Guest Recognition Control Accompanied by Stepwise Gate Closing and Opening of a Macrocyclic Metallohost

Abstract

Host–guest binding behavior of macrocyclic hosts is significantly influenced by the shapes and sizes of the hosts. In particular, closing/opening the apertures of the hosts controls the guest uptake/release. A post‐metalation modification method was used to achieve the open/close conversions. The starting open complex, [LCo2(pip)4](OTf)2, was efficiently converted to the closed complex, [LCo2(hda)2](OTf)2, which has a doubly bridged structure. The conversion of this closed complex to the open complex [LCo2(hda)2(OAc)]+ was too slow to be completed, but this gate‐opening was dramatically accelerated by the addition of Na+. The Na+ binding was also significantly enhanced by the gate‐opening, that is, conversion of [LCo2(hda)2]2+ to [LCo2(hda)2(OAc)]+.

Enhancement of Alkali Metal Ion Recognition by Metalation of a Tris(saloph) Cryptand Having Benzene Rings at the Bridgeheads

“…We then determined the binding constants of LNi 3 with other alkali metal ions by competition experiments (Figure ), which gave the affinity order of Na + < K + < Rb + ≈ Cs + (Table ). The selectivity for larger metal ions is again demonstrated as seen in the corresponding nickel-free host H 6 L. It is noteworthy that LNi 3 has 14–240 times higher binding constants than H 6 L for all the alkali metal ions, which clearly demonstrated the enhancement of the binding affinity by the metalation of H 6 L. This enhancement can be explained by the more polarized phenoxo groups in the [Ni­(saloph)] structures (Figure b) as seen in the analogous monocyclic hosts …”

“…We have investigated the synthesis and functions of a series of noncyclic and macrocyclic host molecules containing two or more salen- and saloph-type coordination sites (H 2 salen = N , N′ -disalicylideneethylenediamine; H 2 saloph = N , N′ -disalicylidene- o -phenylenediamine). − When these host structures are metalated with transition metal ions, their guest binding abilities are significantly enhanced due to the negatively polarized phenoxo groups of the [M­(salen)] or [M­(saloph)] units, which can act as a better cation-interacting motif than the corresponding nonmetalated species (Figure b). As a typical example, a bis­(saloph) macrocyclic host in the metalated form can strongly bind cationic guests in its crown ether like cavity, whereas the corresponding nonmetalated form showed a significantly lower affinity …”

Transient chirality inversion during racemization of a helical cobalt(III) complex

“…This complex has a dynamic helical structure, whose right- and left-handed forms ( P and M forms, respectively, hereafter) are in slow equilibrium on the timescale of minutes to hours. The P / M ratios of the [LCo 3 X 6 ] 3+ helix can be shifted by the introduction of a chiral ligand at the six exchangeable sites ( Scheme 1 ) ( 54 ) because each of the three hexacoordinate cobalt(III) centers can undergo ligand exchange at the two axial sites ( 55 – 59 ). This means that [LCo 3 X 6 ] 3+ will undergo racemization via a six-step process, if all the chiral ligands X are replaced with achiral ones ( Fig.…”

Ternary host-guest complexes with rapid exchange kinetics and photoswitchable fluorescence

“…This deficiency is most likely due to the closed structure of many molecular hosts, which necessitates a partial disassembly of the host for guest encapsulation/release to take place. [31][32][33][34][35][36][37] The ability to combine fast guest exchange with encapsulationinduced change in physicochemical properties could facilitate the development of new stimuli-responsive supramolecular systems-and ultimately materials-with rapid response times.…”