Insights into the Benign, Selective Catalytic Oxidation of HMF to HMFCA in Water Using [MnIV₂(μ-O)₃(tmtacn)₂]²⁺ and Hydrogen Peroxide

5-Hydroxymethylfurfural (HMF) is a biobased platform chemical with a lot of potential to be a key chemical in a future chemical industry. Oxidized derivatives of HMF are explored in many emerging chemical products. However, selective oxidation toward 5-hydroxymethyl-2-furan carboxylic acid (HMFCA) is challenging. Furthermore, this conversion has been hardly explored with homogeneous catalysts. We show here the selective oxidation toward HMFCA using the industrially relevant catalyst [MnIV2(μ-O)3(tmtacn)2][(X)2] (Mncat, X = CH3COO/PF6/SO4). Moreover, this is achieved in water, under mild conditions (room temperature, pH 11), using low loadings of a nonprecious metal catalyst and hydrogen peroxide, reaching TONs of up to 200 molHMFCA/molMncat. We show that the oxidation under these conditions behaves distinctively from the oxidation with Mncat in MeCN and allows selective oxidation of aldehydes in the presence of alcohols. Kinetic and ESI-MS studies were used to study the change in selectivity and to give an explanation for changes in catalytic behavior.