THE REACTIVITY OF (1,5-CYCLOOCTADIENE)RH(BETA-DIKETONATE) COMPOUNDS TOWARDS TRIORGANOPHOSPHINE LIGANDS

The complexes (1,5-COD)Rh(beta-diketonate) have been prepared by the reaction between [(1,5-COD)RhCl]2 and Na(beta-diketonate), where beta-diketonate = acetylacetonate (acac), trifluoroacetylacetonate (tfac), and hexafluoroacetylacetonate (hfac) and 1,5-COD = 1,5-cyclooctadiene. These compounds were characterized by elemental analysis, FTIR, mass spectroscopy, H-1 and C-13 NMR spectroscopy and (1,5-COD)Rh(tfac) was characterized, in the solid state, by single crystal X-ray diffraction. The compounds (1,5-COD) Rh(beta-diketonate) were reacted with a variety of triorganophosphines to investigate the substitutional lability of the 1,5-COD versus the triorganophosphine ligands. In all cases the 1,5-COD ligand was displaced. Reaction with two equivalents of PR3, R = Me, Et gave the species (PR3)2Rh(beta-diketonate). The compound (PMe3)2Rh(tfac) was characterized in the solid state by single crystal X-ray diffraction. Reaction of (1,5-COD)Rh(tfac) or (PMe3)2Rh(tfac) with an excess of PMe3 gave two species, A and B with the same empirical formula (Me3P)4Rh(tfac), but different structures in solution as determined by multinuclear and variable temperature NMR spectroscopy. Species A exhibited NMR data consistent with a trigonal-bipyramidal Rh coordination environment, while species B exhibited NMR data consistent with a square planar coordination environment consistent with the presence of an ion pair [(Me3P)4Rh]+ [tfac]-. Reaction of (1,5-COD)Rh(tfac) with one and two equivalents of bis(diethylphosphino)ethane, DEPE, gave (DEPE)Rh(tfac) and (DEPE)2Rh(tfac), respectively. The reaction of (Me3P)2Rh(tfac) with one equivalent of DEPE resulted in the formation of (Me3P)2(DEPE)Rh(tfac) which exhibited P-31 NMR data more consistent with a square planar than a trigonal-bipyramidal Rh coordination environment. These results are in contrast to the reactions between (1,5-COD)Rh(beta-diketonate) and analogous tertiary amines in which the beta-diketonate ligand is displaced in preference to the olefin ligand.