Reaction of Co(NCS)(2) with pyridine (pyr) in aqueous solution at room temperature leads to the formation of the pyridine-rich 1:4 compound of composition [Co(NCS)(2)(pyridine)(4)] (1) reported recently. On heating, the pyridine-rich 1:4 compound transforms into its corresponding pyridine-deficient 1:2 compound of composition [Co(NCS)(2)(pyridine)(2)](n) (2), which decomposes on further heating. In the crystal structure of compound 2 the metal cations are coordinated by four N-atoms of two pyridine ligands and two N-bonded thiocyanato anions, each in mutually trans orientation, and by two S-atoms of two adjacent thiocyanato anions in a slightly distorted octahedral geometry. The thiocyanato anions bridge the metal cations forming one-dimensional polymeric chains. IR spectroscopic investigations on the pyridine-deficient 1:2 compound prepared in thermal decomposition are in accordance with bridging thiocyanato anions. Magnetic measurements of the pyridine-rich 1:4 compound and pyridine-deficient 1:2 compound reveal different behaviour with Curie-Weiss paramagnetism for compound 1 and single chain magnetic behaviour for compound 2, with a Mydosh-parameter φ = 0.12 and an effective energy barrier (-U(eff)/k(B)) of 62.5 K for the spin relaxation.
Abstract. In this report a rational route to coordination polymers that can show cooperative magnetic phenomena is presented. In this approach compounds based on transition metal cations, small sized terminal N-bonded anionic ligands and additional neutral N-donor coligands are heated, which lead to the formation of intermediates, in which the metal cations are linked by the anionic ligands. Predominantly, the use of this method for the synthesis of bridged thio-and selenocyanato coordination compounds is described in this article but it can also be extended for the preparation of other compounds. In most cases the intermediates are formed in very pure form and in quantitative yields. Thus, compounds, which are not or at least very difficult to obtain, can be prepared if the synthesis is performed in solution. This is especially valid for thio-and selenocyanato coordination compounds, which mostly prefer terminal bonding instead of bridging co-
Recently, strategies for the design of coordination polymers, hybrid compounds, or metal-organic frameworks (MOFs) that show cooperative magnetic phenomena have become of increasing interest.[1] Because of their great potential for possible applications as storage materials or in molecular electronics, 1D materials with a large magnetic anisotropy, slow relaxation of the magnetization M, and a hysteresis of molecular origin, for example, "single-chain magnets" (SCMs) are of special interest.[2] Moreover, for future applications multifunctional materials are needed, in which different physical properties can be tuned or switched as a function of external parameters.[3] These criteria also apply to metamagnetic compounds, which show different magnetic properties below and above a critical field H C . [1c, 4] Unfortunately, because of strong interchain interactions most of these compounds show only 3D ordering above H C .[5] Therefore, only a very few metamagnetic coordination compounds have been reported in which slow relaxation of the magnetization is observed. [5,6] In our research we have developed an alternative method for the synthesis of compounds that show cooperative magnetic interactions. [7] In this approach transition-metal coordination compounds with terminally bound anions and neutral co-ligands are heated leading to a stepwise removal of the co-ligands and the formation of intermediates with bridging anions and modified magnetic interactions. We have found that a large number of different compounds can be prepared by this route and that the dimensionality of the networks can easily be adjusted. [7a,b,d, 8] In this context we have reported on the directed synthesis of a compound that shows SCM behavior.[9] Such a behavior usually occurs only in 1D coordination networks, but should, in principle, also be observed in 2D networks if the magnetic chains are separated by magnetically inactive ligands. To investigate this possibility, precursor compounds based on cobalt(II) thiocyanate and the bidentate co-ligand 1,2-bis(4-pyridyl)ethylene (bpe) were prepared, and the intermediates formed by thermal decomposition were characterized for their magnetic properties.The reaction of Co(NCS) 2 with an excess of bpe leads to the formation of [Co(NCS) 2 (bpe)(bpe)] n (1).[10] In its crystal structure the cobalt cations are octahedrally coordinated by four bpe ligands and two terminal N-bonded thiocyanato anions (Figure 1, top). The metal cations are linked by the bpe ligands into chains that are further connected by the coligands into layers. This arrangement leads to the formation of cavities in which additional bpe ligands are trapped. In further experiments using slightly different reaction conditions the hydrate [Co(NCS) 2 (bpe) 2 (H 2 O) 2 ][10] (2) could be obtained, in which the cobalt(II) cations are surrounded by two bpe ligands, two water molecules, and two terminal N-bonded thiocyanato anions in an octahedral coordination environment (Figure 1, bottom). These complexes are linked into layers by O À H···...
Reaction of FeCl(2)·4H(2)O with KNCSe and pyridine in ethanol leads to the formation of the discrete complex [Fe(NCSe)(2)(pyridine)(4)] (1) in which the Fe(II) cations are coordinated by two N-terminal-bonded selenocyanato anions and four pyridine co-ligands. Thermal treatment of compound 1 enforces the removal of half of the co-ligands leading to the formation of a ligand-deficient (lacking on neutral co-ligands) intermediate of composition [Fe(NCSe)(2)(pyridine)(2)](n) (2) to which we have found no access in the liquid phase. Compound 2 is obtained only as a microcrystalline powder, but it is isotypic to [Cd(NCSe)(2)(pyridine)(2)](n) and therefore, its structure was determined by Rietveld refinement. In its crystal structure the metal cations are coordinated by two pyridine ligands and four selenocyanato anions and are linked into chains by μ-1,3 bridging anionic ligands. Magnetic measurements on compound 1 show only paramagnetic behavior, whereas for compound 2 an unexpected magnetic behavior is found, which to the best of our knowledge was never observed before for a iron(II) homospin compound. In this compound metamagnetism and single-chain magnetic behavior coexist. The metamagnetic transition between the antiferromagnetically ordered phase and a field-induced ferromagnetic phase of the high-spin iron(II) spin carriers is observed at a transition field H(C) of 1300 Oe and the single-chain magnetic behavior is characterized by a blocking temperature T(B), estimated to be about 5 K.
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