Models for B12-conjugated radiopharmaceuticals. Cobaloxime binding to new fac-[Re(CO)3(Me2bipyridine)(amidine)]BF4 complexes having an exposed pyridyl nitrogen.

New mononuclear amidine complexes, fac-[Re(CO)3(Me2bipy)(HNC(CH3)(pyppz))]BF4 [(4,4′-Me2bipy (1), 5,5′-Me2bipy (2), and 6,6′-Me2bipy (3)] (bipy = 2,2′-bipyridine), were synthesized by treating the parent fac-[ReI(CO)3(Me2bipy)(CH3CN)]BF4 complex with the C2-symmetrical amine 1-(4-pyridyl)piperazine (pyppzH). The axial amidine ligand has an exposed, highly basic pyridyl nitrogen. The reaction of complexes 1–3 with a B12 model, (py)Co(DH)2Cl (DH = monoanion of dimethylglyoxime), in CH2Cl2 yielded the respective dinuclear complexes, namely, fac-[Re(CO)3(Me2bipy)(μ-(HNC(CH3)(pyppz)))Co(DH)2Cl]BF4 [(4,4′-Me2bipy (4), 5,5′-Me2bipy (5), and 6,6′-Me2bipy (6)]. 1H NMR spectroscopic analysis of all compounds and single-crystal X-ray crystallographic data for 2, 3, 5, and 6 established that the amidine had only the E configuration in both the solid and solution states and that the pyridyl group is bound to Co in 4–6. Comparison of the NMR spectra of 1–3 with spectra of 4–6 reveals an unusually large “wrong-way” upfi...