Molecular mechanism of acid-triggered aryl-halide reductive elimination in well-defined aryl-Cu(III)-halide species

Well-defined aryl-Cu(III)-halide species undergo reductive elimination upon acid addition resulting in the formation of strong aryl-halide bonds. The computationally studied mechanism points towards ligand protonation as the rate-determining step, in agreement with previous experimental data.     

Droplet States in the XXZ Heisenberg Chain

We consider the ground states of the ferromagnetic XXZ chain with spin up boundary conditions. The ground state of this model, restricted to a sector with a fixed number of down spins, describes a droplet of down spins in an environment of up spins. We find the exact energy and the states that describe these droplets in the limit of an infinite number of down spins. We prove that there is a gap in the spectrum above the droplet states. As the XXZ Hamiltonian has a gap above the fully magnetized ground states as well, this means that the droplet states (for sufficiently large droplets) form an isolated band. The width of this band tends to zero in the limit of infinitely large droplets. We also prove the analogous results for finite chains with periodic boundary conditions and for the infinite chain. Received: 5 September 2000 / Accepted: 8 December 2000     

Finite-Volume Excitations of the¶111 Interface in the Quantum XXZ Model

We show that the ground states of the three-dimensional XXZ Heisenberg ferromagnet with a 111 interface have excitations localized in a subvolume of linear size R with energies bounded by O(1/R²). As part of the proof we show the equivalence of ensembles for the 111 interface states in the following sense: In the thermodynamic limit the states with fixed magnetization yield the same expectation values for gauge invariant local observables as a suitable grand canonical state with fluctuating magnetization. Here, gauge invariant means commuting with the total third component of the spin, which is a conserved quantity of the Hamiltonian. As a corollary of equivalence of ensembles we also prove the convergence of the thermodynamic limit of sequences of canonical states (i.e., with fixed magnetization).     

Mechanistic analysis of trans C-N reductive elimination from a square-planar macrocyclic aryl-copper(III) complex

A net trans C-N reductive elimination reaction is observed from a macrocyclic aryl-Cu(III) complex, and a mechanistic study of this reaction indicates that coordinating ligands play a role in mediating this unusual transformation.     

Benzoquinone-promoted reaction of O2 with a Pd(II)-hydride

Benzoquinone (BQ) and O(2) are among the most common stoichiometric oxidants in Pd-catalyzed oxidation reactions. The present study provides rare insights into mechanistic differences between BQ and O(2) in their reactivity with a well-defined Pd-hydride complex, Pd(IMes)(2)(H)(O(2)CPh) (1). BQ promotes the reductive elimination of PhCO(2)H from 1 and catalyzes the formation of a Pd(II)-OOH complex when this reaction is carried out under aerobic conditions. These results have important implications for Pd-catalyzed oxidation reactions.     

Coordination-induced bond weakening of water at the surface of an oxygen-deficient polyoxovanadate cluster

Hydrogen-atom (H-atom) transfer at the surface of heterogeneous metal oxides has received significant attention owing to its relevance in energy conversion and storage processes. Here, we present the synthesis and characterization of an organofunctionalized polyoxovanadate cluster, (calix)V₆O₅(OH₂)(OMe)₈ (calix = 4-tert-butylcalix[4]arene). Through a series of equilibrium studies, we establish the BDFE(O–H)_avg of the aquo ligand as 62.4 ± 0.2 kcal mol⁻¹, indicating substantial bond weaking of water upon coordination to the cluster surface. Subsequent kinetic isotope effect studies and Eyring analysis indicate the mechanism by which the hydrogenation of organic substrates occurs proceeds through a concerted proton–electron transfer from the aquo ligand. Atomistic resolution of surface reactivity presents a novel route of hydrogenation reactivity from metal oxide surfaces through H-atom transfer from surface-bound water molecules.

Coordination-induced bond weakening facilitates H-atom transfer from polyoxovanadate surface.     

Two atom insertions into the silacyclopropane and silacyclopropene rings: mechanistic considerations

Novel two atom insertion reactions into the SiC₂ rings of hexamethylsilirane and 1,1-dimethyl-2,3-bis(trimethylsilyl)-1-silirene involving the C bonds of styrene and α-methylstyrene, 1,3-butadiene and its 2-methyl and 2,3-dimethyl derivatives, and, in the presence of ultraviolet radiation, of aliphatic aldehydes and ketones, are reported. A radical mechanism is suggested tobe operative in these reactions. Also described are novel palladium-catalyzed reactions of the silirene with terminal and internal acetylenes.