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Química Nova

Print version ISSN 0100-4042On-line version ISSN 1678-7064

Quím. Nova vol.28 no.3 São Paulo May/June 2005

http://dx.doi.org/10.1590/S0100-40422005000300007 

ARTIGO

 

The role of fast atom bombardment mass spectroscopy (FABMS) in cluster characterization

 

 

Adriana F. Sotelo; Anna Maria P. Felicissimo*

Instituto de Química, Universidade de São Paulo, CP 26077, 05513-970 São Paulo - SP

 

 


ABSTRACT

Fast atom bombardment mass spectroscopy has been used to study a large number of cationic phosphine-containing transition-metal-gold clusters, which ranged in mass from 1000 to 4000. Many of these clusters have been previously characterized and were examined in order to test the usefulness of the FABMS technique. Results showed that FABMS is excellent in giving the correct molecular formula and when combined with NMR, IR, and microanalysis gave a reliable characterization for cationic clusters1. Recently FABMS has become one of the techniques employed as routine in cluster characterization2,3 and also is an effective tool for the structure analysis of large biomolecules4. Some results in the present work reinforce the importance of these data in the characterization of clusters in the absence of crystals with quality for X-ray analysis.

Keyword: cluster; gold; palladium.


 

 

INTRODUCTION

Although single crystal X-ray crystallography remains the only definitive technique for structural characterization of heteronuclear gold cluster compounds other techniques, and in particular FABMS, have made available valuable information especially concerning the nature of these species in solution3.

The advent of FABMS in which ion generation is achieved by bombardment of the sample by a beam of fast rare gas atoms5 resolved the limitation problem that the conventional electron impact mass spectrometry used to have in the characterization of high molecular weight cluster compounds. The FABMS spectra of these species are presented as a number of peaks with well resolved fine structures arising from the various possible isotopic combinations for a given molecular formula.

FABMS is also extremely useful in determining the molecular formula of the parent cluster6. Assignments of the clusters fragments provide additional information about cluster composition. As an example the FAB spectrum for the supra cluster [Pt2(AuPPh3)10Ag13Cl 7] shows in the 4500 to 7000 range some masses assigned to molecular fragments7. In general the most abundant peak in this range is due to the parent cluster molecular ion. In many cases the use of this technique provides the best information about the nature of a new or novel cluster when the X-Ray quality crystals are impossible to obtain. Then, associated to other spectroscopic techniques as NMR, IR and with microanalysis gives a reliable characterization for compounds with high molecular weight. Many examples are given in order to emphasize the power of this technique for cluster characterization.

FABMS and cationic clusters

The synthesis of new cationic mixed gold clusters with transition metals and triphenylphosphine (PPh3) as ligand is a field of great interest due to their potential utilization in catalysis8,9.

Research involving ligand stabilized metal clusters has increased considerably in the last two decades and great progress has been achieved in this field. These compounds have gained attention among chemists and physicists due to their fascinating geometries, unique fluxional behavior, catalytic potential and interesting chemical reactivity6,10,11.

Originally the studies of metal clusters dealt primarily with their synthesis and structure determination. More recently efforts have turned towards a systematic study of chemical reactivity and catalytic properties6.

The report on the synthesis and characterization including a single crystal X-ray crystallographic analysis of the first cluster compound containing Au-Pd bond was published in 198910. Since then only some new and novel clusters of this type have been synthesized and well characterized12.

The cationic cluster [(PPh3)Pd(AuPPh3)6 ](NO3)2 (1), a promising complex for reactivity studies was first isolated by Ito et al.10 as a minor product of the reaction between [Pd(PPh3)4] and AuPPh3NO3 in CH3OH/CH2Cl2 using inert atmosphere. The minor product of this reaction was identified as (1) and separated from the major product [Pd(AuPPh3)8]2+ by HPLC . The cluster was later prepared with better yield using palladium acetate as precursor11. Another faster synthesis of this cluster has been carried out with Pd(PPh3)Cl2 and AuPPh3NO313. The addition reaction of NH4PF6 salt to the cluster [(PPh3)Pd(AuPPh3)6 ](NO3)2, provided a new cluster with PF6- as counterion, and the crystal and molecular structure of this compound has been reported12. The characterization was also made by 31P {1H} NMR, microanalysis, IR and FABMS. The FABMS data are the subject of the present article.

A new phosphine stabilized Au-Pd cluster compound [(dmpi)2Pd(AuPPh6)3 ](NO3)2 was prepared by direct reaction of dmpi (2,6 dimethyl phenyl isonitrile) with the cluster (1) under nitrogen atmosphere. The structure for this cluster was obtained14 in agreement with FAB data (two isonitrile groups substituted the PPh3 group present in cluster (1)).

The addition reaction of Hgº or Hg22+ to the cluster (1) was also reported15. 31P{1H} NMR, IR, and FAB-MS data were very useful in confirming the structure of this trimetallic cluster.

Another trimetallic cluster containing Au-Pd and Sn, [Pd(AuPPh3)6(SnCl3 )2](NO3)2 was prepared from the reaction of (1) and SnCl2.2H2O under nitrogen atmosphere. This new compound has been characterized by 31P{1H} NMR, IR, FAB-MS and TGA/DTG16.

This paper reports the results obtained with the use of FABMS, for some Pd-Au clusters emphasizing the utility of this technique in giving the correct molecular formula when combined with NMR, IR and elemental analysis.

 

EXPERIMENTAL PART

Physical measurements

FABMS experiments were carried out with use of a VG Analytical, Ltd., 7070-HF high resolution double focusing mass spectrometer equipped with VG11-250 data system.

Reagents

All manipulations concerning the syntheses were carried out under a purified N2 atmosphere with use of standard Schlenk techniques unless otherwise noted. Solvents were dried and distilled prior to use. NaBH4 was purchased from Aldrich Chemical S. A.

Preparation of the compounds

  1. [(PPh3)Pd(AuPPh3)6 ](PF6)2 was prepared as described in the literature. The analytical data reported below is for the PF6-derivative. 31P NMR (H3PO4, 20 ºC): d 50.0 (doublet with 3JP-P = 31 Hz), d 62.1 (multiplet with 1JP-F = 712 Hz). Anal. Cald. for Au6PdP9C126H105 F12: C, 44.3; H, 3.08. Found: C, 44.04; H, 3.23. IR (KBr): n (cm-1) = 840 (PF6)12.
  2. [(dmpi)2Pd(AuPPh3)6 ](NO3)2 was prepared as described in literature. 31P{1H} NMR (CD2Cl2, 25 ºC): AuP d 45.0 (singlet), IR: n (CNR) 2093cm-1. Conductance (CH3CN solution): 321 S cm2 mol-1(1:2 electrolyte)14.
  3. [Pd(AuPPh3)6(HgNO3 )](NO3) was obtained from nucleophilic addition and substitution reaction between either Hgº or Hg22+ and cluster (1) as reported in the literature. 31P{1H} NMR (CDCl3, 25 ºC): AuP d 43.0 (singlet) with 199Hg satellites (3JHg-P = 690 Hz) due to the AuPPh3 ligands bound to Pd atom. The quantitative determination of metals Pd and Au: calc. 1 Pd : 6 Au (0.92 Pd : 6.32 Au). Anal. Calc. PdAu6P6C108H90 HgN2O6, Pd = 3.33%, Au = 37.12% (found: Pd = 3.08%, Au = 39.11%)15.
  4. [Pd(AuPPh3)6(SnCl3 )2](NO3)2 was prepared from the substitution reaction (PPh3 to SnCl3-) between (1) and SnCl2.2H2O . 31P{1H} NMR (CD2Cl2, 25 ºC): AuP d 49.0 (singlet). IR (KBr): n (cm-1) = 1186 (NO3).Raman: n (cm-1) = 291.6(SnCl) 16.

 

RESULTS AND DISCUSSION

The reaction of [(PPh3)Pd(AuPPh3)6 ](NO3)2 (1) with excess of NH4PF6 gave the cluster compound [(PPh3)Pd(AuPPh3)6 ](PF6)2 in 40% yield. The compound has been characterized by NMR, IR, microanalysis, and X-Ray crystal and molecular structure12.

The molecular composition of PF6- derivative was also confirmed by FABMS. These results turned possible the determination of the cluster ion formula despite not related in that publication (previously not available). Positive-ion FABMS analysis (m-nitrobenzyl alcohol matrix) of this cluster gave a spectrum with well-resolved peaks. An analysis of the isotopic distribution pattern for the most abundant mass ion in 3006.4 corresponded to the ion {[Pd(AuPPh3)6](PF6 )}+. Other results showed fragmentation at: m/z 3413.8 (fragment [M + 2X]+, where M = [Pd(AuPPh3)6(PPh3)] and X = PF6); 3269.0 (fragment [M + X]+); 2861.5 (fragment [M – PPh3]+); 2664.7 (fragment [M – (AuPPh3)]+); 2547.3 (fragment [M –AuPPh3 – PPh3 + X]+).

Although the FABMS of these mixed metal-gold clusters gave accurate information on the molecular formula, it is also very important to analyze other spectroscopic and analytical data. NMR gives information on the relative number of non-equivalent phosphine ligands. IR assignment confirms the presence of PF6- as counterion. A complete analysis of the fragmentation pattern suggested that the neutral compound was [(PPh3)Pd(AuPPh3)6 ](PF6)2 (Figure 1). These data can be compared with the analogous cluster with NO3- as counterion11, and these results shows, once more, the reliability of this technique.

The FABMS analysis of the cluster [(dmpi)2Pd(AuPPh3)6 ](NO3)2 gave direct evidence for the new cluster composition. The spectrum presented well-resolved peaks. The (M + NO3) ion pair was the highest mass peak and the loss of dmpi and PPh3 was noted. The results of FABMS (m-nitrobenzyl alcohol matrix) where assigned as follow: m/z 3185.3 (fragment [M + NO3]+, where M = [Pd(AuPPh3)6(dmpi)2 ]); 3056.2 (fragment [M + NO3 - dmpi]+); 2992.9 (fragment [M - dmpi]+); 2924.6 (fragment [M + NO3 – 2 dmpi]+); 2924.6 (fragment [M + NO3 – PPh3]+); 2861.5 (fragment [M – 2 dmpi]+); 2861.5 (fragment [M - PPh3]+); 2793.3 (fragment [M + NO3 – dmpi – PPh3]+); 2599.9 (fragment [M – 2 dmpi – PPh3]+); 2599.9 (fragment [M – 2 PPh3]+). Just every expected peak was present and they were all major peaks. The proposed molecular formula for this cluster was confirmed later by single crystal X-ray structure14.

High purity crystals of the cluster [Pd(AuPPh3)6(HgNO3 )](NO3) were obtained but, unfortunately, inappropriate for X-ray analysis. FABMS, in this case, was an extremely useful technique in the determination of the molecular formula. The highest mass peak was assigned as MX = [Pd(AuPPh3)6(HgNO3 )](NO3) . Other results FABMS (m-nitrobenzyl alcohol matrix) are presented: m/z 3182.8 (fragment [M + X]+, where M = [Pd(AuPPh3)6(HgNO3 )] and X = NO3); 3120.7 (fragment [M]+); 3057.7 (fragment [M – 2 X]+); 2980.9 (fragment [M - Hg]+).

The trimetallic cluster [Pd(AuPPh3)6(SnCl3 )2](NO3)2 was obtained in high purity state but the crystals were inappropriate for X-ray analysis. In this case, FABMS and TGA/DTG were techniques extremely useful in the determination of the molecular formula. Positive-ion FABMS analysis of this cluster gave a spectrum with well-resolved peaks. The highest mass peak (FABMS) was assigned as [M – Cl] = [Pd(AuPPh3)6(SnCl3 )2 – Cl] in m/z 3276.2. All the assignments are presented and results show all the usual fragmentations (Table 1). Other technique used in order to achieve the molecular formula was TGA/DTG.

 

 

The thermal decomposition process observed in TGA/DTG curves started at 24 ºC, was pronounced at 196 ºC and finalized at 895 ºC. The mass loss was of 56.3% against the theoretical result 55,5% (calculated from the mass of remaining metals). The proposed stoichiometry based on these data is 1Pd: 6 Au: 2 Sn, that reinforces the argument of the molecular formula [Pd(AuPPh3)6(SnCl3 )2](NO3)2 for this cluster.

NMR spectrum of the cluster [Pd(AuPPh3)6(SnCl3 )2](NO3)2 showed one peak at 49 ppm (singlet), interpreted as due to the presence of equivalent phosphorus atoms linked to the gold atoms (P-Au). The absence of other couplings in the spectrum confirms that there are no phosphorus atoms linked to palladium, as it occured with the precursor cluster [Pd(AuPPh3)6(PPh3)](NO3) 2. The presence of Sn and of NO3- as counterion in this compound was respectively confirmed by Raman and IR spectra.

 

ACKNOWLEDGEMENTS

CNPq and FAPESP are thanked for financial support. We are grateful to Professor L. H. Pignolet for the achievement of the FABMS results at the University of Minnesota.

 

REFERENCES

1. Boyle, P. D.; Johnson, B. J.; Alexander, B. D.; Casanuovo, J. A.; Gannon, P. R.; Johnson, S. M.; Larka, E. A.; Mueting, A. M.; Pignolet, L. H.; Inorg. Chem., 1987, 26, 1346.        [ Links ]

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7. Kappen, T. G .M. M.; Schlebos, P P. J.; Bour, J. J.; Bosman, W. P.; Smits, J. M. M.; Beurskens, P. T.; Steggerda, J. J.; Inorg. Chem. 1994, 33, 754.        [ Links ]

8. Ito, L. N.; Sweet, J. D.; Mueting, A. M.; Pignolet, L. H.; Shoondergang, M. F. J.; Steggerda, J. J.; Inorg. Chem. 1989, 28, 3696;         [ Links ]Krogstad, D. A.; Felicissimo, A. M. P.; Schoondergang, M. F. J.; Pignolet, L. H.; Abstr. Pap. Am. Chem Soc. 1993, 206, 425-INOR.        [ Links ]

9. Felicissimo, A. M. P.; Gusevskaya, E. V.; Quim. Nova 1994, 17, 381, and references cited therein.        [ Links ]

10. Ito, L. N.; Johnson, B. J.; Mueting, A. M.; Pignolet, L. H.; Inorg. Chem. 1989, 28, 2026.        [ Links ]

11. Ito, L. N.; Felicíssimo, A. M. P.; Pignolet, L. H.; Inorg Chem. 1991, 30, 988.        [ Links ]

12. Sotelo, A. F.; Felicíssimo, A. M. P.; Gómez-Sal, P.; Inorg. Chim. Acta 2003, 348, 63.        [ Links ]

13. Sotelo, A. F.; Quintilio, W.; Felicissimo, A. M. P.; Spectrosc. Lett. 1994, 27, 605.        [ Links ]

14. Takata, N. H.; Young Jr, V. G.; Felicissimo, A. M. P.; Inorg. Chim. Acta 2001, 325, 79.        [ Links ]

15. Sotelo, A. F.; Felicíssimo, A. M. P.; J. Chromatogr., A 1999, 862, 29.        [ Links ]

16. Sotelo, A. F.; PhD Thesis, University of S. Paulo, Brazil, 2004.        [ Links ]

 

 

Recebido em 13/2/04; aceito em 15/10/04; publicado na web em 2/2/05

 

 

* e-mail: ampfelic@iq.usp.br

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