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Rhodium Indenyl NHC and Fluorenyl-Tethered NHC Half-Sandwich Complexes: Synthesis, Structures and Applications in the Catalytic C−H Borylation of Arenes and Alkanes

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Author(s)

  • Kieren J. Evans
  • Paul A. Morton
  • Christian Luz
  • Callum Miller
  • Olivia Raine
  • Jason M. Lynam
  • Stephen M. Mansell

Department/unit(s)

Publication details

JournalChemistry - A European Journal
DateAccepted/In press - 15 Oct 2021
DateE-pub ahead of print (current) - 18 Nov 2021
Number of pages11
Early online date18/11/21
Original languageEnglish

Abstract

Indenyl (Ind) rhodium N-heterocyclic carbene (NHC) complexes [Rh(η5-Ind)(NHC)(L)] were synthesised for 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr) with L=C2H4 (1), CO (2 a) and cyclooctene (COE; 3), for 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene (SIMes) with L=CO (2 b) and COE (4), and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) with L=CO (2 c) and COE (5). Reaction of SIPr with [Rh(Cp*)(C2H4)2] did not give the desired SIPr complex, thus demonstrating the “indenyl effect” in the synthesis of 1. Oxidative addition of HSi(OEt)3 to 3 proceeded under mild conditions to give the Rh silyl hydride complex [Rh(Ind){Si(OEt)3}(H)(SIPr)] (6) with loss of COE. Tethered-fluorenyl NHC rhodium complexes [Rh{(η5-C13H8)C2H4N(C)C2HxNR}(L)] (x=4, R=Dipp, L=C2H4: 11; L=COE: 12; L=CO: 13; R=Mes, L=COE: 14; L=CO: 15; x=2, R=Me, L=COE: 16; L=CO: 17) were synthesised in low yields (5–31 %) in comparison to good yields for the monodentate complexes (49–79 %). Compounds 3 and 1, which contain labile alkene ligands, were successful catalysts for the catalytic borylation of benzene with B2pin2 (Bpin=pinacolboronate, 97 and 93 % PhBpin respectively with 5 mol % catalyst, 24 h, 80 °C), with SIPr giving a more active catalyst than SIMes or IMes. Fluorenyl-tethered NHC complexes were much less active as borylation catalysts, and the carbonyl complexes were inactive. The borylation of toluene, biphenyl, anisole and diphenyl ether proceeded to give meta substitutions as the major product, with smaller amounts of para substitution and almost no ortho product. The borylation of octane and decane with B2pin2 at 120 and 140 °C, respectively, was monitored by 11B NMR spectroscopy, which showed high conversions into octyl and decylBpin over 4–7 days, thus demonstrating catalysed sp3 C−H borylation with new piano stool rhodium indenyl complexes. Irradiation of the monodentate complexes with 400 or 420 nm light confirmed the ready dissociation of C2H4 and COE ligands, whereas CO complexes were inert. Evidence for C−H bond activation in the alkyl groups of the NHC ligands was obtained.

Bibliographical note

Funding Information:
The authors thank the UK National Mass Spectrometry Facility at Swansea University for sample analysis by mass spectrometry. Drs. Georgina Rosair and Mairi Haddow (HWU) are thanked for assistance with X‐ray crystallography, and Dr. Gary Nichol (University of Edinburgh) is gratefully acknowledged for collecting X‐ray diffraction data for . The EPSRC is thanked for funding (DTP studentships to K.J.E. and P.A.M.), and the RSC for a Travel Grant to K.J.E. 1

© 2021 The Authors

    Research areas

  • C−H activation, C−H borylation, Rh N-heterocyclic carbene catalysts, rhodium indenyl, tethered NHC

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