Synthesis of Inaccessible Oligopyrrolic Ligands and Their Bis ‐Ru(II) and Tris ‐Ru(II) Complexes

ABSTRACT The oligopyrrolic ligands containing more than four pyrrole rings are rare because of the unavailability of proper synthetic protocols and also their inherent instability. A series of oligopyrrolic ligands containing five, six, and seven pyrrole rings were generated in situ for the first time by employing a Lewis acid‐catalyzed strategy to demask the BF 2 unit from their corresponding BF 2 complexes. These in situ generated ligands were subsequently employed for the synthesis of bis ‐Ru(II) complexes 4–7 and a tris ‐Ru(II) complex 8 by reaction with [Ru( p ‐cymene) 2 Cl 2 ] 2 in refluxing toluene. These Ru(II) complexes 4–8 were freely soluble in all common organic solvents and thoroughly characterized by 1D and 2D NMR spectroscopy. The crystal structure obtained for one of the bis ‐Ru(II) complexes 6 revealed coordination of two Ru(II) centers to α ‐pyrrolyl dipyrrin and dipyrrin nitrogen donors, forming six‐membered metallacycles with pseudo‐octahedral geometry around each Ru(II) ion. The Ru(II) complexes absorb strongly in the visible region, and the absorption bands shift bathochromically from mono ‐Ru(II) to bis ‐Ru(II) and then to tris ‐Ru(II) complexes. The electrochemical studies revealed that the complexes 4–8 exhibit Ru(II) to Ru(III) oxidation, and no reductions were observed. The DFT/TD‐DFT studies were in agreement with the experimental observations.