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

Abstract

A cryptand derivative, H6L, which has three H2saloph arms connected by two benzene ring bridgeheads, was synthesized and converted into the trinuclear metallocryptand, LNi3. The nonmetalated host, H6L, was found to bind to alkali metal ions (Na+, K+, Rb+, Cs+; logK a = 3.37–6.67) in its well-defined cavity in DMSO/chloroform (1:9). The binding affinity was enhanced by 1–2 orders of magnitude upon the conversion into the metallocryptand, LNi3, which can be explained by the more polarized phenoxo groups in the [Ni­(saloph)] arms. The guest binding affinity of Na+ < K+ < Rb+ ≈ Cs+ was clearly demonstrated by the 1H NMR competition experiments. The DFT calculations suggested that the Rb+ ion most suitably fit into the benzene–benzene spacing with a cation−π interaction and that only the largest Cs+ ion can almost equally interact with all six phenoxo oxygen donor atoms. The metallocryptand, LNi3, also showed a strong binding affinity to Ag+ by taking advantage of cation−π interactions, which was confirmed by spectroscopic titrations and crystallographic analysis as well as DFT calculations. Thus, the well-defined three-dimensional cavity of LNi3 was found to be suitable for strong binding with alkali metal ions as well as Ag+.

Acceleration and deceleration of chirality inversion speeds in a dynamic helical metallocryptand by alkali metal ion binding

“…At first, the binding constant of ( R 6 )-LNi 3 with K + was determined by the competition experiment with 18-crown-6, which has a known binding constant of log K a = 4.98 with K + in CDCl 3 /DMSO- d 6 (9:1) (fig. S35) ( 57 ). The experiment revealed that ( R 6 )-LNi 3 can bind to K + 291 times stronger than 18-crown-6, which gave the binding constant of ( R 6 )-LNi 3 with K + to be log K a = 7.44.…”

“…1C), because it has both a dynamic helical structure and a guest-binding site. This molecule can adopt a triple helical conformation in which the three metallosalen arms are twisted due to the rigid triphenylbenzene framework as seen in the previously reported tris(saloph) analog L 0 Ni 3 (H 2 saloph = N,N 0 -disalicylidene-ophenylenediamine) (46,57). This helical framework undergoes a dynamic chirality inversion on a timescale of minutes to hours, which can be easily monitored by spectroscopic techniques (46)(47)(48).…”

“…Thus, the ratio of the binding constants, K a(LNi3) / K a(18C6) , toward K + is calculated to be 291. The binding constant of LNi 3 with K + was determined to be log K a(LNi3) = 7.44 on the basis of the binding constant log K a(18C6) = 4.98 for K + in CDCl 3 /DMSO- d 6 (9:1) ( 57 ). Then, the binding constants for all the alkali metal ions were determined by competition experiments among alkali metal ions (Fig.…”

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

“…The helical complex [LCo 3 A 6 ](OTf) 3 was synthesized by the reaction of the corresponding free ligand ( 60 , 61 ) with cobalt(II) acetate under aerobic conditions in the presence of chiral amine A according to our previous report ( 54 ). The right-handed helix was dominant in a methanol solution, and the P / M diastereomeric ratio was 88:12 ( SI Appendix , Fig.…”

Synthesis of Large [2+3] Salicylimine Cages with Embedded Metal‐Salphen Units

“…Furthermore, by using chiral amines on the same cage scaffold the helix to helix interconversion could also be controlled. [21] For all examples of salphen based cages discussed above, the salphen units are formed during the cage synthesis. [22] Due to the irreversible nature of the formation of the metal salphen units, it may be difficult to construct larger cages, by having this reaction as key-step.…”