Influence of the Size of the Metallacycle of P‐NHC Mn Complexes for the Hydrogenation of Esters

Chelate ring size is a key but underexplored parameter in the design of manganese‐based hydrogenation catalysts. Here, we report the synthesis and characterization of Mn(I) NHC–phosphine complexes [Mn‐5] C7 featuring a seven‐membered chelate ring and their application to the hydrogenation of carboxylic esters. The N‐mesityl‐substituted complex [Mn‐5 Mes ] C7 , with markedly improved activity relative to its five‐ and six‐membered analogs, achieves quantitative yields of the corresponding alcohols (0.25 mol%, 2.5 mol% KHBEt 3 , 50 bar H 2 , 2‐MeTHF, 100–120 °C, 5 h), including halogenated aromatic esters, aliphatic esters, and poly(ε‐caprolactone), which is fully depolymerized to hexan‐1‐ol. Systematic comparison of the C5, C6, and C7 series by X‐ray crystallography, multinuclear NMR, IR, and cyclic voltammetry shows that ring enlargement progressively increases the bite angle, reduces steric strain, and enhances electron donation at the metal, structural and electronic changes that collectively account for the improved catalytic performance.