The low-valent U(III) complexes [((t-BuArO)3mes)U] and [((AdArO)3N)U] react with CO2 to form the bridging carbonate complexes [{((t-BuArO)3mes)U}2(mu-kappa2:kappa2-CO3)] and [{((AdArO)3N)U}2(mu-eta1:kappa2-CO3)]. Uranium(IV) bridging oxo complexes have been determined to be the intermediate in these transformations.

Synthetic studies on the redox chemistry of trivalent uranium monoarene complexes were undertaken with a complex derived from the chelating tris(aryloxide)arene ligand ((Ad,Me) ArO)3 mes(3-) . Cyclic voltammetry of [{((Ad,Me) ArO)3 mes}U(III) ] (1) revealed a nearly reversible and chemically accessible reduction at -2.495 V vs. Fc/Fc(+) -the first electrochemical evidence for a formally divalent uranium complex. Chemical reduction of 1 indicates that reduction induces coordination and redox isomerization to form a uranium(IV) hydride, and addition of a crown ether results in hydride insertion into the coordinated arene to afford uranium(IV) complexes. This stoichiometric reaction sequence provides structural insight into the mechanism of arene functionalization at diuranium inverted sandwich complexes.

NÀH bond activation of amines has attracted increasing attention owing to its applicability to the synthesis of various amino compounds, and the development of a new and effective reaction system for the activation of ammonia must be one of the most important research targets in connection with the transformation of abundant and inexpensive ammonia into a useful amino compound.[1] However, successful examples of activation of the NÀH bond of ammonia are still rare because of both the high NÀH bond dissociation energy ( % 104 AE 2 kcal mol À1 ) [2] and the difficulty in forming an NÀH s complex.[3]The groups of Milstein [1g, i, j] and Hartwig [1k, q] showed independently that some mononuclear iridium(i) complexes exhibited activity towards oxidative addition of ammonia. A highly unsaturated 14 e species with T-shaped geometry was proposed as a reactive intermediate for the NÀH bond cleavage on the basis of kinetic studies.[1q] Some complexes containing a d 0 metal center, such as [Cp* 2 MH 2 ] (M = Zr, Hf; Cp* = pentamethylcyclopentadiene), [1b, c, e] [Cp* 2 ScR], [1f] and [(neopentyl) 3 Ta = C(H)(tBu)], [1h] also activated ammonia to generate amido and nitrido complexes. There have, thus far, been examples of bimetallic oxidative addition to ammonia. A trinuclear carbonyl cluster, [Os 3 (CO) 11 (L)] (L = c-C 6 H 8 or CH 3 CN), effectively activates ammonia with the participation of the two osmium centers to produce the m-amido complex [Os 3 (CO) 10 (m-H)(m-NH 2 )]. [1a, d] A multimetallic system may work more efficiently for bond activation than a monometallic complex owing to the cooperative action of the metal centers. Each metal center would be allotted a part as a binding site and an activation site, and the transition state of the bond-activation step may, therefore, be stabilized.

Synergistic effects between a transition metal and an appropriate ligand are required to promote a desired catalytic reaction. Ancillary ligands, provided by the versatile functionality of certain elements, give rise to an almost infinite potential for catalytic applications. Recently, the study of the synergistic effect between transition metals and boron has become easy on account of the development of various rigid multidentate frameworks. In this Review, we mainly focus on the chemistry of σ-acceptor (Z-type) borane ligands, particularly the key achievements of their unique reactivity and catalytic applications. Conceptually, the unique character of σ-acceptor borane ligands provides a new strategy for developing remarkable reactivity and novel catalytic applications. This study discusses recent developments in the field in this context. The chemistry of boron-based multidentate ligands that involve a covalent M-B bond such as the boryl ligand (-BR2), in which a boron moiety serves as a strong electron-donating ligand, is also rapidly developing. The effect of the boryl ligand on a metal center is totally different from that of the borane (-BR3) ligand, and different boron-based functionalities confer opposing electronic properties to the metal center. The interesting character of boryl-based chelating ligands augments their unique coordination chemistry, which is also summarized in this context.