Coordination or Oxidative Addition? Activation of N-H with [Tp'Rh(PMe₃)]

A thermal reaction of amines, anilines, and amides with Tp'Rh(PMe₃)(CH₃)H (1, Tp' = tris(3,5-dimethyl-pyrazolyl)borate) is described in this report. No N-H bond cleavage was observed for reactions between ammonia or unsubstituted aliphatic amines with the reactive fragment [Tp'Rh(PMe₃)]. Instead, amine coordination products (κ²-Tp')Rh(PMe₃)(NHR¹R²) (R¹ = H, R² = H, ⁿPr, ⁱPr, octyl; R¹ = R² = Et; R¹, R²: pyrrolidine) were observed, and the crystal structure of (κ²-Tp')Rh(PMe₃)(NH₂ ⁱPr) is reported. No coordination products were observed when 1 was reacted with 1,1,1,3,3,3-hexafluoropropan-2-amine, anilines, and amides. Instead, the oxidative addition products (κ³-Tp')Rh(PMe₃)(NHR)H (R = CH(CF₃)₂, C₆H₅, 3,5-dimethylbenzyl, C₆F₅, C(O)CH₃, C(O)CF₃) were observed. Both Rh^I-N coordination products (κ²-Tp')Rh(PMe₃)(NH₂CH₂CF₃) and Rh^III N-H addition products (κ³-Tp')Rh(PMe₃)(NHCH₂CF₃)H were generated when 1 was reacted with 2,2,2-trifluoroethylamine. Coordination products dissociate ammonia and amines in benzene much faster than oxidative addition products eliminate anilines and amides. The relative metal-nitrogen bond energies were studied using established kinetic techniques. Analysis of the relationship between the relative M-N bond strengths and N-H bond strengths showed a linear correlation with a slope = R_M-N/N-H of 0.91 (10), indicating that the Rh-N bond strength varies in direct proportion to the N-H bond strength.