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

Single-scorpionates of nickel(II), Tp(R)NiX or Tpm(R)NiX, are kinetic products whose preparation has generally required considerable steric constraints on the ligands (i.e., R = phenyl, terf-butyl, or isopropyl) to prevent formation of intractable two-ligand products like (Tp(R))(2)Ni. 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 TpmNiX(2)(OH(2))(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*NiCl(2)(OH(2))(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). TpmNiX(2)(OH(2))(n) are kinetic products, and although they are stable indefinitely in the solid state, they readily convert to the thermodynamic product (Tpm)(2)Ni(2+) in solution over the course of several hours at room temperature and in a matter of minutes at 100 degrees C, The new nitromethane/NiX(4)(2-) method offers an alternative route to monoscorpionates of first row transition metals, for which tetrahalometallate ions are common. HOCH(2)Tpm (2,2,2-tris(pyrazolyl)ethanol) was covalently attached to polystyrene synthesis beads and found to bind nickel(II) (from NiX(4)(2-)) in a manner similar to Tpm. Solid state electronic spectra of supported-TpmNiCl(2) 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.