Structural Variety and Magnetic Properties of Tetranuclear Nickel(II) Complexes with a Central μ₄-Azide

Abstract

Using a set of pyrazolate-based dinucleating ligands with thioether sidearms and a set of different carboxylates, seven tetranuclear nickel(II) complexes of types [L2Ni4(N3)3(O2CR)2](ClO4) (1) and [L2Ni4(N3)(O2CR)4](ClO4) (2) featuring an unprecedented central mu4-1,1,3,3-azide could be isolated and fully characterized. X-ray crystal structures are discussed for 1a,b,e and 2b. The mu4-1,1,3,3-azide is symmetric in all cases except 1a but exhibits distinct binding modes with significantly different Ni-N(azide)-Ni angles and Ni-NNN-Ni torsions in type 1 and 2 complexes, which indicates high structural flexibility of this novel bridging unit. Also, IR-spectroscopic signatures and magnetic properties are distinct for type 1 and 2 complexes. Magnetic data for 1a,b,d,e and 2a,b were investigated and analyzed in a three-J approach. The only model that gave a satisfactory fit for all type 1 complexes includes one dominant antiferromagnetic coupling and two ferromagnetic interactions (one large and one smaller), indicating some degree of frustration. On the basis of magneto-structural correlations for end-on and end-to-end azide linkages, it is reasonable to assign the antiferromagnetic interaction to the intradimer exchange along the pyrazolate and the end-to-end linkage of the mu4-azide. Overall, the magnitude of the coupling constants differs significantly for the two distinct types of compounds, 1 or 2, and depends on the individual geometric details of the Ni4 array and the mu4-1,1,3,3-azide.

Structures and Magnetic Properties of Tetranuclear Nickel(II) Complexes with Unusual μ3‐1,1,3 Azido Bridges

“…Synthesis and structural characterization of the complexes: We have previously shown that pyrazolate-based bimetallic complexes with accessible coordination sites can serve as suitable building blocks for the assembly of polynuclear [11,17] aggregates or 1D extendedchain compounds with alternating bridges. [19,20] The first example (2 a) of the new type 2 tetranuclear complexes was obtained serendipitously when we set out to assemble a 1D alternating chain from dinickel(ii) building blocks [L 1 Ni 2 (OAc)] 2 + , which was anticipated to occur upon replacement of the labile acetone ligands in 1 [21] by potentially bridging azide ions (Scheme 1).…”

“…Further experiments revealed that various type 2 complexes can be prepared directly from appropriate amounts of the respective components (Scheme 2). Pyrazolate-based ligands with different thioether substituents (L 1 : R = Et; L 2 : R = iPr) [17,22] as well as different carboxylates have been used in order to evaluate the scope of these systems.…”

“…We tentatively assign the former to the m-1,3 azide and the latter to the m 3 -1,1,3 bridges, in accordance with the trends in IR absorption observed for other pyrazolate-based dinuclear and tetranuclear nickel(ii) complexes with azido ligands in various binding modes. [23,11,17] Magnetic properties: Magnetic susceptibility measurements were carried out for powdered samples of complexes 2 a, 2 b, and 2 c in two different fields (2000 G and 5000 G), in a temperature range from 2.0 to 295 K. In no case was any significant field dependence of the magnetic data observed. Plots of the magnetic susceptibility c M versus T and of the product c M T versus T for 2 a-2 c measured at 2000 G are depicted in Figure 4.…”

“…In the absence of any constraining ligand scaffold, Ni-N-N angles in dinuclear complexes or 1D extended systems with one m-1,3 azido bridge tend to lie in the 1158-1458 range with f = 1408-1808, resulting in J values in the À8 to À55 cm À1 range. [8,17] Particularly strong antiferromagnetic coupling of around À50 cm À1 has been observed for some 1D chain complexes with a trans-{Ni-(m-1,3-N 3 )-Ni} motif featuring acute Ni-N-N angles (120.9 or 115.6/116.88, respectively) and a large f (1808 or 175.58, respectively), [27] or in a dinickel(ii) complex with extremely obtuse Ni-N-N angles of 109.98. [8,28] In the case of complexes 2 a-2 c, Ni-N-N angles for the intradimer linkages involving the m 3 -1,1,3-azide are found in the narrow 115.6-121.78 range, and Ni1-NNN-Ni2 and Ni3-NNN-Ni4 torsion angles are not very far from zero and 2 b (10.4-20.78).…”

“…[11,12] In most cases, however, at least one of the MÀN distances is very long, which limits their description as genuine m 3 -1,1,3 type azido ligands. [11,15] Novel m 4 -1,1,1,1 (C) [16] and m 4 -1,1,3,3 (F) [12,17] azide binding modes have only recently been reported. The paucity of complexes with the more unusual azide binding modes C, E, and F is highly unsatisfactory, since these types of azide bridges are particularly suited for the linking of several metal ions in high-nuclearity clusters and for the construction of 2D and 3D networks of paramagnetic metal centers.…”