Jan Fiedler

Valence‐State Analysis through Spectroelectrochemistry in a Series of Quinonoid‐Bridged Diruthenium Complexes [(acac)2Ru(μ‐L)Ru(acac)2]n (n=+2, +1, 0, −1, −2)

The quinonoid ligand-bridged diruthenium compounds [(acac)(2)Ru(mu-L(2-))Ru(acac)(2)] (acac(-)=acetylacetonato=2,4-pentanedionato; L(2-)=2,5-dioxido-1,4-benzoquinone, 1; 3,6-dichloro-2,5-dioxido-1,4-benzoquinone, 2; 5,8-dioxido-1,4-naphthoquinone, 3; 2,3-dichloro-5,8-dioxido-1,4-naphthoquinone, 4; 1,5-dioxido-9,10-anthraquinone, 5; and 1,5-diimido-9,10-anthraquinone, 6) were prepared and characterized analytically. The crystal structure analysis of 5 in the rac configuration reveals two tris(2,4-pentanedionato)ruthenium moieties with an extended anthracenedione-derived bis(ketoenolate) pi-conjugated bridging ligand. The weakly antiferromagnetically coupled {Ru(III)(mu-L(2-))Ru(III)} configuration in 1-6 exhibits complicated overall magnetic and EPR responses. The simultaneous presence of highly redox-active quinonoid-bridging ligands and of two ruthenium centers capable of adopting the oxidation states +2, +3, and +4 creates a large variety of possible oxidation state combinations. Accordingly, the complexes 1-6 exhibit two reversible one-electron oxidation steps and at least two reversible reduction processes. Shifts to positive potentials were observed on introduction of Cl substituents (1-->2, 3-->4) or through replacement of NH by O (6-->5). The ligand-to-metal charge transfer (LMCT) absorptions in the visible region of the neutral molecules become more intense and shifted to lower energies on stepwise reduction with two electrons. On oxidation, the para-substituted systems 1-4 exhibit monocation intermediates with intervalence charge transfer (IVCT) transitions of Ru(III)Ru(IV) mixed-valent species. In contrast, the differently substituted systems 5 and 6 show no such near infrared (NIR) absorption. While the first reduction steps are thus assigned to largely ligand-centered processes, the oxidation appears to involve metal-ligand delocalized molecular orbitals with variable degrees of mixing.

Valence-State Alternatives in Diastereoisomeric Complexes [(acac)2Ru(μ-QL)Ru(acac)2]n (QL2− = 1,4-Dioxido-9,10-anthraquinone,n = +2, +1, 0, −1, −2)

The title complexes were obtained in neutral form (n = 0) as rac (1) and meso isomers (2). 2 was crystallized for X-ray diffraction and its temperature-dependent magnetism studied. It contains two antiferromagnetically coupled ruthenium(III) ions, bridged by the quinizarine dianion QL(2-) (quinizarine = 1,4-dihydroxy-9,10-anthraquinone). The potential of both the ligand (QLo --> QL4-) and the metal complex fragment combination [(acac)2RuII]2 --> ([(acac)2RuIV]2)4+ to exist in five different redox states creates a large variety of combinations, which was assessed for the electrochemically reversibly accessible 2+, 1+, 0, 1-, 2- forms using cyclic voltammetry as well as EPR and UV-vis-NIR spectroelectrochemistry. The results for the two isomers are similar: Oxidation to 1+ or 2+ causes the emergence of a near-infrared band (1390 nm), without revealing an EPR response even at 4 K. Reduction to 1- or 2- produces an EPR signal, signifying metal-centered spin but no near-infrared absorption. Tentatively, we assume metal-based oxidation of [(acac)2RuIII(mu-QL2-)RuIII(acac)2] to a mixed-valent intermediate [(acac)2RuIII(mu-QL2-)RuIV(acac)2]+ and ligand-centered reduction to a radical complex [(acac)2RuIII(mu-QL.3-)RuIII(acac)2 (-) with antiferromagnetic three-spin interaction.

The semiquinone-ruthenium combination as a remarkably invariant feature in the redox and substitution series [Ru(Q)(n)(acac)(3-n)](m), n = 1-3; m = (-2), -1, 0, +1, (+2); Q = 4,6-Di-tert-butyl-N-phenyl-o-iminobenzoquinone.

Three new compounds, [Ru(Q(*-))(acac)(2)] = 1, [Ru(Q(*-))(2)(acac)] = 2, and [Ru(Q(*-))(3)] = 3, were obtained and characterized as Ru(III) complexes with 4,6-di-tert-butyl-N-phenyl-o-iminobenzosemiquinone (Q(*-)) ligands. All three systems show multiple electron transfer behavior, which was analyzed using electron paramagnetic resonance (EPR) and UV-vis-near-infrared (NIR) spectroelectrochemistry. (1)H NMR spectroscopy and a crystal structure analysis suggest antiferromagnetically spin-spin coupled Ru(III) and Q(*-) in 1, similar to that in the related compound 4 with unsubstituted o-iminobenzosemiquinone. However, in contrast to 4(n) (Remenyi, C.; Kaupp, M. J. Am. Chem. Soc. 2005, 127, 11399), the system 1(m) exhibits unambiguously metal-centered electron transfer, producing ions [Ru(IV)(Q(*-))(acac)(2)](+) = 1(+) and [Ru(II)(Q(*-))(acac)(2)](-) = 1(-), both with EPR-evidenced ligand-based spin, as also supported by DFT calculations. Compared with the related redox system [Ru(Q)(bpy)(2)](k) (5(k)) (k = 0-3), the spectroelectrochemical similarity suggests corresponding electronic structures except for the 1(+)/5(3+) pair (Ru(IV)(Q(*-))(acac)(2)](+) (1(+)) versus [Ru(III)(Q(0))(bpy)(2)](3+) (5(3+))). Compound 2, a three-spin system [Ru(III)(Q(*-))(2)(acac)] obtained in the all-cis configuration, possesses a complicated magnetic behavior including strong intramolecular antiferromagnetic coupling (J(Ru-Q), on the order of -10(3) cm(-1) and J(Q-Q), -10(2) cm(-1)) and weak intermolecular antiferromagnetic and ferromagnetic interactions. Strong intramolecular coupling leads to one unpaired electron at low temperatures, as also supported by the radical-type EPR signal of the solid and of solutions, which diminishes at higher temperatures. The up-down-up spin arrangement for the ground state of {(Q(*-))-Ru(III)-(Q(*-))} (S = 1/2) is confirmed by DFT calculations for 2. Oxidation to 2(+) leaves the UV-vis-NIR spectrum almost unchanged, whereas reduction to 2(-) and 2(2-) produces low-energy absorptions. The ligand-centered spin for 2(2-) = [Ru(II)(Q(*-))(Q(2-))(acac)](2-) suggests the [Ru(II)(Q(*-))(2)(acac)](-) formulation for 2(-). Compound 3, obtained as a structurally characterized mer isomer, has a predominantly ligand-centered highest occupied molecular orbital (HOMO), as evident from the EPR signal of the intermediate 3(+) and as supported by DFT calculations. In contrast, electron addition proceeds to yield a metal/ligand mixed spin intermediate 3(-) according to EPR, in agreement with ca. 25% calculated metal character of the lowest unoccupied molecular orbital (LUMO). The near-infrared absorption of 3 at 1280 nm corresponds to the HOMO-LUMO transition (ligand-to-metal/ligand-to-ligand charge transfer). Oxidation to 3(+) produces a weak broad band at about 2500 nm, while the reduction to 3(-) gives rise to an intense absorption feature at 816 nm. The valence state alternatives are being discussed for all spectroelectrochemically accessible species, and the individual results are compared across this unique substitution and redox series involving a highly noninnocent ligand/metal combination. All established oxidation state formulations involve the iminosemiquinone-ruthenium entity, illustrating the remarkable stability of that arrangement, which corroborates the use of this combination in water oxidation catalysis.

Structures and spectroelectrochemistry (UV–vis, IR, EPR) of complexes [(OC)3ClRe]n(abpy), n=1, 2; abpy=2,2′-azobispyridine

Abstract The complexes (OC)3ClRe(abpy) (1) and [(OC)3ClRe]2(abpy) (2) with abpy=2,2′-azobispyridine were structurally characterized and studied by spectroelectrochemistry in the UV–vis and in the IR carbonyl stretching region. Compound 1 exhibits a s-cis/E/s-trans configurated abpy ligand with metal coordination to one pyridyl and one azo function and one non-bonding ReN interaction at 3.293 A to the second pyridyl nitrogen atom. The dinuclear complex 2 with two azoimine-coordinated metal centers is distinguished by the trans arrangement of the chloride ligands with respect to the Re2(μ-abpy) plane, the ReRe distance is 5.033(7) A. The azo bond lengths at 1.272(9) (1) and 1.304(10) A (2) indicate substantial π back donation from the rhenium(I) centers into the π*(abpy) orbital. Spectroelectrochemistry reveals the successive occupation of the π*(abpy) orbital by electrons; metal-to-ligand charge transfer features are shifted to higher energies and carbonyl stretching bands to lower wavenumbers. The EPR spectra of both radical anion intermediates 1 − and 2 − are dominated by very similar 185,187Re hyperfine splitting, ruling out a mixed-valent formulation. The second electron uptake is followed by the rapid loss of one chloride, leading to an unsymmetrical species in the dinuclear system.

A Five-Center Redox System: Molecular Coupling of Two Noninnocent Imino-o-benzoquinonato-Ruthenium Functions through a π Acceptor Bridge

Combining the concepts of noninnocent behavior of metal/ligand entities and the coupling of redox-active moieties via an electronically mediating bridge led to the synthesis and the structural, electrochemical, and spectroscopic characterization of [Cl(Q)Ru(mu-tppz)Ru(Q)Cl](n) where Q(o) is 4,6-di-tert-butyl-N-phenyl-o-iminobenzoquinone and tppz(o) is 2,3,5,6-tetrakis(2-pyridyl)pyrazine, the available oxidation states being Ru(II,III,IV), Q(o,*-,2-), and tppz(o,*-,2-). One-electron transfer steps between the n = (2-) and (4+) states were studied by cyclic voltammetry and by EPR, UV-vis-NIR spectroelectrochemistry for the structurally characterized anti isomer of [Cl(Q)Ru(mu-tppz)Ru(Q)Cl](PF(6))(2), 2(PF(6))(2), the only configuration obtained. The combined investigations reveal that 2(2+) is best described as [Cl(Q(*-))Ru(III)(mu-tppz(o))Ru(III)(Q(*-))Cl](2+) with antiferromagnetic coupling between the ruthenium(III) and the iminosemiquinone components at each end. A metal-based spin as evident from large g factor anisotropy (EPR) and an intense intervalence absorption band at 1850 nm in the near-infrared (NIR) suggest that oxidation occurs at both iminosemiquinones to yield two Ru(II,III)-bonded quinones, implying redox-induced electron transfer. Reduction takes place stepwise at the metal centers yielding iminosemiquinone complexes of Ru(III,II) as evident from radical complex EPR spectra with small (99,101)Ru hyperfine contributions. After complete metal reduction to ruthenium(II) the bridging ligand tppz is being reduced stepwise as apparent from typical NIR absorption bands around 1000 nm and from small g anisotropy of the monoanion [Cl(Q(*-))Ru(II)(mu-tppz(*-))Ru(II)(Q(*-))Cl](-). A structure-based DFT calculation confirms the Ru-Cl character of the HOMO and the iminoquinone-dominated LUMO and illustrates the orbital interaction pattern of the five electron transfer active components in this new system.

Cationic Heteroleptic Cyclometalated Iridium Complexes with 1‐Pyridylimidazo[1,5‐α]pyridine Ligands: Exploitation of an Efficient Intersystem Crossing

Luminescent ligands in Ir(III) cyclometalated complexes. The photophysical and photochemical properties of Ir-cyclometalated complexes containing luminescent ligands are evaluated (see figure). Significant admixture between Ir and ligand orbitals induces an efficient intersystem crossing. Photochemical reactions performed in the presence of oxygen lead to new Ir-cyclometalated complexes containing N(amido) groups directly bound to Ir.A series of phosphorescent cyclometalated heteroleptic iridi um(III) phenylpyridinato (ppy) complexes containing luminescent 1-pyridylimidazo[1,5-alpha]pyridine (pip) ligands, namely [Ir(ppy)(2)(pip)](+), have been synthesised, characterised and their electrochemical, photophysical and electronic properties studied. Seven X-ray structures have been resolved. Excitation of [Ir(ppy)(2)(pip)](+) in acetonitrile at room temperature results in a dual luminescence, strongly quenched by O(2). Four complexes show, in absence of O(2), a high-energy emission (assigned to a (3)MLLCT transition) with two maxima in the blue region of the visible spectra, and a second structured emission (assigned largely to a (3)LC pi-pi* transition) centred around lambda=555 nm. Lifetimes of high-energy emissions are between 0.6 and 1.3 mus. Time-dependent density functional calculations combined with the conductor-like polarisable continuum model method, with acetonitrile as solvent, have been used to calculate a series of ground and excited states of the derivatives under investigation, and the transitions compared with the experimental UV/Vis absorption spectra. A quick and efficient photochemical reaction has been observed for these iridium derivatives that leads to the formation of a new class of cyclometalated iridium complexes containing a stable deprotonated amide unusually coordinated to the metal through a nitrogen bond. The synthesis of a (15)N enriched selected ligand has been performed to investigate, by means of NMR, the particular facile route to these new set of derivatives. The electrochemical behaviour of all complexes is also reported.

Straightforward Synthesis of Substituted p‐Quinones: Isolation of a Key Intermediate and Use as a Bridging Ligand in a Diruthenium Complex

Quinones are naturally occurring redox active molecules that function in vital electron-transport processes, often in conjugation with a transition-metal center. p-Quinones, such as vitamin K derivatives, ubiquinones or plastoquinones, play important roles in photosynthesis, respiration, and information-transfer processes. Substituted p-quinones have been extensively used as ligands in coordination chemistry in recent years. The remarkable properties that such ligands impart to their metal complexes make such compounds useful in a variety of fields, such as homogenous catalysis, supramolecular chemistry, coordination polymers, and as bridging ligands in combination with redox active metal centers such as ruthenium. The last field has gained tremendous attention in recent years because of ambiguities arising in oxidation-state formulations also with “organometallic-type” non-innocent ligands. In addition, the creation of quinone-based molecular magnets is a lively field of research that has produced many interesting results. 2,5-Diamino-1,4-benzoquinone 1 and its substituted derivatives have been known for decades. The synthesis of 1 has previously been reported and such syntheses are rarely straightforward one-pot reactions. Some years back Braunstein et al. reported a straightforward and “green” synthesis of a new class of molecules that occurs through transamination. Such molecules, which are isomers of 1 and its derivatives, are best described as zwitterions, 6. Inspired by this process we looked for an elegant synthesis for the parent compound 1 and for possible intermediates formed during the transamination process. Herein we report a simple one-pot synthesis of 1 and its mono(2 and 3) and dialkyl (4 and 5) derivatives (Scheme 1). To elucidate

Electron transfer and chloride ligand dissociation in complexes [(C5Me5)ClM(bpy)]+/[(C5Me5)M(bpy)]n (M=Co, Rh, Ir;n = 2+, +, 0, −): A combined electrochemical and spectroscopic investigation

Abstract In contrast to the rapid and chemically reversible two-electron ECE′ reductive elimination reaction[(C 5 Me 5 )ClM(bpy)] + + 2e − → (C 5 Me 5 )M(bpy)+Cl − ,M=Rh or Ir, the analogous cobalt system exhibits two separate one-electron steps (EC + E′ process) with a persistent, EPR-spectroscopically characterized cobalt(II) intermediate [(C 5 Me 5 )Co(bpy)] + . Within the series of coordinatively unsaturated homologous species (C 5 Me 5 )M(bpy), the cobalt derivative exhibits the smallest and the iridium homologue the largest metal(I)-to-bpy electron transfer in the ground state, as evident from electrochemical potentials and long-wavelength absorption data. A comparison within that homologous series indicates why the rhodium system, with its intermediate position, is most suitable for hydride transfer catalysis.

An Odd‐Electron Complex [Ruk(NOm)(Qn)(terpy)]2+ with Two Prototypical Non‐Innocent Ligands

Six combinations of oxidation states are conceivable for the paramagnetic title complex. Single-crystal X-ray diffraction, spectroscopic analysis (IR, EPR at conventional and high frequency), and DFT calculations establish that it is the iminosemiquinone radical structure that is formed: [Ru(k)(NO(m))(Q(n))(terpy)](2+) (k = 2+, m = 1+, n = 1-).

Singlet diradical complexes of ruthenium and osmium: geometrical and electronic structures and their unexpected changes on oxidation.

Reaction of HL, HLa (2-[(2-N-phenylamino)phenylazo]pyridine), HLb (2-[{2-N-(4-methylphenyl)amino}phenylazo]pyridine), or HLc (2-[{2-N-(4-chlorophenyl)amino}phenylazo]pyridine), with KRuO4 or OsO4 and PPh3 under exhaustive deoxygenation (PPh3 --> OPPh3) yields diamagnetic compounds ML2. Crystal structure determination for M(La)2 indicates the radical dianion state, L2(.-), for the ligands as evident from the typical N-N bond length of about 1.33 A for a one-electron reduced azo function. The resulting spin-coupled complexes, MIV(L(2.-))2, can be oxidized in two reversible one-electron steps, as probed by cyclic voltammetry and UV-vis-NIR spectroelectrochemistry. The paramagnetic intermediates, [M(La)2]+, are distinguished by intense NIR absorption, largely metal-centered spin as revealed by EPR, and, in the case of [Os(La)2]I3, by crystallographically determined shortening of the NN bond to about 1.30 A, corresponding to a coordinated unreduced azo function. Thus, oxidation of the complex MIV(L(2.-))2 involves partial reduction of the metal in [MIII(L-)2]+ because intramolecular double electron transfer is offsetting the external charge removal. Density-functional theory calculations were employed to confirm the structural features and to support the spectroscopic assignments.