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“True” Inorganic Heterocycles:  Structures and Stability of Group 13−15 Analogues of Benzene and Their Dimers

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journal contribution
posted on 11.12.2002, 00:00 by Alexey Y. Timoshkin, Gernot Frenking
Group 13−15 inorganic analogues of benzene, [HMYH]3 (M = B, Al, Ga; Y = N, P, As), mixed heterocycles of the type [BAlGaNPAs]H6 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]H6. B−N bonding strongly (about ∼90−100 kJ mol-1) stabilizes the mixed heterocycles, followed by the preference of the Al−N bonded structures over Ga−N bonded (∼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 H3MYH3 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 C6H6 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]H6. The dimerization enthalpies of [BAlGaNPAs]H6 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]2H12 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.