Synthesis of Cyclic and Cage Borosilicates Based on Boronic Acids and Acetoxysilylalkoxides. Experimental and Computational Studies of the Stability Difference of Six- and Eight-Membered Rings.

A series of borosilicates was synthesized, where the structure of the borosilicate core was easily modulated using two strategies: blocking of condensation sites and controlling the stoichiometry of the reaction. Thus, on the one hand, the condensation of phenylboronic or 3-hydroxyphenylboronic acid with diacetoxysilylalkoxide [(tBuO)(Ph3CO)Si(OAc)2] led to the formation of borosilicates (tBuO)(Ph3CO)Si{(μ-O)BPh}2(μ-O) (1), [{(tBuO)(Ph3CO)Si(μ-O)BPh(μ-O)}2] (2), and [{(tBuO)(Ph3CO)Si(μ-O)B(3-HOPh)(μ-O)}2] (3) with a cyclic inorganic B2SiO3 or B2Si2O4 core, respectively. On the other hand, the reaction of phenylboronic acid with triacetoxysilylalkoxide (Ph3CO)Si(OAc)3 in 3:2 ratio resulted in the formation of a cagelike structure [{(Ph3CO)Si(μ-O)2BPh(μ-O)}2] (4) with B4Si4O10 core, while the reaction of the boronic acid with silicon tetraacetate generated an unusual 1,3-bis(acetate)-1,3-diphenyldiboraxane PhB(μ-O)(μ-O,O'-OAc)2BPh (5). Additionally, compound 1 was used to evaluate the possibility to form N→B donor-acceptor bond between the boron atom in the borosilicates and a nitrogen donor. Thus, coordination of 1 with piperazine yielded a tricyclic [{(tBuO)(Ph3CO)Si(OBPh)2(μ-O)}2·C4H10N2] compound 6 with two borosilicate rings bridged by a piperazine molecule. Finally, the processes involved in the formation of the six- and eight-membered rings (B2SiO3 and B2Si2O4) in compounds 1 and 2 were explored using solution 1H NMR studies and density functional theory calculations. These molecules represent to the best of our knowledge first examples of cyclic molecular borosilicates containing SiO4 units.