Characterization of high nuclearity close-packed anionic platinum carbonyl clusters by 252Cf plasma desorption mass spectrometry

International Journal of Mass Spectrometry and Ion Processes 126 (1993) 197-210 0168-l 176/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

197

Characterization of high nuclearity close-packed anionic platinum carbonyl clusters by 252Cf plasma desorption mass spectrometry’ Janita M. Hufhesa, Yong Huanga, Ronald D. Macfarlaneal*, Catherine J. McNeala, Greg J. Lewis , Lawrence F. Dahlb aDepartment of Chemistry, Texas A & A4 University, College Station, TX 77843, USA bDepartment of Chemistry, University of Wisconsin-Madison, Madison, WI 53706, USA (Received 15 November 1992; accepted 18 March 1993) Abstract *‘*Cf-plasma desorption mass spectrometry has been used to investigate a series of high nuclearity, di- and tetraanionic platinum carbonyl clusters containing Pt 19, Pt24, Pt26 and Ptss closest-packed metal cores. Abundant singly-charged negative ions produced by loss of an electron, form an envelope of peaks corresponding to successive losses of carbonyl ligands from the intact metal core. A remarkable series of oligomeric negative ions formed by self-condensation of the metal core are also observed for the dianionic clusters that extend beyond m/z 50000. Slightly less abundant positive parent and oligomer ions are also observed for these clusters. The oligomer peaks extend beyond m/z 100000 in the spectrum of the Pt2s cluster. The astonishing formation of these positively charged aggregates that contain in excess of 500 Pt atoms is even more remarkable because there is no incorporation of the associated cation despite the strong presence of the cation in the positive ion spectrum. These investigations have established that 252Cf-PDMS can be used to unequivocally identify the platinum stoichiometry of these high nuclearity clusters, thus providing a new tool in the vast array of spectroscopic techniques used to structurally characterize solution-soluble metal clusters. The unusual ions formed by this class of compounds represent the largest positive and negative ions observed by “*Cf-PDMS. They may also provide a probe of the complex reactions that occur in the fission fragment track and in the ejected plume of neutrals and ions. These large Pt clusters give quite good intensities even at m/z > 50000 where the mechanism for detection must involve potential electron emission. The electronic structure of these metal clusters may be revealing a new aspect of potential emission where the Fermi level structure of the incident ion plays a role. Key words: Plasma desorption mass spectrometry;

Platinum carbonyl clusters; High mass; Fragmentation.

Introduction

The introduction of 252Cf-PDMS in 1976 by Macfarlane and Torgerson [l] was one of the first events in a large cascade that resulted in the rapid incorporation of mass spectrometry as a tool to characterize large, involatile molecules. The developments that have occurred in mass spectrometry *Corresponding

author.

’ Paper presented at the 6th Texas Symposium on Mass Spectrometry Gasp&, Que., Canada, 15-19 June, 1992.

over the last two decades, most notably in the introduction of new desorption-ionization techniques, have extended the mass range from less than 5000 to a point approaching 500 kDa. During this time, a parallel surge in biotechnology, molecular biology and related fields has occurred. It is perhaps a result of this environment that a primary focus in mass spectrometry has been the analysis of biopolymers and, in particular, proteins. The application of 252Cf-PDMS to the analysis of high nuclearity metal clusters (clusters containing

198

J.M.

more than 13 metal atoms) represents a new marriage of interests in both fields. 252Cf-PDMS provides a much needed tool to supplement the existing array of spectroscopic techniques which are currently used to characterize large metal clusters. Conversely, these systems provide a source of abundant high mass ions that can be used to investigate the detection of large molecules. Furthermore, the unusual gas-phase reactions of these clusters may provide insight into the complex chemical environment generated as a fission fragment interacts with a thin, solid film. The structure of a large metal cluster can generally be considered as a metal polyhedral skeleton surrounded by a stabilizing ligand sheath. The interest in these types of clusters stems in part from the challenge to develop new synthetic strategies and because of their unique physical-chemical properties and bonding modes. Because of the high ratio of metal atoms to ligands, large clusters closely resemble the smallest naked metal crystallites and therefore provide excellent models of uniform size to investigate the active metal site in catalysis [2] and the evolution from the properties of metal atoms to those properties associated with the bulk metal [3]. The complete characterization of polyhedral metal clusters relies on an extensive arsenal of techniques including X-ray crystallography, Fourier transform NMR and IR, Miissbauer spectroscopy, transmission electron microscopy and EXAFS. The characterization of complexes containing up to 13 metal atoms by these techniques, as well as mass spectrometry [4], is fairly routine. Difficulties in the structural characterization of larger organometallic clusters are often encountered because of their proclivity to convert to the metallic state and/or problems in purification and crystallization. The use of 252Cf-PDMS to examine several types of high nuclearity metal clusters represents a new application of the high-mass capabilities of mass spectrometry. In addition to a preliminary account [5] of the results obtained for the types of clusters described in this report, a gold super cluster originally formulated as AuS5(PPh3)r2C16(Ph =

CeH5) has been extensively studied [6-81. These investigations established that 252Cf-PDMS can be used effectively to identify the cluster formulation and, in some cases, provide information concerning the bonding arrangement and cluster architecture. The continuing development and refinement of synthetic routes to produce large organometallic clusters as well as the purification of these clusters is benefitted by the availability of mass spectrometry as a rapid screening technique and to provide confirmation of sample purity in advance of or in lieu of other spectroscopic investigations. This study describes the results we have obtained for various close-packed, di- and tetraanionic platinum carbonyl clusters with metal cores ranging in size from Ptis to Ptss [9]. For most of these clusters, abundant positive and negative parent ions and oligomer ions are observed. An unusual feature of the mass spectra is the absence of any ion pair containing the associated cation. The formation of singly-charged positive and negative ions of di- and tetraanions that exclude the counterion are addressed. In addition, the presence of oligomer ions which extend, in the case of a Pt26 cluster, beyond m/z 100000 are described.

Experimental The synthesis and characterization of the platinum carbonyl clusters utilized in this study have been reported [lO,l 11. A general description of the 252Cf-plasma desorption mass spectrometer that was constructed at Texas A & M, as well as the associated electronics and computer processing programs has been published [12]. The data that is reported herein was obtained on one of three 252CfPD instruments which differ slightly in the time-offlight path length, the distance of the 252Cf source from the target, the fission fragment flux through the sample and the acceleration voltages. These parameters are summarized in Table 1. The secondary ion detector consisted of two microchannel plates (Galileo ElectoOptics) in a chevron conlig-

J.M. Hughes et aLlInt. J. Mass Spectrom. Ion Processes 126 (1993)

Table 1 Instrumental

197-210

199

parameters

Parameter

System

Distance from sample Fluxa Flight path Accel. voltage Sample analyzed a Flux is defined as the number

1

2

3

4mm 1800 s-1 50 cm +12/-10

4mm 15oos-’ 32cm +10/-10

2mm 1500 s-1 73cm +15/-10

Pt,4>

Pt26

Ph9

Pt38

of fission fragments

per second passing

uration; an impedance matched conical anode was used to collect the electron cascade. A grounded, 90% transmission Ni grid was placed in front of the first microchannel plate. Secondary ions were accelerated into the front face of the detector which was maintained at -2.2kV. The exception to this configuration was the data obtained on system 2 in which the front face of the secondary ion detector was at ground potential and the collector was maintained at high voltage. Because of the air-sensitive nature of these clusters, samples were prepared in a controlled atmosphere glovebox (Vacuum Atmospheres), transferred from the glovebox to an inflatable glovebag (AtmosBag) that was attached to the inlet port of the mass spectrometer. The glovebag was purged with N2 or Ar prior to the sample introduction. Goldcoated (75 nm) aluminized Mylar (1.5 pm, AtlanTol) films were used as the sample backing. Gold was sputter-coated

(Hummer-X,

Technics,

Inc.) on

the aluminum suface to minimize inteference from Al2O3 peaks. The concentration of the solutions, with the exception of the Ptis cluster, could only be estimated to be between lop2 and 10e3 M because of the inability to accurately weigh small amounts of the available material inside the glovebox. All samples, with the exception of the Ptis cluster, were prepared by dissolving the sample in degassed acetone (Burdick & Jackson, “Distilled in Glass”) and evaporating a small aliquot (20-50 ~1) to dryness on the sample backing. The Ptis cluster was electrosprayed from a lop3 M acetonitrile solution.

through

the sample foil.

Results and discussion All four of the platinum carbonyl clusters described in this report are complex salts consisting of a large, singly charged cation and the polyanionic platinum carbonyl cluster. The architecture of the platinum cores closely resembles the cubicand hexagonal-closest packing (ccp and hcp) found in the bulk metal. All carbonyl groups, both terminal and bridging, are bonded to exterior Pt atoms. The structures and electrochemical behavior of these clusters has been recently described [lo]. Since the positive and negative ion peaks that are characteristic of the variable platinum core are formed without inclusion of the cation (vide infra), the term “parent ion”, although technically incorrect, is used to represent the platinum carbonyl anion. The positive and negative ion mass spectra of the hcp [(C6Hs)3PCH3]2[Pt26(C0)32] cluster shown in Fig. 1 are representative of the types of parent ion patterns that are generally observed for the dianionic clusters. Singly-charged negative ions are formed that correspond to successive carbonyl ligand losses from a species containing the intact Pt26 core, e.g., [Pt26(C0)x]-. This results in a relatively broad, bell-shaped distribution of peaks that extends almost from the fully decarbonylated ion (x = 0) to the fully carbonylated parent ion (x = 22). The maximum corresponds to the loss of approximately half of the ligands. Peaks are not resolved in the upper and lower limits of the distribution but there does not

J.M. Hughes et aLlInt. J. Mass Spectrom. Ion Processes 126 (1993) 197-210

200

l9oc0 lx””

I 1 “““‘,

““‘c NEGATIVE

‘I?% of Flight tns) 2KKlo

20500

”

”

21ooo

”

“‘I

IONS

cf ? 10

I’

(

I’

5500 Mass (m/z)

Time of Flight (ns) 2cKKlo 19500

20500

21ooo

Mass (m/z) Fig.

1.Positive and negative parent ion envelopes for [Pt26(CO),,]2-. spectrum

was acquired

The negative ion spectrum was acquired for 10 h. Data is plotted at 4 ns per channel (ch = channel).

appear to be any overlapping distributions containing fewer and/or greater numbers of Pt atoms as we observed for a small, [Pts(CO)ts12-, triangular platinum carbonyl cluster [13]. It is difficult to obtain accurate masses of the component peaks of this parent ion envelope because of the inability to resolve adjacent peaks. However, a comparison of the observed and calculated masses for

for 3 h; the positive ion

[Ptz6(C0)J ions ranging from x = 11 to 21 (Table 2) reveal that the observed masses agree, on the average, to within 0.05% of the calculated masses. This agreement is closer for the smaller Pts cluster where the peaks are better resolved [13]. The broadness of the negative parent ion distribution in Fig. 1 is believed to be almost entirely the result of unresolvable ligand losses rather than from any

J.M. Hughes et al./Int. J. Mass Spectrom. Ion Processes I26 (1993)

Table 2 Comparison of observed and calculated x

Observed

11 12 13 14 15 16 17 18 19 20 21

5383.1 5412.5 5441 .o 5467.4 5495.5 5522.5 5551.2 5580.0 5608.9 5636.8 5664.7

m/z

peak masses for [Pt26(C0)x]Calculated

m/z

5380.4 5408.5 5436.8 5464.5 5492.5 5520.5 5548.5 5576.5 5604.5 5632.5 5660.6

metastable contribution based upon the result we obtained for the [Pt9(CO)ts]2- cluster. The widths of the individual peaks in the parent ion envelope of this smaller cluster were virtually superimposable on the calculated width of the isotopic distribution. Normally, negative ions of polyanions would be formed by the addition of one less cation than the number of negative charges (assuming the cation is monovalent). For example, ions of this type are observed by PDMS for nucleic acids and polysulfated carbohydrates [14,15] and by fast atom bombardment (FAB) MS for several highly charged polyoxoanions [ 161. The negative parent ions of the Pt26 dianion appear to be formed by dissociation of the salt, followed by the loss of an electron, possibly by transfer to the substrate during the ion desorption. The facile dissociation of the cation-anion pair in the case of the platinum carbonyl clusters reflects the relatively diffuse and large charge separation of this ion pair compared with compounds such as the polyoxoanions. Several structurally related metal carbonyl dianions have been investigated by FAB-MS and, in these examples, singly-charged negative ions formed by the loss of one electron from the dianion were also observed [ 171. Abundant singly-charged positive ions of the Pt26 dianionic cluster are also observed (Fig. 1 (bottom)). There is little evidence of the discrete component structure in this distribution that was observed in the negative parent ion envelope of the

201

197-210

ions Per cent deviation,

(0-C)/C

x 100%

+0.050 +0.074 +0.077 +0.053 +0.055 +0.036 +0.049 +0.063 +0.079 +0.076 +0.072

cluster. We are not certain how positive ions of this cluster are formed. We would expect to observe positive ions by addition of three of the associated monovalent cations to the dianion. If three triphenylmethylphosphine cations (278 u) were added, the center of the distribution would be shifted by 834~ assuming that there was the same degree of carbonylation as the negative ions. There is no evidence for this type of ion despite the prominent presence of the cation in the lower mass range of the positive ion spectrum. The centroid is shifted to higher mass compared with the negative ion distribution but only by z 200 u. A similar shift to higher mass was observed for the positive parent ion distribution of the [Pt9(CO)ts]2- cluster. In the case of the Pts cluster, we attributed this shift to a higher degree of carbonylation as well as the presence of unresolvable, higher nuclearity species [13]. We proposed that the positive ion might be formed either by loss of three electrons, or by proton addition and/or hydrogen displacement, but clearly not by addition of the associated cation. Comparisons with the dianionic clusters described herein as well as the dianionic Pts cluster we have previously reported reveal that a constant mass difference between the positive and negative ion parent envelopes of close to 200u is always observed. This difference is independent of the cation identity or cluster nuclearity. This observation supports our earlier conclusion that neither the associated cation, or a fragment of the cation,

202

J.M. Hughes et al./Int. J. Mass Spectrom.

Time 3ocoo

of Flight 4occo

~-~_LL__-A~L

Ion Processes 126 (1993) 197-210

(ns)

I

5oooo

1

6oooo

-l---Y

Mass (m/z)

Fig. 2. Positive and negative ion monomers and oligomers of [Ptz6(CO),,]*- plotted at 32 ns per channel.

adds to the dianionic metal cluster to form the singly-charged positive ion. It is unlikely that the 200 u shift corresponds to a more highly carbonylated positive parent ion compared with the negative parent ion since the number of CO ligands varies for the complexes we have investigated. A constant shift in the masses of the positive and negative parent ion distributions is, however, indicative of the addition of some species that is common to all clusters (possibly even the addition of Ptf2 ions, although this is not supported by any observation of free Pt ions). Further evidence to support this observation comes from a comparison of the mass shifts between the positive and

negative oligomer ion distributions which is described below. The detection of an extraordinary series of positive and negative oligomer ions of the [Pt26 (CO)32]2- cluster was the most unexpected result we observed during our investigation of this class of metal clusters. Ions that extend beyond m/z 100000 [5] in the positive ion spectrum and m/z 60000 in the negative ion spectrum were observed. These species represent the largest singlycharged positive and negative ions that have been observed by PDMS. The oligomer series are shown in Fig. 2. All the more remarkable is the fact that these spectra were obtained with an accelerating

J.M. Hughes et aLlInt. J. Mass Spectrom.

Ion Processes

126 (1993)

voltage of f10 kV and no post-acceleration. Under these conditions, the ion detection efficiency for these low velocity ions is very low, possibly only a few percent. The positive ion spectrum shown in Fig. 2 was recorded for 10 h, while the more abundant negative ions were recorded for 3 h. The observation of oligomer ions by PDMS is not uncommon for organic molecules such as proteins. Dimers and trimers are generally observed when pure samples are prepared from concentrated solutions. Typically, these oligomer ion intensities are much lower than the intensity of the monomeric parent ion. The oligomer ions of the PtZ6 cluster are unusual because the intensitites of successively larger aggregates decrease very gradually, particularly in the positive ion spectrum. This motif of gradually diminishing aggregate intensities more closely resembles the pattern of peak intensities displayed by inorganic salts such as CsI which form a series of positive and negative ions corresponding to Cs(CsI),f and (CsI),I-. However, for the platinum carbonyl cluster, there is no evidence for phosphonium counterion incorporation in either the positive or negative ion oligomer peaks. Instead, the oligomers, at least in the negative ion spectrum, appear to correspond to multiples of the intact metal core containing a distribution of carbonyl ligands such as that displayed by the parent ion peaks, e.g. [PtZ6(CO)J;. The observed centroids and upper and lower mass limits of the negative ion oligomer distributions are listed in Table 3. The centroid masses are significantly lower than the values that would be calculated based upon a simple multiplication of the respective monomeric ion masses. There is, however, a relatively constant mass difference between neighboring distributions. Formulations corresponding to the best agreement with the centroid masses are also given. The ratio of Pt atoms to CO ligands as a function of oligomer number for these formulations is plotted in Fig. 3. This ratio increases very minimally throughout the oligomer series for the PtZ6 cluster. The lower mass limits of the positive and negative oligomer ion distributions correspond closely to the naked

197-210

203

metal cores, e.g. [Pt& except for the dimer and trimer envelopes. In these cases, the lower limits of the positive and negative ion distributions are slightly less than the masses of the naked PtSz dimer and Pt7s trimer cores. As the aggregate size increases, there is a gradual shift in the lower mass limit to a structure containing a small number of CO ligands. In the closest packed structures, all of the CO ligands are on the surface of the cluster. If two or more metal cores are joined, we would expect a small decrease in the total number of CO ligands. The maximum value of the upper mass limit of each distribution should therefore correspond to a value somewhat less than the mass of [Pt26(C032)]n. Only the dimer and trimer distributions deviate from this pattern. As in the parent ion distributions, the positive ion oligomer peaks are all shifted to higher masses relative to their negative ion conjugate. The centroids and mass limits of these distributions are listed in Table 4. The mass difference between the positive and negative oligomer ions steadily increases with increasing aggregate size. We observe an increase of approximately 100~ for each successively larger [PtZ6(CO),]~ envelope except for the IZ= 6-9 (inclusive) oligomers. However, in these cases, the anomalous values may reflect the difficulties in accurately calculating the centroids of the peaks in the negative ion spectrum which are not well defined. For example, the mass difference between the positive and negative parent ion monomers is about 200~. This increases to M 300 u for the dimeric species, M 400 u for the trimeric species, etc. This pattern suggests that the positive ion species may correspond to the addition of two cations (each having a mass of approximately 100 u) to the monoanionic cluster. This would be represented by C{[C][Pt,,(CO),]}~ where C is the unknown species. This argument assumes that (1) the number of CO ligands (x) represented by the maximum of the distribution remains constant between the positive and negative ions for the same value of n and (2) that the negative ion oligomers can be represented as ions corresponding to [Pt26(CO)x]i (i.e., there is no

J.M. Hughes et al./Int. 1. Mass Spectrom. Ion Processes 126 (1993) 197-210

204

Table 3 Observed centroids and mass limits (u) of negative ion oligomer peaks n

Lower limit=

Calc. ‘m/.2

Centroid

Upper limit

2 3 4 5 6 7 8 9 10 11

9765 14967 20 240 25 362 30 525 35 623 40 846 46 199 51511 56432

10 145 15217 20 289 25 362 30 434 35 506 40 579 45651 50 723 55 796

Calc. m/z [Pt26(C0)321n

LPt261n

10802 16 124 21409 26 680 31964 37419 42 695 48 206 54 072 59 257

12300 17364 23412 28 973 34 287 39 520 45 181 50445 56431 62631

11937 17906 23 875 29 843 35812 41781 47 749 53718 59 687 65 655

The mass values reported for the upper and lower limits correspond approximately to the minima in the baseline. The estimated error is &l-2%. b Values correspond to chemically averaged masses. a

inclusion of any cation). This type of positive ion cluster would be more consistent with the types of ions we would expect to observe. The Pt/CO formulations that best correspond to the observed centroid masses of the distributions based upon ions of the above type show very close to the same degree of carbonylation that is observed for the negative ion oligomers. Alternatively, if we assume that the oligomer peaks correspond to [Pt26(CO),]z ions, the centroids of the positive ion oligomers represent formulations that have a higher degree of carbonylation than the negative ion oligomers. The incorporation of the unknown cation C is attractive because it explains many anomalies ‘in the mass spectra. A significant weakness of this premise is that the presence of such a species is not supported by the observations of any significant peak at m/z 100 in the positive ion spectrum. Hopefully, a comparison with similar nickel carbonyl clusters will help to clarify this matter. The positive and negative ion spectra of the ccp [(C6H5)4As]z[Pt24(C0)30] cluster are shown in Fig. 4. Many of the same spectral characteristics are observed for this ccp cluster including the 200~ shift between the positive and negative parent ions despite the different associated cations.

Furthermore, the mass difference between the centroids of the Ptz6 and Ptz4 positive and negative parent ion peaks corresponds closely to Pt&O [18]. The mass differences between the positive and negative ion dimers and trimers of these two clusters conforms respectively to Pt,(CO), and Pt6(C0)4. This comparison provides conclusive verification that the oligomers do not form clusters with the associated cation. Furthermore, the same mass shift between the positive and negative ion clusters confirms that the process leading to the formation of the positive ions (including whether an extraneous cation is incorporated) is the same for the two clusters. There is somewhat better distinction of peaks contained within the Ptz4 positive parent ion envelope compared to the Pt26 distribution. These peaks are not separated by sequential CO ligand losses as in the negative ion spectrum. The identities of these components have not been confirmed. The negative ion spectrum of the largest cluster we investigated, a ccp, dianionic [Pt3s(CO),]2cluster, is shown in Fig. 5. The cation, [Ph3PCH2 is a bulky phosphonium GH4)Fe(CsHs)lf, ion, bridged to a ferrocene group. The positive parent ions of this cluster were not very intense. However, the negative ion spectrum displays the usual pattern of parent and oligomer ion peaks

J&f. Hughes et al./Int. J. Mass Spectrom. Ion Processes 126 (1993)

0 000, NEGAWE 5 000

Table 4 Observed peaks

1 0

5 500 i

IONS

q

opt19

A Pt24

3 000

/

1 POSITIVE

2 500

IONS

000 0

$ :: 3-

a

2 000 --

0

0

0 0

oOooOooOo

00°

1 500

0

1 ooo--

KEY

0 Pt26

0 500

00004 0

:

:

2

4

I

(

:

:

6 8 10 12 NUMBER OF OLIGOMCR

I 14 IONS

; 16

1 IR

: 20

’

Fig. 3. Ratios of the number of Pt atoms to CO ligands for the Pt19, Ptz4, Ptz6 and Pt3s negative ion oligomer peaks and for the positive ion oligomer peaks of P&.

extending beyond m/z 50000.The ratio of Pt atoms to carbonyl ligands increases much more steeply as a function of the oligomer number than for any of the other clusters reported in this study (see Fig. 3). The reason for this unusual dependence may reflect the smaller surface area of this cluster. The metal atom packing is similar to the Pt, cluster. However, the faces of the Ptz4 metal polyhedron form significantly larger “minisurfaces”. Furthermore the Ptz4 cluster does not contain any encapsulated (interior) metal atoms, whereas the Ptss cluster contains six completely encapsulated Pt atoms [5]. The parent and oligomer peaks in the negative ion spectrum are shifted to higher mass relative to the Ptz4 and Ptz6 clusters by a difference corresponding only to the increased number of metal atoms and ligands. Positive ion peaks corresponding to the cation and fragments of the cation are also abundant. The lower mass range

205

197-210

centroids

and mass limtis (u) of positive ion oligomer

n

Lower limit

Centroid

Upper limit

2 3 4 5 6 I 8 9 10 11 12 13 14 15 16 17 18 19 20

10019 15 199 20 508 25 660 31063 36 394 41704 47 174 52413 57 854 63410 68 487 74 261 78 538 84718 89 552

11097 16478 21970 27 350 32731 38 139 43 548 48 900 54 197 59 602 65 154 70758 75719 80952 86451 91132 97 240 103 377 107 043

12805 18413 23 510 29 678 35 571 40933 46531 51741 57 072 62 605 68 006 73 743 78 538 83 463 89 552 94 521

of the positive ion spectrum is identical to that reported for the phosphonium salt of the Ptg cluster [ 131. All of the platinum carbonyl clusters we have described are dianionic complexes. The positive and negative ion PD mass spectra of a tetraanionic Pt,s cluster, [(C6H5)4As]4[Pt19(C0)22], in the region of the parent ion peaks are shown in Fig. 6. Despite the significantly increased charge, similar types of positive and negative ions are observed that permit the facile identification of the cluster nuclearity. Some distinctions are however notable. Most significantly, the only oligomeric peaks that were observed were dimers which were much lower in intensity than the dimeric species observed for the dianionic clusters. In additon, the distribution of peaks in the negative parent ion envelope is not uniformly decreasing on the high mass side of the distribution. Also, the center of the envelope represents a structure that retains a greater number of CO ligands than the dianionic clusters (approximately 65% compared with nearly 50%). The component peaks in this distribution are separated by values close to 28~ and the lower mass limit

206

J.M. Hughes et al./Int. J. Mass Spectrom. Ion Processes 126 (1993) 197-210

Fig. 4. Positive and negative ion monomers and dimers of the Ptz4 cluster. Spectra were accumulated for 1 h and data are plotted at 16 ns per channel.

conforms

well to the mass of the fully decarbony-

lated cluster leading us to conclude that the negative ions still correspond to the types of ions observed in the spectra of the dianionic clusters e.g., [Ptts(CO),]-, where, in this case, x ranges from nearly 10 to 18 or 21. This distribution has a high mass tail that extends significantly above the mass of the fully carbonylated structure but slightly below the mass of a cluster containing three Ph4As+ cations. The masses of the negative ion dimers of the dianionic clusters were substantially lower than a value corresponding to twice the monomer mass. However, in the case of the Ptts cluster, the dimer mass is almost exactly double

that of the monomer. This mass is also approximately 250 u higher than the mass of the Pt3s parent ion. This mass shift suggests that there has been addition of a cation of some type. The Ptts positive parent ion distribution is composed of a cluster of at least five prominent, resolvable peaks. The mass difference between the peaks is not constant, varying from 68 to 76 u. These values are possibly suggestive of cation addition followed by As and/or Ph losses although no reasonable formulations have been identified. Since both the charge and the solvent and the method of sample preparation have changed, it is not possible to determine if the effects we observe are due entirely to the increased charge.

J.M. Hughes et aLlInt. J. Mass Spectrom. Ion Processes 126 (1993)

207

197-210

Mass (m/z) Fig.

5. Negative

ion monomer

and

oligomer

ion peaks of [Pt~s(CO)&. 32 ns per channel.

The mechanism of the formation of the oligomer ions of these clusters is a key aspect of this study. In previous studies, we have observed oligomer ions of salts such as Cs+(CsI), where both the positive and negative ions of the molecule comprise the oligomer cluster ion. This is not the case for these Pt carbonyl oligomer ions where the counter ion is absent. Also, the original anion produces both positively and negatively charged Pt carbonyl oligomer ions. We propose that these oligomer ions are actually synthesized in the heavy ion track formed by the 252Cf-fission fragments when they pass through the sample. The track has two components: an infra track extending to a 1 nm diameter where a high energy and particle density plasma-like condition prevails for a few femtoseconds and the ultra tracking extending out to a diameter of 20 nm formed by the &electrons that are produced in the primary interaction [19]. We propose that in the infra track the Pt carbonyl anions are dissociated to

Data

was accumulated

for

12h

and

is plotted

at

the bare Pt cluster. Because of the high density and reactivity of these clusters they begin to self associate forming a series of oligomer Pt clusters with up to 12 clusters attached (as in the case of Pt2J. As these clusters cool further, they begin to emerge from the infra track attaching the strongly binding CO ligands in the process. The spectra observed, for both positive and negative ions, represents what the clusters were able to form during the time that the infra track was highly excited. Another important aspect of this study is that these very high mass ions are detected with quite high measured intensity and offer an excellent opportunity to study the mechanism of their detection using microchannel plates (MCPs). We showed previously that the response of MCP electron multipliers is velocity dependent with a sharp rise in efficiency near the kinetic ejection threshold (2 x lo4 ms-‘) [20]. Below that velocity, electron

208

J.M. Hughes et aLlInt. J. Mass Spectrom. Ion Processes I26 (1993) 197-210

Time of Flinht (ns) 3aHt -

Fig. 6. Positive and negative parent ion peaks of [Pt&CO)z&.

The spectra were acquired 8 ns per channel.

with decreasing velocity. In this velocity regime, electron ejection due to potential emission The ionization potential of incident ion plays role in process and the higher is,

for 1 and 4.5 h respectively.

Data is plotted at

greater the probability for upon impact. The that high mass ions be detected such a impact velocity is that the So, what the ionization potential of a

J.M. Hughes et aLlInt. J. Mass Spectrom. Ion Processes 126 (1993)

P& cluster ion; or, how does potential emission operate when an incident metal ion cluster is suffciently complex to have its own conduction band and Fermi level? Conclusions

We have demonstrated that a variety of high nuclearity platinum clusters, both di- and tetraanionic, can be characterized by 252Cf-PDMS. In all cases, singly-charged positive and negative ions, characteristic of the intact metal cores, were observed. These ions comprised a distribution of species that differ by the number of retained carbonyl ligands attached to the metal core. In the negztive ion spectrum, these contributions were partially resolvable for the Pt24 and Pt26 clusters. The mechanism of the positive ion formation is not fully understood. By some process, positive ions of the di- and tetraanionic clusters are formed without addition of the cationic counterion. In all cases however, prominent ions of the cations are observed in the lower mass range of the positive ion spectra. With the exception of the Ptts cluster, these complexes demonstrated a remarkable tendency to form self-aggregates during the desorption-ionization process. The presence of these aggregates that extend to exceptionally high masses is probably a consequence of the high surface concentrations produced when films are prepared by solvent evaporation. This aspect has been discussed more completely in a report of a smaller, Pt, dianionic cluster [13]. The presence of these aggregates actually provide multiple confirmations of the cluster nuclearity. The pattern of oligomers is also different for these close-packed clusters compared with the linear, triangular framework of the Pts cluster and therefore provides a quick confirmation of this rudimentary feature of the cluster architecture. In cases where the oligomer peaks might obscure the possible presence of higher nuclearity impurities, the use of more dilute solutions would probably eliminate this possible problem. The Pt/CO ratio of the aggregates also appears to reflect the surface area

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209

of the cluster. This indicates that the oligomer ion spectrum may also provide a rapid and unique method for assessing the relative surface areas of these clusters. The ability to use mass spectrometry to provide rapid, conclusive information concerning the formulation of high nuclearity metal clusters will greatly facilitate the optimization of synthetic and chromatographic procedures. We have demonstrated, for example, that the change in cluster nuclearity during oxidation-reduction reactions can be monitored by 252Cf-PDMS. It is not clear if changes in the cluster charge can be identified from our results. Clearly, further experiments concerning the effects of sample preparation including the use of isolating matrices, concentration and the choice of solvents must be established. How the parent and aggregate ions of these clusters are formed and why such abundant high-mass oligomers are produced remains an intriguing mystery whose solution may provide some revealing insights into the processes occurring in the fission fragment track and in the ejected plume of neutrals and ions. The detection of ions exceeding m/z 60000 to 100000 having an energy of only 10KeV gives hope that if the right conditions can be found, 252Cf-PDMS may be competitive with matrix-assisted laser desorption or electrospray/ ion spray MS for the analysis of very high mass species. In conclusion, we hope that the marriage of high nuclearity solution-phase clusters and mass spectrometry will provide fruitful results for both fields of investigation.

Acknowledgments

This work was supported by National Science Foundation grants awarded to C.J. McNeal (CHE 9101855) and L.F. Dahl (CHE 8616697 and CHE 9013059) and grants from the National Institutes of Health (GM 26096) and Robert A. Welch Foundation (A-258) both of which were awarded to R.D. Macfarlane. J.M. Hughes was supported by a National Merit Minority Grant.

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