Synthesis, Properties, and Chemoselective Reactions of an AlH–BH Functional Group

Abstract

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 k²-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 k¹ and k 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 rationale framework for further developments.