Charge delocalization of 1,4-benzenedicyclometalated ruthenium: a comparison between tris-bidentate and bis-tridentate complexes

A dimetallic biscyclometalated ruthenium complex, [(bpy)(2)Ru(dpb)Ru(bpy)(2)](2+) (bpy = 2,2'-bipyridine; dpb = 1,4-di-2-pyridylbenzene), with a tris-bidentate coordination mode has been prepared. The electronic properties of this complex were studied by electrochemical and spectroscopic analysis and DFT/TDDFT calculations on both rac and meso isomers. Complex [(bpy)(2)Ru(dpb)Ru(bpy)(2)](2+) has a similar 1,4-benzenedicyclometalated ruthenium (Ru-phenyl-Ru) structural component with a previously reported bis-tridentate complex, [(tpy)Ru(tpb)Ru(tpy)](2+) (tpy = 2,2';6',2″-terpyridine; tpb = 1,2,4,5-tetra-2-pyridylbenzene). The charge delocalizations of these complexes across the Ru-phenyl-Ru array were investigated and compared by studying the corresponding one-electron-oxidized species, generated by chemical oxidation or electrochemical electrolysis, with DFT/TDDFT calculations and spectroscopic and EPR analysis. These studies indicate that both [(bpy)(2)Ru(dpb)Ru(bpy)(2)](3+) and [(tpy)Ru(tpb)Ru(tpy)](3+) are fully delocalized systems. However, the coordination mode of the metal component plays an important role in influencing their electronic properties.     

Marked differences in light-switch behavior of Ru(II) complexes possessing a tridentate DNA intercalating ligand

The tridentate ligand 3-(pyrid-2'-yl)dipyrido[3,2-a:2',3'-c]phenazine (pydppz) has been prepared in two steps by elaboration of 2-(pyrid-2'-yl)-1,10-phenanthroline. Both homoleptic [Ru(pydppz)(2)](2+) and heteroleptic [Ru(tpy)(pydppz)](2+) (tpy = 2,2';6',2' '-terpyridine) complexes have been prepared and characterized by (1)H NMR. The absorption and emission spectra are consistent with low-lying MLCT excited states, which are typical of Ru(II) complexes. Femtosecond transient absorption measurements show that that the (3)MLCT excited state of the heteroleptic complex [Ru(tpy)(pydppz)](2+) (tau approximately 5 ns) is longer-lived than that of the homoleptic complex [Ru(pydppz)(2)](2+) (tau = 2.4 ns) and that these lifetimes are significantly longer than that of the (3)MLCT state of the parent complex [Ru(tpy)(2)](2+) (tau = 120 ps). These differences are explained by the lower-energy (3)MLCT excited state present in [Ru(tpy)(pydppz)](2+) and [Ru(pydppz)(2)](2+) compared to [Ru(tpy)(2)](2+), resulting in less deactivation of the former through the ligand-field state(s). DFT and TDDFT calculations are consistent with this explanation. [Ru(tpy)(pydppz)](2+) and [Ru(pydppz)(2)](2+) bind to DNA through the intercalation of the pydppz ligand; however, only the heteroleptic complex exhibits luminescence enhancement in the presence of DNA. The difference in the photophysical behavior of the complexes is explained by the inability of [Ru(pydppz)(2)](2+) to intercalate both pydppz ligands, such that one pydppz always remains exposed to the solvent. DNA photocleavage is observed for [Ru(tpy)(pydppz)](2+) in air, but not for [Ru(pydppz)(2)](2+). The DNA damage likely proceeds through the production of small amounts of (1)O(2) by the longer-lived complex. Although both complexes possess the intercalating pydppz ligand, they exhibit different photophysical properties in the presence of DNA.     

A novel heteroditopic terpyridine-pincer ligand as building block for mono- and heterometallic Pd(II) and Ru(II) complexes

A palladium-catalyzed Stille coupling reaction was employed as a versatile method for the synthesis of a novel terpyridine-pincer (3, TPBr) bridging ligand, 4'-{4-BrC6H2(CH2NMe2)2-3,5}-2,2':6',2' '-terpyridine. Mononuclear species [PdX(TP)] (X = Br, Cl), [Ru(TPBr)(tpy)](PF6)2, and [Ru(TPBr)2](PF6)2, synthesized by selective metalation of the NCNBr-pincer moiety or complexation of the terpyridine of the bifunctional ligand TPBr, were used as building blocks for the preparation of heterodi- and trimetallic complexes [Ru(TPPdCl)(tpy)](PF6)2 (7) and [Ru(TPPdCl)2](PF6)2 (8). The molecular structures in the solid state of [PdBr(TP)] (4a) and [Ru(TPBr)2](PF6)2 (6) have been determined by single-crystal X-ray analysis. Electrochemical behavior and photophysical properties of the mono- and heterometallic complexes are described. All the above di- and trimetallic Ru complexes exhibit absorption bands attributable to (1)MLCT (Ru --> tpy) transitions. For the heteroleptic complexes, the transitions involving the unsubstituted tpy ligand are at a lower energy than the tpy moiety of the TPBr ligand. The absorption bands observed in the electronic spectra for TPBr and [PdCl(TP)] have been assigned with the aid of TD-DFT calculations. All complexes display weak emission both at room temperature and in a butyronitrile glass at 77 K. The considerable red shift of the emission maxima relative to the signal of the reference compound [Ru(tpy)2]2+ indicates stabilization of the luminescent 3MLCT state. For the mono- and heterometallic complexes, electrochemical and spectroscopic studies (electronic absorption and emission spectra and luminescence lifetimes recorded at room temperature and 77 K in nitrile solvents), together with the information gained from IR spectroelectrochemical studies of the dimetallic complex [Ru(TPPdSCN)(tpy)](PF6)2, are indicative of charge redistribution through the bridging ligand TPBr. The results are in line with a weak coupling between the {Ru(tpy)2} chromophoric unit and the (non)metalated NCN-pincer moiety.     

Iridium cyclometalated complexes with axial symmetry: time-dependent density functional theory investigation of trans-bis-cyclometalated complexes containing the tridentate ligand 2,6-diphenylpyridine

A new series of iridium cyclometalated complexes with a C/N/C dppy-type ligand and a N/N/N tpy-type ligand have been synthesized and characterized by various techniques such as mass spectrometry, 1H and 13C NMR, cyclic voltammetry, both steady-state and time-resolved emission and absorption studies, and time-dependent DFT (TDDFT) calculations. The complexes exhibit strong visible absorptions and long-lived (1.6-2.0 micros) emissions (lambdamax, ca. 680 nm) in room-temperature solution. DFT calculations on the ground-state geometry match that of an X-ray crystal structure. TDDFT calculations give accurate predictions of the electronic absorption energies and intensities, while geometry optimizations on the lowest energy triplet state give accurate energies for the emission. Examination of the relevant molecular orbitals shows that the inherent asymmetry of the coordination environment offers a unique directional character to the emitting excited state, which is predominately LLCT (dppy --> tpy) in nature.     

On the role of the bridging dicyanamidobenzene ligand in a new binuclear ruthenium complex: [{Ru(tpy)(thd)}2(mu-dicyd)][PF6] with tpy = 2,2':6',2' '-terpyridine and thd = 2,2,6,6-tetramethyl-3,5-heptanedione

The dicyanamidobenzene-bridge diruthenium complex [{Ru(tpy)(thd)}(2)(mu-dicyd)][PF(6)] ([3][PF(6)]) (dicyd = 1,4-dicyanamidobenzene, tpy = 2,2':6',2' '-terpyridine, thd = 2,2,6,6-tetramethyl-3,5-heptanedione) and its mononuclear counterpart [Ru(tpy)(thd)(Ipcyd)] (2) [Ipcyd = 4-iodophenylcyanamide anion (Ipcyd(-))] were synthesized and fully characterized. Cyclic voltammetry of 3 showed the presence of four reversible one-electron redox couples. UV-vis-NIR spectroelectrochemistry and EPR spectroscopy of the electrogenerated paramagnetic intermediates were used to ascertain the oxidation-state distribution. The stable starting dinuclear complex 3(+) is found to be a ligand-centered anion radical as shown by EPR spectroscopy, magnetic susceptibility measurements, and DFT calculations. Oxidation of 3(+) to 3(2+) led to an EPR silent system due to substantial intramolecular antiferromagnetic interaction of the electron spins carried by the low spin ruthenium(III) atom and the bridging anion radical dicyanamido (dicyd(*)(-)), an observation which was supported by UV-vis-NIR, X-ray structure, and DFT calculations. Complex 3(3+) presented an EPR spectra consistent with a total effective spin S = (1)/(2) issued from an antiferromagnetic interaction of electron spins carried by two low spin ruthenium(III) atoms and the bridging anion radical dicyd(*)(-) in accordance with UV-vis-NIR. This study shows that the dicyanamidobenzene bridging ligand has indubitably a noninnocent behavior.     

Water oxidation catalysis: influence of anionic ligands upon the redox properties and catalytic performance of mononuclear ruthenium complexes

Aiming at highly efficient molecular catalysts for water oxidation, a mononuclear ruthenium complex Ru(II)(hqc)(pic)(3) (1; H(2)hqc = 8-hydroxyquinoline-2-carboxylic acid and pic = 4-picoline) containing negatively charged carboxylate and phenolate donor groups has been designed and synthesized. As a comparison, two reference complexes, Ru(II)(pdc)(pic)(3) (2; H(2)pdc = 2,6-pyridine-dicarboxylic acid) and Ru(II)(tpy)(pic)(3) (3; tpy = 2,2':6',2"-terpyridine), have also been prepared. All three complexes are fully characterized by NMR, mass spectrometry (MS), and X-ray crystallography. Complex 1 showed a high efficiency toward catalytic water oxidation either driven by chemical oxidant (Ce(IV) in a pH 1 solution) with a initial turnover number of 0.32 s(-1), which is several orders of magnitude higher than that of related mononuclear ruthenium catalysts reported in the literature, or driven by visible light in a three-component system with [Ru(bpy)(3)](2+) types of photosensitizers. Electrospray ionization MS results revealed that at the Ru(III) state complex 1 undergoes ligand exchange of 4-picoline with water, forming the authentic water oxidation catalyst in situ. Density functional theory (DFT) was employed to explain how anionic ligands (hqc and pdc) facilitate the 4-picoline dissociation compared with a neutral ligand (tpy). Electrochemical measurements show that complex 1 has a much lower E(Ru(III)/Ru(II)) than that of reference complex 2 because of the introduction of a phenolate ligand. DFT was further used to study the influence of anionic ligands upon the redox properties of mononuclear aquaruthenium species, which are postulated to be involved in the catalysis cycle of water oxidation.     

Charge delocalization in a cyclometalated bisruthenium complex bridged by a noninnocent 1,2,4,5-tetra(2-pyridyl)benzene ligand

Two ruthenium atoms are covalently connected to the para positions of a phenyl ring in 1,2,4,5-tetra(2-pyridyl)benzene (tpb) to form a linear Ru-tpb-Ru arrangement. This unique structure leads to appealing electronic properties for the biscyclometalated complex [(tpy)Ru(tpb)Ru(tpy)](2+), where tpy is 2,2';6',2″-terpyridine. It could be stepwise oxidized at substantially low potential (+0.12 and +0.55 V vs Ag/AgCl) and with a noticeably large comproportionation constant (1.94 × 10(7)). In addition to the routinely observed metal-to-ligand charge-transfer transitions, [(tpy)Ru(tpb)Ru(tpy)](2+) displays a separate and distinct absorption band at 805 nm with appreciable absorptivity (ε = 9000 M(-1) cm(-1)). This band is assigned to the charge transition from the Ru-tpb-Ru motif to the pyridine rings of tpb with the aide of density functional theory (DFT) and time-dependent DFT calculations. Complex [(tpy)Ru(tpb)Ru(tpy)](2+) was precisely titrated with 1 equiv of cerium ammonium nitrate to produce [(tpy)Ru(tpb)Ru(tpy)](3+), which shows intense multiple NIR transitions. The electronic coupling parameters H(ab) of individual NIR components are determined to be 5812, 4942, 4358, and 3560 cm(-1). DFT and TDDFT calculation were performed on [(tpy)Ru(tpb)Ru(tpy)](3+) to elucidate its electronic structure and spin density population and the nature of the observed NIR transitions. Electron paramagnetic resonance studies of [(tpy)Ru(tpb)Ru(tpy)](3+) exhibit a discernible rhombic signal with the isotropic g factor of ?g? = 2.144. These results point to the strong orbital interaction of tpb with metal centers and that tpb behaves as a redox noninnocent bridging ligand in [(tpy)Ru(tpb)Ru(tpy)](2+). Complex [(tpy)Ru(tpb)Ru(tpy)](3+) is determined to be a Robin-Day class III system with full charge delocalization across the Ru-tpb-Ru motif.     

Mixed-metal supramolecular complexes coupling phosphine-containing Ru(II) light absorbers to a reactive Pt(II) through polyazine bridging ligands

Supramolecular bimetallic Ru(II)/Pt(II) complexes [(tpy)Ru(PEt(2)Ph)(BL)PtCl(2)](2+) and their synthons [(tpy)Ru(L)(BL)](n)()(+) (where L = Cl(-), CH(3)CN, or PEt(2)Ph; tpy = 2,2':6',2'-terpyridine; and BL = 2,2'-bipyrimidine (bpm) or 2,3-bis(2-pyridyl)pyrazine (dpp)) have been synthesized and studied by cyclic voltammetry, electronic absorption spectroscopy, mass spectral analysis, and (31)P NMR. The mixed-metal bimetallic complexes couple phosphine-containing Ru chromophores to a reactive Pt site. These complexes show how substitution of the monodentate ligand on the [(tpy)RuCl(BL)](+) synthons can tune the properties of these light absorbers (LA) and incorporate a (31)P NMR tag by addition of the PEt(2)Ph ligand. The redox potentials for the Ru(III/II) couples occur at values greater than 1.00 V versus the Ag/AgCl reference electrode and can be tuned to more positive potentials on going from Cl(-) to CH(3)CN or PEt(2)Ph (E(1/2) = 1.01, 1.55, and 1.56 V, respectively, for BL = bpm). The BL(0/-) couple at -1.03 (bpm) and -1.05 V (dpp) for [(tpy)Ru(PEt(2)Ph)(BL)](2+) shifts dramatically to more positive potentials upon the addition of the PtCl(2) moiety to -0.34 (bpm) and -0.50 V (dpp) for the [(tpy)Ru(PEt(2)Ph)(BL)PtCl(2)](2+) bridged complex. The lowest energy electronic absorption for these complexes is assigned as the Ru(d pi) --> BL(pi*) metal-to-ligand charge transfer (MLCT) transition. These MLCT transitions are tuned to higher energy in the monometallic synthons when Cl(-) is replaced by CH(3)CN or PEt(2)Ph (516, 452, and 450 nm, for BL = bpm, respectively) and to lower energy when Pt(II)Cl(2) is coordinated to the bridging ligand (560 and 506 nm for BL = bpm or dpp). This MLCT state displays a broad emission at room temperature for all the dpp systems with the [(tpy)Ru(PEt(2)Ph)(dpp)PtCl(2)](2+) system exhibiting an emission centered at 750 nm with a lifetime of 56 ns. These supramolecular complexes [(tpy)Ru(PEt(2)Ph)(BL)PtCl(2)](2+) represent the covalent linkage of TAG-LA-BL-RM assembly (TAG = NMR active tag, RM = Pt(II) reactive metal).     

Temperature-induced solid-state valence tautomeric interconversion in two cobalt-Schiff base diquinone complexes

The mixed-ligand complexes [Co(III)(tpy)(Cat-N-SQ)]Y and [Ni(II)(tpy)(Cat-N-BQ)]PF(6) (tpy = 2,2':6',2' '-terpyridine; Cat-N-BQ, Cat-N-SQ = mononegative and radical dinegative Schiff base diquinone ligand; Y = PF(6), BPh(4)) were prepared. Structural and spectroscopic data support the different charge distribution of the two compounds. The temperature-dependent electronic and spectral properties of solutions containing the [Co(III)(tpy)(Cat-N-SQ)](+) suggest that this compound undergoes a thermally driven valence tautomeric interconversion to [Co(II)(tpy)(Cat-N-BQ)](+) complex, the metal ion being in high-spin configuration. The comparison of the electrochemical properties of the cobalt and nickel derivatives supports the observed behavior. The same interconversion process was found to occur also in the solid state with a significant higher T(c) value than in solution. It was found that the previously reported [Co(III)(Cat-N-BQ)(Cat-N-SQ)] shows a similar behavior. The large difference between the interconversion T(c) in the solid state and in solution is suggested to come from the entropy changes associated with the modifications of vibronic interactions.     

Physical, photophysical and structural properties of ruthenium(II) complexes containing a tetradentate bipyridine ligand

The focus of this report is the synthesis and properties of two new analogues of ruthenium(ii) tris-bipyridine, a monomer and dimer. The complexes contain the ligand 6,6'-(ethan-1,2-diyl)bis-2,2'-bipyridine (O-bpy) which contains two bipyridine units bridged in the 6,6' positions by an ethylene bridge. Crystal structures of the two complexes formulated as [Ru(bpy)(O-bpy)](PF6)2 and [(Ru(bpy)2)2(O-bpy)](PF6)4 reveal structures of lower symmetry than D3 which affects the electronic properties of the complexes as substantiated by density functional theory (DFT) and time dependent density functional theory (TDDFT) calculations. The HOMO lies largely on the ruthenium center; the LUMO spreads its electron density over the bipyridine units, but not equally in the mixed O-bpy-bpy complexes. Calculated Vis/UV spectra using TDDFT methods agree with experimental spectra. The lowest lying triplet excited state for [Ru(bpy)(O-bpy)](PF6)2 is 3MC resulting in a low emission quantum yield and a large chloride ion photosubstitution quantum yield.     

Synthesis, characterization, spectroscopic, and electrochemiluminescence properties of a solvatochromic azacrown-containing cyanoruthenate(II): potential applications in separation and indirect photometric detection of cations and amino acids in HPLC

A new anionic ruthenium(II) complex, (Et4N)[Ru(tpyA18C6)(CN)3] (tpyA18C6=N-[4'-(2,2':6',2'-terpyridyl)]-1,4,7,10,13-pentaoxa-16-azacyclohexadodecane), has been synthesized and characterized. The complex was found to show pronounced solvatochromic behavior and, when dissolved in solution, changed its color from purple to yellow when the solvent system was varied from pure acetonitrile to pure water. Its absorption and emission energies in various solvents showed a linear dependence of the Gutman's acceptor number. The characteristic photoluminescence and electrochemiluminescence (ECL) of the complex were also found to be progressively quenched as the proportion of water in a water/acetonitrile mixture increased. Large changes in the chemical shifts of the 1H NMR and 13C NMR signals of [Ru(tpyA18C6)(CN)3]- in different solvents were observed. The complex has also been demonstrated to serve as a mobile-phase additive in high-performance liquid chromatography for separation of metal cations and amino acids. Comparison studies with the crown-free analogue, (Et4N)[Ru(tpy)(CN)3] (tpy = 2,2':6',2'-terpyridine), showed that other than the ion-pair effect, the allosteric host-guest interaction provided by the presence of the pendant crown was essential to the separation performance of the complex. Indirect detection of nonabsorbing analytes has been achieved by monitoring the absorbance changes of the eluent at the metal-to-ligand charge-transfer (MLCT) absorption band maximum of the complex at 445 nm. The effects of pH, ionic strength, and polarity of the mobile phase as well as the complex concentration on the selectivity and resolution have also been studied.     

Electron donor-acceptor dyads based on ruthenium(II) bipyridine and terpyridine complexes bound to naphthalenediimide

Two series of photosensitizer-electron acceptor complexes have been synthesized and fully characterized: ruthenium(II) tris(bipyridine) ([Ru(II)(bpy)(2)(bpy-X-NDI)], where X = -CH(2)-, tolylene, or phenylene, bpy is 2,2'-bipyridine, and NDI is naphthalenediimide) and ruthenium(II) bis(terpyridine) ([Ru(II)(Y-tpy)(tpy-X-NDI)], where Y = H or tolyl and X = tolylene or phenylene, and tpy = 2,2':6',2' '-terpyridine). The complexes have been studied by cyclic and differential pulse voltammetry and by steady state and time-resolved absorption and emission techniques. Rates for forward and backward electron transfer have been investigated, following photoexcitation of the ruthenium(II) polypyridine moiety. The terpyridine complexes were only marginally affected by the linked diimide unit, and no electron transfer was observed. In the bipyridine complexes we achieved efficient charge separation. For the complexes containing a phenyl link between the ruthenium(II) and diimide moieties, our results suggest a biphasic forward electron-transfer reaction, in which 20% of the charge-separated state was formed via population of the naphthalenediimide triplet state.     

Photoisomerization and proton-coupled electron transfer (PCET) promoted water oxidation by mononuclear cyclometalated ruthenium catalysts

Photoisomeric transformations in ruthenium polypyridyl complexes have been rarely reported. Herein we report the geometrical transformation of cyclometalated trans-[Ru(tpy)(PAD)(OH(2))](+) ([1](+)) to the cis-[Ru(tpy)(PAD)(OH(2))](+) ([1a](+)) (tpy = 2,2';6',2"-terpyridine, PAD = 2-(pyrid-2'-yl)acridine) isomer upon irradiation of visible light (λ ≥420 nm). Due to a proton-induced tautomeric equilibrium between the Ru-C bond and Ru═C coordination, the π* energy levels of PADH are lower than those of tpy by 12.61 and 12.24 kcal mol(-1), respectively, in [1](+) and [1a](+). Isomers [1](+) and [1a](+) both act as catalytic oxygen-evolving complexes (OECs) chemically as well as electrochemically.     

Designing molecular bistability in ruthenium dimethyl sulfoxide complexes

Compounds of the type [Ru(tpy)(L2)(dmso)](z+) (tpy is 2,2':6',2' '-terpyridine; L2 can be 2,2'-bipyridine (bpy), N,N,N',N'-tetramethylethylenediamine (tmen), 2-pyridine carboxylate (pic), acetylacetonate (acac), malonate (mal), or oxalate (ox)) have been studied by X-ray crystallography, electrochemistry, NMR, IR, and UV-vis spectroscopy. When L2 is bpy, tmen, or pic, the dmso ligand can be intramolecularly isomerized either electrochemically or photochemically. Isomerization is not observed when L2 is acac, mal, or ox. Isomerization results in a drastic change in the absorption spectrum, as well as in the voltammetry. Absorption maxima shift by 3470 (419-490 nm), 4775 (421-527 nm), and 4440 cm(-)(1) (429-530 nm) for the bpy, pic, and tmen complexes, respectively. Reduction potentials for S-bonded and O-bonded complexes differ by 0.57, 0.75, and 0.62 V for the bpy, pic, and tmen complexes, respectively. Quantum yields of isomerization (phi(S)(-->)(O)) were determined for the bpy (0.024 +/- 1), pic (0.25 +/- 1), and tmen (0.007 +/- 1) complexes. In comparison of these data to photosubstitution quantum yields, it appears that the isomerization mechanism does not involve the ligand field states. This result is surprising given the importance of these states in the photochemistry of ruthenium and osmium polypyridine complexes. These results and details of the mechanism are discussed.     

Complex series [Ru(tpy)(dpk)(X)]n+ (tpy = 2,2':6',2''-terpyridine; dpk = 2,2'-dipyridyl ketone; X = Cl-, CH3CN, NO2(-), NO+, NO*, NO-): substitution and electron transfer, structure, and spectroscopy

The complex framework [Ru(tpy)(dpk)]2+ has been used to study the generation and reactivity of the nitrosyl complex [Ru(tpy)(dpk)(NO)]3+ ([4]3+). Stepwise conversion of the chloro complex [Ru(tpy)(dpk)(Cl)]+ ([1]+) via [Ru(tpy)(dpk)(CH3CN)]2+ ([2]2+) and the nitro compound [Ru(tpy)(dpk)(NO2)]+ ([3]+) yielded [4]3+; all four complexes were structurally characterized as perchlorates. Electrochemical oxidation and reduction was investigated as a function of the monodentate ligand as was the IR and UV-vis spectroscopic response (absorption/emission). The kinetics of the conversion [4]3+/[3]+ in aqueous environment were also studied. Two-step reduction of [4]3+ was monitored via EPR, UV-vis, and IR (nu(NO), nu(CO)) spectroelectrochemistry to confirm the {RuNO}7 configuration of [4]2+ and to exhibit a relatively intense band at 505 nm for [4]+, attributed to a ligand-to-ligand transition originating from bound NO-.     

Complexes of functionalized dipyrido[3,2-a:2',3'-c]-phenazine: a synthetic, spectroscopic, structural, and density functional theory study

The ligands 11-bromodipyrido[3,2-a:2',3'-c]phenazine and ethyl dipyrido[3,2-a:2',3'-c]phenazine-11-carboxylate have been prepared and coordinated to ruthenium(II), rhenium(I), and copper(I) metal centers. The electronic effects of substitution of dipyrido[2,3-a:3',2'-c]phenazine (dppz) have been investigated by spectroscopy and electrochemistry, and some photophysical properties have been studied. The crystal structures of [Re(L)(CO)(3)Cl] (L = ethyl dipyrido[3,2-a:2',3'-c]phenazine-11-carboxylate or 11-bromodipyrido[3,2-a:2',3'-c]phenazine) are presented. Density functional theory calculations on the complexes show only small deviations in bond lengths and angles (most bonds within 0.02 Angstroms, most angles within 2 degrees) from the crystallographic data. Furthermore, the vibrational spectra of the strongest Raman and IR bands are predicted to within an average 6 cm(-1) for the complexes [Re(L)(CO)(3)Cl] and [Cu(L)(triphenylphosphine)(2)]BF(4) (in the 1000-1700 cm(-1) region). Spectroscopic and electrochemical evidence suggest that reduction of the complex causes structural changes across the entire dppz ligand. This is unusual as dppz-based ligands typically have electrochemical properties that suggest charge localization with reduction on the phenazine portion of the ligand. The excited-state lifetimes of the complexes have been measured, and they range from ca. 200 ns for the [Ru(L)(2,2'-bipyridine)(2)](PF(6))(2) complexes to over 2 mus for [Cu(11-bromodipyrido[3,2-a:2',3'-c]phenazine)(PPh(3))(2)](BF(4)) at room temperature. The emission spectra suggest that the unusually long-lived excited states of the copper complexes result from metal-to-ligand charge transfer (MLCT) transitions as they are completely quenched in methanol. Electroluminescent films may be fabricated from these compounds; they show MLCT state emission even at low doping levels [<0.1% by weight in poly(vinylcarbazole) polymer matrix].     

Towards catenanes using π-stacking interactions and their influence on the spin-state of a bis(2,2':6',2"terpyridine)iron(II) domain

The ligand 6,6"bis(4-methoxyphenyl)-4'-phenyl-2,2':6',2"terpyridine (2) has been prepared and characterized; deprotection using pyridinium chloride leads to the formation of 6,6"bis(4-hydroxyphenyl)-4'-phenyl-2,2':6',2"terpyridinium chloride ([H3]Cl). Treatment of the latter with 3-(2-(2-bromoethoxy)ethoxy)prop-1-ene under basic conditions yields ligand 4 containing pendant, alkene-terminated chains. Whereas direct complexation of 4 with ruthenium(II) proved problematical, the homoleptic complexes [Fe(2)(2)][PF(6)](2) and [Ru(2)(2)][PF(6)](2) were prepared in good to moderate yields. In the solid state, both complexes exhibit multiple face-to-face π-stacking of arene and pyridine rings which influences the coordination geometry about the metal ion. Consequential weakening of the ligand field results in [Fe(2)(2)][PF(6)](2) being high-spin. Variable temperature solution (1)H NMR spectroscopic studies confirm the iron(ii) centre remains high-spin between 200 and 295 K. The paramagnetically shifted (1)H NMR spectrum exhibits signals in the range δ 109.7 to -66.5 ppm and has been fully assigned. Paramagnetic relaxation enhancement (PRE) has been used to correlate the observed proton line-widths to the distances of the protons from the metal centre and these are in good agreement with the Fe···H separations observed in the solid state. The [Fe(2)(2)](2+) ion undergoes two dynamic processes (i) rotation of the pendant phenyl rings which is fast on the NMR timescale at 200 K, and (ii) twisting and sliding of the aromatic rings of the tpy and anisyl units which interconverts the two enantiomers of [Fe(2)(2)](2+) at 295 K.     

Synthesis and study of the spectroscopic and redox properties of Ru(II),Pt(II) mixed-metal complexes bridged by 2,3,5,6-tetrakis(2-pyridyl)pyrazine

The mixed-metal supramolecular complexes [(tpy)Ru(tppz)PtCl](PF6)3 and [ClPt(tppz)Ru(tppz)PtCl](PF6)4 (tpy = 2,2':6',2'-terpyridine and tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine) were synthesized and characterized. These complexes contain ruthenium bridged by tppz to platinum centers to form stereochemically defined linear assemblies. X-ray crystallographic determinations of the two complexes confirm the identity of the metal complexes and reveal intermolecular interactions of the Pt sites in the solid state for [(tpy)Ru(tppz)PtCl](PF6)3 with a Pt...Pt distance of 3.3218(5) A. The (1)H NMR spectra show the expected splitting patterns characteristic of stereochemically defined mixed-metal systems and are assigned with the use of (1)H-(1)H COSY and NOESY. Electronic absorption spectroscopy displays intense ligand-based pi --> pi* transitions in the UV and MLCT transitions in the visible. Electrochemically [(tpy)Ru(tppz)PtCl](PF6)3 and [ClPt(tppz)Ru(tppz)PtCl](PF6)4 display reversible Ru (II/III) couples at 1.63 and 1.83 V versus Ag/AgCl, respectively. The complexes display very low potential tppz (0/-) and tppz(-/2-) couples, relative to their monometallic synthons, [(tpy)Ru(tppz)](PF6)2 and [Ru(tppz)2](PF6)2, consistent with the bridging coordination of the tppz ligand. The Ru(dpi) --> tppz(pi*) MLCT transitions are also red-shifted relative to the monometallic synthons occurring in the visible centered at 530 and 538 nm in CH3CN for [(tpy)Ru(tppz)PtCl](PF6)3 and [ClPt(tppz)Ru(tppz)PtCl](PF6)4, respectively. The complex [(tpy)Ru(tppz)PtCl](PF6)3 displays a barely detectable emission from the Ru(dpi) --> tppz(pi*) (3)MLCT in CH 3CN solution at RT. In contrast, [ClPt(tppz)Ru(tppz)PtCl](PF6)4 displays an intense emission from the Ru(dpi) --> tppz(pi*) (3)MLCT state at RT with lambda max(em) = 754 nm and tau = 80 ns.     

Primary amino-functionalized N-heterocyclic carbene ligands as support for Au(I)···Au(I) interactions: structural, electrochemical, spectroscopic and computational studies of the dinuclear [Au2(NH2(CH2)2imMe)2][NO3]2

The N-heterocyclic carbene (NHC) precursor, 1-(2-aminoethyl)-3-methylimidazolium nitrate, [NH(2)(CH(2))(2)imMe)]NO(3) ([3][NO(3)]) reacted with Ag(2)CO(3) in dimethyl sulfoxide readily yielding a Ag(I)-(NHC-NH(2)) complex presenting limited stability in solution. The in situ carbene transfer reaction of the latter with [Au(tht)Cl] afforded the first example of a dinuclear gold(I) complex [Au(2)(NH(2)(CH(2))(2)imMe)(2)][NO(3)](2) ([5][NO(3)](2)) bearing a primary amino-functionalized NHC ligand. The complex has been characterized by NMR, mass spectrometry, X-ray crystallography and cyclic voltammetry; the electrochemical behaviour and photophysical properties of [5][NO(3)](2) have been also investigated and the experimental data have been compared with density functional theory (DFT) and Time Dependent (TDDFT) calculations. Single-crystal structural studies showed that the Au(I)-carbene compound contains dinuclear (AuL)(2) cations in which pairs of gold(I) centres are linked by a pair of bridging ligands, with a Au···Au aurophilic contact of 3.2332(17) ? that is maintained in solution as documented by the DFT calculations. Complex [5][NO(3)](2) is photoluminescent in solution at room temperature and the high energy emission peak at 410 nm is remarkably shifted with respect to the absorption band centered at 260 nm.     

Near-infrared absorbing and emitting Ru(II)-Pt(II) heterodimetallic complexes of dpdpz (dpdpz = 2,3-di(2-pyridyl)-5,6-diphenylpyrazine)

The reaction of 2,3-di(2-pyridyl)-5,6-diphenylpyrazine (dpdpz) with K(2)PtCl(4) in a mixture of acetonitrile and water afforded mono-Pt complex (dpdpz)PtCl(2)4 in good yield, with two lateral pyridine nitrogen atoms binding to the metal center. Two types of Ru(II)-Pt(II) heterodimetallic complexes bridged by dpdpz, namely, [(bpy)(2)Ru(dpdpz)Pt(C≡CC(6)H(4)R)](2+) (7-9, R = H, NMe(2), or Cl, respectively) and [(tpy)Ru(dpdpz)Pt(C≡CPh)] (+) (12), were then designed and prepared, where bpy = 2,2'-bipyridine and tpy = 2,2';6',2'-terpyridine. In both cases, the platinum atom binds to dpdpz with a C(∧)N(∧)N tridentate mode. However, the coordination of the ruthenium atom with dpdpz could either be noncyclometalated (N(∧)N bidentate) or cyclometalated (C(∧)N(∧)N tridentate). The electronic properties of these complexes were subsequently studied and compared by spectroscopic and electrochemical analyses and theoretical calculations. These complexes exhibit substantial absorption in the visible to NIR (near-infrared) region because of mixed MLCT (metal-to-ligand-charge-tranfer) transitions from both the ruthenium and the platinum centers. Complexes 7 and 9 were found to emit NIR light with higher quantum yields than those of the mono-Ru complex [(bpy)(2)Ru(dpdpz)](2+) (5) and bis-Ru complex [(bpy)(2)Ru(dpdpz)Ru(bpy)(2)](4+) (13). However, no emission was detected from complex 8 or 12 at room temperature in acetonitrile.