Heavier Alkaline Earth Elements Research Articles

Advances in heavy alkaline earth chemistry provide insight into complexation of weakly polarizing Ra2+, Ba2+, and Sr2+ cations.

Numerous technologies-with catalytic, therapeutic, and diagnostic applications-would benefit from improved chelation strategies for heavy alkaline earth elements: Ra2+, Ba2+, and Sr2+. Unfortunately, chelating these metals is challenging because of their large size and weak polarizing power. We found 18-crown-6-tetracarboxylic acid (H4COCO) bound Ra2+, Ba2+, and Sr2+ to form M(HxCOCO)x-2. Upon isolating radioactive 223Ra from its parent radionuclides (227Ac and 227Th), 223Ra2+ reacted with the fully deprotonated COCO4- chelator to generate Ra(COCO)2-(aq) (log KRa(COCO)2- = 5.97 ± 0.01), a rare example of a molecular radium complex. Comparative analyses with Sr2+ and Ba2+ congeners informed on what attributes engendered success in heavy alkaline earth complexation. Chelators with high negative charge [-4 for Ra(COCO)2-(aq)] and many donor atoms [≥11 in Ra(COCO)2-(aq)] provided a framework for stable complex formation. These conditions achieved steric saturation and overcame the weak polarization powers associated with these large dicationic metals.

Transition-Metal Chemistry of the Heavier Alkaline Earth Atoms Ca, Sr, and Ba.

ConspectusAlkaline earth elements beryllium, magnesium, calcium, strontium, and barium with an ns2 valence-shell configuration are usually classified as main-group elements that belong to the s-block atoms. For a long time, the elements were considered to be rather chemically uninteresting atomic species due to preconceived ideas about bonding, structure, and reactivity. They typically use the two ns valence electrons in forming ionic salt compounds with the metal in a formal oxidation state of +2. For the heavier alkaline earth atoms, calcium, strontium, and barium, their (n - 1)d atomic orbitals (AOs) are empty but lie close in energy to the valence np orbitals. Earlier theoretical investigations have already suggested that these elements can employ the (n - 1)d AOs to some extent to form polar bonds in divalent species in which the alkaline earth metal centers are sufficiently positively charged. The d orbital involvement increases from Ca to Sr and markedly in Ba. Thus, barium has been termed an honorary transition metal.Recently, molecular complexes of Ca, Sr, and Ba were prepared in the gas phase and in a low-temperature solid neon matrix and were detected by infrared spectroscopy. An analysis of the electronic structures of [Ba(CO)]+, [Ba(CO)]-, saturated coordinated octacarbonyls [M(CO)8] and [M(CO)8]+, isoelectronic dinitrogen complexes [M(N2)8] and [M(N2)8]+, and the tribenzene complexes [M(Bz)3] (M = Ca, Sr, Ba) revealed that the metal-ligand bonding can be straightforwardly discussed using the traditional Dewar-Chatt-Duncanson (DCD) model as in classical transition-metal complexes. The metal-ligand bonds can be explained with metal → ligand π back donation from occupied metal (n - 1)d AOs to vacant antibonding π molecular orbitals of the ligands with concomitant σ donation from occupied MOs of the ligands to vacant metal d orbitals of the alkaline earth atoms. In addition, heteronuclear Ca-Fe carbonyl cation complexes were also produced in the gas phase. Bonding analysis of the coordination saturated [CaFe(CO)10]+ complex implies that it can be described by the bonding interactions between a [Ca(CO)6]2+ fragment and an [Fe(CO)4]- anion fragment in forming a Fe → Ca d-d dative bond. The nature of metal-ligand and metal-metal bonding was quantitatively elucidated by the energy decomposition analysis in conjunction with the natural orbitals for the chemical valence (EDA-NOCV) method, which indicate that the (n - 1)d AOs of the alkaline earth metals are the dominant orbitals participating in the covalent interactions, just as typical transition metals. The results indicate that the heavier alkaline earth elements have a much richer covalent chemistry than previously thought. These findings, along with earlier studies, suggest that the heavier alkaline earth atoms Ca, Sr, and Ba should be classified as transition metals rather than main group atoms in the periodic table of the elements. This interesting structural chemistry, together with some recently reported examples of spectacular reactivity, establishes these elements as exciting and promising research targets in current research.

Incorporation of Incompatible Strontium and Barium Ions into Calcite (CaCO3) through Amorphous Calcium Carbonate

Calcite is a ubiquitous mineral in nature. Heavy alkaline-earth elements with large ionic radii such as Sr2+ and Ba2+ are highly incompatible to calcite. Our previous study clarified that incompatible Sr2+ ions can be structurally incorporated into calcite through crystallization from amorphous calcium carbonate (ACC). In this study, we synthesized Sr-doped calcite with Sr/(Sr + Ca) up to 30.7 ± 0.6 mol% and Ba-doped calcite with Ba/(Ba + Ca) up to 68.6 ± 1.8 mol%. The obtained Ba-doped calcite samples with Ba concentration higher than Ca can be interpreted as Ca-containing barium carbonates with the calcite structure which have not existed so far because barium carbonate takes the aragonite structure. X-ray diffraction (XRD) patterns of the Sr-doped and Ba-doped calcite samples obtained at room temperature showed that reflection 113 gradually weakened with increasing Sr/(Sr + Ca) or Ba/(Ba + Ca) ratios. The reflection 113 disappeared at Ba/(Ba + Ca) higher than 26.8 ± 1.6 mol%. Extinction of reflection 113 was reported for pure calcite at temperatures higher than 1240 K, which was attributed to the rotational (dynamic) disorder of CO32− in calcite. Our Molecular Dynamics (MD) simulation on Ba-doped calcite clarified that the CO32− ions in Ba-doped calcites are in the static disorder at room temperature. The CO32− ions are notable tilted and displaced from the equilibrium position of pure calcite.

Influence of oil and stratal water contamination on the ash composition of oligotrophic peat soils in the oil-production area (the Ob’ region)

The mineral contamination of peat soils in the oil-production area differs considerably in the places of oil or stratal water spills. The time elapsed since the spill occurred is also an important factor of the changes in the chemical composition of the peat ash. The ash content rises drastically in the oil-contaminated peat, and the peat ash becomes rich in heavy metals (Mn, Ni, and Sr) and lanthanides (La and Ce). The content of K and Fe decreases, and that of P, S, Mg, Ca, Ni, and Pb increases with time at the site of old oil contamination. In the course of the self-rehabilitation of oil-contaminated peat, the content of Cl decreases more intensely than that of the heavier halogen Br. The ash content rises to a lesser extent in the peat contaminated with stratal water. The ash of the salinized peat is enriched in heavy alkaline-earth elements, i.e., Ba and Sr. Although most of the elements are leached with time, the content of Ba and Sr still remains 4–6 times higher than the background one even after long-term (more than 10 years) leaching. The concentrations of halogens rise considerably in the salinized peat, that of Cl in the peat ash decreases by 10 times, and the content of Ba virtually remains the same.

Light shifts and magic wavelengths for heavy alkaline earth elements: Ba and Ra

In this paper, we investigate light shifts of heavy alkaline earth elements barium (Ba) and radium (Ra), which are interesting for optical lattice clocks and for permanent electric dipole moment searches. Detailed knowledge is required in the design of efficient loading of atoms from a magneto-optical trap into an optical dipole trap, to facilitate Doppler cooling while trapping and to achieve longer lifetimes of the trapped atoms with reduced heating rates. The wavelength dependence of light shifts of the ns2 1S0 ground state, the nsnp 3P1 and ns(n − 1)d 1D2 excited states in barium (n = 6) and the n s2 1S0 ground state, the nsnp 3P1 and ns(n − 1)d 3D2 excited states in radium (n = 7) are calculated. Several magic wavelengths in the visible and infrared regions accessible with commercial lasers for optical dipole trapping of Ba and Ra are identified.

N,N′-Bis(2,6-diisopropylphenyl)formamidinate complexes of the heavy alkaline earths

The heavy alkaline earth elements calcium, strontium and barium react with two equivalents of N,N′-bis(2,6-diphenyl)formamidine (DippFormH) in the presence of bis(pentafluorophenyl)mercury to afford the bis(formamidinate) species [M(DippForm)2(THF)n] (M = Ca, n = 1 (1), M = Sr (2) or Ba (3), n = 2) in good to moderate yield. Use of one equivalent of DippFormH during the preparation of 1–3 putatively results in the formation of the organoamide heavy alkaline earth Grignard species “[(X)M(DippForm)]” (M = Ca, Sr or Ba, X = F or C6F5). These compounds participate in the nucleophilic substitution of the para-fluorine of pentafluorobenzene, a by-product of prior ligand exchange, to render DippForm(4-HC6F4).

Binding energies for p electrons in negative alkaline-earth elements.

Multiconfiguration Hartree-Fock (MCHF) calculations recently predicted a positive binding energy for the p electron in the ${s}^{2}$${p}^{2}$P state of ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$, in good agreement with experiment. The present paper explores the binding of electrons in several alkaline-earth elements. Fixed-core Hartree-Fock potentials demonstrate the angular momentum barrier. From a simple correlation study it is shown that the 3${d}^{2}$4p configuration relative to the 3${s}^{2}$3p configuration in ${\mathrm{Mg}}^{\mathrm{\ensuremath{-}}}$ lies much higher than the (n-1)${d}^{2}$np configurations relative to ${\mathrm{ns}}^{2}$np in the heavier alkaline-earth elements. Accurate MCHF correlation studies are reported for two and three electrons outside a core from which binding energies are predicted for ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$, ${\mathrm{Sr}}^{\mathrm{\ensuremath{-}}}$, and ${\mathrm{Ba}}^{\mathrm{\ensuremath{-}}}$. Not included is correlation with the core (core polarization) and correlation within the core. The same procedures were also applied to Sc. Theoretical results with relativistic shift corrections are compared with experiment in the case of ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$ and Sc.

ChemInform Abstract: GLASS SURFACE CRACKS CAUSED BY ALKALINE SOLUTION CONTAINING AN ALKALINE‐EARTH ELEMENT

AbstractBehandlung von Gläsern auf SiO2‐Basis mit heißen BaCl2‐ oder SrCl2‐haltigen NaOH‐Lösungen führt zur Ausbildung scharfer Risse an der Glasoberfläche.

Glass Surface Cracks Caused by Alkaline Solution Containing an Alkaline‐Earth Element

When glass was treated in a hot alkaline solution containing a small amount of heavy alkaline earth elements such as Ba or Sr, sharp cracks formed on the glass surface. The severity of the crack formation was greater, the heavier the alkaline earth element. The cracks caused a drastic reduction in the mechanical strength of the glass.

Effect of alkaline earth ion as an inhibitor to alkaline attack on silica glass

Inhibitor action of Be, Mg, Ca, Sr, Ba, Zn and Al to the alkaline solution attack on silica glass was investigated. Metal silicate hydrated compound was formed on silica glass surface when a minute quantity of alkaline earth element was added to sodium hydroxide solution, while no such film was formed when Zn or Al was added. In addition, extensive crack growth was observed in the case of heavy alkaline earth elements such as Ba and Sr.