Paul A. Grutsch

Use of transition metal compounds to sensitize a photochemical energy storage reaction

Abstract A solar energy storage system based upon the valence isomerization of norbornadiene to quadricyclene possesses several attractive features, including high specific energy storage capacity, kinetic stability of the energy rich photo-product in the absence of suitable catalysts, and relatively inexpensive reactants. An inherent difficulty with the system is the lack of absorption of solar energy by norbornadiene. Attempts to overcome this shortcoming have focused upon the use of transition metal compounds to sensitize the desired energy storage step. Results to date indicate that complexation of norbornadiene to the metal provides a potentially useful route to sensitization. Several copper(I) compounds have thus far been found to be effective. The facors which affect the ability of a transition metal compound to function as a sensitizer via a complexation mechanism are discussed.

Charge-transfer spectroscopy and photochemistry of alkylamine cobalt(III) complexes

Abstract Spectroscopic and photochemical studies are reported for Co(NH2R)63+, Co(NH2R)5OH23+, and Co(NH2R)5X2+ (where R is methyl, ethyl, or n-propyl, and X is Cl− or Br−). These complexes absorb strongly in the ultraviolet region owing to the presence of one or more ligand-to-metal charge transfer (LMCT) band(s). Irradiation into this charge transfer region results in redox decomposition of the complex with the release of Co2+, alkylamine, and a radical. Photoredox quantum yields measured in fluid solution are generally high and dependent upon excitation wavelength, oxygen concentration, temperature, and solution viscosity. In contrast to their well-studied ammine analogues, the alkylamine complexes (1) possess a N → Co charge transfer band at wavelengths well above 200 nm, (2) undergo redox decomposition from the X → Co and N → Co charge transfer excited states, and (3) avoid an intermolecular decomposition pathway involving photooxidation of the solvent. Reasons for these differences between the two families of complexes are discussed.

Photoinduced arene deligation of a dicationic bis(arene)—iron (II) complex

Abstract Continuous photolysis, flash photolysis and photosensitization studies are reported for Fe( η 6 -mes) 2 2+ , where mes = 1,3,5-trimethylbenzene (mesitylene). The complex absorbs in the visible and ultraviolet spectral regions owing to the presence of ligand field, mes → Fe charge transfer, and intraligand π-π ∗ absorption bands. Selective irradiation of the various bands causes substitution of both mesitylene ligands by solvent. The quantum efficiency of arene deligation ranges from 0.6 to 1.0 depending upon the excitation wavelength, but is independent of complex concentration, solvent, oxygen content and excess PF 6 − counterion. Reaction occurs from both ligand field and mes → Fe charge transfer excited states, with no evidence of communication between the states.

Mechanistic photochemistry of transition metal β-diketonate complexes. 3. Detection of a new reaction pathway in the ultraviolet photochemistry of trans-Tris(1,1,1-trifluoro-2,4-pentanedionato)cobalt(III)

Abstract Near-ultraviolet flash photolysis of trans-Co(tfa)3 (tfa is the anion of 1,1,1-trifluoro-2,4-pentanedione) in cyclohexane generates a long-lived (t 1 2 ∼ 3 sec) strongly-absorbing transient. The yield of this species decreases upon addition of good hydrogen-atom donors such as 2-propanol and tri(n-butyl)stannane. In pure 2-propanol the formation of the transient is completely suppressed, and only a bleaching of the absorbance, due to production of weakly-absorbing Co(II) products, is observed. Flashing a methanol solution of trans-Co(tfa)3 produces a transient similar (but not identical) to that observed in cyclohexane; this species subsequently decays to Co(II) products. Based upon their spectral characteristics and reactivities, the flash-induced transients are assigned as Co(II) complexes containing a carbon-bonded ligand. These species result from an intramolecular rearrangement of the reduced metal-oxidized ligand radical pair formed in the primary photochemical step. Good hydrogen-atom donors can intercept this radical pair and thus inhibit transient formation. Previous mechanistic proposals about the ultraviolet photochemistry of tris(β-diketonate)cobalt(III) complexes need to be modified in view of the present results.

Detection of reactive rhodium containing intermediates following flash photolysis of a tris(β-diketonato)rhodium(III) complex

Flash photolysis of trans-tris(1,1,1-trifluoropentane-2,4-dionato)rhodium(III) in alcoholic solvents produces a short-lived RhII-containing species which decays to a longer-lived intermediate tentatively identified as a RhII-hydride.

X-Ray crystallographic characterization of the single hydrogen bridge attachment of the tetrahydroborate group in tris(methyldiphenylphosphine)tetrahydroboratecopper

The X-ray crystal structure of [(MePh2P)3CuBH4] has revealed that the copper atom exists in a distorted tetrahedral environment consisting of the phosphorus atoms of the three phosphine ligands and a single hydrogen atom of the tetrahydroborate group.

A silica-supported inorganic photosensitizer

Adsorption of[Ir(bipy)3OH]2+(bipy = 2,2′-bipyridine) onto silica gel does not adversely affect the efficiency with which this transition metal complex photosensitizes the valence isomerization of norbornadiene to quadricyclene.