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

摘要

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 = l,l-bis(4-aminophenyl)cyclohexane,L2;L = l,l-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: {[Fe_2(Ll)_3](BF_4)_4,1;[Fe_2(Ll)_3](ClO_4)_4,2;[Fe_2(Ll)_3]Cl_4,3;[Fe_2(Ll)_3](SO_4)_2,4;[Fe_2(L2)_3](BF_4)_4,5;[Fe_2(L2)_3]Cl_4,6;[Fe_2(L3)_3](BF_4)_4;7;[Fe_2(L3)_3]Cl_4,8;and [Fe_2(L3)_3](SO_4)_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(BF_4)_2 established conclusively a [Fe_2(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 Bu_4NCl showed significant downfield shifts in the amide and bipyridyl H~6 proton resonances to yield a species of 1 : 2 host to guest stoichiometry.Moreover,addition of Bu_4NCl 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.