[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

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Abstract
A series of single-chain magnets, [Mn2(saltmen)2Ni(pao)2(L)2](A)2 (saltmen2- = 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 Å for 4, 10.51 Å for 6, and 10.30 Å for 7, and there is no significant π−π 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 < JMn-Ni/kB < −20.8 K) connected through Mn···Mn ferromagnetic interaction (via the phenolate bridge, JMn-Mn/kB ≈ +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: τ = τ0 exp(Δ/kBT). Similar values of τ0 ≈ 1 × 10-10 s and Δ/kB ≈ 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.

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