Synthetic and Mechanistic Aspects of a New Method for Ruthenium-Metalation of Porphyrins and Schiff-Bases

A new method is presented for metalation of a wide range of free-base, neutral, cationic, and anionic porphyrins in refluxing dimethylformamide (DMF) using an easily prepared [Ru(DMF)6](OTf)3 complex, and comparisons are made with the more familiar metalation procedure using Ru3(CO)12. Both procedures generate RuII(porp)(CO)L complexes (L = solvent); use of the RuIII-triflate precursor gives yields comparable to, or greater than, those obtained with the carbonyl, and generates no Ru-chlorin impurities. Mechanistic studies on the meso-tetraphenylporphyrin system reveal that the DMF furnishes the CO, which in the presence of essential water reduces the metal, and metalation likely occurs via a RuII−CO species. Corresponding metalation of tetradentate Schiff-bases gives trans-[RuIII(Schiff- base)(DMF)2]OTf complexes in yields of ∼50%, a limitation being the accompanying hydrolysis of the Schiff-base through the presence of trace water.