Barbara Olbrich-Deussner

What determines the solvatochromism of metal-to-ligand charge transfer transitions? A demonstration involving 17 tungsten carbonyl complexes

Abstract A consistent model which permits rationalization and estimation of the solvatochromic behaviour of coordination compounds with metal-to-ligand charge transfer absorption bands is described. The model shows how the changing relationship between metal-ligand bond polarities in the ground and MLCT excited state determines whether negative, positive, or no solvatochromism results. Data for seventeen mononuclear and binuclear tetra- and pentacarbonyltungsten complexes are analyzed in order to illustrate and substantiate different electronic situations leading to various degrees of solvatochromism. Ligand basicities, calculated Huckel molecular orbital coefficients, ESR coupling constants, and metal fragment oxidation potentials are used to estimate the ability of metal fragments and ligands for charge transfer in the excited state and the resulting solvatochromism of complexes.

Elektrontransfer-katalysierte carbonyl-substitution: I. Synthese und spektroskopie von phosphantricarbonylmetall-komplexen der bidiazine

Abstract New phosphane tricarbonylmetal complexes fac-(R3P)(CO)3M(bdz) (R = n Bu, iPr; M  Mo, W) of the four isomeric bidiazine (bdz) chelate ligands 3,3′-bipyridazine, 2,2′-bipyrazine, 2,2′- and 4,4′-bipyrimidine were obtained via electron transfer catalyzed CO substitution of tetracarbonylmetal precursors in good yields. The preparative procedure involves the use of sub-stoichiometric amounts (10–20 mol%) of potassium metal to generate ESR-detectable anion radical intermediates, which then undergo selective substitution of one cis carbonyl group by way of hyperconjugative charge transfer from the reduced bidiazine ligand to the metal fragment. A catalytic cycle results because the ESR-detectable tricarbonyl anion radical complexes can reduce the tetracarbonyl precursors, seen from electrochemistry. Ligand-centered ETC substitution is fairly slow but proceeds by at least one order of magnitude faster than the daylight-induced process which can lead to dissociation of the partially sensitive tricarbonyls. The compounds are distinguished by long-wavelength metal-to-ligand charge transfer (MLCT) absorption bands resulting from transitions between the electron-rich metal and the low-lying π* orbitals of the bidiazines. Advantages and disadvantages of the anion radical ligand-induced activation of metal fragments are discussed.

Elektrontransfer-katalysierte Carbonyl-Substitution: III. Synthese und Spektroskopie von ein- und zweikernigen cis-Phosphantetracarbonylwolfram(0)-Komplexen elektronenarmer Pyridine☆

New mixed donor/acceptor-substituted tetracarbonyltungsten(0) complexes [cis-(CO)4W(PR3)]1,2(py′) have been prepared by electron transfer catalyzed carbonyl substitution of the precursor complexes (CO)5W(py′) containing the 4-acceptor (COMe, COPh, COOMe, CN) substituted pyridines py′. The synthesis involves the generation of substitutionally labile, ESR-detectable anion radical complexes by the addition of a sub-stoichiometric (10–20%) quantity of potassium metal; hyperconjugation between the primarily reduced pyridine π-system and the W(CO)5 fragment is responsible for substitution of one (and only one) CO group at each metal fragment in cis position. Electrochemical data illustrate the differences in reduction potentials which are crucial for electron transfer catalysis; the substituted anion radical complexes must be able to reduce the starting materials. Carbonyl substitution via this ligand-based electron transfer catalysis is rather slow, but the complexes cannot be obtained in comparable yields by the usual thermal or photochemical routes. The intermediate redox “umpolung” during this kind of electron transfer catalysis produces interesting compounds because donor/acceptor substitution at the tungsten carbonyl fragment causes small frontier orbital differences and, as a result, long-wavelength metal-to-ligand charge transfer transitions.

Resonance raman study of (η2-TCNE)M(CO)5 (M = Cr, W; TCNE = tetracyanoethene). Evidence for an olefin-complex → metallacyclopropane transition upon low-energy metal-to-ligand charge-transfer excitation

Abstract A solid state resonance Raman investigation of the non-hydrogen-containing title complexes was performed with long-wavelength laser excitation (514.5-611 nm). The spectra confirmed the η 2 -coordination of the π-acceptor olefin tetracyanoethene (TCNE) and showed that the absorption bands near 700 nm arise from transitions between the metal and TCNE. The metal-to-ligand charge-transfer (MLCT) character of these transitions is much more pronounced for the tungsten complex. These observations confirm the higher degree of metal ( d )/ligand(π ∗ ) orbital mixing in the chromium complex and also provide evidence for a structural change from that of an alkene π-complex towards a metallacyclopropane arrangement upon MLCT excitation

Electron transfer catalyzed substitution in carbonyl complexes. VI: Highly variable rates of substitution by tetracyanoethylene

Rates of substitution of tetrahydrofuran (THF) or trimethyl phosphite by tetracyanoethylene (TCNE) in organometallic complexes (THF)W(CO)5, (THF)Cr(CO)5, [P(OMe)3]Cr(CO)2(C6Me6), (THF)Mn(CO)2(C5Me5) and (THF)Mn(CO)2(C5H4Me) in THF solution, have been determined. As indicated by the calculated second-order rate constants k, the manganese complexes (k > 1900 M −1 s −1) react by a factor of at least 104 more rapidly than the W(CO)5 complex (k = 0.043 M −1 s −1). Measurements of recombination kinetics of photodissociated (acpy)Mn(CO)2(C5Me5) (acpy = 4-acetylpyridine) in THF show that the substitution by TCNE of THF in the corresponding solvent complex proceeds faster by a factor of 2 × 104. Of the two chromium complexes, the pentacarbonyl/THF system has a value of k of 0.39 M−1 s−1 whereas the phosphite ligand in [P(OMe)3]Cr(CO)2(C6Me6) is substituted by TCNE with k = 1.59 M −1 s−1. The results and their correlation with electrochemical data support a self-induced homogeneous electron transfer mechanism: it is proposed that electron transfer between the reaction partners TCNE and the organometallic precursor leads to substitutionally-labile 17 valence electron complexes as essential intermediates in the catalytic chain. Efficient oxidation of the precursors by the TCNE-substituted 17 VE species is possible because of intramolecular metal-to-TCNE electron transfer, especially in the σ-coordinated products with mixed carbonyl/carbocycle ligands. For Part V, see ref. 1.

Electron Transfer Induced Metal Addition and Ligand Exchange in Organometallic Anion Radical Complexes

Transition metal complexes containing anion radical ligands display a strong tendency towards full coordinative saturation at the singly reduced ligand and towards substitutional activation of coligands at the metal center. Examples involving metal carbonyls show how both of these reactivities can be employed either separately or in a combined fashion for electron transfer catalyzed substitution processes and for the construction of new polynuclear complexes with unusual properties.

Using Electrochemical Information for Organometallic Coordination Chemistry: Stoichiometric and Catalytic Reactions

Following a brief comment on the particular electrochemical behaviour of transition and main group organometallics we illustrate several synthetically relevant applications of cyclic voltammetry, mainly by example of carbonyl complexes of group 6 and 7 metals with it acceptor ligands. Such applications include the discovery of stable oxidation states and guidelines to their chemical generation as well the determination of electronic structures. Synthetically useful consequences of electron transfer such as the tendency towards coordinative saturation, isomerization reactions, and (catalytic) substitutional activation can be evident from cyclic voltammetry and may be correlated with the electrochemical data in ways that allow the rational planning of non-electrochemical reactions.