Spectrochimica Acta, 1964, Vol. 20, pp. 1835 to 1842. Pergamon Press Ltd. Printed in ~orthern Ireland

Phosphorus-31 nuclear magnetic resonance studies o~ phosphorus-fluorine compounds

JOHN F. NIXON* and ~ E I N t I A R D SCHMUTZLER

University Chemical Laboratory, Lensfield Road, Cambridge

(Received 13 April 1964)

Abstract--:P 8~ NMR chemical shifts and phosphorus-fluorine coupling constants are reported for a wide variety of phosphorus fluorides, in which the coordination number of phosphorus can have the values three, four, five, and six. The chemical shift, s extend over a wide range (~350 ppm) but are relatively constant for any particular class of compounds, and become more positive as the coordination number of phosphorus increases. The pa~ NMR spectra of the fluorophosphoranes, R,PFs_~, confirm the previous conclusions regarding their stereo- chemistry.

THE l i t e r a tu re conta ins phosphorus chemical shif t measu remen t s for a large n u m b e r of inorganic and organic phosphorus compounds [1-9], and several a t t e m p t s have been m a d e to corre la te the d a t a wi th such bond p roper t i e s as ionic cha rac te r , the degree of h y b r i d i z a t i o n and the e x t e n t of mul t ip le bonding [2-4, 6, 10, 11]. Very , l i t t le d a t a are ava i lab le , however , on phosphorus - f luor ine compounds , where add i t i ona l i n fo rma t ion can be ob t a ined f rom measu remen t s of the p h o s p h o r u s - fluorine, F a l - - F 19 s p i n - s p i n coupl ing cons tan t . W e have therefore measu red the pal N M R spec t ra of a n u m b e r of wide ly differing phosphorus fluorides in order to see how the chemical shif ts and coupl ing cons tan t s are r e l a t ed to the coord ina t ion n u m b e r of the phosphorus a tom.

* Present address: Department of Chemistry, St. SMvators College, University of St. Andrews, Fife, Scotland.

[1] H. S. GuTowsKY, D. W. McCALL, and C. P. SLICHrER, J. Chem. Phys. 21, 279 (1953). [2] I-I. S. GUTOWSKY and D. W. McCALL, J. Chem. Phys. 22, 162 (1954). [3] N. MULLER, P. C. LAUTERBUR, and J. GOLDENSON, J. Am. Chem. Soc. 78, 3557 (1956). [4] J . ]:~. VAN WAZER, C. F. CALLIS, J-. N. SHOOLERY, a n d R. C. JONES, J . A m . Chem. Soc.

78, 5715 (1956). [5] H. FINEGOLD, Ann. N . Y . Aead. Sci. 70, 875 (1958). [6] C. F. CALLIS, J. R. VAN WAZER, J. N. SHOOLERY, and W. A. ANDERSON, J. Am. Chem. Soc.

79, 2719 (1957). [7] K. MOEDRITZER, L. MAIER, and L. C. D. GROENWEGHE, J. Chem. Eng. Data 7, 307

(1962). [8] R. A. ~ff. JONES and A. R. KATRITZK¥, J. lnorg. Nuclear Chem. 15, 193 (1961). [9] M. L. NIELSEN and J. V. PUSTI~GER, JR., J. Phys. Chem. 68, 152 (1964).

[10] J. R. :PARKS, J. Am. Chem. Soc. 79, 757 (1957). I l l ] L. C. D. GROENWEGItE, L. MAIER, and K. MOEDRITZER, J. Phys. Chem. 66, 901 (1962).

1835

1836 JOHN F. NIxoN and REINHARD SCHMUTZLER

EXPERIMEI~TAL

Preparations. Synthetic details for nearly all the compounds reported in this study have already appeared in the literature [12], those for the heptafluoropropyl com- pounds will be published separately elsewhere [12p]. The volatile derivatives were purified by distillation or by fractional condensation in a high-vacuum system. In many cases a small amount of sodium fluoride was placed in the bottom of the NMR tubes to remove possible traces of hydrogen fluoride.

pal NMI~ spectra. The phosphorus-31 NMR spectra were obtained at 16.2 Mc/s using a Varian Associates V4300 B nuclear magnetic resonance spectrometer and 12" electromagnet with flux stabilization and a field homogeneity control unit. The chemical shifts reported are in parts per million (ppm) of the applied field, and are relative to 85 per cent phosphoric acid as the standard. Upfield shifts are denoted by a positive sign, downfield shifts by a negative sign. Samples were sealed or stoppered in Pyrex tubes of l0 mm outside diameter. The 85 per cent phosphoric acid was contained in sealed capillaries of ~ 2 mm diameter which were placed inside the 10 mm sample tubes. The calibration of the spectra was carried out using the normal sideband technique of either the standard or the compound. In some cases, in order to obtain more accurate values for the p31 chemical shifts, the phosphorus-fluorine spin-spin coupling constant JP-F, measured from the F 19 NMI% spectrum, was used for calibration. The reported chemical shift values are the average of several determinations.

RESULTS AND DlSCUSSlOX The tricoordinate phosphorus-fluorine compounds employed in this study, viz.

fluorophosphines, dialkylaminofluorophosphines and fluorophosphites, all exhibit large negative p~1 chemical shifts.

(a) Dialkylaminofluorophosphines (R2N)nPF~_ ~ 1 )31 chemical shifts for three dialkylaminodifluorophosphines, R~NPF~, were

found between -- 139.0 and -- 144.0 ppm, P - - F coupling constants were ranging

[12] a. R. SCHMUTZLER, .4dvances in Chemistry Series (American Chemical Society) 87, 150 (1963).

b. L . M . YAGUPOL'SKII and ZH. M. IVA~OVA, Zhur. Obsheh. Khim. 29, 3766 (1959). c. W . C . S•[TH, J. Am. Chem. Soc. 82, 6176 (1960). d. I . P. KOMKOV, S. Z. IvIl~, K. ~,~,r. KARAWANOV and L. JE. SMIR~-OV, Zhur. Obshch.

Khim. 32, 301 (1962). e. R. SCHMUTZLER, Chem. and 1rid. 1868 (1962). f. A . B . BURG and G. BRENDEL, J. Am. Chem. Soc. 80, 3198 (1958). g. V. N. KULAKOW~, YU. M. ZI~OV'EV and L. Z. SOBOR0VSKI~, Zhur. Obshch. Khim.

29, 3957 (1959). h. J . F . NlxoN, Chem. and Ind. 1555 (1963). i. R. SCHMTJTZLER, Inorg. Chem. 3, 410 (1964). j. It . SCH~UTZLER, Inorg. Chem. 3, 416 (1964). k. R. SCH=~UTZLER, Inorg. Chem. 3, 421 (1964). 1. R. SCHr~UTZLER, Chem. Ber. 96, 2435 (1963). m. R. SCHMUTZLER, J. Inorg. Nuclear Chem. 25, 335 (1963). n. ~V. MAHLER, Inorg. Chem. 2, 230 (1963). o. R. SCH~TZLER, to be published. p. J . F . NIxo~ , J. Chem. Soc., in press.

Phosphorus-31 nuclear magnetic resonance studies of phosphorus-fluorine compounds 1837

b e t w e e n 1194 to 1197 c/s. T he spe c t r a cons i s ted o f basic 1 -2 -1 t r ip le ts [13b]. F o r [(CH3)~N]~PF, a 1-1 doub l e t was o b s e r v e d ; chem. shif t - -150 .8 p p m ; J P - F =- 1042 c/s.

(b) Fluorophosphites ( I~O),PF3_ .

A n u m b e r o f a l ipha t ic a n d a r o m a t i c d i f luorophosphi tes , R O P F 2, a n d bis- d i f luorophosphi tes , F 2 P O - - R ' - - O P F ~, showed p31 chemica l shif ts in t he n a r r o w r ange - -109 .8 to - -112 .0 p p m , the P - - - F coupl ing c o n s t a n t s va r i ed be tween 1278 to 1328 e/s. [13b]. T h e p31 chemica l sh i f t was a t lower field in the he te rocyc l i c c o m p o u n d s , 1 .3 .2-d ioxa-2- f luorophosphole , C~H402PF ( - -123 .8 p p m ) , a n d 1.3.2- d ioxa-4 .5 -benzo-2- f luorophospho le , C6H402PF ( - -123 .1 p p m ) . The P - - F coup l ing

Table 1. Tricoordinate phosphorus-fluorine compounds

(c) Fluorophosphines, RnPFa_ n Compound (~(ppm) ,lp_ F (e/s) Remarks

CICH2PF 2 --201-8 1203 1-2 1 triplet; P--H coupling apparent

CaF~PF ~ --167.8 1250 1-2-1 triplet, each line split into broad 1-2-1 triplet; JP-CFa ~ 90'5 e/s

CFaPF 2 -- 158.3 1245 1-2-1 triplet, each line split into a 1-3-3-1 quartet; JP-CFS ~ 87'2 c/s

(CaF~)~]?F -- 138.8 1020 very broad 1-1 doublet CClaPF, ., -- 130-9 1290 1-2-1 triplet (CFa)2PF [14] -- 123.9 1013 1-1 doublet, each line

split into a septet; JP-eF3 ~ 89.6 c/s

PF a [2] --97"0 1410 1-3 3-1 quartet

c o n s t a n t s were 1226 a n d 1307 c/s, r e spec t ive ly [13b]. The r e p o r t e d mu] t i p l c t s t r u c t u r e s arise f r o m s p i n - s p i n coup l ing be t ween p h o s p h o r u s a n d f luorine nuclei . The coup l ing c o n s t a n t s on the whole t e n d to be l a rger t h a n those o b s e r v e d in c o m p o u n d s where p h o s p h o r u s has a h igher coo rd ina t i on n u m b e r . I t is n o t e w o r t h y t h a t the p h o s p h o r u s nuc leus is s o m e w h a t m o r e shie lded in the f luo rophosph i t e s t h a n in t he o t h e r t r i c o o r d i n a t e species, a n d the chemica l shif ts o f t he dif luoro- p h o s p h i t e s are r e m a r k a b l y cons t an t .

O n l y a l imi ted n u m b e r o f well es tab l i shed f luorophosph ines h a v e been r e p o r t e d in the l i t e ra tu re [] 2e -h i , all o f wh ich h a v e an e l ec t ronega t ive s u b s t i t u e n t a t t a c h e d to p h o s p h o r u s (e.g. pe rha loa lky l ) . I t has been f o u n d t h a t f luor ina t ion o f a lky l or a ry l ch lo rophosph ines a lways leads to f luo rophosphoranes , RnPFs_~, wh ich con t a in p e n t a v a l e n t p h o s p h o r u s [12b,d,e,i] . T he c o m p o u n d s p r ev ious ly be l ieved to be C~HsPF 2 and C~HsPCIF have recently been shown to be in fact C~HsPF 4 and C2HsPOF2, respectively [13a], and their reported pal chemical shift values do fall well within the ranges observed for other related members of their class (vide infra).

The nonavailability of alky] or aryl fluorophosphines precludes a direct compari- son of their chemical shifts with those of the perhaloalkyl derivatives, but it can he

[13] a. R. SCHMUTZLER, J. Chem. Soc., in press. b. G. S. ]=~EDDY and R. SCHMUTZLER, tO be published.

1838 JOHN F. NIxoN and I:~EINHARD SCHI~IUTZLER

seen that chloromethyldifluorophosphine which is the nearest approach to an alkyl- difluorophosphine has the lowest field shift of all the compounds studied, and that replacement of both hydrogen atoms by chlorine does in fact lead to a shift to high field of about 70 ppm. This trend is borne out in other systems described in this paper where a direct comparison can be made. Phosphorus trifiuoride is included for comparative purposes, and clearly has an anomalously high pal chemical shift, compared with the other fluorophosphines.

An empirical equation for the chemical shift 5p in PX a molecules has been described [3, 10], which is based on the observation by SAIKA and SLICHTER that fluorine chemical shifts and chemical shifts of nuclei having other than s electrons associated with them are determined largely by the paramagnetic shielding term. In order to find the number of "unbalanced p-electrons" in the trivalent phosphorus halides, both the ionic character of the P - - X bond and an accurate knowledge of the orbital hybridization of the phosphorus atom are required. The bond angles for the fluorophosphines listed in Table 1 are as yet unknown, but the variation of chemical shifts within the RPF 2 series (~-- 70 ppm) suggests that other factors may be important.

In the dialkylaminofluorophosphines and fluorophosphites there is an additional factor to consider, namely the extent of supplementary p,--d~ bonding arising from the interaction of the lone electron pairs on nitrogen or oxygen with vacant orbitals on the phosphorus atom. This effect will be enhanced by the electronegative fluorine atoms which will lower the energy of the 3d orbitals, and will thus reduce the basic character of nitrogen or oxygen and increase the electron density around the phosphorus atom. Supporting evidence is provided by infrared [12j] and X-ray crystallographic studies [15] on adducts of dialkylamino-difluorophosphines with transition metals, which confirm that coordination occurs via phosphorus and not via nitrogen.

The p31 chemical shifts of the tetracoordinate phosphorus-fluorine compounds shown in Table 2 occur at much higher fields than those of the tricoordinate compounds. The values for the alkylphosphonic difluorides, RPOF2, which are more positive than for the phosphinic fluorides, R~POF, are clustered in the narrow range from --27 to --29 ppm, and show little variation in their P - - F coupling constants. The chloromethyl and phenylphosphonic difluorides have higher field shifts, as do the dialkylamino- and alkoxy derivatives. The observed order of decreasing chemical shift, POF3 > ROPOF~ > RPOF 2 parallels the order observed for the tricoordinate series, PFa > ROPF 2 > RPF 2.

The alkyl and aryl fluorophosphoranes, RnPFs_n, all have positive or slightly negative chemical shifts, and the phosphorus-fluorine coupling constants are less than 1000 c/s. The rather low chemical shift of the heterocyclic C4HsPF a may be a consequence of the phosphorus being part of a strained ring system. Successive replacement of a methyl group by phenyl in the R2PF a system leads to a steady increase in the shielding of phosphorus : (C~Hs)~PF a > (C~Hs)CH3PFa > (CHa)2PF a, and the same trend occurs in the RPF4 system. I t is interesting to note that the

[14] K. J. PACKER, J. Chem. Soc. 960 (1963). [15] B. GREENBERG, A. AMENDOLA, and R. SCHMUTZLER, Naturwissenschaften 50, 593 (1963).

Phosphorus-31 nuclear magnet ic resonance studies of phosphorus-f luorine compounds 1839

T a b l e 2. Tetracoordinate phosphorus-f luorine c o m p o u n d s

Compound ($p (ppm) JP F (c/s) Remarks

CH3POF a [5] -- 27.4 - - - - C2HsPOF 2 --29.2 1130 1-2.-1 triplet n C4HgPOF~ --29.3 l l40 1-2-1 triplet i -CsHHPOF 2 -- 27.2 1126 1-2-1 triplet CICH2POF 2 -- 12.0 1142 1 2-1 triplet C~IIaPOF e -- 11.4 1 115 1 -2-1 triplet POF a [7] @ 35"5 1080 1-3-3-1 quartet POF~C1 [2] .-z 15.0 1138 1-2-1 triplet POFC12 [2] 0.0 1180 1 1 doublet (CHa)2POF --67.3 990 1-1 doublet (C6Hs)2POF --40.5 1020 1-1 doublet

C2H~P. --30"7 1040 1-1 doublets, II \CI-I(CHa)z --30.4 1050 observed on two O independent

[ 5 ] preparations / F /

C H A P \ li \OCH(CI-[3)2 - 16. I O

/ F

[C6HsNH3] IC~H5 \ * p / ~ --9.1

(C~Ha)2NPOF 2 @ 3.6 (CeI-IsO)POF ~ @20.9 (C~HsO)POF ~ @27.1

1 1 doublet

955 1-1 doublet

(compound dissolved in dimethylsulfoxid e)

1004 1-2-1 triplet 1015 1-2-1 triplet 1030 1-2-1 triplet

chemical shift for PF 5 is not particularly high, and in fact lies between the values observed for CH3PF a and C6HsPF 4.

Once again the perhaloalkyl derivatives have much more positive chemical shifts than their alkyl counterparts, and there is also a marked increase in their P - - F coupling constants, viz.

CF3PF 4 Jp_v~ = 1103 c/s t CH3PF 4 Jp_~ 965 c/s J

(CF3)3PF~ J P - F = 9 8 8 C/S 1

(n-C4Hg)3PF2 JP-F = 553 C/S J

(CFa)2PF3 JP-F = 1260 C/S ] (CH3)2PF3 JP--F (eq.)= 975 c/s

J P - F ( a x . ) - - 7 8 7 c / s

The pattern of two overlapping 1-2-1 triplets, at room temperature, is shown by all the I~2PF a systems studied, except cyclotetramethylenetrifluorophosphorane, (CH2)4PF 3, and (CFa)2PF 3, and indicates that two of the fluorine atoms are in an environment different from the third. This confirms the result of a previous F la NMR study [17] which established that the R groups in fluorophosphoranes, R~PF5_., occupy equatorial positions of a trigonal bipyramid, and that the apparent equiv- alence of fluorine atoms in (CH2)4PF a arises from an intramolecular exchange process. The same type of exchange would also account for the apparent equivalence of fluorine atoms observed in all the RPF 4 systems investigated.

[16] E . L . MUE~YrERTIES a n d W . D . PHILLIPS, J. Am. Chem. Soc. 81, 1084 (1959) . [17] E . L . MUETTERTIES, %V. MAHLER, a n d R . SCHMUTZLER, Inorg. Chem. 2, 613 (1963) .

1 8 4 0 Jo~N F. NIxoN and ]:~EINIIARD SCHMUTZLER

Table 3. Pentacoordinate phosphorus-fluorine compounds

(a) Fluoropho~phoranes, RnPFs_ ~

Compound 61" (ppm) JP-F (e/s) Remarks

(CI-I3)~PF a - -8 .0 975 (equatorial) two over lapping 787 (axial) 1-2-1 t r ip le t s

(C2Hs)2pF 3 - -6 ' 3 975 (equ.) two over lapping 827 (ax.) 1-2-1 t r ip le t s

CH z ~ p F a -~- 13-4 955 (equ.) two over lapping

C6I-I5 / 813 (ax.) 1 2-1 t r ip le t s

(C6Hs)2PF 3 -~ 34'8 968 (eq.) two over lapping 834 (ax.) 1-2 1 t r ip le ts

p F a --29.8 915 1-3-3-1 qua r t e t

CH3PF a -~- 29.9 965

C1CI-IupF 4 -~- 43" 7 1008

C~H~PF a Jr 30.2 995

C6HsPF 4 -}- 51.7 973 CI-IaC6H4pF 4 ~- 49.7 950 (n.C4Hg)sPF~ ~ 15.4 553

P F 5 [7] + 35-1 1010 916

(b) Dihalodifluorophosphoranes, R P F 2 X 2

CClaPF~C1 ~ -~- 7.4 1106 CClaPF2Br 2 -~ 27" 0 1109 CaF7PF2C1 ~ ~- 14.2 1095

(c) Perhaloalkyl Fluorophosphoranes, (Rx)nPF5_n

CFaPF 4 -~ 66.4 1103

CClaPF 4 -~- 66-9 1120 (CFa)~pF s ~- 50.9 1260

(CFs)apF 2 -~ 59' 8 988

1-4-6 4-1 qu in te t ; each component spl i t fur ther into 1-3-3 1 quar te t ; JP-H == 19 c/s 1 -4 -6 -4 -1 qu in te t ( P - - H coupl ing apparent ) 1 -4 -6 -4 -1 qu in te t ( P - - H coupl ing apparent ) 1 -4 -6 -4 -1 qu in te t 1 4 -6 -4 -1 qu in te t 1 2-1 t r ip le t (broadened by

P - - H coupling) sex te t (value from F 19

NMR spec t rum [16])

1-2-1 t r ip le t 1 -2-1 t r ip le t 1-2-1 t r ip le t ; fur ther spl i t in to 1 2-1

t r ip le t ; J r cFa = 129 c/s

1-4-6 4-1 quin te t , each component fur ther

spl i t in to 1 -3-3-1 quar te t ; JP-cra = 172 c/s 1 -4 -6 -4 -1 qu in te t

1-3-3-1 quar te t , fur ther spl i t in to septets ;

JP-CFa -- 174 c/s 1-2-1 t r ip le t , fur ther spl i t

into over lapping deeets (not comple te ly resolved); J p _ c F a ~ 1 6 7 c/s

The hexacoordinate phosphorus-fluorine compounds show the largest 1 )31 chemical shifts to high field, and the value observed for the compound [p-C1CsH4N2] + 1)F 6- ( + 143-7 ppm) agrees more closely with the higher value ( + 148 ppm [1]) of the two previous reports for HPF 6 mentioned in the literature [1, 2].

Removal of the perfect octahedral symmetry of the 1)F 6- anion by replacement of one fluorine atom by a phenyl group causes a downfield shift of ~ 8 ppm, and the

Phosphorus-31 nuclear magnet ic resonance studies of phosphorus-f luor ine compounds 1841

Table 4. Hexacoord ina te phosphorus-f luor ine compounds

Compound 6p (ppm) J?-F (c/s) Remarks

H P F 6 [1, 2] ÷ 148 [1] 708 septet ÷ 118 [2]

[p-C1C6H~N2]+PF6- ÷ 143-7 707 septet (in acetonitr i le solution) [C6HsPF[N(CHa)~]2] + / * ÷ 136-0 Ja = 697~ two over lapping

[CsHsPFs] - t J , = 816 1 -4 -6 -4 -1 quinte ts (in acetonitr i le solution)

* The cation p31 spec t rum consisted of a 1-1 doublet ; Jp_r = 1037 c/s; 6 = - 5 6 ppm. t See text .

pa~ NMR spectrum shows two partially overlapping quintets with the recorded coupling constants JP-F,~ and JP-Fe, consistent with the structure shown in Fig. 1 :

Fa

Fg

Fe

/ _/

C~H5

CONCLUSIONS

- ( - )

Fe

/ /

(1) I t can be seen from the data presented in the Tables that the phosphorus chemical shift values in phosphorus-fluorine compounds extend over a range of about 350 ppm, and are progressing towards high field as the coordination number of phosphorus increases, reaching a maximum at the spherically symmetrical hexafluorophosphate anion,

RnPF3_ n ~. (R2N)nPF3_ n < (RO)nPF3_~ ~ RnPOF~_n < R2PF a < R P F 4 < RaPF2 RPF~X2

< (Rx)~PFs_~ < P F 6-

(2) The phosphorus-fluorine spin-spin coupling constants vary in magnitude by almost a factor of three and no clear relationship between JP-F and the coordination number of phosphorus is apparent. There is a general decrease in JP-F in going from left to right in the above series, but the lowest value is observed in RaPF2, and the perhaloalkyl derivatives always have much larger coupling constants than the other members of the respective class.

(3) Alkyl derivatives of any particular class of compounds always have very similar chemical shifts, but substitution of a phenyl or polyhaloalkyl group leads to a high field shift, the effect being particularly marked for the latter.

1842 JOHN F. NIXON and REINHARD SCHMUTZLER

(4) The p31 NMI~ spec t ra of I~3PF 2 and R2PF a sys tems confirm the resul ts p rev ious ly ob ta ined f rom an F 19 N M R s tudy , t h a t the l~ groups occupy equator ia l sites in a t r igonal b ipyramid , while in R P F 4 sys tems there is an a p p a r e n t equivalence of the fluorine a toms, which p r o b a b l y arises f rom an in t ramolecu la r exchange process.

The chemical shift t rends agree fair ly well wi th the theoret ical concept t h a t the shielding increases wi th the e lect ros ta t ic po ten t i a l a t the nucleus which is reduced b y a second order p a r a m a g n e t i c t e r m arising f rom the lack of spherical s y m m e t r y in t roduced b y the bonding electrons. I t is clear, however , t h a t a fuller knowledge of bond angles and of the ex ten t of 7r bonding is needed before an exac t in t e rp re ta t ion of the da t a can be expected.

Acknowledgements--The authors wish to thank Dr. N. SKEPrAnD for helpful discussions, and Mr. E. LIDDELL for recording the spectra. We are indebted to I.C.I. for the award of a fellow- ship (to J.F.N.), and to Deutscher Akademischer Austausehdienst for awarding a N.A.T.O. Fellowship (to R.S.).