Comparison of Redox Activity between 2‑Aminothioether and 2‑Aminothiophenol: Redox-Induced Dimerization of 2‑Aminothioether via C–C Coupling

Three
chemical reactions of two 2-aminothioethers and 2-aminothiophenol
with CpRuIICl­(PPh3)2 (Cp– = cyclopentadienyl anion), under identical reaction conditions,
are reported. While 2-(methylthio)­aniline, H2L1 and an analogous substrate, 2-(phenylthio)­aniline yielded dicationic
dinuclear complexes [(PPh3)­CpRuII(L3/L4)­RuIICp­(PPh3)]­Cl2 (where
L3 = (4E)-4-(4-imino-3-(methylthio)­cyclohexa-2,5-dienylidene)-2-(methylthio)­cyclohexa-2,5-dienimine
([1a]­Cl2) and L4 = (4E)-4-(4-imino-3-(phenylthio)­cyclohexa-2,5-dienylidene)-2-(phenylthio)­cyclohexa-2,5-dienimine
([1b]­Cl2)), the reaction with 2-aminothiophenol
(H2L2) produced a mononuclear complex [(PPh3)­CpRuII(L2)]Cl (where L2 =
6-iminocyclohexa-2,4-dienethione) ([2]­Cl). All these
complexes are obtained in high yields (65%–75%). Formations
of the products from the above reactions involve a similar level of
oxidation of the respective substrate, although their courses are
completely different. A comparison between the above two chemical
transformations are scrutinized thoroughly. Characterizations of these
complexes were made using a host of physical methods: X-ray crystallography,
nuclear magnetic resonance (NMR), cyclic voltammetry, ultraviolet–visible
(UV-vis), electron paramagnetic resonance (EPR) spectroscopy, and
density functional theory (DFT). The complexes [1a]­Cl2 and [1b]­Cl2 showed intense metal-to-ligand
charge transfer transition in the long wavelength region of the spectrum,
at 860 and 895 nm, respectively, and displayed two reversible electron
transfer (ET) processes at [1a]2+: −0.28
and −0.52 V; [1b]2+: −0.13 and
−0.47 V, along with an irreversible ET process at 0.76 and
0.54 V, respectively. The ET processes at negative potentials are
due to successive reductions of the bridging ligand, which are characterized
by EPR and UV-vis spectroscopy. The one-electron reduced compound,
[1a]+, showed a intraligand charge transfer
transition (ILCT) at 1530 nm. The complex [2]+ showed a reversible ET process at −0.36 V and two irreversible
ET processes at −1.04 and 1.18 V, respectively. DFT calculations
were used to support the spectral and redox properties of the complexes
and also to throw light on the difference of redox behavior between
thioether and thiophenol substrates.