T‐shaped 14 Electron Rhodium Complexes: Potential Active Species in C−H Activation

Two T-shaped 14-electron rhodium complexes 2 a and 2 b, "framed" and thus stabilized by PNP pincer ligands have been synthesized. The bis(t-butyl)phosphine derived PNP^tBu-rhodium complex 2 a was isolated from pentane as the more stable cyclometalated Rh(III) hydrido complex and found to be in equilibrium with the T-shaped 14e^- Rh(I) complex 2 aT which itself could be directly crystallized upon change of the solvent. The cyclometallation is suppressed using an adamantyl substituted PNP^Ad ligand to give the analogous T-shaped Rh(I) species 2 b, stabilized through an agostic interaction with one of the adamantyl C-Hs. Depending on the solvent, complex 2 a reacted with ethylene either by π-coordination (4 a) or C-H activation giving a hydrido-vinyl Rh(III) species 4 b, both isomers being in equilibrium in solution. Complex 2 b was found to reversibly C-H activate arenes to form the hydrido-aryl Rh(III) complexes.