Studies of the reactions of tripodal pyridine-containing ligands with Re(CO)5Br leading to rheniumtricarbonyl complexes with potential biomedical applications

The complexes formed from the reaction of N-acylated tris-(pyridin-2-yl)methylamine (LH) with [Re(CO)5Br] depend on the structure of the ligand and the reaction conditions. Thus, while N-[1,1,1-tris-(pyridin-2-yl)methyl]acetamide coordinates through the three pyridine nitrogens to give a stable cationic complex [LHRe(CO)3Br], the analogous N-benzoyl ligand reacts under similar conditions to give a neutral complex [LRe(CO)3] with coordination through two pyridine nitrogens and a deprotonated amide. To try to explain these different outcomes, the reactions of some structurally related N-acylated [1,1-bis(pyridin-2-yl)]methylamines (L[prime or minute]H) with [Re(CO)5Br] have been studied and the reaction pathways identified. These studies indicate that a neutral complex [L[prime or minute]HRe(CO)3Br] is initially formed in which the amide portion of the ligand is uncoordinated, but that this complex under appropriate conditions then rearranges to give a cationic complex [L[prime or minute]HRe(CO)3]Br in which the coordinated amide nitrogen either remains protonated or is present in its imidic acid tautomeric form. Elimination of HBr from these complexes either thermally or in the presence of base then gives stable neutral complexes [L[prime or minute]Re(CO)3]. The impact of the N-acyl group and any substituent at the apex of the tripodal ligands (L[prime or minute][prime or minute]H) on the relative stabilities of intermediate complexes on the reaction pathway helps provide an explanation for the observed difference in behaviour of the N-acylated tris(pyridin-2-yl)methylamines (LH).