Bis-Argentivorous Molecules Bridged by Phenyl and 4,4'-Biphenyl Groups: Structural and Dynamic Behavior of Silver Complexes

Bis-argentivorous molecules (L^a and L^b), which have phenyl and 4,4'-biphenyl groups as linkers, have been prepared. The structures of Ag⁺ complexes with the new ligands (L^a and L^b) were investigated in solution and the solid state. The CSI-MS and ¹H NMR titration of L^a and L^b with Ag⁺ show 1:1 and 1:2 complexes depending on the [Ag⁺]:[L] ratios. In the solid-state structures, single crystals of L^a and L^b with 2 equiv of Ag⁺ were prepared. X-ray crystallography of the silver(I) complexes with L^a and L^b showed that an intramolecular racemic structure (Δ(δδδδ)Λ(λλλλ) form) and a racemic mixture of Δ(δδδδ)Δ(δδδδ) and Λ(λλλλ)Λ(λλλλ) forms were formed, respectively. The dynamic ¹H NMR studies suggest the following: (i) the activation entropies (ΔS^⧧) of the side arm rotations in the Ag⁺ complex with L^a were all negative, indicating restricted rotation of the side arms due to their shortness, and (ii) the ΔS^⧧ values of the Ag⁺ complexes with L^b were negative only when the side arms of both cyclens rotated simultaneously, and the ΔS^⧧ values for the 1:1 and 1:2 complexes were positive when one cyclen side arm was rotated. These values of ΔS^⧧ indicate that the biphenyl side arms between the two cyclens are not long enough to rotate the ring freely.

Cosmosen: Octa-Armed 24-Membered Cyclic Octaamine Synthesized from a Byproduct in the Preparation of 4-Benzyl-2,6-dioxocyclen

The synthesis of an octa-armed 24-membered cyclic octaamine (1) is reported. When 4-benzyl-1,4,7,10-tetraazacyclododecane-2,6-dione (3a) was prepared by the reaction of diethylenetriamine with diethyl N-benzyliminodiacetate (2), a dimeric macrocycle (3b) was obtained as a byproduct in a 5% yield. An octa-armed 24-membered cyclic octaamine (1), named Cosmosen, was prepared via the reductive amination and reduction of 3b. The binding constants for the 1:1 and 2:1 (Ag⁺/1) complexation of 1 were estimated to be ca. 7.9 and 13.9, respectively, by titration experiments using UV-vis spectrometry in methanol and chloroform (v/v, 9:1) solutions at 298 K.

Argentivorous Molecules with Oxyethylene Chains in Side-Arms: Silver Ion-Induced Selectivity Changes toward Alkali Metal Ions

Argentivorous molecules with mono, di, tri, tetra, and penta-oxyethylene chains in aromatic side-arms were prepared (L1-L5). Titration experiments using proton nuclear magnetic resonance and cold electrospray ionization (cold-spray ionization, CSI) mass spectrometry showed that silver ions were trapped in the cyclen moiety and the arranged oxyethylene chains of the side-arms when two equivalents of silver ions were added. The silver complexes formed by adding one equivalent of silver ion to L2-L5 bind alkali metal ions using the oxyethylene chains; alkali metal ion-induced CSI mass spectral changes of L2-L5 were measured in the absence and presence of silver ions to compare the binding properties of the ligand for Li⁺, Na⁺, and K⁺ ions. As a result, the intensity ratios of [L + H + M]²⁺/[L + H]⁺ in L1-L3 were almost zero or very low. L4 and L5, which have tetra(oxyethylene) and penta(oxyethylene) chains, respectively, bind a larger size of alkali metal ions. On the other hand, in the presence of silver ions, the ratio for [L + Ag + M]²⁺/[L + H]⁺ (M = Li, Na, K) in L2-L5 was increased. The highest [L + Ag + M]²⁺/[L + H]⁺ ratios for K⁺ were observed in L4 and L5, while selectivity for Na⁺ was observed in the case of L2 and L3. These results indicate that the increased binding ability and selectivity by L2-L5 are due to the arrangement of oxyethylene chains by the conformational change of the aromatic side-arms. The Ag⁺-induced carbon-13 nuclear magnetic resonance spectral changes suggested that the second and third oxyethylene units, close to the benzene, are involved in the coordination of the second metal ion.

Argentivorous Molecules with Chromophores in Side Arms: Silver Ion-Induced Turn On and Turn Off of Fluorescence

The synthesis of argentivorous molecules (L¹ and L²) having two chromophores (4-(anthracen-9-yl)benzyl or 4-(pyren-1-yl)benzyl groups) and two benzyl groups and the fluorescence properties of their silver complexes in a solution and the solid state are reported. A crystallographic approach for the Ag⁺ complexes with L¹ and L² revealed that the observed fluorescence changes stem from the excimer formation and extinction of fluorescent. Furthermore, binding stabilities of L¹ and L² toward Ag⁺ ions were estimated by the Ag⁺-induced UV-vis and PL spectral changes.

Cyclobenzoin Esters as Hosts for Thin Guests

Cyclotetrabenzoin esters can host terminal triple bonds of alkynes and nitriles in their cavities, as revealed by cocrystal structures of four such complexes. Within cyclotetrabenzoin cavities, π-clouds of triple bonds establish favorable and virtually equidistant interactions with the four aromatic walls of the cyclotetrabenzoin skeleton. Binding is selective for aliphatic nitriles and terminal alkynes, with their aromatic counterparts residing outside of the cyclotetrabenzoin cavity.