Synthesis, Coordination Behavior, and Reduction Chemistry of Cymantrenyl-1,3-bis(2,3,4,5-tetraphenyl)borole

We describe the synthesis of base-free bisborole [Cym dagger(BC4Ph4)2]Cym dagger=(OC)3Mn(?5-C5H3)and its transformation into two fully characterized Lewis acidbase adducts with pyridine bases of the type 4-R?NC5H4 (R=tBu, NMe2). The results of electrochemical, as well as NMR and UV/Vis spectroscopic studies on [Cym dagger(BC4Ph4)2] and the related monoborole derivative [Cym(BC4Ph4)]Cym=(OC)3Mn(?5-C5H4)provided conclusive evidence for 1) the enhanced Lewis acidity of the two boron centers that result from conjugation of two borole fragments, and 2) the fact that Mn?B bonding interactions between the Lewis acidic borole moieties and the Mn center are considerably less pronounced for bisborole [Cym dagger(BC4Ph4)2]. In addition, the reduction chemistry of [Cym dagger(BC4Ph4)2] has been studied in detail, both electrochemically and chemically. Accordingly, chemical reduction of [Cym dagger(BC4Ph4)2] with magnesium anthracene afforded the corresponding tetraanion, which features a rare Mg?OC bonding mode in the solid state.