"True" inorganic heterocycles: structures and stability of group 13-15 analogues of benzene and their dimers

Group 13-15 inorganic analogues of benzene, HMYH](3) (M = B, Al, Ga; Y = N, P, As), mixed heterocycles of the type BAlGaNPAs]H(6) and their dimers have been theoretically examined at the B3LYP/TZVP level of theory. Six different isomers have been structurally characterized for the mixed compounds BAlGaNPAs]H(6). B-N bonding strongly (about approximately 90-100 kJ mol(-)(1)) stabilizes the mixed heterocycles, followed by the preference of the Al-N bonded structures over Ga-N bonded ( approximately 30-40 kJ mol(-1)), while B-P bonding is slightly (5-10 kJ mol(-1)) more favorable compared to B-As. Thus, the bonding pattern is predicted to be the most stable, followed by the core. Processes of HMYH](3) formation from donor-acceptor complexes H(3)MYH(3) are predicted to be thermodynamically favorable for all MY combinations. Dimerization reactions of the coordinationally unsaturated HMYH](3) heterocycles yielding hexamer clusters HMYH](6) are found to be exothermic, with the exception of borazine, for which, as for benzene, dimerization is strongly endothermic due to the aromaticity of C(6)H(6) and HBNH](3). Despite the high endothermicity of HBNH](3) dimerization, the B-N bond formation is the driving force of the dimerization of mixed species BAlGaNPAs]H(6). The dimerization enthalpies of BAlGaNPAs]H(6) may be both exo- and endothermic, depending on the bonding pattern of the isomers. A complete set of mean MY bond energies in four- and six-membered cycles of HMYH](6) was derived. The MY energies were found to be transferable quantities and may serve for a qualitative prediction of the relative stability of different isomers of mixed cluster compounds. BAlGaNPAs](2)H(12) clusters are promising synthetic targets, they are expected to serve as single-source precursors for the stoichiometry-controlled CVD processes of the group 13-15 composites. A strategy of their synthesis and the most suitable starting systems have been also predicted.