Can Ultrasound or pH Influence Pd Distribution on the Surface of HAP to Improve Its Catalytic Properties in the Dry Reforming of Methane?

The influence of ultrasound and different pH pre-treatments during the metal doping/modification of a hydroxyapatite (HAP) support is investigated. HAP is first synthesised via a hard-template synthetic route using carbon nanorods followed by their full physiochemical characterisation. The HAP was found to be crystalline and comprised a mesoporous structure as observed via XRD and nitrogen adsorption with a BET surface area of 97.57 (±1.16) m2 g−1. Ultrasound-assisted ion exchange (IE) and incipient wetness impregnation (IW) methodologies were employed to decorate the surface of HAP with Pd0 and are compared to previous procedures. The influence of pH upon the distribution of Pd0 throughout the samples during the doping process is also studied. All the prepared samples were evaluated for their catalytic activity towards dry reforming of methane (DRM) and the reaction was monitored via a thermal conductivity detector, coupled with gas chromatography (GC-TCD). It was found that ultrasound-assisted IE significantly accelerated the process from 3 days to 3 h and with the Pd0 metal remaining highly distributed upon the HAP with minor changes in catalytic conversions. Moreover, the ultrasound-assisted IW method successfully improved the Pd0 distribution and catalytic performance. On the other hand, the dispersion of the metal was unaffected after pH treatments in IE with no catalytic improvements observed, in contrast to IW, where considerable increase in metal distribution and subsequently catalytic performance was observed

Insights into tungsten catalyzed ring expansion polymerization

The pincer tungsten (formal IV) alkylidene complex depicted in Figure 1 is a potent catalyst for ring-expansion polymerization of alkynes, e.g. reaching TONs of > 17,000 and activities in excess of 5,000 kg mol-1 h-1 (with phenylactylene) and degrees of polymerization, Pn, of up to 1000-2000 (with propene and 1-decene).1-4 Monomer dependent polymer dispersity and the known sensitivity of the system to changes in the alkylidene R-group hint at the complexity of the polymerization mechanism. Several mechanistic pathways are possible, including an initial insertion of the monomer into the catalyst backbone tungsten-aryl bond, which for example has been noted to lead to the active species in Hf(IV) pyridylamido catalyzed olefin polymerization.5 The key question is how the catalyst avoids unproductive benzene formation. In this regard, the flexibility of the alkylidene ligand to change from being part of the backbone to being part of the growing ring appears crucial. Using a combination of DFT mechanism exploration and experimental insight we will highlight recent developments

Isolation of an Elusive Phosphametallacyclobutadiene and Its Role in Reversible Carbon-Carbon Bond Cleavage

The reactivity of phosphaalkynes, the isolobal and isoelectronic congeners to alkynes, with metal alkylidyne complexes is explored in this work. Treating the tungsten alkylidyne [t BuOCO]W≡Ct Bu(THF)2 (1) with phosphaalkyne (10) results in the formation of [O2 C(t BuC=)W{η2 -(P,C)-P≡C-Ad}(THF)] (13-t BuTHF ) and [O2 C(AdC=)W{η2 -(P,C)-P≡C-t Bu}(THF)] (13-AdTHF ); derived from the formal reductive migratory insertion of the alkylidyne moiety into a W-Carene bond. Analogous to alkyne metathesis, a stable phosphametallacyclobutadiene complex [t BuOCO]W[κ2 -C(t Bu)PC(Ad)] (14) forms upon loss of THF from the coordination sphere of either 13-t BuTHF or 13-AdTHF . Remarkably, the C-C bonds reversibly form/cleave with the addition or removal of THF from the coordination sphere of the formal tungsten(VI) metal center, permitting unprecedented control over the transformation of a tetraanionic pincer to a trianionic pincer and back. Computational analysis offers thermodynamic and electronic reasoning for the reversible equilibrium between 13-t Bu/AdTHF and 14

A New ONO^3‑ Trianionic Pincer-Type Ligand for Generating Highly Nucleophilic Metal–Carbon Multiple Bonds

Appending an amine to a CC double bond drastically increases the nucleophilicity of the β-carbon atom of the alkene to form an enamine. In this report, we present the synthesis and characterization of a novel CF₃–ONO^3‑ trianionic pincer-type ligand, rationally designed to mimic enamines within a metal coordination sphere. Presented is a synthetic strategy to create enhanced nucleophilic tungsten–alkylidene and −alkylidyne complexes. Specifically, we present the synthesis and characterization of the new CF₃–ONO^3‑ trianionic pincer tungsten–alkylidene [CF₃–ONO]­WCH­(Et)­(O^<i>t</i>Bu) (<b>2</b>) and −alkylidyne {MePPh₃}­{[CF₃–ONO]­WC­(Et)­(O^<i>t</i>Bu)} (<b>3</b>) complexes. Characterization involves a combination of multinuclear NMR spectroscopy, combustion analysis, DFT computations, and single crystal X-ray analysis for complexes <b>2</b> and <b>3</b>. Exhibiting unique nucleophilic reactivity, <b>3</b> reacts with MeOTf to yield [CF₃–ONO]­WC­(Me)­(Et)­(O^<i>t</i>Bu) (<b>4</b>), but the bulkier Me₃SiOTf silylates the <i>tert</i>-butoxide, which subsequently undergoes isobutylene expulsion to form [CF₃–ONO]­WCH­(Et)­(OSiMe₃) (<b>5</b>). A DFT calculation performed on a model complex of <b>3</b>, namely, [CF₃–ONO]­WC­(Et)­(O^<i>t</i>Bu) (<b>3</b>′), reveals the amide participates in an enamine-type bonding combination. For complex <b>2</b>, the Lewis acids MeOTf, Me₃SiOTf, and B­(C₆F₅)₃ catalyze isobutylene expulsion to yield the tungsten–oxo complex [CF₃–ONO]­W­(O)­(^<i>n</i>Pr) (<b>6</b>)