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Photoelectron Velocity-Map Imaging and Theoretical Studies of Heterotrinuclear Metal Carbonyls VNi(CO) (n = 6-10) 2

n-

Jumei Zhang, Hua Xie, Gang Li, Xiangtao Kong, Hongjun Fan, and Ling Jiang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b09230 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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The Journal of Physical Chemistry

Photoelectron Velocity-Map Imaging and Theoretical Studies of Heterotrinuclear Metal Carbonyls V2Ni(CO)n− (n = 6− −10)

Jumei Zhang,1,2 Hua Xie,1 Gang Li,1 Xiangtao Kong,1 Hongjun Fan,1 and Ling Jiang1,* 1

State Key Laboratory of Molecular Reaction Dynamics, Collaborative Innovation

Center of Chemistry for Energy and Materials (iChEM), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China 2

University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049,

China

*E-mail address: [email protected].

1

ACS Paragon Plus Environment

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Abstract: Photoelectron velocity-map imaging spectroscopy was conducted for the heterotrinuclear metal carbonyls V2Ni(CO)n− (n = 6−10). Electronic structure calculations were performed to understand the experimental spectral features. The binding motif of a V-V-Ni chain with two side-on-bonded carbonyls and two bridging carbonyls is favored in the n = 6–9 clusters. A V2Ni triangle core structure is formed at n = 10 with the involvement of two carbonyls with the carbon atom triply coordinated to metal atoms, three bridging carbonyls, and five terminal carbonyls, in which CO bonding configurations mirror the adsorption features in the 3-fold hollow, bridging, and atop sites on the closely packed surface, respectively. The present study provides a stepwise picture for molecular-level understanding of CO bonding on heteronuclear metal clusters, which is directly relevant to the elementary processes of CO on the alloy surfaces/interfaces.

1.

Introduction Metal clusters, laser-vaporized from the surface of bulk materials and efficiently

cooled by supersonic expansion, may represent the low-coordinate surface sites, which could provide accurate descriptions of the interaction between well-defined single crystal copper surfaces and small molecules (i.e., CO, CO2, H2O, etc.).1-5 The study of the interaction of metal clusters with carbon monoxide is of considerable interest because of its important role in many industrial processes such as hydroformylation, alcohol synthesis, and acetic acid synthesis.4,

6-9

CO is mainly

bonded in a terminal or bridging coordination mode to metals with very few exceptions for side-on-bonded CO on some transition metal surfaces10, 11 and in a limited number of organometallic complexes12-14. The metal-carbonyl bonding is suggested to involve a synergistic interaction between σ-donor bonding from the occupied 5σ molecule orbital of CO into an empty metal orbital with σ symmetry and π-backbonding from occupied metal orbitals into the π* molecular orbital of CO. Various experimental and theoretical approaches have been used to explore the homoleptic metal carbonyls.1,

6, 9, 15-23

Recently, the study of the heteronuclear 2


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transition metal carbonyls has gained increasing interest due to their excellent chemical reactivity of heteronuclear transition metal cluster in many important processes, such as the chemisorptions of the molecules on alloy surface and the enhancement catalytic effects in heteronuclear metallic nanoparticles.24 The heterobinuclear main group metal-nickel carbonyls MNi(CO)3− (M = Mg, Ca, Al) have been studied by photoelectron velocity-map imaging (VMI), which reveal the M-Ni(CO)3 structure with the involvement of the Ni(CO)3 building block.24 The CO molecules are bonded terminally to the Ni atom in CuNi(CO)3– cluster or the Fe atom in CuFe(CO)4– and PbFe(CO)4–.25-27 The MFe(CO)8+ (M = Co, Ni, Cu) carbonyls have eclipsed (CO)5Fe−M(CO)3+ structures and MCu(CO)7+ (M = Co, Ni) contain staggered (CO)4M−Cu(CO)3+ structures, in which all the carbonyl ligands are bonded terminally to the metal atoms.28 Recent investigation of MNi(CO)n– (M = Ti, Zr, Hf)29 has shown that the side-on-bonded, bridging, and terminal metal−CO bonding modes are simultaneously involved in the n = 3 cluster. In particular, UFe(CO)3– and OUFe(CO)3– were characterized by infrared photodissociation spectroscopy and quantum chemical calculations to have triple bonds between U 6d/5f and Fe 3d orbitals, featuring one covalent σ bond and two Fe-to-U dative π bonds.30 Vanadium compounds are widely applied in various fields such as homogeneous and heterogeneous catalytic reactions, biology, and medicine.31,

32

Nickel

tetracarbonyl, Ni(CO)4, was the first metal carbonyl to be prepared in 1890,33 which is a prototypical example of the 18-electron rule in inorganic and organometallic chemistry. Nickel group carbonyls are among the most studied metal carbonyls both experimentally and theoretically.7 It would be of fundamental interest to capture and characterize the intermediates in the catalytic activation of CO by the vanadium−nickel heteronuclear clusters, which is already considered of great importance for a better understanding of the elementary processes of CO activation on the alloy surfaces/interfaces and offering design criteria for catalyst engineering.1, 4, 8 Herein, we report a study of the chemical bonding and electronic structure of mass-selected V2Ni(CO)n− (n = 6–10) using photoelectron velocity-map imaging spectroscopy and ab initio calculations. Experimental and theoretical results reveal 3



that the binding motif of a V-V-Ni chain with two side-on-bonded carbonyls and two bridging carbonyls is favored in the n = 6–9 clusters and a V2Ni triangle core structure is formed at n = 10, in which CO bonding configurations are reminiscent of CO adsorption fashions in the 3-fold hollow, bridging, and atop sites on the well-defined surface, respectively.

2.

Experimental and Computational Methods The experiment was performed using a homemade instrument with a laser

vaporization source, a dual-channel time-of-flight mass spectrometer (D-TOFMS) coupled with velocity-map photoelectron imaging analyzer. The details of the apparatus have been described elsewhere,34 and only a brief outline is given below. The V2Ni(CO)n− complexes were generated by pulsed laser vaporization of metal target in an expansion of 5% CO seeded in helium. The 1064 nm fundamental of a Nd:YAG laser (Continuum, MiniliteⅡ) was employed to ablate a rotating target. The typical backing pressure of the carrier gas was ~3 atm. The ions were cooled and expanded through a clustering channel. The ions were mass-selected by a TOF mass spectrometer. After separation in space, the anionic clusters of interest were mass selected into the photodetachment region and crossed with a laser beam. A photon energy of 266 nm (4.661 eV) was used for the photodetachment of these anionic clusters. The resulting photoelectrons were extracted by a charge-coupled device camera. Each image was obtained with 10000-50000 laser shots at 10 Hz repetition rate. The raw image symbolized the projection of the photoelectron density in 3D laboratory frame onto the 2D imaging detector. The original 3D distribution was reconstructed using the Basis Set Expansion (BASEX) inverse Abel transform method,35 and the photoelectron spectrum was acquired by integrating one central slice of the 3D distribution. The photoelectron kinetic energy spectra were calibrated by the known spectrum of Au¯. The energy resolution of photoelectron spectrum was better than 5%, which corresponded to 50 meV at electron kinetic energy of 1 eV. Density functional theory calculations were performed to elucidate the chemical bonding and electronic structure of V2Ni(CO)n− (n = 6–10) using the BP86 functional 4


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with the Gaussian 09 program package.36 The 6-311+G(2d) basis set was selected for the C and O atoms and the SDD (SC-RECP, MWB28) basis set was used for the V and Ni atoms. All possible spin states and structures were considered. Harmonic vibrational frequency analyses were performed to make sure that the structures were real minima. Theoretical value of vertical detachment energy (VDE) was calculated by the difference in the energy between the anionic and neutral species at the optimized geometry of the anionic species, and that of adiabatic detachment energy (ADE) was computed by the difference in the energy between the anionic and neutral the species based on their optimized geometries. The simulated photoelectron spectra were obtained by fitting the computed VDEs with Gaussian functions with a width of 0.1 eV.

3.

Experimental Results The typical mass spectrum of V2Ni(CO)n− anions is given in Figure S1 in the

Supporting Information. Figure 1 shows the photoelectron imaging and photoelectron spectra of V2Ni(CO)n− (n = 6–10). The VDE and ADE values of V2Ni(CO)n− (n = 6–10) are summarized in Table 1. As shown in Figure 1, the photoelectron spectrum of each cluster has one broad ground band. The experimental VDE value of each cluster was determined from the maximum of the first peak in the respective photoelectron spectrum. The VDEs of the n = 6–10 clusters were measured to be 3.15 ± 0.08, 3.46 ± 0.06, 3.80 ± 0.04, 3.81 ± 0.04, and 3.93 ± 0.04 eV, respectively. The ADEs of the n = 6–10 clusters were estimated to be 2.77 ± 0.09, 3.08 ± 0.08, 3.34 ± 0.07, 3.41 ± 0.06, and 3.65 ± 0.05 eV, respectively. Note that the low resultion of photoelectron spectra of V2Ni(CO)n− clusters might be due to the warm source under the present experimental conditions. Under the efficient cooling by cryogenic ion trap or liquid nitrogen, the ions could be vibrationally cold and their photoelectron spectra could be well resolved.37-40 The incorporation of cryogenic cooling into our apparatus is in progress.

4.

Comparison between experimental and theoretical results 5



Optimized structures of V2Ni(CO)n− (n = 6–10) are shown in Figure 2. The comparison of experiment and calculated VDEs and ADEs for V2Ni(CO)n− (n = 6–10) is listed in Table 1. The comparison of experimental photoelectron spectrum of V2Ni(CO)6− to the simulated spectra of the 6A–6E isomers is shown in Figure S2. Since the band positions of ground state and excited state in the photoelectron spectra of V2Ni(CO)n− (n = 7−10) are not well-seperated, the simulated spectra of the low-lying isomers are not shown here. The following sections summarize the theoretical and experimental results for each individual cluster.

V2Ni(CO)6−. In Figure 2, the lowest-energy isomer, labeled 6A, is a C1 structure with an 2A ground state, which contains a V-V-Ni chain with the involvement of two side-on-bonded carbonyls coordinated to the V2 unit, two bridging carbonyls bonded to the VNi unit, one carbonyl terminally bonded to the V atom, and one carbonyl terminally bonded to the Ni atom. The second isomer (6B) is 0.40 eV higher than 6A, in which the coordination modes of CO are similar to 6A. In the 6C isomer (+0.42 eV), two carbonyls are terminally bonded to the Ni atom, which is different from isomers 6A and 6B. The 6D and 6E isomers lie 0.69 and 0.72 eV above 6A, respectively, in which the unit of V2Ni triangle is formed. As listed in Table 1, the calculated VDE value of 6A is 3.32 eV, which agrees with the experimental value of 3.15 ± 0.08 eV. The calculated ADE value of 6A is also consistent with the experimental value (Table 1). The calculated VDE and ADE values of 6B and 6C are different from the experimental values. The isomers 6D−6E should lie too high in energy, which are not readily probed in the present experiment. As illustrated in Figure S2, the band positions and overall pattern of simulated spectrum of 6A agree best with experiment.

V2Ni(CO)7−. The 7A isomer could be viewed as being derived from 6A by terminally bonding the seventh CO molecule to the V atom. The 7B isomer (+0.42 eV) consists of four bridging carbonyls and three terminal carbonyls bonded to the V2Ni triangle. The 7C isomer (+0.60 eV) has three bridging carbonyls and four terminal 6


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carbonyls bonded to the V2Ni triangle. The 7D (+0.63 eV) isomer has two side-on-bonded carbonyls, three bridging carbonyls, and two terminal carbonyls. The 7E isomer (+0.79 eV) has four bridging carbonyls and three terminal carbonyls coordinated to the V2Ni triangle. As listed in Table 1, the VDE and ADE of 7A is calculated to be 3.61 and 3.25 eV, respectively, which is in accord with the experimental value of 3.46 ± 0.06 and 3.08 ± 0.08 eV. The calculated VDE and ADE values of 7B are 2.85 and 2.67 eV, respectively, which are much lower than the experimental values.

V2Ni(CO)8−. The lowest-energy isomer for V2Ni(CO)8− (8A) could be viewed as being derived from 7A by terminally bonding the eighth CO molecule to the V atom. The 8B isomer (+0.37 eV) consists of a V2Ni triangle with the involvement of three bridging carbonyls and five terminal carbonyls bonded to two V atoms. The 8C isomer (+0.49 eV) contains three bridging carbonyls and five terminal carbonyls coordinated to a V2Ni triangle, in which one terminal carbonyl is bonded to Ni atom, the other four terminal carbonyls are bonded to the two V atoms. The 8D isomer (+0.72 eV) consists of two side-on-bonded carbonyls, two bridging carbonyls, and four terminal carbonyls, in which one terminal carbonyl is bonded to Ni atom, the other three terminal carbonyls are bonded to two V atoms. The 8E isomer has a V-V-Ni chain with involvement of two side-on-bonded carbonyls, one bridging carbonyl, and five terminal carbonyls. It can be seen from Table 1 that the calculated VDE and ADE for 8A (3.81 and 3.51eV) shows good agreement with experiment (3.80 ± 0.04 and 3.34 ± 0.07eV), respectively.

V2Ni(CO)9−.

In

the

lowest-energy

isomer

for

V2Ni(CO)9−,

9A,

two

side-on-bonded carbonyls are coordinated to the V2 unit, two bridging CO molecules coordinated to the VNi unit, four terminal CO molecules coordinated to the two V atoms, and one terminal CO molecule coordinated to the Ni atom. Isomer 9B consists of a V2Ni triangle with the involvement of three bridging carbonyls and six terminal carbonyls, which lies +0.15 eV above 9A. The 9C-9E isomers consist of the V-V-Ni 7



chain with involvement of three different coordination modes of the CO molecules. The calculated values of VDE and ADE for 9A (3.71 and 3.36 eV) are in good agreement with the experimental values (3.81 ± 0.04 and 3.41 ± 0.06 eV) (Table 1). Note that the calculated VDE value of 9B (3.84 eV) is also consistent with the experimental value, the coexistence of 9A and 9B is likely here, implying a structural change for the upcoming larger cluster.

V2Ni(CO)10−. The lowest-energy isomer of V2Ni(CO)10− (10A) has a V2Ni triangle core with the involvement of three bridging carbonyls and five terminal carbonyls. Interestingly, the C atom of one bridging carbonyl is coordinated to all the three metal atoms. In the 10B isomer (+0.56 eV), three bridging CO ligands and six terminal CO ligangs are bonded to the V-V-Ni chain. Similar to the 10B isomer, the 10C−10E isomers consist of the V-V-Ni chain with involvement of three different coordination modes of the CO molecules. The calculated VDE and ADE values of 10A (4.00 and 3.78 eV) agree with the experimental values (3.93 ± 0.04 and 3.65 ± 0.06 eV).

5.

Discussion It can be inferred from the above analysis that the agreement between the

experimental and theoretical results was obtained, which allows for exploring the general trend of chemical bonding of the V2Ni(CO)n− (n = 6−10) clusters. The VDE of V2Ni(CO)n− (n = 6−10) increases with the increase of cluster size, indicating that the sequential bonding of the CO molecules stabilizes the negative electron. It can be seen from Table 1 that the experimental VDE and ADE values of the n = 8 cluster are almost the same as those of the n = 9 cluster. As shown in Figure 3, the trend of VDE values for the n = 6−9 clusters tend to converge around n = 8, suggesting that the structural motif of n = 8 is similar to that of n = 9. The experimental values of VDE and ADE for n = 10 are observably larger than those for n = 9, implying that the building block of the n = 10 cluster is different from that of the n = 9 cluster. It can be found from the above structural analysis that the binding motif of a V-V-Ni chain is 8


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favored in the n = 6–9 clusters and a new binding motif of a V2Ni triangle core structure is preferred at n = 10. These findings are consistent with the trend of VDE value as a function of cluster size (Figure 3). The V2Ni(CO)6− complex consists of two side-on-bonded, two bridging, and two terminal carbonyls. In contrast, TiNi(CO)6− includes three bridging and three terminal carbonyls.29 Ni2(CO)6 has two bridging and four terminal carbonyls.41 Experimental and theoretical studies on the interaction of carbon monoxide with 3d metal dimers (scandium through copper) have demonstrated that on going from left to right across the 3d series, the preference for bonding mode of CO to the metal dimer is from side-on-bonded (Sc2 and Ti2) to bridging (V2 and Cr2), and then to terminal (Mn2 and Cu2), whereas Ni2CO prefers bridging configuration.20, 41-48 Interestingly, the V2 unit is found to accommodate two side-on bonded carbonyls in each cluster of the V2Ni(CO)n− (n = 6−9) series. This exhibits that the C−O bonding is significantly weakened by the incorporation of nickel into the V2 system, implying that the CO activation could be appropriately adjusted via the selection of different transition metals. Figure 4 shows the highest occupied molecular orbitals (HOMO) of V2Ni(CO)n− down to the fourth valence molecular orbital from the HOMO. The HOMO of the most stable isomer for V2Ni(CO)6− (6A) features π-type bonds with striking metal to carbonyl donation. The HOMO-1, HOMO-2, HOMO-3, and HOMO-4 of 6A consist of delocalized π orbitals, in which the metal-carbon unit is mainly involved. Analogous chemical bonding was also observed in the most stable isomers for V2Ni(CO)n− (n = 7–10) (7A–10A) (Figure 4). This bonding mechanism is similar to that of M2[η2(µ2-C, O)] (M = Sc, Ti, Y, La, Ce, Gd),29, 43-45, 49-51 in which the C−O bonding is significantly weakened. In general, there are four different adsorption sites on the closely packed surface (i.e., Cu(111)): the atop site which resides above a surface atom, two 3-fold hollow sites which correspond to the ‘‘fcc site’’ and the ‘‘hcp site’’ (the hcp site resides above an atom of the second substrate layer; the fcc site does not), and the ‘‘bridge’’ site which lies halfway between the fcc and hcp sites. Interestingly, the V2Ni(CO)10− 9



complex is found to have two carbonyls with the carbon atom triply coordinated to metal atoms, three bridging carbonyls, and five terminal carbonyls, in which CO bonding configurations are reminiscent of adsorption fashions in the 3-fold hollow, bridging, and atop sites on the well-defined surface, respectively. This feature exhibits the effect of stepwise CO bonding on the structure and bonding mechanism of heteropolynuclear metal clusters, which would advance the understanding of the multifaceted mechanisms of the adsorption of small molecules (i.e., CO2, N2, H2O, CH4, NH3, etc.) on the alloy surfaces/interfaces and catalysts.

6.

Conclusion Mass-selected photoelectron velocity-map imaging spectroscopy combined with

quantum chemical calculation was used to probe the chemical bonding and electronic structures of heterotrinuclear carbonyls V2Ni(CO)n− (n = 6–10). Experimental and theoretical results indicate that the binding motif in the most stable isomer of the n = 6 cluster, a V-V-Ni chain with two side-on-bonded carbonyls and two bridging carbonyls, is retained in the lowest-energy isomers of n = 7–9. A V2Ni triangle core structure is formed at n = 10 with the involvement of two carbonyls with the carbon atom triply coordinated to metal atoms, three bridging carbonyls, and five terminal carbonyls, which are reminiscent of adsorption features in the 3-fold hollow, bridging, and atop sites on the closely packed surface, respectively. The present findings reveal the effect of stepwise CO bonding on the structure and bonding mechanism of heteropolynuclear metal clusters, which would advance the understanding of the multifaceted mechanisms of the adsorption of small molecules (i.e., CO2, N2, H2O, CH4, NH3, etc.) on the alloy surfaces/interfaces and catalysts.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 21327901, 21503222, 21673231, and 21688102), the Key Research Program (Grant No. KGZD-EW-T05), and the Strategic Priority Research Program (Grant No. XDB17010000) of the Chinese Academy of Science. 10


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Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Mass spectrum of the ions produced by pulsed laser vaporization of metal target in an expansion of 5% CO seeded in helium (Figure S1) and Comparison of experimental photoelectron spectrum of V2Ni(CO)6− to the simulated spectra of the 6A–6E isomers (Figure S2).

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Table 1. Comparison of Experimental and BP86 Calculated VDE and ADE Values for V2Ni(CO)n– (n = 6–10).

Cluster

Isomer

n=6

6A 6B 6C 6D 6E 7A 7B 7C 7D 7E 8A 8B 8C 8D 8E 9A 9B 9C 9D 9E 10A 10B 10C 10D 10E

n=7

n=8

n=9

n = 10

a

VDE (eV) Exp.a Calc. 3.15(8) 3.32 2.70 3.40 3.25 3.26 3.46(6) 3.61 2.85 3.57 3.39 2.98 3.80(4) 3.81 3.28 3.53 3.30 3.68 3.81(4) 3.71 3.84 3.80 4.01 3.81 3.93(4) 4.00 3.75 3.79 3.72 3.75

ADE (eV) Exp.a Calc. 2.77(9) 2.96 2.34 3.21 2.72 2.75 3.08(8) 3.25 2.67 2.92 3.17 2.62 3.34(7) 3.51 2.92 3.37 2.95 2.96 3.41(6) 3.36 3.64 3.43 2.87 2.91 3.65(5) 3.78 3.16 3.40 3.49 3.42

Numbers in parentheses represent the uncertainty in the last digit.

17



E

Figure 1. Photoelectron images of V2Ni(CO)n– (n = 6–10) at 266 nm (4.661 eV). The raw image (upper) and the reconstructed image (bottom) after inverse Abel transformation are shown on the left side. The double arrow indicates the direction of the laser polarization. Photoelectron spectra are shown on the right side.

18


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6A (C1, 2A) 0.00

6B (C1, 2A) +0.40

6C(C1, 2A) +0.42

6D (C1, 2A) +0.69

6E (C1, 2A) +0.72

7A (C1, 2A) 0.00

7B (C1, 2A) +0.42

7C (C1, 2A) +0.60

7D (C1, 2A) +0.63

7E (C1, 2A) +0.79

8A (C1, 2A) 0.00

8B (C1, 2A) +0.37

8C (C1, 2A) +0.49

8D (C1, 2A) +0.72

8E (C1, 2A) +0.73

9A (C1, 2A) 0.00

9B (C1, 2A) +0.15

9C (C1, 2A) +0.29

9D (C1, 4A) +0.79

9E (C1, 2A) +1.01

10A (C1, 2A) 0.00

10B (C1, 2A) +0.56

10C (C1, 2A) +0.84

10D (C1, 2A) +1.03

10E (C1, 2A) +1.32

Figure 2. Optimized structures of the V2Ni(CO)n− (n = 6−10) anions (V, yellow; Ni, blue; C, black; O, red). Relative energies are given in eV.

19



Figure 3. The vertical detachment energy (VDE) of V2Ni(CO)n− (n = 6−10) as a function of cluster size.

20


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6A

HOMO

HOMO-1

HOMO-2

HOMO-3

HOMO-4

-0.03361

-0.03966

-0.05211

-0.07108

-0.07594

-0.03413

-0.03878

-0.06828

-0.07133

-0.05686

-0.06362

-0.08105

-0.08350

-0.04830

-0.05692

-0.07632

-0.07955

-0.06227

-0.07185

-0.08567

-0.09335

Alpha MOs

Beta MOs

7A

Alpha MOs -0.04991 Beta MOs

8A

Alpha MOs -0.06078 Beta MOs -0.05895

9A

-0.06130

-0.08121

-0.08866

Alpha MOs -0.05735

-0.06024

-0.07788

-0.09212

-0.09640

-0.05250

-0.06978

-0.09166

-0.09514

-0.07854

-0.08056

-0.09507

-0. 10111

-0.07391

-0.07698

-0.09072

-0. 09393

Beta MOs

10A Alpha MOs -0.07537 Beta MOs

Figure 4. Molecular orbital pictures of the most stable isomers for V2Ni(CO)n− (n = 6−10) (6A−10A), showing the highest occupied molecular orbitals (HOMO) down to the fourth valence molecular orbital from the HOMO. The orbital energies are given in hartree.

21



TOC Graphic

+ nCO

V2Ni(CO)n− (n = 6− −10)

22


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Photoelectron Velocity-Map Imaging and Theoretical Studies of

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