Variable Ca-Caryl hapticity and its Consequences in Arylcalcium Dimers

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