Synthesis and Structural Variation of Iron, Rhodium, Palladium, and Silver Complexes of a Chiral N-Heterocyclic Carbene−Phenoxyimine Hybrid Ligand

Abstract

A new class of chiral N-heterocyclic carbene (NHC) precursors has been prepared incorporating a phenoxyimine moiety. Reaction between 3-((1R,2R)-2-{[1-(3,5-di-tert-butyl-2-hydroxyphenyl)meth-(E)-ylidene]amino}cyclohexyl)-1-isopropyl-4-phenyl-3H-imidazol-1-ium bromide [C(H)NO(H))][Br] (2a) and Pd(OAc)2 gives the complex [Pd(κ3-CNO)Br] (3), which exhibits tridentate ligand coordination via double deprotonation of 2a. The NHC ligand transfer agents [Ag(κ1-CNO(H))X] (X = Br (5a), Cl (5b)) are prepared from 2a, chloride analogue 2b, and Ag2O. Reaction between 5b and [Rh(COD)Cl]2 gives the NHC complex [Rh(κ1-CNO(H))Cl] (6) and, on chloride abstraction, the bidentate NHC-imine-coordinated complex [Rh(κ2-CNO(H))][B{3,5-(CF3)2C6H3}4] (7). For complexes 5 and 6 coordination of the phenoxyimine and deprotonation of the phenoxy moiety appear to be modified by intramolecular hydrogen bonding between hydroxyl and imine nitrogen groups. Reaction between 5a and Fe{N(SiMe3)2} gives an unusual bimetallic complex, [Fe(κ2-C(AgBr)NO)2] (8), that exhibits a 1-D polymeric chain motif in the solid state. Preliminary catalytic properties of an acetonitrile adduct of 3, [Pd(κ3-CNO)MeCN][BF4] (4), and 7 are reported for hydroamination and phenylation of enones, respectively, that show good activity but poor enantioselectivity.