A Zwitterionic Spirocyclic Pentacoordinate Silicon Compound Synthesized in Water by SiO and SiC Bond Cleavage

Abstract

The chemistry of higher-coordinate silicon compounds is currently one of the main areas of research in silicon chemistry. [1] We have contributed to this field by systematic studies on zwitterionic l 5 Si-silicates that contain a pentacoordinate (formally negatively charged) silicon atom and a tetracoordinate (formally positively charged) nitrogen atom.[1d] The zwitterions 1, [2] 2, [3] 3, [4] 4, [5] 5, [6] 6, [7] 7, [8] 8, [9] and 9 [10] are examples of this type of compound. Generally, these zwitterionic l 5 Si-silicates are quite sensitive to water and easily undergo hydrolytic SiÀOC, SiÀSC, and SiÀNC bond cleavage. We have now succeeded in synthesizing a zwitterionic l 5 Si-silicate, namely, bis[meso-oxolane-3,4-diolato(2À)][(morpholinio)methyl]silicate (10), that shows remarkable stability in water.[11] Surprisingly, this pentacoordinate silicon compound could even be prepared in aqueous solution by SiÀC bond cleavage. We report herein on the synthesis of 10 and its hydrate 10·H 2 O and their structural characterization in the solid state and in solution.Compound 10 was synthesized according to Scheme 1, either through cleavage of three SiÀO bonds (method a) or through cleavage of two SiÀO bonds and one SiÀC bond (method b), starting with trimethoxy[(morpholino)methyl]-silane (11) [12] (yield 87 %) and dimethoxy[(morpholino)-methyl]phenylsilane (12) [12] (yield 53 %), respectively. As the synthesis according to method b involves a relatively slow SiÀC cleavage reaction as the rate-determining step, [13] product formation was significantly slower than with method a (see Experimental Section). The hydrate 10·H 2 O was synthesized analogously to 10 (Scheme 1, method a) by using water instead of acetonitrile as the solvent and was isolated, after crystallization from water, in 93 % yield as a colorless crystalline solid.The chiral l 5 Si-silicates crystallize in the space groups P2 1

Not Unhydrolyzed at pH 8

“…Starting from the well‐known area of pentacoordinate oxolanediolatosilicate anions under non‐hydrolyzing conditions,3 the experiments eventually approach the conditions reported in Ref. 1. The NMR spectra of the anionic morpholinomethyl homologue of a bis(oxolanediolato)phenylsilicate or ‐hydroxosilicate in methanolic solution are shown in Figure 2 a.…”

“…The search for silicon complexes that exhibit hydrolytic stability in neutral aqueous solution is a major challenge in contemporary silicon chemistry and has both technical and biochemical relevance. The recently published claim by one of us1 that a zwitterionic bis(diolato)silicate, derived from anhydroerythritol ( meso ‐oxolane‐3,4‐diol; AnEryt) is hydrolytically stable at pH 7–8, thus appeared to be a milestone. The compound in question is bis[ meso ‐oxolane‐3,4‐diolato(2−)](morpholiniomethyl)silicate ( 1 (originally labeled 10 in Ref.…”

“…The compound in question is bis[ meso ‐oxolane‐3,4‐diolato(2−)](morpholiniomethyl)silicate ( 1 (originally labeled 10 in Ref. 1); see Figure 1). It was reported that no tetracoordinate silicon species could be detected upon dissolution of 1 in water; that is, no hydrolytic decomposition occurred.…”

“…The assignment of signals, the scale, and compounds labels 10 a – c , according to Ref. 1 are shown in blue (n.a.=not assigned in Ref. 1).…”

“…1 are shown in blue (n.a.=not assigned in Ref. 1). e) Aqueous solution of 1 (0.1 mol kg −1 ) at pH 7.8.…”