Nucleophilic Nickel and Palladium Pincer Hydroxides: A Study of Their Reactions with Dimethyl Carbonate and Other Non-Alkylating Organic Electrophiles

Abstract

The reactions of the pincer hydroxide complexes [(iPrPCP)M(OH)] (M=Ni, Pd) with dimethyl carbonate (DMC), and a set of organic electrophiles including benzaldehyde have been investigated in the context of our ongoing investigation on the synthesis of alkyl carbonates from CO2 and alcohols. The final outcome of such reactions is diverse, but for PhCHO and DMC the first step is a mechanistically similar addition of the [M]−OH linkage across the carbonyl functionality, that leads to unstable adducts. DMC is cleaved irreversibly by both Ni and Pd hydroxides, affording the corresponding methylcarbonates [(iPrPCP)MOCO2Me] and methanol, whereas PhCHO affords benzoate complexes [[(iPrPCP)MOCO2Ph]. The main kinetic and thermodynamic features of these reactions were reproduced satisfactorily by computational DFT models. The calculations throw light on the true causes of irreversibility of DMC cleavage by nucleophilic hydroxide complexes, which is the primary cause that prevents methanol carboxylation from being catalysed by the Ni or Pd pincers.