Mechanistic Elucidation of the Formation of the Inverse Ca(I) Sandwich Complex [(thf)3Ca(μ-C6H3-1,3,5-Ph3)Ca(thf)3] and Stability of Aryl-Substituted Phenylcalcium Complexes

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Abstract
The formation of the stable inverse Ca(I) sandwich complex [(thf)3Ca(μ-C6H3-1,3,5-Ph3)Ca(thf)3] (1) has been investigated mechanistically by the reaction of bromo-2,4,6-triphenylbenzene with calcium in varying stoichiometric ratios. The key intermediate consists of a solvent-separated ion pair consisting of a dinuclear calcium cation with a bridging doubly deprotonated triphenylbenzene and a triphenylbenzene radical counteranion [(thf)3Ca(μ-C6H2−C6H4Ph2)(μ-O−CH═CH2)Ca(thf)3][C6H3Ph3] (4). A precondition of the formation of 1 is the lability of the heavy Grignard reagent [{2,4,6-Ph3C6H2}Ca(thf)3Br] (2), which has been studied along with the role of ether degradation reactions. The strong reducing reagent 1 is stable in THF solution, and ether cleavage does not occur. However, toluene is metalated in good yields, and the dibenzylcalcium complex [(tmta)2Ca(CH2C6H5)2] (5) is generated after addition of 1,3,5-trimethyl-1,3,5-triazinane (tmta). The substitution pattern of arylcalcium halides was modified, and it was found that phenyl substituents at the para position induce lability, leading to an enhanced tendency to cleave ethers. Kinetic stabilization of the Ca−Cipso bond can be achieved by ortho substitution using m-terphenyl-based ligands. Direct reaction of iodo-2,6-di(4-tolyl)benzene (6) with activated calcium in THF at low temperatures yielded the first example of a stable m-terphenylcalcium halide, namely, [{2,6-(4-tol)2C6H3}Ca(thf)3I] (8). The latter reacts via insertion of carbon dioxide to form the dimeric benzoate [{2,6-(4-tol)2C6H3CO2}Ca(thf)3I]2 (9).

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