Structural characterization of mono- and bisphosphonium salts by fast atom bombardment mass...

ORGANIC MASS SPECTROMETRY, VOL. 28, 71-82 (1993)

Structural Characterization of Mono- and Bisphosphonium Salts by Fast Atom Bombardment Mass Spectrometry and Tandem Mass Spectrometry

J. Claereboudt and W. Baeten Department of Pharmaceutical Sciences, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Antwerp, Belgium

H. Geise Department of Chemistry, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Antwerp, Belgium

M. Claeys* Department of Pharmaceutical Sciences, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Antwerp, Belgium

Positive-ion fast atom bombardment (FAB) mass spectra are reported for a representative series of mono- and bisphosphonium halides derived from triphenylphosphine. The mass spectra of the monoalkyltriphenylphosphonium salts typically contain abundant intact cations that can be used to establish the cationic relative molecular mass and diagnostic fragment ions that allow the characterization of structural subgroups. Depending on the functional group substitution on the alkyl group, additional fragment ions are observed which are formed by loss of small neutral molecules from the intact cation and that can be used for the differentiation of isomeric phosphonium salts. Molec- ular dications are typically observed in the FAB mass spectra of the bisphosphonium salts when they are analysed in Initrobenzyl alcohol. In addition, production of singly charged ions by clustering with a counter ion, decomposition involving removal of one of the charge centres and one-electron reduction are generally observed. Structurally diagnostic fragments are also obtained. The fragmentation pathways of the ions derived from the phosphonium salts were elucidated by precursor ion and product ion tandem mass spectrometric experiments. For the phosphonium salts containing a long-chain hydrocarbon alkyl group, high-energy collision-induced decomposition of the intact cation is needed to obtain unambiguous structural information.

INTRODUCTION

Organophosphorus salts are important intermediates in synthetic processes.' In particular, alkyltriphenylphosphonium salts are frequently used in the synthesis of alkenes from carbonyl compounds by the Wittig reaction.' During the preparation of these versatile synthetic intermediates, rearrangement reactions can occur which may lead to unexpected products with ambigu- ous structure^.^ Thus, a rapid and reliable method, such as mass spectrometry (MS), for the structural characterization of these compounds is desirable. This study deals from the structural analysis of a series of mono- and bisphosphonium halides derived from triphenylphosphine, which are intermediates in the Wittig synthesis of conjugated oligomers of poly( 1,4- phenylene-ethenylene) and other poly(ary1ene- ethenylene) derivative^.^

Phosphonium salts resist ionization by traditional gas-phase methods because of their poor volatility and thermal instability. Attempts to volatilize these compounds prior to electron impact (EI) or chemical ionization (CI) generally result in thermal degradation of the salt. The mass spectra obtained by these conventional techniques contain ions corresponding to thermal deg-

radation products, which in some cases can still be related to the unperturbed structure. On the other hand, desorption ionization (DI) methods such as field desorption (FD),3,5-7 thermal desorption (TD),' laser desorption (LD),'-' secondary ion mass spectrometry (SIMS)" and fast atom bombardment (FAB)'0*'3-'5 have proven to be particularly useful for the direct analysis of thermally fragile phosphonium salts. In producing ions directly from the condensed phase, DI methods do not result in thermal decomposition of the sample by circumventing the requirement for vapor- ization. Organic onium salts almost invariably produce excellent DI mass spectra which contain not only abundant intact cations (C') and anions (A-), but frequently fragment ions which can be used to characterize their structure, and in some cases cluster ions such as [C,A] + and [CA,] -.

In this work we investigated the potential of fast atom bombardment for the analysis of an extended series of alkyltriphenylphosphonium salts (Tables 1 and 2). The goal of this study was to develop the spectrum- structure correlations necessary for full mass spectral characterization of these synthetic intermediates. The usefulness of FABMS for the structural differentiation of isomeric alkyltriphenylphosphonium salts will be demonstrated. This study also documents the potentials

0030-493X/93/020071-12 $1 1.00 0 1993 by John Wiley & Sons, Ltd.

Received 23 July 1992 Revised manuscript received 14 October 1992

Accepted 19 October 1992

J. CLAEREBOUDT ET ,415. 12

Table 1. Structures of the monophosphonium salts (1-26) studied

Ph3&H2 -&Xz A

I. X , : H . X , : H . A : C I

2. XI : H , X2 : -CH,, A : CI

3. X, . -OH. X,: H . A : Br

4. X , : H . X , : F . A : C I

5. XI : H , X2 : -NO2. A : Br

6. XI : H , X2 : - C H = C H e C H , .

A : C l

+ 14. Ph$'CH2(CH2),CH3 Br-

15. Ph,PCH2(CH2)&H3 BT

16. Ph3PCH,(CH&,CH3 BT

17. m,PCH,(CH,),Br Br

IS. Ph,PCH,CH,OH CI-

19. Ph3PCH,0CH3 CI

0 + 20. Ph3PCH,(CH3,COH BT D D

7. Ph3&D2 4 D ~ r - D D 0

+ I1 21. Ph3PCH2CCH, C1-

Ph3;CH2 q x Br

8. X : H 22. Ph3kH2:-@ Br 9. X : -CH3

10. X : -CH=CH* 0 + II

23. Ph,PCH2CC€H2CH3 B r

24. Ph&H2+'] BT

Ph3&!H2 4 A-

l l n . A : CI

Ilb. A : Br "3 25. Ph3PCH2CH2 + Br

of tandem mass spectrometry (MS/MS) for establishing ionic relationships and for ion structure characterization. One of our objectives was to determine the additional structural information attainable from collision-induced dissociation (CID) of the organic salt cations; more specifically, high- and low-collision energy CID spectra will be compared.

Table 2. Structures of the bisphosphonium salts (27-40) studied

P h & I , -@ CH;Ph3 2C1

27. X. H 37. P h 3 k H 2 --@ CH$Ph, 2 C1

28. X . -CH,

38. P h 3 k H 2 CH2:Ph3 2 C1 29. X : -OCH3

+ 2 Br- 39. Ph3PCH2 M). X : -0-propyl

31. X : -0-isobutyl m + CHZPPh, 32. X : -0-pentyl

40. 33. X : Oisopenty l

34. X : -0-hexyl

35. X : -0-heptyl

36. X : -0-octyl

+ CHZPPh3 I

' + CH2PPh3

EXPERIMENTAL

Mass spectrometry

All mass spectral measurements were performed using a VG 70-SEQ hybrid mass spectrometer equipped with an Ion Tech saddle field atom gun. The FAB gun was operated with xenon gas, producing Xe atoms with 8 keV kinetic energy at a discharge current of 1 mA. Similar results could be obtained when the caesium ion gun was used (i.e. LSIMS) instead of the FAB gun. For FABMS the mass resolution of MS-1 was set to 1000 (10% valley definition). Mass spectra were recorded under computer control with an exponential down scan of 2 s per decade. The intermediate detector was used for data acquisition. The FAB mass spectra shown in the figures represent the data averaged from 20 scans. The spectra were corrected for contributions from the solvent by applying background subtraction for each matrix used.

Accurate mass measurements in FABMS were performed at 10000 resolution (10% valley definition) by linear accelerating voltage scanning. The high- resolution (HR) data were acquired in the multichannel analyser (MCA) mode, measuring the mass of an unknown peak against two mass references originating from the matrix or the sample itself. The accuracy of this linear voltage scan procedure at lo4 resolution was usually +_2 ppm. Product ion and precursor ion tandem mass spectra were obtained by passing ions selected by the sector mass spectrometer (MS-1) into the quadrupole collisions cell (Q and quadrupole mass analyser (MS-2). No collision gas was present in Q1 when metastable ion decompositions (MID), i.e. unimolecular decomposition processes, were studied. Low-energy (i.e. 1-500 eV, laboratory frame) CID spectra of mass-selected precursor ions were taken by activating the ions in the quadrupole collision cell using argon as collision gas and by scanning the quadrupole mass analyser to separate the collision products. The argon pressure measured with an ionization gauge outside Q 1 was 8 x mbar (1 bar = lo5 Pa), rep- resenting & mbar in the 254 mm long collision quadrupole. For most FABMS/MS experiments the double-focusing part (MS-1) of the hybrid instrument was operated at a mass resolution of 1000-1500, with the quadrupole mass filter (MS-2) operated at unit mass resolution. High energy (i.e. 8 keV, laboratory frame) CID spectra were obtained by using the collision cell in the first field-free region (1st FFR) and performing linked scans at constant B/E to study product ions. Helium was used as the collision gas at pressures which attenuated the precursor ion beam by 50-70%. All MSfMS scan data were acquired in the MCA mode of the V G 11-250 J data system to facilitate evaluation of the quality of raw data, with 10-15 scans typically being accumulated.

Materials and sample preparation

Several of the phosphonium salts studied (1-13, 27-40) were prepared in the laboratory of organic synthesis

STRUCTURAL CHARACTERIZATION OF MONO- A N D BISPHOSPHONIUM SALTS 73

using standard procedure^.^.'^ All other phosphonium halides (14-26) and the FAB matrices glycerol and 3- nitrobenzyl alcohol (3-NBA) were obtained com- mercially (Janssen Chimica).

For FABMS the salts were dissolved in either glycerol or 3-NBA, with 2 pI of matrix containing 2-5 pg of salt placed on the FAB probe. The use of two different matrices was necessary in order to identify fragment ions of the phosphonium salts which interfere with matrix related ions. For example, glycerol (G) forms positive ions at m/z 93, 185, 277 and 369, i.e. [Gn + HI+ ions. Fragment ions at m/z 185 and 277, which originate from the fragmentation of phosphonium salts (e.g. salt 10) were therefore identified by using 3-NBA as matrix.

RESULTS AND DISCUSSION

Monophosphonium salts

The monophosphonium salts have the general formula [Ph3PR]+A- (or C'A-) with C' = [Ph3PR]+ (=phosphonium cation), A- = C1- or Br- (=salt anion) and R = alkyl substituent.

FABMS analysis

The positive-ion FAB mass spectra of a series of 26 alkyltriphenylphosphonium salts (1-26) are discussed.

c' 353

(D Jrc - wj' I

Figure 1. Positive-ion FAB mass spectrum of benzyltriphenylphosphonium chloride (1). Matrix: 3-NBA.

Negative-ion FABMS data will not be reported here, because the only feature of interest in the negative-ion spectra of the phosphonium salts is the peak which corresponds to the halide anion (A-). The mass of this peak can be used to characterize the anionic counter ion.

The positive-ion FAB mass spectrum of benzyltriphenylphosphonium chloride (1) is illustrated in Fig. 1. The base peak in the spectrum corresponds to the intact cation (C'). The peaks at m/z 741 and 743 represent the singly charged cluster ions, composed of two salt cations and one chlorine anion ([C,A]'). The intensities of these peaks show the appropriate isotope ratio for the [C,Cl]+ cluster ions. The most important fragmentation reactions for salt 1 are outlined in Scheme 1. The fragmentation pathways were supported

[(Ph3PCH,Ph)2CI]+ mlz 741 ~ - [Ph,PCH,Ph Cl-1 cF mlz Ph3PI'. 262

I + 1 l -ph . \ desorption of

+ IPh$CHzPh]+ intact cation

mlz 353

I I Ph3PCH2Ph CI-

/ \ 1 A&

Ph3P=CHPh

mlz 352

\ -Ph,P \ L

PhCH-J

Ph / \ CH,Ph &@ mlz91 - I

P mlz 351 Ph

PhCHPh]+

m/z 167

mlz 261

[ph2Pl+ FW+.

mlz 185 mlz 108

1 -H2 1 - H

mlz 183

[@I@]+.

mlz 152

Scheme 1. AT= thermal degradation; El =electron impact ionization

14 J. CLAEREBOUDT ET AL.

by performing the appropriate product ion and precursor ion MS/MS experiments. A striking feature of the FAB mass spectrum is the simple nature of the fragmentation processes observed. Complementary fragment ions, characteristic of the functional subgroups of the phosphonium salt, are detected at m/z 91 and 262. The abundant [32% relative abundance (RA)] fragment ion at m/z 91 is formed by direct cleavage of the phosphorus-methylene bond with charge retention on the benzyl substituent (i.e. R+). The fragment ion at m/z 262 (16% RA) corresponds to the triphenylphosphine radical ion [Ph,P] + * which is formed by homolytic cleavage of the bond between the phosphorus and the benzyl substituent. The occurrence of this odd-electron fragment ion was also noted in the FDMS3 and LMMS' of benzyltriphenylphosphonium chloride (1). The [Ph,P]+' ion, which is indicative of the triphenylphosphonium structure, also serves as the precursor of the fragment ions at m/z 261, 185, 183, 152, 108 and 107 (Scheme 1). A review of all spectral data reveals that the latter ions, together with some other ions, which are listed in Table 3, are common to the FAB mass spectra of all phosphonium salts investigated (1-40). Ions such as those at m/z 183 [C12H8P]+ and 185 [C12HloP]+ are also observed in SIMS," LMMS" and FD-CADMS7 of triphenylphosphonium salts, and are commonly observed in electron impact (EI) ionization mass spectra of triphenylphos- p h i n e ~ . ' ~ , ' ~ Deuterium-labelling experiments in the latter studies demonstrated that the loss of a phenyl radical from the triphenylphosphine radical ion at m/z 262 produces the ion at m/z 185 (Scheme 1). Loss of one hydrogen from each of the two phenyl rings was shown to yield the bridged 9-phosphafluorenyl ion at m/z 183.

The FAB mass spectrum of salt 1 also shows an ion at m/z 352 (14% RA), which can be rationalized by loss of a hydrogen radical from the intact cation. The very low abundance (<0.1%) of the [C - 13" ions in the product ion tandem mass spectrum of the intact cation, however, indicates that loss of a hydrogen radical is not a major fragmentation reaction. It is therefore suggested that the majority of the [C - 11 +' ions at m/z 352 result from a beam-induced surface reaction, rather than from a gas-phase fragmentation. A possible explanation is that the ions at m/z 352 are formed by EI ionization of the major thermal degradation product, resulting in [M - HCl]" or [C - 13'' ions (M = salt molecule). The high stability of the [C - 11'' ion obviously results from a participation of the d orbitals from phosphorus

Table 3. Ions common to the positive-ion FAB mass spectra of the alkyltriphenylphosphonium salts investigated'

m h Ion mlz Ion mlz Ion

275 [C,,H,,P]+ 165 [C,,H,]+ 109 [C,H,P]+ 263 [C,,H,,P]+ 152 [C,,H,]+' 108 [C,H,P]+' 262 [C,,H,,P]+' 141 [C,,H,]+ 91 [C,H,]+ 261 [C,,H,,P]+ 133 [C,H,P]+ 77 [C,H,]+ 185 [C,,Hl0P]+ 121 [C,H,P]+ 183 [C,,H,P]+ 115 [C,H,]+

a Empirical formulae were determined by accurate mass measurements (i.e. high-resolution FABMS).

in the bond formation, leading to ylid-type ions, e.g. [Ph,P=CHPH] +*. The product ion tandem mass spectrum of [C - 11'' (m/z 352) shows that this ion is the precursor of two other fragment ions, detected at m/z 351 and 167 in the FAB mass spectrum of salt 1. The fragment ion at m/z 351 (10% RA) is formed by loss of a hydrogen radical from the [C - 11" ions, leading to formation of a bridged [C - 23 + ion, shown in Scheme 1. A similar fragmentation reaction was reported by Williams et a1." in the case of the EI mass spectrum obtained for methylene-triphenylphosphorane, which was generated in the ion source by pyrolysis of methyltriphenylphosphonium bromide. Formation of the fragment ion at m/z 167 (5% RA) implies expulsion of a Ph2P' radical from the [C - 11 + * ion with subsequent recombination of the aromatic nuclei, yielding the diphenylmethyl cation (Scheme 1). This fragment is generally formulated as the [PhR - HI' ion (Table 4). Similar rearrangement reactions were reported by Bud- zikiewicz et d.'* in EIMS of organophosphorus compounds.

The FAB mass spectrum of benzyltriphenylphosphonium chloride (1) is representative of the FAB mass spectra of the monophosphonium salts investigated, which usually consist of the intact cation as the base peak and a few structurally informative fragment ions with relative abundances <30%. This mass spectral behaviour is characteristic of most classes of pre- charged onium compounds, reflecting the efficiency of the sputtering process for preformed ions and the relative softness of the ionization method. In most cases, two cation-one anion cluster ions, i.e. [C2A]', are also observed. The relative abundance of these cluster ions is higher in the spectra obtained with 3-NBA than with glycerol as the FAB matrix. The detection of both ions, C + and [C2A]', permits the determination of the relative molecular masses of the salt molecule and its preformed components, the cation and the anion.

The FABMS data obtained for the deuterated analogue (7) of benzyltriphenylphosphonium chloride, a series of substituted benzyltriphenylphosphonium halides (2-6) and six other monophosphonium salts (8-13) support the mass spectral pattern described for benzyltriphenylphosphonium chloride (1) (Table 4). A common structural feature of these salts is that the substituent R corresponds to a -CH2-aryl group. Table 4 only lists the structurally informative ions. For the other ions, which are generally observed in the spectra of the triphenylphosphonium salts, e.g. m/z 185, 183 and 165 ions, reference is made to Table 3. Typically, the intact cation is the base peak in the spectra (except for 10 and 13). Other important ions correspond to the diagnostic ions previously reported for salt 1, i.e.

and R+ (Table 4). A comparison of the results obtained for the various substituted benzylphosphonium salts (1-6) also indicates that the functional groups on the benzyl substituent can trigger alternative fragmentation routes. This is demonstrated in the FAB mass spectrum of 2-hydroxytriphenylphosphonium bromide (3) (Table 4), showing two additional fragment ions at m/z 351 and 263 which originate from loss of a H 2 0 and a C,H,O molecule, respectively, from the intact cation. The formation of both fragment ions is probably induced by

[C2A]+, [C - l]", [C - 2]+, [Ph,P]+', [PhR - HI+

STRUCTURAL CHARACTERIZATION OF MONO- AND BISPHOSPHONIUM SALTS 15

Table 4. FABMS of monophosphonium salts 1-13 (R = --CH,aryl)'

Compound [ C p ] '

1 741 (6) 2 769 (2) 3 817 (2)

4 777 (6) 5 875 (1)

6 973 (2)

7' 799 (3)

8 765 (3)

9 793 (4) 10 969 (2) l l a 753 (6) l l b 797 (4) 12 841 (3)

13 941 (4)

353 (100) 367 (100) 369 (100)

371 (100) 398 (100)

469 (1 00)

360 (100)

343 (1 00)

357 (loo) 445 (87) 359 (1 00) 359 (loo) 403 (1 00)

453 (91)

[C - 1]+'

352 (15) 366 (9) 368 (18)

370 (12) 397 (8)

468 (15)

359 (42)

342 (1 1)

356 (13) 444 (1 6) 358 (13) 358 (1 5) 402 (10)

452 (13)

[ C - 2 ] +

351 (10) 365 (7) 367 (6)

369 (8) 396 (6)

467 (9)

358 (17)

341 (3)

355 (5) 443 .( 3) 357 (5) 357 (6) 401 (6)

451 (7)

[Ph,P]+'

262 (1 6) 262 (11) 262 (22)

262 (14) 262 (17)

262 (24)

262 (15)

262 (18)

262 (20) 262 (34) 262 (1 8) 262 (1 9) 262 (12)

262 (27)

[PhR -HI'

167 (5)

183 (23)e

185 (14)e

181 (3)

283 (2)

173 (3)

157 (1)

171 (1) 259 (1) 173 (4) 173 (3) 217 (5)

267 (5)

91 (3)2 105 (57) 107 (24)

109 (31) -

207 (68)

192 (11). 191 (11):

98 (20)

81 (41)

95 (87)

97 (48) 97 (53)

141 (67)

191 (100)

a FA8 matrix 3-nitrobenzyl alcohol. m/z with RA values (Yo) in parentheses. blon masses referenced to isotopes 35CI and

Ions common to the FA8 spectra of all salts investigated are listed in Table 3 and are, therefore, not repeated here. Extraneous ions in the spectra can be due to contaminants from synthesis.

e lom at m/z 183 and 185 overlap with the [Ph,P - H,]' and the [Ph,P]+ fragment ions of triphenylphosphine (cf. Table 3) 'Isotopic purity of salt 7 is d, (71 Yo), d, (25%). d, (4%).

Others',d

369 (5); 197 (3) 383 (3) 475 (4): 383 (6): 351 (4);

387 (5) 414 (5): 382 (12);

263 (24)

352 (14): 279 (3): 277 (4). 201 (4); 90 (3)

485 (6). 209 (9); 205 (7); 202 (41): 193 (5);

189 (4): 179 (5); 178 (6)

277 (4): 97 (7)

279 (8): 201 (4)

367 (7); 375 (3): 357 (7).

359 (4); 313 (4); 311 (5):

373 (4). 313 (5); 43 (4) 461 (5) 455 (3): 375 (4): 371 (7) 455 (2): 375 (5). 371 (3) 419 (3): 216 (3); 215 (7):

469 (4); 265 (8); 252 (5): 202 (4)

215 (4); 207 (5). 203 (4): 202 ( 6 ) ; 193 (4); 190 (9); 189 (11); 178 (6)

the phenolic hydroxyl group on the benzyl substituent. In the spectrum of 4-nitrobenzyltriphenylphosphonium bromide (S), additional fragment ions correspond to the [C - 01' ion at m/z 382 and the [C - NO2]+' ion at m/z 352. The fact that the R+ ion (m/z 136) is not observed, but instead an [R - NO2]+' ion (m/z 90) is detected, indicates that the stability of the R' ion is lowered by the presence of a nitro group. The ability to detect the R+ ions of the monophosphonium salts appears to be related to their stability in the gas phase. High-abundance R+ ions are usually detected when R corresponds to a stable alkyl structure without specific functional groups which can trigger the fragmentation. This is the case for most of the salts listed in Table 4 (except salt 5). For salts 10 and 13, the R+ ion even represent the base peak in the FAB mass spectra.

Comparison of the data obtained for (thiophenyl-2- y1)- methyltriphenylphosphonium chloride (l la) and its bromide analogue (llb) illustrates that the halide counter ion has little effect on the ion abundances (Table 4).

Another point of interest is the detection of [C + 161' ions (3-5% RA) in the FAB mass spectra of several of the phosphonium salts analysed in 3-NBA (Table 4). Accurate mass measurements and product ion MS/MS experiments reveal that these ions consist of the intact cation and one oxygen. Formation of [C + 16]+ adduct ions probably involves a reaction of the organic salt cations with the liquid matrix, and is related to the energy deposition by particle bombardment. Adduct ions are also observed when glycerol is used as the FAB matrix. In addition to [C + 16]+ ions, [C + 14]+ adduct ions are detected in glycerol, which are probably formed by a methyl/hydrogen substitution reaction.

Because it was not the aim of this study to investigate the exact mechanism of the formation of these adduct ions, attention is drawn only to the existence of these ion species and to the fact that care must be exercised during interpretation of mass spectral data, since fragment ions originating from these artifact ions are sometimes observed. Detection of [C + 141' and [C + 161' adduct ions in the FAB mass spectra (matrix = glycerol or butane- 1,2,4-triol) of phosphonium salts was reported by Kroha and Busch,', who postulated that these ions are products of beam-catalysed intermolecular interactions between intact cations and fragments of the salt or the matrix solvent. For an excellent review describing the most prominent types of chemical reactions induced upon energy deposition by particle bombardment, reference is made to the study by Detter et ~ 1 . ' ~ Other so-called artifact ions, encountered when the phosphonium salts are analysed in glycerol correspond to the [Ph,POH]+ ion at m/z 279 and the [Ph,POCH,]+ ion at m/z 293. The empirical formulae of the latter ions were determined by accurate mass measurements. The lower abundance of artifact ions and the higher abundance of the [C2A]+ ions in the spectra obtained with 3-NBA make this matrix pre- ferred to glycerol for the analysis of phosphonium salts. For the other monophosphonium salts analysed in this study (1426), the substituent R on the triphenylphosphonium charge centre varies from an aliphatic hydrocarbon group (14-16) to an alkyl group containing various functional groups, including halide (17), alcohol (18), ether (19), acid (20), keto (21, 22), ester (23), dioxolane (24), dioxane and dithiane (26) functions. The influence of the difference in functional group substitution on the fragmentation behaviour of the corre-

16 J. CLAEREBOUDT ET A L

sponding phosphonium salts will be discussed in the following section.

The positive-ion FAB mass spectra of the three phosphonium salts containing an aliphatic hydrocarbon substituent (14-16) have a similar appearance. The spectrum of n-dodecyltriphenylphosphonium bromide (15) is representative of the spectra of this class of compounds (Fig. 2). The signal of the intact phosphonium cation [Ph3PC1,H2J+ (m/z 431) is intense. Elimination of the alkyl radical yields the stable triphenylphosphine molecular ion (m/z 262). The triphenylphosphine ion shows further loss of a phenyl radical (m/z 185) and H, (m/z 183), which is in agreement with the fragmentation described for salt 1 (Scheme 1). The fragment ion at m/z 263 corresponds to protonated triphenylphosphine [Ph,PH]+, which can be rationalized by a hydrogen rearrangement process. The ions detected in the low mass range correspond to the commonly observed fragment ions listed in Table 3. An additional fragment ion is seen at m/z 199, which results from loss of a phenyl radical, followed by a-cleavage of the dodecyl substituent yielding a diphenylmethylene ion, i.e. [Ph,P=CH,]+. The R+ ion, corresponding to the entire alkyl substituent of the salt, is not observed. Instead, the FAB spectrum contains a series of low- abundance fragment ions which result from losses of elements of methane through undecane, i.e. [C - CnH2,,+,]+ with n = 1 (m/z 415), n = 2 (m/z 401) to n = 11 (m/z 275). This set of unique fragment ions, which originate from the fission of each C-C bond in the cation, can be directly correlated with the structure of the aliphatic alkyl group (R). The significant higher abundances of the ions formed by loss of C,,H,, (m/z 289) and Cl,H,, (m/z 275) compared with elimination of the shorter chain alkanes can be explained by the fact that these fragmentation reactions, which occur proximate to the site of ionization, are directed by the charge (see below). The other ions of the [C - CnH2n+2]+ series (i.e. n = 1-9) are of such a low abundance that they are sometimes obscured by the background chemical noise (including matrix ions). We therefore considered it of interest to evaluate whether FAB MS/MS in combination with CID of the phosphonium cations allows the detection of these structurally informative fragment ions. The results of the MS/MS study are illustrated in Fig. 3. The high collision energy (8 keV) CID product ion spectrum of the

'7 [Ph,P - HJ'

Figure 2. Positive-ion FAB mass spectrum of n-dodecyltriphenylphosphonium bromide (1 5).

intact cation [Ph,PC,,H,,]+ (m/z 431) shows extensive fragmentation, which is consistent with its structure [Fig. 3(a)]. All possible alkane losses (n = 1-11) are observed. An interesting aspect of the fragmentation in the high-collision energy CID spectrum is the appearance of two regions of ion stability. This pattern is also seen in the FD-CID spectra reported by Veith.' The four fragment ions at m/z 262, 263, 215 and 289 com- prise the more abundant group, which are produced by fragmentations occurring proximate to and are directed by the charge site. The other alkane losses, [C - CH,]' through [C - Cn-3H2n-4]+ (for n > 4) occur remote from the charge site and are probably not charge stabil- ized. Consequently, these ions show a lower relative abundance. This new class of CID reactions, which are not initiated by either charge- or radical-localized sites, are called charge-remote fragmentations (CRF). CRF were extensively studied by Gross and co -worke r~ .~ ' -~~ They were reported for the first time by Lyon and co- workers in the MS/MS of anionic2' and cationic" sur- factants using CID at 8 keV. CRF also proved to be particularly versatile in determining structural features of other classes of compounds containing long chain alkyl groups.22 For a detailed discussion on the analyti- cal applications and mechanisms of CRF, reference is made to an excellent review paper by Adams.,, CRF are important for the analysis of alkyltriphenylphosphonium salts because they provide structural information on the part of the organic cations which is not directly associated with the phosphonium charge centre.

Ill/.?

Figure 3. Product ion tandem mass spectra of the n - dodecyltriphenylphosphonium cation (m/z 431 ) : (a) CID, 1st FFR, He. E , , , , = 8 keV; (b) CID. Q,, Ar, E,,,, =20 eV; (c) CID, 0,. Ar, E,,,, = 200 eV.

CRF appear to occur mainly for high-energy (keV) collisions, which may explain why they are not present in the low collision energy (20 eV) CID product ion spectrum of the intact cation [Fig. 3(b)]. The product ions detected in the low collision energy CID spectrum are formed by charge site initiated mechanisms. Figure 3(c) demonstrates that it is possible to induce CRF of the phosphonium cations when the collision energy in the quadrupole collision cell is increased above 100 eV. Fragments resulting from losses of CnH2"+, for n = 9-4 are clearly visible at a collision energy of 200 eV. The collision energy onset for the formation of these fragments appeared to be in the region of 100 eV. This is lower than previously reported values for the CRF of quaternary ammonium cations (200 eV),' and stearate anions (400 eV),26 which were also analysed with a hybrid instrument. The different values indicate that the energy requirement for CRF appears to be compound dependent.27 It is also of interest that in the case of low collision energy data [Fig. 3(c)] the envelope of CRF peaks is considerably less intense, in addition to being weighted towards low mass compared with the 8 keV data shown in Fig. 3(a). The altered envelope of product ions using Q-scans was previously noted and may represent an instrumental artifact, which discriminates against product ions in the upper mass range.25,27

The FABMS data for the substituted alkyltriphenylphosphonium salts (17, 20, 21 and 25) are summarized in Table 5. The mass spectra of the other substituted salts (18, 19, 22-24, 26) are discussed in the next two sections. The intact cation is the base peak in all spectra (except for 26). Each mass spectrum also contains structurally relevant fragment ions which are related to the functional group substitution. These fragment ions are formed by loss of small neutral molecules from the intact cation (in analogy to the loss of C,H,, + , characteristic of the aliphatic hydrocarbon group of salts 1416). Rationalization of the fragment ions as the result of loss of neutral molecules is illustrated by the mass spectral behaviour of (Ccarboxy- buty1)triphenyLphosphonium bromide (20): expulsion of H 2 0 , CO,, HCOOH, CH,COOH, C,H,COOH and C,H,COOH from the intact cation (m/z 363) yields the fragment ions at m/z 245, 319, 317, 303, 289 and 275,

respectively (Table 5). The relative softness of the FAB ionization method is reflected in the propensity for these rearrangement reactions rather than direct cleav- ages. An exception to the systematic loss of neutral molecules from the intact cation is the formation of the [Ph,P]+' ion (m/z 262). This ion, found in the mass spectra of all the triphenylphosphonium salts investigated, is rationalized by a direct cleavage of the phosphorus-alkyl substituent bond. The driving force for the cleavage of the even-electron cation to form this odd-electron fragment ion is the stability of the tri- arylphosphine ion formed. The occurrence of this cleavage reaction was also noted in F D mass spectra,, but not in EI spectra of phosphorus compounds, where transitions from odd- to even-electron ions predomi- nate. '

The ion at m/z 303 in the spectrum of 3- bromopropyltriphenylphosphonium bromide (17) can be explained by loss of HBr from the intact cation (m/z 383) Table 5). A corresponding fragment ion at m/z 303 is also observed in the spectrum of n-butyltri- phenylphosphonium bromide (14) (not shown), and can be rationalized by loss of methane from the intact cation (m/z 319). The higher abundance of the fragment ion at m/z 303 in the spectrum of salt 17 (i.e. 25% RA vs. 1% RA for salt 14) indicates that the bromine atom substituted on the alkyl group directs the fragmentation of the phosphonium cation. Other fragment ions that are characteristic of the halide substituent (R) are detected at m/z 289 (loss of CH,Br) and m/z 275 (loss of C,H,Br). The R+ ion itself is not detected in the spectrum.

The spectral pattern of abundant intact cations and fragmentation via losses of neutral molecules is also found for salts 21 and 25 (Table 5). The R+ ions, corresponding to the alkyl substituent on the triphenylphosphonium structure, are not detected in the spectra. This is probably due to the fact that the functional groups on the alkyl substituent trigger the fragmentation of the intact cations, and to the low stability of the R+ ions in the gas phase. Evidence for this low stability is provided by the presence of low-m/z fragments of the alkyl group, e.g. ions at m/z 87, 59, 57, 41 and 31 in the spectrum of salt 25 (Table 5).

Table 5. Diagnostic ions in the FAB mass spectra of monophosphonium salts 17, 20, 21 and 25'

17 20 21 25

845 (2) [C,Br]+ 805 (1) [C,[Br]+ 319 (100) C+ 833 (2) [C,Br]+ 383 (100) C+ 363 (100) C' 303 (7) [C - CH,]+ 377 (100) C+ 303 (25) [C - HBr]+ 345 (2) [C - H,O]+ 277 (5) [C-CH,CO]+ 319 (21) [C-CH,O-C,H,]+ 289 (7) [C - CH,Br]+ 319 (3) [C - CO,]+ 276 (4) [C - CH,CO] +' 289 (9) [C - C4H802]+ 275 (16) [C - C,H,Br]+ 317 (6) [C - HCO,H]+ 275 (13) [C - CH,CHO]+ 275 (8) [C - C5H,,,02]+ 262 (11) [Ph,P]+' 303 (2) [C - CH,CO,H]+ 262 (5) [Ph,P]+' 262 (16) [PH,P]+' 199 (12) [Ph,PCH,]+ 289 (11) [C-C,H,CO,H]+ 201 (6) [Ph,PO]+ 199 (4) [Ph,PCH,]+

no R + ion 275 (8) [C-C,H,CO,H]+ 199 (5) [Ph,PCH,]+ no R + ion 262 (14) [Ph,P]+' no R + ion 87 (6) [C4H,OzI+ 199 (4) [Ph,CH,]+ 43 (2) [CH,CO]+ 59 (5) [C,H,OI'

no R + ion 57 (4) [C,H501' 41 (3) [C,H,I+ 31 (4) [CH,O]+

a FAB matrix 3-nitrobenzyl alcohol. m/z with RA values (Yo) in parentheses. Ion masses referenced to isotopes to the FAB spectra of all salts investigated are listed in Table 3 and are, therefore, not repeated here.

and ,'Br. Ions cornmon

Differentiation of isomeric phosphonium salts

The positive-ion FAB mass spectra of two sets of isomeric phosphonium salts are presented (Figs 4 and 5) to demonstrate the usefulness of FABMS for the structural differentiation of isomers. Two phosphonium salts (23 and 24) showing an intact cation at m/z 349 with the same empirical formula, C22H22P02, were examined. The intact cation is the base peak in both spectra. Ions common to the spectra of the two salts are those at m/z 185, 183, 165, 152, 141, 133, 121, 108, 107, 91 and 77 (Table 3). Each mass spectrum, however, also contains distinctive fragment ions which reflect the specific functional group substitution of the alkyltriphenylphosphonium salt. Most of these fragment ions are formed by loss of small neutral molecules from the intact cation. The ions at m/z 321, 303 and 275 in the spectrum of salt 23 [Fig. 4(a)] can be explained by losses of ethylene, ethanol and ethyl formate, respectively, from the intact cation. Because these ions are of low relative abundance or completely absent in the spectrum of the other isomeric salt (24) examined, they represent the specific fragmentation of the carbethoxymethyl substituent (R). The interpretation of characteristic fragment ions as the result of loss of neutral molecules is also apparent in the spectrum of the isomeric salt 24 [Fig. 4(b)], where the following fragmentations are observed : loss of ethylene oxide to yield the fragment ion at m/z 305, loss of two molecules of formaldehyde to give the fragment ion at m/z 289, combined loss of ethylene and

‘7 c

’i’

c 349 I

Figure 4. Positive-ion FAB mass spectra of two isomeric phosphonium salts (23 and 24) with an intact cation at m/z 349. Matrix: 3-NBA.

‘“1

T g Y ee l I

Figure 5. Positive-ion FAB mass spectra of two isomeric phosphonium salts (18 and 19) with an intact cation at rn/z 307. Matrix: 3-NBA.

carbon dioxide to yield the fragment ion at m/z 277 and loss of dioxolane to give the fragment ion at m/z 275. The ion at m/z 73 in the spectrum corresponds to the 1,3-dioxolan-2-y1 group, which is a part of the (1,3- dioxolan-2-y1)methyl substituent (R). A comparison of the mass spectral data of the salts 23 and 24 clearly indicates that different sets of characteristic fragment ions or characteristic neutral losses are obtained for their isomeric cations, which allow the characterization of their structures.

The feasibility of isomer differentiation is also demonstrated in the FAB mass spectra of a second set of isomeric phosphonium salts, 18 and 19 (Fig. 5). The intact cation at m/z 307 is the base peak in each of the mass spectra. Also here, most of the characteristic fragment ions can be attributed to the loss of small neutral molecules. Structure-specific fragment ions of salt 18 are detected at m/z 289 and 277, corresponding to loss of water and formaldehyde, respectively, from the intact cation. The spectrum of salt 19 is different from that of salt 18. Loss of methane (16 u) yields the fragment ion at m/z 291, whereas loss of methanol gives rise to the fragment ion at m/z 275. The abundance ratio for the fragment ions at m/z 277 us. m/z 275, i.e. 1277/1275, is 2.6 for salt 18 but only 0.6 for salt 19. The change in the pattern of neutral losses from the intact cation, is clearly affected by the nature of the alkyl substituent. Another difference between both spectra relates to the detection of the R+ ion at m/z 45 for salt 19, which corresponds to the methoxymethyl substituent.

Bisphosphonium salts

The bisphosphonium salts have the general formula [(Ph3P)zR]2' 2A- (or C2+2A-) with C2+ = [(Ph3P)2R]2' (= bisphosphonium dication) and A- = C1- or Br- (=salt anion).

All desorption methods generally produce ions corresponding to the cationic or anionic parts of the organic salts. However, when organic salts contain divalent cations (or anions), the doubly charged cationic (or anionic) species are not so commonly observed, in spite of the likely presence of these ionic species on the surface or in the liquid matrix. Multiply charged ions are rarely detected because (i) they are intrinsically unstable, owing to intramolecular coulombic repul- sions; (ii) the increase in the internal energy accompany- ing their production promotes fragmentation; and (iii) of the occurrence of competitive processes, such as clustering of the doubly charged ion with a singly charged counter ion or one-electron reduction. Consequently, the available literature on the detection of doubly charged and multiply charged intact cations by desorption ionization is still limited. Doubly charged ions have been observed for diquaternary ammonium salts analysed by

et u1." noticed that the occurrence of ammonium dications in SIMS correlates with intercharge distances. Heller et d3' emphasized the importance of the field for extraction of multiply charged ions from surfaces.

The FAB mass spectra of bisphosphonium halides 2740 (listed in Table 2) show intact organic dications when 3-NBA is used as a liquid matrix. The positive-ion FAB mass spectrum of 1,4-~ylylenebis(triphenylphos- phonium chloride) (27) is illustrated in Fig. 6. The fragmentation pathways which account for all the abundant ions in the spectrum are summarized in Scheme 2. The ionic relationships were confirmed by performing appropriate MS/MS experiments The FAB mass spec-

SIMS,28 EHDMS,29 FDMS3' and FABMS.29,30 R Yan

trum provides relative molecular mass information which is inferred from the doubly charged intact dication (C") at m/z 314. Emission of the doubly charged species is confirmed by detection of the 13C isotope peak 0.5 u above the dication peak (i.e. m/z 314.5). The [C2+ + Cl-1' cluster ion at m/z 663 (35Cl isotope peak) can be rationalized by clustering of the doubly charged cation with the singly charged chloride counter ion or direct loss of a chloride anion from the intact bisphosphonium chloride molecule. The product ion tandem mass spectrum of the ion at m/z 663 indicates that this cluster ion dissociates, via the expulsion of HCl, to the [C - 11' fragment ion at m/z 627. Sub- sequent loss of triphenylphosphine from this fragment ion gives rise to the [Ph3PCH(C,H,)CH,]' ion at m/z 365, which corresponds to the base peak in the spectrum.

Another ion that is characteristic of the dication of the bisphosphonium salt is the singly charged C " ion at m/z 628. The formation of this intact monocation is rationalized by a one-electron reduction of the dication during the bombardment process. Electron attachment is expected to occur when the electron affinity of the C z + ion is positive, i.e. the compound must have available low-lying unoccupied orbitals into which electrons can be donated. Formation of radical C" ions by one electron reduction of dications was previously reported for SIMS and FAB mass spectra of diquaternary ammonium salts;29~31*32 Williams et ~ 1 . ' ~ suggested that facile reduction of dications in FAB is due to the bombardment process. The keV bombarding particles produce electrons in the matrix via ionization of target molecules (matrix or sample) and these electrons can then be effective as reducing agents. A product ion tandem mass spectrum of the C" ion (m/z 628) reveals that this ion is the precursor for the fragment ions at m/z 551 and 366 which are formed by expulsion of a phenyl radical and triphenylphosphine, respectively

- x rn a,

Figure 6. Positive-ion FAB mass spectra of 1,4-xylylenebis(triphenylphosphonium chloride) (27). Matrix (a) 3-NBA and (b) glycerol.

J. CLAEREBOUDT ET AL. 80

mlz 551

[ Ph3P=CH(C6H4)CH]'.

[Ph,P=CH(C&)CHPh]+ m l z 364

mlz441

-Ph3P I -CH3(C,H4)CH2-

-Ph2PCH2(C6H4)CH2.

W [Ph3Pl+

d z 262

fragment ions at m/z 261.

185,183,152,108,107

(cf. Scheme I)

Scheme 2. AT= thermal degradation; El =electron impact ionization; EC =electron capture.

(Scheme 2). The presence of the [C - Ph]' and [C - Ph,P] +' fragment ions in the FAB mass spectrum of the bisphosphonium salts indicate that one-electron reduction of the dication occurs, even when the odd- electron C+' ion itself is observed with a very low relative abundance. Further fragmentation of the [C - Ph]' ion (m/z 551) yields the fragment ion at m/z 366 (loss of (Ph,P), the [Ph,P]+' ion (m/z 262) and the [Ph,P]+ ion (m/z 185). Another ion worth noting in the spectrum of salt 27 is the [C - 21"' ion at m/z 626. A precursor ion tandem mass spectrum of this [C - 2]+' ion indicates that it does not originate from gas-phase fragmentation of a higher m/z ion. A possible explanation is that the ions at m/z 626 are formed by electron ionization of the major thermal degradation product of the salt, resulting in [M - 2HC1]+' or [C - 21'' ions (M = salt molecule). This reaction probably occurs at the surface, prior to desorption. A similar process was reported for the monophosphonium salts (see above). A product ion tandem mass spectrum of the [C - 21" ion (m/z 626) shows that this ion dissociates to the fragments at m/z 625, 549, 441 and 364, resulting from loss of H', Ph', Ph ,P and Ph,P, respectively (Scheme 2).

The MS/MS experiments indicate that the majority of the fragment ions in the spectrum of salt 27 originate from unimolecular gas-phase decompositions of singly charged C", [C - 13' and [C - 2]+' ions. However, the ions at m/z 381 and 567 cannot be postulated as arising from fragmentation of these ion species. Precur- sor ion MS/MS suggests that both ions originate from a more complex process which involves dissociation of adduct ions with masses above that of the C" ion. The

ion at m/z 381 appears to be formed by loss of Ph,P from the [(C - 1) + 161' ion at m/z 643, whereas the ion at m/z 567 results from expulsion of a phenyl radical from the [C + 161" ion at m/z 644. These adduct ions are observed with a very low (< 1 %) relative abundance in the original FAB mass spectrum. The existence of these extraneous adduct ions, which are formed by beam-catalysed intermolecular reaction in the matrix, was also noticed for the FAB mass spectra of monophosphonium salts (see above).

Changing the counter ion does not affect the FAB mass spectra of the bisphosphonium halides; for example, the dibromide derivative of salt 27 gives a similar spectrum to the dichloride derivative shown in Fig. 6. However, significant differences are observed when the matrix is changed from 3-NBA to glycerol (Fig. 6). Ions related to the dication, i.e. (C2', C", [C2+ + CI-]' and [C - l]', are observed with much lower abundance or are completely absent in the spectra obtained with glycerol. The finding that dications are more easily observed in 3-NBA was previously experienced by Miller et aZ.,,, who reported that abundant doubly charged cations of organoruthenium compounds could be detected when using 3-NBA as a matrix but not with any other matrix. The detection of high-abundance fragment ions at m/z 367 and 105 in the spectrum obtained with glycerol is also noticed (Fig. 6). Formation of the ions at m/z 367 cannot be rationalized by a gas-phase unimolecular fragmentation process. The absence of any particular ion in the precursor ion spectrum of m/z 367 rules out a unimolecular mechanism and indicates that this ion is formed at the surface, prior

Table 6. Diagnostic ions in the FAB mass spectra of bisphosphonium salts 27-4V Compound

27 28 29 30

37 38 39 40'

663(1)

723(1) 779 (1)

653 (1 )

757 (1) -

314(17) 328 (5) 344 (6) 372(17)

386 (5)

400 (2)

400 (4)

414 (2)

418(1)

442 (1)

309 (5) 31 7 (4) 339 (5) 364 (9)

C+' [ C - l ] +

628(6) 627 (13) 656 (3) 855 (5) 688 (2) 887 (3) 744(2) 743(4)

772 (2) 771 (3)

800(1) 799(3)

828 (1) 827 (3)

856 (1 ) 855 (3)

884 (1) 883 (3)

618(1) 617(3) 634(l) 633(3) 678 (3) 677 (7) 728 (1) 727 (2)

[C - 21 *.

626 (5) 854 (4) 686 (4) 742 (3)

770 (3)

798 (4)

826 (4)

854 (3)

882 (3)

816 (6) 632 (6) 676 (4) 726 (2)

[C- Phl * [C - Ph,P] *. [(C-1 ) - Ph,P]' [Ph,P]"

551 (9) 579 (5) 611 (4) 667 (3)

695 (3)

723 (3)

751 (3)

779 (3)

807 (3)

541 (3) 557 (3) 601 (6) 651 (2)

366 (75) 394 (54) 426 (49) 482 (49)

510(53)

538 (51 )

538 (55)

566 (55)

594 (54)

622 (55)

356 (45) 372 (44) 41 6 (63) 466 (60)

365 (1 00) 282 (65) 393 (1 00) 262 (42) 425 (100) 262 (34) 481 (100) 262 (30)

509 (100) 262 (34)

537 (1 00) 262 (45)

537 (100) 262 (40)

565 (1 00) 262 (52)

593 (1 00) 262 (55)

621 (100) 262 (59)

355 (1 00) 262 (41 ) 371 (100) 282 (42) 415(100) 262 (32) 485 (1 00) 262 (80)

a FAB matrix 3-nitrobenzyl alcohol m p with RA values 1%) in Darentheses 'Ion masses referenced to isotopes 3"CI and 79B1

'Ions common to the FAB spectra of all salts investigated are listed in Table 3 and are. therefore. not repeated here 'Sample IS contaminated with the related monophosphonium salt (1 3)

Others'

567(2);441 (2).381 (11).367(24).353(2):362(2) 469 (2) ;411 (6) ,409 (7); 395 (16). 381 (1 2) 501 (1);441 (6);427(14);411 (9);409(4) 683 (1);527 (4) ,497 (5);483 (14);469 (7).

453(15):451 (4).439(10).438(3).437(3); 409 (6), 397 (4)

555 (10). 525 (5): 51 1 (15):497 (12);467 (14) 465 (4):453 (11 ) . 452 (4);451 (3);409 (7). 397 (6), 395 (3)

583 (16); 553 (5 ) ; 539 (14); 525 (36) ;481 (1 5) 479 (6) :467 (14); 486 (4).465 (5). 409 (9). 397 (8), 395 (4): 383 (4). 366 (8)

481 (14),479 (5);467 (13);409 (8); 397 (7). 396 (3) ; 395 (4)

495 (16);493 (7): 481 (15):480 (5).479 (6). 409(12):397(10):396(4);395(5)

509 (15). 507 (7) ;495 (14);494 (5); 493 (6). 409(11).397 (10):396(4).395(5)

523 (15); 521 (7);509 (14); 508 (5), 507 (6): 409 (11); 397 (1 1 );396 (4); 395 (5)

583 (6).553 (6);539 (16). 525 (17); 495 (5);

767 (1 ) , 61 1 (9):581 (6); 567 (1 7);553 (7).

795 (1 ).a39 (2) ;609 (5); 595 (16):581 (3).

823 (1 ); 667 (1 ) ; 637 (5); 623 (1 7) ; 609 (7),

557(1);373(7).371 (8).357(16):95(5)

491 (1):431 (10).417(27):403(2):402(2) 481 (10);467 (24).453 (52).d387 (4); 191 (381"

447 (1 ); 387 (7); 373 (20); 359(2).337 (3)

to desorption. A similar observation was reported by Williams et u Z . , ' ~ who suggested that this ion originates from recombination of the [C - Ph,P]+' species (m/z 366) with a hydrogen radical from the matrix (Scheme 2). An analogous fragmentation reaction, involving replacement of an ammonium group by a hydrogen of the glycerol matrix, was reported for the EHD and FAB spectra of diquaternary ammonium salts.,' Further fragmentation of the ions at m/z 367 by expulsion of triphenylphosphine yields the fragment ion at m/z 105 (Scheme 2).

The FABMS data obtained for the other bisphosphonium salts (28-40) are summarized in Table 6. The doubly charged cations are always encountered in the spectra obtained with 3-NBA. In addition, production of singly charged species by clustering with a counter ion, decomposition and one-electron reduction are generally observed. All the salts give spectra which are rich in singly charged fragment ions, whose formation can be rationalized in terms of simple fragmentation pathways, previously described for salt 27 (Scheme 2). It is worth noting that the intensities of these fragment ion peaks are high compared with those of the molecular ion-like species, i.e. C2+, C" and [C - 13". This is in contrast to the behaviour of monophosphonium salts, for which the intact cations usually correspond to the base peak. The more extensive fragmentation of the bisphosphonium salts can be explained by the relatively high charge density of the dication and the resulting strong interactions with the matrix. The energy needed to overcome these interactions can be substantial and probably promotes fragmentation.

For the bisphosphonium salts containing alkoxy sub- stituents (29-36), additional fragment ions are detected

which correspond to loss of alkene molecules from the [(C - 1) - Ph,P]+ ion (Table 6). For example for salt 30, which contains two propoxy groups, the major fragments of the [(C - l)/Ph,P]+ ion (m/z 481) originate from loss of C2H, (m/z 453) and C,H, (m/z 439), where, for salt 32, which contains two pentoxy groups, the major fragments of the [(C - 1) - Ph,P]+ ion (m/z 537) originate from loss of C,H, (m/z 481) and CfjH,, (m/z 467).

~~ ~~ ~

CONCLUSIONS

This study has shown that FABMS is a useful method for the structural characterization of mono- and bisphosphonium halides formed in synthetic reactions. The FAB spectra of the phosphonium salts not only show abundant intact cations, but also contain fragment ions characteristic of their structures. A 3-NBA matrix yields spectra for the phosphonium salts which are superior in quality to those obtained with glycerol. FABMS can also be applied to the differentiation of isomeric !salts. The results obtained for a series of related salts provide further insight in the behaviour of these thermolabile molecules under FAB conditions, information which is essential if this technique is to have wide applicability, as a supplement to EIMS, for structure determination of organic onium salts. The results also provide evidence of the striking similarities between ionization by FAB and other DI techniques such as FD, SIMS and LD. The advantages of the combination of FAB with tandem mass spectrometry were also demonstrated. The FABMS/MS technique was successfully applied to the determination of ionic relationships in spectra and

enhanced characterization of phosphonium cations Acknowledgements using low- and high-energy collisional activation. The MS/MS results also confirm that most fragment ions observed in FABMS originate from unimolecular gas- phase decompositions.

Financial support for the mass spectrometer by the Belgian Fund for Medical Research (Grant 3.0089.87) and the Belgian Government (Grant 87/92-102) i s gratefully acknowledged.

REFERENCES

1. H. J. Bestmann, in Newer Methods of Preparafive Organic Chemisfry, ed. by W. Forest, pp. 1-60. Academic Press, New York (1968).

2. L. Fieser and M. Fieser, Advanced Organic Chemisfry, pp. 482-486. Reinhold, New York (1961).

3. G. W. Wood, J. M. Mclntosh and P.-Y. Lau, J. Org. Chem. 40, 636 (1975).

4. W. Baeten, PhD Thesis, University of Antwerp (UIA) (1988). 5. M. C. Sammons, M. M. Bursey and C. K. White, Anal. Chem.

6. R. Weber, F. Borchers, K. Levsen and F. W. Rollgen, Z. Natur-

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Am. Chem. SOC. 109,1980 (1 987). 15. C. S. Creaser, S. Crosland, H. R. Hudson, A. Kow, L. Pow-

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16. C. F. Hauser, T. W. Brooks, M. L. Miles, M. A. Raymond and G. B. Butler, J. Org. Chem. 28, 372 (1963).

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19. L. D. Detter, 0. W. Hand, R. G. Cooks and R. A. Walton, Mass Specfrom. Rev. 7,465 (1 988).

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Structural characterization of mono- and bisphosphonium salts by fast atom bombardment mass...

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Determination of ink photoinitiators in packaged beverages by gas chromatography–mass spectrometry and liquid chromatography–mass spectrometry

Application of nanomaterials for desorption mass spectrometry

Fast atom bombardment mass spectrometry of theD-aldohexoses and some deoxyaldohexoses

Modeling Mass Spectrometry-Based Protein Analysis