Steric and electronic factor comparisons in hydrotris(3-phenylpyrazolyl)borate nickel(II) aryloxides

Abstract

Hydrotris(pyrazolyl)borate (Tp) ligands, also known as scorpionates, are potent tridentate donors that effectively bind metal ions in a face-capping array. Hydrotris(3-phenylpyrazolyl)borate enforces a tetrahedral environment on Ni^II to model metalloenzymes. The syntheses and structural characterizations of a number of [hydrotris(3-phenylpyrazolyl)borato]nickel(II) aryloxides were performed to provide insight into the environment of the model active site; these compounds are chlorido[hydrotris(3-phenylpyrazolyl-κN²)borato](3-phenyl-1H-pyrazole-κN²)nickel(II) chloroform monosolvate, [Ni(C₂₇H₂₂BN₆)Cl(C₉H₈N₂)]·CHCl₃, (2), [hydrotris(3-phenylpyrazolyl-κN²)borato](phenolato-κO)nickel(II), [Ni(C₂₇H₂₂BN₆)(C₆H₅O)], (3), (2,6-dimethylphenolato-κO)[hydrotris(3-phenylpyrazolyl-κN²)borato]nickel(II) [Ni(C₂₇H₂₂BN₆)(C₈H₉O)], (4), (4-tert-butylphenolato-κO)[hydrotris(3-phenylpyrazolyl-κN²)borato]nickel(II), [Ni(C₂₇H₂₂BN₆)(C₁₀H₁₃O)], (5), and [hydrotris(3-phenylpyrazolyl-κN²)borato](phenolato-κO)(tetrahydrofuran-κO)nickel(II) tetrahydrofuran monosolvate, [Ni(C₂₇H₂₂BN₆)(C₆H₅O)(C₄H₈O)]·C₄H₈O, (6). Alkyl groups, e.g. tert-butyl in (5) and methyl in (4), electronically activate the aryloxide group to intramolecular π-π stacking but can be frustrated by steric encumbrance at the interacting ring faces. The flexibility at the nickel coordination site, afforded by the uncrowded B atom of the Tp^Ph ligand, allows tetrahydrofuran coordination in the phenolate compound.

Keywords: aryloxide; crystal structure; nickel; scorpionates; steric; tris(pyrazolyl)borate; π–π stacking.