Transition State Characterization for the Reversible Binding of Dihydrogen to Bis(2,2'-bipyridine)rhodium(I) from Temperature- and Pressure-Dependent Experimental and Theoretical Studies

• Published in: Inorganic chemistry Vol. 45; no. 4; pp. 1595 - 1603
• Main Authors: Fujita, Etsuko, Brunschwig, Bruce S, Creutz, Carol, Muckerman, James T, Sutin, Norman, Szalda, David, van Eldik, Rudi
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 20.02.2006
• Subjects: 10 SYNTHETIC FUELS; ACETONE; ENTHALPY; ENTROPY; IRRADIATION; KINETICS; METHANOL; MIXTURES; national synchrotron light source; NSLS; PARTICLE ACCELERATORS; SOLVENTS; THERMODYNAMICS
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic0515498

Thermodynamic and kinetic parameters for the oxidative addition of H2 to [RhI(bpy)2]+ (bpy = 2,2‘-bipyridine) to form [RhIII(H)2(bpy)2]+ were determined from either the UV−vis spectrum of equilibrium mixtures of [RhI(bpy)2]+ and [RhIII(H)2(bpy)2]+ or from the observed rates of dihydride formation following visible-light irradiation of solutions containing [RhIII(H)2(bpy)2]+ as a function of H2 concentration, temperature, and pressure in acetone and methanol. The activation enthalpy and entropy in methanol are 10.0 kcal mol-1 and −18 cal mol-1 K-1, respectively. The reaction enthalpy and entropy are −10.3 kcal mol-1 and −19 cal mol-1 K-1, respectively. Similar values were obtained in acetone. Surprisingly, the volumes of activation for dihydride formation (−15 and −16 cm3 mol-1 in methanol and acetone, respectively) are very close to the overall reaction volumes (−15 cm3 mol-1 in both solvents). Thus, the volumes of activation for the reverse reaction, elimination of dihydrogen from the dihydrido complex, are approximately zero. B3LYP hybrid DFT calculations of the transition-state complex in methanol and similar MP2 calculations in the gas phase suggest that the dihydrogen has a short H−H bond (0.823 and 0.810 Å, respectively) and forms only a weak Rh−H bond (1.866 and 1.915 Å, respectively). Equal partial molar volumes of the dihydrogenrhodium(I) transition state and dihydridorhodium(III) can account for the experimental volume profile found for the overall process.