Dependence of an intramolecular aromatic-ring stacking interaction in mixed-ligand copper(II) complexes in aqueous solution on the distance of the stacking moiety from the coordinating group and the solid-state structure of the ternary copper(II)-1, 10-phenanthroline-phenylpropionate complex
Dubler, E; Haring, UK; Scheller, KH; Baltzer, P; Sigel, H
The stability constants of mixed-ligand complexes of the type Cu(phen)(PheCA)+, where phen = 1, 10-phenanthroline and PheCA- = benzoate, 2-phenylacetate, 3-phenylpropionate, 4-phenylbutyrate, and 5-phenylvalerate, have been determined by potentiometric pH titration inaqueous solution and compared with the stabilities of the corresponding ternary complexes formed with formate or acetate. The ternary complexes containing phenylalkanecarboxylates (PheCA-) are significantly more stable because of the intramolecular stacking between the phenyl moiety of the PheCA- ligands and the phenanthroline molecule. This type of stacking is confirmed by 1H NMR shift measurements of the Zn2+/phen/benzyl alcohol system in water: the stability of the resulting unbridged adduct was determined (K = 2.2 M-1). In addition, by measuring the upfield shifts of the Zn2+/phen/2-phenylacetate system, the formation of stacks was alsoshown and the stability constant determined by potentiometric pH titration was confirmed; similarly, the stability constant of the corresponding Cu2+ system was confirmed by UV difference spectrophotometry. The extent of the intramolecular stacking in the ternary Cu2+ complexes was calculated; the stacked isomers occur between about 20 and 50%, depending on the number of methylene groups between the phenyl moiety and the coordinated carboxylate groups; theâbest fitâ is reached with 2-phenylacetate and 3-phenylpropionate. From an aqueous solution of the Cu2+/phen/ 3-phenylpropionate (PPr-) system crystals of Cu(phen)(PPr)(NO3)·2H2O were isolated and subjected to an X-ray analysis. Crystal data: monoclinic, a = 22.876 (3), b = 12.284 (2), c = 16.320 (6) Ã
; β = 111.03 (2)°; space group C2/c, Z = 8. In the solid state the complex consists of dimeric [Cu2(phen)2(PPr)2(H2O)2]2+ cations, NO3- anions with a considerable amount of disorder, and H2O molecules hydrogen bonded to the H2O molecules of the dimers. The dimers result from bridging of two Cu(phen)2+ units by two carboxylate groups and a phen-phen stack. The Cu-Cu distance is 3.054 Ã
; antiferromagnetic coupling of the two unpaired electrons on each copper center is evidenced by magnetic susceptibility measurements. Each copper atom has an approximate square-pyramidal coordination geometry consisting of two nitrogen and three oxygen atoms. Aromatic ring-ring interactions are dominated by the âhomoâ stacks of the phenanthroline ligands within the dimers, but weak intramolecular ligand-ligand interactions between the phenyl moiety of PPr- and an edge of a phenanthroline, both partners being linked to the same Cu2+, are also observed, as well as the corresponding in/ermolecular interactions. The solid-state structure and the structures of the complexes in solution are compared and discussed. © 1984, American Chemical Society. All rights reserved.