Vibrational spectra of pyrylium, pyrylium-2-d, pyrylium-3-d, and pyrylium-d5 cations

Spectmchimics Acts.Vol.28A,pp.1001to 1005.Pergamon Press 1972. Printed inNorthern Ireland

Vibrational spectra of pyrylium, pyrylium-2-d, pyrylium-8-d, and pyrylium-d, cations I. I. ST~~NOIU, M. PARASCHIV, E. ROMAN and A. T. BALABAN Institute of Atomic Physics, P.O.B. 35, Bucharest, Roumania (Received 2 April

1971)

AbstractPyrylium-2-d (VI) and pyrylium-3-d (VII) perchlorates were prepared from the corresponding deuterated pyridines. The ix. absorption spectra of these salts and of the completely deuterated (VIII) and non-deuteratedparent system (I) are discussedin comparison with the vibrational spectra of pyridine analogues. CHEMICAL data [l] and electronic absorption spectra [2] give evidence that the pyrylium cation (I), despite its structural similarity with benzene, behaves quite This is due to the large electronic perturbation caused by the oxygen differently. heteratom; it is in fact the highest perturbation attainable through one heteratom in a benzenic ring [3]. The study of pyrylium salts is therefore interesting insofar as they represent borderline cases of six-membered aromatic systems. Though their aromaticity is manifest in their stability relative to aliphatic oxonium salts and in their ring current [a], they would not be classified as aromatic on the basis This indicates that the reactivity does not constitute a reliable of their reactivity. basis for the definition of aromaticity. On the other hand, physical properties of the pyrylium ring (I) are closely similar to those of benzene (IV) or heteroanologues with heteroatoms possessing progressively varying electronegativity (II, III). A metallic complex of another member of this series, V, was recently reported [6].

Q

@

R II

I

Q

$3 $3 R

III

R V

IV

The present

cation

paper is a study of the i.r. absorption spectra of the pyrylium I and three deuterated analogues VI-VIII in comparison with related

a, VI

@fD VII

D)& D

G VIII

D -

[I] A. T. BALABAN,W. SCHROTH and G. FISCHER,in: Adwllaces in Heterocyclic Chemistq, Vol. 10,p. 241. (Edited by A. R. KATRITZKYand A. J. BOULTON.Academic Press, New York (1969); K. DIMROTHand K. H. WOLF, in: NewerMethods of Preparative Organic Chemistry, Vol. 8, p. 367 (Edited by W. FOERST). Academic Press, New York (1964). [2] A. T. B-AN, V. E. SAEINIand E. KEPLINOER,Tetrahedm 9,163 (1900). [3] A. T. B-AN and Z. SIMON,Tetrahedron 18,315 (1962); Rev. Roumuke Chinz. 9,99 (1964). [4] A. T. BALABAN,G. R. BEDRORDand A. R. KATRITZKY,J. Chem.Sot. 1646 (1964). [S] G. E. HERBE~CH, G. GREISSand H. F. HEIL, Angew. C&m. 82, 838 (1970); Recently, alkaline salts of V were prepared: A. J. ASHE and P. SW, J. Am. C&m. Sot. 93, 1804 (1971); P. JUTZI,Aragew. Chem. 83, 912 (1971). 13

1001

1002

I. I. SThOIU,

M. P-CHIP,

E. ROBSAY and A. T. BALABAN

species II or III and their deuterated analogues. The perohlorio anion was selected beoause of convenience, though its intense absorption bands at 1100 (broad) and 625 cm-l obscured part of the spectrum. The i.r. spectra of I [6] and VIII [7] have already been reported briefly; in

4ca

300

600

700700

IM)o

1200

1400

1600

1800

I I2200I I2460 I I 2600 I

2000

2600

3WO

Fig. 1. Infrared speotre, of I, VI, VII and VIII in KBr pellets: main trace, 2 mg; upper trace (2200-2600 cm-l), 4 mg; lower trace (loo@-1200 cm+), 0.6 mg compound in 300 mg KBr. [6] A. T. BAXABAN, G. D. MATEESCU [?I I. I. STXNOIU and A. T. BuB~.

and M. ELIAN, Tetrahedronl&1083 (1962). Rev. Rotmaine Chim. 18,127 (1968).

3200

Vibrationalspectraof pyrylium,pyrylium-2-d,pyrylium-3-d,and pyrylium-db cations 1003 the present paper these spectra have been redetermined with greater accuracy and some reassignments have been made. EXPERIMENTAL A three-prism (KBr for 400-700 cm- 1, NaCl for 700-2000 cm-l and LiF for 2OOO-4OOO om-1) double beam i.r. spectrophotometer Jena UR-10 was employed. Wavenumbers were calibrated by employing polystyrene film standards and spectra with an expanded wavenumber scale. Potassium bromide pellets (300 mg) with at least three concentrations of perchlorates (O-5, 2 and 4 mg) were prepared. Perchlorates I [6], VI, VII and VIII [7] were prepared from the corresponding pyridines by the procedure of KLAGES and !I!R&ER [S]. Pyridine-2-d and pyridine3-d were commercial samples produced in USSR with isotopic contents of 97.5 y0 VI, and 90.7% VII, respectively (from the mass spectra, peak with m/e 80). The crude pyrylium perchlorates were recrystallized with charcoal from glacial acetic (20 vol.) to which 1 vol. acetic anhydride and 1 vol. 70 y0 perchloric acid had been added; after filtration of the hot solution through strong filter paper, anhydrous ether was added into the filtrate to assist in the crystallization. The white perchlorates can be stored indefinitely in the absence of moisture and light in the refrigerator. Little or no deuterium exchange appears to take place on recrystallization. RESULTS AND DISCUSSION The spectra of the four perchlorates

I, VI, VII and VIII

are presented in Fig.

In the spectrum of VIII, weak bands corresponding to the strongest bands of I can be noted at 1620 and 3130 cm-l: they are due to the non-deuterated I originating in the initial non-deuterated pyridine in pentadeuteropyridine as well as in some deuterium exchange on preparation and/or recrystallization of VIII. Non-deuterated I is also present in VII as can be inferred from the shoulders at 652 and 998 om-l. On the basis of previous assignments of the vibrational modes for I [6] and VIII [7], as well as of the related pyridine (Py) [g-21] pyridine-2-d and pyridine-3d [11-13, 173, as well as pyridine-d, [lo-12, 14-211, (literature data for these 1.

[S] F. KLA~ES and H. TRXUER,Cimw Ber. 86, 1327 (1963). [9] C. H. I&NE and J. TtJRKEvICH, J. Chem. Phye. U, 300 (1944). [lo] L. CORRSIN,B. J. FAX md R. C. LORD, J. Chem. Phys. 21, 1170 (1963). [ 1I] J. P. MOCULLOUQH,D. R. DOUSLIN, J. F. MESSERLY,I. A. HOSSENLOPP,T. C. KINCEELOE and G. WADDINGTON,J. Am. C?wn. Sot. 79,4289 (1967). [12] F. A. ANDERSEN, B. BAK, S. BRODEB~ENand J. R. ANDERSEN,J. Chem. Phye. f& 1047 (1966).

[13] [14] [16] [IS] [I73

[IS] [19] [20] [21]

J. H. 8. GIUZEN,W. KYN~TON and H. M. PAISLEY, Spctrochim. Actu 19, 649 (1963). J. K. WILMBEVRSTand H. J. BERNSTEIN,Can. J. Chem. 35, 1183 (1967). E. WA(IH-N and E. W. SCHMID,2. Phgeik. Chem. (Frankfwt) 27, 146 (1901). M. A. KO~XER, Yu. S. KOROSTELEVsmdV. I. BEREZIN, Opt. iSpetroekopz$a 10,467 (1961). V. I. BEREZIN,Opt. i Spektroekopiya 15, 310 (1963). D. A. LONG and F. S. MAIN, T*ans. li’araday Sot. &&I171 (1967). D. A. LONG, F. S. MUIU-IN md E. L. THOMAS,Tmm. Faraduy Sot. 59,12 (1963). D. A. LONU and E. L. THOUS, Twms. Faraday Sot. 59,783 (1963). C. ZERBI, B. CIUWPORD, JR. and J. OVIEBEND, J. Chtm. Phys. 88,127 (1963).

1004

I. I.

ST&OIlJ,

M. PARASU~,

E. ROMANand A. T. BALABAN

Table 1. Vibrrttionalassignmentsproposed by various authors* Py-2-d

PY Vibr*tion

[IO, 121

[ll]

[13]

Py-3-d

[141

Cl11

[121

Cl31

[ll]

[12]

[131

Py_d, UO, 121 WI

[14]

1 2 60 80 9a 12 13 180 190 20a

992 3054 605 1580 1218 1029 3054 1068 1482 3036

992 3055 120 1583 1218 1030 3055 1068 1483 3035

992 3054 605 1583 1218 1030 3036 1068 1482 3054

992 3054 605 1583 1218 1030 3054 1068 1482 3036

989 3040 148 1576 1149 1029 3051 1059 1461 2258

989 3040 600 1576 1149 1029 3051 1059 1461 2258

989 980 980 3051 3038 3038 600 717 599 1576 1570 1570 1149 1195 1195 1029 1033 1033 2258 3053 3053 1059 1050 1050 1462 1468 1468 3040 3022 3022

3 6b 7b 8b 14 15 18b 19b 20b

1217 652 3054 1572 1375 1148 1068 1439 3083

1200 1288 653 652 3055 3036 1571 1572 1317 1375 1146 1148 1085 1085 1440 1439 3080 3080

1218 652 3036 1572 1375 1148 1085 1439 3083

1212 640 3068 1570 1357 1112 834 1424 3075

1212 640 3068 1510 1357 1112 834 1424 3015

1294 640 3068 1570 1378 834 1112 1424 3075

lOf3 16a 170

886 374 981

885 374 1030

891 375 986

886 374 981

814 360 997

814 360 910

886 360 970

823 360 965

823 350 965

885 355 965

690 329 798

713 329 798

690 329 798

4 5 lob 11 16b

675 942 749 703 405

675 981 940 605 405

149 942 886 700 405

749 942* 886 700 405

638 970 901 600 405

748 932 814 600 405

632 965 902 599 404

670 902 717 632 404

717 938 823 632 404

625 762 567 530 371

582 690 762 530 371

567 823 762 530 371

660-70 901 748 638 405

980 3053 599 1570 1195 1033 3089 1050 1476 3038

962 2293 582 1530 887 1006 2270 823 1340 2254

962 962 2293 2393 690 582 1530 1530 887 886 1006 1006 2270 2270 823 823 1340 1340 2254 2254

1217 1217 1262 650 650 650 2289 2289 3030 1570 1570 1570 1337 1337 1390 1108 1108 848 848 848 1108 1416 1416 1416 3077 3077 3077

908 625 2285 1542 1322 887 833 1301 2293

908 908 625 625 2285 2285 1542 1542 1322 1322 887 887 833 833 1301 1301 2293 2293

* Refs. [I61 and [19] confirm by caloulstion assignments made by Refs. [13, 141,except that Ref. [19] propmed to repleoe 942 by 981 cm-’ for Py. Therefore the assignments in the last oolumm 82‘8considered BB the moat pleusible.

Table 2. Vibrational assignmentsproposed for pyrylium species* Vibration 1 2 60 8a 9a 12 13 180 190 200 3 6b 7b 8b 14 15 186 19b 20b 4 6 lob 11 16b l

Il symmetry

-41

B1

B,

H-2-d VI

II-3-a VII

I-I-d, VIII

998 (3061) 582 1620 1214 1021 (3061) 1040 1473 (3063)

990 (3057) 578 1609 1169 1029 2308 1051 1466 3046

984 3053 578 1612 1198 1028 3115 1038 1469 (3031)

960 (2392) 561 1571 893 1008 (2350) 809 1397 2304

1216 652 (3053) 1557 1349 1170 1120 1472 3125

1216

950

(3057, 1552 1295 823 1120 1435 3121

1211 639 (3031) 1549 1337 842 1122 1436 3125

(2350, 1516 1309 862 821 1360 (2392)

179 959 755 652 (400

761 968 890 604 <400

783 958 850 <400

839 795 518 <400

I

Bands enclosed in brackets are assigned to more then one vibretion.

Vibrational

Spectra of pyqdium,

pyrylium-l-d,

ppylium-3-d,

and pyrylium-do

cstione

1005

pyridines are summarized in Table l),* the assignments presented in Table 2 are considered as the most plausible for the pyrylium cations (II). The strong bands at 625 and 1100 cm-l are due to the perchlorate anions. By recording spectra at various concentrations of pyrylium perchlorates in the pellets it is possible to distinguish the bands which are partly obscured by the 1100 cm-l perchlorate band (not, however, by the 625 cm-l perchlorate band which may obscure the Vibrations are indicated missing vibrations, 6b and 11, in the spectra of VI-VIII). by WILSON notations [22, 231. (Corresponding alternative notations are indicated in Refs. [19, 20 and 241.) In Table 2, planar vibrations of A, symmetry which are inactive in the i.r. have been omitted. Consideration of Tables 1 and 2 shows a fairly close similarity between i.r. spectra of the pyridine and pyrylium ring systems. It is noteworthy that both the C-H and C-D stretching frequencies are appreciably higher in pyrylium than in pyridine. This effect can be ascribed to the higher electronegativity of the heteroatom, and to the positive charge of the ring. In the vibrational assignment of the N-methyl-pentadeuteropyridinium ion, a similar but smaller effect is apparent for vibration 20~ [25]. * Further literature data for pyridine and pyridine-de were reviewed in Ref. [7]. [22] [23] [24] [25]

E. A. F. E.

B. WILSON, JR., Phys. Rev. 45, 706 (1934). LANUSETH and R. C. LORD, KgZ. Dmeke VkZe&xzb. Mat. Fys. Medd. 16, No. 6 (1938). B~L~ERSON, Rev. Mod. P?y8. 19, 87 (1947). SPINNER, Au&&an J. C&ma. 20, 1805 (1967).