Infrared Photodissociation Spectroscopy and Density Functional

Infrared photodissociation spectra are measured for mass-selected cation complexes with a chemical formula [MC7O6]+ (M = Fe, Co, Ni) formed via pulsed...
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Infrared Photodissociation Spectroscopy and Density Functional Theory Study of Carbon Suboxide Complexes [M(CO)4(C3O2)]+ (M = Fe, Co, Ni) Hui Qu, Guanjun Wang, and Mingfei Zhou* Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: Infrared photodissociation spectra are measured for massselected cation complexes with a chemical formula [MC7O6]+ (M = Fe, Co, Ni) formed via pulsed laser evaporation of metal target in expansions of helium gas seeded by CO. The geometries of the complexes are determined by comparison of the experimental spectra with the simulated spectra from density functional calculations. All of these complexes are identified to have [M(CO)4(C3O2)]+ structures involving a carbon suboxide ligand, which binds the metal center in an η1 fashion. The antisymmetric CO stretching vibration of C3O2 is slightly red-shifted upon coordination. The donor−acceptor bonding interactions between C3O2 and the metal centers are analyzed using the EDA-NOCV method. The results show that M ← C3O2 σ donation is stronger than the M → C3O2 π back-donation in these cation complexes.

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of C3O2 and Ni(C8H12)(PPh3)2.19 Solid-state IR and multinuclear (13C, 31P) CP-MAS NMR spectroscopic studies showed that the carbon suboxide ligand is bound to nickel in an η2olefin-like fashion through two carbon atoms.19 The molecular and electronic structures of [M(η2(C,C′)-C3O2)(PPh3)2] (M = Ni, Pd, Pt) have been investigated by means of quasi-relativistic gradient-corrected density functional calculations.20 The theoretical results are in good agreement with the bonding scheme emerging from experimental IR and NMR data.20 The geometries of the complexes in which C3O2 binds as a ligand to one and two AuCl moieties have been theoretically studied.21 It was found that carbon suboxide binds one AuCl preferentially in the η1 mode, whereas the equilibrium structures of the η1and η2-bonded diaurated complex are nearly degenerate.21 In this study, transition metal−carbon suboxide cation complexes in the form of [M(CO)4(C3O2)]+ (M = Fe, Co, Ni) are prepared in the gas phase using the pulsed laser vaporization supersonic expansion technique. These ions are mass-selected and investigated with infrared photodissociation spectroscopy in the CO stretching frequency region. On the basis of comparison of the experimental spectra with simulated spectra from density functional calculations, the carbon suboxide ligand is determined to bind to the metal centers in an η1 fashion. The nature of the metal−C3O2 interactions are investigated by means of energy decomposition analysis.

INTRODUCTION Carbon suboxide, first synthesized in 1906, is a stable oxide of carbon with a chemical formula C3O2.1 Early Raman and infrared spectroscopic and electron diffraction investigations suggest that the molecule has a symmetrical linear structure.2−9 The bonding situation was described as OCCCO involving four conventional double bonds, which makes it a cumulene, but the infrared fine structure of C3O2 in 1954 already cast some doubt on whether the molecule is linear in the gas phase.10 Later high-resolution infrared spectroscopic investigations indicated that the molecule is bent with an angle of 156° at the central carbon atom.11,12 This value matches excellently with the high-level ab initio calculations at the CCSD(T)/cc-pVQZ level, which gave a value of 155.9°.13 The bending potential of C3O2 was predicted to be very flat (10% and loses up to three CO ligands with focused laser beam. The resulting infrared photodissociation spectrum in the 1800−2280 cm−1 frequency region is shown in Figure 1. The

Figure 1. Experimental (black) and simulated (red) vibrational spectra of the [Fe(CO)4(C3O2)]+ cation complex in the carbonyl stretching frequency region. The experimental spectrum was detected by the elimination of one CO channel. The simulated spectrum was obtained from scaled harmonic vibrational frequencies and intensities for the most stable structure calculated at the B3LYP/aug-cc-pVDZ level.

spectrum consists of five bands centered at 2068, 2078, 2139, 2165, and 2199 cm−1, which are originated from terminally bonded carbonyl ligands. Considering the fact that the 5-fold coordinated Fe(CO)5+ was characterized to have a completed coordination sphere with the iron center having a 17-electron configuration,38 it is reasonable to assume that the [FeC7O6]+ cation has a [Fe(CO)4(C3O2)]+ structure involving four carbonyl ligands and a C3O2 ligand. Density functional theory calculations are performed to gain insight into the structure and bonding of the [FeC7O6]+ cation complex. Geometry optimizations at both the B3LYP and BP86 levels are performed on various possible structures including [Fe(CO)4(C3O2)]+, [Fe(CO)5(CCO)]+ involving five carbonyl ligands and a CCO fragment, and the carbonyl carbide structure [FeC(CO)6]+. These calculations all converge to the [Fe(CO)4(C3O2)]+ structure, which has a 2A′ electronic ground state with Cs symmetry involving four terminally bonded CO ligands and a C3O2 ligand, as shown in Figure 1. In this structure, the C3O2 ligand is bent and is coordinated to the

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RESULTS AND DISCUSSION The mass spectra of the cation complexes in the m/z range of 100−300 from the laser ablation of iron, cobalt, and nickel metal targets in expansions of helium gas seeded with 5% CO are shown in Figure S1 of the Supporting Information. The series of most intense peaks at an interval of m/z = 28 are due to metal carbonyl cation complexes, whose infrared photodissociation spectra have been previously reported.31−33,38−40 Weak peaks due to species with a chemical formula of [MC7O6]+ (m/z = 236, 239, 238 for M = Fe, Co, and Ni respectively) are observed. These ions are mass-selected and subjected to infrared photodissociation. When the frequency of B

DOI: 10.1021/acs.jpca.5b12716 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A iron center via a symmetrical η1 fashion. The cation can be regarded as replacing one equatorial CO ligand of the trigonal bipyramidal Fe(CO)5+ ion by the C3O2 ligand. The structure with the C3O2 ligand coordinated in an η2-C,C′ fashion is not a local minimum. Starting with the initial structure with an η2C,C′ coordinated C3O2 without symmetry constraint, the optimization ends up with the η1-coordinated structure. The computed spectrum of the η1-coordinated structure shown in Figure 1 is in excellent agreement with the experimental one. The [Fe(CO)4(C3O2)]+ complex with Cs symmetry has six CO stretching modes that are all IR-active (Table 1). The 2165 and Table 1. Comparison of the Vibrational Frequencies (cm−1) of the [M(CO)4(C3O2)]+ Cation Complexes Measured in the Present Work to Those Calculated at the B3LYP/aug-ccpVDZ Levela complex

exptl

[Fe(CO)4(C3O2)]+

2068 2078 2139 2165 2199 2113 2135 2175 2204 2159

[Co(CO)4(C3O2)]+

[Ni(CO)4(C3O2)]+

2179 2210

Figure 2. Experimental (black) and simulated (red) vibrational spectra of the [Co(CO)4(C3O2)]+ cation complex in the carbonyl stretching frequency region. The experimental spectrum was detected by the elimination of one CO channel. The simulated spectra were obtained from scaled harmonic vibrational frequencies and intensities calculated at the B3LYP/aug-cc-pVDZ level. The relative energies are given in kcal/mol at both the B3LYP and CCSD(T) (in parentheses) levels of theory.

calcd 2079(525) 2090(996), 2094(290) 2135(246) 2177(1554) 2204(365) 2089(543) 2115(796), 2126(1) 2169(201), 2176(1641) 2199(370) 2132(181), 21333(486), 2144(8), 2164(455) 2194(1392) 2192(721)

the singlet equatorial structure is 4.6 kcal/mol more stable than the triplet axial structure. As shown in Figure 2, the simulated vibrational spectrum of the singlet equatorial structure matches the experiment better than that of the triplet axial structure. Therefore, the experimentally observed [Co(CO)4(C3O2)]+ cation can be confidentially assigned to the singlet equatorial structure. According to the predicted spectrum, the bands at 2175 and 2204 cm−1 are due to the antisymmetric and symmetric CO stretching modes of the bent C3O2 ligand. The other bands are attributed to the CO stretching vibrations of the four carbonyl ligands (Table 1). [Ni(CO)4(C3O2)]+. The [NiC7O6]+ cation dissociated quite efficiently (up to 40% dissociation efficiency at 2170 cm−1 with a laser energy of 1.1 mJ/pulse). The observation of the fragmentation channel via the loss of a C3O2 ligand suggests that the cation may also have a [Ni(CO)4(C3O2)]+ structure. The infrared photodissociation spectrum shown in Figure 3 exhibits three bands centered at 2159, 2179, and 2210 cm−1. Two stationary points are found on the potential energy surface of [Ni(CO)4(C3O2)]+ at the B3LYP level (Figure 3). Both structures have a 2A′ ground state with Cs symmetry and can be regarded as having slightly distorted trigonal bipyramidal geometries with the C3O2 ligand occupying either an equatorial or an axial vertex. Thus, both structures can be regarded as being formed by replacing one equatorial or axial CO ligand of the trigonal bipyramidal Ni(CO)5+ ion by a bent C3O2 ligand. The axial structure with the C3O2 ligand occupying the axial position is predicted to be 1.8 kcal/mol more stable than the equatorial structure with the C3O2 ligand lying in the equatorial plane. At the BP86 level, only the axial structure is stable. The equatorial structure is not a minimum with an imaginary frequency. The CCSD(T)//B3LYP calculations predict that the axial structure is 1.7 kcal/mol more stable than the equatorial structure. The simulated spectra of these two structures at the B3LYP level are compared with the experimental spectrum in Figure 3. Neither spectrum is in excellent agreement with the experimental one. Considering that the equatorial structure is not a minimum at the BP86 level and that the axial structure is more stable than the equatorial

a

Scaled by a factor of 0.97; the IR intensities are listed in parentheses in km/mol.

2199 cm−1 bands are assigned to the antisymmetric and symmetric CO stretching modes of the bent C3O2 ligand. The remaining three bands are originated from the Fe(CO)4+ moiety. The antisymmetric CO stretching frequency of the C3O2 ligand is only slightly red-shifted from that of free C3O2 (2258 cm−1).3 [Co(CO)4(C3O2)]+. The infrared photodissociation spectrum of the [CoC7O6]+ cation complex is shown in Figure 2. The spectrum exhibits four bands at 2113, 2135, 2175, and 2204 cm−1. The strongest one at 2175 cm−1 is comparatively broad and asymmetric, suggesting the involvement of more than one unresolved vibrations. Similar to [Fe(CO)4(C3O2)]+, the [CoC7O6]+ cation complex is also predicted to have a [Co(CO)4(C3O2)]+ structure involving a bent η1-coordinated carbon suboxide ligand. Two stationary points are found that are close in energy, as shown in Figure 2. The first structure (a) has a 1A1 electronic ground state with C2v symmetry. This structure can be regarded as replacing one equatorial CO ligand of the trigonal bipyramidal Co(CO)5+ ion by the C3O2 ligand with the bent C3O2 ligand lying in the equatorial plane (singlet equatorial structure). Another structure (b) has a 3A′ electronic state with Cs symmetry. This structure can be regarded as replacing one axial CO ligand of the trigonal bipyramidal Co(CO)5+ ion by the C3O2 ligand (triplet axial structure). The singlet equatorial structure is slightly less stable (0.5 kcal/mol) than the triplet axial structure at the B3LYP level. In contrast, the BP86 functional predicts that the singlet equatorial structure is 21.4 kcal/mol more stable than the triplet axial structure. The CCSD(T)//B3LYP calculations indicate that C

DOI: 10.1021/acs.jpca.5b12716 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Table 2. Dissociation Energies Calculated at the B3LYP/ aug-cc-pVDZ Level and the EDA-NOCV Results of the Interaction between C3O2 and Metal Carbonyls in the [M(CO)4(C3O2)]+ Cation Complexes at the B3LYP/TZ2P Levela,b complex

[Fe(CO)4(C3O2)]+

[Co(CO)4(C3O2)]+

DE1 DE2 ΔEint ΔEPauli ΔEelstat ΔEorb ΔEorb(1) ΔEorb(2) ΔEorb (3)

10.3 22.2 −31.5 59.7 −42.9 −48.3 −11.4 −16.3 −15.2

2.0 9.7 −24.8 61.1 −45.5 −40.3 −25.9 −7.3 −2.1

[Ni(CO)4(C3O2)]+ 5.5 −14.0 36.2 −18.6 −31.6 −26.4 −1.7

a

DE1 value is the dissociation energy from the overall complex to the equilibrium structure of the two fragments without the change of electronic states. DE2 Value is the dissociation energy from the overall complex to the electronic ground state of the fragments. bEnergy values are given in kcal/mol. ΔEint = ΔEPauli + ΔEelstat + ΔEorb.

Figure 3. Experimental (black) and simulated (red) vibrational spectra of the [Ni(CO)4(C3O2)]+ cation complex in the carbonyl stretching frequency region. The experimental spectrum was detected by the elimination of one CO channel. The simulated spectra were obtained from scaled harmonic vibrational frequencies and intensities calculated at the B3LYP/aug-cc-pVDZ level. The relative energies are given in kcal/mol at both the B3LYP and CCSD(T) (in parentheses) levels of theory.

calculated at the same (B3LYP) level of theory (5.8 kcal/ mol).40 The ground-state C3O2 has been classified as a carbonyl complex of carbon in its 1D excited state with an electron configuration 2s2, 2p(σ)0, 2p(π∥)0, 2p(π⊥)2.15,16 The frontier molecular orbitals of C3O2 are shown in Figure 4. The

structure at both the B3LYP and CCSD(T) levels of theory, we assign the experimentally observed cation complex to possess the axial structure. It is worth noting that the bonding between Ni(CO)4+ and C3O2 in [Ni(CO)4(C3O2)]+ is quite weak, as evidenced by the predicted quite long Ni−C3O2 distance of 2.556 Å. It is well known that density functional theory methods such as B3LYP have difficulty in predicting weak bonding systems. High-level calculations are required to provide a better description of this complex, but such calculations are too expensive to be practical for us. Discussion. On the basis of previous discussions, the experimentally observed [MC7O6]+ cations are characterized to have the [M(CO)4(C3O2)]+ (M = Fe, Co, Ni) structures involving an η1-coordinated C3O2 ligand, which can be viewed as being formed via the interactions between the ground-state bent C3O2 ligand and the M(CO)4+ fragments. The 2A′ ground-state [Fe(CO)4(C3O2)]+ cation correlates to an electronic excited doublet state of Fe(CO)4+, which is predicted to be 11.9 kcal/mol less stable than the quartet ground state at the B3LYP level. The bond dissociation energy is 22.2 kcal/mol with respect to the dissociation limit Fe(CO)4+ (doublet) + C3O2 calculated at the B3LYP level (Table2). This value is very close to the experimentally determined bond dissociation energy of the fifth CO of Fe(CO)5+ (26.8 ± 0.9 kcal/mol).41 The [Co(CO)4(C3O2)]+ cation is determined to have a 1A1 ground state, which correlates to an electronic excited singlet state of Co(CO)4+. The Co(CO)4+ cation was reported to have a triplet ground state with C2v symmetry,39,42 which is predicted to be 7.7 kcal/mol more stable than the singlet state at the B3LYP level. The bond dissociation energy of [Co(CO)4(C3O2)]+ is predicted to be 9.7 kcal/mol for the dissociation into the singlet state Co(CO)4+ and C3O2 (Table 2). This value is ∼8.3 kcal/mol lower than the bond dissociation energy of Co(CO)5+ determined experimentally (18.0 ± 1.2 kcal/mol).43 The dissociation energy of [Ni(CO)4(C3O2)]+ is calculated to be only 5.5 kcal/mol, which is very close to the fifth CO binding energy of Ni(CO)5+

Figure 4. Frontier molecular orbitals of (a) C3O2 and (b) Fe(CO)4+.

HOMO−1 orbital with a1 symmetry is primarily a 2s orbital of the central carbon, but the wave function is mixed with some in-plane 2p character. This sp hybridization helps to reduce the σ repulsion between the central carbon atom and CO and also increases the central C to CO π back-donation. The HOMO (b1) is primarily the central C out-of-plane 2p orbital, which comprises significant C 2p to CO 2π* back bonding. The LUMO and LUMO+1 orbitals are out-of-plane and in-plane π orbitals of C3O2, which are highly localized on the peripheral C atoms and are C−O antibonding in character. On the basis of symmetry arguments and overlap considerations, the HOMO− 1 (a1) orbital of C3O2 serves as a σ donor orbital, while the LUMO+1 orbital acts as an π acceptor orbital in the bonding interactions in the [M(CO)4(C3O2)]+ complexes. The M ← C3O2 σ donation should imply a lengthening of the C−C bonds D

DOI: 10.1021/acs.jpca.5b12716 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A and a shortening of the C−O bonds of the C3O2 fragment, while the M → C3O2 π back-donation should shorten the C−O bonds and scarcely affect the C−C bonds. The valence molecular orbitals of the doublet state Fe(CO)4+ fragment that are involved in the bonding interactions with the C3O2 ligand are also shown in Figure 4. The LUMO is predominantly iron-based sd(σ) hybrid orbital, which is the primary acceptor orbital for donation from the bent C3O2 fragment. The singly occupied orbital (SOMO) is a metal-based d orbital. This orbital also serves as an acceptor orbital for donation from C3O2. The highest doubly occupied orbital (HOMO) is a metal-based dπ orbital that is the principal back-donation orbital. The C3O2 ligand serves as a two-electron donor in these [M(CO)4(C3O2)]+ (M = Fe, Co, Ni) complexes, with the metal centers having 17, 18, and 19 valence electrons, respectively. The nickel species with 19 valence electrons is oversaturated and is expected to be not very stable. This may explain why the nickel complex has a much weaker Ni−C3O2 bond with long Ni−C bond distance and small dissociation energy. The nature of the (OC)4M+-C3O2 donor−acceptor interactions in [M(CO)4(C3O2)]+ are analyzed using the EDA (energy decomposition analysis) in conjunction with the NOCV (natural orbitals for chemical valence) method, which gives a detailed insight into the bonding situation. The numerical results at the B3LYP/TZP level are listed in Table 3. The calculated interaction energies ΔEint between the frozen

Figure 5. Plots of deformation densities Δρ of the pairwise orbital interactions between C3O2 and metal carbonyls and the associated energies (in kcal/mol) in each complex. The direction of the charge flow is red to blue.

shapes of deformation densities Δρ that are associated with the orbital energies shown in Figure 5 clearly indicate the charge flow into the Fe−C bond region. On the basis of the donor− acceptor bonding model, the first interaction favors the C2v geometry, whereas the other two interactions prefer the Cs symmetry. The calculated Cs structure of [Fe(CO)4(C3O2)]+ is the result of a compromise between the first and the other two interactions. The orbital interactions of [Co(CO)4(C3O2)]+ come mainly from the donation of the doubly occupied HOMO−1 orbital of C3O2 to the LUMO orbital of Co(CO)4+ (ΔEorb(1) = −25.9 kcal/mol), which is significantly stronger than that of [Fe(CO) 4 (C 3 O 2 )]+ . The stronger donation interaction ΔEorb(1) is due to better orbital overlap in the higher C2v symmetry [Co(CO)4(C3O2)]+ than that in the Cs symmetry [Fe(CO)4(C3O2)]+. The other two interactions between [Co(CO)4]+ and C3O2 (ΔEorb(2) = −7.3 kcal/mol and ΔEorb(3) = −2.1 kcal/mol) are much weaker than those of the Fe(CO)4+ and C3O2 interactions. In the case of [Ni(CO)4(C3O2)]+, the orbital interactions are originated mainly from the donation of the doubly occupied HOMO−1 orbital of C3O2 to the SOMO orbital of Ni(CO)4+ (ΔEorb(1) = −26.4 kcal/mol), with negligible contribution from the π backdonation interaction (ΔEorb(2) = −1.7 kcal/mol). The decreased π back-donation interaction from Fe to Ni is owing to the decreased orbital overlap as the stability of the π back-donation orbital of the M(CO)4+ fragment increases from Fe to Ni.

Table 3. Equilibrium Molecular Parameters of the C3O2 Fragment in the [M(CO)4(C3O2)]+ Cation Complexes (M = Fe, Co, Ni) Calculated at the B3LYP/aug-cc-pVDZ Levela

a

complex

Fe

Co

Ni

r(M−C) ∠(C−C−C) r(C−C) r(C−O) ∠(C−C−O)

2.183 122.6 1.336 1.147 174.4

2.172 123.5 1.333 1.149 174.5

2.556 127.4 1.321 1.152 173.2

Bond lengths are given in angstroms and the bond angles in degrees.

fragments indicate that the [Fe(CO)4(C3O2)]+ complex has the strongest attraction (ΔEint = −31.5 kcal/mol), while the [Ni(CO)4(C3O2)]+ complex has the weakest attraction (ΔEint = −14.0 kcal/mol) among the three complexes. The weaker attraction in [Ni(CO)4(C3O2)]+ is due mainly to its weaker orbital interaction (ΔEorb = −31.6 kcal/mol) than those of [Fe(CO)4(C3O2)]+ (ΔEorb = −48.3 kcal/mol) and [Co(CO)4(C3O2)]+ (ΔEorb = −40.3 kcal/mol). Inspection of the individual orbital interactions that contribute to ΔEorb gives insight into the nature of the MC3O2 bonds. There are three major contributions to the orbital interactions of Fe+-C3O2. The first interaction consists of the donation of the doubly occupied HOMO−1 orbital of C3O2 to the LUMO orbital of Fe(CO) 4+, which contributes a stabilization of ΔEorb(1) = −11.4 kcal/mol. The second interaction is found for the reverse back-donation (OC)4Fe+→C3O2, where the electronic charge of the HOMO of Fe(CO)4+ is donated back into the vacant in-plane π∥* orbital of C3O2, which has a contribution of ΔEorb(2) = −16.3 kcal/mol. In addition to these two classical Dewar−Chatt− Duncanson interactions, there is another σ donation from the HOMO−1 orbital of C3O2 to the SOMO of Fe(CO)4+, which provides a stabilization of ΔEorb(3) = −15.2 kcal/mol. The

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CONCLUSIONS Transition-metal−carbon suboxide cation complexes in the form of [M(CO)4(C3O2)]+ with M = Fe, Co, and Ni are produced via a laser vaporization supersonic cluster source in the gas phase. The cation complexes are each mass-selected and studied by infrared photodissociation spectroscopy in the carbonyl stretching frequency region. The geometric structures of the complexes are determined with the aid of density functional calculations. All of these complexes are characterized to have slightly distorted trigonal bipyramidal geometries with the C3O2 ligand occupying either an equatorial or an axial vertex position. The C3O2 ligand binds to the metal center in E

DOI: 10.1021/acs.jpca.5b12716 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A an η1 fashion with its antisymmetric CO stretching vibrational frequency slightly red-shifted from that of free C3O2. The donor−acceptor bonding interactions between C3O2 and the metal centers are analyzed using the EDA-NOCV method. The results show that both Fe ← C3O2 σ donation and Fe → C3O2 π back-donation contribute to the Fe−C3O2 bonding, with the former being much stronger that the later; however, in the case of Co and Ni, the bonding is originated mainly from the M ← C3O2 donation interaction with negligible contribution from the π back-donation interaction.

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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.5b12716. Calculated geometries, vibrational frequencies, and intensities; complete ref 24. (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from National Natural Science Foundation (Grant Nos. 21433005 and 21273045) and Ministry of Science and Technology of China (2013CB834603 and 2012YQ220113-3). The calculations are performed at the Fudan University High-End Computing Center.

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DOI: 10.1021/acs.jpca.5b12716 J. Phys. Chem. A XXXX, XXX, XXX−XXX