Inorganic chemists have long believed that anything carbon can do, silicon shoul
ID: 575132 • Letter: I
Question
Inorganic chemists have long believed that anything carbon can do, silicon should be able to do. Carbenes are divalent carbon species R2C: (1), for which the simplest bonding scheme has a sp2 hybridized C, with a empty unhybridized 2p atomic orbital. Stable N-heterocyclic carbenes (2) derived from imidazole are now known, where the ability of the N atoms to -bond N N-Heterocyclic carbene NHC (1 carbene with that empty orbital leads to stabilization. This leaves the carbene C with a pair of electrons and the ability to function as a Lewis base and donate the C lone pair to Lewis acids. There are analogous Si based silylenes (3) with stabilizing large bulky silyl groups attached to the silylene Si to help stabilize it. In this example, the sily group is Si'Bus. Recently a NHC carbene fragment has been used to stabilize the silylene species and permitted the isolation of the adduct (4) 'BuzSi 0 (3) silylene Even more impressive was the fact that the silylene-NHC adduct (4) underwent a one electron oxidation to produce the corresponding NHC stabilized silylene radical cation (5). The radical cation was stable with a tetraphenylborate anion, and the crystal structure shown below was obtained, along with its epr spectrum x20 350 352 (mT) 356 Is the observed epr spectrum consistent with the structure of (5)? Explain the spectrum then, including the weak signals observed on the x20 spectrum.Explanation / Answer
In the ESR spectrum of the given radical cation, the radical couples with two 14N isotopes (which is having the highest abundance out of nitrogen isotopes, i.e. 99.6%).
The I value of 14N = 1
The EPR signal corresponding to the radical that is coupling with two 14N's split into (2nI+1) lines, where n = 2
i.e. The no. of lines = 2*2*1 + 1 = 5
This is the intense signal in the given spectrum.
The weak signals correspond to the coupling of radical with i) 13C (abundance = 1.1%; I = 1/2) and (ii) 29Si (Abundance = 4.7%; I = 1/2).