One-Pot, Template Syntheses of a New Class of Metallomacrocycles with a Tetraoxime Cyclic Skeleton

Abstract

Most common macrocycles such as crown ethers, porphyrins, and macrocyclic polyamines show versatile, excellent functions in a variety of research fields such as coordination chemistry, supramolecular chemistry, analytical chemistry, and material sciences. Thus, the rational design of a new class of readily synthesized macrocyclic metal ligands is a key to the development of a new category of functionalized macrocyclic metal complexes. Herein, we report one-pot, template syntheses of a new class of metallomacrocycles with a 16-membered tetraoxime cyclic skeleton from a dioxime in the presence of a template transition-metal ion such as Fe(2+), Ni(2+), Cu(2+), Co(2+), or Ag(+). These metal ions can be easily removed from the mononuclear metallomacrocycles, and, for example, the metal-free tetraoxime ligand was obtained in 90% yield when Fe(2+) was used as the template. It would appear that this one-pot, metal-template cyclization efficiently proceeds through continual metal-mediated oxime exchange reactions. Interestingly, Pd(2+), which does not afford any cyclized products, formed a 1:1 complex with the macrocyclic ligand. The molecular structures of the metal-free ligand, its 1:1 metal complexes with Fe(2+), Ni(2+), Cu(2+), Pd(2+), and Ag(+), and a dinuclear complex with Ag(+) were fully determined by single-crystal X-ray analyses. UV-visible absorption spectra and cyclic voltammetry measurements of these complexes are also reported.

Discrete and Polymeric, Mono- and Dinuclear Silver Complexes of a Macrocyclic Tetraoxime Ligand with AgI–AgI Interactions

“…In contrast to the discrete complexes, the present polymeric complexes, [Ag( 1 )] n (BF 4 ) n and [Ag 2 ( 1 )(NO 3 ) 2 ] n , and previously reported [Ag 2 ( 1 )(CF 3 CO 2 ) 2 ] n [16], possess a stair-like conformation of 1 , in which two nitrogen atoms and the other two atoms of 1 are oriented to upper and lower faces, respectively (Figure 7(c)). As the result, one macrocycle 1 can bind two Ag I ions on both faces simultaneously to form polymeric structures.…”

“…In these cases, Ag I ions sit on the four nitrogen atoms of 1 due to the large ionic radius of Ag I . Interestingly, a mononuclear complex, [Ag( 1 )CF 3 CO 2 ], previously reported [16] has a similar but significantly different conformation of 1 , that is regarded as a flipped shuttlecock-like conformation in which one nitrogen atom is oriented to the opposite face (Figure 7(b)). The differences in the crystal packing, counter anions and crystallisation conditions might cause the partial flipping of the shuttlecock-like conformation of 1 .…”

“…The ligand 1 was synthesised and isolated as colorless crystals via formation of the ferrous complex, [Fe( 1 )(CH 3 CN) 2 ] (ClO 4 ) 2 , according to our previous report [16]. Crystallisation of silver complexes was carried out in the dark.…”

“…Previously we reported the synthesis of a tetraoxime-type tetraazamacrocycle, 3,7,11,15-tetra-methyl-1,5,9,13,2,6,10,14-tetraoxatetraazacyclohexa-2,6,10,14-tetraene ( 1 ), by facile metal-template one-pot cyclization of a simple dioxime-type compound [16]. The macrocyclic ligand 1 can complex with various divalent transition metal ions such as Fe II , Ni II , Cu II and Pd II to form six-coordinate octahedral and four-coordinate square-planar structures.…”

“…Moreover, the oxime nitrogen donors of 1 generally show lower Lewis basicity than nitrogen donors such as amine, imine and hydrazone. A unique coordination structure based on these features was preliminarily found in a crystalline, linear polymeric Ag I complex in which Ag 2 L 1 -type dinuclear complexes are bridged by counter anions [16]. …”