Infrared Photodissociation Spectroscopy of Saturated Group IV (Ti

Article pubs.acs.org/JPCA

Infrared Photodissociation Spectroscopy of Saturated Group IV (Ti, Zr, Hf) Metal Carbonyl Cations A. D. Brathwaite and M. A. Duncan* Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States S Supporting Information *

ABSTRACT: Group IV metal carbonyl cations of the form M(CO)n+ (M = Ti, Zr, Hf; n = 6−8) are produced in a supersonic molecular beam via laser vaporization in a pulsed nozzle source. The ions are mass selected in a reflectron time-of-flight spectrometer and studied with infrared laser photodissociation spectroscopy in the carbonyl stretching region. The number of infrared active bands, their relative intensities, and their frequency positions provide insight into the structure and bonding of these complexes. Density functional theory calculations are employed to aid in the analysis of the experimental spectra. The n = 6 species is found to be the fully coordinated complex for each metal, and all analogues have a D3d structure. This symmetric structure and the resulting simple spectra facilitate the investigation of trends in the bonding and infrared band positions of these complexes. The carbonyl stretching frequencies of the M(CO)6+ species are all red-shifted with respect to the gas phase CO vibration at 2143 cm−1, occurring at 2110, 2094, and 2075 cm−1 for titanium, zirconium and hafnium. The magnitude of the red shift increases systematically going from titanium to hafnium.

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frequency. In most metal carbonyls, π back-bonding is the dominant interaction, and C−O stretches are lower (i.e., redshifted) than that of the free CO molecule (fundamental = 2143 cm−1).12 Stable neutral transition metal carbonyls have been characterized using infrared spectroscopy in the gas phase,13−15 and various ions have been produced in the condensed phase as salts with counterions.16−19 Photoelectron spectroscopy of mass-selected anions has provided information about the ground states of corresponding neutrals.20−23 In addition, both saturated and unsaturated ions have been produced in cryogenic rare gas matrices and studied with infrared spectroscopy.24−33 Metal carbonyl ions have recently been studied in the gas phase with infrared photodissociation spectroscopy,34−39 and their structures and vibrational frequencies have been investigated with theory.7−11 Group IV metal carbonyl systems have been investigated previously via both experiment and theory. Bond dissociation energies were measured with collision-induced dissociation methods.40 Unsaturated, neutral and anionic carbonyls of Ti, Zr, Hf were isolated and studied with infrared spectroscopy in rare gas matrices.33 Equilibrium geometries and vibrational frequencies of neutral saturated carbonyls of group IV metals were investigated with density functional theory.41 In very recent work, Zhou and co-workers measured infrared photodissociation spectra for selected carbonyls of Ti+ and TiO+.42

INTRODUCTION Transition metal carbonyls are prototypes for metal−ligand bonding in inorganic and organometallic chemistry, they play a pivotal role in catalytic processes, and they provide models for metal surface−adsorbate interactions.1−6 The structure and bonding of first row transition metal carbonyls have been investigated extensively via experiment and theory.1−8 In these systems, the CO stretching frequencies provide sensitive probes for bonding geometries and electronic structure. Among the first row carbonyls, early transition metals make up a small fraction of the available data. These metals are relatively electron deficient, and require higher coordination numbers to achieve stable 18-electron configurations, but electronic effects may conflict with steric crowding. Second- or third-row metals are larger, and may accommodate higher coordination numbers more readily. In this work, we investigate these issues in the group IV transition metal carbonyl cations, using infrared photodissociation spectroscopy and computational chemistry. Transition metal carbonyl bonding is generally discussed using the Dewar−Chatt−Duncanson complexation model.1−11 In this scheme, vibrational frequencies are influenced by two synergistic charge-transfer interactions. In σ donation, the carbonyl donates electron density from its HOMO along the metal−CO bond axis into empty metal d orbitals. This HOMO has partial antibonding character, and this interaction strengthens the C−O bond, increasing its vibrational frequency. In π back-bonding, partially filled metal d orbitals donate charge into the antibonding LUMO on CO. Back-bonding is efficient because the metal d orbitals overlap spatially with the CO antibonding orbital. The additional electron density in this orbital weakens the C−O bond and reduces its vibrational © 2013 American Chemical Society

Special Issue: Curt Wittig Festschrift Received: January 23, 2013 Revised: March 13, 2013 Published: March 13, 2013 11695

dx.doi.org/10.1021/jp400793h | J. Phys. Chem. A 2013, 117, 11695−11703

The Journal of Physical Chemistry A

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RESULTS AND DISCUSSION A mass spectrum of the Ti(CO)n+ ions formed via laser vaporization of a titanium rod in a carbon monoxide expansion is shown in Figure 1. The major peaks correspond to titanium

The possible higher coordination in the early transition metal carbonyls has been examined for the group V metal cations. Seven-coordinate carbonyl structures were proposed by theory to be stable for the vanadium group cations,41,43 which would form 18-electron complexes. Bond dissociation energy studies by Armentrout found stable V(CO)7+ cations, consistent with this.40,44 However, until recently, there was no spectroscopic evidence for these structures. The first spectroscopic measurements to confirm seven-coordinate structures for these metals were described recently by our research group.38d Vanadium was found to form a six-coordinate (but not the sevencoordinate) species, niobium formed both six- and sevencoordinate structures, and tantalum formed the sevencoordinate species exclusively. In the present study, we extend this investigation to the carbonyls of the group IV metal cations. With even fewer valence electrons, these metals could conceivably form similar high-coordinate carbonyls. However, because they have an odd number of valence electrons, it is not possible to form 18-electron complexes. Instead, six-, seven-, or eight-coordinate complexes would have 15, 17, or 19 electrons, respectively.

Figure 1. Mass spectrum of Ti(CO)n+ complexes produced via laser ablation of a titanium rod.

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EXPERIMENTAL SECTION Ti(CO)n+, Zr(CO)n+, Hf(CO)n+ ions are produced in a pulsed nozzle laser vaporization source using the third harmonic of a Nd:YAG laser (355 nm; Spectra Physics INDI). The laser is focused onto a rotating and translating 1/4 in. diameter metal rod mounted on a pulsed valve (General Valve, Series 9) in the so-called “offset” configuration.45 Cluster ions are produced using a pure carbon monoxide expansion at a backing pressure of 150 psig. The resulting gas expansion is skimmed into a second chamber where positive ions are pulse-extracted into a homemade reflectron time-of-flight mass spectrometer.46 Specific ions are mass selected by their flight time using pulsed deflection plates located at the end of the first flight tube. These ions are excited in the turning region of the reflectron with the tunable output of an infrared Optical Parametric Oscillator/ Amplifier (OPO/OPA) system (LaserVision) pumped by a Nd:YAG laser (Spectra Physics Pro 230). This system provides infrared light in the 2000−4000 cm−1 region with a line width of about 1 cm−1. When the output of the infrared laser is on resonance with an IR-allowed vibration, absorption and intramolecular vibrational energy relaxation (IVR) take place on a time scale much smaller than the residence time in the reflectron (1−2 μs), leading to dissociation of the complex. Infrared spectra are recorded by monitoring the appearance of one or more fragment ions as a function of the laser wavelength. In support of the experimental work, Density Functional Theory (DFT) calculations are carried out to determine the structures and bonding patterns of these complexes. The calculations are performed using the B3LYP functional47,48 as implemented in the Gaussian 2003 computational package.49 The Los Alamos effective core potential (ECP) double-ζ (LanL2DZ) basis set50,51 is used for the metal atoms, and the DZP basis set52 is used for carbon and oxygen. The computed carbonyl frequencies are scaled by a factor of 0.971, and bands are given a 10 cm−1 fwhm Lorentzian line shape for comparison to the experimental spectra. The scaling factor was derived by calculating the CO stretching frequencies of the stable neutral carbonyls Ni(CO)4, Fe(CO)5, and Cr(CO)6, and finding the average value needed to make their experimental and theoretical frequencies agree.

carbonyl cations, whereas the minor peaks correspond to titanium oxide carbonyls. The most prominent peak corresponds to Ti(CO)6+, suggesting that this ion has enhanced stability. Ti(CO)n+ complexes with as many as 19 carbonyl ligands are produced. All these ligands cannot possibly be coordinated directly to the central metal ion. Instead, larger complexes are believed to consist of a strongly bound metal− ligand core ion, with additional “external” ligands attached around this via weaker electrostatic and/or van der Waals interactions. This clustering to produce larger complexes is possible because of the cold supersonic expansion employed. Because the external ligands are weakly bound, they are likely to be eliminated efficiently upon absorption of IR photons, as shown below. The mass spectra for the corresponding zirconium and hafnium systems are very similar to that of titanium, with the n = 6 species having enhanced intensity, as shown in the Supporting Information (SI) for this article. Mass spectrometry alone is not a reliable way to determine cluster stability, as ion intensities can vary with the conditions of the cluster source and mass spectrometer focusing. To gain better insight into the stability of these species, infrared photodissociation studies were conducted on larger Ti(CO)n+, Zr(CO)n+ and Hf(CO)n+ clusters, where n = 7−11. The socalled “breakdown” spectra for the Ti(CO)n+ ions are presented in Figure 2 (those for zirconium and hafnium are provided in the SI). These spectra are obtained by subtracting the mass spectrum of a selected complex with the fragmentation laser off from one obtained with it on. Negative peaks are due to depletion of the parent ion, whereas the positive peaks are from the photofragments. Ti(CO)n+ clusters smaller than n = 6 did not photodissociate. This is not surprising, as these complexes are expected to have strong covalent bonding. The bond energy of the Ti(CO)5+ −CO complex has been measured previously to be 6200 cm−1.43 Although this is greater than the energy of an infrared photon in the CO stretching region (∼2100 cm−1), a small amount of fragmentation was actually observed for the M(CO)6+ (M = Ti, Zr, Hf) species. The signal is attributed to multiphoton absorption and/or the presence of a small fraction of ions with some unquenched internal energy from the growth process. Larger clusters, beginning with the n = 7 species, 11696



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whose frequency is red-shifted from that of free CO. However, the magnitude of this red shift increases noticeably for the larger metals. The band in the titanium spectrum is at 2110 cm−1, and those for zirconium and hafnium are at 2094 and 2075 cm−1. The observation of a single infrared band usually indicates a high symmetry structure, and octahedral symmetry is most common for hexacoordinate species. To investigate the geometries of these clusters and their electronic structures, we employ DFT computations using the B3LYP functional on the doublet and quartet spin states for the M(CO)n+ (n = 6−8) clusters of each metal. The results of this computational work are provided in Tables 1−3 and in the SI. As expected, our Table 1. Structures, Electronic Ground States and Relative Energies for M(CO)n+ Complexes, Where M = Ti, Zr, Hf, Computed Using DFTa Figure 2. Photodissociation breakdown spectra of Ti(CO)n+ (n = 7− 11) complexes. These spectra were obtained by subtracting a mass spectrum of a mass selected cluster with the laser off from one with it on.

complex

spin state

symmetry

relative energy (kcal/mol)

Ti(CO)6+

doublet (6C) quartet (6C) doublet (7C) quartet (6C+1) doublet (7C+1) quartet (6C+2) doublet (6C) quartet (6C) doublet (7C) quartet (6C+1) doublet (7C+1) quartet (6C+2) doublet (6C) quartet (6C) doublet (7C) quartet (6C+1) doublet (7C+1) quartet ((6C+2)

C2v Oh C3v C1 C1 C2v C2v Oh C3v C1 C1 C2v C2v Oh C3v C1 C1 C2v

+13.0 0.0 0.0 +3.6 0.0 +4.2 +4.6 0.0 0.0 +14.1 0.0 +15.2 +3.5 0.0 0.0 +17.7 0.0 +18.5

Ti(CO)7+ Ti(CO)8+

fragment efficiently with IR excitation. As shown in Figure 2, all of these undergo sequential ligand loss terminating at n = 6. This suggests that the n = 6 complex is resistant to further fragmentation, i.e., that it is relatively more stable than larger species. This is evidence that the strongly bound core ion has only six ligands. In all of the larger complexes, the most prominent fragmentation channel is the loss of two CO ligands from the parent ion. If these are single photon processes, as we expect, this indicates that the binding energy of the external CO molecules is less than about 1000 cm−1. To examine the spectroscopy of these systems, we measure the wavelength dependence of these fragmentation processes. Figure 3 presents the infrared photodissociation spectra for the

Zr(CO)6+ Zr(CO)7+ Zr(CO)8+ Hf(CO)6+ Hf(CO)7+ Hf(CO)8+ a

Coordination is given in parentheses. Data for higher spin states for these complexes are presented in the SI.

Table 2. Computed Binding Energies in kcal/mol for the Last CO in M(CO)n+ Complexes, Where M = Ti, Zr and Hfa complex

spin state

E[M(CO)n−1+−CO]

Ti(CO)6+

doublet (6C) quartet (6C) doublet (7C) quartet (6C+1) doublet (7C+1) quartet (6C+2) doublet (6C) quartet (6C) doublet (7C) quartet (6C+1) doublet (7C+1) quartet (6C+2) doublet (6C) quartet (6C) doublet (7C) quartet (6C+1) doublet (7C+1) quartet (6C+2)

25.1 25.1 19.9 3.2 3.7 3.2 29.5 25.8 21.8 3.1 4.1 3.1 31.7 30.4 24.4 3.3 4.0 3.2

Ti(CO)7+ Ti(CO)8+ Zr(CO)6+ Zr(CO)7+

Figure 3. IR photodissociation spectra of the M(CO)6+ (M = Ti, Zr, Hf) measured via CO elimination.

Zr(CO)8+

n = 6 complexes of titanium, zirconium and hafnium cations, obtained in each case via the elimination of a single CO ligand. Although the fragmentation is not efficient, it is enough so that signal averaging can produce spectra with reasonable quality. The dashed line represents the free-CO vibration at 2143 cm−1. Each spectrum has a single broad band (12−14 cm−1 fwhm)

Hf(CO)7+

Hf(CO)6+

Hf(CO)8+

a

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Coordination is given in parentheses. dx.doi.org/10.1021/jp400793h | J. Phys. Chem. A 2013, 117, 11695−11703


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Table 3. Measured Vibrational Frequencies and Representative Computed Frequencies (Scaled by 0.971) for the Doublet and Quartet States of M(CO)n+ complexesa complex

experiment

theory (quartet)

Ti(CO)6+

2110

2102 (3421)

Ti(CO)7+

2114, 2164

2100 (2229)

2112, 2164

2101 (1201), 2114 (19) 2154 (68) 2098 (1129), 2099 (2343) 2114 (43), 2153 (140)

Ti(CO)8+

Zr(CO)6+

Zr(CO)7+

Zr(CO)8+

2094

2097, 2164

2095, 2163

2079 (4646)

2076 (3052), 2077 (1621) 2094 (38), 2154 (75) 2074 (1541), 2075 (3186) 2092 (48), 2153 (153)

Hf(CO)6+

2075

2062 (5303)

Hf(CO)7+

2080, 2163

2059 (2465), 2060 (1872) 2081 (37), 2155 (77)

Hf(CO)8+

2079, 2163

2057 (1765), 2058 (3630) 2080 (45), 2155(155)

theory (doublet) 2071 (661), 2072 2085 (1404) 2099 (786), 2150 2066 (707), 2071 2078 (1009) 2092 (524), 2093 2105 (373) 2152 (81) 2062 (707), 2067

(844), (10) (816), (556),

(1097)

2082(1056) 2094 (575), 2098 (291) 2108 (296), 2150 (72), 2156 (74) 2049 (716), 2055 (961), 2074 (1740) 2082 (1062), 2141 (3) 2059 (750), 2062 (1201)

Figure 4. IR photodissociation spectra of Ti(CO)6+ and its argontagged analogue. The spectra predicted by DFT calculations for quartet and doublet spin states are shown in the lower traces.

2076 (1578), 2083 (761), 2084 (190) 2100 (400), 2149 (51) 2053 (740), 2061 (1240)

theory for the bare complex is shown here. As shown in the SI, the computed frequencies for the argon complexes are barely shifted from those for the bare complexes. As shown in the figure, the tagged spectrum has a better signal level and a narrower line width (12 cm−1 fwhm for Ti(CO)6+ vs 7 cm−1 for Ti(CO)6+Ar). This is consistent with the argon atom being weakly bound and easily eliminated. The band positions of the neat and tagged spectra are essentially the same at 2110 cm−1 for Ti(CO)6+ and 2109 cm−1 for Ti(CO)6Ar+. This suggests that the spectral shift from multiphoton dissociation is insignificant. As shown in the figure, the single band pattern and the frequencies measured are consistent with those predicted by theory for the lowest energy quartet spin state of this complex. The higher energy doublet state is predicted to have a more complex multiplet pattern. The Ti(CO)6+ complex was also studied recently by Zhou and co-workers using similar IR photodissociation and computational methods. The Zhou group measured spectra without tagging and obtained a broader line width (>20 cm−1) than that seen here, resulting in a slightly different reported band position (2118 cm−1), but otherwise their data is consistent with ours. The spectra for the M(CO)7+ complexes measured by COloss are presented in Figure 5. These complexes fragment efficiently via the elimination of a CO ligand, producing spectra with good signal levels. Similar to the n = 6 spectra, each n = 7 spectrum has a main band that is red-shifted from the free CO vibration, with increasing red shift going down the group. These bands are detected at 2114, 2097, and 2080 cm−1 for titanium, zirconium and hafnium. For each metal, the main band in the n = 7 spectrum is within 5 cm−1 of the corresponding band in the n = 6 spectrum. In addition to the main band, each spectrum contains a very weak blue-shifted band near 2164 cm−1. This band is a signature for weakly bound second sphere CO ligands, as discussed previously.38 The overall behavior here is therefore consistent with structures having an n = 6 core and a weakly bound second-sphere CO ligand. The lower two traces in the figure show the spectra predicted by theory for the six-coordinate/one-external CO (6C+1) structure versus that for the seven-coordinate (7C) species for Ti(CO)7+. The 6C+1 structure has a near-

2067 (1223), 2080 (660), 2088 (544) 2092 (452), 2144 (15), 2163 (89) 2036 (801), 2045 (1069) 2063 (1909), 2071 (1151) 2072 (6), 2140 (5) 2112 (816), 2115 (1228) 2127 (1857), 2134 (792), 2154 (435) 2212 (49) 2047 (800), 2051 (1289) 2058 (1400), 2071 (820), 2077 (499) 2088 (475), 2146 (35), 2161 (82)

a

All frequencies are in wavenumbers, and computed IR intensities (km/mol) are listed in parentheses. The coordination numbers for doublets and quartets are the same as those indicated in Tables 1 and 2.

DFT computations find octahedral structures for the n = 6 complexes for each metal, resulting in a single degenerate CO stretch. Each of the C−O stretches has high IR intensity, explaining how multiphoton absorption can occur to dissociate these complexes. Because of the high ligand binding energies computed for the n = 6 complexes, their dissociation is expected to involve multiphoton absorption, and this may shift the positions of one-photon vibrational absorption bands. To further explore these spectra, we employ the method of rare gas “tagging” to enhance the dissociation yields of these ions.53−58 M(CO)6+Ar complexes, formed by expansion gas mixtures containing both argon and CO, fragment by elimination of the argon atom following excitation of a CO vibration. The tagged spectrum for the Ti(CO)6+ complex is compared to its spectrum measured by CO loss in Figure 4. Additionally, the spectrum predicted by 11698



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Figure 5. IR photodissociation spectra of the M(CO)7+ (M = Ti, Zr, Hf) measured via CO elimination along with structures and predicted spectra for the 6 + 1 quartet and 7-C doublet species of titanium.

Figure 6. Infrared spectra of the M(CO)8+ (M = Ti, Zr, Hf) measured via CO elimination along with a representative 6 + 2 structure.

was found to be 0.54 eV, which is much greater than expected for a second sphere ligand. In our previous studies of the vanadium group carbonyls, we found a six-coordinate complex for V(CO)n+ when the 18-electron seven-coordinate species was predicted to be stable, but the larger niobium and tantalum complexes did form the seven-coordinate complexes. However, here we find no evidence for a stable seven-coordinate species for any of the metals in the titanium group. Instead, these metals all form the 15-electron, six-coordinate complexes. Although some inconsistency with the predictions of DFT computations is not totally surprising, our apparent disagreement with the previous experiments of Armentrout and coworkers causes more concern. Indeed, we encountered similar issues in our previous study on V(CO)n+.38i In that system also, Armentrout found evidence for a seven-coordinate complex, consistent with the stability trend predicted by theory, but we did not. Although the reason for this discrepancy is not completely clear, we suggested that differences in ion production methods employed by the two laboratories might be the source of the problem. In the Armentrout experiment, ions are thermalized to room temperature in a flowtube; as a result, ions with weakly bound ligands do not survive to be studied. On the other hand, our ions produced by laser ablation are quickly cooled via supersonic expansion. These cold conditions favor the production of ions with weakly bound external ligands. It is then conceivable that both experiments produce only a small absolute amount of seven-coordinate complexes. In our experiment, this signal is insignificant relative to the larger amount of six-coordinate-plus-external species. In the Armentrout experiment, only the strongly bound sevencoordinate species would survive to be studied. If both experiments produce only small amounts of sevencoordinate species, then we can rationalize the disagreement in our observations. However, it is still not clear why these sevencoordinate species that are predicted to be stable do not form efficiently. Again, we discussed this same issue in our study of the vanadium-group carbonyls,38i suggesting that it might involve the kinetics of cluster growth and the role of metal spin states. The 7C complex for V(CO)n+ is most stable as a singlet, but the 6C complex is a triplet. A spin change is therefore required in cluster growth to go from the six-ligand complex to the strongly bound seven-coordinate species. If this spin change

octahedral core ion, like the ground state for the n = 6 species, with a quartet spin state, while the 7C structure lies slightly lower in energy with a doublet ground state. As shown, the 7C structure produces a multiplet of C−O stretches rather than the single band observed. Although the exact position of the single band for Ti(CO)7+ is not as strongly red-shifted as predicted, there is no trace of multiplet structure. The spectra here are therefore more consistent with the presence of the 6C+1 structure, even though the 7C structure is computed to have a slightly lower energy. The corresponding zirconium and hafnium species follow the same pattern, with spectra indicating the 6 + 1 structure (see SI). Higher energy spin states for both the 6C+1 and 7C structures for all three metals were also investigated (see SI). It is conceivable that a seven-coordinate complex like that predicted to be more stable by theory forms for these complexes, but that we are unable to detect it with this photodissociation measurement because it is strongly bound. We therefore examined cluster sizes beyond n = 7 to see if there is any evidence for such a cluster configuration. Figure 6 shows the spectra for the M(CO)8+ complexes, again measured in the CO-loss mass channel. If there were any strongly bound sevencoordinate structures, they should be detected here by elimination of at least one external CO ligand that would be present. However, similar to the n = 7 spectra, the n = 8 spectra all consist of a single strong band that is red-shifted and a weak one near 2164 cm−1. Again, the red shift of the main band increases from titanium to hafnium. Furthermore, the intensity of the blue-shifted bands assigned to external ligands is greater here than it was for the n = 7 spectra. The main band position in the n = 8 spectra varies only slightly (∼4 cm−1) compared to the corresponding band in the n = 6 spectra for each metal. These spectra are therefore again consistent with a sixcoordinate core ion solvated by external ligands. There is no evidence for the seven-coordinate species predicted by theory. The 6-fold coordination of Ti(CO)n+ and its group IV analogues seen here experimentally is unexpected and a little disappointing. Our computational studies found a stable sevencoordinate species for all three metals, and the binding energy studies of Armentrout and co-workers found a stable Ti(CO)7+ species.42 In that study, the CO binding energy in Ti(CO)7+ 11699



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explanation for the back-bonding trend. However, another criterion sometimes used in inorganic chemistry to express available electron density is the Pauling electronegativity.2 Titanium, zirconium and hafnium have electronegativity values of 1.54, 1.33, and 1.3. The decrease down the period of this parameter is consistent with greater available electron density for the heavier metals, consistent with the pattern of backdonation ability indicated by our spectra. As shown in Table 3, DFT also reproduces this experimental trend, with progressively lower carbonyl frequencies predicted for the M(CO)6+ complexes of Ti, Zr and Hf (2102, 2094, and 2075 cm−1). One final point of interest is the positions of the C−O stretches here compared to those measured previously for other transition metal cations. Table 4 provides this data for the fully

inhibits the ligand addition, the heavier metals with stronger spin−orbit coupling might be better able to accomplish the reaction, and indeed the heavier metals of the vanadium group (niobium and tantalum) did produce the seven-coordinate species more efficiently. This spin-change bottleneck in cluster growth is also consistent with the results obtained by Weitz and co-workers in their studies of partially coordinated neutral metal carbonyls undergoing recombination with gaseous CO ligands.59 These researchers found that spin-conserving recombination was efficient, occurring at nearly the collision rate, whereas spin-changing recombination reactions were much (400X) slower. Theoretical models of curve crossing dynamics in these systems by Harvey and co-workers confirmed these observations, finding so-called “spin barriers” for ligand additions that required a spin change.60 In the present system, the relevant spin states are the quartet and doublets of the titanium group metals. The relative energetics, ligand binding energies and vibrational frequencies of the complexes in these two spin states are presented in Tables 1, 2 and 3, respectively. As indicated in Table 1, a spin change is observed between the n = 6 and n = 7 complexes for all three metals. For each metal, the n = 6 cluster has the quartet as its lowest energy structure, whereas the lowest energy structures for the n = 7 and n = 8 complexes are doublets. The quartet spin state is confirmed for the n = 6 complex of each metal by the spectral patterns, as discussed above. If cluster growth involves sequential ligand addition, then a spin change is required to go from the M(CO)6+ quartet to the M(CO)7+ doublet. Such a spin change may be the limiting kinetic factor that inhibits the formation of the seven-coordinate species. However, unlike the vanadium group behavior, even the heavier metals of the titanium group metals are unable to form the seven coordinate species efficiently. The high symmetry and resulting simple spectra for these complexes facilitate the observation of an interesting trend among the saturated group IV metal carbonyl cations. The magnitude of the red-shift in the CO stretching frequency increases smoothly going from titanium to hafnium. This trend was observed previously by Andrews and co-workers for the neutral, unsaturated carbonyls of these metals studied in a neon matrix.33 The carbonyl stretching frequencies reported in that study were 1920, 1900, and 1869 cm−1 for TiCO, ZrCO and HfCO, respectively. A similar trend was also reported for theoretically computed neutral complexes of these metals.40 As discussed previously, π back-bonding is the most important interaction in metal−carbonyl complexes, and the observed trend must be the result of an increase in the back-bonding in these carbonyl systems going from titanium to hafnium. In an attempt to rationalize this trend, we consider the ionization potentials and covalent radii of these metals, which influence the available valence electron density through the binding energy and spatial extent of the orbitals. The first ionization potentials of the group IV metals are 6.83, 6.63, and 6.83 eV, listed in order of increasing atomic number, while the values for the second IP’s are 13.6, 13.1, and 14.9 eV.61 Although there is a decrease in the IP’s between titanium and zirconium, the values for the hafnium atom are greater than those for zirconium. Consideration of the covalent radii yields similar results. The covalent radii are 1.6, 1.75, and 1.75 Å for titanium, zirconium and hafnium.62 Although the covalent radius increases from titanium to zirconium, the values for zirconium and hafnium are identical. These IP and covalent radii data therefore do not by themselves provide a simple

Table 4. Comparison of the Band Positions (cm−1) of Core CO Ligands for Various Metal−Carbonyl Cations

a

complex

band positions

complex

band positions

Ti(CO)6+ Zr(CO)6+ Hf(CO)6+

2110 2094 2075

V(CO)6+ Mn(CO)6+ Co(CO)5+ Cu(CO)4+

2097a 2115b 2140, 2150c 2198d

Reference 38i. bReference 38e. cReference 38c. dReference 38h.

coordinated ions. As shown, the frequencies measured here for the group IV metals are among the lowest for any metal carbonyl cations. We have discussed previously that earlier transition metal carbonyls generally have greater red shifts than those of later transition metals.38i This trend was attributed to a decrease in the availability of electrons for π back-bonding going across the periodic table. Although the number of valence electrons increases across the period, the nuclear charge also increases, producing the well-known effect of d orbital contraction. This d contraction limits the π back-donation across the period, resulting in decreasing red shifts in the carbonyl frequencies. The data measured here for the group IV carbonyls complement those obtained previously and help to further illustrate this effect.

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CONCLUSION Coordinatively saturated group IV metal−carbonyl cations of the form M(CO)n+ (M = Ti, Zr, Hf; n = 6, 7, 8) are produced in a molecular beam via laser vaporization in a pulsed nozzle source. The ions are mass selected in a reflectron time-of-flight spectrometer and studied with infrared laser photodissociation spectroscopy in the carbonyl stretching region. The number of infrared active bands, their relative intensities and their frequency positions provide insights into the structure and bonding of these complexes. DFT calculations were conducted in support of the experimental data. Although seven-coordinate complexes are predicted to be stable, and have been observed under other conditions, the n = 6 species was found to be the fully coordinated complex for each metal in this study. Ion formation conditions, and a possible spin-change barrier in cluster growth, are suggested to account for the lack of sevencoordinate species. All six-coordinate species have a nearoctahedral structure and a characteristic single (degenerate) strong C−O stretching band. These simple spectra facilitate the investigation of trends in the bonding and infrared band positions of these complexes. The carbonyl stretching frequencies of the fully coordinated M(CO)6+ species are all 11700



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lower than that of free CO, with a systematic increase in the red shift going from titanium to hafnium. Additionally, the red shifts seen here are among the largest measured for transition metal cation carbonyls. Both effects can be rationalized on the basis of available d electrons for π back-bonding for different metals.

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ASSOCIATED CONTENT

S Supporting Information *

The full citation for reference 49 along with full details of the DFT computations done in support of the spectroscopy presented here. Structures, energetics, and vibrational frequencies for each of the complexes considered are included. 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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Infrared Photodissociation Spectroscopy of Saturated Group IV (Ti

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