Controlling metal-ligand-metal oxidation state combinations by ancillary ligand (L) variation in the redox systems [L2Ru(mu-boptz)RuL2](n), boptz=3,6-bis(2-oxidophenyl)-1,2,4,5-tetrazine, and L = acetylacetonate, 2,2 '-bipyridine, or 2-phenylazopyridine

The new compounds [(acac)(2)Ru(mu-boptz) Ru(acac)(2)] (1) [(bpy)(2)Ru(mu-boptz)Ru(bpy)(2)] (ClO4)(2) (2-(ClO4)(2)), and [(pap)(2)Ru(mu-boptz)Ru(pap)(2)](ClO4)(2) (3-(ClO4)(2)) were obtained from 3,6-bis(2-hydroxyphenyl)1,2,4,5-tetrazine (H(2)boptz), the crystal structure analysis of which is reported. Compound I contains two antiferromagnetically coupled (J=-36.7cm(-1)) Ru-III centers. We have investigated the role of both the donor and acceptor functions containing the boptz(2-) bridging ligand in combination with the electronically different ancillary ligands (donating acac(-), moderately it-accepting bpy, and strongly pi-accepting pap; acac=acetylacetonate, bpy-2,2'-bipyridine pap =2-phenylazopyridine) by using cyclic voltammetry, spectroelectrochemistry and electron paramagnetic resonance (EPR) spectroscopy for several in situ accessible redox states. We found that metal-ligand-metal oxidation state combinations remain invariant to ancillary ligand change in some instances; however, three isoelectronic paramagnetic cores Ru( mu-boptz)Ru showed remarkable differences. The excellent tolerance of the bpy co-ligand for both Ru-III and Ru-II is demonstrated by the adoption of the mixed-valent form in [L2Ru( mu-boptz)RuL2](3+), L=bpy, whereas the corresponding system with pap stabilizes the Ru-II states to yield a phenoxyl radical ligand and the compound with L= acac(-) contains two Ru-III centers connected by a tetrazine radical-anion bridge

Valence-state alternatives in diastereoisomeric complexes [(acac)(2)ru(mu-ql)ru(acac)(2)](n) (ql(2-)=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 (QL(0) -> OL4-) and the metal complex fragment combination [(acac)(2)RUII](2) -> {[(acac)(2)Ru-IV](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)(2)Ru-II(mu-QL(2-))Ru-III(acac)(2)] to a mixed-valent intermediate [(acac)(2)Ru-III(mu- QL(2-))Ru-IV(acac)(2)](+) and ligand-centered reduction to a radical complex [(acac)(2)Ru-III(mu-QL(3-))Ru-III(acac)(2)](-) with antiferromagnetic three-spin interaction

Diruthenium complexes [{(acac)(2)Ru-III}(2)(mu-OC2H5)(2)], [{(acac)(2)Ru-III}(2)(mu-L)](ClO4)(2), and [{(bpy)(2)Ru-II}(2)(mu-L)](ClO4)(4) [L=(NC5H4)(2)-N-C6H4-N-(NC5H4)(2), acac = acetylacetonate, and bpy=2,2 '-bipyridine]. Synthesis, structure, magnetic, spectral, and photophysical aspects

Paramagnetic diruthenium(III) complexes (acac)(2)Ru-III(mu-OC2H5)(2)Ru-III(acac)(2) (6) and [(acac)(2)Ru-III(mu-L)Ru-III(acac)(2)]-(ClO4)(2), [7](ClO4)(2), were obtained via the reaction of binucleating bridging ligand, N,N,N',N'-tetra(2-pyridyl)-1,4-phenylenediamine [(NC5H4)(2)-N-C6H4-N-(NC5H4)(2), L] with the monomeric metal precursor unit (acac)(2)Ru-II(CH3CN)(2) in ethanol under aerobic conditions. However, the reaction of L with the metal fragment Ru-II(bpy)(2)(EtOH)(2)(2+) resulted in the corresponding [(bpy)(2)Ru-II (mu-L) Ru-II(bpy)(2)](ClO4)(4), [8](ClO4)(4), Crystal structures of L and 6 show that, in each case, the asymmetric unit consists of two independent half-molecules. The Ru-Ru distances in the two crystallographically independent molecules (F and G) of 6 are found to be 2.6448(8) and 2.6515(8) Angstrom, respectively. Variable-temperature magnetic studies suggest that the ruthenium(III) centers in 6 and [7](ClO4)(2) are very weakly antiferromagnetically coupled, having J = -0.45 and -0.63 cm(-1), respectively. The g value calculated for 6 by using the van Vleck equation turned out to be only 1.11, whereas for [7](ClO4)(2), the g value is 2.4, as expected for paramagnetic Ru(III) complexes. The paramagnetic complexes 6 and [7](2+) exhibit rhombic EPR spectra at 77 K in CHCl3 (g(1) = 2.420, g(2) = 2.192, g(3) = 1.710 for 6 and g(1) = 2.385, g(2) = 2.177, g(3) = 1.753 for [7](2+)). This indicates that 6 must have an intermolecular magnetic interaction, in fact, an antiferromagnetic interaction, along at least one of the crystal axes. This conclusion was supported by ZINDO/1-level calculations. The complexes 6, [7](2+), and [8](4+) display closely spaced Ru(III)/Ru(II) couples with 70, 110, and 80 mV separations in potentials between the successive couples, respectively, implying weak intermetallic electrochemical coupling in their mixed-valent states. The electrochemical stability of the Ru(II) state follows the order: [7](2+) < 6 < [8](4+). The bipyridine derivative [8](4+) exhibits a strong luminescence [quantum yield (phi) = 0.18] at 600 nm in EtOH/MeOH (4:1) glass (at 77 K), with an estimated excited-state lifetime of approximately 10 mus

Intramolecular Valence and Spin Interaction in meso and rac Diastereomers of a p-Quinonoid-Bridged Diruthenium Complex

The complexes meso- and rac-[(acac)(2)Ru(mu-L)Ru(acac)(2)](n), 1 and 2, where L(2-) = 1,4-dioxido-2,3-bis(3,5-dimethylpyrazol-1'-yl)benzene and acac = 2,4-pentanedionato. were characterized structurally, magnetically, electrochemically, and spectroscopically as well as spectroelectrochemically (UV-vis-NIR, EPR) in the accessible redox states (n = 0, +, -, 2-). Due to steric interference, the neutral compounds contain a severely twisted L(2) bridging ligand with 43-48 degrees dihedral angles between the planes of the hydroquinone dianion and those of the ortho positioned pyrazolyl substituents. The difference between meso and rac isomers is rather pronounced in terms of the redox potentials (easier oxidation and reduction of the rac form 2) and with respect to the absorption spectra of the oxidized states. Susceptibility and EPR measurements confirm the {Ru(III)(mu-L(2))Ru(III)} configuration of the neutral species, showing J values of -37 and 21 cm (1) for the spin-spin interaction between the ca. 7.75 angstrom separated metal centers in 1 and 2, respectively. Two-step reduction involves the metals and produces Ru(III)Ru(II) mixed-valent monoanions with comproportionation constants of ca. 10(4), with Ru(III)-type EPR signals, and with broad intervalence charge transfer bands at about 1200-1500 nm absorption maximum, suggesting localized valence (class 11). Oxidation produces intense near-infrared absorption at 892 (1(+)) or 1027 nm (2(+)) and narrow isotropic EPR spectra at g approximate to 2.005, signifying unprecedented spin localization at the p-semiquinone bridge. These results are not compatible with an (L(2))-bridged {Ru(IV)Ru(III}) situation nor with an {Ru(III)(mu-L(center dot))Ru(III)} three-spin arrangement with up-down-up spin configuration in the ground state, which would result in metal-centered spin through antiferromagnetic coupling between the adjacent individual spins. Only the {Ru(III)(mu-L(center dot))Ru(III)} situation, with up-up-down spin configuration, leads to ligand-centered resulting spin through the strong antiferromagnetic coupling between the remote metal spins, an unusual situation which is favored here because of weakened metal-radical coupling resulting from the pyrazolyl/p-semiquinone twist

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

Using the [RuCl(mu-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-ditert-butyl-substituted, DTBQ(n)), we have obtained the stable complexes [(Q(center dot-))Ru(II)Cl(mu-tppz)ClRuII(Q(center dot-))] (1) and the structurally characterized [(DTBQ(center dot-))Ru(II)Cl(mu-tppz)ClRu(II)(DTBQ(center dot-))] (2). The compounds exhibit mostly quinone-ligand-based redox activity within a narrow potential range, high-intensity near-IR absorptions (lambda(max) approximate to 920 nm; epsilon > 50 000 M(-1) cm(-1)), and variable intra- and intermolecular spin-spin interactions. Density functional theory calculations, electron paramagnetic resonance (EPR), and spectro-electrochemical 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-phenyhminoquinone 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

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

The diruthenium(III) complex [(acac)(2)Ru(mu-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 (Delta Delta,Lambda Lambda) diastereomer. The Ru(III)Ru(III) configuration in 1 (d(Ru-au) = 6.799 angstrom) results in a triplet ground state (mu = 2.82/2 mu(B) at 300 K) with a density functional theory (DFT) calculated triplet-singlet gap of 10840 cm-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 delta values ranging from -10 to -65 -65 ppm and an anisotropic electron paramagnetic resonance (EPR), spectrum ( = 2.133, g(1) - g(3) = Delta g = 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 Run Rum mixed-valent state 1(-) (K(c) approximate to 10(7)). The intermediate 1(-) shows a weak, broad Ru(II)-> Ru(III) intervalence charge transfer (IVCT) band at about 1040 nm (epsilon = 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 frequencies of the uncoordinated C=O groups of L(2-) in 1" 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 ( = 2.239 and Delta g = 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

Charged, but Found "Not Guilty": Innocence of the Suspect Bridging Ligands [RO(O)CNNC(O)OR](2-) = L2- in [(acac)(2)Ru(mu-L)Ru(acac)(2)](n), n = +,0,-,2-

Neutral diastereoisomeric diruthenium(III) complexes, meso- and rae-[(acac)(2)Ru(mu-adc-OR)Ru(acac)(2)) (acac(-) = 2,4-pentanedionato and adc-OR2- = dialkylazodicarboxylato = [RO(O)CNNC(O)OR](2-), R = tert-butyl or isopropyl), were obtained from electron transfer reactions between Ru(acac)(2)(CH3CN)(2) and azodicarboxylic acid diallcyl esters (adc-OR). The mew,P isomer 3 with R = isopropyl was structurally characterized, revealing two deprotonated and N-N coupled carbamate functions in a reduced dianionic bridge with d(N-N) = 1.440(5) angstrom. A rather short distance of 4.764 angstrom has been determined between the two oxidized, antiferromagnetically coupled Ru-III centers. The rac isomer 4 with R = isopropyl exhibited stronger antiferromagnetic coupling. While the oxidation of the neutral compounds was fully reversible only for 3 and 4, two well-separated (10(8) < K-c < 10(10)) reversible one-electron reduction steps produced monoanionic intermediates 1(-)-4(-) with intense (epsilon approximate to 3000 M-1 cm(-1)), broad (Delta nu(1/2) approximate to 3000 cm(-1)) absorptions in the near-infrared (NIR) region around 2000 nm. The absence of electron paramagnetic resonance (EPR) signals even at 4 K favors the mixed-valent formulation Ru-II(adc-OR2-)Ru-III with innocently behaving bridging ligands over the radical-bridged alternative Ru-II(adc-OR center dot-)Ru-II, a view which is supported by the metal-centered spin as calculated by density functional theory (DFT) for the methyl ester model system. The second reduction of the complexes causes the NIR absorption to disappear completely, the EPR silent oxidized forms 3(+) and 4(+), calculated with asymmetrical spin distribution, do not exhibit near infrared (NIR) activity. The series of azo-bridged diruthenium complex redox systems [(acac)(2)Ru(mu-adc-R)Ru(acac)(2)](n) (n = +,0,-,2-), [(bpy)(2)Ru(mu-adc-R)Ru(bpy)(2)](k) (k = 4+,3+,2+,0,2-), and [(acac)(2)Ru(mu-dihR)Ru(acac)(2)](m) (m = 2+,+,0,-,2-; dih-R2- = 1,2-diiminoacylhydrazido(2-)) is being compared in terms of electronic structure and identity of the odd-electron intermediates, revealing the dichotomy of innocent vs noninnocent behavior

A new coordination mode of the photometric reagent gilyoxalbis(2-hydroxyanil) (H(2)gbha): Bis-bidentate bridging by gbha(2-) in the redox series {(mu-gbha)[Ru(acac)(2)](2)}(n) (n =-2,-1, 0,+1,+2), including a radical-bridged diruthenium(III) and a Ru-III/Ru-IV intermediate

The bis-bidentate bridging function of gbha(2-) with N,O-/N,O- coordination was observed for the first time in the complex (mu-gbha)[Ru-III(acac)(2)](2) (1). Density functional theory calculations of 1 yield a triplet ground state with a large (Delta E > 6000 cm(-1)) singlet-triplet gap. Intermolecular antiferromagnetic coupling was observed (J approximate to -5.3 cm-1) for the solid. Complex 1 undergoes two one-electron reduction and two one-electron oxidation steps; the five redox forms {(mu-gbha)[Ru(acac)2]2}(n) (n = -2, -1, 0, +1, +2) were characterized by UV-vis-NIR spectroelectrochernistry (NIR = near infrared). The paramagnetic intermediates were also investigated by electron paramagnetic resonance (EPR) spectroscopy. The monoanion with a comproportionation constant K-c of 2.7 x 10(8) does not exhibit an NIR band for a Ru-III/Ru-II mixed-valent situation; it is best described as a 1,4-diazabutadiene radical anion containing ligand gbha(center dot 3-), which binds two ruthenium(III) centers. A Ru-III-type EPR spectrum with g(1) = 2.27, g(2) = 2.21, and g(3) = 1.73 is observed as a result of antiferromagnetic coupling between one Ru-III and the ligand radical. The EPR-active monocation (K-c = 1.7 x 10(6)) exhibits a broad (Delta v(1/2) = 2600 cm(-1)) intervalence charge-transfer band at 1800 nm, indicating a valence-averaged (Ru-3.5)(2) formulation (class III) with a tendency toward class II (borderline situation)

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

The diruthenium(III) compound [(mu-oxa){Ru(acac)2}2] [1, oxa2-=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?C7H8 revealed an intramolecular metalmetal distance of 5.433 angstrom 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 (Kc=105.9) with broad NIR absorption bands (e ca. 2000?M-1?cm-1) and maxima at 1800 (1-) and 1500 nm (1+). TD-DFT calculations support a RuIIIRuII formulation for 1- with a doublet ground state. The 1+ ion (RuIVRuIII) was calculated with an S=3/2 ground state and the doublet state higher in energy (?E=694.6 cm-1). The Mulliken spin density calculations showed little participation of the ligand bridge in the spin accommodation for all paramagnetic species [(mu-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 RuIIIRuII (4d5/4d6) system 1-, the RuIVRuIII (4d4/4d5) form 1+ exhibited extended absorbance over the UV/Vis/NIR range

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(center dot-))(acac)(2)] = 1, [Ru(Q(center dot-))(2)(acac)] = 2, and [Ru(Q(center dot-))(3)] = 3, were obtained and characterized as Ru(III) complexes with 4,6-di-terbul-N-phenyl-o-iminobenzosemiquinone (Q(center dot-)) 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(center dot-) 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(center dot-))(acac)(2)](+) = 1(+) and [Ru(II)(Q(center dot-))(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(center dot-))(acac)(2)](+) (1(+)) versus [Ru(III)(Q(0))(bpy)(2)](3+) (5(3+))). Compound 2, a three-spin system [Ru(III)(Q(center dot-))(2)(acac)] obtained in the all-cis configuration, possesses a complicated magnetic behavior including strong intramolecular antiferro-magnetic 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(center dot-))-Ru(III)-(Q(center dot-))} (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(center dot-))(Q(2-))(acac)](2-) suggests the [Ru(II)(Q(center dot-))(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-meta/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 iscussed 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 catalysi