Photoelectron spectra of dithiaspirocyclobutane molecules. Electronic spectra of their ruthenium complexes and evidence from CNDO/2 calculations for a hyperconjugating electron-transfer mechanism

Abstract

Intervalence electron transfer was found to occur with surprising efficiency through the ruthenium dinuclear complexes based on the dithiaspiro bridging ligands 2,6-dithiaspiro(3,3)heptane (I), 2.8-dithiaspiro(3.1.3.1)decane (II), and 2,10-dithiaspiro(3.1.1.3.1.1)tridecane (III). It was found from CNDO/1 calculations that the highest occupied bonding molecular orbital of each molecule was ideally set up with p/sub z/ orbitals on the S and C atoms for ''sideways ..pi.. overlap'', thus providing a hyperconjugating electron-transfer pathway between the ruthenium atoms bonded to the terminal sulfurs.The CNDO/2 calculations predict this hyperconjugating orbital to be the HOMO whereas photoelectron spectroscopy indicates that the HOMO is an almost degenerate set of orbitals largely comprised of a nonbonding pair in the p/sub x/ orbital of each sulfur atom. These nonbonding p/sub x/ orbitals are presumably used to form coordinate covalent bonds to the ruthenium atoms in the corresponding dinuclear complexes. The electronic spectra show ligand-to-metal charge-transfer bands in the visible region for either mononuclear or dinuclear ruthenium(III) complexes of the dithiaspiro ligands. The values of the highest energy visible transitions do not vary as the PES ionization energies of the appropriate orbital as ring number increases, indicating that the spiro ligands having an odd number of rings interact in a different mannermore » with the metal atom than do those ligands having an even number of rings.« less