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. Author manuscript; available in PMC: 2020 Apr 28.
Published in final edited form as: J Am Chem Soc. 2019 Dec 5;141(50):19859–19869. doi: 10.1021/jacs.9b10320

Isolation and Study of Ruthenium–Cobalt Oxo Cubanes Bearing a High-Valent, Terminal RuV–Oxo with Significant Oxyl Radical Character

Jaruwan Amtawong †, David Balcells ‡, Jarett Wilcoxen §, Rex C Handford †, Naomi Biggins †, Andy I Nguyen †,‖, R David Britt §, T Don Tilley †,‖,*
PMCID: PMC7187899  NIHMSID: NIHMS1579721  PMID: 31697896

Abstract

High-valent RuV–oxo intermediates have long been proposed in catalytic oxidation chemistry, but investigations into their electronic and chemical properties have been limited due to their reactive nature and rarity. The incorporation of Ru into the [Co3O4] subcluster via the singlestep assembly reaction of CoII(OAc)2(H2O)4 (OAc = acetate), perruthenate (RuO4−), and pyridine (py) yielded an unprecedented Ru(O)Co3(μ3-O)4(OAc)4(py)3 cubane featuring an isolable, yet reactive, RuV–oxo moiety. EPR, ENDOR, and DFT studies reveal a valence-localized [RuV(S = 1/2)CoIII3(S = 0)O4] configuration and non-negligible covalency in the cubane core. Significant oxyl radical character in the RuV–oxo unit is experimentally demonstrated by radical coupling reactions between the oxo cubane and both 2,4,6-tri-tert-butylphenoxyl and trityl radicals. The oxo cubane oxidizes organic substrates and, notably, reacts with water to form an isolable μ-oxo bis-cubane complex [(py)3(OAc)4Co3(μ3-O)4Ru]–O–[RuCo3(μ3-O)4(OAc)4(py)3]. Redox activity of the RuV–oxo fragment is easily tuned by the electron-donating ability of the distal pyridyl ligand set at the Co sites demonstrating strong electronic communication throughout the entire cubane cluster. Natural bond orbital calculations reveal cooperative orbital interactions of the [Co3O4] unit in supporting the RuV–oxo moiety via a strong π-electron donation.

Graphical Abstract

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INTRODUCTION

Ruthenium is prominent among the elements associated with catalytic oxidation chemistry, with substrates ranging from organic compounds to water.1 In these oxidations, high-valent RuV–oxo intermediates are commonly invoked in substrate bond activation steps such as those involving H atom abstraction or O–O bond formation.2–4 Interestingly, several computational studies predict that the d3 RuV–oxo intermediates possess considerable spin density on the terminal oxo ligand and attribute their reactivity to this radical character, as in O–O bond formation via the radical coupling of two RuV–oxo moieties.5–11 However, the reactive nature of these oxo species has generally hindered their observation, study, and experimental confirmation as intermediates, despite significant effort.12–16 Indeed, transient intermediate RuV–oxo species have only been implicated by electrochemical and spectroscopic methods.4,6,8,9,17–24 Because of their reactive nature, isolable RuV–oxo complexes are rare, and investigations into their electronic and chemical properties have been limited.25–28 The isolation and detailed investigation of RuV–oxo species should provide insight into the nature of these important chemical intermediates and the factors that influence their reactivity.

The incorporation of Ru into heterometal oxides or hydroxides (e.g., Mn,29 Co,30,31 Ni32) often results in superior catalytic oxidation activity, which suggests a significant influence of metal–metal cooperative effects in this chemistry. Such synergism is also observed in nature where water oxidation by the oxygen-evolving complex (OEC) in photosystem II, a [Mn4CaO5] oxo cluster, involves the cooperative accumulation of redox equivalents to access formally high oxidation states and metal–oxo/oxyl species.33,34 Mechanistic analysis and characterization of relevant intermediates are quite challenging for these multimetallic systems; thus, the origins of cooperativity remain considerably unresolved. While binuclear oxo complexes of Ru, such as Meyer’s “blue dimer” [(bpy)2(H2O)RuIIIORuIII(H2O)(bpy)2]4+ (bpy = 2,2′-bipyridine), have been extensively studied,35–38 the only examples of Ru incorporation into heterobimetallic oxo clusters for oxidation chemistry involve ruthenium–polyoxotung-states.39–41 The inherent complexity of these polyoxometalate frameworks makes understanding the exact roles of each metal center difficult. Thus, there is a strong need for heterometallic oxo models that can contribute to an understanding of synergistic mechanisms that enable efficient oxidations.

Previously, this laboratory has employed the [Co3O4] subcluster to support high valent CoIV and MnIV centers in oxo–cubane complexes,42–44 and the related [Co4O4] cubane has been well studied for oxygen evolution.44 Thus, it seemed that incorporation of Ru into an oxo cubane framework could perhaps generate a well-defined and reactive heterometallic oxo cubane suitable for supporting a high-valent Ru moiety (Figure 1).

Figure 1.

Figure 1.

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High-valent CoIV, MnIV, and RuV centers supported by a [Co3O4] subcluster.

This report describes the first mixed-metal [RuCo3O4] oxo cubane featuring a rare RuV–oxo fragment with substantial oxyl radical character. The oxo cubane is highly active in the oxidation of various organic substrates. The reactivity of the oxo cubane with water was also examined. The synthetic versatility and simple preparation allow modulation of redox chemistry, predicted to occur at the Ru center, by tuning the donor properties of distal ligands at the Co sites. Spectroscopic and computational methods provide insight into structure, bonding, and reactivity for these new complexes.

RESULTS AND DISCUSSION

Synthesis and Characterization.

A potential route to heterobimetallic oxo cubane complexes was established with the mixed-metal oxo cubane Mn(OAc)Co3(μ3-O)4(OAc)4(py)3 via an oxidative assembly reaction of CoII(OAc)2·4H2O, MnVIIO4−, and pyridine.43 Analogous assembly with RuVIIO4− and pyridine or a para-substituted pyridine in acetonitrile afforded Ru(O)Co3(μ3-O)4(OAc)4(4-R-py)3 cubanes (la–d, R = H, CF3, Me, and OMe, respectively) with terminal Ru–oxo bonds in moderate yields (Scheme 1). In these reactions, a RuVII perruthenate ion oxidizes 3 equiv of CoII to CoIII to provide the [RuCo3O4] cubane core. An additional equivalent of RuO4− serves as a source of the terminal oxo ligand. The molecular compositions of 1a–d were verified by elemental analysis and high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) (Figure S1). These complexes are stable in air when kept at room temperature. The stability and solubility of 1a–d in organic solvents such as chloroform, dichloromethane, and o-difluorobenzene allow further characterizations and reactivity studies.

Scheme 1.

Scheme 1.

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Synthesis of [RuCo3O4] Oxo Cubanes

Analysis by 1H NMR spectroscopy indicated that 1a–d are Cs-symmetric (Figures S2–S5), consistent with the solid-state molecular structures (Figure 2). The bond distances within the cubane cores of 1a,b and 1d are essentially identical. The Co–μ3–O bond lengths span the range of 1.850(5)–1.904(6) Å, while the Ru–μ3–O bond lengths are longer (1.933(6)–2.013(5) Å) to accommodate the larger Ru center. The relatively short Ru1–O1 bond distances of 1.699(4)–1.719(5) Å are consistent with the presence of terminal RuV–oxo bonds.45 Four other RuV–oxo compounds have been isolated and structurally characterized, and the Ru–oxo bond distances are all between 1.676(6) and 1.756(4) Å.25–28 Notably, the O1–Ru1–O2 and O1–Ru1–O4 angles (108.8(3)°–110.3(2)°) indicate a Jahn–Teller distortion that likely results from a minimization of the π-antibonding character in the SOMO vide infra). A similar type of distortion occurs in a binuclear ruthenium–oxo complex supported by a cobalt-based tripodal ligand.28

Figure 2.

Figure 2.

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Solid-state molecular structures of Ru(O)Co3(μ3-O)4(OAc)4(4-R-py)3 cubanes, R = H (1a), CF3 (1b), and OMe (1d). An additional molecule of 1a in the asymmetric unit, solvent molecules, and hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at 50% probability.

The vibrational frequencies of 853–857 cm−1 for 1a–d are within the reported range for other RuV–oxo complexes, 848–900 cm−1,28,45 and thus support the existence of RuV–oxo bonds in 1a–d (Figure 3 and Figure S6). The relatively strong bands below 840 cm−1 in the spectra of 1b–d are tentatively assigned to the bending vibrations associated with para-substituted pyridines.46 As further evidence for the peak assignments, a comparison of the IR spectra of 1a and the closely related, Cs-symmetric Mn(Cl)Co3(μ3-O)4(OAc)4(py)3 cubane,43 was made (Figure S7). While there is considerable correspondence between the two spectra, there is a notable absence of the 853 cm−1 band in the spectrum of the [MnCo3O4] cubane.

Figure 3.

Figure 3.

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Partial solution-state IR spectra of 1a–d in dichloromethane demonstrating intense RuV–oxo absorptions.

Continuous wave (CW) electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopies were used to interrogate the electronic structure of 1a. The CW X-band EPR spectrum of 1a exhibits a rhombic signal with g values of [2.032 1.923 1.740], diagnostic of an S = 1/2 system (Figure 4a), with no resolvable 59Co hyperfine interactions (HFIs). However, coupling to the 99Ru (I = 5/2, 13%) and 101Ru (I = 5/2, 17%) isotopes is observed, with estimated hyperfine coupling constants of A = [140 50 200] MHz, and is evident by broad shoulders near 320 and 395 mT. The g values and HFI are similar in magnitude to the S = 1/2 species of the “blue dimer” for which the unpaired spin is localized on a single Ru site.13 The relatively strong Ru HFI from the CW data suggests an S = 1/2 RuV center with three S = 0 59CoIII centers. This valence localized assignment (Robin–Day class I) is also supported by UV–vis–NIR measurements (Figure S8) and DFT calculations discussed in the Supporting Information (Figure SCD1).

Figure 4.

Figure 4.

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(a) EPR spectrum (black) of 1a taken at 20 K and simulation of the data (red) using g-values of [2.032 1.923 1.740]. Simulations include contributions of natural abundance 99Ru (I = 5/2, 13%) and 101Ru (I = 5/2, 17%) isotopes (*). (b) Davies ENDOR spectrum of 1a recorded at 6 K (black). The peak at ~23 MHz can be simulated (red) using three isotropic 59Co centers with hyperfine couplings of 17, 20, and 24 MHz, individual components shown in blue.

Q-band Davies ENDOR experiments were performed to probe the contributions of 59Co HFIs (Figure 4b). The peak at ~25 MHz arises from the coupling of 59Co nuclei and varies slightly across the absorption envelope from ~1200 to ~1400 mT (Figure S9), suggesting highly isotropic couplings. Considering the number of 59Co nuclei and the symmetry of 1a, the 59Co HFIs were simulated with isotropic hyperfine coupling values of 17, 20, and 24 MHz, which are comparable to the coupling observed in the Mn(OAc)Co3(μ3-O)4(OAc)4(py)3 cluster.43 The peak at ~55 MHz is assigned to weak HFIs to nearby protons centered at the proton Larmor frequency (vL = 54.1 MHz at 1270 mT). The relatively weak–yet still significant–59Co HFI from the ENDOR experiment indicates a non-negligible degree of covalency in the [RuVCoIII3O4] core.

Electrochemical Studies.

Investigation of the electrochemical properties of 1a–d in o-difluorobenzene by cyclic voltammetry revealed a reversible one-electron oxidation and an irreversible reduction for each cluster (Figures 5a and S10). Cyclic voltammograms of 1d in acetonitrile and dichloromethane show redox events similar to those measured in o-difluorobenzene, but the potentials are more positively shifted. The effect of solvent on redox potentials is discussed in the Supporting Information (Figure S11). The observed redox activity of 1a–d displayed a strong dependence on the electron-donating property of the ligand set as evidenced by a linear correlation between the redox potentials and the conjugate acid pKa values of 4-R-pyridine in water47 (Figure 5b). The simple modulation of the donor properties of the ligand allows predictable redox tunability of the cubane clusters, such that the oxidation is facilitated by electron-donating substituents, whereas the reduction is rendered more facile by electron-withdrawing substituents. The related [Co4O4]48 and [Mn4O4]49 cubane series also exhibit similar correlations between redox potentials and ligand basicity. The observed correlations in these clusters, despite the asymmetric coordination of ligands, suggest strong electronic communication within the cubane cores.

Figure 5.

Figure 5.

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(a) Cyclic voltammograms of 1a–d in o-difluorobenzene (v = 100 mV/s, 0.1 M [nBu4N]PF6 electrolyte). Oxidation potentials are 0.53, 0.63, 0.48, and 0.46 V vs Fc/Fc+, and reduction potentials are −1.06, −0.99, −1.09, and −1.12 V vs Fc/Fc+ for 1a–d, respectively. (b) Plot of redox potentials of 1a–d vs conjugate acid pKa values of 4-R-pyridine with linear regression (R2 = 0.9940 for oxidation and R2 = 0.9884 for reduction).

Because of the comparatively low oxidation potential and reversibility of the redox event of 1d, chemical oxidation and isolation of the oxidized cubane were attempted. Reactions of 1d with various chemical oxidants yielded unidentifiable product mixtures as indicated by NMR spectroscopy. However, treatment of 1d with thianthrenium tetrafluoroborate in acetonitrile resulted in formation of an acetonitrile-bound complex (2-MeCN) and thianthrene oxide in nearly quantitative yields (Scheme 2). The solid-state molecular structure of 2-MeCN (Scheme 2, inset) reveals structural similarities to 1d. The Ru1–N4 distance of 2.068(11) Å is consistent with a RuIV–NCCH3 bond.50 Notably, the N4–Ru1–O2 and N4–Ru1–O4 bond angles span the range of 93.8(3)–95.6(3)° indicating that the Jahn–Teller distortion is diminished, in comparison with 1d, as is expected for a low-spin d4 configuration. The IR-active Ru–oxo band of 1d at 853 cm−1 is absent in 2-MeCN (Figure S12). The formation of 2-MeCN and thianthrene oxide suggests that the direct oxidation product of 1d is a reactive intermediate and acts as an O atom donor to thianthrene in this reaction. The question of whether the oxidation produces a RuVI or CoIV center will be addressed in the DFT section (vide infra).

Scheme 2. Synthesis of 2-MeCNa.

Scheme 2.

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aInset: Solid-state molecular structure of 2-MeCN. Solvents and hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at 50% probability.

Experimental Evidence for Oxyl Radical Character in the RuV–Oxo Moiety.

In oxidative catalysis, the RuIV–oxyl valence tautomer is implicated as an important contributor to the ground state of RuV–oxo intermediates.5,7,8,11,51 However, experimental evidence for validating these computational proposals is quite limited,9,52 and such radical properties are unprecedented in the few isolated RuV–oxo complexes. Thus, we sought to determine the reactivity of 1a by performing reactions with radical organic substrates.

Treatment of 1a with both 2,4,6-tri-tert-butylphenoxyl and trityl radicals in o-difluorobenzene afforded radical-coupled cubane complexes 3 and 4 in 95% and 92% yields, respectively (Scheme 3). Analysis by 1H NMR spectroscopy revealed that both 3 and 4 are Cs symmetric, in agreement with the solid-state molecular structures (Scheme 3, inset). The Rul–O13 distances of 1.908(3) and 1.922(10) Å for 3 and 4 are consistent with RuIV–μ2–OR bonds.53 While most bond distances within the cubane cores remain largely unchanged from those of 1a, the Rul–μ3–O3 bond lengths of 3 and 4 are slightly shorter (by ~0.06 Å) due to a weaker trans influence of an alkoxide versus an oxo ligand. For 3, radical coupling via the 4-position is not unexpected because the cyclohexa-2,5-dien-1-one-4-yl structure is the largest resonance contributor to the 2,4,6-tri-tert-butylphenoxyl radical.54,55 Infrared spectroscopic data corroborate the structural assignments of 3 and 4 (Figures S13 and S14).

Scheme 3. Coupling Reactions of Organic Radicals and Terminal RuV–Oxo Cubane (1a)a.

Scheme 3.

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aInset: Solid-state molecular structures of compounds 3 and 4. Solvents and hydrogen atoms are removed for clarity. Thermal ellipsoids are shown at 50% and 30% probability for 3 and 4, respectively.

The apparent radical nature in the RuV–oxo fragment suggests that 1a is a potential H atom abstractor. Indeed, treatment of 1a with 1 equiv of 2,4,6-tri-tert-butylphenol in o-difluorobenzene gave 3 in 57% yield (91% and 60% conversions with respect to 1a and 2,4,6-tri-tert-butylphenol, by 1H NMR spectroscopy), after 1 h at 23 °C (Scheme 3). The formation of 3 strongly indicates that H atom abstraction of 2,4,6-tri-tert-butylphenol by 1a generated the 2,4,6-tri-tert-butylphenoxyl radical in situ, which was subsequently and quickly trapped by 1a. The observed yield and conversions are consistent with a 1:2 mol ratio of 1a to phenol in generating two moles of phenoxyl radical. This stoichiometry suggests the formation of a formally RuIII–OH2 cubane as a byproduct. Though this species was not isolated, HR-ESI-MS analysis of the reaction mixture in acetonitrile revealed the presence of a [RuIII(MeCN)Co3(μ3-O)4(OAc)4(py)3]·H+ ion (Figure S15). These results prompted further investigations into the oxidative reactivity of 1a.

Reactivity Studies.

Given the importance of RuV–oxo species as postulated intermediates in catalytic water oxidation, the reactivity of 1a with water was examined. Addition of ≤100 equiv of water to solutions of 1a in dichloromethane resulted in very slow reaction, but dissolution of 1a in water at ambient temperature gave rapid gas evolution, and headspace analysis by gas chromatography identified CO2 as the primary gaseous component (Figure S16). The 1H NMR spectrum of the reaction mixture after 1 h exhibits a new diamagnetic species, which was isolated and characterized as the μ-oxo bis-cubane complex [(py)3(OAc)4Co3(μ3-O)4Ru]–O–[RuCo3(μ3-O)4(OAc)4(py)3] (5). The chemical composition of 5 was established by elemental analysis and HR-ESI-MS (Figure S17). The solid-state structure reveals that complex 5 is C2-symmetric (Figure 6), with Ru–O13 bond distances of 1.807(3) and 1.808(3) Å and a Ru–O–Ru bond angle of 162.0(2)°. Compound 5 gives a well-resolved 1H NMR spectrum (Figure S18), consistent with the solid-state structure and a RuIV–O–RuIV formulation.56

Figure 6.

Figure 6.

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Solid-state structure of 5. Solvent molecules and hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at 50% probability.

Complex 5 was isolated in 30% yield after a reaction time of 1 h, but this species is gradually consumed in the reaction mixture (with concomitant formation of CO2) such that it is completely absent after 16 h. Additional byproducts of this reaction, observed by HR-ESI-MS, are Co4(μ3-O)4(OAc)4(py)4 and the RuIII(H2O)CoIII3O4 cubane species (Figure S19). The formation of CO2 and minor quantities of the aforementioned oxo cubanes imply the presence of an oxidizing agent that decomposes organic ligands (e.g., acetate).

Attempts to identify the oxidant were made, as this might give valuable insights into the role of water in generating 5. The formation of 5 from the reaction of 1a with water suggests possible O atom transfer to H2O to produce H2O2, though analysis of the reaction mixture of 1a in water by 1H NMR spectroscopy did not show detectable quantities of H2O2 (Figure S20). Nevertheless, H2O2 could be a reactive intermediate in this chemistry, since it rapidly reacts with compound 1a in organic solvents to generate a product mixture consistent with those observed in the reaction of 1a and water (by HR-ESI-MS, Figures S20 and S21). Another potential oxidant is O2, but the reaction of 1a with water showed no observable O2 production (monitored by a calibrated O2 sensing probe). Additionally, dissolution of 1a in O2 saturated water did not change the rates of decomposition of 1a and 5 compared with reactions in degassed water. Thus, O2 is not likely responsible for oxidation of organic ligands to CO2. Lastly, compound 1a itself could also be the reactive oxidant in the presence of water, as H-bonding to terminal oxo moieties can regulate the reactivity of metal-oxo complexes by reducing the π-basicity of the oxo ligands.57,58 The weakening of the RuV–oxo bond may enhance O atom transfer reactivity leading to self-oxidation and decomposition.

Compound 1a functions as a stoichiometric oxidant toward organic substrates via O atom transfer and C–H activation (Figure 7). These oxidation reactions are accompanied by concomitant reduction of the RuV–oxo to a mixture of formally RuIII cubane species, such as Ru(L)Co3(μ3-O)4(OAc)4(py)3 (L = H2O, py, vacant), and compound 5 (by HR-ESI-MS, Figures S22–S26). An O atom transfer reaction from 1a to triphenylphosphine in acetonitrile resulted in quantitative formation of triphenylphosphine oxide (by 1H and 31P NMR spectroscopy). Analysis of an aliquot of the product mixture by HR-ESI-MS revealed the presence of two main ions containing formally RuIII species, [Ru(L)Co3(μ3-O)4(OAc)4(py)3]·H+ (L = MeCN, py) (Figure S27). An EPR spectrum of the reaction mixture reveals an axial signal, as expected for a RuIII (S = 1/2) species (Figure S28). However, attempts to isolate the RuIII(L)Co3(μ3-O)4(OAc)4(py)3 (L = H2O, MeCN, or py) cubane products of these oxidation reactions in pure form have not been successful.

Figure 7.

Figure 7.

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Oxidation reactions of Ru(O)Co3(μ3-O)4(OAc)4(py)3 (1a) with organic substrates in CDCl3 at 23 °C under an atmosphere of N2. Yields were determined per 1 equiv of 1a by 1H NMR analysis at 24 h using tetrakis(trimethylsilyl)silane as an internal standard. Reaction mixtures were heated to 50 °C for isopropyl alcohol and styrene.

The formation of 5 in organic oxidation reactions suggests a loss of the terminal oxo from the Ru center either via O atom transfer or dissociation of a subsequent aqua ligand. The resulting RuIII–cubane complex may then be intercepted with an equivalent of 1a to form 5 (Scheme 4). Further confirmation of the proposed mechanistic step involving RuIII–cubane formation in the generation of 5 was found by treatment of 2 equiv of 1a with 1 equiv of triphenylphosphine in dichloromethane. This reaction afforded 5 in 28% isolated yield, along with quantitative formation of triphenylphosphine oxide (Scheme 5), confirmed by 1H and 31P NMR spectroscopy and HR-ESI-MS (Figure S29). A similar scenario may also describe the formation of 5 in the reaction of 1a with water in which the RuIII–cubane is an intermediate, presumably generated through water activation by 1a followed by liberation of H2O2 (Scheme 5). This type of dimerization has been observed in Fe–porphyrin peroxidase mimics and Fe complexes with the tetraamido macrocyclic ligand (TAML) in which formation of the FeV–oxo intermediate is followed by comproportionation with the FeIII monomer.59–62

Scheme 4.

Scheme 4.

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Proposed Mechanism for Transformation of 1a to 5

Scheme 5.

Scheme 5.

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Synthetic Routes To Form the μ-Oxo Bis-cubane Complex (5)

Density Functional Theory and Natural Bond Orbital Calculations.

In agreement with the experimental data, optimization of the doublet state of 1a yielded an energy minimum in which the spin density of the unpaired electron (ρ) is mostly located on Ru (ρ = 0.55α). In line with the magnetic measurements, the calculations predict ρ = 0.00 for all three Co centers. The optimization of the quartet state also converged into a stable energy minimum, with most of the spin density localized over the Ru–oxo moiety (ρ = 2.56α), but this configuration is less stable than the doublet state by 6.8 kcal/mol. The fully optimized geometry of the doublet state is in good agreement with the solid-state molecular structure, with root-mean-square (RMSD) and maximum (MaxD) deviations of 0.042 and 0.014 Å, respectively, over all metal–ligand bond distances; deviations were higher for the quartet state (RMSD = 0.067 Å and MaxD = 0.027 Å). In the doublet ground state, the terminal oxo ligand also contains a significant portion of the spin density (ρ = 0.38α), which suggests significant oxyl character (Figure 8a). Consistently, the shapes of the spin density and the SOMO are nearly identical (Figure 8b). The rest of the spin density is delocalized over the structure, with the bridging oxo ligands of the cubane core having the largest contributions (i.e., 0.01–0.02α each). The antibonding nature of the SOMO accounts for the observed bending of the terminal oxo relative to μ3-O2 and μ3-O4, as seen in both the DFT (111.1°) and solid-state structures (108.8(3)–110.3(2)°; Figure 2). This bending reduces the repulsive antibonding interaction arising from the out-of-phase combination of the Ru(d) and O(p) orbitals. The analytic calculation of the frequencies by DFT shows that the stretching mode of the terminal RuV–oxo bond is IR-active (IR intensity = 372 km mol−1), with a predicted frequency of 882 cm−1. In line with the experiments, the calculations also show that the para-substituents of the pyridine ligands have a minor effect on the Ru–O stretching frequency (Figure S30). The strongest shift relative to 1a was found for 1b, with Δν = 3 cm−1 (vs Δν = 4 cm−1 in the experimental spectra). These calculations support a doublet ground state with a localized [RuV(S = 1/2)CoIII3(S = 0)O4] configuration and a RuV–oxo bond with significant oxyl character.

Figure 8.

Figure 8.

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(a) Spin density (ρ(Ru:O) = 0.55:0.38α) and (b) SOMO of 1a for the doublet ground state. The SOMO is a combination of Ru(d) and π-oxo(p) orbitals. Isovalues are 0.007 (ρ) and 0.05 (SOMO).

The one-electron oxidized and reduced derivatives of 1a seen by cyclic voltammetry, 1a+ and 1a−, respectively, were also examined with DFT calculations. The geometry of the oxidized cubane 1a+ was fully optimized, converging into a closed-shell singlet ground state with S = 0 at all atomic centers. The triplet state also yielded an energy minimum, though it was 3.9 kcal mol−1 less stable than the singlet state.63 Considering the ground states of both 1a (doublet) and 1a+ (singlet), the calculations predict an oxidation potential of 0.41 V vs Fc/Fc+, which is similar to the experimental value (0.53 V). For the oxidation of the substituted cubanes, 1b–d, DFT predicts oxidation potential shifts of +160 (CF3), −50 (Me), and −70 (OMe) mV, in good agreement with the cyclic voltammetry experiments (+100, −50, and −70 mV, respectively). These data and the nature of the SOMO of 1a (Figure 8b) are consistent with the oxidation of the [RuVCoIII3O4] valence to [RuVICoIII3O4]. The nature of the para substituents of the pyridine ligands has a significant influence on the energy of the SOMO in which the energy increases from −0.1909 Ha (1b) to −0.1817 Ha (1d) (Figure SCD2), in line with the decreasing oxidation potentials. For 1a−, calculations gave a triplet ground state with a [RuIVCoIII3O4] configuration. Upon reduction of 1a, there is a significant elongation of the Ru–O bond and localization of the spin density over the metal center (1.10α) and the oxo ligand (0.76α), consistent with an oxyl [RuIV(S = 1/2)O(S = 1/2)CoIII3(S = 0)O4]− configuration. These structural and electronic characteristics suggest that 1a− should be highly reactive, which may explain the observed irreversible reduction by electrochemistry. These results also suggest that the formally RuIV and RuVI oxo species may be accessible, with the Ru–O center acting as the reservoir for electrons in this system.

The accessibility of high-valent states of the Ru center prompted an investigation into the role of the [Co3O4] framework in supporting the Ru–oxo moiety. Specifically, natural bond orbital (NBO) calculations were carried out in the neutral 1a and oxidized 1a+, including second-order perturbation analysis to quantify the donor–acceptor interactions. The strongest interactions involve π-electron donation from the 2p orbitals of the μ3-oxos to the 4d orbitals of Ru. These interactions are divided into two groups, cis-π and trans-π, depending on the position of the donor π-O(p) orbitals relative to the Ru–oxo bond axis (shown in Figure 9 for 1a). In contrast with the cis-π interactions, the trans-π interactions have a significant contribution from the μ-oxo ligand and, in one case (trans-π2), there is a relevant contribution from a Co 3d orbital. These interactions were also observed for 1a+, though with larger stabilization energies and stronger contributions from the Co 3d orbitals due to the higher oxidation state of the Ru–O core (Figure SCD3). This NBO analysis shows that, despite the spin densities suggesting a redox-innocent character of the [Co3O4] subcluster, it in fact stabilizes the Ru–oxyl/oxo moieties by strong π(p → d) donation, facilitating the 1 → 1+ oxidation, in line with the relatively low potentials measured in the experiments (Figure 5).

Figure 9.

Figure 9.

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Selected NLMOs (natural localized molecular orbitals) associated with the covalent interactions between the [Co3O4] (donor) and Ru–oxo (acceptor) moieties. Orbitals were plotted for 1a, with a surface isovalue of 0.02.

The above orbital analysis also reflects the contribution of the [Co3O4] subcluster to the stabilization of the RuIV–oxyl moiety of 1a. One key characteristic for RuIV–oxyl intermediates implicated in oxidative catalysis is that the metal center is supported by electron-rich ligands.3 The high electron density introduced by these ligands facilitates the occupancy of the antibonding Ru–O orbitals. Similarly, the strong π(ρ → d) donation from the [Co3O4] framework stabilizes the π-Ru(dxz/dyz) orbitals and promotes the population of the antibonding π*-Ru–oxo(d–p) orbitals, which have a strong contribution from the terminal O(px/py) orbitals, consistent with the oxyl character of 1a. In comparison to other transient RuV–oxo intermediates;4,5,9–11,24,37 however, 1a possesses a significantly lower spin density at the terminal oxo, which may explain why 1a is relatively stable and isolable. The [Co3O4] subcluster appears to provide the optimal electron density to the dopant metal–oxo unit enabling the isolation of the RuV–oxo 1a complex, with oxyl character.

CONCLUDING REMARKS

The incorporation of Ru into the [Co3O4] subcluster has allowed isolation and full characterization of the first heterobimetallic oxo cubane containing Ru and Co ions, Ru(O)Co3(μ3-O)4(OAc)4(py)3 (1a). This complex is also the first example of an isolable terminal metal–oxo incorporated into an oxo cubane core, the key structural motif of reactive intermediates proposed in biological and synthetic water oxidation systems.34,44 Structural, spectroscopic, and computational studies support a spin localized [RuV(S = 1/2)CoIII3(S = 0)O4] configuration of 1a with a rare RuV–oxo moiety possessing significant oxyl radical character. The radical nature of 1a is evidenced by the propensity of the terminal oxo to accept an electron via direct coupling with organic radicals. This remarkable experimental demonstration supports a longstanding computational proposal of oxyl radical contribution to the ground state of RuV–oxo species which may be the key intermediates in oxidative catalysis.3 In accordance with this, complex 1a is active in the oxidation of organic substrates. The reactivity of 1a toward water implicates formation of a reactive oxidizing intermediate. Though the mechanism for this reaction remains unclear, isolation of the μ-oxo bis-cubane complex [(py)3(OAc)4Co3(μ3-O)4RuIV]–O–[RuIVCo3(μ3-O)4(OAc)4(py)3] (5) suggests that RuV–oxo species are reactive with water. The reactivity of 1a established in this report will allow future mechanistic studies to analyze the role of the oxyl character in oxidation chemistry.

The strong dependence of the redox chemistry, predicted to occur at the Ru–oxo center, on the donor properties of pyridine ligands at the Co sites demonstrates a significant electronic communication throughout the entire cubane cluster. Natural bond orbital (NBO) analysis revealed a strong π-electron donation from the [Co3O4] subcluster to stabilize the RuV–oxo fragment. Taken together, these results indicate that the [Co3O4] unit can be regarded as an ancillary ligand that provides electronic stabilization for the RuV–oxo center. Similar donor–acceptor interactions may be operative in Ru-incorporated cobalt oxides for enhanced oxidative reactivity,30,31 with the oxidative transformations occurring at the Ru sites while the cobalt oxides help provide an electronic environment optimally tuned for the chemical and redox steps. The strong orbital overlap between the metal and the oxide framework has been observed to promote water oxidation reactivity in Co-based spinel oxides64 and could be a critical factor for high-valent metal–oxo formation in both synthetic catalysts and natural enzymes.

Supplementary Material

Amtawong SI

ACKNOWLEDGMENTS

This work was primarily funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, under Contract No. DE-AC02-05CH11231. We acknowledge a National Institutes of General Medical Sciences of the National Institutes of Health grant for funding the EPR investigations, under Contract No. 1R35GM12696101. (awarded to R.D.B.), the UC Berkeley College of Chemistry NMR facility, which is supported by the National Institutes of Health under grant No. S10OD024998, and the Catalysis Facility of Lawrence Berkeley National Laboratory. This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH1123 1. J.A. is supported by a scholarship from the Development and Promotion of Science and Technology (DPST), Thailand. D.B. acknowledges the support from the Norwegian Research Council through the Hylleraas Centre for Quantum Molecular Sciences (Project No. 262695) and the Norwegian Metacenter for Computational Science (NOTUR; Grant No. nn4654k). We thank Prof. Craig Williams of the University of Queensland for advice on the synthesis of perruthenates. We thank Peter T. Smith for gas chromatography advice, Dr. Miao Zhang for HR-ESI-MS and IR advice, and Khetpakorn Chakarawet, Dr. Addison N. Desnoyer, Amélie Nicolay, Dr. Yang-Jin Cho, Benjamin A. Suslick, and Dr. Gavin R. Kiel for useful discussions.

Footnotes

Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.9b10320.

X-ray crystallographic data (CIF)

Experimental synthetic, spectroscopic, electrochemical, and computational details (PDF)

The authors declare no competing financial interest.

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