Mononuclear and dinuclear complexes of dibenzoeilatin: synthesis, structure, and electrochemical and photophysical properties

This work describes a study of Ru(II) and Os(II) polypyridyl complexes of the symmetrical, fused-aromatic bridging ligand dibenzoeilatin (1). The synthesis, purification, and structural characterization by NMR of the mononuclear complexes [Ru(bpy)(2)(dbneil)](2+) (2), [Ru(tmbpy)(2)(dbneil)](2+) (3), and [Os(bpy)(2)(dbneil)](2+) (4), the homodinuclear complexes [[Ru(bpy)(2)](2)[micro-dbneil]](4+) (5), [[Ru(tmbpy)(2)](2)[micro-dbneil]](4+) (6), and [[Os(bpy)(2)](2)[micro-dbneil]](4+) (7), and the heterodinuclear complex [[Ru(bpy)(2)][micro-dbneil][Os(bpy)(2)]](4+) (8) are described, along with the crystal structures of 4, 6, and 7. Absorption spectra of the mononuclear complexes feature a low-lying MLCT band around 600 nm. The coordination of a second metal fragment results in a dramatic red shift of the MLCT band to beyond 700 nm. Cyclic and square wave voltammograms of the mononuclear complexes exhibit one reversible metal-based oxidation, as well as several ligand-based reduction waves. The first two reductions, attributed to reduction of the dibenzoeilatin ligand, are substantially anodically shifted compared to [M(bpy)(3)](2+) (M = Ru, Os), consistent with the low-lying pi orbital of dibenzoeilatin. 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 conjugated dibenzoeilatin ligand. Luminescence spectra, quantum yield, and lifetime measurements at room temperature in argon-purged acetonitrile have shown that the complexes exhibit (3)MLCT emission, which occurs in the IR-region between 950 and 1300 nm. The heterodinuclear complex 8 exhibits luminescence only from the Ru-based fragment, the intensity of which is less than 1% of that observed in the corresponding homodinuclear complex 5; no emission from the Os-based unit is observed, and an intramolecular quenching constant of k(q) > or = 3 x10(9) s(-)(1) is evaluated. The nature of the quenching process is briefly discussed.     

Ruthenium(II) dendrimers containing carbazole-based chromophores as branches

Three new luminescent and redox-active Ru(II) complexes containing novel dendritic polypyridine ligands have been synthesized, and their absorption spectra, luminescence properties (both at room temperature in fluid solution and at 77 K in rigid matrix), and redox behavior have been investigated. The dendritic ligands are made of 1,10-phenanthroline coordinating subunits and of carbazole groups as branching sites. The first and second generation species of this novel class of dendritic ligands (L1 and L2, respectively; see Figure 1 for their structural formulas) have been prepared and employed. The metal dendrimers investigated are [Ru(bpy)(2)(L1)](2+) (1; bpy = 2,2'-bipyridine), [Ru(bpy)(2)(L2)](2+) (2), and [Ru(L1)(3)](2+) (3; see Figure 2). For the sake of completeness and comparison purposes, also the absorption spectra, redox behavior, and luminescence properties of L1 and L2 have been studied, together with the properties of 3,6-di(tert-butyl)carbazole (L0) and [Ru(bpy)(2)(phen)](2+) (4, phen = 1,10-phenanthroline). The absorption spectra of the free dendritic ligands show features which can be assigned to the various subunits (i.e., carbazole and phenanthroline groups) and additional bands at lower energies (at lambda > 300 nm) which are assigned to carbazole-to-phenanthroline charge-transfer (CT) transitions. These latter bands are significantly red-shifted upon acid and/or zinc acetate addition. Both L1 and L2 exhibit relatively intense luminescence at room temperature in fluid solution (lifetimes in the nanosecond time scale, quantum yields of the order of 10(-2)-10(-1)) and at 77 K in rigid matrix (lifetimes in the millisecond time scale). Such a luminescence is assigned to CT states at room temperature and to phenanthroline-centered pi-pi triplet levels at 77 K. The room-temperature luminescence of L1 and L2 is totally quenched by acid or zinc acetate. The metal dendrimers exhibit the typical absorption and luminescence properties of Ru(II) polypyridine complexes. In particular, metal-to-ligand charge-transfer (MLCT) bands dominate the visible absorption spectra, and formally triplet MLCT levels govern the excited-state properties. Excitation spectroscopy evidences that all the light absorbed by the dendritic branches is transferred with unitary efficiency to the luminescent MLCT states in 1-3, showing that the new metal dendrimers can be regarded as efficient light-harvesting antenna systems. All the free ligands and metal dendrimers exhibit a rich redox behavior (except L2 and 3, whose redox behavior was not investigated because of solubility reasons), with clearly attributable reversible carbazole- and metal-centered oxidation and polypyridine-centered reduction processes. The electronic interaction between the carbazole redox-active sites of the dendritic ligands is affected by Ru(II) coordination.     

Photophysical properties of TiO(2) surfaces modified with dinuclear RuRu and RuOs polypyridyl complexes

The photophysical properties of nanoporous TiO(2) surfaces modified with two new Ru(II)-(bpt)-Ru(II) and Ru(II)-(bpt)-Os(II) polypyridyl complexes are reported. These dyads have been prepared by a two-step synthetic pathway. In the first step, [Ru(dcbpy)(2)Cl(2)], where dcbpy is 4,4'-dicarboxy-2,2-bipyridyl, was reacted with the bridging ligand 3,5-bis(pyridin-2-yl)-1,2,4-triazole (Hbpt) to yield the mononuclear precursor Na(3)[Ru(dcbpy)(2)(bpt)].3H(2)O. Subsequent reaction of this compound with either [Ru(bpy)(2)Cl(2)] or [Os(bpy)(2)Cl(2)] yields the Ru(II)-Ru(II) and Ru(II)-Os(II) dyads. Electrochemical data, together with time-resolved transient absorption spectroscopy and the investigation of the incident-photon-to-current-efficiency (IPCE), have been used to obtain a detailed picture of the photoinduced charge injection properties of these dyads. These measurements indicate that for the heterosupramolecular triad based on Ru(II)-(bpt)-Ru(II), the final product species obtained upon charge injection is TiO(2)(e)-Ru(II)Ru(III). For the mixed metal Ru(II)-(bpt)-Os(II) dyad, both metal centers inject efficiently into the semiconductor surface and as a result TiO(2)(e)-Ru(II)Os(III) is obtained as a single charge-separated product.     

A Tetranuclear Ruthenium(II) Complex Containing both Electron-Rich and Electron-Poor Bridging Ligands. Absorption Spectrum, Luminescence, Redox Behavior, and Intercomponent Energy Transfer

The first luminescent and redox active multinuclear Ru(II) compound containing both electron-poor (2,3-bis(2-pyridyl)pyrazine, 2,3-dpp) and electron-rich (3,5-bis(pyridyn-2-yl)-1,2,4-triazole, Hbpt) polypyridine bridging ligands has been synthesized. The novel compound is [(bpy)(2)Ru(&mgr;-bpt)Ru{(&mgr;-2,3-dpp)Ru(bpy)(2)}(2)](7+) (1; bpy = 2,2'-bipyridine). Its absorption spectrum, luminescence properties, and redox behavior have been studied and are compared with the properties of the parent complexes [Ru{(&mgr;-2,3-dpp)Ru(bpy)(2)}(3)](8+) (2) and [(bpy)(2)Ru(&mgr;-bpt)Ru(bpy)(2)](3+) (3). The absorption spectrum of 1 is dominated by ligand-centered bands in the UV region and by metal-to-ligand charge transfer bands in the visible region. Excited states and oxidation and reduction processes are localized in specific sites of the multicomponent structure. However, perturbations of each component on the redox and excited states of the others, as well as electronic interactions between the chromophores, can be observed. Intercomponent energy transfer from the upper-lying (&mgr;-bpt)(bpy)Ru-->bpy CT excited state of the Ru(bpy)(2)(&mgr;-bpt)(+) component to the lower-lying (bpy)(2)Ru-->&mgr;-2,3-dpp CT excited state of the Ru(bpy)(2)(&mgr;-2,3-dpp)(2+) subunit(s) is efficient in 1 in fluid solution at room temperature, whereas this process is not observed in a rigid matrix at 77 K. A two-step energy transfer mechanism is proposed to explain the photophysical properties of the new compound.     

Ruthenium complexes of easily accessible tridentate ligands based on the 2-aryl-4,6-bis(2-pyridyl)-s-triazine motif: absorption spectra, luminescence properties, and redox behavior

A family of tridendate ligands 1 a-e, based on the 2-aryl-4,6-di(2-pyridyl)-s-triazine motif, was prepared along with their hetero- and homoleptic Ru(II) complexes 2 a-e ([Ru(tpy)(1 a-e)](2+); tpy=2,2':6',2"-terpyridine) and 3 a-e ([(Ru(1 a-e)(2)](2+)), respectively. The ligands and their complexes were characterized by (1)H NMR spectroscopy, ES-MS, and elemental analysis. Single-crystal X-ray analysis of 2 a and 2 e demonstrated that the triazine core is nearly coplanar with the non-coordinating ring, with dihedral angles of 1.2 and 18.6 degrees, respectively. The redox behavior and electronic absorption and luminescence properties (both at room temperature in liquid acetonitrile and at 77 K in butyronitrile rigid matrix) were investigated. Each species undergoes one oxidation process centered on the metal ion, and several (three for 2 a-e and four for 3 a-e) reduction processes centered on the ligand orbitals. All compounds exhibit intense absorption bands in the UV region, assigned to spin-allowed ligand-centered (LC) transitions, and moderately intense spin-allowed metal-to-ligand charge-transfer (MLCT) absorption bands in the visible region. The compounds exhibit relatively intense emissions, originating from triplet MLCT levels, both at 77 K and at room temperature. The incorporation of triazine rings and the near planarity of the noncoordinating ring increase the luminescence lifetimes of the complexes by lowering the energy of the (3)MLCT state and creating a large energy gap to the dd state.     

Synthesis, structural characterization, and photophysical, electrochemical, intercomponent energy-transfer, and anion-sensing studies of imidazole 4,5-bis(benzimidazole)-bridged Os(II)Os(II) and Ru(II)Os(II) bipyridine complexes

Homo- and heterobimetallic complexes of composition [(bpy)(2)M(II)(H(2)Imbzim)M'(II)(bpy)(2)](ClO(4))(3)·nH(2)O, where M(II) = M'(II) = Os (1), M(II) = Ru and M'(II) = Os (2), H(3)Imbzim = 4,5-bis(benzimidazole-2-yl)imidazole, and bpy = 2,2'-bipyridine, have been synthesized and characterized using standard analytical and spectroscopic techniques. Both of the complexes crystallized in monoclinic form with the space group P2(1)/m for 1 and P2(1)/n for 2. The absorption spectra, redox behavior, and luminescence properties of the complexes have been thoroughly investigated. The complexes display very intense, ligand-centered absorption bands in the UV region and moderately intense metal-to-ligand charge-transfer (MLCT) bands in the visible region. The bimetallic complexes show two successive one-electron reversible metal-centered oxidations. The strong fluorescence of free H(3)Imbzim is completely quenched in the metal complexes by energy transfer to the metal-based units, which exhibit their characteristic MLCT phosphorescence. The luminescence data of the heterometallic complex 2 show that electronic energy transfer takes place from the ruthenium center to the osmium-based component. The anion binding properties of the complexes have been studied in solutions using absorption, emission, and (1)H NMR spectral measurements. The metalloreceptors act as sensors for F(-) and AcO(-) ions. Sensing studies indicate the presence of two successive anion-induced deprotonation steps, leading to the formation of [(bpy)(2)M(HImbzim)M'(bpy)(2)](2+) and [(bpy)(2)M(Imbzim)M'(bpy)(2)](+) species. Double deprotonation is also observed in the presence of hydroxide. The binding affinities of different anions toward the receptors have been evaluated. Cyclic voltammetry measurements carried out in acetonitrile have provided evidence in favor of anion-dependent electrochemical responses of the bimetallic metalloreceptors with F(-) and AcO(-) ions.     

Photophysics and redox behavior of chiral transition metal polymers

The absorption and emission spectra, excited-state lifetimes, quantum yields, and electrochemical measurements have been obtained for a new series of chiral complexes based on three different chiral 2,2':6',2' '-terpyridine ligands, (-)-ctpy, (-)-[ctpy-x-ctpy], and (-)-[ctpy-b-ctpy], with one, two, or multiple Ru metal centers. The room-temperature absorption and emission maxima of [[((-)-ctpy)Ru]-(-)-[ctpy-b-ctpy]-[Ru((-)-ctpy)]](PF(6))(4) and ((-)-[ctpy-b-ctpy])-[[Ru((-)-[ctpy-b-ctpy])](PF(6))(2)](n) were shifted to lower energies and also exhibited significantly longer luminescence lifetimes when compared to [Ru((-)-ctpy)(2)](PF(6))(2), [[((-)-ctpy)Ru]-(-)-[ctpy-x-ctpy]-[Ru((-)-ctpy)]](PF(6))(4), and ((-)-[ctpy-x-ctpy])-[[Ru((-)-[ctpy-x-ctpy])](PF(6))(2)](n). In terms of their electrochemical behavior, all of the complexes studied exhibited one Ru-centered and two ligand-centered redox waves and the [[((-)-ctpy)Ru]-(-)-[ctpy-x-ctpy]-[Ru((-)-ctpy)]](PF(6))(4), ((-)-[ctpy-x-ctpy])-[[Ru((-)-[ctpy-x-ctpy])](PF(6))(2)](n), and ((-)-[ctpy-b-ctpy])-[[Ru((-)-[ctpy-b-ctpy])](PF(6))(2)](n)() complexes were found to electrodeposit upon ligand-based reduction. The difference between the formal potentials of the Ru-centered and the first ligand-centered (least negative) waves corresponded linearly with the changes in the observed emission energies. The shifts in energy are discussed using a particle-in-a-box model, and the luminescence lifetimes are discussed in terms of the structure of the excited-state manifold.     

Absorption, Circular Dichroism, and Luminescence Spectroscopy of Electrogenerated Delta-[Ru(bipy)(3)](+/0/)(-) and Delta-[Os(bipy)(3)](+/0/)(-) (bipy = 2,2'-Bipyridine)

The electronic absorption and circular dichroism (CD) spectra of the complexes produced by the one, two, and three electron reduction of Delta-[Ru(bipy)(3)](2+) and Delta-[Os(bipy)(3)](2+) are reported. The CD spectra give unequivocal proof that the added electrons are localized on individual bipiridine ligands and thus that the complexes are correctly formulated [M(bipy)(2)(bipy(-))](+), [M(bipy)(bipy(-))(2)](0), and [M(bipy(-))(3)](-). The absorption spectra of the triply reduced species [M(bipy(-))(3)](-) (M = Ru, Os) are compared to those of the Fe(II) and Ir(III) analogs. The luminescence spectra of the two triply reduced complexes [Ru(bipy(-))(3)](-) and [Os(bipy(-))(3)](-). are also presented. The MLCT luminescence found in the parent complexes is completely quenched and is replaced by a weak luminescence attributed to the pi(10) --> pi(7) transition of the (coordinated) [bipy](-) ion.     

Fe(II), Ru(II) and Re(I) complexes of endotopic, sterically non-hindering, U-shaped 8,8'-disubstituted-3,3'-biisoquinoline ligands: syntheses and spectroscopic properties

The redox behaviour, optical-absorption spectra and emission properties of U-shaped and elongated disubstituted biisoquinoline ligands and of derived octahedral Fe(ii), Ru(ii), and Re(i) complexes are reported. The ligands are 8,8'-dichloro-3,3'-biisoquinoline (1), 8,8'-dianisyl-3,3'-biisoquinoline (2), and 8,8'-di(phenylanisyl)-3,3'-biisoquinoline (3), and the complexes are [Fe(3)(3)](2+), [Fe(2)(3)](2+), [Ru(1)(phen)(2)](2+), [Ru(2)(3)](2+), [Ru(3)(3)](2+), [Re(2)(py)(CO)(3)](+), and [Re()(py)(CO)(3)](+). For the ligands, the optical properties as observed in dichloromethane are in line with expectations based on the predominant (1)pipi* nature of the involved excited states, with contributions at lower energies from (1)npi* and (1)ILCT (intraligand charge transfer) transitions. For all of the Fe(ii), Ru(ii), and Re(i) complexes, studied in acetonitrile, the transitions associated with the lowest-energy absorption band are of (1)MLCT (metal-to-ligand charge transfer) nature. The emission properties, as observed at room temperature and at 77 K, can be described as follows: (i) the Fe(ii) complexes do not emit, either at room temperature or at 77 K; (ii) the room-temperature emission of the Ru(ii) complexes (phi(em) > 10(-3), tau in the micros range) is of mixed (3)MLCT/(3)LC character (and similarly at 77 K); and (iii) the room-temperature emission of the Re(i) complexes (phi(em) approximately 3 x 10(-3), tau < 1 ns) is of (3)MLCT character and becomes of (3)LC (ligand-centered) character (tau in the ms time scale) at 77 K. The interplay of the involved excited states in determining the luminescence output is examined.     

Experimental and theoretical study of the regiospecific coordination of RuII and OsII fragments on the lacunary polyoxometalate [alpha-PW11O39]7-

New Ru(II) and Os(II) derivatives of the monovacant [alpha-PW(11)O(39)](7-) anion ([PW(11)O(39){M(DMSO)(3)(H(2)O)}](5-) (M = Ru (1), Os (2)) and [PW(11)O(39){Os(eta(6)-p-cym)(H(2)O)}](5-) (3)) have been synthesized and characterized. The binding mode of the d(6)-{M(II)L(3)(H(2)O)}(2+) moieties in these compounds is similar to that in the previously described [PW(11)O(39){Ru(eta(6)-p-cym)(H(2)O)}](5-) (4) complex: bidentate, on two nonequivalent oxygen atoms of the lacuna, leading to a loss of the C(s) symmetry of the parent anion, which thus plays the role of a prochiral bidentate ligand. The density functional theory (DFT) (B3PW91) computation of the lowest unoccupied molecular orbitals of the {ML(3)(H(2)O)}(2+) (M = Os, Ru; L(3) = fac-(DMSO)(3), eta(6)-C(6)H(6)) fragments reveals the similarities between their electrophilic properties. The origin of the regioselectivity of the grafting was investigated through a DFT (B3PW91) analysis of (i) the highest occupied molecular orbital of [alpha-PW(11)O(39)](7-) and (ii) the relative energies of the different potential regioisomers obtained by a bidentate grafting of the {ML(3)(H(2)O)}(2+) moiety onto the lacuna of [alpha-PW(11)O(39)](7-). The role of the water ligand in the stabilization of this peculiar structure was studied.     

Spectroelectrochemical studies on mixed-valence states in a cyanide-bridged molecular square, [Ru(II)(2)Fe(II)(2)(mu-CN)(4)(bpy)(8)](PF6)(4).CHCl(3).H(2)O

A cyanide-bridged molecular square of [Ru(II) (2)Fe(II) (2)(mu-CN)(4)(bpy)(8)](PF(6))(4).CHCl(3).H(2)O, abbreviated as [Ru(II) (2)Fe(II) (2)](PF(6))(4), has been synthesised and electrochemically generated mixed-valence states have been studied by spectroelectrochemical methods. The complex cation of [Ru(II) (2)Fe(II) (2)](4+) is nearly a square and is composed of alternate Ru(II) and Fe(II) ions bridged by four cyanide ions. The cyclic voltammogram (CV) of [Ru(II) (2)Fe(II) (2)](PF(6))(4) in acetonitrile showed four quasireversible waves at 0.69, 0.94, 1.42 and 1.70 V (vs. SSCE), which correspond to the four one-electron redox processes of [Ru(II) (2)Fe(II) (2)](4+) right arrow over left arrow [Ru(II) (2)Fe(II)Fe(III)] (5+) right arrow over left arrow [Ru(II) (2)Fe(III) (2)](6+) right arrow over left arrow [Ru(II)Ru(III)Fe(III) (2)](7+) right arrow over left arrow [Ru(III) (2)Fe(III) (2)](8+). Electrochemically generated [Ru(II) (2)Fe(II)Fe(III)](5+) and [Ru(II) (2)Fe(III) (2)](6+) showed new absorption bands at 2350 nm (epsilon =5500 M(-1) cm(-1)) and 1560 nm (epsilon =10 500 M(-1) cm(-1)), respectively, which were assigned to the intramolecular IT (intervalence transfer) bands from Fe(II) to Fe(III) and from Ru(II) to Fe(III) ions, respectively. The electronic interaction matrix elements (H(AB)) and the degrees of electronic delocalisation (alpha(2)) were estimated to be 1090 cm(-1) and 0.065 for the [Ru(II) (2)Fe(II)Fe(III) (2)](5+) state and 1990 cm(-1) and 0.065 for the [Ru(II) (2)Fe(III) (2)](6+) states.     

Electrochemical, spectroscopic, and photophysical properties of structurally diverse polyazine-bridged Ru(II),Pt(II) and Os(II),Ru(II),Pt(II) supramolecular motifs

Five new tetrametallic supramolecules of the motif [{(TL)(2)M(dpp)}(2)Ru(BL)PtCl(2)](6+) and three new trimetallic light absorbers [{(TL)(2)M(dpp)}(2)Ru(BL)](6+) (TL = bpy = 2,2'-bipyridine or phen = 1,10-phenanthroline; M = Ru(II) or Os(II); BL = dpp = 2,3-bis(2-pyridyl)pyrazine, dpq = 2,3-bis(2-pyridyl)quinoxaline, or bpm = 2,2'-bipyrimidine) were synthesized and their redox, spectroscopic, and photophysical properties investigated. The tetrametallic complexes couple a Pt(II)-based reactive metal center to Ru and/or Os light absorbers through two different polyazine BL to provide structural diversity and interesting resultant properties. The redox potential of the M(II/III) couple is modulated by M variation, with the terminal Ru(II/III) occurring at 1.58-1.61 V and terminal Os(II/III) couples at 1.07-1.18 V versus Ag/AgCl. [{(TL)(2)M(dpp)}(2)Ru(BL)](PF(6))(6) display terminal M(dπ)-based highest occupied molecular orbitals (HOMOs) with the dpp(π*)-based lowest unoccupied molecular orbital (LUMO) energy relatively unaffected by the nature of BL. The coupling of Pt to the BL results in orbital inversion with localization of the LUMO on the remote BL in the tetrametallic complexes, providing a lowest energy charge separated (CS) state with an oxidized terminal Ru or Os and spatially separated reduced BL. The complexes [{(TL)(2)M(dpp)}(2)Ru(BL)](6+) and [{(TL)(2)M(dpp)}(2)Ru(BL)PtCl(2)](6+) efficiently absorb light throughout the UV and visible regions with intense metal-to-ligand charge transfer (MLCT) transitions in the visible at about 540 nm (M = Ru) and 560 nm (M = Os) (ε ≈ 33,000-42,000 M(-1) cm(-1)) and direct excitation to the spin-forbidden (3)MLCT excited state in the Os complexes about 720 nm. All the trimetallic and tetrametallic Ru-based supramolecular systems emit from the terminal Ru(dπ)→dpp(π*) (3)MLCT state, λ(max)(em) ≈ 750 nm. The tetrametallic systems display complex excited state dynamics with quenching of the (3)MLCT emission at room temperature to populate the lowest-lying (3)CS state population of the emissive (3)MLCT state.     

Synthesis, characterization, and detailed electrochemistry of binuclear ruthenium(III) complexes bridged by bisacetylacetonate. Crystal and molecular structures of [[Ru(acac)2]2(tae)] (acac = 2,4-pentanedionate ion, tae = 1,1,2,2-tetraacetylethanate dianion)

Binuclear beta-diketonatoruthenium(III) complexes [[Ru(acac)(2)](2)(tae)], [[Ru(phpa)(2)](2)(tae)], and [(acac)(2)Ru(tae)Ru(phpa)(2)] and binuclear and mononuclear bipyridine complexes [[Ru(bpy)(2)](2)(tae)](PF(6))(2) and [Ru(bpy)(2)(Htae)]PF(6) (acac = 2,4-pentanedionate ion, phpa = 2,2,6,6-tetramethyl-3,5-heptanedionate ion, tae = 1,1,2,2-tetraacetylethanate dianion, and bpy = 2,2'-bipyridine) were synthesized. The new complexes have been characterized by (1)H NMR, MS, and electronic spectral data. Crystal and molecular structures of [[Ru(acac)(2)](2)(tae)] have been solved by single-crystal X-ray diffraction studies. Crystal data for the meso isomer of [[Ru(acac)(2)](2)(tae)] have been confirmed by the dihedral angle result that two acetylacetone units of the bridging tae ligand are almost perpendicular to one another. A detailed investigation on the electrochemistry of the binuclear complexes has been carried out. The electrochemical behavior details of the binuclear complexes have been compared with those of the mononuclear complexes obtained from the half-structures of the corresponding binuclear complexes. Studies on the effects of solvents on the mixed-valence states of Ru(II)-Ru(III) and Ru(III)-Ru(IV) complexes have been carried out by various voltammetric and electrospectroscopic techniques. A correlation between the comproportionation constant (K(c)) and the donor number of the solvent has been obtained. The K(c) values for the binuclear complexes have been found to be low because of the fact that two acetylacetone units of the bridging tae ligand are not in the same plane, as revealed by the crystal structure of [[Ru(acac)(2)](2)(tae)].     

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.     

Homoleptic, sigma-bonded octahedral [M(CO)(6)](2+) cations of iron(II), ruthenium(II), and osmium(II). Part 1: Syntheses, thermochemical and vibrational characterizations, and molecular structures as [Sb(2)F(11)](-) and [SbF(6)](-) salts. A comprehensive, comparative study

Homoleptic octahedral, superelectrophilic sigma-bonded metal carbonyl cations of the type [M(CO)(6)](2+) (M = Ru, Os) are generated in the Bronsted-Lewis conjugate superacid HF/SbF(5) by reductive carbonylation of M(SO(3)F)(3) (M = Ru, Os) or OsF(6). Thermally stable salts form with either [Sb(2)F(11)](-) or [SbF(6)](-) as anion, just as for the previously reported [Fe(CO)(6)](2+) cation. The latter salts are generated by oxidative (XeF(2)) carbonylation of Fe(CO)(5) in HF/SbF(5). A rationale for the two diverging synthetic approaches is provided. The thermal stabilities of [M(CO)(6)][SbF(6)](2) salts, studied by DSC, range from 180 degrees C for M = Fe to 350 degrees C for M = Os before decarbonylation occurs. The two triads [M(CO)(6)][SbF(6)](2) and [M(CO)(6)][Sb(2)F(11)](2) (M = Fe, Ru, Os) are extensively characterized by single-crystal X-ray diffraction and vibrational and (13)C NMR spectroscopy, aided by computational studies of the cations. The three [M(CO)(6)][SbF(6)](2) salts (M = Fe, Ru, Os) crystallize in the tetragonal space group P4/mnc (No. 128), whereas the corresponding [Sb(2)F(11)](-) salts are monoclinic, crystallizing in space group P2(1)/n (No. 14). In both triads, the unit cell parameters are nearly invariant of the metal. Bond parameters for the anions [SbF(6)](-) and [Sb(2)F(11)](-) and their vibrational properties in the two triads are completely identical. In all six salts, the structural and vibrational properties of the [M(CO)(6)](2+) cations (M = Fe, Ru, Os) are independent of the counteranion and for the most part independent of M and nearly identical. Interionic C...F contacts are similarly weak in all six salts. Metal dependency is noted only in the (13)C NMR spectra, in the skeletal M-C vibrations, and to a much smaller extent in some of the C-O stretching fundamentals (A(1g) and T(1u)). The findings reported here are unprecedented among metal carbonyl cations and their salts.     

Homoleptic, sigma-bonded octahedral superelectrophilic metal carbonyl cations of iron(II), ruthenium(II), and osmium(II). Part 2: Syntheses and characterizations of [M(CO)(6)][BF(4)](2) (M = Fe, Ru, Os)

As the first examples of homoleptic, sigma-bonded superelectrophilic metal carbonyl cations with tetrafluoroborate [BF(4)](-) as the counter anions three thermally stable salts of the composition [M(CO)(6)][BF(4)](2) (M = Fe, Ru, Os) have been synthesized and extensively characterized by thermochemical, structural, and spectroscopic methods. A common synthetic route, the oxidative carbonylation of either Fe(CO)(5) (XeF(2) as the oxidizer) or M(3)(CO)(12) (M = Ru, Os) (F(2) as the oxidizer) in the conjugate Bronsted-Lewis superacid HF/BF(3), was employed. The thermal behavior of the three salts, studied by differential scanning calorimetry (DSC) and gas-phase IR spectroscopy of the decomposition products, has been compared to that of the corresponding [SbF(6)](-) salts. The molecular structures of [M(CO)(6)][BF(4)](2) (M = Fe, Os) were obtained by single-crystal X-ray diffraction at 100 K. X-ray powder diffraction data for [M(CO)(6)][BF(4)](2) (M = Ru, Os) were obtained between 100 and 300 K in intervals of 50 K. All three salts are isostructural and crystallized in the tetragonal space group I4/m (No. 87). As for the corresponding [M(CO)(6)][SbF(6)](2) salts (M = Fe, Ru, Os), similar unit cell parameters and vibrational fundamentals were also found for the three [BF(4)](-) compounds. For the structurally characterized salts [M(CO)(6)][BF(4)](2) (M = Fe, Os), very similar bond parameters for both cations and anions were found. Hence, the invariance of structural and spectroscopic properties of [M(CO)(6)](2+) cations (M = Fe, Ru, Os) extended from the fluoroantimonates [Sb(2)F(11)](-) and [SbF(6)](-) as counteranions also to [BF(4)](-).     

DNA mismatch detection by resonance energy transfer between ruthenium(II) and osmium(II) tris(2,2'-bipyridyl) chromophores

Octahedral tris-chelate complexes [M(II)(bpy)(3)](2+) (M = Ru or Os, bpy = 2,2'-bipyridyl), covalently attached to the 3'- and 5'-phosphates of two oligonucleotides, are juxtaposed when hybridized contiguously to a fully complementary DNA target. Visible metal-to-ligand charge-transfer (MLCT) excitation of the [Ru(II)(bpy)(3)](2+) unit leads to resonance energy transfer to the MLCT state of the [Os(II)(bpy)(3)](2+) moiety, with the energy transfer efficiency depending on the degree of hybridization. The extent of attenuation of the intense red luminescence from the Ru(II) chromophore hence allows highly sensitive structural probing of the assembly and constitutes a novel approach to DNA sensing which is capable of detecting mutations.     

Fused donor-acceptor ligands in RuII chemistry: synthesis, electrochemistry and spectroscopy of [Ru(bpy)3-n(TTF-dppz)n](PF6)2.

Three ruthenium(II) polypyridine complexes of general formula [Ru(bpy)(3-n)(TTF-dppz)n](PF6)2 (n=1-3, bpy=2,2'-bipyridine), with one, two or three redox-active TTF-dppz (4',5'-bis(propylthio)tetrathiafulvenyl[i]dipyrido[3,2-a:2',3'-c]phenazine) ligands, were synthesised and fully characterised. Their electrochemical and photophysical properties are reported together with those of the reference compounds [Ru(bpy)3](PF6)2, [Ru(dppz)3](PF6)2 and [Ru(bpy)2(dppz)](PF6)2 and the free TTF-dppz ligand. All three complexes show intraligand charge-transfer (ILCT) fluorescence of the TTF-dppz ligand. Remarkably, the complex with n=1 exhibits luminescence from the Ru(2+)-->dppz metal-to-ligand charge-transfer ((3)MLCT) state, whereas for the other two complexes, a radiationless pathway via electron transfer from a second TTF-dppz ligand quenches the (3)MLCT luminescence. The TTF fragments as electron donors thus induce a ligand-to-ligand charge-separated (LLCS) state of the form TTF-dppz- -Ru(2+)-dppz-TTF(+). The lifetime of this LLCS state is approximately 2.3 micros, which is four orders of magnitude longer than that of 0.4 ns for the ILCT state, because recombination of charges on two different ligands is substantially slower.     

Nearest- and next-nearest-neighbor Ru(II)/Ru(III) electronic coupling in cyanide-bridged tetra-ruthenium square complexes

Electrochemical properties of cyanide-bridged metal squares, [Ru(4)](4+) and [Rh(2)-Ru(2)](6+), clearly demonstrate the role of the nearest (NN) metal moiety in mediating the next-nearest neighbor (NNN) metal-to-metal electronic coupling. The differences in electrochemical potentials for successive oxidations of equivalent Ru(II) centers in [Ru(4)](4+) are ΔE(1/2) = 217 mV and 256 mV and are related to intense, dual metal-to-metal-charge-transfer (MMCT) absorption bands. This contrasts with a small value of ΔE(1/2) = 77 mV and no MMCT absorption bands observed to accompany the oxidations of [Rh(2)-Ru(2)](6+). These observations demonstrate NN-mediated superexchange mixing by the linker Ru of NNN Ru(II) and Ru(III) moieties and that this mixing results in a NNN contribution to the ground state stabilization energy of about 90 ± 20 meV. In contrast, the classical Hush model for mixed valence complexes with the observed MMCT absorption parameters predicts a NNN stabilization energy of about 6 meV. The observations also indicate that the amount of charge delocalization per Ru(II)/Ru(III) pair is about 4 times greater for the NN than the NNN couples in these CN-bridged complexes, which is consistent with DFT modeling. A simple fourth-order secular determinant model is used to describe the effects of donor/acceptor mixing in these complexes.     

Photophysical and novel charge-transfer properties of adducts between [RuII(bpy)3]2+ and [S2Mo18O62]4-

The photophysical properties of acetonitrile solutions of [Ru(bpy)(3)](2+) and [S(2)Mo(18)O(62)](4-) are described. We discuss evidence for ion cluster formation in solution and the observation that despite the strong donor ability of the excited state of [Ru(bpy)(3)](2+) and its inherent photolability, adducts with [S(2)Mo(18)O(62)](4-) were photostable. Photophysical studies suggest that the quenching of the [Ru(bpy)(3)](2+) excited state by [S(2)Mo(18)O(62)](4-) occurs via a static mechanism and that binding is largely electrostatic in nature. Evidence is provided from difference spectroscopy and luminescence excitation spectroscopy for good electronic communication between [Ru(bpy)(3)](2+) and [S(2)Mo(18)O(62)](4-) with the presence of a novel, luminescent, inter-ion charge-transfer transition. The identity of the transition is confirmed by resonance Raman spectroscopy.