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

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)]ClO4.1.5H(2)O (3), where L-phe = L-phenylalanine, L-MS' = 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) Angstrom, b = 11.397(5) Angstrom, c = 12.610(2) Angstrom, beta = 102.10(5)degrees, V = 1043.0(11) Angstrom (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) Angstrom, b = 10.563(4) Angstrom, c = 11.039(3) Angstrom, beta = 115.23(2)degrees, V = 1986.6(11) Angstrom (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 Angstrom. X-ray structures and H-1 NMR studies reveal Conformational isomerism in both solid and liquid states of complexes I and 2. The aromatic side chain Of L-phe in I and 2 adopts either A "folded" (A) or an "extended" (13) 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 I 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 pi-pi stacking interactions are the electronic pathways for magnetic interactions in 1-3.