Horst Hennig

Quantum chemical investigations of the π-acceptor ability of α-diimine ligands

Abstract The bonding properties of α-diimine complexes of transition metals in low oxidation states are essentially determined by π-back donation. The energies of the lowest unoccupied molecular orbitals of the ligands have been used to estimate their acceptor ability. MO calculations of selected α-diimines are reported in the present paper. N-unsubstituted and apliphatic diimines are found to be better π-acceptors than 2,2′-bipyridine which is in agreement with experimental results. It turns out that the LUMO energies calculated by the NDDO method are a good measure for the π-acceptor behaviour of ligands with similar structure towards transition metal ions.

Photoinduced electron transfer in ion pairs

Electrostatic attraction by oppositely charged ions leads to ion pairing. The influence of solvent polarity, the size and the charge of the ions as well as the electron-pair donor and acceptor strength on the equilibrium constant of ion-pair formation is discussed.

Beziehungen zwischen polarographischem und spektralem verhalten einiger substituierter ferrocene

Zusammenfassung Die Halbstufenpotentiale verschiedener elektrophil substituierter Ferrocene wurden bestimmt und in Abhangigkeit von Substituenten diskutiert. Des weiteren wurde der Substituenteneinflus auf die Lage der langwelligen Absorptionsbanden bei 440 und 324 nm untersucht. Zusammenhange zwischen der Verschiebung von E case1 2 und λ in Abhangigkeit vom elektrophilen Charakter des Substituenten konnten mit Hilfe der MO-Theorie qualitativ gedeutet werden.

Quantum chemical calculations for the determination of the molecular structure of conjugated compounds: Part XIV. On the conformational structure of α-diimine ligands

Abstract The conformational structure of some typical α-diimine ligands (2,2′-bipyridyl, glyoxaldiimine, glyoxal-N,N′-dimethyldiimine) is examined using the NDDO method. An analysis of the potential energy curves indicates the influence of specific lone-pair effects on the molecular structure. Based on a continuum model, changes of the molecular arrangement caused by solvent effects are estimated.

HOMOGENEOUS PHOTO CATALYSIS BY TRANSITION METAL COMPLEXES

Abstract The photochemistry of coordination compounds has become an increasingly popular technique for controlling and tuning the reactivity of transition metal complexes and organometallic compounds as well. Rather uncommon reaction pathways can be initiated by electronic excitation of diverse spectroscopic states of these compounds. Besides stoichiometric electron-transfer, dissociation, substitution as well as rearrangement reactions induced by light, photocatalytic processes and photoinduced chain reactions are of particular significance. Examples of photoinduced catalytic and photoassisted reactions as well as photoinduced chain processes will be reviewed to illustrate the advantages of such reaction pathways. Particular attention is given to the spectral sensitization of photocatalytic systems. A summarizing evaluation as well as perspectives for future developments are given.

Photocatalytic systems. LXIII: Intervalence transfer (IT) behaviour and IT photochemistry of mixed-valence compounds with cyanometallates

Abstract Cyanometallates such as octacyanomolybdate(IV), octacyanotungstate(IV), hexacyanoruthenate(II), and hexacyanoferrate(II) form mixed-valence compounds with appropriate copper(II), iron(III), uranium(VI) and vanadium(IV) compounds. Detailed spectroscopic investigations in solution show that the mixed-valence compounds under discussion are characterized by changes in the IT transitions. The IT behaviour is discussed within the framework of the conceptual approach of Hush. Photochemical investigations of the mixed-valence compounds described show low photoreactivity only, caused by fast back electron transfer processes. Both chemical and physical scavenging processes, the latter in the form of a new type of sequential two-photon processes, may be helpful in quenching the back electron transfer. The results obtained enable us to realize the concept of static spectral sensitization of photocatalytic systems based on light-sensitive coordination compounds.

Voltammetric and spectroelectrochemical studies on 4-aminophenol at gold electrodes in aqueous and organic media

Abstract Voltammetric and UV–Vis spectroelectrochemical studies on 4-aminophenol (4-AP) at gold electrodes have been carried out in conventional electrochemical and in novel spectroelectrochemical cells. As base electrolytes acetonitrile with 10−2 M tetrabutylammonium hexafluorophosphate as supporting electrolyte and acetate buffer (pH 4.6) solutions have been used. 4-AP was electrochemically oxidized at gold electrodes at potentials >+0.1 V in aqueous solutions and at potentials >+0.5 V in acetonitrile solutions. The formation of an oxidation product has been characterized in situ by UV–Vis spectroscopy in a novel optical thin-layer spectroelectrochemical cell.

On the electronic structure and spectroscopic properties of d8-metal mixed-ligand complexes with S/O-donor and N-acceptor ligands

Abstract The structural and spectroscopic properties of d 8 -metal mixed-ligand complexes with sulphur- or oxygen-containing donor ligands (dithiolates and catecholates) and nitrogen acceptor ligands of the α-diimine type are studied by qualitative MO considerations. Whereas the HOMO is located mainly on the sulphur ligands with only small metal contributions, the LUMO is located exclusively on the diimines. As the characteristic property of the complexes under discussion an interligand charge-transfer from the donor to the acceptor site has been found. The dependence of the interligand charge-transfer band on the energies of the acceptor and the donor niveau is investigated, resulting in a maximum spectral shift of Δ v =6480 cm −1 for [Ni(mnt) N,N ] mixed-ligand complexes, where mnt=maleonitriledithiolate and N,N =aliphatic or aromatic diimines.

Spectroscopic studies on the mechanism of photolysis of Pt(N3)2(P(C6H5)3)2

Pt(N3)2(P(C6H5)3)2 has been irradiated in different media between r.t. and 10 K mainly with the 351, 364 nm lines of an argon ion laser. The photoreactions taking place have been studied by following the changes of the νas(N3) IR band. The intensity decrease of this band indicates that a photoredox reaction between the metal and azide ligands has occurred partly via dd band absorption. An azide containing intermediate was detected, which was stable in the dark but also disappeared upon further irradiation. It has been assigned to the trans form of the parent complex. A different intermediate, PtN3(P(C6H5)3)2, was detected by laser flash photolysis with 308 nm, having λmax=460 nm and a lifetime of (75±25)10−6 s in CH2Cl2, and (210±60)10−6 s in 2-MeTHF, respectively. The azide free photoproduct Pt0P(C6H5)3)2(2-MeTHF)n was stabilized in 2-MeTHF at 133 K showing an absorption band with λmax=409 nm.

Optical ligand-to-ligand charge transfer of Zn (2,2′-bipyridyl) (3,4-toluenedithiolate)

The electronic structure and the optical ligand-to-ligand charge transfer (LLCT) of the pseudotetrahedral complex Zn (2,2′-bipyridyl) (3,4-toluenedithiolate) is interpreted on the basis of EHT calculations. By comparison with square-planar complexes, it is shown that the extent of electron delocalization between donor and acceptor ligand and charge separation by LLCT excitation depends on the mutual orientation of both ligands.