[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, [Mn-2(saltmen)(2)Ni(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 Mn-III dimer units, i.e., [Mn-2(saltmen)(2)(H2O)(2)](A)(2)(A(-) = ClO4-, BF4-, PF6-) or Mn-2(saltmen)(2)(ReO4)(2), and Ni-II 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 [Mn-2(saltmen)(2)Ni(pao)(2)(py)(2)](ClO4)(2) (1) (Clerac, 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 Mn-III dimer units and Ni-II 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 Angstrom for 4, 10.51 Angstrom for 6, and 10.30 Angstrom 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)/k(B) < -20.8 K) connected through Mn...Mn ferromagnetic interaction (via the phenolate bridge, J(Mn-Mn)/k(B) approximate to +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 = tau(0) exp(Delta/k(B)T). Similar values of tau(0) approximate to 1 x 10(-10) s and Delta/k(B) approximate to 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.