P(CH)P Pincer Rhodium(I) Complexes: The Key Role of Electron-Poor Imidazoliophosphine Extremities

The coordination chemistry of a potentially pincer-type dicationic meta-phenylene-bis­(imidazoliophosphine) ligand 3 to neutral and cationic carbonylrhodium­(I) centers has been investigated. Similarly to what was observed previously for its ortho-phenylene counterpart, 3 was found to bind to the RhCl­(CO) fragment in a trans-chelating manner that makes possible a weak Rh–C­(H) interaction, inferred from the nonbonding but relatively short Rh–C and Rh–H contacts observed in the solid state structure of the dicationic adduct (3)­RhCl­(CO) (5). Formation of the target PCP-type pincer complex could not be triggered despite multiple attempts to deprotonate the central C–H moiety in the initial dicationic adduct 5, or in the tricationic species [(3)­Rh­(CO)]+ (8) generated by abstraction of the chloride ion from 5. Complex 8 was identified on the basis of NMR and IR analyses as a Rh­(I)-stabilized P­(CH)­P-intermediate en route to the anticipated classical PCP-type pincer complex. Analysis of the electronic structure of this intermediate computed at the density functional level of theory (DFT level) revealed a bonding overlap between a Rh d-orbital and π-orbitals of the m-phenylene ring. NBO analyses and calculated Wiberg indices confirm that this interaction comprises an η1-C–Rh bonding mode, with only secondary contributions from the geminal C and H atoms. Although the target PCP-type pincer complex could not be generated, treatment of the tricationic intermediate with methanol induced a P–CN2 bond cleavage at both imidazoliophosphine moieties, resulting in the formation of a dicationic “open pincer” species, that is, a nonchelated bis­((MeO)­PPh2)-stabilized aryl-Rhodium complex that is the κC-only analogue of the putative κP,κC,κP-PCP complex sought initially. Theoretical studies at the DFT level of experimental or putative species relevant to the final C–H activation process ruled out the oxidative addition pathway. Two alternative pathways are proposed to explain the formation of the “open pincer” complex, one based on an organometallic α-elimination step, the other based on an organic aromatization-driven β-elimination process.