Synthesis, properties, and chemoselective reactions of an AlH-BH functional group

A new alumaborane compound containing a {AlH-BH} functional group has been prepared by reaction of the aluminium(I) complex [{(ArCMeN)₂CH}Al] (Ar = 2,6-i-Pr₂C₆H₃) with a boron dihydride species supported by an anionic chelating κ²-N,N ligand. Spectroscopic analysis of the product suggests that it exists as a mixture of isomers in solution with the ligand on boron coordinating in both a κ¹ and κ ² fashion. The dialumane analogue, containing a {AlH-AlH} functional group, was also prepared. Comparison of the structure and bonding of these species by DFT calculations suggests that reactivity of the AlH-BH moiety should be governed by the nucleophilicity of the Al-H and Al-B bonds along with the Lewis acidity imparted by the partially available 2p of boron. Chemoselective reactions are observed with CO₂, CNXyl (Xyl = 2,6-Me₂C₆H₃), and PhCN. While CO₂ inserts selectively into the Al-H bond to generate a formate, the more Lewis basic substrates CNXyl and PhCN react through initial coordination at boron and insertion into the Al-B bond. In the case of PhCN, an unusual pathway to generate 1,3,2-diazaborole compound is reported. Our work delivers the first insight into the reactivity of the {AlH-BH} functional group and provides a rational framework for further developments.