The controlled construction of defined oxidation patterns is one of the key aspects in the synthesis of natural products and bioactive molecules. Towards this goal, we herein report a novel protocol for the Pd-catalyzed direct β-C­(sp3)–H acetoxylation of aliphatic carboxylic acids. The protocol enables the use of free carboxylic acids in one step and without the need of introducing specialized strong directing groups. In our studies, we found that the use of a “traceless base” was crucial for the development of a synthetically useful transformation. Furthermore, the synthetic utility of the products obtained was demonstrated by their use in subsequent transformations.

The synthesis of kinetically stabilized, i.e., metastable, dielectric semiconductors, represents a major frontier within technologically important fields as compared to thermodynamically stable solids that have received considerably more attention. Of long-standing theoretical interest are Sn(II) perovskites [e.g., Sn-(Zr 1/2 Ti 1/2 )O 3 (SZT)], which are isoelectronic Pb-free analogues of Pb(Zr 1/2 Ti 1/2 )O 3 (PZT), a commercial piezoelectric composition that is dominant in the electronics industry. Herein, we describe the synthesis of this metastable SZT dielectric through a low-temperature flux reaction technique. The SZT has been found, for the first time, to grow and to be stabilized as a nanoshell at the surfaces of Ba(Zr 1/2 Ti 1/2 )O 3 (BZT) particles, i.e., forming as BZT−SZT core−shell particles, as a result of Sn(II) cation exchange. In situ powder X-ray diffraction (XRD) and transmission electron microscopy data show that the SZT nanoshells result from the controlled cation diffusion of Sn(II) cations into the BZT particles, with tunable thicknesses of ∼25−100 nm. The SZT nanoshell is calculated to possess a metastability of approximately −0.5 eV atom −1 with respect to decomposition to SnO, ZrO 2 , and TiO 2 and cannot currently be prepared as stand-alone particles. Rietveld refinements of the XRD data are consistent with a two-phase BZT−SZT model, with each phase possessing a generally cubic perovskite-type structure and nearly identical lattice parameters. Mossbauer spectroscopic data ( 119 Sn) are consistent with Sn(II) cations within the SZT nanoshells and an outer ∼5−10 nm surface region comprised of oxidized Sn(IV) cations from exposure to air and water. The optical band gap of the SZT shell was found to be ∼2.2 eV, which is red-shifted by ∼1.2 eV compared to that of BZT. This closing of the band gap was probed by X-ray photoelectron spectroscopy and found to stem from a shift of the valence band edge to higher energies (∼1.07 eV) as a result of the addition of the Sn 5s 2 orbitals forming a new higher-energy valence band. In summary, a novel synthetic tactic is demonstrated to be effective in preparing metastable SZT and representing a generally useful strategy for the kinetic stabilization of other predicted, metastable dielectrics.

The stannide BaRhSn2 was synthesized by induction melting of an arc-melted RhSn2 precursor compound with barium in a sealed tantalum ampoule. The structure of BaRhSn2 was refined from single-crystal X-ray diffractometer data: MgCuAl2 type, Cmcm, a = 437.56(4), b = 1242.35(10), c = 767.30(6) pm, wR2 = 0.0845, 469 F 2 values and 16 variables. The rhodium and tin atoms form a two-dimensional [RhSn2] δ− polyanionic network with short Rh–Sn (273–274 pm) and Sn–Sn (303–312 pm) distances. The large barium atoms lead to a substantial orthorhombic distortion of the (lonsdaleite-related) tin substructure, forcing a break of the Sn–Sn bond in b direction. This change in the tin substructure is reflected in the 119Sn Mössbauer spectrum. The tin atoms exhibit a higher s electron density which is expressed in an increased isomer shift of δ = 2.08(1) mm s−1 as compared to the previously reported stannide CaRhSn2 with a three-dimensional [RhSn2] δ− polyanionic network and δ = 1.96(4) mm s−1.

The Ca2Pd2Ge-type stannides Sr2Pd2Sn and Eu2Pd2Sn were synthesized by reaction of the elements in sealed tantalum ampoules in a high-frequency furnace and characterized by powder X-ray diffraction. The structure of Sr2Pd2Sn (Fdd2, a = 1063.95(5), b = 1623.22(9), c = 594.63(14) pm, wR2 = 0.0472, 972 F 2 values and 26 variables) was refined from single-crystal X-ray diffractometer data. The striking structural motif features equidistant chains formed by the palladium atoms (304.7 pm Pd–Pd), which are interlinked by the tin atoms (266.9 and 268.7 pm Pd–Sn). Together, the palladium and tin atoms form a three-dimensional [Pd2Sn] δ– polyanionic network in which the strontium atoms reside in larger cavities. The divalent character of europium in Eu2Pd2Sn was manifested by 151Eu Mössbauer spectroscopy. The isomer shift is δ = −9.48(1) mm s−1 at room temperature. The results of 119Sn Mössbauer-spectroscopic experiments have confirmed the tin site determined by the single-crystal study, the isomer shifts being δ = 1.71(1) mm s−1 for Eu2Pd2Sn and δ = 1.73(1) mm s−1 for Sr2Pd2Sn. Sr2Pd2Sn is a Pauli paramagnet with a susceptibility of 2.2(1) × 10−5 emu mol−1 at room temperature. Eu2Pd2Sn shows Curie-Weiss paramagnetism with an experimental magnetic moment of 7.85(1) µB per Eu atom, confirming divalent europium. The europium magnetic moments order antiferromagnetically at T N = 14 K.

The ternary Laves phases Sr2Pd3Sn, Eu2Pd3Sn and Eu2Pd3In were synthesized by induction melting of the elements in sealed tantalum ampoules. The polycrystalline products were characterized through their powder X-ray diffraction patterns. The structure of Eu2Pd3Sn was refined from single crystal X-ray diffractometer data: Mg2MnGa3 type, Cmcm, a = 583.36(5), b = 908.31(7), c = 958.06(8) pm, wR2 = 0.0366, 557 F 2 values, 23 variables. The palladium and tin atoms show the inverse coloring on the network of condensed tetrahedra of Mg2MnGa3, i.e., MnGa3 versus Pd3Sn. Refinement of the occupancy parameters revealed small defects for the europium site, leading to composition Eu1.962(6)Pd3Sn for the studied crystal. Sr2Pd3Sn is a Pauli paramagnet and Eu2Pd3Sn shows Curie-Weiss paramagnetism (7.86(1) µB Eu atom−1 and Θ P = 48.1(1) K). Ferromagnetic ordering is observed below T C = 46.1(1) K. The 119Sn and 151Eu Mössbauer spectra of Sr2Pd3Sn and Eu2Pd3Sn are discussed with respect to electron density changes as a function of the tin content and the ionicity in the sequence of the stannides Sr2Pd3Sn/Eu2Pd3Sn → Sr2Pd2Sn/Eu2Pd2Sn → EuPdSn → EuPdSn2.