NUCLEAR MAGNETIC RESONANCE AND ULTRAVIOLET SPECTROSCOPY OF SUBSTITUTED AROMATIC ORGANOMETALLIC COMPOUNDS OF LITHIUM MAGNESIUM AND CALCIUM

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, J2.6 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 organometallic 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 mμ (ε ≃1200) assigned to π → π* excitation, (2) weak absorption which tails off toward 350 mμ, attributed to σC-M → π* analogous to n → π* in pyridine, and (3) a shoulder (ε ≃1000) 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, σC-M → π*, is responsible for C-M bond anisotropy and thus deshielding the ortho hydrogens in ArM. Other effects - inductive, π 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 electronegativities and 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.