Equilibrium Thermodynamic Studies in Water: Reactions of Dihydrogen with Rhodium(III) Porphyrins Relevant to Rh−Rh, Rh−H, and Rh−OH Bond Energetics

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Abstract
Aqueous solutions of rhodium(III) tetra p-sulfonatophenyl porphyrin ((TSPP)Rh(III)) complexes react with dihydrogen to produce equilibrium distributions between six rhodium species including rhodium hydride, rhodium(I), and rhodium(II) dimer complexes. Equilibrium thermodynamic studies (298 K) for this system establish the quantitative relationships that define the distribution of species in aqueous solution as a function of the dihydrogen and hydrogen ion concentrations through direct measurement of five equilibrium constants along with dissociation energies of D2O and dihydrogen in water. The hydride complex ([(TSPP)Rh−D(D2O)]-4) is a weak acid (Ka(298 K) = (8.0 ± 0.5) × 10-8). Equilibrium constants and free energy changes for a series of reactions that could not be directly determined including homolysis reactions of the RhII−RhII dimer with water (D2O) and dihydrogen (D2) are derived from the directly measured equilibria. The rhodium hydride (Rh−D)aq and rhodium hydroxide (Rh−OD)aq bond dissociation free energies for [(TSPP)Rh−D(D2O)]-4 and [(TSPP)Rh−OD(D2O)]-4 in water are nearly equal (Rh−D = 60 ± 3 kcal mol-1, Rh−OD = 62 ± 3 kcal mol-1). Free energy changes in aqueous media are reported for reactions that substitute hydroxide (OD-) (−11.9 ± 0.1 kcal mol-1), hydride (D-) (−54.9 kcal mol-1), and (TSPP)RhI: (−7.3 ± 0.1 kcal mol-1) for a water in [(TSPP)RhIII(D2O)2]-3 and for the rhodium hydride [(TSPP)Rh−D(D2O)]-4 to dissociate to produce a proton (9.7 ± 0.1 kcal mol-1), a hydrogen atom (∼60 ± 3 kcal mol-1), and a hydride (D-) (54.9 kcal mol-1) in water.

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