Pentacoordinate silicon complexes with dynamic motion resembling a pendulum on the SN2 reaction pathway

Abstract

A series of glutarimide derivatives which has two carbonyl coordination sites for intramolecular pentacoordination at silicon with a X(1+n)SiC(3-n)O moiety have been synthesised and characterized. The substituent (leaving group) effects on the Si-O bond exchange between the two coordination sites (resembling a pendulum) have been studied by comparison of the differently substituted (X = F, Cl, OTf, Br and I) structures. The activation parameters for the Si-O bond exchange process were measured by NMR and separately computed and are consistent with the strength of Si-O bond coordination and the nature of the leaving group, X. The temperature-dependent (29)Si NMR spectroscopy is supported by X-ray crystallography and shows that the tetrahedral reactant is converted into pentacoordinate intermediates by intramolecular O-Si association followed by reversion to a tetrahedral geometry by Si-X dissociation. The two association/dissociation patterns offer a model for nucleophilic substitution at a silicon atom. A continuum of structures on the S(N)2 reaction profile from the glutarimide derivatives correlates reasonably well with the structural data obtained from derivatives of lactams, diketopiperazines and quinolones.

Not Only Hydrogen Bonds: Other Noncovalent Interactions

“…The initial investigations were basically theoretical; [104][105][106][107][108][109][110] however, experimental research on TrB has rapidly grown in the last decade. Actually, there are plenty of examples in the literature reporting experimental [111,112] investigations on tetrel bonding, which was named as such in 2013 [113][114][115][116]. A differential feature of tetrel bonding compared to halogen, chalcogen and pnictogen bonding interactions is that the charge density distribution on the tetrel atom is not anisotropic (absence of lone pairs).…”

Tetrel bonds, penta‐ and hexa‐coordinated tin and lead centres

“…For example, the pentavalent carbon atom and the hypervalency of other elements were discussed by Martin, and numerous moieties possessing such properties were classified as ‘frozen’ transition states . Penta‐coordinate silicon complexes were analysed and the corresponding S N 2 reaction pathways were discussed; the hypervalency of silicon and carbon was discussed in terms of the proposed ball‐in‐a‐box model and it was explained that the silicon centres could be characterized by pentavalency while the carbon ones could not . Penta‐coordinate carbon compounds were analysed experimentally and theoretically by Akiba and co‐workers, and also discussed was whether hexavalent carbon can exist .…”

Complementary bonding analysis of the N–Si interaction in pentacoordinated silicon compounds using quantum crystallography

“…In turn, the Si -F bond in compound 2 is slightly elongated. With respect to the simulation of S N 2 reactions according to references [13][14][15][16][17][18][19] , the shorter N -Si distance and elongated A natural bond orbital (NBO) analysis can provide both a qualitative and quantitative description of the bonding situation based on a localized orbital point of view. The N -Si interaction can be linked to the interaction of the nitrogen lone pair NBO (donor orbital) and the Si -F localised antibonding orbital as acceptor orbital, that is, negative hyperconjugation.…”

“…[4][5][6][7][8][9][10][11] Pentacoordinated silicon compounds in the solid state have, e.g., been used as model systems for the experimental simulation of S N 2 reactions at silicon atoms according to the structure correlation 12 philosophy. [13][14][15][16][17][18][19] Moreover, N-Si interactions in high-coordinated silicon compounds were subject of experimental charge-density analyses. [20][21][22] In the present paper, we analyze the bonding situation in two silicon compounds (1 and 2), which are depicted in Figure 1, by quantum crystallography.…”