A stepwise coordination-reduction pathway to benchmark cis-[Ru(L)2Cl2] complexes.

Serving as the key precursor to benchmark photoactive ruthenium polypyridyl complexes, the comprehensive formation mechanism of cis-[Ru(II)(L)2Cl2] from polymeric RuCl3 has long remained elusive. Recently, we have identified 2-methoxyethanol (MOE) as an optimal solvent for the synthesis of cis-[Ru(II)(L)2Cl2] owing to its moderate boiling point, low viscosity, and intrinsic mild reducing capability (MOE conditions). This approach contrasts with the widely used Meyer's method, which relies on the in situ decomposition of DMF to reduce the Ru(III) center. As a critical step toward elucidating the detailed mechanism underlying the transformation of polymeric RuCl3 into crystalline cis-[Ru(II)(L)2Cl2], we demonstrate herein that 2-methoxyethanol provides a unique platform for systematically mapping the complete conversion pathway. Notably, both cis-[Ru(III)(L)2Cl2]+ and trans-[Ru(II)(L)2Cl2] are identified as key intermediates resolving a century-old ambiguity surrounding precursor formation. We anticipate that this comprehensive mechanistic understanding will enable the rational design of advanced ruthenium-based functional materials and the detailed characterization of the new intermediates will offer valuable insights into the catalytic processes involving ruthenium polypyridyl complexes.