Electrochemical Generation and Spectroscopic Characterization of the

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

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Electrochemical Generation and Spectroscopic Characterization of the Key Rhodium(III) Hydride Intermediates of Rhodium Poly(bipyridyl) H2‑Evolving Catalysts Carmen E. Castillo,† Thibaut Stoll,† Martina Sandroni,†,‡,⊥ Robin Gueret,† Jérôme Fortage,† Megumi Kayanuma,§,# Chantal Daniel,§ Fabrice Odobel,∥ Alain Deronzier,† and Marie-Noëlle Collomb*,† Downloaded via KAOHSIUNG MEDICAL UNIV on August 21, 2018 at 21:28:16 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Univ. Grenoble Alpes, CNRS, DCM, F-38000 Grenoble, France Univ. Grenoble Alpes, CEA, CNRS, INAC-SyMMES 38000 Grenoble, France § Laboratoire de Chimie Quantique, Institut de Chimie Strasbourg, UMR 7177 CNRS/UdS, 1-4 Rue Blaise pascal, 67037 Strasbourg, France ∥ CEISAM, Université de Nantes, CNRS, 2 rue de la Houssinière, 44322 Nantes Cedex 3, France ‡

S Supporting Information *

ABSTRACT: We previously reported that the [RhIII(dmbpy)2Cl2]+ (dmbpy = 4,4′-dimethyl-2,2′-bipyridine) complex is an efficient H2-evolving catalyst in water when used in a molecular homogeneous photocatalytic system for hydrogen production with [RuII(bpy)3]2+ (bpy = 2,2′bipyridine) as photosensitizer and ascorbic acid as sacrificial electron donor. The catalysis is believed to proceed via a twoelectron reduction of the Rh(III) catalyst into the squareplanar [RhI(dmbpy)2]+, which reacts with protons to form a Rh(III) hydride intermediate that can, in turn, release H2 following different pathways. To improve the current knowledge of these key intermediate species for H 2 production, we performed herein a detailed electrochemical investigation of the [Rh III (dmbpy) 2 Cl 2 ] + and [RhIII(dtBubpy)2Cl2]+ (dtBubpy = 4,4′-di-tert-butyl-2,2′-bipyridine) complexes in CH3CN, which is a more appropriate medium than water to obtain reliable electrochemical data. The low-valent [RhI(Rbpy)2]+ and, more importantly, the hydride [RhIII(Rbpy)2(H)Cl]+ species (R = dm or dtBu) were successfully electrogenerated by bulk electrolysis and unambiguously spectroscopically characterized. The quantitative formation of the hydrides was achieved in the presence of weak proton sources (HCOOH or CF3CO3H), owing to the fast reaction of the electrogenerated [RhI(Rbpy)2]+ species with protons. Interestingly, the hydrides are more difficult to reduce than the initial Rh(III) bis-chloro complexes by ∼310−340 mV. Besides, 0.5 equiv of H2 is generated through their electrochemical reduction, showing that Rh(III) hydrides are the initial catalytic molecular species for hydrogen evolution. Density functional theory calculations were also performed for the dmbpy derivative. The optimized structures and the theoretical absorption spectra were calculated for the initial bis-chloro complex and for the various rhodium intermediates involved in the H2 evolution process.

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number versus catalyst in water were reported these last five years. Rhodium,3o platinum,4 but also H2-evolving catalysts based on more earth-abundant metal such as cobalt,2c,5 nickel, iron, and molybdenum were employed,6 mostly based on macrocyclic and polypyridinyl ligands. H2 evolution with such complexes generally proceeds via the formation of a metal hydride intermediate, which are often elusive species and therefore difficult to demonstrate experimentally.3p,7 For example, the common mechanism of H2 production proposed for cobalt H2-evolving catalysts, extensively studied

INTRODUCTION

Molecular hydrogen is an important energy carrier and a potential clean-burning fuel for sustainable energy technologies such as electrochemical fuel cells.1 In this context, it is very important to develop processes for producing molecular hydrogen from a noncarbon source as electrochemical or photoinduced reduction of water (or protons). In both approaches, the hydrogen evolution reaction requires catalysts able to operate efficiently in purely aqueous solutions in view to be further implemented in water-splitting devices.1a,b,2 While homogeneous molecular photocatalytic systems with molecular compounds for H2 production have been extensively studied,3 most of the systems that exhibit high turnover © XXXX American Chemical Society

Received: June 29, 2018

A

DOI: 10.1021/acs.inorgchem.8b01811 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Possible Catalytic Mechanisms for the [RhIII(Rbpy)2Cl2]+ Complexes (R = dtBu or dm)a

a

For hydrogen generation in the presence of H+, via formation of Rh(III) and Rh(II) hydride species.

the release of one chloride after each one-electron reduction step of the Rh catalyst (Scheme 1, steps (A) and (B)), rather than a disproportionation of the intermediate Rh(II) species. The [RhI(dmbpy)2]+ species reacts then with protons to give the putative hexacoordinated hydride [RhIII(dmbpy)2(H)(X)]n+ (X = Cl (n = 1) or H2O (n = 2)) as key intermediate for H2 production (Scheme 1, steps (C)). From this RhIII−H species, H2 can be released following two different pathways: (i) reaction with another RhIII−H to generate H2 and two Rh(II) species (homolytic route, step (D)) or (ii) proton attack to form H2 and a Rh(III) complex (heterolytic route, step (E)), this latter pathway being thermodynamically favored in water according to our theoretical calculations.14 In parallel to this mechanism, the reduction of the RhIII−H into the pentacoordinated [RhII(dmbpy)2(H)]+ by [RuII(bpy)2(bpy·−)]+ (step (F)) is also possible, and from this RhII−H species, the homolytic (step (G)) and heterolytic (step (H)) pathways are both thermodynamically favorable to produce H2 (generation of Rh(I) and Rh(II) species, respectively).14,15 By contrast, in CH3CN, photoelectrochemical measurements with a ruthenium-sensitized p-type NiO photoelectrode evidenced that the reduction potential of the ruthenium is not sufficiently negative to ensure the reduction of the RhIII−H species into RhII−H, preventing the catalytic H2 production.16 Since hydride species plays a pivotal role in the H2-evolving mechanism, it appears important to fully characterize experimentally the hydride complex [RhIII(dmbpy)2(H)Cl]+. Such hydride species, whose existence was initially assumed by Sauvage et al.,17 was isolated and spectroscopically characterized later by Sutin18 for the bpy derivative.19 Rhodium hydrides of H2-evolving catalysts therefore appear more stable than the corresponding cobalt hydrides (see above). Nevertheless, the electrochemical behavior of such Rh(III) hydrides species, and hence their reduction potential, has never been reported even in organic solvent, which is a more appropriate medium than water to obtain reliable electrochemical data. Our group has previously reported the elaboration of modified electrodes with polypyrrole films functionalized by such Rh(III) complexes for electrocatalytic hydrogen evolution20 and hydrogenation of organic compounds, 21 but the

by electrochemical techniques and theoretical calculations, is the formation of a Co(III) hydride intermediate (denoted CoIII−H) by protonation of the Co(I) species.7a,8 CoIII−H can then release hydrogen following heterolytic and/or homolytic pathways, or it undergoes a further reduction leading to the more reactive CoII−H species. However, discrimination between these different pathways is rather difficult owing to the transient nature of cobalt hydride intermediates of these H2-evolving catalysts,5e,7a,9 and several pathways can coexist depending on the experimental conditions. Among the photocatalytic molecular systems that operate in water, we reported in 2013 an efficient three-component system based on the rhodium H 2 -evolving catalyst [RhIII(dmbpy)2Cl2]+ (dmbpy = 4,4′-dimethyl-2,2′-bipyridine), in association with the [RuII(bpy)3]2+ (bpy = 2,2′-bipyridine) photosensitizer and sodium ascorbate as sacrificial electron donor.10 This photocatalytic system is very active, with more than 1000 turnover numbers per molecule of catalyst achieved under optimal conditions. [RhIII(dmbpy)2Cl2]+ was also found to be the most efficient rhodium-based H2-evolving catalyst in water, when compared to the previously reported Wilkinson catalyst, Na3[RhI(dpm)3Cl]11 (dmp = diphenylphosphinobenzene-m-sulfonato) or [RhIII(bpy)Cp*(H2O)]SO4 (Cp* = η5C5Me5).12 Additionally, we have shown that the catalytic performances of this system can be significantly improved when the catalyst and the photosensitizer are linked by a nonconjugated bridge.13 Such design allows a faster photoinduced electron transfer from the photosensitizer to the catalyst and thus increases the stability of the photocatalytic system. The possible rhodium intermediates and mechanistic pathways for the H2 production with this photocatalytic system were also investigated by theoretical calculations in water solvent.14 The most favorable pathways are summarized in Scheme 1. The initial step of the H2-evolution mechanism is a reductive quenching of the [RuII(bpy)3]2+ excited state by the sacrificial electron donor into [RuII(bpy)2(bpy·−)]+. The latter is then able to reduce the Rh(III) catalyst into the distorted square-planar [RhI(dmbpy)2]+ species. This two-electron reduction by the reduced [RuII(bpy)2(bpy·−)]+ species appears to be sequential according to an electrochemical chemical electrochemical chemical (ECEC) mechanism that involves B


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Inorganic Chemistry

reduction process (Rh(III) → Rh(I), Epc = −1.33 and −1.28 V versus Ag/AgNO3 10 mM, for the dtBubpy and dmbpy derivatives, respectively, Table 1) is observed, which is coupled to the release of the two chloro ligands to form the squareplanar [RhI(Rbpy)2]+ complexes (Scheme 1 and Figure 1A for

corresponding hydrides were never electrochemically characterized. The determination of the redox properties of the hydride species should provide valuable information about the mechanism involved in the catalytic hydrogen evolution. In this line, we report herein the electrochemical generation of the hydrides of the two following Rh(III) complexes, [Rh(dmbpy)2Cl2]+ and [Rh(dtBubpy)2Cl2]+ (Scheme 2, Scheme 2. Structures of H2-Evolving Catalysts Investigated in This Study

dtBubpy = 4,4′-di-tert-butyl-2,2′-bipyridine) in acidic CH3CN, their in situ spectroscopic characterization by UV− visible absorption and 1H NMR spectroscopies, as well as their reactivity toward H2 production. The substitution of the methyl substituents on the bipyridine ligand by tert-butyl allowed a better solubility of the electrogenerated species in CH3CN. The electrochemical generation and UV−visible spectroscopic characterization of the low-valent precursor species [RhI(Rbpy)2]+ (R = dtBu or dm) from bulk electrolysis of initial Rh(III) solutions are also reported. This experimental study was supplemented by a theoretical study based on density functional theory (DFT) to investigate the electronic properties of the initial complexes and of the various intermediates depicted in Scheme 1, through calculation of the optimized structures and UV−visible absorption spectra.

Figure 1. Cyclic voltammograms at a glassy carbon electrode (ν = 100 mV s−1) of a 1 mM solution of [RhIII(dtBubpy)2Cl2]+ in CH3CN, 0.1 M [Bu4N]ClO4 (A), after an exhaustive reduction at −1.40 V leading to the formation of [RhI(dtBubpy)2]+ (B), and after an exhaustive reoxidation at 0.0 V (regeneration of [Rh(dtBubpy)2Cl2]+ (C).

dtBubpy). This irreversible peak is followed at more negative potentials by two reversible one-electron ligand-centered reduction processes. On the reverse scan, the poorly defined irreversible oxidation peaks at ca. −0.5 V for R = dtBubpy and −0.7 V for R = dmbpy correspond to the two-electron reoxidation of the metallic center (Rh(I) → Rh(III)) and the recoordination of two exogenous ligands, which can be the solvent (CH3CN) or chloride ion(s). Fully consistent with our recent calculations,14 it was suggested by DeArmond et al.22a that the two-electron metal-centered reduction process (Rh(III) → Rh(I)) occurs according to an ECEC mechanism, that is, a first one-electron reduction followed by the release of one Cl−, then a second one-electron reduction followed by another Cl− loss (Scheme 1, steps (A and B)). The [RhII(Rbpy)2Cl]+ being easier to reduce than the starting Rh(III) bis-chloro complex, only one irreversible two-electron reduction process is observed on the cyclic voltammogram. The purple [RhI(bpy)2]+ species was previously prepared and isolated by

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RESULTS AND DISCUSSION Electrochemical Generation of [Rh I(Rbpy) 2] + in CH3CN. The cyclic voltammograms of [RhIII(Rbpy)2Cl2]+ (R = dtBu or dm) were recorded in CH3CN in the presence of [Bu4N]ClO4 (0.1 M). As previously observed for this class of complexes,22 the cyclic voltammograms display no signal in the positive potentials region, consistent with the fact that the two chloro ligands remain coordinated to the metal center upon dissolution of the complexes (free chloride anions exhibit a typical irreversible oxidation peak at ca. +0.7 V; see below). In the negative potentials region, an irreversible two-electron Table 1. Electrochemical Potentials for the Rh(III) Complexesa complexes [RhIII(dmbpy)2Cl2]+ in CH3CN, 0.1 M [Bu4N]ClO4 [RhIII(dtBubpy)2Cl2]+ in CH3CN, 0.1 M [Bu4N]ClO4 [RhIII(dmbpy)2Cl2]+ in CH3CN, 0.05 M NaCF3SO3 [RhIII(dtBubpy)2Cl2]+ in CH3CN, 0.05 M NaCF3SO3 [RhIII(dmbpy)2(H)Cl]+ [RhIII(dtBubpy)2(H)Cl]+

metal-centered reduction processes Rh(III)→Rh(I)/Rh(I)→Rh(III) (Epc (V)/Epa (V) −1.28/ca. −0.7 (−1.38/ca. −0.8 vs Fc+/Fc) −1.33/ca. −0.5 (−1.43/ca. −0.6 vs Fc+/Fc) −1.30/ca. −0.5 (−1.40/ca. −0.6 vs Fc+/Fc) −1.37/ca. −0.25 (−1.47/ca. −0.35 vs Fc+/Fc) Rh(III)→Rh(II) Epc (V) −1.62 (−1.72 vs Fc+/Fc) −1.64 (−1.74 vs Fc+/Fc)

ligand-centered reduction processes Rbpy/Rbpy·− E1/2/V (ΔEp/mV) −1.74 (60), −1.98 (60) (−1.84, −2.08 −1.72 (60), −1.97 (60) (−1.82, −2.07 −1.76 (60), −2.03 (70) (−1.86, −2.13 −1.74 (40), −2.03 (80) (−1.84, −2.13

vs vs vs vs

Fc+/Fc) Fc+/Fc) Fc+/Fc) Fc+/Fc)

In CH3CN, 0.1 M [Bu4N]ClO4 vs Ag/Ag+ (0.01 M AgNO3 in CH3CN, 0.1 M [Bu4N]ClO4) at a scan rate of 100 mV s−1. E1/2 = (Epa + Epc)/2. In these experimental conditions, the reversible system of the ferrocene/ferricenium (Fc+/Fc) couple appears at +100 mV. Potentials referred to Ag/ AgNO3 10 mM can be converted to standard calomel electrode by adding 298 mV. a

C


Article

Inorganic Chemistry Table 2. UV/Visible Data for the Rh Complexes in Various Solvents complexesref III

+ this work

[Rh (dmbpy)2Cl2] [RhIII(dtBubpy)2Cl2]+ this work [RhIII(bpy)2Cl2]+ 25 [RhI(dmbpy)2]+ this work,a [RhI(dtBubpy)2]+ this work,a [RhI(bpy)2]+ 24 [RhI(bpy)2]+ 24 [RhI(bpy)2]+ 18a [RhIII(dmbpy)2(H)Cl]+ this work,a [RhIII(dtBubpy)2(H)Cl]+ this work,a [RhIII(bpy)2(H)Cl]+ 18a

solvent

λabs, nm (ε, M−1 cm−1)

CH3CN CH3CN H2O CH3CN CH3CN methanol acetone H2O 0.1 M NaOH CH3CN CH3CN methanol

251 (21 700), 263 (sh, 17 700), 300 (21 000), 310 (25 300) 251 (26 500), 263 (sh, 19 700), 300 (23 400), 310 (27 400) 252 (23 200), 302 (sh, 23 500), 311 (27 300), 384 (sh, 100) 300 (n.d.),b 365 (8500), 504 (sh, 6800), 548 (14 680), 633 (3950) 300 (20 900), 364 (7500), 504 (sh, 6200), 546 (14 000), 633 (3400) 246 (24 500), 298 (32 100), 362 (6800), 518 (sh, 8500), 552 (13 100), 656 (sh, 2500), 750 (sh, 500) 514 (sh, 5100), 552 (12 500), 648 (sh, 2500), 750 (sh, 500) 523 (9100) 252 (29 700), 298 (sh, 19 600), 307 (19 900), 330 (sh, 5000), 360 (sh, 2500) 252 (30 400), 298 (sh, 20 000), 307 (21 000), 330 (sh, 4800), 360 (sh, 2000) 247 (35 000), 303 (sh, 29 000), 310 (32 000)

a

The molar absorption coefficients (ε) are calculated from electrogenerated solutions assuming that the species are quantitatively generated. bn.d.: not determined.

chemical reduction of [RhIII(bpy)2Cl2]+,23 and its tetrahedral square-planar geometry was unambiguously evidenced later by X-ray crystallography.24 Its UV−visible spectroscopic behavior in various solvents was also investigated (Table 2).17,18,24,25 In acetone and methanol, only the monomeric [RhI(bpy)2]+ is present, while in degassed aqueous solutions, several other species can be formed depending on the pH and rhodium concentration, such as the dimer [RhI(bpy)2]22+ or, in acidic conditions, terminal/bridging hydride species [RhIII(bpy)2(H2O)(H)]2+ and [Rh(bpy)2]2H3+.18a The electrochemical generation of [RhI(dtBubpy)2]+ was also previously reported in dimethylformamide (DMF) solution, but its spectroscopic characterization was prevented by the poor stability of the electrogenerated purple solution.22c To further characterize these species and evaluate their stability in this solvent, bulk electrolysis of [RhIII(Rbpy)2Cl2]+ in CH 3 CN solutions were performed. An exhaustive electrolysis of a 1 mM [RhIII(dtBubpy)2Cl2]+ (1.1 × 10−5 mol) solution at E = −1.40 V consumed two electrons per molecule of initial rhodium complex and led to the formation of the Rh(I) species in agreement with the resulting cyclic voltammogram (Figure 1B). The chloro ligands release can be evidenced by the appearance of an irreversible system at E ≈ +0.7 V (Figure 1B), fully consistent with the formation of the square-planar [RhI(dtBubpy)2]+ species. In the cathodic potential region, the initial irreversible reduction peak Rh(III) → Rh(I) has fully disappeared, and only the two successive reversible waves of the ligand-centered reduction processes are now observed on the cyclic voltammogram. Besides, the irreversible oxidation peak of the [RhI(dtBubpy)2]+ species (Rh(I) → Rh(III)) at −0.5 V is accompanied, on the reverse scan, by the appearance of the Rh(III) → Rh(I) reduction peak at Epc ≈ −1.32 V attesting to the regeneration of the initial bis-chloro complex and thus the overall reversibility of the process (see below; Figure 1B). While the initial solution of Rh(III) is colorless as it displays a strong absorption band at 300−310 nm and only a very weak absorption between 320 and 350 nm, the electrogenerated solution of Rh(I) exhibits an intense pink-purple coloration (Figures 2 and 3A,B and Table 2). This low-valent species displays two intense absorption bands centered at 364 and 546 nm (shoulder at 504 nm) and one less intense at 633 nm. Another intense UV absorption band is also observed at 300 nm (Figure 3B). This spectrum, very similar to the one previously obtained in methanol (Table 2), tends to confirm again the formation of the monomeric [RhI(dtBubpy)2]+

Figure 2. UV−Visible spectral changes in CH3CN, 0.1 M [Bu4N]ClO4 of a 1 mM solution of [Rh(dtBubpy)2Cl2]+ during a twoelectron reduction at −1.40 V vs Ag/AgNO3 (formation of [RhI(dtBubpy)2]+). Optical path length 1 mm.

species. Assuming that the reaction is nearly quantitative, the molar extinction coefficients of the absorption bands were estimated and are reported in Table 2. Because of their very negative oxidation potential, solutions of [RhI(dtBubpy)2]+ are immediately reoxidized into the initial Rh(III) species under air. Besides, if solutions are maintained under strictly anaerobic condition, the evolution of the UV− visible spectra over time shows that the [RhI(dtBubpy)2]+ species are not stable for a long period and slowly decomposed with a half-life time of ∼1.5 h. Anyway, an electrolysis time scale being ∼10−15 min, a back electrolysis at E = −0.20 V of a freshly electrogenerated solution of [RhI(dtBubpy)2]+ restores almost quantitatively the initial amount of Rh(III) complex (Figure 1C), showing also the recoordination of the two chlorides to the Rh(III) complex. The reduction of the [RhIII(dmbpy)2Cl2]+ derivative also leads to the formation of a pink-purple solution of [RhI(dmbpy)2]+ that exhibits a UV−vis absorption spectrum very similar to that found for the [RhI(dtBubpy)2]+ derivative (Figure S1A, and Table 2). However, [RhI(dmbpy)2]+ is poorly soluble in CH 3 CN, 0.1 M [Bu 4 N]ClO 4 , and approximately half of this species has precipitated at the end of the electrolysis. It is possible to fully solubilize [RhI(dmbpy)2]+ by using NaCF3SO3 as supporting electrolyte, and the resulting spectrum presents absorption bands with a quite similar intensity as those of [RhI(dtBubpy)2]+ (Figure D


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Inorganic Chemistry

substituted, exhibiting two shoulders at 365 and 490 nm, slightly differs from the initial bis-choro complex (Figure S3). Electrochemical Generation of Rh(III) Hydrides in the Presence of Weak Acids. The hydride complexes [RhIII(L)2(H)Cl]+ (L = bpy and phen) have been previously chemically isolated by addition of concentrated HCl to an ethanolic solution of [RhI(L)2]Cl, and the obtained yellow product has been characterized by NMR18b and UV−visible absorption spectroscopy (Table 2).18a However, this chemical synthesis did not allow obtaining a pure sample of the Rh(III) hydride compounds, the dimer [Rh(L)(H)Cl2]2 being also formed in parallel as a green compound. Here, we succeeded to quantitatively generate the mononuclear hydride species [RhIII(Rbpy)2(H)(X)]+ (X = Cl or CF3SO3, see below) by a simple electrochemical procedure that consists in the electrochemical reduction of the initial complexes in the presence of a weak proton source in CH3CN. Basically, the Rh(I) species electrogenerated at the electrode reacts immediately with protons to form the [RhIII(Rbpy)2(H)Cl]+ hydrides (Scheme 1, step (C)). This method was employed in early works to electrogenerate hydride species for the [M(L)(Cp*)Cl]+ and [M(L)(P)2Cl2]+ complexes (M = Ir(III) and Rh(III), L = bipyridyl ligand, P = PPh3 or PPh2)26,27 and for rhodium porphyrin28 using the weak acid HCOOH.29 Interestingly, recent studies have shown that if [RhI(Cp*)(L)] (L = bpy, 1,10-phenanthroline (phen)) can be first protonated at the metal by weak acid (Et3NH+, Br−; pKa = 18.8 in CH3CN), then this proton migrates to the Cp* ligand generating a Rh(I) with a protonated Cp*H ligand, rather than a Rh(III)−H hydride complex.30 Herein we first performed experiments with HCOOH and then with a stronger acid, namely, CF3CO2H. In the Presence of HCOOH. The addition of 1 mol equiv of HCOOH (1.1 × 10−5 mol) in a 1 mM CH3CN, 0.1 M [Bu4N]ClO4 solution of [RhIII(Rbpy)2Cl2]+ induces the emergence, on the cyclic voltammograms, of a new, very poorly reversible reduction peak located at a more negative potential than the irreversible reduction of the metal center (Rh(III) → Rh(I)) and just before the first reduction of the bipyridyl ligand (Epc = −1.64 and −1.62 V for the dtBubpy and dmbpy derivatives, respectively, Figures 4A and S4A and Table 1). The intensity of this peak increases progressively upon addition of larger amounts of HCOOH. We assigned this new peak to the electrochemical signature of the [RhIII(Rbpy)2(H)Cl]+ hydrides produced in situ and, more specifically, to their one-electron reduction into [RhII(Rbpy)2(H)]+. We also note that, at the time scale of the cyclic voltammetry, on the reverse scan, the reoxidation wave of the Rh(I) species disappears, indicating that this species undergoes a fast protonation reaction to form the Rh(III) hydride. This is confirmed by a bulk electrolysis of the [RhIII(Rbpy)2Cl2]+ solutions at −1.40 and −1.35 V for the dtBubpy and dmbpy derivatives (i.e., at a potential just beyond the irreversible two-electron reduction of the Rh(III) into Rh(I)). The quantitative formation of the [RhIII(Rbpy)2(H)Cl]+ hydrides requires an excess of HCOOH, 3 mol equiv versus the amount of initial Rh(III) complex, as shown by electrochemical (Figures 4B and S4B) and spectroscopic features (Figures 3C and S5). In the presence of only 1.5 or 2 mol equiv of HCOOH, a mixture of hydride and Rh(I) (∼80/ 20 and 95/5 ratio for 1.5 and 2 equiv, respectively) is obtained, the amount of Rh(I) being quantified by its typical visible absorption at 546 nm. Similarly to the reduction of

Figure 3. Comparison of the experimental and TD-DFT calculated UV−visible absorption spectra in CH3CN, 0.1 M [Bu4N]ClO4, of [RhIII(Rbpy)2Cl2]+ (R = dtBu or dm) (A), [RhI(Rbpy)2]+ (B), and [RhIII(Rbpy)2(H)Cl]+ (C) (optical path length 1 mm).

S1B and Figure 2, respectively). In presence of NaCF3SO3, the two chloride ligands released during the reduction process precipitate in solution as insoluble white powder of NaCl, and thus no signal is observed at +0.7 V on the cyclic voltammogram after exhaustive reduction at −1.40 V (Figure S2B). Thus, after reoxidation at 0.0 V, a shift of the twoelectron irreversible reduction process Rh(III) into Rh(I) to more positive potential by ∼60 mV (Epc = −1.22 V; Figure S2C) is observed in accordance with the generation of a Rh(III) complex in which the chloro ligands have been exchanged by less electron-donating ligands that can be CH3CN and/or triflate anion(s). The UV−vis spectrum of the Rh(III) species in which the chloro ligands have been E


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Inorganic Chemistry

Figure 5. UV−Visible absorption spectra in CH3CN (0.1 M [Bu4N]ClO4) of a 1 mM solution of [RhIII(dtBubpy)2Cl2]+ after a two-electron exhaustive electrolysis at −1.40 V in the presence of 3 equiv of formic acid to yield quasi-quantitatively the [RhIII(dtBubpy)2(H)Cl]+ hydride. (inset) Enlarged 320−450 nm range (optical path length 1 mm).

CD3CN ([Bu4N]ClO4, 0.1 M), and a similar electrochemical behavior and UV−visible features were obtained in this solvent. The 1H NMR spectra exhibit in the hydride region the typical doublet signal18b centered at −14.61 and −14.46 ppm, respectively, for [Rh I I I (dmbpy) 2 (H)Cl] + and [RhIII(dtBubpy)2(H)Cl]+ (Figures S7 and S9). These doublet peaks are consistent with the spin−spin interaction between the hydride and the rhodium(III) metal possessing a nuclear spin of 1/2. The Rh−H coupling constants (JRh−H) of 11.6 (dmbpy) and 11.2 Hz (dtBubpy) are similar to the values reported in literature for Rh(III) hydride complexes.18b,31,32 In the aromatic region between 7 and 9.5 ppm, in addition to the two protons ascribed to the two equivalents of HCOOH in excess (observed between 8.25 and 8.30 ppm), 1H NMR spectra of the hydride species display eight doublets and four singlets for [RhIII(dmbpy)2(H)Cl]+, and two doublets, six doublet of doublets, and two doublets of triplets for [RhIII(dtBubpy)2(H)Cl]+, which are attributed to the 12 protons of the bipyridine ligands (Figures S7 and S9). The protons of methyl groups of [RhIII(dmbpy)2(H)Cl]+ and of the tert-butyl groups of [RhIII(dtBubpy)2(H)Cl]+ are also observed in the aliphatic region between 1.0 and 3.0 ppm (Figures S8 and S10); 18 protons of two tBu groups being buried below the massive peak of [Bu4N]ClO4 electrolyte salt at ca. 1.4 ppm (Figure S10). The perfect correspondence between the peak integrations of aromatic, aliphatic, and hydride regions (i.e., 12 aromatic protons for 1 hydride proton and 12 (or 36) aliphatic protons) confirms that the hydride species were quantitatively electrogenerated. Regarding the electroactivity of the hydrides, the irreversibility of their reduction peak (Figures 4B and S4B) is most probably a consequence of the instability of the hydride reduced forms [RhII(Rbpy)2(H)]+, which can react following the homo- and/or heterolytic pathways to evolve hydrogen (Scheme 1, steps (G) and (H)). The intensity of the irreversible reduction peak is however higher than that expected for a one-electron process. This can be due to the presence of the additional 2 equiv of HCOOH present in solution whose electroactivity is superimposed and may also lead to some catalytic effect. Indeed, a further exhaustive electrolysis of the [RhIII(dtBubpy)2(H)Cl]+ solution (1.1 ×

Figure 4. Cyclic voltammograms in CH3CN, 0.1 M [Bu4N]ClO4, at a glassy carbon electrode (ν = 100 mV s−1) of a 1 mM solution of [Rh(dtBubpy)2Cl2]+ (black) and after successive additions of HCOOH: (A) 1 equiv (blue), 2 equiv (red), and 3 equiv (green), (B) after an exhaustive reduction at −1.40 V in the presence of 3 equiv of HCOOH (formation of [RhIII(dtBubpy)2(H)Cl]+).

[RhIII(Rbpy)2Cl2]+ without acid, the exhaustive electrolyses in the presence of acid require the exchange of two electrons per molecule of initial Rh(III) complexes. At the end of the electrolysis, the initial Rh(III)/Rh(I) irreversible reduction peak has fully disappeared on the resulting cyclic voltammogram (Figures 4B and S4B), and the irreversible reduction wave of the hydride (at Epc = −1.64 V (dtBubpy)) and −1.62 V (dmbpy)) is thus the only reduction wave present. However, the reduction peak of the hydride partially overlaps with that of the formic acid (ca. −1.8 V; see Figure S6A). Hydrides species are colorless, in agreement with Rh(III) species and, for instance, with the absorption of a Rh(III) monohydride terpyridine complex in CH3CN.31 Although the spectra obtained after electrolysis in the presence HCOOH are very close to the spectra of the starting Rh(III) bis-chloro complexes, the hydride species exhibit a more intense absorption between 320 and 390 nm with the two shoulders at 330 and 360 nm, in close correspondence with the absorption spectra of [RhIII(bpy)2(H)Cl]+ previously reported in methanol (Figures 3C, 5, and S5 and Table 2).18a Two strong absorption bands at 252 and 307 nm are also observed for [RhIII(Rbpy)2(H)Cl]+ as well as a residual band at 546 nm resulting from the presence of traces of Rh(I) species in the hydride solutions. The formation of the Rh(III) hydrides is also supported by DFT calculation. The calculated spectrum of [RhIII(dmbpy)2(H)Cl]+ displays two intense bands at 267 and 286 nm associated with two weak absorption bands at 323 and 373 nm (vide infra) that match well with the experimental absorption spectrum (Figure 3C). Finally, the hydrides were unambiguously characterized by 1H NMR spectroscopy (Figures S7−S10). Electrolyses were performed in distilled F


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Inorganic Chemistry

Figure 6. Cyclic voltammograms in CH3CN, 0.1 M [Bu4N]ClO4 at a glassy carbon electrode (ν = 100 mV s−1) of a 1 mM solution of [Rh(dtBubpy)2Cl2]+ (black) and after addition of 1.2 mol equiv of CF3CO2H (red) (A), after a two-electron exhaustive reduction at −1.40 V in the presence of 1.2 equiv of CF3CO2H (formation of [RhIII(dtBubpy)2(H)Cl]+) (B), after a one-electron exhaustive reduction at −1.62 V (formation of [RhI(dtBubpy)2]+) (C), and a after two-electron exhaustive reoxydation at 0.0 V (regeneration of [Rh(dtBubpy)2Cl2]+ (D).

10−5 mol) performed at E = −1.58 V (consumption of three additional electrons) gave 1.62 × 10 −5 mol of H 2 , corresponding to the quantitative reduction of the initially introduced amount of H+ (3 equiv, 3.3 × 10−5 mol) and a Faradaic yield of 98%. The amount of H2 produced after the electrolysis was determined by gas chromatography (see Supporting Information). At the end of the electrolysis, the Rh(I) regeneration is attested to by the dark purple coloration of the solution. Note that in all these experiments, H2 is the only gas detected, without any formation of CO2 (see below).16 However, during the reduction processes, we cannot exclude that a part of the hydrogen produced originates from the direct reduction of HCOOH present in solution due to the proximity of its reduction (Epa ≈ −1.80 V (Figure S6A)) to those of the hydrides species, especially for the dtBubpy derivative (Epc = −1.64 V). To obtain a clear electrochemical behavior and reactivity of Rh(III) hydrides species, we thus explore the formation of such species in the presence of a stronger acid, namely, CF3CO2H (pKa = 12.7), in view to be able to quantitatively electrogenerate them without an excess of acid. In the Presence of CF3CO2H. Electrochemical studies with CF3CO2H were performed in CH3CN, 0.1 M of [Bu4N]ClO4, and in CD3CN, 0.1 M of [Bu4N]ClO4, to characterize the electrogenerated hydride species by 1H NMR. Electrochemical behavior and UV−visible data were found to be the same in both solvents. With the dtBubpy complex, similarly to the electrochemical behavior of this complex in the presence of HCOOH, when CF3CO2H is added, the typical irreversible peak of the hydride species appears at −1.64 V (Figures 6A and S11). In these stronger acidic conditions, only ∼1.1 to 1.5 mol equiv of CF3CO2H is required to fully generate the hydride species by a bulk electrolysis at −1.40 V (Figure 6B).

After the two-electron electrolysis in the presence of 1.2 mol equiv of CF3CO2H, since almost no excess of acid is present, the electroactivity of the hydride is better seen on the cyclic voltammogram with its reduction process at −1.64 V, followed by four successive bpy-centered reversible waves. In addition to the regular bpy-centered reductions of [RhI(dBubpy)2]+ resulting from the reduction of the hydride species, the two additional waves negatively shifted by ∼150 mV probably correspond to the bpy-centered reductions of the hydride species. A further exhaustive reduction of the hydride at −1.62 V requires the exchange of only one electron and generates quantitatively the Rh(I) species as shown by UV−vis spectroscopy (Figures 6C and 7) with the simultaneous production of 0.5 equiv of hydrogen. We can thus propose that the Rh(II) hydride species generated from the electrochemical reduction of the Rh(III) hydride is not stable in CH3CN and can generate hydrogen through different mechanisms. The generation of hydrogen through a disproportionation of the Rh(II) hydride (Scheme 1, step (G), homolytic pathway) is possible. The other possibility is that the Rh(II) hydride species that has lost one chloride ligand is thus reduced by the Rh(III) hydride leading to a Rh(I) hydride species from which hydrogen is produced (this pathway is not shown in Scheme 1). Indeed, according to our previous calculations,14 the pentacoordinated structure for [RhII(Rbpy)2(H)]+ was found to be more stable than the hexacoordinated one, [Rh II (Rbpy) 2 (H)Cl], and thus easier to reduce than [Rh I I I (Rbpy) 2 (H)Cl] + . A further oxidation of the [RhI(dtBubpy)2]+ solution at 0.0 V, as expected, fully regenerates the starting bis-choro Rh(III) complex (Figure 6D). Quite similar results were obtained for the dmbpy derivative when CF3CO2H is used. The hydride complex is formed in the same way as with HCOOH with this stronger acid from a bulk G


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Inorganic Chemistry

electrolysis at −1.40 V. The reduction of the hydride by an electrolysis at −1.60 V also generates a 0.5 equiv of hydrogen concomitantly to Rh(I), although this latter species has partially precipitated at the end of the electrolysis (Figure S12). The formation of the [RhIII(Rbpy)2(H)Cl]+ hydrides generated with CF3CO2H were also further confirmed for both complexes from 1H NMR spectroscopy, revealing spectra similar to those obtained with HCOOH (Figures S7−S10). Besides, the [RhIII(Rbpy)2(H)Cl]+ formulation for the hydride species, that is, a chloride ligand coordinated to the metal center, was firmly established by conducting an experiment with [RhIII(dtBubpy)2Cl2]+ using NaCF3SO3 as supporting electrolyte. In this case, an exhaustive reduction at −1.40 V in the presence of 1.2 mol equiv of CF3CO2H generates a mixture of two hydride species, namely, the regular [RhIII(dtBubpy)2(H)Cl]+ and a new one, assigned to [RhIII(dtBubpy)2(H)X]n+ (X = CF3SO3 (n = 1) or CH3CN (n = 2)) in a 60:40 molar ratio according to the 1H NMR spectra (Figures S13 and S14). Accordingly, two irreversible reduction peaks are observed on the resulting cyclic voltammogram at Epc = −1.64 V for the choro hydride and

Figure 7. UV−Visible absorption spectra in CH3CN, 0.1 M [Bu4N]ClO4, of a 1 mM solution of [Rh(dtBubpy)2Cl2]+ (orange, (a)), after a two-electron exhaustive reduction at −1.40 V in the presence of 1.2 equiv of CF3CO2H (formation of [RhIII(dtBubpy)2(H)Cl]+) (green, (b)), after a one-electron exhaustive reduction at −1.62 V (formation of [RhI(dtBubpy)2]+) (purple, (c)). Optical path length 1 mm.

Figure 8. Optimized structures of [RhIII(dmbpy)2Cl2]+ (C2 symmetry) (a), [RhII(dmbpy)2Cl]+ (C1 symmetry) (b), [RhIII(dmbpy)2(H)Cl]+ (C1 symmetry) (c), [RhII(dmbpy)2(H)]+ (C1 symmetry) (d), and [RhI(dmbpy)2]+ (D2 symmetry) (e) in the electronic ground state in CH3CN. H


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Inorganic Chemistry at Epc = −1.50 V, a less negative potential for the triflate or acetonitrile derivative (Figure S15B). Thus, the generation of [RhIII(dtBubpy)2(H)Cl]+ is not quantitative, since some chloride ligands partially precipitate as NaCl during the reduction process in the presence of NaCF3SO3. A further reduction at −1.62 V of the hydride species regenerates the [RhI(dtBubpy)2]+ species along with the release of 0.5 equiv of hydrogen (Figures S15C and S16). Finally we tested the addition of a stronger acid as pcyanoanilinium tetrafluoroborate (pKa = 7.0)33 to electrogenerated solutions of Rh(III) hydride complexes from HCOOH or CF3CO2H to see if an H2 production could occur following a heterolytic pathway (Scheme 1, step (E)) in these more acidic conditions. However, no formation of H2 was observed. This tends to confirm that the heterolytic pathway is not promoted in these experimental conditions and that, in CH3CN, it is necessary to reduce Rh(III) hydrides into their Rh(II) forms to obtain an effective hydrogen evolution. Indeed, Rh(II) hydrides can disproportionate or be reduced again into the much more reactive Rh(I) hydrides to release hydrogen. The Rh(II) hydride, probably pentacoordinated (the pentacoordinated [RhII(bpy)2(H)]+ structure was found to be more stable than that of the six-coordinated [RhII(bpy)2(H)Cl]+ according to our theoretical calculations; see below), can be reduced by the Rh(III) hydride. Anyway, it was not possible to further study the electrocatalytic reduction of protons by these Rh(III) complexes in the presence of an excess of p-cyanoanilinium or even with an excess of CF3CO2H, since the direct reduction of such acids8i,34 (see Figure S6B for CF3CO2H) occurs at a potential too close to that of the complexes. Anyway, it is interesting to compare the electrochemical behavior of rhodium(III) H2-evolving catalysts, such as [RhIII(Rbpy)2Cl2]+ and rhodium porphyrin,28 for which key rhodium hydride intermediates are easily detected in CH3CN by cyclic voltammetry and fully generated by bulk electrolysis in the presence of acids, to that of the most widely used H2evolving cobalt catalysts, for which most cobalt hydride intermediates in H2 evolution elude detection.30a Such families of Rh(III) complexes are reduced directly into Rh(I) in polar solvents according to a two-electron ECEC or ECEdisproportionation process. Electrogenerated Rh(I) species react readily with acids affording Rh(III) hydrides that are reduced into Rh(II) hydrides at a more negative potential than the starting Rh(III) complexes. The significant difference in potential (∼310−340 mV) between the two-electron reduction process Rh(III)/Rh(I) of the starting complexes [RhIII(Rbpy)2Cl2]+, compared to the reduction process RhIII(H)/RhII(H) of the hydrides [RhIII(Rbpy)2(H)Cl]+, allows their clear observance and their electrochemical generation by bulk electrolysis. By contrast, the electrochemical behavior of H2-evolving cobalt catalysis is characterized by two successive well-separated (quasi)reversible oneelectron processes, namely, Co(III)/Co(II) and Co(II)/Co(I). When moderate-to-strong acids are employed, many cobalt catalysts show an electrocatalytic wave near the Co(II)/Co(I) couple, which implies that the Co(I) species is protonated to afford the CoIII−H as an intermediate.3p,5e,8d,35 This also suggests that the potential of the CoIII(H)/CoII(H) couple coincides with the Co(II)/Co(I) couple or is less cathodic,8i,9b,36 indicating that the CoIII−H intermediate will be spontaneously reduced by the Co(I) species to form the more reactive CoII−H. In some of the few cases where cobalt(III)

hydride intermediates were isolated or generated and characterized in situ, the reduction of CoIII−H to CoII−H is readily identified by cyclic voltammetry and, similarly to the aforementioned rhodium complexes, occurs at potentials more negative than the parent Co(II)/Co(I) couple.7d,8d,12,37 Benchmarking of Structural and Optical Properties. Optimized Structures of the Rh Complexes. The most important calculated bond lengths and angles of the electronic ground states of [RhIII(dmbpy)2Cl2]+, [RhII(dmbpy)2Cl]+, [RhI(dmbpy)2]+, [RhIII(dmbpy)2(H)Cl]+, and [RhII(dmbpy)2(H)]+ after complete DFT geometry optimization in CH3CN solvent are reported in Tables S1 and S2 along with those of [RhIII(bpy)2Cl2]+ and [RhI(bpy)2]+ obtained from X-ray crystallography.24,38 The optimized structures are depicted in Figure 8. The initial [RhIII(dmbpy)2Cl2]+ catalyst exhibits an octahedral geometry with a C2 symmetry. The Rh−N and Rh−Cl bond distances are slightly overestimated by the calculation (∼2% and 4%, respectively) for this complex (Table S1) as compared to the crystallographic data obtained for the cationic [RhIII(bpy)2Cl2]+ complex.38 This is a general trend observed in similar studies.24 The initial catalyst releases one Cl− ligand when it is reduced to [RhII(dmbpy)2Cl]+ in which the Rh−Cl bond undergoes a weak elongation and the N−Rh−N″ angle slightly increased (Table S1 and Figure 8). The pentacoordinated structure is more stable than the hexacoordinated structure at the Rh(II) oxidation state by −21.5 kJ mol,14 since an electron, which is added to [RhIII(dmbpy)2Cl2]+, occupies a metal-localized orbital directed to the sixth-coordination site in [RhII(dmbpy)2Cl]+. Then the reduction of [RhII(dmbpy)2Cl]+ to [RhI(dmbpy)2]+ occurs in conjunction with the loss of the last Cl− ligand and a significant change of geometry. As previously observed by Fujita et al.24 in the crystal structure and by DFT calculation of the analogue complex [RhI(bpy)2]+, [RhI(dmbpy)2]+ adopts a tetrahedrally distorted square-planar geometry with a D2 symmetry. While the Rh−N bond distances are slightly overestimated by the calculation (∼4%), the agreement of the calculated angles for [RhI(dmbpy)2]+ with the experimental data obtained for [RhI(bpy)2]+ 24 is rather good (Table S1). The opened coordination sphere of [RhI(dmbpy)2]+ allows the protonation concomitantly with the coordination of a chloride ligand to yield the octahedral hydride [RhIII(dmbpy)2(H)Cl]+. The Rh(III) hydride formation induces thus a drastic change of geometry from distorted tetrahedral to octahedral environment with a geometry very similar to that of the initial [RhIII(dmbpy)2Cl2]+ complex (Figure 8). As illustrated in Table S2, while the optimized bond lengths for Rh−N and Rh−N′ are both very similar in [RhIII(dmbpy)2(H)Cl]+ and in [RhIII(dmbpy)2Cl2]+, the Rh−N″ bond trans to the hydride undergoes an elongation by 0.18 Å due to the weakening of the dπRh/πbpy bonding interaction by formation of the σRh−H bond compared to the Rh−N″ bond trans to the chloride. The calculated Rh−H bond distance of 1.542 Å in [RhIII(dmbpy)2(H)Cl]+ is comparable to those reported for other Rh(III) hydride complexes.19a The reduction of the hexacoordinated [RhIII(dmbpy)2(H)Cl]+ hydride into the pentacoordinated [RhII(dmbpy)2(H)]+ one is accompanied by an increase of bond angles of N−Rh−N‴ by 9° (Figure 8 and Table S2). In addition, a small elongation of the Rh−H bond by 0.042 Å, a slight decrease of Rh−N″ bond by 0.039 Å, and a strong elongation of the Rh−N bond by 0.224 Å are observed as a consequence of the lacking of π*bpy/dπRh (Rh− I


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Inorganic Chemistry N) and π (Rh−Cl) bonding interactions during the reduction process (Table S2). As observed for the [RhII(dmbpy)2Cl]+, the pentacoordinated structure is more stable than the hexacoordinated structure for the Rh(II) hydride. This tendency to adopt a pentacoordinated geometry is confirmed by our calculations, which indicate that coordination of Cl− to [RhII(dmbpy)2(H)]+ is not thermodynamically favorable (ΔG = +51.9 kJ mol−1). Examples of Rh(II) hydride are scarce, but, for instance, Savéant and co-workers28 reported a pentacoordinated geometry for a Rh(II) porphyrin hydride, which can adopt also a hexacoordinated structure in the presence of strong electron-donating ligands such as tertiary phosphines. Theoretical Absorption Spectra of Rh Complexes. The time-dependent (TD) DFT absorption spectra of the initial catalyst [RhIII(dmbpy)2Cl2]+ and of key intermediates [RhII(dmbpy)2Cl]+, [RhI(dmbpy)2]+, and [RhIII(dmbpy)2(H)Cl]+ calculated in CH3CN are represented in Figure 9. The

transition calculated at 289 nm, an additional peak at 286 nm assigned to a mixed IL/MC state, and a MLCT state calculated at 267 nm (Figure 3B and Table S3). The theoretical absorption spectrum of the intermediate [RhII(dmbpy)2Cl]+ (Figure 9 and Table S4) is characterized, in the visible energy domain between 450 and 325 nm, by broad bands of low intensities due to the presence of large MLCT/IL states mixing and large MLCT/IL/LLCT states mixing in the UV energy domain between 325 and 250 nm. This is the result of an important electronic density delocalization that characterizes this Rh(II) intermediate. The theoretical spectrum of [RhI(dmbpy)2]+ presents two remarkable intense and large visible bands, the first one between 650 and 475 nm with a maximum calculated at 544 nm and the second one between 425 and 325 nm, centered at 364 nm. The highest transition calculated at 544 nm is assigned to a MLCT state, while the one calculated at 364 nm has a MLCT/MC mixed character. This calculated transition fits well with the large visible bands measured in CH3CN for [RhI(Rbpy)2]+ and centered at 364 and 546 or 548 nm (Figures 2 and S1). In agreement with the experimental spectrum, the theoretical absorption spectrum of [RhI(dmbpy)2]+ is characterized by two other bands centered at 298 and 269 nm. The transition calculated at 298 nm is mainly IL, whereas the lowest transition at 269 nm corresponds to a mixture of IL and MLCT states.

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CONCLUSION In this work, a detailed electrochemical investigation of the bischloro [RhIII(Rbpy)2Cl2]+ complexes (R = dm or dtBu) in CH3CN solvent was performed in view to electrogenerate key intermediates for H2 evolution, Rh(III) hydrides, as well as their low-valent precursor Rh(I) species. Both species were successfully generated by bulk electrolyses in quantitative yield and spectroscopically characterized. The [RhIII(Rbpy)2(H)Cl]+ hydrides were obtained by electrochemical reduction of the initial bis-chloro complexes in the presence of HCOOH or CF3CO2H as a weak proton source. These hydrides are formed by fast reaction of the electrogenerated [RhI(Rbpy)2]+ with a proton. Interestingly, this study allows for the first time the determination of the [RhIII(Rbpy)2(H)Cl]+ hydride species reduction potentials. Potentials appear to be ∼310−340 mV more negative than those of the initial Rh(III) bis-chloro complexes (Epc = −1.62/−1.64 V versus −1.28/−1.33 V, for the dmbpy/dtBubpy derivatives), as expected for the higher electron-donating character of the hydrido versus chloro ligand. While CH3CN solutions of Rh(III) hydrides are colorless, Rh(I) species exhibit a dark pink-purple coloration with a visible absorption spectra dominated by an intense visible band at ∼546 nm. Interestingly, electrochemical reduction of this hydride released 0.5 equiv of hydrogen along with the quantitative generation of the [RhI(Rbpy)2]+ species, demonstrating that the Rh(III) hydride is the molecular species responsible for the generation of hydrogen. The [RhII(Rbpy)2(H)]+ hydride electrogenerated is not stable in CH3CN and can disproportionate or be reduced again into the much more reactive Rh(I) hydride to release hydrogen. This experimental study was supplemented by DFT calculations with the dmbpy derivative. The optimized structures and the theoretical absorption spectra were calculated for the initial complex [RhIII(dmbpy)2Cl2]+ and for the various rhodium intermediates, [RhII(dmbpy)2Cl]+, [RhI(dmbpy)2]+, [RhIII(dmbpy)2(H)Cl]+, and

Figure 9. TD-DFT absorption spectra of [RhIII(dmbpy)2Cl2]+, [RhII(dmbpy)2Cl]+, [RhI(dmbpy)2]+, and [RhIII(dmbpy)2(H)Cl]+ in CH3CN.

corresponding transition energies are reported in Tables S3 and S4. A comparison of the calculated and experimental absorption spectra of [RhIII(Rbpy)2Cl2]+, [RhI(Rbpy)2]+, and [RhIII(Rbpy)2(H)Cl]+ recorded in CH3CN, 0.1 M [Bu4N]ClO4, are also shown in Figure 3. Overall, the agreement between the calculated and experimental absorption spectra is rather good. The theoretical absorption spectrum of the initial catalyst [RhIII(dmbpy)2Cl2]+ (Figure 9) is characterized by one weak metal-to-ligand charge transfer (MLCT) absorption at 343 nm, one intense peak centered at ∼290 nm, and a lessintense band at 270 nm. The absorption calculated around 290 nm and measured experimentally at ∼300−310 nm for both [RhIII(Rbpy)2Cl2]+ complexes in CH3CN solution (Table 2 and Figure 3A) covers several intraligand (IL) and ligand-tometal charge transfer (LMCT) states. Two other MLCT states are calculated at 274 and 254 nm, the latter one being observed. The theoretical spectrum of the [RhIII(dmbpy)2(H)Cl]+ hydride (Figure 9) is similar to the one of the initial catalyst [RhIII(dmbpy)2Cl2]+ with two weak absorption bands at 323 nm (MLCT and metal-centered (MC) states) and at 373 nm (MLCT/MC state). These two bands experimentally observed as shoulders at ∼330 and 360 nm in CH 3 CN for [RhIII(Rbpy)2(H)Cl]+ are characteristic of the Rh(III) hydride species (Figure 3B). In contrast, as observed for the initial Rh(III) complexes, the UV part of the calculated spectrum is blue-shifted compared to the experimental one, with an IL J


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Inorganic Chemistry [RhII(dmbpy)2(H)]+. Agreement of the optimized calculated structures of [RhIII(dmbpy)2Cl2]+ and [RhI(dmbpy)2]+ with the experimental ones available for the bpy derivative are rather good. Besides, the calculated spectra in CH3CN solvent of [RhIII(dmbpy)2Cl2]+, [RhI(dmbpy)2]+, and [RhIII(dmbpy)2(H)Cl]+ match well with those experimentally obtained in this solvent, and the transitions were attributed. This theoretical study also evidenced that the Rh(II) species transiently formed, [RhII(dmbpy)2Cl]+ and [RhII(dmbpy)2H]+, are preferentially pentacoordinate rather than hexacoordinate. Finally, the results of this work, beyond improving knowledge of the key species involved in the mechanism of hydrogen production by the [RhIII(Rbpy)2Cl2]+ complexes, confirm that electrochemistry can be a powerful method to experimentally detect and also generate key hydride species by bulk electrolysis. Electrochemistry allows for a fine control of the reduction potential in contrast to reducing agents usually used for a chemical synthesis of hydrides. Given the importance of hydride complexes as key intermediates for hydrogen production but also for other reactions, such as hydrogenation reactions, this work provides a potentially simple procedure to obtain and study transition-metal hydride complexes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jérôme Fortage: 0000-0003-2673-0610 Chantal Daniel: 0000-0002-0520-2969 Marie-Noëlle Collomb: 0000-0002-6641-771X Present Addresses ⊥

ESRFThe European Synchrotron CS 40220−38043 Grenoble Cedex 9, France. # Center for Computational Sciences, University of Tsukuba, 1−1−1 Tennodai, Tsukuba, Ibaraki 305−8577, Japan. Author Contributions

The manuscript was written through contributions of all authors. 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 T.S. and R.G. thank, respectively, the “Département de Chimie Moléculaire de Grenoble” and the University of Grenoble Alpes for their Ph.D. grants. The authors wish to thank, for financial supports including the postdoctoral fellowships to M.K., C.E.C., and M.S., the French National Research Agency for the project HeteroCop (ANR-09-BLAN-0183-01) and the LABEX ARCANE (ANR-11-LABX-0003-01) for the projects H2Photocat and QDPhotocat. This work was also supported by ICMG FR 2067 and COST CM1202 program (PERSPECT H2O).

EXPERIMENTAL SECTION

Computational Method. The optimized structures and the theoretical absorption spectra were only calculated for rhodium complexes with the dmbpy ligand to minimize computation time. The structures of the initial complex [RhIII(dmbpy)2Cl2]+ and of the various rhodium intermediates depicted in Scheme 1, [RhII(dmbpy)2Cl]+, [RhI(dmbpy)2]+, [RhIII(dmbpy)2(H)Cl]+, and [RhII(dmbpy)2(H)]+ were optimized by means of DFT with the B3LYP functional.39 Los Alamos ECP plus DZ (LanL2DZ) basis sets were used for the rhodium atom,40 and 6-31G(d,p) basis sets41 were used for the others. Solvent corrections were included using the Polarizable Continuum Model (PCM) within the integral equation formalism variant (IEFPCM) for acetonitrile (ε = 36.62 at 293 K42).43 Vibrational analysis was performed for all the optimized structures to confirm true energy minima. The theoretical absorption spectra of [RhIII(dmbpy)2Cl2]+, [RhII(dmbpy)2Cl]+, [RhI(dmbpy)2]+, and [RhIII(dmbpy)2(H)Cl]+ were calculated by means of TD-DFT44 using Stuttgart relativistic small-core effective core potential (Stuttgart RSC 1997 ECP) with the corresponding double-ζ valence basis sets on the metal atom45 and Dunning double-ζ polarized (DZP) basis sets46 for the others including solvent corrections (CH3CN). The calculations were performed with Gaussian 09 quantum chemistry software.47

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hydride species of the dmbpy derivative, and TD-DFT transition energies for [RhII(dmbpy)2Cl]+ (PDF)

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01811. Materials and general complexes preparation, electrochemistry, electrocatalytic hydrogen production. Additional electrochemical, spectroscopic, and theoretical data for the [RhIII(Rbpy)2Cl2]+, [RhII(dmbpy)2Cl]+, and [RhIII(Rbpy)2(H)Cl]+ complexes, electrochemical data for HCOOH and CF3CO2H, 1H NMR spectra of the Rh(III) hydrides. Calculated bond lengths and angles of the optimized structures and TD-DFT absorption wavelengths to the low-lying singlet excited states in CH3CN of the Rh(III), Rh(II), Rh(I), and Rh(III) K


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Inorganic Chemistry

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