Comparative Study of the Catalytic Amination of Benzylic C–H Bonds Promoted by Ru(TPP)(py) 2 and Ru(TPP)(CO)

A combined experimental and DFT‐based theoretical analysis elucidated the influence of the axial ligand L on the catalytic activity of Ru(porphyrin)L complexes in promoting the amination of benzylic C–H bonds by organic azides (RN 3 ). Experimental data indicated that the catalytic activity of Ru(TPP)(CO) ( 1 ) (TPP = dianion of tetraphenylporphyrin) is comparable to that of Ru(TPP)(py) 2 ( 2 ) (py = pyridine). DFT modelling disclosed that 2 can be regarded as a precatalyst that becomes active after the endergonic loss of one pyridine ligand to give the unsaturated species [Ru](py) ( 11 ) {[Ru] = Ru(porphine)}. This complex would react with RN 3 to give the mono‐imido singlet complex [Ru](py)(NR) S ( 6 S ), which can be easily transformed into the triplet isomer 6 T having diradical character at the imido N atom. The subsequent formation of the benzylic amine PhCH 2 NHR occurs through a radical homolytic activation of one C–H bond of the toluene substrate (PhCH 3 ). Conversely, by staying on the singlet potential‐energy surface, 6 S can undergo dissociation of the pyridine ligand to form [Ru](NR). This complex can activate another RN 3 molecule to form the bis‐imido compound [Ru](NR) 2 , which is also catalytically active. At this point, the mechanism becomes independent of the nature of the original ligand L coordinated to [Ru].