Coordination of arenes and phosphines by charge separated alkaline earth cations

Generation of β-diketiminato group 2 cations, [(^MeBDI)Ae]+ and [(^t-BuBDI)Ae]+ (^MeBDI = HC{(Me)CN-2,6-iPr₂C₆H₃}₂; ^t-BuBDI = HC{(t-Bu)CN-2,6-i-Pr₂C₆H₃}₂; Ae = Mg or Ca), in conjunction with the weakly coordinating
anion, [Al{OC(CF₃)₃}₄] −, allows the characterisation of charge separated alkaline earth η6-π adducts to toluene or benzene when crystallised from the arene solvents. Addition of 1,4-difluorobenzene to [(^MeBDI)Mg]+ results in the isolation of [(^MeBDI)Mg(1,4-F₂C₆H₄)₃]+ in which the fluorobenzene molecules coordinate via κ1
-F–M interactions. Although DFT analysis indicates that the polyhapto arene binding to Mg is effectively electrostatic in origin, the interactions with Ca (Sr and Ba) are observed to invoke small but significant π overlap of the arene HOMOs with the alkaline earth valence nd orbitals.
Reaction of triphenylphosphine with [(^MeBDI)Mg]+ and [(^t-BuBDI)Mg]+ in toluene solvent allows the isolation
of the respective terminally coordinated magnesium-phosphine adducts. The resultant Mg–P bond
lengths [2.5972(13), 2.6805(12) Å] are comparable to those previously observed in magnesium derivatives
of terminal but formally anionic phosphide ligands, while the effectively electrostatic nature of the
bonding is supported by DFT calculations.