Identification of 2-Methoxyethanol as an Alternative Reducing Solvent for the Preparation of cis -[Ru(L)2Cl2]

Abstract As a key precursor to benchmark photoactive ruthenium polypyridyl complexes, the synthesis of cis-[Ru(II)(L)2Cl2] (L = diimine ligand) from polymeric RuCl3 has long been the subject of debate. The widely used Meyer method, which relies on in situ DMF decomposition, often suffers from poor reproducibility and strong sensitivity to the quality of the ruthenium source and reaction conditions. Here, we systematically compare the major literature protocols by monitoring product yields and side reactions using combined thin-layer chromatography (TLC) analysis and mass spectrometric characterization. This study identified 2-methoxyethanol as an optimal solvent, offering a moderate boiling point, low viscosity, and mild reducing capability, thereby enabling reliable formation of cis-[Ru(II)(L)2Cl2]. The method is robust across a broad range of diimine ligands with diverse electronic and steric properties. Moreover, we demonstrate the successful synthesis of eight bioactive ruthenium complexes that have previously been reported as challenging to access by using existing methods. Overall, this work exemplifies a systematic strategy for revisiting classical synthetic routes to establish a more reliable pathway to an essential ruthenium precursor.