No abstract

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 NbNb unit [NbNb = 2.659(1) Å; 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) Å, N−N = 1.390(17) Å; 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−NaS n ]. 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 Nb−Nb bond [N−N = 1.403(8) Å, Nb−Nb, 2.635(1) Å]. Complex 12 contains a μ-η2:η2 side-on N2 group, bonded between two niobiums and preorganized to be transformed into two nitrido anions. Heating in pyridine (90 °C) converts 12 to 11, with simultaneous cleavage of the Nb−Nb and N−N bonds, the heating causing two-electron transfer from the Nb−Nb to the N−N bond. The reaction of 4 with N2, carried out in toluene instead of THF or DME, gives a nitrido complex, [{p-But-calix[4]-(O)4}3Nb3(μ-N)2Na3(THF)2], 15, which can be formed equally well from the reaction of 4a with 7a carried out in a 1:2 molar ratio in toluene. Complex 15 breaks up in CHCl3 or toluene, upon addition of TMEDA, into [{p-But-calix[4]-(O)4}2Nb2(μ-N)2{μ-Na2(TMEDA)2}Na2], 16, and [{p-But-calix[4]-(O)4}2Nb2(μ-N)][Na(TMEDA)2], 17.

The complex Ru(TPP)(NAr)(2) inserts a nitrene group into allylic and benzylic C-H bonds and is the key intermediate in the ruthenium porphyrin-catalyzed amination of hydrocarbons by aryl azides.

Palladium-phenanthroline complexes catalyze both the nitroarene carbonylation reaction and the amine oxidative carbonylation reaction to give, depending on the conditions, carbamates and ureas. There is evidence that the key step in both processes is the amine carbonylation. Here, we show that when the reaction is run in methanol key intermediate compounds have the general formula [Pd(RPhen)(COOMe)(2)] (1) (RPhen = 1,10-phenanthroline or one of its substituted derivatives). The kinetics of the reaction of 1 with toluidine in the presence of a carboxylic or phosphorus acid is first-order with respect to complex, acid, and toluidine. A CO atmosphere is also required for the reaction to proceed. Acid dimerization was shown not to be influential under the concentration conditions examined, but reaction between the acid and toluidine is not negligible and a correction has to be applied. Diphenylphosphinic acid is more effective than any carboxylic acid in promoting this reaction, as also observed under catalytic conditions. A series of equilibria and an irreversible acid-assisted proton transfer explain the observed data. Formation of an adduct between complexes of the kind 1 and CO was spectroscopically observed when RPhen = 2,9-Me(2)Phen. Several analogous complexes were also spectroscopically characterized and the X-ray structure of [Pd(2,9-Me(2)Phen)Cl(2)(CO)] was solved. This shows an asymmetric coordination of the nitrogen ligand. Kinetic measurements were also conducted under catalytic conditions. An Eyring plot shows that the effect of the acidic promoter is to decrease the DeltaS(double dagger) value, whereas no positive effect is observed on DeltaH(double dagger). A temperature-dependent correction for the reaction between the acid and aniline and phenanthroline present under the reaction conditions has to be applied. Comparison of the results obtained under stoichiometric and catalytic conditions strongly supports the view that 1 is involved even in the latter and that the acid is acting as a bifunctional promoter.

This feature article provides an overview of the application of organic azides for the intermolecular amination of sp(3) and sp(2) C-H bonds. The catalytic activity of several metal complexes was reviewed underlining both synthetic and mechanistic aspects of the C-H amination. The majority of the aminated compounds reported in literature have been collected in this paper to provide a compendium of published procedures. In addition, the discussion of involved mechanisms has been included to assist the reader to envisage the future potential of organic azides in the synthesis of aza-derivatives.

Nitrene transfer reactions represent a useful methodology to synthesize in a few steps high added-value compounds used as organic intermediates. Herein, we describe the catalytic activity of metal porphyrin complexes in a wide range of reactions such as C-H hydrocarbon amination and olefin aziridination to synthesize nitrogen containing molecules. All the most important nitrene sources have been reviewed stressing the potentiality and limits of each one in the particular class of chemical transformation.

This paper describes the synthesis of allylic amines by aryl azides (ArN 3 ) catalysed by [Ru(TPP)CO] (TPP = dianion of tetraphenylporphyrin). The employment of aryl azides renders the methodology sustainable as the formation of molecular nitrogen is the only stoichiometric byproduct. The isolation of catalytic intermediates and spectroscopic and kinetic studies revealed interesting information about the reaction mechanism, which could improve its catalytic efficiency in future research. An important result is the X-ray characteri-

The synthesis of Ar,Ar'-BIAN ligands (Ar,Ar'-BIAN = bis(aryl)acenaphthenequinonediimine) having different aryl groups bound to the two nitrogen atoms is reported for the first time. The ligands were obtained by two different strategies: (i) by a transimination reaction starting from a symmetric Ar,Ar-BIAN ligand having aryl groups bearing strongly electron-withdrawing substituents or (ii) by a two-step-one-pot sequence. The ligands synthesized have been chosen so that the electronic difference between the two aryl groups is very large, but the steric difference is variable and, in one case, the ligand is almost sterically symmetric. The coordination strength of the new ligands towards three palladium complexes has been measured by a competition experiment following a protocol previously described by us. The coordination strength of the mixed ligands is the mean of those of the corresponding symmetric counterparts. The X-ray crystal structure of a palladium pi-allyl complex of the electronically asymmetric-sterically symmetric ligand (3,5-(CF(3))(2)C(6)H(3)),(3,5-Me(2)C(6)H(3))-BIAN has been solved, together with those of the two symmetric analogues to allow a comparison. Despite the fact that the dodecafluorinated ligand has a K(eq) value about three orders of magnitude lower than the non-fluorinated counterpart, no notable difference is observed in the N-Pd and Pd-C(allyl) distances in the three complexes. Calculations at the density functional level confirm that Pd-BIAN distances are not strictly correlated to the coordination energies, which are in qualitative agreement with the spectroscopic evidence. The bond length is thus not a good indication of the bond strength in these cases.