J. Phys. Chem. 1996, 100, 14281-14288
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Reactivities of Metallocarbohedrenes: Nb8C12+ Y. G. Byun, S. A. Lee, S. Z. Kan, and Ben S. Freiser* H. C. Brown Laboratory of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907 ReceiVed: April 23, 1996; In Final Form: May 28, 1996X
A compact supersonic source is employed with a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer to generate and investigate the metallocarbohedrene, Nb8C12+. Nb8C12+ reacts with water and alcohols via sequential attachments and dehydrogenations leading to the initial truncation products Nb8C12(OR)4+ (R ) H, CH3, C2H5, C4H9). Also, Nb8C12+ reacts with NH3 via sequential attachments to produce the initial truncation product Nb8C12(NH3)4+. Nb8C12L5-8+ peaks for ammonia and alcohols are also observed, but they grow in slowly. A measurement of the relative rate constants for the sequential addition reactions of NH3 confirms that addition of the fifth NH3 is at least an order of magnitude slower than the first four attachments. In reactions with acetonitrile and benzene, a maximum of four attachments of these ligands is observed. Sequential halide abstractions with CH3X yield Nb8C12Xn+ truncating at n ) 4 for X ) Cl and n ) 5 for X ) Br and I. The “titration” results and the relative rate constants for the sequential addition reactions of NH3 provide supporting evidence that the geometric structure of Nb8C12+ is the theoretically more stable Td or D2d symmetry having two sets of four equivalent metal sites, as opposed to the Th symmetry with all eight equivalent metals. Finally, the results for Nb8C12+ are compared to those previously obtained for V8C12+ and Ti8C12+.
1. Introduction Since the report of their discovery by Castleman and co-workers in early 1992, metallocarbohedrenes, M8C12 (M ) Ti, V, Zr, Hf, Cr, Mo, and Fe)1-8 have been studied intensely. A pentagonal dodecahedron cage structure with Th symmetry was originally proposed by Castleman and co-workers,1-5 and several additional structures have been proposed to account for the special stability of these species.9-25 Most theoretical studies have indicated that distorted9-21 structures are substantially more stable than the originally proposed pentagonal dodecahedron cage structure with Th symmetry. Rohmer et al., for example, estimated that the Td and D2d structures of Ti8C12+ are more stable than the Th structure by 190 and 154 kcal/mol, respectively.21 Also, Khan has reported calculations on an interesting bicapped antiprism structure in which a 12-carbon cage structure is surrounded by eight Ti atoms.22 Bowers and co-workers obtained evidence for a hollow cage structure for Ti8C12+ using elegant ion chromatography experiments.26 While their results provided support for the Th structure, the Td and D2d structures could not be unambiguously ruled out. However, their results did rule out a few other structures. Castleman and co-workers have found that Ti8C12+ sequentially associates with up to eight polar molecules such as water, ammonia, methanol, and 2-butanol and up to four π-bonding molecules such as C6H6, CH3CN, C2H4, and pyridine.8,27 They explained that the polar molecules attach to eight similar metal sites, and the π-ligands each bond to the two titanium atoms in one pentagonal surface, by a d-π interaction, in accordance with the Th structure. Furthermore, Castleman and co-workers suggested that the number of unpaired electrons on the metcars could be determined directly from the number of sequential iodine atom abstractions from methyl iodide.23 They observed that Ti8C12+ abstracts one iodine atom, Ti7NbC12+ four iodine X
Abstract published in AdVance ACS Abstracts, August 1, 1996.
S0022-3654(96)01166-5 CCC: $12.00
atoms, and Nb8C12+ five iodine atoms from methyl iodide, for example, indicating that these ions contain one, four, and five unpaired electrons, respectively. We recently reported the reactions of V8C12+ with water, ammonia, alcohols, CH3CN, benzene, and CH3X (X ) Cl, Br, I).24,28 For polar molecules, four “fast” sequential attachment reactions occurred leading to the initial truncation product V8C12L4+, followed in all cases except water by additional much slower association reactions. These results were proposed to be consistent with the theoretically calculated more stable Td or D2d symmetry with two sets of four equivalent metal sites, as opposed to the Th symmetry. In reactions with methyl halide, V8C12+ underwent sequential halide abstraction reactions, exclusively, yielding mass spectra that truncated sharply at V8C12X4+ (X ) Cl, Br, I). These results were interpreted as indicating that V8C12+ has four accessible unpaired electrons, and one less accessible or less reactive unpaired electron. Dehydrogenation was observed for the secondary reactions with water and alcohols, ROH (R ) H, CH3, C2H5, C3H7, C4H9), yielding V8C12(OR)2+ intermediates. The previous studies on Ti8C12+ and V8C12+ indicate that the two met-cars might have different structures.8,24,27,28 It is certainly not necessarily the case that all met-cars should have the same structure and, therefore, it is important to study the structures of other met-cars and by a variety of techniques. Thus, in this paper, the reactions of Nb8C12+ with water, ammonia, alcohols, acetonitrile, benzene, and methyl halide are reported. Once again, under the conditions used in our experiments, a clear truncation is observed at Nb8C12L4+with both polar and π-bonding molecules. While additional polar ligands are added to Nb8C12L4+, they grow in very slowly compared to the first four. These results suggest that Nb8C12+ has Td or D2d symmetry with two sets of metal sites consisting of four metal atoms each. As observed previously by Castleman and coworkers for CH3I,23 Nb8C12+ undergoes sequential halide abstractions in its reactions with CH3X (X ) Cl, Br, I). © 1996 American Chemical Society
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Figure 1. Reaction of Nb8C12+ with H2O (∼0.7 × 10-8 Torr). The reaction products correspond to Nb8C12+-L, L as labeled on the spectra at various reaction times: (a) 5 s; (b) 10 s; (c) 25 s; (d) 40 s.
Figure 2. Reaction of Nb8C12+ with NH3 (∼1 × 10-7 Torr). The numbers correspond to the number of NH3 attached to Nb8C12+ at various reaction times: (a) 2 s; (b) 4 s; (c) 6 s; (d) 8 s.
2. Experimental Section
Ar cooling gas was present and ammonia pressure was measured using standard procedures for calibrating the ion gauge for the sensitivity toward ammonia.37
All experiments were performed on a Nicolet FTMS-2000 (Finnigan FT/MS, Madison, WI) dual-cell Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer29,30 equipped with a 3 T superconducting magnet and combined with a compact supersonic source developed by Smalley and coworkers.31 Laser desorption using the second harmonic of a Nd:YAG laser (532 nm) was used to generate the niobiumcarbon clusters in a manner similar to Castleman and co-workers by seeding the He expansion gas with ∼5% methane.5,32 Reagents were introduced at a static pressure (10-8-10-5 Torr) using Varian leak valves or pulsed into the vacuum chamber using General Valve Corp. Series 9 solenoid pulsed valves.33 Ar was used as the collision gas at a static pressure of ∼1.7 × 10-5 Torr. An uncalibrated Bayard-Alpert ionization gauge was used to monitor pressure. Ion isolation,29 collision-induced dissociation (CID),34 and multiple excitation collisional activation (MECA)35 were accomplished either by standard FT-ICR radio frequency pulses of variable frequency and power or by using SWIFT excitation.36 For the rate constant measurements,
3. Results and Discussion A. Collisional Activation. Duncan and co-workers found that the major photofragments of Nb8C12+ were Nb+, Nb2C2+, Nb3C3+, Nb4C4+, Nb5C6+, Nb6C7+, and Nb7C9+.38 This fragmentation pattern is dramatically different from that of other met-cars. The dominant dissociation process of other met-cars such as Ti8C12+, V8C12+, Cr8C12+, and Fe8C12+ is the loss of metal atoms.6,7,24 Interestingly, neither collision-induced dissociation (CID)34 nor multiple excitation collisional activation (MECA)35 was able to promote fragmentation of Nb8C12+. These results are in contrast to MECA on V8C12+, which yielded several products24 and imply that the binding energy of Nb8C12+ is stronger than that of V8C12+. Niobium and vanadium are in the same group and isoelectronic, but the 4d electrons on niobium are more diffuse than the 3d electrons on vanadium and, therefore, niobium can have stronger bonding with carbon.
Reactivities of Metallocarbohedrenes: Nb8C12+
J. Phys. Chem., Vol. 100, No. 34, 1996 14283
Figure 3. Reaction profile of Nb8C12+ with NH3 (∼1 × 10-7 Torr). The numbers correspond to the number of NH3 attached to Nb8C12+ at various reaction times.
TABLE 1: Relative Rate Constants for the Sequential Addition Reactions of Nb8C12+ with NH3a species +
Nb8C12 Nb8C12L+ Nb8C12L2+ Nb8C12L3+ Nb8C12L4+ A
relative reactivity 1 0.59 0.41 0.29 0.03
rates obtained from the decay slopes in Figure 3.
Interestingly, the bond dissociation energy for Nb+-C (134.8 kcal/mol)39 is considerably greater than for V+-C (89.2 kcal/ mol).40 The average binding energy per atom in V8C12 is predicted to be 5.7 eV.17 Therefore, the average binding energy of Nb8C12 should be larger than 5.7 eV. B. Water Reactions. Nb8C12+ reacts with a trace background of water (∼0.7 × 10-8 Torr) in the ICR cell, as shown in Figure 1, by an initial addition of H2O. Reaction of Nb8C12(H2O)+ with a second molecule of water proceeds by elimination of H2 yielding, presumably, Nb8C12(OH)2+. The analogous processes have also been observed in the reactions of Nb4C4+,41 V8C12+,28 and V14C12-13+ 42 with H2O. Two water molecules on adjacent metal atoms of Nb8C12+ may interact, each losing one hydrogen atom, to release a hydrogen molecule without the hydrogen atoms actually binding to the metal atoms, by analogy to the proposed decomposition mechanism for the reaction of water with bare iron clusters involving hydrogen molecule loss.43 Nb8C12(OH)2+ then reacts further to coordinate an additional water to yield Nb8C12(OH)2(H2O)+. This intermediate reacts with an additional water followed by elimination of H2, yielding Nb8C12(OH)4+. These processes have also been observed in the reactions of V14C13+ with H2O.42 Interestingly, the Nb8C12(OH)4+ fails to react further with H2O even at 40 s reaction time and higher pressure, ∼1.7 × 10-7 Torr. Two competitive pathways were found for the reactions of Nb4C4+ with H2O and CH3OH which are dependent on the pressure of the background argon cooling gas.41 In contrast, there was only one pathway observed for Nb8C12+ either with or without argon cooling gas. C. Ammonia Reactions. Nb8C12+ reacts with a static pressure of NH3 at ∼1.5 × 10-7 Torr as shown in Figure 2 by sequential association reactions, exclusively, yielding mass spectra that truncate sharply at Nb8C12(NH3)4+. As the reaction time was increased to up to 45 s, higher order peaks such as Nb8C12(NH3)5+ and Nb8C12(NH3)6+ were also observed to grow
Figure 4. Reaction of Nb8C12+ with CH3OH (∼3.1 × 10-7 Torr). The numbers correspond to the number of OCH3, OH, CH3OH, and H2O attached to Nb8C12+ at various reaction times: (a) 0.5 s; (b) 1 s; (c) 1.5 s; (d) 2 s.
in slowly. No dehydrogenation was observed for the reactions with ammonia, in contrast to those with water. Pseudo-first-order kinetics are observed for the reaction of Nb8C12+ with NH3, Figure 3, indicating, but not unequivocally, that the ions are thermalized and consist predominantly of a single isomeric structure. The slope of the pseudo-first-order plot for Nb8C12+ is used with the estimated pressure to obtain the observed rate constant, kob, for ammonia of 4.9 × 10-10 cm3 molecule-1 s-1. The reaction efficiency is calculated by comparing kob to the average dipole orientation rate constant, kADO.44,45 The parameters for NH3, such as the polarizability (R) and the dipole moment (µD), used for calculating kADO are mentioned in ref 46. kADO and the reaction efficiency (kob /kADO) × 100% are 1.9 × 10-9 cm3 molecule-1 s-1 and 26%, respectively. The relative rate constants for the sequential addition reactions of NH3 are summarized in Table 1. As shown in Table 1 and Figures 2 and 3, the first four reactions of ammonia with Nb8C12+ are relatively fast while subsequent reactions are slower. Interestingly, Dance11 predicted that distorted met-cars should exhibit first four fast association
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Figure 5. Reaction of Nb8C12+ with CH3CH2OH (∼2.3 × 10-7 Torr). The numbers correspond to the number of OCH2CH3, OH, CH3CH2OH, and H2O attached to Nb8C12+ at various reaction times: (a) 1 s; (b) 1.5 s; (c) 2 s; (d) 3 s.
reactions and then four slow association reactions with ammonia, and Lin and Hall12 expected that the four outer metal atoms have larger open coordinate sites in the distorted met-cars. Thus, the first four fast reactions are believed to be associated with the four outer metals, while the four inner metal sites bind ligands less strongly or perhaps are blocked or less exposed to the incoming ligands. D. Alcohol Reactions. Alcohols react with Nb8C12+ in the exact pattern as the reactions with water. Nb8C12+ reacts with methanol, ethanol, and 2-butanol by an initial addition of alcohol, as shown in Figures 4-6. Nb8C12(ROH)+ (R ) CH3, C2H5, C4H9) reacts with the second alcohol by elimination of H2, yielding Nb8C12(OR)2+. This intermediate reacts further to coordinate two additional alcohols followed by elimination of H2 truncating at four attachments, Nb8C12(OR)4+. The presence of trace amounts of background moisture in the ICR cell is evidenced by the observation of combination products, such as Nb8C12(OR)(OH)+ and Nb8C12(OR)3(OH)+. Higher order peaks such as Nb8C12(OR)4(ROH)1-4+ are also observed at longer reaction times. Although the last four attachments
Byun et al.
Figure 6. Reaction of Nb8C12+ with 2-butanol (∼2.9 × 10-7 Torr). The numbers correspond to the number of 2-butoxide, OH, 2-butanol, and H2O attached to Nb8C12+ at various reaction times: (a) 1.5 s; (b) 2.5 s; (c) 3.5 s; (d) 4 s.
are slow, there is a trend toward increasing rates with increasing size of the alcohols. In other words, the initial truncation observed at Nb8C12(OR)4+ is less pronounced going from water and methanol to butanol. These trends were also observed with V8C12+ and can be attributed to the increased degrees of freedom afforded by the larger alcohols.28 E. Reactions with Acetonitrile and Benzene. At ∼5.8 × 10-7 Torr, a maximum uptake occurred for acetonitrile at four attachments, as shown in Figure 7, despite its stronger dipole moment. Nb8C12(CH3CN)1-4(H2O)+ are also observed. Nb8C12+ reacts with a static pressure of benzene at ∼1.3 × 10-6 Torr, as shown in Figure 8. As the reaction proceeds, Nb8C12(C6D6)4+ grows in as the base peak. Minor peaks of Nb8C12(C6D6)3(H2O)+ and Nb8C12(C6D6)2(OH)2+ were also observed because of the presence of background water. No evidence, however, was seen for more than four attachments of CH3CN or C6D6. While truncation for CH3CN and benzene at Nb8C12L4+ is consistent with the distorted met-car structure and analogous to the results obtained for Ti8C12+, Castleman and co-workers have suggested the alternative explanation that these ligands actually π-bond to two metal atoms in each of four pentagonal
Reactivities of Metallocarbohedrenes: Nb8C12+
Figure 7. Reaction of Nb8C12+ with CH3CN (∼5.8 × 10-7 Torr). The reaction products correspond to Nb8C12+-L, L as labeled on the spectra at various reaction times: (a) 2 s; (b) 4 s; (c) 6 s.
faces.27 While this elegant explanation is not unreasonable, it may be problematic for benzene where the nonbonded carbons would interact unfavorably causing rearrangement and possible loss of the aromatic stabilization of benzene. We also consider some additional information here. Nb4C4+,41 Nb6C7+,47 and V14C12-13+ 42 react in a sequential fashion with CH3CN and/or background water to yield the truncation products Nb4C4(CH3CN)2(OH)2+, Nb6C7(CH3CN)4+, V14C12-13(CH3CN)7(H2O)+, and V14C12-13(CH3CN)8+. In addition Nb4C4+,41 Nb4C5+, Nb5C6+,48 and Nb6C7+ 47 all react with benzene (C6D6) sequentially to yield a four ligand association truncation product. Truncation for Nb4C4+ at four attached ligands is consistent with the theoretically calculated cubic structure in which each metal is in equivalent positions. Theoretical calculations on Nb6C7+ indicate truncation at Nb6C7L4+ occurs because four of the six niobium atoms of
J. Phys. Chem., Vol. 100, No. 34, 1996 14285 Nb6C7+ are in active sites, while the other two are in less active sites. The reactivity of V14C12-13+ was found to be consistent with a proposed 3 × 3 × 3 fcc cube in which the eight corner metal sites are less coordinated and, therefore, more active than the six face-centered sites. The shortest metal-metal distances in Nb4C4+, Nb6C7+, Ti8C12+, V8C12+, and V14C12-13+ are ∼3 Å.22,41,47 If each ligand (acetonitrile and benzene) were to bond to two metal centers, truncation should be observed at Nb4C4L2+, Nb4C5L2+, Nb6C7L3+, and V14C12-13L7+. However, these product ions are intermediates, not initial or final truncation products. While these cubic structures are different than the metallocarbohedrene structure, π-ligands should also be able to bond to two metals in the Nb2C2 face in Nb4C4+, for example, because it is a similar environment to the planar Nb2C3 face of the metallocarbohedrene. Acetonitrile has two possible types of coordination to a metal atom center, an end-on interaction via the nitrogen lone pair orbital and a side-on coordination via the π-system of the CN group. Also, side-on coordination can involve one metal center or two metal centers. For Nb4C4+, Nb6C7+, and V14C12-13+, our results indicate that acetonitrile bonds to one metal center not two by either end-on or side-on coordination. Similarly, each benzene bonds to one metal center of Nb4C4-5+, Nb5C6+, and Nb6C7+. Therefore, an interpretation of our results is that each C6D6 or CH3CN bonds to one of the four outer metals of the distorted met-cars, Nb8C12+ and V8C12+, to produce the truncation peak of M8C12L4+. We are in the process of modeling end-on vs side-on bonding theoretically. F. Reactions with CH3X (X ) Cl, Br, I). The reactions of Nb8C12+ with methyl halides yield sequential halide abstraction products. Mass spectra were obtained after cooled Nb8C12+ was isolated and allowed to react with CH3Cl, CH3Br, and CH3I, respectively, introduced through a pulsed valve or a leak valve into the ICR cell. For CH3Cl, the reaction products truncate at Nb8C12(Cl)4+, as shown in Figure 9. For CH3Br, the initial truncation occurs at Nb8C12(Br)4+ (reaction time, 100 ms) and the final truncation at Nb8C12(Br)5+ (reaction time, 350 ms), as shown in Figure 10. Thus, the Nb8C12(Br)5+ product grows in very slowly compared to the rate in which Nb8C12(Br)4+ grows in. As shown in Figure 11, Nb8C12(I)3(H2O)+ and Nb8C12(I)4+ are produced quickly from CH3I in the pressure of a trace amount of background water, but they are converted into Nb8C12(I)4(H2O)+ and Nb8C12(I)5+ very slowly. Even though the reaction time is quite long, 14 s, the Nb8C12(I)3(H2O)+ and Nb8C12(I)4+ precursor ions remain abundant. These results can be rationalized by having four of the unpaired electrons localized onto the four outer metals in the distorted structure, which are involved in the four fast halide abstractions. The remaining
Figure 8. Reaction of Nb8C12+ with C6D6 (∼1.3 × 10-6 Torr). The reaction products correspond to Nb8C12+-L, L as labeled on the spectra at reaction time of 4 s.
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Figure 9. Reaction of Nb8C12+ with CH3Cl introduced through a pulsed valve. The reaction time is 50 ms.
the larger CH3-Cl bond energy relative to the other methyl halides. Interestingly, V8C12+ was observed to abstract only four iodine atoms from methyl iodide,28 in contrast to the five iodine abstractions of Nb8C12+. V8C12+, like Nb8C12+, should have five unpaired electrons, since V and Nb are isoelectronic. However, the 3d electrons on V and the 4d electrons on Nb have different diffuse characters. This might result in different electronic structures for these two met-cars. As shown in Figure 12, once formed, Nb8C12(I)5+ reacts with pulsed-in CH3I (∼5.0 × 10-5 Torr) further to yield the association product, Nb8C12(I)5(CH3I)+. This indicates that Nb8C12(I)5+ has no more unpaired electrons and cannot abstract more iodine from CH3I. This behavior was first predicted and observed by the Castleman group.23 4. Conclusions
Figure 10. Reaction of Nb8C12+ with CH3Br introduced through a pulsed valve. The reaction products correspond to Nb8C12+-L, L as labeled on the spectra at various reaction times: (a) 100 ms; (b) 140 ms; (c) 170 ms; (d) 350 ms.
one unpaired electron could be localized onto the four inner metals and involved in one more slower halide abstraction (X ) Br, I). The absence of a fifth Cl abstraction is attributed to
The initial reactivity studies on met-cars to date have suggested that these 20-atom clusters have either symmetric Th structures with all eight metals in equivalent sites or distorted structures, Td or D2d, in which there are two sets of four equivalent metal sites. Generally, however, if there is more than one degenerate highest occupied orbital in a cluster and there is an odd number of unpaired electrons, then the cluster is likely to distort to achieve a more stable geometric structure.49 Since M8C12+ (M ) Ti, V, Nb) should all have an odd number of unpaired electrons, it is favorable for these met-cars to distort from a Th structure to the distorted structures, such as a Td or D2d structure calculated by theoretical chemists.9-21 Furthermore, in a recent theoretical study, Dance has found that for Ti8C12, there is a barrierless transformation of the Th isomer to the Td isomer.50 Isomerizations were also found for Zr8C12, V8C12, and Nb8C12. In our work, for all of the reagents studied, the first four reactions with Nb8C12+ were observed to be relatively fast leading to a buildup or truncation of the complex with four ligands attached. Subsequent reactions, if observed, are markedly slower than the first four. These results provide support for the distorted structures. In contrast, Castleman and co-workers saw eight attachments to Ti8C12+ occurring at roughly the same rate with no noticeable buildup of Ti8C12L4+ with 2-butanol, supportive of the Th structure.27 The number of unpaired electrons on Nb8C12+ and Ti8C12+ may be 5 and 1, respectively, based on the number of iodine abstractions from CH3I. Thus, with different valence electrons, different numbers of unpaired electrons, and d electrons with different diffuse characters, it is reasonable that Nb8C12+ and Ti8C12+ might have different structures. Alternatively, significant differences in the
Reactivities of Metallocarbohedrenes: Nb8C12+
Figure 11. Reaction of Nb8C12+ with leaked-in CH3I (∼2.9 × 10-7 Torr). The reaction products correspond to Nb8C12+-L, L as labeled on the spectra at various reaction times: (a) 3 s; (b) 7 s; (c) 10 s; (d) 14 s.
experimental parameters (time, pressure, kinetic energy) could account for the discrepancy in the results from the two laboratories.
J. Phys. Chem., Vol. 100, No. 34, 1996 14287 It is also conceivable that the RO ligands in essence change the symmetry of the met-car yielding an apparent truncation at four ligands. A similar effect has been reported by Castleman and co-workers on the iodine abstraction product Ti8C12I+,27 where attachment of the I reduced the symmetry of the eight metals down to two groups of four metals. In this case a buildup at Ti8C12I(ROH)4+ was observed in the distribution leading ultimately to the formation of Ti8C12I(ROH)7+. While certainly the presence of the RO group on Nb8C12(OR)n+ (n ) 2 or 4) has an effect on the local electronic structure at the metals, this should not be a problem with NH3 where a truncation at Nb8C12(NH3)4+ was observed. In addition, unlike CH3CN, NH3 is strictly a σ-bonding polar ligand. A related complication, however, would be if the heat released by attachment of a ligand, such as NH3, would cause a rearrangement of the met-car. While this cannot be completely ruled out, we believe that it is unlikely. In particular, according to the theory discussed above, the distorted structures are lower in energy by as much as 190 kcal/ mol. Thus, with increasing internal energy, rearrangement to the more symmetric Th structure should occur, and this is not observed. Although attachment of ligands in general can alter the structure of the cluster being studied, it is believed that the strong bonds in the met-cars make them quite rigid structures. Thus, there is good reason to believe that the indirect evidence presented here reflects the true structure of Nb8C12+. Interestingly, for the π-ligands CH3CN and benzene, a clear truncation at M8C12L4+ has been observed for all three metcars studied to date, M ) Ti, V, and Nb. This observation has been explained for Ti8C12+, based on a Th structure, as each π-ligand bonded to two metals in a pentagonal face. For V8C12+ and Nb8C12+, however, we believe that each π-ligand bonds to only one of four “active” metal centers on the distorted structure. There is precedence for both types of bonding in the organometallic and surface literature.51,52 Finally, while V8C12+ and Nb8C12+ react in nearly the same fashion, we note one striking difference. The initial truncation product for V8C12+ with water and alcohols is V8C12(OR)2(ROH)2+ 28 while for Nb8C12+ it is Nb8C12(OR)4+, R ) H, alkyl. Thus, a second dehydrogenation is observed for Nb8C12+ and not V8C12+. In looking at the dehydrogenation reactions of these and other metal-carbon clusters, there is a trend that emerges that the number of dehydrogenations goes up with the number of accessible unpaired electrons, as determined by the reactions with CH3I. For example, for M8C12+ (M ) Ti, V, Nb), the number of accessible electrons are determined to be 1, 4, and 5, respectively, and the number of dehydrogenations are 0, 1, and 2, respectively. This might suggest that a mechanism, such as oxidative addition which requires at least two electrons, is
Figure 12. Reaction of Nb8C12+ with CH3I introduced through a pulsed valve. The reaction products correspond to Nb8C12+-L, L as labeled on the spectra at reaction time of 250 ms.
14288 J. Phys. Chem., Vol. 100, No. 34, 1996 involved. Additional data and theoretical studies are underway on other systems to test this trend. Acknowledgment. Acknowledgment for support of this research is made by B.S.F. to the National Science Foundation (CHE-9224476) and to the Division of Chemical Sciences in the Office of Basic Energy Sciences in the United States Department of Energy (DE-FG02-87ER13766); Y.G.B. acknowledges Lubrizol for Fellowship support. We also acknowledge the reviewers for their helpful comments. References and Notes (1) Guo, B. C.; Kerns, K. P.; Castleman, A. W., Jr. Science 1992, 255, 1411. (2) Guo, B. C.; Wei, S.; Purnell, J.; Buzza, S.; Castleman, A. W., Jr. Science 1992, 256, 515. (3) Wei, S. ; Guo, B. C.; Purnell, J.; Buzza, S.; Castleman, A. W., Jr. Science 1992, 256, 818. (4) Wei, S.; Guo, B. C.; Purnell, J.; Buzza, S.; Castleman, A. W., Jr. J. Phys. Chem. 1992, 96, 4166. (5) Chen, Z. Y.; Guo, B. C.; May, B. D.; Cartier, S. F.; Castleman, A. W., Jr. Chem. Phys. Lett. 1992, 198, 118. (6) Pilgrim, J. S.; Duncan, M. A. J. Am. Chem. Soc. 1993, 115, 4395. (7) Pilgrim, J. S.; Duncan, M. A. J. Am. Chem. Soc. 1993, 115, 6958. (8) Guo, B. C.; Kerns, K. P.; Castleman, A. W., Jr. J. Am. Chem. Soc. 1993, 115, 7415. (9) Reddy, B. V.; Khanna, S. N.; Jena, P. Science 1992, 258, 1640. (10) Hay, P. J. J. Phys. Chem. 1993, 97, 3081. (11) Dance, I. J. Chem. Soc., Chem. Commun. 1992, 1779. (12) Lin, Z.; Hall, M. B. J. Am. Chem. Soc. 1993, 115, 11165. (13) Chen, H.; Feyereisen, M.; Long, X. P.; Fitzgerald, G. Phys. ReV. Lett. 1993, 71, 1732. (14) Rohmer, M. M.; de Vaal, P.; Benard, M. J. Am. Chem. Soc. 1992, 114, 9696. (15) Gale, J. D.; Grimes, R. W. J. Chem. Soc., Chem. Commun. 1992, 1222. (16) Ceulemans, A.; Fowler, P. W. J. Chem. Soc., Faraday Trans. 1992, 88, 2797. (17) Reddy, B. V.; Khanna, S. N. Chem. Phys. Lett. 1993, 209, 104 (18) Reddy, B. V.; Khanna, S. N. J. Phys. Chem. 1994, 98, 9446. (19) Methfessel, M.; van Schilfgaarde, M.; Scheffler, M. Phys. ReV. Lett. 1993, 71, 209. (20) Rohmer, M. M.; Benard, M.; Henriet, C.; Bo, C.; Poblet, J. M. J. Chem. Soc., Chem. Commun. 1993, 1182. (21) Rohmer, M. M.; Benard, M.; Bo, C.; M. Poblet, J. J. Am. Chem. Soc. 1995, 117, 508. (22) Khan, A. J. Phys. Chem. 1995, 99, 4923. (23) Deng, H. T.; Guo, B. C.; Kerns, K. P.; Castleman, A. W., Jr. J. Phys. Chem. 1994, 98, 13373.
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