Row Transition Metal Research Articles

Synthesis and Structural Characterization of Divalent Transition Metal Alkynylamidinate Complexes

AbstractA series of new alkynylamidinate complexes of selected first and second row transition metals has been synthesized and fully characterized. Treatment of MCl2 precursors (M=Mn, Fe, Co) with 2 equiv. of the lithium alkynylamidinates Li[c‐C3H5−C≡C−C(NR′)2] ⋅ THF (R′=iPr (2), Cy (cyclohexyl) (2)) afforded a series of binuclear complexes of the type M2[c‐C3H5−C≡C−C(NR)2‐κN:κN′]2[c‐C3H5−C≡C−C(NR)2‐κ2N,N′]2 (3: M=Mn, R=Cy; 4 a: M=Fe, R=iPr; 4 b: M=Fe, R=Cy; 5: M=Co, R=iPr) with no significant metal‐metal bonding. In marked contrast, a similar reaction of CrCl2 with 2 equiv. of 1 afforded the homoleptic dinuclear chromium(II) complex Cr2[c‐C3H5−C≡C−C(NiPr)2‐κN:κN′]4 (6) which supposedly comprises a Cr−Cr quadruple bond. Complex 6 could also be prepared in a more rational way and in better yield (61 %) by using dichromium(II) tetraacetate, Cr2(OAc)4, as starting material. Related reactions employing dimolybdenum(II) tetraacetate, Mo2(OAc)4, and 2 or 3 equiv. of 1 afforded the mixed‐ligand paddle wheel‐type complexes trans‐Mo2(OAc‐κO:κO′)2([c‐C3H5−C≡C−C(NiPr)2‐κN:κN′]2 (7) and Mo2(OAc‐κO:κO′)([c‐C3H5−C≡C−C(NiPr)2‐κN:κN′]3 (8). All title compounds were structurally characterized through single‐crystal X‐ray diffraction and spectroscopic techniques (NMR, IR, Raman).

Hydroxybenzoate salts of cobalt(II), manganese(II), and nickel(II): synthesis, crystal structure, Hirshfeld surface analyses, and thermal behavior

The reaction of hydroxybenzoates with divalent first row transition metal ions afforded four mononuclear complexes, [M(OH2)6](dhbzc)2·2H2O (M = Co2+, 1 or Mn2+, 2), [Ni(OH2)6](hbzc)2·2H2O (3) and [Ni(OH2)5(hbzc)](hbzc)·3H2O (4) (where dhbzc− = 2,6-dihydroxybenzoate and hbzc− = 4-hydroxybenzoate). The complexes were characterized by single crystal and powder X-ray diffraction analyses, infrared spectroscopy, and thermogravimetric analysis. Results showed that the hydrogen bonds present in dhbzc− impaired its coordination to Co2+ and Mn2+. Thereafter, Hirshfeld surface analyses (HS) and 2D fingerprint plots were used to investigate the intermolecular contacts in 1–4. The offset π···π stacking of the dhbzc− counterions in 1 and 2 lead to C···C close contacts over C···H/H···C ones. Complexes 3 and 4 presented the usual herringbone aromatic-based packing for the hbzc− rings with a predominance of C···H/H···C close contacts. The existence of water molecules in the crystal lattices of 1–4 was determined by TG analyses, which proved the presence of coordinated and uncoordinated water molecules. The thermal decomposition patterns were analyzed in light of the percentage of the strongest intermolecular close contacts.

Recent Developments in Manganese, Iron and Cobalt Homogeneous Catalyzed Synthesis of Primary Amines via Reduction of Nitroarenes, Nitriles and Carboxamides

AbstractThe preparation of primary amines attracts huge attention in the molecular synthetic community. Indeed, amines play a crucial role in chemistry as they are important substructures in a huge amount of drug molecules, agrochemicals, dyes and molecular materials. Compared to stoichiometric reductions using metal hydride based reagents, the catalyzed reduction of nitroarenes, nitriles and carboxamides appears to be an attractive option, mainly when associated to first row transition metals such as manganese, iron, or cobalt. In this short review, we illustrate the progress achieved in homogeneous hydrogenation, hydrogen transfer, hydrogen borrowing and hydrosilylation of nitroarenes, nitriles and carboxamides.magnified image

Thermal Atomic Layer Etching of CoO, ZnO, Fe2O3, and NiO by Chlorination and Ligand Addition Using SO2Cl2 and Tetramethylethylenediamine

Thermal atomic layer etching (ALE) of CoO, ZnO, Fe2O3, and NiO was achieved using chlorination and ligand-addition reactions at 250 °C. This two-step process was accomplished by first chlorinating the metal oxide with SO2Cl2. Subsequently, ligand addition to the metal chloride was performed using tetramethylethylenediamine (TMEDA). In situ quadrupole mass spectrometry (QMS) studies on metal oxide powders revealed that CoO, ZnO, Fe2O3, and NiO all formed stable and volatile MCl2(TMEDA) compounds (M = Co, Zn, Fe, Ni) as etch products at 250 °C. These QMS studies of the sequential SO2Cl2 and TMEDA exposures were facilitated by a new reactor design with two nested inlet lines that transport the reactants separately to the powder substrate. The time-dependence of the reactants and products could also be monitored by the QMS investigations. The large SO2+ ion intensity observed at the beginning of the SO2Cl2 exposure was consistent with the chlorination reaction MO + SO2Cl2 → MCl2 + SO2 + (1/2)O2. The time-dependent QMS studies also observed the MClx(TMEDA)+ ion intensity peaking at the beginning of the TMEDA exposures. The subsequent decay of the MClx(TMEDA)+ ion intensity, while the (TMEDA)+ ion intensity remained constant, was evidence for a self-limiting ligand-addition reaction. The mass loss of the metal oxide powders was confirmed after sequential SO2Cl2 and TMEDA exposures. The etching of two of these metal oxides was also verified using separate experiments on flat substrates using SO2Cl2 and TMEDA exposures at 250 °C. For CoO thermal ALE, an etch rate of 4.1 Å/cycle at 250 °C was measured using X-ray reflectivity (XRR) studies. For ZnO thermal ALE, an etch rate of 0.12 Å/cycle at 250 °C was measured using quartz crystal microbalance (QCM) investigations. Other first row transition metal oxides were surveyed in addition to CoO, ZnO, Fe2O3, and NiO. QMS studies of TiO2, Cr2O3, and MnO2 showed no volatile species formation during sequential SO2Cl2 and TMEDA exposures at 250 °C. In contrast, V2O5 and CuO were spontaneously etched using SO2Cl2 at 250 °C, as determined by the observation of volatile VOCl3 and CuCl3 etch products, respectively. Calculated Gibbs free energy changes for the various etching reactions also supported the experimental observations for the first row transition metal oxides. These studies illustrate that the chlorination and ligand-addition reaction mechanism can provide a new avenue for the thermal ALE of a variety of transition metal oxides that have nonvolatile metal chlorides.

First-Principles Study of the Doping Effect in Half Delithiated LiNiO2 Cathodes

A systematic theoretical study of dopants in a half-delithiated lithium nickel oxide (Li0.5NiO2) cathode is performed to determine the preferred occupation sites, dopant ion migration, and suppression of oxygen evolution. Dopants considered include Li, B, Na, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Y, Zr, Nb, Mo, In, Sn, Sb, La, Ce, Ta, and W. For the nontransition metal dopants, the energy barrier governing dopant migration is correlated with the number of valence shell electrons so that it increases from left to right across a row of the periodic table. For these dopants, the energy barrier also decreases moving down a column of the periodic table. For transition metal dopants, the energy barrier depends on the number of unpaired valence electrons of the dopant, so that the energy barrier is maximum near the middle of a transition metal row. Oxygen stability is studied by calculating the gap between O-2p and Ni-3d band centers. Most dopants that prefer occupying octahedral sites in the transition metal layer bring higher oxygen stability. In particular, oxygen stability is enhanced mostly by Mo, Sb, and W doping.

First row transition metal doped B12P12 and Al12P12 nanocages as excellent single atom catalysts for the hydrogen evolution reaction

The hydrogen evolution reaction (HER) is a promising process to produce high purity hydrogen gas. However, the overpotential of this reaction hinders its practical applications. Single atom catalysts (SACs) are recently investigated by the scientific community to facilitate the HER. Herein, we studied the doping of late first-row transition metals on the B12P12 and Al12P12 nano-cages as SACs via density functional theory (DFT) calculations. Results show that all transition metals are chemisorbed on the support, with interaction energies ranging from −0.65 to −3.85 eV. The calculated Gibbs free energies of hydrogen evolution are −0.01, −0.06 and −0.20 eV for Ni@Al12P12, Ni@B12P12, and Co@B12P12, respectively, which are close to the optimum value of 0.00 eV, and comparable to the highly active Pt-based catalysts in literature. Our results indicate that the designed Ni@Al12P12, Ni@B12P12, and Co@B12P12 SACs are excellent candidates as noble metal-free, sufficiently stable, and highly efficient electrocatalysts for HER.

Earth-Abundant Transition Metal Catalyzed Asymmetric Hydrogenation of Minimally Functionalized Alkenes

AbstractTransition-metal-catalyzed asymmetric hydrogenation (AH) is a growing field and a fundamental tool for the construction of chiral compounds. The use of earth-abundant transition metals in AH reactions remains generally limited but has received increased attention in recent years due to cost, sustainability, and environmental concerns. Here, we will summarize progress in first row transition metal catalyzed AH of minimally functionalized alkenes, including scope, mechanism, and challenges in this field.1 Introduction2 Ti-Catalyzed AH of Minimally Functionalized Alkenes3 Zr-Catalyzed AH of Minimally Functionalized Alkenes4 Co-Catalyzed AH of Minimally Functionalized Alkenes5 Fe-Catalyzed AH of Minimally Functionalized Alkenes6 Summary and Outlook

Ab Initio Calculations of XUV Ground and Excited States for First-Row Transition Metal Oxides

Transient X-ray spectroscopies have become ubiquitous in studying photoexcited dynamics in solar energy materials due to their sensitivity to carrier occupations and local chemical or structural dynamics. The interpretation of solid-state photoexcited dynamics, however, is complicated by the core–hole perturbation and the resulting many-body dynamics. Here, an ab initio, Bethe–Salpeter equation (BSE) approach is developed that can incorporate photoexcited state effects for solid-state materials. The extreme ultraviolet (XUV) absorption spectra for the ground, photoexcited, and thermally expanded states of first row transition metal oxides─TiO2, α-Cr2O3, β-MnO2, α-Fe2O3, Co3O4, NiO, CuO, and ZnO─are calculated to demonstrate the accuracy of this approach. The theory is used to decompose the core–valence excitons into the separate components of the X-ray transition Hamiltonian for each of the transition metal oxides investigated. The decomposition provides a physical intuition about the origins of XUV spectral features as well as how the spectra will change following photoexcitation. The method is easily generalized to other K, L, M, and N edges to provide a general approach for analyzing transient X-ray absorption or reflection data.

Electron transfer catalysis mediated by 3d complexes of redox non-innocent ligands possessing an azo function: a perspective.

It was first reported almost two decades ago that ligands with azo functions are capable of accepting electron(s) upon coordination to produce azo-anion radical complexes, thereby exhibiting redox non-innocence. Over the past two decades, there have been numerous reports of such complexes along with their structures and diverse characteristics. The ability of a coordinated azo function to accept one or more electron(s), thereby acting as an electron reservoir, is currently employed to carry out electron transfer catalysis since they can undergo redox transformation at mild potentials due to the presence of energetically accessible energy levels. The cooperative involvement of redox non-innocent ligand(s) containing an azo group and the coordinated metal centre can adjust and modulate the Lewis acidity of the latter through selective ligand-centred redox events, thereby manipulating the capacity of the metal centre to bind to the substrate. We have summarized the list of first row transition metal complexes of iron, cobalt, nickel, copper and zinc with redox non-innocent ligands incorporating an azo function that have been exploited as electron transfer catalysts to effectuate sustainable synthesis of a wide variety of useful chemicals. These include ketazines, pyrimidines, benzothiazole, benzoxazoles, N-acyl hydrazones, quinazoline-4(3)H-ones, C-3 alkylated indoles, N-alkylated anilines and N-alkylated heteroamines. The reaction pathways, as demonstrated by catalytic loops, reveal that the azo function of a coordinated ligand can act as an electron sink in the initial steps to bring about alcohol oxidation and thereafter, they serve as an electron pool to produce the final products either via HAT or PCET processes.

Slow magnetic relaxation in 8-coordinate Mn(II) compounds.

The design and synthesis of high-spin Mn(II)-based single-molecule magnets (SMMs) have not been well developed to a great extent, as compared with a large number of SMMs based on the other first row transition metal complexes. In light of our success in designing Fe(II), Co(II) and Fe(III)-based SMMs with a high coordination number of 8, it is of great interest to design Mn(II) analogues with such a strategy. In this contribution, four Mn(II) compounds, [MnII(Ln)2](ClO4)2 (1-4) were obtained from reactions of neutral tetradentate ligands, L1-L4, with hydrated MnII(ClO4)2 (L1 = 2,9-bis(carbomethoxy)-1,10-phenanthroline, L2 = 2,9-bis(carbomethoxy)-2,2'-dipyridine, L3 = N2,N9-dibutyl-1,10-phenanthroline-2,9-dicarboxamide, L4 = 6,6'-bis(2-(tert-butyl)-2H-tetrazol-5-yl)-2,2'-bipyridine). Their crystal structures have been determined by X-ray crystallography and it clearly shows that the Mn(II) centers in these compounds have an oversaturated coordination number of 8. Their magnetic properties have been investigated in detail; to our surprise, all of these Mn(II) compounds show interesting slow magnetic relaxation behaviors under an applied direct current field, although they have very small negative D values.

Synthesis, antioxidant activity, antimicrobial efficacy and molecular docking studies of 4-chloro-2-(1-(4-methoxyphenyl)-4,5-diphenyl-1H-imidazol-2-yl)phenol and its transition metal complexes.

Herein, a one-pot synthesis of tetra-substituted imidazole, 4-chloro-2-(1-(4-methoxyphenyl)-4,5-diphenyl-1H-imidazol-2-yl)phenol (HL), is reported by the reaction of benzil, 5-bromosalicylaldehyde, ammonium acetate and anisidine. The synthesized imidazole was reacted with salts of 1st row transition metals (Co(ii), Ni(ii), Cu(ii), Mn(ii) and Zn(ii)) to obtain metal complexes. The structure of the compounds was confirmed using various spectroscopic and analytical techniques. HL, which is crystalline, was characterized by SC-XRD. Subsequently, the synthesized compounds were evaluated for their antioxidant and antimicrobial activities. Antimicrobial studies revealed the more noxious nature of metal complexes compared to ligand against various strains of bacteria and fungi. Molecular docking results based on the binding energy values also supported the experimental results of the antioxidant activities of the compounds. HL was found to be a better antioxidant than metal complexes. For a better insight into the structure, computational studies of the compounds were also carried out. A clear intra-molecular charge transfer was perceived in the ligand and its metal complexes. The transfer integral values for holes (36.48 meV) were found to be higher than the electron transfer integrals (24.76 meV), which indicated that the ligand would be a better hole transporter. According to the frontier molecular orbitals of the dimer, the charge transfer within the molecule is found from monomer 1 to 2.

Palladium-anchored donor-flexible pyridylidene amide (PYA) electrocatalysts for CO2 reduction.

The conversion of CO2 into CO as a substitute for processing fossil fuels to produce hydrocarbons is a sustainable, carbon neutral energy technology. However, the electrochemical reduction of CO2 into a synthesis gas (CO and H2) at a commercial scale requires an efficient electrocatalyst. In this perspective, a series of six new palladium complexes with the general formula [Pd(L)(Y)]Y, where L is a donor-flexible PYA, N2,N6-bis(1-ethylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide, N2,N6-bis(1-butylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide, or N2,N6-bis(1-benzylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide, and Y = OAc or Cl-, were utilized as active electrocatalysts for the conversion of CO2 into a synthesis gas. These palladium(ii) pincer complexes were synthesized from their respective H-PYA proligands using 1,8-diazobicyclo[5.4.0]undec-7-ene (DBU) or sodium acetate as a base. All the compounds were successfully characterized by various physical methods of analysis, such as proton and carbon NMR, FTIR, CHN, and single-crystal XRD. The redox chemistry of palladium complexes toward carbon dioxide activation suggested an evident CO2 interaction with each Pd(ii) catalyst. [Pd(N2,N6-bis(1-ethylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide)(Cl)]Cl showed the best electrocatalytic activity for CO2 reduction into a synthesis gas under the acidic condition of trifluoracetic acid (TFA) with a minimum overpotential of 0.40 V, a maximum turnover frequency (TOF) of 101 s-1, and 58% FE of CO. This pincer scaffold could be stereochemically tuned with the exploration of earth abundant first row transition metals for further improvements in the CO2 reduction chemistry.

Desorption of Ammonia Adsorbed on Prussian Blue Analogs by Washing with Saturated Ammonium Hydrogen Carbonate Solution

Prussian blue analogs (PBAs) have been reported as promising ammonia (NH3) adsorbents with a high capacity compared to activated carbon, zeolite, and ion exchange resins. The adsorbed NH3 was desorbed by heating and washing with water or acid. Recently, we demonstrated that desorption was also possible by washing with a saturated ammonium hydrogen carbonate solution (sat. NH4HCO3aq) and recovered NH3 as an NH4HCO3 solid by introducing CO2 into the washing liquid after desorption. However, this has only been proven for copper ferrocyanide and the relationship between the adsorption/desorption behavior and metal ions in PBAs has not been identified. In this study, we investigated the adsorption/desorption behavior of PBAs that are complexes of first row transition metals with hexacyanometalate anions. Six types of PBAs were tested in this study and copper ferricyanide exhibited the highest desorption/adsorption ratio. X-ray diffraction results revealed high structural stability for cobalt hexacyanocobaltate (CoHCC) and nickel ferricyanide (NiHCF). The Fourier transform infrared spectroscopy results showed that the NH3 adsorbed on the vacancy sites tended to desorb compared to the NH3 adsorbed on the interstitial sites as ammonium ions. Interestingly, the desorption/adsorption ratio exhibited the Irving-Williams order.

Femtosecond Extreme Ultraviolet Spectroscopy of an Iridium Photocatalyst Reveals Oxidation State and Ligand Field Specific Dynamics

Femtosecond X-ray absorption spectroscopy at the Ir O3-edge and N6,7-edges is performed on the photocatalyst iridium(III) tris(2-phenylpyridine), Ir(III)(ppy)3 using a tabletop high-harmonic source. Extreme ultraviolet (XUV) absorption between 44 and 76 eV measures transitions from the Ir 5p3/2 and 4f5/2,7/2 core to 5d valence orbitals, and the position of these spectral features is shown to be sensitive to the oxidation state and ligand field of the metal center. Upon excitation of the singlet metal-to-ligand charge transfer (1MLCT) band at 400 nm, a shift in the spectra due to the formation of the Ir(IV) center is observed, as is the creation of a new spectral feature corresponding to transitions into the t2g hole. Vibrational cooling of the MLCT state on the 3 and 16 ps time scales is measured as changes in the intensity of the transient features. This work establishes XUV spectroscopy as a useful tool for measuring the electronic structure of third row transition metal photosensitizers and catalysts at ultrafast time scales.

Boosting the Donor Effect of Side‐On W(II) Alkyne Complex Based Diphosphines

AbstractRedox‐active tungsten alkyne complex‐based phosphines of the type [Tp*W(CO)Br(η2‐C2(PR2)x] (x=1, 2; R=Ph, i‐Pr) and coordination experiments with the corresponding diphosphines are described. The pKB values of the terminal free phosphine groups were estimated indirectly by the 1JP/Se coupling constants in the associated P‐selenides. A ten orders of magnitude higher basicity for the bis(diisopropyl)phosphine derivatives compared to the diphenyl congener as well as a significant difference in basicity of the phosphine depending on its position at the alkyne was determined. The diphosphines were utilized for comparative coordination experiments with Ni(II) and Cu(I) as first row transition metals. The heterobimetallic complexes {[Tp*W(CO)Br(μ‐η2‐C,C′‐κ2‐P,P′‐C2(PR2)2]NiCl2} (R=Ph, i‐Pr) were synthesized and fully characterized, the large difference in basicity and donor strength being underlined by a different degree of stability. Furthermore, two Cu(I) complexes of the isopropyl diphosphine derivative are presented and discussed; one of both being a dinuclear phenanthroline Cu(I) complex and the other a tetranuclear complex exhibiting an angled Cu2Br+ core.

Anchoring the late first row transition metals with B12P12 nanocage to act as single atom catalysts toward oxygen evolution reaction (OER)

The high over-potential associated with water splitting hinders the wide production of hydrogen and oxygen gases. Recent advancements in this field are being made by exploring novel, low-cost and highly efficient catalysts to lower the over-potential of the water splitting reaction. Herein, we studied the novel single atom catalysts (SACs) based on late first-row transition metal doped boron phosphide (B12P12) nano-cages for the electrocatalysis of the oxygen evolution reaction (OER) via density functional theory (DFT) calculations. The choice of using boron phosphide nano-cages as the support is based on the highly desirable properties within it. Namely, having many defects, excellent electrical conductivity, large surface area, and high chemical stability. The Ni@B12P12 and Co@B12P12 SACs exhibit high chemical stability, having interaction energies of −1.70 and −2.55 eV, respectively. Moreover, the results of quantum theory of atoms in molecules (QTAIM) analysis confirmed that the transition metals are covalently chemisorbed on the nano-cages, such strong interactions are desirable in SACs to ensure they withstand harsh chemical environments and are active for longer time period. Frontier molecular orbitals (FMOs) analysis indicates that the designed catalysts have semi-conducting capabilities which facilitate the transfer of electrons. The calculated FMOs energy gap (H-L Egap) values range from 2.01 to 2.88 eV. Results of OER activity analysis indicate that Ni@B12P12 and Co@B12P12 are promising OER SACs with low overpotentials (1.01 and 1.06 V, respectively). The result of this study highlights the viability of B12P12 nano-cages as supports in SACs and encourage the exploration of other nano-cages in the catalysis field.

Base-Free Catalytic Hydrogen Production from Formic Acid Mediated by a Cubane-Type Mo3S4 Cluster Hydride.

Formic acid (FA) dehydrogenation is an attractive process in the implementation of a hydrogen economy. To make this process greener and less costly, the interest nowadays is moving toward non-noble metal catalysts and additive-free protocols. Efficient protocols using earth abundant first row transition metals, mostly iron, have been developed, but other metals, such as molybdenum, remain practically unexplored. Herein, we present the transformation of FA to form H2 and CO2 through a cluster catalysis mechanism mediated by a cuboidal [Mo3S4H3(dmpe)3]+ hydride cluster in the absence of base or any other additive. Our catalyst has proved to be more active and selective than the other molybdenum compounds reported to date for this purpose. Kinetic studies, reaction monitoring, and isolation of the [Mo3S4(OCHO)3(dmpe)3]+ formate reaction intermediate, in combination with DFT calculations, have allowed us to formulate an unambiguous mechanism of FA dehydrogenation. Kinetic studies indicate that the reaction at temperatures up to 60 °C ends at the triformate complex and occurs in a single kinetic step, which can be interpreted in terms of statistical kinetics at the three metal centers. The process starts with the formation of a dihydrogen-bonded species with Mo-H···HOOCH bonds, detected by NMR techniques, followed by hydrogen release and formate coordination. Whereas this process is favored at temperatures up to 60 °C, the subsequent β-hydride elimination that allows for the CO2 release and closes the catalytic cycle is only completed at higher temperatures. The cycle also operates starting from the [Mo3S4(OCHO)3(dmpe)3]+ formate intermediate, again with preservation of the cluster integrity, which adds our proposal to the list of the infrequent cluster catalysis reaction mechanisms.

Structure and magnetism across the Sr2−xBaxNiOsO6 phase diagram: Understanding magnetic superexchange between first and third row transition metal ions

The crystal chemistry and magnetism of compositions in the Sr2−xBaxNiOsO6 phase diagram have been investigated to better understand the superexchange interactions between 3d and 5d transition metal ions. Compositions with x ​< ​1.35 crystallize with the double perovskite structure, while those with x ​> ​1.83 crystallize in a trigonal structure that is a cation ordered variant of the 6H hexagonal perovskite structure. The Sr-rich double perovskites undergo a cubic to tetragonal distortion upon cooling that is driven by out-of-phase octahedral tilting. The tetragonal distortion occurs upon cooling below 650 ​K in Sr2NiOsO6. The temperature of this transition decreases as the barium content increases and is completely suppressed for x ​≥ ​1.2. All compositions in the double perovskite region become spin glasses below Tg ​≈ ​40 ​K, due to the competition between ferromagnetic Ni–Os and antiferromagnetic Ni–Ni and Os–Os superexchange interactions. The double perovskite compositions that are rich in barium, like Sr0.8Ba1.2NiOsO6 show some signs of magnetic ordering in small clusters, but the presence of Ba/Sr disorder inhibits long-range magnetic order. Trigonal Ba2NiOsO6 orders ferrimagnetically below TC ​= ​108 ​K. The magnetic structure of this compound contains ferromagnetic ∼180° Ni–O–Os interactions and antiferromagnetic ∼90° Ni–O–Os interactions, both in agreement with the Goodenough-Kanamori rules.

(Student Award, 1st Place, Invited) Thermal Etching of Metal Oxides: Mechanisms Revealed By Quadrupole Mass Spectrometry

Thermal etching of metal oxides can occur spontaneously or by sequential surface reactions during atomic layer etching (ALE). This thermal etching does not require the use of plasmas or energetic species to remove material. Thermal etching can be studied by measuring the decrease of film thickness. However, the mechanisms of thermal etching still require the identification of volatile etch products. To address this issue, we have used quadrupole mass spectrometry (QMS) to detect volatile etch products during thermal etching. A custom reactor was built to measure the etch products with high sensitivity. This reactor uses molecular beam techniques to direct the etch products with line-of-sight to the QMS ionizer. Figure 1 shows a schematic of the QMS reactor. Nested inlet lines allow two reactants to be transported to the sample holder.This talk will explore the thermal etching of metal oxides using three different approaches: (1) chlorination followed by ligand addition; (2) exposure to acetylacetone; and (3) fluorination followed by ligand exchange or conversion. For the first approach, chlorination followed by ligand addition will be utilized for CoO thermal ALE with SO2Cl2 as the chlorination reactant and tetramethylethylenediamine (TMEDA) as the ligand-addition reactant at 250°C. The proposed mechanism for CoO thermal ALE is shown in Figure 2. SO2Cl2 converts the CoO surface to a CoCl2 layer and yields SO3. Then the TMEDA adds to CoCl2 and forms volatile CoCl2(TMEDA) etch products. QMS studies were able to confirm the formation of CoCl2(TMEDA) etch products. Figure 3 displays the CoCl2(TMEDA) etch products observed for the 5th CoO ALE cycle during the TMEDA exposure at 250°C. The natural abundance of the Cl isotopes affirmed that CoCl2(TMEDA) was the volatile etch product.Additional time-resolved experiments were able to monitor the reactants and volatile etch products. Figure 4 displays the ion intensities of C3N8N+ (largest crack of TMEDA) and CoCl(TMEDA)+ (largest crack of CoCl2TMEDA) during the TMEDA exposure at 250°C. The self-limiting nature of the ligand-addition reaction is revealed by the decrease of the CoCl(TMEDA)+ ion intensity with time while the TMEDA ion intensity remains constant. Chlorination followed by ligand addition was also utilized for the thermal ALE of other first row transition metal oxides (Fe2O3, NiO, and ZnO). M(TMEDA)Cl2 was observed as the volatile etch product for M= Fe and Ni. Zn(TMEDA)Cl was observed for ZnO.For the second approach, acetylacetone (Hacac) and hexafluoroacetylacetone (Hhfac) have been known to spontaneously etch a variety of metal oxides including ZnO, CuOx and Fe2O3. In addition, some thermal ALE procedures have been developed using Hacac together with an oxidation reactant to clean the metal oxide surface after the Hacac exposure. This talk will highlight results for ZnO etching using sequential Hacac and O3 exposures and also consecutive Hacac exposures. Zn(acac)2 was observed by the QMS studies during the alternating Hacac and O3 exposures on ZnO throughout the Hacac exposures at 250°C. Additional experiments using Hacac exposures with no O3 exposures also observed the continuous yield of Zn(acac)2. These experiments provide evidence for the spontaneous etching of ZnO by Hacac according to the reaction: ZnO + 2Hacac -> Zn(acac)2 +H2O. This reaction is believed to proceed spontaneously through a ligand substitution/hydrogen transfer mechanism. Other metal oxides were also etched using sequential Hacac and O3 exposures including V2O5, Fe2O3, CoO and CuO at 250°C. M(acac)2 species were observed for M=Co, Fe and Cu. VO(acac)2 was observed for V2O5.Lastly, for the third approach, Al2O3 thermal ALE was explored using sequential BCl3 and HF exposures at 300°C. This thermal ALE process could occur by (A) conversion of Al2O3 to B2O3 by BCl3 followed by the spontaneous etching of B2O3 by HF or (B) fluorination of Al2O3 to AlF3 followed by the ligand exchange of AlCl3 with BCl3 to produce volatile AlCl3. QMS analysis revealed that initial exposures of BCl3 on Al2O3 led to a conversion reaction forming B2O3 and volatile AlCl3. Subsequently, the HF exposure on B2O3 released BF3 and H2O. After removing the B2O3 conversion layer, the HF proceeded to fluorinate Al2O3 to form a AlF3 layer and volatile H2O. Additional BCl3 exposures then underwent ligand exchange with the AlF3 layer to form BFCl2 and AlCl3. The BCl3 continued to convert the underlying Al2O3 to B2O3. Moreover, the BCl3 exposures were also observed to spontaneously etch the B2O3 conversion layer. QMS studies revealed that the reaction of BCl3 with the B2O3 conversion layer or initial B2O3 substrates produced B3O3Cl3, a boroxine ring species. BCl3 has the unusual ability to convert Al2O3 to B2O3 and then spontaneously etch the B2O3. Figure 1

Developing First Row Transition Metal Antimonate Oxynitride and Oxysulfide Nanoparticles As Oxygen Reduction Electrocatalysts

Hydrogen fuel cells (FCs) are a promising avenue for replacing global dependence on carbon-intensive fuels especially in the transportation sector. The cathode of a FC carries out the oxygen reduction reaction (ORR), and the kinetics of the ORR is one of the primary sources of inefficiency in commercial FC devices. Commercial FC cathodes are based on expensive platinum-based catalyst architectures owing to their high activity, and therefore identifying highly active, non-precious ORR catalysts could accelerate the deployment of hydrogen fuel cells into the energy landscape. Theory calculations and experiments indicate first-row transition metal antimonates (MSbOx, M = Mn, Fe, Cr, Ni) have highly desirable ORR activity and stability characteristics. Furthermore, the material systems in this family have been extended to ternary and quaternary compositions for the ORR through the inclusion of additional transition metals to identify further opportunities for activity enhancement. In this work, we develop (oxy)sulfides and (oxy)nitrides of first-row transition metals such as manganese (MnSbOxSy and MnSbOxNz, respectively) to extend this class of materials towards enhanced activity when compared to previously reported antimonate materials. These nanoscale electrocatalysts are synthesized via colloidal means followed by high temperature surface modification under various reactive atmospheres (NH3, H2S, O2). Precise control of temperature ramps and holds enables modulation of the degree of oxidation, sulfidation, or nitridation, resulting in fully oxidized manganese antimonate nanocrystal cores with thin polycrystalline shells of (oxy)sulfide and (oxy)nitride. We tune the synthetic parameters of these materials and evaluate their enhanced electrochemical ORR activity in both alkaline and acidic conditions that are relevant to the operating conditions of anion-exchange membrane and proton-exchange membrane FCs, respectively. We perform extensive bulk and surface characterization with techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and gas adsorption BET surface area analysis. We find that converting the pure oxide into oxysulfide or oxynitride causes significant alterations to crystallinity and is accompanied by the appearance of additional crystalline phases that were not present previously as seen in XRD diffractograms. Transmission electron micrographs reveal polycrystalline nanocrystals of roughly 50 nm diameter for all compositions and show varying internal contrast patterns. The pure oxide nanocrystals form a thin shell of ~4 nm thickness after sulfidation or nitridation, and this shell may be converting into an active phase exposed to electrolyte during ORR testing and is expected to be significant for understanding the observed activity enhancement. XPS spectra indicate shifts in Sb and Mn peaks that are suggestive of changing surface-level oxidation state. We also utilize nanometer-resolution elemental mapping with TEM coupled to energy dispersive X-ray spectroscopy (EDS) to visualize the distribution of elements both before and after catalysis to understand how degradation affected observed activity trends. Lastly, on-line degradation measurements during potential cycling on an electrochemical flow cell coupled to inductively coupled plasma-mass spectrometry (ICP-MS) reveal potential and current density dependence of corrosion to provide insight into degradation mechanisms. Utilization of a diverse set of physical and electrochemical characterization techniques provides a thorough description of how manganese antimonate (oxy)sulfides and (oxy)nitrides carry out the ORR and could inform the future development of advanced non-precious FC cathode catalysts to drive down device cost.