Tuning of chain chirality by interchain stacking forces and the structure-property relationship in coordination systems constructed by meridional Fe-III cyanide and Mn-III Schiff bases

Abstract

We synthesized six Fe(III) Mn(III) bimetallic compounds by self-assembling the newly developed mer-Fe cyanide PPh4[Fe(Clqpa)(CN)(3)]center dot H2O (1) and PPh4[Fe(Brqpa)(CN)(3)]center dot H2O (2) with Mn Schiff baseMn(5-Xsalen)(+) cations. These compounds include [Fe(Xqpa)(CN)(3)][Mn(5-Ysalen)]center dot pMeOH center dot qH(2)O [qpaH(2) = N-(quinolin-8-yl) picolinamide; salen = N,N'-ethylenebis(salicylideneiminato) dianion; X = Cl, Y = H (3); X = Cl, Y = Br (4); X = Br, Y = H (5); X = Br, Y = F (6); X = Br, Y = Cl (7); X = Br, Y = Br (8)]. When precursor 1 was used, compounds 3 and 4 were isolated to give a dinuclear entity and a linear chain structure, respectively. The reaction of precursor 2 with the Schiff bases afforded four linear Fe(III)-Mn(III) chain complexes. Chain chirality with P-and M-helicity emerges in 4, 7, and 8, while 5 exhibits chain helicity opposite to the previous chain complexes and 6 presents no chain helicity. Such a structural feature is heavily dependent on the interchain pi-pi contacts and the Fe precursor bridging unit. Chiral induction from a local ethylenediamine link of Y-salen is propagated over the chain via noncovalent pi-pi interactions. All the bimetallic compounds show antiferromagnetic interactions transmitted by the cyanide linkage. A field-induced metamagnetic transition is involved in 4, 7, and 8, while a field-induced two-step transition is evident in 6. From a magnetostructural viewpoint, the coupling constant is primarily governed by the Mn-N-ax-C-ax angle (ax = axial) in the bimetallic chain complexes composed of mer-Fe(III) tri-cyanides, although the torsion angle plays a role.