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

Abstract

The 9-fluorenone complexes MX 3(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 AlBr 3(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 MgCl 2 ( 8), HgCl 2 ( 9), ZrCl 4 ( 10) and SnCl 4 ( 11) were characterized by IR and UV–vis spectroscopy, while the unusual mixed vanadium(IV,V) compound, VO(Cl)(9-fluorenone) 2(H 2O)(μ-O)(VOCl 2) ( 12) has been structurally characterized. Solution ligand dissociation energies for MX 3(9-fluorenone) were determined by variable temperature 1H-NMR spectroscopy. For all the compounds, Δ S D is large and positive, as would be expected from a dissociative process. A correlation of the results for the five structurally characterized compounds MX 3(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 ν CO, UV λ max, and 13C-NMR δ CO, K eq at 298 K, Δ H, Δ G, 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 ν CO band upon coordination provides a reasonable ordering for a specific metal (i.e. AlI 3≈AlBr 3>AlCl 3), but does differentiate simply between 1:1 and 1:2 complexes. Unexplainably, the shift in the δ C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 K eq at 298 K and Δ G would appear to be the best measures of the ability of a Lewis acid to bind a specific Lewis base, while the Δ λ max for the 9- fluorenone complexes provides a good indication as to the effect of a Lewis acid on a particular Lewis base.