Organocalcium chemistry

Organocalcium chemistry is the chemistry of compounds containing a calcium to carbon bond, or in broader definitions, organic compounds that contain calcium. Although discovered around the same time as the now commonly utilized organomagnesium compounds, organocalcium compounds were subject to greatly reduced interest due to drastic differences in stability. Because calcium metal is less reactive to organic reagents than magnesium and the organocalcium compounds are more reactive than organomagnesium compounds, synthesis of novel compounds still poses a significant challenge. Calcium also has access to empty d orbitals that the lighter alkaline earth metals cannot access, and the degree to which this affects bonding and reactivity has sparked a fundamental debate. Lastly, despite the inherent instability of most organocalcium complexes, the unique basicity and size of the calcium ion together with the highly polarized bonds formed has opened up applications for organocalcium compounds in organic transformations and catalytic cycles.

Calcium carbide

In terms of scale (millions of kilograms) and practicality, calcium carbide is the dominant organocalcium compound. It is produced by reaction of carbon and calcium oxide:

CaO + 3 C → CaC2 + CO It is used to prepare acetylene, which is widely used in welding. It reacts with nitrogen gas to give calcium cyanamide, a versatile synthetic intermediate.

Compounds In general, organocalcium synthesis is complicated by relatively unreactive calcium metal (compared to magnesium or the alkali metals due to a high atomization energy) and high reactivity of most organocalcium compounds to oxygen, water, and even ethereal solvents. To sustain the highly electropositive calcium center, the vast majority of compounds have anionic ligands by which they can be categorized, with neutral coordinating ligands utilized for increased stability.

History

In 1905 by Ernst Beckmann claimed the synthesis of phenylcalcium iodide by stirring of calcium shavings with iodobenzene in diethyl ether (Et2O). Subsequent study by Henry Gilman and Ferdinand Schulze argued that the isolated product in this report was actually the Et2O adduct of CaI2, and, although phenylcalcium halides have been reported numerous times, they were usually characterized through subsequent derivatization products. It took a full century until, in 2005, Matthias Westerhausen and colleagues obtained the first structural characterization of an arylcalcium compound, crystallizing phenylcalcium iodide as an adduct of tetrahydrofuran (THF) and calcium oxide. A consistent challenge in the formation of organocalcium compounds has been the activation of calcium metal. Mechanochemistry (ball-milling) has allowed the use of unactivated calcium. The history of alkyl calcium compounds is also checkered until Lappert et al.'s synthesis of Ca[CHSi(CH3)3)]2]2.

Aryl, allyl, and alkyl derivatives

Bis(allyl)calcium complexes are stabilized by sterically large, silyl substituents. These syntheses use salt metathesis reactions, involving allyl potassium and CaI2. This strategy has been used to synthesize the unsubstituted complex Ca(η3-C3H5)2 as a soluble triglyme adduct. and related species. The carbon atom in the calcium-carbon bond takes on a significant negative charge. Because of the greater nucleophilicity of alkyl ligands, the alkylcalcium reagents are in general harder to synthesize than the arylcalcium compounds. substituents to stabilize this negative charge. When targeting a Grignard analogue, the decreased reactivity from this method and the poor stability of the less protected methyl- and ethylcalcium halides has led to in situ generation of reactive alkylcalcium halides as the preferred method over the synthesis of isolable compounds. Because of this poor stability, the pure organometallic dimethylcalcium was only isolated in 2018 by Reiner Anwander and colleagues as an insoluble, amorphous solid, with the THF adduct being structurally characterizable as a heptametallic cluster.

Metallocenes

The first synthesis of Cp2Ca (Cp = cyclopentadienyl) involved combining calcium metal and cyclopentadiene in THF, producing an insoluble, polymeric product. According to X-ray crystallography calcocene is polymer. This bent structure is observed in related compounds. For example, two butenyl-substituted Cp ligand will coordinate to Ca through both the five-membered rings and the olefins, in contrast to related magnesium compounds

Low-oxidation-state compounds Few organocalcium(I) compounds exist. The first and only report of an isolable Ca(I) compound came in 2009, where two THF-coordinated Ca(I) ions sit on either side of an arene ring. The π-antibonding orbitals of the sandwiched arene help stabilize the two calcium ions, which are further stabilized by the coordinating solvent. Other studies of Ca(I) were done at low temperatures in exotic conditions or examine formally Ca(II) compounds that imply Ca(I)-containing intermediates either during synthesis or further reactivity. A landmark example of this from Sjoerd Harder and coworkers is the reported reduction of arenes and N2 by a bridged Ca(I)-Ca(I) species generated in situ. The ease of activating the normally inert N2 to turn it into a strong reductant even at room temperature highlights the instability of Ca(I) species. Although not isolable as a Ca(I)-Ca(I) dimer, it possesses similar reactivity as a stronger reducing agent than a Mg(I) dimer.

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