Gas-Phase Bond Strength and Atomic Connectivity Studies of the Unsymmetrical Two-Center Three-Electron Ion, [Et2S∴SMe2]+

1,2-dithiin annelated with bicyclo[2.2.2]octene frameworks. One-electron and two-electron oxidations and formation of a novel 2,3,5,6-tetrathiabicyclo[2.2.2]oct-7-ene radical cation with remarkable stability owing to a strong transannular interaction

A stable derivative of 1,2-dithiin annelated with bicyclo[2.2.2]octene frameworks 4 was synthesized as red crystals by the reaction of a dilithiated dimer of bicyclo[2.2.2]octene with elemental sulfur in 59% yield. The cyclic voltammetry of 4 in CH(2)Cl(2) at -78 degrees C showed two reversible oxidation waves at E(1/2) +0.18 V and +0.72 V versus Fc/Fc(+), indicating that the radical cation and dication of 4 are stable under these conditions. Upon chemical one-electron oxidation of 4 in a rather low concentration (4.0 x 10(-4) M) with a 1.5 equiv of SbCl(5) in CH(2)Cl(2), a radical cation 4.+ was formed, whose spin distribution was determined by ESR spectroscopy and by the results of theoretical calculations (UB3LYP/6-31G). The electronic absorption spectrum of 4.+ in CH(2)Cl(2) exhibited a maximum absorption at 428 nm (epsilon = 2.3 x 10(3)), which was hypsochromically shifted from that of neutral 4 (469 nm). When the radical cation 4.+ was produced in higher concentration (0.06 M) in CH(2)Cl(2), a disproportionation was found to take place to give a SbCl(6)(-) salt of remarkably stable radical cation 5.+ having a novel 2,3,5,6-tetrathiabicyclo[2.2.2]oct-7-ene structure. In the X-ray structure of 5.+SbCl(6)(-), the transannular distance (2.794(3) A) between the sulfur atoms was found to be less than the sum of the van der Waals radii of a sulfur atom (3.70 A), suggesting the existence of a bonding interaction between the two disulfide linkages. The theoretical calculations (UB3LYP/6-31G) suggested that this transannular interaction could be described as the resonance between the limiting structures, each of them having a two-center three-electron bond between two sulfur atoms belonging to two different disulfide linkages: thus, both the spin and positive charge are equally delocalized to the four sulfur atoms, causing a great stabilization of 5.+. On the other hand, the 1,2-dithiin radical cation 4.+ was found to readily react with triplet oxygen with subsequent rearrangement to give the 1,2-dithiolium derivative 6+ having a carboxyl group. Finally, the reaction of 4 with an excess amount of SbF(5) gave the corresponding dication 4(2+), which was found to be a 6pi aromatic system on the basis of the results of NMR measurement and theoretical calculations.

Structure, bonding, and spectra of cyclic dithia radical cations: a theoretical study

Ab initio molecular orbital and hybrid density functional theory methods are employed to characterize the structure, bonding and properties of several cyclic dithia radical cation systems, particularly in the context of intra molecular two-center three-electron (2c-3e) bonding between two sulfur atoms. The calculated results are able to interpret the time-resolved transient optical spectra obtained from pulse radiolysis technique for these positively charged dithia systems in aqueous solution. Visualization of the appropriate molecular orbital (MO) in the systems is able to depict the presence of a 2c-3e bond between two sulfur atoms and its sigma character. Geometry optimizations of these doublet systems are carried out at restricted open shell Becke's half-and-half (BHH) nonlocal exchange and Lee-Yang-Parr (LYP) nonlocal correlation functionals (BHHLYP) with 6-311+G(d,p) basis set including solvent effects adopting Onsager's reaction field model. Hessian calculations are done at the same level to check the nature of the equilibrium geometry. Energy data are further improved by performing MP2/6-311+G(d,p) calculations on these radical cation systems. Excited-state calculations are done following configuration interaction with single-electron excitation (CIS) method and the optical transition wavelength from the highest doubly occupied molecular orbital (HDOMO) to the lowest singly occupied molecular orbital (LSOMO) is seen to correspond and match to the position of the absorption maxima (lambda(max)) obtained from the experimental spectra for all these radical cation systems in aqueous solution. These calculations are able to resolve a long-standing ambiguity in the assignment of intra molecular 2c-3e bonding in the case of the 3-methyl-2,4-dithiapentane radical cation system and to provide new insights into bonding features of this odd electron system as well as of other cyclic dithia systems studied.