ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Activation of c-h bonds by metal complexes

Schiff base ligands are considered “privileged ligands” because they are easily prepared by the condensation between aldehydes and imines. Stereogenic centres or other elements of chirality (planes, axes) can be introduced in the synthetic design. Schiff base ligands are able to coordinate many different metals, and to stabilize them in various oxidation states, enabling the use of Schiff base metal complexes for a large variety of useful catalytic transformations. Practical guidelines for the preparation and use of different Schiff base metal complexes in the field of catalytic transformations are discussed in this tutorial review.

Metal–Salen Schiff base complexes in catalysis: practical aspects

Recent Developments in the Preparation and Chemistry of Metallacycles and Metallacages via Coordination

Frustrated Lewis pairs (FLPs) are combinations of Lewis acids and Lewis bases in solution that are deterred from strong adduct formation by steric and/or electronic factors. This opens pathways to novel cooperative reactions with added substrates. Small-molecule binding and activation by FLPs has led to the discovery of a variety of new reactions through unprecedented pathways. Hydrogen activation and subsequent manipulation in metal-free catalytic hydrogenations is a frequently observed feature of many FLPs. The current state of this young but rapidly expanding field is outlined in this Review and the future directions for its broadening sphere of impact are considered.

Frustrated Lewis pair chemistry: development and perspectives.

6.1.2. Aqueous V(III) Chemistry 877 6.1.3. Oxidation State of Vanadium in Tunicates 878 6.1.4. Uptake of Vanadate into Tunicates 879 6.1.5. Vanadium Binding Proteins: Vanabins 879 6.1.6. Model Complexes and Their Chemistry 880 6.1.7. Catechol-Based Model Chemistry 880 6.1.8. Vanadium Sulfate Complexes 881 6.2. Fan Worm Pseudopotamilla occelata 883 7. Vanadium Nitrogenase 883 7.1. Nitrogenases 883 7.2. Biochemistry of Nitrogenase 884 7.3. Clusters in Nitrogenase and Model Systems: Structure and Reactivity 885

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The chemistry and biochemistry of vanadium and the biological activities exerted by vanadium compounds.

Four-electron reduction of dinitrogen has been achieved in the reaction of N2 with [{p-But-calix[4]-(O)4}2Nb2(μ-M)2] (M = Li, 3; M = Na, 4; M = K, 5), which contain a very reactive NbNb unit [NbNb = 2.659(1) A; 4], in THF. The reaction leads to the dinuclear complexes [{p-But-calix[4]-(O)4}2(Nb⋮N−N⋮Nb)]2- M+2 (M = Li, 6; M = Na, 7; M = K, 8) [Nb⋮N = 1.747(12) A, N−N = 1.390(17) A; 7], containing the hydrazido [N2]4- anion. Two-electron reduction of the hydrazido tetraanion in 7 using sodium metal leads to the nitrido species 11, [{p-But-calix[4]-(O)4}2Nb2(μ-N)2(μ-Na)2Na2], which in solution is in equilibrium with the monomeric form 18, [{p-But-calix[4]-(O)4}Nb⋮N−NaSn]. Two distinct pathways have been identified as a function of the solvent used (THF or DME). In the case of DME, the key intermediate 12 has been intercepted as green crystals. The formation of 12, [{p-But-calix[4]-(O)4}2Nb2(μ-η2:η2-N2)(μ-Na)3Na], results from the reduction of Nb(V) to Nb(IV), which temporarily stores the two electrons in an ...

Dinitrogen Rearranging over a Metal−Oxo Surface and Cleaving to Nitride: From the End-On to the Side-On Bonding Mode, to the Stepwise Cleavage of the N⋮N Bonds Assisted by NbIII-calix[4]arene

It is possible to control the geometry and the composition of metallasupramolecular assemblies via the aspect ratio of their ligands. This point is demonstrated for a series of iron- and palladium-based coordination cages. Functionalized clathrochelate complexes with variable aspect ratios were used as rod-like metalloligands. A cubic FeII8L12 cage was obtained from a metalloligand with an intermediate aspect ratio. By increasing the length or by decreasing the width of the ligand, the self-assembly process resulted in the clean formation of tetrahedral FeII4L6 cages instead of cubic cages. In a related fashion, it was possible to control the geometry of PdII-based coordination cages. A metalloligand with a large aspect ratio gave an entropically favored tetrahedral PdII4L8 assembly, whereas an octahedral PdII6L12 cage was formed with a ligand of the same length but with an increased width. The aspect ratio can also be used to control the composition of dynamic mixtures of PdII cages. Out of two metalloli...

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Ligand Aspect Ratio as a Decisive Factor for the Self-Assembly of Coordination Cages

Magnetic Properties Diagnostic for the Existence of Iron(II)-Iron(II) Bonds in Dinuclear Complexes Which Derive from Stepwise Insertion Reactions on Unsupported Iron-Aryl Bonds

Stepwise Reduction of Dinitrogen to Nitride Assisted by Niobium Bonded to Oxygen Donor Atoms: The Potential of Reduced Forms of Niobium Calix[4]arene

We report on the synthesis and optical characterization of hydrazone derivatives developed for second-order nonlinear optics. Electric-field-induced second harmonic generation (EFISH) as well as semiempirical quantum-chemical computations have been carried out to investigate and explain the nonlinear optical properties of these molecules. We obtained values of up to 220 x 10(-40) m(4)/V for the molecular hyperpolarizability beta at lambda = 1907 nm. The investigation of a series of 39 hydrazone derivatives with the second harmonic generation powder test according to Kurtz and Ferry revealed that an extraordinarily large fraction of these hydrazones show efficient second-order nonlinear activity. Among them, 4-(dimethylamino)-benzaldehyde-4-nitrophenylhydrazone (DANPH) and 4-(1-azacycloheptyl)-benzaldehyde-4-nitrophenylhydrazone (ACNPH) were selected for more detailed investigations. Their crystalline structures are presented, and the role played by hydrogen bonds in their molecular packing is emphasized. Finally, preliminary linear and nonlinear optical data of DANPH crystals are reported. [References: 42]

Euro Solari

Papers

Dinitrogen Rearranging over a Metal−Oxo Surface and Cleaving to Nitride: From the End-On to the Side-On Bonding Mode, to the Stepwise Cleavage of the N⋮N Bonds Assisted by NbIII-calix[4]arene

Ligand Aspect Ratio as a Decisive Factor for the Self-Assembly of Coordination Cages

Magnetic Properties Diagnostic for the Existence of Iron(II)-Iron(II) Bonds in Dinuclear Complexes Which Derive from Stepwise Insertion Reactions on Unsupported Iron-Aryl Bonds

Stepwise Reduction of Dinitrogen to Nitride Assisted by Niobium Bonded to Oxygen Donor Atoms: The Potential of Reduced Forms of Niobium Calix[4]arene

Hydrazone Derivatives, an Efficient Class of Crystalline Materials for Nonlinear Optics