Article pubs.acs.org/JPCA
IR Spectroscopy of Gas Phase V(CO2)n+ Clusters: Solvation-Induced Electron Transfer and Activation of CO2 Allen M. Ricks, Antonio D. Brathwaite, and Michael A. Duncan* Department of Chemistry University of Georgia, Athens, Georgia 30602 S Supporting Information *
ABSTRACT: Ion−molecule complexes of vanadium and CO2, i.e., V(CO2)n+, produced by laser vaporization are mass selected and studied with infrared laser photodissociation spectroscopy. Vibrational bands for the smaller clusters (n < 7) are consistent with CO2 ligands bound to the metal cation via electrostatic interactions and/or attaching as inert species in the second coordination sphere. All IR bands for these complexes are consistent with intact CO2 molecules weakly perturbed by cation binding. However, multiple new IR bands occur only in larger complexes (n ≥ 7), indicating the formation of an intracluster reaction product whose nominal mass is the same as that of V(CO2)n+ complexes. Computational studies and the comparison of predicted spectra for different possible reaction products allow identification of an oxalate-type C2O4 anion species in the cluster. The activation of CO2 producing this product occurs via a solvation-induced metal→ligand electron transfer reaction.
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INTRODUCTION Global warming issues have stimulated intense interest in the chemistry of CO2 sequestration and activation. Related catalytic processes are therefore of widespread interest in many subfields of chemistry.1−7 Although gas phase systems that activate CO2 are unlikely candidates for practical applications, they may serve as model systems demonstrating useful principles of this chemistry. In the present report, we investigate gas phase ion− molecule clusters of atomic vanadium ions (V+) interacting with CO2 that exhibit fascinating intracluster chemistry leading to CO2 activation. The structures, bonding, and mechanism of this chemistry are explored with mass spectrometry, infrared spectroscopy, and computational chemistry. Metal ion−molecular clusters that can be size-selected with mass spectrometry provide intriguing reaction systems whose composition is known and for which the exact numbers of reactant and/or solvent molecules can be controlled.8−16 Many examples are known for the reactions of isolated metal ions with molecular partners and their energy dependence.17−23 However, when these ions are produced within molecular clusters, new potential energy landscapes are accessed, influenced by the solvation of both reactants and products. Previous mass spectrometry measurements and fragmentation patterns have documented reactive processes in metal ions clustered with water.24−26 These systems undergo insertion chemistry producing hydroxide ions, after a certain number of water molecules needed to stabilize the charge-separated system are present. Metal ion−CO2 complexes have been observed to form oxide species upon UV laser excitation.27−30 Although many metal ion−molecular reactions have been suggested on the basis of mass spectrometry data, there are few systems with specific spectroscopic identification of the reaction © 2013 American Chemical Society
products. Our group has previously reported IR spectra for nickel cation complexes with cyclobutadiene, formed from the coupling of acetylene molecules after a critical cluster size was reached.31 Likewise, we found metal ion hydroxide bands resulting from the insertion chemistry of doubly charged metal cation complexes with water.32−35 In our previous studies of M(CO2)n+ complexes,36−42 the Ni+, Si+, and V+ systems exhibited unusual vibrational bands suggesting intracluster reactions, but the exact reaction product could not be identified.40,42,43 In the present work, we use expanded laser tuning ranges to provide a clearer picture of the intracluster chemistry of vanadium cations with CO2. Gas phase metal cation−CO2 complexes have been studied with mass spectrometry, providing bonding energetics,8,11,22,23 and with computational chemistry, providing predicted structures and bonding patterns.44,45 These systems have been studied with electronic spectroscopy, using solvated atomic transitions of the metal cation.27−30,46−49 More recently, our research group reported infrared spectroscopy and computational studies of several metal cation−carbon dioxide complexes (M+ = Fe+, Ni+, V+, Al+, Mg+, Si+)36−42 in the region of the CO2 asymmetric stretch vibration (2349 cm−1).50 In each of these systems, the metal−ligand binding was concluded to arise primarily from the charge-quadrupole electrostatic interaction, leading to linear M +OCO bonding configurations. The CO2 asymmetric stretch vibration in these complexes shifted to frequencies higher than that of the isolated CO2 molecule. These blue-shifted vibrations were Received: September 5, 2013 Revised: October 17, 2013 Published: October 21, 2013 11490
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documented for several metal ions and found to vary only slightly from one system to another, consistent with the electrostatic interaction. Because CO2 forms clusters efficiently at the low temperatures of the experiments, larger M(CO2)n+ complexes were produced having many second-sphere molecules. These outer CO2 molecules were observed to have asymmetric stretch vibrations at almost exactly the frequency of the free molecule, and to undergo efficient photoelimination because of their weaker binding interactions. Related to these metal cation complexes, Weber and coworkers have used similar IR methods to study metal anion− CO2 complexes.51,52 While most M(CO2)n+ spectra were understood in terms of the vibrations of the ″core″ and ″second-sphere″ CO2 species, a puzzling observation was made for the larger complexes of certain metal ions (including vanadium) that contained more than 7−8 CO2 molecules.40,42,43 In these larger systems, a new vibrational band was found near the asymmetric stretch that was more blue-shifted than any of the core ligand vibrations. This band became quite strong as cluster size increased, eventually dominating the spectrum for some systems.40 Its appearance was interpreted as indirect evidence for an intracluster reaction promoted by solvation, with a product having the same mass as one or more CO2 units. The greater vibrational shift for CO2 molecules in ″reacted″ clusters, compared to that for core ligands interacting with a singly charged cation, suggested that the metal in these species had a greater positive charge, indicating oxidation chemistry. However, the exact reaction product could not be determined at that time with the limited tuning range of the IR lasers available. More recently, new IR lasers with broader tuning range have become available, providing access to almost the entire infrared region.53,54 In the present study, we employ this improved IR coverage to investigate the V(CO2)n+ cluster system, revealing the intracluster chemistry that leads to CO2 activation.
FWHM Lorentzian line width for comparison to the experimental spectra.
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RESULTS AND DISCUSSION Figure 1 shows the mass spectrum of V(CO2)n+ complexes produced by laser vaporization. Also shown are oxide masses
Figure 1. Mass spectrum of the cations produced from laser vaporization of vanadium in an expansion of CO2.
resulting from either CO2 decomposition in the laser plasma or vaporization of oxides from the metal rod surface. This mass distribution, which extends out to clusters having 10 or more CO2 molecules, is typical of those seen for many metal cationCO2 complexes in the past.27−30,36−42 Because the number of CO2 molecules exceeds reasonable expectations for metal coordination, it is understood that these masses represent a subset of ligands attached directly to the metal that form the core ion, and another set of weakly bound second-sphere (or greater) species. The large clusters are produced efficiently because of the cold supersonic beam conditions. Throughout the discussion below, we make the convenient distinction between ″ligands″, indicating CO2 molecules cooordinated directly to the metal ion, and second-sphere, or ″solvent″ species. However, we recognize that this distinction is somewhat arbitrary, as all of these species are bound by pure electrostatic interactions. However, as shown below, the binding energetics are significantly different for these two types of CO2 molecules. Because the density of mass-selected ions is far too low for absorption spectroscopy, we use photodissociation measurements to obtain the infrared spectra of these clusters.13−16 To do this, we mass-select the ions from this distribution and excite them with tunable infrared radiation from the OPO laser system. As discussed in our previous report on this system,43 V(CO2)n+ clusters for n > 4 dissociate efficiently by the elimination of one or more CO2 molecules when they are excited near the asymmetric stretch vibration. Larger clusters eliminate multiple CO2’s, terminating their ″breakdown″ at the n = 4 complex. This behavior indicates that the coordination of V(CO2)n+ complexes is complete with four ligands, i.e., the core ion is V(CO2)4+. Theory and spectral evidence discussed previously43 indicates that the core ligands are bound primarily by charge−quadrupole interactions, with computed bond energies in the 15−25 kcal/mol range (5250−8750 cm−1; see Supporting Information). The structure of this core ion is predicted by theory to be square-planar (see SI). Complexes
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EXPERIMENTAL SECTION V(CO2)n+ clusters produced in a pulsed molecular beam by laser vaporization were mass-analyzed and size-selected in a reflectron time-of-flight mass spectrometer. The cluster source employed the so-called ″cutaway″ configuration for laser vaporization.55 The mass spectrometer is a specially designed reflectron instrument optimized for laser photodissociation.56 Mass-selected cluster ions were investigated with infrared laser photodissociation spectroscopy,13−16 using a tunable infrared optical parametric oscillator system (LaserVision) pumped by a Nd:YAG laser (Spectra Physics Pro-230). The OPO employs a KTP crystal oscillator and a set of four KTA crystals using parametric amplification to cover the 2000−4500 cm −1 region;53 a second stage of difference frequency generation in a AgGaSe2 crystal provides light in the 600−2300 cm−1 region.54 Fragment ions produced by elimination of CO2 from V(CO2)n+ species are recorded as a function of the laser frequency with a digital oscilloscope (LeCroy) to obtain a spectrum. Computational studies on these clusters are carried out using density functional theory (DFT) with the B3LYP functional, using the Gaussian03 suite of programs.57 The Def2-TZVP basis set was used for vanadium and the DZP basis set for carbon and oxygen. No scaling was done for these calculations. The bands in the predicted IR spectra were given a 10 cm−1 11491
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with more than four molecules have at least one of these in the second sphere where they are more weakly bound. The quadrupole−quadrupole interactions are predicted to give bond energies in the 3−5 kcal/mol range (1050−1750 cm−1; see SI). These computed values are not expected to be highly accurate because of the well-known problems of density functional theory for weak electrostatic interactions.58 The binding of external CO2 molecules can be compared to the dissociation energy of the neutral CO2 dimer, which is computed to be 484 cm−1.59 Because our systems are charged, this value is likely a lower limit on the binding energy of external molecules. When the single photon IR excitation energy exceeds the externalmolecule binding energy, efficient photodissociation is expected. Efficient photodissociation is detected for all the clusters with n > 4 down into the 630 cm−1 region, indicating that the actual binding energies are lower than this value. Figure 2 shows the photodissociation spectra of the V(CO2)n+ clusters for n = 8−11 in the region of the CO2
clusters that have been studied. Similar band patterns have been found for the M(CO2)n+ complexes of other metals.36−43 In addition to these two vibrations, which do not change substantially after n = 8, two other weaker bands gradually emerge in the larger clusters. A feature near 2359 cm−1 is seen for the M(CO2)n+ complexes of several different metals in the size range greater than about n = 7−8.36−43 A band at this position has also been reported in the spectra of large pure-CO2 clusters having no metal present.60 As discussed previously,36−43 this band is assigned to the asymmetric stretch of second-sphere CO2 molecules ″caged″ by surrounding CO2 molecules. The repulsion at the outer turning point of the CO2 stretching potential for caged molecules pushes the frequency slightly higher than it is for second-sphere CO2 molecules near the surface of the cluster that have no confinement. This band is therefore understandable on the basis of simple cluster growth, and has no obvious connection to an intracluster reaction. The more interesting new vibrational band for the larger clusters occurs near 2402 cm−1.43 It was also seen previously in M(CO2)n+ clusters for M = Ni and Si,40,42 and was particularly intense for nickel complexes.40 In the only other transition metal system for which such large CO2 clusters were studied, Fe(CO2)n+ complexes did not produce this feature.37 The behavior of this band in the previous studies led to its assignment as a CO2 asymmetric stretch vibration in clusters whose core ion had become more highly charged from some sort of intracluster reaction.40,42,43 In many metal ion cluster systems, higher charge states have been documented to cause greater vibrational band shifts,32−35 although this trend is not produced well by theory (see SI). The position of this band indicates a blue shift with respect to the isolated CO2 vibration, like that seen for core ligands in the 2377−2380 cm−1 region, but the magnitude of the shift is greater. Because M(CO2)+ complexes excited electronically were known to form oxides,27−30 and because the mass of the selected clusters corresponds to M(CO2)n+ units, an intracluster reaction was proposed that formed an ″oxide-carbonyl″ unit in the cluster.40,42,43 Oxidation of the core metal ion to a species with partial M2+,O− character could explain the higher charge on the metal and the consequently larger blue shift of the nearby CO2 molecules. However, the evidence for this was only circumstantial, as no vibrations of the reaction product itself were detected. Because of this, other reaction products having mass of one or more CO2 units were also noted to be possible, such as ″carbonyl−carbonate″ (OC-M-CO3)+ or ″oxalate″ (MC2O4)+ species.40,42,43 At the time of these previous studies, no further measurements were possible because of the limited tuning range of the available laser system, and the true nature of the proposed intracluster reactions could not be determined. As noted above, infrared OPO laser technology has improved dramatically in recent years, allowing much wider spectral coverage for these experiments. Figure 3 shows the spectroscopy now possible in scans of V(CO2)6+ and V(CO2)7+ over the range of 600−4000 cm−1. Because the vibrations of CO2 are well-known,50 the main bands in these spectra can be assigned to CO2 molecules interacting with metal whose vibrations are more or less shifted from those of the free molecule. Bands near 630 cm−1 correspond to the bending mode (ν2 = 667 cm−1 in isolated CO2); those near 1363 cm−1 to the symmetric stretch (ν1 = 1333 cm−1 in isolated CO2); those near 2350 cm−1 to the asymmetric stretch (ν3 = 2349 cm−1 in isolated CO2, as already shown in Figure 2); and finally, those near 3600−3800 cm−1 to
Figure 2. Infrared spectra of V(CO2)n+ clusters in the region of the CO2 asymmetric stretch vibration (monomer frequency indicated with dashed vertical line). The bands near 2380 cm−1 are assigned to the vibrations of CO2 molecules attached to the metal ion, while those near 2350 cm−1 are for second-sphere molecules. The bands near 2402 cm−1 are attributed to the presence of an intracluster reaction, producing a more highly charged core metal ion.
asymmetric stretch vibration (monomer value indicated with dashed vertical line). These spectra were reported in our original study of this system,43 and have been remeasured and reproduced here. The n = 8 cluster has two main bands, illustrating the pattern for the smaller clusters. The higher frequency feature at 2380 cm−1 is assigned to the asymmetric stretch of the core CO2 ligands. All of the smaller clusters (n = 1−4) have only this kind of vibration. The blue shift arises from the electrostatic charge−quadrupole bonding in this system and the effect of the cation on the CO2 vibrational potential.36−43 Clusters larger than n = 4 have the additional lower frequency band (2351 cm−1 for n = 8) near the position of the asymmetric stretch vibration in the isolated CO2 molecule (2349 cm−1).50 Based on the photodissociation behavior noted above, and the comparison to spectra predicted by theory, this band is assigned to the vibrations of second-sphere CO2 molecules. Both of these two main bands are present for all the larger V(CO2)n+ 11492
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Figure 3. Infrared spectra of V(CO2)6+ and V(CO2)7+ across the full infrared region of this experiment. The bands at 1140, 1800, and 3008 cm−1 marked in red are those from the intracluster reaction product.
Fermi resonance/combination bands (ν1 + ν3; 2ν2 + ν3). These spectra are more complex than that of isolated gas phase CO2 because of the presence of the metal ion. These spectra are also narrower than that of ambient CO2 because the rotational temperature is low here. The bending vibration has a doublet at 637/664 cm−1 in both of these spectra. The 664 cm−1 feature is close to the isolated molecule vibration at 667 cm−1, and therefore this can be associated with second-sphere molecules not affected strongly by metal binding. The 637 cm−1 feature then corresponds to the bend of the core ions; this is shifted by metal binding about 30 cm−1 to lower frequency than the vibration in the free molecule. A doublet corresponding to the overtone of this bending mode doublet is found at 1237/1267 cm−1. The asymmetric stretch has a similar doublet (2348/2376 cm−1) for the second-sphere and core ion molecules, as discussed already. The symmetric stretch of isolated CO2 is not IR active, but it becomes active in the metal complex with the V+OCO configuration. As a result, we see only a single band corresponding to the core ions; second-sphere molecules are not perturbed enough to have significant IR intensity. The band seen at 1363 cm−1 is blue-shifted 34 cm−1 from the vibration in isolated CO2. We do not make any attempt at a detailed assignment of the structure in the combination band region near 3600−3800 cm−1, as this is beyond the scope of the present report. The vibrations noted so far are found in the spectra of both V(CO2)6+ and V(CO2)7+ and can be assigned to intact CO2 molecules present in the core ion or the second sphere. However, three new vibrations at 1140, 1800, and 3008 cm−1 appear in the spectrum of V(CO2)7+ that are not present for V(CO2)6+. The onset of these vibrations is quite similar to that of the ″reaction″ feature at 2402 cm−1 mentioned above in Figure 2. The 2402 cm−1 band emerged gradually for clusters in the n = 8−9 range, while those here arise more suddenly at n = 7. However, this difference can easily be attributed to different IR intensities and experimental sensitivity in the two regions of the spectrum. As shown in Figure 4, the 1140 and 1800 cm−1 bands are also present in all clusters larger than n = 7, and grow in intensity as the cluster size increases, just like the 2402 cm−1 feature. The 3008 cm−1 band (not shown here) has the same size dependence. These new bands cannot be assigned to CO2 molecules, and therefore represent the presence of a different
Figure 4. Fingerprint region IR spectra for V(CO2)n+ (n = 5−10) showing the appearance of the 1140 and 1800 cm−1 bands for different cluster sizes.
molecular moiety in these clusters that is produced and/or stabilized only in the larger clusters. Consistent with our earlier proposal,43 these bands suggest the appearance of the product from an intracluster reaction. However, unlike the previous work, we now have enough information to identify this product. We first consider the possibility that the reaction product is an oxide−carbonyl species, as proposed in the original study.43 A complex of the form VO(CO)(CO2)n‑1+ would have the same mass as a V(CO2)n+ species. This suggestion makes sense because of the known oxidation that occurs for the V(CO2)+ complex upon excitation with UV light.27 Formation of the oxide would produce a metal center with partial M2+,O− character, thus explaining the additional shift on nearby CO2 molecule vibrations. To test this proposal, we made both V(CO)n+ and VO(CO)n+ clusters in separate experiments employing carbon monoxide and oxygen instead of CO2.61,62 The infrared spectra of oxide−carbonyl complexes was found to contain a strong carbonyl stretch near 2200 cm−1 and a strong metal−oxygen stretch near 1000 cm−1. Both assignments are validated by theory.62 A representative example of these spectra is shown for the VO(CO)6+ complex in the middle trace of Figure 5, where it is compared to the pattern measured here for the reacted V(CO2)10+ cluster. The lower trace shows the spectrum for this same complex predicted by theory. Differences between the experimental spectrum and that predicted by theory are understandable, because the theory here is not scaled. As shown in this comparison, the M−O and C−O stretches occur at significantly lower and higher frequencies, respectively, than the bands seen here for the CO2 complexes, and the overall appearance of the oxide− carbonyl spectra does not match the experiment for the larger V(CO)n+ clusters. Because it is conceivable that the presence of CO2 ligands could affect these frequencies, we have also computed the spectrum for the VO(CO)(CO2)5+ complex, and 11493
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Figure 6. Comparison of the spectrum measured for a reacted cluster, V(CO2)10+, to that measured for the CO3− anion in matrix isolation experiments (red stick spectrum) and to that predicted by theory for a V(CO)(CO3)− complex.
Figure 5. Comparison of the spectrum measured for a reacted cluster, V(CO2)10+, to the spectra measured (middle) and predicted by theory (lower traces) for VO(CO)6+ and OV(CO)(CO2)5+.
represents a challenging multireference problem. As an alternative, we instead attempted computations on the corresponding anion species, V(CO)(CO3)−. This complex does not have the desired ion-pair character, but instead has a neutral metal interacting with the carbonate anion. However, it does provide some information about how the vibrations might shift from a metal interaction. For this anion species, we find a C−O stretch of the carbonate moiety at 1755 cm−1, one for the carbonyl at 1910 cm−1, and a M−C stretch just below 1200 cm−1; the carbonate vibrations shift to higher frequency than those for the isolated moiety seen in the matrix. The simulated spectrum for this ion is shown in the bottom trace of Figure 6. The M−C stretch falls in approximately the same region as the 1180 cm−1 band in the V(CO2)10+ spectrum, but the strong doublet does not match the single band seen at 1800 cm−1. Therefore, the measured spectrum does not match that seen for the carbonate ion in a matrix, nor that for a metal carbonate species. The V(CO2)n+ complexes here do not contain carbonate, and the carbonyl−carbonate reaction product can also be ruled out. A final possible reaction product with the same mass as V(CO2)n+ is a metal-oxalate species, V(C2O4)(CO2)n‑2+, in which two CO2 molecules couple through the formation of a C−C bond. The actual oxalate anion is doubly charged, and so an oxalate-like moiety would also be stabilized by negative charge. A singly charged oxalate-like C2O4− anion was seen previously in matrix isolation infrared studies by Zhou and Andrews66,67 and also by Thompson and Jacox.68 Additionally, a similar oxalate species was concluded to form with an interesting size dependence on gas phase (CO2)n− anion clusters by Johnson and co-workers on the basis of photoelectron spectroscopy69 and infrared spectroscopy.70 The matrix spectra had IR bands at 1185 and 1857 cm−1 assigned to the ν5 symmetric (b2) and ν7 asymmetric (e) fundamentals of C2O4−.66−68 The gas phase clusters containing the oxalate anion had a doublet near 3100 cm−1 assigned to a combination band involving the ν5 + ν7 fundamentals.70 Unfortunately, this
this predicted spectrum is also presented in Figure 5. It includes a V−O stretch, a C−O stretch, and the expected bands from the CO2. Again, the spectrum shown is not scaled, so the actual frequencies are expected to be slightly lower than those shown. The presence of the CO2 causes the V−O stretch to shift to a slightly higher frequency than in the VO(CO)n+ complexes, but again it does not agree with the experimental spectrum and there is no band near 1800 cm−1. Based on all these data, we can therefore conclude that the reaction product is not an oxide carbonyl species. A second possible reaction product having the same mass as V(CO2)n+, and that could conceivably be formed, is a carbonylcarbonate species, V(CO)(CO3)(CO2)n‑2+. A carbonate moiety is stabilized with negative charge, thus producing an ion with V2+(CO3)− character. Again, this could explain the shift of nearby CO2 molecule vibrations producing the 2402 cm−1 band. Unfortunately, we were not able to produce such a metal carbonyl−carbonate species experimentally. However, the infrared spectroscopy of isolated CO3− has been reported in both matrix isolation63 and gas phase photodissociation studies.64 The matrix spectrum for CO3− contained two main vibrations at 1307 and 1494 cm−1 corresponding to the symmetric and asymmetric C−O stretches.63 A stick spectrum showing these bands is presented in the middle trace of Figure 6 for comparison to our experimental spectrum. The massselected-ion infrared photodissociation spectrum contained a complex pattern of bands due to combination vibrations with the attached argon tag atoms, but the main feature was at 1575 cm−1.64 Computational studies on the CO3− anion, which is isoelectronic to the NO3 neutral, are well-known to be severely affected by its complex electronic structure and vibronic interactions.64,65 We therefore have not attempted to predict a spectrum for this species with theory. We attempted a computational study of the V(CO)(CO3)+ species, but unfortunately this calculation did not converge. Because of its ionic bonding and partial ion-pair character, this system also 11494
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provide some useful information about metal bonding on the oxalate species. The lower trace in the figure therefore shows the IR spectrum predicted for the V(C2O4)− species. Again, the key vibrational bands agree reasonably well with those seen for the V(CO2)n+ clusters. The new experimental bands in the larger V(CO2)n+ clusters are therefore consistent with those seen in previous experiments for an isolated oxalate anion species and with those predicted for a metal complex containing this. The reaction product in these clusters therefore can be identified as an oxalate anion resulting from the coupling two CO2 molecules. Several aspects of this intracluster chemistry are interesting. The 2402 cm−1 band indicates that nearby unreacted CO2 molecules are interacting with a core metal cation that is more highly charged than the singly charged V+ species. The C2O4 oxalate species is stable as an anion, and our spectra match those seen previously for anions. This suggests that these clusters having a net +1 charge actually consist of configurations resembling a V2+, C2O4− ion pair. This ion pair must form by electron transfer from the singly positive vanadium ion to the CO2 ligands, which in turn initiates the coupling of two of these to form oxalate. It is then interesting to consider whether this is a ″contact″ ion pair as opposed to ″solvent-separated″. The main experimental evidence relevant for this is the size dependence of the reaction; it does not occur for small cluster sizes, but instead the onset occurs after at least seven CO2 molecules are present, and grows in intensity in larger clusters. These seven molecules represent the original inner coordination of the V+ ion and three second-sphere species. The larger clusters studied with 10 or more molecules have more in the second sphere, providing additional solvation. The size dependence indicates that solvation is essential for the stabilization of these ions, which in turn suggests a solventseparated structure for the ion pair. Our computational studies did not converge when we considered the V(C2O4)+ cation, which would represent the contact ion pair. The V(C2O4)− species that we were able to compute had a spectrum with vibrational bands shifted noticeably from those in the experiment, whereas the experimental bands were closer to those measured for the isolated oxalate species. We also attempted calculations on V2+(CO2)n(C2O4−) clusters, i.e., the solvent-separated ion pair. These also did not converge with the methods available, but this is not too surprising. Calculations for many of our metal−ligand complexes fail to converge when there are more than one or two second-sphere molecules because of the many possible configurations that lie close in energy. Considering all of this, it is not possible to make a definitive conclusion, but the size-dependent evidence favors the formation of a solvent-separated ion pair. Further insight into this proposed intracluster reaction can be achieved by a more detailed consideration of the structures and energetics involved. For this, we have conducted extensive computational studies of various cluster sizes and charge states (see Supporting Information). The double ionization of vanadium (second IP 14.7 eV71) must be offset by significant solvent and ion−ion stabilization energies for this process to be possible. The O2C−CO2 bond formation and electron affinity of the oxalate species produced (estimated energy of −3.65 kcal/mol or −0.158 eV relative to two separated CO2 molecules; see SI) offsets this only slightly. However, significant changes in the ligand/solvent structures also promote the reaction. The V+(CO2)n core ion is found experimentally to be four-coordinate,43 with computed CO2 binding energies of
gas phase study did not investigate the region of the vibrational fundamentals for oxalate.70 The bands seen previously for oxalate are therefore close to the positions of the new features seen here. To investigate this in more detail, we have conducted more experiments on pure (CO2)n− anion clusters and computational studies on the isolated C2O4− ion and its corresponding metal complex. Figure 7 presents a comparison of the experimental V(CO2)10+
Figure 7. Comparison of the spectrum measured for a reacted cluster, V(CO2)10+, to the spectrum measured for (CO2)16− and those predicted by theory for the oxalate anion and its metal complex. The red stick markers superimposed on the predicted oxalate anion spectrum represent the vibrations measured for this anion in matrix isolation experiments.
spectrum (top trace) to the resulting spectra. The second trace of the figure shows the spectrum of the (CO2)16− anion cluster containing no metal, measured here in the fingerprint region for the first time. This is one of the cluster sizes having the 3100 cm−1 bands and assigned by Johnson and co-workers to have the oxalate anion at its core.70 As shown, this spectrum has bands at 674, 1190, and 1858/1878 cm−1. The lower frequency band is in the region of the CO2 bending mode and the latter two match well with the oxalate species seen in matrix isolation studies at 1185 and 1857 cm−1.66−68 Apparently, both the matrix studies and the gas phase clusters contain the same oxalate anion, allowing for small matrix shifts. The third trace shows the IR spectrum predicted from our computations for C2O4−. The red lines superimposed on this predicted spectrum indicate the band positions measured in the matrix isolation experiments.66−68 The match to the three key bands is again acceptable, considering that the computed vibrations are not scaled. The match between the computed spectrum for the oxalate anion and that for the (CO2)16− species is quite good, as could perhaps be expected, since neither of these contains any metal. Finally, computations on the corresponding V(C2O4)+ cation were attempted, but these calculations again did not converge. As before, however, we were able to complete calculations on the corresponding anion. Although this anion species is again not the exact ion-pair species desired, it can 11495
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CONCLUSION Infrared photodissociation spectroscopy was employed to study mass-selected V(CO2)n+ clusters in the fingerprint region. These studies document the presence of intact CO2 molecules in these clusters and the effects of metal binding on their vibrational modes. CO2 molecules attached directly to the metal ion have vibrations shifted significantly from those of the isolated molecule, whereas second-sphere CO2 molecules have spectra close to those of the free molecule. In addition to CO2 molecules present in different clustering environments, IR spectroscopy reveals the presence of reacted CO2 molecules in V(CO2)n+ clusters beyond a critical size range of about seven molecules. The relative concentration of the reaction product increases with cluster size. Computational studies and the comparison of predicted spectra for different possible reaction products allow identification of an oxalate-type C2O4 anion species in the cluster. The activation of CO2 producing this product occurs via a metal→ligand electron transfer reaction, promoted by the combined effects of an expanded coordination sphere around the V2+ dication, Coulombic attraction between the V2+(CO2)6 core ion and the newly formed C2O4− anion, and the solvation of these species by excess CO2 molecules. The system resembles a solvent-separated V2+, C2O4− ion pair, with intervening CO2 ligands interacting strongly with the metal. The interactions producing this intracluster chemistry are likely to be found for other transition metal-CO2 complexes.
24.6, 24.0, 16.0, 16.1, 3.8, and 3.8 kcal/mol for the n = 1−6 species (see SI). However, formation of the metal dication changes both the binding energies and the coordination. The V2+(CO2)n core ion is found in computations to be sixcoordinate, with computed binding energies of 63.3, 50.5, 36.9, 35.9, 27.2, and 23.6 kcal/mol for the first six ligands. (These energetics are computed at the DFT/B3LYP level of theory, which underestimates the binding energies slightly for similar systems compared to the same calculations with dispersioncorrected functionals.) This provides roughly 149 kcal/mol (6.5 eV) of additional stabilization for the dication. Second-sphere CO2 molecules also bind about 3 kcal/mol (0.13 eV) each more strongly to the dication (see SI). Finally, once the charge is transferred, there is significant dication−anion Coulombic interaction between the V2+(CO2)6 core cation and the oxalate anion. A simple point charge electrostatic calculation with ions separated by 3−4 Å (distance estimated from computed structures) provides binding energies of 9.6−7.2 eV. These numbers are certainly too high, as there would be screening from the ligands and the actual charges are delocalized over molecular volumes. However, the sum of stabilizing interactions seems to be enough to offset the metal ionization, making the energetics of this process feasible. The shift of external solvent molecules into an expanded ligand coordination sphere around the metal ion, and the corresponding much stronger V2+−CO2 binding energies, appears to be a key factor in this system. Energetically, this is of about equal importance to the dication− anion binding in promoting the reaction. This solvent/ligand rearrangement also helps to explain the size dependence of the reaction, which has an onset with seven CO2 molecules and occurs more efficiently beyond this size. Since six CO2 molecules are required to complete the expanded metal coordination, and two more are needed to make the oxalate species, the reaction is apparently first possible just before the full coordination shift has occurred. The present results immediately raise questions about the similar reactions suggested previously for nickel and silicon cation complexes with CO2.40,42 Both of these systems also had bands in the 2400 cm−1 region that appeared only in the larger M(CO2)n+ clusters. As described here, this band indicates CO2 molecules interacting with a more highly charged core metal ion, but this is not reaction specific. It will be critical to investigate the fingerprint region for these clusters to see if the same chemistry occurs. The reaction here depends on electron transfer from the singly charged cation to the CO2 ligands, which may be facilitated by metals with low second ionization potentials. Vanadium has a relatively low second IP (14.7 eV), and so other metals in this category (Ti: 13.6 eV; Y: 12.2 eV)71 may have similar chemistry. However, preliminary experiments indicate that these other early transition metals are too reactive toward oxide formation, leading to insertion rather than electron transfer. However, the effect of expanded coordination seen here upon formation of the dication may be expected for other metals. In related studies on anions, Weber and coworkers found that Au(CO2)n− species exhibited charge transfer to the CO2 ligands for some cluster sizes, but no evidence for subsequent reactions was reported.52 It is conceivable that a variety of CO2 reactions may be found in clusters with other metals or with other added ligands. This chemistry is now accessible for study with the new infrared lasers available. Future studies of other M(CO2)n cations and anions in the fingerprint region will explore this fascinating new chemistry.
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ASSOCIATED CONTENT
S Supporting Information *
Full citation for references 7 and 57. Additional details are provided on the DFT computations, including the structures, energetics and vibrational frequencies for each of the structures considered. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge generous support for this work from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Chemical, Geological, and Biosciences (grant no. DE-FG02-96ER14658) and the Air Force Office of Scientific Research (grant no. FA95509-1-0166).
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IR Spectroscopy of Gas Phase V(CO2)n+ Clusters: Solvation-Induced Electron Transfer and Activation of CO2 Allen M. Ricks, Antonio D. Brathwaite, and Michael A. Duncan* Department of Chemistry University of Georgia, Athens, Georgia 30602 S Supporting Information *
ABSTRACT: Ion−molecule complexes of vanadium and CO2, i.e., V(CO2)n+, produced by laser vaporization are mass selected and studied with infrared laser photodissociation spectroscopy. Vibrational bands for the smaller clusters (n < 7) are consistent with CO2 ligands bound to the metal cation via electrostatic interactions and/or attaching as inert species in the second coordination sphere. All IR bands for these complexes are consistent with intact CO2 molecules weakly perturbed by cation binding. However, multiple new IR bands occur only in larger complexes (n ≥ 7), indicating the formation of an intracluster reaction product whose nominal mass is the same as that of V(CO2)n+ complexes. Computational studies and the comparison of predicted spectra for different possible reaction products allow identification of an oxalate-type C2O4 anion species in the cluster. The activation of CO2 producing this product occurs via a solvation-induced metal→ligand electron transfer reaction.
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INTRODUCTION Global warming issues have stimulated intense interest in the chemistry of CO2 sequestration and activation. Related catalytic processes are therefore of widespread interest in many subfields of chemistry.1−7 Although gas phase systems that activate CO2 are unlikely candidates for practical applications, they may serve as model systems demonstrating useful principles of this chemistry. In the present report, we investigate gas phase ion− molecule clusters of atomic vanadium ions (V+) interacting with CO2 that exhibit fascinating intracluster chemistry leading to CO2 activation. The structures, bonding, and mechanism of this chemistry are explored with mass spectrometry, infrared spectroscopy, and computational chemistry. Metal ion−molecular clusters that can be size-selected with mass spectrometry provide intriguing reaction systems whose composition is known and for which the exact numbers of reactant and/or solvent molecules can be controlled.8−16 Many examples are known for the reactions of isolated metal ions with molecular partners and their energy dependence.17−23 However, when these ions are produced within molecular clusters, new potential energy landscapes are accessed, influenced by the solvation of both reactants and products. Previous mass spectrometry measurements and fragmentation patterns have documented reactive processes in metal ions clustered with water.24−26 These systems undergo insertion chemistry producing hydroxide ions, after a certain number of water molecules needed to stabilize the charge-separated system are present. Metal ion−CO2 complexes have been observed to form oxide species upon UV laser excitation.27−30 Although many metal ion−molecular reactions have been suggested on the basis of mass spectrometry data, there are few systems with specific spectroscopic identification of the reaction © 2013 American Chemical Society
products. Our group has previously reported IR spectra for nickel cation complexes with cyclobutadiene, formed from the coupling of acetylene molecules after a critical cluster size was reached.31 Likewise, we found metal ion hydroxide bands resulting from the insertion chemistry of doubly charged metal cation complexes with water.32−35 In our previous studies of M(CO2)n+ complexes,36−42 the Ni+, Si+, and V+ systems exhibited unusual vibrational bands suggesting intracluster reactions, but the exact reaction product could not be identified.40,42,43 In the present work, we use expanded laser tuning ranges to provide a clearer picture of the intracluster chemistry of vanadium cations with CO2. Gas phase metal cation−CO2 complexes have been studied with mass spectrometry, providing bonding energetics,8,11,22,23 and with computational chemistry, providing predicted structures and bonding patterns.44,45 These systems have been studied with electronic spectroscopy, using solvated atomic transitions of the metal cation.27−30,46−49 More recently, our research group reported infrared spectroscopy and computational studies of several metal cation−carbon dioxide complexes (M+ = Fe+, Ni+, V+, Al+, Mg+, Si+)36−42 in the region of the CO2 asymmetric stretch vibration (2349 cm−1).50 In each of these systems, the metal−ligand binding was concluded to arise primarily from the charge-quadrupole electrostatic interaction, leading to linear M +OCO bonding configurations. The CO2 asymmetric stretch vibration in these complexes shifted to frequencies higher than that of the isolated CO2 molecule. These blue-shifted vibrations were Received: September 5, 2013 Revised: October 17, 2013 Published: October 21, 2013 11490
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documented for several metal ions and found to vary only slightly from one system to another, consistent with the electrostatic interaction. Because CO2 forms clusters efficiently at the low temperatures of the experiments, larger M(CO2)n+ complexes were produced having many second-sphere molecules. These outer CO2 molecules were observed to have asymmetric stretch vibrations at almost exactly the frequency of the free molecule, and to undergo efficient photoelimination because of their weaker binding interactions. Related to these metal cation complexes, Weber and coworkers have used similar IR methods to study metal anion− CO2 complexes.51,52 While most M(CO2)n+ spectra were understood in terms of the vibrations of the ″core″ and ″second-sphere″ CO2 species, a puzzling observation was made for the larger complexes of certain metal ions (including vanadium) that contained more than 7−8 CO2 molecules.40,42,43 In these larger systems, a new vibrational band was found near the asymmetric stretch that was more blue-shifted than any of the core ligand vibrations. This band became quite strong as cluster size increased, eventually dominating the spectrum for some systems.40 Its appearance was interpreted as indirect evidence for an intracluster reaction promoted by solvation, with a product having the same mass as one or more CO2 units. The greater vibrational shift for CO2 molecules in ″reacted″ clusters, compared to that for core ligands interacting with a singly charged cation, suggested that the metal in these species had a greater positive charge, indicating oxidation chemistry. However, the exact reaction product could not be determined at that time with the limited tuning range of the IR lasers available. More recently, new IR lasers with broader tuning range have become available, providing access to almost the entire infrared region.53,54 In the present study, we employ this improved IR coverage to investigate the V(CO2)n+ cluster system, revealing the intracluster chemistry that leads to CO2 activation.
FWHM Lorentzian line width for comparison to the experimental spectra.
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RESULTS AND DISCUSSION Figure 1 shows the mass spectrum of V(CO2)n+ complexes produced by laser vaporization. Also shown are oxide masses
Figure 1. Mass spectrum of the cations produced from laser vaporization of vanadium in an expansion of CO2.
resulting from either CO2 decomposition in the laser plasma or vaporization of oxides from the metal rod surface. This mass distribution, which extends out to clusters having 10 or more CO2 molecules, is typical of those seen for many metal cationCO2 complexes in the past.27−30,36−42 Because the number of CO2 molecules exceeds reasonable expectations for metal coordination, it is understood that these masses represent a subset of ligands attached directly to the metal that form the core ion, and another set of weakly bound second-sphere (or greater) species. The large clusters are produced efficiently because of the cold supersonic beam conditions. Throughout the discussion below, we make the convenient distinction between ″ligands″, indicating CO2 molecules cooordinated directly to the metal ion, and second-sphere, or ″solvent″ species. However, we recognize that this distinction is somewhat arbitrary, as all of these species are bound by pure electrostatic interactions. However, as shown below, the binding energetics are significantly different for these two types of CO2 molecules. Because the density of mass-selected ions is far too low for absorption spectroscopy, we use photodissociation measurements to obtain the infrared spectra of these clusters.13−16 To do this, we mass-select the ions from this distribution and excite them with tunable infrared radiation from the OPO laser system. As discussed in our previous report on this system,43 V(CO2)n+ clusters for n > 4 dissociate efficiently by the elimination of one or more CO2 molecules when they are excited near the asymmetric stretch vibration. Larger clusters eliminate multiple CO2’s, terminating their ″breakdown″ at the n = 4 complex. This behavior indicates that the coordination of V(CO2)n+ complexes is complete with four ligands, i.e., the core ion is V(CO2)4+. Theory and spectral evidence discussed previously43 indicates that the core ligands are bound primarily by charge−quadrupole interactions, with computed bond energies in the 15−25 kcal/mol range (5250−8750 cm−1; see Supporting Information). The structure of this core ion is predicted by theory to be square-planar (see SI). Complexes
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EXPERIMENTAL SECTION V(CO2)n+ clusters produced in a pulsed molecular beam by laser vaporization were mass-analyzed and size-selected in a reflectron time-of-flight mass spectrometer. The cluster source employed the so-called ″cutaway″ configuration for laser vaporization.55 The mass spectrometer is a specially designed reflectron instrument optimized for laser photodissociation.56 Mass-selected cluster ions were investigated with infrared laser photodissociation spectroscopy,13−16 using a tunable infrared optical parametric oscillator system (LaserVision) pumped by a Nd:YAG laser (Spectra Physics Pro-230). The OPO employs a KTP crystal oscillator and a set of four KTA crystals using parametric amplification to cover the 2000−4500 cm −1 region;53 a second stage of difference frequency generation in a AgGaSe2 crystal provides light in the 600−2300 cm−1 region.54 Fragment ions produced by elimination of CO2 from V(CO2)n+ species are recorded as a function of the laser frequency with a digital oscilloscope (LeCroy) to obtain a spectrum. Computational studies on these clusters are carried out using density functional theory (DFT) with the B3LYP functional, using the Gaussian03 suite of programs.57 The Def2-TZVP basis set was used for vanadium and the DZP basis set for carbon and oxygen. No scaling was done for these calculations. The bands in the predicted IR spectra were given a 10 cm−1 11491
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with more than four molecules have at least one of these in the second sphere where they are more weakly bound. The quadrupole−quadrupole interactions are predicted to give bond energies in the 3−5 kcal/mol range (1050−1750 cm−1; see SI). These computed values are not expected to be highly accurate because of the well-known problems of density functional theory for weak electrostatic interactions.58 The binding of external CO2 molecules can be compared to the dissociation energy of the neutral CO2 dimer, which is computed to be 484 cm−1.59 Because our systems are charged, this value is likely a lower limit on the binding energy of external molecules. When the single photon IR excitation energy exceeds the externalmolecule binding energy, efficient photodissociation is expected. Efficient photodissociation is detected for all the clusters with n > 4 down into the 630 cm−1 region, indicating that the actual binding energies are lower than this value. Figure 2 shows the photodissociation spectra of the V(CO2)n+ clusters for n = 8−11 in the region of the CO2
clusters that have been studied. Similar band patterns have been found for the M(CO2)n+ complexes of other metals.36−43 In addition to these two vibrations, which do not change substantially after n = 8, two other weaker bands gradually emerge in the larger clusters. A feature near 2359 cm−1 is seen for the M(CO2)n+ complexes of several different metals in the size range greater than about n = 7−8.36−43 A band at this position has also been reported in the spectra of large pure-CO2 clusters having no metal present.60 As discussed previously,36−43 this band is assigned to the asymmetric stretch of second-sphere CO2 molecules ″caged″ by surrounding CO2 molecules. The repulsion at the outer turning point of the CO2 stretching potential for caged molecules pushes the frequency slightly higher than it is for second-sphere CO2 molecules near the surface of the cluster that have no confinement. This band is therefore understandable on the basis of simple cluster growth, and has no obvious connection to an intracluster reaction. The more interesting new vibrational band for the larger clusters occurs near 2402 cm−1.43 It was also seen previously in M(CO2)n+ clusters for M = Ni and Si,40,42 and was particularly intense for nickel complexes.40 In the only other transition metal system for which such large CO2 clusters were studied, Fe(CO2)n+ complexes did not produce this feature.37 The behavior of this band in the previous studies led to its assignment as a CO2 asymmetric stretch vibration in clusters whose core ion had become more highly charged from some sort of intracluster reaction.40,42,43 In many metal ion cluster systems, higher charge states have been documented to cause greater vibrational band shifts,32−35 although this trend is not produced well by theory (see SI). The position of this band indicates a blue shift with respect to the isolated CO2 vibration, like that seen for core ligands in the 2377−2380 cm−1 region, but the magnitude of the shift is greater. Because M(CO2)+ complexes excited electronically were known to form oxides,27−30 and because the mass of the selected clusters corresponds to M(CO2)n+ units, an intracluster reaction was proposed that formed an ″oxide-carbonyl″ unit in the cluster.40,42,43 Oxidation of the core metal ion to a species with partial M2+,O− character could explain the higher charge on the metal and the consequently larger blue shift of the nearby CO2 molecules. However, the evidence for this was only circumstantial, as no vibrations of the reaction product itself were detected. Because of this, other reaction products having mass of one or more CO2 units were also noted to be possible, such as ″carbonyl−carbonate″ (OC-M-CO3)+ or ″oxalate″ (MC2O4)+ species.40,42,43 At the time of these previous studies, no further measurements were possible because of the limited tuning range of the available laser system, and the true nature of the proposed intracluster reactions could not be determined. As noted above, infrared OPO laser technology has improved dramatically in recent years, allowing much wider spectral coverage for these experiments. Figure 3 shows the spectroscopy now possible in scans of V(CO2)6+ and V(CO2)7+ over the range of 600−4000 cm−1. Because the vibrations of CO2 are well-known,50 the main bands in these spectra can be assigned to CO2 molecules interacting with metal whose vibrations are more or less shifted from those of the free molecule. Bands near 630 cm−1 correspond to the bending mode (ν2 = 667 cm−1 in isolated CO2); those near 1363 cm−1 to the symmetric stretch (ν1 = 1333 cm−1 in isolated CO2); those near 2350 cm−1 to the asymmetric stretch (ν3 = 2349 cm−1 in isolated CO2, as already shown in Figure 2); and finally, those near 3600−3800 cm−1 to
Figure 2. Infrared spectra of V(CO2)n+ clusters in the region of the CO2 asymmetric stretch vibration (monomer frequency indicated with dashed vertical line). The bands near 2380 cm−1 are assigned to the vibrations of CO2 molecules attached to the metal ion, while those near 2350 cm−1 are for second-sphere molecules. The bands near 2402 cm−1 are attributed to the presence of an intracluster reaction, producing a more highly charged core metal ion.
asymmetric stretch vibration (monomer value indicated with dashed vertical line). These spectra were reported in our original study of this system,43 and have been remeasured and reproduced here. The n = 8 cluster has two main bands, illustrating the pattern for the smaller clusters. The higher frequency feature at 2380 cm−1 is assigned to the asymmetric stretch of the core CO2 ligands. All of the smaller clusters (n = 1−4) have only this kind of vibration. The blue shift arises from the electrostatic charge−quadrupole bonding in this system and the effect of the cation on the CO2 vibrational potential.36−43 Clusters larger than n = 4 have the additional lower frequency band (2351 cm−1 for n = 8) near the position of the asymmetric stretch vibration in the isolated CO2 molecule (2349 cm−1).50 Based on the photodissociation behavior noted above, and the comparison to spectra predicted by theory, this band is assigned to the vibrations of second-sphere CO2 molecules. Both of these two main bands are present for all the larger V(CO2)n+ 11492
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Figure 3. Infrared spectra of V(CO2)6+ and V(CO2)7+ across the full infrared region of this experiment. The bands at 1140, 1800, and 3008 cm−1 marked in red are those from the intracluster reaction product.
Fermi resonance/combination bands (ν1 + ν3; 2ν2 + ν3). These spectra are more complex than that of isolated gas phase CO2 because of the presence of the metal ion. These spectra are also narrower than that of ambient CO2 because the rotational temperature is low here. The bending vibration has a doublet at 637/664 cm−1 in both of these spectra. The 664 cm−1 feature is close to the isolated molecule vibration at 667 cm−1, and therefore this can be associated with second-sphere molecules not affected strongly by metal binding. The 637 cm−1 feature then corresponds to the bend of the core ions; this is shifted by metal binding about 30 cm−1 to lower frequency than the vibration in the free molecule. A doublet corresponding to the overtone of this bending mode doublet is found at 1237/1267 cm−1. The asymmetric stretch has a similar doublet (2348/2376 cm−1) for the second-sphere and core ion molecules, as discussed already. The symmetric stretch of isolated CO2 is not IR active, but it becomes active in the metal complex with the V+OCO configuration. As a result, we see only a single band corresponding to the core ions; second-sphere molecules are not perturbed enough to have significant IR intensity. The band seen at 1363 cm−1 is blue-shifted 34 cm−1 from the vibration in isolated CO2. We do not make any attempt at a detailed assignment of the structure in the combination band region near 3600−3800 cm−1, as this is beyond the scope of the present report. The vibrations noted so far are found in the spectra of both V(CO2)6+ and V(CO2)7+ and can be assigned to intact CO2 molecules present in the core ion or the second sphere. However, three new vibrations at 1140, 1800, and 3008 cm−1 appear in the spectrum of V(CO2)7+ that are not present for V(CO2)6+. The onset of these vibrations is quite similar to that of the ″reaction″ feature at 2402 cm−1 mentioned above in Figure 2. The 2402 cm−1 band emerged gradually for clusters in the n = 8−9 range, while those here arise more suddenly at n = 7. However, this difference can easily be attributed to different IR intensities and experimental sensitivity in the two regions of the spectrum. As shown in Figure 4, the 1140 and 1800 cm−1 bands are also present in all clusters larger than n = 7, and grow in intensity as the cluster size increases, just like the 2402 cm−1 feature. The 3008 cm−1 band (not shown here) has the same size dependence. These new bands cannot be assigned to CO2 molecules, and therefore represent the presence of a different
Figure 4. Fingerprint region IR spectra for V(CO2)n+ (n = 5−10) showing the appearance of the 1140 and 1800 cm−1 bands for different cluster sizes.
molecular moiety in these clusters that is produced and/or stabilized only in the larger clusters. Consistent with our earlier proposal,43 these bands suggest the appearance of the product from an intracluster reaction. However, unlike the previous work, we now have enough information to identify this product. We first consider the possibility that the reaction product is an oxide−carbonyl species, as proposed in the original study.43 A complex of the form VO(CO)(CO2)n‑1+ would have the same mass as a V(CO2)n+ species. This suggestion makes sense because of the known oxidation that occurs for the V(CO2)+ complex upon excitation with UV light.27 Formation of the oxide would produce a metal center with partial M2+,O− character, thus explaining the additional shift on nearby CO2 molecule vibrations. To test this proposal, we made both V(CO)n+ and VO(CO)n+ clusters in separate experiments employing carbon monoxide and oxygen instead of CO2.61,62 The infrared spectra of oxide−carbonyl complexes was found to contain a strong carbonyl stretch near 2200 cm−1 and a strong metal−oxygen stretch near 1000 cm−1. Both assignments are validated by theory.62 A representative example of these spectra is shown for the VO(CO)6+ complex in the middle trace of Figure 5, where it is compared to the pattern measured here for the reacted V(CO2)10+ cluster. The lower trace shows the spectrum for this same complex predicted by theory. Differences between the experimental spectrum and that predicted by theory are understandable, because the theory here is not scaled. As shown in this comparison, the M−O and C−O stretches occur at significantly lower and higher frequencies, respectively, than the bands seen here for the CO2 complexes, and the overall appearance of the oxide− carbonyl spectra does not match the experiment for the larger V(CO)n+ clusters. Because it is conceivable that the presence of CO2 ligands could affect these frequencies, we have also computed the spectrum for the VO(CO)(CO2)5+ complex, and 11493
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Figure 6. Comparison of the spectrum measured for a reacted cluster, V(CO2)10+, to that measured for the CO3− anion in matrix isolation experiments (red stick spectrum) and to that predicted by theory for a V(CO)(CO3)− complex.
Figure 5. Comparison of the spectrum measured for a reacted cluster, V(CO2)10+, to the spectra measured (middle) and predicted by theory (lower traces) for VO(CO)6+ and OV(CO)(CO2)5+.
represents a challenging multireference problem. As an alternative, we instead attempted computations on the corresponding anion species, V(CO)(CO3)−. This complex does not have the desired ion-pair character, but instead has a neutral metal interacting with the carbonate anion. However, it does provide some information about how the vibrations might shift from a metal interaction. For this anion species, we find a C−O stretch of the carbonate moiety at 1755 cm−1, one for the carbonyl at 1910 cm−1, and a M−C stretch just below 1200 cm−1; the carbonate vibrations shift to higher frequency than those for the isolated moiety seen in the matrix. The simulated spectrum for this ion is shown in the bottom trace of Figure 6. The M−C stretch falls in approximately the same region as the 1180 cm−1 band in the V(CO2)10+ spectrum, but the strong doublet does not match the single band seen at 1800 cm−1. Therefore, the measured spectrum does not match that seen for the carbonate ion in a matrix, nor that for a metal carbonate species. The V(CO2)n+ complexes here do not contain carbonate, and the carbonyl−carbonate reaction product can also be ruled out. A final possible reaction product with the same mass as V(CO2)n+ is a metal-oxalate species, V(C2O4)(CO2)n‑2+, in which two CO2 molecules couple through the formation of a C−C bond. The actual oxalate anion is doubly charged, and so an oxalate-like moiety would also be stabilized by negative charge. A singly charged oxalate-like C2O4− anion was seen previously in matrix isolation infrared studies by Zhou and Andrews66,67 and also by Thompson and Jacox.68 Additionally, a similar oxalate species was concluded to form with an interesting size dependence on gas phase (CO2)n− anion clusters by Johnson and co-workers on the basis of photoelectron spectroscopy69 and infrared spectroscopy.70 The matrix spectra had IR bands at 1185 and 1857 cm−1 assigned to the ν5 symmetric (b2) and ν7 asymmetric (e) fundamentals of C2O4−.66−68 The gas phase clusters containing the oxalate anion had a doublet near 3100 cm−1 assigned to a combination band involving the ν5 + ν7 fundamentals.70 Unfortunately, this
this predicted spectrum is also presented in Figure 5. It includes a V−O stretch, a C−O stretch, and the expected bands from the CO2. Again, the spectrum shown is not scaled, so the actual frequencies are expected to be slightly lower than those shown. The presence of the CO2 causes the V−O stretch to shift to a slightly higher frequency than in the VO(CO)n+ complexes, but again it does not agree with the experimental spectrum and there is no band near 1800 cm−1. Based on all these data, we can therefore conclude that the reaction product is not an oxide carbonyl species. A second possible reaction product having the same mass as V(CO2)n+, and that could conceivably be formed, is a carbonylcarbonate species, V(CO)(CO3)(CO2)n‑2+. A carbonate moiety is stabilized with negative charge, thus producing an ion with V2+(CO3)− character. Again, this could explain the shift of nearby CO2 molecule vibrations producing the 2402 cm−1 band. Unfortunately, we were not able to produce such a metal carbonyl−carbonate species experimentally. However, the infrared spectroscopy of isolated CO3− has been reported in both matrix isolation63 and gas phase photodissociation studies.64 The matrix spectrum for CO3− contained two main vibrations at 1307 and 1494 cm−1 corresponding to the symmetric and asymmetric C−O stretches.63 A stick spectrum showing these bands is presented in the middle trace of Figure 6 for comparison to our experimental spectrum. The massselected-ion infrared photodissociation spectrum contained a complex pattern of bands due to combination vibrations with the attached argon tag atoms, but the main feature was at 1575 cm−1.64 Computational studies on the CO3− anion, which is isoelectronic to the NO3 neutral, are well-known to be severely affected by its complex electronic structure and vibronic interactions.64,65 We therefore have not attempted to predict a spectrum for this species with theory. We attempted a computational study of the V(CO)(CO3)+ species, but unfortunately this calculation did not converge. Because of its ionic bonding and partial ion-pair character, this system also 11494
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provide some useful information about metal bonding on the oxalate species. The lower trace in the figure therefore shows the IR spectrum predicted for the V(C2O4)− species. Again, the key vibrational bands agree reasonably well with those seen for the V(CO2)n+ clusters. The new experimental bands in the larger V(CO2)n+ clusters are therefore consistent with those seen in previous experiments for an isolated oxalate anion species and with those predicted for a metal complex containing this. The reaction product in these clusters therefore can be identified as an oxalate anion resulting from the coupling two CO2 molecules. Several aspects of this intracluster chemistry are interesting. The 2402 cm−1 band indicates that nearby unreacted CO2 molecules are interacting with a core metal cation that is more highly charged than the singly charged V+ species. The C2O4 oxalate species is stable as an anion, and our spectra match those seen previously for anions. This suggests that these clusters having a net +1 charge actually consist of configurations resembling a V2+, C2O4− ion pair. This ion pair must form by electron transfer from the singly positive vanadium ion to the CO2 ligands, which in turn initiates the coupling of two of these to form oxalate. It is then interesting to consider whether this is a ″contact″ ion pair as opposed to ″solvent-separated″. The main experimental evidence relevant for this is the size dependence of the reaction; it does not occur for small cluster sizes, but instead the onset occurs after at least seven CO2 molecules are present, and grows in intensity in larger clusters. These seven molecules represent the original inner coordination of the V+ ion and three second-sphere species. The larger clusters studied with 10 or more molecules have more in the second sphere, providing additional solvation. The size dependence indicates that solvation is essential for the stabilization of these ions, which in turn suggests a solventseparated structure for the ion pair. Our computational studies did not converge when we considered the V(C2O4)+ cation, which would represent the contact ion pair. The V(C2O4)− species that we were able to compute had a spectrum with vibrational bands shifted noticeably from those in the experiment, whereas the experimental bands were closer to those measured for the isolated oxalate species. We also attempted calculations on V2+(CO2)n(C2O4−) clusters, i.e., the solvent-separated ion pair. These also did not converge with the methods available, but this is not too surprising. Calculations for many of our metal−ligand complexes fail to converge when there are more than one or two second-sphere molecules because of the many possible configurations that lie close in energy. Considering all of this, it is not possible to make a definitive conclusion, but the size-dependent evidence favors the formation of a solvent-separated ion pair. Further insight into this proposed intracluster reaction can be achieved by a more detailed consideration of the structures and energetics involved. For this, we have conducted extensive computational studies of various cluster sizes and charge states (see Supporting Information). The double ionization of vanadium (second IP 14.7 eV71) must be offset by significant solvent and ion−ion stabilization energies for this process to be possible. The O2C−CO2 bond formation and electron affinity of the oxalate species produced (estimated energy of −3.65 kcal/mol or −0.158 eV relative to two separated CO2 molecules; see SI) offsets this only slightly. However, significant changes in the ligand/solvent structures also promote the reaction. The V+(CO2)n core ion is found experimentally to be four-coordinate,43 with computed CO2 binding energies of
gas phase study did not investigate the region of the vibrational fundamentals for oxalate.70 The bands seen previously for oxalate are therefore close to the positions of the new features seen here. To investigate this in more detail, we have conducted more experiments on pure (CO2)n− anion clusters and computational studies on the isolated C2O4− ion and its corresponding metal complex. Figure 7 presents a comparison of the experimental V(CO2)10+
Figure 7. Comparison of the spectrum measured for a reacted cluster, V(CO2)10+, to the spectrum measured for (CO2)16− and those predicted by theory for the oxalate anion and its metal complex. The red stick markers superimposed on the predicted oxalate anion spectrum represent the vibrations measured for this anion in matrix isolation experiments.
spectrum (top trace) to the resulting spectra. The second trace of the figure shows the spectrum of the (CO2)16− anion cluster containing no metal, measured here in the fingerprint region for the first time. This is one of the cluster sizes having the 3100 cm−1 bands and assigned by Johnson and co-workers to have the oxalate anion at its core.70 As shown, this spectrum has bands at 674, 1190, and 1858/1878 cm−1. The lower frequency band is in the region of the CO2 bending mode and the latter two match well with the oxalate species seen in matrix isolation studies at 1185 and 1857 cm−1.66−68 Apparently, both the matrix studies and the gas phase clusters contain the same oxalate anion, allowing for small matrix shifts. The third trace shows the IR spectrum predicted from our computations for C2O4−. The red lines superimposed on this predicted spectrum indicate the band positions measured in the matrix isolation experiments.66−68 The match to the three key bands is again acceptable, considering that the computed vibrations are not scaled. The match between the computed spectrum for the oxalate anion and that for the (CO2)16− species is quite good, as could perhaps be expected, since neither of these contains any metal. Finally, computations on the corresponding V(C2O4)+ cation were attempted, but these calculations again did not converge. As before, however, we were able to complete calculations on the corresponding anion. Although this anion species is again not the exact ion-pair species desired, it can 11495
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CONCLUSION Infrared photodissociation spectroscopy was employed to study mass-selected V(CO2)n+ clusters in the fingerprint region. These studies document the presence of intact CO2 molecules in these clusters and the effects of metal binding on their vibrational modes. CO2 molecules attached directly to the metal ion have vibrations shifted significantly from those of the isolated molecule, whereas second-sphere CO2 molecules have spectra close to those of the free molecule. In addition to CO2 molecules present in different clustering environments, IR spectroscopy reveals the presence of reacted CO2 molecules in V(CO2)n+ clusters beyond a critical size range of about seven molecules. The relative concentration of the reaction product increases with cluster size. Computational studies and the comparison of predicted spectra for different possible reaction products allow identification of an oxalate-type C2O4 anion species in the cluster. The activation of CO2 producing this product occurs via a metal→ligand electron transfer reaction, promoted by the combined effects of an expanded coordination sphere around the V2+ dication, Coulombic attraction between the V2+(CO2)6 core ion and the newly formed C2O4− anion, and the solvation of these species by excess CO2 molecules. The system resembles a solvent-separated V2+, C2O4− ion pair, with intervening CO2 ligands interacting strongly with the metal. The interactions producing this intracluster chemistry are likely to be found for other transition metal-CO2 complexes.
24.6, 24.0, 16.0, 16.1, 3.8, and 3.8 kcal/mol for the n = 1−6 species (see SI). However, formation of the metal dication changes both the binding energies and the coordination. The V2+(CO2)n core ion is found in computations to be sixcoordinate, with computed binding energies of 63.3, 50.5, 36.9, 35.9, 27.2, and 23.6 kcal/mol for the first six ligands. (These energetics are computed at the DFT/B3LYP level of theory, which underestimates the binding energies slightly for similar systems compared to the same calculations with dispersioncorrected functionals.) This provides roughly 149 kcal/mol (6.5 eV) of additional stabilization for the dication. Second-sphere CO2 molecules also bind about 3 kcal/mol (0.13 eV) each more strongly to the dication (see SI). Finally, once the charge is transferred, there is significant dication−anion Coulombic interaction between the V2+(CO2)6 core cation and the oxalate anion. A simple point charge electrostatic calculation with ions separated by 3−4 Å (distance estimated from computed structures) provides binding energies of 9.6−7.2 eV. These numbers are certainly too high, as there would be screening from the ligands and the actual charges are delocalized over molecular volumes. However, the sum of stabilizing interactions seems to be enough to offset the metal ionization, making the energetics of this process feasible. The shift of external solvent molecules into an expanded ligand coordination sphere around the metal ion, and the corresponding much stronger V2+−CO2 binding energies, appears to be a key factor in this system. Energetically, this is of about equal importance to the dication− anion binding in promoting the reaction. This solvent/ligand rearrangement also helps to explain the size dependence of the reaction, which has an onset with seven CO2 molecules and occurs more efficiently beyond this size. Since six CO2 molecules are required to complete the expanded metal coordination, and two more are needed to make the oxalate species, the reaction is apparently first possible just before the full coordination shift has occurred. The present results immediately raise questions about the similar reactions suggested previously for nickel and silicon cation complexes with CO2.40,42 Both of these systems also had bands in the 2400 cm−1 region that appeared only in the larger M(CO2)n+ clusters. As described here, this band indicates CO2 molecules interacting with a more highly charged core metal ion, but this is not reaction specific. It will be critical to investigate the fingerprint region for these clusters to see if the same chemistry occurs. The reaction here depends on electron transfer from the singly charged cation to the CO2 ligands, which may be facilitated by metals with low second ionization potentials. Vanadium has a relatively low second IP (14.7 eV), and so other metals in this category (Ti: 13.6 eV; Y: 12.2 eV)71 may have similar chemistry. However, preliminary experiments indicate that these other early transition metals are too reactive toward oxide formation, leading to insertion rather than electron transfer. However, the effect of expanded coordination seen here upon formation of the dication may be expected for other metals. In related studies on anions, Weber and coworkers found that Au(CO2)n− species exhibited charge transfer to the CO2 ligands for some cluster sizes, but no evidence for subsequent reactions was reported.52 It is conceivable that a variety of CO2 reactions may be found in clusters with other metals or with other added ligands. This chemistry is now accessible for study with the new infrared lasers available. Future studies of other M(CO2)n cations and anions in the fingerprint region will explore this fascinating new chemistry.
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ASSOCIATED CONTENT
S Supporting Information *
Full citation for references 7 and 57. Additional details are provided on the DFT computations, including the structures, energetics and vibrational frequencies for each of the structures considered. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge generous support for this work from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Chemical, Geological, and Biosciences (grant no. DE-FG02-96ER14658) and the Air Force Office of Scientific Research (grant no. FA95509-1-0166).
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