pubs.acs.org/IC Article Aqueous Persistent Noncovalent Ion-Pair Cooperative Coupling in a Ruthenium Cobaltabis(dicarbollide) System as a Highly Efficient Photoredox Oxidation Catalyst Isabel Guerrero, Clara Viñas, Xavier Fontrodona, Isabel Romero,* and Francesc Teixidor* Cite This: Inorg. Chem. 2021, 60, 8898−8907 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: An original cooperative photoredox catalytic system, [RuII(trpy)(bpy)(H2O)][3,3′-Co(1,2-C2B9H11)2]2 (C4; trpy = terpyridine and bpy = bipyridine), has been synthesized. In this system, the photoredox metallacarborane catalyst [3,3′- Co(1,2-C2B9H11)2]− ([1]−) and the oxidation catalyst [RuII(trpy)- (bpy)(H2O)]2+ (C2′) are linked by noncovalent interactions and not through covalent bonds. The noncovalent interactions to a large degree persist even after water dissolution. This represents a step ahead in cooperativity avoiding costly covalent bonding. Recrystallization of C4 in acetonitrile leads to the substitution of water by the acetonitrile ligand and the formation of complex [RuII(trpy)(bpy)(CH3CN)][3,3′-Co(1,2-C2B9H11)2]2 (C5), struc- turally characterized. A significant electronic coupling between C2′ and [1]− was first sensed in electrochemical studies in water. The CoIV/III redox couple in water differed by 170 mV when [1]− had Na+ as a cation versus when the ruthenium complex was the cation. This cooperative system leads to an efficient catalyst for the photooxidation of alcohols in water, through a proton-coupled electron- transfer process. We have highlighted the capacity of C4 to perform as an excellent cooperative photoredox catalyst in the photooxidation of alcohols in water at room temperature under UV irradiation, using 0.005 mol % catalyst. A high turnover number (TON = 20000) has been observed. The hybrid system C4 displays a better catalytic performance than the separated mixtures of C2′ and Na[1], with the same concentrations and ratios of Ru/Co, proving the history relevance of the photocatalyst. Cooperative systems with this type of interaction have not been described and represent a step forward in getting cooperativity avoiding costly covalent bonding. A possible mechanism has been proposed. ■ INTRODUCTION The development of new photocatalytic methods for organic structural modification upon ET. The challenge is to generate charge separation by ET and maintain it by avoiding regeneration of the initial reagent. transformations is an important challenge given the significant Cooperative photoredox catalysis supposes a new progress in environmental and economic impact that this entails. Solar this field and refers to a first catalyst that is photochemically energy combined with water are widely considered to be good active, while the second is redox-active in the absence of light.9 alternative energy sources for the development of nonfossil- This strategy allows multielectron photoredox catalysis with based fuels.1,2 Great effort has been dedicated to the catalysis the help of a second redox catalyst, avoiding the single electron activation of organic molecules by light.3−5 The latter can be transfer (SET) imposed by the photoredox catalyst. The union considered to be a good reagent for environmentally friendly, in one single complex of a photoredox catalyst and a transition- green chemical synthesis; unlike many conventional reagents, metal catalyst could result in higher efficiency and may lead to light is an abundant energy form for driving chemical reactions new selective light-induced organic transformations. in a sustainable future. The activation of molecules by light depends on the ability of metal complexes and organic dyes to participate in electron- Received: March 15, 2021 transfer (ET) processes with organic molecules upon Published: June 7, 2021 excitation by light.6,7 The octahedral compound [Ru(bpy)3]2+ (bpy = bipyridine)8 has been one of the most studied photoredox catalysts in the literature; it participates in ET by an outer-sphere mechanism that supposes no significant © 2021 American Chemical Society https://doi.org/10.1021/acs.inorgchem.1c00751 8898 Inorg. Chem. 2021, 60, 8898−8907 Inorganic Chemistry pubs.acs.org/IC Article Currently, systems that are used for the light-induced oxidation catalyst would lead to the formation of a stable and oxidation of substrates are based on chromophore−catalyst efficient cooperative photoredox catalyst. dyads of ruthenium polypyridyl compounds,10−12 where one With all this in mind, in this work we present the synthesis compound acts as the light-harvesting antenna and the second of an air-stable ruthenium cobaltabis(dicarbollide) compound, metal complex is used as catalyst, to activate a water molecule [RuII(trpy)(bpy)(H2O)][3,3′-Co(1,2-C2B9H11)2]2 (C4, where or an organic substrate. In these cases, the problem is the trpy = terpyridine and bpy = bipyridine; Scheme 1), where the charge trapping between the two moieties influenced by a [Ru-OH2] cation belongs to the family of redox oxidation bridging ligand that connects both parts, leading to a reduction catalysts,35,36 together with their spectroscopic and electro- of the catalytic activity observed in some systems. Moreover, chemical characterization. The recrystallization of C4 in the synthesis of these systems can be laborious and expensive. acetonitrile leads to formation of the complex [RuII(trpy)- Oxidized raw materials are needed as bulk chemicals in (bpy)(CH3CN)][3,3′-Co(1,2-C2B9H11)2]2 (C5), which has different fields, including polymer and fine chemicals, among been structurally characterized. C4 has been tested as a others.13 So, the selective oxidation of alcohols is an essential cooperative photoredox catalyst in the oxidation of aromatic process in organic chemistry14−17 because this oxidation can be and aliphatic alcohols in water showing high performance, used to produce aldehydes, acids, ketones, etc. using very low catalyst loads. It is also relevant in hydrogen-based energy technologies because this reaction involves a two-electron proton-coupled Scheme 1. Schematic Representation of the Ruthenium process that could be recombined on a cathode for hydrogen Cobaltabis(dicarbollide) Complex C4 production.18,19 It is well-known that carboranes interact with light20 and that they have been studied as catalysts in different processes.21 The use of carboranes in supramolecular chemistry is a field to be explored for their particular characteristics.22−25 The metallacarborane sandwich compound [3,3′-Co(1,2- C2B9H11)2]− ([1]−) has many possibilities of forming hydrogen bonds, e.g., Cc−H···O or Cc−H···X (X = halogen) as well as dihydrogen bonding Cc−H···H−B and B−H···H−N (Cc stands for the cluster C atoms).26−28 It is well-known that [1]− is highly stable in water but at low concentration forms ■ aggregates (vesicles and micelles).29,30 Dihydrogen interactions participate in the self-assembling formation.31 These supra- RESULTS AND DISCUSSION molecular interactions appear to be significant in the processes of ET and therefore in the performance and efficiency of Synthetic Strategy, Structure, and Redox Character- photocatalytic systems.32 Recently, we have shown that ization. The synthetic process used for preparation of the cobaltabis(dicarbollide), [1]−, and its chloro derivatives, acting compounds is outlined in Scheme 2. The aqua complex C4 is as both catalyst and photosensitizer, are highly efficient in the obtained by dissolving the chloridoruthenium(II) complex [RuII(trpy)(bpy)(Cl)]Cl (C2), obtained following the method photooxidation of alcohols in water, through SET processes.33 described in the literature,37 in a mixture of water/acetone A high performance of Na[1] in the photooxidation of alcohols (1:1) in the presence of Ag[1], C3, under reflux. This latter is observed, using a catalyst load of 0.01 mol % and reaching compound was prepared by a cationic exchange resin from TON = 10000, in some cases.33 We have also supported the Cs[1], as described in the literature.38 After filtration of AgCl, cobaltabis(dicarbollide) catalyst on silica-coated magnetite the complex was isolated. The recrystallization of C4 in nanoparticles.34 This system has proven to be a green and acetonitrile led to the substitution of water by the acetonitrile sustainable heterogeneous catalytic system, highly efficient and ligand, and then the formation of complex C5 took place. It is easily reusable for the photooxidation of alcohols in water. In worth mentioning that, to the best of our knowledge, this is the general, the SET processes carry out the transformation of a first example of a molecular aqua ruthenium(II) complex limited number of substrates, and in some cases undesirable containing two cobaltabis(dicarbollide) anions as counterions. byproducts are generated. As we have indicated, the results Suitable single crystals of C5 were grown by the offered by [1]− in SET have been excellent, taking into account recrystallization of C4 in acetonitrile. All attempts to obtain the low catalyst loading, the selectivity, and the high degree of crystals of C4 in water or noncoordinating solvents were conversion. However, we wanted to know more about their unsuccesful. behavior, whether cooperative action offered any advantage, X-ray diffraction analysis was used to solve the crystal whether a cooperative effect really existed, and whether the structure of complex C5 (Figure 1). The main crystallographic noncovalent interactions demonstrated in water had a history. data, together with selected bond distances and angles, are To this end, the studies with alcohol oxidations already studied reported in Tables S1 and S2. The cationic moiety of only with [1]− would allow us to prove the features that have compound C5 is formed by a ruthenium(II) complex, where not been demonstrated or found in other related systems. the Ru center displays an octahedral distorted type of Once these points are known and demonstrated, we will be in coordination, in which the trpy ligand is bonded to the RuII the position to develop unprecedented systems. cation in a meridional manner and the bpy ligand is As mentioned above, [1]− can form ion-pair complexes coordinated in a bidentate fashion. The sixth coordination through hydrogen and dihydrogen interactions.31 Then, the site is occupied by the monodentate acetonitrile ligand. All design of a new hybrid system where cobaltabis(dicarbollide) bond distances and angles are within the expected values for can be linked by noncovalent interactions to a redox-active these types of complexes.39−41 For instance, it is interesting to 8899 https://doi.org/10.1021/acs.inorgchem.1c00751 Inorg. Chem. 2021, 60, 8898−8907 Inorganic Chemistry pubs.acs.org/IC Article Scheme 2. Synthetic Strategy for the Preparation of Complexes C4 and C5 The C1A−C2A (1.661 Å) and C1B−C2B (1.642 Å) bond lengths are different in the cisoid anion, while the C1C−C2C bond lengths are similar (1.649 Å) in the transoid rotamer anion. The packing in Figure S1b displays the arrangement adopted between the metallacarborane anions and cationic moieties. The IR spectrum of complex C4 shows vibrations around 2900 cm−1 that can be assigned to the νC−H stretching modes for the aromatic rings, while the bands around 3030 cm−1 correspond to the νC−H stretching of the Cc−H bonds in the different rotamers. A band over 3500 cm−1 can be seen that Figure 1. Crystal structure of complex C5. corresponds to the νO−H stretching of the aqua ligand. We have also observed a significant vibration around 2530 cm−1 that note that the Ru−N2 bond length in the bpy ligand, where the corresponds to the νB−H stretching mode for the B−H bond N2 atom is placed trans to the pyridyl N4 atom of trpy, is (Figure S2). The 1D and 2D 1H NMR spectra of complexes longer (2.069 Å) than the analogous distance Ru−N1 (2.033 C3 and C4 were recorded in acetone-d6 and are displayed in Å), where the N1 atom is trans to the N6 atom of the Figures S3 and S4. The 1H NMR spectrum of C4 shows two acetonitrile ligand. This evidence highlights the stronger trans sets of signals (Figure S4a): one in the aromatic region influence of the pyridine ring with respect to the acetonitrile corresponding to the protons of the bipyridine and terpyridine ligand. The N3−Ru−N4 and N4−Ru−N5 angles are 79.88° and 79.13°, respectively, less than the 90° expected for an ideal ligands of the [Ru−OH2]2+ cation and the second set in the octahedral geometry. This may be due to the geometry of the aliphatic region that can be assigned to the Cc−H of the trpy ligand, which defines the equatorial plane of the structure; cobaltacarborane anions, whose resonances appear near δ = as a consequence, the other two equatorial angles, N3−Ru−N2 3.98 ppm. The 1H{11B} NMR spectrum exhibits the and N2−Ru−N5, are larger than 90°. The anionic moiety is resonances of H atoms bonded to B atoms over a wide formed by two anionic metallabis(dicarbollide) units, which chemical-shift range in the region from δ = 1.50 to 3.5 ppm. display different rotamers: one cisoid and the other transoid. These resonances appear as broad bands (Figure S4b). 8900 https://doi.org/10.1021/acs.inorgchem.1c00751 Inorg. Chem. 2021, 60, 8898−8907 Inorganic Chemistry pubs.acs.org/IC Article The 11B{1H} NMR resonances featured the typical pattern of the pristine cobaltabis(dicarbollide) cluster in the range from δ = 15 to −22 ppm (Figure S4c).42 The UV−vis spectra of Ag[1], C3, and C2′, together with C4, are displayed in Figure S5a,b. The UV−vis spectrum of Ag[1], C3 (Figure S5a), shows one strong absorption band at 286 nm and others with less intensity at 210, 333, and 450 nm, in agreement with the literature.43,44 The spectrum of the aqua complex C2′ (Figure S5b) shows ligand-based π−π* bands and dπ(Ru)−π*(L) metal-to-ligand charge-transfer (MLCT) transitions, as is usual for these complexes.45 In Figures S5b and 2 is shown the UV−visible spectrum corresponding to Figure 3. CV for complexes C4 (red line) and Na[1] (blue line) registered in a phosphate buffer (pH = 7.12) versus Ag/AgCl. The right red and blue insets show dI/dE to better appreciate the position of the couple CoIV/CoIII. Further proof of this will be shown later when dealing with the C4 precedents that we will henceforth call the C4 history. Figure 2. UV−vis spectra of complex C4 in CH2Cl2 (dotted line), phosphate buffer at 7.02 pH (dash-dotted line), and CH3CN (solid line). complex C4, which exhibits the ligand-based π−π* bands of the cationic part below 350 nm that are partially eclipsed by strong absorptions corresponding to the cobaltabis- (dicarbollide) anionic moiety. Above 350 nm, the spectra show less intense bands that correspond to dπ(Ru)−π*(L) MLCT transitions. Figure 2 displays the UV−vis spectra of C4 in CH2Cl2, phosphate buffer, and CH3CN. It is worth mentioning the shift to higher energy absorptions observed Figure 4. DPV for complex C4 registered in a phosphate buffer (pH = 7.12) versus Ag/AgCl. for C4 in acetonitrile with regard to the same complex in water or dichloromethane. This is expected because of substitution of the aqua ligand by acetonitrile; this ligand shows a higher π- acceptor capacity with respect to the aqua ligand, which coupled electron-transfer (PCET) processes (eqs 1 and 2). provokes stabilization of the dπ(Ru) donor orbitals. These values have been assigned to the RuIII/RuII and RuIV/ The electrochemical behavior of complexes C3, Na[1], and RuIII couples of the catalytic unit, based in the values presented C4 have been studied by means of cyclic voltammetry (CV) by the mononuclear [Ru II (trpy)(bpy)(H 2 O)] 2+ com- and differential pulse voltammetry (DPV) (Figures S6−S8). pound.46,47 The CV curve is followed by another two-electron The CV curve of C3 in acetonitrile shows two reversible one- oxidation wave at 1.19 V (vs Ag/AgCl), which was attributed electron-redox processes at E1/2 = −1.69 and 1.27 V versus Ag/ to the CoIVRuIV/CoIIIRuIV redox couple. In this case, this value AgCl as reference electrode, which can be assigned to CoIII/ is assigned to the CoIV/CoIII couple of the photoactive unit CoII and CoIV/CoIII, respectively (Figure S6a). The CV curve given the similarity with the potential value of Na[1], whose of the aqua complex C4 in dichloromethane exhibits different CV curve presents a CoIV/CoIII redox wave at E1/2 = 1.36 V redox processes because of the Ru and Co ions. Two are (Figure 3, blue line). For C4, this value is 170 mV more almost reversible monoelectronic CoIIIRuIII/CoIIIRuII and cathodic with respect to the potential of Na[1], which CoIIIRuIV/CoIIIRuIII redox waves at E1/2 around 0.68 and indicates a certain and significant electronic coupling between 0.96 V versus Ag and two monoelectronic CoIIIRuII/CoIIRuII the two units. To check this out, we have measured the redox and CoIVRuIV/CoIIIRuIV redox waves at E1/2 = −1.57 and 1.38 potentials of Na[1] after adding different concentrations of V Versus Ag/AgCl, respectively (Figure S6b). two divalent salts, CaCl2 and ZnCl2 (Figure S9 and Table S3), The electrochemical behavior of C4 was also studied in to mimic the dipositive charge of the ruthenium complex, and aqueous solution. The CV curve of C4 (Figure 3, red line) and no substantial change in the redox potential of Na[1] has been DPV curve (Figure 4) exhibit two successive one-electron- observed. This is consistent with a coupling between the cation oxidation waves at 0.55 and 0.67 V (vs Ag/AgCl), which and anion in C4. Equations 1−4 contain the electrochemical correspond to CoIIIRuIII/CoIIIRuII and CoIIIRuIV/CoIIIRuIII transition reactions corresponding to each redox couple redox couples, respectively, and are assigned to two proton- processes at pH = 7.12 with reference electrode Ag/AgCl. 8901 https://doi.org/10.1021/acs.inorgchem.1c00751 Inorg. Chem. 2021, 60, 8898−8907 Inorganic Chemistry pubs.acs.org/IC Article Table 1. Photooxidation Tests Performed with Ruthenium Cobaltabis(dicarbollide) Complex C4a a Conditions: C4 (0.001 mM), substrate (20 mM), Na2S2O8 (40 mM), 5 mL of a potassium carbonate solution at pH = 7. Ratio 1:20000:40000: b8 h of reaction, with previous neutralization after 4 h. c4 h of reaction, with previous neutralization after 3 h. d4 h of reaction, without neutralization. e Ratio 1:20000:20000; after 4 h of reaction. Cop IIIRuc III‐OH + 1e− + 1H+ F Cop IIIRuc II‐OH 2 Cop IV Ruc IV O + 1e− F Cop IIIRuc IV O E1/2 = 0.55 V (1) E1/2 = 1.19 V (3) CoIV + 1e− F CoIII E = 1.36 V (4) Therefore, the observed oxidation potentials indicate that Cop IIIRuc IV =O + 1e− + 1H+ F Cop IIIRuc III‐OH the photogenerated CoIV is able to easily oxidize [RuIII-OH]2+ to [RuIVO]2+, with a driving force of 520 mV, because the E1/2 = 0.67 V (2) potential for CoIV/CoIII is high enough compared to the RuIV/ 8902 https://doi.org/10.1021/acs.inorgchem.1c00751 Inorg. Chem. 2021, 60, 8898−8907 Inorganic Chemistry pubs.acs.org/IC Article Table 2. Photooxidation Tests Performed with C4 and C2′ + Na[1]a entry photocatalyst substrate product TON (yield, selectivity %) 1 C4 1-phenylethanol acetophenone 16200 (81, ≥99) 2 C2′ 1-phenylethanol acetophenone 4400 (22, ≥99) 3 Na[1] 1-phenylethanol acetophenone 6200 (62, ≥99) 4 C2′ + Na[1](1:1) 1-phenylethanol acetophenone 12800 (64, ≥99) 5 C2′ + Na[1](1:2) 1-phenylethanol acetophenone 15200 (76, ≥99) 6 C4 4-methylbenzyl alcohol 4-methylbenzaldehyde 19600 (98, ≥99) 7 C2′ 4-methylbenzyl alcohol 4-methylbenzaldehyde 5200 (25, ≥96) 8 C2′ + Na[1](1:1) 4-methylbenzyl alcohol 4-methylbenzaldehyde 17200 (70, 82) 9 C2′ + Na[1] (1:2) 4-methylbenzyl alcohol 4-methylbenzaldehyde 18000 (86, 96) a Conditions: C4 (0.001 mM); C2′ and Na[1] (0.001 mM) for ratio 1:1 (C2′/Na[1]); C2′ (0.001 mM) and Na[1] (0.002 mM) for ratio 1:2 (C2′/Na[1]), substrate (20 mM), Na2S2O8 (40 mM), 5 mL of a potassium carbonate solution at pH = 7. Light irradiation during 4 h. RuIII couple. This would be thermodynamically possible in 1-Hexanol (entry 6), 1-butanol (entry 7), isobutyl alcohol water via PCET processes.48 Therefore, on the basis of these (entry 8) and 2-ethoxyethanol (entry 9) were converted to the studies, it is expected that, for photocatalytic oxidation of corresponding acids with yields of 82% (4 h) and 92 % (8 h) organic substrates, [1]− could act as a good photosensitizer. (entry 6), 89% (4 h) and 98 % (8 h) (entry 7), 83% (4 h) and Figure 3 shows that there is an ion-pair contact effect, which 92 % (8 h) (entry 8), and 76% (4 h) and 93 % (8 h) (entry 9), causes the CoIV/CoIII redox couple potential to vary near 170 respectively. Conversely, cyclohexanol converted to cyclo- mV under the influence of the ruthenium complex. hexanone in 79% yield after 4 h and 95% after 8 h for 10. Photocatalytic Oxidation. C4 is a [Ru][Co]2 ion-pair When the amount of oxidizing agent was lowered (ratio complex in which the photocatalytic system is based on cobalt, applied 1:20000:20000), we observed that the degree of an abundant transition metal, whereas the redox part is the conversion for 1-phenylethanol and benzyl alcohol, after 4 h, is ruthenium complex. The photocatalytic oxidation experiments on the same order as the values obtained using a ratio were all carried out by exposing the reaction quartz vials to UV 1:20000:40000 (entries 1e and 2e). It should be noticed that irradiation (2.2 W, λ ∼ 300 nm), for different times, at the conversion was a little bit less because of the kinetics of the atmospheric pressure and room temperature. The samples reaction. These results are consistent because 1 mol of S2O82− were 5 mL of water, a K2CO3 solution at pH = 7, and a mixture is needed for each 1 mol of alcohol to achieve the oxidizing of C4 as a substrate, Na2S2O8 as a sacrificial oxidant (air could reaction. We have no evidence that the catalyst has undergone also do the work),33 and 0.005 mol % catalyst. The blank any transformation following catalysis, as has been evidenced experiments were performed in the absence of catalyst, in other cases with ruthenium complexes containing trpy.36 We sacrificial oxidant, or light for 8 h. The results evidenced that performed matrix-assisted laser desorption ionization time-of- a negligible amount of oxidation product was formed, less than flight mass spectrometry (MALDI-TOF MS) of C4 after the 8% in all cases. After irradiation for a specified time (Table 1), catalytic experiments, and it can be seen that the catalyst the reaction products were extracted with dichloromethane remains unchanged after catalysis (Figure S10). three times, quantified by means of 1H NMR spectroscopy, Relevance of the History of C4. As mentioned in the and confirmed by gas chromatography with flame ionization Introduction, Na[1] tends to self-aggregate in water, producing detection (GC-FID) analysis. micelles and/or vesicles.29,30 Upon the process of dissolving Table 1 displays the catalytic results obtained in the C4, the constituent ions would become free; for the vast photooxidation of aromatic and aliphatic alcohols, after majority of salts, this is completely independent. However, this different times of reaction. In general, high yields of conversion allegedly would not be the case here; see also the electro- and selectivity have been obtained, even when short reaction chemical section in which the [Ru−OH2]2+ affects the redox times have been used. After 8 h of irraditation and timely couple of [1]−. Persistence even in the water ion-pair coupling neutralization at 4 h, the yields observed in the photooxidation between the cation and anion would be possible, which would of 1-phenylethanol (entry 1) and diphenylmethanol (entry 5), prevent full dissociation and therefore maintain cooperation. both secondary aromatic alcohols, were remarkably >99%, with Indeed, no noticeable changes have been observed at the UV− selectivity values of >99% and turnover numbers equal to vis spectra in catalytic conditions near 10−6 M upon a 10-fold TON = 20000 in both cases. However, the yield values increase in concentration (Figure S11). To evidence the observed for benzyl alcohol are slightly less than the secondary noncovalent bonding relevance between the photosensitizer ones. It is worth mentioning that, after 4 h of irradiation, the [1]− and the redox catalyst, we took advantage of the achieved high selectivity values were to obtain only the photooxidation process and studied the yields with the same corresponding aldehydes. The conversion values enhanced by concentrations of cations and anions but whose origins were the presence of electron-donating substituents, such as methyl, different. Thus, we have studied the behavior of a mixture of on the aromatic ring of the benzyl alcohol (entry 3d) and two separate compounds, [RuII(trpy)(bpy)(H2O)](ClO4)2 decreased with the presence of chloro, an electron-withdrawing (C2′) and Na[3,3′-Co(1,2-C2B9H11)2] (Na[1]), after 4 h of substituent (entry 4d). In the case of both benzyl alcohol and catalysis and using the Co/Ru ratios 1:1 and 2:1, maintaining the substituted substrates, the selectivity decreases after 8 h of the same ruthenium concentrations as that for the C4 irradiation, observing alcohol overoxidation toward formation photocatalyst. Table 2 displays the results obtained for two of the corresponding acids. C4 displays high efficiency by substrates, 1-phenylethanol and 4-methylbenzyl alcohol. Table primary and secondary aliphatic alcohols, with yields larger 2 shows that the noncovalent ion pair C4 displays a better than 76% after 4 h of irradiation and selectivity values of >99%. catalytic performance than the equal 1:2 mixture of C2′ and 8903 https://doi.org/10.1021/acs.inorgchem.1c00751 Inorg. Chem. 2021, 60, 8898−8907 Inorganic Chemistry pubs.acs.org/IC Article Na[1], evidencing the advantage of the coupling history two electrons and two protons takes place in the photo- between the two compounds, where nonbonding interactions oxidation of alcohols. between [RuII(trpy)(bpy)(H2O)]2+ and [1]− could facilitate or stimulate ET and, consequently, the efficiency of the cooperative system in the alcohol oxidation. For comparison ■ CONCLUSIONS In this work, an air-stable cooperative ion-pair photoredox purposes, the 1:1 ratio of C2′ and Na[1] as well as C2′ and system, C4, formed by cobaltabis(dicarbollide) [1]− as a light Na[1] alone was also studied. All of these are reported in collector, and the aqua ruthenium complex [RuII(trpy)(bpy)- Table 2, showing a distinct and lower yield than C4. Dynamic (OH2)]2+ as an ET agent is reported. Both cobaltabis- light scattering (DLS) measurements of catalytic mixtures were (dicarbollide) and the ruthenium aqua complex are linked by done, and the results are displayed in Figure S12. The noncovalent interactions, which to a large degree persist even hydrodynamic radius value of C4 in the catalytic mixture is D after water dissolution. This represents a step ahead in = 140.9 nm, which is of the same order as that presented by cooperativity avoiding costly covalent bonding. C4 displays Na[1],30 which indicates the absence of larger aggregates in excellent photoredox catalyst properties in water through the medium, despite the presence of the [Ru−OH2]2+. On the PCET. C4 has been easily made by reaction of the chlorido other hand, the value of D = 128.4 nm, in the case of using a ruthenium (II) complex with Ag[1] in water/acetone (1:1) mixture of a 1:2 ratio of C2′ and Na[1] as catalysts, is less than under reflux. The recrystallization of the aqua complex C4 in that for C4, indicating to what extent the history of the acetonitrile yielded C5 in which the aqua ligand was catalytic medium influences the generated aggregates and substituted by CH3CN, as shown in X-ray diffraction. The hence, although limited, the results. electrochemical studies evidence a significant electronic This study represents the first work, to the best of our coupling between the two units in the noncovalent ion-pair knowledge, in which a photoredox catalyst based on an ruthenium cobaltabis(dicarbollide) compound and that the abundant first transition metal (Co3+) is noncovalently bonded photogenerated CoIV can easily oxidize [RuIII-OH]2+ to to an active oxidation catalyst ([RuII(trpy)(bpy)(H2O)]2+), [RuIVO]2+ in water via PCET processes; therefore, the producing a persistent ion-pair interaction even in water and metallacarborane can act as a good photosensitizer for the showing high activity in the photooxidation of alcohols. photocatalytic oxidation of organic substrates in this The observed values of conversion obtained using C4, cooperative system. together with the previous electrochemical study, are We have highlighted the capacity of C4 to perform as an consistent and support the proposed mechanism shown in excellent cooperative photoredox catalyst in the oxidation of Figure 5. Under irradiation, photons are absorbed by the alcohols in water, using a catalyst load of 0.005 mol %, achieving high yields even when short reaction times of irradiation have been used. In some cases, a high turnover number (TON = 20000) has been observed. The CoIV/III redox couple of [1]− in water differed by 170 mV when [1]− had Na+ as the cation versus when [Ru−OH2]2+ was the cation. In solution, having one or the other cation should not influence the potential, unless there were bonds that lasted even after dissolution. This led us to study two identical solutions in [1]− and [Ru]: one resulting from dissolving C4 and the other from dissolving the precursors in water. It was remarkable to find that C4 displays a better catalytic performance than the mixtures of C2′ and Na[1], evidencing that the noncovalent bonding existing in C4 facilitates or stimulates ET between the photosensitizer and catalyst and, consequently, efficiency of the cooperative system in the photooxidation process. To the best of our knowledge, cooperative systems with this type of interaction have not been described and represent a step forward in getting cooperativity avoiding covalent bonding. In most of the systems displayed in the literature, both the photoredox catalyst and the catalyst are connected through an organic linker or the metals of both share the same ligand. A possible pathway has been proposed. Figure 5. Proposed mechanism for alcohol photooxidation using C4. ■ EXPERIMENTAL SECTION Materials, Instrumentation, and Measurements. Na[Co(1,2- photosensitizer CopIII that experiences excitation to form C2B9H11)2] (Na[1]),31 and Ag[Co(1,2-C2B9H11)2] (Ag[1], C3)38 CopIII*, which undergoes oxidative quenching by the oxidizing were synthesized from commercial Cs[Co(1,2-C2B9H11)2] (Cs[1]; agent S2O82−, generating CopIV. This photogenerated strong Katchem Spol.sr.o), following the methods described in the oxidizing CoIV is able to oxidize RucII-OH2 to RucIII-OH. Then literature.38 [RuIIICl3(trpy)] (C1),49 [RuIICl(trpy)(bpy)]Cl (C2), and [RuII (trpy)(bpy)(OH) 2](ClO4)2 (C2′)46,47 were also prepared the SO4•− radical oxidizes a new CopIII to CopIV, which oxidizes according to literature procedures. All synthetic manipulations were RucIII-OH to RucIVO species. The RucIVO species reacts performed under a nitrogen atmosphere using vacuum-line with the corresponding alcohols to afford the oxidized techniques. products, with regeneration of the corresponding catalyst All reagents used in the present work were obtained from Aldrich RucII-OH2. With the proposed mechanism, the exchange of Chemical Co. and used without further purification. Reagent-grade 8904 https://doi.org/10.1021/acs.inorgchem.1c00751 Inorg. Chem. 2021, 60, 8898−8907 Inorganic Chemistry pubs.acs.org/IC Article organic solvents were obtained from SDS, and high-purity deionized 13 C{1H} NMR (acetone-d6, 100 MHz): δ 159.42, 158.98, 153.69, water was obtained by passing distilled water through a Nanopure 153.55, 150.66, 138.74, 138.46, 128.32, 127.91, 126.68, 126.50, Milli-Q water purification system. 124.27, 124.17, 123.63, 123.47 (C Ru trpy-bpy), 51.11 and 50.89 UV−vis spectroscopy was performed on a Cary 50 Scan (Varian) (Cc). IR (ν, cm−1): 3039, 2922, 2854, 2531, 1601, 1446, 1464,1384, UV−vis spectrophotometer with 1 cm quartz cells or with an 1095, 1016, 980, 884, 761. E1/2 (CH2Cl2 + 0.1 M TBAH): CoIII/II, immersion probe of 5 mm path length. NMR spectra have been −1.37 V; CoIV/III, 1.38 V; RuIII/II, 0.70 V; RuIV/III, 1.06 V (vs Ag/ recorded with a Bruker ARX 300 instrument, and 1H NMR spectra AgCl). UV−vis [CH2Cl2, 1.16 × 10−5 M; λmax, nm (ε, M−1 cm−1)]: were recorded in acetone-d6. Chemical shift values were referenced to 279 (37297), 292 (42727), 325 (66493), 392 (9686), 477 (8362). SiMe4. Elemental analyses were performed using a CHNS-O EA-1108 ESI-MS: m/z 814.4 (100%, [M − cosane − H2O]+), 831.4 (31%, [M elemental analyzer from Fisons. Electrospray ionization mass − cosane]+). spectrometry (ESI-MS) experiments were performed on a Navigator Synthesis of [Ru II(trpy)(bpy)(CH3CN)][3,3′-Co(1,2-C2B9H11)2]2 liquid chromatography (LC)/MS chromatograph from Thermo (C5). By recrystallization of C4 in an acetonitrile solution, yellow Quest Finnigan, using acetonitrile as the mobile phase. CV or DPV needles suitable for X-ray diffraction were obtained corresponding to was performed on an IJ-Cambria 660C potentiostat using a three- complex C5. UV−vis [CH3CN, 1.16 × 10−5 M; λmax, nm (ε, M−1 electrode cell. A glassy carbon electrode (3 mm diameter) from BAS cm−1)]: 287 (59852), 308 (31314), 333 (11997), 464 (5774). was used as the working electrode and Ag/AgCl as the reference Photocatalytic Studies. A quartz tube containing an aqueous electrode. All cyclic voltammograms presented in this work were solution (5 mL) at pH = 7 (K2CO3) with C4 or C2′ as the catalyst, recorded under a nitrogen atmosphere. The complexes were dissolved alcohol as the substrate, and Na2S2O8 as the sacrificial acceptor was in deoxygenated solvents containing the necessary amount of [n- exposed to UV light (2.2 W, λ = 300 nm) for different times. The Bu4N][PF]6 (TBAH) as the supporting electrolyte to yield a 0.1 M complex/substrate/sacrificial oxidant ratios used (1:20000:40000 and ionic strength solution. All E1/2 values reported in this work were 1:20000:20000 corresponding to concentrations of 0.001:20:40 mM estimated from CV experiments as an average of the oxidative and and 0.001:20:20 mM) were varied according to the study. For each reductive peak potentials [(Epa + Epc)/2]. Unless explicitly mentioned, experiment, a light reactor supplied light illumination with 12 lamps the concentration of the complexes was approximately 1 mM. that produce UVA light at room temperature. The resulting solutions GC was performed with a GC-2010 gas chromatograph from were extracted with CH2Cl2 three times. The solution was dried with Shimadzu, equipped with an Astec CHIRALDEX G-TA column [30 anhydrous sodium sulfate, and the solvent was evaporated under m × 0.25 mm (i.d.); FID detector, 250 °C; injection, 250 °C; carrier reduced pressure. To check the reproducibility of the reactions, all of gas, helium; rate, 1.57 mL min−1; area normalization]. Product the experiments were carried out in triplicate. The reaction products analyses in the catalytic experiments were performed by GC with were quantified and characterized by 1H NMR spectroscopy using biphenyl as the internal standard. tetramethylsilane as the internal standard in the case of primary Crystallographic Data Collection and Structure Determination aromatic alcohols and confirmed by gas chromatography. ■ of C5. Measurement of the C5 crystals was performed on a Bruker Smart Apex CCD diffractometer using graphite-monochromated Mo ASSOCIATED CONTENT Kα radiation (λ = 0.71073 Å) from an X-ray tube: data collection, SMART, version 5.631 (Bruker AXS 1997−2002); data reduction, * sı Supporting Information SAINT+, version 6.36A (Bruker AXS 2001); absorption correction, The Supporting Information is available free of charge at SADABS, version 2.10 (Bruker AXS 2001); structure solution, https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00751. SHELXTL, version 6.14 (Bruker 2003); structure refinement, SHELXL-2018/3 (Sheldrick, 2018). The crystallographic data as Supplementary crystallographic information, spectro- well as details of the structure solution and refinement procedures are scopic characterization (IR, NMR, UV−vis, and ESI- reported in the Supporting Information. CCDC 2058809 for C5 MS spectra) and additional electrochemical character- contains the supplementary crystallographic data for this paper. ization, MALDI-TOF-MS of C4 after photooxidation, Synthesis of [RuII(trpy)(bpy)(H2O)][3,3′-Co(1,2-C2B9H11)2]2 (C4). A and UV−vis and DLS of catalytic mixtures (PDF) 90 mg sample of C2 and a 173 mg sample of Ag[1] were dissolved in 60 mL of acetone.water (1:1), and the resulting solution was refluxed Accession Codes for 3 h. Then, AgCl was filtered off through a frit containing Celite. The volume of the solution was reduced, and the mixture was chilled CCDC 2058809 contains the supplementary crystallographic in a refrigerator for 48 h. The dark orange precipitate was collected on data for this paper. These data can be obtained free of charge a frit, washed with cold water and anhydrous ethyl ether, and then via www.ccdc.cam.ac.uk/data_request/cif, or by emailing vacuum-dried. Yield: 134.69 mg (89.93%). Anal. Found (calcd) for data_request@ccdc.cam.ac.uk, or by contacting The Cam- C33 H65B36N5Co2ORu·1.5H2O·2Et2O: C, 37.04 (36.99); H, 6.24 bridge Crystallographic Data Centre, 12 Union Road, (6.66); N, 5.19 (5.26). 1H NMR (acetone-d6, 400 MHz): δ 9.87 (d, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. ■ 1H, 3JH−H = 5.6 Hz, 1H, H1), 8.98 (d, 1H, 3JH−H = 8.1 Hz, 1H, H7), 8.91 (d, 3JH−H = 8.0 Hz, 2H, H17, H19), 8.76 (d, 3JH−H = 8.1 Hz, 2H, AUTHOR INFORMATION H14, H22), 8.64 (d, 3JH−H = 8.1 Hz, 1H, H4), 8.52 (t, 3JH−H = 8.1 Hz, 1H, H8), 8.43 (t, 3JH−H = 8.1 Hz, 1H, H18), 8.23 (t, 3JH−H = 7.1 Hz, Corresponding Authors 1H, H9), 8.16 (t, 3JH−H = 8.1 Hz, 2H, H13, H23), 8.04(d, 3JH−H = 5.6 Isabel Romero − Departament de Química and Serveis Hz, 2H, H11, H25), 7.88 (t, 3JH−H = 8.4 Hz, 1H, H3), 7.55 (ddd, Tècnics de Recerca, Universitat de Girona, E-17003 Girona, 3 JH−H = 7.9 Hz, 8.0 Hz, 4JH−H = 1.3 Hz, 3H, H10, H12, H24), 7.17 (t, Spain; orcid.org/0000-0003-4805-8394; 3 JH−H = 7.0 Hz, 1H, H2), 5.83 (s, 2H, Ru−OH2), 3.98 (s, 8H, Cc−H). Email: marisa.romero@udg.edu 1 H{11B} NMR (acetone-d6, 400 MHz): δ 3.98 (s, 8H, Cc−H), 3.41 (s, Francesc Teixidor − Institut de Ciencia de Materials de 4B−H, B8, B8′), 3.16 (s, 4B−H, B10, B10′), 2.75(s, 8B−H, B4, B4′, Barcelona, Consejo Superior de Investigaciones Científicas, E- B7, B7′), 1.96 (s, 8B−H, B9, B9′, B12, B12′), 1.68 (s, 4B−H, B6, B6′), 1.59 (s, 8B−H, B5, B5′, B11, B11′). 11B NMR (acetone-d6, 128 08193 Bellaterra, Spain; orcid.org/0000-0002-3010- MHz): δ 6.31 (d, 4B, JB−H = 144.3 Hz, B−H), 1.13 (d, 4B, JB−H = 2417; Email: teixidor@icmab.es 143.5 Hz, B−H), −5.87 (m, 16B, B−H), −17.45 (d, 8B, JB−H = 156.0 Hz, B−H), −22.90 (d, 4B, JB−H = 165.3 Hz, B−H). 11B{1H} NMR Authors (acetone-d6, 128 MHz): δ 6.33 (s, 4B, B8, B8′), 1.19 (s, 4B, B10, Isabel Guerrero − Institut de Ciencia de Materials de B10′), −5.87 (d, JB−B = 95.4 Hz 16B, B4, B7, B4′, B7′, B9, B12, B9′, Barcelona, Consejo Superior de Investigaciones Científicas, E- B12′), −17.40 (s, 8B, B5′, B11′, B5, B11), −22.84 (s, 4B, B6′, B6). 08193 Bellaterra, Spain; Departament de Química and 8905 https://doi.org/10.1021/acs.inorgchem.1c00751 Inorg. Chem. 2021, 60, 8898−8907 Inorganic Chemistry pubs.acs.org/IC Article Serveis Tècnics de Recerca, Universitat de Girona, E-17003 to the synthesis of furandicarboxylic acid and terephthalic acid. Green Girona, Spain Chem. 2017, 19, 5548−5552. Clara Viñas − Institut de Ciencia de Materials de Barcelona, (17) Jiang, X.; Zhang, J.; Ma, S. Iron Catalysis for Room- Consejo Superior de Investigaciones Científicas, E-08193 Temperature Aerobic Oxidation of Alcohols to Carboxylic Acids. J. Am. Chem. Soc. 2016, 138, 8344−8347. Bellaterra, Spain; orcid.org/0000-0001-5000-0277 (18) Esswein, A. J.; Nocera, D. G. Hydrogen Production by Xavier Fontrodona − Departament de Química and Serveis Molecular Catalysis. Chem. Rev. 2007, 107, 4022−4047. Tècnics de Recerca, Universitat de Girona, E-17003 Girona, (19) Luconi, L.; Tuci, G.; Giambastiani, G.; Rossin, A.; Peruzzini, M. Spain H2 production from lightweight inorganic hydrides catalyzed by 3d Complete contact information is available at: transition metals. Int. J. Hydrogen Energy 2019, 44 (47), 25746− https://pubs.acs.org/10.1021/acs.inorgchem.1c00751 25776. (20) Shi, C.; Sun, H.; Jiang, Q.; Zhao, Q.; Wang, J.; Huang, W.; Yan, H. Carborane tuning of photophysical properties of phosphorescent Notes iridium(III) complexes. Chem. Commun. 2013, 49, 4746−4748. The authors declare no competing financial interest. ■ (21) Shen, H.; Xie, Z. In Boron Science, New Technologies and Applications; Hosmane, N. S., Ed.: CRC Press: Boca Raton, FL, 2012; ACKNOWLEDGMENTS Chapter 21; pp 517−528. This research has been financed by MINECO (Grants (22) Grimes, R. N. Carboranes, 3rd ed.; Elsevier Inc., 2016. (23) Bauer, S.; Hey-Hawkins, E. In Boron Science, New Technologies PID2019-106832RB-I00 and CTQ2015-66143-P) and Gen- and Applications; Hosmane, N. 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