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Formation of vanadium-arene complex anions and their photoelectron spectroscopy
16May 1997
CHEMICAL PHYSICS LETTERS
ELSEVIER
Chemical Physics Letters 270 (1997) 23-30
Formation of vanadium-arene complex anions and their photoelectron spectroscopy Ken Judai a, Masaaki Hirano a, Hiroshi Kawamata a, Satoshi Yabushita a, Atsushi Nakajima a,b, Koji Kaya a,b,* a Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223, Japan b The Institute of Physical and Chemical Research (RIKEN), Wako 351-01, Saitama, Japan
Received 7 February 1997; in final form 3 March 1997
Abstract
Vanadium-arene complex anions, V~(arene)~ (m = 1, 2; arene = benzene, fluorobenzene, and toluene) were synthesized by the gas phase reaction of laser vaporized metal atoms with aromatic compound vapors. According to the 18-electron rule, it was anticipated that bis(benzene)vanadium should be stabilized by an electron attachment. However, it was experimentally concluded that bis(benzene)vanadium has negative electron affinity. The photoelectron spectra for the observed complex anions were measured, and the electronic configurations of metal-arene complexes and their structures are discussed.
1. I n t r o d u c t i o n Organometallic chemistry has been developed rapidly since the discovery of ferrocene [1], and various kinds of complexes were synthesized from many metal atoms and many organic molecules. The stability of the complexes is qualitatively explained by counting a number of valence electrons. The electron counting is useful in the explanation for transition metal complexes whose stoichiometries correspond to what is called the 18-electron rule [2]. For ferrocene, indeed, Fe(Cp) 2, Fe 2÷ atom has 6 valence electrons, and cyclopentadienyl anion, C p - ,
* Corresponding author. At: Department of Chemistry., Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223, Japan.
has 6, and then the total number of valence electrons is 18. Since it is phenomenologically deduced that the 18 electrons occupy all its valence orbitals, namely, the five nd, the (n + 1)s, and the three (n + 1)p orbitals as fully as possible in metal-ligand bonding, organometallic complexes having 18 valence electrons are expected to be very stable. These organometallic complexes have been experimentally characterized in solution and in the solid state with the combined efforts of synthetic and spectroscopic studies. The molecular beam technique [3-7] or the matrix isolation technique [8-15] makes it possible to investigate the bonding nature of complexes under little or no influence of the surrounding environmental factors. Bersohn et al. have measured the absorption spectrum of bis(benzene)chromium in the molecular beam condition [3]. Armentrout et al.
0009-2614/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. Pll S0009-261 4(97)00336-9
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K. Judai et al. / Chemical Physics Letters 270 (1997) 23-30
have determined the binding energies of metal ionbenzene complexes in their collision induced dissociation experiments [4,5]. Ozin et al. have synthesized vanadium-benzene complexes under Ar matrix isolation condition and analyzed their physical properties by electron paramagnetic resonance (EPR) spectroscopy and so on [8-10], and very recently, Mattar et al. have refined the EPR experiment [15]. Recently, vanadium-benzene clusters, Vn(benzene) m, (Ref. [16]) and cobalt-benzene clusters, Co~(benzene) m, (Ref. [17]) were synthesized by the gas phase reaction of laser vaporized metal atoms with benzene vapor in our group, and were characterized by mass spectroscopy and photoionization spectroscopy. The mass spectrum of vanadium-benzene clusters, V~(benzene) m, exhibited magic number behavior at m = n + l ( n = 1 - 5 ) . It was concluded that the V~(benzene) m clusters take multiple-decker sandwich structures, where vanadium atoms and benzene molecules are piled up alternatively. In this Letter, we report the photoelectron spectra of vanadium-arene complex anions, Vj(arene)~ (m = 1, 2; a r e n e = b e n z e n e (C6H6; Bz), toluene (C6HsCH3; Tol), fluorobenzene (C6HsF; FBz), using a magnetic-bottle type photoelectron spectrometer, in order to clarify electronic structures of the vanadium-arene complex. Photoelectron spectroscopy of a mass-selected cluster anion is a powerful method to investigate the electronic structure of neutral clusters because the neutral electronic states are observed as final states. On the contrary to the prediction of the 18-electron rule, bis(benzene) vanadium anion was not generated. We will discuss the stability of the 18-electron compound, bis(benzene)vanadium anion, and will reveal the geometrical and the electronic structures of the organometallic complexes from their photoelectron spectra.
2. Experimental Details of experimental setup have been provided elsewhere [16,18]. Organometallic cluster anions were prepared by two methods; (1) the complex anions production in the reaction between laser vaporized metal anion and arene molecules and (2) slow electron attachment to neutral organometallic
clusters. In both two methods, V atoms were vaporized by the irradiation of the second harmonic of a pulsed Nd 3+ :YAG laser (532 nm), and vaporized hot metal species of atoms, clusters, and their charged species were cooled to room temperature by a pulsed He carrier gas (10 atm). Then the metal species were sent into a flow-tube reactor where benzene (or fluorobenzene or toluene) vapor seeded in He (1-2 atm) was injected. The laser vaporization method can generate neutral, cationic and anionic clusters, and the produced anions were directly used for the target anions in method (1). In method (2), slow electrons were additionally attached to neutral organometallic clusters which were synthesized by the above laser vaporization method. The irradiation of the second harmonic of the other Nd3+:YAG laser (2.34 eV) onto Y203 plate (work function; 2.0 eV) produces slow electrons (0-0.5 eV) by a photoelectric effect. The electrons generated by this method were attached to neutral organometallic clusters just after the nozzle expansion. The cluster anions produced by either method (1) or (2) were analyzed by time-of-flight (TOF) mass spectrometer. The cluster anions was accelerated by applying a pulsed electric field (1-3 keV). After a 1.5 m flight path, the cluster anions were detected by a dual microchannel plate and the signal was digitized and averaged in a digital oscilloscope (LeCroy 9400). In the case of measurement of photoelectron spectra, the accelerated clusters were mass-selected by pulsed deflection plates, and were decelerated by a potential elevator before photodetachment. The kinetic energy of the photodetached electrons with the third harmonic of Nd3+:YAG laser (355 nm; 3.49 eV) were analyzed by a magnetic bottle type photoelectron spectrometer. The repetition rate was 10 Hz, and the photoelectron signal was typically accumulated to 15000-30000 shots by a multichannel scaler/averager (Stanford Research System, SR430). The two types of the anion sources gave the same photoelectron spectra for all the species. The energy of the photoelectron was calibrated by measuring photoelectron spectra of noble metal atom anions, e.g. C u - , A g - , Au-. The power density of the detachment laser was in the range of 10-20 m J / c m 2, and no power dependent processes for the spectrum shape were observed.
K. Judai et al. / Chemical Physics Letters 270 (1997) 23-30
25
Vn(benzene)m(fluorobenzene)k" = (n,m,k)
3. Results and discussion
(1,1,1)
3.1. Production of vanadium-arene complex anions
(1,o,2) Fig. 1 shows a T O F mass spectrum o f v a n a d i u m - b e n z e n e complex anions produced by the laser vaporization method without any additional electron attachment in method (1). The peaks are labeled according to the notation (n, m), denoting the number of vanadium atoms ( n ) and benzene molecules (m). All o f the complex anions contain one benzene molecule, as indicated by (n, m) = (1, 1), (2, 1), (3, 1), and (4, 1) in Fig. 1. However, the (3, 1), (4, 1) complexes were mainly observed as dehydrogenated species, as indicated by (3, 1)-4H, (4, I)-6H, where (3, 1)-4H corresponds to 4 H-atoms elimination from (3, 1). It is deduced that these dehydrogenated complex anions resulted from the reactions o f vanadium cluster anions toward a benzene molecule. In fact, the H atom elimination reaction has been reported also in the reaction of vanadium cluster cations, V~+ ( n / > 2), toward a benzene molecule [19,20]. In the elimination of H atoms, moreover, the H atoms are seemingly eliminated as H 2 molecule, because only even number of the H atom loss were observed. Here, it should be noted that bis(benzene)vanadium anion (1, 2) was not observed even in the higher concentration of benzene vapor. Since the
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number o f valence electrons o f vanadium is 5 and that of benzene is 6, bis(benzene)vanadium has 17 valence electrons. The 18-electron rule predicts that bis(benzene)vanadium anion should form a stable complex due to the full occupation of 18 electrons. However, the bis(benzene)vanadium anion was not observed at all. The mass spectra o f neutral and cationic v a n a d i u m - b e n z e n e clusters in the same method exhibited the abundant production of bis(benzene)vanadium [16,21]. Therefore it seems contradictory that bis(benzene)vanadium anion was not generated irrespective of satisfying the 18-electron rule. Based on the results under method (1), it is presumed that the bis(benzene)vanadium anion is unstable and has a negative electron affinity (EA). To make sure o f its negative EA, slow electrons were attached to neutral bis(benzene)vanadium. The laser vaporized vanadium atoms were reacted toward benzene and fluorobenzene mixed vapor, and slow electrons generated by photoelectric effect of Y203 were attached; by method (2). The benzene and fluorobenzene mixed vapor was used for presenting reliable evidence for cluster anions formation. Fig. 2 shows a T O F mass spectrum o f v a n a d i u m b e n z e n e - f l u o r o b e n z e n e complex anions produced by this method (2). The notation (n, m, k) means the number o f vanadium atoms (n), benzene (m), and fluorobenzene (k). In the measurement of the mass spectrum, almost no anions were detected without
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K. Judai et al. / Chemical Physics Letters 270 (1997) 23-30
the slow electron attachment. By the aid of the slow electron attachment, the intensities of the peaks of (1, 1, 1) and (1, 0, 2) were drastically enhanced, whereas those of the bis(benzene)vanadium anion (1, 2, 0) were not observed at all, although both their neutral and their cationic complexes were produced. In a similar experiment, slow electron attachment was found to generate successfully a cyanobenzene anion despite an extremely small EA (0.0 ___0.1 eV) [22]. This indicates that the attachment of slow electrons can generate anions, when the corresponding neutral has positive EA and when an anionic state is vertically accessible from the neutral ground state. When the excess electron is acceptable to the neutral without serious geometric distortion, its anion is expected to be produced by the electron attachment. Since a vacant molecular orbital (MO) for the excess electron in the neutral bis(benzene)vanadium is nonbonding MO (a~), its occupation of the excess elec-
tron seemingly occurs without any serious geometric change. Indeed, the photoionization efficiency curve for Vl(benzene) ~- production from its neutral, which corresponds to one electron removal from the nonbonding MO, shows a sharp rise, implying little geometric change in the electron ejection from the non-bonding orbital. Nevertheless, bis(benzene)vanadium anion was not generated as shown in Figs. 1 and 2. Therefore, we concluded that bis(benzene)vanadium has negative EA.
3.2. Negative electron affinity of bis(benzene)vanadium Bis(benzene)vanadium has a negative EA contrary to the prediction based on the 18-electron rule; the bis(benzene)vanadium anion cannot exist as a stable species in the gas phase. The negative EA of bis(benzene)vanadium has been predicted by an elec-
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K. Judai et al. / Chemical Physics Letters 270 (1997) 23-30
trochemical study [23,24]. Elschenbroich and coworkers have measured the electrochemical redox potential of bis(benzene)vanadium by cyclic voltammetry in a strong polar solvent [23]. Although the bis(benzene)vanadium anion is produced under electrochemical reduction at - 2 . 7 1 eV, the reduction potential of - 2 . 7 1 V is fairly negative, and a sequence of decreasing the reduction potential is naphthalene > biphenyl = bis(benzene)vanadium > benzene. Based on this electrochemical experiment, the EA of bis(benzene)vanadium in the gas phase is expected to be negative, because the EAs of naphthalene, biphenyl, and benzene in the gas phase were determined by electron transmission spectroscopy, as - 0 . 2 , - 0 . 3 , - 1.11 eV, respectively [25]. This prediction agrees well with our conclusion from the mass spectra and the electron attachment experiment. Fig. 3a and 3b show schematic MO diagrams for Vl(benzene) l and Vl(benzene) 2 under C6v and D6h symmetry, respectively. Metal 3d orbitals split into three levels with benzene ligand(s) under C6v or D6h symmetry. The 3dxz and 3dy z orbitals (elg) interact with benzene rr orbital, forming antbbonding orbitals. The 3dxy and 3dx2_y2 orbitals (e2g) interact with the benzene -rr * orbital, forming bonding orbitals. The 3dz2 orbital (a~g) does not interact with benzene orbitals and remains a non-bonding orbital. The 5 valence electrons of the vanadium atom occupy two bonding orbitals (ezg) and one non-bonding orbital (a~g), making the non-bonding orbital half-filled. To produce the bis(benzene)vanadium anion, one more electron occupies the half-filled nonbonding orbital, and completes a closed shell. According to the one electron orbital model, the total electronic energy does not depend on occupation of the non-bonding orbital, and it is predicted that the anion is stably formed due to pairing energy. Then, non-formation of the bis(benzene)vanadium anion shows that the one electron orbital model is insufficient to predict its electronic properties, although it is actually useful to describe the electronic structures qualitatively. Namely, negative electron affinity of bis(benzene)vanadium means a breakdown of the one electron orbital model in this system, where it seems crucial to take account of repulsive interaction between one excess electron in the non-bonding orbital and the other orbital electrons. In order to confirm the negative electron affinity,
27
we have performed ab initio calculations, namely, the geometry optimizations of bis(benzene)vanadium and its anion at open-shell and closed-shell RHF levels, respectively, with the MIDI basis sets for C and H atoms [26] and valence triple zeta basis set for V atom [27,28]. The obtained EA was - 1 . 9 9 eV. Furthermore, the optimized V-benzene distances of the neutral and anion structures were 1.797 and 1.781 A, respectively, supporting the previous discussion on the negligible geometric change in the neutral and anion forms. These theoretical results are also in accord with the recent theoretical calculations by Hada et al. with the SAC-CI method [29]. Behavior contrary to the 18-electron rule is not only the case for anions but also for neutral complexes. The 18-electron rule suggests that the ionization energy of bis(benzene)chromium should be large because the neutral bis(benzene)chromium stably takes a closed shell structure of 18 electrons, whereas the bis(benzene)chromium cation is a open shell compound. However, the ionization energy of the bis(benzene)chromium (5.43 eV) is smaller than those of bis(benzene)vanadium (5.75 eV) and bis(benzene)titanium (5.71 eV) [3,16,30]. The relatively small ionization energy of the bis(benzene)chromium cannot be explained by the 18-electron rule under the one electron orbital model. o
3.3. Photoelectron spectra of vanadium-arene complex anions Although bis(benzene)vanadium has negative EA, bis(fluorobenzene)vanadium anion was observed in the mass spectra (see Fig. 2). There is a question whether only the substituent effect of a fluorine atom changes the EA of bis(fluorobenzene)vanadium into a positive value. To understand the substituent effect, we measured the photoelectron spectra of complex anions of benzene, fluorobenzene, and toluene. It should be noted that a fluorine atom works as an electron-withdrawing group, accepting electron from benzene -rr orbitals. On the other hand, a methyl group works as an electron-donating one.
3.3.1. Vl(arene)f- anions Fig. 4 shows the photoelectron spectra of mono(arene)vanadium anions, VI(Tol)~-, Vl(Bz)~-, and VI(FBz)[-. In the spectra, the horizontal axis
K. Judai et al./ Chemical Physics Letters 270 (1997) 23-30
28
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corresponds to the electron binding energy, Eb, defined as E b = h ~ ' - E k , where E k is the kinetic energy of the photoelectron. Arrows indicate threshold energies (ET), and the E T corresponds to the upper limit of the adiabatic EA. The values of E r (EA) are listed in Table 1. Compared with the spectra, the peak positions of the photoelectron spectrum of Vt(toluene) 1 were close to that of Vl(benzene)~-. This implies that toluene binds to vanadium atom under the same structure of benzene which is a half sandwich structure, and methyl group of toluene causes only a trivial shift of peak positions. It is reasonably presumed that the substitution of benzene into another arene makes the electronic structures similar around the HOMO, when the arene binds vanadium atom through d-Tr interaction similarly to benzene.
It should be noticed that the first peak is very sharp in both spectra (Fig. 4a and 4b). In the photodetachment process, the Franck-Condon factor depends on the spatial overlap of the vibrational wave functions of the anion and the neutral, under the assumption that the electronic transition dipole moment does not change significantly. Then, the features in the photoelectron spectra become broader by the large geometrical change upon the photodetachment. This is because the degrees of freedom of bending or rocking vibration having low frequency increase with the dimensions of the structure and the peaks of these low-frequency vibration excited cannot be observed separately. Namely, the most plausible explanation for the first sharp peak is that the photodetached electron is ejected from a non-bonding orbital. Otherwise the peak should become broader due to geometric changes. Under one electron orbital model, the HOMO of Vl(benzene) 1 is certainly non-bonding orbital (a l) as shown in Fig. 3(a), and the first peak is assigned to the photodetachment from the non-bonding orbital (a~). Indeed, this explanation is in agreement with the EPR experiment; the neutral V~(benzene)~ has the half-filled non-bonding orbital (al), taking a 2A l ground state
[15]. In the case of fluorobenzene, however, the spectrum of V~(fluorobenzene) 1 was entirely different from that of Vl(benzene)i-. This difference cannot be explained only by the substituent effect of a fluorine atom on the benzene ring. It is conceivable that V~(fluorobenzene)~- does not take the half sandwich structure, but takes a different structure. Although
Table 1 Electron affinities (EA) of Vn(arene),, complexes in eV Composition
EA a
(i) n - m = 1 - 1
Vl(benzene) l Vl(toluene) l Vl(flu°r°benzene) i
0.62(7) b 0.76(7) 1.15(12)
(ii) n - m = 1 - 2
Vl(benzene) 2 Vl(benzene) t(fluorobenzene) I Vl(fluorobenzene) 2
negative 1.28(13) 1.74(37)
a The threshold energies of photoelectron spectra correspond to the adiabatic electron affinities. b Numbers of parentheses are uncertainties; 0.62(7) represents 0.62 _+0.07.
K. Judai et a l . / Chemical Physics Letters 270 (1997) 2 3 - 3 0
29
we have no definite ideas about this different structure, we suggest the plausible structure in which a fluorine atom and a vanadium atom are bonded directly. In the reaction of a vanadium atom anion and a fluorobenzene, the oxidative addition reaction should occur [31]. In this reaction, V atom should also interact with the F atom of fluorobenzene, and then the V atom is coordinated with both the phenyl ring and the F atom, resulting in the structure as shown in Fig. 4(c). Thus, it is rationalized that the candidate for the structure of V~(fluorobenzene)]gives a different electronic structure from that of V~(Bz)~-. In fact, the fluorine atom adducts of (1, 1 ) + F and (2, 1 ) + F were produced as a minor product, in which a V - F bond is formed. These species should be generated by the elimination reaction of a phenyl group which was caused by the oxidative addition.
compared to the negative EA of bis(benzene)vanadium. If the substitution of fluorobenzene to benzene has a cumulative effect on EA [32], we can estimate the EA of bis(benzene)vanadium, Vl(Bz) z, to be about 0.8 eV. As concluded above, however, its EA is negative. Therefore, this result also leads us to the conclusion that fluorobenzene does not coordinate with the V atom at the benzene ring. A c o m p a r i s o n b e t w e e n the spectra of Vl(Bz)l(FBz) 1 and VI(FBz)2, shows similar spectral features. In VI(Bz)I(FBz)~-, it is deduced that the benzene molecule coordinates with the V atom at the benzene ring. Then, their spectral similarity suggests that one of fluorobenzene molecules also coordinates with the V atom at the benzene ring in V~(FBz) 2 whereas the other fluorobenzene coordinates with both the F atom and a part of phenyl ring.
3.3.2. Vl(arene) ~- anions
Acknowledgements
Fig. 5 shows the photoelectron spectra of bis(arene)vanadium anions, VI(Bz)I(FBz) ~- and VI(FBz) 2. The E T (EA) of Vl(BZ)l(FBz)l and VI(FBz) 2 were determined to be 1.29 eV and 1.74 eV from the threshold, respectively, and the values are listed in Table 1. These EAs are fairly large
.-~~. ~::D(a) -' Vl(BZ)l~
We are grateful to Mr. Takasuke Hayase, Mr. Yuichi Negishi, and Mr. Fumitaka Hayakawa for technical assistance and to Mr. Tomokazu Yasuike for quantum chemical calculation. This work is partially supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science, and Culture.
References
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'
,
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CHEMICAL PHYSICS LETTERS
ELSEVIER
Chemical Physics Letters 270 (1997) 23-30
Formation of vanadium-arene complex anions and their photoelectron spectroscopy Ken Judai a, Masaaki Hirano a, Hiroshi Kawamata a, Satoshi Yabushita a, Atsushi Nakajima a,b, Koji Kaya a,b,* a Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223, Japan b The Institute of Physical and Chemical Research (RIKEN), Wako 351-01, Saitama, Japan
Received 7 February 1997; in final form 3 March 1997
Abstract
Vanadium-arene complex anions, V~(arene)~ (m = 1, 2; arene = benzene, fluorobenzene, and toluene) were synthesized by the gas phase reaction of laser vaporized metal atoms with aromatic compound vapors. According to the 18-electron rule, it was anticipated that bis(benzene)vanadium should be stabilized by an electron attachment. However, it was experimentally concluded that bis(benzene)vanadium has negative electron affinity. The photoelectron spectra for the observed complex anions were measured, and the electronic configurations of metal-arene complexes and their structures are discussed.
1. I n t r o d u c t i o n Organometallic chemistry has been developed rapidly since the discovery of ferrocene [1], and various kinds of complexes were synthesized from many metal atoms and many organic molecules. The stability of the complexes is qualitatively explained by counting a number of valence electrons. The electron counting is useful in the explanation for transition metal complexes whose stoichiometries correspond to what is called the 18-electron rule [2]. For ferrocene, indeed, Fe(Cp) 2, Fe 2÷ atom has 6 valence electrons, and cyclopentadienyl anion, C p - ,
* Corresponding author. At: Department of Chemistry., Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223, Japan.
has 6, and then the total number of valence electrons is 18. Since it is phenomenologically deduced that the 18 electrons occupy all its valence orbitals, namely, the five nd, the (n + 1)s, and the three (n + 1)p orbitals as fully as possible in metal-ligand bonding, organometallic complexes having 18 valence electrons are expected to be very stable. These organometallic complexes have been experimentally characterized in solution and in the solid state with the combined efforts of synthetic and spectroscopic studies. The molecular beam technique [3-7] or the matrix isolation technique [8-15] makes it possible to investigate the bonding nature of complexes under little or no influence of the surrounding environmental factors. Bersohn et al. have measured the absorption spectrum of bis(benzene)chromium in the molecular beam condition [3]. Armentrout et al.
0009-2614/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. Pll S0009-261 4(97)00336-9
24
K. Judai et al. / Chemical Physics Letters 270 (1997) 23-30
have determined the binding energies of metal ionbenzene complexes in their collision induced dissociation experiments [4,5]. Ozin et al. have synthesized vanadium-benzene complexes under Ar matrix isolation condition and analyzed their physical properties by electron paramagnetic resonance (EPR) spectroscopy and so on [8-10], and very recently, Mattar et al. have refined the EPR experiment [15]. Recently, vanadium-benzene clusters, Vn(benzene) m, (Ref. [16]) and cobalt-benzene clusters, Co~(benzene) m, (Ref. [17]) were synthesized by the gas phase reaction of laser vaporized metal atoms with benzene vapor in our group, and were characterized by mass spectroscopy and photoionization spectroscopy. The mass spectrum of vanadium-benzene clusters, V~(benzene) m, exhibited magic number behavior at m = n + l ( n = 1 - 5 ) . It was concluded that the V~(benzene) m clusters take multiple-decker sandwich structures, where vanadium atoms and benzene molecules are piled up alternatively. In this Letter, we report the photoelectron spectra of vanadium-arene complex anions, Vj(arene)~ (m = 1, 2; a r e n e = b e n z e n e (C6H6; Bz), toluene (C6HsCH3; Tol), fluorobenzene (C6HsF; FBz), using a magnetic-bottle type photoelectron spectrometer, in order to clarify electronic structures of the vanadium-arene complex. Photoelectron spectroscopy of a mass-selected cluster anion is a powerful method to investigate the electronic structure of neutral clusters because the neutral electronic states are observed as final states. On the contrary to the prediction of the 18-electron rule, bis(benzene) vanadium anion was not generated. We will discuss the stability of the 18-electron compound, bis(benzene)vanadium anion, and will reveal the geometrical and the electronic structures of the organometallic complexes from their photoelectron spectra.
2. Experimental Details of experimental setup have been provided elsewhere [16,18]. Organometallic cluster anions were prepared by two methods; (1) the complex anions production in the reaction between laser vaporized metal anion and arene molecules and (2) slow electron attachment to neutral organometallic
clusters. In both two methods, V atoms were vaporized by the irradiation of the second harmonic of a pulsed Nd 3+ :YAG laser (532 nm), and vaporized hot metal species of atoms, clusters, and their charged species were cooled to room temperature by a pulsed He carrier gas (10 atm). Then the metal species were sent into a flow-tube reactor where benzene (or fluorobenzene or toluene) vapor seeded in He (1-2 atm) was injected. The laser vaporization method can generate neutral, cationic and anionic clusters, and the produced anions were directly used for the target anions in method (1). In method (2), slow electrons were additionally attached to neutral organometallic clusters which were synthesized by the above laser vaporization method. The irradiation of the second harmonic of the other Nd3+:YAG laser (2.34 eV) onto Y203 plate (work function; 2.0 eV) produces slow electrons (0-0.5 eV) by a photoelectric effect. The electrons generated by this method were attached to neutral organometallic clusters just after the nozzle expansion. The cluster anions produced by either method (1) or (2) were analyzed by time-of-flight (TOF) mass spectrometer. The cluster anions was accelerated by applying a pulsed electric field (1-3 keV). After a 1.5 m flight path, the cluster anions were detected by a dual microchannel plate and the signal was digitized and averaged in a digital oscilloscope (LeCroy 9400). In the case of measurement of photoelectron spectra, the accelerated clusters were mass-selected by pulsed deflection plates, and were decelerated by a potential elevator before photodetachment. The kinetic energy of the photodetached electrons with the third harmonic of Nd3+:YAG laser (355 nm; 3.49 eV) were analyzed by a magnetic bottle type photoelectron spectrometer. The repetition rate was 10 Hz, and the photoelectron signal was typically accumulated to 15000-30000 shots by a multichannel scaler/averager (Stanford Research System, SR430). The two types of the anion sources gave the same photoelectron spectra for all the species. The energy of the photoelectron was calibrated by measuring photoelectron spectra of noble metal atom anions, e.g. C u - , A g - , Au-. The power density of the detachment laser was in the range of 10-20 m J / c m 2, and no power dependent processes for the spectrum shape were observed.
K. Judai et al. / Chemical Physics Letters 270 (1997) 23-30
25
Vn(benzene)m(fluorobenzene)k" = (n,m,k)
3. Results and discussion
(1,1,1)
3.1. Production of vanadium-arene complex anions
(1,o,2) Fig. 1 shows a T O F mass spectrum o f v a n a d i u m - b e n z e n e complex anions produced by the laser vaporization method without any additional electron attachment in method (1). The peaks are labeled according to the notation (n, m), denoting the number of vanadium atoms ( n ) and benzene molecules (m). All o f the complex anions contain one benzene molecule, as indicated by (n, m) = (1, 1), (2, 1), (3, 1), and (4, 1) in Fig. 1. However, the (3, 1), (4, 1) complexes were mainly observed as dehydrogenated species, as indicated by (3, 1)-4H, (4, I)-6H, where (3, 1)-4H corresponds to 4 H-atoms elimination from (3, 1). It is deduced that these dehydrogenated complex anions resulted from the reactions o f vanadium cluster anions toward a benzene molecule. In fact, the H atom elimination reaction has been reported also in the reaction of vanadium cluster cations, V~+ ( n / > 2), toward a benzene molecule [19,20]. In the elimination of H atoms, moreover, the H atoms are seemingly eliminated as H 2 molecule, because only even number of the H atom loss were observed. Here, it should be noted that bis(benzene)vanadium anion (1, 2) was not observed even in the higher concentration of benzene vapor. Since the
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Fig. 1. Time-of-flight mass spectra of organometallic cluster anions of vanadium-benzene. V,,(benzene)~- (n = 1-4) were generated by laser vaporization of a vanadium rod and reaction with benzene vapor. The peaks are labeled according to the notation (n, m), denoting the number of vanadium atoms (n) and benzene molecules (m). The label of(3, 1)- 4H means 4 H-atoms elimination from (3, 1).
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Fig. 2. Time-of-flight mass spectrum of V,(benzene)m(fluorobenzene)~- generated by a slow electron attachment to the neutral organometallic clusters. The notation (n, m, k) means the number of vanadium atoms (n), benzene (m), and fluorobenzene (k). As indicated by an arrow, (1, 2, 0) could not be generated.
number o f valence electrons o f vanadium is 5 and that of benzene is 6, bis(benzene)vanadium has 17 valence electrons. The 18-electron rule predicts that bis(benzene)vanadium anion should form a stable complex due to the full occupation of 18 electrons. However, the bis(benzene)vanadium anion was not observed at all. The mass spectra o f neutral and cationic v a n a d i u m - b e n z e n e clusters in the same method exhibited the abundant production of bis(benzene)vanadium [16,21]. Therefore it seems contradictory that bis(benzene)vanadium anion was not generated irrespective of satisfying the 18-electron rule. Based on the results under method (1), it is presumed that the bis(benzene)vanadium anion is unstable and has a negative electron affinity (EA). To make sure o f its negative EA, slow electrons were attached to neutral bis(benzene)vanadium. The laser vaporized vanadium atoms were reacted toward benzene and fluorobenzene mixed vapor, and slow electrons generated by photoelectric effect of Y203 were attached; by method (2). The benzene and fluorobenzene mixed vapor was used for presenting reliable evidence for cluster anions formation. Fig. 2 shows a T O F mass spectrum o f v a n a d i u m b e n z e n e - f l u o r o b e n z e n e complex anions produced by this method (2). The notation (n, m, k) means the number o f vanadium atoms (n), benzene (m), and fluorobenzene (k). In the measurement of the mass spectrum, almost no anions were detected without
26
K. Judai et al. / Chemical Physics Letters 270 (1997) 23-30
the slow electron attachment. By the aid of the slow electron attachment, the intensities of the peaks of (1, 1, 1) and (1, 0, 2) were drastically enhanced, whereas those of the bis(benzene)vanadium anion (1, 2, 0) were not observed at all, although both their neutral and their cationic complexes were produced. In a similar experiment, slow electron attachment was found to generate successfully a cyanobenzene anion despite an extremely small EA (0.0 ___0.1 eV) [22]. This indicates that the attachment of slow electrons can generate anions, when the corresponding neutral has positive EA and when an anionic state is vertically accessible from the neutral ground state. When the excess electron is acceptable to the neutral without serious geometric distortion, its anion is expected to be produced by the electron attachment. Since a vacant molecular orbital (MO) for the excess electron in the neutral bis(benzene)vanadium is nonbonding MO (a~), its occupation of the excess elec-
tron seemingly occurs without any serious geometric change. Indeed, the photoionization efficiency curve for Vl(benzene) ~- production from its neutral, which corresponds to one electron removal from the nonbonding MO, shows a sharp rise, implying little geometric change in the electron ejection from the non-bonding orbital. Nevertheless, bis(benzene)vanadium anion was not generated as shown in Figs. 1 and 2. Therefore, we concluded that bis(benzene)vanadium has negative EA.
3.2. Negative electron affinity of bis(benzene)vanadium Bis(benzene)vanadium has a negative EA contrary to the prediction based on the 18-electron rule; the bis(benzene)vanadium anion cannot exist as a stable species in the gas phase. The negative EA of bis(benzene)vanadium has been predicted by an elec-
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K. Judai et al. / Chemical Physics Letters 270 (1997) 23-30
trochemical study [23,24]. Elschenbroich and coworkers have measured the electrochemical redox potential of bis(benzene)vanadium by cyclic voltammetry in a strong polar solvent [23]. Although the bis(benzene)vanadium anion is produced under electrochemical reduction at - 2 . 7 1 eV, the reduction potential of - 2 . 7 1 V is fairly negative, and a sequence of decreasing the reduction potential is naphthalene > biphenyl = bis(benzene)vanadium > benzene. Based on this electrochemical experiment, the EA of bis(benzene)vanadium in the gas phase is expected to be negative, because the EAs of naphthalene, biphenyl, and benzene in the gas phase were determined by electron transmission spectroscopy, as - 0 . 2 , - 0 . 3 , - 1.11 eV, respectively [25]. This prediction agrees well with our conclusion from the mass spectra and the electron attachment experiment. Fig. 3a and 3b show schematic MO diagrams for Vl(benzene) l and Vl(benzene) 2 under C6v and D6h symmetry, respectively. Metal 3d orbitals split into three levels with benzene ligand(s) under C6v or D6h symmetry. The 3dxz and 3dy z orbitals (elg) interact with benzene rr orbital, forming antbbonding orbitals. The 3dxy and 3dx2_y2 orbitals (e2g) interact with the benzene -rr * orbital, forming bonding orbitals. The 3dz2 orbital (a~g) does not interact with benzene orbitals and remains a non-bonding orbital. The 5 valence electrons of the vanadium atom occupy two bonding orbitals (ezg) and one non-bonding orbital (a~g), making the non-bonding orbital half-filled. To produce the bis(benzene)vanadium anion, one more electron occupies the half-filled nonbonding orbital, and completes a closed shell. According to the one electron orbital model, the total electronic energy does not depend on occupation of the non-bonding orbital, and it is predicted that the anion is stably formed due to pairing energy. Then, non-formation of the bis(benzene)vanadium anion shows that the one electron orbital model is insufficient to predict its electronic properties, although it is actually useful to describe the electronic structures qualitatively. Namely, negative electron affinity of bis(benzene)vanadium means a breakdown of the one electron orbital model in this system, where it seems crucial to take account of repulsive interaction between one excess electron in the non-bonding orbital and the other orbital electrons. In order to confirm the negative electron affinity,
27
we have performed ab initio calculations, namely, the geometry optimizations of bis(benzene)vanadium and its anion at open-shell and closed-shell RHF levels, respectively, with the MIDI basis sets for C and H atoms [26] and valence triple zeta basis set for V atom [27,28]. The obtained EA was - 1 . 9 9 eV. Furthermore, the optimized V-benzene distances of the neutral and anion structures were 1.797 and 1.781 A, respectively, supporting the previous discussion on the negligible geometric change in the neutral and anion forms. These theoretical results are also in accord with the recent theoretical calculations by Hada et al. with the SAC-CI method [29]. Behavior contrary to the 18-electron rule is not only the case for anions but also for neutral complexes. The 18-electron rule suggests that the ionization energy of bis(benzene)chromium should be large because the neutral bis(benzene)chromium stably takes a closed shell structure of 18 electrons, whereas the bis(benzene)chromium cation is a open shell compound. However, the ionization energy of the bis(benzene)chromium (5.43 eV) is smaller than those of bis(benzene)vanadium (5.75 eV) and bis(benzene)titanium (5.71 eV) [3,16,30]. The relatively small ionization energy of the bis(benzene)chromium cannot be explained by the 18-electron rule under the one electron orbital model. o
3.3. Photoelectron spectra of vanadium-arene complex anions Although bis(benzene)vanadium has negative EA, bis(fluorobenzene)vanadium anion was observed in the mass spectra (see Fig. 2). There is a question whether only the substituent effect of a fluorine atom changes the EA of bis(fluorobenzene)vanadium into a positive value. To understand the substituent effect, we measured the photoelectron spectra of complex anions of benzene, fluorobenzene, and toluene. It should be noted that a fluorine atom works as an electron-withdrawing group, accepting electron from benzene -rr orbitals. On the other hand, a methyl group works as an electron-donating one.
3.3.1. Vl(arene)f- anions Fig. 4 shows the photoelectron spectra of mono(arene)vanadium anions, VI(Tol)~-, Vl(Bz)~-, and VI(FBz)[-. In the spectra, the horizontal axis
K. Judai et al./ Chemical Physics Letters 270 (1997) 23-30
28
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corresponds to the electron binding energy, Eb, defined as E b = h ~ ' - E k , where E k is the kinetic energy of the photoelectron. Arrows indicate threshold energies (ET), and the E T corresponds to the upper limit of the adiabatic EA. The values of E r (EA) are listed in Table 1. Compared with the spectra, the peak positions of the photoelectron spectrum of Vt(toluene) 1 were close to that of Vl(benzene)~-. This implies that toluene binds to vanadium atom under the same structure of benzene which is a half sandwich structure, and methyl group of toluene causes only a trivial shift of peak positions. It is reasonably presumed that the substitution of benzene into another arene makes the electronic structures similar around the HOMO, when the arene binds vanadium atom through d-Tr interaction similarly to benzene.
It should be noticed that the first peak is very sharp in both spectra (Fig. 4a and 4b). In the photodetachment process, the Franck-Condon factor depends on the spatial overlap of the vibrational wave functions of the anion and the neutral, under the assumption that the electronic transition dipole moment does not change significantly. Then, the features in the photoelectron spectra become broader by the large geometrical change upon the photodetachment. This is because the degrees of freedom of bending or rocking vibration having low frequency increase with the dimensions of the structure and the peaks of these low-frequency vibration excited cannot be observed separately. Namely, the most plausible explanation for the first sharp peak is that the photodetached electron is ejected from a non-bonding orbital. Otherwise the peak should become broader due to geometric changes. Under one electron orbital model, the HOMO of Vl(benzene) 1 is certainly non-bonding orbital (a l) as shown in Fig. 3(a), and the first peak is assigned to the photodetachment from the non-bonding orbital (a~). Indeed, this explanation is in agreement with the EPR experiment; the neutral V~(benzene)~ has the half-filled non-bonding orbital (al), taking a 2A l ground state
[15]. In the case of fluorobenzene, however, the spectrum of V~(fluorobenzene) 1 was entirely different from that of Vl(benzene)i-. This difference cannot be explained only by the substituent effect of a fluorine atom on the benzene ring. It is conceivable that V~(fluorobenzene)~- does not take the half sandwich structure, but takes a different structure. Although
Table 1 Electron affinities (EA) of Vn(arene),, complexes in eV Composition
EA a
(i) n - m = 1 - 1
Vl(benzene) l Vl(toluene) l Vl(flu°r°benzene) i
0.62(7) b 0.76(7) 1.15(12)
(ii) n - m = 1 - 2
Vl(benzene) 2 Vl(benzene) t(fluorobenzene) I Vl(fluorobenzene) 2
negative 1.28(13) 1.74(37)
a The threshold energies of photoelectron spectra correspond to the adiabatic electron affinities. b Numbers of parentheses are uncertainties; 0.62(7) represents 0.62 _+0.07.
K. Judai et a l . / Chemical Physics Letters 270 (1997) 2 3 - 3 0
29
we have no definite ideas about this different structure, we suggest the plausible structure in which a fluorine atom and a vanadium atom are bonded directly. In the reaction of a vanadium atom anion and a fluorobenzene, the oxidative addition reaction should occur [31]. In this reaction, V atom should also interact with the F atom of fluorobenzene, and then the V atom is coordinated with both the phenyl ring and the F atom, resulting in the structure as shown in Fig. 4(c). Thus, it is rationalized that the candidate for the structure of V~(fluorobenzene)]gives a different electronic structure from that of V~(Bz)~-. In fact, the fluorine atom adducts of (1, 1 ) + F and (2, 1 ) + F were produced as a minor product, in which a V - F bond is formed. These species should be generated by the elimination reaction of a phenyl group which was caused by the oxidative addition.
compared to the negative EA of bis(benzene)vanadium. If the substitution of fluorobenzene to benzene has a cumulative effect on EA [32], we can estimate the EA of bis(benzene)vanadium, Vl(Bz) z, to be about 0.8 eV. As concluded above, however, its EA is negative. Therefore, this result also leads us to the conclusion that fluorobenzene does not coordinate with the V atom at the benzene ring. A c o m p a r i s o n b e t w e e n the spectra of Vl(Bz)l(FBz) 1 and VI(FBz)2, shows similar spectral features. In VI(Bz)I(FBz)~-, it is deduced that the benzene molecule coordinates with the V atom at the benzene ring. Then, their spectral similarity suggests that one of fluorobenzene molecules also coordinates with the V atom at the benzene ring in V~(FBz) 2 whereas the other fluorobenzene coordinates with both the F atom and a part of phenyl ring.
3.3.2. Vl(arene) ~- anions
Acknowledgements
Fig. 5 shows the photoelectron spectra of bis(arene)vanadium anions, VI(Bz)I(FBz) ~- and VI(FBz) 2. The E T (EA) of Vl(BZ)l(FBz)l and VI(FBz) 2 were determined to be 1.29 eV and 1.74 eV from the threshold, respectively, and the values are listed in Table 1. These EAs are fairly large
.-~~. ~::D(a) -' Vl(BZ)l~
We are grateful to Mr. Takasuke Hayase, Mr. Yuichi Negishi, and Mr. Fumitaka Hayakawa for technical assistance and to Mr. Tomokazu Yasuike for quantum chemical calculation. This work is partially supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science, and Culture.
References
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