Monomeric Platinum(II) Hydroxides Supported by Sterically Dominant α-Diimine Ligands

The use of two new highly sterically bulky α-diimine ligands for the stabilization of neutral, monomeric platinum­(II) hydroxo complexes is described. Halide abstraction from LPtCl₂ complexes of these ligands in the presence of water, followed by deprotonation of the cationic aquo complex, leads to LPt­(OH)Cl and LPt­(OH)₂. The latter can be reprotonated with HNTf₂ to yield a highly fluxional hydroxoaquoplatinum­(II) cation

Monomeric Platinum(II) Hydroxides Supported by Sterically Dominant α-Diimine Ligands

The use of two new highly sterically bulky α-diimine ligands for the stabilization of neutral, monomeric platinum­(II) hydroxo complexes is described. Halide abstraction from LPtCl₂ complexes of these ligands in the presence of water, followed by deprotonation of the cationic aquo complex, leads to LPt­(OH)Cl and LPt­(OH)₂. The latter can be reprotonated with HNTf₂ to yield a highly fluxional hydroxoaquoplatinum­(II) cation

β-Elimination-Immune PC_carbeneP Iridium Complexes via Double C–H Activation: Ligand–Metal Cooperation in Hydrogen Activation

The synthesis of two members of a new PCP pincer ligand family and their complexation with iridium has been developed. Double C–H activation results in a ligand anchored by a carbene donor; hydrogen reversibly adds to the (PC_carbeneP)­IrCl complexes <b>3</b>^<b>R</b> (R = ⁱPr, ^tBu). Aryl, amido, and phenoxy derivatives are available from the chlorides via salt metathesis. These compounds add H₂ and eliminate H–X; the iridium compound is trapped as a PC_alkylP iridium polyhydride

Ligand Cooperation in the Formal Hydrogenation of N₂O Using a PC_sp2P Iridium Pincer Complex

A new PC_carbeneP pincer ligand with 2,3-benzo­[<i>b</i>]­thiophene linkers connecting the flanking dialkyl phosphine donors to the central carbene can be attached to Ir­(I). The chloro derivative reacts with N₂O with loss of N₂ to form an iridaepoxide species by addition of an oxygen atom to the IrC linkage. This compound reacts with H₂ to afford the oxidative addition product, in which the hydride ligands are <i>trans</i> to the Ir–O bond. Heating this dihydride results in slow release of H₂O; kinetic and spectroscopic studies show that conversion of the dihydride to its isomer, in which the hydrides are <i>cis</i> to the Ir–O bond, is required for H₂O elimination to take place. Together, these reactions constitute the stoichiometric conversion of N₂O and H₂ to N₂ and H₂O; further mechanistic studies suggest ways to make the system catalytic

Photogeneration of a Phosphonium Alkylidene Olefin Metathesis Catalyst

Treatment of ruthenium carbide (H₂IMes)­(Cl)₂(PCy₃)­RuC (<b>1</b>) with the photoacid generator (PAG) [Ph₃S]­[OTf] (<b>3</b>) under 254 nm light results in a highly efficient catalyst for ring-closing metathesis (RCM) and ring-opening metathesis polymerization (ROMP) reactions. The reactions proceed via formation of the ruthenium phosphonium alkylidene complex [(H₂IMes)­(Cl)₂RuC­(H)­PCy₃]­[OTf] as the active catalytic species. In the case of ROMP of cycloalkenes, reactions do not require addition of PAG and protonation of <b>1</b> proceeds via allylic C–H bond activation of the substrate under UV light

Reactions of Neutral Cobalt(II) Complexes of a Dianionic Tetrapodal Pentadentate Ligand: Cobalt(III) Amides from Imido Radicals

Neutral cobalt­(II) complexes of the dianionic tetrapodal pentadentate ligand B₂Pz₄Py, in which borate linkers supply the anionic charges, are reported. Both the six-coordinate THF adduct <b>1-THF</b> and the five-coordinate THF-free complex <b>1</b> are in a high-spin <i>S</i> = 3/2 configuration in the ground state and have been structurally characterized by X-ray crystallography. These two Co­(II) starting materials react rapidly with aryl azides of moderate steric bulk. The thermodynamic products of these reactions are low-spin, diamagnetic, Co­(III) amido complexes that are either monomeric, when an external hydrogen atom source such as 1,4-cyclohexadiene is present, or dimeric products formed via C–C coupling of the azide aryl group and internal transfer of H^• to the nitrogen. These products are fully characterized and are rare examples of octahedral Co amido compounds; structural determinations reveal significant pyramidalization of the amido nitrogens due to π–π repulsion wherein the amido ligand is primarily a σ donor. The amido products arise from highly reactive Co­(III) imido radical intermediates that are the kinetic products of the reactions of <b>1</b> or <b>1-THF</b> with the azide reagents. The imido radicals can be detected by X-band EPR spectroscopy and have been probed by density functional theory computations, which indicate that this doublet species is characterized by a high degree of spin localization on the imido ligand, accounting for the reactivity with hydrogen atom sources and dimerization chemistry observed. The high coordination number and the electron-rich nature of the dianionic B₂Pz₄Py ligand framework render the imido ligand formed highly reactive

An Empirical Analysis of Pruning Techniques: Performance, Retrievability and Bias

Prior work on using retrievability measures in the evaluation of information retrieval (IR) systems has laid out the foundations for investigating the relation between retrieval performance and retrieval bias. While various factors influencing retrievability have been examined, showing how the retrieval model may influence bias, no prior work has examined the impact of the index (and how it is optimized) on retrieval bias. Intuitively, how the documents are represented, and what terms they contain, will influence whether they are retrievable or not. In this paper, we investigate how the retrieval bias of a system changes as the inverted index is optimized for efficiency through static index pruning. In our analysis, we consider four pruning methods and examine how they affect performance and bias on the TREC GOV2 Collection. Our results show that the relationship between these factors is varied and complex - and very much dependent on the pruning algorithm. We find that more pruning results in relatively little change or a slight decrease in bias up to a point, and then a dramatic increase. The increase in bias corresponds to a sharp decrease in early precision such as NDCG@10 and is also indicative of a large decrease in MAP. The findings suggest that the impact of pruning algorithms can be quite varied - but retrieval bias could be used to guide the pruning process. Further work is required to determine precisely which documents are most affected and how this impacts upon performance

Reactivity of Scandium β-Diketiminate Alkyl Complexes with Carbon Dioxide

The reactions of two highly air- and moisture-sensitive scandium bis­(alkyls) supported by a bulky β-diketiminato (nacnac) ligand with carbon dioxide are described. [κ²-ArNC­(^<i>t</i>Bu)­CHC­(^<i>t</i>Bu)­NAr]­ScR₂ (Ar = 2,6-^<i>i</i>Pr₂C₆H₃; R = CH₃, <b>1a</b>; R = CH₂SiMe₃, <b>1b</b>) react rapidly with CO₂ to give mixtures of mono- and bis­(carboxylato) insertion products <b>2a</b>/<b>2b</b> and <b>3a</b>/<b>3b</b> depending on the stoichiometry and conditions of the reaction. Compound <b>2a</b> (R = CH₃) is a dimeric complex with bridging acetato groups, as determined by X-ray crystallography. These compounds were characterized by NMR spectroscopy, and <b>3a</b> could be isolated in pure form. Treatment of these compounds with excess CO₂ resulted in addition to the central carbon of the Sc­(nacnac) six-membered ring and displacement of the nitrogen donors to yield dimeric scandium carboxylates <b>4a</b>/<b>4b</b>; compound <b>4b</b> was characterized by X-ray crystallography. Reactions of the nacnac scandium cations formed upon abstraction of one or two methides from <b>1a</b> using B­(C₆F₅)₃ with CO₂ were also explored. Although the products were qualitatively more thermally robust, eventually ligand displacement occurred in these cationic acetato complexes as well. Nevertheless, insertion products were characterizable in solution using NMR spectroscopy. Overall, this study shows the facility with which CO₂ is taken up by scandium alkyls but that the nacnac ligand framework is too reactive to support chemistry aimed at catalytic conversion of CO₂ into other products

Ring Expansion Reactions of Electron-Rich Boron-Containing Heterocycles

The potassium salts of the dianions of isomeric compounds bis-benzocycloborabutylidene, <b>1</b>, and the ladder diborole <b>2</b> were reacted with carbon dioxide (CO₂) and carbon monoxide (CO) and the ring expansion products fully characterized. Both dianions <b>1</b> and <b>2</b> react rapidly with carbon dioxide to form the same insertion product, <b>3</b>, in which the boron-containing rings are expanded to six-membered rings. Compound <b>3</b> is a B–O analogue of binaphtholate. Only dianion <b>2</b> reacts cleanly with the weaker electrophile CO, producing a product (<b>4</b>) in which only one of the boron-containing rings is expanded through formal insertion of CO into a B–C bond. The X-ray structures of both <b>3</b> and <b>4</b> are reported, and reasonable paths to their formation are proposed

Synthesis and Reactivity of a Terminal Scandium Imido Complex

Preparation of a terminal scandium imido complex, <b>2</b>·DMAP, was accomplished through thermolysis of an arylamido methyl complex, <b>1</b>, stabilized by a bulky β-diketiminato ligand in the presence of 4-<i>N</i>,<i>N</i>-dimethylaminopyridine (DMAP). Mechanistic studies revealed that the reaction proceeds by initial metalation of <b>1</b>, followed by rapid DMAP-promoted alkane elimination to generate the scandium imido complex. Kinetic studies of the reaction between separately synthesized metalate <b>3</b> and DMAP under pseudo-first-order conditions yielded activation parameters of Δ<i>H</i>^⧧ = 73.5(2) kJ mol^–1 and Δ<i>S</i>^⧧ = −70.4(5) J K^–1 mol^–1. The reaction of <b>2</b>·DMAP with <i>tert</i>-butyl amine or phenylacetylene resulted in addition of the N–H or C–H bond across the scandium imide linkage, respectively, to furnish complexes <i>endo-</i>/<i>exo</i><b>-4</b> and <i>endo</i><b>-5</b>. These compounds were fully characterized, including via structural analysis, providing further evidence for the terminal scandium imido derivative <b>2</b>·DMAP