Organophosphorus chemistry

233 Jotwmal of Molecufar Strttmtt~~, 23 (1974) 233-240 g. Elsevier Scientific Publishing Company, Amsterdam -

~R~ANOP~OSPHORUS

Printed in The Netherlands

CHEMiSTRY

PART IV, VIBRATlGNAL SPECTROSCOPIC STUDY OF THE P-O, P-S AND P-Se BONDS IN TRIARYLPHOSPHOROUS CHALCOGENIDES

SUMMARY

The infrared and Raman spectra of the series (4”XC6Hj)+.,(C6Hs),PY, (IXC,I-E,j,_, (C6Hsj,PY (X = Cl or F, II = 0,1,2) and (4-FC,H,ja-. (3-FC,H,j,PY (ir = 1,2j (Y = 0, S and Se) compounds have been studied and ass~g~mentsfor the v(P&), v(P-S) and v(P-Se) stretching frequencies are proposed. The results indicate that the frequency shifts caused by the substituents bonded to phosphorus are more important in the P-S and P-Se bond than in the P-O bond. Mechanisms for the changes in PY bond order as a result of changing the aromatic substituent are discussed.

iNTRODUCTION

When the bonding in triphenyiphospbine chalcogenides (X&H&PY (X = H, F, CI; Y = 0, S, Se) is studied the question rises to what extent the P-C bonds are partaking in extended double bonding from the IEelectron system of the ring on the one hand and on the other hand how strong the P-Y z-bonding is and how this is infiuenced by changes in the rings e.g. by changing the substituent X. A series of compounds comprising the molecules (4-XCsHQ)3_,iC6Hs),PY, (3-XCBH4)3_-n(C6H5),$?Y (X = CI,F; n = 0,1,2) and (4-FC6H&_,(3-FC&H&PY (82 = 1,2) (Y = 0, S, Se) could offer suitable models. The stretching frequency of the PY moiety in these .molecules, which can be derived from combined * To whom correspondence &o&d

be addressed.

234 ifLfrared and Raman

spectra offks

complete

analysis of the corresponding

vibrational

a good

criterion for discussion. phosphines

Thereforz

was made

a

[la].

Starting with that information and observing that the vibrational behaviour of the R3P-group in R,PY molecules is much the same as it is in the free R3P molecules, the v(P-Y) modes could be easily assigned. This procedure was first applied by us to a related series of R,As and R,AsO compounds [Z].

RESULTS

AND

DISCUSSION

Phosphorus-oxygen Extensive

stretching.

frequency

studies on hundreds

of compounds

containing

the P-O

group

is to be found between 1 I50 and 1350 cm- 1 depending on the other substituents [4,8,9, 10-131. Thomas and Chittenden [4] assigned the phosphorylabsorption in di- and tri-substituted phosphineoxides in the region 1150-1210 cm-‘. ln Table 1 the wavenumbers assigned to th-, stretching frequency l*(P=O) in the IR and Raman spectra of the R3P0 compounds are given. The frequency differences between the values of the solid and liquid state could be explained have shown

TABLE r(P-0)

that the stretching vibration

of this group

I OF RxPO

COMPOUNDS

(Cm-‘)

Rnrnan

IR Solution

Solid

Solution

(C.52)

Solid

tcffcr31

1201 1202 1101 1201

1189

1178/1187

4

(4-F&H&PO (4-FCsH,),C6HsP0 (4-FCsH~)(CsH&P0

1171/1186

1189

I165/1174

5 6 7

(4-CIC6H,)3P0 (4-CICsH,)&H5P0 (4-CICsHd(Cd-W2PO

1207 1205 1204

1190 1190 1190

1190 1191 1189

1189 1197 1191

8 9 10

(3-FCJ&)3P0 (3-FCsI!L&CsH5P0 (3-FCsH.&CsH&PO

1202 1201 1201

1182 1184 1186

1190 1190 1189

1182 1177 1191

1I I2 I3

(3-CICsH.J3P0 (3-CICaH&C6H~P0 (3-CICsHa)(CsH&PO

1208 1206 1203

1190 1190 1189

1195 1195 1191

1187 I183 1182

I4 15

(4-FCsH&(3-FCsHJ)PO (3-FC,H,),(4_FCsH,)PO

I202 1202

X179/1184 1188

1191 1190

1193 1200

1

2 3

GHs)3PO

1190 1179/1183 1185/l

194

1189 1190

1184 1172/l188

235

by lattice effects. The frequency shifts in the

IR

solution

spectra versus the solu-

to Boiselle [3] hydrogenhydrogen bonding as well as dipole-dipole association between solvent and the oxide molecules should be taken into account. Both effects should be stronger with CHC13 as a solvent than with CS2 and so the lower frequencies observed in rhe solution Raman spectra could be explained. The compounds were not sufficiently soluble in CS2 to allow the recording of good Raman spectra. Similar observations can be made for the data of v(P=S) and v(P-Se) in Tables 3 and 4 and the interpretation given above will also be valid for these compounds. For the para-fluorine substituted phenyl compounds two v(P=O) frequencies are found. This is probably caused by inter- or intramolecular forces in the solid state. The fact that we observed splitting of rhe v(P=O) only for a few compounds shows that this is not a general rule, contrary to the opinion of Thomas \ and Chittenden [4] and Goubeau [5]. Even for -P=S and for -P-Se we have > / found only one phosphorus-chalcogen bond stretching frequency. From all these observations it can be concluded that only the frequencies obtained from the IR-absorption spectra of CSI solutions could give reliable material for a discussion of the bonding in the free molecules. It is a general rule that in the absence of external residual bonding of the phosphoryl group, the v(P=O) is chiefly influenced by the electronegativity of the other substituents bonded to the P-atom. For the symmetrical substituted compounds 1,5, 8 and 11 we have calculated the group-electronegativities z and phosphorus-inductive constants l7 using eqns. (1) and (2) published by other authors [6,4]. tion Raman spectra should be solvent effects. According

Y(P=O)

= 950.1+

37.5 Z%

v(P=O)

= 930 +40

(1)

1n

The results are summarized

(2) in Table

2.

TABLE 2 GROUP-ELECTRONEGATIVITIES PHOSPHORUS

1

AND

PHOSPHORUS-INDUCTIVE

CONSTANTS

fl

FOR

SUBSTITUTED

COMPOUNDS

Compound

Y(p-0)

%

l7

N15)

&J-b 12’0 M-FC,H,)aPO !4-CIC6H4)JPO C3-F&H&PO i3-C&H&PO

1201

1202

2.230 2.239

2.258 2.266

f0.07

1207

2.283

2.308

to.23

1202 1208

2.239

2.266

$-o-34

2.292

2.316

+0.37

0

ARYL-

236

From Table 2 the trend for z and L? is found to be (3-C&H,-)

> (4-C&H,-)

> (4FC,H,-)

z (3-FC6H,-)

> (C,H,)

(3)

In (3-FC6Hb)3P=0 however we had evidence for a strong coupling between v(C-F) (I 230 cm- ’ ) and v(P-0) (1202 cm- ‘). In (3-FCGH,),P, Y(C-F) was found at 1212cm”. As a result of anharmonic coupling with v(P=O) the frequency of this fundamental could have been shifted upwards, with a concomitant downfield shift of r(P=O). In the absence of coupling v(P=O) might have been found at about 1209 cm- r and then the sequence of z and n would become (3-CIC,H,)

> (3-F&H,)

L- (4CICsHq)

> @FC,H,)

This trend for group-etectronegativities is in accordance mett G values found by DanieI and Brown [7]. The pftosphorrrs-slcfpftrrr

> (C,H,)

(4)

with the trend for Ham-

srretcftitzg freqrtetzcy

A literature survey of the frequency for the phosphorus-sulphur stretching of P=S in widely different groups of chemical compounds yields a possible region from 550 to 940 cm- ’ [9, II, 13, 14, 151. From the series of compounds studied in this report oniy (CgH5)3PS has afready been investigated by Zingaro [I61 and by Jensen and Nielson [I 7J and the v(P=S) assigned to 627 cm-’ and 637 cm-’ TABLE NPd)

3 OF

RjPS

COMPOUNDS

16 17 18 19

(4-FCd-Ld3PS

‘0 21 22

(cm-*)

IK

Raman

Sulurion Solid (C&)

Solution (CffCf~)

Solid

642 584 595

634 576 590 608

648 584 598 617

633 580 595 612

(4-CIC,H,),PS @-CIC,H+),C,HSPS @-C&d-U(C,H,M’S

672 666 654

668 655 649

679 667 657

672 657 647

23 24 25

(3-F&H&PS

W=C~Ho)rCsH&5 (3-FGI-LIGH512PS

645 642 642

639 638 634

647 647

638 638 636

26 27 28

(3-ClC6H4)3PS (33_CIC,H&CsHsPS (3-CGHd(CsH5)rPS

640 640 641

643 639 637

445

643 641

645 642 639

29 30

(4-FC,H&(3-FCaI&)PS (4-FC,H,)(3-FC6H.&PS

596 613

595 608

602 622

601 619

GH&PS (4-FCsHa)ZC6HSPS (4-FCeHa)(C&ISjrPS

647

237 respectively.

The v(P-S)

in R,PS

molecules is found

as a strong IR and Raman

band in the frequency region 510-700 cm-l. The data for the molecules investigated in this study are collected in Table 3. Again the IR spectral data in solution are considered for the discussion. For the v(P=S) a range from 677 to 884 cm -I is found. A comparison of the data of Table 3 with corresponding data of Table 1 shows that v(P=S) is much more sensitive to substituent effects than v(P=O). Comparison of literature values for v(P-0) in (CH3),P0 and in (C,H,),PO respectively, 1228 1181 and 1 I95 cm-’ [19], with the literature values for v(P-S) in (CH,)3PS and in (C6H5)3PS respectively, 570 and 627 cm- ‘, leads to the same conclusion [ll]. In the literature, however, no agreement was reached about the bonding parameter(s) affecting the v(P=S). Chittenden and Thomas [15] conclude from their data that v(P-S) does not depend on the group-electronegativity, whereas Popov et al. [ 111 and Zingaro [ 161 claim that v(P=S) should be more dependent on the group-electronegativity than the P-O frequency. Since the frequency shifts as a function of R are much more pronounced than for v(P=O) we can draw a sequence for R using compounds 16,20,23 and 26 for increasing v(P-S) and we find 4-Cl&H,

> 3-F&H4

On comparing

z C6H5 2 3-C1C6H4 of v(P=S)

(5)

for 23 and 16 it is obvious that compounds, if each substituent has its own contribution to v(P=S), the values for ths v(P-S) for 29 and 18 shoufd be the same, and this is confirmed by the data in Table 3. A similar remark holds for the data for 19 and 30 in Table 3; the near equality of v(P-S) for 23,24,25 and for 26,27, 28 is also explained by this hypothesis. The nature of substituent effects on v(P-S) was also discussed by Goubeau [20] and he considers four parameters: hybridizatioi? on phosphorus, ion charge of the groups e-g_ SP(SCH,), versus PSa3-, inductive and x-bond effects. Hybridisation and ionic charge can be considered constant in the compounds investigated. If only group electronegativities or the inductive effect alone should inAuence the P=S bond frequency a sequence parallel to that for the v(P=O) (sequence (4)) would be found. The sequences (4) and (5) are however differentBecause x0 > zs the reIative ionic character of the P-O bond is greater than in the P-S bond and the P-O bond will be less polarizable than the P-S bond. For a given change in the inductive effect of the substituent R the relative change in z electron density will therefore be smaller in the P-O bond and the v(P-0) shift will also be smaller_ In R,PS molecules therefore x electron feedback towards phosphorus will show up and this is determined by electronic effects of the substituent. indeed, comparison of the values of v(P=S) for 17 and 20, respectively 584 and 672 cm-‘, shows that the inductive effect alone could not explain this large difference. On the other hand n back donation, 5~(p --, d), phenyl + phospho-

3-FC,H,-

the frequency

> 4-FCsH4

and CsH,- have about the same influence_ In unsymmetrical

238 rus, is also possible in (4-XC6H4)3P [22].This second n-donation will counteract the pn + &, sulphur - phosphorus, back donation and so could lead to a decrease in P-S bond order. In this respect the findings of Gosling and Burg [21] are very interesting. They measured the IR spectra of bis(trifiuoromethyi)dithiophosphinic acid and related derivatives and concluded that the v(P=S) decreases as the adjacent group feeds x-electron density more effectively towards phosphorus. We therefore think that our data should be explained by the assumption that the double bond character of the P=S bond in RJPS compounds is chiefly influenced by polarisation and mesomeric effects of the R group. Comparison of the v(P=S) literature values for PSFJ, PSC13 and PSBr,, 1231, versus the v(P=O) values which are respectively 719, 770 1223 and 825 cm-’ for POF3, POC13 and POBr3, respectively 1415, 1300 [22] and 1261 [23], also indicates that the v(P=0) is strongly affected by th=e electronegativity of the R group. The opposite trend observed for v(P=S) would result from considering an increasing n-donor bonding of the adjacent group R: weakest for bromine and strongest for fluorine, as explained above. The phospJlo]-us-seienitrrtl

stretching

frequency

Very few data on phosphineselenide compounds are available. Only Zingaro [I61 and Jensen and Nielsen [I73 studied the IR spectrum of (C,H,),PSe and TABLE

4

v(P-Se) OF RjPSe

COMPOUNDS

(cm-‘)

IR Solution

Roman Solid

(C&l

31 32 33 34

K6HAJPSe

Solid

(CffC~,)

560

568

542

543

538

551 557

554 561

548 556

585 582 577

583 576 576

586 584 583

587 586 581

G-FC6H4)3PSe (3-FCsH&C6H5PSe (3-FCsHd(CGH&PSe

591 585 579

586 583 581

595 586

585 585

584

585

(3-CIC,H,)3PSe (3-CICsH4)+ZeH5PSe (3-CIC~H+)(CsH&PSe

579

575

581

571

575 569

573 567

576 574

571 572

572

574

577

-

581

580

587

-

(4-FCeHa)(C6H&PSe

564 543 551 558

35 36 37

(4CICsH,)JPSe (4-C1C6H4 j&HSPSe (4-CIC6H4)(C~H~)~PSe

38 39 40 41 42 43 44 4s

Solution

(4-FCeH,)3PSe

(4-FCsHrKsHsPSe

555

239 assigned an intense band at 560 and 568 cm-’ respectively to the v(P=Se). We observed in the IR spectrum (solution) for (C6H5)sPSe an intense band at 564 cm-‘. In Table 4 the data for a series of R,PSe compounds are given. The v(P=Se) values in Table 4 with changing R, yield the sequence 3-F&Ha

z 4-Cl&H4

z 3-CICsHj

> C6H5 > 4-FCsHJ

which is the same as sequence (5) except for 3FC6H,. Again coupling of v(P=Se) in (3FCsHJ),PSe with the fundamental +(C-C)l6a 1161 could be suggested. Changing R creates for v(P-Se) less pronounced shifts than for v(P=S). These results suggest that the x effects in the P=Se bond are less dominant than P=S bond. This observation is consistent with available phosphorus-31 data 1221 and with selenium-77 chemical shifts [24].

in the NMR

EXPERIMENTAL IR spectra were recorded on a Perkin-Elmer 225 grating spectrometer using CsI windows for the 4000 cm-r to 200 cm- ’ region, all frequencies being measured with an accuracy of + 1 cm-l. In the solid state the spectra were taken using mulls in perfluorocarbon in the region 4000-1800 cm-’ and in nujol below In the liquid state CSt was used as solvent. 1300 cm-‘. Raman spectra were recorded on a Coderg PH1 spectrometer with the 6.328 A line from a 200 mV O.I.P. He/Ne laser. CHCls was used as solvent. The compounds were synthesized in this laboratory and details about the synthesis, purification and methods of identification have been submitted for publication elsewhere [ 1b].

ACKNOWLEDGMENT

The authors wish to thank Mr. F. Persyn and Mr. T. Haemers tral measurements reported in this study.

for the spec-

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