2724 Boron Trifluoride Complexes of Aromatic Aldehydes. V. The CHO :BF, Pseudosubstituentl Mordecai Rabinovitz* and Amiram Grinvald Contribution from the Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel. Received April 27, 1971 Abstract: Boron trifluoride forms solid, stable 1 : 1 u-u complexes with aromatic aldehydes. The CHO :BFa pseudosubstituent proved to be an extremely strong electron-withdrawing group. The ir, uv, and nmr spectra of 17 such complexes were correlated with a new data matrix technique and compared with NOz, CHO, and CHOH+ benzene derivatives. n 1878 Landolf reported the formation of solid com- correlated with the respective properties of the free I plexes when aromatic aldehydes are treated with aldehydes, the protonated aldehydes (bearing the boron trifluoride.2 Almost 80 years later, Lombard pseudosubstituent CHOH+), and the analogous nitro- and Stephan prepared a series of such complexes in benzene derivatives. The energy differences between an analogous manner and studied their thermal be- the ground- and excited-state levels of the complexes, havior., No attempt has been made to characterize their electron-withdrawing properties, and the nature these complexes and no structure was suggested. In of complexation have been determined. The large 1965, Mhyre and coworkers isolated a stable complex volume of data accumulated in the present study of boron trifluoride and mesitaldebde and studied prompted us to develop a new method for its rapid its ir and uv ~pectra.~The scarcity of information evaluation and for the correlation of our experimental regarding these interesting complexes prompted us results with other data taken from the literature (the to start a detailed study of these compounds with a data matrix); vide supra. view to establishing their structure and ground- and excited-state properties. We thought that if these Experimental Section complexes would prove to be of a 1:l composition, aldehyde :BF3 and the boron trifluoride directly linked Table I shows the uv spectral data of the 17 complexes and Table I1 shows some ground-state properties of these complexes in to the aldehyde function, the resulting group CHO: BF, methylene chloride solution. The complexes are sensitive to might be considered as a pseudosubstituent, e.g., Ia. humidity, and they have to be prepared and maintained under The formyl (CHO) substituent by itself is a strong elec- strict anhydrous conditions. All complexes except those derived tron-withdrawing group, and we expected that the from p-nitrobenzaldehyde and p-cyanobenzaldehyde can be pre- pared in a carbon tetrachloride solution of the aldehyde, by addi- CHO:BF, group would be a much stronger electron- tion of boron trifluoride etherate. All these complexes are pre- withdrawing group. Such a pseudosubstituent would cipitated by the addition of boron trifluoride gas to the free alde- stabilize resonance forms favoring a high carbon- hyde in carbon tetrachloride solution at room temperature. Reg- carbon n-bond character in the ground state, e.g., Ib. ular crystals are obtained from methylene chloride solutions satu- rated with the complexes. The complexes can be preserved for The first indication to this effect came from the nmr long periods of time. Some of them, especially those bearing an studies which demonstrated the relatively high bar- electron-donating substituent, could be kept as solids for over 1 rier to rotation about the Caryl-Cformylbond.6 We year in sealed ampoules. The only unstable complexes are the ones with an electron-withdrawing substituent. Some of the more H, H, @O:BF, stable complexes can be sublimed. Elementary analyses of the C complexes were within experimental error and all complexes gave I the ratio BFa/aldehyde = 1. Nmr spectra were recorded at high settings of amplitude, and the purity of the complexes at low tem- peratures (lower than their exchange temperatures) was shown to be >99%, and no free aldehyde could be detected. We assume, there- X x+ fore, that these are stable 1 : 1 boron trifluoride-aromatic aldehyde Q 0 complexes. The spectra of both the isolated complexes and of the Ia Ib complexes prepared in sifu (Le., by passing gaseous boron tri- fluoride through the solution) are identical, although in the latter report the ground- and excited-state properties and the there is an excess of boron trifluoride. nature of bonding of these complexes and their rela- The aromatic aldehydes were distilled twice in uucuo immediately before use. Methylene chloride of analytical grade was washed tion to the parent free benzaldehydes. with concentrated sulfuric acid, distilled into a flask containing cal- This Study. We have prepared 17 complexes of sub- cium hydride from which it was redistilled, and preserved with mag- stituted aromatic aldehydes and boron trifluoride and nesium perchlorate (Anhydrone). The uv spectra were determined determined their ultraviolet, infrared, and nmr spectra. in 1-cm pathway cells with the aid of a Unicam SP-800 spectro- The absorption bands and extinction coefficients were photometer. The extinction coefficients of the free aldehydes were determined at two or more concentrations, and the difference be- (1) Part IV: M. Rabinovitz and A. Grinvald, TetrahedronLett.,4325 tween the two determinations was no greater than f2x. The ex- (1971). tinction of the solution was determined before and after passage of (2) M. Landolf, C.R. Acad. Sci., 86, 671 (1878). gaseous boron trifluoride through the solution. Subsequently, a (3) R. Lombard and J. P. Stephan, Bull. SOC.Chim. Fr., 1369 (1957); third measurement was obtained, by the addition of 1 drop of dilute 1458 (1958); R. Lombard and J. P. Stephan, C. R. Acad. Sci. Paris, 239, ethanol, giving a spectrum which superimposed the original alde- 887 (1954). (4) P. C. Myhre, C. D. Fisher, A. T. Nielsen, and W. M. Schubert, hyde. Solutions of the precipitated complex always showed the J. Amer. Chem. SOC.,81, 29 (1965). superimposed spectra of both the complex and the aldehyde. (5) A. Grinvald and M. Rabinovitz, Chem. Commun., 642 (1969). Therefore, dilutions were made in methylene chloride saturated with Journal of the American Chemical Society 1 94:8 j April 19, 1972 2725 Table I. Spectral Data of Benzaldehydes and Their Boron Trifluoride Complexes r -B band . _____C band - -Aband-- -E----- -Amax, nmb- -- F Amax e Transitionenergy -A,,,, -A,,,, nmn- Com- Alde- Com- Alde- Com- Alde- Corn- Com- difference, AET Substituent Complex Aldehyde plexc hyded plex hyde plexe hyde plex plexe AET~ AET~ 282 1. 4-H 281.6 246.5 18,800 12,800 330. 1330 1240 -290s 14.5 14.8 294a 235 9,100 14.1 2411 17.1 3. 4-CH(CH& 298.0 258.4e 25,800 17;500 -2910 14.7 4. 4-CH3 285.7 251.2~ 18,300 12,400 339 295 3150 1850 13.7 12.6 2831 5. 4-F 285.3 249.0 16,900 12,600 800 14.6 2748 f 6. 4-C1 298.5 257.8 23,600 18,100 2901 1300 15.1 281. f 291O 7. 4-Br 307.0 262.0 24,000 19,300 -1700 16.0 2828 f 1900 13.3 11.3 8. 4-CN 251'5 31,100 :::Ef 3258 2918 284S 260.01 299' f -1600 1700f 9.5 8.5 9. 4-NOz 278.4 265 21,600 15,500 320' 304 -3000 2450 5.0 7.4 401 i 1 13.2 10. 4-N(CH& 338.Y 32,800 -265 i 389 =t If 10.8 12.6 268 343i 35,500 i 10.4 12. 3,5-CH3 294.5 256.4 18,400 12,300 342 299 1960 1670 14.4 12.0 13. 3,5-OCH3 320.0 269.9 10,800 7,600 391 325.5 2290 2750 237 17,500 16.6 14.7 14. 3,4,5-OCH, 368 + 1 287.5 17,600 12,100 -3178 234 16,300 21.8 15. 2,643 287.5k 252.6k 11,200 6,900 362 306 2940 1810 13.7 14.3 16. 2-OCH3 287.5 253.6 15,200 10,500 380 319 6020 4950 13.3 14.3 17. 2,4,6-CHa 305.4 265.7 24,800 14,400 347 299 3860 2160 14.0 13.2 i0.2 nm. * =tl nm. Superscript s denotes shoulder. c &5z. &2%. e =tlOz;.f Doublet. 0 Nonsymmetrical unresolved dou- blet. Nonsymmetrical peak probably due to an absorption at 305 nm. E <2500. j Nonsymmetrical probably due to an absorption at 310 nm. Nonsymmetrical toward higher wavelengths. Table n. Ground-State Properties of Benzaldehydes and Their BF3 Complexes -1r Nmr - Ionization --Yc,o, cm-l- d,cps- potential of Subst it uent Aldehyde Complex Av, cm-l Aldehyde Complex A& cps hC-H, cps aldehydea 1. 4-NO2 1712 1642 70 1017.8 999.9 17.6 179.3 10.27 2. 4-CN 1711 1642 69 1011.4 992.2 19.2 178.9 3. 2,6-C1 1711 1631 80 1048.3 1022.2 26.1 187.7 4. 3,5-OMe 1703.5 1622 81.5 988.5 956.9 31.6 175.5 5. 4-CI 1703.5 1627 76.5 1003.1 968.1 35 175.9 9.61 6. 4-Br 1703 1626 77 997.1 970.3 26.5 175.7 7. 4-H 1703 1626 77 998.4 970.5 21.9 174.2 9.73 8. 4-F 1702 1627 75 997.3 966.1 31.2 175.1 9. 4-i-Pr 1701.5 1625 76.5 993.4 957.8 35.6 173 10. 3,5-Me 1698 1624 74 989.9 956.2 33.7 172.9 11. 4-Me 1695 1619 76 992.1 955.7 36.4 173.1 9.33 12.