Spin-canted antiferromagnetic phase transitions in alternating phenoxo and carboxylato-bridged Mn^III -salen complexes

Abstract

Three new Mn^III complexes, {[Mn₂(salen) ₂(OCn)](ClO₄)}_n (1), {[Mn₂(salen) ₂(OPh)](ClO₄)}_n (2) and {[Mn₂(salen) ₂(OBz)](ClO₄)}₂ (3) (where salen = N,N′-bis(salicylidene)-1,2-diaminoethane dianion, OCn = cinnamate, OPh = phenylacetate and OBz = benzoate), have been synthesized and characterized structurally and magnetically. The crystal structures reveal that all three structures contain syn-anticarboxylate-bridged dimeric [Mn₂ (salen) ₂(OOCR)]⁺ cations (OOCR = bridging carboxylate) that are joined together by weak Mn•••O(phenoxo) interactions to form infinite alternating chain structures in 1 and 2, but the relatively long Mn•••O(phenoxo) distance [3.621(2) Å] in 3 restricts this structure to tetranuclear units. Magnetic studies showed that 1 and 2 exhibited magnetic long-range order at T_N = 4.0 and 4.6 K (T_N = Neel transition temperature), respectively, to give spin-canted antiferromagnetic structures. Antiferromagnetic coupling was also observed in 3 but no peaks were recorded in the Field-cooled Magnetization (FCM) or Zero-Field-Cooled Magnetization (ZFCM) data, indicating that 3 remained paramagnetic down to 2 K. This dominant antiferromagnetic coupling is attributed to the carboxylate bridges. The ferromagnetic coupling expected due to the Mn–O(phenoxo)•••Mn bridge plays an auxiliary role in the magnetic chain, but is an essential component of the bulk magnetic properties of the material.