Steric control of the reactivity of moderately hindered tris(pyrazolyl)borates with copper(II) salts

Abstract

The co-ordination chemistry of the anions tris(3-phenylpyrazolyl)borate ([Tp^Ph]⁻), tris(3-cyclohexylpyrazolyl)borate ([Tp^Cy]⁻) and tris(3,5-diphenylpyrazolyl)borate ([Tp^Ph₂]⁻) with Cu(O₂CMe)₂·H₂O, CuCl₂ and Cu(BF₄)₂·6H₂O has been investigated. The complex [Cu(O₂CMe)(Tp^Ph)] (1) transforms in solution to the B–N cleavage product [Cu(O₂CMe)(Hpz^Ph)(Tp^Ph)] (2), whose crystal structure shows two square pyramidal Cu(II) centres in the asymmetric unit, each with a monodentate acetate ligand. The analogous complex [Cu(O₂CMe)(Tp^Cy)] (3) does not undergo this reaction. Reaction of K[Tp^Cy] or K[Tp^Ph₂] with one equivalent of CuCl₂ in CH₂Cl₂ yields mixtures of [CuCl(Tp^R)] (R = Cy, Ph₂) and [CuCl(Hpz^R)(Tp^R)] (R = Cy, 5; R = Ph₂, 6). Reaction of Cu(BF₄)₂·6H₂O with one equivalent of K[Tp^Ph] in CH₂Cl₂ gives [Cu(Hpz^Ph)₄](BF₄)₂ (7) in low yield as the only isolable product. An identical reaction with K[Tp^Cy] affords [Cu(Hpz^Cy)₂(Tp^Cy)]BF₄ (8) in moderate yield. The single crystal structure of 8·CHCl₃ contains a square pyramidal complex with two N–H⋯FBF₃ and one Cl₃C–H⋯FBF₃ hydrogen bonds within each formula unit. Complexation of CuCl₂ by two equivalents of K[Tp^Ph] in MeOH affords [Cu(Tp^Ph)₂] (9) in high yield. In contrast, identical reactions employing K[Tp^Cy] or K[Tp^Ph₂] yield [Cu(pz^Cy)(Hpz^Cy)(Tp^Cy)] (10) or 6 as the major products. The single crystal structure of 10 shows a square pyramidal Cu(II) centre with metric parameters very similar to 8; although not crystallographically located, the presence of a N–H⋯N hydrogen bond between the Hpz^Cy and [pz^Cy]⁻ ligands can be inferred from the close approach of the pz^Cy pyrrolic N atoms. All complexes were characterised by FAB mass spectrometry, microanalysis, IR, UV/vis and EPR spectroscopies.