Alkaline‐Earth Metal Mediated Benzene‐to‐Biphenyl Coupling

Complex [(DIPePBDI)Ca]2(C6H6), with a C6H62− dianion bridging two Ca2+ ions, reacts with benzene to yield [(DIPePBDI)Ca]2(biphenyl) with a bridging biphenyl2− dianion (DIPePBDI=HC[C(Me)N‐DIPeP]2; DIPeP=2,6‐CH(Et)2‐phenyl). The biphenyl complex was also prepared by reacting [(DIPePBDI)Ca]2(C6H6) with biphenyl or by reduction of [(DIPePBDI)CaI]2 with KC8 in presence of biphenyl. Benzene‐benzene coupling was also observed when the deep purple product of ball‐milling [(DIPPBDI)CaI(THF)]2 with K/KI was extracted with benzene (DIPP=2,6‐CH(Me)2‐phenyl) giving crystalline [(DIPPBDI)Ca(THF)]2(biphenyl) (52 % yield). Reduction of [(DIPePBDI)SrI]2 with KC8 gave highly labile [(DIPePBDI)Sr]2(C6H6) as a black powder (61 % yield) which reacts rapidly and selectively with benzene to [(DIPePBDI)Sr]2(biphenyl). DFT calculations show that the most likely route for biphenyl formation is a pathway in which the C6H62− dianion attacks neutral benzene. This is facilitated by metal‐benzene coordination. Attempts to isolate low‐valent (BDI)Ae‐Ae(BDI) complexes led to reduction of the benzene solvent and formation of (BDI)Ae‐(C6H6)‐Ae(BDI); BDI=β‐diketiminate ligand, Ae=Ca or Sr. Reaction of bridging C6H62− with benzene gave (biphenyl)2− complexes. Ball‐milling is an efficient synthetic tool for this dehydrogenative benzene‐benzene coupling. Calculations suggest attack of C6H62− at benzene. This is facilitated by Ae2+⋅⋅⋅benzene coordination.