Switching between Stepwise and Concerted Proton-Coupled Electron

Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Switching between Stepwise and Concerted Proton-Coupled Electron Transfer Pathways in Tungsten Hydride Activation Tao Huang, Eric S. Rountree, Andrew P. Traywick, Magd Bayoumi, and Jillian L. Dempsey* Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States

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S Supporting Information *

ABSTRACT: Catalytic processes to generate (or oxidize) fuels such as hydrogen are underpinned by multiple proton-coupled electron transfer (PCET) steps that are associated with the formation or activation of metal−hydride bonds. Fully understanding the detailed PCET mechanisms of metal hydride transformations holds promise for the rational design of energyefficient catalysis. Here we investigate the detailed PCET mechanisms for the activation of the transition metal hydride complex CpW(CO)2(PMe3)H (Cp = cyclopentadienyl) using stopped-flow rapid mixing coupled with time-resolved optical spectroscopy. We reveal that all three limiting PCET pathways can be accessed by changing the free energy for elementary proton, electron, and proton−electron transfers through the choice of base and oxidant, with the concerted pathway occurring exclusively as a secondary parallel route. Through detailed kinetics analysis, we define free energy relationships for the kinetics of elementary reaction steps, which provide insight into the factors influencing reaction mechanism. Rate constants for proton transfer processes in the limiting stepwise pathways reveal a large reorganization energy associated with protonation/ deprotonation of the metal center (λ = 1.59 eV) and suggest that sluggish proton transfer kinetics hinder access to a concerted route. Rate constants for concerted PCET indicate that the concerted routes are asynchronous. Additionally, through quantification of the relative contributions of parallel stepwise and concerted mechanisms toward net product formation, the influence of various reaction parameters on reactivity are identified. This work underscores the importance of understanding the PCET mechanism for controlling metal hydride reactivity, which could lead to superior catalyst design for fuel production and oxidation.

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INTRODUCTION Electrochemical oxidation of dihydrogen using catalysts based on inexpensive transition metals is a promising avenue for the development of hydrogen fuel cells. Several systems have been extensively explored to probe the reaction mechanisms (Scheme 1A)1−3 of hydrogen oxidation, in which the essential roles of transition metal hydrides have been highlighted. Studies have shown that activation of a metal−hydride bond introduces a large kinetic barrier in the overall catalytic cycle.4−6 As a one-proton, one-electron proton-coupled electron transfer (PCET) event, metal−hydride bond activation may proceed either through stepwise (electron transfer followed by proton transfer, ET-PT, or proton transfer followed by electron transfer, PT-ET) pathways or in a single kinetic step (concerted proton−electron transfer, CPET). Unlike the extensively studied PCET mechanisms involving organic functional groups (N−H,7 O−H,8 S−H9), the activation of metal−hydride bonds remains comparatively underexplored.10−12 Stepwise ET-PT pathways have been reported for M−H activation in a number of systems,6,13−16 and recently the oxidation of a tungsten hydride complex was revealed to proceed via a concerted PCET pathway.10 The third major pathway in the PCET square scheme (Scheme © XXXX American Chemical Society

1A)the stepwise PT-ET mechanismwas only recently reported and involves a complex with a covalently tethered proton acceptor.17 Understanding the factors that influence the PCET reaction pathways for the activation of metal hydride complexesand realizing how PCET reactivity can be controlledcould help elucidate the relative efficiencies of stepwise and concerted pathways and aid in the design of more efficient systems for hydrogen oxidation. In this work, we explore the PCET activation of CpW(CO)2(PMe3)H (Cp = cyclopentadienyl) and establish how the reaction pathway can be modulated systematically through the choice of oxidant and base. The related species CpW(CO)3H was previously recognized by Hammarström and co-workers to react through a concerted PCET pathway upon activation with appropriate oxidant and base.10,17 Interested in examining if this intriguing reactivity could be generalized across other metal hydride complexesand across a wider range of oxidants and baseswe identified CpW(CO)2(PMe3)H as a model system because its well-established thermochemical properties18 informed us that both the PT and Received: July 5, 2018

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DOI: 10.1021/jacs.8b07102 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

can give rise to the 1e−/1H+ product, including PT-ET, ETPT, and CPET, while ET-PT-ET, ET-CPET, and CPET-ET are the major routes to the 2e−/1H+ product (Scheme 2). In

Scheme 1. (A) Catalytic Dihydrogen Activation Cycles Proceeding through a Transition Metal Hydride Intermediate (B) CpW(CO)2(PMe3)H is a Mixture of the Cis (58%) and Trans (42%) Isomers in CD3CN at 298 Ka

Scheme 2. Summary of the Different PCET Pathways for the Activation of CpW(CO)2(PMe3)H

the experiments described below, we have sought to establish the operating pathways as a function of oxidant and base strength and to understand how these reaction mechanisms can be modulated through choice of reagent. Oxidants and bases were chosen such that the first step of the stepwise PCET pathways is always endergonic (or close to thermoneutral), as it has previously been shown for the related complex CpW(CO)3H that stepwise pathways are dominant when strong oxidants or bases trigger initial ET and PT, respectively.10 Characterization of CpW(CO)2(PMe3)H Activation with Weak Oxidants and Strong to Moderate Bases. Upon rapid mixing of CpW(CO)2(PMe3)H (pKa = 26.6)18 with the weak oxidant [Me10Fc][PF6] (E°′ = −0.51 V, vs Fc+/ Fc in CH3CN) and a moderate base piperazine (4, pKa (BH+) = 18.69 in CH3CN for conjugate acid BH+), the rapid decay of [Me10Fc][PF6] absorbance (λmax 775 nm) was observed (Figure 1A). In parallel, quantitative formation of the 1e−/ 1H+ product CpW(CO)2(PMe3)• was observed through detection of the rapidly formed dimer [CpW(CO)2(PMe3)]2 (λmax 467 nm).19 The consumption of [Me10Fc][PF6] and the concomitant quantitative formation of dimeric [CpW(CO)2(PMe3)]2 were also observed when bases with conjugate acid pKa values spanning 17.95−25.98 were employed, including 4-dimethylaminopyridine (DMAP, 3, pKa(BH+) = 17.95), triethylamine (Et3N, 5, pKa(BH+) = 18.83), pyrrolidine (7, pKa(BH+) = 19.56), N,N′-dimethylpropane-1,3-diamine (DMPA, 8, pKa(BH+) = 20.39), 1,5-diazabicyclo(4.3.0)non-5ene (DBN, 9, pKa(BH+) = 23.89) 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU, 10, pKa(BH+) = 24.34), 7-methyl-1,5,7triazabicyclo[4.4.0]dec-5-ene (MTBD, 11, pKa(BH+) = 25.49), 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD, 12, pKa(BH+) = 25.98). In contrast, little change in absorbance was observed when only two of the three reagentsCpW(CO)2(PMe3)H and [Me10Fc][PF6] or base and [Me10Fc][PF6]were mixed (Figure S3A shown for pyrrolidine (3)). The lack of reactivity between CpW(CO)2(PMe3)H and [Me10Fc][PF6] suggests that the stepwise ET-PT mechanism is not a dominant pathway, which agrees with the thermochemical calculations, where the initial ET step (ΔG°ET2 = +15.5 kcal mol−1) is more endergonic than the initial PT reaction (ΔG°PT1 = +9.6 kcal mol−1 with pyrrolidine (3)) and the concerted pathway

Thermochemical properties: E°′(CpW(CO)2(PMe3)H•+/0) = 0.16 V (vs Fc+/Fc), E°′(CpW(CO)2(PMe3)−/•) = −1.12 V (vs Fc+/Fc), pKa(CpW(CO)2(PMe3)H) = 26.6, and pKa([CpW(CO)2(PMe3)H]•+) = 5.1, reported in CH3CN. a

ET intermediates of the stepwise pathways are relatively stable. The relatively lower oxidation potential (0.16 V in CH3CN vs Fc+/Fc) in comparison to CpW(CO)3H (0.74 V) allows reaction with ferrocenium-type oxidants that provide spectral handles for monitoring electron transfer with optical properties distinct from the tungsten complex and its activation products, including [CpW(CO)2(PMe3)]219 and [CpW(CO)2(PMe3)(CH3CN)][PF6],18 enabling deconvolution of complicated PCET mechanisms. Through stopped-flow rapid mixing experiments, we establish a complete mechanistic picture of the rich reactivity of this transition metal hydride complex and reveal that the reactivity can be tuned across all three limiting PCET mechanisms by the judicious choice of oxidant and base.10 Further, through this work we quantify the intrinsic barrier for protonation/deprotonation of the metal center to help realize how proton transfer kinetics can influence the PCET mechanism, examine the synchronicity of the concerted proton−electron transfer reactivity, and identify the experimental parameters that influence the PCET reaction pathway.

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RESULTS Upon reaction with base and oxidant, CpW(CO)2(PMe3)H can undergo one of two PCET reactions depending on the strength of the reagents employed. The first involves loss of one proton and one electron (1e−/1H+) to form a radical species (CpW(CO)2(PMe3)•) which undergoes rapid dimerization20 to form [CpW(CO)2(PMe3)]2. The second involves the loss of two electrons and one proton (2e−/1H+) followed by solvent association to form the cationic species [CpW(CO)2(PMe3)(CH3CN)]+. Several limiting PCET pathways B


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Figure 1. (A) (top) Representative spectra recorded at various time delays upon the rapid mixing of CpW(CO)2(PMe3)H (4.4 mM, WH) with [Me10Fc][PF6] (0.45 mM) and piperazine (4, 124 mM, pKa(BH+) = 18.69) in CH3CN with 0.1 M [Bu4N][PF6] and (bottom) molar absorptivity of [CpW(CO)2(PMe3)]2, CpW(CO)2(PMe3)H, [Me10Fc][PF6], and Me10Fc. All of the bases and corresponding conjugate acids studied in this work only absorb in the UV and are not depicted. (B) Change in [Me10Fc][PF6] (0.33 mM) absorbance at 775 nm in the presence of DMAP (3, 110 mM, pKa(BH+) = 17.95) and WH (4.9 mM) and CpW(CO)2(PMe3)D (4.8 mM, WD) following first-order decay kinetics. The inset shows the quadratic relationship of kobs′ vs [DMAP], where kobs is defined by kobs[Ox]0/[WH]. An inverse KIE (kH/kD) = 0.6 was observed. (C) Change in [Me10Fc][PF6] (0.26 mM) absorbance at 775 nm in the presence of DBU (10, 13.3 mM, pKa(BH+) = 24.43) with WH (2.7 mM) and WD (2.7 mM) following zero-order decay kinetics. The inset shows the linear relationship of kobs′ vs [DBU] with KIE = 1.6.

(ΔG° CPET1 = −4.4 kcal mol−1 with pyrrolidine (3)). Interestingly, no formation of [CpW(CO)2(PMe3)]2 was observed with weaker bases such as lutidine (pKa(BH+) = 14.13) and pyridine (1, pKa(BH+) = 12.33) over a period of 2 h. Although the net PCET reaction is endergonic under these conditions (ΔG°CPET1 = 3.0 and 5.5 kcal mol−1, respectively), the favorable dimerization of CpW(CO)2(PMe3)• (ΔG°Dim of the analogous CpW(CO)3• ≈ − 40 kcal mol−1)21 should make the overall reaction spontaneous. However, the uphill PCET steps are too slow with lutidine and pyridine to observe reactivity in our experimental window. To ascertain whether reactivity proceeds through a PT-ET or CPET pathway, we performed comprehensive kinetics analysis of the PCET reactivity observed. By monitoring the consumption of [Me10Fc][PF6] (λmax 775 nm, probed at 700 nm in some experiments) under pseudo-first-order conditions with excess CpW(CO)2(PMe3)H and relatively weak bases (DMAP (3), piperazine (4), Et3N (5), pyrrolidine (7) and DMPA (8)), we observed clean single-exponential decay kinetics (e.g., DMAP (3) in Figure 1B, see the Supporting Information for other bases), confirming an apparent firstorder dependence on [Me10Fc][PF6]. A similar kinetic profile was observed by monitoring the formation of [CpW(CO)2(PMe3)]2 (λmax 467 nm, Figure S2), indicating that dimerization is rapid, consistent with previous reports (kdim ≈ 1 × 109 M−1 s−1).20 With pyrrolidine (7, pKa(BH+) = 19.56) as a base, first-order rate constants (kobs) extracted from the kinetics traces monitoring [Me10Fc][PF6] consumption at 700 nm have a first-order dependence on the concentration of CpW(CO)2(PMe3)H (Figure 2A) and a first-order dependence on the concentration of base at relatively low concentration (2.5−10 mM, Figure 2B, red). Interestingly, the relationship between kobs and base is quadratic at higher base concentrations (10−125 mM, Figure 2B, blue). Further, the kobs value exhibits an inverse-order dependence on the initial concentration of the conjugate acid added ([BH+]0, Figure 2C) with a nonzero intercept and inverse-order dependence on the initial [Me10Fc][PF6] concentration ([Ox]0, Figure 2D). The inverse-order dependence on acid concentrations suggests a pre-equilibrium PT-ET (PTeq-ET) pathway, which should be

Figure 2. Apparent first-order rate constant kobs for the reaction of CpW(CO)2(PMe3)H, Me10Fc+, and pyrrolidine plotted as a function of (A) [CpW(CO)2(PMe3)H] (3.0−10.1 mM), (B) [pyrrolidine] (7, 2.5−125 mM), (C) [pyrrolidinium]0 ([BH+]0, 2.5−7.5 mM), and (D) [Me10Fc+]0 ([Ox]0, 0.17−0.80 mM).

influenced by either the addition of acid or by acid generated upon deprotonation of CpW(CO)2(PMe3)H. The quadratic dependence of kobs on base at higher concentrations is also consistent with a PTeq-ET model, as the pyrrolidine (7) used in the reaction order studies is known to have a moderate homoconjugation constant (KHC = 32 M−1),22 which should shift PT pre-equilibrium extensively at high base concentrations, yielding second-order dependence on base. In contrast to the second-order dependence on base observed for proton acceptors with moderate homoconjugation constants, a first-order dependence is seen for bases with small homoconjugation constants (e.g., Et3N (5), pKa (BH+) = 18.83, KHC = 0 M−1, Figure S11).22 Moreover, kobs appears to approach a limiting value with Et3N, indicative of saturation kinetics, supporting the formation of a hydrogen-bonded WH···base adduct (see below),23−25 consistent with computaC


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between CpW(CO)2(PMe3)H and base, kCPET1 is the bimolecular rate constant for the reaction of [Me10Fc][PF6] and the hydrogen-bonded hydride−base adduct, kPT1 and kPT‑1 are the deprotonation and reprotonation rate constants for the hydride complex, and kET1 is the bimolecular rate constant describing the ET reaction between anionic CpW(CO)2(PMe3)− and oxidant Me10Fc+. KHC1 and KHC2 are the primary and secondary homoconjugation constants of the base and its conjugate acid BH+ and of base with B2H+ (necessary to model reactions with bases that undergo homoconjugation; KHC2 is used only for kinetics simulations of systems in which a secondary homoconjugation constant is reported, namely pyrrolidine (7)),22 and kdim is the rate constant for dimerization of CpW(CO)2(PMe3)•.20 From the PTeq-ET/CPET reaction model presented in Scheme 3, the apparent first-order rate constant kobs, which depicts the consumption of [Me10Fc][PF6] under pseudo-firstorder conditions, can be described by eq 1 (see the Supporting Information for derivation).

tional investigations of the related species CpW(CO)3H which showed that the hydride undergoes hydrogen bonding with Me3N.26 Preassociation between WH and base via hydrogen bonding is likely a necessary first step of the postulated PTeqET reaction sequence. While the reaction order analysis supports a PTeq-ET mechanism, careful inspection of the reaction order data recorded for pyrrolidine (7) suggests that a second parallel reaction pathway may be operating, as indicated by the nonzero intercept for the relationship between kobs and 1/ [BH+]0 in Figure 2C. We posit that a concerted PCET pathway is operative, especially when PT pre-equilibrium favors CpW(CO)2(PMe3)H. To account for the contribution from both a PT eq-ET and CPET pathway and take homoconjugation into consideration, we have modified the parallel PT-ET and CPET kinetic model of tyrosine oxidation and deprotonation by Fecenko, Thorp, and Meyer27 (Scheme 3). In this paradigm, KA1 is the hydrogen-bonding constant

[B] ji zyz + kPT‐ET[WH] kobs ≈ KA1k CPET1[WH]jjj j 1 + K [B] zzz A1 k { ij yzji [B] + KHC[B]2 zy 1 z jj zj jj [BH+] + [Ox] zzzjjj 1 + K [B] zzz 0 0 A1 k {k {

Scheme 3. Parallel PT-ET and CPET Mechanism for Reaction of CpW(CO)2(PMe3)H with [Me10Fc][PF6] and Base

(1)

kPT‑ET is the PT-gated-ET rate constant, which is defined as the product of PT pre-equilibrium constant (KA1KPT1) and bimolecular rate constant kET1: kPT‐ET = KA1KPT1kET1 =

KA1kPT1kET1 kPT‐1

(2)

For data collected with pyrrolidine (7), this rate law is supported by the observation of first-order dependence of kobs on CpW(CO)2(PMe3)H, first- and second-order dependence on base, and an inverse-order dependence on the conjugate Table 1. Summary of Equilibrium and Reaction Rate Constants for the Reaction of WH with Various Bases and [Me10Fc][PF6] Determined from Regression Analysis base (no.)

pKaa

DMAP (3) piperazine (4) Et3N (5) pyrrolidine (7) DMPA (8) DBN (9) DBU (10) MTBD (11) TBD (12)

17.95 18.69 18.83 19.56 20.39 23.89 24.34 25.49 25.98

KA1kPT1 (M−1 s−1)b,c

KA1kCPET1 (M−2 s−1)d 21 ± 3 9.5 ± 1.5 (2.2 ± 0.2) × 102

KA1 (M−1)

KHC (M−1)f

kPT‑ET (M−1 s−1)g

3.9 ± 1.4 1e 1.1 ± 0.3 1e 1.8 ± 2.5

8.2 ± 0.4 36.4 ± 3.4 0 32 7.9 ± 3.4

(1.9 ± 0.1) × 10−2 (KIE = 0.6) (8.3 ± 0.5) × 10−2 (8.2 ± 0.2) × 10−2 0.76 ± 0.06 3.2 ± 0.2

31.6 ± 0.5 (KIE = 1.8) 20.1 ± 0.6 (KIE = 1.6) 3.7 ± 0.2 (KIE = 2.9) 392 ± 3

a pKa of conjugate acid. bFor the cases of DMAP, piperazine, Et3N, pyrrolidine, and DMPA, KA1kPT1 cannot be determined by regression analysis. These are determined from kinetics modeling results instead (see Table 2 for details). cFor DBN, DBU, MTBD, and TBD, KA1kPT1 is determined from initial rate analysis from eq 3. dFor piperazine and DMAP, KA1kCPET1 was not determined confidently by regression analysis. Kinetics modeling results are used instead (see Table 2 for details). eFor piperazine and pyrrolidine, KA1 was not determined confidently by regression analysis and KA1 = 1 M−1 is used. fKHC1 values for DMAP are determined independently in this work (see Figure S50). Literature values of KHC1 in CH3CN are used for Et3N and pyrrolidine.22 In the case of piperazine and DMPA, KHC1 was determined from regression analysis. gThe PTeq-ET rate constant is defined as kPT‑ET = KA1KPT1kET1 (eq 2), where kET1 = 7 × 106 M−1 s−1 (see Figure 3 for details). These values are the average of values determined from regression analyses varying (1) base concentration and (2)

1 + KHC[B] [BH+]0 + [Ox]0 (Figures S4−S14). D


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Journal of the American Chemical Society acid [BH+]0 with a nonzero intercept (Figure 2A−C). This expression also rationalizes the inverse-order dependence on initial [Me10Fc][PF6] concentrations (Figure 2D), and careful analysis has shown that increasing [BH+]0 or [Ox]0 has similar effect on the observed rate constants and can be treated as the combined term [BH+]0 + [Ox]0 (Figure S5). Regression analysis of kobs as a function of [B], [WH], and [BH+]0 + [Ox]0 provides KA1, kCPET1, and kPT‑ET (Table 1) with input of known values of KHC, and the rate constants kPT‑ET were found to be self-consistent across all the measurements under different conditions (Figures S4−S6). However, as the PT-ET pathway is dominant over CPET under most of the conditions studied, kCPET1 cannot be determined as confidently as kPT‑ET. This is because the contribution of the PT-gated ET component is inversely proportional to [BH+]0 + [Ox]0, which makes the stepwise component of eq 1

S8) derived from the kinetics model based on Scheme 3 (completely described in Scheme S7). The rate constants were solved numerically using an ordinary differential equation solver. The initial concentrations of all involved species, the time interval of reaction, reported homoconjugation constants, hydrogen-bonding equilibrium constants (KA1) determined by nonlinear regression analysis described above, and the previously reported rate constant for W• dimerization (kdim = 1 × 109 M−1 s−1)20 were used as fixed constants. The rate constants kPT1, kPT‑1, kCPET1, and kET1 were independently and systematically adjusted through iterations of the simulation to generate time-dependent concentration profiles, which were converted to kinetics traces (Figure S15) on the basis of known molar extinction coefficients for each species absorbing at 700 (εMe10Fc+ = 270 M−1 cm−1) or 775 nm (εMe10Fc+ = 530 M−1 cm−1). Using this method, the kET1 rate constant was found to be (7.7 ± 1.2) × 106 M−1 s−1, which is close to the value determined from the nonlinear regression analysis (Figure 3). Other rates constants (kPT1, kPT‑1, and kCPET1) obtained in this approach (Table 2) are consistent with those determined from regression analysis. In addition to the analytical efforts to deconvolute contributions from stepwise and concerted PCET mechanisms to the observed reactivity, isotopic labeling experiments were performed by mixing CpW(CO) 2 (PMe 3 )(H/D) with [Me10Fc][PF6] and DMAP (3) in order to ascertain the influence of the PT component on the observed kinetics. As shown in Figure 1B, the consumption of [Me10Fc][PF6] with CpW(CO)2(PMe3)D is faster than in the case with CpW(CO)2(PMe3)H, with kH/kD = 0.6 based on the kPT‑ET rate constants obtained as described above. As suggested by the definition kPT‑ET = KA1KPT1kET1, such an inverse isotope effect should originate from an equilibrium isotope effect (EIE) of the proton transfer equilibrium between tungsten hydride and the base DMAP (3)

ij yzij [B] + KHC[B]2 yz 1 zz zzjj kPT‐ET[WH]jjj j [BH+]0 + [Ox]0 zzjj 1 + KA1[B] zz k {k {

ij yz [B] z KA1k CPET1[WH]jjj j 1 + K [B] zzz A1 k { under nonbuffered conditions. Similar relationships among kobs and [WH], [B], [BH+], and [Ox]0 were observed for reactions of CpW(CO)2(PMe3)H, [Me10Fc]+, and moderate bases, including DMAP (3), piperazine (4), Et3N (5), and DMPA (7), yielding PTeq-ET rate constants kPT‑ET of 1.9 × 10−2 to 3.2 M−1 s−1 (Figures S7− S14). From these data, we observe a log10−log10 free energy relationship between the PT-gated ET rate constant kPT‑ET and the PT equilibrium constant KA1KPT1, with a slope of unity (Figure 3). This linear free energy relationship is consistent

significantly larger than the concerted part

kH K K (H) = A1 PT1 kD KA1KPT1(D)

An EIE can be predicted on the basis of the corresponding stretching frequencies of W−H(D) and N−H(D) as described by Norton.28 The EIE calculated through this method (EIE = 0.5) is consistent with our measured value, supporting assignment of a PT pre-equilibrium process. A detailed derivation of this relationship is given in Scheme S11. When CpW(CO)2(PMe3)H is reacted with stronger bases (DBN (9), DBU (10), MTBD (11), and TBD (12), pKa(BH+) = 23.89−25.98, Figures S16−S22) and [Me10Fc][PF6], the decay kinetics of [Me10Fc][PF6] are zero order in oxidant (Figure 1C). This suggests a change in mechanism to a PT-ET process with an irreversible PT step. With stronger bases, reprotonation (described by k PT‑1 ) of [CpW(CO)2(PMe3)]− by the weak acids produced is slow. The ET step (kET1 = 7 × 106 M−1 s−1) competes with kPT‑1, making PT (kPT1) an irreversible, rate-limiting step for the formation of W•. The consumption of [Me10Fc]+ can be approximated by eq 3 (see the Supporting Information for derivation). The observed first-order dependences of k obs on CpW(CO)2(PMe3)H and base (Figures S16−S18) further support this rate law.

Figure 3. Linear correlation between kPT‑ET (M−1 s−1) (defined as KA1KPT1kET1, see eq 2 and Table 1) and proton transfer equilibrium constant KA1KPT2 (KA1KPT2 = 10ΔpKa, where ΔpKa = pKa(BH+) − pKa(CpW(CO)2(PMe3)H)) with a slope of 1 for bases 3 (DMAP), 4 (piperazine), 5 (Et3N), 7 (pyrrolidine), and 8 (DMPA). The intercept is defined as log10(kET1) (7 × 106 M−1 s−1).

with the relationship log10(kPT‑ET) = log10(KA1KPT1) + log10(kET1) defined by eq 2. From these data, the ET rate constant can be extrapolated (kET1 = 7 × 106 M−1 s−1). To further support the proposed reaction mechanism and obtain detailed kinetic information that cannot be determined from regression analysis (e.g. kPT1), simulations of kinetics traces monitoring the consumption of [Me10Fc]+ were performed with a series of differential equations (Scheme

− E

d[Ox] ≈ KA1kPT1[WH][B] dt

(3) DOI: 10.1021/jacs.8b07102 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Table 2. Rate Constants of Elementary Reaction Steps of the Reaction of WH with [Me10Fc]+ and Various Bases, Determined from Kinetic Modeling as a Function of Base (pKa of Conjugate Acid)a rate constant

DMAP (3, 17.95)

piperazine (4, 18.69)

Et3N (5, 18.83)

pyrrolidine (7, 19.56)

DMPA (8, 20.39)

kA1 (M−1 s−1) kA‑1 (s−1) kPT (s−1) kPT‑1 (M−1 s−1) kET1 (M−1 s−1) kdim (M−1 s−1)c kHC1 (M−1 s−1)d kHC‑1 (s−1)d kHC2 (M−1 s−1)d kHC‑2 (M−1 s−1)d kCPET1 (M−1 s−1)

3.9 × 109 1 × 109 8.7 × 10−3 1.5 × 107 7.5 × 106 1 × 109 8.2 × 109 1 × 109 0 0 2.4

1 × 109b 1 × 109b 0.24 2.0 × 107 8.5 × 106 1 × 109 3.6 × 1010 1 × 109 0 0 17

1.1 × 109 1 × 109 8.9 × 10−2 5.2× 106 6 × 106 1 × 109 0 0 0 0 8.6

1 × 109b 1 × 109b 0.75 8.3 × 106 7.5 × 106 1 × 109 3.2 × 1010 1 × 109 2 × 109 1 × 109 3.5 × 102

1.8 × 109 1 × 109 1.7 4.8 × 106 9 × 106 1 × 109 7.5 × 109 1 × 109 0 0 1 × 102

For comparison to Table 1, second-order PT rate constants: KA1kPT1 = kA1 × kPT1/kA‑1. Third-order CPET rates: KA1kCPET1 = kA1 × kCPET1/kA‑1. KA1 = 1 M−1 was fixed in the cases of piperazine and pyrrolidine since regression analysis did not provide a confident hydrogen-bonding constant. The rate constants kA1 and kA‑1 are set to close to the diffusion limit. cEstimated value from a previous report.20 dKHC1 value for DMAP is determined independently in this work (see Figure S50). Literature values of KHC1 in CH3CN are used for Et3N and pyrrolidine.22 In the case of piperazine and DMPA, KHC1 from regression analysis is used. a

b

Initial-rate studies were used to extract the second-order PT rate constants (KA1kPT1, Table 1), and these values are consistent with numerically simulated results (Table S2 and Figure S19). In contrast to the inverse EIE observed for DMAP (3), primary KIE values of 1.6−2.9 (Figures S24 and S25) of the KA1kPT1 rate constants were found for DBN (9), DBU (10), and MTBD (11), further supporting a PT-ET mechanism with rate-limiting PT for these stronger bases. Charge Transfer between CpW(CO)2(PMe3)− and Me10Fc+. In order to further confirm the rate constant (kET1) determined from the PT-ET model, kinetic studies between the isolated tungsten anion [CpW(CO)2(PMe3)][Na] and [Me10Fc][PF6] were carried out (Figure 3 and Figures S26−S28). Upon mixing, decay of [Me10Fc][PF6] absorbance (λmax 775 nm) was observed, consistent with prior experiments (Figure 1). Notably, an intermediate was observed, detected as a newly absorbing species with λmax 480 nm formed on the 0−0.01 s time scale. On longer time scales (0.01−2 s), the absorbance blue-shifted, consistent with the formation of [CpW(CO)2(PMe3)]2 (λmax 467 nm), with two clean isosbestic points at 436 and 490 nm. When the change in [Me10Fc][PF6] absorbance was monitored at 775 nm, the fast decay kinetics could be fit with equal concentration, second-order decay kinetics, to give a rate constant of 6.6 × 106 M−1 s−1. This second-order rate constant is consistent with the kET1 rate constant extrapolated from the free energy relationship plot in Figure 3 and numerical simulations in Table 2. Absorbance features attributed to the reaction intermediate were formed within 0.02 s of mixing (λobs 550 nm), suggesting that formation of the intermediate is concomitant with the consumption of [Me10Fc][PF6]. The rapid appearance of these features is followed by a slower firstorder decay (kfirst = 1.8 s−1) (Figures S27 and S28). A similar first-order rate constant (kfirst = 1.9 s−1) was obtained by monitoring the appearance of the tungsten dimer [CpW(CO)2(PMe3)]2 at 467 nm (Figure 4). These observations suggest that simple outer-sphere ET between [CpW(CO)2(PMe3)][Na] and [Me10Fc][PF6] is not sufficient to rationalize this redox reaction. The fast second-order rate constant (6.6 × 106 M−1 s−1) suggests a bimolecular reaction between [CpW(CO)2(PMe3)][Na] and [Me10Fc][PF6] to form an intermediate adduct that undergoes inner-sphere

Figure 4. Stopped-flow absorbance profile of [CpW(CO)2(PMe3)][Na] (0.36 mM) reacting with [Me10Fc][PF6] (0.36 mM) in CH3CN with 0.1 M [Bu4N][PF6]. The inset shows the change of absorbance at 467, 550, and 775 nm. The second-order rate kET = (6.6 ± 0.4) × 106 M−1 s−1 was determined from the second-order equal concentration fit (red), and the slower component was identified from the representative absorbance trances at 467 nm (pink) and 550 nm (green), showing first-order kinetics with rate constants of kfirst = 1.8−1.9 s−1 on the basis of the single-exponential fits (black).

electron transfer,29−31 followed by dissociation and rapid dimerization that proceeds with an observed first-order rate constant (1.8 s−1). Interestingly, the decay of the putative adduct to form [CpW(CO)2(PMe3)]2 is accelerated by excess [Me10Fc][PF6] (Figure S28). A proposed mechanism is presented in Scheme S3, where the initially formed adduct (W−Fe) may either undergo inner-sphere electron transfer or react with another 1 equiv of [Me10Fc][PF6] to produce CpW(CO)2(PMe3)• with a faster rate. This helps rationalize why no reaction intermediates are detected in the previously described three-component PCET reactions, as those studies were performed on a significantly longer time scale (10−300 s), during which the reactivity is gated by slow PT reactions (KA1kPT1 = 0.034−392 M−1 s−1), and the unreacted [Me10Fc][PF6] further shortens the lifetime of this inner-sphere ET intermediate. To further probe this putative inner-sphere reactivity, stopped-flow experiments between [CpW(CO)2(PMe3)][Na] and the tropylium cation ([C7H7][BF4], E°′ = −0.65 V vs Fc+/Fc) were performed under analogous conditions, and similar reactivity was observed (Figures S29 F


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Figure 5. (A) (top) Representative spectra recorded at various time delays upon the rapid mixing of CpW(CO)2(PMe3)H (4.0 mM, WH) with [Me2Fc][PF6] (0.30 mM) without an external base in CH3CN with 0.1 M [Bu4N][PF6]. (bottom) Molar absorptivity of [CpW(CO)2(PMe3)(CH3CN)][PF6], CpW(CO)2(PMe3)H, [Me2Fc][PF6], and Me2Fc. (B) Representative spectra showing the change in [Me2Fc][PF6] (0.43 mM) absorbance at 650 nm in the presence of WH or WD (4.0 mM). A two-parameter (k1, k2) nonlinear regression (black curve) described by eq 4 was used to extract the rate constants k1(H) = 6080 M−1, k1(D) = 4037 M−1, k2(H) = 0.079 s−1, and k2(D) = 0.081 s−1, respectively. (C) Rate constant k2 extracted from multiple stopped-flow experiments of WH/WD (4.0 mM) with [Me2Fc][PF6] (0.43 mM) and [Me2Fc] (0−11.1 mM). An inverse-order dependence on [Me2Fc] was observed for parameter k2..

suggesting a ET pre-equilibrium (ETeq) between CpW(CO)2(PMe3)H and [Me2Fc][PF6] is established. Coupled to the observation of [CpW(CO)2(PMe3)(CH3CN)][PF6] and [CpW(CO)2(PMe3)H2][PF6] products, these data inspired us to consider both ETeq-PT-ET and ETeq-CPET pathways to yield the two-electron one-proton transfer products (Scheme 4).

and S30). Overall, this unprecedented observation on the elementary PCET mechanisms underscores the rich pathways that could potentially lead to different reactivities of metal hydride mediated reactions, setting the stage for new chemical applications. Characterization of CpW(CO)2(PMe3)H Activation Using a Moderate Oxidant in the Absence of External Bases. In contrast to the reactivity with [Me10Fc][PF6], CpW(CO)2(PMe3)H readily reacts with the stronger oxidant [Me2Fc][PF6] (E°′ = −0.12 V, Figure 5A) in the absence of base. Reaction of equimolar CpW(CO)2(PMe3)H and [Me2Fc][PF6] leads to complete consumption of [Me2Fc][PF6] and formation of 1/2 equiv each of cationic [CpW(CO) 2 (PMe 3 )(CH 3 CN)][PF 6 ] and dihydride [CpW(CO)2(PMe3)H2][PF6], as detected via 1H NMR spectroscopy. This is consistent with a prior report, where a stronger oxidant Fc+ (E°′ = 0 V) was shown to oxidize CpW(CO)2(PMe3)H and generate equimolar amounts of the 2e−/ 1H+ product [CpW(CO)2(PMe3)(CH3CN)]+ and [CpW(CO)2(PMe3)H2]+.18 It is known that the oxidation of metal hydrides can yield the dihydride species through intermolecular proton transfer from the acidic metal hydride radical cation to the neutral hydride, from which dihydrogen could be released.11,18 Despite the widespread observation of this interhydride proton transfer reaction, only a few kinetic studies32 have been reported. Here we have utilized stoppedflow rapid mixing and kinetics modeling to fully understand the reactivity (Figures S31−S33). As illustrated by the representative data in Figure 5A and Figure S47, rapid mixing of [Me2Fc][PF6] and excess CpW(CO)2(PMe3)H yields quantitative formation of 1/2 equiv of the cationic species [CpW(CO)2(PMe3)(CH3CN)][PF6], consistent with the 1H NMR data described above (the dihydride absorbs in the ultraviolet, outside the detection window). However, the decay of the [Me2Fc][PF6] absorbance at 650 nm cannot be fit to simple first-order or second-order kinetics. A reasonable fit can be achieved with a sum of two single-exponential functions, suggesting that parallel pathways might be operating. Through systematic experiments, we found that the consumption rate of [Me2Fc][PF6] is inversely proportional to the concentration of [Me2Fc] in the solution,

Scheme 4. Parallel ETeq-PT-ET and ETeq-CPET Mechanism for the Reaction of CpW(CO)2(PMe3)H with [Me2Fc][PF6] in the Absence of an External Base

The analytical expression describing the kinetics for the reaction pathways described in Scheme 4 is given as eq 4 (see the Supporting Information for derivation) ε [Ox]Abs = k2t e + k1(e k 2t − 1) [Ox] (4) 0

where k1 =

k CPET2 kPT2

k2 =

2KA2kPT2KET2[WH]2 [Red]

KET2 =

kET2 kET‐2

ε is the molar absorptivity of the wavelength monitored, KA2 is the hypothetical hydrogen-bonding constant between CpW(CO)2(PMe3)H and the one-electron-oxidized [CpW(CO)2(PMe3)H]•+, kPT2 is the first-order PT rate constant from the hydrogen-bound intermediate [WH···HW]•+ to form [CpW(CO)2(PMe3)H2]+ and CpW(CO)2(PMe3)•, KET2 is the G


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Figure 6. (A) (top) Representative spectra recorded at various time delays upon the rapid mixing of CpW(CO)2(PMe3)H (4.1 mM, WH) with [Me2Fc][PF6] (0.43 mM), [Me2Fc] (9.2 mM) and MeO-pyridine (2, 12.3 mM). (bottom) Molar absorptivity of [CpW(CO)2(PMe3)(CH3CN)][PF6], CpW(CO)2(PMe3)H, [Me2Fc][PF6] and Me2Fc. (B) Representative spectra showing the change in [Me2Fc][PF6] (0.43 mM) absorbance at 650 nm in the presence of WH/WD (4.1 mM), [Me2Fc] (16.5 mM), and MeO-pyridine (2, 12.3 mM). The single-exponential fit is shown as the black curve. (C) Observed rate constant kobs (s−1) found to have inverse-order dependence on [Me2Fc]. KIE = 2.4 was found on the second-order PT rate constants KA2kPT2, as extracted from the slope of linear fit according to eq 5.

H and an inverse-order dependence on Me2Fc were found, consistent with the proposed mechanism described in Scheme 4. Isotopic labeling experiments using CpW(CO)2(PMe3)D were performed under identical conditions, showing a primary KIE = 1.46 for KA2kCPET2 and a much smaller KIE = 0.97 for KA2kPT2. As the determination of the hydrogen-bonding constant KA2 is experimentally inaccessible, the apparent second-order PT rate constant KA2kPT2 = 2.4 × 104 M−1 s−1 and the apparent third-order concerted rate constant KA2kCPET2 = 1.5 × 108 M−2 s−1 are determined. Kinetics modeling of multiple data sets (Figure S33) yielded rate constants consistent with an analysis of reaction order experiments (KA2kPT2 = 4.0 × 104 M−1 s−1 and KA2kCPET2 = 1.5 × 108 M−2 s−1). The electron transfer rate constant (kET2) between CpW(CO)2(PMe3)H and [Me2Fc][PF6] determined in kinetics modeling (kET2 = 3.0 × 103 M−1 s−1) is also in good agreement with the ET rates of the analogous complex CpW(CO)3H with oxidant at similar driving force.10 The excellent consistency between these experiments and analyses strongly suggests that parallel reaction pathways are indeed viable for the reaction of CpW(CO)2(PMe3)H and [Me2Fc][PF6] without an external base. We propose that the preequilibrium electron transfer is followed sequentially by irreversible PT and ET through an overall ETeq-PT-ET pathway. The parallel ETeq-CPET pathway involves a concerted PCET reaction involving a nonconventional hydrogen-bound intermediate.36,37 Characterization of CpW(CO)2(PMe3)H Activation with Bases and a Moderate Oxidant. To further explore the factors influencing the PCET reactivity of WH complexes, reactions between CpW(CO)2(PMe3)H and the stronger oxidant [Me2Fc][PF6] were carried in the presence of external bases with conjugate acid pKa values spanning 12.33−23.89, including pyridine (1), MeO-pyridine (2), DMAP (3), piperazine (4), piperidine (6), pyrrolidine (7), DMPA (8), and DBN (9). Rapid mixing of [Me2Fc][PF6] with excess CpW(CO)2(PMe3)H and base yields quantitative formation of 0.5 equiv of the cationic species [CpW(CO)2(PMe3)(CH3CN)][PF6] (Figure 5A and Figure S47 shown for MeO-pyridine). Separate 1H NMR experiments monitoring the reactivity show no dihydride formation under these conditions. This lies in contrast to reactivity with the weaker

ET equilibrium constant between CpW(CO)2(PMe3)H and [Me2Fc][PF6], and kCPET2 is the bimolecular electron transfer rate constant between [Me2Fc][PF6] and [WH···HW]•+. The apparent second-order PT rate constant and third-order concerted PCET rate constant are expressed as KA2kPT2 and KA2kCPET2, respectively. The rate constant for further oxidation of CpW(CO)2(PMe3)• by a second equivalent of [Me2Fc][PF6] to produce the cationic species [CpW(CO)2(PMe3)(CH3CN)][PF6] is described as kET3. This rate constant must be large in order to compete with the rapid radical dimerization to form [CpW(CO)2(PMe3)]2 (kdim = 1 × 109 M−1 s−1).20 In addition, it is known that the oxidation of CpW(CO)2(PMe3)• is highly coupled to the coordination of CH3CN,18 with an irreversible Ep,c ≈ − 1.9 V (vs Fc+/Fc in CH3CN) for the CpW(CO)2(PMe3)•/CpW(CO)2(PMe3)(CH3CN)+ couple, suggesting that the oxidation by Me2Fc+ (E°′ = −0.12 V vs Fc+/Fc in CH3CN) must be very exergonic. Thus, we have treated kET3 as a fast-irreversible diffusional limited reaction (kET3 = 1 × 1010 M−1 s−1) and the solvation process was not further deconvoluted. In this mechanistic paradigm, the PT event is proposed to proceed through a [WH···HW]•+ intermediate. It is known that, in a dry CH3CN environment, deprotonation of the radical cation [CpW(CO)2(PMe3)H]•+ can only be accomplished by 1 equiv of CpW(CO)2(PMe3)H.16 The relatively small PT driving force (ΔG°PT = −0.69 kcal mol−1)33 suggests that such reaction may not be fast. As nonconventional hydrogen bonding has been observed between hydride complexes and acidic protons,34,35 the hydrogen-bound intermediate WH···HW+• may be involved. Recent computational work has suggested such an intermediate in a related metal hydride system.36 Such hydrogen-bound species formed between the initial oxidation product WH+• and excess WH can react with another 1 equiv of [Me2Fc][PF6] in a concerted manner or via a stepwise PT-ET pathway to yield the reaction products [CpW(CO)2(PMe3)(CH3CN)][PF6] and [CpW(CO)2(PMe3)H2][PF6], which also rationalize the aforementioned complex kinetics observed for [Me2Fc][PF6] consumption. Application of this analytical expression yields excellent fits of experimental data under different conditions (Figures S31 and S32). A second-order dependence on CpW(CO)2(PMe3) H


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Journal of the American Chemical Society oxidant [Me10Fc]+, which yields the dimeric tungsten radical coupling product, and reactivity with [Me2Fc]+ in the absence of base, which yields 0.5 equiv each of the cationic species and the dihydride. The [Me2Fc][PF6] signal decays more rapidly in comparison to that when only two of the three reagents CpW(CO) 2 (PMe 3 )H and [Me 2 Fc][PF 6 ] or base and [Me2Fc][PF6]are mixed (Figure S3B). Further, decay of the [Me2Fc][PF6] signal is concomitant with the appearance of the cationic species [CpW(CO)2(PMe3)(CH3CN)][PF6] (λmax 420 nm) (Figure 6A). Additionally, when Me2Fc is present, kinetics monitoring the consumption of [Me2Fc][PF6] are attenuated (Figure S34), suggestive of an ET preequilibrium between CpW(CO)2(PMe3)H and [Me2Fc][PF6], as observed for the reactivity of these same reactants in the absence of base. WH•+ is deprotonated to form W•, and subsequent rapid oxidation yields [CpW(CO)2(PMe3)(CH3CN)][PF6] (an ETeq-PT-ET pathway). Reaction order studies based on the single-exponential decay kinetics of the oxidant [Me2Fc][PF6] as a function of substrate concentration reveal that the observed rate constant has first-order dependence on [CpW(CO)2(PMe3)H] and the base and an inverseorder dependence on Me2Fc (Figures S34−S36 for pyrrolidine). No quadratic relationship on base in high concentration was observed. This is in contrast with the case for PTeq-ET, suggesting that homoconjugation plays a nonsignificant role. Notably, nonzero intercepts were found for plots of kobs vs [Me2Fc]−1, suggesting a parallel rate-limiting concerted PCET reaction between CpW(CO)2(PMe3)H and [Me2Fc][PF6] (followed by rapid oxidation of the W• product, CPET-ET), which should be independent of [Me2Fc] (Scheme 5, purple).

Together, these observations are consistent with the parallel ETeq-PT-ET/CPET-ET mechanism in Scheme 5, where CpW(CO)2(PMe3)H can either be oxidized and deprotonated concertedly by [Me2Fc][PF6] and the external base or react with [Me 2 Fc][PF 6 ] via ET-gated-PT to form CpW(CO)2(PMe3)•, which is further oxidized by a second equivalent of [Me2Fc][PF6]. An analytical expression can be derived (eq 5; see the Supporting Information for derivation) describing the observed rate constant for [Me2Fc]+ consumption.

Scheme 5. Parallel ETeq-PT-ET and CPET-ET Mechanism for the Reaction of CpW(CO)2(PMe3)H with [Me2Fc][PF6] and Base

■

kobs =

2KA2kPT2KET2[WH][B] + 2k CPET1KA1[WH][B] [Red] (5)

Self-consistent second-order PT rate constants KA2kPT2 and third-order concerted rate constants KA1kCPET1 were obtained from the fits of kobs vs [Red], [B], and [WH] (Figures S34− S36 shown for pyrrolidine). Isotopic labeling experiments by reacting [CpW(CO)2(PMe3)D] with [Me2Fc][PF6] and pyridine (1) or 4-MeO-pyridine (2) were performed and compared to the analogous reactivity with [CpW(CO)2(PMe3)H]. A primary KIE (2.3−2.4) was found for the ratio of the second-order PT rate constants KA2kPT2, supporting assignment of the ETeq-PT-ET mechanism and our kinetics analysis. kobs values for the reaction of CpW(CO)2(PMe3)H and [Me2Fc][PF6] with DMAP (3), piperazine (4), piperidine (6), pyrrolidine (7), and DMPA (8) showed similar relationships with [B], [WH], and [red], yielding PT rates KA2kPT2 in a range from 7.6 × 104 to 5.3 × 106 M−1 s−1 (Table 3 and Figures S37−S42). To the best of our knowledge, these are the first direct measurements of PT rates from an unstable 17e− transition metal radical cation.11

DISCUSSION Influence of Oxidant Strength on Product Selectivity. The observation of different tungsten hydride activation products ([CpW(CO)2(PMe3)]2 vs [CpW(CO)2(PMe3)(CH3CN)][PF6]) formed as a function of oxidant strength (Scheme 2) suggests that the product formation step is governed by the competition between kdim and kET3. Although the reduction potential of the CpW(CO)2(PMe3)•/+ couple has not been established in CH3CN, it is known that the oxidation of CpW(CO)2(PMe3)• is highly coupled to the coordination of CH3CN, forming the strongly reducing 19e− CpW(CO)2(PMe3)(CH3CN)• intermediate18 (Epc ≈ − 1.9 V for CpW(CO)2(PMe3)•/CpW(CO)2(PMe3)(CH3CN)+ vs

Table 3. Summary of Equilibrium and Reaction Rate Constants for Reaction of WH with Different Bases and [Me2Fc][PF6] Determined from Regression Analyses base (no.) Pyridine (1) MeO-pyridine (2) DMAP (3) piperazine (4) piperidine (6) pyrrolidine (7) DMPA (8) DBN (9) CpW(CO)2(PMe3)H (13)b

KA2kPT2 (M−1 s−1)

pKaa 12.33 14.23 17.95 18.69 19.29 19.56 20.39 23.89 5.6c

(7.6 ± (2.1 ± (1.5 ± (3.9 ± (5.3 ± (3.8 ± (3.8 ± 107d (2.4 ±

0.3) 0.1) 0.1) 0.1) 0.4) 0.2) 0.2)

× × × × × × ×

4

10 (KIE = 2.4) 105 (KIE = 2.3) 106 106 106 106 106

0.2) × 104 e (KIE = 0.97)

KA1kCPET1 (M−2 s−1)

KA2kCPET2f (M−2 s−1)

(0.4 ± 0.1) × 10 (1.1 ± 0.4) × 102 (4 ± 3) × 102 (3.0 ± 0.3) × 103 (2.7 ± 0.6) × 103 (1.3 ± 0.1) × 104 (1.1 ± 0.1) × 104 2

(1.5 ± 0.1)× 108 e (KIE = 1.46)

a

pKa of conjugate acid. bExcess CpW(CO)2(PMe3)H functions as a base when no external base is used. cReference 33. dEstimated upper limit; see Figure S46 for details. eThe reaction pathway is defined in Scheme 4. fFrom regression analysis using eq 4; See Figure 5 for details. I


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Figure 7. Rate constants for deprotonation of CpW(CO)2(PMe3)H (kPT = KA1kPT1, blue) and [CpW(CO)2(PMe3)H•+] (kPT = KA2kPT2, red) as a function of PT reaction free energy in CH3CN, where − ΔG°PT (kcal mol−1) is calculated using 1.37 × [pKa(BH+) − pKa(CpW(CO)2(PMe3)H)] and 1.37 × [pKa(BH+) − pKa([CpW(CO)2(PMe3)H•+])] for blue and red points, respectively. The slopes of solid lines through linear fit are 0.47 (blue) and 0.23 (red). The dashed lines are obtained through nonlinear regression based on eqs 6 and 7 using λW→N = 36.8 kcal mol−1, |C| = 2.11 × 10−2 kcal mol−1 (blue), and λW→N = 28.4 kcal mol−1, |C| = 4.84 × 10−4 kcal mol−1 (red).

Fc+/0). This intermediate is strong enough to reduce either [Me2Fc][PF6] (E°′ = −0.12 V) or [Me10Fc][PF6] (E°′ = −0.51 V). However, the fact that no [CpW(CO)2(PMe3)(CH3CN)][PF6] was observed with the weaker oxidant [Me10Fc][PF6] suggests that kET3 ≪ kdim for [Me10Fc][PF6] and kET3 ≫ kdim with [Me2Fc][PF6]. Such product selectivity influenced by the choice of reagent would be crucial to determine the optimized conditions in which catalysts can achieve better performance. Free Energy Relationships of PT Rate Constants from CpW(CO)2(PMe3)H and CpW(CO)2(PMe3)H•+. Marcus-like quadratic free energy relationships38,39 are predicted for proton transfer reactions. Using the framework of nonadiabatic PT theory developed by Kuznetsov40 and Hynes,41−44 we can rationalize the observed free energy relationships for PT rate constants kPT1 and kPT2, which show a clear dependence on base strength (Figure 7). In Hynes’ model, the PT transfer reaction is described by an initial solvent fluctuation that brings the reactant and product states into resonance, at which proton tunnel can occur through the adiabatic electronic barrier in the proton transfer coordinate to form the product state.45 The relationship between PT rate constants and driving force at a fixed H-bond distance is described by eq 6:45−47 ÄÅ É ÅÅ ΔG⧧ ÑÑÑ π C2 Å ÑÑ Å kPT = expÅÅ− Ñ ÅÅÇ RT ÑÑÑÖ h/2π λRT (6)

observation of inverted region behavior. In spite of this complication in the formulation of theoretical expressions, there are reported examples45,48,49 that are consistent with proton tunneling through only ground vibrational states, from which both the normal and inverted regions of PT are observed. However, in most cases, quadratic behavior in both the “normal” and inverted regions is expected to be obscured by contributions from proton tunneling through excited vibrational states.47 With this context, we consider the relationship between second-order rate constants for the deprotonation of CpW(CO)2(PMe3)H (kPT = KA1kPT1, obtained from regression analysis and kinetics modeling; Tables 1 and 2) and CpW(CO)2(PMe3)H•+ (kPT = KA2kPT2, obtained from regression analysis; Table 3) and their PT free energies (−ΔG°PT, Figure 7). For deprotonation of neutral CpW(CO)2(PMe3)H, the PT rate constants (plotted as RT ln(kPT)) with bases DMAP (3), piperazine (4), Et3N (5), pyrrolidine (7), DMPA (8), DBN (9), and TBD (12) show a linear free energy relationship (LFER) (Figure 7, blue). In the limit of low driving force |ΔG°PT|/λ ≪ 1, eq 7 predicts a Brønsted α of 0.5 per eq 8, which is observed for these data (α = 0.47). α≅

(ΔG°PT + λ)2 4λ

(8)

PT rate constants for the deprotonation of other neutral metal hydrides have been quantified using NMR techniques, showing similar α values in the range 0.4−0.6.28 Interestingly, with bases DBU (10) and MTBD (11), the rate constants determined are lower than those predicted from the linear extrapolation of bases DMAP (3), piperazine (4), Et3N (5), pyrrolidine (7), DMPA (8), DBN (9), and TBD (12). The attenuation of PT kinetics between bases DBN (9) and MTBD (11) can be understood by recognizing the steric hindrance of the basic site (11 > 10 > 9). The influence of steric hindrance is especially striking when MTBD (11) and TBD (12) are compared, which have similar basicities (pKa(BH+) = 25.49 vs 25.98 for MTBD (11) and TBD (12), respectively), as the PT rate constant is 2 orders of magnitude smaller for the more sterically bulky MTBD (11). These observations are consistent with previous results from

where the proton coupling element C describes the PT tunneling probability and activation energy ΔG⧧ is a function of the solvent reorganization free energy λ and PT reaction driving force ΔGPT per eq 7. ΔG⧧ =

ΔG°PT δ ΔG⧧ 1 = + δ ΔG°PT 2 2λ

(7)

Equations 6 and 7 only describe the simplest case of nonadiabatic PT transfer reaction with a fixed hydrogen-bond length, and only the ground vibrational state of reagent and product are considered in the proton tunneling process. In this form, eq 6 predicts inverted region behavior for PT. However, Hynes and co-workers43,46,47 have shown that tunneling through an excited vibrational state is possible when the driving force is large, and these contributions override J


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the CPET rate constants with the free energy of the reaction is clear from the Brønsted plot in Figure 8, in which RT ln kCPET

our group recognizing the effect of sterics on PT rate constants for the formation of a cobalt hydride complex.50 The free energy relationship can be fit to eq 6 using an independently determined value for reorganization energy λ. Of note, Hynes initially only explicitly considered solvent reorganization energy for eq 6, but Peters has shown that this factor encompasses the vibrational reorganization energy and thus we expect this parameter to encompass inner- and outersphere reorganization energies.47,48 We estimate a value of λW→N = 36.8 kcal mol−1 (1.59 eV) from the self-exchange reaction between CpW(CO) 2 (PMe 3 )D and [CpW(CO)2(PMe3)]− (obtained via 2D EXSY, Figures S48 and S49) and the Marcus cross relation. This value is a lower limit calculated by assuming self-exchange of λN→N = 0 kcal mol−1. By fixing λW→N = 36.8 kcal mol−1 in eqs 6 and 7, we obtain a reasonable fit of our data in Figure 7 with |C| = 2.11 × 10−2 kcal mol−1 (Figure 7, blue dashed line, points 10 and 11 not included in fit). The goodness of fit is consistent with Hynes’s prediction that contributions from the 0−0 transitions are expected to dominate near ΔG° = 0.47 Similar to the free energy correlations observed for the deprotonation of CpW(CO)2(PMe3)H, an LFER was found for the rate constants for the deprotonation of cationic [CpW(CO)2(PMe3)H]•+ (pKa = 5.1) (kPT = KA2kPT2, obtained from regression analysis; Table 3),18 but with a shallower slope (α = 0.23) (Figure 7, red). This line is constructed on the basis of pyridine-type bases that have similar structures: pyridine (1), MeO-pyridine (2), and DMAP (3). This trend can be further extended to other non-pyridine bases, including piperazine (4), piperidine (6), pyrrolidine (7), and DMPA (8). The differences in the Brønsted slopes for CpW(CO)2(PMe3)H and [CpW(CO)2(PMe3)H]•+ are due to the relative driving forces for the deprotonation reactions. Whereas for CpW(CO)2(PMe3)H, |ΔG°PT|λ ≪ 1, deprotonation of [CpW(CO)2(PMe3)H]•+ with the same series of bases is substantially more exergonic, due to the differences in their pKa values. As such, |ΔG°PT| likely approaches the reorganization energy, and a shallower slope is expected from a tangent line near the top of the parabolic free energy relationship. These PT rate constants can also be correlated with a quadratic function with both λ and |C| as variables (eq 6), yielding λW→N = 28.4 kcal mol−1 (1.23 eV) and |C| = 4.84 × 10−4 kcal mol−1. In this region, where PT is relatively exergonic, increased contributions from the 0−1 transition are expected, skewing the goodness of fit to eq 6.47 As such, a large error is expected for λW→N, hindering comparisons with the larger value obtained for deprotonation of CpW(CO)2(PMe3)H. Still though, we found the proton coupling element of CpW(CO)2(PMe3)H (|C| = 2.11 × 10−2 kcal mol−1) is almost 2 orders of magnitude larger than that of [CpW(CO)2(PMe3)H]•+ (|C| = 4.84 × 10−4 kcal mol−1), suggesting a significant decrease of proton tunneling probability upon oxidation on tungsten center. While continuous efforts in both experimental and theoretical aspects will be needed to fully elucidate the physical meaning of λ and |C| for PT reactions, these free energy relationships observed for the deprotonation of CpW(CO)2(PMe3)H and [CpW(CO)2(PMe3)H]•+ can help us better understand nonadiabatic PT reactions. Free Energy Relationship of CPET Rate Constants. The rate constants for CPET of CpW(CO)2(PMe3)H (kCPET1) are observed to vary as a function of both oxidant and base, spanning 3 orders of magnitude (Tables 1−3). Correlation of

Figure 8. Relationship between the second-order PCET rate constant (kCPET, obtained from regression analysis and kinetics simulation, see Tables 1−3) as a function of CPET reaction free energy. The estimated KA1 values from Table 2 were used to calculate the secondorder rate constant kCPET1. For other cases that KA1 was not measured, KA1 = 1 M−1 was used.

is plotted vs −ΔG°CPET. Taken together, the data correlate with a slope (Brønsted α) of 0.33. However, the fit is poor and more clear correlations are observed when kCPET1 values determined with different oxidants are distinguised. The kCPET1 values have a correlation with α = 0.49 from reaction of CpW(CO)2(PMe3)H with [Me10Fc]+ (and base), while for reaction with [Me2Fc]+ (and base) α = 0.29. Marcus theory for electron transfer predicts a slope of 0.5 for reactions in the limit of low driving force, and PCET reactions have been widely analyzed in the context of Marcus theory.23,51 Indeed, the observed Brønsted slopes are similar to others reported for three-component concerted PCET reactions.10,23,51 The observation that the kCPET1 values exhibit no strong linear relationship when data for the two oxidants [Me2Fc]+ and [Me10Fc]+ are plotted together is notable (Figure 8). The synchronicity of proton and electron transfer components in CPET reactions can be assessed by comparing the free energy relationships (α values) obtained by varying the energetics of proton transfer vs electron transfer; when kCPET values respond equally to changes in ΔG°PT and ΔG°ET, the CPET reaction is interpreted as synchronous.51 Our data reveal that the CPET oxidation CpW(CO)2(PMe3)H is inequivalently influenced by ΔG°PT and ΔG°ET, indicating that CPET is asynchronous. While Mayer has uncovered synchronous oxidation of 1hydroxy-2,2,6,6-tetramethylpiperidine (TEMPOH) by ferrocenium oxidants and pyridine bases,51 asynchronous CPET reactions have been reported by Shaik and co-workers,52 who showed C−H, N−H, and O−H bond activation by non-heme iron−oxo complexes can proceed through concerted but asynchronous pathways. Moreover, data reported by Hammarström for the PCET oxidation CpW(CO)3H show a linear correlation between ln(kCPET) and ΔG°PCET through variation of both oxidant and base, insinuating that the CPET oxidation of CpW(CO)3H is synchronous,10 in contrast to our findings for CpW(CO)2(PMe3)H. Competition between Stepwise and Concerted PCET Reactivity. When the oxidant is switched from [Me10Fc]+ to [Me2Fc]+, the dominant reaction mechanism for the PCET oxidation of CpW(CO)2(PMe3)H switches from PTeq-ET to ETeq-PT, invariant of the base strength. It is especially notable K


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oxygen or nitrogen acids. Norton and others have attributed the slow PT kinetics of metal complexes to high kinetic barriers associated with the changes in coordination geometry and electronic configuration that accompany PT.28,33,53−57 While these data are for PT reactions, PT and PCET reactions of metal hydrides may undergo similar changes in structure and coordination associated with proton addition/removal. On the basis of these data, we hypothesize that proton transfer kineticsand thus reorganization energiesare correlated with the competition between concerted reactivity and stepwise PCET processes. Indeed, the self-exchange proton transfer rate constant determined here for CpW(CO)2(PMe3)H is 3 orders of magnitude smaller than that reported for the related complex CpW(CO)3H (0.24 M−1 s−1 vs 650 M−1 s−1, Table S7), and we find second-order PT rate constants for CpW(CO)2(PMe3)H (kPT = 3.4 × 10−2 to 3.9 × 102 M−1 s−1) are 2 orders of magnitude smaller than for CpW(CO)3H (kPT = (3.0−8.0) × 104 M−1 s−1) at similar ΔG°PT values (Figure S51). Can coordination complexes only access CPET reactivity if they can also undergo rapid PT reactions? If so, this helps rationalize why Hammarström and co-workers have observed a dominant CPET reaction pathway for CpW(CO)3H with the appropriate oxidants and bases,10 while our findings show stepwise reactivity is generally favored for CpW(CO)2(PMe3)H across similar ranges of free energy differences. Toward this, Hammarström and co-workers very recently demonstrated a tremendous enhancement of both PT and CPET rate constants in the oxidation of metal hydrides with appended proton accepting sites.17 This acceleration is presumably due to a reduction in reorganization energy (λ) and/or increase in proton coupling element. Indeed, theoretical work by both Miller58 and Hammes-Schiffer59 has shown that, for general PCET reactions, increasing the barrier for PT has a marked influence on reactivity, leading to a crossover from CPET reactivity to the ET-PT stepwise mechanism. While these studies tuned the PT barrier by changing the proton donor−acceptor distance, similar trends are expected when the PT barrier is influenced by structural reorganization. While much work must be done to test this hypothesis, comparison of our data to that of Hammarström10,17 suggests that indeed rates of metal hydride PT reactions may correlate with contributions from the CPET pathway in the related PCET reactivity. Second, our data suggest that the CPET reactivity observed for CpW(CO)2(PMe3)H is asynchronous, as evidenced by the lack of correlation for kCPET1 in the Brønsted plot across free energy changes induced by changes in both base and oxidant. As CPET reactivity occurs exclusively in parallel with a stepwise process under the conditions explored, this asynchronicity is not entirely surprising. Likely, the CPET reaction observed for Me10Fc+ has a transition structure dominated by proton transfer character, while for Me2Fc+ the CPET reaction electron transfer proceeds at a transition structure involving little change along the proton transfer coordinate, consistent with their relative Brønsted slopes, which is steeper (more sensitive to the change in basicity) for Me10Fc+. For comparison, Hammarström and co-workers observed correlations between kCPET1 and ΔG°CPET1 across changes in both oxidant and base,10 under conditions where the CPET mechanism dominated the reactivity. These data insinuate that the asynchronicity of CPET reactions is correlated with increased competition between stepwise and concerted routes.

that the dominant reaction pathway is dictated by the oxidant strength, and not the free energy of the net PCET reaction (Table S6 and Figure 8), yet contributions for a CPET pathway are observed for both systems. Moreover, while the rate constants for the concerted reaction (kCPET1 ≈ 101−104 M−1 s−1 Tables 1 and 3) can be larger than the rate constants for the initial processes in the stepwise routes (kPT1 ≈ 10−2 to 1 M−1 s−1, Table 2; kET2 = 3 × 103 M−1 s−1, Table S3), the stepwise reactions dominate. To better understand these observations, we evaluated the relative contributions of the stepwise vs the concerted pathways to the observed reactivity by quantifying the initial rates of formation of the one-electron, one-proton product CpW(CO)2(PMe3)• through the two pathways (d[W•]/dt; eq 9 for Me10Fc+ and eq 10 for Me2Fc+). d[W •] = kET1[W −][Me10Fc+] dt + KA1k CPET1[WH][B][Me10Fc+]

(9)

d[W •] = KA2kPT2[WH•+][B] dt + KA1k CPET1[WH][B][Me2Fc+]

(10) −

From the initial equilibrium concentrations of [W ] and [WH•+] estimated from KPT1, KET1, and reactant concentrations (Tables S9 and S10 and Figure S52), we find that the rate of product formation via a stepwise PTeq-ET pathway is 3−4 orders of magnitude larger than that via the concerted reaction with the weaker oxidant Me10Fc+. With the stronger oxidant Me2Fc+, product formation via the stepwise ETeq-PT route is also 3−4 orders of magnitude faster for the concerted reaction. These relative rates are strongly influenced by the equilibrium concentrations of W−, WH•+, and the hydrogenbonded WH···B adduct. Indeed, when Me2Fc reductant is added to reactions with Me2Fc+, the equilibrium concentration of [WH•+] is attenuated substantially and the concerted pathway becomes increasingly competitive. For example, with ca. 6−8 mM Me2Fc and 0.4 mM Me2Fc+, product formation from the concerted pathway becomes dominant with stronger bases such as pyrrolidine (7) and DMPA (8). The competition is clearly detected in experimental data (Figures S37−S42) when the relative kobs values are compared to kCPET1 − concerted reactivity is observed to play an increasingly dominant role in product formation as the reductant concentration and base strength are increased. Together, these data indicate that product formation is primarily dictated by the equilibrium concentrations of W−, WH•+, and the hydrogen-bonded WH···B adduct, and not simply the relative rate constants for the elementary steps. These data, collectively with the reorganization energy for proton transfer and analysis of CPET synchronicity described above, help us better understand the factors that influence the dominance of a stepwise reactivity for CpW(CO)2(PMe3)H over the concerted reaction in the systems studied in this work. Moreover, they inform us of the criteria necessary to engender concerted PCET reactivity in the activation (and formation) of metal hydride complexes. First, the large reorganization energies determined for proton transfer reactions between CpW(CO)2(PMe3)H (λ = 36.8 kcal mol−1) or [CpW(CO)2(PMe3)H]•+ (λ = 28.4 kcal mol−1) and nitrogen bases are consistent with those of Norton, who after extensive study of PT reactions to metals and from metal hydrides determined that these reactions are slow in comparison to those involving L


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Journal of the American Chemical Society Third, when the initial PT or ET step of a stepwise PCET reaction is endergonic or close to isoenergetic, the equilibrium concentrations of the PT or ET product influence the observed rates of product formation. This provides a means to perturb the contributions of stepwise and concerted reactions through experimental parameters, including concentrations of reagents and hydrogen bond strength in reactive adducts.

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CONCLUSION The PCET oxidation of CpW(CO)2(PMe3)H can proceed via all three limiting PCET reaction pathwaysstepwise ETeq-PT, PTeq-ET, and CPET pathwaysdepending on the strength of oxidant and base employed. Under most conditions explored in this work, the stepwise pathway dominates the observed reactivity while the concerted route operates in parallel and makes minor contributions to overall product formation. This reactivity lies in stark contrast to that reported for CpW(CO)3H, where the concerted mechanism was identified as the primary mode of reactivity at similar driving forces.10 Analysis of the proton transfer kinetics in the limiting stepwise reactions reveals a large reorganization energy associated with deprotonation of CpW(CO)2(PMe3)H (λ = 36.8 kcal mol−1, 1.59 eV). The reorganization is manifested in the proton transfer kinetics; kPT values for CpW(CO)2(PMe3)H are 3 orders of magnitude smaller than for CpW(CO)3H. We rationalize the divergence of primary reactivity for CpW(CO)2(PMe3)H (stepwise) versus CpW(CO)3H (concerted) by acknowledging that intrinsic barriers to proton transfer likely correlate with access to a concerted PCET pathway, as metal hydrides undergo similar structural and coordination geometry changes with loss of a proton or a proton and electron, and large reorganization energies for a CPET reaction inhibit its competitiveness with stepwise processes. We hypothesize that, in order for concerted reactivity to be competitive with stepwise routes, the related proton transfer reaction must be rapid. Future work will focus on testing this prediction. Analysis of rate constants for the concerted PCET pathway as a function of proton transfer vs electron transfer energetics indicates that the concerted PCET reaction is asynchronous for CpW(CO)2(PMe3)H and synchronous for CpW(CO)3H. Because CPET reactivity is only observed as a secondary pathway for CpW(CO)2(PMe3)H, asynchronicity may be anticipated when stepwise and concerted pathways operate in parallel. Finally, experimental conditions such as concentrations of reagents can influence the contribution between one way or another. This offers an additional strategy for controlling PCET reactivity. Overall, these data better provide a deeper understanding of the properties intrinsic to the coordination complex and the factors engendered through the system parameters that influence PCET reactivity in the activation (and formation) of metal hydrides. This insight will guide the development of efficient catalysts for fuel cell and related applications.

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analyses, thermochemical details of reactions studied in this work, kinetics modeling details, 2D EXSY 1H NMR spectra investigating self-exchange rate constant and intrinsic barrier between CpW(CO)2(PMe3)− and CpW(CO)2(PMe3)H, measurement of the homoconjugation constant for DMAP, and theoretical estimation of the H/D KIE of CpW(CO)2(PMe3)H PTeq-ET reaction (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail for J.L.D.: [email protected]. ORCID

Jillian L. Dempsey: 0000-0002-9459-4166 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (CHE-1452615). J.L.D. acknowledges support from a Packard Fellowship in Science and Engineering and a Sloan Research Fellowship. We gratefully acknowledge the lab of Prof. Thomas Meyer for the use of their stopped-flow instrumentation and Sharon Hammes-Schiffer, Marshall Newton, Leif Hammarström, Yan Choi Lam, and Zachary Goldsmith for insightful discussions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b07102. Further experimental details, stopped-flow spectral and kinetic data, derivation of equations for regression M


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