Carol Creutz

Metal—lingad and metal—metal coupling elements

Abstract The electronic matrix element coupling a ground and charge-transfer excited state can be calculated from the energy intensity of the appropriate charge-transfer transition. An expression for the electronic coupling element widely used for this purpose is based on equations derived by Mulliken and Hush for an effective two-state model and is frequently assumed to be valid only in the perturbation limit. This expression is shown to be exact within a two-state model. Provided that overlap can be neglected and that the spectroscopic transition is polarized along the donor—acceptor axis, it can be applied to system ranging from those which are very weakly coupled to those which are very strongly coupled. Application of the Mulliken—Hush expression to (NH3)5RuL2+ complexes, for which metal—lingand backbonding is important, yields metal—lingand coupling elements of 5000–6000 cm−1 with pyridyl lingands (donor—acceptor separation 3.5 A), in very good agreement with estimates obtained from a molecular orbital analysis of the band energies. With use of the superexchange formalism, the metal—lingand coupling elements were used to calculate metal—metal coupling elements for binuclear mixed-valence complexes. Comparison of these values with those obtained from the Mulliken—Hush expression applied directly to the metal-to-metal charge-transfer transition yields agreement within a factor of two or better.

The role of inner-sphere configuration changes in electron-exchange reactions of metal complexes

Metal-ligand distances were determined for various complexes of Ru, Co, Fe, and Cr. Changes in metal-oxygen or metal-nitrogen bond lengths which occur upon oxidation were alo determined. These changes correlate strongly with electron exchange rates. (DLC)

Photo-Induced Generation of Dihydrogen and Reduction of Carbon Dioxide Using Transition Metal Complexes

Abstract Homogeneous and microheterogeneous transition-metal-based systems that generate dihydrogen and/or reduce carbon dioxide upon irradiation with visible light are considered. Most of the systems involve polypyridine complexes of the d6 centers cobalt(III), rhodium(III), iridium(III), ruthenium(II) and rhenium(I). Complexes with diimine ligands serve as photosensitizers and/or catalyst precursors. The corresponding d8 metal centers and d6 hydrides are important intermediates: bimolecular reactions of the hydrides or their reactions with H2O/H3O+ are responsible for formation of dihydrogen. When carbon dioxide is also present, it may insert into the metal-hydride bond to yield formate. Mechanistic schemes for some dual-acting photoconversion systems that generate both dihydrogen and carbon monoxide or formate are considered.

Electroabsorption spectroscopy of charge transfer states of transition metal complexes

Abstract The use of electroabsorption spectroscopy to determine the dipole-moment changes that occur in the metal-to-ligand, ligand-to-metal, and metal-to-metal charge-transfer transitions in mononuclear and binuclear transition metal complexes is reviewed. The ground-excited state dipole-moment differences are much smaller than expected for the transfer of unit electronic charge between the donor and acceptor centers. The results are discussed in terms of a model in which two factors, electron delocalization and polarization of the acceptor, donor or bridging ligand electrons in response to the changed charge on the metal centers, are considered to be primarily responsible for the relatively small dipole-moment changes. The implications of the results for electronic coupling elements and reorganization energies are also discussed.

Binding of Catechols to Mononuclear Titanium(IV) and to 1- and 5-nm TiO2 Nanoparticles

The binding of catechol derivatives (LH 2 = catechol, 4-methyl catechol, 4-t-butyl catechol, and dopamine) to 1- and 4.7-nm TiO2 nanoparticles in aqueous, pH 3.5 suspensions has been characterized by UV-vis spectroscopy. The binding constants derived from Benesi-Hildebrand plots are (2-4) x 10(3) M(-1) for the 1-nm nanoparticles and (0.4-1) x 10(4)M(-1) for the 4.7-nm particles. Ti(IV)L3 complexes were prepared from the same catechols. The L = methyl catechol, and dopamine complexes are reported for the first time. The TiL3 reduction potentials are not very sensitive to the nature of the catechol nor evidently are the binding constants to TiO2 nanoparticles. The intense (epsilon > or = 10(3) M(-1)cm(-1)), about 400-nm, ligand-to-metal charge-transfer (LMCT) absorptions of the nanoparticle complexes are compared with those of the TiL 3 complexes (epsilon approximately 10(4)M(-1) cm(-1)) which lie in the same spectral region. The nanoparticle colors are attributed (as are the colors of the Ti(IV)L3 complexes) to the tails of the about 400-nm LMCT bands.

Hydricities of d6 Metal Hydride Complexes in Water

Hydricities of d(6) metal hydride complexes in water have been calculated from redox properties and acidities (cobalt and rhodium) or by equilibration with carbon dioxide/formate ion (ruthenium).

Reduction of carbon dioxide by tris(2,2′-bipyridine)cobalt(I)

Abstract Preliminary stoichiometric and kinetic results bearing on the mechanism of the reduction of HCO3− to CO by tris(2,2′-bipyridine)cobalt(I) in aqueous media are reported. The results indicate that CO (not formate) is the dominant carbon product and that it is scavenged by Co(bpy)3+ to give insoluble [Co(bpy)(CO)2]2. At pH ∼ 9, bicarbonate reduction occurs in competition with H2O reduction. Both processes are inhibited by bpy and promoted by H+, suggesting the common intermediate Co(bpy)2(H2O)H2+. The bicarbonate reaction itself branches to give H2 and CO in ∼ 3:1 ratio.

Bipyridine Radical Ions

Abstract A survey of the spectral, electrochemical, and chemical properties of organic and inorganic 2,2′-bipyridine (bpy) radical derivatives leads to the conclusion that these species form a common family whose properties are largely determined by those of bpy, the one-electron reduction product of bpy.

Calculation of thermodynamic hydricities and the design of hydride donors for CO2 reduction

We have developed a correlation between experimental and density functional theory-derived results of the hydride-donating power, or “hydricity”, of various ruthenium, rhenium, and organic hydride donors. This approach utilizes the correlation between experimental hydricity values and their corresponding calculated free-energy differences between the hydride donors and their conjugate acceptors in acetonitrile, and leads to an extrapolated value of the absolute free energy of the hydride ion without the necessity to calculate it directly. We then use this correlation to predict, from density functional theory-calculated data, hydricity values of ruthenium and rhenium complexes that incorporate the pbnHH ligand—pbnHH = 1,5-dihydro-2-(2-pyridyl)-benzo[b]-1,5-naphthyridine—to model the function of NADPH. These visible light-generated, photocatalytic complexes produced by disproportionation of a protonated-photoreduced dimer of a metal-pbn complex may be valuable for use in reducing CO2 to fuels such as methanol. The excited-state lifetime of photoexcited [Ru(bpy)2(pbnHH)]2+ is found to be about 70 ns, and this excited state can be reductively quenched by triethylamine or 1,4-diazabicyclo[2.2.2]octane to produce the one-electron-reduced [Ru(bpy)2(pbnHH)]+ species with half-life exceeding 50 μs, thus opening the door to new opportunities for hydride-transfer reactions leading to CO2 reduction by producing a species with much increased hydricity.

Photogeneration and reactions of cobalt(I) complexes

Cobalt(I) polypyridine complexes (which are capable of reducing H/sup +/ to H/sub 2/ and CO/sub 2/ to CO) may be generated from polypyridineruthenium(II) excited-state reactions by a variety of routes. The relation between the energetics and the rate constants for these routes are considered. In addition, factors leading to loss of cobalt(I) and the mechanisms of substrate reduction are discussed.