Are Two Metal Ions Better than One? Mono- and Binuclear α-Diimine-Re(CO)3 Complexes with Proton-Responsive Ligands in CO2 Reduction Catalysis

Here, the reduction chemistry of mono- and binuclear α-diimine-Re(CO)₃ complexes with proton responsive ligands and their application in the electrochemically-driven CO₂ reduction catalysis are presented. The work was aimed to investigate the impact of 1) two metal ions in close proximity and 2) an internal proton source on catalysis. Therefore, three different Re complexes, a binuclear one with a central phenol unit, 3 , and two mononuclear, one having a central phenol unit, 1 , and one with a methoxy unit, 2 , were utilised. All complexes are active in the CO₂-to-CO conversion and CO is always the major product. The catalytic rate constant k_cat for all three complexes is much higher and the overpotential is lower in DMF/water mixtures than in pure DMF (DMF=N,N-dimethylformamide). Cyclic voltammetry (CV) studies in the absence of substrate revealed that this is due to an accelerated chloride ion loss after initial reduction in DMF/water mixtures in comparison to pure DMF. Chloride ion loss is necessary for subsequent CO₂ binding and this step is around ten times faster in the presence of water [ 2 : k^Cl(DMF)≈1.7 s⁻¹; k^Cl(DMF/H₂O)≈20 s⁻¹]. The binuclear complex 3 with a proton responsive phenol unit is more active than the mononuclear complexes. In the presence of water, the observed rate constant k_obs for 3 is four times higher than of 2 , in the absence of water even ten times. Thus, the two metal centres are beneficial for catalysis. Lastly, the investigation showed that the phenol unit has no impact on the rate of the catalysis, it even slows down the CO₂-to-CO conversion. This is due to an unproductive, competitive side reaction: After initial reduction, 1 and 3 loose either Cl⁻ or undergo a reductive OH deprotonation forming a phenolate unit. The phenolate could bind to the metal centre blocking the sixth coordination site for CO₂ activation. In DMF, O−H bond breaking and Cl⁻ ion loss have similar rate constants [ 1 : k^Cl(DMF)≈2 s⁻¹, k^OH≈1.5 s⁻¹], in water/DMF Cl⁻ loss is much faster. Thus, the effect on the catalytic rate is more pronounced in DMF. However, the acidic protons lower the overpotential of the catalysis by about 150 mV.     

Ultrafast excited state dynamics controlling photochemical isomerization of N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium coordinated to a {Re I(CO)3(2,2'-bipyridine)} chromophore

Two multifunctional photoactive complexes [Re(Cl)(CO)(3)(MeDpe(+))(2)](2+) and [Re(MeDpe(+))(CO)(3)(bpy)](2+) (MeDpe(+)=N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium, bpy=2,2'-bipyridine) were synthesized, characterized, and their redox and photonic properties were investigated by cyclic voltammetry; ultraviolet-visible-infrared (UV/Vis/IR) spectroelectrochemistry, stationary UV/Vis and resonance Raman spectroscopy; photolysis; picosecond time-resolved absorption spectroscopy in the visible and infrared regions; and time-resolved resonance Raman spectroscopy. The first reduction step of either complex occurs at about -1.1 V versus Fc/Fc(+) and is localized at MeDpe(+). Reduction alone does not induce a trans-->cis isomerization of MeDpe(+). [Re(Cl)(CO)(3)(MeDpe(+))(2)](2+) is photostable, while [Re(MeDpe(+))(CO)(3)(bpy)](2+) and free MeDpe(+) isomerize under near-UV irradiation. The lowest excited state of [Re(Cl)(CO)(3)(MeDpe(+))(2)](2+) has been identified as the Re(Cl)(CO)(3)-->MeDpe(+ 3)MLCT (MLCT=metal-to-ligand charge transfer), decaying directly to the ground state with lifetimes of approximately 42 (73 %) and approximately 430 ps (27 %). Optical excitation of [Re(MeDpe(+))(CO)(3)(bpy)](2+) leads to population of Re(CO)(3)-->MeDpe(+) and Re(CO)(3)-->bpy (3)MLCT states, from which a MeDpe(+) localized intraligand (3)pipi* excited state ((3)IL) is populated with lifetimes of approximately 0.6 and approximately 10 ps, respectively. The (3)IL state undergoes a approximately 21 ps internal rotation, which eventually produces the cis isomer on a much longer timescale. The different excited-state behavior of the two complexes and the absence of thermodynamically favorable interligand electron transfer in excited [Re(MeDpe(+))(CO)(3)(bpy)](2+) reflect the fine energetic balance between excited states of different orbital origin, which can be tuned by subtle structural variations. The complex [Re(MeDpe(+))(CO)(3)(bpy)](2+) emerges as a prototypical, multifunctional species with complementary redox and photonic behavior.     

β-榄香烯三羰基铼配合物的合成、放射化学合成及抗癌活性研究

以中草药有效成分β-榄香烯为起始原料, 经烯丙位的氯代反应及亲核取代反应在β-榄香烯母体上成功地引入含吡啶基的三齿螯合剂, 并与稳定的三羰基铼配位, 得到了一种新的铼(I)三齿配合物, 在此基础上利用铼的放射性同位素Re-188进行了放射性标记. 反应中间体及最终化合物分别用IR, ¹H NMR, HRMS, HPLC或元素分析进行表征, 并对该化合物进行了初步的体外抗癌活性研究. β-榄香烯三羰基铼配合物的合成、放射化学合成及体外抗癌活性评价, 为探讨β-榄香烯体内靶点和作用机制提供了可能, 并为最终开发基于β-榄香烯的放射性药物奠定了基础.     

trans-cis Photoisomerization of the styrylpyridine Ligand in [Re(CO)3(2,2'-bipyridine)(t-4-styrylpyridine)]+: role of the metal-to-ligand charge-transfer excited states

The trans-cis isomerization of the styrylpyridine carbon-carbon double bond induced by visible light irradiation in fac-[Re(CO)(3)(bpy)(stpy)](+) (bpy = 2,2'-bipyridine; stpy = t-4-styrylpyridine) has been investigated by means of quantum-chemical methods. The structures of the various cis and trans conformers of [Re(CO)(3)(bpy)(stpy)](+) have been optimized at the density functional theory (DFT) level. Three rotational conformers for the most stable trans isomer lie within 2.3 kJ mol(-1) each other. The energy difference between the cis and trans isomers is 27.0 kJ mol(-1). The electronic spectroscopy of the most stable conformers has been investigated by time-dependent DFT (TD-DFT) and complete active space self-consistent field/CAS second order perturbation theory (CASSCF/CASPT2) calculations. The lowest absorption bands are dominated by metal-to-ligand charge-transfer (MLCT, d(Re)-->pi*(bpy)) transitions calculated at about 25,000 cm(-1) and by a strong intraligand (1)IL (pi(stpy)-->pi*(stpy)) transition in the near UV region. On the basis of CASSCF potential energy curves (PECs) calculated as a function of the torsion angle of the C=C bond of the styrylpyridine ligand, it is shown that the role of the low-lying MLCT states is important in the photoisomerization mechanism. In contrast to the free organic ligand, in which the singlet mechanism is operational via the (1)IL (S(1)) and electronic ground (S(0)) states, coordination to the rhenium steers the isomerization to the triplet PEC corresponding to the (3)IL state. From the (3)IL(t) (t = trans) the system evolves to the perpendicular intermediate (3)IL(p) (p = perpendicular) following a 90 degrees rotation around the styrylpyridine C=C bond. The metal center acts as a photosensitizer because of the presence of photoactive MLCT states under visible irradiation. The position of the crossing between the (3)IL and electronic ground state PEC determines the quantum yield of the isomerization process.     

Novel Cyanide-Bridged Three-Dimensional Coordination Polymer Derived from an Octahedral Rhenium Cluster [Re6Te8(CN)6]4?: Synthesis and Crystal Structure of [{Mn(H2O)2(DMF)} 2Re6Te8(CN)6]·2H2O

The reaction of a water solution of K₄Re₆Te₈(CN)₆ with a solution of Mn(NO₃)₂ in 0.02M hydrochloric acid in the presence of DMF gave crystals of a cluster rhenium complex [{ Mn(H₂O)₂(DMF)}₂Re₆Te₈(CN)₆]·2H₂O. The structure of the compound was determined by single crystal X-ray diffraction (a = 12.6679(9) Å, b = 17.4524(12) Å, c = 9.7882(6) Å, β = 105.570(6)°, V = 2084.6(3) ų, Z = 2, space group P2₁/n, R = 0. 0389). In the complex, the [Re₆Te₈(CN)₆]⁴⁻ cluster anions are linked to Mn²⁺ cations by the cyanide bridges, the manganese cations being additionally coordinated by the DMF molecule and two water molecules. The neighboring clusters are joined by Re-C-N-Mn bridges into a three-dimensional framework possessing cavities filled with doubly disordered water molecules.Original Russian Text Copyright © 2004 by Yu. V. Mironov, S. F. Solodovnikov, V. E. Fedorov, and Yu. V. Gatilov__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 918–922, September–October, 2004.     

Hydrido-Carbonyl Rhenium Clusters with a Square Geometry of the Metal Core. Synthesis and X-Ray Characterization of the Novel [Re4(μ-H)3(CO)16]− Anion

Three and tetranuclear ring clusters have been obtained by treatment of [Re₂(CO)₈(THF)₂] with carbonyl-rhenates containing two terminal hydrides. The reaction with [ReH₂(CO)₄]^- provided a selective route to the previously known [Re₃(-H)₂(CO)₁₂]^- triangular cluster anion 1. The reaction with [Re₂H₂(-H)(CO)₈]^- gave the novel [Re₄(-H)₃(CO)₁₆]^- anion 2, containing a rare example of a puckered-square metal cluster. Protonation of 1 is known to afford the neutral [Re₃(-H)₃(CO)₁₂] species 3. Analogously the reaction of 2 with a strong acid afforded the previously known square metal clusters [Re₄(-H)₄(CO)₁₆] 4. The reaction could not be reversed by treatment with bases. Photolysis of 4 gave the unsaturated complex [Re₂(-H)₂(CO)₈] 5: this is the reverse of the dimerization reaction, that in THF at room temperature produces 4 from 5. Thermal treatment (reflux in cyclohexane for 24 h) left 4 almost unchanged. A single crystal X-ray analysis of [NEt₄]2 showed a s/e/s/s (e=eclipsed, s=staggered) conformation of the Re(CO)₄ units, leading to a puckered geometry of the ring, at variance with the square-planar geometry of 4 (all eclipsed). Two of the three hydrides of 2 have been located as bridging the Re–Re edges from inside the metal ring, as previously observed in 4. Density functional computations indicated a puckered conformation as the most stable for both 2 and 4, with very low activation energies for ring inversion (6.6 and 2.2 kcal·mol^-1, respectively), but ruled out solid state fluxionality for 4, whose observed planar geometry must be attributed to packing stabilization.     

Rhenium(I)‐Catalyzed ortho‐C−H Addition to Bicyclic Alkenes

Hydroarylation of bicyclic alkenes has been developed using a low‐valent Re^I‐catalyzed, directing group‐assisted C?H bond activation strategy. The addition of sodium acetate significantly improves the reaction efficiency; moreover, bicyclic alkenes such as 7‐oxa and aza benzonorbornadienes worked efficiently under this reaction condition. Preliminary mechanistic studies suggest that, after the alkene insertion, the rhenacycle preferentially undergoes protonolysis rather than reductive elimination.     

Effect of the rhenium additive to VOx/TiO2 and MoOx/TiO2 catalysts on their properties in oxidative dehydrogenation of propane

VO_x/TiO₂ and MoO_x/TiO₂ catalysts with the addition of Re (Re/V or Mo = 0.5) were synthetized and tested in oxidative dehydrogenation of propane and in reduction by propane. XPS measurements showed depletion of the surface in Re. The Re additive does not affect the total conversion of propane, but increases the selectivity to propene. The effect is more pronounced for the MoO_x/TiO₂ catalyst. The increase in the selectivity to propene is accompanied with the increase in the reducibility of the catalysts.     

Activation of Hydrogen Peroxide by Ionic Liquids: Mechanistic Studies and Application in the Epoxidation of Olefins

Imidazolium‐based ionic liquids that contain perrhenate anions are very efficient reaction media for the epoxidation of olefins with H₂O₂ as an oxidant, thus affording cyclooctene in almost quantitative yields. The mechanism of this reaction does not follow the usual pathway through peroxo complexes, as is the case with long‐known molecular transition‐metal catalysts. By using in situ Raman, FTIR, and NMR spectroscopy and DFT calculations, we have shown that the formation of hydrogen bonds between the oxidant and perrhenate activates the oxidant, thereby leading to the transfer of an oxygen atom onto the olefin demonstrating the special features of an ionic liquid as a reaction environment. The influence of the imidazolium cation and the oxidant (aqueous H₂O₂, urea hydrogen peroxide, and tert‐butyl hydrogen peroxide) on the efficiency of the epoxidation of cis‐cyclooctene were examined. Other olefinic substrates were also used in this study and they exhibited good yields of the corresponding epoxides. This report shows the potential of using simple complexes or salts for the activation of hydrogen peroxide, owing to the interactions between the solvent medium and the active complex.