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Dihydropteridine/Pteridine as a 2H/2e Redox Mediator for the Reduction of CO to Methanol: A Computational Study 2

Chern-Hooi Lim, Aaron M. Holder, James T. Hynes, and CHARLES B. MUSGRAVE J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b01224 • Publication Date (Web): 04 Apr 2017 Downloaded from http://pubs.acs.org on April 6, 2017

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The Journal of Physical Chemistry

Dihydropteridine/Pteridine as a 2H+/2e- Redox Mediator for the Reduction of CO2 to Methanol: A Computational Study Chern-Hooi Lim1,2, Aaron M. Holder1,3, James T. Hynes2,4, and Charles B. Musgrave1,2* 1

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Department of Chemical and Biological Engineering and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States 3

National Renewable Energy Laboratory, Golden, CO 80401, United States

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Chemistry Department, Ecole Normale Supérieure-PSL Research University, Sorbonne Universités-UPMC University Paris 06, CNRS UMR 8640 Pasteur, 24 rue Lhomond, 75005 Paris, France ABSTRACT: Conflicting experimental results for the electrocatalytic reduction of CO2 to CH3OH on a glassy carbon electrode by the 6,7-dimethyl-4-hydroxy-2-mercaptopteridine have been recently reported [J. Am. Chem. Soc. 2014, 136, 1400714010, J. Am. Chem. Soc. 2016, 138, 1017-1021]. In this connection, we have used computational chemistry to examine the issue of this molecule’s ability to act as a hydride donor to reduce CO2. We first determined that the most thermodynamically stable tautomer of this aqueous compound is its oxothione form, termed here PTE. It is argued that this species electrochemically undergoes concerted 2H+/2e- transfers to first form the kinetic product 5,8-dihydropteridine, followed by acid-catalyzed tautomerization to the thermodynamically more stable 7,8-dihydropteridine PTEH2. While the overall conversion of CO2 to CH3OH by three successive hydride and proton transfers from this most stable tautomer is computed to be exergonic by 5.1 kcal/mol, we predict high activation free energies (∆G‡HT) of 29.0 and 29.7 kcal/mol for the homogeneous reductions of CO2 and its intermediary formic acid product by PTE/PTEH2, respectively. These high barriers imply that PTE/PTEH2 is unable, by this mechanism, to homogeneously reduce CO2 on a time scale of hours at room temperature.

1. Introduction The development of renewable energy technologies has been the focus of intensive recent effort, with one aspect of considerable recent interest being the reduction of atmospheric CO2 concentrations.1 One particularly attractive approach that addresses both of these challenges involves using solar energy to power the production of “solar fuels” by either splitting H2O2-3 or reducing CO2.4-5 These technologies could form the basis of, respectively, a hydrogen or a methanol economy.6 In either case, the required discovery of capable redox catalysts lies at the heart of these visions but remains a grand challenge. Methanol (CH3OH) possesses several desirable attributes that motivate efforts for its production by reduction of CO2: it is a liquid at ambient conditions and is relatively easy to handle, transport, and store; it is compatible with existing infrastructure and combustion-based transportation systems; and it is an important feedstock for chemical processes.6 While CH3OH is a proven fuel and feedstock, chemical reduction of CO2 to efficiently produce it has long been and remains an outstanding challenge. Although important recent experimental advances

in reducing CO2 to CH3OH --- including reduction via frustrated Lewis pairs,7-10 photocatalysts,11-12 and electrocatalysts13-16 --- have been reported, many obstacles remain. Dyer and coworkers recently reported the use of the 6,7-dimethyl-4-hydroxy-2-mercaptopteridine (PTE) as an electrocatalyst for the reduction of CO2 to CH3OH at low overpotentials and Faradaic efficiencies of 10-23%; intermediate 2e- (formic acid, HCOOH) and 4e- (formaldehyde, OCH2) reduction products were also observed.17 The reaction was performed using an inert glassy carbon electrode and no metals, suggesting that this room temperature transformation would likely involve exclusively solution phase catalysis. However, this conclusion of a catalytic role of this pteridine has been challenged by Saveant and Tard, who reported that it does not reduce CO2 on the time scale of hours.18 In an effort to shed light on the question of whether PTE catalyzes CO2 reduction, we have employed quantum chemistry to study the process. As discussed in detail within, we find that the most stable tautomer of the pteridine in question is molecule 3 in Scheme 1 and a concert-

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ed 2H+/2e- transfer to 3 transforms it predominantly into the dihydropteridine tautomer 3a, in agreement with ref. 18 . To proceed further, we investigate a single mechanism which is similar to that found in our recent work on 1,2dihydropyridine (PyH2) - catalyzed reduction of CO2 to CH3OH.19 In this mechanism, tautomer 3a reduces CO2 to CH3OH via three successive hydride (H-) and proton (H+) transfers (HTPT) (Scheme 1). There exists, however, a key difference from the PyH2 case; for the present PTE-based reactions, we predict that the HT steps that reduce CO2 and its intermediary reduction product HCOOH, despite their thermodynamic feasibility, unfortunately have sufficiently high activation free energies (~29-30 kcal/mol) to render these steps too sluggish to proceed on the time scale of hours at room temperature. Scheme 1. Mechanism for Reduction of CO2 to Methanol via HTPT Steps Catalyzed by the 3/3a Redox Couple.

From one perspective, a pteridine tautomer such as 3 is a plausible organic molecular candidate to participate in the catalytic reduction of CO2 to CH3OH. Pyridine, a onering heterocycle related to 3’s pyrazine moiety, has been experimentally shown to catalyze the same reductions of CO2 to CH3OH under certain conditions.13 In this connection, we have previously reported quantum chemical calculations that predict that pyridine undergoes 2H+/2etransfers (in the sequence PT-ET-PT-ET; ET is electron transfer) to produce PyH2, which we predicted --- as noted above --- reduces CO2 to CH3OH via three consecutive HTPTs.19 Recent electrochemical studies provide support for partial hydrogenation of pyridine20 and involvement of dihydropyridine-like species21 in the reduction of CO2 to CH3OH on platinum and glassy carbon electrodes. Here, we report that the pteridine tautomer 3 and pyridine are reduced to their respective dihydride forms --the dihydropteridine tautomer 3a in Scheme 1 and PyH2, respectively --- by analogous redox processes, and that they possess intrinsically similar HTPT chemistry, albeit with importantly different activation free energies. In particular, they both undergo 2H+/2e- transfers to store energy via dearomatization;22 this ultimately drives reduc-

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tions by HT that recover aromaticity, and the reduced substrate is stabilized by subsequent PT23 mediated by proton relays composed of chains of water molecules.19, 22, 24-25

A number of the properties of pteridines --- which serve various important biological functions --- suggest that they might catalyze CO2 reduction. For example, the 4H+/4e- reduced pteridine tetrahydrobiopterin acts as an enzymatic co-factor: to reduce NADP+ to NADPH with the aid of the enzyme pteridine reductase;26-27 in nitric oxide synthase;28 in the synthesis of neurotransmitters;29 and in the O-H functionalization of aromatic amino acids.30-31 More directly relevant to the present study --- in particular in connection with the 2H+/2e- conversion of 3 to 3a in Scheme 1 --- are the electro-analytical studies of pteridine’s redox properties in efforts to understand its biological role.32-37 It is notable that concerted 2H+/2etransfers are typically observed in electrochemical reductions of pteridines rather than the alternative 1H+/1etransfers to pteridine that would produce open-shell radical species.36 Under typical electrochemical conditions, pteridine is reduced via a concerted 2H+/2e- transfer, producing first the kinetically favored species 5,8dihydropteridine (5,8-DHP), which subsequently tautomerizes to the more thermodynamically favorable 7,8dihydropteridine (7,8-DHP).32, 35 Depending on the electrochemical conditions, the relatively stable 7,8-DHP undergoes a further concerted 2H+/2e- transfer to form the 5,6,7,8-tetrahydropteridine (5,6,7,8-THP).33 All of these aspects will be relevant for our efforts in Section 3. The outline of the remainder of this paper is the following. We first provide the computational details in Section 2. In Section 3, we examine and explain that the principle pteridine tautomer in aqueous solution is the oxothione species 3 in Scheme 1. We then compute the reduction potentials of six different 2H+/2e- products derived from 3, and identify that 3a in Scheme 1 is the most likely produced dihydropteridine species as a result of 2H+/2etransfers. We then show that this product can perform three successive HTPTs to CO2, and its intermediaries HCOOH and OCH2, to ultimately form CH3OH while recovering the aromatic catalyst 3, albeit with high activation free energies, via a mechanism analogous to what we found in ref. 19. Concluding remarks are given in Section 4. 2. Computational Details Tautomerization reaction free energies of pteridines (Figure 1), 2H+/2e- tautomeric products of pteridine 3 (Figure 2), and 4H+/4e- tautomeric products of the same pteridine were determined at the rM06/6-31+G** level of theory with the aqueous solvent described by the implicit solvation dielectric continuum model (CPCM: Conductorlike Polarizable Continuum Model).38-40 As discussed below, calculation of the reaction free energies with rM06/6-31+G** is sufficiently accurate to reproduce the high-level CBS-QB3 results to within ~ 2 kcal/mol.

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Reduction potentials E0 (V) vs. Ag/AgCl, KCl (sat’d) at pH 6.3 were similarly computed with rM06/631+G**/CPCM-H2O. We employed the following values for computation of E0s at 298 K: Gibbs free energy of H+ = -6.28 kcal/mol,41 the solvation free energy -264.0 kcal/mol for H+,42 a potential shift of -0.0592 V per pH unit, and 4.44 eV for the absolute standard hydrogen electrode potential,43 and a shift of -0.197 V to convert the reference from SHE to Ag/AgCl, KCl (sat’d). The last two entries of Table 1 indicate that our methods can predict the E0s of the SHE and the reduction of CO2 to HCOOH to within ~0.04 V. Other studies provide further details of E0 computations.44-45 Our selected DHT-7H2O-CPCM core model system, comprising the 2H+/2e- tautomer species 3a, a substrate (CO2, HCOOH or OCH2) and seven explicit quantum waters, is computed with rM06/6-31+G**, with this entire system embedded in a CPCM dielectric continuum aqueous solvent. In Supporting Information (SI), Section 1, we show that rM06/6-31+G** is sufficiently accurate to reproduce the high-level rCCSD(T)/aug-ccPVDZ activation energy results to within ~3 kcal/mol. Next, we explain the important reason for inclusion of these seven QM described waters in our calculations. We first determined that the transition state (TS) for the reduction of CO2, HCOOH and OCH2 by 3a is predominantly of HT character, where the intermediary anionic species HCOO-, (HCOOH)H- and OCH3- are formed prior to PT to form the neutral products of formic acid, methanediol and methanol. Of course, for accurate prediction of TS free energies, the stabilization of these anionic species by the polar solvent must be correctly described. However, we found that representation of the aqueous environment solely by the dielectric continuum CPCM water solvent model does not accurately describe the stabilization of these important anionic species. For example, with only the CPCM description of the aqueous environment, the pKa of HCOOH (important for completion of the first HTPT step in the CO2 reduction) was predicted to be 11.9, which is unacceptably far from the experimental value of 3.8.46 This failure is mainly due to the absence of explicit H-bonding by solvating H2O molecules that stabilize the anionic HCOO- conjugate base. Addition of two explicit waters H-bonded to HCOO-, along with CPCM, only improves the pKa value to 9.8, while adding seven explicit waters and embedding in CPCM water yields a pKa of 4.4, which approaches HCOOH’s experimental pKa of 3.8.46 Our inclusion of seven explicit waters in our DHT-7H2O-CPCM model was motivated by this and related results. As a further improvement, we also employ the more explicit DHT-7H2O-QM/MM model, which similarly includes seven explicit quantum waters described at the rM06/6-31+G** level of theory, but instead of embedding this model in CPCM, we now model the external aqueous environment using explicit molecular mechanics (MM) waters. GROMACS MM software47 was used to configure

H-bonded waters surrounding the core model system (3a, the substrate to be reduced, and the seven explicit waters) centered in a 2nm × 2nm × 2nm cube. To ensure the proper water density (~1 g/cm3), ~200 MM waters were placed in the cube along with the core model system. Both the DHT-7H2O-QM/MM and DHT-7H2O-CPCM models were computed with the Gaussian 09 computational chemistry package, Revision D.01.48 In particular, for the former model, a two-layer ONIOM QM/MM model49 was used. The high-level layer consists of the core model system and is QM-described at the rM06/6-31+G** level of theory; while the low-level layer, which consists of ~200 MM waters, was described by the Universal Force Field (UFF).50 To calculate energies using the DHT-7H2OQM/MM model, stationary geometries were first obtained without electronic embedding, followed by single-point energy calculations with electronic embedding. The results of our calculations are summarized in Table 2. Activation free energies and enthalpies are referenced to complexed reactants, which are determined from IRC calculations; reaction free energies and enthalpies are referenced to separated reactants in solution; all values are reported in kcal/mol at 298 K and 1 atm. Because the reactants and products, in contrast to the reaction TSs, do not involve anionic species, the reaction free energies and enthalpies are determined in the absence of explicit waters, with the solvent environment described by CPCM. For example, the reaction free energy to form HCOOH and 3 from CO2 and 3a are determined by: ∆G0rxn = G0(HCOOH) + G0(3) - G0(CO2) - G0(3a). In order to obtain our ∆G‡HT values for our employed models and for each step, we add our calculated ∆H‡HT to the entropic contribution value -T∆S‡exp = 2.3 kcal/mol experimentally determined for an analogous HT reaction.51 The motivation and the details for this approach, which we apply to each reduction step, are discussed in ref. 19. A key condition for its appropriateness is that the TS for each of the three reduction steps is dominated by the HT aspect of the reaction;19 we have confirmed that this condition is satisfied in the present cases. 3. Results and Discussion Pteridine tautomers in aqueous solution. We begin by examining, via the computational method described in Section 2, the tautomers of 6,7-dimethyl-4-hydroxy-2mercaptopteridine (species 1 in Figure 1) to determine the stable structure(s) potentially responsible for the electrocatalysis of CO2. 15 possible tautomers exist in which the two labile H atoms reside at six possible sites ( = 15), namely: the S (position 2), the O (position 4), and the four Ns (positions 1, 3, 5, and 8); we report the relative thermodynamic stabilities of these 15 species in Section 2 of the SI. Species 2, which has been proposed to be the stable pteridine tautomer responsible for electrocatalysis,17 has a free energy of reaction from 1 that we predict to be ∆G0(298 K) = -5.3 kcal/mol relative to 1. However, we find that the transfer of 2’s thiol H to protonate the N at position 1 of the molecule and thus to yield

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tautomer 3 (the pteridine oxothione form) is favorable; indeed, we predict that 3 lies -12.2 kcal/mol below 2, and is the most stable of the 15 possible PTE tautomers. The ∆G0 values calculated with the highly accurate CBS-QB3 level of theory result in ∆G0 values for 2 (relative to 1) and for 3 (relative to 2) of -5.2 and -10.0 kcal/mol respectively, further supporting the conclusion that 3 is the dominant pteridine tautomer in aqueous solution.

Figure 1. Tautomerism of 6,7-dimethyl-4-hydroxy-2mercaptopteridine where the oxothione form 3 is argued to be the principal tautomer. Calculated reaction free energies, 0 ∆G (298 K) values obtained by the rM06/6-31+G**/CPCMH2O method: -5.3 and -12.2 kcal/mol, for the free energies of 0 2 relative to 1 and 3 relative to 2, respectively. Similar ∆G values are obtained by high-level CBS-QB3/CPCM-H2O calculations: -5.2 and -10.0 kcal/mol, respectively.

It is relevant to our prediction to observe that computational and experimental studies of 4-hydroxy-2mercaptopyrimidine (2-thiouracil),52-55 a close analog of pteridine 1, demonstrate that the analogous oxothione form is also the principle tautomer of 2-thiouracil. In particular, characteristic vibrations --- expected for 2thiouracil’s tautomer analogue of 1 --- of νO-H and νS-H in the range of 3750-3550 cm-1 and 2650-2550 cm-1, respectively, are absent, while strong bands assigned to νC=O and νC=S at 1780-1680 cm-1 and 1240-1110 cm-1, respectively, are both clearly visible in the vibrational spectra.54 Thus, 2thiouracil exists predominantly in the oxothione form, supporting our prediction that the PTE analogue exists almost entirely as species 3.56

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2 shows six possible 2H+/2e- dihydropteridine tautomers (3a-f) produced from 3 ( = 6, where the two additional Hs occupy four possible sites: the two Ns at positions 5 and 8 and the Cs at positions 6 and 7) where the Hs at Ns 1 and 3 are conserved. (For completeness, in SI, Section 3, we report the relative stabilities of all 70 possible 2H+/2eproducts of pteridine tautomer 3 (four Hs on eight sites). In Table 1, we report the calculated standard reduction potentials for the 2H+/2e- transfer processes to form 3a-f (see Section 2, paragraph 2 for the method). These E0 values are computed at pH 6.3 and reported in volts vs. Ag/AgCl (saturated KCl), similar to the electrochemical conditions employed.17 As shown in Table 1 for reactions 1 and 2, displayed below in Figure 3,32, 35 our computational approach accurately predicts the E0 values of the related pteridine species I and II that are available for comparison.57 In further support of our calculated potentials, our approach also correctly predicts the Standard Hydrogen Electrode (SHE) potential and the potential to reduce CO2 to HCOOH (last two rows of Table 1).

Figure 3. Computational predictions and experimental reduction potentials of related pteridine species.

Our Table 1 results predict that reductions of 3 to 3a-d require E0 values of -0.83 to -0.65 V, assuming there are no overpotentials from kinetically-limited processes. These values are in close proximity to the experimentally observed E0 values of ~ -0.68 V (vs. Ag/AgCl, 3M KCl and pH 6.3) from cyclic voltammetry (CV) of PTE.17 Moreover, we predict that 3a-d are among the most stable tautomers of the 70 possible 2H+/2e- products of pteridine tautomer 3 (SI, Section 3). But we rule out 3e and 3f in the participation of electrocatalysis because their E0 values of -1.03 and -1.31 V are noticeably farther from the observed E0s.17 Thus, tautomers 3a-d are predicted to be the four most likely candidates produced by 2H+/2e- transfers to reduce 3 at the glassy carbon cathode. We now determine the most important candidate among these four species.

Figure 2. Tautomeric 2H /2e products of species 3.

Table 1. Calculated Reduction Potentials

2H+/2e- tautomeric products of pteridine tautomer 3. We now examine possible 2H+/2e- products that can be derived from the predominant pteridine tautomer species 3 and that can serve as a catalyst for CO2 reduction. As we noted in the Introduction, the reduction of pteridine and related species normally proceeds via concerted 2H+/2etransfers to produce closed-shell intermediates.33, 36 Figure

Reaction

+

-

0

E (V) vs. Ag/AgCl, KCl (sat’d)

+

-

3a

-0.65

+

-

3b

-0.72

+

-

3c

-0.83

3 + 2H + 2e 3 + 2H + 2e 3 + 2H + 2e

[a]

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The Journal of Physical Chemistry +

-

3d

-0.83

+

-

3e

-1.03

+

-

3f

-1.31

+

-

Ia

-0.83 (-0.90)

3 + 2H + 2e 3 + 2H + 2e 3 + 2H + 2e I + 2H + 2e +

II + 2H + 2e +

2H + 2e

-

-

IIa

-0.77 (-0.75)

H2 +

CO2 + 2H + 2e

-

[b]

[c]

-0.61 (-0.57) HCOOH

-0.74 (-0.77) 0

[a] Reduction potentials E are calculated at pH 6.3 using the rM06/6-31+G**/CPCM-H2O level of theory (Section 2). 0 Available experimental values of E are shown in parenthe0 ses. [b] E calc= -0.83 V vs. SCE at pH 9, in comparison to E1/2= 32 0 -0.90 V vs. SCE at pH 9 as reported in ref. . [c] E calc= -0.77 V vs. SCE at pH 6.95, in comparison to Ep = -0.75 V vs. SCE at 35 pH 6.95, as reported in ref. .

As we noted in the Introduction, the less stable tautomer 5,8-DHP (3d) is typically produced first as a kinetic product, and has been observed spectroscopically.35 We now argue that 3d then undergoes acid-catalyzed tautomerization to the thermodynamically more stable 7,8DHP (3a) as follows. The half-lives for conversion of 5,8DHP to 7,8-DHP (derived from species II of equation 2) are 10.7 s and 220 s at pH 6.87 and 8.34, respectively.35 Thus, at pH 6.3 where the electrocatalytic reduction of CO2 to CH3OH was stated to occur,17 the kinetic 2H+/2eproduct 3d likely tautomerizes to the thermodynamically more stable dihydropteridine 3a, given that the observed timescale of a few hours for the reported catalytic CO2 reduction by the pteridine/dihydropteridine couple is much longer than these reaction times.17 Further, we have previously reported the stability of 3 and its subsequent reduction to 3a,58 and more recently Saveant and Tard have reported the production of 3a from 3 by preparativescale electrolysis.18 Consequently, the 2H+/2e- tautomer 3a is the most likely predominant species potentially responsible for any catalytic reduction of CO2 to CH3OH.59 Before proceeding to the discussion of the CO2 to MeOH conversion, we remark that we consider that 4H+/4e- transfer products of pteridine tautomer 3 are not likely to form at electrocatalytic conditions where only one reduction peak occurs in CV, because this current corresponds instead to reductions to produce 2H+/2etransfer products.17 Although as we noted earlier, 5,6,7,8THP is the normally observed 4H+/4e- product,33 for completeness we report the 28 possible 4H+/4e- transfer tautomers of pteridine tautomer 3 in SI, Section 4. In the remainder of this paper, we will adopt for ease of discussion the following notation. Unless otherwise indicated, we employ the acronym PTE for the oxothione tautomer 3 (Scheme 1 and Figure 1) and employ the acronym PTEH2 for the dihydropteridine tautomer 3a (Scheme 1 and Figure 2). HTPT steps to convert CO2 to methanol. We now examine in detail the elementary steps for the possible catalytic reduction of CO2 to CH3OH by species PTEH2 in

aqueous solution. Figure 4 displays the direct hydride transfer core reaction model with seven explicit quantum waters (DHT-7H2O-CPCM)19, with further dielectric continuum solvation described below, that we employ here to examine the energetics of PTEH2’s HTPT steps from CO2 to CH3OH. We define “direct HT” as transfer of an Has a single entity (effectively 2e-/H+) from the same chemical bond, as for example in Figure 4 where H- transfer involving one of the PTEH2 isomer’s hydridic Hs occurs to the carbon atom of CO2. (Such a direct HT differs fundamentally from 2e-/H+ transfers that involve multiple chemical bonds, e.g. 2e- transfers from a π-system while a H+ transfers from a σ-bond.60) As will be seen, the explicit quantum waters may or may not participate in a proton relay chain of water(s) which results in the protonation of the HT product and the deprotonation of the hydridedeprived PTEH2 isomer. Recall that the DHT-7H2O-CPCM core model system consists of the 2H+/2e- tautomer PTEH2, a substrate (CO2, HCOOH, or OCH2, depending on the reduction step considered) and seven explicit quantum waters modeled at the rM06/6-31+G** level of theory; that rM06/6-31+G** is sufficiently accurate to reproduce the high-level rCCSD(T)/aug-ccPVDZ results to within ~3 kcal/mol is shown in SI, Section 1. In addition to these explicit waters, the continuum CPCM description of the remaining aqueous solvent is also included to solvate the core model system.

Figure 4. Direct hydride transfer (DHT) model with seven explicit quantum waters (DHT-7H2O-CPCM); CO2 is shown here as the first of the three substrates treated via the DHT model. This core reactive system is embedded in implicit CPCM water in the DHT-7H2O-CPCM model and in explicit MM-described water in the DHT-7H2O-QM/MM model.

The quantum mechanically (QM)- described explicit waters stabilize the ionic TS as the hydride transfers from position 7 of PTEH2 (3a) to the C of the substrate; as noted above, these explicit waters may also act as a proton relay chain to aid the PT of the H+ at position 8 of PTEH2

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(3a). The more intensive DHT-7H2O-QM/MM model calculation involving both QM and MM calculations is simulated using a two-layer ONIOM model,49 where the QMdescribed core model system constitutes the high-level layer which is electronically embedded in the MMdescribed aqueous solvent which comprises the low-level layer; see Section 2 for a detailed discussion. The discussion of the detailed mechanism which could lead from CO2 to CH3OH will begin in the next subsection. Here we preview the results of this discussion for overview purposes, and present Table 2, which summarizes the results for both the DHT-7H2O-CPCM and DHT7H2O-QM/MM models: activation and reaction free energies and enthalpies for HTPT steps from the 2H+/2e- tautomer PTEH2 to CO2 and to the intermediaries formic acid (HCOOH) and formaldehyde (OCH2) in order to ultimately form CH3OH. First HTPT step PTEH2 + CO2 PTE+ HCOOH. Using the DHT-7H2O-CPCM model, we predict that the first HTPT step producing formic acid from CO2 has a reaction free energy ∆G0rxn of 3.9 kcal/mol. The endoergonic character of this reaction is expected, because the E0calc(3/3a) of -0.65 V is less negative than E0calc(CO2/HCOOH) of 0.74 V (E0expt = -0.77 V), see Table 1. With the same model, we also predict a significant free energy barrier ∆G‡HT of 29.0 kcal/mol for HT to CO2 by PTEH2 to produce formate (HCOO-) and PTE-H+. We find that the formation of formic acid HCOOH from CO2 is a sequential HTPT step:19 after the HT to CO2 --- which determines the reaction TS (see Figure 4) --- a separate PT step from PTE-H+ to the formate anion HCOO- is required to form HCOOH and recover the PTE catalyst. No proton relay, which would have generated a concerted, asynchronous reaction,19 is observed in this step. All of these features are quite analogous to our previous findings for the first step of CO2 reduction to HCOO- by PyH2,19 and can be understood as follows. HCOOH’s pKa of 3.8 is sufficiently low such that HCOO- is not basic enough to initiate PT via the proton relay which would strongly couple the PT to the HT. The

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character of the separate PT is strongly influenced by the experimental conditions. Although the PTE-H+ cation produced by the HT reaction is sufficiently acidic (pKa,calc = -8.4) to protonate the formate anion directly, deprotonation of any formic acid so formed results in absorption of its proton by the experimental system buffer (the solution is buffered17-18 at pH 6.3). The low pKa of PTE-H+ cation can be rationalized as follows. The N’s on the pyrazine moiety of PTE are not particularly basic, e.g. protonated pyrazine has a pKa of 0.6; in comparison, the N on pyridine is more basic: the protonated pyridine has a pKa of 5.3. The presence of C=O and C=S electron withdrawing groups on the ring adjacent to the pyrazine moiety in PTE will further lower the basicity of the N’s on pyrazine. Thus, in the solution whose bulk pH is 6.3, very little HCOO- is protonated to form HCOOH; this is a potential short circuit, since HCOOH is an intermediate oxidant required for the subsequent HTPT steps to form CH3OH. However, as we have previously argued in a similar case,19 the effective pH near the negatively biased cathode in the experimental system17 will be considerably lower, such that HCOOH will exist at considerably higher concentrations in this region and allow the subsequent HTPT steps. Our more sophisticated DHT-7H2O-QM/MM model predicts ∆G‡HT = 29.0 kcal/mol for the CO2 reduction step, a result identical to that predicted by the DHT-7H2OCPCM model (see Table 2). We preview here the fact that, in addition, the ∆G‡HT values predicted for the 2nd and 3rd HT steps to HCOOH and OCH2 (vide infra) by these two models differ by only 1.2 kcal/mol for each step. This level of agreement for the predicted barriers for all three HT steps indicates that the DHT-7H2O-CPCM model, with its seven explicit quantum waters augmented with implicit water solvent is sufficient to describe the polar TSs for these reactions. The computed TS structures (for reductions of CO2, HCOOH and OCH2 by PTEH2) using the DHT-7H2O-QM/MM model are shown in Table 2. Before leaving this first step, we remark that its high 29.0 kcal/mol barrier already casts doubt on the viability of the overall reaction path.

Table 2. Activation and reaction free energies and enthalpies for HTPT steps from dihydropteridine PTEH2 (3a) to CO2, HCOOH and OCH2.

6


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Modela

CO2b

HCOOHc

∆G‡HT

∆H‡HT

∆G0rxn

∆H0rxn

∆G‡HT ∆H‡HT

DHT-7H2O-CPCM

29.0

26.7

3.9

8.3

30.9

DHT-7H2O-QM/MM

29.0

26.7

3.9

8.3

29.7

OCH2d

∆G0rxn

∆H0rxn

∆G‡HT

∆H‡HT

∆G0rxn

∆H0rxn

28.6

2.9

3.5

19.3

17.0

-16.4

-15.8

27.4

2.9

3.5

20.5

18.2

-16.4

-15.8

a

Activation free energies and enthalpies for the three reduced species CO2, HCOOH and OCH2, referenced to the complexed reactants, which are determined from Intrinsic Reaction Coordinate (IRC) calculations. Reaction free energies and enthalpies for + b the reduction reactions are referenced to separated reactants in aqueous solution. 2e /2H transfer products: formic acid, c d methanediol and methanol. Both DHT-7H2O-CPCM and DHT-7H2O-QM/MM results are listed in the Table, with all values reported in kcal/mol at 298K and 1 atm. In the Figure, PTE is the tautomer 3 in Scheme 1 and PTEH2 is the dihydropteridine tautomer 3a in Scheme 1 and Figure 2. The schematic free energy surface (FES) shown is for the direct hydride transfer (DHT) model with seven explicit waters embedded in explicit MM-described water (DHT-7H2O-QM/MM). The transition state structures are shown for the reduction of CO2, HCOOH, and OCH2, respectively, and each is characterized by the anionic HT unstable species produced; RC-H signifies the carbon-hydrogen bond distance at the transition state. The ball-and-stick and wireframe models show the quantum mechanically (QM)-described core system and explicit waters described by molecular mechanics (MM), respectively.

Second HTPT step PTEH2 + HCOOH PTE + CH2(OH)2. We now examine the next HTPT step, producing methanediol CH2(OH)2 from formic acid. The free energy surface (FES) diagram associated with Table 2 reflects the challenge of this HCOOH reduction: we predict a free energy barrier ∆G‡HT of 30.9 kcal/mol using the DHT-7H2O-CPCM model; the DHT-7H2O-QM/MM model again predicts a similar barrier of 29.7 kcal/mol. Both calculations have explicit hydration of the lone pairs on the HCOOH reactant, so there is no lack of stabilization of the HT anionic moiety (HCOOH)H- produced (see the step 2 TS in Table 2). For the thermodynamics, the endergonic ∆G0rxn of 2.9 kcal/mol for this step reflects that E0calc (PTE/PTEH2) = -0.65 V is less negative than E0calc (HCOOH/CH2(OH)2) = -0.71 V. Finally, just as with the first step, a high barrier raises the issue of the overall mechanism’s viability. In a fashion similar to the analogous reduction step in the PyH2 system that we examined previously,19 the

2H+/2e- tautomer PTEH2 reduces HCOOH via a coupled HTPT step in which both HT and PT occur with a single TS. The details could be followed for the simpler calculation with only one quantum water. (In contrast to ref. 19, the present seven quantum waters calculations do not allow any control to examine different numbers of waters in a proton relay chain). For this system, PTEH2 first performs HT to HCOOH to form an unstable intermediate (HCOOH)H- and PTE-H+ product complex, which essentially determines the HTPT step TS and thus the activation free energy; past this TS, the basic carbonyl oxygen of the (HCOOH)H- anion then abstracts a H+ from its Hbonded water to initiate a proton relay that effectively transfers the H+ from PTE-H+ to form CH2(OH)2 and recover 3 (the PTE catalyst). The HTPT process is a coupled, asynchronous concerted reaction. Third HTPT step PTEH2 + OCH2 PTE + CH3OH. In order for the final HTPT step to proceed, the diol CH2(OH)2 produced in the second HTPT step just dis-

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cussed must first be dehydrated to form the more reactive unsaturated formaldehyde OCH2. Just as in ref. 19 --where the same issue arose--- we do not perform calculations for this step, but employ the experimental dehydration free energy barrier of CH2(OH)2 ∆G‡ = 20.0 kcal/mol and its reaction free energy ∆G0rxn = 4.5 kcal/mol;61-62 these free energies are shown in the FES diagram associated with Table 2. It is of interest to note that, in contrast to the situation found in ref. 19, the dehydration step here is not the largest free energy barrier step in the calculated mechanism. For the final, methanol-forming HTPT step involving the reduction by PTEH2 of formaldehyde, we calculate the barrier ∆G‡HT = 19.3 kcal/mol (DHT-7H2O-CPCM model) and reaction free energy ∆G0 = -16.4 kcal/mol; the DHT7H2O-QM/MM model predicts a similar barrier of ∆G‡HT = 20.5 kcal/mol. Again, the TS character is determined by the production of an unstable anionic HT complex (see Table 2). The exergonicity of this reaction reflects that E0calc (PTE/PTEH2) = -0.65 V is more negative than E0calc (OCH2/CH3OH) = -0.29 V. In a fashion similar to that of the second HTPT to HCOOH discussed above, a coupled HTPT asynchronous concerted process occurs in the OCH2 reduction: the TSdetermining HT from PTEH2 to OCH2 forms an unstable anionic intermediate OCH3- and PTE-H+ complex, which is followed by PT (mediated by a proton relay) that effectively transfers PTE-H+’s proton to OCH3- to form the desired methanol product while recovering the catalyst 3. The result reported in Table 2 has a relay chain consisting of two waters among the seven quantum waters. For a simpler system facilitating illustration, SI section 6 displays diagrams detailing the coupled, asynchronous concerted HTPT steps, mediated by a proton relay, for the reduction of OCH2 to CH3OH. Experimental viability of the three HTPT step mechanism. With the preceding results in hand, we can now turn to the important issue discussed in the Introduction of whether these results support the experimental feasibility of PTE homogeneous catalysis, via this three HTPT step mechanism, of CO2 reduction to methanol. (Here and hereafter, we use the term “homogeneous” in the chemical sense employed in the discussion of the similar mechanism in ref. 19). We first discuss the barriers for the mechanism. We have stressed that the barriers for each of these three steps are dominated by the HT aspect of the HTPT process, i.e. the reduction by PTEH2 of CO2, HCOOH and OCH2. It is especially notable that PTEH2 reduces CO2 to produce HCOOH at calculated barriers of ∆G‡HT of 29.0 and 29.7 kcal/mol, respectively, for the first two HTPT steps of the methanol generation; these are barriers which markedly exceed those found for the corresponding reductions by 1,2-dihydropyridine PyH2 in ref. 19. These higher barriers result because PTEH2 is a significantly weaker hydride donor than is PyH2, which we pre-

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viously demonstrated to be a strong hydride donor for the CO2 to CH3OH reduction.19 For example, we previously calculated that Mayr’s nucleophilicity parameter N63-64 for PyH2 was 11.4;19 N is a kinetic property proportional to the activation free energy, such that the reaction barrier increases with decreasing N. Using the same method, we predict PTEH2 to be a much weaker hydride donor, with a kinetic hydride nucleophilicity value of 6.4 (and thus a higher HT barrier). Evidently more energy is required to dissociate the C-H bond of the transferring hydride from PTEH2 than for PyH2. A qualitative argument for this feature is that the presence of a fused ring adjacent to the pyrazine moiety causes electronic delocalization over a larger π space and thus the reduced electron density on the transferring hydride. In addition, heteroatoms in the ring adjacent to the pyrazine core of PTEH2 (see 3a in e.g. Scheme 1) --- especially the C=S and the C=O moieties which act as electron withdrawing groups --- weakens PTEH2 as a hydride donor. We explicitly support this argument in SI, Section 5 by showing that when the ring adjacent to the pyrazine ring of PTEH2 is replaced by a heteroatom-free phenyl to yield 2,3-dimethyl-1,2dihydroquinoxaline, the barrier ∆G‡HT for hydride transfer to CO2 is lowered by ~ 8 kcal/mol compared to the HT barrier from PTEH2. As Table 2 and its associated FES diagram illustrate, the overall reaction that converts CO2 to CH3OH catalyzed by the 2H+/2e- tautomer species PTEH2 (CO2 + 3 PTEH2 CH3OH + H2O + 3 PTE) is exergonic by 5.1 kcal/mol. Despite the thermodynamic feasibility of the overall conversion of CO2 to CH3OH by PTE/PTEH2, our calculations predict that the HT steps to reduce CO2 and HCOOH in the mechanism have high activation free energies ∆G‡HT ~ 29-30 kcal/mol, such that these reactions would be very slow at 298 K. For comparison, such barriers result in reaction times about a factor of 106 longer than that resulting from the key free energy barrier of ~ 21 kcal/mol found by our calculations for the reduction of CO2 to CH3OH via the Py/PyH2 couple19, 65. Thus, our predicted high ∆G‡HT for the present mechanism are consistent with recent experimental findings that the PTE/PTEH2 couple does not homogeneously reduce CO2 on the time scale of hours.18 4. Concluding Remarks We have presented a quantum chemical computational study of the conversion of CO2 to methanol in aqueous solution catalyzed via a dihydropteridine/pteridine couple, a reaction which has received recent experimental attention, with conflicting conclusions about the reaction’s viability.17-18 In this effort, we have examined a three step HTPT mechanism similar to that found in our recent work on PyH2-catalyzed reduction of CO2 to CH3OH.19 We have first determined the thermodynamically most stable tautomer of 6,7-dimethyl-4-hydroxy-2mercaptopteridine (PTE) to be its oxothione form (3, in Scheme 1), a result supported by spectroscopic observa-

8


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tions.52-55 In accordance with previous electroanalytical studies,32-37 we conclude that this tautomer electrochemically undergoes concerted 2H+/2e- transfers to first form the kinetic product 5,8-dihydropteridine (3d in Figure 2), followed by tautomerization to the thermodynamically more stable 7,8-dihydropteridine (PTEH2 3a in Scheme 1 and Figure 2); a similar conclusion was reached concerning the viability of electrochemical formation of 3a via bulk electrolysis.18 Our results indicate that PTE’s catalytic driving force arises from a dearomatization-aromatization cycle, similar to that of 1,2-dihydropyridine.66 In particular, 3 is dearomatized upon its reduction to form PTEH2. In analogy with the mechanism found in our previous work involving a dihydropyridine/pyridine catalytic couple,19 the 2H+/2e- tautomer PTEH2’s proclivity to recovery its aromaticity provides a driving force for it to transfer a hydride H-, which is followed by transfer of a proton H+ from the resulting cation. This HTPT process provides a potential mechanism to reduce CO2 successively to CH3OH for our investigation. With the use of models that embed both the quantum reacting system and seven explicit quantum waters in an implicit continuum aqueous solvent or in an explicit molecular mechanical aqueous solvent, we predict that the 2H+/2e- tautomer PTEH2 is thermodynamically able to reduce, via three HTPT steps, CO2 and its two reduction intermediates formic acid and formaldehyde to ultimately form the desired methanol product; the overall conversion of CO2 to CH3OH by PTE/PTEH2 is predicted to be exergonic by 5.1 kcal/mol. However, we predict that activation free energies for the reduction steps of CO2 and HCOOH in the mechanism are high: 29.0 and 29.7 kcal/mol, respectively. In summary, our predicted barriers for the present homogeneous mechanism are too high for the reactions to proceed at reasonable rates at 298 K. This conclusion is consistent a recent experimental report that concludes that PTE/PTEH2 does not reduce CO2 on the time scale of hours,18 but does not support the opposite conclusion of an earlier experimental study.17

ASSOCIATED CONTENT Benchmarking of Electronic Structure Calculations; Pteridine + Tautomers in Aqueous Solution; 2H /2e tautomeric products of pteridine; 4H+/4e- tautomeric products of pteridine; Coordinates of Molecular Structures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Charles B. Musgrave ([email protected])

ACKNOWLEDGMENT This work was supported in part by NSF grants CHE-1214131 (CBM and AMH) and CHE-1112564 (JTH). We also gratefully acknowledge use of XSEDE supercomputing resources (NSF ACI-1053575) and the Janus supercomputer, which is sup-

ported by NSF (CNS-0821794) and the University of Colorado Boulder.

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Mechanics and Molecular Dynamics Simulations. J. Am. Chem. Soc. 1992, 114, 10024-10035. (51) Srinivasan, R.; Medary, R. T.; Fisher, H. F.; Norris, D. J.; Stewart, R., The Pyridinium-Dihydropyridine System. Reduction Potentials and the Mechanism of Oxidation of 1,4Dihydropyridines by a Schiff Base. J. Am. Chem. Soc. 1982, 104, 807-812. (52) Yekeler, H., Ab Initio Study on Tautomerism of 2Thiouracil in the Gas Phase and in Solution. J. Comput. Aided Mol. Des. 2000, 14, 243-250. (53) Marino, T.; Russo, N.; Sicilia, E.; Toscano, M., Tautomeric Equilibria of 2- and 4-Thiouracil in Gas Phase and in Solvent: A Density Functional Study. Int. J. Quantum Chem. 2001, 82, 44-52. (54) Lapinski, L.; Rostkowska, H.; Nowak, M. J.; Kwiatkowski, J. S.; Leszczyński, J., Infrared Spectra of Thiouracils: Experimental Matrix Isolation and Ab Initio Hartree-Fock, Post-Hartree-Fock and Density Functional Theory Studies. Vibration. Spectrosc. 1996, 13, 23-40. (55) Rostkowska, H.; Szczepaniak, K.; Nowak, M. J.; Leszczynski, J.; KuBulat, K.; Person, W. B., Thiouracils. 2. Tautomerism and Infrared Spectra of Thiouracils. Matrix-Isolation and Ab Initio Studies. J. Am. Chem. Soc. 1990, 112, 2147-2160. (56) We note that although the conclusion that 2-thiouracil resides predominantly as the oxothione tautomer was drawn from vibration spectra taken in low temperature matrices, PTE in aqueous solution (where the electrocatalysis takes place) is also expected to exist dominantly in the intrinsically stable oxothione form; for example, in SI, Section 2, we show that 3 is significantly more stable than all 14 competing tautomers by >10 kcal/mol in free energy at 298 K. (57) However, this statement requires justification because experimental reduction potentials were reported as E1/2 (half-wave potential) and Ep (peak potential) vs. SCE instead of E0 values. But we can assert that E0 ≈ E1/2 ≈ Ep because the 2H+/2e- reduc-

tion wave of pteridine rises sufficiently sharply so as not to introduce significant differences in the characteristic potentials. (58) Lim, C.-H.; Kuo, Y.-C.; Holder, A. M.; Hynes, J. T.; Musgrave, C. B. In Dihydropteridine/Pteridine as a 2H+/2e- Redox Mediator for the Catalytic Reduction of CO2 to Methanol Via Hydride-Proton Transfer, 249th ACS National Meeting, AMER CHEMICAL SOC 1155 16TH ST, NW, WASHINGTON, DC 20036 USA: 2015. (59) For the authors of ref. 18, the 3 3a reaction is the only reaction occurring. (60) Huynh, M. H. V.; Meyer, T. J., Proton-Coupled Electron Transfer. Chemi. Rev. 2007, 107, 5004-5064. (61) Winkelman, J. G. M.; Ottens, M.; Beenackers, A. A. C. M., The Kinetics of the Dehydration of Methylene Glycol. Chem. Eng. Sci. 2000, 55, 2065-2071. (62) Bell, R. P.; Evans, P. G., Kinetics of the Dehydration of Methylene Glycol in Aqueous Solution. Proc. R. Soc. A. 1966, 291, 297-323. (63) Horn, M.; Schappele, L. H.; Lang-Wittkowski, G.; Mayr, H.; Ofial, A. R., Towards a Comprehensive Hydride Donor Ability Scale. Chem. Eur. J. 2013, 19, 249-263. (64) Richter, D.; Mayr, H., Hydride-Donor Abilities of 1,4Dihydropyridines: A Comparison with Π Nucleophiles and Borohydride Anions. Angew. Chem. Int. Ed. 2009, 48, 1958-1961. (65) Since we have used the same activation entropy values here and in ref 19 (see Section 2), the same conclusion follows if activation enthalpy values are employed for the estimate. (66) Lim, C.-H.; Holder, A. M.; Hynes, J. T.; Musgrave, C. B., Catalytic Reduction of CO2 by Renewable Organohydrides. J. Phys. Chem. Lett. 2015, 5078-5092. .

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