Roland Benedix

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.

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.

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.

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.

Electronic structure and spectroscopic properties of copper catecholate complexes with interligand charge-transfer behavior

The structural and spectroscopic properties of copper catecholate complexes with nitrogen ligands of the α- diimine type are studied by qualitative MO considerations. The characteristic feature of these donor acceptor systems is an interligand charge-transfer from the catecholate donor to the α-diimine acceptor site. The dependence of the interligand charge-transfer absorption band on the energies of the acceptor and the donor levels as well as on the solvent polarity is discussed.

Synthesis, Crystal Structure, Spectroscopy, and Theoretical Investigations of Tetrahedrally Distorted Copper(II) Chelates with [CuN2S2] Coordination Sphere

A series of tetrahedrally distorted copper(II) complexes with thiolate and imine coordination were synthesized. Schiff bases derived from 4-benzoyl-3-methyl-1-phenyl-2-pyrazoline-5-thione and various diamines were used as tetradentate ligands to obtain tetrahedrally distorted metal chelates with [CuN2S2] complex units. Crystal structures of the complexes 1, 2, 5 and 6 and of ligand H25 have been determined by means of single-crystal X-ray structure analysis. The structure data show a strong influence of the diamine building blocks on the tetrahedral distortion of the copper(II) complexes. Results of Extended Huckel LCAO calculations correlate strongly with structural, electrochemical, UV/Vis- and EPR-spectroscopic features obtained experimentally. The calculations confirm for the whole complex series a strong delocalization of the frontier orbitals. The highest fully occupied molecular orbital shows a weak contribution, resulting from thiolate donor atoms, whereas the antibonding singly occupied molecular orbitals (SOMOs) are distributed between the copper(II) centre (ca. 35–40%) and the N2S2 donor set. The SOMO energy significantly lowers with increasing tetrahedral distortion of the coordination sphere. The influence of the tetrahedral distortion of copper(II) complexes on redox potentials, UV/Vis and EPR spectra is discussed.

Ion-pair charge-transfer complexes of a dithiooxalate zinc donor component with viologens. Synthesis, structural and electronic characterization

Abstract Bipyridinium and phenanthrolinium acceptors of different reduction potentials form with zinc 1,2-dithiooxalates (dto) ion-pair charge-transfer complexes of the general formula {A 2+ [Zn(dto) 2 ] 2− }. The contact ion pairs exhibit absorptions in the range 390–490 nm which can be attributed to the ion-pair charge-transfer (IPCT) type. On the base of spectroscopic, electrochemical and quantum-chemical investigations the relation between optical and thermal electron transfer within the ion pair applying the Hush theory is discussed. The mean reorganization energy of 12 complexes is 180 U mol −1 and exceeds the values found for the dithiolene systems. Due to the diminished donor ability of the dithiooxalate unit, a hypsochromic shift of the position of the IPCT band, compared with the metal dithiolenes, results. The extent of electron delocalization from [Zn(dto) 2 ] 2− to pQ 2+ , as described by the parameter α 2 , is calculated as 4.1 × 10 −6 . X-ray analyses of BQ[Zn(dto) 2 ] and DP[Zn(dto) 2 ] reveal that the solid-state structure is largely determined by the geometry of the acceptor component.

Extended hückel calculation on diimine substituted oxalate cobalt(III) complexes

Abstract Tris(oxalato)cobaltate(III) is known a typical thermo- and photoreactive compound. Substitution of oxalato by α-diimines reduces the thermally induced redox decomposition. EHT calculation on diimine substituted Co(III) complexes with different aliphatic and aromatic diimine ligands have been carried out. It results in the EHT-LUMO energies being able to be used to estimate the thermodynamic redox stability of the mixed ligand complexes and are helpful for planned syntheses of photocatalytically interesting systems.

Spectroscopic and theoretical investigations of Schiff base metal complexes with intraligand charge-transfer behavior

Abstract Nickel and zinc complexes of hydroxy or mercapto substituted glyoxaldianiles exhibit unusual long-wavelength absorptions. The electronic structure and spectroscopic properties of the compounds are investigated by means of electron spectroscopic studies and qualitative MO considerations. The deep color of the complexes is caused by intraligand charge-transfer transitions from the oxygen or sulphur-containing donor part to the diimine substructure of the ligand. The transition energies depend sensitively on the nature of the donor part of the ligand, and on the solvent polarity. The close structural relationship between these complexes and the d 8 mixed-ligand complexes with S/O-donor and N-acceptor ligands is discussed.

Dreiparameterbeschreibung von Lösungsmitteleinflüssen auf Ionenpaar-Charge-Transfer-Banden

Solvent effects on the position of ion-pair charge-transfer absorption bands are described using a three parameter approach. The values of calculated coefficients are discussed considering specific and non-specific interactions.