Preparation, properties, and reactivity of carbonylrhodium(I) complexes of di(2-pyrazolylaryl)amido-pincer ligands

A series of six carbonylrhodium(I) complexes of three new and three previously reported di(2-3R-pyrazolyl)-p-Z/X-aryl)amido pincer ligands, (^RZX)Rh(CO), (R is the substituent at the 3-pyrazolyl position proximal to the metal; Z and X are the aryl substituents para- to the arylamido nitrogen) were prepared. The metal complexes were studied to assess how their properties and reactivities can be tuned by varying the groups along the ligand periphery and how they compared to other known carbonylrhodium(I) pincer derivatives. This study was facilitated by the discovery of a new CuI-catalyzed coupling reaction between 2-(pyrazolyl)-4-X-anilines (X = Me or CF₃) and 2-bromoaryl-1H-pyrazoles that allow the fabrication of pincer ligands with two different aryl arms. The NNN-pincer scaffolds provide an electron-rich environment for the carbonylrhodium(I) fragment as indicated by carbonyl stretching frequencies that occur in the range of 1948-1968 cm⁻¹. As such, the oxidative addition (OA) reactions with iodomethane proceed instantaneously to form trans-(NNN-pincer)Rh(Me)(CO)(I) in room temperature acetone solution. The OA reactions with iodoethane proceeded at a convenient rate in acetone near 45 °C which allowed detailed kinetic studies. The relative order of reactivity was found to be (CF₃CF₃)Rh(CO) < (^iPrMeMe)Rh(CO) < (^MeMeMe)Rh(CO) ∼ (CF₃Me)Rh(CO) < (MeH)Rh(CO) < (MeMe)Rh(CO) with the second order rate constant of the most reactive in the series, k₂ = 8 × 10⁻³ M⁻¹ s⁻¹, being about three orders of magnitude greater than those reported for Rh(CO)₂I₂]⁻ or CpRh(CO)(PPh₃). After oxidative addition, the resultant rhodium(III) complexes were found to be unstable. Although a few trans-(^RMeMe)Rh(E = Me, Et, or I)(CO)(I) could be isolated in pure form, all were found to slowly decompose in solution to give different products depending on the 3R-pyrazolyl substituents. Those with unsubstituted pyrazolyls (R = H) decompose with CO dissociation to give insoluble dimeric (^RMeMe)Rh(E)(μ-I)]₂ while those with 3-alkylpyrazolyls (R = Me, ⁱPr) decompose to give soluble, but unidentified products.