[Mn2(saltmen)2Ni(pao)2(L)2](A)2 with L = Pyridine, 4-Picoline, 4-tert-Butylpyridine, N-Methylimidazole and A = ClO4-, BF4-, PF6-, ReO4-: A Family of Single-Chain Magnets

A series of single-chain magnets, [Mn2(saltmen)2Ni(pao)2(L)2](A)2 (saltmen(2-)=N,N'-(1,1,2,2-tetramethylethylene) bis(salicylideneiminate), pao-=pyridine-2-aldoximate; A-=ClO4- with L=4-picoline; 2, 4-tert-butylpyridine; 3, N-methylimidazole; 4, and L=pyridine with A-=BF4-; 5, PF6-; 6, ReO4-; 7), was prepared by reactions between MnIII dimer units, i.e., [Mn2(saltmen)2(H2O)2](A)2 (A-=ClO4-, BF4-, PF6-) or Mn2(saltmen)2(ReO4)2, and NiII monomeric units, i.e., Ni(pao)2(L)2, in methanol/water media. The crystal structures of 4, 6, and 7 were established by single-crystal X-ray crystallography. These three compounds are isostructural with [Mn2(saltmen)2Ni(pao)2(py)2](ClO4)2 (1) (Clérac, R.; Miyasaka, H.; Yamashita, M.; Coulon, C. J. Am. Chem. Soc. 2002, 124, 12837) and crystallize in monoclinic space group C2/c. The linear arrangement of MnIII dimer units and NiII building blocks leads to an alternating chain having a repeating unit, [-(O)2-Mn-ON-Ni-NO-Mn-]. The chains are well separated with the nearest interchain intermetallic distance of 10.36 A for 4, 10.51 A for 6, and 10.30 A for 7, and there is no significant pi-pi interchain interaction between ligands. The void space between the chains is occupied by counteranions, which control the three-dimensional organization of the chains. The X-ray diffraction analysis (XRD) on a powder sample was also performed for all compounds. The XRD patterns for 1, 2, and 4-7 are very similar, emphasizing the isostructural nature of these materials although they have individually slight different interchain distances. Inversely, the XRD pattern for 3 reveals a completely different shape being indicative of the peculiar crystal packing compared to the others. Nevertheless, the one-dimensional nature of the structure is also kept in 3 as indicated by magnetic measurements. The whole family of compounds exhibits quasi-identical magnetic behavior compared to that described for 1. Above 30 K, the heterometallic chain can be described as an assembly of antiferromagnetically coupled Mn...Ni....Mn trimers (via oximate bridge, -24.2 K<J(Mn-Ni)/kB<-20.8 K) connected through Mn...Mn ferromagnetic interaction (via the phenolate bridge, J(Mn-Mn)/kB approximately +0.7 K), assuming a ferromagnetic chain with S=3 units. In the low temperature region, combined ac and dc magnetic measurements revealed for the whole series the systematic presence of a magnet behavior exhibiting coercivity and slow relaxation of magnetization below 3.5 K. This behavior was analyzed on the basis of Glauber's theory for an Ising one-dimensional system which predicts an activated dependence (Arrhenius law) of the relaxation time: tau=tau0 exp(Delta/kBT). Similar values of tau0 approximately 1x10(-10) s and Delta/kB approximately 70 K have been found along the series. The described compounds constitute the first example of the rational design of a single-chain magnet family and clearly demonstrate the unique behavior of these heterometallic chains independently of their interchain environments.