Christian Bessenbacher

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.

Koordinationsambivalenz und fluktuierendes Verhalten bei ein- und zweikernigen Wolframpentacarbonyl-Komplexen des Pteridins und seiner Deazaderivate

Abstract The mono- and binuclear pentacarbonyltungsten complexes of highly ambidentate pteridine (1,3,5,8-tetraazanaphthalene), the parent heterocycle of the biochemically important pterins, and of its deaza derivatives 1,5-naphthyridien, 1,4,5- and 1,4,6-triazanaphthalene have been synthesized. Unambiguous identification of coordination sites was only possible by use of high resolution 1 H NMR because the chemical shifts and spin-spin coupling constants are characteristically affected by W(CO) 5 -coordination to neighbouring nitrogen centers. Fluxional behaviour with respect to the peri coordination sites was observed for the mononuclear complex of 1,4,5-triazanaphthalene and for the binuclear pteridine system, competition between the more basic pyridine or pyrimidine and the better π backdonating pyrazine N centers leads to various positions for the non-degenerate equilibria between linkage isomers. Cyclic voltammetry and solvent-dependent charge transfer absorption spectra of the complexes reveal low-lying π* levels which can become populated after chemical or electrochemical reduction, the spin distribution in the centrosym-metric binuclear complex of 1,5-naphthyridine anion radical has been fully characterized using ESR and results from molecular orbital calculations.

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.

Stabilisierung biochemisch interessanter Zwischenstufen durch Metallkoordination: II. 5,8-Bis(trimethylsilyl)-5,8-dihydropteridin und seine Deaza-Derivate☆

Abstract Reductive trimethylsilylation of pteridine, of its 1- and 3-deaza derivatives 1,4,6- and 1,4,5-triazanaphthalene, and of the 1,3-dideaza derivative quinoxaline yields the primary reduced forms of these heterocycles, which contain the 1,4-dihydro-1,4-diazine ring with 8 conjugated π-electrons as the only low molecular weight products. Although the organometallic substituents stabilize these biochemically important yet normally short-lived dihydro forms and so allow unambiguous characterization by NMR, the non-crystalline, coloured compounds are still highly reactive. Unexpectedly the deaza derivatives prove to be less electron-rich than the silylated 5,8-dihydropteridine despite a clear increase in the electron density in the aromatic non-1,4-diazine ring. The characteristic conformational flexibility of these intermediates is responsible for this “inverse” annelation effect. Reductive trimethylsilylation of 1,5-naphthyridine (3,8-dideazapteridine) yields a 1,4-dihydro derivative as major product.

Ein- und zweikernige Bis(2,2′-bipyridyl)ruthenium-Komplexe mit N,O-Modelliganden für Dehydrogenase-Cofaktoren / Mono- and Binuclear Bis(2,2′-bipyridine)ruthenium Complexes with N,O-Ligands Modelling Dehydrogenase Cofactors

Abstract Coordination of substitutionally inert [Ru(bpy)2]2+ fragments (bpy: 2,2′-bipyridine) to the a-iminoketone chelate ligands pyrazine-2-dimethylcarboxamide (4) and 4,7-phenanthroline-5,6-dione (5) yields the complexes [(N,O-4)Ru(bpy)2]2⊕, [(O,O′-5⊖)Ru(bpy)2]⊕ and {(N,O; N′,O′-5)[Ru(bpy)2]2}4⊕ which exhibit a rich electrochemistry. The distinctly different electronic structures of the complexes are evident from the ESR behaviour of paramagnetic intermediates: N.O-coordinated complexes have the unpaired electron residing in the ligand n system upon reduction, albeit with g<2 for the binuclear complex of 5. The paramagnetic O,O′-coordinated mononuclear complex with 5 has its redox potentials shifted positively relative to that of the binuclear system. These results are particularly noteworthy because 4 and 5 can be regarded as model compounds for the flavin and methoxatin dehydrogenase cofactors.

Stabilization of biochemically interesting intermediates by metal coordination. Part 5.—Complexes of ZnII, CuI, ReI and RuII with singly reduced 2,5-diacetylpyrazine

Homobinuclear chelate complexes of the 2,5-diacetylpyrazine anion radical with the d10 and d6 metal fragments Zn2+, [(PPh3)2Cu]+, (OC)3ClRe and [(bpy)2Ru]2+ have been studied by ESR and cyclic voltammetry. The radical state of this centrosymmetric ligand with two biochemically relevant α-carbonylpyrazine chelate sites is considerably stabilized by coordination.

Elektronenreiche Olefine, 2 Struktur und Elektrontransfer-Reaktivität des cyclisch σ/π-hyperkonjugierten Carbosilans 3,3,6,6-Tetrakis(trimethylsilyl)-1,4-cyclohexadien

Abstract Crystal and molecular structure analysis of the title compound 1, a most electron rich carbosilane, exhibits a shallow boat conformation for the cyclohexadiene ring which is shielded by four bulky Me3Si groups. Multiple hyperconjugative interaction occurs between the two non-conjugated olefinic π systems and the four rather long (192 pm) carbon-silicon o bonds which form an angle of about 34° with the assumed π axis. The HOMO destabilization caused by this unique structural arrangement explains the energetically facile formation and subsequent reactivity of the cation radical 1+ which was found to undergo oxidative desilylation to the aromatic 1,4-bis(trimethylsilyl) benzene precursor in the single electron transfer reaction with TCNE.

Persistent cation radicals containing carboxamide and thiophosphinic amide functions. The one electron oxidation of acceptor stabilized 1,4-dihydropyrazines

Even when it is N,N′-disubstituted by the acceptor groups C(O)Me and P(S) Me2, the 1,4-dihydropyrazine system remains sufficiently electron rich to undergo facile one-electron oxidation to give persistent radical cations with acetamide and thiophosphinic amide functionality.

Elektronenreiche Olefine, 3 / Electron Rich Olefins, 3

Exhaustive reductive silylation of 2-trimethylsilyl-1,2,3-triazole (1) using Me,SiCl/K yields the new E-1,1,4,4-tetrakis(trimethylsilyl)-l,4-diazabutene(2) (4) as final product together with N(SiMe3)3. Initial reductive 1,4-addition to 1 leads to formation of 1.2.3-tris(trimethylsilyl)-1,2,3-triazoline(4) (2) as an intermediate which is rapidly reduced and silylated further to give the butene 4. Partial π overlap within the five-membered ring 2 containing 8 π electrons and three neighbouring unshared electron pairs is evident from NMR shifts and electron transfer reaction with TCNE. Photoelectron spectroscopy of 4 and the lability of its radical cation 4+. as studied by ESR show that this system cannot adopt a planar conformation in contrast to the tetrazene(2) analogue.

Evidence for the 1-thia-3-sila-2,4-diazetidine heterocyclic system with three parallel lone pairs from an e.s.r. spectroscopic study of its cation radical

Formation of the new electron-rich heterocyclic system (But)[graphic omitted]i, an intermediate during the reductive cyclosilylation of ButNSNBut with Me2SiCl2/2Li, was established by an e.s.r. spectroscopic analysis of its persistent cation radical after oxidation with AlCl3/CH2Cl2.