The role of three-center/four-electron bonds in superelectrophilic dirhodium carbene and nitrene catalytic intermediates

Three-center/four-electron (3c/4e) bonds are important bonding motifs that dictate the electronic structure, and thereby the reactivity, of metal-metal bonded carbene and nitrene intermediate complexes that are crucial to the dirhodium-catalyzed functionalization of hydrocarbons. In this Perspective article, general features of the 3c/4e bond are presented and discussed in comparison to two-center/two-electron (2c/2e) bonds. Specifically, 3c/4e bonding interactions lead to longer distances between the atoms involved and measurably weaker bonds. Additionally, excited states derived from the 3c/4e bonding manifold are lower in energy than those derived from a 2c/2e manifold, signifying a greater degree of reactivity in the former case. Three coterminous 3c/4e Ru-Ru-N bonds are present in metal-metal/metal-ligand multiply bonded diruthenium terminal nitrido compounds. This bonding situation results in an unusual superelectrophilic character of the nitride nitrogen atom, exemplified by its insertion into aryl C-H bonds via an electrophilic aromatic substitution mechanism. The key catalytic intermediates in dirhodium-catalyzed C-H functionalization reactions, dirhodium carbene and dirhodium nitrene complexes, may also be described as superelectrophilic by virtue of 3c/4e Rh-Rh-C(or N) σ and π bonds. These 3c/4e bonding interactions set apart dirhodium carbene and nitrene intermediates from other, less electrophilic, carbene or nitrene species.     

A new three-center model for diatomic bonds based on the spherical Gaussian bonding function

A new three-center model for diatomic bonds is proposed based on the spherical Gaussian bonding function. The characteristics of the model are that the magnitude of effective bond-charge described by the function is different for different observation points, and that the position of the bond-charge on the bond axis satisfies the energy variation principle.

According to the model and two additional assumptions, some theoretical formulas for bond properties, such as the total energy of system, dissociation energy, dipole moment, force constant and field gradients at nuclei, are derived. The calculated results are basically in agreement with the observed ones.

In addition, a comparison between the new model and the electrostatic three-center model proposed in a previous paper is discussed in brief.     

Island formation in chemisorption 1. Chemisorption model

A model of dissociative chemisorption on a surface with a square lattice was studied using the Monte Carlo method. The model is based on two chemisorption pathways: “direct”—nucleation of adsorption islands, and “indirect”—their growth. The development of the surface distribution picture of chemisorbed particles was found to depend significantly on the contribution of these two pathways (S_indir/S_dir).     

Island formation in chemisorption 2. Chemisorption kinetics

Influence of “direct” and “indirect” pathways of dissociative chemisorption on the form of kinetic dependences ϕ(MCS) and S(ϕ) was studied by the Monte Carlo method. Langmuir adsorption observed at S_indir/S_dir≤0.1 gradually changes to island-mediated adsorption with an increase of the “indirect” adsorption contribution at 0.1≤S_indir/S_dir≤1.0. At S_indir/S_dir≥1.0 the island-mediated adsorption dominates: large adsorption islands arise and gradually grow.     

Metal-mediated formation of carbon-halogen bonds

Organic halides represent basic starting materials for numerous metal-catalyzed organic transformations. Generally, the carbon-halogen is broken in the first step, that is, an oxidative addition reaction, of the catalytic cycle. On the other hand, very little is known about the reverse reaction, carbon-halogen reductive elimination from a transition-metal center. In this Concept article, we describe the examples of C(sp(3))-halide and C(sp(2))-halide reductive-elimination reactions which demonstrate that this type of reactivity can be quite common in organometallic chemistry. Although the thermodynamic driving force for the formation of carbon-halogen bonds is relatively small, the kinetic barrier for these reactions can also be low. Thus, C-halide reductive elimination can compete favorably with the more established organic transformations, such as C-C reductive elimination.     

The effect of anions on hydrogen chemisorption and oxide formation on Pt in aqueous acids

An approximate treatment shows that the superficial area of a mercury pool overlain by an aqueous electrolyte solution and contained within a cylindrical vessel of radius R (not less than about 12 mm) is given by πR²+2.603 aR+0.26 a², where a is a constant in the range 2.4±0.2 mm.     

Hydrogen bonding. Part 84. Infrared spectral evidence for three-center NHF bonds in a variety of tertiary amine-hydrogen fluoride compounds

N,N-dimethyl-1-adamantamine-hydrogen fluoride (2MAAHF) was previously shown by infrared spectral comparison to known compounds, ab initio molecular orbital treatment, and FT-NMR studies to contain a covalent three-center NHF bond which does not dissociate in aqueous solution. We have now examined a number of 1:1 hydrogen fluoride compounds of other tertiary amines, including monocyclic, bicyclic, aromatic, and trialkyl examples. All give the distinctive infrared absorptions of NHF bonds similar to those of 2MAAHF. Since these compounds were isolated from—and may well be stable in—aqueous solution, such species could play a role in the interaction of hydrogen fluoride or fluoride ion with biological material.     

Controlled formation of peptide bonds in the gas phase

Photoexcitation (using 157 nm vacuum ultraviolet radiation) of proton-bound peptide complexes leads to water elimination and the formation of longer amino acid chains. Thus, it appears that proton-bound dimers are long-lived intermediates along the pathway to peptide formation. Product specificity can be controlled by selection of specific complexes and the incorporation of blocking groups at the N- or C-termini. The product peptide sequences are confirmed using collision-induced dissociation.     

Redox-dependent formation of disulfide bonds in human replication protein A

Human replication protein A (RPA) is a single-stranded DNA (ssDNA)-binding protein with three subunits. The largest subunit, p70, contains a conserved (cysteine)(4)-type zinc-finger motif that has been implicated in the regulation of DNA replication and repair. Previous studies indicated that the ssDNA-binding activity of RPA could be redox-regulated via reversible oxidation of cysteines in the zinc-finger motif. We exposed recombinant human RPA to hydrogen peroxide and characterized the oxidized protein by liquid chromatography/tandem mass spectrometric (LC/MS/MS) analyses. Our results demonstrated that, upon H(2)O(2) treatment, four cysteines, which reside at the zinc-finger motif of the p70 subunit, could result in the formation of two pairs of intramolecular disulfides, Cys481-Cys486 and Cys500-Cys503; no cysteine sulfinic acid or cysteine sulfonic acid could be found. Moreover, the other 11 cysteines in this protein remained intact. The results demonstrated that the formation of disulfide bonds at the zinc-finger site was responsible for the redox regulation of the DNA-binding activity of RPA.     

Parallelization of three-center electron repulsion integrals

The parallelization of the three-center electron repulsion integrals arising from the variational fitting of the Coulomb potential is presented. A scheme for dynamical load balancing of the corresponding loop structure is discussed. The implementation in the density functional theory program deMon using the message passing interface is described. The efficiency of the parallelization is analyzed by selected benchmark calculations     

Semi-asymptotic evaluation of certain three-center two-electron integrals

Two-electron repulsion integrals between a two-center charge distribution and a charge distribution about a third center, which do not appreciably interpenetrate, are shown to be given to useful accuracy by numerical differentiation of certain three-center one-electron integrals. This method also may be used to evaluate integrals of this type for which the Mulliken or Sklar approximations are inapplicable.Supported by Contract SD-102 with the Advanced Research Projects Agency.     

The stabilization of centers with six valence electrons through electron-high three-center bonding

A possible method to stabilize a singlet state of a nitrene (or any other 6-valence electron center) is to approach it in a symmetrical manner with two lone pairs. The resultant electron-rich 3-center bond is generally unstable with respect to a 2-center bond. The 3-center geometry may however be stabilized when the atoms involved carry low lying d orbitals. The electronic structure of thiothiophthenes is investigated in context and a general analysis of the feasibility of electron-rich 3-center bonding in the first and second rows of the periodic table is presented.     

One-step CN, CO bonds cleavage and CO, CN bonds formation over supported ionic liquid in water

At ambient reaction temperature, the silica gel confined ionic liquid catalysts were perfectly combined with water as an effective catalytic system for simultaneous CN and CO bonds transformation with a TONs exceeding 300 mol mol⁻¹.