Effect of CO on the Oxidative Addition of Arene CH Bonds by Cationic Rhodium Complexes

Abstract

Sequential addition of CO molecules to cationic aryl-hydrido Rh(III) complexes of phosphine-based (PCP) pincer ligands was found to lead first to C-H reductive elimination and then to C-H oxidative addition, thereby demonstrating a dual role of CO. DFT calculations indicate that the oxidative addition reaction is directly promoted by CO, in contrast to the commonly accepted view that CO hinders such reactions. This intriguing effect was traced to repulsive pi interactions along the aryl-Rh-CO axis, which are augmented by the initially added CO ligand (due to antibonding interactions between occupied Rh d(pi) orbitals and occupied pi orbitals of both CO and the arene moiety), but counteracted by the second CO ligand (due to significant pi back-donation). These repulsive interactions were themselves linked to significant weakening of the pi-acceptor character of CO in the positively charged rhodium complexes, which is concurrent with an enhanced sigma-donating capability. Replacement of the phosphine ligands by an analogous phosphinite-based (POCOP) pincer ligand led to significant changes in reactivity, whereby addition of CO did not result in C-H reductive elimination, but yielded relatively stable mono- and dicarbonyl aryl-hydrido POCOP-Rh(III) complexes. DFT calculations showed that the stability of these complexes arises from the higher electrophilicity of the POCOP ligand, relative to PCP, which leads to partial reduction of the excessive pi-electron density along the aryl-Rh-CO axis. Finally, comparison between the effects of CO and acetonitrile on C-H oxidative addition revealed that they exhibit similar reactivity, despite their markedly different electronic properties. However, DFT calculations indicate that the two ligands operate by different mechanisms.

From para‐Benziporphyrin to Rhodium(III) 21‐Carbaporphyrins: Imprinting Rh⋅⋅⋅η2‐CC, Rh⋅⋅⋅η2‐CO, and Rh⋅⋅⋅η2‐CH Coordination Motifs

“…Such structuralm otivesa fford the stabilization of ap eculiar rhodium-carbon bonding situation, outstandingly reflected in the rhodiumo rganometallic chemistry in the surroundings of pincer-typeligands. [35][36][37][38][39][40][41] Herein, we report the formation and characterization of rhodium(III) para-benziporphyrin (1-Rh) and its contraction to rhodium(III)2 1-carbaporphyrin 2-Rh, the CNNN macrocyclic coordination cavity of which assists the creation of peculiar Rh···h 2 -CC, Rh···h 2 -CO, or Rh···h 2 -CH coordination motifs.…”

Synthesis and Characterization of 3,5-Bis(di-tert-butylphosphinito)pyridine Pincer Complexes

“…C–H activation of ligand 2 would be expected to give a six-coordinate rhodium hydrido-chloride carbonyl complex. Prior results from the groups of Milstein and Heinekey have demonstrated the high acidity of similar Rh­(III) hydrides, leading to fast elimination of HCl in the case of ( t Bu POCOP)­Rh­(CO)­HCl. , N-Coordination of B­(C 6 F 5 ) 3 in 4 should cause even greater Rh–H acidity, therefore rationalizing the overall fast reaction observed.…”

Preise der israelischen chemischen Gesellschaft: Y. Apeloig, S. Shaik, J. M. L. Martin, D. Meyerstein, S. Ruthstein und M. E. van der Boom / NAS Award in Chemical Sciences: J. A. Doudna / Remsen Award und Ralph N. Adams Award: C. A. Mirkin

“…Martins Forschungsinteressen umfassen Computer‐ und Quantenchemie unter Anwendung von DFT‐ und wellenfunktionsbasierten Methoden. Chemistry – A European Journal veröffentlichte eine Arbeit von ihm über die oxidative Addition von Aren‐C‐H‐Bindungen an kationische Rhodiumkomplexe …”