Variable Ca‐C_aryl Hapticity and its Consequences in Arylcalcium Dimers

Abstract The dimeric β‐diketiminato calcium hydride, [(^DippBDI)CaH]₂ (^DippBDI = HC{(Me)CN‐2,6‐i‐Pr₂C₆H₃}₂), reacts with ortho‐, meta‐ or para‐tolyl mercuric compounds to afford hydridoarylcalcium compounds, [(^DippBDI)₂Ca₂(μ‐H)(μ‐o‐,m‐,p‐tolyl)], in which dimer propagation occurs either via μ₂‐η¹‐η¹ or μ₂‐η¹‐η⁶ 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‐Bu₂C₆H₃}, facilitates the selective formation of [(^DippBDI)Ca(μ‐H)(μ‐3,5‐t‐Bu₂C₆H₃)Ca(^DippBDI)], which can be converted into the unsymmetrically‐substituted σ‐aryl calcium complexes, [(^DippBDI)Ca(μ‐Ph)(μ‐3,5‐t‐Bu₂C₆H₃)Ca(^DippBDI)] and [(^DippBDI)Ca(μ‐p‐tolyl)(μ‐3,5‐t‐Bu₂C₆H₃)Ca(^DippBDI)] by reaction with the appropriate mercuric diaryl. Conversion of [(^DippBDI)Ca(H)(Ph)Ca(^DippBDI)] to afford [{{(^DippBDI)Ca}₂(μ₂‐Cl)}₂(C₆H₅‐C₆H₅)], 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.