Patrick E. Hoggard

Picosecond flash photolysis and spectroscopy: Transition metal coordination compounds

Abstract Lifetimes of excited electronic states of a number of transition metal compounds have been investigated with picosecond time resolution by following excited state absorption or ground state bleaching and repopulation. The lowest excited quarret state of octahedral chromium(III) compounds is found to have a lifetime with respect to intersystem crossing to a doublet state too short to be followed by this technique. The lifetimes of excited states of some non-luminescent compounds of iron and ruthenium have been established in the sub-nanosecond range.

Solvent-initiated photochemistry of transition metal complexes

Abstract Although it is generally assumed that photoreactions of transition metal complexes proceed through metal complex excited states, some reactions in halogenated solvents occur instead as radical processes, following carbon-halogen bond homolysis. Oxidation, substitution, or oxidative addition can occur by either a metal-centered or a solvent-initiated photoreaction, and they can be hard to distinguish. Under some circumstances even the kinetic rate laws can be the same. However, with proper choice of irradiation wavelength the dependence of the initial rate on light intensity and metal complex concentration suffices to discriminate between the two possibilities.

The photolysis of iron(III) chloride in chloroform

In air-saturated solutions, iron(III) chloride functions as a long wavelength sensitizer of the photolysis of chloroform. In the presence of oxygen, irradiation of chloroform solutions of iron(III) chloride at wavelengths above 340 nm leads to the formation of HFeCl4, followed by continued production of HCl and C2Cl6 as chlorine photodissociates from HFeCl4, while the Fe(II) species formed is reoxidized. When iron(III) chloride is photolyzed in the absence of oxygen, conversion to HFeCl4 and reduction to colloidal FeCl2 occur simultaneously, ending in complete reduction. Both CCl3OO and CCl3OOH appear to participate in the reoxidation of iron(II) chloride. # 2002 Elsevier Science B.V. All rights reserved.

Ligand field properties of the peptide nitrogen: The sharp-line electronic spectrum of mer-di(3-aminopropyl)amine(glycylglycinato-O,N,N′)-chromium(III)

Abstract An Angular Overlap Model calculation was used to fit the electronic spectrum, particularly the sharp-line positions, of [Cr(dpt)(glygly)]ClO 4 ·0.5C 4 H 6 N 2 O 2 ·2.5H 2 O [dpt = di(3-aminopropyl)amine, H 2 glygly = glycylglycine] at 77 K. An e π value of 502 cm −1 for the peptide nitrogen indicates that it is a weak net π-donor to chromium(III), presumably because donation from the filled π-orbital and back-donation to the empty π-antibonding orbital are almost in balance.

Photocatalytic degradation of dichloromethane by chlorocuprate(II) ions.

Near UV irradiation of aerated solutions of (Et 4N) 2[CuCl 4] in dichloromethane causes the decomposition of CH 2Cl 2, as evidenced by the buildup of HCl, C 2H 2Cl 4, and peroxides. A net reduction to [CuCl 2] (-) occurs in the early stages, but is later reversed. In CH 2Cl 2, [CuCl 4] (2-) is in equilibrium with [Cu 2Cl 6] (2-), and only the latter species is photoactive. The decomposition is initiated by the photodissociation of chlorine atoms, which propagate to peroxy radicals, CHCl 2OO. Experimental evidence, including a linear dependence of the decomposition rate on the incident light intensity and on the fraction of light absorbed by [Cu 2Cl 6] (2-), is consistent with a mechanism in which CHCl 2OO is reduced by electron transfer from [CuCl 2] (-), following which protonation yields CHCl 2OOH. The hydroperoxide accumulates during irradiation and it too can reoxidize [CuCl 2] (-). The quantum yield for HCl production at the outset of irradiation at 313 nm is 1.3 mol/einstein, based on the fraction of light absorbed by [Cu 2Cl 6] (2-).

The photooxidation of tetrachloroplatinate(II) in chloroform

Abstract [PtCl 6 ] 2− is known to be photoreduced by UV light to [PtCl 4 ] 2− in chloroform, but at irradiation wavelengths of 254 nm or lower the reverse reaction, photooxidation of [PtCl 4 ] 2− to [PtCl 6 ] 2− , occurs. This is a solvent-initiated process, proceeding through absorption of light by CHCl 3 and subsequent bond homolysis. The quantum yield, based on the light absorbed by CHCl 3 , is 0.20. Tetrachloroplatinate(II) and hexachloroplatinate(IV) may be cycled back and forth several times by changing the irradiation wavelength.

Kinetics of the photooxidation of tris(bipyridine)iron(II) in chloroform

Abstract In chloroform/acetonitrile solution, [Fe(bpy) 3 ] 2+ , bpy=2,2′-bipyridine, reacts under 254 nm irradiation to yield Hbpy + and [FeCl 4 ]. The reaction rate is faster the lower the concentration of [Fe(bpy) 3 ] 2+ . The reaction proceeds predominantly through light absorption by chloroform, creating HCl and CCl 3 radicals, both of which participate in the reaction. A lesser part of the reaction is initiated through light absorption by the tris(bipyridine) complex. The variation of the reaction rate with light intensity and on the fraction of light absorbed by the solvent and the metal complex is between a square root and a linear dependence, and is consistent with a mechanism passing through a [Fe(bpy) 3 Cl] 2+ intermediate, in which one bipyridine is presumably monodentate.

THE SOLVENT-INITIATED PHOTOCHEMISTRY OF TRIS(N,N-DIETHYLDITHIOCARBAMATO)IRON(III)

Abstract The photolysis of [Fe(Et2dtc)3], Et2dtc = diethyldithiocarbamate to yield [Fe(Et2dtc)2Cl] proceeds under 313 nm irradiation through a metal complex excited state, as expected. Under 254 nm irradiation, however, the dominant pathway is through a solvent-initiated reaction in which radicals formed after absorption of light by CHCl3 react thermally with [Fe(Et2dtc)3]. The initial rate varies linearly with the light intensity at 313 nm, but at 254 nm varies with the square root of the intensity.

Kinetics of the photolysis of bis(triethylphosphine)dichloroplatinum(II) in chloroform

Abstract Under 254 nm irradiation [Pt(Et 3 P) 2 Cl 2 ] in chloroform is converted to H 2 PtCl 6 . Absorption of light by both the metal complex and the solvent contributes to the first of this process, suggested to form HPt(Et 3 P)Cl 3 . The rate law for the reaction can be expressed as af R + bf R f S t, bf R fst, where f R and f s represent the fraction of light absorbed by the metal complex and the solvent, respectively, and t is the elapsed time. The direct dependence of the rate on the time of reaction was proposed to arise from an increasing concentration of HCl, which can react with the excited state metal complex to yield HPt(Et 3 P)Cl 3 .

A kinetic study of the photoreduction of hexachloroplatinate(IV) in chloroform

The photoreduction of (Bu4N)2[PtCl6] in CHCl3 under 297 nm irradiation to yield [PtCl4]2− follows a rate law proportional to Io12f12[PtCl6]2−, where f is the fraction of light absorbed by [PtCl6]2−, consistent with a chain mechanism in which the termination step is the dimerization of an intermediate radical, suggested to be CCl3. The same rate law appears to apply in CH3OH.