Coordination or Oxidative Addition? Activation of N–H with [Tp′Rh(PMe3)

Abstract

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 RhI-N coordination products (κ2-Tp')Rh(PMe3)(NH2CH2CF3) and RhIII 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 = RM-N/N-H of 0.91 (10), indicating that the Rh-N bond strength varies in direct proportion to the N-H bond strength.