2652
Elizabeth V. Patton
H
and
Robert West
conclusion is further supported by the earlier observation t h a t there is close association between the hydrogen-bonded HMPA and the cation. A similar association is observed here with the ammonia HMPA complex and the cation involved in ion pairing. Acknowledgment. The authors are grateful t o the Research Corporation for the support of this work. References and Notes (1) G . R . Stevenson, L. Echegoyen, and L. R . Lizardi, J , Phys. Chem., 76, 1439 (1972). (2) (a) G. R. Stevenson, L. Echegoyen, and L. R. Lizardi, J, Phys. Chem.. 76, 2058; ( b ) G. R. Stevenson, and L. Echegoyen, ibid.. in press. (3) G. R . Stevenson and H.Hidalgo, J . Phys. Chem., 77, 1027 (1973). (4) M. T. Watts, M . L. Lu, and M . P. Eastman, J. Phys. Chem., 77, 625 (1973). (5) F . J. Smentowski and G. R. Stevenson, J. Amer. Chem. Soc.. 90, 4661 (1968)
New Aromatic Anions. IX. Anion Radicals of the Monocyclic Oxocarbons' Elizabeth V. Patton and Robert West* DePdrtment of Chemistry University ot Wisconsin, Madison, Wisconsin 53706
(Received June 27, 7973)
Publication costs assisted by the National Science Foundation
Electrolytic oxidation of bis(tripheny1phosphine)iminium salts of squarate, croconate. and rhodizonate anions in dichloromethane gives electron spin resonance spectra attributed to C 4 0 4 .-, C 5 0 5 .-, and c606' -. respectively. Electrolytic reduction of the same species gave a radical only for rhodizonate, believed to be & & - . A 13c hyperfine splitting constant of 4.10 G was found for C 5 0 5 - - . The implications of this value, and of t h e g values for all the radicals, for electron distribution in these species is discussed.
The properties of the monocyclic aromatic anions, C,O,-", have been studied in some detail over the past 10 years.2 Four members of t h e series are known. The dianions with n, = 4 (squarate), n = 5 (croconate), and n = 6 (rhodizonate) are all extremely weak bases which form very stable, high-melting salts. The tetraanion C60s4has also been isolated as the tetrapotassium salt,3 but i t is much more reactive, undergoing rapid oxidation in air. The existence of intermediate one-electron oxidation and reduction products of the oxocarbon dianions has been predicted.2 These radicals could be stabilized by electron delocalization in the same way as the parent dianions. Two groups have reported tentative evidence for the existence of oxocarbon anion radicals in the solid state. Air oxidation of the tetrapotassium salt of C6Oa4--, which leads ultimately to C6O&, produces a n intermediate paragmagnetic solid surmised to be CSO6.3-.3 Also, Buchner and Lucken treated potassium metal with carbon monoxide a t 250" and obtained a mixture of products containing a radical giving a single-line esr ~ p e c t r u m .This ~ was attributed to the species c606.5 - . The Journal of Physical Chemistry, Vol 77, NO. 22, 1973
We wish to report the generation of oxocarbon anion radicals in solution by electrolytic oxidation or reduction of oxocarbon salts in dichloromethane a t -85". Repeated attempts to obtain esr signals of oxocarbon anion radicals upon electrolytic oxidation or reduction in aqueous solution were uniformly unsuccessful. We then wished to study oxocarbon anions in nonaqueous solvents, in which temperatures low enough to stabilize the radicals might be possible. Most oxocarbon salts are highly insoluble in solvents other than water, or water-alcohol mixtures, and it proved quite difficult to find oxocarbon salts with sufficient solubility in nonhydroxylic solvents. Ultimately, however, salts of the oxocarbon dianions with bis(tripheny1phosphine)iminium cation5 were prepared and found to be soluble in organic solvents. Upon electrolytic oxidation, the squarate, croconate, and rhodizonate salts all formed radicals giving single-line electron spin resonance spectra which we attribute to c404* -, ' 2 5 0 5 - -, and C6O6. -, respectively. Confirmation of this assignment for C505.- is given by the observation of 13C sidebands with a hyperfine splitting of 4.10
2653
Anion Radicals of the Monocyclic Oxocarbons
I i
i /
.2 8
.24
.20
-A
.I6
(Pi
Figure 2. Differences in g value vs. h (in units of 6 ) for oxocarbon monoanion radicals. Figure 1. Esr spectrum of C 5 0 s . - radical obtained by oxidation of bis(tripheny1phosphine)iminium croconate, showing sidebands attributed to hyperfine splitting by 13C.
G (Figure 1). The area under each sideband was 2.5
f
0.5% of the area under the central peak; for a species with electron delocalization over five equivalent carbon atoms, the sideband intensities are predicted to be 2.970. The species produced by oxidation of c404'- and c60S2- gave only singlets. These oxidations were repeated many times under slightly different conditions b u t 13C sidebands were never observed. It is curious t h a t 13C hyperfine splitting could be detected only for C505, -, and the reasons are not. fully understood. It is possible t h a t the anion radicals are in equilibrium with the two-electron oxidation product and the dianion
c,o,," + ct,03,>=+= 2c,o,;or electron exchange may take place between the anion radical and the urioxidized dianion present in the solution. Either exchange process, if rapid, would destroy the I3C sidebands. Apparently whatever processes are averaging I3C sidebands for C404--. and C 6 0 6 . - are slow for c505. - . The I3C coupling found from the sidebands of the CjO5'- spectrum allows estimation of t h e spin density distribution in the radical. Using the PT polarization constants of Karplus and Fraenkel,6 the theoretical equation for the coupling constant may be written a,. = 1 6 . 1 ~ ~.-" 13.9(pc!
+ pC") +
QcOCpc"
+
QOcc po"
where C ' and C" are the carbons attached to t h e one in question. In this case, all of the carbons are equivalent, and p c , = pes' = pc. Broze and Luz7 have measured values of QcoC and QocC from a large number of carbonyl radicals and found the best values to be QcoC = 36.0 G and QocC = -24.3 G. Substituting these in t h e above equation, a n ac of 4.10 G predicts Bpc = 0.92, Bpo = 0.08; a n a(: of -4.10 G gives B p c = 0.08:Zpo = 0.92. Huckel molecular orbital calculationsS predict Zpc = 0.24 and Zpo = 0.76. Maclachlan calculations: which add a small perturbation t o HMO's to take into account the effect of the spin of the unpaired electron on the energy of electrons of like spin, were also carried out.8 These predict a total spin density on oxygen of 0.90:quite consistent with the experimental result of ac = -4.10. The 13C peaks show no measurable difference in broadness so the sign of the coupling cannot be determined empirically.
TABLE I: g Values and HMO Unpaired Electron Energy Levels Radical c404.
c505.-
cs0s.CE06'
-
gvalue 2 00584 2.00624 2.00652 2.00457
Zp0 (Mac-HMOIU
0 900
h(Mac-HMO) - 0 1746
0.898
-- 0 , 2 3 6 8
0.895
-0.278p
0.671
-0.5298
Reference 8
Attempts were made to reduce all three oxocarbon anions electrolytically, An electron spin resonance signal was obtained only for C6Oe2-; it was again a simple singlet with no sidebands, probably due to &Os3-. KOsignals were observed from the reduction of c 4 0 4 ' - or c&5'-. The energy levels which would contain the unpaired electrons for the trianion radicals C4O4.3- a n d c 5 0 5 e 3 - are predicted. in HMO calculations,8+9 to be much higher t h a n t h a t for C60e3-. The g values measured for the four radicals are listed in Table I. T h e shift from the value for a free electron, 2.00232, is caused by coupling of the electron spin to the angular momenta of the nuclei in the molecule. This shift is expressed semiempirically by Stone's relationshiplo
in which n is the index for the nucleus in the radical, ?, is the energy level, in units, of the orbital containing the unpaired electron, a n d h and c are empirical constants related to the magnitude of spin-orbit coupling between the electron and n. For radicals which contain oxygen and carbon nuclei, Ag is found to be mainly a function of the spin density on oxygen, since the spin orbit coupling constant for oxygen is much larger than that for carbon.11 Values of h and Bpo for each radical, predicted by the Maclachlan calculations, are given in Table I. The last two electrons of the ground-state dianions are slightly a n t i b ~ n d i n g , thus ~ , ~ X values for the unpaired electrons in the oxidation products, as well as the reduction product, are negative. A plot of Ag us. X for the monoanions is presented in Figure 2. The plot is linear, with 6 = 0.0026 and c = -0.0072. Lg for t h e C 6 0 6 a 3 - radical does not lie on this line, probably because the unpaired electron is in a doubly degenerate energy level, subject to Jahn-Teller distortions from planarity. The Journal of Physicai Chemistry, VG/. 7 7 , NG 22, 7973
2654
L. M. Toth and G . E. Boyd
Experimental Section Synthesis of Bis(tripheny1phosphine)iminium Salts. The bis(tripheny1phosphine)iminium salts were prepared by combination of 5 : l molar ratios of the sodium or potassium oxocarbon salt and triphenylphosphineiminium chloride. respectively. These were dissolved separately in boiling water and combined. The product precipitated immediately upon mixing of the two solutions, and was recrystallized from CHzClz and dried under vacuum. (It was necessary in the case of the rhodizonate anion to carry out the reaction and filtration quickly to prevent oxidative decarboxylation to croconate anion.) 2[(c&5)3P]zA~+ C4042-.2Hzo. Electronic spectrum , , ,A 275 nm (6 2.8 X lo4),268 nm (2.9 X lo4). (CHzCl.2): Anal. Calcd for [(Ph3P)zX]zC404.2HzO: C, 74.50; H, 5.26; P, 10.11; S , 2.29; 0, 7.38. Found: C, 74.37; H, 5.11; P, 10.30; N, 2.42; 0, 7.58. 2[(C&5)3P]zN* C5052-. Electronic spectrum 373 nm ( t 3.5 x 104), 275 nm (1.0 x IO4), (CH2C12): A,,, 268 nm (1.3 x lo4), 262 nm (1.1 x 104). Anal. Calcd: C, 75.8; H, 5.0; P, 10.8; N, 2.3. Found: C, 75.8; H , 5.1; P, 10.3; N, 2.6. 2[(C6H5)3P]&+ C S O ~ ~ - . Electronic spectrum (CHzC12):,,,A, 487 ( e 4.0 X 1 O 4 ) , 275 nm (1.0 X lo4), 268 nm (1.3 X lo4): 262 nm (1.1 x 104). Anal. Calcd: C, 75.2; H. 4.9; P, 9.9; K, 2.2. Found: C, 75.0; H, 5.0;P,9.9; N, 2.4. Electrolytic Oxidation and Reduction. Solutions in dichloromethane were made up to be approximately 0.001 M in sample and 0.01 M in tetra-n-butylammonium perchlorate, used as a supporting electrolyte. The electrolytic cells, sample preparation. and apparatus have been described elsewhere.12s13 Electrolysis was performed at the lowest potential necessary for passage of current through
the cell. On oxidation the C,O,.- radicals appeared a t a potential of about 5 V with a current of 0.3 PA. The signals were stable to -60", but slowly decreased in intensity at higher temperatures. g values were measured with a dual cavity using peroxglamine disulfonate anion (g = 2.00550 f 0.0001) as a reference.14 Acknowledgment. This work was partially supported by a grant from the Kational Science Foundation. We are indebted to Professor John Ruff for suggesting the preparation of bis(tripheny1phoshine)iminium salts of the oxocarbon anions. References and Notes (1) Previous paper in this series: E. Patton and R. West, J. Phys. Chem.. 74, 2512 (1970) (2) For reviews, see R. West and J, Niu in "Non-Benzenoid Aromatic Compounds," Vol: I , J. Snyder, Ed., Academic Press, New York, N, Y . , 1969. p 312; R . West and J. Niu in "The Chemistry of the Carbonyl Group," Vol. / / , J. Zabicky, Ed., Interscience, New York. N. Y., 1970. pp 241-275. (3) R. West and H. Y. Niu, J. Amer. Chem. SOC.,84, 1324 (1962). (4) W. Buchner and E. A. C. Lucken, Helv Chim Acta, 47, 2113 (1 964) (5) R . Appel and A. Huass. 2. Anorg. Alig. Chem.. 311. 290 (1961); J. K . Ruff, lnorg. Chem., 6, 2080 (1967). (6) M.Karplusand G. K. Fraenkel, J. Chem. Phys,. 35,1312 (1961). (7) M. Broze and Z . L u 2 . J . Chem. Phys.. 51, 749 (1969). (8) E. V. Patton, Ph.D. Thesis. University of Wisconsin. 1971. These calculations used ho = 0.3, k c o = 1.4. Maclachlan calculations were carried out with the same ho and k c o parameters, and an adjustable parameter. A, of 1.0. For earlier Huckel M O studies see N. C. Baenziger and J , J. Hegenbarth, J. Amer. Chem. Soc.. 86. 3250 (1964); R. West and D. L. Powell, i b i d , 85, 2577 (1963). (9) K . Sakamoto and Y . 1 . I'Haya, Bull. Chem. SOC J a p , 44, 1201 (1971). (IO) A. J. Stone, Moi. Phys., 6, 505 (1963). (11) B. G . Segal. M. Kaplan, and G. K . Fraenkel, J. Chem. Phys.. 43. 4171 (1965). (12) E . Carberry. R . West. and G . E. Glass. J. Amer. Chem. Soc., 91 5446 (1969). (131 G. E. Glassand R . West, lnorg. Chem.. 11, 2847 (1972) (14) M. K. Carter and G. Vincow, J. Chem Phys.. 47, 292 (1967).
Raman Spectra of Thorium(lV) Fluoride Complex Ions in Fluoride Melts' L. M. TothX and G. E. Boyd Oak Ridge National Laboratory. Oak Ridge, Tennessee 37830 (ReceivedJuly 11, 7973) Publication costs assisted by the Oak Ridge National Laboratory
Raman spectra of LiF-NaF-ThF4 mixtures a t 650" have been examined to establish the coordination behavior of Th(1V) in molten fluorides. Eight coordinated Th(1V) has been identified in melts with excess fluoride ion by comparing their spectra with that of crystalline K5ThFg. Seven coordinated Th(IV), present in fluoride ion deficient melts, was identified by shifts in the frequency of V I with melt composition changes. These results are compared with previous coordination studies of U(1V) and Zr(1V).
Introduction The presence of either alkaline earth or transition metal complex ions in molten salts frequently has been inferred from Raman spectral measurements. However, there has been little attempt to compare the spectra of various catThe Journal of Physical Chemistry, Vol. 77, No. 22, 1973
ions which are expected to have similar coordination geometries in the liquid state. The ions, Th(IV), U(IV), and Zr(IV), have been regarded as structurally similar in molten salts because they form many analogous crystalline compounds. A consequence of considering these ions as similar in molten salt solutions is that their thermody-
