Syntheses and Crystal Structures of a Novel 20‐Membered N₃O₂‐Donor Macrocycle and Its Disilver(I) Bis(Macrocycle) and Monocadmium(II) Complexes

Abstract

In macrocyclic coordination chemistry, the cation to cavity size ratio is an important parameter for understanding complexation-based metal ion recognition.1 For example, 16-to 19-membered ring ligands incorporating dibenzo subunits have been widely investigated because these ligands generally favor stable 1:1 complexation and show intermediate flexibility which limits the number of observed complex conformations. 1,2Following these earlier studies, new synthetic results and applications for the related macrocyclic systems have continued up to the present. 3 In terms of macrocyclic cavity sizes, macrocyclic rings greater than 24-membered are strong candidates for forming dinuclear complexes in both the synthetic and bioinorganic chemistry areas. 4 However, their preparation and purification tend to be difficult, often leading to the formation of the target ring in low yield.On the other hand, the preparation and structural characterization of the complexes with 20-to 23-membered macrocycles have been less investigated because their cavity sizes are too large for planar complexation with one metal ion but too small for coordination with two metal ions simultaneously. 5 However, macrocycles with the 20-23 membered rings are expected to give the unusual coordination topologies, while another advantage is that the latter category is usually relatively easier to prepare than larger analogs. Recently, several kinds of 20-to 23-membered oxathia-tribenzo macrocycles and their discrete and polymeric complexes showing less common stoichiometric ratios have been reported. 5 To the best of our knowledge, such 20-membered O 2 N 3 -donor analogs have not been reported so far.In this work, we present a 20-membered O 2 N 3 -macrocycle L incorporating a tribenzo-subunit (Scheme 1); L is a potentially pentadentate macrocyclic ligand which is expected to react with a wide range of metal species. In particular, d 10 metal ions are free from formation of specific coordination geometries and often show favorable reactivity toward polyoxaaza ligand systems. Here we report the synthesis of L along with an X-ray study of its complexes incorporating the d 10 -metal ions Cd(II) and Ag(I). The novel tribenzo-O 2 N 3 -macrocycle L was obtained directly in 75% yield from reaction of dialdehyde 6 with diethylenetriamine in methanol followed by sodium borohydride reduction. The 1 H and 13 C NMR spectra together with elemental analysis and mass spectra were all in agreement with the proposed structure.Reaction of L with Cd(NO 3 ) 2 Á4H 2 O in dichloromethane/ methanol afforded a colorless crystalline product 1 suitable for X-ray analysis (Table 1). The crystal structure of 1 is shown in Figure 1 and its selected geometric parameters are presented in Table 2. The crystal structure of 1 has an anion-coordinated mononuclear arrangement with the formula being [Cd(L) (NO 3 ) 2 ]. Unlike our expectation, the Cd(II) center locates outside the macrocyclic cavity (Figure 1(a)). The Cd(II) center is in a hexa-coordinated environment, being bound to th...