The Reaction of Triphenylphosphine with Alkali Metals in

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REACTION OF TRIPHENYLPHOSPHINE WITH ~ K A L METALS I

effects would be comparable with the experimental error of temperature measurement ( Alo). Therefore, no corrections are necessary for the temperatures with PFz = 13.2 torr, and the calculated activation

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Table 11: Surface Temperatures of Graphite in Various Fluorine Partial Pressures a t a Furnace Temperature of 790' PF?, torr

Surface temp. , O C .

Temp. rise, OC.

0 2.8

790 795

0 5

6.5

10

13.2

800 810

52.5 74.7

890 940

20 100 150

energies are valid for the fluorination of graphite a t this partial pressure. At higher fluorine pressures, there will be correspondingly greater surface effects, and temperature corrections may be significant. However, the lack of thermochemical data on C,F, precludes quantitative corrections, and one expects them to be small since the monofluoride detaches itself from the graphite surface as soon as it is formed a t the higher temperatures.

Acknowledgment. This work was supported by the Aeronautical Research Laboratory, Wright Air Development Division, United States Air Force, in part, under a subcontract with A. D. Little, Inc., administered by Dr. Leslie A. McClaine.

The Reaction of Triphenylphosphinewith Alkali Metals in Tetrahydrofuran'

by A. D. Britt and E. T. Kaiser2 Department of Chemktry, Univmsity of Chicago, Chicago 67,Illinois (Received March 89, 1966)

The first reaction of triphenylphosphine with alkali metals which has been detected is a phenyl cleavage rather than formation of the triphenylphosphine negative ion. Radical formation occurs in a second reaction when the metal diphenylphosphine produced by the f i s t reaction is reduced by additional metal. The resultant radical has the formula (CeHs)2PM-,as identified by electron spin resonance and chemical tests.

Introduction The reaction of triphenylphosphine ((CsHs)&?) and alkali metals in tetrahydrofuran (THF) has been reported to produce the mononegative ion ((C&3&€"), identsed by its electron spin resonance (e.s.r.) spect r w n 8 Since the phenyl cleavage of (C&&)J? by alkali metals in T H F is a well-known r e a ~ t i o n , ~the -~ nature of this reported radical has been reinvestigated. The reaction was studied as a function of the concentration of (CaH&P for each of the following alkali metals: K, Na, and Li. The course of the reaction was determined by chemical tests and e.s.r.

Experimental

Materials. The solvents and metals were purified and the solutions prepared in the usual Com(1) Grateful acknowledgment is made to the donors of the Pe troleum Research Fund for support of this research. (2) To whom inquiries regarding this article should be made. (3) M. W. Hanna, J. Chem. Phua., 37, 685 (1962). (4) D.Wittenberg and H. Gilman, J. Org. C h m . , 2 3 , 1063 (1958). (5) I(.Isdeib and H. 0. Frohlich, 2.Naturfwsch., 14b, 349 (1959). (6) A. M.Aguair, J. Beialer, and A. Mills, J. 0 ~ g Chem., . 27, 1001 (1962). (7) D.E.Paul, D. Lipkin, and S. I. Weissman, J. Am. Chem. SOC., 78, 116 (1956).

Volume 69,Number 8

August 1966


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mercial triphenylphosphine was recrystallized from diethyl ether to yield white plates melting a t 80" m.p. SO'). Diphenylphosphine was prepared from diphenylchlorophosphine ((CeH~)2PC1) .lo The diphenylphosphine was obtained as a colorless oil, b.p. 69-71' a t 0.3 mm. Samples were stored in evacuated break-seals. Determination of Biphenyl Concentration. The possibility that biphenyl might be formed in the reaction of (C6H5)3Pwith alkali metals was investigated. For the determination of biphenyl, the following procedure was used. The solutions resulting from the reaction of (CeH5)3Pwith alkali metals were opened to air and transferred to 25-ml. flasks. Five milliliters of 0.1 N HC1 was added dropwise to each solution followed by 10 pl. of diphenylmethane. The THF was slowly evaporated at 40' until the volume of the mixture corresponded t o approximately 5 d. The solutions were then extracted with benzene, and the benzene extract was dried over NazSOIand slowly evaporated to a small volume (1 or 2 drops). Aliquots were then analyzed by vapor phase chromatography. Two standard samples were also analyzed by this procedure: one consisted of 10.5 mg. of biphenyl and the other was 10.5 mg. of biphenyl which was first reduced in elacuo to the biphenyl negative ion (determined by e.s.r.). Results on both samples showed that biphenyl and diphenylmethane are carried through the analysis procedure in the same weight ratio as their initial weight ratio. Therefore, the final ratio of biphenyl to diphenylmethane together with the known initial concentration of diphenylmethane can be used to provide a good estimate for the maximum concentration of biphenyl in the triphenylphosphine-alkali metal reaction mixtures.

Results and Discussion The reaction of (C6H5)3Pwith K in THF was first studied as a function of the concentration of (C&5)3P. Variation of the initial concentration of (C6HS)iP over the range 7 X to 2 X 10-1 M established the presence of two distinct reactions. Below M (C6H5)3PIthe reaction yields products which are yellow-green in solution and which give no e.s.r. signal. From to M (C6H5)3P1a second reaction occurs, yielding products which are orange-red in solution and which give the e.s.r. spectrum previously reported for (C6H6),P-.3 This spectrum was recorded a t -20" and is shown in Figure 1. The concentration range of (CeH5)3P which was to found to be most convenient for study was 10-1 M . Careful treatment with the alkali metal permits the reaction either to be stopped a t the first The Journal of Physical Chemistry

A. D. BRITTAND E. T. KAISER

Figure 1. E.s.r. spectrum of (CsH&PKT a t

-2OO.

stage or carried further to the second stage. If the reaction is stopped a t the &st stage and the yellowgreen solution is removed from the metal, then the orange-red color cannot be obtained either by concentration of the solution or by addition of unreacted (ceHs)&'. Therefore, the second reaction appears to occur between the metal and the products of the first reaction, rather than with (C&&)aPdirectly. The possibility that a one-step reduction might occur a t a lower reaction temperature was investigated. Independent variation of the reaction temperature over the range -60 to 25" showed no change in the two-reaction sequence apart from longer reaction times a t lower temperatures. The products of the first reaction did not appear unstable with respect to temperature. The radical obtained in the second stage of K reduction was relatively stable a t -20°, but slowly decomposed above that temperature. Opening the solutions to air caused rapid conversion of the orange-red color back to the yellow-green color, which was thereby intensified. Treatment with aqueous acid and subsequent analysis yielded the types of products described for the phenyl cleavage reaction. 4-6 The phenyl cleavage products were obtained regardless (8) P. Balk, G. J. Hoijtink, and J. W. H. Schreurs, Rec. trau. chim., 76, 813 (1957). (9) I. Heilbron, Ed., "Dictionary of Organic Compounds," Oxford University Press, London, 1953. (IO) F.G. Mann and J. T. Millar, J. Chem. Soc., 4453 (1952).

REACTION OF TRIPHENYLPHOSPHINE WITH ALKALIMETAM


of either the initial concentration of (c&&P or the extent of the reaction. A reaction sequence which is consistent with the information given above is

(M denotes the metal.) 111, like I, can produce (C6H5)2PH and its oxidized derivatives upon aqueous acid treatment. Therefore, reaction 2 does not affect the nature of the final products which are used to demonstrate the phenyl cleavage reaction. On the other hand, a t initial concentrations of 10-1M (CeH&P and higher, the alkali metal reaction cannot be stopped a t the first stage and only the single orange-red color change is visually observed, in agreement with observations made in ref. 3. As mentioned in ref. 3, decomposition of (CeH5)aP into smaller fragments is a possible interpretation of the alkali metal reaction. There are several ways to test the nature of the radical and the validity of the two-reaction sequence. The tests which were chosen are the following: (1) The (CeH&PK- structure should be consistent with the e.s.r. spectrum in Figure 1. (2) Use of other alkali metals should cause changes in any metal splittings seen by e.s.r. (3) It should be possible to prepare the radical from (C6H5)2PMdirectly, with complete exclusion of (CaH5)aP. According to the first test then, the e.s.r. spectrum of (CSH~)~PK(Figure 1) should be consistent with: one P atom per radical (two lines of intensity ratio 1:1) ; one K atom per radical (4 lines of intensity ratio 1: 1 : 1 : 1) ; and an electron distribution pattern from two benzene rings connected through a trivalent phosphorus. The primary feature of the e.s.r. spectrum is the appearance of two broad lines of equal intensity, separated by 8.4 gauss and associated with one P atom per radical. With increased resolution, each of these two broad lines is seen to be composed of a set of four or five lines separated by about 2 gauss, with considerable overlap in the center of the spectrum. This had led to the conclusion that the five lines in each set arise from splittings from four equivalent protons (intensity ratio 1:4:6:4:1) in the postulated (CaH&P-species.a Greatly increased amplification of the signal showed the presence of an additional line of low intensity a t each extreme of the spectrum. This additional peak is sharper a t the high-field side of the spectrum and is

2777

barely discernible in Figure 1, 2 gauss above the last intense peak. Each set is therefore composed of seven lines, arising from splittings of six equivalent protons (intensity ratio 1 :6 :15 :20 :15 :6 :1). Under high resolution, each of these seven lines is seen to be composed of 10-12 lines separated by about 0.2 gauss. Expansion of the spectrum showed that these additional lines were disposed as three groups of four lines each. Within each of the three groups, the four lines are of equal intensity and are split by 0.24 gauss. The splitting between the three groups is about 0.8 gauss; the first and third group are of equal intensity, and the middle group is about 1.5 times as intense as either the first or third. These three groups of lines are considered to be the most intense lines of a set of five lines from splittings by four equivalent protons (intensity ratio 1 :4 :6 :4 : 1). The weak first and fifth lines are considered to overlap with corresponding lines of adjacent sets. Within each group, the four lines of equal intensity are considered to be due to splittings from one K per radical. The electron distribution pattern in the two benzene rings indicates an equivalence between the protons in the para positions and those in either the ortho or meta positions (total of six equivalent protons), with the four protons in the nonequivalent positions furnishing the five-line set split by 0.8 gauss. The proof of which position is equivalent to the para position awaits deuterium labeling or some similar process. However, the pattern bears a striking resemblance to that of the diphenyl nitroxide system in which the two benzene rings are connected through a trivalent nitrogen. In the diphenyl nitroxide case a, = a, = 1.94 gauss and a, = 0.82 gauss.11 If an analogous assignment is made for the (CeH&PK- species, a, = a, = ca. 2 gauss and a, = ca. 0.8 gauss. Construction of a theoretical spectrum with the diphenyl nitroxide values, together with the P and K splittings, shows a close match with Figure 1 in terms of number of lines, splittings, and intensities. The e.s.r. spectrum is therefore consistent with the formulation of the radical as a metal diphenylphosphine negative ion, as stated in reaction 2. A second way of testing the nature of the radical involves use of other alkali metals and observation of changes in metal splitting in the e.s.r. spectrum. For this test, Li and Na were used. Our results were in accord with the well-established fact that the nature of the triphenylphosphine-alkali metal reaction is independent of the particular metal emp10yed.~~~ The colors, reaction sequence, concentra(11) J. Pitnnell, Mol. Phys., 5, 291 (1962).

Volume 69,Number 8 August 1966


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tion level of (CBHS)SP necessary for radical production, and final products were the same with Li, Na, and K. However, the lithium and sodium diphenylphosphine negative ions differed from the potassium diphenylphosphine negative ion in two important respectstemperature stability and e.s.r. spectra. Both (c6H&PLi- and ( C O H ~ ) ~ P N appeared ~unstable above -50". The e.s.r. spectrum of (C6H5)2PLiT at -65" showed two main groups of three broad lines each. The splitting between the centers of the main groups was about 8 gauss; the splitting between the three broad lines in each group was about 2 gauss. The crude outline of the (C6H5)2PK- spectrum is therefore seen with (C6H5)PLi-. Attempts to resolve the spectrum further showed additional unresolved splittings in each of these six lines. To the extent to which these additional splittings could be resolved, great variations in line width were observed (0.2-0.5 gauss). The e.s.r. spectrum of (C6H&PNaT a t -65" showed essentially one broad derivative curve covering the entire spectrum. The peak-to-trough splitting was about 8 gauss. Under high resolution, 8-9 unresolved lines were seen on each side of the spectrum. To the extent to which these 16-18 lines could be resolved, great variations in line width were observed (0.2-1.0 gauss). The nature of this effect remains to be established. However, we would speculate on the effect in the following way. The K radical derivative is relatively stable at -20" ; simple replacement by Na or Li at the metalphosphorus bond might alter the temperature stability by a few degrees, but not as drastically as is observed. This might indicate the possibility of intramolecular rearrangement to different configurations with the metal at a different location, for example, one of the para positions. With the K derivative, the principal configuration involves a P-K bond; with the smaller Li and Na, other configurations might also be present in appreciable percentages. These other configurations could serve as an avenue for temperature inst& bility. In addition, such intramolecular exchange of the metal atom to positions in the two rings could produce great variations in the proton line widths, as is observed.12 Once the e.8.r. spectra of the Li, Na, and K derivatives were obtained, the nature of the radical could be subjected to a third test, direct preparation by means of reaction 2 in the absence of (C6H5)SP. For this test, (C6H5)21'H was prepared and treated with Li, Na, and K. The initial reaction of (C6H5)2PHwith the alkali metal led to formation of a yellow-green solution which gave no e.s.r. signal. The JOUTW.Z~ of Physical Chemistry

A. D. BRITTAND E. T. KAISER

The solution was outgassed to eliminate Hz and then concentrated. No orange-red color or e.s.r. signal was obtained. The concentrated solution was then diluted back to its original volume and treated further with the alkali metal. An orange-red solution was obtained which gave an e.s.r. spectrum identical with the e.8.r. spectrum of the corresponding metal derivative of (C6H5)SP. These results prove that the radical is produced from (CaH&PM, not ( C G H ~ ) ~and P , therefore support the two-reaction sequence. An additional point was investigated concerning the nature of the (C6H5)2PM-radical. It is known that the metal is closely associated in (C6H5)2PM.1aOne might reasonably assume that the metal is even more strongly associated in the corresponding negative ion -(C6H5)2PMT. Therefore, it should be difficult to dissociate the species and obtain the e.s.r. spectrum of the (c6H5)2P7ion. Two experiments were performed to explore this point. In the fist experiment, a K-(C6H5)3Preaction was performed in 1,Zdimethoxyethane, and a radical spectrum identical with Figure 1was obtained at -20". The second experiment was more extreme, involving a Na-(C6H&P reaction in liquid NH3. Na was chosen because it was the most effective alkali metal in terms of obscuring the basic spectrum. Correspondingly, the greatest difference should be seen between (C6H5)2PM- and (C6H5)2l+ if Na is used. The e.s.r. spectrum of the radical produced in liquid NIfa was recorded at -65" and was identical with the (C6H5)SNa- spectrum in THF at -65". Use of other alkali metals or different temperatures might disclose appreciable dissociation, but this does not appear likely. We found no alterations in e.s.r. spectra over temperature ranges consistent with the stability of the (C6Hs)2PMTradicals. Attempts were made to investigate possible intermolecular exchange by measurements of line broadening as a function of temperature. The e.s.r. spectra of the Na and Li derivatives were scanned from -50 to 25" and the e.s.r. spectrum of the K derivative from -20 to 25". Broadening occurred in all cases, but the intensity of the signal also decreased rapidly with increasing temperature. For this reason, the broadening cannot be attributed solely to some intermolecular exchange process. In any event, the narrow lines in Figure 1 demonstrate that for the (C6H5)2PK' system a t -20", any intermolecular exchange reaction possesses a half-time longer than the range of halftimes which may be observed by e.s.r. line broadening. (12) E. DeBoer and E. L. Mackor, J. Am. Chem. SOC.,8 6 , 1513 (1964). (13) K.Issleib and A. Taschach, Chem. Ber., 92, 1118 (1959).

NOTES


Finally, since the formation of biphenyl from the reaction of (C6H5),& species (where X is a heteroatom) with alkali metals is a fairly common occurrence and can cause complications in the interpretation of e.s.r. spectra, it was considered of interest to analyze several of the radical-containing solutions for biphenyl. An estimate of the maximum concentration of biphenyl which might be present was obtained. It was in the

2779

range 2-5 X loV5M for the various metal radical solutions. Therefore, the effect of biphenyl was considered to be negligible.

Acknowledgments. We wish to express our appreciation to Professor S. I. Weissman for his help in obtaining several of the spectra. We also thank Mr. T. F. Wulfers and Mr. P. L. Hall for their aid in the vapor phase chromatographicanalysis.

NOTES

On the Chlorophyllide- Sensitized Reduction of Azobenzene and Other Compounds

by G. R. Seely Contribution No. 186 from the Charles F. Kettering Research Laboratory, Yellow Spring& Ohw (Received February 8, 1966)

We have found1 that the reduction of phenosafranine by hydrazobenzene in ethanol-pyridine mixtures, photosensitized by ethyl chlorophyllide a, is not retarded by one of its products, azobenzene, a t 1.7 X 10-4 M . This suggested that excited chlorophyllide, Chl*, reacts much faster with phenosafranine than with azobenzene. We have now found that azobenzene a t the same concentration almost completely prevents the photochemical reduction of ethyl chlorophyllide by ascorbic acid in ethanol-pyridine mixtures.2pa At a lower azobenzene concentration (4 X 10" M ) , where the reduction of chlorophyllide is still strongly retarded, the loss of azobenzene can be followed by the decline of absorptivity at 335 mp. As the azobenzene disappears, the reduction of chlorophyllide is accelerated. Without ascorbic acid there is no loss of azobenzene; there is no reaction in the dark. Livingston and Pariser reported that chlorophyll sensitized the photoreduction of azobenzene by phenylhydrazine, but not by ascorbic acid, in methanol s ~ l u t i o n . ~ It seemed likely that azobenzene was reduced to hydrazobenzene in this reaction. To see whether this was so, or whether azobenzene was converted to

some other product, we tried hydrazobenzene as a reductant, in 1:6 ethano1:pyridine containing 0.05 M benzoic acid. (Hydrazobenzeneis inert as a reductant for Chl* in ethanol-pyridine mixtures, unless acid is present.) As expected, the absorption a t 335 mp did not remain constant or decrease but slowly increased by an amount commensuratewith the amount of chlorophyllide reduced. We believe that azobenzene is reduced by the radical ChlH. formed in the reduction of chl~rophyllide~ for the following reasons. Azobenzene and ChlH2, the stable photoreduction product of chlorophyllide (absorption maximum a t 525 mp),2 coexist in the dark without reaction, which rules out ChlH2 as the effective reducing agent. Increasing the ascorbic acid concentration from 2 X to 2 X M accelerates both the reduction of chlorophyllide and the reduction of azobenzene, but the former more than the latter. This is compatible with the mechanism (I) Chl*

+ AH2

4ChlH.

+ AH-

+ ChlH2 ChlH. + D +Chl + DH. 2ChlH. +Chl

(1) (2)

(3)

D and AH2 represent the oxidant and the reductant, in this case azobenzene and ascorbic acid. The product of step 1 is written ChlH. because transfer of a (1) G. R. Seely,J. Phys. Chem., 69,2633 (1965). (2) A. A. Kraanovskii, DON.Akad. Nauk SSSR, 60, 421 (1948). (3) G. R. Seely and A. Folkmanis, J. Am. Chem. SOC.,86, 2763 (1964). (4) R.Livingston and R.Pariser, ibid., 78, 2948 (1956). . (5) V. B. Evstigneev and V. A. Gavrilova, Dokl. Akad. Nauk SSSR, 92, 381 (1953).

Volume 69,Number 8 August 1966

The Reaction of Triphenylphosphine with Alkali Metals in

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