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

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(H2L)]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)2·4H2O or NiCl2·6H2O, 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(H2L)]Br single crystal XRD. Complexes 2–5 were used in homogeneous catalysis, in the cycloaddition reaction between CO2 and epoxides. The Fe(III) complex 2, [Fe(acac)2(H2L)]Br, outperformed the other catalysts prepared in the frame of this study, showing a good activity under mild reaction conditions (i.e. pCO2 = 1 bar) and in the absence of co-catalysts.