J. Phys. Chem. 1995, 99, 3053-3055
3053
Structures and Ionization Energies of Sandwich Clusters (V,(benzene),) Kuniyoshi Hoshino, Tsuyoshi Kurikawa, Hiroaki Takeda, Atsushi Nakajima, and Koji Kaya* Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223, Japan Received: September I , 1994; In Final Form: December 2, 1994@
Metal-molecule clusters V,(benzene), were synthesized by the reaction of laser-vaporized vanadium atoms with benzene vapor. The clusters exhibit magic number behavior at m = n - 1, n, and n 1 (n = 1-5), which is explicable in terms of the multiple decker sandwich structure for these stable clusters. The ionization energies of these sandwich clusters decrease significantly with increasing cluster size.
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1. Introduction Since the development of the synthesis of metal clusters by laser vaporization of the metal rod, various kinds of transitionmetal clusters which cannot be produced by a crucible method because of their high boiling temperatures have been studied under molecular beam conditions. Several groups independently have succeeded in the synthesis of metal-molecule complexes in the gas phase by modifying the laser vaporization method.'-' Metal-molecule complexes in the gas phase have been studied in order to investigate the interaction between the metal atom and ligand molecules without the influence of the solvent. Recently, Duncan and his co-workers and Freiser and his coworkers reported the photofragmentation processes of metal ion-benzene c o m p l e ~ e s . ~Armentrout -~ and co-workers determined the binding energy of a metal ion-benzene complex in their collision-induced fragmentation experiments.I0 Most of these studies have been concerned with mononuclear complexes. In this study, we successfully produced polynuclear metalbenzene complexes by the laser vaporization method. In the present paper, we will discuss the stability and ionization energy (Ei)of the complexes between vanadium atoms and benzene molecules which were produced under molecular beam conditions by the use of the laser vaporization method. Evidence that the structures of these complexes are multiple decker sandwiches of alternating vanadium and benzene molecules is provided.
2. Experimental Section V,(benzene), clusters were synthesized by the following procedure. First, V atoms were vaporized by the irradiation of the second harmonic of a pulsed Nd3+: YAG laser (532 nm), and vaporized hot V atoms were cooled to room temperature by a pulsed high-pressure (10-atm) He carrier gas. Then, the V atoms were sent into a flow-tube reactor where benzene vapor seeded in a He gas was injected in synchronization with the flowing of the V atoms. The VJbenzene), clusters thus generated were sent into the ionization chamber through a skimmer and were ionized by an ArF excimer laser (6.42 eV) or second harmonic of a dye laser. The photoions were mass analyzed by a reflectron time-of-flight mass spectrometer. In order to obtain information on the structure of V,(benzene), clusters, V,(benzene), clusters were further reacted with CO gas inside the second flow-tube reactor, which is added downstream of the benzene addition, and the V,(benzene),(CO)k clusters produced were also analyzed. @
Abstract published in Advance ACS Abstracts, February 1, 1995.
0022-365419512099-3053$09.00/0
In the Ei measurement, the second harmonic of a dye laser pumped by a XeCl excimer laser was used as the ionization laser. The photon energy was changed at a 0.03-0.05-eV interval in the range of 5.9-3.5 eV, while the abundance and composition of the VJbenzene), clusters were monitored by the ionization of an ArF laser. The fluences of both the tunable ultraviolet (W)laser and the ArF laser were monitored by a pyroelectric detector (Molectron 5-3) and were kept at -200 pJlcm2 to avoid multiphoton processes. To obtain photoionization efficiency curves, the ion intensities of the mass spectra ionized by the tunable UV laser were plotted as a function of photon energy with the normalization by both the laser fluence and the ion intensities of ArF ionization mass spectra. The El's of the V,(benzene), clusters were determined from the final decline of the photoionization efficiency curves. The uncertainty of the El's is estimated to be typically f0.05eV. 3. Results and Discussion
3.1. Multiple Decker Sandwich Structure of the V,(benzene),,, Cluster. Parts a and b of Figure 1 show typical examples of the mass spectra of the V,(benzene), clusters produced by the above-described procedure. The fluence of the vaporization laser is relatively low in Figure l a and high in Figure lb. Zakin et al. have reported the reactivity study of V,+ clusters toward benzene." Their experimental setup is almost the same as ours. A distinct difference in the experiments is that Zakin et al. used low concentrations of benzene in order to estimate the reaction rate, whereas higher concentrations of benzene were injected upon V atoms in our case. Under our experimental conditions, all the V, clusters completely underwent reaction. Therefore, the ion intensities of the respective V,(benzene), clusters in Figure 1 should reflect the stabilities of the clusters if the Ei's of the clusters are lower than the photon energy of the ionization laser (6.42 eV). As will be discussed later, in fact, the Ei's of the V,(benzene), clusters are well below 6.42 eV. The prominent clusters in Figure l a and l b are thus the stable clusters formed at both low and high concentrations of V atoms. The main products in the figures are V,(benzene),+l (Figure l a and lb), V,(benzene), (Figure lb), and V,+l(benzene), (Figure lb), with n extending from 1 to 5. It should be noted that no dehydrogenated species were observed in the mass spectra. The structure of Vl(benzene)e in an Ar matrix was determined to take the sandwich structure by EPR s p e c t r o ~ c o p y . ' ~Duff ~'~ et al. reported the triple decker sandwich structure of (C5H5)V(benzene)V(CsH5).l4 In analogy with these facts, multiple decker sandwich structures can explain the magic numbers observed in Figure la. The assumed structures of the stable 0 1995 American Chemical Society
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3054 J. Phys. Chem., Vol. 99, No. 10, 1995
(c) V + benzene + CO VJbenzene) ,(CO),= n-rn-k
(b) V + benzene
I
Hoshino et al. 1.2,
I
Photon Energy / eV
Figure 3. Photoionization efficiency curve of the Vl(benzene)* cluster normalized by the power of ionization laser. The photon energy was changed at an 0.015-eV interval in the range of 5.85-5.65 eV. From the final decline of the curve, the Ei of the cluster was determined to be 5.75 & 0.03 eV.
Mass number (mlz)
Figure 1. Time-of-flight mass spectra of V,(benzene), clusters (a and b) and V,(benzene),(CO)k clusters (c). V,(benzene), clusters were produced from a mixture of vaporized V atoms and benzene. ((b); at higher concentration of V atoms). V,(benzene),(CO)k clusters are products from a reaction of V,(benzene), with CO gas. (See text.) Peaks of the clusters are labeled according to the notations n - rn and n - rn - k, denoting the number of V atoms (n),benzene molecules (m),and CO molecules (k).
(a)
V,(benzene)
V2(benzene)
-
V,(benzene)
CD V4(benzene)
Figure 2. Proposed structures of V,(benzene),+l clusters (n = 1-4) (a) and V,(benzene),(CO)k clusters (2-2-3 and 3-2-6)
(b).
V,(benzene),+t clusters are shown in Figure 2a. Then, oneand two-V atoms should be on the exterior sides of the sandwiches for V,(benzene), and Vn+l(benzene),, clusters, as shown Figure lb. Two mechanisms are possible for the production of V,(benzene), clusters. They are (i) benzene molecules reacted with V clusters and (ii) benzene molecules reacted with V atoms. Morse and Spain have reported the binding energy of the V dimer to be 2.735 eV,I5 and moreover, Armentrout et al. have reported the binding energy of larger V, clusters to be 3-4 eV.I6 In consideration of these values, the former mechanism can be energetically ruled out for the sandwich cluster formation. The formation of the sandwich clusters by mechanism i needs the fragmentation of the V, clusters into V atoms. In the reaction with the V,+ clusters toward C6D6, furthermore, sequential dehydrogenation channels for VflC6Hk+( k 5 5) have been observed;lI the reaction of the cluster proceeds via facile initial chemisorption followed by activated C-D cleavage and DZelimination. As described above, however, no dehydroge-
nated species were observed in our condition. Then, mechanism i is again unfavorable. Since a large portion of laser-ablated V vapor exists in the form of atoms, it is reasonable that the V atoms cooled to room temperature react with benzene and, sequentially, form sandwich clusters. In the reaction between V atoms and benzene, no dehydrogenationprocess has been reported:" the mass spectrum observed in this work is consistent with this result. Mechanism ii seems to be the main route of the sandwich clusters. On the other hand, the reacted V, clusters with benzene could not be observed, probably because the products were distributed over a wide range of compositions of V,C,Hk under high concentrations of benzene. If the exterior V atom(s) of V,(benzene), and V,+l(benzene), is really uncoordinated, then it can be easily coordinated by CO molecules with the coordination number being 3, in consideration of the stable carbonyl complexes such as (benzene)Cr(C0)3.I7 Figure IC exhibits the mass spectrum of V,(benzene), clusters when CO gas is injected into the reaction tube under the same conditions as in Figure l b (high laser fluence). The mass peaks of V,(benzene),+l which have no exterior V atoms remain in the spectrum, indicating no reaction with CO. In contrast to this, the mass peaks for the V,(benzene), and V,+l(benzene), clusters are depleted completely by the reaction with CO molecules and the mass peaks corresponding to V,(benzene),(C0)3 (n = 2-4) and V,+l(benzene),(C0)6 (n = 2 and 3) newly appear in the spectrum. These carbonyl clusters in which three CO molecules coordinate to each exterior V atom should take the structures as shown in Figure 2b. All of these experimental facts bring about the consensus that V,(benzene), clusters are stabilized by taking the multiple decker sandwich structure. 3.2. Ionization Energy and Binding Energy of the V,(benzene),,, Cluster. The ionization energies of the V,(benzene), clusters were determined by the photoionization method using the tunable W laser. Figure 3 shows a photoionization efficiency curve for Vl(benzene)2. The E, of the Vl(benzene)z cluster was determined to be 5.75 f 0.03 eV from the final decline of the curve. Similarly, The Ei's of the V,(benzene), clusters were determined as listed in Table 1. The Ei value of Vl(benzene)l (5.11 eV) is lower than the E, values of the V atom (6.74 eV) and benzene (9.24 eV). This enormous decrement cannot be explained in a simple oneelectron treatment of the complex. As listed in the table, the Ei value decreases greatly as n is increased. Theoretical treatment for the explanation of the dramatic change in Ei values by the complex formation is in progress in our laboratory. From the observed Ei values of Vl(benzene)l (Ei = 5.11 eV) and Vl(benzene)z (E1 = 5.75 eV) combined with the known binding energies of Vl(benzene)l+ (DO= 2.69 eV) and
Structures and Energies of Sandwich Clusters
J. Phys. Chem., Vol. 99, No. 10, 1995 3055 combination of vanadium atoms and benzene molecules is not clear at the present stage.
TABLE 1: Ionization Energies (eV) of V,(benzene), Clusters composition structure'
n
m
E!
no ext V atoms
1 2 3 4 1 2 3 3 4
2 3 4 5 1 2 3 2 3
5.75(3) 4.70(4) 4.14(5) 3.83(5) 5.11(4) 4.49(4) 4.27(6) 4.67(5) 4.23(6)
1 ext V atom
2 ext V atoms
Acknowledgment. This work is supported by a Grant-inAid for Scientific Research on Priority Areas from the Ministry of Education, Science and Culture. K. H. expresses his gratitude to the Japan Society for the Promotion of Science for Japanese Junior Scientists. References and Notes
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In the V,(benzene), clusters, the clusters of m = n 1, n , and n - 1 take the structures having 0, 1, and 2 exterior V atoms, respectively. (See text.) The value in parentheses indicates an experimental uncertainty; 5.75(3) represents 5.75 f 0.03.
Vl(benzene)z+ (DO= 2.74 eV)18 and the ionization energy of V atom (Ei = 6.74 eV), the binding energies of the neutral complexes, Vl(benzene)l and Vl(benzene)z, were evaluated by the following formulas: D0(V-Bz) = Ei(VBz) D,(B~V-BZ) = E,(VBZ,)
+ D0(V+-Bz)
+ D,(BZV+-BZ)
- EJV)
(1)
- E,(VBZ) (2)
The obtained binding energies are 1.14 eV for Vl(benzene)l and 3.03 eV for Vl(benzene)z. The value for Vt(benzene)2 is as large as the binding energy of Fe-GH5 in ferrocene (3.18 eV).I9 A qualitative explanation for the stability of Vl(benzene)2 may be given by the 18-electron rule because Vl(benzene)z has 17 valence electrons which are close to, but less than, the closing of 18 electrons. As well-known already, however, the binding energy of Cr-benzene in Crl(benzene)z is relatively small (1.716 eV)20irrespective of the completeness of the 18-electron rule. Although we could produce Crl(benzene)z in a molecular beam, multiple decker sandwich clusters, Cr,(benzene), (n 2 2 and m = n f 1 or n), could not be generated in our experimental conditions. These results reflect on the fact that the one-electron lacking in the 18-electron system is a key to form the sandwich V,(benzene), clusters. The reason why preferential formation of the sandwich clusters occurs in the
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