Trends in benzene inverse sandwich complexes of the alkaline-earth metals Mg, Ca, Sr and Ba

Mechanochemical reduction of beta-diketiminate (BDI) barium iodide precursors with K/KI resulted in the first barium inverse sandwich complexes containing the benzene dianion in yields of up to 54%. This most challenging isolation of highly reactive (BDI)Ba-(C$_6$H$_6$)-Ba(BDI) complexes, completes the family of heavier benzene inverse sandwich complexes and allows for a comparison of trends in the series from Mg, Ca, Sr to Ba. Syntheses, stabilities, structures, electronic states and reactivities of the full range are compared. Crucial for isolation of the Ba inverse sandwich complexes are the tBu-substituents in the ligand backbone which push the bulky aryl rings towards the large Ba metal cations. These secondary Ba & ctdot;(pi-Ar) interactions result in an unexpected high stability. Another trend is found for the ring puckering in the bridging benzene2- dianion which steadily increases from Ba to Mg. DFT calculations show the general ionic character of (BDI)Ae-(C$_6$H$_6$)-Ae(BDI) complexes (Ae = Mg, Ca, Sr, Ba) and reveal only small energy differences between closed-shell singlet or open-shell triplet states. The most reactive (BDI)Ba-(C$_6$H$_6$)-Ba(BDI) complexes could be considered the first BaI synthons. ... mehrThey reduce a range of polyaromatic hydrocarbons, H$_2$ or even convert (BDI)MgI precursors into well-known (BDI)Mg-Mg(BDI) complexes. Reactions with heavier (BDI)AeI (Ae = Ca, Sr) gave (BDI)Ae-(C6H6)-Ae(BDI) and (BDI)BaI.