Supramolecular metallomacrocycles based on trans-dicyanoferrite(III) building blocks: synthesis, crystal structure and magnetic properties

Abstract

The reaction of trans-[Fe(R-bpb)(CN)₂]⁻ (R-bpb²⁻ = R-substituted-1,2-bis(pyridine-2-carboxamido)benzenate) with trans-Mn(III) Schiff base complexes [Mn(5-X-saltn)]ClO₄ (5-X-saltn²⁻ = N,N′-propanolbis(5-X-substituted-salcylideneiminato) dianion) gave rise to cyanide-bridged neutral binuclear [MnFe] compounds [Mn(saltn)(MeOH)][Fe(bpb)(CN)₂]·3H₂O (1), [Mn(saltn)(H₂O)Fe(bpmb)(CN)₂]·H₂O (2), [Mn(saltn)(MeOH)Fe(bpClb)(CN)₂]·2H₂O (3), and ionic [Mn₂Fe]⁺-[Fe]⁻ complexes [Mn₂(5-Br-saltn)₂(H₂O)(EtOH)Fe(bpb)(CN)₂][Fe(bpb)(CN)₂]·6H₂O (4) and [Mn₂(5-Cl-saltn)₂(CH₃OH)(EtOH)Fe(bpb)(CN)₂][Fe(bpb)(CN)₂]·5H₂O·MeCN (5). Four binuclear units of complexes 1–3 assemble in a head-to-tail way viahydrogen bonding giving rise to a metallo-supramolecular [MnFe]₄ square, while two [Mn₂Fe]⁺-[Fe]⁻ units of complexes 4–5 form a metallo-supramolecular macrocyclic structure. Magnetic studies reveal that complexes 1–3 and 5 exhibit intermetallic ferromagnetic coupling, while complex 4 displays antiferromagnetic interaction between low-spin Fe(III) and high-spin Mn(III) through the cyanide bridges. Complexes 1, 4 and 5 display frequency dependent of current-alternating (ac) magnetic susceptibility, typical of the presence of slow magnetization relaxation. Because of the existence of intermolecular magnetic interaction, complex 4 shows an exchange-biased single-molecule magnet (SMM) behavior below 0.5 K.