ESR of A l k a l i Radical Ion Pairs and Triple Ions
in that more diverse ion/molecule reactions can be used in the structural characterization. It seems likely that negative ions will prove to be the preferred method for analyzing some classes of compounds (e.g., carboxylic acids) while the selection of positive ions will continue to form the basis for the analysis of others, such as amines. Acknowledgments. This work was supported by the National Science Foundation (MPS 77-01 295).
605 1 (17) R. G. Cooks, J. H. Beynon, and J. F. Litton, Org. Mass Specfrom., 10,503 (1975). (18) A. Maquestiau, Y. Van Haverbeke, R. Flammang, C. DeMeyer, and A. Menu, Org. Mass Specfrom., 12, 706 (1977). (19) J. H. Bowie and T. Blumenthal, J. Am. Chem. Soc.,97, 2959 (1975). (20) J. H. Beynon, R. G. Cooks,J. W. Amy, W. E. Baiiinger. and T. Y. Ridley, AM/. Chem., 45, 1023A (1973). (21) J. F. Litton, Ph.D. Thesis, Purdue University, 1976. (22) This reaction type has been observed previously; see J. H. Bowie and J. A. Benbow, Org. Mass Specfrom., 13, 103 (1978). (23) D. L. Kemp and R. G. Cooks, “Collision Spectroscopy”, R. G. Cooks. Ed.. Plenum Press, New York, N.Y., 1978, Chapter 5. The ion velocity is ca. 1X cm s-’ so that interaction with the target occurs in less than 10-13 S.
References and Notes (1) R. W. Kondrat and R. G. Cooks, Anal. Chem., 50, 81A (1978). (2) F. W. McLafferty and F. M. Bockhoff, Anal. Chem., 50, 69 (1978). (3) R . W. Kondrat, R . G. Cooks, and J. L. McLaughlin, Science, 199, 978 (1978). (4) T. L. Kruger, R. G. Cooks, J. L. McLaughlin, and R. L. Ranieri, J. Org. Chem., 42, 4161 (1977). (5) T. L. Kruger, J. F. Litton, R. W. Kondrat. and R. G. Cooks, Anal. Chem., 48, 2113(1976). (6) Alternatives include field ionization7and electron impactst9for ionization, spontaneous dissociationsi0,” instead of CID, and quadrupoles for mass analysis,12or various combinations of these features. (7) J. H. McReynolds and M. Anbar, lnf. J. Mass Specfrom. /on Phys., 24, 37 (1977). (8) T. L. Kruger, J. F. Litton, and R . G. Cooks, Anal. Lett., 9, 533 (1976). (9) K. Levsen and H. R. Schulten, Biomed. Mass Spectrom., 3, 137 (1976). (IO) D. H. Smith, C. Djerassi, K. H. Maurer, and V. Rapp, J. Am. Chem. Soc., 96, 3482 (1974). (1 1) R . G. Cooks, J. H. Beynon, R . M. Caprioli, and G. R. Lester, “Metastable Ions”, Elsevier. Amsterdam, 1973, Chapter 6. (12) C. Enke and R. Yost, J. Am. Chem. SOC., 100,2274 (1978). (13) For a preliminary communication of the present work, see R. W. Kondrat, G. A. McClusky. and R . G. Cooks, Anal. Chem., 50, 1222 (1978). (14) J. H. Bowie and B. D. Williams, “International Review of Science, Mass Spectrometry”, A. Maccoll, Ed., Butterworth, London, 1975, Chapter 2. (15) H. Knof, R. Large, and G. Albers, Anal. Chem., 48, 2120 (1976). (16) R. C. Dougherty, J. D. Roberts, and F. J. Biros, Anal. Chem., 47, 54 ( 1975).
(24) Compare Figure 4 with that of benzoyl chloride (E. Stenhagen. S. Abrahamsson, and F. W. McLafferty. Ed., “Atlas of Mass Spectra Data”, Vol. 1. Interscience, New York, N.Y.. 1969, p 735) and Figure 7 with that of phydroxyacetophenone (ibid., p 692) or that of phydroxybenzoic acid (ibid., p 718). (25) J. H. Bowie and T. Blumenthal, Ausf. J. Chem., 29, 115 (1976). (26) Suggested by a referee. (27) The occurrence of methane loss is noteworthy; for other examples of alkane loss from protonated molecules, see M. L. Sigsby, R. J. Day, and R. G. Cooks, submitted for publication. (28) See, for example, H. Budzikiewicz. C. Djerassi, and D. H. Williams, “Mass Spectrometryof Organic Compounds”, HoldenDay, San Francisco, Calif.. 1967, pp 115-1 18. (29) Major ions in the MIKE spectrum of protonatedbenzoic acid are due to loss of H20 (80% relative abundance), COP(40%), H20 CO (loo%),and H20 CO C2H2 (30%). (30) Note, for example, the peak dce to loss of 0 and H20 which is present only in the isomer with a para placement of OH and COzH, just as was found in the & vs. phydroxybenzoic acid comparison. (31) J. H. Pardanini, J. L. McLaughlin, R. W. Kondrat. and R. G. Cooks, Lloydia, 40, 585 (1977). (32) T. L. Kruger, R. W. Kondrat, K. T. Joseph, and R . G. Cooks, submitted for publication. (33) E. Soltero-Rigau, T. L. Kruger, and R. G. Cooks, Anal. Chem., 49, 435 (1977). (34) R. Richterich, “Clinical Chemistry, Theory and Practice”, Academic Press, New York. N.Y.. 1969, p 235. (35) H. P. Tannenbaum, J. D. Roberts, and R. C. Dougherty, Anal. Chem., 47, 49 (1975).
+
+
+
Electron Spin Resonance Studies of Alkali Radical Ion Pairs and Triple Ions of 1,2- and 1,4-Dicarbonyl Compounds K. S. Chen and J. K. S. Wan* Contribution from the Department of Chemistry, Queen’s Uniuersity, Kingston, C a n a d a K 7 L 3N6. Receiued April 13, 1 9 7 8
Abstract: Relatively stable alkali radical ion pairs and triple ions of furil (1,2-diketone) and of di-tert-butyl azodicarboxylate ( 1,4-diketone) have been generated in tetrahydrofuran (THF) and examined by electron spin resonance spectroscopy. For ion pairs, intramolecular migration of the metal ion between the two carbonyl groups was observed in the furil system but not in the di-tert-butyl azodicarboxylate (BADC) system. The latter is probably one of the tightest ion pair systems and the effect of complexation of this ion pair by the polyether 18-crown-6 and N,N-dimethylformamide (DMF) was clearly demonstrated by the ESR analysis of the complex spectrum. The alkali cationic triple ions of both furil and BADC exhibited “unusual” thermal stability and thus afforded very high resolution in their ESR spectra which have been fully interpreted. The strong coordination is partly due to the nature of the binding of the cations by both carbonyl and “ethereal” oxygens. The interesting reactions of these ion pair systems with group 4B organometals are also reported.
The interesting spectroscopic features and chemistry of the alkali radical ion pairs, triple ions, and ion quadruplets’ have been attracting much attention. The earlier ESR investigations of cationic triple ions mainly centered around radical anion systems having the two polar groups in an axially symmetrical position,2-s e.g., the 1,4-semiquinones and the pyrazines. Although other 1,2- and aliphatic 1,4-substituted diketones such as furil and di-tert-butyl azodicarboxylate (BADC) with the carbonyl groups strategically placed near the “ethereal oxygens” are admirably suited for triple ion formation, none has been reported. W e wish to demonstrate 0002-7863/78/1S00-605l$01 .OO/O
here that these diketones can form stable ion pairs and triple ions with a high degree of selectivity among the alkali metal series and the tight ion pairs of BADC can further complex with crown ether and survive in DMF. Ion pairs of both systems can also react readily with group 4B organometals leading to interesting and stable organometal spin adducts. The formation of the triple ion by metal reduction is a two-step process, e.g. furil
- M
THF, vacuum
M+ furil-(ion pair)
M+BPhd-
0 1978 American Chemical Society
-
M+ furil- M+ (triple ion)
(I)
6052
Journal of the American Chemical Society
/
100:19
/
September I S . 1978
e
Figure 2. ESR spectra of di-rrrt-butyl azodicarboxqlate: (a) anion radical in 2-propanol at -3 "C; (b) sodium ion pair in T H F at 22 'C; (c) sodium triple ion in T H F at 22 "C; (d) 18-crown-6 complex sodium ion pair i n T H F at 20 'C: and (e) modulation diagram for the line width alternation observed in (d).
Figure 1. ESR spectra of furil: (a) anion radical in 2-propanol at -20 "C; (b) sodium ion pair in T H F at 24 "C: (c) sodium triple ion in T H F at -20 "C; and (d) triphenyltin adduct in T H F at 20 "C.
The first step leading to an ion pair is well known among the 1,2- and 1,4-dicarbonyl compounds such as benziL6 9,lOphenanthrenequinone (PQ),' acenaphthenequinone,8 and trans-ox in dig^.^ Upon addition of an alkali metal salt such as NaBPh4, none of the above-mentioned ion pairs proceeds in the second step to form the triple ion. However, both the furil ( 1,2-diketone) and the BADC (1,4-diketone) readily interact with MBPh4 to form the corresponding triple ions. The striking resolution of their ESR spectra owing to their high stability in T H F revealed many valuable features which shed some light on the structure and the dynamics of ion migrations in these systems.
Experimental Section Alkali metal was supplied by Alfa Inorganic and was vacuum sublimated. Fisher reagent grade T H F was dried repeatedly over Na-K alloy and vacuum distilled into sample tubes. Furil was received from PCR and BADC and sodium tetraphenylborate was received from Aldrich. The polyether 18-crown-6 was a gift from Professor E. Buncel. All were used as received. The preparations of potassium and lithium tetraphenylborates and samples of the ion pairs and triple ions in T H F in a high vacuum line were accomplished according to the previously described procedure^.^ Electron spin resonance measurements were recorded on a Bruker 420 spectrometer with 100-kHz modulation. The temperature was varied by flowing thermostated nitrogen past the sample. g factors were measured using the Bruker NMR oscillator and the potassium naphthalene ion pair in T H F as an external standard.I0
Results and Discussion I. The Alkali Radical Ion Pairs of Furil and BADC. A. ESR Spectra. The ESR spectra of the radical anions of furil and BADC are given in Figures l a and 2a, respectively. They were generated by photochemical means in the presence of triethylamine. Their ESR parameters are reported in Tables I and I1 for comparison purposes. It is interesting to point out that the photochemically generated BADC anion (Figure 2a) exhibited CEDEP (EA polarization)" and a detailed polarization study is now in progress. The typical ESR spectra of sodium ion pairs of furil and BADC are illustrated in Figures l b and 2b, respectively, and the ESR parameters, including all K, Na, and Li ion pairs, are given in Tables I and 11. B. Intramolecular Cation Migration in Ion Pairs. The dynamic processes of cation migration in various p-semiquinones and the triple ions as possible intermediates in the intermolecular cation transfer have been reviewed by Chen and Hirota.I2 In the alkali-furil radical ion pairs, marked line-width alternation due to intramolecular cation migration was observed. Surprisingly such line-width effect was not observed
in the alkali ion pairs with other 1,2-dicarbonyl compounds, as we have repeated the previously reported alkali ion pairs of benzil, PQ, and acenaphthenequinone over a temperature range of -80 to 20 OC. In all probability, the failure to observe the line-width effect is due to the extremely fast cation migration rateXain these ''loose'' ion pairs. Thus, marked linewidth alternation has also been observed in the Ba2+-acenaphthene semiquinone ion pair in DME8b when the jumping of the Ba2+ between the two sites was substantially slowed down compared to the alkali ion analogue. In the Ba(ACQ)2 case, the four oxygen atoms adopting a "tetrahedral-like" configuration provide four equivalent sites all of which have the Ba2+ unsymmetrically located with respect to the two oxygen atoms of any one anion and are off the two planes of symmetry. The "jumping" is visualized as the wandering of the Ba2+ between these four equivalent sites. This interpretation is consistent with our conclusion that the alkali-furil ion pair is relatively tighter bound owing to the extra coordination with the strategically situated ethereal oxygen.
6053
E S R of Alkali Radical Ion Pairs and Triple Ions Table I. ESR Parameters of Radical Anion, Ion Pairs, Triple Ions, and Group 4B Organometal Adducts of Furil (FQ) in T H F
(y
’\ 1
-\o
1
system
T,“C
FQ-* K’FQ-. Na+FQ-. Li+FQ-. K+FQ-*K+ Na+FQ-.Na+ Li+FQ-.Li+ Ph3SnFQ. Pb3PbFQ.
-20
g factor
20 20 20 20 20 20 20 20
ai3,,Gn
2.004 82 2.004 76 2.004 61 2.004 72 2.004 67 2.004 57 2.003 72 2.003 42
6.25 6.56 6.48
OM,
G
al
= a ] ’ ,G c
1.86 1.49 1.62 1.37 1.62 1.72 1.82 2.26 1.79
ns. 0.50 0.64
