Competition studies of oxidative addition of aryl halides to the (PNP)Rh fragment

The (PNP)Rh fragment (2) can be conveniently accessed by dissociation of L from the four-coordinate complexes (PNP)Rh(L) (L = SPrⁱ ₂, 3; L = H₂C - CHBu^t, 10), which contain the tridentate PNP pincer ligand (2-ⁱPr₂P-4-Me-C₆H ₄)₂N^-. A new and a more straightforward synthesis of 10 is reported, yielding 65% of 10 based on RhCl₃(H ₂O)_x. A number of new aryl halide oxidation addition products (AHOAP) of the general formula (PNP)Rh(Ar)(Hal) (Hal = Cl, Br, I) have been synthesized through oxidative addition (OA) reactions of meta- and para-substituted aryl halides with 3 or 10. The rotation about the Rh-C _aryl bond is restricted, resulting in rotamers for the meta-substituted aryls that are distinct on the NMR spectroscopy time scale. Reactions of some aryl halides containing a p-NO₂ or p-CO ₂Me with 3 or 10 led to the observation of products of C-H OA that are ostensibly stabilized by coordination to the NO₂ or CO ₂Me group. Analogous C-H OA products were observed for the halide-free nitrobenzene and ethyl benzoate, as well. However, the C-H OA products are thermodynamically unstable with respect to the isomeric AHOAP, to which they convert upon thermolysis. A Hammett-style analysis of the relative electronic effects of the para substituents X in the OA reactions of p-HalC ₆H₄X with 10 was carried out. The positive values of ρ obtained (ρ = 1.51(15) for Ar-Cl, ρ = 0.70(9) for Ar-Br, and ρ = 0.92(9) for Ar-I) illustrate the increase in the OA rate with increasing electron-withdrawing effect of X. Issues relating to the usefulness of the Hammett parameters in comparing various OA reactions are discussed. Comparison with analogous studies on the OA of aryl halides to Pd(0) complexes leads to the notion that the electronic effects have an impact on the rate similar to, but less pronounced than, that of the (PNP)Rh system, possibly indicative of an earlier transition state for the OA of aryl halides with (PNP)Rh.