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

The preparation of a 1,3-bis-imidazolium functionalized 2,4-dihydroxyl p-tert-butylcalix[4]arene ligand [tBuC(4)(OH)(2)(NHC)(2)]Br-2 (1) is reported. Treatment of ligand 1 with excess of silver oxide allows for the straightforward preparation of the silver complex [tBuC(4)(OH)2(NHC<middle dot>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 [tBuC(4)(OH)(2)(NHC<middle dot>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 [tBuC(4)(OH)(2)(NHC<middle dot>AuBr)(2)] (6) in moderate yields. Characterization of the new complexes 3-6 was determined by spectroscopic methods and by elemental analysis. Complex [tBuC(4)(OH)(2)(NHC<middle dot>Ru(cymene)Cl)(2)] demonstrated good catalytic efficiency for the transfer hydrogenation of ketones and aldehydes under low catalyst loadings, while complex [tBuC(4)(OH)(2)(NHC<middle dot>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.