Sulfur-Sulfur Lone Pair and Sulfur-Naphthalene Interactions in Naphtho[1, 8-b,c]-1,5-dithiocin

Naphtho[1,8-b,c]-1,5-dithiocin (4), which has a unique geometry so constrained that the sulfur atoms are held close to one another and oriented such that their p orbitals are almost colinear and orthogonal to the naphthalene π-system, has been synthesized. Its crystal and molecular structure was determined by single-crystal X-ray analysis. It crystallizes in the orthorhombic space group Pbca with a = 8.140 (2) Å, b = 9.866 (1) Å, c = 28.302 (3) Å, and Z = 8. The structure was solved by direct methods. Full-matrix least-squares refinement led to a conventional R factor of 0.046 after several cycles of anisotropic refinement. For comparison purposes the crystal and molecular structure of the previously reported 1,8-bis-(methylthio)naphthalene (5) was also determined by X-ray techniques. Semiempirical molecular orbital methods (MNDO and AM1) were used to analyze the five highest occupied molecular orbitals in l,8-bis(methylthio)naphthalene as a function of the C(1)-S and C(8)-S torsion angles and to analyze the molecular orbitals of compound 4. Of particular interest is the result that the energy of the highest occupied molecular orbital in 1,8-bis(methylthio)naphthalene is nearly independent of the C-S torsion angle and that the lowest ionization potential for 4 is predicted to be 7.75 eV and its lone pair-lone pair splitting due to transannular S-S interaction is 1.6-2.0 eV. The computations were correlated with the experimentally measured He I and He II photoelectron spectra of 4 and the AM1 method provided reasonable agreement with the experimental data. The electrochemical oxidation of 4 and 5 in acetonitrile was studied by cyclic voltammetry. They undergo irreversible oxidation with peak potentials of 0.70 and 0.47 V, respectively, versus a Ag/0.1 M AgNO₃ in acetonitrile reference electrode. Controlled-potential electrolysis of 4 gives the corresponding sulfoxide (12), which is consistent with removal of an electron from the highest occupied molecular orbital which is sulfur lone pair in character.