Polynuclear Nickel(II) Complexes:  Preparation, Characterization, Magnetic Properties, and Quantum-Chemical Study of [Ni₅(OH)(Rbta)₅(acac)₄(H₂O)₄] (RbtaH = Benzotriazole and 5,6-Dimethylbenzotriazole)

The preparation and properties are described of two related Ni II 5 clusters. The reactions of [Ni(acac) 2 (H 2 O) 2 ] (acacH = acetylacetone) with benzotriazole (btaH) and 5,6-dimethylbenzotriazole (5,6diMebtaH) in refluxing Me 2 CO in the presence of H 2 O leads to the isolation of [Ni 5 (OH)(bta) 5 (acac) 4 (H 2 O) 4 ] ( 1 ) and [Ni 5 (OH)(5,6diMebta) 5 (acac) 4 (H 2 O) 4 ] ( 2 ) in 70−75 and 40−50% yields, respectively. Complex 1 ·4Me 2 CO·0.5C 6 H 14 crystallizes in the triclinic space group P 1̄ with (at 25 °C) a = 13.885(1) Å, b = 12.013(1) Å, c = 25.611(2) Å, α = 89.02(1)°, β = 104.76(2)°, γ = 111.78(1)°, and Z = 2. Complex 2 ·4Me 2 CO crystallizes in the monoclinic space group C 2/ c with (at 25 °C) a = 19.085(3) Å, b = 20.142(3) Å, c = 22.574(4) Å, β = 103.30(1)°, and Z = 4. The Ni II assemblies of 1 and 2 are composed of a tetrahedral arrangement of four six-coordinate metal ions centered on the fifth. Each of the five η 1:η 1:η 1:μ 3 benzotriazolate ligands spans an edge of the Ni 4 tetrahedron. The OH - ion bridges three Ni II ions and spans the sixth edge of the tetrahedron. A chelating acac - ion and a terminal H 2 O molecule complete the coordination sphere of each peripheral metal. Variable-temperature magnetic susceptibility data (5.0−295 K), obtained for 1 and 2, show antiferromagnetic interactions for both of them. The data are interpreted using a five- J model, which is based upon the hierarchy of algebras approach. Least-squares fitting of the data gives exchange parameters J for the interactions between the benzotriazolate-bridged peripheral Ni II ions in the range from −3.9 to −10.7 cm - 1, and for those between the central ion and the peripheral ones in the range from −3.1 to −6.1 cm - 1; the J value for the interaction between the O-bridged peripheral nickels is −39.6 cm - 1 for 1 and −43.2 cm - 1 for 2 . Magnetization data are in line with an intermediate spin ground state [ S = 0, S = 1] for both clusters. An orbital interpretation of the coupling is proposed.