The Mass Spectra of Some para Substituted Triarylphosphines and Triarylphosphine Oxides

Greg Marshall Materials Quality Assurance Directorate, Woolwich, London, SE18 6TD, UK

Stephen Franks and Frank R. HartleyP Department of Chemistry and Metallurgy, The Royal Military College of Science, Shrivenham, Swindon, Wiltshire SN6 8LA. UK

The mass spectra are reported for a series of tris(palkylary1)phosphines and the corresponding phosphine oxides. The phosphines all give [MI" as the base peak except when the phenyl groups are not para substituted. For the oxides [M-H]+ gives the base peak with one exception for which [MI" is the most abundant ion.

Triarylphosphines are good ligands for transition met- als, and triarylphosphine oxides for lanthanide and actinide elements. We have recently described the preparation and complexes of a series of tris(p- alkylary1)phosphines which were synthesized in order to prepare metal complexes with rather different solu- bility properties to triphenylphosphine complexes.132 In the course of characterizing the new phosphines their mass spectra were found to show significant differences from the mass spectrum of triphenyl- phosphine. These differences are described in the pre- sent paper. A detailed examination of the correspond- ing phosphine oxides is also included because, with the exception of triphenylphosphine oxide itself, these ox- ides have not been investigated by mass spectrometry.

Extensive mass spectral studies of triphenyl- phosphine and its oxide have been carried out previ- 0us1y.~ Studies with deuterated analogues of triphenyl- phosphine revealed that scrambling on the ring sites does not occur. Cyclization can occur to give heterocyclic ions through the loss of a hydrogen atom ortho to the phosphorus atom. The effect of a methyl substituent on the aromatic ring of triphenylphosphine has been i~ivestigated.~ The mass spectrum of the para derivative shows many of the features of triphenyl- phosphine itself in the formation of heterocyclic ions. The mass spectrum of tris(p-chloropheny1)phosphine has been examined by other workers but no spectral assignments are p~bl ished.~

The compounds under investigation in this work can be represented by structures 1 and 2 for the phos- phines and phosphine oxides respectively, where X = H, CI, CH,, C2H5, n-C,H,, n-C,HI5, n-C,H1, and OCH,. Initial studies at medium resolution for X = CI and CzH5 coupled with the results outlined above for

1 2

t Author to whom correspondence should be addressed.

X = H and CH, suggest that common fragmentation pathways exist for both sets of compounds. The pres- ence of the related ions can also be inferred from the low resolution studies of the remaining compounds.

The general structures of ions obtained with the phosphines are shown in Table 1. The normalized intensities of each set of ions are also shown. The most notable feature is that the base peak is the molecular ion [MI" in all cases except when X = H . Both electron-withdrawing substituents (X = C1) and electron-donating substituents (X = alkyl or OCH,) give [MIf' as the base peak, suggesting that the elec- tron densities on the phosphorus atoms are similar in both cases. The 31P nuclear magnetic resonance (NMR) chemical shifts of these compounds are all similar in agreement with this deduction.' The anomalous behaviour observed in the phosphine series when X = H is not observed with the phosphine ox- ides. For the alkyl substituents X = C2H5 to n-C,H,, the length of the substituent alkyl chain has little effect on the distribution of the ions in Table 1.

A similar treatment for the phosphine oxides in terms of general ion structures is shown in Table 2. With the exception of n-C,H,, the base peak involves the loss of a hydrogen radical from the molecular ion [M-HI'. Care must be taken to account for the isotopic contribution of [M-HI' to [MI". The loss of a single substituent group from the molecular ion [M- XI' is more favourable for the phosphine oxides than for the corresponding phosphines. This suggests a weak- ening of the X-aryl bond in the former, which may be due to the lower resonance stabilization of the aromatic ring coupled to phosphorus(V). However, 31P NMR studies suggest that the X-substituent does not affect the electron density on the phosphorus atom to any degree.' Some of the ions in Table 2 are also seen in the phosphines themselves. Thus, the oxygen atom may be undergoing new bond formation with the aromatic rings to produce some of these ions.

The accurate masses of some of the ions in the spectrum of tris(p-chloropheny1)phosphine oxide are of interest. A strong peak at m/z 170 is observed

CCC-0030-493X/81/0016-0272$01.50

272 ORGANIC MASS SPECTROMETRY, VOL. 16, NO. 6, 1981 @Heyden & Son Ltd, 1981

MASS SPECTRA OF SOME TRIARYLPHOSPHINES AND TRIARYLPHOSPHINE OXIDES

Table 1. Ions produced in the mass spectra of triarylphosphines (X +3P

lon\X H CI CH, CZHS n-GH, n-C7Hl, n-GHlS OCH,

[MI+' 33 100 100 100 100 100 100 100 [M-XI+ 11 3 2 1 2 1 1 1

(x+2P+ 6 27 29 6 2 3 13 26

xQ-0. 100 20 55 1 1 1 1 5

p+

( 4 3 2 1 4 17 4 2 1 2 16

0 76 [XC,H,Pl+' 26 91 76 26 4 4

Table 2. Ions produced in the mass spectra of triarylphosphine oxides (X - O + , P = O

lon\X

[MI+' [M - HI+ [M-XI+

(x-@;P=o

( X Q T O X / + /

II 0

H CI

26 45 100 100

24 -

CH3

36 100

4

CzHs

41 100 10

n-GH7

37 100 19

n - W l 5 n-GHlS

60 100 100 63

7 7

OCH.

54 100

1

19 42 17 18 20 32 28 33

15 14 9 4 2 4 2 8

33 19 10 58 23 21 25 73

4 8 3 6 2 12 19 5

which shows no chlorine isotope peaks at low resolu- tion. The accurate mass of this ion suggests CI1H7P to be the elemental composition. Two possible structures are a and b. The origin of this ion is unclear, although its formation is thought to involve the oxygen atom of the starting molecular ion since it is not seen in the spectrum of the corresponding phosphine. Accurate

+. d, m/z +. 224 c, mlz 225

+ e, m/z 149

EXPERIMENTAL a, mlz 170 b, mlz 170

Triphenylphosphine, triphenylphosphine oxide (Al- drich Chemical Co Ltd), tri(p-toly1)phosphine and tri- (p-methoxypheny1)phosphine (Kodak Ltd) were ex- amined as supplied. A range of triarylphosphines (p- XC,H,),P, where X = C1, C2H5, n-C,H7, n-C7HI5 and n-C,H,,, were synthesized by a Grignard method as

mass studies on (P-C~H$~H,)~P show evidence of reactions occurring at the sidechain. The ions c, d and e at mlz 225, 224 and 149 respectively show how sidechain reactions may produce heterocyclic ions.

@ Heyden & Son Ltd, 1981 ORGANIC MASS SPECTROMETRY, VOL. 16, NO. 6, 1981 273

G. MARSHALL, S. FRANKS AND F. R. HARTLEY

described previously.' The oxides (p-XC6H,),P=0, where X = C2H5, n-C3H7, n-C7H15 and n-CgH19 were prepared by oxidation of the corresponding phosphine in acetone using 6% w/v hydrogen peroxide.' The oxides with X = C1, OCH, and CH, were also prepared in this fashion: their 31P NMR chemical shifts were 22.7, 24.9 and 25.9 ppm downfield from trimethyl- phosphate respectively.

Low resolution mass spectra (M/AM = 1000) were recorded using either a Perkin-Elmer-Hitachi RMU7 M mass spectrometer or a VG Micromass 305F mass spectrometer, operating at 70eV with an ion source temperaturle of 250 "C. The accelerating voltages were 3.2 and 4.0 kV respectively. The data were collected using a VG 2035 data system. Observed metastable

transitions were within k0.2 u of the calculated values throughout.

Medium resolution mass spectra (M/AM = 7500) were recorded using a Kratos MS 50 mass spectrome- ter operating at 70 eV; the data were collected using a Kratos DS 50 data system.

31P NMR spectra were recorded in deutero- chloroform solution using a JEOL FX 90Q NMR spectrometer operating at 36.2 MHz.

Acknowledgements The authors would like to thank Mr D. Johnston for running the medium resolution mass spectra. Financial assistance by the Euro- pean Office of the US Army under the auspices of grant DAERO- 79-G-0033 is gratefully acknowledged.

REFERENCES

1. S. Franks and F. R. Hartley, J. Chem. SOC., Perkin Trans. 1

2. S. Franks and F. R. Hartley, Inorg. Chim. Acta 47,235 (1981).

5. R. F. de Ketelaere, G. P. van der Kelen and 2. Eeckhaudt, 2233 (1980). Phosphorus 5, 43 (1974).

3. D. H. Wi\\iamS, R. s. Ward and R. G . Cooks, J. Am. Chem. ~ ~ ~ ~ i ~ ~ d 11 M ~ ~ ~ . , 1981; accepted 26 M ~ ~ ~ . , 1981 SOC. SO, 966 (1968).

4. T. R. Spalding, Org. Mass Spectrom. 11, 1019 (1976). 0 Heyden lk Son Ltd, 1981

274 ORGANIC MASS SPECTROMETRY, VOL. 16, NO. 6, 1981 @ Heyden & Son Ltd, 1981