Coordination and Spin States in Vanadium Carbonyl Complexes (V

Article pubs.acs.org/JPCA

Coordination and Spin States in Vanadium Carbonyl Complexes (V(CO)n+, n = 1−7) Revealed with IR Spectroscopy Allen M. Ricks, Antonio D. Brathwaite, and Michael A. Duncan* Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States S Supporting Information *

ABSTRACT: The vibrational spectra of vanadium carbonyl cations of the form V(CO)n+, where n = 1−7, were obtained via mass-selected infrared laser photodissociation spectroscopy in the carbonyl stretching region. The cations and their argon and neon “tagged” analogues were produced in a molecular beam via laser vaporization in a pulsed nozzle source. The relative intensities and frequency positions of the infrared bands observed provide distinctive patterns from which information on the coordination and spin states of these complexes can be obtained. Density functional theory is carried out in support of the experimental spectra. Infrared spectra obtained by experiment and predicted by theory provide evidence for a reduction in spin state as the ligand coordination number increases. The octahedral V(CO)6+ complex is the fully coordinated experimental species. A single band at 2097 cm−1 was observed for this complex red-shifted from the free CO vibration at 2143 cm−1.

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filled metal d orbitals donate charge into the antibonding lowest unoccupied molecular orbital (LUMO) on CO. The addition of electron density to this orbital weakens the CO bond and reduces the vibrational frequency. In “classical” transition metal carbonyls, the effects of π back-bonding tend to outweigh those of σ donation. This results in a C−O stretch that is lower (i.e., red-shifted) in frequency compared to that of the free CO molecule (fundamental = 2143 cm−1).31 These effects are well documented for neutral carbonyls,1−7,32−34 and have recently been studied for several gas phase cations.27 It is interesting to investigate how the limited d orbital occupation of vanadium cation (d4 ground state) affects the carbonyl stretches, and to compare these carbonyls to those of other transition metals. In transition metal−ligand complexes, the orbital occupation and spin multiplicity on the metal greatly influence the bonding and reactivity of the system.35 The ground state of vanadium cation is a d4 quintet. However, both the triplet and quintet multiplicities have been observed in its cation-ligand complexes.36−39 Definitive experimental determination of the electronic states and structural properties of metal−ligand systems is often only achieved using high-resolution electronic spectroscopy. Unfortunately, in gas-phase metal complexes, the excited electronic states often predissociate on a short time scale, broadening spectra so that vibrational state information is lost. However, investigation of vibrational resonances in the ground electronic state can yield well-resolved spectra. These

INTRODUCTION Transition metal−carbonyl complexes are an integral component of inorganic and organometallic chemistry, and they play a vital role in several catalytic processes.1−5Metal carbonyl systems have been studied extensively because the frequency of the carbonyl stretch vibration is a sensitive indicator of the type of bonding present.1−7Transition metal−carbonyl ions have been studied traditionally in the condensed phase, stabilized with appropriate counterions.8−12However, the condensed phase environment may perturb the carbonyl vibrations. Gas phase studies are important, as they reveal the intrinsic properties of metal carbonyls in an isolated environment. In addition, by studying small unsaturated metal− carbonyl systems, we can systematically investigate the effects of successive ligand coordination on the electronic structure of the central metal atom. Simple cationic metal carbonyls have been characterized using mass spectrometry13−17 or infrared spectroscopy of ions trapped in rare gas matrixes.7,18−26Our group has recently developed methods for studying the infrared spectroscopy of small cationic metal−carbonyl systems using new laser technology and mass spectrometry.27We present here an infrared spectroscopy study of vanadium carbonyl cations, beginning with one CO ligand and progressing up to and beyond the first coordination sphere. The bonding in metal carbonyls is often explained using the Dewar−Chatt−Duncanson complexation model.1−7,28−30In this paradigm of metal−carbonyl bonding, two dominant interactions influence the vibrational frequencies. In σ donation, the carbonyl donates electron density from its highest occupied molecular orbital (HOMO) along the metal-CO axis into empty metal d orbitals. Because the HOMO has partial antibonding character, the removal of electron density increases the bond order as well as the vibrational frequency. In π back-bonding, partially © 2012 American Chemical Society

Special Issue: Peter B. Armentrout Festschrift Received: February 20, 2012 Revised: April 5, 2012 Published: April 9, 2012 1001

dx.doi.org/10.1021/jp301679m | J. Phys. Chem. A 2013, 117, 1001−1010

The Journal of Physical Chemistry A

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infrared spectroscopy on these complexes. For larger clusters with weakly bound ligands, we detect the elimination of one or more CO’s following laser excitation. For more strongly bound species, we employ the rare gas “messenger” or “tagging” technique.47−52 In this scheme, mixed ions of the form V(CO)n+-RG are produced, where RG = Ar or Ne, and absorption is detected via the elimination of the rare gas atom. Spectra are recorded by monitoring the appearance of the fragment ion(s) as a function of infrared laser wavelength. In support of this work, DFT calculations were performed using the B3LYP functional40,41 as implemented in the PCGAMESS53 computational suite. The Def2-TZVP basis set was used for vanadium, the DZP basis set for carbon and oxygen and the 6-311G(d) basis set for argon and neon. Calculated spectra are scaled by 0.968 and given a 10 cm−1 FWHM Lorentzian line shape for comparison to the experimental data. The scaling factor was derived by calculating the carbonyl stretches of neutral Fe(CO)5 and Cr(CO)6 complexes and comparing them to their known values.1−7,32−34

spectra can be used in conjunction with computational chemistry to give insight into the electronic structures of these systems. The most common theoretical method used for transition metal complexes is Density Functional Theory (DFT). The popular B3LYP hybrid functional40,41 provides accurate structural and frequency information,42 but generally overestimates the stability of high spin states.43Because of the efficient collisional cooling in the supersonic expansion, ions in our molecular beam experiments are generally produced in their ground electronic states. However, it is possible for a small fraction of these to be trapped in an excited state; this anomaly may be manifested as one or more low intensity bands in the experimental spectrum.27e By comparing the structural and vibrational information obtained by DFT methods to experimental spectra in the gas phase, the structure and electronic configuration of metal ligand complexes can be determined. The ligand binding energies in vanadium cation carbonyls have been studied previously by Armentrout and co-workers.16 The V(CO)n+-CO binding energies were found to decrease for n = 1−3 and increase for n = 3−6. This observation was attributed to a change in the electronic configuration of the central metal ion beginning at n = 4. The binding energy of CO in the V(CO)6+-CO complex was found to be much higher than expected, providing evidence for a strongly coordinated n = 7 species. This result is not surprising because the seven-coordinate carbonyl cation would be an 18-electron complex, and could be stable despite the steric crowding associated with seven ligands. A recent DFT and ab initio study of vanadium carbonyl cations suggested that the V(CO)7+ complex is indeed a stable seven-coordinate species with a singlet multiplicity and a capped octahedral structure.44 Our group recently published a communication detailing the investigation of the M(CO)7+ (M = V, Nb, Ta) series.27d In contrast to previous work, our results indicated that the vanadium cation did not form the seven-coordinate species, but instead formed a six coordinate complex with a weakly bound external CO ligand. On the other hand, the niobium cation produced a mixture of six- and seven-coordinate species, while the tantalum cation formed the seven-coordinate complex exclusively. The present work extends the study of vanadium cation carbonyls, beginning with the single ligand system and building up to the complete coordination.

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RESULTS AND DISCUSSION Figure 1 shows a mass spectrum of the V(CO)n+ ions formed via laser vaporization of a vanadium rod in a carbon monoxide

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EXPERIMENTAL SECTION Gas phase vanadium carbonyl cations are generated by laser vaporization of a rotating and translating metal rod using the third harmonic of a Nd:YAG laser (355 nm; Spectra Physics INDI).27 The sample rod is mounted in an offset position,45 about 1 cm to the side of the gas flow from the pulsed beam valve (General valve, Series 9). The expansion gas is either pure CO, or a mixture of CO with a few percent of either neon or argon, at a total backing pressure of 5−10 atm. Ions generated by ablation are picked up and entrained in the gas flow, where efficient evaporative and collisional cooling produce cold ion− molecule complexes. The gas expansion is skimmed into a second chamber, where cations are mass analyzed and sizeselected for photodissociation in a custom-built reflectron timeof-flight mass spectrometer.46At the turning point in the reflectron, ions are excited with an infrared Optical Parametric Oscillation/Optical Parametric Amplification system (LaserVision, pumped by Spectra Physics Pro 230 Nd:YAG laser) which has a tuning range of 2000−4600 cm−1 and a line width of approximately 1 cm−1. The density of ions produced in our experiment is too low to utilize absorption methods. We therefore employ mass-selected photodissociation to conduct

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

expansion. The major peaks correspond to V(CO)n+ ions whereas the minor peaks correspond to vanadium oxidecarbonyl ions. The most prominent peak in the mass spectrum corresponds to V(CO)6+, suggesting that this ion has enhanced stability. V(CO)n+ complexes up to n = 24 are produced. All these ligands cannot possibly be coordinated directly to the central metal ion. Instead, larger complexes are thought to consist of a strongly coordinated metal−ligand core ion with additional external ligands attached via electrostatic and/or van der Waals interactions. These external or second-sphere ligands would likely be weakly bound, and their elimination is likely to be efficient upon absorption of infrared photons. Figure 2 shows the infrared photodissociation “breakdown” spectra of larger V(CO)n+ clusters (n = 6−10). For each of these fragmentation spectra, the IR laser wavelength was 1002



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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. In all of the larger complexes the most prominent fragmentation channel is the loss of two CO ligands from the parent ion. This indicates that the binding energy of the external CO molecules is less than 1000 cm−1. The apparent doubling of the fragmentation peaks in the breakdown spectra of larger complexes is an artifact of the experimental technique. The photodissociation laser was slightly misaligned, crossing the ion beam just below its turning point, producing two groups of fragment ions with slightly different positions, and hence arrival times. To obtain vibrational spectra, we record the fragmentation yield as a function of the infrared laser wavelength. In smaller complexes with more strongly bound CO ligands, we employ rare gas tagging, as noted above, to enhance the dissociation yields.47−52 Mixed V(CO)n+-Ar and V(CO)n+-Ne complexes fragment by elimination of the rare gas atom following photoexcitation of a CO vibration. To aid in the assignment of the experimental spectra, DFT calculations using the B3LYP functional were performed on the singlet, triplet, and quintet spin states for multiple V(CO)n+ and V(CO)n+-RG isomers. In each case, only a single minimum was found for each cluster in each electronic state. The full details of these calculations are presented in the Supporting Information for this Article. As shown in Table 1, the ground state of the V+ ion is a quintet, with the triplet and singlet states +24.6 and +51.2 kcal/mol

Figure 2. Infrared photodissociation breakdown spectra of V(CO)n+ (n = 6−10) complexes. These spectra were obtained by subtracting a mass spectrum of a mass selected cluster with the laser off from one with it on.

Table 1. Calculated and Experimental Energetics of V+ Ions

adjusted to coincide with the strongest vibrational resonance in the spectrum (see below). These spectra were obtained by subtracting the mass spectrum of a selected complex with the fragmentation laser off from one obtained with it on. The negative peaks are due to depletion of the parent ion whereas the positive peaks are its photofragments. Ideally, the integrated area of the parent ion depletion should be equal to the sum of the areas of the fragment ions. However, this is not the case here because of mass discrimination in our instrument; we cannot focus simultaneously on the parent and fragment ions. As shown in this data, V(CO)n+ clusters smaller than n = 6 do not fragment efficiently. The n = 6 complex fragments weakly, as indicated by the low signal/noise of the spectrum. The bond energy of the V(CO)5+-CO complex has been previously measured to be 8170 cm−1.16 This is significantly more than the energy of an infrared photon in the CO stretching region (∼2100 cm−1). Therefore the observed fragmentation must be due to multiphoton absorption or the presence of a small fraction of ions with some unquenched internal energy from the growth process. In principle, a multiphoton process could be verified with power dependence. In practice, however, this is not easily done because of the shot-to-shot noise of the cluster source and the fact the IR laser spot changes shape with power, thus changing the overlap with the ion beam. Residual internal energy from incomplete collisional relaxation in the expansion is conceivable in the form of either unquenched vibrations or metastable excited states. Hot vibrations lead to dissociation below the “cold” threshold, and may cause additional broadening in the spectra. Metastable excited states are also possible for transition metal ions, but these would usually lead to vibrational resonances in different positions than those observed for the ground state (see below). The simplest explanation for the dissociation seen here therefore is a multiphoton process, which is likely because of the high oscillator strengths computed for these vibrations (see below). Beginning with the n = 7 complex, efficient fragmentation is observed. These clusters undergo sequential

species

calculated energy kcal/mol (absolute)

experimental53

V D J = 0 (3d4) V F J = 2 (3d34s) 1 +1 V G J = 4 (3d4)

+0.0 (−943.6746600) +36.1 (−943.6171752) +57.8 (−943.5824980)

+0.0 +24.6 +51.2

5 +5 3 +3

higher in energy, respectively.54 Our single reference DFT calculations reproduce the relative energetics of the singlet and triplet states reasonably well, but find the energy of the quintet state to be much lower relative to these than it actually is. This favoring of the high spin states is a known consequence of the inclusion of Hartree−Fock exchange in the B3LYP hybrid functional.43 Although the level of theory employed here overestimates the stability of high spin species, it is also known to give accurate structural and vibrational parameters for metalmolecular complexes.42 Table 2 lists the calculated energies and infrared band positions of the V(CO)n+ complexes and their argon and neon tagged analogues in different spin states. Figure 3 shows the experimental infrared spectra obtained for the V(CO)+-Ar2 and V(CO)+-Ne ions. The spectrum predicted for the V(CO)+-Ar2 ion is shown in the figure, while the predicted vibrations for all of the n = 1 complexes are provided in Table 2. No fragmentation was observed for the V(CO)+ or V(CO)+-Ar complexes. This is not surprising, because the binding energies in these systems are computed to be relatively high. Efficient elimination of neon from the V(CO)+-Ne complex was observed, and the spectrum obtained is shown in the upper trace of Figure 3. It consists of an intense peak at 2159 cm−1 and a very weak feature much further to the red at 2045 cm−1. As illustrated in Table 2, the quintet species of both V(CO)+-Ar2 and V(CO)+-Ne is calculated to be lowest in energy, and the most intense band seen for V(CO)+-Ne at 2159 cm−1 is in reasonable agreement with the frequency 1003



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Table 2. Calculated Relative Energies and Scaled Vibrational Frequencies of V(CO)n+ Complexesa species 5

+

relative energy (kcal/mol)

V(CO) V(CO)+ 1 V(CO)+

0.0 +23.5 +52.3

2145.1(412.1) 2049.3(510.6) 2043.4(506.8)

5

V(CO)+Ne 3 V(CO)+Ne 1 V(CO)+Ne

0.0 +22.8 +52.4

2126.7(419.2) 2051.2(536.6) 2042.5(527.0)

5

0.0 +22.1 +52.9

2119.9(487.2) 2108.3(529.9) 2119.9(583.1)

3

V(CO)+Ar V(CO)+Ar 1 V(CO)+Ar 3

5

V(CO)+Ar2 V(CO)+Ar2 1 V(CO)+Ar2 3

relative energy (kcal/mol)

calculated IR frequencies (cm−1)

5

V(CO)4+ 1 V(CO)4+

+0.3 +17.5

2114.2(1135.3), 2114.3(1134.7), 2124.5(65.8) 2076.9(1111.5), 2076.9(1111.5), 2076.9(1111.5)

3

0.0 +3.5

2082.6(1330.1), 2092.6 (0.9), 2150.5(4.7) 2098.9(1210.1), 2116.9(67.5), 2122.8(1045.1), 2162.3(7.7) 2054.2(596.9), 2063.5(1005.9), 2079.9(1066.8), 2138.9(99.0)

calculated IR frequencies (cm−1)

species

V(CO)4+Ar V(CO)4+Ar

5

1

+18.3

3

0.0

5

+5.5 +7.1

V(CO)4+Ar

V(CO)5+ V(CO)5+ V(CO)5+

1

0.0 +17.9 +50.4

2131.5(482.3) 2021.2(579.8) 2088.0(554.5)

3

0.0

1

+7.0

5

+9.3

3

0.0

1

+14.0

5

+29.8

2095.3(889.5), 2096.9(813.4), 2115.9(8.2), 2130.5(802.5), 2150.8(68.8), 2160.0(0.7)

3

0.0

1

+6.1

5

+29.8

2070.8(0.21), 2073.2(0.26), 2095.9(1266.2), 2098.9(992.5), 2099.4(1008.4) 2079.6(0.1), 2086.7(1050.2), 2087.7(133.4), 2098.6(1127.6), 2101.1(968.0) 2095.0(882.0), 2096.8(812.6), 2115.6(7.8), 2130.1(801.2), 2150.4(69.2), 2159.8(0.7)

1

0.0

2052.5(629.4), 2061.8(1025.3), 2061.9(1025.9), 2084.8(582.8), 2108.2(355.5), 2108.2(354.9), 2147.9(99.3)

3

+7.2

2056.5(3.9), 2072.9(1258), 2089.1(8.2), 2100.7(1116.4), 2103.7(888.3), 2153(65.5), 2154.9(4.4)

5

+35.5

2089.3(902.7), 2090.8(822.8), 2110.4(1.5), 2126.0(807.5), 2150.5(43.7), 2150.9(103), 2154.7(3.3)

V(CO)5+Ar V(CO)5+Ar

5

V(CO)2+ 3 V(CO)2+ 1 V(CO)2+

0.0 +19.0 +43.3

2111.3(788.3), 2145.1(236.5) 2034.7(2230.5) 2056.0(1747.2)

5

V(CO)2+Ne 3 V(CO)2+Ne 1 V(CO)2+Ne

0.0 +19.0 +43.5

2108.0(777.2), 2142.1(248.7) 2057.6(2000.3), 2126.8(0.7) 2052.4(1770.6), 2118.5(0.5)

5

V(CO)2+Ar 3 V(CO)2+Ar 1 V(CO)2+Ar

0.0 +19.1 +39.2

2101.0(793.4), 2135.9(293.2) 2049.1(2053.9), 2119.7(6.5) 2056.8(1742), 2111.2(7.1)

5

V(CO)3+ 3 V(CO)3+ 1 V(CO)3+

0.0 +14.0 +18.7

2110.6(557.5), 2127.2(1176.7), 2166.7(1) 2079.8(1667.1),2100.9(356.4),2147.7(137.3) 2085.1(561.4), 2096.6(1429.9), 2143.3(27.4)

5

V(CO)3+Ne V(CO)3+Ne 1 V(CO)3+Ne

0.0 +7.1 +25.4

2109.4(540.5), 2124.8(1182.4), 2164.9(3.6) 2063.4(590.3), 2081.3(1554), 2142.2(5) 2063.6(1171.3), 2063.6(1171.3), 2129.6(73.1)

5

V(CO)3+Ar 3 V(CO)3+Ar 1 V(CO)3+Ar

0.0 +6.9 +23.6

2104.3(553.2), 2119.5(1203.9), 2160.5(11.5) 2059.8(601.4), 2076.2(1535.1), 2137.7(14.9) 2056.7(1196.6), 2056.8, 2124(104.3)

3

0.0

2082.6(1331.5), 2083.0(1332.3), 2150.5(4.7)

3

V(CO)4+

a

2082.5(1297.3), 2082.6(1303.1), 2093(6), 2132.1(269.6), 2159.9(72.1) 2100.0(842.9), 2100.0(841.5), 2132.9(799.2) 2082.7(1281.06), 2082.7(1281.7), 2092.6(0.9), 2122.2(348.5), 2155.9(36.8)

V(CO)5+Ar V(CO)6+ V(CO)6+ V(CO)6+

V(CO)6+Ar V(CO)6+Ar V(CO)6+Ar

V(CO)7+

V(CO)7+

V(CO)7+

2078.2(1305.9), 2078.3(1307.8), 2088.5(0.3), 2128.1(239.3), 2156.2(108.1) 2077.9(1279.2), 2078.3(1282.8), 2088.4(1.4), 2118.0(324.9), 2152.1(66.5) 2099.6(848.4), 2100.1(828.2), 2131.1(796.5) 2072.1(0.3), 2073.8(0.5), 2097(1272.8), 2096.5(1272.8), 2099.7(999.2), 2099.7(996.5) 2081.9(0.2), 2090.2(1278.5), 2090.4(1274.8), 2098.4(774.9)

IR intensities (km/mol) are shown in parentheses.

predicted for this state (2127 cm−1). We therefore assign this band to the C−O stretch of the quintet ground state. However, it should be noted that the computed band position is redshifted with respect to the vibration of isolated CO, while the experimental band is slightly blue-shifted. The appearance of a second weaker band for this complex, which has only one CO group, is at first puzzling. However, previous experiments have shown that during laser vaporization it is possible to produce atomic vanadium ions having more than one spin state.55,56 The spin state of the metal is known to influence the nature of the metal−ligand bond and if different spin states are also present in these molecular complexes, this can lead to different CO stretching vibrations. As shown in Table 2, the band at 2045 cm−1 is in nearly the same position as the vibrations predicted for either the triplet

(2051 cm−1) or singlet (2043 cm−1) excited states of V(CO)+-Ne. We therefore assign this weak feature to a very small amount of unquenched excited states having one of these different spin configurations. This detection of unquenched excited states is similar to our previous observations for the Mn(CO)n+ system, where both the quintet and the septet states were assigned.27e Elimination of a single argon atom from the V(CO)+-Ar2 species is observed, and the spectrum obtained by monitoring this loss channel is shown as the second trace in Figure 3. It consists of a single peak at 2130 cm−1, which is red-shifted from the free molecular CO stretch by 13 cm−1. As noted above, the quintet state is computed to have the lowest energy for this complex, and its predicted vibration at 2132 cm−1 agrees nicely with the experiment. The frequencies predicted for the excited 1004



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Figure 3. Infrared spectra of the V(CO)+ ion obtained using neon and argon tagging. Calculated relative energies of the isomers are given in kcal/mol.

Figure 4. Infrared spectra of the V(CO)2+ ion obtained using neon and argon tagging. Calculated relative energies of the isomers are given in kcal/mol.

states of this complex are again much further to the red, but no signal is detected in this region. We therefore assign V(CO)+Ar2 to have the quintet ground state. It is evident that the Ar2 complex has a spectrum shifted to the red from that of the neon complex. This observation is not surprising, and is predicted by theory. In small complexes such as this, the argon binding is strong enough to change the electronic environment around the metal, thus inducing a shift in the ligand vibration. The neon binding is much weaker and therefore it is expected to cause much less of a perturbation to the system. The spectra of the V(CO)2+ ion obtained using argon and neon tagging are shown in Figure 4. The V(CO)2+-Ne spectrum has two bands at 2129 and 2153 cm−1, while that of the argon tagged species has bands at 2122 and 2145 cm−1. Here the difference between the argon and neon complexes is smaller than it was for the n = 1 complex. In general, we have found that vibrational shifts induced by rare gas atoms, and also the differences between argon and neon, become smaller for ions that are more fully coordinated with strongly bound ligands.27DFT calculations predict a quintet ground state with a bent structure for the V(CO)2+ ion, giving rise to two IR active CO stretches. The argon-tagged ion has this same kind of bent OC-V+-CO configuration, with argon attached to the metal ion. The positions computed for the two IR bands(2100 and 2136 cm−1) match reasonably well with those in the spectra. Contrasting with this, the triplet and singlet spin states are each predicted to have only a single IR band shifted much further to the red. Attachment of rare gas for these spin states causes the normally linear V(CO)2+ ion to undergo a small distortion so that there is still only one strong IR-active band and the in-phase carbonyl stretch remains essentially IR inactive. We therefore assign the experimental spectra to be representative of ions in their quintet ground states having the bent structure. Unlike the V(CO)+-Ne system, there is no evidence for the presence of any excited states for the V(CO)2+-Ne system. Figure 5 shows the spectra for the argon and neon tagged V(CO)3+ ions. These spectra have two bands each at 2115/ 2136 cm−1 (argon) and 2122/2146 cm−1 (neon), respectively.

Figure 5. Infrared spectra of the V(CO)3+ ion obtained using neon and argon tagging. Calculated relative energies of the isomers are given in kcal/mol.

Again, the difference between the argon and the neon complexes is small. The spectra resulting from theory for the argon complex in different spin states are also shown; the spectra for the neon complex are essentially the same as these. Both the quintet and the triplet isomers are calculated to have structures with C2v symmetry, in which the rare gas atom (not shown in the inset structures) binds to the vanadium ion on the C2 axis. However, the quintet is calculated to be lower in energy than the triplet by 6.9 kcal/mol. 1005



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ion.16Our assignment of a quintet state for V(CO)3+ and a triplet for V(CO)4+ is consistent with their proposal. Figure 7 shows the experimental and theoretical spectra for the V(CO)5+-Ar species. The experimental spectrum has a

The singlet species has a C3v structure lying much higher in energy. Both the quintet and the triplet have two IR bands with about the same intensity pattern. The more intense peak at higher frequency corresponds to the asymmetric stretch of the two horizontal CO ligands, while the peak at lower frequency corresponds to the stretch of the CO ligand along the C2 axis. The doublets predicted for the quintet and triplet ions both reproduce the relative intensity and splitting of those in the experimental spectra, but the triplet bands are shifted further to the red than those in the spectra. The singlet state spectrum is dominated by a single intense red-shifted peak, shifted even further to the red. We therefore assign the spectra for both the V(CO)3+-Ne and V(CO)3+-Ar complexes to be representative of the “T” structures and the quintet ground states. Figure 6 shows the infrared spectrum of the V(CO)4+-Ar complex and the spectra predicted by theory for this complex in

Figure 7. Infrared spectrum of the V(CO)5+ ion obtained using argon tagging. Calculated relative energies of the isomers are given in kcal/mol.

single intense band at 2075 cm−1, in nearly the same position as the band in the n = 4 spectrum. The triplet is predicted to be the ground state with C4v symmetry, while the singlet lies only +7.0 kcal/mol higher also with a C4v structure. The quintet has a D3h trigonal bipyramidal structure and a relative energy of +9.3 kcal/mol. The single intense band in the experimental spectrum matches well with the single main band predicted for the triplet, and it is assigned as such. The two minor features in the calculated spectrum are not observed in the experiment, presumably because of their low oscillator strengths. The quintet state is predicted to have a doublet band pattern, which is not seen, and can be clearly ruled out. However, the singlet species also has essentially one main band, and it also agrees with the experimental band position. We therefore cannot rule this out with absolute certainty. However, because the triplet is predicted to be the ground state, and the larger n = 6 clusters have spectra inconsistent with a singlet (see below), we believe that the triplet is the correct assignment here. It is interesting that the main band seen here at 2075 cm−1 appears at virtually the same position as that for the n = 4 complex (2076 cm−1). These bands are also predicted to have essentially the same positions by theory if both have the triplet ground state configuration, consistent with our assignment. The spectrum of the V(CO)6+Ar ion obtained by the elimination of argon is shown at the top of Figure 8, along with the spectra predicted by theory for the different spin states of this ion. The experimental spectrum consists of one intense peak at 2097 cm−1. DFT calculations indicate that the triplet species is again the ground state, and that it has D3d symmetry. This structure is slightly displaced from Oh symmetry because of Jahn−Teller distortion. The singlet state lies at +6.1 kcal/mol,


different spin states. In this case, the neon complex was not produced with enough intensity to study. The experimental spectrum contains a single intense band at 2076 cm−1 and a barely noticeable feature at 2112 cm−1. The triplet state is predicted to be lowest in energy, but only 3.5 kcal/mol lower than the quintet. The triplet has a D4h structure, and the quintet and singlet have D2d and Cs structures, respectively. Corresponding to these structures, the triplet is expected to have only one IR-active mode, while the quintet and singlet produce multiplet patterns. The calculated spectrum of the triplet reproduces both the single band observed and its frequency position, and therefore we assign our spectrum to the triplet ground state and the D4h structure. The weak feature near 2112 cm−1 likely represents a small amount of the quintet excited state. Both theory and experiment therefore suggest that the quintet spin configuration seen for all the smaller complexes changes over to a triplet at the n = 4 complex. Consistent with such a sharp change in the electronic structure, the single band seen here at 2076 cm−1 is red-shifted by approximately 50 cm−1 from those of the n = 1−3 complexes. Previously, the Armentrout group noted a similar sharp change in the binding energy for these V(CO)n+ complexes between n = 3 and n = 4, and attributed this to a spin change of the vanadium 1006



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Figure 9. Infrared spectra of the V(CO)7+ ion obtained via argon tagging and by elimination of a single CO molecule. V(CO)7+-Ar was found to fragment by concerted elimination of both CO and Ar. Calculated relative energetics of the isomers of V(CO)7+ are given in kcal/mol.

also with a structure distorted from Oh symmetry. A quintet species at much higher energy (+29.8 kcal/mol) is predicted to have a structure similar to that of the n = 5 quintet, with a secondsphere carbonyl. If this isomer were present, the neat V(CO)6+ complex would fragment efficiently via the elimination of this second-sphere CO ligand, but this was not observed. The quintet species is therefore eliminated from consideration because of this fragmentation behavior and because the number of bands is inconsistent with the observed spectrum. The triplet state species produces a spectrum consisting of essentially one main band, although this is the result of more than one band overlapping closely. The multiplet arises from a small distortion from the ideal octahedral structure. The singlet state has a greater distortion from octahedral, with a resolved doublet band structure. Because the triplet is predicted to lie at lowest energy and because the position and single band match the predicted spectrum, we assign the spectrum of the V(CO)6+-Ar species to the triplet ground state. This assignment is relevant for the assignment of the V(CO)5+-Ar species mentioned earlier. If ligand-electron repulsion is not yet strong enough to induce the n = 6 complex to pair its spins to produce the singlet state, it is also unlikely that this has happened for the n = 5 species. This supports the triplet state assignment for the n = 5 species. The neat V(CO)7+ complex fragments efficiently via the elimination of a single CO ligand, and its spectrum consists of a broad peak at 2096 cm−1 and a weak band at 2156 cm−1, as illustrated in Figure 9. The band at 2096 cm−1 is in virtually the same position as the single band seen in the spectrum of V(CO)6+-Ar(2097 cm−1). Surprisingly, the V(CO)7+-Ar complex fragments by concerted loss of both CO and Ar, producing a slightly noisier spectrum whose main band is only slightly shifted to the blue at 2103 cm−1. Unlike the V(CO)6+-Ar species, both V(CO)7+ and V(CO)7+-Ar have a weak band at high frequency (2156 and 2166 cm−1, respectively). All of these experimental data indicate that the core ion detected here for both V(CO)7+ and V(CO)7+-Ar is the same, and that it is also the same core ion seen in the V(CO)6+-Ar spectrum. It is the

V(CO)6+ species with near-octahedral symmetry, which produces one main band because of its symmetry. The main band seen in each of these spectra lies at the position predicted by theory for the V(CO)6+-Ar ion in its triplet ground state. The features at 2156 and 2166 cm−1 for these are consistent with the presence of a second-sphere CO ligand, as discussed previously.27The presence of such external ligands also likely explains the width of the main band for V(CO)7+, which is noticeably greater than the one in the V(CO)6+-Ar spectrum. Theory shows that different placements for the external CO lie close in energy, and then inhomogeneous broadening could result from the multiple isomeric structures like this, each having slightly different spectra. These experimental observations on the nature of the V(CO)7+ ion do not agree with our expectations for this ion, nor our computational results for it. On the basis of simple electron counting, the V(CO)7+ ion should have 18 electrons and thus be stable as an unusual seven-coordinate (7-C) carbonyl species. The V(CO)7+ species is computed to be stable in a capped octahedral structure, and to have a ground state singlet configuration that is 7.2 kcal/mol lower in energy than the V(CO)6+ + external CO (i.e., 6 + 1) structure, in agreement with the previous work of Schaefer and coworkers.44 Indeed, previous experimental work by Armentrout and co-workers found evidence for a stable seven-coordinate V(CO)7+ species, with a bond energy of 12.7 kcal/mol (4440 cm−1) for the loss of CO.16 On the other hand, the spectrum predicted for the 7-C species (top theory trace, Figure 9) has a multiplet of five CO stretch bands resulting from the lower symmetry, and this pattern definitely does not match our measured spectrum. If a strongly bound V(CO)7+ ion were present, it would likely not be detected in our IR photodissociation experiment by the elimination of CO because of the bond energy. However, the V(CO)7+-Ar ion or larger V(CO)n+ ions would not have this problem and should reveal the spectral pattern for the core 7-C species if it were 1007



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roughly the same red shifts are observed for the n = 5 and 6 complexes, and the qualitative picture for this is not as simple. It is also interesting to see how the CO stretching frequency observed here for the (nearly) closed-shell V(CO)6+ ions compares to those of other transition metal carbonyls. Table 3 lists the C−O

present, but no such patterns were detected. Why then do we not see the expected seven-coordinate V(CO)7+ species? We have discussed this issue previously in our communication on the carbonyls of vanadium, niobium, and tantalum carbonyls.27d In that report, we showed that although V(CO)7+ does not produce a spectrum matching the 7-C species, both Nb(CO)7+ and Ta(CO)7+ do. In fact, the Nb(CO)7+ ions produced different spectra in different fragmentation channels, indicating that there were two different structural isomers present, both the 6 + 1 and the 7-C structures, while the Ta(CO)7+ ion was found to exist only as the 7-C species. The 6 + 1 niobium isomer has a spectrum like that of the V(CO)7+ species, while the 7-C isomers for both niobium and tantalum have the five-band pattern predicted by theory. Apparently, seven coordinate ions are formed for the larger vanadium group cations, but not for vanadium itself, even though all have the same valence electron count. To rationalize these seemingly inconsistent results, we consider the differences between our experiment and that of Armentrout. In the Armentrout experiment, ions are produced by laser ablation, but then thermalized to room temperature in a higher pressure (several torr) flowtube and studied much farther downstream by collisional excitation. Under these conditions, ions having weakly bound external ligands, such as those that we see as the major component in our V+(CO)7 experiment, would not survive. This explains why Armentrout does not observe the weakly bound ions that we see. Armentrout’s source apparently produced at least some strongly bound V(CO)7+ ions which survived to be detected, but we did not. We speculate that because of the continuous (rather than pulsed) gas flow, the growth conditions in his source may be hotter than in ours. If cluster growth involves sequential addition of CO ligands, then production of V(CO)7+ (singlet ground state) from V(CO)6+ (triplet ground state) requires a spin change, and a kinetic “bottleneck” for this process is not at all unreasonable to expect. Under the rapid cooling in our supersonic expansion, V(CO)6++ CO ions may not be able to form singlets, but given longer time under warmer growth conditions in the Armentrout experiment they might be able to do so. Once formed, they could survive to be detected. The spin change in cluster growth might be easier for heavier ions like niobium or tantalum with stronger spin−orbit interactions, thus explaining how these ions did form the stable 7-C species in our experiment. It is clear from this study that the number of carbonyl stretches is related to the structure of these complexes, and that the position of the C−O bands varies with the electronic structure of the metal. Higher symmetry complexes have fewer IR-active bands, such as the single band spectra seen for the V(CO)4+ and V(CO)6+ complexes, while bent or asymmetric structures produce more bands, such as those seen in the spectra for V(CO)2+ and V(CO)3+. An extreme case of multiplet CO bands occurs for the sevencoordinate species not seen here, but detected for niobium and tantalum.27d Five CO bands were seen for those systems. The positions of the CO bands here are quite close to the frequency of the isolated CO molecule in the small clusters, but much more strongly red-shifted for the V(CO)4+ and larger species. As additional CO’s are added around the metal, ligand-electron repulsion eventually leads to spin coupling to form the triplet state. Apparently, this configuration is much more effective at back-bonding, because there is a sharp increase in the red shift for those clusters having triplet ground states. It is easy to see how this might happen for the V(CO)4+ complex, whose D4h structure would naturally leave electron density in the dz2 orbital out of the plane of the ligands. Electron density here lines up spatially with the π* orbitals of the ligands, facilitating the back-bonding. However,

Table 3. Comparison of the Band Positions (cm−1) of Core CO Ligands for Various Metal−Carbonyl Cation Complexes Published by Our Group Previously along with Those of Stable Neutral Metal−Carbonyls

a

complex

band positions

complex

band positions

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

2097 2115b 2140, 2150d 2198f

V(CO)6 Cr(CO)6 Fe(CO)5 Ni(CO)4

1985,1991a 2002.9c 2013, 2034e 2056g

Ref 57. bRef 27e. cRef 33. dRef 27c. eRef 32. fRef 27h. gRef 34.

stretches for various charged metal−carbonyl systems measured by our group, as well as those for well-known stable neutrals. As discussed widely in the literature,1−7 both σ donation and π back-bonding are understood to govern the metal carbonyl bonding interaction. The π back-bonding from metal d orbitals into the empty π* orbital on CO plays a dominant role in the C−O stretching frequencies, and for most metal carbonyls these occur at values lower than that of isolated CO (2143 cm−1). The red shifts of the C−O stretches are clearly seen in Table 3 for the well-known Cr(CO)6, Fe(CO)5, and Ni(CO)4 complexes. Metals for which π backbonding is efficient have larger red shifts, while those for which it is less efficient have small red shifts or even no red shift at all (i.e., a blue shift), such as the so-called “nonclassical” carbonyls of copper. In comparing the C−O stretch vibrations for these different metals, it is immediately apparent that the red shifts are much greater for the neutral species than they are for the corresponding ions. This is evident for V(CO)6 versus the V(CO)6+cation, which is not an exact isoelectronic comparison. It is also clear for the comparison of Co(CO)5+ versus Fe(CO)5, Mn(CO)6+ versus Cr(CO)6, and Cu(CO)4+ versus Ni(CO)4, which are all isoelectronic pairs. The smaller red shifts for cations are understood to result from their reduced ability to undergo π backbonding.27g An additional trend is also evident across the V, Cr, Fe, Ni neutral series and the V, Mn, Co, Cu cation series of metals. Earlier transition metal carbonyls generally have greater red shifts than those of the later metals. For example, vanadium neutral and cation carbonyls have the largest red shifts for both series. The number of metal d electrons of course increases across the period, so there are more electrons available for back-bonding in the late metals, but the nuclear charge also increases for these systems. The latter is responsible for the well-known d contraction of these electrons, which leads to greater ionization potentials for the late transition metals. Although this effect has not been discussed widely in the literature, it appears that the d contraction also contributes to reduced back-bonding across the period, and therefore greater red shifts for the early transition metals. The cation data measured recently complements the well-known neutral data to illustrate these effects.

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CONCLUSION V(CO)n+ complexes and their neon and argon tagged analogues were produced in a molecular beam via laser vaporization in a pulsed nozzle source. The ions were mass-selected and studied using infrared photodissociation spectroscopy in the carbonyl stretching region. DFT computations were carried out to aid in 1008



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(11) McLean, R. A. N. Infrared and Raman spectra of Mn(CO)6+ in CH3CN solutions and solids. Can. J. Chem. 1974, 52, 213−215. (12) Rack, J. J.; Webb, J. D.; Strauss, S. H. Polycarbonyl cations of Cu(I), Ag(I), and Au(I): [M(CO)n]+. Inorg. Chem. 1996, 35, 277− 278. (13) Meyer, F.; Chen, Y. M.; Armentrout, P. B. Sequential bond energies of Cu(CO)x+ and Ag(CO)x+(x = 1−4). J. Am. Chem. Soc. 1995, 117, 4071−4081. (14) Meyer, F.; Armentrout, P. B. Sequential bond energies of Ti(CO)x+, x = 1−7. Mol. Phys. 1996, 88, 187−197. (15) Khan, F. A.; Clemmer, D. E.; Schultz, R. H.; Armentrout, P. B. Sequential bond energies of chromium carbonyls (Cr(CO)x+, x = 1−6). J. Phys. Chem. 1993, 97, 7978−7987. (16) Sievers, M. R.; Armentrout, P. B. Collision-induced dissociation studies of V(CO)x+, x = 1−7: Sequential bond energies and the heat of formation of V(CO)6. J. Phys. Chem. 1995, 99, 8135−8141. (17) Zhang, X. G.; Armentrout, P. B. Sequential bond energies of Pt+ (CO)x (x = 1−4) determined by collision-induced dissociation. Organometallics 2001, 20, 4266−4273. (18) Liang, B.; Zhou, M.; Andrews, L. Reactions of laser-ablated Ni, Pd, and Pt atoms with carbon monoxide: Matrix infrared spectra and density functional calculations on M(CO)n (n = 1−4), M(CO)n− (n = 1−3), and M(CO)n+ (n = 1−2), (M = Ni, Pd, Pt). J. Phys. Chem. A 2000, 104, 3905−3914. (19) Gutsev, G. L.; Andrews, L.; Bauschlicher, C. W., Jr. 3d-Metal monocarbonyls MCO, MCO+ and MCO− (M = Sc to Cu): Comparative bond strengths and catalytic ability to produce CO2 in reactions with CO. Chem. Phys. 2003, 290, 47−58. (20) Liang, B.; Andrews, L. Reactions of laser-ablated Ag and Au atoms with carbon monoxide: Matrix infrared spectra and density functional calculations on Ag(CO)n (n = 2, 3), Au(CO)n (n = 1, 2), and M(CO)n+ (n = 1−4), (M = Ag, Au). J. Phys. Chem. A 2000, 104, 9156−9164. (21) Zhou, M.; Andrews, L. Infrared spectra of RhCO+, RhCO, and RhCO− in solid neon: A scale for charge support in catalyst systems. J. Am. Chem. Soc. 1999, 121, 9171−9175. (22) Zhou, M.; Andrews, L. Infrared spectra and density functional calculations of CuCOn+ (n = 1−4), CuCOn (n = 1−3), and CuCOn− (n = 1−3), in solid neon. J. Chem. Phys. 1999, 111, 4548−4557. (23) Zhou, M.; Andrews, L. Infrared spectra and density functional calculations of RuCO+, OsCO+, RuCOx, OsCOx, RuCO−, and OsCO− (x = 1−4) in solid neon. J. Phys. Chem. A 1999, 103, 6956−6968. (24) Zhou, M.; Andrews, L. Reactions of laser-ablated iron atoms and cations with carbon monoxide: Infrared spectra of FeCO+, Fe(CO)2+, Fe(CO)x and Fe(CO)x− (x=1−4) in solid neon. J. Chem. Phys. 1999, 110, 10370−10379. (25) Zhou, M.; Andrews, L. Matrix infrared spectra and density functional calculations of ScCO, ScCO−, and ScCO+. J. Phys. Chem. A 1999, 103, 2964−2971. (26) Zhou, M.; Andrews, L. Infrared spectra and density functional calculations for OMCO, and OM-(η2-CO−), OMCO+, and OMOC+ (M = V, Ti) in solid argon. J. Phys. Chem. A 1999, 103, 2066−2075. (27) (a) Velasquez, J., III; Duncan, M. A. IR photodissociation spectroscopy of gas phase Pt+(CO)n (n = 4−6). Chem. Phys. Lett. 2008, 461, 28−32. (b) Velasquez, J., III; Njegic, B.; Gordon, M. S.; Duncan, M. A. IR photodissociation spectroscopy and theory of Au+(CO)ncomplexes: Nonclassical carbonyls in the gas phase. J.Phys. Chem. A 2008, 112, 19071913. (c) Ricks, A. M.; Bakker, J. M.; Douberly, G. E.; Duncan, M. A. Infrared spectroscopy and structures of cobalt carbonyl cations. J. Phys. Chem. A 2009, 113, 4701−4708. (d) Ricks, A. M.; Reed, Z. D.; Duncan, M. A. Seven-coordinate homoleptic metal carbonyls in the gas phase. J. Am. Chem. Soc. 2009, 131, 9176−9177. (e) Reed, Z. D.; Duncan, M. A. Infrared spectroscopy and structures of manganese carbonyl cations, Mn(CO)n+ (n = 1−9). J. Am. Soc. Mass Spectrom. 2010, 21, 739−749. (f) Ricks, A. M.; Gagliardi, L.; Duncan, M. A. Infrared spectroscopy of extreme coordination: The carbonyls of U+ and UO2+. J. Am. Chem. Soc. 2010, 132, 15905−15907. (g) Ricks, A. M.; Duncan, M. A. Infrared spectroscopy of metal carbonyl cations. J. Mol. Spectrosc. 2011,

the interpretation of the experimental spectra. The number of infrared active bands, their relative intensities, and frequency positions provide insight into the structure and bonding of these complexes, particularly with respect to the spin configurations that result for complexes with different numbers of ligands. Evidence for a reduction in the spin state of the central metal ion was observed, changing from a quintet in the V(CO)3+ and smaller complexes to a triplet in the V(CO)4+ and larger complexes. The triplet states of the larger complexes have greater CO red shifts than the quintets in the smaller complexes. A single band at 2097 cm−1 was observed for the n = 6 complex, which is strongly redshifted from the free CO vibration at 2143 cm−1 compared to other transition metal cation carbonyls. This n = 6 species was found to be the fully coordinated complex under our conditions and to have a structure slightly distorted from octahedral. We were not able to observe the 18-electron seven-coordinate V(CO)7+ complex predicted to be stable by theory, presumably because of the unusual dynamics of cluster growth in our source.

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ASSOCIATED CONTENT

S Supporting Information *

The geometric and energetic parameters for the structures of V(CO)n+ and V(CO)n+-RG complexes in different spin states. 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 the generous support for this work from the U.S. Department of Energy (Grant DE-FG02-96ER14658), and the Air Force Office of Scientific Research (Grant FA95509-10166).

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REFERENCES

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