Structural and Oxidation State Alternatives in Platinum and Palladium Complexes of a Redox-Active Amidinato Ligand.

Reaction of [Pt(DMSO)2 Cl2 ] or [Pd(MeCN)2 Cl2 ] with the electron-rich LH=N,N'-bis(4-dimethylaminophenyl)ethanimidamide yielded mononuclear [PtL2 ] (1) but dinuclear [Pd2 L4 ] (2), a paddle-wheel complex. The neutral compounds were characterized through experiments (crystal structures, electrochemistry, UV-vis-NIR spectroscopy, magnetic resonance) and TD-DFT calculations as metal(II) species with noninnocent ligands L- . The reversibly accessible cations [PtL2 ]+ and [Pd2 L4 ]+ were also studied, the latter as [Pd2 L4 ][B{3,5-(CF3 )2 C6 H3 }4 ] single crystals. Experimental and computational investigations were directed at the elucidation of the electronic structures, establishing the correct oxidation states within the alternatives [PtII (L- )2 ] or [Pt. (L )2 ], [PtII (L0.5- )2 ]+ or [PtIII (L- )2 ]+ , [(PdII )2 (µ-L- )4 ] or [(Pd1.5 )2 (µ-L0.75- )4 ], and [(Pd2.5 )2 (µ-L- )4 ]+ or [(PdII )2 (µ-L0.75- )4 ]+ . In each case, the first alternative was shown to be most appropriate. Remarkable results include the preference of platinum for mononuclear planar [PtL2 ] with an N-Pt-N bite angle of 62.8(2)° in contrast to [Pd2 L4 ], and the dimetal (Pd2 4+ →Pd2 5+ ) instead of ligand (L- →L ) oxidation of the dinuclear palladium compound.

Near-Infrared-Absorbing Organometallic Diruthenium Complex Intermediates: Evidence for Bridging Anthrasemiquinone Formation and against Mixed Valency.

The new redox-active complexes [RuH(CO)(EPh3 )2 (µ-Q2- )RuH(CO)(EPh3 )2 ], E=P (1) and E=As (2) with the bis-chelate bridging ligand Q2- =1,4-dioxido-9,10-anthraquinone were prepared and characterised. The related compound [RuCl(CO)(PPh3 )2 (µ-Qx2- )RuCl(CO)(PPh3 )2 ] (4) with E=P and Qx2- =5,8-dioxido-1,4-naphthoquinone 4 revealed trans-positioned PPh3 groups. The electrogenerated one-electron oxidised states 1+ and 2+ were examined using spectroelectrochemical techniques (EPR, IR and UV/Vis/NIR). In situ EPR studies gave spectra with 31 P or 75 As hyperfine splitting of about 16 Gauss, small 99, 101 Ru coupling and small g-anisotropy in the frozen solution state. The 31 P and 75 As hyperfine values reflect axial positioning of the four Ru-E bonds relative to the plane of an anthrasemiquinone bridge. Single CO stretching bands around 1910 cm-1 of the precursors 1 and 2 shift by about 25 cm-1 to higher energies on oxidation. The direction, uniformity and the extent of the shifts confirm ligand bridge-based oxidation. Absorbance by the cations in the near infrared region is thus assigned to intra-ligand transitions of ruthenium(II)-bonded anthrasemiquinones and not to intervalence charge transfer of mixed-valent species. Ruthenium(II) stabilisation by CO and EPh3 is made responsible for the anthrasemiquinone formation instead of metal-centered oxidation.

Metal-Induced Thiophene Ring Opening and C-C Bond Formation To Produce Unique Hexa-1,3,5-trienediyl-Coupled Non-Innocent Ligand Chelates.

Ring opening of thiophenes containing an azo function in 2-position and subsequent dimerization through C-C coupling were observed on reaction with [Ru(acac)2 (CH3 CN)2 ] (acac=acetylacetonate) to produce two 1,3,5-hexatriene-linked redox-active azothiocarbonyl chelate systems. Interaction of the non-innocent chelate ligands and of the metals at a nanoscale distance of 1.45 nm via the conjugated hexatriene bridge was studied by magnetic and electron spectroscopic measurements in conjunction with DFT calculations, revealing four-center magnetic interactions of this unique setting and weak intervalence coupling after reduction.

Metal-Chelating N,N'-Bis(4-dimethylaminophenyl)acetamidinyl Radical: A New Chromophore for the Near-Infrared Region.

A new non-innocent ligand redox system, N,N'-bis(4-dimethylaminophenyl) substituted acetamidinato/acetamidinyl, has been designed and described by example of structurally and spectroscopically characterized ruthenium complexes. The hitherto unreported ligand is responsible for rather intense and narrow absorptions in the near-infrared region of the one- and two-electron oxidized forms. The spectroscopic, computational, and first structural characterization of an amidinyl radical complex adds to the list of established N-based radical ligands.

Combining [arene-Ru] with azocarboxamide to generate a complex with cytotoxic properties.

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.

Tetra-µ3-iodido-tetra-kis-[(tri-n-butyl-phosphane-κP)copper(I)].

The title complex, [Cu4I4(C12H27P)4], crystallizes with six mol-ecules in the unit cell and with three independent one-third mol-ecule fragments, completed by application of the relevant symmetry operators, in the asymmetric unit. The tetranuclear copper core shows a tetrahedral geometry (site symmetry 3..). The I atoms also form a tetra-hedron, with I⋯I distances of 4.471 (1) Å. Both tetra-hedra show an orientation similar to that of a pair of self-dual platonic bodies. The edges of the I-tetra-hedral structure are capped to the face centers of the Cu-tetra-hedron and vice versa. The Cuface⋯I distances are 2.18 Š(averaged) and the Iface⋯Cu distances are 0.78 Š(averaged). As a geometric consequence of these properties there are eight distorted trigonal-bipyramidal polyhedra evident, wherein each trigonal face builds up the equatorial site and the opposite Cu⋯I positions form the axial site. As expected, the n-butyl moieties are highly flexible, resulting in large elongations of their anisotropic displacement parameters. Some C atoms of the n-butyl groups were needed to fix alternative discrete disordered positions.

Correlated coordination and redox activity of a hemilabile noninnocent ligand in nickel complexes.

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.

Varying electronic structural forms of ruthenium complexes of non-innocent 9,10-phenanthrenequinonoid ligands.

Bis(acetylacetonato)ruthenium complexes [Ru(acac)2(Q1-3)], 1-3, incorporating redox non-innocent 9,10-phenanthrenequinonoid ligands (Q1 = 9,10-phenanthrenequinone, 1; Q2 = 9,10-phenanthrenequinonediimine, 2; Q3 = 9,10-phenanthrenequinonemonoimine, 3) have been characterised electrochemically, spectroscopically and structurally. The four independent molecules in the unit cell of 2 are involved in intermolecular hydrogen bonding and π-π interactions, leading to a 2D network. The oxidation state-sensitive bond distances of the coordinated ligands Q(n) at 1.296(5)/1.289(5) Å (C-O), 1.315(3)/1.322(4) Å (C-N), and 1.285(3)/1.328(3) Å (C-O/C-N) in 1, 2 and 3, respectively, and the well resolved (1)H NMR resonances within the standard chemical shift range suggest DFT supported variable contributions from valence formulations [Ru(III)(acac)2(Q˙(-))] (spin-coupled) and [Ru(II)(acac)2(Q(0))], respectively. Complexes 1-3 exhibit one oxidation and two reduction steps with comproportionation constants Kc∼ 10(7)-10(22) for the intermediates. The electrochemically generated persistent redox states 1(n) (n = 0, 1-, 2-) and 2(n)/3(n) (n = 1+, 0, 1-, 2-) have been analysed by UV-vis-NIR spectroelectrochemistry and by EPR for the paramagnetic intermediates in combination with DFT and TD-DFT calculations, revealing significant differences in the oxidation state distribution at the {Ru-Q} interface for 1(n)-3(n). In particular, the diminished propensity of the NH-containing systems for reduction results in the preference for Ru(II)(Q(0)) relative to Ru(III)(Q˙(-)) (neutral compounds) and for Ru(II)(Q˙(-)) over the Ru(III)(Q(2-)) alternative in the case of the monoanionic complexes.

A diruthenium complex of a "Nindigo" ligand.

The compound {(µ-Nindigo)[Ru(acac)2]2} = 1, H2(Nindigo) = indigo-N,N'-diphenylimine and acac(-) = 2,4-pentanedionate, has been structurally characterized in the rac form, which exhibits two edge-sharing six-membered chelate rings involving ruthenium, and the former ß-diketiminato functions with a twist angle of 33.9° around the central C-C bond. The metric parameters suggest a neutral π acceptor bridge containing coupled s-trans configurated α-diimines, which are coordinated by two ruthenium(II) centers. DFT calculations confirm the experimental structure and oxidation state assignment of the rac form; both diastereoisomers are present in solution according to (1)H NMR spectroscopy. A very intense long-wavelength MLCT absorption at 630 nm (ε = 66 800 M(-1) cm(-1)) and a weaker near-IR band at 1120 nm (ε = 3000 M(-1) cm(-1)) are observed for the CH3CN solution. Reversible one-electron reduction and oxidation steps were studied by cyclic voltammetry, differential pulse voltammetry, EPR, and UV-vis-NIR spectroelectrochemistry to exhibit metal-centered oxidation and mixed metal/ligand-centered reduction. These results are supported by TD-DFT calculations of the species rac- or meso-1(n), n = 3+, 2+, +, 0, -, 2-.

Discovering more non-innocence: triazenido versus triazenyl radical ligand function, and a comment on [NO2]n as a "suspect" ligand.

The unusual suspects: Depending on co-ligands L(n) and the effects of substituents (R), the well-known triazenides [N(NR)2](-) may act as EPR detectable coordinated triazenyl ligands, [N(NR)2](·). They are thus new non-innocent ligands and are related to the hitherto unused non-innocent nitrogen dioxide ligand, [NO2](·).

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 = 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.

(α-Diimine)tricarbonylhalorhenium complexes: the oxidation side.

The electrochemical behaviour of the complexes [Re(CO)(3)X(α-diimine)], X = Cl or Br, α-diimine = 1,4-di-tert-butyl-1,4-diaza-1,3-diene, was reinvestigated using cyclic voltammetry accompanied by IR and UV-vis spectroelectrochemistry. While the reduction results in the loss of halide, as necessary for the electrocatalytic activity of related diimine compounds, a reversible oxidation could be observed for the chloro complex 1 (X = Cl). The conversion of 1 to 1(+) in CH(2)Cl(2) or CH(3)CN is accompanied by high-frequency shifts (Δν = 73-114 cm(-1)) of the three carbonyl stretching bands, by a considerable change in carbonyl stretching modes, and by changed absorption in the visible region. DFT calculations support the spectroelectrochemical results and suggest an unusually large g anisotropy (g(1) 1.38, g(2) 2.06, g(3) 3.20) which explains the absence of detectable EPR signals for 1(+) under normal X band conditions. Frontier orbitals calculated by DFT for 1 reveal two close lying occupied orbitals (HOMO, HOMO-1) with Re-Cl character and a diimine based LUMO.