Nees S., Fantuzzi F., Wellnitz T., Fischer M., Siewert JE., Goettel JT., Hofmann A., Härterich M., Braunschweig H., Hering-Junghans C.
Using the AlI precursor Cp3tAl in conjunction with triphosphiranes (PAr)3 (Ar=Mes, Dip, Tip) we have succeeded in preparing Lewis base-free cyclic diphosphadialanes with both the Al and P atoms bearing three substituents. Using the sterically more demanding Dip and Tip substituents the first 1,2-diphospha-3,4-dialuminacyclobutanes were obtained, whereas with Mes substituents [Cp3tAl(μ-PMes)]2 is formed. This divergent reactivity was corroborated by DFT studies, which indicated the thermodynamic preference for the 1,2-diphospha-3,4-dialuminacyclobutane form for sterically more demanding groups on phosphorus. Using Cp*Al we could extend this concept to the corresponding cyclic diarsadialanes [Cp*Al(μ-AsAr)]2 (Ar=Dip, Tip) and additionally add the phosphorus variants [Cp*Al(μ-PAr)]2 (P=Mes, Dip, Tip). The reactivity of one variant [Cp3tAl(μ-PPh)]2 towards NHCs was tested and resulted in double NHC-stabilised [Cp3t(IiPr2)Al(μ-PPh)]2.