The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of β-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

Azobenzene undergoes trans → cisisomerization when irradiated with light tuned to an appropriate wavelength. The reverse cis →transisomerization can be driven by light or occurs thermally in the dark. Azobenzene's photochromatic properties make it an ideal component of numerous molecular devices and functional materials. Despite the abundance of application-driven research, azobenzene photochemistry and the isomerization mechanism remain topics of investigation. Additional substituents on the azobenzene ring system change the spectroscopic properties and isomerization mechanism. This critical review details the studies completed to date on the 3 main classes of azobenzene derivatives. Understanding the differences in photochemistry, which originate from substitution, is imperative in exploiting azobenzene in the desired applications.

Photoisomerization in different classes of azobenzene

The ability of platinum and gold catalysts to effect powerful atom-economic transformations has led to a marked increase in their utilization. The quite remarkable correlation of their catalytic behavior with the available structural data, coordination chemistry, and organometallic reactivity patterns, including relativistic effects, allows the underlying principles of catalytic carbophilic activation by π acids to be formulated. The spectrum of reactivity extends beyond their utility as catalytic and benign alternatives to conventional stoichiometric π acids. The resulting reactivity profile allows this entire field of catalysis to be rationalized, and brings together the apparently disparate electrophilic metal carbene and nonclassical carbocation explanations. The advances in coupling, cycloisomerization, and structural reorganization—from the design of new transformations to the improvement to known reactions—are highlighted in this Review. The application of platinum- and gold-catalyzed transformations in natural product synthesis is also discussed.

Catalytic Carbophilic Activation: Catalysis by Platinum and Gold π Acids

Living ring-opening metathesis polymerization

Publisher Summary This is one of two articles in this volume that is concerned with the borane-carborane structural pattern. In the other, Williams has shown how the pattern reflects the coordination number preferences of the various atoms involved. The purpose of the present article is to note some bonding implications of the pattern, and to show its relevance to a wide range of other compounds, including metal clusters, metal-hydrocarbon 7∼ complexes, and various neutral or charged hydrocarbons. Boranes and carboranes may be regarded as cluster compounds in the sense defined by Cotton; they contain a finite group or skeleton of atoms held together entirely, mainly, or at least to a significant extent by bonding directly between those atoms, even though some other atoms may be associated intimately with the cluster. Examples of their structural pattern, however, can be found far beyond the confines of what is normally regarded as cluster chemistry, so this survey includes many systems not commonly referred to as clusters.

Structural and Bonding Patterns in Cluster Chemistry

Zur Frage eines Wolfram‐Carbonyl‐Carben‐Komplexes

Das äußerst stabile, unpolare Dicyclopentadienyl-eisen, FeII(C5H5)2, stellt sich als neuartiger Typ eines Durchdringungskomplexes dar. Das zentrale Eisen(II)-Ion besitzt hier durch oktaedrisch-koordinative Bindung an die beiden dreizähligen, aromatisierten, parallel gegenüberstehenden Cyclopentadienyl-Anionen Kryptonkonfiguration. Chemische,magnetische und röntgenographische Befunde bestätigen diese Auffassung. Auch in den komplexen Kationen [FeIII(C5H5)2]+ und [CoIII(C5H5)2] +, welche als schwerlösliche Reinedceate isoliert wurden, erscheint das Cyclopentadienyl gleichartig gebunden.

Cyclopentadien-Metallkomplexe, ein neuer Typ metallorganischer Verbindungen

Alfred Nobel hat einmal im Gesprach zu einem Freund gesagt, er wolle den Traumern unter den Wissenschaftlern helfen. Das den Traumern im Reiche der Chemie auch in Zukunft ein Platz erhalten bleibe, ist der Wunsch der Nobel-Preistrager fur Chemie des Jahres 1973.

Auf dem Weg zu Carben‐ und Carbin‐Komplexen (Nobel‐Vortrag)

In Weiterentwicklung der zuerst an FeII(C5H5)2 gebildeten Vorstellung, daß aromatische Systeme mit Übergangselementen Durchdringungskomplexe zu bilden vermögen, gelang die Darstellung eines gelben, wasserlöslichen Di-benzol-chrom(I)-Kations [Cr(C6H6)2]+. Es läßt sich zu ungeladenem, schwarzbraunem Di-benzol-chrom(0) Cr(C6H6)2 reduzieren. Letzteres ist in organischen Medien löslich, sublimierbar und bei Luftausschluß bis 300° beständig. Sein Diamagnetismus entspricht der Auffüllung des zentralen Metalls zur Kryptonkonfiguration durch die je drei π-Elektronenpaare beider Ringe. Es liegt nach allem ein raum- und bindungsisosteres Molekül zu FeII(C5H5)2 vor.

Di-benzol-chrom

trans-Halogeno[alkyl(aryl)carbyne]tetracarbonyl Complexes of Chromium, Molybdenum, and Tungsten —A New Class of Compounds Having a Transition Metal-Carbon Triple Bond†

Ernst Otto Fischer

Papers

Zur Frage eines Wolfram‐Carbonyl‐Carben‐Komplexes

Cyclopentadien-Metallkomplexe, ein neuer Typ metallorganischer Verbindungen

Auf dem Weg zu Carben‐ und Carbin‐Komplexen (Nobel‐Vortrag)

Di-benzol-chrom

trans-Halogeno[alkyl(aryl)carbyne]tetracarbonyl Complexes of Chromium, Molybdenum, and Tungsten —A New Class of Compounds Having a Transition Metal-Carbon Triple Bond†