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

The dimeric β-diketiminato calcium hydride, [( Dipp BDI)CaH] 2 ( Dipp BDI = HC{(Me)CN-2,6-i-Pr 2 C 6 H 3 } 2 ), reacts with ortho-, meta- or para-tolyl mercuric compounds to afford hydridoarylcalcium compounds, [( Dipp BDI) 2 Ca 2 (μ-H)(μ-o-,m-,p-tolyl)], in which dimer propagation occurs either via μ 2 -η 1 -η 1 or μ 2 -η 1 -η 6 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, [( Dipp BDI)Ca(m-tolyl)] 2 and [( Dipp BDI)Ca(p-tolyl)] 2 , 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 2 C 6 H 3 }, facilitates the selective formation of [( Dipp BDI)Ca(μ-H)(μ-3,5-t-Bu 2 C 6 H 3 )Ca( Dipp BDI)], which can be converted into the unsymmetrically-substituted σ-aryl calcium complexes, [( Dipp BDI)Ca(μ-Ph)(μ-3,5-t-Bu 2 C 6 H 3 )Ca( Dipp BDI)] and [( Dipp BDI)Ca(μ-p-tolyl)(μ-3,5-t-Bu 2 C 6 H 3 )Ca( Dipp BDI)] by reaction with the appropriate mercuric diaryl. Conversion of [( Dipp BDI)Ca(H)(Ph)Ca( Dipp BDI)] to afford [{{( Dipp BDI)Ca} 2 (μ 2 -Cl)} 2 (C 6 H 5 -C 6 H 5 )], 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.