Three-dimensional antiferromagnetic order of single-chain magnets: A new approach to design molecule-based magnets

Two one-dimensional compounds composed of a 1:1 ratio of Mn^III salen-type complex and Ni^II oximato moiety with different counter anions, PF₆ ^- and BPh₄ ^-, were synthesized : [Mn(3,5-Cl₂saltmen)Ni(pao)_2- (phen)]PF ₆ (1) and [Mn(5Clsaltmen)Ni(pao)₂(phen)]BPh₄ (2), where 3,5-Cl₂saltmen^2- =N,N'-(1,1,2, 2tetramethylethylene)bis(3,5-dichlorosalicylideneiminate) ; 5-Clsaltmen ^2- = N,N'-(1,1,2,2-tetramethylethylene)bis(5- chlorosalicylideneiminate); pao^-=pyridine-2-aldoximate ; and phen = 1,10-phenanthroline. Singlecrystal X-ray diffraction study was carried out for both compounds. In 1 and 2, the chain topology is very similar forming an alternating linear chain with a [-Mn^III-ON-Ni^IINO-] repeating motif (where -ON- is the oximate bridge). The use of a bulky counteranion, such as BPh₄ ^-, located between the chains in 2 rather than PF₆ ^-in 1, successfully led to the magnetic isolation of the chains in 2. This minimization of the interchain interactions allows the study of the intrinsic magnetic properties of the chains present in 1 and 2. While 1 and 2 possess, as expected, very similar paramagnetic properties above 15 K, their ground state is antiferromagnetic below 9.4 K and paramagnetic down to 1.8 K, respectively. Nevertheless, both compounds exhibit a magnet-type behavior at temperatures below 6 K. While for 2, the observed magnetism is well explained by a Single-Chain Magnet (SCM) behavior, the magnet properties for 1 are induced by the presence in the material of SCM building units that order antiferromagnetically. By controlling both intra- and interchain magnetic interactions in this new [Mn^IIINi^II] SCM system, a remarkable AF phase with a magnet-type behavior has been stabilized in relation with the intrinsic SCM properties of the chains present in 1. This result suggests that the simultaneous enhancement of both intrachain (J) and interchain (J') magnetic interactions (with keeping J ≫ J'), independently of the presence of AF phase might be an efficient route to design high temperature SCM-based magnets.