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Charge distributions in multiple-decker sandwich clusters of lanthanide–cyclooctatetraene: Application of Na-atom doping
18 June 1999
Chemical Physics Letters 306 Ž1999. 256–262
Charge distributions in multiple-decker sandwich clusters of lanthanide–cyclooctatetraene: Application of Na-atom doping Ken Miyajima, Tsuyoshi Kurikawa, Masatomo Hashimoto, Atsushi Nakajima ) , Koji Kaya Department of Chemistry, Faculty of Science and Technology, Keio UniÕersity, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Received 12 March 1999; in final form 12 April 1999
Abstract In gas phase, Na-atom adducts were produced for organometallic lanthanide complexes of Ln nŽC 8 H 8 . nq1 wLn s europium ŽEu. and holmium ŽHo.; C 8 H 8 s 1,3,5,7-cyclooctatetraene ŽCOT.x by a combination of two-laser vaporization and a molecular beam method. The number of attached Na atoms can be reasonably explained by charge distributions of the complex, where Eu and Ho atoms exist as Eu2q and Ho 3q, respectively, and C 8 H 8 molecules as COT 2y, accepting two excess electrons. The behavior of the Na adducts chemically demonstrates that the organometallic lanthanide complex is formed through ionic bonding, implying that it is possible to be isolated as its Na salt. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Since UŽC 8 H 8 . 2 was discovered by Streitwieser and Muller-Westerhoff in 1968 w1x, studies of ¨ organo-rare earth metal chemistry in condensed phase have been an active area of experimental research w2x. Several classes of compounds with the rare earth elements were typically found in their oxidation state of Ln3q, and occasionally in Ln0 , Ln2q, and Ln4q. These organometallic compounds have recently been characterized and used as reagents in organic synthesis and as active catalysts. Specific among these are questions regarding the degree of metal–ligand cova-
)
Corresponding author. E-mail: [email protected]
lency. It is now generally agreed that there is less involvement of f orbitals in lanthanide bonding than is generally assumed to operate in the actinides. Therefore, organolanthanide compounds have long been expected as optical and magnetic materials for such 4f orbitals that are shielded largely from external interactions by the outer 5s and 5p orbitals w3,4x. Numerous experimentalists and theoreticians have been prompted to describe the bonding of organolanthanide compounds as wholly ionic w5–8x. Owing to their large radius and ionic nature, lanthanide ions can easily form many complexes with the 1,3,5,7cyclooctatetraene ŽCOT. molecule. Many of them have been found as sandwich complexes with the planar aromatic COT dianion ŽCOT 2y . occasionally taking counter ions. In the condensed phase, furthermore, the triple-decker sandwich complexes have
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 4 8 9 - 3
K. Miyajima et al.r Chemical Physics Letters 306 (1999) 256–262
also been prepared for Ln s Ce, Nd as the alkali w 3q ŽCOT 2y . 2 x w9,10x. metal salts of Mq alkali Ln Our recent development of methodology combining a laser vaporization method and a molecular beam method has opened up a new way for facile investigation of the organometallic clusters w11,12x. In gas phase, it is possible to investigate interactions with the metal–ligand molecule without influence of the solvent. We have successfully produced multipledecker sandwich clusters of Ln–COT from a mixture of vaporized metal atoms and aromatic molecules w13x. In this previous report, the electronic and geometric properties of the sandwich Ln nŽCOT. m clusters were discussed by a spectroscopic approach, such as photoionization spectroscopy and photoelectron spectroscopy. The magic-numbered clusters having Ž n, m. s Ž n, n q 1. for n s 1–5 take multiple-decker sandwich structures in which Ln atoms and COT molecules are alternately piled up. The obtained spectroscopic data show that these sandwich complexes are formed through ionic bonds and are charge transfer complexes. In this Letter, we will discuss the charge distributions of LnŽII. –COT and LnŽIII. –COT clusters by a chemical probe method with Na atoms as electron donors. The number of the Na atoms attached to the Ln–COT clusters enables us to examine the electron orbit, which is filled with a valence 3s electron of each Na atom.
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chronously with the flow of the metal vapors. The organometallic clusters thus generated were sent into the second chamber through a skimmer Ž3 mm diameter.. Neutral clusters in the beam were intersected with the light of an ionization laser, an ArF excimer laser Ž193 nm, 6.42 eV., to ionize the clusters, and the photoions were accelerated in a static electric field. On the other hand, cluster ions in the beam were directly accelerated with a fast pulsed electric field. The ion signal detected by a dual micro-channel plate was accumulated in a transient oscilloscope ŽLeCroy 9400A. coupled with a personal computer, and the mass spectra of the organometallic clusters were measured by averaging 700–800 outputs.
3. Results and discussion Fig. 1 shows a mass spectrum of Ho nŽCOT. m Na k photoionized by the ArF laser. The peaks are labeled according to the notation Ž n, m, k ., denoting the numbers of Ho atoms Ž n., COT molecules Ž m., and Na atoms Ž k .. With a set of deflection plates, ions
2. Experimental The Na-atom adducts of lanthanide–cyclooctatetraene clusters, Ln nŽCOT. m Na k wLn s Eu and Ho; COT s 1,3,5,7-cyclooctatetraene ŽC 8 H 8 .x, were produced by the combination of the laser-vaporization method and molecular beam method using a flow tube reactor ŽFTR. w14x. The experimental setup used in this work is described elsewhere w13,15x. Briefly, Ln and Na atoms were vaporized independently by two Q-switched Nd 3q:YAG lasers Ž532 nm, ; 10 mJrpulse.. After the metal vapors were cooled to room temperature with He carrier gas Ž5 atm stagnation pressure. through a channel Ž3 mm diameter and 4 cm length., COT vapor Ž; 70 Torr, 708C. diluted with He Ž1.5 atm. was injected into the FTR syn-
Fig. 1. Time-of-flight mass spectrum of holmium ŽHo. –1,3,5,7cyclooctatetraene ŽC 8 H 8 ; COT. clusters mixed with sodium ŽNa. atoms, Ho nŽCOT. m Na k , which is obtained by the photoionization of the ArF laser Ž6.42 eV.. Peaks are labeled according to the notation Ž n, m. or Ž n, m, k ., denoting the number of Ho atoms Ž n., COT molecules Ž m., and Na atoms Ž k .. Although the peaks of Ho and Na atoms were observed, the alloy clusters of Ho–Na could not be generated either without COT or with COT. The position of HoNa dimer is indicated by a downward arrow, but no peak appears.
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having around 200 amu were emphasized in the figure. In the photoionization mass spectrum, clusters containing COT were observed together with Na and Ho atoms, but no alloy clusters of Ho n Na k were observed, neither Ho n Na k clusters without COT. Since no cationic clusters of Ho n Naq k were observed in the mass spectrum of Ho–Na cations, the non observation of neutral Ho n Na k clusters is not due to high ionization energy above 6.42 eV. Therefore, it is reasonably presumed that mixed clusters of Ho–Na are not produced in this setup, which is important to deduce the structure of the Ho nŽCOT. m Na k clusters as mentioned later. Once the C 8 H 8 molecules were injected, Ho and Na atoms form their complexes with COT molecules. This is ascribed to the acceptability of COT molecules for taking up two electrons w16,17x, which is understandable by the well-known 4 n q 2 rule of aromatics. As shown in the spectrum, indeed, the clusters of Na–COT were prominently formed at NaCOT and Na 2 COT, which is due to electron transfer from Na to COT. Therefore, the two Na atoms in Na 2 COT exist as an atom on both sides of COT, and the most probable structure can be expressed as NaqŽCOT 2y .Naq. Indeed, this is consistent with the result that the abundance of Na 3 COT is very poor compared to those of NaCOT and Na 2 COT. Under the beam rich in Ho and Na atoms, COT molecules work as glue in the formation of the organometallic clusters. In the following sections the mass spectra of Eu nŽCOT. m Na k and Ho nŽCOT. m Na k are presented, whose features are explicable in terms of ionic organometallic clusters. 3.1. Eu–COT–Na Fig. 2 shows the mass spectra of neutral or anionic Eu nŽCOT. m Na0ry clusters; Fig. 2a and b gives k photoionization mass spectra of the neutral with the ArF laser, whereas Fig. 2c is that of anionic clusters, Eu nŽCOT. m Nay k . To enhance the ion intensity of larger clusters, the transmission efficiency of the mass spectrometer was optimized around 1000–1500 amu. The peaks are labeled according to the notation Ž n, m, k ., denoting the numbers of Eu atoms Ž n., COT molecules Ž m., and Na atoms Ž k . along with the superscript showing the total charge of the clus-
Fig. 2. Time-of-flight mass spectra of: Ža. neutral europium ŽEu. –COT; Žb. neutral Eu–COT–Na; and Žc. anionic Eu–COT– Na clusters. Ža. and Žb. were obtained by the photoionization of the ArF laser Ž6.42 eV.. Peaks are labeled according to the notation Ž n, m. or Ž n, m, k ., denoting the number of Eu atoms Ž n., COT molecules Ž m., and Na atoms Ž k .. Peaks unlabeled can be assigned to contaminated species with O 2 or H 2 O.
ter. Without Na atoms, the neutral Eu nŽCOT. m clusters are produced prominently at Ž n, m. s Ž n, n q 1., as shown in Fig. 2a, and no dehydrogenated species were accompanied with these clusters. Even when
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the concentration of COT vapor was increased, these main peaks remained unchanged in the mass spectra. Therefore, these clusters are stable clusters in the saturated concentration of COT. These regular progressions show the formation of the sandwich clusters of Eu–COT, where Eu and COT are alternately piled up w13x. With co-vaporization of Na atoms, the Na adducts were produced. However, the number of Na atoms attached to the neutral Eu nŽCOT. nq1 clusters are restricted up to two: either one or two Na atoms are efficiently added to the cluster. Even under higher density of Na atoms with higher fluence of the vaporization laser, the third Na atom cannot be added to the Eu nŽCOT. nq1 clusters. In the formation of the Eu nŽCOT. m Nay anions, furthermore, only k Ž one Na atom can be attached to Eu nŽCOT.y nq1 n s 1–3., although the intensity of the Na adduct is low due to poor production efficiency. These features in the mass spectra of Na adducts can be explained by the charge distribution of ionic Eu nŽCOT. m Na k clusters, assuming that the Eu atom prefers to take the oxidation state of q2 in the Eu nŽCOT. m cluster, where the COT molecule accepts up to two electrons. This charge distribution appears particularly applicable in the Eu nŽCOT. m Na k clusters, because the Na atom prefers the oxidation state of q1, donating one valence electron. As shown in Fig. 3, neutral Eu–COT clusters have a terminal COT ligand accepting one electron, and then the COT ligands at both ends can afford to accept one more valence electron, satisfying aromaticity as COT 2y. Therefore, the Eu nŽCOT. nq1 Na 2 clusters should have two Na atoms at both the ends: NaqŽ COT 2y Eu2q . nCOT 2y Naq – which is shown in Fig. 3a. According to the theoretical calculation of Dolg and co-workers w18–20x, indeed, the Ln–COT cluster is a charge transfer cluster from Ln atoms to COT molecules. As pointed out in the preceding section, in fact, since Ln and Na atoms do not make alloy clusters in this condition, it should reasonably be presumed that both Eu and Na atoms exist as atoms in the cluster. In the saturation condition of the Na atoms, only the Eu nŽCOT. nq1 Na 2 cluster should be formed. However, it was difficult to form them selectively without any hydroxide and oxide species, because the higher laser fluence for the Na rod generates their contamination simultaneously from oxygen andror moisture in the He carrier gas.
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Fig. 3. Proposed charge distributions of valence electrons of Eu nŽCOT. nq1 Na k clusters Ž ns1–3.: Ža. neutral and Žb. anionic clusters. These schematics are based on the assumption of twoelectron acceptability of COT and multiple-charged positive ions of Eu2q. Neutral and anionic Eu nŽCOT. nq1 take two and one Na atoms, respectively, where the charge distributions of Eu2q, COT 2y, and Naq are satisfied.
The formation of Na adduct of Eu nŽCOT. nq1 Na 2 implies that it is chemically possible to synthesize the sandwich Eu nŽCOT. nq1 clusters as their Na salts in bulk w21x. As shown in Fig. 3b, moreover, the anionic Eu nŽCOT.y nq1 cluster can accept one electron where the electron of the excess charge can be regarded as one of two accepted electrons. Thus, only one Na atom can be attached to Eu nŽCOT.y nq1, completing the charged states of Eu2q and COT 2y in the sandwich structure. This charge distribution of the highly ionic Eu–COT clusters was consistently discussed by using the spectroscopic methods of both photoionization and photoelectron spectroscopies w13x. The photoelectron spectrum of Eu 1ŽCOT.y 2 exhibits almost the same feature with that of Yb1ŽCOT.y 2 , but is different from those of Nd 1ŽCOT.y and 2 Ho 1ŽCOT.y 2 , where both Eu and Yb prefers the oxidation state of q2, while Nd and Ho do prefer
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that of q3. Moreover, the n dependence of the ionization energy for Ž n, n q 1. clusters shows a common feature with that for Yb–COT; the ionization energy is almost constant for n. This is because all of the Eu atoms can take the oxidation state of q2 in the cluster, where the ionizing state of the cluster for Ž n, n q 1. is a non-bonding e 3u state of Ln2q under the symmetry of D 8h . These spectroscopic findings on the charge distributions are consistent with the work of the chemical probe by Na atoms. It should be noted that NaŽCOT.y 2 was prominently observed in Fig. 2c without the formation of ŽCOT.y 2 . This is probably attributed to the sandwich structure of ŽCOT.NaŽCOT.y, although the aromaticity of COT 2y is not satisfied in it. The photoelectron spectra of NaŽCOT.y 2 will be published elsewhere, including those of the larger Na nŽCOT.y m clusters w22x. 3.2. Ho–COT–Na Fig. 4 shows the mass spectra of neutral or cationic Ho nŽCOT. m Na0rq clusters; Fig. 4a and b gives phok toionization mass spectra of the neutral with the ArF laser, whereas Fig. 4c is the mass spectrum of cationic clusters, Ho nŽCOT. m Naq k . The peaks are labeled according to the notation Ž n, m, k., denoting the numbers of Ho atoms Ž n., COT molecules Ž m., and Na atoms Ž k . along with the superscript showing the total charge of the cluster. Without Na atoms, the neutral Ho nŽCOT. m clusters are produced prominently at Ž n, m. s Ž n, n q 1., as shown in Fig. 4a, and no dehydrogenated species were found with these clusters. Similarly to Eu–COT, these progressions show the formation of the sandwich clusters of Ho–COT, where Ho and COT are alternately piled up. With co-vaporization of Na atoms, the Na adduct was produced only for Ho 1ŽCOT. 2 , and the higher Ho nŽCOT. nq1 clusters Ž n G 2. do not exhibit the Na-atom addition. In the Ho nŽCOT. m Naq k cations, furthermore, up to two Na atoms can be attached to Ž1, 2.q, and one Na atom can also be attached Ž . to Ž2, 3.q, forming Ho 1ŽCOT. 2 Naq 2 and Ho 2 COT 3 q q Ž . Na 1 , respectively. Larger n, n q 1 clusters for n G 3 do not exhibit the Na-atom addition. These features in the mass spectra of the Na adduct can also be explained by the charge distribu-
Fig. 4. Time-of-flight mass spectra of: Ža. neutral Ho–COT; Žb. neutral Ho–COT–Na; and Žc. cationic Ho–COT–Na clusters. Ža. and Žb. were obtained by the photoionization of the ArF laser Ž6.42 eV.. Peaks are labeled according to the notation Ž n, m. or Ž n, m, k ., denoting the number of Ho atoms Ž n., COT molecules Ž m., and Na atoms Ž k .. Many peaks unlabeled can be assigned to contaminated species with O 2 andror H 2 O.
tion of ionic Ho nŽCOT. m Na k clusters, which is very similar to Eu–COT. A marked difference between them is the oxidation state of the metal atom, because the Ho atom prefers to take the oxidation state of q3 in Ho nŽCOT. m , as reported previously. As shown in Fig. 5, the neutral Ho 1ŽCOT. 2 cluster has a valence vacancy in the terminal COT ligands, and the addition of one Na atom can strictly satisfy
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take Na atoms until EurHo and COT satisfy the charged states of Eu2qrHo 3q and COT 2y. This chemical probe experiment of Na atoms shows the number of electron vacancies in the cluster, implying that some Eu–rHo–COT clusters should be prepared as their Na salts in bulk.
Acknowledgements This work is supported by the program entitled ‘‘Research for the Future ŽRFTF.’’ of the Japan Society for the Promotion of Science Ž98P01203. and by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science, Sports, and Culture. TK expresses his gratitude to Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. Fig. 5. Proposed charge distributions of valence electrons of Ho nŽCOT. nq1 Na k clusters: Ža. neutral and Žb. cationic clusters. These schematics are based on the assumption of two-electron acceptability of COT and multiple-charged positive ions of Ho 3q. For Ž1, 2., one Na atom can be added, while Ž1, 2.q and Ž2, 3.q can take two and one Na atomŽs., respectively, where the charge distributions of Ho 3q, COT 2y, and Naq are satisfied.
aromaticity as COT 2y. However, larger clusters of neutral Ho nŽCOT. nq1 Ž n G 2. have no vacancy to accept the valence electron of Na atoms, so that no Na adducts were formed. These results for Ho–COT, therefore, suggest that Ž1, 2. can be synthesized as the Na salt of Ho 1ŽCOT. 2 Na 1 , and Ž2, 3. itself can be synthesized without any counter-ions in bulk. As reported in the previous work about the formation of Nd 3 ŽCOT.4 Cl 1 w13x, furthermore, Ž3, 4. is expected to be synthesized as halogenated species, because an halogen atom can remove one electron from the cluster, completing the charge balance between three Nd 3q ions and four COT 2y. As shown in Fig. 5b, moreover, the cationic Ho nŽCOT.q nq1 cluster can accept one more electron compared to the corresponding neutral, because the positive charge makes electrons acceptable: Ž1, 2.q and Ž2, 3.q become Ž1, 2.q– Na 2 and Ž2, 3.q–Na, respectively. In summary, the chemical methodology of the Na-atom addition can reveal the charge distribution of the lanthanide organometallic clusters successfully. The highly ionic Eu–rHo–COT cluster can
References w1x A. Streitwieser Jr., U. Muller-Westerhoff, J. Am. Chem. Soc. ¨ 90 Ž1968. 7364. w2x H. Schumann, J.A. Meese-Marktscheffel, L. Esser, Chem. Rev. 95 Ž1995. 865. w3x J.V. Ortiz, R. Hoffmann, Inorg. Chem. 24 Ž1985. 2095. w4x H. Rabaa, J.-Y. Saillard, R. Hoffmann, J. Am. Chem. Soc. 108 Ž1986. 4327. w5x L.J. Nugent, P.G. Laubereau, G.K. Werner, K.L. Vander Sluis, J. Organomet. Chem. 27 Ž1971. 365. w6x K.O. Hogson, F. Mares, D.F. Starks, A. Streitwieser Jr., J. Am. Chem. Soc. 95 Ž1973. 8650. w7x K.N. Raymond, C.W. Eigenbrot, Acc. Chem. Res. 13 Ž1980. 276. w8x R.A. Andersen, J.M. Bonsella, C.J. Burns, J.C. Green, D. Hohl, N. Rosch, J. Chem. Soc., Chem. Commun. Ž1986. 405. w9x A. Greco, S. Cesca, G. Bertolini, J. Organomet. Chem. 113 Ž1976. 321. w10x C.W. DeKock, S.R. Ely, T.E. Hopkins, M.A. Brault, Inorg. Chem. 17 Ž1978. 625. w11x K. Hoshino, T. Kurikawa, H. Takeda, A. Nakajima, K. Kaya, J. Phys. Chem. 99 Ž1995. 3053. w12x T. Kurikawa, M. Hirano, H. Takeda, K. Yagi, K. Hoshino, A. Nakajima, K. Kaya, J. Phys. Chem. 99 Ž1995. 16248. w13x T. Kurikawa, Y. Negishi, F. Hayakawa, S. Nagao, K. Miyajima, A. Nakajima, K. Kaya, J. Am. Chem. Soc. 120 Ž1998. 11766. w14x M.E. Geusic, M.D. Morse, S.C. O’Brien, R.E. Smalley, Rev. Sci. Instrum. 56 Ž1985. 2123. w15x T. Kurikawa, H. Takeda, M. Hirano, K. Judai, T. Arita, S. Nagao, A. Nakajima, K. Kaya, Organometallics Ž1999, in press..
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w16x M.J.S. Dewar, G.J. Gleicher, J. Am. Chem. Soc. 87 Ž1965. 685. w17x G.I. Fray, R.G. Saxton, The Chemistry of Cyclooctatetraene and Its Derivatives, Cambridge University Press, Cambridge, 1978. w18x M. Dolg, P. Fulde, W. Kuchle, C. Neumann, H. Stoll, J. ¨ Chem. Phys. 94 Ž1991. 3011.
w19x M. Dolg, P. Fulde, H. Stoll, H. Preuss, A. Chang, R.M. Pitzer, Chem. Phys. 195 Ž1995. 71. w20x W. Liu, M. Dolg, P. Fulde, J. Chem. Phys. 107 Ž1997. 3584. w21x W.J. Evans, J.L. Shreeve, J.W. Ziller, Polyhedron 14 Ž1995. 2945. w22x K. Miyajima, M. Hashimoto, A. Nakajima, K. Kaya Žto be published..
Chemical Physics Letters 306 Ž1999. 256–262
Charge distributions in multiple-decker sandwich clusters of lanthanide–cyclooctatetraene: Application of Na-atom doping Ken Miyajima, Tsuyoshi Kurikawa, Masatomo Hashimoto, Atsushi Nakajima ) , Koji Kaya Department of Chemistry, Faculty of Science and Technology, Keio UniÕersity, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Received 12 March 1999; in final form 12 April 1999
Abstract In gas phase, Na-atom adducts were produced for organometallic lanthanide complexes of Ln nŽC 8 H 8 . nq1 wLn s europium ŽEu. and holmium ŽHo.; C 8 H 8 s 1,3,5,7-cyclooctatetraene ŽCOT.x by a combination of two-laser vaporization and a molecular beam method. The number of attached Na atoms can be reasonably explained by charge distributions of the complex, where Eu and Ho atoms exist as Eu2q and Ho 3q, respectively, and C 8 H 8 molecules as COT 2y, accepting two excess electrons. The behavior of the Na adducts chemically demonstrates that the organometallic lanthanide complex is formed through ionic bonding, implying that it is possible to be isolated as its Na salt. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Since UŽC 8 H 8 . 2 was discovered by Streitwieser and Muller-Westerhoff in 1968 w1x, studies of ¨ organo-rare earth metal chemistry in condensed phase have been an active area of experimental research w2x. Several classes of compounds with the rare earth elements were typically found in their oxidation state of Ln3q, and occasionally in Ln0 , Ln2q, and Ln4q. These organometallic compounds have recently been characterized and used as reagents in organic synthesis and as active catalysts. Specific among these are questions regarding the degree of metal–ligand cova-
)
Corresponding author. E-mail: [email protected]
lency. It is now generally agreed that there is less involvement of f orbitals in lanthanide bonding than is generally assumed to operate in the actinides. Therefore, organolanthanide compounds have long been expected as optical and magnetic materials for such 4f orbitals that are shielded largely from external interactions by the outer 5s and 5p orbitals w3,4x. Numerous experimentalists and theoreticians have been prompted to describe the bonding of organolanthanide compounds as wholly ionic w5–8x. Owing to their large radius and ionic nature, lanthanide ions can easily form many complexes with the 1,3,5,7cyclooctatetraene ŽCOT. molecule. Many of them have been found as sandwich complexes with the planar aromatic COT dianion ŽCOT 2y . occasionally taking counter ions. In the condensed phase, furthermore, the triple-decker sandwich complexes have
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 4 8 9 - 3
K. Miyajima et al.r Chemical Physics Letters 306 (1999) 256–262
also been prepared for Ln s Ce, Nd as the alkali w 3q ŽCOT 2y . 2 x w9,10x. metal salts of Mq alkali Ln Our recent development of methodology combining a laser vaporization method and a molecular beam method has opened up a new way for facile investigation of the organometallic clusters w11,12x. In gas phase, it is possible to investigate interactions with the metal–ligand molecule without influence of the solvent. We have successfully produced multipledecker sandwich clusters of Ln–COT from a mixture of vaporized metal atoms and aromatic molecules w13x. In this previous report, the electronic and geometric properties of the sandwich Ln nŽCOT. m clusters were discussed by a spectroscopic approach, such as photoionization spectroscopy and photoelectron spectroscopy. The magic-numbered clusters having Ž n, m. s Ž n, n q 1. for n s 1–5 take multiple-decker sandwich structures in which Ln atoms and COT molecules are alternately piled up. The obtained spectroscopic data show that these sandwich complexes are formed through ionic bonds and are charge transfer complexes. In this Letter, we will discuss the charge distributions of LnŽII. –COT and LnŽIII. –COT clusters by a chemical probe method with Na atoms as electron donors. The number of the Na atoms attached to the Ln–COT clusters enables us to examine the electron orbit, which is filled with a valence 3s electron of each Na atom.
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chronously with the flow of the metal vapors. The organometallic clusters thus generated were sent into the second chamber through a skimmer Ž3 mm diameter.. Neutral clusters in the beam were intersected with the light of an ionization laser, an ArF excimer laser Ž193 nm, 6.42 eV., to ionize the clusters, and the photoions were accelerated in a static electric field. On the other hand, cluster ions in the beam were directly accelerated with a fast pulsed electric field. The ion signal detected by a dual micro-channel plate was accumulated in a transient oscilloscope ŽLeCroy 9400A. coupled with a personal computer, and the mass spectra of the organometallic clusters were measured by averaging 700–800 outputs.
3. Results and discussion Fig. 1 shows a mass spectrum of Ho nŽCOT. m Na k photoionized by the ArF laser. The peaks are labeled according to the notation Ž n, m, k ., denoting the numbers of Ho atoms Ž n., COT molecules Ž m., and Na atoms Ž k .. With a set of deflection plates, ions
2. Experimental The Na-atom adducts of lanthanide–cyclooctatetraene clusters, Ln nŽCOT. m Na k wLn s Eu and Ho; COT s 1,3,5,7-cyclooctatetraene ŽC 8 H 8 .x, were produced by the combination of the laser-vaporization method and molecular beam method using a flow tube reactor ŽFTR. w14x. The experimental setup used in this work is described elsewhere w13,15x. Briefly, Ln and Na atoms were vaporized independently by two Q-switched Nd 3q:YAG lasers Ž532 nm, ; 10 mJrpulse.. After the metal vapors were cooled to room temperature with He carrier gas Ž5 atm stagnation pressure. through a channel Ž3 mm diameter and 4 cm length., COT vapor Ž; 70 Torr, 708C. diluted with He Ž1.5 atm. was injected into the FTR syn-
Fig. 1. Time-of-flight mass spectrum of holmium ŽHo. –1,3,5,7cyclooctatetraene ŽC 8 H 8 ; COT. clusters mixed with sodium ŽNa. atoms, Ho nŽCOT. m Na k , which is obtained by the photoionization of the ArF laser Ž6.42 eV.. Peaks are labeled according to the notation Ž n, m. or Ž n, m, k ., denoting the number of Ho atoms Ž n., COT molecules Ž m., and Na atoms Ž k .. Although the peaks of Ho and Na atoms were observed, the alloy clusters of Ho–Na could not be generated either without COT or with COT. The position of HoNa dimer is indicated by a downward arrow, but no peak appears.
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having around 200 amu were emphasized in the figure. In the photoionization mass spectrum, clusters containing COT were observed together with Na and Ho atoms, but no alloy clusters of Ho n Na k were observed, neither Ho n Na k clusters without COT. Since no cationic clusters of Ho n Naq k were observed in the mass spectrum of Ho–Na cations, the non observation of neutral Ho n Na k clusters is not due to high ionization energy above 6.42 eV. Therefore, it is reasonably presumed that mixed clusters of Ho–Na are not produced in this setup, which is important to deduce the structure of the Ho nŽCOT. m Na k clusters as mentioned later. Once the C 8 H 8 molecules were injected, Ho and Na atoms form their complexes with COT molecules. This is ascribed to the acceptability of COT molecules for taking up two electrons w16,17x, which is understandable by the well-known 4 n q 2 rule of aromatics. As shown in the spectrum, indeed, the clusters of Na–COT were prominently formed at NaCOT and Na 2 COT, which is due to electron transfer from Na to COT. Therefore, the two Na atoms in Na 2 COT exist as an atom on both sides of COT, and the most probable structure can be expressed as NaqŽCOT 2y .Naq. Indeed, this is consistent with the result that the abundance of Na 3 COT is very poor compared to those of NaCOT and Na 2 COT. Under the beam rich in Ho and Na atoms, COT molecules work as glue in the formation of the organometallic clusters. In the following sections the mass spectra of Eu nŽCOT. m Na k and Ho nŽCOT. m Na k are presented, whose features are explicable in terms of ionic organometallic clusters. 3.1. Eu–COT–Na Fig. 2 shows the mass spectra of neutral or anionic Eu nŽCOT. m Na0ry clusters; Fig. 2a and b gives k photoionization mass spectra of the neutral with the ArF laser, whereas Fig. 2c is that of anionic clusters, Eu nŽCOT. m Nay k . To enhance the ion intensity of larger clusters, the transmission efficiency of the mass spectrometer was optimized around 1000–1500 amu. The peaks are labeled according to the notation Ž n, m, k ., denoting the numbers of Eu atoms Ž n., COT molecules Ž m., and Na atoms Ž k . along with the superscript showing the total charge of the clus-
Fig. 2. Time-of-flight mass spectra of: Ža. neutral europium ŽEu. –COT; Žb. neutral Eu–COT–Na; and Žc. anionic Eu–COT– Na clusters. Ža. and Žb. were obtained by the photoionization of the ArF laser Ž6.42 eV.. Peaks are labeled according to the notation Ž n, m. or Ž n, m, k ., denoting the number of Eu atoms Ž n., COT molecules Ž m., and Na atoms Ž k .. Peaks unlabeled can be assigned to contaminated species with O 2 or H 2 O.
ter. Without Na atoms, the neutral Eu nŽCOT. m clusters are produced prominently at Ž n, m. s Ž n, n q 1., as shown in Fig. 2a, and no dehydrogenated species were accompanied with these clusters. Even when
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the concentration of COT vapor was increased, these main peaks remained unchanged in the mass spectra. Therefore, these clusters are stable clusters in the saturated concentration of COT. These regular progressions show the formation of the sandwich clusters of Eu–COT, where Eu and COT are alternately piled up w13x. With co-vaporization of Na atoms, the Na adducts were produced. However, the number of Na atoms attached to the neutral Eu nŽCOT. nq1 clusters are restricted up to two: either one or two Na atoms are efficiently added to the cluster. Even under higher density of Na atoms with higher fluence of the vaporization laser, the third Na atom cannot be added to the Eu nŽCOT. nq1 clusters. In the formation of the Eu nŽCOT. m Nay anions, furthermore, only k Ž one Na atom can be attached to Eu nŽCOT.y nq1 n s 1–3., although the intensity of the Na adduct is low due to poor production efficiency. These features in the mass spectra of Na adducts can be explained by the charge distribution of ionic Eu nŽCOT. m Na k clusters, assuming that the Eu atom prefers to take the oxidation state of q2 in the Eu nŽCOT. m cluster, where the COT molecule accepts up to two electrons. This charge distribution appears particularly applicable in the Eu nŽCOT. m Na k clusters, because the Na atom prefers the oxidation state of q1, donating one valence electron. As shown in Fig. 3, neutral Eu–COT clusters have a terminal COT ligand accepting one electron, and then the COT ligands at both ends can afford to accept one more valence electron, satisfying aromaticity as COT 2y. Therefore, the Eu nŽCOT. nq1 Na 2 clusters should have two Na atoms at both the ends: NaqŽ COT 2y Eu2q . nCOT 2y Naq – which is shown in Fig. 3a. According to the theoretical calculation of Dolg and co-workers w18–20x, indeed, the Ln–COT cluster is a charge transfer cluster from Ln atoms to COT molecules. As pointed out in the preceding section, in fact, since Ln and Na atoms do not make alloy clusters in this condition, it should reasonably be presumed that both Eu and Na atoms exist as atoms in the cluster. In the saturation condition of the Na atoms, only the Eu nŽCOT. nq1 Na 2 cluster should be formed. However, it was difficult to form them selectively without any hydroxide and oxide species, because the higher laser fluence for the Na rod generates their contamination simultaneously from oxygen andror moisture in the He carrier gas.
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Fig. 3. Proposed charge distributions of valence electrons of Eu nŽCOT. nq1 Na k clusters Ž ns1–3.: Ža. neutral and Žb. anionic clusters. These schematics are based on the assumption of twoelectron acceptability of COT and multiple-charged positive ions of Eu2q. Neutral and anionic Eu nŽCOT. nq1 take two and one Na atoms, respectively, where the charge distributions of Eu2q, COT 2y, and Naq are satisfied.
The formation of Na adduct of Eu nŽCOT. nq1 Na 2 implies that it is chemically possible to synthesize the sandwich Eu nŽCOT. nq1 clusters as their Na salts in bulk w21x. As shown in Fig. 3b, moreover, the anionic Eu nŽCOT.y nq1 cluster can accept one electron where the electron of the excess charge can be regarded as one of two accepted electrons. Thus, only one Na atom can be attached to Eu nŽCOT.y nq1, completing the charged states of Eu2q and COT 2y in the sandwich structure. This charge distribution of the highly ionic Eu–COT clusters was consistently discussed by using the spectroscopic methods of both photoionization and photoelectron spectroscopies w13x. The photoelectron spectrum of Eu 1ŽCOT.y 2 exhibits almost the same feature with that of Yb1ŽCOT.y 2 , but is different from those of Nd 1ŽCOT.y and 2 Ho 1ŽCOT.y 2 , where both Eu and Yb prefers the oxidation state of q2, while Nd and Ho do prefer
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that of q3. Moreover, the n dependence of the ionization energy for Ž n, n q 1. clusters shows a common feature with that for Yb–COT; the ionization energy is almost constant for n. This is because all of the Eu atoms can take the oxidation state of q2 in the cluster, where the ionizing state of the cluster for Ž n, n q 1. is a non-bonding e 3u state of Ln2q under the symmetry of D 8h . These spectroscopic findings on the charge distributions are consistent with the work of the chemical probe by Na atoms. It should be noted that NaŽCOT.y 2 was prominently observed in Fig. 2c without the formation of ŽCOT.y 2 . This is probably attributed to the sandwich structure of ŽCOT.NaŽCOT.y, although the aromaticity of COT 2y is not satisfied in it. The photoelectron spectra of NaŽCOT.y 2 will be published elsewhere, including those of the larger Na nŽCOT.y m clusters w22x. 3.2. Ho–COT–Na Fig. 4 shows the mass spectra of neutral or cationic Ho nŽCOT. m Na0rq clusters; Fig. 4a and b gives phok toionization mass spectra of the neutral with the ArF laser, whereas Fig. 4c is the mass spectrum of cationic clusters, Ho nŽCOT. m Naq k . The peaks are labeled according to the notation Ž n, m, k., denoting the numbers of Ho atoms Ž n., COT molecules Ž m., and Na atoms Ž k . along with the superscript showing the total charge of the cluster. Without Na atoms, the neutral Ho nŽCOT. m clusters are produced prominently at Ž n, m. s Ž n, n q 1., as shown in Fig. 4a, and no dehydrogenated species were found with these clusters. Similarly to Eu–COT, these progressions show the formation of the sandwich clusters of Ho–COT, where Ho and COT are alternately piled up. With co-vaporization of Na atoms, the Na adduct was produced only for Ho 1ŽCOT. 2 , and the higher Ho nŽCOT. nq1 clusters Ž n G 2. do not exhibit the Na-atom addition. In the Ho nŽCOT. m Naq k cations, furthermore, up to two Na atoms can be attached to Ž1, 2.q, and one Na atom can also be attached Ž . to Ž2, 3.q, forming Ho 1ŽCOT. 2 Naq 2 and Ho 2 COT 3 q q Ž . Na 1 , respectively. Larger n, n q 1 clusters for n G 3 do not exhibit the Na-atom addition. These features in the mass spectra of the Na adduct can also be explained by the charge distribu-
Fig. 4. Time-of-flight mass spectra of: Ža. neutral Ho–COT; Žb. neutral Ho–COT–Na; and Žc. cationic Ho–COT–Na clusters. Ža. and Žb. were obtained by the photoionization of the ArF laser Ž6.42 eV.. Peaks are labeled according to the notation Ž n, m. or Ž n, m, k ., denoting the number of Ho atoms Ž n., COT molecules Ž m., and Na atoms Ž k .. Many peaks unlabeled can be assigned to contaminated species with O 2 andror H 2 O.
tion of ionic Ho nŽCOT. m Na k clusters, which is very similar to Eu–COT. A marked difference between them is the oxidation state of the metal atom, because the Ho atom prefers to take the oxidation state of q3 in Ho nŽCOT. m , as reported previously. As shown in Fig. 5, the neutral Ho 1ŽCOT. 2 cluster has a valence vacancy in the terminal COT ligands, and the addition of one Na atom can strictly satisfy
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take Na atoms until EurHo and COT satisfy the charged states of Eu2qrHo 3q and COT 2y. This chemical probe experiment of Na atoms shows the number of electron vacancies in the cluster, implying that some Eu–rHo–COT clusters should be prepared as their Na salts in bulk.
Acknowledgements This work is supported by the program entitled ‘‘Research for the Future ŽRFTF.’’ of the Japan Society for the Promotion of Science Ž98P01203. and by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science, Sports, and Culture. TK expresses his gratitude to Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. Fig. 5. Proposed charge distributions of valence electrons of Ho nŽCOT. nq1 Na k clusters: Ža. neutral and Žb. cationic clusters. These schematics are based on the assumption of two-electron acceptability of COT and multiple-charged positive ions of Ho 3q. For Ž1, 2., one Na atom can be added, while Ž1, 2.q and Ž2, 3.q can take two and one Na atomŽs., respectively, where the charge distributions of Ho 3q, COT 2y, and Naq are satisfied.
aromaticity as COT 2y. However, larger clusters of neutral Ho nŽCOT. nq1 Ž n G 2. have no vacancy to accept the valence electron of Na atoms, so that no Na adducts were formed. These results for Ho–COT, therefore, suggest that Ž1, 2. can be synthesized as the Na salt of Ho 1ŽCOT. 2 Na 1 , and Ž2, 3. itself can be synthesized without any counter-ions in bulk. As reported in the previous work about the formation of Nd 3 ŽCOT.4 Cl 1 w13x, furthermore, Ž3, 4. is expected to be synthesized as halogenated species, because an halogen atom can remove one electron from the cluster, completing the charge balance between three Nd 3q ions and four COT 2y. As shown in Fig. 5b, moreover, the cationic Ho nŽCOT.q nq1 cluster can accept one more electron compared to the corresponding neutral, because the positive charge makes electrons acceptable: Ž1, 2.q and Ž2, 3.q become Ž1, 2.q– Na 2 and Ž2, 3.q–Na, respectively. In summary, the chemical methodology of the Na-atom addition can reveal the charge distribution of the lanthanide organometallic clusters successfully. The highly ionic Eu–rHo–COT cluster can
References w1x A. Streitwieser Jr., U. Muller-Westerhoff, J. Am. Chem. Soc. ¨ 90 Ž1968. 7364. w2x H. Schumann, J.A. Meese-Marktscheffel, L. Esser, Chem. Rev. 95 Ž1995. 865. w3x J.V. Ortiz, R. Hoffmann, Inorg. Chem. 24 Ž1985. 2095. w4x H. Rabaa, J.-Y. Saillard, R. Hoffmann, J. Am. Chem. Soc. 108 Ž1986. 4327. w5x L.J. Nugent, P.G. Laubereau, G.K. Werner, K.L. Vander Sluis, J. Organomet. Chem. 27 Ž1971. 365. w6x K.O. Hogson, F. Mares, D.F. Starks, A. Streitwieser Jr., J. Am. Chem. Soc. 95 Ž1973. 8650. w7x K.N. Raymond, C.W. Eigenbrot, Acc. Chem. Res. 13 Ž1980. 276. w8x R.A. Andersen, J.M. Bonsella, C.J. Burns, J.C. Green, D. Hohl, N. Rosch, J. Chem. Soc., Chem. Commun. Ž1986. 405. w9x A. Greco, S. Cesca, G. Bertolini, J. Organomet. Chem. 113 Ž1976. 321. w10x C.W. DeKock, S.R. Ely, T.E. Hopkins, M.A. Brault, Inorg. Chem. 17 Ž1978. 625. w11x K. Hoshino, T. Kurikawa, H. Takeda, A. Nakajima, K. Kaya, J. Phys. Chem. 99 Ž1995. 3053. w12x T. Kurikawa, M. Hirano, H. Takeda, K. Yagi, K. Hoshino, A. Nakajima, K. Kaya, J. Phys. Chem. 99 Ž1995. 16248. w13x T. Kurikawa, Y. Negishi, F. Hayakawa, S. Nagao, K. Miyajima, A. Nakajima, K. Kaya, J. Am. Chem. Soc. 120 Ž1998. 11766. w14x M.E. Geusic, M.D. Morse, S.C. O’Brien, R.E. Smalley, Rev. Sci. Instrum. 56 Ž1985. 2123. w15x T. Kurikawa, H. Takeda, M. Hirano, K. Judai, T. Arita, S. Nagao, A. Nakajima, K. Kaya, Organometallics Ž1999, in press..
262
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w16x M.J.S. Dewar, G.J. Gleicher, J. Am. Chem. Soc. 87 Ž1965. 685. w17x G.I. Fray, R.G. Saxton, The Chemistry of Cyclooctatetraene and Its Derivatives, Cambridge University Press, Cambridge, 1978. w18x M. Dolg, P. Fulde, W. Kuchle, C. Neumann, H. Stoll, J. ¨ Chem. Phys. 94 Ž1991. 3011.
w19x M. Dolg, P. Fulde, H. Stoll, H. Preuss, A. Chang, R.M. Pitzer, Chem. Phys. 195 Ž1995. 71. w20x W. Liu, M. Dolg, P. Fulde, J. Chem. Phys. 107 Ž1997. 3584. w21x W.J. Evans, J.L. Shreeve, J.W. Ziller, Polyhedron 14 Ž1995. 2945. w22x K. Miyajima, M. Hashimoto, A. Nakajima, K. Kaya Žto be published..