Thermodynamics of addition of CO, isocyanide, and H2 to Rh(PR3)2Cl

The enthalpies of the addition of CO, H-2, or Bu(f)NC to [Rh((PPr3)-Pr-i)(2)Cl](2) (1) to give the mononuclear complexes Rh((PPr3)-Pr-i)(2)Cl(CO) (2), Rh((PPr3)-Pr-i)(2)ClH2 (3), and Rh((PPr3)-Pr-i)(2)Cl(CNBu(t)) (4) are reported. 2-Ethylhexanal is decarbonylated by 1 to give n-heptane and 2; solution-phase calorimetric measurement of this reaction enables calculation of the enthalpy of addition of CO to 1. The coordinatively unsaturated dihydride 3 reacts with Bu(t)NC to give 4; measurement of the enthalpy of this reaction, and the direct reaction of 1 with Bu(t)NC, permits calculation of the enthalpy of addition of H-2 to 1. These results afford the relative enthalpies of addition to the hypothetical fragment Rh((PPr3)-Pr-i)(2)Cl. Although 1 was previously formulated as monomeric in solution, the complex is exclusively dimeric. Based on the observation that no measurable concentration of Rh((PPr3)-Pr-i)(2)Cl monomer exists in solution, a lower limit for the bridge strength of 1 is calculated which, in turn, affords lower limits for the exothermicity of additions to the hypothetical monomer: 48.2 kcal/mol for addition of CO (i.e, the Rh-CO BDE of 1), 42.4 kcal/mol for addition of Bu(t)NC, and 32.5 kcal/mol for addition of H-2. Although these values represent lower limits, the Rh-CO BDE and particularly the exothermicity of H-2 addition are quite high compared with previously reported values for second-row transition metals. These results are consistent with and help to explain the unusual ability of Rh(PMe(3))(2)Cl(CO) to efficiently catalyze photo- and thermochemical alkane functionalization reactions