Tris(tert-butoxy)siloxy Derivatives of Boron, Including the Boronous Acid HOB[OSi(OtBu)3]2 and the Metal (Siloxy)boryloxide Complex Cp2Zr(Me)OB[OSi(OtBu)3]2:  A Remarkable Crystal Structure with 18 Independent Molecules in Its Asymmetric Unit

Silanolysis of B(OtBu)3 with 2 and 3 equiv of HOSi(OtBu)3 led to the formation of tBuOB[OSi(OtBu)3]2 (1) and B[OSi(OtBu)3]3 (2), respectively. Compounds 1 and 2 are efficient single-source molecular precursors to B/Si/O materials via thermolytic routes in nonpolar media, as demonstrated by the generation of BO1.5·2SiO2 (BOSi2xg) and BO1.5·3SiO2 (BOSi3xg) xerogels, respectively. Use of a block copolymer template provided B/Si/O materials (BOSi2epe and BOSi3epe) with a broad distribution of mesopores (by N2 porosimetry) and smaller, more uniform particle sizes (by TEM) as compared to the nontemplated materials. Hydrolyses of 1 and 2 with excess H2O resulted in formation of the expected amounts of tBuOH and HOSi(OtBu)3; however, reaction of 1 with 1 equiv of H2O led to isolation of the new boronous acid HOB[OSi(OtBu)3]2 (3). This ligand precursor is well suited for the synthesis of new metal (siloxy)boryloxide complexes via proton-transfer reactions involving the BOH group. The reaction of 3 with Cp2ZrMe2 resulted in formation of Cp2Zr(Me)OB[OSi(OtBu)3]2 (4) in high yield. This rare example of a transition metal boryloxide complex crystallizes in the triclinic space group P1̄ and exhibits a crystal structure with an unprecedented number of independent molecules in its asymmetric unit (i.e., Z‘ = 18 and Z = 36). This unusual crystal structure presented an opportunity to perform statistical analyses of the metric parameters for the 18 crystallographically independent molecules. Complex 4 readily converts to Cp2Zr[OSi(OtBu)3]2 (5) upon thermolysis or upon dissolution in Et2O at room temperature.