Proton-Coupled Electron Transfer between 4-Cyanophenol and Photoexcited Rhenium(I) Complexes with Different Protonatable Sites

Two rhenium(I) tricarbonyl diimine complexes, one of them with a 2,2'-bipyrazine (bpz) and a pyridine (py) ligand in addition to the carbonyls ([Re(bpz)(CO)(3)(py)](+)), and one tricarbonyl complex with a 2,2'-bipyridine (bpy) and a 1,4-pyrazine (pz) ligand ([Re(bpy)(CO)(3)(pz)](+)) were synthesized, and their photochemistry with 4-cyanophenol in acetonitrile solution was explored. Metal-to-ligand charge transfer (MLCT) excitation occurs toward the protonatable bpz ligand in the [Re(bpz)(CO)(3)(py)](+) complex while in the [Re(bpy)(CO)(3)(pz)](+) complex the same type of excitation promotes an electron away from the protonatable pz ligand. This study aimed to explore how this difference in electronic excited-state structure affects the rates and the reaction mechanism for photoinduced proton-coupled electron transfer (PCET) between 4-cyanophenol and the two rhenium(I) complexes. Transient absorption spectroscopy provides clear evidence for PCET reaction products, and significant H/D kinetic isotope effects are observed in some of the luminescence quenching experiments. Concerted proton-electron transfer is likely to play an important role in both cases, but a reaction sequence of proton transfer and electron transfer steps cannot be fully excluded for the 4-cyanophenol/[Re(bpz)(CO)(3)(py)](+) reaction couple. Interestingly, the rate constants for bimolecular excited-state quenching are on the same order of magnitude for both rhenium(I) complexes.     

Density functional theory calculations of selected Ru(II) two ring diimine complex dications

Geometry optimization for a series of ten, two-ring diimine Ru(II) complexes was effected using the Gaussian 98 protocol at density functional theory (DFT) B3LYP level with basis sets 3-21G*and 3-21G**. HOMO-LUMO energy difference values compared favorably to the experimental data from electrochemistry [Delta E(1/2) = (E(1/2ox) - E(1/2red))] and the lowest energy absorption maxima, which for these complexes correspond to the metal-to-ligand charge transfer (MLCT) band. The HOMO and LUMO distributions from DFT support the idea that the lowest energy transitions are metal-to-ligand charge transfer and that the lowest energy LUMO for the mixed ligand complexes is located on 2,2'-bipyrazine (bpz), followed by 2,2'-bipyrimidine (bpm) and then 2,2'-bipyridine (bpy).     

Synthesis and study of Ru,Rh,Ru triads: modulation of orbital energies in a supramolecular architecture

Supramolecular trimetallic complexes [((tpy)RuCl(BL))(2)RhCl(2)](3+) where tpy = 2,2':6',2' '-terpyridine and BL = dpp or bpm [dpp = 2,3-bis(2-pyridyl)pyrazine and bpm = 2,2'-bipyrimidine] have been synthesized and characterized. The mixed-metal complexes couple a reactive rhodium(III) center to two ruthenium(II) light absorbers to form a light absorber-electron collector-light absorber triad. The variation of the bridging (dpp and bpm) and terminal (tpy in lieu of bpy) ligands has some profound effects on the properties of these complexes, and they are remarkably different from the previously reported [((bpy)(2)Ru(bpm))(2)RhCl(2)](5+) system. The electrochemical data for both title trimetallics consist of overlapping Ru(III/II) couples for both terminal metals at 1.12 V versus the Ag/AgCl reference electrode. Cathodically an irreversible Rh(III/I) reduction followed by bridging ligand reductions is seen. This is indicative of highest occupied molecular orbitals (HOMO) localized on the terminal ruthenium metal centers and a lowest unoccupied molecular orbital (LUMO) residing on the rhodium. This rhodium-based LUMO is in contrast to the bpy analogue [((bpy)(2)Ru(bpm))(2)RhCl(2)](5+), which has a bpm(pi) localized LUMO. This orbital inversion by terminal ligand variation illustrates the similar energy of these Rh(dsigma) and bpm(pi) orbitals within this structural motif. Both title trimetallics possess broad, low-energy Ru --> BL charge transfer absorbances at 540 nm (dpp) and 656 nm (bpm). A comparison of the spectroscopic, electrochemical, and spectroelectrochemical properties of these trimetallic complexes is presented.     

Protonation studies of reduced ruthenium(II) complexes with polypyridyl ligands

The pKa values associated with protonation of the one-electron reduced forms of series of [L'2Ru(II)L]2+ complexes [L' = bidentate polypyridyl ligand; L = bidentate polypyridyl ligand with additional uncoordinated N atoms in the aromatic ring system: e.g., dpp = 2,3-bis(2-pyridyl)pyrazine, bpz = 2,2'-bipyrazine] were assessed using pulse radiolysis techniques by the measurement of spectral variations as a function of pH. A linear correlation was observed between pKa and E (RuL'2L2+/+) for complexes in which the protonatable ligand was at the same time the site of reduction. In complexes where one or more of the nonprotonatable ligands (L') had very low pi* energy levels [e.g. (CF3)4bpy], reduction occurs on a nonprotonatable ligand and a dramatic decrease in the pKa values was observed for the reduced species. In complexes where the energies of the protonatable and nonprotonatable ligands were comparable, the protonation behavior was consistent with some orbital mixing/ delocalization of the electronic charge.     

Ab initio calculations and normal coordinate analysis of ruthenium tris-alpha-diimine complexes

For the first time, a full scaled quantum chemical normal coordinate analysis has been performed on [Ru(LL')(3)](2+) complexes, where LL' = 2,2'-bipyrazine (bpz) or 2,2'-bipyrimidine (bpm). Geometric structures were fully optimized using density functional theory and an effective core potential basis set. The infrared and Raman spectra were calculated using the optimized geometries. The results of the calculations provide a highly satisfactory fit to the experimental infrared and Raman spectra, and the potential energy distributions allow a detailed understanding of the vibrational bands therein.     

Reduced and excited states of the intermediates (alpha-diimine)(C5R5)Rh in hydride transfer catalysis schemes: EPR and resonance Raman spectroscopy, and comparative DFT calculations of Co, Rh and Ir analogues

The electronic structures of the highly air-sensitive intermediates (N[caret]N) (C(5)Me(5))Rh, (N[caret]N = 2,2'-bipyridine (bpy), 2,2'-bipyrimidine (bpym), 2,2'-bipyrazine (bpz) and 3,3'-bipyridazine (bpdz)) of hydride transfer catalysis schemes were studied through resonance Raman (rR) spectroscopy and through EPR of the reduced forms [(N[caret]N) (C(5)Me(5))Rh](.-). The rR results are compatible with a predominant MLCT character of the lowest excited states [ (N[caret]N) (C(5)Me(5))Rh]*, and the EPR spectra of the reduced states reveal the presence of anion radical ligands, (N[caret]N) (.-), coordinated by unusually electron rich rhodium(i) centres. The experimental results, including the assignments of electronic transitions, are supported by DFT calculations for the model compounds [(N[caret]N)(C(5)H(5))Rh](o)/(.-), (N[caret]N) = bpy or bpym. The calculations confirm a significant but not complete mixing of metal and ligand orbitals in the lowest unoccupied MO which still retains about 3/4 pi* (N[caret]N) character. DFT calculations on (bpy)(C(5)H(5))M and [(bpy)(C(5)H(5))ClM](+), M = Co, Rh, Ir, agree with the experimental results such as the differences between the homologues, especially the different LUMO characters of the precursor cations in the case of Co-->d(M)) and Rh or Ir (-->pi*(bpy)).     

Synthesis, crystal structures and magnetic properties of tricyanomethanide-containing copper(II) complexes

The preparation, crystal structures and magnetic properties of the copper(II) complexes of formula [Cu(pyim)(tcm)(2)](n) (1), [Cu(bpy)(tcm)(2)](n) (2), [Cu(4)(bpz)(4)(tcm)(8)] (3), {[Cu(terpy)(tcm)].tcm}(n) (4) and {[Cu(2)(tppz)(tcm)(4)].3/2H(2)O}(n) (5) [pyim = 2-(2-pyridyl)imidazole, tcm = tricyanomethanide, bpy = 2,2'-bipyridine, bpz = 2,2'-bipyrazine, terpy = 2,2':6',2'-terpyridine and tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine] are reported. Complexes, 1, 2 and 4 are uniform copper(II) chains with single- (1 and 4) and double-(2) micro-1,5-tcm bridges with values of the intrachain copper-copper separation of 7.489(1) (1), 7.520(1) and 7.758(1) (2) and 7.469(1) A (4). Each copper atom in 1, 2 and 4 is five-coordinate with bidentate pyim (1)/bpy (2) and tridentate terpy (4) ligands and nitrile-nitrogen atoms from bridging (1,2 and 4) and terminal (1) tcm groups building a distorted square pyramidal surrounding. The structure of 3 is made up of neutral centrosymmetric rectangles of (2,2'-bipyrazine)copper(II) units at the corners, the edges being built by single- and double-micro-1,5-tcm bridges with copper-copper separations of 7.969(1) and 7.270(1) A, respectively. Five- and six-coordinated copper atoms with distorted square pyramidal and elongated octahedral environments occur in . Compound 5 is a neutral copper(II) chain with regular alternating bis-tridentate tppz and double micro-1,5-tcm bridges, the intrachain copper-copper distances being 6.549(7) and 7.668(1) A, respectively. The two crystallographically independent copper atoms in 5 have an elongated octahedral geometry with three tppz nitrogen atoms and a nitrile-nitrogen atom from a bridging tcm group in the equatorial positions, and two nitrile nitrogen atoms from a terminal and a bridging tcm ligand occupying the axial sites. The investigation of the magnetic properies of 1-5 in the temperature range 1.9-295 K has shown the occurrence of weak ferro- [J = +0.11(1) cm(-1) (2)] and antiferromagnetic interactions [J = -0.093(1) (1), -0.083(1) (4), -0.04(1) and 1.21(1) cm(-1) (3)] across the micro-1,5-tcm bridges and intermediate antiferromagnetic coupling [-J = 37.4(1) cm(-1) (5)] through bis-tridentate tppz. The values of the magnetic interactions are analyzed through simple orbital symmetry considerations and compared with those previously reported for related systems.     

Bipyrimidine-Bridged Mixed-Metal Trimetallic Complexes of Ruthenium(II) with Rhodium(III) or Iridium(III), {[(bpy)(2)Ru(bpm)](2)MCl(2)}(5+)

Bipyrimidine-bridged trimetallic complexes of the form {[(bpy)(2)Ru(bpm)](2)MCl(2)}(5+), where M = Rh(III) or Ir(III), bpy = 2,2'-bipyridine, and bpm = 2,2'-bipyrimidine, have been synthesized and characterized. These complexes are of interest in that they couple catalytically active rhodium(III) and iridium(III) metals with light-absorbing ruthenium(II) metals within a polymetallic framework. Their molecular composition is a light absorber-electron collector-light absorber core of a photochemical molecular device (PMD) for photoinitiated electron collection. The variation of the central metal has some profound effects on the observed properties of these complexes. The electrochemical data for the title trimetallics consist of a Ru(II/III) oxidation and sequential reductions assigned to the bipyrimidine ligands, Ir or Rh metal centers, and bipyridines. In both trimetallic complexes, the first oxidation is Ru based and the bridging ligand reductions occur prior to the central metal reduction. This illustrates that the highest occupied molecular orbital (HOMO) is localized on the ruthenium metal center and the lowest unoccupied molecular orbital resides on the bpm ligand. This bpm-based LUMO in {[(bpy)(2)Ru(bpm)](2)RhCl(2)}(5+) is in contrast with that observed for the monometallic [Rh(bpm)(2)Cl(2)](+) where the Rh(III)/Rh(I) reduction occurs prior to the bpm reduction. This orbital inversion is a result of bridge formation upon construction of the trimetallic complex. Both the Ir- and Rh-based trimetallic complexes exhibit a room temperature emission centered at 800 nm with tau = 10 ns. A detailed comparison of the spectroscopic, electrochemical, and spectroelectrochemical properties of these polymetallic complexes is described herein.     

Prolonging the Triplet State Lifetimes of Rhenium Complexes with Imidazole-Pyridine Framework for Efficient CO2 Photoreduction

The photocatalytic reduction of CO₂ into fuels offers the prospect for creating a new CO₂ economy. Harnessing visible light-driven CO₂-to-CO reduction mediated by the long-lived triplet excited state of rhenium(I) tricarbonyl complexes is a challenging approach. We here develop a series of new mononuclear rhenium(I) tricarbonyl complexes ( Re-1 − Re-4 ) based on the imidazole-pyridine skeleton for photo-driven CO₂ reduction. These catalysts are featured by combining pyridyl-imidazole with the aromatic ring and different pendant organic groups onto the N1 position of 1,3-imidazole unit, which display phosphorescence under Ar-saturated solution even at ambient conditions. By contrast, {Re[9-(pyren-1-yl)-10-(pyridin-2-yl)-9H-pyreno[4,5-d]imidazole)](CO)₃Cl} ( Re-4 ) by introducing pyrene ring at the N1 position of pyrene-fused imidazole unit exhibits superior catalytic performance with a higher turnover number for CO (TON_CO=124) and >99.9 % selectivity, primarily ascribed to the strong visible light-harvesting ability, long-lived triplet lifetimes (164.2 μs) and large reductive quenching constant. Moreover, the rhenium(I) tricarbonyl complexes derived from π-extended pyrene chromophore exhibit a long lifetime corresponding to its ligand-localized triplet state (³IL) evidenced from spectroscopic investigations and DFT calculations.     

Structural and pH dependence of excited state PCET reactions involving reductive quenching of the MLCT excited state of [RuII(bpy)2(bpz)]2+ by hydroquinones

The proton-coupled electron transfer (PCET) reaction between the bpz-based photoexcited (3)MLCT state of [Ru(II)(bpy)(2)(bpz)](2+) (bpy = 2,2'-bipyridine, bpz = 2,2'-bipyrazine) and a series of substituted hydroquinones (H(2)Q) has been studied by transient absorption (TA) and time-resolved electron paramagnetic resonance (TREPR) spectroscopy at X-band. When the reaction is carried out in a CH(3)CN/H(2)O mixed solvent system with unsubstituted hydroquinone, the neutral semiquinone radical (4a) and its conjugate base, the semiquinone radical anion (4b), are both observed. Variation of the acid strength in the solvent mixture allows the acid/base dependence of the PCET reaction to be investigated. In solutions with very low acid concentrations, TREPR spectra exclusively derived from radical anion 4b are observed, while at very high acid concentrations, the spectrum is assigned to the protonated structure 4a. At intermediate acid concentrations, either a superposition of spectra is observed (slow exchange between 4a and 4b) or substantial broadening in the TREPR spectrum is observed (fast exchange between 4a and 4b). Variation of substituents on the H(2)Q ring substantially alter this acid/base dependence and provide a means to investigate electronic effects on both the ET and PT components of the PCET process. The TA results suggest a change in mechanism from PCET to direct ET quenching in strongly basic solutions and with electron withdrawing groups on the H(2)Q ring system. Changing the ligand on the Ru complex also alters the acid/base dependence of the TREPR spectra through a series of complex equilibria between protonated and deprotonated hydroquinone radicals and anions. The relative intensities of the signals from radical 4a versus 4b can be rationalized quantitatively in terms of these equilibria and the relevant pK(a) values. An observed equilibrium deuterium isotope effect supports the conclusion that the post-PCET HQ(?)/Q(?-) equilibrium is the most important in determining the 4a/4b ratio at early delay times.     

Zeolite-based organized molecular assemblies. photophysical characterization and documentation of donor oxidation upon photosensitized charge separation

An organized molecular assembly composed of two ruthenium polypyridine complexes, Ru(bpy)(2)(bpz)(2+) and Ru(bpy)(2)(H(2)O)(2)(2+) (where bpy = 2, 2'-bipyridine and bpz = 2, 2'-bipyrazine), has been prepared in adjacent supercages of Y-zeolite. This material has been characterized by diffuse reflectance, electronic absorption, electronic emission, and resonance Raman (RR) spectroscopy, as well as lifetime measurements. The spectral results confirm the identity of the entrapped complexes and resonance Raman measurements show that the relative concentrations of the two complexes within the zeolite particles are identical. A dramatic decrease in emission intensity observed for the adjacent cage assembly, relative to that observed for an appropriate reference material composed of a mixture of zeolite particles containing the separated complexes, indicates strong interaction between the adjacent complexes which provides an additional nonradiative decay pathway. The excited state lifetime measurements implicate a very short-lived component, dominating the decay curve at early times, which is most reasonably attributed to excited-state electron-transfer quenching of the adjacent cage pair. More importantly, analysis of diffuse reflectance spectra acquired during selective (sensitizer) irradiation of a sample of this material, wherein the remaining cages are filled with a suitable acceptor (MV(2+)), provides direct evidence for oxidation of the Ru(bpy)(2)(H(2)O)(2)(2+) donor complex, confirming the targeted synergy of the adjacent cage assembly.     

Luminescent rhenium(I) polypyridine fluorous complexes as novel trifunctional biological probes

We present the synthesis, characterization, and photophysical properties of three luminescent rhenium(I) polypyridine fluorous complexes [Re(Me(2)bpy)(CO)(3)(L)](PF(6)) (Me(2)bpy = 4,4'-dimethyl-2,2'-bipyridine; L = 3-amino-5-(N-((3-perfluorooctyl)propyl)aminocarbonyl)pyridine (py-Rf-NH(2)) (1), 3-isothiocyanato-5-(N-((3-perfluorooctyl)propyl)aminocarbonyl)pyridine (py-Rf-NCS) (2), 3-ethylthioureidyl-5-(N-((3-perfluorooctyl)propyl)aminocarbonyl)pyridine (py-Rf-TU-C(2)H(5)) (3)). The isothiocyanate complex 2 has been used to label bovine serum albumin (BSA) and glutathione (GSH). The photophysical properties of the resultant bioconjugates have been studied. The isolation of the luminescent fluorous rhenium-GSH conjugate from a mixture of 20 amino acids has been demonstrated using fluorous solid-phase extraction (FSPE). Additionally, the cytotoxicity of complexes 1 and 3 toward HeLa cells has been examined by the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) assay. The cellular uptake properties of complex 3 have also been investigated by laser-scanning confocal microscopy.     

Toward more efficient photochemical CO2 reduction: Use of scCO2 or photogenerated hydrides

Rhenium(I) and ruthenium(II) complexes have been successfully used for photochemical CO₂ reduction to CO or formate. However, a typical turnover frequency for such reactions is <20 h^?1 and the formation of reduced species beyond CO or formate is very limited. In the case of the rhenium(I) bipyridyl tricarbonyl system, the key intermediate has been shown to decay with a first-order dependence on [CO₂] to produce CO, which is the rate-determining step. The limited concentration of dissolved CO₂ in organic solvents results in extremely slow CO₂ reduction. To improve the reaction rate, we prepared new CO₂-soluble rhenium(I) bipyridine complexes bearing fluorinated alkyl ligands and investigated their photophysical properties in CH₃CN and supercritical CO₂. We also investigated the properties of a metal complex with an NAD⁺ model ligand, [Ru(bpy)₂(pbn)]²⁺ (bpy = 2,2′-bipyridine, pbn = 2-(2-pyridyl)-benzo[b]-1,5-naphthyridine), and prepared the corresponding NADH-like complex [Ru(bpy)₂(pbnHH)]²⁺ upon MLCT excitation followed by reductive quenching. This species can be used as a renewable hydride donor. The electrochemical and photochemical properties, and the reactivity of these species toward CO₂ reduction were investigated.     

Structural, electronic, and acid/base properties of [Ru(bpy)2(bpy(OH)2)]2+ (bpy = 2,2'-bipyridine, bpy(OH)2 = 4,4'-dihydroxy-2,2'-bipyridine)

We have synthesized the complex [Ru(bpy)(2)(bpy(OH)(2))](2+) (bpy =2,2'-bipyridine, bpy(OH)(2) = 4,4'-dihydroxy-2,2'-bipyridine). Experimental results coupled with computational studies were utilized to investigate the structural and electronic properties of the complex, with particular attention paid toward the effects of deprotonation on these properties. The most distinguishing feature observed in the X-ray structural data is a shortening of the CO bond lengths in the modified ligand upon deprotonation. Similar results are also observed in the computational studies as the CO bond becomes double bond in character after deprotonating the complex. Electrochemically, the hydroxy-modified bipyridyl ligand plays a significant role in the redox properties of the complex. When protonated, the bpy(OH)(2) ligand undergoes irreversible reduction processes; however, when deprotonated, reduction of the substituted ligand is no longer observed, and several new irreversible oxidation processes associated with the modified ligand arise. pH studies indicate [Ru(bpy)(2)(bpy(OH)(2))](2+) has two distinct deprotonations at pK(a1) = 2.7 and pK(a2) = 5.8. The protonated [Ru(bpy)(2)(bpy(OH)(2))](2+) complex has a characteristic UV/Visible absorption spectrum similar to the well-studied complex [Ru(bpy)(3)](2+) with bands arising from Metal-to-Ligand Charge Transfer (MLCT) transitions. When the complex is deprotonated, the absorption spectrum is altered significantly and becomes heavily solvent dependent. Computational methods indicate that the deprotonated bpy(O(-))(2) ligand mixes heavily with the metal d orbitals leading to a new absorption manifold. The transitions in the complex have been assigned as mixed Metal-Ligand to Ligand Charge Transfer (MLLCT).     

Bimetallic Ru(II)–Ir(III) complexes with bridging 2,2′‐bipyrimidine: Synthesis,structures and characterization

In this work, four bimetallic Ru(II)–Ir(III) complexes with the general formula [(bpy)₂Ru(bpm)Ir(C^N)₂](PF₆)₃ (bpy = 2,2‐bipyridine, bpm = 2,2′‐bipyrimidine, C^N = 2‐phenylpyridinato ( 2 ), (2‐p‐tolyl)pyridinato ( 3 ), 2‐(2,4‐difluorophenyl)pyridinato ( 4 ), and 2‐thienylpyridinato ( 5 )) were synthesized. Complexes 2 – 5 were characterized by NMR spectroscopy, high‐resolution mass spectrometry, and elemental analysis. The structures of the complexes 2 and 4 were further confirmed by single‐crystal X‐ray diffraction analysis. All the complexes display strong absorption in the high‐energy UV region assigned to intraligand (IL) transitions, and the lower energy bands are ascribed to metal‐to‐ligand charge transfer (MLCT) transitions. The reduction and oxidation behavior of the complexes 2 – 5 were examined by cyclic voltammetry. Variation of the ligands on Ir(III) center resulted in significant changes in electrochemical properties.     

Synthesis, characterization and photochromic studies of spirooxazine-containing 2,2'-bipyridine ligands and their rhenium(I) tricarbonyl complexes

A series of spirooxazine-containing 2,2'-bipyridine ligands and their rhenium(i) tricarbonyl complexes has been designed and synthesized, and their photophysical, photochromic and electrochemical properties have been studied. The X-ray crystal structures of two of the complexes have been determined. Detailed studies showed that the emission properties of the complexes could readily be switched through photochromic reactions.     

Synthesis, characterization and photophysics of alkyne bridged bimetallic rhenium(I) and ruthenium(II) complexes

Six new homobimetallic and heterobimetallic complexes of rhenium(I) and ruthenium(II) bridged by ethynylene spacer [(CO)₃(bpy)Re(BL)Re(bpy)(CO)₃]²⁺ [Cl(bpy)₂Ru(BL)Ru(bpy)₂Cl]²⁺ and [(CO)₃(bpy)Re(BL)Ru(bpy)₂Cl]²⁺ (bpy = 2,2′-bipyridine, BL = 1,2-bis(4-pyridyl)acetylene (bpa) and 1,4-bis(4-pyridyl)butadiyne (bpb) are synthesized and characterized. The electrochemical and photophysical properties of all the complexes show a weak interaction between two metal centers in heterobimetallic complexes. The excited state lifetime of the complexes is increased upon introduction of ethynylene spacer and the transient spectra show that this is due to delocalization of electron in the bridging ligand. Also, intramolecular energy transfer from *Re(I) to Ru(II) in Re–Ru heterobimetallic complexes occurs with a rate constant 4 × 10⁷ s⁻¹.     

Tuning the redox potentials of dinuclear tungsten oxo complexes [(Cp*W(4,4'-R,R-2,2'-bpy)(mu-O))2][PF6]2 toward photochemical water splitting

A series of novel dinuclear tungsten(IV) oxo complexes with disubstituted 4,4'-R,R-2,2'-bipyridyl (R(2)bpy) ligands of the type [(Cp*W(R(2)bpy)(mu-O))(2)][PF(6)](2) (R=NMe(2), tBu, Me, H, Cl) was prepared by hydrolysis of the tungsten(IV) trichloro complexes [Cp*W(R(2)bpy)Cl(3)]. Cyclic voltammetry measurements for the tungsten(IV) oxo compounds provided evidence for one reversible oxidation and two reversible reductions leading to the oxidation states W(V)W(IV), W(IV)W(III) and W(III)W(III). The corresponding complexes [(Cp*W(R(2)bpy)(mu-O))(2)](n+) [PF(6)](n) (n=0 for R=Me, tBu, and 1, 3 for both R=Me) could be isolated after chemical oxidation/reduction of the tungsten(IV) oxo complexes. The crystal structures of the complexes [(Cp*W(R(2)bpy)(mu-O))(2)][BPh(4)](2) (R=NMe(2), tBu) and [(Cp*W(Me(2)bpy)(mu-O))(2)](n+)[PF(6)](n) (n=0, 1, 2, 3) show a cis geometry with a puckered W(2)O(2) four-membered ring for all compounds except [(Cp*W(Me(2)bpy)(mu-O))(2)] which displays a trans geometry with a planar W(2)O(2) ring. Examining the interaction of these novel tungsten oxo complexes with protons, we were able to show that the W(IV)W(IV) complexes [(Cp*W(R(2)bpy)(mu-O))(2)][PF(6) (-)](2) (R=NMe(2), tBu) undergo reversible protonation, while the W(III)W(III) complexes [(Cp*W(R(2)bpy)(mu-O))(2)] transfer two electrons forming the W(IV)W(IV) complex and molecular hydrogen.     

Ruthenium(II)-polyazine light absorbers bridged to reactive cis-dichlororhodium(III) centers in a bimetallic molecular architecture

Bimetallic complexes of the form [(bpy)(2)Ru(BL)RhCl(2)(phen)](PF(6))(3), where bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, and BL = 2,3-bis(2-pyridyl)pyrazine (dpp) or 2,2'-bipyrimidine (bpm), were synthesized, characterized, and compared to the [{(bpy)(2)Ru(BL)}(2)RhCl(2)](PF(6))(5) trimetallic analogues. The new complexes were synthesized via the building block method, exploiting the known coordination chemistry of Rh(III) polyazine complexes. In contrast to [{(bpy)(2)Ru(dpp)}(2)RhCl(2)](PF(6))(5) and [{(bpy)(2)Ru(bpm)}(2)RhCl(2)](PF(6))(5), [(bpy)(2)Ru(dpp)RhCl(2)(phen)](PF(6))(3) and [(bpy)(2)Ru(bpm)RhCl(2)(phen)](PF(6))(3) have a single visible light absorber subunit coupled to the cis-Rh(III)Cl(2) moiety, an unexplored molecular architecture. The electrochemistry of [(bpy)(2)Ru(dpp)RhCl(2)(phen)](PF(6))(3) showed a reversible oxidation at 1.61 V (vs Ag/AgCl) (Ru(III/II)), quasi-reversible reductions at -0.39 V, -0.74, and -0.98 V. The first two reductive couples corresponded to two electrons, consistent with Rh reduction. The electrochemistry of [(bpy)(2)Ru(bpm)RhCl(2)(phen)](PF(6))(3) exhibited a reversible oxidation at 1.76 V (Ru(III/II)). A reversible reduction at -0.14 V (bpm(0/-)), and quasi-reversible reductions at -0.77 and -0.91 V each corresponded to a one electron process, bpm(0/-), Rh(III/II), and Rh(II/I). The dpp bridged bimetallic and trimetallic display Ru(dpi)-->dpp(pi*) metal-to-ligand charge transfer (MLCT) transitions at 509 nm (14,700 M(-1) cm(-1)) and 518 nm (26,100 M(-1) cm(-1)), respectively. The bpm bridged bimetallic and trimetallic display Ru(dpi)-->bpm(pi*) charge transfer (CT) transitions at 581 nm (4,000 M(-1) cm(-1)) and 594 nm (9,900 M(-1) cm(-1)), respectively. The heteronuclear complexes [(bpy)(2)Ru(dpp)RhCl(2)(phen)](PF(6))(3) and [{(bpy)(2)Ru(dpp)}(2)RhCl(2)](PF(6))(5) had (3)MLCT emissions that are Ru(dpi)-->dpp(pi*) CT in nature but were red-shifted and lower intensity than [(bpy)(2)Ru(dpp)Ru(bpy)(2)](PF(6))(4). The lifetimes of the (3)MLCT state of [(bpy)(2)Ru(dpp)RhCl(2)(phen)](PF(6))(3) at room temperature (30 ns) was shorter than [(bpy)(2)Ru(dpp)Ru(bpy)(2)](PF(6))(4), consistent with favorable electron transfer to Rh(III) to generate a metal-to-metal charge-transfer ((3)MMCT) state. The reported synthetic methods provide means to a new molecular architecture coupling a single Ru light absorber to the Rh(III) center while retaining the interesting cis-Rh(III)Cl(2) moiety.     

Reactions of ruthenium complexes having pyridyl-containing ligands, 2-pyridinecarboxylato and 2,2'-bipyridine, with an azide ion: formation of nitrido-bridged diruthenium complexes

Reactions of ruthenium complexes having 2-pyridinecarboxylato and 2,2'-bipyridine ligands with sodium azide in alcohol afforded nitrido-bridged diruthenium complexes, [{Ru(OR)(pyca)(bpy)}2(mu-N)](+) (R = CH3, C2H5). Diruthenium complexes showed diamagnetic properties, a linear Ru-N-Ru coordination configuration, and two irreversible oxidation waves and two reversible reduction waves.