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A: Spectroscopy, Molecular Structure, and Quantum Chemistry 2

Clean and Efficient Transformation of CO to Isocyanic Acid: The Important Role of Triatomic Cation ScNH +

Ming Wang, Chuanxin Sun, Jia-Tong Cui, Yun-Hong Zhang, and Jia-Bi Ma J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b02133 • Publication Date (Web): 14 May 2019 Downloaded from http://pubs.acs.org on May 14, 2019

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The Journal of Physical Chemistry

Clean and Efficient Transformation of CO2 to Isocyanic Acid: The Important Role of Triatomic Cation ScNH+ Ming Wang,[a] Chuanxin Sun,[a] Jiatong Cui,[a] Yunhong Zhang,[a] Jiabi Ma*[a] Key Laboratory of Cluster Science of Ministry of Education, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100190, China AUTHOR INFORMATION Corresponding Author *J.-B. M.: Email: [email protected]

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ABSTRACT

Achieving the desired selective transformations of the very stable CO2 into useful chemicals is quite important for the development of economically and environmentally sustainable synthetic methods. Herein, mass spectrometric experiments and quantum-chemical calculations have identified that ScNH+ reacts quite efficiently with CO2 under thermal collision conditions to exclusively yields ScO+ and isocyanic acid (HNCO). This is a novel reaction type in CO2 activation reactions mediated by gas-phase ions. In this reaction, the C=N double bond has also been firstly formed in gas phase. The mechanism of “migratory insertion” is proposed. Coupled with the previously reported reaction of Sc+ with NH3, HNCO can be synthesized under mild conditions from NH3 and CO2 in quite simple reactions. The mechanistic information gained in this gas-phase model reaction can offer fundamental insights relevant to corresponding processes, and further guide how to design brand new catalysts.

1. INTRODUCTION Chemical conversion and utilization of CO2 is one of the effective ways to reduce this global greenhouse gas, and CO2 also offers the possibility to create a renewable carbon economy to produce C1-molecules of higher value.1-3 However, the thermodynamic stability of the most oxidized form of carbon raises a challenge for this purpose, and the activation as well as subsequent conversion are energy demanding. As one of major value-added products, isocyanates (R-N=C=O) can be synthesized through the reactions of metal silylamides with CO2,1,4-6 as shown in Scheme 1a. However, due to the isoelectronic nature of isocyanates and CO2, metals that strongly activate CO2 also activate the isocyanate, leading to metal isocyanates, silyl ether and other poisonous side products. Holland et al. developed a new methodology to synthesize isocyanates with high selectivity(>95%),6-7 in which the compound Fe[N(SiMe3)2]2 is reactive toward CO2. The cyanate ion, NCO−, is a quite stable functional group, and salts containing this ambidentate ligand are commonly found in organic chemistry. The isocyanates are potentially useful as carbamoyl synthons for organic synthesis.8-9 In addition, the NCO unit is also an important building block in 2 ACS Paragon Plus Environment

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the field of pseudohalogen chemistry.10-11 Moreover, the conjugate acid of NCO−, isocyanic acid HNCO, is the simplest compound, possessing the four main biogenic elements, and it was the one of the first polyatomic molecules observed in the interstellar gas. It is highly desirable to develop a simple HNCO-producing reaction with high selectivity and reactivity by using CO2 as sources, and to understand the reaction mechanism. In gas-phase chemistry, ion-molecule reactions have been served to discover mechanisms of bond activation and formation, and the finding sheds light on the preparation of related catalysts.1218

Among the numerous studies of the CO2 activation and transformation mediated by gas-phase

metal-containing ions, several kinds of organic bonds are generated with the formation of typical organic compounds:19 1) translocate a hydride from metal hydrides (PtH3‒, FeH‒, and Cu2H2‒) to generate a formate ligand HCO2‒;20-22 2) form a C‒C single bond by inserting CO2 in an M‒C single bond to generate cyanoformate (NCCO2‒), mediated by FeCN+ and CoCN+ cations,23 or to form benzoate salt of yttrium (C6H5CO2Y+) by YC6H5+;24 3) form C=C double bonds (ketene) in CH4 and CO2 coupling reactions by Ta+ and CuB+.25-27 In addition, CO2 reduction to CO via oxygen-atom transfer to a suitable oxygen acceptor has also been extensively reported.19,25,28-37 It is noteworthy that examples of C–N coupling process in the gas phase are rather scarce.38-43 To the best of our knowledge, there is no gas-phase report about the C=N bond formation in CO2 activation reactions or the generation of isocyanic acid. Herein, we report the first example of triatomic cation ScNH+ that activates CO2 to form HNCO under thermal collision conditions.

Scheme 1. Reaction products from the reactions of metal silylamides (a) and ScNH+ (b) with CO2.



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2. METHODS 2.1. Experimental Methods Ion/molecule reactions were examined with the time-of-flight mass spectrometry (TOF-MS) equipped with a laser ablation ion source, a quadrupole mass filter (QMF), and the LIT reactor, which has been described previously in detail.44-46 Only a brief outline of the apparatus is given below. The ScNH+ ions were generated by laser ablation of a scandium disk (99.999%) in the presence of about 0.02% NH3 seeded in a He carrier gas with a backing pressure of 4 atm. A 532 nm (second harmonic of Nd3+: yttrium aluminum garnet-YAG) laser with an energy of 5–8 mJ/pulse and a repetition rate of 10 Hz was used. The ions of interest were mass-selected by the QMF and entered into the LIT reactor, where they were confined and thermalized by collisions with a pulse of He gas and then interacted with a pulse of reactants (CO2 or C18O2) for a period of time. It has been proved that the ions are thermalized to (or close to) room temperature before reactions in previous works.47 The ions ejected from the LIT were detected by a reflection TOFMS. The method to derive the rate constants was described in detail in Ref. 47. 2.2 Computational Methods. All theoretical work was performed using the Gaussian 09 D.01 program.48 The TPSS functional,49-50 including DFT-D3 dispersion correction,51-52 was used for the structural optimization and frequency analysis. Among the 20 tested methods, TPSS gives the best prediction of Sc+‒O, Sc+‒NH, Sc+‒N, N=C, O=C, and N‒H bonds, as listed in Table S1. The 6-311+G(d) basis set53 was employed for the carbon, nitrogen, hydrogen and oxygen atoms, and the def2QZVP54-56 was used for the Sc atom. More accurate single-point energies were obtained at the CCSD(T)57-59/ (Stuttgart RSC 1997 ECP60-61 for and basis set Sc atom; aug-cc-pVTZ62-63 for C, N, H and O atoms) levels of theory. All T1 diagnostics are well below 0.04. Stationary points were optimized without symmetry constraint, and their nature was confirmed by vibrational frequency analysis. Intrinsic reaction coordinate (IRC)64-66 calculations were also performed to make sure 4 ACS Paragon Plus Environment

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that a TS connects two appropriate minima. Gibbs free energies at 298 K (∆G298K in eV) are reported in this work. Natural bond orbital (NBO) analysis was performed using NBO 6.0,67 and the program Multiwfn68 is employed to analyze orbital compositions by natural atomic orbital method. The multicoordinate driven (MCD) method69 as applied to obtain the potential‐energy curve for the step of I2→P. 3. RESULTS 3.1 Experimental Results The laser-ablation-generated ScNH+ cations were generated, mass-selected, confined and cooled, and then reacted with CO2 in a linear ion trap (LIT) reactor. Figure 1a shows the mass spectrum of ScNH+ cations when the reactor is filled with inert bath gas He, and two weak peaks, corresponding to ScN+ and ScO+, are present, which may result from the reactions with water or other trace impurities. Upon the interaction of ScNH+ with CO2, Figure 1b and c, the peak intensity of ScO+ increases remarkably, indicating the following reaction channel (Reaction 1); the isotopic labeling experiment with C18O2 (Figure 1d) confirms this channel. It is a clean reaction (>99%), and no other products, such as the adsorbate ScNHCO2+, were observed in the entire mass spectrum. ScNH+ + CO2 → ScO+ + HNCO

(1)

The pseudo first-order rate constant (k1) for the reaction of ScNH+ with CO2 were estimated to be (8.5 ±1.7) × 10-10 cm3 molecule-1 s-1, corresponding to the reaction efficiency (Φ) of (107 ± 20%).70-71 The signal dependence of product ions on CO2 pressures was derived and fitted with the experimental data (Figure S1). The single exponential decay of the ScNH+ intensities with respect to the CO2 pressures imply that there is no inert component in the generated ScNH+ cations. Since it is not possible to detect the neutral product in our experimental apparatus, and to rationalize Reaction 1 proposed above, density functional theory (DFT) calculations thus play an important role in gaining insights into potential neutral products and mechanism of reaction.



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Figure 1. Time-of flight mass spectra for the reactions of mass-selected ScNH+ with He (a), with CO2 (b,c), and with C18O2 (d) for 0.7 ms. The effective reactant gas pressures are shown. No adsorption products are observed. 3.2 Computational Results The most stable isomer has a linear shape and possesses a singlet ground state; other isomers with different spin states or shapes are much higher in energy (>2 eV). To further support the conclusion that the observed neutral product is HNCO, thermodynamics of other reactions, involving the isomers of isocyanic acid and other possible products, were calculated (Table S1, SI). The other twelve reactions are all endothermic, except for the generation of HNCO. The potential-energy surface (PES) of Reaction 1 is shown in Figure 2, and Gibbs free energies at 298 K and zero-point vibration-corrected energies (∆G298K/∆H0K in eV) of the reaction intermediates, transition states, and products with respect to the separated reactants are given.


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Figure 2. TPSS-D3(BJ)-calculated potential energy surfaces (PESs) for the reactions of ScNH+ with CO2. Some bond lengths are given in pm. Gibbs free energies at 298 K and zero-point vibration-corrected energies (∆G298K/∆H0K in eV) of the reaction intermediates, transition states, and products with respect to the separated reactants are given. The values in the square brackets are the single point energies calculated at the CCSD(T) level. The superscripts indicate the spin states. The transformation starts from an encounter complex I1, and through the newly formed C–O bond, the two linear reactant molecules are bonded together, with the ∠N–Sc–O of 105.7°. This coplanar structure is sterically favorable for the following HNCO unit formation in I2. During the step of I1→TS1→I2, the N‒C bond is generated concomitantly with the mild activation of the stable C=O double bond in CO2. The Sc‒N bond is also weakened in this step, evidenced by the 7 ACS Paragon Plus Environment


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elongated bond, 170 pm in the free ScNH+ compared to 223 pm in I2. By TS2, the atom bonded with the Sc atom is shifted from the N atom in I2 to the O atom in I3, and the distance between the Sc to the O atom in the isocyanic acid moiety (217 pm) is much longer than the bond length of Sc‒O single bond (192-208 pm).44,72 Liberation of the HNCO unit can happen from I2 and I3, bringing about the final products ScO+ and isocyanic acid, and these steps are accompanied by a significant entropy increase. The zero-point energy corrected relative energy of products is slightly lower than the entrance channel (ΔE = -0.05 eV); however, when the entropy contribution (-TΔS) to the free energy is taken into consideration, the free energy of products is lowered to -0.19 eV. The single-point energies of reactants, intermediates, transition states, and products located on the PES were recalculated by the high-level coupled cluster method with single, double, and perturbative triple excitations method [CCSD(T)]. In addition, the calculated bond dissociation energy of D(ScN+‒H) is 4.93 eV. Since the peak ScN+ in Figure 1a is quite weak and some trace impurities may exist in the LIT reactor, it is difficult to provide the exact reaction of the ScN+ generation. Wiberg bond index (WBI)73-74 is a good indicator for bond formation and breaking. Natural bond orbital analysis shows that the bond orders of Sc-N and N-H bonds in ScNH+ are 2 and 1, respectively. As given in Table 1, along the sequence I1→TS1→I2, the WBI of Sc-O2 is increased significantly from 0.11 to 1.88, concomitant with the dramatically decreased WBI of ScN. In contrast, the bond orders of C-O1 and N-H almost do not change. Furthermore, charge analysis shows that the N and C atoms in I1 are negatively (-1.06e) and positively (1.04e) charged, respectively (Figure S3, SI), and the opposite charges attract, which assists the reactivity. A detailed molecular orbital analysis for I1→TS1→I2 has been performed to provide more insights (Figure S4). Most of the frontier orbitals of I1 and TS1 possess similar shapes, which are six doubly occupied π orbitals of Sc-N bond and CO2 unit as well as one δ(Sc-N-H) orbital. The obvious orbital interactions between N and C atoms are in two rather low-lying orbitals; therefore, the reaction barrier is easy to surmount (ΔE‡ = 0.40 eV). In I2, the seven doubly occupied orbitals are one δ(Sc‒O), two π(Sc‒O), and four π orbitals of HNCO moiety. The frontier orbital energies 8 ACS Paragon Plus Environment

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in I2 is lower than those in I1, which is consistent with the fact that the bond dissociation energy D(Sc+‒O) is stronger than D(Sc+‒NH). Table 1. Wiberg bond indexes for bonds in I1, TS1, and I2. Bond

I1

TS1

I2

Sc‒O2

0.11

0.35

1.88

Sc‒N

2.02

1.63

0.21

C‒O1

2.10

2.01

2.14

C‒O2

1.66

1.43

0.02

N‒H

0.84

0.82

0.76

The collision rates of the intermediates I1, I2 and I3 with the cooling gas He (kcollision) is around 105 s−1 (See SI for details). The calculated rates of the processes I1→TS1 and I2→TS2 are 7.7 ×1010 and 3.2 ×1011 s−1, respectively, and the dissociation rates of HNCO desorption from I2 and I3 are around 108 s−1. These data indicate that the I1-I3 have short lifetimes and have a little chance to be stabilized. Also note that for this six-atom system [ScNHCO2+], the vibrational degree of freedoms is only 12. Although He was applied as cooling gas in the LIT reactor, the binding energy (ΔEb = 0.82 eV, based on CCSD(T) calculations) can heat up the reaction complex significantly, leading to a high vibrational temperature. The Sc‒N and Sc‒O bonds in I2 and I3 are rather electrostatic in nature, reflected by the quite small values of WBI(Sc‒N) = 0.21 and WBI(Sc‒O2) = 0.02, respectively. From I3, (I2), the releasing of the products is an entropy-driven dissociation process. These factors are responsible for the observed products HNCO and ScO+. 4. DISCUSSIONS In organometallic chemistry, the addition of the N‒H bond of ammonia across carbon-carbon multiple bonds,75 the hydroamination reaction,41-43,76 is an important synthetic method for the C‒ N bond formation. Two strategies, alkene activation and N‒H activation, are reported. The 9 ACS Paragon Plus Environment


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proposed alkene activation mechanism is that an alkene first coordinates to an electrophilic metal center, followed by a nucleophilic attack of the amine to the polarized olefin.77 Interestingly, the similar mechanism is adopted for Reaction 1: In I1, CO2 is adsorbed on the positively charged Sc atom and polarized to increase the electrophilicity of the C+ atom in CO2; then, the negatively charged N‒ atom in ScNH+ nucleophilicly attacks the “activated” C atom via TS1. The CO2 activation mechanism can be summarized as “ migratory insertion”. Therefore, electrostatic effect is quite important to the formation of HNCO in the investigated reaction. In literature, the reaction of Sc+ with NH3 has been studied experimentally and theoretically.78-79 The dominant process at thermal energies is the formation of ScNH+ and H2 (Reaction 2),78 and a spin crossing from the triplet to the singlet PES occurs, leading to the production of singlet ScNH+.79 This efficient reaction was also studied in our apparatus (Figure S5, SI); ScNH+ and ScNH2+ were observed as products, with branching ratios of 95% and 5%, respectively, which are consistent with the previously reported results. The rate constant was estimated to be (2.1 ±0.5) ×10-10 cm3 molecule-1 s-1, corresponding to the reaction efficiency (Φ) of 21%. Combined with Reactions 1 and 2, isocyanic acid can be formed by using NH3 and CO2 as feedstock (Reaction 3). Step1:

3

Sc+ + NH3 → 1ScNH+ + H2

(2)

Step2: ScNH+ + CO2 → ScO+ + HNCO Overall reaction: NH3 + CO2 + Sc+ → HNCO + H2 + ScO+

(3)

In industry, isocyanic acid is made by protonation of the cyanate anion (H+ + NCO− → HNCO),80 or by the high temperature thermal decomposition of cyanuric acid (C3H3N3O3 → 3HNCO), and so on. Notably, NCO unit already exists in the reactants of these methods. In the organic synthesis mentioned above, isocyanates (Me3Si‒NCO) and metal isocyanato complexes (M‒NCO) coexist as products, which contains the conjugate base NCO − rather than HNCO. Interestingly, in order to achieve visible light activity, a mixture of titania and urea is heated at 400°C, and the isocyanic acid, released from urea, can react with the OH groups in pristine titania, 10 ACS Paragon Plus Environment

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liberating CO2 molecules: [TiO2]‒OH + O=C=NH → [TiO2]‒NH2 + CO2,81 which can be viewed as a kind of the reverse reaction of Reaction 1. Although gas-phase investigations never account for the many features that prevail at a surface or in solution, the information gained in this gasphase model reaction can offer fundamental mechanistic insights relevant to corresponding processes and catalysis, and further guide how to design brand new catalysts. 5. CONCLUSIONS In conclusion, the generation of isocyanic acid in the reaction of ScNH+ with CO2 has been described. To the best of our knowledge, this is the first example of forming HNCO in CO 2 activation reactions mediated by gas-phase ions, and the formation of C=N double bond has also been identified for the first time. This reaction is with quite excellent selectivity (>99%) and reactivity, and only ScO+ and HNCO were observed as products. The mechanism of “migratory insertion” is adopted. When combination with the previously reported reaction of Sc+ with NH3, HNCO can be synthesized from NH3 and CO2, with the formation of the stable ScO+ cations. Further work on the catalytic synthesis of HNCO with higher atom economy by using NH 3 and CO2 as feedstock and applying cheaper catalysts than scandium is ongoing. ASSOCIATED CONTENT Supporting Information. Tables giving related bond dissociation energies by experiments and DFT calculations as well as thermodynamics of all possible reactions; figures giving additional mass spectra, DFT-calculated isomers, NBO charge developments, and schematic orbital diagrams based on a frontier orbital analysis. AUTHOR INFORMATION ORCID Jiabi Ma: 0000-0002-7428-0231 Notes 11 ACS Paragon Plus Environment


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The authors declare no competing financial interests. ACKNOWLEDGMENT M. Wang and C.-X. Sun contributed equally to this work and should be considered as co-first authors. This work was supported by National Key R&D Program of China (No. 2016YFC0203000), the National Natural Science Foundation of China (No.21503015), and the Fundamental Research Funds for the Central Universities (No. (2 2050205) 2017CX01008). REFERENCES (1) Peterson, A. A.; Norskov, J. K. Activity Descriptors for CO2 Electroreduction to Methane on Transition-Metal Catalysts. J. Phys. Chem. Lett. 2012, 3, 251-258. (2) Vidal, A. B.; Feria, L.; Evans, J.; Takahashi, Y.; Liu, P.; Nakamura, K.; Illas, F.; Rodriguez, J. A. CO2 Activation and Methanol Synthesis on Novel Au/TiC and Cu/TiC Catalysts. J. Phys. Chem. Lett. 2012, 3, 2275-2280. (3) Liu, Q.; Wu, L. P.; Jackstell, R.; Beller, M. Using Carbon Dioxide as a Building Block in Organic Synthesis. Nat. Commun. 2015, 47, 5933. (4) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. Mechanism of the Alternating Copolymerization of Epoxides and CO2 Using Beta-Diiminate Zinc Catalysts: Evidence for a Bimetallic Epoxide Enchainment. J. Am. Chem. Soc. 2003, 125, 11911-11924. (5) Reiter, M.; Vagin, S.; Kronast, A.; Jandl, C.; Rieger, B. A Lewis Acid Beta-Diiminato-ZincComplex as All-Rounder for Co- and Terpolymerisation of Various Epoxides with Carbon Dioxide. Chem. Sci. 2017, 8, 1876-1882. (6) Broere, D. L. J.; Mercado, B. Q.; Holland, P. L. Selective Conversion of CO2 into Isocyanate by Low-Coordinate Iron Complexes. Angew. Chem. Int. Ed. 2018, 57, 6507-6511. (7) Broere, D. L. J.; Mercado, B. Q.; Bill, E.; Lancaster, K. M.; Sproules, S.; Holland, P. L. Alkali Cation Effects on Redox-Active Formazanate Ligands in Iron Chemistry. Inorg. Chem. 2018, 57, 9580-9591. (8) Pudovik, M. A.; Krepysheva, N. E.; Kharitonov, D. I.; Pudovik, A. N. Reactions of 12 ACS Paragon Plus Environment

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