In situ Investigations of the Formation Mechanism of Metastable γ-BiPd Nanoparticles in Polyol Reductions.

Synthesizing intermetallic phases containing noble metals often poses a challenge as the melting points of noble metals often exceed the boiling point of bismuth (1560 °C). Reactions in the solid state generally circumvent this issue but are extremely time consuming. A convenient method to overcome these obstacles is the co-reduction of metal salts in polyols, which can be performed within hours at moderate temperatures and even allows access to metastable phases. However, little attention has been paid to the formation mechanisms of intermetallic particles in polyol reductions. Identifying crucial reaction parameters and finding patterns are key factors to enable targeted syntheses and product design. Here, we chose metastable γ-BiPd as an example to investigate the formation mechanism from mixtures of metal salts in ethylene glycol and to determine critical factors for phase formation. The reaction was also monitored by in situ X-ray diffraction using synchrotron radiation. Products, intermediates and solutions were characterized by (in situ) X-ray diffraction, electron microscopy, and UV-Vis spectroscopy. In the first step of the reaction, elemental palladium precipitates. Increasing temperature induces the reduction of bismuth cations and the subsequent rapid incorporation of bismuth into the palladium cores, yielding the γ-BiPd phase.

Crystal structure of bis-(N,N'-di-methyl-thio-urea-κS)bis-(thio-cyanato-κN)cobalt(II).

During systematic investigations on the synthesis of coordination polymers with Co(NCS)2 involving different thio-urea derivatives as coligands, crystals of the title compound Co(NCS)2(N,N'-di-methyl-thio-urea)2, or [Co(C3H8N2S)2(NCS)2], were obtained. These crystals were non-merohedric twins and therefore, a twin refinement using data in HKLF-5 format was performed. In the crystal structure of this compound, the CoII cations are coordinated by two N-terminally bonded thio-cyanate anions as well as two S-bonding N,N'-di-methyl-thio-urea mol-ecules, forming two crystallographically independent discrete complexes each with a strongly distorted tetra-hedral geometry. An intricate network of inter-molecular N-H⋯S and C-H⋯S hydrogen bonds can be found between the complexes. The thermogravimetric curve of the title compound shows two discrete steps in which all coligand mol-ecules have been emitted, which is also accompanied by partial decomposition of the cobalt thio-cyanate. If the measurement is stopped after the first mass loss, only broad reflections of CoS can be found in the XRPD pattern of the residue, which proves that this compound decomposes completely upon heating. However, at lower temperatures an endothermic signal can be found in the DTA and DSC curve, which corresponds to melting, as proven by thermomicroscopy.

Crystal structure of bis-(tetra-methyl-thio-urea-κS)bis(thio-cyanato-κN)cobalt(II).

In the course of systematic investigations on the synthesis of Co(NCS)2 coordination compounds with different thio-urea ligands, the title compound, [Co(NCS)2(C5H12N2S)2], was obtained. In this compound the CoII cations are coordinated by two crystallographically independent N-bonded thio-cyanate anions and two tetra-methyl-thio-urea ligands into discrete complexes that are located in general positions and show a strongly distorted tetra-hedral geometry. Inter-molecular C-H⋯S hydrogen bonds of different strength can be observed between the discrete complexes, which are connected by pairs of hydrogen bonds into zigzag-like chains that elongate in the b-axis direction. These chains are additionally linked by strong C-H⋯S hydrogen bonds along the a-axis direction, resulting in the formation of layers that are parallel to the ab plane. There is also one weak intra-molecular C-H⋯S hydrogen bond between two neighbouring thio-urea ligands within the complexes. Comparison of the experimental PXRD pattern with that calculated from the single-crystal data prove that a pure phase has been obtained. Thermoanalytical investigations reveal that this compound melts at 364 K and decomposes upon further heating.

Crystal structure of bis-(tetra-methyl-thio-urea-κS)bis-(thio-cyanato-κN)nickel(II).

In the course of our investigations regarding transition-metal thio-cyanates with thio-urea derivatives, the title compound, [Ni(NCS)2(C5H12N2S)2], was obtained. The asymmetric unit consists of one thio-cyanate anion and one tetra-methyl-thio-urea mol-ecule on general positions, as well as one NiII cation that is located on a twofold rotational axis. In this compound, discrete complexes are formed in which the NiII cations are surrounded by two trans-N-bonding thio-cyanate anions as well as two trans-S-bonding tetra-methyl-thio-urea mol-ecules within a distorted square-planar coordination geometry. The discrete complexes are linked by pairs of weak C-H⋯S hydrogen bonds between the thio-cyanate S and one of the tetra-methyl-thio-urea methyl hydrogen atoms into chains along the crystallographic a- and c-axis directions, which are combined into layers parallel to the ac plane. X-ray powder diffraction proves that a pure crystalline phase was obtained and measurements using thermogravimetry and differential thermoanalysis reveal that the compound decomposes at about 408 K, where all tetra-methyl-thio-urea mol-ecules are lost.