Group 13 trihalide complexes of 9-fluorenone: a comparison of methods for assigning relative Lewis acidity

The 9-fluorenone complexes MX3(9-fluorenone) [M=B, X=Cl (1); M=Al, X=Cl, Br (2), I (4), M=Ga; X=Cl (5), Br (6), I (7) and AlBr3(9-fluorenone)2 (3) have been prepared and characterized by NMR, IR and UV–vis spectroscopy and X-ray crystallography (1, 2, 3, 5 and 6). Complexes of MgCl2 (8), HgCl2 (9), ZrCl4 (10) and SnCl4 (11) were characterized by IR and UV–vis spectroscopy, while the unusual mixed vanadium(IV,V) compound, VO(Cl)(9-fluorenone)2(H2O)(μ-O)(VOCl2) (12) has been structurally characterized. Solution ligand dissociation energies for MX3(9-fluorenone) were determined by variable temperature 1H-NMR spectroscopy. For all the compounds, ΔSD is large and positive, as would be expected from a dissociative process. A correlation of the results for the five structurally characterized compounds MX3(9-fluorenone) (M=B, X=Cl; M=Al, X=Cl, Br; M=Ga, X=Cl, Br) was used to examine the suitability of the following parameters to measuring Lewis acidity: IR νCO, UV λmax, and 13C-NMR δCO, Keq at 298 K, ΔH, ΔG, and a variety of structural parameters. Parameters determined from X-ray crystallography appear to be controlled by inter-ligand repulsion or the ionic radii of the metal atom. The change in the νCO band upon coordination provides a reasonable ordering for a specific metal (i.e. AlI3≈AlBr3>AlCl3), but does differentiate simply between 1:1 and 1:2 complexes. Unexplainably, the shift in the δCO in the 13C-NMR appears to be the complete opposite of other trends. It appears important to differentiate, the ability of a Lewis acidic compound to bind a Lewis base from the effect the metal compound has upon that Lewis base once coordinated. In this regard, the Keq at 298 K and ΔG would appear to be the best measures of the ability of a Lewis acid to bind a specific Lewis base, while the Δλmax for the 9- fluorenone complexes provides a good indication as to the effect of a Lewis acid on a particular Lewis base.