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

Abstract

Mechanochemical reduction of β-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–(C₆H₆)–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⋯(π-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–(C₆H₆)–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–(C₆H₆)–Ba(BDI) complexes could be considered the first Ba^I synthons. They reduce a range of polyaromatic hydrocarbons, H₂ 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.