Kinetics of Absorbed Chromophore Exchange on Metal Oxide Electrodes

Exchange experiments utilizing Ru(bpy)2(4,4‘-(CO2H)2bpy)2+ and Os(bpy)2(4,4‘-(CO2H)2bpy)2+ (4,4‘-(CO2H)2bpy is 4,4‘-dicarboxy-2,2‘-bipyridine) provide evidence for a distribution of binding sites on ITO electrodes by electrochemical monitoring and on optically transparent, nanoparticle TiO2 films by optical monitoring. Using the Albery model of dispersed kinetics for the analysis of exchange, the results are consistent with dissociative loss from the surface with k1 = 0.1 s-1 for the dissociative step in CH3CN on ITO. Equilibrium binding data after exchange suggest that a site or distribution of sites is also present that undergoes exchange on a very slow time scale. Complete exchange takes place in EtOH, and the rate is greatly enhanced compared to that in CH3CN, suggesting that EtOH labilizes the surface to exchange. On TiO2 the time scale for exchange is far longer than that on ITO, and in CH3CN it scales with film thickness

Metal-to-Ligand Charge Transfer Excited-State ν(CO) Shifts in Rigid Media

The results of ν(CO) infrared measurements on the ground and metal-to-ligand charge transfer (MLCT) excited states of fac-[ReI(4,4‘-X2bpy)(CO)3(4-Etpy)]+ (X = H, CH3, and CO2Et; 4-Etpy is 4-ethylpyridine) in both CH3CN and poly(methyl methacrylate) (PMMA) films at 298 K are reported. In PMMA, the shifts in ν(CO) between the excited and ground states (+18 to +68 cm-1) are noticeably less than in solution (+33 to +88 cm-1). Our work was stimulated by the observation by Turner and co-workers (Chem. Commun. 1996, 1587−1588) that ν(CO) excited-to-ground-state shifts for fac-[Re(bpy)(CO)3Cl] are much less in a rigid butyronitrile/propionitrile glass at 77 K than in CH3CN at 298 K. For the Re complexes, a single correlation exists between excited-state ν(CO) shifts and the MLCT ground-to-excited-state energy gap (E0) regardless of whether the energy gap is changed by varying the substituent X or the medium. The substituent and rigid medium effects appear to have a common orbital origin arising from π*(4,4‘-X2bpy•-)−π*(CO) mixing. This provides the orbital basis for mixing higher lying dπ(Re)−π*(CO) MLCT states with the emitting dπ(Re)−π*(4,4‘-X2bpy•-) MLCT excited state(s)

Mechanism of Oxidation of Benzaldehyde by Polypyridyl Oxo Complexes of Ru(IV)

The oxidation of benzaldehyde and several of its derivatives to their carboxylic acids by cis-[RuIV(bpy)2(py)(O)]2+ (RuIVO2+; bpy is 2,2‘-bipyridine, py is pyridine), cis-[RuIII(bpy)2(py)(OH)]2+ (RuIII−OH2+), and [RuIV(tpy)(bpy)(O)]2+ (tpy is 2,2‘:6‘,2‘ ‘-terpyridine) in acetonitrile and water has been investigated using a variety of techniques. Several lines of evidence support a one-electron hydrogen-atom transfer (HAT) mechanism for the redox step in the oxidation of benzaldehyde. They include (i) moderate kC-H/kC-D kinetic isotope effects of 8.1 ± 0.3 in CH3CN, 9.4 ± 0.4 in H2O, and 7.2 ± 0.8 in D2O; (ii) a low kH2O/D2O kinetic isotope effect of 1.2 ± 0.1; (iii) a decrease in rate constant by a factor of only ∼5 in CH3CN and ∼8 in H2O for the oxidation of benzaldehyde by cis-[RuIII(bpy)2(py)(OH)]2+ compared to cis-[RuIV(bpy)2(py)(O)]2+; (iv) the appearance of cis-[RuIII(bpy)2(py)(OH)]2+ rather than cis-[RuII(bpy)2(py)(OH2)]2+ as the initial product; and (v) the small ρ value of −0.65 ± 0.03 in a Hammett plot of log k vs σ in the oxidation of a series of aldehydes. A mechanism is proposed for the process occurring in the absence of O2 involving (i) preassociation of the reactants, (ii) H-atom transfer to RuIVO2+ to give RuIII−OH2+ and PhĊO, (iii) capture of PhĊO by RuIII−OH2+ to give RuII−OC(O)Ph+ and H+, and (iv) solvolysis to give cis-[RuII(bpy)2(py)(NCCH3)]2+ or the aqua complex and the carboxylic acid as products

Preparation of Coordinatively Asymmetrical Ruthenium(II) Polypyridine Complexes

A series of salts of the type cis-[RuII(bpy‘)(bpy‘ ‘)(CO)2](PF6)2 (bpy‘ and bpy‘ ‘ represent different bipyridine derivatives) have been prepared by using literature procedures and utilized as precursors for the preparation of highly functionalized complexes of RuII incorporating neutral and anionic, mono- and bidentate, nitrogen-, phosphorus-, sulfur-, and oxygen-donor ligands. The new synthetic approach builds upon previous work with the trimethylamine N-oxide-assisted removal of the carbonyl ligand. Difficulties with the use of this potent oxidant in the presence of reducing ligands such as dppe and nitrite have been overcome by the use of acetonitrile complexes of the type cis-[Ru(bpy‘)(bpy‘ ‘)(CH3CN)2]2+ and pyridine complexes of the type cis-[Ru(bpy‘)(bpy‘ ‘)(py)2]2+ as highly versatile intermediates. Strategies for the selective removal of a single carbonyl ligand from the precursors have been developed and used to synthesize the highly asymmetrical complexes cis-[RuII(bpy‘)(bpy‘ ‘)(py)(CO)]2+ and cis-[RuII(bpy‘)(bpy‘ ‘)(py)(NO)]3+. The synthetic chemistry has been extended by using either of these complexes and the complex-as-ligand strategy to prepare the pyrazine-bridged complex cis,cis-[(RuII(bpy‘)(bpy‘ ‘)(py))2(pz)](PF6)4 (pz is pyrazine). Finally, a methodology for the preparation of isothiocyanate complexes such as cis-Ru(bpy‘)(bpy‘ ‘)(NCS)2 has been developed. For the pyridyl/carbonyl, pyridyl/nitrosyl, and ligand-bridged complexes, geometrical isomers were formed in statistical yields. For the isothiocyanate complex cis-Ru(dmb)(4,4‘-(COOEt)2bpy)(NCS)2, the majority (N,N-bound) isomer was isolated from the other three linkage isomers. For all the syntheses reported, yields were high, 44−96%, and each procedure appears to be both general and redundant in that multiple schemes are possible for the preparation of most targets

Oxidation of Benzyl Alcohol by a Dioxo Complex of Ruthenium(VI)

The kinetics and mechanism of reduction of trans-[RuVI(tpy)(O)2(L)]2+ (L is H2O or CH3CN; tpy is 2,2‘:6‘,2‘ ‘-terpyridine) by benzyl alcohol have been studied in water and acetonitrile. The reactions are first order in alcohol and complex in both solvents and give benzaldehyde as the sole oxidation product. In acetonitrile, sequential RuVI → RuIV and RuIV → RuII‘ steps occur. As shown by FTIR and UV−visible measurements, RuII‘ solvolyzes to give [RuII(tpy)(CH3CN)3]2+ and benzaldehyde. With 18O-labeled RuVI, ∼50% of the label ends up in the aldehyde product for both the RuVI → RuIV and RuIV → RuII steps as shown by FTIR. In water, RuVI → RuIV reduction is followed by rapid dimerization by μ-oxo formation. Kinetic parameters for the individual redox steps in 0.1 M HClO4 at 25 °C are kVI→IV = 13.3 ± 0.8 M-1 s-1 (ΔH⧧ = 11.4 ± 0.2 kcal/mol, ΔS⧧ = −15.0 ± 1 eu, kH/kD = 10.4 for α,α-d2 benzyl alcohol). In CH3CN at 25 °C, kVI→IV = 67 ± 3 M-1 s-1 (ΔH⧧ = 7.5 ± 0.3 kcal/mol, ΔS⧧ = −33 ± 2 eu, kH/kD = 12.1) and kIV→II‘ = 2.4 ± 0.1 (ΔH⧧ = 5.1 ± 0.3 kcal/mol, ΔS⧧ = −47 ± 2 eu, kH/kD = 61.5). On the basis of the 18O labeling results in CH3CN, the O atom of the oxo group transfers to benzyl alcohol in both steps. Mechanisms are proposed involving prior coordination of the alcohol followed by O insertion into a benzylic C−H bond

Stepwise Oxidation of Anilines by <i>cis</i>-[Ru^IV(bpy)₂(py)(O)]²⁺

The net six-electron oxidation of aniline to nitrobenzene or azoxybenzene by cis-[RuIV(bpy)2(py)(O)]2+ (bpy is 2,2‘-bipyridine; py is pyridine) occurs in a series of discrete stages. In the first, initial two-electron oxidation is followed by competition between oxidative coupling with aniline to give 1,2-diphenylhydrazine and capture by H2O to give N-phenylhydroxylamine. The kinetics are first order in aniline and first order in Ru(IV) with k(25.1 °C, CH3CN) = (2.05 ± 0.18) × 102 M-1 s-1 (ΔH⧧ = 5.0 ± 0.7 kcal/mol; ΔS⧧ = −31 ± 2 eu). On the basis of competition experiments, kH2O/kD2O kinetic isotope effects, and the results of an 18O labeling study, it is concluded that the initial redox step probably involves proton-coupled two-electron transfer from aniline to cis-[RuIV(bpy)2(py)(O)]2+ (RuIVO2+). The product is an intermediate nitrene (PhN) or a protonated nitrene (PhNH+) which is captured by water to give PhNHOH or aniline to give PhNHNHPh. In the following stages, PhNHOH, once formed, is rapidly oxidized by RuIVO2+ to PhNO and PhNHNHPh to PhNNPh. The rate laws for these reactions are first order in RuIVO2+ and first order in reductant with k(14.4 °C, H2O/(CH3)2CO) = (4.35 ± 0.24) × 106 M-1 s-1 for PhNHOH and k(25.1 °C, CH3CN) = (1.79 ± 0.14) × 104 M-1 s-1 for PhNHNHPh. In the final stages of the six-electron reactions, PhNO is oxidized to PhNO2 and PhNNPh to PhN(O)NPh. The oxidation of PhNO is first order in PhNO and in RuIVO2+ with k(25.1 °C, CH3CN) = 6.32 ± 0.33 M-1 s-1 (ΔH⧧ = 4.6 ± 0.8 kcal/mol; ΔS⧧ = −39 ± 3 eu). The reaction occurs by O-atom transfer, as shown by an 18O labeling study and by the appearance of a nitrobenzene-bound intermediate at low temperature

The Cyanoimido Ligand as an Oxo Analogue. Novel Approaches to the Preparations of Cyano(imino)-aza-phosphorus(V) and <i>N</i>-Cyanoaziridine

The known Os(IV)−cyanoimido complexes, mer-Et4N[OsIV(bpy)(Cl)3(NαCNβ)] (mer-[OsIVN−C⋮N]-) (bpy = 2,2‘-bipyridine) and trans-[OsIV(tpy)(Cl)2(NαCNβ)] (trans-[OsIVN−C⋮N]) (2,2‘:6‘,2‘ ‘-terpyridine), have formal electronic relationships with high oxidation state Ru and Os-oxo and -dioxo complexes. These include multiple bonding to the metal, the ability to undergo multiple electron transfer, and the availability of nonbonding electron pairs for donation. Thermodynamic, oxo-like behavior is observed for mer-[OsIVN−C⋮N]- in the pH-dependence of its Os(VI/V) to Os(III/II) redox couples in 1:1 (v/v) CH3CN:H2O. Oxo-like behavior is also observed in the reaction between mer-[OsVI(bpy)(Cl)3(NαCNβ)]PF6 and benzyl alcohol to give mer-[OsIV(bpy)(Cl)3(NαCNβH2)]PF6 and benzaldehyde. The reaction is first order in each reactant with kbenzyl(CH3CN, 25.0 ± 0.1 °C) = (8.6 ± 0.2) × 102 M-1 s-1. Formal NCN° transfer, analogous to O-atom transfer, occurs in reactions with tertiary phosphine and hexenes. In CH3CN under N2, a rapid reaction occurs between trans-[OsIVN−C⋮N] and PPh3 (kPPh3(DMF, 25.0 ± 0.1 °C) = 4.06 ± 0.02 M-1 s-1) to form the nitrilic N-bound Os(II)−(N-cyano)iminophosphorano product, trans-[OsII(tpy)(Cl)2(NαCNβPPh3)] (trans-[OsII−Nα⋮C−NβPPh3]). It undergoes solvolysis at 45 °C after 24 h to give trans-[OsII(tpy)(Cl)2(NCCH3)] and (N-cyano)iminophosphorane (Nα⋮C−NβPPh3). The analogue to epoxidation, N-cyanoaziridination of cyclohexene and 1-hexene by mer-[OsIVN−C⋮N]- and trans-[OsIVN−C⋮N], occurs at Nβ to give the Os(IV)−N-cyanoaziridino complexes, mer-Et4N[OsII(bpy)(Cl)3(NαCNβC6H10)] and trans-[OsII(tpy)(Cl)2(NαCNβC6H11)], respectively. Oxidation to mer-[OsV(bpy)(Cl)3(NαCNβ)]- greatly accelerates N-cyanoaziridination of cyclohexene, which is followed by slow solvolysis to give mer-[OsIII(bpy)(Cl)3(NCCH3)] and N-cyanoaziridine (N⋮C−NC6H10). The Os−(N-cyano)aziridino complexes are the first well-characterized examples of coordinated cyanoaziridines

The Role of Free Energy Change in Coupled Electron−Proton Transfer

The kinetics of oxidation of tyrosine by the series of metal complex oxidants, M(bpy)33+ (M = Os, Fe, Ru), in the presence of added bases (acetate, succinate monoanion, histidine, phosphate, and tris), were investigated in 0.5 M buffer with 0.8 M NaCl at 25 °C by utilizing a catalytic cyclic voltammetry technique. As reported in an earlier study, oxidation occurs by a series of pathwayselectron transfer followed by proton transfer (ET−PT), proton transfer followed by electron transfer (PT−ET), and concerted electron−proton transfer (EPT). The latter two occur within H-bonded association complex between tyrosine and the added base. Kinetic isolation was used to focus on the EPT pathway, in which multiple site-electron proton transfer (MS-EPT) occurs with electron transfer to the oxidant and proton transfer to the base. Measured rate constants varied from kEPT = 5.0 × 103 to 9.8 × 107 M-1 s-1. Systematic variations in RT ln kEPT with ΔG°‘ were observed with RT lnkEPT increasing with −ΔG°‘ with a slope of ∼0.6 regardless of whether ΔG°‘ was varied either by varying E°‘ for the oxidant or the strength of the base. Over the ∼0.7 eV variation in ΔG°‘ for the complete data set the variation with ΔG°‘ is consistent with the importance of quantum effects arising from the O−H transfer mode with possible appearance of a novel “quantum beat” effect at ∼3000 cm-1

Henry Taube: Inorganic Chemist Extraordinaire

The numerous innovative contributions of Henry Taube to modern inorganic chemistry are briefly reviewed. Highlights include the determination of solvation numbers and lability, elucidation of substitution mechanisms, discovery and documentation of inner-sphere electron transfer, and discovery of the remarkable coordination chemistry of ruthenium and osmium ammine complexes with unsaturated ligands and mixed-valence complexes and their fundamental relationship to intramolecular electron transfer

Visible Region Photooxidation on TiO₂ with a Chromophore−Catalyst Molecular Assembly

Visible Region Photooxidation on TiO2 with a Chromophore−Catalyst Molecular Assembl