Infrared Photodissociation Spectroscopy of Mononuclear Iron

Article pubs.acs.org/JPCA

Infrared Photodissociation Spectroscopy of Mononuclear Iron Carbonyl Anions Guanjun Wang,† Chaoxian Chi,† Jieming Cui,† Xiaopeng Xing,‡ and Mingfei Zhou*,† †

Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai 200433, China ‡ Graduate University of Chinese Academy of Sciences, College of Materials Sciences and Opto-Electronic Technology, Beijing 100049, China ABSTRACT: The infrared photodissociation spectroscopy of mass-selected mononuclear iron carbonyl anions Fe(CO)n− (n = 2−8) were studied in the carbonyl stretching frequency region. The FeCO− anion does not fragment when excited with infrared light. Only a single IR active band was observed for the Fe(CO)2− and Fe(CO)3− anions, consistent with theoretical predictions that these complexes have linear D∞h and planar D3h symmetry, respectively. The Fe(CO)4− anion is the most intense peak in the mass spectra and was characterized to have a completed coordination sphere with high stability. Anion clusters larger than n = 4 were determined to involve a Fe(CO)4− core anion that is progressively solvated by external CO molecules. Three CO stretching vibrational fundamentals were observed for the Fe(CO)4− core anion, indicating that the Fe(CO)4− anion has a C3v structure. All the carbonyl stretching frequencies of the Fe(CO)n− anion complexes are red-shifted with respect to those of the corresponding neutrals.

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Fe(CO)3− and Fe(CO)4− anions were characterized to have D3h and C3v symmetry, respectively.20,29,30 The Fe(CO)n− anions have also been studied in the gas phase.31−34 The CO binding energies were experimentally determined via energy resolved collision-induced dissociation.33 Photoelectron spectroscopic study in the gas phase indicated that FeCO− has a quartet ground state,32 in agreement with theoretical calculations.30 The Fe−C stretching and bending vibrational frequencies of FeCO− were measured.32 Gas-phase investigations suggested that Fe(CO)x− (x = 2−4) all have doublet ground states,34 which were supported by theoretical calculations.30

INTRODUCTION Transition-metal carbonyl complexes are of considerable structural interest in coordination chemistry and organometallic chemistry.1 They are often considered as models for CO binding to the metal surface, and they play important roles in catalytic and other reactions.2−4 Neutral transition metal carbonyls have been intensively studied in both the gas phase and the condensed phase environments using various spectroscopic techniques.5−10 Their structures and bonding have also been the subject of a large number of theoretical investigations.11−15 Besides neutral complexes, transition metal carbonyl cations have been produced in the condensed phase as salts with counterions, and studied using spectroscopy.16 Various unsaturated transition metal carbonyl cations have also been prepared in noble gas matrixes and studied using infrared absorption spectroscopy.17−20 Recently, infrared photodissociation spectroscopy is successfully employed in studying transition metal carbonyl cations and multinuclear metal carbonyl clusters in the gas phase.21−28 In the present article, infrared photodissociation spectroscopy is used to investigate mass selected mononuclear iron carbonyl anion complexes in the gas phase. The iron carbonyl anions Fe(CO)x− (x = 1−4) have been prepared in solid noble gas matrixes and characterized by infrared absorption spectroscopy.20,29 The Fe(CO)2− anion was characterized to possess a linear geometry in solid neon, but it adopts a bent C2v geometry in solid argon.20 DFT calculations predicted that the Fe(CO)2− anion has a linear 2∏u ground state with a bent 2A1 state slightly higher in energy. The © 2012 American Chemical Society

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EXPERIMENTAL METHOD The experiments were carried out using a collinear tandem time-of-flight mass spectrometer equipped with a laser vaporization supersonic cluster source, which is similar to those used previously in the literature.35,36 A schematic diagram of the experimental apparatus is shown in Figure 1. The 1064 nm fundamental of a Nd:YAG laser (Continuum, Minilite II; 10 Hz repetition rate and 6 ns pulse width) was used to vaporize a rotating iron metal target. The Fe(CO)n− anion complexes were produced from the laser vaporization process in expansions of helium gas seeded with 1−4% CO using a pulsed valve (General Valve, Series 9) at 3−5 MPa backing Received: December 12, 2011 Revised: January 9, 2012 Published: February 23, 2012 2484

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Figure 1. Schematic diagram of the experimental apparatus consisting of a laser vaporization supersonic cluster source and a collinear tandem timeof-flight mass spectrometer.

mass selection as well as the resolution of the second time-offlight mass spectrometer. The anions were detected with a dual microchannel plate detector. The mass signals were amplified with a broadband amplifier, digitized, and transferred to a computer. Infrared photodissociation spectra were obtained by monitoring the fragment anion yield as a function of the dissociation IR laser wavelength. Typical spectra were recorded by scanning the dissociation laser in steps of 2 cm−1 and averaging over 250 laser shots at each wavelength. The tunable infrared source used in this study is generated by an KTP/KTA/AgGaSe2 optical parametric oscillator/amplifier system (OPO/OPA, Laser Vision) pumped by a Continuum Powerlite 8000 Nd:YAG laser. The laser pulse energies range from 0.2 to 1.5 mJ/pulse with an approximate line width of 1 cm−1. The infrared laser is either unfocused (for weakly bound anion complexes) or loosely focused by a CaF2 lens (for strongly bound complexes). The wavenumber of the OPO laser is calibrated by a commercial wavemeter (Coherent, WaveMaster). Quantum chemical calculations were performed to determine the molecular structures and to support the assignment of vibrational frequencies of the anion species studied. The calculations were performed with the B3LYP density functional theory (DFT) method, where Becke’s three-parameter hybrid functional and the Lee−Yang−Parr correlation functional were used.37 The 6-311+G(d,p) basis sets were used for C and O atoms, and the all-electron basis set of Wachter−Hay as modified by Gaussian was used for the iron atom.38 The geometries were fully optimized, and the harmonic vibrational frequencies were calculated with analytic second derivatives. All these calculations were performed by using the Gaussian03 program.39

pressure. After free expansion, the anions were skimmed and analyzed using a Wiley−McLaren time-of-flight mass spectrometer. The Fe(CO)n− anion species of interest were each mass selected and decelerated into the extraction region of a second collinear time-of-flight mass spectrometer, where they were dissociated by a tunable IR laser. The fragment and parent anions were reaccelerated and mass analyzed by the second time-of-flight mass spectrometer. Different from the photodissociation apparatus in the literature,35,36 the mass gate, deceleration, and reacceleration were integrated into an assembly, which consists of an array of five grid plates designated as P1, P2, P3, P4, and P5, as shown in Figure 1. The ion packet enters the array at P1, passes through P2, P3, and P4, and leaves the array from P5. The combination of P1, P2, and P3 acts as a mass gate to select the ion of interest. P3 also serves as a deceleration plate. The assembly of P3, P4, and P5 provides the two-stage acceleration field for the second time-of-flight mass spectrometer. In the experiments, the P1 and P5 plates are always grounded; a constant DC voltage that is higher than the ion energy is applied in the second plate; both P3 and P4 are applied with a constant DC voltage that is slightly lower than the ion energy. When the ion packet with the mass of interest arrives at P1, the P2 plate is pulsed to ground. The ions are decelerated by the field between P2 and P3. When the ion packet of interest just passes through P3, an additional pulsed voltage is added to P3 to deflect the higher mass ions. The pulsed IR photodissociation laser is nearly simultaneously fired and irradiated to the selected ion packet. Infrared photodissociation takes place in the region between P3 and P4. The fragment and parent anions are extracted and accelerated by the fields provided by P3, P4, and P5. The constant and pulsed voltages applied to P3 and P4 are adjusted to optimize the efficiency of 2485

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RESULTS AND DISCUSSION A typical mass spectrum of the Fe(CO)n− complexes produced by the laser vaporization supersonic cluster source is shown in Figure 2. It is composed of a progression of mass peaks that are

mass spectra for a selected complex recorded with the photodissociation laser on versus off. The IR wavelength is adjusted to the most intense resonance for each anion complex. The negative peak indicates depletion of the parent ion via photodissociation, while the positive peaks represent the fragment ions. It was found that the FeCO− anion does not dissociate when excited with focused infrared light in the 1600−2100 cm−1 region even at the highest laser pulse energy. The CO binding energy of FeCO − was determined experimentally to be 33.1 ± 3.5 kcal/mol.33 Therefore, it is not surprising that no fragmentation is observed for FeCO− as the binding energy is much greater than the energy of IR photons in the CO stretching region. The difficulty in fragmenting such small complexes has been found previously for many systems due to relatively high bond strengths and low vibrational density of states.25−28 The second, third, and fourth CO binding energies for Fe(CO)n− (n = 2−4) were determined to be even larger than that of FeCO−,33 suggesting that the n = 2−4 anion complexes are not expected to dissociate. However, it was found that all of the n = 2−4 anion complexes were able to dissociate one CO ligand by a loosely focused IR laser though with very low efficiency. The dissociation detected is very likely due to the multiphoton absorption process, which is not expected to be very efficient at the laser pulse energies used here (about 1 mJ/pulse). However, the multiphoton absorption process can be detected if the IR oscillator strength is high, as reported previously.25−28 As shown in Table 1, the predicted IR

Figure 2. Mass spectrum of the iron carbonyl anion complexes produced by pulsed laser vaporization of an iron metal target in an expansion of helium seeded by carbon monoxide.

attributed to the 56Fe(CO)n− complexes with n up to 15. All the peaks are accompanied by three weak peaks with −2, +1, and +2 mass difference, showed natural abundance Fe isotopic intensity distributions, and clearly indicate one iron atom involvement. The peak intensity increases smoothly from n = 1 to 3 and increases dramatically from n = 3 to 4. The Fe(CO)4− peak is the most intense one, indicating that this anion complex is formed preferentially with high stability. The infrared induced fragmentation mass spectra for the Fe(CO)n− (n = 3−7) complexes are shown in Figure 3. These spectra were obtained by taking the difference between the

Table 1. Comparison of the Band Positions of the Fe(CO)n− Anions Measured in the Present Work to Those Computed by Density Functional Theory (IR Intensities in Parentheses in km/mol) and to Those of the Corresponding Neutrals exptl n n n n

= = = =

1 2 3 4

n = 4 Ar tagged n=5 n=6 n=7 n=8

1782a 1735 1780 1849

calcdb 1842(2742) 1861(4862) 1913(4092) 1928(1259) 2022(27)

1861, 1872, 1978 1861, 1872, 1975, 2131 1861, 1977, 2133 1863, 1977, 2133 1864, 1977, 2133

neutral 1946.5c 1928.2c 1950d 1984, 2000c

1913(4056) 1928(1263) 2022(28) 1914(1985) 1918(1934) 1931(1185) 2022(78) 2182(166)

a

Reference 20, neon matrix value. bUnscaled values. cReferences 47 and 48. dReference 49.

intensities of the CO stretching modes of the Fe(CO)n− anion complexes increase with the coordination number. Complexes larger than n = 4 were observed to dissociate very efficiently even using an unfocused laser beam with very low laser pulse energies (about 0.2−0.3 mJ/pulse). Apparently, the dissociation proceeded via a single photon process. The fragmentation of all of these larger complexes terminates at n = 4, indicating that Fe(CO)4− has a completed coordination sphere with high stability. The extra CO(s) are coordinated to the core anion via weaker electrostatic forces. The photodissociation spectra for the strongly bound Fe(CO)n− (n = 2−4) anions obtained by monitoring the loss of one CO ligand are shown in Figure 4. The parent anions are

Figure 3. Photofragmentation mass spectra of the Fe(CO)n− anion complexes (difference between the mass spectra for a selected complex recorded with the photodissociation laser on versus off). The IR wavelength is adjusted to the most intense resonance for each anion complex. The negative peak represents the depletion of the massselected parent ion, while the positive peaks represent the resulting fragment anions. 2486

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the donation to the CO 2π* orbital also reduces the Fe−CO repulsion.30 The spectrum of the Fe(CO)4− complex exhibits a single broad band centered at 1849 cm−1, about the same as that reported previously using the FTICR apparatus with infrared free electron laser.22 The band is slightly narrower than that recorded using free electron lasers presumably because our cluster source provides colder anion complexes. Density functional calculations predicted a 2A1 ground state for Fe(CO)4− with C3v symmetry,30 as is the isoelectronic Co(CO)4 molecule.41 The anion is derived from Fe(CO)3− by adding an axial CO molecule along the molecular axis. The three equatorial CO molecules that are in the same plane in Fe(CO)3− bend out of the plane by about 10°. The 2A1 ground state of Fe(CO)4− has a 17-electron configuration. The unpaired electron is located opposite to the axial CO and between the three equatorial CO molecules to reduce the repulsion with the axial CO. The Fe(CO)4− anion with C3v symmetry is expected to have four IR active modes in the CO stretching frequency region, namely, the antisymmetric stretching of the three equatorial CO units, which is doubly degenerate, the symmetric stretching of the three equatorial CO units, and the stretching of the axial CO unit. These modes were predicted at 1913, 1928, and 2022 cm−1 at the B3LYP/6311+G* level of theory. The doubly degenerate antisymmetric stretching mode was previously observed at 1859.7 cm−1 in solid neon and at 1853.5 cm−1 in solid argon.20 The broad band observed in the IR photodissociation spectra was assumed to be an unresolved doublet involving both the doubly degenerate antisymmetric stretching and the axial CO stretching modes, which were predicted to split by only about 15 cm−1. To obtain high quality spectra for the strongly bound anion complexes, the method of rare gas tagging is used. As shown previously,24,42−46 the rare gas tagging technique makes it possible to overcome low fragmentation yields by attaching weakly bound rare gas atom(s) to otherwise strongly bound complexes. The tagged complexes fragment by the loss of a weakly bound rare gas atom. Attaching a rare gas atom such as argon usually results in a weak perturbation on the complex, causing little or no spectral shift in the vibrational spectrum. In this regard, argon seeded with CO is used as the carrier gas. It was found that the large Fe(CO)n− anions (n ≥ 4) are able to form argon-tagged complexes, whereas the small Fe(CO)n− anions (n = 1−3) failed to form argon complexes. Photodissociation of the Fe(CO)4−(Ar) complex results in the elimination of argon, as expected. Thus, a high quality spectrum of Fe(CO)4− is able to be obtained by single photon excitation to avoid power broadening. The infrared photodissociation spectra of the Fe(CO)4−(Ar) and Fe(CO)5− complexes are illustrated in Figure 5. These complexes dissociate very efficiently. The parent anions can be depleted by more than 80% with an unfocused laser beam. The tagged Fe(CO)4−(Ar) complex spectrum has lower signal level as the densities of the tagged anion complex are much less than that of the Fe(CO)4− anion. The band position in the tagged spectrum is blue-shifted by approximately 12 cm−1 from that in Fe(CO)4−. The band of the tagged Fe(CO)4−(Ar) complex is narrower than that of without tagging, as expected. As can be seen in Figure 5, two bands centered at 1861 and 1872 cm−1 can clearly be resolved, which are attributed to the doubly degenerate antisymmetric stretching of the three equatorial CO ligands and the stretching of the axial CO ligand, respectively. A much weaker band centered at 1978 cm−1 was also observed in

Figure 4. Infrared photodissociation spectra of the Fe(CO)n− anion complexes with n = 2−4, detected by the elimination of one CO molecule.

depleted by about 6% at the laser pulse energy of about 1 mJ/ pulse. The spectrum of Fe(CO)2− has a very low signal level because the density of this anion is low, and its photodissociation via loss of CO is relatively inefficient. The spectrum of the Fe(CO)2− complex exhibits a single band centered at 1735 cm−1, which is about 408 cm−1 red-shifted from the free CO stretching (fundamental at 2143 cm−1).40 The band is quite broad due to power broadening, as the dissociation is a multiphoton process because of the high binding energy of the anion complex. This band is assigned to the antisymmetric CO stretching vibration of the complex. The observation of only one band in the CO stretching frequency region suggests that the complex has a linear structure. The Fe(CO)2− anion was previously produced via codeposition of laser ablated metal atoms and electrons with CO in solid noble gas matrixes.20 In solid argon, the anion was characterized to have a bent geometry with antisymmetric and symmetric C−O stretching vibrations at 1721.9 and 1815.0 cm−1, respectively. In solid neon, the anion was suggested to have a linear structure with the antisymmetric stretching vibration observed at 1732.9 cm−1, which is just 2 cm−1 lower than the present gas phase value. Previous density functional theory calculations predicted that the Fe(CO)2− anion has a linear 2Πu doublet ground state with a bent 2A1 state and a linear 4Πu state slightly higher in energy.20,30As has been discussed,30 both the linear 2Πu and 4Πu states of Fe(CO)2− have a Fe 3d74s1 occupation. That is, the bonding is Fe°(CO)2− in character, where the electron on the (CO)2 subunit is in the CO 2π* orbital. The dissociation efficiency increases by a factor of 2 in going from Fe(CO)2− to Fe(CO)3−. The spectrum of Fe(CO)3− also shows only one band centered at 1780 cm−1. This mode corresponds to the doubly degenerate antisymmetric CO stretching vibration of the anion with D3h symmetry. The same mode was observed at 1786.5 cm−1 in solid argon and at 1794.5 cm−1 in solid neon.20 The Fe(CO)3− anion was predicted to have a 2A1′ ground state with D3h symmetry, which was confirmed from the infrared spectrum of the mixed isotopically substituted molecule.20,30 The ground state complex can be viewed as being derived from a 3d9 occupation of Fe−. The D3h structure is very favorable as it minimizes the ligand−ligand repulsion, and the polarization of the 3d orbitals that maximizes 2487

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with an antisymmetric stretching frequency of 1950 cm−1 in the gas phase.49 The Fe(CO)4 molecule has C2v symmetry with three IR-active absorptions. Two of them were observed at 1984 and 2000 cm−1 in the gas phase.49 The band positions measured for the Fe(CO)n− anions (n = 2−4) in the present study are about 130−200 cm−1 lower than those of the corresponding neutrals. As discussed earlier,8 the carbonyl frequency shift results from the combined effects of σ donation, electrostatic polarization, and π back-bonding. σ donation and electrostatic interaction are expected to be strong for cationic carbonyl complexes, as these cause charge transfer from the CO toward the metal. In contrast, π back-donation plays a more important role for the neutral and anionic carbonyl complexes. The transfer of metal charge to the CO π* antibonding orbital results in a weakening of the CO bond and hence a decrease in the CO stretching vibrational frequency. Clearly, an anion is expected to donate more electrons than a neutral, which in turn should donate more electrons than a cation. Cationic carbonyls have very small red shifts, and nonclassical carbonyls can also have blue-shifted CO stretching frequencies.8,16,24,25

Figure 5. Infrared photodissociation spectra of the Fe(CO)4−(Ar) and Fe(CO)n− (n = 5−8) anion complexes, detected by the elimination of Ar or CO molecule(s).

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the tagged spectrum. This band is due to the symmetric stretching mode of the three equatorial CO ligands in Fe(CO)4−. This mode was predicted to have very low IR intensity (27 km/mol) relative to the other two modes (4092 and 1259 km/mol, respectively, see Table 1). The spectrum for the Fe(CO)5− anion complex obtained by monitoring the loss of one CO is about the same as that of the tagged Fe(CO)4−(Ar) complex, indicating that both argon and the fifth CO are weakly bound to the Fe(CO)4− core anion. The three CO stretching modes of the Fe(CO)4− core in the Fe(CO)5− complex are located at 1861, 1872, and 1975 cm−1, respectively. Besides the core absorptions, a weak band at 2131 cm−1 was also observed for the Fe(CO)5− complex. This band is slightly red-shifted from that of free CO and is due to the absorption of the weakly bound CO ligand. The observation of a weakly bound CO ligand in Fe(CO)5− is in accord with a tetra-coordinated Fe(CO)4− as the core anion. DFT calculations on the Fe(CO)5− complex predicted a weakly bound Fe(CO)4−−CO complex with the fifth CO loosely bound to the iron center opposite to the axial CO of the Fe(CO)4− core. The Fe−CO distance was predicted to be 3.78 Å. The structure of Fe(CO)4− remains almost unchanged upon the weak CO coordination. The spectra for the complexes larger than n = 5 are also shown in Figure 5. Similar to Fe(CO)5−, these complexes have the spectrum of the Fe(CO)4− core anion that is progressively solvated by external CO molecules. All the spectra of the complexes with n = 6−8 are very similar. The two modes at 1870 cm−1 cannot be resolved. There is a gradual blue-shift of the band positions with increased CO number due to increased solvation interaction. A comparison of the band positions of the anions measured in the present work to those computed by density functional theory and to those of the corresponding neutrals is presented in Table 1. The 3Σ− ground state FeCO neutral was determined to have a CO stretching frequency at 1946.5 cm−1 in the gas phase.47 The spectrum of the FeCO− anion was not obtained in the present study. This anion has been identified by 1782 cm−1 frequency in solid neon,20 about 174 cm−1 lower than that of the FeCO neutral. The antisymmetric stretching vibration of linear dicarbonyl neutral Fe(CO)2 was measured at 1928.2 cm−1 in the gas phase. Fe(CO)3 is trigonal with C3v symmetry

CONCLUSIONS Mononuclear iron carbonyl anion complexes of the form Fe(CO)n− with n = 2−8 as well as the argon tagged Fe(CO)4−(Ar) complex were produced and studied using the infrared photodissociation spectroscopy in the carbonyl stretching frequency region. The FeCO− anion does not fragment when excited with infrared light. Only a single IR active band was observed for the Fe(CO)2− and Fe(CO)3− anions, which were characterized to have linear D∞h and planar D3h symmetry, respectively. The 17-electron Fe(CO)4− anion is the most intense peak in the mass spectra and was characterized to have a completed coordination sphere with high stability. Anion clusters larger than n = 4 were determined to involve a Fe(CO)4− core anion that is progressively solvated by external CO molecules. Although only one band was observed in the photodissociation spectrum of Fe(CO)4−, all three IR-active CO stretching vibrational fundamentals of the Fe(CO)4− core anion were resolved in the spectra of the argon-tagged Fe(CO)4−(Ar) complex as well as the larger Fe(CO)5− complex, and these confirmed that the Fe(CO)4− anion has a C3v structure. All of the carbonyl stretching frequencies of the Fe(CO)n− anion complexes are red-shifted with respect to those of the corresponding neutrals.

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AUTHOR INFORMATION

Corresponding Author

*Fax: (+86) 21-6564-3532. E-mail: [email protected].

ACKNOWLEDGMENTS This work was supported by NKBRSF (2010CB732306) and NSFC (20933030 and 21173053). REFERENCES

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dx.doi.org/10.1021/jp211936b | J. Phys. Chem. A 2012, 116, 2484−2489