SPECTROSCOPY OF SUBSTITUTED AROMATIC ORGANOMETALLIC COMPOUNDS
953
Nuclear Magnetic Resonance and Ultraviolet Spectroscopy of Substituted Aromatic Organometallic Compounds of Lithium, Magnesium, and Calcium by Gideon Fraenkel, Shlomo Dayagi, and Sadao Kobayashi Evans Chemical Laboratory, The Ohio State University, Columbus, Ohio 4WlO
(ReceCed August 24, 1967)
Nmr and uv spectra have been obtained for a variety of aromatic organometallic reagents of lithium, magnesium, and calcium, including two bifunctional reagents. m-Dilithiobenzene is reported for the first time. The nmr parameters for these compounds depend little on solvent or concentration and are much more similar to those of the corresponding heterocycles than to values for covalently substituted benzenes. In particular, Jz,*lies in the region 0.8-1.0 Hz and the shift of hydrogen ortho to the carbon-metal bond is unusually low, decreasing with metal electronegativity. Among these analyses, the coupling constants are found to be all of the same sign. Low concentrations, 0.001 M , of organoinetallic compounds are oxidized by the peroxide impurities in the solvents. However, reproducible uv spectra have been obtained for several ArM compounds in higher concentrations, 0.04-0.2 M . These spectra consist of (1) strong absorption around 260 mp ( E -1200) assigned to R T * excitation, (2) weak absorption which tails off toward 350 mp, attributed to B C - X R* analogous to n R* in pyridine, and (3) a shoulder ( E .ulOOO) which moves to higher wavelengths with increasing solvent polarity. The latter is assigned to a transition in which the ionic character of the carbon-metal bond increases. The close resemblance of the nmr and uv data for ArM compounds and pyridines leads to the conclusion that magnetic mixing of the ground and the nearest excited electronic states, UC-11 T*,is responsible for C-M bond anisotropy and thus deshielding the ortho hydrogens in ArM. Other effects-inductive, T density, and ring-current variations-need not be considered. This postulate for paramagnetic shifts is supported by the dependence of the ortho shifts on metal electronegativitiesand the observation that the ring shifts vary around the rings qualitatively according to the trend predicted by the geometrical factor in the point-anisotropy approximation. -+
-+
-.+
-.+
Introduction Aromatic organometallic compounds of magnesium and lithium exhibit carbanionic character in their chemical properties’ and it might be expected that these materials would reflect useful information about carbanions in their physical properties. I n fact, the conclusions from infrared s p e c t r o ~ c o p y ~and - ~ magnetic susceptibility measurements8 support the postulate of partially ionic carbon-metal bonds. Recent nmr9 and uv9l1o studies of phenyllithium, phenylmagnesium bromide, and pyridine reveal that there is a surprising similarity among the nmr parameters for these materials, compared to values for ordinary covalently substituted benzenes,l’ in particular, the low values of J2,6and T ~ . It was suggested that since phenyl anion (I) and pyridine (11) are isoelectronic
these species should have similar electronic structures. To the extent that the carbon-metal bonds in phenyllithium and phenylmagnesium bromide are ionic, proton shifts in these materials should have similar origins to those in pyridine. The currently accepted explanation for the LY shift in pyridines is that they are of paramagnetic origin
due to the low-lying n + T* excited state.12 Therefore, on the basis of the nmr data, similar effects should operate also in the phenylorganometallic compounds. Furthermore, there should be a lotv-energy electronic excitation, analogous to n T* in pyridine, available to ArRf compounds. I n this paper, we have further explored the similarities in electronic structure among pyridines and a (1) M. S. Kharasch and 0. Reinmuth, “Grignard Reactions of Nonmetallic Substances,” Prentice-Hall, Inc., New York, N. Y., 1954; G. E. Coates, “Organometallic Compounds,” 2nd ed, John Wiley and Sons, Inc., New York, N. Y., 1960. (2) E. J. Lanpher, J . Org. Chem., 21, 830 (1956). (3) A. N. Rodionov, D. Shigorin, T. V. Talalaeva, and K. Kocheshkov, Izu. A k a d . N a u k S S S R O f d . Khim. Nauk, 120 (1958). (4) A. N. Rodionov, D. N. Shigorin, E. N. Gur’yanova, and K. A. Kocheshkov, Dokl. A k a d . ATauk S S S R , 125, 562 (1959); 123, 113 (1958); 128, 728 (1959). (5) A. N. Rodionov, D. N. Shigorin, and K. A. Kocheshkov, ibid., 136, 369 (1961). (6) T. V. Talalaeva, A. N. Rodionov, and K. A . Kocheshkov, Izv.A k a d . N a u k SSSR Otd. Khim. N a u k , 1990 (1961). (7) R. West and W. H. Glaze, J. Am. Chem. Soc., 83, 3580 (1961).
(8) I. B. Golovanov and A. K. Piskunov, Zh. Fiz. Khim., 38, 2063 (1965). (9) J. A . Ladd, Spectrochim. Acta, 22, 1157 (1966). (10) G. Fraenkel and D. G. Adams, Abstracts, 147th National
Meeting of the American Chemical Society, Philadelphia, Pa., April 1964; G. Fraenkel, D. G. Adams, and R. R. Dean, J . P h y s . Chem., 72, 944 (1968).
(11) J. S. Martin and B. P. Dailey, J . Chem. Phys., 37, 2594 (1962). (12) V. M. S. Gil and J. N. Murrel, Trans. Faraday Soc., 60, 248 (1964).
Volume 72,Number S March 1968
954
G. FRAENKEL, S. DAYAGI, AND S. KOBAYASHI
Table I : Nmr Parameters for Aromatic Lithium and Magnesium Compounds Species, solvent
p-Tolylmagnesium bromide, ether p-Tolylmagnesium bromide, T H F p-Tolyllithium, ether p-Tolylcalcium iodide, T H F p- Anisylmagnesium bromide, ether p- Anisylmagnesium bromide, T H F p- Anis yllithium, ether p-Dimethylaminophenyllithium, ether Mesit yllithium, THF 1,4-Bis(bromomagnesio)benxene, THF p-Chlorophenylmagnesium bromide, ether p-chlorophenyllithium, ether
b G : T, HF +
a
Conon, M
To0
7m
J*,Sb
J2,S
J8,6
Jz,a
0.76
2.515
3.143
7.14
0.91
2.31
0.62
0.44
2.584
3.298
7.33
0.73
1.72
0.48
0.50
2.090
3.080
7.04
0.63
1.68
0.57
0.50
1.755
3.040
7.00
C
C
C
0.58
2.596
3.340
7.69
1.13
2.62
0.51
0.68
2.505
3 I395
7.60
1.09
2.59
0.53
0.50
2.030
3.200
7.42
1.11
2.50
0.48
0.50
2.093
3.313
7.58
1.14
2.58
0.61
0.50
3.489
0.50
2.300
0.50
2.461
2.907
7.07
1.70
1.70
0.56
0.50
2.545
2.744
8.10
2.20
2.20
0.73
72
74
76
0.396
2.383
3.325
0.50
J2,C
