| Title
| - Conformational Isomerism and Weak Molecular and Magnetic Interactions in TernaryCopper(II) Complexes of [Cu(AA)L‘]ClO4·nH2O, Where AA = l-Phenylalanine andl-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
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| 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 cellparameters a = 7.422(7) Å, b = 11.397(5) Å, c = 12.610(2) Å, β = 102.10(5)°, V = 1043.0(11) Å3, Z = 2, R= 0.0574, and Rw = 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) Å3, Z = 4, R = 0.0466, and Rw = 0.1211.Molecules of 2, in the solid state, are self-assembled via weak intra- and intermolecular π−π stacking and H-bondinginteractions. Molecules of 3 exhibit intermolecular dimeric association with the Cu···Cu separation being 3.811Å. X-ray structures and 1H NMR studies reveal conformational isomerism in both solid and liquid states ofcomplexes 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 interconversionbetween conformations A and B at temperatures above 323 K. The change in molecular conformation induces achange 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 NMRspectroscopic 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 of3 correspond to weak magnetic interactions between associated dimeric units. The structural and magnetic resonanceinvestigations together reveal that the weak π−π stacking interactions are the electronic pathways for magneticinteractions in 1−3.
- Weak molecular and magnetic interactions in ternary copper(II) complexes have been investigated. The complexes containing l-phenylalanine exhibited conformational isomerism in both solid and solution states. Intra- and intermolecular π−π stacking and H-bonding interactions stabilize the molecules in the solid state. Interestingly, the complexes are amenable to both EPR and 1H NMR studies. The weak molecular interactions are the electronic pathways for the magnetic interactions in these complexes.
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