Donation Of Electron Density Research Articles

The gas-phase bis-uranyl nitrate complex [(UO 2) 2(NO 3) 5] −: Infrared spectrum and structure

The infrared spectrum of the bis-uranyl nitrate complex [(UO 2) 2(NO 3) 5] − was measured in the gas phase using multiple photon dissociation (IRMPD). Intense absorptions corresponding to the nitrate symmetric and asymmetric vibrations, and the uranyl asymmetric vibration were observed. The nitrate ν 3 vibrations indicate the presence of nitrate in a bridging configuration bound to both uranyl cations, and probably two distinct pendant nitrates in the complex. The coordination environment of the nitrate ligands and the uranyl cations were compared to those in the mono-uranyl complex. Overall, the uranyl cation is more loosely coordinated in the bis-uranyl complex [(UO 2) 2(NO 3) 5] − compared to the mono-complex [UO 2(NO 3) 3] −, as indicated by a higher O–U–O asymmetric stretching ( ν 3) frequency. However, the pendant nitrate ligands are more strongly bound in the bis-complex than they are in the mono-uranyl complex, as indicated by the ν 3 frequencies of the pendant nitrate, which are split into nitrosyl and O–N–O vibrations as a result of bidentate coordination. These phenomena are consistent with lower electron density donation per uranyl by the nitrate bridging two uranyl centers compared to that of a pendant nitrate in the mono-uranyl complex. The lowest energy structure predicted by density functional theory (B3LYP functional) calculations was one in which the two uranyl molecules bridged by a single nitrate coordinated in a bis-bidentate fashion. Each uranyl molecule was coordinated by two pendant nitrate ligands. The corresponding vibrational spectrum was in excellent agreement with the IRMPD measurement, confirming the structural assignment.

A Density Functional Theory and Quantum Theory of Atoms-in-Molecules Analysis of the Stability of Ni(II) Complexes of Some Amino Alcohol Ligands

The structure of the complexes of the type [Ni(L)(H(2)O)(2)](2+), where L is an amino alcohol ligand, L = N,N'-bis(2-hydroxyethyl)-ethane-1,2-diamine (BHEEN), N,N'-bis(2-hydroxycyclohexyl)-ethane-1,2-diamine (Cy(2)EN), and N,N'-bis(2-hydroxycyclopentyl)-ethane-1,2-diamine, (Cyp(2)EN) were investigated at the X3LYP/6-31+G(d,p) level of theory both in the gas phase and in solvent (CPCM model) to gain insight into factors that control the experimental log K(1) values. We find that (i) analyses based on Bader's quantum theory of atoms in molecules (QTAIM) are useful in providing significant insight into the nature of metal-ligand bonding and in clarifying the nature of weak "nonbonded" interactions in these complexes and (ii) the conventional explanation of complex stability in these sorts of complexes (based on considerations of bond lengths, bite angles and H-clashes) could be inadequate and indeed might be misleading. The strength of metal-ligand bonds follows the order Ni-N > Ni-OH ≥ Ni-OH(2); the bonds are predominantly ionic with some covalent character decreasing in the order Ni-N > Ni-OH > Ni-OH(2), with Ni-OH(2) being close to purely ionic. We predict that the cis complexes are preferred over the trans complexes because of (i) stronger bonding to the alcoholic O-donor atoms and (ii) more favorable intramolecular interactions, which appear to be important in determining the conformation of a metal-ligand complex. We show that (i) the flexibility of the ligand, which controls the Ni-OH bond length, and (ii) the ability of the ligand to donate electron density to the metal are likely to be important factors in determining values of log K(1). We find that the electron density at the ring critical point of the cyclopentyl moieties in Cyp(2)EN is much higher than that in the cyclohexyl moieties of Cy(2)EN and interpret this to mean that Cyp(2)EN is a poorer donor of electron density to a Lewis acid than Cy(2)EN.

Kinetics and molecular orbital calculations of desulfonation of strong acid cation-exchange resin

This paper deals with the desulfonation properties of some strong acid cation-exchange resins. The sulfate concentration in solution is continuously increased when a strong acid cation-exchange resin is mixed with water. The leaching of sulfate results from the desulfonation of the fixed group, and the amount of leached sulfate depends on the counter ion charge, the crosslinking degree and the exchanger matrix. The effects of the counter ion charge on the desulfonation rate suggested that the counter ion induces the nucleophilic attack of a water molecule on the sulfo group. This interpretation was supported by semiempirical molecular orbital calculations for the C 6H 5SO 3 −M m+ (M m+ = Na +, Mg 2+ and Al 3+) systems, and the transition state of the Na + system was successfully predicted by DFT calculations. The crosslinking degree influenced the desulfonation rate, which can be related to the decreasing hydration number of each counter ion in the resin phase with the increasing crosslinking degree. Furthermore, different exchanger matrices produced the differences in the rates, which may be derived from the electron-density donation from the exchanger matrix to the sulfo group. The desulfonation is governed by the electron-density of the sulfur atom and the water activity in the solid phase.

Rhodium carbonyl complexes of bulky, anionic nitrogen-based ligands: A comparison of donating ability

Phosphinimine-imine ((C 6H 3 i-Pr 2)NC(Me)CH 2PPh 2(NC 6H 3 i-Pr 2))Rh(CO) 2 – ( 1) and β-diketiminate CH(C(Me)(N i-Pr 2C 6H 3)) 2Rh(CO) 2 – ( 2) rhodium dicarbonyl complexes were prepared as to elucidate any difference among these anionic, nitrogen-based ligands regarding donating ability to the rhodium center. Utilizing infrared spectroscopy and single crystal structural comparisons, differences in electron density donation by the ligands to the rhodium center were not observed. The carbonyl stretching frequencies of the aforementioned rhodium complexes were ν CO = 2055, 1987 and 2055, 1988 for the phosphinimine-imine ( 1) and β-diketiminate ( 2) respectively.

Heterogeneous Mercury Reaction Chemistry on Activated Carbon

ABSTRACT Experimental and theory-based investigations have been carried out on the oxidation and adsorption mechanism of mercury (Hg) on brominated activated carbon (AC). Air containing parts per billion concentrations of Hg was passed over a packed-bed reactor with varying sorbent materials at 140 and 30 °C. Through X-ray photoelectron spectroscopy surface characterization studies it was found that Hg adsorption is primarily associated with bromine (Br) on the surface, but that it may be possible for surface-bound oxygen (O) to play a role in determining the stability of adsorbed Hg. In addition to surface characterization experiments, the interaction of Hg with brominated AC was studied using plane-wave density functional theory. Various configurations of hydrogen, O, Br, and Hg on the zigzag edge sites of graphene were investigated, and although Hg-Br complexes were found to be stable on the surface, the most stable configurations found were those with Hg adjacent to O. The Hg-carbon (C) bond length ranged from 2.26 to 2.34 Å and is approximately 0.1 Å shorter when O is a nearest-neighbor atom rather than a next-nearest neighbor, resulting in increased stability of the given configuration and overall tighter Hg-C binding. Through a density of states analysis, Hg was found to gain electron density in the six p-states after adsorption and was found to donate electron density from the five s-states, thereby leading to an oxidized surface-bound Hg complex. IMPLICATIONS The design of Hg capture sorbents from air has application to Hg removal from dentist offices because of the elevated elemental mercury vapor concentrations in these environments from amalgam use or replacement. This work serves as the foundation for potential optimization of AC's properties to make this capture technology more effective and potentially less costly

Edge- versus Vertex-Inversion at Trigonal Pyramidal Ge(II) Centers—A New Aromatic Anchimerically Assisted Edge-Inversion Mechanism

Theoretical calculations reveal that the model phosphagermylenes {(Me)P(C₆H₄-2-CH₂NMe₂)}GeX [X = F (1F), Cl (1Cl), Br (1Br), H (1H), Me (1Me)], which are chiral at both the phosphorus and pyramidal germanium(II) centers, may be subject to multiple inversion pathways which result in interconversion between enantiomers/diastereomers. Inversion via a classical vertex-inversion process (through a trigonal planar transition state) is observed for the phosphorus center in all compounds and for the germanium center in 1H, although this latter process has a very high barrier to inversion (221.6 kJ mol⁻¹); the barriers to vertex-inversion at phosphorus increase with decreasing electronegativity of the substituent X. Transition states corresponding to edge-inversion at germanium (via a T-shaped transition state) were located for all five compounds; for each compound two different arrangements of the substituent atoms [N and X axial (1X(N-X)) or P and X axial (1X(P-X))] are possible, and two distinct transition states were located for each of these arrangements. In the first of these (1X(N-X)(Planar) and 1X(P-X)(Planar)), inversion at germanium is accompanied by simultaneous planarization at phosphorus; these transition states are stabilized by pπ-pπ interactions between the phosphorus lone pair and the vacant p(z)-orbital at germanium. In the alternative transition states (1X(N-X)(Folded) and 1X(P-X)(Folded)), the phosphorus atoms remain pyramidal and inversion at germanium is accompanied by folding of the phosphide ligand such that there are short contacts between germanium and one of the ipso-carbon atoms of the aromatic ring. These transition states appear to be stabilized by donation of electron density from the π-system of the aromatic rings into the vacant p(z)-orbital at germanium. The barriers to inversion via 1X(P-X)(Planar) and 1X(P-X)(Folded) are rather high, whereas the barriers to inversion via 1X(N-X)(Planar) and 1X(N-X)(Folded) are similar to those for inversion at phosphorus, clearly suggesting that the most important factor in stabilizing these transition states is the σ-withdrawing ability of the substituents, rather than π-donation of lone pairs or donation of π-electron density from the aromatic rings into the vacant p(z)-orbital at germanium.

The influence of the structural and electronic characteristics of palladium on the activity and selectivity of Pd/Al2O3 and Pd-Co/Al2O3 catalysts for the hydrogenation of acetylene hydrocarbons

The influence of the structural and electronic characteristics of nonpromoted and cobalt-promoted Pd catalysts on their adsorption and catalytic properties is studied. It is shown that the conversion of vinylacetylene depends on the dispersion of palladium for both types of catalysts synthesized from acetate and acetylacetonate complexes. The palladium acetylacetonate catalysts have a higher palladium dispersion than the samples obtained from acetate complex solutions, thus leading to a higher conversion of vinylacetylene. It is established that the selectivity of vinylacetylene conversion into 1,3-butadiene on palladium acetate and acetylacetonate catalysts depends on the state of the 3d orbitals of surface Pd atoms. The palladium acetate catalysts are characterized by a higher electron density on the 3d orbital in comparison with the acetylacetonate samples, thus producing higher selectivities of vinylacetylene conversion into 1,3-butadiene. The introduction of cobalt into Pd/δ-Al2O3 catalyst synthesized from acetylacetonate complex leads to the formation of bimetallic Pd-Co particles, in which Pd atoms have higher electron density than those in the nonpromoted Pd/δ-Al2O3 catalyst, due probably to the donation of electron density from promoter atoms, with a resulting decline in the adsorption ability of bimetallic particles with regard to 1,3-butadiene and hydrogen. As a consequence, the selectivity of vinylacetylene conversion into 1,3-butadiene increases. Requirements for the size, dispersion, and electronic characteristics of the active component in the catalysts for the selective hydrogenation of vinylacetylene are formulated, and two techniques for their synthesis are proposed.

Palladium(II) and platinum(II) complexes containing benzimidazole ligands: Molecular structures, vibrational frequencies and cytotoxicity

(1H-benzimidazol-2-ylmethyl)-(4-methoxyl-phenyl)-amine (L 1), (1H-benzimidazol-2-ylmethyl)-(4-methyl-phenyl)-amine (L 2) and their Pd(II) and Pt(II) complexes have been synthesized as potential anticancer compounds and their structures were elucidated using a variety of physico-chemical techniques. Theoretical calculations invoking geometry optimization, vibrational assignments, 1H NMR, charge distribution and molecular orbital description HOMO and LUMO were done using density functional theory. Natural bond orbital analysis (NBO) method was performed to provide details about the type of hybridization and the nature of bonding in the studied complexes. Strong coordination bonds (LP(1)N11 → σ *(M Cl22)) and (LP(1)N21 → σ *(M Cl23)) (M = Pd or Pt) result from donation of electron density from a lone pair orbital on the nitrogen atoms to the acceptor metal molecular orbitals. The experimental results and the calculated molecular parameters revealed square-planar geometries around the metallic centre through the pyridine-type nitrogen of the benzimidazole ring and secondary amino group and two chlorine atoms. The activation thermodynamic parameters were calculated using non-isothermal methods. The synthesized ligands, in comparison to their metal complexes were screened for their antibacterial activity. In addition, the studied complexes showed activity against three cell lines of different origin, breast cancer (MCF-7), Colon Carcinoma (HCT) and human heptacellular carcinoma (Hep-G2) comparable to cis-platin.

Unprecedented binding and activation of CS2 in a dinuclear copper(i) complex

The first structural characterisation of a copper-carbondisulfide complex revealed a hitherto unknown binding mode for CS(2): it interacts with two metal centres (Cu(I)) simultaneously via both C=S π bonds. DFT calculations showed that complex formation occurs mainly due to a donation of electron density from the copper centres into the C=S π* orbitals.

Transition metals as Lewis bases: “Z-type” boron ligands and metal-to-boron dative bonding

While the vast majority of inorganic chemistry involves electron donation from main-group atoms to metals, an intriguing yet flip-side exists: where Lewis-basic metals donate electron density to Lewis-acidic main-group atoms (most often boron). These so-called "Z-type" ligands, along with other less clear-cut complexes, are examples of this metal-ligand role reversal. This perspective article offers an introduction to metal-to-boron dative bonding, and attempts to correlate spectroscopic and structural data from the complexes.

Nucleophilic Scandium Carbene Complexes

Using a geminal dianion as precursor, a nucleophilic scandium carbene complex (2) has been synthesized by salt metathesis on ScCl(3)(THF)(3) in 52% isolated yield. The X-ray structure as well as an NBO analysis points to a double interaction between the carbon and the scandium atoms. Quantification of the electron density donation from the carbon to the metal center, from both σ and π symmetry orbitals, predicts a "nucleophilic carbene" behavior. Addition of benzophenone on complex 2 results in the formation of the expected alkene derivative and the trapping of a rare, μ(3)-oxo-Sc species via a "scandia-Wittig" reaction.

Inorganic and Transition Metal Azides

Experimental and theoretical studies show that all covalent azides possess a nonlinear azide group. They also rationalize this remarkable structural feature. We have seen that the most important non-covalent contributions in the covalently bound azides system (X-N1-N2-N3) are the π-delocalization over the entire molecule and a strong negative hyperconjugation which donates electron density from the filled σ (X-N1) orbital into the unfilled, antibonding π* (N2-N3) orbital. For transition metal azide complexes, a bent configuration and a small difference between the N-N bond lengths, generally the longer one being adjacent to the transition metal, were observed.

Synthesis of neutral gold(III) pyrimidine-complexes and theoretical studies on the proton affinity of the coordinated ligands

The neutral gold(III) complexes AuCl 3(pm) (pm = pyrimidine, 2-methylpyrimidine, 4-methylpyrimidine, 5-methylpyrimidine, 4,6-dimethylpyrimidine, 2-aminopyrimidine) have been synthesised and characterised. The proton affinity (PA) values of the free and coordinated N-heterocycles have been theoretically estimated on the basis of DFT calculations. The coordination to the AuCl 3 metal fragment causes a strong lowering of the basicity of the residual protonation sites, with PA variations around −25 kcal mol −1. The lowering of PA caused by the bonding to the gold centre results related to the presence of space-demanding groups near the coordinating nitrogen atom and to the π-back donation of electron density from the metal to the protonated ligand.

In alkaline media, Fremy’s salt oxidizes alkanols by a hydrogen atom transfer mechanism

In aqueous alkali, Fremy’s salt (potassium nitrosodisulfonate dimer), homolyses nearly exclusively to the monomer radical anion, nitrosodisulfonate (NDS). In this media, NDS almost quantitatively oxidizes benzyl alcohol (PhCH 2OH) to benzaldehyde (PhCHO), itself being reduced to hydroxylamine disulfonate (HNDS). The reaction is very nearly first-order in [NDS], [alkanol] and in [OH −]. However, with progressive addition of HNDS, decay kinetics of NDS gradually deviates from first-order. Ultimately, with sufficient excess of HNDS, the reaction becomes second-order in [NDS]. The consumption ratio, (ΔPhCH 2OH]/Δ[NDS]), is ∼2. PhCD 2OH manifests a large primary kinetic isotope effect ( k H/ k D = 11.6). Substituted benzyl alcohols (RBzCH 2OH) with R-groups withdrawing electron density from the O–H bond accelerated the reaction; those with R-groups donating electron density to the O–H bond retarded the reaction. The conversion of 2-propanol to 2-propanone is much slower compared to that of benzyl alcohol to benzaldehyde. An alpha-H atom transfer mechanism seems logical.

Stereodirecting Effect of the Pyranosyl C-5 Substituent in Glycosylation Reactions

The stereodirecting effect of the glycosyl C-5 substituent has been investigated in a series of d-pyranosyl thioglycoside donors and related to their preferred positions in the intermediate (3)H(4) and (4)H(3) half-chair oxacarbenium ions. Computational studies showed that an axially positioned C-5 carboxylate ester can stabilize the (3)H(4) half-chair oxacarbenium ion conformer by donating electron density from its carbonyl function into the electron-poor oxacarbenium ion functionality. A similar stabilization can be achieved by a C-5 benzyloxymethyl group, but the magnitude of this stabilization is significantly smaller than for the C-5 carboxylate ester. As a result, the preference of the C-5 benzyloxymethyl to occupy an axial position in the half-chair oxacarbenium ions is much reduced compared to the C-5 carboxylate ester. To minimize steric interactions, a C-5 methyl group prefers to adopt an equatorial position and therefore favors the (4)H(3) half-chair oxacarbenium ion. When all pyranosyl substituents occupy their favored position in one of the two intermediate half-chair oxacarbenium ions, highly stereoselective glycosylations can be achieved as revealed by the excellent beta-selectivity of mannuronate esters and alpha-selectivity of 6-deoxygulosides.

Interaction of Organoplatinum(II) Complexes with Monovalent Coinage Metal Triflates

The organoplatinum(II) complexes [(NN)PtMe(2)] and [(NN)PtPh(2)] (NN = ArNC(Me)C(Me)NAr, Ar = 2,6-dichlorophenyl) can act as donor ligands for copper(I) and silver(I) triflates, affording a series of homo- and heteroleptic complexes which were characterized by X-ray diffraction. [(NN)PtMe(2)] binds to the coinage metals through short, ligand-unsupported d-d(10) contacts that are best described as Pt-->M dative bonds (M = Cu, Ag), in which the d(z(2)) orbital of the square-planar Pt(II) center donates electron density to the Lewis-acidic metal. Spectroscopic studies in solution and DFT calculations corroborate this description. [(NN)PtPh(2)] binds preferentially by eta(1) or eta(2) complexation of the ipso carbon atoms of the phenyl groups to the coinage metal, affording homoleptic complexes {[(NN)PtPh(2)](2)M}(+){TfO}(-) in the solid state. The 1:1 adducts of formula {[(NN)PtPh(2)]M(OTf)}(n) (M = Cu, n = 1; M = Ag, n = 2) are observed in solution, and a 1:2 adduct of formula {[(NN)PtPh(2)]Ag(2)(OTf)(2)(C(6)H(6))}(n) was characterized in the solid state, showing that unsupported Pt-->M bonds are also accessible for [(NN)PtPh(2)]. The thermolyses of the complexes [(NN)PtMe(2)]MOTf in benzene affords moderate yields of [(NN)PtPh(2)] through an oxidatively induced double C-H activation process.

Low-temperature synthesis of supported hydrodesulfurization catalysts based on chevrel phases

A new method has been developed for the synthesis of finely dispersed, highly active, supported hydrodesulfurization catalysts based on Chevrel phases. It is hypothesized that the modification of MoS2 with cobalt or nickel, which enhances the catalytic activity, and the same modification of Chevrel-type systems are of the same nature. The modifiers act through electron density donation into the conduction band of the active component. The increase in catalytic activity is due to the decrease of the effective charge of the molybdenum ion. The catalysts undergo partial restructuring during the reaction.

Acceleration effect of sulfate ion on the dissolution of amorphous silica

The dissolution rate of amorphous silica is enhanced by sulfate ions. The zeta potential for silica particles in Na 2SO 4 solution was lower than that in NaCl solution with the same ionic strength. These facts indicate that the specific adsorption of sulfate ions occurred by overcoming repulsion between negative charges of the SO 2− 4 ion and Si O − on the surface of silica. The dissolution rate of amorphous silica may be accelerated by the specific adsorption of SO 2− 4 ions because of a decrease in the strength of the Si O Si bond in amorphous silica due to donation of electron density from the adsorbed SO 2− 4 ions.

A structural survey of metal⋯π heteroaromatic supramolecular synthons for metal = tellurium, tin, and gold

A survey of tellurium, tin and gold structures containing metal⋯π heteroaromatic interactions has been conducted, and their occurrence correlated with previously described metal⋯π aryl interactions. Metal⋯π heteroaromatic interactions are found for all three metals when the heteroaromatic ring was a pyridine/pyridine-derivative, and these contacts lead usually to 0-D or 1-D aggregates with rare examples of 2-D and 3-D architectures. For each of tellurium and tin, metal⋯π aryl interactions are more likely to form over metal⋯π pyridine interactions, while the reverse is true in the case of gold. These observations are readily correlated with the influence of increasing electron density in the pyridine compared with an aryl ring. In the case of tellurium and tin, for which the metal⋯π interaction involves a metal-based lone pair of electrons being donated to the LUMO of the aryl group, increasing the electron density of the aromatic ring naturally decreases the probability of forming this type of interaction. In the case of gold, where the metal⋯π interaction involves the donation of electron density from the aromatic ring to the gold centre, increasing the electron density of the aromatic ring favours such an interaction. At least for the metals surveyed, metal⋯π interactions are observed in a significant number of their crystal structures, for example up to 6% of all tellurium structures feature Te⋯π aryl interactions as a supramolecular synthon.

Carbenes Stabilized by Ylides: Pushing the Limits

Being neighborly: The concept of placing two reactive intermediates next to each other to gain stability is borne out in amino(ylide)carbenes (AYCs). Different kinds of ylides stabilize an adjacent singlet carbene by donation of electron density from the ylidic carbon atom into the empty p orbital to give AYCs of various structural formats (see picture) that are distinguished by their exceptionally strong σ-donor qualities.