Synthesis, characterization and kinetics of the metallation of the intermediate compound [Rh2(μ-O2CCH3)3(O2CCH3){η2-(P)under-bar(2-(CH3(O)under-bar)C6H4)Ph2}]•H2O to produce compound [Rh2(μ-O2CCH3)3-{μ-((C)under-barC5H4)(P)under-bar(2-(CH3O)C6H4)Ph}]•(H2O) from Rh(II) tetraacetate

Photochemical irradiation of mixtures of [Rh-2(mu-O2CCH3)(4)] . (HOCH3)(2) and P(2-(CH3O)C6H4)Ph-2 (P-OMe) in CH2Cl2 solution (1:1 molar ratio) yields quantitatively compound [Rh-2(mu-O2CCH3)(3)(O2CCH3) {eta(2)-(P) under bar(2-(CH3(O) under bar)C6H4)Ph-2}] . (H2O) (1) containing a P-OMe acting as a chelating equatorial ((P) under bar) axial ((O) under bar) donor ligand. This compound undergoes intramolecular C-H activation reaction at one of the phenyl rings in CHCl3 or CH3COOH/toluene solution at room temperature to form the monometallated compound [Rh-2(mu-O2CCH3)(3){mu- ((C) under bar C5H4)(P) under bar(2-(CH3O)C6H4)Ph}] . (H2O) (2). The kinetics of the 1 --> 2 reaction have been studied in acid media (CH3COOH, CH3COOD, CH3COOH/CF3SO3H, CH3COOH/toluene) and aprotic solvents (chloroform, toluene) at variable temperatures and pressures; the results are interpreted in view of those published for similar dinuclear Rh(II) compounds. The values found for Delta H-not equal, Delta S-not equal and Delta V-not equal (84 kJ mol(-1), -51 J K-1 mol(-1), 1.3 cm(3) mol(-1) in CH3COOH; and 68 kJ mol(-1), -121 J K-1 mol(-1), -15 cm(3) mol(-1) in aprotic solvents) agree with those found for similar monometallation reactions studied on Rh(II)(2) core compounds. The more positive values found in this study for Delta S-not equal and Delta V-not equal could be attributed both to a bond-breaking expansion of the axial Rh-O(phosphine) bond and to the transfer of the monodentated acetato ligand from a tight inert equatorial coordination position to a labile axial position. Compound 1 has to undergo these processes in order to reach the highly ordered transition state needed for the C-H bond activation to take place if the steric demands of the phosphine ligand are taken into account. (C) 1998 Elsevier Science S.A. All rights reserved.