Redox Behavior of a Dinuclear Ruthenium(II) Complex Bearing an

Article Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX

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Redox Behavior of a Dinuclear Ruthenium(II) Complex Bearing an Uncommon Bridging Ligand: Insights from High-Pressure Electrochemistry Maximilian Dürr,† Johannes Klein,§ Axel Kahnt,‡ Sabine Becker,∥ Ralph Puchta,† Biprajit Sarkar,§ and Ivana Ivanović-Burmazović*,† †

Lehrstuhl für Bioanorganische Chemie and ‡Lehrstuhl für Physikalische Chemie I, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany § Institut für Chemie und Biochemie, Freie Universität Berlin, Fabeckstraße 34/36, 14195 Berlin, Germany ∥ Fachbereich Chemie, Technische Universität Kaiserslautern, Erwin-Schrödinger-Straße 52/416a, 67663 Kaiserslautern, Germany S Supporting Information *

ABSTRACT: A dinuclear ruthenium complex bridged by 2,3,5,6pyrazinetetracarboxylic acid (μ-LH22−) was synthesized and characterized by X-ray crystallography, cyclic voltammetry under ambient and elevated pressures, electron paramagnetic resonance (EPR) and UV/vis-NIR (NIR = near-infrared) spectroelectrochemistry, pulse radiolysis, and computational methods. We probed for the first time in the field of mixed-valency the use of high-pressure electrochemical methods. The investigations were directed toward the influence of the protonation state of the bridging ligand on the electronic communication between the ruthenium ions, since such behavior is interesting in terms of modulating redox chemistry by pH. Starting from the [RuII(μ-LH22−)RuII]0 configuration, which shows an intense metal-to-ligand charge transfer absorption band at 600 nm, cyclic voltammetry revealed a pH-independent, reversible one-electron reduction and a protonation-state-dependent (proton coupled electron transfer, PCET) reversible oxidation. Deeper insight into the electrode reactions was provided by pressure-dependent cyclic voltammetry up to 150 MPa, providing insight into the conformational changes, the protonation state, and the environment of the molecule during the redox processes. Spectroelectrochemical investigations (EPR, UV/vis-NIR) of the respective redox reactions suggest a ligand-centered radical anion [RuII(μ-LH2•3−)RuII]− upon reduction (EPR Δg = 0.042) and an ambiguous, EPR-silent one-electron oxidized state. In both cases, the absence of the otherwise typical broad intervalence charge transfer bands in the NIR region for mixed-valent complexes support the formulation as radical anionic bridged compound. However, on the basis of high-pressure electrochemical data and density functional theory calculations the oneelectron oxidized form could be assigned as a charge-delocalized [RuII.5(μ-LH22−)RuII.5]+ valence tautomer rather than [RuIII(μLH2•3−)RuIII]+. Deprotonation of the bridging ligand causes a severe shift of the redox potential for the metal-based oxidation toward lower potentials, yielding the charge-localized [RuIII(μ-LH3−)RuII]0 complex. This PCET process is accompanied by large intrinsic volume changes. All findings are supported by computational methods (geometry optimization, spin population analysis). For all redox processes, valence alternatives are discussed.

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INTRODUCTION The concept of mixed-valency known for several decades, with the Creutz−Taube ion1 as the first example (Figure 1a) examined in 1969, has excessively been investigated and reviewed in recent years.2 These multinuclear complexes bearing metal centers in different oxidation states offer the unique possibility to investigate the fundamentals of intramolecular electron transfer. Such studies are relevant to understand the working principle of polynuclear metalloenzymes or to develop new molecular materials with unique physicochemical features.3 In many cases, bridging ligands such as pyrazines,4 bipyridines,3a,4b as well as polypyridines5 were used to mediate the electronic coupling between the ruthenium(II) ions and were shown to be redox-active. One© XXXX American Chemical Society

electron oxidation of such a bis-ruthenium(II) complex leads to a mixed-valent compound that can be categorized by the Robin-Day classification depending on the strength of the electronic interaction between the metal ions.6 In the last years, ortho- and para-quinones,7 azo-compounds,8 TCNX-type ligands (tetracyanoethene TCNE; 1,2,4,5-tetracyanobenzene TCNB),9 and verdazyls10 as explicit redox-active, noninnocent bridging ligands were applied to gain access to a new generation of mixed-valent compounds. The additional redox-active site makes their investigation and understanding more challenging. Thus, the radical anionic bridged [RuIII(μ-L•−)RuIII] state with Received: August 25, 2017

A

DOI: 10.1021/acs.inorgchem.7b02192 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Schematic representation of the Creutz−Taube ion (a), the bridging ligand (b) and the synthesized complex [(PPh3)2(Cl)Ru(pztcH2)Ru(Cl)(PPh3)2] (abbreviated with [RuII(μ-LH2)RuII] for clarity) [1] (c).

reaction volume can be determined by measuring the change in the redox potential by either cyclic voltammetry or differential pulse voltammetry (ΔE1/2 is identical with ΔE0 in a first approximation) in dependence of the pressure (p) at a constant temperature T (see (1))

a three-spin situation represents a new possible option besides the conventional valence-localized RuIII/RuII or the valencedelocalized RuII.5/RuII.5 states upon one-electron oxidation of corresponding dinuclear ruthenium(II) complexes, as comprehensively studied in literature.2a,b,7−11 It was proposed for symmetrical, acceptor bridged dinuclear complexes that their ground state can have a nature of a radical anionic bridged mixed-valent [RuIII(μ-L•−)RuII] valence tautomer, as in their corresponding metal-to-ligand charge transfer (MLCT) excited state (see Scheme 1).11a,12 Furthermore, competitive redox

ΔVcell 0 = −nF(δ ΔE 0 /δp)T

(1)

with n as the number of moles of electrons transferred (usually n = 1) and F being the Faraday constant.20 The ΔVcell0 value determined in such way consists of three parts (see (2)),19c which involve the contribution of the volume change related to the reference electrode, ΔVref0, the contribution of the volume change associated with the studied complex, ΔVcomplex0, and the partial molar volume of protons, mVH+0, with m as the number of transferred protons (m = 0 for pure electron transfer):

Scheme 1. Formation of the MLCT Excited-State Valence Tautomer and Subsequent Chemical Oxidation

ΔVcell 0 = ΔVref 0 + ΔVcomplex 0 − mVH +0

(2)

Furthermore, the volume change of the complex, ΔVcomplex0, can be subdivided into the intrinsic volume changes, ΔVintr0, representing bond length and conformational changes in the structure of the complex, and electrostrictive volume changes, ΔVelec0, corresponding to the volume change of the solvent hull responding on the change in overall charge of the complex (see (3)).

reactions between a noninnocent bridging ligand and the metal ions also may trigger new reaction pathways by intramolecular redox-induced electron transfer (RIET), as reviewed by Miller and Min.13 External oxidation processes leading to intramolecular reduction events and vice versa are possible consequences. Here, we used 2,3,5,6-pyrazinetetracarboxylic acid (pztcH4; Figure 1b) in its dianionic form (pztcH22−) as a bridging ligand to gain access to a new dinuclear ruthenium(II) complex [1] (Figure 1c). So far, examples for discrete dinuclear metal complexes bearing this bridging ligand are rare,14 whereas its use within metal−organic frameworks has been more common.15 The structural relation of our complex to the Creutz−Taube ion promises interesting electronic properties, which have been studied herewith. The protons between the ortho-positioned carboxylic acid groups are known to be strongly bound, and even at pH = 14 they do not necessarily dissociate.16 Thus, they are assumed to have substantial influence on the redox properties of the whole complex. As a result of this influence such ligand systems have attracted increasing attention in context of their use to probe proton-coupled electron transfer (PCET) processes.17 The possibility to switch the redox activity of a complex by modulating pH has already been demonstrated18 and is a promising approach for molecular switches.2a,3a A more unusual method to gain insight into PCET mechanisms is pressure-dependent cyclic voltammetry. The effect of pressure on the thermodynamics of the electrode reactions yields information about the changes in reaction volume ΔVcell0 of the studied redox process.19 For PCET reactions as well as for pure electron-transfer reactions, the cell

ΔVcomplex 0 = ΔVelec 0 + ΔVintr 0

(3)

It was found that the electrostrictive reaction volume changes depend linearly on the change in the square of the charge Δz2 (see (4)): ΔVelec 0 = k Δz 2 = k(zox 2 − zred 2)

(4)

In this equation, k is an empirical factor that was found to be k = 4.3 in the case of pure electron transfer.21 All electrochemical investigations implemented in this work suggest that one-electron oxidation of our complex does not necessarily lead to a classical mixed-valent Ru(II)/Ru(III) state but rather to a radical anion bridged Ru(III)/Ru(III) complex. Conversely, deprotonation of the bridging ligand and subsequent oxidation yields a classical mixed-valent Ru(II)/ Ru(III) complex, as suggested by density functional theory (DFT) calculations. The approach presented below consists of employing a wide range of analytical methods on this unique dinuclear ruthenium(II) complex, including high-pressure electrochemical studies, to clarify its (electronic) structure and electrochemical behavior under different conditions.

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthesis of the bistridentate bridging ligand, 2,3,5,6-pyrazinetetracarboxylic acid, B


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Inorganic Chemistry

(Tables S1−S6 and Figure S11). The neutral complex [RuII(μLH2)RuII]0 [1] yields deep blue, air-stable crystals and crystallizes in the monoclinic space group P21/n with half a complex molecule and one free CHCl3 molecule in the asymmetric unit (see Figure 2).

was performed according to a textbook modified version of the original synthetic procedure of Ludwig Wolff.22 The oxidation of the methyl groups by KMnO4 to carboxylic acid groups was improved by addition of a base and Aliquat 336, a phasetransfer catalyst, prior to the reaction. The product is obtained as the potassium salt pztcH3K. Several workup procedures to gain the potassium-free tetracarboxylic acid were considered. The use of perchloric acid to precipitate the excess potassium as KClO4 bears a potential explosion hazard and is strongly disapproved. Wolff precipitated and analyzed the tetracarboxylic acid as Ag+ salt.22 However, this approach is too expensive, and a more economic synthetic procedure would be appreciated. Thus, conversion of KpztcH3 into insoluble Ca2pztc by adding a slight excess of Ca(NO3)2 and subsequent conversion into pztcH4 by addition of H2SO4 was a quite practical approach. 13C NMR analysis of the product in D2O (Figure S1) revealed two signals due to its D2h symmetry, one at 165.4 ppm corresponding to the four carboxylic acid groups, and one at 146.1 ppm corresponding to the four pyrazine carbon atoms. Ultrahigh-resolution electrospray ionization mass spectrometry (UHR-ESI-MS; Figure S2) confirmed unambiguously the formation of pztcH4 and its high affinity toward alkali metal ions, as the ligand was detected as [pztcH4 + Na]+ and [pztcH4 + K]+. The dinuclear ruthenium complex was obtained by a standard reaction of the bis-tridentate bridging ligand with 2 equiv of the commercially available metal precursor [Ru(PPh3)3Cl2] under nitrogen atmosphere in an ethanol/water mixture bearing moderate yields of the complex. The recrystallized deep blue product is air stable and was characterized by 1H-, 13C-, and 31P NMR, UHR-ESI-MS, elemental analysis, and X-ray crystallography. The 1H NMR (Figure S3) shows the typical multiplet signals for PPh3 in the aromatic chemical shift range from 7.25 to 6.68 ppm, overlapping with the CDCl3 residual signal. 13C NMR analysis of the complex (Figure S4) reveals a peak at 177.1 ppm, corresponding to the deprotonated carboxylic acid groups. The peak at 173.7 ppm indicates the presence of protonated carboxylic groups, as well. The carbon atoms of the pyrazine core are detected at 148.7 ppm, the remaining carbon atoms of the PPh3 groups appear in the range from 134.6 to 127.8 ppm. The 31P NMR (Figure S5) shows two doublets, one at 44.56 ppm and one at 34.79 ppm, which are formed due to geminal coupling of the axially and equatorially coordinated PPh3 groups. The coupling constant is 1JP,P = 34.02 Hz. Positive ion mode UHR-ESI-MS of [1] shows a signal at m/z = 1541.1245, corresponding to [M − Cl] + (M = C80H62Cl2N2O8P4Ru2), with a distinct isotope pattern that is in good agreement with the simulated isotope pattern for the complex (Figure S6) bearing a twofold protonated bridging ligand in solution. Negative ion mode UHR-ESI-MS in dichloromethane (DCM) reveals a signal at m/z = 1576.0954, which could be assigned as the radical anion [RuII(μ-LH2)RuII]•− (Figure S8), corresponding to [M]•−. Such radical anions can be formed during the ionization process in the negative ion mode if the redox potential of the analyte is above −0.8 V versus saturated calomel electrode (SCE).23 Thus, their existence already indicates an easy reductibility of the complex. X-ray Structural Analysis and DFT calculations. Crystals suitable for X-ray diffraction were obtained through diffusion of Et2O into a saturated CHCl3 solution at low temperatures. Details and parameters of the X-ray structure determination can be found in the Supporting Information

Figure 2. Molecular structure of [1]. Thermal ellipsoids are drawn at 50% probability; phenyl-H atoms and solvent molecules are omitted for clarity.

The μ-LH2 ligand chelates the two ruthenium centers in a bis-tridentate O,N,O-fashion. Each ruthenium center is chelated by a carbonyl oxygen of a carboxylic acid group O(1), with a Ru(1)−O(1) distance of 2.1264(17) Å and an oxygen of a hydroxyl group O(3), with a Ru(1)−O(3) distance of 2.1393(17) Å. The C(4)−O(3) (1.258(3) Å) and C(4)− O(4) (1.267(3) Å) bond lengths are in the range between a C− OH single (1.34 Å) and a CO double bond (1.20 Å).24 This suggests the presence of a carboxylate anion with a delocalized negative charge. Noticeably, the two remaining protons of the bridging ligand are located between the hydroxyl groups, with an O(2)−H distance of 1.10(4) Å, and the carbonyl groups, with a H···O(4) distance of 1.33(4) Å. This O−H···−O arrangement could be classified as a negative charge-assisted hydrogen bond, (−)CAHB.16b The short O(2)···O(4) distance of 2.429 Å, a typical value for (−)CAHB, indicates very strong intramolecular hydrogen bonds in this molecule, with a quasicovalent nature and an energy gain in the covalent part of the hydrogen bond of ∼30−37 kcal mol−1.16b The remaining coordination sites in the distorted octahedral coordination sphere of the ruthenium centers are occupied by PPh3 and Cl− in axial positions and another PPh3 in equatorial position. The bridging ligand is nearly planar and exhibits a slight fold along the N(1)−N(1) axis by ∼6°. The O(1)−Ru(1)−O(3) angle is 154.06(6)°, showing a significant distortion of the octahedral coordination sphere. The distortion is also reflected by the P(1)−Ru(1)−P(2) angle of 97.37(2)° and the significantly elongated Ru(1)−P(2) bond of 2.3925(7) Å compared to the Ru(1)−P(1) bond of 2.3090(7) Å. With a value of 2.4098(6) Å, the Ru(1)−Cl(1) bond length suggests the ruthenium(II) oxidation state.25 The intramolecular Ru−Ru distance of 6.7153(5) Å in combination with the planar μ-LH2 ligand should enable an intramolecular electron transfer. Furthermore, the almost parallel orientation of two phenyl groups above and below the central pyrazine bridging ligand suggests intramolecular π−π interactions with an average Ph−(μ-LH2) distance of 3.741 Å that stabilizes the complex. The Ph ring is C


Article

Inorganic Chemistry Table 1. Selected Bond Lengths [Å] and Angles [deg] of [RuII(μ-LH22−)RuII]0 [1] X-raya

DFT-calculated (ωB97XD/def2svp)b •−

[1]

[1]

[1 ]

[1+]

6.7153(5) 2.3090(7) 2.3925(7) 2.4098(6) 2.1264(17) 2.1393(17) 1.993(2) 1.250(3) 1.276(3) 1.258(3) 1.267(3) 1.10(4) 1.33(4) 175.96 154.06(6) 97.37(2) 81.02(2) 87.64(6) 180.00

6.73 2.34 2.39 2.43 2.14 2.15 2.02 1.24 1.27 1.26 1.24 1.08 1.33 173.9 153.6 95.1 83.7 87.0 180.0

6.76 2.31 2.36 2.45 2.14 2.15 2.04 1.26 1.30 1.27 1.24 1.03 1.45 173.2 153.7 96.0 82.5 89.0 180.0

6.72 2.41 2.43 2.36 2.08 2.08 2.03 1.27 1.28 1.28 1.23 1.02 1.52 170.1 153.2 93.0 84.9 87.6 180.0

[1+-H+]b L

Ru(1)−Ru(1) Ru(1)−P(1) Ru(1)−P(2) Ru(1)−Cl(1) Ru(1)−O(1) Ru(1)−O(3) Ru(1)−N(1) C(2)−O(1) C(2)−O(2) C(4)−O(3) C(4)−O(4) O(2)−H(1) O(4)−H(1) O(2)−H(1)−O(4) O(1)−Ru(1)−O(3) P(1)−Ru(1)−P(2) P(2)−Ru(1)−Cl(1) Cl(1)−Ru(1)−N(1) P(1)−Ru(1)−Ru(1)-(P1)

6.70 2.33 2.39 2.43 2.18 2.11 2.01 1.23 1.29 1.29 1.21 1.03 1.48 17.3 154.2 94.6 82.8 88.1 170.1

R 2.41 2.41 2.37 1.99 2.11 2.04 1.33 1.20 1.28 1.23

153.8 93.9 84.5 87.5

Symmetry transformations used to generate equivalent atoms: No. 1 −x + 2, −y + 1, −z + 1. bCorresponding DFT (ωB97XD/def2svp)-based values are given for the species [1], [1·−], [1+], and [1+-H+]. L and R = values for different Ru centers in asymmetrical molecule; see Figure 3c for assignment. a

Figure 3. DFT-optimized (ωB97XD/def2svp) core geometries for [1·−] (a), [1+] (b), and [1+-H+] (c); Phenyl groups are omitted for clarity.

indicated by our electrochemical investigations under elevated pressure (vide supra). The HOMO is now located on the pyrazine bridge, supporting the formulation of the one-electron reduced species as the ligand-centered radical anion [RuII(μLH2•3−)RuII]−. Upon one-electron oxidation, from [1] to [1+], the calculated geometry slightly changes (Table 1, Figure 3b, Table S8−S9, Figure S22). With an identical decrease of Ru− Cl bonds, from 2.4098(6) Å (in the crystal structure) or 2.43 Å (in the calculated neutral structure) to 2.36 Å (calculated), shortened Ru−O bonds (2.08 Å) and overall symmetrical coordination environment around two Ru centers, the chargedelocalized [RuII.5(μ-LH22−)RuII.5]+ or [RuIII(μ-LH2•3−)RuIII]+ forms of the oxidized complex could be suggested.26 Highpressure electrochemical data and the calculated HOMO/ LUMO (ωB97XD/def2svp; see Figure S23) speak in favor of the charge-delocalized [RuII.5(μ-LH22−)RuII.5]+ valence form. The intramolecular H-bonding is slightly weakened as indicated by the elongated O(2)···H(1) bridge of 1.52 Å and the smaller O−H···O angle of 170.1°, whereas the intramolecular Ru−Ru distance remains unchanged. The optimized (ωB97XD/def2svp) geometry for the oxidized and deprotonated molecule [1+-H+], however, shows interesting changes (Figure 3c) due to the loss of one proton

offset by 21.5° toward the central pyrazine bridging motif. Further π−π interactions are observed between the PPh3 groups, one in equatorial and one in axial position, as indicated by two parallel orientated phenyl rings with an intramolecular distance of 3.488 Å. Selected bond lengths and angles as well as the corresponding DFT-calculated (ωB97XD/def2svp) values for the complex in the different oxidation states, allowing conclusions on conformational changes, are given in Table 1. To understand a pressure effect on electrochemical redox processes and corresponding volume changes (vide infra), the geometries for the dinuclear complex in different oxidation states were calculated by DFT methods (ωB97XD/def2svp) (Tables S8−S13 and Figures S18−S25). The experimentally determined structural parameters for neutral [RuII(μ-LH22−)RuII]0 [1] and calculated ones are in good agreement (Tables 1, S3, and S4 and Figure S20). The calculated highest occupied molecular orbital (HOMO) and HOMO−1 are located on the metal centers, whereas the lowest unoccupied molecular orbital (LUMO) and LUMO+1 are mainly located on the bridging ligand. The calculated (ωB97XD/def2svp) geometry of the one-electron reduced species [1·−] (Table 1, Figure 3a, Tables S8 and S9, Figure S18) exhibits only insignificant changes in bond lengths. Hence, the molecule retains its geometry, as also D


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Inorganic Chemistry

metal-based one-electron oxidation of [1] into its mixed-valent state [RuII(μ-LH22−)RuIII]+ [1+], which gets stabilized by either an MLCT process (i.e., inner-sphere electron transfer) in the form of [RuIII(μ-LH2•3−)RuIII]+ or by charge delocalization in [RuII.5(μ-LH22−)RuII.5]+. The CV of the complex upon deprotonation of the bridging ligand using 6 equiv of 2,6dimethylpyridine as a sterically hindered base exhibits a significantly shifted oxidation wave Vb (Figure 4b and Table 2). The ligand-centered reduction IV remains unaffected. Vb is shifted by −0.56 V toward lower redox potentials, from 1.13 to 0.57 V. In addition, the nature of the redox process has changed into a quasi-reversible process as indicated by the increased peak separation of 240 mV. A similar behavior of a dinuclear ruthenium complex bridged by 2,2′-bis(2-pyridyl)dibenzimidazole was observed by Haga et al., who reported that the redox potential of this complex was also shifted significantly toward lower potentials upon deprotonation.27b The deprotonation of the carboxylic acid group results in an increased donor ability of the ligand, which in turn results in a better stabilization of higher oxidation states of the metal center, and thus, its oxidation is facilitated.2a,11d This oxidation step seems to be a proton-coupled redox reaction and can be described more precisely as a one-electron oxidation from [RuII(μ-LH22−)RuII] to the deprotonated mixed-valent [RuII(μLH3−)RuIII] in the presence of a base. This assignment is supported by DFT calculations (ωB97XD/def2svp). Despite the increased anodic current for process Vb compared to that of process IV, differential pulse voltammetry (DPV) suggests a one-electron redox process, as the peak area of the peak Vb is the same order of magnitude as for IV (Figure S13). Further DPV investigations with 1,1′-dimethylferrocene used in equimolar amounts as an internal redox standard confirm these assignments (Figures S14 and S15). This observation is typical for PCET processes (a sort of an electrochemical reaction coupled with a homogeneous chemical step, ECmechanism), where the current measured by cyclic voltammetry is dependent on the amount of base present in solution as well as the rate constant of the proton transfer relative to the electron transfer.28 The reduction process III could be assigned to a second reduction of the bridging ligand (Figure S12), leading to the dianionic complex [RuII(μ-LH24−)RuII]2−. Further evidence for this assignment is delivered by DFT calculations (ωB97XD/def2svp) that predict localization of LUMO and LUMO+1 orbitals of [Ru II (μ-LH 2 2− )Ru II ] 0 predominantly on the μ-LH22− moiety (Table S11 and Figure S21). Further oxidations yielding the [RuIII(μ-LH22−)RuIII]2+ [12+] or [RuIII(μ-LH3−)RuIII]+ [12+-H+]+ species were not observable within the chosen potential range. Chemical oxidation of [1] using ferrocenium tetrafluoroborate (FcBF4) and a 30 min exposure to ambient light yields observable amounts of the oneelectron oxidized species (m/z = 1576.0913), as demonstrated by ESI-MS (Figures 5c and S9). This is interesting knowing that this process is thermodynamically impossible, since the redox potential of the Fc+/0 couple (E0 = 0.41 V vs Ag/AgCl)29 is way below the measured redox potential for the first oxidation (Vb = 1.13 V). Repetition of this experiment under strict exclusion of light shows no change in the ESI-MS spectrum after 30 min, and thus, the complex remains in its [RuII(μ-LH22−)RuII]0 oxidation state (Figure 5b). However, significant amounts of the oneelectron oxidized species [1+] could be detected after a 30 min period (Figures 5d and S9 inset) if the solution is irradiated

and thus, one intramolecular H-bond. Consequently, electrostatic repulsions between two ortho-positioned carboxylate groups rise. This results in a torsion along the otherwise linear (180°) P(1)/Cl(1)−Ru(1)−Ru(1)−P(1)/Cl(1) axis, resulting in dihedral angles of 163.5° (for Cl) and 170.1° (for P), respectively (see Table S9). Most interestingly, the calculated structure suggests a clearly defined Ru(III)/Ru(II) mixedvalent state, as the molecule is unsymmetrical, and the bond lengths around the two metal centers are different. The predicted HOMO is located only on the ruthenium(II) metal center, whereas the LUMO and LUMO+1 are located on the pyrazine moiety (Figure S25). Higher oxidation states, such as a Ru(III)/Ru(III) arrangement of the molecule, were not considered for DFT calculations, since their redox potential seems to be located outside the chosen potential range. The one-electron oxidized, twofold deprotonated form [RuII(μL4−)RuIII]− was also not considered here, since the intramolecular hydrogen bonds in this molecule are presumed to be extremely stable (vide supra),16 and furthermore, complete deprotonation would lead to a torsion to such an extent leading to dissociation of the ligand. Additional MS experiments also suggest the absence of a completely deprotonated state. In the first place, the negative-mode MS spectrum of the complex in acetone (Figure S7) only shows the signal corresponding to the onefold deprotonated complex [M − H]− (m/z = 1575.0664) but not the signal for the twofold deprotonated, twofold negatively charged complex [M − 2H]2− with an expected m/z = 787.0400. Electrochemistry. The redox behavior of the complex is of particular interest to identify a potential mixed-valent state.2a,c As it is apparent from UHR-ESI-MS measurements as well as from crystallographic data, the bridging ligand comprises two acidic protons. There are several examples in the literature that describe the influence of proton-transfer reactions of bridging ligands on the electrochemical behavior of mixed-valent compounds.18,27 To reveal the influence of the residual protons, the electrochemical behavior of [1] was tested in the absence and in the presence of 2,6-dimethylpyridine, a sterically hindered base. The cyclic voltammogram (CV) of [1] in dichloromethane exhibits two reversible redox processes and several irreversible reduction processes (Figure 4a and Table 2). The one-electron redox process at −0.32 V (IV) represents the reduction of [1] into its radical-anionic species [RuII(μ-LH2•3−)RuII]•− [1•−], whereas the redox process at 1.13 V (Va) is presumed to be the

Figure 4. CVs and DPVs of [1] (a) and [1] after deprotonation with 2,6-dimethylpyridine (b) in CH2Cl2/0.1 M Bu4NBF4 vs Ag/AgCl; scan rate 50 mV s−1. E


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Inorganic Chemistry Table 2. Electrochemical Dataa for [1] in Its Protonated State (a) and Deprotonated State (b) (a) (b)

Ib

IIb

IIIc

IVc

Vc

−1.77 −1.76

−1.52 −1.52

−0.96 (100) −0.95 (120)

−0.32 (80) −0.32 (80)

1.13 (90) 0.57 (240)

From cyclic voltammetry in CH2Cl2/0.1 M Bu4NBF4 with a scan rate of 50 mV s−1; potential is given vs Ag/AgCl reference electrode. bIrreversible process; estimated from peak potential. cReversible process; the potential values (E1/2) were calculated using the following equation E1/2 = (Epc + Epa)/2, where Epc and Epa correspond to the cathodic and anodic peak potentials, respectively. Peak potential differences ΔE [mV] (in parentheses). a

Figure 5. (a) UHR-ESI-MS (+MS) spectrum of the stock solution of [1] before addition of FcBF4. (b) Spectrum after addition of FcBF4 and 30 min reaction in the dark. (c) Spectrum after addition of FcBF4 and 30 min reaction at ambient light. (d) Spectrum after addition of FcBF4 and 30 min reaction while irradiated with a 150 W xenon lamp (with UV filter).

with a 150 W xenon lamp. These observations could be brought in accordance with the existence of an MLCT excitedstate valence tautomer [1*] for the neutral molecule in solution, followed by chemical oxidation (Scheme 1).11a,12a,30 The radical anionic bridging ligand present in such [RuIII(μLH2•3−)RuII] form can easily be chemically oxidized by Fc+, considering that its redox potential is expected to be close to the observed redox process IV at −0.32 V versus Ag/AgCl. In our heterogeneous electrochemical setup, oxidation of the MLCT excited state [1*] present in solution at ambient light would not be observed, presumably due to kinetic reasons. Application of a strong oxidant such as H2O2 results in the twoelectron oxidized, deprotonated species [12+-H+]+ as shown by ESI-MS (Figure S10). However, this species only plays a tangential role, as it exhibits only limited stability and seemingly only exists in the deprotonated state. Pressure-Dependent Electrochemistry. As mentioned above, measuring the half-wave potential in dependence of pressure provides additional insights into the occurring redox processes. The ΔV cell 0 values of the redox reactions corresponding to IV, Va, and Vb (Figure 6) were calculated according to (1). The pressure-dependent CVs (for Va and Vb) as well as details on the shifts of the corresponding redox potentials can be found in the Supporting Information (Table S7 and Figures S16 and S17). As pointed out, ΔVcell0 consists of several factors (see (2) and (3)). The contribution of the ΔVcomplex0 value can be expressed by (5), obtained by combination of (2) and (3):

Figure 6. (a) Pressure dependence of the different half-wave potentials IV (reduction; black), Va (oxidation; red), and Vb (oxidation in the presence of a base; blue) written as ΔE1/2 = E1/2(P) − E1/2(P0) (P0 = 0.5 MPa) vs the pressure [MPa]. Scan rate = 25 mV s−1, T = 21 °C. (b) CVs (potentials are given vs Ag/AgCl reference electrode) obtained for IV at different pressures in CH2Cl2/0.1 M Bu4NBF4; scan rate 25 mV s−1.

ΔVcomplex 0 = ΔVcell 0 − ΔVref 0 + mVH +0 = ΔVintr 0 + ΔVelec 0

(5)

As defined by convention, all processes are considered as a reduction (Scheme 2). Sun et al. determined the contribution of the Ag(s)/AgCl(s) reference electrode as ΔVref0 = −9.0 ± 1.0 cm3 mol−1.31 The partial molar volume of a proton VH+0 was estimated by different groups in the past, and the values range from −4.2 ± 1.5 to −7.7 ± 0.8 cm3 mol−1 with the latter being a widely Scheme 2. Diagrammatic Representation of the Molar Volume Contributions of Two Half-Cells to the Measured Reaction Volume19a

F


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Inorganic Chemistry accepted value.19c However, almost all reported values in literature are valid for aqueous systems. Reports about pressuredependent cyclic voltammetry in organic solvents, especially in dichloromethane, are extremely rare.29b,32 Assuming that the values for ΔVref0 and mVH+0 are in the same order of magnitude for dichloromethane, the calculated values for ΔVintr0 show the same tendency as the observed ΔVcell0 (Table 3). Thus, the obtained data are sufficient for a qualitative discussion. Table 3. Data Obtained from Pressure-Dependent Cyclic Voltammetrya,b

IV Va Vb

ΔVcell0, [cm3 mol−1]

number of protons, m

Δz2

ΔVelec0,c [cm3 mol−1]

ΔVintr0, [cm3 mol−1]

−12.4 ± 1.1 +4.4 ± 1.7 +18.5 ± 1.4

0 0 1

−1 1 0

−4.3 +4.3 0

+0.9 +9.1 +19.8

Calculated from (5). bΔVref0 = −9.0 cm3 mol−1; VH+0 = −7.7 cm3 mol−1. cCalculated from (4). a

The overall volume change for the first redox reaction IV yields a negative value of ΔVcell0(IV) = −12.4 ± 1.1 cm3 mol−1. With the reduction from [RuII(μ-LH22−)RuII]0 to [RuII(μLH2•3−)RuII]−, minor intrinsic volume changes of +0.9 cm3 mol−1 appear. The measured value ΔVcell0(IV) mostly comprises of the reference electrode contribution and electrostrictive volume changes. DFT calculations on the ωB97XD/ def2svp level (see Discussion above) support this value, as only insignificant changes in the calculated bond lengths are visible, and thus, the molecule retains its overall structure. The second redox reaction Va yields a positive volume change of ΔVcell0(Va) = +4.4 ± 1.7 cm3 mol−1 upon reduction. The calculated intrinsic volume change of ΔVintr0(Va) = +9.1 cm3 mol−1 indicates changes in the geometry of the molecule. It is expected that the oxidation of a metal center within a complex predominantly results in a shortening of the metal−ligand bonds, and consequently, the oxidized form has a smaller molar volume. This is consistent with positive values of ΔVintr0 (= Vred0 − Vox0). Since the value of ΔVintr0(Va) = +9.1 cm3 mol−1 is quite moderate, it indicates that the additional positive charge, generated upon oxidation, is delocalized between the two ruthenium centers in the form of [RuII.5(μ-LH22−)RuII.5]+ (vide infra). In the case of a higher positive charge, localized on one [RuIII(μ-LH22−)RuII]+ or two [RuIII(μ-LH2•3−)RuIII]+ ruthenium centers, one would expect a larger impact on the intrinsic volume changes of the complex. Such a larger volume change, with ΔVintr0(Vb) = +19.8 cm3 mol−1, was observed by us for the redox reaction Vb that is coupled with deprotonation of the bridging ligand. This large volume change agrees with an increased positive charge on one ruthenium center and an increased negative charge on the bridging ligand, supporting the formulation as [RuIII(μLH3−)RuII]0 [1+-H+] (vide supra). In addition, because of the deprotonation and the resulting loss of one intramolecular H bond, the structure gets less rigid, and a slight twist along the Ru−Ru axis causes a more relaxed and compact configuration. EPR Spectroelectrochemistry and Spin Density Calculations. The one-electron reduced complex [1•−] with an S = 1/2 state could be investigated by EPR spectroelectrochemistry (Figure 7a). However, the one-electron oxidized species [1+] showed no EPR activity at all. The EPR spectrum of [1•−], which was obtained upon electrochemical reduction at 113 K, yields a small g factor

Figure 7. (a) EPR spectrum of the electrochemically generated reduced form [1·−] (IV) in CH2Cl2/0.1 M Bu4NPF6 at 113 K. (b) EPR spectrum of the electrochemically generated oxidized species [1+H+] (Vb) in CH2Cl2/0.1 M Bu4NPF6 at 113 K. The asterisk indicates the signal of an organic impurity.

anisotropy of Δg = 0.042, proving a ligand-centered radical without ruthenium interaction. Further evidence is given by spin population analysis (B3LYP-D3/TZVP), indicating a bridging ligand-centered spin with a minor ruthenium contribution of ∼1.5% (Figure 8). Thus, the formulation of the one-electron reduced form as [RuII(μ-LH2•3−)RuII]− is confirmed.

Figure 8. Spin population analysis (B3LYP-D3/TZVP) of [1·−].

The one-electron oxidized complex [1+], also with an S = 1/ 2 state, is EPR silent at room temperature and at 113 K. EPR silence of d5/d6 mixed-valent complexes is known in literature for weak-interacting, valence-averaged systems with π-accepting bridging ligands, which display fast relaxation due to close-lying excited states.2a,b,33 This suggests the formulation as the chargedelocalized mixed-valent complex [RuII.5(μ-LH22−)RuII.5]+. Another possibility for the EPR silence of the ruthenium(III) metal centers is the lability of chloride ligands, resulting in rapid EPR relaxation of the solvated complex at higher temperatures.2a,5 However, the calculated HOMO (ωB97XD/ G


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Inorganic Chemistry

Figure 9. (a) Changes in the UV/vis-NIR absorption of [1] during in situ spectroelectrochemical reduction (IV) in CH2Cl2/0.1 M Bu4NPF6 at room temperature. (b) Changes in the UV/vis-NIR absorption of [1] during in situ spectroelectrochemical oxidation (Va) in CH2Cl2/0.1 MBu4NPF6 at room temperature. (c) Changes in the UV/vis-NIR absorption in the presence of a base (2,6-dimethylpyridin) of [1] during in situ spectroelectrochemical oxidation (Vb) in CH2Cl2 /0.1MBu4NPF6 at room temperature.

Scheme 3. Redox Reactions and Valence Tautomers of [RuII(μ-LH22−)RuII]

(RuII(d)/μ-LH22−(π) → μ-LH2•3−(π*)). No spectral changes are observable in the near-infrared (NIR) region. One-electron oxidation of the complex into its cationic form [1+] (Figure 9b) reveals a weakened MLCT band at 580 nm. Here, the absence of any visible intervalence charge-transfer (IVCT) bands implies the radical anion-bridged [RuIII(μ-LH2•3−)RuIII]+ valence tautomer.11a,30 However, several dinuclear and trinuclear mixed-valent compounds connected by neutral acceptor bridging ligands are known in literature, that also do not exhibit any IVCT absorption bands in the NIR region.34 This would be in accordance with the mixed-valent nature of [1+], either in the charge-localized [RuIII(μ-LH22−)RuII]+ or charge-delocalized [RuII.5(μ-LH22−)RuII.5]+ state. Addition of 2,6-dimethylpyridine to a DCM solution of [1] does not change the absorption spectra. Thus, the strong MLCT absorption band at 611 nm (RuII(d) → μ-LH22−(π*)/μLH3−(π*)) cannot solely be affected by the presence of a base. However, electrochemical oxidation in the presence of a base results in the [1+-H+] species, observed by complete dissipation of the MLCT band at 611 nm and an increase in absorption below 500 nm (Figure 9c). Noticeably, we have a distinct charge-localized mixed-valent complex (spectroelectrochemical EPR measurements, DFT calculations (ωB97XD/def2svp)), but detectable IVCT absorptions are still absent. To further characterize the reduced form of [1], [RuII(μLH2•3−)RuII]−, pulse radiolysis experiments under reducing conditions were performed. This necessitates to dissolve [1] into a mixture of 80% toluene, 10% acetone, and 10% 2propanol, saturated with N2, and irradiated with high-energy electron pulses. These conditions led to the formation of (CH3)2·COH35 known as a strong reductant36 capable of

def2svp) is largely delocalized across the metal centers, the PPh3 groups, and the chloride ligands, excluding the bridging ligand (Figure S23). Also, such a delocalization can explain the EPR-silent nature of the oxidized complex and is consistent with identical coordination spheres around the ruthenium centers as well. Thus, the oxidation state of [1+] can rather be considered as a charge-delocalized [RuII.5(μ-LH22−)RuII.5]+ mixed-valent complex. The spectroelectrochemical EPR investigation of the one-electron oxidized, deprotonated complex [1+-H+] clearly shows a metal-centered oxidation with significant g factor anisotropy of Δg = 0.556 (Figure 7b), suggesting the formulation as a charge-localized mixed-valent complex [RuII(μ-LH3−)RuIII]0. Further support comes from DFT calculations (ωB97XD/def2svp) (Figures S24 and S25) indicating an unsymmetrical molecule with different coordination settings for each ruthenium center, suggesting different metal oxidation states. UV/Vis-NIR Spectroelectrochemistry and Pulse Radiolysis. The [RuII(μ-LH22−)RuII]0 complex in its neutral form shows an intense MLCT absorption band in the visible region at 611 nm with a molar extinction coefficient of ε611 = 21 150 L mol−1 cm−1 (Figure 9a,b). This band is assigned to a transition from the metal-based HOMO to the bridging ligand-centered LUMO (RuII(d) → μ-LH22−(π*)). This also implies a possible coexistence of a radical anion bridged mixed-valent valence tautomer (MLCT excited state) as mentioned above.11a,12,30 Upon reduction to [RuII(μ-LH2•3−)RuII]− (Figure 9a) the intense MLCT band at 611 nm diminishes, as another band at 500 nm rises. Here, the MLCT/LLCT absorptions originate from transitions between the metal-based HOMO−1, the bridging-ligand-based HOMO, and the ligand-centered LUMO H


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Inorganic Chemistry

SAINT.38 Empirical absorption corrections were calculated with SADABS.39 The space group was determined by XPREP40 through analysis of metric symmetry and systematic absences. The structure was solved with SHELXT41 and refined by full-matrix least-squares based on F2 using SHELXL (Re. 770).42 The structure was checked for higher symmetry using PLATON.43 All non-hydrogen atoms were located and refined anisotropically. The isotropic displacement parameters of the hydrogen atoms were fixed to 1.2 times the Ueq value of the atoms they are linked. Coordinates for the hydrogen atom H1 were taken from the difference Fourier synthesis. H1 was subsequently refined freely without the use of any restraints. Mass Spectrometry. Electrospray-ionization mass spectrometric (ESI-MS) measurements were performed on a UHR-qToF Bruker Daltonik (Bremen, Germany) maXis plus, an ESI-quadrupole time-offlight (qToF) mass spectrometer capable of a resolution of at least 60.000 (full width at half-maximum (fwhm)). Detection was in positive ion mode as well as in negative ion mode; the source voltage was 3.8 kV (positive ion mode) or 2.0 kV (negative ion mode). The flow rates were 180 μL/h. The drying gas (N2), to aid solvent removal, was held at 180 °C. The MS was calibrated prior to every experiment via direct infusion of the Agilent ESI-TOF low concentration tuning mixture, which provided an m/z range of singly charged peaks up to 2700 Da in both ion modes. NMR Spectroscopy. NMR measurements were performed on a Bruker AVANCE DRX400 WB instrument. Pulse Radiolysis. The samples were dissolved in a mixture of 80% toluene, 10% acetone, and 10% isopropanol, saturated with N2, and irradiated with high-energy electron pulses (1 MeV, 15 ns duration) by a pulse transformer type of electron accelerator (Elit−Institute of Nuclear Physics, Novosibirsk, Russia). The dose delivered per pulse was measured by electron dosimetry.44 A dose of 100 Gy was employed. The optical detection of the transients was performed with a detection system consisting of a pulsed (pulser MSP 05−Müller Elektronik Optik) xenon lamp (XBO 450, Osram), a SpectraPro 500 monochromator (Acton Research Corporation), an R9220 photomultiplier (Hamamatsu Photonics), and a 500 MHz digitizing oscilloscope (TDS 640, Tektronix). Electron Paramagnetic Resonance Spectroelectrochemistry. EPR spectra at X-band frequency (ca. 9.5 GHz) were obtained with a Magnettech MS-5000 benchtop EPR spectrometer equipped with a rectangular TE 102 cavity and TC HO4 temperature controller. The measurements were performed in synthetic quarz glass tubes. For EPR spectroelectrochemistry a three-electrode setup was employed using two Teflon-coated platinum wires (0.005″ bare, 0.008″ coated) as working and counter electrode and a Teflon-coated silver wire (0.005″ bare, 0.007″ coated) as pseudoreference electrode. Electrochemistry/Pressure-Dependent Electrochemistry. Electrochemical measurements (CV/DPV) were performed with an Autolab PGSTAT 101 device (Metrohm). A conventional three-electrode arrangement was employed consisting of a glassy-carbon disk working electrode (geometric area: 0.07 cm2) (Metrohm), a platinum wire auxiliary electrode (Metrohm), and a Ag/AgCl, LiCl (3 M; in EtOH) (Metrohm) reference electrode. All electrochemical measurements were performed in CH2Cl2 with 0.1 M Bu4NBF4 as supporting electrolyte. All solutions were thoroughly degassed with N2 before use, and a stream of N2 was maintained throughout the measurement. The solutions were thermostated at 21 °C. Electrochemical measurements under elevated pressure were performed in a homemade cell.21,45 A conventional three-electrode system was employed. It consisted of a platinum working disk electrode (geometric area: 0.02 cm2) (BAS), a platinum wire (Sigma-Aldrich) as an auxiliary electrode, and a AgClcoated Ag-wire (Sigma-Aldrich) pseudoreference electrode. The set of electrodes was placed into the Teflon cup, which was screwed into the electrochemical cell body. The cell was filled with the abovementioned solution, containing 1 mM of the analyte, and thoroughly degassed with N2 (15 min.). Then, the Teflon cell was closed with a Teflon plug and a screw. The closed pressure vessel containing the cell was thermostated at 21 °C. After each change in pressure, the vessel was given a time span of 20 min to reach thermal equilibrium (21.0 ± 0.1 °C). The pressure runs were performed in an ascending and

reducing fullerenes.35b The radiolytic reduction of [1] with (CH3)2·COH to [RuII(μ-LH2·3−)RuII]− results in an transient absorption band with a maximum at 500 nm (Figures S26 and S27), similar to the one observed during spectroelectrochemical reduction.

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CONCLUSION We expanded the known area of Creutz−Taube analogues with a dinuclear ruthenium complex bearing a hitherto largely unused pyrazine derivative. Because of the strong π-acceptor characteristics of the bridging ligand, the complex offers a variety of controllable redox processes and possible valence tautomers (Scheme 3). The complex can easily be reduced into its anionic form [1•−] with a bridging ligand-centered radical, as experimentally (negative mode ESI-MS, EPR, pulse radiolysis) and computationally confirmed. The neutral form of the complex [RuII(μLH22−)RuII]0 persists in the crystal structure. However, the concept of an MLCT excited-state type of configuration such as [RuIII(μ-LH2•3−)RuII]* [1*] should be considered in solution, since a strong MLCT band at 600 nm indicates its existence.7b,10,11,30 Consequently, the one-electron oxidation leads to three possible valence tautomers. The charge-localized [RuIII(μ-LH22−)RuII]+ or charge-delocalized [RuII.5(μ-LH22−)RuII.5]+ mixed-valent forms are consistent with their EPR-silent nature, whereas the absence of IVCT bands in the NIR region is more consistent with the [RuIII(μ-LH2•3−)RuIII]+ alternative. High-pressure electrochemical studies and the calculated MOs (ωB97XD/def2svp) support the [RuII.5(μ-LH22−)RuII.5]+ valence tautomer form. Thus, the assignment of a certain formulation is rather ambiguous. Deprotonation facilitates the one-electron oxidation that leads to the asymmetrical, chargelocalized formulation [RuIII(μ-LH3−)RuII]0 [1+-H+] as suggested by EPR, UV/vis-NIR, and DFT calculations on the ωB97XD/def2svp level. Furthermore, electrochemical investigations under elevated pressure allowed us to gain valuable insight into conformational changes during the redox reactions, being consistent with the corresponding DFT calculations (ωB97XD/def2svp). Thus, the new dinuclear ruthenium complex synthesized and characterized in the present study offers a variable and easily controllable redox chemistry including formation of a stable and easily accessible distinct anion radical state, making it suitable as a model complex for PCET processes.3 Investigations of the mechanisms of PCET processes of complexes bearing the central [RuII(μ-LH22−)RuII]0 moiety are in progress in our laboratories. Furthermore, the simple possibility to switch between a diffuse, radical anionbridged or charge-delocalized and a distinct RuIII/RuII mixedvalent configuration by a simple deprotonation is, to the best of our knowledge, unique.

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EXPERIMENTAL SECTION

Chemicals. All chemicals and solvents used were of practical grade (p.a.) quality and were purchased from Sigma-Aldrich, Fischer, Roth, or Merck if not mentioned otherwise. The syntheses were performed either under aerobic conditions or under nitrogen atmosphere using standard Schlenk techniques. X-ray Crystallography. The single crystal was mounted on a loop with mineral oil and transferred to a N2 cold stream (100 K) by an Oxford Cryosystem low-temperature apparatus. Data were collected on a Bruker D8 Venture Dual I\mS fixed chi instrument equipped with a Bruker PHOTON100 detector with Mo Kα radiation (λ = 0.710 73 Å) controlled by the APEX2 software package.37 The data set was collected at 100 K. Reduction of the data was performed by I


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Inorganic Chemistry

534.9838 ([2 M + Na]+; calcd. m/z = 534.9828) Anal. Calcd C8H3N2O8K (294.22 g mol−1): calcd. C 32.66, H 1.03, N 9.52; found: C 32.72, H 1.12, N 9.29%. Synthesis of [(PPh3)2(Cl)Ru(pztcH2)Ru(Cl)(PPh3)2] ([Ru(μ-LH2)Ru]). Under nitrogen atmosphere, [Ru(PPh3)3Cl2] (958 mg, 1.00 mmol) and pztcH4 (128 mg, 0.50 mmol) were dissolved in 50 mL of degassed EtOH−H2O (80/20 v/v) and refluxed for 16 h. Afterward, the solution was cooled to RT and left at −20 °C overnight. The dark blue precipitate was filtered off, washed three times with 15 mL of EtOH−H2O (80/20 v/v) each and air-dried. Purification was achieved by recrystallization from CHCl3−Et2O and yielded 614 mg (78%) of a deep blue crystalline powder. 1H NMR (CDCl3, 300 MHz) δ [ppm] = 7.25 (m, 18H, PPh3), 7.10 (t, 6H, J = 6.00 Hz, PPh3), 6.68 (t, 6H, J = 9.00 Hz, PPh3). 31P{1H}-NMR (CDCl3, 121.5 MHz) δ [ppm] = 44.56 (d, 1JP,P = 34.02 Hz, PPh3(AX)), 34.79 (d, 1JP,P = 34.02 Hz, PPh3(EQ)). 13 C{1H}-NMR (CDCl3, 75 MHz) δ [ppm] = 177.1, 173.7, 148.7, 134.7, 134.5, 134.4, 134.0, 133.8, 130.4, 129.5, 128.7, 128.6, 128.0, 127.8, 124.9. UHR-ESI-MS (CH2Cl2, +MS) m/z = 1541.1245 ([M − Cl]+; calcd. m/z = 1541.1258); (CH2Cl2, −MS) m/z = 1575.0664 ([M − H]−; calcd. m/z = 1575.0874)

descending pressure cycle, and each measurement was repeated at least two times. UV/Vis Spectroscopy and Spectroelectrochemistry. UV/vis spectra were recorded with an Avantes spectrometer consisting of a light source (AvaLight-DH-S-Bal), a UV/vis detector (AvaSpecULS2048), and an NIR detector (AvaSpec-NIR256-TEC). Spectroelectrochemical measurements were performed in an optically transparent thin-layer electrochemical (OTTLE)46 cell (CaF2 windows) with a platinum-mesh working electrode, a platinum-mesh counter electrode, and a silver-foil pseudoreference electrode. Anhydrous and degassed dichloromethane (H2O ≤ 0.005%, puriss., Sigma-Aldrich) distilled from CaH2 with 0.1 M NBu4PF6 as electrolyte was used as the solvent. Elemental Analysis. Elemental analyses were performed on Euro EA 3000 (Euro Vector) and EA 1108 (Carlo Erba) instruments (σ ± 1% of the measured content). Quantum Chemical Methods. The Gaussian09 suite of programs was used.47 All structures were fully preoptimized with the BP8648 density functional and the basis set def2SVP49 in combination with the density fitting approximation SVPFit.50 The structures were evaluated to be local minima by computing the vibrational frequencies. The final optimization was performed using the long-range corrected functional ωB97XD51 and the basis set def2SVP.52 All wave functions were successfully tested to be stable.53 For calculating the spin populations according to the Löwdin population analysis,54 the program package ORCA 3.0.355 was used. Therefore, single-point calculations were performed on the optimized geometries using the B3LYP functional.56 These calculations were run with empirical van der Waals correction (D3).57 The restricted and unrestricted DFT methods were employed for closed- and open-shell molecules, respectively. Relativistic effects were included with the zeroth-order regular approximation (ZORA).58 Triple-ζ valence basis sets (TZVP-ZORA)59 were employed for all atoms. Calculations were performed using the RIJCOSX (combination of the resolution of the identity and chain of spheres algorithms) approximation.60 Photochemistry. For the photochemical experiment, 15 mL of a 1.0 μM stock solution of the complex in DCM was prepared and partitioned into three gastight vials under strict exclusion of light. Into each vial 5 equiv of FcBF4 (stock solution in DCM) were given. The first batch was kept in the dark, the second batch was kept at ambient light, and the third was irradiated with a 150 W xenon lamp (BioLogic). After 30 min, each sample was investigated by UHR-ESIMS. Synthesis of pztcH4 (μ-LH4): Pyrazine-2,3,5,6-tetracarboxylic acid (pztcH4) was synthesized according to a modified known procedure.22 2,3,5,6-Tetramethylpyrazine (1.50 g, 11.00 mmol), Na2CO3 (2.12 g, 20.00 mmol), and the phase-transfer catalyst Aliquat 336 (0.60 mL) were suspended in 40 mL of cold H2O. Then, KMnO4 (23.30 g, 147.00 mmol) was added slowly over 1 h, and the mixture was carefully heated to 100 °C. The solution was heated under reflux, until all KMnO4 was consumed. The formed MnO2 was filtered off, and the filtrate was washed three times with 25 mL of hot water. The resulting clear solution was concentrated under reduced pressure. Subsequently, the solution was cooled in an ice bath and carefully acidified with concentrated HCl until the crude product, the potassium salt KpztcH3, appeared as a colorless precipitate. The precipitate was filtered off, washed with cold water, and dried in vacuum. To generate the potassium-free and easily soluble pztcH4, the crude product was dissolved in 50 mL of hot H2O and reprecipitated as the Ca-salt (Ca2pztc) by addition of a slight excess Ca(NO3)2. The solution was cooled to room temperature, and the Ca2pztc was filtered off, washed with cold water, and dried in vacuum. Again, the Ca-salt was suspended in 50 mL of H2O and slowly acidified with H2SO4; the precipitating CaSO4 filtered off, washed with low amounts of cold water, and the resulting filtrate was concentrated under reduced pressure and left for crystallization at 0 °C. The final product pztcH4 was obtained as white crystals in a moderate yield of 1.29 g (46%). 13 C{1H}-NMR (D2O, 75 MHz) δ [ppm] = 165.4 (−COOH), 146.1 (Cpz). UHR-ESI-MS (MeCN, + MS) m/z = 278.9867 ([M + Na]+; calcd. m/z = 278.9860), 294.9606 ([M + K]+; calcd. m/z = 294.9599),

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02192. Mass spectra, 1H NMR, 13C NMR, 31P NMR, crystallographic details, DFT-optimized structures, calculated bond lengths, angles, MOs, additional CV and pressure-dependent CV, pulse radiolysis (PDF) Accession Codes

CCDC 1531006 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Biprajit Sarkar: 0000-0003-4887-7277 Ivana Ivanović-Burmazović: 0000-0002-1651-3359 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was generously supported by “Solar Technologies go Hybrid”, an initiative of the Bavarian State Ministry for Science, Research and Art. I.I.-B. acknowledges support through the Deutsche Forschungsgemeinschaft (DFG) Project No. IV80/ 12-1 “Pressure effects on thermal and photochemical protoncoupled electron transfer reactions with metal complexes”. We also thank Prof. T. Clark for hosting this work at the Computer Chemistry Center (CCC) and the Regionales Rechenzentrum Erlangen (RRZE) for a generous allotment of computer time. We would like to thank Prof. Abel and his group from Leipzig Univ., Leipzig, Germany, and the Leibniz Institute of Surface J


Article

Inorganic Chemistry

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Modification (IOM) in Leipzig, Germany, for support during the pulse radiolysis measurements.

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DEDICATION Dedicated to Prof. Dr. Günter Grampp on the occasion of his 70th birthday.

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Inorganic Chemistry

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[(acac)2RuII (-bpytz)RuII(acac)2] and [(acac)2RuII(-bpytz)RuIII(acac)2](ClO4) (acac = acetylacetonate and bpytz = 3,6-bis(3,5dimethylpyrazolyl)-1,2,4,5-tetrazine): synthesis, spectroelectrochemical, and epr investigation. Inorg. Chem. 2004, 43, 6108−6113. (c) Ghumaan, S.; Sarkar, B.; Patil, M. P.; Fiedler, J.; Sunoj, R. B.; Kaim, W.; Lahiri, G. K. Multiple one-electron oxidation and reduction of trinuclear bis(2,4-pentanedionato)ruthenium complexes with substituted diquinoxalino[2,3-a:2′,3′-c]phenazine ligands. Polyhedron 2007, 26, 3409−3418. (35) (a) Nakayama, T.; Ushida, K.; Hamanoue, K.; Washio, M.; Tagawa, S.; Tabata, Y. Pulse radiolyses of anthraquinone and anthraquinone−triethylamine in acetonitrile and toluene at room temperature. J. Chem. Soc., Faraday Trans. 1990, 86, 95−103. (b) Guldi, D. M.; Hungerbuehler, H.; Asmus, K.-D. Redox and Excitation Studies with C60-Substituted Malonic Acid Diethyl Esters. J. Phys. Chem. 1995, 99, 9380−9385. (c) Porter, G.; Dogra, S. K.; Loutfy, R. O.; Sugamori, S. E.; Yip, R. W. Triplet state of acetone in solution. Deactivation and hydrogen abstraction. J. Chem. Soc., Faraday Trans. 1 1973, 69, 1462. (36) Wardman, P. Reduction Potentials of One-Electron Couples Involving Free Radicals in Aqueous Solution. J. Phys. Chem. Ref. Data 1989, 18, 1637−1755. (37) APEX2 v.4.0; Bruker AXS, Inc.: Madison, WI, 2008. (38) SAINT: SAX Area-Detector Integration Program; University of Göttingen: Göttingen, Germany, 2008. (39) Sheldrick, G. M. SADABS: Area-Detector Absorption Correction; University of Göttingen: Göttingen, Germany, 2008. (40) XPREP, 2nd ed., Bruker AXS, Inc.: Madison, WI, 2008. (41) Sheldrick, G. M. SHELXT - Integrated space-group and crystalstructure determination. Acta Crystallogr., Sect. A: Found. Adv. 2015, 71, 3−8. (42) Sheldrick, G. Crystal structure refinement with SHELXL. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8. (43) Spek, A. L. PLATON: A Multipurpose Crystallographic Tool; University of Utrecht: Utrecht, The Netherlands, 2008. (44) Fielden, E. M. Chemical Dosimetry of Pulsed Electron and XRay Sources in the 1−20 MeV Range. InThe Study of Fast Processes and Transient Species by Electron Pulse Radiolysis, Proceedings of the NATO Advanced Study Institute, Capri, Italy, Sept 7−18, 1981; Baxendale, J. H., Busi, F., Eds.; Springer Netherlands: Dordrecht, 1982; pp 49−62. (45) Górecki, J.; Gołaś, J. Stand for high pressure electrochemical measurements. Electroanalysis 1996, 8, 630−634. (46) Krejčik, M.; Daněk, M.; Hartl, F. Simple construction of an infrared optically transparent thin-layer electrochemical cell. J. Electroanal. Chem. Interfacial Electrochem. 1991, 317, 179−187. (47) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A. V.; Bloino, J.; Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J. V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams; Ding, F.; Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson, T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.; Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Throssell, K.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M. J.; Heyd, J. J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Keith, T. A.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M.; Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J. Gaussian 09, Revision D.01; Gaussian Inc.: Wallingford, CT, 2013. (48) (a) Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3100. (b) Perdew, J. P. Density-functional approximation for the correlation energy of the inhomogeneous electron gas. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 33, 8822−8824. (49) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: M


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