Francisco A. Urbanos

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.

Reductive Approach to Mixed Valency (n = 1−) in the Pyrazine Ligand-Bridged [(acac)2Ru(μ-L2–)Ru(acac)2]n (L2– = 2,5-Pyrazine-dicarboxylate) through Experiment and Theory

The diruthenium(III) complex [(acac)(2)Ru(μ-L(2-))Ru(acac)(2)] (1) with acac(-) = acetylacetonato = 2,4-pentanedionato and a 2,5-pyrazine-dicarboxylato bridge, L(2-), has been obtained and structurally characterized as the rac (ΔΔ,ΛΛ) diastereomer. The Ru(III)Ru(III) configuration in 1 (d(Ru-Ru) = 6.799 Å) results in a triplet ground state (μ = 2.82 μ(B) at 300 K) with a density functional theory (DFT) calculated triplet-singlet gap of 10840 cm(-1) and the metal ions as the primary spin-bearing centers (Mulliken spin densities: Ru, 1.711; L, 0.105; acac, 0.184). The paramagnetic 1 exhibits broad, upfield shifted (1)H NMR signals with δ values ranging from -10 to -65 ppm and an anisotropic electron paramagnetic resonance (EPR) spectrum (g = 2.133, g(1) - g(3) = Δg = 0.512), accompanied by a weak half-field signal at g = 4.420 in glassy frozen acetonitrile at 4 K. Compound 1 displays two closely spaced oxidation steps to yield labile cations. In contrast, two well separated reversible reduction steps of 1 signify appreciable electrochemical metal-metal interaction in the Ru(II)Ru(III) mixed-valent state 1(-) (K(c) ≈ 10(7)). The intermediate 1(-) shows a weak, broad Ru(II)→Ru(III) intervalence charge transfer (IVCT) band at about 1040 nm (ε = 380 M(-1) cm(-1)); the DFT approach for 1(-) yielded Mulliken spin densities of 0.460 and 0.685 for the two metal centers. The monitoring of the ν(C═O) frequencies of the uncoordinated C═O groups of L(2-) in 1(n) by IR spectroelectrochemistry suggests valence averaging (Ru(2.5)Ru(2.5)) in 1(-) on the vibrational time scale. The mixed-valent 1(-) displays a rhombic EPR signal (g = 2.239 and Δg = 0.32) which reveals non-negligible contributions from the bridging ligand, reflecting a partial hole-transfer mechanism and being confirmed by the DFT-calculated spin distribution (Mulliken spin density of -0.241 for L in 1(-)). The major low energy electronic transitions in 1(n) (n = 0,-,2-) have been assigned as charge transfer processes with the support of TD-DFT analysis.

Bis(acetylacetonato)ruthenium Complexes of Noninnocent 1,2‐Dioxolene Ligands: Qualitatively Different Bonding in Relation to Monoimino and Diimino Analogues

Coordination compounds [Ru(acac)(2)(Q)] (acac=acetylacetonate; Q=o-benzoquinone) were prepared as complexes 1 (Q=o-benzoquinone), 2 (Q=3-methoxy-o-benzoquinone), 3 (Q=4-methyl-o-benzoquinone), and 4 (Q=3,5-di-tert-butyl-o-benzoquinone). The structures of 1 and 2 were determined to reveal a Ru(III)/o-benzosemiquinone formulation, supported by analysis of experimental data (spectroscopy, magnetism of 1) and by DFT calculations. The S=1 ground state calculated for 1 stands in contrast to the spin-paired analogues with arylimino-o-benzosemiquinonato and diimino-o-benzoquinone ligands. The close contacts of about 5.3 Å possible between semiquinone O atoms of different molecules in the crystal allow for intermolecular spin-spin interactions and an overall complex magnetic behavior. One quasireversible oxidation and two reversible one-electron reductions yielded the corresponding molecular ions, which were characterized by UV-visible-NIR and EPR spectroelectrochemistry in terms of [Ru(III)(acac)(2)(Q(0))](+) , [Ru(III)(acac)(2)(Q(2-))](-), and [Ru(II)(acac)(2)(Q(2-))](2-) descriptions in agreement with DFT results. The use of acceptor-substituted 1,2-dioxolenes resulted in the isolation of ionic species Na[Ru(acac)(2)(Q)] (Na(5); Q=4-chloro-o-benzoquinone) and Na(6) (Q=4-nitro-o-benzoquinone), which were similarly investigated as compounds 1-4. Magnetic susceptibility and EPR results confirm an S=1/2 ground state based on ruthenium(III). The combined studies reveal a remarkable substituent sensitivity, and in comparison to recently analyzed Ru(acac)(2) complexes with o-benzoquinone monoimine and diimine ligands, the all-O-donor-containing new systems are distinguished by a qualitatively different metal-ligand interaction based on closer intermolecular radical-radical contacts and on weaker intramolecular dπ-π* interactions.

Reaction of diruthenium(II,III) acetate with triphenylphosphine

Abstract The reaction of [Ru2(μ-O2CCH3)4(thf)2]BF4 with triphenylphosphine in thf or toluene is described. In this reaction, a yellow compound of formula Ru(O2CCH3)2 (PPh3)·CH3C6H5 (1), and a violet, diamagnetic oxo-bridged compound, formulated as Ru2(μ-O)(μ-O2CCH3)2(O2CCH3)2(PPh3)2 (2) are obtained. The compounds have been characterized by elemental analysis, magnetic susceptibility measurements and spectroscopic techniques. The structure of 2 has been determined by X-ray crystallography. Two pseudo-octahedral ruthenium(III) atoms are bridged by an oxo and two acetate ligands. The hexacoordination of each ruthenium atom is completed with a bidentate acetate and a triphenylphosphine ligand. Some spectroscopic evidence for the formation of a tetranuclear intermediate compound is also presented.

Synthesis and characterization of new carboxylate dimers of ruthenium

A series of Ru2Cl(μ-O2CR)4 [R = C2H5 (I), cyclo-C6H11 (II), C6H5 (III), o-ClC6H4 (IV), o-HOC6H4 (V)] complexes has been synthesized by reaction of Ru2Cl(μ-O2CCH3)4 with the appropriate carboxylic acid. The reaction of these insoluble compounds with AgBF4 in thf gives [Ru2(μ-O2CR)4(thf)2]BF4 (VI)-(X) which are soluble in polar organic solvents. The substitution in these complexes of the thf axial ligands in toluene by OPPh3 or pyridine leads to new adducts of the type [Ru2(μ-O2CR)4L2]BF4 [L = OPPh3 (XI)-(XV), L = py (XVI)-(XIX)] which are not easily accessible by other methods. The compounds have been characterized by elemental analysis, electrical conductance, magnetic susceptibility measurements and spectral data.

Synthesis and properties of some diruthenium acetate compounds

Abstract The compound [Ru 2 (μ-O 2 CCH 3 ) 4 (THF) 2 ]BF 4 ( I ) containing the Ru 2 5+ unit was prepared by reaction of Ru 2 Cl(μ-O 2 CCH 3 ) 4 with AgBF 4 in THF. This compound, in contrast with Ru 2 Cl(μ-O 2 CCH 3 ) 4 , is soluble in several polar organic solvents and reacts in THF with OPPh 3 and PPh 3 giving [Ru 2 (μ-O 2 CCH 3 ) 4 (OPPh 3 ) 2 ]BF 4 ·CH 2 Cl 2 ( II ) and [Ru(μ-O 2 CCH 3 )(O 2 CCH 3 )(PPh 3 )] n ( III ), respectively. The complex II has been also obtained as hexafluorophosphate [Ru 2 (μ-O 2 CCH 3 ) 4 (OPPh 3 2 ]PF 6 ·CH 2 Cl 2 ( IV ) by treatment of Ru 2 Cl(μ-O 2 CCH 3 ) 4 with an excess of NOPF 6 and PPh 3 in methanol. In this reaction the triphenylphosphine oxide is generated by oxidation of the triphenylphosphine.

Redox-Rich Spin-Spin-Coupled Semiquinoneruthenium Dimers with Intense Near-IR Absorption

Using the [RuCl(μ-tppz)ClRu](2+) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine] platform for bridging two o-quinone/catecholate two-step redox systems (unsubstituted, Q(n), or 3,5- di-tert-butyl-substituted, DTBQ(n)), we have obtained the stable complexes [(Q(•-))Ru(II)Cl(μ-tppz)ClRu(II)(Q(•-))] (1) and the structurally characterized [(DTBQ(•-))Ru(II)Cl(μ-tppz)ClRu(II)(DTBQ(•-))] (2). The compounds exhibit mostly quinone-ligand-based redox activity within a narrow potential range, high-intensity near-IR absorptions (λ(max) ≈ 920 nm; ε > 50,000 M(-1) cm(-1)), and variable intra- and intermolecular spin-spin interactions. Density functional theory calculations, electron paramagnetic resonance (EPR), and spectroelectrochemical results (UV-vis-near-IR region) for three one-electron-reduction and two one-electron-oxidation processes were used to probe the electronic structures of the systems in the various accessible valence states. EPR spectroscopy of the singly charged doublet species showed semiquinone-type response for 1(+), 2(+), and 2(-), while 1 exhibits more metal based spin, a consequence of the easier reduction of Q as compared to DTBQ. Comparison with the analogous redox series involving a more basic N-phenyliminoquinone ligand reveals significant differences related to the shifted redox potentials, different space requirements, and different interactions between the metals and the quinone-type ligands. As a result, the tppz bridge is reduced here only after full reduction of the terminal quinone ligands to their catecholate states.

Correspondence of RuIIIRuII and RuIVRuIII Mixed Valent States in a Small Dinuclear Complex

The diruthenium(III) compound [(μ-oxa){Ru(acac)(2)}(2)] [1, oxa(2-) =oxamidato(2-), acac(-) =2,4-pentanedionato] exhibits an S=1 ground state with antiferromagnetic spin-spin coupling (J=-40 cm(-1)). The molecular structure in the crystal of 1⋅2 C(7)H(8) revealed an intramolecular metal-metal distance of 5.433 Å and a notable asymmetry within the bridging ligand. Cyclic voltammetry and spectroelectrochemistry (EPR, UV/Vis/NIR) of the two-step reduction and of the two-step oxidation (irreversible second step) produced monocation and monoanion intermediates (K(c) =10(5.9)) with broad NIR absorption bands (ε ca. 2000 M(-1)cm(-1)) and maxima at 1800 (1(-)) and 1500 nm (1(+)). TD-DFT calculations support a Ru(III)Ru(II) formulation for 1(-) with a doublet ground state. The 1(+) ion (Ru(IV)Ru(III)) was calculated with an S=3/2 ground state and the doublet state higher in energy (ΔE=694.6 cm(-1)). The Mulliken spin density calculations showed little participation of the ligand bridge in the spin accommodation for all paramagnetic species [(μ-oxa){Ru(acac)(2)}(2)](n), n=+1, 0, -1, and, accordingly, the NIR absorptions were identified as metal-to-metal (intervalence) charge transfers. Whereas only one such NIR band was observed for the Ru(III)Ru(II) (4d(5)/4d(6)) system 1(-), the Ru(IV)Ru(III) (4d(4)/4d(5)) form 1(+) exhibited extended absorbance over the UV/Vis/NIR range.