Article pubs.acs.org/JPCA

Infrared Photodissociation Spectroscopy of Mass-Selected Heteronuclear Iron−Copper Carbonyl Cluster Anions in the Gas Phase Ning Zhang,† Mingbiao Luo,† Chaoxian Chi,*,† Guanjun Wang,‡ Jieming Cui,‡ and Mingfei Zhou*,‡ †

Department of Applied Chemistry, East China Institute of Technology, Nanchang, Jiangxi Province 330013, China Department of Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai 200433, China

‡

S Supporting Information *

ABSTRACT: Mass-selected heteronuclear iron−copper carbonyl cluster anions CuFe(CO)n− (n = 4−7) are studied by infrared photodissociation spectroscopy in the carbonyl stretching frequency region in the gas phase. The cluster anions are produced via a laser vaporization supersonic cluster ion source. Their geometric structures are determined by comparison of the experimental spectra with those calculated by density functional theory. The experimentally observed CuFe(CO)n− (n = 4−7) cluster anions are characterized to have (OC)4Fe−Cu(CO)n−4 structures, each involving a C3v symmetry Fe(CO)4− building block. Bonding analysis indicates that the Fe− Cu bond in the CuFe(CO)n− (n = 4−7) cluster anions is a σ type single bond with the iron center possessing the most favored 18-electron configuration. The results provide important new insight into the structure and bonding of hetronuclear transition metal carbonyl cluster anions.

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INTRODUCTION Transition metal−carbonyl complexes are of great importance in inorganic and organometallic chemistry.1−3 They are prototypical examples of metal−metal and metal−ligand bonding and play a vital role in many catalytic processes.4−6 Homonuclear iron and copper carbonyls have been extensively studied both experimentally and theoretically. The Fe(CO)5, Fe2(CO)9, and Fe3(CO)12 compounds are well-known stable iron carbonyls.7−15 The radical anion Fe3(CO)11−, the dianions Fe3(CO)112− and Fe4(CO)132−, and the cations Cu(CO)n+ (n = 1−4) have been prepared in the condensed phase as salts with counterions and characterized spectroscopically.16−22 Unsaturated homoleptic iron and copper carbonyl neutrals as well as ions have been prepared in solid noble gas matrices at cryogenic temperatures and characterized by various spectroscopic methods including infrared absorption (IR), UV−visible, and electron spin resonance (ESR) spectroscopy.23−40 Their geometric and electronic structures are determined, and their bonding is discussed. Theoretical calculations indicate that CuCO has a bent structure and that the bond parameters and CO binding energies of copper carbonyls are very sensitive to the functionals and the size of basis sets employed.41,42 Sequential bond dissociation energies of Fe(CO)n− (n = 1−4), Fe(CO)n+ (n = 1−5), and Cu(CO)n+ (n = 1−4) were determined by collision-induced dissociation in the gas phase.43−45 The bond energies of neutral carbonyls Fe(CO)n (n = 1−4) were estimated from the bond energies and electron detachment energies of the anions.43 Metal−carbonyl bond © XXXX American Chemical Society

dissociation energies of multinuclear iron carbonyl cations Fe1−3(CO)n+ (n = 1−6) and Fe2(CO)n+ (n = 1−9) were also measured by laser ion-beam photodissociation studies.46,47 The dissociation energy of the CuCO complex was estimated via kinetic study in the gas phase.48 These studies indicate that the copper−carbonyl bonds are very weak relative to the iron− carbonyl bonds. The electron affinities of Fe(CO)n (n = 1−4) and Cu(CO)n (n = 2−3) were determined by negative anion photoelectron spectroscopy in the gas phase.49−51 Although the electron binding energy of Cu is much higher than that of Fe, the electron affinities of iron carbonyls are higher than those of copper carbonyls.52 Recently, mass-selected homoleptic mononuclear and multinuclear iron and copper carbonyl cluster cations and anions were investigated by infrared photodissociation spectroscopy in the gas phase. The electronic and geometric structures of these carbonyl ions were determined by comparison of the experimental spectra with simulated spectra derived from density functional calculations.53−58 The results show that both Cu+ and Fe− can coordinate up to four CO ligands and that there is no metal− metal multiple bonding in the multinuclear metal carbonyl ions. Compared to the homonuclear iron and copper carbonyls, less attention has been paid to the heteronuclear iron−copper carbonyls. Clusters of the composition [Cu3{Fe(CO)4}3]3−, Received: March 13, 2015 Revised: April 11, 2015

A

DOI: 10.1021/acs.jpca.5b02442 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A [Cu5{Fe(CO)4}3]3−, and [Cu6{Fe(CO)4}3]2− have been synthesized in the condensed phase and characterized by the X-ray and NMR methods.59−61 Their electronic and geometric structures have been analyzed by density functional calculations.62 It was found that all the CO ligands are bound to Fe atoms. The Fe(CO)4 groups act as μ2- or μ3-ligands which formally correspond to two or four electron donors, respectively. The Fe(CO)4 groups can also act as Lewis acids by taking up two electrons due to the presence of low-lying empty levels. These Fe−Cu heterobimetallic carbonyl anions can be classified as Fe−Cu bimetallic clusters stabilized by CO ligands or as copper cores surrounded by Fe(CO)4 moieties. The CO ligands strongly affect the electronic structure of the bimetallic clusters and induce a noticeable charge transfer from Cu to Fe. As far as we know, there is no spectroscopic report on heterobimetallic iron−copper carbonyls in the gas phase. In the present paper, heteronuclear Fe−Cu carbonyl cluster anions are produced in the gas phase. The anions of interest are each mass-selected and studied by infrared photodissociation spectroscopy. The cluster structures are assigned, and structural and bonding characters are identified by comparison of the experimental spectra with simulated spectra derived from density functional calculations.

Quantum chemical calculations were performed to determine the molecular structures and to support the assignment of vibrational frequencies of the cluster anions studied. Geometry optimization and harmonic vibrational frequency analysis were performed using the hybrid B3LYP density functional theory (DFT) method in combination with the 6-311+G(d) basis sets.64−67 The B3LYP functional is the most popular density functional method and can provide reliable predictions on the structures and vibrational frequencies of transition-metalcontaining compounds.68,69 It was found that this functional provides the best agreement between theory and the available experimental data for the copper carbonyl complexes.42 Geometry optimizations were performed on various possible structures for each species. All the calculations were performed using the Gaussian 09 program.70 The computed harmonic vibrational frequencies were scaled by a factor of 0.9754 that was determined to give the vibrational frequencies closest to the experimental vibrational frequencies of the CuFe(CO)n− cluster anions studied. Theoretical predicted IR spectra were obtained by applying Lorentzian functions with the scaled theoretical harmonic vibrational frequencies and IR intensities and giving a 5 cm−1 full width at half-maximum (fwhm).

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RESULTS AND DISCUSSION A typical mass spectrum of iron−copper carbonyl cluster anions produced by laser vaporization supersonic cluster source in the m/z range of 100−450 amu is shown in Figure 1. The

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EXPERIMENTAL AND COMPUTATIONAL METHODS The infrared photodissociation spectra of the heteronuclear Fe−Cu carbonyl cluster anions were measured using a collinear tandem time-of-flight mass spectrometer equipped with a laser vaporization supersonic cluster source. The experimental apparatus has been described in detail previously.63 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 copper metal target. The laser beam with 5−8 mJ/ pulse is focused by a lens with a focal length of 250 mm. The heteronuclear Fe−Cu carbonyl cluster anion complexes were produced from the laser vaporization process in expansions of helium gas seeded with 2−4% CO using a pulsed valve (General Valve, series 9) at 0.6−0.8 MPa backing pressure. The iron carbonyl species were most likely formed in the ion source which is made of stainless steel via scatted light vaporization or ion sputtering. After free expansion, the anions were skimmed and pulse-extracted and analyzed using a Wiley−McLaren timeof-flight mass spectrometer. The cluster anions of a specific mass were selected by their flight time 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 collinear time-of-flight mass spectrometer. Infrared photodissociation spectra were obtained by monitoring the total fragment ion yields 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 is generated by a KTP/KTA/AgGaSe2 optical parametric oscillator/amplifier system (OPO/OPA, Laser Vision) pumped by the fundamental of a pulsed Nd:YAG laser (1064 nm; Surelite EX). The infrared laser with pulse energies ranging from 0.2 to 1.5 mJ/pulse and a line width of about 2 cm−1 is either unfocused (for weakly bound anion complexes) or loosely focused by a CaF2 lens.

Figure 1. Mass spectrum of heteronuclear iron−copper carbonyl cluster anions formed by pulsed laser vaporization of copper metal target in an expansion of helium seeded by carbon monoxide.

mass spectrum is mainly composed of progressions of mass peaks due to mononuclear iron carbonyl anion Fe(CO)4−, bimetallic carbonyl anions CuFe(CO)n− (n = 4−7), and** Cu2Fe(CO)m− (m = 6−8). Peaks due to complexes with an H 2 O impurity such as Fe(CO) 4 (H 2 O) − and CuFe(CO)5(H2O)− as well as metal carbide carbonyl anions FeC(CO)4− and CuFeC(CO)5− are also presented in the spectrum. For each species, the isotopic splittings of iron and copper can clearly be resolved and their relative intensities match the natural abundance isotopic intensity distributions. The most notable feature of the spectrum is that no homonuclear copper carbonyl cluster anions were observed. All the metallic carbonyl cluster anions contain one or more iron atoms, and the most intense peak in the spectrum corresponds to Fe(CO)4−. The preferred formation of iron carbonyls can be rationalized by the quite different metal− B

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The Journal of Physical Chemistry A carbonyl bond strength between iron and copper in the carbonyl anion species. The binding energies of Fe(CO)n− (n = 1−4) were experimentally determined to be larger than 120 kJ· mol−1,43 while those of Cu(CO)n− (n = 1−3) were predicted to be only 13.8, 92.0, and 77.3 kJ·mol−1, respectively.42 The heteronuclear carbonyl cluster anions of interest are each mass-selected and subjected to infrared photodissociation. For each cluster, only the mass peak corresponding to the 56 63 Fe Cu isotopomer is selected. When the IR laser is on resonance with the CO stretching of the cluster anions, photofragmentation of the anions involving the loss of one or more CO ligands is observed. The infrared induced fragmentation mass spectra for selected cluster anion complexes are shown in Figure 2. These spectra were obtained by taking

Figure 3. Experimental and simulated vibrational spectra of the CuFe(CO)4− cluster anion in the carbonyl stretching frequency region. The experimental spectrum (Exptl.) was measured by monitoring the fragmentation channels leading to the formation of CuFe(CO)2−, CuFe(CO)3−, and Fe(CO)4−. The simulated spectra (a−c) were obtained from the scaled harmonic frequencies and intensities for the three low-lying energy structures (Figure 4, 1a−1c) calculated at the B3LYP/6-311+G(d) level.

1870 cm−1.59−61 The [Cu6{Fe(CO)4}3]2− salts have higher carbonyl stretching vibrational frequencies than those of CuFe(CO)4− because of the fact that each Fe(CO)4 unit in [Cu6{Fe(CO)4}3]2− has less negative charge than that in CuFe(CO)4−. The most stable structure of CuFe(CO)4− was predicted to have a singlet ground state (1A1) with C3v symmetry, in which all the carbonyl ligands are bonded to the Fe center (Figure 4, 1a). A second structure with C3v symmetry (1b) having one carbonyl bonded to the Cu center and three carbonyls bonded to the Fe center was predicted to be 142.0 kJ·mol−1 higher in energy than the most stable isomer. A third isomer with Cs symmetry (1c) involving one bridging carbonyl lies 180.2 kJ· mol−1 higher in energy than the most stable structure. The second and third structures (1b and 1c) with both metal centers bonded by carbonyl can be ruled out because of their relatively high energy. The calculated infrared spectra for these stable structures are compared with the experimental spectrum in Figure 3. Obviously, only the calculated spectrum of the most stable isomer (1a) matches the experiment. The observed and calculated band positions are listed in Table 1. The 1841 cm−1 band is assigned to the doubly degenerate antisymmetric stretching vibration of the three equatorial CO ligands. The 1937 cm−1 band is attributed to the corresponding symmetric stretching mode. The axial CO stretching mode was predicted to be only 14 cm−1 higher than the antisymmetric stretching mode. This mode cannot be well-resolved because of band overlap. CuFe(CO)5−. The infrared spectrum of the CuFe(CO)5− cluster anion obtained by monitoring the loss of one CO ligand is shown in Figure 5. The parent anions can be depleted by about 15% at the laser pulse energy of 1 mJ/pulse. The spectrum exhibits three well-resolved bands centered at 1837, 1943, and 2055 cm−1, together with a partially resolved shoulder at 1873 cm−1. The spectral feature suggests that the CuFe(CO)5− cluster anion has a (CO)4Fe−CuCO structure by adding the fifth CO to the copper center of the most stable structure of CuFe(CO)4−. The 1837, 1873, and 1943 cm−1 bands are attributed to the CO stretching vibrations of the

Figure 2. Photofragmentation mass spectra of the CuFe(CO)n− (n = 4−7) cluster anions (difference between the mass spectra for a selected complex recorded with the photodissociation laser on vs off). The negative peak represents the depletion of the mass-selected parent ion, while the positive peaks represent the resulting fragment anions.

the difference between the mass spectra for a selected complex recorded with the photodissociation laser on versus off. The negative peak indicates depletion of the parent ion via photodissociation, while the positive peaks represent the fragment ions. The larger complexes fragment more efficiently than the smaller complexes. The FeCu(CO)4− and FeCu(CO)5− anions fragment only under focused IR laser irradiation, indicating that multiphoton absorption is necessary. As shown in Figure 2, the FeCu(CO)4− complex is able to lose up to two CO ligands or one copper atom, while the FeCu(CO)5− complex only loses one CO. The FeCu(CO)6− and FeCu(CO)7− complexes dissociate quite efficiently even with an unfocused IR laser. CuFe(CO)4−. Infrared photodissociation spectrum for the CuFe(CO)4− anion is shown in Figure 3. The spectrum has very low signal level because of poor photodissociation efficiency. The parent anions can be depleted by only about 6% at the laser pulse energy of about 1 mJ/pulse. The observation of the fragmentation channel via the loss of a copper atom suggests that the CuFe(CO)4− cluster anion has a (CO)4Fe−Cu structure, in which all the four CO ligands are coordinated on the Fe center. The infrared spectrum exhibits two broad bands centered at 1937 and 1841 cm−1. The observed spectrum is similar to that of [Cu6{Fe(CO)4}3]2− salts, in which the Fe(CO)4 group has nearly C3v symmetry with the carbonyl stretching frequencies observed at 1970 and C

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Table 1. Comparison of the Band Positions of the CuFe(CO)n− (n = 4−7) Cluster Anions Measured in the Present Work to Those Computed by Density Functional Theory at the B3LYP/6-311+G(d) Level (IR Intensities Are Listed in Parentheses in km·mol−1) calcda (cm−1) −1

exptl (cm ) n=4

1841

n=5

1937 1837

n = 6b

1873 1943 2055 1844

n=7

1875 1941 1952 2043 2073 2089 2134 1842 1860 1944 2050 2093

1870 1884 1957 1841 1848 1873 1941 2059 1848 1851 1869 1936

(1735 × 2) (423) (1071) (1862) (1702) (470) (1626) (1009) (1679) (1267) (69) (2125)

1834 (2001) 1848 (1703) 1873 (500) 1942 (1587)

2021 (1850) 2062 (986) 2069 (402) 2152 (92) 1861 1876 1941 2023 2076

(1343 × 2) (33) (2147) (1421 × 2) (241)

a

The calculated harmonic vibrational frequencies were scaled by a factor of 0.9754. bThe calculated values are for the CuFe(CO)6− (Cs, 1 A′, left column) and CuFe(CO)6− (C1, 1A, tagged isomer, right column), respectively. Figure 4. Geometries and selected bond lengths (in angstrom) for the low-lying CuFe(CO)n− (n = 4−7) isomers calculated at the B3LYP/6311+G(d) level. The relative energies (ΔE) are given in kJ·mol−1.

Fe(CO)4 fragment, while the 2055 cm−1 band is due to the carbonyl stretching vibration of the CuCO subunit. Quantum chemical calculations were performed on various possible structures for CuFe(CO)5−. Four stable isomers were found, as shown in Figure 4 (2a−2d). The first three isomers each have four carbonyls on the Fe center and one carbonyl on the Cu center. The first structure (2a) is the most stable one among these isomers. It was predicted to have a singlet ground state without symmetry, which can be viewed as being built from the C3v (1A1) ground state structure of CuFe(CO)4− by adding the fifth CO to the copper center along the Fe−Cu bond. The Fe(CO)4 moiety retains a slightly distorted C3v structure. The second structure (2b) with C2v symmetry involving a C2v symmetry Fe(CO)4 moiety was predicted to be only 2.0 kJ·mol−1 higher in energy than the first structure. The third isomer (2c) can be viewed as a distorted C2v structure. It was predicted to have a triplet ground state with Cs symmetry, which lies 98.5 kJ·mol−1 higher in energy than the first structure. The last isomer (2d) with three carbonyls on the Fe center and two carbonyls on the Cu center was predicted to be 212.2 kJ·mol−1 less stable than the most stable isomer. The simulated spectra are compared with the experimental spectrum in Figure 5. Apparently, the spectrum of the first isomer fits the experiment better than the other structures. The observed and calculated band positions are listed in Table 1.

Figure 5. Experimental and simulated vibrational spectra of the CuFe(CO)5− cluster anion in the carbonyl stretching frequency region. The experimental spectrum was measured by monitoring the CO fragmentation channel leading to the formation of CuFe(CO)4−. The simulated spectra (a−d) were obtained from scaled harmonic frequencies and intensities for the structures (Figure 4, 2a−2d) calculated at the B3LYP/6-311+G(d) level.

CuFe(CO)6−. Figure 6 shows the infrared photodissociation spectrum of the CuFe(CO)6− cluster anion obtained by monitoring the loss of one CO ligand using an unfocused laser beam. The parent anions were depleted by about 40%. The bands centered at 1844, 1875, 1941, 1952, 2043, 2073, D

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vibration of the weakly bound CO. The remaining four bands are originated from the CuFe(CO)5− core ion, which are slightly shifted from those of CuFe(CO)5−. The only disagreement between the experimental and simulated spectra is the pattern around 1844 cm−1. The tagged structure was predicted to have two separated antisymmetric stretching modes of the three equatorial CO ligands at 1834 and 1848 cm−1. These two modes are degenerate in the C3v symmetry CuFe(CO)5− anion. As a result of symmetry reduction by CO tagging, the double degeneracy is lifted, and the degenerate mode splits into two distinct modes. Calculations at the B3LYP level gave a splitting of 14 cm−1. Experimentally, only one broad band centered at 1844 cm−1 was observed, indicating that the experimental mode splitting is smaller than the calculated value. The disagreement is due to computational difficulties in describing this weakly bound complex at the DFT level. CuFe(CO)7−. The CuFe(CO)7− cluster anion fragments by losing CO ligands very efficiently with more than 50% depletion of the parent anions using an unfocused laser beam. The resulting infrared spectrum is shown in Figure 7,

Figure 6. Experimental and simulated vibrational spectra of the CuFe(CO)6− cluster anion in the carbonyl stretching frequency region. The experimental spectrum was measured by monitoring the CO fragmentation channel leading to the formation of CuFe(CO)5−. The simulated spectrum (a−d) were obtained from the scaled harmonic frequencies and intensities for the structures (Figure 4, 3a− 3d) calculated at the B3LYP/6-311+G(d) level.

2089, and 2134 cm−1 can be clearly resolved. The observation of eight bands indicate that more than one isomer is experimentally observed, as any CuFe(CO)6− structure should at most have six vibrational fundamentals in the CO stretching region. The observation of a weak band at 2134 cm−1 that is slightly red-shifted from free CO (2143 cm−1) suggests the involvement of a weakly bound CO···CuFe(CO)5− complex, i.e., CO tagged CuFe(CO)5− anion complex. Four structures were found starting with various possible geometries of CuFe(CO)6−, and the results are shown in Figure 4. The most stable structure (3a) was found to have an asymmetric unbridged (OC)4Fe−Cu(CO)2 structure with Cs symmetry. In this structure, the Fe(CO)4 moiety has slightly distorted C3v symmetry and the Cu(CO)2 moiety lies in the molecular mirror plane. The second lowest structure (3b) is characterized to be a weakly bound CO···CuFe(CO) 5 − complex with the sixth CO loosely tagged on the Fe(CO)4 moiety. This structure was predicted to be 11.3 kJ·mol−1 higher in energy than the first isomer. The CuFe(CO)5− core anion has a very similar structure as the most stable structure of CuFe(CO)5−. Both the third and fourth structures (3c and 3d) have Cs symmetry with each metal center bonded by three carbonyl ligands. The third isomer with the Fe(CO)3 moiety bending away from the Cu(CO)3 fragment has a triplet ground state (3A″) and lies 113.7 kJ·mol−1 higher in energy than the most stable structure. The fourth isomer with a singlet ground state (1A′) lies 192.7 kJ·mol−1 above the most stable isomer. The calculated infrared spectra for these low-lying isomers are compared with the experimental spectrum in Figure 6. Apparently, the coexistence of the first two isomers can explain the observed spectrum. The bands centered at 1844, 1941, 2043, and 2089 cm−1 are attributed to different CO stretching vibrational modes of the most stable structure of CuFe(CO)6−. The 1844 and 1941 cm−1 bands are originated from the vibrations of the Fe(CO)4 moiety, while the bands centered at 2043 and 2089 cm−1 are attributed to the antisymmetric and symmetric stretching vibrations of the Cu(CO)2 fragment (Table 1). The experimentally observed bands centered at 1844, 1875, 1952, 2073, and 2134 cm−1 are assigned to the vibrational fundamentals of the CO tagged isomer (the second structure). The 2134 cm−1 band is due to the stretching

Figure 7. Experimental and simulated vibrational spectra of the CuFe(CO)7− cluster anion in the carbonyl stretching frequency region. The experimental spectrum was measured by monitoring the CO fragmentation channels leading to the formation of CuFe(CO)5− and CuFe(CO)6−. The simulated spectrum (a) was obtained from the scaled harmonic frequencies and intensities for the lowest energy structure (Figure 4, 4a) calculated at the B3LYP/6-311+G(d) level.

which has four bands at 1842, 1944, 2050, and 2093 cm−1. The CuFe(CO)7− cluster anion was predicted to have a staggered C3v structure containing a Fe(CO)4 fragment and a Cu(CO)3 group (Figure 4, 4a). The eclipsed C3v structure, which was calculated to have an imaginary frequency, is not a minimum. The calculated infrared spectrum of the staggered C3v structure is shown in Figure 7, which agrees well with the experimental spectrum. The observed and calculated band positions are listed in Table 1. The 1842 and 1944 cm−1 bands belong to the antisymmetric and symmetric stretching vibrations of the three equatorial CO units in the C3v Fe(CO)4 group. The axial CO stretching mode of the Fe(CO)4 group was predicted to be only 15 cm−1 higher than the antisymmetric stretching mode with quite low IR intensity (33 km·mol−1, Table 1) and thus cannot be resolved. The bands centered at 2050 and 2093 cm−1 belong to the antisymmetric and symmetric stretching vibrations of the Cu(CO)3 group. Discussion. The dissociation energies of the CuFe(CO)n− (n = 4−7) cluster anions calculated at the B3LYP/6-311+G(d) level are listed in Table 2. The energies required to remove one E

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Table 2. Bond Dissociation Energies at 0 K (with ZPE Correction, in kJ·mol−1) Calculated at the B3LYP/6-311+G(d) Level of Theory for the Most Stable Structure of the CuFe(CO)n− (n = 4−7) Cluster Anions structure CuFe(CO)4− CuFe(CO)5− CuFe(CO)6− CuFe(CO)7−

1

(C3v, A1) (C1, 1A) (Cs, 1A′) (C3v, 1A1)

ΔE(−CO)a

ΔE(−CO)b

ΔE(Fe−Cu)c

ΔE(Fe−Cu)d

164.7 223.6 226.1 148.8

81.9 14.2 (2.9e) 35.1

160.9 211.9 135.9 104.1

263.3 335.2 305.1 221.1

a

Energy to dissociate one CO from the Fe(CO)4 moiety. bEnergy to dissociate one CO from the Cu(CO)n−4 moiety. cEnergy to break the Cu−Fe bond to form Fe(CO)4− + Cu(CO)n−4. dEnergy to break the Cu−Fe bond to form Fe(CO)4 + Cu(CO)n−4−. eThe calculated values are for the COtagged CuFe(CO)6− (C1, 1A) cluster anion.

Table 3. Bonding Character of the CuFe(CO)n− (n = 4−7) Cluster Anions NBO group charge structure CuFe(CO)4− CuFe(CO)5− CuFe(CO)6− CuFe(CO)7−

(C3v, 1A1) (C1, 1A) (Cs, 1A′) (C3v, 1A1)

natural charge

Cu−Fe bond distance (Å)

Fe(CO)4

Cu(CO)n−4

Fe

Cu

2.337 2.290 2.442 2.628

−1.26 −1.42 −1.25 −1.09

0.26 0.42 0.25 0.09

−2.21 −2.30 −2.16 −2.04

0.26 0.37 0.09 −0.36

Duncanson model, the metal carbonyl bonding involves two main interactions.1,3,23 The first is the σ donation, in which the CO ligand donates its lone pair 5σ electrons into the metal vacant orbital. The second is π back-donation, in which the metal donates filled d electron density into the empty 2π antibonding orbitals of CO. The π back-donation tends to weaken the CO bond and decreases the CO stretching frequency. The σ donation has a much weaker impact on the carbonyl stretching than π back-donation, as the 5σ orbital of CO is largely nonbonding in character. In the CuFe(CO)n− (n = 4−7) cluster anions, all the carbonyl stretching frequencies are red-shifted from that of free CO (Table 1). The carbonyl stretching frequencies of the Fe(CO)4 moiety are much lower than those of the Cu(CO)n−4 moiety, implying that the π back bonding of Fe-CO is stronger than that of Cu−CO. The CO stretching vibrational frequencies of the Fe(CO)4 moiety are very close (red-shifted by 0−40 cm−1) to those of the corresponding modes of the gas phase Fe(CO)4− anion (1861, 1872, and 1978 cm−1).57 In contrast, the vibrational frequencies of the Cu(CO)n−4 subunit are slightly blue-shifted from those of the neutral Cu(CO)x (x = 1−3) complexes in a neon matrix (CuCO, 2029.7 cm−1; Cu(CO)2, 1904.4 cm−1; Cu(CO)3, 1994.3 cm−1).40 Accordingly, the CuFe(CO)n− (n = 4−7) cluster anions can be regarded as being formed via the interactions between a Fe(CO)4− fragment and a Cu(CO)n−4 fragment. Consistent with this notation, natural bond orbital analysis76 shows that the negative charge is mainly located on the Fe(CO)4 group and that the Cu(CO)n−4 group exhibits some positive charge (Table 3). This can be rationalized in terms of different electron affinities of the Fe(CO)4 and Cu(CO)n (n = 0−3) complexes. The electron affinity of Fe(CO)4 is experimentally determined to be 2.4 ± 0.3 eV,49 which is significantly higher than those of Cu (1.23 eV), Cu(CO)2 (0.95 eV), and Cu(CO)3 (1.02 eV).51 The experimental electron affinity is unavailable for CuCO. Theoretical calculations gave a value of 0.73 eV, which is also smaller than that of Fe(CO)4. Note that the natural charge of Fe center is about 1e larger than the group charge of the Fe(CO)4− moiety. Thus, there is net electron density transfer from CO to Fe, which implies a strong Fe−CO σ donation interaction in the Fe(CO)4− moiety. The charge at copper is close to that in the Cu(CO)n−4 moiety, indicating that σ

CO ligand coordinated to iron or copper and the energies required to break the Fe−Cu bond for the experimentally observed structures of the CuFe(CO)n− (n = 4−7) cluster anions are summarized. In general, the dissociation energy for the elimination a CO ligand decreases with increasing the cluster size, consistent with the experimental observation that the larger cluster anions dissociate more efficiently than the small anions. For the CuFe(CO)4− anion, the dissociation energy for loss of a Cu atom was predicted to be 160.9 kJ·mol−1 (Table 2), slightly lower than that of the CO fragmentation channel (164.7 kJ·mol−1), in agreement with the experimental observation of the Cu atom fragmentation channel. For the other anions, the energy required to dissociate a CO ligand is lower than that of Fe−Cu bond dissociation. The calculated dissociation energies of CuFe(CO)n− (n = 4−5) are higher than the IR photon energies in the CO stretching frequency region. Therefore, the dissociation signal detected is likely due to multiphoton processes. Multiphoton absorption is not expected to be very efficient at the low laser energy used here (about 1 mJ/pulse) but can be detected if the IR oscillator strength is high, as reported previously.54−57 The vibrational frequencies recorded via multiphoton process are expected to be slightly red-shifted from those obtained by single photon dissociation. Our previous study found that the band position in the multiphoton dissociation spectrum of Fe(CO)4− is redshifted by approximately 12 cm−1 from that in the single photon dissociation spectrum of argon-tagged Fe(CO)4−(Ar) complex.57 The energy required to remove a CO ligand from CuFe(CO)n− (n = 6, 7) is comparable to the mid-IR photon energies, suggesting single photon processes for these anions. The CuFe(CO)n− (n = 4−7) cluster anions are characterized to have (OC)4Fe−Cu(CO)n−4 structures each involving a Fe(CO)4 moiety, which are similar to those of Fe2(CO)n− (n = 4−7) cluster anions.55 The Fe(CO)4 moiety is a common structural unit in these metal carbonyl cluster anions. In the mixed iron−coinage metal carbonyl cluster compounds in the condensed phase, Fe(CO)4 is also characterized to be a common building block that stabilizes the whole clusters.59−61,71−75 The CO stretching vibrational frequencies in metal carbonyls are associated with the strength of the bonding interactions between metal and CO. According to the Dewar−Chatt− F

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The Journal of Physical Chemistry A donation is comparable with π back-donation in the Cu−CO bonding, which is much weaker than that of the Fe−CO bonding. The Fe(CO)4− anion was experimentally determined to have a metal carbonyl bond dissociation energy of 174.3 ± 10.3 kJ·mol−1,43 significantly larger than that of CuCO (25.1 ± 5.0 kJ·mol−1).48 The experimentally observed CuFe(CO)n− (n = 4−7) cluster anions are characterized to have Fe−Cu bonded structures, which also correspond to the computed lowest energy structures. (For n = 6, the slightly less stable weakly bound CO tagged complex structure is also observed.) The Cu−Fe bond can be regarded as a formal single bond with the iron center possessing the most favored 18-electron configuration. In this regard, the Cu−Fe single bonded CuFe(CO)7− cluster anion has a completed coordination sphere with both the iron and copper centers having the 18-electron configurations. The frontier bonding molecular orbitals accounting for the Cu−Fe bonding in the CuFe(CO)n− (n = 4−7) cluster anions are shown in Figure 8. Each anion has one doubly occupied Fe−Cu

= 6 and 7 anions were predicted to have a Fe−Cu bond length of 2.443 and 2.628 Å, respectively. In these two anions, the Fe− Cu bonding orbital is formed between the hybrid orbital of Fe and a Cu 4s and 4p hybrid orbital. The sp hybridization can efficiently reduce the σ repulsion and increase both the CO to Cu σ donation and the Cu (4p) to CO π back-donation. As shown in Figure 8, the Fe−Cu bonding orbitals for the n = 6 and 7 anions are delocalized, comprising notable copper to CO back bonding. Therefore, the electron density on the Fe−Cu bond is reduced, which leads to the weakening of the Fe−Cu bond. For the n = 5 anion, such sp hybridization was not observed, as the cost of s to p promotion cannot be paid by a single Cu-CO bonding.

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CONCLUSIONS Heteronuclear iron−copper carbonyl cluster anions of the form CuFe(CO)n− (n = 4−7) are produced via a laser vaporization supersonic cluster ion source. The anions are each massselected and their infrared spectra are measured via infrared photodissociation spectroscopy in the carbonyl stretching frequency region. Density functional calculations at the B3LYP/6-311+G(d) level have been performed and the calculated vibrational spectra are compared to the experimental data to identify the gas phase structures of the clusters. The experimentally observed CuFe(CO)n− (n = 4−7) cluster anions are characterized to have the Fe−Cu bonded (OC)4Fe− Cu(CO)n−4 structures, which also correspond to the computed lowest energy structures. The CuFe(CO)n− (n = 4−7) cluster anions can be regarded as being formed via the interactions between a Fe(CO)4− fragment and a Cu(CO)n−4 fragment. The Fe−Cu bond can thus be regarded as a formal single bond to satisfy the 18-electron configuration of the iron center. The structure of the CuFe(CO)5− core ion tagged by a weakly bound CO is also observed for the CuFe(CO)6− cluster anion. The CuFe(CO)5− cluster anion has the strongest Cu−Fe bond, consistent with experimental observation that the CuFe(CO)5− cluster anion is the most intense peak in the mass spectrum. The CuFe(CO)6− and CuFe(CO)7− cluster anions are more weakly bound than the smaller cluster anions. The Fe(CO)4− group with C3v symmetry serves as a building block in these hetronuclear cluster anions. The results provide important new insight into the structure and bonding of hetronuclear transition metal carbonyl clusters.

Figure 8. Frontier molecular orbitals account for the Cu−Fe bonding in the CuFe(CO)n− (n = 4−7) cluster anions.

σ bonding orbital. Population analysis shows that for the Fe(CO)4− moiety the orbital used for Fe−Cu bonding is primarily a hybrid of the Fe 3dz2, 4s, and 4p orbitals. In the small n = 4 and 5 complexes, the orbital involved in the Fe−Cu bonding at the copper center is predominately the Cu 4s atomic orbital. The Fe−Cu bonding orbital is quite localized for the n = 4 and 5 anions. Both anions have very similar Fe−Cu bond lengths (2.337 versus 2.290 Å, Table 3). These values are very close to the sum of single-bond covalent radius of Fe and Cu (2.28 Å).77 Note that the Fe−Cu bond length in the mixed iron−copper carbonyl cluster compounds in the condensed phase is experimentally determined to be around 2.40 Å, which is assigned to be a formal single bond.59−61 The predicted Cu− CO bond dissociation energy of the n = 5 anion (81.9 kJ· mol−1) is much larger than that of the CuCO neutral determined experimentally (25.1 ± 5.0 kJ·mol−1). In CuCO, the σ donation is weak. The complex possesses a bent structure to minimize the Cu−CO σ repulsion. The Fe−Cu σ bonding in CuFe(CO)5− markedly reduces the s population on the carbonyl bonding side of the copper center. Thus, the CuCO moiety is nearly linear and the Cu−CO bonding is enhanced by the reduction of the σ repulsion and the increase in the CO σ donation. The predicted Fe−Cu bond length increases with the number of CO ligands bonded on the copper center. The n

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ASSOCIATED CONTENT

S Supporting Information *

Calculated geometries, vibrational frequencies and intensities, and complete listing of ref 70. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*M.Z.: e-mail: [email protected]; fax, (+86) 21-65643532. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from National Natural Science Foundation (Grant 21433005) and Ministry of Science and Technology of China (Grants 2013CB834603 and 2012YQ220113). G

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