Synthesis and catalytic applications of NHC–metal complexes supported on p-tert-butylcalix[4]arene frameworks

Abstract

The preparation of a 1,3-bis-imidazolium functionalized 2,4-dihydroxyl p-tert-butylcalix[4]arene ligand [^tBuC4(OH)₂(NHC)₂]Br₂ (1) is reported. Treatment of ligand 1 with excess of silver oxide allows for the straightforward preparation of the silver complex [^tBuC4(OH)₂(NHC·AgBr)₂] (2) which is successfully employed as a transmetallation agent for the synthesis of a series of bimetallic complexes (3–5) of the type [^tBuC4(OH)₂(NHC·M)₂] [M = Ru(cymene)Cl, Rh(COD)Cl, Pd(allyl)Cl]. Furthermore, the selective deprotonation of proligand 1 with KHMDS in the presence of AuCl(SMe₂) permits the isolation of the gold derivative [^tBuC4(OH)₂(NHC·AuBr)₂] (6) in moderate yields. Characterization of the new complexes 3–6 was determined by spectroscopic methods and by elemental analysis. Complex [^tBuC4(OH)₂(NHC·Ru(cymene)Cl)₂] demonstrated good catalytic efficiency for the transfer hydrogenation of ketones and aldehydes under low catalyst loadings, while complex [^tBuC4(OH)₂(NHC·Pd(allyl)Cl)₂] performs well in the Suzuki coupling of aryl chlorides and boronic acids for the generation of di- and tri-ortho substituted biaryls.