Reactive collisions in quadrupole cells 5. Reactions of singly charged transition metal ions with cyclopentene

Reactive collisions in quadrupole cells 5. Reactions of singly charged transition metal ions with cyclopentene

Mass Spectrometry a n d [oll Processes ELSEVIER International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 251 260 Reactive collis...

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Mass Spectrometry a n d [oll Processes

ELSEVIER

International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 251 260

Reactive collisions in quadrupole cells . Reactions of singly charged transition metal ions with cyclopentene J i n s o n g Ni, Alex G. H a r r i s o n * Department of Chemisto,, University o[" Toronto, Toronto, Ont. M5S IA 1, Canada

Received 28 December 1994; accepted 22 March 1995

Abstract

The development of a tandem mass spectrometric method, involving reactive collisions in a quadrupole collision cell, to study the reactions of metal ions with organic substrates is reported. This approach is evaluated by studying the reactions of singly charged transition metal ions with cyclopentene. The transition metal ions Sc + through Cu" were produced by fast atom bombardment of the appropriate solid metal chloride. After mass selection of these metal ions by the double-focusing (BE) stage of a hybrid BEqQ mass spectrometer, the metal ions were introduced into the r.f.-only quadrupole collision cell (q) at low injection energy where they underwent reaction with cyclopentene. The reaction products were mass analyzed by the final quadrupole (Q). The Mn + ion was largely unreactive with cyclopentene. The major primary product for the metal ions Sc +, Ti +, V +, Fe +, Co ~ and Ni* was M+(CsH6) arising by reductive H2 elimination from the M+(c-CsH8) complex. Cr + and Cu + showed only clustering to form M + (CsH~). This difference in reaction mode has been rationalized in terms of the energetics of insertion of the metal ion into a C H bond. The M ~ (C5H6) product reacted further with cyclopentene to produce M +(C5H5) 2, presumably the ionized metallocene, except for C0+(C5H6) which also produced C0+(C5H6)2 and Ni ~ (C5H6) which produced only Ni" (CsH6)(C5Hs). Cr* (C5H8). Cr+(CsHs) andCu+(CsHs) underwent further clustering with cyclopentene. Where comparison is possible, the results obtained are in good agreement with results obtained in earlier studies, indicating that the present tandem mass spectrometric method is a viable approach to the study of metal ion/organic molecule reactions. Kevwords. Cyclopentene; Quadrupole cells; Reactive collisions: Transition metal ions

I. Introduction

Tandem mass spectrometry (MS/MS) is a well-developed technique for the analysis of complex mixtures and for structure elucidation '~ Dedicated to the memory of Professor Alfred O. Nier. Taken, in part, from the Ph.D. thesis of J. Ni, University of Toronto, 1994. * Corresponding author.

of gaseous ions [1,2]. In these uses the technique, for the most part has involved collision of a mass-selected ion with an inert gas (He, Ar, N2) at collision energies that result in fragmentation or charge inversion (or both) of the incident ion. With the development of quadrupole collision cells, as part of either triple quadrupole or hybrid sector/quadrupole instruments, it is possible to decrease the energy of

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J. Ni, A.G. Harrison~International Journal of Mass Spectrometo, and hm Processes 146/147 (1995) 251-260

the incident ion to values such that chemical reactions occur between the ion and a reactive collision gas. Such reactive collisions in quadrupole cells have been used to explore fundamental aspects of gas-phase ion/molecule reactions [3-12], to characterize gaseous ion structures through ion/molecule reactions [13-20] and to probe gas-phase H/D exchange reactions of massselected ions with appropriate deuterium-containing reagent gases [21-26]. Of more interest in the present context is the approach of introducing the analyte into the collision cell and identifying it by characteristic reactions with mass-selected reactant ions. Thus, Meyerhoffer and Bursey [27] showed that (CH3)3Si + reacted selectively with cis-cyclopentane-l,2-diol at low collision energies; the reaction with the trans isomer is endothermic and does not occur until higher collision energies are reached. Hail et al. [28] have shown that the aromatic components of a test mixture, introduced by gas chromatography into the collision cell, can be selectively ionized using charge exchange with mass-selected benzene molecular ions. Usypchuk et al. [29] have shown that the isomeric butenes and pentenes can be characterized to a considerable extent by reaction of the neutral molecule with mass-selected CH3NH2 + ions in the quadrupole collision cell of a hybrid BEqQ mass spectrometer. Numerous studies have shown that chemical ionization using metal ions as reagents can provide useful analytical information [30-39]. Therefore it appeared of interest to evaluate the usefulness of studying reactive collisions of metal ions with organic substrates in the quadrupole collision cell of a hybrid BEqQ mass spectrometer [40] as an alternative to metal ion chemical ionization studies. Although the ultimate goal of our studies is to use metal ion reactions to distinguish among isomeric molecules, we present here the study of the reactions of the singly

charged first row transition metal ions with cyclopentene. A comparison of the results obtained in the present study with literature data provides an indication of the usefulness of the tandem mass spectrometric approach to the study of the chemistry of gaseous metal ions. There has been one brief report [41] of the study of the reactions of mass-selected metal ions with 2-methylpropane in the quadrupole cell of a triple quadrupole mass spectrometer but no previous studies employing a hybrid sector-quadrupole instrument. There is a continuing interest in the chemistry of the interaction of gas-phase metal ions with organic molecules [42-46], not only because of the analytical applications but also because a knowledge of the intrinsic properties of gaseous metal ions provides valuable data relevant to the mechanisms operative in condensed phase catalytic processes. Most of the studies of metal ion/organic molecule reactions have been carried out using ion cyclotron resonance (ICR), particularly Fourier transform ICR, techniques [47,48] or ion-beam techniques [49-51]; an alternative approach has been the study by tandem mass spectrometry of the metastable or collision-induced decomposition reactions of organometallic complexes formed in the gas phase [52,53]. A variety of methods has been employed to produce gaseous metal ions [45] including electron impact on volatile organometallic complexes [46,54], laser vaporization/ionization of pure metals [55], surface ionization [56] and sputtering by bombardment of solid metal salts by energetic particle beams [57-59]. Of particular interest in the present context is the use of fast atom bombardment (FAB) of metal salts as a convenient source of singly charged metal ions [41,57,59-62]. Charge stripping studies [60,61] suggest that, for the transition metals, FAB produces ground state ions, although, as Armentrout [45] has pointed out, the resolution and the sensitivity of the charge stripping method do not preclude

.I. Ni, A.G. Harrison~International Journal o l Mass Spectrometry and h,n Processes 146,147 ( 1995 ; 251 260

Table I Relative sensitivities for production of singly charged metal ions Salt

Relative sensitivity"

CrCl~ Cr(NO~ 2 Cr2 S() 4 )1 MnCI2 Mn(NO3)2 MnSoa Fe('l~ Fe(71,

4 2 3 1 I 14 14

Fe~ N O 313

3

t-e~ SO4 3 FeSO~ CoCL Co(NO~ )2 CoSO4 NiCE Nil N O 3 )2 NiSO4

3 1 11 3 1 7

1

1

~' Relative to metal sulfate sensitivity of 1.

the presence of up to 10% of excited state ions. The majority of studies of metal ion/neutral molecule reactions using FAB ionization have involved tandem mass spectrometric studies of the complexes formed in a high pressure ion source. With the addition of a FAB source to our tandem hybrid BEqQ mass spectrometer [40], we have undertaken a study of the reactions of mass-selected singly charged transition metal ions with organic molecules in the quadrupole collision cell of the tandem hybrid instrument using fast atom bombardment to produce the metal ions.

2. Experimental All experimental work was carried out using a VG Analytical ZAB-2FQ [63] hybrid BEqQ mass spectrometer which has been described in detail previously [40]. Briefly, the instrument is a reversed-geometry (BE) double-focusing mass spectrometer which is followed by a third stage consisting of a deceleration lens system, an r.f.-only quadrupole collision cell (q) and a quadrupole mass analyzer (Q). In

253

the present study, the appropriate metal ion beam was mass selected by the BE doublefocusing mass spectrometer at 6 keV ion kinetic energy, decelerated to I 10 eV kinetic energy (laboratory scale) and injected into the quadrupole collision cell which contained cyclopentene at pressures ranging from 5 × 10 6 to 3 × 10 5 Torr, as read by an ionization gauge attached to the pumping line for the quadrupole region. It is estimated that the actual pressure in the quadrupole cell is 100-1000 times that read by the ionization gauge. The reaction products were analyzed by scanning the final quadrupole. Approximately 20 40 2 s scans were accumulated on a multichannel analyzer. The singly charged metal ions were prepared by bombardment, with Ar atoms of 7 8 keV energy, of the appropriate metal salt deposited on a stainless steel probe. An aqueous solution of the salt was applied to the probe and evaporated to dryness before insertion into the ion source. Singly charged metal ions were the predominant ion species observed from metal sulfates, metal nitrates and metal chlorides with the chlorides giving the most intense ion signals as indicated by the relative sensitivities listed in Table 1 for several salts ofCr, Mn, Fe, Co and Ni. It appears that the oxidation state of the metal has little effect on the singly charged metal ion signal observed. The metal ion signals were stable for at least 30 rain, providing more than adequate time to carry out the experiments. In related experiments, M' (C5H5) (M = Fe, Co, Ni) ions were prepared by electron ionization of the appropriate metallocene using an EI/CI source operating in the El mode at a source temperature of 250C and 70 eV ionizing electron energy. The M ~(C5H5) ions were mass selected, decelerated and reacted with cyclopentene in the quadrupole collision cell at 3 eV incident ion energy and an indicated cyclopentene pressure of 3 x 10.5 Torr.

J. Ni, A.G. Harrison~International Journal o f Mass Spectrometry and Ion Processes 146/147 (1995) 251 260

254

IOC

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{

{

{

8Cz

_o i.-

Fe+(C~Hs) CsHACsHe)

....... L;.._

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o+

~ 6o _

........

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z

~4C

-

Co*(CsHe)z

N

Co*(C~H~

o

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5

lO

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15 20 25 OUADRUPOLE CELL PRESSURE (IO-6TORR)

30

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Fig. 2. I o n signals as a function of cell pressure at 3 eV incident ion energy.

,,

LliC'J4

C~(CsHa c~(c~-~

Fig. 1. Spectra obtained for reaction of Fe + and Co + with cyclopentene.

3. Results and discussion

Fig. 1 shows typical traces illustrating the spectra obtained when Fe + and Co + are reacted with cyclopentene at an incident ion energy of 3 eV and an indicated cyclopentene pressure of 3 x 10-5 Torr in the quadrupole collision cell. A minor reaction channel involves hydride abstraction from cyclopentene to form CsH~-; separate experiments showed that the major reaction channel of CsH~- with cyclopentene involved clustering to form CsH+(CsHs) (m/z 135). For Fe + the major reaction product observed under these conditions is Fe+(CsH5)2 with very minor yields of Fe+(CsH6) and Fe+(CsHs). The ion signal one mass unit higher than Fe+(CsHs)2 is more intense than would be expected for the 13C isotope contribution (Fe+(13CCaHs)(CsHs)), indicating formation of Fe+(CsHs)(CsH6) in minor yield. In contrast, the reaction of Co + with cyclopentene

under these conditions produces Co+(C5H5)2 and Co+(C5H6)2 in nearly equal yields with a minor yield of Co + (C5H6). Fig. 2 shows a typical plot of relative ion signals as a function of the indicated quadrupole cell pressure at an incident ion energy of 3 eV. For all metal ions it was observed that there was very little reaction of the metal ion at pressures below about 7 × 10 -6 Torr, following which the metal ion signal decreased in essentially an exponential fashion. We believe that this indicates that the incident metal ion must be cooled translationally by non-reactive collisions before effective reaction occurs. At 3 eV incident ion energy and a pressure of 5 × 10 -6 Torr we estimate that the metal ion undergoes 5-15 collisions with cyclopentene in the collision cell, although clearly the number of collisions will increase as the incident ion energy is decreased by non-reactive collisions. The necessity for translational cooling has been noted previously [25] in a study of the H/D exchange reactions of protonated aromatic amines with ND3 in the quadrupole collision cell. Fig. 3 shows the variation in ion signals



J. Ni, A.G. Harrison/International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 251 260 '

I

'

I

8C

J <

6c

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'

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z 0

~< 4o ©

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o

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V+(CsHs)

2

4

I 6

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I 8

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INCIDENT ION ENERGY(eV)

Fig. 3. Ion signals as a function of incident ion energy at 3 x l0 -5 Torr indicated cyclopentene pressure.

observed for the V +-cyclopentene system with incident ion energy at 3 x 10-5 Torr indicated pressure of cyclopentene. A decrease in extent of reaction with increasing incident ion energy is expected; however, the discontinuity between 5 and 6 eV incident ion energy is unexpected. A similar discontinuity in the region of 4-7 eV incident ion energy was observed for all systems with the exception of Mn +-cyclopentene where the Mn + ion showed only a very low reactivity at all ion energies. It appears that above incident ion energies in the 4-7 eV range the metal ion

cannot be sufficiently translationally cooled to react effectively. The majority of experiments were carried out at 3 eV incident ion energy and 3 x 10 -5 Torr indicated cell pressure. Table 2 summarizes the results obtained for the reaction of the first row transition metal ions Scthrough Cu + with cyclopentene under these conditions. We have not included in the table the minor yield of CsH~- observed nor its condensation product CsHv(CsHs) +. The results obtained for Sc + are in agreement with the ICR results of Lech and Freiser [64] who reported Sc+(C5H6) as the dominant (>98%) primary reaction product with further reaction of this product to give Sc+(C5H5)2 . Jacobson and Freiser [65] have reported that Fe +, Co ~ and Ni + react with cyclopentene to give M+(CsH6); the minor yield of Fe+(CsHs) observed in the present study may represent reaction of electronically excited Fe + or may be the result of conversion of kinetic energy to internal energy to overcome the reaction endothermicity Fe + + c-C5H8 --, Fe+(CsHs) + H 2 + H A H ° = 14 kcal tool-t

( 1)

derived using AH~(Fe +) = 280 kcal tool -1 [66], AH~(c-CsHs) = 8 kcal mol -I [66], AH~(H) = 52 kcal tool -t [66] and AH~(Fe+)CsHs))= 250 kcal mol -I [67]. In related studies, Peake

Table 2 Products from M+/cyclopentene reaction (% of base peak) Product M+ M~(CsHs) M ~ (C5H6) M ' (CsH 8) M " (C5H5)2 M + (CsHs)(CsH6) M+(CsH6)2 M ~(CsHs)(CsH8) M +(C5H6) (C5H8) M + ((75H8)2

Sc

Ti

26 8 100

73 10 12 100 16

V 40 10 5

Cr

Mn

64

100 1

12

6

100 37

Fe

Co

Ni

Cu

29 2 4

9

4

23

7

4

6 16

100 7

73

100

8 100

100 5 100

255

20

256

J. Ni, A.G. Harr&on/lnternational Journal of Mass Spectromet O, and Ion Processes 146/147 (1995) 251 260

M+..--~ + H2 ,~

H H.I~/I+--~

Scheme1. et al. [68] have reported that high energy collision-induced dissociation of the Fe + cyclopentene complex yields primarily Fe+(CsH6) with a minor yield of Fe+(CsHs). Jacobson and Freiser [65] reported that Fe+(CsH6) reacted further with cyclopentene to yield Fe+(CsHs)2 which we also see as a major product; the small yield of Fe+(CsHs)(CsH6) observed in the present study arises by further reaction of the primary product Fe+(CsHs) (see below). Jacobson and Freiser [65] also reported that reaction of Co+(C5H6) with cyclopentene gave Co+(C5H6)2 and Co+(C5H5)2 with relative yields of 19% and 81% respectively. We observe both products but with relative yields of 58% and 42%. However, as shown in Fig. 2, the relative yields are pressure dependent, with Co+(C5H5)2 being more abundant than Co+(C5H6)2 at lower pressures. This is consistent with collisional deactivation of Co+(C5H6)~ which would be expected to occur more readily in our system than in the low pressure ICR study. It is interesting to note that, in ion-beam studies, Armentrout and Beauchamp [69] observed that Co + dehydrogenated cyclopentane to yield Co+(C5H6). Jacobson and Freiser [65] reported that reaction of Ni+(CsH6) with cyclopentene gave a variety of products including Ni+(C6H6) (7%), Ni+(CsH6)(CzH2) (38%), Ni+(CsH6)(C3H6) (9%), Ni+(CsH6)2 (15%) and Ni+(CsH6)(CsHs) (31%) and noted that this was surprising since Ni+(CsHs) has been found [70] to react with cyclic alkanes to give dehydrogenation products only. As

noted from the results in Table 2 we observe only clustering of Ni+(CsH6) to form Ni+(CsH6)(CsH8); again it is possible that the higher pressures in the present study lead to more efficient collisional stabilization of the addition complex. The reactions of Ti +, V +, Cr +, Mn + and Cu + with cyclopentene do not appear to have been studied previously. The present results show that Cr + and Cu + predominantly cluster with cyclopentene to form M+(CsHs) and M+(CsH8)2 while Yi+ and V + react to form both M+(CsHs) and M+(CsH6) as primary products with M+(CsH5)2 being the dominant secondary product arising, presumably, by further reaction of M+(CsH6). The lower yield of M+(CsHs)(CsH6) (M = Yi, V) probably originates by reaction of M+(CsHs) which appears to be less reactive than M+(CsH6). The Mn + ion is quite unreactive but does react to a slight extent to give mainly the cluster ions Mn+(CsHs) and Mn+(CsH~)2. Under the same experimental conditions the unreacted Mn + ion signal represents 76% of the total ion signal compared to 4% (Ni +) to 36% (Cr +) for the other transition metal ions. The results of Table 2 show that the singly charged transition metal ions, with the exception of Cr +, Mn + and Cu +, effect dehydrogenation of cyclopentene. With the exception of Ni+(CsH6), the resultant M+(CsH6) species react further with cyclopentene to produce the metallocene ion M+(CsHs)2; Ni+(CsH6) only clusters with cyclopentene while Co+(C5H6) reacts in part to produce Co+(C5H6)2 . A mechanism for the initial dehydrogenation reaction is proposed in Scheme 1 [71,72]. Initially, an M + cyclopentene adduct is formed. Although M + cyclopentene bond strengths are not known, M+-C2H4 bond strengths [31,73-79] range from 26 kcal tool -1 for Cu+-C2H4 [31] to 51 kcal tool -1 for V +-C2H4 [73]. Although theoretical calculations by Sodupe et al. [80] suggest that the Cu+-C2H4 bond energy is low and the

J. Ni, A.G. Harrison~International Journal ql" Mass Spectrometry and 1ot~ Processes 146; 147 ( 1995 J 2.51 260 Table 3 Bond energies and estimated AH:, values M

D(M* -CH~) ~

D(M+-H) ~

A H ; ~'

Sc Ti V Cr Mn Fe Co Ni Cu

59 58 50 30 51 58 51 47 30

56 54 48 33 48 50 47 40 22

-33 30 16 +19 -17 -26 -16 -5 +30

~' Bond energies (kcal t o o l i ) from Refs. [82,83].

M ~ + c-CsH 6 --+ HM+(CsH7)

(2)

the standard enthalpy change is given by AH~ = D(c-CsHT-H) = D(M+-c-CsH7) - D(c-CsH7M+-H)

addition, the strengths of the second bond to the central metal ion appears [82,83] to be similar to the strength of the first bond and we have approximated D(c-CsHTM+-H) as D ( M + - H ) for which values are known [82,83] (Table 3). Within these approximations we estimate the AH~ values given in the final column of Table 3. For Sc + , Ti +, V +, Fe + and Co + , the estimated values of AH~, are substantially negative and provided the energy barrier for the process M+-c-CsH~, ~ H - M + - c - C s H 7

V ! - C 2 H 4 bond energy is too high, the clustering reaction should be substantially exothermic and should occur readily. Within this activated complex, transfer of a fl-hydrogen results in a hydridoallylic species which rearranges by a second H-transfer to produce H2M+(CsH6) which reductively eliminates He. We believe that it is the inability of the Cr+-c-CsH6 and the Cu+-c-CsH8 complexes to undergo rearrangement to the hydridoallylic species which prevents these metal ions from ultimately dehydrogenating cyclopentene and thus distinguishes their reaction from that of the other transition metal ions. For the reaction

(3)

where D(c-CsHv-H) = 82.3 kcal tool -i [81]. Unfortunately, metal ion-CsH7 bond strengths are not known nor are D ( c - C s H 7 M ÷ - H ) values available, so only approximate estimates of AH.~ can be made. In general, metal ion--carbon single-bond strengths appear to be relatively independent of the nature of the carbon [82,83] and we have approximated D(M+-c-CsH7) by D ( M + - C H 3 ) for which values are known [82,83] (Table 3). In

257

(4)

does not exceed the M * - c - C s H s bond strength (i.e. the internal energy contained by the M+-c-CsH8 complex), metal ion insertion should occur readily. This has been shown [71,72] to be the case for the Fe v cyclopentene reaction. On the other hand, A H ) is decidedly positive for the reaction of Cr + and Cu +, indicating that the activation barrier for reaction (4) is greater than the internal energy of the initial metal ion cyclopentene complex and that formation of the hydridoallylic complex will not occur significantly under our experimental conditions. The lack of reactivity of Cr + and Cu + arises from the weak M ~ - C H 3 and M ~- H bonds formed by these metal ions. The rationale for these weak bonds has been discussed elsewhere [82 84] and is clearly related to the d 5 configuration for Cr + and the d m configuration for Cu ~ . Our estimate of AH~ for the Ni ~ cyclopentene system is only slightly negative; however, the observation of Ni+(CsHD as the primary reaction product both in this work and in the ICR study [65] indicates that A H ; truly must be negative. Finally, although A H ) is quite negative for the Mn +-cyclopentene system, Mn + is quite unreactive, as noted above, and what little reaction is observed is primarily clustering rather than dehydrogenation. We do not have a satisfactory explanation for the overall unreactivity of Mn + compared to the other transition metal ions.

258

J. Ni, A.G. Harrison/International Journal o f Mass Spectrometry and 1on Processes 146/147 (1995) 251-260

Table 4 Reactions of M+(c-CsHs) with cyclopentene (% of total product signal) Product

Fe+(c-CsHs) Co+(c-CsH5) Ni+(c-CsHs)

M+(CsHs)2 41 M+(CsHs)(CsH6) 59 M + (C5H6)2 M+(CsHs)(CsH8) M + (C5H6)(C5H8) M+(CsHs)2

59 2 67

1 53 13 29 5

3.1. Reactions of M+ (c-C5H5) with cyclopentene The M+(c-CsHs) (M = Fe, Co, Ni) ions were prepared by dissociative ionization of the appropriate metallocene in the electron impact ion source. The mass-selected ions were introduced into the quadrupole collision cell containing cyclopentene at 3 x 10-5 Torr indicated pressure; the incident ion energy was 3 eV. The products observed and their relative yields are recorded in Table 4. The reaction of Fe+(c-CsHs) with cyclopentene under these conditions leads to the two products Fe+(CsHs)(CsH6) and Fe+(CsHs)2 in the ratio 59 : 41, presumably formed by the reactions

4. Conclusions

Fe+(c-CsHs) + c-C5H8 Fe+(CsHs)(CsH6) + H2 Fe+(CsHs)2 + H2 + H

Co+(C5H5)2 may originate, in part, by elimination of an H atom followed by elimination of H2 rather than the reverse sequence and that collisional stabilization of the H-loss product, Co + (C5H5) (C5H7), occurs. The reactions of Ni+(c-CsHs) with cyclopentene are complex. We observe the simple cluster product Ni+(CsHs)(CsHs) (13%) as well as the dehydrogenation product Ni+(CsHs)(CsH6) (53%). It is interesting to note that Ni+(CsH6) is unable to dehydrogenate cyclopentene (Table 2) while the reaction of Ni+(c-CsHs) leads to significant dehydrogenation. Beauchamp et al. [70] have reported that Ni+(c-CsHs) reacts with cyclopentane to give the dehydrogenated products Ni+(CsHs)(CsHs) (49%) and Ni+(CsHs)(CsH6) (51%). In addition to the cluster and dehydrogenation products, we observe Ni+(CsH6)(CsHs) (29%) and in minor yield (5%) Ni+(CsH8)2 . The mechanisms by which these products are formed are not clear but may involve displacement of the c-CsH 5 ligand in Ni+(CsHs)(CsH6) and Ni + (C5H5) (C5Hs) by cyclopentene.

(5) (6)

Jacobson and Freiser [85] have observed the same two reactions and reported that the yield of Fe+(CsHs)2 decreased as the pressure of argon in the ICR cell increased; they attributed this effect to collisional stabilization of Fe+ (C5H5) (C5H6). Jacobson and Freiser [85] observed only Co+(CsHs)z in the reaction of Co+(c-CsHs) with cyclopentene. Although this is the major product that we observe (Table 4), we also observe C o + ( C s H s ) ( C s H 7 ) ( o r C 0 + ( C 5 H 6 ) 2 ) in 39% yield. The observation of the latter product suggests that formation of

The present work has shown that singly charged transition metal ions may be produced in good yield by fast atom bombardment of metal salts. After mass selection these ions can be introduced into an r.f.-only quadrupole collision cell to react with organic substrates at low incident ion energies. The results obtained in the present study for the reactions of Sc+, Fe + and Co + with cyclopentene are in good agreement with results obtained in earlier ICR studies. This agreement lends credence to the use of the tandem mass spectrometric technique to study metal ion reactions. The present tandem mass spectrometric approach provides an alternative method for

J. Ni, A.G. Harrison~International Journal of Mass Spectrometry and hm Processes 146/147 (1995) 251 260

establishing the reactions of metal ions with organic molecules. However, this approach does not provide the control of incident ion energy which is achieved by ion-beam techniques [49,51] nor does it provide the capability of CID studies to establish the structures of the product ions which is possible with the FT-ICR approach [47,48]. Thus, the results obtained by the tandem MS approach are more qualitative in nature, but should prove capable of distinguishing between isomeric molecules. A further paper will explore this use to characterize the C5H8 isomers.

Acknowledgments We are indebted to the Natural Sciences and Engineering Research Council of Canada for financial support and to Fisons Instruments, Inc. for the loan of the fast atom gun and FAB source.

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