Spin-canted antiferromagnetic phase transitions in alternating phenoxo- and carboxylato-bridged MnIII-salen complexes

Article abstract of DOI:10.1002/ejic.201001215

Three new Mn^IIIcomplexes₂(OCn)](ClO ₄)}_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-anti carboxylate-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) A] 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.