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., [(Mn 2(saltmen)2(H2O)2])(A)2 (A- = ClO4-, BF4-, PF6-) or Mn2(saltmen)2(ReO 4)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 [(Mn 2(saltmen)2Ni(pao)2(py)2](ClO 4)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 = τo exp(Δ/kBT). Similar values of τo ≈ 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry