Infrared Multiple Photon Dissociation Spectroscopy of Hydrated Cobalt Anions Doped with Carbon Dioxide CoCO2(H2O)n −, n=1–10, in the C−O Stretch Region

Abstract We investigate anionic [Co,CO2,nH2O]− clusters as model systems for the electrochemical activation of CO2 by infrared multiple photon dissociation (IRMPD) spectroscopy in the range of 1250–2234 cm−1 using an FT‐ICR mass spectrometer. We show that both CO2 and H2O are activated in a significant fraction of the [Co,CO2,H2O]− clusters since it dissociates by CO loss, and the IR spectrum exhibits the characteristic C−O stretching frequency. About 25 % of the ion population can be dissociated by pumping the C−O stretching mode. With the help of quantum chemical calculations, we assign the structure of this ion as Co(CO)(OH)2 −. However, calculations find Co(HCOO)(OH)− as the global minimum, which is stable against IRMPD under the conditions of our experiment. Weak features around 1590–1730 cm−1 are most likely due to higher lying isomers of the composition Co(HOCO)(OH)−. Upon additional hydration, all species [Co,CO2,nH2O]−, n≥2, undergo IRMPD through loss of H2O molecules as a relatively weakly bound messenger. The main spectral features are the C−O stretching mode of the CO ligand around 1900 cm−1, the water bending mode mixed with the antisymmetric C−O stretching mode of the HCOO− ligand around 1580–1730 cm−1, and the symmetric C−O stretching mode of the HCOO− ligand around 1300 cm−1. A weak feature above 2000 cm−1 is assigned to water combination bands. The spectral assignment clearly indicates the presence of at least two distinct isomers for n ≥2.


Introduction
Carbon dioxide as the most important greenhouse gas in the Earth's atmosphere is currently intensely investigated. [1] The electrochemical route of activation involves the carbon dioxide radicala nion CO 2 À as as hort-lived intermediate. [2,3] It is well known that CO 2 À is metastable and undergoes autodetachment with am easuredl ifetime of up to milliseconds. [4][5][6][7] This has been repeatedly confirmed by quantum chemical calculations. [3,[7][8][9] In interaction with ar are gas matrix [10] or as olvation shell such as (CO 2 ) n À [6,11,12] or CO 2 (H 2 O) n À , [13,14] the radical anion is stabilized. [4] The same is true in as alt environment where the interaction with positive chargec enters is responsible for the stabilization. [15,16] In the interaction of CO 2 with metal ions, electront ransfer from the metal to the electrophilic carbon atom can occur spontaneously,l eadingt oc omplexes of the metal centerw ith CO 2 À . [4,[17][18][19][20] When as ingle bond is formed between them etal and the carbon atom, as observed for example,w ith the nickel group, coinage metal, or bismuth anions, the excess chargei nt his metalloformate h 1 -(C) complex, MCO 2 À ,i sd elocalized over the whole molecular ion. [21,22] Organometallic complexes of transition metals like cobalt can play an important role in catalytic reductionso fC O 2 , [23] a key step in carbon capturea nd usage (CCU) processes. In the gas phase, the reverse reaction, CO oxidation leading to CO 2 , has been observed with anionic cobalt oxide clusters. [24] Decomposition reactions of copperf ormate revealed important elementary steps in the transformation of CO 2 to HCOOH. [25] Schwarz has recently summarized the mechanistic insighti nto CO 2 activation derived from gas-phase studies, combining experiment and theory. [26] Vibrational spectroscopy is ap owerful methodf or structural analysis in the gas phase. [4] Vibrational spectra of Co n + (CH 3 OH) 3 (n = 1-3) were measured by IR photodissociation spectroscopy. [27] Anionic cobaltc lusters dopedw ith methanol, ethanol, or propanol molecules were probedb yI Rs pectroscopy in the OÀHs tretch region. [28] Cobalt carbonylc ations Co(CO) n + (n = 1-9) were investigated in an Ar tagging experiment by the group of Duncan, finding one stronga bsorption for n = 1a t2 156 cm À1 . [29] Cationic metal-CO 2 complexes M + (CO 2 ) n (M = Mg, Al, Si, V, Fe, Co, Ni, Rh, Ir) have been exten-sively investigatedi nt he past decades, [30][31][32][33][34][35][36][37][38][39][40] and also anionic speciesM À (CO 2 ) n (M = Ti,M n, Fe, Co, Ni, Cu, Ag, Au, Sn, Bi) have received considerable attention,f oremost by the group of Weber. [41][42][43][44][45][46][47][48][49][50][51] Generally,t he anionic CO 2 À stretching vibrations shift to the red compared to neutral CO 2 vibrations. [4,52] CO 2 as al igand was also investigated as metal oxides, NbO 2 + (CO 2 ) n and Ta O 2 + (CO 2 ) n by Mackenzie and co-workers. [53] Photoelectron spectroscopy by the Bowengroup revealed CO 2 activation upon attachment to anionic cobalt pyridine complexes [54] and providedadifferent look on anionic coinage metal complexes with CO 2 . [55] The above-mentionedI Rs tudy of Co(CO 2 ) n À showed that Co forms ac ore with two negatively charged CO 2 molecules attached via ab identate motif, formingatwisted butterfly arrangement. Further CO 2 molecules surround this core. [43] Avery interesting study on cooperative effects which are operative during metal insertion into the C=Obond of CO 2 has been performed recently by the group of Weber with Ti À (CO 2 ) n . [51] Insertion of neutralT ii nto the C=Ob ond of CO 2 had been predicted by quantum chemical calculations. [56] In an environmentally benign chemical process, water is the ideal solvent. It is, therefore, important to understand cooperative effects during the activation of CO 2 in the presence of water molecules. We have recently demonstrated CÀH, [57] CÀC, [58][59][60] CÀS [61] bond formation and protonation reactions [62] with CO 2 À (H 2 O) n clusters in the gas phase. Nanocalorimetry revealed important details about the thermochemistry of the carbon dioxide radical anion,i np articular,i ts hydration enthalpy. [63,64] Raman spectroscopy of CO 2 À in bulk aqueous solution [65] places the symmetric stretching mode of hydrated CO 2 À at 1298 cm À1 .I no ur recent IR study on gas phase clusters CO 2 À (H 2 O) n ,w eo bservedv ery similarv alues already around n = 20. [14] Some hydrated metal ions M + (H 2 O) n ,M= Mg, Cr,C o, pick up exactly one CO 2 molecule, indicating that electron transfer from the metal to carbon dioxide takes place. [66][67][68] In the case of magnesium, the electron is already present in the hydration shell, detached from the metal center, as recently confirmed by electronic spectroscopy of Mg + (H 2 O) n . [69] For the structural analysis of hydrated metal ions M(H 2 O) n (M = Li + ,N a + ,M g + ,M g 2 + ,A l + ,C a 2 + ,C o + ,C o 2 + ,C u + ,A g + , Cs + ,B a 2 + ,T m 3 + ,L a 3 + ), as eries of infrared photodissociation studies are available. [70][71][72][73][74][75][76][77] Pure cationic cobalt clusters Co n + Ar were investigated spectroscopicallyb ya rgon tagging. [78] Herein, we report the first IR multiphoton dissociation( IRMPD) study investigating CO 2 attached to am etal anion solvated with water.T he spectra of isolated CoCO 2 (H 2 O) n À , n = 1-10, clusters along with quantum chemical calculations provide clear evidence of CO 2 and H 2 Ob ond rearrangementsa lready for the CoCO 2 H 2 O À ion.

Experimental and Theoretical Methods
The experiments were performed on am odified 4.7 TF T-ICR Bruker/Spectrospin CMS47X mass spectrometer [64,[79][80][81][82] equipped with aB ruker infinity cell. [83] Ions are produced in an external laser vaporization source [84,85] with a3 0Hzp ulsed frequency doubled Nd:YAG laser (Litron Nano S6 0-30). Ag as mixture of He, H 2 O, and CO 2 is expanded through ah omebuilt piezoelectric valve. The laser is focused on ar otating Co target, producing ah ot plasma, which is cooled by supersonic jet expansion. These ions are guided through as ystem of electrostatic lenses passing three differential pumping stages to the center of the ICR cell [86] where they are stored and mass selected in a4 .7 Tm agnetic field [87] under ultrahigh vacuum ( % 10 À10 mbar) conditions. Ac opper shield, which is cooled by liquid nitrogen to T % 80 K, surrounds the cell [88,89] to minimize the amount of black body infrared radiative dissociation (BIRD). [90][91][92][93][94][95][96][97][98][99] From the rear side of the magnet, at unable IR OPO laser system (EKSPLA NT273-XIR) is coupled into the cell through aC aF 2 window. [100] When absorption events lead to photodissociation, [101] they are detected by the experiment. The measurements were performed in the range of 1250-2234 cm À1 where characteristic CÀO stretching modes are typically observed. Details on the experimental laser setup can be found elsewhere. [14,100] The present experiments are lacking information on the number of photons required for dissociation, thus we determine the IRMPD yield, which is total photofragment intensity divided by total ion intensity,i rradiation time and laser power.I nc ontrast to the usual definition of IRMPD yield, [102] we also include the irradiation time, since we adjust it to avoid saturation effects and to increase the signal-to-noise ratio of weak bands. As already mentioned above, fragments like CO 2 C À and CO 2 C À (H 2 O) cannot be detected, [103] because the excess electron undergoes autodetachment. However,n os ignal loss was detected in the present experiment, implying that the decomposition into fragments like CO 2 C À and CO 2 C À (H 2 O) does not take place to asignificant extent. Structure and properties of CoCO 2 (H 2 O) n À , n = 1-10, were studied using methods of theoretical chemistry at the B3LYP/def2TZVPP level of theory.B enchmark calculations with respect to CCSD(T) results for the most stable isomers of n = 1c an be found in Tables S1 and S2.
The CoCO 2 À ion exhibits either am etalloformate h 1 -(C) motif or the linear OCoCO À inserted structure. Starting with those, we added a water molecule and constructed several isomers with both, an intact and an activated water molecule, resulting in 14 stable structures for the CoCO 2 H 2 O À ion. By adding successive water molecules to various positions and optimizing the structures, we created structures for clusters with up to four water molecules. For seven selected structures, further solvation with up to at otal of 10 water molecules was performed. Vibrational spectra are modeled by using Gaussian broadening with af ull width at half maximum (FWHM) of 20 cm À1 and scaled by af actor of 0.96. Wavefunction stabilization was performed for every calculation, with internal instability issues found in more than 20 %o fc alculated structures. All considered structures represent local minima. Transition states are verified through intrinsic reaction coordinate (IRC) calculations. For some transition states, starting points with as mall offset along the normal vector of the corresponding imaginary frequency with subsequent steepest decent optimization had to be used to make the IRC calculations work. The Gaussian 16 software was employed for all calculations. [104] Results and Discussion Bare CoCO 2

À À
We start our discussion with the non-hydrated ion, CoCO 2 À .I n the experiment, no fragments are observed in the investigated wavelength regione ven after irradiating for 20 s. Thisi si n  [43] Dissociation to Co À and CO 2 ,t he lowest energy fragmentation pathway,r equires 73 kJ mol À1 ,c alculated at the B3LYP/def2TZVPP level. IRMPD is inefficient in smalls ystems with highb inding energy, since the molecule undergoes radiative cooling before dissociation.

Monohydrated CoCO 2 H 2 O À À
The absorption spectrum of the monohydrated ion, (1), in which m is the number of photons: In the measured IRMPD spectrum of CoCO 2 H 2 O À ,t he absorptionm aximum appears at 1881cm À1 .Aless intense broad band was observed in the 1570-1730 cm À1 region. The absorption saturates upon longer irradiation at the maximum, but only 25 %o ft he precursor ions dissociate. Laser misalignment can be ruled out, since otheri ons could be almostf ully depleted with the same laser alignment.This indicates that additional isomersa re presentw ith an abundance of % 75 %, which do not absorb at this wavelength.
Quantum chemical calculations of CoCO 2 H 2 O À reveal ar ich structurald iversity.T he most stable structure is isomer Ia,w ith Co(OH)(HCO 2 ) À structure, in which both H 2 Oa nd CO 2 are activated,s ee Figure 1. Isomer Ib with cobalt inserted in the C=O bond is less stable by 41 kJ mol À1 .F urtheri somers with activated H 2 O, HCo(HCO 3 ) À (Ic), HOCo(HOCO) À (Id, Ie), and HCoOH(CO 2 ) À (If)l ie even higher in energy.T wo isomersw ith intact H 2 O( Ig)a nd both intact H 2 Oa nd CO 2 (Ih)l ie about 180 kJ mol À1 above Ia. Figure 2s hows the potential energy surface of possible CO loss reactions for the CoCO 2 H 2 O À ion. Figure 2a reveals al ow water activation energy on CoCO 2 À of 24 kJ mol À1 relative to the entrance channel, transferring ah ydrogen atom metalmediated to CO 2 and eventually creating the most stable Co(OH)(HCO 2 ) À structure (Ia), with If as an intermediate. Another possible pathway can be seen in Figure 2b,inw hich CO 2 activation in the absence of water requires 167 kJ mol À1 relative Figure 1. Comparison of a) measured IRMPD spectrum for CoCO 2 H 2 O À resulting in CO loss with b-d) the calculatedabsorption cross section s theo for isomers Ia-h.T he main band in the experiment was fitted with aG aussian to determine the peak position. Geometryo ptimization and frequencycalculation for each isomer was performed at the B3LYP/def2TZVPP level of theory. Relative energy of isomers is giveni nkJmol À1 including zero-pointc orrection. to the entrance channel. As soon as water is added, the OCCoO(H 2 O) À structure (Ig)i sf ormed.F rom there, water activation proceeds readily over as mall barrier,a nd the path opens to form the OCCo(OH) 2 À ion (Ib). Water activationo n bare Co À requires 119kJmol À1 (Figure 2c). CO 2 can then be further activated over ab arriero fa bout 80 kJ mol À1 formingi somer Ic.Apotentiale nergy barrier of 243 kJ mol À1 needs to be overcome for isomerization to Id with Co(OH)(HCO 2 ) 2 À structure. The mostp rominents pectralf eature in the experiment at 1881 cm À1 can be reproduced by the C=Ov ibration in both Ib and Ig isomers (Figure 1). However,i somerization of Ig to Ib faces ab arrier of only 8kJmol À1 (Figure 2b)a nd is thus not expected to survivei nt he ICR cell. In the Ib structure, aC O group is present and will readily dissociate after absorption of 3-4 photons at 1881 cm À1 .W et hus assignt he 1881 cm À1 band exclusively to isomer Ib.T he experiment indicates that this isomer forms about 25 %ofthe total ion abundance,estimated from the IRMPD yield in saturation.
The weaker absorption band observed experimentally at 1570-1730 cm À1 lies in the range of the H 2 Ob endingm ode and the antisymmetric stretching mode of CO 2 À . [14] The presence of an intact water molecule, isomers Ig and Ih,c an be ruled out. According to Figure 2t hese ions are expected to dissociate by loss of water,which is not observed in the experiment. The remaining calculated isomers Ia, Ic-f all exhibit vibrational modes in this region. The presenceo ft he most stable isomer Ia is probable,a lso due to itsv ibration at % 1300 cm À1 observed for n > 1( see below). The CO loss energy is calculated to be 126 kJ mol À1 with respect to isomer Ia,b ut it requires ar earrangement with ab arrier of 291 kJ mol À1 .F or that reason, it is not plausible that Ia contributes to the observed photodissociation spectrum, as approximately 15 photonsw ould be required. Similarly,i somer Ic is topologically well separated from the CO loss pathway and CO 2 loss would be the most probablechannel here.
Only isomers Id and Ie can thusa ccount for the broad weak feature. These isomersf eature aH OCO ligand,w ith absorptions in the relevant spectral region.B oth face ab arrier around 40 kJ mol À1 against rearrangement to isomer Ib,a nd the barriers lie above the CO loss channel, Figure 2b.I somerization to Ib will, therefore, be immediately followed by CO loss. The barrier corresponds to the absorption of 2-3 photons. Depending on the orientation of the ligand in Id and Ie and dynamic effects, the spectrum may exhibit the observed broad structure, given the high conformational flexibility of the HOCO ligand. Since relativelyf ew photonsa re required for dissociation of Id and Ie,alowa bundance of these isomers is sufficient to cause the observed features.
We therefore conclude that from the calculated isomers, only Ib, Id and Ie contribute to the observed spectrum. Isomer Ia is very likely present, even as the most abundant isomer, but it does not lead to an IRMPD signal under the conditions of our experiment.

À À
For clusters with two water molecules,w ater evaporation is exclusivelyo bserved,r eaction(2) with n = 2. The most intense absorption band shifts to the blue, and additional bands arise at both ends of the spectrum.
The features from the monohydrated speciesa re again observed, Figure 3. The absorption maximum in the IRMPD spectrum lies at 1898 cm À1 ,s hiftedb ya bout 18 cm À1 to the blue, and roughlya no rder of magnitude more intense comparedt o the n = 1s pectrum.T he higheri ntensity is due to the fact that H 2 Ol oss requires less energy than the loss of aC Om olecule, that is, only about two photons. At longer irradiationt imes, CoOH 2 O À is formed by secondary fragmentation of CoCO 2 H 2 O À .T oa void saturation effects and secondary fragmentation,t his strong band around 1900 cm À1 is measured with shorter irradiation time than the rest of the spectrum.
In the region of 1500-1700 cm À1 ,two clearly visible bands at % 1622 and % 1665 cm À1 are observedf or n = 2. Further,t wo new absorption bands are observed, av ery weak transition between 1272 and 1314 cm À1 and ab and around 2060 cm À1 .T he isomer absorbing in the former region seemst ob ep resent only in very little amount in our experiment. Even after irradia- tion times as long as 10 s, only % 2% of the ions dissociate due to laser irradiation. This band might arise due to the symmetric CÀOs tretching mode of an HCO 2 À ligand, which lies at 1314 cm À1 in HCOO À (Ar). [105,106] In ar ecent study by Weber on [Ti(CO 2 ) n ] À ,i nw hich titanium insertsi nto aC =Ob ond, as mall band observed at 2056 cm À1 was assigned to oxalato ligands, which can be ruled out here. [51] DFT calculations predict very similar structures compared to the case of one water molecule.T he most stable isomer IIa has a( H 2 O)(OH)Co(HCO 2 ) À structure, that is, CO 2 and one H 2 Oa re activated. Isomer IIb with an inserted metal in the C=Ob ond and an activated H 2 Oi sl ess stable by only 31 kJ mol À1 .F urther isomerslie at least % 70 kJ mol À1 higher in energy.
As seen in Figures 3, 4, and Figure S1, Supporting Information, calculated IR spectra do not change much when passing from one to two water molecules.T he most intense band in the experiment at 1898 cm À1 results from the C=Ov ibration in isomer IIb.T he absorption at % 1580-1700 cm À1 is due to a mixture of the bending mode of the intact H 2 Om olecule and the antisymmetricC ÀOs tretching mode in the HCO 2 ligand, with contributions from various isomers, for example, the CÀO stretch in IIc-e as well as the water bend in IIa or IIb.I ni somers IIc and IIf,t he frequencies corresponding to the CoÀHv ibrationlie between 1650 and 1800 cm À1 .
The small absorption at low energies can be assigned to either isomer IIa or IIc.D epending on the angle of the (HCO 2 ) complexi nIIa,t he absorption might shift even more to higher energiesa ss een in Figure S2, Supporting Information. The presence of an exotic Co(OH)(H 2 O)···HCO 2 À complex with ar elative energy of 24 kJ mol À1 could also account for the observed band, see Figure S2, Supporting Information. However,f ormation of such an isomer does not correspondt ot he observed water loss within the IRMPD process.
With respectt ot he experimentally measured band at 2060 cm À1 ,n oc alculated isomer features harmonic vibrational modesn ear this wavenumber.O ur excited states calculations at the equation of motion-coupled clusters ingles and doubles (EOM-CCSD) level show that there are also no low-lying electronically excited states in the IR region. Such states are calculated in the OCoCO À ion but disappear upon water activation. Most likely,t he band origins from overtones and combination bandso fl ower-lying transitions.

Larger hydrated species
Clusters with n > 2a lso evaporate as ingle water molecule upon resonantI Ri rradiation, reaction (2). Saturation effects become more evident with increasing clusters ize, and the effect on the band shape of the absorption at 1900 cm À1 is shown for n = 3w ith two different irradiation times t IR in Figure S3, Supporting Information.
The spectra for n = 1-10 are shown in Figure 5, with the spectra for n = 1, 2i ncluded for comparison. Generally,o ne can see ab lueshift of the bands at % 1300 and % 1900 cm À1 , whereas the other two bands do not exhibit as ystematic shift.  These shifts are compared in Figure 6, in which the evolution of absorption maximaw ith respectt ot he clusters ize is shown. For the band at % 1300 cm À1 ,e xperimental data shows an average shift of % 4cm À1 per water molecule. This shift is reproduced in the calculations by the vibrations of isomers Ia-Xa with an average shifto f% 3cm À1 per water molecule (Figure 6a). The corresponding structures are showni nF igure S4, Supporting Information.
The most intense absorption is found for all clusters izes between 1860 and 1960 cm À1 and shifts to the blue with increasing n. As mentioned above,t his band corresponds to the C=O vibration in isomer b,a nd its shiftc an be wellr eproducedb y our calculations, see Figure 6b.F or n 6, an early linear blue shift is observed. As seen in Figure S5, Supporting Information, as houlder arises on the low-energy side for n = 4, which becomes more and more dominantf or higher n,a nd two data points are included for these clusters izes in Figure 6b.T his band is also seen in CO adsorption experiments on aC os urface. [107] It is also seen as av ery weak feature for inserted isomers in Co(CO 2 ) n À by the group of Weber. [43] We interpret it here as the emergence of an ew isomer,m ost likely involving a hydratedC Og roup.
The wavenumber region of 1550-1750 cm À1 is composed mainly of water vibrations,w ith minor contributionsf rom the antisymmetric stretching mode of formate. No clear trends can be identified due to several isomersc ontributing to the spectral envelope. Theoretical calculations do not show any clear trend with respectt ot he clusters ize for any isomer,F igure 4.
The last feature at % 2060 cm À1 exhibits ap ronounced band only for 2 n 5. It does not shift to the blue with increasing n. However,t he band broadens with increasing n so that the band is smeared out for n ! 6, resulting in ar aised baselinea s seen in Figure 5. For larger clusters, the band might be explained by ac ombination of H 2 Ob ending n 2 ,H 2 Ol ibration n L2 , and bending of H 2 Ot riplets n T2 , [108] as seen before in the spectra of CO 2 À (H 2 O) n . [14] Conclusions We measured IR multiple photon dissociation spectra of the CoCO 2 (H 2 O) n À systems. As reported before, [43] the non-hydrated speciesC oCO 2 À does not show an IRMPD signali nt he wavelength region investigated. Alreadyf or n = 1, the most prominent absorption is characteristico fametal-coordinated CO group, which shows that the Co atom has inserted into the C=Ob ond of CO 2 .H owever,t he spectra also show that multiple isomersa re present, and those without am etal coordinated CO seem to prevail. Twoi somersf eaturing aH OCO ligand are most likely responsible for the weak, broad transition around1 570-1730 cm À1 ,s ince they have absorptions in that region and simple rearrangements allow for the release of CO.
For n ! 2, all primary IRMPD signals are due to loss of one H 2 Om olecule. The probably most abundant isomer class that features af ormate ligand is directly evidenced by ab and the position of which shifts from 1303 to 1337 cm À1 upon hydration with up to 10 H 2 Om olecules.T he region, which could be indicative of aH OCO ligand,h owever,i sn ow smeared out by overlapping absorptions due to the water bending and antisymmetric HOCO À or HCOO À stretching modes. The most intense absorption of the CÀOs tretching mode in the metal inserted isomer shifts to the blue with increasing n,from 1881 to 1938 cm À1 .Aweak feature at roughly2 060 cm À1 ,w hich is assignedt oac ombination band of low-lyingw ater modes, smears out with increasing solvation,l eading to an elevated baselinefor large clusters in this region.
We rationalize the presence of different isomers by the pronounced non-equilibrium conditions in the ion source. Due to the specific nature of the potential energy surfaceo ft he and thus persist under the experimentalc onditions.