The infrared spectra of organoarsenic compounds

Spectrochimica Acta, Vol. 24A, pp. 999 to 1006. PergamonPress 1908. Printed in Northern Ireland

The infrared spectra of organoarsenic compounds L. D. PETTIT a n d D. TURNER Department of Inorganic and Structural Chemistry, The University, Leeds

(Received 28 September 1967)

Abstract--Theinfrared

spectra of a wide range of organoarsenic compounds, both aromatic and aliphatie, arsenic (III) and arsenic (V), have been interpreted qualitatively. Factors considered include: (a) the dependence of absorption bands associated with phenyl ring vibrations on the presence of the arsenic atom and its substituents, and on the substitution pattern of the ring; (b) the arsenic-carbon (aliphatic) and related methylene, carbonyl and hydroxyl vibration frequencies; (c) the bands characteristic of arsonic and arsinic acids.

INTRODUCTION

VARIOUS authors have included data gathered from the infrared spectra of particular classes of organoarsenic compounds in more broadly based studies but there has been no attempt to compare the spectra of a wide range of organoarsenicals. For instance arsenic has been included in studies of the spectra of compounds containing X( :O ) O H groups [l], of the phenyl derivatives of group IVb, Vb and VIIb elements [2, 3], of compounds of the type (CeHs)3MX where X ~- O, S or Se [4], and a systematic study has been made of the As~-S bond [5]. In this communication the infrared spectra of a large range of organoarsenie compounds, both aliphatic and aromatic and including arsenic (III) and arsenic (V), are discussed and compared with those already reported so that further assignments of absorption bands can be made. The presence of phenyl groups complicates the spectra of aromatic organoarsenie compounds considerably. Interpretation of the spectra has therefore been divided into two sections. In Section 1 bands resulting from vibrations in the phenyl ring or the As--C (aromatic) bond are considered and in Section 2 bands resulting from the As--C (alkyl) bonds and associated carbonyl, methylene, oxygen or hydroxyl groups. On account of the large number of compounds studied it is not practical to tabulate each spectrum. Instead particular vibration frequencies of assigned bands are tabulated, together with any data available in the literature to support the assignments made and demonstrate the frequencies over which the particular vibration m a y be found. EXPERIMENTAL

Spectral data A b s o r p t i o n s d o w n to 400 c m -1 were m e a s u r e d on a G r u b b P a r s o n s DB1, GS4 i n s t r u m e n t . All s p e c t r a were o b t a i n e d f r o m t h i n films (liquids) or K B r discs (solids). [1] [2] [3] [4] [5]

J. T. BRAUNHOLTZ,G. E. HALL, F. G. M A n and N. SHEPPARD, J. Chem. Soe. 868 (1959). C. N. R. RAO, J. RAIW-ACHANDRANand A. BALASUBRAMANIAN,Can. J. Chem. 39, 171 (1961). L. A. HARRAIL M. T. RYA~ and C. TAMBORSKI,Speetroehim. Aeta 18, 21 (1962). K. A. JENSE~ and P. H. NIELSEN, Acta Chem. Scand. 17, 1875 (1963). R. A. ZINGARO, R. E. McGLOTHIN and R. M. HED(~ES, Trans. FaradaySoe. 59, 798 (1963). 999

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Nujol was used for a comparison with butyl-arsenic acid but the absorption bands were not so well defined as those with a K B r disc. Absorptions below 400 cm -~ were measured on a Grubb Parsons DB3 Radiation Unit with a monochromator unit using caesium iodide optics. The spectra were obtained as thin films or nujol mulls between polythene plates.

Compounds used All compounds used were prepared by published methods, or modifications based on these [6, 7]. They are listed in Table 1. In all 39 compounds were obtained in an analytical pure state. Many were readily oxidized or were powerful vesicants or lachrymators so required special handling. For convenience, the number given to each compound in Table 1 is used to identify t h a t compound in later tables. Table 1 Compound

Identffication number

n-BuaAs MeAsC12 EtAsI, n-BuAsC12 n-BuAsI, EtAs(:O)(OH)2 (n-Pr)2As(:O)OH n-BuAs (: O) (OH) 2 CH 2:CH'CH,As(: O)(OH), (CeHs)3As (2-Me.C6Ha)aAs (C6Hs.As)e CeHsAsC12 2-CI'C6H4AsC1~ 3-CI'CeHaAsC1 ~ 4-CI'CsHaAsCI~ 3-Mo'CaH~AsCI~ 4-Me-C~HaAsCI~ CaHsAs( : O) (OH)~ 2-Me'C~HaAs ( : O}(OH)~

I II III IV V VI VII VIII IX X XI XII XIII XIV XV XVI XVII XVIII XIX XX

Compound

Identification number

3-Mc'CaH4As( : O)(OH)z 4-Me'C6H4As ( : O) (OH)2 2-NO,'5-Me'CaH3As {: O)(OH), 2-Me'CeH4As ( : O) (OH)CHaCOOH 4-Me'C6HaAs ( : O ) (OH)CH2COOH C6HsAs(CI)CH,COOH 2-Me.C6H4As(CI)CH2COOH CeI-IsAs(CHaCOOH) 2 2-Me-CeH4As(CH2COOH )2 3-Me-C6H4As(CH2OOH)2 4-Me.C6HaAs (CH,CO OH) ~ (C6Hs)2As( : O)OH (C6HsAs( : O)(OH)CH2)~ CsHsAs : O (C6Hs)aAs : 0 CsH5As(CH~CH~NH~)~ 2-COOH'C~HaAs (Me) ~ (2-COOH.C~H4)~AsMe (2-COOH-C~Ha)aAs

XXI XXII XXIII XXIV XXV XXVI XXVII XXVIII XXIX XXX XXXI XXXII XXXIII XXXIV XXXV XXXVI XXXVII XXXVIII XXXIX

The compounds 37-39 were prepared by oxidising the parent tolyl arsines with aqueous alkaline permanganate solution [8] and reducing the resulting arsenic (V) to arsenic (III) with SO S in HC1. Compound 36 was prepared by reacting phenylarsine with ethyleneimine in ether [9]. I)ISCUSSION

Sectio~ 1 Arsenic-phenyl interactions. I f the phenyl ring in a monophenylarsine contains no additional substituents the molecule will approximate to the general formula CeHsX as far as arsenic-phenyl interactions are concerned, and so should show [6] D . R . LYON a n d F . C. MA.wlv, J. Chem. £*oc. 30 (1945); E . R . F . JONES a n d F . C. M A ~ , ibid. 401 (1955), Inorg. Syn. 6, 113, 116; a n d r e f e r e n c e s t h e r e i n . [7] J . T. B R A V ~ O L T Z a n d F. G. MANN; J . Chem. Soc. 3285 (1957). [8] Org. Syn. 8, 740 (1955). [9] W . C. F~.RN~LIVS, p r i v a t e c o m m u n i c a t i o n .

The infrared spectra of organoarseriic compounds

1001

the fundamental infrared active vibrations of halobenzenes which have been studied in detail b y W~:IFFE~ [10]. Assuming that deviations from the C~.~ symmetry of halobenzenes are small, Whiffen's assignments should provide a basis for interpretation. The halobenzenes should show 27 infrared active fundamental vibrations although only a small number of these would be markedly X-sensitive. In their study of phenyl derivatives of elements from groups IVb, Vb and VIIb, I-IARRXH et al. [3] found seven bands which were either distinctly X-sensitive or distinctive Table 2 B a n d range [3] (cm -1) 1451-1428 1190-1050 755-725 701-675 507-433 520-190 (520-314 in halobenzenes)

Band range (this work) (cm -1) 1451-1429 1100-1071 752-730 699-673 518-458 463-407

Assignment and symbol [3.10] (C--C) stretch (C--X) stretch (C--H) deformation (C--C) deformation (C---X) deformation Uncertain m a y be ( C - - C - - C ) deformation

Bff(C--C) Aft(C--X) B27(C--H ) B2~b(C--C) B2~(C---X) 7(C--C--C)

of a benzene ring bonded to a heavy atom. Of these seven bands we have omitted the band between 806-654 cm -1 [assignment b y W ~ F F ~ N to a (C--X) vibration] since the range is large for any certainty in assignment in this region and HARRAH et al. found it of little qualitative value. RX~r])LE and WHIFFE~ [11] point out that this frequency is more easily determined from R a m a n spectra. The other six X-sensitive bands have been assigned b y WHIFFE~ and HARRAH et aI. as shown in Table 2. HARRAH et al. [3] assign the 1451-1428 cm -1 band to a planar ring deformation but W~rrFFE~ [10] and KATRITSKY and LAOOWSKI [12] assign it to a stretching vibration. Clearly the band is associated with the nature of X b u t it does not appear to be specific for any particular C - - X linkage as long as only heavy atoms are involved. Rather, assuggested b y Harrahet al., it isassociated with a phenyl group perturbed b y a heavy substituent atom. I t has been assigned to a (C--Si) [13], a (C--P) [14] and a (C--As) vibration [15]. The splitting in this band in the spectrum of (2-carboxylphenyl)dimethylarsine, also found in other parts of the spectrum, is presumably due to the gem-dimethyl groups. The assignments to these six Xsensitive bands in the spectra of the compounds studied are shown in Table 3. A question mark indicates that the band was either obscured or that assignment was very unreliable. [1O] D. H. WHrFFE~r, J . Chem. See. 1350 (1956). [11] R. R. RANDLE and D. H. WHIFFEN, Molecular Spectroscopy, p. 111. Institute of Petroleum

(1955). [12] A. R. KATRITSKY and J. M. LAGOWSKY,J . Chem. Soc. 4156 (1958). [13] C. W. YOUNG, P. C. SERVAIS, C. C. CURRIE and M. J. HU~TER, J. A m . Chem. Soc. 70~ 3758 (1948). [14] L. J. BELLAMY, The Infra-red Spectra of Complex Molecules. John Wiley (1958). [15] L. HOm~ER and H. 0~.DIOER, Ann. 627, 142 (1959).

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T a b l e 3. X - s e n s i t i v e b a n d s in c o m p o u n d s c o n t a i n i n g a r s e n i c - - p h e n y l b o n d s Frequency Compound Halobenzene X XI XII XIII XIV XV XVI XVII XVIII XIX XX XXI XXII XXIII XXIV XXV XXVIII XXIX XXX XXXI XXXII XXXIII XXXIV XXXV(a) XXXVI XXXVII XXXVIII XXXIX

(C--C)

(C--As)

stretch

stretch

1451-1428 1429 s 1447 s 1429 s 1429 s 1429 s 1451 s absent 1445 w 1443 w 1439 s 1449 m 1447 w 1443 w absent 1433 s 14887 1429 s 1429 s 1429 s 1427 s 1437 s 1429 s 1429 m 1434 w 1428 s 1428 s 1447 m 1447 m

1190-1050 1072 m 1053 w 1092 w 1071 s 1096 s 1096 m 1087 s 1093 m 1087 w 1092 s 1055 w 1100 s 1089 s 1042 w 1063 w 1085 s 1078 s 1099 s 1092 s 1074 s 1087 s 1085 s 1075 s 1086 re(b} 1086 s 1053 w 1042 w 1080 w

(C--H)

deformahon 755-725 739 s, 731 s 746 s, 741 s 732 s 736 s 757 s ? 729 s ? absent 744 s 752? absent absent ? ? 757 s 748 s 742 s 757 w absent 746 s, 735 s ? 725 s 738 s 741 s 739 s 746 s 746 s

(era -1) (C--C)

deformation 701-675 692 s 709 w 690 s 686 s ? 673 s absent 684 s 698 w 688 s 699 w 683 s 699 m 680 w 699 m 699 m 694 s 707 w 693 m 714 w 689 s 689 s 689 s 693 w 690 s 683 m 671 m 671 m

(C--As)

deformatmn 507-433 474 s, 466 s ? 463 s, 458 m 459 m ? 514 w 488 m ? 486 m, 483 m 465 s 494 w 509 m 487 s 515 s? 505 m 485 s 465 m 496 w, 476 w 459? 483 m, 465 m 476 s, 456 s 469? 518 m 512 m 472 s 485 w 485 m, 468 m 490 s, 474 m

(C--C--C)

deformation 520-314 ? 439 m ? ? 429 m 422 m ? 420 w ? 407 m 433 s 430 s ? 410 w 427 m 408 w 412 w 432 w 427 w ? ? ? 463 s 479, 460 m ? 416 w 437 w 420 w

(a) Data from BERI~STEIN e$ al., J. Chem. Soc. 821 (1964). (b) Assigned in (a) to a (C---H) deformation.

Aromatic substitution patterns. Absorption bands characteristic of the type of substitution (o% m- or p-) in the region 2000-1650 cm -1 are obscured in all the acids studied by bands associated with carbonyl and hydroxyl vibrations. In the region 1225-735 em -1 bands characteristic of substitution patterns often overlap with the X-sensitive band between 1190-1050 cm -1 but the (C--H) out of plane deformation bands (860-730 cm -1) were readily characterised by their intense absorption and are listed in Table 4. These bands show that mono- and di-substituted phenyl-arsenic compounds conform to the analogous substituted benzenes. Hexaphenylcyclohexaarsine (HPCHA). HPCHA has been shown to have the formula (Cells)eAse, having a ring of arsenic atoms in the chair configuration [16]. The u.v. spectrum of HPCHA (in 0.005% chloroform since it is insoluble in methanol) was obtained to compare it with that published for triphenylarsine in methanol [ 17]. The spectra (Table 5) are in very close agreement apart from the strong absorption at about 2000 A in the triphenylarsine spectrum which is totally absent in that of HPCHA. This is likely to be a solvent effect. This similarity indicates similar bonding between arsenic and the phenyl rings in both types of compound. When HPCHA is reacted with Mo(C0)e in dibutyl ether a brown compound, [16] K . HEDBERG, E. W . HUGHES a n d J. WASER, Acta Cryst. 14, 369 (1961). [17] W . R . CtrLLEN a n d R. M. HOCHSTRASSER, J. Mol. Spectry. 5, 118 (1960).

The infrared spectra of organoarsenic compounds

1003

Table 4. Aromatic substitution bands--(C--H) out of plane deformations F r e q u e n c y (era -1) Compound

Disubstituted Monosubstituted ortho 770-730 739 S, 731 S 733 S 736 S 758 W 735 S 767 S 725 S 740 S

X XII XIII XXVIII XXXII XXXlII XXXIV XXXVI XI XIV XXXVII XXXVI1T XXXIX

meta

770-735

746 758 738 746 746

para

810-750

860-800

s, 741 s s s s s

XV XVII XXI XXX

775 s, 752 m 774 787 s, 766 s 781 s, 757 w

XVI XVlII XXlI XXXI

810s 801 s 862 s, 801 s 852 m , 800 s

recrystallised from benzene, is obtained (m.p. 210 ° to a blue liquid). Analysis for carbon and hydrogen is in agreement with the formula (CsHa)eAse.~o(CO)3. The infrared spectrum of this compound is in close agreement with that for HPCHA alone but, in addition, shows peaks of high intensity at 1923 and 1859 cm -1 and of medium intensity at 603, 576 and 459 cm -1. The first two absorption bands are Table 5. Ultra-violet absorption maxima (A) (CeHs)aAs (CeHs)6Asa

~2000 --

2465 2480

2610 2570

2670 2640

2730 2700

comparable with the published data for [(CsHs)aAs]al~Io(CO)3and (AsCla)3Mo (CO)3 [18]. Thus it is reasonable to assume that a complex has been formed between HPCHA and Mo(CO)e without disruption of the arsenic ring system. FOWLES and Jv.~KINS assume the ring system to be (CeH5)4As 4 in a study predominantly based on the phosphorus analogue [19]. The crystallographic evidence does not support this. Section 2

Carbonyl stretching vibrations, ~co, occur, in most compounds, at about 1700 em -1 [6]. The carbonyl derivatives studied here the CO group either ~ or fl to the arsenic atom and, in similar compounds studied [7], it is found that the arsenic atom, being less electronegative than Other interactions.

[18] E. W. ABEL, M. A. BE~-E~r and G. W~g_rNsoN, J. Chem. Soc. 2323 (1959). [19] G. W. A. FOWLESand D. K. J~.~g_r~s, Chem. Comm. 61 (1965). 4

organic contain already carbon,

1004

L . D . PETTIT and D. TURNER

tends to decrease the frequency. I n all the compounds studied this absorption is of high intensity and very readily identified. Small variation in wave number are found but t h e y do not conform to any simple pattern. Values are shown in Table 6. Related vibrations found in carboxylic acids are in the region of 1250 and a broad based band around 920 cm -1. These vibrations have been identified as coupled (O--H) stretch and an (O--H) out of plane deformation respectively. Bands which Table 6. Bands not associated with the phenyl rings F r e q u e n c y ( c m -1) Compound (C--O) stretch (1700) I II III IV V VII VIII IX XXIV XXV XXVI[7] XXVII XXVIII XXIX XXX XXXI XXXVI XXXVII XXXVIII XXXIX

(O--I-I) c o u p l e d stretch (1300)

(O--H) bend (935)

(CH2) b e n d (1465) 1453 s 1449 m

1695 1709 1683 1678 1670 1681 1689 1681

s s

1280 s 1309 s, 1252 s

893 s? 852 m ?

s s s s s

1290 1282 1276 1295 1297

? 924 s, 902 s ? 913 s 912 m

1669 s 1695 s 1695 s

s s s s s

1311 s, 1290 s 1319 m 1330 s

841, 824 w ? 833? 837?

1456 1460 1464 1428

m s m w

1389 1385 1389 1389 1370

m m m m m

(As---C) s t r e t c h (570, 583) 556 w, 578 m 515 w, 565 w, 523 w, 584 w, absent 550 m

546 505 511 540 518 567 ? ?

w, w, m, w, w, m,

641 w 543 641 603 590

w w w vr

645 w 649 w 649 w 654 w 534 w 572 m

can be assigned with reasonable confidence are shown in Table 6. Some occur as doublets and are listed as such while in other cases shoulders are present indicating doublet character but only the main peak has been tabulated. This is found in m a n y dicarboxylic acids. The deformation vibration, in particular, is very sensitive to environment and shows large variations in both position and intensity so the assignments made, particularly for the 2-carboxyphenylarsine derivatives, should be regarded as tentative. Certainly no other bands are present in this region which cannot be assigned reliably elsewhere but it m a y be t h a t the absorption is of very low intensity or obscured. The CH 2 deformation (scissoring) vibration is usually at about 1465 cm -1 but in the compounds studied is generally of lower frequency, particularly in the substituted phenylarsine derivatives which show an absorption band at 1390 cm -1. The bands assigned to this vibration are all sharp and well-defined. Other bands in the region can be assigned with confidence, mostly to (C--C) vibrations in the phenyl ring, giving reasonable confidence to the assignments to the CH2 bend in Table 6. Arsenic, particularly bonded to heavy groups such as phenyl, would be expected to decrease the frequency of the deformation; in fact the decrease found increases as the total mass increases. For comparison, the chlorine atoms in CH2C12 decrease the frequency from the expected 1465 to 1429 cm -1.

The infrared spectra of organoarsenic compounds

1005

The CH9 rocking bands, expected at about 720 cm -1, are also clearly defined in the aliphatic compounds studied and show little mass dependence, all having frequencies between 720 and 710 cm -1. Identification of twisting and wagging vibrations is difficult due to strong absorption bands characteristic of arsenic and arsinic acids [1]. BRAU~HOLTZ et al. have studied the spectra of acids containing the group M(:0)OH, where 1~I = P, As, S, Se or C and observed three broad bands, called Table 7. Arsenic and arsinic acid absorption bands

Compound

Apprommate frequency (em-1) A B (2700-2600) (2300-2000)

C (1720-1600)

Arsomc acids VI VIII IX XIX XX XXI XXII XXIII

2760 2760 2780 2740 2780 2700 2740 2790

2240 2320 2310 2270 2300 2270 2280 2290

1610 1580 1650 (peak at1631) 1700 1620 (peak at1590) 1700 1660 (peak at 1587) 1700

Arsinic acids VII XXXII XXXIII

2700 2670 2700

2260 2270 2230

1670 1700 1610

A, B and C, in the region 3000-1500 cm -1, which they consider typical of the above group [1]. All the arsenic and arsinic acids studied, both aromatic and aliphatic, are in broad agreement with this. The bands are all very broad so t h a t wavenumbers given in Table 7 for the absorption maxima are only very approximate. The A (2700-2600 cm -1) and B (2300-2000 cm -1) bands, assigned to (O--H) stretching vibrations, are both clear and characteristic in all cases. The C (1720-1600 cm -1) band, assigned to an (O--H) deformation, is present in all cases but is of variable intensity and, in some cases, had sharp bands superimposed on it. I n all the arsinic acids studied the A band had some sharp absorption peaks superimposed. These were absent in the arsenic acids. In addition to the compounds listed in Table 7, the arsinic acids of general formula R.CsHa.As(O)0H.CH~C00H appeared to show a similar pattern of A and B bands but these, and the region of the C band in particular, were generally confused by bands resulting from the carboxylic group, hence t h e y are not so useful for diagnostic purposes. Strong absorption was also found in the region 1220-1200 cm -1. In organophosphonates absorption in this region has been attributed to a (P--O) stretching vibration [20]. Since this absorption was also found in both aromatic, and aliphatic arsenic acids it must be associated with the (As--O) bond also but it seems unlikely t h a t (P--O) and (As--O) bonds would have the same stretching frequency. In addition to the frequencies shown [20] D. F. PEPPARD, J. R. FERRAROand G. W. MASON,J. Inorg. Nucl. Chem. 12, 60 (1959).

1006

L . D . PETrIT and D. TURNER

in Table 7, both the arsonic and arsinic acids showed broad absorption peaks in the region 940-740 cm -1. There appeared to be about five main bands in the region but overlap made resolution difficult. They presumably include the (As--0) out of plane vibration, found at about 840 em -1 [21], but, since interpretation would be very unreliable, they have been omitted. A comparison of the absorption spectra of phenyl arsonic acid and diphenyl arsinic acid (0.01% solution in alchol) in the near ultraviolet shows t h a t the ratio of the extinction coefficients, e, of the absorption bands is approximately equal to the ratio of the number of phenyl groups. This extends the observations of JAFF~ who has studied the spectra of phenyl substituted phosphines and arsines Table 8. Arsenic--chlorlne stretching vibrations Compound Frequency (cm-1)

AsCla(25) 370

IV 388 368

XIII 388 365

XVI 400 357

XVIII 390 365

XXVII 385 370

[22] and shows t h a t there is little conjugation between the phenyl groups in diphenylarsinic acid since each contributes approximately equally to the total extinction coefficient. Assignment of bands to (As--C) (aliphatic) vibrations proved more difficult, partly on account of the physical problem involved in handling m a n y of the simpler compounds studied. SIEBERT [23] and ROSE~BAVM et al. [24] have given full assignments to the (As--C) vibration modes in As(CH3) 3 on the basis of data obtained from R a m a n spectra. They found main absorption bands (both doublets) at about 580 cm -1 [(C--As) stretch] and 230 cm -1 [(C--As) bend]. We only studied the higher frequency region and made the assignments shown in Table 6. In almost all cases the two peaks expected (often these were themselves unresolved doublets) were found although the intensities were usually medium to weak and the separations appeared to vary considerably. In most eases it was found t h a t the separation tended to increase as the mass of the molecule increased, the ~e vibration going to lower and the ~2o to higher frequencies. The (As--C1) vibrations in AsC13 have been studied and a broad band at about 370 cm -1 assigned to the (As--C1) stretch [25]. Broad bands of high intensity were found in this region in all the chloroarsines studied in this part of the spectrum. Frequencies are listed in Table 8. The bands appeared to be doublets. This could result from a coupling of the two (As--C1) vibrations in the dichloroarsines giving in phase and out phase vibrations, one on either side of the expected (As--C1) stretching band. [21] F. A. MILLER,G. L. CARLSON,F. F. BENTLEYand W. H. JONES,Spectrochim. Acta 16, 135 (1960). [22] H. H. JAFFa, J. Chem. Phys. 22, 1430 (1954). [23] H. SIEBERT, Z. Anorg. Allgem. Chem. 278, 161 (1953). [24] E. J. ROSENBAUM,D. J. RUBIlVand C. R. SANDBERG,J. Chem. Phys. 8, 366 (1940). [25] J. CABANNESand A. ROUSSET,Ann. Phys. 19, 229 (1933).