Isolation and Structural Characterisation of Monoprotonated 1.2-Bis(diphenylphosphino)benzene in its Tetrabromogallate(III) Salt

In a recent study of the coordination chemistry of gallium and indium trihalides with chelating ditertiary phosphines a rich series of 1:1 and 1:2 com­ plexes has been isolated and structurally charac­ terised [11. The metals appear in a tetra-, pentaand hexacoordinated state both in neutral molecules and in ionic crystals. The compounds are often fluctional in solution and various sets of isomers have been discovered [2 ]. These compounds show tailored Lewis acid re­ activity depending on the nature of the metal (Ga, In), the halogen (Cl, Br, I), and the phosphine ligand [3], They are also sensitive to hydrolysis, and this is particularly true for the gallium(III) complexes [41. It therefore was of no surprise that in many experiments small amounts of the corresponding hydrolysis products have also been observed even if moisture was rigorously excluded. A typical ex­ ample has been the title salt, which appeared when­ ever gallium tribromide was reacted with 1 ,2 -bis(diphenylphosphino)benzene. After its identity had been confirmed by standard methods, the compound was prepared deliberately by controlled hydrolysis as described below (eq. (1)). Following the same procedure, but using gallium tri iodide, the analo­ gous tetra/odogallate(III) has also been obtained. A protonation of both phosphorus atoms was never observed. The formation of a second onium centre in cis position seems to be hindered by electrostatic repulsion.