Theoretical Designs for Organoaluminum C₂Al₄R₄ with Well-Separated Al(I) and Al(III)

It is well-known that the chemistry of aluminum is dominated by Al(III) in the +3 oxidation state. Only during the past 2 decades has the chemistry of Al(I) and Al(II) been rapidly developed. However, if Al(I) and Al(III) are combined, the inherently high reactivities of Al(I) and Al(III) mostly result in their coupling with each other or interacting with surrounding elements, which easily results in significant deactivation or quenching of the desired oxidation states, as in the case of reported mixed valent Al-compounds. In this article, we report an unprecedented type of organoaluminum system, C₂Al₄R₄ (R = H, SiH₃, Si(C₆H₅)₃, SiiPrDis₂, SiMe(SitBu₃)₂), whose lowest-energy structure, C₂Al₄R₄-01, contains two Al(I) and two Al(III) atoms. The global nature and bonding motif of the parent C₂Al₄R₄-01 (R = H) were supported by an extensive global isomeric search, CBS-QB3 energy calculations, adaptive natural density partitioning, and bond order analysis. Interestingly and in sharp contrast to most organoaluminum species, C₂Al₄R₄-01 is associated with little multicenter bonding. C₂Al₄R₄-01 has a high feasibility of being observed either in the gas or condensed phases (with suitable substitutents). With well-separated Al(I) and Al(III), C₂Al₄R₄-01 (with suitable substitutents) could serve as the first Al/Al frustrated Lewis pair.