Conformational Isomerism and Weak Molecular and Magnetic Interactions in Ternary Copper(II) Complexes of [Cu(AA)L‘]ClO₄·nH₂O, Where AA =l-Phenylalanine andl-Histidine, L‘ = 1,10-Phenanthroline and 2,2-Bipyridine, andn= 1 or 1.5:  Synthesis, Single-Crystal X-ray Structures, and Magnetic Resonance Investigations

Abstract

Weak molecular and magnetic exchange interactions in ternary copper(II) complexes, viz., [Cu(L-phe)(phen)(H(2)O)]ClO(4) (1), [Cu(L-phe)(bpy)(H(2)O)]ClO(4) (2), and [Cu(L-his)(bpy)]ClO(4).1.5H(2)O (3), where L-phe = L-phenylalanine, L-his = L-histidine, phen = 1,10-phenanthroline, and bpy = 2,2'-bipyridine, have been investigated. Single-crystal X-ray structures reveal that complex 2 crystallizes in a monoclinic space group P2(1), with unit cell parameters a = 7.422(7) A, b = 11.397(5) A, c = 12.610(2) A, beta = 102.10(5) degrees, V = 1043.0(11) A(3), Z = 2, R = 0.0574, and R(w) = 0.1657. Complex 3 crystallizes in a monoclinic space group C2, with a = 18.834(6) A, b = 10.563(4) A, c = 11.039(3) A, beta = 115.23(2) degrees, V = 1986.6(11) A(3), Z = 4, R = 0.0466, and R(w) = 0.1211. Molecules of 2, in the solid state, are self-assembled via weak intra- and intermolecular pi-pi stacking and H-bonding interactions. Molecules of 3 exhibit intermolecular dimeric association with the Cu.Cu separation being 3.811 A. X-ray structures and (1)H NMR studies reveal conformational isomerism in both solid and liquid states of complexes 1 and 2. The aromatic side chain of L-phe in 1 and 2 adopts either a "folded" (A) or an "extended" (B) conformation. Variable-temperature (1)H NMR and spin lattice relaxation measurements point out interconversion between conformations A and B at temperatures above 323 K. The change in molecular conformation induces a change in the electron density at the site of copper and band gap energy between HOMO and LUMO orbitals. Interestingly, in spite of paramagnetic nature, complexes 1 and 2 are amenable for both EPR and (1)H NMR spectroscopic studies. Single-crystal EPR spectra of 2 in three orthogonal planes are consistent with three-dimensional magnetic behavior. Intramolecular exchange dominates the dipolar interactions. The EPR spectra of 3 correspond to weak magnetic interactions between associated dimeric units. The structural and magnetic resonance investigations together reveal that the weak pi-pi stacking interactions are the electronic pathways for magnetic interactions in 1-3.

Supramolecular Helical Architectures Dictated by Folded and Extended Conformations of the Amino Acid in Ternary CuII/Diamine/Racemic Amino Acid Complexes

“…In the structure of the complexes [Cu(,-phe)(bpy)]·ClO 4 (1), [Cu(,-phe)-(phen)(H 2 O)]·ClO 4 ·H 2 O (2), and [Cu(-phe)(bpy)(H 2 O)]· ClO 4 ·H 2 O (3) that we describe here (bpy = 2,2Ј-bipyridyl; phen = 1,10-phenanthroline), the difference in the orientation of the phenylalanine side chain results in folded and extended conformations of the amino acid chelate ring that lead to noncovalently propagated supramolecular helical architectures. Folded and extended forms of ternary aromatic amino acid complexes have been reported by many researchers, [2,5,[7][8][9] and here we show how these conformations can influence the helicity of the polymeric structure of the ternary complexes. Complexes 1 and 2, which contain racemic phenylalanine, undergo spontaneous resolution in the crystalline state and their different supramolecular helical structures result in interesting differences in chiroptical properties in the solid state.…”

“…We have also recently reported a folded conformation of -phe in the ternary complex [Cu(-phe)(bpy)(H 2 O)]·ClO 4 . [9] The coordination around Cu II remains almost the same for both 2a and 2b, although the phenylalanine side chain varies somewhat in its orientation. The apical coordination of the Cu II square pyramid is fulfilled by long bonds with water molecules [Cu1-O1w 2.285 (2a), Cu2-O2w 2.351 Å (2b)] in both cases, while the square base resembles that present in complexes 1 and 3.…”

“…[1] A number of ternary Cu II /diamine (DA) complexes with optically active -and -amino acids (AA), where the latter residues can be present in folded and extended conformations, are known, and their optical properties in solution have been studied extensively. [2][3][4] Many of the investigations on such [Cu II (DA)(AA)] [5][6][7][8][9] complexes reported so far have focused mainly on structure/reactivity correlations and conformational studies; no correlation between their structural architecture and solid-state optical properties has been reported. Inorganic metallohelicates [10] are known to have interesting chiroptical properties.…”

Antibacterial Activities and Nuclease Properties of Two New Ternary Copper(II) Complexes with 2‐(4′‐Thiazolyl)‐benzimidazole and 2,2′‐Bipyridine/1,10‐phenanthroline

“…23,24 Crystal structure Complex 2 has been structurally characterized by single-crystal X-ray diffraction. The selected bond lengths and angles are collected in Table 2.…”

Synthesis, Characterization and Nuclease Activity of Ternary Copper(II) Complexes Containing Dipyrido[3,2‐a:2′,3′‐c]‐phenazine and L‐α‐Amino Acids

“…Electronic absorption spectroscopy The d-d transitions with higher energy (at about 610 nm) of the complexes under the experimental conditions indicate that all the complexes may have a distorted squarepyramidal geometry, 40 as observed in the solid state. The absorption spectra of the complexes in the range of 250-300 nm in the absence and presence of CT-DNA were shown in Figure 2.…”