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 & sdot;4H2O or NiCl2 & sdot;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.