A Unique Barium−Carbon Bond: Mechanism of Formation and Crystallographic Characterization

Coordination of arenes and phosphines by charge separated alkaline earth cations

Generation of ‘naked’ β-diketiminato magnesium and calcium cations allows the isolation of benzene, toluene, 1,4-difluorobenzene and terminal phosphine adducts.

Intramolecular C-F and C-H bond cleavage promoted by butadienyl heavy Grignard reagents

Organomagnesium compounds (Grignard reagents) are among the most useful organometallic reagents and have greatly accelerated the advancement of synthetic chemistry and related sciences. Nevertheless, heavy Grignard reagents based on the metals calcium, strontium or barium are not widely used, mainly due to their rather inert heavy alkaline-earth metals and extremely high reactivity of their corresponding Grignard-type reagents. Here we report the generation and reaction chemistry of butadienyl heavy Grignard reagents whose extremely high reactivity is successfully tamed. Facile synthesis of perfluoro-π-extended pentalene and naphthalene derivatives is realized by the in situ generated heavy Grignard reagents via intramolecular C-F/C-H bond cleavage. These obtained perfluorodibenzopentalene and perfluorodinaphthopentalene derivatives show low-lying LUMO levels, with one being the lowest value so far among all pentalene derivatives. Our results set an exciting example for the meaningful synthetic application of heavy Grignard reagents.

Heterofunctionalization catalysis with organometallic complexes of calcium, strontium and barium

Despite the routine employment of Grignard reagents and Hauser bases as stoichiometric carbanion reagents in organic and inorganic synthesis, a defined reaction chemistry encompassing the heavier elements of Group II (M = Ca, Sr and Ba) has, until recently, remained unreported. This article provides details of the recent progress in heavier Group II catalysed small molecule transformations mediated by well-defined heteroleptic and homoleptic complexes of the form LMX or MX 2 ; where L is a mono-anionic ligand and X is a reactive σ-bonded substituent. The intra- and intermolecular heterofunctionalization (hydroamination, hydrophosphination, hydrosilylation and hydrogenation) of alkenes, alkynes, dienes, carbodiimides, isocyanates and ketones is discussed.

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.

Aryl Calcium Compounds: Syntheses, Structures, Physical Properties, and Chemical Behavior

Organocalcium chemistry is still in its infancy. The direct synthesis of activated calcium and (substituted) iodobenzenes allows for the large-scale and high-yield synthesis of aryl calcium iodides. The influence of the substitution patterns of the phenyl group, halogen atom, and solvent is discussed. Aryl calcium iodides show a Schlenk equilibrium that enables the isolation of diaryl calcium derivatives. Owing to the high reactivity of aryl calcium halides, low temperatures have to be maintained throughout the preparative procedures in order to avoid side reactions. A decrease of reactivity and, hence, an enhanced stability at higher temperatures can be achieved by shielding of the calcium atom by increasing the coordination number of the metal center or by substitution of the iodide anion by bulky groups.

Calcium-Mediated Intramolecular Hydroamination Catalysis

The calcium-catalyzed intramolecular hydroamination of alkenes and alkynes is reported. The beta-diketiminato complex [{HC(C(Me)2N-2,6-iPr2C6H3)2}-Ca{N(SiMe3)2}(THF)] affects catalytic cyclization of a range of aminoalkenes and aminoalkynes with activities that are broadly commensurate to those of established rare earth catalysts.

Complexes of the Heavier Alkaline Earth Metals Ca, Sr, and Ba with O-Functionalized Phosphanide Ligands

Metathesis reactions between either SrI(2) or BaI(2) and 2 equiv of the potassium phosphanide [[(Me(3)Si)(2)CH]-(C(6)H(4)-2-OMe)P]K yield, after recrystallization, the complexes [[([Me(3)Si](2)CH)(C(6)H(4)-2-OMe)P](2)M(THF)(n)] [M = Sr, n = 2 (5); Ba, n = 3 (6)]. Similar metathesis reactions between MI(2) and 2 equiv of the more sterically demanding potassium phosphanide [[(Me(3)Si)(2)CH](C(6)H(3)-2-OMe-3-Me)P]K yield the chemically isostructural complexes [[([Me(3)Si](2)CH)(C(6)H(3)-2-OMe-3-Me)P](2)M(THF)(2)] [M = Ca (9), Sr (7), Ba (8)]. Compounds 5-9 have been characterized by multi-element NMR spectroscopy and X-ray crystallography. Complex 9 is thermally unstable and decomposes at room temperature to give the tertiary phosphane [(Me(3)Si)(2)CH](C(6)H(3)-2-OMe-3-Me)P(Me) and an unidentified Ca-containing product. Compounds 5 and 6 also decompose at elevated temperatures to give the corresponding tertiary phosphane [(Me(3)Si)(2)CH](C(6)H(4)-2-OMe)P(Me) and intractable metal-containing products. The decomposition of 5, 6, and 9 suggests that these compounds undergo an intramolecular methyl migration from the O atom in one phosphanide ligand to the P atom of an adjacent phosphanide ligand to give species containing dianionic alkoxo-phosphanide ligands.

Alkyl complexes of strontium and barium: synthesis and structural characterization of [(Me3Si)2(MeOMe2Si)C]2Sr(THF) and [(Me3Si)2(MeOMe2Si)C]2Ba(MeOCH2CH2OMe)

Metathesis between either SrI2 or BaI2 and 2 equiv of {(Me3Si)2(MeOMe2Si)C}K in THF yields the novel heavier alkali metal dialkyls {(Me3Si)2(MeOMe2Si)C}2M(L) [M(L) = Sr(THF) (2), Ba(DME) (3) (DME = 1,2-dimethoxyethane)] after recrystallization.

100 years after grignard: where does the organometallic chemistry of the heavy alkaline Earth metals stand today?

Even a century after the discovery of the Grignard reagents, similar compounds of the heavy alkaline earth metals are still lacking. Owing to the industrial need for such compounds (in particular as anionic polymerization catalysts), the last few years have been marked by a rapid, extensive development which has culminated in the first structurally characterized alkylbarium derivatives such as the crown ether complex of bis(triphenylmethyl)barium (1). hmpt=hexamethylphosphoric acid triamide.