Reactivity of the latent 12-electron fragment [Rh(PiBu3) 2]+with aryl bromides: Aryl-br and phosphine ligand C-H activation

Nell S. Townsend, Adrian B. Chaplin, M Abu Naser, Amber L. Thompson, Nicholas H. Rees, Stuart A. Macgregor, Andrew S. Weller

Research output: Contribution to journalArticle

Abstract

Oxidative addition of aryl bromides to 12-electron [Rh(PiBu 3)2]-[BArF4] (ArF = 3,5-(CF3)2C6H3) forms a variety of products. With p-tolyl bromides, RhIII dimeric complexes result [Rh(PiBu3)2(o/p-MeC6H4)(µ-Br)] 2-[BArF4]2. Similarly, reaction with p-ClC6H4Br gives [Rh(PiBu3) 2(p-ClC6H4)(µ-Br)]2[BAr F4]2. In contrast, the use of o-BrC 6H4Me leads to a product in which toluene has been eliminated and an isobutyl phosphine has undergone C-H activation: [Rh(PiBu 2-(CH2CHCH3CH2)}(PiBu 3)(µ-Br)]2-[BArF4]2. Trapping experiments with ortho-biomo anisole or ortho-bromo thioanisole indicate that a possible intermediate for this process is a low-coordinate RhIII complex that then undergoes C-H activation. The anisole and thioanisole complexes have been isolated and their structures show OMe or SMe interactions with the metal centre alongside supporting agostic interactions, [Rn(PiBu3)2(C6H4OMe)Br]-[BA F4] (the solid-state structure of the 5-methyl substituted analogue is reported) and [Rh(PiBu3)2(C6H 4SMe)-Br][BArF4]. The anisole-derived complex proceeds to give [Rh-{PiBu2(CH2CHCH3CH 2)}(PiBu3)(µ-Br)]2[BArF4]2, whereas the thioanisole complex is unreactive. The isolation of [Rh(PiBu3)2(C6H4OMe)Br] [BArF4] and its onward reactivity to give the products of C-H activation and aryl elimination suggest that it is implicated on the pathway of a s-bond metathesis reaction, a hypothesis strengthened by DFT calculations. Calculations also suggest that C-H bond cleavage through phosphine-assisted deprotonation of a non-agostic bond is also competitive, although the subsequent protonation of the aryl ligand is too high in energy to account for product formation. C-H activation through oxidative addition is also ruled out on the basis of these calculations. These new complexes have been characterised by solution NMR/ESIMS techniques and in the solid-state by X-ray crystallography. © 2010 Wiley-VCH. Verlag GmbH & Co. KGaA, Weinheim.

Original languageEnglish
Pages (from-to)8376-8389
Number of pages14
JournalChemistry - A European Journal
Volume16
Issue number28
DOIs
Publication statusPublished - 26 Jul 2010

Keywords

  • Agostic interactions
  • C-H activation
  • Density functional calculations
  • Transition metals
  • X-ray diffraction

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