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 [(BuO)(PhCO)Si(OAc)] led to the formation of borosilicates (BuO)(PhCO)Si{(μ-O)BPh}(μ-O) (1), [{(BuO)(PhCO)Si(μ-O)BPh(μ-O)}] (2), and [{(BuO)(PhCO)Si(μ-O)B(3-HOPh)(μ-O)}] (3) with a cyclic inorganic BSiO or BSiO core, respectively. On the other hand, the reaction of phenylboronic acid with triacetoxysilylalkoxide (PhCO)Si(OAc) in 3:2 ratio resulted in the formation of a cagelike structure [{(PhCO)Si(μ-O)BPh(μ-O)}] (4) with BSiO 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)BPh (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 [{(BuO)(PhCO)Si(OBPh)(μ-O)}·CHN] 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 (BSiO and BSiO) in compounds 1 and 2 were explored using solution H NMR studies and density functional theory calculations. These molecules represent to the best of our knowledge first examples of cyclic molecular borosilicates containing SiO units.