Ab initio computational studies of heterocycloalkynes: Structures, natural bond orders, ring strain energies, and isomerizations of cyclic iminoboranes and iminoalanes

Ab initio calculations of series of cyclic iminoboranes (CH2)(n)B=N and iminoalanes (CH2)(n)- Al=N show that they adopt angularly distorted structures compared to the isoelectronic cycloalkynes. Natural bond order analyses indicate that the larger ring iminoboranes contain BN triple bonds, but the smaller ring iminoboranes and all the iminoalanes contain MN double bonds and a lone pair on the nitrogen atom. Group equivalent calculations confirm that cyclic iminoboranes and iminoalanes contain less ring strain energy than the corresponding cycloalkynes, but the data do not distinguish between doubly and triply bonded systems. Examination of the rearrangements of isomeric azacycloalkane borines to cyclic iminoboranes shows that they are exothermic, with moderate activation barriers. Thus, a way by which the heterocycloalkynes might be synthesized is suggested.