α-Amino Isobutyric Acid-Derived Silacyclopentane Complexes with Penta- and Hexacoordinate Si Atoms

Pyrrolidinyl-substituted silacyclopentane (CH2)4Si(Pyr)2 and α-amino isobutyric acid (H2Aib) react with the release of one equivalent pyrrolidine (HPyr) and the formation of the pentacoordinate silicon bis-chelate (Aib)Si(CH2)4(HPyr), which features the di-anion of the amino acid as an (O,N)-chelator and one equivalent of pyrrolidine as an additional lone-pair donor. Crystallographic analyses of the chloroform solvate (Aib)Si(CH2)4(HPyr)·(CHCl3), which undergoes a phase transition at 200 K, and a solvent-free modification (Aib)Si(CH2)4(HPyr), which features two crystallographically independent molecules of the complex, revealed that the N atom of the HPyr ligand, as well as the carboxylate of Aib, occupy the axial positions in the trigonal bipyramidal Si coordination sphere; the Si–C bonds of the silacyclopentane rest on equatorial sites. For the isolated molecule in a solvent environment, computational analyses revealed that the energy difference between this configuration and the related isomer with an equatorial HPyr and equatorial–axial positioning of the silacyclopentane motif is marginal. In DMSO solution, the adduct (Aib)Si(CH2)4(HPyr) decomposed, forming the hexacoordinate Si complex (HAib)2Si(CH2)4 as one of the decomposition products. In a deliberate manner, this compound was accessible from the diethylamino-substituted silacyclopentane (CH2)4Si(NEt2)2 and H2Aib with the liberation of diethylamine. (HAib)2Si(CH2)4 features two mono-anions of the α-amino acid as (O,N)-chelators, their carboxylate O atoms are trans-disposed to silacyclopentane, and their NH2 groups are mutually trans.