Synthesis, Physical Properties, and Crystal Structure of the Cubane Compound Bis(µ-acetato)-tetra-g-methoxo-bis[µ-(2,5-dimethyl-2,5-diisocyanohexane)]-tetranickel-(II) Tetraphenylborate

Reaction of nickel(II) acetate with 2,5-dimethyl-2,5-diisocyanohexane (TMB) in methanol followed by precipitation with sodium tetraphenylborate produces a compound having the formulation [Ni4(OCH3)4(OAc)2(TMB)4](BPh4)2. A single-crystal X-ray structural analysis reveals that the complex cation adopts a cubane arrangement of nickel(II) and methoxide ions; the acetates bridge two nickel ions on opposite faces of the cube, and the isocyanides bridge the remaining four faces, with each nickel(II) in a distorted octahedral ligand environment. At room temperature [Ni4(OCH3)4(OAc)2(TMB)4](BPh4)2 is paramagnetic, but variable-temperature (286-4.2 K) magnetic susceptibility measurements show that the compound has a diamagnetic ground state. The magnetic exchange among the four 3A2 Ni(II) centers has been modeled with use of three exchange parameters, two ferromagnetic interactions (J2 = 17.7 and J3 = 17.2 cm-1) and one antiferromagnetic interaction (J1 = -9.1 cm-1). It is proposed that these magnetic exchange interactions are propagated by the bridging methoxides and are critically dependent on the Ni-O-Ni angles. The presence of the acetate bridges leads to two types of Ni-O-Ni angles, those that are ca. 93° and those that are ca. 101°. It is suggested that the Ni-O-Ni units possessing the larger angles are coupled antiferromagnetically and that the ferromagnetic interaction is associated with the smaller angles.