Functionalization of RhIII–Me Bonds: Use of “Capping Arene” Ligands to Facilitate Me–X Reductive Elimination

We show how to improve the yield of MeX from CH4 activation catalysts from 12% to 90% through the use of “capping arene” ligands. Four (FP)­RhIII(Me)­(TFA)2 {FP = “capping arene” ligands, including 8,8′-(1,2-phenylene)­diquinoline (6-FP), 8,8′-(1,2-naphthalene)­diquinoline (6-NPFP), 1,2-bis­(N-7-azaindolyl)­benzene (5-FP), and 1,2-bis­(N-7-azaindolyl)­naphthalene (5-NPFP)} complexes. These complexes and (dpe)­RhIII(Me)­(TFA)2 (dpe = 1,2-di-2-pyridylethane) were synthesized and tested for their performance in reductive elimination of MeX (X = TFA or halide). The FP ligands were used with the goal of blocking a coordination site to destabilize the RhIII complexes and facilitate MeX reductive elimination. On the basis of single-crystal X-ray diffraction studies, the 6-FP and 6-NPFP ligated Rh complexes have Rh–arene distances shorter than those of the 5-FP and 5-NPFP Rh complexes; thus, it is expected that the Rh–arene interactions are weaker for the 5-FP complexes than for the 6-FP complexes. Consistent with our hypothesis, the 5-FP and 5-NPFP RhIII complexes demonstrate improved performance (from 12% to ∼60% yield) in the reductive elimination of MeX. The reductive elimination of MeX from (FP)­RhIII(Me)­(TFA)2 can be further improved by the use of chemical oxidants. For example, the addition of 2 equiv of AgOTf leads to 87(2)% yield of MeTFA and can be achieved in CD3CN at 90 °C using (5-FP)­Rh­(Me)­(TFA)2.