The compound [Ni(QM)2], QM = 4,6-di-tert-butyl-N-(2-methylthiomethylphenyl)-o-iminobenzoquinone, is a singlet diradical species with approximately planar configuration at the tetracoordinate metal atom and without any Ni-S bonding interaction. One-electron oxidation results in additional twofold Ni-S coordination (dNi-S ≈2.38 Å) to produce a complex cation of [Ni(QM)2](PF6) with hexacoordinate Ni(II) and two distinctly different mer-configurated tridentate ligands. The O,O'-trans arrangement in the neutral precursor is changed to an O,O'-cis configuration in the cation. The EPR signal of [Ni(QM)2](PF6) has a very large g anisotropy and the magnetic measurements indicate an S = 3/2 state. The dication was structurally characterized as [Ni(QM)2](ClO4)2 to exhibit a similar NiN2O2S2 framework as the monocation. However, the two tridentate (O,N,S) ligands are now equivalent according to the formulation [Ni(II)(QM(0))2](2+). Cyclic voltammetry reflects the qualitative structure change on the first, but not on the second oxidation of [Ni(QM)2], and spectroelectrochemistry reveals a pronounced dependence of the 800-900 nm absorption on the solvent and counterion. Reduction of the neutral form occurs in an electrochemically reversible step to yield an anion with an intense near-infrared absorption at 1345 nm (ε = 10,400 M(-1)  cm(-1)) and a conventional g factor splitting for a largely metal-based spin (S = 1/2), suggesting a [(QM(·-))Ni(II)(QM(2-))](-) configuration with a tetracoordinate metal atom with antiferromagnetic Ni(II)-(QM(·-)) interactions and symmetry-allowed ligand-to-ligand intervalence charge transfer (LLIVCT). Calculations are used to understand the Ni-S binding activity as induced by remote electron transfer at the iminobenzoquinone redox system.

Azocarboxamide (azcH) has been combined for the first time with [Ru-Cym] to generate metal complexes with N,N- and N,O-coordination mode, [(Cym)Ru(azc)Cl] and [(Cym)Ru(azcH)Cl](+) [PF6 ](-). Geometric and electronic structures of the complexes are reported along with their in vitro activities against different tumour cell lines and preliminary results on solution chemistry. Compound [(Cym)Ru(azc)Cl] exhibited remarkable cytotoxic properties. It was cell-type specific and had comparable IC50 values towards both cancer cells and their drug-resistant subline. A tenfold increase in the sensitivity towards [(Cym)Ru(azc)Cl] was noted for the tumour cells with depleted intracellular glutathione (GSH) level, suggesting the essential role of GSH in cell response to this compound.

Azocarboxamides were used as chelating ligands in ruthenium half-sandwich complexes. The synthesis and characterization of two new complexes with an unprecedented coordination motif are presented together with an in-depth investigation of two recently published complexes. Three different coordination modes of the ligands were realized, as evident by NMR spectroscopy and single-crystal X-ray diffraction. The use of base during the synthesis leads to a coordination of a deprotonated ligand, while the introduction of additional donor atoms results in a noncoordinated amide group. The first systematic experimental (cyclic voltammetry and UV–vis–NIR and EPR spectroelectrochemistry) and theoretical (DFT) investigation of the electronic structure of metal complexes bearing this redox-active ligand class is presented, revealing redox processes with ligand contribution. The absorption spectra and electrochemistry are mainly determined by the protonation state of the ligand. While complexes 2­[PF 6 ], 3­[PF 6 ], and 4­[PF 6 ] with neutral azocarboxamides show similar electronic spectra and cyclovoltammograms, the incorporation of a deprotonated monoanionic ligand in complex 1 leads to significant changes of these properties. In contrast, the catalytic activity in the base-free transfer hydrogenation reaction is mainly dependent on the coordination of the amide group, with only minor effects of the protonation state. While complexes 3­[PF 6 ] and 4­[PF 6 ], with an uncoordinated amide group, are inactive without the addition of base, complexes 1 and 2­[PF 6 ], with a metal-bound amide group, show activity under base-free conditions. The impact of the position of the amide group together with the detection of metal hydride species in 1H NMR spectroscopy suggests the operation of metal–ligand bifunctional catalysis to take place when no base is added.

The new compounds [Ru(R-DAB)(acac) 2 ] (R-DAB = 1,4-diorganyl-1,4-diazabuta-1,3-diene; R = tert-butyl, 4-methoxyphenyl, 2,6-dimethylphenyl; acac -= 2,4-pentanedionate) exhibit intrachelate ring bond lengths 1.297 Ͻ d(CN) Ͻ 1.344 Å and 1.382 Ͻ d(CC) Ͻ 1.425 Å, which suggest a Ru III (R-DAB ·-) oxidation state formulation. This notion is confirmed by the negligible solvatochromism of the intense (ε ≈ 10 4 M -1 cm -1 ) charge-transfer absorption band in the visible region and by DFT calculations. Oxidation of the compounds occurs mainly at the R-DAB ·radical ligand to [a] 110 produce UV/Vis/NIR and electron paramagnetic resonance (EPR) spectroelectrochemically detectable Ru III species, whereas the reduction proceeds less reversibly and yields predominantly (R-DAB)-ligand-based spin for the 4-methoxyphenyl derivative, measured at low temperature. The results are discussed with respect to metal-to-ligand chargetransfer (MLCT) excited states of conventional (α-diimine)ruthenium(II) complexes and in view of other (α-diimine)metal complexes with ambiguous oxidation-state assignments.

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 = L(O), and N-thiobenzoyl-N'-phenyldiazene = L(S), (obtained in situ) form structurally characterized compounds [(acac)(2)Ru(L)], 1 with L = L(O), and 3 with L = L(S), and [(bpy)(2)Ru(L)](PF(6)), 2(PF(6)) with L = L(O), and 4(PF(6)) with L = L(S) (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 Ru(III) leads to diamagnetic ground states of the neutral complexes, whereas the cations 2(+) and 4(+) are EPR-active radical ligand complexes of Ru(II). 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)(2)Ru(III)(L(0))](+) ⇌ [(acac)(2)Ru(III)(L(•-))] ⇌ [(acac)(2)Ru(III)(L(2-))](-)/[(acac)(2)Ru(II)(L(•-))](-), and [(bpy)(2)Ru(III)(L(•-))](2+)/[(bpy)(2)Ru(II)(L(0))](2+) ⇌ [(bpy)(2)Ru(II)(L(•-))](+) ⇌ [(bpy)(2)Ru(II)(L(2-))](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.