COMMUNICATIONS TO THE EDITOR
Sept. 5, 1964
3587
interesting among them being the one a t 1890 crn.-l. I t appears in the three other methylenecyclopropene derivatives known4-6 a t 1828, 1835, and 1852 cm.-l, respectively, and represents, in our opinion, the shifted frequency (1818 cm.-l) of the cyclic double bond observed in 1,2-diphenylcycloprop-l-enederivatives. l o This would tend to show that the classical form I makes a significant contribution to the ground state of the molecule. The n.m.r. spectrum in anhydrous trifluoroacetic acid“ showed two unresolved multiplets a t 7 1.95and 2.32 (tetramethylsilane as standard) with relative areas 2 : 3. As we found for diphenylcyclopropenone in trifluoroacetic acid (T 2.05 and 2.37) and CDC13 (3) R. B Turner. D . E . Nettleton. and M. Perelman, J A m Chem Soc , 80, 1430 (1958) (T 2.20 and 2.55) the same ratio of areas (2:3),we RICHARD B. TURNER assume that the two signals correspond to the orthoDEPARTMENT OF CHEMISTRY RICEUNIVERSITY and the meta- plus para-hydrogen atoms of the phenyl TEXAS HOUSTON, rings. l 2 RECEIVED July 9, 1964 It is noteworthy that C-1 of I is not protonated under these circumstances as only the above multiplets have been found. 1-Dicyanomethylene-2,3-diphenylcyclopropene The ultraviolet spectrum was measured in dioxane (“l,l-Dicyano-3,4-diphenyltriafulvene”)1 (longest band: 363 mp (log e 4.00)),acetonitrile (352 Sir: (4.07)),trifluoroacetic acid (335 (4.23)),and mesitylene Among the nonalternant pseudoaromatic systems (373 (3.91)). The dependence of the wave length on that of methylenecyclopropene? (“triafulvene”) was the solvent seems to indicate that in the excited state unknown until recently, when three compounds of the “fulvenic” (dipolar) form of the molecule (11) this type have been We wish to report makes a significant contribution; of interest is the another representative of this group, l-dicyanomethylconsiderable shift observed in concentrated sulfuric ene-2,3-diphenylcyclopropene, in which both the classiacid: 386 mCc (log e 3.64). This may be due to a cal (I) and the dipolar (11) structure make a contribuchemical reaction of the solvent with the solute. This tion to the ground state of the molecule. We exhypothesis is supported by the observation that heating of I with 90% sulfuric acid a t 100’ (15 min.) gives a product (from acetic acid, m.p. 199-200’ (dec.)) which, according to the analysis, is the corresponding diamide plus one molecule of H2S04. C-3 Anal. Calcd. for C I ~ H ~ ~ N ~c, O 55.7; ~ S : H, 4.2; N , NC CN NC - \,CN 7.2;S,8.2. Found: C, 55.9; H, 4.1;N, 7.0; S,7.8. I I1 Finally, we wish to report the dipole moment 7.9 f pected the dipolar form of the triafulvene, in which the 0.1 D. of I (in dioxane a t 30’). This figure points to negative charge is localized mostly a t the exocyclic the contribution of the dipolar form (11) to the ground carbon atom (C-l)>to be enhanced by such electrostate of the molecule as shown by comparison with the negative substituents as the nitrile group. moments of cyclohexylidenemalononitrile (5.45 Refluxing diphenylcycl~propenone~ and malononitrile 0.02 D.in benzene a t 30’), diphenylmethylenemalonoin freshly distilled acetic anhydride and recrystallizanitrile (5.85f 0.05D . in benzene a t 30’), w,w-dicyanotion from benzene gave a 4,8y0 yield of yellowish dibenzofulvene (5.53 f 0.07 D. in benzene a t 30’), crystals, m.p. 294’ dec. Anal. Calcd. for C18H10N2: and w,w-dicyanoheptafulvene (7.49 D. in dioxane a t C, 85.0; H, 4.0; N , 11.0; mol. wt., 254. Found: C, 25’) . I 3 Thus, l,l-dicyan0-3,4-diphenyltriafulvenein 85.2;H, 4.2; N , 10.8; mol. wt., 254 (by mass specits ground state is a resonance hybrid of I and 11. troscopy).* The infrared spectrum of the compound (10) See, e.g.. R . Breslow, J. Lockhart, and H. W. Chang, J . A m . C h e m . (in Nujol) showed strong peaks a t 2227, 1890, 1618, Soc., 83, 2375 (1961); R . Breslow and H. W. Chang, ibid , 83, 2367 (1961). (11) T h e substance proved insoluble in deuteriochloroform. 1522, 1488, 1466, 1397, 777, and 693 cm.-’, the most R . Breslow, H. Hover, and H. W. Chang, (12) For this explanation, 6.
is 37.6 kcal./mole. This represents the highest value we have yet encountered for the heat of hydrogenation of an ethylenic linkage (cyclohexene, -27.1 kcal./ mole2; cis-di-t-butylethylene, -36.2 kcal./mole3). The incorporation of a high degree of strain in the barrelene molecule is evident from examination of Dreiding models. Although the present result does not rigorously exclude the possibility of ground-state delocalization in barrelene, it i s clear that such stabilization, if present, is swamped by steric strain. Acknowledgment.-The support of the National Science Foundation is gratefully acknowledged.
/e,
,and by the 60 Mc. proton magnetic resonance spectrum which showed only a unique propyl group. The resonance positions of the methylene CY and 13 to the ring appear a t t 7.13 and 8.14, respectively, in CDCl3 solution. Addition of 20 vol. % trifluoroacetic acid to the CDCla solution had no effect on the resonance spectrum in the region T 0 to 10, consistent with the observation by Bergmann and Agranate that the triafulvene IIc remains unprotonated under similar conditions. The negligible basicity of 111 is paralleled by its failure to react with bromine in methylene chloride a t room temperature' and by its inertness to tetracyanoethylene in cold acetonitrile. * The proton chemical shift of a methyl group directly attached to cationoid carbon has been employed as a probe of the ?r-electrondensity on the cationoid enter.^ Alternatively, the internal chemical shift between the a- and @-methylenesof a propyl chain attached to an electron-withdrawing group has been used as a rough measure of group electronegativity. lo Table I lists TABLE I
w
Model
CHaCHzCHn
"yy 0
Solvent
Ta-cm2
C F ~ C O I H ' O ~6 72
18-ca, 7 99
qn
ca + O 33
CHICHIC Hz a 8
CHzCHsCHs
CHsCHiCHi
A
CHzCHzCHs S
50% HISO,
6 58'
7 88O
7.13 7 16
8.14 8.15)
+O
33
B
COOCH3
k"
11a, b, c
I
XN
CN
NC
CH,CHsCH,
CN
A
CH&H,C& U
111
Treatment of 2.5 g. of di-n-propylcyclopropenone with 1.25 g. of nialononitrile in 30 ml. of acetic anhydride a t 140' for 2 hr. under a nitrogen atmosphere converted the ketone into a mixture of products, as indicated by infrared maxima (after removal of acetic anhydride) a t 5.32, 5.51, and 5.76 1.1 (CCl, solution). Short-path distillation at 62- 7 8 O (0.2 mm.) removed the fractions responsible for the two higher bands and continued distillatioq a t 110-115' (0.1 mrn.) produced a straw-colored liqdid (18% yield) which exhibited the strikingly simple infrared spectrum (Fig. 1) expected for the symmetrical triafulvene 111. This structure was further confirmed by combustion data (3) M. A. Battiste. J . A m . Chem. Sor., 86, 942 (1964). ( 4 ) W. M Jones and J. M. Denham. ibid.. 86, 944 (1964). ( 5 ) E I) Bergmann and I . Agranat. ibid., 86, 3587 (1964). We are indebted to these authors for communicating their d a t a t o us prior t o publication. (6) Compare. for example, the stability of 8.8-dicyanoheptafulvene [T. Nozoe, T . Mukai. K . Osaka, and N. Shishido, Bull. C h e m . Soc. J o p o n , 84, 1384 (1961)l with t h a t of the very unstable parent. heptafulvene [W. von E. Doering and I) W. Wiley, Telrohedron, 11, 183 (1960)l.
lCHCli lCFaC02H
Calcd. +0.16b + O 14c
B
r-Values given are extrapolated to CF3C02Hsolvent by addition of 0.97 p.p.m. to the resonances reported in ref. lob. b'Net s-electron density on each propyl-bearing ring carbon interpolated from the chemical shift of the a-methylene. Net relectron density on each propyl-bearing ring carbon interpolated from the internal chemical shift between a- and @-methylenes.
the relevant parameters for three model compounds and for the triafulvene 111. To the extent that these values reflect charge density on the ring carbon rather than a ring-current effect, both methods indicate a net charge of La. +0.15 for the propyl-bearing ring carbons in 111." (7) In contrast we find t h a t the methyl ester of 2,3-dipropyl-2-cyclopropene-1-carboxylic acid rapidly decolorizes bromine in C H C l r . (8) T h e triafulvene I I a is reported' to react "immediately" with t e t r a cyanoethylene under these conditions. (9) (a) C. MacLean and E. L. Mackor, M o l . Phys.. 4, 241 (1961). ( b ) T. J. K a t z and E. H. Gold. J . A m . Chem. Soc., 86, 1600 (1964). (10) (a) J. R Cavanaugh and B P. Dailey, J Chem. P h y s . , 34, 1099 (1961); (b) R. Breslow, H. Hover, and H W. Chang, J . A m Chem. S o t . , 84, 3168 (1962). (11) A linear relation between electron density and chemical shiit within this series is assumed, as, for example, in ref. 9b. This assumption gives f o r 2 p.p.m. per t h e present case a proportionality constant for r a - ~ of~ 2.94 unit charge, a s compared with 3.27 p p.m. per unit charge in ref. 9b.
Sept. 5 , 1964
3589
COMMUNICATIONS TO THE EDITOR
Finally, the ultraviolet spectrum of the triafulvene 111 in cyclohexane shows an essentially symmetric single maximum a t 246 mw (log t 4 30), in methanol the maximum is a t 245 mp (log e 4.33). There is evidence neither for absorption above 300 mp nor for a solvent effect as is found for other triafulvenes, and the position of the maximum suggests qualitative agreement with Julg's prediction'* that the longest wave length (V + N) absorption for methylenecyclopropene will fall near 200 mp ( 1 2 ) A Julg J rhrm p h y s
6 0 , 652 (1953)
ORGAXIC CHEMICAL RESEARCH SECTION LEDERLE LABORATORIES DIVISION AMERICAU C YANAMID COMPANY PEARL RIVER,SEW YORK
ANDREW S KENDE T IZZO PATRICK
RECEIVED JULY 15, 1964
The Tropylium-Iron Tricarbonyl Cation Sir: Organometallic cationic complexes of the type LM(C0)3+ in which L is a tropylium ring a-bonded to a metal ( M ) are known for the group VI-B metals chromium, molybdenum,* and tungsten. The presumed structure of these complexes is indicated in formula I . The seven carbon atoms of the ring are symmetrically bonded to the metal by means of the six x-electrons, and the metal attains the effective atomic number of the next inert g a s 4
Treatment of methyl tropyl ether with iron enneacarbonyl gave the corresponding iron tricarbonyl complex (11) which reacted with fluoroboric acid to give the fluoroborate salt of the tropylium-iron tricarbonyl cation. The salt crystallizes in yellow needles from nitromethane, m.p. 112' dec. (Anal. Calcd. for C7H7Fe(C0)3BF4: C, 37.78; H , 2.22; B, 3.18. Found: C, 37.44; 2.03; B, 3.32). I t is stable in inert atmosphere but hydrolyzes rapidly in water to give tropyl alcohol-iron tricarbonyl from which the cation can be regenerated by treatment with fluoroboric acid. As with the chromium, molybdenum, and tungsten complexes, the n.m.r. spectrum of the tropylium-iron tricarbonyl cation shows a single sharp absorption, but further comparison of these complexes reveals several significant differences. The ~ K Rvalues + listed in Table I indicate that the tropylium-iron tricarbonyl cation is slightly less stable than the tropylium cation, whereas the group VI-B metal complex cations are considerably more stable. TABLE I Complex
P
PKR+~
0.72 4.7 CjH7+BF4C7H7+Fe(C0)3BF,4.20 4 5 C?Hs+Fe(C0)3BFa.. 4.7 C?H,+Cr(C0)sBFb3.42 6 3 CjHj +Mo(C0)zBFd3 82 6.2 C?H,'W( C 0 ) s B F C 3.83 -6' * Measa Position of single n.m.r. peak measured in liquid SOz. ured in HzO a t 30.0". c Approximate value only because of secondary decomposition.
The most significant differences are seen in the infrared absorption spectra. The group VI-B metal complexes each show a single strong absorption band in the C-H stretching region,? but the corresponding L i iron complex shows three strong bands which presumably reflect a lower symmetry of the C7H7 ligand in the 1 iron complex.8 Furthermore, the group VI-B metal The analogous cationic complex containing iron is of complexes are transparent in the 650-800 ern.-' region some theoretical interest for, if the bonding in such a while the iron complex shows an intense absorption complex were to be similar, the effective atomic number band a t 731 cm.-' suggestive of the presence of a cis of iron would exceed that of krypton by two. double bond. Previous attempts to prepare salts of the tropyliumOn the basis of this evidence, we propose that the iron tricarbonyl cation have been unsuc~essful.~ structure of the tropylium-iron tricarbonyl cation is Whereas the triphenylmethyl cation abstracts hydride as shown in IIIa. The iron atom is bonded to five ion from cycloheptatriene-M(C0)3 complexes (M = carbons of the ring, leaving one double bond not inCr, Mo, and W) to give t r o p y l i ~ m - M ( C 0 )complexes, ~ volved in coordination to the metal. The ligand-metal in the case of cycloheptatriene-iron tricarbonyl addibonding is then analogous to that found in other pentation occurs to give a substituted cycloheptadienyl-iron dienyl-iron tricarbonyl cationic complexes. The ~ K R + tricarbonyl cation.6 value of the C7H7-Fe(C0)3cation is very similar to that We now wish to report the synthesis of the tropyliumof the cycloheptadienyl-iron tricarbonyl cation (see tricarbonyl cation by the following route. Table I ) . The salt displays carbonyl absorption typical of dienyl-iron tricarbonyl cations (strong absorption peaks a t 2070 and 2120 cm.-l).g C7H?Fe(CO)*+BF; I1 (1) J , D . Munro and P . I,. Pauson, J . C h e m . S O L . 3475 , (1961). ( 2 ) H . J . Dauben, Jr., a n d L. R . Honnen, J A m . C k c m . SOC.,80, 5570 (1958). ( 3 ) H . J. Dauben. J r . , I,. R . Honnen a n d D. J Bertelli, Abstracts, 15th Southwest Regional Meeting. American Chemical Society, Baton Rouge, La , llec. 3, 1959, p. 89. ( 4 ) I> A . Brown, J . Inorg. N u c l . C k e m . , 10, 39 (1959). ( r ? ) An account of these is given in ref 6 ( 6 ) H . J Dauben. J r , and D J. Bertelli. J . A m Chem Soc , 83, 497 (1961)
(7) H P. Fritz. "Advances in Organometallic Chemistry," Vol 1 . Academic Press. Inc , New York. N Y.. 1964. p 240. (8) Y C - ~ 3086, 3076, and 3086 c m - 1 f o r the C r . M o . and W complexes. respectively. and 3093, 3070. a n d 3056 cm. - 1 for t h e Fe complex measured as hexachlorobutadiene mulls using a Beckman I R-7 spectrometer ( 9 ) R Pettit and G. F Emerson, ref 7 p 1
