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Charge stripping as a probe to determine the effects of ligation in metal ion complexes produced in a fast atom bombardment source
International Journal of Mass Spectrometry and Ion Processes, 107 (1991) 545-552
545
Elsevier Science Publishers B.V., Amsterdam
Short Communication CHARGE STRIPPING AS A PROBE TO DETERMINE THE EFFECTS OF LIGATION IN METAL ION COMPLEXES PRODUCED IN A FAST ATOM BOMBARDMENT SOURCE
SESI M. MCCULLOUGH ‘, A. DANIEL JONES b and CARLITO
B. LEBRILLA ‘.*
aDepartment of Chemistry, University of California. Davis, CA 9.5616 (USA) bFacility for Advanced Instrumentation, University of California, Davis, CA 95616 (USA)
(First received 14 February
1991; in final form 8 April 1991)
ABSTRACT Fast atom bombardment ionisation is combined with charge stripping (CS) to produce bare and ligated metal dications. Abundant quantities of ions resulting from the bombardment of transition and lanthanide metal salts are produced and charge stripped. Qminvalues obtained from the CS spectra agree within + 0.5 eV of the respective second ionisation energies for the bare metal ions. Effects of ligands such as Cl and OH are observed as an increase in Qminvalues relative to the bare metal ions, e.g. AQ,,,, = 0.9eV for CoCl+ and AQ,,,,, = 3.2eV for HoCl+.
Continuing interest in the gas-phase chemistry of metals necessitates the development of alternative methods for the formation and characterisation of metal ions. We examine the use of matrix-free fast atom bombardment (FAB)** [l-3] as a general method for the formation of various groups of metal and ligated metal ions. This method is used in combination with charge stripping (CS) [4-61 to probe the effects of ion formation and ligation on the electronic state of the formed ionic species. In addition, CS provides access to the second ionisation energies of the ligated metal ions which we observe to vary significantly from the second ionisation energies of the corresponding bare metal ion. Metal ion formation for a variety of transition metals by FAB has recently been used in the gas-phase ion/molecule chemistry of several transition metal ions with simple organic compounds [7-l 11.Although not as extensively used as other ionisation methods such as laser ablation [12] and electron impact,
* Author to whom correspondence should be addressed. **The term FAB will refer to both secondary ion mass spectrometry (SIMS) and fast atom bombardment and is used in this report to mean the bombardment of a neat substrate with neutral atoms.
016%1176/91/$03.50
0 1991 Elsevier Science Publishers B.V.
546
FAB offers a simple and convenient alternative and overcomes many of the problems associated with the other two techniques. Electron impact ionisation, for example, relies heavily on thermally stable and volatile organometallit precursors. Laser ablation needs specific instrumentation and is not easily applicable to all types of mass spectrometer. Conversely, FAB allows the formation of metal ions from readily available metal salts. A FAB source is available in many sector instruments and recently in trapped ion mass spectrometers such as Fourier transform mass spectrometers. By ejecting a second electron to produce the dication, the CS process has been extensively used to probe the electronic energy levels of several simple inorganic and organic ions [13-151. Many theoretically interesting compounds, for example C,Hi+, the smallest Hiickel aromatic dication [6], and some organic ions of high complexity such as the dication of [2.2.1.1] pagodane with the molecular formula C,,Hi: [6,16], have been formed by CS. Recent studies on the formation and reactions of gas-phase dipositive transition metal ions such as Nb2+ [17,18], Ti2+ [ 191 and the mixed metal dimer LaFe’+ [20] have also generated new interest in doubly charged metal ions. The combination of FAB ionisation-CS allows a readily accessible source of doubly charged bare and ligated metal ions. We have used FAB to produce a variety of bare transition metal ions including Fe+, Co+, Ni+, and Pd+, lanthanide metal ions, Eu+ and Ho+, and several ligated metal ions including MgCl+, CoCl+, HoCl+ and HoOH+. Sufficient quantities of ions are obtained allowing charge stripping to be performed. The charge stripping of metal ions as well as several ligated alkaline earth metal ions has previously been reported [21]. There have been, to our knowledge, no reports on the charge stripping of FAB produced ions of ligated transition metals. The charge stripping of lanthanides and, particularly, ligated lanthanide metal ions, has also not been performed despite their general importance in the area of materials science research [22,23]. The experiments were performed in a VG-Analytical ZAB-2F mass spectrometer with BE geometry. Samples are obtained by dissolving the appropriate metal salt in deionised water or other suitable solvent, and placing a 2 ~1 aliquot of the resulting solution on a stainless steel probe tip. MgCl, was added to each sample for calibration and to ensure identical experimental conditions for both the reference and the sample. All metal salts are commercially available (Aldrich, Fisher, Strem) and used without further purification. The solvent is evaporated prior to bombardment of the sample by a 9 keV Xe atom beam. Bombarding the sample produces several ionic species including bare metal ions and often more than one ligated species. The ionic species are extracted from the source region by an 8 keV accelerating potential, and the desired species are selected by the magnetic sector. To form the dications, M2+, nitrogen gas at an indicated pressure of approxi-
547
mately 5 x low6 Torr was introduced into the collision chamber located in the second field-free region of the spectrometer. At this pressure an approximately 50% attenuation of the main beam is observed. The charge stripping spectra are obtained by scanning the electric sector voltage about E/2 where E is the energy necessary to transmit the singly charged ion. In general, 3-10 scans are accumulated for the parent peak at E, and 5-25 scans are summed to obtain the charge stripping peak about E/2. Data were acquired and averaged using the VG 1l/250 data system in multichannel analysis (MCA) mode. The value of the minimum translational energy loss in the charge stripping process, Qmin, is obtained by extrapolating to the baseline of the high energy side of the charge stripping peak. A correction factor, 6, based on an assigned value of Qmin(Mg+) = 15.1 eV, is applied to calculate Qminvalues using the method described previously by both Lammertsma et al. [6] and Rabrenovic et al. [14]. An additive calibration method was used to determine the Qminvalues which varied typically to within & 0.5 eV. RESULTS
AND DISCUSSION
Average charge stripping values (Qmi”) for metal ions belonging to several groups including alkaline earth (Mg+), transition metal (Fe+, Ni+, Co+, and Pd+) and rare earth (Ho+ and Eu+) are shown in Table 1. In general, Qmin values agree very well with second ionisation values obtained from photoelectron spectroscopy [24]. Deviations between the two values ranged from less than 0.1 eV for Ni+ to 0.4 eV for Co+ which are well within the experimental error limits of the charge stripping method. The charge stripping spectra of monocations produced by FAB ionisation contain a single major intensity without lesser intensity signals corresponding to lower Qminvalues. These signals, when observed, are due to the charge stripping of electronically excited ions and have been observed with the charge stripping of ions produced by electron impact [25,26]. The absence of these signals has also been reported by Rabranovic et al. [14] for the charge stripping of FAB ions and provides evidence that FAB ionisation is a “softer” ionisation technique than electron impact ionisation. However, the comparison of Qminvalues for ions obtained with EI and FAB produces nearly identical values. For example, Qminvalues of Fe+ and Co+ from EI are reported to be 16.6 eV and 17.5 eV respectively, which are nearly identical to our respective values of 16.3 eV and 17.5 eV. The Qminvalues for Eu+ and Ho+ in Table 1 are the first results obtained for the investigation of lanthanides by charge stripping. Lanthanides in solution are typically observed in the + 3 oxidation state, however, + 2 ions can be formed and stabilised [27]. Many well-known examples of divalent lanthanides exist, including Eu2+ and Ho2+. The second ionisation values of
548 TABLE 1 Qminvalues of metal ions from the corresponding ionisation energy are provided for comparison
metal salts. Literature values for second
Ion
Precursor compound
QminW
IE(M+ + MZf )” (eV)
Mg+ Fe+ co+ Ni + Pd+ Eu+ Ho+
MgClz FeCl, CoCl, NiCI, PdCl, Eu(thd), HoCl,
15.1 b 16.3 17.5 18.2 19.2 11.3 12.2
15.04 16.18 17.06 18.17 19.43 11.25 11.80
a Data taken from ref. 24. bValue obtained from ref. 21.
this group are relatively low and comparable with the first ionisation energies of many known thermally stable compounds. Thus the charge stripping spectra show intense dication signals, generally with excellent signal-to-noise ratios. As with the other metals in the study, dication signals from the charge stripping of electronically excited states are not readily observed. Good agreement is also obtained between Qminvalues and known second ionisation energies for this group of metals. A major emphasis of our group has been in the formation of ligated metal ions and the effect of ligation on the reactivity of the metal ions. Knowing the effect of ligation on the electronic structure of the metal center is the key to determining the effects of ligation on reactivity [28]. We have thus begun a program of developing methods for producing ligated metal ions and to determine the effects of ligation via charge stripping. Ligated metal ions are obtained from the same sample used to form the bare metal ions. A FAB mass spectrum of an evaporated aqueous solution of a metal salt such as MCl, typically contains bare metal ions as well as several ligated species such as MO+, MOH+, and MCl+. A mixture of two metal salts does not produce the mixed metal complex. The charge stripping spectra of ligated metal ions, Fig. 1, also consist of only a single peak and are devoid of smaller intensities corresponding to lower Qminvalues due to excited species. Charge stripping spectra of ligated metal ions are generally as intense or nearly as intense as the corresponding bare metal ions. Similarly, reproducibility is nearly identical to the bare metal ions and is generally within f GSeV. We have found that ion intensities as low as 20% relative to the bare metal ion, which is often the base peak, can be charge stripped. A full scale mass-analyzed ion kinetic energy (MIKE) spectrum yields the charge stripping peak and collisionally induced dissociation (CID) peaks. The CID peaks are used to confirm the identity of the ion selected by the magnetic analyzer. Intensities due to CID consist pri-
H&l’+
M&C 12*
E/2
d)
b) \
coc1*+
Fig. 1. Charge stripping spectra of ligand metal ions produced from the FAB ionisation of their respective metal chloride salts: (a) MgCl*+ ; (b) CoC12+; (c) HoCl*+; (d) HoOH2+. The E/2 position of the main beam is marked. Each division on the horizontal coordinate equals IOeV. HoOH’ is a product of HoCl, and H20.
1
z v)
5.50
TABLE 2 Qminvalues of selected ligated metal ions produced from the corresponding salt. The differences between Qmin(Mf ) and Qhn (M-L+), AQmin, are provided for comparison Ion
Precursor
QminW)
AQminCW = Qmin(M-L)+
MgCl+ coc1+ HoCl+ HoOH+
MgCl, CoCl, HoCl, HoCl,
16.5 18.4 15.4 13.2
+ 1.4 +0.9 + 3.2 +0.9
- Qmin(Mf )
marily of the metal ion and the ionised ligand. For species such as HoOH+ the presence of Ho+ and HoO+ in the CID spectra confirm the identity of the ion. Table 2 lists Qminvalues for several ligated metal ions and illustrates the general utility of this method to probe ligation effects. The value of 16.5eV for Qminof MgCl+ corresponds well with the value of 16.3 eV reported by Caldwell and Gross [21b]. Ligation, in all cases, increases the Qmin value relative to that of the bare metal ion. This trend is expected since both Cl and OH ligands are electron withdrawing and hence expected to stabilise metal electron orbitals resulting in increased Qminvalues. A similar trend has been reported in the charge stripping of TiCl, (n = l-3) where the Qmin values increase with increasing values of n [25]. Ligation of rare earth metal ions to form metal halides and oxides also produces the same effect [21]. We have, as yet, too few values to comment on the effect of ligation on various metal groups. However, it is clear that the greatest effect is observed for the lanthanides. This is probably due to the large diffuse atomic orbitals of Ho+ which are greatly stabilised by the presence of the Cl atom. The larger Qmin value obtained for MgCl+ compared with CoCl+ is also interesting. The valence electron of Mg+ has a greater s character than the valence electrons of Co+. If the s orbital is strongly involved in the M-Cl bonding then it follows that the larger effect should be observed for Mg, which is indeed the case. Finally, the large difference in Qminvalues between Cl and OH ligands when bound to Ho+ illustrates the large variation in ligation effect induced by various ligands on the same metal. A comparison between HoCl+ (AQmi, = 3.2 eV) and HoOH+ (AQmin= 0.9 eV) shows a difference of 2.3 eV between the AQmin values of the two species. The chlorine atom is a much stronger stabilising ligand than the hydroxyl group. This large difference between the two species indicates that the lanthanide metals may provide a good system for the study of ligation. CONCLUSION
The formation of bare and ligated metal ions by FAB of their simple salts is shown to be a general method of producing many different types of metal
551
ions for ion/molecule chemistry. The charge stripping of ligated metal ions can be used to determine the effect of complexation on the electron energy levels of metal ions. From the Qminvalues of the bare and ligated metal ions obtained in this work, the observed effect is shown not only to be measurable, but also to be quite significant. In addition, the charge stripping of ligated species allows investigation into the nature and strengths of metal-ligand bonds. ACKNOWLEDGMENTS
Funding from the Department of Chemistry, University of California, Davis, the University of California University-Wide Energy Grant and the Petroleum Research Fund is gratefully acknowledged. We would like to thank the groups of Dr. Susan Kauzlarich and Dr. Alan Balch for providing some of the metal salts. We would also like to thank Kei Miyano of the Facility for Advanced Instrumentation for his assistance with this work. REFERENCES 1 R.J. Day, S.E. Unger and R.G. Cooks, Anal. Chem., 52 (1980) 557A. 2 M. Barber, R.S. Bordoli, G.J. Elliott, R.D. Sedgwick and A.N. Tyler, J. Chem. Sot., Chem. Commun., (1981) 325. 3 M. Barber, R.S. Bordoli, R.D. Sedgwick and A.N. Tyler, Anal. Chem., 54 (1982) 645A. 4 T. Ast, J.H. Beynon and R.G. Cooks, J. Am. Chem. Sot., 94 (1972) 6611. 5 D.L. Kemp, J.H. Beynon and R.G. Cooks, Org. Mass Spectrom., 11 (1976) 857. 6 K. Lammertsma, P. von Rag& Schleyer and H. Schwarz, Angew. Chem., Int. Ed. Engl., 28 (1989) 1321. 7 H. Mestdagh, N. Morin and C. Rolando, Tetrahedron Len., 27 (1986) 33. 8 T. Drewello, K. Ekart, C.B. Lebrilla and H. Schwarz, Int. J. Mass Spectrom. Ion Processes, 76 (1987) Rl. 9 C.B. Lebrilla, T. Drewello and H. Schwarz, Int. J. Mass Spectrom. Ion Processes, 79 (1987) 287. 10 C.B. Lebrilla, T. Drewello and H. Schwarz, Organometallics, 6 (1987) 2450. 11 C. Schulze and H. Schwarz, J. Am. Chem. Sot., 110 (1988) 67. 12 B.S. Freiser, Talanta, 32 (1985) 697. 13 C.J. Proctor, C.J. Porter, T. Ast, P.D. Bolton and J.H. Beynon, Org. Mass Spectrom., 16 (1981) 454. 14 M. Rabrenovic, C.J. Proctor, C.G. Herbert, A.G. Brenton and J.H. Beynon, J. Phys. Chem., 87 (1983) 3305. 15 C.J. Porter, C.J. Proctor, T. Ast and J.H. Beynon, Croat. Chem. Acta, 54 (1981) 407. 16 T. Drewello, W.-D. Fessner, A.J. Kos, C.B. Lebrilla, H. Prinzbach, P. von Rag& Schleyer and H. Schwarz, Chem. Ber., 121 (1988) 187. 17 SW. Buckner and B.S. Freiser, J. Am. Chem. Sot., 109 (1987) 1247. 18 J.R. Gord, B.S. Freiser and S.W. Buckner, J. Chem. Phys., 91 (1989) 7530. 19 R. Tonkyn and J.C. Weisshaar, J. Am. Chem. Sot., 108 (1986) 7128. 20 Y. Huang and B.S. Freiser, J. Am. Chem. Sot., 110 (1988) 4434. 21 (a) M. Rabrenovic, T. Ast and J.H. Beynon, Int. J. Mass Spectrom. Ion Processes, 61 (1984) 31.
552
22
23 24 25 26 27 28
(b) K.A. Caldwell and M.L. Gross, Proc. 37th ASMS Conf. Mass Spectrometry and Allied Topics, Miami Beach, FL, 1989, p. 529. (a) M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. Lett., 58 (1987) 908. (b) P.H. Hor, L. Gao, R.L. Meng, Z.J. Huang, Y.Q. Wang, K. Forster, J. Vassilious, C. W. Chu, M.K. Wu, J.R. Ashburn and C.J. Torng, Phys. Rev. Lett., 58 (1987) 911. E.J. Munson and J.F. Haw, Anal. Chem., 62 (1990) 2532. R.C. Weast (Ed.), CRC Handbook of Chemistry and Physics, 69th edn., CRC Press, Boca Raton, FL, 1988/1989. C.J. Porter, C.J. Proctor, T. Ast and J.H. Beynon, Int. J. Mass Spectrom. Ion Phys., 41 (1982) 265. D. Stahl, S. Beaudet, T. Drewello and H. Schwarz, Int. J. Mass Spectrom. Ion Processes, 101 (1990) 121. F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 4th edn., Wiley-Interscience, New York, 1980. M.L. Mandich, M.L. Steigerwald and W.D. Reents, Jr., J. Am. Chem. Sot., 108 (1986) 6197.
545
Elsevier Science Publishers B.V., Amsterdam
Short Communication CHARGE STRIPPING AS A PROBE TO DETERMINE THE EFFECTS OF LIGATION IN METAL ION COMPLEXES PRODUCED IN A FAST ATOM BOMBARDMENT SOURCE
SESI M. MCCULLOUGH ‘, A. DANIEL JONES b and CARLITO
B. LEBRILLA ‘.*
aDepartment of Chemistry, University of California. Davis, CA 9.5616 (USA) bFacility for Advanced Instrumentation, University of California, Davis, CA 95616 (USA)
(First received 14 February
1991; in final form 8 April 1991)
ABSTRACT Fast atom bombardment ionisation is combined with charge stripping (CS) to produce bare and ligated metal dications. Abundant quantities of ions resulting from the bombardment of transition and lanthanide metal salts are produced and charge stripped. Qminvalues obtained from the CS spectra agree within + 0.5 eV of the respective second ionisation energies for the bare metal ions. Effects of ligands such as Cl and OH are observed as an increase in Qminvalues relative to the bare metal ions, e.g. AQ,,,, = 0.9eV for CoCl+ and AQ,,,,, = 3.2eV for HoCl+.
Continuing interest in the gas-phase chemistry of metals necessitates the development of alternative methods for the formation and characterisation of metal ions. We examine the use of matrix-free fast atom bombardment (FAB)** [l-3] as a general method for the formation of various groups of metal and ligated metal ions. This method is used in combination with charge stripping (CS) [4-61 to probe the effects of ion formation and ligation on the electronic state of the formed ionic species. In addition, CS provides access to the second ionisation energies of the ligated metal ions which we observe to vary significantly from the second ionisation energies of the corresponding bare metal ion. Metal ion formation for a variety of transition metals by FAB has recently been used in the gas-phase ion/molecule chemistry of several transition metal ions with simple organic compounds [7-l 11.Although not as extensively used as other ionisation methods such as laser ablation [12] and electron impact,
* Author to whom correspondence should be addressed. **The term FAB will refer to both secondary ion mass spectrometry (SIMS) and fast atom bombardment and is used in this report to mean the bombardment of a neat substrate with neutral atoms.
016%1176/91/$03.50
0 1991 Elsevier Science Publishers B.V.
546
FAB offers a simple and convenient alternative and overcomes many of the problems associated with the other two techniques. Electron impact ionisation, for example, relies heavily on thermally stable and volatile organometallit precursors. Laser ablation needs specific instrumentation and is not easily applicable to all types of mass spectrometer. Conversely, FAB allows the formation of metal ions from readily available metal salts. A FAB source is available in many sector instruments and recently in trapped ion mass spectrometers such as Fourier transform mass spectrometers. By ejecting a second electron to produce the dication, the CS process has been extensively used to probe the electronic energy levels of several simple inorganic and organic ions [13-151. Many theoretically interesting compounds, for example C,Hi+, the smallest Hiickel aromatic dication [6], and some organic ions of high complexity such as the dication of [2.2.1.1] pagodane with the molecular formula C,,Hi: [6,16], have been formed by CS. Recent studies on the formation and reactions of gas-phase dipositive transition metal ions such as Nb2+ [17,18], Ti2+ [ 191 and the mixed metal dimer LaFe’+ [20] have also generated new interest in doubly charged metal ions. The combination of FAB ionisation-CS allows a readily accessible source of doubly charged bare and ligated metal ions. We have used FAB to produce a variety of bare transition metal ions including Fe+, Co+, Ni+, and Pd+, lanthanide metal ions, Eu+ and Ho+, and several ligated metal ions including MgCl+, CoCl+, HoCl+ and HoOH+. Sufficient quantities of ions are obtained allowing charge stripping to be performed. The charge stripping of metal ions as well as several ligated alkaline earth metal ions has previously been reported [21]. There have been, to our knowledge, no reports on the charge stripping of FAB produced ions of ligated transition metals. The charge stripping of lanthanides and, particularly, ligated lanthanide metal ions, has also not been performed despite their general importance in the area of materials science research [22,23]. The experiments were performed in a VG-Analytical ZAB-2F mass spectrometer with BE geometry. Samples are obtained by dissolving the appropriate metal salt in deionised water or other suitable solvent, and placing a 2 ~1 aliquot of the resulting solution on a stainless steel probe tip. MgCl, was added to each sample for calibration and to ensure identical experimental conditions for both the reference and the sample. All metal salts are commercially available (Aldrich, Fisher, Strem) and used without further purification. The solvent is evaporated prior to bombardment of the sample by a 9 keV Xe atom beam. Bombarding the sample produces several ionic species including bare metal ions and often more than one ligated species. The ionic species are extracted from the source region by an 8 keV accelerating potential, and the desired species are selected by the magnetic sector. To form the dications, M2+, nitrogen gas at an indicated pressure of approxi-
547
mately 5 x low6 Torr was introduced into the collision chamber located in the second field-free region of the spectrometer. At this pressure an approximately 50% attenuation of the main beam is observed. The charge stripping spectra are obtained by scanning the electric sector voltage about E/2 where E is the energy necessary to transmit the singly charged ion. In general, 3-10 scans are accumulated for the parent peak at E, and 5-25 scans are summed to obtain the charge stripping peak about E/2. Data were acquired and averaged using the VG 1l/250 data system in multichannel analysis (MCA) mode. The value of the minimum translational energy loss in the charge stripping process, Qmin, is obtained by extrapolating to the baseline of the high energy side of the charge stripping peak. A correction factor, 6, based on an assigned value of Qmin(Mg+) = 15.1 eV, is applied to calculate Qminvalues using the method described previously by both Lammertsma et al. [6] and Rabrenovic et al. [14]. An additive calibration method was used to determine the Qminvalues which varied typically to within & 0.5 eV. RESULTS
AND DISCUSSION
Average charge stripping values (Qmi”) for metal ions belonging to several groups including alkaline earth (Mg+), transition metal (Fe+, Ni+, Co+, and Pd+) and rare earth (Ho+ and Eu+) are shown in Table 1. In general, Qmin values agree very well with second ionisation values obtained from photoelectron spectroscopy [24]. Deviations between the two values ranged from less than 0.1 eV for Ni+ to 0.4 eV for Co+ which are well within the experimental error limits of the charge stripping method. The charge stripping spectra of monocations produced by FAB ionisation contain a single major intensity without lesser intensity signals corresponding to lower Qminvalues. These signals, when observed, are due to the charge stripping of electronically excited ions and have been observed with the charge stripping of ions produced by electron impact [25,26]. The absence of these signals has also been reported by Rabranovic et al. [14] for the charge stripping of FAB ions and provides evidence that FAB ionisation is a “softer” ionisation technique than electron impact ionisation. However, the comparison of Qminvalues for ions obtained with EI and FAB produces nearly identical values. For example, Qminvalues of Fe+ and Co+ from EI are reported to be 16.6 eV and 17.5 eV respectively, which are nearly identical to our respective values of 16.3 eV and 17.5 eV. The Qminvalues for Eu+ and Ho+ in Table 1 are the first results obtained for the investigation of lanthanides by charge stripping. Lanthanides in solution are typically observed in the + 3 oxidation state, however, + 2 ions can be formed and stabilised [27]. Many well-known examples of divalent lanthanides exist, including Eu2+ and Ho2+. The second ionisation values of
548 TABLE 1 Qminvalues of metal ions from the corresponding ionisation energy are provided for comparison
metal salts. Literature values for second
Ion
Precursor compound
QminW
IE(M+ + MZf )” (eV)
Mg+ Fe+ co+ Ni + Pd+ Eu+ Ho+
MgClz FeCl, CoCl, NiCI, PdCl, Eu(thd), HoCl,
15.1 b 16.3 17.5 18.2 19.2 11.3 12.2
15.04 16.18 17.06 18.17 19.43 11.25 11.80
a Data taken from ref. 24. bValue obtained from ref. 21.
this group are relatively low and comparable with the first ionisation energies of many known thermally stable compounds. Thus the charge stripping spectra show intense dication signals, generally with excellent signal-to-noise ratios. As with the other metals in the study, dication signals from the charge stripping of electronically excited states are not readily observed. Good agreement is also obtained between Qminvalues and known second ionisation energies for this group of metals. A major emphasis of our group has been in the formation of ligated metal ions and the effect of ligation on the reactivity of the metal ions. Knowing the effect of ligation on the electronic structure of the metal center is the key to determining the effects of ligation on reactivity [28]. We have thus begun a program of developing methods for producing ligated metal ions and to determine the effects of ligation via charge stripping. Ligated metal ions are obtained from the same sample used to form the bare metal ions. A FAB mass spectrum of an evaporated aqueous solution of a metal salt such as MCl, typically contains bare metal ions as well as several ligated species such as MO+, MOH+, and MCl+. A mixture of two metal salts does not produce the mixed metal complex. The charge stripping spectra of ligated metal ions, Fig. 1, also consist of only a single peak and are devoid of smaller intensities corresponding to lower Qminvalues due to excited species. Charge stripping spectra of ligated metal ions are generally as intense or nearly as intense as the corresponding bare metal ions. Similarly, reproducibility is nearly identical to the bare metal ions and is generally within f GSeV. We have found that ion intensities as low as 20% relative to the bare metal ion, which is often the base peak, can be charge stripped. A full scale mass-analyzed ion kinetic energy (MIKE) spectrum yields the charge stripping peak and collisionally induced dissociation (CID) peaks. The CID peaks are used to confirm the identity of the ion selected by the magnetic analyzer. Intensities due to CID consist pri-
H&l’+
M&C 12*
E/2
d)
b) \
coc1*+
Fig. 1. Charge stripping spectra of ligand metal ions produced from the FAB ionisation of their respective metal chloride salts: (a) MgCl*+ ; (b) CoC12+; (c) HoCl*+; (d) HoOH2+. The E/2 position of the main beam is marked. Each division on the horizontal coordinate equals IOeV. HoOH’ is a product of HoCl, and H20.
1
z v)
5.50
TABLE 2 Qminvalues of selected ligated metal ions produced from the corresponding salt. The differences between Qmin(Mf ) and Qhn (M-L+), AQmin, are provided for comparison Ion
Precursor
QminW)
AQminCW = Qmin(M-L)+
MgCl+ coc1+ HoCl+ HoOH+
MgCl, CoCl, HoCl, HoCl,
16.5 18.4 15.4 13.2
+ 1.4 +0.9 + 3.2 +0.9
- Qmin(Mf )
marily of the metal ion and the ionised ligand. For species such as HoOH+ the presence of Ho+ and HoO+ in the CID spectra confirm the identity of the ion. Table 2 lists Qminvalues for several ligated metal ions and illustrates the general utility of this method to probe ligation effects. The value of 16.5eV for Qminof MgCl+ corresponds well with the value of 16.3 eV reported by Caldwell and Gross [21b]. Ligation, in all cases, increases the Qmin value relative to that of the bare metal ion. This trend is expected since both Cl and OH ligands are electron withdrawing and hence expected to stabilise metal electron orbitals resulting in increased Qminvalues. A similar trend has been reported in the charge stripping of TiCl, (n = l-3) where the Qmin values increase with increasing values of n [25]. Ligation of rare earth metal ions to form metal halides and oxides also produces the same effect [21]. We have, as yet, too few values to comment on the effect of ligation on various metal groups. However, it is clear that the greatest effect is observed for the lanthanides. This is probably due to the large diffuse atomic orbitals of Ho+ which are greatly stabilised by the presence of the Cl atom. The larger Qmin value obtained for MgCl+ compared with CoCl+ is also interesting. The valence electron of Mg+ has a greater s character than the valence electrons of Co+. If the s orbital is strongly involved in the M-Cl bonding then it follows that the larger effect should be observed for Mg, which is indeed the case. Finally, the large difference in Qminvalues between Cl and OH ligands when bound to Ho+ illustrates the large variation in ligation effect induced by various ligands on the same metal. A comparison between HoCl+ (AQmi, = 3.2 eV) and HoOH+ (AQmin= 0.9 eV) shows a difference of 2.3 eV between the AQmin values of the two species. The chlorine atom is a much stronger stabilising ligand than the hydroxyl group. This large difference between the two species indicates that the lanthanide metals may provide a good system for the study of ligation. CONCLUSION
The formation of bare and ligated metal ions by FAB of their simple salts is shown to be a general method of producing many different types of metal
551
ions for ion/molecule chemistry. The charge stripping of ligated metal ions can be used to determine the effect of complexation on the electron energy levels of metal ions. From the Qminvalues of the bare and ligated metal ions obtained in this work, the observed effect is shown not only to be measurable, but also to be quite significant. In addition, the charge stripping of ligated species allows investigation into the nature and strengths of metal-ligand bonds. ACKNOWLEDGMENTS
Funding from the Department of Chemistry, University of California, Davis, the University of California University-Wide Energy Grant and the Petroleum Research Fund is gratefully acknowledged. We would like to thank the groups of Dr. Susan Kauzlarich and Dr. Alan Balch for providing some of the metal salts. We would also like to thank Kei Miyano of the Facility for Advanced Instrumentation for his assistance with this work. REFERENCES 1 R.J. Day, S.E. Unger and R.G. Cooks, Anal. Chem., 52 (1980) 557A. 2 M. Barber, R.S. Bordoli, G.J. Elliott, R.D. Sedgwick and A.N. Tyler, J. Chem. Sot., Chem. Commun., (1981) 325. 3 M. Barber, R.S. Bordoli, R.D. Sedgwick and A.N. Tyler, Anal. Chem., 54 (1982) 645A. 4 T. Ast, J.H. Beynon and R.G. Cooks, J. Am. Chem. Sot., 94 (1972) 6611. 5 D.L. Kemp, J.H. Beynon and R.G. Cooks, Org. Mass Spectrom., 11 (1976) 857. 6 K. Lammertsma, P. von Rag& Schleyer and H. Schwarz, Angew. Chem., Int. Ed. Engl., 28 (1989) 1321. 7 H. Mestdagh, N. Morin and C. Rolando, Tetrahedron Len., 27 (1986) 33. 8 T. Drewello, K. Ekart, C.B. Lebrilla and H. Schwarz, Int. J. Mass Spectrom. Ion Processes, 76 (1987) Rl. 9 C.B. Lebrilla, T. Drewello and H. Schwarz, Int. J. Mass Spectrom. Ion Processes, 79 (1987) 287. 10 C.B. Lebrilla, T. Drewello and H. Schwarz, Organometallics, 6 (1987) 2450. 11 C. Schulze and H. Schwarz, J. Am. Chem. Sot., 110 (1988) 67. 12 B.S. Freiser, Talanta, 32 (1985) 697. 13 C.J. Proctor, C.J. Porter, T. Ast, P.D. Bolton and J.H. Beynon, Org. Mass Spectrom., 16 (1981) 454. 14 M. Rabrenovic, C.J. Proctor, C.G. Herbert, A.G. Brenton and J.H. Beynon, J. Phys. Chem., 87 (1983) 3305. 15 C.J. Porter, C.J. Proctor, T. Ast and J.H. Beynon, Croat. Chem. Acta, 54 (1981) 407. 16 T. Drewello, W.-D. Fessner, A.J. Kos, C.B. Lebrilla, H. Prinzbach, P. von Rag& Schleyer and H. Schwarz, Chem. Ber., 121 (1988) 187. 17 SW. Buckner and B.S. Freiser, J. Am. Chem. Sot., 109 (1987) 1247. 18 J.R. Gord, B.S. Freiser and S.W. Buckner, J. Chem. Phys., 91 (1989) 7530. 19 R. Tonkyn and J.C. Weisshaar, J. Am. Chem. Sot., 108 (1986) 7128. 20 Y. Huang and B.S. Freiser, J. Am. Chem. Sot., 110 (1988) 4434. 21 (a) M. Rabrenovic, T. Ast and J.H. Beynon, Int. J. Mass Spectrom. Ion Processes, 61 (1984) 31.
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