Several Novel N-Donor Tridentate Ligands Formed in Chemical Studies of New <i>fac</i>-Re(CO)₃ Complexes Relevant to <i>fac</i>-^99mTc(CO)₃ Radiopharmaceuticals: Attack of a Terminal Amine on Coordinated Acetonitrile

To evaluate syntheses of fac-[Re(CO)3L]+ complexes in organic solvents, we treated fac-[Re(CO)3(CH3CN)3]PF6/BF4 in acetonitrile with triamine ligands (L). When L had two primary or two tertiary terminal amine groups, the expected fac-[Re(CO)3L]+ complexes formed. In contrast, N,N-dimethyldiethylenetriamine (N,N-Me2dien) formed an unusual compound, fac-[Re(CO)3(DAE)]BF4 {DAE = (Z)-N′-(2-(2-(dimethylamino)ethylamino)ethyl)acetimidamide = (Me2NCH2CH2)NH(CH2CH2NC(NH2)Me)}. DAE is formed by addition of acetonitrile to the N,N-Me2dien terminal primary amine, converting this sp3 nitrogen to an sp2 nitrogen with a double bond to the original acetonitrile sp carbon. The three Ns bound to Re derive from N,N-Me2dien. The pathway to fac-[Re(CO)3(DAE)]BF4 is suggested by a second unusual compound, fac-[Re(CO)3(MAE)]PF6 {MAE = N-methyl-N-(2-(methyl-(2-(methylamino)ethyl)amino)ethyl)acetimidamide = MeN(H)CH2CH2N(Me)CH2CH2N(Me)C(Me)NH}, isolated after treating fac-[Re(CO)3(CH3CN)3]PF6 with N,N′,N′′-trimethyldiethylenetriamine (N,N′,N′′-Me3dien). MAE chelates via a terminal and a central sp3 N from N,N′,N′′-Me3dien and via one sp2 NH in a C(Me)NH group. This group is derived from acetonitrile by addition of the other N,N′,N′′-Me3dien terminal amine to the nitrile carbon. This addition creates an endocyclic NMe group within a seven-membered chelate ring. The structure and other properties of fac-[Re(CO)3(MAE)]PF6 allow us to propose a reaction scheme for the formation of the unprecedented DAE ligand. The new compounds advance our understanding of the spectral and structural properties of Re analogues of 99mTc radiopharmaceuticals.