A Simple Route to Single-Scorpionate Nickel(II) Complexes with Minimum Steric Requirements

Single-scorpionates of nickel(II), Tp^RNiX or Tpm^RNiX, are kinetic products whose preparation has generally required considerable steric constraints on the ligands (i.e., R = phenyl, tert-butyl, or isopropyl) to prevent formation of intractable two-ligand products like (Tp^R)₂Ni. It is well established that the facial tridentate chelates hydrotris(3,5-dimethylpyrazolyl)borate (Tp*⁻), tris(3,5-dimethylpyrazolyl)methane (Tpm*), and trispyrazolylmethane (Tpm), all readily form two-ligand complexes as thermodynamic products. For the first time we report a route to the single-ligand complex TpmNiX₂(OH₂)_n (X = Cl and Br). We also report a novel method for making single-ligand nickel(II) scorpionate complexes using preformed tetrahalonickelate(II) ion in nitromethane. The complex Tpm*NiCl₂(OH₂)_n was also prepared here for the first time utilizing an alternative method first reported by Zargarian and co-workers (Inorg. Chim. Acta 2006, 2592). TpmNiX₂(OH₂)_n are kinetic products, and although they are stable indefinitely in the solid state, they readily convert to the thermodynamic product (Tpm)₂Ni²⁺ in solution over the course of several hours at room temperature and in a matter of minutes at 100 °C. The new nitromethane/NiX₄²⁻ method offers an alternative route to monoscorpionates of first row transition metals, for which tetrahalometallate ions are common. HOCH₂Tpm (2,2,2-tris(pyrazolyl)ethanol) was covalently attached to polystyrene synthesis beads and found to bind nickel(II) (from NiX₄²⁻) in a manner similar to Tpm. Solid state electronic spectra of supported-TpmNiCl₂ are comparable to those measured for their homogeneous complexes. Covalently supported scorpionates are expected to further extend the utility of this rich ligand class in areas of heterogeneous catalysis and metal-protein interactions.