Combined Effects of Hemicolligation and Ion Pairing on Reduction Potentials of Biphenyl Radical Cations

Formal reduction potentials of highly oxidizing and short-lived radical cations of substituted biphenyls generated by pulse radiolysis in 1,2-dichloroethane (DCE) were measured using a redox equilibrium ladder method. The effect of halide ion–radical interactions on reduction potentials of biphenyls was examined by utilizing the ability of DCE to release Cl– in the vicinity of the radical cation. The Hammett correlation of measured potentials across a range of over 700 mV shows saturation at high Hammett sigma values. This effect has been explained by both ion-pairing and hemicolligation interactions between biphenyl radical cations and Cl– and appears to modulate reduction potentials by as much as 400 mV. This finding offers a convenient way to manipulate the energetics of electron transfer involving organic redox species

Combined Effects of Hemicolligation and Ion Pairing on Reduction Potentials of Biphenyl Radical Cations

Formal reduction potentials of highly oxidizing and short-lived radical cations of substituted biphenyls generated by pulse radiolysis in 1,2-dichloroethane (DCE) were measured using a redox equilibrium ladder method. The effect of halide ion–radical interactions on reduction potentials of biphenyls was examined by utilizing the ability of DCE to release Cl– in the vicinity of the radical cation. The Hammett correlation of measured potentials across a range of over 700 mV shows saturation at high Hammett sigma values. This effect has been explained by both ion-pairing and hemicolligation interactions between biphenyl radical cations and Cl– and appears to modulate reduction potentials by as much as 400 mV. This finding offers a convenient way to manipulate the energetics of electron transfer involving organic redox species

Observation of Triplet Intraligand Excited States through Nanosecond Step-Scan Fourier Transform Infrared Spectroscopy

Nanosecond step-scan Fourier transform infrared spectroscopy permits the observation of triplet intraligand (3IL) character in the excited states of [Ru(bpy)2(PNI-phen)]2+ and [Ru(PNI-phen)3]2+ where PNI is 4-piperidinyl-1,8-naphthalimide. After pulsed 355-nm laser excitation, the two ground-state imide CO bands in each compound are bleached and two substantially lower energy vibrations are produced; the lower energy feature appears as two distinct bands split by an overlapping transient bleach. Model studies confirm that the time-resolved vibrational data are consistent with photoinduced sensitization of the 3IL excited state. Density functional theory calculations also support these assignments because localization of triplet electron density on the PNI moiety is expected to lead to red-shifted CO vibrations of magnitude similar to those measured experimentally. The current results illustrate that triplet electron density can be directly tracked by time-resolved infrared measurements in metal−organic chromophores and that frequency shifts comparable to those observed in charge-transfer systems can be realized

Influence of a Gold(I)−Acetylide Subunit on the Photophysics of Re(Phen)(CO)₃Cl

The synthesis and photophysical properties of two new Re(I) complexes are reported:  fac-Re(phenC⋮CH)(CO)3Cl (where phenC⋮CH is 5-ethynyl-1,10-phenanthroline) and its Au(I)-acetylide analogue (fac-Re(phenC⋮CAuPPh3)(CO)3Cl). Also reported are the photophysical measurements obtained for the benchmark fac-Re(phen)(CO)3Cl chromophore, as well as the phenC⋮CAuPPh3 and phenC⋮CH ligands. The unstable nature of the precursor gold-containing ligand illustrates the advantage of using the “chemistry on the complex” approach, which facilitated preparation of the Re−Au binuclear complex. Where possible, all compounds were studied by static and transient absorption (TA), as well as steady-state and time-resolved photoluminescence (TRPL), at room temperature (RT) and 77 K, as well as nanosecond time-resolved infrared (TRIR) spectroscopy. The spectroscopic information provided by these techniques enabled a thorough evaluation of excited-state decay in most cases. In fac-Re(phenC⋮CH)(CO)3Cl, the RT excited-state decay is most consistent with a metal-to-ligand charge transfer (MLCT) assignment, whereas at 77 K, the lowest excited state is dominated by the triplet intraligand (3IL) state, localized within the diimine ligand. The lowest excited state in fac-Re(phenC⋮CAuPPh3)(CO)3Cl seems to result from an admixture of Re-based MLCT and 3IL states resident on the phenC⋮CAuPPh3 moiety. TA and TRIR methods indicate that these excited states are thermally equilibrated at room temperature. At 77 K, the MLCT energy of fac-Re(phenC⋮CAuPPh3)(CO)3Cl is increased as a result of the glassy medium and the resulting excited state can be considered to be ligand-localized

Water Oxidation by a Ruthenium Complex with Noninnocent Quinone Ligands: Possible Formation of an O−O Bond at a Low Oxidation State of the Metal

Tanaka and co-workers reported a novel dinuclear Ru complex, [Ru2(OH)2(3,6-Bu2Q)2(btpyan)](SbF6)2 (3,6-Bu2Q = 3,6-ditert-butyl-1,2-benzoquinone, btpyan = 1,8-bis(2,2′:6′,2″-terpyrid-4′-yl)anthracene), that contains redox active quinone ligands and has an excellent electrocatalytic activity for water oxidation when immobilized on an indium-tin-oxide electrode (Inorg. Chem., 2001, 40, 329–337). The novel features of the dinuclear and related mononuclear Ru species with quinone ligands, and comparison of their properties to those of the Ru analogues with the bpy ligand (bpy = 2,2′-bipyridine) replacing quinone, are summarized here together with new theoretical and experimental results that show striking features for both the dinuclear and mononuclear species. The identity and oxidation state of key mononuclear species, including the previously reported oxyl radical, have been reassigned. Our gas-phase theoretical calculations indicate that the Tanaka Ru-dinuclear catalyst seems to maintain predominantly Ru(II) centers while the quinone ligands and water moiety are involved in redox reactions throughout the entire catalytic cycle for water oxidation. Our theoretical study identifies [Ru2(O2−)(Q−1.5)2(btpyan)]0 as a key intermediate and the most reduced catalyst species that is formed by removal of all four protons before four-electron oxidation takes place. While our study toward understanding the complicated electronic and geometric structures of possible intermediates in the catalytic cycle is still in progress, the current status and new directions for kinetic and mechanistic investigations, and key issues and challenges in water oxidation with the Tanaka catalyst (and its analogues with Cl- or NO2-substituted quinones and a species with a xanthene bridge instead an antheracene) are discussed

Unexpected Roles of Triethanolamine in the Photochemical Reduction of CO₂ to Formate by Ruthenium Complexes

A series of 4,4′-dimethyl-2,2′-bipyridyl ruthenium complexes with carbonyl ligands were prepared and studied using a combination of electrochemical and spectroscopic methods with infrared detection to provide structural information on reaction intermediates in the photochemical reduction of CO2 to formate in acetonitrile (CH3CN). An unsaturated 5-coordinate intermediate was characterized, and the hydride-transfer step to CO2 from a singly reduced metal-hydride complex was observed with kinetic resolution. While triethanolamine (TEOA) was expected to act as a proton acceptor to ensure the sacrificial behavior of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo­[d]­imidazole as an electron donor, time-resolved infrared measurements revealed that about 90% of the photogenerated one-electron reduced complexes undergo unproductive back electron transfer. Furthermore, TEOA showed the ability to capture CO2 from CH3CN solutions to form a zwitterionic alkylcarbonate adduct and was actively engaged in key catalytic steps such as metal-hydride formation, hydride transfer to CO2 to form the bound formate intermediate, and dissociation of formate ion product. Collectively, the data provide an overview of the transient intermediates of Ru­(II) carbonyl complexes and emphasize the importance of considering the participation of TEOA when investigating and proposing catalytic pathways

Unexpected Roles of Triethanolamine in the Photochemical Reduction of CO₂ to Formate by Ruthenium Complexes

A series of 4,4′-dimethyl-2,2′-bipyridyl ruthenium complexes with carbonyl ligands were prepared and studied using a combination of electrochemical and spectroscopic methods with infrared detection to provide structural information on reaction intermediates in the photochemical reduction of CO2 to formate in acetonitrile (CH3CN). An unsaturated 5-coordinate intermediate was characterized, and the hydride-transfer step to CO2 from a singly reduced metal-hydride complex was observed with kinetic resolution. While triethanolamine (TEOA) was expected to act as a proton acceptor to ensure the sacrificial behavior of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo­[d]­imidazole as an electron donor, time-resolved infrared measurements revealed that about 90% of the photogenerated one-electron reduced complexes undergo unproductive back electron transfer. Furthermore, TEOA showed the ability to capture CO2 from CH3CN solutions to form a zwitterionic alkylcarbonate adduct and was actively engaged in key catalytic steps such as metal-hydride formation, hydride transfer to CO2 to form the bound formate intermediate, and dissociation of formate ion product. Collectively, the data provide an overview of the transient intermediates of Ru­(II) carbonyl complexes and emphasize the importance of considering the participation of TEOA when investigating and proposing catalytic pathways

Role of Bimetallic Interactions in the Enhancement of Catalytic CO₂ Reduction by a Macrocyclic Cobalt Catalyst

The use of two metal centers in a CO2 reduction catalyst that work together synergistically, with similar or complementary functions, can potentially lead to a significant reduction in overpotential, enhance catalytic activity and/or selectivity, and/or enable access to cascade strategies where each metal center catalyzes a different step in the conversion of CO2 to a fuel. Here, the bimetallic reactivity of two metal centers has been identified as the primary route for the reduction of CO2 to CO promoted by the macrocycle, [Co­(HMD)]2+ (HMD = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene), based on the experimental characterization of all major steps of the proposed catalytic cycle using pulse radiolysis time-resolved IR (PR-TRIR) spectroscopy, corroborated by density functional theory (DFT) calculations and IR spectroelectrochemistry (IR-SEC). A bimetallic intermediate is formed in situ from two singly reduced [Co­(HMD)]+ species bridged by a CO2 molecule, and the presence of a coordinating species, e.g., formate anion, appears to assist in the formation of such an intermediate. It has been demonstrated that this reactivity enables access to elementary steps with lower energy requirements, resulting in overall catalysis being kinetically more facile compared to the mononuclear pathway. A two-step approach that combines chemical reduction followed by PR-TRIR has been successfully used for probing the structure and reactivity of reactive intermediates involved in the advanced stages of a catalytic cycle, which are rarely interrogated using experimental techniques

Unexpected Roles of Triethanolamine in the Photochemical Reduction of CO₂ to Formate by Ruthenium Complexes

A series of 4,4′-dimethyl-2,2′-bipyridyl ruthenium complexes with carbonyl ligands were prepared and studied using a combination of electrochemical and spectroscopic methods with infrared detection to provide structural information on reaction intermediates in the photochemical reduction of CO2 to formate in acetonitrile (CH3CN). An unsaturated 5-coordinate intermediate was characterized, and the hydride-transfer step to CO2 from a singly reduced metal-hydride complex was observed with kinetic resolution. While triethanolamine (TEOA) was expected to act as a proton acceptor to ensure the sacrificial behavior of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo­[d]­imidazole as an electron donor, time-resolved infrared measurements revealed that about 90% of the photogenerated one-electron reduced complexes undergo unproductive back electron transfer. Furthermore, TEOA showed the ability to capture CO2 from CH3CN solutions to form a zwitterionic alkylcarbonate adduct and was actively engaged in key catalytic steps such as metal-hydride formation, hydride transfer to CO2 to form the bound formate intermediate, and dissociation of formate ion product. Collectively, the data provide an overview of the transient intermediates of Ru­(II) carbonyl complexes and emphasize the importance of considering the participation of TEOA when investigating and proposing catalytic pathways