The synthesis and structures of chiral N-heterocyclic carbene (NHC)-N-donor complexes of silver(I) and palladium(II) are reported. The X-ray structure of an NHC-imine silver(I) complex [((nPr)CN(CHPh))AgBr](2) exhibits an Ag(2)Br(2) dimer motif where the imine group is not coordinated to the silver atom. Reaction between 2 and [PdCl(2)(MeCN)(2)] gives the palladium(II) complex [(kappa(2)-(nPr)CN(CHPh))PdCl(2)](3) that contains a chelating NHC-imine ligand as shown by single-crystal X-ray diffraction. Slow hydrolysis of related complexes [(kappa(2)-(nPr)CN(CHPh))PdCl(2)](3) and [(kappa(2)-((Ph)(2)CH)CN(CHPh))PdCl(2)](4) using triethylammonium chloride and water lead to the precipitation of single crystals of insoluble NHC-amino palladium(II) complexes [(kappa(2)-(nPr)CN(H(2)))PdCl(2)](6) and [(kappa(2)-((Ph)(2)CH)CN(H(2)))PdCl(2)](7), respectively. In the solid state, complexes 6 and 7 both exhibit intermolecular hydrogen bonding between chlorine and an amino-hydrogen atom resulting in an infinite chain structure. Substitution of an amino hydrogen for an ethyl group gives the soluble complex [(kappa(2)-(iPr)CN((H)Et))PdCl(2)](12). Reaction between two equivalents of 2 and [PdCl(2)(MeCN)(2)] gives the di-NHC complex [(kappa(1)-(nPr)CN(CHPh))(2)PdCl(2)](5) that does not contain a coordinated imine as shown by single crystal X-ray diffraction. Conproportionation between 5 and an equivalent of [PdCl(2)(MeCN)(2)] to does not occur at temperatures up to 100 degrees C in CD(3)CN.

Reactions between imidazolium-imine salts and base result in C-C bond formation via intermediate N-heterocyclic carbenes. In the presence of a proximal OH moiety, carbene formation occurs via intramolecular deprotonation by phenoxide. For simple imines, a reactive Breslow-type intermediate gives access to new heterocycles with the formation of six- and seven-member rings.

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.

No abstract

Copper(I) bromide complexes (2a and 2b) of NHC–phenolimine ligand precursors {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 (1a) and 3‐[(1R,2R)‐2‐{[1‐(2‐hydroxyphenyl)meth‐(E)‐ylidene]amino}cyclohexyl]‐1‐isopropyl‐4‐phenyl‐3H‐imidazol‐1‐ium bromide (1b), respectively} have been prepared. Complexes 2a and 2b exhibit copper coordination only through the carbene carbon atom (C) and do not spontaneously eliminate HBr to give additional phenoxyimine (NO) bonds, which is attributed to intramolecular hydrogen bonding. Crystallisation of 2a and 2b gives 2a′ and 2b′, respectively, that contain (C) copper(I) bromide and (NO)2 copper(II) coordination. Complex 2b′ also exhibits intermolecular CuIBr interactions giving a Cu2Br2 bridge that links two molecules of 2b′ resulting in an ellipse motif. Reduction of the ligand precursor imine group of 1a allows synthesis of silver(I) and copper(I) NHC–phenolamine complexes 6 and 7, respectively, that also retain the phenol hydrogen atom. Attempts to selectively prepare 2a′ gave a copper(II) complex 9 that exhibits an (NO)2CuII structure with pendant imidazolium salt groups. Reaction between the silver(I) bromide derivative of 1a and CuCl2·2H2O gives a complex derived from a Cu6(O)(OH)4Cl3 core and two (NO) and one (CNO) ligands, respectively. The use of 2a and 7 as precatalysts for 1,4‐conjugate addition to enones and aziridination of alkenes was studied, showing that, whilst both catalysts are active, enantioselectivities are low, which is attributed to the lack of Cu‐(NO) coordination. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)