Organocalcium chemistry
The first of these was reported in 1905 by Ernst Beckmann, where synthesis of phenylcalcium iodide was claimed following stirring of calcium shavings with iodobenzene in diethyl ether (Et2O).
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.
Recent advancements in mechanochemistry have opened up simpler synthetic setups, with unactivated calcium being used to form arylcalcium reagents in situ during ball-milling.
[15] Allylcalcium compounds have also seen recent synthetic success, beginning with Timothy Hanusa and colleagues’ synthesis of a bis(allyl)calcium complex stabilized by sterically large, silyl substituents.
[17] This has been proven to be a versatile strategy, with a full series of substituted allylcalcium complexes of different sizes also characterized through a salt metathesis pathway.
[21] 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.
[24] A crystal structure showed that, unlike most transition metal metallocenes, the Cp-Ca-Cp angle is significantly bent and Cp2Ca has an opening that can be utilized to access derivatives.
[33] Additional electronic and kinetic stabilization can be provided through carbenes, despite lacking the π-backbonding that other main group elements are capable of.
[36] Inspired by the well-studied and useful solid-state CaH2, several molecular calcium hydrides have been synthesized with the hope of interesting small molecule activation.
[41][42] The changes in properties going down the alkaline earth group causes calcium to possess qualitatively entirely distinct bonding characteristics than the lighter beryllium and magnesium ions.
[23][43] This has been highly debated, however, with other explanations invoking the polarizability of the larger Ca core[44] and a stabilizing van der Waals interaction between the two ligands.
This is largely due to differences in electronegativity, which allow organocalcium compounds to function as a base more often than typical magnesium-based Grignard reagents do.
[49] The previously mentioned in situ generation of reactive alkylcalcium species has also been successfully used to react with amines to form calcium amides.
Non-Grignard alkylcalcium complexes have also shown unique reactivity, such as alkylation of benzene driven by the formation of a calcium hydride.
[51] This is not only enabled by the previously discussed electronic and electrostatic differences, but also by the larger size of calcium in comparison to the alkali metals or magnesium.
