Crystal Structures and Stereodynamics of Neutral Hexacoordinate Silicon Chelates:  Use of an Optically Active Ligand for Assignment of an Intramolecular Ligand Exchange Process

Abstract

Crystal structures determined for several neutral hexacoordinate bis(N→Si) chelates revealed in all cases near octahedral geometries with the nitrogen ligands in relative trans and the monodentate ligands in cis positions. To assign the two consecutive intramolecular ligand-site exchange processes (reported earlier), a bis-chelate was prepared containing a chiral carbon center in each of the chelate rings (14). By means of a phase sensitive NOESY NMR spectrum it was possible to conclude that the lower-barrier process involved exchange of diastereotopic groups only within each diastereomer, and not between them, resulting in the assignment of this process to the direct nondissociative interchange of the monodentate ligands (X, Y). The Si−Cl bond lengths were found to inversely correlate with the lower of the activation barriers. Dissociation−recombination of the N→Si dative bond was also observed by the high-temperature NMR spectra. Lack of correlation between Si−N distances in the crystals and activation barriers led to the conclusion that Si−N dissociation was not involved in the measured rate processes, but followed, at slightly higher temperature, the epimerization at the silicon center by exchange of the two oxygen sites.

Ligand‐site exchange in intramolecular complexes of silicon: substituent effects

“…Various mechanisms have been advanced to explain the ligand‐site exchange in the hypervalent compounds . A unified mechanism seems to be lacking.…”

“…For our purposes an important point is that the experiments have provided a considerable body of data on permutational isomerization . The ligand‐site exchange is a stereodynamic process; its rate ranges from 10 −1 to 10 −5 s .…”

“…Therefore, these processes are conveniently studied by using the dynamic nuclear magnetic resonance (NMR) spectroscopy (an examination of coalescence phenomena and that of the temperature dependence of the line shape). This method is widely applied to determine the free energies of activation for ligand exchange Δ G ≠ …”

Die E.‐coli‐Siderophore Enterobactin und Salmochelin bilden sechsfach koordinierte Siliciumkomplexe bei physiologischen pH‐Werten

“…Hypervalente Siliciumkomplexe und insbesondere sechsfach koordinierte Siliciumkomplexe wurden in der Vergangenheit durch Anorganiker intensiv erforscht. [6,7] In der Literatur wurde über die Synthese kationischer, [8] anionischer [7] und neutraler [9,10] sechsfach koordinierter Siliciumkomplexe berichtet. Diese Verbindungen wurden in einem biologischen Kontext bisher nicht gründlich erforscht.…”

The E. coli Siderophores Enterobactin and Salmochelin Form Six‐Coordinate Silicon Complexes at Physiological pH

“…[2,5] Structures of synthetic model siderophores 5 and 6 a-c. Hypervalent silicon complexes, especially the hexacoordinate complexes, have been widely studied by inorganic chemists. [6,7] The syntheses of cationic, [8] anionic, [7] and also neutral [9,10] hexacoordinated silicon complexes have been reported. However, these compounds have not been thoroughly studied in a biological context.…”