Electronic Fine‐tuning of Ruthenium(II) Transfer Hydrogenation Catalysts with Ethanol as the Hydrogen Source

Abstract

AbstractA series of ruthenium(II) complexes bearing tridentate N,N’‐diallyl‐2,6‐di(5‐butylpyrazol‐3‐yl)pyridine ligands was synthesized and characterized. Introduction of substituents in the 4‐position of the pyrazole rings tune the electron density at the ruthenium center, which was proved by correlation of the 31P NMR chemical shifts with the σp parameters of the Hammett equation. The structural elucidation of a phosphine‐free ruthenium(II) complex proves that one of the allyl side‐chains undergoes chelating coordination to the ruthenium site to realize a 18 VE center. This compound is the starting point for complexes of the type (N,N,N)Ru(L)(Cl)2 bearing ligands L other than triphenylphosphine. The ruthenium(II) complexes were investigated for their activity in the transfer hydrogenation with ethanol as the hydrogen source. Here the logarithms of the measured turn‐over frequencies (TOF) correlate with the σp parameters of the Hammett equation in terms of a linear free‐energy relationship.