Conformational isomerism and weak molecular and magnetic interactions in ternary copper(II) complexes of [Cu(AA)L‘]ClO₄·nH₂O, where AA =l-phenylalanine and l-histidine, L‘ = 1,10-phenanthroline and 2,2-bipyridine, and n= 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)(H₂O)]ClO₄ (1), [Cu(l-phe)(bpy)(H₂O)]ClO₄ (2), and [Cu(l-his)(bpy)]ClO₄·1.5H₂O (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 P2₁, with unit cell parameters a = 7.422(7) Å, b = 11.397(5) Å, c = 12.610(2) Å, β = 102.10(5)°, V = 1043.0(11) ų, Z = 2, R = 0.0574, and R_w = 0.1657. Complex 3 crystallizes in a monoclinic space group C2, with a = 18.834(6) Å, b = 10.563(4) Å, c = 11.039(3) Å, β = 115.23(2)°, V = 1986.6(11) ų, 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 π−π stacking and H-bonding interactions. Molecules of 3 exhibit intermolecular dimeric association with the Cu···Cu separation being 3.811 Å. X-ray structures and ¹H 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 ¹H 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 ¹H 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 π−π stacking interactions are the electronic pathways for magnetic interactions in 1−3.