A 19F nuclear magnetic resonance study of a series of pentafluorochromarenes (C6F5X)Cr(C6H6)

The 19F NMR spectra of a series of complexes (C6F5X)Cr(C6F6) show marked differences from the spectra of the corresponding C6F5X molecules. On complexation the 19F chemical shifts are shielded by 30–50 ppm, the correlation of the para fluorine shift with the Taft parameters of the substiutent no longer obtains nor does the correlation of the para shift with J(2,4). The two ortho coupling constants, J2,3) and J3,4), in the chromium complex are no longer close to -20Hz but are now very large (70–100 Hz) and are of opposite sign. An internal compensation mechanism is discussed by which excess electron density in the fluorinated ring of the sandwich is buffered by the C6F6 ring.