The reactivity of (1.5-cyclooctadiene)Rh(β-diketonate) compounds towards triorganophosphine ligands

Abstract

The complexes (1,5-COD)Rh(β-diketonate) have been prepared by the reaction between[(1,5-COD)RhCl] 2 and Na(β-diketonate), where β-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. 1H and 13C 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(β-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 PR 3, R = Me, Et gave the species (PR 3) 2Rh(β-diketonate). The compound (PMe 3) 2Rh(tfac) was characterized in the solid state by single crystal X-ray diffraction. Reaction of (1,5-COD)Rh(tfac) or (PMe 3) 2Rh(tfac) with an excess of PMe 3 gave two species, A and B with the same empirical formula (Me 3P) 4Rh(tfac), but different 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 [(Me 3P) 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 (Me 3P) 2Rh(tfac) with one equivalent of DEPE resulted in the formation of (Me 3P) 2(DEPE)Rh(tfac) which exhibited 31P 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(β-diketonate) and analogous tertiary amines in which the β-diketonate ligand is displaced in preference to the olefin ligand.