Experimental and theoretical investigations of structural trends for selenium(IV) imides and oxides: X-ray structure of Se3(NAd)2.

The thermal decomposition of Se(NAd)(2) (Ad = 1-adamantyl) in THF was monitored by (77)Se NMR and shown to give the novel cyclic selenium imide Se(3)(NAd)(2) as one of the products. An X-ray structural determination showed that Se(3)(NAd)(2) is a puckered five-membered ring with d(Se-Se) = 2.404(1) A and |d(Se-N)| = 1.873(4) A. On the basis of (77)Se NMR data, other decomposition products include the six-membered ring Se(3)(NAd)(3), and the four-membered rings AdNSe(micro-NAd)(2)SeO and OSe(micro-NAd)(2)SeO. The energies for the cyclodimerization of E(NR)(2) and RNEO (E = S, Se; R = H, Me, (t)Bu, SiMe(3)), and the cycloaddition reactions of RNSeO with E(NR)(2), RNSO(2) with Se(NR)(2), and S(NR)(2) with Se(NR)(2) have been calculated at MP2, CCSD, and CCSD(T) levels of theory using the cc-pVDZ basis sets and B3PW91/6-31G* optimized geometries. Sulfur(IV) and selenium(IV) diimide monomers are predicted to be stable, the sole exception being Se(NSiMe(3))(2) that shows a tendency toward cyclodimerization. The cyclodimerization energy for RNSeO and the cycloaddition reaction energies of RNSeO with Se(NR)(2) as well as that of RNSO(2) with Se(NR)(2) are negative, consistent with the observed formation of OSe(micro-N(t)Bu)(2)SeO, OSe(micro-N(t)Bu)(2)SeN(t)Bu, and O(2)S(micro-N(t)Bu)(2)SeN(t)Bu, respectively. Cycloaddition is unlikely when one of the reactants is a sulfur(IV) diimide.