New Monodentate Amidine Superbasic Ligands with a Single Configuration in fac-[Re(CO)3(5,5′- or 6,6′-Me2bipyridine)(amidine)]BF4 Complexes

Treatment of two precursors, fac-[Re­(CO)3(L)­(CH3CN)]­BF4 [L = 5,5′-dimethyl-2,2′-bipyridine (5,5′-Me2bipy) (1) and 6,6′-dimethyl-2,2′-bipyridine (6,6′-Me2bipy) (2)], with five C2-symmetrical saturated heterocyclic amines yielded 10 new amidine complexes, fac-[Re­(CO)3(L)­(HNC­(CH3)­N­(CH2CH2)2Y)]­BF4 [Y = CH2, (CH2)2, (CH2)3, NH, or O]. All 10 complexes possess the novel feature of having only one isomer (amidine E configuration), as established by crystallographic and 1H NMR spectroscopic methods. We are confident that NMR signals of the other possible isomer (amidine Z configuration) would have been detected, if it were present. Isomers are readily detected in closely related amidine complexes because the double-bond character of the amidine C–N3 bond (N3 is bound to Re) leads to slow E to Z isomer interchange. The new fac-[Re­(CO)3(L)­(HNC­(CH3)­N­(CH2CH2)2Y)]­BF4 complexes have C–N3 bonds with essentially identical double-bond character. However, the reason that the Z isomer is so unstable as to be undetectable in the new complexes is undoubtedly because of unfavorable clashes between the equatorial ligands and the bulky N­(CH2CH2)2Y ring moiety of the axial amidine ligand. The amidine formation reactions in acetonitrile (25 °C) proceeded more easily with 2 than with 1, indicating that the distortion in 6,6′-Me2bipy resulting from the proximity of the methyl substituents to the inner coordination sphere enhanced the reactivity of the coordinated CH3CN. Reaction times for 1 and 2 exhibited a similar dependence on the basicity and ring size of the heterocyclic amine reactants. Moreover, when the product of the reaction of 1 with piperidine, fac-[Re­(CO)3(5,5′-Me2bipy)­(HNC­(CH3)­N­(CH2CH2)2CH2)]­BF4, was challenged in acetonitrile-d3 or CDCl3 with a 5-fold excess of the strong 4-dimethylaminopyridine ligand, there was no evidence for replacement of the amidine ligand after two months, thus establishing that the piperidinylamidine ligand is a robust ligand. This chemistry offers promise as a suitable means for preparing isomerically pure conjugated fac-[99mTc­(CO)3L]n± imaging agents, including conjugates with known bioactive heterocyclic amines.