Polymer compounds with a group 13/15 element backbone are valence isoelectronic to polyethylene, but exhibit completely different functional properties. [R2PBH2]n oligomers and polymers can be used as precursors for ceramic materials1, thus, investigation of possibilities for controlled synthesis of well-defined [R2PBH2]n oligomers is an actual scientific task.
In present report, we will discuss results of computational studies of chain and ring [H2PBH2]n oligomers ( E=B,Al and n=1-15). We show that the difference in the electronegativities of the constituent atoms leads to substantial differences between oligomerization of phosphinoboranes and phospholanes. Chains with E=Al have a pronounced donor-acceptor character with profound bond length alternation and exhibit much stronger tendency for the cyclization. Only helix conformation is theoretically possible for long (n>7) Al-containing chain oligomers, while B-containing chains can adopt different (synclinal and anti) conformations.
Rings with E=B are analogous to cyclic oligoethylene structures, but rings with E=Al adopt a different honeycomb structure. Subsequent oligomerization enthalpies become very close for the ring and chain oligomers with the increase of n. However, these oligomers demonstrate very different electronic properties with band gaps below 0.2 eV for the chain [H2PBH2]15 and more than 5 eV for the ring isomer [H2PEH2]15. The MOs in the rings are evenly delocalized along the oligomer. In case E=B the frontier orbitals are localized at the P-B bonds and in case E=Al they are more localized at the atomic centers. In the chain isomers HOMO is localized at the P-terminated end with the main contribution from the lone pair orbital of P atom, and LUMO is strictly localized at the opposite end with the main contribution from p-orbital of B/Al atom. Overall [H2PBH2]n oligomers are found to be more stable.
This work was supported by the joint SPSU-DFG grant 188.8.131.527.