Influence of the Ionic Liquid on the Activity of a Supported Ionic Liquid Phase Fe(II) Pincer Catalyst for the Hydrogenation of Aldehydes

The catalytic hydrogenation of different aldehydes to the corresponding alcohols was investigated using an Fe(II) hydride pincer complex as catalyst in the supported ionic liquid phase (SILP) reaction mode. Two different ionic liquids of the type [X(4441)][NTf(2)] with X=N or P were applied with mesoporous silica gel as support, which was coated first with a chemisorbed monolayer of the corresponding modified IL to remove acidic surface OH‐groups and to prevent IL leaching. Quantitative conversion with turn‐over frequencies in the order of 1000 h(–) (1) were obtained for various aromatic and heteroaromatic aldehydes and highly selective aldehyde reduction was observed also for substrates containing reducible C=C bonds. Aldehydes with longer aliphatic chains or cycloalkyl substituents, however, showed no conversion here, in contrast to a previous study with an imidazolium‐based ionic liquid. These differences were ascribed primarily to differences in substrate/ionic liquid interactions. Whereas [N(4441)][NTf(2)] and [P(4441)][NTf(2)] gave essentially identical results for different substrates in single‐batch reactions, prolonged use of the catalyst in repeated reaction cycles lead to a quick drop‐off in catalyst activity in [P(4441)][NTf(2)], but a continuous, quantitative conversion in [N(4441)][NTf(2)].