INVESTIGATION OF THE INTERMEDIATES IN THE OXIDATION OF A BULKY ARYLPHOSPHINE LIGAND WITH FERRIC-CHLORIDE

The syntheses, spectroscopic properties and structures of iron(II) and iron(III) complexes prepared by reactions of ferric chloride with the tertiary phosphine ligand tris(2,4,6-trimethoxyphenyl)phosphine (TMPP) are described. The novel diferrous chloride salt [H-TMPP]2[Fe2Cl6] (1) was prepared by the reaction of anhydrous ferric chloride with 1 equivalent of TMPP in benzene or diethyl ether. This redox reaction also produces [TMPP-Cl][FeCl4] (2) as the oxidized by-product. The structure of 1, determined by single-crystal X-ray diffraction, revealed that the [Fe2Cl6]2- anion consists of two edge-sharing tetrahedra with two terminal and two bridging chlorides per metal atom. Dissolution of complex 1 in ethanol produces [H-TMPP]2[Fe(II)Cl4] (3), which was identified on the basis of a preliminary X-ray investigation. Compound 3, independently prepared by the reaction of [Fe(NCCH3)6][AlCl4]2 with 2 equivalents of TMPP in methanol, reacts with molecular oxygen to initially form [H-TMPP][Fe(III)Cl4] (4) and finally the phosphine oxide species FeCl3(O=TMPP) (5). The structure of 4 was determined by single-crystal X-ray diffraction. To test the importance of compound 4 as an intermediate in the production of TMPP=O, it was prepared directly by reaction of [Fe(NCCH3)6][SbCl6]2 with 2 equivalents of TMPP in methanol and reacted with oxygen to give 5 in high yield. Compounds 1-5 were further characterized by IR and electronic spectroscopies, as well as by elemental analyses. The reactions of the salts 1, 3 and 4 suggest that the oxidation of tertiary arylphosphines with FeCl3 in alcohols does not proceed via the formation of dioxygen complexes, but rather via quaternarization of the phosphine ligand.