Synthesis and electrochemical properties of bipyrimidine bridged triruthenium complexes

A new μ(4)-bpym-bridged dimer of an oxoacetao-triruthenium complex with carbonyl, [{Ru(3)O(CH(3)COO)(5)(CO)(py)}(2)(μ(4)-bpym)], was synthesized. The complex possesses two stable mixed-valence states associated with Ru(3)(III,III,II)/Ru(3)(III,II,II) and Ru(3)(III,III,III)/Ru(3)(III,III,II). The IR-spectroelectrochemistry reveals ν(CO) spectra in five oxidation states, Ru(3)(III,III,III)-Ru(3)(III,III,III) to Ru(3)(III,II,II)-Ru(3)(III,II,II) and both the mixed-valence states show a spectrum indicating medium interaction between the Ru(3) units.     

An unusual dinuclear ruthenium(III) complex with a conjugated bridging ligand derived from cleavage of a 1,4-dihydro-1,2,4,5-tetrazine ring. Synthesis,structure, and UV-vis-NIR spectroelectrochemical characterization of a five-membered redox chain incorporating two mixed-valence states

Reaction of [Ru(acac)(2)(CH(3)CN)(2)] with 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,4-dihydro-1,2,4,5-tetrazine (H(2)L) results in formation of an unexpected dinuclear complex [(acac)(2)Ru(III)(L(1))Ru(III)(acac)(2)] (1) in which the bridging ligand [L(1)](2)(-) contains an (-)HN[bond]C[double bond]N[bond]N[double bond]C[bond]NH(-) unit arising from two-electron reduction of the 1,4-dihydro-1,2,4,5-tetrazine component of H(2)L. The crystal structure of complex 1 confirms the oxidation assignment of the metal ions as Ru(III) and clearly shows the consequent arrangement of double and single bonds in the bridging ligand, which acts as a bis-bidentate chelate having two pyrazolyl/amido chelating sites. Cyclic voltammetry of the complex shows the presence of four reversible one-electron redox couples, assigned as two Ru(III)/Ru(IV) couples (oxidations with respect to the starting material) and two Ru(II)/Ru(III) couples (reductions with respect to the starting material). The separation between the two Ru(III)/Ru(IV) couples (Delta E(1/2) = 700 mV) is much larger than that between the two Ru(II)/Ru(III) couples (Delta E(1/2) = 350 mV) across the same bridging pathway, because of the better ability of the dianionic bridging ligand to delocalize an added hole (in the oxidized mixed-valence state) than an added electron (in the reduced mixed-valence state), implying some ligand-centered character for the oxidations. UV-vis-NIR spectroelectrochemical measurements were performed in all five oxidation states; the Ru(II)-Ru(III) mixed-valence state of [1](-) has a strong IVCT transition at 2360 nm whose parameters give an electronic coupling constant of V(ab) approximately 1100 cm(-1), characteristic of a strongly interacting but localized (class II) mixed-valence state. In the Ru(III)-Ru(IV) mixed-valence state [1](+), no low-energy IVCT could be detected despite the strong electronic interaction, possibly because it is in the visible region and obscured by LMCT bands.     

Electronic Structure and Substitution and Redox Reactivity of Imidazolate-Bridged Complexes of Pentacyanoferrate and Pentaammineruthenium

The mixed-valence compound [(NC)(5)Fe(II)-Im-Ru(III)(NH(3))(5)](-),M(i), was prepared in solution and as a solid sodium salt from [Fe(CN)(5)H(2)O](3)(-) and [Ru(NH(3))(5)Im](2+). The binuclear complex shows two bands at 366 nm (epsilon = 3350 M(-)(1) cm(-)(1)) and 576 nm (epsilon = 1025 M(-)(1) cm(-)(1)), assigned as LMCT transitions, as well as a near-IR band at 979 nm (epsilon = 962 M(-)(1) cm(-)(1)) associated with an intervalence transition. By calculation of the Hush model parameters alpha(2) and H(ab) (delocalization and electronic coupling factors, respectively), the complex is defined as a valence-trapped Fe(II)-Ru(III) system; this is confirmed by the measured redox potentials at -0.20 V and 0.30 V, associated with redox processes at the ruthenium and iron center, respectively. The formation stability constant of the mixed-valence ion was obtained through independent measurements of k(f) and k(d), the formation and dissociation specific rate constants, respectively. The stabilization of M(i) with respect to disproportionation into the isovalent states, as well as toward the formation of the electronic isomer, Fe(III)-Im-Ru(II), was also estimated. The fully reduced (R(i)) and fully oxidized (O(i)) binuclear complexes were prepared in solution and characterized by UV-vis spectroscopy. The kinetics of the reactions of R(i) and M(i) with peroxydisulfate were measured and a mechanistic analysis was performed, showing the relevance of electronic isomerization in completing the full conversion to O(i), through the assistance of the Ru(II)(NH(3))(5)(2+) center in the oxidation of the neighboring Fe(II)(CN)(5)(3)(-) moiety. The latter results are compared with those obtained with related complexes comprising different X(5)M-L moieties bound to Ru(II)(NH(3))(5)(2+). A linear correlation is displayed by plotting ln k(et) against E degrees (Ru), associated with the intramolecular oxidation rate constant of Ru(II) in the ion pair (binuclear species + peroxydisulfate) and the reduction potential of the corresponding Ru(III,II) couple in the ion pair.     

Synthesis and properties of [Ru(2)(acac)(4)(bptz)](n+) (n=0,1) and crystal structure of [Ru(2)(acac)(4)(bptz)

The neutral complex [Ru(2)(acac)(4)(bptz)] (I) has been prepared by the reaction of Ru(acac)(2)(CH(3)CN)(2) with bptz (bptz = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine) in acetone. The diruthenium(II,II) complex (I) is green and exhibits an intense metal-ligand charge-transfer band at 700 nm. Complex I is diamagnetic and has been characterized by NMR, optical spectroscopy, IR, and single-crystal X-ray diffraction. Crystal structure data for I are as follows: triclinic, P1, a = 11.709(2) A, b = 13.487(3) A, c = 15.151(3) A, alpha = 65.701(14) degrees, beta = 70.610(14) degrees, gamma = 75.50(2) degrees, V = 2038.8(6) A(3), Z = 2, R = 0.0610, for 4397 reflections with F(o) > 4sigmaF(o). Complex I shows reversible Ru(2)(II,II)-Ru(2)(II,III) and Ru(2)(II,III)-Ru(2)(III,III) couples at 0.17 and 0.97 V, respectively; the 800 mV separation indicates considerable stabilization of the mixed-valence species (K(com) > 10(13)). The diruthenium(II,III) complex, [Ru(2)(acac)(4)(bptz)](PF(6)) (II) is prepared quantitatively by one-electron oxidation of I with cerium(IV) ammonium nitrate in methanol followed by precipitation with NH(4)PF(6). Complex II is blue and shows an intense MLCT band at 575 nm and a weak band at 1220 nm in CHCl(3), which is assigned as the intervalence CT band. The mixed valence complex is paramagnetic, and an isotropic EPR signal at g = 2.17 is observed at 77 and 4 K. The solvent independence and narrowness of the 1200 nm band show that complex II is a Robin and Day class III mixed-valence complex.     

Dinuclear cyano-bridged CoIII-FeII complexes as precursors for molecular mixed-valence complexes of higher nuclearity

The preparation and characterization of a series of trinuclear mixed-valence cyano-bridged Co(III)-Fe(II)-Co(III) compounds derived from known dinuclear [[L(n)Co(III)(mu-NC)]Fe(II)(CN)(5)](-) complexes (L(n)() = N(5) or N(3)S(2) n-membered pendant amine macrocycle) are presented. All of the new trinuclear complexes were fully characterized spectroscopically (UV-vis, IR, and (13)C NMR). Complexes exhibiting a trans and cis arrangement of the Co-Fe-Co units around the [Fe(CN)(6)](4-) center are described (i.e., cis/trans-[{L(n)Co(III)(mu-NC)](2)Fe(II)(CN)(4)](2+)), and some of their structures are determined by X-ray crystallography. Electrochemical experiments revealed an expected anodic shift of the Fe(III/II) redox potential upon addition of a tripositively charged [Co(III)L(n)] moiety. The Co(III/II) redox potentials do not change greatly from the di- to the trinuclear complex, but rather behave in a fully independent and noncooperative way. In this respect, the energies and extinction coefficients of the MMCT bands agree with the formal existence of two mixed-valence Fe(II)-CN-Co(III) units per molecule. Solvatochromic experiments also indicated that the MMCT band of these compounds behaves as expected for a class II mixed-valence complex. Nevertheless, its extinction coefficient is dramatically increased upon increasing the solvent donor number.     

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.     

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)].     

Multisite effects on intervalence charge transfer in a clusterlike trinuclear assembly containing ruthenium and osmium

The intervalence charge transfer (IVCT) properties of the mixed-valence forms of the diastereoisomers of the dinuclear [[Ru(bpy)2](mu-HAT)[M(bpy)2]]5+ (M = Ru or Os) complexes and the trinuclear heterochiral [[Ru(bpy)2]2[Os(bpy)2](mu-HAT)]n+ (n = 7, 8; HAT = 1,4,5,8,9,12-hexaazatriphenylene; bpy = 2,2'-bipyridine) species display a marked dependence on the nuclearity and extent of oxidation of the assemblies, while small differences are also observed for the diastereoisomers of the same complex in the dinuclear cases. The mixed-valence heterochiral [[Ru(bpy)2]2[Os(bpy)2](mu-HAT)]n+ (n = 7, 8) forms exhibit IVCT properties that are intermediate between those of the diastereoisomeric forms of the localized hetero-dinuclear complex [[Ru(bpy)2](mu-HAT)[Os(bpy)2]]5+ and the borderline localized-to-delocalized homo-trinuclear complex [[Ru(bpy)2]3(mu-HAT)]n+ (n = 7, 8). The near-infrared (NIR) spectrum of the +7 mixed-valence species exhibits both interconfigurational (IC) and IVCT transitions which are quantitatively similar to those in [[Ru(bpy)2](mu-HAT)[Os(bpy)2]]5+ and are indicative of the localized mixed-valence formulation [[Ru(II)(bpy)2]2[Os(III)(bpy)2](mu-HAT)]7+. The +8 state exhibits a new band attributable to an IVCT transition in the near-infrared region.     

Synthesis, structure, and electronic properties of a dimer of Ru(bpy)2 doubly bridged by methoxide and pyrazolate

The heterobridged dinuclear complex cis,cis-[(bpy) 2Ru(mu-OCH 3)(mu-pyz)Ru(bpy) 2] (2+) ( 1; bpy = 2,2'-bipyridine; pyz = pyrazolate) was synthesized and isolated as a hexafluorophosphate salt. Its molecular structure was fully characterized by X-ray crystallography, (1)H NMR spectroscopy, and ESI mass spectrometry. The compound 1.(PF 6) 2 (C 44H 38F 12N 10OP 2Ru 2) crystallizes in the monoclinic space group P2 1/ c with a = 13.3312(4) A, b = 22.5379(6) A, c = 17.2818(4) A, beta = 99.497(2) degrees , V = 5121.3(2) A (3), and Z = 4. The meso diastereoisomeric form was exclusively found in the crystal structure, although the NMR spectra clearly demonstrated the presence of two stereoisomers in solution (rac and meso forms at approximately 1:1 ratio). The electronic properties of the complex in acetonitrile were investigated by cyclic voltammetry and UV-vis and NIR-IR spectroelectrochemistries. The stepwise oxidation of the Ru (II)-Ru (II) complex into the mixed-valent Ru (II)-Ru (III) and fully oxidized Ru (III)-Ru (III) states is fully reversible on the time scale of the in situ (spectro)electrochemical measurements. The mixed-valent species displays strong electronic coupling, as evidenced by the large splitting between the redox potentials for the Ru(III)/Ru(II) couples (Delta E 1/2 = 0.62 V; K c = 3 x 10 (10)) and the appearance of an intervalence transfer (IT) band at 1490 nm that is intense, narrow, and independent of solvent. Whereas this salient band in the NIR region originates primarily from highest-energy of the three IT transitions predicted for Ru(II)-Ru(III) systems, a weaker absorption band corresponding to the lowest-energy IT transition was clearly evidenced in the IR region ( approximately 3200 cm (-1)). The observation of totally coalesced vibrational peaks in the 1400-1650 cm (-1) range for a set of five bpy spectator vibrations in Ru (II)-Ru (III) relative to Ru (II)-Ru (II) and Ru (III)-Ru (III) provided evidence for rapid electron transfer and valence averaging on the picosecond time scale. Other than a relatively short Ru...Ru distance (3.72 A for the crystalline Ru (II)-Ru (II) complex), the extensive communication between metal centers is attributed mostly to the pi-donor ability of the bridging ligands (pyz, OMe) combined with the pi-acceptor ability of the peripheral (bpy) ligands.     

Interfacial electron transfer in FeII(CN)6 4- -sensitized TiO2 nanoparticles: a study of direct charge injection by electroabsorption spectroscopy

Electroabsorption (Stark) spectroscopy has been used to study the dye sensitized interfacial electron transfer in an Fe(II)(CN)(6)(4)(-) donor complex bound to a TiO(2) nanoparticle. The average charge-transfer distance determined from the Stark spectra is 5.3 A. This value is similar to the estimated distance between the Fe(II) center of the complex and the Ti(IV) surface site coordinated to the nitrogen end of a bridging CN ligand in (CN)(5)Fe(II)-CN-Ti(IV)(particle). This finding suggests that the electron injection is to either an individual titanium surface site or a small number of Ti centers localized around the point of ferrocyanide coordination to the particle and not into a conduction band orbital delocalized over the nanoparticle. The polarizability change, Tr(Deltaalpha), between the ground and the excited states of the Fe(II)(CN)(6)(4)(-)-TiO(2)(particle) system is approximately 3 time larger than normally observed in mixed-valence dinuclear metal complexes. It is proposed that the large polarizability of the excited state increases the dipole-moment changes measured by Stark spectroscopy.     

Molecular QCA cells. 2. Characterization of an unsymmetrical dinuclear mixed-valence complex bound to a Au surface by an organic linker

Utilization of binary information encoded in the charge configuration of quantum-dot cells (the quantum-dot cellular automata, QCA, paradigm) requires surface-bound molecule-sized dots for room temperature operation. Molecular QCA cells are mixed-valence complexes, and the evaluation of a surface-bound unsymmetrical, heterobinuclear, two-dot, Fe-Ru molecular QCA cell is described. The tailed complex, trans-[Ru(dppm)(2)(C[triple bond]CFc)(N[triple bond]CCH(2)CH(2)-NH(2))][PF(6)] (dppm = methylbis(diphenylphosphane), Fc = (eta(5)-C(5)H(5))Fe(eta(5)-C(5)H(4))), is covalently modified with the molecular adapter, HS(CH(2))(10)COOH, for binding to a Au surface. Preparation and characterization of the films by AFM, XPS, and electrochemical techniques are reported. Cyclic voltammetric techniques are used to assess film growth, coverage and uniformity, effects of thiol diluents on areal densities of the complex, and stabilities of the accessible redox states. Amperometric techniques are used to investigate the efficiency of both chemical and electrochemical oxidation in producing the mixed-valence dication on the surface.     

p-Carborane: a new cage spacer for photoactive metal polypyridine dyads

The first example of a binuclear ruthenium complex involving the p-carborane framework in the bridging ligand is reported. The bridging ligand is a symmetric linear array comprising a central p-carborane unit, two p-phenylene spacers, and two 5-yl-2,2'-bipyridine coordinating units. A homobinuclear Ru(II) complex, with 2,2'-bipyridine as peripheral ligands, was synthesized and characterized. The Ru(II)-Ru(III) mixed-valence species, obtained by partial oxidation, has been investigated with steady-state and time-resolved techniques in CH3CN. The rate of photoinduced electron transfer is 2.3 x 10(8) s(-1).     

Application of circular dichroism spectroscopy in the study of mixed-valence asymmetric ruthenium polypyridyl complexes

Circular dichroism (CD) spectroelectrochemistry is used to determine the extent of singly occupied molecular orbital delocalization in mixed-valence multinuclear complexes, specifically the mixed-valence Ru(II)Ru(III) states of the four diastereoisomers of [(Ru(bpy)(2))(2)(bpt)](3+) [1; bpy = 2,2'-bipyridyl and bpt = 3,5-bis(pyrid-2'-yl)-1,2,4-triazolato anion]. The complex was found to be stable to thermal racemization in the three oxidation states, but photoracemization in the Ru(II)Ru(II) state was observed.     

Mono- and dinuclear ruthenium carbonyl complexes with redox-active dioxolene ligands: electrochemical and spectroscopic studies and the properties of the mixed-valence complexes

The mononuclear complex [Ru(PPh(3))(2)(CO)(2)(L(1))] (1; H(2)L(1) = 7,8-dihydroxy-6-methoxycoumarin) and the dinuclear complexes [[Ru(PPh(3))(2)(CO)(2)](2)(L(2))][PF(6)] [[2][PF(6)]; H(3)L(2) = 9-phenyl-2,3,7-trihydroxy-6-fluorone] and [[Ru(PBu(3))(2)(CO)(2)](2)(L(3))] (3; H(4)L(3) = 1,2,3,5,6,7-hexahydroxyanthracene-9,10-dione) have been prepared; all complexes contain one or two trans,cis-[Ru(PR(3))(2)(CO)(2)] units, each connected to a chelating dioxolene-type ligand. In all cases the dioxolene ligands exhibit reversible redox activity, and accordingly the complexes were studied by electrochemistry and UV/vis/NIR, IR, and EPR spectroscopy in their accessible oxidation states. Oxidation of 1 to [1](+) generates a ligand-centered semiquinone radical with some metal character as shown by the IR and EPR spectra. Dinuclear complexes [2](+) and 3 show two reversible ligand-centered couples (one associated with each dioxolene terminus) which are separated by 690 and 440 mV, respectively. This indicates that the mixed-valence species [2](2+) has greater degree of electronic delocalization between the ligand termini than does [3](+), an observation which was supported by IR, EPR, and UV/vis/NIR spectroelectrochemistry. Both [2](2+) and [3](+) have a solution EPR spectrum consistent with full delocalization of the unpaired electron between the ligand termini on the EPR time scale (a quintet arising from equal coupling to all four (31)P nuclei); [3](+) is localized on the faster IR time scale (four CO vibrations rather than two, indicative of inequivalent [Ru(CO)(2)] units) whereas [2](2+) is fully delocalized (two CO vibrations). UV/vis/NIR spectroelectrochemistry revealed the presence of a narrow, low-energy (2695 nm) transition for [3](+) associated with the catecholate --> semiquinone intervalence transition. The narrowness and solvent-independence of this transition (characteristic of class III mixed-valence character) coupled with evidence for inequivalent [Ru(CO)(2)] termini in the mixed-valence state (characteristic of class II character) place this complex at the class II-III borderline, in contrast to [2](2+) which is clearly class III.     

Switching of the electronic communication between two {Ru(trpy)(bpy)} (trpy = 2,2':6',2' '-terpyridine and bpy = 2,2'-bipyridine) centers by protonation on the bridging dimercaptothiadiazolato ligand

A stable Ru(II)/Ru(III) mixed-valence state was observed in acetonitrile for the ruthenium binuclear complex bridged by dimercaptothiadiazolate (DeltaE(1/2) = 220 mV for Ru(2)(II,II)/Ru(2)(II,III) and Ru(2)(II,III)/Ru(2)(III,III) processes; K(com) = 5.3 x 10(3)). Upon protonation of the bridging ligand by the addition of equimolar p-toluenesulfonic acid, however, the mixed-valence state diminished (DeltaE(1/2) = 0 mV). The bridging ligand operates as a proton-induced switch of the electronic communication in the dimeric complex.     

Molecular QCA cells. 1. Structure and functionalization of an unsymmetrical dinuclear mixed-valence complex for surface binding

Utilization of binary information encoded in the charge configuration of quantum-dot cells (the quantum-dot cellular automata, QCA, paradigm) requires molecule-sized dots for room temperature operation. Molecular QCA cells are mixed-valence complexes, and the evaluation and functionalization of an unsymmetrical heterobinuclear, two-dot, Fe-Ru molecular QCA cell is described. The solid state structures of trans-RuCl(dppm)(2)(C[triple bond]CFc) (1) (dppm = methylbis(diphenylphosphane), Fc = (eta(5)-C(5)H(5))Fe(eta(5)-C(5)H(4))) and mixed-valence [trans-RuCl(dppm)(2)(C[triple bond]CFc)][BF(4)] (1a) as well as XPS and spectroscopic data suggest class II behavior suitable for the intended application. Utilization of the trans-Cl position of 1 permits functionalization for surface binding. Two "tailed" complexes of 1, trans-Ru(dppm)(2)(C[triple bond]CFc)(C[triple bond]CPhOCH(3)) (2) and trans-[Ru(dppm)(2)(C[triple bond]CFc)(N[triple bond]CCH(2)CH(2)NH(2))][PF(6)] (3), have been prepared and characterized. The solid state structure of 3 and multinuclear NMR experiments define the structures. In addition, the spectroscopic properties of all complexes and their mixed-valence species are used to define the effect of the substituent "tail" on mixed-valence properties. Further, the electrochemistry of these compounds permits assessment of the extent of perturbation of the substituents on the comproportionation constants and overall electrochemical stability. The complexes possess properties necessary for candidate QCA molecules.     

Discrete Rh(III)/Fe(II) and Rh(III)/Fe(II)/Co(III) cyanide-bridged mixed valence compounds

The heterotrinuclear complexes trans- and cis-[{cis-VI-L(15)Rh(III)(μ-NC)}{trans-III-L(14S)Co(III)(μ-NC)}Fe(II)(CN)(4)](2+) are unprecedented examples of mixed valence complexes based on ferrocyanide bearing three different metal centers. These complexes have been assembled in a stepwise manner from their {trans-III-L(14S)Co(III)}, {cis-VI-L(15)Rh(III)}, and {Fe(II)(CN)(6)} building blocks. The preparative procedure follows that found for other known discrete assemblies of mixed valence dinuclear Cr(III)/Fe(II) and polynuclear Co(III)/Fe(II) complexes of the same family. A simple slow substitution process of [Fe(II)(CN)(6)](4-) on inert cis-VI-[Rh(III)L(15)(OH)](2+) leads to the preparation of the new dinuclear mixed valence complex [{cis-VI-L(15)Rh(III)(μ-NC)}Fe(II)(CN)(5)](-) with a redox reactivity that parallels that found for dinuclear complexes from the same family. The combination of this dinuclear precursor with mononuclear trans-III-[Co(III)L(14S)Cl](2+) enables a redox-assisted substitution on the transient {L(14S)Co(II)} unit to form [{cis-VI-L(15)Rh(III)(μ-NC)}{trans-III-L(14S)Co(III)(μ-NC)}Fe(II)(CN)(4)](2+). The structure of the final cis-[{cis-VI-L(15)Rh(III)(μ-NC)}{trans-III-L(14S)Co(III)(μ-NC)}Fe(II)(CN)(4)](2+) complex has been established via X-ray diffraction and fully agrees with its solution spectroscopy and electrochemistry data. The new species [{cis-VI-L(15)Rh(III)(μ-NC)}{trans-III-L(14S)Co(III)(μ-NC)}Fe(II)(CN)(4)](2+) and [{cis-VI-L(15)Rh(III)(μ-NC)}Fe(II)(CN)(5)](-) show the expected electronic spectra and electrochemical features typical of Class II mixed valence complexes. Interestingly, in the trinuclear complex, these features appear to be a simple addition of those for the Rh(III)/Fe(II) and Co(III)/Fe(II) moieties, despite the vast differences existent in the electronic spectra and electrochemical properties of the two isolated units.     

Fine tuning of the electronic coupling between metal centers in cyano-bridged mixed-valent trinuclear complexes

We report the synthesis, characterization, and spectroscopic properties of a family of trinuclear cyano-bridged mixed-valent compounds, trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-), trans-[Ru(II)L(4)[CNFe(III)(CN)(5)](2)](4-), and cis-[Ru(II)(bpy)(2)[NCFe(III)(CN)(5)](2)](4-) (L = pyridine, 4-tert-butylpyridine, and 4-methoxypyridine). Tetraphenylphosphonium salts of complexes trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-) (L = pyridine and 4-tert-butylpyridine) crystallize in the space groups C2 and P2(1)/c, respectively, and show a linear arrangement of the metal units and an almost completely eclipsed configuration of the equatorial ligands. An intense band (epsilon approximately 2000-9000 M(-1) cm(-1)) is observed for all of the compounds in the NIR region of the spectrum, not present in the separated building blocks, and strongly solvent dependent. We assign it as a metal-to-metal charge transfer (MMCT) from the Ru(II) to the terminal Fe(III) moieties in the context of a simplified three-center model. The electrochemistry measurements reveal a splitting of the redox waves for the reduction of the iron centers for some of the complexes with a trans configuration between the metal units, ranging from 100 to 260 mV, depending on the substituting pyridine ligand and the solvent, suggesting long-range metal-metal interactions. These interactions are rationalized in terms of the energy matching between the pi-symmetry orbitals of the metals and the cyanide bridge. The one- and two-electron reduced species derived from compounds trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-,5-,6-) were characterized in methanolic solution. The mixed-valent Fe(II)-Ru(II)-Fe(III) system exhibits an intense red shifted band in the NIR region of the spectrum, arising from the superposition of MMCT bands from the central Ru(II) to the terminal Fe(III) fragments and from the 1 nm distant Fe(II) to Fe(III) centers.     

Efficient energy transfer via the cyanide bridge in dinuclear complexes containing Ru(II) polypyridine moieties

We report the synthesis, structure and properties of the cyanide-bridged dinuclear complex ions [Ru(L)(bpy)(μ-NC)M(CN)(5)](2-/-) (L = tpy, 2,2';6',2'-terpyridine, or tpm, tris(1-pyrazolyl)methane, bpy = 2,2'-bipyridine, M = Fe(II), Fe(III), Cr(III)) and the related monomers [Ru(L)(bpy)X](2+) (X = CN(-) and NCS(-)). All the monomeric compounds are weak MLCT emitters (λ = 650-715 nm, ? ≈ 10(-4)). In the Fe(II) and Cr(III) dinuclear systems, the cyanide bridge promotes efficient energy transfer between the Ru-centered MLCT state and a Fe(II)- or Cr(III)-centered d-d state, which results either in a complete quenching of luminescence or in a narrow red emission (λ ≈ 820 nm, ? ≈ 10(-3)) respectively. In the case of Fe(III) dinuclear systems, an electron transfer quenching process is also likely to occur.