Theoretical investigation of the Friedländer reaction catalysed by CuBTC: Concerted effect of the adjacent Cu2+ sites

The possible mechanisms of Friedländer reaction were investigated computationally for an uncatalysed reaction and for the reaction catalysed by Lewis or Brønsted acids. Calculations were performed at the density functional theory level employing cluster and periodic models. A particular emphasis was put on the mechanism of Friedländer reaction catalysed by Lewis acid Cu2+ sites in CuBTC MOF; aldolization path was favoured over imination path in the case of CuBTC catalyst. The efficiency of CuBTC catalyst was due to two complementary effects: (i) the barriers of annulation step were significantly lowered due to the interaction of reactants with the coordinatively unsaturated Cu2+ site (cus) and (ii) barriers of dehydration reaction steps were decreased due to a concerted effect of two adjacent Cu2+ cus sites (separated just by 8.1 Å) interacting simultaneously with the corresponding reaction intermediates. It is proposed that the simultaneous involvement of two adjacent cus sites (concerted effect) in the catalysis can be more general phenomenon in CuBTC and perhaps in other MOF materials having large concentration of catalytically active sites as well.