New Monodentate Amidine Superbasic Ligands with a Single Configuration in fac-Re(CO)3(55′- or 66′-Me2bipyridine)(amidine)BF4 Complexes

摘要

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 ten new amidine complexes, fac-[Re(CO)3(L)(HNC(CH3)N(CH2CH2)2Y)]BF4 [Y = CH2, (CH2)2, (CH2)₃, NH or O]. All ten 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 fivefold 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.