Monomeric alkoxide and alkylcarbonate complexes of nickel and palladium stabilized with the iPrPCP pincer ligand: a model for the catalytic carboxylation of alcohols to alkyl carbonates

Abstract

Monomeric alkoxo complexes of the type [(^iPrPCP)M-OR] (M = Ni or Pd; R = Me, Et, CH₂CH₂OH; ^iPrPCP = 2,6-bis(diisopropylphosphino)phenyl) react rapidly with CO₂ to afford the corresponding alkylcarbonates [(^iPrPCP)M-OCOOR]. We have investigated the reactions of these compounds as models for key steps of catalytic synthesis of organic carbonates from alcohols and CO₂. The MOCO-OR linkage is kinetically labile, and readily exchanges the OR group with water or other alcohols (R′OH), to afford equilibrium mixtures containing ROH and [(^iPrPCP)M-OCOOH] (bicarbonate) or [(^iPrPCP)M-OCOOR′], respectively. However, [(^iPrPCP)M-OCOOR] complexes are thermally stable and remain indefinitely stable in solution when these are kept in sealed vessels. The constants for the exchange equilibria have been interpreted, showing that CO₂ insertion into M-O bonds is thermodynamically more favorable for M-OR than for M-OH. Alkylcarbonate complexes [(^iPrPCP)M-OCOOR] fail to undergo nucleophilic attack by ROH to yield organic carbonates ROCOOR, either intermolecularly (using neat ROH solvent) or in intramolecular fashion (e.g., [(^iPrPCP)M-OCOOCH₂CH₂OH]). In contrast, [(^iPrPCP)M-OCOOMe] complexes react with a variety of electrophilic methylating reagents (MeX) to afford dimethylcarbonate and [(^iPrPCP)M-X]. The reaction rates increase in the order X = OTs < IMe ≪ OTf and Ni < Pd. These findings suggest that a suitable catalyst design should combine basic and electrophilic alcohol activation sites in order to perform alkyl carbonate syntheses via direct alcohol carboxylation.