Thermodynamic Studies of Competitive Adduct Formation: Single- and Double-Insertion Reactions of Carbon Monoxide with Rhodium Octaethylporphyrin Dimer

Article abstract of DOI:10.1021/ja00226a022

Reaction of carbon monoxide with (octaethylporphyrin)rhodium dimer, <(OEP)Rh>2, in toluene produce an equilibrium system involving four species: <(OEP)Rh>2 (1), <(OEP)Rh>2(CO) (2), (OEP)RhC(O)(Rh(OEP) (3), and (OEP)RhC(O)C(O)Rh(OEP) (4).No evidence is found for the formation of a dicarbonyl adduct of 1, which is accounted for on the basis of required structural features of metal-metal-bonded porphyrin dimers. 1H and 13C NMR evidence is presented for double insertion of CO into the Rh-Rh bond.NMR methods are used to determine enthalpy and entropy changes for the reaction of 1 with CO to form the following: 2, ΔH₂⁰ = -10 +/- 1 kcal/mol (-41 +/- 4 kJ/mol), ΔS₂⁰ = -26 +/- 4 cal/K*mol (-109 +/- 17 J/K*mol); 3, ΔH₃⁰ = -12 +/- 2 kcal/mol (-50 +/-8 kJ/mol), ΔS₃⁰ = -31 +/- 5 cal/K*mol (-130 +/- 21 J/K*mol); 4, ΔH₈⁰ = -21 +/- 2 kcal/mol (-88 +/- 8 kJ/mol), ΔS₈⁰ = -62 +/- 5 cal/K*mol (-259 +/- 30 J/K*mol).Thermodynamic criteria for the double insertion of CO into metal-metal bonds are presented and applied to the (OEP)Rh system.