Redox induced reactions of transition metal π- and σ,π-complexes

Structural Changes in the Carbon Sphere of a Dirhodium Complex Induced by Redox or Deprotonation Reactions

A carbon-rich molecule is synthesized, which mainly contains conjugated sp2 and sp hybridized carbon centers. Alkenyl and alkynyl binding sites are arranged such that this compound serves as ligand to a binuclear metal unit with a RhI─RhI bond. Furthermore, CH units are placed in proximity to the metal centers. The dicationic complex [Rh2(bipy)2{Ph2Ptrop(C≡CCy)2}]2+(OTf-)2 allows to study possible responses of the carbon-framework to redox reactions as well as deprotonation reactions. All products are, whenever possible, characterized by X-ray diffraction (XRD) methods, NMR and EPR spectroscopy as well as electrochemical methods. It is shown that the carbon skeleton of the ligand framework undergoes C─C bond rearrangement reactions of remarkable diversity. In combination with DFT (density functional theory) studies, these results allow to gain insight into the electronic structure changes caused by metal sites in a carbon-rich environment, which may be of relevance for the properties of metal particles on carbon support materials when they are exposed to hydrogen, electrons, or protons.

Electronic communication in phosphine substituted bridged dirhenium complexes – clarifying ambiguities raised by the redox non-innocence of the C4H2- and C4-bridges

Dinuclear rhenium complexes with C₄H₂- or C₄- bridges can show open or closed shell structures.

Redox noninnocence of carbene ligands: carbene radicals in (catalytic) C-C bond formation

In this Forum contribution, we highlight the radical-type reactivities of one-electron-reduced Fischer-type carbenes. Carbene complexes of group 6 transition metals (Cr, Mo, and W) can be relatively easily reduced by an external reducing agent, leading to one-electron reduction of the carbene ligand moiety. This leads to the formation of "carbene-radical" ligands, showing typical radical-type reactivities. Fischer-type carbene ligands are thus clearly redox-active and can behave as so-called "redox noninnocent ligands". The "redox noninnocence" of Fischer-type carbene ligands is most clearly illustrated at group 9 transition metals in the oxidation state II+ (Co(II), Rh(II), and Ir(II)). In such carbene complexes, the metal effectively reduces the carbene ligand by one electron in an intramolecular redox process. As a result, the thus formed "carbene radicals" undergo a variety of radical-type C-C and C-H bond formations. The redox noninnocence of Fischer-type carbene ligands is not just a chemical curiosity but, in fact, plays an essential role in catalytic cyclopropanation reactions by cobalt(II) porphyrins. This has led to the successful development of new chiral cobalt(II) porphyrins as highly effective catalysts for asymmetric cyclopropanation with unprecedented reactivity and stereocontrol. The redox noninnocence of the carbene intermediates results in the formation of carbene-radical ligands with nucleophilic character, which explains their effectiveness in the cyclopropanation of electron-deficient olefins and their reduced tendency to mediate carbene dimerization. To the best of our knowledge, this represents the first example in which the redox noninnocence of a reacting ligand plays a key role in a catalytic organometallic reaction. This Forum contribution ends with an outlook on further potential applications of one-electron-activated Fischer-type carbenes in new catalytic reactions.