Oxide Ligands Research Articles

Gas-Phase Hydrogenation with Parahydrogen Over Immobilized Vaska’s Complex

Abstract Generating parahydrogen-induced polarization (PHIP) of nuclear spins with immobilized transition metal complexes as hydrogenation catalysts allows one to produce pure hyperpolarized substances, which can open new revolutionary perspectives for PHIP applications. A major drawback of immobilized complexes is their low stability under reaction conditions. In the present work we studied an immobilized iridium complex, Ir/SiO2 P, synthesized by a covalent anchoring of Vaska’s complex on phospine-modified silica gel. This complex was used to obtain hyperpolarized gasses in the gas phase hydrogenation of propene, propyne and 1-butyne with parahydrogen in PASADENA and ALTADENA experiments. It was found that, in contrast to other immobilized complexes, Ir/SiO2 P is stable under reaction conditions at up to 140°C, and the reduction of iridium does not occur according to XPS analysis. Moreover, the application of Ir/SiO2 P catalyst allowed us to generate continuous flow of hyperpolarized propene and 1-butene with (300–500)-fold NMR signal enhancement which is significantly higher than commonly observed for most supported metal catalysts. The shape of polarized propene signals in PASADENA experiment has indicated that parahydrogen addition to propyne occurs non-stereospecifically, i.e. PHIP was observed for all protons of the vinyl fragment of propene. The analysis of the polarized signals has shown that syn pairwise addition dominates, which was confirmed by spectra simulations. It was found that storage of Ir/SiO2 P under Ar atmosphere leads to a decrease in PHIP amplitude and an increase in the activity of the catalyst. This observation is discussed in terms of the interaction of Ir/SiO2 P with trace amounts of oxygen in Ar, leading to partial oxidation of triphenylphosphine ligand to triphenylphosphine oxide accompanied by the activation of the immobilized complex. It was also found that the interaction of Ir/SiO2 P with alkenes likely leads to formation of stable monohydride complexes, decreasing the production of PHIP in hydrogenations. At the same time, stable substrate complexes are likely formed in alkyne hydrogenations, leading to a significant decrease in the monohydride complex formation and to an increased production of PHIP.

Optical Properties of Heavily Fluorinated Lanthanide Tris β-Diketonate Phosphine Oxide Adducts

The construction of lanthanide(III) chelates that exhibit superior photophysical properties holds great importance in biological and materials science. One strategy to increase the luminescence properties of lanthanide(III) chelates is to hinder competitive non-radiative decay processes through perfluorination of the chelating ligands. Here, the synthesis of two families of heavily fluorinated lanthanide(III) β-diketonate complexes bearing monodentate perfluorinated tris phenyl phosphine oxide ligands have been prepared through a facile one pot reaction [Ln(hfac)3{(ArF)3PO}(H2O)] and [Ln(F7-acac)3{(ArF)3PO}2] (where Ln = Sm3+, Eu3+, Tb3+, Er3+ and Yb3+). Single crystal X-ray diffraction analysis in combination with photophysical studies have been performed to investigate the factors responsible for the differences in the luminescence lifetimes and intrinsic quantum yields of the complexes. Replacement of both bound H2O and C–H oscillators in the ligand backbone has a dramatic effect on the photophysical properties of the complexes, particularly for the near infra-red emitting ion Yb3+, where a five fold increase in luminescence lifetime and quantum yield is observed. The complexes [Sm(hfac)3{(ArF)3PO}(H2O)] (1), [Yb(hfac)3{(ArF)3PO}(H2O)] (5), [Sm(F7-acac)3{(ArF)3PO}2] (6) and [Yb(F7-acac)3{(ArF)3PO}2] (10) exhibit unusually long luminescence lifetimes and attractive intrinsic quantum yields of emission in fluid solution (ΦLn = 3.4% (1); 1.4% (10)) and in the solid state (ΦLn = 8.5% (1); 2.0% (5); 26% (6); 11% (10)), which are amongst the largest values for this class of compounds to date.

Polymeric Materials for the Separation of f-Elements Utilizing Carbamoylmethylphosphine Oxide Chelating Ligands

The ability of carbamoylmethylphosphine oxide (CMPO) ligands to selectively chelate actinides has been shown to be enhanced by preorganizing the ligands on molecular scaffolds. To increase the preorganization, a new polymeric material with a polyoxanorbornene backbone and CMPO ligand pendant groups has been synthesized, and the ability of the material to selectively extract actinides utilizing a biphasic extraction strategy has been tested. In liquid-liquid extractions of f-block metals from acidic aqueous media into an organic solution containing chelating materials, the polymeric material exhibited a significantly higher ability to extract Th4+ ions than the monomer, with an order of magnitude greater affinity for the polymeric material in most cases. Due to the unique properties of polymeric materials versus small molecules, additional questions arose as to the effect of molecular weight on extraction efficiency. Extraction and separation efficiencies varied for materials of differing molecular weights, with the largest molecular weight polymer extracting near quantitative amounts of thorium ions, and the smaller molecular weight polymers showing separation factors for Th4+ ions over Ce3+, La3+, and Eu3+ ions ranging from 124 to 328.

Synthesis of a low-coordinate erbium (III) β-diketonate complex assembled by opto-electronically active 1,3-diphenyl-1,3-propanedione and triphenylphosphine oxide ligands

A novel Er(III) β-diketonate complex, tris(dibenzoylmethanate)mono(triphenylphosphine oxide) erbium(III) or [Er(dbm)3(tppo)] was synthesised with dbm as a main sensitizer and tppo as a synergic ligand. The single crystal X-ray analysis reveals that the crystal structure of the complex is a triclinic centrosymmetric where Er(III) ion is coordinated to seven oxygen atoms; one O-atom from one tppo and six O-atoms from three dbm ligands. Shape-measure criterion measurements show that the complex possesses a distorted monocapped octahedron geometry, which facilitates the radiative transitions that is an indicative of efficient energy transfer from coordinated ligands to Er ion. The crystal structure is stabilised by the π–π stacking and CH/π interactions which lead to its high photo- and thermal stabilities. The complex was doped into a polymer matrix and the resulted material shows an enhanced luminescence intensity as well as full width at half maximum (fwhm) which allow its possible use for optical-amplification applications. Finally, a single layered organic diode was fabricated to determine charge carrier mobility (μ0) inside the complex. For this purpose, the Mott–Gurney model was used which gave sizable μ0 of 3.39×10−9cm2/Vs (at <5V).

Higher Quantum State Transitions in Colloidal Quantum Dot with Heavy Electron Doping

Electron occupation in the lowest quantized state of the conduction band (1Se) in the colloidal quantum dot leads to the intraband transition in steady-state (1Se-1Pe). The intraband transition, solely originating from the quantum confinement effect, is the unique property of semiconducting nanocrystals. To achieve the electron occupation in 1Se state in the absence of impurity ions, nonthiol ligand passivated HgS colloidal quantum dots are synthesized. The nonthiol ligand passivated HgS quantum dot exhibits strong steady-state intraband transition in ambient condition and enables a versatile ligand replacement to oxide, acid, and halide functional ligands, which was not achievable from conventional HgS or HgSe quantum dots. Surprisingly, the atomic ligand passivation to HgS colloidal quantum dot solution efficiently maintains the electron occupation at 1Se of HgS CQDs in ambient condition. The electron occupation in 1Se of HgS CQD solid film is controlled by surface treatment with charged ions, which is ...

Understanding the Role of Surface Capping Ligands in Passivating the Quantum Dots Using Copper Dopants as Internal Sensor

The role of ligands in passivating quantum dots has been studied in this work using Cu doping as internal sensors. This has been elucidated using the example of dodecanethiol, 3-mercaptopropionic acid, trioctylphosphine, trioctylphosphine oxide, and primary amines for the passivation of CdSe quantum dots by exchanging the original ligands. Steady state and time dependent photoluminescence spectra of the Cu emission have provided the basis for determining the role of ligands. The surface of the quantum dots with ligand exchange has been monitored using nuclear magnetic resonance spectroscopy. The results suggest that the presence of trioctylphosphine, trioctylphosphine oxide, and oleylamine ligands on CdSe quantum dot surface lead to better photoluminescence efficiency. Further, increase in the chain length of the primary amines increases the effectiveness of passivation on the CdSe quantum dot surface. We have also extended this method to the study of oleylamine capping in CdS quantum dots.

A Triad of Highly Reduced, Linear Iron Nitrosyl Complexes: {FeNO}8–10

AbstractGiven the importance of Fe–NO complexes in both human biology and the global nitrogen cycle, there has been interest in understanding their diverse electronic structures. Herein a redox series of isolable iron nitrosyl complexes stabilized by a tris(phosphine)borane (TPB) ligand is described. These structurally characterized iron nitrosyl complexes reside in the following highly reduced Enemark–Feltham numbers: {FeNO}8, {FeNO}9, and {FeNO}10. These {FeNO}8–10 compounds are each low‐spin, and feature linear yet strongly activated nitric oxide ligands. Use of Mössbauer, EPR, NMR, UV/Vis, and IR spectroscopy, in conjunction with DFT calculations, provides insight into the electronic structures of this uncommon redox series of iron nitrosyl complexes. In particular, the data collectively suggest that {TPBFeNO}8–10 are all remarkably covalent. This covalency is likely responsible for the stability of this system across three highly reduced redox states that correlate with unusually high Enemark–Feltham numbers.

A Triad of Highly Reduced, Linear Iron Nitrosyl Complexes: {FeNO}(8-10).

Given the importance of Fe-NO complexes in both human biology and the global nitrogen cycle, there has been interest in understanding their diverse electronic structures. Herein a redox series of isolable iron nitrosyl complexes stabilized by a tris(phosphine)borane (TPB) ligand is described. These structurally characterized iron nitrosyl complexes reside in the following highly reduced Enemark-Feltham numbers: {FeNO}(8) , {FeNO}(9) , and {FeNO}(10) . These {FeNO}(8-10) compounds are each low-spin, and feature linear yet strongly activated nitric oxide ligands. Use of Mössbauer, EPR, NMR, UV/Vis, and IR spectroscopy, in conjunction with DFT calculations, provides insight into the electronic structures of this uncommon redox series of iron nitrosyl complexes. In particular, the data collectively suggest that {TPBFeNO}(8-10) are all remarkably covalent. This covalency is likely responsible for the stability of this system across three highly reduced redox states that correlate with unusually high Enemark-Feltham numbers.

Ligand "Brackets" for Ga-Ga Bond.

The reactivity of digallane (dpp-Bian)Ga-Ga(dpp-Bian) (1) (dpp-Bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) toward acenaphthenequinone (AcQ), sulfur dioxide, and azobenzene was investigated. The reaction of 1 with AcQ in 1:1 molar ratio proceeds via two-electron reduction of AcQ to give (dpp-Bian)Ga(μ2-AcQ)Ga(dpp-Bian) (2), in which diolate [AcQ](2-) acts as "bracket" for the Ga-Ga bond. The interaction of 1 with AcQ in 1:2 molar ratio proceeds with an oxidation of the both dpp-Bian ligands as well as of the Ga-Ga bond to give (dpp-Bian)Ga(μ2-AcQ)2Ga(dpp-Bian) (3). At 330 K in toluene complex 2 decomposes to give compounds 3 and 1. The reaction of complex 2 with atmospheric oxygen results in oxidation of a Ga-Ga bond and affords (dpp-Bian)Ga(μ2-AcQ)(μ2-O)Ga(dpp-Bian) (4). The reaction of digallane 1 with SO2 produces, depending on the ratio (1:2 or 1:4), dithionites (dpp-Bian)Ga(μ2-O2S-SO2)Ga(dpp-Bian) (5) and (dpp-Bian)Ga(μ2-O2S-SO2)2Ga(dpp-Bian) (6). In compound 5 the Ga-Ga bond is preserved and supported by dithionite dianionic bracket. In compound 6 the gallium centers are bridged by two dithionite ligands. Both 5 and 6 consist of dpp-Bian radical anionic ligands. Four-electron reduction of azobenzene with 1 mol equiv of digallane 1 leads to complex (dpp-Bian)Ga(μ2-NPh)2Ga(dpp-Bian) (7). Paramagnetic compounds 2-7 were characterized by electron spin resonance spectroscopy, and their molecular structures were established by single-crystal X-ray analysis. Magnetic behavior of compounds 2, 5, and 6 was investigated by superconducting quantum interference device technique in the range of 2-295 K.

Bis(carbamoylmethylphosphine oxide) ligands fixed on the arene core via 1,2,3-triazole linkers: Novel effective extractants for palladium, lanthanides and actinides

GRAPHICAL ABSTRACT

Direct Copper(III) Formation from O2 and Copper(I) with Histamine Ligation.

Histamine chelation of copper(I) by a terminal histidine residue in copper hydroxylating enzymes activates dioxygen to form unknown oxidants, generally assumed as copper(II) species. The direct formation of copper(III)-containing products from the oxygenation of histamine-ligated copper(I) complexes is demonstrated here, indicating that copper(III) is a viable oxidation state in such products from both kinetic and thermodynamic perspectives. At low temperatures, both trinuclear Cu(II)2Cu(III)O2 and dinuclear Cu(III)2O2 predominate, with the distribution dependent on the histamine ligand structure and oxygenation conditions. Kinetics studies suggest the bifurcation point to these two products is an unobserved peroxide-level dimer intermediate. The hydrogen atom reactivity difference between the trinuclear and binuclear complexes at parity of histamine ligand is striking. This behavior is best attributed to the accessibility of the bridging oxide ligands to exogenous substrates rather than a difference in oxidizing abilities of the clusters.

Well‐Defined Dinuclear Gold Complexes for Preorganization‐Induced Selective Dual Gold Catalysis

The synthesis, reactivity, and potential of well-defined dinuclear gold complexes as precursors for dual gold catalysis are explored. Using the preorganizing abilities of the ditopic PN(H) P(iPr) (L(H) ) ligand, dinuclear Au(I) -Au(I) complex 1 and mixed-valent Au(I) -Au(III) complex 2 provide access to structurally characterized chlorido-bridged cationic species 3 and 4 upon halide abstraction. For 2, this transformation involves unprecedented two-electron oxidation of the redox-active ligand, generating a highly rigidified environment for the Au2 core. Facile reaction with phenylacetylene affords the σ,π-activated phenylacetylide complex 5. When applied in the dual gold heterocycloaddition of a urea-functionalized alkyne, well-defined precatalyst 3 provides high regioselectivities for the anti-Markovnikov product, even at low catalyst loadings, and outperforms common mononuclear Au(I) systems. This proof-of-concept demonstrates the benefit of preorganization of two gold centers to enforce selective non-classical σ,π-activation with bifunctional substrates.

Multiple-Sized Amphiphilic Janus Gold Nanoparticles by Ligand Exchange at Toluene/Water Interface

Amphiphilic Janus gold nanoparticles (GNPs) are prospected to encapsulate drug molecules in cancer therapy and to serve as heterogeneous catalysts at oil/water interfaces, where Janus GNPs with different sizes are required. In this work, multiple-sized precursor GNPs were synthesized by seeded growth method protected with tris(hydroxymethyl)phosphine oxide (THPO) ligand molecule, and a ligand exchange reaction with triphenylphosphine (PPh3) at the toluene/water interface was employed to prepare amphiphilic Janus GNPs. UV-vis and transmission electron microscopy (TEM) analyses indicate that the as-prepared GNPs are nanocrystals with average diameters of 2.3 nm, 9.5 nm, 16.1 nm and 18.8 nm, respectively. Contact angle, Raman and X-ray photonic spectroscopy (XPS) analyses reveal that the self-assembled GNP films exhibit hydrophilic on one side and hydrophobic on another, owing to the adsorption of hydrophilic ligands (THPO and THP) and a similar amount of hydrophobic ligands (PPh3 and PPh3O). Angle-resolved XPS analysis further demonstrates that the individual GNPs actually possess hydrophilic and hydrophobic compartments on the surface, which regularly packed by supramolecular interactions at toluene/water interface to form the self-assembled GNP films.

Reactivity of [WCl6] with Ethers: A Joint Computational, Spectroscopic and Crystallographic Study

The reactions of [WCl6] with a series of ethers have been performed in chlorinated solvent and elucidated by means of analytical, spectroscopic and DFT methods. The addition of tetrahydropyran (thp) or 1,4‐dioxane to [WCl6] resulted in the reversible formation of the adducts WCl6···L [L = thp (1a), 1,4‐dioxane (1b)], detected in solution by NMR spectroscopy. The reaction of [WCl6] with thp in a molar ratio of 1:2 in chloroform at reflux afforded [WOCl4(thp)] (2a), which was isolated in 51 % yield. [WOCl4(OMe2)] (2b) and [WOCl3(OMe2)2] (3a) were isolated in yields of 53 and 18 %, respectively, from the reaction of [WCl6] with an excess of dimethyl ether. [WOCl3(OEt2)2] (3b) was the only identified metal compound produced from the reaction of [WCl6] and OEt2 (1:2 molar ratio). According to NMR studies, the oxide ligand in 2a,b and 3a,b was generated by double C–O bond cleavage involving one equivalent of organic reactant. The 1:1 reaction of [WCl6] with 1,2‐diethoxyethane led to [WCl5(κ1‐OCH2CH2OEt)] (4) and a minor amount of [WCl4(κ2‐EtOCH2CH2OEt)] (5). The aryl oxide compound [WCl5(OPh)] (6) was prepared in 62 % yield from the reaction of [WCl6] and anisole by selective Csp3–O bond activation. The prolonged heating of a mixture of [WCl6] and diphenyl ether in 1,2‐dichloroethane led to the isolation of the WV complex [WCl5(OPh2)] (7). The molecular structures of 2a and 3a were ascertained by X‐ray diffraction.

Synthesis and Characterization of 1,1′-Diphosphaplumbocenes: Oxidative Ligand Transfer Reactions with Divalent Thulium Complexes

The new 1,1′-diphosphaplumbocenes Pb(Dtp)2 (Dtp = 2,5-di-tert-butyl-3,4-dimethylphospholyl) and Pb(Dsp)2 (Dsp = 2,5-ditrimethylsilyl-3,4-dimethylphospholyl) have been synthesized and characterized by multinuclear NMR spectroscopy. 207Pb NMR spectra of both complexes, as well as for the known complex Pb(Htp)2 (Htp = 2,5-di-tert-butylphospholyl), show that the phospholyl ligands lead to an important downfield shift compared with that of cyclopentadienyl based plumbocenes. X-ray diffraction studies of the Pb(Dtp)2 complex revealed a structure with two slightly bent η5-bound ligands. This complex was employed in the oxidative ligand transfer reaction with bulky divalent thulium complexes. In the case of Tm(Dtp)2, the first example of a monomeric trivalent tris(phospholyl)lanthanide complex Tm(Dtp)3 was obtained. X-ray diffraction studies and low-temperature 1H NMR studies show the η1 coordination mode of the third Dtp ligand. In contrast, oxidation of Tm(Cpttt)2 (Cpttt is for tris-tert-butylcyclopentadienyl) led presumably to the formation of the ion pair complex [Tm(Cpttt)2][Dtp], in which no interaction between the free Dtp ligand and Tm was observed, as shown by variable temperature 1H and 31P NMR studies. The new trivalent complexes could be reduced back to the original divalent complexes with potassium graphite.

Preparation of Secondary Phosphine Oxide Ligands through Nucleophilic Attack on Imines and Their Applications in Palladium‐Catalyzed Catellani Reactions

Several new amino‐type secondary phosphine oxide (SPO) pre‐ligands (3a–3h) that contain P–N bonds were synthesized and characterized. SPOs 3a–3h can tautomerize to phosphinous acids (PA, 3a–3h) as genuine ligands. The formation of SPOs 3a–3h occurred first through nucleophilic attack on the imine carbon atom, then by the addition of RPCl2 (R = Ph, Cy, tBu, or iPr), and work‐up under acidic conditions. The P–N bond in the newly prepared SPOs is evident from the crystal structures of SPOs 3d and 3h. Reactions of SPOs 3f, 3g, or 3h with Pd(COD)Cl2 (COD = cyclooctadiene) yielded palladium complexes 6f, 6g, or 6h. In these crystal structures, PAs 3f, 3g, and 3h act as didentate ligands through P and N donors. Intriguingly, the reaction of SPO 3f with Pd(COD)Cl2 also gave rise to palladium complexes 7fa, 7fb, and 8f. The crystal structures of 7fa and 7fb show that the palladium atom is chelated by PA 3f and coordinated by a phosphine‐like ligand fragmented from SPO 3f through C–N bond dissociation. Finally, the syntheses of carbazole derivatives were pursued in Catellani reactions with SPOs 3g and 3h as pre‐ligands. A mechanism is proposed to account for the catalytic reaction (see the Supporting Information). The optimized conditions for Suzuki reactions using selected SPO ligands are also reported.

Electrophilic and nucleophilic pathways in ligand oxide mediated reactions of phenylsulfinylacetic acids with oxo(salen)chromium(V) complexes

The mechanism of oxidative decarboxylation of phenylsulfinylacetic acids (PSAA) by oxo(salen)Cr(V)+ ion in the presence of ligand oxides has been studied spectrophotometrically in acetonitrile medium. Addition of ligand oxides (LO) causes a red shift in the λmax values of oxo(salen) complexes and an increase in absorbance with the concentration of LO along with a clear isobestic point. The reaction shows first-order dependence on oxo(salen)-chromium(V)+ ion and fractional-order dependence on PSAA and ligand oxide. Michaelis–Menten kinetics without kinetic saturation was observed for the reaction. The order of reactivity among the ligand oxides is picoline N-oxide>pyridine N-oxide>triphenylphosphine oxide. The low catalytic activity of TPPO was rationalized. Both electron-withdrawing and electron-donating substituents in the phenyl ring of PSAA facilitate the reaction rate. The Hammett plots are non-linear upward type with negative ρ value for electron-donating substituents, (ρ−=−0.740 to −4.10) and positive ρ value for electron-withdrawing substituents (ρ+=+0.057 to +0.886). Non-linear Hammett plot is explained by two possible mechanistic scenarios, electrophilic and nucleophilic attack of oxo(salen)chromium(V)+-LO adduct on PSAA as the substituent in PSAA is changed from electron-donating to electron-withdrawing. The linearity in the logk vs. Eox plot confirms single-electron transfer (SET) mechanism for PSAAs with electron-donating substituents.

Synthesis, Radical Reactivity, and Thermochemistry of Monomeric Cu(II) Alkoxide Complexes Relevant to Cu/Radical Alcohol Oxidation Catalysis

Two new monomeric Cu(II) alkoxide complexes were prepared and fully characterized as models for intermediates in copper/radical mediated alcohol oxidation catalysis: Tp(tBuR)Cu(II)OCH2CF3 with Tp(tBu) = hydro-tris(3-tert-butyl-pyrazol-1-yl)borate 1 or Tp(tBuMe) = hydro-tris(3-tert-butyl-5-methyl-pyrazol-1-yl)borate 2. These complexes were made as models for potential intermediates in enzymatic and synthetic catalytic cycles for alcohol oxidation. However, the alkoxide ligands are not readily oxidized by loss of H; instead, these complexes were found to be hydrogen atom acceptors. They oxidize the hydroxylamine TEMPOH, 2,4,6-tri-t-butylphenol, and 1,4-cyclohexadiene to the nitroxyl radical, phenoxyl radical, and benzene, with formation of HOCH2CF3 (TFE) and the Cu(I) complexes Tp(tBuR)Cu(I)-MeCN in dichloromethane/1% MeCN or 1/2 [Tp(tBuR)Cu(I)]2 in toluene. On the basis of thermodynamics and kinetics arguments, these reactions likely proceed through concerted proton-electron transfer mechanisms. Thermochemical analyses give lower limits for the "effective bond dissociation free energies (BDFE)" of the O-H bonds in 1/2[Tp(tBuR)Cu(I)]2 + TFE and upper limits for the free energies associated with alkoxide oxidations via hydrogen atom transfer (effective alkoxide α-C-H BDFEs). These values are summations of the free energies of multiple chemical steps, which include the energetically favorable formation of 1/2[Tp(tBuR)Cu(I)]2. The effective alkoxide α-C-H bonds are very weak, BDFE ≤ 38 ± 4 kcal mol(-1) for 1 and ≤44 ± 5 kcal mol(-1) for 2 (gas-phase estimates), because C-H homolysis is thermodynamically coupled to one electron transfer to Cu(II) as well as the favorable formation of the 1/2[Tp(tBuR)Cu(I)]2 dimer. Treating 1 with the H atom acceptor (t)Bu3ArO(•) did not result in the expected alkoxide oxidation to an aldehyde, but rather net 2,2,2-trifluoroethoxyl radical transfer occurred to generate an unusual 2-substituted dienone-ether product. Treating 2 with (t)Bu3ArO(•) gives no reaction, despite evidence that overall ligand oxidation and formation of 1/2[Tp(tBuMe)Cu(I)]2 is significantly exoergic. The origin of this lack of reactivity may be due to insufficient weakening of the alcohol α-C-H bond upon complexation to copper.

Computation and Experiment: A Powerful Combination to Understand and Predict Reactivities.

Computational chemistry has become an established tool for the study of the origins of chemical phenomena and examination of molecular properties. Because of major advances in theory, hardware and software, calculations of molecular processes can nowadays be done with reasonable accuracy on a time-scale that is competitive or even faster than experiments. This overview will highlight broad applications of computational chemistry in the study of organic and organometallic reactivities, including catalytic (NHC-, Cu-, Pd-, Ni-catalyzed) and noncatalytic examples of relevance to organic synthesis. The selected examples showcase the ability of computational chemistry to rationalize and also predict reactivities of broad significance. A particular emphasis is placed on the synergistic interplay of computations and experiments. It is discussed how this approach allows one to (i) gain greater insight than the isolated techniques, (ii) inspire novel chemistry avenues, and (iii) assist in reaction development. Examples of successful rationalizations of reactivities are discussed, including the elucidation of mechanistic features (radical versus polar) and origins of stereoselectivity in NHC-catalyzed reactions as well as the rationalization of ligand effects on ligation states and selectivity in Pd- and Ni-catalyzed transformations. Beyond explaining, the synergistic interplay of computation and experiments is then discussed, showcasing the identification of the likely catalytically active species as a function of ligand, additive, and solvent in Pd-catalyzed cross-coupling reactions. These may vary between mono- or bisphosphine-bound or even anionic Pd complexes in polar media in the presence of coordinating additives. These fundamental studies also inspired avenues in catalysis via dinuclear Pd(I) cycles. Detailed mechanistic studies supporting the direct reactivity of Pd(I)-Pd(I) with aryl halides as well as applications of air-stable dinuclear Pd(I) catalysts are discussed. Additional combined experimental and computational studies are described for alternative metals, these include the discussion of the factors that control C-H versus C-C activation in the aerobic Cu-catalyzed oxidation of ketones, and ligand and additive effects on the nature and favored oxidation state of the active catalyst in Ni-catalyzed trifluoromethylthiolations of aryl chlorides. Examples of successful computational reactivity predictions along with experimental verifications are then presented. This includes the design of a fluorinated ligand [(CF3)2P(CH2)2P(CF3)2] for the challenging reductive elimination of ArCF3 from Pd(II) as well as the guidance of substrate scope (functional group tolerance and suitable leaving group) in the Ni-catalyzed trifluoromethylthiolation of C(sp(2))-O bonds. In summary, this account aims to convey the benefits of integrating computational studies in experimental research to increase understanding of observed phenomena and guide future experiments.

Direct Cross-Linking of Au/Ag Alloy Nanoparticles into Monolithic Aerogels for Application in Surface-Enhanced Raman Scattering

The direct cross-linking of Au/Ag alloy nanoparticles (NPs) into high surface area, mesoporous Au/Ag aerogels via chemical oxidation of the surface ligands is reported. The precursor alloy NPs with composition-tunable morphologies were produced by galvanic replacement of the preformed Ag hollow NPs. The effect of Au:Ag molar ratio on the NP morphology and surface plasmon resonance has been thoroughly investigated and resulted in smaller Au/Ag alloy NPs (4-8 nm), larger Au/Ag alloy hollow NPs (40-45 nm), and Au/Ag alloy hollow particles decorated with smaller Au NPs (2-5 nm). The oxidative removal of surfactant ligands, followed by supercritical drying, is utilized to construct large (centimeter to millimeter) self-supported Au/Ag alloy aerogels. The resultant assemblies exhibit high surface areas (67-73 m(2)/g), extremely low densities (0.051-0.055 g/cm(3)), and interconnected mesoporous (2-50 nm) networks, making them of great interest for a number of new technologies. The influence of mesoporous gel morphology on surface-enhanced Raman scattering (SERS) has been studied using Rhodamine 101 (Rd 101) as the probe molecule. The alloy aerogels exhibit SERS signal intensities that are 10-42 times higher than those achieved from the precursor Au/Ag alloy NPs. The Au/Ag alloy aerogel III exhibits SERS sensing capability down to 1 nM level. The increased signal intensities attained for alloy aerogels are attributed to highly porous gel morphology and enhanced surface roughness that can potentially generate a large number of plasmonic hot spots, creating efficient SERS substrates for future applications.