Infrared Photodissociation Spectroscopy of Heterodinuclear Iron–Zinc

Article pubs.acs.org/JPCA

Infrared Photodissociation Spectroscopy of Heterodinuclear Iron− Zinc and Cobalt−Zinc Carbonyl Cation Complexes Hui Qu, Fanchen Kong, Guanjun Wang, and Mingfei Zhou* Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: Fe−Zn and Co−Zn heteronuclear carbonyl cation complexes are produced via a laser vaporization supersonic cluster source in the gas phase. The dinuclear FeZn(CO)5+ and CoZn(CO)7+ cation complexes are observed to be the most intense heterodinuclear carbonyl cation species in the mass spectra. The infrared spectra are obtained via mass selection and infrared photodissociation spectroscopy in the carbonyl stretching frequency region. Their geometric and electronic structures are assigned with the support of density functional calculations. The FeZn(CO)5+ complex is determined to have a (OC)5Fe−Zn structure with a Fe−Zn half bond. The CoZn(CO)7+ ion is established to have a staggered (OC)4Co−Zn(CO)3 structure involving a Co−Zn σ single bond.

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INTRODUCTION Zinc-containing complexes have wide applications in heterogeneous catalysis.1 Due to its fully filled 3d and 4s orbitals, zero oxidation state zinc complexes are rare. Most of the stable zinc complexes are based on +I and +II oxidation states of zinc.1−3 A combined matrix isolation infrared spectroscopic and DFT study claimed the existence of the 18-electron zinc tricarbonyl complex Zn(CO)3 in solid noble gas matrices.4,5 However, later theoretical calculations reveal that the Zn(CO)3 complex is both kinetically and thermodynamically unstable and should not exist.6 In contrast, [Zn(CO)1−2]2+ were calculated to be among the strongest bonded transition metal carbonyls7 and can be formed in zinc-substituted zeolites or on ZnO surfaces under CO pressure.8,9 The synergy effects of multiple transition metals in the chemical reactions are expected to be the reason for high catalytic performance for heteronuclear transition metal complexes.10 Metal carbonyl complexes are appropriate models for studying the synergy effects of different transition metals on the catalytic performance.11−13 The interactions between zinc and other transition metals have been extensively studied.14 Iron and cobalt are among the most commonly utilized metals on the surface of the catalyst and the active site because of their unsaturated coordination structure.1,14 The isoelectronic species, Zn[Co(CO)4]2 and Zn[Fe(CO)4]22−, were prepared decades ago, which were characterized to have a (CO)4M− Zn−M(CO)4 (M = Fe and Co) linear structure with two M− Zn single bonds.15,16 The bond lengths were determined to be 2.317 and 2.305 Å, respectively. All of the carbonyl ligands are coordinated on the iron and cobalt centers. Iron and cobalt carbonyls were also used as building blocks in synthesis of complexes with [Fe2Zn4] and [Co2Zn3] cores.17 In these © XXXX American Chemical Society

complexes, zinc with positive charges usually coordinated with the ligands to stabilize the complexes.17,18 Here, we report the production and infrared photodissociation spectroscopic study of Fe−Zn and Co−Zn heteronuclear carbonyl cation complexes in the form of FeZn(CO)5+ and CoZn(CO)7+ in the gas phase. Their geometric and electronic structures are determined by comparison of the experimental IR spectra with those derived from quantum chemical calculations.

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EXPERIMENTAL AND COMPUTATIONAL METHODS A tandem time-of-flight mass spectrometer (TOFMS) was employed to measure the infrared spectra of heteronuclear transition metal carbonyl cluster cations, which has been described in detail in earlier publications.19,20 The cluster cations were produced with a uniformly mixed metal target, which was prepared by pressing mixtures of metal powders. The fundamental of a Nd:YAG laser (1064 nm, Continuum, Minilite II) with 10−20 mJ/pulse was employed to ablate the metal target. Heterodinuclear transition metal carbonyl cation complexes were produced during the laser ablation process in pulsed supersonic expansions of helium/CO gas mixtures at about 0.8 MPa backing pressure. The cations were skimmed and mass separated by a TOFMS. After mass selection and deceleration, the cations were then subjected to infrared photodissociation. The fragment ions together with the undissociated parent ions were reaccelerated and detected by a second collinear TOFMS. IR spectra were recorded by Received: December 28, 2016 Revised: February 6, 2017 Published: February 10, 2017 A

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dinuclear iron and cobalt carbonyl cation complexes, including Fe(CO)5+, Co(CO)5+, Fe2(CO)8+, and Co2(CO)8+, which have been studied by infrared photodissociation spectroscopy previously.32−35 It is interesting to note that homoleptic zinc carbonyl complexes are barely observed. As discussed previously,36−40 the preferred formation of iron and cobalt carbonyls over the zinc carbonyls suggests that the iron− and cobalt−carbonyl bond strengths are larger than that of zinc. Besides the homoleptic carbonyls, heteronuclear carbonyl complexes with chemical formulas of FeZn(CO)5+ and CoZn(CO)7+ are also observed with appreciable peak intensities. The isotopic splitting of iron and zinc can clearly be resolved, with their relative intensities in general matching the natural abundance isotopic distributions. FeZn(CO)5+. The most intense heteronuclear FeZn(CO)5+ cation complex observed in the mass spectrum is selected for infrared photodissociation. It is found that the FeZn(CO)5+ cation dissociates by losing a CO ligand when excited with infrared light around the 2100 cm−1 frequency region. However, the dissociation efficiency (less than 3%) is too low to achieve an effective spectrum. This suggests that the FeZn(CO)5+ cation is a strongly bound carbonyl complex. To obtain the infrared spectrum of FeZn(CO)5+, the FeZn(CO)6+ cation complex is formed by adjusting the timing to favor the formation of weakly bound complexes due to cold supersonic beam conditions. The FeZn(CO)6+ cation complex is able to dissociate via loss of a CO ligand with very high efficiency (about 30% at 2101 cm−1). This indicates that FeZn(CO)6+ is a weakly bound complex and can be regarded as a CO “tagged” cation complex involving a FeZn(CO)5+ core ion. The infrared photodissociation spectrum of FeZn(CO)6+ thus represents the spectrum of the FeZn(CO)5+ core ion that is weakly perturbed by the tagged CO ligand. The tagging effect is expected to change the position of the FeZn(CO)5+ band only slightly, as discussed previously.41−43 The infrared photodissociation spectrum of FeZn(CO)6+ is shown in Figure 2. Four bands centered at 2101, 2125, 2163, and 2191 cm−1 are observed. Geometric optimizations were performed on various possible structures in order to determine the geometric and electronic structure of the experimentally observed cation complex. The most stable structure of FeZn(CO)5+ involves a bare zinc atom and a Fe(CO)5 fragment, as shown in Figure 3. This structure has a doublet ground state with C4v symmetry with the zinc atom coordinated to the iron center along the molecular axis. The second isomer has a (OC)4Fe−ZnCO structure with four carbonyls terminally bonded on the iron center and one carbonyl terminally bonded on the zinc center. This structure can be regarded as replacing an equatorial CO of the squarepyramidal Fe(CO)5 moiety by ZnCO. It is predicted to be 6.4 kcal/mol higher in energy than the most stable structure. The third isomer is also Fe−Zn bonded with three terminal carbonyl ligands on the iron center and two carbonyls on the zinc center. The Fe(CO)3 and Zn moiety is planar. This structure is calculated to lie 23.7 kcal/mol above the most stable isomer. The fourth isomer with a (OC)2Fe−Zn(CO)3 structure lies much higher in energy than the most stable isomer. Similar calculations were also performed on the FeZn(CO)6+ cation complex. The most stable structure is a weakly bound complex involving a ZnFe(CO)5+ core ion (Figure 2). The sixth CO is weakly coordinated to the zinc center with a quite long Zn−CO distance of 3.278 Å. The binding energy of the sixth CO is predicted to be only 1.5 kcal/

monitoring the relative yield of fragment ions as a function of the photodissociation IR laser wavelength. The infrared laser was generated by an OPO/OPA system (Laser Vision) pumped by a Nd:YAG laser (Continuum Powerlite 8000), which provides tunable infrared light with energies from 0.6 to 1.0 mJ/pulse in the wavelength range from 1600 to 2200 cm−1. The spectra were recorded by scanning the dissociation laser in steps of 2 cm−1 and averaging over 300 laser shots at each step. The wavenumber of the laser output was calibrated by the photoacoustic spectrum of CO. In order to have deep insight into the geometric and electronic structures of the observed complexes and to support the assignments of the spectra, density functional theory (DFT) calculations were performed using the B3LYP functional with the Gaussian 09 program package.21−23 The all-electron 6311+G(d) basis set was used for all of the elements.24,25 All possible spin states and structures were considered. The harmonic vibrational frequencies were scaled by a factor of 0.97 and were given an 8 cm−1 full width at half-maximum.26,27 The dissociation energies were zero-point-energy corrected. Bonding analyses in terms of energy-decomposition analysis (EDA) in conjunction with the natural orbitals for chemical valence (NOCV) method were performed using the B3LYP functional with the ADF2014 program package.28−31 The basis sets for all elements had triple-ζ quality augmented by two sets of polarization functions (ADF-basis set TZ2P).

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RESULTS AND DISCUSSION Mass Spectra. The mass spectra of the carbonyl cation complexes produced by laser vaporization of the 1:1 mixed Fe/ Zn and Co/Zn targets in expansions of helium gas seeded with 5% CO are shown in Figure 1 in the m/z range of 180−360.

Figure 1. Mass spectra of the cation complexes in the m/z range of 180−360 produced by pulsed laser vaporization of the mixed (a) iron−zinc and (b) cobalt−zinc metal targets in expansion of 5% CO in helium.

The mass spectrum depends strongly on the conditions of the ion source, in particular, the timing between the vaporization laser and supersonic expansion. The mass spectra shown in Figure 1 were obtained with long-time delay between the pulsed valve opening and vaporization laser pulse. Under this condition, the saturated coordinated carbonyl complexes with high thermal stability are favored. In both spectra, the most intense peaks are saturated coordinated homoleptic mono- and B

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Figure 2. Experimental (black) infrared spectrum of FeZn(CO)6+ and the simulated (red) vibrational spectra of the most stable FeZn(CO)5+ and FeZn(CO)6+ cation complexes in the carbonyl stretching frequency region.

polarization leads to the blue shift of the CO stretching frequency. CoZn(CO)7+. The CoZn(CO)7+ cation complex is the only heteronuclear cation complex observed in the mass spectrum shown in Figure 1, suggesting that CoZn(CO)7+ is a coordination saturated cation. It is found that the CoZn(CO)7+ cation dissociates by losing a CO ligand with a focused laser beam. The resulting infrared photodissociation spectrum is shown in Figure 4, which consists of three well-resolved bands

Figure 3. Optimized structures and relative energies (kcal/mol) of the FeZn(CO)5+ cation isomers at the B3LYP/6-311+G(d) level.

mol. Geometry optimizations on other possible structures converged to the most stable structure. The simulated spectra of the FeZn(CO)6+ cation complex together with that of the FeZn(CO)5+ core ion are compared with the experimental spectrum in Figure 2. The spectra clearly show that the sixth tagged CO has very little effect on the spectrum of the ZnFe(CO)5+ core ion. The simulated spectrum of ZnFe(CO)6+ matches well with the experimental spectrum. On the basis of frequency calculations, the experimentally observed 2101, 2125, and 2163 cm−1 bands can be attributed to the carbonyl stretching vibrations of the ZnFe(CO)5+ core ion, while the 2191 cm−1 band belongs to the tagged CO. This tagged CO stretching frequency is blue-shifted by 48 cm−1 with respect to the free CO vibration at 2143 cm−1, suggesting incipient bonding interaction between the ZnFe(CO)5+ core ion and the tagged CO. This CO is weakly coordinated to the zinc center that is positively charged (+0.46 e), and the charge-induced

Figure 4. Experimental (black) and simulated (red) vibrational spectra of the CoZn(CO)7+ cation complex in the carbonyl stretching frequency region.

centered at 2030, 2132, and 2210 cm−1. The observation of only three CO bands in the carbonyl stretching vibrational region suggests that the cation has high symmetry. Only one stable structure was found for CoZn(CO)7+. As shown in Figure 4, the cation has a Co−Zn-bonded (OC)4Co− Zn(CO)3 structure with all of the carbonyl ligands terminally bonded. It has a closed-shell singlet ground state with staggered C3v symmetry. The eclipsed (CO)4Co−Zn(CO)3 structure is not a minimum. The simulated vibrational spectrum matches C

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The Journal of Physical Chemistry A the experimental spectrum quite well. The 2030 cm−1 band is due to the doubly degenerate antisymmetric stretching mode of the three equatorial CO ligands of the C3v Co(CO)4 subunit, which is predicted at 2029 cm−1. The 2132 cm−1 band is attributed to vibration of the axial CO ligand of the Co(CO)4 subunit, which is predicted at 2122 cm−1. The symmetric stretching mode of the Co(CO)4 subunit is predicted at 2062 cm−1 with very low IR intensity and is not observed experimentally. The 2210 cm−1 band is assigned to the stretching vibrations of the Zn(CO)3 moiety. The doubly degenerate antisymmetric stretching and the symmetric stretching modes of the Zn(CO)3 fragment are predicted at 2213 and 2217 cm−1 and cannot be resolved (Table 1).

The ground-state CoZn(CO)7+ cation has a Co−Zn bond distance of 2.346 Å, significantly shorter than the Fe−Zn bond in FeZn(CO)5+. The bond distance is quite close to the sum of the single-bond covalent radii of Co and Zn (Co + Zn = 2.29 Å),45 suggesting a single bond. Population analysis indicates that the Co(CO)4 moiety is slightly negatively charged (−0.26 e) and the positive charge is located on the Zn(CO)3 subunit (+1.26 e); therefore, the CoZn(CO)7+ cation can be regarded as being formed via interactions between a Co(CO)4 neutral fragment and a Zn(CO)3+ cation fragment. The binding energy is predicted to be 41.1 kcal/mol. We analyzed the nature of the Fe−Zn and Co−Zn bonds in FeZn(CO)5+ and CoZn(CO)7+ with the EDA-NOCV method in order to obtain insight into the bonding situation. The numerical results of the calculations where the fragments Fe(CO)5+ (2A1) + Zn (1S) are used for FeZn(CO)5+ and Co(CO)4 (2A1) + Zn(CO)3+ (2A1) are used for CoZn(CO)7+ are listed in Table 3. It becomes obvious that the orbital

Table 1. Comparison of the Experimental Vibrational Frequencies (cm−1) of the Complexes to the Calculated Values at the B3LYP/6-311+G(d) Levela complex

exptl.

FeZn(CO)6+

2101 2125 2163 2191 2030 2132 2220

CoZn(CO)7+

calcd 2104 2114 2155 2178 2029 2122 2213

Table 3. EDA-NOCV Results of the Interaction between the Two Metals in the MZn(CO)n+ (M = Fe and Co) Complexes at the B3LYP/TZ2P Levela

(958), 2104 (975) (19), 2119 (297) (263) (79) (1060 × 2), 2062 (13) (803) (128 × 2), 2217 (104)

a

The calculated values are scaled by a factor of 0.97, and the IR intensities are listed in parentheses in km/mol. a

Discussion. The features observed in the mass spectrum and infrared photodissociation spectrum indicate that FeZn(CO)5+ is a coordination saturated cation, implying that the heterodinuclear FeZn+ cation has a coordination number of five toward CO. In contrast, the saturation limit of CO coordination on CoZn+ is seven, although cobalt has one more electron than iron. This suggests a different bonding situation in FeZn(CO)5+ and CoZn(CO)7+. The FeZn(CO)5+ cation is predicted to have a very long Fe−Zn bond distance of 2.844 Å, which is shorter than the sum of the van der Waals radii of iron and zinc (Fe + Zn = 4.15 Å)44 but is longer than the sum of the single-bond covalent radii of Fe and Zn (Fe + Zn = 2.34 Å) reported by Pyykko et al.45 The Fe(CO)5 neutral molecule is a closed-shell 18-electron species; the FeZn(CO)5+ cation can thus only be regarded as being formed via the interaction between a Fe(CO)5+ cation and a bare neutral zinc atom. Natural population analysis indicates that the positive charge is almost evenly distributed on the Fe(CO)5 and Zn fragments (in Table 2), which implies some charge transfer from Fe(CO)5+ to zinc. The dissociation energy of FeZn(CO)5+ is calculated to be 10.2 kcal/mol with respect to ground-state Fe(CO)5+ (2A1) and Zn (1S).

complex

(OC)5Fe+−Zn

(OC)4Co−Zn(CO)3+

ΔEint ΔEPauli ΔEelstat ΔEorb ΔEorb(σ)

−17.1 57.3 −25.7 −48.7 −49.3

−81.6 165.6 −62.4 −184.7 −115.0

Energy values are given in kcal/mol. ΔEint = ΔEPauli + ΔEelstat + ΔEorb.

interactions come mainly from σ-bonding between the two fragments and that the σ-bonding interaction in FeZn(CO)5+ is much weaker than that in CoZn(CO)7+. The EDA-NOCV method is able to graphically display the change in the electronic structure that is associated with the pairwise orbital interactions.46−48 Figure 5 shows the most important pairs of molecular orbitals of the Fe(CO)5+, Co(CO)4, Zn, and Zn(CO)3+ fragments and the connected deformation densities Δρ. The color code of the charge flow is red → blue. The singly occupied molecular orbital (SOMO) of

Table 2. Fe−Zn and Co−Zn Bond Distances (in Å), Natural Charges, and Binding Energies (in kcal/mol) of FeZn(CO)5+ and CoZn(CO)7+ NBO group charge

bonding energy

complex

M−Zn distance

M(CO)4or5

Zn(CO)0or3

M−Zn

Zn−CO

FeZn(CO)5+ CoZn(CO)7+

2.844 2.346

0.54 −0.26

0.46 1.26

10.2 41.1

7.2

Figure 5. Plots of the frontier molecule orbitals and the deformation densities Δρ of the pairwise orbital interactions in the FeZn(CO)5+ and CoZn(CO)7+ complexes. D

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the square-pyramidal Fe(CO)5+ fragment is primarily an iron 3d orbital (dz2) in character that comprises some iron to equatorial CO π-back-donation. The dominated orbital interaction between Fe(CO)5+ and Zn comes from the combination of the SOMO of Fe(CO)5+ and the doubly occupied 4s atomic orbital of Zn in forming a σ-bonding orbital and an antibonding orbital. Because the bonding orbital is doubly occupied (HOMO) and the antibonding orbital is singly occupied (SOMO), the Fe−Zn bond can thus be regarded as a half-bond. Figure 5 shows the deformation density Δρ of this σ-orbital interaction between Fe(CO)5+ and Zn in FeZn(CO)5+. The shape of Δρ clearly shows electron concentration (blue) between Fe and Zn and an area of charge concentration (red) on the right side of the zinc atom. The Co(CO) 4 neutral has a doublet ground state with C 3v symmetry.49 The SOMO is a predominantly metal-based sd hybrid orbital that comprises cobalt to equatorial CO π-backdonation. The Zn(CO)3+ fragment is predicted to have a doublet ground state with C3v symmetry. The SOMO of Zn(CO)3+ is the s orbital of zinc that comprises zinc to CO πback-donation. The dominant orbital interaction between Co(CO)4 and Zn(CO)3+ involves the combination of the SOMO of Co(CO)4 and the SOMO of Zn(CO)3+ in forming a σ-bonding orbital (HOMO−2 of CoZn(CO)7+), which is doubly occupied to give a Co−Zn single bond. Both the cobalt and zinc centers have the most favorable 18-electron configuration in the closed-shell singlet ground state. The Co−Zn σ-bonding markedly reduces the σ population on the zinc center trans to the Co−Zn bond side, as shown by the deformation density Δρ. The positively charged zinc center is thus able to bind three CO ligands due to the reduction of σ repulsion between zinc and the CO ligands. The lack of zinc to CO π-back-donation and the charge-induced polarization50 lead to a blue shift of the CO stretching vibrations of the Zn(CO)3 moiety in the CoZn(CO)7+ cation complex.

AUTHOR INFORMATION

Corresponding Author

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

Mingfei Zhou: 0000-0002-1915-6203 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation (Grants 21688102 and 21433005) and the Ministry of Science and Technology of China (2013CB834603 and 2012YQ220113-3).

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CONCLUSIONS Coordination saturated heterodinuclear transition metal carbonyl cluster cations with chemical formulas of FeZn(CO)5+ and CoZn(CO)7+ are produced in the gas phase via a laser vaporization supersonic cluster ion source. The infrared spectra of both complexes are recorded via mass selection and infrared photodissociation spectroscopy in the carbonyl stretching frequency region. The geometric and electronic structures are determined on the basis of theoretical calculations at the DFT/ B3LYP/6-311+G(d) level. The experimentally observed FeZn(CO)5+ cation is determined to have a (OC)5Fe−Zn structure, which can be regarded as being formed through the interaction between a Fe(CO)5+ cation fragment and a bare Zn atom in forming an electron-sharing Fe−Zn half-bond with a quite long bond distance. The CoZn(CO)7+ cation is determined to have a staggered (CO)4Co−Zn(CO)3 structure formed via the interaction between a neutral Co(CO)4 fragment and a Zn(CO)3+ cation. The Co−Zn bond is characterized to be an electron-sharing σ-type single bond.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b13025. Calculated geometries and complete refs 21 and 32 (PDF) E

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DOI: 10.1021/acs.jpca.6b13025 J. Phys. Chem. A XXXX, XXX, XXX−XXX