title: P(CH)P pincer rhodium(I) complexes: the key role of electron-poor imidazoliophosphine extremities.
abstract: 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 η(1)-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-CN(2) bond cleavage at both imidazoliophosphine moieties, resulting in the formation of a dicationic "open pincer" species, that is, a nonchelated bis((MeO)PPh(2))-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.
authors.affiliation: CNRS, LCC (Laboratoire de Chimie de Coordination), 205, route de Narbonne, F-31077 Toulouse, France.
authors.forename: Cécile
authors.initials: C
authors.lastname: Barthes
authors.forename: Christine
authors.initials: C
authors.lastname: Lepetit
authors.forename: Yves
authors.initials: Y
authors.lastname: Canac
authors.forename: Carine
authors.initials: C
authors.lastname: Duhayon
authors.forename: Davit
authors.initials: D
authors.lastname: Zargarian
authors.forename: Remi
authors.initials: R
authors.lastname: Chauvin
mesh_terms: D018360:Crystallography, X-Ray; D004583:Electrons; D007093:Imidazoles / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D010720:Phosphines / Q000737:chemistry*; D011789:Quantum Theory; D012238:Rhodium / Q000737:chemistry*
chemical_list: D007093:Imidazoles; D008024:Ligands; D009942:Organometallic Compounds; D010720:Phosphines; D012238:Rhodium