Copper pyridinedicarboxylates: assembly, structures, and catalytic oxidation of terpenes

Abstract

Three new copper(II) compounds, formulated as [Cu(pdc)(H₃tea)(H₂O)] (1), [Cu₂(μ₃-pdc)₂(DMF)(H₂O)₂]_n·2nH₂O (2) and [Cu₂(μ₄-pdc)₂(H₂O)₃]_n·3nH₂O (3), were generated from copper(II) nitrate, 3,4-pyridinedicarboxylic acid (H₂pdc), and triethanolamine (H₃tea). Their crystal structures revealed a discrete 0D complex (1) as well as 2D (2) and 3D (3) coordination polymers (CPs). The obtained compounds were screened as precatalysts for the oxidation of terpenes with molecular oxygen as the oxidizing agent and t-BuOOH as the radical initiator. The relationship between the precatalyst structures and the accumulation of α-pinene oxidation products was investigated. Among the three compounds, compound 2 revealed the best activity in α-pinene oxidation. Under the optimized conditions, the conversion of α-pinene reached ∼90%, yielding pinene oxide, verbenyl hydroperoxide, verbenol, and verbenone as the main products. Furthermore, compound 2 can catalyze the oxidation of (+)-valencene with O₂ to give (+)-nootkatone. This study showcases the successful synthesis of three new copper-pyridinedicarboxylate derivatives, among which the 2D coordination polymer 2 acts as a particularly efficient precatalyst for upgrading terpenes into value-added oxyfunctionalized products.