Mechanistic Insights into the Stepwise Assembly of Ruthenium(II) Tris-heteroleptic Compounds

The ruthenium(II) tris-heteroleptic compounds cis-[Ru(NN)(dcbH₂)(NCS)₂], NN = polypyridyl ligand and dcbH₂ = 2,2'-bipyridine-4,4'-dicarboxylic acid, can be synthesized by a one-pot route starting from [Ru( p-cymene)Cl₂]₂, followed by the sequential addition of ligands. In this work, each synthetic step for the cis-[Ru(R-phen)(dcbH₂)(NCS)₂] (R = H, Me, Ph, MeO, or Cl) preparation was individually investigated, aiming to identify reaction intermediates and to establish correlations among temperature, reaction time, reactant concentration, and the identity of the substituent of the polypyridyl ligand with the kinetics of the reactions and distribution of the products. The first step is the cleavage of [Ru( p-cymene)Cl₂]₂, followed by the coordination of R-phen via an associative mechanism and establishment of a direct correlation between the electron-donating or electron-withdrawing character of R and the reaction rates. The second step is the conversion of [Ru(R-phen)( p-cymene)Cl]Cl to cis-[Ru(R-phen)(dcbH₂)Cl₂], and the rate-determining step is the formation of the intermediate [Ru(R-phen)Cl₂], which exhibits a low dependence on R. The last step is the substitution of Cl^- by NCS^-, and the N-bound isomer is the major product. The reaction temperature, time, and identity of R influence the relative distribution of the linkage isomers. The comprehension of each of these processes is a key factor to develop new strategies to optimize the one-pot synthetic route.