Copper complexes with bibracchial lariat ethers: from mono- to binuclear structures

Abstract

Copper(II) complexes with a series of bibracchial lariat ethers are described. Independently of the nature of the counterion present (nitrate or perchlorate), the lariat ether N, N%-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 (L 1 ) always forms mononuclear complexes, whereas the lariat ethers N, N%-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L 2 ) and N,N%-bis(2-salicylaldiminobenzyl)-4,13-diaza-18-crown-6 (L 3 ) only give binuclear compounds. The X-ray crystal structure of [CuL 1 ](ClO 4 ) 2 shows a seven-coordinated copper(II) ion in a distorted (axially compressed) pentagonal-bipyramidal geometry. The X-ray crystal structure of [Cu 2 (L 3 -2H)](ClO 4 ) 2 confirms the binuclear nature of the compound with both metal ions having identical coordination environments and each one placed out of the crown hole but efficiently encapsulated by the corresponding pendant arm; each copper(II) ion is five-coordinated with an intermediate geometry between trigonal-bipyramidal and square-pyramidal (~= 0.40). The EPR spectra in frozen solution samples are in accordance with a stable coordinate pattern for the metal centre of ligand L 1 , yielding a rhombic distorted complex with axial compression in solution, in agreement with the X-ray crystal structure of [CuL 1 ](ClO 4 ) 2 . For the binuclear complexes of L 2 and L 3 , the Cu(II) centres in solution can be distorted from their tetragonally elongated structures via interaction with ethanol and/or the nitrate counterion, leading to more than one species.

A merged experimental and theoretical conformational study on alkaline-earth complexes with lariat ethers derived from 4,13-diaza-18-crown-6

“…We have carried out studies to asses their different complexation capabilities towards post-transitional divalent metal ions such as Zn(II) [17], Pb(II) and Cd(II) [18][19][20][21] as well as transition metal ions. With first-row transition metal ions such as Ni(II), Co(II) and Cu(II), L 2 and L 4 are able to form binuclear complexes [22,23], whereas the related receptors L 1 and L 3 only form mononuclear complexes [24][25][26]. Interestingly, these mononuclear complexes represent unusual examples of structurally characterized seven coordinate (pentagonal bipyramidal) complexes of first-row transition metal ions.…”

Anion Coordination Effect on the Nuclearity of CoII, NiII, CuII, and ZnII Complexes with a Benzimidazole Pendant‐Armed Crown

“…The Zn II ion behaves in a different way, and addition of chloride anions to the Zn II perchlorate in the presence of L 4 does not lead to the formation of a binuclear complex. The formation of these binuclear systems was followed by spectrophotometric ti- We recently demonstrated that the pendant-armed crown ethers N,NЈ-bis(2-aminobenzyl)-1-10-diaza-15-crown-5 (L 1 , Scheme 1) [5,6] and N,NЈ-bis(benzimidazol-2-yl-methyl)-1,10-diaza-15-crown-5 (L 3 ) [7] form mononuclear complexes in acetonitrile solution with divalent first-row transition-metal ions that represent unusual examples of structurally characterized seven-coordinate (pentagonal bipyramidal) complexes. [8] Particularly interesting are also the coordinative properties towards these divalent metal ions of the larger related receptor N,NЈ-bis (2-aminobenzyl with the metal ions being five-or six-coordinate, [9] whereas the corresponding mononuclear complexes could not be isolated.…”

Evolution of the Coordination‐Sphere Symmetry in Copper(II), Nickel(II), and Zinc(II) Complexes with N,N′‐Double‐Armed Diaza‐Crown Ethers: Experimental and Theoretical Approaches

“…The few examples of X-ray structure of the [15]crown-5-Cu 2+ systems found in the literature led us to present here the partially resolved structure of [Cu(L2)] 2+ (Figure 2). [6,10,12,20] 2+ in solution, which is different from the asymmetric structure found in the solid state.…”

“…The [15]crown-5-based ligands usually lead to heptacoordinated complexes with metal ions, the most common polyhedron being the pentagonal bipyramid [19] with either O 7 , [11,12] N 2 O 5 , [10,20] or N 4 O 3 [6,8,9,14] coordination spheres. The pentagonal equatorial plane is probably imposed by the rigid structure of the [15]crown-5 ligand.…”

“…[8,[10][11][12][13][14] The size of chelate rings formed upon coordination of the pendant arms to the metal ion influences the nuclearity of the complexes. For instance, in the case of disubstituted diaza [18]crown-6 ethers, the six-membered chelate rings (and beyond) tend to form binuclear complexes, [6,7] whereas the five-membered chelate rings lead to mononuclear complexes in the presence of non-coordinating counterion. [4,[15][16][17][18] In the present study, we focus on mononuclear complexes formed by the N,NЈ-disubstituted diaza [18]crown-6 and diaza [15]crown-5 macrocyclic compounds with first-row transition-metal ions (Cu 2+ , Ni 2+ , Zn 2+ ) and, specifically, [4,16] Zn 2+ acetate / L5 / N 2 O 4 octahedral / asymmetric [17] Cu 2+ / ClO 4 -(1-benzyl-1H-1,2,3-triazol-4-yl)methyl / L1 / N 4 O 2 octahedral / symmetric [15] N,NЈ-Disubstituted diaza [15]crown-5…”