[AlCl4] -is comparable to py-AlCl3 with a greater degree of deactivation in the Given the rather hard character of the Al-based Lewis acids, precursors based on B and Zn were screened as softer alternatives. Borosilicate Lewis acids differ from their aluminosilicate counterparts by the tendency of the [BO3] moiety to assume planar geometry with π-electron delocalization across the whole moiety. This results in much weaker, more reversible coordination to ligands and the fact that, in contrast to [Al(OSi≡)3], [B(OSi≡)3] is a well-defined and stable species. To facilitate maximally relevant comparison, py-BCl3 (1A/1B) was chosen as the starting precursor, and the results show that its condensation with CUBE is highly exoergic (-99 to -77 kJ mol -1 )