ETA(3)-ALLYL AND ETA(3)-BENZYL RHODIUM COMPLEXES - SYNTHESIS, STRUCTURE DYNAMIC, AND REACTIONS WITH CARBOXYLIC-ACIDS

The (eta(3)-allyl)rhodium complexes [Rh(eta(3)-2-RC(3)H(4))(PiPr(3))(2)] (3-5) have been prepared in 70-90% yield from the in situ generated chlororhodium precursor [RhCl(PiPr(3))(2)] (2) and 2-RC(3)H(4)MgX. On a similar route, the corresponding ethene(phosphane) derivative [Rh(eta(3)-2-MeC(3)H(4)) (C2H4)(PiPr(3))] (9) has been obtained. Compound 9 smoothly reacts with PiPr(3) and PMe(3) to give 4 (R = Me) and [Rh(eta(3)-2-MeC(3)H(4))(PMe(3))(PiPr(3))] (10), respectively. In contrast to the eta(3)-allyl complexes which are configurationally stable, the eta(3)-benzyl analogues [Rh(eta(3)-CH(2)C(6)H(4)R)(PiPr(3))(2)] (11, 12), prepared from 2 and 4-RC(6)H(4)CH(2)MgCl, are highly fluctional in solution. At room temperature, an antarafacial (pi-sigma-pi) as well as a suprafacial rearrangement occurs, the first one of which is frozen out at 263 K. On cooling to 193 K, the faster process (equally designated as a metallotropic shift) is also slowed down, and the rigid structure of 11 and 12 is observed. The Delta G(not equal) values for the antara and suprafacial rearrangements of 11 and 12 which have been determined from the H-1- and P-31-NMR spectra at variable temperature are 60.0 +/- 1.5 and 39.5 +/- 1.0 kJ/mol, respectively. The eta(3)-benzyl compound 11 reacts even at -78 degrees C with CO to give the monocarbonylrhodium(I) complex trans-[Rh(eta(1)-CH2C6H5)(CO)(PiPr3)2] (13) Treatment of 3, 4, or 11 with RCO(2)H (R = CF3, CH3, C6H5, C6H4-4-OMe, C6H4-4-NO2) affords the monomeric eta(2)-carboxylatorhodium(I) compounds [Rh(eta(2)-O(2)CR) (PiPr(3))(2)] (14-18) almost quantitatively. If the reaction of 3 or 4 with CF3CO2H is performed at -20 degrees C in pentane, the octahedral (eta(3)-allyl)hydrido complexes [Rh(eta(3)-2-RC(3)H(4)) (H) (eta(1)-O2CCF3)(PiPr(3))(2)] (19, 20) are isolated. The crystal structures of 12 (at 223 K) and of 15 (at 298 K) have been determined by X-ray diffraction studies. They confirm that the eta(3)-benzyl Ligand is highly unsymmetrically and the acetato ligand completely symmetrically bound to the [Rh(PiPr(3))(2)] unit.