The enthalpies of the addition of CO, H2, or ButNC to [Rh(PiPr3)2Cl]2 (1) to give the mononuclear complexes Rh(PiPr3)2Cl(CO) (2), Rh(PiPr3)2CIH2 (3), and Rh(PiPr3)2Cl(CNBut) (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 ButNC to give 4; measurement of the enthalpy of this reaction, and the direct reaction of 1 with ButNC, permits calculation of the enthalpy of addition of H2 to 1. These results afford the relative enthalpies of addition to the hypothetical fragment Rh(PiPr3)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(PiPr3)2Cl 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 ButNC, and 32.5 kcal/mol for addition of H2. Although these values represent lower limits, the Rh-CO BDE and particularly the exothermicity of H2 addition are quite high compared with previously reported values for secondrow transition metals. These results are consistent with and help to explain the unusual ability of Rh(PMe3)2Cl(CO) to efficiently catalyze photo- and thermochemical alkane functionalization reactions.
|Number of pages||7|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - May 10 1995|
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