Tuning the metal-to-metal charge transfer energy of cyano-bridged dinuclear complexes

The metal-to-metal charge transfer (MMCT) transitions of a series of Class II mixed valence dinuclear complexes bearing cyano bridging ligands may be varied systematically by variations to either the hexacyanometallate(II) donor or Co-III acceptor moieties. Specifically, the new dinuclear species trans-[(LCoNCFe)-Co-14S(CN)(5)](-) (L-14S = 6-methyl-1,11-diaza-4,8-dithia- cyclotetradecane-6-amine) and trans-[(LCoNCRu)-Co-14(CN)(5)]-(L-14 = 6-methyl-1,4,8,11-tetraazacyclotetradecane-6-amine) have been prepared and their spectroscopic and electrochemical properties are compared with the relative trans-[(LCoNCFe)-Co-14(CN)(5)](-). The crystal structures of Na{trans-[(LCoNCFe)-Co-14S(CN)(5)]}.51/2H(2)O.1/2EtOH, Na{trans-[(LCoNCRu)-Co-14(CN)(5)]}.3H(2)O and Na{trans-[(LCoNCRu)-Co-14(CN)(5)]}.8H(2)O are also reported. The ensuing changes to the MMCT energy have been examined within the framework of Hush theory, and it was found that the free energy change between the redox isomers was the dominant effect in altering the energy of the MMCT transition

Chemical, electrochemical and photochemical catalytic oxidation of water to dioxygen with mononuclear ru complexes

Four new RuII[BOND]Cl and RuII[BOND]H2O complexes containing the meridional 2,2’:6’,2”-terpyridine (trpy) ligand and the chelating 2-(5-phenyl-1H-pyrazol-3-yl)pyridine (H3p) ligand of general formula in- and out-[RuII(trpy)(H3p)(X)]n+ (X=Cl, n=1; X=H2O, n=2) have been prepared, isolated and thoroughly characterized both in solution and in the solid state. In solution, all the complexes are characterized spectroscopically by UV/Vis and NMR, and their redox properties investigated by means of cyclic voltammetry, square wave voltammetry, and coulometry. In the solid state, monocrystal X-ray diffraction analysis was carried out for the in and out Ru[BOND]Cl complexes. The capacity of the Ru-aqua complexes to act as water oxidation catalysts (WOCs) was also investigated chemically, electrochemically, and photochemically. The performance of these complexes has also been compared to two previously described complexes of general formula in- and out-[RuII(trpy)(Hbpp)(H2O)]2+ (Hbpp is 2,2′-(1H-pyrazole-3,5-diyl)dipyridine)), the capacity of which as WOCs had not been previously described.SOLAR-H2 (EU 212508) and MICINN (CTQ2010–21497 and Consolider Ingenio 2010 CSD2006–0003) are gratefully acknowledged. SR is grateful for the award of doctoral grant from MICINN

Tailoring mixed-valence CoIII/FeII complexes for their potential use as sensitizers in dye sensitized solar cells

Dinuclear class II CoIII/FeII mixed-valence complexes, [LnCoIII(μ-NC)FeII(CN)3L2]m-, where Ln represents a pentadentate macrocycle and L2 a bpy ligand or 2 cyanides, have electronic properties that make them possible sensitizers in dye sensitized solar cells (DSSCs). For this purpose the new complex Na2[{trans-L14COOCoIII(μ-NC)}FeII(CN)5] was prepd. and characterized by the usual methods and its sensitizer properties compared with those of the known [{trans-L15CoIII(μ-NC)}FeII(CN)3(bipy)eq,ax](ClO4). [{Trans-L14COOCoIII(μ-NC)}FeII(CN)5]2- was designed for electron injection from the Co center on MMCT and the [{trans-LnCoIII(μ-NC)}FeII(CN)3(bipy)]+ structure can produce tuned injection from the Fe center via a MLCT electronic state, as described for similar systems. The characterization on solid oxide semiconductor supports was carried out for these 2 complexes and the results were compared with the behavior obsd. in aq. soln. and in solvents of varying polarities. Their use in a DSSC was tested - although a sensitizer response was obsd. for [{trans-L14COOCoIII(μ-NC)}FeII(CN)5]2-, the complex [{trans-L15CoIII(μ-NC)}FeII(CN)3(bipy)eq,ax]+ does not produce any elec. current on illumination. The low efficiency of such a DSSC is due to the low value of the extinction coeff. of the MMCT band responsible for the electron injection and with the small driving force for the redn. of the complex with the std. I2/I3- system used after electron injection

Mononuclear Ruthenium–Water Oxidation Catalysts: Discerning between Electronic and Hydrogen-Bonding Effects

New mononuculear complexes of the general formula [Ru­(trpy)­(<i>n</i>,<i>n</i>′-F₂-bpy)­X]^<i>m</i>+ [<i>n</i> = <i>n</i>′ = 5, X = Cl (<b>3⁺</b>) and H₂O (<b>5²⁺</b>); <i>n</i> = <i>n</i>′ = 6, X = Cl (<b>4⁺</b>) and H₂O (<b>6²⁺</b>); trpy is 2,2′:6′:2″-terpyridine] have been prepared and thoroughly characterized. The 5,5′- and 6,6′-F₂-bpy ligands allow one to exert a remote electronic perturbation to the ruthenium metal center, which affects the combination of species involved in the catalytic cycle. Additionally, 6,6′-F₂-bpy also allows through-space interaction with the Ru–O moiety of the complex via hydrogen interaction, which also affects the stability of the different species involved in the catalytic cycle. The combination of both effects has a strong impact on the kinetics of the catalytic process, as observed through manometric monitoring

Isomeric distribution and catalyzed isomerization of cobalt(III) complexes with pentadentate macrocyclic ligands. Importance of hydrogen bonding

We have investigated the isomeric distribution and rearrangement of complexes of the type [CoXLn](2+,3+) (where X = Cl-, OH-, H2O, and L-n represents a pentadentate 13-, 14-, and 15-membered tetraaza or diaza-dithia (N-4 or N2S2) macrocycle bearing a pendant primary amine). The preparative procedures for chloro complexes produced almost exclusively kinetically preferred cis isomers (where the pendant primary amine is cis to the chloro ligand) that can be separated by careful cation-exchange chromatography. For L-13 and L-14 the so-called cis-V isomer is isolated as the kinetic product, and for L-15 the cis-VI form (an N-based diastereomer) is the preferred, while for the L-14(S) complex both cis-V and trans-I forms are obtained. All these complexes rearrange to form stable trans isomers in which the pendent primary amine is trans to the monodentate aqua or hydroxo ligand, depending on pH and the workup procedure. In total 11 different complexes have been studied. From these, two different trans isomers of [CoCIL14S](2+) have been characterized crystallographically for the first time in addition to a new structure of cis-V-[CoCIL14S](2+); all were isolated as their chloride perchlorate salts. Two additional isomers have been identified and characterized by NMR as reaction intermediates. The remaining seven forms correspond to the complexes already known, produced in preparative procedures. The kinetic, thermal, and baric activation parameters for all the isomerization reactions have been determined and involve large activation enthalpies and positive volumes of activation. Activation entropies indicate a very important degree of hydrogen bonding in the reactivity of the complexes, confirmed by density functional theory studies on the stability of the different isomeric forms. The isomerization processes are not simple and even some unstable intermediates have been detected and characterized as part of the above-mentioned 11 forms of the complexes. A common reaction mechanism for the isomerization reactions has been proposed for all the complexes derived from the observed kinetic and solution behavior

Electrochemical and Resonance Raman Spectroscopic Studies of Water-Oxidizing Ruthenium-Terpyridyl-Bipyridyl Complexes

The irreversible conversion of single-site water oxidation catalysts (WOC) into the more rugged catalysts structurally related to [(trpy)(5,5&rsquo;-X2-bpy)RuIV(&mu;-O)RuIV(trpy)(O)(H2O)]4+ (X = H, 1-dn4+; X = F, 2-dn4+) represents a critical issue in developing active and durable WOC. In this work, the electrochemical and acid-base properties of 1-dn4+ and 2-dn4+ were evaluated. In-situ resonance Raman spectroscopy was employed to characterize species formed upon stoichiometric oxidation of single-site catalysts demonstrating the formation of high oxidation states mononuclear Ru=O and RuO-O complexes. Under turnover conditions, the dinuclear intermediates, 1-dn4+ and 2-dn4+, as well as the previously proposed [RuVI(trpy)(O)2(H2O)]2+ complex (32+) are formed. 32+ is a pivotal intermediate that provides access to the formation of dinuclear species. Single crystal X-ray diffraction analysis of the isolated complex [RuIV(O)(trpy)(5,5&rsquo;-F2-bpy)]2+ reveals a clear elongation of the Ru-N bond located in the trans position to the oxo group, documenting the weakness of this bond which promotes the release of the bpy ligand and the subsequent formation of 32+. &nbsp;</p

Water Oxidation Catalysis with Ligand Substituted Ru-bpp-type Complexes

A series of symmetric and non-symmetric dinuclear Ru complexes of general formula {[Ru(R2-trpy)(H2O)][Ru(R3-trpy)(H2O)](&mu;-R1-bpp)}3+ where trpy is 2,2&prime;:6&prime;,2&prime;&prime;-terpyridine, bpp&minus; is 3,5-bis(2-pyridyl)-pyrazolate and R1, R2 and R3 are electron donating (ED) and electron withdrawing (EW) groups such as Me, MeO, NH2 and NO2 have been prepared using microwave assisted techniques. These complexes have been thoroughly characterized by means of analytical (elemental analysis), spectroscopic (UV-vis, NMR) and electrochemical (CV, SQWV, CPE) techniques. The single crystal X-ray structures for one acetate- and one chloro-bridged precursor have also been solved. Kinetic analysis monitored by UV-vis spectroscopy reveals the electronic effects exerted by the ED and EW groups on the substitution kinetics and stoichiometric water oxidation reaction. The catalytic water oxidation activity is evaluated by means of chemically (CeIV), electrochemically, and photochemically induced processes. It is found that, in general, ED groups do not strongly affect the catalytic rates whereas EW groups drastically reduce catalytic rates. Finally, DFT calculations provide a general and experimentally consistent view of the different water oxidation pathways that can operate in the water oxidation reactions catalyzed by these complexes.</p

CF ₃-Ph reductive elimination from [(Xantphos)Pd(CF ₃)(Ph)]

CF 3-Ph reductive elimination from [(Xantphos)Pd(Ph)(CF 3)] (1) and [(i-Pr-Xantphos)Pd(Ph)(CF 3)] (2) has been studied by experimental and computational methods. Complex 1 is cis in the solid state and predominantly cis in solution, undergoing degenerate cis-cis isomerization (ΔG * exp = 13.4 kcal mol -1; ΔG * calc = 12.8 kcal mol -1 in toluene) and slower cis-trans isomerization (ΔG calc = +0.9 kcal mol -1; ΔG * calc = 21.9 kcal mol -1). In contrast, 2 is only trans in both solution and the solid state with trans-2 computed to be 10.2 kcal mol -1 lower in energy than cis-2. Kinetic and computational studies of the previously communicated (J. Am. Chem. Soc. 2006, 128, 12644), remarkably facile CF 3-Ph reductive elimination from 1 suggest that the process does not require P-Pd bond dissociation but rather occurs directly from cis-1. The experimentally determined activation parameters (ΔH * = 25.9 ± 2.6 kcal mol -1; ΔS * = 6.4 ± 7.8 e.u.) are in excellent agreement with the computed data (ΔH * calc = 24.8 kcal mol -1; ΔG * calc = 25.0 kcal mol -1). ΔG * calc for CF 3-Ph reductive elimination from cis-2 is only 24.0 kcal mol -1; however, the overall barrier relative to trans-2 is much higher (ΔG * calc = 34.2 kcal mol -1) due to the need to include the energetic cost of trans-cis isomerization. This is consistent with the higher thermal stability of 2 that decomposes to PhCF 3 only at 100 °C and even then only in a sluggish and less selective manner. The presence of excess Xantphos has a minor decelerating effect on the decomposition of 1. A steady slight decrease in k obs in the presence of 1 and 2 equiv of Xantphos then plateaus at [Xantphos]:1 = 5, 10, and 20. Specific molecular interactions between 1 and Xantphos are not involved in this kinetic effect (NMR, T 1 measurements). A deduced kinetic scheme accounting for the influence of extra Xantphos involves the formation of cis-[(η 1- Xantphos) 2Pd(Ph)(CF 3)] that, by computation, is predicted to access reductive elimination of CF 3-Ph with ΔG * calc = 22.8 kcal mol -1.</p

CF3-Ph reductive elimination from [(Xantphos)Pd(CF3)(Ph)]

CF 3-Ph reductive elimination from [(Xantphos)Pd(Ph)(CF 3)] (1) and [(i-Pr-Xantphos)Pd(Ph)(CF 3)] (2) has been studied by experimental and computational methods. Complex 1 is cis in the solid state and predominantly cis in solution, undergoing degenerate cis-cis isomerization (ΔG * exp = 13.4 kcal mol -1; ΔG * calc = 12.8 kcal mol -1 in toluene) and slower cis-trans isomerization (ΔG calc = +0.9 kcal mol -1; ΔG * calc = 21.9 kcal mol -1). In contrast, 2 is only trans in both solution and the solid state with trans-2 computed to be 10.2 kcal mol -1 lower in energy than cis-2. Kinetic and computational studies of the previously communicated (J. Am. Chem. Soc. 2006, 128, 12644), remarkably facile CF 3-Ph reductive elimination from 1 suggest that the process does not require P-Pd bond dissociation but rather occurs directly from cis-1. The experimentally determined activation parameters (ΔH * = 25.9 ± 2.6 kcal mol -1; ΔS * = 6.4 ± 7.8 e.u.) are in excellent agreement with the computed data (ΔH * calc = 24.8 kcal mol -1; ΔG * calc = 25.0 kcal mol -1). ΔG * calc for CF 3-Ph reductive elimination from cis-2 is only 24.0 kcal mol -1; however, the overall barrier relative to trans-2 is much higher (ΔG * calc = 34.2 kcal mol -1) due to the need to include the energetic cost of trans-cis isomerization. This is consistent with the higher thermal stability of 2 that decomposes to PhCF 3 only at 100 °C and even then only in a sluggish and less selective manner. The presence of excess Xantphos has a minor decelerating effect on the decomposition of 1. A steady slight decrease in k obs in the presence of 1 and 2 equiv of Xantphos then plateaus at [Xantphos]:1 = 5, 10, and 20. Specific molecular interactions between 1 and Xantphos are not involved in this kinetic effect (NMR, T 1 measurements). A deduced kinetic scheme accounting for the influence of extra Xantphos involves the formation of cis-[(η 1- Xantphos) 2Pd(Ph)(CF 3)] that, by computation, is predicted to access reductive elimination of CF 3-Ph with ΔG * calc = 22.8 kcal mol -1.</p