Rational fabrication of three imidazole-functionalized ionic-type Pd-Polyoxovanadates for aerobic oxidation of 5-Hydroxymethulfurfural to 2,5-Diformylfuran

The controlled synthesis and exploration of the biomass valorization properties of organo-functionalized ionic-type polyoxometalates has been attracting considerable interest, but it still poses a challenge. Herein, three imidazole-functionalized ionic-type Pd-polyoxovanadates, [Pd(eIM)4][V6O16(eIM)4] (IPP-1), [Pd(mIM)4]2(H2V10O28)·3H2O (IPP-2) and [Pd(mIM)4]2(V4O12)·4H2O (IPP-3) (mIM = 1-methylimidazole, eIM = 1-ethylimidazole) with [V6O16(eIM)4]2-, [H2V10O28]4- and [V4O12]4- clusters were skillfully designed and successfully isolated, respectively. Importantly, IPP-1 represents the first ionic-type combination of a Pd-complex with an organo-functionalized {V6O16(eIM)4} cluster, and it exhibits remarkable catalytic activity towards the aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF), achieving an impressive conversion of 93 % and selectivity of 98 % under atmospheric O2. It can be reused at least five times without any loss of activity. Control experiments confirm that such exceptional catalytic activity originate from the cooperative effect between Pd-complex and {V6O16(eIM)4}. Based on spectroscopic results and density functional theory calculations, a plausible mechanism involving two parallel pathways has been proposed.