2525
SUBSTITUTED MALONONITRILE ANIONRADICALS
Substituted Malononitrile Anion Radicals'
by F. J. Smentowski and Gerald R. Stevenson Chemistry Department, Texas A and M University, College Station, Texas 77843 (Received October 14, 1060)
Monitoring by electron spin resonance the successive additions of sodium-potassium alloy to benzylidene malononitrile in 1,2-dimethoxyethane, two different anion radicals have been observed. ilt intermediate stages of the reduction of benzylidene malononitrile, line width variation spectra are noted. An electron spin resonance study of the reduction of substituted malononitriles reveals equilibria 1 to 6 adequately explain the data.
Introduction Extensive kinetic and equilibrium studies have been made of monomers which undergo anionic polymerization.2~3 Electron spin resonance (esr) has verified the presence of a paramagnetic species during the anionic polymerization of monomers such as 9-vinylanthracene4" and a ~ e n a p h t h y l e n e ,but ~ ~ to our knowledge definite identification of several different anion radicals during the anionic polymerization of a monomer has not been accomplished because of instability of the system and/or overlapping lines4J or unfavorable rates. This report communicates the successful identification of two different anion radicals observed during different stages of the reduction of benzylidene malononitrile (I) preceding its polymerization. It is worthwhile noting that dimer cation radicals of aromatic derivatives such as naphthalenej6 anthracenej6 aryl ether^,^ coronene,8 and pyreneg have been observed by esr.
Results Benzylidene Malononitrile ( I ) . Addition of a small amount of sodium-potassium alloy to a 0.01 1lf solution of I in 1,2-dimethoxyethane(DnIE) a t -90" led to a red anion radical R1. Addition of sodium-potassium alloy to 0.001 M solutions of I in DNE leads to R1 concentrations barely detectable by electron spirl resonance. Further addition of metal alloy to R1gave a blue monomer anion radical, Rz (Figure 1). If the sample is carefully reduced, other spectra are seen after that of R1 and before that of Rz. As with R1, addition of sodium-potassium alloy to 0.001 M solutions of I in DME lead to Rz concentrations barely detectable by esr. Both the spectra of RI and R2 are almost first order. R1 has four equivalent nitrogens, leading to nine main lines, each of which is split into a quintet by four equivalent hydrogens. The nitrogen splittings in R1 and R2 are easily differentiated from proton splittings by their intensity ratios. The assigned
Experimental Section (1) Temperature Dependent Electron Spin Resonance Studies. 11'. X-Band esr spectra were recorded using a Varian Part 111, F. J. Smentowski, R . M. Owens, and B. D. Faubion, Jr., V 4502-15 esr spectrometer with a 12-in. magnet. TeniJ . Amer. Chem. SOC.,90, 1537 (1968). perature was controlled within A 1' by a Varian V-4557 (2) M. Szwarc, "Carbanions, Living Polymers, and ElectronTransfer Processes," John Wiley and Sons, New York, N. Y . , 1968. variable temperature controller. A copper-constantan (3) (a) R. Lipman, J. Jagur-Grodzinski, and M. Szwarc, J . Amer. thermocouple was used to calibrate the variable-temChem. Soc., 87, 3005 (1965); (b) A . Eisenberg and A. Rembaum, perature controller. Coupling constants and line Polym. Lett., 2, 157 (1964); (c) F. Bahstetter, J. Smid, and M. Szwarc, J. Amer. Chem. SOC.,85, 3909 (1963); (d) D. N. Bhattacharyya, widths were taken directly from the calibrated chart C. L. Lee, J. Smid, and M . Szwarc, ibid., 85,533 (1963) ; (e) C. L. Lee, paper. J. Smid, and M. Sewarc, ibid., 85, 912 (1963); (f) G. Spach, H. Monteiro, M. Levy, and M. Szwarc, Trans. Faraday SOC.,58, 1809 Benzylidene malononitrile was prepared by the con(1962); (g) J. Jagur, M. Levy, M. Feld, and M.Sewarc, ihid., 58, densation of benzaldehyde with malononitrile using 2168 (1962); (h) D. Gill, J. Jagur-Grodzinslri, and M. Szwarc, ihid., 60, 1424 (1964); (i) M. Matsuda, J. Jagur-Grodzinslri, and piperidine as a catalyst. Benzylidene malononitrile was hl. Szwarc, Proc. Roy. SOC.,A288, 212 (1965) ; fj)J. Jagur-Grodzinski found to be greater than 99.9% pure as determined and M. Szwarc, ibid.,A288, 244 (1965); (lr) M. Shima, D. N. Bhattacharyya, J. Smid, and NI. Szwarc, J . Amer. Chem. SOC.,85, 1306 by glc analysis. (1963). a-Methylbenzylidene malorionitrile was prepared by (4) (a) A. Eisenberg and A. Rembaum, Polym. Lett., 2, 157 (1964); the condensation of acetophenone with malononitrile (b) J. Moacanin and A. Rembaum, ihid., 2, 979 (1964). using piperidine as a catalyst. l,l-Dicyano-2-(o-chloro- (5) (a) K. Hirota, J . Polymer Sei., 60, 552 (1962); (b) K. Kuwata, H. Kawaeura, and K . Hirota, Chem. Abstr., 56, 4928 (1962); (e) pheny1)ethylene was prepared by the condensation of H. P. Leftin and W. K. Hall, J . Phys. Chem., 64, 382 (1960). o-chlorobenzaldehyde with malononitrile using piper(6) (a) I. C. Lewis and L. S . Singer, J . Chem. Phys., 43, 2712 (1965) ; (b) 0. W. Haworth and G. K. Fraenkel, J . Amer. Chem. SOC.,88, idine as a catalyst. a-Phenylbenzylidene malononi4514 (1966). trile was obtained from Aldrich Chemical Co., Inc., (7) W.F. Forbes and P. D. Sullivan, Can. J . Chem., 46, 325 (1968). and recrystallized to constant melting point. 1,l-Di(8) €1. van Willigan, E. de Boer, J. T. Cooper, and W. F. Forbes, phenylethylene was obtained from K and K LaboraJ . Chem. Phys., 49, 1190 (1968). tories, Inc. (9) J. T . Cooper and W.F. Forbes, Can. J . Chem., 46, 1158 (1968). The Journal of Physical Chemistry, Vol. 74, No. 12, 1970
2526
F. J. SMENTOWSKI AND G. R. STEVENSON
Figure 1. First-derivative esr spectrum of the anion radical of benzylidene malononitrile reduced by NaKz in DME; The first three of the five main groups of recorded at -90'. lines are shown.
coupling constants of both anion radicals are given in Table I. The calculated and experimental peak positions agree within 11%. The structures of R1 and R2 are the dimer anion radical (.X-34-) and monomer anion radical (RI --)7 respectively. No temperature dependence of the esr spectral parameters is observed for the anion radical .M--l!I- between
Table I : Hyperfine Coupling Constants for Anion Radicals RI and Rz Formed by Sodium-Potassium Alloy Reduction of I in DME at -80'
Both .M-T\.I- and Me- are not stable at higher temperatures. Above -lo', .M-M- undergoes a rapid irreversible polymerization. ;\I - was not as stable as .A'I-I!t- and depending upon the concentration of I, the anion radical rapidly polymerized irThe esr spectra of reversibly between -60 to--40'. both .M--NI- and M . - disappeared too rapidly at these higher temperatures, so a kinetic determination of the polymerization by esr was not possible. Reduction of I in liquid ammonia with potassium using techniques previously described, lo gives a diamagnetic solution, which subsequently polymerized, even at -78". a-Phenylbenzylidene Malononitrile ( I I ) . The anion radical of I1 (111) was prepared by alkali metal reduction (Li, Na, K) of I1 in DME. I11 could also be formed by electrolytic reduction using tetra-nbutylammonium perchlorate in DME. The coupling constants of I11 are given in Table 11. Addition of Table 11: Hyperfine Coupling Constants for the Anion Radical of a-Phenylbenzylidene Malononitrile Reduced by the Indicated Methods at -60' Metal
Anion radical
Position assignment
M . -a3b
Nitrogens Ortho protons Meta protons Para protons Vinyl protons Nitrogens Protons
*M-M-
No. of protons or nitrogens
2 2 2 1 1 41 4
K Na ai,
CX
5.21 1 0 . 0 1 0.601&00.01 0 . 6 0 1 2 ~0.01 0.224i0.01 0.224 10.01 2.61 2 0 . 0 5 0.40 1 0 . 0 1
a Benzylidene malononitrile anion radical. b HMO calculations confirm the relative order of the coupling constants, and show the equivalence of the ortho and the meta positions and the equivalence of the vinyl and para positions.
-90" and -lo", as indicated by the invariance of coupling constants and line intensities over this range. Below are the structures for vinylidene monomers and
Rz.
L
Li Electrolytic
Solvent
AN
A~(o,m)
DME DME DME DME
2.47 & 0.03 2.46 =I= 0.03 2.46 i 0.03 2.50 & 0.03
1.47 i 0.03 1.49 f 0.03 1.47 i 0.03 1.44 i:0.03
AH(P)
0.17 i 0.03 0.17 =I= 0.03 0.17 i 0.03 0.17 & 0.03
the alkali metal to a 0.01 M solution of I1 in DME a t -90" leads to a green anion radical characterized as 111 (Figure 2). At the beginning of the reduction, the concentration of I11 is low, and the hfs line width is broad. As more metal is added, the concentration of I11 increases, and its esr hfs sharpens (150 mG). As more metal is added, the spin concentration of I11 is reduced, and the hfs become broad again. AS more metal is added, the solution becomes blue and diamagnetic. Throughout the reduction, only one anion radical, 111, is observed. This contrasts with the reduction of I which gives two anion radicals, *I!I-M- and M a - . a-Methylbenzylidene Malononitrile ( I V ) . A 0.001 ik? solution of IV in DME upon contact with alkali metal even at -90" polymerized irreversibly. No esr spectrum was observed. I ,I-Dicyano-2-(0-chloropheny1)ethylene and 1 J -Diphenylethytene. Addition of sodium-potassium alloy to 0.01 M solution of 1,l-dicyano-2(o-chlorophenyl)ethylene in DME a t -90' gave a pink anion radical characterized by one line (line width 10 G). Further addition of metal gave a blue monomer anion radical, (10) F. J. Smentowski and G. R. Stevenson, J. Amer. Chem. Soc., 90, 4661 (1968).
The Journal of Physical Chemistry, Vol. 74, N o , 12, lQ7O
SUBSTITUTED MALONONITRILE ANIONRADICALS
2527 coupling constant is one half that of the monomer anion radical, 11.- (vide infra). Possible explanations for the line width alternation spectra observed for the I-NaK2-D1\IE system are equilibria 7, 8, 9, or 10.
a-11- = M
m-w + - n m -
Figure 2. First-derivative esr spectrum of hl,. - formed by NaKe in DME; recorded at -60'.
-nI-w
with U N = 5.2 G (with intensity ratios for two nitrogens). Also, addition of potassium to a 0.01 M solution of 1,l-diphenylethylene in DATE a t -90" led to a blue anion radical characterized by one line (line width 10 GI). Repeated attempts to further resolve the possible dimer anion radicals of l,l-dicyano-2- (o-chloropheny1)ethylene and 1,l-diphenylethylene were unsuccessful.
Discussion For these vinylidene systems, the esr results indicate we are observing (1) through (6), with the relative importance of the various equilibria dependent upon the particular monomer. Thus, at temperatures below - 60°, the system I-KaI
