N-Heterocyclic Carbene Boranes: Dual Reagents for the Synthesis of Gold Nanoparticles.

N-Heterocyclic carbenes (NHCs) have drawn considerable interest in the field of nanomaterials chemistry as highly stabilizing ligands enabling the formation of strong and covalent carbon-metal bonds. Applied to gold nanoparticles synthesis, the most common strategy consists of the reduction of a preformed NHC-AuI complex with a large excess of a reducing agent that makes the particle size difficult to control. In this paper, we report the straightforward synthesis of NHC-coated gold nanoparticles (NHC-AuNPs) by treating a commercially available gold(I) precursor with an easy-to-synthesize NHC-BH3 reagent. The latter acts as both the reducing agent and the source of surface ligands operating under mild conditions. Mechanistic studies including NMR spectroscopy and mass spectrometry demonstrate that the reduction of gold(I) generates NHC-BH2 Cl as a by-product. This strategy gives efficient control over the nucleation and growth of gold particles by varying the NHC-borane/gold(I) ratio, allowing unparalleled particle size variation over the range of 4.9±0.9 to 10.0±2.7 nm. Our strategy also allows an unprecedented precise and controlled seeded growth of gold nanoparticles. In addition, the as-prepared NHC-AuNPs exhibit narrow size distributions without the need for extensive purification or size-selectivity techniques, and are stable over months.

Benchmarking higher energy collision dissociation (HCD) by investigation of binding energies of gas-phase host-guest complexes of hemicryptophane cages.

Synthesis of host molecules that feature well-defined characteristics for molecular recognition of guest molecules is often a major aim of synthetic host-guest (H-G) chemistry. A key consideration in evaluating the selectivity of hosts and the affinities of guests is the measurement of binding energies of obtained H-G complexes. In contrast to nuclear magnetic resonance (NMR) or fluorescence measurements that are capable of measuring binding strengths in solution, mass spectrometry offers the opportunity to measure gas-phase binding energies. Presented in this article is a higher energy collision dissociation (HCD) approach for determining critical energies of dissociation of H-G complexes. Experiments were performed on electrospray ionization (ESI)-generated H-G pairs in an LTQ-XL/Orbitrap hybrid instrument. The presented HCD approach requires preliminary calibration of the internal energy distribution of generated ions that was achieved by the use of activation parameters that were known from previous low-energy collision-induced dissociation (low-energy CID) experiments. Internal energy deposition was modeled based on a truncated Maxwell-Boltzmann distribution and characteristic temperature (Tchar ). Using this method, critical energies of dissociation were determined for 10 H-G biologically relevant complexes of the heteroditopic hemicryptophane cage host (Host). Obtained results are compared with those found previously by low-energy CID. The use of this HCD technique is relatively straightforward, although its implementation does require knowledge (or a presumption) about the Arrhenius pre-exponential factor of the complexes to obtain their critical energies of dissociation.

Racemization mechanism of lithium tert-butylphenylphosphido-borane: A kinetic insight.

The racemization mechanism of tert-butylphenylphosphido-borane is investigated experimentally and theoretically. Based on this converging approach, it is shown, first, that several phosphido-borane molecular species coexist at the time of the reaction and, second, that one particular of both initially assumed reactive routes most significantly contribute to the overall racemization process. From our converging modeling and experimental measurement, it comes out that the most probable species to be here encountered is a phosphido-borane-Li (THF)2 neutral solvate, whose P-stereogenic center monomolecular inversion through a Y-shaped transition structure (Δr G°≠ : 81 kJ mol-1 ) brings the largest contribution to the racemization process.

Exploring Phosphine Electronic Effects on Molybdenum Complexes: A Combined Photoelectron Spectroscopy and Energy Decomposition Analysis Study.

In organometallic chemistry, especially in the catalysis area, accessing the finest tuning of a catalytic reaction pathway requires a detailed knowledge of the steric and electronic influences of the ligands bound to the metal center. Usually, the M-L bond between a ligand and metal is depicted by the Dewar-Chatt-Duncanson model involving two opposite interactions, σ-donor and π-acceptor effects of the ligand. The experimental evaluation of these effects is essential and complementary to in-depth theoretical approaches that are able to provide a detailed description of the M-L bond. In this work, we present a study of LMo(CO)5 complexes with L being various tertiary phosphine ligands by means of mass-selected high-resolution photoelectron spectroscopy (PES) performed with synchrotron radiation, DFT, and energy decomposition analyses (EDA) combined with the natural orbitals for chemical valence (NOCV) analysis. These methods enable a separated access of the σ-donor and π-acceptor effects of ligands by probing either the electronic configuration of the complex (PES) or the interaction of the ligand with the metal (EDA). Three series of PR3 ligands with various electronic influences are investigated: the strong donating alkyl substituents (PMe3, PEt3, and PiPr3), the intermediate PPhxMe(3-x) (x = 0-3) set, and the PPhxPyrl(3-x) set (x = 0-3 with Pyrl being the strong electron withdrawing pyrrolyl group C4H4N). For each complex, their adiabatic and vertical ionization energies (IEs) could be determined with a 0.03 eV precision. Experiment and theory show an excellent agreement, either for the IE determination or electronic effect analysis. The ability to interpret the spectra is shown to depend on the character of the ligand. "Innocent" ligands provide the spectra that are the most straightforward to analyze, whereas the "non-innocent" ligands (which are ionized prior to the metal center) render the analysis more difficult due to an increased number of molecular orbitals in the energy range considered. A very good linear correlation is finally found between the measured adiabatic ionization energies and the interaction energy term obtained by EDA for each of these two types of ligands, which opens interesting perspective for the prediction of ligand characters.

Investigation of activation energies for dissociation of host-guest complexes in the gas phase using low-energy collision induced dissociation.

A low-energy collision induced dissociation (CID) (low-energy CID) approach that can determine both activation energy and activation entropy has been used to evaluate gas-phase binding energies of host-guest (H-G) complexes of a heteroditopic hemicryptophane cage host (Zn (II)@1) with a series of biologically relevant guests. In order to use this approach, preliminary calibration of the effective temperature of ions undergoing resonance excitation is required. This was accomplished by employing blackbody infrared radiative dissociation (BIRD) which allows direct measurement of activation parameters. Activation energies and pre-exponential factors were evaluated for more than 10 H-G complexes via the use of low-energy CID. The relatively long residence time of the ions inside the linear ion trap (maximum of 60 s) allowed the study of dissociations with rates below 1 s-1 . This possibility, along with the large size of the investigated ions, ensures the fulfilment of rapid energy exchange (REX) conditions and, as a consequence, accurate application of the Arrhenius equation. Compared with the BIRD technique, low-energy CID allows access to higher effective temperatures, thereby permitting one to probe more endothermic decomposition pathways. Based on the measured activation parameters, guests bearing a phosphate (-OPO3 2- ) functional group were found to bind more strongly with the encapsulating cage than those having a sulfonate (-SO3 - ) group; however, the latter ones make stronger bonds than those with a carboxylate (-CO2 - ) group. In addition, it was observed that the presence of trimethylammonium (-N(CH3 )3 + ) or phenyl groups in the guest's structure improves the strength of H-G interactions. The use of this technique is very straightforward, and it does not require any instrumental modifications. Thus, it can be applied to other H-G chemistry studies where comparison of bond dissociation energies is of paramount importance.

Investigation of Hemicryptophane Host-Guest Binding Energies Using High-Pressure Collision-Induced Dissociation in Combination with RRKM Modeling.

In advancing host-guest (H-G) chemistry, considerable effort has been spent to synthesize host molecules with specific and well-defined molecular recognition characteristics including selectivity and adjustable affinity. An important step in the process is the characterization of binding strengths of the H-G complexes that is typically performed in solution using NMR or fluorescence. Here, we present a mass spectrometry-based multimodal approach to obtain critical energies of dissociation for two hemicryptophane cages with three biologically relevant guest molecules. A combination of blackbody infrared radiative dissociation (BIRD) and high-pressure collision-induced dissociation (high-pressure CID), along with RRKM modeling, was employed for this purpose. For the two tested hemicryptophane hosts, the cage containing naphthyl linkages exhibited stronger interactions than the cage bearing phenyl linkages. For both cages, the order of guest stability is choline > acetylcholine > betaine. The information obtained by these types of mass spectrometric studies can provide new insight into the structural features that most influence the stability of H-G pairs, thereby providing guidance for future syntheses. Graphical Abstract.

Bond dissociation energies of carbonyl gold complexes: a new descriptor of ligand effects in gold(i) complexes?

Ligand electronic effects in gold(i) chemistry have been evaluated by means of the experimental determination of M-CO bond dissociation energies for 16 [L-Au-CO]+ complexes, bearing L ligands widely used in gold catalysis. Energy-resolved analyses have been made using tandem mass spectrometry with collision-induced dissociation. Coupled with DFT calculations, this approach enables the quantification of ligand effects based on the LAu-CO bond strength. A further energy decomposition analysis gives access to detailed insights into this bond's characteristics. Whereas small differences are observed between phosphine- and phosphite-containing gold complexes, carbene ligands are shown to stabilize the gold-carbonyl bond much more efficiently.

Stereospecific Synthesis of α- and ß-Hydroxyalkyl P-Stereogenic Phosphine-Boranes and Functionalized Derivatives: Evidence of the P=O Activation in the BH3 -Mediated Reduction.

Access to hydroxy-functionalized P-chiral phosphine-boranes has become an important field in the synthesis of P-stereogenic compounds used as ligands in asymmetric catalysis. A family of optically pure α and ß-hydroxyalkyl tertiary phosphine-boranes has been prepared by using a three-step procedure from readily accessible enantiopure adamantylphosphinate, obtained by semi-preparative HPLC on multigram scale. Firstly, a two-step one-pot transformation affords the enantiopure hydroxyalkyl tertiary phosphine oxides in good yields and enantioselectivities. The third step, BH3 -mediated reduction, allows the formation of the desired phosphine-boranes with excellent stereospecifity. The mechanistic study of this reduction provides new evidence to elucidate the crucial role of the pendant hydroxy group and the subsequent activation of the P=O bond by the boron atom.

Preparation of Vinylcyclopropanes by Sodium Mediated Reductive Isomerization of Methylenecyclopropanes.

We disclosed therein a new reaction of reductive isomerization of methylenecyclopropanes (MCPs) to vinylcyclopropanes (VCPs). On treatment with sodium metal in liquid ammonia, MCPs bearing a C-O bond at allylic position undergo both a reductive cleavage of the C-O bond and an isomerization of the C-C double bond giving rise to VCPs. The scope of the reductive isomerization was investigated and showed a broad applicability since various functional groups are tolerated. MCP substrates were straightforwardly prepared by a palladium-promoted [2 + 1] cycloaddition between norbornene derivatives with alkynes.

H-adamantylphosphinates as universal precursors of P-stereogenic compounds.

A new family of H-adamantylphosphinates as universal precursors of P-stereogenic ligands was obtained in one step from commercial chlorophosphines. Both enantiomers of these air- and moisture-stable intermediates can easily be separated by semipreparative chiral HPLC on a gram scale and individually undergo stereoselective transformations to afford each enantiomer of a set of P-stereogenic compounds such as secondary phosphine oxides and boron-protected monophosphines.

Making Fe(BPBP)-catalyzed C-H and C[double bond, length as m-dash]C oxidations more affordable.

The limited availability of catalytic reaction components may represent a major hurdle for the practical application of many catalytic procedures in organic synthesis. In this work, we demonstrate that the mixture of isomeric iron complexes [Fe(OTf)2(mix-BPBP)] (mix-1), composed of Λ-α-[Fe(OTf)2(S,S-BPBP)] (S,S-1), Δ-α-[Fe(OTf)2(R,R-BPBP)] (R,R-1) and Δ/Λ-ß-[Fe(OTf)2(R,S-BPBP)] (R,S-1), is a practical catalyst for the preparative oxidation of various aliphatic compounds including model hydrocarbons and optically pure natural products using hydrogen peroxide as an oxidant. Among the species present in mix-1, S,S-1 and R,R-1 are catalytically active, act independently and represent ca. 75% of mix-1. The remaining 25% of mix-1 is represented by mesomeric R,S-1 which nominally plays a spectator role in both C-H and C[double bond, length as m-dash]C bond oxidation reactions. Overall, this mixture of iron complexes displays the same catalytic profile as its enantiopure components that have been previously used separately in sp(3) C-H oxidations. In contrast to them, mix-1 is readily available on a multi-gram scale via two high yielding steps from crude dl/meso-2,2'-bipyrrolidine. Next to its use in C-H oxidation, mix-1 is active in chemospecific epoxidation reactions, which has allowed us to develop a practical catalytic protocol for the synthesis of epoxides.

N-heterocyclic carbene-initiated hydrosilylation of styryl alcohols with dihydrosilanes: a mechanistic investigation.

Investigation of the mechanism of the NHC-initiated hydrosilylation of styryl alcohols in the presence of a dihydrosilane suggests a general base catalysis mechanism and not the activation of the dihydrosilane by the NHC.

Bulky, optically active P-stereogenic phosphine-boranes from pure H-menthylphosphinates.

The transformation of readily available pure-H-menthylphosphinates into chiral phosphinous acid-boranes permits the elaboration of bulky P-stereogenic secondary phosphine-boranes. Taking advantage of the synthetic potential of these compounds, a broad range of hindered P-chiral tertiary phosphine-boranes has been prepared with excellent enantiomeric excesses. The utility of bulky o-tolylphosphines was illustrated by the synthesis of a rare enantiopure phosphapalladacycle (S(P),S(P))-12.

A simple route to chiral phosphinous acid-boranes.

We report a simple one-pot synthesis of enantiomerically enriched alkyl- and arylphenylphosphinous acid-borane starting from readily available (R(P))-(-)-menthylhydrogenophenylphosphinate and organolithium reagents.