Variable Ca-C Hapticity and its Consequences in Arylcalcium Dimers.

The dimeric β-diketiminato calcium hydride, [( BDI)CaH] ( BDI = HC{(Me)CN-2,6-i-Pr C H } ), reacts with ortho-, meta- or para-tolyl mercuric compounds to afford hydridoarylcalcium compounds, [( BDI) Ca (μ-H)(μ-o-,m-,p-tolyl)], in which dimer propagation occurs either via μ -η -η or μ -η -η bridging between the calcium centers. In each case, the orientation and hapticity of the aryl units is dependent upon the position of the methyl substituent. While wholly organometallic meta- and para-tolyl dimers, [( BDI)Ca(m-tolyl)] and [( BDI)Ca(p-tolyl)] , can be prepared and are stable, the ortho-tolyl isomer is prone to isomerization to a calcium benzyl analog. Computational analysis of this latter process with density functional theory (DFT) highlights an unusual mechanism invoking the generation of an intermediate dicalcium species in which the group 2 centers are bridged by a toluene dianion formed by the formal attachment of the original hydride anion to the initially generated ortho-tolyl substituent. Use of a more sterically encumbered aryl substituent, {3,5-t-Bu C H }, facilitates the selective formation of [( BDI)Ca(μ-H)(μ-3,5-t-Bu C H )Ca( BDI)], which can be converted into the unsymmetrically-substituted σ-aryl calcium complexes, [( BDI)Ca(μ-Ph)(μ-3,5-t-Bu C H )Ca( BDI)] and [( BDI)Ca(μ-p-tolyl)(μ-3,5-t-Bu C H )Ca( BDI)] by reaction with the appropriate mercuric diaryl. Conversion of [( BDI)Ca(H)(Ph)Ca( BDI)] to afford [{{( BDI)Ca} (μ -Cl)} (C H -C H )], comprising a biphenyl dianion, is also reported. Although this latter transformation is serendipitous, AIM analysis highlights that, in a related manner to the ortho-tolyl to benzyl isomerization, the requisite C-C coupling may be facilitated in an "across dimer" fashion by the experimentally-observed polyhapto engagement of the aryl substituents with each calcium.