(BNAH-H+)] (3 b); this deprotonation underlies the enhancement in reducing ability. The deprotonated forms 3 a and 3 b can efficiently reduce other NAD(P) models to give the corresponding 1,4-dihydro form, resulting in the deprotonated BNA+ being coordinated to the metal complexes [Ru(tpy)(bpy)(BNA(+)-H+)]2+ (2 a) and [Re(bpy)(CO)3(BNA+-H+)]+ (2 b); "free" BNAH and the protonated adducts 1 a and 1 b cannot act in this way. X-ray crystallography was performed on the PF6- salt of 2 a, and showed that the deprotonated nitrogen atom on the carbamoyl group coordinates to the ruthenium(II) metal center with a bond length of 2.086(3) Angstroms. Infrared spectral data suggested that the deprotonated carbamoyl group on the reduced forms 3 a and 3 b is converted to the imido group, and that the oxygen atom coordinates to the metal center.     

Role of ruthenium oxidation states in ligand-to-ligand charge transfer processes

We describe in this paper the properties of [Ru(II/III)(bpy)(2)ClL](+1/+2) and [Ru(II/III)(bpy)(2)L(2)](+2/+3). L = ditolyl-3-pyridylamine (dt3pya) is a redox active ligand related to triarylamines, which is very similar to 3-aminopyridine except for the reversible redox behavior. The monosubstituted complex shows a metal-to-ligand charge-transfer (MLCT) at 502 nm, and reversible waves in acetonitrile at E(0)(Ru(III/II)) = 1.07 V, E(0)(L(+/0)) = 1.46 V (NHE). The disubstituted complex shows an MLCT at 461 nm, a photorelease of dt3pya with quantum yield of 0.11 at 473 nm, and two reversible one-electron overlapped waves at 1.39 V associated with one of the ligands (1.37 V) and Ru(III/II) (1.41 V). Further oxidation of the second ligand at 1.80 V forms a 2,2'-bipiridine derivative, in an irreversible reaction similar to dimerization of triphenylamine to yield tetraphenylbenzidine. In the dioxidized state, the spectroelectrochemistry of the disubstituted complex shows a ligand-to-ligand charge transfer at 1425 nm, with a transition moment of 1.25 ? and an effective two-state coupling of 1200 cm(-1). No charge transfer between ligands was observed when Ru was in a 2+ oxidation state. We propose that a superexchange process would be involved in ligand-metal-ligand charge transfer, when ligands and metals are engaged in complementary π interactions, as in metal-ligand-metal complexes. Best orbital matching occurs when metallic donor fragments are combined with acceptor ligands and vice versa. In our case, Ru(III) bridge (an acceptor) and two dt3pya (donors, one of them being oxidized) made the complex a Robin-Day Class II system, while the Ru(II) bridge (a donor, reduced) was not able to couple two dt3pya (also donors, one oxidized).     

Strongly Coupled Cyclometalated Ruthenium–Triarylamine Hybrids: Tuning Electrochemical Properties,Intervalence Charge Transfer,and Spin Distribution by Substituent Effects

Nine cyclometalated ruthenium complexes with a redox‐active diphenylamine unit in the para position to the Ru?C bond were prepared. MeO, Me, and Cl substituents on the diphenylamine unit and three types of auxiliary ligands—bis(N‐methylbenzimidazolyl)pyridine (Mebip), 2,2′:6′,2′′‐terpyridine (tpy), and trimethyl‐4,4′,4′′‐tricarboxylate‐2,2′:6′,2′′‐terpyridine (Me₃tctpy)—were used to vary the electronic properties of these complexes. The derivative with an MeO‐substituted amine unit and Me₃tctpy ligand was studied by single‐crystal X‐ray analysis. All complexes display two well‐separated redox waves in the potential region of +0.1 to +1.0 V versus Ag/AgCl, and the potential splitting ranges from 360 to 510 mV. Spectroelectrochemical measurements show that these complexes display electrochromism at low potentials and intense near‐infrared (NIR) absorptions. In the one‐electron oxidized form, the complex with the Cl‐substituted amine unit and Mebip ligand shows a moderate ligand‐to‐metal charge transfer at 800 nm. The other eight complexes show asymmetric, narrow, and intense intervalence charge‐transfer transitions in the NIR region, which are independent of the polarity of the solvent. The Mebip‐containing complexes display rhombic or broad isotropic EPR signals, whereas the other seven complexes show relatively narrow isotropic EPR signals. In addition, DFT and time‐dependent DFT studies were performed to gain insights into the spin distributions and NIR absorptions.     

Metal-assisted unusual hydroxylation at the carbon atom of the triazine ring in dinuclear ruthenium(II) and osmium(II) complexes bridged by 2,4,6-tris(2-pyridyl)-1,3,5-triazine: synthesis, structural characterization, stereochemistry, and electrochemical studies

The reaction of cis-[M(bpy)2Cl2] (M = Ru(II), and Os(II) with 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) in refluxing ethanol-water resulted in the formation of dinuclear complexes of the composition [(M(bpy)2)2(tptz-OH)](PF6)3.nH2O (n = 1 for Ru and n = 0 for Os). In this reaction an unusual metal-induced hydroxylation at the carbon atom of the triazine ring of bridged tptz occurred. However, hydroxylation did not occur in the corresponding mononuclear complexes under similar reaction condition. A comparative study revealed that sufficient electrophilicity on the carbon atom and free movement of the attached pyridyl ring promoted the hydroxylation reaction. The hydroxylated dinuclear complexes exist in two stereoisomeric forms, a rac form (delta delta/lambda lambda) and a meso form (delta lambda/lambda delta). Both diastereoisomers have been isolated in pure form and characterized. The molecular structures of the rac form of Ru(II) complex (3-II) and meso form of the Os(II) complex (4-I) have been established by single-crystal X-ray studies. Crystal data: complex 3-II, monoclinic, C2/c, a = 24.584(7) A, b = 14.309(4) A, c = 41.044(13) A, beta = 92.84(2) degrees, V = 14420.0(7) A3, Z = 8, R = 0.179, wR2 = 0.479; complex 4-I, triclinic, P1, a = 13.444(7) A, b = 14.576(5) A, c = 19.641(7) A, alpha = 98.21(3) degrees, beta = 101.67(4) degrees, gamma = 105.80(4) degrees, V = 3546.0(3) A3, Z = 2, R = 0.093, wR2 = 0.279. The poor data quality of 3-II did not allow anisotropic refinement of non-hydrogen atoms except Ru and P. A PLUTO drawing of this compound is given only to support the molecular structure. 1H NMR data have been used to characterize the diastereoisomers. The dinuclear complexes exhibit unusual electrochemical behavior; cathodic shifts of the metal-centered oxidations and ligand-based first reduction compared to mononuclear complexes have been observed. There is a splitting in the metal-centered oxidation potentials, indicating strong electronic communication between the metal centers. Comproportionation constants (Kcom) of the mixed-valence species have been calculated; the values are in the range 6.03 x 10(4)-4.7 x 10(6). It appears that a metal-metal interaction occurred by an electron-transfer mode across the low-lying pi* orbital of the bridged tptz.