A new twist in anion binding: metallo-helicate hosts for anionic guests

Abstract

The condensation of 5-chlorocarbonyl-2,2′-bipyridine with a variety of rigid aromatic diamines, L, gave a series of new bisamido-2,2′-bipyridine based ligands (L = 4,4′-methylenediamine, L1; L = 1,1-bis(4-aminophenyl)cyclohexane, L2; L = 1,1-bis(4-amino-3,5-dimethylphenyl)cyclohexane, L3) capable of forming dinuclear triple helicate complexes on coordination to Fe(II). The reaction of various Fe(II) salts with L1–L3 gave: {[Fe₂(L1)₃](BF₄)₄, 1; [Fe₂(L1)₃](ClO₄)₄, 2; [Fe₂(L1)₃]Cl₄, 3; [Fe₂(L1)₃](SO₄)₂, 4; [Fe₂(L2)₃](BF₄)₄, 5; [Fe₂(L2)₃]Cl₄, 6; [Fe₂(L3)₃](BF₄)₄; 7; [Fe₂(L3)₃]Cl₄, 8; and [Fe₂(L3)₃](SO₄)₂, 9, as determined by UV-Vis, IR and ¹H NMR spectroscopy, electrospray mass spectrometry (ESMS) and elemental analyses. A UV-Vis complexometric titration experiment between L3 and Fe(BF₄)₂ established conclusively a [Fe₂(L3)₃]⁴⁺ product species. ¹H NMR spectroscopy showed that the complexes exist as both rac-(helical) and meso-(non-helical) isomers in DMSO-d6 solution at 298 K. L1–L3 were designed such that following complexation, six amide hydrogen atoms would line an inter-strand intrahelical cavity of sufficient size to facilitate the binding of guest species within it. Indeed, ESMS studies showed characteristic peaks typical of complex–anion species in solution. Furthermore, ¹H NMR titration experiments showed that anions bind within the intrahelical cavity as titration of 1, 5 and 7 with Bu₄NCl showed significant downfield shifts in the amide and bipyridyl H⁶ proton resonances to yield a species of 1 : 2 host to guest stoichiometry. Moreover, addition of Bu₄NCl to 1, 5 and 7 shifted the rac-/meso-species distribution from 1 : 2 in favour of the meso- to 100% in favour of the rac-isomer, showing that Cl⁻ ions favour the formation of the triple helicate species in DMSO-d6 solution.