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(?)(BC(4)Ph(4))(2)]-Cym(?)=(OC)(3)Mn(η(5)-C(5)H(3))-and its transformation into two fully characterized Lewis acid-base adducts with pyridine bases of the type 4-R-NC(5)H(4) (R=tBu, NMe(2)). The results of electrochemical, as well as NMR and UV/Vis spectroscopic studies on [Cym(?)(BC(4 Ph(4))(2)] and the related monoborole derivative [Cym(BC(4)Ph(4))]-Cym=(OC)(3)Mn(η(5)-C(5) H(4))-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(?)(BC(4)Ph(4))(2)]. In addition, the reduction chemistry of [Cym(?)(BC(4)Ph(4))(2)] has been studied in detail, both electrochemically and chemically. Accordingly, chemical reduction of [Cym(?)(BC(4)Ph(4))(2)] with magnesium anthracene afforded the corresponding tetraanion, which features a rare Mg-OC bonding mode in the solid state.