Alkaline Earth Calcium Research Articles

Biodiesel Prepared Through Silver Doped Blended Heterogeneous Catalyst and Soybean Oil

Biodiesel amalgamate with silver doped nickel and calcium blended oxide catalyst & soybean oil. Alkaline earth metal calcium and the use of nickel blended oxide integrated and optimized throughout a blending process that after silver doped in differing percentages to research the outcome of prepared catalysts on the change oil to FAME. Catalytic characteristics such as surface area, energy band difference, crystallinity, morphology, and the composition of prepared samples explored utilizing different BET, FTIR, XRD, SEM, and EDX methods. Silver doped 20 percent NiO-CaO blended oxide catalyst defined with a distinctive weight percentage of silver 2wt percent Ag found to be most viable with 80 percent transformation of oil. The optimized response variable for the change of biodiesel was alcohol to oil proportion 15:1, response temperature 64oC, and catalytic converter stacking 05wt percent. This prepared catalyst has added significance as a blended catalyst for biodiesel generation. Key Words: FAME, Transesterification, Silver, Mixed Oxide, Catalyst

Study on Fluorescence Sensing of 2D Calcium Metal-Organic Frameworks to Tetracycline Hydrochloride

Detection methods based on changes in fluorescence intensity have been widely studied because of their convenience, fast response, and high sensitivity. In many fluorescent materials, luminescent metal-organic frameworks(LMOF) exhibit the characteristics of simple preparation and recyclability, which has become a new type of detection material. Thus, a two-dimensional structure is prepared by combining amide functionalization ligand with alkaline earth metal calcium (namely CUST-620), not only the fluorescent intensity is improved, but also the thermal stability is ideal. The excellent luminescence properties and fluorescent sensing of antibiotics are commonly used in aqueous environments. The results present significant quenching occurring when tetracycline hydrochloride is added to CUST-620. It is expected to become a novel antibiotic fluorescent sensor.

Calcium poisoning mechanism on the selective catalytic reduction of NOx by ammonia over the γ-Fe2O3 (001) surface.

γ-Fe2O3 has an excellent low-temperature selective catalytic reduction (SCR) deNOx performance, but its resistance to alkaline earth metal calcium (Ca) is poor. In particular, the detailed mechanism of Ca poisoning on the γ-Fe2O3 catalyst at the atomic level is not clear. Hence, the density functional theory method was used in this research to investigate the influence mechanism of Ca poisoning on the NH3-SCR over the γ-Fe2O3 catalyst surface. The findings reveal that NH3, NO, and O2 molecules can bind to the γ-Fe2O3 (001) surface to generate coordinated ammonia, monodentate nitroso, and adsorption oxygen species, respectively. The main active site is Fe1-top. For the γ-Fe2O3 with Ca poisoning, the Ca atom has a high adsorption energy on the surface of γ-Fe2O3 (001), which covers the catalyst surface and reduces the active sites. The presence of Ca atom decreases the adsorption performance of NH3, while slightly improving the NO and O2 adsorption. In particular, the Ca atom restrains the NH3 activation and NH2 formation, which is detrimental to the NH3-SCR process.

CALCIUM SULFATE SCALE IN THE PETROLEUM INDUSTRY

Oilfield scale is defined as the precipitation of hard, adherent deposits of inorganic solid originating from aqueous media. This constitutes sulfate and carbonate of the alkaline earth metals calcium, barium and strontium and complex salts of iron. Generally, the process of the scale deposition occurs when the product solubility of a compound considered is exceeded. The formation of scale, such as calcium sulfate, has long recognised as one of the serious problems in oil and gas production leading to reduced production rates as flow becomes restricted. Calcium sulfate scale found in the oilfield is in the form of gypsum (CaSO4, 2H2,0) which is the most stable form at temperatures of 40 °C or less at atmosphere pressure. Above this temperature, anhydrite (CaSO4,) may be found, although hemihydrate (CaSO. 1/2H,O) may form under certain conditions. The reaction for precipitation of calcium sulfate is as follows: Ca+2 (aq) + so4-2 (aq) = CaSO4 (solid) The solubility of calcium sulfate in distilled water is 2080 mg/l at 25 "C. Calcium sulfate scale arises from several causes, such as temperature, dissolved salts, pressure, and time. The main points of this paper are focused on nomenclature, chemical structure, the occurrence of calcium sulfate scale, example of calcium sulfate scale in the petroleum industry, and calculation of calcium sulfate solubility in brine.

Metal-CO Bonding in Mononuclear Transition Metal Carbonyl Complexes.

DFT calculations have been carried out for coordinatively saturated neutral and charged carbonyl complexes [M(CO)n]q where M is a metal atom of groups 2–10. The model compounds M(CO)2 (M = Ca, Sr, Ba) and the experimentally observed [Ba(CO)]+ were also studied. The bonding situation has been analyzed with a variety of charge and energy partitioning approaches. It is shown that the Dewar–Chatt–Duncanson model in terms of M ← CO σ-donation and M → CO π-backdonation is a valid approach to explain the M–CO bonds and the trend of the CO stretching frequencies. The carbonyl ligands of the neutral complexes carry a negative charge, and the polarity of the M–CO bonds increases for the less electronegative metals, which is particularly strong for the group 4 and group 2 atoms. The NBO method delivers an unrealistic charge distribution in the carbonyl complexes, while the AIM approach gives physically reasonable partial charges that are consistent with the EDA-NOCV calculations and with the trend of the C–O stretching frequencies. The AdNDP method provides delocalized MOs which are very useful models for the carbonyl complexes. Deep insight into the nature of the metal–CO bonds and quantitative information about the strength of the [M] ← (CO)8 σ-donation and [M(d)] → (CO)8 π-backdonation visualized by the deformation densities are provided by the EDA-NOCV method. The large polarity of the M–CO π orbitals toward the CO end in the alkaline earth octacarbonyls M(CO)8 (M = Ca, Sr, Ba) leads to small values for the delocalization indices δ(M–C) and δ(M···O) and significant overlap between adjacent CO groups, but the origin of the charge migration and the associated red-shift of the C–O stretching frequencies is the [M(d)] → (CO)8 π-backdonation. The heavier alkaline earth metals calcium, strontium and barium use their s/d valence orbitals for covalent bonding. They are therefore to be assigned to the transition metals.

Solvothermal synthesis, structure and properties of heterometallic coordination polymer based on metalloligand and alkaline-earth metal calcium

A new 3D heterometallic coordination polymer (HCP), which formulated as {[Ca(H2O)2][LCo]·DMSO·2H2O}n (1) (DMSO = dimethyl sulfoxide), has been solvothermally synthesized from metalloligand LCo = [Co(2,4-pydca)2]2– (2,4-pydca = pyridine-2,4-dicarboxylate) and alkaline-earth ion CaII. HCP 1 has been well characterized by Fourier-transform infrared spectroscopy, elemental analysis and single-crystal X-ray diffraction analysis. The crystallographic analysis indicates that HCP 1 crystallizes in the monoclinic system with space group of C2 and possesses the 3D framework structure, which is constructed from metalloligands LCo connecting with CaII ions via the carboxyl groups. The 3D framework of HCP 1 can be defined as (4,4,4)-connected 3-nodal topology with the point symbol of {42·84}{42·84}{42·84}. Thermogravimetric analysis exhibits that the skeleton of HCP 1 collapses after 375 °C. Magnetic susceptibility study reveals the presence of antiferromagnetic interaction between the spin centers. The solid-state luminescence property of HCP 1 has also been investigated.

Observation of alkaline earth complexes M(CO)8 (M = Ca, Sr, or Ba) that mimic transition metals.

The alkaline earth metals calcium (Ca), strontium (Sr), and barium (Ba) typically engage in chemical bonding as classical main-group elements through their ns and np valence orbitals, where n is the principal quantum number. Here we report the isolation and spectroscopic characterization of eight-coordinate carbonyl complexes M(CO)8 (where M = Ca, Sr, or Ba) in a low-temperature neon matrix. Analysis of the electronic structure of these cubic Oh -symmetric complexes reveals that the metal-carbon monoxide (CO) bonds arise mainly from [M(dπ)] → (CO)8 π backdonation, which explains the strong observed red shift of the C-O stretching frequencies. The corresponding radical cation complexes were also prepared in gas phase and characterized by mass-selected infrared photodissociation spectroscopy, confirming adherence to the 18-electron rule more conventionally associated with transition metal chemistry.

Combustion of Biosolids in a Bubbling Fluidized Bed, Part 1: Main Ash-Forming Elements and Ash Distribution with a Focus on Phosphorus

This is the first in a series of three papers describing combustion of biosolids in a 5-kW bubbling fluidized bed, the ash chemistry, and possible application of the ash produced as a fertilizing agent. This part of the study aims to clarify whether the distribution of main ash forming elements from biosolids can be changed by modifying the fuel matrix, the crystalline compounds of which can be identified in the raw materials and what role the total composition may play for which compounds are formed during combustion. The biosolids were subjected to low-temperature ashing to investigate which crystalline compounds that were present in the raw materials. Combustion experiments of two different types of biosolids were conducted in a 5-kW benchscale bubbling fluidized bed at two different bed temperatures and with two different additives. The additives were chosen to investigate whether the addition of alkali (K2CO3) and alkaline-earth metal (CaCO3) would affect the speciation of phosphorus, so the molar ratios targeted in modified fuels were P:K = 1:1 and P:K:Ca = 1:1:1, respectively. After combustion the ash fractions were collected, the ash distribution was determined and the ash fractions were analyzed with regards to elemental composition (ICP-AES and SEM-EDS) and part of the bed ash was also analyzed qualitatively using XRD. There was no evidence of zeolites in the unmodified fuels, based on low-temperature ashing. During combustion, the biosolid pellets formed large bed ash particles, ash pellets, which contained most of the total ash content (54%–95% (w/w)). This ash fraction contained most of the phosphorus found in the ash and the only phosphate that was identified was a whitlockite, Ca9(K,Mg,Fe)(PO4)7, for all fuels and fuel mixtures. With the addition of potassium, cristobalite (SiO2) could no longer be identified via X-ray diffraction (XRD) in the bed ash particles and leucite (KAlSi2O6) was formed. Most of the alkaline-earth metals calcium and magnesium were also found in the bed ash. Both the formation of aluminum-containing alkali silicates and inclusion of calcium and magnesium in bed ash could assist in preventing bed agglomeration during co-combustion of biosolids with other renewable fuels in a full-scale bubbling fluidized bed.

Tris(pyrazolyl)borate Complexes of the Alkaline-Earth Metals: Alkylaluminate Precursors and Schlenk-Type Rearrangements

A series of 3,5-substituted tris(pyrazolyl)borate (TpR,Me; R = Me, Ph, tBu) complexes of the alkaline-earth metals (Mg, Ca, Ba) was synthesized by salt metathesis reactions. The influence of different organometallic precursors on Schlenk-type rearrangement reactions was studied, putting emphasis on the metal size and the steric encumbrance of the Tp ligands. Magnesium alkyls MgR2 (R = AlMe4, CH3) react with KTpR,Me to form the heteroleptic complexes (TpMe,Me)Mg(CH3), (TptBu,Me)Mg(CH3), and (TpMe,Me)Mg(AlMe4). The latter tetramethylaluminate complex can also be obtained by treatment of TpMe,MeMg(CH3) with an excess of trimethylaluminum. The formally six-coordinate cyclopentadienyl derivative (C5Me5)Mg(Me)(thf)2 is synthesized from MeMgBr and 1 equiv of K(C5Me5). Equimolar reactions of the tetraethylaluminates [M(AlEt4)2]n of the heavier alkaline-earth metals calcium and barium with KTpR,Me give the homoleptic complexes of Ca(TpR,Ph)2 and Ba(TpR,Me)2. Heterotrimetallic [BaK(AlEt4)3]n is identified as a ligand rearrangement product and can be independently obtained by adding [K(AlEt4)]n to [Ba(AlEt4)2]n. Treatment of Ba[N(SiMe3)2]2(thf)2 with KTpMe,Me generates the heteroleptic complex (TpMe,Me)Ba[N(SiMe3)2](thf)2. All complexes are fully characterized including X-ray structure analyses.

Alkaline Earth Metal Catalysts for Asymmetric Reactions

The group 2 alkaline earth metals calcium (Ca), strontium (Sr), and barium (Ba) are among the most common elements on Earth, abundant in both the sea and the Earth's crust. Although they are familiar in our daily lives, their application to organic synthesis has, so far, been limited. Some particularly useful properties of these elements include (i) low electronegativity, (ii) a stable oxidation state of +2, meaning that they can potentially form two covalent bonds with anions, and (iii) the ability to occupy a variety of coordination sites due to their large ionic radius. Furthermore, the alkaline earth metals, found between the group 1 and group 3 elements, show mild but significant Lewis acidity, which can be harnessed to control coordinative molecules via a Lewis acid-base interaction. Taken together, these characteristics make the metals Ca, Sr, and Ba very promising components of highly functionalized acid-base catalysts. In this Account, we describe the development of chiral alkaline earth metal catalysts for asymmetric carbon-carbon bond-forming reactions. Recently prepared chiral alkaline earth metal complexes have shown high diastereo- and enantioselectivities in fundamental and important chemical transformations. We chose chiral bisoxazoline (Box) derivatives bearing a methylene tether as a ligand for chiral modification. These molecules are very useful because they can covalently coordinate to alkaline earth metals in a bidentate fashion through deprotonation of the tether portion. It was found that chiral calcium-Box complexes could successfully promote catalytic asymmetric 1,4-addition and [3 + 2] cycloaddition reactions with high diastereo- and enantioselectivities. Both the calcium-Box complexes and chiral strontium-bis-sulfonamide and chiral barium-BINOLate complexes could catalyze asymmetric 1,4-addition reactions with high enantioselectivities. Furthermore, we designed a calcium-neutral coordinative ligand complex as a new type of chiral alkaline earth metal catalyst. We found that pyridinebisoxazolines (Pybox) worked well: they served as excellent ligands for calcium compounds in 1,4-addition reactions and Mannich reactions. Moreover, they were successful in 1,4-additions in concert with enantioselective protonation, affording the desired products in good to high enantioselectivities. Our results demonstrate that alkaline earth metals are very useful and attractive catalysts in organic synthesis. Moreover, their ubiquity in the environment is a distinct advantage over rare metals for large-scale processes, and their minimal toxicity is beneficial in both handling and disposal.

Tunable, Ligand-Based Emission from Inorganic−Organic Frameworks: A New Approach to Phosphors for Solid State Lighting and Other Applications

Two dense inorganic−organic frameworks were prepared using the intrinsically luminescent organic ligand 9-fluorenone-2,7-dicarboxylic acid (H2FDC) in combination with the alkaline earth metals calcium and strontium. Ca(FDC)(H2O)2 (1) and Sr(FDC)(H2O)5·2H2O (2) were prepared hydrothermally and both adopt three-dimensional framework structures. They absorb strongly between 380 and 460 nm and show broad visible emission with peaks at 503 and 526 nm, respectively. Structure 1 shows a quantum yield at room temperature of 7.8% which increases to 15% at −196 °C, while structure 2 shows a room temperature quantum yield of 2.8%, increasing to 3.3% at −196 °C.

Effects of adding calcium to lithium on the discharge and hydrogen evolution of lithium anodes

The effects of the alkaline earth metal calcium on the electrochemical behavior of lithium anode in 4 mol L −1 LiOH and NaOH solutions were investigated via hydrogen collection, X-ray diffraction (XRD) and scanning electron microscope (SEM). It was found that adding minor calcium to lithium decreased the hydrogen evolution on the surface of lithium–calcium alloy in the alkaline electrolyte containing corrosion inhibitors. The SEM image and XRD analysis showed that calcium formed calcium hydride on the surface of lithium–calcium alloy anode. The hypothesis has been proposed that the hydrogen inhibition effect is caused by calcium hydride combined with LiOH and LiOH·H 2O formed on the anode surface. The formation of calcium hydride and LiOH and LiOH·H 2O reduced the porosity of the film and water penetration onto the active sites of the lithium–calcium alloy.

Synthesis and structural variations of substituted phenylamide complexes of the heavy alkaline earth metals calcium, strontium and barium

Starting material KN(H)C(6)H(3)-2,6-F(2) was prepared via a transamination reaction from KNH(2) and 2,6-F(2)C(6)H(3)NH(2) in THF and crystallized from 1,4-dioxane (diox) as the three-dimensional polymer [(diox)(1.5)K{N(H)-2,6-F(2)C(6)H(3)}.diox(0.5)](infinity) (1). The metathesis reaction of (THF)(4)CaI(2) with KN(Me)Ph in THF yields monomeric (THF)(4)Ca[N(Me)Ph](2) (2) with a nearly linear N-Ca-N moiety of 179.84(8) degrees . The metathesis reaction of (THF)(4)CaI(2) with KN(H)Mes yields trinuclear (THF)(6)Ca(3)[N(H)Mes](6) (3) with a linear Ca(3) fragment and bridging 2,4,6-trimethylphenylamido groups. The reaction of 1 with (THF)(4)CaI(2) gives dinuclear (THF)(5)Ca(2)[N(H)-2,6-F(2)C(6)H(3)](4).2THF (4) with three bridging and one terminally bound 2,6-difluorophenylamide. A similar reaction of (THF)(5)SrI(2) with KN(H)-2,6-F(2)C(6)H(3) yields dinuclear (THF)(6)Sr(2)[N(H)-2,6-F(2)C(6)H(3)](3)I.THF (5) in which the iodide anion binds terminally. This iodide ligand cannot be substituted as easily by excess KN(H)-2,6-F(2)C(6)H(3). The metathesis reaction of (THF)(5)BaI(2) with KN(H)-2,6-F(2)C(6)H(3) leads to the formation of [(THF)(2)Ba{N(H)-2,6-F(2)C(6)H(3)}(2)](infinity) (6) which crystallizes as a one-dimensional polymer with bridging 2,6-difluorophenylamide anions and additional Ba-F-bonds.

Cover Picture: Aromaticity: Molecular‐Orbital Picture of an Intuitive Concept (Chem. Eur. J. 22/2007)

π Electrons decide about aromatic delocalization versus antiaromatic localization of the double bonds in benzene and cyclobutadiene, respectively. In their Full Paper on page 6321 ff., S. C. A. H. Pierrefixe and F. M. Bickelhaupt present a molecular-orbital (MO) model of aromaticity that augments the modern valence-bond picture available. Organometallic Chemistry In their Concept article on page 6292 ff., M. Westerhausen et al. discuss the organometallic chemistry of the heavy alkaline earth metals calcium, strontium, and barium. Due to the existence of straightforward procedures for the synthesis of organic calcium compounds, it is envisaged that the importance of this substance class will be enhanced and a wide range of applications will be developed.1 Lanthanide Complexes In their Full Paper on page 6308 ff., Z. Pikramenou et al. demonstrate that aromatic CF substitution in an imidodiphosphinate binding site leads to complexes with unparalleled photophysical behaviour in the NIR and visible region. The HF20tpip ligand provides a ligand framework structure that leads to stable, luminescent [Ln(F20tpip)3] complexes for all lanthanides and provides a ligand framework, with no high-energy XH vibrations and aryl sensitiser groups, that is able to assemble in a shell arrangement around the metal.1 Oxo–Vandium Clusters In their Full Paper on page 6329 ff., E. J. L. McInnes, R. H. Laye et al. describe the synthesis and characterisation of several new members of the family of polyoxovanadate clusters, which have unprecedentedly low oxidation states. Solvothermal conditions are essential for the formation of these species, and the level of oxidation of the isolated clusters is in part controlled by the crystallisation time.1

Heavy Grignard Reagents: Challenges and Possibilities of Aryl Alkaline Earth Metal Compounds

Compounds of the type aryl--M--X, with M=Ca, Sr, Ba and X as any kind of ligand (such as halide, phosphanide, amide, aryl), are presented. The low reactivity of the heavy alkaline earth metals calcium, strontium, and barium enforces an activation prior to use for the direct synthesis. The insertion of these metals into C--I bonds of aryl iodides (direct synthesis) yields aryl metal iodides and has to be performed at low temperatures and in THF. Aryl alkaline-earth-metal compounds show some characteristics: 1) the ease of ether cleavage enforces low reaction temperatures, 2) for Sr and Ba the Schlenk equilibrium is shifted towards homoleptic MI2 and MPh2, 3) high solubility of diaryl alkaline-earth-metal derivatives in THF even at low temperatures initiated quantum chemical investigations on the aggregation behavior, and 4) a strong low field shift of the 13C resonances of the ipso carbon atoms in NMR spectra was observed. First results from quantum chemical calculations on diaryl dicalcium(I) suggest a long Ca--Ca bond with a considerable Ca--Ca bond dissociation energy. Initial results on a selection of applications such as metallation, metathesis, and addition reactions of aryl calcium compounds are presented as well.

Simulation of Ca2+ and Mg2+ Solvation Using Polarizable Atomic Multipole Potential

The alkaline earth metals calcium and magnesium are critically involved in many biomolecular processes. To understand the hydration thermodynamics of these ions, we have performed molecular dynamics simulations using a polarizable potential. Particle-mesh Ewald for point multipoles has been applied to the calculation of electrostatic interactions. The parameters in this model have been determined from an ab initio quantum mechanical calculation of dimer interactions between ions and water. Two methods for ion solvation free energy calculation, free energy perturbation, and the Bennett acceptance ratio have been compared. Both predict results consistent with other theoretical estimations while the Bennett approach leads to a much smaller statistical error. Based on the Born theory and the ion-oxygen radial distribution functions, we estimate the effective size of the ions in solution, concluding that K(+) > Na(+) congruent with Ca(2+) > Mg(2+). There appears to be much stronger perturbation in water structure, dynamics, and dipole moment around the divalent cations than the monovalent K(+) and Na(+). The average water coordination numbers for Ca(2+) and Mg(2+) are 7.3 and 6, respectively. The lifetime of water molecules in the first solvation shell of Mg(2+) is on the order of hundreds of picoseconds, in contrast to only few picoseconds for Ca(2+), K(+), or Na(+).

Co-intercalation into graphite of lithium and sodium with an alkaline earth metal

After a short overview on the co-intercalation into graphite of light alkali metals––lithium and sodium––with a third element, novel results concerning the co-intercalation of these alkali metals with an alkaline earth metal are given. The intercalation into graphite of lithium together with magnesium, calcium, strontium and barium is investigated, and the reaction products are described and characterised by X-ray diffraction. Ternary intercalation compounds were synthesised only in the graphite–lithium–calcium system. In the other systems, binary compounds were obtained. The co-intercalation of sodium and the alkaline earth metals calcium, strontium and barium is studied too. No ternary compounds were obtained. However, in the case of the sodium–barium system, a discussion on ternary graphite–sodium–barium compounds previously published is opened. Several data are in favour of a graphite–sodium–oxygen compound instead of a graphite–sodium–barium one.

Focus Issue: Metals in Biology: Coin for the Signaling Realm?

From the time of the protoscientists who learned to extract metals from ores and mix copper with tin, catapulting our ancestors from the Neolithic to the Bronze Age, metals have been an integral part of our lives. The alchemists' attempts to transform lead and other base metals to gold laid the foundations for modern chemistry and, today, metals provide the skeletons of our skyscrapers, constitute a medium for our currency, and add color to our lives in pigments and glazes. Metals are also critical to our biology, and the current issues of Science and Science's STKE bring together a series of articles on metals in biology and in the environment. Science's STKE highlights two metals with essential biological roles in signaling: iron and zinc. Iron, which is required for human respiration both during oxygen transport by hemoglobin and during oxidative phosphorylation, is also implicated in cytotoxic effects of macrophage-produced nitric oxide (NO); zinc, which is an important structural element in many enzymes and transcription factors, is beginning to be recognized as a signaling messenger. As discussed in the Science article by Finney and O'Halloran, various transition metals--including zinc, copper, and iron--are not only essential components of proteins but can also be cytotoxic. As a rule, these metals are tightly regulated at very low free concentrations inside cells, necessitating an intricate array of transporters, metal sensors, and diffusible protein chaperones to guide them to their protein targets. In an STKE Perspective, Bouton and Drapier discuss the complex effects of NO on two iron regulatory proteins (IRP1 and IRP2) and, consequently, on iron uptake and metabolism. NO, produced secondary to inflammatory responses, promotes the conversion of IRP1 from an aconitase to an RNA binding regulator of transferrin receptor and ferritin synthesis and simultaneously decreases the expression of IRP2. Compared to target cells that respond to macrophage-produced NO, macrophages respond differently to NO than would be expected based on the effects of IRP activity alone. Bouton and Drapier discuss the basis for these differences in cellular response to NO with regard to iron handling and cytotoxicity. For more on NO signaling, see the Review by Lane et al ., the Protocol by Jaffrey and Snyder, and the Perspective by Helmann in the STKE Archives. Among signaling molecules, the alkaline earth metal calcium has long reigned as the prima donna of metals; zinc is now beginning to take the stage as a signaling star as well. Zinc's emerging role as both an intercellular and intracellular signal is described in two complementary STKE Perspectives. This role depends upon an exception to the above rule of extremely low concentrations of free (or loosely bound) zinc: A pool of chelatable zinc that is sequestered in presynaptic vesicles in some regions of the brain and released following physiological stimuli. Li et al . discuss research that implicates zinc released from hippocampal mossy fibers in the heterosynaptic regulation of N -methyl-D-aspartate (NMDA) receptors and as a trans-synaptic second messenger that enters postsynaptic neurons to modulate various signal transduction pathways. Frederickson describes the zinc-sensitive dyes--from dithizonate to the new fluorescence resonance energy transfer (FRET)-based sensors--and imaging techniques that freed zinc from its protein shackles by identifying sequestered pools of releasable zinc in synaptic vesicles and secretory granules that can be released into the extracellular space. The STKE Archives are rich in articles related to calcium signaling and various imaging techniques. For example, calcium signaling and gene regulation in neurons is the topic of two Perspectives (by Dolmetsch and by Impey and Goodman). Frequency-dependent calcium signaling in plants is the topic of the Review by Allen and Schroeder. The Perspective by Gaits and Hahn in the STKE Archives describes the various applications of biosensors, including the use of biosensors to perform FRET analysis. Calcium imaging methods can be found in the Protocols by Fiala and Spall and Shimonozo et al . As new methods are developed and novel regulatory mechanisms are uncovered, more metals may become recognized as players in the signaling realm. Featured in this Focus Issue Related Resources

Not Just Heavy “Grignards”: Recent Advances in the Organometallic Chemistry of the Alkaline Earth Metals Calcium, Strontium and Barium

AbstractFor Abstract see ChemInform Abstract in Full Text.

Not Just Heavy “Grignards”: Recent Advances in the Organometallic Chemistry of the Alkaline Earth Metals Calcium, Strontium and Barium

Since their initial development in the early 1900’s, Grignard reagents have proven to be immensely useful and are among the most common organometallic reagents. The nature of the reagents in solution is complex and depends on substituents, solvent, concentration and temperature. Despite continuing questions about their solid state and solution composition and conformation, organomagnesium reagents find new applications continually. In contrast, little information about the heavier alkaline earth organometallic compounds RMX and R2M (R = alkyl, aryl; M = Ca, Sr, Ba, X = halide) exists. High reactivity due to the predominantly ionic character of the metal-ligand bond and increased lability complicates synthetic access. Recent interest in the organometallic chemistry of the alkaline earth metals calcium, strontium and barium has been sparked by the realization that heavy alkaline earth metal organometallics play unique roles in synthetic applications, such as polymerization initiators and as reagents to modify polymers. As a result, the organometallic chemistry of calcium, strontium and barium has received significant attention over the last few years. This article summarizes recent results in this emerging area of chemistry. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)