Spin Coupling through the N→Si Dative Bond in Penta- and Hexacoordinate Hydrido- and Fluorosilicon Complexes; Coupling through a Rapidly Dissociating−Recombining N→Si Bond

Abstract

Penta- and hexacoordinate silicon complexes with spin 1/2 nuclei (1H and 19F) directly attached to silicon have been prepared and used to study NMR spin−spin interactions across the N→Si dative bond. In both hydrido and fluoro complexes, two-bond coupling constants were found to drop sharply as the geminal bond angle deviated from 90°. Three-bond coupling constants showed a Karplus-type dependence upon the corresponding dihedral angles for both 1H and 19F complexes. Four-bond coupling constants 4 J(19F−Si−N−C−1H) across the dative bond were observed. The correlations can be used as tools for the interpolation of bond and dihedral angles from NMR data, in cases where crystal structures are unavailable. They also demonstrate that the N→Si dative bond in hypercoordinate silicon complexes behaves essentially in the manner expected from normal covalent bonds. Rapid dissociation and recombination of the N−Si dative bond was established in several pentacoordinate complexes; however, one-, two-, and three-bond coupling constants could nevertheless be measured across this rapidly dissociating bond.

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

“…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 ≠ …”

Dimethyl-, diphenyl- and cyclotetramethylene silylchloride derivatives of guanidine, their synthesis and structure. Formation of pentacoordinated silicon compounds and an unexpected diionic disila-diguanidinium dichloride

“…Compounds with cyclic frameworks, strained silacarbon rings, spirobisilafluorenes, silaphenanthrolines, silanaphthyls and amidinates with k 5 Si-coordination can readily be assembled [8][9][10][11][12][13][14][15][16]. Additionally structures with zwitterionic frameworks and k 5 Si atoms have been made [17,18].…”

8-oxyquinolinate and 8-oxyquinaldinate in silicon complex chemistry — a new face of old ligands

“…The equatorial situation of both diazasilolidine N atoms (Scheme 5, b) would result in such a molecule bearing even higher ring strain due to upscaled bond angle discrepancy (aimed angle N(1)-Si(1)-N(2): 120°). The preference of axial positions within a trigonal bipyramid for setting up "dative" N→Si bonds was demonstrated with various examples [19][20][21][22][23][24][25]. A major effect preventing the 8-oxyquinolinate from coordinating the Si atom in the aimed manner (Scheme 5, a) should be the potential repulsion emerging from the H atom in oposition at the phenyl group of the equatorially located diazasilolidine N atom, which points to the 8-oxyquinolinate moiety.…”