Hybrid bis(N-heterocyclic carbene)-bis(phenolate) ligands: Coordination chemistry and catalytic application in CO2 valorization.

[Display omitted] • Synthesis of new tetradentate bis(N -heterocyclic carbene)-bis(phenolate) ligands L. • Synthesis of a new Cr(III) complex: [Cr(H 2 L)(acac) 2 ]Br. • Synthesis of a new Fe(III) complex: [Fe(H 2 L)(acac) 2 ]Br. • Synthesis of two new Ni(II) complexes: [NiL]. • [Fe(H 2 L)(acac) 2 ]Br is a good catalyst in the CO 2 cycloaddition with epoxides. In the frame of a coordination chemistry study on hybrid bis(N -heterocyclic carbene)-bis(phenolate) ligands (L), the synthesis of novel Fe(III), Cr(III) and Ni(II) complexes is reported. The Fe(III) and Cr(III) complexes are obtained starting from their acetylacetonates [M(acac) 3 ] (M = Fe or Cr, acac = acetylacetonate), and are characterized by a general formula [M(acac) 2 (H 2 L)]Br (2 , M = Fe and 3 , M = Cr), in which only the phenolate donors of the ligand are coordinated to the metal center. Differently, the Ni(II) complexes are prepared starting from Ni(OAc) 2 ·4H 2 O or NiCl 2 ·6H 2 O, and the corresponding products are characterized by the general formula [NiLx] (4 , x = 1 and 5 , x = 2), in which both the N -heterocyclic carbene and the phenolate donors of the ligand are coordinated to the metal center. The synthesized compounds were characterized by means of elemental analysis, ESI-MS and in the case of [Fe(acac) 2 (H 2 L) ]Br single crystal XRD. Complexes 2 – 5 were used in homogeneous catalysis, in the cycloaddition reaction between CO 2 and epoxides. The Fe(III) complex 2 , [Fe(acac) 2 (H 2 L) ]Br , outperformed the other catalysts prepared in the frame of this study, showing a good activity under mild reaction conditions (i.e. p CO2 = 1 bar) and in the absence of co-catalysts. [ABSTRACT FROM AUTHOR]