Wade C. Henke,Yun Peng,Alex A. Meier,Etsuko Fujita,David C. Grills,Dmitry E. Polyansky,James D. Blakemore
Mechanistic roles of metal- and ligand-protonated species in hydrogen evolution with [Cp*Rh] complexes
catalysis,redox chemistry,stopped-flow,pulse radiolysis,energy
Significance Catalytic energy storage in chemical bonds is a goal in renewable energy science, but catalyst design depends on identifying and understanding the protonated intermediates involved in multielectron and multiproton reactivity, like that needed for hydrogen evolution. Here, the elementary proton- and electron-transfer steps in hydrogen evolution catalyzed by Cp*Rh(bpy) (where Cp* is η5-pentamethylcyclopentadienyl and bpy is κ2-2,2'-bipyridyl) have been mapped with time-resolved spectroscopies. Initial protonation of Cp*Rh(bpy) results in the formation of a hydride complex, long thought to form but never before fully characterized; this species then undergoes tautomerization to generate an uncommon η4-pentamethylcyclopentadiene (Cp*H) intermediate, [(Cp*H)Rh(bpy)]+. Revealing how the Cp*H moiety opens and drives catalysis adds to knowledge of the rich reactivity patterns accessible to complexes supported by cyclopentadienyl-type ligands.
Mechanistic roles of metal- and ligand-protonated species in hydrogen evolution with [Cp*Rh] complexes

Catalytic energy storage in chemical bonds is a goal in renewable energy science, but catalyst design depends on identifying and understanding the protonated intermediates involved in multielectron and multiproton reactivity, like that needed for hydrogen evolution. Here, the elementary proton- and electron-transfer steps in hydrogen evolution catalyzed by Cp*Rh(bpy) (where Cp* is η5-pentamethylcyclopentadienyl and bpy is κ2-2,2'-bipyridyl) have been mapped with time-resolved spectroscopies. Initial protonation of Cp*Rh(bpy) results in the formation of a hydride complex, long thought to form but never before fully characterized; this species then undergoes tautomerization to generate an uncommon η4-pentamethylcyclopentadiene (Cp*H) intermediate, [(Cp*H)Rh(bpy)]+. Revealing how the Cp*H moiety opens and drives catalysis adds to knowledge of the rich reactivity patterns accessible to complexes supported by cyclopentadienyl-type ligands.

Cyclopentadienyls are among the most common ligands in organometallic chemistry, capable of supporting metallocene and half-sandwich type complexes across a wide range of metals and oxidation states (1–4). Cyclopentadienyl-type ligands are distinguished by their planarity and aromaticity as well as their tendency to “slip” to coordination modes, or hapticities, of η5, η3, or η1 (5–8). The accessibility of these various coordination modes contributes to the tremendous variety of reactions that can be catalyzed by complexes supported by this ligand type. With few possible exceptions (9, 10), the aromatic [C5H5] core is maintained and noninteracting with substrates in these different hapticities, leading to the usefulness of cyclopentadienyls as flexible but commonly innocent ligands that reliably promote metal-centered reactivity. Indeed, the commonly innocent nature of cyclopentadienyl-type ligands has enabled extensive studies of proton transfer to/from transition metals and their ligands, including classic work on ferrocene from Meyer (11) and other notable systems from Heinekey (12), Morris (13), Lapinte (14–16), Brookhart (17), and Norton (18). On the other hand, the pentamethylcyclopentadienyl (Cp*) ligand has recently been shown to undergo ligand-centered protonation in certain cases by direct proton transfer from acids or by tautomerization of transition metal hydrides resulting from metal-centered protonation reactions (19, 20). These reactions result in the generation of isolable complexes bearing the uncommon η4-pentamethylcyclopentadiene (Cp*H) ligand (Fig. 1), a species that can serve as a net H-atom (H•) or hydride (H–) donor via cleavage of the unique C–H bond on the Cp*H ring (21, 22). The orientation of the proton on the Cp*H ligand can give rise to unique isomers; the endo-isomer has the proton facing toward the metal center, while the exo-isomer has the proton facing away from the metal center. In the work of Chalkley, Peters, and coworkers, Cp*2Co was shown to serve as a catalytic H• donor, or “PCET mediator,” via the formation of [(exo-Cp*H)CoCp*]+ in the presence of strong acid, enabling enhanced N2 reduction to NH3 (23). Building on the work of Lapinte (15, 16), Peters and coworkers also showed that exposure of [Cp*Fe(dppe)H]+ (dppe = 1,2-bis(diphenylphosphino)ethane) (15) to CO results in the formation of [(endo-Cp*H)Fe(dppe)CO]+, a species with a weak C–H bond capable of transfer of H• (24). In all these examples, the metal center and Cp*H ligand appear to work in concert to provide H• or H– equivalents and to promote multiproton and multielectron reactivity. In the course of investigating the mechanism of H2 generation with Cp*Rh(bpy), we found that the addition of weak acids, such as triethylammonium ([HNEt3]+, pKa = 18.8 in acetonitrile (MeCN)), results in the generation of an isolable Cp*H complex formulated as [(endo-Cp*H)Rh(bpy)]+ (19, 21). X-ray diffraction studies and spectroscopic evidence confirmed the exclusive generation of [(endo-Cp*H)Rh(bpy)]+ (hereafter denoted as [(Cp*H)Rh(bpy)]+) when using weak acids. Catalytic proton reduction using Cp*Rh(bpy), a system notable for its ease of preparation and longevity, was first described by Grätzel and Kölle in 1987 (25, 26). At that time, the mechanism of catalysis was proposed to involve a first protonation that leads to a [Cp*Rh–H] species that could undergo protonolysis to generate H2, although in subsequent years, this hydride has eluded direct characterization. This elusive hydride has broader importance, however, as it has been repeatedly proposed as the active catalyst in certain bioelectrochemical systems for the formation of NADH from NAD+ (27, 28). Despite attempts, the putative [Cp*Rh–H] intermediate has been observed only in mixtures of products that are generated under very specific reaction conditions involving, variously, photochemical reactors, buffered solutions, or low temperatures (–40 °C) (20, 29). On the other hand, our group has recently developed reliable synthetic procedures to access analogous phosphine-supported [Cp*Rh] monohydrides, but these species are remarkably resistant to protonolysis due to enhanced stabilization provided by phosphine ligands (30–32). Contrasting with these monohydrides, isolated [(Cp*H)Rh(bpy)]+ reacts with even moderate acids such as anilinium ([PhNH3]+, pKa = 10.6 in MeCN) to quantitatively yield H2 and regenerate the starting Rh(III) complex. Building on these results, Nozaki et al. and Ishitani et al. have utilized [Cp*Rh]-based complexes to promote catalytic dehydrogenation of dimethylamine-borane and photocatalytic reduction of CO2 to formate, respectively (33, 34). In these investigations, among others (35, 36), the identity of the species driving the observed reactivity was proposed to be either [Cp*Rh–H] or [(Cp*H)Rh], fueling debate regarding the precise roles of these protonated intermediates during catalytic turnover. Considering the challenge in distinguishing reactive intermediates, a lingering ambiguity in the literature suggests that the Cp*H reactivity manifold merits further investigation. In particular, the mechanistic details surrounding the conversion of the proposed metal hydride and observed Cp*H species via hydride-to-proton tautomerism are not yet clear even for Cp*Rh(bpy); understanding the reaction kinetics and the possible involvement of concerted vs. multistep pathways could be especially enlightening. In the case of the [Cp*Rh] systems, the ability to access [Cp*H]-type compounds appears to be essential for catalytic turnover, a finding in agreement with the apparent transient nature of the corresponding [Cp*Rh] monohydride. However, considering that reliable syntheses are now available for Cp*Rh(bpy) and [(Cp*H)Rh(bpy)]+, contrasting with the more reactive and temperature-sensitive nature of analogous first-row metal systems supporting Cp*H species, we anticipated that time-resolved spectroscopic studies under conditions relevant to catalysis could be useful to investigate the elementary steps involved in hydrogen evolution with Cp*Rh(bpy). Here, we report time-resolved pulse radiolysis and stopped-flow measurements that map the elementary electron- and proton-transfer steps leading to hydrogen evolution with Cp*Rh(bpy) in MeCN. In particular, stopped-flow measurements with UV-visible detection reveal spectra that show isosbestic behavior, indicating clean conversion from Cp*Rh(bpy) to a new molecular species; complementary stopped-flow IR measurements confirm that the sole product of this initial protonation is the hydride, [Cp*Rh(H)(bpy)]+, based on a characteristic hydride stretch at 1,920 cm−1. The initial protonation was found to be followed by a concomitant tautomerization that generates [(Cp*H)Rh(bpy)]+. Variable temperature kinetic measurements, as well as isotopic labeling experiments, show that the activation barriers of these reactions are sufficiently modest to allow for the hydride to form and tautomerize to the Cp*H complex. Such involvement of Cp* in a catalytic system gives this workhorse ligand a possible role in catalytic applications that was not previously believed to be accessible. Taken together, our results suggest that protic Cp* noninnocence could play a role in catalysis and should be actively considered in mechanistic investigations.

Long-standing work from Kölle, Grätzel, and coworkers describes a synthetic procedure for the clean and facile generation of [Cp*Rh(OH2)(bpy)]2+ (1) (see Fig. 2, where L = H2O); 1 is a well-known precatalyst for the chemical and electrochemical reduction of protons to dihydrogen (37). Using Na(Hg), 1 can be reduced by 2e– to generate the electron-rich, neutral species, Cp*Rh(bpy) (3) (38, 39). Addition of two equivalents of strong acid, such as dimethylformamidinium ([DMF⋅H]+, pKa ≈ 6.1 in MeCN) (40), to 3 results in the quantitative formation of dihydrogen and regeneration of 1. However, more recently, the addition of weak acid, such as [HNEt3]+, to 3 was shown to result in the unexpected generation of [(Cp*H)Rh(bpy)]+ (5) instead of the anticipated metal hydride complex (19, 41). We have since also developed a synthetic procedure that enables the clean isolation of 5 (21). With these procedures, we prepared 1, 3, and 5 (SI Appendix, Figs. S1–S3) and, with careful handling, used these complexes to map the sequential electron and proton transfer events leading to hydrogen evolution with time-resolved pulse radiolysis and stopped-flow experiments.

Prior cyclic voltammetry (CV) data for 1 show a single, quasireversible 2e– redox event centered at –1.05 V vs. ferrocenium/ferrocene (37). This process can be confidently assigned as the Rh(III)/Rh(I) redox couple based on previous synthetic work and CV measurements of similar [Cp*Rh] complexes (22, 38). Since the potential of the Rh(II)/Rh(I) couple is similar to or more positive than that of the Rh(III)/Rh(II) couple, isolation of the Rh(II) species cannot be achieved by electrochemical or chemical reduction. This prompted us to use the technique of pulse radiolysis (PR), which allows for the rapid generation of singly reduced intermediates and their characterization by time-resolved spectroscopic methods (42–46). In 1989, Kölle and Grätzel investigated the reduction events of 1 in aqueous solutions using PR (37); in their aqueous study, absorbance vs. time kinetic data were used to support a disproportionation mechanism that involves the generation of 1 and 3, from 2. Building on this work, we performed PR in concert with time-resolved UV-visible detection, starting with 1 in dry MeCN. The absorption spectrum of 1 is unremarkable, with a feature at approximately 370 nm that tails slightly into the visible region and no absorption beyond 500 nm (see Fig. 3, gold line). Following pulse radiolysis, a dramatic change in absorption occurs, with the observation of a UV-visible spectrum 1 ms after the electron pulse that is consistent with the generation of 3 (Fig. 3, purple squares). Taking a cue from the known absorption profile of 3 (Fig. 3, light-purple line), kinetic traces were monitored near the prominent absorption feature with λmax = 511 nm in an attempt to examine the rates of the two reduction events. The resulting absorbance vs. time data (Fig. 3, Inset) revealed two distinct processes; the first process is very rapid (occurring over a timescale of ~10 µs) and is assigned as the reduction of 1 to [Cp*Rh(bpy)]+ (2), while the second, comparatively slower, process (occurring over a timescale of ~1 ms) is assigned as the reduction of 2 to 3 (SI Appendix, Figs. S8–S13). Examining the earliest absorbance vs. wavelength data reveals a UV-visible profile 10 µs after the electron pulse that is unique (Fig. 3, green squares) from those of both 1 and 3. This spectrum is assigned to the 1e– reduced species, 2, and has absorbance features that span the range of the visible region, with prominent transitions at 474 (3,100 M−1 cm−1) and 723 nm (4,900 M−1 cm−1, vide supra). The spectrum of 2 is consistent with other [Cp*Rh(II)] complexes examined by us (47, 48); in particular, the UV-visible spectrum of 2 obtained by PR is comparable to the RhII complex, [Cp*Rh(Me2daf)]+, which exhibits similar, but less intense, transitions at 462 (1,400 M−1 cm−1) and 716 nm (3,200 M−1 cm−1; Me2daf is 9,9′-dimethyldiazafluorene) (48). With the spectrum of 2 in hand, we then moved to examine the second reduction event. In the work performed by Kölle and Grätzel, PR was carried out with 1 under aqueous conditions where they observed the generation of 3, which they attribute to a disproportionation event involving 2 equiv. of 2 (37). However, our work in dry MeCN does not support a kinetic model involving disproportionation of 2 because, when this is considered, the resulting extinction coefficients are too large for 3. To further confirm this observation, initial rate analysis (49) was performed to determine the reaction order with respect to 2. PR experiments were performed at time delays where 2 has already formed, and by controlling the dose, we generated different concentrations of 2. Plotting the logarithm of the initial rate of formation of 3 as a function of the logarithm of the concentration of generated 2 (log(dose) = log([2])) reveals a line with a slope of 1.03 ± 0.07, confirming a first-order dependence on 2 in the conversion of 2 to 3 (SI Appendix, Fig. S13). Thus, a disproportionation mechanism can be ruled out here under our specific conditions. Instead, the reduction of 2 to 3 here most likely results from the reaction of 2 with formate. A biexponential fit of the kinetic trace monitored at 520 nm (Fig. 3, inlay) results in two observed rates of 2.6 × 104 s−1 and 3.2 × 103 s−1, with the former being attributed to the reduction of 1 by CO2•– (since reduction of 1 by the solvated electron will occur on a much faster timescale that is not resolved in our data) and the latter corresponding to the conversion of 2 to 3 by formate anion; regarding the faster rate constant, reduction by CO2•– is expected to be much slower than by e–solv, meaning the reaction with CO2•– should be rate limiting. Lending support to this kinetic model, the extinction coefficients for 3 obtained during PR agree well with those obtained from isolated material (Fig. 3); the UV-visible spectrum of 3 exhibits absorption bands that span the visible region (ε ≈ 4 to 17 × 103 M−1 cm−1 in MeCN) and displays strong absorption features near λmax = 511 nm (ε ≈ 1.7 × 104 M−1 cm−1) as well as longer wavelength transitions with vibronic structure between 650 and 850 nm arising from delocalization of electron density into the bpy ligand (vide infra) (50). Following this interrogation of the reduction events leading to 3, we moved to explore the reactivity of 3 with protons.

Complex 3 reacts readily with weak acids, an observation established in prior work (19). With this in mind, we set out to determine the rate constant, the reaction order, and molecularity of this protonation reaction using stopped-flow equipped with UV-visible or infrared (IR) spectroscopic detection. Control reactions rapidly mixing 3 with pure MeCN did not show degradation over the duration of the experiment (SI Appendix, Figs. S14 and S15), confirming that our apparatus was air-free and suitable for the desired experiments. Thus, 3 was rapidly mixed with [HNEt3]+ at various concentrations under pseudo-first-order conditions (SI Appendix, Figs. S16–S25 and S36–S39). The prominent absorption feature at approximately 511 nm (vide supra) was used to monitor the consumption of 3 after rapid mixing with [HNEt3]+ (Fig. 4A). The spectral changes observed after the addition of acid illustrate the rapid decay of 3 with isosbestic behavior at 379 and 431 nm, indicating clean conversion to a new molecular species. The decay of the absorbance at 511 nm occurs monoexponentially, and the resulting observed rate constants (kobs) were thus plotted as a function of acid concentration to extract the overall second-order rate constant, k3 = (6.4 ± 0.2) × 103 M−1 s−1 (Fig. 4B) (49). Notably, after initial protonation (~100 ms), the resulting UV-visible spectrum did not resemble the experimental spectrum of [(Cp*H)Rh(bpy)]+ (Fig. 4C). Eager to further interrogate this species, we repeated this protonation experiment with IR monitoring, providing unequivocal evidence from vibrational data for the formation of [Cp*Rh(H)(bpy)]+ (4) based on a characteristic Rh–H stretch at 1,920 cm−1 (Fig. 5). Notably, this value is similar to other Rh–H stretches (1,986 to 1,936 cm−1) previously reported by our group for analogous [Cp*Rh] monohydride complexes ligated by bisphosphine ligands, albeit slightly lower in energy, and is in good agreement with our DFT-calculated spectrum (1,920 vs. 1,902 cm−1, respectively; SI Appendix, Fig. S19) (30, 32). The UV-visible spectrum of 4 displays a prominent absorption at 400 nm and weaker features that tail into the visible region. As a result, the full reaction sequence could be monitored at 400 nm between 1 to 10 s following the rapid mixing of 3 with [HNEt3]+ (SI Appendix, Figs. S20–S25). The resulting absorbance vs. time data reveal two distinct chemical steps, where the first step corresponds to the initial protonation of 3 to generate 4. Following the generation of 4, the evolution of the absorbance vs. wavelength spectra results in an isosbestic point at 370 nm, supporting the formation of a new species (Fig. 4C). The monoexponential decay of 4 was monitored as a function of time at 400 nm, and the corresponding kobs values were plotted as a function of [HNEt3]+ concentration, revealing a second step that is zero-order in acid (k4 = 870 ± 30 s−1, see Fig. 4D). Monitoring the rate of tautomerization out to 1 s or 10 s returns values for the zero-order rate constant that are indistinguishable within the uncertainty of the measurements (SI Appendix, Figs. S22–S25). The zero-order dependence was further confirmed by monitoring the decay of the Rh–H stretch at 1,920 cm−1 during stopped-flow IR (SI Appendix, Figs. S20 and S21). These kinetic data are consistent with 4 undergoing hydride-to-proton tautomerization to generate [(Cp*H)Rh(bpy)]+ (5) and provide tantalizing kinetic and spectroscopic characterization of the species involved in these proton transfer reactions. Having identified the sequence of the elementary protonation and tautomerization reactions, we next set out to further characterize the steps leading to Cp*H formation using variable temperature kinetic experiments. Variable temperature kinetic data were collected between 20 and 40 °C under pseudo-first-order conditions following rapid mixing of 3 with either [HNEt3]+ or [DNEt3]+ (D incorporation >95%, SI Appendix, Fig. S4). Eyring and Arrhenius plots of ln(kobs/T) as a function of T−1 using 3 reveal the enthalpy of activation (ΔH‡), entropy of activation (ΔS‡), free energy of activation (ΔG‡), and activation energy (Ea), respectively, for the elementary protonation and tautomerization steps that convert 3 to 4(D) (our notation 4(D) refers to the protonated (4) or deuterated (4D) species) and 4(D) to 5(D) (Table 1 and SI Appendix, Figs. S27–S31 and S36–S47). Experimentally derived activation parameters for the protonation of 3 with [HNEt3]+ or [DNEt3]+ ([H(D)–NEt3]+) reveal values for ΔH‡ (for H+, 8.8 kcal/mol; for D+, 9.7 kcal/mol) and ΔS‡ (for H+, –22.3 cal mol−1 K−1; for D+, –21.0 cal mol−1 K−1) at 25 °C that are in good agreement with activation parameters observed for other proton transfer reactions (51), and with prior calculations for the protonation of 3 with [HNEt3]+ (41). Our experimentally determined ΔH‡ and ΔS‡ parameters support the swift protonation of 3 in an associative bimolecular fashion (Table 1). The separation between the lines in the Eyring plots (SI Appendix, Fig. S50) here demonstrates the attenuation of the reaction rate when using [DNEt3]+, supporting the larger ΔH‡. The kinetic isotope effect (KIE = kobsH/kobsD) measured for the protonation of 3 is 2.2 ± 0.2 at 298 K, a value that corresponds to a normal primary KIE. Taken together, our activation entropy and KIE data support a three-centered transition state involving the cleavage of the [H(D)–NEt3]+ bond and formation of the [Rh–H(D)] bond in a concerted fashion, enabling handoff of the proton (or deuteron) to the electron-rich Rh(I) during the generation of 4(D). As an alternative way to analyze this reactivity, we have included data in the SI Appendix associated with this paper for both [HNEt3]OTf and [HNEt3]BF4 to explore the influence of the counteranion of the acid on reactivity. The data for [HNEt3]BF4 were collected under the same conditions as the data using [HNEt3]OTf and [DNEt3]OTf (20 to 40 °C under pseudo-first-order conditions). The resulting data confirm that swapping these counteranions does not substantially affect the rate of protonation or tautomerization and that OTf– and BF4– essentially behave as spectator ions. The similarity of the resulting rate constants, activation energies, and activation parameters among the respective protonation and tautomerization events is unsurprising due to the similar noncoordinating nature of both the triflate and tetrafluoroborate counteranions. See SI Appendix, Tables S1 and S2 for details. Perhaps more interesting, however, are the Eyring plots describing the tautomerization process of 4(D) to 5(D). To further examine the tautomerization event, the ΔH‡ for the conversion of 4 to 5 was determined experimentally to be approximately 2 kcal/mol higher than the corresponding transformation involving deuterons (4D to 5D, see Table1). Remarkably, the ΔS‡ for the tautomerization of 4 to 5 is 10 cal mol−1 K−1 more positive in comparison to the initial protonation step. The large negative ΔS‡ values suggest that the transition state leading to tautomerization involves associative behavior (49, 52). However, we note that this is only one possible explanation and that there are multiple contributions to the activation entropy that must be considered, including solvation and translational, rotational, and vibrational degrees of freedom (53, 54). The transition state describing the migration of the hydride (deuteride) to the Cp* ring during tautomerization may be less symmetrical than that for the initial hydride formation (55). In such a scenario, lower rotational and vibrational degrees of freedom could be consistent with the negative ΔS‡ values measured. However, to provide further support for the proposed bimolecular behavior during the tautomerization step, we performed a KIE analysis. Superficially, the tautomerization of 4(D) to 5(D) involves hydride (deuteride) migration, dearomatization of the Cp* ligand, and the formation of a new sp3-hybridized C–H(D) bond. The measured KIE, which was determined from the activation parameters measured for the tautomerization of 4(D) to 5(D), is 0.87 ± 0.09 at 298 K. Intriguingly, inverse KIEs of 0.8 to 0.9 have been measured for reactions in which a carbon atom undergoes a hybridization change from sp2- to sp3-hybridized, much like the situation encountered in this work when η5-Cp* is transformed to η4-Cp*H (56–60). Our measured inverse KIE, in combination with the experimentally determined ΔS‡, is also consistent with a plausible mechanism involving NEt3 playing the role of a proton shuttle, effectively facilitating tautomerization of the [Rh–H] to form (Cp*H) (61). Within this proposed mechanism, the atomistic picture for the hand-off of the proton from the metal center to the Cp* ring includes a role for an exogenous conjugate base, resembling traditional base-catalyzed pathways for tautomerization reactions in organic chemistry (62, 63).

In prior work, in situ NMR studies demonstrated that addition of excess [HNEt3]+ to 3 results in the generation of 5, with virtually no hydrogen evolution (19). On the other hand, addition of 2 equivalents of dimethylformamidinium ([DMF·H]+, pKa = 6.1) to 3 results in quantitative dihydrogen formation and regeneration of 1. We have also previously shown that anilinium ([PhNH3]+) reacts with 5 to readily generate dihydrogen (22). Here, to confirm dihydrogen evolution from 5 and [DMF·H]+, 1 equiv. of [DMF·H]+ was added to 5 in MeCN solution, and after 10 min, the headspace of the reaction was injected into a gas chromatograph. The resulting chromatogram confirms quantitative generation of dihydrogen, closing the catalytic cycle, and poising the system for further turnover (SI Appendix, Fig. S52). To more closely examine the role of 5 in hydrogen evolution, 5 was rapidly mixed with either [PhNH3]+ or [DMF·H]+ at various concentrations under pseudo-first-order conditions using a stopped-flow apparatus equipped with a UV-visible detector (Fig. 6 A and B). When using [PhNH3]+ as the proton source, consumption of 5 was monitored at 400 nm following rapid mixing with [PhNH3]+. The spectral changes observed after the addition of [PhNH3]+ illustrate a monoexponential decay of 5 with isosbestic behavior at 325 nm, which ultimately results in a UV-visible spectrum consistent with 1. In a similar fashion, when [DMF·H]+ is rapidly mixed with 5, isosbestic behavior is observed at 326 nm, and the absorbance at 400 nm decays monoexponentially, resulting in a UV-visible spectrum consistent with 1. The resemblance of the spectra and isosbestic points when using either [PhNH3]+ or [DMF·H]+ suggests the final product(s) absorbing in this region to be the same in both instances. While both hydrogen evolution and the regeneration of 1 are observed following addition of [PhNH3]+ or [DMF·H]+ to 5, when the kobs values are plotted as a function of acid concentration, two pathways to dihydrogen evolution become apparent. When the values of kobs measured for [PhNH3]+ were examined, a zero-order acid dependence (k-4 = 260 ± 10 s−1, see Fig. 1) on [PhNH3]+ is observed. On the other hand, plotting the resulting kobs values as a function of [DMF·H]+ reveals a first-order dependence on the acid (k5 = 230 ± 10 M−1 s−1, see Fig. 1). Taking into account the use of a much weaker acid ([HNEt3]+) when determining the rates for the initial protonation and tautomerization events (vide supra), the second protonation event or events are comparatively slower, suggesting that either step k-4 or k5 could represent the rate-determining step in dihydrogen evolution when using [PhNH3]+ or [DMF·H]+ with this system. The defining characteristics distinguishing the reactivity of the second protonation event are the observed zero-order and first-order dependences on [PhNH3]+ and [DMF·H]+, respectively, implicating the involvement of two pathways with different intermediates leading to dihydrogen evolution. When using [PhNH3]+ to protonate 5, the observed zero-order dependence is consistent with a reaction sequence involving relatively slow back-tautomerization of 5 to the more reactive hydride species, 4, followed by comparatively fast reactivity with [PhNH3]+ to generate 1 and dihydrogen. In contrast, when [DMF·H]+ is used, our kinetic data are consistent with protonation of 5 at the relatively electron-rich Rh(I) center to generate a putative, highly reactive intermediate, [(Cp*H)Rh(H)(bpy)]2+. (6). This species could then undergo fast intramolecular reactivity to generate dihydrogen and 1. Notably, prior computational work regarding the protonation of 5 with [DMF·H]+ postulated the accessibility of this pathway (41); the experimentally determined first-order dependence of H2 generation on [DMF·H]+ concentration shows that 5 is indeed not an off-cycle intermediate but instead can play a direct role in opening an additional pathway to dihydrogen evolution.

Our work here has elucidated the mechanistic details underpinning the elementary electron- and proton-transfer steps involved in the generation of dihydrogen using the model system Cp*Rh(bpy). Variable temperature experiments examining the proton-transfer steps have provided insight into the atomistic details involved in the initial handoff of the proton to the metal center, followed by the migration of the proton to the Cp* ring. In particular, the experimentally determined KIEs and activation parameters describing the tautomerization event provide direct evidence for the involvement of a proton shuttle, despite the fact that the Cp* ligand is closely associated with the metal center. Based on the known geometry of 5 from X-ray crystallography (19), and the computed structure of 4 (41), the distance (ca. 1.57 Å) between the hydride and an sp2-hybridized carbon in the ring would likely be too great for an unassisted proton transfer, highlighting the necessity of the proton shuttle (61). Notably, the requirement of a proton shuttle is not problematic because the typical synthetic or bioelectrochemical conditions used to access 4 or 5 involve either organic acid/conjugate base pairs or water (19, 22, 28, 29, 37). Beyond the initial protonation and tautomerization, the mechanistic path of the second protonation event is governed by the strength of the acid. Our study provides kinetic evidence that the identity of the acid influences the roles played by the hydride and Cp*H species, the intermediates that form, and the rate of dihydrogen evolution. In particular, the hydride complex, 4, has been an elusive and hotly pursued intermediate in organometallic rhodium chemistry, a situation promoted by the catalytic systems in which this species could play a role. In a sense, clear identification of the hydride here as an intermediate in dihydrogen evolution confirms the intuition of the field over the past 35 y. However, our work provides evidence that the hydride is but one possible protonated intermediate in this system and suggests the Cp*H moiety should be actively considered in mechanistic investigations when both hydrides and Cp* ligands could be involved in reactivity. Indeed, Kölle and Grätzel previously investigated the protonation of 3 with electrochemical measurements in aqueous phosphate buffer (37); following electrolysis, spectrophotometric data revealed an absorption spectrum that was not consistent with 1, 2, or 3, and instead proposed 4 as a plausible electrogenerated species. Upon reflection, the observation of the Cp*H complex, 5, was likely missed in the early work because the absorption spectra of 4 and 5 are very similar. The fact that these species are similarly colored can be attributed to the noninnocence of the bpy π-system, which is intimately involved in management of redox load in these compounds. To this point, 3 exhibits vivid pigmentation, attributable to delocalization of electron density by π-backbonding into the bpy π-system (50). The properties of bpy thus strongly influence the catalytic reactivity paradigm observed, reactivity that is not echoed in systems supported by bidentate bisphosphine ligands (30, 32) or even the closely related 9,9′-dimethyl-4,5-diazafluorene ligand (48), as well as impart spectral uniqueness to the [Cp*Rh] system. The Cp*Rh(bpy) system represents one of the sole examples of a case where a metal-bound protonated cyclopentadienyl-type ligand can be isolated and studied at room temperature. Cp* is not merely a spectator ligand, a generalizable conclusion in light of the reactivity observed here and in other systems by Peters and coworkers (23, 24). The five methyl groups of Cp* appear to play a key role, imparting stability in the sense that clean protonation/deprotonation reactivity is observed across the various proton transfer events. In the Cp*Rh(bpy) system, the involvement of both initial metal-centered protonation and the requirement of a proton shuttle in hydride-to-proton tautomerization appear to engender exclusively endo-protonation, a point of difference from related iron and cobalt systems (23, 24). For robust catalysis to occur with this system, the endo-protonated species is presumably required for both of the H–H bond–forming reaction channels proposed here; if [PhNH3]+ is utilized, back-tautomerization is needed, and if [DMF·H]+ is used to form a second hydride with adjacent Cp*H, an acidified C–H bond pointing downward toward the hydride to promote intramolecular reactivity is needed. On the other hand, since both endo- and exo-protonated species are active in the work with cobalt for H-atom transfer catalysis, stereochemical considerations of Cp* ligand-centered H-management could find further applications and appear worthy of further study.

In summary, we have described the elementary steps leading to dihydrogen evolution using Cp*Rh(bpy). Using pulse radiolysis, we measured a first-order dependence on 2 when 3 is generated in MeCN in the presence of excess formate, excluding the disproportionation mechanism proposed for this system in aqueous solution. Following the reaction of 3 with weak acids, strong kinetic and spectroscopic evidence confirms that the initial site of protonation is metal centered, forming 4; this is followed by subsequent tautomerization to generate 5. To further support our assignments, variable temperature kinetic data using isotopically labeled acid have been used to show that NEt3 plays an essential role as a proton shuttle, facilitating tautomerization of the proton (in the hydride ligand) to form the unusual η4-Cp*H moiety. 5 reacts further with stronger acids, demonstrating that Cp*H can be an active participant in dihydrogen evolution, opening viable reaction pathways that depend on the strength of the acid. Our findings provide key insight into the rates and elementary reduction and protonation steps that promote dihydrogen evolution with this system; taken together with the experimental activation parameters and mechanistic insights gleaned into metal-mediated hydride-proton tautomerism, these findings could be a springboard for further investigations involving the uncommon but apparently quite promiscuous η4-Cp*H moiety.

All manipulations were carried out in dry N2-filled gloveboxes or under an N2 atmosphere using standard Schlenk techniques unless otherwise noted. Pulse radiolysis experiments were carried out with rapid anaerobic mixing using the 2 MeV van de Graaf accelerator at Brookhaven National Laboratory using electron pulses with widths of 40 to 500 ns. Stopped-flow experiments with UV-visible detection were performed using an Applied Photophysics SX 18MV-R Stopped-flow Reaction Analyzer, and the stopped-flow experiments with infrared detection were performed with a modified FMA-10 FT-IR rapid mixing accessory from Hi-Tech Scientific. Gas chromatography experiments were carried out with a Shimadzu 2014 Custom-GC gas chromatograph equipped with a thermal conductivity detector and dual flame-ionization detectors. Detailed descriptions of the materials and methods used in this work are given in SI Appendix, pp. S5 and S6.

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