Chalcogen diimides

The accessibility of the +4 and +6 oxidation states for sulfur and, to a lesser extent, selenium gives rise to both acyclic and cyclic molecules that have no parallels in N-O chemistry. Thus there is an extensive chemistry of chalcogen diimides RN=E=NR (E = S, Se, Te) (Section 10.4). In the case of Te these unsaturated molecules form dimeric structures reflecting the increasing reluctance for the heavier chalcogens to form multiple bonds to nitrogen. The acyclic molecule N=Sp3,... 

Three different conformations are possible for monomeric chalcogen diimides (Eig. 10.1). Variable-temperature NMR spectra indicate that the cis,trans isomer of S(NR)2 is most stable in solution for small organic groups (R = Me, Bu). With bulkier organic substituents, small amounts of the trans,trans isomer exist in equilibrium with the cis,cis isomer. " The cis,cis isomer is observed in solutions of certain sulfur diimides with... 

The geometries and relative energies of the different conformations of model chalcogen diimides E(NR)2 (E = S, Se R = H, Me, Bu and SiMe3) have been investigated by using ab initio and DET molecular orbital methods.The cis,trans conformation is predicted to be most stable with the exception of the parent molecules E(NH)2 and the unsymmetrical systems RNSNH, for which the cis,cis conformation is slightly more stable than the cis, trans isomer. 

Scheme 8 Coordination modes for monomeric chalcogen diimides...

Dianions of the type [E(NlBu)3]2 (E=S,161 Se,162 Te163) are made by the nucleophilic addition of LiNIl Bu to the appropriate chalcogen diimide followed by deprotonation with a second equivalent of LiNI I Bu. 

Figure 25 The calculated energy change in the [2 + 2] cyclodimerization of chalcogen diimides E(NMe)2 (E = S, Se, Te). The calculations have been performed at LDA as well as at MP2/ECP, CCSD/ECP, and CCSD(T)/ECP levels of theory ( 2005 from Ref. 165 Reproduced by permission of Taylor Francis, Inc. http //www.taylorandfrancis.com)...

Physical Properties. - Naphtho[2,3-c][1,2,5]thiadiazol-4,9-dione and its selenium analogue have been studied by X-ray crystallography. Both show significant ir interaction of the chalcogen diimide with the it system of the naphthoquinone. ... 

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