Computational probing of all-boron Li2nB2nH2n+2 polyenes

We tested if the analogues of the polyacetylene chains with the general formula Li2nB2nH2n+2 (n = 2–7) are viable species. We performed searches for the global minimum structures of two stoichiometries Li4B4H6 and Li6B6H8. It was shown that Li4B4H6 global minimum structure contains cis-planar B 4 H 6 4 - kernel. We probed the stability of two different types of cis-isomers of the all-boron polyenes on the example of Li6B6H8 and Li8B8H10 molecules with the similar spatial arrangement of lithium atoms and kernel of boron atoms as in the case of the smallest model fragment Li4B4H6 global minimum structure. It was established that the trans–cisoid form of these all-boron polyenes is much more stable than the cis–transoid form, though they are all local minima. Though it was proved that trans–cisoid polymer chains Li2nB2nH2n+2 (n = 2–7) are minima on the potential energy surface, the trans–cisoid form of cis-Li6B6H8 was found to be a high-energy isomer. Based on these results we believe that such all-boron polyene structures are not viable species for synthesis since they are not among the most stable structures. Though lithiation of B2nH2n+2 failed to produce proposed polyene chains, we do not exclude neither the use of the other main group metals nor transition metals for producing such polyene nor even the all-boron polyacetylene chains.