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-i-Pr2C6H3}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.