| Summary: | The coordination chemistry of the anionic NHC 1 – based on an imidazo[1,5- a ]pyridin-3-ylidene (IPy) platform substituted at the C5 position by an anionic barbituric heterocycle was studied with d 6 (Ru(II), Mn(I)) and d 8 (Pd(II), Rh(I), Ir(I), Au(III)) transition-metal centers. While the anionic barbituric heterocycle is planar in the zwitterionic NHC precursor 1 ·H, NMR spectroscopic analyses supplemented by X-ray diffraction studies evidenced the chelating behavior of ligand 1 – through the carbenic and the malonic carbon atoms in all of the complexes, resulting from a deformation of the lateral barbituric heterocycle. The complexes were obtained by reaction of the free carbene with the appropriate metal precursor, except for the Au(III) complex 10 , which was obtained by oxidation of the antecedent gold(I) complex [AuCl( 1 )] − with PhICl 2 as an external oxidant. During the course of the process, the kinetic gold(I) intermediate 9 resulting from the oxidation of the malonic carbon of the barbituric moiety was isolated upon crystallization from the reaction mixture. The ν CO stretching frequencies recorded for complex [Rh( 1 )(CO) 2 ] ( 5 ) demonstrated the strong donating character of the malonate-C(sp 3 )/NHC ligand 1 – . The ruthenium complex [Ru( 1 )Cl( p -cymene)] ( 11 ) was implemented as a precatalyst in the dehydrogenative synthesis of carboxylic acid derivatives from primary alcohols and exhibited high activities at low catalyst loadings (25–250 ppm) and a large tolerance toward functional groups.
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