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.