B. Patrick Sullivan

Photochemistry and Photophysics of Coordination Compounds: Rhenium

This review describes recent advances in the photochemistry of complexes of the element rhenium. It covers not only fundamental chemistry but the recent applications of these complexes to supramolecular chemistry, carbon dioxide reduction, bioinorganic chemistry, sensors, and light-emitting devices.

One- and two-electron pathways in the electrocatalytic reduction of CO2 by fac-Re(bpy)(CO)3Cl (bpy = 2,2′-bipyridine)

The cyclic voltammetry of fac-Re(bpy)(CO)3Cl (bpy = 2,2′-bipyridine) and of the intermediate complexes formed by bulk electrolysis in MeCN solution in the presence and absence of CO2 has been interpreted in terms of two different electrocatalytic pathways for the reduction of CO2 to Co, one involving an initial one-electron reduction and the other an initial two-electron reduction.

Application of the energy gap law to the decay of charge transfer excited states, solvent effects

Abstract Values of non-radiative decay rate constants (knr) and emission energies (Ecm) have been obtained for Os(Phen3)2+ in a series of solvents and the results are consistent with the energy gap law. For hydroxylic solvents like water or methanol related studies suggest the existence of strong, specific contributions to the vibrational trapping energy of the solvent.

Photoinduced irreversible insertion of CO2 into a metal–hydride bond

The reactive organometallic trifluoromethanesulphonato (CF3SO3–) complex fac-Re(bpy)(CO)3CF3SO3(bpy = 2,2′-bipyridine) has neen used as a basis for the synthesis of the series of complexes fac-Re(bpy)(CO)3X (X = H, O2CH, and O2COH) all of which are involved in the reduction of CO2; the hydrido complex has been found to undergo a slow thermal reaction with CO2 to give the formato complex, a reaction which is greatly enhanced by visible light.

Application of electron-transfer theory to excited-state redox processes

Abstract Quenching rate constants have been obtained for oxidative electron transfer quenching of Ru(bpy) 2++ 3 by a series of nitroaromatic and bipyridinium quenchers and reductive quenching by a series of aromatic amines. The results are consistent with electron-transfer theory when competing processes which occur following the electron-transfer quenching step are taken into account. The competing processes are back-electron-transfer to give the excited state Ru(bpy) 2++ 3 and net quenching by the sum of redox-product-separation and back-electron-transfer to give Ru(bpy) 2++ 3

Photophysics and photochemistry of rhenium(V)-nitrogen triple bonds

Abstract Photophysical and photochemical studies on new nitrido and imido complexes of the type [NRe(diphos) 2 Cl] + and [PhNRe(diphos) 2 Cl] 2+ ] respectively, enable a direct comparison of excited state properties to be made (diphos is a diphosphine ligand). The two main findings follow are; 1) that the excited states are dramatically more nucleophilic than the ground state, and, 2) that the “ligand field” of the phenylimido group is weaker than that of the nitrido ligand.

Synthesis of hydrido–iridium(III) complexes from a trifluoromethanesulphonato intermediate

The versatile new precursor to bis-2,2′-bipyridine complexes of IrIII, cis-[IrIII(bpy)2(OSO2CF3)2][CF3SO3](bpy = 2,2′-biyridine), has been isolated and used for the preparation of the new hydrido complexes of IrIII, cis-[Ir(bpy)2(PPh3)H][PF6]2 and cis-[Ir(bpy)2H2][PF6], and for an improved, high-yield synthesis of [Ir(bpy)3][PF6]3.

In situ surface plasmon study of the electropolymerization of Fe(vbpy)32+ onto a gold surface

Abstract Sequential multilayer electropolymerization of Fe(vbpy)32+ (vbpy=4-vinyl-4′-methyl-2,2′-bipyridine) onto a thin gold electrode was followed in situ with surface plasmon spectroscopy (SPS) using a 1 mW HeNe laser at 6328 A. The robustness of the gold film electrode necessary for electrochemical deposition in 0.10 M tetraethylammonium perchlorate+acetonitrile is imparted by use of a thin film of 3-mercaptopropyl-trimethoxysilane attached to a SF10 slide to which the metal is covalently bonded. As each polymer layer is deposited by cycling a potentiostat from 0.0 to −1.75 and back to 0.0 V, a plasmon spectrum (reflectivity versus prism angle) is obtained. SP analysis of the angular shift of the spectrum, which increases as the polymer layer thickens, yields an estimate of both the thickness and index of refraction of the polymer film. We found that the plasmon spectrum shifts to higher angles as the polymer layer thickens, along with a progressive decrease in the depth of the resonance minimum. Our modeling shows this unusual spectral behavior involving the resonance minimum is consistent with a Fe(vbpy)32+ chromophore absorption at 6328 A, along with thickening of the polymer film. This work demonstrates that SPS is a viable in situ technique for obtaining thickness measurements of electrodeposited thin films.

The use of flow-injection analysis with chemiluminescence detection of aqueous ferrous iron in waters containing high concentrations of organic compounds.

An evaluation of flow-injection analysis with chemiluminescence detection (FIA-CL) to quantify Fe2+(aq) in freshwaters was performed. Iron-coordinating and/or iron-reducing compounds, dissolved organic matter (DOM), and samples from two natural water systems were used to amend standard solutions of Fe2+(aq). Slopes of the response curves from ferrous iron standards (1 – 100 nM) were compared to the response curves of iron standards containing the amendments. Results suggest that FIA-CL is not suitable for systems containing ascorbate, hydroxylamine, cysteine or DOM. Little or no change in sensitivity occurred in solutions of oxalate and glycine or in natural waters with little organic matter.

An excited state photoelectrochemical cell for the production of H2O2 and Br2

Abstract The design and operation of a series of oxidative excited state photoelectrochemical half-cells is described, as is the application of one of them to the production of H 2 O 2 and Br 2 in separate cell compartments in acetonitrile solution. The basis for the half-cells is in the series of steps: (1) optical excitation of the metal-to-ligand charge transfer chromophore in complexes such as (bpy)Os(das) 2 2+ (bpy is 2,2′-bipyridine; das is 1,2-bis-dimethylarsinobenzene), (2) reductive quenching of the excited states by N( p -C 6 H 4 Br) 3 (NAr 3 ), (3) scavenging of the reduced metal complex in acidic acetonitrile by O 2 which gives H 2 O 2 and builds up NAr 3 + and (4) transfer of the oxidizing equivalents in NAr 3 + through an external circuit to a dark anode compartment where oxidation occurs. In the H 2 O 2 /Br 2 cell, Br − is oxidized to Br 2 in the second compartment. A kinetic model developed earlier has been successfully applied to the (bpy)Os(das) 2 2+ half-cell; this successfully predicts variations in photocharge with variations in chromophore, quencher and O 2 concentrations, incident light intensity and added acid. In an air-saturated solution saturated with quencher the cell efficiency for the production of H 2 O 2 and Br 2 at high limiting concentrations of quencher is φ′ cell = 0.85 ]+- 0.05 (equivalents of photoproducts per photon absorbed) at 25 °C. Under the same air-saturated conditions, the most efficient half-cell is based on the chromophore Ru(4,4′-(Co 2 Et) 2 bpy) 3 2+ (4,4′-(CO 2 Et) 2 bpy is 4,4′-bis(carboethoxy)-2,2′-bipyridine) where φ′ cell = 1.8 ± 0.5. φ cell values in excess of unity provide evidence for a kinetic step in which the one-electron intermediate HO 2 is reduced by NAr 3 to give a second mole of NAr 3 + per photon absorbed.