Filling Gaps in the Series of Noninnocent Hetero-1,3-diene Chelate Ligands: Ruthenium Complexes of Redox-Active α-Azocarbonyl and α-Azothiocarbonyl Ligands RNNC(R′)E, E = O or S

The series of 4-center unsaturated chelate ligands AB–CD
with redox activity to yield –A–BC–D– in two steps has been complemented by two new combinations
RNNC­(R′)­E, E = O or S, R = R′ = Ph. The ligands N-benzoyl-N′-phenyldiazene = LO, and N-thiobenzoyl-N′-phenyldiazene
= LS, (obtained in situ) form structurally characterized
compounds [(acac)2Ru­(L)], 1 with L = LO, and 3 with L = LS, and [(bpy)2Ru­(L)]­(PF6), 2(PF6) with
L = LO, and 4(PF6) with L = LS (acac– = 2,4-pentanedionato; bpy = 2,2′-bipyridine).
According to spectroscopy and the N–N distances around 1.35
Å and N–C bond lengths of about 1.33 Å, all complexes
involve the monoanionic (radical) ligand form. For 1 and 3, the antiferromagnetic spin–spin coupling with electron
transfer-generated RuIII leads to diamagnetic ground states
of the neutral complexes, whereas the cations 2+ and 4+ are EPR-active
radical ligand complexes of RuII. The complexes are reduced
and oxidized in reversible one-electron steps. Electron paramagnetic
resonance (EPR) and UV–vis–NIR spectroelectrochemistry
in conjunction with time-dependent density functional theory (TD-DFT)
calculations allowed us to assign the electronic transitions in the
redox series, revealing mostly ligand-centered electron transfer:
[(acac)2RuIII(L0)]+ ⇌
[(acac)2RuIII(L•–)]
⇌ [(acac)2RuIII(L2–)]−/[(acac)2RuII(L•–)]−, and [(bpy)2RuIII(L•–)]2+/[(bpy)2RuII(L0)]2+ ⇌ [(bpy)2RuII(L•–)]+ ⇌ [(bpy)2RuII(L2–)]0. The differences
between the O and S containing compounds are rather small in comparison
to the effects of the ancillary ligands, acac– versus
bpy.