Improved carrier concentration control in Zn-doped Ca_5Al_2Sb_6

Ca_5Al_2Sb_6 is an inexpensive, Earth-abundant compound that exhibits promising thermoelectric efficiency at temperatures suitable for waste heat recovery. Inspired by our previous study of p-type Ca_(5−x)Na_xAl_2Sb_6, this work investigates doping with Zn^(2+) on the Al^(3+) site (Ca_5Al_(2−x)Zn_xSb_6). We find Zn to be an effective p-type dopant, in contrast to the low solubility limit and poor doping efficiency of Na. Seebeck coefficient measurements indicate that the hole band mass is unaffected by the dopant type in the high-zT temperature range. Band structure and density of states calculations are employed in order to understand the carrier concentration-dependent effective mass. Ca_5Al_(2−x)Zn_xSb_6 has a low lattice thermal conductivity that approaches the predicted minimum value at high temperature (1000 K) due to the complex crystal structure and high mass contrast

Mechanism of the Iron(II)-Catalyzed Hydrosilylation of Ketones: Activation of Iron Carboxylate Precatalysts and Reaction Pathways of the Active Catalyst

A detailed mechanistic study of the catalytic hydrosilylation of ketones with the highly active and enantioselective iron­(II) boxmi complexes as catalysts (up to >99% ee) was carried out to elucidate the pathways for precatalyst activation and the mechanism for the iron-catalyzed hydrosilylation. Carboxylate precatalysts were found to be activated by reduction of the carboxylate ligand to the corresponding alkoxide followed by entering the catalytic cycle for the iron-catalyzed hydrosilylation. An Eyring-type analysis of the temperature dependence of the enantiomeric ratio established a linear relationship of ln­(<i>S</i>/<i>R</i>) and <i>T</i>^–1, indicating a single selectivity-determining step over the whole temperature range from −40 to +65 °C (ΔΔ<i>G</i>^‡_sel, 233 K = 9 ± 1 kJ/mol). The rate law as well as activation parameters for the rate-determining step were derived and complemented by a Hammett analysis, radical clock experiments, kinetic isotope effect (KIE) measurements (<i>k</i>_H/<i>k</i>_D = 3.0 ± 0.2), the isolation of the catalytically active alkoxide intermediate, and DFT-modeling of the whole reaction sequence. The proposed reaction mechanism is characterized by a rate-determining σ-bond metathesis of an alkoxide complex with the silane, subsequent coordination of the ketone to the iron hydride complex, and insertion of the ketone into the Fe–H bond to regenerate the alkoxide complex

Iron Achieves Noble Metal Reactivity and Selectivity: Highly Reactive and Enantioselective Iron Complexes as Catalysts in the Hydrosilylation of Ketones

Chiral iron alkyl and iron alkoxide complexes bearing boxmi pincers as stereodirecting ligands have been employed as catalysts for enantioselective hydrosilylation reactions with unprecedented activity and selectivity (TOF = 240 h^–1 at −40 °C, ee up to 99% for alkyl aryl ketones), which match the performance of previously established noble-metal-based catalysts. This shows the potential of earth-abundant metals such as iron for replacing platinummetals without any drawbacks for the reaction design

Enantioselective Iron-Catalyzed Azidation of β‑Keto Esters and Oxindoles

The first example of Fe-catalyzed enantioselective azidations of β-keto esters and oxindoles using a readily available N₃-transfer reagent is reported. A number of α-azido-β-keto esters were obtained with up to 93% ee, and this methodology also generates 3-substitued 3-azidooxindoles with high enantioselectivities (up to 94%)

Enantioselective Iron-Catalyzed Azidation of β‑Keto Esters and Oxindoles

The first example of Fe-catalyzed enantioselective azidations of β-keto esters and oxindoles using a readily available N₃-transfer reagent is reported. A number of α-azido-β-keto esters were obtained with up to 93% ee, and this methodology also generates 3-substitued 3-azidooxindoles with high enantioselectivities (up to 94%)

Iridium Half-Sandwich Complexes with Di- and Tridentate Bis(pyridylimino)isoindolato Ligands: Stoichiometric and Catalytic Reactivity

A series of κ²-(<i>N</i>,<i>N</i>)-coordinated bis­(2-pyridylimino)­isoindolato (BPI) complexes [Cp*Ir­(BPI)­Cl], which possess “three-legged piano-stool” structures, with the iridium atom being coordinated by the Cp* ligand 2 × N and Cl, were prepared via deprotonation of the BPIH ligands with either potassium hydride or LDA and subsequent reaction with [Cp*IrCl₂]₂ in THF. Cationic complexes [Cp*Ir­(BPI)]⁺ containing κ³-(<i>N</i>,<i>N</i>,<i>N</i>)-coordinated BPI ligands were prepared as well as complexes with bidentate-coordinated BPI ligands, where the chloride ligand was substituted by either neutral or anionic ligands. Substitution in the <i>ortho</i>-position of the PBI ligands led to the formation of cycloiridated κ³-(<i>N</i>,<i>N</i>,<i>C</i>) species. Upon substitution of the anionic ligand by triphenylphosphine, a product was obtained with a hitherto unobserved κ²-(<i>N</i>,<i>N</i>) coordination of <i>o</i>Me-BPI to the metal center via the deprotonated nitrogen atom of the isoindole unit and one of the imine nitrogen atoms of the BPI ligand. A series of (<i>para</i>-cymene) osmium half-sandwich complexes with analogous structures and reactivities to their isoelectronic Cp*Ir­(BPI) congeners were also prepared. Finally, it has been demonstrated that both Ir and Os complexes are catalytically active in the transfer hydrogenation of various ketones and imines

Iridium Half-Sandwich Complexes with Di- and Tridentate Bis(pyridylimino)isoindolato Ligands: Stoichiometric and Catalytic Reactivity

A series of κ²-(<i>N</i>,<i>N</i>)-coordinated bis­(2-pyridylimino)­isoindolato (BPI) complexes [Cp*Ir­(BPI)­Cl], which possess “three-legged piano-stool” structures, with the iridium atom being coordinated by the Cp* ligand 2 × N and Cl, were prepared via deprotonation of the BPIH ligands with either potassium hydride or LDA and subsequent reaction with [Cp*IrCl₂]₂ in THF. Cationic complexes [Cp*Ir­(BPI)]⁺ containing κ³-(<i>N</i>,<i>N</i>,<i>N</i>)-coordinated BPI ligands were prepared as well as complexes with bidentate-coordinated BPI ligands, where the chloride ligand was substituted by either neutral or anionic ligands. Substitution in the <i>ortho</i>-position of the PBI ligands led to the formation of cycloiridated κ³-(<i>N</i>,<i>N</i>,<i>C</i>) species. Upon substitution of the anionic ligand by triphenylphosphine, a product was obtained with a hitherto unobserved κ²-(<i>N</i>,<i>N</i>) coordination of <i>o</i>Me-BPI to the metal center via the deprotonated nitrogen atom of the isoindole unit and one of the imine nitrogen atoms of the BPI ligand. A series of (<i>para</i>-cymene) osmium half-sandwich complexes with analogous structures and reactivities to their isoelectronic Cp*Ir­(BPI) congeners were also prepared. Finally, it has been demonstrated that both Ir and Os complexes are catalytically active in the transfer hydrogenation of various ketones and imines

Iridium Half-Sandwich Complexes with Di- and Tridentate Bis(pyridylimino)isoindolato Ligands: Stoichiometric and Catalytic Reactivity

A series of κ²-(<i>N</i>,<i>N</i>)-coordinated bis­(2-pyridylimino)­isoindolato (BPI) complexes [Cp*Ir­(BPI)­Cl], which possess “three-legged piano-stool” structures, with the iridium atom being coordinated by the Cp* ligand 2 × N and Cl, were prepared via deprotonation of the BPIH ligands with either potassium hydride or LDA and subsequent reaction with [Cp*IrCl₂]₂ in THF. Cationic complexes [Cp*Ir­(BPI)]⁺ containing κ³-(<i>N</i>,<i>N</i>,<i>N</i>)-coordinated BPI ligands were prepared as well as complexes with bidentate-coordinated BPI ligands, where the chloride ligand was substituted by either neutral or anionic ligands. Substitution in the <i>ortho</i>-position of the PBI ligands led to the formation of cycloiridated κ³-(<i>N</i>,<i>N</i>,<i>C</i>) species. Upon substitution of the anionic ligand by triphenylphosphine, a product was obtained with a hitherto unobserved κ²-(<i>N</i>,<i>N</i>) coordination of <i>o</i>Me-BPI to the metal center via the deprotonated nitrogen atom of the isoindole unit and one of the imine nitrogen atoms of the BPI ligand. A series of (<i>para</i>-cymene) osmium half-sandwich complexes with analogous structures and reactivities to their isoelectronic Cp*Ir­(BPI) congeners were also prepared. Finally, it has been demonstrated that both Ir and Os complexes are catalytically active in the transfer hydrogenation of various ketones and imines