The enthalpies of the addition of CO, H2, or ButNC to [Rh(PiPr3)2Cl2] (1) to give the mononuclear complexes Rh(PiPr3)2Cl(CO) (2), Rh(PiPr3)2ClH2(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 BuNC 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 H2addition 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(PMe3)2Cl(CO) to efficiently catalyze photo- and thermochemical alkane functionalization reactions.
ASJC Scopus subject areas
- Colloid and Surface Chemistry