Complexes of the platinum metals. Part 23. Synthesis of the nitrosyl carboxylate complexes [M(O2CR)2(NO)(PPh3)2](M = Rh or Ir; R = CF3, C2F3, or C6F5): crystal and molecular structures of the trifluoroacetate derivatives [M(O2CCF3)2(NO)(PPh3)2]

Abstract

The rhodium complex [Rh(NO)(PPh₃)₃] reacts with perfluorocarboxylic acids, RCO₂H, in the presence of dioxygen to afford emerald green complexes [Rh(O₂CR)₂(NO)(PPh₃)₂](R = CF₃, C₂F₃, or C₆F₅) in good yield. In contrast, the corresponding iridium precursor, [Ir(NO)(PPh₃)₃] reacts with the same acids under aerobic or anaerobic conditions to yield brown complexes [Ir(O₂CR)₂(NO)(PPh₃)₂]. The trifluoroacetate derivatives have been examined by X-ray diffraction methods and shown to possess dissimilar structures. Crystals of the rhodium complex are monoclinic, space group Cc, with a= 12.593(2), b= 15.466(3), c= 20.479(3)Å, β= 100.03(4)°, and Z= 4. The structure, which has been refined to R= 0.06 for 2 918 observed reflections, consists of tetragonal-pyramidal molecules with angular apical nitrosyl ligands and trans phosphine ligands. Crystals of the iridium complex (solvated with one molecule of acetone) are orthorhombic, space group Pbcn, with a= 19.171(2), b= 22.684(3), c= 20.169(3)Å, and Z= 8. The structure, which has been refined to R= 0.081 for 3 791 observed reflections, consists of trigonal-bipyramidal molecules with linear equatorial nitrosyl groups and axial trans phosphine ligands. The angle subtended at the iridium by the co-ordinated oxygen atoms of the unidentate trifluoroacetate ligands is remarkably small [75.0(6)°]. Preliminary calculations performed using a steric energy calculation computer program, EENY2, suggest that in each case the structure adopted corresponds to a stereochemical energy minimum.