Rhodium porphyrins, thermodynamic

The addition of metal hydrides to C—C or C—O multiple bonds is a fundamental step in the transition metal catalyzed reactions of many substrates. Both kinetic and thermodynamic effects are important in the success of these reactions, and the rhodium porphyrin chemistry has been important in understanding the thermochemical aspects of these processes, particularly in terms of bond energies. For example, for first-row elements. M—C bond energies arc typically in the range of 2, i-. i() kcal mol. M—H bond energies are usually 25-30 kcal mol. stronger, and as a result, addition of M—CH bonds to CO or simple hydrocarbons is thermodynamically unfavorable. 

Rhodium(III) tetra(/ -sulfonatophenyl)porphyrin [(TSPP)Rh] aquo and hydroxo complexes react with a series of alkenes in water to form /3-hydroxyalkyl coordination compounds. Addition reactions of (TSPP)Rh-OH to unactivated terminal alkenes CH2=CHR invariably occur with both kinetic and thermodynamic preferences to place rhodium on the terminal carbon to form (TSPP)Rh-CH2CH(OH)R complexes. Acrylic and styrenic alkenes initially react to place rhodium on the terminal carbon to form [Rh]-CH2CH(OH)R as the kinetically preferred isomer, but subsequently proceed to an equilibrium distribution of regioisomers where [Rh]-CH(CH2OH)R is the predominant thermodynamic product. Equilibrium constants for reactions of the diaquo rhodium(III) compound [(TSPP)Rhm(H20)2]3 in water with a series of terminal alkenes that form /9-hydroxyalkyl complexes were directly evaluated and used in deriving thermodynamic values for addition of the Rh-OH unit to alkenes. The AG° for reactions of the Rh-OH unit with alkenes in water was found to be approximately 3 kcalmol-1 less favourable than the comparable Rh-H reactions in water.100... 

Thermodynamic Studies of the Hydrogenation and Reductive Coupling of Carbon Monoxide by Rhodium (II) Porphyrins... 

Fu X, Wayland BB (2004) Equilibrium thermodynamic studies in water reactions of dihydrogen with rhodium(lll) porphyrins relevant to Rh-Rh, Rh-H, and Rh-OH bond energetics. J Am Chem Soc 126(8) 2623-2631... 

Fu, X., Li, S., Wayland, B.B. (2006) Reactivity and equilibrium thermodynamic smdies of rhodium tetrakis(3,5-disulfonatomesityl) porphyrin species with H2, CO, and olefins in water. Inorganic Chemistry, 45, 9884-9889. 

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