Vibrational Characterization of Radical Ion Adducts between

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Vibrational Characterization of Radical Ion Adducts Between Imidazole and CO Stephanie M. Craig, Christopher J Johnson, Duminda S Ranasinghe, Ajith Perera, Rodney J. Bartlett, Michael R Berman, and Mark A. Johnson J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b01883 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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The Journal of Physical Chemistry

Vibrational Characterization of Radical Ion Adducts between Imidazole and CO2 Stephanie M. Craig,1 Christopher J. Johnson,2 Duminda S. Ranasinghe,3 Ajith Perera,3 Rodney J. Bartlett,3 Michael R. Berman,4 and Mark A. Johnson1* 1.

Sterling Chemistry Laboratory, Yale University, New Haven, CT 06520, USA

2.

Department of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA

3.

Department of Chemistry, University of Florida, Gainesville, FL 32611, USA

4.

Air Force Office of Scientific Research, Arlington, VA 22203, USA * corresponding author: [email protected]

Abstract We address the molecular level origins of the dramatic difference in the catalytic mechanisms of CO2 activation by the seemingly similar molecules pyridine (Py) and imidazole (Im). This is accomplished by comparing the fundamental interactions of CO2 radical anions with Py and Im in the isolated, gas phase PyCO2¯ and ImCO2¯ complexes. These species are prepared by condensation of the neutral compounds onto a (CO2)n¯ cluster ion beam by entrainment in a supersonic jet ion source. The structures of the anionic complexes are determined by theoretical analysis of their vibrational spectra, obtained by IR photodissociation of weakly bound CO2 molecules in a photofragmentation mass spectrometer. Although the radical PyCO2¯ system adopts a carbamate-like configuration corresponding to formation of an N-C covalent bond, the ImCO2¯ species is revealed to be best described as an ion-molecule complex in which an oxygen atom in the CO2¯ radical anion is H-bonded to the NH group. Species that feature a covalent N-C interaction in ImCO2¯ are calculated to be locally stable structures, but are much higher in energy than the largely electrostatically bound ion-molecule complex. These results support the suggestion from solution phase electrochemical studies (Bocarsly, A. B., et al., 2012, 2, 1684-1692) that the N atoms are not directly involved in the catalytic activation of CO2 by Im.

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I. Introduction The reduction chemistry of CO2 is currently under intensive investigation, largely because of its importance in CO2 capture and chemical transformation into liquid fuels.1-3 Here we are concerned with the activation of Fig. 1. Structures and electrostatic potentials, calculated at the B3LYP/aug-cc-pVDZ level of theory, of imidazole (Im) and pyridine (Py) with red and blue indicating negative and positive domains, respectively.

CO2 by the heterocyclic compounds pyridine and imidazole (Py and Im, Fig. 1), which have both shown to be effective in the electrocatalytic conversion of CO2 to CO and

formic acid.4 The mechanisms of their actions are, however, quite different. Specifically, Bocarsly et al. suggested that the Py system appears to involve direct interaction with the N atom, while the Im system more likely works through attack at the C(2) carbon center (the carbon atom between the two N atoms, labeled with a 2 in Fig. 1).4 This raises the issue of how to prepare and study the key intermediates present when CO2 encounters heterocyclic ring systems under highly reducing conditions. One route that is particularly useful to understand the molecular level aspects of such processes is through the reactions of the gas phase (CO2)n¯ cluster anions,5-13 which enables direct access to species that are highly unstable in condensed media. Although the primary molecular anion, CO2¯, is unstable with respect to electron autodetachment,14 the homogeneous cluster ions with n ≥ 2 are readily prepared using supersonic jet ion sources, and their physical properties and reaction chemistries have been well documented over the past three decades.8-9, 15-16 In fact, cluster ion chemistry was employed previously to generate the key carbamate-like17-19 intermediate invoked2023

to explain the catalytic mechanism of Py activation: (CO2)n¯ + Py → PyCO2¯ + (n-1)CO2,

(1)

which requires the formation of a covalent N-C bond to the Fig. 2. The reaction of CO2¯ with Im can follow two pathways, forming either a) a hydrogen bond to the NH group or b) a carbamate group with the N:.

CO2¯ radical anion. Here we extend the earlier study to the more interesting case involving CO2¯ complexation with

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imidazole (Im), which presents potential interaction sites at the N: atom, similar to the situation in Py, but also at the acidic NH group. These two points of contact are evident in the electrostatic potentials displayed over the structures in Fig. 1, where the most negative site (red) resides on the N: atom while the positive region (blue) occurs, as expected, near the NH group. There is also a possible role for interaction of CO2¯ at the C(2)H group in the Im case, as this motif has, for example, been shown to be active in the reductive CO2 chemistry of the alkylated Im derivatives used extensively in ionic liquds.24-26 Electronic structure calculations indicate that attack by CO2¯ at both the NH group and N: atom yield locally stable species with structures indicated in Fig. 2, while attack at the C(2)H group does not converge (vide infra) to an energetically stable minimum at the B3LYP/aug-cc-pVDZ level of theory. Here we compare the nature of the product ions created by condensation of Im and Py with (CO2)n¯ clusters by analyzing their vibrational predissociation spectra with the aid of electronic structure calculations. II. Experimental The experimental spectra were taken using the Yale double-focusing, tandem time-offlight photofragmentation mass spectrometer previously described.8, 27 Anionic (CO2)m¯ clusters were produced by an expansion of neat CO2 vapor through a pulsed nozzle (Parker-Hannifin General Valve) and bombarded by a counterpropagating 1 keV electron beam. Solid imidazole (melting point = 90° C) was heated to 100-120° C to produce trace Im vapor, which was introduced into the ion source close to the orifice on the high-vacuum side of the free jet. This created clusters of both (CO2)m¯ and Im(CO2)n¯, as shown in the product ion mass spectrum included in Fig. S1. The ions of interest were mass selected and intersected with an IR OPO/OPA laser (LaserVision) tuned over the 1000-3800 cm-1 range. Vibrational spectra were recorded by monitoring the photoinduced mass loss arising from subsequent photoevaporation of one CO2 molecule. III. Results and Discussion IIIA. Review of (CO2)n¯ cluster physics Several aspects of excess electron capture by carbon dioxide clusters are relevant to the present discussion of their reaction chemistry and vibrational spectra arising from incorporation of potentially reactive partners. Most importantly, the n = 2 cluster adopts a covalently bound C2O4¯ arrangement that is best considered a singly charged oxalate ion.28 This radical molecular 3



anion forms the charged core upon addition of the next four neutral CO2 molecules. At n = 6, however, the system abruptly changes to a scenario where the excess charge is localized on the CO2¯ molecular ion. This is due do a solventinduced “core ion switching” phenomenon in which the covalent C-C bond in the anomalously polarizable C2O4¯ species is broken in response to the electric field induced by the formation of an asymmetrical, incomplete solvation shell around the excess charge center.5-6, 8, 28 The C2O4¯ core ion is restored at n = 14, however, as the solvent electric field at the ion is reduced upon formation of a completed solvation shell with 12 CO2 molecules. Incorporation of non-reactive solvent molecules can also affect this switching process by adding

Fig. 3. Vibrational predissociation spectra of a) (CO2)5¯, b) (CO2)7¯ (both reproduced from Ref. 17), c) Im(CO2)2¯, e) Im(CO2)3¯, g) Im(CO2)6¯ and h) Py(CO2)2¯ (reproduced from Ref. 17). Harmonic predictions for the vibrational spectra are included as inverted traces for d) Im(CO2)2¯, and f) Im(CO2)3¯ for the respective lowest energy isomers, which can be found in the Supplementary Information. Calculations were performed at the B3LYP/aug-cc-pVDZ level of theory and scaled by 0.97.

asymmetrically to the solvent shell. Indeed, a single water molecule is observed to induce the C2O4¯ → CO2¯ + CO2 rearrangement (i.e., with charge localization onto one CO2 molecule) at n = 4 through the formation of a strong H-bond to the smaller molecular anion. Thus, the Im molecule creates the additional possibility (beyond H-bond-induced localization onto a CO2 monomer) of forming a covalent N-C bond in analogy with the behavior observed previously for Py.17 Analogous reactive uptake of neutrals has also been reported in the formation of M-CO2¯ (M= Mg, Mn, Fe, Co, Cu, Ni, Ag, Au) 29-37 and R-CO2¯ motifs.13, 38-40

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IIIB. Vibrational predissociation spectra of [Im(CO2)n]¯ cluster ions Figure 3 presents the vibrational spectra of the Im(CO2)n¯ clusters (Fig. 3c, 3e and 3g), along with those of relevant species from earlier studies:17 (CO2)n¯ with n=5 (Fig. 3a), n = 7 (Fig. 3b) and the carbamate system, Py(CO2)2¯ (Fig. 3h). The key features in the homogeneous (CO2)5¯ and (CO2)7¯spectra are the bands corresponding to the CO2¯ monomer anion (green) and those arising from the C2O4¯ dimer (pink), which occur at 1660 and 1865 cm-1 respectively.17 These signature bands are presented without contamination in the n = 5 and 7 spectra because the core ion switch occurs promptly at n = 6,6, 9 with n = 5 built on a dimer core ion, while n = 7 is formed exclusively with a monomer core. The strong feature at 2349 cm-1 (blue) occurs at the location of the ν3 fundamental in bare CO2, establishing the presence of at least one neutral CO2 molecule in all of these anionic clusters. Note that the covalently bound CO2¯ moiety in the Py(CO2)2¯ spectrum (Fig. 3h) displays a similar antisymmetric CO stretch to that of the isolated CO2¯ anion (1660 vs 1705 cm-1, respectively, for (CO2)7¯ and Py(CO2)2¯) while the second CO2 contributes the strong feature at 2349 cm-1, again indicating that it is a weakly bound neutral molecule. Interestingly, the symmetric CO stretch of the –CO2¯ moiety in the carbamate at 1258 cm-1 is much stronger than those in isolated CO2¯, which is due to a large participation of the N-C stretching displacement in this normal mode.17 All three Im(CO2)n¯ clusters display very similar features (Fig. 3c, 3e and 3g), indicating that they share a common charge accommodation motif. As such, the Im(CO2)2¯ spectrum is most informative, as all the key features are presented in this smallest assembly. The intact CO2¯ ν3 band (green) immediately rules out the formation of the C2O4¯ species, raising the question of whether the remaining CO2¯ anion is intact as a solvated molecular anion, or attacks the N: position to form a carbamate as was observed in the Py system earlier. Although the very small (45 cm-1) shift between the CO antisymmetric stretches in the –CO2¯ motif in the carbamate (Fig. 3h) relative to the bare ion ((CO2)7¯, Fig. 3b) complicates definitive structural assignment from this band location, it is compelling that the ν3 CO2¯ feature in the Im(CO2)2¯ spectrum (Fig. 3c) lies very close (5 cm-1) from (CO2)7¯ as well as (7 cm-1) to that of CO2¯ in solid neon observed by Jacox and Thompson.41 This conclusion is supported by the fact that the corresponding band close to the expected location of the –CO2 symmetric stretch (purple) is very weak, while this fundamental is enhanced upon formation of a covalent N-C bond as described above.

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More evidence that the Im(CO2)2¯ ion occurs with a CO2¯ radical ion core derives from the fact that the NH stretch associated with the Im neutral molecule near 3518 cm-1 is completely missing in the Im(CO2)2¯ spectrum (Fig. 3c)27 despite being calculated at the harmonic level to dominate the higher energy region of the spectrum. This is indeed the case for the Ar-tagged H+·(Im)n=1-3 clusters, with the observed and calculated spectra presented in Fig. S5. Table S1 displays the frequencies and intensities of this NH stretch for both the protonated Im clusters and the isomers of Im(CO2)2¯ clusters shown in Fig. 4, which shows that the relative intensities of the NH stretches are calculated to be much larger (about a factor of 10) in the case of Im(CO2)2¯ isomers. This suggests that the NH group

Fig. 4. Calculated structures of a-f) six low energy isomers of Im(CO2)2¯. Harmonic calculations were performed at the B3LYP/aug-cc-pVDZ level of theory and the relative single point energies computed were computed using SCF/aug-cc-pv(d,t)z extrapolated (λ= 1.3476302) + ∆CCSD/aug-cc-pv(d,t)z extrapolated (λ= 1.5877616)+(T)/aug-cc-pvtz and are BSSE corrected. The energies are relative to the global minimum, correctd for the harmonic vibrational zero-point energies. The a) structure in the red box is the lowest energy isomer and its calculated spectrum concluded as best fits the experimental trends observed.

could be strongly interacting with the CO2¯ moiety that is evident in the spectrum, raising the possibility that the broad structure around 2600 cm-1 arises from a red-shifted fundamental labeled IHB (ionic hydrogen bond) in Fig. 3, associated with the NH-(CO2¯) interaction. Fig. 4 presents six low lying minimum energy structures of Im(CO2)2¯, with the (scaled) harmonic spectrum calculated for the lowest energy isomer (Fig. 4a) displayed as the inverted trace in Fig. 3d. Note that, although the carbamate analogue (Fig. 4e and 4f) is a local minimum, it is much higher in energy than structures that have most of the excess charge on a single CO2 molecule. The C2O4¯ “dimer core” species is also locally stable (Fig. 4d), but energetically far above the structure corresponding to a CO2¯ anion H- bonded to the NH group on the Im ring (Fig. 4a). A curious aspect of these structures is that the N-bound CO2 does not appear to draw significant excess charge from the CO2¯ anion bound to the NH group. The spectra of these higher energy

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complex arrangements (Supp. Fig. S2) are not consistent with the observed pattern (Fig. 3c), however, as they exhibit the characteristic 2349 cm-1 feature associated with a neutral, weakly bound CO2 molecule. This behavior is best recovered by the calculated spectrum for the structure in Fig. 4a (as well as the structures in Fig. 4b and 4c), in which the second CO2 molecule attaches to the CO2¯ anion roughly along its symmetry axis. The broad features in the Im(CO2)n¯ spectra, which are presented on an expanded scale in Fig. 5, occur about 200 cm-1 below the predicted H-bonded NH stretch, with fine structure that is dependent on the number of attached neutral CO2 molecules. Of these, the ν1+ν3 band of the CO2¯ anion at 2900 cm-1 (green in Fig. 5), which is not dependent on cluster size, is readily assigned, as are the CH stretches on the Im moiety. It is useful to consider the anharmonic redshift of the observed diffuse features (red in Fig. 3) in the context of the spectral signatures of other systems with strong H-bonds to CO2¯. Previous studies have looked at the complexation of CO2¯ to both water42and methanol.43 A single water molecule actually binds in two motifs: one in which both OH groups bind to oxygen atoms of CO2¯ (a double H-bond donor or DD motif) and another with only one (a D motif). Although the red-shift of the OH stretches in the DD form is modest (~100 cm-1), the D configuration in which one OH group binds to an oxygen atom of the anion, displays a much larger (~400 cm-1) redshift with a diffuse envelope spanning about 350 cm-1.42 Interestingly, the dimer core ion persists in the H2O(CO2)n¯ clusters until n = 3, while in the case of CH3OH(CO2)n¯, all the clusters are based on a monomer core ion.43 This implies that Im and CH3OH, which both feature a single ionic H-bonding motif, appear to provide sufficient stabilization of the monomer ion to break up the C-C covalent bond in C2O4¯. The diffuse

Fig. 5. An expansion of the vibrational predissociation spectra of a) Im(CO2)2¯, b) Im(CO2)3¯ and c) Im(CO2)6¯ from the region of 2400 cm-1 to 3200 cm-1. The band in the IHB labeled * blue shifts with increasing CO2 solvation.

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nature of strong H-bonds in small molecular anions has been reported extensively, and in general arises from anharmonic couplings between the bound NH stretch and soft modes associated with the frustrated rotations and translations of the molecule relative to the NH-O bond axis. There is typically also a role for Fermi-type coupling to the NH bending modes,44 but these effects are beyond the

Fig. 6. Calculated minimum energy structures of the a) MIm, b) MIm(CO2)¯ where the CO2¯ transfers its charge to the MIm. All calculations were performed at the B3LYP/aug-cc-pVDZ level of theory.

scope of this survey work to determine the qualitative nature of the interactions. Larger Im(CO2)n¯ clusters are calculated to form with the essentially neutral CO2 molecules associated mostly with the anionic domain near the H-bonded CO2¯ anion,42, 45 as displayed in Figs. S3 and S4. In this general configuration, they are expected to act to solvate the anion, which in turn slightly weakens the H-bond and accounts for the small sequential blue shift of the diffuse bands arising from the NH stretch.45-47 These results raise the question of why the Im system is not observed to adopt a Im(CO2)¯ structure with a covalent N-C bond to the electron lone pair on the ring N atom, which is calculated to be a minimum energy structure (Fig. S3c) located 2.44 kJ/mol above the global minimum (Fig. S3a). Such higher energy isomers are routinely observed in the rapid cooling conditions at play in supersonic expansions.48-49 In the limit that the observed products are controlled by overall energetics rather than kinetic trapping, then one might suspect that the observed H-bonded species falls sufficiently below the carbamate that the ion-molecule complex dominates the cluster chemistry. To address this, we calculated the energies associated with CO2¯ attachment to the methylated derivative, MIm, displayed in Fig. 6a, which does not have the possibility of forming the H-bonded structure. Interestingly, the minimum energy structure identified for the MIm(CO2) ¯ system (Fig. 6b) features a CO2 moiety proximal to the N: atom, but with a nearly linear OCO axis. Closer inspection of the charge densities associated with this structure indicate that the excess charge now resides mostly on the MIm scaffold, so that it is essentially a charge-transfer system with neutral CO2 solvating the MIm¯ anion. The MIm¯

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anion has a positive electron affinity (1.987 eV),50 which has been established with negative ion photoelectron spectroscopy. These results all point to the fact that in the Im case the presence of the N-R functionality acts to dramatically deactivate the lone pair of electrons on the N: atom in the ring. Further work is therefore warranted to test this prediction by direct observation of the structures adopted by reactive condensation of MIm onto a (CO2)n¯ cluster ion beam, or alternatively condensation of CO2 onto MIm¯, which present promising future directions for this study. IV. Summary The chemical pathways available for attack of the CO2¯ radical anion on neutral imidazole (Im) are considered with theoretical calculations and structural determinations of the ionic products arising from Im incorporation into (CO2)n¯ cluster ions in an ionized supersonic expansion. Calculations indicate that two classes of low lying structures are available, the lowest of which involves simple attachment of the CO2¯ monomer ion with an H-bond to the NH group, while a second class higher in energy involves a partial covalent bond between the CO2 carbon atom and the open N: atom position on the ring. Vibrational spectra of the Im(CO2)2¯ cluster indicates that only one CO2 molecule interacts strongly interacts with the Im ring, while the second (and subsequent, in larger clusters) CO2 molecules are weakly bound as a neutral solvent molecule(s). This pattern is consistent with the lowest energy isomer, in which the strongly interacting NH group accounts for a diffuse, red-shifted pattern about 1000 cm-1 below the free NH stretch. The binding motif is calculated to occur by attachment of one of the CO2¯ oxygen atoms to the NH group. The diffuse nature of the NH stretch is rationalized in context of the floppy nature of this directional H-bond, and the expected strong anharmonic coupling between the high frequency NH stretch and the soft modes of the ion-molecule complex. These results provide a microscopic picture of the very different interaction motifs for CO2¯ to the seemingly similar pyridine and imidazole molecules.

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Supporting Information Mass spectra of Im(CO2)n¯ clusters; calculated harmonic spectra for isomers of Im(CO2)2¯, Im(CO2)3¯ and Im(CO2)6¯; imidazole cluster vibrational spectra Acknowledgements MAJ gratefully acknowledges the Air Force Office of Scientific Research (Grant FA9550-13-1-0007). We additionally thank Conrad T. Wolke for his confirmation of measurements and improvement of the signal to noise of the Im(CO2)n¯ cluster spectra. AP also thanks the Air Force Office of Scientific Research (Grant FA9550-11-1-0065) for funding on this work.

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V. References 1. Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Electrocatalytic and Homogeneous Approaches to Conversion of CO2 to Liquid Fuels. Chem. Soc. Rev. 2009, 38 (1), 89-99. 2. Costentin, C.; Robert, M.; Saveant, J. M. Catalysis of the Electrochemical Reduction of Carbon Dioxide. Chem. Soc. Rev. 2013, 42 (6), 2423-2436. 3. Yu, K. M. K.; Curcic, I.; Gabriel, J.; Tsang, S. C. E. Recent Advances in CO2 Capture and Utilization. Chemsuschem 2008, 1 (11), 893-899. 4. Bocarsly, A. B.; Gibson, Q. D.; Morris, A. J.; L'Esperance, R. P.; Detweiler, Z. M.; Lakkaraju, P. S.; Zeitler, E. L.; Shaw, T. W. Comparative Study of Imidazole and Pyridine Catalyzed Reduction of Carbon Dioxide at Illuminated Iron Pyrite Electrodes. ACS Catal. 2012, 2 (8), 1684-1692. 5. Saeki, M.; Tsukuda, T.; Nagata, T. Ab Initio Study of (CO2)n-: Structures and Stabilities of Isomers. Chem. Phys. Lett. 2001, 340, 376-384. 6. Tsukuda, T.; Johnson, M. A.; Nagata, T. Photoelectron Spectroscopy of (CO2)n- Revisited: Core Switching in the 2

Vibrational Characterization of Radical Ion Adducts between

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