Synthesis, structure and magnetism of a new ferromagnetic hexanuclear nickel cluster with a dicubane-like core (original) (raw)

A new ferromagnetic nickel(II) hexameric structure based on the versatile N,O 3-ligand 2,2 ,2-nitrilotribenzoic acid has been prepared and characterised by X-ray crystallography and magnetic measurements; the compound represents a rare example of a nickel cluster with a dicubane-like core having only oxygen bridges. During the past decade molecule-based magnets have attracted considerable interest. Presently it is understood that the important prerequisites for molecules to show single-molecule magnet (SMM) behaviour are the presence of a high spin ground state and a large zero field splitting parameter D. 1 Single-molecule magnets offer the possibility of information storage at the molecular level and there is an increasing interest to develop synthetic approaches to larger, more complex molecules to further investigate the relationship between structure and molecular properties. By use of simple organic ligands, i.e. carboxylates, clusters containing up to 24 transition metals have been isolated. 2 The number of such complexes containing polydentate ligands such as polycarboxylates 3 is limited, but should produce new polynuclear topologies. We have recently investigated the coordination chemistry of the tripodal N,O 3 ligand 2,2 ,2-nitrilotribenzoic acid (H 3 L) and have isolated monomeric and dimeric complexes, but no compounds of higher nuclearity have been obtained so far. 4 Here we present a new hexameric nickel(II) complex of H 3 L underlining the versatility of this ligand to adopt a variety of coordination modes. While we reported a dimeric nickel(II) complex of H 3 L showing a trigonal-bipyramidal coordination mode, we found that this ligand also supports octahedral coordination. The reaction of six equivalents Ni II with four equivalent H 3 L, deprotonated by triethylamine in acetonitrile, gave the reported {Ni 6 } cluster 1. † The single crystal structure of 1 [HNEt 3 ] 2 [Ni 6 L 4 (l 3-OH) 2 (l-OH 2) 2 ]·5CH 3 CN·2.6H 2 O (Fig. 1) displays a centrosymmetric complex anion, containing six nickel(II) atoms, bridged by the carboxylate groups of four L 3− ligands, two hydroxide and two water molecules. ‡ The two nickel(II) ions Ni1 and Ni3 have a distorted octahedral coordination sphere and bind to the deprotonated ligand L 3− by the central nitrogen atom and the three carboxylate groups. The other two coordination sites are occupied by a carboxylate group of a second ligand and a water molecule in the case of Ni3 and a hydroxide in the case of Ni1. Furthermore these two [NiL] units are joined by the two Ni2 and Ni2 ions, forming a dicubane-like core. The carboxylic groups exhibit three different coordination modes within the cluster: monodentate, monoatomic oxygen bridging (l 1-COO) of two Ni ions and triatomic carboxylic bridging OCO(l 3-COO) of three Ni ions. The Ni · · · Ni distances are in the range 2.987(1)–3.128(1)A ˚. Magnetic susceptibility data for 1 were acquired from a microcrystalline sample in the 2–300 K temperature range at a constant magnetic field of 1 T. The temperature dependence of the reciprocal susceptibility is linear above 50 K and obeys the Curie–Weiss law with a Weiss constant h = 6.0 K and C = Fig. 1 Molecular structure of [Ni 6 L 4 (l 3-OH) 2 (l-OH 2) 2 ] 2− , 1. Hydrogen atoms and solvent molecules have been omitted for clarity. Atoms with a prime () character are at equivalent positions (1 − x,1 − y,1 − z). 7.29 cm 3 K mol −1. The C value corresponds to isolated high-spin Ni II (S = 1) ions with g = 2.21. A plot of temperature dependence of the experimental v m T product is presented in Fig. 2 (where v m is the molar paramagnetic susceptibility corrected for the diamagnetic contribution of the sample). The value of v m T product at room temperature (7.48 cm 3 K mol −1) is close to that expected for six noninteracting Ni II ions with S = 1 and g = 2.21 (7.33). On cooling the v m T product starts to increase slowly up to ca. 100 K, and then its increase becomes faster and below 50 K is quite steep. The curve reaches a maximum at 10 K (9.79 cm 3 K mol −1) and then drops rapidly down to 5.60 cm 3 K mol −1. Such behaviour suggests the presence of ferromagnetic interactions within the cluster which is conditioned by increasing population of low energy level high spin multiplicity states with decreasing temperature. The steep decrease of the v m T product below 6 K often occurring in polynuclear nickel complexes is probably due to the effect of zero-field splitting and might be dependent on the molecular effect of the crystal and antiferromagnetic ordering of the ferromagnetic aggregates. Similar behaviour and characteristic shape of the curve was reported for other nickel(II) clusters of different nuclearity with predominant ferromagnetic interactions, in particular, 4-nuclear dicubane-type complexes, 5 hexanuclear 2 × [Ni 3 ], 6 4-and 5-nuclear clusters. 7 Although 1 evidently represents a new topology of Ni clusters, it contains a tetranuclear dicubane-like core to which two terminal Ni ions are attached. Several dicubane-like Ni clusters are described in the literature. 5,8,9 In most reported cases the Ni ions are linked by monatomic end-on azido and oxo-bridged, and the clusters exhibit global ferromagnetic exchange giving S = 4. Only in one case 9 the antiferromagnetic coupling dominates, however, in the cluster the (N,O)-bridging oximino T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5