Silicon chelation in aqueous and nonaqueous media: the furanoidic diol approach

Anhydroerythritol (AnEryt) shares some of its ligand properties with furanosides and furanoses. Its bonding to silicon centers of coordination number four, five, and six was studied by X-ray and NMR methods, and compared to silicon bonding of related compounds. Diphenyl(cycloalkylenedioxy)silanes show various degrees of oligomerization depending on the diol component involved. For example, Ph(2)Si(cis-ChxdH(-2)) (1) and Ph(2)Si[(R,R)-trans-ChxdH(-2))] (2; Chxd = cyclohexanediol) are dimeric, Ph(2)Si(AnErytH(-2)) (3) is monomeric, and Ph(2)Si(L-AnThreH(-2)) (4; AnThre = anhydrothreitol) is trimeric both in the solid state and in solution. Ph(2)Si(cis-CptdH(-2)) (5) (Cptd = cyclopentanediol) is monomeric in solution but dimerizes on crystallization. Si(AnErytH(-2))(2) (6) and Si(cis-CptdH(-2))(2) (7) are monomeric spiro compounds in solution but are pentacoordinate dimers in the crystalline state. Pentacoordinate silicate ions are found in A[Si(OH)(AnErytH(-2))(2)] (A = Na, 8 a; Rb, 8 b; Cs, 8 c). Related compounds are formed by substitution of the hydroxo by a phenyl ligand. K[SiPh(AnErytH(-2))(2)]1/2 MeOH (9) is a prototypical example as it shows the two most significant isomers in one crystal structure: the syn/anti and the anti/anti form (syn and anti define the oxolane ring orientation close to, or apart from, the monodentate ligand, respectively). syn/anti Isomerism generally rules the appearance of the NMR spectra of pentacoordinate silicates of furanos(id)e ligands. NMR spectroscopic data are presented for various pentacoordinate bis(diolato)silicates of adenosine, cytidine, methyl-beta-D-ribofuranoside, and ribose. In even more basic solutions, hexacoordinate silicates are enriched. Preliminary X-ray analyses are presented for Cs(2)[Si(CydH(-2))(3)] 21.5 H(2)O (10) and Cs(2)[Si(cis-InsH(-3))] cis-Ins8 H(2)O (11) (Cyd = cytidine, Ins = inositol).