Inorg. Chem. 1993, 32, 1931-1940
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Gas Phase Metal Chalcogenide Cluster Ions: A New [Co#,]- Series Up to [Co3&4]- and Two [Fe$,]- Series John El Nakat, Keith J. Fisher,' Ian G. Dance,' and Gary D. Willett School of Chemistry, University of New South Wales, PO Box 1, Kensington, N S W 2033, Australia
Received September 18, 1992 Laser (1064 nm) ablation of COS yields 83 gaseous ions [Co,S,]-, detected and characterized by FTICR mass spectrometry. These ions, ranging in size from [COS,]- to [CO&&, possess a slightly curved composition distribution on the x / y plot, with x / y 1.6. The same technique applied to FeS and KFeS2, yields 45 [Fe,S,]- ions, which are distributed in two distinct regions of the x / y composition map. Series a is a linear progression of triplets of ions with the compositions [Fe,Sn-']-, [Fens,]-, and [Fensn+']-,for n = 3-10, while ions in the unprecedented series b contain an additional SS,namely and [FenSn+6]-,for n = 1-7. Ions in series a are probably globular clusters, while each ion in series b could contain an additional chelating polysulfide ligand or could evince a structural principle of extended chains or ribbons of linked tetrahedra. Collisionally activated dissociation measurements for the smaller ions revealstabilityfor [Fe&6]-, [co&]-, and [Co&-. Cluster structures are postulated for representative ions throughout the composition range. Although there are analogies between probable gas-phase structures of the clusters and core structures for clusters in crystals, the much greater range and number of compositions observed for gaseous clusters presage possibilities for synthesis of new and unexpected clusters in condensed phases. Electronic structures are considered in comparison with those of [Ni,S,]- and [Cu,S,]-, revealing that the valence electron population per metal is largely independent of the metal identity, and decreases from about 15 a t x = 5 to about 12.5 for clusters with 37 metal atoms. This is consistent with increased concentration of metal atoms and metalmetal bonding in the cores of larger clusters.
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Introduction We report new collections of cobalt and iron sulfide clusters, generated as monoanions [Fe,S,]- and [Co,S,]- in the gas phase by laser ablation. The compositions of the [Fe,S,]- ions occur as two distinct populations with no intermediate ions, a characteristic unprecedented in gas phase metal cluster chemistry. The clusters we describe are possibly relevant to all of the frontiers and applications of metal chalcogenides,14 including semiconductor material^,^ photoresponsive nanoclusters and quantum dots: electrooptic and nonlinearly optic compound^,^ and biomineralizations and to the active site of the enzyme nitrogenase and compounds which model it.9 Recent years have seen a renaissance in gas-phase inorganic ( I ) Muller, A,, Krebs, B., Eds. Suljiur, its Significance f o r Chemistry, for the Geo-, Bio- and Cosmosphere and Technology; Studies in Inorganic Chemistry 5; Elsevier: Amsterdam, 1984. (2) Stucky, G. D. Clusters and Cluster-Assembled Materials. Mater. Res. SOC.Symp. Proc. 1991, 206, 507-520. (3) Steigerwald, M. L.; Brus, L. E. Annu. Rev. Mater. Sci. 1989, 19, 471-
495. (4) Averback, R. S.; Bernholc, J.; Nelson, D. L. Clusters and Cluster AssembledMaterials; Materials Research Society: Pittsburgh, PA, 199 1. (5) Jarrold, M. F. Metal and Semiconductor Cluster Ions. In Russell, D. H. Gas Phase Inorganic Chemistry; Russell, D. H., Ed.; Plenum Press: New York, 1989; Chapter 5, pp 137-192. (6) (a) Henglein,A.Chem. Rev. 1989,89,1861-1873. (b) Wang,Y.;Herron, N. J . Phys. Chem. 1991, 95, 525-532. (c) Spanhel, L.; Hasse, M.; Weller, H.; Henglein, A. J . Am. Chem. SOC.1987,109,5649-5655. (d) Variano, B. F.; Hwang, D. M.;Sandroff, C. J.; Wiltzius, P.; Jing,T. W.; Ong,N. P. J . Phys. Chem. 1987,91,6455-6458. ( e ) Wang, Y.; Mahler, S. W.; Kasowski, R. J . Chem. Phys. 1987,87, 7315-7322. (f) Nozik, A. J.; Williams, F.; Nenadovic, M. T.; Rajh, T.; Micic, 0. I. J . Phys. Chem. 1985, 89, 397-399. (7) (a) Wang, Y.; Mahler, W.; Herron, N. J . Opt. SOC.Am. 1989, B6,808. (b) Cheng, L-T.; Herron, N.; Wang, Y. J . Appl. Phys. 1989,66, 34173419. (8) (a) Dameron, C. T.; Reese, R. N.; Mehra, R. K.; Kortan, A. R.; Carroll, P. J.; Steigerwald, M. L.; Brus, L. E.; Winge, D. R. Nature 1989, 338, 596-597. (b) Dameron, C. T.; Smith, B. R.; Winge, D. R. J . Biol. Chem. 1989, 264, 17355-17360. (c) Dameron, C. T.; Winge, D. R. Inorg. Chem. 1990, 29, 1343-1348. (9) (a) Berg, J. M.; Holm, R. H. In Iron-Sulfur Proteins; Spiro, T. G., Ed.; Wiley-Interscience: New York, 1982; Vol. 4, Chapter I . (b) Coucouvanis, D. Acc. Chem. Res. 1991, 24, I . (c) Holm, R. H.; Ciurli, S.; Weigel, J. A. Prog. Inorg. Chem. 1990,38,1 and references cited therein.
0020-1669/93/1332-1931$04.00/0
and cluster chemistry, driven by the major advances in mass spectrometry and ionization techniques.lOJI The technique we use to obtain new metal sulfide clusters is laser ablation (LA), coupled with Fourier transform ion cyclotron resonance (FTICR) mass spectrometry for characterization. Laser ablation is now well established for the generation of gaseous clusters of metals and nonmetallic elements.12 Similarly, FTICR mass spectrometric techniques are invaluable for post-detection characterization of these ionized clusters, because the ions can be trapped, separated, and stored in the FTICR cell for some time, during which time it is possible to monitor their fragmentation on collision with inert gases and their reactions with any volatile reactant.' l a , 1 3 Gas-phase metal oxide clusters have been generated and studied by a variety of methods.llaJk17 Metal sulfide chemistry in the (IO) Russell, D. H. Gas Phase Inorganic Chemistry; Plenum Press: New York, 1989; p 412. (1 1) (a) Irion, M. P.; Selinger, A,; Wendel, R. In?. J . Mass Spectrom. Ion
Processes 1990, 96, 27-47. (b) Bernstein, E. R. Studies Phys. Theor. Chem. 1990,68,806. (c) Eller, K.; Schwarz, H. Chem. Rev. 1991, 91,
1121-1177. (12) Lubman, D. M. Lasers and Mass Spectrometry; Oxford University Press: London, 1990. (13) (a) Comisarow,M. B.Adu. MassSpectrom. 1980,8,1698. (b) Wanczek, K. P. Int. J . Mass Spectrom. Ion Processes 1984,60, 1 1. (c) Gross, M. L.; Renpel, D. L. Science 1984,226,261. (d) Freiser, B. S. In Techniques in Ion Molecule Reactions, Farrar, J. M., Saunders, W. H., Eds.; Wiley: New York, 1989; pp 61-118. ( e ) Marshall, A. G. Acc. Chem. Res. 1985,18,316. (f) Wilkins, C. L.; Chowdhury, A. K.; Nuwaugir, L. M.; Coates, M. L. Mass Spectrom. Rev. 1989, 8, 67. (g) Russell, D. H. Mass Spectrom. Rev. 1986, 5, 167. (h) Marshall, A. G.Adu. Mass Spectrom. 1989,11A,651-668. (i) Sharpe, P.; Richards0n.D. E. Coord. Chem. Rev. 1989,93,59. 6 ) Irion, M. P.; Selinger, A.; Wendel. R. Int. J . Mass Spectrom. Ion Processes 1990, 96, 27. (k) Marshall, A. G.; Verdun, F. R. In Fourier Transforms in N M R . Optical and Mass Spectrometry, a Users Handbook; Elsevier: Amsterdam, 1989. (14) (a) Gord, J. R.; Bemish, R. J.; Freiser, B. S. Int. J . Mass Spectrom. Ion Processes 1990, 102, 115-132. (b) Mele, A.; Consalvo, D.; Stranges, D.; Giardini-Guidoni, A.; Teghil, R. In?. J . Mass Spectrom. & Ion Processes 1990,95,359-373. (c) Fisher, E. R.; Elkind, J . L.; Clemmer, D. E.; Georgiadis, R.; Loh, S. K.; Aristov, N.; Sunderlin, L. S.; Armentrout, P. B. J . Chem. Phys. 1990, 93, 2676-91. (d) Maleknia, S . ; Brcdbelt, J.; Pope, K. J . Am.Soc. MassSpectrom. 1991,2,212. ( e ) Fisher, K. J.;Smith, D. R.; Willett,G. D. Proceedingsof the39th ASMS Conferenceon MassSpectrometry andAlIied Topics;ASMS: Nashville, TN, 1991; p 37.
0 1993 American Chemical Society
1932 Inorganic Chemistry, Vol. 32, No. 10, 1993
gas phase is less well understood. Subsequent to some early using classical mass spectrometric and vaporization techniques on metal chalcogenide compounds, Freiser et al. have examined the ions [MS,]+ ( M = Fe, Co, V, Ti; y = 1-8) formed by reaction of M+ with ethylene sulfide,21while Parent22 has observed sixteen [Al,S,]+ clustersup to [A17Slo]+by LA-FTICR of Martin23reports [As,S,]+ clusters and Schild et al.24 report [Pb,E,]+ (E = Se, Te) clusters, formed in both cases by thermal evaporation. Linton et have demonstrated the formation of small [Ni,S,]+ ions by laser ablation of NiS. Attempts to deposit clusters formed by laser evaporation of FeS2 in helium yielded only FeS.26 Our previous reports of gaseous metal chalcogenide cluster ions formed by laser ablation have covered nickel, copper, and silver. The nickel sulfides NiS and Ni3S2 yielded a series of 27 negativeions [Ni,S,]-, rangingin size from [NiSI-to [Ni,$3l0]-.~~ Laser ablation of various solid copper chalcogenide compounds, and of mixtures of Cu with Se or Te, yielded collections of anions [Cu,E,]- (E = S, Se, Te) ranging in size up to [ C U ~ ~ Sand ~~]In a demonstrating a dominant series in which x = 2y similar investigation with silver chalcogenides, the most intense large ions also follow the general progression [AgZy-lEy]-up to y = 11.29 In this paper we describe our LA-FTICR experiments on FeS, KFeS2, and COS. Under our conditions COSyields more clusters in theclass [M&- than any other metal chalcogenide investigated so far. The iron compounds yield two series of [Fe,S,]- clusters, distinctly separate in composition, which we believe represent different structural principles.
El Nakat et al. Nd-YAG laser (1064 nm, Spectra Physics DCR-11) focused to an area of 0.1 mm2. In the Q-switched mode the pulse width was 8 ns and the laser power was varied between 5 and 56 mJ using neutral density filters. Time delays ranging from 0.005 to 0.1 s were introduced between the laser pulse and the measurement of the FTICR spectra and caused some variation in the relative intensities of the ions due to the varying periods for gas-phase reactions and formation of clusters. Narrow-band and high-resolution spectra were used to identify ions up to m / z 1800. As cobalt is monoisotopic and sulfur and iron have one isotope in large abundance, the ions are reported with the mass of the major peak. In thecollisionalactivationexperiments theionof interest wasgenerated by the laser pulse and allowed to relax in the collision gas argon at 1 X lo-’ mbar, for periods of 0.1-1 s. Then all other ions were ejected from the cell, and the remaining ion was activated by an rf pulse of between 30 and 100 ps. After collisional activation for 0.01-0.1 s, the product ions were excited and observed.
Results
The positive ion spectra of FeS and KFeS2 and of COS were dominated by the naked metal ion M+ and contained only a few metal sulfide ions of small mass and low intensity. In contrast, prolificnegativeion spectra were readily obtained fromallsamples. Figure 1 shows part of the broadband negative ion spectrum of KFeS2, while Table I lists the ions and their relative intensities when obtained from KFeS2 and FeS. Ions up to [FeloS9]- have been identified by high resolution measurements, while ions with m / z >900 were very weak and could not be identified unambiguously. Attempts to investigate FeSe were thwarted by the ferromagnetism of the sample. Figure 2 shows a broadband LA FTICR negative ion spectrum of COS,starting at m / z 500, with Experimental Section [Co,S,]- ions extending up to m / z 3100. The base peak for COS The FeS used was the commercial solid, whilc the KFeS2 was prepared was [Co3S3]- (not shown in Figure 2), while [COS]- was not by the fusion method.30 Since the spectra are largely independent of the observed. Using the high-resolution mode it has been possible sample composition, low grade samples can be used. Our LA-FTICR to assign unambiguously the compositions of the ions ranging up technique does not at this stage use an external ion source, and therefore to m / z 1776 ( [ C O ~ ~ S ~ and ~ ] - these ), ions are listed with their ferromagnetic iron sulfide samples have not been investigated. COS was relative intensities in Table 11. At masses higher than m / z 1780, prepared by the reaction of aqueous cobalt nitrate solution with H2S and the decreasing intensities of the ions preclude the good narrow was washed and dried in vacuum. band spectra that would be required for unambiguous assignment. Mass spectra were obtained from pressed samples as previously r e p ~ r t e d , ~using ’ a Spectrospin CMS-47 FT-ICR mass spectrometer, However there is little doubt about the assigned compositions of equipped with a 4.7-T magnet. Ion generation by laser ablation used a most of them. They progress in a pattern similar to that shown by the ions identified unequivocally. It can be noted that in the (15) (a) King, F. L.; Parent, D. C.; Ross, M. M. J . Chem. Phys. 1991, 94, progression of [Co,S,]- up to m / z -3000 there are in general 2578. (b) Parent, D. C. Chem. Phys. Lett. 1991, 183, 51-54. two values of y for each value of x : for example the last four ions (16) Radi, P. P.; von Helden, G.; Hsu, M.-T.; Kemper, P. R.; Bowers, M. T. identified are [Co37S23]-, [Coj7S24]-, [co38s23]-, and [co38s24]-. Int. J . Mass Spectrom. Ion Processes 1991, 109, 49-73. (17) (a) Sone, Y.; Hoshino, K.; Naganuma, T.; Nakajima, A.; Kaya, K. J . The details of the ion distributions are dependent on the laser Phys. Chem. 1991,95,6830-6832. (b)Nakajima,A.;Kishi,T.;Sugioka, power, and on the delay between the laser pulse and the T.; Sone, Y.; Kaya, K. J . Phys. Chem. 1991, 95, 6833-6835. (18) (a) Goldfinger, P.; Jeunehomme, M. Trans.Farad.Soc. 1963,59,285 1measurement of the mass spectrum. For both metals, higher 2867. (b) De Maria, G.; Goldfinger, P.; Malaspina, L.; Piacente, V . power yields larger ions, and longer delays decrease the relative Trans. Farad. Sci. 1965,61, 2146-2152. abundances of the larger ions. Spectra for COS observed at (19) (a) Colin, R.; Drowart, J. J . Chim. Phys. 1962, 37, 1120-1121. (b) Coppens, P.; Smoes, S.; Drowart, J. Trans. Farad. SOC.1967,63,2140maximum laser power (56 mJ) contained more ions at high mass 2147. (c) Colin, R.; Drowart, J. Trans. Farad. SOC.1968, 64, 261 1( m / z >1400) than the spectra observed at lower laser powers. 2621. (d) Uy, 0. M.; Drowart, J. Trans. Farad. SOC.1968.65, 3221Delays of 0.005-0.01 s between the laser pulse and acquisition 3230. (20) Uy, 0. M.; Muenow, D. W.; Ficalora, P. J.; Margrave, J. L. Trans. gave spectra with more abundant high mass ions with m / z > 1400, Farad. SOC.1968,64, 2998-3005. while delays greater than 0.01 s gave spectra with low intensity (21) (a) Carlin, T.J.; Wise, M. B.; Freiser, B. S. Inorg. Chem. 1981, 20, for ions with m / z > 1400 and greater relative intensity of ions 2745. (b) MacMahon, T. J.; Jackson,T. C.; Freiser, B. S. J . A m . Chem. SOC.1989. 111. 421-427. with m / z
