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.

A Cobalt(I) Pincer Complex with an η2‐Caryl−H Agostic Bond: Facile C−H Bond Cleavage through Deprotonation, Radical Abstraction, and Oxidative Addition

“…[3,4] In this respect, intramolecular and directed C À Ha ctivation reactions of Ru, Rh, and Ir PCP pincer complexes led to the formation of stable complexes that contain h 2 -C-H agostic arene bonds. [5][6][7][8][9][10][11] Agostic square-planar Rh I pincer complexes reported by Milstein and co-workers are particularly intriguing.[5] These four-coordinate species were unable to cleave the C À Hbond and no oxidative addition to form Rh III hydride complexes took place.However,addition of CO,despite of being aweak s-donor,s ufficiently increased the electron density at the metal center and triggered the oxidative addition process (Scheme 1). Based on DFT calculations,t he aromatic CÀH cyclometalation process was suggested to procced through an elusive agostic-like trigonal bipyramidal dicarbonyl Rh I intermediate.…”

Influence of electrostatic repulsive force and electron-withdrawing effect in ionic diphosphine on regioselectivity of rhodium-catalyzed hydroformylation of 1-octene

“…1 and 2 were prepared according to the procedures reported by Canac and Chauvin [21] with some modifications.…”

“…In this paper, the effect of electric charges in diphosphines was considered for the first time to control the bite angle to tailor the regioselectivity of hydroformylation. For this purpose, the neutral and ionic diphosphines of 1 and 2 was synthesized according the method reported by Canac and Chauvin [21]. 2 was the resultant compound of 1 after quaternization 1 by MeOTf (methyl-trifluoromethane sulfonate) at 3N-position of imidazolyl ring, in which the two -PPh 2 groups were vicinal to the two positive charged imidazolium rings (Scheme 1).…”

The Sound of the Invisible

“…I will merely present this narrative in order to distinguish the three levels of meaning that Barthes saw. The first 'informational' level, the one that communicates, 2 sets the scene, introduces the characters, placing them in time by their clothes and perhaps their haircuts. There is a grown man, a young girl, some prawning nets, a river and, amidst all of it somewhere, a message.…”

“…There is a grown man, a young girl, some prawning nets, a river and, amidst all of it somewhere, a message. The second 'symbolic' level, the one that, according to Barthes, signifies, 3 is no longer open to the science of the message but to the sciences of the symbol, the symbols here being that of love, attraction, age and youth, the light of my mother' s youthful hair and complexion versus the dark brooding looks of my father, and beside them, the ageless river winding through the landscape down towards the inevitable sea. And the third level?…”