Trigonal Bipyramidal Structure Research Articles

SYNTHESIS AND CHARACTERIZATION OF NOVEL ORGANOBISMUTH FOR ANTIMICROBIAL AND ANTITUMOR STUDIES

Objectives: The major objective of this manuscript is to present synthesis and biomedical screening of some organic derivatives of bismuth having general formula (R3BiL2) by the method reported and characterized with the help of M.P., elemental, I.R., and NMR spectral analysis along with their antimicrobial and in vitro antitumor activity against human breast (MCF-7) and mammary cancer (EVSA-7) cell line.
 Methods: All the newly organobismuth having general formula [R3BiL2] were synthesised by the method reported especially using oxidative addition and complexation reactions.
 Results: It was found that organobismuth compounds have trigonal bipyramidal structure as per their elemental and spectral analysis and show potentiality as antimicrobial and antitumor agents.
 Conclusion: The newly synthesized organobismuth(V)substituted carboxylates were fully characterized chemically to ascertain their structure by sophisticated instrumental and spectral analysis resulted as trigonal bipyramidal structure. The compounds were also screened 1st time for antitumor and antimicrobial studies. The observations clearly indicated that organobismuth carboxylates show potent antimicrobial and antitumor activity.

Synthesis, structural, spectroscopic, and thermal studies of some transition‐metal complexes of a ligand containing the amino mercapto triazole moiety

A new series of transition‐metal complexes of Schiff base ligand containing the amino mercapto triazole moiety (HL) was prepared. The Schiff base and its metal complexes were elucidated by different spectroscopic techniques (infrared [IR], 1H nuclear magnetic resonance, UV–Visible, mass, and electron spin resonance [ESR]), and magnetic moment and thermal studies. Quantum chemical calculations have been carried out to study the structure of the ligand and some of its complexes. The IR spectra showed that the ligand is chelated with the metal ion in a neutral, tridentate, and bidentate manner using NOS and NO donors in complexes 1–6, 10–12, and 7 and 8, respectively, whereas it behaves in a monobasic tridentate fashion using NOS donor sites in copper(II) nitrate complex (9). The magnetic moment and electronic spectra data revealed octahedral and square pyramidal geometries for complexes 2, 11, 12, and 5–8 and 10, respectively. However, the other complexes were found to have tetrahedral (4), trigonal bipyramidal (1 and 3), and square planar (9) structures. Thermal studies revealed that the chelates with different crystallized solvents undergo different types of interactions and the decomposition pathway ended with the formation of metal oxygen (MO) and metal sulfur (MS) as final products. The ESR spectrum of copper(II) complex 10 is axial in nature with hyperfine splitting with 2B1g as a ground state. By contrast, complexes 7 and 8 undergo distortion around the Cu(II) center, affording rhombic ESR spectra. The HL ligand and some of its complexes were screened against two bacterial species. Data showed that complex 12 demonstrated a better antibacterial activity than HL ligand and other chelates.

Synthesis, characterization and biological screenings of 5-coordinated Organotin(IV) complexes based on carboxylate ligand

This article focus on the synthesis of three 5-coordinated Sn(IV) complexes based on carboxylate ligand: 4-((4-methoxy-2-nitrophenyl)amino)-4-oxobutanoic acid. The complexes were characterized both in solid state (FT-IR, single crystal XRD and micro elemental analysis) as well as in solution state (1H, 13C and 119Sn NMR) and their results demonstrate that the carboxylate moiety of the ligand is linked with Sn atom. A 5-coordinated trigonal bipyramidal structure was obtained from crystallographic data for the reported complexes with little bit distortion from the ideal structure. Preliminary antifungal screening showed that the tested compounds have 100% fungal inhibition capability as shown by the well known antifungal drug, Terbinafine. Bacterial inhibition results showed that these compounds have outstanding inhibition capability against various pathogens as compared to the market available standard antibacterial drugs, Cefixime and Roxithromicine. The antiproliferative/anticancer result against H157 cancer cell line revealed that compounds 1 and 2 have shown the maximum activity (49–69% cytotoxicity even at 2 μg/mL final concentration). The tested compounds were found active against human corneal epithelial cells (HCEC). The synthesized compounds were almost non-toxic towards HCEC. Compounds 1 and 2 have shown 6.9% and 7.9% antiproliferative activity, respectively when used at the 2.0 μg/mL final concentration while compound 3 is totally non-toxic towards HCEC like the standard drug, Methotrexate. The antileishmanial results of these compounds are good though their activity is lower than that of the Amphotericin B. An intercalative mode of interaction was observed during the interaction of the titled compounds with DNA.

Evaluation of Zn2+ Coordination Structures in Chiral Zn2+ Complexes Based on Shape Measurement Factors: Relationships between Activity and the Coordination Structure

We previously reported on enantioselective aldol reactions of acetone and cyclic ketones with benzaldehyde derivatives catalyzed by chiral Zn2+ complexes that mimic class II aldolases. The results and mechanistic studies indicated that the catalytic activity of Zn2+ complexes is dependent not only on the coordination numbers of Zn2+ but also on its coordination structure. In this study, we report on shape measures (abbreviated as S values in this manuscript) of such Zn2+ complexes with respect to the 5‐coordination structure based on calculations with continuous shape measures (CShM) using their crystal structures. The S values of Zn2+ ions in Zn2+ complexes for trigonal bipyramidal (D3h) or square pyramidal (C4v) structures suggest that Zn2+ ions in the catalytically active Zn2+ complexes possess D3h and C4v structures, while Zn2+ ions in the unreactive Zn2+ complex has a C4v structure. Moreover, the S values of Zn2+ were calculated with respect to 6‐coordination geometry, based on the assumption that aldol reactions proceed via 6‐membered transition states, and it was implied that 6‐coordinated Zn2+ ions in aldol‐active Zn2+ complexes prefer an octahedron structure (Oh). These results for the S values of Zn2+ ions in the active Zn2+ complexes and those for complexes reported by other research groups suggest that the transition of 5‐coordinated D3h to the 6‐coordinated Oh structure is essential for achieving catalytic stereoselective aldol reactions.

On the Stability of Cu5 Catalysts in Air Using Multireference Perturbation Theory.

An ab initio study of the interaction of O2, the most abundant radical and oxidant species in the atmosphere, with a Cu5 cluster, a new generation atomic metal catalyst, is presented. The open-shell nature of the reactant species is properly accounted for by using the multireference perturbation theory, allowing the experimentally confirmed resistivity of Cu5 clusters toward oxidation to be investigated. Approximate reaction pathways for the transition from physisorption to chemisorption are calculated for the interaction of O2 with quasi-iso-energetic trapezoidal planar and trigonal bipyramidal structures. Within the multireference approach, the transition barrier for O2 activation can be interpreted as an avoided crossing between adiabatic states (neutral and ionic), which provides new insights into the charge-transfer process and gives better estimates for this hard to localize and therefore often neglected first intermediate state. For Cu5 arranged in a bipyramidal structure, the O–O bond cleavage is confirmed as the rate-determining step. However, for planar Cu5, the high energy barrier for O2 activation, related to a very pronounced avoided crossing when going from physisorption to chemisorption, determines the reactivity in this case.

Structure of the SiCl4←O=C[N(CH3)2]2 Complex and Electron Density Redistribution upon Its Formation according to ab initio Calculations

To determine the structure of the SiCl4 complex with tetramethylurea, its possible structures of composition 1 : 1 and 1 : 2 have been calculated by the RHF, B3LYP, and МР2 methods with the 6-31G(d) basis set. It has been demonstrated that this complex has the 1 : 1 complex and a trigonal-bipyramidal structure with an uncommon equatorial rather than axial location. The formation of the complex is accompanied by electron density transfer from the hydrogen atoms, carbonyl carbon, and nitrogen atoms the electron donor to the chlorine atoms of the acceptor, as well as to the carbonyl oxygen atom and methyl carbon atoms of the electron donor itself. No electron density transfer from the coordination center of the donor to the coordination center of the acceptor is observed.

Frustration phenomenon in the spin-1/2 Ising–Heisenberg planar model of inter-connected trigonal bipyramid structures

Ground-state and finite-temperature properties of the exactly solvable mixed spin-1/2 Ising–Heisenberg planar model composed of identical trigonal bipyramids that are arranged into a regular archimedean lattice are examined with the aim to clarify the frustration phenomenon at zero and finite temperatures. It is shown that the ground-state spin frustration persists even far above the second-order phase transition. If the interaction ratio between the Heisenberg and Ising exchange interactions is close enough to the ground-state boundaries between the neighboring phases, a remarkable re-entrance of the (non-)frustrated spin arrangement of the Heisenberg spins can be observed around the critical temperature of the model. It is also evidenced that entropy and specific heat show pronounced temperature variations not only around the critical temperature, but also in low-temperature regime if values of the interaction parameters are taken from neighborhood of the ground-state phase transitions, where energies of the neighboring phases are very close.

Jahn-Teller effect versus spin-orbit coupling: The structure of the free molybdenum pentafluoride molecule

The complete active space self-consistent field (CASSCF) and multiconfigurational quasi-degenerate second-order perturbation (MCQDPT2) calculations have been used to study the ground and low-lying excited electronic states of MoF5. Scalar-relativistic effects and spin-orbit coupling (SOC) have been taken into account employing the third-order Douglas-Kroll-Hess (DKH) Hamiltonian and full Breit-Pauli operator, respectively. The optimization and the calculation of the Hessian matrix have been performed by non-routine procedure employing the symmetry coordinates for all considered (D3h, C2v and C4v) geometrical configurations. The trigonal bipyramidal D3h structure of molybdenum pentafluoride MoF5 possesses two-fold degenerate ground first excited electronic orbital state and undergoes Jahn-Teller distortion to the C2v configurations. Accounting of SOC quenches the JT effect. As a result the D3h structure of MoF5 is no longer the conical intersection of two adiabatic potential energy surfaces (APES) but two separated APES corresponding to ground (12E1/2) and nearest excited (12E3/2) spin-orbit states are appeared instead. The D3h trigonal bipyramidal structure in the ground spin-orbit state corresponds to the 2-nd order saddle point on APES and lies above the minima (C2v structure, 12E1/2 state) by 244 cm−1. The warping barrier (another C2v structure) in the trough of APES and the Berry pseudorotation barrier (C4v structure, ground spin-orbit state 2E3/2) are 19 and 310 cm−1, respectively. Therefore MoF5 molecule is characterized in the gas phase by a large-amplitude motion of fluorine atoms in the equatorial plane and non-rigid intramolecular rearrangement by Berry mechanism. Despite such non-rigid nuclei dynamic, the calculated harmonic frequencies of MoF5 molecule do not contradict to available experimental IR spectrum. The repeat refinement of MoF5 gas electron diffraction data has been carried out using static and pseudoconformer models based on the SO-MCQDPT2 results. Corresponding to only extreme points on APES the configurations have been included into the pseudoconformer model. The both models have been fitted to the experimental data by the same quality.

4-(2-Methoxyphenyl)-5-(thiophen-2-ylmethyl)-2,4-Dihydro-1H-1,2,4-Triazole-3-Thione and Its Complex with Cadmium Chloride: Molecular and Crystal Structures

Compound 4-(2-methoxyphenyl)-5-(thiophen-2-ylmethyl)-2,4-dihydro-1H-1,2,4-triazole-3-thione (LН) and its complex with cadmium(II) C42.12H39.50N9O3.12S6Cl2Cd (I) are synthesized. Their molecular and crystal structures are determined by X-ray diffraction analysis (СIF files CCDC nos. 1835776 (LН) and 1547251 (I)). Free ligand LН is thione and in the crystal forms H-bonded centrosymmetric dimers by two hydrogen bonds N–H···S. Complex I has a distorted trigonal bipyramidal structure: the sulfur atoms lie in the base and the chlorine atoms occupy the axial positions. Compound LН manifests itself as a monodentate ligand (S) when forming complex I and retains the thione form. The molecule of complex I is stabilized due to strong intramolecular contacts N–H···Cl. The crystal packing of compounds LН and I is formed due to the intermolecular contacts C–H···π, H···S, H···O, and H···N.

Theoretical prediction of Ni(I)-catalyst for hydrosilylation of pyridine and quinoline.

Catalytic synthesis of dihydropyridine by transition-metal complex is one of the important research targets, recently. Density functional theory calculations here demonstrate that nickel(I) hydride complex (bpy)NiI H (bpy = 2,2'-bipyridine) 1 is a good catalyst for hydrosilylation of both quinoline and pyridine. Two pathways are possible; in path 1, substrate reacts with 1 to form stable intermediate Int1. After that, N3 ─C1 bond of substrate inserts into Ni─H bond of 1 via TS1 to afford N-coordinated 1,2-dihydroquinoline Int2 with the Gibbs activation energy (ΔG°‡ ) of 21.8 kcal mol-1 . Then, Int2 reacts with hydrosilane to form hydrosilane σ-complex Int3; this is named path 1A. In the other route (path 1B), Int1 reacts with phenylsilane in a concerted manner via hydride-shuttle transition state TS2 to afford Int3. In TS2, Si atom takes hypervalent trigonal bipyramidal structure. Formation of hypervalent structure is crucial for stabilization of TS2 (ΔG°‡ = 17.3 kcal mol-1 ). The final step of path 1 is metathesis between Ni─N3 bond of Int3 and Si─H bond of PhSiH3 to afford N-silylated 1,2-dihydroproduct and regenerate 1 (ΔG°‡ = 4.5 kcal mol-1 ). In path 2, 1 reacts with hydrosilane to form Int5, which then forms adduct Int6 with substrate through Si-N interaction between substrate and PhSiH3 . Then, N-silylated 1,2-dihydroproduct is produced via hydride-shuttle transition state TS5 (ΔG°‡ = 18.8 kcal mol-1 ). The absence of N-coordination of substrate to NiI in TS5 is the reason why path 2 is less favorable than path 1B. Quinoline hydrosilylation occurs more easily than pyridine because quinoline has the lowest unoccupied molecular orbital at lower energy than that of pyridine. © 2019 Wiley Periodicals, Inc.

Scalable Geometrically Designed Protein Cages Assembled via Genetically Encoded Split Inteins

Engineering proteins to assemble into user-defined structures is key in their development for biotechnological applications. However, designing generic rather than bespoke solutions is challenging. Here we describe an expandable recombinant assembly system that produces scalable protein cages via split intein-mediated native chemical ligation. Three types of component are used: two complementary oligomeric "half-cage" protein fusions and an extendablemonomeric "linker" fusion. All are composed of modular protein domains chosen to fulfill the required geometries, with two orthogonal pairs of split intein halves to drive assembly when mixed. This combination enables both one-pot construction of two-component cages and stepwise assembly of larger three-component scalable cages. To illustrate the system's versatility, trimeric half-cages and linker constructs comprising consensus-designed repeat proteins were ligated in one-pot and stepwise reactions. Under mild conditions, rapid high-yielding ligations were obtained, from which discrete proteins cages were easily purified and shown to form the desired trigonal bipyramidal structures.

Spectroscopic, vibrational and structural properties analysis of CaXen (n = 1–4) clusters

A systematic ab initio calculations based on a complete Configuration Interaction (CI) including the pseudo-potential model where the core valence correlation was presented by the Core Polarization Potential (CPP), were employed to determine the geometric structures of CaXen (n = 1–4) neutral clusters. For the diatomic (CaXe) and tri-atomic (CaXe2) systems, the potential energy curves for the ground and several excited states are determined and the different spectroscopic constants are illustrated. Interested structures were seen in the PECs which their effects in the vibrational levels' spacing and the electric dipole moments variations were shown. The agreement with the experimental and the theoretical constants for n = 1 was excellent. Accordingly, for the higher clusters the predicted geometries should be reliable. The bent geometry of CaXe2 ground state with (C2v) symmetry was performed and hence was concluded to be the most adaptable with a bond angle of 54.84° and bond distance dXe-Xe of 8.08 a0. As well as, the bent isomer for the excited states was also studied to illustrate the most compact geometry for each state. For the ground state of CaXe3, the most stable geometry was trigonal-pyramidal with (C3v) symmetry with dXe-Xe = 8.13 a0. And concerned n = 4, four configurations were treated; the tetrahedral structure, the square pyramid structure, the trigonal bipyramid structure with (C3v) symmetry and the trigonal bipyramid structure with (C2v) symmetry. The comparison between their minimal energies showed that the most compact geometry for CaXe4 is the trigonal bipyramid with (C3v) symmetry with dXe-Xe = 8.17 a0. Consequently, this work illustrates that the mechanism of cluster forming is consistent with the calcium location on the xenon clusters' surface model.

B3Si12: A hexagonal prism with three short B[sbnd]B single bonds at the center coordinated by a Si12 cage and reinforced by aromaticity

The structures and properties of B3Si12ˉ/0/+ clusters were investigated by density functional theory calculations combined with mass-selected anion photoelectron spectroscopy. The experimental photoelectron spectrum of cluster anion is reasonably reproduced by theoretical simulations. The global minimum of B3Si12ˉ anion has a B3Si2 trigonal bipyramidal structure with ten capping Si atoms, whereas those of B3Si12 neutral and B3Si12+ cation adopt hexagonal prismatic structures with the B3 unit at the center. In particular, B3Si12ˉ anion is an outstanding closed-shell superatom cluster according to the jellium model (1S21P61D102S21F142P61G18). Furthermore, the NICS values of B3Si12ˉ, B3Si12, and B3Si12+ calculated at points 1 Å above the cage centers are to be −16.8, −56.8, and −51.1 ppm, respectively, suggesting that they are all aromatic.

First-principles study of NaxTiO2 with trigonal bipyramid structures: an insight into sodium-ion battery anode applications.

Developing efficient anode materials with low electrode voltage, high specific capacity and superior rate capability is urgently required on the road to commercially viable sodium-ion batteries (SIBs). Aiming at finding a new SIB anode material, we investigate the electrochemical properties of NaxTiO2 compounds with unprecedented penta-oxygen-coordinated trigonal bipyramid (TB) structures by using first-principles calculations. Identifying the four different TB phases, we perform the optimization of their crystal structures and calculate their energetics such as sodium binding energy, formation energy, electrode potential and activation energy for Na ion migration. The computations reveal that the TB-I phase is the best choice among the four TB phases for a SIB anode material due to a relatively low volume change of under 4% upon Na insertion, low electrode voltage under 1.0 V with a possibility of realizing the highest specific capacity of ∼335 mA h g-1 from full sodiation at x = 1, and reasonably low activation barriers under 0.35 eV at the Na content from x = 0.125 to x = 0.5. Through the analysis of electronic density of states and charge density difference upon sodiation, we find that the NaxTiO2 compounds in TB phases change from electron insulating to electron conducting materials due to the electron transfer from Na atoms to Ti ions, offering the Ti4+/Ti3+ redox couple for SIB operation.

Alkoxy(tetraaryl)silicates bearing 9,10‐disilatriptycene skeleton

AbstractAs anionic five‐coordinate silicon species (five‐coordinate silicates) exhibit high reactivity and unique fluxional behavior, their structure and property have received substantial attention. We report herein synthesis, structure, and fluxional behavior of alkoxy(tetraaryl)silicates having the 9,10‐disilatriptycene skeleton (6− and 7−). The molecular structure of the cryptate salt [Li(crypt‐222)]+7− shows that the five‐coordinate silicon adopts a distorted trigonal bipyramidal structure with oxygen atom occupying an equatorial position. In the 1H and 13C{1H} NMR spectra of [Li(crypt‐222)]+7−, a set of signals due to o‐phenylene blades in 7− was observed at room temperature, which indicates fluxional behavior of 7− on the NMR timescale. Computational study suggests that the observed process proceeds via a 120° rotation of bidentate ligand around the Si‐Si axis that consists of two sequential Berry pseudorotations.

Scalable Geometrically Designed Protein Cages Assembled via Genetically Encoded Split Inteins

Engineering proteins to assembly into user-defined structures is key in their development for biotechnological applications. However, designing generic rather than bespoke solutions is a challenging task. Here, we describe an expandable recombinant assembly system that produces scalable protein cages via split intein-mediated native chemical ligation. Three types of construct are used: a pair of oligomeric mirror-image “half-cages” and an extendable “linker. All constructs are composed of modular protein domains chosen to fulfil the required geometries, fused to two orthogonal pairs of split-intein halves to drive assembly when mixed. This combination enables both one-pot construction of two-component cages and stepwise assembly of larger three-component scalable cages. To illustrate the system’s versatility, tetrahedral half-cages and linker constructs comprising consensus-designed repeat proteins were ligated in one-pot and stepwise reactions. Under mild conditions rapid high yielding ligations were obtained, from which discrete proteins cages were easily purified and shown to form the desired trigonal bipyramidal structures.

DFT studies of isomerization in palladium(IV) chemistry and alkyl halide transfer from palladium(IV) to palladium(II)

A DFT study of alkyl halide exchange from Pd(IV) to Pd(II) centers supports proposals for an SN2 process in which nucleophilic Pd(II) square-planar complexes attack the axial alkyl group in electrophilic square-pyramidal Pd(IV) centers, leading to transfer of R+ from Pd(IV) to Pd(II) involving a transition structure of the form [R2(bpy)··R··PdR2 (bpy)]+ (bpy = 2,2′-bipyridine). Computation accounts for the selectivity in benzyl (Bn) over methyl transfer from PhMeBnPdIV(bpy)Br to Me2PdII(bpy), giving PhMePdII(bpy) and BnMe2PdIV(bpy). Transition structures are formed by the interaction of a dz2-like HOMO at the Pd(II) nucleophile and the Pd-C σ* LUMO at the cationic Pd(IV) electrophile. Isomerization of octahedral palladium(IV) complexes via halide loss followed by involvement of trigonal-bipyramidal transition structures to facilitate isomerization is also examined. A lower barrier for Bn+ than Me+ transfer is attributed to stabilisation of the transition structure by delocalisation of positive charge within the benzyl group, reflected in a higher total charge density for the benzyl group than found for the methyl group.

Secondary Oxide Phosphines to Promote Tandem Acyl-Alkyl Coupling/Hydrogen Transfer to Afford (Hydroxyalkyl)rhodium Complexes. Theoretical and Experimental Studies.

Acyl(σ-norbornenyl)rhodium(III) dimer [Rh(μ-Cl)(C9H6NCO)(C7H9)L]2 (1) (C7H9 = σ-norbornenyl; L = 4-picoline, isoquinoline) reacts with diphenylphosphine oxide (SPO) to undergo a one-pot reaction involving (i) cleavage of the chloride bridges and coordination of the phosphine, (ii) C-C bond coupling between acyl and norbornenyl in a 18e species, and (iii) ligand-assisted outer-sphere O(P)-to-O(C) hydrogen transfer, to afford mononuclear 16e species [RhCl{(C9H6NC(O)C7H9)(Ph2PO)H}(L)] (2) containing a quinolinyl-(norbornenylhydroxyalkyl) fragment hydrogen-bonded to a κ1- P-phosphinite ligand. Pentacoordinated 2, which adopt a distorted trigonal bipyramidal structure, are kinetic reaction products that transform into the thermodynamic favored isomers 3. Structures 3 contain an unusual weak η1-C anagostic interaction involving the rhodium atom and one carbon atom of the olefinic C-H bond of the norbornenyl substituent in the chelating quinolinyl-hydroxyalkyl moiety. Their structure can be described as pseudoctahedral, through a 5 + 1 coordination, with the anagostic interaction in a trans disposition with respect to the phosphorus atom of the phosphinite ligand. Complexes were characterized in solution by NMR spectroscopy and electrospray ionization mass spectrometry. Complex [RhCl{(C9H6NC(O)C7H9)(Ph2PO)H}(4-picoline)] (3a) was also identified by X-ray diffraction. Density functional theory calculations confirm the proposed structures by a plausible set of mechanisms that accounts for the 1 (monomer) → 2 → 3 transformation. Lowest-energy pathways involve reductive elimination of quinolinylnorbornenylketone, still coordinated in the rhodium(I) species thus formed, followed by O-to-O hydrogen transfer from κ1- P-SPO to the sp3 hybridized carbonyl group (formal alkoxide) avoiding the otherwise expected classical release of ketone. Theoretical 13C NMR studies also confirm the experimental spectral data for the considered structures.

Ni(I)-Hydride Catalyst for Hydrosilylation of Carbon Dioxide and Dihydrogen Generation: Theoretical Prediction and Exploration of Full Catalytic Cycle

We wish to report here the first theoretical prediction that a unique nickel(I) hydride complex NiIH(bpy) (bpy = 2,2′-bipyridine) 1 is a good catalyst for hydrosilylation of CO2 with hydrosilane (HSiR3) and dihydrogen (H2) generation via alcoholysis/hydrolysis of hydrosilane. Full catalytic cycles of these reactions were elucidated by density functional theory (DFT) calculations. In the CO2 hydrosilylation, the first step is CO2 insertion into the NiI–H bond of 1, which occurs easily to afford NiI(η1–OCOH)(bpy) species, in which charge transfer (CT) occurs from singly occupied molecular orbital (SOMO) corresponding to Ni–H antibonding orbital to LUMO of CO2. The next step is metathesis between NiI(η1–OCOH)(bpy) and HSiR3 to produce H(CO)OSiR3 and regenerate 1. This is rate-determining (ΔG°⧧ = 19.5 kcal mol–1). In the transition state of the metathesis, the Si atom takes a hypervalent trigonal-bipyramidal structure, suggesting that hydrosilane with electron-withdrawing substituents is good for this reaction. The use of 6,6′-dimethyl-2,2′-bipyridine (dmbpy), which has methyl groups at the ortho position to the N of bpy, substantially decreases the ΔG°⧧ value of the metathesis, suggesting that NiIH(dmbpy) is a better catalyst. In the dihydrogen generation via alcoholysis of hydrosilane by 1, the first step is metathesis between the O–H bond of ROH (R= Me or H) and the NiI–H bond of 1 to afford NiI(OR)(bpy) and dihydrogen molecule. Next step is the second metathesis between the NiI–OR bond and the Si–H bond of HSiR3 to yield R3Si(OR) and regenerate 1. The second metathesis occurs with a negligibly small ΔG°⧧ value, indicating that various hydrosilanes can be employed for this reaction. On the basis of these computational findings, it is predicted that 1 can catalyze two completely different reactions, CO2 hydrosilylation with hydrosilane and dihydrogen generation via alcoholysis/hydrolysis of hydrosilane.

Z-Scan Study on a Silver Nanoparticles Embedded TeO<sub>2</sub>-SiO<sub>2</sub> Glass Prepared by Sol-Gel Method

An inserted type Ag/TeO2-SiO2composite gel glass was prepared through silver nanoparticles embedded in TeO2-SiO2composite gel. The silver nanoparticles were prepared previously by ascorbic acid deoxygenates AgNO3solution and mixed into TeO2-SiO2composite sol which was prepared in aqueous system using TeO2and tetraethoxysilane as precursors, and then the inserted type Ag/TeO2-SiO2composite gel glass was produced through sol aging and molding. The silver nanoparticles which dispersed in the TeO2-SiO2gel substrate made the composite glass having excellent nonlinear optical properties. The nonlinear optical parameters was tested by Z-scan technique using 15 ps laser pulses at the wavelength of 532 nm. The surface morphology was characterized by SEM. The results identified that spherical Ag nanoparticles with the size about 50-100 nm dispersed in the TeO2-SiO2gel. In the Ag/TeO2-SiO2material, the doped structure and the trigonal bipyramid structure of TeO2was the important factor for the excellent nonlinear optical performance.