J. Phys. Chem. 1992, 96, 4166-4168
4166
Metallocarbohedrenes as a Class of Stable Neutral Clusters: Formation Mechanism of M& (M = Ti and V) S. Wei, B. C. Guo, J. Purnell, S. Buzza, and A. W. Castleman, Jr.* Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: April 14, 1992)
Evidence for the existence of metallocarbohedrenes [MacI2(M = Ti and V)] as a general class of stable neutral molecular clusters is reported. Studies of the formation and growth of metal-carbon clusters M,C, reveal the general mechanisms responsible for the formation of the prominent cagelike structure, M6C12,using a laser-based timeof-flight mass spectrometer. Investigation of the effect of the ionizing laser fluence over at least 4 orders of magnitude and studies at 1064, 532, 355, and 266 nm establish that the prominence of the M6CI2cluster arises due to its presence as a neutral rather than as the photofragment of clusters of large size. The unusual stability of this species for both Ti and V is consistent with the proposed cagelike structure being a pentagonal dodecahedron (point group symmetry Th).
Introduction It is well-known that Ca is very stable due to its unique cagelike structure.l Recently, it was found in our laboratory that the ionic species TisC12+displays an exceptional abundance (“super” magic) in the distribution of metal-carbon clusters formed from reactions of titanium with hydrocarbom2 In accordance with the findings, it was speculated that the enhanced abundance of Ti&+ was due to an unusually stable cagelike structure in the form of a dodecahedron and that thiscluster ion might comprise one member of a new class of material^,^ referred to as metallocarbohedrenes. We also suggested that if the model of a dodecahedral structure is correct, the neutral form of this species should exist and that other members of the class should be stable, as well. Very recently, we have made efforts to produce and investigate the neutral metal-carbon clusters of the metallocarbohedrene class. As discussed herein, our latest results on neutral clusters bear out our original speculation. Moreover, studies concerning their mechanism of formation suggest that these metal-carbon clusters first develop through multiple-ring structures as shown in Figure la, via the successive addition of MC2 units; then a very stable cage suddenly closes at MsC12(drawn in Figure lb). Discussion of these new findings is the subject of the present Letter.
Experimental Section The apparatus used in these experiments is a time-of-flight (TOF) mass spectrometer coupled with a laser vaporization source described in detail el~ewhere.~In brief, the second-harmonic output (532 nm, 20 mJ/pulse) of a Nd:YAG laser (DCR-1A) is used to irradiate the metal surface. Both ionic and neutral metal-carbon clusters are produced through plasma reactions between metals and various small hydrocarbons (e.g., CH4, CzH4, C2HZ,C3H6,etc.). After passing a 5-mm-diameter skimmer, the cluster beam enters the TOF lens region. The ionic cluster spectrum (e.g., Figure 2a) is obtained when a pulsed TOF field is used to accelerate the ionic species. Alternatively, the photoion spectrum of the neutral clusters (e.g., Figure 2b) is obtained when the neutral clusters are ionized using a second Nd:YAG laser (GCR-5) and the photoions are accelerated in a static TOF field. The ions are then detected by a microchannel plate detector, and ~~
(1) Kroto, H. W.; Heath, J. R.; OBrien, S. C.; Curl, R. F.; Smally, R. E. Nature 1985,318, 162. Kroto, H. W.; Allaf, A. W.; Balm, S . P. Chem. Reo. 1991, 91, 1213 and references therein. (2) Guo,B. C.; Kerns, K. P.; Castleman, Jr., A. W. Science 1992, 255, 1411. (3) Guo, B. C.; Wei, S.; Purnell, J.; Buzza, S.;Castleman, Jr., A. W. Science 1992, 256, 515. (4) Guo, B. C.; Wei, S.; Purnell, J.; Buzza, S.; Castleman, Jr., A. W. Manuscript in preparation.
Figure 1. (a, top) A possible structure of M4C8(M = dark balls, C = light balls) with four pentagon rings. (b, bottom) A proposed cagelike pentagonal dodecahedron structure of Mac,*.
the signal is analyzed by a LeCroy digital oscilloscope (Model 7200).
Results and Discussion Figure 2a displays a typical ionic cluster distribution of Ti&,+ using CH4 as the reactant gas. It is evident that TiSCl2+is “super” magic in the mass spectrum, in agreement with the earlier studies using a double-quadrupole apparatw2 The photoion spectrum obtained through photoionization of the neutral metal-carbon clusters Ti$, at 355 nm is presented in Figure 2b. It is clear that neutral TiaC12also displays prominence. Besides, other combinations at (4,8), (5,10), (6,12), and (7,13) are noticeably more abundant than the proximate metal-carbon clusters. It should be noted that the major isotope ( m / e = 48) of Ti coincides with four carbon ( m / e = 12) units, and the assignment of the spectrum is based on experiments using 13CH4and CD4 as the reactant gases. As for the smaller cluster size, the most dominate peak appears at Tic,; and clusters with combinations of (2,4) and (3,6) also display prominence. Interestingly, similar distributions are also observed in vanadium-carbon clusters. Figure 2c displays the photoion distributions of the neutral clusters, V,C,; a magic number at V& is clearly seen in the spectrum. Similar to Ti$, distributions, the peaks corresponding to (4,8), (5,10), (6,12), and (7,13) are also the local maxima of V,C,. Besides VC2, which is the most abundant peak
0022-365419212096-4166%03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4167
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Figure 2. (a) A typical mass spectrum of ionic clusters Ti$,,+. (c) A typical photoion spectrum of neutral clusters V,C, at 355 nm ( 1
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Figure 3. Photoion spectra of titanium-carbon clusters obtained at differing laser fluence. The peaks labeled by (m,n) correspond to Ti$,+, respectively. A 355-nm output was used as an ionizing laser and C2H4as the hydrocarbon reactant. Laser fluence for figures: (a) =5 X lo6 W/cm2, (b) =5 X IO9 W/cm2, and (c) =4X 1Olo W/cm2.
in the smaller cluster size range, the combinations (2,4) and (3,6) are also prominent. It should be noted that the peak intensities corresponding to M3C7and M3C8become comparable with MoCs at higher ionization laser power (possibly due to the contribution from fragmentation process at higher laser powers). However, all other features observed in both the Ti& and V,C, systems do not change under different experimental conditions, e.g., over a range of ionization laser powers (from lo6 to 10" W/cm2) and at different laser wavelengths (266, 355, 532, and 1064 nm). These findings are discussed in what follows. Since formation of the photoions may involve multiphoton processes, before definitively attributing the magic peaks to their
neutral species, it is necessary to conduct a series of experiments to determine whether the observed distribution is power and/or wavelength dependent. In order to address this point, we examined the effect of laser fluence on the photoion distribution of titanium-carbon clusters as shown in Figure 3. The spectrum shown in Figure 3a was taken at the minimum fluence leading to the appearance of Ti&+, while in Figure 3, b and c, the laser power density was increased by more than 3 and 4 orders of magnitude, respectively. Obviously the distribution does not change appreciably; in particular, the magic number patterns remain the same, despite the substantial change in fluence. Figure 4 shows the photoion spectra of titanium-carbon clusters obtained with a 266-,
4168 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
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Figure 4. Photoion spectra of titanium-carbon clusters obtained with a (a) 266 nm, (b) 532 nm, and (c) 1064 nm ionizing laser. Laser fluence for figures: (a) =l X lo7 W/cm2, (b) -1 X lo* W/cm2, and (c) = l X lo9 W/cm2. The peak labeled “+” corresponds to pump oil.
532-, and 1064-nm ionizing laser. As in Figure 3a, the spectra were taken at the minimum laser fluence of the Ti&,+ appearance so that the possibility of photofragmentation was minimized. As seen from Figures 3 and 4, in the case of all four wavelengths 266, 355, 532, and 1064 nm (one-photon energy corresponding to 4.7, 3.5,2.3, and 1.2 eV, respectively), these photoion spectra are very similar; also, the enhanced abundance of Ti8CI2+is apparent in all spectra. Hence, it is virtually certain that the distribution (magic number pattern) is neither laser power nor wavelength dependent. The fact that MC, (M = Ti and V) is the most abundant peak and that metal-carbon clusters with combinations of (MC,),, n < 7, always show enhanced abundance strongly points to MC, as the main building unit in the formation of these metal-carbon clusters. However, these “magic” combinations break down at n = 7 and more explicitly at n = 8. The findings are consistent with a cage closing to a pentagonal dodecahedron structure (Figure lb), where a minimum of 20 atoms are required to construct the cage. For the smaller cluster size, there are always enough sites for MC2 units to be filled; e.g., the possible M4C8 structure is drawn in Figure la. Therefore, no breakdown of the “magic” combinations of (Tic2), is expected, in accordance with the experimental findings. The combination of (7,13) marks the first closure of the cage when one of the eight metal sites (dark balls in Figure 1b) is occupied by one additional carbon atom. With one more addition of a MC2 unit, all of the required metal and
carbon sites are filled as shown in Figure lb. Due to the exceptional stability of this highly symmetrical structure, a part or all of the excess carbon atoms will “boil off“ during the formation process. This mechanism also accounts for observation that one or two additional carbon atoms could still remain attached to the cage (i.e., M8CI3and M8CI4in the spectra). Our preliminary studies4 on the metastable decomposition processes of these metal-carbon clusters do indeed show that one carbon atom is loosely bonded in MBCIj, while two carbons are loosely bonded in M8CI4.This result suggests that the abundant peaks at Mac13 and M8CI4observed in the spectra result from the stability of M8CI2,in agreement with the proposed formation mechanism. In the unique structure of M&, each of the 12 pentagonal rings contains two metals and three carbons. The fact that no magic numbers at (9, 11) or (10, 10) are present in the spectra indicates that the maximum number of metals which can be incorporated in this particular cage is eight, in accordance with the stronger metal-carbon bonds than metal-metal bonds in these system^.^
Acknowledgment. Financial support by the US. Department of Energy, Grant DE-FG02-ER-60668, is gratefully acknowledged. We thank Mr. Z. Shi,Dr. Z. Chen, Mr. B. May, and Mr. S. Cartier for helpful discussions during the course of the work. ( 5 ) Organometallic Chemistry of the Transition Elements; Pruchnik, F.
P., Ed.; Plenum Press: New York, 1990.
