Trialkylarsine oxides: Donor properties toward iodine

J. inorg,nucl. Chem..1966.Vol. 28, pp. 2981 to 2985. PergamonPress Ltd. Printedin NorthernIreland

TRIALKYLARSINE OXIDES: DONOR PROPERTIES TOWARD IODINE F. L. KOLAR, R. A. ZINGARO and K. IRGOLIC Department of Chemistry of Texas A & M University, College Station, Texas

(Received 6 June 1966) Abstract--Equilibrium quotients and extinction coefficients have been measured in carbon tetrachloride solutions, at 25, 35 and 45 °, for charge-transfer complexes which form between iodine and tris(n-octyl)arsine oxide, or tris(cyclohexyl)arsine oxide. These molecules, using the equilibrium constants as a criterion, possess a donor strength toward iodine considerably greater than that of triphenylarsine oxide, tdphenylarsinc or the trialkylphosphine oxides. INTRODUCTION

THE DONORability of the arsine oxides has been demonstrated in the study of their formation of charge-transfer complexes with halogens and interhalogens and of co-ordination compounds with transition metal ions. The overwhelming number of investigations has dealt with triphenylarsine oxide ~ but the donor properties oftrimethylarsine oxide C2~and diphenylmethylarsine oxide ~8~have also been studied. In a recent report/4~ the exceedingly hygroscopic nature of the trialkylarsine oxides has been discussed. It is because of this very extreme sensitivity to moisture that, despite continued efforts during the past several years, the first reproducible data on two trialkylarsine oxide-iodine systems are being reported. The donors used in this study, tris(cyclohexyl)- and tris(n-octyl)arsine oxides are, relatively speaking, less moisture sensitive than the lower homologues. Nevertheless, great care in the exclusion of moisture and the performance of many experiments was necessary to obtain reproducible results. EXPERIMENTAL

Reagents. The arsine oxides were prepared and purified as described elsewhcre:*~ The carbon tetrachloride was Matheson, Coleman & Bell Spectroquality Reagent grade. J. T. Baker rcsublimed iodine was used. Apparatus. A Beckman model DK-I double beam recording spectrophotometer with a thermostatically-controlled cell compartment (=[=0-1°C)was used for recording the spectra. Preparation of solutions. All attempts to prepare solutions of the arsine oxides in the ordinary atmosphere resulted in contamination by atmospheric moisture. Therefore, stock solutions of the ~1~~,~ D. J. PmLL~S and S. Y. TYRO, JR., 3". Am. chem. Soc. 83, 1806 (1961); ¢~ F. A. CoTtON, D. M. L. GOODOAMEand M. GOODGAME,Inorg. Chem. 1, 239 (1962); tc~ R. A. ZINGAROand E. A. M~Y~RS, ibid. 1, 771 (1962); ta~ S. M. HORNERand S. Y. TYP~E,JR., ibid. 2, 568 (1963); ¢,~F. A. COTTON. D. M. L. GOODGAMEand R. H. SODERnER6,ibid. 2, 1162(1963); ¢1~C. I. BRANDEN, Acta chem. scand. 17, 1363 (1963); ~g~D. FORST~Rand D. M. L. GOODGAMe,J. chem. Soc. 458 (1965); th~ D. M. L. GOODOAMEand M. GOODGAME,Inorg. Chem. 4, 139 (1965); co D. M. L. GOODGAMEand M. A. H r r ~ N , ibid. 4, 721 (1965); o~ L. CHANDRAS~ARANand G. A. RODLeY, ibid. 4, 1360 (1965); c~ D. B. COPLEY,F. FAn~ROTrIZRand A. THOMPSON,J. less-common Metals 8, 256 (1965). t2) F. SCHINDI_~, H. SeHMIDeAtreRand G. JONAS, Angew. Chem. 77, 170 (1965). ~*~J. Lewis, R. S. NYHOLMand G. A. RODLeY, Nature, Lend. 207, 72 (1965). t4~ A. MERJAmANand R. A. ZINC3AItO,lnorg. Chem. 5, 187 (1966). 2981

2982

F . L . KOLAR, R. A. Z~oA]~o and IC IRc,o u c

arsine oxides were prepared inside a nitrogen filled dry-box. An analytical balance was also in the dry-box so that all weighing, dilutions and transfers took place therein. These stock solutions had concentrations of 2-5 x 10-8 M. The stock iodine solutions of concentrations 4-5 × 10-8 M were not prepared in the dry-box. For each arsine oxide, a series of solutions was prepared in which the iodine concentration was maintained constant while the donor-aeceptor ratios ranged from 1 : 1 to 2:1. Standard ground glass sealed 1-cm silica ceils containing the solutions were placed in the temperature controlled compartment of the instrument. Sufficient time was allowed for the solutions to come to equilibrium at the desired temperature as indicated by the reproducibility of the spectra. The concentrations at the higher temperatures were calculated from the 25° concentra tions using the known density changes of carbon tetrachloride, tsl CALCULATIONS

AND

RESULTS

At high donor to acceptor ratios (>20:1), the only absorption bands observed were at about 365 and 295 m/z with an absorbance ratio of c a . 1:2. The intensity of both peaks decreased with time and on the basis of published valuesIs~ they could confidently be attributed to the presence of Is'- ion. It was found that if the concentrations of arsine oxide and iodine in carbon tetrachloride were about equal, the spectra displayed the typical "blue-shift" band at 410-415 nap. These spectra were reproducible over a period of several hours and then slowly shifted with the concurrent appearance of the intense 365 nap (Is-) peak. The charge-transfer band could not be located because it was probably beyond the cut-off point of the solvent. For the reasons just presented, viz., formation of I a- ion at high donor to acceptor 7

7o 3

:~ 5 0

"~

40

/////

r - -I . , .

J

\

o

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Y 0

400

450

Wavelength,

500

m/J.

FIG. 1 . - - S l ~ t r a l absorption curves for the system tris(n-octyl)arsine oxide-iodine in carbon tetrachloride at 25 °. The initial concentration of iodine in each solution is 4.82 x 104 M; the arsine oxide concentrations are as follows: (1) 0; (2) 3"43 × 10-4 M; (3) 4.58 × 1 0 - ' M ; (4) 5.72 × 10-4M; (5) 6.87 × 1 0 - ' M ; (6) 8.01 × 1 0 - ' M ; (7) 9-16 × 10-4 M. m E. W. WAsnntn~N, (Ed.), International Critical Tables of Numerical Data, Physics, Chemistry and Technology, Vol. 3, p. 28. McGraw-Hill, New York (1928). ts~ A. I. PoPOV and R. F. SV~NSEN,J. Am. chem. Soc. 77, 3724 (1955).

Trialkylarsine oxides: donor properties toward iodine

2983

ratios, the method of BENESIand HILDEBRANDtT) was not applicable to the calculation of equilibrium constants for this system. Instead, the graphical method of Rose and DRAGOts~ was used. The presence of a good isosbestic point (Fig. 1) can be used as evidence to indicate that only two absorbing species are present. It should be pointed out that the calculated equilibrium constants are, in fact, concentration equilibrium quotients since the activity coefficients are not generally known. The standard free energy, AG~98, was calculated from the relationship A G ° = - - R T l n K and the enthalpy change, AH, was calculated by plotting R In K vs. 1/T, in the conventional manner. The entropy change was calculated from the relationship AG = A H - T AS. The results obtained for the two systems studied are given in Table 1. TABLE 1.--RESULTS

FOR

1:1 ARSINE OXIDE-IODINE COMPLEXES IN CARBON TETRACHLORIDE

Temp.

~max, blueshift band (m/z) A~1/9 (cm -I) e~ x 10-s 0/mole. cm) K x 10-s (I/mole) --AH (keal/mole) --AG~98 (kcal/mole) --AS

(°C)

(C6H,x)sAsO.I~

(CsH10aAsO.Iz

25 35 45 25 35 45 25 35 45 25 35 45

410 411 412 5296 5344 5593 2'06 4- 0"01 2.12 q- 0.03 2.09 4- 0.02 15-7 4- 0.6 8.22 4- 0.44 6"36 4- 0.19 8"55

411 412 410 5058 5386 5565 1'91 4- 0"03 2.00 4- 0.01 2.00 4- 0.02 12.0 4- 0.9 6"07 + 0.09 5.46 4- 0.25 7"12

5.72

5.56

9"50

5.23

(e.u.) DISCUSSION

In Table 2 are listed a series of donors in which a group VA or group VIA element functions as the a-donor atom towards iodine. Because the thermodynamic data are incomplete, comparisons can only be made using the equilibrium constants as a criterion of donor strength. The trialkylphosphine and trialkylarsine ehalcogenides are stronger donors than the corresponding triphenyl derivatives: a~ This is to be expected in view of the greater electron releasing ability of the alkyl groups. The nature of the organic substituent is not as critical as the chalcogen or the group VA atom in determining the stability of the complex. This is readily observed when the t~ H. A. BENESIand J. H. HILB~RAND, d. Am. chem. Soc. 71, 2703 (1949). ~s~N. J. ROSE and R. S. DRAGO, d. Am. chem. Soe. 81, 6138 (1959). t0) For the tris(n-butyl)phosphine sulphide-iodine equilibrium in chloroform, K = 59 (R. A. ZINGARO R. E. McGLOTHLIN and E. A. MEYERS,J. phys. Chem. 66, 2579 (1962)). This is obviously not consistent with the general observation.

2984

F.L. KOLAn,R. A. ZINOAROand I(. IR(~m.zc TAnLe2.--EQummRIUMcol,~rAlcrs FOR 1:1 IODINECOMPLF.X~ Donor (C~H6)lO (C2Hs)iS (CHs)2S (CHa)2Se (CeHs)aPS (CeHs)sPSe (CsHI~)aPO

(CoHlx)aPO (CsHu)sPS (CeHu)sPSe (CeHs)aAs (CeHs)sAsO (C6Hx08AsO (CaH17)3AsO

Solvent eel, Heptane CCI~ CCl4 CHCIs CHEla Heptane CHCls CHCls CHC1s CC1, CHEla CCI~ CC14

K(1/mole) 8'7 210 71 472 106 3370 588 39 1820 46,600 1400 41.2 15,700 12,000

Temp. (°C) 25 20 25 25 25 25 25 25 25 25 20 25 25 25

Reference 10 10 11 11 12 12 12 12 12 12 13 ...............l(c) . . . .

values of the equilibrium constants involving the following sets of donors are compared: (CeHu)aPSe > (C6H10aPO; (Cells)aPSe > (CeHs)3PS; (CH3)zSe > (CHs)2S and (C2H5)~S > (CzHs)20. The observed sequences are to be expected since the e-donor strengths are expected to follow the order Se > S > O. This has been discussed in greater detail elsewhere c14) both in terms of ionization potentials and ,r-interactions. The results reported in this investigation furnish the first experimental data which suggest that among the R3MX molecules, where M is a group VA and X a group VIA atom, the donor strength increases as the electronegativity of M decreases. Thus, again comparing the values of the equilibrium constants, we find that they increase in the order (CeHn)aPO <(CsHn)aAsO and (CaHI~)sPO < (CsHI~)aAsO. This trend is not unexpected. In the R3MX molecules, the electron density around the g-donor ~Itom, X, is expected to decrease upon complexation with iodine acceptor. The greater the electronegativity of M, the less will be the tendency of X to function as a g-donor. Thus, among the M - - O sequences which have been experimentally observed, we find the RaA~----O molecules to be considerably stronger bases than the corresponding R3P--O molecules. The obvious inadequacy of this explanation is immediately recognized in consideration of KUBOTA'S recent work (15) on iodine-amine oxide equilibria. He reports for the systems tribenzylamine N oxide-iodine in CH2C12 and for trimethylamine N-oxide-iodine in the same solvent the K values 3479 (20°) and 5660 (22°) 1/mole and AH = --10.5 and --10"0 kcal/mole, respectively. It is apparent that while the tertiary arsine oxides are stronger donors than the tertiary amine oxides, the latter are stronger than the tertiary phosphine oxides. Another important observation worthy of note is that AH for the triphenylarsine-iodine system has been found to be --9"4 kcal/mole, (xa) a value somewhat (10)H. TSUBOMURAand 17,.p. LANG,J. Am. chem. Soc. 83, 2085 (1961). (1x)N. W. TmESWELLand J. D. McCuLLOOGH,J. Am. chem. Soe. 79, 1031 (1957). (t~) R. A. ZINGARO,R. E. McGI.OTHLINand E. A. MEYERS,J. phys. Chem. 66, 2579 (1962). (tt) E. AUGDAI-IL,J. GRUNDNESand P. KLABOE,Inorg. Chem. 4, 1475 (1965). (14t}W. TEFTELLER,JR., Ph.D. Dissertation, Texas A & M University (1966). (Is) T. KUBOTA,d. Am. chem. Soe. 87, 458 (1965).

Trialkylarsine oxides: donor properties toward iodine

2985

larger than that measured in this work for the two trialkylarsine oxides. The latter are, based on the values of the equilibrium constants and free energies of formation, much stronger donors. The greater negative enthalpy measured in the former case is very likely due to the greater stability of the very probable tetrahedral geometry which results on formation of the Ph3As.I2 complex. In view of the obvious inadequacies resulting from most of the conventional explanations, there seems little justification for an extended discussion concerning the various inconsistencies in donor strengths. As with so many other attempts at classification, they depend largely upon the definition one chooses to apply. There are so many factors involved, including geometry, solvent effects, ~r-bonding or lack of it, relative electronegativities, etc., that any consistent order appears to be unattainable since they apply in varying degrees to different situations.

Acknowledgements--Wewish to express our appreciation for financial support to the United States Atomic Energy Commision, Contract At-(40-1)-2733 and to the Robert H. Welch Foundation of Houston, Texas. Mr. KOLARhas also been a recipient of a NASA fellowship. The technicalassistance of Mr. DONALDE. LINOERis also appreciated.