CO2 Hydrogenation Catalysts with Deprotonated Picolinamide

Letter pubs.acs.org/acscatalysis

CO2 Hydrogenation Catalysts with Deprotonated Picolinamide Ligands Ryoichi Kanega,† Naoya Onishi,*,† David J. Szalda,‡ Mehmed Z. Ertem,§ James T. Muckerman,§ Etsuko Fujita,*,§ and Yuichiro Himeda*,† †

Research Institute of Energy Frontier, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ‡ Department of Natural Science, Baruch College, CUNY, New York, New York 10010-5585, United States § Chemistry Division, Brookhaven National Laboratory, Upton, New York 11973-5000, United States S Supporting Information *

ABSTRACT: In an effort to design concepts for highly active catalysts for the hydrogenation of CO2 to formate in basic water, we have prepared several catalysts with picolinic acid, picolinamide, and its derivatives, and we investigated their catalytic activity. The CO2 hydrogenation catalyst having a 4-hydroxy-N-methylpicolinamidate ligand exhibited excellent activity even under ambient conditions (0.1 MPa, 25 °C) in basic water, exhibiting a TON of 14700, a TOF of 167 h−1, and producing a 0.64 M formate concentration. Its high catalytic activity originates from strong electron donation by the anionic amide moiety in addition to the phenolic O− functionality. KEYWORDS: CO2 hydrogenation, deprotonated picolinamide ligand, water-soluble Ir catalysts, formate production, DFT calculations, hydrogen storage

T

Scheme 1. Mononuclear Catalysts for Selective CO2 Hydrogenation to Generate Formate under Ambient Conditions in Basic Water (pH ∼ 8.2)

he concentration of atmospheric CO2 continues to increase, causing major climate change.1 Hence, it is important to develop CO2 utilization technologies for converting CO2 into formic acid (formate) or methanol as has already been reported.2,3 Especially, formic acid and formate have recently attracted attention as a liquid-based hydrogen storage medium.2,4 Many homogeneous catalysts for CO2 hydrogenation to formate with a variety of ligands have been reported.2 Remarkably, a high catalytic activity was obtained using Ir(H)3 and Ru(H)Cl(CO) complexes both with PNP-pincer ligands under severe conditions (>4 MPa, >100 °C).5 Furthermore, non-noble metal catalysts with phosphinetype ligands have been successfully used for CO2 hydrogenation in organic solvents with some durability problems and/or requirement of strong organic bases.6 To achieve CO2 hydrogenation under ambient conditions without using any organic bases, we have developed mononuclear Cp*Ir catalysts having N,N-bidentate ligands such as i−v (Scheme 1) according to the following four catalyst-design concepts: (I) enhancement by the electrondonating effect explained by a Hammett plot,7a (II) a strong electron-donating effect resulting from the oxyanion7b formed by deprotonation of a hydroxyl group (Scheme 2a); (III) a second coordination sphere interaction such as pendent base effects;7c and (IV) the formation of more electron-rich N atoms through the resonance structure of the nonaromatic imidazoline (Scheme 2b) than that with imidazole.7h © XXXX American Chemical Society

On the basis of concepts (I)−(III),7c−e,g we designed catalysts i and ii containing an oxyanion from the Received: July 11, 2017 Revised: August 16, 2017 Published: August 21, 2017 6426

DOI: 10.1021/acscatal.7b02280 ACS Catal. 2017, 7, 6426−6429

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ACS Catalysis Scheme 2. Catalyst-Design Concepts for CO2 Hydrogenation

Figure 1. ORTEP diagram of 5. The counterion, HSO4−, is omitted for clarity.

deprotonation of an OH group. Especially, an ortho-positioned OH group accelerates H2 heterolysis through a proton relay involving a bridging water molecule.7f When using catalyst iii, the catalytic performance was improved significantly due to enhancement through the effect of concepts (II) and (III) by additional numbers of hydroxyl groups.7e Concept (IV) is the nonaromatic electron delocalization7h in the amidine moiety without deprotonation of the NH proton (pKa 8.94) (Scheme 2b). We have reported that nonaromatic imidazoline moieties were more efficient ligands than aromatic imidazole moieties due to the restricted delocalization of π-electrons within the amidine moieties. Therefore, catalyst iv having the imidazoline ligand exhibited efficient catalytic activity in CO2 hydrogenation (TOF: 43 h−1). By the combination of these effects, we have recently reported the highly efficient catalyst v, which showed a TOF of 106 h−1 and a TON of 7280.7i To the best of our knowledge, as a catalyst that can hydrogenate CO2 in water under ambient conditions without any organic additives, only one (complex vi, Scheme 1)8 is known in addition to ours.7 Under the CO2 hydrogenation conditions we used (1 M NaHCO3, pH 8.2), the catalytic efficiency of iv seems to rely on the electronic resonance of the neutral amidine ligand.7h On the other hand, in catalysts i−iii the electron-donating effect of an oxyanion generated from deprotonation of a phenolic OH group seems to weaken through the delocalization of electrons over the entire aromatic ligand. In this Letter, we present a new concept for ligand design with the use of picolinic acid, picolinamide, or a related ligand that deprotonates upon coordination and produces an anionic coordinating atom (Scheme 2c).9 By combining the functional noncyclic amine moiety with the proton-responsive function, we have successfully developed the most highly active catalyst to date under ambient conditions. Density functional theory (DFT) calculations for the most active catalyst, [Cp*Ir(4-hydroxy-Nmethyl-picolinamidate)-(H2O)]2+ (7), and the less active catalyst with a picolinic acid ligand (L2), point toward the importance of the H2 binding kinetics and equilibrium for the catalytic CO2 hydrogenation mechanism. Figure 1 shows the ORTEP diagram of [Cp*Ir(Nmethylpicolinamidate)(H2O)](HSO4) (5) without the counteranion. Selected bond distances and angles are summarized in Table S2. A pyridine N atom and an amide N atom of the L5 ligand constitute, along with a H2O ligand, the three legs of a

piano stool geometry. Taking account of the report of the coordinated form of N,N-donor and N,O-donor type ligands as in picolinamide derivatives,10a the L5 ligand was found to have an N,N-donor type coordinated form. Since the dihedral angle between the planes Ir(1), N(2), C(6) and Ir(1), N(2), C(7) is only 4.94°, the four atoms are almost in the same plane. Moreover, the sum of angles Ir(1)−N(2)−C(6), Ir(1)−N(2)− C(7) and C(6)−N(2)−C(7) is 359.6°. These results indicate that the N(2) atom has sp2 hybridization, suggesting that the proton of (Me)NH was transferred to SO42−.10b,c Furthermore, the C(6)−N(2) and C(6)−O(2) bonds exhibit lengths of 1.324(9) Å and 1.257(7) Å (Table S2), respectively, indicating that these bonds possess double-bond character. The X-ray analysis indicates that there is a resonance between the two structures shown in Scheme 2c. Hence, it should be noted that amide (RHN−CO) was converted to RN−−CO by deprotonation, and this species functions as a strong electron donating group. For screening purposes, we initially evaluated 0.2 μmol in situ prepared catalysts (1:1 mixture of [Cp*Ir(H2O)3][SO4] and pyridyl derivatives L1-L8 as ligands, Figure S1−S7) for the catalytic hydrogenation of CO2 (Table 1). The TOF of [Cp*Ir(H2O)3]+ with the amidine ligand (L1, TOF: 130 h−1) was approximately equal to that of the “isolated” and wellcharacterized Ir catalyst having a pyridyl-imidazoline ligand (TOF: 168 h−1).7h This result indicates that amidine moieties, in which the NH of amidine may not deprotonate at pH 8.2, possesses the same functionality as imidazoline. Next, since the more acidic carboxylic acid moiety (L2, pKa 1.1) will deprotonate and coordinate to the Ir center, the TOF with L2 was examined. Unfortunately, the performance of the L2 catalyst decreased significantly. Therefore, thioamide (L3) and amide (L4) moieties, which will have properties intermediate between the amidine and carboxylic acid, were evaluated. The complex having thioamide (L3) hardly exhibited catalytic activity, attributable to poor coordination to Ir as evidenced by the existence of multiple species in NMR. On the other hand, the in situ complex with L4 exhibited excellent activity (TOF of 1230 h−1). This extraordinary enhancement of catalytic activity seems to be attributable to the electronic effect of the anion generated by deprotonation of the NH in the amide moiety (L4, pKa 1.8).11 The N-methyl analogue with L5 exhibited similar catalytic ability. For further improvement, the OH group 6427

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ACS Catalysis Table 1. TOFs in the Hydrogenation of CO2 to Formatea

spectroscopy under H2 (1 MPa) and basic conditions (Figure S11). It seems that the H/D exchange of Ir−H in D2O is slow under basic conditions. An addition of D2SO4 into the solution caused bubble formation together with disappearance of the −11.80 ppm signal, suggesting the generation of H2 . Furthermore, KIE studies were conducted to obtain information on the rate-determining step of the hydrogenation of CO2 using 7 (Table S4). Under the condition of H2/D2O/NaDCO3, the KIE was 1.05 and it seems that there was only a slight influence of D2O due to slow Ir−H/Ir−D exchange. On the other hand, the KIEs increased to 1.69 by replacing H2 with D2 consistent with H2 heterolysis for the formation of the Ir−H species as the rate-determining step. Finally, we performed DFT calculations at the M06 level of theory12 with the SMD aqueous continuum solvation model13 to investigate the CO2 hydrogenation mechanism of complexes 2 and 7. The proposed mechanisms are summarized as free energy diagrams in Scheme S1 and S2, respectively. The first step of the proposed mechanism for 7 involves the binding of H2 to a vacant site on the Ir complex (ΔG = 18.8 kcal/mol), which becomes less favorable upon deprotonation of the ligand. Upon binding, three possible transition state structures for heterolysis of H2, which span a narrow free energy range, were located (Scheme S2, Figure S12). The first one involves direct H2 heterolysis (TS-1a, ΔG‡ = 33.1 kcal/ mol) with amide as the proton acceptor facilitating the formation of the iridium hydride (Ir−H) intermediate. The second and third optimized transition state structures feature H2O (TS-1b, ΔG‡ = 33.6 kcal/mol) and HCO3− (TS-1c, ΔG‡ = 28.6 kcal/mol) as external bases for the H2 heterolysis step. The formation of Ir−H is followed by electrophilic attack by CO2 (TS-2, ΔG‡ = 15.8 kcal/mol) to generate an iridium formate (Ir−OCHO) intermediate and dissociation of formate regenerates the catalyst (Scheme S2). The computed ΔG‡s indicate heterolysis of H2 as the rate-determining step and comparison of computed and experimental deuterium KIEs support the direct heterolysis as the preferred pathway (Table S4). The direct H2 heterolysis is predicted to be even more favored at elevated temperatures given the additional entropic cost for other pathways involving external bases (e.g., H2O or HCO3−). The comparison of activation free energies of H2 heterolysis for 2 and 7 indicates that the former involves lower ΔG‡s contradicting its observed lower activity toward CO2 hydrogenation. As a next step, we investigated the H2 binding and dissociation kinetics for both complexes and found that the dissociation of H2 features a very low activation energy (ΔE‡ = 1.0 kcal/mol) for 2 compared to that of 7 (ΔE‡ = 8.3 kcal/mol) (Figure S13). Even more intriguing is the difference in ΔE‡s for H2 dissociation versus H2 heterolysis which are ΔΔE‡ = 16.7 and 8.3 kcal/mol, respectively, for 2 and 7 showing that heterolysis is much less favored from the H 2 bound intermediate compared to dissociation of H2 for 2 (Scheme 3). These results indicate that H2 binding and dissociation kinetics compared to those of H2 heterolysis could play a significant role in determining the activities of the catalysts and will be the focus of future work. In conclusion, we found that amide moieties effectively act as ligands in CO2 hydrogenation. Single-crystal X-ray structural analysis indicated that the amide moieties were converted to anionic species by deprotonation through a resonance stabilization effect and functioned as anionic ligands which exhibited a strong electron donation property to the Ir center.

a

Values following Ln (n = 1−8) indicate the TOF obtained with in situ-prepared catalyst (1:1 mixture of [Cp*Ir(H2O)3][SO4] and pyridyl derivatives Ln). bIn situ-prepared catalyst, but not a single component. cUsing synthesized and isolated Ir complexes with L4-L7.

as a proton-responsive moiety was introduced into pyridine at the 4-position (L6 and L7) according to our catalyst design concept (II).7b As expected, significantly greater activity of these Cp*Ir catalysts was observed owing to the electronic property of the oxyanion generated by deprotonation of the OH group under basic conditions. The in situ complex with L7 gave the highest TOF of 2640 h−1 among complexes with L1− L7. On the other hand, ethyl picolinimidate (L8) showed low catalytic activity (TOF: 86 h−1), probably because L8 was not able to form anionic species due to the presence of the substituent (OMe). Thus, picolinamide moieties realized our desired ligand properties. Subsequently, in order to further identify the factors contributing to this significant improvement, we synthesized complexes 4−7 and tested their catalytic activity using 0.2 μmol of catalyst. As shown in Table 1, the catalytic activity of these complexes was identical to that of the corresponding in situprepared species within experimental error. Figure S8 shows Arrhenius plots of 4−7. The most efficient catalyst 7 was further evaluated for CO2 hydrogenation under ambient conditions. Figure S9 shows the time course of the TON based on the Ir catalyst. The TON increased with reaction time and reached 14 700 (formate 0.16 M) after 328 h. This result was twice that for v (TON: 7280) that we previously reported.7i Furthermore, the TOF with 7 reached 167 h−1 during the initial 1 h, exceeding the maximum TOF of 106 h−1 obtained with v. The time course of the formate concentration (with 100 μM catalyst 7) is shown in Figure S10. The formate concentration gradually reached a peak at 0.63 M after 312 h. When the CO2 hydrogenation was carried out with increased catalyst concentration (250 μM), the final formate concentration was 0.64 M, i.e., there was no significant change. The formation of formate was considered to have reached equilibrium because it was comparable under any conditions. CO2 + H 2 + OH− ⇌ HCO3− + H 2 ⇌ HCO2− + H 2O

Previously, we have proposed a reaction mechanism via the formation of an Ir−H intermediate as the active species for CO2 hydrogenation.2 We detected the Ir−H species (δ − 11.80 ppm) derived from the reaction of catalyst 7 by 1H NMR 6428

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ACS Catalysis

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Scheme 3. Energetic Diagram Displaying Relative Energies (ΔE) of H2 Dissociation and H2 Heterolysis

Furthermore, 7 which incorporated a proton-responsive pyridinol moiety, achieved a TON of 14 700 (TOF: 167 h−1) and a formate concentration of 0.64 M under ambient conditions in the catalytic hydrogenation of CO2. These results represent the highest TON and formate concentration among the previous reports under ambient conditions in water. Moreover, amide derivatives have the advantage of being easy to modify and evaluate. This work has produced new design concepts for realizing highly active catalysts for the hydrogenation of CO2.

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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/acscatal.7b02280. NMR spectra, additional CO2 hydrogenation data, proposed mechanisms, optimized transition-state structures, KIEs, and coordinates of DFT structures (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for N.O.: [email protected]. *E-mail for E.F.: [email protected]. *E-mail for Y.H.: [email protected]. ORCID

Naoya Onishi: 0000-0003-4501-1742 Etsuko Fujita: 0000-0002-0407-6307 Yuichiro Himeda: 0000-0002-9869-5554 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JST ACT-C Grant Number JPMJCR12Z0, Japan. The work at BNL was carried out with support from the U.S. Department of Energy, Office of Science, Division of Chemical Sciences, Geosciences & Biosciences, Office of Basic Energy Sciences under contract DE-SC0012704.

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REFERENCES

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